Transmission System, Transmission Method, and Program

Abstract

A transfer system according to an embodiment of the present invention is for transferring an optical signal and comprises: a phase-conjugated light generation unit that generates phase-conjugated light of the optical signal having a frequency within a first band, in a second band different from the first band; and an optical phase sensitive amplification unit that performs phase sensitive amplification on the optical signal and the phase-conjugated light. The first band is, among frequency bands in which a transmitter for outputting the optical signal can output the optical signal at a prescribed intensity or more, a frequency band at which a light-reception sensitivity of a receiver for receiving the optical signal is equal to or higher than a prescribed sensitivity. The second band is a band having a frequency higher or lower than the first band.

Description

Technical Field

[0001] The present invention relates to a transmission system, a transmission method, and a program.

Background Art

[0002] Due to factors such as an increase in the opportunity of working from home due to the impact of COVID-19 and the prosperity of e-Sports, the importance of optical communication technology has been increasing in recent years. Although optical communication has attracted such attention, in optical communication, wavelength division multiplexing (WDM) that uses the amplification band of an erbium-doped fiber amplifier (EDFA) as a transmission band is generally used. Specifically, the amplification band of the EDFA is a band of about 4 THz within the wavelength band of the C-band or the L-band.

[0003] An EDFA is an optical amplifier that amplifies an optical signal attenuated by transmission as light, and is used for signal relaying and improvement of reception sensitivity. An optical amplifier whose operation does not depend on the phase of a signal, such as an EDFA, is called a phase-insensitive amplifier (PIA). Since the PIA can amplify an attenuated optical signal as it is, it is very useful for optical communication. However, the PIA also has problems.

[0004] Specifically, the problem of the PIA is that the SNR (signal-to-noise ratio) of the input light in the coherent state is inevitably degraded by 3 dB or more due to the mixing of ASE (amplified spontaneous emission) noise, which is noise derived from spontaneous emission light. That is, the PIA has a problem that the signal-to-noise ratio (SNR: optical signal-to-noise ratio) is degraded by ASE noise.

[0005] This is actually one of the essential factors that limit the transmission capacity and distance in optical fiber transmission among various noise factors.

[0006] There are other essential factors that limit the transmission capacity and distance in optical fiber transmission. It is a non-linear phenomenon caused by an increase in the energy density in the optical fiber. To ensure a high SNR, it is necessary to make the transmission power of the optical signal relatively strong against noise. However, when the energy density in the optical fiber increases, waveform distortion due to non-linear optical effects becomes apparent and the characteristics deteriorate.

[0007] Under such circumstances, for the further extension of transmission distance and increase in transmission capacity of optical fiber transmission, reduction of ASE noise of optical amplifiers and compensation for non-linear distortion are important, and several technologies aiming at solving these problems have been studied so far.

[0008] As a means to break through the noise limit of conventional PIA, a phase-sensitive amplifier (PSA) using optical parametric amplification (OPA) has been studied. OPA is a non-linear optical process in which signal light is amplified by inputting signal light and high-power excitation light into a medium with high optical non-linearity.

[0009] As non-linear optical media, those using second-order non-linearity and those using third-order non-linearity exist, and lithium niobate, dispersion-shifted optical fiber, etc. are typical respectively. As a secondary effect accompanying signal amplification by OPA, there is an effect that idler light, which is the phase conjugate light of the signal light, is generated. By utilizing this idler light, OPA can cause various optical phenomena.

[0010] PSA is one of the devices that use OPA. In PSA, the generated phase-conjugated light and the original input signal light are overlapped within the same band. As a result, in PSA, the ASE of the orthogonal components is suppressed. Since the ASE of the orthogonal components is suppressed, the noise generated in PSA is below the noise limit of PIA. That is, PSA is a device for low-noise amplification. PSA also has the effect of compensating for the distortion in the phase direction due to non-linear optical effects and the like.

[0011] One type of PSA is a degenerate PSA that occurs when the channel to be amplified is arranged at the degenerate frequency, which is the center of the amplification band of PSA. In degenerate PSA, an idler light is generated within the same band as the signal light by the interaction between the signal light and the pump light in the non-linear optical medium. As a result, in degenerate PSA, the idler light and the signal light overlap to cause a phase-sensitive amplification effect. However, there are problems in the amplification by degenerate PSA, such as the need to amplify in parallel with multiple devices when amplifying a WDM signal, and the inability to amplify a signal having signal points on both the real axis and the imaginary axis in the complex plane, such as a QAM (quadrature amplitude modulation) signal.

[0012] Therefore, for the amplification of WDM signals and high-order QAM signals, research on non-degenerate PSA (ND-PSA: non-degenerate PSA) in which signals are arranged at frequencies shifted from the degenerate frequency in PSA has been conducted (see, for example, Patent Document 1). In ND-PSA, signal light and idler light whose frequencies are symmetric with respect to the degenerate frequency are used. Such signal light and idler light are generated in advance on the transmission side. In ND-PSA, the signal light and the idler light co-propagate through the transmission path. Then, a phase-sensitive amplification operation occurs due to the interaction among the signal light, the idler light, and the pump light with different frequencies in the non-linear optical medium. This is ND-PSA.

[0013] When the frequency relationship between the signal light and the idler light is symmetric about the degenerate frequency, the phase-conjugated conversion light of the idler light is generated within exactly the same band as the signal light. The same applies to the phase-conjugated conversion light of the signal light. By generating and propagating the idler light for the number of wavelength-division multiplexed signal lights, phase-sensitive amplification of the WDM signal by ND-PSA is realized.

[0014] In the band of the signal light in ND-PSA, the phase-conjugated conversion light of the input idler light (i.e., light having the same complex amplitude distribution as the original signal light) is superimposed. Therefore, in ND-PSA, the information in the phase direction is retained even after amplification. Thus, ND-PSA can amplify QAM signals.

[0015] The idler light that has been pre-generated on the transmission side is generally optically generated by an OPA that takes only the signal light as input after modulating the signal light as in normal optical transmission. Such a device that generates the idler light, which is the phase-conjugated light of the signal light, using an OPA is called an optical phase conjugator (OPC).

[0016] Note that since ND-PSA does not need to receive this idler light for data demodulation, a transceiver for the idler light is not essential.

Prior Art Documents

Patent Documents

[0017]

Patent Document 1

Non-Patent Documents

[0018]

Non-Patent Document 1

[0019] As described above, for the application of PSA to high-capacity information communication using WDM and higher-order QAM signals, a non-degenerate configuration is required. The non-degenerate configuration means using signal light and idler light whose frequencies are symmetric with respect to the degenerate frequency as the center.

[0020] Conventionally, OPA has been developed and demonstrated as an amplification method for amplifying signal light in the C-band, and many have the center of the amplification band near the center of the C-band. Therefore, similarly for PSA, the low-noise performance and the like have been demonstrated for signal light within the C-band. However, in the conventional configuration, it is necessary to divide the band of the existing C-band transmission system into two parts for signal light and idler light.

[0021] Therefore, even if the SNR of the signal is improved by PSA, the number of WDM channels is halved compared to a transmission system that uses the entire existing C-band as the signal light. As a result, even if the SNR of the signal is improved by PSA, the total transmission capacity per system in optical fiber transmission does not necessarily increase and may decrease in some cases.

[0022] In view of the above circumstances, an object of the present invention is to provide a technique for increasing the transmission capacity of a transmission system that transmits an optical signal.

Means for Solving the Problems

[0023] One aspect of the present invention is a transmission system for transmitting an optical signal, comprising: a phase conjugate light generation unit that generates phase conjugate light of the optical signal whose frequency is within a first band in a second band different from the first band; and an optical phase sensitive amplification unit that phase sensitively amplifies the optical signal and the phase conjugate light, wherein the first band is a frequency band in which the sensitivity of light reception of a receiver that receives the optical signal is equal to or higher than a predetermined level among the frequency bands in which a transmitter that outputs the optical signal can output the optical signal at a predetermined intensity or higher, and the second band is a band having a higher or lower frequency than the first band.

[0024] One aspect of the present invention is a transmission method executed by a transmission system, comprising: a phase conjugate light generation unit that generates phase conjugate light of an optical signal whose frequency is within a first band in a second band different from the first band; and an optical phase sensitive amplification unit that phase sensitively amplifies the optical signal and the phase conjugate light, wherein the first band is a frequency band in which the sensitivity of light reception of a receiver that receives the optical signal is equal to or higher than a predetermined level among the frequency bands in which a transmitter that outputs the optical signal can output the optical signal at a predetermined intensity or higher, and the second band is a band having a higher or lower frequency than the first band, the method having an optical signal generation step of generating the optical signal.

[0025] One aspect of the present invention is a program for causing a computer to function as the above-described transmission system.

Advantages of the Invention

[0026] According to the present invention, it becomes possible to increase the transmission capacity of a transmission system for transmitting optical signals.

Brief Description of the Drawings

[0027]

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Modes for Carrying Out the Invention

[0028] (Embodiment) For the sake of simplicity in the following description, a fiber optic transmission system will be described by taking as an example the case where the frequency band of an optical signal is the C-band and idler light is generated on the high-frequency side. However, the frequency band of the optical signal does not necessarily have to be the C-band. The frequency band of the optical signal may be, for example, the L-band. Also, the idler light may be generated on the low-frequency side of the optical signal.

[0029] FIG. 1 is a diagram showing an example of the configuration of a transmission system 100 according to an embodiment. The transmission system 100 is a transmission system that transmits an optical signal.

[0030] The transmission system 100 includes a transmitter 1, a receiver 2, a transmission line 3, an OPC 4, one or more optical power adjusters 5, and one or more PSAs 6. The transmitter 1 outputs N types of optical signals from a signal of frequency f1 to a signal of frequency f N where N is a natural number. Also, the frequencies are in the relationship of f1 < ··· < f N / 2 < f (N / 2+1) < ··· < fN. That is, f n has a higher frequency as the numerical value of the subscript is larger. n is a natural number.

[0031] To describe the transmission system 100 by taking as an example the case where the frequency band of the optical signal is the C-band as described above, the frequency f1 is the lowest frequency of the C-band, and the frequency f N is the highest frequency of the C-band. If the frequency band of the optical signal is the L-band, then the frequency f1 is the lowest frequency of the L-band, and the frequency f N is the highest frequency of the L-band.

[0032] The transmitter 1 includes N signal sources 10 of signal sources 10-1 to signal sources 10-N and a WDM 11. The signal source 10-m outputs an optical signal of frequency f m For example, the signal source 10-(N / 2) outputs an optical signal of frequency f (N / 2) and the signal source 10-(N / 2 + 1) outputs an optical signal of frequency f (N / 2+1) .

[0033] WDM11 is a WDM coupler. WDM11 multiplexes the optical signals output from signal sources 10-1 to 10-N and outputs the composite wave obtained by the multiplexing. More specifically, the optical signal output by transmitter 1 is the composite wave output by WDM11. That is, more specifically, the optical signal output by transmitter 1 is the composite wave of N types of optical signals from the signal with frequency f1 to the signal with frequency f N of. Hereinafter, the composite wave of N types of optical signals from the signal with frequency f1 to the signal with frequency f N of is referred to as the signal light.

[0034] Note that signal source 10-m may be a device capable of generating an optical signal such as a laser, or may be a device that outputs the optical signal input from a device outside transmission system 100 toward WDM11.

[0035] FIG. 2 is an explanatory diagram for explaining the frequency spectrum of the signal light output by transmitter 1 in the embodiment. FIG. 2 shows that in transmission system 100, any frequency in the C-band is used as the frequency of the optical signal. That is, the transmission band in transmission system 100 is the entire C-band.

[0036] Returning to the description of FIG. 1, receiver 2 is connected to transmission line 3 and receives the optical signal propagated through transmission line 3. Transmission line 3 is a transmission line that transmits the optical signal from transmitter 1 to receiver 2 via OPC 4, one or more optical power adjusters 5, and one or more PSAs 6. Therefore, the optical signal received by receiver 2 is specifically the optical signal that the optical signal transmitted by transmitter 1 has propagated to receiver 2 through the process of amplitude attenuation and amplification. That is, the optical signal received by receiver 2 is the signal light. Note that transmission line 3 is specifically an optical fiber.

[0037] OPC4 is an optical phase conjugator (OPC). Signal light is input to OPC4. Therefore, OPC4 receives the signal light and outputs the signal light and the idler light (i.e., the phase-conjugated light). More specifically, OPC4 outputs the combined wave of the signal light and the idler light. Hereinafter, the light output by OPC4 (i.e., the combined wave of the received light and the idler light) is referred to as OPC light. The OPC light propagates from OPC4 to the receiver 2 while being amplified or attenuated by the optical power adjuster 5 or the PSA 6.

[0038] FIG. 3 is an explanatory diagram for explaining the frequency spectrum of the OPC light in the embodiment. FIG. 3 shows that the band of the OPC light is from the frequency f1 to the frequency (2f n -f1). From the frequency f1 to the frequency f N is the frequency of the signal light, and from the frequency f N to the frequency (2f n -f1) is the frequency of the idler light.

[0039] Returning to the description of FIG. 1. The optical power adjuster 5 adjusts the power of the input OPC light. That is, the optical power adjuster 5 adjusts the power of the input signal light and idler light. Specifically, the adjustment means performing power amplification or attenuation. An example of the specific configuration of the optical power adjuster 5 will be described later.

[0040] PSA6 is a phase-sensitive amplifier (PSA). PSA6 receives the OPC light. PSA6 amplifies and outputs the input OPC light.

[0041] FIG. 4 is a diagram showing an example of the configuration of OPC4 in the embodiment. OPC4 may have any configuration as long as it is an optical phase conjugator (OPC), and FIG. 4 is an example thereof. In explaining the configuration of OPC4, first, optical parametric amplification will be described.

[0042] <Optical Parametric Amplification> Since optical parametric amplification generally has polarization dependence, a polarization diversity configuration is used in which the input light is split into orthogonal polarization components, processed separately, and then recombined. As the nonlinear optical medium for performing optical parametric amplification, a third-order nonlinear optical medium typified by an optical fiber or a second-order nonlinear optical medium typified by periodically poled inverted lithium niobate is used.

[0043] Signal light is incident on OPC4. OPC4 includes a polarization beam splitter 41, a pump combiner 42, an optical amplifier 43, a pump separator 44, and a polarization combiner 45. The polarization beam splitter 41 separates the incident light into two lights with orthogonal polarization planes. The light incident on the polarization beam splitter 41 is signal light. The polarization beam splitter 41 is, for example, a polarization beam splitter. The pump combiner 42 combines the signal light separated by the polarization beam splitter 41 and the pump light incident from the outside. Hereinafter, each signal light separated by the polarization beam splitter 41 is referred to as polarized signal light. The pump combiner 42 is, for example, a WDM coupler. The pump combiner 42 may be, for example, a dichroic mirror. The light output from the pump combiner 42 is incident on the optical amplifier 43.

[0044] The optical amplifier 43 includes a nonlinear optical medium. The light incident on the optical amplifier 43 is incident on the nonlinear optical medium. The light incident on the optical amplifier 43 is amplified by optical parametric amplification by the nonlinear optical medium. Here, the nonlinear optical medium used in the optical amplifier 43 will be described.

[0045] <Regarding the Nonlinear Optical Medium Used in OPC4> The nonlinear optical medium used in OPC4 is a nonlinear optical medium that is designed in advance such that the center frequency of the amplification band exists at the end of the first band, rather than having the center frequency of the amplification band within the transmission band. The first band is the band supported by the transmitter 1 and the receiver 2. More specifically, the band supported by the transmitter 1 and the receiver 2 is the frequency band in which the light receiving sensitivity of the receiver 2 is higher than a predetermined level among the frequency bands in which the transmitter 1 can output an optical signal with a predetermined intensity or higher.

[0046] The first band is, for example, the C-band. Note that the nonlinear optical medium used in OPC4 is the nonlinear optical medium included in the optical amplification unit 43. The center frequency of the amplification band is determined by the phase matching condition of the nonlinear optical medium and is a frequency predetermined by, for example, the wavelength dispersion of the medium and the frequency of the excitation light.

[0047] The nonlinear optical medium in OPC4 is, for example, an OPA medium in which the center of the amplification band is at the edge of the transmission band of an existing single-band transmission system such as the C-band. The amplification bandwidth of this OPA medium covers, for example, 8 THz or more. Note that Non-Patent Documents 1 and 2 describe an example of a nonlinear optical medium with an amplification bandwidth of 8 THz or more.

[0048] Note that the center of the amplification band of the nonlinear optical medium used in OPC4 does not necessarily have to be at the edge of the first band. The center of the amplification band of the nonlinear optical medium used in OPC4 may be located between the first band and the second band. Note that the second band is a band with a higher or lower frequency than the first band.

[0049] The excitation light will be described in more detail. The frequency of the excitation light is f when using a third-order nonlinear optical medium N and is the second harmonic 2 fN when using a second-order nonlinear optical medium. In the case of a second-order nonlinear optical medium, a configuration is used, for example, in which continuous light of f N is converted to 2 fN by second harmonic generation (SHG) using the nonlinear optical medium. It is desirable that the excitation light used for each polarization component be output from the same light source from the viewpoints of frequency synchronization and phase control with the excitation light in PSA6.

[0050] The pump light separation unit 44 separates the light amplified by the optical amplification unit 43 into polarization signal light and pump light. The pump light separation unit 44 is, for example, a WDM coupler. The pump light separation unit 44 may be, for example, a dichroic mirror. The pump light separation unit 44 outputs the polarization signal light toward the polarization multiplexing unit 45.

[0051] The polarization multiplexing unit 45 multiplexes and outputs the incident polarization signal lights having two mutually orthogonal polarization planes. The polarization multiplexing unit 45 is, for example, a polarization beam splitter.

[0052] In this way, the OPC 4 generates the phase conjugate light of the input signal light by an optical parametric amplification process using a nonlinear optical medium having the center of the amplification band at the edge of the first band.

[0053] FIG. 5 is a diagram showing an example of the configuration of the PSA 6 in the embodiment. The PSA 6 may have any configuration as long as it is a phase-sensitive amplifier (PSA), and FIG. 5 shows an example thereof. The PSA 6 also uses a nonlinear optical medium, but in the PSA 6 as well, the center of the amplification band of the nonlinear optical medium is designed in advance so as to be at the edge of the signal light band, similar to the OPC 4. The PSA 6 includes a synchronization light separation unit 61, a pump light generation unit 62, a polarization demultiplexing unit 63, a phase adjustment unit 64, a pump light multiplexing unit 65, an optical amplification unit 66, a pump light separation unit 67, and a polarization multiplexing unit 68.

[0054] The synchronization light separation unit 61 separates the input light (i.e., OPC light) into a plurality of lights having different propagation directions. The synchronization light separation unit 61 is, for example, a half mirror. The pump light generation unit 62 generates pump light. The pump light generation unit 62 is, for example, a laser. The pump light generated by the pump light generation unit 62 is light that satisfies the pump light conditions. The pump light conditions will be described later.

[0055] The polarization beam splitting unit 63 separates the incident light into two lights with orthogonal polarization planes. The light incident on the polarization beam splitting unit 63 is OPC light. The polarization beam splitting unit 63 is, for example, a polarization beam splitter. Hereinafter, each OPC light separated by the polarization beam splitting unit 63 is referred to as polarized OPC light.

[0056] The phase adjuster 64 adjusts the phase of the incident OPC light. Adjusting the phase of the incident OPC light specifically means changing the phase of the incident OPC light by a predetermined amount. Note that the OPC light incident on the phase adjuster 64 is the polarized OPC light output from the polarization beam splitting unit 63. Therefore, the light output from the phase adjuster 64 is polarized OPC light.

[0057] In order to perform phase-sensitive amplification, the phase relationship among the signal light, the idler light, and the pump light needs to be an appropriate phase relationship. The appropriate phase relationship is a phase relationship such that the phase-conjugate light of the idler light generated at the frequency of the signal light as a result of the non-linear interaction between the idler light and the pump light and the signal light interfere constructively. The phase adjuster 64 makes the phase relationship among the signal light, the idler light, and the pump light an appropriate phase relationship by changing the phase of the signal light by a predetermined amount.

[0058] In FIG. 5, the phase adjuster 64 is located on the signal light line. However, the PSA 6 does not necessarily need to include the phase adjuster 64. When the PSA 6 does not include the phase adjuster 64, the adjustment of the phase relationship among the signal light, the idler light, and the pump light may be performed in advance by the transmitter 1.

[0059] Note that in order to make the phase relationship among the signal light, the idler light, and the pump light an appropriate phase relationship, the relative phase may be adaptively controlled using a piezo-driven fiber stretcher or the like. The phase adjuster 64 may be, for example, a waveguide-type phase modulator.

[0060] The pump light combining unit 65 combines the polarization OPC light output from the phase adjustment unit 64 and the pump light generated by the pump light generation unit 62. The pump light combining unit 65 is, for example, a WDM coupler. The pump light combining unit 65 may be, for example, a dichroic mirror. The light output from the pump light combining unit 65 is incident on the optical amplification unit 66.

[0061] The optical amplification unit 66 includes a nonlinear optical medium. The light incident on the optical amplification unit 66 is incident on the nonlinear optical medium. The light incident on the optical amplification unit 66 is amplified by optical parametric amplification by the nonlinear optical medium. Here, the nonlinear optical medium used in the optical amplification unit 66 will be described.

[0062] <Regarding the nonlinear optical medium used in PSA6> The nonlinear optical medium used in PSA6 is a nonlinear optical medium that is not designed such that the center frequency of the amplification band exists within the transmission band, but is designed in advance such that the center frequency of the amplification band exists at the edge of the first band. Note that the nonlinear optical medium used in PSA6 is the nonlinear optical medium included in the optical amplification unit 66. The center frequency of the amplification band is determined by the phase matching condition of the nonlinear optical medium and is a frequency determined in advance by the wavelength dispersion of the medium, the frequency of the pump light, etc.

[0063] The nonlinear optical medium in PSA6 is, for example, the nonlinear optical medium described in Non-Patent Documents 1 and 2. The nonlinear optical media described in Non-Patent Documents 1 and 2 are OPA media in which the center of the amplification band is at the edge of the transmission band of an existing single-band transmission system such as the C-band. The amplification bandwidth of this OPA medium covers, for example, 8 THz or more.

[0064] Note that the center of the amplification band of the nonlinear optical medium used in PSA6 does not necessarily have to be at the edge of the first band. The center of the amplification band of the nonlinear optical medium used in PSA6 may be located between the first band and the second band.

[0065] The pump light will be described in more detail. The frequency of the pump light is f when using a third-order nonlinear optical medium. Nand is the second harmonic when using a second-order nonlinear optical medium fN is used. In the case of a second-order nonlinear optical medium, f N of the continuous light is converted to 2 fN by second harmonic generation (SHG) using a nonlinear optical medium. For example, a configuration is used in which the excitation light used for each polarization component is output from the same light source.

[0066] The excitation light separation unit 67 separates the light amplified by the optical amplification unit 66 into polarization OPC light and excitation light. The excitation light separation unit 67 is, for example, a WDM coupler. The excitation light separation unit 67 may be, for example, a dichroic mirror. The excitation light separation unit 67 outputs the polarization OPC light toward the polarization multiplexing unit 68.

[0067] The polarization multiplexing unit 68 multiplexes and outputs the incident polarization OPC lights having two mutually orthogonal polarization planes. The polarization multiplexing unit 68 is, for example, a polarization beam splitter.

[0068] In PSA6, the frequency of the excitation light needs to be synchronized with the carrier components of the pair of the signal light and the idler light by, for example, optical injection synchronization. The carrier component coincides with the excitation light in OPC4. Therefore, for frequency synchronization, in PSA6, a part of the input light is tapped at the time of input. As a specific example of tapping a part of the input light, it is the separation by the above-described synchronization light separation unit 61.

[0069] In the example of FIG. 5, the tapped component is the pair of the signal light and the idler light (that is, the OPC light) itself. However, the tapped component may be a previously prepared pilot light. When using a pilot light, a part of the excitation light having the frequency f N used in OPC4 is tapped and co-propagated with the signal light. When a second-order nonlinear optical medium is used as the nonlinear optical medium, the pilot light is preferably the original continuous light before being converted to the second harmonic.

[0070] <Pumping light conditions> The pumping light conditions are such that they are frequency - synchronized with the components tapped by optical phase synchronization or optical injection synchronization. Therefore, in the pumping light generation unit 62, pumping light that is frequency - synchronized with the components tapped by optical phase synchronization or optical injection synchronization is generated.

[0071] With PSA6 configured in this way, the signal light and the idler light that have propagated through different transmission bands are coherently combined in PSA6. As a result, PSA6 can obtain phase - sensitive amplification characteristics.

[0072] In this way, PSA6 performs phase - sensitive amplification through the interaction among three optical waves, namely the input signal light, the phase - conjugate light, and the pumping light, by means of an optical parametric amplification process using a non - linear optical medium having the center of the amplification band at the end of the first band.

[0073] <Details of the optical power adjustment unit 5> Here, the optical power adjustment unit 5 will be described in more detail. Generally, in the transmission path through which an optical signal is transmitted, there is a wavelength - dependence of transmission loss. In a general optical fiber, it is known that within the C - band, the variation with respect to wavelength is relatively gentle, while within the S - band, it varies greatly.

[0074] On the other hand, in ND - PSA, it is known that if there is a difference in optical power between the signal light and the idler light input to the amplification unit, the noise figure deteriorates according to the magnitude of the difference.

[0075] Therefore, when the signal light in the transmission system 100 is light in the C - band and the idler light is light in the S - band, especially the farther away from the center of the amplification band, the power difference between the signal light and the idler light at the input end of PSA6 (that is, the synchronization optical separation unit 61) is large.

[0076] FIG. 6 is a diagram showing an example of the frequency spectrum of the OPC light in the synchronization optical separation unit 61 of the embodiment. FIG. 6 shows that there is a difference between the power of the idler light and the power of the signal light.

[0077] Therefore, the optical power adjuster 5 reduces this power difference (i.e., the difference in power between the signal light and the idler light). Specifically, the optical power adjuster 5 reduces the difference in power between the signal light and the idler light by adjusting the transmission power to the transmission line using an optical amplifier or an optical attenuator. Note that this difference in power is caused by the difference in transmission loss due to the different transmission bands of the signal light and the idler light.

[0078] FIG. 7 is a diagram showing an example of the configuration of the optical power adjuster 5 in the embodiment. More specifically, FIG. 7 is an example of the configuration of the optical power adjuster 5 that reduces the difference in power between the idler light and the signal light using an optical amplifier.

[0079] The optical power adjuster 5 includes a band demultiplexer 51, a first band optical amplifier 52, a second band optical amplifier 53, a first gain equalization filter 54, a second gain equalization filter 55, and a band multiplexer 56.

[0080] The OPC light incident on the optical power adjuster 5 first enters the band demultiplexer 51. The band demultiplexer 51 is a band demultiplexer that propagates the light within a first band having a predetermined frequency along a first path and propagates the light within a second band having a predetermined frequency along a second path different from the first path. That is, the band demultiplexer 51 is a band demultiplexer that demultiplexes the incident light according to the frequency. When the first band is the C-band, the second band is, for example, the S-band.

[0081] The first band optical amplifier 52 amplifies the light demultiplexed by the band demultiplexer 51 and propagated along the first path. The second band optical amplifier 53 amplifies the light demultiplexed by the band demultiplexer 51 and propagated along the second path.

[0082] The first gain equalization filter 54 is a gain equalization filter that flattens the gain curve in the first band. The first gain equalization filter receives the light amplified by the first band optical amplifier 52 and propagating through the first path. Therefore, the first gain equalization filter 54 has an inverse characteristic of the wavelength dependence of the transmission loss so that the power spectrum is uniform at the input end of the next PSA6 located in the subsequent stage.

[0083] The second gain equalization filter 55 is a gain equalization filter that flattens the gain curve in the second band. The second gain equalization filter receives the light amplified by the second band optical amplifier 53 and propagating through the second path. Therefore, the second gain equalization filter 55 has an inverse characteristic of the wavelength dependence of the transmission loss so that the power spectrum is uniform at the input end of the next PSA6 located in the subsequent stage.

[0084] The band multiplexer 56 multiplexes the light output from the first gain equalization filter 54 and the light output from the second gain equalization filter 55.

[0085] Note that when the amplification gain in PSA6 is sufficient and the gain equalization filter can flatten the gain curve for the entire band of the total band of the signal light frequency band and the idler light frequency band, it is not necessary for the OPC light to be split into two transmission bands in the optical power adjustment unit 5. In such a case, it is only necessary to have a gain equalization filter immediately after PSA6. That is, the optical power adjustment unit 5 may simply be one gain equalization filter.

[0086] The transmission system 100 configured in this way generates idler light in a band different from the band previously defined as the transmission band by the OPC4. Therefore, it is not necessary to use a part of the previously defined transmission band as idler light, and it can be used entirely for the transmission of signal light. Therefore, the transmission system 100 can increase the transmission capacity of the transmission system for transmitting optical signals as compared with the case where idler light is generated in a previously defined band.

[0087] Moreover, in the case of the transmission system 100 configured as described above, since the transmission capacity increases compared to the case where the idle light is generated in a predetermined band, the transmission distance can also be increased to a long distance.

[0088] (Modification example) Note that the optical power adjustment unit 5 does not necessarily have to exist. Also, it is not necessarily required that the optical power adjustment unit 5 exists for each of the OPC 4 and all the PSA 6 as shown in FIG. 1. The optical power adjustment unit 5 may exist only for a part of the OPC 4 and all the PSA 6.

[0089] Note that the first band is, for example, a band with a frequency lower than the frequency at the boundary between the C-band and the S-band, and the second band is, for example, a band with a frequency higher than the frequency at the boundary between the C-band and the S-band. Therefore, as described above, the first band is, for example, the C-band, and the second band is, for example, the S-band. Specifically, the frequency at the boundary between the C-band and the S-band is 1530 nm.

[0090] Note that the frequencies of the signal light and the idle light do not necessarily have to be located in the C-band and the S-band as long as they are located in different defined bands from each other. For example, when there are two different frequency bands named "X1-band" and "X2-band", the frequency of the signal light may be located in the "X1-band" and the frequency of the idle light may be located in the "X1-band". In such a case, the "X1-band" is the first band, and the "X2-band" is the second band. That is, the first band may be one of two different frequency bands with different names from each other, and the second band may be the other of the different frequency bands with different names from each other.

[0091] Note that when the signal sources 10-1 to 10-N are devices capable of controlling the operation of the output of light such as a laser, the transmission system 100 may include a device for controlling the operations of the signal sources 10-1 to 10-N. Hereinafter, the transmission system 100 including a device for controlling the operations of the signal sources 10-1 to 10-N is referred to as the transmission system 100a.

[0092] FIG. 8 is a diagram showing an example of the configuration of the transmission system 100a in a modified example. The transmission system 100a includes a transmitter 1, a receiver 2, a transmission line 3, an OPC 4, one or more optical power adjusters 5, and one or more PSAs 6, and further includes a signal generation control device 7. That is, the transmission system 100a is different from the transmission system 100 in that it includes the signal generation control device 7. The signal generation control device 7 controls the operations of the signal sources 10-1 to 10-N. More specifically, the signal generation control device 7 controls the operations of the signal sources 10-1 to 10-N to control the timing, frequency, or waveform of the signals generated by the signal sources 10-1 to 10-N.

[0093] FIG. 9 is a diagram showing an example of the hardware configuration of the signal generation control device 7 in a modified example. The signal generation control device 7 includes a control unit 71 including a processor 91 such as a CPU (Central Processing Unit) connected by a bus and a memory 92, and executes a program. The signal generation control device 7 functions as a device including a control unit 71, an input unit 72, a communication unit 73, a storage unit 74, and an output unit 75 by executing the program.

[0094] More specifically, the processor 91 reads out the program stored in the storage unit 74 and stores the read program in the memory 92. By the processor 91 executing the program stored in the memory 92, the signal generation control device 7 functions as a device including a control unit 71, an input unit 72, a communication unit 73, a storage unit 74, and an output unit 75.

[0095] The control unit 71 controls the operations of various functional units included in the signal generation control device 7, such as the input unit 72, the communication unit 73, the storage unit 74, and the output unit 75. The control unit 71 records various information in the storage unit 74, for example. The control unit 71 controls the operations of the signal sources 10-1 to 10-N via the communication unit 73, for example.

[0096] The input unit 72 is configured to include input devices such as a mouse, a keyboard, and a touch panel. The input unit 72 may be configured as an interface for connecting these input devices to the signal generation control device 7. The input unit 72 receives the input of various information to the signal generation control device 7. The waveforms of the respective lights generated by the control unit 71 controlling the operations of the signal sources 10-1 to 10-N and causing the signal sources 10-1 to 10-N to generate are, for example, waveforms indicating the information input to the input unit 72 and intended to be transmitted to the receiver 2. That is, the control unit 71 controls the operations of the signal sources 10-1 to 10-N, for example, to generate an optical signal indicating a waveform indicating the information input to the input unit 72.

[0097] The communication unit 73 is configured to include a communication interface for connecting the signal generation control device 7 to an external device. The communication unit 73 communicates with the external device via wired or wireless means. The external device is, for example, the signal sources 10-1 to 10-N.

[0098] The storage unit 74 is configured using a computer-readable storage medium device such as a magnetic hard disk device or a semiconductor storage device. The storage unit 74 stores various information related to the signal generation control device 7. The storage unit 74 stores, for example, the information input via the input unit 72 or the communication unit 73.

[0099] The output unit 75 outputs various information. The output unit 75 is configured to include a display device such as a CRT (Cathode Ray Tube) display, a liquid crystal display, or an organic EL (Electro-Luminescence) display. The output unit 75 may be configured as an interface for connecting these display devices to the signal generation control device 7. The output unit 75 outputs, for example, the information input to the input unit 72.

[0100] FIG. 10 is a flowchart showing an example of the flow of processing executed in the transmission system 100 in the modification. The control unit 71 controls the operations of the signal sources 10-1 to 10-N to generate optical signals having frequencies in the first band for each of the signal sources 10-1 to 10-N (step S101). Next, the receiver 2 receives the optical signals that have reached via the OPC 4, one or more optical power adjusters 5, and one or more PSAs 6, and receives each optical signal generated in step S101 (step S102).

[0101] Note that the OPC 4 is an example of a phase conjugate light generation unit. Note that the PSA 6 is an example of an optical phase sensitive amplification unit.

[0102] As described above, for phase sensitive amplification, the phase relationship among the signal light, the idler light, and the pump light needs to be an appropriate phase relationship. And the state of an appropriate phase relationship is realized, for example, by adaptively controlling the relative phase among the signal light, the idler light, and the pump light. Here, an example of an apparatus for performing adaptive control will be described. Hereinafter, the transmission system 100 including an apparatus for performing adaptive control is referred to as a transmission system 100b.

[0103] FIG. 11 is a diagram showing an example of the configuration of the transmission system 100b in the modification. The transmission system 100b further includes an adaptive control device 8 in addition to the transmitter 1, the receiver 2, the transmission line 3, the OPC 4, one or more optical power adjusters 5, and one or more PSAs 6. That is, the transmission system 100b is different from the transmission system 100 in that it includes the adaptive control device 8.

[0104] The adaptive control device 8 performs adaptive control of the relative phase among the signal light, the idler light, and the pump light (hereinafter referred to as "phase adaptive control"). That is, the adaptive control device 8 controls the relative phase among the signal light, the idler light, and the pump light to be controlled so as to be an appropriate phase relationship.

[0105] The phase adaptive control is specifically performed based on the result of tapping and monitoring a part of the output of PSA6. More specifically, the phase adaptive control is a process of performing phase control so that the monitored value becomes maximum based on the result of tapping and monitoring a part of the output of PSA6. The monitored value is the intensity of the light of a part of the output of PSA6, which is monitored after being tapped. The reason for performing phase control so that the monitored value becomes maximum is that the output of PSA6 becomes the largest when an appropriate phase relationship is satisfied. The phase adaptive control is executed by the adaptive control device 8. An example of the configuration of the adaptive control device 8 will be described later.

[0106] FIG. 12 is a diagram showing an example of the hardware configuration of the adaptive control device 8 in the embodiment. The adaptive control device 8 includes a control unit 81 including a processor 93 such as a CPU (Central Processing Unit) and a memory 94 connected by a bus, and executes a program. The adaptive control device 8 functions as a device including a control unit 81, an input unit 82, a communication unit 83, a storage unit 84, an output unit 85, and a light receiving unit 86 by executing a program.

[0107] More specifically, the processor 91 reads out the program stored in the storage unit 84 and stores the read program in the memory 92. By the processor 91 executing the program stored in the memory 92, the adaptive control device 8 functions as a device including a control unit 81, an input unit 82, a communication unit 83, a storage unit 84, an output unit 85, and a light receiving unit 86.

[0108] The control unit 81 controls the operations of various functional units included in the adaptive control device 8 such as the input unit 82, the communication unit 83, the storage unit 84, the output unit 85, and the light receiving unit 86. The control unit 81 records various information in the storage unit 84, for example. The control unit 81 executes, for example, phase adaptive control. The control unit 81 controls the operation of the phase adjustment unit 64 via the communication unit 83 to perform adaptive control of the relative phase among the signal light, the idler light, and the excitation light. In such a case, the phase adjustment unit 64 controls the relative phase among the signal light, the idler light, and the excitation light under the control of the control unit 81.

[0109] The input unit 82 is configured to include an input device such as a mouse, a keyboard, a touch panel, etc. The input unit 82 may be configured as an interface for connecting these input devices to the adaptation control device 8. The input unit 82 receives the input of various information to the adaptation control device 8.

[0110] The communication unit 83 is configured to include a communication interface for connecting the adaptation control device 8 to an external device. The communication unit 83 communicates with the external device via wire or wirelessly. The external device is, for example, the phase adjuster 64.

[0111] The storage unit 84 is configured using a computer-readable storage medium device such as a magnetic hard disk device or a semiconductor storage device. The storage unit 84 stores various information related to the adaptation control device 8. The storage unit 84 stores, for example, the information input via the input unit 82 or the communication unit 83.

[0112] The output unit 85 outputs various information. The output unit 85 is configured to include a display device such as a CRT display, a liquid crystal display, an organic EL display, etc. The output unit 85 may be configured as an interface for connecting these display devices to the adaptation control device 8. The output unit 85 outputs, for example, the information input to the input unit 82.

[0113] The light receiving unit 86 receives the OPC light and the excitation light generated by the excitation light generation unit 62. Hereinafter, the excitation light generated by the excitation light generation unit 62 is referred to as PSA excitation light. The light receiving unit 86 is, for example, a half mirror installed on the optical path in the PSA6 where the OPC light propagates, or a part of the OPC light tapped by an optical coupler that distributes the power of the input light to two optical fibers at a predetermined ratio by fusing two optical fibers or a dielectric multilayer film. The light receiving unit 86 receives a part of the PSA excitation light tapped by, for example, a half mirror installed on the optical path in the PSA6 where the PSA excitation light propagates or an optical coupler. The light receiving unit 86 outputs a signal indicating the result of the light reception to the control unit 81. The result of the light reception is specifically the above-described monitor value. The control unit 81 performs phase control so that the monitor value becomes maximum based on the result of the light reception by the light receiving unit 86.

[0114] In this way, the adaptive control device 8 controls the relative phases among the signal light, the idler light, and the excitation light, which are the control targets, to have an appropriate phase relationship.

[0115] Note that the transmission system 100b may include the signal generation control device 7.

[0116] Note that each of the signal generation control device 7 and the adaptive control device 8 may be implemented using a plurality of information processing devices communicably connected via a network. In this case, each functional unit included in each of the signal generation control device 7 and the adaptive control device 8 may be implemented in a distributed manner in a plurality of information processing devices.

[0117] Note that the signal generation control device 7 and the adaptive control device 8 do not necessarily have to be implemented as different devices. The signal generation control device 7 and the adaptive control device 8 may be implemented, for example, as one device having both functions.

[0118] Note that all or part of the functions of the signal generation control device 7 and the adaptive control device 8 may be realized using hardware such as an ASIC (Application Specific Integrated Circuit), a PLD (Programmable Logic Device), or an FPGA (Field Programmable Gate Array). The program may be recorded on a computer-readable recording medium. A computer-readable recording medium is, for example, a portable medium such as a flexible disk, a magneto-optical disk, a ROM, a CD-ROM, or a storage device such as a hard disk incorporated in a computer system. The program may be transmitted via a telecommunication line.

[0119] As described above, the embodiments of the present invention have been described in detail with reference to the drawings. However, the specific configuration is not limited to this embodiment, and designs and the like within the scope not departing from the gist of the present invention are also included.

Explanation of Reference Numerals

[0120] 100... Transmission system, 1... Transmitter, 2... Receiver, 3... Transmission line, 4... OPC, 5... Optical power adjustment unit, 6... PSA, 10-1 to 10-N... Signal sources, 11... WDM, 41... Polarization demultiplexing unit, 42... Pump light multiplexing unit, 43... Optical amplifier unit, 44... Pump light separation unit, 45... Polarization multiplexing unit, 51... Band demultiplexer, 52... First band optical amplifier, 53... Second band optical amplifier, 54... First gain equalization filter, 55... Second gain equalization filter, 56... Band multiplexer, 61... Synchronization light separation unit, 62... Pump light generation unit, 63... Polarization demultiplexing unit, 64... Phase adjustment unit, 65... Pump light multiplexing unit, 66... Optical amplifier unit, 67... Pump light separation unit, 68... Polarization multiplexing unit, 7... Signal generation control device, 71... Control unit, 72... Input unit, 73... Communication unit, 74... Storage unit, 75... Output unit, 8... Adaptive control device, 81... Control unit, 82... Input unit, 83... Communication unit, 84... Storage unit, 85... Output unit, 86... Light receiving unit, 91... Processor, 92... Memory, 93... Processor, 94... Memory

Claims

1. A transmission system for transmitting an optical signal, comprising: a phase conjugate light generation unit that generates phase conjugate light of the optical signal having a frequency within a first band in a second band different from the first band; an optical phase-sensitive amplification unit that phase-sensitively amplifies the optical signal and the phase conjugate light; wherein the first band is a frequency band in which the sensitivity of light reception of a receiver that receives the optical signal is equal to or higher than a predetermined level among the frequency bands in which a transmitter that outputs the optical signal can output the optical signal at a predetermined intensity or higher, and the second band is a band having a higher or lower frequency than the first band; the phase conjugate light generation unit generates the phase conjugate light of the input optical signal by an optical parametric amplification process using a nonlinear optical medium having a center of an amplification band between the first band and the second band; the optical phase-sensitive amplification unit performs phase-sensitive amplification by an interaction between three optical waves of the input optical signal, the phase conjugate light, and pump light by an optical parametric amplification process using a nonlinear optical medium having a center of an amplification band between the first band and the second band; a transmission system.

2. An optical power adjustment unit that adjusts the power of the optical signal and the phase conjugate light; The transmission system according to claim 1, further comprising the optical power adjustment unit.

3. The optical power adjustment unit reduces a power difference between the optical signal and the phase conjugate light caused by a difference in transmission loss resulting from a difference in transmission band between the optical signal and the phase conjugate light; The transmission system according to claim 2.

4. a signal source that generates the optical signal; a control unit that controls the operation of the signal source; The transmission system according to any one of claims 1 to 3, further comprising the signal source and the control unit.

5. A phase conjugate light generation unit that generates phase conjugate light of an optical signal whose frequency is within a first band in a second band different from the first band, and an optical phase sensitive amplification unit that phase-sensitively amplifies the optical signal and the phase conjugate light, wherein the first band is a frequency band in which the sensitivity of light reception of a receiver that receives the optical signal is equal to or higher than a predetermined level among the frequency bands in which a transmitter that outputs the optical signal can output the optical signal at a predetermined intensity or higher, the second band is a band having a higher or lower frequency than the first band, the phase conjugate light generation unit generates the phase conjugate light of the input optical signal by an optical parametric amplification process using a nonlinear optical medium having a center of an amplification band between the first band and the second band, and the optical phase sensitive amplification unit performs phase sensitive amplification by an interaction between three optical waves of the input optical signal, the phase conjugate light, and excitation light by an optical parametric amplification process using a nonlinear optical medium having a center of an amplification band between the first band and the second band, which is a transmission method executed by a transmission system. An optical signal generation step of generating the optical signal. A transmission method having the above.

6. A program for causing a computer to function as the transmission system according to claim 4.

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