Optical transmitter

An optical transmitter operates on the DPSH-IM (direct phase-shift and self-homodyne intensity modulation) principle. The optical transmitter slightly amplitude-modulates the modulation current of a laser diode using a low frequency signal and detects the modulated component from the optical output. These features stabilize the operating point and improve the transmission waveform.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an optical transmitter for use with an 
optical fiber communication system. 
Recently, the so-called DPSH-IM (direct phase-shift and self-homodyne 
intensity modulation) has been proposed as a modulation method which is 
highly immune to the effect of wavelength dispersion and which dissipates 
low levels of modulation driving power compared with other methods. The 
DPSH-IM works as follows: Varying the injection current of a laser diode 
first modulates the oscillated light waves therefrom in phase. The 
phase-modulated light is then converted into intensity-modulated light by 
a self-homodyne arrangement. With the DPSH-IM, a Mach-Zehnder 
interferometer provides the self-homodyne capability. This requires 
stabilizing the operating point involved in order to prevent waveform 
distortion. Furthermore, to address the fluctuation in the frequency 
modulation efficiency of the laser diode requires controlling the driving 
amplitude involved. 
Meanwhile, optical transmitters for coherent light wave communication, to 
which the so-called CP-FSK is applied, also require control over the 
driving amplitude. Optical transmitters in which the light from a 
constantly driven laser diode is intensity-modulated by a Mach-Zehnder 
optical modulator require control over the operating point and driving 
amplitude. 
2. Description of the Related Art 
Under the DPSH-IM method, a laser diode fed with a bias current larger than 
a threshold current is supplied with modulation current pulses of a small 
amplitude for phase-modulating the oscillated light. The phase-modulated 
light is converted by passage through an optical interferometer into 
intensity-modulated light. The operating principle of the DPSH-IM is 
described in more detail in "Fibre Transmission Properties of Optical 
Pulses Produced Through Direct Phase Modulation of DFB Laser Diode" by 
Shirasaki M., Nishimoto H., Okiyama T and Touge T (ELECTRONICS LETTERS, 
14th Apr., 1988, Vol. 24, No. 8, pp. 486-488). One PCT application related 
to the DPSH-IM is PCT/JP89/00220. 
The DPSH-I permits small amplitude modulation of the laser diode under 
large bias currents. This makes it possible to construct a system highly 
immune to the adverse effects of chirping and operating on low driving 
voltages. 
In a DPSH-IM setup, slight changes in the oscillation frequency of the 
laser diode or in the delay time difference of the optical interferometer 
cause the optical signal from an optical interferometer to be distorted or 
inverted in polarity. The oscillation frequency of the laser diode and the 
delay time difference of the optical interferometer are known to vary 
depending on temperature and aging characteristics. Thus where an optical 
transmitter based on the DPSH-IM is used for practical purposes, it is 
necessary to control the oscillation frequency of the laser diode or the 
delay time difference of the optical interferometer in order to stabilize 
the operating point. Stabilization of the operating point is needed to 
prevent waveform distortion and polarity inversion in signals. A simple 
prior art description about how to stabilize the operating point is found 
in "Field Demonstration of FSK Transmission at 2.488 Gigabits/second over 
a 132 km Submarine Cable Using an Erbium Power Amplifier" by E. G. Bryant 
et al. (Topical Meeting on Optical Amplifiers and Their Applications, 
1990, pp. 152-155). According to the above publication, the operating 
point is stabilized apparently by having the bias current of a laser diode 
modulated with a small signal of 10 kHz and by having the output light 
from an optical interferometer monitored in order to control the bias 
current. 
The rate of fluctuation in the oscillation frequency (FM efficiency) with 
respect to the amplitude of the current for driving the laser diode is 
predictably varied depending on the temperature and aging characteristics 
of the laser diode. Thus, in order to prevent waveform distortion, it is 
necessary to ensure continuous control of the oscillation frequency based 
on the driving currents of an appropriate amplitude. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide an optical 
transmitter which operates on the DPSH-IM principle and which stabilizes 
the operating point to prevent waveform distortion and polarity inversion 
of signals. 
It is another object of the invention to provide an optical transmitter 
which operates on the DPSH-IM principle and which controls driving 
amplitude to prevent waveform distortion. 
It is a further object of the invention to provide an optical transmitter 
which is constructed for coherent light wave communication and which 
stabilizes driving amplitude. 
It is an even further object of the invention to provide an optical 
transmitter which comprises a constantly driven laser diode and a 
Mach-Zehnder optical modulator and thereby stabilizes the operating point 
and driving amplitude. 
In carrying out the invention and according to one aspect thereof, there is 
provided an optical transmitter comprising: a laser diode; a bias circuit 
for supplying a bias current to the laser diode; a driving circuit for 
supplying the laser diode with a modulation current in accordance with an 
input signal, the modulation current being supplied in such a manner that 
the integral value of the oscillation frequency of the laser diode, varied 
in one time slot by the modulation current, reaches a phase amount of one 
of k.pi. and -k.pi., "k" being at least 1; a Mach-Zehnder optical 
interferometer for converting angle-modulated light coming from the laser 
diode into intensity-modulated light, the interferometer having a delay 
time difference corresponding to 1/k of one time slot; an optical 
branching circuit for branching the signal light from the Mach-Zehnder 
interferometer; a first oscillator for slightly amplitude-modulating the 
modulation current using a first low frequency signal; a first amplitude 
detector for detecting the amplitude and polarity of the frequency 
component of the first low frequency signal contained in the signal light 
branched by the optical branching circuit; and an operating point control 
circuit for increasing or decreasing, depending on the polarity detected 
by the first amplitude detector, the phase difference given upon 
interference by multiplying the oscillation frequency by the delay time 
difference, whereby control is effected so that the amplitude detected by 
the first amplitude detector reaches zero. 
In a preferred structure according to the invention, the modulation current 
is amplitude-modulated using the first low frequency signal in such manner 
that in the waveform of the modulation current, the envelope on the space 
side and the envelope on the mark side are opposite to each other in phase 
and are the same in amplitude. 
In a further preferred structure according to the invention, the optical 
transmitter further comprises: a second oscillator for slightly modulating 
the bias current using a second low frequency signal having a frequency 
different from that of the first low frequency signal; a second amplitude 
detector for detecting the amplitude and polarity of the frequency 
component of the second low frequency signal contained in the signal light 
branched by the optical branching circuit; and a driving amplitude control 
circuit for increasing or decreasing the amplitude of the modulation 
current depending on the polarity detected by the second amplitude 
detector, whereby control is effected so that the amplitude detected by 
the second amplitude detector reaches zero. 
According to another aspect of the invention, there is provided an optical 
transmitter comprising: a laser diode; a bias circuit for supplying a bias 
current to the laser diode; a driving circuit for supplying the laser 
diode with a modulation current in accordance with an input signal, the 
modulation current being supplied in such a manner that the laser diode is 
frequency-modulated o phase-modulated; an optical branching circuit for 
branching the signal light from the laser diode; a Mach-Zehnder optical 
interferometer for converting the signal light branched by the optical 
branching circuit into intensity-modulated light; a first oscillator for 
slightly amplitude-modulating the modulation current using a first low 
frequency signal; a first amplitude detector for detecting the amplitude 
and polarity of the frequency component of the first low frequency signal 
contained in the intensity-modulated light from the Mach-Zehnder optical 
interferometer; an operating point control circuit for increasing or 
decreasing the bias current depending on the polarity detected by the 
first amplitude detector, whereby control is effected so that the 
amplitude detected by the first amplitude detector reaches zero; a second 
oscillator for slightly modulating the bias current using a second low 
frequency signal having a frequency different from that of the first low 
frequency signal; a second amplitude detector for detecting the amplitude 
and polarity of the frequency component of the second low frequency signal 
contained in th intensity-modulated light from the Mach-Zehnder optical 
interferometer; and a driving amplitude control circuit for increasing or 
decreasing the amplitude of the modulation current depending on the 
polarity detected by the second amplitude detector, whereby control is 
effected so that the amplitude detected by the second amplitude detector 
reaches zero. 
According to a further aspect of the invention, there is provided an 
optical transmitter comprising: a laser diode; a bias circuit for 
supplying a bias current to the laser diode; a Mach-Zehnder optical 
modulator for intensity-modulating the light from the laser diode; a 
driving circuit for supplying the Mach-Zehnder optical modulator with a 
modulation signal in accordance with an input signal; an optical branching 
circuit for branching the signal light from the Mach-Zehnder optical 
modulator; a first oscillator for slightly amplitude-modulating the 
modulation signal for the Mach-Zehnder optical modulator using a first low 
frequency signal; a first amplitude detector for detecting the amplitude 
and polarity of the frequency component of the first low frequency signal 
contained in the signal light branched by the optical branching circuit; 
an operating point control circuit for increasing or decreasing the bias 
voltage of the Mach-Zehnder optical modulator depending on the polarity 
detected by the first amplitude detector, whereby control is effected so 
that the amplitude detected by the first amplitude detector reaches zero; 
a second oscillator for slightly modulating the bias voltage of the 
Mach-Zehnder optical modulator using a second low frequency signal having 
a frequency different from that of the first low frequency signal; a 
second amplitude detector for detecting the amplitude and polarity of the 
frequency component of the second low frequency signal contained in the 
signal light branched by the optical branching circuit; and a driving 
amplitude control circuit for increasing or decreasing the amplitude of 
the modulation signal fed to the Mach-Zehnder optical modulator depending 
on the polarity detected by the second amplitude detector, whereby control 
is effected so that the amplitude detected by the second amplitude 
detector reaches zero. 
The invention modulates the amplitude of the modulation current supplied to 
the laser diode for operating point control. This is a feature that 
clearly separates the invention from the prior art espoused by Bryant et 
al., the latter modulating the bias current of the laser diode to 
stabilize the operating point. 
The above and other objects, features and advantages of the present 
invention and the manner of realizing them will become more apparent, and 
the invention itself will best be understood from a study of the following 
description and appended claims with reference to the attached drawings 
showing some preferred embodiments of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The preferred embodiments of the invention will now be described in detail 
with reference to the accompanying drawings. 
FIG. 1 is a block diagram of the optical transmitter practiced as the first 
embodiment of the invention. In FIG. 1, reference numeral 2 is a laser 
diode; 4 is a bias circuit that supplies a bias current to the laser diode 
2; 6 is a driving circuit that supplies the laser diode 2 with a 
modulation current (pulses) in accordance with an input signal; and 8 is a 
Mach-Zehnder (MZ) optical interferometer that converts the phase-modulated 
light coming from the laser diode 2 to intensity-modulated light. 
The bias current is set to be larger than the oscillation threshold value 
of the laser diode 2. The amplitude and pulse width of the modulation 
current are set in such a manner that the integral value of the 
oscillation frequency of the laser diode 2, varied in one time slot by the 
modulation current, reaches a phase amount of .pi. or -.pi.. 
As depicted in FIG. 2, the MZ optical interferometer 8 receives light 
through an input port 8A and gets the light branched into a first and a 
second optical path. Light portions passing through the first and the 
second optical paths are made to interfere with each other before being 
output via an output port 8B. The first optical path is traveled by the 
light that enters the input port 8A, is transmitted through half mirrors 
8C and 8F, in that order, and reaches the output port 8B. The second 
optical path is traveled by the light that enters the input port 8A, is 
reflected by the half mirror 8C, mirrors 8D and 8E and the half mirror 8F, 
in that order, and reaches the output port 8B. The propagation delay time 
difference between the first and the second optical paths is 
illustratively set for a time period corresponding to one time slot of the 
input signal. 
In FIG. 1, reference numeral 10 is an optical branching circuit that 
branches the signal light coming from the MZ optical interferometer 8. One 
of the branched light portions is sent over an optical transmission path, 
not show, and the other branched light portion is used for control over 
the operating point and driving amplitude. 
In a feedback loop for operating point control, reference numeral 12 is a 
first oscillator that slightly amplitude-modulates the modulation current 
using a first low frequency signal; 14 is a first amplitude detector that 
detects the amplitude and polarity of the frequency component of the first 
low frequency signal contained in the signal light; and 16 is an operating 
point control circuit that increases or decreases the bias current 
depending on the polarity detected by the first amplitude detector 14, 
whereby control is effected in such a manner that the amplitude detected 
by the first amplitude detector 14 reaches zero. 
In a feedback loop for driving amplitude control, reference numeral 18 is a 
second oscillator that slightly amplitude-modulates the bias current using 
a second low frequency signal having a frequency different from that of 
the first low frequency signal; 20 is a second amplitude detector that 
detects the amplitude and polarity of the frequency component of the 
second low frequency signal contained in the signal light; and 22 is a 
driving amplitude control circuit that increases or decreases the 
amplitude of the modulation current depending on the polarity detected by 
the second amplitude detector 20, whereby control is effected in such a 
manner that the amplitude detected by the second amplitude detector 20 
reaches zero. The period of the first low frequency signal and that of the 
second are each set for a time period sufficiently longer than one time 
slot of the modulation current. 
Described below is the operating principle of the DPSH-IM with reference to 
FIGS. 3A through 3D. 
The output of the MZ optical interferometer depends on the phase difference 
between the light waves that occur when the light portion from the first 
optical path interferes with that from the second. If the phase difference 
were fixed to .pi. (or (2n +1).pi., n being an integer), no power would be 
output. When the phase of the optical waves entering the MZ optical 
interferometer 8 varies within one time slot, the output intensity also 
varies. 
The oscillation frequency of the laser diode 2 varies in keeping with the 
injection current applied thereto. Thus the waveform of the modulation 
current pulses is reflected in the frequency change of the light waves 
from the laser diode 2. FIG. 3A is a waveform chart illustrating the 
frequency transition in the form of an RZ type waveform. The RZ type 
simply means that the signal level returns to zero in each time slot of 
modulation; the duty ratio may be arbitrary. 
The light wave phase is an integral of frequency with time. Thus where the 
amplitude of the modulation current pulses is appropriately determined, 
the phase change in a time slot corresponding to a signal "1" is .pi., as 
shown in FIG. 3B. The MZ optical interferometer 8 causes this light to 
interfere in the same amplitude with the other portion of light delayed by 
one time slot. The phase of the light delayed by one time slot is 
represented by broken lines in FIG. 3B. In the above setup, the initial 
phase of the delayed light is set to zero or to .pi. (in FIG. 3B, the 
initial phase is set to .pi.). 
The output intensity of the MZ optical interferometer 8 is determined by 
the relative phase difference between the two interfering portions of 
light. In FIG. 3B, the relative phase difference is the difference between 
solid and broken lines. The phase difference thus turns out to be as 
illustrated in FIG. 3C, varying between zero and .pi. in accordance with 
the input signal. As a result of the interference, the optical output is 
maximized when the phase difference is zero; the optical output is zero 
when the phase difference is .pi.. Thus the intensity waveform of the 
optical output is as depicted in FIG. 3D. 
While the output waveform varies in keeping with the input signal, there 
exists a time delay corresponding to a half time slot between the output 
and input signals. The output waveform is an NRZ type regardless of the 
duty ratio of the RZ input signal. The duty ratio of the input signal 
determines the rise and fall times for the output waveform. 
FIGS. 4A through 4C serve to explain how the operating point is controlled. 
In these figures, the ordinate represents optical power levels, and the 
abscissa denotes th product of the oscillation frequency (fo) of the laser 
diode 2 and the delay time difference between the first and the second 
optical paths (.DELTA..tau.=.tau..sub.2 -.tau..sub.1) in the MZ optical 
interferometer 8. FIG. 4A shows a case in which the operating point is 
appropriate; FIG. 4B is a case in which the operating point has drifted in 
the negative direction; and FIG. 4C is a case wherein the operating point 
has drifted in the positive direction. 
Where the DPSH-IM is applied, the optical power responds periodically to 
the product (f.sub.o..DELTA..tau.). Specifically, the response takes place 
as follows: 
If it is assumed that the electric field of the light entering the MZ 
optical interferometer 8 is given as 
EQU E.sub.in (t)=E.sub.o COS(2.pi. f.sub.o t) 
that the electric field of the light output by the MZ optical 
interferometer 8 is 
EQU E.sub.out (t) 
and that the power of the light output by the MZ optical interferometer is 
EQU S.sub.out 
Then one gets 
##EQU1## 
As indicated, the optical output power S.sub.out responds periodically in 
the form of a sine wave curve to the product of the oscillation frequency 
f.sub.o and the delay time difference .DELTA..pi.. 
FIGS. 4A through 4C show that the amplitude of high speed modulation 
current pulses is modulated using the first low frequency signal 
(frequency: f.sub.1) so that the envelope on the space side is 
symmetrically positioned relative to the envelope on the mark side. In 
other words, amplitude modulation is carried out in such a manner that in 
the modulation current waveform, the envelope on the space side and that 
on the mark side are opposite to each other in phase and have the same 
amplitude. In this setup, a drift of the operating point causes the low 
frequency component having the frequency f.sub.1 to emerge in the optical 
output waveform. The polarity of the low frequency component is determined 
by the direction of drift of the operating point. If the operating point 
is optimum, the low frequency component is zero. Thus an optimum operating 
point is maintained by first extracting the low frequency component from 
the output signal light, and by controlling the oscillation frequency of 
the laser diode and/or the delay time difference of the MZ optical 
interferometer so as to bring the amplitude of the low frequency component 
to zero. The control direction is known from the polarity of the low 
frequency component extracted. 
The oscillation frequency of the laser diode is varied using parameters on 
which that frequency depends, such as the bias current supplied to the 
laser diode or the temperature of that diode. The delay time difference of 
the MZ optical interferometer is varied illustratively in two ways: by 
applying asymmetrical temperature changes to the first and the second 
optical paths of the interferometer; or by forming the first and the 
second optical paths as branch wave guides on an electro-optical crystal 
substrate and applying asymmetrical electric fields to these branch wave 
guides. 
FIGS. 5A through 5C are used to illustrate how driving amplitude is 
controlled. The axes of ordinate and abscissa along which operation 
characteristic curves are given in FIGS. 5A through 5C are the same as 
those in FIGS. 4A through 4C. FIG. 5A represents a case wherein the 
driving amplitude is smaller than optimum; FIG. 5B is a case in which the 
driving amplitude is optimum; and FIG. 5C is a case where the driving 
amplitude is greater than optimum. When the bias current of the laser 
diode 2 is modulated using the second low frequency signal (frequency: 
f.sub.2), a drift of the driving amplitude from its optimum value causes 
the low frequency component having the frequency f.sub.2 (not the same as 
f.sub.1) to appear in the optical output waveform. If the driving 
amplitude is too large or too small, the polarity of the detected low 
frequency component is inverted. Where the driving amplitude is optimum, 
no low frequency component is detected. Thus if the low frequency 
component contained in the signal light is extracted therefrom, and the 
driving amplitude is controlled so as to bring the component to zero, then 
the driving amplitude is maintained at its optimum value. As a result, 
waveform distortion and other adverse effects are prevented. The direction 
of control is known from the polarity of the extracted low frequency 
component. 
FIG. 6 is a more detailed block diagram of the optical transmitter shown in 
FIG. 1. The laser diode 2, bias circuit 4, driving circuit 6, MZ optical 
interferometer 8, optical branching circuit 10, first oscillator 12 and 
second oscillator 18 in FIG. 6 are the same in terms of connection and 
function as those discussed with reference to FIG. 1, and any repetitive 
description thereof will be omitted. 
Reference numeral 24 is a temperature stabilizing circuit containing a 
Peltier device or the like. The temperature stabilizing circuit 24 keeps 
the temperature of the laser diode 2 constant. An optical isolator 26 
prevents reflected feedback light from entering the laser diode 2. An 
optical amplifier 28, located upstream of the optical branching circuit 
10, has its gain controlled by a gain control circuit 30. A photodetector 
32 detects the intensity of the light branched by the optical branching 
circuit 10. The detected signals by the photodetector 32 are forwarded to 
an amplifier 34 and to the gain control circuit 30. The gain control 
circuit 30 controls the gain of the optical amplifier 28 in such a way 
that the level of light reception by the photodetector 32 is kept 
constant. An optical fiber amplifier or a laser diode type optical 
amplifier may be used as the optical amplifier 28. 
The output of the amplifier 34 is input to band-pass filters 36 and 38. The 
band-pass filter 36 allows the frequency component having the frequency 
f.sub.1 to pass, and the band-pass filter 38 lets the frequency component 
having the frequency f2 pass therethrough. A multiplier 40 multiplies the 
output of the band-pass filter 36 by the low frequency signal coming from 
the first oscillator 12. The polarity (positive or negative) of the output 
from the multiplier 40 represents the direction of drift of the operating 
point. The absolute value of the output level of the multiplier 40 is 
approximately proportionate to the amount of drift of the operating point. 
The output of the multiplier 40 is input to the negative side input port 
of an operation amplifier 44 via a resistance 42. The positive side input 
port of the operation amplifier 44 is connected to ground, A control 
signal from the operation amplifier 44 is input to the bias circuit 4. A 
resistance 46 and a capacitor 48 are connected in parallel between the 
negative side input and output ports of the operation amplifier 44. The 
capacitor 48 determines the time constant of the feedback loop formed by 
the operating point control circuit. 
Meanwhile, the signal that has passed through the band-pass filter 38 is 
multiplied in a multiplier 50 by the low frequency signal from the second 
oscillator 18, The output of the multiplier 50 is input to the negative 
side input port of an operation amplifier 54 via a resistance 52. The 
polarity (positive or negative) of the output from the multiplier 0 
indicates whether the driving amplitude is too large or too small. The 
absolute value of the level of output from the multiplier 50 is 
appropriately proportionate to the amount of drift from the optimum 
driving amplitude. A control signal from the operation amplifier 54 is 
input to the driving circuit 6. The positive side input port of the 
operation amplifier 54 is connected to ground. A resistance 56 and a 
capacitor 58 are connected in parallel between the negative side input and 
output ports of the operation amplifier 54. The capacitor 58 determines 
the time constant of the feedback loop formed by the driving amplitude 
control circuit. 
In the first embodiment described, the photodetector 32, amplifier 34, 
band-pass filter 36 and multiplier 40 perform the function of the first 
amplitude detector 14; the operation amplifier 44 and the circuits 
associated therewith perform the function of the operating point control 
circuit 16; the photodetector 32, amplifier 34, band-pass filter 38 and 
multiplier 50 perform the function of the second amplitude detector 20; 
and the operation amplifier 54 and the circuits associated therewith 
perform the function of the driving amplitude control circuit 22. 
Driving amplitude control presupposes that the operating point is 
controlled relative to an optimum value. Thus the time constant of the 
feedback loop formed by the operating point control circuit should 
preferably be set to be smaller than the time constant of the feedback 
loop by the driving amplitude control circuit. 
FIG. 7 is a circuit diagram of the bias circuit 4. In FIG. 7, reference 
numeral 60 is an input port that receives the control signal from the 
operation amplifier 44; 62 is an input port that receives the second low 
frequency signal; and 64 is an output port that outputs the bias current. 
The bias current is supplied to the laser diode via an inductor 66. The 
output port 64 is connected to the collector of a transistor 68. The 
emitter of the transistor 68 is connected to a power input port 72 via a 
resistance 70. The second low frequency signal that has entered the input 
port 62 is input to the base of the transistor 68 via a capacitor 74. The 
control signal that has entered the input port 60 is input to the base of 
the transistor 68 via a resistance 76. A resistance 78 is interposed 
between the base of the transistor 68 and the power input port 72. The 
base of the transistor 68 is connected to ground via a resistance 80. 
FIG. 8 is a circuit diagram of the driving circuit 6. In FIG. 8, reference 
numeral 82 is a signal input port; 84 is an input port that receives the 
first low frequency signal; 86 is an input port that receives the control 
signal from the operation amplifier 54; and 88 is a output port that 
outputs the modulation current. A signal that has entered the signal input 
port 82 is input to the gate of an FET 90. The gate of an FET 92 receives 
a reference voltage from a power input port 94. A current source 96 is 
positioned between the source of the FET 90, the source of the FET 92, and 
a power input port 97. The first low frequency signal that has entered the 
input port 84 is input to the current source 96 via a capacitor 98. The 
control signal that has entered the input port 86 is input to the current 
source 96 via a resistance 100. The drain of the FET 90 is connected to 
ground via a resistance 102. The drain of the FET 92 is connected to a 
power input port 104 via an inductor 106. 
The modulation current output by the output port 88 is fed to the laser 
diode 2 via a decoupling capacitor 108. The first function of the 
decoupling capacitor 108 is to disconnect the bias current directed to the 
driving circuit 6. The second function of the decoupling capacitor 108 is 
to amplitude-modulate the modulation current using the first low frequency 
signal in such a manner that the space side envelope and mark side 
envelope of the modulation current will become symmetrical. That is, the 
decoupling capacitor 108 is used to remove the frequency component of the 
first low frequency signal from the asymmetrically amplitude-modulated 
modulation current. This causes the space side and mark side envelopes in 
the modulation current waveform to become opposite to each other in phase 
and to have the same amplitude. 
In the setup of FIG. 6, the optical amplifier 28 is located upstream of the 
optical branching circuit 10, wherein the controlled gain is used to 
amplify the signal light. This provides not only the benefit of high 
output but also the advantages. One such advantage is this: With 
conventional bias control in which the bias current of the laser diode is 
generally controlled so as to maintain a constant optical output level, an 
optimum operating point cannot be obtained by the DPSH-IM method because 
the oscillation frequency of the laser diode varies in keeping with 
changes in the bias current. By contrast, the invention maintains an 
optimum operating point and ensures a constant optical output level 
illustratively by carrying out APC through the gain of the optical 
amplifier. 
FIG. 9A is a block diagram of another optical transmitter practiced as the 
second embodiment of the invention. What differentiates the second 
embodiment from the first is that the second embodiment adopts, as the 
target of control by the operating point control circuit 16 (16'), the 
delay time difference of the MZ optical interferometer 8 in place of the 
bias current of the laser diode. In this setup, the delay time difference 
of the optical interferometer may be varied illustratively in three ways: 
by utilizing the electro-optical effect, by relying on dynamics involving 
a piezoelectric device, or by using thermal expansion with a Peltier 
device. 
FIG. 9B is a more detailed block diagram of the optical transmitter shown 
in FIG. 9A. FIG. 9B corresponds with FIG. 6 of the first embodiment with 
respect to all circuit elements and the interconnection thereof with the 
exception of the control signal from the operation amplifier 44. In FIG. 6 
of the first embodiment, the control signal from the operation amplifier 
44 was input to the bias circuit 4. However, in FIG. 9B of the second 
embodiment, the control signal from the operation amplifier 44 is input to 
the Mach-Zehnder optical interferometer 8. A repetitive description of 
these circuit elements and interconnection thereof which does correspond 
with FIG. 6 will be omitted. 
In this manner, the second embodiment optimally controls the operating 
point and amplitude in an optical transmitter operating on the DPSH-IM 
principle. Although neither the optical amplifier nor the APC control loop 
is shown, these components may be included in the second embodiment as in 
the first. In the first and the second embodiments, the optical amplifier 
may be positioned upstream of the MZ optical interferometer 8. 
Below is a description of how this invention may be applied to coherent 
light wave communication. FIG. 10 is a block diagram of another optical 
transmitter practiced as the third embodiment for coherent light wave 
communication. The third embodiment comprises a laser diode 110; a bias 
circuit 112 that feeds a bias current to the laser diode 110; a driving 
circuit 114 that supplies the laser diode 110 with a modulation current as 
per an input signal in order to frequency- or phase-modulate that diode; 
an optical branching circuit 116 that branches the signal light from the 
laser diode 110; an MZ optical interferometer 118 that converts the signal 
light branched by the optical branching circuit 116 into 
intensity-modulated light; a first oscillator 120 that slightly 
amplitude-modulates the modulation current using a first low frequency 
signal; a first amplitude detector 122 that detects the amplitude and 
polarity of the frequency component of the first low frequency signal 
contained in the intensity-modulated light from the MZ optical 
interferometer 118; an operating point control circuit 124 that increases 
or decreases the bias current depending o the polarity detected by the 
first amplitude detector 122, whereby control is effected in such a manner 
that the amplitude detected by the first amplitude detector 122 reaches 
zero; a second oscillator 126 that slightly modulates the bias current 
using a second low frequency signal having a frequency different from that 
of the first low frequency signal; a second amplitude detector 128 that 
detects the amplitude and polarity of the frequency component of the 
second low frequency signal contained in the intensity-modulated light 
from the MZ optical interferometer 118; and a driving amplitude control 
circuit 130 that increases or decreases the amplitude of the modulation 
current depending on the polarity detected by the second amplitude 
detector 128, whereby control is effected in such a manner that the 
amplitude detected by the second amplitude detector 128 reaches zero. 
The method of modulation applicable to the driving circuit 114 may be the 
CPFSK, DPSK, or equivalent. As with the first and the second embodiments, 
the third embodiment may have an optical amplifier located upstream or 
downstream of the optical branching circuit 116 to form an APC loop. 
The construction of the third embodiment stabilizes the driving amplitude 
of the optical transmitter for coherent light wave communication. As a 
result, the optical transmitter keeps the modulation factor constant for 
frequency or phase modulation. In the third embodiment, both the operating 
point and the driving amplitude are controlled. This is because making the 
feedback loop for driving amplitude control properly function presupposes 
stabilization of the operating point. 
FIG. 11 is a block diagram of yet another optical transmitter practiced as 
the fourth embodiment of the invention. This optical transmitter comprises 
a laser diode 132; a bias circuit 134 that feeds a bias current to the 
laser diode 132; a Mach-Zehnder (MZ) optical modulator 136 that 
intensity-modulates the light from the laser diode 132; a driving circuit 
138 that supplies the MZ optical modulator with a modulation signal as per 
an input signal; an optical branching circuit 140 that branches the signal 
light from the MZ optical modulator 136; a first oscillator 142 that 
slightly amplitude-modulates the modulation signal of the MZ optical 
modulator using a first low frequency signal; a first amplitude detector 
144 that detects the amplitude and polarity of the frequency component of 
the first low frequency signal contained in the signal light branched by 
the optical branching circuit 140; an operating point control circuit 146 
that increases or decreases the bias voltage of the MZ optical modulator 
136 depending on the polarity detected by the first amplitude detector 
144, whereby control is effected in such a manner that the amplitude 
detected by the first amplitude detector 144 reaches zero; a second 
oscillator 148 that slightly modulates the bias voltage of the MZ optical 
modulator 136 using a second low frequency signal having a frequency 
different from that of the first low frequency signal; a second amplitude 
detector 150 that detects the amplitude and polarity of the frequency 
component of the second low frequency signal contained in the signal light 
branched by the optical branching circuit 140; and a driving amplitude 
control circuit 152 that increases or decreases the amplitude of the 
modulation signal fed to the MZ optical modulator depending on the 
polarity detected by the second amplitude detector 150, whereby control is 
effected in such a manner that the amplitude detected by the second 
amplitude detector 150 reaches zero. 
In the fourth embodiment, the optical output responds periodically to the 
bias voltage of the MZ optical modulator 136. This is what differentiates 
the fourth embodiment from the DPSH-IM type optical transmitter in which 
the optical output responds periodically to the product of the oscillation 
frequency of the laser diode and the delay time difference of the MZ 
optical interferometer. In the fourth embodiment, the object of control by 
the operating point control circuit 146 is th bias voltage of the MZ 
optical modulator 136, and the object of control by the driving amplitude 
control circuit 152 is the amplitude of the modulation signal to the MZ 
optical modulator 136. These features derive from the above-mentioned 
differentiating point between the fourth embodiment and the other 
embodiments. Where an APC loop is added to the fourth embodiment, the 
target of control thereby may be the bias current of the laser diode 132. 
Furthermore, the ability of the fourth embodiment to control the operating 
point and driving amplitude in an optimum manner effectively prevents 
transmission waveform deterioration. 
Although the description above contains many specificities, these should 
not be construed as limiting the scope of the invention but as merely 
providing illustrations of some of the presently preferred embodiments of 
this invention. For example, the optical transmitter of FIG. 1 and that of 
FIG. 6 are each provided with both the feedback loop for operating point 
control and the feedback loop for driving amplitude control. An 
alternative to this arrangement is to omit the feedback loop for driving 
amplitude control if there is no possibility that the frequency modulation 
efficiency of the laser diode would vary. 
Thus the scope of the invention should be determined by the appended claims 
and their legal equivalents, rather than by the examples given.