U.S. patent application number 08/932222 was filed with the patent office on 2001-08-09 for multi-wavelength light source and discrete-wavelength-variable light source.
Invention is credited to EDAGAWA, NOBORU, MIYAZAKI, TETSUYA, YAMAMOTO, SHU.
Application Number | 20010012144 08/932222 |
Document ID | / |
Family ID | 17117874 |
Filed Date | 2001-08-09 |
United States Patent
Application |
20010012144 |
Kind Code |
A1 |
MIYAZAKI, TETSUYA ; et
al. |
August 9, 2001 |
MULTI-WAVELENGTH LIGHT SOURCE AND DISCRETE-WAVELENGTH-VARIABLE
LIGHT SOURCE
Abstract
In a light source for generating light containing multiple
wavelengths substantially uniform in intensity, a wavelength
demultiplexing element 10 (for example, waveguide-type wavelength
selecting filter) demultiplexes input light into a plurality of
wavelengths .lambda.1 through .lambda.32. Optical amplifiers 14-1
through 14-32 amplify outputs of the element 10 and applies them to
input ports of a wavelength multiplexing element 12. The wavelength
multiplexing element 12 wavelength-multiplexes their input. Output
of the wavelength multiplexing element 12 is applied to a fiber
coupler 16 which, in turn, applies one of its outputs to the
wavelength demultiplexing element 10. The optical amplifiers 14
have a gain larger by approximately 10 dB than the loss in the
optical loop made of the element 10, optical amplifier 14, element
12 and fiber coupler 16. The other output of the fiber coupler 16
is wavelength-multiplex light containing wavelengths .lambda.1
through .lambda.32.
Inventors: |
MIYAZAKI, TETSUYA; (TOKYO,
JP) ; EDAGAWA, NOBORU; (TOKYO, JP) ; YAMAMOTO,
SHU; (TOKYO, JP) |
Correspondence
Address: |
CHRISTIE PARKER & HALE
P O BOX 7068
PASADENA
CA
911097068
|
Family ID: |
17117874 |
Appl. No.: |
08/932222 |
Filed: |
September 17, 1997 |
Current U.S.
Class: |
398/194 |
Current CPC
Class: |
H01S 3/2383 20130101;
H04B 2210/258 20130101; H04J 14/02 20130101; H01S 3/08086 20130101;
H01S 3/067 20130101; H01S 3/06791 20130101 |
Class at
Publication: |
359/188 ;
359/133; 359/132; 359/179 |
International
Class: |
H04B 010/04 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 17, 1996 |
JP |
HEISE8-244383 |
Claims
1. A multi-wavelength light source for outputting laser light
containing different wavelengths simultaneously, comprising:
wavelength demultiplexing means for demultiplexing input light into
a plurality of predetermined different wavelengths; a plurality of
optically amplifying means for amplifying said predetermined
different wavelengths from said wavelength demultiplexing means
individually; wavelength multiplexing means for
wavelength-multiplexing optical outputs of respective said
optically amplifying means; connecting means for connecting an
output of said wavelength multiplexing means to an input of said
wavelength demultiplexing means; and output take-out means for
taking out light circulating in a loop comprising of said
wavelength demultiplexing means, said optically amplifying means,
said wavelength multiplexing means and said connecting mean to the
exterior of said loop.
2. The multi-wavelength light source according to claim 1 wherein
said wavelength demultiplexing means demultiplexes said input light
into a plurality of predetermined different wavelengths in
predetermined wavelength intervals.
3. The multi-wavelength light source according to claim 2 wherein
said wavelength demultiplexing means is a waveguide-type wavelength
selecting filter.
4. The multi-wavelength light source according to claim 1 wherein
said wavelength multiplexing means wavelength-multiplexes optical
inputs having predetermined wavelengths which enter in respective
said inputs.
5. The multi-wavelength light source according to claim 4 wherein
said wavelength multiplexing means is a waveguide-type wavelength
selecting filter.
6. The multi-wavelength light source according to claim 1 wherein
said output take-out means is light splitting means for taking out
light travelling through said connecting means.
7. The multi-wavelength light source according to claim 1 further
comprising optical band pass filter means provided on said
connecting means and transparent only to light within a
predetermined wavelength band.
8. The multi-wavelength light source according to claim 1 further
comprising optical modulating means provided on said optical loop
for intensity-modifying circulating light in said optical loop in
accordance with a modification signal, said modification signal
having a frequency corresponding to an integer multiple of the
circular frequency of said optical loop, and said light taken out
by said output take-out means is pulsating light.
9. The multi-wavelength light source according to claim 8 wherein
said means on said optical loop is of a polarization holding
type.
10. The multi-wavelength light source according to claim 8 further
comprising polarization adjusting means located in an input side of
said optical modulating means.
11. The multi-wavelength light source according to claim 1 wherein
each of said optically amplifying means can selectively supply or
block its output to said wavelength multiplexing means.
12. The multi-wavelength light source according to claim 10 wherein
each of said optically amplifying means includes an optical
amplifier for amplifying corresponding one of optical outputs from
said wavelength demultiplexing means and optical switching means
for supplying or blocking an output of said optical amplifier.
13. A multi-wavelength light source for outputting laser light
containing a plurality of collectively modified wavelengths,
comprising: wavelength demultiplex/amplify/multiplexing means for
demultiplexing input light into a plurality of predetermined
different wavelengths, then optically amplifying them individually,
and wavelength-multiplexing them; polarizing means for extracting
predetermined polarized components from output light of said
wavelength demultiplex/amplify/multiplexing means; light splitting
means for splitting output light of said polarizing means;
depolarizing means for depolarizing one of optical outputs of said
light splitting means and for applying it to wavelength
demultiplex/amplify/multiplexing means; and modulating means for
modulating the other optical output of said splitting means in
accordance with a modulation signal.
14. The multi-wavelength light source according to claim 13 further
comprising first polarization adjusting means located between one
of outputs of said splitting means and input of said depolarizing
means.
15. The multi-wavelength light source according to claim 13 further
comprising second polarization adjusting means located between the
other output of said splitting means and input of said modulating
means.
16. The multi-wavelength light source according to claim 13 wherein
said wavelength demultiplex/amplify/multiplexing means comprises
wavelength demultiplexing means for demultiplexing input light into
predetermined different wavelengths in predetermined wavelength
intervals, a plurality of optically amplifying means for amplifying
said wavelengths demultiplexed by said wavelength demultiplexing
means individually, and wavelength multiplexing means for
wavelength-multiplexing optical outputs from respective said
optically amplifying means.
17. The multi-wavelength light source according to claim 16 wherein
said wavelength demultiplexing means and said wavelength
multiplexing means are waveguide-type wavelength selecting
filters.
18. A multi-wavelength light source for outputting ASE light
containing a plurality of wavelengths, comprising: wavelength
demultiplex/amplify/mult- iplexing means for demultiplexing input
light into a plurality of predetermined different wavelengths, then
optically amplifying them individually, and wavelength-multiplexing
them; wavelength shifting means for slightly shifting wavelengths
in output light from said wavelength
demultiplex/amplify/multiplexing means and for returning it back to
input of said wavelength demultiplex/amplify/multiplexing means;
and output take-out means for taking out light which is circulated
by said wavelength demultiplex/amplify/multiplexing means and said
wavelength shifting means.
19. The multi-wavelength light source according to claim 18 wherein
said wavelength demultiplex/amplify/multiplexing means and said
output take-out means are of a polarization holding type.
20. The multi-wavelength light source according to claim 18 further
comprising first polarization adjusting means for adjusting
polarization of output light from said wavelength
demultiplex/amplify/multiplexing means and for supplying it to said
wavelength shifting means.
21. The multi-wavelength light source according to claim 18 further
comprising depolarizing means for depolarizing output light from
said wavelength shifting means and for supplying it to said
wavelength demultiplex/amplify/multiplexing means.
22. The multi-wavelength light source according to claim 21 wherein
said depolarizing means comprises second polarization adjusting
means for adjusting polarization of output light from said
wavelength shifting means, and a depolarizing element for
depolarizing output light from said second polarization adjusting
means.
23. The multi-wavelength light source according to claim 21 wherein
said output take-out means is located between output of said
depolarizing means and input of said wavelength
demultiplex/amplify/multiplexing means.
24. The multi-wavelength light source according to claim 18 wherein
said wavelength shifting means is an acousto-optic modulator.
25. The multi-wavelength light source according to claim 18 wherein
said wavelength demultiplex/amplify/multiplexing means comprises
wavelength demultiplexing means for demultiplexing input light into
predetermined different wavelengths in predetermined wavelength
intervals, a plurality of optically amplifying means for amplifying
said wavelengths demultiplexed by said wavelength demultiplexing
means individually, and wavelength multiplexing means for
wavelength-multiplexing optical outputs from respective said
optically amplifying means.
26. The multi-wavelength light source according to claim 25 wherein
said wavelength demultiplexing means and said wavelength
multiplexing means are waveguide-type wavelength selecting
filters.
27. A multi-wavelength light source for outputting light containing
a plurality of individually modified wavelengths, comprising:
wavelength demultiplexing means for demultiplexing input light into
a plurality of different wavelengths; a plurality of optically
amplifying means each for amplifying one of said different
wavelengths from said wavelength demultiplexing means individually;
a plurality of splitting means each for splitting optical output
from associated one of said optically amplifying means; first
wavelength multiplexing means for wavelength-multiplexing first
optical outputs of said splitting means and supplying the
multiplexed output to said wavelength demultiplexing means; a
plurality of optical modulating means capable of modulating second
optical output of said splitting means individually; and second
wavelength multiplexing means for wavelength-multiplexing outputs
of said optical modulating means.
28. The multi-wavelength light source according to claim 27 wherein
said wavelength demultiplexing means demultiplexes said input light
into different predetermined wavelengths in predetermined
wavelength intervals.
29. The multi-wavelength light source according to claim 27 wherein
said wavelength demultiplexing means is a waveguide-type wavelength
selective filter.
30. The multi-wavelength light source according to claim 27 wherein
said first and second wavelength demultiplexing means
wavelength-multiplex optical inputs entering in its inputs and
having predetermined wavelengths.
31. The multi-wavelength light source according to claim 30 wherein
said first and second wavelength multiplexing means are
waveguide-type wavelength selecting filters.
32. A discrete-wavelength-variable light source for selectively
supplying one or more of discrete wavelengths contained in output
laser light, comprising: selective demultiplex/amplifying means for
selectively demultiplexing one or more predetermined wavelengths
from input light and optically amplifying them; and optical
splitting means for part of optical output from said selective
demultiplex/amplifying means to an input of said selective
demultiplex/amplifying means and for externally supplying the
remainder of said optical output from said selective
demultiplex/amplifying means.
33. The discrete-wavelength-variable light source according to
claim 31 wherein said selective demultiplex/amplifying means
comprises wavelength demultiplexing means for demultiplexing input
light into a plurality of predetermined wavelengths, first optical
switch means for selecting one of said wavelength components from
said wavelength demultiplexing means, and optically amplifying
means for amplifying optical output of said first optical switch
means.
34. The discrete-wavelength-variable light source according to
claim 33 wherein said selective demultiplex/amplifying means
further comprises wavelength multiplexing means for
wavelength-multiplexing a plurality of optical inputs with
wavelength multiplexing characteristics consistent with input
ports, and second optical switch means for supplying optical output
of said optically amplifying means to one of input ports of said
wavelength multiplexing means corresponding to the wavelength
selected by said first optical switch means.
35. The discrete-wavelength-variable light source according to
claim 35 wherein said wavelength demultiplexing means demultiplexes
said input light into predetermined wavelengths in predetermined
wavelength intervals.
36. The discrete-wavelength-variable light source according to
claim 35 wherein said wavelength demultiplexing means is a
waveguide-type wavelength selecting filter.
37. The discrete-wavelength-variable light source according to
claim 34 wherein said wavelength multiplexing means is a
waveguide-type wavelength selecting filter.
38. The discrete-wavelength-variable light source according to
claim 32 further comprising optical modulating means provided on an
optical loop made of said selective demultiplex/amplifying means
and said optical splitting means to intensity-modulate circulating
light in said optical loop in accordance with a modulation signal,
said modulation signal having a frequency which is an integer
multiple of a frequency circulating in said optical loop, said
remainder of said optical output from said optical splitting means
being pulsating light.
39. The discrete-wavelength-variable light source according to
claim 38 wherein said selective demultiplex/amplifying means and
said optical splitting means are of a polarization holding
type.
40. The discrete-wavelength-variable light source according to
claim 38 further comprising polarization adjusting means located in
an input side of said optical modulating means.
Description
FIELD OF THE INVENTION
[0001] This invention relates to a multi-wavelength light source
and a discrete-wavelength-variable light source, and more
particularly, to a multi-wavelength light source for supplying one
or more optical outputs with different wavelengths concurrently or
selectively and a discrete-wavelength-variable light source capable
of selecting one of a plural of wavelengths, which are suitable for
transmission or tests of a wavelength division/multiplex
transmission system.
BACKGROUND OF THE INVENTION
[0002] In wavelength division multiplex transmission systems, it is
essential to reliably obtain laser lights with a number of close
wavelengths. For transmission tests or tests of optical components
used in wavelength division/multiplex transmission systems, there
is the need for a laser light source highly stable in wavelengths
and outputs.
[0003] ITU has recommended 0.8 nm (100 GHz) as the wavelength
interval in wavelength division multiplex transmission systems.
While temperature coefficients of wavelength changes of
semiconductor lasers are approximately 0.1 nm/.degree. C. That is,
semiconductor lasers are very sensitive to temperature fluctuation.
Therefore, it is difficult to maintain wavelength intervals of 0.8
nm in a number of semiconductor laser light sources over a long
period. Moreover, in ordinary laser sources, injected current is
used to stabilize optical outputs. Control current for
stabilization of optical outputs causes changes in temperature, and
it results in changes in wavelength. That is, control of optical
outputs affects wavelengths, and makes it difficult to stabilize
wavelengths.
[0004] A prior proposal to cope with the problem is to connect an
optical filter and an optically amplifying element in a ring to
form a multi-wavelength light source for collectively supplying
multiple wavelengths. FIG. 15 is a schematic block diagram showing
a prior example A Fabry-Perot optical filter 210, erbium-doped
optical fiber amplifier 212 and optical fiber coupler 214 are
connected to form a ring.
[0005] FIG. 16 show characteristic diagrams of the prior example of
FIG. 15. FIG. 16(1) shows transparent wavelength characteristics of
the Fabry-Perot optical filter 210, FIG. 16(2) shows amplifying
characteristics of the optical fiber amplifier 212, and FIG. 16(3)
shows the spectral waveform of output wavelength. The Fabry-Perot
optical filter 210 is a kind of wavelength selecting optical
filters having wavelength transparent characteristics which permit
specific wavelengths in certain wavelength intervals called FSR
(Free Spectral Range) to pass through as shown in FIG. 16(1).
Individual transparent wavelengths of the Fabry-Perot optical
filter 210 are selected from the spontaneous emission light
generated in the optical fiber amplifier 212. The output spectral
waveform coincides with that obtained by multiplying the
transparent wavelength characteristics of the optical filter 210 by
the amplifying characteristics of the optical fiber amplifier 212.
Theoretically, laser oscillation outputs are obtained in
wavelengths where the gain of the optical fiber amplifier 212
surpasses the loss of the optical loop.
[0006] In the prior art example shown in FIG. 15, the output
intensity is large near the gain center wavelength within the
amplifying range of the optical fiber amplifier 212, where
oscillation is most liable to occur, and largely decreases in
peripheral portions, as shown in FIG. 16(3). That is, the prior art
example cannot realize simultaneous oscillation in multiple
wavelengths in substantially uniform output levels.
[0007] Moreover, wavelength interval in output light in the prior
art example exclusively depends on transparent characteristics of
the Fabry-Perot optical filter 210. When the wavelength interval is
0.8 nm (100 GHz), the wavelength interval FSR of the transparent
wavelength characteristics of the Fabry-Perot optical filter 210 is
less than the uniform extension width of the erbium-doped optical
fiber amplifier 212. Therefore, even when a plurality of
oscillation wavelengths are obtained near the gain center
wavelength of the erbium-doped optical fiber amplifier 212, mode
competition occurs, and results in unstable output intensities and
wavelength fluctuations of respective wavelengths.
[0008] A Fabry-Perot semiconductor lasers is a multi-wavelength
light source, other than the fiber ring light source. However, it
involves unacceptable fluctuations in oscillation wavelengths due
to mode competition or mode hopping, and fails to uniform
intensities of respective oscillated wavelength components.
SUMMARY OF THE INVENTION
[0009] It is therefore an object of the invention to provide a
multi-wavelength light source and a discrete-wavelength-variable
light source capable of simultaneously or selectively outputting
one or more wavelengths with a uniform intensity.
[0010] Another object of the invention is to provide a
multi-wavelength light source capable of selecting one or more
wavelengths among a plurality of wavelengths.
[0011] Another object of the invention is to provide a
multi-wavelength light source and a discrete-wavelength-variable
light source immune to temperature fluctuations.
[0012] The invention uses a wavelength
demultiplex/amplify/multiplexing unit for demultiplexing input
light into a plurality of different predetermined wavelengths,
optically amplifying individual wavelengths, and multiplexing the
wavelengths, and connects its output to its input to form an
optical loop. Since individual wavelengths are optically amplified
by the wavelength demultiplex/amplify/multiplexing unit, laser
oscillation in a plurality of wavelengths with substantially the
same intensity is promised in the optical loop. Since the structure
is simple and the most elements are passive ones, it is highly
stable against temperature fluctuations.
[0013] When using wavelength demultiplexing means for
demultiplexing input light into a plurality of predetermined
wavelengths in predetermined wavelength intervals, the light
containing multiple wavelengths in substantially constant
wavelength intervals can be obtained. Usable as the wavelength
demultiplexing means is a waveguide-type wavelength selecting
filter, for example.
[0014] When using the optical band pass filter means which is
transparent only to light within a predetermined wavelength band,
the light source can prevent that light beyond the desired
wavelength band circulates in the optical loop. This contributes
not only to stabilization of laser oscillation but also to reliably
preventing that the output contains undesirable wavelengths.
[0015] By using optical modulation means for intensity-modulating
circulating light in the optical loop with a modulation signal
having a frequency, which is an integer multiple of the circulation
frequency (namely, c/nL) in the optical loop, the light source can
conjoin multiple-wavelength light into pulsating light synchronous
with the modulation signal. Location of the optical modulation
means may be either posterior to wavelength division or posterior
to wavelength multiplexing. When it is located after wavelength
division, fine adjustment of individual wavelengths is easier, but
a plurality of optical modulating means for individual wavelengths
must be used. When it is located after wavelength multiplexing,
optical modulating means may be only one, but adjustment of
individual wavelengths must be done in another portion.
Polarization adjusting means may be provided in the input side of
the optical modulating means to previously adjust polarization so
as to ensure optimum operations of the optical modulating means.
If, of course, necessary means is of a polarization holding type,
polarization adjusting means may be omitted to reduce elements.
[0016] When individual optically amplifying means are capable of
selectively supplying or blocking outputs to the wavelength
multiplexing means, multiplex output light containing one or more
selected wavelengths can be obtained. If each of the optically
amplifying means comprises an optical amplifier for amplifying
corresponding one of optical outputs from the wavelength
demultiplexing means and an optical switch means for feeding or
blocking the optical output of the optical amplifier, undesired
noise light is prevented from entering into the wavelength
multiplexing means while the optical switch means blocks the
path.
[0017] In another aspect of the invention, an output of wavelength
demultiplex/amplify/multiplexing means for demultiplexing input
light into a plurality of predetermined wavelengths, then optically
amplifying them individually, and thereafter multiplexing them is
connected to the input of the same wavelength
demultiplex/amplify/multiplexing means via polarization means,
optical dividing means and depolarization means to form an optical
loop. There is also provided modulation means for modulating
divisional optical outputs from the optical dividing means in
accordance with a modulation signal.
[0018] With this arrangement, light components with multiple
wavelengths which are simultaneously oscillated in the optical loop
can be modulated collectively by the modulation means.
[0019] Since the polarization means suppresses fluctuations in
plane of polarization, fluctuations in the ring cavity mode are
less likely to occur, and simultaneous oscillation in multiple
wavelengths is stabilized. Since the polarization adjusting means
in an appropriate location selects and maintains an appropriate
plane of polarization for each element, behaviors of individual
elements are stabilized. If essential means are of a polarization
holding type, polarization means and polarization adjusting means
may be omitted to reduce elements.
[0020] In another aspect of the invention, an output of wavelength
demultiplex/amplify/multiplexing means for demultiplexing input
light into a plurality of predetermined wavelengths, then optically
amplifying them individually, and thereafter multiplexing them is
connected to the input of the same wavelength
demultiplex/amplify/multiplexing means to form an optical loop and
wavelength shifting means is provided in the optical loop to
slightly shift the wavelengths. As a result, laser oscillation is
suppressed, and an ASE (Amplified Spontaneous emission) light
source for multiple wavelengths can be realized.
[0021] Since the polarization adjusting means and depolarization
means in appropriate locations select and maintain an appropriate
plane of polarization for each element, behaviors of individual
elements are stabilized. By taking out the light from the optical
loop after depolarization, output light independent from or less
dependent on polarization can be obtained. If essential means are
of a polarization holding type, polarization adjusting means may be
omitted to reduce elements.
[0022] In another aspect of the invention, an optical loop is
formed such as demultiplexing input light into a plurality of
predetermined wavelengths, then optically amplifying them
individually, thereafter multiplexing them and feedback to the
input, and it is activated for simultaneous oscillation in multiple
wavelengths. Lights which are wavelength-demultiplexed and
individually amplified are divided, individually modulated outside
and thereafter wavelength-multiplexed. As a result,
multi-wavelength light containing individually modified wavelengths
can be obtained.
[0023] In another version of the invention, an output of
selective-demultiplex/amplifying means for selectively
demultiplexing a predetermined wavelength from input light and
optically amplifying it is connected to the input of the
selective-demultiplex/amplifying means to form an optical loop.
Thus, a single wavelength selected by the
selective-demultiplex/amplifying means can be supplied as output
light. That is, any one of a plurality of discrete wavelengths can
be selected. Since it is selected from predetermined wavelengths,
output with a stable wavelength can be obtained. Since the
polarization adjusting means in an appropriate location selects and
maintains an appropriate plane of polarization for each element,
behaviors of individual elements are stabilized. If essential means
are of a polarization holding type, polarization adjusting means
may be omitted to reduce elements.
DESCRIPTION OF THE DRAWINGS
[0024] FIG. 1 is a schematic block diagram showing a general
construction of a first embodiment of the invention;
[0025] FIG. 2 shows waveform characteristics of the same
embodiment;
[0026] FIG. 3 is a schematic block diagram of a general
construction of a modified embodiment;
[0027] FIG. 4 shows waveform diagrams of a version using AWGs as a
wavelength demultiplexing element 10 and a wavelength multiplexing
element 12, and containing two FSRs of these AWGs in the band of
amplification of the optical amplifier 14;
[0028] FIG. 5 shows a waveform obtained by an experiment;
[0029] FIG. 6 is a schematic block diagram showing a general
construction of a version commonly using two optical amplifiers
14-1 and 14-2;
[0030] FIG. 7 is a schematic block diagram showing a general
construction of an embodiment configured to modulate
multi-wavelength light collectively;
[0031] FIG. 8 is a schematic block diagram showing a general
construction of an embodiment of the invention applied to a
multi-wavelength ASE light source;
[0032] FIG. 9 is a schematic block diagram showing a general
construction of an embodiment configured to modulate each
wavelength component individually;
[0033] FIG. 10 is a schematic block diagram showing a general
construction of an embodiment configured to extract one or more
desired wavelengths among a plurality of wavelengths in given
wavelength intervals;
[0034] FIG. 11 is a schematic block diagram showing a general
construction of an embodiment applied to a wavelength-variable
light source for outputting a single discrete wavelength;
[0035] FIG. 12 shows distribution of wavelengths in output of the
embodiment shown in FIG. 11;
[0036] FIG. 13 is a schematic block diagram showing a general
construction of an embodiment applied to a multi-wavelength mode
lock pulse light source;
[0037] FIG. 14 is a schematic block diagram showing a general
construction of a wavelength converting apparatus using the
wavelength-variable light source shown in FIGS. 10 and 12 as its
pump light source;
[0038] FIG. 15 is a schematic block diagram showing a general
construction of a conventional multi-wavelength light source;
and
[0039] FIG. 16 is a characteristics diagram of the conventional
light source shown in FIG. 15.
DETAILED DESCRIPTION OF THE INVENTION
[0040] Embodiments of the invention are explained in detail with
reference to the drawings.
[0041] FIG. 1 is a schematic block diagram showing a general
construction of a first embodiment of the invention. FIG. 2 shows
wavelength characteristics of this embodiment.
[0042] In FIG. 1, reference numeral 10 denotes a wavelength
demultiplexing element for demultiplexing input light through an
input port #1 into a plurality of predetermined wavelength
components (in this embodiment, components of wavelengths .lambda.1
to .lambda.2). Numeral 12 denotes a wavelength multiplexing element
for wavelength-multiplexing the light components with multiple
wavelengths (in this embodiment, wavelengths .lambda.1 to
.lambda.32). Namely, these elements are waveguide-type wavelength
selecting filters (AWG). Other than AWG, known as another optical
element for demultiplexing and multiplexing a plurality of
wavelengths collectively is the optical demultiplex/multiplexing
filter developed by Optical Corporation of America, U.S.A. Also
this type of optical element can be used as the wavelength
demultiplexing element 10 and the wavelength multiplexing element
12.
[0043] Output ports #1 through #32 of the wavelength demultiplexing
element 10 are connected to input ports #1 through #32 of the
wavelength multiplexing element 12 via optical amplifiers 14 (14-1
through 14-32). Output port #1 of the wavelength multiplexing
element 12 is connected to a fiber coupler 16, and one of two
outputs of the fiber coupler 16 is connected to input port #1 of
the wavelength demultiplexing element 10. Thus, the other output of
the fiber coupler 16 is extracted as desired multi-wavelength
light. The unused output end of the fiber coupler 16 is made as a
non-reflective end. As a result, instable oscillation by Fresnel
reflection can be prevented. The same also applies to all
embodiments shown below.
[0044] Each optical amplifier 14 includes an erbium-doped optical
fiber amplifier, pumping light source and wavelength
demultiplex/multiplexing (WDM) coupler for supplying output light
from the pumping light source to the optical fiber amplifier. The
optical amplifier 14 may be made of a semiconductor laser amplifier
and a Raman amplifier.
[0045] Briefly explained below are functions of AWG used as the
wavelength demultiplexing element 10 and the wavelength
multiplexing element 12. AWG is an optical element in which
wavelengths .lambda.1 through .lambda.32 entering in the input port
#1 are output from output ports #1 through #32, and wavelengths
.lambda.1 through .lambda.32 entering in the input port #2 are
output from output ports #2 through #32 and #1. Those entering in
the subsequent ports are output from output ports with
corresponding and subsequent numbers, and wavelengths .lambda.1
through .lambda.32 entering in the input port #32 are output from
output ports #32 and #1 to #31. Wavelength intervals of wavelengths
.lambda.1 through .lambda.32 are determined by the inner
interference structure. Therefore, when wavelength-division
multiplexed light containing wavelengths .lambda.1 through
.lambda.32 enters in the input ports #1, these wavelengths
.lambda.1 through .lambda.32 are wavelength-demultiplexed and
output from corresponding output ports #1 through #32. In contrast,
when lights of wavelengths .lambda.1 through .lambda.32 enter in
respective corresponding input ports #1 through #32,
wavelength-multiplexed light containing the entered wavelengths
.lambda.1 through .lambda.32 is output from the output port #1.
[0046] AWGs have a periodicity, and these wavelengths .lambda.1
through .lambda.32 are so-called basic waves. Also longer
wavelengths .lambda.1' through .lambda.32' and shorter wavelengths
.lambda.31"' and .lambda.32"' are wavelength-demultiplexed and
wavelength-multiplexed.
[0047] FIG. 2 (1) shows composite transparent wavelength
characteristics obtained when output ports of the wavelength
demultiplexing element 10 are connected to common-numbered input
ports of the wavelength multiplexing element 12, respectively. In
this embodiment, in which the wavelength demultiplexing element 10
and the wavelength multiplexing element 12 are 32.times.32 type
AWGs, the transparent wavelength characteristics are periodic, and
32 wavelengths form one cycle, as explained above. In general, this
is defined as FSR (Free Spectral Range) of AWG. There are
wavelengths .lambda.1', .lambda.2', . . . and . . . , .lambda.31"'
and .lambda.32"' outside .lambda.1 through .lambda.32 used in the
embodiment, as shown in FIG. 2(1). For example, .lambda.1' and
.lambda.32"' entering in the input port #1 of the wavelength
demultiplexing element 10 are output from output ports #1 and
#32.
[0048] If the composite transparent wavelength characteristics of
the wavelength demultiplexing element 10 and the wavelength
multiplexing element 12 are such that the transparent wavelength
width for each wavelength is sufficiently narrow, longitudinal
modes of ring resonance, described later, can be decreased to a few
or only one. This is attained by narrowing the transparent
wavelength width of each wavelength in the transparent wavelength
characteristics of the wavelength demultiplexing element 10 and the
wavelength multiplexing element 12, respectively, or by slightly
shifting the transparent wavelength characteristics of the
wavelength demultiplexing element 10 from those of the wavelength
multiplexing element 12. The latter is advantageous for obtaining a
desired wavelength width more easily although the loss is
larger.
[0049] Ideally, each of the optical amplifiers 14-1 through 14-32
has amplifying wavelength characteristics covering one cycle of
FSR, namely, the wavelength range of .lambda.1 through .lambda.32,
and preferably exhibiting a drastic decrease in gain beyond the
range. Actually, in accordance with the amplifying wavelength
characteristics of available optical amplifiers, AWGs having FSRs
consistent with the amplifying wavelength characteristics are used
as the wavelength demultiplexing element 10 and the wavelength
multiplexing element 12. The gain of amplification by each optical
amplifier 14 is determined larger by approximately 10 dB than the
loss in one circulation of the loop made of the wavelength
demultiplexing g element 10, optical amplifier 14, wavelength
multiplexing element 12 and fiber coupler 16.
[0050] Basically, it is sufficient for each of the optical
amplifiers 14-1 through 14-32 that its gain center wavelength can
cover a single wavelength assigned to it. However, the use of
different optical amplifiers with different gain center wavelengths
makes the process of producing and assembling respective element
more troublesome, and it is preferable to use optical amplifiers
14-1 to 14-32 with the same amplifying wavelength range. From this
point of view, the amplifying wavelength characteristics in which
the gain is flat throughout one cycle, namely one FSR OF AWGs 10
AND 12, and drastically decreases outside this range is
preferable.
[0051] Due to the composite transparent wavelength characteristics
of the wavelength demultiplexing element 10 and the wavelength
multiplexing element 12 (FIG. 2(1)) and the amplifying wavelength
characteristics of the optical amplifiers 14-1 through 14-32 (FIG.
2(2)), the loop gain of the embodiment shown in FIG. 1 draws peaks
at wavelengths .lambda.1, . . . .lambda.32, and light output from
the fiber coupler 16 to the exterior of the optical loop results in
the spectrum shown in FIG. 2(3). Since the wavelength
demultiplexing element 10 and the wavelength multiplexing element
12 have the same transparent center wavelength and uniform
transmissivity to respective wavelengths, and the optical
amplifiers 14-1 through 14-32 have substantially the same gain,
respective wavelengths .lambda.1 to .lambda.32 in output light
extracted by the fiber coupler 16 have substantially the same
optical intensity. In AWG, variance in loss among different
wavelengths upon wavelength division and wavelength multiplexing
can be readily reduced to 3 through 4 dB or less in the process of
fabrication, and this degree of variance can be compensated by fine
adjustment of amplification gains of respective optical amplifiers
14-1 thorough 14-32.
[0052] Since each optical amplifier 14-1 to 14-32 amplifies a
single wavelength alone, mode competition does not occur, and
stable amplification of input light is promised. Therefore, this
embodiment can realize multi-wavelength oscillation in wavelengths
.lambda.1 to .lambda.32, and can substantially equalize intensities
of respective wavelengths.
[0053] It is no problem, provided that one of the wavelength
demultiplexing element 10 and the wavelength multiplexing element
12, preferably the wavelength demultiplexing element 10, does not
have a wavelength periodicity. However, under the conditions where
the wavelength demultiplexing element 10 (and the wavelength
multiplexing element 12) has a wavelength periodicity, and FSR of
the wavelength demultiplexing characteristics is narrower than the
amplification band of the optical amplifiers 14, which results in
containing two FSRs in the amplification band of the optical
amplifiers 14, the loop gain happens to exist also for wavelengths
outside .lambda.1 to .lambda.32, e.g. wavelengths .lambda.1' and
.lambda.32"', and possibly causes mode competition or instable
oscillation.
[0054] This can be prevented by locating an optical band pass
filter in the loop to pass wavelengths .lambda.1 to .lambda.32
alone. FIG. 3 is a schematic block diagram showing a general
construction of another embodiment taken for this purpose.
Reference numeral 20 denotes a wavelength
demultiplex/amplify/multiplexing unit containing the wavelength
demultiplexing element 10, wavelength multiplexing element 12 and
optical amplifiers 14 of FIG. 1. Numeral 22 denotes an optical band
pass filter (optical BPF) transparent to wavelengths .lambda.1
through .lambda.32 alone in output light from the wavelength
demultiplex/amplify/multiplexing unit 20. Numeral 24 denotes a
fiber coupler which demultiplexes output light of the optical BPF
22 into two components, and supplies one to the wavelength
demultiplex/amplify/multip- lexing unit 20 and extracts the other
as multi-wavelength output.
[0055] FIG. 4 shows waveforms of a version using AWGs as the
wavelength demultiplexing element 10 and the wavelength
multiplexing element 12, and containing two FSRs of AWGs in the
amplification band of the optical amplifiers 14. FIG. 4(1) shows
transparent wavelength characteristics of AWGs used as the
wavelength demultiplexing element 10 and the wavelength
multiplexing element 12. FIG. 4(2) shows amplification
characteristics of the optical amplifiers 14. FIG. 4(3) shows
wavelength characteristics of light passing through the optical
amplifier 14-1 when the optical BPF 22 is not provided. As shown,
since three wavelengths .lambda.1, .lambda.1' and .lambda.1" pass
through the optical amplifier 14-1 and are amplified, competition
of these wavelengths invites instable oscillation of the target
wavelength .lambda.1.
[0056] FIG. 4(4) shows transparent characteristics of the optical
BPF 22. FIG. 4(5) shows wavelength characteristics of light passing
through the optical amplifier 14-1 when the optical BPF 22 is
provided. Since the optical BPF 22 permits the basic waves
(.lambda.1 to .lambda.32) alone to circulate in the loop,
wavelength .lambda.1 alone, here, can enter the optical amplifier
14-1 and is amplified there.
[0057] In this manner, in the embodiment shown in FIG. 3,
wavelengths other than basic waves (wavelengths .lambda.1 through
.lambda.32) are removed by the optical BPF 22, and do not circulate
in the loop. Therefore, even if the wavelength demultiplexing
characteristics of the wavelength demultiplexing element 10 (and
the wavelength multiplexing element 12) are periodic such that it
demultiplexes (or they demultiplex) wavelengths other than basic
waves as well, and the optical amplifiers 14 can amplify these
undesired waves sufficiently, stable multi-wavelength laser
oscillation containing basic waves alone is ensured.
[0058] Wavelength intervals of wavelengths contained in output
light extracted from the fiber couplers 16 and 24 are determined by
wavelength selectivities of the wavelength demultiplexing element
10 and the wavelength multiplexing element 12. It is easy to design
AWGs such that the wavelength intervals be 100 GHz (0.8 nm) or its
integer multiple. Therefore, it is sufficiently possible to realize
multi-wavelength oscillation with wavelength intervals of
approximately 0.8 nm.
[0059] FIG. 5 shows waveforms confirmed by an actual experiment.
Used in the experiment are AWGs of wavelength intervals of 0.7 nm
as the wavelength demultiplexing element 10 and the wavelength
multiplexing element 12. Four ports in every other sequences are
connected to counterpart ports having common numbers via optical
amplifiers. It is known that four wavelengths in intervals of 1.4
nm are oscillated simultaneously in substantially the same optical
intensity. The side mode suppression ratio was as good as 35 dB,
and the ratio of the signal level to the background noise level was
as good as approximately 60 dB.
[0060] A quartz AWG has a temperature coefficient of approximately
0.01 nm/.degree. C. which is smaller by one digit than that of a
semiconductor laser. Therefore, the accuracy for temperature
control of two AWGs used as the wavelength demultiplexing element
10 and the wavelength multiplexing element 12 can be alleviated to
{fraction (1/10)} of the accuracy required for a signal-generating
semiconductor laser for generating signal light used in wavelength
multiplexing. Considering that the temperature controlling accuracy
of the pumping light source of the optical amplifiers 14 (for
example, a semiconductor laser for a wavelength around 1.48 nm)
need not be so high as that required for a signal-generating
semiconductor laser, temperature control of the pumping light
source can be simplified. That is, this embodiment makes the entire
temperature control easier and simpler, and can be manufactured
economically.
[0061] This embodiment also makes it easy to adjust and modify
wavelengths in output light because, by selecting appropriate
temperatures of AWGs used as the wavelength demultiplexing element
10 and the wavelength multiplexing element 12, .lambda.1 to
.lambda.32 can be shifted to longer or shorter wavelengths while
maintaining the same wavelength intervals.
[0062] In most cases, the optical amplifiers 14 have their own
pumping light sources. However, erbium-doped optical fibers of a
plurality of optical amplifiers can be pumped by a single pumping
light source. FIG. 6 is a schematic block diagram showing a general
construction of the modified part of a modified embodiment in this
respect. Parts or elements common to those of FIG. 1 are labelled
with common reference numerals. Output port #1 of the wavelength
demultiplexing element 10 is connected to input port #1 of the
wavelength multiplexing element 12 via an optical isolator 30-1,
erbium doped optical fiber 32-1 and wavelength
demultiplex/multiplexing (WDM) coupler 34-1. Similarly, output port
#2 of the wavelength demultiplexing element 10 is connected to
input port #2 of the wavelength multiplexing element 12 via an
optical isolator 30-2, erbium-doped optical fiber 32-2 and
wavelength demultiplex/multiplexing coupler 34-2.
[0063] Output light of a 1.48 .mu.m pumping semiconductor laser 36
is divided into two parts by a 3 dB coupler 38, and one of them is
supplied to the erbium-doped optical fiber 32-1 via the WDM coupler
34-1 while the other is supplied to the erbium-doped optical fiber
32-2 via the WDM coupler 34-2. The optical isolators 30-1, 30-2
prevent that pumping light to the erbium-doped optical fibers 32-1,
32-2 enter the output ports #1 and #2 of the wavelength
demultiplexing element 10.
[0064] In this manner, the optical amplifiers 14-1 and 14-2 can
share a single pumping light source. By using this arrangement also
for other optical amplifiers 14-3 through 14-32, the total number
of pumping light sources can be reduced to a half.
[0065] It is convenient to use light containing collectively
modified multiple wavelengths in transmission tests of wavelength
division multiplex optical transmission systems. Explained below is
an embodiment in which multi-wavelength light is modified
collectively. FIG. 7 is a schematic block diagram of its general
construction. Numeral 40 denotes a wavelength
demultiplex/amplify/multiplexing unit containing the wavelength
demultiplexing element 10, optical amplifiers 14 and wavelength
multiplexing element 12 of FIG. 1 (and optical BPF 22 of FIG. 3).
Output light of the unit 40 enters in a fiber coupler 44 via a
polarizer 42. One of outputs of the fiber coupler 44 is fed to the
wavelength demultiplex/amplify/multiplexing unit 40 through a
polarization adjuster 46 and a depolarizer 48. The other output of
the fiber coupler 44 enters into the external optical modulator 52
through a polarization adjuster 50.
[0066] In a fiber ring or loop formed by the wavelength
demultiplex/amplify/multiplexing unit 40, polarizer 42, fiber
coupler 44, polarization adjuster 46 and depolarizer 48, laser
oscillation of multiple wavelengths occur simultaneously in
substantially the same intensity in the same manner as the
embodiment shown in FIG. 1. The multi-wavelength light is extracted
from the fiber ring by the fiber coupler 50.
[0067] Also in the wavelength demultiplex/amplify/multiplexing unit
40, the composite transparent wavelength characteristics of the
wavelength demultiplexing and the wavelength multiplexing are
chosen to sufficiently narrow the transparent wavelength widths for
individual wavelengths so that longitudinal modes can be decreased
to a few or only one. As explained with reference to FIG. 1, this
is attained by narrowing the transparent wavelength width of each
wavelength in the transparent wavelength characteristics of each of
the wavelength demultiplexing element and the wavelength
multiplexing element, or by slightly shifting the transparent
wavelength characteristics of the wavelength demultiplexing element
from those of the wavelength multiplexing element.
[0068] The use of the polarizer 42 contributes to suppression of
polarization fluctuations in the fiber ring. In order to prevent
interference in the wavelength demultiplex/amplify/multiplexing
unit 40, the depolarizer 48 depolarizes the input light. If the
polarized condition by the polarizer 42 is maintained, interference
or other undesirable effects may occur in the external modulator
52. To remove such trouble in the external modulator 52, the
polarization adjuster 50 adjusts the polarization. Additionally,
for more effective depolarization by the depolarizer 48, the
polarization adjuster 46 adjusts polarization of the input
light.
[0069] While light circulates in the fiber ring made of the
wavelength demultiplex/amplify/multiplexing unit 40, polarizer 42,
fiber coupler 44, polarization adjuster 46 and depolarizer 48,
simultaneous laser oscillation in multiple wavelengths occurs in
the same manner as the embodiment of FIG. 1. The multi-wavelength
light by the simultaneous laser oscillation is extracted by the
fiber coupler 44, and applied to the external modulator 52 via the
polarization adjuster 50. The external optical modulator 52
modulates the applied multi-wavelength light collectively in
accordance with an externally applied modulation signal. The
modulated light is supplied to transmission optical fibers,
etc.
[0070] Polarization fluctuation in the fiber ring causes
fluctuation of the ring cavity mode, and makes simultaneous
oscillation of multiple wavelengths instable. In the embodiment,
however, since the polarizer 42 suppresses fluctuations in planes
of polarization, instable oscillation can be suppressed. If,
however, the wavelength demultiplex/amplify/multip- lexing unit 40
(wavelength demultiplexing element 10, wavelength multiplexing
element 12 and optical amplifiers 14) and the fiber coupler 16 are
of a polarization holding type, the polarizer 42, depolarizer 48
and polarization adjusters 46, 50 may be omitted.
[0071] Also in the embodiment shown in FIG. 7, if looping of light
of undesired wavelengths outside the target wavelength band should
be previously prevented, an optical BPF similar to the optical BPF
22 in the embodiment shown in FIG. 3 is placed at a desired
location in the fiber ring (inside or outside the wavelength
demultiplex/amplify/multiplexing unit 20).
[0072] To test characteristics of optical components, it is
desirable to use an ASE (Amplified Spontaneous Emission) light
source for multiple wavelengths, which does not laser-oscillate.
Such a multi-wavelength ASE light source can be readily obtained
according to the invention. FIG. 8 is a schematic block diagram
showing a general construction of an embodiment taken for this
purpose.
[0073] Explained below is the construction of the embodiment of
FIG. 8. Numeral 60 denotes a wavelength
demultiplex/amplify/multiplexing unit similar to the wavelength
demultiplex/amplify/multiplexing unit 40. Output light from the
unit 60 enters in an acousto-optic modulator 64 via a polarization
adjuster 62. Output of an A/O modulator 64 enters in a fiber
coupler 70 via a polarization adjuster 66 and a depolarizer 68. One
of outputs of the fiber coupler 70 enters in the wavelength
demultiplex/amplify/multiplexing unit 60, and the other output of
the fiber coupler 70 is extracted as multi-wavelength ASE
light.
[0074] The A/O modulator 64 slightly shifts and outputs wavelengths
in the input light. Therefore, light circulating in the fiber ring
made of the wavelength demultiplex/amplify/multiplexing unit 60,
polarization adjuster 62, A/O modulator 64, polarization adjuster
66, depolarizer 68 and fiber coupler 70 is slightly shifted in
wavelength by the A/O modulator 64. As a result, laser oscillation
does not occur, and amplified spontaneous emission light, that is,
ASE light is obtained. Since the multi-wavelength state is not lost
even after passing the A/O modulator 64, the light extracted from
the fiber coupler 70 is ASE light containing multiple
wavelengths.
[0075] In order to prevent interference or other undesired events
in the A/O modulator 64, the polarization adjuster adjusts
polarization of input light to the A/O modulator 64. If the output
light of the A/O modulator 64 remains in a specific polarized
state, undesirable effects may occur in the wavelength
demultiplex/amplify/multiplexing unit 60. To deal with the matter,
the polarization adjuster 66 and depolarizer 68 previously cancel
the specific polarized state. The polarization adjuster 66 and the
depolarizer 68 may be located between the fiber coupler 70 and the
wavelength demultiplex/amplify/multiplexing unit 60. However, as
shown in FIG. 8, when they are located between the A/O modulator 64
and the fiber coupler 70, polarization dependency is removed from
multi-wavelength ASE light extracted from the fiber coupler 70, and
this light can be used more conveniently for examining various
characteristics (such as amplification characteristics or loss
characteristics) of optical components to wavelength-division
multiplexed light.
[0076] In the embodiment shown in FIG. 7, multi-wavelength light is
modified collectively. However, it is preferable that individual
wavelengths can be data-modulated independently for use in actual
transmission tests or transmission.
[0077] FIG. 9 is a schematic block diagram showing a general
construction of an embodiment configured to modify respective
wavelengths individually. Numeral 80 denotes a wavelength
demultiplexing element similar to the wavelength demultiplexing
element 10, and 82 denotes a wavelength multiplexing element
similar to the wavelength multiplexing element 12. Output ports of
the wavelength demultiplexing element 80 are connected to
common-numbered input ports of the wavelength multiplexing element
82 through optical amplifiers 84 (84-1 through 84-32) similar to
the optical amplifiers 14. Wavelength multiplex output of the
wavelength multiplexing element 82 is connected to the input of the
wavelength demultiplexing element 80. Here again, if necessary, an
optical BPF similar to the optical BPF 22 used in the embodiment of
FIG. 3 may be provided, for example, between the output of the
wavelength multiplexing element 82 and the input of the wavelength
demultiplexing element 80.
[0078] Since this embodiment does not take out multi-wavelength
light directly, it does not use a fiber coupler similar to the
fiber coupler 16. Instead, fiber couplers 86 (86-1 through 86-32)
are provided for dividing outputs of the optical amplifiers 84-1
through 84-32. Optical outputs extracted by the fiber couplers 86-1
to 86-32 are applied to external modulators 88 (88-1 through
88-32). The external modulators 88 (88-1 through 88-32) are
supplied with different modulation signals #1 through #32. Optical
outputs from the external modulators 88-1 through 88-32 are applied
to a wavelength multiplexing element 90 which is identical to the
wavelength multiplexing element 82.
[0079] The wavelength demultiplexing element 80, wavelength
multiplexing element 82, optical amplifiers 84 and fiber coupler 86
are of a polarization holding type. If not, additional elements
corresponding to the polarizer 42, polarization adjuster 46 and
depolarizer 48 used in the embodiment of FIG. 7 must be provided in
the loop made of the wavelength demultiplexing element 80, optical
amplifiers 84 and wavelength multiplexing element 82.
[0080] Composite transparent wavelength characteristics of the
wavelength demultiplexing element 80 and the wavelength
multiplexing element 82 are chosen to sufficiently narrow the
transparent wavelength widths for individual wavelengths so that
longitudinal modes can be decreased to a few or only one. As
explained with reference to FIG. 1, this is attained by narrowing
the transparent wavelength width for each wavelength of the
transparent wavelength characteristics of each of the wavelength
demultiplexing element 80 and the wavelength multiplexing element
82, or by slightly shifting the transparent wavelength
characteristics of the wavelength demultiplexing element 80 from
those of the wavelength multiplexing element 82.
[0081] Explained below are behaviors of the embodiment shown in
FIG. 9. In the loop made of the wavelength demultiplexing element
80, optical amplifiers 84 and wavelength multiplexing element 82,
laser oscillation of multiple wavelengths occur simultaneously in
substantially the same intensity in the same manner as the
embodiment shown in FIG. 1. Respective wavelengths by laser
oscillation are taken out individually by the fiber couplers 86-1
through 86-3, and applied to the external modulators 88-1 through
88-32. External modulators 88-1 and 88-32 modulate their optical
inputs by modulation signals #1 to #32, respectively. As a result,
modulated optical outputs containing different wavelengths
modulated by different modulation signals #1 to #32 can be
obtained. Then, the wavelength multiplexing element 90
wavelength-multiplexes the outputs of the external modulators 88-1
through 88-32, and supplies the multiplexed light to an external
element such as optical fiber transmission path, for example. Thus,
the transmission test can be executed in practical conditions for
transmission.
[0082] It is essential for the wavelength multiplexing element 90
only to compose or multiplex optical outputs of the external
modulators 88-1 through 88-32, and it need not have the same
wavelength multiplexing function as that of the wavelength
multiplexing element 82.
[0083] In some applications, it is desired to take out one or some
wavelengths from a number of wavelengths in certain wavelength
intervals. Such requirement is attained by modifying the embodiment
of FIG. 1 in the manner as shown in FIG. 10. That is, optical
switches 92 (92-1 through 92-32) are inserted between outputs of
optical amplifiers 14-1 through 14-32 and input ports of the
wavelength multiplexing element 12. When one or more of the optical
switches 92-1 through 92-32 are turned on, corresponding
wavelengths alone circulate in the fiber ring, and laser oscillated
outputs with corresponding wavelengths are taken out from the fiber
coupler 16. For example, when only the optical switch 92-4 is
turned on, only the wavelength .lambda.4 stimulates laser
oscillation, and the laser light is taken out from the fiber
coupler 16. If optical switches in every two intervals are turned
on among optical switches 92-1 through 92-32, then multi-wavelength
light containing wavelengths in wavelength interval twice that of
the wavelength demultiplexing element 10 (and the wavelength
multiplexing element 12) can be obtained.
[0084] In the same manner as the embodiment shown in FIG. 3, which
is a modified version of the embodiment of FIG. 1, an optical BPF
similar to the optical BPF 22 in the embodiment of FIG. 3 may be
provided, if necessary.
[0085] According to the embodiment shown in FIG. 10, light
containing only one or some of a plurality of predetermined
wavelengths can be obtained. That is, this light source can be
operated as a discrete-wavelength-varia- ble light source or as a
multi-wavelength light source capable of selecting any desired
wavelength interval.
[0086] The modification in the embodiment shown in FIG. 10 is
applicable also to embodiments shown in FIGS. 7, 8 and 9.
[0087] FIG. 11 is a schematic block diagram showing a general
construction of an embodiment realizing a wavelength-variable light
source for a discrete single wavelength. Numeral 110 denotes a
wavelength demultiplexing element similar to the wavelength
demultiplexing element 10, and 112 denotes a wavelength
multiplexing element similar to the wavelength multiplexing element
12. 114 designates a 32.times.1 optical switch for selecting one of
plural output ports (32 output ports in this embodiment) of the
wavelength demultiplexing element 110. 116 denotes an optical
amplifier for amplifying output light from the optical switch 114.
118 denotes a 1.times.32 optical switch for switching an output of
the optical amplifier 116 to one of plural input ports (32 input
ports in this embodiment) of the wavelength multiplexing element
112.
[0088] Optical switches 114, 118 can be turned ON and OFF by using
a common switching signal. That is, optical switches 114, 118
select an output port and an input port with a common number among
plural output ports of the wavelength demultiplexing element 110
and plural input ports of the wavelength multiplexing element
112.
[0089] Since the optical amplifier 116 amplifies one of wavelengths
.lambda.1 through .lambda.32 demultiplexed by the wavelength
demultiplexing element 110, its amplification band is wide enough
to cover wavelengths .lambda.1 through .lambda.32 and need not be
wider. No problem of FSR occurs.
[0090] Explained below are behaviors of the embodiment shown in
FIG. 11. Among wavelengths .lambda.1 through .lambda.32
demultiplexed by the wavelength demultiplexing element 110, a
wavelength selected by the optical switch 114 is amplified by the
optical amplifier 116. Output of the optical amplifier 116 enters
in one of input ports of the wavelength multiplexing element 112,
having a number common to the output port selected by the optical
switch 114. Therefore, the wavelength multiplexing element 112
outputs light amplified by the optical amplifier 116 from its
output port to the fiber coupler 120. The fiber coupler 120 divides
the light from the wavelength multiplexing element 112 into two
components, and supplies one to the wavelength demultiplexing
element 110 and externally outputs the other as output light.
[0091] The light of the wavelength selected by the optical switches
114, 118 circulates in the fiber ring made of the wavelength
demultiplexing element 110, optical switch 114, optical amplifier
116, optical switch 118, wavelength multiplexing element 112 and
fiber coupler 120, and stimulates laser oscillation.
[0092] FIG. 12 shows an example of wavelength distribution in
output of the embodiment shown in FIG. 11. In this example, an
output port #i of the wavelength demultiplexing element 110 and an
input port #i of the wavelength multiplexing element 112, which
correspond to wavelength .lambda.i, are selected by the optical
switches 114, 118. In FIG. 12, the actually laser-oscillated
wavelength is shown by the bold solid line, and wavelengths that
can be selected are shown by the thin solid line.
[0093] If a sufficient wavelength selectivity is ensured only with
the wavelength demultiplexing element 110, the system may omit the
wavelength multiplexing element 112 and hence the optical switch
118.
[0094] FIG. 13 is a schematic block diagram showing a general
construction of a multi-wavelength mode-locked pulse light source
taken as another embodiment of the invention. In a pulse light
source, it is desirable that the pulse phase is stable on the time
domain. In this embodiment, mode-locked pulse light for plural
wavelengths can be obtained collectively.
[0095] Numeral 130 denotes a wavelength
demultiplex/amplify/multiplexing unit comprising the wavelength
demultiplexing element 10, optical amplifiers 14 and wavelength
multiplexing element 12, all of FIG. 1, wavelength demultiplexing
element 10, optical amplifiers 14-1 through 14-32, optical switches
92-1 through 92-32 and wavelength multiplexing element 12, all of
FIG. 10, or wavelength demultiplexing element 110, optical switch
114, optical amplifier 116, optical switch 118 and wavelength
multiplexing element 112, all of FIG. 11. When the wavelength
demultiplex/amplify/multiplexing unit 120 comprises the wavelength
demultiplexing element 10, optical amplifiers 14 and wavelength
multiplexing element 12 of FIG. 1, laser oscillation occurs
simultaneously in multiple wavelengths. When the unit 120 comprises
the wavelength demultiplexing element 10, optical amplifiers 14-1
through 14-32, optical switches 92-1 through 92-32 and wavelength
multiplexing element 12 of FIG. 10, or the wavelength
demultiplexing element 110, optical switch 114, optical amplifier
116, optical switch 118 and wavelength multiplexing element 112,
laser oscillation occurs in selected one or some wavelengths.
[0096] Numeral 134 denotes an electroabsorption optical modulator
for modulating output light of the wavelength
demultiplex/amplify/multiplexin- g unit by a sinusoidal modulation
signal, and 136 denotes a fiber coupler for dividing output light
of the electroabsorption optical modulator 134 into two parts to
supply one to the wavelength demultiplex/amplify/multip- lexing
unit 130 and to externally output the other as output light.
[0097] When L is the ring length of the ring or loop made of the
wavelength demultiplex/amplify/multiplexing unit 130,
electroabsorption optical modulator 134, and fiber coupler 136, n
is the effective refractive index, and c is the light velocity, a
sinusoidal voltage of a frequency corresponding to an integer
multiple of the basic frequency fo=c/(nL) is applied as a
modulation signal to the electroabsorption optical modulator 134.
Transparent band widths of the wavelength demultiplexing element
(and wavelength multiplexing element) in the wavelength
demultiplex/amplify/multiplexing unit 130 for respective
wavelengths are determined to be sufficiently narrower than the
circulating basic frequency fo.
[0098] Under these conditions of frequency, light circulating in
the fiber ring or loop made of the wavelength
demultiplex/amplify/multiplexing unit 130, electroabsorption
optical modulator 134 and fiber coupler 136 is mode-locked to the
sinusoidal modification signal applied to the electroabsorption
optical modulator 134, and has the form of a pulse rising at an
apex or nadir of the sinusoidal modulation signal. As a result, a
sequence of pulses containing multiple wavelengths and mode-locked
can be obtained.
[0099] Since the ring length L and the effective refractive index n
vary for different wavelengths, it is necessary, in a strict sense,
to adjust effective optical path lengths for individual wavelengths
in the wavelength demultiplex/amplify/multiplexing unit 130.
However, it is sufficient to connect an electroabsorption optical
modulator (and, if necessarily, a polarization adjuster) in a
location anterior to the optical path for each wavelength, more
preferably, anterior to the optical amplifier 14, in the wavelength
demultiplex/amplify/multiplexing unit 130 and to apply a sinusoidal
modulation signal in corresponding phase and frequency to the
electroabsorption optical modulator to modulate it there. Then, the
phase and frequency of one sinusoidal signal may be adjusted
independently, and may be applied as a modulation signal to each
electroabsorption optical modulator. In this case, however, A
number of electroabsorption optical modulators (and polarization
adjusters) corresponding to respective wavelengths are needed and
the light source becomes more expensive than the embodiment of FIG.
13.
[0100] In the embodiment of FIG. 13, when looping of undesired
wavelengths other than the target wavelengths should be prevented,
an optical BPF similar to the optical BPF 22 in the embodiment
shown in FIG. 3 is provided at an appropriate location, for
example, between the output of the wavelength
demultiplex/amplify/multiplexing unit 130 and the optical modulator
134.
[0101] By using the wavelength variable light source according to
the embodiment shown in FIG. 10 or FIG. 11 as a pump light source
of a wavelength converting apparatus, any wavelength acceptable in
a network can be used efficiently. FIG. 14 is a schematic block
diagram of a general construction of an embodiment taken for this
purpose.
[0102] In FIG. 14, numeral 140 denotes a wavelength variable light
source shown in FIG. 10 or FIG. 11, which is designed and
fabricated so that wavelengths acceptable in a wavelength-division
multiplexing optical network can be selected. Output light from the
wavelength variable light source is applied as pumping light
.lambda.p to a semiconductor laser amplifier 142. On the other
hand, input modified light .lambda.s enters into a terminal A of an
optical circulator 144. The optical circulator 144 is an optical
element which outputs the light entering through the terminal A
from another terminal B and outputs the light entering through the
terminal B from a terminal C. Output light from the terminal B of
the optical circulator 144 (modulated light .lambda.s) is fed to an
end surface of the semiconductor laser amplifier 142 opposite from
the end surface into which the pumping light .lambda.p is
entered.
[0103] The pumping light .lambda.p and the modulated light
.lambda.s travel in opposite direction within the semiconductor
laser amplifier 142. If the intensity of the pump light .lambda.p
is held at a value where the gain of the semiconductor laser
amplifier 142 is saturated, the pumping light .lambda.p is
waveform-modified in accordance with the intensity waveform of the
modulated light .lambda.s due to their mutual gain modulation
effect. That is, waveform of the pumping light .lambda.p becomes
substantially opposite from the waveform of the modulated light
.lambda.s. The waveform-modified pumping light .lambda.p enters
into the optical circulator 144 through the terminal B, and it is
output from the terminal C. The light output from the terminal C of
the optical circulator 144 has a form in which the input modulated
light .lambda.s has been wavelength-converted to the wavelength of
the pump light .lambda.p.
[0104] In the discrete-wavelength-variable light source 140 to
which the invention is applied, its available wavelengths can be
readily set to coincide with wavelengths acceptable in the
wavelength-division multiplexing optical network. Once the
wavelengths are set so, the wavelength of the optical signal
obtained by wavelength conversion of the input modified signal
.lambda.s is an acceptable wavelength of the network, and the
acceptable wavelength in the network can be re-used. When a light
source capable of varying continuous wavelengths, such as
conventional multi-electrode semiconductor laser, for example, is
used as the wavelength variable light source 140, precisely
accurate control must be made to adjust the wavelength of its
output light to one of wavelengths acceptable in the network, and
this invites a much complicated construction and a high cost. In
contrast, according to the invention, the
discrete-wavelength-variable light source can select an appropriate
wavelength through the switch, and can remove the need for
wavelength control and severe accuracy therefor.
[0105] In addition to the foregoing examples, there are
arrangements for four-wave-mixing, for example, as wavelength
converting mechanisms, and a fiber amplifier is also usable in lieu
of the semiconductor laser amplifier and the arrangement using an
absorption-type optical modulator is disclosed in a patent
application by the same Applicant, entitled Waveform Converting
Apparatus, (Japanese Patent Application Heisei 8-233796).
[0106] As readily understandable from the above explanation,
according to the invention, laser output containing multiple
wavelengths with substantially uniform intensity can be obtained.
By using as wavelength demultiplexing means an element for
wavelength-demultiplexing input light into multiple wavelengths in
predetermined wavelength intervals, a multi-wavelength light source
for generating light containing multiple wavelengths in certain
intervals can be realized. Since the light source has a simple
structure and is mostly of passive elements, it is stable against
changes in temperature.
[0107] By intensity-modifying circulating light in an optical loop
with a modification signal having an integer multiple frequency of
the circular frequency of the optical loop, multi-wavelength pulse
light locked with the modulation signal can be obtained.
[0108] By locating means posterior to wavelength division
(preferably, posterior to optical amplification) for selectively
supplying light to or blocking light from wavelength multiplexing
means, multiplex output light containing any selected one or more
wavelengths can be obtained.
[0109] According to the invention, it is also easy to modify
multi-wavelength laser light either collectively or
individually.
[0110] When wavelength shifting means is placed in an optical loop,
an ASE light source for multiple wavelengths can be realized.
[0111] When the output of selectively demultiplexing and amplifying
means for selectively demultiplexing a predetermined wavelength
from input light and amplifying the demultiplexed light is
connected to input of the same means so as to form an optical loop,
one of a plurality of discrete wavelengths can be used as output
light. That is, one of discrete wavelengths can be selected. Since
the wavelength is selected from predetermined wavelengths, output
containing stable wavelengths can be obtained.
* * * * *