JP2019074642A - Optical modulator - Google Patents

Abstract

To provide an optical modulator capable of reducing a time required to correct a bias voltage, by reducing the number of minimum points to prevent erroneous detection of a minimum point.SOLUTION: The optical modulator of the present invention includes: a multiplexing part for multiplexing output light beams output by two respective Mach-Zehnder optical modulators: an output waveguide for guiding beams multiplexed by the multiplexing part; a first monitor part for photo-electrically converting a first monitor light beam as a part of the beams multiplexed by the multiplexing part to produce a first electric output indicating the light intensity of a first monitor beam; a second monitor part for photo-electrically converting a second monitor beam as a part of the light guided in the output waveguide to produce a second electric output indicating the light intensity of the second monitor beam; and a combined monitor output part for adding the first and second electric outputs or adding after multiplication with a correction coefficient to produce a combined monitor output.SELECTED DRAWING: Figure 4

Description

  The present invention relates to optical modulators, and more particularly to optical modulators used in optical fiber communication technology.

  With the increase of smartphones and the development of the cloud, the amount of communication including movement and fixation keeps on increasing, and the optical communication network connecting the world is always required to increase its capacity. In addition, with the development of IoT and artificial intelligence technology, the demand for huge data centers handling big data etc. is increasing. In order to connect devices dispersed at multiple locations into one data center, the demand for large capacity communication devices at relatively short distances of less than 100 km is also increasing.

  Conventionally, digital coherent communication technology has been used only for trunk lines, but because it has superior characteristics such as high dispersion tolerance, high sensitivity, and network flexibility, it can be used in such short-distance communication devices as well. It has come. For short range communication applications, the number of communication devices required is far greater than long distance trunk devices and the cost requirements are also much more stringent.

Silicon photonics technology is focused on as a technology that responds to such demands for cost reduction and miniaturization. In silicon photonics technology, optical devices such as light wave circuits, light modulators, and light receiving circuits are formed and integrated on a silicon substrate using a matured and inexpensive silicon processing technology as an electronic circuit. Silicon photonics technology can make the radius of curvature of the optical waveguide very small due to the difference in refractive index more than twice between the Si core and the SiO ₂ cladding, so it is very useful for making small, highly integrated and inexpensive optical components. Suitable for

  Among the optical components, a high-speed light modulator is an essential component to realize a transmitter for digital coherent communication. In digital coherent communication, a polarization multiplexing (PM) quadrature phase light modulator is used to make maximum use of optical information. In the past, a four-phase modulation scheme called QPSK (Quadrature Phase Shift Keying) has been used, but recently it has been decided to use a quadrature amplitude modulation scheme with more communication capacity such as 16 QAM (Quadrature Amplitude Modulation) or 64 QAM. It has become to.

  As such a modulator, it is common to use a PM quadrature phase modulator, in which four Mach-Zehnder type (hereinafter MZ type) optical modulators are integrated to realize orthogonal phase and polarization multiplexing. One MZ-type optical modulator realizes an amplitude / binary phase modulator with one polarization and one orthogonal phase, and combines them to form a quadrature phase amplitude modulator.

  When using such an MZ optical modulator, it is necessary to adjust the signal voltage range so as to obtain a correct output.

FIG. 1 (a) illustrates the specific output characteristics of a single MZ-type optical modulator as an example, and FIG. 1 (b) illustrates the specifics of the MZ-type optical modulator used in FIG. 1 (a). An example of the configuration. FIG. 1B shows an MZ optical modulator 100 including a pair of phase modulation electrodes 101 for applying differential high frequency signals ± V _rf and a bias electrode 102 for applying V _dc . .

As shown in FIG. 1A, the output light intensity with respect to V _rf fluctuates with the voltage of V _dc , and as shown in waveform 1, the output light intensity has a local minimum value at 0 V, or the waveform 2 As a result, the minimum value of the output light intensity may deviate from 0V. When used as a phase modulator, it is necessary to adjust V _dc so that the output light intensity has a minimum value at 0 V as shown in waveform 1. By adjusting the bias in this manner, by applying an oscillating voltage centered at 0 V as the signal voltage _Vrf, as a phase modulator that outputs a binary optical signal having the same intensity and the phase reversed from the two arm waveguides. It can be used.

However, in the MZ type optical modulator, the phase difference of the light propagating between the two arm waveguides changes due to the minute change of the waveguide refractive index accompanying the temperature change. Since the wavelength of light propagating through the MZ optical modulator is usually 1/1000 or less of the length of the waveguide, a slight change in the refractive index causes a shift in the light interference state through a change in the phase difference. For this reason, even if the phase modulation electrode 101 is first correctly adjusted, it is necessary to keep adjusting V _dc so that the output light intensity has a minimum value at 0V. That is, in order to confirm that the output light intensity takes a minimum value at 0 V, it is necessary to monitor the light interference state.

As a method of monitoring the interference state of light, there is a method of using a monitor photodiode (MPD). In this method, a part of output light is taken out for monitoring and made to enter the MPD, or light leaked from the combining part that combines the light of the two arm waveguides of the MZ optical modulator is made to enter the MPD In addition, V _dc is adjusted so that the monitor light intensity takes a minimum value at 0 V, utilizing that these monitor lights have bias-dependent extreme values as well as the output light.

The specific method utilizing the monitor output, for example, by superimposing an oscillating voltage called dither signal V _dc, V _dc by the light intensity component of the monitor light having the same frequency as the oscillating voltage is fed back so as to minimize There are techniques such as adjusting the Thus, the monitor light intensity can be adjusted to always take the minimum value at 0V.

As a method of utilizing the leak mode, for example, there is a method described in Non-Patent Document 1. In the method described in Non-patent Document 1, in addition to the propagation mode output to the output waveguide, in the combining section, there is a radiation (leakage) mode not output from the output waveguide. By monitoring the radiation mode with MPD, it is possible to monitor the condition of V _dc without reducing the output light.

N. Miyazaki et al., "LiNbO3 Optical Intensity Modulator Packaged with Monitor Photodiode," IEEE Photonics Technology Letters, Vol. 13, No. 5, pp. 442-444, (2001)

  However, when MPD is used in a quadrature IQ modulator, there is a problem that the minimum value when sweeping the modulator bias is not single. This situation will be described using FIG. 2 and FIG.

  FIG. 2 illustrates a conventional IQ quadrature MZ light modulator. In FIG. 2A, the light input from the input waveguide 201, the output waveguide 202, the first and second MZ optical modulators 210 and 220, and the input waveguide 201 is first and second The light beams guided by the branching unit 203 branched into the two MZ optical modulators 210 and 220 and the first and second MZ optical modulators 210 and 220 are multiplexed and output to the output waveguide 202 The light intensity of the leaked light of the combining unit 204, the monitoring waveguide 205 for inputting and guiding leaked light in the combining unit 204, and the combining unit 204 for guiding the monitoring waveguide 205 is monitored An IQ quadrature MZ optical modulator 200 with an MPD 206 is shown.

The first MZ-type optical modulator 210 includes a branch portion 211 which further branches the light branched by the branch portion 203, arm waveguides 212 and 213 which respectively guide the light branched by the branch portion 211, and an arm It includes a wave combining portion 214 for multiplexing the light guided through the waveguides 212 and 213, a phase modulation electrode 215, and a bias electrode 216 for applying a bias voltage _VA .

The second MZ-type optical modulator 220 further includes a branch portion 221 which further branches the light branched by the branch portion 203, arm waveguides 222 and 223 which respectively guide the light branched by the branch portion 221, and an arm the waveguides 222 and 223 and multiplexing section 224 that multiplexes the light waveguide, and applies the phase modulation electrode 225, and the bias electrode 226 for applying a bias voltage V _B, the bias voltage V _C for quadrature phase adjustment And a bias electrode 227.

  Further, as described above, instead of monitoring the leaked output of the multiplexing unit 205 by the MPD 206, part of the light can be branched and taken out and monitored from the output waveguide. The example is shown in FIG.2 (b). In the IQ orthogonal MZ type optical modulator 250 shown in FIG. 2B, the output waveguide 202 is not provided with the monitor waveguide 205 and the MPD 206 in the IQ orthogonal MZ type optical modulator 200 shown in FIG. 2A. The configuration corresponds to the configuration in which the monitor waveguide 207 and the MPD 208 are provided. In the IQ orthogonal MZ type optical modulator 250, part of light from the output waveguide 202 is branched and taken out by the monitoring waveguide 207 and monitored by the MPD 208.

FIG. 3A shows the output dependency of the light monitored by the MPD 206 in the IQ orthogonal MZ type optical modulator 200 with respect to the bias voltages V _A and V _B , and FIG. 3B shows the IQ orthogonal MZ type optical modulator the monitor light by MPD208 at 250, shows the output dependence on the bias voltage V _a and V _B.

It turns out that two minimum points, a star 301 and a circle 302, exist in any of FIG. 3 (a) and FIG. 3 (b). However, since convergence to the local minimum of the circle 302 among the two local minimums causes a malfunction, the minimum point to be truly converged is the part of the star 301. Even if adjustment is made with V _A and V _B so as to reach the minimum point at startup, if the bias point is largely deviated due to temperature or the like after that, or when the minimum point is erroneously detected If the points are caught, there is a problem that it takes time to fix in V _a and V _B to converge truly minimum point you want to converge.

  The optical modulator according to the present invention has been made to address such a problem, and by reducing the number of local minima, it becomes possible to prevent the misdetection of the misdetection of the minimum point, whereby bias The purpose is to reduce the time required to correct the voltage.

  The quadrature-phase optical modulator according to an aspect of the present invention is a quadrature-phase optical modulator in which two Mach-Zehnder optical modulators are integrated, and the output light respectively output from the two Mach-Zehnder optical modulators is combined The first monitor light, which is a portion of the light multiplexed in the wave combining section, the output waveguide for guiding the light multiplexed in the wave combining section, and the light, is photoelectrically converted A first monitor unit generating a first electrical output indicating the light intensity of the first monitor light, and photoelectrically converting a second monitor light that is part of the light guided through the output waveguide And a second monitor unit that generates a second electrical output indicating the light intensity of the second monitor light, and a composite monitor output that is generated by adding the first and second electrical outputs. And a monitor output unit.

  The quadrature-phase optical modulator according to an aspect of the present invention is a quadrature-phase optical modulator in which two Mach-Zehnder optical modulators are integrated, and the output light respectively output from the two Mach-Zehnder optical modulators is combined The first monitor light, which is a portion of the light multiplexed in the wave combining section, the output waveguide for guiding the light multiplexed in the wave combining section, and the light, is photoelectrically converted A first monitor unit generating a first electrical output indicating the light intensity of the first monitor light, and photoelectrically converting a second monitor light that is part of the light guided through the output waveguide And a second monitor unit for generating a second electrical output indicating the light intensity of the second monitor light, and multiplying the first and second electrical outputs by first and second correction coefficients, respectively. To generate first and second weighted electrical outputs, and adding the first and second weighted electrical outputs Characterized by comprising a combining monitor output unit for generating a composite monitor output, a by.

  According to the light modulator of the present invention, it is possible to prevent the erroneous detection of the minimum point by reducing the number of the minimum points, and it is thereby possible to reduce the time required to correct the bias voltage.

It is a figure which illustrates (a) output intensity characteristics and (b) composition of a common single MZ type light modulator. It is a figure which illustrates together two types of monitor PD structure (a) and (b) about the structure of the conventional IQ orthogonal MZ type | mold optical modulator. Is a diagram showing an output dependency on V _A and V _B of the monitor light in two kinds of monitor PD shown in FIG. 2 in a conventional IQ quadrature MZ type optical modulator (a) and (b). It is a figure which illustrates the structure of IQ orthogonal MZ type | mold light modulator based on Example 1 of this invention. In IQ quadrature MZ type optical modulator according to the present invention, showing the dependence on (a) the first monitoring unit, (b) V _A and V _B of the monitor output of the second monitoring unit and (c) Synthesis monitor FIG. It is a figure which illustrates the structure of the polarization multiplexing IQ orthogonal MZ type | mold optical modulator based on Example 2 of this invention.

Example 1
FIG. 4 is a diagram illustrating the configuration of an IQ orthogonal MZ type optical modulator according to a first embodiment of the present invention. In FIG. 4, the light input from the input waveguide 401, the output waveguide 402, the first and second MZ optical modulators 410 and 420, and the input waveguide 401 is referred to as the first and second MZ. Type light modulators 410 and 420, and a combining section for combining the light guided by the first and second MZ type optical modulators 410 and 420 and outputting the combined light to the output waveguide 402 404, a first monitor waveguide 405 from which the leaked output of the multiplexing unit 404 is taken out, and a first monitor to monitor the light intensity of the first monitor light guided through the first monitor waveguide 405 Unit 406, a second monitor waveguide 407 for branching and guiding a part of the output light guided through the output waveguide 402, and a second monitor waveguide for guiding the second monitor waveguide 407 A second monitor unit 408 for monitoring the light intensity of the light; A force unit 409, the IQ quadrature MZ type optical modulator 400 with are shown.

As shown in FIG. 4, the first MZ optical modulator 410 includes a branch 411 that further branches the light branched by the branch 403 and an arm that guides the light branched by the branch 411. Waveguides 412 and 413, a wave combining portion 414 for multiplexing the light guided in the arm waveguides 412 and 413, and a phase modulation electrode 415 for applying a pair of differential modulation voltages to the arm waveguides 412 and 413; including the arm waveguide 412 and the bias electrode 416 for applying a bias voltage V _a, a.

The second MZ optical modulator 420 further includes a branch portion 421 that further branches the light branched by the branch portion 403, and arm waveguides 422 and 423 that respectively guide the light branched by the branch portion 421. A multiplexing unit 424 for multiplexing the light guided through the arm waveguides 422 and 423, a phase modulation electrode 425 for applying a pair of differential modulation voltages to the arm waveguides 422 and 423, and a bias for the arm waveguide 423; It includes a bias electrode 426 for applying a voltage V _B, the bias electrode 427 for applying a bias voltage V _c for quadrature phase adjustment to the waveguide between the multiplexing unit 404 and 424, a.

  Bias electrodes 416 and 426 are provided along arm waveguides 412 and 423, respectively. The bias electrode 427 is provided along the waveguide between the couplers 404 and 424.

  In the first embodiment, the combining unit 404 is configured by a combining unit with two inputs and one output, and the first monitor unit 406 uses the first leaked light leaked from the combining unit 404 with two inputs and one output. Detection is performed via the monitoring waveguide 405. At this time, as described in Non-Patent Document 1, the first monitor light, which is the leaked output of the branch unit 403 used in the first monitor unit 406, is one of the output light guided through the output waveguide 402. The phase differs by π at the time of monitor output combining in the combining monitor output unit 409 as compared with the second monitor light that is the unit.

  In addition, as the first and second monitor units 406 and 408, for example, a photodiode can be used as a monitor photodiode (MPD).

In the IQ orthogonal MZ type optical modulator 400 according to the first embodiment, the first and second monitor lights at two points of the multiplexing part 404 and the output waveguide 402 are transmitted to the first and second monitoring parts 406 and 408. Each is incident. First and second monitor section 406 and 408, by photoelectrically converting the first and second monitor light incident respectively, the first and second electrical output EA _L and EA _T indicates the light intensity They are respectively generated and output to the combined monitor output unit 409.

Synthesis monitor output unit 409, the weighting of the first and second electrical output EA _L and the first and second weighting factors needed to erase the effects of metastable point for EA _T beta _L and beta _T Then, the combined monitor output MG is generated by adding. In this case, for example, monitor the ratio to the first main signal electrical output EA _L the first and second at 1 / 2-1 / 10 of the monitor ratio main signal in the second electrical output EA _T The weighting factors β _L and β _T can be determined.

FIGS. 5 (a) and 5 (b), the IQ quadrature MZ type optical modulator 400, when changing the V _A and V _B, the first representing the light intensity of the first and second monitor light and it illustrates a second electrical output EA _L and EA _T. Further, FIG. 5 (c) illustrates the synthesis monitor output MG produced by adding and weighted by the first and second electrical output EA _L and EA _T beta _L and beta _T. Synthesis monitor output MG shown in FIG. 5 (c), multiplied by beta _L = 0.2 with respect to EA _L, is generated by adding after it has been multiplied by the beta _T = 0.8 against EA _T .

In the results shown in FIGS. 5A and 5B, there are two minimum points, which are the minimum point and the metastable point. On the other hand, it can be seen that the result shown in FIG. 5C differs from the results shown in FIGS. 5A and 5B in that the minimum point is one and only the minimum point. This is in the vicinity of the maximum point in the result shown in FIG. 5B in the voltage range of V _A and V _{B at} the metastable point indicated by the circle mark in FIG. 5A, and similarly, FIG. In the voltage range of V _A and V _{B at} the metastable point indicated by the circle mark in (b), since it is near the maximum point in the result shown in FIG. 5A, after weighting by β _L and β _T of when the sum of the EA _L and EA _T because the metastable point is canceled. On the other hand, since the minimum points respectively indicated by star marks in FIG. 5A and FIG. 5B exist in the same voltage range of V _A and V _{B respectively} , after weighting by β _L and β _T it can remain as the minimum point also adds the EA _L and EA _T without being canceled with each other.

Note that the first and second weighting coefficients β _L and β _{T at} the time of combining can be set to any value in a range in which the effect of the metastable point can be eliminated or substantially ineffective. For the _{example, (β L: β T)} = (0.2: 0.8) ratio, relative to the EA _T, corresponds to the case of a 4-fold higher weighting than EA _L. This is because the have the same phase as the main signal a second monitor light constituting the EA _T is outputted from the output waveguide 402, the first monitor light the main signal constituting the EA _L in order to approximate the stable point of the main signal output from to have a different phase, the detailed location of the larger weighting stable point synthetic monitor output MG of EA _T from the output waveguide 402 and is there.

  By using the combined monitor output MG as a monitor output for bias adjustment, it is possible to realize an optical modulator that can perform bias adjustment stably by only one minimum point.

As a method of maintaining the combined monitor output MG at the minimum value, for example, a dither signal is superimposed on V _A and V _{B so} that the light intensity of the combined monitor output MG of the same frequency component as the frequency of the dither signal is minimized. there are methods such as adjusting the V _a and V _B to. To adjust the V _A and V _B, for example, by connecting to the optical modulator according to the present invention further provided or the bias voltage adjustment circuit to the optical modulator according to the invention or an external bias voltage adjustment circuit , it is possible to adjust the V _a and V _B based on the combined monitor output MG. More specifically, the bias voltage adjustment circuit, so that the resultant monitor output MG is minimized, it is possible to adjust the V _A and V _B.

  In addition, in order to detect a minute monitor output, it is possible to amplify with a semiconductor optical amplifier and then detect with a photodiode, or to use an avalanche photodiode (APD). In these cases, the correction coefficient can be adjusted by the multiplication factor by the amplifier or APD and used for weighting.

However, in order to actually carry out weighting, it may be necessary to carry out in consideration of the ratio to the signal strength. Since the first and second monitor lights respectively incident on the first and second monitor parts 406 and 408 are leaked or branched at different points, the light for the main signal SigX output from the output waveguide 402 The intensity ratio is not necessarily the same. For example, the light intensity of the leaked light as the first monitor light detected by the first monitor unit 406 is the light intensity of the second monitor light branched from the output waveguide 402 detected by the second monitor unit 408 It is different. Therefore, even if adding the weighting by the first and second electrical output EA _L and EA _T as it beta _L and beta _T bias voltage adjusting circuit, not to the weighting designed, only one minimum point is It becomes difficult to appear noticeably. Therefore, the EA _L and EA _T first and second correction coefficient alpha _L and alpha _T first and second corrected electricity conducted respectively corrected by the output EA _L1 and EA _T1 in line with the signal intensity ratio for After that, by weighting with the first and second weighting factors β _L and β _T , it is possible to obtain a synthesized output at the intended effective ratio with respect to the main signal. is there.

By performing this correction with α _L and α _T in advance, the substantial ratio of the first and second electrical outputs after weighting with β _L and β _T can be expressed as the desired ratio (β _L : β _T ). can do. To derive the coefficients alpha _L and alpha _T, for example a first and second electrical output EA _L and EA _T of sweeping V _A and V _B when not applied modulation voltage, then the maximum when made, the first corrected electric output EA _L1 = α _L EA _L and the second corrected electric output EA _T1 = α _T EA first and second correction coefficients such that the value becomes the same _T alpha there are methods such as previously calculated _L and alpha _T.

  In addition, as means for combining two monitor outputs, A / D conversion is performed on the outputs of the first monitor unit 406 and the second monitor unit 408, and the above correction is performed in the digital domain and then the sum is taken. Alternatively, it is possible to use a method of performing correction and addition processing as it is without analog-to-digital conversion. In addition, adjustment of weighting can be performed by an optical method using an optical amplifier or an optical attenuator, and two monitor outputs can be input to one photodiode after processing by this optical method. It is.

Here, in the first embodiment, the correction and weighting by respective the EA _L and EA _T multiplied by a predetermined correction coefficient and weighting factors, an example for generating a composite monitor output MG, the output signal If the ratio is in advance required ratio may be configured to generate a composite monitor output MG by directly adding the EA _L and EA _T. The same applies to Example 2 below.

  Further, in the first embodiment, the combining unit 404 is configured by a combining unit with two inputs and one output, and the first monitor unit 406 detects leaked light leaked from the combining unit 404 with two inputs and one output. An example is shown, but the invention is not limited thereto. For example, the multiplexing unit 404 is configured of a 2-input 2-output multiplexing unit, one of the output ends is connected to the output waveguide 402, and the other output end is The first monitor light may be detected by the first monitor unit 406 by connecting to the first monitor waveguide 405. The same applies to Example 2 below.

(Example 2)
FIG. 6 illustrates the configuration of a polarization multiplexing IQ orthogonal MZ optical modulator according to an embodiment of the present invention. Figure 6 is provided with a first IQ quadrature MZ type optical modulator 400 _1, a second IQ quadrature MZ type optical modulator 400 _2, the polarization rotator 601, the polarization beam combiner 602, a A polarization multiplexed IQ orthogonal MZ optical modulator 600 is shown.

The first IQ quadrature MZ type optical modulator 400 _first and second IQ quadrature MZ type optical modulator 400 ₂ has the same configuration as that of the IQ quadrature MZ type optical modulator 400 according to each embodiment 1. For example, a first IQ quadrature MZ type optical modulator 400 first and second MZ type light modulators 410 ₁ and 420 ₁ of ₁ corresponds to the I-channel and Q-channel of polarization X, the second IQ quadrature MZ type optical modulator 400 ₂ of the first and second MZ type light modulator 410 ₂ and 420 ₂ may correspond to I-channel and Q-channel of the polarization Y.

As shown in FIG. 6, the output of the second IQ quadrature MZ type optical modulator 400 ₂ is polarization rotated by the polarization rotator 601, a first IQ quadrature MZ type optical modulator in the polarization beam combiner 602 vessels 400 is _first output and polarization combining.

  The polarization multiplexing IQ orthogonal MZ optical modulator 600 according to the second embodiment can be used for polarization multiplexing quadrature phase amplitude modulation such as DP (Dual Polarization) -64 QAM, DP-16 QAM, for example.

Claims (6)

A quadrature-phase optical modulator in which two Mach-Zehnder optical modulators are integrated,
A multiplexer that multiplexes output lights respectively output from the two Mach-Zehnder optical modulators;
An output waveguide for guiding the light combined in the combining unit;
A first monitor unit that photoelectrically converts a first monitor light, which is a part of the light multiplexed by the multiplexing unit, and generates a first electrical output indicating a light intensity of the first monitor light. When,
A second monitor unit photoelectrically converting a second monitor light which is a part of light guided through the output waveguide, to generate a second electric output indicating a light intensity of the second monitor light; ,
A combined monitor output that generates a combined monitor output by adding the first and second electrical outputs;
A quadrature phase light modulator comprising: A quadrature-phase optical modulator in which two Mach-Zehnder optical modulators are integrated,
A multiplexer that multiplexes output lights respectively output from the two Mach-Zehnder optical modulators;
An output waveguide for guiding the light combined in the combining unit;
A first monitor unit that photoelectrically converts a first monitor light, which is a part of the light multiplexed by the multiplexing unit, and generates a first electrical output indicating a light intensity of the first monitor light. When,
A second monitor unit photoelectrically converting a second monitor light which is a part of light guided through the output waveguide, to generate a second electric output indicating a light intensity of the second monitor light; ,
The first and second electrical outputs are respectively multiplied by a first and second weighting factor to produce first and second weighted electrical outputs, and the first and second weighted electrical outputs are summed. A synthetic monitor output unit that generates a synthetic monitor output by
A quadrature phase light modulator comprising:   The first and second weighting factors are determined such that the effective monitoring rate of the first electrical output is 1/2 to 1/10 of the effective monitoring rate of the second electrical output. The quadrature phase light modulator according to claim 2, characterized in that:   The quadrature phase light modulator according to any one of claims 1 to 3, wherein the bias voltage is controlled by a bias voltage adjustment unit so as to minimize the combined monitor output. The combining unit is a combining unit with two inputs and one output,
The quadrature phase light modulator according to any one of claims 1 to 4, wherein the first monitor unit inputs the leaked light leaked from the combining unit as the first monitor light. 6. The quadrature optical modulator according to any one of claims 1 to 5,
A polarization rotator for rotating polarization at one output of the quadrature optical modulator according to any one of claims 1 to 5.
The other output of the quadrature optical modulator according to any one of claims 1 to 5, and a polarization beam combiner for polarization combining light polarized and rotated by the polarization rotator.
What is claimed is: 1. A polarization multiplexing quadrature phase optical modulator comprising: