Light source for wavelength division multiplexing communications

Disclosed is a light source for wavelength division multiplexing communications which has: a unit for outputting light with a wavelength band having a constant width; a unit for dividing the light into first dividied light and second divided light; at least one selective wavelength blocking unit which blocks selectively light with a specific wavelength of the first divided light to output light that the light with a specific wavelength is removed; a unit for conducting such a phase control that the phase difference between the selectively-wevelength-blocked light and the second divided light is 180.degree. to each other and for outputting first phase-controlled light and second phase-controlled light; and a coupling for coupling the first phase-controlled light and the second phase-controlled light to output coupled light.

FIELD OF THE INVENTION 
This invention relates to a light source used in wavelength division 
multiplexing communications. 
BACKGROUND OF THE INVENTION 
Wavelength division multiplexing technologies, where multiple wavelengths 
are simultaneously transmitted for achieving high-speed and large-capacity 
communications, have rapidly developed with the progress in optical 
communication technology. 
In the construction or experiment of a wavelength division multiplexing 
transmission system, multiple light sources with different wavelengths are 
necessary corresponding to the number of transmission channels. In 
general, wavelength selection in wavelength division multiplexing 
transmission cannot be freely conducted since there are some limitations 
such as to avoid a combination of wavelengths troubling the transmission, 
e.g., mixing of four light waves. Namely, an arbitrariness, accuracy and 
sureness in wavelength selection are desired. In general, such a light 
source is difficult to fabricate and is costly due to the lowered 
production yield. Further, one light source is needed for one wavelength, 
thus, in case of eight-channel wavelength division multiplexing 
transmission, eight light sources are necessary. 
On the other hand, when lights from multiple light sources are transmitted 
through one transmission line, a wavelength division multiplexing device 
called coupler or multiplexer is necessary. As the wavelength division 
multiplexing device, for example, a N.times.1 coupler of fiber fusion 
type, a N.times.1 coupler of waveguide type, or AWG (arrayed waveguide 
grating) is used. In case of eight channels, for example, the insertion 
loss for a 8.times.1 coupler is about 10 to 12 dB and that for AWG is 
about 6 to 9 dB. Thus, the insertion loss in coupling is so big. 
In order to provide arbitrarily different light source wavelengths, it is 
necessary to fabricate separately laser diodes (LDs) for light source 
corresponding to the necessary wavelengths. Therefore, the manufacturing 
cost must be increased. Further, the production yield is lowered due to 
the accuracy and sureness needed for providing desired wavelengths. Also, 
the insertion loss in coupling must be so big. 
For example, in a conventional N.times.1 coupler composed of several 
2.times.1 couplers, a 2.times.1 coupler theoretically causes an insertion 
loss of 3 dB at minimum. Namely, a 8.times.1 coupler causes an insertion 
loss of 9 dB theoretically. In fact, there occurs an insertion loss of 
about 10 to 12 dB. 
Meanwhile, the insertion loss can be reduced by using a wavelength coupler 
called AWG. For example, in case of AWG with eight channels, the insertion 
loss is about 6 to 9 dB, which is smaller than that of the 8.times.1 
coupler. However, the insertion loss in coupling is still big. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the invention to provide a light source for 
wavelength division multiplexing communications where the insertion loss 
in coupling can be reduced. 
It is a further object of the invention to provide a light source for 
wavelength division multiplexing communications where an arbitrariness, 
accuracy and sureness in wavelength selection can be achieved with a 
reduced cost. 
According to the invention, a light source for wavelength division 
multiplexing communications, comprises: 
means for outputting light with a wavelength band having a constant width; 
means for dividing the light into first divided light and second divided 
light; 
at least one selective wavelength blocking means which blocks selectively 
light with a specific wavelength of the first divided light to output 
light that the light with a specific wavelength is removed; 
means for conducting such a phase control that the phase difference between 
the selectively-wavelength-blocked light and the second divided light is 
180.degree. to each other and for outputting first phase-controlled light 
and second phase-controlled light; and 
means for coupling the first phase-controlled light and the second 
phase-controlled light to output coupled light.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Before explaining a light source for wavelength division multiplexing 
communications in the preferred embodiments, the aforementioned 
conventional light sources will be explained in FIGS. 1 and 2. 
FIG. 1 shows the conventional light source with a 8.times.1 coupler. Lights 
output from eight light-source LDs (laser diodes) 61 to 68 are coupled 
into four lights at the first stage of the 8.times.1 coupler, and then the 
four lights are coupled into two lights at the second stage of the 
8.times.1 coupler, and then the two lights are coupled into single light, 
i.e., the eight lights are finally multiplexed. FIG. 2 shows the 
conventional light source with AWG (arrayed waveguide grating). Lights 
output from eight light-source LDs (laser diodes) 61 to 68 are coupled 
into single light, i.e., the eight lights are directly multiplexed. 
Next, the operation principle of a light source for wavelength division 
multiplexing communications of the invention will be explained below. 
In the light source for wavelength division multiplexing communications of 
the invention, light with a constant width and relatively broad wavelength 
band is output from a light source, and then the light is divided into two 
lights. One light is kept unaltered, and the other light is processed by 
blocking specific light with a narrow band wavelength to be used as the 
light source wavelength. Then, the divided lights are phase-controlled to 
have a phase difference of .pi. (180.degree.) to each other, and then are 
optically coupled to conduct the differential amplification. As a result, 
neither of the divided lights is output and only the blocked specific 
light is output as it is. 
In this case, by disposing a plurality of selective wavelength blocking 
means for blocking specific wavelength lights in series and setting the 
wavelengths of the blocked lights to be different from each other, the 
blocked lights can be output as lights for wavelength division 
multiplexing after the optical coupling. 
The light source to output light with the relatively broad and constant 
wavelength band may be an optical fiber amplifier to output amplified 
spontaneous emission (ASE) light. Specifically, an optical fiber with a 
core doped with a rare earth element, e.g., an erbium-doped fiber 
(hereinafter also referred to as `EDF`) that erbium is doped into the core 
part of a quartz-system optical fiber, may be used. When light power with 
a specific wavelength, e.g., 1.48 .mu.m band or 0.98 .mu.m band, is input 
to EDF, amplified spontaneous emission (ASE) light with a wavelength band 
from 1.53 .mu.m band to 1.56 .mu.m band can be output. 
Meanwhile, the selective wavelength blocking means for blocking specific 
wavelength lights may be dielectric multilayer film, optical waveguide, 
fiber grating etc. 
Now, taken is an example that four wavelength filters (band-pass filters) 
with different blocking wavelengths are inserted into path B corresponding 
to the other light described above. FIGS. 8A to 8C show spectra output at 
several parts in this composition. FIG. 8A shows a light spectrum after 
passing the band-pass filters, and FIG. 8B shows a light spectrum after 
the optical dividing on path A corresponding to one light described above. 
Both the output lights are controlled to have a phase difference of .pi. 
(180.degree.) to each other, and then are optically coupled. Thereby, 
after the coupling, a light spectrum shown in FIG. 8C can be obtained. 
Thus, the light output differentially amplified can be obtained. 
Next, light sources for wavelength division multiplexing communications in 
the first to sixth preferred embodiments will be explained in FIGS. 3 to 7 
and 9. 
In the first embodiment having a basic composition, as shown in FIG. 3, an 
excitation laser diode (LD) 1 is connected to one end of an erbium-doped 
fiber (HDF) 2. Thereby, from the other end of the erbium-doped fiber (EDF) 
2, amplified spontaneous emission (ASE) light is output. 
The amplified spontaneous emission (ASE) light obtained has the same 
profile as that in FIG. 8B. As show in FIG. 3, the amplified spontaneous 
emission (ASE) light is divided into two lights by an optical divider 3, 
and one of the two lights is input to a phase modulator 41 and the other 
is transmitted through a wavelength filter 51 and then is input to a phase 
modulator 42. 
The wavelength filter 51 has a characteristic of blocking only a specific 
wavelength. The output spectra of the phase modulators 41, 42 are 
different from each other only as to the wavelength characteristic of the 
wavelength filter 51, and the phase difference between the phase 
modulators 41, 42 is controlled to be .pi. (180.degree.) by the phase 
modulators 41, 42. Then, the divided lights that are controlled to have 
the phase difference of .pi. to each other are coupled by an optical 
coupler 9, and then the difference of their spectra is output from the 
optical coupler 9. 
A light source for wavelength division multiplexing communications in the 
second preferred embodiment will be explained in FIG. 4. The second 
embodiment, which is a modification of the first embodiment, is 
characterized in that there are disposed a plurality of wavelength filters 
51 to 5N. Therefore, it can obtain a plurality of wavelength lights. 
Light sources for wavelength division multiplexing communications in the 
third and fourth preferred embodiments will be explained in FIGS. 5 and 6. 
The third and fourth embodiments, which are modifications of the first and 
second embodiments, are characterized in that there are disposed variable 
light attenuators (ATT) 81 and/or 82 after the phase modulator. Thereby, a 
dispersion or insertion loss in the optical divider 3 or phase modulator 
41, 42 can be corrected. Therefore, light other than specific wavelength 
light to be used for the optical communications can be eliminated from the 
output light. 
A light source for wavelength division multiplexing communications in the 
fifth preferred embodiment will be explained in FIG. 7. The fifth 
embodiment is characterized in that there are provided polarization 
controllers 71, 72, which function to control polarized wave input to the 
phase modulators 41, 42, before the phase modulators 41, 42. Thereby, the 
change of polarized wave that may occur on the transmission line can be 
corrected. Therefore, the operation of the phase modulators can be further 
stabilized. 
A light source for wavelength division multiplexing communications in the 
sixth preferred embodiment will be more specifically explained in FIG. 9. 
The excitation laser diode (LD) 1 uses an excitation wavelength of 1.48 
.mu.m band, and Al co-doped EDF is used as the erbium-doped fiber (EDF) 2. 
A 3 dB fiber fused coupler is use as the optical divider 3. Fiber gratings 
are used as the wavelength filters 51 to 54, which have blocking 
wavelengths of 1545, 1548, 1551 and 1554 nm, respectively. The insertion 
loss of each of the fiber gratings is 15 dB at the blocking wavelength and 
is 0.1 dB at the other wavelengths. The phase modulators 41, 42 are of 
lithium niobate into which titanium is thermally diffused. The insertion 
loss of both the phase modulators 41, 42 is 3 dB. The variable ATTs 81, 82 
are of fiber type, and the insertion loss of the variable ATT 81 is set to 
be 0.4 dB greater than that of the variable ATT 82. 
In this composition, a four-wavelength output shown in FIG. 10 can be 
obtained by adjusting the control voltages to the phase modulators 41, 42 
while monitoring the output spectra. The insertion loss of this 
composition is calculated by summing those of the wavelength filters and 
the phase modulators. In this embodiment, it is 3.4 dB. Thus, the 
insertion loss can be significantly reduced comparing with the 
conventional case using, in particular, AWG. 
Further, the wavelength filters can be provided with a blocking wavelength 
in a wide wavelength range. Therefore, it is easy to select an arbitrary 
wavelength. 
Though, in the above embodiments, the fiber gratings are used as the 
wavelength filters, the wavelength filter may be of another 
wavelength-blocking type filter, e.g., a filter using waveguide or 
dielectric multilayer film. When the invention is applied to a light 
source for wavelength division multiplexing communications, a plurality of 
wavelength filters can be used. Otherwise, by using the composition with a 
single wavelength filter, unnecessary light can be completely removed by 
that composition. Thus, it can be also used as a light source for optical 
communications that serves to reduce the effect caused by wavelength 
diffusion. 
As the phase modulators, other than a device using electro-optic effect, a 
waveguide type phase modulator using thermo-optic effect or a phase 
generator that uses a bulk type optical element, such as a prism, to 
physically change an optical path length may be used. 
Further, EDF with back excitation can bring the same result. 
Although the invention has been described with respect to specific 
embodiment for complete and clear disclosure, the appended claims are not 
to be thus limited but are to be construed as embodying all modification 
and alternative constructions that may be occurred to one skilled in the 
art which fairly fall within the basic teaching here is set forth.