JP6774382B2 - Optical amplifier and optical transmission system using it - Google Patents

Description

The present invention relates to an optical amplifier used in an optical communication system or an optical measurement system, and an optical transmission system using the same.

In a conventional optical transmission system, an identification reproduction optical repeater is used to convert an optical signal into an electric signal and reproduce the optical signal after identifying the digital signal in order to reproduce the signal propagated through the optical fiber and attenuated. .. The identification / regeneration optical repeater has problems such as limitation of response speed of electronic components for optical-electric conversion and increase in power consumption. Therefore, fiber laser amplifiers and semiconductor laser amplifiers have been introduced, in which excitation light is incident on an optical fiber to which a rare earth element is added to amplify the signal light as it is. These laser amplifiers did not have the function of shaping the deteriorated signal light waveform. On the contrary, the spontaneously emitted light that is unavoidably and randomly generated is mixed completely independently of the signal component, and the S / N of the signal light is reduced by at least 3 dB before and after amplification. The decrease in S / N increases the transmission code error rate during digital signal transmission and lowers the transmission quality.

A phase sensitive amplifier (PSA) is being studied as a means of overcoming the limitations of a laser amplifier. The PSA has a function of shaping a deteriorated signal light waveform and a phase signal due to the influence of the dispersion of the transmission fiber. Naturally emitted light having a quadrature phase unrelated to the signal can be suppressed, and naturally emitted light of the same phase can be minimized. Therefore, in principle, the S / N of the signal light can be kept the same before and after amplification without deterioration.

FIG. 1 is a diagram showing a basic configuration of a PSA of the prior art. As shown in FIG. 1, the PSA100 includes a phase-sensitive optical amplification unit 101 using optical parametric amplification, an excitation light source 102, an excitation optical phase control unit 103, a first optical branching unit 104-1 and a second optical branching. A unit 104-2 is provided. As shown in FIG. 1, the signal light 110 input to the PSA 100 is bifurcated by the optical branching unit 104-1, one incident on the phase-sensitive optical amplification unit 101 and the other incident on the excitation light source 102. .. The excitation light 111 emitted from the excitation light source 102 is phase-adjusted via the excitation light phase control unit 103, and is incident on the phase-sensitive light amplification unit 101. The phase sensitive light amplification unit 101 outputs the output signal light 112 based on the input signal light 110 and the excitation light 111.

The phase-sensitive light amplification unit 101 has a characteristic that the signal light 110 is amplified when the phase of the signal light 110 and the phase of the excitation light 111 match, and the signal light 110 is attenuated when the two phases have an orthogonal phase relationship of 90 degrees. Have. When the phases of the excitation light 111 and the signal light 110 are matched so that the amplification gain is maximized by utilizing this characteristic, the spontaneous emission light having a phase orthogonal to the signal light 110 is not generated, and the components having the same phase are also included. It does not generate more naturally emitted light than the noise of signal light. Therefore, the signal light 110 can be amplified without deteriorating the S / N ratio.

In order to achieve the phase synchronization of the signal light 110 and the excitation light 111, the excitation light phase control unit 103 uses the excitation light 111 to synchronize with the phase of the signal light 110 branched by the first optical branching unit 104-1. Control the phase of. The excitation optical phase control unit 103 detects a part of the output signal light 112 branched by the second optical branching unit 104-2 with a narrow band detector so that the amplification gain of the output signal light 112 is maximized. The phase of the excitation light 111 is controlled.

Non-linear optical media that perform parametric amplification as described above include a second-order nonlinear optical material represented by a periodic polarization inversion LiNbO ₃ (PPRN) waveguide and a third-order nonlinear optical material represented by a quartz glass fiber.

FIG. 2 is a diagram showing a configuration of a PSA of a prior art using a PPLN waveguide (see Non-Patent Document 1). The PSA200 shown in FIG. 2 includes an erbium-added fiber optic laser amplifier (EDFA) 201, a first second-order nonlinear optical element 202, a second second-order nonlinear optical element 204, a first optical branching portion 203-1 and A second optical branching portion 203-2, a phase modulator 205, an optical fiber extender 206, a polarization holding fiber 207, an optical detector 208, and a phase synchronization loop (PLL) circuit 209 are provided. The first second-order nonlinear optical element 202 includes a first spatial optical system 211, a first PPLN waveguide 212, a second spatial optical system 213, and a first dichroic mirror 214. The second second-order nonlinear optical element 204 also has a similar configuration, and details thereof will be omitted.

The signal light 250 incident on the PSA 200 in FIG. 2 is branched by the optical branching portion 203-1 and one is incident on the second second-order nonlinear optical element 204. The other of the branched lights is phase-controlled and incident on the EDFA 201 via the phase modulator 205 and the optical fiber extender 206 as the excitation fundamental wave light 251. The EDFA 201 sufficiently amplifies the incident excitation fundamental wave light 251 and incidents on the first second-order nonlinear optical element 202. With the EDFA201, sufficient power can be obtained from the weak excited fundamental wave light 251 to obtain a nonlinear optical effect. In the first second-order nonlinear optical element 202, a second harmonic (hereinafter referred to as SH light) 252 is generated from the incident excitation fundamental wave light 251. The generated SH light 252 is incident on the second second-order nonlinear optical element 204 via the polarization-holding fiber 207. In the second second-order nonlinear optical element 204, phase-sensitive optical amplification is performed by performing degenerate parametric amplification with the incident signal light 250 and SH light 252, and the output signal light 253 is output.

In the PSA, in order to amplify only the light whose phase is in phase with the signal light, it is necessary that the phase of the signal light and the phase of the excitation light match or are deviated by π radians as described above. That is, when the second-order nonlinear optical effect is used, it is necessary that the phase φ 2 _ωs of the excitation light having a wavelength corresponding to the SH light and the phase φ _{ω s} of the signal light satisfy the relationship of the following equation (1). Become. Here, n is an integer.
_{Δφ = 1/2 (φ 2ωs} -φ ωs) = nπ formula (1)

FIG. 3 is a diagram showing the relationship between the phase difference Δφ between the input signal light and the excitation light and the gain in the PSA using the second-order nonlinear optical effect of the prior art. It can be seen that the gain (dB) on the vertical axis is maximum when the phase difference Δφ on the horizontal axis is −π, 0, or π.

For phase synchronization between the signal light 250 and the excitation fundamental wave light 251 first, the phase modulator 205 applies phase modulation to the excitation fundamental wave light 251 with a weak pilot signal, and a part of the output signal light 253 is branched. Detected by detector 208. This pilot signal component becomes the minimum when the phase difference Δφ shown in FIG. 3 is the minimum and the phase is synchronized. Therefore, feedback is performed to the optical fiber extender 206 by using the PLL circuit 209 so that the pilot signal is the minimum, that is, the amplified output signal is the maximum. The phase of the excitation fundamental wave light 251 can be controlled by the optical fiber extender 206 to achieve phase synchronization between the signal light 250 and the excitation fundamental wave light 251.

As shown in FIG. 3, the phase-sensitive optical amplifier using the degenerate parametric process described above has a property of attenuating orthogonal phase components. For this reason, the intensity modulation signal (OK) used in ordinary optical communication, the intensity modulation / direct detection (IMDD: Intensity Modulation-Direct Detection) method using binary phase modulation, or the binary phase modulation (BPSK) or difference. It can only be used for amplification of modulated signals such as dynamic phase shift keying (DPSK). Moreover, only the signal light of one wavelength can be amplified with phase sensitivity. In order to widely apply the phase-sensitive optical amplifier to optical communication technology, it is necessary to support various optical signals such as multi-value modulation formats and wavelength division multiplexing signals. As an example of this correspondence, Non-Patent Document 3 reports a phase-sensitive optical amplifier capable of relay-amplifying a plurality of wavelengths at once, supporting an arbitrary modulation format by using a configuration based on non-degenerate parametric amplification. ..

T. Umeki, O. Tadanaga, A. Takada and M. Asobe, "Phase sensitive degenerate parametric amplification using directly-bonded PPLN ridge waveguides," Optics Express, 2011, Vol.19, No.7, p.6326-6332 Takeshi Umeki, Masaki Asobe, and Hirokazu Takenouchi, “In-line phase sensitive amplifier based on PPLN waveguides,” Optics Express, May 2013, Vol.21, No.10, p.12077-12084 M. Asobe, T. Umeki, H. Takenouchi, and Y. Miyamoto, “In-line phase-sensitive amplifier for QPSK signal using multiple QPM LiNbO3 waveguide,” In Proceedings of the OptoElectronics and Communications Conference, OECC, 2013, PDP paper PD2-3 Masaki Asobe, Takeshi Umeki, Kouji Enbutsu, Osamu Tadanaga, and Hirokazu Takenouchi, "Phase squeezing and dispersion tolerance of phase sensitive amplifier using periodically poled LiNbO3 waveguide," J. Opt. Soc. Am. B 31, 3164-3169 2014 D. McGhan, C. Laperle, A. Savchenko, C. Li, G. Mak, and M. O'Sullivan, "5120 km RZ-DPSK Transmission over G652 Fiber at 10Gb / s with No Optical Dispersion Compensation," in Optical Fiber Communication Conference and Exposition and The National Fiber Optic Engineers Conference, Technical Digest (CD) (Optical Society of America, 2005, paper PDP27 Koji Enbutsu, Takeshi Umeki, Osamu Tadanaga, Masaki Asobe, Hirokazu Takenouchi, "PPLN-Based Low-Noise In-Line Phase Sensitive Amplifier with Highly Sensitive Carrier-Recovery System," IEICE Transactions on Communications, vol. E99.B 2016 No . 8 pp. 1727-1733

However, the conventional phase-sensitive optical amplifier has the following problems. As shown in FIG. 3, in the phase-sensitive optical amplifier, the gain changes depending on the phase relationship between the signal and the excitation light. Therefore, it has an amplification characteristic that is very sensitive to the phase fluctuation of the signal. When a phase-sensitive optical amplifier is used as a relay amplifier, if the phase fluctuation depending on the frequency component of the signal light is large due to the influence of dispersion in the optical fiber, the signal quality after phase-sensitive optical amplification deteriorates (non-). Patent Document 4). Since the frequency spectrum spread of the signal light differs depending on the modulation rate of the signal, the yield strength to dispersion also differs. For example, it has been pointed out that the signal quality after phase-sensitive light amplification deteriorates due to the addition of cumulative dispersion of about 40 ps / nm to the signal light modulated at a symbol rate of 40 GHz. Assuming that the dispersion value of a commonly used single mode fiber (SMF) is 18 ps / nm · km with respect to 1.5 μm band light, the cumulative dispersion value of 40 ps / nm corresponds to a length of about 2.2 km. Given that one span of a transmission fiber usually has a length of about 80 km, the phase sensitive optical amplifier cannot amplify the signal light that has transmitted the SMF.

As a transmission fiber when using a phase-sensitive optical amplifier, a method of optically suppressing the dispersion of a transmission line by using a dispersion compensating fiber has been adopted (Non-Patent Documents 2 and 3). By using the dispersion compensating fiber, the dispersion value in the wavelength band used can be suppressed to several ps / nm, and low noise amplification can be realized without deteriorating the signal quality even through the phase sensitive optical amplifier. However, it is not realistic to use dispersion compensating fibers for all the transmission fibers for arranging the phase-sensitive optical amplifier, because the existing optical communication equipment is significantly renewed.

The present invention has been made in view of the above-mentioned problems of the prior art, and an object of the present invention is to provide a configuration in which a phase-sensitive optical amplifier can be applied to a signal after transmitting an SMF.

In order to achieve such an object, the invention according to claim 1 is connected to a transmitter that generates a modulated signal light group and the transmitter via an optical fiber transmission path. An optical transmission system including an optical amplification device, wherein the transmitter is digital signal data with pre-dispersion compensation according to the wavelength dispersion generated in the optical fiber transmission line, and has a phase-conjugated relationship. A digital signal generator that generates two sets of digital signal data, an idler light generator that generates idler light that has a phase conjugate relationship with the carrier light from the carrier light and locally oscillated light from the signal light source, and A first optical modulation unit that generates modulated signal light by changing the optical phase and light intensity of the carrier light based on one of the two sets of digital signal data having a phase conjugate relationship, and the phase conjugate The modulated signal light, said to include a second optical modulator that changes the optical phase and intensity of the idler light to generate modulated idler light based on the other of the two sets of digital signal data in relation to each other. The modulated idler light and the locally oscillating light are simultaneously transmitted, and the optical amplification device is connected to a first secondary nonlinear optical element that generates excitation light based on the locally oscillating light transmitted through the optical fiber transmission path. A second second-order nonlinear optical element that performs non-reduced parametric amplification of the modulated signal light and the modulated idler light based on the excitation light, the phase of the modulated signal light, and the phase of the excitation light. It is an optical transmission system characterized by including means for synchronization. This optical transmission system corresponds to the optical transmission system of the second embodiment.

The invention according to claim 2 is the optical transmission system according to claim 1 , wherein the transmitter includes a third second-order nonlinear optical element that generates second harmonic light from the locally oscillating light, and the first. It is characterized by further including a fourth second-order nonlinear optical element that generates the idler light, which is the difference frequency light between the second harmonic light and the carrier light.

The invention according to claim 3 is a signal light group generated in a transmitter by a modulated signal to which pre-dispersion compensation is added according to the wavelength dispersion generated in the optical fiber transmission line, and transmits the optical fiber transmission line. It is an optical amplification device that photosensitizes and amplifies the signal light group, and the signal light group is a carrier light from a signal light light source based on one of two sets of digital signal data having a phase conjugate relationship. Based on the modulated signal light modulated by changing the optical phase and light intensity of the above and the other of the two sets of digital signal data having a phase-coupled relationship, the carrier light is generated from the carrier light and the locally oscillated light. With respect to the idler light having a phase conjugate relationship with the idler light, the idler modulated light modulated by changing the optical phase and the light intensity, the modulated signal light, and the locally oscillating light transmitted together with the idler modulated light. Including, the optical amplification device includes a first secondary nonlinear optical element that generates excitation light based on the locally oscillating light transmitted through the optical fiber transmission path, and the modulation signal light based on the excitation light. The light includes a second secondary nonlinear optical element that performs non-reduced parametric amplification with respect to the idler-modulated light, and means for synchronizing the phase of the modulated signal light with the phase of the excitation light. It is an amplification device. This optical amplifier corresponds to the optical amplifier of the second embodiment.

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to realize low-noise phase-sensitive optical amplification without characteristic deterioration due to dispersion even for a signal after transmitting SMF.

FIG. 1 is a diagram showing a basic configuration of a PSA of the prior art. FIG. 2 is a diagram showing a configuration of a prior art PSA using a PPLN waveguide. FIG. 3 is a diagram showing the relationship between the phase difference between the input signal photoexcited light of PSA and the gain. FIG. 4 is a diagram showing a configuration of an optical transmission system according to the first embodiment of the present invention. FIG. 5 is a diagram showing a configuration of PSA of the optical transmission system of the first embodiment. FIG. 6 is a diagram illustrating the relationship of each signal of the relay type PSA on the wavelength axis. FIG. 7 is a diagram showing a spectrum of signal light amplified by the relay type PSA. FIG. 8 is a diagram showing a signal optical constellation of each part of the optical transmission system. FIG. 9 is a diagram showing a configuration of an optical transmission system according to a second embodiment of the present invention. FIG. 10 is a diagram showing a configuration of a transmitter of the optical transmission system of the second embodiment.

In the optical transmission system of the present invention, the transmitter side performs pre-dispersion compensation that gives the modulation signal applied to the modulator a wavelength dispersion opposite to the wavelength dispersion generated in the transmission line. By transmitting the pre-dispersion-compensated modulated light to the transmission line, the deterioration of signal quality due to the wavelength dispersion occurring in the transmission line can be reduced or canceled. An optical transmission system that combines a signal light modulated by a modulated signal with pre-dispersion compensation for a BPSK signal and a reduced PSA is disclosed. Further, an optical transmission system in which a signal light modulated by a modulated signal in which a QPSK signal is pre-distributed compensated and a non-reduced PSA are combined is disclosed. According to the present invention, it is possible to realize low-noise phase-sensitive optical amplification without characteristic deterioration due to dispersion even for a signal after SMF transmission, which has been impossible until now. Not only can low-noise optical amplification be possible, but the phase noise received by the optical signal in the transmission line can be compensated for as it is, and the SNR, which is essential for large-capacity optical transmission, can be improved. .. Hereinafter, embodiments of the optical transmission system and the optical amplifier device of the present invention will be described in detail with reference to the drawings.

[Embodiment 1]
FIG. 4 is a diagram showing a configuration of an optical transmission system according to the first embodiment of the present invention. In the optical amplification device and the optical transmission system according to the present invention, the configuration of PSA using the PPLN waveguide is shown. The optical transmission system according to the present embodiment enables low-noise phase-sensitive optical amplification even for an optical signal after optical fiber transmission by compensating for the dispersion of the transmission line in advance by pre-digital signal processing. In the first embodiment of FIG. 4, a degenerate PSA that amplifies only components in phase with the excitation light shows a configuration that realizes signal amplification after fiber transmission by using pre-dispersion compensation. The degenerate PSA can amplify an intensity-modulated signal or a signal in which only in-phase components are modulated. In this embodiment, a configuration example for amplifying a signal light subjected to binary phase modulation (BPSK) will be described.

The optical transmission system 400 of FIG. 4 roughly includes a transmitter 401, an optical fiber 430, a polarization compensator 440, and a PSA 500 that transmit signal light pre-dispersed compensated by digital signal processing. The details will be described later together with the operation, but the transmitter 401 includes a signal light source 409, a digital signal generator 402, a digital-to-analog converter (DAC) 405 and 406, drive amplifiers 407 and 408, and an IQ which is an optical modulation unit. It is equipped with a modulator (IQM) 410 and an EDFA 420. In the optical transmission system of this embodiment, the transmitter 401 produces a pre-dispersion compensated BPSK signal.

FIG. 5 is a diagram showing a configuration of a relay type PSA in the optical transmission system of the first embodiment. The relay type PSA500 of FIG. 5 is roughly classified into an excitation light generation unit 530 and a phase-sensitive light amplification unit 540 that perform a method of extracting a carrier wave phase from a BPSK signal. The excitation light generation unit 530 and the phase sensitive light amplification unit 540 are each configured by using two PPLN waveguide modules. In the excitation light generation unit 530, the first PPLN waveguide module 504 is used for the generation of the second harmonic (SH light) (SHG) that becomes the excitation light, and the second PPLN waveguide module 505 is used. , Used for Difference Frequency Generation. The first PPLN waveguide module 504 includes a spatial optical system for both input and output, a PPLN waveguide, and a dichroic mirror on the output side. The second PPLN waveguide module 505 includes both input / output spatial optical systems, a PPLN waveguide, and both input / output dichroic mirrors. In the phase-sensitive optical amplification unit 540, the third PPLN waveguide module 516 is used to generate SH light as excitation light, and the fourth PPLN waveguide module 517 is used for parametric amplification (OPA: Optical Parametric Amplification). Be done. The configuration of each PPLN waveguide module is the same as that of the excitation light generator.

Here, the method of manufacturing the PPLIN waveguide used in the PSA500 of FIG. 5 is illustrated below. First, a periodic electrode having a period of about 17 μm was formed on LiNbO3 to which Zn was added. Next, a polarization reversal grating corresponding to the above electrode pattern was formed in Zn: LiNbO3 by the electric field application method. Next, the Zn: LiNbO3 substrate having this periodic polarization inversion structure was directly bonded onto the clad LiTaO3, and the two substrates were firmly bonded by heat treatment at 500 ° C. Next, the core layer was thinned to about 5 μm by polishing, and a ridge type optical waveguide was formed by using a dry etching process. The temperature of this waveguide can be adjusted by a Peltier element. The length of the waveguide was set to 50 mm. The second-order nonlinear optical element having the PPLN waveguide formed in this way has a modular configuration in which light can be input and output with a polarization-holding fiber in the 1.5 μm band. In this embodiment, LiNbO3 to which Zn is added is used, but other non-linear materials such as KNbO3, LiTaO3, LiNbxTa1-xO3 (0 ≦ x ≦ 1) or KTIOPO4, or Mg, Zn, Sc, In to them are used. A material containing at least one selected from the group consisting of the above as an additive may be used.

The operation of the transmitter 401 that transmits the pre-dispersion-compensated signal light in the optical transmission system 400 will be further described with reference to FIG. 4 again. In the transmitter 401 of FIG. 4, the digital signal generation unit 402 performs an operation of giving a shift opposite to the optical phase shift caused by the dispersion received in the optical fiber transmission line, and performs an operation on the modulated signal based on the transmission data. Add compensation. That is, the digital signal generation unit 402 outputs the pre-dispersion compensated first data signal 404 and the second data signal 403. These data signals were input to the first DAC 406 and the second DAC 405 to be converted into analog signals, and further amplified to a required level by the first drive amplifier 408 and the second drive amplifier 407. As a result, the two modulation signals I and Q input to the IQ modulator 410 become signals with pre-compensation for wavelength dispersion. The carrier light from the signal light source 409 is intensity and phase-modulated by the above-mentioned modulated signals I and Q in the IQ modulator 410, and a BPSK signal having a symbol rate of 10 GHz with pre-dispersion compensation is output.

The distortion given by the digital signal generation unit 402 is given so as to be offset by receiving signal distortion due to dispersion accumulated in the signal light after transmission through the optical fiber 430 in the subsequent stage. In order to compensate for the level drop in the IQ modulator 410, the signal light was amplified using the EDFA 420 and then input to the 80 km optical fiber 430. The optical fiber 430 has a dispersion characteristic of 16.7 ps / nm / km, and the total dispersion value after 80 km transmission is 1336.0 ps. As shown in the signal optical constellations 421 and 422 before and after the optical fiber 430 in FIG. 4, the distortion of the signal light is compensated by propagating through the optical fiber 430. An ideal BPSK signal light can be obtained by transmitting an 80 km optical fiber 430 and receiving signal distortion due to dispersion accumulated in the signal light. In the pre-dispersion compensation process, compensation can be performed in consideration of the slope of the dispersion. Therefore, it is possible to perform highly accurate dispersion compensation for a fixed wavelength as compared with the optical compensation means of the prior art, and it is possible to obtain signal light without distortion due to dispersion due to optical fiber transmission. .. By compensating for the cumulative dispersion value of the signal light input to the PSA in this way, it is possible to perform phase-sensitive light amplification with low noise even for the signal light after transmission through the optical fiber. The signal light transmitted through the optical fiber 430 is compensated for polarization in the optical fiber by the polarization compensator 440, and then the signal light 501 is input to the PSA 500. Next, the operation of the PSA500 will be described with reference to FIG.

Referring to FIG. 5, in PSA500, a part of the input signal light 501 is branched by the optical coupler 502 and incident on the excitation light generation unit 530. When a retractable PSA is used as a relay amplifier in optical transmission, the average phase is extracted from the modulated signal light, and the excitation light generator 530 generates excitation light synchronized with the carrier phase of the signal. There is a need to. In the following description, the operation of the PSA500 will be described with reference to FIG. 6 with reference to the relationship of the light of each part of the relay type PSA500 on the wavelength axis.

6 (a) to 6 (d) are diagrams for explaining the relationship of each signal on the wavelength axis in the relay type PSA500 of FIG. In each of the figures of FIG. 6, the relative positional relationship of each signal on the wavelength axis is conceptually shown with the horizontal axis as the wavelength. Note that light with a higher frequency is shown as light with a shorter wavelength in FIG. 6 because the horizontal axis is the wavelength axis in each figure of FIG. Therefore, the higher the frequency of light, the closer it is to the short wavelength side on the left side of the horizontal axis in FIG.

Returning to FIG. 5 again, in the excitation light generation unit 530, after the branched signal light 501 is amplified by the EDFA 503, the second PPLN waveguide module 504 has a second wavelength that is half the wavelength of the signal light. Generates harmonic light (SH light). On the other hand, in the excitation light generation unit 530, the excitation light (Lo1) 551 generated by the local oscillation light source 507 and the SH light from the above-mentioned first PPLN waveguide module 504 are input to the second PPLN module 505. .. In the second PPLN module 505, the difference frequency light (Lo2) between the excitation light (Lo1) and the SH light is generated.

As for the signal light, the excitation light (Lo1) and the differential frequency light (Lo2) output from the second PPLN waveguide module 505, only the differential frequency light (Lo2) is extracted by the bandpass filter 506. After passing through the optical circulator 513, the differential frequency light (Lo2) is input to the semiconductor laser 512 via, for example, an array waveguide grating (AWG) type wavelength duplexer 511. The semiconductor laser 512 generates a second excitation light having the same phase and the same wavelength as the difference frequency light (Lo2) by photoinjection synchronization. Since the light intensity of the second excitation light is determined by the output of the semiconductor laser 512, a weak difference frequency light (Lo2) of several tens of μW from the second PPLN waveguide module 505 is used to obtain a second excitation light of several tens of mW or more. The excitation light (Lo2) of 2 can be obtained. From the demultiplexing side port of the wavelength demultiplexer 511, the excitation light (Lo1) branched by the optical coupler 508 from the local oscillation light source 507 is incident on the demultiplexing side port of the wavelength demultiplexer 511, and the semiconductor laser 510 The excitation light output 552 is output from the circulator 513 by merging with the second excitation light (Lo2) synchronized with the injection from.

Here, referring to FIG. 6A, the excitation light (Lo1) p1 is shown on the short wavelength side on the wavelength axis with respect to the signal light s. Since the signal light s is modulated, its spectrum is widened. With reference to FIG. 6B, the difference frequency light (Lo2) p2 is shown on the opposite side of the excitation light (Lo1) p1 with respect to the signal light s on the wavelength axis. Intensity noise of the difference frequency light (Lo2) p2 is removed by injection synchronization (injection lock) into the semiconductor laser 512.

Phase φs of the signal light s in the relay-type PSA of FIG. 5, between the phase phi _p2 of the pump light (Lo1) p1 phase phi _p1 and difference frequency light (Lo2) p2, satisfies the following expression.
2φs − φ _p1 − φ _p2 = 0 equation (2)

Therefore, the phase φ _p2 of the differential frequency light (Lo2) p2 is expressed by using the phase φ s of the signal light s and the phase φ _p1 of the excitation light (Lo1) p1 as shown in the following equation.
φ _p2 = ₂ φ s − φ _p1 formula (3)

As can be seen from the equations (2) and (3), the phase φs of the signal light s can be doubled by generating SH light from the signal light s in the first PPLN waveguide module 504. .. Normally, since the signal light is modulated by the information data to be transmitted, it is difficult to extract the carrier phase. However, the binary phase modulation can be removed by doubling the phase φs of the signal light s as in the excitation light generation unit 530 of FIG. Further, as shown in FIG. 6B, the intensity noise of the second excitation light (Lo2) p2, which is the difference frequency light between the SH light and the excitation light (Lo1), is removed by injection synchronization. In the PSA500 of FIG. 5, a pure second excitation light (Lo2) 552 having a good S / N ratio holding the phase information of the carrier wave of the signal light 501 is obtained.

The excitation light (Lo1) 551 from the local oscillation light source 507 and the injection-synchronized second excitation light (Lo2) 552 are amplified by the EDFA 514 in the phase-sensitive optical amplification unit 540. Further, after removing unnecessary ASE light using BPF515, it is input to the third PPLN waveguide module 516. In the third PPLN waveguide module 516, as shown in FIG. 6 (c), excitation having a sum frequency (SF: Sum Frequency) of the excitation light (Lo1) p1 and the second excitation light (Lo2) p2. Optical SF is generated. At this time, the following equation is satisfied between the phase φ _p1 of the excitation light (Lo1) p1, the phase φ _{p2 of} the second excitation light (Lo2) p2, and the phase φ _SF of the sum frequency excitation light SF.
φ _SF = φ _p1 + φ _p2
= 2φs equation (4)

The sum frequency excitation light 553 from the third PPLN waveguide module 516 in FIG. 5 is incident on the fourth PPLN waveguide module 517. Parametric amplification of the signal light 501 via the optical coupler 502 and the sum frequency excitation light 553 is performed in the fourth PPLN waveguide module 517. For phase-sensitive optical amplifier, as shown in (d) of FIG. 6, between the phase phi _SF phase φs and the sum frequency light SF of the signal light s, it is necessary to satisfy the following equation.
ΔΦ = φ _SF − 2φs = nπ (n is an integer) Equation (5)

That is, the gain is maximized when the phase difference ΔΦ between the input signal light and the sum frequency excitation light is −π, 0, or π. In order to synchronize the phase between the signal light and each excitation light, the excitation light 551 is subjected to phase modulation with a weak signal using a phase modulator 509. A part of the output light that has passed through the fourth PPLN waveguide module 517 is branched by the optical coupler 518. The photodetector 519 detects a change in the light intensity of the branched light, and feedback is applied to the optical fiber extender 521 using the PLL circuit 520 so that the excitation light 551 and the signal light 501 are phase-synchronized. By the above-mentioned phase synchronization, the relay type PSA500 of FIG. 5 can be stably operated. Therefore, the loop including the optical coupler 518, the PLL circuit 520 and the optical fiber extender 521 constitutes a means for synchronizing the phase of the signal light with the phase of the second excitation light.

To give an example of the wavelength of each signal, the signal light s is 1535.8 nm, the excitation light p1 (Lo1) is 1534.25 nm, the difference frequency p2 (Lo2) is 1537.4 nm, and the sum frequency SF is 767.9 nm.

In the optical transmission system 400 of FIG. 4, in order to confirm the optical amplification characteristics of the PSA500, the signal optical frequency spectrum and the modulation constellation before and after the amplification were confirmed under different conditions of the pre-dispersion compensation.

FIG. 7 is a diagram illustrating a frequency spectrum of signal light amplified by the phase-sensitive optical amplifier of the first embodiment. In any of FIGS. 7A to 7C, three types of spectra are superimposed and displayed, one is the input of PSA500 and the other is the output of two PSA500s (position of arrow C in FIG. 4). PSA indicates a normal operating state (PSA amp) and PSA indicates a decayed state (PSA deamp). The PSA attenuation state (PSA deamp) is obtained by making the phases of the input signal light 501 to the PSA 500 and the sum frequency excitation light 553 orthogonal (shift by π / 2). For reference, FIG. 7A shows a spectrum when a BPSK signal “without pre-dispersion compensation” is input to the PSA500 without transmitting the optical fiber 430. That is, the spectrum (Input) of the BPSK signal without pre-dispersion compensation (no distortion) at the position of arrow A in FIG. 4 and the spectrum in the ideal state (PSA amp, PSA) in which PSA is not affected by the channel dispersion at all. deamp) is shown.

FIG. 7B shows a spectrum when a BPSK signal “without pre-dispersion compensation” is input to the PSA after 80 km of optical fiber transmission. That is, the input spectrum (Input) is at the position of arrow B in FIG. FIG. 7 (c) shows a spectrum when a “pre-dispersion compensated” BPSK signal is input to the PSA after 80 km of optical fiber transmission.

As shown in FIG. 7B, in a signal without pre-dispersion compensation, the phase shifts for each frequency component due to the influence of the dispersion of the optical fiber, so that the phase sensitivity synchronized for each frequency component No characteristics have been obtained. In particular, the spectrum in the sensitive attenuation region is deteriorated due to the phase error, and the distortion in the adjacent channel band is increased. On the other hand, as shown in FIG. 7 (c), when a signal with pre-dispersion compensation is used, no spectral deterioration is observed in the phase-sensitive light amplification / attenuation operation, and FIG. 7 (a) shows. The spectrum equivalent to that of the input signal in the ideal state and the signal light amplified in the ideal state was obtained.

FIG. 8 is a diagram showing a constellation of signal light amplified by the phase sensitive optical amplifier of the first embodiment. In order to confirm the signal quality after the phase-sensitive optical amplification, the constellation diagram of the signal light before and after the amplification was acquired. As a reference, FIG. 8A shows a constellation in the PSA output of a BPSK signal that is not subjected to optical fiber transmission without pre-dispersion compensation. FIG. 8B shows the constellation at the PSA output after 80 km optical fiber transmission of the BPSK signal “without pre-dispersion compensation”. FIG. 8 (c) shows the constellation of the "pre-dispersion compensated" BPSK signal at the PSA output after 80 km fiber transmission.

In (a), (b), and (c) of FIG. 7, the Q values are 17.6 dB, 6.9 dB, and 17.3 dB, respectively. Here, the Q value in the BPSK signal is defined by the difference between the standard deviation and the average amplitude of the amplitudes of "1" and "0" of the binary signal. In the signal without the pre-dispersion compensation, the Q value of the signal light is deteriorated by the phase sensitive light amplification of the signal light affected by the dispersion in the optical fiber. On the other hand, by using the pre-dispersion compensation, it was possible to realize phase-sensitive optical amplification without deterioration of the Q value even when the single-mode fiber was transmitted and affected by the dispersion of the fiber.

In the PSA500 of the present embodiment, optical injection synchronization is used for phase synchronization between the signal light 501 and the local oscillation light 551, but other methods may be used. For example, phase synchronization may be performed by feeding back a high-speed electric signal feedback represented by a costus loop to the local oscillation light source 507.

In the optical transmission system of the first embodiment described above, the phase error caused by the dispersion of the optical fiber transmission line is compensated in advance at the stage of the transmitter 401 digital signal generation unit. As a result, it is possible to realize phase-sensitive optical amplification without characteristic deterioration due to dispersion even for signal light after transmitting an optical fiber having dispersion. The optical transmission system of the present invention makes it possible to apply PSA, which could only be applied to a transmission line composed of dispersion-compensated fibers in the prior art, to a signal after normal SMF transmission. The application area of PSA, which is the key to improving the SNR required for large-capacity optical transmission, can be significantly expanded.

[Embodiment 2]
Hereinafter, the optical amplifier and the optical transmission system according to the second embodiment of the present invention will be described. The optical transmission system of the first embodiment has a configuration in which a degenerate PSA is used for the BPSK modulated signal. The optical transmission system of the first embodiment has no problem in application to a system in which the transmission capacity is sufficiently satisfied even when a modulation method such as a BPSK modulated signal is used. However, in a system requiring a larger capacity transmission, it is difficult to apply a degenerate PSA having a limited amplification format and a limited number of wavelengths. The optical transmission system of the present embodiment can support signals of any modulation format such as quadrature amplitude modulation (QAM) signals including a data modulation component as well as a quadrature phase component, and collectively amplifies a plurality of wavelengths. A non-retractable PSA configuration that can be presented. In this embodiment in which pre-dispersion compensation is applied to a modulated signal including a data modulation component as well as a quadrature phase component, a quadrature phase shift keying (QPSK) signal, which is a quadrature QAM, is taken as an example. explain.

FIG. 9 is a diagram showing a configuration of an optical transmission system according to a second embodiment of the present invention. The optical transmission system 900 according to the present embodiment pre-compensates the dispersion of the transmission line for the modulated signal by digital signal processing in the transmitter 1000, so that the optical signal after the optical fiber transmission is also low in noise. Enables phase-sensitive light amplification. The optical transmission system 900 of FIG. 9 uses a transmitter 1000 for transmitting signal light, idler light, and pilot signal, an optical fiber (single mode fiber: SMF) 910 and a polarization compensator 920, and a pilot signal to generate excitation light. It consists of a non-retractable PSA930 to be produced.

FIG. 10 is a diagram showing a detailed configuration of the transmitter 1000 of the optical transmission system of the second embodiment. Hereinafter, the configuration and operation of the transmitter will be described with reference to FIG. 10, and then the configuration and operation of the non-degenerate PSA930 will be described with reference to FIG. The transmitter 1000 of FIG. 10 is roughly composed of a signal light source 1001, a local oscillation light source 1003, an EDFA1005, 1022, a PPLN waveguide modules 1002, 1007, an optical coupler 1004, 1021, 1023, and a bandpass filter (BPF). ) 1006 and a wavelength selection type optical switch (WSS: Wavelength Selective Switch) 1020. In order to generate a pre-dispersion compensated modulated signal, digital signal generators 1010, 1011, DACs 1012 to 1015, drive amplifiers 1016 to 1019, and IQ modulators 1008 and 1009 are further provided.

In the transmitter 1000, idler light paired with the signal light 1030 is generated by a difference frequency generation process (DFG: Difference Frequency Generation) using a PPLN waveguide. The DFG process is operated by a first PPLN waveguide module 1007 for generating SH light as excitation light and a second PPLN waveguide module 1002 for idler light generation. The configuration of the PPLN waveguide module and the method of manufacturing the PPLN waveguide in the module are the same as those in the first embodiment.

The excitation light (Lo1) from the local oscillation light source 1003 generates SH light 1038 in the first PPLN waveguide module 1007 via the optical coupler 1004, EDFA1005 and BPF1006. The signal optical carrier 1030 from the SH light 1038 and the signal light source 1001 generates idler light corresponding to the difference frequency between the SH light 1038 and the signal light carrier 1030 in the second PPLN waveguide module 1002. Therefore, the output light 1031 from the second PPLN waveguide module 1002 includes a carrier wave of signal light and idler light. Here, the frequency of the signal light and the frequency of the idler light have a symmetrical positional relationship with respect to the frequency of the locally oscillated light on the frequency axis. Further, the carrier wave of the signal light and the idler light are in a phase conjugation relationship.

The first digital signal generation unit 1010 and the second digital signal generation unit 1011 perform an operation to give a shift opposite to the optical phase shift caused by the dispersion received in the optical fiber transmission line, and are based on the transmission data. Compensate for the modulated signal. The first data signal 1034 and the second data signal 1035 from the first digital signal generation unit 1010 generate a modulated signal for the signal light. Similarly, the third data signal 1036 and the fourth data signal 1037 from the second digital signal generator 1011 generate a modulated signal for idler light. The data signals 1034 to 1037 were input to the DACs 1012 to 1015, converted into analog signals, and further amplified to the required level by the drive amplifiers 1016 to 1019. The two modulation signals I and Q input to the first IQ modulator 1008, which is an optical modulation unit, are modulation signals for the signal light to which the wavelength dispersion is pre-compensated. Similarly, the two modulation signals I and Q input to the second IQ modulator 1009, which is the optical modulation unit, are modulation signals for the idler light to which the wavelength dispersion is pre-compensated. The distortion due to the pre-dispersion compensation given by the digital signal generation units 1010 and 1011 is offset by receiving the signal distortion due to the dispersion accumulated in the signal light and the idler light after transmission through the optical fiber 910 in the subsequent stage. Further, the first data signal 1034 and the second data signal 1035 and the third data signal 1036 and the fourth data signal 1037 are data signals having a phase conjugation relationship with each other.

The signal light and idler light from the second PPLN waveguide module 1002 are demultiplexed by WSS1020 and their intensity and phase are modulated by IQ modulators 1008 and 1009, respectively. That is, the IQ modulator 1008 outputs a signal light of QPSK modulation with a symbol rate of 10 GHz with pre-dispersion compensation, and the IQ modulator 1009 outputs a QPSK with a symbol rate of 10 GHz with pre-dispersion compensation. Modulated idler light is output. The QPSK signals from the two IQ modulators 1008 and 1009 are combined by the optical duplexer (optical coupler) 1021. As described above, the modulation signals to the two IQ modulators 1008 and 1009 are signals that are in phase conjugation with each other. Further, the signal light and the idler light as the carrier waves to be modulated also have a phase conjugation relationship. That is, the modulated signal light and idler light from the IQ modulators 1008 and 1009 have a phase conjugation relationship with each other in both the modulation data and the optical phase of the carrier wave.

To compensate for the level drop in the IQ modulator, the EDFA1022 is used to amplify the modulated signal. Then, the excitation light demultiplexed by the optical coupler 1004 was combined with the signal light and the idler light by the optical coupler 102, and input to the 80 km optical fiber 910 of FIG. The transmission fiber and its dispersion characteristics are the same as those in the first embodiment. When performing phase-sensitive light amplification, excitation light having information on the reference phase of the signal light to be amplified and the idler light is required. In the optical transmission system of the present embodiment, the excitation light 1032 output from the local oscillation light source 1033, which is the reference phase of the signal light and the idler light, is used as a pilot tone that is simultaneously transmitted with the signal light and the idler light.

Referencing FIG. 9 again, the relay type PSA930 roughly includes a wavelength duplexer 931, a fourth optical loop duplexer, BPF933, 938, 942, a circulator 934, and a local oscillation light source (semiconductor laser) 935. The phase modulator 936, the EDFA937, the PPLN waveguide modules 939 and 940, the photodetector 943, the PLL circuit 944, and the optical fiber extender 945 by the PZT are provided. In the relay type PSA930, as in the transmitter 1000, the phase sensitive light amplification process involves a third PPLN waveguide module 939 for SH light generation (SHG), which is the excitation light, and a fourth PPLN lead for OPA. It is operated by the waveguide module 940. The configuration of each PPLN waveguide module and the method of manufacturing the PPLN waveguide in the module are the same as those in the first embodiment.

The polarization compensator 920 compensates for the polarization fluctuation in the optical fiber of the signal light group that has transmitted the optical fiber 910 including the signal light, the idler light, and the pilot tone. In the PSA930, only the pilot tone in the signal light group 951 is demultiplexed by the wavelength duplexer 931 and the signal light and idler light are input to the fourth PPLN waveguide module 940. The pilot tone was subjected to light injection synchronization to the second local oscillation light source 935 via the optical circulator 934 after removing unnecessary ASE optical components by BPF933. By optical injection synchronization, locally oscillated light having the same wavelength and the same phase information as the pilot tone was generated. The local oscillation light source 935 can be realized by, for example, a semiconductor laser.

The injection-synchronized output from the local oscillation light source 935 is taken out by the optical circulator 934, and the local oscillation light 953 is amplified to 3W by the EDFA937. Further, after removing the naturally emitted light of EDFA by BPF938, the light was incident on the third PPLN waveguide module 939. The excitation light 954 of the SH light was generated by the SHG operation in the PPLN waveguide in the third PPLN waveguide module 939. Excitation light 954 with an intensity of 1 W was obtained by locally oscillating light of 3 W. This excitation light 954 was incident on the fourth PPLN waveguide module 940. In the PPLN waveguide in the fourth PPLN waveguide module 940, a non-degenerate parametric process is caused for the signal light and the idler light.

The signal light and idler light are synchronized with the reference phase of the excitation light 1038 in the transmitter 1000. Therefore, even in the relay type PSA930, by synchronizing the excitation light 954 with the reference phase of the transmitter 1000, batch phase sensitive light amplification can be realized at all wavelengths. In order to synchronize the phase of the excitation light and the signal light in the PSA930, the phase modulator 936 is used to perform phase modulation on the locally oscillated light with a weak signal. A part of the output light 955 that passed through the fourth PPLN module 940 was branched by the optical coupler 941. Only the wavelength of the signal light of the output light 955 is cut out by the BPF 942, and the change in the light intensity is detected by the photodetector 943. Using the PLL circuit 944, feedback was applied to the optical fiber extender 945 so that the phase of the excitation light and the phase of the signal light were synchronized. Here, only the signal light of the output light 955 is cut out in order to apply phase synchronization, but even if the idler light of the output light 955 is cut out and feedback is applied to the optical fiber extender 945, the phase sensitive light is similarly cut out. Amplification is possible.

In the optical transmission system 900 of the present embodiment, optical injection synchronization is used for phase synchronization between the pilot tone transmitted from the transmitter 1000 and the locally oscillated light 953 of the PSA930. As another method, phase synchronization may be performed by feeding back the feedback of a high-speed electric signal represented by a costus loop to the local oscillation light source 935, for example.

Also in the above-described optical transmission system of the present embodiment, the phase error caused by the dispersion of the optical fiber transmission line is compensated in advance at the stage of the digital signal generation units 1010 and 1011 of the transmitter 1000. As a result, it is possible to realize phase-sensitive optical amplification without characteristic deterioration due to dispersion even for signal light after transmitting an optical fiber having dispersion. The optical transmission system of the present invention makes it possible to apply PSA, which could only be applied to a transmission line composed of dispersion-compensated fibers in the prior art, to a signal after normal SMF transmission. It is possible to support signals in modulation formats such as QAM signals containing both quadrature phase components and data modulation components, and PSA capable of batch amplification of multiple wavelengths can expand the application range of PSA required for large-capacity optical transmission.

The present invention can be used in an optical transmission system. In particular, it can be used for relay-type optical amplifiers.

100, 200, 500, 930 Phase Sensitive Optical Amplifier (PSA)
102 Excitation light source 103 Excitation light phase control unit 201, 420, 503, 514, 937, 1005, 1022 EDFA
202, 204, 504, 505, 516, 517, 939, 940, 1002, 1007 PPLN waveguide module 203-1, 203-2, 502, 508, 518, 941, 1004, 1021, 1023 Optical coupler (optical branch) )
205, 936 Phase Modulators 206, 207, 521, 945 Optical Fiber Elongers 209, 420, 520, 944 PLL Circuits 400, 900 Optical Transmission Systems 401, 1000 Transmitters 402, 1010, 1011 Digital Signal Generators 405, 406, 1012-1015 Digital-to-analog converter (DAC)
409, 1001 Signal light source 410, 1008, 1009 IQ modulator (IQM)
430, 910 Optical Fiber 440, 920 Dispersion and Polarization Compensator 506, 515, 933, 938, 942, 1006 BPF
507, 1003 Local oscillation light source 512, 935 Semiconductor laser (local oscillation light source)
513, 934 Optical circulator 530 Excitation light generator 540 Phase sensitive optical amplifier 931 Wavelength duplexer 1020 Wavelength selection type optical switch

Claims (3)

A transmitter that produces a modulated signal light group,
An optical transmission system including an optical amplifier connected to the transmitter via an optical fiber transmission line.
The transmitter
A digital signal generator that generates two sets of digital signal data having a phase conjugate relationship, which is digital signal data to which pre-dispersion compensation is added according to the wavelength dispersion generated in the optical fiber transmission line.
An idler light generator that generates idler light that has a phase conjugate relationship with the carrier light from the carrier light and locally oscillated light from the signal light source.
A first optical modulation unit that generates modulated signal light by changing the optical phase and light intensity of the carrier wave light based on one of the two sets of digital signal data having a phase conjugation relationship.
It includes a second optical modulation section that generates modulated idler light by changing the optical phase and light intensity of the idler light based on the other of the two sets of digital signal data having a phase conjugation relationship.
The modulated signal light, the modulated idler light, and the locally oscillated light are simultaneously transmitted.
The optical amplifier
A first second-order nonlinear optical element that generates excitation light based on the locally oscillating light transmitted through the optical fiber transmission line.
A second second-order nonlinear optical element that performs non-degenerate parametric amplification on the modulated signal light and the modulated idler light based on the excitation light.
An optical transmission system comprising a means for synchronizing the phase of the modulated signal light with the phase of the excitation light. The transmitter
A third second-order nonlinear optical element that generates second harmonic light from the locally oscillated light,
The optical transmission system according to claim 1 , further comprising a fourth second-order nonlinear optical element that generates the idler light, which is the difference frequency light between the second harmonic light and the carrier light. A signal light group generated in the transmitter by a modulated signal with pre-dispersion compensation according to the wavelength dispersion generated in the optical fiber transmission line, and the signal light group transmitted through the optical fiber transmission line is photosensitively amplified. It is an optical amplification device that
The signal light group is
Modulated signal light modulated by changing the optical phase and intensity of carrier light from a signal light source based on one of two sets of digital signal data in a phase-conjugated relationship.
Based on the other of the two sets of digital signal data having a phase conjugation relationship, the optical phase and light of the idler light generated from the carrier wave light and the locally oscillating light and having a phase conjugation relationship with the carrier wave light. Idler-modulated light modulated by changing the intensity,
Including the modulated signal light and the locally oscillated light transmitted together with the idler modulated light.
The optical amplifier
A first second-order nonlinear optical element that generates excitation light based on the locally oscillating light transmitted through the optical fiber transmission line, and
A second second-order nonlinear optical element that performs non-degenerate parametric amplification of the modulated signal light and the idler-modulated light based on the excitation light.
An optical amplifier that includes means for synchronizing the phase of the modulated signal light with the phase of the excitation light.

Images