Optical amplifier for amplifying a signal light and a continuous light having a wave length different from the signal light

In an optical amplifier a signal light of a wave length .lambda. a is coupled with a continuous light of a wave length .lambda. b and amplified by an optical fiber amplifier. Then, the amplified output light is subjected to filtration to select the light of the wave length .lambda. a alone, so that the ASE power is reduced and the dynamic range of the optical fiber amplifier from the minimum light receipt power to the saturation after amplification can be enlarged, and, in the case that the signal light is pulse-modulated, the deterioration of the extinction ratio is minimized.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an optical amplifier for amplifying a signal 
light and a continuous light having a different wave length. More 
particularly, the present invention relates to an optical amplifier which 
functions to couple a signal light and a continuous light and amplifying 
the coupled light so as to reduce the effect of ASE (Amplified Spontaneous 
Emission) generated in the optical fiber amplifier, to expand dynamic 
range of the output light and to alleviate the deterioration of the 
extinction ratio when the signal light is pulse-modulated light. 
2. Prior Art 
The conventional optical amplifier will first be briefly explained by 
making reference to FIG. 7. In this figure, 1 is an optical source 1 which 
outputs a signal light 11 having a wave length .pi. a, 4 is an optical 
fiber amplifier and 5 is an optical filter. The optical fiber amplifier 4 
amplifies the signal light 11 from the optical source 1. The output of the 
optical fiber amplifier 4 includes ASE generated in the signal light 11 
and the optical fiber amplifier 4. Detected light involves beat noise 
between them as well as that among the spectral components of the ASE. The 
optical filter 5 extracts the light component having a wavelength of .pi. 
a from the output of the optical fiber amplifier 4, whereby the beat noise 
resulting from the spectral components of the ASE is reduced. 
Next, the input/output characteristics of the system in FIG. 7 will be 
explained in reference to FIG. 8. In this figure, the abscissa is the 
input optical power and the ordinate is the output optical power. When the 
input optical power is small the output optical power increases with the 
increase in the input optical power. When the input optical power becomes 
sufficiently large the output light power reaches a saturation as depicted 
by a power curve 16. The level 17 of the curve 16 is the minimum light 
receipt optical power which is restricted by the beat noise between the 
signal light 11 and the beat noise between the spectral components of the 
ASE, and the level 18 is the maximum receipt optical power right prior to 
the saturation of the output signal 11 of the optical fiber amplifier 4. 
Incidentally, beat noise is described in FIG. 9 of Masataka Nakazawa 
"Optical Fiber Amplification with Er-Doped Optical Fiber and Its 
Application "Applied Physics of Japan",Vol. 59, No.9 (1990). 
Problem to be solved by the present invention 
With the construction in FIG. 7, even if it is desired to use the signal 
light 11 in a wide dynamic range, the power of the ASE is large when the 
power of the signal light 11 is small and, accordingly, it is not possible 
to expand the dynamic range, that is, the range between the level 17 and 
the level 18 shown in FIG. 8. In addition, if the signal light 11 has been 
pulse-modulated, the extinction ratio is deteriorated by the ASE in the 
pass band of the optical filter 5. 
Accordingly, a principal object of the invention is to reduce the effect of 
ASE generated in the optical fiber amplifier 4 so as to expand the dynamic 
range of the output light, and, in the case of the signal light being 
pulse-modulated, deterioration of the extinction ratio is suppressed. 
Another object of the present invention is to provide an optical amplifier 
which amplifies the signal light 11 having a wavelength .pi. a 
simultaneously with another continuous light having a wavelength .pi. b in 
the optical fiber amplifier 4 and only the light having the wavelength 
.pi. a is selected by the optical filter 5, which achieves the 
above-mentioned object of the invention. 
Means to solve the problem. 
To achieve the objects, the present invention provides an optical fiber 
amplifier comprising an optical source for generating a signal light 
having a wavelength .pi. a, an optical source for generating a continuous 
light having a wavelength .pi. b, an optical coupler for coupling the 
signal light with the continuous light, an optical fiber amplifier for 
amplifying the output from the optical coupler, and a optical filter for 
extracting only the light having the wavelength .pi. a.

PREFERRED EMBODIMENT OF THE INVENTION 
A preferred embodiment of the present invention will now be explained in 
detail by making reference to the attached drawings. 
Referring to FIG. 1, an optical amplifier according to the present 
invention comprises an optical source 1 which generates a signal light 11 
having a wavelength .pi. a, an optical source 2 which generates a 
continuous light 12 having a wavelength .pi. b which is different from 
.pi. a, an optical coupler to couple or combine the signal light 11 with 
the continuous light 12, an optical fiber amplifier 4 which amplifies the 
output from the optical coupler 3, and an optical filter 5 for extracting 
only the light having a wavelength .pi. a. As seen from the comparison 
between FIG. 1 and FIG. 7, the optical amplifier according to the present 
invention is obtained by modifying the conventional amplifier of FIG. 7 by 
adding the light source 2 and the optical coupler 3. 
The optical source 1 is a laser optical source and the optical source 2 is 
a laser optical source which continuously outputs a continuous light 12 
having a wavelength .pi. b different from that of the signal light 11 
within the band width of the optical fiber amplifier 4. It should be noted 
that the light 12 from the optical source 2 is deemed continuous even if 
it is intensity-modulated with a cycle which is sufficiently shorter than 
the life of photons in the optical fiber amplifier 4. 
The optical coupler 3 couples the signal light 11 and continuous light 12. 
The optical fiber amplifier 4 amplifies the output of the coupler 13. The 
optical fiber amplifier 4 is described in the above-cited literature as 
well as in Tatsuya Takada "Optical Fiber Amplification Modules" OQE 90-80 
and Tetsuya Sakai et al "High Conversion 1.48 .mu.m Excited .EPSILON. 
.GAMMA.-Doped Optical Fiber Amplifier", 1991, Denshi Joho Tsushin Gakkai, 
Spring Meeting C-306. 
An example of the embodiment of the present invention will now be described 
in connection with FIG. 2. In this example, as the optical source 1 used 
is a laser diode of a wavelength of 1.554 .mu.m the optical source 2 used 
is a laser diode having a wavelength of 1.533 .mu.m, and as the optical 
coupler 3 used is an optical fiber coupler. 
Next, output spectra of the optical coupler 3 of FIG. 2 will be explained 
with reference to FIG. 3 in which the spectrum 21 is that from the optical 
source 2 and the spectrum 22 is from the optical source 1. 
Next, the spectra of the output of the optical fiber amplifier 4 in FIG. 2 
will be explained with reference to FIG. 4 in which the spectrum 21 is 
that from the optical source 2 and the spectrum 22 is from the optical 
source 1. The spectrum 23 is due to ASE. Amplification of the signal light 
11 in the optical fiber amplifier 4 gives rise to an output of the signal 
light 11 as well as the ASE. 
According to FIG. 11(a) of Masataka Nakazawa "Optical Fiber Amplification 
with Er-Doped Optical Fiber and Its Application "Applied Physics of 
Japan", Vol. 59, No. 9 (1990), with increase in the input signal light 
level the beat noise of ASE is reduced, which means that the power of the 
ASE per se is decreased with the increase in the input signal light level. 
The present invention reduces the ASE from the output of the optical fiber 
amplifier 4 by coupling the signal 11 of a wavelength .pi. a and the 
continuous light 12 of a wavelength .pi. b by the optical coupler 3 prior 
to inputting the signal light 11 into the optical fiber amplifier 4, 
thereby reducing the ASE from the output of the optical fiber amplifier 4. 
Then, the signal light 1 alone is outputted from the system after passing 
through the optical filter 5, with the result that a broader dynamic range 
is obtained or the extinction rate is suppressed when the signal light 1 
is pulse-modulated. 
FIG. 5 illustrates the output spectra of from the optical filter 5. By 
setting the pass band of the optical filter 5 near 1.554 .mu.m, only the 
spectrum 22 around the wave length 1.554 .mu.m can be taken out as shown. 
The optical filter 5 may be IFOS-1560AL(tradename) sold by Kogaku Giken 
K.K.in Japan. 
Next, the input/output characteristics of FIG. 2 is explained in reference 
to FIG. 6. The curve 24 shows the characteristic of the system in FIG. 2 
and the curve 25 shows that of FIG. 7. Comparison between these curves 24 
and 25 shows that the output light levels are almost the same toward the 
saturation at a greater light power input. Comparison between the ASE 
powers of the curve 24 and the curve 25 shows that the curve 24 has an ASE 
power less by about 10 dB than the curve 25. That is, the energy used for 
amplifying the natural emission light is used for amplification of the 
output light of the optical source 2. Accordingly, when the signal light 1 
is pulse-modulated, deterioration of the extinction rate can be suppressed 
by 10 dB corresponding to the reduction in the ASE power. The dotted line 
26 in FIG. 6 is the maximum output power level at which no distortion is 
observed. 
The level 17 is the minimum light receipt power of the conventional system 
and the level 27 is the minimum light receipt power of FIG. 2. When the 
minimum light receipt power is restricted by the beat noise between the 
signal light 11 and the ASE or beat noise among the spectral components of 
the ASE, the curve 24 of FIG. 6 has an improved minimum light power by 10 
dB or so lower than the curve 25. Accordingly, the dynamic range, i.e., 
the range from the minimum light receipt power up to the distortion area 
of the signal after amplification, is enlarged by about 10 dB. In other 
words, the dynamic range for the curve 24 is broader by 10 dB than for the 
curve 25. 
Advantage resulting from the present invention. 
According to the present invention, a signal light of a wave length .pi. a 
is coupled with a continuous light of a wave length .pi. b and amplified 
by an optical fiber amplifier and the amplified output light is subjected 
to filtration to select the light of the wave length .pi. a alone, so that 
the ASE power is reduced and the dynamic range of the optical fiber 
amplifier from the minimum light receipt power to the saturation after 
amplification can be enlarged, and, in the case that the signal light is 
pulse-modulated, the deterioration of the extinction ratio is minimized.