U.S. patent application number 10/079209 was filed with the patent office on 2002-10-31 for thulium-doped fiber amplifier using pump light for improving conversion efficiency in a s-band.
Invention is credited to Aozasa, Shinichi, Masuda, Hiroji, Nishida, Yoshiki, Sakamoto, Tadashi, Shimizu, Makoto.
Application Number | 20020159137 10/079209 |
Document ID | / |
Family ID | 26609782 |
Filed Date | 2002-10-31 |
United States Patent
Application |
20020159137 |
Kind Code |
A1 |
Aozasa, Shinichi ; et
al. |
October 31, 2002 |
Thulium-doped fiber amplifier using pump light for improving
conversion efficiency in a S-band
Abstract
An optical fiber amplifier with a high conversion efficiency,
which is capable of using the pump light in the wavelength band
that can be emitted by the laser diode, is formed by an
amplification optical fiber containing thulium at least in a core,
to which a signal light is to be entered, and a pump light input
unit configured to enter at least one pump light with a wavelength
in a range of 1320-1520 nm, or more preferably 1320-1480 nm, into
the amplification optical amplifier.
Inventors: |
Aozasa, Shinichi;
(Ibaraki-ken, JP) ; Masuda, Hiroji; (Ibaraki-ken,
JP) ; Sakamoto, Tadashi; (Ibaraki-ken, JP) ;
Shimizu, Makoto; (Ibaraki-ken, JP) ; Nishida,
Yoshiki; (Ibaraki-ken, JP) |
Correspondence
Address: |
JOHN S. PRATT, ESQ
KILPATRICK STOCKTON, LLP
1100 PEACHTREE STREET
SUITE 2800
ATLANTA
GA
30309
US
|
Family ID: |
26609782 |
Appl. No.: |
10/079209 |
Filed: |
February 20, 2002 |
Current U.S.
Class: |
359/341.5 |
Current CPC
Class: |
H01S 3/0941 20130101;
H01S 3/06708 20130101; H01S 3/06754 20130101; H01S 3/06758
20130101; H01S 3/094011 20130101; H01S 3/094003 20130101; H01S
3/1616 20130101; H01S 3/2333 20130101; H01S 2303/00 20130101 |
Class at
Publication: |
359/341.5 |
International
Class: |
H01S 003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 21, 2001 |
JP |
2001-44477 |
Mar 19, 2001 |
JP |
2001-79017 |
Claims
What is claimed is:
1. An optical fiber amplifier, comprising: an amplification optical
fiber containing thulium at least in a core, to which a signal
light is to be entered; and a pump light input unit configured to
enter at least one pump light with a wavelength in a range of
1320-1520 nm into the amplification optical amplifier.
2. The optical-fiber amplifier of claim 1, wherein the pump light
Input unit enters the at least one pump light with a wavelength in
a range of 1320-1480 nm.
3. The optical fiber amplifier of claim 1, wherein the pump light
input unit enters the at least one pump light with a wavelength in
a range of 1370-1460 nm.
4. The optical fiber amplifier of claim 1, wherein the
amplification optical fiber contains thulium at least in the core
at a concentration not less than 2000 ppmwt.
5. The optical fiber amplifier of claim 1, wherein the
amplification optical fiber contains thulium at least in the core
at a concentration not less than 3000 ppmwt.
6. The optical fiber amplifier of claim 1, further comprising: an
auxiliary pump light input unit configured to enter at least one
auxiliary pump light with a wavelength in at least one of ranges
630-720 nm, 740-830 nm, 1100-1300 nm. and 1500-2000 nm, into the
amplification optical fiber.
7. The optical fiber amplifier of claim 1, further comprising: a
signal light turnaround propagation unit configured to cause the
signal light to propagate forward and backward through the
amplification optical fiber.
8. The optical fiber amplifier of claim 7, wherein the signal light
turnaround propagation unit is a reflection mirror provided at one
end of the amplification optical fiber.
9. The optical fiber amplifier of claim 7, wherein the signal light
turnaround propagation unit causes only the signal light, or only
the signal light and the at least one pump light, to propagate
forward and backward through the amplification optical fiber.
10. The optical fiber amplifier of claim 9, wherein the signal
light turnaround propagation unit through which amplified
spontaneous emission lights pass.
11. The optical fiber amplifier of claim 1, further comprising; an
optical isolator or an optical circulator connected to the
amplification optical fiber for causing the signal light to
propagate only in one direction through the amplification optical
fiber.
12. An optical fiber amplifier, comprising: a plurality of
amplification optical fibers each containing thulium at least in a
core, to which a signal light is to be entered, the plurality of
amplification optical fibers being connected in series or in
parallel; and a plurality of pump light input units each configured
to enter at least one pump light with a wavelength in a range of
1320-1520 nm into a respective one of the amplification optical
amplifiers.
13. The optical fiber amplifier of claim 12, further comprising: at
least one gain equalizer connected to the amplification optical
fibers.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a wideband optical fiber
amplifier having an amplification bandwidth at 1480-1520 nm which
is a low loss region of an optical fiber.
[0003] 1. Description of the Related Art
[0004] In conjunction with the spread of the Internet, etc., the
communication capacity has been increasing rapidly so that the use
of a communication system in the WDM (Wavelength Division
Multiplexing) scheme as a large capacity optical communication
system is becoming popular. In this WDM system, it is indispensable
to use an EDFA (Erbium-Doped Fiber Amplifier) as a repeater, and
the WDM system with the EDFA having an amplification bandwidth of
1.53-1.60 .mu.m has been available.
[0005] However, in order to realize a larger capacity for the
communication facility, there is a need to enlarge the
amplification bandwidth of the optical fiber amplifier, and there
are great demands for a development of an optical fiber amplifier
capable of covering a low loss region (1.45-1.65 .mu.m) of a silica
fiber.
[0006] To this end, an optical fiber amplifier adapted to an S-band
(1480-1520 nm) in which the silica fiber has low loss and low
dispersion equivalent to the already available C-band (1530-1560
nm) has been developed. There are currently three types of such an
optical fiber amplifier in the S-band.
[0007] The first one is a Raman optical fiber amplifier that
utilizes the induced Raman scattering that occurs when a signal
light is entered into a silica fiber in a state of having an
intense pump light incident thereon (see J. Kani, et al.,
Electronics Letters, Vol. 34, No, 18, pp. 1745-1747, September
1998, for example).
[0008] The second one is a dual wavelength pumped TDFA
(Thulium-Doped Fiber Amplifier) in which the population inversion
is low and the amplification bandwidth is shifted to the long
wavelength side by adding wavelengths with a high efficiency for
pumping from the ground level to the amplification final energy
level In the TDFA with 1000 nm band upconversion pumping which has
the amplification bandwidth at S.sup.--band (1450-1480 nm) (see, T.
Kasamatsu, et al., Optical Amplifiers and their Applications '99.
Optical Society of America Trends In Optics and Photonics Series
Vol. 30, pp. 46-50, June 1999, for example).
[0009] FIG. 1A shows the energy levels of Tm and the amplification
state of the dual wavelength pumped TDFA. For the amplification of
the S-band, the stimulated emission from .sup.3H.sub.4 to
.sup.3SF.sub.4. In the case of the dual wavelength pumping, tile
signal light is pumped from the ground level .sup.3H.sub.6 to the
amplification final energy level .sup.3F.sub.4 by the 1560 nm pump
light, and then pumped from the amplification final energy level
.sup.3F.sub.4 to the amplification initial energy level
.sup.3F.sub.2 by the 1000 nm pump light. By controlling the powers
of the pump lights in two wavelengths so as to control the number
of Tm ions (Tm.sup.3+) at each level, a low population inversion
state is formed and the amplification bandwidth of the TDFA in the
S.sup.+-band is shifted to the S-band.
[0010] Moreover, as shown in FIG. 1B, when the pumping wavelength
from the amplification final energy level .sup.2F.sub.4 to the
amplification initial energy level .sup.3F.sub.2 is changed from
the 1000 nm band to the 1400 nm band which has the higher pumping
efficiency, it is possible to realize the high efficiency S-band
optical fiber amplifier (see, T. Kasamatsu, et al., Electronics
Letters, Vol. 36, No. 19, pp. 1607-1609, September 2000, for
example).
[0011] The third one is a high Tm.sup.3+ concentration TDFA in
which the low population inversion is formed by the cross
relaxation among Tm.sup.3- generated In the pumping state and the
amplification bandwidth is shifted to the S-band on the long
wavelength region, by adding Tm which is the additive ions to the
optical fiber core which is the amplification medium at a high
Tm.sup.3+ concentration, in the 1000 nm band upconversion pumping
TDFA (see, S. Aozasa, et al., Electronics Letters, Vol. 36, No. 5,
pp. 418-419, March 2000, for example).
[0012] FIG. 2 shows the energy levels of Tm and the amplification
state of the high Tm.sup.3 concentration TDFA. The signal light is
pumped once from the ground level .sup.3H.sub.6 to the
amplification final energy level .sup.3F.sub.4 by the 1000 nm pump
light, and then pumped further from the amplification final energy
level .sup.3F.sub.4 to the amplification initial energy level
.sup.3F.sub.2 by the pump light of the same wavelength.
[0013] Now, in the low Tm.sup.3+ concentration TDFA, the high
population inversion is formed because the absorption of Tm.sup.3-
with respect to the pump light at a time of pumping from the
amplification final energy level .sup.3F.sub.4 to the amplification
initial energy level .sup.3F.sub.3 is higher than the absorption of
Tm.sup.3+ with respect to the pump light at a time of pumping from
the ground level .sup.3H.sub.3 to the amplification final energy
level .sup.3F.sub.4.
[0014] Such a low Tm.sup.3+ concentration TDFA has the
amplification bandwidth mostly in the S.sup.+-band described above
as a result of the high population inversion state, and the
amplification operation can be realized even in the S-band although
it is deviated from the peak wavelength of the gain spectrum. The
amplification efficiency in the S-band by this low Tm.sup.3+
concentration TDFA is less than or equal to that of the high
Tm.sup.3+ concentration TDFA.
[0015] In contrast, in the high Tm.sup.3+ concentration TDFA, the
interaction among Tm.sup.3+ occurs so that, as shown in FIG. 2,
Tm.sup.3+ pumped to the amplification initial energy level
.sup.3F.sub.2 is relaxed to the amplification final energy level
.sup.3F.sub.4 by causing the energy transfer to the neighboring
Tm.sup.3- at the ground level .sup.3H.sub.6, while Tm.sup.3+ that
received the energy is pumped to the amplification final energy
level .sup.3F.sub.4. As a result, the number of Tm.sup.3+ pumped to
the amplification final energy level .sup.3F.sub.4 is increased so
that tile low population inversion is formed and the gain shirt
occurs.
[0016] However, in the high Tm.sup.3+ concentration TDFA described
above, the laser diode (LD) for emitting the 1000 nm wavelength
band to be used for the pump light has not been developed yet, so
that its practical realization has been difficult because it is
difficult to realize a low cost and a compact size, and the
conversion efficiency is not very good (approximately 5%).
[0017] Also, even in tile low Tm.sup.3+ concentration TDFA, the
laser diode (LD) for emitting the 1000 nm wavelength band to be
used for the pump light has not been developed yet, so that its
practical realization has been difficult because it is difficult to
realize a low cost and a compact size, and the conversion
efficiency in tile S-band is less than or equal to the high
Tm.sup.3, concentration TDFA.
BRIEF SUMMARY OF THE INVENTION
[0018] It is therefore an object of the present invention to
provide an optical fiber amplifier with a high conversion
efficiency which is capable of using the pump light in the
wavelength band that can be emitted by the laser diode.
[0019] According to one aspect of the present invention there is
provided an optical fiber amplifier, comprising: an amplification
optical fiber containing thulium at least in a core, to which a
signal light is to be entered; and a pump light input unit
configured to enter at least one pump light with a wavelength in a
range of 1320-1520 nm into the amplification optical amplifier.
[0020] According to another aspect of the present invention there
is provided an optical fiber amplifier, comprising: a plurality of
amplification optical fibers each containing thulium at least in a
core, to which a signal light is to be entered, the plurality of
amplification optical fibers being connected in series or in
parallel; arid a plurality of pump light input units each
configured to enter at least one pump light with a wavelength in a
range of 1320-1520 nm into a respective one of the amplification
optical amplifiers.
[0021] Other features and advantages of the present invention will
become apparent from the following description taken in conjunction
with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIGS. 1A and 1B are energy level diagrams for explaining an
operation principle of a conventional dual wavelength pumped TDFA
(Thulium-Doped Fiber Amplifier)
[0023] FIG. 2 is an energy level diagram for explaining an
operation principle of a conventional high Tm.sup.3+ concentration
TDFA.
[0024] FIGS. 3A, 3B and 3C are schematic diagrams respectively
showing forward pumping type, backward pumping type, and
bidirectional pumping type configurations of an optical fiber
amplifier according to the first embodiment of the present
invention.
[0025] FIG. 4 Is a table showing conversion efficiencies obtained
by using exemplary optical fiber amplifiers of the first embodiment
and conventional optical fiber amplifiers.
[0026] FIG. 5 is a graph showing a relationship between a
wavelength of a pump light and a conversion efficiency in the
optical fiber amplifier or the first embodiment.
[0027] Figs, 6A, 6B and 6C are schematic diagrams respectively
showing forward pumping type, backward pumping type, and
bidirectional pumping type configurations of an optical fiber
amplifier according to the second embodiment of the present
invention.
[0028] FIG. 7 is a graph showing an absorption spectrum of thulium
utilized in the optical fiber amplifier of the second
embodiment.
[0029] FIG. 8 is a table showing conversion efficiencies obtained
by using exemplary optical fiber amplifiers of the second
embodiment and exemplary optical fiber amplifiers of the first
embodiment.
[0030] FIGS. 9A, 9B and 9C are schematic diagrams respectively
showing forward pumping type, backward pumping type, and
bidirectional pumping type configurations of an optical fiber
amplifier according to the third embodiment of the present
invention.
[0031] FIG. 10 is a graph showing gain spectra in the case of using
an amplification optical fiber containing 2000 ppmwt of Tm, for the
optical fiber amplifiers shown in FIG. 3C and FIG. 9C.
[0032] FIG. 11 is a graph showing gain spectra in the case of using
an amplification optical fiber containing 6000 ppmwt of Tm, for the
optical fiber amplifiers shown in FIG. 3C and FIG. 9C.
[0033] FIG. 12 is a graph showing a relationship between a length
of an amplification optical fiber and a conversion efficiency in
the optical fiber amplifier of the third embodiment.
[0034] FIGS. 13A, 13B. 13C. 13D, 13E and 13F are schematic diagrams
showing exemplary forms of a reflection mirror that can be used in
the optical fiber amplifier of the third embodiment.
[0035] FIGS. 14A. 14B and 14C are schematic diagrams respectively
showing forward pumping type, backward pumping type, and
bidirectional pumping type configurations of an optical fiber
amplifier according to the fourth embodiment of the present
invention.
[0036] FIG. 15 is graphs showing signal light spectra in the
optical fiber amplifier of the fourth embodiment and the optical
fiber amplifier of the third embodiment.
[0037] FIGS. 16A. 16B and 16C are schematic diagrams respectively
showing forward pumping type, backward pumping type, and
bidirectional pumping type configurations of an optical fiber
amplifier according to the fifth embodiment of the present
invention.
[0038] FIG. 17 is a schematic diagram showing an exemplary
configuration of an optical fiber amplifier according to the sixth
embodiment of the present invention.
[0039] FIG. 18 is a schematic diagram showing an exemplary
configuration of an optical fiber amplifier according to the
seventh embodiment of the present invention.
[0040] FIG. 19 is a schematic diagram showing an exemplary
configuration of an optical fiber amplifier according to the eighth
embodiment of the present invention.
[0041] FIG. 20 is a schematic diagram showing one exemplary
configuration of an optical fiber amplifier according to the ninth
embodiment of the present invention.
[0042] FIG. 21 is a schematic diagram showing another exemplary
configuration of an optical fiber amplifier according to the ninth
embodiment of the present invention.
[0043] FIG. 22 is a schematic diagram showing another exemplary
configuration of an optical fiber amplifier according to the ninth
embodiment of the present invention.
[0044] FIG. 23 is a schematic diagram showing another exemplary
configuration of an optical fiber amplifier according to the ninth
embodiment of the present invention.
[0045] FIG. 24 is a schematic diagram showing another exemplary
configuration of an optical fiber amplifier according to the ninth
embodiment of the present invention.
[0046] FIG. 25 is a schematic diagram showing another exemplary
configuration of an optical fiber amplifier according to the ninth
embodiment of the present invention.
[0047] FIG. 26 is a schematic diagram showing another exemplary
configuration of an optical fiber amplifier according to the ninth
embodiment of the present invention.
[0048] FIG. 27 is a schematic diagram showing another exemplary
configuration of an optical fiber amplifier according to the ninth
embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
FIRST EMBODIMENT
[0049] Referring now to FIG. 3A to FIG. 5, the first embodiment of
an optical fiber amplifier according to the present invention will
be described in detail.
[0050] FIGS. 3A to 3C show schematic configurations of the optical
fiber amplifier of the first embodiment.
[0051] FIG. 3A shows a schematic configuration of a forward pumping
type optical fiber amplifier 110 in which a pump light 2 is entered
along the same direction as a propagating direction of a signal
light 1. As shown in FIG. 3A, this optical fiber amplifier 110 has
a configuration in which optical isolators 112 are connected at
both ends of an amplification optical fiber 111 that contains
thulium (Tm) (preferably not less than 2000 ppmwt, more preferably
not less than 3000 ppmwt) in its core, a wavelength division
multiplexing type coupler 113 for multiplexing the signal light 1
and the pump light 2 is connected between the optical isolator 112
on the input side of the signal light 1 (1480-1520 nm)-and the
amplification optical fiber 111, and a pump light source 114 for
generating the pump light (1320-1480 nm) is connected to that
coupler 113.
[0052] FIG. 3B shows a schematic configuration of a backward
pumping type optical fiber amplifier 120 in which a pump light 2 is
entered along the opposite direction of a propagating direction of
the signal light 1. As shown in FIG. 3B, this optical fiber
amplifier 120 has a configuration in which the coupler 113 and the
pump light source 114 are connected between the optical isolator
112 on the output side of the signal light 1 and the amplification
optical fiber 111, rather than between the optical isolator 112 on
the input side of the signal light 1 and the amplification optical
fiber 111.
[0053] FIG. 3C shows a schematic configuration of a bidirectional
pumping type optical fiber amplifier 130 in which pump lights 2 are
entered along both the same direction and the opposite direction of
a propagating direction of the signal light 1. As shown in FIG. 3C,
this optical fiber amplifier 130 has a configuration in which two
couplers 113 are connected respectively between the optical
isolator 112 on the input side of the signal light 1 and the
amplification optical fiber 111 and between the optical isolator
112 on the output side of the signal light 1 and the amplification
optical fiber 111, and two pump light sources 114 are connected
respectively to these couplers 113.
[0054] In the amplification optical fiber 111, a host glass to
which Tm is added can be a fluoride type glass (ZBLAN glass having
ZrF.sub.4, BaF.sub.2, LaF.sub.3 or the like as a main component, or
In-Pb glass having InF.sub.2, BaF.sub.2, PbF.sub.3 or the like as a
main component, for example) in which the non-radiative transition
is hard to occur, or a tellurite glass having TeO.sub.2 or the like
as a main component.
[0055] In this amplification optical fiber 111, Tm is contained at
least in its core so that it is possible to cause the transition
due Lo the stimulated emission of Tm ions and therefore it is
possible to realize the amplification in the S-band.
[0056] Here, it is preferable for the amplification optical fiber
111 to contain Tm at least in its core at the concentration not
less than 2000 ppmwt because then it becomes possible to realize a
compact size by suppressing the length of the amplification optical
fiber 111. In particular, it is more preferable for the
amplification optical fiber 111 to contain Tm at least In its core
at the concentration not less than 3000 ppmwt because then it
becomes possible to reduce the fluorescent lifetime of
amplification initial energy level to be not greater than 90% as
the influence of the cross relaxation appears prominently. Note
that the above described concentration should preferably be not
greater than 10 wt % (or more preferably be not greater than 6 wt
%) from a viewpoint of the currently available glass and fiber
manufacturing techniques.
[0057] The coupler 113 can be a fused tapered type coupler, a
dielectric multi-layer film type coupler, a circulator combined
with a fiber grating, etc.
[0058] The pump light source 114 can be a fiber Raman laser, a
laser diode (LD), etc.
[0059] In this embodiment, the coupler 113 and the pump light
source 114 constitute a pump light input unit.
[0060] The conversion efficiencies obtained in the cases of pumping
the signal light 1 (1480-1520 nm) by the pump light 2 (1320-1480
nm) using any one of ZBLAN glass (Zr type). In-Pb glass and the
tellurite glass as the host glass, and using the amplification
optical fiber 111 to which Tm is added at the concentration of
either one of 2000 ppmwt and 3000 ppmwt (altogether six cases) in
the optical fiber amplifier 110, 120 or 130 of the above described
configuration are shown in FIG. 4. Here, the conversion
efficiencies in the cases of using the conventional pump light
(1047 nm) are also shown for the purpose of comparison.
[0061] As can be seen from FIG. 4, it is possible to improve the
conversion efficiency for all types of the host glass for the
amplification optical fiber 111. In particular, the conversion
efficiency can be further improved in the case of the Tm
concentration of 3000 ppmwt compared with the case of the Tm
concentration or 2000 ppmwt.
[0062] Now, the relationship between the wavelength of the pump
light 2 and the conversion efficiency is shown in FIG. 5. The
conversion efficiency of the amplification optical fiber 111 that
contains Tm at least in its core increases when the wavelength of
the pump light 2 becomes greater than 1320 nm, becomes maximum in a
vicinity of 1400 nm, and mostly disappears when the wavelength of
the pump light 2 exceeds 1520 nm. For this reason, the wavelength
of the pump light 2 should preferably be 1320-1520 nm, but in order
to set the amplification bandwidth to the S-band (1480-1520 nm), it
is preferably be in a range of 1320-1480 nm, or more preferably in
a range of 1370-1460 nm because the conversion efficiency can be
made sufficiently high (over 20% for 2000 ppmwt, over 25% for 3000
ppmwt) in this case.
[0063] Thus, according to the first embodiment, the pump light 2
with the wavelength in the 1400 nm hand which is the same as that
of the signal light 1 is used so that the conversion efficiency of
the signal light 1 can be improved.
SECOND EMBODIMENT
[0064] Referring now to FIG. 6A to FIG. 8, the second embodiment of
an optical fiber amplifier according to the present invention will
be described in detail. Here, those elements which are
substantially similar to the corresponding elements in the first
embodiment described above will be given the same reference
numerals in the figures and their description will be omitted.
[0065] FIGS. 6A to 6C show schematic configurations of the optical
fiber amplifier of the second embodiment.
[0066] FIG. 6A shows a schematic configuration of a forward pumping
type optical fiber amplifier 210 in which the pump light 2 and a
auxiliary pumping light are entered along the same direction as the
propagating direction of the signal light 1. As shown in FIG. 6A,
this optical fiber amplifier 210 has a configuration in which the
amplification optical finer 110 of the first embodiment described
above is modified by connecting an auxiliary coupler 213 between
the optical isolator 112 on the input side of the signal light 1
and the coupler 113, and connecting an auxiliary pump light source
214 for generating an auxiliary pump light 3 with the wavelength in
a range of at least one of ranges 630-720 nm, 740-830 nm, 1100-1300
nm, and 1500-2000 nm to the auxiliary coupler 213. Here, the ranges
630-720 nm, 740-830 nm, 1100-1300 nm, and 1500-2000 nm are the high
absorption ranges of Tm as shown in FIG. 7,
[0067] Note that, in the forward pumping type optical fiber
amplifier 210, the auxiliary coupler 213 and the auxiliary pump
light source 214 are provided at a position A1 between the optical
isolator 112 on the input side of the signal light 1 and the
coupler 113 so that the auxiliary pump light 3 is entered along the
same direction as the propagating direction of the signal light 1
as shown in FIG. 6A, but it is also possible to provide the
auxiliary coupler 213 and the auxiliary pump light source 214 at a
position A2 between the coupler 113 and the amplification optical
fiber 111 so that the auxiliary pump light 3 is entered along the
same direction as the propagating direction of the signal light 1,
or provide the auxiliary coupler 213 and the auxiliary pump light
source 214 at a position B between the amplification optical fiber
111 and the optical isolator 112 on the output side of the signal
light 1 so that the auxiliary pump light 3 is entered along the
opposite direction of the propagating direction of the signal light
1, or enter the auxiliary pump light 3 by combining any of these
configurations.
[0068] FIG. 6B shows a schematic configuration of a backward
pumping type optical fiber amplifier 220 in which the pump light 2
and the auxiliary pump light 3 are entered along the opposite
direction of the propagating direction of the signal light 1. As
shown in FIG. 6B, this optical fiber amplifier 220 has a
configuration in which the optical fiber amplifier 120 of the first
embodiment described above is modified by connecting the auxiliary
coupler 213 at a position 82 between the optical isolator 112 on
the output side of the signal light 1 and the coupler 113, and
connecting the auxiliary pump light source 214 to the auxiliary
coupler 213.
[0069] Note that, in the backward pumping type optical fiber
amplifier 220, the auxiliary coupler 213 and the auxiliary pump
light source 214 are provided at a position B2 between the optical
isolator 112 on the output side of the signal light 1 and the
coupler 113 so that the auxiliary pump light 3 is entered along the
opposite direction of the propagating direction of the signal light
1 as shown in FIG. 6B, but it Is also possible to provide the
auxiliary coupler 213 and the auxiliary pump light source 214 at a
position B1 between the amplification optical fiber 111 and the
coupler 113 so that the auxiliary pump light 3 is entered along the
opposite direction of the propagating direction of the signal light
1, or provide the auxiliary coupler 213 and the auxiliary pump
light source 214 at a position A between the optical isolator 112
on the input side of the signal light 1 and the amplification
optical fiber 111 so that the auxiliary pump light 3 is entered
along the same direction as the propagating direction of the signal
light 1, or enter the auxiliary pump light 3 by combining any of
these configurations.
[0070] FIG. 6C shows a schematic configuration of a bidirectional
pumping type optical fiber amplifier 230 in which the pulp lights 2
and the auxiliary pump lights 3 are entered along both the same
direction and the opposite direction of the propagating direction
of the signal light 1. As shown In FIG. 6C, this optical fiber
amplifier 230 has a configuration in which the amplification
optical fiber 130 of the first embodiment described above is
modified by connecting the auxiliary couplers 213 respectively at a
position A2 between the optical isolator 112 on the input side of
the signal light 1 and the amplification optical fiber 111 and at a
position B1 between the amplification optical fiber 111 and the
optical isolator 112 on the output side of the signal light 1, and
connecting the auxiliary pump light sources 214 respectively to the
auxiliary couplers 213.
[0071] Note that, in the bidirectional pumping type optical fiber
amplifier 230, the auxiliary couplers 213 and the auxiliary pump
light sources 214 are provided at a position A2 between the optical
isolator 112 on the input side of the signal light 1 and the
amplification optical fiber 111 and at a position B1 between the
amplification optical fiber 111 and the optical isolator 112 on the
output side of the signal light 1 so that the auxiliary pump lights
3 are entered along both the same direction and the opposite
direction of the propagating direction of the signal light 1 as
shown in FIG. 6C, but it is also possible to enter the auxiliary
pump light 3 by providing the auxiliary coupler 213 and the
auxiliary pump light source 214 at at least one of the above
described positions A1, A2, B1 and B2.
[0072] In this embodiment, the coupler 213 and the pump light
source 214 constitute an auxiliary pump Light input unit.
[0073] The conversion efficiencies obtained in the cases of pumping
the signal light 1 by two wavelengths of the pump light 2 and the
auxiliary pump light 3 using any one of ZBLAN glass (Zr type),
In-Pb glass and the tellurite glass as the host glass, and using
the amplification optical fiber 111 to which Tm is added at the
concentration of either one of 2000 ppmwt and 3000 ppmwt
(altogether six cases) in the optical fiber amplifier 210, 220 or
230 of the above described configuration are shown in FIG. 8. Here,
the wavelength of the auxiliary pump light is set to be any one of
650 nm, 800 nm, 1200 nm, and 1560 nm. Also, the conversion
efficiencies in the cases of the first embodiment described above
(cases of using one wavelength of the pump light 2 alone) are also
shown for the purpose of comparison.
[0074] As Can be seen from FIG. 8, it is possible to improve the
conversion efficiency in the S-band for all types of the host glass
for the amplification optical fiber 111, in the case of using two
wavelengths of the pump light 2 and the auxiliary pump light 3
compared with the case of using one wavelength of the pump light 2
alone. In particular, the conversion efficiency in the S-band can
be further improved in the case of the Tm concentration of 3000
ppmwt compared with the case of the Tm concentration of 2000
ppmwt.
[0075] Thus, according to the second embodiment, the auxiliary pump
light 3 with the wavelength for which the ground level absorption
is greater is entered in addition to the pump light 2 with the
wavelength in the 1400 nm band so that the conversion efficiency of
the signal light 1 can be improved even more than the case of the
first embodiment described above.
THIRD EMBODIMENT
[0076] Referring now to FIG. 9A to FIG. 13F, the third embodiment
of an optical fiber amplifier according to the present invention
will be described in detail. Here, those elements which are
substantially similar to the corresponding elements in the first
and second embodiments described above will be given the same
reference numerals in the figures and their description will be
omitted.
[0077] FIGS. 9A to 9C show schematic configurations of the optical
fiber amplifier of the third embodiment.
[0078] FIG. 9A shows a schematic configuration of a forward pumping
type optical fiber amplifier 310 in which the pump light 2 is
entered along the same direction as the propagating direction of
the signal light 1. As shown in FIG. 9A, this optical fiber
amplifier 310 has a configuration in which the amplification
optical fiber 110 of the first embodiment described above is
modified by replacing the optical isolator 112 on the output side
of the signal light 1 with a reflection mirror 315 for reflecting
at least the signal light 1 among the signal light 1 and the pump
light 2, and replacing the optical isolator 112 on the input side
of the signal light 1 with an optical circulator 316 for separating
an input signal light 1 and an output signal light 1.
[0079] FIG. 9B shows a schematic configuration of a backward
pumping type optical fiber amplifier 320 in which the pump light 2
is entered along the opposite direction of the propagating
direction of the signal light 1. As shown in FIG. 9B, this optical
fiber amplifier 320 has a configuration In which the optical fiber
amplifier 120 of the first embodiment described above is modified
by replacing the optical isolator 112 on the output side of the
signal light 1 with the reflection mirror 315, and replacing the
optical isolator 112 on the input side of the signal light 1 with
the optical circulator 316.
[0080] FIG. 9C shows a schematic configuration of a bidirectional
pumping type optical fiber amplifier 330 in which the pump lights 2
are entered along both the same direction and the opposite
direction of the propagating direction of the signal light 1. As
shown in FIG. 9C, this optical fiber amplifier 330 has a
configuration in which the amplification optical fiber 130 of the
first embodiment described above is modified by replacing the
optical isolator 112 on the output side of the signal light 1 with
the reflection mirror 315, and replacing the optical isolator 112
on the input side of the signal light 1 with the optical circulator
316.
[0081] In this embodiment, the reflection mirror 315 constitutes a
signal light turnaround propagation unit.
[0082] The gain spectra obtained in the cases of pumping the signal
light 1 by the pump light 2 using the double pass type optical
fiber amplifier 330 of the above described configuration in which
the amplification optical fiber 111 has ZBLAN glass (Zr type) as
the host glass and the Tm concentration of either one of 2000 ppmwt
and 6000 ppmwt are shown in FIG. 10 (case of 2000 ppmwt) and FIG.
11 (case of 6000 ppmwt). Here, the input power of the signal light
1 is -13 dbm/ch(.times.4ch), the wavelength of the pump light 2
from one pump light source 114 is 1400 nm, the wavelength of the
pump light 2 from another pump light source 114 is 1415 nm,, the
total output power of the pump light sources 114 is 500 mW, and the
Length of the amplification optical fiber 11l is optimized such
that a high gain can be obtained in the S-band. Also, the gain
spectra in the cases of FIG. 3C of the first embodiment described
above (single pass type using the length of the amplification
optical fiber 111 is 16 m) are also shown for the purpose of
comparison.
[0083] As can be seen from FIG. 10 and FIG. 11, it is possible to
improve the gain In the S-band (1480-1520 nm) by the third
embodiment (double pass type) compared with the first embodiment
(single pass type) described above. In particular, the gain in the
S-band can be further improved and the flatness of the gain can be
improved in the case of the Tm concentration of over 3000 ppmwt
(6000 ppmwt in this example) compared with the case of the Tm
concentration of 2000 ppmwt.
[0084] Also, the relationships between the length of the
amplification optical fiber 111 (Tm concentration: 6000 ppmwt) and
the conversion efficiency in the S-band in the cases of the first
embodiment (single pass type) described above and the third
embodiment (double pass type) are shown in FIG. 12. Note that the
conditions on the signal light 1 are the same in both cases.
[0085] As shown in FIG. 12, the single pass type can obtain a
characteristic in which the gains at the 1480 nm and 1510 nm
approximately coincide when the fiber length is 15.5 to 17 m,
whereas the double pass type can obtain a characteristic in which
the gains at the 1480 nm and 1510 nm approximately coincide when
the fiber length is 6 to 8 m. The conversion efficiency of this
fiber length for which the gains approximately coincide is about
24% for the single pass type and about 35% for the double pass
type.
[0086] In the configurations of FIGS. 9A to 9C of the third
embodiment, the reflection mirror 315 can be provided in various
forms such as those shown in FIGS. 13A to 13F. More specifically,
FIG. 13A slows a reflection mirror realized by applying a gold
vapor deposition to an end face of a vertically cut optical fiber.
FIG. 13B shows a reflection mirror realized by attaching a
dielectric multi-layer film at an end face of a vertically cut
optical fiber. FIG. 13C shows a reflection mirror realized by a
metallic mirror for reflecting lights emitted from a fiber end face
back into the fiber. FIG. 13D shows a reflection mirror realized by
inserting a Faraday rotator between tile fiber end face and the
metallic mirror in the reflection mirror of FIG. 13C. FIG. 13E
shows a fiber grating type reflection mirror realized by providing
periodic refractive index variations along a length direction of
the optical fiber such that the entered lights will be reflected by
the Bragg reflection. FIG. 13F shows a loop mirror type reflection
mirror in which the lights are propagated through a looped
polarization-preserving optical fiber and re-coupled by a 3 dB
coupler.
[0087] Thus, according to the third embodiment, the conversion
efficiency of the signal light 1 can be improved even more than the
case of the first embodiment described above.
FOURTH EMBODIMENT
[0088] Referring now to FIG. 14A to FIG. 15, the fourth embodiment
of an optical fiber amplifier according to the present invention
will be described in detail. Here, those elements which are
substantially similar to the corresponding elements in the first to
third embodiments described above will be given the same reference
numerals in the figures and their description will be omitted.
[0089] FIGS. 14A to 14C show schematic configurations of the
optical fiber amplifier of the fourth embodiment.
[0090] FIG. 14A shows a schematic configuration of a forward
pumping type optical fiber amplifier 410 In which the pump light 2
is entered along the same direction as the propagating direction of
the signal light 1. As shown in FIG. 14A, this optical fiber
amplifier 410 has a configuration in which the amplification
optical fiber 310 of the third embodiment described above is
modified by replacing the reflection mirror 315 with a reflection
mirror 415 through which ASE (Amplified Spontaneous Emission)
lights pass.
[0091] FIG. 14B shows a schematic configuration of a backward
pumping type optical fiber amplifier 420 in which the pump light 2
is entered along the opposite direction of the propagating
direction of the signal light 1. As shown in FIG. 14B, this optical
fiber amplifier 420 has a configuration in which the optical fiber
amplifier 320 of the third embodiment described above is modified
by replacing the reflection mirror 315 with the reflection mirror
415 through which ASE (Amplified Spontaneous Emission) lights can
be transmitted.
[0092] FIG. 14C shows a schematic configuration of a bidirectional
pumping type optical fiber amplifier 430 in which the pump lights 2
are entered along both the same direction and the opposite
direction of the propagating direction of the signal light 1. As
shown in FIG. 14C, this optical fiber amplifier 430 has a
configuration in which the amplification optical fiber 330 of the
third embodiment described above is modified by replacing the
reflection mirror 315 with the reflection mirror 415 through which
ASE (Amplified Spontaneous Emission) lights can be transmitted.
[0093] The reflection mirror 415 can be provided in various forms
including a reflection mirror in which a metal vapor deposition is
applied to a silica glass, a dielectric multi-layer film type
reflection mirror, a fiber grating type reflection mirror, etc.
[0094] In this embodiment, the reflection mirror 415 constitutes a
signal light turnaround propagation unit.
[0095] The signal light spectrum obtained in the case of pumping
the signal light 1 by the pump light 2 using the optical fiber
amplifier 410, 420 or 430 of the above described configuration is
shown in a part (a) of FIG. 15. Also, the signal light spectrum in
the case of using the optical fiber amplifier 310, 320, or 330 of
the first embodiment described above (using the reflection mirror
315 for reflecting at least the signal light 1 among the signal
light 1 and the pump light 2) is also shown for the purpose of
comparison in a part (b) of FIG. 15.
[0096] As can be seen in FIG. 15, compared with the case of using
the optical fiber amplifier 310, 320, or 330 of the third
embodiment (a part (b) of FIG. 15), when the optical fiber
amplifier 410, 420 or 430 of the fourth embodiment is used (a part
(a) of FIG. 15), the signal-to-noise ratio becomes larger so that
the noise characteristic is improved and the output intensity of
the signal light 1 increases because the wasteful consumption of
the pump light 2 by the ASF is reduced.
[0097] Thus, according to the fourth embodiment, the conversion
efficiency of the signal light 1 can be improved even more than the
case of the third embodiment described above.
FIFTH EMBODIMENT
[0098] Referring now to FIG. 16A to FIG. 16C, the fifth embodiment
of an optical fiber amplifier according to the present invention
will be described in detail. Here, those elements which are
substantially similar to the corresponding elements in the first to
fourth embodiments described above will be given the same reference
numerals in the figures and their description will be omitted.
[0099] FIGS. 16A to 16C show schematic configurations of the
optical fiber amplifier of the fifth embodiment.
[0100] FIG. 16A shows a schematic configuration of a forward
pumping type optical fiber amplifier 510 in which the pump light 2
is entered along the same direction as the propagating direction of
the signal light 1. As shown in FIG. 16A, this optical fiber
amplifier 510 has a configuration in which the amplification
optical fiber 210 of the second embodiment described above is
modified by replacing the optical isolator 112 on the output side
of the signal light 1 with the reflection mirror 315 used in the
optical fiber amplifier 310 of the third embodiment described
above, and replacing the optical isolator 112 on the input side of
the signal light 1 with the optical circulator 316 used in the
optical fiber amplifier 310 of the third embodiment described
above.
[0101] FIG. 16B shows a schematic configuration of a backward
pumping type optical fiber amplifier 520 in which the pump light 2
is entered along the opposite direction of the propagating
direction of the signal light 1. As shown in FIG. 16B, this optical
fiber amplifier 520 has a configuration in which the optical fiber
amplifier 220 of the second embodiment described above is modified
by replacing the optical isolator 112 on the output side of the
signal light 1 with the reflection mirror 315, and replacing the
optical isolator 112 on the input side of the signal light 1 with
the optical circulator 316.
[0102] FIG. 16C shows a schematic configuration of a bidirectional
pumping type optical fiber amplifier 530 in which the pump lights 2
are entered along both the same direction and the opposite
direction of the propagating direction of the signal light 1. As
shown in FIG. 16C, this optical fiber amplifier 530 has a
configuration in which the amplification optical fiber 230 of the
second embodiment described above Is modified by replacing the
optical isolator 112 on the output side of the signal light 1 with
the reflection mirror 315, and replacing the optical isolator 112
on the input side of the signal light 1 with the optical circulator
316.
[0103] In other words, the optical fiber amplifier 510, 520, or 530
of the fifth embodiment has a configuration that combines features
of the optical fiber amplifier 210, 220, or 230 of the second
embodiment described above and features of the optical fiber
amplifier 310, 320, or 330 of the third embodiment described
above.
[0104] Thus, according to the fifth embodiment, the effect of the
second embodiment described above and the effect of the third
embodiment described above can be obtained together so that the
conversion efficiency of the signal light 1 can be improved
further.
SIXTH EMBODIMENT
[0105] Referring now to FIG. 17, the sixth embodiment of an optical
fiber amplifier according to the present invention will be
described in detail. Here, those elements which are substantially
similar to the corresponding elements in the first to fifth
embodiments described above will be given the same reference
numerals in the figures and their description will be omitted.
[0106] FIG. 17 shows a schematic configuration of the optical fiber
amplifier of the sixth embodiment.
[0107] FIG. 17 shows a schematic configuration of a bidirectional
pumping type optical fiber amplifier 630 in which the pump lights 2
are entered along both the same direction and the opposite
direction of the propagating direction of the signal light 1. As
shown in FIG. 17, this optical fiber amplifier 630 has a two stage
configuration in which two of the amplification optical fiber 130
of the first embodiment described above are connected in series,
that is, a plurality of the amplification optical fibers 111 are
connected in series and the pump light sources 114 are connected to
these amplification optical fibers 111 through the couplers 113.
Note here that the optical isolator 112 on the output side of the
optical fiber amplifier 130 of the first stage to which the signal
light 1 is entered and the optical isolator 112 on the input side
of the optical fiber amplifier 130 of the second stage from which
the signal light 1 is outputted are provided by a single optical
isolator 112.
[0108] In the optical fiber amplifier 630 of the above described
configuration, the signal light 1 amplified by the optical fiber
amplifier 130 of the first stage can be further amplified and
outputted at the optical fiber amplifier 130 of the second
stage.
[0109] Consequently, according to the sixth embodiment, it becomes
possible to realize the signal light amplification with a higher
gain and a higher output power than the case of the first
embodiment described above.
[0110] Note that the sixth embodiment described above is directed
to the case of using a configuration in which two bidirectional
pumping type optical fiber amplifiers 130 are connected in series,
but it is also possible to replace at least one of the optical
fiber amplifiers of the first stage and the second stage with the
forward pumping type optical fiber amplifier 110 or the backward
pumping type optical fiber amplifier 120.
SEVENTH EMBODIMENT
[0111] Referring now to FIG. 18, the seventh embodiment of an
optical fiber amplifier according to the present invention will be
described in detail. Here, those elements which are substantially
similar to the corresponding elements in the first to sixth
embodiments described above will be given the same reference
numerals in the figures and their description will be omitted.
[0112] FIG. 18 shows a schematic configuration of the optical fiber
amplifier of the seventh embodiment.
[0113] FIG. 18 shows a schematic configuration of a bidirectional
pumping type optical fiber amplifier 730 in which the pump lights 2
are entered along both the same direction and the opposite
direction of the propagating direction of the signal light 1. As
shown in FIG. 18, this optical fiber amplifier 730 has a two stage
configuration in which two of the amplification optical fiber 330
of the third embodiment described above are connected in series,
that is, a plurality of the amplification optical fibers 111 are
connected in series and the pump light sources 114 are connected to
these amplification optical fibers 111 through the couplers
113.
[0114] In the optical fiber amplifier 730 of the above described
configuration, the signal light 1 amplified by the optical fiber
amplifier 330 of the first stage can be further amplified and
outputted at the optical fiber amplifier 330 of the second
stage.
[0115] Consequently, according to the seventh embodiment, it
becomes possible to realize the signal light amplification with a
higher gain and a higher output power than the case of the third
embodiment described above. In addition, by suppressing the signal
light 1 outputted from the optical fiber amplifier 330 of the first
stage within several dBm, the population inversion state at the
front end portion of the amplification optical fiber 111 in the
optical fiber amplifier 330 of the second stage can be made higher,
so that it is possible to suppress the lowering of the noise
figure.
[0116] Note that the seventh embodiment described above is directed
to the case of using a configuration in which two bidirectional
pumping type optical fiber amplifiers 330 are connected in series,
but it Is also possible to replace at least one of the optical
fiber amplifiers of the first stage and the second stage with the
forward pumping type optical fiber amplifier 310 or the backward
pumping type optical fiber amplifier 320.
EIGHTH EMBODIMENT
[0117] Referring now to FIG. 19, the eighth embodiment of an
optical fiber amplifier according to the present invention will be
described in detail. Here, those elements which are substantially
similar to the corresponding elements in the first to seventh
embodiments described above will be given the same reference
numerals in the figures and their description will be omitted.
[0118] FIG. 19 shows a schematic configuration of the optical fiber
amplifier of the eighth embodiment.
[0119] FIG. 19 shows a schematic configuration of a bidirectional
pumping type optical fiber amplifier 830 in which the pump lights 2
are entered along both the same direction and the opposite
direction of the propagating direction of the signal light 1. As
shown in FIG. 19, this optical fiber amplifier 830 has a
configuration in which the amplification optical fiber 730 of the
seventh embodiment described above is modified by connecting a gain
equalizer 817 between the optical fiber amplifier 330 of the first
stage and the optical fiber amplifier 330 of the second stage.
[0120] In the optical fiber amplifier 830 of the above described
configuration, the signal light 1 amplified b)y the optical fiber
amplifier 330 of the first stage will be equalized to have a flat
gain characteristic by the gain equalizer 817 and then further
amplified and outputted by the optical fiber amplifier 330 of the
second stage.
[0121] Consequently, according to tile eighth embodiment, it
becomes possible to output the signal light 1 which is amplified by
a more equalized gain characteristic in the 1480-1520 nm band than
the case of the seventh embodiment described above.
[0122] Note that the eighth embodiment described above is directed
to the case of using a configuration in which two bidirectional
pumping type optical fiber amplifiers 330 are connected in series,
but it is also possible to replace at least one of the optical
fiber amplifiers of the first stage and the second stage with the
forward pumping type optical fiber amplifier 310 or the backward
pumping type optical fiber amplifier 320.
[0123] Note also that the eighth embodiment described above is
directed to the case of using one gain equalizer 817, but it is
also possible to use more than one gain equalizers 817.
NINTH EMBODIMENT
[0124] Referring now to FIG. 20 to FIG. 27, the ninth embodiment of
an optical fiber amplifier according to the present invention will
be described in detail. Here, those elements which are
substantially similar to the corresponding elements in the first to
eighth embodiments described above will be given the same reference
numerals in the figures and their description will be omitted.
[0125] FIGS. 20 to 27 show schematic configurations of the optical
fiber amplifier of the ninth embodiment.
[0126] In the sixth to eighth embodiments described above, a
plurality of the amplification optical fibers 111 are connected in
series, and the pump light sources 114 are connected to these
amplification optical fibers 111 through the to couplers 113. In
contrast, in the ninth embodiment, a plurality of amplification
optical fibers 111 are connected in parallel through optical
splitter/combiner. In this way, it is also possible to realize the
signal light amplification and the high output power.
[0127] More specifically, FIG. 20 shows schematic configuration of
a bidirectional pumping type optical fiber amplifier in which the
pump lights 2 are entered along both the same direction and the
opposite direction of the propagating direction of the signal light
1, and which has a configuration in which two of the amplification
optical fiber 130 of the first embodiment described above are
connected in parallel.
[0128] Similarly, FIG. 21 shows a schematic configuration of a
bidirectional pumping type optical fiber amplifier in which the
pump lights 2 arc entered along both the same direction and the
opposite direction of the propagating direction of the signal light
1, and which has a configuration in which two sets of two
amplification optical fibers 130 of the first embodiment described
above that are connected in series are connected in parallel.
[0129] Similarly, FIG. 22 shows a schematic configuration of a
bidirectional pumping type optical fiber amplifier in which the
pump lights 2 are entered along both the same direction and the
opposite direction of the propagating direction of the signal light
1, and which has a configuration in which two of the amplification
optical fiber 130 of the first embodiment described above are
connected in parallel, and two gain equalizers 817 are added
respectively at output side of these two amplification optical
fibers 130.
[0130] Similarly, FIG. 23 shows a schematic configuration of a
bidirectional pumping type optical fiber amplifier in which the
pump lights 2 are entered along both the same direction and the
opposite direction of the propagating direction of the signal light
1, and which has a configuration in which two sets of two
amplification optical fibers 130 of the first embodiment described
above that are connected in series are connected in parallel, and
two gain equalizers 817 are added respectively between the optical
fiber amplifier of the first stage and the optical fiber amplifier
of the second stage in each of these two sets of two amplification
optical fibers 130.
[0131] Similarly, FIG. 24 shows a schematic configuration of a
bidirectional pumping type optical fiber amplifier in which the
pump lights 2 are entered along both the same direction and the
opposite direction of the propagating direction of the signal light
1, and which has a configuration in which two of the amplification
optical fiber 330 of the third embodiment described above are
connected in parallel.
[0132] Similarly, FIG. 25 shows a schematic configuration of a
bidirectional pumping type optical fiber amplifier in which the
pump lights 2 are entered along both the same direction and the
opposite direction of the propagating direction of the signal light
1, and which has a configuration in which two sets of two
amplification optical fibers 330 of the third embodiment described
above that are connected in series are connected in parallel.
[0133] Similarly, FIG. 26 shows a schematic configuration of a
bidirectional pumping type optical fiber amplifier in which the
pump lights 2 are entered along both the same direction and the
opposite direction of the propagating direction of the signal light
1, and which has a configuration in which two of the amplification
optical fiber 330 of the third embodiment described above are
connected in parallel, and two gain equalizers 817 are added
respectively at output side of these two amplification optical
fibers 330.
[0134] Similarly, FIG. 27 shows a schematic configuration of a
bidirectional pumping type optical fiber amplifier In which the
pump lights 2 are entered along both the same direction and the
opposite direction of the propagating direction of the signal light
1, and which has a configuration in which two sets of two
amplification optical fibers 330 of the third embodiment described
above that are connected in series are connected in parallel, and
two gain equalizers 817 are added respectively between the optical
fiber amplifier of the first stage and the optical fiber amplifier
of the second stage in each of these two sets of two amplification
optical fibers 330.
OTHER EMBODIMENTS
[0135] In each of the first to ninth embodiments described above,
it is also possible to provide a coupler for separating the signal
light 1 and the pump light 2.
[0136] Also, in each of the first to ninth embodiments described
above, the amplification optical fiber 111 having Tm in its core is
used, but instead of that, it is also possible to use an
amplification optical fiber having Tm in its core and cladding, and
the similar effects as in the cases of the first to ninth
embodiments described above can be obtained as long as the
amplification optical fiber that has Tm at least in its core.
[0137] As described, according to the optical fiber amplifier of
the present invention, the signal light is pumped by using the pump
light of the wavelength in 1400 nm band that is the same as the
wavelength band of the signal light that can be emitted by the
laser diode (LD), so that it is possible to suppress the occurrence
of ESA (Excited State Absorption) from .sup.3H.sub.4, so that it is
possible to realize a high efficiency in the S-band, a compact
size, and a low cost. Also, by using a double pass type
configuration or a dual wavelength pumping type configuration, it
is possible to realize a higher efficiency.
[0138] It is also to be noted that, besides those already mentioned
above, many modifications and variations of the above embodiments
may be made without departing from the novel and advantageous
features of the present invention. Accordingly, all such
modifications and variations are intended to be included within the
scope of the appended claims.
* * * * *