U.S. patent application number 09/911418 was filed with the patent office on 2002-01-24 for optical amplifier for amplifying light in a long wavelength band.
This patent application is currently assigned to FUJITSU LIMITED. Invention is credited to Kinoshita, Susumu, Onaka, Hiroshi, Sugaya, Yasushi.
Application Number | 20020008900 09/911418 |
Document ID | / |
Family ID | 15930504 |
Filed Date | 2002-01-24 |
United States Patent
Application |
20020008900 |
Kind Code |
A1 |
Sugaya, Yasushi ; et
al. |
January 24, 2002 |
Optical amplifier for amplifying light in a long wavelength
band
Abstract
An optical amplifier for amplifying light in a longer wavelength
band. The optical amplifier includes first and second optical
fibers doped with a rare earth element and optically connected so
that a signal light travels through the first optical fiber and
then through the second optical fiber. Excitation light causes the
signal light to be amplified in both the first and second optical
fibers. Spontaneous emission lights are generated in the first
optical fiber. An oscillation generator causes the spontaneous
emission lights to oscillate in the first optical fiber, to thereby
generate laser oscillation. Light generated by the laser
oscillation is supplied to the second optical fiber as excitation
light in the second optical fiber. The oscillation generator can be
formed by fiber gratings along the first optical fiber and which
reflect spontaneous emission light at a predetermined wavelength,
thereby causing spontaneous emission lights to oscillate between
the fiber gratings in the first optical fiber. Alternatively, the
oscillation generator can be a traveling- wave type optical
oscillator. In an additional embodiment, an optical amplifier
includes first and second optical amplifiers amplifying first and
second signal lights, respectively, the first and second signal
lights being in different wavelength bands. An excitation light
providing device branches a portion of the signal light amplified
by the first optical amplifier, and provides the branched portion
to the second optical amplifier as excitation light in the second
optical amplifier.
Inventors: |
Sugaya, Yasushi;
(Kawasaki-shi, JP) ; Kinoshita, Susumu;
(Kawasaki-shi, JP) ; Onaka, Hiroshi;
(Kawasaki-shi, JP) |
Correspondence
Address: |
STAAS & HALSEY LLP
700 11TH STREET, NW
SUITE 500
WASHINGTON
DC
20001
US
|
Assignee: |
FUJITSU LIMITED
Kawasaki
JP
|
Family ID: |
15930504 |
Appl. No.: |
09/911418 |
Filed: |
July 25, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09911418 |
Jul 25, 2001 |
|
|
|
09334622 |
Jun 17, 1999 |
|
|
|
6288834 |
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Current U.S.
Class: |
359/341.1 ;
359/341.3 |
Current CPC
Class: |
H01S 3/0677 20130101;
H04B 10/2972 20130101; H01S 3/06758 20130101; H01S 3/06766
20130101; H01S 3/094003 20130101; H01S 3/06754 20130101; H04B
10/291 20130101; H01S 3/094023 20130101 |
Class at
Publication: |
359/341.1 ;
359/341.3 |
International
Class: |
H01S 003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 18, 1998 |
JP |
10-171828 |
Claims
What is claimed is:
1. An apparatus comprising: a first optical fiber doped with a rare
earth element and supplied with an excitation light, spontaneous
emission lights being generated in the first optical fiber; a
second optical fiber doped with a rare earth element; and an
oscillation generator causing the spontaneous emission lights to
oscillate in the first optical fiber, and supplying light generated
by the oscillation of the spontaneous emission lights to the second
optical fiber as excitation light in the second optical fiber.
2. An apparatus as in claim 1, further comprising: a light source
supplying excitation light to the second optical fiber, the
excitation light supplied by the light source being different that
the light generated by the oscillation of the spontaneous emission
lights and supplied to the second optical fiber as excitation
light.
3. An apparatus as in claim 1, wherein a signal light travels
through the first optical fiber and then through the second optical
fiber, the excitation light and the oscillating spontaneous
emission lights in the first optical fiber cause the signal light
to be amplified as the signal light travels through the first
optical fiber, and the light generated by the oscillation of the
spontaneous emission lights and supplied to the second optical
fiber causes the signal light to be amplified as the signal light
travels through the second optical fiber.
4. An apparatus as in claim 1, wherein the oscillation generator
comprises: reflectors at different points along the first optical
fiber and which reflect spontaneous emission light at a
predetermined wavelength, the reflectors causing spontaneous
emission lights to oscillate in the first optical fiber.
5. An apparatus as in claim 4, wherein the reflectors are fiber
gratings.
6. An apparatus as in claim 1, wherein a signal light travels
through the first optical fiber from a first end to a second end of
the first optical fiber, and then through the second optical fiber,
the oscillation generator includes a first reflector at the first
end of the first optical fiber and a second reflector at the second
end of the first optical fiber, the first and second reflectors
reflecting spontaneous emission light at a predetermined wavelength
so that spontaneous emission lights oscillate in the first optical
fiber between the first and second reflectors.
7. An apparatus as in claim 6, wherein the first and second
reflectors are fiber gratings.
8. An apparatus as in claim 6, wherein the first reflector is a
fiber grating having approximately 100% reflectance for spontaneous
emission light at the predetermined wavelength, and the second
reflector is a fiber grating having less than 100% reflectance for
spontaneous emission light at the predetermined wavelength.
9. An apparatus as in claim 8, wherein the first and second
reflectors are fiber gratings.
10. An apparatus as in claim 6, wherein a signal light travels
through the first optical fiber from a first point to a second
point along the first optical fiber, and then through the second
optical fiber, and the oscillation generator includes a first fiber
grating positioned at the first point and having approximately 100%
reflectance for spontaneous emission light at the predetermined
wavelength, and a second fiber grating positioned at the second
point and having less than 100% reflectance for spontaneous
emission light at the predetermined wavelength, the first and
second fiber gratings causing spontaneous emission lights to
oscillate in the first optical fiber between the first and second
fiber gratings, the light generated by the oscillation of the
spontaneous emission lights passing through the second fiber
grating into the second optical fiber.
11. An apparatus as in claim 1, wherein the excitation light
supplied to the first optical fiber is forward excitation
light.
12. An apparatus as in claim 1, wherein the excitation light
supplied to the first optical fiber is bi-directional excitation
light.
13. An apparatus as in claim 1, wherein the excitation light
supplied to the first optical fiber is bi-directional excitation
light, and the second optical fiber is provided with bi-directional
excitation light, in addition to the light generated by the
oscillation of the spontaneous emission lights and supplied to the
second optical fiber as excitation light.
14. As apparatus as in claim 1, wherein the first and second
optical fibers each have first and second ends, light travels
through the first optical fiber from the first end to the second
end of the first optical fiber, and then through the second optical
fiber from the first end to the second end of the second optical
fiber, the apparatus further comprises a first light source
providing excitation light to the first end of the first optical
fiber, and a second light source providing excitation light to the
second end of the second optical fiber, so that the first and
second light sources together provide bi directional excitation
light to both the first and second optical fibers.
15. An apparatus as in claim 1, wherein a signal light travels
through the first optical fiber from a first end to a second end of
the first optical fiber, and then through the second optical fiber,
the apparatus further comprising a light source providing the
excitation light to one of the first and second ends of the first
optical fiber, and an excitation light reflector at the other of
said one of the first and second ends of the first optical fiber,
the excitation light reflector reflecting excitation light back
into the first optical fiber and passing the signal light.
16. An apparatus as in claim 1, wherein a signal light travels
through the first optical fiber from a first end to a second end of
the first optical fiber, and then through the second optical fiber,
the apparatus further comprising a light source providing the
excitation light to the first end of the first optical fiber as
forward excitation light, and an excitation light reflector at the
second end of the first optical fiber, the forward excitation light
traveling through the first optical fiber and out the second end of
the first optical fiber, and being reflected back into the first
optical fiber through the second end by the excitation light
reflector.
17. An apparatus as in claim 1, wherein signal light travels from
the first optical fiber to the second optical fiber, the excitation
light supplied to the first optical fiber is bi-directional
excitation light, the second optical fiber is provided with bi
directional excitation light, in addition to the light generated by
the oscillation of the spontaneous emission lights and supplied to
the second optical fiber as excitation light, and the apparatus
further comprises an excitation light reflector optically connected
between the first and second optical fibers, the excitation light
reflector reflecting forward excitation light traveling out of the
first optical fiber back into the first optical fiber, reflecting
backward excitation light traveling out of the second optical fiber
back into the second optical fiber, and passing signal light from
the first optical fiber to the second optical fiber.
18. An apparatus as in claim 17, wherein the excitation light
reflector is a fiber grating.
19. An apparatus as in claim 1, further comprising: a third optical
fiber doped with a rare earth element and optically connected to
the first and second optical fibers so that light travels through
the third optical fiber, then through the first optical fiber, and
then through the second optical fiber.
20. An apparatus as in claim 16, further comprising: a third
optical fiber doped with a rare earth element and optically
connected to the first and second optical fibers so that light
travels through the third optical fiber, then through the first
optical fiber, and then through the second optical fiber.
21. An apparatus as in claim 19, further comprising: a light
intercepting device intercepting light traveling from the first
optical fiber to the third optical fiber.
22. An apparatus as in claim 20, further comprising: a light
intercepting device intercepting light traveling from the first
optical fiber to the third optical fiber.
23. An apparatus as in claim 21, wherein the light intercepting
device is an optical isolator.
24. An apparatus as in claim 1, wherein a signal light travels
through the first optical fiber and then through the second optical
fiber, the signal light being in a wavelength band from 1.57 to
1.62 .mu.m, and the spontaneous emission lights are in a wavelength
band from 1.53 to 1.57 .mu.m
25. An apparatus as in claim 1, wherein a signal light travels
through the first optical fiber and then through the second optical
fiber, the signal light being in a wavelength band from 1.57 to
1.62 .mu.m, the spontaneous emission lights are in a wavelength
band from 1.53 to 1.57 .mu.m, the excitation light and the
oscillating spontaneous emission lights in the first optical fiber
cause the signal light to be amplified as the signal light travels
through the first optical fiber, and the signal light is amplified
as the signal light travels through the second optical fiber.
26. An apparatus as in claim 1, wherein the oscillation generator
is a traveling-wave type optical oscillator.
27. An apparatus as in claim 1, wherein the first and second
optical fibers each have first and second ends, light travels
through the first optical fiber from the first end to the second
end of the first optical fiber, and then through the second optical
fiber from the first end to the second end of the second optical
fiber, the oscillation generator is a traveling-wave type optical
oscillator comprising a branching device branching a portion of
light traveling out of the second end of the second optical fiber,
a wavelength selection device selecting spontaneous emission light
of a predetermined wavelength band from said branched portion, the
selected spontaneous emission light being provided to the first
optical fiber through the first end of the first optical fiber.
28. An apparatus as in claim 1, wherein the oscillation of the
spontaneous emission lights in the first optical fiber generates
laser oscillation, and the light generated by the oscillation of
the spontaneous emission lights and supplied to the second optical
fiber is light generated by the laser oscillation.
29. An apparatus comprising: first and second optical fibers each
doped with a rare earth element and optically connected so that a
signal light travels through the first optical fiber and then
through the second optical fiber; at least one light source
providing excitation light to the first and second optical fibers
so that the signal light is amplified in both the first and second
optical fibers, spontaneous emission lights being generated in the
first optical fiber; and an oscillation generator causing the
spontaneous emission lights to oscillate in the first optical
fiber, and supplying light generated by the oscillation of the
spontaneous emission lights to the second optical fiber as
excitation light in the second optical fiber.
30. An apparatus as in claim 29, wherein the oscillation generator
comprises: reflectors at different positions along the first
optical fiber and which reflect spontaneous emission light at a
predetermined wavelength, the reflectors causing spontaneous
emission lights to oscillate between the reflectors in the first
optical fiber.
31. An apparatus as in claim 30, wherein the reflectors are fiber
gratings.
32. An apparatus as in claim 29, wherein the signal light travels
through the first optical fiber from a first point to a second
point along the first optical fiber, and then through the second
optical fiber, the oscillation generator includes a first reflector
at the first point and a second reflector at the second point, the
first and second reflectors reflecting spontaneous emission light
at a predetermined wavelength so that spontaneous emission lights
oscillate in the first optical fiber between the first and second
reflectors.
33. An apparatus as in claim 32, wherein the first and second
reflectors are fiber gratings.
34. An apparatus as in claim 32, wherein the first reflector is a
fiber grating having approximately 100% reflectance for spontaneous
emission light at the predetermined wavelength, and the second
reflector is a fiber grating having less than 100% reflectance for
spontaneous emission light at the predetermined wavelength so that
the light generated by the oscillation of the spontaneous emission
lights passes through the second reflector into the second optical
fiber.
35. An apparatus as in claim 29, wherein a signal light travels
through the first optical fiber and then through the second optical
fiber, the signal light being in a wavelength band from 1.57 to
1.62 .mu.m, and the spontaneous emission lights are in a wavelength
band from 1.53 to 1.57 .mu.m.
36. An apparatus as in claim 29, wherein the oscillation of the
spontaneous emission lights in the first optical fiber generates
laser oscillation, and the light generated by the oscillation of
the spontaneous emission lights and supplied to the second optical
fiber is light generated by the laser oscillation.
37. An optical amplifier comprising: a first amplification stage
amplifying a signal light and generating spontaneous emission
lights as the signal light is amplified; a second amplification
stage amplifying the signal light after being amplified by the
first amplification stage; and an oscillation generator causing the
spontaneous emission lights to oscillate in the first amplification
stage and thereby generate laser oscillation, and causing light
generated by the laser oscillation to be supplied to the second
amplification stage for amplification of the signal light in the
second amplification stage.
38. An apparatus comprising: a first optical fiber doped with a
rare earth element and supplied with an excitation light so that
spontaneous emission lights are generated in the first optical
fiber; a second optical fiber doped with a rare earth element; and
means for causing the spontaneous emission lights to oscillate in
the first optical fiber, and for supplying light generated by the
oscillation of the spontaneous emission lights to the second
optical fiber as excitation light in the second optical fiber.
39. A method comprising: causing a signal light to travel through a
first optical fiber and then through a second optical fiber, the
first and second optical fibers each doped with a rare earth
element, providing excitation light to the first and second optical
fibers so that the signal light is amplified in both the first and
second optical fibers, spontaneous emission lights being generated
in the first optical fiber; causing the spontaneous emission lights
to oscillate in the first optical fiber; and supplying light
generated by the oscillation of the spontaneous emission lights to
the second optical fiber as excitation light in the second optical
fiber.
40. A method as in claim 39, wherein the oscillation of the
spontaneous emission lights in the first optical fiber generates
laser oscillation, and the light generated by the oscillation of
the spontaneous emission lights and supplied to the second optical
fiber is light generated by the laser oscillation.
41. An apparatus for amplifying a signal light, comprising: an
optical fiber doped with a rare earth element and provided with
excitation light so that the signal light is amplified as the
signal light travels through the optical fiber, thereby generating
spontaneous emission light in the optical fiber; and an oscillation
generator causing the spontaneous emission lights to oscillate in
the optical fiber.
42. An apparatus as in claim 41, wherein the oscillation generator
comprises: reflectors at ends of the optical fiber and which
reflect spontaneous emission light at a predetermined wavelength,
the reflectors causing spontaneous emission lights to oscillate
between the reflectors in the first optical fiber.
43. An apparatus as in claim 42, wherein the reflectors are fiber
gratings.
44. An apparatus as in claim 41, wherein the oscillation generator
is a traveling-wave type optical oscillator.
45. An apparatus as in claim 41, wherein the optical fiber has
first and second ends, the signal light travels through the optical
fiber from the first end to the second end, and the oscillation
generator is a traveling-wave type optical oscillator comprising a
branching device branching a portion of light traveling out of the
second end of the optical fiber, a wavelength selection device
selecting spontaneous emission light of a predetermined wavelength
band from said branched portion, the selected spontaneous emission
light being provided to the optical fiber through the first end of
the optical fiber.
46. An apparatus as in claim 41, wherein the oscillation of the
spontaneous emission lights in the optical fiber generates laser
oscillation.
47. A method comprising: providing an optical fiber doped with a
rare earth element; provided excitation light to the optical fiber
so that a signal light is amplified as the signal light travels
through the optical fiber, thereby generating spontaneous emission
light in the optical fiber; and causing the spontaneous emission
lights to oscillate in the optical fiber.
48. An apparatus comprising: first and second optical amplifiers
amplifying first and second signal lights, respectively, the first
and second signal lights being in different wavelength bands; and
an excitation light providing device branching a portion of the
signal light amplified by the first optical amplifier, and
providing said portion to the second optical amplifier as
excitation light in the second optical amplifier.
49. An apparatus as in claim 48, wherein the excitation light
providing device comprises: a branching device branching a portion
of the signal light amplified by the first optical amplifier; and a
coupler providing said portion to the second optical amplifier as
excitation light in the second optical amplifier.
50. An apparatus as in claim 48, wherein each of the first and
second optical amplifiers is a fiber amplifier employing an optical
fiber doped with a rare earth element.
51. An apparatus as in claim 48, wherein the first wavelength band
is a conventional band, and the second wavelength band is a longer
wavelength band.
52. An apparatus comprising: a first optical fiber doped with a
rare earth element and provided with excitation light to amplify a
signal light in a first wavelength band as the signal light travels
through the first optical fiber; a second optical fiber doped with
a rare earth element and provided with excitation light to amplify
a signal light in a second wavelength band, different from the
first wavelength band, as the signal light travels through the
second optical fiber; and an excitation light providing device
branching a portion of the signal light amplified by the first
optical fiber, and providing said portion to the second optical
fiber as excitation light in the second optical fiber.
53. An apparatus as in claim 52, wherein the excitation light
providing device comprises: a branching device branching a portion
of the signal light amplified by the first optical fiber; and a
coupler providing said portion to the second optical fiber as
excitation light in the second optical fiber.
54. An apparatus as in claim 52, wherein the first wavelength band
is a conventional band, and the second wavelength band is a longer
wavelength band.
55. An apparatus comprising: a first optical fiber doped with a
rare earth element and provided with excitation light to amplify a
signal light in a first wavelength band as the signal light travels
through the first optical fiber; a second optical fiber doped with
a rare earth element and provided with excitation light to amplify
a signal light in a second wavelength band, different from the
first wavelength band, as the signal light travels through the
second optical fiber; and means for branching a portion of the
signal light amplified by the first optical fiber, and for
providing said portion to the second optical fiber as excitation
light in the second optical fiber.
56. An apparatus as in claim 55, wherein the first wavelength band
is a conventional band, and the second wavelength band is a longer
wavelength band.
57. A method comprising: amplifying a signal light in a first
wavelength band with a first optical fiber doped with a rare earth
element; amplifying a signal light in a second wavelength band,
different from the first wavelength band, with a second optical
fiber doped with a rare earth element; branching a portion of the
signal light amplified by the first optical fiber; and providing
said portion to the second optical fiber as excitation light in the
second optical fiber.
58. A method as in claim 57, wherein the first wavelength band is a
conventional band, and the second wavelength band is a longer
wavelength band.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is based on, and claims priority to,
Japanese application number 10-171828, filed Jun. 18, 1998, in
Japan, and which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to an optical amplifier and
method for amplifying optical signal light using an optical fiber
doped with a rare earth element, and especially for amplifying
optical signal light in a long wavelength band.
[0004] 2. Description of the Related Art
[0005] With the advancing development of multimedia networks,
demand for information is drastically increasing. Therefore, trunk
optical transmission systems, which have relatively high
information transmission capacity, will be required to have even
higher information transmission capacity and will be required to
form flexible networks.
[0006] To provide higher transmission capacity, wavelength division
multiplexing (WDM) optical transmission systems are being used. The
commercialization of WDM optical transmission systems has already
been advanced mainly in North America.
[0007] Moreover, WDM optical amplifiers have been used to amplify
WDM optical signals. A WDM optical amplifier can collectively
amplify signal lights having two or more different wavelengths in,
for example, a wavelength range of 1.53 to 1.57 .mu.m (hereinafter
referred to as the "conventional band"). Therefore, the use of WDM
optical amplifiers in a WDM optical transmission system can enable
high-capacity, long-distance optical transmission with a relatively
simple configuration.
[0008] Furthermore, by expanding the wavelength band of an optical
amplifier, a system has been proposed which employs a long
wavelength band, for example, of 1.57 to 1.62 .mu.m (hereinafter
referred to as the "longer wavelength band"), as a new transmission
band.
[0009] The following is a description of the use of an optical
amplifier employing an erbium doped fiber (EDF) for the
amplification of signal light in the longer wavelength band.
[0010] FIG. 1 is a graph showing the gain per unit length versus
wavelength characteristics of an EDF corresponding to the degrees
of population inversion (ranging from 0.0 to 1.0). As shown in FIG.
1, in the conventional band, the gain characteristic of an EDF is
flat in the case that the degree of population inversion is 70% or
so. Namely, the gain of an EDF in the conventional band is
dominant.
[0011] Conversely, in the longer wavelength band, the gain
characteristic of an EDF is flat in the case that the degree of
population inversion is low, namely, 40% or so. Thus, the gain of
an EDF in the longer wavelength band is dominant. Therefore, in the
case of optical amplification of signal light in the longer
wavelength band, an excitation light of a wavelength in a 0.98
.mu.m band or a 1.48 .mu.m band is supplied to the EDF by setting
the degree of population inversion at a low level. In this case,
the amplification factor per unit length of the EDF decreases in
principle because of the low degree of population inversion.
[0012] In the forward excitation case of supplying the excitation
light from a signal light input terminal of the EDF to a signal
output terminal thereof, the degree of population inversion
corresponds to the excitation light power. Therefore, the degree of
population inversion is high at the signal light input terminal,
but is low at the signal light output terminal. In the backward
excitation case of supplying an excitation light from the signal
light output terminal of the EDF to the signal input terminal
thereof, there is a relation between the degrees of population
inversion at the signal input and output terminals thereof opposite
to that of the forward excitation case.
[0013] Thus, generally, a required total gain of the EDF in the
case of the optical amplification of signal light in the longer
wavelength band is obtained by elongating a conventional EDF used
for the optical amplification of signal light of the conventional
band, to thereby lower the degree of population inversion. This can
be understood by referring to FIG. 2, which illustrates gain
distribution in the longitudinal direction of an EDF.
[0014] Further, when the degree of population inversion is set at a
low level, the absorption of excitation light is increased. For
example, if the wavelength of the excitation light is in the 1.48
.mu.m band in FIG. 1, the gain per unit length of the EDF is about
-0.2 dB when the degree of population inversion is 0.4. This
indicates that, in such a case, the excitation light is more likely
to be absorbed in the EDF, as compared with the case where the gain
per unit length thereof is about 0 dB when the degree of population
inversion is 0.7. When the excitation light is largely absorbed
therein, the absorption of excitation light is performed at a
biased position in the EDF. Consequently, the excitation power
propagation efficiency in the longitudinal direction of the EDF is
reduced. Thus, the optical amplification of signal light in the
longer wavelength band has a feature that the excitation efficiency
of the entire EDF is limited to a low value in comparison with that
of the entire EDF in the case of the optical amplification of
signal light in the conventional band. A conventional optical
amplifier for amplifying signal light of a long wavelength band
having such a feature is described in, for example, the article
titled "Gain Flattened Er.sup.30+Doped Fiber Amplifier for a WDM
Signal in the 1.57-1.60 .mu.m Wavelength Region," Ono et al., IEEE
Photon. Tech. Lett., Vol. 9, pp. 596-598, May, 1997.
[0015] FIG. 3 is a diagram showing such a conventional long
wavelength band optical amplifier. In the optical amplifier of FIG.
3, an incident longer wavelength band signal light L.sub.s passes
through an optical isolator 2.sub.1 and is multiplexed with an
excitation light Lp.sub.1 emitted from an excitation light source
4.sub.1 by a wavelength division multiplexing (WDM) coupler
3.sub.1. Then, the multiplexed light enters an EDF 1. An excitation
light Lp.sub.2 is emitted from an excitation light source 4.sub.2.
At an emitting terminal of EDF 1, excitation light LP.sub.2 is
multiplexed by a WDM coupler 3.sub.2 and is then propagated through
EDF 1 in the opposite direction than excitation light Lp.sub.1,
thereby contributing to optical amplification. The longer
wavelength band signal light L.sub.s passes through WDM coupler
3.sub.2 and optical isolator 2.sub.2 after passing through EDF 1.
Then, signal light L.sub.s is emitted from the amplifier.
[0016] FIG. 4 is a diagram showing another conventional long
wavelength band optical amplifier. Such a long wavelength band
optical amplifier is described, for example, in the article titled
"Amplification Characteristics of 1.58 .mu.m Band Er.sup.3+ Doped
Fiber Amplifier," Ono et al., TECHNICAL REPORT OF IEICE, Vol.
OCS97-5, pp. 25-30, 1997.
[0017] In the optical amplifier of FIG. 4, an incident longer
wavelength band signal light L.sub.s passes through an optical
isolator 2.sub.1. A forward excitation light Lp.sub.1 is emitted
from an excitation light source 4.sub.1. Signal light L.sub.s and
excitation light Lp.sub.1 are multiplexed by a WDM coupler 3.sub.1.
Then, the multiplexed light enters a pre-stage EDF 1.sub.1. After
passing through pre-stage EDF 1.sub.1, the longer wavelength band
signal light L.sub.s passes through an optical isolator 2.sub.3 and
enters a post-stage EDF 1.sub.2. Further, a backward excitation
light Lp.sub.2 is emitted from an excitation light source 4.sub.2.
Excitation light Lp.sub.2 enter post-stage EDF 1.sub.2 through a
WDM coupler 3.sub.2. Excitation light Lp.sub.2 propagates through
post-stage EDF 1.sub.2 in the opposite direction than excitation
light Lp.sub.1, and thereby contributes to the optical
amplification by the post-stage portion. Then, the longer
wavelength band signal light L.sub.s passes through WDM coupler
3.sub.2 and an optical isolator 2.sub.2 after passing through
post-stage EDF 1.sub.2. Finally, signal light L.sub.s reaches the
emitting terminal of the amplifier. In this case, one of
wavelengths (ranging from 960 to 1000 nm) in the 0.98 .mu.m band is
used as that of the forward excitation light LP.sub.2. Moreover,
one of wavelengths (ranging from 1450 to 1490 nm) in the 1.48 .mu.m
band is used as that of the backward excitation light Lp.sub.2.
With this configuration, a low noise optical amplifier is
realized.
[0018] FIG. 5 is a diagram showing another conventional long
wavelength band optical amplifier. Conventional long wavelength
band optical amplifiers, such as that in FIG. 5, are described, for
example, in the article titled "Low Noise Operation of Er.sup.3+
Doped Silica Fiber Amplifier around 1.6 .mu.m," Massicott et al.,
Electron. Lett., Vol. 28, pp. 1924-1925, September 1992, and U.S.
Pat. No. 5,500,764 Official Gazette.
[0019] In the optical amplifier of FIG. 5, an incident longer
wavelength band signal light L.sub.s is multiplexed with
conventional band signal light Lp.sub.3 through a multiplexer 5,
passes through an isolator 2.sub.1, and is then multiplexed at a
WDM coupler 3.sub.1 with an excitation light Lp.sub.1 emitted from
an excitation light source 4.sub.1. Then, the multiplexed signal
light L.sub.s enters in EDF 1. At an emitting terminal of EDF 1, an
excitation light Lp.sub.2 emitted from an excitation light source
4.sub.2 is multiplexed at a WDM coupler 3.sub.2. Excitation light
Lp.sub.2 propagates through EDF 1 in the opposite direction than
excitation light Lp.sub.1, and thereby contributes to
amplification. The longer wavelength band signal light L.sub.s
passes through WDM coupler 3.sub.2 and an optical isolator 2.sub.2
after passing through EDF 1. Finally, signal light L.sub.s reaches
the emitting terminal of the amplifier. This optical amplifier
improves the excitation efficiency by adding the conventional band
signal light Lp.sub.3 to the signal light L.sub.s at a low level.
Namely, the more the excitation light having a wavelength close to
that of signal light is used, the higher the excitation efficiency.
Therefore, the conventional band light is used as excitation light
for amplifying the longer wavelength band signal light.
[0020] However, the conversion efficiency of the conventional
amplifier in FIG. 3 is only 37.7% or so when the gain flattens in
the case that the wavelength of the excitation light is set at, for
example, one of wavelengths of 1450 to 1490 nm. Therefore, this
conventional optical amplifier has a problem that properties, such
as excitation efficiency and noise factor, are inferior to those in
the case of optical amplification of signal light of the
conventional band.
[0021] To cope with this problem, the conventional optical
amplifier in FIG. 4 reduces the length of pre-stage EDF 1.sub.1,
increases the length of post-stage EDF 1.sub.2 and uses the 0.98
.mu.m band excitation light, which has good noise characteristics,
for the pre-stage portion amplification. Thus, the noise level in
the case of this conventional optical amplifier is low, as compared
with the optical amplifier of FIG. 3.
[0022] However, although the optical amplifier in FIG. 4 is
effective in reducing noise, this optical amplifier has problems in
that the excitation power transmission efficiency is low and that
the excitation efficiency is still low.
[0023] In contrast, the optical amplifier in FIG. 5 obtains high
excitation efficiency by supplying the conventional band light
Lp.sub.3 to EDF 1, and reduces the energy consumption thereof.
However, this conventional optical amplifier has problems in that
an additional light source for generating the conventional band
light is required and that active optical parts, such as a light
source, are expensive and thus the cost of the amplifier is
increased.
SUMMARY OF THE INVENTION
[0024] Accordingly, it is an object of the present invention to
provide a low-cost optical amplifier and optical amplification
method which realize high excitation efficiency optical
amplification of signal light of new bands, such as the longer
wavelength band, by adding only passive optical parts to a
conventional doped fiber optical amplifier.
[0025] Additional objects and advantages of the invention will be
set forth in part in the description which follows, and, in part,
will be obvious from the description, or may be learned by practice
of the invention.
[0026] Objects of the present invention are achieved by providing
an optical amplifier for amplifying signal light of a predetermined
wavelength band. The amplifier includes first and second rare earth
element doped fibers cascaded together so that signal light travels
through the first rare earth element doped fiber and then through
the second rare earth element doped fiber. At least one excitation
light source generates excitation light, and at least one
excitation light supplying device supplies the excitation light
from the excitation light source to the first rare earth element
doped fiber. A laser oscillation light generating device laser
oscillates spontaneous emission light of a predetermined wavelength
band among spontaneous emission lights generated in the first rare
earth element doped fiber, so that laser oscillation is generated.
The laser oscillation light generating device then supplies light
generated by the laser oscillation to the second rare earth element
doped fiber as an excitation light.
[0027] With such an optical amplifier, when a signal light of a
predetermined wavelength band, for example, the longer wavelength
band, enters this optical amplifier, the signal light is sent to
the first rare earth element doped fiber doped with, for example,
erbium. An excitation light output from the excitation light source
is supplied to this first rare earth element doped fiber through
the excitation light supplying device. Then, a spontaneous emission
light is generated, and the signal light is amplified. The laser
oscillation light generating device oscillates a light of the
predetermined wavelength band, for example, the conventional band,
among the generated spontaneous emission lights, so that laser
oscillation is generated. Subsequently, the light generated by the
laser oscillation is supplied to the second rare earth element
doped fiber.
[0028] The signal light having passed through the first rare earth
element doped fiber enters in the second rare earth element doped
fiber. Light generated by the laser oscillation serves as an
excitation light in the second rare earth element doped fiber.
Thus, the signal light is amplified with high excitation
efficiency.
[0029] Consequently, the high excitation efficiency amplification
of signal light of the longer wavelength band is achieved without
adding active optical parts, such as a light source, differently
from the aforementioned conventional long wavelength band optical
amplifiers. Moreover, the cost of the optical amplifier is
reduced.
[0030] The laser oscillation light generating device may be, for
example, a fiber resonator type device or a traveling-wave type
optical oscillator. In the case that the laser oscillation light
generating device is the fiber resonator type device, the laser
oscillation light generating device includes a pair of wavelength
selective reflection type optical devices, one at each end of the
first rare earth element doped fiber, which can reflect the
spontaneous emission light of the predetermined wavelength band and
transmit the signal light and the excitation light. With such a
configuration, the spontaneous emission light of the predetermined
wavelength band generated in the first rare earth element doped
fiber is reflected by the wavelength selective reflection type
optical devices to resonate, so that laser oscillation occurs in
the first rare earth element doped fiber.
[0031] In the case that the laser oscillation light generating
device is a traveling-wave type optical oscillator, the laser
oscillation light generating device has a branching portion for
branching a part of light emitted from the signal light emitting
terminal of the first rare earth element doped fiber, and a
wavelength selection portion for receiving the branched light from
the branching portion and for selecting and emitting only a
spontaneous emission light of the predetermined wavelength band.
Moreover, the laser oscillation light generating device has a
multiplexing portion for entering a light emitted from the
wavelength selection portion, in a signal light incidence terminal
of the first rare earth element doped fiber.
[0032] With this configuration, a part of light emitted from the
first rare earth element doped fiber is branched at the branching
portion. A spontaneous emission light of the predetermined
wavelength band included in the branched light is selected by the
wavelength selection portion and is then sent to the signal light
incidence terminal of the first rare earth element doped fiber
through the multiplexing portion. Thus, the spontaneous emission
light of the predetermined wavelength band becomes traveling waves.
Consequently, laser oscillation occurs.
[0033] The optical amplifier may be a forward excitation type
device in which the excitation light supplying device is placed at
the signal light incidence terminal side of the first rare earth
element doped fiber. Alternatively, the optical amplifier may be a
bi-directional excitation type device in which excitation light
supplying devices are placed at the signal light incidence terminal
side of the first rare earth element doped fiber and at the signal
light emission terminal side of the second rare earth element doped
fiber, respectively.
[0034] Moreover, it is preferable that the optical amplifier of the
present invention has an excitation light reflecting device for
reflecting the excitation light and for transmitting the signal
light. Practically, in the case that the optical amplifier of the
present invention is of the forward excitation type, the excitation
light reflecting device is provided at the signal light emission
side of the second rare earth element doped fiber. In the case that
the optical amplifier of the present invention is of the
bi-directional excitation type, the excitation light reflecting
device is provided between the first and second rare earth element
doped fibers.
[0035] As a result of providing the excitation light reflecting
device in the optical amplifier of the present invention, the
excitation light emitted from the excitation light source is
propagated reciprocatingly through the first and second rare earth
element doped fibers, so that the conversion efficiency of the
excitation is enhanced. Consequently, the present invention
provides high excitation efficiency optical amplifier.
[0036] Furthermore, in the case of the optical amplifiers of the
forward excitation type and the bi directional excitation type, it
is preferable that a third rare earth element doped fiber be
cascaded with the signal light incidence terminal side of the first
rare earth element doped fiber. In the optical amplifier of such a
configuration, an incident signal light is first sent to the third
rare earth element doped fiber. Then, the third rare earth element
doped fiber is supplied with a forward excitation light of
relatively high power and thus amplifies a signal light at a high
degree of population inversion. Then, the signal light and
excitation light having passed through the third rare earth element
doped fiber are sent to the first rare earth element doped fiber.
Thus, the signal light is amplified.
[0037] Moreover, the laser oscillation of spontaneous emission
light of a predetermined wavelength band occurs. Then, the laser
oscillation light and the signal light enter the second rare earth
element doped fiber, so that the signal light is more greatly
amplified. Consequently, the low level signal light is effectively
amplified at the pre-stage portion. Thus, noise is reduced in the
entire optical amplifier.
[0038] Further, preferably, the optical amplifier has a light
intercepting device provided between the first and third rare earth
element doped fibers for intercepting a light being propagated from
the first rare earth element doped fiber to the third rare earth
element doped fiber.
[0039] When among spontaneous emission light generated in the first
rare earth element doped fiber, a component being propagated in a
direction opposite to the direction of propagation of the signal
light enters in the third rare earth element doped fiber, the
signal-light gain of the third rare earth element doped fiber
decreases. Thus, the population inversion in the third rare earth
element doped fiber, namely, the gain thereof, is maintained at a
high level by using the light intercepting device to intercept the
spontaneous emission light propagating in a direction opposite to
the direction of this signal light. Consequently, the noise
characteristics of the entire optical amplifier are further
improved.
[0040] According to another aspect of the present invention, there
is provided an optical amplifier for amplifying a signal light of a
first wavelength band and a signal light of a second wavelength
band. The amplifier includes a first optical amplifying device for
amplifying the signal light of the first wavelength band by using a
rare earth element doped fiber, a second optical amplifying device
for amplifying the signal light of the second wavelength band by
using a rare earth element doped fiber, a light branching device
for branching a part of the signal light amplified by the first
optical amplifying device, and a branch light supplying device for
supplying a branch light from the light branching device to the
second optical amplifying device as an excitation light. With such
a configuration, when a signal light of a first wavelength band,
such as the conventional band, and a signal light of a second
wavelength band, such as the longer wavelength band, are amplified
by using a rare earth element doped fiber, a part of signal light
of the conventional band amplified by the first optical amplifying
device is branched by the light branching device and is then
supplied to the second optical amplifying device through the branch
light supplying device. This branch light sent from the first
optical amplifying device serves as an excitation light for the
second optical amplifying device. Thus, the signal light of the
longer wavelength band is amplified with high excitation
efficiency. Consequently, the present invention provides an optical
amplifier having excellent amplification characteristics at a low
cost.
[0041] According to still another aspect of the present invention,
there is provided an optical amplification method for amplifying a
signal light of a predetermined wavelength band by using a rare
earth element doped fiber. The method includes (a) oscillating a
spontaneous emission light of a predetermined wavelength band among
spontaneous emission lights generated in the rare earth element
doped fiber, so that laser oscillation is generated and (b)
amplifying the signal light of the predetermined wavelength band by
supplying light generated by the laser oscillation to a rare earth
element doped fiber as an excitation light.
[0042] According to yet another aspect of the present invention,
there is provided an optical amplification method for amplifying a
signal light of a first wavelength band and a signal light of a
second wavelength band by first and second amplifying devices,
respectively, each of which uses a rare earth element doped fiber.
The method includes (a) branching a part of signal light amplified
by the first optical amplifying device, and (b) supplying the
branched part to the second optical amplifying device as an
excitation light.
BRIEF DESCRIPTION OF THE DRAWINGS
[0043] These and other objects and advantages of the invention will
become apparent and more readily appreciated from the following
description of the preferred embodiments, taken in conjunction with
the accompanying drawings of which:
[0044] FIG. 1 (prior art) is a graph showing the gain per unit
length versus wavelength characteristics of an EDF corresponding to
the degrees of population inversion.
[0045] FIG. 2 (prior art) is a graph illustrating gain distribution
in the longitudinal direction of an EDF.
[0046] FIG. 3 (prior art) is a diagram showing a conventional long
wavelength band optical amplifier.
[0047] FIG. 4 (prior art) is a diagram showing a conventional long
wavelength band optical amplifier.
[0048] FIG. 5 (prior art) is a diagram showing a conventional long
wavelength band optical amplifier.
[0049] FIG. 6 is a diagram showing an optical amplifier, according
to an embodiment of the present invention.
[0050] FIG. 7 is a diagram showing an optical amplifier, according
to an embodiment of the present invention.
[0051] FIG. 8 is a diagram showing an optical amplifier, according
to an embodiment of the present invention.
[0052] FIG. 9 is a diagram showing an optical amplifier, according
to an embodiment of the present invention.
[0053] FIG. 10 is a diagram showing an optical amplifier, according
to an embodiment of the present invention.
[0054] FIG. 11 is a diagram showing an optical amplifier, according
to an embodiment of the present invention.
[0055] FIG. 12 is a diagram showing an optical amplifier, according
to an embodiment of the present invention.
[0056] FIG. 13 is a diagram showing an optical amplifier, according
to an embodiment of the present invention.
[0057] FIG. 14 is a diagram showing an optical amplifier, according
to an embodiment of the present invention.
[0058] FIG. 15 is a diagram showing an optical amplifier, according
to an embodiment of the present invention.
[0059] FIG. 16 is a diagram showing an optical amplifier, according
to an embodiment of the present invention.
[0060] FIG. 17 is a diagram showing an optical amplifier, according
to an embodiment of the present invention.
[0061] FIG. 18 is a diagram showing an optical amplifier, according
to an embodiment of the present invention.
[0062] FIG. 19 is a diagram showing an optical amplifier, according
to an embodiment of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0063] Reference will now be made in detail to the present
preferred embodiments of the present invention, examples of which
are illustrated in the accompanying drawings, wherein like
reference numerals refer to like elements throughout.
[0064] FIG. 6 is a diagram showing an optical amplifier, according
to an embodiment of the present invention. As shown in FIG. 6, a
signal light L.sub.s of the longer wavelength band enters and
passes through an optical isolator 2.sub.1. An excitation light
source (LD) 4.sub.1 generates an excitation light Lp.sub.1. A WDM
coupler 3.sub.1 multiplexes the signal light L.sub.s with the
excitation light Lp.sub.1 and sends the multiplexed light to a
pre-stage erbium doped fiber (EDF) 1.sub.1. A post-stage EDF
1.sub.2 is cascaded together with pre-stage EDF 1.sub.1. An optical
isolator 2.sub.2 is connected to an emission terminal of EDF
1.sub.2. Wavelength selective reflection type optical devices
6.sub.1 and 6.sub.2 are provided at ends of pre-stage EDF 1.sub.1,
where EDF 1.sub.1 is located at the signal light incidence side in
a forward excitation optical amplifier. Thus, wavelength selective
reflection type optical devices 6.sub.1 and 6.sub.2 and EDF 1.sub.1
form a fiber resonator type structure as a laser oscillation light
generating device. High efficiency in optical amplification of the
longer wavelength band is achieved by performing laser oscillation
of light of the conventional band in the fiber resonator type
structure.
[0065] Preferably, EDFs 1.sub.1 and 1.sub.2 are ordinary rare earth
element doped optical fibers each of which has a core portion doped
with a rare earth element which is, for example, erbium
(Er.sup.3+).
[0066] Each of optical isolators 2.sub.1 and 2.sub.2 has an
incidence terminal and an emission terminal and is a passive
optical part adapted to provide low loss to a forward light
traveling from the incidence side to the emission side and a high
loss to a backward light returning from the emission side to the
incidence side, thereby allowing light to pass through only in a
predetermined direction.
[0067] WDM coupler 3.sub.1 is an optical coupler having input ports
which receive signal light L.sub.s and excitation light L.sub.p,
respectively. WDM coupler 3.sub.1 multiplexes signal light L.sub.s
and excitation light L.sub.p and emits the multiplexed light from
an output port.
[0068] As an example, an ordinary light source for generating an
excitation light having a wavelength band such as the 0.98 .mu.m
band or the 1.48 .mu.m band is used as excitation light source
4.sub.1.
[0069] As an example, fiber gratings and optical bulk filters and
the like can be used as wavelength selective reflection type
optical devices 6.sub.1 and 6.sub.2. Each of wavelength selective
reflection type optical devices 6.sub.1 and 6.sub.2 has a required
reflection bandwidth. Further, in the case of this embodiment, the
center wavelength of the reflection wavelength band is set at 1.55
.mu.m. The reflectance of each of these wavelength selective
reflection type optical devices will be described later.
[0070] Next, an operation of the amplifier in FIG. 6 will be
described below.
[0071] The signal light L.sub.s of the longer wavelength band
having entered the amplifier passes through optical isolator
2.sub.1 and is then multiplexed by WDM coupler 3.sub.1 with
excitation light Lp.sub.1 emitted from excitation light source
4.sub.1. Subsequently, the multiplexed light passes through
wavelength selective reflection type optical device 6.sub.1 and
then enters EDF 1.sub.1. In EDF 1.sub.1, erbium is excited by
excitation light Lp.sub.1 having passed through wavelength
selective reflection type optical device 6.sub.1, so that the
required degree of population inversion is obtained. Signal light
L.sub.s passing through EDF 1.sub.1, is amplified therein.
Moreover, an amplified spontaneous emission (ASE) light of the
conventional band is generated in EDF 1.sub.1. The amplified signal
light L.sub.s passes through wavelength selective reflection type
optical device 6.sub.2 and then enters EDF 1.sub.2. Signal light
L.sub.s is then amplified in EDF 1.sub.2. Thereafter, this
amplified signal light L.sub.s is emitted to the outside through
optical isolator 2.sub.2.
[0072] With the fiber resonator type structure formed of wavelength
selective reflection type optical device 6.sub.1 and 6.sub.2 and
EDF 1.sub.1, the light of the conventional band being propagated
through EDF 1.sub.1 is selectively reflected by each of wavelength
selective reflection type optical devices 6.sub.1 and 6.sub.2, and
laser oscillation is generated. The light generated by the laser
oscillation of the conventional band enters post-stage EDF 1.sub.2
and thus serves as excitation light. As described above, in
EDF.sub.2, the light of the conventional band is amplified at a
portion having a high degree of population inversion, while such
light is absorbed at a portion having a low degree of population
inversion and serves as the excitation light. Therefore, the light
generated by the laser oscillation of the conventional band can be
used as the excitation light by adjusting the degree of population
inversion of post-stage EDF 1.sub.2.
[0073] To realize the laser oscillation of light of the
conventional band, the reflectance R.sub.1 and R.sub.2 of
wavelength selective reflection type optical devices 6.sub.1 and
6.sub.2 and the operational gain G.sub.1 of EDF 1.sub.1 are set in
such a manner as to meet the relationship expressed by the
following equation.
[0074] Equation (1):
G.sub.1.multidot.(R.sub.1.multidot.R.sub.2).sup.1/2>1
[0075] Further, regarding the reflectance R.sub.1 and R.sub.2 of
wavelength selective reflection type optical devices 6.sub.1 and
6.sub.2, it is assumed that the reflectance R.sub.1 is set at
approximate 100% and the reflectance R.sub.2 is set to be less than
100% so that the light of the conventional band is output only in
the direction of the emission terminal (namely, only to the right
in FIG. 6).
[0076] Incidentally, the reflection factor R.sub.2 can be set at a
suitable value being less than 100% according to the optical power
of the light of the conventional band needed as the excitation
light.
[0077] As described above, a fiber resonator type structure is
formed in the pre-stage portion of the optical amplifier so that
light of the conventional band being propagated through the
pre-stage EDF 1.sub.1 is oscillated, to generate laser oscillation.
Then, light generated by the laser oscillation is supplied to the
post-stage EDF 1.sub.2. Thus, the amplification of the signal light
L.sub.s of the longer wavelength band is achieved with high
excitation efficiency by adding only inexpensive passive optical
parts (namely, wavelength selective reflection type optical devices
6.sub.1 and 6.sub.2) without adding active optical parts, such as a
light source, to the optical amplifier. This operation is
significantly different than the conventional long wavelength band
optical amplifiers in FIGS. 3-5. Consequently, the signal light
L.sub.s of the longer wavelength band may be amplified by the
optical power of excitation light, which is lower than the optical
power required by the conventional optical amplifiers. Moreover,
reduction in the cost of the optical amplifier is achieved.
[0078] As an example, gratings can be written into EDF.sub.1 to
provide the operation of wavelength selective reflection type
optical devices 6.sub.1 and 6.sub.2.
[0079] Further, although the optical amplifier of the forward
excitation type is described above, the present invention is not
limited thereto. The present invention may be applied to the
optical amplifiers of the backward excitation type and the
bi-directional excitation type.
[0080] For example, FIG. 7 is a diagram showing an optical
amplifier of the bi-directional excitation type, according to an
embodiment of the present invention. As shown in FIG. 7, a WDM
coupler 3.sub.2 is inserted between post-stage EDF 1.sub.2 and
optical isolator 2.sub.2. An excitation light Lp.sub.2 generated in
an excitation light source 4.sub.2 is sent to EDF 1.sub.2 through
WDM coupler 3.sub.2. Backward excitation light Lp.sub.2 having
entered EDF 1.sub.2 is propagated through EDFs 1.sub.1 and 1.sub.2
in the direction opposite to the direction of propagation of
forward excitation light Lp.sub.1. For example, the 0.98 .mu.m band
and the 1.48 .mu.m band may be employed as the wavelength band of
backward excitation light LP.sub.2.
[0081] Even in the case of the optical amplifier of the
bi-directional excitation type, a fiber resonator type structure is
formed of wavelength selective reflection type optical devices
6.sub.1 and 6.sub.2 and EDF 1.sub.1. The light of the conventional
band is selectively reflected by each wavelength selective
reflection type optical devices 6.sub.1 and 6.sub.2, so that laser
oscillation is generated. Light generated by the laser oscillation
enters post-stage EDF 1.sub.2 and serves as an excitation light.
Consequently, advantageous effects similar to those of in FIG. 6
are obtained. Moreover, high excitation efficiency may be achieved
since the optical amplifier is of the bi-directional excitation
type.
[0082] FIG. 8 is a diagram showing an optical amplifier according
to an additional embodiment of the present invention. As shown in
FIG. 8, an excitation wavelength selective reflection type optical
device 7 is inserted between EDFs 1.sub.1 and 1.sub.2 as an
excitation light reflecting device in, for example, the optical
amplifier of the bi-directional excitation type illustrated in FIG.
7.
[0083] For example, a fiber grating, an optical bulk filter and the
like can be used as excitation wavelength selective reflection type
optical device 7. Excitation wavelength selective reflection type
optical device 7 is assumed to have characteristics to reflect each
of excitation lights Lp.sub.1 and Lp.sub.2 and to transmit lights
of other wavelength bands. In the case where the wavelength band of
excitation light Lp.sub.1 is different from that of excitation
light Lp.sub.2, excitation wavelength selective reflection type
optical device 7 is set in such a manner as to have two reflection
bands respectively corresponding to the excitation wavelength
bands. Alternatively, excitation wavelength selective reflection
type optical device 7 is set in such a manner as to have one
reflection band containing the respective excitation wavelength
bands.
[0084] In FIG. 8, wavelength selective reflection type optical
device 6.sub.2 and excitation wavelength selective reflection type
optical device 7 are provided separately from each other. However,
the functions of these devices may be implemented by using a single
wavelength selective reflection type optical device. In this case,
the optical amplifier employs a wavelength selective reflection
type optical device that has a reflection wavelength band of which
reflectance is less than 100% in the conventional band, and further
has reflection wavelength bands respectively corresponding to the
wavelength bands of excitation light Lp.sub.1 and excitation light
Lp.sub.2.
[0085] In FIG. 8, signal light L.sub.s of the longer wavelength
band having entered in this amplifier passes through optical
isolator 2.sub.1 and is then multiplexed by WDM coupler 3.sub.1
with excitation light Lp.sub.1 emitted from excitation light source
4.sub.1. Subsequently, the multiplexed light passes through
wavelength selective reflection type optical device 6.sub.1 and
then enters EDF 1.sub.1 to thereby be amplified. Further, signal
light L.sub.s of the longer wavelength band having passed through
EDF 1.sub.1 travels through wavelength selective reflection type
optical device 6.sub.2 and excitation wavelength selective
reflection type optical device 7 and then enters EDF 1.sub.2.
Excitation light Lp.sub.2 emitted from excitation light source
4.sub.2 is multiplexed at WDM coupler 3.sub.2 and then is supplied
to EDF 1.sub.2 in a direction opposite to the direction of
propagation of signal light L.sub.s. Subsequently, signal light
L.sub.s is amplified in EDF 1.sub.2. Thereafter, this amplified
signal light L.sub.s having passed through EDF 1.sub.2 is output to
the outside through WDM coupler 3.sub.2 and optical isolator
2.sub.2.
[0086] Excitation light Lp.sub.1, which is propagated from the
signal light incidence terminal side in the forward direction, and
excitation light Lp.sub.2, which is propagated from the signal
light emission terminal side in the opposite direction,
wavelength-selectively undergo total reflection at excitation
wavelength selective reflection type optical device 7 and are then
put back into EDFs 1.sub.1 and 1.sub.2, respectively. This results
in an increase in the conversion efficiencies of excitation lights
Lp.sub.1 and Lp.sub.2 in EDFs 1.sub.1 and 1.sub.2, and thus
contributes to the high efficiency of an excitation operation.
Further, similarly as in the optical amplifier of FIG. 6, the light
of the conventional band is selectively reflected by each of
wavelength selective reflection type optical devices 6.sub.1 and
6.sub.2 with a fiber resonator type structure formed of the
wavelength selective reflection type optical devices 6.sub.1 and
6.sub.2 and EDF 1.sub.1, so that laser oscillation is generated.
The light of the conventional band is then sent to EDF 1.sub.2 and
serves as an excitation light therefor.
[0087] As described above, as shown in FIG. 8, excitation
wavelength selective reflection type optical device 7 is provided
between EDFs 1.sub.1 and 1.sub.2. Thus, the excitation lights
Lp.sub.1 and Lp.sub.2 reciprocate in EDFs 1.sub.1 and 1.sub.2, so
that the excitation lights are effectively used. Consequently, the
excitation efficiency of the optical amplifier is enhanced.
[0088] Incidentally, although the optical amplifier of the
bi-directional excitation type is shown in FIG. 8, the excitation
efficiency is increased by providing an excitation wavelength
selective reflection type optical device in an optical amplifier
even in the case that the optical amplifier is of the forward or
backward excitation type. For example, in the case of the optical
amplifier of the forward excitation type shown in FIG. 6, an
excitation wavelength selective reflection type optical device may
be inserted between EDF 1.sub.2 and optical isolator 2.sub.2.
[0089] FIG. 9 is a diagram showing an optical amplifier according
to a further embodiment of the present invention. As shown in FIG.
9, an EDF 1.sub.3 is provided between WDM coupler 3.sub.1 and
wavelength selective reflection type optical device 6.sub.1 with,
for example, the optical amplifier of the forward excitation type
illustrated in FIG. 6. When amplifying a signal light at a high
degree of population inversion, this optical amplifier performs a
low-noise operation. More specifically, this optical amplifier
realizes a low-noise operation by amplifying a signal light in the
vicinity of the signal light incidence terminal thereof at a high
degree of population inversion.
[0090] Preferably, EDF 1.sub.3 is an ordinary rare earth element
doped fiber which has a core portion doped with a rare earth
element, such as erbium, similarly as EDFs 1.sub.1 and 1.sub.2.
[0091] In FIG. 9, signal light L.sub.s of the longer wavelength
band passes through optical isolator 2.sub.1 and is then
multiplexed by WDM coupler 3.sub.1 with excitation light Lp.sub.1
emitted from excitation light source 4.sub.1. Subsequently, the
multiplexed light is enters EDF 1.sub.3. Because a relatively high
power excitation light Lp.sub.1 is supplied to EDF 1.sub.3, the
degree of population inversion therein is higher than those of
population inversion in post-stage EDFs 1.sub.1 and 1.sub.2. When
the signal light L.sub.s of the longer wavelength band having
passed through EDF 1.sub.3 of which degree of population inversion
is high, the signal light L.sub.s is amplified at a relatively high
gain, though the gain flatness is impaired as illustrated in FIG.
4. Then, the amplified signal light L.sub.s and excitation light
Lp.sub.1 which has not been absorbed in EDF 1.sub.3, pass through
wavelength selective reflection type optical device 6.sub.1 and
then enters EDF 1.sub.1.
[0092] In EDF 1.sub.1, similarly as in FIG. 6, the signal light
L.sub.s is amplified. Moreover, the light of the conventional band
is selectively reflected between wavelength selective reflection
type optical devices 6.sub.1 and 6.sub.2, so that laser oscillation
is generated. Subsequently, signal light L.sub.s and the light of
the conventional band pass through wavelength selective reflection
type optical device 6.sub.2 and are then sent to EDF 1.sub.2.
Further, excitation light Lp.sub.1, which has not been absorbed in
EDF 1.sub.1, passes through wavelength selective reflection type
optical device 6.sub.2 and is sent to EDF 1.sub.2.
[0093] EDF 1.sub.2 is put into an excited state by the light of the
conventional band and excitation light Lp.sub.1 at a relatively low
degree of population inversion. Signal light L.sub.s is further
amplified by EDF 1.sub.2 and is then emitted to the outside through
optical isolator 2.sub.2.
[0094] In this case, signal light L.sub.s of the longer wavelength
band is amplified by each of EDFs 1.sub.3, 1.sub.1 and 1.sub.2
which are different in the degree of population inversion from one
another. The gain flatness in each of EDFs 1.sub.3, 1.sub.1 and
1.sub.2 is impaired. However, signal light L.sub.s having flat gain
characteristics is obtained in the entire optical amplifier by
preliminarily setting the optical amplifier so that the average of
the degrees of population inversion of EDFs 1.sub.3, 1.sub.1 and
1.sub.2 is about 40%.
[0095] As above described, as illustrated in FIG. 9, the optical
amplifier has the following advantageous effect in addition to the
effects of the optical amplifier in FIG. 6. Namely, because EDF
1.sub.3, of which degree of population inversion is set at a high
level, is provided at a signal light incidence portion of the
optical amplifier and thus signal light L.sub.s is effectively
amplified at the pre-stage portion where the optical power level is
low, the noise is reduced in the entire optical amplifier.
[0096] Incidentally, although the optical amplifier of the forward
excitation type is shown in FIG. 9, the present invention may be
applied to the optical amplifier of the bi-directional excitation
type.
[0097] For example, FIG. 10 is a diagram showing an optical
amplifier of the bi-directional excitation type, according to an
embodiment of the present invention.
[0098] Further, similar to the case in FIG. 8, an excitation
wavelength selective reflection type optical device for reflecting
excitation light may be provided in the optical amplifier of FIGS.
9 or 10.
[0099] For example, FIG. 11 is a diagram showing an optical
amplifier of the forward excitation type, and employing an
excitation wavelength selective reflection type optical device 7,
according to an embodiment of the present invention. As shown in
FIG. 11, in the case of the optical amplifier of the forward
excitation type, excitation wavelength selective reflection type
optical device 7 may be inserted between EDF 1.sub.2 and optical
isolator 2.sub.2.
[0100] FIG. 12 is a diagram shown an optical amplifier of the
bi-directional excitation type, and employing an excitation
wavelength selective reflection type optical device 7, according to
an embodiment of the present invention. As shown in FIG. 12, in the
case of the optical amplifier of the bi-directional excitation
type, the excitation wavelength selective reflection type optical
device 7 may be inserted between EDF 1.sub.1 and EDF 1.sub.2.
[0101] Incidentally, in the case of the optical amplifier of the
bi-directional excitation type, an excitation wavelength selective
reflection type optical device (not shown) may be inserted between
EDFs 1.sub.3 and 1.sub.1.
[0102] Therefore, by providing an excitation wavelength selective
reflection type optical device in a bi-direction excitation type
optical amplifier, excitation lights Lp.sub.1 and Lp.sub.2 will be
effectively used, and the excitation efficiency will be
enhanced.
[0103] FIG. 13 is a diagram showing an optical amplifier according
to a still further embodiment of the present invention. The optical
amplifier in FIG. 13 is similar to that in FIG. 9, but includes an
optical isolator 2.sub.3 serving as a light intercepting device
between EDF 1.sub.3 and wavelength selective reflection type
optical device 6.sub.1. The optical amplifier in FIG. 13 realizes a
lower noise operation by using optical isolator 2.sub.3 to
intercept spontaneous emission light (ASE) generated in EDF 1.sub.1
and propagating in a direction opposite to the direction of
propagation of signal light L.sub.s.
[0104] Optical isolator 2.sub.3 is similar to optical isolators
2.sub.1 and 2.sub.2. Optical isolator 2.sub.3 has characteristics
to transmit a light traveling from EDF 1.sub.3 to EDF 1.sub.1 and
to intercept a light traveling from EDF 1.sub.1 to EDF 1.sub.3.
[0105] Therefore, in the optical amplifier of FIG. 13, a component
propagating in a direction opposite to that of signal light in the
spontaneous emission light (ASE) generated in EDF 1.sub.1 is
intercepted by optical isolator 2.sub.3. Wavelength selective
reflection type optical device 6.sub.1 is provided between EDFs
1.sub.1 and 1.sub.3. Thus, the spontaneous emission light having a
wavelength within the reflection band is reflected by wavelength
selective reflection type optical device 6.sub.1.
[0106] However, the spontaneous emission light having a wavelength
outside the reflection band of wavelength selective reflection type
optical device 6.sub.1 is also generated in EDF 1.sub.1. This
spontaneous emission light passes through wavelength selective
reflection type optical device 6.sub.1 and propagates in the
direction of EDF 1.sub.3. When the spontaneous emission light
propagating in a direction opposite to this signal light enters EDF
1.sub.3, the gain corresponding to the light of the longer
wavelength band in EDF 1.sub.3 is lowered. Therefore, the gain of
EDF 1.sub.3 is maintained at a high level by intercepting the
spontaneous emission light that passes through wavelength selective
reflection type optical device 6.sub.1 and propagates in the
direction opposite to the direction of propagation of the signal
light.
[0107] As described above, according to an optical amplifier as
illustrated in FIG. 13, the gain of signal light L.sub.s in EDF
1.sub.3 is increased by providing optical isolator 2.sub.3 in the
optical amplifier. Consequently, excellent noise characteristics of
the entire optical amplifier are obtained.
[0108] Incidentally, although the optical amplifier of the forward
excitation type is shown in FIG. 13, optical isolator 2.sub.3 may
be provided between EDF 1.sub.3 and wavelength selective reflection
type optical device 6.sub.1 even in the case of the optical
amplifier of the bi-directional excitation type shown in FIG.
10.
[0109] For example, FIG. 14 is a diagram showing an optical
amplifier according to another embodiment of the present invention.
FIG. 14 illustrates a bi-directional excitation type optical
amplifier.
[0110] Alternatively, as in FIG. 13, an optical isolator 2.sub.3
may be provided in an optical amplifier having an excitation
wavelength selective reflection type optical device as illustrated
in FIGS. 11 or 12.
[0111] For example, FIG. 15 is a diagram showing an optical
amplifier of the bi-directional excitation type, according to an
embodiment of the present invention. In FIG. 15, excitation
wavelength selective reflection type optical device 7 is provided
between EDF 1.sub.3 and wavelength selective reflection type
optical device 6.sub.1. Furthermore, optical isolator 2.sub.3 is
inserted between excitation wavelength selective reflection type
optical device 7 and wavelength selective reflection type optical
device 6.sub.1. In the case of the optical amplifier of the
bi-directional excitation type, the insertion position of
excitation wavelength selective reflection type optical device 7
may be established between optical isolator 2.sub.3 and wavelength
selective reflection type optical device 6.sub.1 or between EDFs
1.sub.1 and 1.sub.2, in addition to the aforementioned insertion
position.
[0112] FIG. 16 is a diagram showing an optical amplifier according
to an additional embodiment of the present invention. As shown in
FIG. 16, the optical amplifier includes multiplexing/demultiplexing
devices 5.sub.1 and 5.sub.2 and an optical band-pass filter (BPF)
8, instead of wavelength selective reflection type optical devices
6.sub.1 and 6.sub.2 in the optical amplifier of the bi-directional
excitation type in FIG. 7. The remaining components of the optical
amplifier in FIG. 16 are similar to the corresponding components of
the optical amplifier in FIG. 7.
[0113] Multiplexing/demultiplexing device 5.sub.1 is connected to,
for example, the pre-stage of optical isolator 2.sub.1 and is
operative to multiplex the incident signal light L.sub.s of the
longer wavelength band with a light transmitted by BPF 8 and to
then emit the multiplexed light to optical isolator 2.sub.1.
Further, multiplexing/demultiplexing device 52 is connected between
EDFs 1.sub.1 and 1.sub.2 and is operative to bifurcate the light
emitted from EDF 1.sub.1 and to then emit branch lights to EDF
1.sub.2 and BPF 8. It is assumed that the branching ratio of
multiplexing/demultiplexing device 5.sub.2 is suitably set on the
basis of an oscillation condition (to be described later).
[0114] BPF 8 is preferably an ordinary optical band-pass filter
connected between one of the emission terminals of
multiplexing/demultiplexing device 5.sub.2 and one of the incidence
terminals of multiplexing/demultiplexing device 5.sub.1. BPF 8 has
a predetermined pass-band. The center wavelength of the pass-band
is set, for example, at 1.55 .mu.m.
[0115] In FIG. 16, multiplexing/demultiplexing device 5.sub.1
functions as a multiplexing portion. Multiplexing/demultiplexing
device 5.sub.2 serves as a branching portion. BPF 8 acts as a
wavelength selecting portion.
[0116] In the optical amplifier of FIG. 16, the incident signal
light L.sub.s of the longer wavelength band passes through
multiplexing/demultiplexing device 5.sub.1 and optical isolator 2,
and is then multiplexed at WDM coupler 3.sub.1 with excitation
light Lp.sub.1 sent from excitation light source 4.sub.1.
Subsequently, the multiplexed light enters EDF 1.sub.1 and is then
amplified therein.
[0117] Furthermore, the light emitted from EDF 1.sub.1 is
bifurcated by multiplexing/demultiplexing device 5.sub.2. Then,
resultant lights are sent to EDF 1.sub.2 and BPF 8, respectively.
The signal light L.sub.s included in the light sent to EDF 1.sub.2
is amplified in EDF 1.sub.2 and then passes through WDM coupler
3.sub.2 and optical isolator 2.sub.2 and is subsequently emitted to
the outside.
[0118] At WDM coupler 3.sub.2, excitation light Lp.sub.2 sent from
excitation light source 4.sub.2 is multiplexed. The multiplexed
light Lp.sub.2 is supplied to EDF 1.sub.2 as a backward excitation
light.
[0119] Therefore, a traveling-wave type optical oscillator (namely,
in this example, a fiber ring laser) is comprised of
multiplexing/demultiple- xing devices 5.sub.1 and 5.sub.2, optical
isolator 2.sub.1, WDM coupler 3.sub.1, EDF 1.sub.1 and BPF 8.
However, in other embodiments, all of these components might not be
necessary to form the traveling-wave type optical oscillator. The
light of the conventional band included in the light, which is
branched by multiplexing/demultiplexing device 5.sub.2 and is sent
to BPF 8, is selected by BPF 8 in this traveling-wave type optical
oscillator, so as to generate laser oscillation. Then, light
generated by the laser oscillation enters EDF 1.sub.2 through
multiplexing/demultiplexing device 5.sub.2 and serves as an
excitation light for EDF 1.sub.2.
[0120] To realize the aforementioned traveling-wave type laser
oscillation of light of the conventional band, a total sum L.sub.1
of losses caused at the wavelength of 1.55 .mu.m in
multiplexing/demultiplexing device 5.sub.2, optical isolator
2.sub.1, WDM coupler 3.sub.1 and BPF 8, and the operational gain
G.sub.1 of EDF 1.sub.1 at the wavelength of 1.55 .mu.m meet the
following relation expressed by the following inequality.
[0121] Equation 2:
G.sub.1/L.sub.1>1
[0122] As described above, as illustrated in FIG. 16, the
amplification of signal light L.sub.s of the longer wavelength band
may be achieved with high efficiency by adding only inexpensive
passive optical parts to the optical amplifier. Such operation is
achieved even if the traveling-wave optical oscillator is provided
at the pre-stage portion of the optical amplifier and the light of
the conventional band selected in the pass-band of BPF 8 is
oscillated to generate laser oscillation, whereby light generated
by the laser oscillation is supplied to the post-stage EDF 1.sub.2.
Consequently, a reduction in the cost of the optical amplifier is
attained.
[0123] Although multiplexing/demultiplexing device 5.sub.1 is
provided at the pre-stage of optical isolator 2.sub.1 in FIG. 16,
multiplexing/demultiplexing device 5.sub.1 may be placed at an
arbitrary position at the pre-stage side of the signal light
incidence terminal of EDF 1.sub.1. In the case where
multiplexing/demultiplexing device 5.sub.1 is provided at the
post-stage side of optical isolator 2.sub.1, an optical isolator
for determining the direction of a traveling-wave should be added
to a loop including BPF 8.
[0124] FIG. 17 is a diagram showing an optical amplifier according
to a further embodiment of the present invention. As shown in FIG.
17, EDF 1.sub.3 of which population inversion is set at a high
value is provided at the pre-stage of the traveling-wave type
optical oscillator of FIG. 16, thereby reducing noise in the
optical amplifier.
[0125] Practically, the optical amplifier in FIG. 17 is configured
as follows. EDF 1.sub.3 is connected between WDM coupler 3.sub.1
and EDF 1.sub.1. Multiplexing/demultiplexing device 5.sub.1, which
was provided at the pre-stage of optical isolator 2.sub.1 in FIG.
16, is placed between EDFs 1.sub.3 and 1.sub.1 in FIG. 17. Further,
in FIG. 17, optical isolator 2.sub.3 is provided between
multiplexing/demultiplexing device 5.sub.1 and EDF 1.sub.1.
[0126] In the optical amplifier of FIG. 17, the incident signal
light L.sub.s of the longer wavelength band passes through optical
isolator 2.sub.1 and is then multiplexed at WDM coupler 3.sub.1
with excitation light Lp.sub.1 sent from excitation light source
4.sub.1. Subsequently, the multiplexed light enters EDF 1.sub.3.
Because a relatively high power excitation light Lp.sub.1 is
supplied to EDF 1.sub.3, the degree of population inversion therein
is high. When the signal light L.sub.s of the longer wavelength
band passes through EDF 1.sub.3 of which degree of population
inversion is high, signal light L.sub.s is amplified at a
relatively high gain, though the gain flatness is impaired. Then,
the amplified signal light L.sub.s and excitation light Lp.sub.1,
which has not been absorbed in EDF 1.sub.3, pass through
multiplexing/demultiplexin- g device 5.sub.1 and optical isolator
2.sub.3 and then enter EDF 1.sub.1.
[0127] In EDF 1.sub.1, similarly as in the case of FIG. 16, signal
light L.sub.s is amplified. Moreover, a spontaneous emission light
is generated.
[0128] Light output from EDF.sub.1 is bifurcated by
multiplexing/demultiplexing device 5.sub.2. Then, resultant lights
are sent to EDF 1.sub.2 and BPF 8, respectively. Signal light
L.sub.s included in the light sent to EDF 1.sub.2 is amplified in
EDF 1.sub.2. Subsequently, the amplified signal light L.sub.s
passes through WDM coupler 3.sub.2 and optical isolator 2.sub.2 and
is then emitted to the outside.
[0129] A traveling-wave type optical oscillator (namely, for
example, a fiber ring laser) is formed of
multiplexing/demultiplexing devices 5.sub.1 and 5.sub.2, optical
isolator 2.sub.3, EDF 1.sub.1 and BPF 8. The light of the
conventional band included in the light, which is branched by
multiplexing/demultiplexing device 5.sub.2 and is sent to BPF 8, is
selected by BPF 8 in this traveling-wave type optical oscillator,
so that laser oscillation is generated. Then, a part of the light
generated by the laser oscillation of the conventional band enters
EDF 1.sub.2 through multiplexing/demultiplexing device 5.sub.2 and
serves as an excitation light for EDF 1.sub.2. Optical isolator
2.sub.3 has the functions of limiting the direction of propagation
of light of the conventional band only to one direction to thereby
make such light as a traveling wave, and preventing the light of
the conventional band from entering EDF 1.sub.3.
[0130] The total sum L.sub.2 of losses caused at the wavelength of
1.55 .mu.m in multiplexing/demultiplexing devices 5.sub.1 and
5.sub.2, optical isolator 2.sub.3 and BPF 8, and the operational
gain G.sub.1 of EDF 1.sub.1 at the wavelength of 1.55 .mu.m meet
the oscillation condition expressed by the following
inequality.
[0131] Equation (3):
G.sub.1/L.sub.2>1
[0132] As described above, as in FIG. 17, in an optical amplifier
having a traveling-wave type optical oscillator, the signal light
L.sub.s of the longer wavelength band is effectively amplified at
the pre-stage portion, in which the optical power level is low, by
providing EDF 1.sub.3, of which degree of population inversion is
set at a high value, at the signal light incidence portion of the
optical amplifier. Thus, noise is reduced in the entire optical
amplifier to low levels.
[0133] Although the optical amplifier of the bi-directional
excitation type is shown in FIGS. 16 and 17, the present invention
is not limited thereto. The present invention may be applied to an
optical amplifier of the forward excitation type from which
excitation light source 4.sub.2 and WDM coupler 3.sub.2 are
removed.
[0134] Further, although an optical amplifier as in FIG. 17 has
multiplexing/demultiplexing device 5.sub.2 and BPF 8 and is adapted
so that the light of the conventional band included in the branch
light sent from multiplexing/demultiplexing device 5.sub.2 is
selected by BPF 8, a multiplexing/demultiplexing device having a
wavelength selecting function may be used instead of
multiplexing/demultiplexing device 5.sub.2 and BPF 8. More
specifically, an optical part having the function of selectively
branching a part of a component of the conventional band included
in the light sent from EDF 1.sub.1, may be used instead of
multiplexing/demultiplexing device 5.sub.2 and BPF 8.
[0135] An optical amplifier adapted to receive a light obtained by
multiplexing a signal light of the conventional band and a signal
light of the longer wavelength band, to amplify these signal lights
and to emit the multiplexed light, will be described below.
[0136] For example, FIG. 18 is a diagram showing an optical
amplifier according to an additional embodiment of the present
invention. As shown in FIG. 18, signal light L.sub.s obtained by
multiplexing lights of two wavelength bands, such as the
conventional band and the longer wavelength band, enters WDM
coupler 3.sub.3. An optical amplifying portion 10 serves as a first
optical amplifying device for amplifying a light of the
conventional band. An optical amplifying portion 11 serves as a
second optical amplifying device for amplifying a light of the
longer wavelength band. A branching coupler 9 serves as a light
branching device for bifurcating a light emitted from optical
amplifying portion 10. A WDM coupler 3.sub.4 serves as a branch
light supplying device for multiplexing a light emitted from
optical amplifying portion 11 with one of branch lights received
from branching coupler 9. It is assumed herein that the direction
of propagation of the signal light of the conventional band is the
same as that of propagation of the signal light of the longer
wavelength band.
[0137] WDM coupler 3.sub.3 has at least two input ports and two
output ports. Signal light L.sub.s obtained by multiplexing a light
of the conventional band with a light of the longer wavelength band
enters in one of the input ports. Signal light L.sub.s is
demultiplexed in accordance with wavelength bands. More
specifically, the light of the conventional band is emitted from an
output port of WDM coupler 3.sub.3 connected to optical amplifying
portion 10. The light of the longer wavelength band is emitted from
an output port of WDM coupler 3.sub.3 connected to optical
amplifying portion 11. Furthermore, one of branch lights obtained
by bifurcating the signal light at branching coupler 9 enters an
input port of WDM coupler 3.sub.3. The light of the conventional
band included in this branch light is emitted from an output port
of WDM coupler 3.sub.3 to optical amplifying portion 11.
[0138] Optical amplifying portion 10 is, for example, an ordinary
optical amplifier for amplifying a light of the conventional band.
More practically, optical amplifying portion 10 is, for example,
the optical amplifier of FIGS. 3 or 4. Further, the conventional
band is preferably set as the optical amplification band of such an
optical amplifier.
[0139] Optical amplifying portion 11 is, for example, an optical
amplifier for amplifying a light of the longer wavelength band.
More practically, optical amplifying portion 11 is, for example,
the optical amplifier used in any of the above embodiments of the
present invention, or a conventional long wavelength optical
amplifier illustrated, for example, in FIGS. 3 or 4.
[0140] Branching coupler 9 is operative to branch a light emitted
from optical amplifying portion 10 (mainly, amplified signal light
of the conventional band) at a previously set branching ratio into
two branch light beams, and to send one of the branch lights to an
input port of WDM coupler 3.sub.4, and to put back the other of the
branch lights to an input port of WDM coupler 3.sub.3.
[0141] WDM coupler 3.sub.4 has at least two input ports and one
output port. The branch light from branching coupler 9 enters in
one of the input ports and the light emitted from optical
amplifying portion 11 (mainly, amplified signal light of the 1.58
.mu.m) enters in the other of the input ports. The lights entered
in the input ports are multiplexed and then emitted to the outside.
WDM couplers 3.sub.3 and 3.sub.4 are known devices and include, for
example, a dielectric filter and a WDM fiber coupler.
[0142] When the signal light L.sub.s obtained by multiplexing
lights of two wavelength bands enters the optical amplifier of FIG.
18, the signal light L.sub.s is demultiplexed by WDM coupler
3.sub.3 into lights of the respective wavelength bands. These
lights are sent to optical amplifying portions 10 and 11
respectively corresponding to the wavelength bands thereof to
thereby be amplified. The light of the conventional band is
bifurcated at branching coupler 9 into two branch lights after
being emitted from optical amplifying portion 10. One of the branch
lights is emitted to the outside through WDM coupler 3.sub.4.
Further, the other of the branch lights is put back to WDM coupler
3.sub.3. Then, a component of the conventional band enters in
optical amplifying portion 11 through WDM coupler 3.sub.3.
[0143] Thus, the signal light of the longer wavelength band and a
part of the light of the conventional band, which has been
amplified by optical amplifying portion 10, are input to optical
amplifying portion 11 through WDM coupler 3.sub.3. Consequently,
the light of the conventional band serves as an excitation light
for the optical amplification of the longer wavelength band. The
amplification of the signal light of the longer wavelength band is
performed with high excitation efficiency. This amplified signal
light of the longer wavelength band is multiplexed at WDM coupler
3.sub.4 with the signal light of the conventional band and is then
emitted to the outside.
[0144] As described above, as in FIG. 18, in the case of
collectively amplifying the signal light of the conventional band
and the signal light of the longer wavelength band, a part of the
light of the conventional band is branched by branching coupler 9.
Then, the branched part is supplied to optical amplifying portion
11 through WDM coupler 3.sub.3. Thus, the light of the conventional
band acts as the excitation light for the optical amplification of
the light of the longer wavelength band. Consequently, the
excitation efficiency of the optical amplification of the light of
the longer wavelength band is enhanced. Further, only passive
optical parts are required to supply the light of the conventional
band to the optical amplifying portion 11 for amplifying the light
of the longer wavelength band.
[0145] Thus, the present invention provides an optical amplifier
having excellent amplification characteristics at a low cost. Such
an optical amplifier is very useful as an optical amplifying
repeater for use, especially, in a superwideband amplifying
repeater transmission system for transmission of light of a
superwideband obtained by multiplexing lights of the conventional
wavelength band and the longer wavelength band.
[0146] Therefore, as illustrated in FIG. 18, an optical amplifier
includes first and second optical amplifiers amplifying first and
second signal lights, respectively, the first and second signal
lights being in different wavelength bands. An excitation light
providing device (such as, for example, WDM couplers 3.sub.3 and
3.sub.4) branches a portion of the signal light amplified by the
first optical amplifier, and provides the branched portion to the
second optical amplifier as excitation light in the second optical
amplifier.
[0147] FIG. 19 is a diagram showing an optical amplifier according
to an additional embodiment of the present invention. More
specifically, FIG. 19 shows an optical amplifier which collectively
amplifies a signal light of the conventional band and a signal
light of the longer wavelength band traveling in different
directions of transmission from each other. As compared to FIG. 18,
the optical amplifier in FIG. 19 inverts the input and output
directions of optical amplifying portion 10 and changes the
placement of branching coupler 9 in such a way as to be located
between the output terminal of optical amplifying portion 10 and
WDM coupler 3.sub.3, in the case where the signal light L.sub.s of
the conventional band is transmitted, for example, to the left, as
viewed in this figure, and the signal light of the longer
wavelength band is transmitted to the right.
[0148] In the optical amplifier of FIG. 19, the signal light of the
conventional band enters optical amplifying portion 10 through WDM
coupler 3.sub.4. Then, the incident signal light is amplified
therein. Subsequently, the light emitted from optical amplifying
portion 10 is branched at branching coupler 9 into two branch
lights. One of the branch lights is emitted to the outside through
WDM coupler 3.sub.3. The other branch light is sent to optical
amplifying portion 11 through WDM coupler 3.sub.3, and serves as an
excitation light. The signal light L.sub.s of the longer wavelength
band enters optical amplifying portion 11 through WDM coupler
3.sub.3. Then, this incident signal light is amplified with high
excitation efficiency. Finally, the amplified signal light is
emitted to the outside through WDM coupler 3.sub.4.
[0149] Thus, the optical amplifier in FIG. 19 obtains advantageous
effects similar to those of the optical amplifier in FIG. 18 even
in the case that the transmission direction of the signal light of
the conventional band is different from that of the signal light of
the longer wavelength band.
[0150] FIGS. 18 and 19 show optical amplifiers which amplify lights
in two different wavelength bands. In these figures, the wavelength
bands are described as being, for example, the conventional band
and the longer wavelength band. However, these wavelength bands are
only intended to be examples, and the embodiments of the present
invention are applicable to other wavelength bands.
[0151] Although the above embodiments employ EDFs as the rare earth
element doped fibers, the embodiments of the present invention are
not limited thereto. For example, optical fiber doped with a rare
earth element other than erbium may be used. Further, in the
foregoing descriptions, the case of using the new wavelength
transmission band as the longer wavelength band, and of utilizing
the light of the conventional band to increase the efficiency of
optical amplification as the excitation light, has been described.
However, the new transmission band and the wavelength band of light
to be utilized as the excitation light according to the embodiments
of the present invention are not limited thereto. More
specifically, other wavelength bands can be used.
[0152] The above embodiments of the present invention provide a
low-cost optical amplifier and optical amplification method which
achieve high excitation efficiency in amplifying signal light (such
as that, for example, in the longer wavelength band). The optical
amplifier and optical amplification method can be achieved by
adding only passive optical parts to a conventional optical
amplifier. Therefore, for example, highly efficient optical
amplification of signal light of the longer wavelength band can be
realized.
[0153] Although a few preferred embodiments of the present
invention have been shown and described, it would be appreciated by
those skilled in the art that changes may be made in these
embodiments without departing from the principles and spirit of the
invention, the scope of which is defamed in the claims and their
equivalents.
* * * * *