JP2010002775A - Optical transmitter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical transmitter having a bias control function of automatically and stably controlling optical phase control without performing superposition of dither to an optical phase adjusting part without depending on the description of an optical modulator and the bias control method. <P>SOLUTION: The optical transmitter includes: optical modulation parts 2a and 2b in which carrier light outputted from a light source 15 is splitted and inputted; the optical phase adjustment part 3 connected to the optical modulation part 2b and shifting an optical phase of an inputted light signal according to an applied DC bias and outputting the shifted optical phase; a 3 dB coupler 4 as a multiplex/interference part; tap couplers 5a and 5b acquiring portions of output of the 3 dB coupler 4 at prescribed ratios; light receivers 7a and 7b respectively receiving light signals taken out by the tap couplers 5a and 5b; and a control part controlling the DC bias applied to the optical phase adjustment part 3 so that the difference of light received amounts detected by the light receivers 7a and 7b is made to be zero. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an optical transmitter having a function for automatically and stably controlling an optical phase difference between optical modulation signals respectively output from a plurality of optical modulators.

  In order to cope with an increase in information communication traffic in recent years, there is a very strong demand for high-speed and large-capacity communication networks. There is a demand for practical use of an optical transceiver that meets this requirement, and as one of the candidates, adoption of a DQPSK (Differential Quadrature Phase Shift Keying) modulation method based on phase modulation is considered promising. As a configuration of an optical transmitter that realizes the DQPSK modulation method, for example, as disclosed in Patent Document 1, a method of combining phase-modulated optical signals generated by two optical modulators is well known.

  When a DQPSK modulated signal is generated by combining two phase modulated optical signals as described above, it is necessary to adjust the optical phase difference between the respective phase modulated optical signals to π / 2. Since this optical phase difference directly affects the inter-symbol distance of the signal to be transmitted, the optical signal quality degradation is very large for the deviation from the optimum adjustment point. For this reason, it is indispensable for the practical use of the DQPSK modulation method to establish and implement an adjustment method for automatically and stably adjusting the optical phase difference between the two phase-modulated optical signals.

  As such a technique, for example, in Patent Document 2, the optical phase difference is adjusted by superimposing a low-frequency dither signal on a variable delay device that adjusts the optical phase.

  On the other hand, in Patent Document 3, a low-frequency dither signal superimposed on each optical modulator is used to optimize the DC bias of each optical modulator, and the optical phase difference between the modulated signals becomes π / 2. By utilizing the fact that the difference frequency or sum frequency between these dither signals does not occur, the optical phase difference between the two modulated optical signals can be controlled without applying a low frequency dither signal to the optical phase adjustment unit. Yes.

JP-T-2004-516743 Japanese Patent Laid-Open No. 10-200512 JP 2007-133176 A

  However, the conventional techniques have the following problems. That is, the conventional technique described in Patent Document 1 has a problem in that the deterioration of the quality of the main signal due to the low frequency dither signal itself cannot be ignored because the low frequency dither signal is superimposed on the variable delay device that adjusts the optical phase. It was.

  The conventional technique described in Patent Document 2 can be applied when it is not necessary to use a low-frequency dither signal as a means for optimizing the DC bias, such as when each optical modulator is a phase modulator. There was a problem that I could not.

  The present invention has been made to solve the above-described problems, and does not depend on the type of optical modulator or the bias control method, and does not perform dither superimposition on the optical phase adjustment unit. It is an object of the present invention to provide an optical transmitter having a bias control function for controlling automatically and stably.

  In order to solve the above-described problems and achieve the object, an optical transmitter according to the present invention includes first and second optical modulation units to which carrier light output from a light source is demultiplexed and input, An optical phase adjusting unit connected to an input side or an output side of the second optical modulation unit and shifting and outputting an optical phase of an optical signal input in accordance with an applied DC bias; and the first light A first input unit to which an output of a modulation unit is input; a second input unit to which an output of the optical phase adjustment unit or an output of the second optical modulation unit is input; and the first and second When the optical signals input to the input unit are in phase with each other, the first output unit that outputs all of the input light and the optical signals input to the first and second input units are opposite to each other. A second output unit that outputs all of the input light when in phase, and Optical signals input to the first and second input units are combined and interfered, and output from the first and second output units, and a predetermined output from the output of the first output unit. An output acquisition unit that acquires a part thereof at a rate and acquires a part of the output from the output of the second output unit at a predetermined rate, and a light receiving unit that receives each optical signal acquired by the output acquisition unit. And the received light amount of the optical signal from the first output unit detected by the light receiving unit and the received light amount of the optical signal from the second output unit are applied to the optical phase adjusting unit. And a control unit that controls the DC bias.

  According to the present invention, the direct current bias applied to the optical phase adjustment unit is determined by the received light amount of the optical signal from the first output unit and the received light amount of the optical signal from the second output unit detected by the light receiving unit. By controlling based on the above, there is an effect that the optical phase difference between the optical signals input to the first and second input units can be controlled automatically and stably. Thus, a desired operation can be realized with a simple circuit configuration without using a low-frequency dither signal.

  Embodiments of an optical transmitter according to the present invention will be described below in detail with reference to the drawings. Note that the present invention is not limited to the embodiments.

Embodiment 1 FIG.
FIG. 1 is a block diagram showing a configuration of a DQPSK modulation type optical transmitter according to the present embodiment. As shown in FIG. 1, in the optical transmitter according to the present embodiment, a Mach-Zehnder (hereinafter abbreviated as MZ) interferometer 1 having upper and lower waveguides is configured. For example, an optical modulation unit 2a made of an optical modulator having an MZ type structure is provided, and an optical modulation unit 2b made of an optical modulator having an MZ type structure, for example, and an optical modulation unit 2b on the lower waveguide. A connected optical phase adjusting unit 3 is provided. In FIG. 1, the optical phase adjusting unit 3 is provided at the subsequent stage (output side) of the optical modulation unit 2b, but may be provided at the previous stage (input side), and not the lower waveguide. It may be provided in the upper waveguide.

  On the input side of the MZ interferometer 1, there is provided an optical demultiplexing unit 16 that bifurcates the carrier optical signal output from the light source 15 and demultiplexes it into the upper and lower waveguides. Further, on the output side of the MZ interferometer 1, for example, a 3 dB coupler 4 is provided as an optical multiplexing / interference unit that multiplexes and interferes the outputs of the optical modulation unit 2 a and the optical phase adjustment unit 3.

  A predetermined direct current bias that determines the operating point is applied to each of the optical modulators 2a and 2b, and a modulation signal for data modulation is applied (not shown). The optical signals output from the light source 15 and divided into two by the optical demultiplexing unit 16 are input to the optical modulation units 2a and 2b, respectively, and output after undergoing phase modulation by the optical modulation units 2a and 2b. The phase difference between the optical signals output from the optical modulators 2a and 2b is set to be 0 or π.

  A predetermined direct current bias that determines the operating point is applied to the optical phase adjustment unit 3, whereby the optical phase difference between the output light of the optical phase adjustment unit 3 and the output light of the optical modulation unit 2 a is π / It is controlled to be 2. The direct current bias applied to the optical phase adjustment unit 3 is supplied from the control unit 9 described later.

  The 3 dB coupler 4 has, for example, an X-type branch structure, and has two input units (first and second input units) and two output units (first and second output units). The optical signals respectively input from the optical modulation unit 2a and the optical phase adjustment unit 3 are combined / interfered by the 3 dB coupler 4 and output after being branched in two. One of the two branches of this output is output as the transmitter output 8 and the other is terminated at the optical termination 6.

  In the 3 dB coupler 4, when the optical phase difference between the optical signals output from the optical modulation unit 2 a and the optical phase adjustment unit 3 is 0 or π, all of the optical signals input to the 3 dB coupler 4 are either one of the branches. Is output from. That is, for example, when the optical phase difference is 0, all the optical signals input to the 3 dB coupler 4 are output to the branch of the transmitter output 8, and when the optical phase difference is π, the optical signals input to the 3 dB coupler 4 Are all output to the branch of the optical termination 6. In other words, when the optical signals output from the optical modulation unit 2a and the optical phase adjustment unit 3 and input to the 3 dB coupler 4 are in phase with each other, all the optical signals input to the 3 dB coupler 4 are, for example, transmitters. When the optical signals output to the branch of the output 8 and respectively output from the optical modulation unit 2a and the optical phase adjustment unit 3 and input to the 3 dB coupler 4 have opposite phases, the optical signal input to the 3 dB coupler 4 is All are output to the branch of the optical terminal 6, for example. On the other hand, when the optical phase difference between the optical signals output from the optical modulation unit 2a and the optical phase adjustment unit 3 is exactly shifted by π / 2, the output light levels output from the 3 dB coupler 4 are the same. That is, the output light output to the branch of the transmitter output 8 and the output light output to the branch of the optical terminal 6 are at the same level.

  On the output side of the 3 dB coupler 4, tap couplers 5a and 5b and light receivers 7a and 7b are provided. The tap coupler 5a extracts a part of the output on the transmitter output 8 side, which is one output of the 3 dB coupler 4, and the tap coupler 5b, a part of the output on the optical terminal 6 side, which is the other output of the 3 dB coupler 4. Take out. Each of the tap couplers 5a and 5b acquires an output from the output of the 3 dB coupler 4 at a predetermined same ratio. For example, if the output light level of the transmitter output 8 side of the 3 dB coupler 4 and the output light level of the optical termination 6 side are the same, the output light levels partially acquired by the tap couplers 5a and 5b are also the same. . The optical signal taken out by the tap coupler 5a is received by the light receiver 7a and is optically / electrically converted. The optical signal taken out by the tap coupler 5b is received by the light receiver 7b and is optically / electrically converted. The tap couplers 5a and 5b constitute an output acquisition unit, and the light receivers 7a and 7b constitute a light receiving unit.

  A control unit 9 is provided that determines and outputs a DC bias that controls the optical phase shift amount of the optical phase adjustment unit 3. The control unit 9 adjusts the direct current bias determined by the control signal input 10 to the control unit 9 based on the difference between the received light amounts detected by the light receivers 7 a and 7 b, and then outputs it to the optical phase adjustment unit 3. . That is, the control unit 9 calculates the difference between the received light amounts detected by the light receivers 7a and 7b, adjusts the DC bias so that the difference decreases and becomes zero, and outputs the difference to the optical phase adjustment unit 3. Thereby, the optical phase difference between the optical signals output from the optical modulation unit 2a and the optical phase adjustment unit 3 is controlled to be π / 2.

  Next, the operation of the present embodiment will be described. The carrier optical signal output from the light source 15 is input to the MZ interferometer 1 and branched into two by the optical demultiplexing unit 16. The signal light traveling through the upper waveguide is subjected to phase modulation by the light modulator 2a. The signal light traveling through the lower waveguide is subjected to phase modulation by the optical modulation unit 2b, and further subjected to an optical phase shift of π / 2 based on the direct current bias input from the control unit 9 in the optical phase adjustment unit 3. . Then, the optical signal output from the optical modulation unit 2a and the optical signal output from the optical phase adjustment unit 3 are input to the 3 dB coupler 4, combined / interfered, and then branched into two to be output. Part of the output of the 3 dB coupler 4 is taken out by the tap couplers 5a and 5b, received by the light receivers 7a and 7b, and optical / electrically converted.

  Here, as described above, when the optical phase difference of the optical signal output from the optical modulation unit 2 a and the optical phase modulation unit 3 and input to the 3 dB coupler 4 is 0 or π, the optical signal is input to the 3 dB coupler 4. All the light is output from one of the branches. On the other hand, when the optical phase difference is shifted by exactly π / 2, the respective output light levels of the 3 dB coupler 4 are the same. Therefore, a desired phase relationship can be obtained by controlling the DC bias of the optical phase adjusting unit 3 so that the received light amounts detected by the light receivers 7a and 7b are the same or the difference between them is zero. .

  Returning to the explanation of the operation, the received light amounts detected by the light receivers 7 a and 7 b are respectively output to the control unit 9. The control unit 9 calculates the difference between the received light amounts detected by the light receivers 7a and 7b, and controls the DC bias applied to the optical phase adjustment unit 3 so that the difference amount becomes zero.

  In the actual transmitter configuration, pre-equalization modulation was performed in addition to various error factors such as the insertion loss of the optical modulation units 2a and 2b, the insertion loss of the optical phase adjustment unit 3, and the coupling efficiency of the 3 dB coupler 4. In some cases, the mark ratio of the modulation signal may be a value different from the normal ½. Accordingly, by controlling these factors by the control unit 9, it is possible to control the optical signal in a desired phase relationship regardless of the aforementioned factors. Specifically, the control unit 9 weights the received light amounts detected by the light receivers 7a and 7b as necessary, and the optical phase is set so that the difference between these weighted received light amounts becomes zero. The DC bias to the adjustment unit 3 can be controlled. This controls the direct current bias to the optical phase adjusting unit 3 so that the amount of received light detected by the light receivers 7a and 7b becomes a predetermined ratio.

  In the above, the 3 dB coupler 4 can be configured as a fiber type, a planar waveguide type, a half mirror type, or the like.

  FIG. 3 is a block diagram showing a configuration of the optical time division multiplexing apparatus described in Patent Document 2. In FIG. In FIG. 3, the optical time division multiplexing apparatus includes a short pulse light source 51, an optical splitter 52, optical modulators 53 and 54, a variable optical delay element 57, an optical coupler 58, a photoelectric converter 59, a band pass filter 60, and a detection. 61, a phase comparator 62, a low frequency transmitter 63, an error amplifier 64, and an adder 65.

  In FIG. 3, the optical short pulse train from the short pulse light source 51 is divided into two systems by the optical branching device 52, and after data modulation by the optical modulators 53 and 54, the phase is adjusted by the variable optical delay element 57 in one system. Then, the optical coupler 58 combines the signals so as to be every other bit. Here, the reference frequency signal generated by the low frequency oscillator 63 is added to the delay amount control signal of the variable optical delay element 57, and the delay amount is minutely modulated. Then, the optical output partially branched by the optical coupler 58 is received by the photoelectric converter 59, the frequency component corresponding to the code speed twice that of the optical short pulse train is extracted by the band pass filter 60, and detected by the detector 61. Thereafter, the phase comparator 62 compares the phase with the reference frequency signal, the phase error signal is amplified by the error amplifier 64, and then added to the output of the low frequency oscillator 63 to adjust the delay amount control signal.

  In this conventional optical time division multiplexing apparatus, an optical coupler is used to superimpose a reference frequency signal generated by the low-frequency oscillator 63, that is, a low-frequency dither signal, on the variable optical delay element 57 that is an optical phase adjustment unit. The low-frequency dither signal is also superimposed on the multiplexed optical output output from 58, and there may be a problem that quality degradation of the main signal due to the low-frequency dither signal itself cannot be ignored. On the other hand, in the present embodiment, it is not necessary to superimpose a low-frequency dither signal on the optical phase adjustment unit 3, so that such a problem does not occur.

  FIG. 4 is a block diagram showing a configuration of the optical modulator described in Patent Document 3. As shown in FIG. In FIG. 4, an optical modulator 105 includes an upper waveguide 106a, a lower waveguide 106b, high frequency modulation signal driving means 110a and 110b, MZ type optical modulation sections 113a and 113b, DC bias control means 111a, 111b, and 111c, and addition. Means 112a and 112b, low frequency oscillators 114a and 114b, optical phase adjustment unit 115, mixing means 116, photoelectric conversion means 117, synchronous detection means 118, optical demultiplexing means 119a and 119b, and optical multiplexing means 120 are provided. .

  The input light is demultiplexed by the optical demultiplexing means 119a and is input to the MZ type optical modulation units 113a and 113b, respectively. A DC bias is input from the DC bias control unit 111a to the MZ type optical modulation unit 113a, and a high frequency modulation signal for data modulation is input from the high frequency modulation signal driving unit 110a. Further, a low frequency dither signal having a frequency f1 is superimposed on the DC bias from the low frequency oscillator 114a. The MZ type optical modulation unit 113b can be described in the same manner. A low frequency dither signal having a frequency f2 is superimposed on the applied DC bias from the low frequency oscillator 114b. The optical signal output from the MZ type optical modulation unit 113b is given an optical phase difference π / 2 by the optical phase adjustment unit 115, and the output light from the optical phase adjustment unit 115 and the output light from the MZ type optical modulation unit 113a. Are multiplexed by the optical multiplexing means 120.

  A part of the optical signal output from the optical multiplexing unit 120 is branched by the optical demultiplexing unit 119b, and is output to the synchronous detection unit 118 via the photoelectric conversion unit 117. The synchronous detection means 118 detects a light amount change in each frequency component of the sum frequency f1 + f2 and the difference frequency f1-f2 by comparison with a reference signal. The DC bias control unit 111 c controls the DC bias output to the optical phase adjustment unit 115 based on the change in the synchronous detection output at the sum frequency and the difference frequency output from the synchronous detection unit 118. When the phase difference applied by the optical phase adjustment unit 115 is π / 2, the sum frequency component and the difference frequency component are 0, and an error signal having a sign is generated when a deviation from π / 2 occurs.

  In this conventional optical modulator, a low frequency dither signal is superimposed on each of the optical modulators of the MZ type optical modulators 113a and 113b as means for optimizing the DC bias. In general, the optical modulator is a phase modulator. In the case where the low frequency dither signal is not used as means for optimizing the DC bias, such as in the case of the above, there is a problem that it cannot be applied. On the other hand, in the present embodiment, since it is not necessary to superimpose a low-frequency dither signal on the optical modulation unit, it can be applied regardless of the type of the optical modulator.

  As described above, according to this embodiment, modulation is performed by controlling the DC bias applied to the optical phase adjustment unit 3 based on the amount of received light detected by the light receiving units 7a and 7b, particularly the difference between them. There is an effect that the optical phase difference between optical signals can be controlled automatically and stably. Thus, in this embodiment, it is not necessary to superimpose a dither signal, and the light receivers 7a and 7b only need to receive an average amount as an output, so there is no need to use a high-speed electric circuit or the like. A desired operation can be realized with a simple circuit.

Embodiment 2. FIG.
FIG. 2 is a block diagram showing the configuration of the DQPSK modulation type optical transmitter according to this embodiment. In FIG. 2, the same components as those in FIG. 1 are denoted by the same reference numerals.

  As shown in FIG. 2, in the optical transmitter according to the present embodiment, an MZ interferometer 18 is configured, and an optical multiplexing / interference unit in the MZ interferometer 18 is configured by, for example, a Y-type branching waveguide 17. In this case, when the optical phase difference of the signal light output from the optical modulation unit 2a and the optical phase adjustment unit 3 is 0 (or π), all the signal light input to the Y-type branching waveguide 17 is output from the transmitter. 8 is output. Conversely, when the optical phase difference of the signal light output from the optical modulation unit 2a and the optical phase adjustment unit 3 is π (or 0), all the signal light input to the Y-type branching waveguide 17 enters the Y-type branching unit. Thus, it becomes a non-waveguide mode and diffuses out of the waveguide. In other words, when the optical signals output from the optical modulation unit 2a and the optical phase adjustment unit 3 and input to the Y-type branch waveguide 17 are in phase with each other, the light input to the Y-type branch waveguide 17 All the signals are output as, for example, the transmitter output 8, and when the optical signals output from the optical modulation unit 2a and the optical phase adjustment unit 3 and input to the Y-type branching waveguide 17 are opposite in phase, the Y-type branching is performed. All the optical signals input to the waveguide 17 become, for example, a non-waveguide mode and are diffused outside the waveguide at the Y-type branching portion. On the other hand, when the optical phase difference between the optical signals output from the optical modulation unit 2a and the optical phase adjustment unit 3 is exactly shifted by π / 2, the output output from the Y-type branching waveguide 17 as the transmitter output 8 The light level is the same as the output light level that diffuses out of the waveguide in the non-waveguide mode at the Y-shaped branch.

  As described above, the Y-type branching waveguide 17 includes two input units (first and second input units) and one output unit (for example, first output unit) each made of a waveguide, and a Y-type branching unit. And another output unit (for example, a second output unit) that diffuses output light as a non-waveguide mode.

  In FIG. 2, as a light receiving means for receiving the diffused light in the non-waveguide mode, the light is received by, for example, a light receiver 11 disposed on the back surface of the waveguide substrate. Also, a part of the optical signal output as guided light from the Y-type branching waveguide 17 is branched out by the tap coupler 5a and taken out as in the case of the first embodiment. The tap coupler 5a and the light receiver 11 each obtain an output at a predetermined ratio from the output of the Y-type branch waveguide 17 (see Embodiment 1). In the present embodiment, a differential light receiver 12 is provided as a light receiving unit, and in this differential light receiver 12, a light reception amount acquired by the tap coupler 5 a and a light reception amount acquired by the light receiver 11. The difference is calculated, and the calculation result is sent to the control unit 9. The controller 9 controls the DC bias based on the obtained difference in the amount of received light, thereby obtaining the same effect as in the case of FIG.

  In the above operation, the Y-type branch waveguide 17 as the multiplexing / interference part of the MZ interferometer 18 is devised as an asymmetric X-type branch waveguide, for example, so that it is output as a waveguide mode from another waveguide. It is also possible to obtain the same effect as the present embodiment by using such an X-type branching waveguide as a multiplexing / interference part.

  In FIG. 2, the differential light receiver 12 is used as the light receiving unit, and the difference between the received light amounts detected by the tap coupler 5 a and the light receiver 11 is directly extracted, and this is used as an error signal to control the control unit 9. Further, it can be adjusted to the optimum phase by controlling the optical phase adjusting unit 3. Other configurations, operations, and effects of the present embodiment are the same as those of the first embodiment. In FIG. 1, the same effect can be obtained even when the light receivers 7 a and 7 b are replaced with the differential light receiver 12.

  The present invention is useful as a technique for automatically and stably controlling an optical phase difference between modulated signals output from an optical modulator in a DQPSK optical transmitter.

1 is a block diagram illustrating a configuration of an optical transmitter using a DQPSK modulation method according to a first exemplary embodiment; FIG. 5 is a block diagram illustrating a configuration of a DQPSK modulation type optical transmitter according to a second exemplary embodiment; 10 is a block diagram showing a configuration of an optical time division multiplexing apparatus described in Patent Literature 2. FIG. 10 is a block diagram showing a configuration of an optical modulator described in Patent Document 3. FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1,18 MZ interferometer 2a, 2b Optical modulation part 3 Optical phase adjustment part 4 3 dB coupler 5a, 5b Tap coupler 6 Optical termination 7a, 7b Light receiver 8 Transmitter output 9 Control part 10 Control signal input 11 Light receiver 12 Differential Light receiver 15 Light source 16 Optical demultiplexing unit 17 Y-type branch waveguide

Claims (7)

First and second light modulators to which carrier light output from the light source is demultiplexed and input,
An optical phase adjustment unit that is connected to the input side or output side of the second optical modulation unit and that shifts and outputs the optical phase of an optical signal input in accordance with an applied DC bias;
A first input unit to which an output of the first optical modulation unit is input; a second input unit to which an output of the optical phase adjustment unit or an output of the second optical modulation unit is input; A first output unit that outputs all of the input light when the optical signals input to the first and second input units are in phase with each other; and the light that is input to the first and second input units A second output unit that outputs all of the input light when the signals are out of phase with each other, and combines and interferes with the optical signals input to the first and second input units, An optical multiplexing / interference unit for outputting from the first and second output units;
An output acquisition unit that acquires a part thereof from the output of the first output unit at a predetermined rate, and acquires a part thereof from the output of the second output unit;
A light receiving unit that receives each optical signal acquired by the output acquisition unit;
Based on the received light amount of the optical signal from the first output unit detected by the light receiving unit and the received light amount of the optical signal from the second output unit, a DC bias applied to the optical phase adjusting unit is A control unit to control;
An optical transmitter comprising:   The control unit is configured so that the received light amount of the optical signal from the first output unit detected by the light receiving unit and the received light amount of the optical signal from the second output unit have a predetermined ratio. The optical transmitter according to claim 1, wherein a DC bias applied to the optical phase adjusting unit is controlled.   The control unit calculates a difference between a received light amount of the optical signal from the first output unit detected by the light receiving unit and a received light amount of the optical signal from the second output unit. 2. The optical transmitter according to claim 1, wherein a direct current bias applied to the optical phase adjusting unit is controlled so as to be zero.   The optical transmitter according to any one of claims 1 to 3, wherein the first and second optical modulation units are configured by Mach-Zehnder type optical modulators.   5. The optical transmitter according to claim 1, wherein the optical multiplexing / interference unit includes an optical coupler.   The optical multiplexing / interference unit is configured by an X-type branching waveguide or a Y-type branching waveguide, and an optical signal output from the first or second output unit is output as a non-waveguide mode. The optical transmitter according to any one of claims 1 to 4.   The light receiving unit is composed of a differential light receiver, and the amount of light received from the first output unit and the amount of light received from the second output unit calculated by the differential light receiver are calculated. The optical transmitter according to claim 1, wherein the difference is output to the control unit.

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