JPH04263215A - Method for controlling optical modulator - Google Patents

Abstract

PURPOSE:To allow the control of an operating point at the time of driving of an optical modulator using a lithium niobate substrate in an optical communication system. CONSTITUTION:The light of the polarization state having a TM polarization component and a TE polarization component is inputted to the optical modulator 3 and after the light is applied with intensity modulation by the optical modulator 3, the light is separated to the TM polarization component and the TE polarization component in a polarized light separating circuit 5 to control the bias voltage to be applied to a modulation signal by monitoring the operating point from the average value power of the TE polarization component 7 to the optical modulator at the time of driving the optical modulator by using the lithium niobate substrate. The average value power of the TE polarization component increases or decreases with a fluctuation in the operating point of the optical modulator and, therefore, the monitoring of the operating point from the average value power of the TE polarization component is possible.

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, optical communication devices with transmission speeds in the gigabit range have been developed as optical communication systems for transmitting large amounts of information. The optical communication unit used in such a large-capacity optical communication system has a small spectral spread even during modulation, and long-distance transmission is possible without being affected by the dispersion of the optical fiber. Lithium niobate (L
A Mach-Zehnder optical modulator using iNbO3 is promising. However, in the Mach-Zehnder optical modulator using lithium niobeta, there is an operating point drift (DC drift) in the optical modulation characteristics with respect to the modulation voltage, so in practical use, a control circuit that stabilizes this operating point is required. Is required. As such a Mach-Zehnder type control circuit, a method has been proposed in which a low-frequency signal is superimposed on the electrical signal to be modulated (for example: "Study of automatic bias control circuit for Mach-Zehnder type optical modulator" by Kuwata et al., 1990 Electronic Spring National Conference of the Institute of Information and Communication Engineers B-9
76) In this control method, a 1KHz low frequency signal is superimposed on an electrical signal with a transmission rate of 2.5Gb/s, the optical signal is modulated, the modulated optical signal is converted into an electrical signal, and the signal is transmitted at 1KHz.
The optimum operating point is controlled by extracting the low frequency signal and detecting its phase.

0003

As described above, by superimposing a low frequency signal on a modulation signal, it is possible to control the operating point of a Mach-Zehnder optical modulator. however,
In the above-mentioned method, since a low frequency signal is superimposed, an unnecessary control signal is added to the modulated optical signal, resulting in deterioration of the optical transmission waveform and a reduction in receiving sensitivity due to the waveform deterioration.

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

[0005]

[Means for Solving the Problems] The optical modulator control method of the present invention includes an optical modulator that performs intensity modulation of an optical signal, a light source that inputs an optical signal to the optical modulator, and an optical modulator that outputs light from the optical modulator. a polarization separation circuit into which the output is input, the polarization separation circuit comprising:
Polarization separation is performed into TE polarized light having an electric field component perpendicular to the normal to the substrate surface of the optical modulator and TM polarized light whose polarization plane is orthogonal to the TE polarized light, and the separated TE
The optical modulator is characterized in that bias control of the optical modulator is performed by detecting the average power using polarized light as monitor light.

The optical modulator control method of the present invention includes an optical modulator;
A first optical beam is input to the optical modulator as TE polarized light having an electric field component perpendicular to the normal direction of the substrate surface, and a first optical beam is input as TM polarized light whose polarization plane is orthogonal to the TE polarized light. 2 beams, and a polarization separation circuit that separates the output light from the optical modulator into TE polarization consisting of the first optical beam and TM polarization consisting of the second beam, The present invention is characterized in that bias control of the optical modulator is performed by detecting the average power of the TE polarized light consisting of the first beam.

[0007]

In the present invention, the operating point is controlled by detecting the average value of the TE polarized light component that has passed through the Mach-Zehnder optical modulator.

The operation of the present invention will be explained below with reference to FIG.

FIG. 3(a) is a diagram showing the switching characteristics of the TM polarization component and the TE polarization component in a Mach-Zehnder optical modulator. Here, TE polarized light is polarized light that has an electric field with respect to the normal to the surface of the substrate of the optical modulator, and TM polarized light is polarized light that is orthogonal to TE polarized light.

Usually, the switching voltage difference Vπ between “0” and “1” of the TE polarization component of a Mach-Zehnder optical modulator is
(TE) is Vπ of the TM polarization component mainly used for intensity modulation.
(TM), and when the applied voltage from OV to Vπ (TM) shown in Fig. 3(b) is used as a signal,
The Mach-Zehnder optical modulator operates normally, and its optical output becomes as shown in FIG. 3(c).

On the other hand, FIG. 3(d) shows a case where the operating point of the Mach-Zehnder optical modulator changes. In this case, it is assumed that the point where the optical output is maximum is not at OV but shifted by 1/4, Vπ(TM). At this time, the optical output when the applied voltage shown in FIG. 3(e) is used as a signal is shown in FIG. 3(f).

The optical output shown in FIG. 3(f) is in a state where the depolarization ratio of the TM polarized light component is deteriorated compared to the optical output at the normal operating point shown in FIG. 3(c). In order to investigate this waveform with a degraded polarization ratio, an optical receiver with a bandwidth equivalent to the transmission speed and high-speed peak detection are required, so the operating point of the Mach-Zehnder optical modulator is monitored from the TM polarization component. is difficult. Also, T in Figures 3(c) and 3(f)
The time average power of the M polarization component is the same, and the operating point cannot be monitored from the average power of the TM polarization component.

On the other hand, when focusing on the TE polarization component of the optical output of the Mach-Zehnder optical modulator, the time average power of the TE polarization component in FIG. 3(f) is increased compared to the average power in FIG. 3(c). . Furthermore, when the operating point of the Mach-Zehnder optical modulator is located opposite to the state shown in FIG. 3(d), T
The average power of the E polarization is further reduced compared to the average power of the TE polarization in FIG. 3(c).

Therefore, by detecting the time average power of the TE polarized wave, the operating point of the Mach-Zehnder optical modulator can be found, so by adding a bias to the applied voltage of the signal (Fig. 3 (e) applied voltage of optimum bias). If the time average power of the TE polarized wave is always kept constant, the Mach-Zehnder optical modulator can be controlled at a good operating point.

[0015]

EXAMPLES The present invention will be explained in detail below with reference to Examples.

FIG. 1 shows control etc. of an optical modulator according to an embodiment of the present invention.

In this embodiment, the first
The output light 2 is input into a Mach-Zehnder type optical modulator 3 as a polarized light having a TM polarization component and a TE polarization component. Here, the second output light 4 that has passed through the optical modulator 3 is an optical signal intensity-modulated by the modulation signal inputted from the modulation signal input 11. This second output light 4 is separated by a polarization separation circuit 5 into third output light 6 which is a TM polarization component and a T
The fourth output light 7 is an E-polarized component. The third output light 6 is used as a transmission signal light, and the fourth output light 7 is used as an operating point monitor of the Mach-Zehnder type optical modulator 3. The average power of the fourth output light 7, which is the TE polarization component, increases or decreases as the operating point of the optical modulator 3 shifts.
By detecting the average value of this light, bias control of the voltage applied to the optical modulator 3 becomes possible.

In this embodiment, the fourth output light 7 is received by the photodetector 8, and after being converted to an average power by the amplifier 17 and low-pass filter 18, the reference voltage adjusted to the optimum bias point is applied. A differential amplifier 9 input to a reference terminal 10 extracts an error signal and inputs the signal to a bias circuit 12 .

With the above configuration, the optical modulator 3 was driven at a transmission rate of 10 Gb/s, and the state of bias control was investigated. The optical modulator 3 used here was of Mach-Zehnder type using a lithium niobate substrate, and the switching voltage Vπ (TM) was 4V.

First, the control system of this embodiment was removed and the optical modulator 3 was operated alone. As a result, after 10 hours had elapsed, the bias point of the applied voltage deviated from the optimum point by about 1 V due to temperature changes, pyrovoltage effects, etc., and the optical modulation waveform deteriorated.

On the other hand, when the control system according to the present invention is used, since the operating point bias is controlled, the optical modulator is driven at the optimum operating point for a long time, resulting in waveform deterioration and extinction ratio deterioration. A good optical modulation waveform with no distortion was obtained.

FIG. 2 shows a control system for an optical modulator according to another embodiment of the present invention.

In this embodiment, two semiconductor lasers 1 and 13 are used to increase the optical output from the optical modulator 3. The first semiconductor laser 1 is used for transmitting signals, and the second semiconductor laser 13 is used for transmitting signals. It is used to monitor the operating point of the modulator 3.

In FIG. 2, the output light 2 from the first semiconductor laser 1 is made to be TM polarized light, and the first polarization separation circuit 5
The signal is inputted to a Mach-Zehnder type optical modulator 3 via. This TM polarized output light 2 is intensity-modulated by an optical modulator 3, and then becomes output light 6 for optical transmission via a second polarization separation circuit 14.

On the other hand, the output light 15 from the second semiconductor laser 13 is adjusted to be TE polarized light and inputted to the optical modulator 3 via the second polarization separation circuit 12. This TE polarized light component is intensity-modulated by the optical modulator 3, and then sent to the first polarization separation circuit 5.
The light is inputted to a photodetector 8 as output light 7 via.

In the control system of this embodiment, the backside output light from the second semiconductor laser 13 is monitored and inputted to the differential amplifier 9 as a reference voltage for stabilizing the operating point. By using this configuration, even if the output power of the second semiconductor laser 13 fluctuates, the operating point of the optical modulator 3 can be stably controlled.

In this embodiment, the drive control of the optical modulator 3 was actually performed. The Mach-Zehnder optical modulator 3 using lithium niobate was driven at a transmission rate of 2.5 Gb/s, and good characteristics were confirmed without any fluctuation in the operating point over a long period of time.

Furthermore, in this embodiment, the output light 7 of the monitor light propagates in the opposite direction compared to the output light 6, which is the transmission signal, and therefore does not mix with the output light 6, which is the transmission signal. Therefore, the waveform of the output light 6 of the transmission signal can achieve a good extinction ratio. In this example, an extinction ratio of 20:1 was confirmed, and FIG.
Compared to the control system using one semiconductor laser 1, the power of the output light 6 of the transmission signal could be increased by about 2 dB.

In the above embodiment, the optical path of the optical output 7 from the second semiconductor laser 2 is taken in the opposite direction to that of the optical output 6 from the first semiconductor laser 1. The positions of the semiconductor laser 2 and the photodetector 8 may be exchanged so that the optical paths of the output lights from the first semiconductor laser 1 and the second semiconductor laser 2 are in the same direction. Further, although the two polarization separation circuits 5 and 14 are used in the configuration of FIG. 2, an optical coupler having no polarization characteristics may be used for either one. Further, the optical modulator may be constructed of a semiconductor substrate,
Furthermore, it is not limited to the Mach-Zehnder type, but may be a directional coupler type.

[0030]

[Effects of the Invention] As explained above, according to the present invention,
Since there is no need to apply an extra modulation signal to the Mach-Zehnder type optical modulator, it is possible to obtain a good optical transmission signal without waveform deterioration, and moreover, it is possible to reliably control the operating point when driving the optical modulator.

[Brief explanation of the drawing]

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

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

FIG. 3 is a diagram for explaining the operation of the present invention.

[Explanation of symbols]

1 Semiconductor laser 2 Output light 3. Optical modulator 4 Output light 5 Polarization separation circuit 6 Output light 7 Output light 8 Photodetector 9 Amplifier 10 Modulation signal input 11 Modulation signal input 12 Bias circuit 13 Semiconductor laser 14 Polarization separation circuit 15 Output light 16 Photodetector 17 Amplifier 18 Low pass filter

Claims (2)

[Claims] [Claim 1] An optical modulator that performs intensity modulation of an optical signal;
The polarization separation circuit includes a light source that inputs an optical signal to the optical modulator, and a polarization separation circuit that receives an optical output from the optical modulator, and the polarization separation circuit is configured to polarization separation into TE polarized light having a perpendicular electric field component and TM polarized light whose polarization plane is orthogonal to the TE polarized light, and detecting the average power of the separated TE polarized light as monitor light. An optical modulator control method characterized by performing bias control of an optical modulator. 2. An optical modulator, a first optical beam input to the optical modulator as TE polarized light having an electric field component perpendicular to the normal direction of the substrate surface, and a first optical beam polarized with respect to the TE polarized light; A second beam input as TM polarized light whose wavefronts are orthogonal to each other, and an output light from the optical modulator made up of the first optical beam.
The optical modulator includes a polarization separation circuit that separates the E-polarized light and the TM-polarized light made of the second beam, and detects the average power of the TE-polarized light made of the polarized-separated first beam. An optical modulator control method characterized by performing bias control.