JPH0667128A - Optical modulator bias control circuit - Google Patents

Abstract

PURPOSE:To obtain an operation point control method at the time of driving an optical modulator using a lithium niobate substrate in an optical communication system. CONSTITUTION:By this circuit, bias voltage applied to the optical modulator 2 is controlled by detecting mean output power of an optical modulation signal from the optical modulator 2 at the time of driving the optical modulator using the lithium niobate substrate and comparing it with reference voltage. Since the mean value power of an optical signal modulated by optical modulation is increased according to increase in a drift amt. in the optical modulator 2, an operation point is monitored from the mean power of the optical signal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of controlling an optical modulator in an optical transmitter in an optical communication system.

[0002]

2. Description of the Related Art In recent years, an optical communication device for transmitting a large amount of information has been developed, and an optical communication device having a transmission speed of gigabit range has been developed. As an optical communication unit used in such a large-capacity optical communication system, the spectrum spread is small even at the time of modulation, and long-distance transmission is possible without being affected by the dispersion of the optical fiber. The Mach-Zehnder type optical modulator used is promising. However, in the Mach-Zehnder type optical modulator using lithium niobate, there is an operating point drift (DC drift) of the optical modulation characteristic with respect to the modulation voltage. Therefore, in practical use, a control circuit for stabilizing this operating point is provided. Is required. As such a Mach-Zehnder type control circuit, a method of superimposing a low-frequency signal on an electric signal to be modulated has been proposed (for example: Kuwata et al., "Study of automatic bias control circuit for Mach-Zehnder type optical modulator", 1990 Electronic IEICE Spring National Convention B-9
76) In this control method, a low-frequency signal of 1 KHz is superimposed on an electric signal having a transmission rate of 2.5 Gb / s, the optical signal is modulated, and the modulated optical signal is converted into an electric signal to obtain 1 KHz.
The optimum operating point is controlled by extracting the low-frequency signal of and detecting the phase.

[0003]

As described above, the operating point of the Mach-Zehnder type optical modulator can be controlled by superimposing the low frequency signal on the modulation signal. However,
In the above-mentioned method, since a low-frequency signal is superposed, an unnecessary control signal is added to the modulated optical signal, which causes the deterioration of the optical transmission waveform and the deterioration of the reception sensitivity due to the waveform deterioration.

An object of the present invention is to provide a method for controlling an operating point of a Mach-Zehnder type optical modulator without using a control signal and without causing deterioration of an optical transmission waveform.

[0005]

An optical modulator bias control circuit according to the present invention comprises a light source for optical transmission, an optical modulator for intensity-modulating the optical output of the light source, and an optical signal from the optical modulator. An optical circuit for separating the optical signal for transmission and the optical signal for control,
A photoelectric conversion circuit for converting the optical signal for control into an electric signal, and a differential amplifier for amplifying a difference between an output of the photoelectric conversion circuit and a reference voltage signal, and the optical modulation of the output signal of the differential amplifier Bias control of the optical modulator is performed as a bias signal of the optical modulator.

The optical modulator bias control circuit of the present invention comprises a light source for optical transmission, an optical circuit for extracting a first optical output and a second optical output from the light source, and an intensity modulation of the first optical output. And an optical circuit for separating the optical signal from the optical modulator into an optical signal for transmission and an optical signal for control, and a first photoelectric converter for converting the second optical output into an electric signal. A conversion circuit, a second photoelectric conversion circuit that converts the intensity-modulated optical signal for control into an electric signal, and a difference between the output of the first photoelectric conversion circuit and the output of the second photoelectric conversion circuit is amplified. And a bias amplifier for controlling the bias of the optical modulator by using the output signal of the differential amplifier as a bias signal of the optical modulator.

The optical modulator bias control circuit of the present invention comprises a light source for optical transmission, an optical circuit for extracting a first optical output and a second optical output from the light source, and an intensity modulation of the first optical output. And an optical circuit for separating the optical signal from the optical modulator into an optical signal for transmission and an optical signal for control, and a first photoelectric converter for converting the second optical output into an electric signal. A conversion circuit, a second photoelectric conversion circuit for converting the intensity-modulated optical signal for control into an electric signal, and a mark rate signal corresponding to a change in the mark rate of the digital signal from the electric signal applied to the optical modulator. , A first differential amplifier for amplifying the difference between the mark ratio signal and the output of the second photoelectric conversion circuit, the output of the first differential amplifier, and the first photoelectric conversion circuit A second differential amplifier that amplifies the difference from the output of And performing bias control of the optical modulator output signal of the second differential amplifier as the bias signal of the optical modulator.

[0008]

In the present invention, the operating point of the optical modulator is controlled by detecting the average value of the optical signal modulated by the Mach-Zehnder type optical modulator.

The operation of the present invention will be described below with reference to FIG. FIG. 4 is a diagram showing the relationship between the applied voltage and the optical output of the Mach-Zehnder interferometer type optical modulator. Here, V _sw is the switching voltage of the optical modulator, the solid cosine curve of FIG. 4 (a) is the characteristic at the start of use of the optical modulator, and the broken line in FIG. 4 (a) is the optical modulator with time. Shows the characteristics when the characteristics of (4) cause drift. In addition, the waveform of the solid line in FIG.
The modulation signal applied in the optimum bias state is shown in the characteristics of the optical modulator shown by the solid line in (a). Here, when the characteristic of the optical modulator causes a β drift from the optimum bias point of the modulation signal, as shown by the broken line in FIG.
The switching characteristic of the optical output O _pt is represented by O _pt = O ₁ (1 + cos ((V / V _sw ) π−β)). Here, V is an applied voltage, V _sw is a switching voltage, and O ₁ is a constant associated with light output.

If the modulation signal applied to the optical modulator is a waveform with a mark ratio of 1/2, the average power P _opt of the modulated optical signal is

[Equation 1] Is approximated by. The average power of the optical signal can be detected as a function of the amount of drift β from the optimum bias point of the modulated signal, β
In the range where is small, the average power of the optical signal is a monotonically increasing function of β. Therefore, the operating point of the Mach-Zehnder optical modulator can be found by detecting the average power of the optical signal. Therefore, if a bias is applied to the applied voltage of the signal so that the average power of the optical signal is always constant, the Mach-Zehnder optical The modulator can be controlled at a good operating point as in the state of the applied voltage indicated by the broken line in FIG. 4 (b).

[0011]

EXAMPLES The present invention will be described in detail below with reference to examples. FIG. 1 shows an optical modulator bias control circuit which is an embodiment of the invention described in claim 1.

In this embodiment, the output light from the semiconductor laser 1 is optically modulated by the Mach-Zehnder type optical modulator 2, and the output light of the optical modulator 2 is transmitted by the optical branching circuit 3 to the optical signal 4 and the monitor optical signal 5. Branch to. The monitor optical signal 5 is converted into an electric signal by the photoelectric conversion circuit 6 including a photodetector and a low-speed amplifier. The output signal of the photoelectric conversion circuit 6 is input to the operation amplifier 7, which amplifies the difference signal from the reference voltage given from the reference voltage input terminal 8 and outputs the error signal to the bias input terminal 9 of the Mach-Zehnder optical modulator 2. Input and configured the control loop. As the optical modulator 2, a switching voltage of 5 V and a band of 11 GHz was used, and a signal voltage of 5.2 V _pp was applied from the drive circuit 10.

With the above configuration, an electric signal having a transmission rate of 10 Gb / s was applied to the drive circuit input terminal 11 to drive the optical modulator 2. When the power was turned on, the optimum bias of the optical modulator 2 was 2.5 V, but after 100 hours, the characteristic change of the optical modulator 2 occurred and a 1.1 V drift occurred.
Even in this case, the bias control circuit of the present invention operated normally, the applied bias voltage was automatically increased by 1.1 V, and an appropriate electric signal could be input to the optical modulator 2.

In the above embodiment, the modulation voltage applied to the optical modulator 2 is 0.2 than the switching voltage of the optical modulator 2.
Although the value of V is larger, the amplitude of the modulation voltage is not limited to this, and may be any voltage as long as the extinction ratio of the optical signal allows. Although the bias to the optical modulator 2 is supplied from the resistance load R side of the optical modulator, the bias may be supplied from the output part of the drive circuit 10. Further, in the present embodiment, the half mirror type optical branch circuit 3 is used for separating the monitor optical signal 5, but a fiber coupler type may be used for the optical branch circuit.

FIG. 2 shows an optical modulator bias control circuit which is an embodiment of the invention described in claim 2. In this embodiment,
A configuration is adopted in which a part of the output light from the semiconductor laser 1 is extracted and used as the reference voltage of the control circuit. Here, in order to monitor the optical output of the semiconductor laser 1, the back surface output light of the semiconductor laser 1 is detected by the photoelectric conversion circuit 12 including a photodetector and a low-speed amplifier. The output of the photoelectric conversion circuit 12 is input to the operation amplifier 7 after the amplitude is adjusted by the variable gain amplifier 17. On the other hand, the monitor optical signal light 5 passing through the optical modulator 2 is converted into an electric signal by the photoelectric conversion circuit 6, the amplitude is adjusted by the variable gain amplifier, and the result is input to the operation amplifier 7.

With the above configuration, an electric signal having a transmission rate of 10 Gb / s was applied to the drive circuit input terminal 11 to drive the optical modulator 2. The optical modulator 2 operated stably by the control circuit even after 500 hours had passed. Further, in the present embodiment, since the optical output of the semiconductor laser 1 is used as a reference, the bias voltage accurately follows the slight change in the optical output of the semiconductor laser 1.

In the above embodiments, the backside output light was used as the optical output of the semiconductor laser 1. However, in the output monitor of the semiconductor laser 1, an optical branch circuit is inserted between the semiconductor laser 1 and the optical modulator 2, The monitor light of the semiconductor laser 1 may be extracted from this optical branch circuit.

FIG. 3 shows an optical modulator bias control circuit which is an embodiment of the invention described in claim 3.

In this embodiment, a mark ratio detection circuit and a control loop for mark ratio comparison are provided so that the control circuit operates more stably even when the mark ratio of the modulation signal changes. The mark ratio detection circuit 16 in FIG. 3 includes a parallel circuit of the load resistance R and the capacitor C ₂ of the drive circuit 10 and the low-speed amplifier 1.
The integration circuit is composed of 4. This mark ratio detection circuit 1
The output of 6 is input to the operation amplifier 7, and the operation amplifier 7
As the other input, an electric signal obtained by photoelectrically converting the monitor optical signal 5 is input to the operation amplifier 7. This operation amplifier 7 mainly absorbs the error of the optical signal due to the mark rate variation. Further, the output of the operation amplifier 7 and the reference electricity from the semiconductor laser 1 are input to the operation amplifier 13, the error signal which is the output of the operation amplifier 13 is input to the bias input terminal 9 of the optical modulator, and the control loop is opened. Configured.

With the above configuration, an electric signal having a transmission rate of 10 Gb / s was applied to the drive circuit input terminal 11 to drive the optical modulator 2. In this embodiment, the mark ratio of the modulation signal is set to 1/4.
It was possible to control while keeping the bias of the optical modulator 2 at an optimum value even when the value was changed from 1 to 3/4. Also, 800
This optical modulator bias control circuit operated stably over a long period of time.

In the above embodiments, the mark ratio of the modulation signal applied to the optical modulator 2 is detected from the load resistance R portion of the optical modulator 2, but it may be detected from inside the drive circuit or from the drive circuit input section. . Further, in the present control circuit, the monitor optical signal 5 and the mark ratio signal are first compared, but the monitor optical signal and the reference signal from the semiconductor laser 1 are first compared and then the mark ratio signal is compared. May be compared.

[0022]

As described above, according to the present invention,
Since it is not necessary to apply an extra modulation signal to the Mach-Zehnder type optical modulator, a good optical transmission signal without waveform deterioration can be obtained, and the operating point control at the time of driving the optical modulator can be surely performed.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an optical modulator bias control circuit that is an embodiment of the present invention.

FIG. 2 is a configuration diagram of an optical modulator bias control circuit according to another embodiment of the present invention.

FIG. 3 is a configuration diagram of an optical modulator bias control circuit which is still another embodiment of the present invention.

FIG. 4 is a diagram for explaining the operation of the present invention.

[Explanation of symbols]

 1 Semiconductor Laser 2 Optical Modulation Path 3 Optical Branch Circuit 4 Optical Signal for Transmission 5 Optical Signal for Monitoring 6 Photoelectric Conversion Circuit 7 Differential Amplifier 8 Reference Voltage Input Terminal 9 Bias Input Terminal 10 Drive Circuit 11 Drive Circuit Input Terminal 12 Photoelectric Conversion Circuit 13 differential amplifier 14 amplifier 15 optical branch circuit 16 mark ratio detection circuit 17 variable gain amplifier 18 variable gain amplifier

Claims (3)

[Claims] 1. A light source for optical transmission, an optical modulator for performing intensity modulation of an optical output of the light source, and an optical signal from the optical modulator is separated into an optical signal for transmission and an optical signal for control. Optical circuit,
A photoelectric conversion circuit for converting the optical signal for control into an electric signal, and a differential amplifier for amplifying a difference between an output of the photoelectric conversion circuit and a reference voltage signal, and the optical modulation of the output signal of the differential amplifier An optical modulator bias control circuit for performing bias control of the optical modulator as a bias signal of the optical modulator. 2. A light source for optical transmission, an optical circuit for extracting a first light output and a second light output from the light source, an optical modulator for performing intensity modulation of the first light output, and the light. An optical circuit for separating an optical signal from the modulator into an optical signal for transmission and an optical signal for control, and a first circuit for converting the second optical output into an electric signal.
Photoelectric conversion circuit, a second photoelectric conversion circuit for converting the intensity-modulated optical signal for control into an electric signal, and the first photoelectric conversion circuit
A differential amplifier that amplifies a difference between the output of the photoelectric conversion circuit and the output of the second photoelectric conversion circuit, and uses the output signal of the differential amplifier as the bias signal of the optical modulator to control the bias of the optical modulator. An optical modulator bias control circuit characterized by performing. 3. A light source for optical transmission, an optical circuit for extracting a first light output and a second light output from the light source, an optical modulator for performing intensity modulation of the first light output, and the light. An optical circuit for separating an optical signal from the modulator into an optical signal for transmission and an optical signal for control, and a first circuit for converting the second optical output into an electric signal.
Photoelectric conversion circuit, a second photoelectric conversion circuit for converting the intensity-modulated optical signal for control into an electric signal, and a mark corresponding to a change in the mark ratio of the digital signal from the electric signal applied to the optical modulator. A mark rate detection circuit for extracting a rate signal, a first operational amplifier for amplifying a difference between the mark rate signal and an output of the second photoelectric conversion circuit, an output of the first differential amplifier and the first photoelectric converter. A second differential amplifier that amplifies a difference from the output of the conversion circuit, and performs bias control of the optical modulator by using an output signal of the second differential amplifier as a bias signal of the optical modulator. Characteristic optical modulator bias control circuit.