JP5207993B2 - Optical transmission apparatus and method - Google Patents

Description

  The present invention relates to an optical transmission apparatus and method, and more particularly to an optical transmission apparatus and method for reducing signal degradation that occurs in a frequency division multiplexed optical signal.

  As the demand for data communication increases, optical transmission networks using optical signal modulation technology and optical signal multiplexing technology that enable transmission of large-capacity traffic are becoming widespread. Realization of high-capacity traffic transmission requires realization of high frequency utilization efficiency, and examples of techniques for realizing it include multilevel modulation techniques and optical frequency orthogonal division multiplexing (OFDM) techniques. Optical OFDM, in particular, is a method that achieves high frequency utilization efficiency by suppressing optical interference from adjacent channels that could otherwise occur by using the orthogonality of optical frequencies that are satisfied between channels. Combination with value modulation is also possible (for example, see Non-Patent Document 1).

  FIG. 13 shows an example of the configuration of a transmitter that realizes optical OFDM.

The transmitter shown in the figure includes light sources 10 ₁ to 10 ₄ , optical modulators 11 _{1 to} 11 ₄ , and an optical coupler 12. Lights output from the respective light sources 10 ₁ to 10 ₄ are called subcarriers and have the same frequency interval (f2-f1 = f3-f2 = f4-f3 = Δf). Here, the subcarriers output from the light sources 10 ₁ to 10 ₄ may be generated by performing optical phase modulation on the light output from the single light source 10. Each subcarrier is individually modulated by each optical modulator 11 at a modulation rate equal to the frequency interval, and becomes a signal having a spectrum spread. By combining these modulated subcarriers by the optical coupler 12, an optical OFDM signal is generated.

  FIG. 14 shows an example of the configuration of a receiver that separates an optical OFDM signal into subcarriers.

The receiver shown in the figure includes a Mach-Zehnder Interferometer (MZI) 20, optical couplers 21 ₁ and 21 ₂ , optical bandpass filters (BPF) 23 ₁ to 23 ₄ , and optical demodulators 24 _{1 to} 24 _4. It has. The optical OFDM signal is separated into even and odd subcarriers by the MZI 20 in which the delay amount is set to half the symbol length. The subcarriers separated evenly and oddly are further separated into subcarriers by optical couplers 21 ₁ and 21 ₂ and optical bandpass filters 23 ₁ to 23 _{4, and} individually demodulated by optical demodulators 24 _{1 to} 24 ₄ . The optical couplers 21 ₁ and 21 ₂ and the optical bandpass filters 23 ₁ to 23 ₄ may use MZIs having different FSRs from the first stage MZI 20.

H. Sanjoh, et. Al., "Optical orthogonal frequency division multiplexing using frequency / time domain filtering for high spectral efficiency up to 1 bit / s / Hz," OFC 2002, ThD1

  An optical OFDM signal is composed of subcarriers that overlap in the frequency spectrum. When demodulating a signal, it is necessary to separate each subcarrier while suppressing optical interference between adjacent subcarriers. In order to realize this, the optical OFDM signal is subjected to delay interference using the MZI in which the delay amount is set to half the symbol length T, that is, set to T / 2, and each subcarrier is separated. At this time, when the orthogonality of the optical frequency is ideally maintained, the optical interference between the subcarriers is suppressed only in the T / 2 time domain where the delayed interference is performed, and each subcarrier is separated without waveform deterioration. Is possible.

  However, in an actual transmission path, the waveform is distorted and intersymbol interference occurs due to the chromatic dispersion by the fiber, the filtering effect by an optical switch, etc., the nonlinear optical effect in the transmission path, and the like. In such a situation, the orthogonality of the optical frequencies is not kept in an ideal state, and the optical interference between the subcarriers is not completely suppressed even in the T / 2 time domain where the delay interference occurs. That is, it is difficult to separate each subcarrier from an optical OFDM signal propagated through an actual transmission path while suppressing optical interference between adjacent subcarriers.

  FIG. 15 shows a signal of an optical OFDM signal without intersymbol interference obtained by a numerical simulation in a state where there is no filter and no polarization beam combiner (PBC) and orthogonal polarization multiplexing is not performed. A spectrum and an eye diagram of each subcarrier are shown. Here, an optical OFDM signal composed of two subcarriers (subcarrier 1 and subcarrier 2) is used. Each subcarrier is modulated as a 10 Gbps NRZ signal, and the frequency interval between the subcarriers is 10 GHz. From the eye diagram, it can be seen that the eye is wide open in the time domain (around 150 ps) where the delay interference occurred. FIG. 16 shows a signal spectrum of an optical OFDM signal in which intersymbol interference occurs due to an optical filter, and an eye diagram of each subcarrier, obtained by numerical simulation in a state with a filter and without a PBC. A primary Gaussian filter having a 3 dB bandwidth of 10 GHz was used as the optical filter. It can be seen from the eye diagram that a sufficient eye opening is not obtained even in the time domain where the delay interference occurs. Thus, the optical OFDM signal propagated through the transmission path receives optical interference from adjacent subcarriers during subcarrier separation s, and waveform deterioration due to optical interference leads to bit errors when demodulating each subcarrier.

  The present invention has been made in view of the above points, and in a transmitter that generates an optical OFDM signal, the polarization state between adjacent subcarriers is orthogonalized, and light power fluctuation due to optical interference is reduced. An object of the present invention is to provide an optical transmission apparatus and method capable of suppressing optical interference that occurs during subcarrier separation of an optical OFDM signal.

  FIG. 1 is a principle configuration diagram of the present invention.

The present invention (Claim 1) is an optical transmission apparatus for transmitting an optical signal multiplexed in optical frequency,
Light modulating means 2 for modulating input light for each frequency;
Polarization orthogonal means 3 for orthogonalizing polarization between optical signals adjacent in frequency space;
Phase synchronization means for synchronizing optical phases between optical signals;
Optical frequency multiplexing means 4 for optical frequency multiplexing optical signals phase-synchronized by the phase synchronization means ;
I have a,
The phase synchronization means is
A reference signal source for generating a signal having a frequency interval between two optical signals adjacent in frequency space;
A polarization beam splitter that separates two optical signals adjacent in frequency space among signals after optical frequency multiplexing by an optical frequency multiplexing means;
A λ / 2 plate for aligning one polarization of the two optical signals separated by the polarization beam splitter with the other polarization;
An optical coupler that combines the two optical signals after the polarizations are aligned;
A light receiver for extracting a beat component of light intensity generated by optical interference of two optical signals combined by an optical coupler;
A mixer that inputs a beat component extracted by a light receiver and a signal generated from a reference signal source and outputs an error frequency signal;
And an optical frequency shifter that inputs an error frequency signal from the mixer and adjusts one of the two optical signals so that the frequency of the signal is reduced.

In the optical transmission apparatus of the present invention (claim 2 ), the optical modulation means 2, the polarization orthogonal means 3, the phase synchronization means, and the optical frequency multiplexing means 4 are arranged on a planar lightwave circuit.

The present invention (Claim 3) is an optical transmission method for transmitting an optical signal multiplexed in optical frequency,
An optical modulation step in which the optical modulation means modulates the input light for each frequency; and
A polarization orthogonal step, wherein the polarization orthogonal means makes the polarization between optical signals adjacent in frequency space orthogonal,
A phase synchronization step in which the phase synchronization means synchronizes the optical phase between the optical signals;
An optical frequency multiplexing means for optical frequency multiplexing the optical signal phase-synchronized in the phase synchronization step;
And
In the phase synchronization step,
A signal having a frequency interval between two optical signals adjacent in the frequency space is generated.
Two optical signals adjacent in the frequency space among the signals after frequency multiplexing are polarized and separated,
Align one polarization of the two separated optical signals with the other polarization,
Combine the two optical signals after aligning the polarization,
Extract the beat component of the light intensity caused by the optical interference of the two combined optical signals,
An error frequency signal is generated by multiplying the extracted beat component by a signal having a frequency interval between two adjacent optical signals in the wave number space ,
One of the two optical signals is adjusted so that the frequency of the error frequency signal is reduced.

In the optical transmission method of the present invention (claim 4 ), the optical modulation step, the polarization orthogonal step, the phase synchronization step, and the optical frequency multiplexing step are realized on a planar lightwave circuit.

  As described above, according to the present invention, an optical transmission device having a phase synchronization means between multiplexed signals is used as a transmitter for generating an optical OFDM signal, thereby orthogonalizing polarization states between adjacent subcarriers. In this way, optical interference that occurs during subcarrier separation is suppressed. By orthogonalizing the polarization state, fluctuations in optical power due to optical interference are reduced, and bit errors that occur when demodulating each subcarrier can be reduced.

It is a principle block diagram of this invention. It is a block diagram of the transmission apparatus in the 1st Embodiment of this invention. It is a figure which shows the relationship between the transmittance | permeability and applied voltage in the 1st Embodiment of this invention. It is a block diagram of the optical transmission apparatus using the polarization controller in the 1st Embodiment of this invention. It is a block diagram (the 1) of the optical transmission apparatus which implement | achieves the optical OFDM signal comprised from three or more subcarriers in the 1st Embodiment of this invention. It is a block diagram (the 2) (example using a polarization maintaining optical coupler) of the optical transmission apparatus which implement | achieves the optical OFDM signal comprised from three or more subcarriers in the 1st Embodiment of this invention. FIG. 5 is a configuration diagram (No. 3) of an optical transmission apparatus that realizes an optical OFDM signal including three or more subcarriers according to the first embodiment of the present invention. It is a simulation result in the 1st Embodiment of this invention. It is a block diagram (the 1) of the optical transmission apparatus in the 2nd Embodiment of this invention. It is a block diagram (the 2) of the optical transmission apparatus in the 2nd Embodiment of this invention. It is a block diagram (the 3) of the optical transmission apparatus in the 2nd Embodiment of this invention. It is a block diagram of the optical transmission apparatus in the 3rd Embodiment of this invention. It is a block diagram of the conventional optical OFDM transmitter. It is a block diagram of the conventional optical OFDM receiver. It is the simulation result (the 1) by a prior art. It is the simulation result (the 2) by a prior art.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[First Embodiment]
FIG. 2 shows the configuration of the optical transmission apparatus according to the first embodiment of the present invention.

  The optical transmission device 100a shown in FIG. 1 includes a light source 110, a Mach-Zehnder Modulator (MZM) 120, a Mach-Zehnder Interferometer (MZI) 130, an optical modulator 140, a polarization beam combiner ( PBC: Polarization Beam Combiner (150). Each optical modulator 140 and PBC 150 are connected by a polarization maintaining fiber.

The optical transmission device 100a according to the present embodiment performs intensity phase modulation on the continuous light output from the single light source 110 using an MZM 120 or a vector modulator configured by arranging the MZM 120 in parallel. One subcarrier is generated. In general, the transmittance of the MZM 120 and the applied voltage have a relationship as shown in FIG. 3. For example, by applying a voltage of ± Vpi around the point N at a frequency of Δf [GHz], 2Δf Two subcarriers having a frequency interval of [GHz] are generated. Since these two subcarriers are generated from the single light source 110, the frequency interval is always kept constant. These two subcarriers are separated by using an MZI 130 or an interleave filter (ILF), and the optical modulators 140 ₁ and 140 ₂ have a modulation rate equal to the frequency interval between the subcarriers for each subcarrier. To apply arbitrary intensity and phase modulation.

  The subsequent stage of the MZI 130 is constructed as a polarization maintaining system by a polarization maintaining fiber (indicated by a dotted line in FIG. 2), and the polarization beam combiner 150 combines the polarization so that the polarization between the subcarriers is in an orthogonal state. I do.

  By generating the optical OFDM signal in this manner, the polarization between adjacent subcarriers is maintained in an orthogonal state, and an optical OFDM signal is realized.

Further, here, it is not always necessary to use polarization maintaining systems in order to realize the orthogonal state of polarization, as shown in FIG. 4, a polarization controller 160 between the optical modulator 140 ₁ and the optical coupler 170 provided, multiplexing may be performed by the optical coupler 170 as polarization between subcarriers output from the subcarrier and the optical modulator 140 ₂ output from the polarization controller 160 is orthogonal states.

  Next, a configuration for realizing an optical OFDM signal composed of three or more subcarriers will be described.

  FIGS. 5, 6, and 7 show the configuration of a transmission apparatus that realizes an optical OFDM signal composed of three or more subcarriers in the first embodiment of the present invention.

The transmission apparatus shown in FIG. 5 includes a light source 210, an MZM 220, an optical demultiplexer 230, optical modulators 240 ₁ 240 ₄ , PBC 250 ₁ , 250 ₂ , and an optical coupler 260.

An MZM 220 or a vector modulator (not shown) performs intensity phase modulation on the continuous light output from the single light source 210 to generate three or more subcarriers that are phase-synchronized. For example, in order to generate three subcarriers, by applying a voltage of about ± Vpi / 2 around the point M in FIG. 3 at a frequency of Δf [GHz], a frequency interval of Δf [GHz] is obtained. Are generated. An optical demultiplexer 230 such as an array waveguide grating (AWG) separates those subcarriers. Each of the optical modulators 240 ₁ to 240 ₄ modulates the respective subcarriers at a modulation rate equal to the frequency interval between the subcarriers so that the polarization between adjacent subcarriers is orthogonal as in the transmission apparatus of FIG. , To PBC 250 ₁ and 250 ₂ . The PBCs 250 ₁ and 250 ₂ multiplex the subcarriers output from the optical modulators 240 ₁ to 240 ₄ , and the optical coupler 260 multiplexes the outputs from the PBCs 250 ₁ and 250 ₂ .

  In this way, an optical OFDM signal composed of three or more subcarriers, in which the polarization between adjacent subcarriers is maintained in an orthogonal state, is realized.

Further, in this case, an optical OFDM signal composed of three or more subcarriers can be realized even using a configuration such as the optical transmission device 200b shown in FIG. 6 includes a light source 210, an MZM 220, an optical demultiplexer 230, optical modulators 240 ₁ to 240 ₄ , polarization maintaining optical couplers 270 ₁ , 270 ₂ , and PBC 280. As shown in the figure, after the polarization maintaining coupler 270 combines subcarriers of the same polarization output from the optical modulators 240 ₁ to 240 ₄ , the PBC 280 combines subcarriers in the orthogonal polarization state. A configuration of multiplexing is also possible.

Furthermore, an optical OFDM signal composed of three or more subcarriers can be realized even by using a configuration such as the optical transmission device 300 shown in FIG. 7 includes a light source 310, an optical coupler 390, MZMs 320 ₁ and 320 ₂ , optical demultiplexers 330 ₁ and 330 ₂ , optical modulators 340 _{1 to} 340 ₅ , optical couplers 370 ₁ and 370 ₂ , and PBC 380. Consists of

The optical coupler 390 branches the continuous light output from the single light source 310, and the MZM 320 ₁ , 320 ₂ applies a voltage of ± Vpi around the point N in FIG. ] Is generated at a frequency of 2Δf [GHz], and the subcarriers are demultiplexed and modulated using AWG or the like and then multiplexed. For the other continuous light, by applying a voltage of about ± Vpi / 2 around the point M in FIG. 3 at a frequency of 2Δf [GHz], a frequency interval of 2Δf [GHz] is obtained. Three subcarriers are generated, and the subcarriers are similarly demultiplexed by using AWG or the like. After the optical modulators 340 _{1 to} 3240 ₄ perform modulation, the polarization maintaining optical couplers 370 ₁ and 370 ₂ perform modulation. The subcarriers combined are multiplexed. Finally, the PBC 380 combines these subcarriers so that the polarization between adjacent subcarriers is maintained in an orthogonal state, and an optical OFDM signal is realized. In this method, since the frequency interval of the subcarriers generated in each MZM is wider than the frequency interval in the example of the optical transmission apparatus 200a shown in FIG. 5, when separating each subcarrier, the channel frequency interval is more A widely designed optical demultiplexer can be used.

  FIG. 8 shows a simulation result in the first embodiment of the present invention. This figure shows a signal after transmission through an optical filter of an optical OFDM signal generated by numerical simulation and using the optical transmission device 100a shown in FIG. 2 and in which the polarization between adjacent subcarriers is maintained in an orthogonal state. A spectrum and an eye diagram are shown. Here, it is assumed that the path lengths from when each subcarrier is demultiplexed by the MZI 130 to when the subcarriers are multiplexed by the PBC 150 are equal. Similarly to FIG. 16, each subcarrier is modulated as a 10 Gbps NRZ signal, and the frequency interval between the subcarriers is 10 GHz. As the optical filter, a primary Gaussian filter having a 3 dB bandwidth was used. Compared with FIG. 16, it can be seen that the signal spectrum is narrowed to the same extent by the optical filter, while the eye is wide open in the time domain (around 150 ps) where delay interference occurred. This indicates that optical interference from adjacent subcarriers during subcarrier separation resulting from waveform degradation due to optical filters is sufficiently suppressed by making the polarization between adjacent subcarriers orthogonal. . Similarly, optical interference from adjacent subcarriers, such as waveform deterioration due to wavelength dispersion and fiber nonlinear optical effects, as well as waveform deterioration due to optical filters, between adjacent subcarriers, regardless of the deterioration factor. It is possible to sufficiently suppress the influence by making the polarization of the two orthogonal. In particular, the optical OFDM signal generally has a large effect of suppressing optical interference due to orthogonal polarization because the time domain in which the eye is open is narrow.

[Second Embodiment]
FIG. 9 shows a configuration of an optical transmission apparatus according to the second embodiment of the present invention.

The optical transmission device 400 shown in the figure includes a light source 410, MZM 420, MZI 431, optical modulators 432 ₁ and 432 ₂ , a λ / 2 plate 433, a phase adjustment unit 434, and an optical coupler 435.

  In the transmission system as shown in the first embodiment, each subcarrier is multiplexed after propagating through different paths. In this case, due to the temporal variation of the respective path lengths, a state occurs in which phase synchronization is not necessarily established between the subcarriers at the time of multiplexing, and this phase fluctuation occurs during subcarrier separation during signal reception. It causes optical interference between subcarriers and waveform deterioration. Therefore, as shown in FIG. 9, a system up to subcarrier multiplexing by MZI 431 is constructed by a planar lightwave circuit (PLC) 430 as a transmitter. By adopting such a configuration, it is possible to strictly control the path length through which each subcarrier passes, and to realize phase synchronization between the subcarriers. In this configuration, a method using a λ / 2 plate 433 can be cited as means for realizing polarization orthogonalization between subcarriers.

  Further, as a method for realizing the phase synchronization between the subcarriers, the configuration shown in FIGS. 10 and 11 can be realized in addition to using the PLC 430 described above. The optical transmission apparatus shown in FIG. 10 includes a light source 510, MZM 520, MZI 530, optical modulators 5401 and 5402, an optical frequency shifter 550, a PBC 560, a phase difference detector 570, and an optical coupler 580. Among them, a phase difference detector 570 for detecting a phase difference between subcarriers includes a reference signal source 571, a mixer 572, a light detector (PD: Photo Detector) 573, a PBS (Polarization Beam Splitter) 575, λ / 2. A plate 574 is provided.

  11 includes a light source 610, MZM 620 and MZI 630, optical modulators 6401 and 6402, a polarization controller 650, an optical frequency shifter 660, an optical coupler 670, a phase difference detector 680, and an optical coupler 690. Is done. Among these, the phase difference detector 680 includes a reference signal source 681, a mixer 682, a PD 683, a λ / 2 plate 685, and a PBS 686.

  As shown in FIGS. 10 and 11, phase synchronization between subcarriers can be realized by using phase difference detectors 570 and 680 and optical frequency shifters 550 and 660.

  Each of the phase difference detectors 570 and 680 in FIG. 10 and FIG. 11 taps the combined OFDM signal and uses PBSs 575 and 686 and optical couplers 580 and 690 to align the polarization of each subcarrier. The PD 573,683 extracts the beat component of the light intensity generated by the optical interference between the subcarriers. When this beat frequency is Δf + δf [GHz] and a desired subcarrier frequency interval is Δf [GhZ], a signal having a subcarrier interval frequency Δf [GHz] is generated from the reference signal source 571,681. The error frequency δf [GHz] is extracted by the mixers 572 and 682. The optical frequency shifters 550 and 660 control the error frequency δf [GHz] to be small. Thereby, the phase synchronization between subcarriers is realizable.

[Third Embodiment]
FIG. 12 shows the configuration of the optical transmission apparatus in the third embodiment of the present invention.

The optical transmission device 700 shown in the figure includes light sources 710 _{1 to} 710 ₄ , optical modulators 720 _{1 to} 720 ₄ , optical frequency shifters 730 _{1 to} 730 ₃ , PBC 740 ₁ and 740 ₂ , an optical coupler 750, a phase difference detector 760. _{1 to} 760 ₃ , an optical demultiplexer 780, and an optical coupler 790. Among these, each of the phase difference detectors 760 _{1 to} 760 ₃ includes a reference signal source 761, a mixer 762, a PD 763, a λ / 2 plate 764, and a PBS 765.

In the optical transmission device 700 of the figure, a plurality of light sources ₇₁₀ 1 to 710 _4, respectively output different continuous light _{f 1, f 2, f 3} , f 4 wavelengths, each of the continuous light, the optical OFDM signal It is assumed that the subcarrier is configured. As a method of realizing phase synchronization between subcarriers, each of the phase difference detectors 760 _{1 to} 760 ₃ extracts a beat component of light intensity due to interference between subcarriers, and the beat frequency Δf + δf [GHz] is A method of controlling each of the optical frequency shifters 730 _{1 to} 730 ₃ to be equal to a desired subcarrier frequency interval Δf [GHz] (= f ₂ −f ₁ = f ₃ −f ₂ = f ₄ −f ₃ ) Is mentioned. The optical frequency shifters 730 _{1 to} 730 ₃ perform arbitrary intensity and phase modulation on the respective subcarriers output from the light sources 710 _{1 to} 710 ₄ at a modulation rate equal to the frequency interval between the subcarriers. Each subcarrier subjected to phase modulation propagates through a polarization maintaining system constituted by a polarization maintaining fiber, and is input to PBCs 740 ₁ and 740 ₂ . The PBCs 740 ₁ and 740 ₂ generate an optical OFDM signal in which the polarization between adjacent subcarriers is maintained in an orthogonal state by multiplexing so that the polarization between adjacent subcarriers is in an orthogonal state.

  Unlike the first and second embodiments, this embodiment is characterized in that it is not necessary to separate subcarriers using an optical demultiplexer prior to modulation of each subcarrier.

  The present invention is not limited to the above-described embodiment, and various modifications and applications can be made within the scope of the claims.

  The present invention can be applied to an orthogonal polarization multiplexing system when transmitting an optical signal multiplexed in optical frequency.

2 Optical modulation means 3 Polarization orthogonal means 4 Optical frequency multiplexing means 100, 200, 300, 400, 500, 600, 700 Optical transmission devices 110, 210, 310, 410, 510, 610, 710 Light sources 120, 220, 320, 420, 520, 620 MZM (Mach-Zehnder Modulator)
130,431,530,630 MZI (Mach-Zehnder Interferometer)
140, 240, 340, 432, 540, 640, 720, 780 Optical modulator 150, 250, 280, 380, 560, 740 PBC (Polarization Beam Combiner)
160, 650 Polarization controller 170, 260, 370, 435, 580, 670, 684, 690, 750, 790 Optical coupler 230, 330 Optical demultiplexer 270 Polarization maintaining optical coupler 433, 574, 685, 764 λ / 2 Plate 434 Phase adjuster 550, 660, 730 Optical frequency shifter 570, 680, 760 Phase difference detector 571, 681, 761 Reference signal source 572, 682, 762 Mixer 573, 683, 763 PD
575,686,765 PBS

Claims (4)

An optical transmission device for transmitting an optical signal multiplexed in optical frequency,
Light modulating means for modulating the input light for each frequency;
A polarization orthogonal means for orthogonalizing polarization between adjacent optical signals in frequency space;
Phase synchronization means for synchronizing the optical phase between the optical signals;
Optical frequency multiplexing means for optical frequency multiplexing optical signals phase-synchronized by the phase synchronization means;
Have
The phase synchronization means includes
A reference signal source for generating a signal having a frequency interval between two optical signals adjacent in frequency space;
A polarization beam splitter that polarization-separates two optical signals adjacent in frequency space among signals after optical frequency multiplexing by the optical frequency multiplexing means;
A λ / 2 plate that aligns one polarization of the two optical signals separated by the polarization beam splitter with the other polarization;
An optical coupler that combines the two optical signals after the polarizations are aligned;
A light receiver for extracting a beat component of light intensity generated by optical interference of two optical signals combined by the optical coupler;
A mixer for inputting the beat component extracted by the light receiver and a signal generated from the reference signal source to output an error frequency signal;
An optical transmission apparatus comprising: an optical frequency shifter that inputs the error frequency signal from the mixer and adjusts one of the two optical signals so that the frequency of the signal is reduced. 2. The optical transmission apparatus according to claim 1, wherein the optical modulation means, the polarization orthogonal means, the phase synchronization means, and the optical frequency multiplexing means are arranged on a planar lightwave circuit. An optical transmission method for transmitting an optical signal multiplexed in optical frequency,
An optical modulation step in which the optical modulation means modulates the input light for each frequency; and
A polarization orthogonal step, wherein the polarization orthogonal means makes the polarization between optical signals adjacent in frequency space orthogonal,
A phase synchronization means for synchronizing the optical phase between the optical signals;
An optical frequency multiplexing means for optical frequency multiplexing the optical signal phase-synchronized in the phase synchronization step;
And
In the phase synchronization step,
A signal having a frequency interval between two optical signals adjacent in the frequency space is generated.
Two optical signals adjacent in the frequency space among the signals after frequency multiplexing are polarized and separated,
Align one polarization of the two separated optical signals with the other polarization,
Combine the two optical signals after aligning the polarization,
Extract the beat component of the light intensity caused by the optical interference of the two combined optical signals,
An error frequency signal is generated by multiplying the extracted beat component by a signal having an interval frequency between two optical signals adjacent in the frequency space ,
One of the two optical signals is adjusted so that the frequency of the error frequency signal is reduced. 4. The optical transmission method according to claim 3, wherein the optical modulation step, the polarization orthogonal step, the phase synchronization step, and the optical frequency multiplexing step are realized on a planar lightwave circuit.

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