WO2022085075A1 - Light transport system, light amplification method and computer program - Google Patents

Light transport system, light amplification method and computer program Download PDF

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Publication number
WO2022085075A1
WO2022085075A1 PCT/JP2020/039400 JP2020039400W WO2022085075A1 WO 2022085075 A1 WO2022085075 A1 WO 2022085075A1 JP 2020039400 W JP2020039400 W JP 2020039400W WO 2022085075 A1 WO2022085075 A1 WO 2022085075A1
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optical
signal
phase
light
amplification
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PCT/JP2020/039400
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French (fr)
Japanese (ja)
Inventor
暁 川合
新平 清水
孝行 小林
拓志 風間
毅伺 梅木
裕 宮本
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日本電信電話株式会社
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Priority to PCT/JP2020/039400 priority Critical patent/WO2022085075A1/en
Priority to JP2022556858A priority patent/JPWO2022085075A1/ja
Publication of WO2022085075A1 publication Critical patent/WO2022085075A1/en

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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/25—Arrangements specific to fibre transmission
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/50—Transmitters
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/29—Repeaters
    • H04B10/291—Repeaters in which processing or amplification is carried out without conversion of the main signal from optical form

Definitions

  • the present invention relates to an optical transmission system, an optical amplification method and a computer program.
  • the main cause of noise in a transmission system using an optical amplifier is noise derived from naturally emitted light (ASE: Amplified Spontaneous Emission) generated by the optical amplifier.
  • the reduction of noise contributes to the increase of OSNR of the system and the increase of communication capacity.
  • EDFA Erbium Doped Fiber Amplifier
  • PIA phase insensitive amplifier
  • PSA Phase Sensitive Amplifier
  • PPA Phase Sensitive Amplifier
  • OPA is an amplifier that performs optical amplification using a non-linear optical effect, and uses a highly non-linear fiber, which is a third-order nonlinear medium, or periodic polarization inversion lithium niobate (PPLN: Periodically), which is a second-order nonlinear medium.
  • PPLN periodic polarization inversion lithium niobate
  • the one using poled lithium niobate) has been realized so far.
  • phase-coupled light of input light called idler light is generated at the time of amplification. When this idler light is generated at the same frequency as the original input light, the above phase-sensitive amplification operation is realized.
  • the channel to be amplified is arranged at the degenerate frequency of the PSA.
  • the degenerate PSA idler light is generated by the interaction between the optical signal and the excitation light in the non-linear medium, and the amplification action is obtained by superimposing the optical signal and the generated idler light.
  • the input light is arranged at the fundamental frequency of the amplification medium (the center frequency of the phase matching condition of the nonlinear optical effect), and the idler light is generated at the same frequency in the amplification medium.
  • the degenerate PSA also has the following problems.
  • ND-PSA Non-Degenerate Phase Sensitive Amplification
  • the optical signal is arranged at a frequency different from the fundamental frequency of the amplification medium.
  • the ND-PSA has a configuration in which an optical signal is arranged at a frequency shifted from the degenerate frequency of the PSA in order to amplify a wavelength division multiplexing signal or a high multivalued signal.
  • an optical signal and an idler light having a complex amplitude coupled to the optical signal are generated at a frequency symmetric with respect to the degenerate frequency, and the transmission fiber is co-propagated. Then, a phase-sensitive amplification operation is obtained by the interaction between three light waves having different frequencies of the optical signal, the frequency of the idler light, and the frequency of the excitation light in the non-linear medium.
  • the idler light is converted into an optical signal frequency by a nonlinear optical effect, and phase-sensitive amplification is realized when the phases of the optical signal and the idler light are aligned. Therefore, in order to realize ND-PSA, it is necessary that the relative phase relationships of the three lights of the optical signal, the idler light, and the excitation light are appropriately synchronized. Since the optical signal and the idler light need to have noise components that are uncorrelated with each other, the idler light needs to be generated on the transmitting side and co-propagated with the optical signal.
  • a method called an optical phase-locked loop (PLL) or a method called injection synchronization is used.
  • PLL optical phase-locked loop
  • injection synchronization is used.
  • the generation of idler light a method of incorporating an OPA different from PSA into the transmission system to optically generate idler light is common.
  • an object of the present invention is to provide a technique capable of realizing non-degenerate phase-sensitive amplification with a simpler configuration.
  • One aspect of the present invention is a signal generation unit that generates a baseband signal used for modulation of the laser light by using a light source that outputs a single laser light, a digital signal to be transmitted, and the signal generation unit.
  • An optical modulator that modulates the laser light using the baseband signal generated by the above to generate an optical signal having a conjugate phase correlation centered on the fundamental frequency of the phase-sensitive amplifier and capable of phase-sensitive amplification.
  • An optical transmission system including an optical amplification unit that performs phase-sensitive amplification by a non-linear optical effect on the optical signal.
  • the digital signal to be transmitted is used to generate a baseband signal used for modulating the laser beam output from a light source that outputs a single laser beam, and the generated baseband is generated.
  • the laser beam is modulated using a signal to generate an optical signal having a conjugate phase correlation centered on the fundamental frequency of the phase-sensitive amplifier and capable of phase-sensitive amplification, and a nonlinear optical effect on the optical signal.
  • This is an optical amplification method for performing phase-sensitive amplification by.
  • One aspect of the present invention is a signal generation step of using a digital signal to be transmitted to generate a baseband signal used for modulation of the laser light output from a light source that outputs a single laser light, and the signal.
  • Light that modulates the laser beam with the baseband signal generated in the generation step to generate an optical signal that has a conjugate phase correlation around the fundamental frequency of the phase sensitive amplifier and is phase sensitive and amplifyable.
  • It is a computer program for causing a computer to execute a modulation step and an optical amplification step of performing phase-sensitive amplification by a nonlinear optical effect on the optical signal.
  • FIG. 1 is a system configuration diagram of the optical transmission system 100 according to the first embodiment.
  • the optical transmission system 100 includes an optical transmitter 10, an optical receiver 20, and an optical amplifier 30.
  • the optical transmitter 10 and the optical receiver 20 communicate with each other via the transmission line 40.
  • the transmission line 40 is a transmission line for transmitting an optical signal transmitted and received between the optical transmitter 10 and the optical receiver 20.
  • the transmission line 40 is, for example, an optical fiber.
  • An optical amplifier 30 is provided on the transmission line 40 between the optical transmitter 10 and the optical receiver 20.
  • the optical transmitter 10 generates a multi-carrier optical signal having a spectrum having a frequency arrangement symmetrical with respect to the frequency of the light source and a phase conjugate relationship.
  • the optical receiver 20 receives an optical signal transmitted from the optical transmitter 10 and amplified by the optical amplifier 30.
  • the optical amplifier 30 amplifies the optical signal transmitted from the optical transmitter 10.
  • the optical amplifier 30 is a phase-sensitive amplifier that performs phase-sensitive amplification by a nonlinear optical effect.
  • the optical amplifier 30 is, for example, a PSA. Further, the optical amplifier 30 has a function of compensating for wavelength dispersion.
  • the optical transmitter 10 includes a light source 11, a signal generation unit 12, an optical modulation unit 13, and a transmission unit 14.
  • the light source 11 continuously outputs a single laser beam.
  • a single laser beam means a laser beam having one wavelength.
  • the laser light output by the light source 11 will be referred to as continuous light.
  • the signal generation unit 12 generates a baseband signal used for modulation of continuous light output from the light source 11 by using a digital signal to be transmitted (hereinafter referred to as “transmission data”).
  • the signal generation unit 12 is composed of a multiplication unit 121 and an addition unit 122.
  • the multiplication unit 121 multiplies the transmission data with a complex number (exp (j ⁇ t)).
  • the addition unit 122 adds the multiplication result of the multiplication unit 121 and the complex conjugate of the multiplication result.
  • the signal generation unit 12 generates a baseband signal by performing such processing.
  • the optical modulation unit 13 modulates the continuous light output from the light source 11 using the baseband signal generated by the signal generation unit 12, has a coupled phase correlation centered on the basic frequency of the optical amplifier 30, and has a coupled phase correlation. , Generates an optical signal that can be phase-sensitive and amplified.
  • the optical modulation unit 13 is a modulator that performs amplitude modulation.
  • the transmission unit 14 transmits the optical signal generated by the optical modulation unit 13 to the transmission line 40.
  • FIG. 2 is a sequence diagram showing a processing flow of the optical transmission system 100 according to the first embodiment.
  • the process shown in FIG. 2 is executed by inputting transmission data to the optical transmitter 10.
  • the signal generation unit 12 generates a baseband signal using the input transmission data (step S101).
  • step S101 a specific method for generating a baseband signal by the signal generation unit 12 will be described.
  • the signal generation unit 12 generates a baseband signal necessary for simultaneously generating an optical signal and idler light by using a light source 11 that outputs a single continuous light and an optical modulator 12. Therefore, in the first embodiment, a method called subcarrier multiplexing is used for simultaneous generation of an optical signal and idler light (see, for example, Reference 1).
  • the signal generation unit 12 generates a signal of an arbitrary modulation format based on the transmission bit string in the input transmission data. Let this be x (t), and then the signal generation unit 12 generates a baseband signal s (t) by performing an operation based on the following equation (1) for a frequency ⁇ that is more than half of the symbol rate. Note that * in Eq. (1) represents the complex conjugate. (Reference 1: GP Agrawal, “Fiber-optic communication systems”, Vol. 222. John Wiley & Sons, 2012.)
  • the signal generation unit 12 inputs the generated baseband signal s (t) to the optical modulation unit 13.
  • the optical modulation unit 13 modulates the continuous light of the frequency ⁇ c output from the light source 11 with the input baseband signal s (t) (step S102). As a result, the optical modulation unit 13 generates a modulation signal.
  • the frequency ⁇ c is assumed to be equal to the fundamental frequency of the amplification medium (optical amplifier 30). Assuming that the time-varying phase fluctuation of continuous light is ⁇ (t), the electric field of the modulated signal is expressed by the following equation (2).
  • the modulated signal is a multi-carrier optical signal that shares a phase fluctuation and has a spectrum having a frequency arrangement symmetric with respect to the frequency of the light source 11 and a phase conjugate relationship.
  • the modulated signal is a signal in which an optical signal and idler light are subcarrier-multiplexed.
  • the optical modulation unit 13 outputs the generated modulation signal to the transmission line 40 via the transmission unit 14 (step S103).
  • the modulated signal output to the transmission line 40 is input to the optical amplifier 30.
  • the optical amplifier 30 amplifies the input modulated signal (step S104). For example, the optical amplifier 30 amplifies the modulated signal by the input modulated signal and the excitation light. The optical amplifier 30 outputs the amplified optical signal to the optical receiver 20 (step S105). The optical receiver 20 receives the modulated signal output from the optical amplifier 30 (step S105).
  • the modulation component has no imaginary component. Therefore, even if the original signal is a complex number such as a QAM signal, it can be realized by using a single amplitude modulator instead of the IQ modulator.
  • the signal generation unit 12 can also be realized by an analog circuit.
  • FIG. 3 is a schematic diagram of a demonstration experiment of the method in the first embodiment.
  • FIG. 4 is a diagram showing the results of a demonstration experiment of the method in the first embodiment.
  • the transmitter 200 shown in FIG. 3 corresponds to the optical transmitter 10 of FIG. 1
  • the PSA 400 corresponds to the optical amplifier 30 of FIG.
  • the fundamental frequency is 193 THz.
  • PPLN was used as the amplification medium of PSA400.
  • the optical signal and the excitation light were subjected to carrier recovery using the sum frequency generation described in Reference 2, and phase-locked using the optical PLL. (Reference 2: Y. Okamura, et al. “Optical pump phase locking to a carrier wave extracted from phase-conjugated twin waves for phase-sensitive optical amplifier repeaters”, Optics express 24.23 (2016))
  • a QAM signal in which an optical signal and idler light were subcarrier-multiplexed was generated, noise was added by attenuation, and then the signal was input to the PSA400.
  • a Nyquist filter was applied to the signal, the modulation speed was 10 Gbaud, and the frequency shift amount was 5.5 GHz.
  • the signal component was extracted and demodulated using a bandpass filter on the digital signal processing.
  • FIG. 5A shows the results of a demonstration experiment when PSA is used as an optical amplifier.
  • constellations obtained from 16QAM, 32QAM, and PS-256QAM signals are shown in order from the left.
  • FIG. 5A With reference to the constellation shown in FIG. 5A, it can be seen that amplification operations with less distortion are realized for each of the 16QAM, 32QAM, and PS-256QAM signals.
  • FIG. 5 (B) shows an example in which a similar experiment was performed by replacing PSA with EDFA for comparison with FIG. 5 (A).
  • the same subcarrier multiplex signal is used as the modulation method, and on the receiving side, only the signal component is extracted by a bandpass filter and demodulated.
  • the SNR was 4.6 dB higher on average when PSA was used in all modulation formats than when EDFA was used.
  • there is a wavelength diversity gain of 3 dB by using two waves of optical signal and idler light in principle but further SNR improvement effect can be confirmed, and it is shown that phase sensitive amplification operation is performed. Has been done.
  • the modulation of one laser beam has a phase relationship coupled to the optical signal and the optical signal without using an additional optical system. It can generate idler light. Therefore, it becomes possible to realize a non-degenerate phase-sensitive amplification operation with a simpler configuration of the optical transmitter 10.
  • PSA was used as the optical amplifier 30.
  • carrier recovery of the optical signal is performed by generating a sum frequency as used in the first embodiment, or a carrier component is transmitted as pilot light from the transmitting side as shown in Reference 3.
  • the method can be mentioned. Therefore, in the second embodiment, a configuration using light injection synchronization will be described.
  • FIG. 5 is a system configuration diagram of the optical transmission system 100a according to the second embodiment.
  • the optical transmission system 100a includes an optical transmitter 10a, an optical receiver 20, an optical amplifier 30, a demultiplexer 50, a circulator 60, a slave laser 61, and a combiner 70.
  • the optical amplifier 30, the duplexer 50, the circulator 60, the slave laser 61 and the combiner 70 may be configured as one PSA module.
  • the optical transmitter 10a generates a multi-carrier modulated signal having a spectrum having a frequency arrangement symmetrical with respect to the frequency of the light source and a phase conjugate relationship. At this time, the optical transmitter 10a generates an optical signal having a guard band in the vicinity of the DC component in the frequency domain, and the generated optical signal is provided with pilot light for phase synchronization of the excitation light of the optical amplifier 30 in the vicinity of the DC component.
  • the demultiplexer 50 demultiplexes the pilot light from the optical signal and the idler light in the modulated signal transmitted from the optical transmitter 10a.
  • the demultiplexer 50 outputs a signal obtained by removing the pilot light from the modulated signal (a signal of the demultiplexed optical signal and the idler light) to the first path to which the duplexer 70 is connected, and the demultiplexing pilot. Light is output to the second path to which the circulator 60 is connected.
  • the circulator 60 is a component having a plurality of ports.
  • the example shown in FIG. 5 shows an example in which the circulator 60 has three ports (first port to third port).
  • the first port of the circulator 60 is connected to the demultiplexer 50, the second port is connected to the slave laser 61, and the third port is directly or indirectly connected to the combiner 70.
  • the pilot light demultiplexed by the demultiplexer 50 is input to the first port, and the laser light output from the slave laser 61 is input to the second port.
  • the circulator 60 outputs the pilot light input from the first port to the slave laser 61.
  • the circulator 60 outputs the laser beam input from the second port to the combiner 70.
  • the third port of the circulator 60 and the combiner 70 may be directly connected by a transmission line or may be connected via a ⁇ 2 medium such as a PPLN waveguide.
  • the slave laser 61 outputs a laser beam having a high OSNR fundamental frequency in injection synchronization using pilot light input via the circulator 60.
  • the combiner 70 combines the optical signal input via the first path and the optical signal output from the slave laser 61.
  • the optical signal output from the slave laser 61 is input to the combiner 70 directly or via the ⁇ 2 medium.
  • the optical signal of the fundamental frequency becomes the desired excitation light as it is.
  • desired excitation light can be generated by generating light having a frequency twice the fundamental frequency by using the nonlinear optical effect.
  • FIG. 6 is a diagram showing a specific example of the first configuration of the optical transmitter 10a in the second embodiment
  • FIG. 7 shows a specific example of the second configuration of the optical transmitter 10a in the second embodiment. It is a figure.
  • each configuration will be described.
  • the optical transmitter 10a includes a light source 11, a signal generation unit 12a, an optical modulation unit 13, a transmitter unit 14, a demultiplexer 15, and a combiner 16.
  • the same components as those of the optical transmitter 10 in the first embodiment are designated by the same reference numerals, and the description thereof will be omitted.
  • the signal generation unit 12a generates a multi-carrier optical signal having a spectrum provided with a guard band by sufficiently increasing the frequency ⁇ .
  • the signal generation unit 12a outputs the generated optical signal to the optical modulation unit 13.
  • the demultiplexer 15 is provided between the light source 11 and the optical modulation unit 13, and demultiplexes the continuous light output from the light source 11.
  • the demultiplexer 15 outputs the demultiplexed continuous light to the first path to which the optical modulation unit 13 is connected and the second path to which the duplexer 16 is connected, respectively.
  • the pilot light is output to the combiner 16.
  • the combiner 16 is provided between the optical modulation unit 13 and the transmission unit 14, and combines the modulation signal output from the optical modulation unit 13 with the optical signal branched by the demultiplexer 15. As a result, the optical transmitter 10a can combine the pilot light with the modulated signal.
  • the optical transmitter 10a includes a light source 11, a signal generation unit 12, an optical modulation unit 13a, and a transmission unit 14.
  • the same components as those of the optical transmitter 10 in the first embodiment are designated by the same reference numerals, and the description thereof will be omitted.
  • the optical modulation unit 13a modulates the continuous light output from the light source 11 using the baseband signal generated by the signal generation unit 12, has a coupled phase correlation centered on the basic frequency of the optical amplifier 30, and has a coupled phase correlation. , Generates an optical signal that can be phase-sensitive and amplified.
  • the carrier frequency component remains due to the imperfections of the optical modulation section 13a.
  • the carrier frequency component remaining due to the imperfections of the optical modulation unit 13a is used as the pilot light.
  • the internal light is modulated by applying various voltages to the crystals in the modulator, and the bias voltage (modulation) applied at this time is applied. If the default voltage applied when not done) deviates from the ideal value, carrier frequency will occur.
  • the carrier frequency component thus generated is used as pilot light.
  • Pilot light is prone to unwanted non-linear effects, such as induced Brillouin scattering, in transmission lines or fiber amplifiers when its spectrum is narrow. Therefore, especially when high power is required, the non-linear effect is suppressed by widening the spectrum by some kind of random modulation (for example, high-frequency phase modulation is performed by demultiplexing the pilot light and incident on the phase modulator). Is also possible.
  • the technique of Reference 4 below may be used as a method of suppressing the non-linear effect by widening the spectrum. (Reference 4: Z. Tong, et al. “Towards ultrasensitive optical links enabled by low-noise phase-sensitive amplifiers.” Nature photonics, 5 (7), 430 (2011))
  • optical transmission system 100a in the second embodiment configured as described above, it is possible to support a method using pilot light. Therefore, it becomes possible to improve convenience.
  • the optical signal and the idler light are subcarrier-multiplexed in the baseband signal, it is possible to perform dispersion prediction by digital signal processing over both bands. Therefore, in the third embodiment, a configuration for predicting the wavelength dispersion generated by transmission at the time of optical modulation on the transmitting side will be described.
  • the optical transmission system 100 in the third embodiment is the same as that in the first embodiment except that the optical transmitter 10b is provided in place of the optical transmitter 10.
  • FIG. 8 is a diagram showing the configuration of the optical transmitter 10b according to the third embodiment.
  • the optical transmitter 10b includes a light source 11, a signal generation unit 12b, an optical modulation unit 13b, and a transmission unit 14.
  • the optical transmitter 10b is different from the optical transmitter 10 in that it includes a signal generation unit 12b and an optical modulation unit 13b in place of the signal generation unit 12 and the optical modulation unit 13.
  • the optical transmitter 10b is the same as the optical transmitter 10 in other configurations. Therefore, the description of the entire optical transmitter 10b will be omitted, and the No. generation unit 12b and the optical modulation unit 13b will be described.
  • the signal generation unit 12b uses the transmission data to generate a baseband signal used for modulating the continuous light output from the light source 11.
  • the signal generation unit 12b includes a multiplication unit 121, an addition unit 122, and a dispersion imparting unit 123.
  • the dispersion imparting unit 123 imparts the wavelength dispersion obtained by inverting the sign of the wavelength dispersion generated in the transmission line 40 between the transmission unit 14 and the optical amplifier 30 to the baseband signal generated by the multiplication unit 121 and the addition unit 122. do. In this way, the dispersion imparting unit 123 can generate an optical signal with a phase change that cancels the wavelength dispersion of the transmission path 40 between the transmission unit 14 and the optical amplifier 30.
  • an IQ modulator is required because an imaginary number component remains due to dispersion preequalization.
  • the optical modulation unit 13b included in the optical transmitter 10b is an IQ modulator, and performs modulation by inputting an optical signal of the I component and an optical signal of the Q component calculated from the dispersion imparting unit.
  • the multiplication unit 121 and the addition unit 122 generate a baseband signal in which the optical signal and the idler light are subcarrier-multiplexed.
  • the dispersion imparting unit 123 applies the wavelength dispersion obtained by inverting the sign of the wavelength dispersion generated in the transmission line to the baseband signal. Gives wavelength dispersion.
  • Examples of the method for calculating the wavelength dispersion include an overlap cut method. A specific method of the overlap cut method is described in Reference 5 below. (Reference 5: R. Kudo, T. Kobayashi, K. Ishihara, Y. Takatori, A. Sano, and Y. Miyamoto, “Coherent Optical Single Carrier Transmission Using Overlap Frequency Domain Equalization for Long-Haul Optical Systems,” Journal of Lightwave Technology, 27, 16, 3721 (2009).)
  • the optical modulation unit 13b modulates the continuous light output from the light source 11 with the baseband signal.
  • the wavelength dispersion imparted by the optical transmitter 10b is offset by the wavelength dispersion associated with the transmission. This eliminates the wavelength dispersion at the amplification point and enables ideal PSA operation without the use of optical dispersion compensation.
  • the optical transmitters 10, 10a, 10b may be configured to include an optical amplifier 30.
  • the optical transmitters 10 and 10b may include an optical amplifier 30 after the transmission unit 14 and may be configured as an optical amplifier.
  • the optical transmitter 10a may include a PSA module after the transmission unit 14 and may be configured as an optical amplifier.
  • optical transmitters 10, 10a, 10b in the above-described embodiment may be realized by a computer.
  • the program for realizing this function may be recorded on a computer-readable recording medium, and the program recorded on the recording medium may be read by a computer system and executed.
  • the term "computer system” as used herein includes hardware such as an OS and peripheral devices.
  • the "computer-readable recording medium” refers to a portable medium such as a flexible disk, a magneto-optical disk, a ROM, or a CD-ROM, and a storage device such as a hard disk built in a computer system.
  • a "computer-readable recording medium” is a communication line for transmitting a program via a network such as the Internet or a communication line such as a telephone line, and dynamically holds the program for a short period of time. It may also include a program that holds a program for a certain period of time, such as a volatile memory inside a computer system that is a server or a client in that case. Further, the above program may be for realizing a part of the above-mentioned functions, and may be further realized for realizing the above-mentioned functions in combination with a program already recorded in the computer system. It may be realized by using a programmable logic device such as FPGA (Field Programmable Gate Array).
  • FPGA Field Programmable Gate Array
  • the present invention can be applied to a technique for performing phase-sensitive amplification.

Abstract

A light transport system comprising: a light source that outputs a single laser light; a signal generating unit that generates, by use of a digital signal to be transmitted, a baseband signal to be used for modulating the laser light; a light modulating unit that modulates the laser light by use of the baseband signal generated by the signal generating unit, thereby generating an optical signal having a phasic correlation that is conjugate with the fundamental frequency of a phase sensitive amplifier being centered, said optical signal being phase-sensitive-amplifiable; and a light amplifying unit that implements a phase sensitive amplification using a nonlinear optical effect with respect to the optical signal.

Description

光伝送システム、光増幅方法及びコンピュータプログラムOptical transmission system, optical amplification method and computer program
　本発明は、光伝送システム、光増幅方法及びコンピュータプログラムに関する。 The present invention relates to an optical transmission system, an optical amplification method and a computer program.
　情報技術の発展を背景に、世界の総通信量は拡大を続けており、その基礎となる光ファイバ通信路の通信容量を拡大する技術の需要も増している。近年では、自由空間中を伝搬する光を用いた空間光通信も衛星通信などへの応用を念頭に研究が進んでいる。通信路で送れる容量の上限は受信時の光シグナル対ノイズ比（ＯＳＮＲ：Optical Signal-to-Noise Ratio）に依存するため、通信容量の向上のためにはＯＳＮＲの改善が必要不可欠である。 With the development of information technology, the total amount of communication in the world continues to expand, and the demand for technology to expand the communication capacity of the optical fiber communication line, which is the basis of the expansion, is also increasing. In recent years, research on spatial optical communication using light propagating in free space has been progressing with the application to satellite communication and the like in mind. Since the upper limit of the capacity that can be sent on the communication path depends on the optical signal-to-noise ratio (OSNR) at the time of reception, it is indispensable to improve the OSNR in order to improve the communication capacity.
　光増幅器を用いる伝送システムにおけるノイズの主因は、光増幅器で生み出される自然放出光（ＡＳＥ：Amplified Spontaneous Emission）由来の雑音である。ノイズの低減は、システムのＯＳＮＲ増加、ひいては通信容量増加に寄与する。ただし、現在主に使われているＥＤＦＡ（Erbium Doped Fiber Amplifier）などは位相不感応増幅器（ＰＩＡ：Phase insensitive amplifier）に分類され、原理的に少なくとも雑音指数３ｄＢに相当する雑音が発生することが知られている。この限界に近い光増幅器は既に実用化されており、ＰＩＡにおけるノイズ性能の改善の余地が少ない。 The main cause of noise in a transmission system using an optical amplifier is noise derived from naturally emitted light (ASE: Amplified Spontaneous Emission) generated by the optical amplifier. The reduction of noise contributes to the increase of OSNR of the system and the increase of communication capacity. However, it is known that EDFA (Erbium Doped Fiber Amplifier), which is mainly used at present, is classified as a phase insensitive amplifier (PIA), and in principle, noise corresponding to at least a noise figure of 3 dB is generated. Has been done. Optical amplifiers near this limit have already been put into practical use, and there is little room for improvement in noise performance in PIA.
　従来のＰＩＡの雑音限界を打破する手段として、位相感応増幅器（ＰＳＡ：Phase Sensitive Amplification）が検討されている。ＰＳＡでは、直交する成分のＡＳＥを抑圧することで、従来のＰＩＡが持つ雑音限界以下の低雑音増幅を実現することができる。ＰＳＡは、光パラメトリック増幅器（ＯＰＡ：Optical Parametric Amplification）の特殊なケースであり、入力された光信号（以下「入力光」という。）の片方の位相成分のみを増幅し、それに直交する位相成分を抑圧することによって原理的には雑音指数０ｄＢを達成可能である。 A phase sensitive amplifier (PSA: Phase Sensitive Amplifier) is being studied as a means of breaking the noise limit of the conventional PIA. In PSA, by suppressing the ASE of orthogonal components, it is possible to realize low noise amplification below the noise limit of the conventional PIA. PSA is a special case of optical parametric amplifier (OPA), which amplifies only one phase component of an input optical signal (hereinafter referred to as "input light") and a phase component orthogonal to it. In principle, a noise index of 0 dB can be achieved by suppressing the noise.
　ＯＰＡは、非線形光学効果を利用して光増幅を行う増幅器であり、三次の非線形媒質である高非線形ファイバを用いたものや、二次の非線形媒質である周期分極反転ニオブ酸リチウム（ＰＰＬＮ: Periodically poled lithium niobate）を用いたものがこれまで実現されている。ＯＰＡでは、増幅時にアイドラ光と呼ばれる入力光の位相共役光が発生する。このアイドラ光が元の入力光と同じ周波数に発生する場合に、上記の位相感応増幅動作が実現される。 OPA is an amplifier that performs optical amplification using a non-linear optical effect, and uses a highly non-linear fiber, which is a third-order nonlinear medium, or periodic polarization inversion lithium niobate (PPLN: Periodically), which is a second-order nonlinear medium. The one using poled lithium niobate) has been realized so far. In OPA, phase-coupled light of input light called idler light is generated at the time of amplification. When this idler light is generated at the same frequency as the original input light, the above phase-sensitive amplification operation is realized.
　ＰＳＡの構成の一つとして、増幅するチャネルをＰＳＡの縮退周波数に配置する縮退ＰＳＡがある。縮退ＰＳＡでは、非線形媒質中における光信号と励起光との間の相互作用によってアイドラ光を生成し、光信号と、生成したアイドラ光との重ね合わせで増幅作用を得る。このように縮退ＰＳＡでは、入力光は増幅媒体の基本周波数(非線形光学効果の位相整合条件の中心周波数)に配置され、アイドラ光は増幅媒質中で同周波数に発生する。しかしながら、縮退ＰＳＡでは、次のような課題もある。１つ目は、波長多重信号を増幅する場合に複数の装置でパラレルに増幅する必要がある。２つ目は、ＱＡＭ（quadrature amplitude modulation）信号のような複素平面上の実数軸と虚数軸の両方に信号点の広がりをもつ信号を増幅できない。 As one of the configurations of PSA, there is a degenerate PSA in which the channel to be amplified is arranged at the degenerate frequency of the PSA. In the degenerate PSA, idler light is generated by the interaction between the optical signal and the excitation light in the non-linear medium, and the amplification action is obtained by superimposing the optical signal and the generated idler light. Thus, in the degenerate PSA, the input light is arranged at the fundamental frequency of the amplification medium (the center frequency of the phase matching condition of the nonlinear optical effect), and the idler light is generated at the same frequency in the amplification medium. However, the degenerate PSA also has the following problems. First, when amplifying a wavelength division multiplexing signal, it is necessary to amplify it in parallel by a plurality of devices. Second, it is not possible to amplify a signal having a spread of signal points on both the real and imaginary axes on the complex plane, such as a QAM (quadrature amplitude modulation) signal.
　それに対し、非縮退位相感応増幅器（ＮＤ－ＰＳＡ：Non-Degenerate Phase Sensitive Amplification）と呼ばれる構成は、波長多重信号やＱＡＭ信号も増幅可能という特性から注目を集めている（例えば、特許文献１参照）。ＮＤ－ＰＳＡにおいて光信号は、増幅媒体の基本周波数とは異なる周波数の位置に配置される。例えば、ＮＤ－ＰＳＡでは、波長分割多重信号や高多値信号の増幅のために、ＰＳＡの縮退周波数からずらした周波数の位置に光信号を配置する構成をとる。さらにＮＤ－ＰＳＡでは、光信号と、光信号に共役な複素振幅を持つアイドラ光とを、縮退周波数に対して対称な周波数に生成し、伝送ファイバを共伝搬させる。そして非線形媒質中での光信号の周波数、アイドラ光の周波数及び励起光の周波数が異なる３光波間の相互作用により位相感応増幅動作を得る。 On the other hand, a configuration called a non-degenerate phase sensitive amplifier (ND-PSA: Non-Degenerate Phase Sensitive Amplification) is attracting attention because of its ability to amplify wavelength division multiplexing signals and QAM signals (see, for example, Patent Document 1). .. In the ND-PSA, the optical signal is arranged at a frequency different from the fundamental frequency of the amplification medium. For example, the ND-PSA has a configuration in which an optical signal is arranged at a frequency shifted from the degenerate frequency of the PSA in order to amplify a wavelength division multiplexing signal or a high multivalued signal. Further, in the ND-PSA, an optical signal and an idler light having a complex amplitude coupled to the optical signal are generated at a frequency symmetric with respect to the degenerate frequency, and the transmission fiber is co-propagated. Then, a phase-sensitive amplification operation is obtained by the interaction between three light waves having different frequencies of the optical signal, the frequency of the idler light, and the frequency of the excitation light in the non-linear medium.
　この時、アイドラ光は非線形光学効果によって光信号周波数に変換され、光信号とアイドラ光の位相が揃っている場合に位相感応増幅が実現される。したがって、ＮＤ－ＰＳＡを実現するには、光信号、アイドラ光及び励起光の三つの光の相対位相関係が適切に同期されている必要がある。光信号とアイドラ光は互いに無相関な雑音成分を有する必要があるため、アイドラ光は送信側で生成し、光信号と共伝搬させる必要がある。励起光の位相同期に関しては、光位相同期回路（ＰＬＬ：Phase Locked Loop）と呼ばれる手法や、注入同期と呼ばれる手法が用いられる。アイドラ光の生成に関しては、ＰＳＡとは別のＯＰＡを送信系に組み込み、光学的にアイドラ光を生成する方法が一般的である。 At this time, the idler light is converted into an optical signal frequency by a nonlinear optical effect, and phase-sensitive amplification is realized when the phases of the optical signal and the idler light are aligned. Therefore, in order to realize ND-PSA, it is necessary that the relative phase relationships of the three lights of the optical signal, the idler light, and the excitation light are appropriately synchronized. Since the optical signal and the idler light need to have noise components that are uncorrelated with each other, the idler light needs to be generated on the transmitting side and co-propagated with the optical signal. Regarding the phase synchronization of the excitation light, a method called an optical phase-locked loop (PLL) or a method called injection synchronization is used. Regarding the generation of idler light, a method of incorporating an OPA different from PSA into the transmission system to optically generate idler light is common.
特開２０１５－１６１８２７号公報Japanese Unexamined Patent Publication No. 2015-161827
　前述のようにＮＤ－ＰＳＡでは、位相相関を持った光信号とアイドラ光を用意する必要がある。レーザ光源は位相揺らぎを持つため、光信号とアイドラ光を生成するために別の独立したレーザ光源を用いると時間経過によって位相相関が崩れてしまう。そのため、ＯＰＡによって光信号から光信号と位相同期したアイドラ光を生成するという手順が一般に行われている。しかしながら、この方法は光送信器内に追加の光学系を必要とする。そのため、光送信器の構成が複雑になり、装置のコストが高くなってしまうという問題があった。 As mentioned above, in ND-PSA, it is necessary to prepare an optical signal and idler light having a phase correlation. Since the laser light source has a phase fluctuation, if another independent laser light source is used to generate the optical signal and the idler light, the phase correlation is broken with the passage of time. Therefore, a procedure of generating idler light phase-locked with an optical signal from an optical signal by OPA is generally performed. However, this method requires additional optics within the optical transmitter. Therefore, there is a problem that the configuration of the optical transmitter becomes complicated and the cost of the device increases.
　上記事情に鑑み、本発明は、より簡易な構成で非縮退位相感応増幅を実現することができる技術の提供を目的としている。 In view of the above circumstances, an object of the present invention is to provide a technique capable of realizing non-degenerate phase-sensitive amplification with a simpler configuration.
　本発明の一態様は、単一のレーザ光を出力する光源と、送信対象となるデジタル信号を用いて、前記レーザ光の変調に用いるベースバンド信号を生成する信号生成部と、前記信号生成部によって生成された前記ベースバンド信号を用いて前記レーザ光を変調して、位相感応増幅器の基本周波数を中心として共役な位相相関を持ち、かつ、位相感応増幅可能な光信号を生成する光変調部と、前記光信号に対して非線形光学効果による位相感応増幅を行う光増幅部と、を備える光伝送システムである。 One aspect of the present invention is a signal generation unit that generates a baseband signal used for modulation of the laser light by using a light source that outputs a single laser light, a digital signal to be transmitted, and the signal generation unit. An optical modulator that modulates the laser light using the baseband signal generated by the above to generate an optical signal having a conjugate phase correlation centered on the fundamental frequency of the phase-sensitive amplifier and capable of phase-sensitive amplification. An optical transmission system including an optical amplification unit that performs phase-sensitive amplification by a non-linear optical effect on the optical signal.
　本発明の一態様は、送信対象となるデジタル信号を用いて、単一のレーザ光を出力する光源から出力される前記レーザ光の変調に用いるベースバンド信号を生成し、生成された前記ベースバンド信号を用いて前記レーザ光を変調して、位相感応増幅器の基本周波数を中心として共役な位相相関を持ち、かつ、位相感応増幅可能な光信号を生成し、前記光信号に対して非線形光学効果による位相感応増幅を行う光増幅方法である。 In one aspect of the present invention, the digital signal to be transmitted is used to generate a baseband signal used for modulating the laser beam output from a light source that outputs a single laser beam, and the generated baseband is generated. The laser beam is modulated using a signal to generate an optical signal having a conjugate phase correlation centered on the fundamental frequency of the phase-sensitive amplifier and capable of phase-sensitive amplification, and a nonlinear optical effect on the optical signal. This is an optical amplification method for performing phase-sensitive amplification by.
　本発明の一態様は、送信対象となるデジタル信号を用いて、単一のレーザ光を出力する光源から出力される前記レーザ光の変調に用いるベースバンド信号を生成する信号生成ステップと、前記信号生成ステップにおいて生成された前記ベースバンド信号を用いて前記レーザ光を変調して、位相感応増幅器の基本周波数を中心として共役な位相相関を持ち、かつ、位相感応増幅可能な光信号を生成する光変調ステップと、前記光信号に対して非線形光学効果による位相感応増幅を行う光増幅ステップと、をコンピュータに実行させるためのコンピュータプログラムである。 One aspect of the present invention is a signal generation step of using a digital signal to be transmitted to generate a baseband signal used for modulation of the laser light output from a light source that outputs a single laser light, and the signal. Light that modulates the laser beam with the baseband signal generated in the generation step to generate an optical signal that has a conjugate phase correlation around the fundamental frequency of the phase sensitive amplifier and is phase sensitive and amplifyable. It is a computer program for causing a computer to execute a modulation step and an optical amplification step of performing phase-sensitive amplification by a nonlinear optical effect on the optical signal.
　本発明により、より簡易な構成で非縮退位相感応増幅を実現することが可能となる。 According to the present invention, it is possible to realize non-degenerate phase-sensitive amplification with a simpler configuration.
第１の実施形態における光伝送システムのシステム構成図である。It is a system block diagram of the optical transmission system in 1st Embodiment. 第１の実施形態における光伝送システムの処理の流れを表すシーケンス図である。It is a sequence diagram which shows the processing flow of the optical transmission system in 1st Embodiment. 第１の実施形態における手法の実証実験の概要図である。It is a schematic diagram of the demonstration experiment of the method in 1st Embodiment. 第１の実施形態における手法の実証実験の結果を示す図である。It is a figure which shows the result of the proof experiment of the method in 1st Embodiment. 第２の実施形態における光伝送システムのシステム構成図である。It is a system block diagram of the optical transmission system in 2nd Embodiment. 第２の実施形態における光送信器の第１の構成の具体例を示す図である。It is a figure which shows the specific example of the 1st structure of the optical transmitter in 2nd Embodiment. 第２の実施形態における光送信器の第２の構成の具体例を示す図である。It is a figure which shows the specific example of the 2nd configuration of the optical transmitter in 2nd Embodiment. 第３の実施形態における光送信器の構成を表す図である。It is a figure which shows the structure of the optical transmitter in 3rd Embodiment.
　以下、本発明の一実施形態を、図面を参照しながら説明する。
（第１の実施形態）
　図１は、第１の実施形態における光伝送システム１００のシステム構成図である。光伝送システム１００は、光送信器１０、光受信器２０及び光増幅器３０を備える。光送信器１０と光受信器２０とは、伝送路４０を介して通信を行う。伝送路４０は、光送信器１０と光受信器２０との間で送受信される光信号を伝送するための伝送路である。伝送路４０は、例えば光ファイバである。光送信器１０と光受信器２０との間の伝送路４０上には、光増幅器３０が設けられる。 Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
(First Embodiment)
FIG. 1 is a system configuration diagram of the optical transmission system 100 according to the first embodiment. The optical transmission system 100 includes an optical transmitter 10, an optical receiver 20, and an optical amplifier 30. The optical transmitter 10 and the optical receiver 20 communicate with each other via the transmission line 40. The transmission line 40 is a transmission line for transmitting an optical signal transmitted and received between the optical transmitter 10 and the optical receiver 20. The transmission line 40 is, for example, an optical fiber. An optical amplifier 30 is provided on the transmission line 40 between the optical transmitter 10 and the optical receiver 20.
　光送信器１０は、光源の周波数に対して対称な周波数配置かつ位相共役関係にあるスペクトルを持つマルチキャリアの光信号を生成する。 The optical transmitter 10 generates a multi-carrier optical signal having a spectrum having a frequency arrangement symmetrical with respect to the frequency of the light source and a phase conjugate relationship.
　光受信器２０は、光送信器１０から送信されて光増幅器３０によって増幅された光信号を受信する。 The optical receiver 20 receives an optical signal transmitted from the optical transmitter 10 and amplified by the optical amplifier 30.
　光増幅器３０は、光送信器１０から送信された光信号を増幅する。光増幅器３０は、非線形光学効果による位相感応増幅を行う位相感応増幅器である。光増幅器３０は、例えばＰＳＡである。さらに、光増幅器３０は、波長分散を補償する機能を有する。 The optical amplifier 30 amplifies the optical signal transmitted from the optical transmitter 10. The optical amplifier 30 is a phase-sensitive amplifier that performs phase-sensitive amplification by a nonlinear optical effect. The optical amplifier 30 is, for example, a PSA. Further, the optical amplifier 30 has a function of compensating for wavelength dispersion.
　次に、光送信器１０の具体的な構成について説明する。
　光送信器１０は、光源１１、信号生成部１２、光変調部１３及び送信部１４を備える。 Next, a specific configuration of the optical transmitter 10 will be described.
The optical transmitter 10 includes a light source 11, a signal generation unit 12, an optical modulation unit 13, and a transmission unit 14.
　光源１１は、単一のレーザ光を連続で出力する。ここで単一のレーザ光とは、１つの波長のレーザ光を意味する。以下、光源１１が出力するレーザ光を連続光と記載する。
　信号生成部１２は、送信対象となるデジタル信号（以下「送信データ」という。）を用いて、光源１１から出力された連続光の変調に用いるベースバンド信号を生成する。 The light source 11 continuously outputs a single laser beam. Here, a single laser beam means a laser beam having one wavelength. Hereinafter, the laser light output by the light source 11 will be referred to as continuous light.
The signal generation unit 12 generates a baseband signal used for modulation of continuous light output from the light source 11 by using a digital signal to be transmitted (hereinafter referred to as “transmission data”).
　信号生成部１２は、乗算部１２１及び加算部１２２で構成される。乗算部１２１は、送信データと複素数（ｅｘｐ（ｊΔωｔ））とを乗算する。加算部１２２は、乗算部１２１の乗算結果と、乗算結果の複素共役とを加算する。信号生成部１２は、このような処理を行うことによってベースバンド信号を生成する。 The signal generation unit 12 is composed of a multiplication unit 121 and an addition unit 122. The multiplication unit 121 multiplies the transmission data with a complex number (exp (jΔωt)). The addition unit 122 adds the multiplication result of the multiplication unit 121 and the complex conjugate of the multiplication result. The signal generation unit 12 generates a baseband signal by performing such processing.
　光変調部１３は、信号生成部１２によって生成されたベースバンド信号を用いて光源１１から出力された連続光を変調して、光増幅器３０の基本周波数を中心として共役な位相相関を持ち、かつ、位相感応増幅可能な光信号を生成する。例えば、光変調部１３は、振幅変調を行う変調器である。
　送信部１４は、光変調部１３によって生成された光信号を伝送路４０に送信する。 The optical modulation unit 13 modulates the continuous light output from the light source 11 using the baseband signal generated by the signal generation unit 12, has a coupled phase correlation centered on the basic frequency of the optical amplifier 30, and has a coupled phase correlation. , Generates an optical signal that can be phase-sensitive and amplified. For example, the optical modulation unit 13 is a modulator that performs amplitude modulation.
The transmission unit 14 transmits the optical signal generated by the optical modulation unit 13 to the transmission line 40.
　図２は、第１の実施形態における光伝送システム１００の処理の流れを表すシーケンス図である。図２に示す処理は、送信データが光送信器１０に入力されることによって実行される。
　信号生成部１２は、入力された送信データを用いてベースバンド信号を生成する（ステップＳ１０１）。以下、信号生成部１２によるベースバンド信号の具体的な生成方法について説明する。 FIG. 2 is a sequence diagram showing a processing flow of the optical transmission system 100 according to the first embodiment. The process shown in FIG. 2 is executed by inputting transmission data to the optical transmitter 10.
The signal generation unit 12 generates a baseband signal using the input transmission data (step S101). Hereinafter, a specific method for generating a baseband signal by the signal generation unit 12 will be described.
　信号生成部１２は、単一の連続光を出力する光源１１と光変調器１２とを用いて光信号とアイドラ光とを同時に発生させるために必要なベースバンド信号を生成する。そこで、第１の実施形態では、光信号とアイドラ光との同時発生にサブキャリア多重と呼ばれる方法を用いる（例えば、参考文献１参照）。まず、信号生成部１２は入力された送信データにおける送信ビット列に基づいて任意の変調フォーマットの信号を生成する。これをｘ（ｔ）とし、次に、信号生成部１２は、シンボルレートの半分以上の周波数Δωについて以下の式（１）に基づく演算を行うことによってベースバンド信号ｓ（ｔ）を生成する。なお、式（１）における*は複素共役を表す。
（参考文献１：G. P. Agrawal, “Fiber-optic communication systems”, Vol. 222. John Wiley & Sons, 2012.） The signal generation unit 12 generates a baseband signal necessary for simultaneously generating an optical signal and idler light by using a light source 11 that outputs a single continuous light and an optical modulator 12. Therefore, in the first embodiment, a method called subcarrier multiplexing is used for simultaneous generation of an optical signal and idler light (see, for example, Reference 1). First, the signal generation unit 12 generates a signal of an arbitrary modulation format based on the transmission bit string in the input transmission data. Let this be x (t), and then the signal generation unit 12 generates a baseband signal s (t) by performing an operation based on the following equation (1) for a frequency Δω that is more than half of the symbol rate. Note that * in Eq. (1) represents the complex conjugate.
(Reference 1: GP Agrawal, “Fiber-optic communication systems”, Vol. 222. John Wiley & Sons, 2012.)
Figure JPOXMLDOC01-appb-M000001
Figure JPOXMLDOC01-appb-M000001
　信号生成部１２は、生成したベースバンド信号ｓ（ｔ）を光変調部１３に入力する。光変調部１３は、入力されたベースバンド信号ｓ（ｔ）で、光源１１から出力される周波数ωｃの連続光を変調する（ステップＳ１０２）。これにより、光変調部１３は、変調信号を生成する。なお、周波数ωｃは増幅媒体（光増幅器３０）の基本周波数に等しいものとする。連続光の時間変動する位相揺らぎをδ（ｔ）とすると、変調信号の電場は以下の式（２）のように表される。 The signal generation unit 12 inputs the generated baseband signal s (t) to the optical modulation unit 13. The optical modulation unit 13 modulates the continuous light of the frequency ω c output from the light source 11 with the input baseband signal s (t) (step S102). As a result, the optical modulation unit 13 generates a modulation signal. The frequency ω c is assumed to be equal to the fundamental frequency of the amplification medium (optical amplifier 30). Assuming that the time-varying phase fluctuation of continuous light is δ (t), the electric field of the modulated signal is expressed by the following equation (2).
Figure JPOXMLDOC01-appb-M000002
Figure JPOXMLDOC01-appb-M000002
　式（２）に示すように、変調信号は、位相揺らぎを共有し、光源１１の周波数に対して対称な周波数配置かつ位相共役関係にあるスペクトルを持つマルチキャリアの光信号である。例えば、変調信号は、光信号とアイドラ光がサブキャリア多重された信号である。この変調信号を光増幅器３０に励起光と共に入射することによって位相感応増幅動作が可能となる。光変調部１３は、生成した変調信号を送信部１４を介して伝送路４０に出力する（ステップＳ１０３）。伝送路４０に出力された変調信号は、光増幅器３０に入力される。 As shown in the equation (2), the modulated signal is a multi-carrier optical signal that shares a phase fluctuation and has a spectrum having a frequency arrangement symmetric with respect to the frequency of the light source 11 and a phase conjugate relationship. For example, the modulated signal is a signal in which an optical signal and idler light are subcarrier-multiplexed. By incidenting this modulated signal on the optical amplifier 30 together with the excitation light, a phase-sensitive amplification operation becomes possible. The optical modulation unit 13 outputs the generated modulation signal to the transmission line 40 via the transmission unit 14 (step S103). The modulated signal output to the transmission line 40 is input to the optical amplifier 30.
　光増幅器３０は、入力された変調信号を増幅する（ステップＳ１０４）。例えば、光増幅器３０は、入力した変調信号と励起光とにより変調信号を増幅する。光増幅器３０は、増幅した光信号を光受信器２０に出力する（ステップＳ１０５）。光受信器２０は、光増幅器３０から出力された変調信号を受信する（ステップＳ１０５）。 The optical amplifier 30 amplifies the input modulated signal (step S104). For example, the optical amplifier 30 amplifies the modulated signal by the input modulated signal and the excitation light. The optical amplifier 30 outputs the amplified optical signal to the optical receiver 20 (step S105). The optical receiver 20 receives the modulated signal output from the optical amplifier 30 (step S105).
　上記のような方法で変調信号を生成することにより、ＯＰＡのような追加の光学系を用いずに、デジタル信号処理と光変調とでアイドラ光を生成することが可能になる。式（１）から明らかなように、変調成分が虚数成分を持たない。そのため、元の信号がＱＡＭ信号のような複素数であった場合でも、ＩＱ変調器ではなく単一の振幅変調器を用いることで実現が可能である。なお、信号生成部１２は、アナログ回路でも実現可能である。 By generating the modulated signal by the above method, it becomes possible to generate idler light by digital signal processing and optical modulation without using an additional optical system such as OPA. As is clear from the equation (1), the modulation component has no imaginary component. Therefore, even if the original signal is a complex number such as a QAM signal, it can be realized by using a single amplitude modulator instead of the IQ modulator. The signal generation unit 12 can also be realized by an analog circuit.
　図３は、第１の実施形態における手法の実証実験の概要図である。図４は、第１の実施形態における手法の実証実験の結果を示す図である。図３に示す送信機２００は図１の光送信器１０に相当し、ＰＳＡ４００は図１の光増幅器３０に相当する。基本周波数は１９３ＴＨｚである。ＰＳＡ４００の増幅媒体としてはＰＰＬＮを使用した。光信号と励起光は参考文献２に記載されている和周波発生を利用したキャリアリカバリを行い、光ＰＬＬを用いて位相同期した。
（参考文献２：Y. Okamura, et al. “Optical pump phase locking to a carrier wave extracted from phase-conjugated twin waves for phase-sensitive optical amplifier repeaters”, Optics express 24.23 (2016)） FIG. 3 is a schematic diagram of a demonstration experiment of the method in the first embodiment. FIG. 4 is a diagram showing the results of a demonstration experiment of the method in the first embodiment. The transmitter 200 shown in FIG. 3 corresponds to the optical transmitter 10 of FIG. 1, and the PSA 400 corresponds to the optical amplifier 30 of FIG. The fundamental frequency is 193 THz. PPLN was used as the amplification medium of PSA400. The optical signal and the excitation light were subjected to carrier recovery using the sum frequency generation described in Reference 2, and phase-locked using the optical PLL.
(Reference 2: Y. Okamura, et al. “Optical pump phase locking to a carrier wave extracted from phase-conjugated twin waves for phase-sensitive optical amplifier repeaters”, Optics express 24.23 (2016))
　上記の信号生成部１２による信号生成の手法に基づいて光信号とアイドラ光がサブキャリア多重されたＱＡＭ信号を生成し、減衰によって雑音を付加した後、ＰＳＡ４００に入力した。信号にはナイキストフィルタが適用されており、変調速度は１０Ｇｂａｕｄ、周波数シフト量は５．５ＧＨｚであった。受信側ではデジタル信号処理上でバンドパスフィルタを用いて信号成分のみを抽出し、復調した。 Based on the signal generation method by the signal generation unit 12 described above, a QAM signal in which an optical signal and idler light were subcarrier-multiplexed was generated, noise was added by attenuation, and then the signal was input to the PSA400. A Nyquist filter was applied to the signal, the modulation speed was 10 Gbaud, and the frequency shift amount was 5.5 GHz. On the receiving side, only the signal component was extracted and demodulated using a bandpass filter on the digital signal processing.
　図５（Ａ）には、光増幅器としてＰＳＡを用いた場合の実証実験の結果を示している。図５（Ａ）に示す例では、左から順に１６ＱＡＭ、３２ＱＡＭ、ＰＳ－２５６ＱＡＭ信号より得られたコンスタレーションを示している。図５（Ａ）に示すコンスタレーションを参照すると、１６ＱＡＭ、３２ＱＡＭ、ＰＳ－２５６ＱＡＭ信号のそれぞれについて歪みの少ない増幅動作が実現されていることがわかる。 FIG. 5A shows the results of a demonstration experiment when PSA is used as an optical amplifier. In the example shown in FIG. 5A, constellations obtained from 16QAM, 32QAM, and PS-256QAM signals are shown in order from the left. With reference to the constellation shown in FIG. 5A, it can be seen that amplification operations with less distortion are realized for each of the 16QAM, 32QAM, and PS-256QAM signals.
　図５（Ａ）との比較のために同様の実験を、ＰＳＡをＥＤＦＡに置き換えて行った例を図５（Ｂ）に示す。ＥＤＦＡの場合でも変調方式は同様のサブキャリア多重信号を用いており、受信側ではバンドパスフィルタで信号成分のみを抽出して復調した。結果として、全ての変調フォーマットでＰＳＡを用いた場合の方がＥＤＦＡを用いた場合よりも、ＳＮＲが平均４．６ｄＢ高くなっていることが確認できた。非縮退ＰＳＡでは光信号とアイドラ光の２波を用いることによる３ｄＢの波長ダイバーシティ利得が原理的に存在するが、それ以上のＳＮＲ改善効果が確認でき、位相感応増幅動作が行えていることが示されている。 FIG. 5 (B) shows an example in which a similar experiment was performed by replacing PSA with EDFA for comparison with FIG. 5 (A). Even in the case of EDFA, the same subcarrier multiplex signal is used as the modulation method, and on the receiving side, only the signal component is extracted by a bandpass filter and demodulated. As a result, it was confirmed that the SNR was 4.6 dB higher on average when PSA was used in all modulation formats than when EDFA was used. In non-degenerate PSA, there is a wavelength diversity gain of 3 dB by using two waves of optical signal and idler light in principle, but further SNR improvement effect can be confirmed, and it is shown that phase sensitive amplification operation is performed. Has been done.
　以上のように構成された第１の実施形態における光伝送システム１００によれば、１つのレーザ光の変調で、追加の光学系を用いずに光信号と、光信号に共役な位相関係を持ったアイドラ光とを生成することができる。そのため、より簡易な光送信器１０の構成で非縮退位相感応増幅動作を実現することが可能になる。 According to the optical transmission system 100 according to the first embodiment configured as described above, the modulation of one laser beam has a phase relationship coupled to the optical signal and the optical signal without using an additional optical system. It can generate idler light. Therefore, it becomes possible to realize a non-degenerate phase-sensitive amplification operation with a simpler configuration of the optical transmitter 10.
（第２の実施形態）
　第１の実施形態では、光増幅器３０としてＰＳＡを用いた。ＰＳＡでは、光ＰＬＬや光注入同期を用いて光信号のキャリア成分と励起光との位相同期を行う必要がある。位相同期を行うためには、第１の実施形態で用いたような和周波発生などによって光信号のキャリアリカバリを行うか、参考文献３に示すような送信側からパイロット光としてキャリア成分を送信する方法が挙げられる。そこで、第２の実施形態では、光注入同期を用いた構成について説明する。
（参考文献３：S. L. I. Olsson, et al. "Injection locking-based pump recovery for phase-sensitive amplified links." Optics express 21.12 (2013)） (Second embodiment)
In the first embodiment, PSA was used as the optical amplifier 30. In PSA, it is necessary to perform phase synchronization between the carrier component of the optical signal and the excitation light by using optical PLL or optical injection synchronization. In order to perform phase synchronization, carrier recovery of the optical signal is performed by generating a sum frequency as used in the first embodiment, or a carrier component is transmitted as pilot light from the transmitting side as shown in Reference 3. The method can be mentioned. Therefore, in the second embodiment, a configuration using light injection synchronization will be described.
(Reference 3: S.L.I. Olsson, et al. "Injection locking-based pump recovery for phase-sensitive amplified links." Optics express 21.12 (2013))
　図５は、第２の実施形態における光伝送システム１００ａのシステム構成図である。光伝送システム１００ａは、光送信器１０ａ、光受信器２０、光増幅器３０、分波器５０、サーキュレーター６０、スレーブレーザ６１及び合波器７０を備える。光増幅器３０、分波器５０、サーキュレーター６０、スレーブレーザ６１及び合波器７０は、１つのＰＳＡモジュールとして構成されてもよい。 FIG. 5 is a system configuration diagram of the optical transmission system 100a according to the second embodiment. The optical transmission system 100a includes an optical transmitter 10a, an optical receiver 20, an optical amplifier 30, a demultiplexer 50, a circulator 60, a slave laser 61, and a combiner 70. The optical amplifier 30, the duplexer 50, the circulator 60, the slave laser 61 and the combiner 70 may be configured as one PSA module.
　光送信器１０ａは、光源の周波数に対して対称な周波数配置かつ位相共役関係にあるスペクトルを持つマルチキャリアの変調信号を生成する。この際、光送信器１０ａは、周波数領域の直流成分付近にガードバンドを持つ光信号を生成し、生成した光信号に直流成分付近に光増幅器３０の励起光の位相同期のためのパイロット光を付加する。 The optical transmitter 10a generates a multi-carrier modulated signal having a spectrum having a frequency arrangement symmetrical with respect to the frequency of the light source and a phase conjugate relationship. At this time, the optical transmitter 10a generates an optical signal having a guard band in the vicinity of the DC component in the frequency domain, and the generated optical signal is provided with pilot light for phase synchronization of the excitation light of the optical amplifier 30 in the vicinity of the DC component. Add.
　分波器５０は、光送信器１０ａから送信された変調信号において、光信号とアイドラ光からパイロット光を分波する。分波器５０は、変調信号からパイロット光を除いた信号（分波した光信号とアイドラ光との信号）を合波器７０が接続されている第１の経路に出力し、分波したパイロット光をサーキュレーター６０が接続されている第２の経路に出力する。 The demultiplexer 50 demultiplexes the pilot light from the optical signal and the idler light in the modulated signal transmitted from the optical transmitter 10a. The demultiplexer 50 outputs a signal obtained by removing the pilot light from the modulated signal (a signal of the demultiplexed optical signal and the idler light) to the first path to which the duplexer 70 is connected, and the demultiplexing pilot. Light is output to the second path to which the circulator 60 is connected.
　サーキュレーター６０は、複数のポートを有する部品である。図５に示す例では、サーキュレーター６０が３つのポート（第１のポート～第３のポート）を有する例を示している。サーキュレーター６０が有する第１のポートは分波器５０に接続され、第２のポートはスレーブレーザ６１に接続され、第３のポートは直接又は間接的に合波器７０に接続される。第１のポートには分波器５０によって分波されたパイロット光が入力され、第２のポートにはスレーブレーザ６１から出力されたレーザ光が入力される。 The circulator 60 is a component having a plurality of ports. The example shown in FIG. 5 shows an example in which the circulator 60 has three ports (first port to third port). The first port of the circulator 60 is connected to the demultiplexer 50, the second port is connected to the slave laser 61, and the third port is directly or indirectly connected to the combiner 70. The pilot light demultiplexed by the demultiplexer 50 is input to the first port, and the laser light output from the slave laser 61 is input to the second port.
　サーキュレーター６０は、第１のポートから入力されたパイロット光をスレーブレーザ６１に出力する。サーキュレーター６０は、第２のポートから入力されたレーザ光を合波器７０に出力する。サーキュレーター６０の第３ポートと、合波器７０との間は、伝送路により直接接続されていてもよいし、ＰＰＬＮ導波路のようなχ２媒質を介して接続されてもよい。 The circulator 60 outputs the pilot light input from the first port to the slave laser 61. The circulator 60 outputs the laser beam input from the second port to the combiner 70. The third port of the circulator 60 and the combiner 70 may be directly connected by a transmission line or may be connected via a χ 2 medium such as a PPLN waveguide.
　スレーブレーザ６１は、サーキュレーター６０を介して入力されたパイロット光を用いた注入同期で高いＯＳＮＲの基本周波数のレーザ光を出力する。
　合波器７０は、第１の経路を介して入力された光信号と、スレーブレーザ６１から出力された光信号とを合波する。なお、スレーブレーザ６１から出力された光信号は、直接又はχ２媒質を介して合波器７０に入力される。 The slave laser 61 outputs a laser beam having a high OSNR fundamental frequency in injection synchronization using pilot light input via the circulator 60.
The combiner 70 combines the optical signal input via the first path and the optical signal output from the slave laser 61. The optical signal output from the slave laser 61 is input to the combiner 70 directly or via the χ 2 medium.
　３次の非線形光学効果を利用したＰＳＡの場合には基本周波数の光信号がそのまま所望の励起光となる。２次の非線形光学効果を利用したＰＳＡの場合には非線形光学効果を用いて基本周波数の２倍の周波数の光を発生させることで所望の励起光が発生可能である。  In the case of PSA using the third-order nonlinear optical effect, the optical signal of the fundamental frequency becomes the desired excitation light as it is. In the case of PSA using a second-order nonlinear optical effect, desired excitation light can be generated by generating light having a frequency twice the fundamental frequency by using the nonlinear optical effect. The
　第２の実施形態における光送信器１０ａの構成について説明する。
　図６は第２の実施形態における光送信器１０ａの第１の構成の具体例を示す図であり、図７は第２の実施形態における光送信器１０ａの第２の構成の具体例を示す図である。以下、各構成について説明する。 The configuration of the optical transmitter 10a in the second embodiment will be described.
FIG. 6 is a diagram showing a specific example of the first configuration of the optical transmitter 10a in the second embodiment, and FIG. 7 shows a specific example of the second configuration of the optical transmitter 10a in the second embodiment. It is a figure. Hereinafter, each configuration will be described.
　図６に示す第１の構成として、光送信器１０ａは、光源１１、信号生成部１２ａ、光変調部１３、送信部１４、分波器１５及び合波器１６を備える。第１の実施形態における光送信器１０と同様の構成については同じ符号を付して説明を省略する。 As the first configuration shown in FIG. 6, the optical transmitter 10a includes a light source 11, a signal generation unit 12a, an optical modulation unit 13, a transmitter unit 14, a demultiplexer 15, and a combiner 16. The same components as those of the optical transmitter 10 in the first embodiment are designated by the same reference numerals, and the description thereof will be omitted.
　信号生成部１２ａは、周波数Δωを十分大きくしてガードバンドを設けたスペクトルを持つマルチキャリアの光信号を生成する。信号生成部１２ａは、生成した光信号を光変調部１３に出力する。 The signal generation unit 12a generates a multi-carrier optical signal having a spectrum provided with a guard band by sufficiently increasing the frequency Δω. The signal generation unit 12a outputs the generated optical signal to the optical modulation unit 13.
　分波器１５は、光源１１と光変調部１３との間に設けられ、光源１１から出力された連続光を分波する。分波器１５は、分波した連続光をそれぞれ光変調部１３が接続されている第１の経路と、合波器１６が接続されている第２の経路に出力する。これにより、パイロット光が合波器１６に出力される。 The demultiplexer 15 is provided between the light source 11 and the optical modulation unit 13, and demultiplexes the continuous light output from the light source 11. The demultiplexer 15 outputs the demultiplexed continuous light to the first path to which the optical modulation unit 13 is connected and the second path to which the duplexer 16 is connected, respectively. As a result, the pilot light is output to the combiner 16.
　合波器１６は、光変調部１３と送信部１４との間に設けられ、光変調部１３から出力された変調信号と、分波器１５により分岐された光信号とを合波する。その結果、光送信器１０ａは、変調信号に対してパイロット光を合波することが可能になる。 The combiner 16 is provided between the optical modulation unit 13 and the transmission unit 14, and combines the modulation signal output from the optical modulation unit 13 with the optical signal branched by the demultiplexer 15. As a result, the optical transmitter 10a can combine the pilot light with the modulated signal.
　図７に示す第２の構成として、光送信器１０ａは、光源１１、信号生成部１２、光変調部１３ａ及び送信部１４を備える。第１の実施形態における光送信器１０と同様の構成については同じ符号を付して説明を省略する。
　光変調部１３ａは、信号生成部１２によって生成されたベースバンド信号を用いて光源１１から出力された連続光を変調して、光増幅器３０の基本周波数を中心として共役な位相相関を持ち、かつ、位相感応増幅可能な光信号を生成する。しかしながら、第２の構成では、光変調部１３ａの不完全性によってキャリア周波数成分が残っている。そこで、第２の構成では、光変調部１３ａの不完全性によって残ったキャリア周波数成分をパイロット光として利用する。例えば、光通信システムで利用されるマッハツェンダ型光変調器では変調器中の結晶に様々な電圧を印加することにより内部の光の変調を行うが、この際に印加されているバイアス電圧(変調を行わない場合にかけるデフォルトの電圧)が理想的な値よりずれているとキャリア周波数が発生する。第２の構成では、このように発生したキャリア周波数成分をパイロット光として利用する。 As the second configuration shown in FIG. 7, the optical transmitter 10a includes a light source 11, a signal generation unit 12, an optical modulation unit 13a, and a transmission unit 14. The same components as those of the optical transmitter 10 in the first embodiment are designated by the same reference numerals, and the description thereof will be omitted.
The optical modulation unit 13a modulates the continuous light output from the light source 11 using the baseband signal generated by the signal generation unit 12, has a coupled phase correlation centered on the basic frequency of the optical amplifier 30, and has a coupled phase correlation. , Generates an optical signal that can be phase-sensitive and amplified. However, in the second configuration, the carrier frequency component remains due to the imperfections of the optical modulation section 13a. Therefore, in the second configuration, the carrier frequency component remaining due to the imperfections of the optical modulation unit 13a is used as the pilot light. For example, in the Machzenda type optical modulator used in an optical communication system, the internal light is modulated by applying various voltages to the crystals in the modulator, and the bias voltage (modulation) applied at this time is applied. If the default voltage applied when not done) deviates from the ideal value, carrier frequency will occur. In the second configuration, the carrier frequency component thus generated is used as pilot light.
　パイロット光は、そのスペクトルが狭線幅であると、伝送路もしくはファイバ増幅器中で誘導ブリルアン散乱を始めとする望ましくない非線形効果が生じやすくなる。そのため、特に高パワーが求められる場合には何らかのランダム変調(例えばパイロット光を分波して位相変調器に入射することにより高周波の位相変調を行う)によりスペクトルを広げることにより非線形効果を抑圧することも可能である。スペクトルを広げることにより非線形効果を抑圧する手法として、下記の参考文献４の技術が用いられてもよい。
（参考文献４：Z. Tong, et al. “Towards ultrasensitive optical links enabled by low-noise phase-sensitive amplifiers. “ Nature photonics, 5(7), 430 (2011)） Pilot light is prone to unwanted non-linear effects, such as induced Brillouin scattering, in transmission lines or fiber amplifiers when its spectrum is narrow. Therefore, especially when high power is required, the non-linear effect is suppressed by widening the spectrum by some kind of random modulation (for example, high-frequency phase modulation is performed by demultiplexing the pilot light and incident on the phase modulator). Is also possible. As a method of suppressing the non-linear effect by widening the spectrum, the technique of Reference 4 below may be used.
(Reference 4: Z. Tong, et al. “Towards ultrasensitive optical links enabled by low-noise phase-sensitive amplifiers.” Nature photonics, 5 (7), 430 (2011))
　以上のように構成された第２の実施形態における光伝送システム１００ａによれば、パイロット光を用いた手法にも対応することができる。そのため、利便性を向上させることが可能になる。 According to the optical transmission system 100a in the second embodiment configured as described above, it is possible to support a method using pilot light. Therefore, it becomes possible to improve convenience.
（第３の実施形態）
　ＰＳＡの増幅特性は、光信号とアイドラ光との間の位相関係によって変化し、理想的な動作のためには光増幅器３０で入力光の波長分散が補償されていることが前提である。そのため、従来のＰＳＡの検討では、分散補償ファイバなどを用いて光学的に分散補償を行っていた。一方で、デジタル信号処理を用いると、伝送によって生じる波長分散を送信側の光変調時に予等化することが可能である。しかしながら、アイドラ光の生成にＯＰＡを用いる従来の送信器構成では、アイドラ光を光変調後に光信号のコピーとして生成するため、光信号とアイドラ光の両帯域にわたってデジタル信号処理による分散予等化を行うことができない。一方で、本発明では、光信号とアイドラ光とをベースバンド信号中にサブキャリア多重するため、両帯域にわたってデジタル信号処理による分散予等化を行うことができる。そこで、第３の実施形態では、伝送によって生じる波長分散を送信側の光変調時に予等化する構成について説明する。 (Third embodiment)
The amplification characteristics of the PSA change depending on the phase relationship between the optical signal and the idler light, and it is premised that the optical amplifier 30 compensates for the wavelength dispersion of the input light for ideal operation. Therefore, in the conventional PSA study, dispersion compensation is optically performed using a dispersion compensation fiber or the like. On the other hand, if digital signal processing is used, it is possible to predict the wavelength dispersion generated by transmission at the time of optical modulation on the transmitting side. However, in the conventional transmitter configuration that uses OPA to generate idler light, since idler light is generated as a copy of the optical signal after optical modulation, dispersion estimation by digital signal processing is performed over both the optical signal and idler light bands. I can't do it. On the other hand, in the present invention, since the optical signal and the idler light are subcarrier-multiplexed in the baseband signal, it is possible to perform dispersion prediction by digital signal processing over both bands. Therefore, in the third embodiment, a configuration for predicting the wavelength dispersion generated by transmission at the time of optical modulation on the transmitting side will be described.
　第３の実施形態における光伝送システム１００は、光送信器１０に代えて光送信器１０ｂを備える点以外は第１の実施形態と同様である。図８は、第３の実施形態における光送信器１０ｂの構成を表す図である。
　光送信器１０ｂは、光源１１、信号生成部１２ｂ、光変調部１３ｂ及び送信部１４を備える。 The optical transmission system 100 in the third embodiment is the same as that in the first embodiment except that the optical transmitter 10b is provided in place of the optical transmitter 10. FIG. 8 is a diagram showing the configuration of the optical transmitter 10b according to the third embodiment.
The optical transmitter 10b includes a light source 11, a signal generation unit 12b, an optical modulation unit 13b, and a transmission unit 14.
　光送信器１０ｂは、信号生成部１２及び光変調部１３に代えて信号生成部１２ｂ及び光変調部１３ｂを備える点で光送信器１０と構成が異なる。光送信器１０ｂは、他の構成については光送信器１０と同様である。そのため、光送信器１０ｂ全体の説明は省略し、号生成部１２ｂ及び光変調部１３ｂについて説明する。 The optical transmitter 10b is different from the optical transmitter 10 in that it includes a signal generation unit 12b and an optical modulation unit 13b in place of the signal generation unit 12 and the optical modulation unit 13. The optical transmitter 10b is the same as the optical transmitter 10 in other configurations. Therefore, the description of the entire optical transmitter 10b will be omitted, and the No. generation unit 12b and the optical modulation unit 13b will be described.
　信号生成部１２ｂは、送信データを用いて、光源１１から出力された連続光の変調に用いるベースバンド信号を生成する。信号生成部１２ｂは、乗算部１２１、加算部１２２及び分散付与部１２３を備える。
　分散付与部１２３は、送信部１４と光増幅器３０との間の伝送路４０で発生する波長分散の符号を反転した波長分散を、乗算部１２１及び加算部１２２により生成されたベースバンド信号に付与する。このように、分散付与部１２３は、送信部１４と光増幅器３０との間の伝送路４０の波長分散を打ち消す位相変化を加えた光信号を生成することができる。 The signal generation unit 12b uses the transmission data to generate a baseband signal used for modulating the continuous light output from the light source 11. The signal generation unit 12b includes a multiplication unit 121, an addition unit 122, and a dispersion imparting unit 123.
The dispersion imparting unit 123 imparts the wavelength dispersion obtained by inverting the sign of the wavelength dispersion generated in the transmission line 40 between the transmission unit 14 and the optical amplifier 30 to the baseband signal generated by the multiplication unit 121 and the addition unit 122. do. In this way, the dispersion imparting unit 123 can generate an optical signal with a phase change that cancels the wavelength dispersion of the transmission path 40 between the transmission unit 14 and the optical amplifier 30.
　第３の実施形態では、分散予等化によって虚数成分が残るため、ＩＱ変調器が必要となる。光送信器１０ｂが備える光変調部１３ｂは、ＩＱ変調器であり、分散付与部から算出されたＩ成分の光信号とＱ成分の光信号を入力することにより変調を行う。 In the third embodiment, an IQ modulator is required because an imaginary number component remains due to dispersion preequalization. The optical modulation unit 13b included in the optical transmitter 10b is an IQ modulator, and performs modulation by inputting an optical signal of the I component and an optical signal of the Q component calculated from the dispersion imparting unit.
　第３の実施形態における信号生成部１２ｂの処理について説明する。
　信号生成部１２ｂでは、まず第１の実施形態と同様に、乗算部１２１及び加算部１２２により光信号とアイドラ光のサブキャリア多重をしたベースバンド信号を生成する。その後、分散付与部１２３は、伝送路で発生する波長分散の符号を反転した波長分散を、ベースバンド信号に付与する。波長分散を付与する。波長分散の計算方法として、オーバーラップカット法などが挙げられる。オーバーラップカット法の具体的な手法は、下記の参考文献５に記載されている。
（参考文献５：R. Kudo, T. Kobayashi, K. Ishihara, Y. Takatori, A. Sano, and Y. Miyamoto, “Coherent Optical Single Carrier Transmission Using Overlap Frequency Domain Equalization for Long-Haul Optical Systems,” Journal of Lightwave Technology, 27, 16, 3721 (2009).） The processing of the signal generation unit 12b in the third embodiment will be described.
In the signal generation unit 12b, first, as in the first embodiment, the multiplication unit 121 and the addition unit 122 generate a baseband signal in which the optical signal and the idler light are subcarrier-multiplexed. After that, the dispersion imparting unit 123 applies the wavelength dispersion obtained by inverting the sign of the wavelength dispersion generated in the transmission line to the baseband signal. Gives wavelength dispersion. Examples of the method for calculating the wavelength dispersion include an overlap cut method. A specific method of the overlap cut method is described in Reference 5 below.
(Reference 5: R. Kudo, T. Kobayashi, K. Ishihara, Y. Takatori, A. Sano, and Y. Miyamoto, “Coherent Optical Single Carrier Transmission Using Overlap Frequency Domain Equalization for Long-Haul Optical Systems,” Journal of Lightwave Technology, 27, 16, 3721 (2009).)
　その後、光送信器１０では、光変調部１３ｂが、光源１１から出力される連続光をベースバンド信号で変調する。 After that, in the optical transmitter 10, the optical modulation unit 13b modulates the continuous light output from the light source 11 with the baseband signal.
　以上のように構成された第３の実施形態における光伝送システム１００によれば、光送信器１０ｂで付与した波長分散が伝送に伴う波長分散によって相殺される。これにより、増幅地点での波長分散が無くなり、光学的な分散補償を用いずに理想的なＰＳＡの動作が可能になる。 According to the optical transmission system 100 according to the third embodiment configured as described above, the wavelength dispersion imparted by the optical transmitter 10b is offset by the wavelength dispersion associated with the transmission. This eliminates the wavelength dispersion at the amplification point and enables ideal PSA operation without the use of optical dispersion compensation.
　以下、各実施形態に共通する変形例について説明する。
　光送信器１０，１０ａ，１０ｂは、光増幅器３０を備えるように構成されてもよい。光送信器１０及び１０ｂが光増幅器３０を備える場合、光送信器１０及び１０ｂは送信部１４の後段に光増幅器３０を備え、光増幅装置として構成されてもよい。光送信器１０ａが光増幅器３０を備える場合、光送信器１０ａは送信部１４の後段にＰＳＡモジュールを備え、光増幅装置として構成されてもよい。 Hereinafter, modification common to each embodiment will be described.
The optical transmitters 10, 10a, 10b may be configured to include an optical amplifier 30. When the optical transmitters 10 and 10b include an optical amplifier 30, the optical transmitters 10 and 10b may include an optical amplifier 30 after the transmission unit 14 and may be configured as an optical amplifier. When the optical transmitter 10a includes an optical amplifier 30, the optical transmitter 10a may include a PSA module after the transmission unit 14 and may be configured as an optical amplifier.
　上述した実施形態における光送信器１０，１０ａ，１０ｂの一部の機能（例えば、信号生成部１２，１２ａ，１２ｂ、光変調部１３，１３ａ，１３ｂ）をコンピュータで実現するようにしてもよい。その場合、この機能を実現するためのプログラムをコンピュータ読み取り可能な記録媒体に記録して、この記録媒体に記録されたプログラムをコンピュータシステムに読み込ませ、実行することによって実現してもよい。なお、ここでいう「コンピュータシステム」とは、ＯＳや周辺機器等のハードウェアを含むものとする。また、「コンピュータ読み取り可能な記録媒体」とは、フレキシブルディスク、光磁気ディスク、ＲＯＭ、ＣＤ－ＲＯＭ等の可搬媒体、コンピュータシステムに内蔵されるハードディスク等の記憶装置のことをいう。 Some functions of the optical transmitters 10, 10a, 10b in the above-described embodiment (for example, signal generation units 12, 12a, 12b, optical modulation units 13, 13a, 13b) may be realized by a computer. In that case, the program for realizing this function may be recorded on a computer-readable recording medium, and the program recorded on the recording medium may be read by a computer system and executed. The term "computer system" as used herein includes hardware such as an OS and peripheral devices. Further, the "computer-readable recording medium" refers to a portable medium such as a flexible disk, a magneto-optical disk, a ROM, or a CD-ROM, and a storage device such as a hard disk built in a computer system.
　さらに「コンピュータ読み取り可能な記録媒体」とは、インターネット等のネットワークや電話回線等の通信回線を介してプログラムを送信する場合の通信線のように、短時間の間、動的にプログラムを保持するもの、その場合のサーバやクライアントとなるコンピュータシステム内部の揮発性メモリのように、一定時間プログラムを保持しているものも含んでもよい。また上記プログラムは、前述した機能の一部を実現するためのものであってもよく、さらに前述した機能をコンピュータシステムにすでに記録されているプログラムとの組み合わせで実現できるものであってもよく、ＦＰＧＡ（Field Programmable Gate Array）等のプログラマブルロジックデバイスを用いて実現されるものであってもよい。 Further, a "computer-readable recording medium" is a communication line for transmitting a program via a network such as the Internet or a communication line such as a telephone line, and dynamically holds the program for a short period of time. It may also include a program that holds a program for a certain period of time, such as a volatile memory inside a computer system that is a server or a client in that case. Further, the above program may be for realizing a part of the above-mentioned functions, and may be further realized for realizing the above-mentioned functions in combination with a program already recorded in the computer system. It may be realized by using a programmable logic device such as FPGA (Field Programmable Gate Array).
　以上、この発明の実施形態について図面を参照して詳述してきたが、具体的な構成はこの実施形態に限られるものではなく、この発明の要旨を逸脱しない範囲の設計等も含まれる。 As described above, the embodiment of the present invention has been described in detail with reference to the drawings, but the specific configuration is not limited to this embodiment, and the design and the like within a range not deviating from the gist of the present invention are also included.
　本発明は、位相感応増幅を行う技術に適用できる。 The present invention can be applied to a technique for performing phase-sensitive amplification.
１０、１０ａ、１０ｂ…光送信器，　２０…光受信器，　３０…光増幅器，　１１…光源，　１２、１２ａ…信号生成部，　１３、１３ａ、１３ｂ…光変調部，　１４…送信部，　１５、５０…分波器，　１６、７０…合波器，　６０…サーキュレーター，　６１…スレーブレーザ，　１２１…乗算部，　１２２…加算部，　１２３…分散付与部 10, 10a, 10b ... Optical transmitter, 20 ... Optical receiver, 30 ... Optical amplifier, 11 ... Light source, 12, 12a ... Signal generator, 13, 13a, 13b ... Optical modulation unit, 14 ... Transmitter, 15, 50 ... demultiplexer, 16, 70 ... combiner, 60 ... circulator, 61 ... slave laser, 121 ... multiplication part, 122 ... addition part, 123 ... dispersion imparting part

Claims (5)

  1. 　単一のレーザ光を出力する光源と、
    　送信対象となるデジタル信号を用いて、前記レーザ光の変調に用いるベースバンド信号を生成する信号生成部と、
    　前記信号生成部によって生成された前記ベースバンド信号を用いて前記レーザ光を変調して、位相感応増幅器の基本周波数を中心として共役な位相相関を持ち、かつ、位相感応増幅可能な光信号を生成する光変調部と、
    　前記光信号に対して非線形光学効果による位相感応増幅を行う光増幅部と、
    　を備える光伝送システム。     A light source that outputs a single laser beam and
    A signal generation unit that generates a baseband signal used for modulation of the laser beam using a digital signal to be transmitted, and a signal generation unit.
    The laser beam is modulated using the baseband signal generated by the signal generation unit to generate an optical signal having a coupled phase correlation centered on the basic frequency of the phase-sensitive amplifier and capable of phase-sensitive amplification. Optical modulator and
    An optical amplification unit that performs phase-sensitive amplification of the optical signal by a non-linear optical effect,
    Optical transmission system.
  2. 　前記信号生成部は、周波数領域の直流成分付近にガードバンドを持つ光信号を生成し、生成した前記光信号に前記直流成分付近に位相感応増幅器の励起光の位相同期のためのパイロット光を付加する、請求項１に記載の光伝送システム。 The signal generation unit generates an optical signal having a guard band near the DC component in the frequency region, and adds pilot light for phase synchronization of the excitation light of the phase-sensitive amplifier to the generated optical signal near the DC component. The optical transmission system according to claim 1.
  3. 　前記信号生成部は、前記光送信器と、前記光送信器から光受信器までの間に設けられた光増幅部との間の伝送路の波長分散を打ち消す位相変化を加えた光信号を前記ベースバンド信号として生成する、請求項１に記載の光伝送システム。 The signal generation unit obtains an optical signal having a phase change that cancels the wavelength dispersion of the transmission line between the optical transmitter and the optical amplification unit provided between the optical transmitter and the optical receiver. The optical transmission system according to claim 1, which is generated as a baseband signal.
  4. 　送信対象となるデジタル信号を用いて、単一のレーザ光を出力する光源から出力される前記レーザ光の変調に用いるベースバンド信号を生成し、
    　生成された前記ベースバンド信号を用いて前記レーザ光を変調して、位相感応増幅器の基本周波数を中心として共役な位相相関を持ち、かつ、位相感応増幅可能な光信号を生成し、
    　前記光信号に対して非線形光学効果による位相感応増幅を行う光増幅方法。     Using the digital signal to be transmitted, a baseband signal used for modulation of the laser beam output from a light source that outputs a single laser beam is generated.
    The laser beam is modulated using the generated baseband signal to generate an optical signal having a coupled phase correlation centered on the fundamental frequency of the phase-sensitive amplifier and capable of phase-sensitive amplification.
    An optical amplification method for performing phase-sensitive amplification of the optical signal by a nonlinear optical effect.
  5. 　送信対象となるデジタル信号を用いて、単一のレーザ光を出力する光源から出力される前記レーザ光の変調に用いるベースバンド信号を生成する信号生成ステップと、
    　前記信号生成ステップにおいて生成された前記ベースバンド信号を用いて前記レーザ光を変調して、位相感応増幅器の基本周波数を中心として共役な位相相関を持ち、かつ、位相感応増幅可能な光信号を生成する光変調ステップと、
    　前記光信号に対して非線形光学効果による位相感応増幅を行う光増幅ステップと、
    　をコンピュータに実行させるためのコンピュータプログラム。     A signal generation step of generating a baseband signal used for modulation of the laser beam output from a light source that outputs a single laser beam using a digital signal to be transmitted, and a signal generation step.
    The laser beam is modulated using the baseband signal generated in the signal generation step to generate an optical signal having a coupled phase correlation around the basic frequency of the phase-sensitive amplifier and capable of phase-sensitive amplification. Optical modulation step and
    An optical amplification step that performs phase-sensitive amplification by a nonlinear optical effect on the optical signal,
    A computer program that lets your computer run.
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Title
OKAMURA YASUHIRO; KONDO KOTARO; SEKI SHINGO; OHMICHI YUYA; KOGA MASAFUMI; TAKADA ATSUSHI: "Frequency nondegenerate optical parametric phase-sensitive amplifier repeater by using recovered pump carrier generated from phase-conjugated twin waves", 2016 OPTICAL FIBER COMMUNICATIONS CONFERENCE AND EXHIBITION (OFC), OSA, 20 March 2016 (2016-03-20), pages 1 - 3, XP032942461 *

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