Optical transmission system

Abstract

This optical transmission system comprises: a plurality of phase conjugate conversion units that phase-rotate an optical signal through an optical parametric amplification process; a plurality of excitation light source units that are provided for each of the phase conjugate conversion units and generate excitation light used in the optical parametric amplification process; an optical transmission unit that transmits, together with the optical signal, pilot light, which is part of the excitation light generated by the excitation light source unit on the upstream side, to the excitation light source unit on the downstream side; and an excitation light synchronization unit that causes the excitation light source unit on the downstream side to generate the excitation light by performing phase synchronization with the pilot light transmitted from the excitation light source unit on the upstream side.

Description

Optical Transmission System

[0001] The present invention relates to an optical transmission system.

[0002] In recent years, communication traffic has been increasing exponentially with the launch of the 5th Generation Mobile Communication System and the widespread use of rich content such as high-resolution video. This has led to a demand for continuous increases in the communication capacity of optical transmission systems. In optical fiber transmission, multiple signal lights are arranged in the wavelength direction (on the frequency axis), and these multiple signal lights are multiplexed and transmitted through a single optical fiber, thereby achieving high-capacity information transmission.

[0003] By using digital coherent technology to convert signals into high-level signals, spectral efficiency is improved and the transmission capacity of long-distance optical fiber communication networks is increased. To further improve spectral efficiency, it is necessary to reduce the noise level in the optical domain. One way to reduce the noise level in the optical domain is to increase the intensity (optical power) of the transmitted signal light.

[0004] However, in optical fiber transmission, when the intensity of the transmitted signal light increases, nonlinear optical effects in the optical fiber become apparent, distorting the waveform of the signal light. Depending on the intensity of the transmitted signal light, signal quality improves in the low-power range, but deteriorates in the high-power range. Therefore, because there is an optimum value for the intensity of the transmitted signal light, spectral efficiency is limited. This limit on spectral efficiency in optical fiber transmission is called the nonlinear Shannon limit.

[0005] Another approach, different from improving spectral efficiency, is to broaden the optical transmission band. Wavelength ranges with low transmission loss in optical fibers are divided into several optical transmission bands. Long-distance optical fiber communication networks primarily use the C-band or L-band, which has the lowest transmission loss among all optical transmission bands, from approximately 4 THz to 5 THz. In this band, an erbium-doped fiber amplifier (EDFA) can be used as an optical amplifier repeater.

[0006] In order to broaden the optical transmission band, there has been active research and development into multi-band wavelength division multiplexing transmission systems that use new optical transmission bands such as the S-band in addition to the combined use of the C-band and L-band. Such a wideband transmission system using multiple optical transmission bands is called a multi-band transmission system. A multi-band transmission system requires the development of new optical transceivers that can handle optical transmission bands that have not been used before.

[0007] Therefore, optical parametric amplification (OPA) has attracted attention as a technology for performing signal light processing such as wavelength conversion and phase conjugate conversion of signal light using phase conjugate light (idler light) (see Patent Document 1). Optical parametric amplification uses four wave mixing (FWM), which is a third-order nonlinear optical effect, and difference frequency generation (DFG), which is a second-order nonlinear optical effect.

[0008] In the optical parametric amplification process, signal light and pump light are injected into a nonlinear optical medium, and the signal is amplified by the nonlinear optical effect of the medium. As the signal is amplified, a phase-conjugated light called an idler light is generated at a wavelength symmetrical to the wavelength of the signal light with respect to the degenerate wavelength (degenerate frequency). The degenerate wavelength is located at the center of the gain band determined by the wavelength of the pump light and the phase matching characteristics of the medium. By utilizing this idler light, a wavelength-multiplexed signal can be generated in a conventionally used band (e.g., C-band) and converted to another transmission band, enabling multi-band transmission without the need for an optical transceiver adapted to the new transmission band (see Non-Patent Document 1).

[0009] Furthermore, the idler light is a phase conjugate of the signal light, and by extracting and transmitting the idler light at the output of an optical parametric amplifier (OPA), it is possible to perform optical phase conjugation (OPC) of the signal light. It is known that by transmitting the idler light while repeatedly performing phase conjugation (OPC) in an optical amplifier repeater (optical node device), it is possible to compensate for phase distortions that occur in the signal light due to nonlinear optical effects in optical fibers and chromatic dispersion signals (see Non-Patent Document 2). The mechanism for compensating for such phase distortions is described below.

[0010] 1) When signal light is transmitted, phase rotation occurs due to nonlinear optical effects and chromatic dispersion in the optical fiber, which is the transmission medium. 2) The signal light is converted into phase conjugate light in the amplifying repeater. This phase conjugate light has a phase rotation with a different sign from that added to the original signal light. 3) In the transmission path after phase conjugation, phase rotation occurs due to nonlinear optical effects and chromatic dispersion, just as before phase conjugation. 4) The phase rotations that occur in the transmission path before and after phase conjugation have different signs, so they cancel each other out.

[0011] As described above, if the nonlinear optical effect in an optical fiber can be compensated for, it will be possible to exceed the conventional nonlinear Shannon limit, further improving the spectral efficiency and extending the transmission distance. Furthermore, in digital coherent transmission, chromatic dispersion in an optical fiber is compensated for by digital signal processing on the receiving side, but by compensating in the optical domain using optical phase conjugation (OPC), it is possible to reduce the power consumption of the digital signal processing unit.

[0012] Japanese Patent Application Laid-Open No. 2020-86031

[0013] T. Kato, H. Muranaka, Y. Tanaka, Y. Akiyama, T. Hoshida, S. Shimizu, T. Kobayashi, T. Kazama, T. Umeki, K. Watanabe, and Y. Miyamoto, “S+C+L-Band WDM Transmission Using 400-Gb / s Real-Time Transceivers Extended by PPLN-Based Wavelength Converter,” Proceeding of European Conference on Optical Communication (ECOC), We4D.4, September 2022.T. Umeki et al., “Simultaneous nonlinearity mitigation in 92 × 180-Gbit / s PDM-16QAM transmission over 3840 km using PPLN-based guard-band-less optical phase conjugation,” Optics EXPRESS, Research Article, Vol.24, No.15, pp.16945-16951, July 2016.

[0014] In optical parametric amplification using four-wave mixing, a third-order nonlinear optical effect, stimulated Brillouin scattering and phase modulation effects of the pump light can cause signal distortion and distortion of the spectrum of the output optical signal. To suppress stimulated Brillouin scattering, a method of modulating phase dither with the pump light is used, but the phase noise and frequency fluctuation of the pump light are transmitted to the idler light, which poses a problem regarding the effect of phase dither on the signal.

[0015] In optical parametric amplification using difference frequency generation (DFR), a second-order nonlinear optical effect, the influence of stimulated Brillouin scattering is small in principle, so phase dithering is generally not necessary. On the other hand, as in optical parametric amplification using four-wave mixing (FWM), a third-order nonlinear optical effect, the phase noise and frequency fluctuations of the pump light are transmitted to the wavelength-converted light, so it is desirable that the linewidth and frequency noise of the pump light source be as small as possible.

[0016] The present invention has been made in view of the above-described technical background, and aims to provide a technology that can cancel out phase noise transmitted from pump light to signal light and suppress excessive degradation of signal quality accompanying phase conjugation (wavelength conversion).

[0017] One aspect of the present invention is an optical transmission system comprising: a plurality of phase conjugate converters that perform phase rotation on an optical signal through an optical parametric amplification process; a plurality of pumping light source units that are provided for each of the phase conjugate converters and generate pumping light to be used in the optical parametric amplification process; an optical transmission unit that transmits pilot light, which is part of the pumping light generated by the upstream pumping light source unit, to the downstream pumping light source unit along with the optical signal; and a pumping light synchronization unit that phase-synchronizes with the pilot light transmitted from the upstream pumping light source unit and causes the downstream pumping light source unit to generate the pumping light.

[0018] According to the present invention, it is possible to cancel out the phase noise transferred from the pump light to the signal light and to suppress excessive deterioration of the signal quality caused by the phase conjugation (wavelength conversion).

[0019] FIG. 1 is a diagram showing wavelength conversion by an optical parametric amplifier (OPA). FIG. 2 is a basic configuration diagram of an optical transmission system 1 in a first embodiment of the present invention. FIG. 3 is a basic configuration diagram of an optical transmission system 1a in a second embodiment of the present invention. FIG. 4 is a basic configuration diagram of an optical transmission system 1b in a third embodiment of the present invention. FIG. 5 is a basic configuration diagram of an optical transmission system 1c in a fourth embodiment of the present invention. FIG. 6 is a basic configuration diagram of an optical transmission system 1d in a fifth embodiment of the present invention. FIG. 7 is a basic configuration diagram of an optical transmission system 1e in a sixth embodiment of the present invention.

[0020] Hereinafter, an optical transmission system according to an embodiment will be described with reference to the drawings.

[0021] As mentioned above, suppressing the degradation of signal characteristics due to the transmission of phase noise is an important issue when applying all-optical wavelength conversion technology using nonlinear optical effects in optical transmission systems. ｉ is expressed as the following equations (1) and (2).

[0022]

[0023]

[0024] Here, "E ｓ " represents the time waveform of the input signal light. ｓ " represents the complex amplitude of the modulating signal. ｏ " represents a phase offset resulting from the relative phase difference between the pump light and the signal carrier, etc. "j" represents the imaginary unit. "*" represents the complex conjugate.

[0025] In addition, the angular frequency ω of the wavelength-converted light ｉ is ω ｉ = 2ω ｐ -ω ｓ Here, "ω ｐ " represents the angular frequency of the excitation light. "ω ｓ " represents the angular frequency of the signal light.

[0026] In the case of wavelength conversion using difference frequency generation, which is a second-order nonlinear optical effect, the pump light here refers to the fundamental pump light before being converted into a second harmonic by the second harmonic generation (SHG) process. Note that here, the effects of noise components originally present in the signal band and vacuum noise amplified during the wavelength conversion process are ignored. Also, here, the effects of pump depletion and optical loss in the nonlinear medium are ignored. After reconversion to the original frequency ω ｓ The complex amplitude E of the electric field of the optical signal returned to ｓ ' is expressed as the following equation (3).

[0027]

[0028] Here, "φ ｏ '" is the phase offset at the time of reconversion added to the original phase offset. On the other hand, when the frequency noise Δω(t) of the fundamental pump light is taken into consideration, Equation (2) can be expressed as the following Equation (4).

[0029]

[0030] Similarly, when the fundamental pump light having frequency noise Δω′(t) is reconverted, Equation (3) can be expressed as Equation (5) below.

[0031]

[0032] Therefore, the signal light is imparted with frequency noise originating from the pump light. This frequency noise not only causes a frequency offset during coherent detection, but is also known to be converted into excess noise such as group delay jitter through interaction with chromatic dispersion occurring in the transmission line.

[0033] In order to solve the above-mentioned problems, the optical transmission system according to the embodiment of the present invention described below has an optical node configuration that suppresses the transmission of phase noise from pump light. From the above equation (5), if Δω(t) = Δω'(t), the frequency noise is canceled out when the optical signal is reconverted, and an optical signal without excess noise can be obtained.

[0034] The correlation of frequency noise of laser light is defined by coherence. There is no correlation between frequency noise of two laser lights emitted from different light sources. Therefore, it is generally not possible to use pump light with correlated frequency noise between optical parametric amplifiers (OPAs) located at different locations.

[0035] Therefore, the optical transmission system of each embodiment of the present invention described below is configured to transmit pilot light having the same wavelength as the pump light together with the signal light from the transmitting side, and synchronize the pump light used in multiple optical parametric amplifiers (OPAs) located at distant locations using an optical injection locking circuit and an optical phase-locked circuit using the pilot light, thereby suppressing signal degradation due to conversion to idler light in the optical transmission system.

[0036] In order to operate an optical injection locking circuit and an optical phase locking circuit, it is necessary to transmit information on pump light, which is continuous light. Therefore, when an optical transmission system first performs optical phase conjugation (OPC), it taps a part of the pump light to use it as pilot light, and then multiplexes this pilot light with signal light (idler light) that has been phase conjugated and wavelength converted, and transmits them together.

[0037] The pilot light is separated in the downstream optical parametric amplifier (OPA), and the optical transmission system synchronizes the pump light and the pilot light using an optical injection locking circuit and an optical phase locking circuit.The optical transmission system then taps a portion of the pump light again, combines it with the phase-conjugated and wavelength-converted signal light as new pilot light, and transmits it.

[0038] With this configuration, the optical transmission system of each embodiment of the present invention can synchronize the pump light used in optical parametric amplifiers (OPAs) located at distant locations, thereby achieving correlated frequency noise characteristics.

[0039] First Embodiment An optical transmission system 1 according to a first embodiment of the present invention will be described below with reference to FIGS.

[0040] FIG. 1 is a diagram showing wavelength conversion by an optical parametric amplifier (OPA). １ ~E５ represents each channel of a wavelength division multiplexing (WDM) signal.

[0041] When signal light is input into an optical parametric amplifier (OPA), idler light, which is phase conjugate light, is generated at a frequency symmetrical to the degenerate frequency. In other words, the idler light is generated with the spectral arrangement of the input wavelength division multiplexed (WDM) signal reversed. While it is sufficient to extract either one of the band components for the transmission signal, extracting the band component of the idler light makes it possible to perform simultaneous wavelength conversion of wavelength division multiplexed (WDM) signals.

[0042] 2 is a basic configuration diagram of an optical transmission system 1 according to a first embodiment of the present invention. As shown in FIG. 2, the optical transmission system 1 includes an optical transmitter 101, a pumping light source 102, a pumping light branching unit 103, a wavelength converter 104, a pilot light multiplexer 105, an optical transmitter 106, a pilot light separator 107, a pumping light synchronizer 108, a pumping light source 109, a wavelength converter 110, and an optical receiver 111.

[0043] In the optical transmission system 1 of this embodiment, wavelength conversion is performed in the optical transmitting unit 101 and the optical receiving unit 111 .

[0044] The optical transmitter 101 generates a wavelength division multiplexed (WDM) signal in a conventionally used general band (e.g., C band or L band) and outputs the generated wavelength division multiplexed (WDM) signal to the wavelength converter 104.

[0045] The wavelength conversion unit 104 receives an input of a wavelength division multiplexed (WDM) signal output from the optical transmission unit 101. The wavelength conversion unit 104 also receives an input of pumping light output from the pumping light branching unit 103, which will be described later. The wavelength conversion unit 104 uses an optical parametric amplifier (OPA) to convert the wavelength of the wavelength division multiplexed (WDM) signal all at once to a band that cannot be directly generated. At this time, the pumping light used in the optical parametric amplification process is continuous light with a degenerate frequency of the nonlinear medium. The wavelength conversion unit 104 outputs the wavelength-converted wavelength division multiplexed (WDM) signal to the pilot light multiplexing unit 105.

[0046] The pumping light source unit 102 generates pumping light and outputs the pumping light to the pumping light branching unit 103.

[0047] The pumping light branching unit 103 receives the pumping light emitted from the pumping light source unit 102. The pumping light branching unit 103 taps a portion of the received pumping light to generate pilot light. The pumping light branching unit 103 is, for example, a 1:10 optical coupler. The pumping light branching unit 103 outputs the pumping light to the wavelength conversion unit 104. In addition, the pumping light branching unit 103 outputs the pilot light to the pilot light multiplexing unit 105.

[0048] The pilot light multiplexer 105 multiplexes the wavelength-converted wavelength division multiplexed (WDM) signal output from the wavelength converter 104 with the pilot light output from the pumping light splitter 103. The pilot light multiplexer 105 is, for example, a 1:10 optical coupler or a wavelength division multiplexed (WDM) coupler. The pilot light multiplexer 105 outputs the wavelength division multiplexed (WDM) signal multiplexed with the pilot light to the optical transmission unit 106.

[0049] The optical transmission section 106 is an arbitrary transmission path. A wavelength division multiplexed (WDM) signal with pilot light multiplexed therein passes through the optical transmission section 106 from the transmitting side and reaches the receiving side.

[0050] On the receiving side, the pilot light separating unit 107 receives a wavelength division multiplexed (WDM) signal into which pilot light has been multiplexed, which has been transmitted from the transmitting side via the optical transmission unit 106. The pilot light separating unit 107 separates the pilot light from the wavelength division multiplexed (WDM) signal. The pilot light separating unit 107 outputs the separated pilot light to the pump light synchronization unit 108. The pilot light separating unit 107 also outputs the wavelength division multiplexed (WDM) signal to the wavelength conversion unit 110.

[0051] The pumping light synchronization unit 108 receives input of the pilot light output from the pilot light separation unit 107. The pumping light synchronization unit 108 synchronizes the pumping light emitted from a pumping light source unit 109 (described later) with the pilot light. The pumping light synchronization unit 108 is configured using, for example, an optical injection locking circuit, an optical phase locking circuit, etc.

[0052] The pumping light source unit 109 generates pumping light and outputs the pumping light synchronized with the pilot light under the control of the pumping light synchronization unit 108 to the wavelength conversion unit 110.

[0053] The wavelength conversion unit 110 receives the wavelength division multiplexed (WDM) signal output from the pilot light separation unit 107. The wavelength conversion unit 110 also receives pumping light synchronized with the pilot light emitted from the pumping light source unit 109. The wavelength conversion unit 110 wavelength-converts the WDM signal to its original wavelength band using the pumping light synchronized with the pilot light. The wavelength conversion unit 110 outputs the WDM signal wavelength-converted to its original wavelength band to the optical receiving unit 111.

[0054] The optical receiving unit 111 receives the wavelength division multiplexed (WDM) signal output from the wavelength converting unit 110 and having been wavelength converted to the original wavelength band.

[0055] With the above-described configuration, the optical transmission system 1 according to the first embodiment of the present invention can cancel out the phase noise transferred from the pump light to the signal light between the upstream (transmitting side) phase conjugate converter (wavelength converter 104) and the downstream (receiving side) phase conjugate converter (wavelength converter 110). As a result, the optical transmission system 1 according to the first embodiment can suppress excessive degradation of signal quality due to phase conjugate conversion (wavelength conversion).

[0056] Second Embodiment An optical transmission system 1a according to a second embodiment of the present invention will now be described with reference to FIG.

[0057] 3 is a basic configuration diagram of an optical transmission system 1a according to a second embodiment of the present invention. As shown in Fig. 3, the optical transmission system 1a includes an optical transmitter 101, a pumping light source 102, a pumping light branching unit 103, a wavelength converter 104, a pilot light multiplexer 105, an optical transmitter 106, a pilot light separator 107, a pumping light synchronizer 108, a pumping light source 109, a wavelength converter 110, an optical receiver 111, and a delay adjuster 121.

[0058] The configuration of the optical transmission system 1a in the second embodiment is a configuration in which a delay adjustment unit 121 is further added to the configuration of the optical transmission system 1 in the first embodiment described above. The following description will focus on the differences from the configuration of the optical transmission system 1 in the first embodiment.

[0059] Whether or not the correlated pump light cancels out the FM (Frequency Modulation) noise depends on the delay difference between the pump light phases imparted by the two wavelength conversion units (i.e., wavelength conversion unit 104 and wavelength conversion unit 110) on the transmitting and receiving sides.

[0060] The pump light phase imparted to the signal light by the wavelength conversion unit 104 on the transmitting side rotates as it passes through the optical transmission unit 106, etc. The pilot light is also transmitted in the same manner as the signal light, and therefore its phase rotates as it passes through the optical transmission unit 106, etc. The pump light source unit 109 on the receiving side performs phase synchronization using the pilot light having a phase rotation of the same propagation amount as the pump light phase of this signal light, thereby generating pump light having a phase rotation of the same propagation amount as the pump light phase of the signal light.

[0061] On the other hand, the pump light phase component and the pilot light phase component added to the signal light by the wavelength converter 104 on the transmitting side have a propagation time offset corresponding to the delay difference between the path from the pump light branching unit 103 to the wavelength converter 104 and the path from the pump light branching unit 103 to the pilot light multiplexer 105. Similarly, on the receiving side, a propagation time offset corresponding to the delay difference between the path from the pilot light separator 107 to the pump light synchronizer 108 and the path from the pilot light separator 107 to the wavelength converter 110 has occurred.

[0062] As these offsets increase, it becomes difficult to cancel out the high-frequency (fast-changing) FM noise components. To completely cancel out the FM noise, this offset needs to be zero. For this reason, the optical transmission system 1a of the second embodiment includes a delay adjustment unit 121 midway along the path from the pumping light source unit 109 to the wavelength conversion unit 110.

[0063] The delay adjustment unit 121 suppresses the above-described offset in propagation time. The delay adjustment unit 121 can impart any delay to the pump light by using an optical fiber, a spatial optical system, or the like. By having such a configuration, the optical transmission system 1a according to the second embodiment of the present invention can cancel out FM noise with higher accuracy.

[0064] Third Embodiment An optical transmission system 1b according to a third embodiment of the present invention will now be described with reference to FIG.

[0065] 4 is a basic configuration diagram of an optical transmission system 1b according to a third embodiment of the present invention. As shown in Fig. 4, the optical transmission system 1b includes an optical transmitter 101, a pilot light multiplexer 105, N optical transmitters 106, N pilot light separators 107, N pump light synchronizers 108, N pump light source units 109, an optical receiver 111, a pilot light source unit 131, N pump light branchers 132, N phase conjugate converters 133, and N pilot light multiplexers 134.

[0066] In the third embodiment, it is assumed that optical phase conjugate conversion (OPC) is applied to signals in an optical node device in an amplifier-repeater transmission system. The optical node device here refers to a device including a set of functional units, namely, a pilot light separation unit 107, a pumping light synchronization unit 108, a pumping light source unit 109, a pumping light branching unit 132, a phase conjugate conversion unit 133, and a pilot light multiplexing unit 134, as shown in FIG.

[0067] The optical transmitting unit 101 generates an optical signal and outputs the generated optical signal to the pilot optical multiplexing unit 105.

[0068] The pilot light source section 131 outputs, as pilot light, continuous light having a wavelength that is the degenerate wavelength of a nonlinear optical medium used in the phase conjugate conversion section 133 (described later), to the pilot light multiplexing section 105 .

[0069] The pilot light multiplexing unit 105 receives an input of an optical signal output from the optical transmitting unit 101. The pilot light multiplexing unit 105 also receives pilot light output from the pilot light source unit 131. The pilot light multiplexing unit 105 multiplexes the optical signal output from the optical transmitting unit 101 with the pilot light output from the pilot light source unit 131. The pilot light multiplexing unit 105 outputs the optical signal multiplexed with the pilot light to the optical transmitting unit 106.

[0070] The optical transmission unit 106 is an arbitrary transmission path. The optical signal with the pilot light multiplexed therein passes through the optical transmission unit 106 and reaches the pilot light separation unit 107 of one of the N optical node devices.

[0071] Here, as in the first embodiment described above, the optical transmission system 1b performs phase synchronization of the pump light using pilot light separated from the optical signal and performs optical phase conjugate conversion (OPC). The optical transmission system 1b branches a part of the pump light to generate pilot light, which is then multiplexed with the signal light after optical phase conjugate conversion (OPC). The optical signal that has passed through the path between the optical node device that performs optical phase conjugate conversion (OPC) and the optical transmission unit 106 an arbitrary number of times (N times) is received by the optical receiving unit 111 on the receiving side.

[0072] The pilot light separating unit 107 receives the optical signal into which the pilot light is multiplexed, transmitted via the optical transmission unit 106. The pilot light separating unit 107 separates the pilot light from the optical signal. The pilot light separating unit 107 outputs the separated pilot light to the pump light synchronization unit 108. The pilot light separating unit 107 also outputs the optical signal to the phase conjugate conversion unit 133.

[0073] The pumping light synchronization unit 108 receives input of the pilot light output from the pilot light separation unit 107. The pumping light synchronization unit 108 synchronizes the pumping light emitted from a pumping light source unit 109 (described later) with the pilot light. The pumping light synchronization unit 108 is configured using, for example, an optical injection locking circuit, an optical phase locking circuit, etc.

[0074] The pumping light source unit 109 generates pumping light and outputs the pumping light synchronized with the pilot light under the control of the pumping light synchronization unit 108 to the pumping light branching unit 132.

[0075] The pumping light branching unit 132 receives the pumping light emitted from the pumping light source unit 109. The pumping light branching unit 132 taps a portion of the received pumping light to generate pilot light. The pumping light branching unit 132 is, for example, a 1:10 optical coupler. The pumping light branching unit 132 outputs the pumping light to the phase conjugate conversion unit 133. In addition, the pumping light branching unit 132 outputs the pilot light to the pilot light multiplexing unit 134.

[0076] The phase conjugate converter 133 receives the optical signal output from the pilot light separator 107. The phase conjugate converter 133 also receives the pumping light synchronized with the pilot light output from the pumping light brancher 132. The phase conjugate converter 133 wavelength-converts the optical signal to its original wavelength band using the pumping light synchronized with the pilot light. The phase conjugate converter 133 outputs the optical signal wavelength-converted to its original wavelength band to the pilot light combiner 134.

[0077] The pilot light multiplexing unit 134 receives input of the optical signal that has been wavelength-converted to the original wavelength band and that has been output from the phase conjugate conversion unit 133. The pilot light multiplexing unit 134 also receives input of the pilot light that has been output from the pumping light branching unit 132. The pilot light multiplexing unit 134 multiplexes the optical signal that has been output from the phase conjugate conversion unit 133 with the pilot light that has been emitted from the pumping light branching unit 132. The pilot light multiplexing unit 134 outputs the optical signal that has been multiplexed with the pilot light to the optical transmission unit 106.

[0078] The optical signal passes through the optical transmission unit 106 and an optical node device that performs optical phase conjugation (OPC) an arbitrary number of times (N times), and then reaches the receiving-side optical receiving unit 111. The optical receiving unit 111 receives the optical signal that has been wavelength-converted to the original wavelength band and output from the pilot optical multiplexing unit 134 of one of the N optical node devices.

[0079] With the above-described configuration, the optical transmission system 1b according to the third embodiment of the present invention can cancel out the phase noise transferred from the pump light to the signal light between the phase conjugate converter 133 of the upstream (previous stage) optical node device and the phase conjugate converter 133 of the downstream (next stage) optical node device. As a result, the optical transmission system 1b according to the third embodiment can suppress excessive deterioration of signal quality due to phase conjugate conversion (wavelength conversion).

[0080] Fourth Embodiment An optical transmission system 1c according to a fourth embodiment of the present invention will now be described with reference to FIG.

[0081] 5 is a basic configuration diagram of an optical transmission system 1c according to a fourth embodiment of the present invention. As shown in Fig. 5, the optical transmission system 1c includes an optical transmitter 101, a pumping light source 102, a pumping light branching unit 103, a wavelength converter 104, a pilot light multiplexer 105, an optical transmitter 106, a pilot light separator 107, a pumping light synchronizer 108, a pumping light source 109, a wavelength converter 110, an optical receiver 111, a delay adjuster 121, and a delay adjuster 141.

[0082] The configuration of the optical transmission system 1c in the fourth embodiment is a configuration in which a delay adjustment unit 141 is further added to the configuration of the optical transmission system 1b in the third embodiment described above. The following description will focus on the differences from the configuration of the optical transmission system 1b in the third embodiment.

[0083] Whether or not FM noise is cancelled out by correlated pump light depends on the delay difference in the pump light phase imparted by the phase conjugate conversion unit 133 of the upstream (previous stage) optical node device and the phase conjugate conversion unit 133 of the downstream (next stage) optical node device.

[0084] The pump light phase imparted to the signal light by the phase conjugate converter 133 of the upstream optical node device rotates in phase as it passes through the optical transmission unit 106, etc. The pilot light is also transmitted in the same manner as the signal light, and therefore its phase rotates as it passes through the optical transmission unit 106, etc. The pump light source unit 109 of the downstream optical node device performs phase synchronization using the pilot light having a phase rotation of the same propagation amount as the pump light phase of this signal light, thereby being able to generate pump light having a phase rotation of the same propagation amount as the pump light phase of the signal light.

[0085] On the other hand, the pump light phase component and the phase component of the pilot light imparted to the signal light by the phase conjugate converter 133 of the upstream optical node device have a propagation time offset equivalent to the delay difference between the path from the pump light branching unit 132 to the phase conjugate converter 133 and the path from the pump light branching unit 132 to the pilot light multiplexing unit 134. Similarly, in the downstream optical node device, the pump light phase component and the phase component of the pilot light imparted to the signal light by the phase conjugate converter 133 of the downstream optical node device have a propagation time offset equivalent to the delay difference between the path from the pump light branching unit 132 to the phase conjugate converter 133 and the path from the pump light branching unit 132 to the pilot light multiplexing unit 134.

[0086] As these offsets increase, it becomes difficult to cancel out high-frequency (fast-changing) FM noise components. To completely cancel out FM noise, this offset must be zero. For this reason, the optical transmission system 1c of the fourth embodiment includes a delay adjustment unit 141 midway along the path from the pumping light source unit 109 to the pumping light branching unit 132.

[0087] The delay adjustment unit 141 suppresses the above-described offset in propagation time. The delay adjustment unit 141 can impart any delay to the pump light by using an optical fiber, a spatial optical system, or the like. With this configuration, the optical transmission system 1c according to the fourth embodiment of the present invention can cancel out FM noise with higher accuracy.

[0088] Fifth Embodiment An optical transmission system 1d according to a fifth embodiment of the present invention will now be described with reference to FIG.

[0089] 6 is a basic configuration diagram of an optical transmission system 1d according to a fifth embodiment of the present invention. As shown in Fig. 6, the optical transmission system 1d includes an optical transmitter 101, a pumping light source 102, a pumping light branching unit 103, a wavelength converter 104, a pilot light multiplexer 105, an optical transmitter 106, a pilot light separator 107, a pumping light synchronizer 108, a pumping light source 109, a wavelength converter 110, an optical receiver 111, a delay adjuster 121, a delay adjuster 141, and an optical amplifier 151.

[0090] The configuration of the optical transmission system 1d in the fifth embodiment is the same as that of the optical transmission system 1c in the fourth embodiment, except that an optical amplifier 151 is further added. The following description will focus on the differences from the configuration of the optical transmission system 1c in the fourth embodiment.

[0091] In optical phase conjugation (OPC) and wavelength conversion using an optical parametric amplifier (OPA), amplification gain occurs along with the conversion. Therefore, the optical phase conjugation unit 133 can also function as an optical amplifier. On the other hand, if the amplification gain associated with the conversion is insufficient for amplification relay, further optical amplification is required. Furthermore, if the required output power is too high, signal distortion due to gain saturation may occur. Therefore, there may be cases where operation at a lower gain is necessary.

[0092] 6, the optical transmission system 1d according to the fifth embodiment includes an optical amplifier 151 in the subsequent stage of the optical phase conjugate converter 133 of each optical node device. This allows the optical transmission system 1d to have a configuration that compensates for the gain and output power described above. From the perspective of increasing the output power, it is desirable that the optical amplifier used in the optical amplifier 151 be a high-output, highly linear optical amplifier, such as an erbium-doped optical fiber amplifier (EDFA).

[0093] Sixth Embodiment An optical transmission system 1e according to a sixth embodiment of the present invention will now be described with reference to FIG.

[0094] 7 is a basic configuration diagram of an optical transmission system 1e according to a sixth embodiment of the present invention. As shown in Fig. 7, the optical transmission system 1e includes an optical transmitter 101, a pumping light source 102, a pumping light branching unit 103, a wavelength converter 104, a pilot light multiplexer 105, an optical transmitter 106, a pilot light demultiplexer 107, a pumping light synchronizer 108, a pumping light source 109, a wavelength converter 110, an optical receiver 111, a delay adjuster 121, a delay adjuster 141, an optical amplifier 151, and an optical amplifier 161.

[0095] The configuration of the optical transmission system 1e in the sixth embodiment is the same as that of the optical transmission system 1d in the fifth embodiment, except that an optical amplifier 161 is further added. The following description will focus on the differences from the configuration of the optical transmission system 1d in the fifth embodiment.

[0096] If the optical power of the converted optical signal becomes smaller than the optical power of the original input optical signal due to low conversion efficiency of the optical phase conjugate converter 133, excessive degradation of the optical signal-to-noise ratio (OSNR) occurs. In order to avoid excessive degradation, it is desirable to amplify the optical signal in advance before conversion.

[0097] 7, the optical transmission system 1e according to the sixth embodiment further includes an optical amplifier 161 in front of the phase conjugate converter 133 of each optical node device. This prevents the optical power of the optical signal output from the phase conjugate converter 133 from becoming weaker, and prevents excessive deterioration of the optical signal-to-noise ratio (OSNR).

[0098] As described above, the optical transmission system in each of the above-described embodiments of the present invention transmits pilot light together with signal light and synchronizes the pilot light with the phase of the pump light, thereby canceling out the transfer of phase noise from the pump light to the wavelength-converted light and suppressing excessive degradation of signal quality due to phase conjugation and wavelength conversion.

[0099] According to the above-described embodiment, the optical transmission system includes a plurality of phase conjugate converters, a plurality of pumping light source units, an optical transmission unit, and a pumping light synchronization unit. For example, the plurality of phase conjugate converters are the wavelength conversion unit 104 and the wavelength conversion unit 110 or the plurality of phase conjugate converters 133 in the embodiments, the plurality of pumping light source units are the pumping light source unit 102 and the pumping light source unit 109 or the plurality of pumping light source units 109 in the embodiments, the optical transmission unit is the optical transmission unit 106 in the embodiments, and the pumping light synchronization unit is the pumping light synchronization unit 108 in the embodiments.

[0100] The phase conjugate converter rotates the phase of the optical signal through an optical parametric amplification process. The pumping light source unit is provided for each phase conjugate converter and generates pumping light used in the optical parametric amplification process. The optical transmission unit transmits pilot light, which is part of the pumping light generated by the upstream pumping light source unit, to the downstream pumping light source unit along with the optical signal. For example, the upstream pumping light source unit and the downstream pumping light source unit are the wavelength conversion unit 104 and the wavelength conversion unit 110 in the embodiment, or the upstream phase conjugate converter unit 133 and the downstream phase conjugate converter unit 133. The pumping light synchronization unit phase-synchronizes with the pilot light transmitted from the upstream pumping light source unit and causes the downstream pumping light source unit to generate pumping light.

[0101] In the above optical transmission system, the phase conjugate converter may include at least one of a rare-earth doped fiber amplifier, a Raman amplifier, a semiconductor optical amplifier, and an optical parametric amplifier.

[0102] The optical transmission system may further include a delay adjustment unit. For example, the delay adjustment unit is the delay adjustment unit 121 or 141 in the embodiment. The pumping light source unit adjusts the propagation delay of the pumping light based on the path length between the pumping light source unit and a phase conjugate conversion unit that uses the pumping light generated by the pumping light source unit.

[0103] The optical transmission system may further include an optical amplifier at least in one of the upstream and downstream stages of the optical phase conjugate converter. For example, the optical amplifier is the optical amplifier 151 or 161 in the embodiments. The optical amplifier amplifies the optical signal.

[0104] Based on the above-described embodiment, the optical node device of the present invention can be configured as follows.

[0105] (1) An optical transmission system for transmitting an optical signal, comprising: an optical transmitter that generates the optical signal; a plurality of phase conjugate converters that phase rotate the optical signal through an optical parametric amplification process; a pilot light generator that generates continuous light of a degenerate wavelength in the optical parametric amplification process and uses the continuous light as pilot light; and a phase synchronization unit that phase-synchronizes the pilot light with pump light used in the optical parametric amplification process.

[0106] (2) The optical transmission system according to (1), wherein the phase conjugate conversion unit includes one or more optical amplification units using a rare-earth doped fiber amplifier, a Raman amplifier, a semiconductor optical amplifier, or an optical parametric amplifier, or a plurality of these.

[0107] (3) The optical transmission system according to (1), wherein the phase conjugate conversion unit includes a delay adjustment unit that adjusts an optical path length between the pump light and the nonlinear optical medium that generates the optical parametric amplification.

[0108] (4) The optical transmission system according to (1), further comprising an optical amplifier section before or after the optical phase conjugate converter section, or before and after the optical amplifier section.

[0109] (5) The optical transmission system according to (1), wherein the optical amplifying section uses a rare-earth doped fiber amplifier, a Raman amplifier, a semiconductor optical amplifier, or an optical parametric amplifier, or a plurality of these.

[0110] A portion of the configuration of the optical node device in the above-described embodiments may be implemented by a computer. In this case, a program for implementing this function may be recorded on a computer-readable recording medium, and the program may be loaded into a computer system and executed. Note that the term "computer system" herein includes hardware such as an OS and peripheral devices. Furthermore, "computer-readable recording medium" refers to portable media such as flexible disks, optical magnetic disks, ROMs, and CD-ROMs, as well as storage devices such as hard disks built into computer systems. Furthermore, "computer-readable recording medium" may also include devices that dynamically store programs for a short period of time, such as communication lines used when transmitting programs via networks such as the Internet or communication lines such as telephone lines, or devices that store programs for a certain period of time, such as volatile memory within the computer system that serves as the server or client. The program may be for implementing a portion of the above-described functions, or may be capable of implementing the above-described functions in combination with a program already recorded in the computer system, or may be implemented using a programmable logic device such as an FPGA (Field Programmable Gate Array).

[0111] Although an embodiment of the present invention has been described above in detail with reference to the drawings, the specific configuration is not limited to this embodiment, and includes designs within the scope of the gist of the present invention.

[0112] 1, 1a to 1e...optical transmission system, 101...optical transmitting section, 102...pumping light source section, 103...pumping light branching section, 104...wavelength converting section, 105...pilot light multiplexing section, 106...optical transmitting section, 107...pilot light separating section, 108...pumping light synchronizing section, 109...pumping light source section, 110...wavelength converting section, 111...optical receiving section, 121...delay adjusting section, 131...pilot light source section, 132...pumping light branching section, 133...phase conjugate converting section, 134...pilot light multiplexing section, 141...delay adjusting section, 151...optical amplifying section, 161...optical amplifying section

Claims

1. An optical transmission system comprising: a plurality of phase conjugate conversion units which rotate the phase of an optical signal through an optical parametric amplification process; a plurality of pumping light source units which are provided for each of the phase conjugate conversion units and generate pumping light used in the optical parametric amplification process; an optical transmission unit which transmits pilot light which is a part of the pumping light generated by the upstream pumping light source unit to the downstream pumping light source unit together with the optical signal; and a pumping light synchronization unit which causes the downstream pumping light source unit to generate the pumping light by phase synchronizing with the pilot light transmitted from the upstream pumping light source unit.

2. The optical transmission system according to claim 1, wherein the phase conjugate conversion section includes at least one of a rare-earth doped fiber amplifier, a Raman amplifier, a semiconductor optical amplifier, and an optical parametric amplifier.

3. The optical transmission system according to claim 1, wherein the phase conjugate conversion section further comprises a delay adjustment section that adjusts the propagation delay of the pumping light based on the path length between the pumping light source section and the phase conjugate conversion section that uses the pumping light generated by the pumping light source section.

4. The optical transmission system according to claim 1, further comprising an optical amplifier for amplifying said optical signal, at least either before or after said optical phase conjugate converter.

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