Optical transmitter system

An optical transmitter system includes a semiconductor laser, an external modulator, a detector and a frequency-modulation controller. The semiconductor laser transmits an optical signal of a predetermined wavelength onto an optical fiber and a backscattering wave caused by the optical signal is detected by the detector. The frequency-modulation controller controls the semiconductor laser so that the amplitude of a frequency-modulation signal applied to the semiconductor laser is varied depending on a detection signal of the backscattering wave.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention generally relates to an optical fiber transmission 
system, and in particular to an optical transmitter system coupled to 
optical fiber for use in the transmission system. 
2. Description of the Related Art 
Nonlinear effects in fiber may have a significant impact on the performance 
of optical fiber communication systems. One of the most significant 
effects may be caused by Stimulated Brillouin Scattering (SBS). When SBS 
occurs, the backscattering wave propagates in the opposite direction of 
the input light, resulting in attenuation of light at the receiving end 
which would not be compensated for by the input light increasing in power. 
Further, SBS may lead to distortion and noise, affecting the bit error 
rate at the receiver. 
It has been known that SBS occurs when the level of input light exceeds a 
certain threshold P.sub.TH which is proportional to (.DELTA.V.sub.SBS 
+.DELTA.V.sub.LD)/.DELTA.V.sub.SBS, where .DELTA.V.sub.SBS is Brillouin 
bandwidth and .DELTA.V.sub.LD is the spectral line width of signal light 
input into an optical fiber. In the case of a system employing an external 
modulator coupled to semiconductor diode laser operating at a wavelength 
of 1.55 .mu.m, the threshold P.sub.TH is on the order of several decibels 
(dB), which probably leads to SBS in optical fiber amplifier systems. 
To suppress SBS, the semiconductor laser is driven by a bias current on 
which a frequency-modulation signal is superimposed as an SBS suppression 
signal. Such a SBS suppression method has been proposed by Y. Aoki et al 
("Stimulated Brillouin Scattering Suppression in IM/DD Optical Fiber 
Communication System with Optical Booster Amplifiers", IEICE, Nov. 21, 
1991, pp75-80). By applying a frequency-modulation signal, chirping occurs 
in the output light of the semiconductor laser. Since chirping causes the 
spectral line width .DELTA.V.sub.LD to be broadened, the threshold level 
P.sub.TH becomes higher (see FIG. 3 of page 76 in the above paper by Y. 
Aoki et al.). That is, the larger the superimposed level of the 
frequency-modulation signal, or the SBS suppression signal, the higher the 
threshold level P.sub.TH at which SBS starts occurring. However, in the 
case of the frequency-modulation signal of great amplitude, the wave form 
of laser light is degraded, leading to a degradation of transmission 
performance. Therefore, it is important to adjust the modulation degree of 
SBS suppression signal. More specifically, the SBS suppression signal 
should be set to a minimum level at which SBS does not occur. 
Several disadvantages exist in the case of a wavelength-division 
multiplexing (WDM) system where a plurality of wavelength signals are 
combined by an optical coupler and then amplified by an optical fiber 
amplifier. The output light of the optical fiber amplifier includes 
variations in signal level from wavelength to wavelength because of 
variations in-output light level from semiconductor laser to semiconductor 
laser, variations in loss from port to port of the optical coupler, and 
the gain characteristic of the optical fiber amplifier. 
In conventional optical transmitters for use in WDM systems, the 
superimposed level of the SBS suppression signal is set for each 
semiconductor laser so as to be matched with the input light having the 
maximum power which is estimated from the above variations and 
characteristics. Therefore, the wave forms of laser signals other than the 
maximum power signal are degraded due to excessive superimposed level of 
the SBS suppression signal, leading to a degradation of transmission 
performance. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an optical transmitter 
system which can improve the transmission performance of an optical fiber 
communication system. 
Another object of the present invention is to provide an optical 
transmitter system which can optimally suppress a degradation of 
transmission optical signals. 
According to the present invention, a circuit is provided with an optical 
light source for transmitting an optical signal of a predetermined 
wavelength onto an optical fiber. A back-propagating wave caused by the 
optical signal is detected by a detector from the optical fiber and a 
detection signal corresponding to the back-propagating wave is produced. 
The light source is controlled to vary in a degree of frequency modulation 
depending on the detection signal. 
According to another aspect of the present invention, a circuit is provided 
with a plurality of optical transmitters each comprising a laser light 
source for producing a light wave of a different wavelength, an optical 
modulator for modulating the light wave according to input data to produce 
an optical signal, and a frequency-modulation driver for driving the laser 
light source in predetermined frequency modulation. The optical signals 
output from the optical transmitters are combined by an optical coupler to 
produce optical wavelength-division multiplexing (WDM) signals. The 
optical WDM signals are amplified by an optical fiber amplifier and are 
transmitted onto an optical fiber. A detector detects a back-propagating 
wave caused by each of the optical WDM signals from the optical fiber to 
produce a detection signal and a controller controls the 
frequency-modulation driver of an optical transmitter transmitting the 
optical signal to vary a degree of frequency modulation depending on the 
detection signal. 
Since the degree of frequency modulation is adjusted according to the 
detection signal of each back-propagating wave, the modulation degree of 
SBS suppression signal can be easily set to a minimum level at which SBS 
does not occur. Therefore, a degradation of transmission optical signals 
can be optimally suppressed, resulting in improved transmission 
performance of an optical fiber communication system. As will be described 
hereinafter, a frequency-modulation signal is also a chirp-generation 
signal for generating chirping of a laser light source or a spectral width 
control signal.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, there is an optical transmitter system for use in an 
optical fiber transmission system using WDM technology. The optical 
transmitter system includes a plurality of optical transmitters TX.sub.1 
-TX.sub.N which transmit optical signals S.sub.1 -S.sub.N with different 
wavelengths, respectively. The optical signals S.sub.1 -S.sub.N are 
combined into optical WDM signals by an optical coupler 101 such as fused 
coupler or waveguide combiner. The optical WDM signals are amplified by an 
optical fiber amplifier 102 such as rare-earth-element-doped fiber 
amplifier, typically Er-doped fiber amplifier (EDFA). The optical fiber 
amplifier 102 has a gain characteristic over a broad band including the 
wavelengths of the optical signals S.sub.1 -S.sub.N. 
The amplified optical WDM signals pass through a beam splitter 103 onto 
optical fiber 104. The beam splitter 103 splits a backscattering wave 
caused by SBS propagating through the optical fiber 104 in the opposite 
direction of the optical WDM signals. An optical circulator may be used in 
place of the beam splitter 103. A part of the backscattering wave is used 
for SBS suppression control. 
A part of the backscattering wave is transferred to a tunable optical 
bandpass filter 105 which is capable of tuning to N different wavelengths 
.lambda..sub.1 -.lambda..sub.N depending on sweep control signal as will 
be described later. The backscattering wave of a wavelength .lambda..sub.i 
passing through the tunable bandpass filter 105 is converted to a voltage 
signal V.sub.DET by a photo detector 106 which is composed of a photodiode 
PD and a current-to-voltage converter. 
A differential amplifier 107 inputs the voltage signal V.sub.DET from the 
photo detector 106 and produces a voltage difference V.sub.SBS (i) between 
the voltage signal V.sub.DET and a reference voltage V.sub.REF. When 
receiving the voltage difference V.sub.SBS (i) from the differential 
amplifier 107, a gain controller 108 generates a gain control signal 
S.sub.Gi based on the voltage difference V.sub.SBS (i) in cooperation with 
a BPF sweep controller 109 and outputs it to the corresponding optical 
transmitter TX.sub.1. The BPF sweep controller 109 outputs a sweep control 
signal to the tunable bandpass filter 105. More specifically, when the BPF 
sweep controller 109 sets the tunable bandpass filter 105 at a wavelength 
.lambda..sub.1, the gain controller 108 generates the gain control signal 
S.sub.Gi base on the voltage difference V.sub.SBS (i) and outputs it to 
the corresponding optical transmitter TX.sub.1. Therefore, the gain 
controller 108 sequentially controls the optical transmitter TX.sub.1 
-TX.sub.N depending on which wavelength is selected by the BPF sweep 
controller 109. 
The optical transmitters TX.sub.1 -TX.sub.N has the same circuit 
configuration. Each optical transmitter TX.sub.1 is provided with a 
semiconductor laser 201 operating at a different wavelength. The output 
light of the semiconductor laser 201 is intensity-modulated to produce an 
optical output signals S.sub.1 according to transmission data. The 
intensity modulation is carried out by an external modulator 202 which is 
a high-speed electro-optic on-off switch. For example, a semiconductor 
electro-absorption modulator or an LN modulator using lithium niobate 
(LiNb0.sub.3) can be used. The semiconductor laser 201 and the 
semiconductor electro-absorption modulator can be integrated 
The output light of the semiconductor laser 201 is kept constant by a 
constant output controller 203 controlling a bias current supplied to the 
semiconductor laser 201 while monitoring its output light. Further, an 
oscillator 204 generates a frequency-modulation signal S.sub.PN on the 
order of several kHz to several MHz. The frequency-modulation signal 
S.sub.FM is amplified by a variable-gain amplifier 205 whose gain is 
varied depending on the gain control signal S.sub.Gi received from the 
gain controller 108. The combiner 206 superimposes the 
frequency-modulation signal S.sub.FM on the bias current to produce a 
driving signal which is applied to the semiconductor laser 201. Since the 
driving signal includes frequency components on the order of several kHz 
to several MHz, chirping occurs in the semiconductor laser 201 and 
therefore the spectral line width .DELTA.V.sub.LD is caused to be 
broadened, resulting in reduced SBS. In other words, by adjusting the 
amplitude of the frequency-modulation signal S.sub.PM, or modulation 
degree, depending on the gain control signal S.sub.Gi, the amount of 
chirping occurring in the semiconductor laser 201 can be optimally 
controlled. 
FIG. 2 shows an illustrative waveform of a voltage signal V.sub.DET 
obtained when the tunable bandpass filter 105 sequentially tunes to 
wavelengths .lambda..sub.i-1, .lambda..sub.1, and .lambda..sub.i+1 and 
FIG. 3 shows the magnitude of gain control signals corresponding to the 
wavelengths .lambda..sub.i-1, .lambda..sub.1 and .lambda..sub.i+1, 
respectively. Here, it is assumed for simplicity that a voltage signal 
V.sub.DET obtained at wavelength .lambda..sub.i-1 is lower than the 
reference voltage V.sub.REF and voltage signals V.sub.DET obtained at 
wavelengths .lambda..sub.1 and .lambda..sub.i+1 are higher than the 
reference voltage V.sub.REF. 
When the tunable bandpass filter 105 tunes to the wavelength 
.lambda..sub.i-1, the backscattering wave of the wavelength 
.lambda..sub.i-1 can pass through the tunable bandpass filter 105 and then 
is converted into the voltage signal V.sub.DET by the photo detector 106. 
Since the voltage signal V.sub.DET obtained at wavelength .lambda..sub.i-1 
is lower than the reference voltage V.sub.REF, the gain controller 108 
does not vary the gain control signal S.sub.G(i-1) which is output to the 
variable-gain amplifier 205 of the corresponding optical transmitter 
TX.sub.i-1 as shown in a) of FIG. 3. 
When the tunable bandpass filter 105 tunes to the wavelength 
.lambda..sub.1, the backscattering wave of the wavelength .lambda..sub.1 
can pass through the tunable bandpass filter 105 and then is converted 
into the voltage signal V.sub.DET by the photo detector 106. Since the 
voltage signal V.sub.DET obtained at wavelength .lambda..sub.1 is higher 
than the reference voltage V.sub.REF, the gain controller 108 receives the 
voltage difference V.sub.SBS (i) from the differential amplifier 107 and 
varies the gain control signal S.sub.G(i) depending on the voltage 
difference V.sub.SBS (i) as shown in b) of FIG. 3. The gain control signal 
S.sub.G(i) is output to the variable-gain amplifier 205 of the 
corresponding optical transmitter TX.sub.i. The amplitude of the 
frequency-modulation signal S.sub.PM is varied according to the gain 
control signal S.sub.G(i) so as to eliminate the occurrence of SBS due to 
the output light signal S.sub.1. 
Similarly, when the tunable bandpass filter 105 tunes to the wavelength 
.lambda..sub.i+1, the backscattering wave of the wavelength 
.lambda..sub.i+1 can pass through the tunable bandpass filter 105 and then 
is converted into the voltage signal V.sub.DET by the photo detector 106. 
Since the voltage signal V.sub.DET obtained at wavelength .lambda..sub.i+1 
is higher than the reference voltage V.sub.REF, the gain controller 108 
receives the voltage difference V.sub.SBS (i+1) from the differential 
amplifier 107 and varies the gain control signal S.sub.G(i+1) depending on 
the voltage difference V.sub.SBS (i+1) as shown in c) of FIG. 3. The gain 
control signal S.sub.G(i+1) is output to the variable-gain amplifier 205 
of the corresponding optical transmitter TX.sub.i+1. The amplitude of the 
frequency-modulation signal S.sub.PM is varied according to the gain 
control signal S.sub.G(i+1) so as to eliminate the occurrence of SBS due 
to the output light signal S.sub.i+1. 
In this manner, the backscattering wave corresponding to each transmission 
signal of a predetermined wavelength is detected and the modulation degree 
of the corresponding semiconductor laser is controlled depending on the 
magnitude of the detected backscattering wave. Such a control is 
repeatedly performed for each sweeping to suppress SBS effectively and 
reliably in WDM systems. 
As an example, consider the case where a 1.55 .mu.m-band semiconductor 
electro-absorption modulator is used to perform modulation of 2.4 Gbps and 
the modulated light signal propagates over an optical fiber having a core 
diameter of 8 .mu.m. In this case, if the above SBS suppression system is 
not employed, SBS occurs when the level of the modulated light signal is 
increased to approximately +5 dBm or more. Contrarily, if the above SBS 
suppression system is employed to apply the gain-controlled 
frequency-modulation signal S.sub.PM to the semiconductor laser 201, the 
spectral line width of the modulated light signal is broadened to 150 MHz 
or more. Therefore, SBS can be suppressed until the level of the modulated 
light signal exceeds +12 dBm.