U.S. patent application number 10/029237 was filed with the patent office on 2002-06-27 for optical fiber splicing structure.
This patent application is currently assigned to NIPPON TELEGRAPH AND TELEPHONE CORPORATION. Invention is credited to Kanamori, Terutoshi, Mori, Atsushi, Ohishi, Yasutake, Ono, Hirotaka, Shimada, Toshiyuki, Yamada, Makoto.
Application Number | 20020080474 10/029237 |
Document ID | / |
Family ID | 27549505 |
Filed Date | 2002-06-27 |
United States Patent
Application |
20020080474 |
Kind Code |
A1 |
Ohishi, Yasutake ; et
al. |
June 27, 2002 |
OPTICAL FIBER SPLICING STRUCTURE
Abstract
A tellurite glass as a glass material of optical fiber and
optical waveguide has a composition of 0<Bi.sub.2O.sub.3
.ltoreq.20 (mole %), 0.ltoreq.Na.sub.2O.ltoreq.35 (mole %),
0.ltoreq.ZnO.ltoreq.35 (mole %), and 55.ltoreq.TeO.sub.2.ltoreq.90
(mole %). The tellurite glass allows an optical amplifier and a
laser device that have broadband and low-noise characteristics. In
a splicing structure of non silica-based optical fiber (as a first
fiber) and a silica-based optical fiber (as a second fiber),
optical axes of the first and second optical fibers are held at
different angles .theta..sub.1 and .theta..sub.2
(.theta..sub.1.noteq..th- eta..sub.2) respectively from a vertical
axis of a boundary surface between their spliced ends, and a
relationship between the angles .theta..sub.1 and .theta..sub.2
satisfies Snell's law represented by an equation of sin
.theta..sub.1/sin .theta..sub.2=n.sub.2 /n.sub.1 (where n.sub.1 is
a refractive index of the first optical fiber and n.sub.2 is a
refractive index of the second optical fiber) at the time of
splicing the first and second optical fibers.
Inventors: |
Ohishi, Yasutake; (Tokyo,
JP) ; Mori, Atsushi; (Tokyo, JP) ; Yamada,
Makoto; (Tokyo, JP) ; Ono, Hirotaka; (Tokyo,
JP) ; Kanamori, Terutoshi; (Tokyo, JP) ;
Shimada, Toshiyuki; (Tokyo, JP) |
Correspondence
Address: |
VENABLE, BAETJER, HOWARD AND CIVILETTI, LLP
P.O. BOX 34385
WASHINGTON
DC
20043-9998
US
|
Assignee: |
NIPPON TELEGRAPH AND TELEPHONE
CORPORATION
19-2 Nishi-Shijuku 3-chome, Shinjuku-ku
Tokyo
JP
163-8019
|
Family ID: |
27549505 |
Appl. No.: |
10/029237 |
Filed: |
December 28, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10029237 |
Dec 28, 2001 |
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09710961 |
Nov 14, 2000 |
|
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09710961 |
Nov 14, 2000 |
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09023210 |
Feb 13, 1998 |
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Current U.S.
Class: |
359/341.5 |
Current CPC
Class: |
C03C 4/0071 20130101;
H04B 10/2971 20130101; H01S 3/0675 20130101; H01S 3/06787 20130101;
H01S 3/06708 20130101; H01S 3/1608 20130101; C03C 3/122 20130101;
G02B 6/382 20130101; G02B 6/3818 20130101; G02B 6/3652 20130101;
H01S 3/2375 20130101; C03C 13/048 20130101; G02B 6/3803 20130101;
H01S 3/06754 20130101; G02B 6/3838 20130101; H01S 3/177 20130101;
G02B 6/3822 20130101; G02B 6/3861 20130101; G02B 6/3636 20130101;
G02B 6/255 20130101; G02B 6/3806 20130101; H01S 3/06716 20130101;
H01S 3/1618 20130101 |
Class at
Publication: |
359/341.5 |
International
Class: |
H01S 003/00; H04B
010/12 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 14, 1997 |
JP |
30430/1997 |
Feb 14, 1997 |
JP |
30122/1997 |
Aug 22, 1997 |
JP |
226890/1997 |
Sep 25, 1997 |
JP |
259806/1997 |
Dec 19, 1997 |
JP |
351538/1997 |
Dec 19, 1997 |
JP |
351539/1997 |
Feb 13, 1998 |
JP |
31874/1998 |
Claims
What is claimed is:
1. A tellurite glass as a glass material for an optical fiber or an
optical waveguide, comprising: 0<Bi.sub.2O.sub.3.ltoreq.20 (mole
%); 0.ltoreq.Na.sub.2O.ltoreq.35 (mole %); 0.ltoreq.ZnO.ltoreq.35
(mole %); and 55<TeO.sub.2<90 (mole %).
2. A tellurite glass as a glass material for an optical fiber or an
optical waveguide, comprising: 1.5<Bi.sub.2O.sub.3.ltoreq.15
(mole %); 0.ltoreq.Na.sub.2O.ltoreq.35 (mole %);
0.ltoreq.ZnO.ltoreq.35 (mole %); and 55.ltoreq.TeO.sub.2.ltoreq.90
(mole %).
3. A tellurite glass as a glass material for an optical fiber or an
optical waveguide, comprising: 0<Bi.sub.2O.sub.3.ltoreq.20 (mole
%); 0.ltoreq.Li.sub.2O.ltoreq.25 (mole %); 0.ltoreq.ZnO.ltoreq.25
(mole %); and 55.ltoreq.TeO.sub.2.ltoreq.90 (mole %).
4. A tellurite glass as a glass material for an optical fiber or an
optical waveguide, comprising: 0<Bi.sub.2O.sub.3.ltoreq.20 (mole
%); 0.ltoreq.M.sub.2O.ltoreq.35 (mole %); 0.ltoreq.ZnO.ltoreq.35
(mole %); and 55.ltoreq.TeO.sub.2.ltoreq.90 (mole %), wherein the M
is at least two univalent metals selected from a group of Na, Li,
K, Rb, and Cs.
5. A tellurite glass as a glass material for an optical fiber or an
optical waveguide, comprising: 1.5<Bi.sub.2O.sub.3.ltoreq.15
(mole %); 0.ltoreq.M.sub.2O.ltoreq.35 (mole %);
0.ltoreq.ZnO.ltoreq.35 (mole %); and 55.ltoreq.TeO.sub.2.ltoreq.90
(mole %), wherein the M is at least two univalent metals selected
from a group of Na, Li, K, Rb, and Cs.
6. A tellurite glass as a glass material for an optical fiber or an
optical waveguide, comprising: 0<Bi.sub.2O.sub.3.ltoreq.20 (mole
%); 0.ltoreq.Li.sub.2O.sub.3.ltoreq.25 (mole %);
0.ltoreq.Na.sub.2O.ltoreq.15 (mole %); 0.ltoreq.ZnO.ltoreq.25 (mole
%); and 60.ltoreq.TeO.sub.2.ltoreq- .90 (mole %).
7. A tellurite glass as a glass material for an optical fiber or an
optical waveguide that contains erbium at least in a core,
consisting of a glass composition that contains
Al.sub.2O.sub.3.
8. A tellurite glass as a glass material for an optical fiber or an
optical waveguide, wherein the glass material has a composition of:
TeO.sub.2--ZnO--M.sub.2O--Bi.sub.2O.sub.3--Al.sub.2O.sub.3 where M
is at least one alkali element.
9. A tellurite glass as a glass material for an optical fiber or an
optical waveguide, comprising: 0<Bi.sub.2O.sub.3.ltoreq.10 (mole
%) 0.ltoreq.Li.sub.2O.sub.3.ltoreq.30 (mole %);
0.ltoreq.ZnO.ltoreq.4 (mole%) 70.ltoreq.TeO.sub.2.ltoreq.90 (mole
%); and 0.ltoreq.Al.sub.2O.sub.3.ltoreq.3 (mole %).
10. A tellurite glass as a glass material for an optical fiber or
an optical waveguide, comprising: 0<Bi.sub.2O.sub.3.ltoreq.15
(mole %); 0.ltoreq.Na.sub.2O.ltoreq.30 (mole %);
0.ltoreq.ZnO.ltoreq.35 (mole %);
60.ltoreq.Ti.sub.2O.sub.2.ltoreq.90 (mole %); and
0.ltoreq.Al.sub.2O.sub.- 3.ltoreq.4 (mole %).
11. A tellurite glass as claimed in any one of claims 1 to 10,
wherein a concentration of the Bi.sub.2O.sub.3 is:
4<Bi.sub.2O.sub.3<7.
12. An optical amplification medium comprised of an optical
amplifier or an optical waveguide having a core glass and a clad
glass, wherein at least one of the core glass and the clad glass is
made of the tellurite glass of any one of claims 1 to 6 or 8 to
11.
13. An optical amplification medium comprised of an optical
amplifier or an optical waveguide having a core glass and a clad
glass, wherein the core glass is made of a tellurite glass having a
composition of: 0<Bi.sub.2O.sub.3.ltoreq.20 (mole %), preferably
1.5<Bi.sub.2O.sub.3.ltoreq.15 (mole %), or more preferably
4<Bi.sub.2O.sub.3.ltoreq.7; 0.ltoreq.Na.sub.2O.ltoreq.35 (mole
%); 0.ltoreq.ZnO .ltoreq.35 (mole %); and
55.ltoreq.TeO.sub.2.ltoreq.90 (mole %), and the clad is made of a
tellurite glass having a composition selected from a group of: a
first composition including 5<Na.sub.2O<35 (mole %),
0.ltoreq.ZnO<10 mole %), and 55<TeO.sub.2<85 (mole %); a
second composition including 5<Na.sub.2O<35 (mole %),
10<ZnO.ltoreq.20 mole %), and 55<TeO.sub.2<85 (mole %);
and a third composition including 0.ltoreq.Na.sub.2O<25 (mole
%), 20<ZnO.ltoreq.30 mole %), and 55<TeO.sub.2<75 (mole
%).
14. An optical amplification medium as claimed in claim 12 or claim
13, wherein at least one of the core glass and the clad glass
contains erbium or erbium and ytterbium.
15. An optical amplification medium as claimed in any one of claims
12 to 14, wherein at least one of the core glass and the clad glass
contains at least one selected from a group consisting of boron
(B), phosphorus (P), and hydroxyl group.
16. An optical amplification medium as claimed in any one of claims
12 to 15, wherein at least one of the core glass and the clad glass
includes an element selected from a group consisting of Ce, Pr, Nd,
Sm, Tb, Gd, Eu, Dy, Ho, Tm, and Yb.
17. An optical amplification medium comprised of an optical
amplifier or an optical waveguide having a core and a clad which
are made of a glass material and at least the core contains erbium,
wherein the glass material consists of a tellurite composition that
contains Al.sub.2O.sub.3.
18. An optical amplification medium comprised of an optical
amplifier or an optical waveguide having a core and a clad which
are made of a glass material and at least the core contains erbium,
wherein the glass material consists of a tellurite composition of:
TeO.sub.2--ZnO--M.sub.2O- --Bi.sub.2O.sub.3--Al.sub.2O.sub.3 where
M is at least one alkali element.
19. An optical amplification medium as claimed in any one of claims
12 to 18, wherein a cut-off wavelength is in the range of 0.4 .mu.m
to 2.5 .mu.m.
20. A laser device comprising an optical cavity and an excitation
light source, wherein at least one of optical amplification media
in the optical cavity is the optical amplification medium of any
one of claims 12 to 19.
21. A laser device having a plurality of optical amplification
media comprised of optical fibers that contain erbium in their
cores and arranged in series, wherein each of the optical
amplification media is the optical amplification medium of any one
of claims 12 to 19.
22. A laser device having an amplification medium and an excitation
light source, wherein the amplification medium is the optical
amplification medium of any one of claims 12 to 19.
23. An optical amplifier having an optical amplification medium, an
input device that inputs an excitation light and a signal light for
pumping the optical amplification medium, wherein the optical
amplification medium is the optical amplification medium of any one
of claims 12 to 19.
24. An optical amplifier having a plurality of optical
amplification media comprised of optical fibers that contain erbium
in their cores and arranged in series, wherein each of the optical
amplification media is the optical amplification medium of any one
of claims 12 to 19.
25. An optical amplifier having a tellurite glass as an optical
amplification medium, comprising: a dispersion medium which is
placed on at least one position in the front of or at the back of
the optical amplification medium, wherein the dispersion medium
compensates for dispersion of wavelengths by a value of chromatic
dispersion that takes a plus or negative number opposite to a value
of chromatic dispersion for the optical amplification medium.
26. An optical amplifier as claimed in claim 25, wherein the
optical amplification medium is an optical waveguide made of a
tellurite glass that contains a rare-earth element or a transition
metal element.
27. An optical amplifier as claimed in claim 25 or 26, wherein the
tellurite glass consists of a composition selected from:
TeO.sub.2--ZnO--M.sub.2O--Bi.sub.2O.sub.3;
TeO.sub.2--ZnO--M.sub.2O--Bi.s- ub.2O.sub.3--Al.sub.2O.sub.3, and
TeO.sub.2--WO.sub.3--La.sub.2O.sub.3--Bi-
.sub.2O.sub.3--Al.sub.2O.sub.3 where M is at least one alkali
element.
28. An optical amplifier as claimed in any one of claims 25 to 27,
wherein the dispersion medium is one selected from an optical fiber
and a fiber-bragg-grating.
29. An optical amplifier having a plurality of stages of optical
amplification portions that include erbium-doped optical fibers as
their optical amplification media, wherein a tellurite glass
optical fiber is used as a material of the optical fiber in at
least one of the optical amplification portions except one at the
front thereof, and an optical amplification portion positioned in
front of the optical amplification portion using the tellurite
glass optical fiber is comprised of an erbium-doped optical fiber,
where a product of an erbium-doping concentration and a
fiber-length of the erbium-doped optical fiber is smaller than that
of the tellurite glass fiber.
30. An optical amplifier as claimed in claim 29, wherein the
tellurite glass consists of a composition selected from:
TeO.sub.2-ZnO-M.sub.2O-Bi.- sub.2O.sub.3; and
TeO.sub.2-ZnO-M.sub.2O-Bi.sub.203-Al.sub.2O.sub.3, where M is at
least one alkali element.
31. An optical amplifier as claimed in claim 29 or 30, wherein a
material of the optical amplification medium is one selected from a
group of a silica optical fiber, a fluoro-phosphate optical fiber,
a phosphate optical fiber, and a chalcogenide optical fiber, in
addition to the tellurite optical fiber.
32. An optical amplifier as claimed in any one of claims 29 to 31,
wherein an optical fiber material except a tellurite optical fiber
is used as at least one optical amplification portion at any given
stage up to the optical amplification portion using the tellurite
glass fiber.
33. An optical amplifier as claimed in claim 29 or 30, wherein a
product of an erbium-addition concentration and a fiber-length of
at least one optical fiber, which is positioned at any given stage
up to the optical amplification portion using the tellurite glass
fiber, is smaller than that of the tellurite optical fiber.
34. An optical amplifier using erbium-doped optical fibers as
optical amplification media, comprising at least one arrangement
configuration wherein at least two tellurite optical fibers each
having a different product of an erbium-doping concentration and a
fiber-length are arranged in series so that the tellurite optical
fiber having a smaller product of an erbium-addition concentration
and a fiber-length is placed at the front stage up to the tellurite
optical fiber having a larger product of an erbium-addition
concentration and a fiber-length.
35. An optical amplifier as claimed in claim 33, wherein the
tellurite glass consists of a composition selected from:
TeO.sub.2--ZnO--M.sub.2O--- Bi.sub.2O.sub.3; and
TeO.sub.2--ZnO--M.sub.2O--Bi.sub.2O.sub.3--Al.sub.2O.- sub.3, where
M is at least one alkali element.
36. An optical-fiber splicing structure for contacting a splicing
end surface of a first housing in which an end of a first optical
fiber is held and an splicing end surface of a second housing in
which an end of a second optical fiber is held in a state of
co-axially centering an optical axis of the first optical fiber and
an optical axis of the second optical fiber, where at least one of
the first optical fiber and the second optical fiber is a
non-silica-based optical fiber, wherein optical axes of the first
and second optical fibers are held in the first and second housings
respectively at angles .theta..sub.1 and .theta..sub.2
(.theta..sub.1.apprxeq..theta..sub.2) from a vertical axis of a
boundary surface between the splicing end surfaces, and a
relationship between the angles .theta..sub.1 and .theta..sub.2
satisfies Snell's law represented by an equation (4) at the time of
splicing the first and second optical fibers: 7 sin 1 sin 2 = n 2 n
1 ( 4 ) where n.sub.1 is a refractive index of the first optical
fiber and n.sub.2 is a refractive index of the second optical
fiber.
37. An optical fiber splicing structure as claimed in claim 36,
wherein the splicing end surface of the first optical fiber is
connected to the splicing end surface of the second optical fiber
through an optical adhesive at the time of splicing the first and
second optical fibers.
38. An optical fiber splicing structure as claimed in claim 36 or
37, wherein the splicing end surface of the first optical fiber and
the splicing end surface of the second optical fiber are kept in
absolute contact with each other at the time of splicing the first
and second optical fibers.
39. An optical fiber splicing structure as claimed in any one of
claims 36 to 38, wherein said first and second optical fibers are
non-silica-based optical fibers.
40. An optical fiber splicing structure as claimed in claim 39,
wherein said non-silica-based optical fibers are selected from Zr-
or In-based fluoride optical fibers, chalcogenide optical fibers,
and tellurite glass optical fibers.
41. An optical fiber splicing structure as claimed in claim 39,
wherein the non-silica-based optical fibers are selected from Zr-
or In-based fluoride optical fibers, chalcogenide optical fibers,
and tellurite glass optical fibers, and furthermore said
non-silica-based optical fibers are doped with a rare-earth
element.
42. An optical fiber splicing structure as claimed in any one of
claims 36 to 38, wherein the first optical fiber is a tellurite
glass optical fiber, the second optical fiber is a silica-based
optical fiber, and said angle .theta..sub.1 is of 8 or more
degrees.
43. An optical fiber splicing structure as claimed in any one of
claims 36 to 38, wherein the first optical fiber is a Zr-based
fluoride optical glass fiber, the second optical fiber is a
silica-based optical fiber, and said angle .theta..sub.1 is of 3 or
more degrees.
44. An optical fiber splicing structure as claimed in any one of
claims 36 to 38, wherein the first optical fiber is an In-based
fluoride optical glass fiber, the second optical fiber is a
silica-based optical fiber, and said angle .theta..sub.1 is of 4 or
more degrees.
45. An optical fiber splicing structure as claimed in any one of
claims 36 to 38, wherein the first optical fiber is a chalcogenide
optical glass fiber, the second optical fiber is a silica-based
optical fiber, and said angle .theta..sub.1 is of 8 or more
degrees.
46. A light source comprising: an optical amplification medium
which is one selected from a group of an erbium-doped tellurite
optical fiber and an optical waveguide; and an optical coupler
arranged on an end of the optical amplification medium, wherein at
least one terminal of the optical coupler is equipped with a
reflector.
47. A light source as claimed in claim 46: the erbium-doped
tellurite optical fiber or the optical waveguide consisting of the
tellurite glass as claimed in any one of claims 1 to 15, 17, and
19.
48. A light source as claimed in claim 46 or 47, wherein the
reflector is comprised of one selected from a group of a
dielectric-multiple-film filter and a fiber-bragg-grating.
49. An optical amplifier using an erbium-doped tellurite optical
fiber or an optical waveguide as an optical amplification medium,
comprising an optical coupler arranged on an end of the optical
amplification medium, wherein at least one terminal of the optical
coupler is equipped with a reflector.
50. An optical amplifier as claimed in claim 49, wherein the
erbium-doped tellurite optical fiber or the optical waveguide
consisting of the tellurite glass as claimed in any one of claims 1
to 15, 17, and 19.
51. An optical amplifier as claimed in claim 49 or 50, wherein the
reflector is comprised of one selected from a group of a
dielectric-multiple-film filter and a fiber-bragg-grating.
Description
[0001] This application is based on Patent Application No.
030,430/1997 filed in Feb. 14, 1997, No. 030,122/1997 filed in Feb.
14, 1997, No. 226,890/1997 filed in Aug. 22, 1997, No. 259,806/1997
filed in Sep. 25, 1997, No. 351,538/1997 filed in Dec. 19, 1997,
and No. 351,539/1997 filed in Dec. 19, 1997 in Japan, the content
of which is incorporated hereinto by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a tellurite glass as a
glass material for an optical fiber and an optical waveguide, and
in particular a broadband optical amplification medium using the
tellurite glass which is capable of working even at wavelengths of
1.5 .mu.m to 1.7 .mu.m. The present invention also relates to a
broadband optical amplifier and a laser device using the broad band
optical amplification medium. Furthermore, the present invention
relates to a method of splicing a non-silica-based optical fiber
and a silica-based optical fiber reliably with the characteristics
of low fiber-loss and low reflection.
[0004] 2. Description of the Related Art
[0005] The technology of wavelength division multiplexing (WDM) has
been studied and developed for expanding transmission volume of
optical communication systems and functionally improving such
systems. The WDM is responsible for combining a plurality of
optical signals and transmitting a combined signal through a single
optical fiber. In addition, the WDM is reversibly responsible for
dividing a combined signal passing through a single optical fiber
into a plurality of optical signals for every wavelength. This kind
of transmitting technology requires a transit amplification just as
is the case with the conventional one according to the distance of
transmitting a plurality of optical signals of different
wavelengths through a single optical fiber. Thus, the need for an
optical amplifier having a broad amplification waveband arises from
the demands for increasing the optical signal's wavelength and the
transmission volume. The wavelengths of 1.61 .mu.m to 1.66 .mu.m
have been considered as appropriate for conserving and monitoring
an optical system, so that it is desirable to develop an optical
source and an optical amplifier for that system.
[0006] In recent years, there has been considerable work devoted to
research and development on optical fiber amplifiers that comprise
optical fibers as optical amplification materials, such as erbium
(Er) doped optical fiber amplifiers (EDFAs), with increasing
applications to the field of optical communication system. The EDFA
works at a wavelength of 1.5 .mu.m where a loss of silica-based
optical fiber decreases to a minimum, and also it is known for its
excellent characteristics of high gain of 30 dB or more, low noise,
broad gain-bandwidth, no dependence on polarized waves, and high
saturation power.
[0007] As described above, one of the remarkable facts to be
required of applying the above EDFA to the WDM transmission is that
the amplification waveband is broad. Up to now, a fluoride EDFA
using a fluoride glass as a host of an erbium-doped optical fiber
amplifier has been developed as a broad amplification band
EDFA.
[0008] In U.S. Pat. Nos. 3,836,868, 3,836,871, and 3,883,357,
Cooley et al. discloses the possibility of laser oscillation to be
caused by tellurite glass containing an rare earth element. In this
case, however, Cooley et al. have no idea of forming tellurite
glass into an optical fiber because there is no description
concerned about the adjustment of refractive index and the thermal
stability of tellurite glass to be required for that formation.
[0009] In U.S. Pat. No. 5,251,062, Snitzer et al. insists that
tellurite glass play an important role in extending the EDFA's
amplification band and it should be formed into a fiber which is
absolutely essential to induction of an optical amplification.
Thus, they disclose the allowable percent ranges of ingredients in
tellurite-glass composition in a concretive manner. The
tellurite-glass composition includes a rare earth element as an
optically active element and can be formed into a fiber. More
specifically, the tellurite-glass composition of Snitzer et al. is
a ternary composition comprising TeO.sub.2, R.sub.2O, and QO where
R denotes a monovalent metal except Li and Q denotes a divalent
metal. The reason why Li is excluded as the monovalent metal is
that Li depresses thermal stability of the tellurite-glass
composition.
[0010] In U.S. Pat. No. 5,251,062, furthermore, Snitzer et al. make
a comparative study of fluorescence erbium spectra of silica and
tellurite glass and find that the tellurite glass shows a broader
erbium spectrum compared with that of the silica glass. They
conclude that the ternary tellurite glass composition may allow a
broadband amplification of EDFA and an optically active material
such as praseodymium or neodymium may be added in that composition
for inducing an optical amplification. In this patent document,
however, there is no concrete description of the properties of
gain, pump wavelength, signal wavelength, and the like which is
important evidence to show that the optical amplification was
actually down. In other words, U.S. Pat. No. 5,251,062 merely
indicate the percent ranges of ingredients of ternary tellurite
glass composition that can be used in an optical fiber.
[0011] Furthermore, Snitzer et al. show that thermal and optical
features of various kinds of tellurite glass except of those
described in U.S. Pat. No. 5,251,062 in a technical literature
(Wang et al., Optical Materials, vol. 3 pages 187-203, 1994;
hereinafter simply referred as "Optical Materials"). In this
literature, however, there is also no concrete description of
optical amplification and laser oscillation.
[0012] In another technical literature (J. S. Wang et al., Optics
Letters, vol. 19 pages 1448-1449, 1994; hereinafter simply referred
as "Optics Letters") published in right after the literature
mentioned above, Snitzer et al. show the laser oscillation for the
first time caused by using a single mode optical fiber of
neodymium-doped tellurite glass. The single mode fiber comprises a
core having the composition of 76.9 % TeO.sub.2 -6.0 % Na.sub.2O
-15.5 % ZnO -1.5 % Bi.sub.2O.sub.3 -0.1 % Nd.sub.2O.sub.3 and a
clad having the composition of 75 % TeO.sub.2 -5.0 % Na.sub.2O
-20.0 % ZnO and allows 1,061 nm laser oscillation by 81 nm pumping.
In this literature, there is no description of a fiber loss. In
Optical Materials, on the other hand, there is a description of
which the loss for an optical fiber having a core composition of
Nd.sub.2O.sub.3-77 % TeO.sub.2 -6.0 % Na.sub.2O -15.5% ZnO -1.5 %
Bi.sub.2O.sub.3 and a clad composition of 75 % TeO.sub.2 -5.0 %
Na.sub.2O -20.0 % ZnO (it is deemed to be almost the same
composition as that of Optics Letters) is 1500 dB/km at a
wavelength of 1.55 .mu.m (see FIG. 1 that illustrates a comparison
between .sup.4I.sub.13/2 to .sup.4I.sub.15/2 Er emission in
tellurite glass and .sup.4I.sub.13/2 to .sup.4I.sub.15/2 Er.sup.3+
emission in fluoride glass). The core composition of this optical
fiber is different from that of a ternary composition disclosed in
U.S. Pat. No. 5,251,062 because the former includes
Bi.sub.2O.sub.3. It is noted that there is no description or teach
of thermal stability of Bi.sub.2O.sub.3-contained glass composition
in the descriptions of Optics Letters, Optical Materials, and U.S.
Pat. No. 5,251,062 mentioned above.
[0013] However, the fluoride based EDFA has an amplification band
of about 30 nm which is not enough to extend an amplification band
of optical fiber amplifier for the purpose of extending the band of
WDM.
[0014] As described above, tellurite glass shows a comparatively
broader fluorescence spectral band width, so that there is a
possibility to extend the amplification band if the EDFA uses the
tellurite glass as its host. In addition, the possibility of
producing a ternary system optical fiber using the composition of
TeO.sub.2, R.sub.2O, and QO (wherein R is a univalent metal except
Li and Q is a divalent atom) has been realized, so that laser
oscillation at a wavelength of 1061 nm by a neodymium-doped single
mode optical fiber mainly comprising the above composition has been
attained. In contrast, EDFA using tellurite glass is not yet
realized. Therefore, we will describe the challenge to realize a
tellurite-based EDFA in the following.
[0015] First, the difference between the objective EDFA and the
neodymium-doped fiber laser (i.e., the difference between 1.5 .mu.m
band emission of erbium and 1.06 .mu.m band emission of neodymium
in glass) should be described in detail.
[0016] An optical transition of the objective EDFA is shown in FIG.
2 where three different energy levels are indicated by Level 1,
Level 2, and Level 3, respectively. For attaining an objective
induced emission from Level 2 to Level 1, a population inversion
between Level 1 and Level 2 is done by pumping from Level 1 to
Level 3 and then relaxing from Level 3 to Level 2. This kind of the
induced emission can be referred as a three-level system.
[0017] In the case of the neodymium, as shown in FIG. 3, a
four-level system can be defined that a final level of the induced
emission is not a ground level but a first level (Level 1) which is
higher than the ground level. Comparing the three-level system with
the four-level system, the former is hard to attain the population
inversion so that an ending level of the induced emission is in a
ground state. Accordingly, the three-level system EDFA requires
enhanced optically pumping light intensity, and also the fiber
itself should be of having the properties of low-loss and high
.DELTA.n. In this case, the high .DELTA.n is for effective
optically pumping.
[0018] Secondly, we will briefly described that an amplification
band cannot be extended even if it is possible to perform an
optical amplification when a transmission loss of fiber is
large.
[0019] Wavelength dependencies of the silica-based EDFA and the
tellurite-based EDFA are illustrated in FIG. 4. As shown in the
figure, it can be expected that the tellurite-based EDFA will
attain a broadband optical amplification broader than that of the
silica-based EDFA. Comparing with the silica-based glass and the
non-silica-based glass, a transmission loss at a communication
wavelength of the latter is substantially larger than that of the
former. In the optical fiber amplifier, therefore, the loss leads
to a substantial decrease in gain.
[0020] As schematically shown in FIG. 5, if the loss is
comparatively small, the amplification band of tellurite glass is
close to the one shown in FIG. 4. If the loss is comparatively
large, on the other hand, the amplification band of tellurite glass
is narrowed.
[0021] In recent technical investigations on WDM transmission, by
the way, it has been made attempts to speed up transmission through
one channel for increasing transmission capacity. To solve this
problem, it is necessary to optimize the chromatic dispersion
characteristics of the Er-doped optical fiber. Up to now, however,
no attention have been given to that characteristics.
[0022] For the tellurite glass, a wavelength at which a chromatic
dispersion value takes zero is in the wavelengths longer than 2
.mu.m. In the case of a high NA (Numerical Aperture) fiber to be
used in EDFA, a chromatic dispersion value is generally -100
ps/km/nm or less at 1.55 .mu.m band. Thus, a chromatic dispersion
of a short optical fiber of about 10 m in length also takes the
large value of -1 ps/nm or less.
[0023] For the use of tellurite EDFA in long-distance and
high-speed WDM transmission, therefore, it is need to bring the
chromatic dispersion close to zero as far as possible. As described
above, however, as the material dispersion value of tellurite glass
takes the value of zero at wavelengths of 2 .mu.m and over.
Therefore, the tellurite-based optical fiber cannot utilize the
technique adopted in the silica-based optical fiber that brings the
chromatic dispersion value at 1.55 .mu.m band close to zero by
optimizing the construction parameters of the fiber.
[0024] Furthermore, the tellurite-based optical fiber can be used
as a host of praseodymium (Pr) for 1.3 .mu.m band amplification. As
described above, however, the tellurite-bade optical fiber has a
large chromatic dispersion value as the absolute value. In the case
of amplifying a high-speed optical signal by using the
tellurite-based optical fiber, a distortion of pulse wavelength can
be induced and thus the chromatic dispersion value should be
corrected for. If not, the use of tellurite glass in an optical
communication system falls into difficulties.
[0025] Next, an optical-fiber splicing between a non-silica-based
optical fiber and a silica-based optical fiber will be described
below.
[0026] For using the above non-silica-based topical fiber such as a
tellurite optical fiber as an optical amplification or nonlinear
optical fiber, there is a necessity to connect to a silica-based
optical fiber to form the junction between these fibers with
low-loss and low reflection. However, these fibers have their own
core refractive indexes which are different from each other. If
these fibers are connected together as shown in FIGS. 6 and 7, a
residual reflection can be observed so that the junction
appropriately adaptable to practical use cannot be implemented. In
FIGS. 6 and 7, reference numeral 1 denotes a non-silica-based
optical fiber, 2 denotes a silica-based optical fiber, 5 denotes an
optical binder, and 6 denotes a binder. In FIG. 6, furthermore,
there is no optical binder applied on a boundary surface between
the fibers. As shown in FIG. 8, therefore, the existence of
residual reflection between the silica-based optical fibers 2a, 2b
and the non-silica-based optical fiber 1 degrades the quality of
signal because of a ghost (which acts as noise) due to a reflected
signal on the connected ends of the fibers. Therefore, the
connected portion between those fibers require -60 dB or over as a
residual reflection factor for an optical amplifier (see Takei et
al. "Optical Amplifier Module", Okidenki Kaihatu, vol. 64, No. 1,
pp 63-66, 1997). For example, a zirconium-doped fluoride fiber, an
indium-doped fluoride fiber, chalcogenide glass fiber (i.e., glass
composition: As-S), and a tellurite glass fiber have their own
core's refractive indexes of 1.4 to 1.5, 1.45 to 1.65 and 2.4 and
2.1, respectively, depending on the variations in their glass
compositions. If one of those fibers is connected to a silica based
optical fiber (core's refractive index is about 1.50 or less), a
return loss R can be obtained by the formula (2) described below.
In this case, the unit of R is dB and the residual reflective index
is expressed in a negative form while the return loss is expressed
in a positive form as an absolute value of the residual reflective
index. The return loss can be obtained by the equation (1) below. 1
R = | 10 log { [ ( n NS - n S ) ( n NS + n S ) ] 2 } | ( d B ) ( 1
)
[0027] where n.sub.NS and ns are core's refractive indexes of
silica and non-silica optical fibers, respectively. The return loss
between the silica-based optical fiber and the zirconium-doped
fluoride optical fiber, indium-doped optical fluoride fiber,
chalcogenide glass fiber (i.e., glass composition: As-S), or
tellurite glass fiber is 35 dB or more, 26 dB or more, 13 dB, or 16
dB, respectively. In the case of Zr-based and In-based fluoride
optical fiber, the return loss can be increased (while the residual
reflection coefficient can be decreased) by bringing their
refractive indexes to that of the silica-based optical fiber's core
by modifying their glass compositions, respectively. However, the
modification of glass composition leads to the formation of
practical optical fiber under the constraint that the glass
composition should be precisely formulated in the process of
forming a fiber in a manner which is consistent with an ideal glass
composition for the process of forming a low-loss fiber). A
coupling between the silica-based optical fiber and the
non-silica-based optical fiber has the following problems. That is,
conventional fusion splicing procedures cannot be applied because
of the difference in softening temperatures of both fibers (i.e.,
1,400.degree. C. for the silica-based optical fiber and less than
500.degree. C. for the non-silica-based one); the conventional
optical connector coupling technologies cannot be applied because
there is no appropriate coupling method for the non-silica-based
optical fiber; and so on. Thus, a general coupling method for
coupling the Zr-based or In-based optical fiber to the silica-based
optical fiber without depending on its glass composition has been
demanded. In addition, a general coupling method for reliably
coupling the chalcogenide glass optical fiber or the tellurite
optical fiber to the silica-based optical fiber with a low-loss and
low-reflection.
[0028] One of the conventional coupling technologies for solving
such problems, Japanese Patent Application Laying-open No. 6-27343,
is illustrated in FIGS. 9 and 10. In this technology, a
non-silica-based optical fiber 1 and a silica-based optical fiber 2
are held in housings 7a and 7b, respectively. The fibers 1, 2 are
positioned in their respective V-shaped grooves on substrates 8a,
8b and fixed on their respective housings 7a, 7b by means of
bonding agents 10a, 10b and fiber-fixing plates 9a, 9b. In
addition, there is a dielectric film 18 applied on a coupling end
of one of the housings for preventing a reflection to be generated
by coupling the fibers together. The coupling between the
non-silica-based optical fiber 1 and the silica-based optical fiber
2 are carried out by using an optical bonding agent 5 made of
ultraviolet-curing region after adjusting the relative positions of
the housings 7a, 7b so as to match their optical axes. At this
moment, the coupling end of the housing 7a is perpendicular to the
optical axis of the non-silica-based optical fiber and also the
coupling end of the housing 7b is perpendicular to the optical axis
of the silica-based optical fiber, so that if the reflection of
light is occurred at a boundary surface of the coupling the
reflected light returns in the reverse direction, resulting in a
falloff in the return loss. Accordingly, the conventional
technology uses the dielectric film 18 to reduce the reflection
from the boundary surface of the coupling. However, the
conventional coupling requires a precision adjustment to a
refractive index of the optical biding agent 5 and a refractive
index and thickness of the dielectric film 18. That is, their
refractive indexes must satisfy the following equations (2) and (3)
if a core's refractive index of the non-silica-based optical fiber
1 is n.sub.1 and a core's refractive index of the silica-based
optical fiber 2 is n.sub.2. A refractive index of the optical
binding agent 5 is adjusted to n.sub.1, while a refractive index
and a thickness of the dielectric film 18 is adjusted to n.sub.1
and t.sub.f, respectively, so as to satisfy the following equations
(2), (3).
n.sub.f={square root}{square root over (n.sub.1.multidot.n.sub.2)}
(2) 2 t f = 4 n 1 n 2 ( 3 )
[0029] wherein .lambda. is a signal wavelength (i.e., the
wavelength to be used).
[0030] In the conventional technology, as described above, there is
the need for precisely adjusting a refractive index of the optical
binder 5 and a refractive index and thickness of the dielectric
film 18 for constructing a coupling portion with the properties of
low-reflection and low-loss by using the dielectric film 18. It
means that the precise adjustments leads to difficulties in
implementing a coupling between the fibers favorably with an
improvement in yield.
[0031] A process of coupling two different optical fibers in
accordance with another conventional technology, as shown in FIG.
11, comprises the steps of: holding an optical fiber 9a on a
housing 7b and also holding an optical fiber 9a on a housing 7b;
positioning these housings 7a, 7b in their right places so that a
coupling end of the housing 7a that holds the optical fiber 19a and
a coupling end of housing 7b that holds the optical fiber 19b are
positioned with a .theta.-degree slant with respect to a direction
perpendicular to the optical axes of the optical fibers 19a, 19b;
and connecting the housing 7a and the housing 7b together after the
positioning of the housings so as to concentrically adjust the
optical axes in a straight line. The process is a so-called slant
coupling method for realizing the coupling with low-reflection and
low fiber-loss. However, this process is only applied to the fibers
when their core refractive indexes are almost the same, so that it
cannot be applied to the coupling between the non-silica-based
optical fiber and the silica-based optical fiber which have
different core refractive indexes with respect to each other.
SUMMARY OF THE INVENTION
[0032] A first object of the present invention is to provide a
tellurite glass fiber of high .DELTA.n and low fiber-loss.
[0033] A second object of the present invention is to provide a
tellurite glass fiber that includes the capability of realizing a
broadband EDFA doped with an optically active rare earth element,
which cannot be realized by the conventional glass
compositions.
[0034] A third object of the present invention is to provide a
broadband optical amplification medium that includes the capability
of acting at wavelengths, especially from 1.5 .mu.m to 1.7 .mu.m,
and also to provide an optical amplifier and a laser device which
use such a medium and act at wavelengths in a broad range and have
low-noise figures.
[0035] A fourth object of the present invention is to provide a
general and practical technique of reliably coupling a
non-silica-based optical fiber and a silica-based optical fiber or
coupling optical fibers having different core refractive indexes
with low fiber-loss and low reflection.
[0036] In a first aspect of the present invention, there is
provided a tellurite glass as a glass material for an optical fiber
or an optical waveguide, comprising:
[0037] 0<Bi.sub.2O.sub.3.ltoreq.20 (mole %);
[0038] 0.ltoreq.Na.sub.2O.ltoreq.35 (mole %);
[0039] 0.ltoreq.ZnO.ltoreq.35 (mole %); and
[0040] 55.ltoreq.TeO.sub.2.ltoreq.90 (mole %)
[0041] In a second aspect of the present invention, there is
provided a tellurite glass as a glass material for an optical fiber
or an optical waveguide, comprising:
[0042] 1.5<Bi.sub.2O.sub.3.ltoreq.15 (mole %);
[0043] 0.ltoreq.Na.sub.2O.ltoreq.35 (mole %);
[0044] 0.ltoreq.ZnO .ltoreq.35 (mole %); and
[0045] 55.ltoreq.TeO.sub.2.ltoreq.90 (mole %).
[0046] In a third aspect of the present invention, there is
provided a tellurite glass as a glass material for an optical fiber
or an optical waveguide, comprising:
[0047] 0<Bi.sub.2O.sub.3.ltoreq.20 (mole %);
[0048] 0.ltoreq.Li.sub.2O.ltoreq.25 (mole %);
[0049] 0.ltoreq.ZnO.ltoreq.25 (mole %); and
[0050] 55.ltoreq.TeO.sub.2<90 (mole %).
[0051] In a fourth aspect of the present invention, there is
provided a tellurite glass as a glass material for an optical fiber
or an optical waveguide, comprising:
[0052] 0<Bi.sub.2O.sub.3.ltoreq.20 (mole %);
[0053] 0.ltoreq.M.sub.2O.ltoreq.35 (mole %);
[0054] 0.ltoreq.ZnO.ltoreq.35 (mole %); and
[0055] 55.ltoreq.TeO.sub.2.ltoreq.90 (mole %), wherein
[0056] the M is at least two univalent metals selected from a group
of Na, Li, K, Rb, and Cs.
[0057] In a fifth aspect of the present invention, there is
provided a tellurite glass as a glass material for an optical fiber
or an optical waveguide, comprising:
[0058] 1.5<Bi.sub.2O.sub.3.ltoreq.15 (mole %);
[0059] 0.ltoreq.M.sub.2O.ltoreq.35 (mole %);
[0060] 0.ltoreq.ZnO.ltoreq.35 (mole %); and
[0061] 55.ltoreq.TeO.sub.2<90 (mole %), wherein
[0062] the M is at least two univalent metals selected from a group
of Na, Li, K, Rb, and Cs.
[0063] In a sixth aspect of the present invention, there is
provided a tellurite glass as a glass material for an optical fiber
or an optical waveguide, comprising:
[0064] 0<Bi.sub.2O.sub.3.ltoreq.20 (mole %);
[0065] 0<Li.sub.2O.sub.3.ltoreq.25 (mole %);
[0066] 0.ltoreq.Na.sub.2O<15 (mole %);
[0067] 0.ltoreq.ZnO<25 (mole %); and
[0068] 60.ltoreq.TeO.sub.2<90 (mole %).
[0069] In a seventh aspect of the present invention, there is
provided a tellurite glass as a glass material for an optical fiber
or an optical waveguide that contains erbium at least in a core,
consisting of a glass composition that contains
Al.sub.2O.sub.3.
[0070] In an eighth aspect of the present invention, there is
provided a tellurite glass as a glass material for an optical fiber
or an optical waveguide, wherein
[0071] the glass material has a composition of:
[0072] TeO.sub.2--ZnO--M.sub.2O--Bi.sub.2O.sub.3--Al.sub.2O.sub.3
where M is at least one alkali element.
[0073] In a ninth aspect of the present invention, there is
provided a tellurite glass as a glass material for an optical fiber
or an optical waveguide, comprising:
[0074] 0<Bi.sub.2O.sub.3.ltoreq.10 (mole %);
[0075] 0.ltoreq.Li.sub.2O.sub.3.ltoreq.30 (mole %);
[0076] 0.ltoreq.ZnO.ltoreq.4 (mole %);
[0077] 70.ltoreq.TeO.sub.2<90 (mole %); and
[0078] 0.ltoreq.Al.sub.2O.sub.3.ltoreq.3 (mole %).
[0079] In a tenth aspect of the present invention, there is
provided a tellurite glass as a glass material for an optical fiber
or an optical waveguide, comprising:
[0080] 0<Bi.sub.2O.sub.3.ltoreq.15 (mole %);
[0081] 0.ltoreq.Na.sub.2O.ltoreq.30 (mole %);
[0082] 0.ltoreq.ZnO.ltoreq.35 (mole %);
[0083] 60.ltoreq.TeO.sub.2.ltoreq.90 (mole %); and
[0084] 0.ltoreq.Al.sub.2O.sub.3.ltoreq.4 (mole %).
[0085] Here, a concentration of the Bi.sub.2O.sub.3 may be:
[0086] 4<Bi.sub.2O.sub.3<7.
[0087] In an eleventh aspect of the present invention, there is
provided an optical amplification medium comprised of an optical
amplifier or an optical waveguide having a core glass and a clad
glass, wherein
[0088] at least one of the core glass and the clad glass is made of
the tellurite glass of one of the novel tellurite glasses described
above.
[0089] In a twelfth aspect of the present invention, there is
provided an optical amplification medium comprised of an optical
amplifier or an optical waveguide having a core glass and a clad
glass, wherein
[0090] the core glass is made of a tellurite glass having a
composition of:
[0091] 0<Bi.sub.2O.sub.3.ltoreq.20 (mole %), preferably
1.5<Bi.sub.2O.sub.3.ltoreq.15 (mole %), or more preferably
4<Bi.sub.2O.sub.3<7;
[0092] 0.ltoreq.Na.sub.2O.ltoreq.35 (mole %);
[0093] 0.ltoreq.ZnO.ltoreq.35 (mole %); and
[0094] 55.ltoreq.TeO.sub.2.ltoreq.90 (mole %), and
[0095] the clad is made of a tellurite glass having a composition
selected from a group of:
[0096] a first composition including
[0097] 5<Na.sub.2O<35 (mole %),
[0098] 0.ltoreq.ZnO<10 mole %), and
[0099] 55<TeO.sub.2<85 (mole %);
[0100] a second composition including
[0101] 5<Na.sub.2O<35 (mole %),
[0102] 10<ZnO.ltoreq.20 mole %), and
[0103] 55<TeO.sub.2<85 (mole %); and
[0104] a third composition including
[0105] 0.ltoreq.Na.sub.2O<25 (mole %),
[0106] 20<ZnO.ltoreq.30 mole %), and
[0107] 55<TeO.sub.2<75 (mole %).
[0108] Here, at least one of the core glass and the clad glass may
contain erbium or erbium and ytterbium.
[0109] At least one of the core glass and the clad glass may
contain at least one selected from a group consisting of boron (B),
phosphorus (P), and hydroxyl group.
[0110] At least one of the core glass and the clad glass may
include an element selected from a group consisting of Ce, Pr, Nd,
Sm, Tb, Gd, Eu, Dy, Ho, Tm, and Yb.
[0111] In a thirteenth aspect of the present invention, there is
provided an optical amplification medium comprised of an optical
amplifier or an optical waveguide having a core and a clad which
are made of a glass material and at least the core contains erbium,
wherein
[0112] the glass material consists of a tellurite composition that
contains Al.sub.2O.sub.3.
[0113] In a fourteenth aspect of the present invention, there is
provided an optical amplification medium comprised of an optical
amplifier or an optical waveguide having a core and a clad which
are made of a glass material and at least the core contains erbium,
wherein
[0114] the glass material consists of a tellurite composition
of:
[0115]
TeO.sub.2--ZnO--M.sub.2O--Bi.sub.2O.sub.3--Al.sub.2O.sub.3
[0116] where
[0117] M is at least one alkali element.
[0118] Here, a cut-off wavelength may be in the range of 0.4 .mu.m
to 2.5 .mu.m.
[0119] In a fifteenth aspect of the present invention, there is
provided a laser device comprising an optical cavity and an
excitation light source, wherein
[0120] at least one of optical amplification media in the optical
cavity is one of the novel optical amplification media described
above.
[0121] In a sixteenth aspect of the present invention, there is
provided a laser device having a plurality of optical amplification
media comprised of optical fibers that contain erbium in their
cores and arranged in series, wherein
[0122] each of the optical amplification media is one of the novel
optical amplification media described above.
[0123] In a seventeenth aspect of the present invention, there is
provided a laser device having an amplification medium and an
excitation light source, wherein
[0124] the amplification medium is one of the novel optical
amplification media described above.
[0125] In an eighteenth aspect of the present invention, there is
provided an optical amplifier having an optical amplification
medium, an input device that inputs an excitation light and a
signal light for pumping the optical amplification medium,
wherein
[0126] the optical amplification medium is one of the novel optical
amplification media described above.
[0127] In a nineteenth aspect of the present invention, there is
provided an optical amplifier having a plurality of optical
amplification media comprised of optical fibers that contain erbium
in their cores and arranged in series, wherein
[0128] each of the optical amplification media is one of the novel
optical amplification media described above.
[0129] In a twentieth aspect of the present invention, there is
provided an optical amplifier having a tellurite glass as an
optical amplification medium, comprising:
[0130] a dispersion medium which is placed on at least one position
in the front of or at the back of the optical amplification medium,
wherein
[0131] the dispersion medium compensates for dispersion of
wavelengths by a value of chromatic dispersion that takes a plus or
negative number opposite to a value of chromatic dispersion for the
optical amplification medium.
[0132] Here, the optical amplification medium may be an optical
waveguide made of a tellurite glass that contains a rare-earth
element or a transition metal element.
[0133] The tellurite glass may consist of a composition selected
from:
[0134] TeO.sub.2--ZnO--M.sub.2O--Bi.sub.2O.sub.3;
[0135] TeO.sub.2--ZnO-M.sub.2O--Bi.sub.2O.sub.3--Al.sub.2O.sub.3,
and
[0136]
TeO.sub.2--WO.sub.3--La.sub.2O.sub.3--Bi.sub.2O.sub.3--Al.sub.2O.su-
b.3
[0137] where M is at least one alkali element.
[0138] The dispersion medium may be one selected from an optical
fiber and a fiber-bragg-grating.
[0139] In a twenty-first aspect of the present invention, there is
provided an optical amplifier having a plurality of stages of
optical amplification portions that include erbium-doped optical
fibers as their optical amplification media, wherein
[0140] a tellurite glass optical fiber is used as a material of the
optical fiber in at least one of the optical amplification portions
except one at the front thereof, and
[0141] an optical amplification portion positioned in front of the
optical amplification portion using the tellurite glass optical
fiber is comprised of an erbium-doped optical fiber, where
[0142] a product of an erbium-doping concentration and a
fiber-length of the erbium-doped optical fiber is smaller than that
of the tellurite glass fiber.
[0143] Here, the tellurite glass may consist of a composition
selected from:
[0144] TeO.sub.2--ZnO--M.sub.2O--Bi.sub.2O.sub.3; and
[0145]
TeO.sub.2--ZnO--M.sub.2O--Bi.sub.2O.sub.3--Al.sub.2O.sub.3,
[0146] where M is at least one alkali element.
[0147] A material of the optical amplification medium may be one
selected from a group of a silica optical fiber, a fluoro-phosphate
optical fiber, a phosphate optical fiber, and a calcogenide optical
fiber, in addition to the tellurite optical fiber.
[0148] An optical fiber material except a tellurite optical fiber
may be used as at least one optical amplification portion at any
given stage up to the optical amplification portion using the
tellurite glass fiber.
[0149] A product of an erbium-addition concentration and a
fiber-length of at least one optical fiber, which is positioned at
any given stage up to the optical amplification portion using the
tellurite glass fiber, may be smaller than that of the tellurite
optical fiber.
[0150] In a twenty-second aspect of the present invention, there is
provided an optical amplifier using erbium-doped optical fibers as
optical amplification media, comprising at least one arrangement
configuration wherein
[0151] at least two tellurite optical fibers each having a
different product of an erbium-doping concentration and a
fiber-length are arranged in series so that the tellurite optical
fiber having a smaller product of an erbium-addition concentration
and a fiber-length is placed at the front stage up to the tellurite
optical fiber having a larger product of an erbium-addition
concentration and a fiber-length.
[0152] Here, the tellurite glass may consist of a composition
selected from:
[0153] TeO.sub.2--ZnO--M.sub.2O--Bi.sub.2O.sub.3; and
[0154]
TeO.sub.2--ZnO--M.sub.2O--Bi.sub.2O.sub.3--Al.sub.2O.sub.3,
[0155] where M is at least one alkali element.
[0156] In a twenty-third aspect of the present invention, there is
provided an optical-fiber splicing structure for contacting a
splicing end surface of a first housing in which an end of a first
optical fiber is held and an splicing end surface of a second
housing in which an end of a second optical fiber is held in a
state of co-axially centering an optical axis of the first optical
fiber and an optical axis of the second optical fiber, where at
least one of the first optical fiber and the second optical fiber
is a non-silica-based optical fiber, wherein
[0157] optical axes of the first and second optical fibers are held
in the first and second housings respectively at angles
.theta..sub.1and .theta..sub.2
(.theta..sub.1.apprxeq..theta..sub.2) from a vertical axis of a
boundary surface between the splicing end surfaces, and a
relationship between the angles .theta..sub.1 and .theta..sub.2
satisfies Snell's law represented by an equation (4) at the time of
splicing the first and second optical fibers: 3 sin 1 sin 2 = n 2 n
1 ( 4 )
[0158] where n.sub.1 is a refractive index of the first optical
fiber and n.sub.2 is a refractive index of the second optical
fiber.
[0159] Here, the splicing end surface of the first optical fiber
may be connected to the splicing end surface of the second optical
fiber through an optical adhesive at the time of splicing the first
and second optical fibers.
[0160] The splicing end surface of the first optical fiber and the
splicing end surface of the second optical fiber may be kept in
absolute contact with each other at the time of splicing the first
and second optical fibers.
[0161] The first and second optical fibers may be non-silica-based
optical fibers.
[0162] The non-silica-based optical fibers may be selected from Zr-
or In-based fluoride optical fibers, chalcogenide optical fibers,
and tellurite glass optical fibers.
[0163] The non-silica-based optical fibers may be selected from Zr-
or In-based fluoride optical fibers, chalcogenide optical fibers,
and tellurite glass optical fibers, and furthermore the
non-silica-based optical fibers may be doped with a rare-earth
element.
[0164] The first optical fiber may be a tellurite glass optical
fiber, the second optical fiber may be a silica-based optical
fiber, and the angle .theta..sub.1 may be of 8 or more degrees.
[0165] The first optical fiber may be a Zr-based fluoride optical
glass fiber, the second optical fiber may be a silica-based optical
fiber, and the angle .theta..sub.1 may be of 3 or more degrees.
[0166] The first optical fiber may be a In-based fluoride optical
glass fiber, the second optical fiber may be a silica-based optical
fiber, and the angle .theta..sub.1 may be of 4 or more degrees.
[0167] The first optical fiber may be a chalcogenide optical glass
fiber, the second optical fiber may be a silica-based optical
fiber, and the angle .theta..sub.1 may be of 8 or more degrees.
[0168] In a twenty-fourth aspect of the present invention, there is
provided a light source comprising:
[0169] an optical amplification medium which is one selected from a
group of an erbium-doped tellurite optical fiber and an optical
waveguide; and
[0170] an optical coupler arranged on an end of the optical
amplification medium, wherein
[0171] at least one terminal of the optical coupler is equipped
with a reflector.
[0172] Here, the erbium-doped tellurite optical fiber or the
optical waveguide may consist of the novel tellurite glasses
described above.
[0173] The reflector may be comprised of one selected from a group
of a dielectric-multiple-film filter and a fiber-bragg-grating.
[0174] In a twenty-fourth aspect of the present invention, there is
provided an optical amplifier using an erbium-doped tellurite
optical fiber or an optical waveguide as an optical amplification
medium, comprising
[0175] an optical coupler arranged on an end of the optical
amplification medium, wherein
[0176] at least one terminal of the optical coupler is equipped
with a reflector.
[0177] Here, the erbium-doped tellurite optical fiber or the
optical waveguide may consist of the novel tellurite glasses
described above.
[0178] The reflector may be comprised of one selected from a group
of a dielectric-multiple-film filter and a fiber-bragg-grating.
[0179] The above and other objects, effects, features and
advantages of the present invention will become more apparent from
the following description of embodiments thereof taken in
conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0180] FIG. 1 is a spectrum diagram that illustrates the
.sup.4I.sub.13/2-.sup.4I.sub.15/2 emission of Er in the tellurite
glass;
[0181] FIG. 2 is an energy-level diagram of 3-level system for
Er.sup.3+ at around 1.54 .mu.m (N.sub.1.apprxeq.0);
[0182] FIG. 3 is an energy-level diagram of 4-level system for
Nd.sup.3+ at around 1.06 .mu.m (N.sub.1=0);
[0183] FIG. 4 is a graphical representation of wavelength
dependencies of the silica-based EDFA (a broken line) and the
tellurite-based EDFA (a solid line);
[0184] FIG. 5 is a graphical representation of the difference in
amplification bands of the tellurite EDFA with respect of large and
small fiber-losses;
[0185] FIG. 6 is a schematic representation of the conventional
splicing between the non-silica-based optical fiber and the
silica-based optical fiber;
[0186] FIG. 7 is a schematic representation of the conventional
splicing between the non-silica-based optical fiber and the
silica-based optical fiber;
[0187] FIG. 8 is a schematic representation for illustrating the
mechanism of ghost generation by the reflection on the spliced
portions;
[0188] FIG. 9 is a schematic representation of the conventional
splicing between the non-silica-based optical fiber and the
silica-based optical fiber;
[0189] FIG. 10 is a schematic representation of the conventional
splicing between the non-silica-based optical fiber and the
silica-based optical fiber;
[0190] FIG. 11 is a schematic representation of the conventional
splicing between the non-silica-based optical fiber and the
silica-based optical fiber;
[0191] FIG. 12 is a schematic representation of the stable glass
formation range for TeO.sub.2--Na.sub.2O--ZnO glass;
[0192] FIG. 13 is a schematic representation of the stable glass
formation range for TeO.sub.2--Li.sub.2O--ZnO glass when
Bi.sub.2O.sub.3=5 mole %;
[0193] FIG. 14 is a graphical representation of the results of DSC
measurement, where the upper line is for
75TeO.sub.2-20ZnO-5Na.sub.2O glass in the absence of
Bi.sub.2O.sub.3, the middle line is for
77TeO.sub.2-15.5ZnO-6Na.sub.2O-1.5Bi.sub.2O.sub.3 glass, and the
lower line is for 73.5TeO.sub.2-15.5ZnO-6Na.sub.2O-5Bi.sub.2O.sub.3
glass;
[0194] FIG. 15 is a graphical representation of the results of DSC
measurement, where the upper line is for
83TeO.sub.2-5ZnO-12Li.sub.2O glass and the lower line is for
78TeO.sub.2-5ZnO-12Li.sub.2O-5Bi.sub.2O.s- ub.3 glass;
[0195] FIG. 16 is a graphical representation of the dependency of a
refractive index (n.sub.D) of
TeO.sub.2--Na.sub.2O--ZnO--Bi.sub.2O.sub.3 glass on Bi.sub.2O.sub.3
content;
[0196] FIG. 17 is a schematic representation of the stable glass
formation range for TeO.sub.2--Na.sub.2O--ZnO glass;
[0197] FIG. 18 is an energy-level diagram of Er.sup.3+;
[0198] FIG. 19 is a schematic block diagram of an optical amplifier
as one of the preferred embodiment of the present invention;
[0199] FIG. 20 is a schematic block diagram of a laser as one of
the preferred embodiment of the present invention;
[0200] FIG. 21 is a schematic block diagram of a laser as one of
the preferred embodiment of the present invention;
[0201] FIG. 22 is a gain-spectrum diagram that illustrates the
gains obtained in Embodiment 8;
[0202] FIG. 23 is a schematic block diagram of another laser as one
of the preferred embodiment of the present invention;
[0203] FIG. 24 is a schematic representation of the stable glass
formation range for TeO.sub.2--Li.sub.2O--ZnO glass when
Bi.sub.2O.sub.3=5 mole % (region A: Tx -Tg>120.degree. C.,
region B: no crystallization peak);
[0204] FIG. 25 is a graphical representation of the results of DSC
measurement, where the upper line (line a) is for
73.5TeO.sub.2-20ZnO-5Na- .sub.2O-1.5Bi.sub.2O.sub.3 glass and the
lower line (line b) is for
73TeO.sub.2-23ZnO-5Na.sub.2O-2Bi.sub.2O.sub.3 glass;
[0205] FIG. 26 is a graphical representation of 1.5 .mu.m-band
emission spectrum of each of
TeO.sub.2--ZnO--Na.sub.2O--Bi.sub.2O.sub.3 glass,
TeO.sub.2--ZnO--Na.sub.2O--Bi.sub.2O.sub.3--Al.sub.2O.sub.3 glass,
and TeO.sub.2--ZnO--Li.sub.2O--Bi.sub.2O.sub.3--Al.sub.2O.sub.3
glass;
[0206] FIG. 27 is a schematic representation of the stable glass
formation range for glass comprising five ingredients as a
composition of
TeO.sub.2--ZnO--Li.sub.2O--Na.sub.2O--Bi.sub.2O.sub.3 glass when
TeO.sub.2=75 mole % and Bi.sub.2O.sub.3=5 mole %;
[0207] FIG. 28 is a schematic representation of the stable glass
formation range for glass comprising five ingredients as a
composition of
TeO.sub.2--ZnO--Li.sub.2O--Na.sub.2O--Bi.sub.2O.sub.3 glass when
TeO.sub.2=80 mole % and Bi.sub.2O.sub.3=5 mole %;
[0208] FIG. 29 is a schematic representation of the stable glass
formation range for glass comprising five ingredients as a
composition of
TeO.sub.2--ZnO--Li.sub.2O--Al.sub.2O--Bi.sub.2O.sub.3 glass when
Al.sub.2O.sub.3=2 mole % and Li.sub.2O.sub.3=12 mole %;
[0209] FIG. 30 is a schematic block diagram of an optical amplifier
using the tellurite glass optical fiber as the amplification medium
as one of the preferred embodiment of the present invention;
[0210] FIG. 31 is a schematic block diagram of an optical amplifier
using the tellurite glass optical fiber as the amplification medium
as one of the preferred embodiment of the present invention;
[0211] FIG. 32 is a schematic diagram of the splicing structure
between the non-silica-based optical fiber and the silica-based
optical fiber in accordance with the present invention;
[0212] FIG. 33 is a schematic diagram of the splicing structure
between the non-silica-based optical fiber and the silica-based
optical fiber in accordance with the present invention;
[0213] FIG. 34 is a schematic diagram of the splicing structure
between the non-silica-based optical fiber and the silica-based
optical fiber in accordance with the present invention;
[0214] FIG. 35 is cross sectional view of the splicing structure
shown in FIG. 34;
[0215] FIG. 36 is a schematic diagram of the splicing structure
between the non-silica-based optical fiber and the silica-based
optical fiber in accordance with the present invention;
[0216] FIG. 37 is cross sectional view of the splicing structure
shown in FIG. 36;
[0217] FIG. 38 is a schematic diagram of the splicing structure
between the non-silica-based optical fiber and the silica-based
optical fiber in accordance with the present invention;
[0218] FIG. 39 is cross sectional view of the splicing structure
shown in FIG. 38;
[0219] FIG. 40 is a schematic diagram of the splicing structure
between the non-silica-based optical fiber and the silica-based
optical fiber in accordance with the present invention;
[0220] FIG. 41 is cross sectional view of the splicing structure
shown in FIG. 40;
[0221] FIG. 42 is a schematic block diagram of an optical amplifier
that uses the splicing structure between the non-silica-based
optical fiber and the silica-based optical fiber in accordance with
the present invention;
[0222] FIG. 43 is a graphical representation of the amplification
characteristics of the optical amplifier in accordance with the
present invention;
[0223] FIG. 44 is a schematic block diagram of the ASE light source
as one of the preferred embodiments of the present invention;
[0224] FIG. 45 is a graphical representation of the relationship
between the reflection of mirror and the intensity of the ASE
spectrum of FIG. 44;
[0225] FIG. 46 is a spectrum diagram of ASE; and
[0226] FIG. 47 is a schematic block diagram of the fiber amplifier
as one of the preferred embodiments of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0227] First of all, we will describe tellurite glass compositions
of Bi.sub.2O.sub.3--Na.sub.2O--ZnO--TeO.sub.2 and
Bi.sub.2O.sub.3--Li.sub.2O- --ZnO--TeO.sub.2.
[0228] The tellurite glass composition can be provided as one of
the following compositions A, B, and C.
[0229] The first composition A consists of:
0<Bi.sub.2O.sub.3.ltoreq.20 (mole %); 0.ltoreq.ZnO.ltoreq.35
(mole %); and 55.ltoreq.TeO.sub.2.ltoreq- .90 (mole %).
[0230] The second composition B consists of:
1.5<Bi.sub.2O.sub.3.ltoreq- .15 (mole %);
0.ltoreq.Na.sub.2O.ltoreq.35 (mole %); and
55.ltoreq.TeO.sub.2.ltoreq.90 (mole %).
[0231] The third composition C consists of:
0<Bi.sub.2O.sub.3.ltoreq.20 (mole %);
0.ltoreq.Li.sub.2O.ltoreq.25 (mole %); 0.ltoreq.ZnO<25 (mole %);
and 55.ltoreq.TeO.sub.2.ltoreq.90 (mole %).
[0232] Each of FIG. 12 and FIG. 13 illustrates a composition range
that shows interrelationship among ingredients of the glass
composition. A stable region (region A) indicates an allowable
range of each ingredient in the composition (i.e., FIG. 12 for the
composition A or B in which Bi.sub.2O.sub.3=5 mole % and FIG. 13
for the composition C in which Bi.sub.2O.sub.3=5 mole %). That is,
the stabilization of glass can be attained when a content of each
ingredient is in that range.
[0233] A thermal stability of the glass composition for preparing a
fiber can be estimated by a technique of differential scanning
calorimetry (DSC) generally used to indicate phase changes, so that
a glass composition that has a larger value of Tx-Tg (Tx:
crystallization temperature, and Tg: glass transition temperature)
is a more stable glass composition. The process for preparing a
single-mode optical fiber includes the steps of elongating and
drawing a glass preform through the addition of heat, so that the
glass preform is subjected to elevated temperatures twice. If a
crystallization temperature (Tx) of the glass preform is almost the
same order of a glass transition temperature (Tg), crystalline
nuclei grow one after another resulting in an increase in
scattering loss of the optical fiber. If a value of Tx-Tg is large,
on the other hand, a low-loss optical fiber can be formed. In the
case of a glass composition defined in that region shown in FIG.
12, a value of Tx-Tg is 120.degree. C. or more and thus the glass
composition can be used in the process for preparing the low-loss
optical fiber. However, the low-loss optical fiber cannot be formed
when a glass composition out of that region is used in the steps of
preparing both of core and clad. Among the ingredients of the
composition, the addition of Bi.sub.2O.sub.3 has the large effect
of stabilizing the glass.
[0234] Referring now to FIG. 14, there is shown the results of DSC
measurements on three different compositions:
75TeO.sub.2-20ZnO-5Na.sub.2- O,
77TeO.sub.2-15.5ZnO-6Na.sub.2O-1.5Bi.sub.2O.sub.3, and
73.5TeO.sub.2-15.5ZnO-6Na.sub.2O-5Bi.sub.2O.sub.3, which are
characterized by their respective contents of Bi.sub.2O.sub.3
(i.e., Bi.sub.2O.sub.3=0, 1.5, and 5 mole %, respectively). Each
measurement is performed by breaking a glass sample, packing the
bulk of broken glass (a piece of glass is 30 mg in weight) in a
sealed container made of silver, and subjecting the glass in the
container to the DSC measurement in an argon atmosphere at a
heat-up rate of 10.degree. C./minute. As is evident from FIG. 14, a
value of Tx-Tg is varied among the glass compositions. That is, the
value for Bi2O.sub.3=0 takes on 119.2.degree. C., the value for
Bi.sub.2O.sub.3=1.5 mole % takes on 121.6.degree. C., and the value
for Bi.sub.2O.sub.3=5 mole % takes on 167.5.degree. C. Among them,
the glass composition of
73.5TeO.sub.2-15.5ZnO-6Na.sub.2O-5Bi.sub.2O.sub.3, which
corresponds to the composition B mentioned above, shows the most
excellent thermal stability compared with other compositions
because a peak of the curve for that composition (Bi.sub.2O.sub.3=5
mole %) shifts from peaks of others about 40.degree. C. or more to
the side of higher temperatures.
[0235] FIG. 15 shows the results of DSC measurements on two
different compositions that contain Li: Bi.sub.2O.sub.3=0 mole %
and Bi.sub.2O.sub.3=5 mole %, where the latter corresponds to the
composition C mentioned above. As shown in the figure, a value of
Tx-Tg for Bi.sub.2O.sub.3=0 mole % takes on 54.6.degree. C. On the
other hand, there is no heating peak of crystallization for the
composition of Bi.sub.2O.sub.3=5 mole %, so that a value of Tx-Tg
for Bi.sub.2O.sub.3=5 mole % is infinity and thus a thermal
stability of the glass can be dramatically increased. This kind of
effect can be also observed when a trivalent metal oxide
(Al.sub.2O.sub.3, La.sub.2O.sub.3, Er.sub.2O.sub.3, Nd.sub.2O.sub.3
or the like) is added instead of Bi.sub.2O.sub.3.
[0236] The addition of Bi.sub.2O.sub.3 also crucially effects on an
adjustment in refractive indexes. FIG. 16 shows that the dependence
of a refractive index (n.sub.D) of TeO.sub.2-glass on an added
amount of Bi.sub.2O.sub.3. As shown in the figure, there is the
direct proportionality between the refractive index (n.sub.D) of
TeO.sub.2-glass and the added amount of Bi.sub.2O.sub.3. The
n.sub.Dvalue varies from 2.04 to 2.2 if the added amount of
Bi.sub.2O.sub.3 varies from 0 to 20 mole %.
[0237] Through the use of such a property, therefore, optical
fibers that have large and small values (from about 0.2% to about
6%) of relative refractive-index difference can be easily designed
by changing the added amount of Bi.sub.2O.sub.3.
[0238] Next, we will describe an optical amplification medium as
one of the preferred embodiments of the present invention.
[0239] The optical amplification medium comprises a core and a
clad.
[0240] The core is provided as an optical fiber made of a tellurite
composition A that consists of: 0.ltoreq.Bi.sub.2O.sub.3.ltoreq.20
(mole %); 0<Na.sub.2O.ltoreq.15 (mole %); 5.ltoreq.ZnO.ltoreq.35
(mole %); and 60 <TeO.sub.2<90 (mole %).
[0241] The clad comprises one of tellurite glass compositions (B1,
C1, or D1) in the form of an optical fiber or an optical waveguide
as a host of rare-earth element, where the composition (B1)
consists of: 5<Na.sub.2O<35 (mole %); 0.ltoreq.ZnO<10
(mole %); and 55<TeO.sub.2<85 (mole %), the composition (C1)
consists of: 5<Na.sub.2O<35 (mole %); 10<ZnO<20 (mole
%); and 55<TeO.sub.2<85 (mole %), and the composition (D1)
consists of: 0.ltoreq.Na.sub.2O<25 (mole %); 20<ZnO.ltoreq.30
(mole %); 55<TeO.sub.2<75 (mole %). The composition rages of
those compositions B1, C1, and D1 that stabilize glass are
illustrated in FIG. 17.
[0242] Glass prepared from the composition defined in the region
shown in FIG. 17 shows that a value of Tx-Tg is 100.degree. C.
Therefore, the glass is not crystallized during the fiber-forming
process including a drawing step, so that it can be used in the
process of forming a low-loss optical fiber.
[0243] By the way, at least one of the tellurite glass compositions
to be used in core and clad formations may be doped with erbium or
erbium and ytterbium.
[0244] A laser device according to the present invention comprises
an optical amplification medium and an excitation light source, and
is mainly characterized by the effective use of induced emission
transition of Er from .sup.4I.sub.13/2 level to .sup.4I.sub.15/2
level.
[0245] FIG. 18 is an energy level diagram of Er.sup.3+, which
illustrates an induced emission from the upper level of
.sup.4I.sub.13/2 to the ground level of .sup.4I.sub.15/2
(hereinafter, referred as .sup.4I.sub.13/2.fwdarw..sup.4I.sub.15/2
emission).
[0246] In addition, as shown in FIG. 1, it is known that a
bandwidth that allows .sup.4I.sub.13/2.fwdarw..sup.4I.sub.15/2
emission of Er.sup.3+ in the fluoride glass is broader than that of
Er.sup.3+ in other glass such as a silica glass. At a wavelength of
over 1.6 .mu.m, however, an emission intensity becomes low and also
the occurrence of optical amplification and laser oscillation
becomes difficult.
[0247] In a case where a predetermined amount of Er is added in
Tellurite glass, Er receives more strong electric field than the
conventional one. As a result, an acceptable level of fluorescent
intensity can be observed at a wavelength over 1.6 .mu.m because of
the increasing range of levels caused by Stokes, effects on the
levels such as .sup.4I.sub.13/2 and .sup.4I.sub.15/2.
[0248] Consequently, an optical amplifier or a laser device
operating at wavelengths from 1.5 .mu.m to 1.7 .mu.m can be
realized if its optical amplification medium is a tellurite glass
fiber where erbium is added in at least a core portion.
[0249] If the tellurite glass contains at least one of boron,
phosphorus, and hydroxyl group, improvements in the properties of
gain coefficient and noise figure can be also attained at the time
of pumping .sup.4I.sub.11/2 level by 0.98 .mu.m light. That is,
vibrational energies of B--O, P--O, and O--H which are
approximately 1,400 cm.sup.-1, 1,200 cm.sup.-1, and 3,700
cm.sup.-1, respectively, while phonon energy of the tellurite glass
free of the above additive is 600 to 700 cm.sup.-1, so that the
tellurite glass containing at least one of boron, phosphorus, and
hydroxyl group generates more than double the energy of the
tellurite glass free of the additive. As a result, if an optical
amplification at 1.5 .mu.m band is caused by a transition of
.sup.4I.sub.13/2.fwdarw..sup.- 4I.sub.15/2 by pumping
.sup.4I.sub.11/2 level of Er by light at a wavelength of near 0.98
.mu.m, a relaxation from .sup.4I.sub.11/2 level to .sup.4I.sub.13/2
level is more likely to be occurred and a reduction in quantum
efficiency is relatively little. Thus, the reduction in pumping
efficiency of .sup.4I.sub.13/2 level is hardly occurred (FIG. 18).
If a relaxation from .sup.4I.sub.11/2 level to .sup.4I.sub.13/2
level is more likely to be occurred, it is preferable to pump
.sup.4I.sub.13/2 level after pumping .sup.4I.sub.11/2 level better
than the direct pumping of .sup.4I.sub.13/2 level by light at a
wavelength of near 1.48 .mu.m because the former is convenient to
obtain a population inversion between .sup.4I.sub.13/2 level and
.sup.4I.sub.15/2 level. Therefore, there is an advantage of having
excellent noise characteristics.
[0250] Hereinafter, we will describe preferred embodiments of an
optical amplification medium, a broadband optical amplifier using
such a medium, and a laser device in accordance with the present
invention.
[0251] (Embodiment 1)
[0252] Raw materials of TeO.sub.2, ZnO, Na.sub.2No.sub.3, and
Bi.sub.2O.sub.3 were formulated so as to be prepared as
compositions of TeO.sub.2 (75 mole %)--ZnO (20 mole %)--Na.sub.2O
(5 mole %), TeO.sub.2 (77 mole %)--ZnO (15.5 mole %)--Na.sub.2O (6
mole %), and TeO.sub.2 (73.5 mole %)--ZnO (15.5 mole %)--Na.sub.2O
(6 mole %)--Bi.sub.2O.sub.3 (5 mole %) after melting. Then, 20 g of
a mixture of the formulated raw materials were filled in a crucible
and melted in an electric furnace at 800.degree. C. for 2 hours in
an oxygen atmosphere. After that, a molten mixture was casted on a
pre-heated plate (200.degree. C.) to obtain glass. The glass was
annealed at 250.degree. C. for 4 hours and then a part of harden
glass was broken. Two samples, a 30 mg bulk of glass and 30 mg
glass fine powder grained in an agate mortar, were filled in a
sealed container made of silver with gold-plating and subjected to
a differential scanning calorimetry (DSC) at a heat-up rate of
10.degree. C./minute in an argon gas atmosphere. In the case of the
bulk glass sample, obtained values of Tx-Tg were 119.2.degree. C.
when Bi.sub.2O.sub.3=0, 121.6.degree. C. when Bi.sub.2O.sub.3=1.5
mole %, and 167.5.degree. C. when Bi.sub.2O.sub.3. Especially, a
heat-stability of the composition within the confines of:
1.5<Bi.sub.2O.sub.3.ltoreq.15 (mole %);
0.ltoreq.Na.sub.2O.ltoreq.35 (mole %); and
55.ltoreq.TeO.sub.2.ltoreq.90 (mole %) as defined as the
composition B described above was improved over 40.degree. C. In
the case of the powdered glass sample, obtained values of Tx-Tg
were 80.2 .degree. C when Bi.sub.2O.sub.3=0, 76.3.degree. C. when
Bi.sub.2O.sub.3=1.5 mole %, and 110.2.degree. C. when
Bi.sub.2O.sub.3=5 mole %, which are smaller than those of the bulk
glass sample but thermal stability of the glass could be estimated
more precisely. In both cases, however, we found that their thermal
stabilities were extremely improved by the addition of 5 mole % of
Bi.sub.2O.sub.3.
[0253] In the present specification, the value of Tx-Tg related to
the thermal stability of glass is based on the measurement carried
out on the bulk glass unless otherwise specified.
[0254] As mentioned above, it is possible to make a low-loss
optical fiber using the glass that shows a value of
Tx-Tg.gtoreq.120.degree. C. with reference to the DSC measurement
value of bulk glass. A fiber-loss of this kind of glass is almost 1
dB/km or less. For performing a high-efficiency optical
amplification using an optical transition of three-level system,
more stable glass will be required for making an optical fiber with
a fiber-loss of lower than that of the above glass by an order of
magnitude. In this case, the DSC measurement value for the powdered
glass is effective as an evaluation standard. Thus, an optical
fiber with a fiber-loss of 0.1 dB/km or less can be obtained if the
glass to be measured as Tx-Tg.gtoreq.100.degree. C. is used.
[0255] (Embodiment 2)
[0256] Tellurite optical fibers were prepared by the following
procedure. For raw materials of core glass and clad glass, the
compositions A or B described above was used. That is, the
composition A consists of: 0<Bi.sub.2O.sub.3<20 (mole %);
0.ltoreq.ZnO.ltoreq.35 (mole %); and 55.ltoreq.TeO.sub.2.ltoreq.90
(mole %), and the composition B consists of:
1.5<Bi.sub.2O.sub.3.ltoreq.15 (mole %);
0.ltoreq.Na.sub.2O.ltoreq.- 35 (mole %); and
55.ltoreq.TeO.sub.2.ltoreq.90 (mole %).
[0257] The glass composition was melted in a crucible made of
platinum or gold in an oxygen atmosphere and then formed into a
preform by a well-known technique of suction-casting. In addition,
a jacket tube was prepared from the same glass composition by a
well-known technique of rotational-casting (cf. Kanamori et al.,
Proceeding of 9th International Symposium on Nonoxide Glasses, page
74, 1994).
[0258] Each tellurite glass optical fiber was obtained as a result
of drawing both the preform and the jacket tube. The obtained fiber
had a minimum fiber-loss of 0.1 dB/m or less, a cut-off wavelength
of 0.5 .mu.m to 2.5 .mu.m, and a relative refractive index
difference between the core and the clad of 0.2% to 6%. In
addition, we could add one of rare-earth elements (such as Er, Pr,
Yb, Nd, Ce, Sm, Tm, Eu, Tb, Ho, and Dy) in the glass composition to
be formed into core or clad glass.
[0259] (Embodiment 3)
[0260] Tellurite optical fibers were prepared as the same way as
that of Embodiment 2 except the glass compositions for core and
clad glasses. In this embodiment, the glass composition A1
described above was used as a core glass and the glass composition
B1, C1, or D1 described above was used as a clad glass. Each of the
obtained tellurite optical fibers was characterized by having a
minimum fiber-loss of 0.1 dB/m or less, a cut-off wavelength of 0.5
to 2.5 .mu.m, and a relative refractive index difference between
the core and the clad of 0.2% to 6%. In addition, we could add
rare-earth elements (such as Er, Pr, Yb, Nd, Ce, Sm, Tm, Eu, Tb,
Ho, and Dy) for 10weight % or less in the core or clad glass.
[0261] (Embodiment 4)
[0262] An optical amplification medium was prepared as an optical
fiber. A core of the optical fiber was made of a glass composition
of TeO, (68.6 mole %)--Na.sub.2O (7.6 mole %)--ZnO(19.0 mole
%)--Bi.sub.2O.sub.3 (4.8 mole % as a core material and doped with
1,000 ppm of erbium. Also, a clad of the optical fiber was made of
a glass composition of TeO.sub.2 (71 mole %) --Na.sub.2O (8 mole
%)--ZnO(21 mole %). Therefore, the optical fiber was characterized
by having a cut-off wavelength of 1.3 .mu.m and a relative
refractive index difference between the core and the clad of
2%.
[0263] Then, an optical amplifier for 1.5 to 1.7 .mu.m band was
assembled using that optical amplification medium. The optical
amplifier was subjected to an amplification at a pump wavelength is
0.98 .mu.m. In this test, a DFB laser was used as a light source
for generating a signal light at 1.5 to 1.7 .mu.m band.
[0264] FIG. 19 is a schematic block diagram that illustrates a
configuration of an optical amplifier as one of preferred
embodiments of the present invention. As shown in the figure, a
signal light source 101 and an excitation light source 102 are
connected to one end of an amplification optical fiber 104 through
an optical coupler 103. In addition, an optical isolator 105 is
connected to the other end of the amplification optical fiber 105.
Any of the connections between the components is implemented
through an optical fiber 106a, 106b, 106c, 106d, or 106e.
[0265] The optical amplifier is subjected to an amplification test,
resulting in amplification gains at wavelengths of 1.5 to 1.7
.mu.m.
[0266] As shown in FIG. 20, a ring laser having a tunable band-pass
filter 107 for a narrow bandwidth is constructed using the same
optical amplifier as that of being subjected to the amplification
test. The ring laser is constructed by forming an optical resonator
shaped like a ring by connecting an output side end of the optical
isolator 105 with an optical coupler 103 instead of connecting with
the signal light source 101 and then inserting the band-pass filter
107 into an appropriate position in the ring. In the figure, that
is, it is placed between the optical isolator 105 and the optical
coupler 103 through the optical fibers 106e, 106d.
[0267] Then, a laser-oscillation test is performed using the ring
laser. During the test, the ring laser receives light from the
excitation light source 102 while a transmission region of the
band-pass filter 107 is varied in the range of 1.5 .mu.m to 1.7
.mu.m. Consequently, a laser-oscillation at 1.5 to 1.7 .mu.m band
is observed.
[0268] In this embodiment, a pump wavelength of 0.98 .mu.m is used
to indirectly pump the .sup.4I.sub.13/2 level through the
.sup.4I.sub.11/2 level. However, it is not limited to such a
wavelength. It is also possible to use 1.48 .mu.m for directly
pumping the .sup.4I.sub.13/2 level. In addition, an energy level
higher than the .sup.4I.sub.13/2 level may be pumped by light at a
wavelength of less than 0.98 .mu.m.
[0269] (Embodiment 5)
[0270] Using the optical amplifier shown in FIG. 19, an optical
amplification test is performed at 1.5 .mu.m band on condition that
a pump wavelength is of 0.98 .mu.m. As a result, an optical
amplification is observed at a wavelength of 1.52 .mu.m or more
with a noise figure of 7 dB or less.
[0271] (Embodiment 6)
[0272] An optical amplifier is prepared using the same optical
fiber as that of Embodiment 3 except that the glass is co-doped
with Er and Yb instead of doping Er alone.
[0273] An optical amplification test and a laser oscillation test
are performed by the same ways as those of Embodiments 4 and 5 on
condition that a pump wavelength of 1.029 .mu.m (Yb-doped YAG
laser), 1047 .mu.m (Nd-doped YLF laser), 1.053 .mu.m (Nd-doped YAG
laser), 1.064 Km (Nd-doped YAG laser), or the like. In a case where
Yb is co-doped with Er in the medium, a laser oscillation at
wavelengths of 1.5 to 1.7 .mu.m and a broadband optical
amplification at 1.5 .mu.m band are observed whether the above pump
wavelength is used if an energy shift from Yb to Er is gained.
[0274] Any of the glass compositions in Embodiments 1 to 6 is only
represented as an example of the allowable compositions. It is also
possible to use a glass composition that includes at least one
selected from the group of, for example Cs.sub.2O, Rb.sub.2O,
K.sub.2O, Li.sub.2O, BaO, SrO, CaO, MgO, BeO, La.sub.2O.sub.3,
Y.sub.2O.sub.3, Sc.sub.2O.sub.3, Al.sub.2O.sub.3, ThO.sub.2,
HfO.sub.2, ZrO.sub.2, TiO.sub.2, Ta.sub.2O.sub.5, Nb.sub.2O.sub.5,
Wo.sub.3, Tl.sub.2O, CdO, PbO, In.sub.2O.sub.3, and
Ga.sub.2O.sub.3, in addition to TeO.sub.2.
[0275] Furthermore, Er or Er and Yb may be contained not only in
the core but also in the clad.
[0276] A configuration of the optical amplifier is not limited to
the one described above. Any of the optical amplifiers having the
optical amplification medium, an excitation light source for
exciting the medium, and input and output means for signal light
can be allowable.
[0277] (Embodiment 7)
[0278] An amplification optical fiber is prepared as a an optical
fiber (4 m in length) doped with 1,000 ppm of erbium in its core
and subjected to a measurement for determining the amplification
characteristics of the fiber at 1.5 .mu.m band. In this case, a
core glass composition is of TeO.sub.2 (68.6 mole %)--ZnO (19 mole
%)-Na.sub.2O (7.6 mole %)--Bi.sub.2O.sub.3 (4.8 mole %) and doped
with 5 weight % of P.sub.2O.sub.5, and a clad composition is of
TeO.sub.2 (71 mole %)--Na.sub.2O (8 mole %)--ZnO (21 mole %). The
optical fiber has a core/clad refractive index difference of 2.5 %
and a cut-off wavelength of 0.96 .mu.m.
[0279] A small signal gain at 1.5 .mu.m band of the amplification
optical fiber is measured using 0.98 .mu.m light as an excitation
light from a light source (a semiconductor laser), resulting in an
increase in a gain efficiency of the optical fiber. That is, the
gain efficiency of the optical fiber reaches a value of 2 dB/mW
which is approximately five times as large as that of an optical
fiber without containing P.sub.2O.sub.5.
[0280] A gain spectrum at a saturated region when an input signal
level is --10 dBm is measured, resulting in a flat gain at a
bandwidth of 90 nm from 1,530 nm to 1,620 nm (an excitation
intensity is 200 mW). A noise figure of 7 dB is observed when the
optical fiber does not contain P.sub.2O.sub.3, while a noise of 4
dB is observed when the optical fiber contains P.sub.2O.sub.3.
[0281] It is noted that improvements in the characteristics of gain
and noise figure of the optical fiber is also observed when it
contains B.sub.2O.sub.3 instead of P.sub.2O.sub.5.
[0282] (Embodiment 8)
[0283] Amplification optical fibers are prepared using a core glass
composition of TeO.sub.2 (68.6 mole %)--ZnO (19 mole %)--Na.sub.2O
(7.6 mole %)--Bi.sub.2O.sub.3 (4.8 mole %) and doped with or
without 5,000 ppm of hydroxyl (OH) radical and 1,000 ppm of Er. The
optical fiber containing the OH-radical shows a gain factor
three-times as large as that of the optical fiber without
containing the OH-radical. The reason is that the OH-radical has a
comparatively large signal energy of 3,700 cm.sup.-1 that causes a
slight relaxation of the .sup.4I.sub.13/2 level which is a starting
level of the amplification by a multiple-phonon emission.
[0284] Referring now to FIG. 21, a laser device according to one of
the preferred embodiments of the present invention is illustrated.
In the figure, reference numerals 111 and 111' denote pumping
semiconductor lasers (wavelength: 1,480 nm), 112 and 112' denote
optical couplers that couple a signal light and an excitation light
together, 113 denotes a first amplification optical fiber, 114
denotes an optical isolator, and 115 denotes a second amplification
optical fiber. In this configuration of the laser device, an input
signal light enters the laser device from a port A and exits from a
port B after passing through the components in the device.
[0285] In this embodiment, the first amplification optical fiber
113 is a ZrF.sub.4-contained fluoride optical fiber doped with
1,000 ppm of erbium (cf. Kanamori et al., Proceeding of 9th
International Symposium on Non-Oxide Glasses, page 74, 1994). The
second amplification optical fiber 115 is a oxidized tellurite
optical fiber having a glass composition of
TeO.sub.2--Na.sub.2O--Bi.sub.2O.sub.3--ZnO doped with 1,000 ppm of
erbium.
[0286] Each of the amplification fibers has a core/clad refractive
index difference of each fiber is 2.5%, a cut-off wavelength of
1.35 .mu.m, and a fiber length of 10 m or 7 m.
[0287] A measurement of gain spectrum at 1.5 .mu.m band is
performed on condition that an output optical intensity of each of
the pumping semiconductor lasers 111, 111' is 150 mW. The resulting
gain spectrum is shown in FIG. 22.
[0288] According to the gain spectrum in the figure, a curve that
indicates variations in signal gain is flattened over 80 nm
bandwidth that corresponds to signal-light wavelengths from 1,530
nm to 1,610 nm. At that wavelengths, that is, the signal gain is
held at approximately 30 dB and the gain tilt is minimized. In the
case of Er-doped fluoride optical fiber, a curve that indicates
variations in signal gain is flatted over 30 nm bandwidth that
corresponds to signal-light wavelengths from 1,530 nm to 1,560 nm.
Therefore, the present embodiment allows that a bandwidth where the
gain is flatted is doubled or more compared with the conventional
Er-doped fluoride optical fiber. In the case of Er-doped silica
optical fiber, furthermore, a bandwidth where the gain is flatted
is only 10 nm. Thus, the present embodiment allows that a bandwidth
where the gain is flatted is eight times as large as that of the
conventional Er-doped silica optical fiber.
[0289] In this embodiment, as shown in the figure, the Er-doped
ZrF.sub.4 fluoride optical fiber is arranged in the downstream of
the device, while the Er-doped tellurite optical fiber is arranged
in the upstream. However, there is no restraint on the arrangement
of these fibers, so that it is possible to arrange them in a
retrograde order. In addition, an InF.sub.3 fluoride optical fiber
may be also used, or an optical fiber of Er-doped oxide
multi-component glass may be included in the amplification optical
fibers. In other words, a matter of great import is that at least
one of the amplification optical fiber must be the Er-doped optical
fiber.
[0290] Furthermore, a composition of the tellurite optical fiber is
not restraint on the composition of the present embodiment.
[0291] It is needless to say that a method for pumping an
amplification optical fiber may be of forward-pumping,
backward-pumping, or bidirectional-pumping.
[0292] (Embodiment 9)
[0293] FIG. 23 is a schematic block diagram that illustrates a
configuration of another laser device as one of preferred
embodiments of the present invention.
[0294] The amplification optical fibers 113, 115 used in Embodiment
8 are connected in series through a wavelength-tunable band-pass
filter 117 (a bandwidth of 3 nm). Also, a mirror 116 is placed on a
free end of the first amplification optical fiber 113 and a mirror
118 is placed on a free end of the second amplification optical
fiber 115 to perform a laser oscillation. The mirror 116 has a
transmission of 99% at 1,480 nm and a reflectivity of 100% at 1,500
nm to 1,630 nm. The mirror 118 has a transmission of 20% at 1,500
nm to 1,630 nm. As a result of the laser oscillation, we find that
the laser device of the present embodiment is able to act as a
broadband-tunable laser to be used at 1.5 .mu.m band.
[0295] As described above, the optical amplification media of the
present invention permit configurations of optical amplifiers and
laser devices to be operated at wavelengths of 1.5 .mu.m to 1.7
.mu.m. On the other hand, the conventional optical fiber amplifier
is not capable of operating at these wavelengths. According to the
present invention, therefore, sophisticated maintenance and
monitoring mechanisms used in an optical communication system at
1.5 .mu.m band can be accomplished. Thus, it becomes possible to
provide the stable management of the optical communication system.
Through the use of the characteristics of broad amplification band,
it becomes possible to amplify a short optical pulse such as in the
order of femto-second, useful as an optical amplifier to be used in
a transmission system of wavelength division multiplexing
(WDM).
[0296] (Embodiment 10)
[0297] In this embodiment, we effect the operation of a
super-luminescence laser using the tellurite optical fiber prepared
in Embodiment 4. An excitation light source is a laser diode of
1.48 .mu.m to introduce light into an end of the optical fiber. The
other end of the optical fiber is beveled at an angle 10.degree. to
prevent Fresnel reflection on the fiber's end surface. Then, an
output spectrum of light passing through the optical fiber is
measured. As a result, a broad emission spectrum of 1.46 .mu.m to
1.64 .mu.m is observed, so that we found that the optical fiber can
be used as a broad band super-luminescent laser.
[0298] (Embodiment 11)
[0299] A filter responsible for equalizing gains is arranged at the
back of the optical isolator 105 of an optical amplifier shown in
FIG. 19, and then the characteristics of optical amplification is
measured. The filter may be a chirped fiber bragg grating, a
programmable filter, a Fabry-Perot etalon type filter, and a
Mach-Zehnder type filter, or the like.
[0300] A peak of gains at 1530 to 1580 nm is observed when a
optical pulse with a signal intensity of -30 dBm is launched into
the optical amplifier which is not equipped with the filer receives
and subjected to 200 mW pumping at 1.48 .mu.m. However, this kind
of gain peak can be canceled by inserting the filter into the
optical amplifier and adjusting its loss. For WDM signals at
wavelengths of 1,530 nm to 1,610 nm, the optical amplifier is able
to operate with a gain deviation of 0.2 dB or less.
[0301] (Embodiment 12)
[0302] An optical waveguide laser and an optical waveguide type
optical amplifier are prepared using the glass composition A
described above for a core glass and the glass composition A with
an addition of Ce, Pr, Gd, Nd, Eu, Sm, Tb, Tm, Dy, Ho, Yb, or Er
for a clad glass. We effect the operation of the laser and the
optical amplifier independently. As a result, a broadband laser
oscillation and a broadband optical amplification can be attained
by the one doped with Ce, Pr, Gd, Nd, Eu, Sm, Tb, Tm, Dy, Ho, Yb,
or Er at 0.3 .mu.m, 1.3 .mu.m, 0.31 .mu.m, 1.07 .mu.m, 0.61 .mu.m,
0.59 .mu.n, 0.54 .mu.m, 1.48 .mu.m, 3.0 .mu.m, 1.49 .mu.m, 1 .mu.m,
or 1.55 .mu.m band, respectively.
[0303] (Embodiment 13)
[0304] An amplification optical fiber is prepared using a glass
composition of TeO.sub.2 (70 mole %)--ZnO (18 mole %)--Na.sub.2O (6
mole %)--Bi.sub.2O.sub.3 (6 mole %) as a core material and doped
with 2,000 ppm of erbium and a glass composition of TeO.sub.2 (68
mole %)-ZnO (22 mole %)--Na.sub.2O (7 mole %)--Bi.sub.2O.sub.3 (3
mole %) as a clad material. The optical fiber has a cut-off
wavelength of 1.1 .mu.m, and a core/clad relative refractive index
difference of 1.8%, and also it shows a fiber-loss at 1.3 .mu.m of
40 dB/km.
[0305] Then, an optical amplifier is constructed using the optical
fiber of 4 m in length as an optical amplification medium and
subjected to an optical amplification test. In this case, a
bidirectional pumping of a forward pump wavelength of 0.98 .mu.m
and a backward pump wavelength of 1.48 .mu.m is used. As a result,
a small signal gain of 5 dB or more is observed at 110 nm bandwidth
of 1,500 nm to 1,630 nm. At this time, in addition, a noise figure
of 5 dB or less is observed at a wavelength of 1,530 nm or
more.
[0306] (Embodiment 14)
[0307] An optical amplifier is constructed using the same optical
fiber as that of Embodiment 13 except that the fiber length of this
embodiment is 15 m.
[0308] An optical amplification test is performed using the optical
amplifier on condition that a bidirectional pump wavelength is 1.48
.mu.m with a coincidence of the front and backward pump
wavelengths.
[0309] As a result, especially at 50 nm bandwidth of 1,580 nm to
1,630 nm, a small signal gain of 35 dB or more is observed. At this
time, a noise figure of 5 dB is observed.
[0310] (Embodiment 15)
[0311] A laser is constructed using the same optical fiber as that
of Embodiment 13 except that the fiber length of this embodiment is
15 m. In addition, a cavity is also constructed using a total
reflection mirror and a fiber-bragg-grating having a reflectivity
of 3%. A bidirectional pump wavelength is 1.48 .mu.m with a
coincidence of the front and backward pump wavelengths. As a
result, an optical-power output of 150 mW at a wavelength of 1,625
nm is attained when an incident pump intensity is 300 mW. This kind
of the high power cannot be generated by the conventional silica-
and fluoride-based optical fibers.
[0312] (Embodiment 16)
[0313] An amplification optical fiber is prepared using a glass
composition of TeO.sub.2 (68 mole %)--ZnO (13 mole %)--Na.sub.2O (4
mole %)--Bi.sub.2O.sub.3 (15 mole %) as a core material and doped
with 3 weight % of erbium and a glass composition of TeO.sub.2 (69
mole %)--ZnO (21 mole %)--Na.sub.2O (8 mole %)--Bi.sub.2O.sub.3 (2
mole %) as a clad material. The optical fiber has a cut-off
wavelength of 1.1 .mu.m, and a core/clad relative refractive index
difference of 5%.
[0314] Then, a small-sized optical amplifier is constructed using a
3-cm piece of the optical fiber as an optical amplification medium
and subjected to an optical amplification test. In this case, a
forward pump wavelength of 1.48 .mu.m is used. In addition, a
wavelength tunable laser operating at 1.5 .mu.m to 1.7 .mu.m bands
is used as a signal light source. As a result, a small signal gain
of 20 dB or more is observed at 180 nm bandwidth of 1,530 nm to
1,610 nm. At this time, in addition, a noise figure of 7 dB or less
is observed at a wavelength of 1,530 nm or more.
[0315] (Embodiment 17)
[0316] 50 glass samples are prepared using different formulations
of a glass composition of quadric system:
TeO.sub.2--ZnO--Li.sub.2O--Bi.sub.2O- .sub.3 is formulated so that
all of them contain Bi.sub.2O.sub.3 with a fixed concentration (5
mole %) and other ingredients with varied concentrations.
[0317] Each of the glass samples is subjected to a technique of
differential scanning calorimetry (DSC) as the same way as that of
Embodiment 1 to estimate its thermal stability. The results are
shown in FIG. 24. As shown in the figure, thermally stable glasses
are obtained if the respective glass compositions are included in
the region A in the figure. In each of the thermally stable
glasses, the difference between crystallization temperature (Tx)
and glass transition temperature (Tg), i.e., Tx--Tg, is 120.degree.
C. or more. In the case of the glass compositions corresponding to
the region B in the figure, extremely stable glasses without
causing a heating peak of crystallization. Therefore, the thermally
stable optical glasses allow optical fibers having the properties
of low fiber-loss and also allow the mass production of the optical
fiber with enhanced yields. Thus, low-priced optical fibers become
feasible.
[0318] Among the compositions allowable in the region B, a glass
composition of TeO.sub.2 (80 mole %)--ZnO (5 mole %)--Li.sub.2O (10
mole %)--Bi.sub.2O.sub.3 (5 mole %) is selected and used as a core
material. The core material is doped with 2,000 ppm of erbium.
Also, a glass composition of TeO.sub.2 (75 mole %)--ZnO (5 mole
%)--Li.sub.2O (15 mole %)--Bi.sub.2O.sub.3 (5 mole %) is selected
and used as a clad material. Then, an optical fiber having a
cut-off wavelength of 1.1 .mu.m and a core/clad relative refractive
index difference of 2.5 % is formed using these materials. In this
embodiment, the resulting optical fiber is used as an amplification
medium with a fiber-loss at 1.2 .mu.m of 20 dB/km.
[0319] An optical amplifier is constructed using the amplification
medium of 3 m in length and subjected to an amplification test on
condition that a bi-directional pumping with a forward pump
wavelength of 0.98 .mu.m and a backward pump wavelength of 1.48
.mu.m is used and a wavelength tunable laser operating at 1.5 .mu.m
to 1.7 .mu.m bands is used as a signal light source. As a result, a
small signal gain of 20 dB or more is observed at 80 nm bandwidth
of 1,530 nm to 1,610 nm. At this time, in addition, a noise figure
of 5 dB or less is observed.
[0320] Among the compositions allowable in the region A, a glass
composition of TeO.sub.2 (70 mole %)--ZnO (10 mole %)--Li.sub.2O
(15 mole %)--Bi.sub.2O.sub.3 (5 mole %) is selected and used as a
core material. The core material is doped with 2,000 rpm of erbium.
Also, a glass composition of TeO.sub.2 (70 mole %)--ZnO (7 mole
%)--Li.sub.2O (18 mole %)--Bi.sub.2O.sub.3 (5 mole %) is selected
and used as a clad material. Then, an optical fiber having a
cut-off wavelength of 1.1 .mu.m and a core/clad relative refractive
index difference of 1.5% is formed using these materials. In this
embodiment, the resulting optical fiber is used as an amplification
medium with a fiber-loss at 1.2 .mu.m of 60 dB/km.
[0321] An optical amplifier is constructed using the amplification
medium of 3 m in length and subjected to an amplification test as
the same way as that of the aforementioned optical amplifier having
the composition of the region B. As a result, a small signal gain
of 20 dB or more is observed at 80 nm bandwidth of 1,530 nm to
1,610 nm. At this time, in addition, a noise figure of 5 dB or less
is observed. Accordingly, the result indicates that the glass
composition in the region A can be also used in the process of
making a practical broadband EDFA.
[0322] (Embodiment 18)
[0323] An optical amplifier is constructed using a 15-meter piece
of the optical fiber of Embodiment 17 and subjected to an
amplification test on condition that a bi-directional pumping with
forward and backward pump wavelengths of 1.48 .mu.m is used and a
wavelength tunable laser operating at 1.5 .mu.m to 1.7 .mu.m bands
is used as a signal light source. As a result, a small signal gain
of 20 dB or more is observed at 50 nm bandwidth of 1,580 nm to
1,630 nm. The noise figure is 5 dB or less.
[0324] (Embodiment 19)
[0325] A laser is constructed using a 15-meter piece of the optical
fiber of Embodiment 17. In addition, a cavity is also constructed
using a total reflection mirror and a fiber-bragg-grating having a
reflectivity of 3 %. A bidirectional pump wavelength is 1.48 .mu.m
with a coincidence of the forward and backward pump wavelengths. An
optical-power output of 150 mW at a wavelength of 1,625 nm is
attained when an incident pump intensity is 300 mW. This kind of
the high power cannot be generated by the conventional silica- and
fluoride-based optical fibers.
[0326] (Embodiment 20)
[0327] An amplification optical fiber is prepared using a glass
composition of TeO.sub.2 (68 mole %)--ZnO (13 mole %)--Na.sub.2O (4
mole %)--Bi.sub.2O.sub.3 (15 mole %) as a core material and doped
with 3 weight % of erbium and a glass composition of TeO.sub.2 (69
mole %)-ZnO (21 mole %)--Na.sub.2O (8 mole %)--Bi.sub.2O.sub.3 (2
mole %) as a clad material. The optical fiber has a cut-off
wavelength of 1.4 .mu.m, and a core/clad relative refractive index
difference of 5%.
[0328] Then, a small-sized optical amplifier is constructed using a
3-cm piece of the optical fiber as an optical amplification medium
and subjected to an optical amplification test. In this case, a
forward pump wavelength of 1.48 .mu.m is used. In addition, a
wavelength tunable laser operating at 1.5 .mu.m to 1.7 .mu.m bands
is used as a signal light source. As a result, a small signal gain
of 20 dB or more is observed at 80 nm bandwidth of 1,530 nm to
1,610 nm. At this time, in addition, a noise figure of 5 dB or less
is observed.
[0329] (Embodiment 21)
[0330] A tellurite glass is prepared by the process including the
following steps. That is, at the start, raw materials of TeO.sub.2,
ZnO, Na.sub.2CO.sub.3, and Bi.sub.2O.sub.3 are formulated so as to
become a formulation of TeO.sub.2 (73.5 mole %)--ZnO (20 mole
%)--Na.sub.2O (5 mole %)--Bi.sub.2O.sub.3 (1.5 mole %) and a
formulation of TeO.sub.2 (73 mole %)--ZnO (20 mole %)--Na.sub.2O (5
mole %)--Bi.sub.2O.sub.3 (2 mole %) after melting. Two different
formulations are filled in respective 90 g volume crucibles and
heated by an electric furnace at 800.degree. C. for 2 hours in an
oxygen atmosphere to melt those formulations, resulting in molten
materials. Subsequently, each of the molten materials is casted in
a cylindrical hollow-mold and an opening of the mold is capped with
a cap without delay. The capped mold is laid in a horizontal
position and left for 2 minutes and then allowed to reach room
temperature, resulting a tellurite glass of 15 mm in outer
diameter, 5 mm in inner diameter, and 130 mm in length in the form
of a cylindrical tube having a bottom surface. In this manner, two
glass tubes are obtained.
[0331] The glass tubes are examined under a microscope. As a
result, they can be distinguished microscopically. That is, the
glass tube containing 1.5 mole % of Bi.sub.2O.sub.3 has many
crystallized portions in the proximity of its outside wall, while
the glass tube containing 2.0 mole % of Bi.sub.2O.sub.3 does not
have any crystallized portion.
[0332] Then, a part of each glass sample is broken into pieces and
powdered in an agate mortar. 30 mg of the obtained powder is filled
into a sealed container made of silver and then subjected into the
DSC measurement in an argon atmosphere at a heat-up rate of
10.degree. C./minute. The results are shown in FIG. 25.
[0333] FIG. 25 is a graphical representation of the results of the
DSC measurement. In the figure, a line (a) is for the glass having
a composition of 73.5TeO.sub.2-20ZnO-5Na.sub.2O-1.5Bi.sub.2O.sub.3
and a line (b) is for the glass having a composition of
73TeO.sub.2-20ZnO-5Na.s- ub.2O-2Bi.sub.2O.sub.3. In the case of the
glass containing 1.5 mole % of Bi.sub.2O.sub.3, a peak of
crystallization is started at a temperature of about 350.degree. C.
and a value of Tx-Tg is 69.2.degree. C. In the case of the glass
containing 2.0 mole % of Bi.sub.2O.sub.3, on the other hand, a peak
of crystallization is started at a temperature of about 390.degree.
C. and a value of Tx-Tg is 110.4.degree. C. It means that a thermal
stability of glass can be dramatically increased by an addition of
2.0 mole % Bi.sub.2O.sub.3 compared with that of 1.5 mole %
Bi.sub.2O.sub.3 .
[0334] The most striking characteristics of the tellurite glasses
of Embodiments 1 to 21 is that each of them is formulated as a
quadric system composition that consists four different ingredients
including Bi.sub.2O.sub.3. This kind of the tellurite glass shows
an excellent thermal stability, so that a fiber-loss can be
minimized at the time of forming a fiber. Furthermore, the
tellurite glass allows easily control of the refractive index, so
that a fiber with a high An can be formed. Therefore, the tellurite
glass allows the scale-up of amplification bandwidth of the EDFA
having the ternary-system composition which leads to a low degree
of efficiency.
[0335] As described above, the ternary-system composition disclosed
in U.S. Pat. No. 5,251,062 is less stable than the quadric system
tellurite glass of the present invention, so that a minimum of the
fiber-loss at 1.5 .mu.m is 1,500 dB/km. In the present invention,
we studies various compositions for the purpose of the reduction in
fiber-loss and finally find that the quadric system tellurite
composition containing Bi.sub.2O.sub.3 allows a dramatic decrease
in the fiber-loss. Also, we find that the tellurite glass allows
easily control of the refractive index and a fiber with a high An
can be formed. Therefore, the present invention realizes the
tellurite EDFA with a low fiber-loss at first. We can easily
understand that the tellurite glass having the ternary system
composition of U.S. Pat. No. 5,251,062 cannot realize the
three-level system EDFA because there is no concrete description
not only in the specification of U.S. Pat. No. 5,241,062 but also
in subsequent reports in technical journals such as Optics Letters
and Optical Materials.
[0336] In U.S. Pat. No. 5,251,062, for more details, Sniitzer et
al. indicate that a composition range of ternary-system tellurite
glass which can be formed into a fiber with a description that a
laser device can be realized by using a bulk glass while an optical
amplifier requires a fiber structure having core and clad
structures. Accordingly, it is clear that the reference discloses
the tellurite glass for the purpose of realizing an optical
amplification. However, Snitzer et al. cannot disclose the way of
solving the problems except the description of a fiber laser using
neodymium in Optics Letters cited above. It is a well-known fact
that neodymium cannot be applied in an optical amplification at 1.3
.mu.m band because of excitation state absorption as described in
Optics Letters cited above in spite that it holds great promise to
be applied in 1.3 .mu.m band amplification in past.
[0337] In Optics Letters cited above, the tellurite glass that
contains Bi.sub.2O.sub.3 has the composition of 78 % TeO.sub.2-18%
Bi.sub.2O.sub.3 and 80 % TeO.sub.2-10% Bi.sub.2O.sub.3-10 %
TiO.sub.2. However, this composition is much different from the
quadric system composition of the present invention. Furthermore,
it is noted that there is no description or teach of thermal
stability of glass and fiber-loss even though these properties are
very important.
[0338] Furthermore, the quadric system tellurite glass having a
core composition of 77 % TeO.sub.2-6.0% Na.sub.2O-; 15.5 % ZnO-1.5%
Bi.sub.2O.sub.3 is described in Optical Materials and Optic Letters
cited above, especially in Optical Materials which is also disclose
a fiber-loss at 1.55 .mu.m band of 1,500 dB/km. However, this
fiber-loss is too high, and also there is no description or teach
that indicates or recalls the improvements in thermal stability by
an addition of Bi.sub.2O.sub.3. In the field of optical fiber, it
is well-known fact that a refractive index of glass is adjusted by
an appropriate adjusting agent. The addition of Bi.sub.2O.sub.3 is
exactly what the optical fiber needs for that adjustment.
[0339] In the present invention, as will be described afterward, we
find that tellurite glasses having Bi.sub.2O.sub.3-contained
quadric system compositions are effective on a reduction in
fiber-loss. A thermal stability of the tellurite glass is
dramatically improved by the addition of Bi.sub.2O.sub.3 in
concentration of over 1.5%, and thus the tellurite glass optical
fiber can be provided as the one having the properties of low
fiber-loss. Secondary, a high An fiber can be formed as a result of
controlling Bi.sub.2O.sub.3 content in the core and clad glasses
without restraint. Consequently, the scale-up of amplification band
region of low-efficient three-level system EDFA by a synergistic
effect of these improvements.
[0340] We are now considering a glass composition that levels a
gain spectrum of tellurite EDFA. In the following embodiments, one
of the most striking characteristics of the following embodiments
is that the tellurite glass or the tellurite optical fiber contains
aluminum (Al) as a host. It is also known that if SiO.sub.2-based
glass contains Al a dented portion between 1.53 .mu.m and 1.56
.mu.m of a cross-section of stimulated emission of Er in the glass
is despaired and also the gain spectrum is flatten at wavelengths
of 1.54 .mu.m to 1.56 .mu.m (Emmanue Desurvire, "Erbium-Doped Fiber
Amplifiers", John Wiley & Sons, 1994). However, this is an
effect of Al on the silica-based optical fiber, so that the effect
on the tellurite glass is still unknown.
[0341] In the present invention, as described in the following
embodiments, the present inventors are finally found the following
facts. That is, the addition of Al in tellurite glass leads to the
effects of disappearing a dented portion between 1.53 .mu.m and
1.56 .mu.m of a cross-section of stimulated emission of Er in the
glass and increasing variations in the cross-section, resulting in
reduction of a gain difference between 1.55 .mu.m band and 1.6
.mu.m band.
[0342] (Embodiment 22)
[0343] FIG. 26 is a spectrum diagram that illustrates each 1.5
.mu.m emission spectrum of Er in glasses having their respective
compositions of:
[0344] TeO.sub.2 (74 mole %)--ZnO (16 mole %)--Na.sub.2O (6
mole--Bi.sub.2O.sub.3 (4 mole %);
[0345] TeO.sub.2 (73 mole %)--ZnO (15 mole %)--Na.sub.2O (6 mole
%)--Bi.sub.2O.sub.3 (3 mole %)--Al.sub.2O.sub.3 (3 mole %); and
[0346] TeO.sub.2 (79 mole %)--ZnO (3 mole %)--Li.sub.2O (12 mole
%)--Bi.sub.2O.sub.3 (3 mole %)--Al.sub.2O.sub.3 (3 mole %).
[0347] As shown in the figure, an intensity of emission spectrum of
the glass containing Al.sub.2O.sub.3 at a wavelength of around 1.6
.mu.m is stronger than that of the glass without Al.sub.2O.sub.3,
and also a depth of the dent between 1.53 .mu.m and 1.56 .mu.m of
the former is disappeared or shallower than that of the latter.
[0348] An erbium-doped tellurite optical fiber is prepared using
the Al.sub.2O.sub.3-contained glass
(TeO.sub.2--ZnO--Na.sub.2O--Bi.sub.2O.sub- .3 system glass). The
obtained fiber has a cut-off wavelength of 1.3 .mu.m. Er-content of
4,000 ppm, and length of 0.9 m. Then, the fiber is optically pumped
at 1.48 .mu.m with the power of 200 mW, resulting in a 10 dB or
less gain difference between 1.56 .mu.m and 1.60 .mu.m.
[0349] Next, EDFA is constructed using the above erbium-doped
optical fiber as an optical amplifier and a fiber-bragg-grating as
a gain-equalizing device. The obtained EDFA showed 1 dB or less
gain difference between 1.53 .mu.m and 1.60 .mu.m.
[0350] On the other hand, an EDFA using the Al.sub.2O.sub.3-absent
fiber showed 15 dB or more gain difference between 1.56 .mu.m and
1.60 .mu.m. In this case, furthermore, the gain deviation at
wavelengths from 1.56 .mu.m to 1.60 .mu.m could not be reduced to 1
dB or less across a width of 70 nm in spite of using the
gain-equalizing device.
[0351] The addition effects of Al.sub.2O.sub.3 to the gain
characteristics of optical fiber is confirmed for the composition
of TeO.sub.2--ZnO--Li.sub.2O--Bi.sub.2O.sub.3
(55.ltoreq.TeO.sub.2.ltoreq.90- , 0.ltoreq.ZnO.ltoreq.35,
0.ltoreq.Na.sub.2O.ltoreq.35, 0.ltoreq.Bi.sub.2O.sub.3.ltoreq.20,
unit: mole %).
[0352] (Embodiment 23)
[0353] In this Embodiment, we confirm an influence of adding
Al.sub.2O.sub.3 to TeO.sub.2--ZnO--Li.sub.2O--Bi.sub.2O.sub.3 on
the gain characteristics.
[0354] A comparative study of 1.5 .mu.m band emission spectra of Er
in an Al.sub.2O.sub.3-absent glass having the composition of
(80)TeO.sub.2-(3)ZnO-(12)Li.sub.2O-(5)Bi.sub.2O.sub.3 and an
Al.sub.2O-contained glass having the composition of
(79)TeO.sub.2-(3)ZnO-(3)Li.sub.2O-(12)Bi.sub.2O.sub.3-(3)Bi.sub.2O.sub.3
is performed. As a consequence, we find that the
Al.sub.2O-contained glass has a large emission strength at 1.6
.mu.m and there is no dent portion between 1.53 .mu.m and 1.56
.mu.m in the spectrum compared with those of the Al.sub.2O-absent
glass. The Al.sub.2O-contained glass is formed into a core of
erbium-doped tellurite optical fiber (a cut-off wavelength of 1.3
.mu.m, an erbium-content of 4,000 ppm, and a length of 0.9 m) and
optically pumped at 1.48 .mu.m with the power of 200 mW, resulting
in a 10 dB or less gain deviation at wavelengths from 1.56 .mu.m
and 1.60 .mu.m.
[0355] Next, an EDFA is constructed using the above erbium-doped
optical fiber as an optical amplifier and a Mach-Zehnder type
filter (a medium for loss of light) as a gain-equalizing device.
The obtained EDFA showed 1 dB or less gain deviation at wavelengths
from 1.53 .mu.m to 1.60 .mu.m.
[0356] On the other hand, an EDFA using the Al.sub.2O.sub.3-absent
fiber showed 15 dB or more gain deviation at wavelengths from 1.56
.mu.m to 1.60 .mu.m. In this case, furthermore, the gain deviation
at wavelengths from 1.56 .mu.m to 1.60 .mu.m could not reduced to 1
dB or less across a width of 70 nm in spite of using a
gain-equalizing device.
[0357] In addition, variations in the gain between 1.53 .mu.m and
1.56 .mu.m are disappeared at the time of measuring the
amplification spectrum when the fiber of 2 m in length doped with
1,000 ppm of Er is used, resulting in uniformity of gain at 1.53
.mu.m to 1.56 .mu.m. Therefore, this fiber could be useful in the
amplification of WDM transmission at 1.53 .mu.m to 1.56 .mu.m. This
phenomenon is also observed when the fiber having the composition
of TeO.sub.2--ZnO--Na.sub.2O--Bi.sub.2O.sub.3--Al.- sub.2O.sub.3 is
used.
[0358] The addition effects of Al.sub.2O.sub.3 to the gain
characteristics of optical fiber is confirmed for the composition
of TeO.sub.2--ZnO--Li.sub.2O--Bi.sub.2O.sub.3
(70.ltoreq.TeO.sub.2.ltoreq.90- , 0.ltoreq.ZnO.ltoreq.4,
0.ltoreq.Li.sub.2O.ltoreq.30, 0<Bi.sub.2O.sub.3<10, unit:
mole %) to be used in the fiber formation with stability.
[0359] In Embodiments 22 and 23, 3 mole % of Al.sub.2O.sub.3 is
used. However, it is not limited to such a concentration. We also
attained the Al.sub.2O.sub.3 addition effect at concentrations of
more than zero mole %. It is not preferable to include
Al.sub.2O.sub.3 in the fiber more than necessary because an
excessive concentration thereof leads to ignore the above-mentioned
composition that allows the stable fiber formation.
[0360] (Embodiment 24)
[0361] In this Embodiment, we confirmed an influence of adding
Al.sub.2O.sub.3 to TeO.sub.2--ZnO--M.sub.2O--Bi.sub.2O.sub.3 (M is
one of alkali elements except Li and Na) on the gain
characteristics. As in the case of Embodiments 22 to 23, a 10 dB or
less gain deviation at wavelengths from 1.56 .mu.m and 1.60 .mu.m
is attained by using K, Cs, or Rb as M in the above composition. In
addition, an EDFA is constructed using the gain-equalizing device.
As a result, the EDFA showed 1 dB or less gain at wavelengths from
1.53 .mu.m to 1.60 .mu.m (i.e., over a bandwidth of 70 nm), and
also provided the uniform gain at that wavelengths.
[0362] (Embodiment 25)
[0363] In this Embodiment, we confirmed an influence of adding
Al.sub.2O.sub.3 to TeO.sub.2--ZnO--M.sub.2O--Bi.sub.2O.sub.3 (M is
at least two of alkali elements except Li and Na) on the gain
characteristics. As in the case of Embodiments 23 and 24, a 10 dB
or less gain deviation at wavelengths from 1.56 .mu.m and 1.60
.mu.m is attained in spite of including two elements in the
composition. In addition, an EDFA is constructed using the
gain-equalizing device. As a result, the EDFA showed 1 dB or less
gain at wavelengths from 1.53 .mu.m to 1.60 .mu.m (i.e., over a
bandwidth of 70 nm), and also provided the uniform gain at that
wavelengths.
[0364] (Embodiment 26)
[0365] We described above that an influence of adding
Al.sub.2O.sub.3 to TeO.sub.2--ZnO--R.sub.2O--Bi.sub.2O.sub.3 (R is
an alkali element) on the gain characteristics. In this Embodiment,
we also confirmed that an addition of Al.sub.2O.sub.3 effected on
the gain deviation on another type of tellurite glass (not depended
on the composition except TeO.sub.2 and Al.sub.2O.sub.31 such as
TeO.sub.2--WO.sub.2 and
TeO.sub.2--ZnO--La.sub.2O--Bi.sub.2O.sub.3--Al.sub.2O.sub.3 (R is
at least one of alkali elements) glass) for realizing EDFA in the
type of a broadband and gain-flattening.
[0366] (Embodiment 27)
[0367] In this Embodiment, 100 glass samples are prepared from a
glass composition of
TeO.sub.2-ZnO-Li.sub.2O--Na.sub.2O--Bi.sub.2O.sub.3. That is, the
glass samples are made of materials containing 75 mole % TeO.sub.2
and 5 mole % Bi.sub.2O.sub.3 or materials containing 80 mole %
TeO.sub.2 and 5 mole % Bi.sub.2O.sub.3. In each of the materials,
furthermore, the contents of other ingredients are varied. Then,
each of the glass samples is broken into pieces and powdered in an
agate mortar. 30 mg of the obtained powder is filled into a sealed
container made of silver and then subjected into the DSC
measurement in an argon atmosphere at a heat-up rate of 10.degree.
C/minute, resulting in a heat-stable glass having 120.degree. C. or
more of Tx-Tg in the region B shown in FIGS. 27 and 28. The
heat-stable glass allows the mass production of optical fibers and
the lower prices that result therefrom.
[0368] Then, an optical amplification medium is prepared as a fiber
having a cut-off wavelength of 1.1 .mu.m and a core/clad relative
refractive index difference of 1.6 %. That is, a core of the fiber
is formed from a glass composition the glass composition of
TeO.sub.2 (75 mole %)-ZnO (5 mole %)-Li.sub.2O (12 mole
%)-Na.sub.2O (3 mole %)-Bi.sub.2O.sub.3 (5 mole %) doped with 2,000
ppm of erbium, and also a clad of the fiber is formed from a glass
composition of TeO.sub.2 (75 mole %)-ZnO (2 mole %)-Li.sub.2O (15
mole %)-Na.sub.2O (3 mole %)-Bi.sub.2O.sub.3 (5 mole %).
[0369] The obtained medium is cut to a fiber of 3 m in length to
construct an optical amplifier. The optical amplifier is subjected
to an amplification test.
[0370] In the amplification test, a bidirectional pumping procedure
with a forward-pump wavelength of 0.98 .mu.m and a backward-pump
wavelength of 1.48 .mu.m is used. In addition, a wavelength tunable
laser that covers from 1.5 .mu.m to 1.7 .mu.m band is used as an
optical signal source. As a result, a small signal gain of 20 dB or
more is obtained at a bandwidth of 80 nm ranging from 1,530 to
1,610 .mu.m. At this time, a noise figure (NF) is 5 dB or less.
[0371] From a region (that indicates the compositions capable of
being used in the fiber formation) shown in FIG. 28, a glass
composition of TeO.sub.2 (80 mole %)-ZnO (6 mole %)-Li.sub.2O (4
mole %)-Na.sub.2O (5 mole %)-Bi.sub.2O.sub.3 (5 mole %) is selected
from among allowable compositions in the region so as to be
provided as a core material. Among the compositions in the region,
furthermore, a glass composition of TeO.sub.2 (80 mole %)-ZnO (2
mole %)-Li.sub.2O (6 mole %)-Na.sub.2O (7 mole %)-Bi.sub.203 (5
mole %) is selected so as to be provided as a clad material. The
core material is doped with 2,000 ppm of erbium. Then, an optical
fiber having a cut-off wavelength of 1.1 .mu.m and a core/clad
relative refractive index difference of 1.5 %. The obtained fiber
is provided as an optical amplification medium. A fiber-loss of the
medium is 0.07 dB/m at 1.2 .mu.m.
[0372] The medium is cut to a fiber of 3 m in length to construct
an optical amplifier. The optical amplifier is also subjected to an
amplification test. As a result, a small signal gain of 20 dB or
more over is obtained at a bandwidth of 80 nm ranging from of 1,510
to 1,630 nm. At this time, a noise figure (NF) is 5 dB or less. The
results indicated that a practical broadband EDFA could be prepared
from any glass composition of the B region without any trouble.
[0373] (Embodiment 28)
[0374] An optical amplifier is constructed by the same way as that
of Embodiment 27 except of using a fiber of 15 m length in this
Embodiment and subjected to an amplification test. For pump
wavelengths to be applied from both side, a bidirectional pumping
procedure in which the front and the backward wavelengths are
identical with each other is used. In addition, a wavelength
tunable laser that covers from 1.5 .mu.m to 1.7 .mu.m band is used
as an optical light source. As a result, a small signal gain of 20
dB or more is obtained at a bandwidth of 70 nm ranging from 1,560
to 1,630 nm. At this time, noise figure is 5 dB or less.
[0375] (Embodiment 29)
[0376] A laser is constructed using a fiber (15 m in length) which
is prepared as the same way as that of Embodiment 27. A cavity is
constructed using a fiber-bragg- grating that has a refractive
index of 3 % at a wavelength of 1,625 nm with respect to a total
reflection mirror. For pump wavelengths to be applied from both
side, a bidirectional pumping procedure in which the forward and
the backward wavelengths are of 1.48 .mu.m. The laser generated a
high-power of 150 mW at 1,625 nm wavelength, which could not be
attained by silica-based and fluoride-based optical fibers.
[0377] (Embodiment 30)
[0378] In this Embodiment, 50 glass samples are prepared from a
glass composition of
TeO.sub.2--ZnO--Li.sub.2O--Al.sub.2O.sub.3--Bi.sub.2O.sub.- 3. The
samples have 2 mole of Al.sub.2O.sub.3 and 12 mole % of Li.sub.2O
except that every sample have its own ratios of other ingredients.
Then, each of the glass samples is broken into pieces and powdered
in an agate mortar. 30 mg of the obtained powder is filled into a
sealed container made of silver and then subjected into the DSC
measurement in an argon atmosphere at a heat-up rate of 10.degree.
C./minute, resulting in a heat-stable glass having 120.degree. C.
or more of Tx-Tg in a region defined as A in FIG. 29. The
heat-stable glass allows a fiber having a lower loss of 0.1 dB/m or
less. In addition, the effects of adding Al.sub.2O.sub.3 into the
glass composition allows a broader area of induced-emission cross
section, so that an amplification bandwidth of EDFA can be
broadened.
[0379] Next, an optical amplification medium is prepared as a fiber
having a cut-off wavelength of 1.1 .mu.m and a core/clad relative
refractive index difference of 1.6 %. A core of the fiber is formed
from a glass composition the glass composition selected from the
region in FIG. 29, that is, TeO.sub.2 (82 mole %)-ZnO (1 mole
%)-Li.sub.2O (12 mole %)-Al.sub.2O.sub.3 (2 mole %)-Bi.sub.203 (3
mole %) doped with 2,000 ppm of erbium. Also, a clad of the fiber
is formed from a glass composition of TeO.sub.2 (75 mole %)-ZnO (3
mole %)-Li.sub.2O (18 mole %)-Bi.sub.2O.sub.3 (4 mole %). A
fiber-loss at 1.2 .mu.m is 0.07 dB/m.
[0380] The obtained medium is cut to a fiber of 3 m in length to
construct an optical amplifier. The optical amplifier is subjected
to an amplification test.
[0381] In the amplification test, a bidirectional pumping procedure
with a forward-pump wavelength of 0.98 .mu.m and a backward-pump
wavelength of 1.48 .mu.m is used. In addition, a wavelength tunable
laser that covers from 1.5 .mu.m to 1.7 .mu.m band is used as an
optical signal source. As a result, a small signal gain of 20 dB or
more is obtained at a bandwidth of 80 nm ranging from 1,530 to
1,610 .mu.m. At this time, a noise figure (NF) is 5 dB or less.
[0382] (Embodiment 31)
[0383] A wavelength tunable ring laser is constructed using the
same fiber (4 m in length) as that of Embodiment 30, and also a
wave tunable filter for wavelengths from 1.5 .mu.m to 1.7 .mu.m is
used as a filter. In addition, a bidirectional pumping procedure in
which the front and the backward wavelengths are identical with
each other is used. At an incident pump strength of 300 mW, the
laser showed its broadband laser characteristic of 5 mW or more at
a bandwidth of 135 nm ranging from 1,500 to 1,635 nm, which could
not be attained by the silica-based and fluoride-based optical
fiber.
[0384] (Embodiment 32)
[0385] Five optical fibers of 800 m in length as a fiber having a
cut-off wavelength of 1.1 .mu.m and a core/clad relative refractive
index difference of 1.3 to 2.2 %. In this Embodiment, a core of the
fiber is formed from a glass composition the glass composition of
TeO.sub.2 (79.5 - x mole %)-ZnO (14.5 mole %)-Na.sub.2O (6 mole
%)-Bi.sub.2O.sub.3 (x mole %) doped with 500 ppm of erbium, and
five fibers took their values of x=4, 4.2, 5.4, 6.8, and 7,
respectively. Also, a clad of the fiber is formed from a glass
composition of TeO.sub.2 (75 mole %)-ZnO (17.5 mole %)-Na.sub.2O (5
mole %)-Bi.sub.2O.sub.3 (2.5 mole %) glass. In the case of fibers
of x=4 and x=7 mole %, the spacing between adjacent scattering
points (i.e., a point where a fiber-loss is remarkably increased by
a scattering of light from particles such as crystals) is 15 m or
less, and also a fiber-loss in the area without the scattering
centers is 0.07 dB/m at a wavelength of 1.2 .mu.m. In the case of
fibers of x=4.2, 5.4, and 6.8 mole %, on the other hand, the
spacing between adjacent scattering centers is 100 m or more, and
also a fiber-loss in the area without the scattering centers is
0.02 dB/m or less.
[0386] In general, by the way, a required fiber length for
constructing EDFA is about 10 m. When the fibers of x=4 or 7 mole %
are used, only 20 or less 10-meter-long fibers are obtained from an
800-meter-long fiber. On the other hand, 70 or more 10-meter-long
fibers are obtained from an 800-meter-long fiber when the fibers of
x=4.2, 5.4, and 6.8 mole % are used, resulting in a dramatic
improvement in yield.
[0387] In the following Embodiments 33 to 39, tellurite EDFAs that
has improved characteristics of chromatic dispersion will be
described in view of the fact that the improved characteristics of
tellurite optical fibers described above.
[0388] An optical amplifier using a tellurite glass composition as
an amplification medium is mainly characterized by having a
configuration where a dispersion medium is placed in the front of
or at the back of the tellurite EDFA. The dispersion medium
compensates for the dispersion of wavelengths by a value of
chromatic dispersion that takes a plus or negative number opposite
to a value of chromatic dispersion for the tellurite EDFA. The
medium that compensates the chromatic dispersion may be an optical
fiber, an fiber black grating, or the like.
[0389] The conventional tellurite EDFAs do not have any medium that
compensates the chromatic dispersion, so that the degree of
dispersion tends to increase. Conventionally, therefore, there is a
problem of that an error rate is increased as a result of
performing a first signal amplification. To solve this problem, the
following Embodiments will provide novel configurations of
tellurite EDFA that retains the qualities of communications by
allowing an decrease in a value of chromatic dispersion in the
optical amplifier to avoid an increase in error rate whether the
high speed signal amplification is performed or not.
[0390] (Embodiment 33)
[0391] FIG. 30 illustrates an optical amplifier as one of the
preferred embodiments of the present invention. In the figure, an
optical signal enters from the left side and exits to the right
side of the optical amplifier. The input signal light passes
through an optical isolator 201a and then combined with an
excitation light from an excitation light source through an optical
coupler 203. Then, the combined signal light is introduced into an
optical fiber 205 for optical amplification after passing through a
dispersion medium 204. The signal light amplified by the optical
fiber 205 is then outputted through an optical isolator lb.
[0392] In the optical amplifier of the present embodiment, a
semiconductor laser having an oscillation wavelength of 1.48 .mu.m
is used as an excitation light source 203 and a signal wavelength
of 1.55 .mu.m is used. In addition, a tellurite optical fiber of 10
m in length is used as an optical fiber for optical amplification.
The tellurite optical fiber is characterized by an erbium-doping
concentration of 200 ppm in its core, a cut-off wavelength of 1.3
.mu.m, and a core/clad relative refractive index difference of 1.4
%. A value of chromatic dispersion is --1.3 ps/nm. Furthermore, a
single-mode silica-based optical fiber of 1.3 .mu.m zero dispersion
(so-called a standard single-mode optical fiber) having a chromatic
dispersion value of 17 ps/km/nm at 1.55 .mu.m is used as a
dispersion medium. A length of the fiber is 76 m.
[0393] A chromatic dispersion of all of the dispersion medium 204
and the amplification optical fiber 205 is measured and resulted in
0.1 ps/nm or less.
[0394] An amplification of a high-speed optical signal of 40
Gbit/sec at a wavelength of 1.55 .mu.m is performed using the
optical amplifier obtained by the above procedure. In this case, we
could not observe any distortion of pulse wavelength caused by the
chromatic dispersion. Therefore, we find that the optical amplifier
of the present embodiment can be used in a booster amp, an in-line
amplifier, or a pre-amplifier without decreasing the qualities of
communications. For the comparison, on the other hand, an
amplification of a high-speed pulse of 40 Gbit/s at 1.55 .mu.m
wavelength is performed without inserting the dispersion medium
204. As a consequence of the amplification, pulse-waveform
distortions are observed. It means that it is very difficult to
apply this configuration to a high-speed communication system.
[0395] In the present embodiment, the dispersion medium 204 is
placed between an optical coupler 203 and Er-doped tellurite
optical fiber 205 but not limited to such an arrangement. The
dispersion medium 204 may be placed in front of the optical
isolator la, between the optical isolator la and the optical
coupler 203, between the amplification medium and the optical
isolator 201b, or at the back of the optical isolator 201b. In this
embodiment, furthermore, a standard single-mode optical fiber is
used but not limited to. It is also possible to use any optical
fibers that have a chromatic dispersion of the tellurite optical
fiber 205 and an oppositely signed value of chromatic dispersion.
For the dispersion medium 204, a chirped fiber grating (K.O.Hill,
CLEO/PACIFC RIM SHORT COURSE '97 "Photosensitivity and bragg
Gratings in Optical Waveguide") may be used instead of the optical
fiber.
[0396] In the above description, by the way, the dispersion medium
204 is placed in front of or at the back of the amplification
optical fiber 205 but not limited to. It is also possible to adopt
another arrangement of the dispersion medium 204. That is, if an
optical fiber is used as the dispersion medium 204, the optical
fiber can be divided into two portions in which one is arranged on
an appropriate position in front of the amplification optical fiber
205 and the other is arranged on an appropriate position at the
back of the fiber 205. Also, a plurality of optical fibers having
different characteristics can be arranged on appropriate
positions.
[0397] (Embodiment 34)
[0398] In this embodiment, an amplification is a tellurite optical
fiber (15 m in length) doped with 500 ppm of Pr (praseodymium)
having a cut-off wavelength of 1.0 .mu.m and .DELTA.n=1.4% and an
amplification optical fiber 205 in FIG. 30. An excitation light
source 202 is a Nd (neodium)-doped YLF laser. For a dispersion
medium 204, a chirped fiber grating is used. At this time, a
chromatic dispersion of the tellurite optical fiber at 1.31 .mu.m
is 3.15 ps/nm. Hence, a chromatic dispersion value of the grating
is adjusted to 3.15 ps/nm and then an amplification of high-speed
signal at 1.31 .mu.m wavelength is performed using an amplifier of
such a configuration. As a result, pulse-waveform distortions are
not observed in spite of performing an amplification of high-speed
pulse of 40 Gbit/s at 1.3 .mu.m wavelength. It means that it is
possible to apply this configuration to a high-speed communication
system. In the case of an optical amplifier which is constructed
without using the dispersion medium 204, pulse-waveform distortions
are occurred if an amplification of high-speed pulse is performed,
resulting in the difficulty in an amplification for the high-speed
communication system.
[0399] (Embodiment 35)
[0400] In this embodiment,
TeO.sub.2--ZnO--M.sub.2O--Bi.sub.2O.sub.3 glass composition (where
M is one or more alkali elements) is used as a preform of an
amplification optical fiber 205. That is, a core of the fiber is
made of the glass composition plus Er, Pr, and Tm as additives for
1.48 .mu.m or 1.65 .mu.m band amplification, and also a clad of the
fiber is made of the glass composition plus Nd as an additive for
1.06 .mu.m or 1.33 .mu.m band amplification.
[0401] An amplification of high-speed pulse is performed by an
optical amplifier using a silica-based fiber or a chirped-fiber
gratings as the dispersion medium 204 and compensating chromatic
dispersions at an amplification wavelength of each earth rare
element. As a result, waveform distortions of optical pulse that
occurred when the dispersion medium 204 is absent are prevented.
Consequently, we confirmed that it could be used in the high-speed
optical communication system.
[0402] (Embodiment 36)
[0403] In this embodiment,
TeO.sub.2--ZnO--Na.sub.2O--Bi.sub.2O.sub.3 glass composition is
used as a preform of an amplification optical fiber 205. A content
of each ingredient in the composition is 55 mole
%.ltoreq.TeO.sub.2.ltoreq.90 mole %, 0 mole %.ltoreq.ZnO.ltoreq.25
mole %, 0 mole %.ltoreq.Li.sub.2O.ltoreq.25 mole %, 0 mole
%.ltoreq.Bi.sub.2O.sub.3.ltoreq.20 mole %. A core of the fiber is
made of the glass composition plus Er, Pr, Tm, or Nd as an additive
for 1.48 .mu.m or 1.65 .mu.m band amplification, and also a clad of
the fiber is made of the glass composition plus Nd as an additive
for 1.06 .mu.m or 1.33 .mu.m band amplification.
[0404] An amplification of high-speed pulse is performed by an
optical amplifier using a silica-based fiber or a chirped-fiber
grating as the dispersion medium 204 and compensating chromatic
dispersions at an amplification wavelength of each earth rare
element. As a result, waveform distortions of optical pulse that
occurred when the dispersion medium 204 is absent are prevented.
Consequently, we confirmed that it could be used in the high-speed
optical communication system.
[0405] (Embodiment 37)
[0406] An amplification optical fiber 205 is constructed using a
glass composition of TeO.sub.2--ZnO--M.sub.2O--Bi.sub.2O.sub.3
(where M is one or more alkali elements) as a preform. In the
composition, a content of each ingredient is 55 mole
%.ltoreq.TeO.sub.2.ltoreq.90 mole %, 0 mole %.ltoreq.ZnO.ltoreq.25
mole %, 0 mole %.ltoreq.Li.sub.2O.ltoreq.25 mole %, 0 mole
%.ltoreq.Bi.sub.2O.sub.3.ltoreq.20 mole %. A core of the fiber is
made of the glass composition plus Er, Pr, Tm, or Nd as an additive
for 1.48 .mu.m or 1.65 .mu.m band amplification, and also a clad of
the fiber is made of the glass composition plus Nd as an additive
for 1.06 .mu.m or 1.33 .mu.m band amplification.
[0407] An amplification of high-speed pulse is performed by an
optical amplifier using a silica-based fiber or a chirped-fiber
gratings the dispersion medium 204 and compensating chromatic
dispersions at an amplification wavelength of each earth rare
element. As a result, waveform distortions of optical pulse that
occurred when the dispersion medium 204 is absent are prevented.
Consequently, we confirmed that it could be used in the high-speed
optical communication system.
[0408] In addition, an optical amplifier that uses an amplification
optical fiber 205 comprising a glass composition of TeO.sub.2
--ZnO--M.sub.2O--Bi.sub.2O.sub.3--Al.sub.2O.sub.3 (where M is one
or more alkali elements) showed the same effects.
[0409] (Embodiment 38)
[0410] Raman amplification is performed using a single-mode
tellurite optical fiber (a cut-off wavelength of 1.3 .mu.m,
.DELTA.n 1.4 %, and a length of 1 km). in this embodiment, the
glass composition of Embodiment 26 is used as a preform of an
amplification optical fiber except that an additive such as a rare
earth element or a transition metal element is not used in the
present embodiment. An optical amplification at 1.5 .mu.m band is
performed by pumping a wavelength of 1.48 .mu.m.
[0411] A chromatic dispersion observed in the single-mode tellurite
optical fiber at a signal wavelength is -130 ps/nm. In this case, a
dispersion medium 204 is a standard single-mode silica optical
fiber.
[0412] The dispersion medium 204 is placed at the back of the
single-mode tellurite optical fiber (the amplification optical
fiber) 205 and then an optical amplification is performed. In a
case where 7.6 km of the standard single-mode silica optical fiber
is used, we could prevent a waveform distortion of the optical
pulse at 1.5 .mu.m band (which could be considered as a result of
chromatic dispersion of the single-mode tellurite optical
fiber).
[0413] (Embodiment 39)
[0414] In this embodiment, an optical amplification procedure is
performed at each of 1.5 .mu.m band, 1.5 .mu.m band, and 1 .mu.m
band by means of an amplification optical fiber 205 which is
constructed by adding Cr, Ni, or Ti into a core of the tellurite
glass optical fiber made of the composition of Embodiment 36 or 37.
In a case where an amplification of high-speed optical pulse is
performed by connecting a standard single-mode silica optical fiber
as a dispersion medium 204 at the back of the amplification optical
fiber 205, the optical amplification could be attained without
causing a waveform distortion.
[0415] By the way, each of the embodiments described above is for
an optical waveguide. We could also obtained the same effects
recognized in the above Embodiments when a flat-type optical
waveguide is used as an optical waveguide.
[0416] (Embodiment 40)
[0417] In this Embodiment, a tellurite glass composition of
TeO.sub.2--ZnO--M.sub.2O--Bi.sub.2O.sub.3 (where M is one or more
alkali elements) is used as a preform for preparing an
amplification medium. The amplification medium is prepared using a
flat-type optical waveguide having a core doped with Er instead of
the optical fiber 205 of FIG. 30. For a dispersion medium 204, a
chromatic dispersion of that optical waveguide is corrected using a
dispersion medium 204 such as an optical fiber or a Bragg grating.
As a result, an optical amplification at 1.5 .mu.m band is
accomplished so as to lessen the likelihood of the dispersion of
optical pulse waveforms due to the chromatic dispersion
characteristics of the tellurite fiber.
[0418] Next, we carried out the following Embodiments 40 to 44 for
the scale-up of an amplification bandwidth of the conventional
tellurite EDFA, that is, shifting from 1.53 .mu.m band to a shorter
band and from 1.56 .mu.m band to a longer band.
[0419] For that purpose, an erbium-doped tellurite optical fiber of
a predetermined length is used as at least one coupled optical
fiber. In addition, a shorter erbium-doped tellurite optical fiber
(or a smaller product of erbium concentration and fiber length) or
an erbium-doped optical fiber that contains a different glass
composition is placed in front of or at the back of the tellurite
optical fiber. The different glass composition may be a fluoride
glass composition such as erbium-doped ZrF.sub.4 based glass or
InF.sub.3 based fluoride glass), a silica glass composition, a
fluorophosphate glass composition, a phosphate glass composition,
or a chalcogenide glass composition.
[0420] Using the optical amplifier constructed as described above,
therefore, a novel EDFA that acts at a broad bandwidth with a low
noise compared with that of the conventional tellurite EDFA.
[0421] (Embodiment 41)
[0422] FIG. 31 is a block diagram that illustrates an optical
amplifier as one of preferred embodiments of the present invention.
In the figure, reference numerals 201a, 201b, and 201c denotes
optical isolators, 202a and 202b denotes an optical coupler for
introducing excitation light into the fibers, 203a and 203b are
excitation light sources, and 204 and 205 are optical fibers for
the amplification.
[0423] In this embodiment, the optical fiber 204 is an
aluminum-added silica-based optical fiber doped with 1,000 ppm of
erbium (2.5 m in length, 1.2 .mu.m cut-off wavelength, and 2,500
m.multidot.ppm in product of content and length). The excitation
light source 203a is a semiconductor laser with an oscillation
wavelength of 1.48 .mu.m.
[0424] In a case that an amount of excitation light from the source
203a is 70 mW and an amount of excitation light from the source
203b is 150 mW, a gain of 20 dB or more and a noise figure of 5 dB
or less are obtained at a bandwidth of 85 nm in the region of 1,525
.mu.m to 1,610 .mu.m.
[0425] The EDFA of the present embodiment attains the ability to
act in such a bandwidth without causing any significant noise,
which is impossible for the conventional EDFA.
[0426] In this embodiment, an operating bandwidth is extended as a
result that the low-noise band is extended toward the side of
shorter wavelengths. The reason is simple: the tellurite optical
fiber that performs an optical amplification after amplifying at
wavelengths of 1.525 .mu.m to 1.54 .mu.m with a high gain and a
low-noise by the amplification optical fiber (the product of
erbium-content and fiber-length) is small) which is arranged in
front of the tellurite optical fiber.
[0427] Referring again FIG. 31, we will describe one of modified
configurations of this embodiment.
[0428] In this modified embodiment, the optical fiber 204 is an
silica-based optical fiber doped with 1,000 ppm of erbium (1.2 m in
length, 1.2 .mu.m cut-off wavelength, and 12,000 m-ppm in product
of content and length. The product is larger than that of the
erbium-doped tellurite fiber). The excitation light source 203a is
a semiconductor laser with an oscillation wavelength of 1.48
.mu.m.
[0429] In a case that an amount of excitation light from the source
203a is 70 mW and an amount of excitation light from the source
203b is 150 mW, a gain of 20 dB or more and a noise figure of 5 dB
or less are obtained at a bandwidth of 75 nm in the region of 1,535
.mu.m to 1,610 .mu.m.
[0430] The EDFA of the present modified embodiment also attains the
ability to act in such a bandwidth without causing any significant
noise, which is impossible for the conventional EDFA.
[0431] (Embodiment 42)
[0432] In this embodiment, the optical fiber 204 is a
ZrF.sub.4-contained fluoride optical fiber doped with 1 00 ppm of
erbium (3.5 m in length, 1.2 .mu.m cut-off wavelength, and 2,500
m-ppm in product of erbium-content and fiber-length). The
excitation light source 203 is a semiconductor laser with an
oscillation wavelength of 1.48 .mu.m. The amplification optical
fiber 205 is a tellurite optical fiber that contains the above
tellurite glass composition of TeO.sub.2--ZnO--Li.sub.-
2O--Bi.sub.2O.sub.3 (55.ltoreq.TeO.sub.2.ltoreq.90,
0.ltoreq.ZnO.ltoreq.25, 0.ltoreq.Li.sub.2O.ltoreq.25,
0<Bi.sub.2O.sub.3<20, unit: mole %) as a preform. The
tellurite optical fiber (12 m in length) is prepared from that
composition doped with 500 ppm of erbium and has a cut-off
wavelength of 1.3 .mu.m (the product of erbium-content and
fiber-length is 6,000 m-ppm). In addition, the excitation light
source 203b is a semiconductor laser with an oscillation wavelength
of 1.48 .mu.m.
[0433] In a case that an amount of excitation light from the source
203a is 70 mW and an amount of excitation light from the source
203b is 150 mW, a gain of 20 dB or more and a noise figure of 5 dB
or less are observed at a bandwidth of 85 nm in the region of 1,525
.mu.m to 1,610 .mu.m. In a case that the amplification fiber is not
used, a noise figure of more than 5 dB is observed at a wavelength
under 1.54 .mu.m and a noise figure of 10 dB or more is observed at
a wavelength of 1.525 .mu.m. In addition, a gain of 20 dB or more
is only observed at a bandwidth of 80 nm in the region of 1.53
.mu.m to 1.61
[0434] (Embodiment 43)
[0435] In this embodiment, the optical fibers 204, 205 are the same
tellurite glass optical fibers except their lengths, that is, the
fiber 204 is 3 m in length, and the fiber 205 is 12 m in length.
Each of these fibers 4, 5 is prepared using the above glass
composition: TeO.sub.2--ZnO--Li.sub.2O--Bi.sub.2O.sub.3
(55.ltoreq.TeO.sub.2.ltoreq.90- , 0.ltoreq.ZnO.ltoreq.25,
0.ltoreq.Li.sub.2O<25, 0.ltoreq.Bi.sub.2O.sub.3.ltoreq.20, unit:
mole %) as a preform and doped with 500 ppm of erbium. In addition,
a cut-off wavelength of 1.3 .mu.m of the fiber is 1.3 m. The light
source 3b is a semiconductor laser with an oscillation wavelength
of 1.48 .mu.m.
[0436] In a case that an amount of excitation light from the source
203a is 100 mW and an amount of excitation light from the source
203b is 150 mW, a gain of 20 dB or more and a noise figure of 5 dB
or less are observed at a bandwidth of 85 nm in the region of 1.525
.mu.m to 1.610 .mu.m. In a case that the amplification fiber is not
used, a noise figure of more than 5 dB is observed at a wavelength
under 1.54 .mu.m and a noise figure of 10 dB or more is observed at
a wavelength of 1.525 .mu.m. In addition, a gain of 20 dB or more
is only observed at a bandwidth of 80 nm in the region of 1.53
.mu.m to 1.61 .mu.m.
[0437] In Embodiments 41, 42, and 43, the amplification optical
fibers 204, 205 are used for the procedures of forward excitation
and backward excitation but not limited to. Another excitation
procedure such as a bidirectional excitation may be applied instead
of those procedures.
[0438] (Embodiment 44) In this embodiment, the amplification
optical fiber 204 is the same one as that of Embodiments 41 to 43.
The amplification optical fiber 205 is a tellurite optical fiber
(14 m in length) doped with 500 ppm of erbium. In this embodiment,
just as in the case of Embodiments 40 to 42, the EDFA using the
amplification optical fiber 204 allows the scale-up of low-noise
amplification bandwidth compared with that of the EDFA which does
not use the fiber 204.
[0439] (Embodiment 45)
[0440] In this Embodiment, the amplification optical fiber 204 is
one of an erbium-doped fluoride phosphate optical fiber, a
phosphate optical fiber and a chalcogenide optical fiber. In a case
that the product of Er-concentration and fiber-length of the
optical fiber 204 is less than that of the tellurite glass (i.e.,
the amplification optical fiber 205), the scale-up of low-noise
amplification bandwidth is observed. It means that a raw material
of the fiber 204 is of little importance to the effects of the
present invention but the product of Er-concentration and
fiber-length is importance thereto.
[0441] In Embodiments 41 to 45, two optical fibers which are
different with each other with respect to the product of Er-content
and fiber-length are used as amplification media but not limited to
that numbers. It is also possible to 3 or more optical fibers are
used as the amplification media. The optical fiber having the
minimum product may be placed in any place except the rear,
preferably it may be placed in the front.
[0442] In the following description, we will disclose a structure
for reliably splicing two different optical fibers (i.e., a
non-silica-based optical fiber and a silica-based optical fiber or
two different non-silica based optical fibers having different core
refractive indexes) with a low-loss and low-reflection.
[0443] FIG. 32 is a schematic diagram of a spliced portion between
a non-silica-based optical fiber and a silica-based optical fiber.
In the figure, reference numeral 301 denotes the non-silica-based
optical fiber, 302 denotes the silica-based optical fiber, 303a
denotes a housing for holding an end portion of the
non-silica-based optical fiber, 303b denotes a housing for holding
an end portion of the silica-based optical fiber, 304a denotes an
end surface of the housing 303a, 303b denotes an end surface of the
housing 303b, and 305 denotes an optical adhesive. The
non-silica-based optical fiber 301 is held in the housing 303a at
an angle of .theta..sub.1 from the vertical axis of the end face
304a, and the silica-based optical fiber 302 is held in the housing
303b at an angle of .theta..sub.2 from the vertical axis of the end
face 304b. In this case, a low-loss coupling between these fibers
301, 302 can be attained if the angles .theta..sub.1, .theta..sub.2
[rad] satisfy the equation (4), i.e., Snell's law. 4 sin 1 sin 2 =
n 2 n 1 ( 4 )
[0444] where n.sub.1 is a refractive index of the first optical
fiber and n.sub.2 is a refractive index of the second optical
fiber.
[0445] Return losses R.sub.1 and R.sub.2, expressed in decibels, on
the end surfaces of non-silica-based optical fiber 301 and
silica-based optical fiber 302 are expressed by the following
equations (5) and (6), respectively, quoted from technical
literature: H. M. Presby, et. al., "Bevelled-microlensed taper
connectors for laser and fiber back-reflections", Electron. Lett.,
vol. 24, pp. 1162-1163, 1988. 5 R 1 ( d B ) = | 10 log { [ ( n 1 -
n UV ) ( n 1 + n UV ) ] 2 } + 43.4 .times. ( 2 n 1 - 1 1 ) 2 | ( 5
) R 2 ( d B ) = | 10 log { [ ( n 2 - n UV ) ( n 2 + n UV ) ] 2 } +
43.4 .times. ( 2 n 2 - 2 2 ) 2 | ( 6 )
[0446] wherein n.sub.uv is a refractive index, .lambda. is a signal
wavelength (wavelength to be used), .omega..sub.1 is a mode field
diameter of the non-silica-based optical fiber 301, and
.omega..sub.2 is a mode field diameter of the silica-based optical
fiber 302.
[0447] Therefore, a low-reflection coupling beyond all expected
return losses can be attained by adjusting the angles .theta..sub.1
and .theta..sub.2 by the equations (5) and (6). An angle
.theta..sub.1 required for attaining a desired return loss R.sub.1
for the non-silica-based optical fiber 301 (such as Zr-fluoride
fiber: core refractive index=1.55, In-fluoride fiber: core
refractive index=1.65, chalcogenide fiber (glass composition As-S):
core refractive index=2.4, or tellurite glass: core refractive
index=2.1) can be calculated by the following equation (7) as a
modification of the equation (5). Also, an angle .theta..sub.2
required for attaining a desired return loss R.sub.2 for the
silica-based optical fiber 302 can be calculated by the following
equation (8) as a modification of the equation (6). 6 1 R 1 - | 10
log { n 1 - n UV n 1 + n UV } 2 | 43.4 .times. ( 2 n 1 1 ) 2 ( 7 )
2 R 2 - | 10 log { n 2 - n UV n 2 + n UV } 2 | 43.4 .times. ( 2 n 2
2 ) 2 ( 8 )
[0448] In a case where a refractive index n.sub.UV of the optical
adhesive 305 is 1.5, a signal wavelengths is 1.3 .mu.m, a spot size
(radius) .omega..sub.1 of the non-silica-based optical fiber 301 is
5 .mu.m, and a spot size (radius) .omega..sub.2 of the silica-based
optical fiber 302 is 5 .mu.m, the angles .theta..sub.1 and
.theta..sub.2 for realizing R.sub.1 =40 dB, 50 dB, and 60 dB and
R.sub.2=40 dB, 50 dB, and 60 dB are listed in Table 1, where the
angle .theta..sub.2 for realizing .R.sub.2=40 dB, 50 dB, and 60 dB
is defined as zero because the optical adhesive used in this
embodiment has the same refractive index as that of the
silica-based optical fiber 302. As a result, for Embodiment, the
low-loss splicing between the tellurite optical fiber and the
silica-based optical fiber with a return loss of 50 dB can be
attained if the angle .theta.1 is 3.2 [deg] and the angle
.theta..sub.2 is 4.5 [deg] (02 is calculated from the equation
(4)).
1 TABLE 1 Angle [deg] that satisfy the following return loss Return
Return Return Core refractive loss: loss: loss: Fiber types index
40 dB 50 dB 60 dB Zr-fluoride optical fiber .about.1.55 (max) 1.6
2.9 3.7 In-fluoride optical fiber .about.1.65 (max) 2.6 3.4 4
Charcogenide optical fiber .about.2.4 2.5 2.9 3.2 (As-S system)
Tellurite glass optical fiber .about.2.1 2.7 3.2 3.6 Silica-based
optical fiber .about.1.5 0 0 0
[0449] Accordingly, the structure for splicing between two optical
fibers in accordance with the present invention is characterized by
the following facts:
[0450] 1) an optical axis of the non-silica-based optical fiber and
an optical axis are not on of the silica-based optical fiber
satisfies Snell's law not on the same straight line;
[0451] 2) there is no need to use a dielectric film for preventing
the reflection, which is required in the prior art; and
[0452] 3) an inclination angle of an optical axis of the
non-silica-based optical fiber from the normal to a splicing end
surface of its housing and that of the silica-based optical fiber
from the normal to a splicing end surface of its housing is
different from each other, constructing to the conventional
one.
[0453] In this embodiment, the splicing of the fibers 301, 302 are
accomplished through the splicing end surfaces 304a, 304b of the
housings 303a, 303b by the optical adhesive 305 but not limited to
such an indirect contact. It is also possible to directly contact
between these end surfaces 304a, 404b by fixing the both sides of
the spliced portion by an adhesive 306 (hereinafter, this kind of
adhesion will be referred as a grip-fixing) as shown in FIG. 33. In
this case, furthermore, the angle .theta..sub.1 required for
realizing the return loss R.sub.1=40 dB, 50 dB, and 60 dB for the
non-silica-based optical fiber 301.
[0454] In the above description, the splicing between the
non-silica-based optical fiber and the silica-based optical fiber
with low fiber-loss and low reflection is explained. According to
the present invention, however, a splicing between two of
non-silica-based optical fibers of different glasses, for example a
chalcogenide glass optical fiber and In-fluoride optical fiber, can
be effectively realized.
[0455] (Embodiment 46)
[0456] FIGS. 34 and 35 illustrate spliced portions of two different
optical fibers, where FIG. 34 is a top view of the spliced portions
and FIG. 35 is a cross sectional view of the spliced portions. In
these figures, reference numeral 301 denotes an erbium-doped
tellurite optical fiber (a glass composition of
TeO.sub.2--ZnO--Na.sub.2O--Bi.sub.2O.sub.3, a core refractive index
of 2.1, a mode-field radius of 5 .mu.m, Er-content of 4,000 ppm, a
fiber-covering material of UV resin), 302 denotes a silica-based
optical fiber (a core refractive index of 1.5, a mode-field radius
of 5 .mu.m, and a fiber-covering material of UV resin), and 307a
and 307b denote housings having V-shaped grooves for holding end
portions of optical fibers 301 and 302, respectively. The fibers
301, 302 are independently set in prescribed positions by a
V-grooved substrate 308 and then fixed on the housings 307a, 308b
by means of a fiber-fixing plate 309 and an adhesive 310,
respectively. In this embodiment, the basic materials of housings
307a, 307b, V-grooved substrate 308, and fiber-fixing plate 309 are
Pyrex glasses. Furthermore, reference numerals 311a, 311b are a
connecting end surfaces of the housings 307a, 307b, 305 denotes an
optical adhesive (in this embodiment, an epoxy-based UV adhesive
with a refractive index of 1.5 is used). The tellurite optical
fiber 301 and the silica-based optical fiber 302 are held at angles
of .theta..sub.1=18 [deg] and .theta..sub.2=25 [deg] to vertical
axes on the splicing end surfaces 311a, 311b. Consequently, the
Er-doped tellurite optical fiber 301 and the silica-based optical
fiber 302 can be spliced at a splicing-loss of 0.2 dB. In this
case, however, the splicing-loss is measured at a wavelength of 1.3
.mu.m where there is no absorption of Er-ions of Er-doped optical
fiber 301. Then, a return-loss at a wavelength of 1.3 .mu.m is
measured using the commercially available return-loss measuring
devise. The return-loss measured from the side of silica-based
optical fiber 302 is greater than 60 dB which is beyond the margin
of measuring limits, resulting in an excellent performance. If
angles of the Er-doped tellurite optical fiber 301 and the
silica-based optical fiber 302 to vertical axes of the splicing end
surface 311a, 311b are selected from {.theta..sub.1=8
[deg],.theta..sub.2=11.2 [deg]} and {.theta..sub.1=14 [deg],
.theta..sub.2=20 [deg]}, the splicing-loss between the Er-doped
optical fiber and the silica-based optical fiber is 0.2 dB
(measured at a wavelength of 1.3 .mu.m) and return-losses measured
from the Er-doped optical fiber 301 and the silica-based optical
fiber 302 are greater than 60 dB which is beyond the margin of
measuring limits.
[0457] Strictly speaking, as easily recognized from the values of
.theta..sub.1 and .theta..sub.2, a value of sin .theta..sub.1/sin
.theta..sub.2 is not always equal to a value of n.sub.2/n, because
of the effects of an equalizing refractive index of the fiber's
core. In this case, by the way, a value of sin .theta..sub.1/sin
.theta..sub.2 may be in the region of n.sub.2/n.sub.1 with errors
of plus or minus 10 %. If angles of the Er-doped tellurite optical
fiber 301 and the silica-based optical fiber 302 to vertical axes
of the splicing end surface 311a, 311b are selected from
{.theta..sub.1=5 [deg] .theta..sub.2=7 [deg]}, the splicing-loss
between the Er-doped optical fiber and the silica-based optical
fiber is 0.2 dB (measured at a wavelength of 1.3 .mu.m) and
return-losses measured from the silica-based optical fiber is 60 dB
or more, while the measurement from the side of Er-doped optical
fiber 301 is 55 dB. As a result, we find that an angle of 8.degree.
or over to a vertical axis on the splicing end surface is required
for the tellurite optical fiber to realize the splicing between the
Er-doped optical fiber and the silica-based option with low
fiber-loss and low reflection in both directions (return loss of 60
dB or over) whether or not sin .theta..sub.1/sin .theta..sub.2 is
in the above region with respect to a value of n.sub.2/n.sub.1.
[0458] By the way, an optical adhesive 305 having a refractive
index of 1.55 effects as the same way as that of using the optical
adhesive without using the optical adhesive of 1.5.
[0459] (Embodiment 47)
[0460] The present embodiments will be described in detail with
reference to FIGS. 36 to 37, where FIG. 36 is a top view of the
spliced portions, and FIG. 37 is a cross sectional view of the
spliced portions. In the figures, reference numeral 301 denotes an
erbium-doped tellurite optical fiber (a glass composition of
TeO.sub.2--ZnO--Na.sub.2O--Bi.sub.2O.sub.3, a core refractive index
of 2.1, a mode-field radius of 5 .mu.m, Er-content of 4,000 ppm,
and a fiber-covering of UV-resin), and 2 denotes a silica-based
optical fiber (a core refractive index of 1.5 or less, a mode-field
radius of 5 .mu.m, and a fiber-covering of UV-resin). End portions
of the optical fibers 301 and 302 are held in the housings 307a and
307b, respectively, as the same way as that of Embodiment 45 except
that the slicing end surfaces 311a and 311b are directly connected
together completely without using any optical adhesive.
Subsequently, the housings 311a, 311b are fixed together by
applying the adhesive from the both sides of slicing portions. The
Er-doped tellurite optical fiber 301 and the silica-based optical
fiber 302 are inclined at angles .theta..sub.1=18 [deg] and
.theta..sub.2=25 [deg] to the vertical axes on the splicing end
surfaces 313a, 313b, respectively. Furthermore, a splicing loss
between the Er-doped tellurite optical fiber 301 and the
silica-based optical fiber 302 are 0.2 dB (measuring wavelength of
1.3 .mu.m). Return losses measured from the side of Er-doped
tellurite optical fiber 301 and the silica-based optical fiber 302
are 60 dB or more, respectively. As in the same way as Embodiment
46, it is experimentally cleared that the Er-doped fiber should be
inclined 8.degree. from a vertical axis on the splicing end surface
of the fiber to splice the Er-doped tellurite glass and
silica-based optical fiber with low fiber-loss (return loss of 60
dB or more).
[0461] (Embodiments 48, 49)
[0462] The present embodiments will be described in detail with
reference to FIGS. 38 to 41, where FIG. 38 and FIG. 40 are top
views of the spliced portions and FIG. 39 and FIG. 41 are cross
sectional views of the spliced portions. In the figures, reference
numeral 301 denotes an erbium-doped tellurite optical fiber (a
glass composition of TeO.sub.2--ZnO--Na.sub.2O- --Bi.sub.2O.sub.3,
a core refractive index of 2.1, a mode-field radius of 5 .mu.m,
Er-content of 4,000 ppm, a fiber-covering is UV-resin). In these
embodiments, furthermore, housings for holding the optical fibers
are glass ferrules 312a, 312b, and splicing end surfaces 713a, 713b
are formed by diagonally grinding glass ferrules 312a, 312b.
[0463] An erbium-doped tellurite optical fiber 301 and the
silica-based optical fiber 302 are fixed on the glass ferrules
312a, 312b, respectively. In the case of Embodiment 48 shown in
FIGS. 38 and 39, the splicing end surfaces 312a, 312b are connected
together through an optical adhesive 305 (refractive indexes of 1.5
and 1.55 are applied). In the case of Embodiment 49 shown in FIGS.
40 and 41, the splicing end surface 313a, 313b are directly
connected together. The splicing end portions 313a, 313b of the
Er-doped tellurite optical fiber 301 and the silica-based optical
fiber 302 are inclined at angles .theta..sub.1=12 [deg] and
.theta..sub.2=17 [deg] to the vertical axis on the splicing end
surfaces 313a, 313b, respectively. Furthermore, a splicing loss
between the Er-doped tellurite optical fiber 301 and the
silica-based optical fiber 302 is 0.2 dB (measuring wavelength of
1.3 .mu.m). Return losses measured from the side of Er-doped
tellurite optical fiber 301 and the silica-based optical fiber 302
are 60 dB or more, respectively. In Embodiment 48, refractive
indexes of the optical adhesive is 1.5 and 1.55 are used, but the
same results are obtained. As in the same way as Embodiment 46 and
47, it is experimentally cleared that the Er-doped fiber should be
inclined 8.degree. from a vertical axis on the splicing end surface
of the fiber to splice the Er-doped tellurite glass and
silica-based optical fiber with low fiber-loss (return loss of 60
dB or more).
[0464] As shown in FIG. 42, furthermore, an optical amplifier is
constructed using one of the splicing methods described in
Embodiments 46 to 49. The optical amplifier comprises an
erbium-doped tellurite optical fiber 301 (a glass composition of
TeO.sub.2--ZnO--Na.sub.2O--Bi.sub.2O.su- b.3, a core refractive
index of 2.1, a mode-field radius of 5 Mm, Er-content of 4,000 ppm,
a fiber-covering is UV-resin) and silica-based optical fibers which
are spliced to respective ends of the tellurite fiber. In the
figure, reference numeral 314a and 314b are semiconductor lasers
(an oscillation wavelength of 1.48 .mu.m and an output power of 200
mW) as excitation light sources for generating pump light to be
provided to the Er-doped tellurite, 315a and 315b are optical
multiplexers for multiplexing the pump light generated from the
excitation light sources 314a, 314b with signal light, and 316a and
316b is an optical isolator for preventing an oscillation of the
optical amplifier. Furthermore, reference numeral 317a and 317b is
a splicing portions of the present invention on which all of the
methods shown in Embodiment 46 (a refractive index of the optical
adhesive is 1.55), Embodiment 47, Embodiment 48 (a refractive index
of the optical adhesive is 1.55), and Embodiment 49. In the
splicing portion of Embodiments 46 and 47, the Er-doped tellurite
optical fiber 301 and the silica-based optical fiber 302 are
inclined at angles .theta..sub.1=14 [deg] and .theta..sub.2=20
[deg] to the vertical axis on the splicing end surfaces 313a, 313b,
respectively. In the splicing portion of Embodiments 48 and 49, the
Er-doped tellurite optical fiber 301 and the silica-based optical
fiber 302 are inclined at angles .theta..sub.1=12 [deg] and
.theta..sub.2=17 [deg] to the vertical axis on the splicing end
surfaces 313a, 313b, respectively.
[0465] Consequently, the optical amplifier realizes a signal gain
of 40 dB or more without generating a ghost. In FIG. 43, the
characteristics of optical amplification of the optical amplifier
is illustrated as one of the embodiments of the present invention.
The present embodiment uses the splicing method of Embodiment
46.
[0466] (Embodiment 50)
[0467] Various kind of non-silica-based optical fibers are spliced
with a silica-based optical fiber in accordance with the present
invention, which are listed in Tables 2 and 3.
[0468] The non-silica-based optical fiber may be selected from the
following fibers:
[0469] 1. tellurite glass optical fiber (in Table 2, indicated as
non-silica-based optical fiber A)
[0470] glass composition:
TeO.sub.2--ZnO--Na.sub.2O--Bi.sub.2O.sub.3
[0471] core refractive index: 2.1
[0472] 2. Zn-based fluoride optical fiber (in Table 2, indicated as
non-silica-based optical fiber B)
[0473] glass composition:
[0474]
ZrF.sub.4--BaF.sub.2--LaF.sub.3--YF.sub.3--AlF.sub.3--LiF--NaF
[0475] core refractive index: 1.55
[0476] mode-field radius: 4 .mu.m
[0477] fiber-covering: UV-resin
[0478] 3. In-based fluoride optical fiber (in Table 3, indicated as
non-silica-based optical fiber C)
[0479] glass composition:
[0480]
InF.sub.3--GaF.sub.3--ZnF.sub.2--PbF.sub.2--BaF.sub.2--SrF.sub.2--Y-
F.sub.3--NaF
[0481] core refractive index: 1.65
[0482] mode-field radius: 4.5 .mu.m
[0483] fiber-covering: UV-resin
[0484] 4. chalcogenide glass optical fiber (in Table 3, indicated
as a non-silica-based optical fiber D)
[0485] glass composition: As-S
[0486] core refractive index: 2.4
[0487] mode-field radius: 3 .mu.m
[0488] fiber-covering: UV resin
[0489] Each of the non-silica-based optical fibers A, B. C, and D
is prepared with or without one or more rare-earth elements
selected from the group of:
[0490] Er (1,000 ppm): Pr (500 ppm); Tm (2,000 ppm), Ho (1,000
ppm), Yb (500 ppm), Tb (2,000 ppm), Nd (1,000 ppm), and Eu (2,000
ppm).
[0491] In addition, the silica-based optical fibers to be spliced
have a core refractive index of 1.5 and the same mode-field radius
as that of the respective non-silica-based optical fibers. The
method of splicing the fibers is selected from Embodiments 45 to
48. If the splicing form of Embodiment 44 or 45 is formed, a
refractive index of the optical adhesive 305 to be applied between
the splicing end surfaces 313a, 313b is 1.5. By the way, the
slicing-loss and return-loss have no relation to the presence or
the kind of a rare-earth element in the glass composition.
2 TABLE 2 Splicing- loss Return loss (dB) Non-silica- (measuring
Non-silica- based optical Splicing wavelength based optical
Silica-based fiber form .theta.1 .theta.2 1.2 .mu.m) fiber optical
fiber A Embodiment 46 8 11.2 0.3 60 dB or more 60 dB or more
Embodiment 47 10 14 0.2 60 dB or more 60 dB or more Embodiment 48
12 17 0.1 60 dB or more 60 dB or more Embodiment 49 10 14 0.2 60 dB
or more 60 dB or more Embodiment 47 12 17 0.2 60 dB or more 60 dB
or more B Embodiment 46 4 4.1 0.2 60 dB or more 60 dB or more
Embodiment 47 4 4.1 0.2 60 dB or more 60 dB or more Embodiment 48 4
4.1 0.1 60 dB or more 60 dB or more Embodiment 49 4 4.1 0.2 60 dB
or more 60 dB or more Embodiment 46 6 6.2 0.2 60 dB or more 60 dB
or more
[0492]
3 TABLE 3 Splicing- loss Return loss (dB) Non-silica- (measuring
Non-silica- based optical Splicing wavelength based optical
Silica-based fiber form .theta.1 .theta.2 1.2 .mu.m) fiber optical
fiber C Embodiment 46 5 5.5 0.2 60 dB or more 60 dB or more
Embodiment 47 5 5.5 0.3 60 dB or more 60 dB or more Embodiment 48 5
5.5 0.2 60 dB or more 60 dB or more Embodiment 49 5 5.5 0.2 60 dB
or more 60 dB or more Embodiment 46 10 11 0.2 60 dB or more 60 dB
or more D Embodiment 46 9 14.5 0.2 60 dB or more 60 dB or more
Embodiment 46 15 24.5 0.2 60 dB or more 60 dB or more Embodiment 47
9 14.5 0.1 60 dB or more 60 dB or more Embodiment 48 9 14.5 0.2 60
dB or more 60 dB or more Embodiment 49 9 14.5 0.2 60 dB or more 60
dB or more Embodiment 49 10 16.1 0.2 60 dB or more 60 dB or
more
[0493] As listed in Table 2 and Table 3, the non-silica based
optical fiber can be spliced with a low-loss and a low-reflection
if the slicing method of the present invention is used. In these
tables, examples of low-reflection (return loss is 60 dB or more)
are shown. However, a return loss of 60 dB or more in both
directions cannot be attained on condition that .theta..sub.1<3
[deg] for the Zr-based fluoride optical fiber, .theta..sub.1<4
[deg] for the In-based fluoride optical fiber, or
.theta..sub.1<8 [deg] for a chalcogenide glass optical fiber.
Therefore, .theta..sub.1 should be larger than those values for
realizing a return loss of 60 dB or more in both directions.
[0494] An optical amplifier having a signal gain of 30 dB or more
is constructed as an optical fiber amplifier operating at 1.3 .mu.m
band using the above Pr-doped In-based fluoride optical fiber (in
Table 3, indicated as non-silica-based optical fiber D). In this
case, the slicing form of Embodiment 47 is applied under the
condition that .theta..sub.1 is 5 [deg], .theta..sub.2 is 5.5
[deg], a pump light source is a ND-YLF laser with 1.047 .mu.m
oscillation. In addition, there is no ghost observed.
[0495] By the way, the above embodiments are for the splicing
between the non-silica-based optical fiber and the silica-based
optical fiber. According to the present invention, it is also
possible to connect two different non-silica-optical fibers. We
list some of the connections between the non-silica-optical fibers
in Table 4 in which four different non-silica-based optical fibers
A, B. C, and D of Embodiment 4 are used. A refractive index and a
splicing angle of each of the fibers are in the range defined
above. As a result, low fiber-loss and low reflection
characteristics of the non-silica-based optical fibers are attained
by the way of the present invention.
4 TABLE 4 Splicing- Non-silica- loss Return loss (dB) based optical
(measuring Non-silica- fiber Splicing wavelength based optical
Silica-based 1 2 form .theta.1 .theta.2 1.2 .mu.m) fiber 1 optical
fiber 2 A B Embodiment 46 8 10.9 0.2 60 dB or more 60 dB or more C
Embodiment 47 9 11.5 0.2 60 dB or more 60 dB or more D Embodiment
48 15 13.1 0.3 60 dB or more 60 dB or more B C Embodiment 46 8 7.5
0.2 60 dB or more 60 dB or more D Embodiment 47 25 15.6 0.3 60 dB
or more 60 dB or more C D Embodiment 46 24 16.2 0.2 60 dB or more
60 dB or more
[0496] According to the present invention, the performance of
wavelength multiplexing transmission systems and optical CATV
systems can be improved by a combination of the characteristics of
optical amplification media, and optical amplifiers and laser
devices using the optical amplification media. Thus, the present
invention has advantages of contributing the economical and
technical improvements of service using those systems.
[0497] Furthermore, a broadband amplifier allows a dramatic
increase in transmission volume if it is used in a wavelength
multiplexing transmission system and contributes a reduction in
costs of data communication. Also, a reduction in costs of optical
CATV can be attained if the optical amplifier having
characteristics of low gain tilt is used so as to allow
distribution and relay of high-quality images by wavelength
multiplexing.
[0498] Practical applications of the optical amplification medium
in the laser device contributes to a reduction of cost of in
various kind of wavelength multiplexing transmission and an
improvement in optical instrumentation.
[0499] (Embodiment 51) Referring now to FIGS. 44 to 46, we will
describe an erbium-doped tellurite optical fiber or optical
waveguide to be used as a light source of amplified spontaneous
emission (ASE) in the present embodiment. FIG. 44 is a schematic
block diagram of the ASE light source as one of the preferred
embodiments of the present invention, FIG. 45 is a graphical
representation of the relationship between the wavelength and the
reflection of reflectors 406, 408 and the ASE spectrum of the fiber
or optical waveguide of FIG. 44, and FIG. 46 is a spectrum diagram
of ASE.
[0500] In general, a spectrum of amplified spontaneous emission
(ASE) is observed when the Er-doped tellurite optical fiber is
pumped. In this case, a spectrum of ASE can be represented by a
solid line in FIG. 46. Thus, it is possible to use the ASE as a
light source of 1.5 to 1.6 .mu.m without any modification. For
extending the boundaries of the applications, the
wavelength-dependency of the spectrum should be eliminated so as to
become flat.
[0501] In this embodiment, therefore, an ASE light source is
constructed as shown in FIG. 44. In the figure, reference numeral
401 denotes an optical coupler for coupling or dividing pump light
at a wavelength of 1.48 .mu.m with another wavelength of 1.5 .mu.m
or more; 402 denotes an erbium-doped tellurite optical fiber which
is prepared using a glass composition of
TeO.sub.2--ZnO--M.sub.2O--Bi.sub.2O.sub.3 (M is at least one alkali
element) or TeO.sub.2--ZnO--M.sub.2O--Bi.sub.2O.sub.3--Al.sub.-
2O.sub.3 (M is at least one alkali element) under the condition
that an erbium content in a core of the fiber is 2,000 ppm, a
fiber-length is 4 m, a cut-off wavelength is 1.3 .mu.m, and An is
1.5 %; 403 denotes an optical coupler that considers 1.56 .mu.m as
a center wavelength for coupling or dividing light at longer and
shorter wavelengths, 404 and 405 denote optical attenuators, and
406 and 408 denote reflectors.
[0502] The Er-doped tellurite optical fiber 402 generates ASE that
includes light at a wavelength of over 1.56 .mu.m. The light passes
through an optical attenuator 404 and then the light is reflected
on the reflector 406. Subsequently, the reflected light propagates
in a reverse direction and enters into the optical fiber 402 where
it is amplified. Then the amplified light is emitted from one end
of the optical coupler 401. Light at a wavelength of shorter than
1,56 .mu.m is reflected on the reflector 408 after passing through
the optical attenuator 405. Furthermore, the light propagates in a
reverse direction and passes through the optical fiber 402 again.
The amplified light is reflected on a reflector 408 and then
emitted from one end of the optical coupler.
[0503] If a reflectivity of the reflector is adjusted as shown in
FIG. 45, that is, a reflectivity of the light (a line B for the
reflector 406 and a line C for the reflector 408 in FIG. 45) is
minimized at a wavelength in proximity to a peak of the ASE and
increased with distance from the peak, a spectrum of ASE unmodified
by the reflector is obtained as indicated by a line A in FIG. 45.
As shown in FIG. 46, therefore, we obtains the ASE spectrum (a
dushed line in FIG. 46) which indicates that light intensity has a
small dependence on wavelengths from 1.53 .mu.m to 1.60 .mu.m. At
this time, furthermore, attenuation of the optical attenuators 404,
405 are optimized. As a result, the ASE spectrum which indicates
that light intensity has a small dependence on wavelengths is
obtained when an optical waveguide is used as an amplification
medium.
[0504] (Embodiment 52)
[0505] In this embodiment, we conduct evaluations of the optical
amplification characteristic of an amplifier shown in FIG. 47. The
amplifier of this embodiment is based on the configuration shown in
FIG. 44. That is, an optical circulator 409 is connected to a
signal input terminal of the optical amplifier 401a, and also a
pumping optical coupler 401b is arranged at the back of the
Er-doped tellurite optical fiber 402. An optical amplification is
performed under the condition that pump light is at a wavelength of
0.98 .mu.m or 1.48 .mu.m, for example 0.98 .mu.m light incident
from the front side, 1.48 .mu.m light incident from the back side,
or 1.48 .mu.m light incident from both sides. As a result, a gain
spectrum indicating that the gain has a small dependency on
wavelengths from 1.53 .mu.m and 1.60 .mu.m. At this time,
attenuation of optical attenuator 404, 405 are optimized.
Conventionally, for obtaining a flattened spectrum of gains that
shows an extremely high dependency on wavelength, a gain peak
observed at 1.53 .mu.m to 1.57 .mu.m becomes flat on a gain peak at
wavelengths of 1.53 .mu.m to 1.57 .mu.m by a filter such as a
fiber-bragg-grating that courses any loss for the purpose of
cutting or leveling the spectrum. However, this method has the
problems of a decrease in the quantum efficiency of the optical
amplifier and unifying the flatten gains into a small value (In
FIG. 43, the gain values are at around 1.58 .mu.m). In this
embodiment, however, a decrease in the quantum efficiency is not in
existence, the gains are standardized into a higher level but not
into a lower level.
[0506] For the reflectors 406, 408, by the way, dielectric multiple
layers, fiber-bragg-grating, or the like can be used. An
amplification optical fiber may be selected not only from the
Er-doped tellurite optical fibers but also from silica-based
optical fibers and fluoride optical fibers. The effects of
gain-flattening is also observed when one of these fibers is
used.
[0507] The present invention has been described in detail with
respect to preferred embodiments, and it will now be apparent from
the foregoing to those skilled in the art that changes and
modifications may be made without departing from the invention in
its broader aspects, and it is the intention, therefore, in the
appended claims to cover all such changes and modifications as fall
within the true spirit of the invention.
* * * * *