JP2010161646A - Optical transmitter and optical communication system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical transmitter outputting signal light of which the residual intensity modulation component is reduced. <P>SOLUTION: The optical transmitter 10A includes a light source 101, a light phase modulation part 102, a light intensity modulation part 103, a predistortion part 104, an offset part 105, a pilot signal generating part 106, a photocoupler 107, a light receiving part 108, an RF signal generating part 109 and a band pass filter 110. The RF signal generating part 109 outputs a first RF signal and a second RF signal of which the phases are inverted from each other. The light source 101 or the light phase modulation part 102 phase-modulates light on the basis of the first RF signal (SBS suppression signal) and expands light spectrum beam width. The light source 101 intensity-modulates light on the basis of the second RF signal (RAM compensation signal) and compensates for a residual intensity modulation component generated in the light phase modulation part 102. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an optical communication system that performs optical communication by transmitting signal light, and an optical transmitter that transmits signal light.

  An optical communication system receives an optical transmitter that transmits signal light, an optical fiber transmission path that transmits the signal light transmitted from the optical transmitter, and signal light that is transmitted through the optical fiber transmission path and arrives An optical receiver. Further, among the optical transmitters, those of the external modulation type include a light source that outputs continuous light, an optical phase modulation unit that outputs a phase-modulated light output from the light source, and an output from the optical phase modulation unit. A light intensity modulation unit that modulates the intensity of the emitted light with a transmission signal and outputs the modulated light. The optical phase modulation unit widens the optical spectral line width in order to improve the stimulated Brillouin scattering (SBS) threshold.

A. Yariv, et al., IEEE Journal of LT, Vol.15, No.3, pp.437-443 (1997) F. W. Willems, et al., IEEE Photon.Technol. Lett., Vol.6, No.12, pp.1476-1478 (1994)

  If the optical phase modulation unit is ideal, the optical phase modulation unit can modulate only the phase of light. However, actually, the light output from the optical phase modulator includes a residual intensity modulation (RAM) component. When signal light including such a residual intensity modulation component is transmitted through an optical transmission line having a reflection point, a beat is generated between the residual intensity modulation component included in the signal light and the intensity modulation component due to the transmission signal. May occur. Such beats cause distortion in transmission characteristics (particularly analog video transmission characteristics).

  The present invention has been made to solve the above problems, and an object of the present invention is to provide an optical transmitter capable of outputting signal light with a reduced residual intensity modulation component. It is another object of the present invention to provide an optical communication system including such an optical transmitter and capable of transmitting signal light with excellent transmission characteristics.

  An optical transmitter according to the present invention includes (1) a light source that outputs light, (2) an optical phase modulation unit that phase-modulates and outputs light output from the light source, and (3) an output from the optical phase modulation unit. A light intensity modulation unit for intensity-modulating the output light with a transmission signal, and (4) an RF signal generation unit for outputting the first RF signal and the second RF signal. Furthermore, in the optical transmitter according to the present invention, the optical phase modulation unit performs phase modulation on the light input from the light source to the phase modulator based on the first RF signal output from the RF signal generation unit, The spectral line width is enlarged, and the light source or the light intensity modulation unit modulates the intensity of the light based on the second RF signal output from the RF signal generation unit to compensate for the residual intensity modulation component generated in the optical phase modulation unit. It is characterized by.

  In this optical transmitter, the light output from the light source is phase-modulated by the optical phase modulation unit, intensity-modulated by the transmission signal by the light intensity modulation unit, and output as signal light. A first RF signal and a second RF signal having a phase inversion relationship with each other are output from the RF signal generator. The first RF signal output from the RF signal generator is supplied to the light source or the optical phase modulator, and the optical spectral line width is expanded. The second RF signal output from the RF signal generator is supplied to the light source or the light intensity modulator, and the residual intensity modulation component generated in the optical phase modulator is compensated.

  Alternatively, the optical transmitter according to the present invention includes (1) a light source that outputs light, (2) an optical phase modulator that modulates and outputs the light output from the light source, and (3) a first light intensity. A first light intensity modulator for intensity-modulating the light input to the modulator or the light output from the first light intensity modulator; and (4) the light input to the first light intensity modulator or the first light A second light intensity modulating unit that modulates and outputs the light output from the one light intensity modulating unit; and (5) an RF signal generating unit that outputs the first RF signal and the second RF signal. . Furthermore, in the optical transmitter according to the present invention, the optical phase modulator modulates the phase of light input from the light source to the phase modulator based on the first RF signal output from the RF signal generator, The width is enlarged, and the second light intensity modulation unit modulates the intensity of light based on the second RF signal output from the RF signal generation unit to compensate for the residual intensity modulation component generated in the optical phase modulation unit. And

  In this optical transmitter, the light output from the light source is phase-modulated by the optical phase modulation unit, intensity-modulated by the transmission signal by the first light intensity modulation unit, and intensity-modulated by the second RF signal by the second light intensity modulation unit. And output as signal light. A first RF signal and a second RF signal having a phase inversion relationship with each other are output from the RF signal generator. The first RF signal output from the RF signal generator is supplied to the light source or the optical phase modulator, and the optical spectral line width is expanded. The second RF signal output from the RF signal generator is supplied to the second light intensity modulator, and the residual intensity modulation component generated in the optical phase modulator is compensated.

  In the optical transmitter according to the present invention, it is preferable that the RF signal generator adjusts at least one of the intensity and phase of the second RF signal.

  In the optical transmitter according to the present invention, it is preferable that the second RF signal includes a component for correcting spurious (distortion component) included in the light output from the optical phase modulation unit.

  The optical transmitter according to the present invention receives an optical coupler that branches a part of the light output from the optical phase modulation unit, the light branched by the optical coupler, and outputs an electrical signal corresponding to the received light amount. It is preferable that the RF signal generator adjusts one of the intensity, phase, and distortion of the second RF signal so that the distortion component included in the electrical signal output from the light receiver is minimized. It is.

  In the optical transmitter according to the present invention, it is preferable that the ratio of the spurious included in the first RF signal to the first RF signal is 65 dB or more.

  In the optical transmitter according to the present invention, the RF signal generation unit includes an RF signal generation source that generates an RF signal. The RF signal output from the RF signal generation source is branched into two, and each of the two branched RF signals is generated. It is preferable that the signals are output as the first RF signal and the second RF signal so as to have a phase inversion relationship with each other.

  An optical communication system according to the present invention includes the optical transmitter according to the present invention that outputs signal light, and performs optical communication by transmitting the signal light output from the optical transmitter.

  According to the present invention, signal light with a reduced residual intensity modulation component can be output, and signal light can be transmitted with excellent transmission characteristics.

1 is a schematic configuration diagram of an optical communication system 1 according to the present embodiment. 1 is a configuration diagram of an optical transmitter 10A according to a first embodiment. FIG. 2 is a diagram illustrating a first configuration example of an RF signal generation unit 109. FIG. 6 is a diagram illustrating a second configuration example of an RF signal generation unit 109. FIG. 6 is a diagram illustrating a third configuration example of an RF signal generation unit 109. FIG. 10 is a diagram illustrating a fourth configuration example of an RF signal generation unit 109. FIG. It is a figure which shows the intensity | strength modulation and frequency spectrum of light in various places of 10 A of optical transmitters which concern on 1st Embodiment. It is a block diagram of the optical transmitter 10B which concerns on 2nd Embodiment. It is a block diagram of 10 C of optical transmitters concerning 3rd Embodiment. It is a block diagram of optical transmitter 10D which concerns on 3rd Embodiment.

  DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments for carrying out the present invention will be described in detail with reference to the accompanying drawings. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted.

  First, an embodiment of an optical communication system according to the present invention will be described. FIG. 1 is a schematic configuration diagram of an optical communication system 1 according to the present embodiment. The optical communication system 1 shown in this figure includes an optical transmitter 10, an optical receiver 20, and an optical fiber transmission line 30. In the optical communication system 1, the signal light transmitted from the optical transmitter 10 is transmitted through the optical fiber transmission line 30, reaches the optical receiver 20, and is received by the optical receiver 20. The optical transmitter 10 is according to the present invention. Hereinafter, embodiments of the optical transmitter according to the present invention will be described.

  Next, a first embodiment of the optical transmitter according to the present invention will be described. FIG. 2 is a configuration diagram of the optical transmitter 10A according to the first embodiment. An optical transmitter 10A shown in this figure is used as the optical transmitter 10 in FIG. 1, and includes a light source 101, an optical phase modulation unit 102, a light intensity modulation unit 103, a predistortion unit 104, and an offset unit 105. A pilot signal generator 106, an optical coupler 107, a light receiver 108, an RF signal generator 109, and a bandpass filter 110.

  The light source 101 outputs continuous light, and a laser diode is generally used. Preferably, a distributed feedback laser (DFB laser), an external resonance laser (ECL), a Fabry-Perot laser (FP laser), and the like are used. Used. The optical phase modulation unit 102 modulates and outputs the light output from the light source 101, and preferably uses a lithium niobate (LN) waveguide. The light intensity modulation unit 103 is for intensity-modulating the light output from the optical phase modulation unit 102 with a transmission signal and outputting it, and a phase control type or distributed coupling type LN waveguide is preferably used. The light output from the light intensity modulator 103 is sent to the optical fiber transmission line 30 as signal light.

  The light intensity modulation unit 103 has a certain relationship between the applied voltage and the light transmittance, and the light transmittance is controlled by applying the voltage, thereby intensity-modulating the light. Can do. However, the light intensity modulator 103 generally has a distortion in the relationship between the applied voltage and the light transmittance. In order to compensate for this, a predistortion unit 104 and an offset unit 105 are provided. That is, the predistortion unit 104 distorts the transmission signal and the offset unit 105 offsets the transmission signal so as to compensate for the distortion included in the relationship between the applied voltage and the light transmittance in the light intensity modulation unit 103. give.

  In addition, a pilot signal generation unit 106, an optical coupler 107, and a light receiving unit 108 are provided in order to determine the offset that the offset unit 105 gives to the transmission signal. That is, the pilot signal generation unit 106 supplies a pilot signal for monitoring to the light source 101 to drive the light source 101 and is also incident on the offset unit 105. The optical coupler 107 branches a part of the light output from the light source 101 driven by the pilot signal and passed through the optical phase modulation unit 102 and the light intensity modulation unit 103. The light receiving unit 108 receives the light branched by the optical coupler 107 and outputs an electrical signal corresponding to the received light amount. The offset unit 105 receives the electrical signal output from the optical coupler 107, extracts a pilot signal from the electrical signal, and optimizes the offset by feedback control.

  The RF signal generation unit 109 supplies an RF signal (SBS suppression signal) to the light source 101 or the optical phase modulation unit 102 to expand the optical spectrum line width. The RF signal intensity can be changed according to the SBS threshold value to be suppressed.

  By the way, if the optical phase modulation unit 102 is ideal, the optical phase modulation unit 102 can modulate only the phase of light. However, in practice, the light output from the optical phase modulation unit 102 includes a residual intensity modulation (RAM) component. This residual intensity modulation component includes the etalon effect due to light scattering inside and on the surface of the optical phase modulation unit, the piezoelectric response of a piezoelectric element (for example, LN) used in the optical phase modulation unit, and the modulation intensity in the piezoelectric element. This is caused by uniformity and the optical path length and birefringence of the piezoelectric element crystal.

  As described above, when the light output from the optical phase modulation unit 102 includes a residual intensity modulation component, the light output from the optical phase modulation unit 102 is photoelectrically converted by the light receiving unit and output from the light receiving unit. When the electric signal is observed with an RF spectrum analyzer, the presence of the residual intensity modulation component can be confirmed.

  When signal light including such a residual intensity modulation component is transmitted through an optical transmission line having a reflection point, a beat is generated between the residual intensity modulation component included in the signal light and the intensity modulation component due to the transmission signal. May occur. In the beat having high coherence, the interference intensity can be observed depending on the coherence length of light depending on the reciprocal of the optical spectrum line width. The frequency of this beat exists in the vicinity of the frequency of the intensity modulation component caused by the transmission signal. Therefore, since such beats cause distortion in transmission characteristics (particularly analog video transmission characteristics), it is necessary to reduce the beats.

  The beat can also be reduced by reducing the spurious component of the RF signal (SBS suppression signal) supplied from the RF signal generator 109 to the light source 101 or the optical phase modulator 102. In this case, it is desirable that the DUR between the spurious component and the RF signal is 65 dB or more. However, in this case, it is difficult to remove beats having the same frequency as the intensity modulation component due to the transmission signal.

  Therefore, the RF signal generation unit 109 in the optical transmitter 10A according to the first embodiment outputs a first RF signal and a second RF signal that are in phase-inverted relationship with each other. Then, the light source 101 or the optical phase modulation unit 102 modulates the phase of the light based on the first RF signal (SBS suppression signal) to expand the optical spectrum line width, and the light source 101 receives the second RF signal (RAM compensation signal). Based on the above, the intensity of the light is modulated, and the residual intensity modulation component generated in the optical phase modulation unit 102 is compensated.

  Preferably, the RF signal generator 109 adjusts at least one of the intensity and phase of the second RF signal. The RF signal generation unit 109 includes a component for correcting a distortion component included in the light output from the optical phase modulation unit 102 in the second RF signal. The band pass filter 110 receives the electrical signal output from the light receiving unit 108, selectively transmits the distortion component included in the electrical signal, and outputs it to the RF signal generation unit 109. In addition, the RF signal generation unit 109 adjusts any one of the intensity, phase, and distortion of the second RF signal so that the distortion component is minimized.

  FIG. 3 is a diagram illustrating a first configuration example of the RF signal generation unit 109. The RF signal generation unit 109 shown in this figure includes an RF generation source 121, a phase inversion unit 123, a variable phase shift unit 124, a variable attenuation unit 125, and a control unit 126. The first RF signal (SBS suppression signal) supplied to the optical phase modulation unit 102 is output from the RF generation source 121. The second RF signal (RAM compensation signal) supplied to the light source 101 is output from the RF generation source 121, the phase is inverted by the phase inversion unit 123, the phase is adjusted by the variable phase shift unit 124, and the variable attenuation unit. The intensity is adjusted by 125. Each of the phase inversion unit 123, the variable phase shift unit 124, and the variable attenuation unit 125 is controlled by the control unit 126 so that the distortion component output from the bandpass filter 110 is minimized.

  FIG. 4 is a diagram illustrating a second configuration example of the RF signal generation unit 109. The RF signal generation unit 109 shown in this figure includes an RF generation source 121, a phase inversion unit 123, a variable phase shift unit 124, a variable attenuation unit 125, and a control unit 126. The first RF signal (SBS suppression signal) supplied to the optical phase modulation unit 102 is output from the RF generation source 121, the phase is inverted by the phase inversion unit 123, and the phase is adjusted by the variable phase shift unit 124. The second RF signal (RAM compensation signal) supplied to the light source 101 is output from the RF generation source 121 and the intensity is adjusted by the variable attenuation unit 125. Each of the phase inversion unit 123, the variable phase shift unit 124, and the variable attenuation unit 125 is controlled by the control unit 126 so that the distortion component output from the bandpass filter 110 is minimized.

  FIG. 5 is a diagram illustrating a third configuration example of the RF signal generation unit 109. The RF signal generation unit 109 shown in this figure includes an RF generation source 121, an RF generation source 122, a phase inversion unit 123, a variable phase shift unit 124, a variable attenuation unit 125, and a control unit 126. The RF generation source 121 and the RF generation source 122 output RF signals having the same frequency. The first RF signal (SBS suppression signal) supplied to the optical phase modulation unit 102 is output from the RF generation source 121. The second RF signal (RAM compensation signal) supplied to the light source 101 is output from the RF generation source 122, the phase is inverted by the phase inversion unit 123, the phase is adjusted by the variable phase shift unit 124, and the variable attenuation unit. The intensity is adjusted by 125. Each of the phase inversion unit 123, the variable phase shift unit 124, and the variable attenuation unit 125 is controlled by the control unit 126 so that the distortion component output from the bandpass filter 110 is minimized.

  FIG. 6 is a diagram illustrating a fourth configuration example of the RF signal generation unit 109. The RF signal generation unit 109 shown in this figure includes an RF generation source 121, an RF generation source 122, a phase inversion unit 123, a variable phase shift unit 124, a variable attenuation unit 125, and a control unit 126. The RF generation source 121 and the RF generation source 122 output RF signals having the same frequency. The first RF signal (SBS suppression signal) supplied to the optical phase modulation unit 102 is output from the RF generation source 121, the phase is inverted by the phase inversion unit 123, and the phase is adjusted by the variable phase shift unit 124. The second RF signal (RAM compensation signal) supplied to the light source 101 is output from the RF generation source 122 and the intensity is adjusted by the variable attenuation unit 125. Each of the phase inversion unit 123, the variable phase shift unit 124, and the variable attenuation unit 125 is controlled by the control unit 126 so that the distortion component output from the bandpass filter 110 is minimized.

  FIG. 7 is a diagram showing light intensity modulation and frequency spectrum at various points in the optical transmitter 10A according to the first embodiment. In this figure, intensity modulation and frequency spectrum of light output from the light source 101 supplied with the second RF signal (RAM compensation signal), output from the optical phase modulation unit 102 supplied with the first RF signal (SBS suppression signal). The intensity modulation and frequency spectrum of the emitted light, and the frequency spectrum of the light output from the light intensity modulation unit 103 are shown. As shown in this figure, the residual intensity modulation component generated in the optical phase modulation unit 102 is offset with the light intensity modulation in the light source 101 based on the second RF signal, and is included in the light output from the light intensity modulation unit 103. The residual intensity modulation component that is generated can be reduced. Further, the optical communication system 1 including such an optical transmitter 10A can transmit signal light with excellent transmission characteristics.

  Next, a second embodiment of the optical transmitter according to the present invention will be described. FIG. 8 is a configuration diagram of an optical transmitter 10B according to the second embodiment. The optical transmitter 10B shown in this figure is used as the optical transmitter 10 in FIG. 1, and includes a light source 101, an optical phase modulation unit 102, a light intensity modulation unit 103, a predistortion unit 104, and an offset unit 105. A pilot signal generator 106, an optical coupler 107, a light receiver 108, an RF signal generator 109, and a bandpass filter 110. Each of these elements is the same as that in the first embodiment.

  The RF signal generator 109 in the optical transmitter 10B according to the second embodiment outputs a first RF signal and a second RF signal that are in a phase-inverted relationship with each other. Then, the light source 101 or the optical phase modulation unit 102 phase modulates the light based on the first RF signal (SBS suppression signal) to expand the optical spectrum line width, and the light intensity modulation unit 103 outputs the second RF signal (RAM The intensity of the light is modulated based on the compensation signal), and the residual intensity modulation component generated in the optical phase modulation unit 102 is compensated. The voltage value applied to the light intensity modulation unit 103 is obtained by adding the offset by the offset unit 105 to the transmission signal distorted by the predistortion unit 104 and further superimposing the second RF signal. In the optical transmitter 10B according to the second embodiment, the residual intensity modulation component generated in the optical phase modulation unit 102 is canceled with the light intensity modulation in the light intensity modulation unit 103 based on the second RF signal, so that the light intensity modulation is performed. The residual intensity modulation component included in the light output from the unit 103 can be reduced.

  Next, a third embodiment of the optical transmitter according to the invention will be described. FIG. 9 is a configuration diagram of an optical transmitter 10C according to the third embodiment. An optical transmitter 10C shown in this figure is used as the optical transmitter 10 in FIG. 1, and includes a light source 101, an optical phase modulation unit 102, a light intensity modulation unit 103, a light intensity modulation unit 103a, a predistortion. Unit 104, offset unit 105, pilot signal generation unit 106, optical coupler 107, light reception unit 108, RF signal generation unit 109, and bandpass filter 110. The third embodiment is different from the second embodiment in that another optical intensity modulator 103 a is further provided between the optical phase modulator 102 and the optical intensity modulator 103.

  The RF signal generator 109 in the optical transmitter 10C according to the third embodiment outputs a first RF signal and a second RF signal that are in a phase-inverted relationship with each other. Then, the light source 101 or the optical phase modulator 102 performs phase modulation on the light based on the first RF signal (SBS suppression signal) to expand the optical spectrum line width, and the light intensity modulator 103a receives the second RF signal (RAM The intensity of the light is modulated based on the compensation signal), and the residual intensity modulation component generated in the optical phase modulation unit 102 is compensated. In the optical transmitter 10C according to the third embodiment, the residual intensity modulation component generated in the optical phase modulation unit 102 is canceled with the light intensity modulation in the light intensity modulation unit 103a based on the second RF signal, so that the light intensity modulation is performed. The residual intensity modulation component included in the light output from the unit 103 can be reduced.

  Next, a fourth embodiment of the optical transmitter according to the invention will be described. FIG. 10 is a configuration diagram of an optical transmitter 10D according to the fourth embodiment. An optical transmitter 10D shown in this figure is used as the optical transmitter 10 in FIG. 1, and includes a light source 101, an optical phase modulation unit 102, a light intensity modulation unit 103, a light intensity modulation unit 103a, a predistortion. Unit 104, offset unit 105, pilot signal generation unit 106, optical coupler 107, light reception unit 108, RF signal generation unit 109, and bandpass filter 110. The fourth embodiment differs from the third embodiment in the arrangement relationship between the light intensity modulator 103 and the light intensity modulator 103a.

  The RF signal generator 109 in the optical transmitter 10D according to the fourth embodiment outputs a first RF signal and a second RF signal that are in phase-inverted relationship with each other. Then, the light source 101 or the optical phase modulator 102 performs phase modulation on the light based on the first RF signal (SBS suppression signal) to expand the optical spectrum line width, and the light intensity modulator 103a receives the second RF signal (RAM The intensity of the light is modulated based on the compensation signal), and the residual intensity modulation component generated in the optical phase modulation unit 102 is compensated. In the optical transmitter 10D according to the fourth embodiment, the residual intensity modulation component generated in the optical phase modulation unit 102 is canceled with the light intensity modulation in the light intensity modulation unit 103a based on the second RF signal, so that the light intensity modulation is performed. The residual intensity modulation component included in the light output from the unit 103 can be reduced.

  The present invention is not limited to the above embodiment, and various modifications can be made. For example, the optical coupler 107 and the light receiving unit 108 for detecting a distortion component included in the light output from the optical phase modulation unit 102 are provided in the subsequent stage of the light intensity modulation unit 103 in the above embodiment, but the optical phase It may be provided at an arbitrary position as long as it is subsequent to the modulation unit 102.

  DESCRIPTION OF SYMBOLS 1 ... Optical communication system 10, 10A, 10B, 10C, 10D ... Optical transmitter, 20 ... Optical receiver, 30 ... Optical fiber transmission line, 101 ... Light source, 102 ... Optical phase modulation part, 103, 103a ... Light intensity Modulation section 104... Predistortion section 105. Offset section 106 106 Pilot signal generation section 107. Optical coupler 108. Light reception section 109. RF signal generation section 110.

Claims (8)

A light source that outputs light;
An optical phase modulation unit for phase-modulating and outputting the light output from the light source;
A light intensity modulation unit that modulates the intensity of light output from the optical phase modulation unit with a transmission signal and outputs the modulated light;
An RF signal generator for outputting a first RF signal and a second RF signal;
Based on the first RF signal output from the RF signal generation unit, the optical phase modulation unit phase-modulates the light input from the light source to the phase modulator, and expands the optical spectrum line width,
The light source or the light intensity modulation unit modulates the intensity of light based on the second RF signal output from the RF signal generation unit and compensates for a residual intensity modulation component generated in the optical phase modulation unit. Optical transmitter. A light source that outputs light;
An optical phase modulation unit for phase-modulating and outputting the light output from the light source;
A first light intensity modulation unit that modulates the intensity of light output from the optical phase modulation unit with a transmission signal and outputs the modulated light;
A second light intensity modulator that modulates the intensity of the light input to the first light intensity modulator or the light output from the first light intensity modulator;
An RF signal generator for outputting a first RF signal and a second RF signal;
Based on the first RF signal output from the RF signal generation unit, the optical phase modulation unit phase-modulates the light input from the light source to the phase modulator, and expands the optical spectrum line width,
The second light intensity modulation unit modulates the intensity of light based on the second RF signal output from the RF signal generation unit to compensate for a residual intensity modulation component generated in the optical phase modulation unit. Optical transmitter.   The optical transmitter according to claim 1, wherein the RF signal generation unit adjusts at least one of the intensity and the phase of the second RF signal.   The optical transmitter according to claim 1, wherein the second RF signal includes a component that corrects spurious (distortion component) included in the light output from the optical phase modulation unit. An optical coupler for branching a part of the light output from the optical phase modulator;
A light receiving unit that receives light branched by the optical coupler and outputs an electrical signal according to the amount of light received;
The RF signal generation unit adjusts any one of an intensity, a phase, and distortion of the second RF signal so that a distortion component included in an electric signal output from the light receiving unit is minimized. Item 3. The optical transmitter according to Item 1 or 2.   The optical transmitter according to claim 1, wherein a ratio of spurious included in the first RF signal to the first RF signal is 65 dB or more.   The RF signal generation unit includes an RF signal generation source that generates an RF signal. The RF signal output from the RF signal generation source is branched into two, and the two branched RF signals have a phase inversion relationship with each other. The optical transmitter according to claim 1 or 2, wherein the optical transmitter outputs the first RF signal and the second RF signal.   An optical communication system comprising the optical transmitter according to any one of claims 1 to 7 for outputting signal light, and performing optical communication by transmitting the signal light output from the optical transmitter. .

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