Laser light generator

A laser light generator includes an adder 12 which adds an optical frequency modulation signal Sm onto a d.c. voltage for driving a laser diode 10 for continuous laser oscillation, and applies the sum signal to the laser diode 10 to drive it. An light output from the laser diode 10 is introduced to an optical intensity controller 14 that is controlled by a control signal prepared by adjusting the optical frequency modulation signal Sm both in phase and in amplitude by a phase adjusting circuit 16 and an amplitude adjusting circuit 18. The optical intensity controller 14 may be an electroabsorption modulator that changes its transmissivity in response to an output voltage of the circuit 18. Quantities of adjustment by the circuits 16 and 18 are determined so that fluctuation in transmissivity of the optical intensity controller 14 suppresses intensity fluctuation of the light output of the laser diode 10.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a laser light generator and, more particularly, 
to a device for generating an optical-frequency-modulated laser. 
2. Related Art 
In optical fiber transmission systems, more particularly in repeaterless 
transmission systems, it is desired to increase optical input power into 
optical fibers in order to increase the transmission distance (or the 
repeater distance in repeating transmission systems). However, high-power 
laser light increases deterioration in transmission characteristics of 
such systems due to stimulated Brillouin scattering (SBS). This disturbs a 
sufficient increase in input power to optical fibers. The threshold value 
of the input power that can prevent an adverse effect of stimulated 
Brillouin scattering in typical optical fibers is considered around 5 mW. 
There is known that the threshold value can be increased by 
frequency-modulating a laser light (carrier wave) in the tone of a single 
frequency, for example. The simplest way is to slightly modulate the 
current injected into a semiconductor laser diode, which results in 
changing the refractive index in the resonance cavity of the semiconductor 
laser diode, and in changing the frequency of oscillated light in the 
semiconductor laser diode in response to a modulation signal contained in 
the injected current. If a large change in optical frequency is desired, 
the amplitude of the modulation signal superposed to the injected current 
must be increased. 
This approach relying on fine modulation of injected current to the 
semiconductor laser diode invites intensity modulation in addition to 
optical frequency modulation, and the intensity-modulated components 
degrade transmission characteristics of the optical transmission system 
(directly, increase the transmission loss and the error rate of the 
system). It is taught that optical intensity-modulated components of 
approximately 42% are produced when fine modulation of injected current to 
a semiconductor laser diode is effected to obtain an optical spectral 
width of approximately 8 GHz (L. Eskildsen, et al., "Residual Amplitude 
Modulation Suppression Using Deeply Saturated Erbium-doped Fiber 
Amplifiers", IEEE Photon. Technol. Lett., vol.7, No.12, pp. 1516-1518, 
1995). The eye pattern deteriorates as shown in FIG. 4(a) of this 
Literature. 
The Literature also teaches a technology for suppressing such 
intensity-modulated components by using response characteristics of 
erbium-doped optical fiber amplifiers. That is, by effecting small 
modulation at frequencies of 5 kHz to 10 kHz while a DFB laser diode is 
driven for continuous laser oscillation, its output light is entered into 
a deeply saturated erbium-doped optical fiber amplifier. The erbium-doped 
optical fiber amplifier has a 3 dB cut-off frequency as high as 25 kHz. 
Intensity-modulated components in the range of 5 kHz to 10 kHz are 
suppressed by a slope portion of response characteristics of the 
erbium-doped optical fiber amplifier. Here, the amount of shift in optical 
frequency is approximately 8 GHz, and the optical power after optical 
amplification is approximately 200 mW. According to the Literature, 
transmission penalties of approximately 3.5 dB without suppression of 
intensity-modulated components could be improved to 0.2 thorough 1.0 dB by 
suppression of intensity components. 
However, the structure taught by the Literature results in restricting the 
modulation frequency of the laser diode below 10 kHz, or below 25 kHz at 
maximum. Higher frequencies are considered preferable to reliably 
compensate degradation of transmission characteristics of optical 
transmission systems caused by stimulated Brillouin scattering, and the 
technology taught by the Literature cannot cope with such demands. 
Moreover, the same prior art needs excitation optical sources as many as 
six pumps (PUMP 1 through PUMP 6 in FIG. 1 of the Literature) because the 
erbium-doped optical fiber amplifiers are used in substantially saturated 
conditions. This is uneconomical because the saturated optical output 
powers used there are as high as 200 mW or more that are not required in 
many typical cases. 
OBJECT AND SUMMARY OF THE INVENTION 
It is therefore an object of the invention to provide a laser light 
generator for generating a laser with no or less fluctuation in optical 
intensity. 
A further object of the invention is to provide a laser light generator for 
generating a data-modulated optical signal with reduced fluctuation in 
optical intensity for use in optical communication or measurement. 
According to the invention, the light output from a laser source which is 
modulated both in optical frequency and in intensity by an optical 
frequency modulation signal Sm, is introduced to an optical intensity 
fluctuation suppressing means using electroabsorption optical modulating 
means or optical amplifying means. The gain or attenuation factor of the 
optical intensity fluctuation suppressing means is controlled 
substantially by the optical frequency modulation signal Sm to suppress 
intensity fluctuation of the light output from the laser source. In this 
manner, intensity fluctuation in the light output from the laser source 
can be suppressed efficiently, which results in producing optical carrier 
waves that are modulated in optical frequency and include no or less 
fluctuation in optical intensity. The light output signal thus obtained is 
useful for communication and for measurement. 
The optical intensity fluctuation suppressing means may comprise 
phase/amplitude adjusting means and optical intensity control means. Then, 
by adjusting the optical frequency modulation signal Sm at least in phase 
or in amplitude and by controlling the intensity of the light output from 
the laser source in accordance with a signal resulting from the 
adjustment, fluctuation in optical intensity can be suppressed more 
accurately. 
The optical intensity fluctuation suppressing means may further include 
residual fluctuation detecting means and correlation detecting means so as 
to detect residual fluctuation after the intensity fluctuation suppression 
and to detect correlation with the optical frequency modulation signal Sm 
such that the amount of amplitude adjustment by the phase/amplitude 
adjusting means be controlled in accordance with the correlation rate 
obtained. Then, residual fluctuation after the intensity fluctuation 
suppression can be automatically minimized even under fluctuation in 
amplitude of the optical frequency modulation signal Sm, for example. 
The optical intensity control means may be optical amplifier means 
controllable in amplification gain by an external control signal or light 
transmission means controllable in attenuation factor by an external 
control signal. More specifically, the optical intensity control means may 
be appropriate one of an electroabsorption type optical modulator, 
Mach-Zehnder interferometer-type modulator, optical filter, optical 
attenuator, semiconductor laser amplifier, optical fiber amplifier, and so 
forth. 
According to another aspect of the invention, the laser source is adjusted 
in intensity by using a composite signal which is obtained by multiplying 
or adding a signal obtained from an optical frequency modulation signal Sm 
for optical frequency-modulating the laser source by or to a data signal 
Sd. Then, an optical carrier wave modulated in optical frequency but 
having less fluctuation in optical intensity can be obtained. At the same 
time, since a common element modulates it by the data signal Sd, a 
qualified optical signal can be obtained. Since a common optical 
modulating means may be used both for suppression of intensity fluctuation 
and for modulation by the data signal Sd, the device is economical.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Embodiments of the invention are described below in detail with reference 
to the drawings. 
FIG. 1 is a block diagram of a general construction of a device embodying 
the invention, and FIG. 2 shows waveforms at different portions in the 
device. Numeral 10 denotes a semiconductor laser diode. An adder 12 
superposes a small optical frequency modulation signal (sinusoidal a.c. 
signal) Sm to a predetermined d.c. bias for continuously laser oscillating 
the laser diode 10, and applies the result to the laser diode 10 as the 
dive current. The frequency of the optical frequency modulation signal Sm 
ranges from d.c.or several hertz to several megahertz when it is used for 
measurement, but ranges from several kilohertz to several megahertz when 
it is used for SBS suppression. FIG. 2(a) shows the waveform of a drive 
current for the laser diode 10. The laser diode 10 is driven by the drive 
current to generate a laser light modulated both in intensity and in 
optical frequency. FIG. 2(b) shows the waveform of the intensity 
fluctuation of the light output from the laser diode 10, and FIG. 2(c) 
shows the optical frequency fluctuation waveform of the light output from 
the laser diode 10. 
The light output from the laser diode 10 is applied to an optical intensity 
controller 14. The optical intensity controller 14 is an optical element 
whose input light intensity is electrically externally controllable, 
namely, an optical element such as semiconductor laser amplifier or 
optical fiber amplifier whose gain can be electrically controlled 
externally, or an optical element such as electroabsorption optical 
modulator, Mach-Zehnder interferometer-type modulator or optical 
attenuator whose attenuation factor can be electrically controlled 
externally. In case of an optical fiber amplifier, its gain of 
amplification is controlled by adjusting the amount of pump light. If an 
optical filter is used, its attenuation factor may be controlled by using 
a slope portion of its transmissivity wavelength characteristics and by 
externally changing the center wavelength. This embodiment uses an 
electroabsorption optical modulator as the optical intensity controller 14 
because it copes with wider frequencies and is commonly usable for 
modulation by a data signal Sd as explained later. FIG. 3 shows 
characteristics of the electroabsorption optical modulator, taking the 
voltage applied to the cathode (control voltage) on the horizontal axis 
and the transmissivity (logarithm) on the vertical axis. As shown in FIG. 
3, the transmissivity of the electroabsorption optical modulator 
exponentially varies with control voltage. 
A phase adjusting circuit 16 adjusts the phase of the optical frequency 
modulation signal Sm and applies it to an amplitude adjusting circuit 18. 
The phase adjusting circuit 16 is a so-called delay circuit interposed to 
match the intensity fluctuation of the light output of the laser diode 10 
in phase with effects of intensity fluctuation suppression by the optical 
intensity controller 14. The amplitude adjusting circuit 18 adjusts the 
amplitude of the output from the phase adjusting circuit 16 (and inverts 
the phase, if necessary), simultaneously adds a predetermined bias, and 
then applies the resulting signal as a control signal to the optical 
intensity controller 14. 
Due to the output signal from the amplitude adjusting circuit 18, the 
transmissivity of the optical intensity controller 14 fluctuates 
synchronously with the optical frequency modulation signal Sm with an 
amplitude responsive to the output voltage of the amplitude adjusting 
circuit 18 so as to cancel the intensity fluctuation of the output laser 
light of the laser diode 10 as shown in FIG. 2(d). In other words, the 
amplitude adjusting circuit 18 adjusts the amplitude of the output signal 
from the phase adjusting circuit 16 such that the transmission 
characteristics of the optical intensity controller 14 varies to cancel 
the intensity fluctuation of the output laser light from the laser diode 
10. 
FIG. 2(e) shows intensity fluctuation of the light output from the optical 
intensity controller 14. The optical intensity controller 14 reduces the 
intensity fluctuation components of the light output from the laser diode 
10 to substantially zero, and the intensity of light output from the 
optical intensity controller 14 becomes substantially constant with time 
as shown in FIG. 2(e). 
An light intensity modulator 20 ON/OFF-modulates the light output from the 
optical intensity controller 14 in response to the communication data 
signal Sd. The frequency of the communication data signal Sd is hundreds 
of megahertz or more. Although the data signal Sd is intended for use in 
communication, it can of course be a data signal for measurement, as well, 
when it is used for measurement of transmission characteristics of optical 
transmission systems, etc. The optical intensity modulator 20 is, for 
example, an electroabsorption modulator. The light output from the optical 
intensity modulator 20 has the waveform as shown in FIG. 2(f). 
Next explained is an embodiment using a single optical modulator both for 
suppression of intensity modulated components caused by optical frequency 
modulation and for modulation by the communication data signal Sd. FIG. 4 
is a block diagram of a general construction of the embodiment. 
In FIG. 4, numeral 30 denotes a branch circuit that branches the optical 
frequency modulation signal Sm into two. One of the outputs from the 
branch circuit 30 is applied to an adder 34 via a delay circuit 32 with a 
predetermined amount of delay. The adder 34 makes addition of output from 
the delay circuit 32 (optical frequency modulation signal Sm) to a d.c. 
voltage necessary for continuous laser oscillation, and applies the 
resulting signal as a drive current to a semiconductor laser diode 36. The 
other output from the branch circuit 30 is applied to an amplitude 
adjusting circuit 40 via a delay circuit 38 with a predetermined amount of 
delay. The signal supplied from the branch circuit 30 to the delay circuit 
32 or 38 is the optical frequency modulation signal Sm or its 
phase-inverted equivalent. Whether the signal is phase-inverted or not 
depends on whichever is preferred to suppress intensity fluctuation 
components of light output of the laser diode 36. 
The amplitude adjusting circuit 40 is a gain variable circuit using a 
variable resistor and operational amplifier. The amplitude adjusting 
circuit 40 adjusts the gain of the output signal from the delay circuit 38 
and delivers its output to one of inputs of a multiplier 42. The 
communication data signal Sd is supplied to the other input of the 
multiplier 42. The multiplier 42 multiplies the communication data signal 
Sd by the output from the gain adjusting circuit 40 (an amplitude-adjusted 
signal of the phase-adjusted optical frequency modulation signal Sm), and 
applies the result of the multiplication to an adder 44. That is, the 
multiplier 42 functions as an amplitude modulator circuit. The 
communication data signal Sd is a voltage signal with two values in which 
+Va (volt) indicates "1" and -Va (volt) indicates "0". 
The adder 44 adds a predetermined d.c. bias to the output from the 
multiplier 42, and applies the resulting signal, as a control voltage, to 
a control terminal (cathode) of an electroabsorption modulator 46 which is 
also supplied with light output from the semiconductor laser diode 36. The 
electroabsorption modulator 46 changes in transmissivity in response to 
the output voltage from the adder 44, and thus modulates the intensity of 
the light output of the laser diode 36 in accordance with the output 
voltage of the adder 44. 
The bias value added to the adder 44 is chosen such that all or 
substantially all of output signals from the multiplier 42 exhibit one of 
the opposite polarities (for example, negative polarity). The bias value 
added by the adder 44 may be, for example, -Va (volt). The amplitude 2 Va 
of the communication data signal Sd is set to a voltage that reduces the 
transmissivity of the electroabsorption modulator 46 to substantially 
zero. Waveforms of the optical frequency modulation signal Sm, 
communication data signal Sd and outputs from the multiplier 42 and the 
adder 44 are shown in FIG. 5, and input/output characteristics of the 
electroabsorption modulator 46 are shown in FIG. 6. 
As understood from FIG. 6, fluctuation in output voltage of the adder 44 
around -Va (volt) (in particular, the fluctuation synchronous with the 
optical frequency modulation signal Sm) causes only a small fluctuation in 
the transmissivity of the electroabsorption modulator 46 at a portion 
where the transmissivity is sufficiently low, and therefore, its affection 
to the light output of the laser diode 36 is very small. In contrast, when 
the output voltage of the adder 44 is around 0 volt, the transmissivity of 
the electroabsorption modulator 46 is high and varies largely, and 
therefore, fluctuation in the output voltage of the adder 44 around 0 volt 
largely affects the light output of the laser diode 36. More specifically, 
intensity modulation components contained in the light output from the 
laser diode 36 are suppressed due to fluctuation in transmissivity of the 
electroabsorption optical modulator 46 caused by intensity fluctuation of 
the optical frequency modulation signal Sm. At the same time, d.c. 
components of the light output of the laser diode 36 are 
intensity-modulated by components corresponding to the communication data 
signal Sd contained in the output from the adder 44. 
In this manner, the light output from the electroabsorption modulator 46 is 
ON/OFF-modulated with the communication data signal Sd, and the intensity 
modulation components having been contained in the light output of the 
laser diode 36 by optical frequency modulation by the optical frequency 
modulation signal Sm is well-suppressed. Thus, a constant and stable 
extinction ratio can be obtained. In an experiment, intensity modulation 
components contained in a light output from the laser diode 36 could be 
suppressed to 1/10. Even this order of suppression sufficiently improves 
the transmission characteristics of the optical transmission system. 
Quantities of delay of the delay circuits 32, 38 and quantities of 
amplitude adjustment in the amplitude adjusting circuit 40 are previously 
adjusted in response to the optical frequency modulation signal Sm used, 
so as to maximize the intensity fluctuation suppression effect in the 
electroabsorption modulator 46. Apparently, it is sufficient to use only 
one of the delay circuits 32, 38 by considering the relations of the 
timing of intensity fluctuation suppression in the electroabsorption 
modulator 46 relative to the phase of intensity fluctuation of the light 
output of the laser diode 36. 
Although the embodiment shown in FIG. 4 employs amplitude modulation of the 
communication data signal Sd, using a phase-and amplitude-modulated signal 
of the optical frequency modulation signal Sm, the same effects can be 
obtained by adding both signals. That is, an adder may be used instead of 
the multiplier 42. FIG. 7 is a block diagram showing a general 
construction of another embodiment modified in this respect. Elements in 
FIG. 7 which are the same as or equivalent to those in FIG. 4 are labeled 
with common reference numerals. 
An adder 48 adds a bias voltage (for example, Va (volt)), that determines 
the operative point of the electroabsorption modulator 46, to an output 
voltage of the amplitude adjusting circuit 40. An adder 50 adds the 
communication data signal Sd to the output voltage of the adder 48. The 
adder 50 applies its output to the control terminal (cathode) of the 
electroabsorption modulator 46. Apparently, the bias voltage may be added 
after the communication data signal Sd is added to the amplitude-adjusted 
optical frequency modulation signal Sm. Also in the construction shown in 
FIG. 7, the electroabsorption modulator 46 functions in exactly the same 
manner as that in the construction of FIG. 4, and its light output has the 
form ON/OFF-modulated by the communication data signal Sd and suppressing 
intensity modulation components having been contained in the light output 
of the laser diode 36 by optical frequency modulation by the optical 
frequency modulation signal Sm. 
The transmission characteristics of the electroabsorption modulator 46 are 
significantly nonlinear as shown in FIG. 3. Taking it into consideration, 
the embodiment shown in FIG. 7 may be modified to include a converter 
circuit that converts the output of the adder 48, for example, so as to 
compensate the nonlinear transmission characteristics of the 
electroabsorption modulator 46 before applying it to the adder 50. The use 
of the converter circuit promises more effective intensity fluctuation 
suppression in the electroabsorption modulator 46. 
Although the embodiments shown in FIGS. 1, 4 and 7 are configured to 
suppress intensity fluctuation of the light output of the semiconductor 
laser diode 10 or 36 by using the optical frequency modulation signal Sm, 
intensity fluctuation components of the light output from the 
semiconductor laser diode 10 or 36 can be detected and extracted by a 
photo diode or other appropriate means to use its output instead of the 
optical frequency modulation signal Sm. This approach is useful when two 
laser outputs can be obtained from the laser diode 10 or 36, and has the 
advantage that the ratio of intensity fluctuation of the laser light 
relative to the optical frequency modulation signal Sm can be taken into 
consideration. Also such construction substantially uses the optical 
frequency modulation signal Sm to suppress fluctuation in optical 
intensity. 
Next explained is a further embodiment for automatically minimizing 
residual fluctuation after intensity fluctuation suppression whenever the 
amplitude of the optical frequency modulation signal Sm may be fluctuated. 
FIG. 8 is a block diagram showing a general construction of the 
embodiment. 
In FIG. 8, the optical frequency modulation signal Sm is divided into two 
by a branch circuit 60, and one of outputs from the branch circuit 60 is 
divided again into two by a branch circuit 62. One of output of the branch 
circuit 62 is applied to an adder 66 through a delay circuit 64 having a 
predetermined amount of delay, and the other output is applied to a delay 
circuit 76. The adder 66 is supplied with a d.c. voltage for causing 
continuous laser oscillation of a semiconductor laser diode 68, and adds 
the output voltage of the delay circuit 64 to the d.c. voltage. The adder 
66 applies its output current to the semiconductor laser diode 68 to drive 
it. As a result, the semiconductor laser diode 68 is modulated both in 
optical frequency and in intensity by the optical frequency modulation 
signal Sm during continuous laser oscillation. 
The other output of the branch circuit 60 is applied to one of inputs of a 
multiplier 72 through a delay circuit 70 having a predetermined amount of 
delay. The multiplier 72 is also supplied to the other input with a signal 
that indicates residual intensity fluctuation components after suppression 
of intensity fluctuation of the light output from the semiconductor laser 
diode 68 (specifically, an output from an optical detector 86 explained 
later). Thus, the multiplier 72 multiplies both inputs and detects 
correlation between the inputs. That is, the output of the multiplier 72 
represents the correlation between the optical frequency modulation signal 
Sm and the residual components after intensity fluctuation suppression, 
and the multiplier 72 thus functions as a correlation detector. That is, 
the magnitude of the correlation output signal from the multiplier 72 
indicates the quantity of the residual intensity fluctuation components, 
and the polarity indicates the direction of excessive suppression or 
insufficient suppression of the intensity fluctuation. 
Signals supplied from the branch circuits 60, 62 to the delay circuits 70, 
76, respectively, are inverted in phase from signals input to the branch 
circuits 60, 62 if necessarily. 
An integrating circuit 74 integrates the output of the multiplier 72, and 
applies its output to a multiplication factor control terminal of a 
multiplier 78. Fluctuation of the correlation rate (output of the 
multiplier 72) is smoothed and integrated by the integrating circuit 74. 
Also applied to the multiplier 78 is the other output of the branch 
circuit 62 via a delay circuit 76 having a predetermined amount of delay. 
The multiplier 78 multiplies the output of the delay circuit 76 (a signal 
delayed from the optical frequency modulation signal Sm by the amount of 
delay of the delay circuit 76) by a factor of a magnitude determined by 
the voltage and the polarity of the output from the integrating circuit 
74. More specifically, the multiplier 78 multiplies the output of the 
delay circuit 76 by a larger multiplication factor if the output voltage 
of the integrating circuit 74 is positive and large, or by a smaller 
multiplication factor if the output voltage of the integrating circuit 74 
is negative and large. The multiplier 78 functions as amplitude adjusting 
means, and the integrating circuit 74 functions as amplitude control means 
for controlling the amplitude adjusting amount of the amplitude adjusting 
means (multiplier 78). 
The multiplier 78 applies its output to an adder 80 which is also supplied 
with a bias voltage that determines the operative point of an 
electroabsorption modulator 82. The adder 80 adds the output of the 
multiplier 78 to the bias voltage, and applies its output to a control 
terminal (cathode) of the electroabsorption modulator 82. In this 
embodiment, since the electroabsorption modulator 82 is used for the 
purpose of suppressing intensity fluctuation of the light output of the 
semiconductor laser diode 68, the bias voltage added by the adder 80 may 
be zero volt. 
The transmissivity of the electroabsorption modulator 82 thus varies to 
cancel intensity fluctuation of the light output of the laser diode 68. 
The amplitude of the fluctuation in transmissivity depends on the residual 
amount after intensity fluctuation suppression (specifically, output of 
the multiplier 72), and is controlled in the direction reducing the 
residual amount after intensity fluctuation suppression. The output signal 
of the integrating circuit 74 converges to a level where the correlation 
signal (output of the multiplier 72) becomes zero, that is, the residual 
intensity components are minimized. This embodiment is the same as the 
embodiment of FIG. 4 in that amounts of delay of the delay circuits 64 and 
76 are previously determined so that the fluctuation in transmissivity of 
the electroabsorption modulator 82 takes a phase that cancels intensity 
fluctuation of the light output from the semiconductor laser diode 68. It 
will be apparent that at least one of the delay circuits 64, 70, 76 can be 
omitted. An optical splitter 84 splits the light output of the 
electroabsorption modulator 82 into two, and applies one of them to an 
optical detector 86 and the other to an electroabsorption modulator 88. 
The optical detector 86 converts an optical signal from the optical 
splitter 84 (optical signal output from the electroabsorption modulator 
82) into an electric signal. The electric signal output from the optical 
detector 86 reflects the residual amount of intensity fluctuation 
suppression of the light output of the semiconductor laser diode 68 by the 
electroabsorption modulator 82. The output of the optical detector 86 is 
applied to the multiplier 72 and used there for detecting correlation with 
the optical frequency modulation signal Sm as explained above. The amount 
of delay of the delay circuit 70 is previously determined to match the 
output of the optical detector 86 in phase with the output of the delay 
circuit 70. 
FIG. 9, FIG. 10 and FIG. 11 show waveforms at different portions in the 
construction shown in FIG. 8. Among these figures, FIG. 9 shows those 
resulting from insufficient intensity fluctuation suppression, FIG. 10 
shows those obtained by optimum control, and FIG. 11 shows those by 
excessive suppression. In these figures, diagrams labeled (a) show those 
of the optical frequency modulation signal Sm, diagrams labeled (b) show 
those of the light output intensity of the semiconductor laser diode 68, 
diagrams labeled (c) show those of the transmission characteristics of the 
electroabsorption modulator 82, diagrams labeled (d) show those of the 
light output of the electroabsorption modulator 82, and diagrams labeled 
(e) show those of the correlation output of the multiplier 72. 
When the effect of intensity fluctuation suppression by the 
electroabsorption modulator 82, for example, is small, waveforms appear as 
shown in FIG. 9, where the light output from the electroabsorption 
modulator 82 still contains intensity fluctuation of the same phase as the 
intensity fluctuation of the light output from the laser diode 68. 
Therefore, the output of the optical detector 86 contains signal 
components having the same phase as the optical frequency modulation 
signal Sm and having an amplitude corresponding to the residual amount of 
intensity fluctuation, and the correlation output of the multiplier 72 
exhibits the positive voltage value responsive to the residual amount of 
intensity fluctuation as shown in FIG. 9(e). As a result, the output 
voltage of the integrating circuit 74 increases in the positive direction, 
and the multiplier 78 multiplies the output of the delay circuit 76 by a 
large factor. As the amplitude of the output from the multiplier 78 
becomes large, the amplitude of fluctuation in transmissivity of the 
electroabsorption modulator 82 also becomes large, and this results in 
stronger suppression of intensity fluctuation of the light output from the 
semiconductor laser diode 68. In this manner, the electroabsorption 
modulator 82 is controlled to decrease the intensity fluctuation remaining 
in the light output from the electroabsorption modulator 82, that is, 
toward the condition shown in FIG. 10. 
In contrast, when the effect of intensity fluctuation suppression by the 
electroabsorption modulator 82 is excessive, waveforms appear as shown in 
FIG. 11 where the light output of the electroabsorption modulator 82 still 
contains intensity fluctuation which is opposite in phase from the 
intensity fluctuation of the light output from the semiconductor laser 
diode 68. Therefore, the output of the optical detector 86 contains 
intensity fluctuation signal components that are opposite in phase from 
the optical frequency modulation signal Sm and have an amplitude 
responsive to the residual amount of intensity fluctuation, and the 
correlation output of the multiplier 72 exhibits a negative value of the 
magnitude corresponding to the residual amount of intensity fluctuation as 
shown in FIG. 11(e). The multiplier 78 multiplies the output of the delay 
circuit 76 by a factor smaller than the optimum condition in response to 
the output of the integrating circuit 74. As a result, the amplitude of 
the fluctuation in transmissivity of the electroabsorption optical 
modulator 82 becomes small, and the effect of intensity fluctuation 
suppression of the electroabsorption modulator 82 that was excessive is 
diminished. In this manner, even under the excessively controlled 
condition shown in FIG. 11, the electroabsorption modulator 82 is 
controlled to decrease intensity fluctuation residual in its light output, 
that is, toward the condition shown in FIG. 10. 
Thus, the electroabsorption modulator 82 is controlled by the 
above-explained control loop to minimize intensity fluctuation of the 
light output from the semiconductor laser diode 68, and the light output 
of the electroabsorption modulator 82 becomes substantially constant 
relative to the time as shown in FIG. 10(d). 
The control terminal (cathode) of the electroabsorption modulator 88 is 
supplied with the communication data signal Sd, and the electroabsorption 
modulator 88 modulates the light signal from the optical splitter 84 with 
the communication data signal Sd. 
Although the embodiment shown in FIG. 8 uses the optical frequency 
modulation signal Sm itself as the reference signal for correlation 
detection by the multiplier 72, intensity fluctuation components in the 
light output from the semiconductor laser diode 68, which can be detected 
and extracted by an additional optical detector (not shown), may be used 
as the reference signal for correlation detection. In this case, the 
intensity fluctuation component signal extracted from the light output of 
the semiconductor laser diode 68 may be used as the multiplicand signal 
for the multipliers 72 and 78. This arrangement is useful especially when 
the optical frequency modulation signal Sm causing optical intensity 
fluctuation or the electrical signal indicating optical intensity 
fluctuation is not obtained or difficult to obtain, and gives the same 
effects as the embodiment shown in FIG. 8. 
Next explained is a further embodiment modified from the embodiment shown 
in FIG. 8 so that a single electroabsorption modulator simultaneously 
executes both suppression of intensity fluctuation and modulation by the 
communication data signal Sd. FIG. 12 is a block diagram of a general 
construction of the modified embodiment. 
In FIG. 12, the optical frequency modulation signal Sm is divided into two 
by a branch circuit 110, and one of outputs of the branch circuit 110 is 
again divided into two by a branch circuit 112. One of outputs of the 
branch circuit 112 is applied to an adder 116 through a delay circuit 114 
having a predetermined amount of delay, and the other output of the branch 
circuit 112 is applied to a delay circuit 126. The adder 116 is also 
supplied with a d.c. voltage that causes continuous laser oscillation of a 
semiconductor laser diode 118, and adds the output voltage of the delay 
circuit 114 to the d.c. voltage. The adder 116 applies its output current 
to the semiconductor laser diode 118 to drive it. As a result, the 
semiconductor laser diode 118 is modulated both in optical frequency and 
in intensity by the optical frequency modulation signal Sm during 
continuous laser oscillation. 
The other output of the branch circuit 110 is applied to one of inputs of a 
multiplier 122 via a delay circuit 120 having a predetermined amount of 
delay. The multiplier 122 is also supplied at its other input with a 
signal indicating residual components after suppression of intensity 
fluctuation of the light output from the semiconductor laser diode 118 
(specifically, an output from an optical detector 138 explained later) to 
multiply both inputs and to detect correlation between them. That is, the 
output of the multiplier 122 indicates the correlation between the optical 
frequency modulation signal Sm and the residual components after intensity 
fluctuation suppression, and the multiplier 122 thus functions as a 
correlation detector. 
Signals supplied from the branch circuits 110, 112 to the delay circuits 
120, 126, respectively, are inverted in phase from signals input to the 
branch circuits 110, 112 if necessarily. 
An integrating circuit 124 integrates the output of the multiplier 122, and 
applies its output to a multiplication factor control terminal of a 
multiplier 128. Fluctuation of the correlation rate is smoothed and 
integrated by the integrating circuit 124. Also applied to the multiplier 
128 is the other output of the branch circuit 112 via a delay circuit 126 
having a predetermined amount of delay. The multiplier 128 multiplies the 
output of the delay circuit 126 (a signal delayed from the optical 
frequency modulation signal Sm by the amount of delay of the delay circuit 
126) by a factor of a magnitude determined by the voltage and the polarity 
of the output from the integrating circuit 124. More specifically, the 
multiplier 128 multiplies the output of the delay circuit 126 by a larger 
multiplication factor if the output voltage of the integrating circuit 124 
is positive and large, or by a smaller multiplication factor if the output 
voltage of the integrating circuit 124 is negative and large. The 
multiplier 128 functions as amplitude adjusting means, and the integrating 
circuit 124 functions as amplitude control means for controlling the 
amplitude adjusting amount of the amplitude adjusting means (multiplier 
128). 
The multiplier 128 applies its output to one of inputs of a multiplier 130. 
The multiplier 130 is also supplied at its other input with the 
communication data signal Sd. The multiplier 130 issues a signal obtained 
by amplitude-modulating the communication data signal Sd with the output 
of the multiplier 128. An adder 132 adds to the output of the multiplier 
130 a bias voltage that determines the operative point of an 
electroabsorption modulator 134. Functions of the multiplier 130 and the 
adder 132 are the same as those of the multiplier 42 and the adder 44 in 
FIG. 4. Here again, the communication data signal Sd is a voltage signal 
with two values in which +Va (volt) indicates "1" and -Va (volt) indicates 
"0". 
The output voltage of the adder 132 is applied to a control terminal 
(cathode) of the electroabsorption modulator 134 which is also supplied 
with the light output from the semiconductor laser diode 118. The 
electroabsorption modulator 134 changes its transmissivity in response to 
the output voltage of the adder 132, and hence modulates the intensity of 
the light output of the laser diode 118 in accordance with the output 
voltage of the adder 132. The electroabsorption modulator 134 behaves in 
exactly the same manner as the electroabsorption modulator 46 in the 
embodiment shown in FIG. 4. That is, intensity fluctuation components 
contained in the light output of the laser diode 118 are suppressed by 
fluctuation in transmissivity of the electroabsorption modulator 134 due 
to frequency fluctuation components of the optical frequency modulation 
signal Sm on one side of the output of the adder 132 nearer to zero volt. 
Simultaneously, d.c. components in the light output of the laser diode 118 
are intensity-modulated by components corresponding to the communication 
data signal Sd contained in the output of the adder 132. 
In this manner, the light output from the electroabsorption modulator 134 
is ON/OFF-modulated with the communication data signal Sd, and the 
intensity modulation components having been contained in the light output 
of the laser diode 118 by optical frequency modulation by the optical 
frequency modulation signal Sm is well-suppressed. Thus, a constant, 
stable extinction ratio can be obtained. Amounts of delay of the delay 
circuits 114, 120, 126 are previously determined so that the phase of 
intensity fluctuation of the light output from the laser diode 118 
coincides with the phase of the fluctuation in transmissivity of the 
electroabsorption modulator 118 and that the correlation detection by the 
multiplier 122 and the amplitude adjustment by the multiplier 128 are 
optimized. It will be apparent that at least one of the delay circuits 
114, 120, 126 can be omitted. 
An optical splitter 136 splits the light output of the electroabsorption 
modulator 134 into two, and applies one of them to the optical detector 
138 and the other to an optical transmission line such as optical fiber. 
The optical detector 138 converts an optical signal from the optical 
splitter 136 (optical signal output from the electroabsorption modulator 
134) into an electric signal. The electric signal output from the optical 
detector 138 reflects the residual amount of intensity fluctuation 
suppression of the light output of the semiconductor laser diode 118 by 
the electroabsorption modulator 134 and the communication data signal Sd. 
Here is needed, however, only the residual amount of intensity fluctuation 
suppression of the light output from the semiconductor laser diode 118. 
Therefore, the optical detector 138 used here is a low-speed optical 
detector element independent from the frequency of the communication data 
signal Sd. Alternatively, a high-speed optical detector element may be 
used to extract from its output only frequency components of the optical 
frequency modulation signal Sm through a low pass filter or a band pass 
filter. By removing undesired components other than Sm, behaviors of the 
control system for optical intensity fluctuation suppression can be 
stabilized. 
The optical detector 138 applies its output to the multiplier 122 which 
uses it for detecting correlation with the optical frequency modulation 
signal Sm as the multiplier 72 does. The amount of delay of the delay 
circuit 120 is previously determined to bring its output into the same 
phase with the output of the optical detector 138. 
The loop made of the optical splitter 136, optical detector 138, multiplier 
122, integrating circuit 124, multiplier 128, multiplier 130 and adder 132 
controls the effect of intensity fluctuation suppression in the 
electroabsorption modulator 134 to minimize the residual fluctuation. This 
function is the same as the function of the control loop of the embodiment 
shown in FIG. 8 made of the optical splitter 84, optical detector 86, 
multiplier 72, integrating circuit 74, multiplier 78 and adder 80. 
Although the embodiment shown in FIG. 12 uses the optical frequency 
modulation signal Sm itself as the reference signal for correlation 
detection by the multiplier 122, intensity fluctuation components in the 
light output from the semiconductor laser diode 68, which can be detected 
and extracted by an additional optical detector (not shown), may be used 
as the reference signal for correlation detection. In this case, the 
intensity fluctuation component signal extracted from the light output of 
the semiconductor laser diode 118 may be used as the multiplicand signal 
for the multiplier 128. This arrangement is useful especially when the 
optical frequency modulation signal Sm causing optical intensity 
fluctuation or the electrical signal indicating optical intensity 
fluctuation is not obtained or difficult to obtain, and gives the same 
effects as the embodiment shown in FIG. 12. 
The embodiment shown in FIG. 12 uses a signal obtained by adjusting the 
optical frequency modulation signal Sm in phase and in amplitude to 
modulate the amplitude of the communication data signal Sd; however, the 
same effects can be obtained by adding both these signals. Namely, the 
multiplier 130 may be replaced by an adder. FIG. 13 is a block diagram of 
a general construction of an embodiment so modified. Elements common to 
those in FIG. 12 are labeled with common reference numerals. 
An adder 140 adds to the output voltage of the multiplier 128 a bias 
voltage (for example, -Va (volt)) that determines the operative point of 
the electroabsorption modulator 134, and an adder 142 adds the 
communication data signal Sd to the output voltage of the adder 140. The 
output of the adder 142 is applied to the control terminal (cathode) of 
the electroabsorption modulator 134. The electroabsorption modulator 134 
in the embodiment shown in FIG. 13 behaves in exactly the same manner as 
that of FIG. 12. The light output of the electroabsorption modulator 134 
is ON/OFF-modulated by the communication data signal Sd, and the intensity 
modulation components having been contained in the light output of the 
laser diode 118 by optical frequency modulation by the optical frequency 
modulation signal Sm is suppressed. 
The adder 140 (addition of the bias voltage) may be located at the output 
side of the adder 142. If a converter circuit for compensating the 
non-linearity of the electroabsorption modulator 134 is additionally 
connected at the output stage of the multiplier 128 or the adder 140 so as 
to compensate amplitude-change of the control signal (intensity 
fluctuation suppression control signal) of the electroabsorption modulator 
134 such that the fluctuation in transmissivity of the electroabsorption 
optical modulator 134 matches the intensity fluctuation of the light 
output from the semiconductor laser diode 118, then the intensity 
fluctuation suppressing effect can be improved. 
Although the foregoing explanation has been made as obtaining a laser light 
for communication, the laser light obtained by these embodiments can be 
used also for measurement. That is, the invention can provide a laser 
light that is modulated in optical frequency but maintains a constant 
optical intensity, and can be utilized in various fields using such a 
laser light. 
Those skilled in the art will readily understand that the invention enables 
stable and effective suppression of intensity fluctuation caused by 
optical frequency modulation of a laser source. It also enables the use of 
higher frequencies for optical frequency modulation. 
The invention can realize the intended device more economically than that 
attained by conventional devices using an optical fiber amplifier as 
taught by the Literature referred to above. Especially when the inventive 
device is used together with a modulator for modulating a data signal for 
communication or for measurement, optical signals with higher qualities 
can be obtained more economically. 
Since the invention enables generation of optical carriers that are 
modulated in optical frequency but includes less or no optical intensity 
fluctuation or signals modulated by a data signal, it contributes to 
realization and long-distance applications of high-power optical 
transmission systems, such as, in particular, repeaterless optical 
transmission systems. 
Devices according to the invention can apparently be used in various 
applications including measurement in addition to communication.