JPH0564490B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical communication device, and more particularly to an optical signal transmitting device that maintains the extinction ratio of an optically transmitted signal at an optimal value and stabilizes the output of the optically transmitted signal. .

In recent years, coherent optical transmission has attracted attention, and the possibility of using an external modulator to modulate the transmitted signal is increasing. Usually, an electro-optic modulator capable of high-speed modulation is used as an external modulator, but the extinction ratio of this electro-optic modulator changes due to temperature changes, pyroelectric effects, etc. The drawback was that it caused deterioration. In addition, in the case of a modulator whose extinction ratio changes, the average level of the output changes as the extinction ratio changes. Therefore, fluctuations in the output of the light source and fluctuations in the coupling between the light source and the optical modulator can be considered as fluctuations due to changes in the extinction ratio. It also had drawbacks such as difficulty in separate detection and difficulty in stabilizing signal light output.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to solve the above-mentioned conventional drawbacks and maintain the extinction ratio of the signal at an optimum value.
An object of the present invention is to provide an optical signal transmitter that maintains stable output of an optical transmission signal.

The optical signal transmitting device of the present invention includes a light source and a waveguide type optical modulator, and includes a first and a second waveguide type optical modulator.
In an optical signal transmitting device that modulates light from a light source by controlling the coupling of waveguides, the output light from the first waveguide of a waveguide optical modulator is used as transmission signal light. an output fluctuation detection section that detects fluctuations in the output of the optical signal transmitter using a portion of the output light from the first waveguide and the output from the second waveguide of the modulator; and a waveguide type optical modulator. an extinction ratio change detection unit that detects a change in the extinction ratio of the optical signal transmission device using at least one of the output light from the first waveguide and the output from the second waveguide; , an output control section that controls the output of the optical signal transmission device using an error signal from the output fluctuation detection section; and an extinction ratio control section that controls the extinction ratio of the optical signal transmission device using the error signal from the extinction ratio change detection section. The structure has been prepared. Next, the principle of the present invention will be briefly explained using the drawings.

FIG. 1A shows the applied voltage versus transmitted output characteristics of the first and second waveguides of a waveguide optical modulator using LiNbO ₃ . Further, FIG. 1B shows the dependence of the total output power, which is the sum of the transmitted outputs of the first and second waveguides of the waveguide type optical modulator, on the applied voltage. As is clear from FIG. 1B, even if the bias point of the waveguide optical modulator changes and the extinction ratio of the output signal changes, the total output power of the waveguide optical modulator hardly changes. Therefore, by using a beam splitter or the like to separate the output from the first and second waveguides in equal proportions, and monitoring the sum of the powers of the two separated lights, it is possible to know the fluctuations in the output. I can do it. By controlling the input power to the waveguide optical modulator in response to this variation, the output power of the optical signal transmitter can be kept constant.

On the other hand, the extinction ratio of the signal output can be maintained at an optimal value as follows. Here, the bias voltage of the waveguide optical modulator is set at point a in FIG.
Let us consider the case where binary amplitude modulation is performed between point and point b. In this case, the extinction ratio is approximately 0, and it is possible to transmit an ideal signal. However, if the bias point changes to point a' due to temperature fluctuations, etc., the transmission output becomes an output corresponding to two points a' and b', and extinction ratio degradation occurs during signal demodulation. At this time, the bias point of the second waveguide has also moved from point c to point c'. FIG. 2 shows how the average output of the second waveguide changes with respect to changes in the bias point. Corresponding to the change in the bias point from point a to point a', the average output of the second waveguide also changes from point e to point e' in FIG. By creating a control signal that returns the bias point of the waveguide optical modulator to its original state based on the difference in output between points e and e', and providing feedback to the bias voltage of the waveguide optical modulator, ambient temperature, etc. The extinction ratio can always be kept at the optimum value. Methods for detecting changes in extinction ratio include methods that use the average value of the first waveguide output, fluctuations in the alternating current component of a part of the first waveguide output or the output of the second waveguide, or its primary It is also possible to use a method using differentiation, a method using fluctuations in both the AC component and the average output, a method using the ratio of the average outputs of the first and second waveguides, and the like. Next, the present invention will be explained in detail with reference to Examples.

FIG. 3 is a block diagram for explaining the first embodiment of the present invention. Output light 2 of semiconductor laser 1
is first incident on the waveguide type optical modulator 3. The waveguide type optical modulator 3 is driven by a modulation signal 4. The first waveguide output 5 and the second waveguide output 6 of the waveguide type optical modulator 3 are connected to a first beam splitter 7 and a second beam splitter 8 with equal reflectance.
are divided into a first reference light 9 and a signal light 10, and a second reference light 11 and a third reference light 12, respectively. The first reference light 9 and the second reference light 11 are photoelectrically converted by a first photodetector 13 and a second photodetector 14, respectively, and a first electrical signal 15 and a second electrical signal 16 are obtained. It will be done. At this time, the relative magnitudes of the two electrical signal levels are determined during photoelectric conversion so that the sum of the first electrical signal 15 and the second electrical signal 16 becomes an output proportional to the total output power of the waveguide optical modulator 3. Adjustments were made. The first electrical signal 15 and the second electrical signal 16 are compared with a first reference signal 18 in an output fluctuation detection circuit 17 to obtain an output error signal 19. This output error signal 19 passes through a temperature control circuit 20, becomes a temperature control signal 21, controls a temperature control element 22, adjusts the output of the semiconductor laser 1, and stabilizes the output of the waveguide optical modulator 3. Ta.

On the other hand, the third reference light 12 is incident on the optical fiber 23 and guided to the third photodetector 24. The average output voltage 25 of the third photodetector 24 is compared by a comparator 26 with a second reference signal 27 for keeping the extinction ratio optimal, resulting in an extinction ratio error signal 28.
becomes a voltage feedback signal 30 through the modulator bias circuit 29, and the bias level of the waveguide optical modulator 3 is set to the optimum value, so that the extinction ratio is always kept at the optimum value.

In this example, a LiNbO ₃ directionally coupled optical modulator having two waveguides was used as the waveguide optical modulator 3 to perform amplitude modulation at 100 Mb/s.
The reflectance for beam splitters 7 and 8 is 5%.
A glass plate was used. In addition, the photodetector 13,1
4 and 24 were used as photodiodes, and as the temperature control element 22, a Peltier element was used. The output fluctuation detection circuit 17 was composed of an adder and a differential amplifier, and the temperature control loop was operated in a low frequency range of 1 KHz or less. Further, the comparator 26 was configured with a differential amplifier, and the modulator bias control loop was operated in a high frequency range of 1 KHz or higher. We actually transmitted signal light at 100 Mb/s using this system.
At this time, the ambient temperature of the optical signal transmitter varied by more than 10°C, but the temperature of the semiconductor laser mount was kept relatively stable by the Peltier element.
As a result, the output fluctuation of the optical signal transmitter was suppressed to 0.1% or less. Also, at this time, the bias voltage of the directional coupling type optical modulator was controlled as the ambient temperature changed, and the bias level of the optical modulator was always kept constant. This confirmed that the extinction ratio of the optical transmission signal was always kept below 0.01.

FIG. 4 is a block diagram for explaining a second embodiment of the present invention. In this example, a He--Ne laser 31 was used as the light source. Also the third
The reference light 12 was guided by a lens 32 to a third photodetector. Furthermore, in this embodiment, the output obtained by receiving the third reference light 12 at the third photodetector 24 is branched as the second electrical signal 16 among the inputs to the output fluctuation detection circuit 17. The branch output 33 was used. However, at this time, the branch output 33 is controlled by the level adjuster 34 to
The level was adjusted so that the relative level matched that of the electrical signal 15. Further, the output level was stabilized by driving the driver 35 with the output error signal 19 obtained by the output fluctuation detection circuit 17 and changing the transmittance of the variable optical attenuator 36.

Furthermore, in this embodiment, the extinction ratio error signal 28 from the comparator 26 passes through the temperature control circuit 20, becomes a temperature control signal 21, and controls the driving current of the Peltier element as the temperature control element 22, thereby controlling the waveguide type optical modulation. The ambient temperature of container 3 was controlled. The other configurations are the same as in the first embodiment.
In this embodiment, the modulated signal 4 is branched, and the resulting signal proportional to the average value of the modulated signal is used as the second reference signal 27. In this case, even if there is a change in the mark rate of the signal, the second reference signal 27 and the average output voltage 25 of the third photodetector 24 change at the same rate in accordance with the mark change, so the mark rate changes. Variations do not affect the extinction ratio error signal 28. Therefore, in this example, even if the modulation signal 4 had a variation in mark rate, the extinction ratio could be stably maintained at 0.01 or less. Also, at this time, the output fluctuation of the optical signal transmitter is the same as in the first embodiment.
It was less than 0.1%.

In the present invention, various modifications other than the above embodiments are possible. For example, waveguide type optical modulator 3
In addition to the directional coupling type optical modulator, it is also possible to use an interference type optical modulator, a mode conversion type optical modulator, etc. In addition, when controlling the output level using the method of the second embodiment, He-Ne is used as the light source.
In addition to the laser 31, semiconductor lasers, various gas lasers, solid state lasers, light emitting diodes, etc. can be used. Furthermore, when a semiconductor laser or a light emitting diode is used as a light source, the output error signal 19 can be fed back to the injected current to stabilize the output of the optical signal transmitter. By changing the combination of control methods, the output and extinction ratio stabilized optical signal transmitter of the present invention can be realized with various configurations.

Furthermore, semiconductor lasers can simultaneously stabilize the output and oscillation wavelength by controlling the ambient temperature and injection current, so if a semiconductor laser is used as a light source, an optical signal transmitter can stabilize the output level, wavelength, and extinction ratio at the same time. You can also get

As described above, according to the present invention, it is possible to provide a stable optical signal transmitter that maintains a stable output level of a transmitted signal and maintains an optimum extinction ratio even if the ambient temperature of an external modulator changes. I can do it.

[Brief explanation of the drawing]

Figures 1A and B are diagrams showing the transmitted output characteristics of a LiNbO ₃ waveguide optical modulator with respect to applied voltage in order to explain the principle of the present invention, and Figure 2 is a diagram showing the optical modulator bias voltage during optical modulation. FIG. 3 is a diagram showing a change in the average output voltage of the second waveguide of the optical modulator with respect to a change in . Further, FIG. 3 is a block diagram for explaining the first embodiment of the present invention, and FIG. 4 is a block diagram for explaining the first embodiment of the present invention.
FIG. 2 is a block diagram for explaining an embodiment of the present invention. In the figure, 1... Semiconductor laser, 3... Waveguide optical modulator, 4... Modulation signal, 7, 8... Beam splitter, 13, 14, 24... Photodetector, 17... Output fluctuation Detection circuit, 20... Temperature control circuit, 22... Temperature control element, 23... Optical fiber, 26... Comparator, 29... Modulator bias circuit, 31... He-Ne laser, 32... Lens, 34... Level adjuster, 35... Driver,
36... Optical variable attenuator.

Claims (1)

[Claims] 1 includes a light source and a waveguide-type optical modulator, modulates the light from the light source by controlling the coupling between the first and second waveguides of the waveguide-type optical modulator, and modulates the light from the light source; In an optical signal transmitter that uses output light from the first waveguide of a modulator as transmission signal light, a part of the output light from the first waveguide of the waveguide-type optical modulator and the second an output fluctuation detection unit that detects a fluctuation in the output of the optical signal transmitting device using the output from the waveguide; a portion of the output light from the first waveguide of the waveguide type optical modulator; an extinction ratio change detection section that detects a change in the extinction ratio of the optical signal transmission device using at least one of the outputs from the second waveguide; and an extinction ratio change detection section that detects a change in extinction ratio of the optical signal transmission device using at least one of the outputs from the second waveguide; An optical signal transmitting device comprising: an output control section that controls the output of the device; and an extinction ratio control section that controls the extinction ratio of the optical signal transmitting device based on an error signal from the extinction ratio change detection section.