JP5437223B2 - Optical receiver, optical communication system, and coherent detection method - Google Patents

Description

  The present invention relates to an optical receiver for coherent synchronous detection of an optical signal, an optical communication system including the optical receiver, and a coherent synchronous detection method thereof.

  With the spread of optical access systems represented by PON (Passive Optical Network), demands for higher-speed communication are increasing. As a means for solving this requirement, OFDMA (Orthogonal Frequency Division Multiple Access) PON in which coherent detection, which is known to be capable of reception with high sensitivity and suppression of out-of-band noise, is applied to an access system (for example, non-patent literature) 1) and Code Division Multiple Access (CDMA) PON (see, for example, Non-Patent Document 2) are being studied.

N. Cvjetic, D.C. Qian, J.A. Hu, and T.H. Wang, “Orthogonal frequency division multiple access PON (OFDMA-PON) for color upstream transmission bead 10 Gb / s,” IEEE J. Select. Areas Commun. , Vol. 28, no. 6, pp. 781-790, Aug. 2010. M.M. Yoshino, S .; Kaneko, T .; Taniguchi, N .; Miki, K .; Kumozaki, T .; Imai, N.I. Yoshimoto, and M.K. Tsubokawa, “Beat noise mitigation of spectral amplification coding OCDMA using heterodyne detection,” J. Am. Lightwave Technol. , Vol. 26, no. 8, pp. 963-970, Apr. 2008. K. Kasai, J .; Hongo, M .; Yoshida, and M.K. Nakazawa, “Optical phase-locked loop for coherent transmission over 500 km using heterodyne detection with fiber lasers”, IEICE Electronics. 4, no. 3, pp. 77-81, Feb. 2007. S. Ristic, A.R. Bhardwaj M.M. J. et al. Rodwell, L.M. A. Coldren, and L.C. A. Johansson, “An optical phase-locked loop photonic integrated circuit,” J. Lightwave Technol. , Vol. 28, no. 4, pp. 526-538, Feb. 2010. M.M. Fujiwara, J. et al. Kani, H .; Suzuki, K .; Araya, and M.M. Teshima, “Flattened optical multi carrier generation of 12.5 GHz spaced 256 channels based on sinusoidal amplification and phase hybrid modulation,”. Lett. , Vol. 37, pp. 967-968, Jul. 2001. M.M. Yoshino, N .; Miki, N .; Yoshimoto, and K.K. Kumozaki, “Multiwavelength optical source for OCDM using sinusoidally modulated laser diode,” J. Lightwave Technol. Vol. 27, no. 20, pp. 4524-4529, Oct. 2009. Takuo Tanemura, Kazuhiro Kato, Kazuo Kikuchi, “Polarization-independent asymmetric four-wave mixing using twisted optical fiber”, 2005 IEICE General Conference C-4-9 Manabu Yoshino, Naoto Yoshimoto, Makoto Tsubokawa, "Optical interference suppression by chirp synchronous reception", 2008 IEICE General Conference B-10-53 Manabu Yoshino, Makoto Kaneko, Junki Miki, Makoto Tsubokawa, “Decoder for SAC-OCDMA that is strong against leakage of adjacent spectrum chips”, 2007 IEICE General Conference B-10-22 O. Ishida and T. Okoshi, “Effect of frequency offset in DPSK phase diversity optical receivers”, ECOC, 1988, pp. 196 155-158.

  In coherent detection, a signal carried by signal light is converted into an intermediate frequency (IF) (frequency corresponding to the frequency difference between two lights) signal by the beat of signal light and local light, and the signal is demodulated. . Coherent detection includes envelope detection and synchronous detection. Among these, in coherent synchronous detection, a dynamic phase synchronization technique of local light emission is important. This is because if the phase of the signal light after transmission and the phase of local light are not synchronized, the phase of the IF signal fluctuates and accurate demodulation cannot be performed (see Non-Patent Document 3, for example). Fluctuations need to be negligible compared to the signal transmission rate, so that higher-speed dynamic phase synchronization is required to receive high-speed signals.

  Dynamic phase synchronization is performed by an optical phase-locked loop (OPLL). The optical phase-locked loop is usually configured so that two lights are detected by an optical detector, and error information of the detected optical phase is fed back to a local light source by an electric circuit. To realize a practical optical phase-locked loop, the line width of the light source should be set so that the optical frequency difference during free running of the two lights is always within the frequency pull-in range of the optical phase-locked loop and the phase error is reduced. It is necessary to reduce the length, shorten the loop length, and widen the bandwidth of the optical phase-locked loop. For example, in the case of signal light having a line width of about 10 MHz, a bandwidth of GHz is required. For the bandwidth of GHz, the loop delay time and the loop length need to be at most 0.1 microsecond and about 2 to 3 cm. Therefore, use of a light source having a narrow line width (for example, see Non-Patent Document 3) or integrating an optical detector and an electric circuit component to shorten an optical phase-locked loop (for example, see Non-Patent Document 4). Etc.) have been proposed. However, further speeding up is difficult.

  Further, in a point-multipoint optical communication system, optical code division multiplexing is expected as a method for providing an occupying optical service excellent in secrecy and interference resistance. In particular, optical code division multiplexing using an intensity code in a spectral region that can use a simple system for a subscriber side apparatus is expected. The intensity code in the spectral region is encoded by a combination of a plurality of lights having different optical frequencies. In the optical code division multiplexing using the intensity code in the spectral region, transmission characteristics are deteriorated due to beat noise between signal lights using the same spectrum. For this reason, application of a coherent reception system using local light emission composed of a plurality of lights having different optical frequencies has been proposed. In this optical code division multiplexing, multiple access interference (MAI) occurs due to variations in phase difference with signal light from other transmitters, and therefore, a plurality of lights constituting the signal light and respective stations corresponding thereto There is a demand for making the phase relationship of light emission constant (for example, see Non-Patent Document 2).

  In order to solve the problem of phase synchronization in the coherent synchronous detection described above, for example, signal light having a constant phase relationship and local light are time-division multiplexed or polarization-division multiplexed (however, polarization division multiplexing is signal light and local light). However, it is possible to make the phase relationship between the signal light and the local light constant by adopting a method of transmitting from the transmitter by limiting to heterodyne detection using different optical frequencies. However, since this method propagates local light along with signal light in the same direction, the utilization efficiency of the transmission path is deteriorated, which is not desirable. Furthermore, in the case of optical code division multiplexing in which multiple signal lights of the same optical frequency may arrive at the same time, local light of the same optical frequency is transmitted simultaneously from multiple transmitters in order to generate coherent crosstalk. I can't. In addition, when there are multiple transmitters in optical code division multiplexing, the phase of signal light arriving from different transmitters is different, so the phase of one local light is synchronized with multiple signal lights of different phases. Is essentially impossible.

  Therefore, the present invention has been made to solve the above-described problems, and transmits local light having a constant phase relationship with signal light together with signal light, or uses a dynamic optical phase-locked loop of local light. It is another object of the present invention to provide an optical receiver, an optical communication system, and a coherent synchronous detection method capable of coherently detecting one or a plurality of signal lights regardless of phase and optical frequency fluctuations.

  In order to solve the above-described problems, an optical receiver, an optical communication system, and a coherent synchronous detection method according to the present invention are configured to pump a plurality of lights having the same initial phase and phase noise term to a four-wave mixing unit. And output a plurality of idler lights that are a four-light mixture of signal light and pump light, and the intermediate frequency signal generated by these output idler lights has an electrical signal and phase when generating the pump light. Synchronous detection was performed using synchronized electrical signals.

  Specifically, in the optical receiver according to the present invention, the signal light and a plurality of pump lights having the same initial phase and phase noise terms and having a predetermined frequency difference are input, and the four light waves of the signal light and the pump light are input. A four-wave mixing unit that generates mixing and outputs at least one set of idler light whose optical frequency interval is an intermediate frequency, and optically detects the idler light from the four-wave mixing unit, and intermediate frequency between idler lights And a synchronous detection unit that performs coherent synchronous detection on the signal with an electric signal having the intermediate frequency and the phase synchronized with the electric signal when generating the pump light.

  The optical receiver according to the present invention generates four-wave mixing of the signal light and the pump light from the signal light and a plurality of pump lights having the same initial phase and phase noise terms and a predetermined frequency difference, and at least one A plurality of idler light whose intermediate optical frequency intervals are intermediate frequencies are output, the idler light is optically detected, an intermediate frequency signal between idler lights is the intermediate frequency and an electric signal for generating the pump light A coherent synchronous detection method is adopted, in which coherent synchronous detection is performed with an electrical signal whose phase is synchronized with the signal.

  An optical communication system according to the present invention includes the optical receiver and an optical transmitter that transmits the signal light to the optical receiver.

  The four-wave mixing unit outputs idler light having the same initial phase and phase noise term. Further, the phase of the electrical signal at the time of synchronous detection is set as the phase difference of the intermediate frequency signal between the idler lights. The phase of the intermediate frequency signal between idler lights having the same initial phase and phase noise term is constant regardless of the phase of the signal light and the optical frequency fluctuation. Therefore, the optical receiver, coherent synchronous detection method and optical communication system according to the present invention can perform coherent synchronous detection using one idler light as signal light and another idler light as local light.

  Therefore, the present invention can transmit local light having a constant phase relationship with signal light together with the signal light, and can also transmit one or more signal lights to the phase and light without using a dynamic optical phase-locked loop of local light. It is possible to provide an optical receiver, an optical communication system, and a coherent synchronous detection method capable of performing coherent synchronous detection regardless of frequency fluctuations.

  In the present invention, the signal light is composed of a plurality of lights having different optical frequencies, and the signal light and the pump light are the signals among the idler lights to be subjected to coherent synchronous detection in the synchronous detection unit. It is preferable that the optical frequency interval between the idler lights based on the light of different optical frequencies constituting the light is an optical frequency that is not less than twice the transmission band and not less than four times the intermediate frequency.

  In the optical receiver and the optical communication system according to the present invention, a set of idler lights synchronized with each other corresponding to each of a plurality of lights constituting the signal light is independently generated, so that coherent synchronization can be performed without any interference. Detection can be performed.

  In the present invention, the signal light is composed of a plurality of lights having different optical frequencies, and the signal light and the pump light are the signals among the idler lights to be subjected to coherent synchronous detection in the synchronous detection unit. The optical frequency interval between the idler lights based on the light of different optical frequencies constituting the light is an optical frequency that is not less than the transmission band and not less than twice the intermediate frequency, and the four-wave mixing unit is provided for each signal light. The idler light may be output, and the synchronous detection unit may perform coherent synchronous detection of the idler light for each signal light.

  The optical receiver and the optical communication system according to the present invention can receive an optical code division multiplexed signal. Further, the optical receiver and the optical communication system according to the present invention each generate a pair of idler lights that are synchronized with each other corresponding to each of the plurality of lights constituting the signal light, so that they do not interfere with each other. Coherent synchronous detection can be performed.

  In the present invention, the signal light is optical code division multiplexed with light of a plurality of optical frequencies, and the four-wave mixing unit outputs or idles the idler light of the signal light branched according to the code to be decoded. The light is branched and output according to the code for decoding the light, and the synchronous detection unit synchronously detects the idler light output according to the code from the four-wave mixing unit, and uses the signal for synchronous detection as the code. It is preferable to add or subtract accordingly.

  The optical receiver and the optical communication system according to the present invention correspond to a plurality of lights (spectrum chips) constituting all of a plurality of optical code multiplexed signal lights, respectively, and signal lights having the same initial phase and phase noise terms. And local light can be generated to perform coherent synchronous detection. The optical receiver and the optical communication system according to the present invention can solve the problem of MAI due to variation in phase difference between signal light and local light.

  In the present invention, the optical frequency of the signal light may change with time, and the optical frequency of the pump light may change in synchronization with the optical frequency change of the signal light.

  The optical receiver and the optical communication system according to the present invention synchronize the optical frequency fluctuation of the spectrum chip and the optical frequency fluctuation of the pump light, and eliminate the problem of MAI due to the phase difference variation between the signal light and the local light. be able to.

  In the present invention, it is preferable that the pump lights are in the same circular polarization state, and the four-wave mixing unit generates the idler light with a circular polarization twisted optical fiber.

  In order to perform coherent synchronous detection, it is necessary to make the polarization of the local light and the signal light coincide with each other to eliminate the polarization dependency. In the optical receiver and the optical communication system according to the present invention, the four-wave mixing unit is a polarization-independent four-wave mixing described in Non-Patent Document 7. That is, when the pump light is input to the four-wave mixing unit, the same circular polarization state is set, and the four-wave mixing unit is a circularly polarized torsion optical fiber, thereby eliminating the polarization dependency in the four-wave mixing. Can do. Therefore, the optical receiver and the optical communication system according to the present invention can eliminate the polarization dependency of coherent synchronous detection.

  In the present invention, the four-wave mixing unit may generate the idler light in a loop including a nonlinear medium unit and a polarization beam combiner (PBC).

  By combining a nonlinear medium and PBC, similar four-wave mixing can be generated for any orthogonal polarization component. As a result, the sum of the outputs of the respective coherent synchronous detections is constant regardless of the polarization. Therefore, the optical receiver and the optical communication system according to the present invention can eliminate the polarization dependence in the four-wave mixing, and can eliminate the polarization dependence in the coherent synchronous detection.

  In the present invention, the pump light may be composed of two lights whose polarizations are orthogonal to each other and have the same light intensity.

  When the polarization of idler light corresponding to the orthogonally polarized pump light is maintained, no beat component is generated between the orthogonally polarized idler lights. Therefore, the optical receiver and the optical communication system according to the present invention can eliminate the polarization dependence in the four-wave mixing, and can eliminate the polarization dependence in the coherent synchronous detection.

  The present invention transmits local light having a constant phase relationship with signal light together with the signal light, and does not use a dynamic optical phase-locked loop of local light. It is possible to provide an optical receiver, an optical communication system, and a coherent synchronous detection method capable of performing coherent synchronous detection regardless of fluctuations.

1 is a schematic configuration diagram of an optical communication system according to the present invention. 1 is a schematic configuration diagram of an optical communication system according to the present invention. 1 is a schematic configuration diagram of an optical communication system according to the present invention. 1 is a schematic configuration diagram of an optical communication system according to the present invention. 1 is a schematic configuration diagram of an optical communication system according to the present invention. It is a schematic diagram of the optical spectrum of the signal light transmitted by the optical communication system according to the present invention. FIG. 7 is a schematic diagram of an electric spectrum output by the optical detector of the optical communication system according to the present invention in the case of FIG. 6. It is a schematic diagram of the optical spectrum of the signal light transmitted by the optical communication system according to the present invention. FIG. 9 is a schematic diagram of an electrical spectrum output by the optical detector of the optical communication system according to the present invention in the case of FIG. 8. It is a figure explaining the nonlinear medium part with which the optical communication system which concerns on this invention is provided. It is a figure explaining the nonlinear medium part with which the optical communication system which concerns on this invention is provided. It is a schematic diagram of the optical spectrum which the nonlinear medium part of the optical receiver which concerns on this invention outputs. (A) It is a schematic diagram of the optical spectrum which the nonlinear medium part of the optical receiver which concerns on this invention outputs. (B) It is a figure explaining the relationship of the optical frequency space | interval D of signal light Ls and pump light Lp1, and the optical frequency space | interval H of signal light Ls and pump light Lp3. It is a schematic diagram of the optical spectrum which the nonlinear medium part of the optical receiver which concerns on this invention outputs. It is a schematic diagram of the optical spectrum which the nonlinear medium part of the optical receiver which concerns on this invention outputs. It is a figure explaining the four-wave mixing part with which the optical communication system which concerns on this invention is provided. It is a figure explaining the experimental system which verified the effect of the Example of the optical communication system which concerns on this invention. It is a figure explaining the spectrum of (A) light and (B) electricity after the optical filter in the experiment system which verified the effect of the Example of the optical communication system which concerns on this invention. It is a figure explaining the change of the signal amplitude with respect to the phase shift in the phase modulator which simulated the phase fluctuation of signal light. It is a schematic diagram of the optical spectrum of the signal light transmitted by the optical communication system according to the present invention. It is a schematic diagram of the optical spectrum of the signal light transmitted by the optical communication system according to the present invention. It is a schematic diagram of the optical spectrum of the signal light transmitted by the optical communication system according to the present invention. It is a schematic diagram of the optical spectrum of the signal light transmitted by the optical communication system according to the present invention. It is a schematic diagram of the optical spectrum of the signal light transmitted by the optical communication system according to the present invention. It is a schematic diagram of the optical spectrum of the signal light transmitted by the optical communication system according to the present invention.

  Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to the following embodiments. In the present specification and drawings, the same reference numerals denote the same components.

(First embodiment)
FIG. 1 is a schematic configuration diagram of an optical communication system 501 of the present embodiment. The optical communication system 501 includes an optical receiver 301, an optical transmitter 401 that transmits signal light to the optical receiver 301, and a pump light source 402 that supplies pump light to the optical receiver 301. The optical transmitter 401 outputs signal light having an optical frequency (fs in the figure). Here, since the pump light source 402 does not need to apply a dynamic optical phase locked loop to the signal light, unlike the optical receiver using the dynamic optical phase locked loop, the electric signal used in the light source is If it can be supplied to the optical receiver, it can be installed outside the optical receiver 301 as shown in FIG. 1, and the pump light source 402 can be shared by a plurality of optical receivers 301. The pump light source 402 is not independent of the optical receiver 301, and may be configured to be built in the optical receiver 301. Here, the description has been made on the assumption that the electrical signal used for synchronous detection in the synchronous detection unit described later is the electrical signal used to generate pump light. As long as the electric signal whose phase is synchronized with the pump light can be generated by detecting the phase of the pulse, etc., the electric signal is not limited to the electric signal used to generate the pump light. It is not necessary to generate the pump light. This is the same in the following description.

  Hereinafter, the case where the pump light is 2 and degenerate four-wave mixing is used will be described. However, as described later, the pump light may be three or more, and four-wave mixing other than degeneration may be used. The pump light output from the pump light source 402 has the same initial phase and phase noise term and a predetermined frequency difference. The pump light source 402 that outputs such pump light includes, for example, a continuous light (CW) light source 101, a sine wave signal generator 102, and an intensity modulator 103. The sine wave signal generator 102 is, for example, a transmitter. The intensity modulator 103 modulates the continuous light having the optical frequency fp from the CW light source 101 with the sine wave signal from the sine wave signal generator 102. Here, the intensity modulator 103 suppresses the carrier component that is the output of the CW light source 101, adjusts it to a state in which only two modulation sidebands are output, and outputs it as pump light. Since the pump light is two modulation sidebands of intensity modulation, their phases are equal. Non-patent documents 5 and 6 show light sources using such modulation sidebands.

  In addition, hereinafter, the case where the phase of the intermediate frequency signal is zero will be described as an example, but the initial phase and the phase noise term of the pump light are equal, and the initial phase and the phase noise term are equal, The initial phase of the idler light and the phase noise term are also constant, and if the difference between the phase of the electrical signal to be synchronously detected is other than 90 degrees, the output of coherent synchronous detection becomes a constant value other than zero. For this reason, the absolute value of the output intensity is also reduced in cases other than 90 degrees, but it is the same as when the phase is zero. Further, although an example in which a modulation sideband is used as a light source has been described, other light sources such as a mode-locked laser and an optical frequency comb may be used as long as the initial phase and the phase noise term are equal.

  The optical receiver 301 includes a four-wave mixing unit 31 and a synchronous detection unit 21. Specifically, the optical receiver 301 receives signal light and a plurality of pump lights having the same initial phase and phase noise terms and having a predetermined frequency difference, and is an idler for four-wave mixing of the signal light and the pump light. The four-wave mixing unit 31 that outputs a plurality of lights, and the idler light from the four-wave mixing unit 31 is optically detected, and the phase of the electrical signal and the phase when generating the pump light at the intermediate frequency of the intermediate frequency signal between the idler lights And a synchronous detection unit 21 that performs synchronous detection with a synchronized electrical signal. Here, in order to set at least one set of optical frequency intervals of idler light as an intermediate frequency, the frequency can be adjusted by the optical frequency interval of pump light. Further, an electrical signal used when synchronous detection is performed by the synchronous detection unit may be adjusted, and at least one set of idler light to be detected by the synchronous detection unit may be output.

  The optical receiver 301 generates a plurality of idler lights of four-wave mixing of the signal light and the pump light from the signal light and the plurality of pump lights having the same initial phase and phase noise term and a predetermined frequency difference. The lights are synchronously detected with an electric signal whose phase is synchronized with the electric signal when generating the pump light at the intermediate frequency. Here, the optical frequency of the plurality of pump lights is a frequency at which at least one set of optical frequency intervals of idler light is an intermediate frequency.

  The four-wave mixing unit 31 includes a nonlinear medium unit 11 that receives signal light and pump light and generates four-wave mixing, and an optical filter 12 that extracts idler light of the four-wave mixing output from the nonlinear medium unit 11. For example, a semiconductor optical amplifier (Semiconductor Optical Amplifier: SOA), a highly nonlinear fiber (High Nonlinear Fiber: HNLF), a zero dispersion-shifted fiber (DSF), or the like can be used as the nonlinear medium unit 11. It is assumed that the nonlinear medium section 11 can have a sufficient interaction length even when wavelength dispersion of signal light and pump light is taken into consideration.

  When the signal light and pump light having the same phase noise term as the initial phase are input, the four-wave mixing unit 31 receives a degenerate four-wave mixing idler light of one pump light and a degenerate four-wave mixing idler of the other pump light. Outputs four-wave mixing idler light of light and both pump lights. The initial phase and phase noise terms of these idler lights are equal.

Here, the signal light, the pump light, and the idler light are combined with the signal light Ls, one pump light Lp1, the other pump light Lp2, the degenerate four-wave mixing idler light La1, and the degenerate four-wave mixing of the pump light Lp2. The idler light La 2, the pump light Lp 1, and the pump light Lp 2 are named and described as four-wave mixing idler light La 3. The signal light Ls, one pump light Lp1, the other pump light Lp2, the degenerate four-wave mixing idler light La1 of the pump light Lp1, the degenerate four-wave mixing idler light La2 of the pump light Lp2, the pump light Lp1 and the pump light Lp2 The phases of the four-wave mixing idler light La3 are assumed to be φ1, φp1, φp2, φ1 ′, φ1 ″, and φ1 ′ ″, respectively. The signal light Ls, one pump light Lp1, the other pump light Lp2, the degenerate four-wave mixing idler light La1 of the pump light Lp1, the degenerate four-wave mixing idler light La2 of the pump light Lp2, the pump light Lp1, and the pump light The optical frequencies of the four-wave mixing idler light La3 of Lp2 are f1, fp1, fp2, f1 ′, f1 ″, and f1 ′ ″, respectively. Further, when the optical frequency interval between the pump lights is df, the following equation is established.
f1 ′ = 2fp1-f1
f1 '' = 2fp2-f1 = 2fp1 + 2df-f1
f1 ′ ″ = fp1 + fp2−f1 = 2fp1 + df−f1
φ1 ′ = 2φp1-φ1
φ1 ″ = 2φp2-φ1 = 2φp1-φ1
φ1 ′ ″ = φp1 + φp2-φ1 = 2φp1-φ1
Here, all of the idler light (La1, La2, La3) have the same initial phase and phase noise term.

  Although the optical frequency relationship between the pump light and the signal light is not shown, it is desirable that the pump light, the signal light, and the idler light do not overlap each other in consideration of the optical frequency spread due to the modulation. When pump light and signal light or pump light and idler light overlap, signal light and idler light also become part of the pump light, and the modulation component is unnecessarily superimposed on idler light generated from the pump light. There is a fear. However, the influence of the overlap of the signal light, idler light and pump light depends on the intensity ratio of the signal light, idler light and pump light, so that the intensity of the pump light is, for example, 30 dB relative to the intensity of the signal light or idler light. If it is large enough, it can be ignored. When idler light to be subjected to synchronous detection described later overlaps with other light, since the idler light alone cannot be extracted by the optical filter 12 as described later, components that cannot be removed by the optical filter 12 are noise. It becomes. In particular, when the overlapped light is pump light having a higher intensity than the idler light, the influence becomes large.

FIG. 12 is an example in which there is one signal light and two pump lights, and the optical frequencies of the pump light, signal light, and idler light do not overlap each other. In FIG. 12, the signal light Ls, the pump light Lp1, the pump light Lp2, the idler light La1, the idler light La3, and the idler light La2 are arranged in order from the lowest optical frequency. Here, the level of the optical frequency may be inverted, and this level inversion can be similarly performed in the following description.
Optical frequency f1 = fs of the signal light Ls,
Optical frequency fp1 = ps + D of pump light Lp1
Optical frequency fp2 = fp1 + df = fs + D + df of the pump light Lp2
Optical frequency f1 ′ of idler light La1
= 2fp1-fs = 2fs + 2D-fs = fs + 2D,
Optical frequency f1 ′ ″ of idler light La3
= Fp1 + fp2-fs = 2fp1 + df-fs = fs + 2D + df,
Optical frequency f1 '' of idler light La2
= 2fp2-fs = 2fp1 + 2df-fs = fs + 2D + 2df
It can be expressed. Here, the optical frequency interval between the signal light Ls and the pump light Lp1 is D, the optical frequency interval df between the pump lights is an intermediate frequency for synchronous detection, and the transmission band B of the signal light is half the intermediate frequency.

  Here, the modulation by the transmission signal of the optical transmitter 401 is limited to the transmission band, and the intermediate frequency is set to twice the transmission band on the premise that the modulation components other than the modulation main lobe can be ignored. In such modulation, the signal itself that modulates the optical modulator or the direct modulation laser in the optical transmitter 401 is passed through an electrical low-pass filter or the like whose 3 dB band is 0.5 to 0.8 times the modulation band. Can be realized by band-limited modulation. When the modulation component having a frequency higher than that of the main lobe cannot be ignored, the intermediate frequency may be set to a frequency obtained by adding the frequency width of the modulation component that cannot be ignored with the higher frequency component than the main lobe to the transmission band. On the other hand, if it can be ignored, the frequency width of the modulation component that can be ignored may be used as the frequency. The same applies to the following description.

At this time, in order for the signal light and the pump light not to overlap in consideration of the modulation,
{Fp1}-{fs + B} = {fs + D}-{fs + B}
= D−B ≧ 0, that is, D ≧ B may be satisfied.
In addition, in consideration of modulation, idler light and pump light do not overlap.
{2fp1-fs-B}-{fp2}
= {Fs + 2D-B}-{fs + D + df} = D−B−df ≧ 0,
That is, D ≧ B + df may be satisfied.
Accordingly, this condition is satisfied if the optical frequency interval D between the signal light Ls and the pump light Lp1 is equal to or greater than the sum of the transmission band B and the pump light interval df. When the transmission band B is half the pump light interval df, this condition is satisfied if the optical frequency interval D between the signal light Ls and the pump light Lp1 is 1.5 times the pump light interval df or more.

  Here, it is assumed that the transmitted light frequency and the cut-off light frequency of the optical filter 12, which will be described later, are switched on the step on the optical frequency axis. When the switching between the transmitted light frequency and the cut-off light frequency is gentle, the interval between the idler light and the pump light is not less than 0.5 df, which is half the pump light interval corresponding to the transmission band B, and the pump light is sufficiently cut off. What is necessary is just to make it a possible interval. The consideration of the characteristics of the optical filter may be similarly considered in the following description. In addition, although the relationship between the transmission band B and the intermediate frequency is doubled, it may be a relationship necessary for extracting an intermediate frequency component by an electric filter described later, and may be double or more. The consideration of the characteristics of the electric filter may be similarly considered in the following description. On the contrary, when SSB (Single Side Band) modulation is performed and the suppression of the modulation sideband is sufficiently small, the transmission band B of the signal light can be set to an intermediate frequency.

Furthermore, an intermediate frequency smaller than the transmission band may be used. Such an intermediate frequency is, for example, an intermediate frequency at which the power penalty can be ignored by homodyne detection. For example, as shown in Non-Patent Document 10, a value that is 2% or less of the transmission band, which is a value at which a power penalty of 1 dB or less is expected in homodyne detection, may be used. However, the value of this non-patent document is an intermediate frequency signal between two light beats, but in the present application, since three lights are involved, it is stricter than the value of the non-patent document, for example, 1%, etc. The value of
At this time, in order for the signal light and the pump light not to overlap in consideration of the modulation,
{Fp1}-{fs + B} = {fs + D}-{fs + B}
= D−B ≧ 0, that is, D ≧ B may be satisfied.
In addition, in consideration of modulation, idler light and pump light do not overlap.
{2fp1-fs-B}-{fp2}
= {Fs + 2D-B}-{fs + D + df} = D−B−df ≧ 0,
That is, D ≧ B + df may be satisfied.
Therefore, this condition is satisfied if the optical frequency interval D between the signal light Ls and the pump light Lp1 is equal to or greater than the frequency obtained by adding the pump light interval df to the transmission band B. The same applies to the following description.

  In the following description, the case where the intermediate frequency is larger than the transmission band is described unless otherwise specified.

  Returning to the description of FIG. The optical filter 12 extracts only idler light (La1, La2, La3) and inputs it to the optical detector 13. Here, if the intensity of the signal light and the pump light is negligible at the time of synchronous detection of the intermediate frequency signal, the optical filter 12 may be omitted. The negligible degree depends on the requirement of the signal-to-noise ratio (SN) required at the time of reception. For example, if the SN degradation accompanying the intensity of the signal light and pump light at this location can be tolerated to 1/1000 or less of the signal, it corresponds to 1/1000 or less. The same applies to the following embodiments.

  The synchronous detector 21 includes an optical detector 13, an electric filter 14 for extracting an intermediate frequency signal from the output of the optical detector 13, an intermediate frequency signal extracted by the electric filter 14, and an electric signal for generating pump light. And a mixer 18 for performing synchronous detection by multiplication. Here, since all of the idler light (La1, La2, La3) have the same initial phase and phase noise terms, the optical detector 13 regards one idler light as signal light and other idler light as local light, thereby coherent synchronization. Detection is possible. Here, the electric filter 14 may be omitted as long as the components other than the desired intermediate frequency component can be ignored in the synchronous detection of the intermediate frequency signal. The same applies to the following embodiments.

As shown in FIG. 1, when the optical filter 12 conducts all idler light (La1, La2, La3), the optical detector 13 causes the idler light La1 to idler light La3, idler light La2 to idler light La3. As a result, an intermediate frequency signal having a pump light interval df (f _IF 1 in the drawing) is generated, and an intermediate frequency signal having 2 df (f _IF 2 in the drawing) twice the pump light interval is generated by the idler light La1 to the idler light La2. For example, the electric filter 14 sufficiently suppresses the intermediate frequency signal of the intermediate frequency 2df and extracts the intermediate frequency signal of the intermediate frequency df, so that the intermediate between the idler light La1-idler light La3 and idler light La2-idler light La3 is obtained. Facilitates coherent synchronization of frequency signals. This configuration is most desirable in terms of signal strength.

  The electric filter 14 may extract an intermediate frequency signal of 2 df that is an intermediate frequency signal of the idler light La1 to the idler light La2. Further, the optical filter 12 may extract any two of idler lights (La1, La2, La3). For example, any one of the idler light La1-the idler light La3 and the idler light La2-the idler light La3 may be extracted. In this case, only the intermediate frequency signal between the extracted idler lights is generated by the optical detector 13. If only the idler light La1 and the idler light La2 are extracted, there is also an effect that the attenuation requirement on the low frequency side of the electric filter 14 is relaxed. That is, the optical receiver 301 can perform coherent synchronous detection even if the electrofilter 14 that extracts the intermediate frequency signal of the optical frequency 2df cannot completely suppress the intermediate frequency signal of the optical frequency df.

The electric signal used for synchronous detection is an electric signal whose phase is synchronized with that of the electric signal when generating the pump light. The electrical signal is an electrical signal that is synchronously detected by the mixer at the same phase as the phase difference of the idler light in consideration of the phase rotation due to propagation by multiplying the frequency of the electrical signal when generating the pump light. desirable. In this case, since the same electric signal source is used, the effect of phase fluctuation of the electric signal source is also canceled. In the figure, the multiplier 104 that multiplies the frequency of the electric signal is provided in the light source 402, but may be provided in another location, for example, the synchronous detection unit 25 or the like. Further, different electrical signal sources may be used in synchronization with each other in the pump light source 402 and the synchronous detection unit 25. In this case, phase synchronization between the electrical signal sources is required, but the limitation on the band product and the like can be ignored in practice as compared with the case where the optical phase locked loop is configured. The phase of the intermediate frequency signal is also constant because the initial phase and phase noise terms of idler light are equal and have a predetermined frequency difference. For this reason, the phase of the electrical signal used for synchronous detection can be synchronized with the electrical signal itself when generating the pump light, or the electrical signal synchronized with the same external clock as the electrical signal source used in the local light source. It is possible to use a signal source or to synchronize with an intermediate frequency signal. Thus, in order to perform synchronous detection with an electric signal from a stable electric signal source having a stability of 10 ⁻⁶ or the like, the envelope detection of the intermediate frequency signal itself generated from the optical signal and In comparison, a signal with less noise can be obtained. Also, unlike envelope detection, the selectivity of the multiplied electric signal is high, and the effect of removing noise components that cannot be removed by the electric filter is also high.

  In the above description, it is assumed that the center optical frequency of the signal light Ls1 is constant. However, as described in Non-Patent Document 8, it may be light whose optical frequency changes with time. When the optical filter 12 is used, the optical frequency of the pump light (Lp1, Lp2) may be changed in synchronization with the optical frequency change with respect to the time of the signal light Ls1 so that idler light is conducted through the optical filter 12.

  In the present embodiment, an optical amplifier is not shown, but may be used as appropriate.

In the present embodiment, the degenerate four-wave mixing using two pump lights has been described as an example, but three or more pump lights may be used. For example, it is assumed that 3 pump light in which 1 pump light is arranged on the low frequency side of the signal light and 2 pump light is arranged on the high frequency side on the optical frequency axis is used. Here, the optical frequency of the pump light on the low frequency side is fp1, and the optical frequencies of the pump light on the high frequency side are fp2 and fp2 + df. At this time, the optical frequency of the idler light with respect to the pump light whose optical frequency is fp1 and fp2 with respect to the signal light with the optical frequency f1 higher than fp2 is f &, and the idler light with respect to the pump light with the optical frequency of fp1 and fp2 + df. Let f && be the frequency. At this time, the following equations hold for the optical frequencies f1 & and f1 && of the idler light.
f1 & = fp2 + f1-fp1 = (fp2-fp1) + f1
f1 && = fp2 + df + f1-fp1 = (fp2-fp1) + f1 + df

  In this case, two idler lights having optical frequencies f1 & and f1 && separated by a frequency interval df are generated. By detecting these two idler lights, synchronous detection at the intermediate frequency df is possible. Here, although two pump lights are arranged on the high frequency side, the same applies even if two pump lights are arranged on the low frequency side. The two pump lights only need to have an interval in which the signal light and the idler light fall between the two pump lights. The interval that falls within the range is an interval in which the influence on the signal is sufficiently small, for example, 30 dB if they do not overlap or overlap each other, taking into account the optical frequency spread due to modulation.

  Is the optical frequency of the idler light of the optical frequencies f1 & and f1 && sufficient to block the idler light of f1 & and f1 && from the signal light, pump light, idler light other than the optical frequencies f1 & and f1 && by the optical filter 12? It is assumed that the optical frequency can be cut off by the electric filter 14 or the optical frequency that can be suppressed by synchronous detection, for example, df or more.

Here, although an example in which signal light enters between pump light is shown, the pump light may be located on the high frequency side or low frequency side of the signal light. FIG. 13 shows an example in which the optical frequencies do not overlap each other when there is one signal light and three pump lights. In FIG. 13, in order from the lowest optical frequency, the signal light Ls, the pump light Lp1, the pump light Lp2, the third pump light Lp3, the idler light La1, La3, the idler light La2, the pump light Lp3 and the pump light Lp1 or the pump. The idler light (referred to as La4 and La5) with the light Lp2 and the idler light Lp6 of the degenerate four-wave mixing of the pump light Lp3 are arranged in this order, but the optical frequency may be reversed. In FIG. 13, the intermediate frequency signals of the idler lights La4 and La5 are targeted for synchronous detection, but the synchronous detection may be performed using the intermediate frequency signals of the idler lights La1, La3, and La2.
Optical frequency of signal light f1 = fs,
Optical frequency fp1 = ps + D of pump light Lp1
Optical frequency fp2 = fp1 + df = fs + D + df of the pump light Lp2
The optical frequency fp3 = fs + H of the pump light Lp3,
The optical frequencies of idler light La1, La3, La2, La4, La5, La6 are respectively f1 ′ = 2fp1-fs = 2fs + 2D−fs = fs + 2D,
f1 ′ ″ = fp1 + fp2−fs = 2fp1 + df−fs = fs + 2D + df,
f1 ″ = 2fp2-fs = 2fp1 + 2df−fs = fs + 2D + 2df,
f1 '''' = fp1 + fp3-fs = fs + D + H,
f1 ′ ″ ″ = fp2 + fp3-fs = fs + D + H + df,
f1 '''''' = 2fp3-fs = 2fs + 2H-fs = fs + 2H
It can be expressed. Here, the optical frequency interval between the signal light Ls and the pump light Lp1 is D, the optical frequency interval between the signal light Ls and the pump light Lp3 is H, and the optical frequency interval df between the pump light Lp1 and the pump light Lp2 is synchronized. The intermediate frequency to be detected and the transmission band B of the signal light are set to half of the intermediate frequency.

At this time, in consideration of modulation, the signal light and the pump light do not overlap. If the modulation sideband is the transmission band B, {fp1} − {fs + B} = {fs + D} − {fs + B}
= D−B ≧ 0, that is, D ≧ B may be satisfied.
In order to prevent the idler light and the pump light from overlapping,
{2fp1-fs-B}-{fp3}
= {Fs + 2D−B} − {fs + H} = 2D−H−B ≧ 0,
That is, H ≦ 2D-B may be satisfied.
In order not to overlap the idler light La4 that is subject to synchronous detection and the idler light La2 that is not subject to synchronous detection in consideration of each modulation,
{Fp1 + fp3-fs-B}-{2fp2-fs + B}
= {Fs + D + H−B} − {fs + 2D + 2df + B} = HD−2df−2B ≧ 0,
That is, H ≧ D + 2df + 2B may be satisfied.
In order not to overlap the idler light La5 that is subject to synchronous detection and the idler light La6 that is not subject to synchronous detection in consideration of the respective modulations,
{2fp3-fs-B}-{fp2 + fp3-fs + B}
= {Fs + 2H-B}-{fs + D + H + df + B}
= HD-df-2B ≧ 0,
That is, H ≧ D + df + 2B may be satisfied.

  Therefore, if the optical frequency interval D between the signal light Ls and the pump light Lp1 and the optical frequency interval H between the signal light Ls and the pump light Lp3 are in the relationship shown by the hatched area in FIG. Fulfill. The same applies when the pump light Lp3 has a lower frequency than the pump light Lp1.

Further, for example, when four-pump light having an initial phase and phase noise term equal to each other with a frequency interval of df is used, the optical frequencies of six idler lights other than degenerate four-wave mixing are set to fa, fb, fc, fd, fe, ff, respectively. Then the following equation holds.
fa = fp1 + fp2-f1 = 2fp1 + df-f1
fb = fp1 + fp3-f1 = 2fp1 + 2df-f1
fc = fp1 + fp4-f1 = 2fp1 + 3df-f1
fd = fp2 + fp3-f1 = 2fp1 + 3df-f1
fe = fp2 + fp4-f1 = 2fp1 + 4df-f1
ff = fp3 + fp4-f1 = 2fp1 + 5df-f1
The optical frequencies of the four idler lights in the degenerate four-wave mixing are f1 ′ = 2fp1-f1, f1 ″ = fb, fe, 2fp1 + 6df−f1. In this case, since intermediate frequency signals of df, 2df, 3df, 4df, 5df, and 6df are generated, any intermediate frequency signal may be selected and synchronously detected.

  Next, in this embodiment, since the phase fluctuation and frequency fluctuation of the signal light can be canceled by the intermediate frequency signal between idler lights, dynamic optical phase synchronization by an optical phase locked loop is unnecessary, and static phase synchronization is achieved. Explain that is possible.

  First, the present application uses four-wave mixing by pump light having the same initial phase and phase noise term and a predetermined frequency difference. The received signal light is input to the nonlinear medium together with pump light having the same initial phase and phase noise term and a predetermined frequency difference. Idler light is generated by four-wave mixing of signal light and pump light in a nonlinear medium. The idler light is input to the optical detector, and one or more sets of idler light act as coherent detection signal light and local light. The output signal of the optical detector is multiplied with an electric sine wave signal by a mixer. Here, the frequency of the electric sine wave signal is the same frequency as the signal of the intermediate frequency to be synchronously detected among the beat signals (intermediate frequency signals) between the idler lights. The phase is in phase with the intermediate frequency signal. Such a sine wave signal can be obtained, for example, by converting an electric sine wave signal supplied to a light source that generates pump light into a signal obtained by multiplying the frequency. Assume that a set of initial phases and phase noise terms that are separated by an intermediate frequency are used for one signal light and pump light having a predetermined frequency difference is used. Furthermore, in the case of using an intermediate frequency signal generated from idler lights generated by degenerate four-wave mixing of the signal light and the respective pump lights, that is, two of the three idler lights, an electric sine wave signal for driving the light source is used. What is necessary is just to perform synchronous detection with a sine wave signal of four times the frequency. Here, the line length is adjusted so that the phases of the intermediate frequency signal and the electric sine wave coincide.

At this time, the intermediate frequency and the initial phase between the two outer idler lights can be expressed as follows.
fIF = {2fp2-fa}-{2fp1-fa}
= {2 (fp1 + fD) -fa}-{2fp1-fa} = 2fD
OIF = {2Op2-Oa}-{2Op1-Oa} = 2 {Op2-Op1}
Here, fIF and OIF are the intermediate frequency and the initial phase of the intermediate frequency signal, fa and fp1 and fp2 are the frequencies of the signal light and the pump light, fD is the frequency difference between the two pump lights, and Oa and Opj (j = 1, 2) means the initial phase of signal light and pump light. As shown in the above equation, the frequency fluctuation and phase fluctuation components of the signal light are canceled by the intermediate frequency signal. If the pump light is generated by modulating a single light, Op1-Op2 is constant, and it can be seen that only the frequency difference between the pump lights is affected. Thus, it can be seen that the phase of the intermediate frequency signal is always constant regardless of the frequency fluctuation and phase fluctuation of the signal light.

  FIG. 17 shows an experimental system for verifying the effect of this example. As the nonlinear medium, HNLF having a small connection loss with a single mode fiber and a response time in the femtosecond order was used. Since the response time is fast, the waveform distortion of the idler light due to the response time can be ignored, and the delay until phase synchronization due to the response time can also be ignored. A pump light source having the same initial phase and phase noise term and a predetermined frequency difference includes a DFB LD (Distributed Feedback Laser Diode), an intensity modulator (IM), and a 625 MHz sine wave signal that drives the intensity modulator. Consists of an output oscillator. The output of the oscillator and the intensity modulator were adjusted to suppress the carrier and generate a dual sideband with 1.25 GHz spacing. The signal light was modulated with a 100 Mbit / s “10” alternating pattern from a PPG (Pulse Pattern Generator) by an intensity modulator. The phase of the signal light was shifted by 2π or more with a phase modulator (PM) in order to verify the effect of phase fluctuation. The signal light and the pump light were combined by an optical coupler (OC), amplified by an optical amplifier, and then input to the HNLF. The output of HNLF was filtered with an optical filter having a transmission width of 1 nm in order to suppress the intensity other than idler light. The spectrum of light after filtering and the spectrum of electricity are shown in FIG. As shown in FIG. 18A, the wavelength interval between the signal light and the pump light, and between the pump light and the idler light is about 1.2 nm. As shown in FIG. 18B, the frequency difference between adjacent idler lights is 1.25 GHz. The filtered light was filtered with an electric filter (Bandpass Filter; BPF) having a transmission frequency of 1.5 GHz to 3.5 GHz in order to extract an intermediate frequency signal of the two outer idler lights from the signal detected by the optical detector. The filtered electrical signal was synchronously detected by a mixer with a 2.5 GHz sine wave signal from another transmitter synchronized with the same 10 MHz external reference clock as the transmitter used in the light source. FIG. 19 shows a change in signal amplitude with respect to a phase shift in a phase modulator simulating the phase fluctuation of signal light. The black circle is the actual measurement value, and the calculation value of the example of the amplitude change when the solid line is not phase-synchronized is shown. The calculation is an example when the phase is matched with the phase shift amount being zero. As shown in FIG. 19, it can be seen that the influence of the phase shift is negligible.

  As described above, the optical receiver 301 and the optical communication system 501 of the present embodiment do not transmit the local light having the same frequency as the signal light, the initial phase, and the phase noise term through the same transmission path as the signal light. In addition, coherent synchronous detection can be realized without using an optical phase locked loop that performs dynamic phase synchronization of local light.

(Second Embodiment)
FIG. 2 is a schematic diagram illustrating an optical communication system 502 in the case where signal light includes a plurality of optical frequencies. The difference between the optical communication system 501 in FIG. 1 and the optical communication system 502 in FIG. 2 is only the signal light. The signal light transmitted by the optical communication system 502 includes a plurality of lights having different optical frequencies.

  FIG. 6 is a schematic diagram of an optical spectrum when the number of lights of different optical frequencies constituting the signal light is 2, the frequency interval is 4 times the intermediate frequency, and 8 times the transmission band. FIG. 7 is a schematic diagram of an electric spectrum output from the optical detector 13 in this case. The horizontal axis in FIG. 6 is the optical frequency, the vertical arrow is the center frequency of the optical spectrum, and the solid line trapezoid means the modulation component. The horizontal axis in FIG. 7 is frequency, the solid line trapezoid means the modulation component, the vertical line in the trapezoid means the center frequency of the electric spectrum, and the broken line trapezoid means the transmission band of the electric filter 14. As shown in FIG. 6, if the interval between two lights having different optical frequencies constituting the signal light is 4 df or more, idler light corresponding to the two lights having different optical frequencies constituting the signal light is pumped at the optical frequency. The distance is 2 df, which is twice the light interval. Therefore, the electrofilter 14 that extracts the intermediate frequency signal of the intermediate frequency df can sufficiently suppress the 2 df intermediate frequency signal that is twice the intermediate frequency as shown in FIG. It is possible to extract only the intermediate frequency component of df. Therefore, the optical receiver 301 in FIG. 2 can suppress noise due to beats between a plurality of lights having different optical frequencies constituting the signal light, and optical filtering among a plurality of lights having different optical frequencies constituting the signal light. Coherent synchronous detection can be performed on idler light corresponding to any light selected by the unit 12. 2, 6, and 7, it is assumed that idler light corresponding to a plurality of lights having different optical frequencies is simultaneously coherently detected by the optical filter 12, but a plurality of lights having different optical frequencies constituting the signal light are simultaneously detected. The idler light corresponding to either one may be selected by the optical filter 13. In this case, since idler light corresponding to light of different optical frequencies constituting the signal light does not have to overlap with each other, the interval between the two lights having different optical frequencies constituting the signal light is 3 df as shown in FIG. That is all you need. Furthermore, when only idler light for coherent synchronous detection is selected, the interval between two lights having different optical frequencies constituting the signal light may be 2 df or more as shown in FIG.

  Here, the modulation by the transmission signal of the optical transmitter 401 is limited to the transmission band, and the intermediate frequency is set to twice the transmission band on the premise that the modulation components other than the modulation main lobe can be ignored. In such modulation, the signal itself that modulates the optical modulator or the direct modulation laser in the optical transmitter 401 is passed through an electrical low-pass filter or the like whose 3 dB band is 0.5 to 0.8 times the modulation band. Can be realized by band-limited modulation. When a modulation component having a frequency higher than that of the main lobe cannot be ignored, the intermediate frequency may be set to at least twice the frequency obtained by adding the frequency width of the modulation component that cannot be ignored with the higher frequency component than the main lobe to the transmission band. Conversely, in the case of SSB modulation, the intermediate frequency may be a transmission band. An example in the case of SSB modulation corresponding to FIG. 6 is shown in FIG.

  In order to extract an intermediate frequency signal of 2 df, which is twice the transmission band and the pump light interval is twice that of the pump light interval, and to detect synchronously by the electric filter 14, the light of different optical frequencies constituting the signal light is used. The frequency interval may be separated by 5 times or more of the pump light interval. In this case, idler light corresponding to light of different optical frequencies constituting the signal light is separated by 3 df or more, which is three times the pump light interval.

  Although the optical frequency relationship between the pump light and the signal light is not shown in the above example, the pump light, the signal light, and the idler light are modulated in addition to the optical frequency interval between the idler lights to be subjected to the synchronous detection. It is desirable that they do not overlap each other in consideration of the optical frequency spread due to. For example, it is desirable that a pair of idler lights to be subjected to coherent synchronous detection can be separated by 2 df or more. Also, assuming that idler light that is not subject to coherent synchronous detection is blocked by the optical filter 12, it is desirable that idler light that is subject to coherent synchronous detection and idler light that is not subject to overlap do not overlap.

FIG. 14 is an example in which there are two signal lights and two pump lights, and the pump light, signal light, idler light, and idler light of different signal lights do not overlap in optical frequency. In FIG. 14, the signal lights Ls1 and Ls2, the pump lights Lp1 and Lp2, the idler light of the signal light Ls1, and the idler light of the signal light Ls2 are arranged in order from the lowest optical frequency. Here, the arrangement of the optical frequency may be reversed.
Optical frequency f1 = fs of the signal light Ls1;
Optical frequency f2 = fs + S of the signal light Ls2;
Optical frequency fp1 = ps + D of pump light Lp1
Optical frequency fp2 = fp1 + df = fs + D + df of the pump light Lp2
The optical frequencies of the idler light of the signal light Ls1 and the pump light Lp1 and Lp2 are respectively f1 ′ = 2fp1-fs = 2fs + 2D−fs = fs + 2D,
f1 ′ ″ = fp1 + fp2−fs = 2fp1 + df−fs = fs + 2D + df,
f1 ″ = 2fp2-fs = 2fp1 + 2df−fs = fs + 2D + 2df,
The optical frequencies of the signal light Ls2 and the idler light of the pump light Lp1 and Lp2 are f2 ′ = 2fp1-f2 = fs + 2D−S, respectively.
f2 '''= fp1 + fp2-f2 = fs + 2D + df-S,
f2 '' = 2fp2-f2 = fs + 2D + 2df-S
It can be expressed. Here, the optical frequency interval between the signal lights is S, the optical frequency interval between the signal light Ls and the pump light Lp1, the intermediate frequency for synchronously detecting the optical frequency interval df between the pump lights, and the transmission band B of the signal light. The half of the intermediate frequency.

  Here, the modulation by the transmission signal of the optical transmitter 401 is limited to the transmission band, and the intermediate frequency is set to twice the transmission band on the premise that the modulation components other than the modulation main lobe can be ignored. In such modulation, the signal itself that modulates the optical modulator or the direct modulation laser in the optical transmitter 401 is passed through an electrical low-pass filter or the like whose 3 dB band is 0.5 to 0.8 times the modulation band. Can be realized by band-limited modulation. When the modulation component having a frequency higher than that of the main lobe cannot be ignored, the intermediate frequency may be set to a frequency obtained by adding the frequency width of the modulation component that cannot be ignored with the higher frequency component than the main lobe to the transmission band.

At this time, the idler light corresponding to the light of the different optical frequencies constituting the signal light is not overlapped with the idler light corresponding to the light of the different optical frequencies constituting the signal light. It suffices if they are separated by 2 df or more. Therefore,
f1'-f2 ''-2df = {2fp1-f1}-{2fp2-f2} -2df
= {Fs + 2D} − {fs + 2D + 2df−S} −2df = S−4df ≧ 0,
That is, it is only necessary to satisfy S ≧ 4df.
In order not to overlap the signal light and the pump light in consideration of the modulation,
{Fp1}-{f2 + B} = {fs + D}-{fs + S + B}
= DSB ≧ 0,
That is, D ≧ S + B may be satisfied.
In order to prevent the idler light and the pump light from overlapping,
{2fp1-f2-B}-{fp2}
= {Fs + 2D-SB}-{fs + D + df}
= DS-df-B ≧ 0,
That is, D ≧ S + df + B may be satisfied.
Accordingly, when the transmission band B is half of df, the optical frequency interval S between the signal lights is 4 df, and the optical frequency interval D between the signal light Ls and the pump light Lp1 is not less than 5.5 times the pump light interval df. This condition is met.

  Here, it is assumed that the transmitted light frequency and the cut-off light frequency of the optical filter 12, which will be described later, are switched on the step on the optical frequency axis. When the switching between the transmitted light frequency and the cut-off light frequency is gentle, the interval between the idler light and the bump light can be sufficiently blocked in place of 0.5 df, which is half the pump light interval corresponding to the transmission band B. An interval may be used. Also. Although the relationship between the transmission band B and the intermediate frequency is doubled, it may be a relationship necessary for extracting an intermediate frequency component by an electric filter and synchronous detection described later, and may be twice or more.

In addition, when the intermediate frequency is smaller than the transmission band, the idle light corresponding to the coherent synchronous detection target and the idler light corresponding to light of different optical frequencies constituting the signal light do not overlap with each other. It suffices that idler lights corresponding to lights of different optical frequencies are separated by 2B or more. Therefore,
f1'-f2 "-2B = {2fp1-f1}-{2fp2-f2} -2B
= {Fs + 2D}-{fs + 2D + 2df-S} -2B = S-2B-2df ≧ 0,
That is, as shown in FIG. 22, it is sufficient that S ≧ 2B + 2df. In the case of SSB modulation, S ≧ B + 2df may be satisfied as shown in FIG.
In order not to overlap the signal light and the pump light in consideration of the modulation,
{Fp1}-{f2 + B} = {fs + D}-{fs + S + B}
= DSB ≧ 0,
That is, D ≧ S + B may be satisfied.
In order to prevent the idler light and the pump light from overlapping,
{2fp1-f2-B}-{fp2}
= {Fs + 2D-SB}-{fs + D + df}
= DS-df-B ≧ 0,
That is, D ≧ S + df + B may be satisfied.
Therefore, this condition is satisfied if the optical frequency interval S between the signal lights is 2B + 2df or more and the optical frequency interval D between the signal light Ls and the pump light Lp1 is 3B + 2df or more.

  Also, the idler light that is the intermediate frequency is larger than the transmission band and is subject to coherent synchronous detection, for each signal, is branched by an optical filter as in the third embodiment to be described later, and coherent synchronous detection is performed by different optical detectors. In this case, it is sufficient that the modulation components of idler light corresponding to different signal lights do not overlap each other. That is, since it is only necessary to separate df that is twice the transmission band, when coherent synchronous detection is performed between idler lights of degenerate four-wave mixing, the optical frequency interval S between the signal lights is S ≧ 3 df, degenerate four-wave mixing and degenerate four-wave In the case of coherent synchronous detection between idler lights other than those mixed, it is clear that the optical frequency interval S between the signal lights only needs to satisfy S ≧ 2df. This relationship is the same in the following description.

  In addition, the idler light to be subject to coherent synchronous detection is branched for each signal by an optical filter, as in the third embodiment to be described later, and coherent synchronous detection is performed by different optical detectors. In this case, it is sufficient that the modulation components of the idler light that is the target of coherent synchronous detection do not overlap with the idler light that corresponds to the light having different optical frequencies constituting the signal light. That is, in the case of the dual sideband, as shown in FIG. 24, it is sufficient to be 2B + 2df or more obtained by adding the double of the pump light interval to the double of the transmission band, so if the pump light interval is ignored, the optical frequency interval S between the signal lights Obviously, it is sufficient to satisfy S ≧ 2B + 2df. In the case of SSB, in order to prevent the modulation components of the idler light to be coherent synchronously detected from overlapping with the idler light corresponding to the light of different optical frequencies constituting the signal light, B + df obtained by adding the pump light interval to the transmission band. It suffices if it is far away. These relationships are the same in the following description.

  When the signal light is 3 or more, the relational expression of the optical frequency interval between the signal light and the pump light is obtained by multiplying the optical frequency interval S between the signal lights by the number obtained by subtracting 1 from the number of the signal lights. If it calculates, it will be found similarly. This relationship is the same in the following description.

  Even when the pump light is 3 or more, idler lights corresponding to light of different optical frequencies constituting the signal light do not overlap with each other, and the same idler light corresponding to light of different optical frequencies constituting the signal light is the same. It is the same that the interval of idler light detected by the optical detector is set to the frequency obtained by subtracting the transmission band from the frequency obtained by subtracting the transmission band from the intermediate frequency used for coherent synchronous detection and the frequency obtained by adding the transmission band from the intermediate frequency used for coherent synchronous detection. For example, it is desirable that light having different optical frequencies constituting the signal light is more than twice the transmission band and separated by an intermediate frequency to be extracted by the electric filter 14 and subjected to synchronous detection.

For example, it is assumed that three pump lights in which one pump light is arranged on the low frequency side of the signal light on the optical frequency axis and two pump lights are arranged on the high frequency side are used. Here, the optical frequency of the pump light on the low frequency side is fp1, and the optical frequencies of the pump light on the high frequency side are fp2 and fp2 + df. At this time, the optical frequencies of the idler light with respect to the pump light with the optical frequencies fp1 and fp2 constituting the signal light of the optical frequencies f1 and f2 positioned on the higher frequency side than fp2 are f1 &, f1 &&, and the optical frequency. Let f2 & and f2 && be the optical frequencies of idler light for the pump light whose optical frequencies for the signal light of f2 are fp1 and fp2 + df. At this time, the following equations hold for the optical frequencies f1 &, f1 &&, f2 &, f2 && of idler light.
f1 & = fp2 + f1-fp1 = (fp2-fp1) + f1
f1 && = fp2 + df + f1-fp1 = (fp2-fp1) + f1 + df
f2 & = fp2 + f2-fp1 = (fp2-fp1) + f2
f2 && = fp2 + df + f2-fp1 = (fp2-fp1) + f2 + df

  In this case, if the interval between the idler light and the adjacent signal light is not less than twice the transmission band and not less than 3 df, the respective signal lights can be independently coherently detected. For this reason, the optical frequency interval of the plurality of lights having different optical frequencies constituting the signal light is set to | f2−f1 | ≧ 3df or more, that is, three times or more of the pump light interval. Here, the two pump lights are arranged on the high frequency side, but the same is true if the two pump lights are arranged on the low frequency side. It should be noted that the two pump lights only need to have an interval in which the signal light and the idler light fall between the two pump lights.

  The optical frequency of the idler light of the optical frequencies f1 & and f1 && is an optical frequency that can be sufficiently blocked by the optical filter 12 from signal light, pump light, idler light other than the optical frequencies f1 & and f1 &&, or can be blocked by the electric filter 14 It is assumed that the optical frequency is different or the optical frequency that can be cut off by synchronous detection, for example, df or more.

  Here, although an example in which signal light enters between pump light is shown, the pump light may be located on the high frequency side or low frequency side of the signal light. FIG. 15 is an example in which there are two signal lights and three pump lights and the optical frequencies do not overlap each other. In FIG. 15, in order from the lowest optical frequency, light Ls1 and Ls2 having different optical frequencies constituting the signal light, pump lights Lp1, Lp2, and Lp3, light Ls2 constituting the signal light, and pump lights Lp1 and Lp2 are corresponded. Idler light, light Ls1 constituting signal light and idler light corresponding to pump light Lp1 and Lp2, light Ls2 constituting signal light, idler light corresponding to pump light Lp3 and Lp1 or 2, and light Ls1 constituting signal light And idler light corresponding to the pump light Lp3 and Lp1 or Lp2, light Ls2 or Ls1 constituting the signal light, and idler light corresponding to the pump light Lp3 are arranged in this order. The optical frequency may be reversed. In FIG. 15, the idler light corresponding to the pump light Lp3 and Lp1 or Lp2 is the target of coherent synchronous detection.

The optical frequencies of the light constituting the signal light are respectively f1 = fs,
f2 = fs + S,
The optical frequencies of the pump lights Lp1, Lp2, and Lp3 are fp1 = fs + D,
fp2 = fp1 + df = fs + D + df,
fp3 = fs + H,
The optical frequencies of idler light are respectively f2 ′ = 2fp1-f2 = 2fs + 2D−fs−S = fs + 2D−S,
f2 ′ ″ = fp1 + fp2−f2 = 2fp1 + df−fs−S = fs + 2D + df−S,
f2 '' = 2fp2-f2 = 2fp1 + 2df-fs-S = fs + 2D + 2df-S, f2 '''' = fp1 + fp3-f2 = fs + D + HS
f2 '''''= fp2 + fp3-f2 = fs + D + H + df-S,
f2 '''''' = 2fp3-f2 = 2fs + 2H-fs-S = fs + 2H-S,
f1 ′ = 2fp1-fs = 2fs + 2D−fs = fs + 2D,
fl ′ ″ = fp1 + fp2−fs = 2fp1 + df−fs = fs + 2D + df,
f1 ″ = 2fp2-fs = 2fp1 + 2df−fs = fs + 2D + 2df,
f1 ″ ″ = fp1 + fp3-fs = fs + 1D + H,
f1 ′ ″ ″ = fp2 + fp3-fs = fs + D + H + df,
f1 '''''''= 2fp3-fs = 2fs + 2H-fs = fs + 2H
It can be expressed. Here, the optical frequency interval between the light constituting the signal light is S, the optical frequency interval between the light Ls1 constituting the signal light and the pump light Lp1, and the light Ls1 constituting the signal light and the pump light Lp3. The optical frequency interval is H, the intermediate frequency for synchronously detecting the optical frequency interval df between the pump light Lp1 and the pump light Lp2, and the transmission band B of the signal light is half of the intermediate frequency.

At this time, in the idler light to be synchronously detected, the intermediate frequency signal of the idler light corresponding to the light of different optical frequencies constituting the signal light does not overlap with the intermediate frequency signal to be coherently detected. The idler light corresponding to the light of different optical frequencies constituting the light source should be 2 df or more away,
{Fp1 + fp3-fs}-{fp2 + fp3-f2} -2df
= {Fs + D + H} − {fs + D + H + df−S} −2df
= S-3df ≧ 0,
That is, it is only necessary to satisfy S ≧ 3df.
Considering modulation, signal light and pump light do not overlap so that the modulation sideband is transmission band B.
{Fp1}-{f2 + B} = {fs + D}-{fs + S + B}
= DSB ≧ 0,
That is, when D ≧ S + B and B = 0.5 df, D ≧ 3.5 df may be satisfied.
In order to prevent the idler light and the pump light from overlapping,
{2fp1-f2-B}-{fp3}
= {Fs + 2D−S−B} − {fs + H} = 2D−H−S−B ≧ 0,
That is, it is sufficient that H ≦ 2D−S−B is satisfied.
In order not to overlap idler light that is subject to synchronous detection and idler light that is not subject to synchronous detection in consideration of each modulation,
{Fp1 + fp3-fs2-B}-{2fp2-fs + B}
= {Fs + D + HSB}-{fs + 2D + 2df + B} = HD-S-2df = 2B ≧ 0,
That is, when H ≧ D + S + 2df + 2B and B = 0.5 df, H ≧ D + 6df
And {2fp3-fs2-B}-{fp2 + fp3-fs + B}
= {Fs + 2H−SB} − {fs + D + H + df + B} = HD−S−df−2B ≧ 0,
That is, when H ≧ D + S + df + 2B and B = 0.5 df, H ≧ D + 5df may be satisfied. The same applies when the pump light Lp3 has a lower frequency than the pump light Lp1.
Note that the signal light may be selected before the idler light corresponding to any one of the plurality of signal lights is input to the four-wave mixing unit by the optical filter 12. In the present application, even if the signal light to be selected is changed, it is not necessary to readjust the synchronous detection unit accordingly. Further, idler light corresponding to each signal light from the four-wave mixing unit may be branched as in a third embodiment to be described later, and the number of synchronized detection units may be provided.

  As described above, in the present optical communication system 502, sets of idler lights synchronized with each other corresponding to each signal light are generated independently, so that coherent synchronous detection can be performed without any interference. Furthermore, in order to receive a plurality of signal lights in the conventional system, it is necessary to adjust the PLL correspondingly. However, in the present optical communication system 502, it is not necessary, and simultaneous reception is also possible.

(Third embodiment)
FIG. 3 is a schematic diagram illustrating an optical communication system 503 according to the third embodiment. A difference between the optical communication system 502 and the optical communication system 503 in FIG. 2 is that a receiver 303 is provided as an alternative to the receiver 302. The optical receiver 303 generates a four-wave mixing unit 33 that outputs idler light for each light having different optical frequencies constituting a signal, and generates idler light and pump light for each light having different optical frequencies constituting the signal. A synchronous detection unit 23 that performs synchronous detection with an electrical signal that is synchronized in phase with the electrical signal.

  The four-wave mixing unit 33 includes a decoder 15 and outputs idler light for each idler light having different optical frequencies. The synchronous detection unit 23 includes optical detectors (13a to 13d) that perform optical detection for each idler light of light having different optical frequencies. In the figure, it is assumed that the optical detector is provided in a one-to-one correspondence with the idler light having different optical frequencies. However, the idler light having different optical frequencies that are optically detected by the same optical detector as described later. On the premise that the beat does not affect, idler light of light having a plurality of optical frequencies may be optically detected by the same optical detector. The other points are the same as in the first embodiment. The case where the signal light has an optical frequency interval of 6 times or more of the band and an optical frequency of 3 times or more of the intermediate frequency, and the pump light has two wavelengths will be described.

  FIG. 8 is a schematic diagram of an optical spectrum when the number of lights of different optical frequencies constituting the signal light is 2, the frequency interval is 3 times the intermediate frequency, and 6 times the transmission band. FIG. 9 is a schematic diagram of the spectrum of the electrical signal output from the optical detectors (13a to 13d) in this case. In FIG. 8, the horizontal axis indicates frequency, the vertical arrow indicates the center frequency in the case of the optical spectrum, the solid trapezoid indicates the modulation component, and the broken trapezoid indicates the transmission band of the filter. As shown in FIG. 8, idler light corresponding to two lights of different optical frequencies constituting the signal light is separated by df or more. For this reason, unlike the second embodiment, the frequency interval between the idler lights of the two lights having different optical frequencies constituting the signal light is df. In the configuration of FIG. 2, when the pump light has two wavelengths that differ by the intermediate frequency df, after optical detection, beats between a plurality of lights having different optical frequencies constituting the signal light cannot be removed by the electric filter 14. Therefore, the noise caused by the beat cannot be ignored. However, the optical receiver 303 of the present embodiment is different because the decoder 15 separates each of a plurality of lights having different optical frequencies constituting the signal light and performs optical detection on each of the optical detectors (13a to 13d). No beat is generated between multiple lights of optical frequency. For example, the decoder 15 is an optical demultiplexer that branches idler light respectively corresponding to a plurality of lights having different optical frequencies. The electrical signals optically detected by the optical detectors (13a to 13d) are extracted only by the intermediate frequency signals required by the electrical filters 14a to 14d, and the intermediate frequency signals extracted by the electrical filters 14a to 14d are respectively pumped. The mixer 18 multiplies the electrical signal whose phase is synchronized with the electrical signal when generating the signal, and performs synchronous detection. Therefore, the present optical communication system can perform coherent synchronous detection for each of a plurality of lights having different optical frequencies constituting the signal light.

  The modulation by the transmission signal of the optical transmitter 401 is limited to the transmission band as in the second embodiment, and assuming that the modulation components other than the modulation main lobe can be ignored, the intermediate frequency is set to twice the transmission band. . Similarly, when the high-frequency modulation component cannot be ignored from the main lobe, the intermediate frequency may be a frequency obtained by adding the frequency width of the modulation component that cannot be ignored by the high-frequency component from the main lobe to the transmission band. Furthermore, the plurality of optical frequency intervals constituting the signal light may be four times or more of the intermediate frequency.

  In this way, the optical communication system 503 receives 2 signal lights that are the minimum value of 3 df of the optical frequency interval of light of different optical frequencies (spectrum chips) constituting the signal light in the configuration shown in Non-Patent Document 2. In the case of pump light, coherent synchronous detection can be performed. When optical code division multiplexing signal light described in Non-Patent Document 2 is received, the electrical signal of each spectrum chip is added and subtracted by an adder / subtracter (not shown) in accordance with the code to be received. Furthermore, as shown in the second embodiment, in the case of three pump lights, it is possible to perform coherent synchronous detection of signal light having an optical frequency interval of 2 df. Further, it is possible to reduce beat components between idlers corresponding to light of different optical frequencies constituting the signal light by thinning out idler light by the optical filter 12.

  As described above, in the optical communication system 503, idler light sets that are synchronized with each other corresponding to each signal light are generated independently, so that coherent synchronous detection can be performed without interference.

(Fourth embodiment)
4 and 5 are schematic diagrams illustrating an optical communication system 504 and an optical communication system 505 according to the fourth embodiment. The optical communication systems (504, 505) receive optical code-multiplexed signal light using spectrum chips constituting signal light. Such optical code division multiplexing signal light uses light of the same optical frequency as a plurality of signal lights.

  The difference between the optical communication system 502 and the optical communication system (504, 505) in FIG. 2 is that the signal light transmitted and received by the transceiver is optical code multiplexed signal light, and the receiver 301 is an alternative to the receiver 301. 304 or a receiver 305. The optical receiver 304 or the receiver 305 includes a four-wave mixing unit (33, 35) that branches and outputs according to a code for decoding idler light, and a branched idler from the four-wave mixing unit (33, 35). It has a synchronous detector 24 or a synchronous detector 25 for adding and subtracting light according to codes decoded after coherent synchronous detection.

  The optical receiver 304 in FIG. 4 includes, after the optical filter 12, for example, a four-wave mixing unit that includes a decoder 15 that branches according to a code for decoding a spectrum chip constituting signal light described in Non-Patent Document 2. The synchronous detector 25 has a differential optical detector 16 that differentially detects the output branched by the decoder 15. The output of the differential optical detector 16 is input to the intermediate frequency electric filter 14. Although not shown in FIG. 4, the synchronous detector 25 includes a decoder 15 and optical detectors (13 a to 13 d) that detect each output as in the synchronous detector 23 of FIG. 3, and outputs the outputs of the respective optical detectors. You may have an adder / subtractor which adds / subtracts according to a code | symbol through the electric filter (14a-14d) of intermediate frequency.

  In view of the fact that the arrangement of the optical frequencies of the idler light and the signal light is inverted, the codes of the decoder 15 are arranged in an inverted manner on the optical frequency axis and the code when encoded by the encoder. In FIG. 4, the decoder 15 corresponds to the code (1100) and the inverted (0011) of the arrangement. Here, the code is an example, and may be another code or a code length other than 4. The frequency interval of each spectrum chip is the interval of idler light to be decoded by each spectrum chip.

  In addition, it is possible to reduce beat components between idlers corresponding to different spectrum chips by thinning out idler light by the decoder 15 or the optical filter 12.

  The optical filter 12 may be disposed after the decoder 15 or may be integrated with the decoder.

  The difference between the optical receiver 304 in FIG. 4 and the optical receiver 305 in FIG. In the case of the optical receiver 305 in FIG. 5, the decoder 15 is positioned in front of the nonlinear medium unit 11 and receives only signal light. The optical receiver 305 includes a plurality of nonlinear medium units 11 to which at least a spectrum chip to be added and a spectrum chip to be subtracted are input. Each nonlinear medium unit 11 four-wave mixes each output of the decoder 15. Note that the nonlinear medium unit 11 may be provided for each spectrum chip. In this configuration, since the signal light is input to the decoder 15 as it is, not the idler light in which the arrangement of the optical frequencies is inverted, the code of the decoder 15 is encoded by the encoder, not the arrangement inverted by the optical frequency axis. It will be the same array of spectrum chips. Further, the frequency interval of the code of the decoder 15 is also equal to the frequency interval of the spectrum chip of the signal light, and a decoder having the same configuration as the encoder mounted on the optical transmitter 401 can be used as it is.

  As described above, in the optical communication system (504, 505), the signal light and the local light having the same initial phase and phase noise term corresponding to the plurality of spectrum chips constituting all the plurality of optical code multiplexed signal lights. Are generated to perform coherent synchronous detection. For this reason, the optical communication system (504, 505) can solve the problem of MAI due to variation in phase difference between signal light and local light.

(Fifth embodiment)
The difference between the present optical communication system and the optical communication system (504, 505) of the fourth embodiment is in the codes used by the signal light, the pump light, and the decoder. Others are the same as in the fourth embodiment. In the signal light of this optical communication system, the frequency of each of the spectrum chips constituting the signal light similarly changes with time. Specifically, each of the plurality of spectrum chips changes as shown in Non-Patent Document 8.

  The pump light has the same frequency in synchronism with the frequency change with respect to the time of the signal light so that the idler light is conducted through the optical filter 12 when the optical filter 12 is not conducted due to the fluctuation of the optical frequency of the desired idler light. The light changes to. Further, the code may be a code in which a spectrum chip that is insufficient for orthogonalization due to leakage in an adjacent spectrum chip proposed in Non-Patent Document 9 may be added. In that case, specifically, a decoder 15 that decodes a code in which a chip corresponding to a leaky spectrum chip is arranged on the opposite side of the code by using the cyclicity of the code is used. For example, when the original code is 0010111 and ± 1 spectrum chip leaks, it is set to 100101110. Interference due to the code that shifts the spectrum chip within the range of the number of leaky spectrum chips to the same code is unavoidable, so these codes are not used at the same time. When the number of codes not used is negligible except for leakage of adjacent spectrum chips, the number is half for spectrum chip-shifted M-sequence codes and one at most for Walsh Hadamard codes.

  The present optical communication system uses this code and adopts the configuration of FIG. 5, so that even the decoder 15 corresponding to the frequency interval of each spectrum chip can perform coherent synchronous detection of the optical code division multiplexed signal. Is possible. This is because, in an optical filter usually used in a decoder, the phase of light rotates when the transmission intensity decreases, so the phase difference between signal light and local light changes. For this reason, in the case of the signal light as described above, the phase relationship fluctuates even within one symbol time. For this reason, the decoder corresponding to the frequency interval of each spectrum chip generates MAI due to the phase difference variation between the signal light and the local light. However, since this optical communication system does not cause phase difference variation, MAI due to phase difference variation between signal light and local light can be eliminated.

(Sixth embodiment)
In this optical communication system, pump light in the same circular polarization state is used, and the idler light is generated by a four-wave mixing unit of a circularly polarized twisted optical fiber. The difference between the present optical communication system and the first to fifth embodiments resides in the polarization of pump light from the pump light source 402 and the nonlinear medium section 11. That is, the pump light source 402 sets the pump light to the same circular polarization state, and the four-wave mixing unit 31 is a circular polarization twisted optical fiber. For this reason, this optical communication system can eliminate the polarization dependence in the four-wave mixing, and can eliminate the polarization dependence of the coherent periodic detection.

(Seventh embodiment)
The present optical communication system includes a nonlinear medium unit 11 including a nonlinear medium 71 and a polarization beam combiner (PBC) 72. The nonlinear medium 71 is, for example, a nonlinear medium fiber. The nonlinear medium unit 11 receives signal light and pump light from both ends of the nonlinear medium fiber, and generates idler light.

  As shown in FIG. 10, the nonlinear medium unit 11 forms a loop with the nonlinear medium 71 and the PBC 72. The PBC 72 inputs pump light having the same light intensity in both orthogonal polarizations from both ends of the nonlinear medium 71. In FIG. 10, the input light is separated into two polarized waves orthogonal to each other by the PBC 72, and returns to the PBC 72 again from the one end of the PBC 72 through the nonlinear medium 71. At this time, the polarization is the same when it is output from the PBC 72 and when it returns to the PBC 72 again. Alternatively, the nonlinear medium unit 11 includes a loop of the nonlinear medium 71 and the PBC 72 and an optical circulator 73 that separates input and output to the PBC 72 as shown in FIG. In the configuration of the nonlinear medium portion 11 in FIG. 11, the polarization is rotated by 90 degrees at the midpoint of the nonlinear medium 71, but may be another part of the loop. In the configuration of FIG. 11, since the polarization is rotated 90 degrees in the middle, the polarization returns to the PBC 72 again with the polarization orthogonal to that output from the PBC 72. Further, the nonlinear medium 71 of FIGS. 10 and 11 may be a concentrated type instead of a distributed type. The horizontal arrow α and the double circle β in FIGS. 10 and 11 mean orthogonal polarizations. As shown in FIGS. 10 and 11, four-wave mixing is similarly generated for any orthogonal polarization component, so that the sum of the outputs of the respective coherent synchronous detections is constant regardless of the polarization.

  For this reason, this optical communication system can eliminate the polarization dependence in the four-wave mixing, and can eliminate the polarization dependence of the coherent synchronous detection.

(Eighth embodiment)
The difference between the present optical communication system and the first to fifth embodiments resides in pump light. The pump light of the present optical communication system is composed of two pump lights having equal intensities with different polarizations orthogonal to one pump light shown in the first to fifth embodiments. When the polarization of idler light is kept the same as that of pump light, beat components between idler lights of different polarizations are not generated.

  The optical frequency difference between the two pump lights having orthogonal polarizations corresponding to one pump light is such that the center frequency signal generated from the idler lights of the respective polarizations enters the conduction band of the electric filter 14. Preferably, the optical frequency difference is such that two pump lights with different polarizations do not become one pump light with a single polarization. There is no need to separate each polarization before optical detection and to merge electrical signals after optical detection.

  On the other hand, when splitting each polarization before optical detection and combining the electrical signals after optical detection, the light is such that two pump lights with different polarizations do not become one pump light with a single polarization. Any frequency difference may be used.

  When two pump lights with different polarizations are combined into one pump light with a single polarization, the four-wave mixing unit separates the signal light into two orthogonal polarizations, and the separated signal light and A four-wave mixing unit is provided that outputs four-wave mixing idler light with pump light of the same polarization. By providing a four-wave mixing unit corresponding to each of the orthogonal polarizations in addition to the pump light having the orthogonal polarizations, the same four-wave mixing can be generated for any orthogonal polarization component. . An example of such a four-wave mixing unit is shown in FIG. In FIG. 16, the polarization is separated into orthogonal polarizations by PBC, and each polarization is recombined by PBC via a nonlinear medium. Also good. In either case, the path length difference until the polarization-separated signal light or its synchronous detection output is merged is suppressed to a length that is sufficiently shorter than the symbol length, for example, about one tenth of the symbol length. As a result, the sum of the outputs of the respective synchronous detections is constant regardless of the polarization of the signal light. The synchronous detection unit may be provided for each polarization. However, when the polarization of the orthogonally polarized signal light and the idler light corresponding to the pump light is maintained, between the orthogonally polarized idler lights Since no beat component is generated, it may be input to a single synchronous detection unit.

  For this reason, this optical communication system can eliminate the polarization dependence in the four-wave mixing, and can eliminate the polarization dependence of the coherent synchronous detection.

11: Non-linear medium section 12: Optical filters 13, 13a, 13b, 13c, 13d: Optical detectors 14, 14a, 14b, 14c, 14d: Electric filter 15: Decoder 16: Differential optical detectors 18, 18a 18b, 18c, 18d: mixers 21, 23, 24, 25: synchronous detection units 31, 33, 35: four-wave mixing unit 71: nonlinear medium 72: polarization beam combiner (PBC)
73: Circulator 101: CW light source 102: Sine wave signal generator 103: Intensity modulator 104: Multiplier 301, 303, 304, 305: Optical receiver 401: Optical transmitter 402: Pump light sources 501 to 505: Optical communication system

Claims (10)

The signal light and a plurality of pump lights having the same initial phase and phase noise terms and having a predetermined frequency difference are input to generate four-wave mixing of the signal light and the pump light, and at least one set of optical frequency intervals is intermediate A four-wave mixing unit that outputs a plurality of idler lights as frequencies;
The idler light from the four-wave mixing unit is optically detected, and an intermediate frequency signal between the idler lights is coherently synchronized with an electric signal that is the intermediate frequency and is in phase with the electric signal when generating the pump light. A synchronous detector for detecting,
An optical receiver. The signal light is composed of a plurality of lights having different optical frequencies,
The signal light and the pump light have an optical frequency interval between the idler lights based on light of different optical frequencies constituting the signal light among the idler lights to be subjected to coherent synchronous detection in the synchronous detection unit. The optical receiver according to claim 1, wherein the optical frequency is an optical frequency that is at least twice the transmission band and at least four times the intermediate frequency. The signal light is composed of a plurality of lights having different optical frequencies,
The signal light and the pump light have an optical frequency interval between the idler lights based on light of different optical frequencies constituting the signal light among the idler lights to be subjected to coherent synchronous detection in the synchronous detection unit. It is an optical frequency that is equal to or higher than the transmission band and double the intermediate frequency,
The four-wave mixing unit outputs the idler light for each signal light,
The optical receiver according to claim 1, wherein the synchronous detection unit performs coherent synchronous detection of the idler light for each signal light. The signal light is optical code division multiplexed with light of a plurality of optical frequencies,
The four-wave mixing unit outputs or outputs the idler light of the signal light branched according to the code to be decoded or branches according to the code to decode the idler light,
The synchronous detection unit synchronously detects the idler light output according to the code from the four-wave mixing unit, and adds or subtracts a signal to be synchronously detected according to the code. The optical receiver in any one of. The signal light has an optical frequency that changes with time,
5. The light according to claim 1, wherein an optical frequency of the pump light changes in synchronization with an optical frequency change of the signal light so that the idler light is not blocked by an optical filter. Receiving machine. The pump lights are in the same circular polarization state,
The optical receiver according to claim 1, wherein the four-wave mixing unit generates the idler light by a circularly polarized twisted optical fiber.   6. The optical receiver according to claim 1, wherein the four-wave mixing unit generates the idler light in a loop including a nonlinear medium unit and a polarization beam combiner.   6. The optical receiver according to claim 1, wherein the pump light includes two lights whose polarizations are orthogonal to each other and have the same light intensity. An optical receiver according to any one of claims 1 to 8,
An optical transmitter for transmitting the signal light to the optical receiver;
An optical communication system including: Four-wave mixing of the signal light and the pump light is generated from the signal light and a plurality of pump lights having the same initial phase and phase noise terms and a predetermined frequency difference, and at least one set of optical frequency intervals is an intermediate frequency. Output multiple idler lights,
A coherent synchronous detection method in which the idler light is optically detected, and an intermediate frequency signal between the idler lights is coherently detected with an electrical signal having the intermediate frequency and having a phase synchronized with the electrical signal when generating the pump light.

Images