JP2019002975A - Light amplifier and transmission system using the same - Google Patents

Abstract

To solve the problem in which in a relay type PSA using a conventional technology, since a carrier wave phase is extracted from BPSK modulation signal light, phase direction modulation is cancelled using a nonlinear process of a PPLN waveguide, but since an intensity modulation component cannot be cancelled for modulation in which signal light is multi-valued in an intensity direction, the intensity modulation component remains, and if excitation light into a PPLN waveguide module of the PSA is generated from the extracted carrier wave, a SN ratio of a light amplifier deteriorates due to energy transfer from a noise component.SOLUTION: A light transmission system and a light amplifier according to the present invention realizes a PSA relay amplification operation for a multi-valued modulation signal in an intensity direction. A pair of pilot tones which have two wavelengths arranged being symmetrically detuned on a frequency axis having signal light at a center, and which have a phase conjugation relationship to each other, are used. A carrier wave phase can be extracted even for a signal multi-valued in the intensity direction without depending on a modulation format. Sum frequency light of the pair of pilot tones is generated, and secondary harmonic wave excitation light performing degenerate phase sensitive optical amplification of the signal light is generated. Multistage connection of PSA can also be performed.SELECTED DRAWING: Figure 6

Description

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

  In a conventional optical transmission system, an identification reproduction optical repeater that converts an optical signal into an electrical signal and regenerates the optical signal after identifying the digital signal is used to reproduce the attenuated signal propagating through the optical fiber. . In the identification reproduction optical repeater, there are problems such as a limitation in response speed of an electronic component for optical-electrical conversion and an increase in power consumption. Therefore, fiber laser amplifiers and semiconductor laser amplifiers that amplify signal light as it is by entering pump light into an optical fiber doped with rare earth elements have appeared. These laser amplifiers have no function of shaping a deteriorated signal light waveform. Conversely, spontaneously emitted light that is unavoidably and randomly generated is mixed in completely irrespective of the signal component, and the S / N of the signal light decreases by at least 3 dB before and after amplification. The reduction in S / N increases the transmission code error rate during digital signal transmission and lowers the transmission quality.

  As means for overcoming the limitations of laser amplifiers, phase sensitive amplifiers (PSAs) have been studied. The PSA has a function of shaping a deteriorated signal light waveform or phase signal due to the influence of dispersion of the transmission fiber. Spontaneous emission light having a quadrature phase irrelevant to the signal can be suppressed, and in-phase spontaneous emission light can be minimized. Therefore, in principle, the S / N of the signal light can be kept the same before and after amplification.

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

  The phase sensitive light amplifying unit 101 amplifies the signal light 110 when the phase of the signal light 110 and the phase of the excitation light 111 coincide with each other, and attenuates the signal light 110 when the phase of the both is 90 degrees. Have. If the phase between the pumping light 111 and the signal light 110 is matched so that the amplification gain is maximized using this characteristic, the spontaneous emission light having the quadrature phase with the signal light 110 is not generated, and the in-phase component is also generated. Does not generate excessive spontaneous emission light beyond the noise of signal light. For this reason, the signal light 110 can be amplified without deteriorating the S / N ratio.

  In order to achieve phase synchronization between the signal light 110 and the pump light 111, the pump light phase control unit 103 is synchronized with the phase of the signal light 110 branched by the first light branching unit 104-1. Control the phase of. The pumping light 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.

Nonlinear optical media that perform the parametric amplification described above include second-order nonlinear optical materials typified by periodically poled LiNbO ₃ (PPLN) waveguides and _third- order nonlinear optical materials typified by quartz glass fibers.

  FIG. 2 is a diagram showing a configuration of a conventional PSA using a PPLN waveguide (see Non-Patent Document 1). The PSA 200 shown in FIG. 2 includes an erbium-doped fiber laser amplifier (EDFA) 201, a first second-order nonlinear optical element 202, a second second-order nonlinear optical element 204, and a first optical branching unit (optical coupler). 203-1 and a second optical branching unit 203-2 (optical coupler), a phase modulator 205, an optical fiber expander 206, a polarization maintaining fiber 207, a photodetector 208, and a phase locked loop (PLL). ) Circuit 209. 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 has the same configuration and will not be described in detail.

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

In PSA, in order to amplify only light in phase with signal light, it is necessary that the phase of the signal light and the phase of the excitation light coincide with each other or be shifted by π radians as described above. That is, when using the second-order nonlinear optical effect, the phase phi _2Omegaesu of the excitation light is a wavelength corresponding to the SH light, and the phase phi _.omega.s of signal light required to satisfy 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 pumping 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.

  In order to synchronize the phase between the signal light 250 and the excitation fundamental light 251, first, the phase modulator 205 performs phase modulation on the excitation fundamental light 251 with a weak pilot signal, and a part of the output signal light 253 is branched. Detection is performed by the detector 208. The pilot signal component is minimized when the phase difference Δφ shown in FIG. 3 is minimized and phase synchronization is established. Therefore, feedback is performed to the optical fiber expander 206 using the PLL circuit 209 so that the pilot signal is minimized, that is, the amplified output signal is maximized. The phase of the excitation fundamental light 251 can be controlled by the optical fiber stretcher 206 to achieve phase synchronization between the signal light 250 and the excitation fundamental light 251.

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 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 Opto Electronics and Communications Conference, OECC, 2013, PDP paper PD2-3 T. Umeki, O. Tadanaga, M. Asobe, Y. Miyamoto and H. Takenouchi., “First demonstration of high-order QAM signal amplification in PPLN-based phase sensitive amplifier,” Optics Express, February 2014, Vol. 22, No.3, p.2473-2482 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

  The prior art PSA described above is based on a degenerate parametric process. The degenerate PSA has a simple configuration compared to a non-degenerate PSA described later, and can be applied to amplification of an intensity modulation signal, a binary phase modulation signal (BPSK), a differential phase shift keying (DPSK) signal, or the like. The degenerate PSA can be expected to improve the signal-to-noise ratio by applying it to an optical transmission system for short and medium distances where the demand for transmission capacity is not so high.

  Since the degenerate PSA attenuates the orthogonal phase component, it cannot amplify a signal modulated by QPSK modulation (Quadrature Phase Shift Keying) in which the orthogonal phase component also includes a data modulation component, 8PSK, QAM, or the like. It has been demonstrated that a non-degenerate PSA can be configured for a modulation format that also includes a data modulation component in the quadrature phase component (Non-patent Documents 2 and 3).

  When PSA is used as a relay amplifier in an optical transmission system, it is necessary to generate pumping light that is synchronized with the carrier phase of signal light. If the signal light source is arranged near the phase sensitive light amplification unit as shown in FIG. 1, a part of the signal light source can be branched and used as excitation light. When the PSA is used as a relay amplifier, a part of the signal light source cannot be branched because an optical fiber having a span of about 80 km is transmitted. It is important to configure the PSA by a method of extracting the carrier phase from the transmitted signal light.

  FIG. 4 is a diagram showing a configuration of a relay type PSA including extraction of a carrier phase in the prior art. 4 is roughly divided into a pump light generation unit 430 and a phase sensitive light amplification unit 440 that perform a method for extracting a carrier phase from a BPSK signal.

  5A to 5D are diagrams for explaining the relationship on the wavelength axis of each signal in the relay type PSA of the prior art of FIG. All the diagrams in FIG. 5 conceptually show the relative positional relationship of each signal on the wavelength axis with the horizontal axis as the wavelength. Hereinafter, an outline of the operation of the relay PSA of FIG. 4 will be described with reference to FIG. It should be noted that light having a higher frequency is shown as light having a shorter wavelength in FIG. 5 because the horizontal axis is the wavelength axis in each diagram of FIG. Therefore, the higher the frequency of light, the closer to the short wavelength side on the left side of the horizontal axis in FIG.

  The signal light 401 transmitted through the optical fiber is input to the PSA 400, and part of the signal light 401 is branched by the optical coupler 402 and is incident on the excitation light generation unit 430. In the excitation light generation unit 430, after the branched signal light is amplified by the EDFA 403, the first PPLN waveguide module 404 generates second harmonic light (SH light) having a half wavelength with respect to the wavelength of the signal light. To do. On the other hand, in the excitation light generation unit 430, the excitation light (Lo1) 451 generated by the local oscillation light source 407 and the SH light from the first PPLN waveguide module 404 described above are input to the second PPLN module 405. . In the second PPLN module 405, difference frequency light (Lo2) between the excitation light (Lo1) and the SH light is generated.

  From the signal light, excitation light (Lo1), and difference frequency light (Lo2) output from the second PPLN waveguide module 405, only the difference frequency light (Lo2) is extracted by the band pass filter 406. The difference frequency light (Lo2) passes through the optical circulator 413 and then is input to the semiconductor laser 412 via, for example, an arrayed waveguide grating (AWG) type wavelength multiplexer / demultiplexer 411. The semiconductor laser 412 generates second excitation light having the same phase and the same wavelength as the difference frequency light (Lo2) by light injection locking. Since the light intensity of the second excitation light is determined by the output of the semiconductor laser 412, the weak difference frequency light (Lo2) of about several tens of μW from the second PPLN waveguide module 405 is used, and the second excitation light is several tens of mW or more. 2 excitation light (Lo2) can be obtained. The pumping light (Lo1) branched by the optical coupler 408 from the local oscillation light source 407 is incident on the multiplexing side port of the wavelength multiplexing / demultiplexing unit 411 from the demultiplexing side port of the wavelength multiplexing / demultiplexing unit 411, and the semiconductor laser 410 The pumping light output 452 is output from the circulator 413 in combination with the second pumping light (Lo2) that is injection-synchronized from the circulator.

  Referring to FIG. 5A, 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, the spectrum is broadened. Referring to FIG. 5B, 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 is removed from the difference frequency light (Lo2) p2 by injection locking (injection lock) to the semiconductor laser 412.

The following expression is satisfied between the phase φs of the signal light s, the phase φ _{p1 of} the pumping light (Lo1) p1, and the phase φ _p2 of the difference frequency light (Lo2) p2 at each location of the relay type PSA of FIG.
_{2φs - φ p1 - φ p2 =} 0 equation (2)

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

  As can be seen from the equations (2) and (3), by generating SH light from the signal light s in the first PPLN waveguide module 404, the phase φs of the signal light s can be doubled. . Usually, since signal light is modulated by information data to be transmitted, it is difficult to extract a carrier wave phase. However, binary phase modulation can be removed by doubling the phase φs of the signal light s as in the excitation light generation unit 430 of FIG. Further, as shown in FIG. 5B, 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 locking. In FIG. 4, the second excitation light (Lo2) 452 having a pure S / N ratio and retaining the phase information of the carrier wave of the signal light 401 is obtained.

The pumping light (Lo1) 451 from the local oscillation light source 407 and the second pumping light (Lo2) 452 injection-locked are amplified by the EDFA 414 in the phase sensitive light amplifying unit 440. Further, unnecessary ASE light is removed using the BPF 415, and then input to the third PPLN waveguide module 416. In the third PPLN waveguide module 416, as shown in FIG. 5C, the excitation having the sum frequency (SF: Sum Frequency) of the excitation light (Lo1) p1 and the second excitation light (Lo2) p2. An optical SF is generated. At this time, the following expression is satisfied among the phase φ _p1 of the pumping light (Lo1) p1, the phase φ _{p2 of} the second pumping light (Lo2) p2, and the phase φ _SF of the sum frequency pumping light SF.
φ _SF = φ _p1 + φ _p2
= 2φs Equation (4)

The sum frequency excitation light 453 from the third PPLN waveguide module 416 in FIG. 4 is incident on the fourth PPLN waveguide module 417. In the fourth PPLN waveguide module 417, phase sensitive optical amplification is performed by parametric amplification of the signal light 401 that has passed through the optical coupler 402 and the sum frequency excitation light 453. For phase-sensitive light amplification, as shown in FIG. 5D, the following equation must be satisfied between the phase φs of the signal light s and the phase φ _SF of the sum frequency light SF.
_{ΔΦ = φ SF - 2φs = nπ} (n is an integer) (5)

  That is, the gain is maximized when the phase difference ΔΦ between the input signal light and the sum frequency pump light is −π, 0, or π. In order to synchronize the phase between the signal light and each pump light, the phase modulator 409 is used to apply phase modulation to the pump light 451 using a weak signal. Part of the output light that has passed through the fourth PPLN waveguide module 417 is branched by the optical coupler 418. A change in the light intensity of the branched light is detected by the optical detector 419, and feedback is applied to the optical fiber expander 421 using the PLL circuit 420 so that the phase is synchronized between the pumping light 451 and the signal light 401. With the above-described phase synchronization, the relay type degenerate PSA 400 of FIG. 4 can be stably operated. An example of the wavelength of each signal is signal light s1535.8 nm, excitation light p1 (Lo1) 1534.25 nm, difference frequency p2 (Lo2) 1537.4 nm, and sum frequency SF767.9 nm. The wavelengths of all signals up to the input to the third PPLN waveguide module 416 are substantially in the same wavelength band as the signal light s, thus avoiding the above-described device availability problem.

  In the relay type PSA of FIG. 4 described above, in order to extract the phase of the carrier wave from the BPSK-modulated signal light, the phase process is modulated using the nonlinear process (SH light generation) of the first PPLN waveguide module 404. Canceled. On the other hand, when using modulation in which the signal light is multi-valued in the intensity direction, the intensity modulation component cannot be canceled. Therefore, an intensity modulation component remains in the carrier wave extracted from the signal light by the configuration of the PSA in FIG. If the pumping light SF453 for the final stage PPLN waveguide module 417 is generated from the extracted carrier wave, the SN ratio of the optical amplifier also deteriorates due to the energy transfer from the remaining noise component.

  The signal light transmitted through the single mode fiber has a phase distortion due to the influence of dispersion in the fiber and the nonlinear optical effect. These phase distortions cannot be canceled at the stage of generation of SH light in FIG. 4 and remain as phase noise of the excitation light SF for the PPLN waveguide module 417 at the final stage. It is difficult to generate excitation light with a good S / N ratio only by using a nonlinear process of SH light generation as in the prior art PSA or using injection locking for excitation light.

  In order to realize a low-noise phase-sensitive optical amplifier, it is necessary to perform phase synchronization with high accuracy by using pumping light having a sufficiently high SNR compared to the SNR of signal light. High quality is required for the excitation light, and high SNR is also required for local oscillation light for generating the excitation light.

  The present invention has been made in view of such problems, and an object of the present invention is to provide a phase-sensitive optical amplifying device capable of extracting a carrier phase even for a signal multi-valued in the intensity direction. It is to provide.

  In order to achieve such an object, the present invention provides a signal light, a pilot signal light detuned to a short wavelength side around the wavelength of the signal light, and a long wavelength side. An optical amplifying apparatus for amplifying a phase-sensitive light of a signal light group including a pair of pilot tones including the idler light of the pilot signal light that is symmetrically detuned to a part of the signal light group. An optical branching device, an optical demultiplexer for taking out and demultiplexing the pilot signal light and idler light from the output light of the optical branching device for each wavelength, and a first local oscillation phase-synchronized with the pilot signal light A first local oscillation light source that outputs light, a second local oscillation light source that outputs a second local oscillation light that is phase-synchronized with the idler light, the first local oscillation light, and the second local oscillation light. Generates sum frequency light of oscillation light The second-order nonlinear optical element 1 and the sum frequency light output from the first second-order nonlinear optical element are used as excitation light to perform degenerate parametric amplification on the signal light, and to the pilot tone pair A second second-order nonlinear optical element that performs non-degenerate parametric amplification, and phase synchronization means that synchronizes the phase of the signal light group and the phase of the excitation light in the second second-order nonlinear optical element. An optical amplifying device including the optical amplifier.

  The invention according to claim 2 is the optical amplifying device according to claim 1, wherein the first local oscillation light source oscillates in synchronization with the pilot signal and is synchronized with the phase of the pilot signal. The second local oscillation light source is a semiconductor laser that oscillates in synchronization with the idler light and synchronizes with the phase of the idler light.

  A third aspect of the present invention is the optical amplifying device according to the first or second aspect, wherein the phase synchronization unit is a light that propagates the signal light group before entering the second second-order nonlinear optical element. A first optical fiber expander that expands and contracts a fiber; first detection means that extracts a part of the amplified light output from the second second-order nonlinear optical element and converts it into an electrical signal; Control means for generating a control signal to the first optical fiber expander so that the reference phase and the phase of the pumping light are synchronized based on an output signal of one detection means. And

A fourth aspect of the present invention is the optical amplifying device according to any one of the first to third aspects, wherein the first second-order nonlinear optical element and the second second-order nonlinear optical element each include an optical waveguide. The optical waveguide is made of any one of LiNbO ₃ , KNbO ₃ , LiTaO ₃ , LiNb _x Ta _(1-x) O ₃ (0 ≦ x ≦ 1), or KTiOPO ₄ , or any of these materials Further, it is composed of a material in which at least one selected from the group consisting of Mg, Zn, Sc, and In is added as an additive.

  The invention according to claim 5 is an optical transmission comprising a transmitter that outputs the signal light group, any one of the above-described optical amplifying devices, and a transmission fiber that connects the transmitter and the optical amplifying device. The transmitter includes a signal light source that outputs the carrier light of the signal light, a pilot signal light source that outputs the pilot signal, and the carrier light that is partially branched as a fundamental pump light. A third second-order nonlinear optical element that inputs and generates the second harmonic light of the fundamental excitation light, and the second harmonic light output from the third second-order nonlinear optical element as the excitation light, A fourth secondary nonlinear optical element that amplifies the signal light modulated by a carrier wave and generates the idler light that is a difference frequency light between the pilot signal light and the second harmonic light; and the fourth secondary light For nonlinear optical elements Second phase synchronization for synchronizing the phase of the signal light group and the phase of the fundamental excitation light in the fourth second-order nonlinear optical element by controlling the phase of the signal light group before radiating. Means. An optical transmission system comprising: means.

  The invention according to claim 6 is the optical transmission system according to claim 5, wherein the second phase synchronization means propagates the signal light group before entering the fourth second-order nonlinear optical element. A second optical fiber expander that expands and contracts the optical fiber, a second detection means that extracts a part of the light output from the fourth second-order nonlinear optical element and converts it into an electrical signal, and the second Second control means for generating a control signal to the second optical fiber expander so that the phase of the signal light group and the phase of the fundamental wave excitation light are synchronized based on the output signal of the detection means; It is characterized by including.

  The invention according to claim 7 is the optical transmission system according to claim 5, wherein the fourth-order nonlinear optical element degenerates the signal light by degenerate parametric amplification and generates the idler light by a non-degenerate parametric process. It is characterized by doing.

  A seventh aspect of the present invention is the optical transmission system according to any one of the fifth to seventh aspects, wherein the optical amplifying devices are connected in multiple stages.

  According to the present invention, low-noise phase-sensitive optical amplification that does not depend on a modulation method can be provided.

FIG. 1 is a diagram showing a basic configuration of a conventional PSA. FIG. 2 is a diagram showing the configuration of a conventional PSA using a PPLN waveguide. FIG. 3 is a diagram showing the relationship between the phase difference between PSA input signal light pumping light and gain. FIG. 4 is a configuration diagram of a relay type PSA including carrier phase extraction according to the prior art. FIG. 5 is a diagram for explaining the relationship on the wavelength axis of each signal of the relay type PSA. FIG. 6 is a diagram showing the configuration of the optical transmission system of the present invention. FIG. 7 is a diagram showing a configuration of a transmitter in the optical transmission system of the present invention.

  In the optical amplifying apparatus and the optical transmission system of the present invention, a configuration of a phase sensitive optical amplifier (hereinafter referred to as PSA) capable of amplifying with low noise even for a signal multi-valued in the intensity direction is disclosed. The repeater amplification operation of the multistage PSA is also realized in the optical transmission system. As a group of optical signals from the transmitter, a pair of pilot tones (pilot signal light and its idler light) having two frequencies symmetrically detuned on the frequency axis around the signal light and having a phase conjugate relationship with each other is used. Regardless of the modulation format, the carrier phase can be extracted even for signal light multi-valued in the intensity direction. By generating the sum frequency light of the pair of pilot tones, second harmonic excitation light for amplifying the degenerate phase sensitive light of the signal light is generated. In the optical transmission system, a pair of pilot tones is sent together with signal light, and propagates after being subjected to relay amplification by low noise PSA like signal light. Since a part of a pair of pilot tones is branched and used as pumping light at the time of phase sensitive light amplification, it is possible to provide an optical transmission system in which the pilot tone also has very little SN ratio degradation. Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[Embodiment]
FIG. 6 is a diagram showing the configuration of the optical transmission system and the optical amplifying device according to the present invention. The optical transmission system 600 of FIG. 6 includes a transmitter 700, a transmission fiber 601, a dispersion / polarization compensator 602, and a relay type PSA, that is, an optical amplification device 603. In the optical transmission system 600, the signal light is further transmitted by the transmission fiber 622 following the optical amplification device 603. The optical amplifying device 603 of the present invention shown in FIG. 6 can be used as a relay type PSA as a multistage configuration. Hereinafter, the detailed configuration of the transmitter 700 and the signal light / pilot tone pair generation operation will be described first, and then the configuration of the optical amplifying device 603 and the carrier phase extraction operation will be described.

  FIG. 7 is a diagram showing a detailed configuration of a transmitter in the optical transmission system of the present invention. Transmitter 700 generates and transmits a pair of signal light and pilot tone. The main components of the transmitter 700 are listed as follows: a signal light source 701, a modulator 703, a first local oscillation light source 714, an EDFA 705, PPLN waveguide modules 707 and 708, and an optical coupler (optical splitter). ), 702, 715, 709, a phase modulator (PM) 704, bandpass filters (BPF) 706, 710, a photodetector 711, a PLL circuit 712, and an optical fiber stretcher 713 using PZT. .

  In the transmitter 700, the carrier wave of the signal light is data-modulated by the modulator 703, and then subjected to optical amplification by a parametric process by the two PPLN waveguide modules 707 and 708 in order to compensate for the loss due to the modulation. That is, a first PPLN waveguide module 707 for second harmonic generation (SH) light generation (SHG) serving as excitation light, and a second PPLN for optical parametric amplification (OPA) A waveguide module 708 is used. The first PPLN waveguide module 707 includes an input-side spatial optical system, a PPLN waveguide, an output-side spatial optical system, and a dichroic mirror. The second PPLN waveguide module 708 includes a spatial optical system on the input side, a PPLN waveguide, a spatial optical system on the output side, and dichroic mirrors on both the input and output sides.

Here, a method for manufacturing a PPLN waveguide used in the transmitter 700 in FIG. 7 and the optical amplification device in FIG. First, a periodic electrode having a period of about 17 μm was formed on LiNbO ₃ to which Zn was added. Next, a polarization inversion grating corresponding to the above electrode pattern was formed in Zn: LiNbO ₃ by an electric field application method. Furthermore, the Zn: LiNbO ₃ substrate having this periodic polarization reversal structure was directly bonded onto LiTaO _{3 serving} as a cladding, and both substrates were firmly bonded by performing 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 using a dry etching process. The temperature of this waveguide can be adjusted by a Peltier element. The length of the waveguide was 50 mm. The second-order nonlinear optical element having the PPLN waveguide formed as described above has a module configuration capable of inputting and outputting light using a 1.5 μm band polarization maintaining fiber. In this embodiment, LiNbO ₃ to which Zn is added is used, but other nonlinear materials such as KNbO ₃ , LiTaO ₃ , LiNb _x Ta _1-x O ₃ (0 ≦ x ≦ 1) or KTiOPO ₄ , or A material containing at least one selected from the group consisting of Mg, Zn, Sc, and In as an additive may be used.

  In the transmitter 700 of FIG. 7, the wavelength of the signal light is 1550 nm for simplicity of explanation. The light output from the signal light source 701 is branched into two paths by an optical coupler (optical splitter) 702. One of the branched lights is quaternary pulse amplitude modulated (PAM4: Pulse Amplitude Modulation 4) signal light 721, and the other is fundamental pump light 725 for generating pump light used in the parametric process. Light used as a pilot signal detuned to a wavelength shorter than 0.8 nm from the wavelength of the signal light is output from the local oscillation light source 714, and is combined with the signal light 721 by the optical coupler 715. The fundamental wave excitation light 725 used for generation of the excitation light was amplified to 3 W by the EDFA 705, the spontaneous emission light of the EDFA 705 was removed by the BPF 706, and then incident on the first PPLN waveguide module 707. With the SH light generation (SHG) mechanism in the first PPLN waveguide 707, excitation light 726 having a frequency of 775 nm, which is twice the frequency of the fundamental excitation light 725 and is half of 1550 nm in terms of wavelength, is generated. An excitation light 726 having an intensity of 1 W was obtained from the fundamental wave light 725 of 3 W.

  The above-described excitation light 726 and signal light including the pilot signal 723 are incident on the second PPLN waveguide module 708. As in the spectrum schematically shown at the right end of FIG. 7, the pilot signal 723 is generated at a position that is substantially symmetrical about the wavelength 1550 nm of the signal light 721 by a parametric process in the second PPLN waveguide module 708. Is generated. Hereinafter, the pilot signal light 723 and its idler light 724 are collectively referred to as a pilot tone pair. The signal light 721 and the pilot tone pair are collectively referred to as a signal light group 722. Transmitter 700 outputs a signal light group 722 including a pair of pilot tones. In FIG. 7, since the pair of pilot tones is drawn on the wavelength axis, the pilot signal light 723 and the idler light 724 are not strictly symmetrical on the wavelength axis. Note that the pair of pilot tones is in a strictly symmetrical position with respect to the signal light 721 when displayed on the linear frequency axis. In the following description, for the sake of simplicity, the description will be made assuming that the positions are symmetrical on the wavelength axis.

  On the other hand, the wavelength of the signal light 721 matches the fundamental wave excitation light 725 obtained by branching. For this reason, the wavelength (frequency) of idler light generated from the signal light 721 is also the same as that of the signal light. That is, the transmitter 700 performs a degenerate parametric process only for the signal light 721. In the degenerate parametric process, since the wavelengths are the same, the signal light and its idler light interfere coherently, and the light intensity varies depending on the phase relationship between the signal light and the idler light. Therefore, in the transmitter 700, phase synchronization is always applied so that the phase of the fundamental wave excitation light 725, which is the reference phase of the degenerate parametric process, and the phase of the signal light group 722 satisfy the phase condition.

  In the transmitter 700 of FIG. 7, in order to synchronize the signal light 721 with the fundamental wave excitation light 725, the fundamental wave excitation light is phase-modulated with a weak signal using the phase modulator 704. Part of the output light that passed through the second PPLN module 708 was branched by the optical coupler 709. Only the wavelength of the signal light 721 is cut out by the BPF 710, and a change in light intensity due to a weak modulation signal is detected by the photodetector 711. As shown in FIG. 3, the weak modulation signal component is minimized when the phase difference Δφ between the signal light and the fundamental wave excitation light is in the minimum phase synchronization. Therefore, the optical fiber stretcher 713 is fed back using the PLL circuit 712 so that the modulation signal is the minimum and the amplifier output signal is the maximum. By controlling the phase of the modulation signal, phase synchronization between the signal light group 722 including the signal light 721 and the fundamental pump light 725 can be achieved. Therefore, the loop including the optical coupler 709, the PLL circuit 712, and the optical fiber expander 713 constitutes a phase synchronization means. That is, the phase synchronization means outputs from the first optical fiber expander that expands and contracts the optical fiber through which the signal light group before entering the second secondary nonlinear optical element propagates, and the second secondary nonlinear optical element. First reference means for extracting a part of the amplified light and converting it into an electrical signal, and based on the output signal of the first detection means, the reference phase and the phase of the excitation light are synchronized Control means for generating a control signal to the first optical fiber expander.

  As described with reference to FIG. 7, on the transmitter 700 side of the optical transmission system 600 of the present invention, parametric amplification of signal light and idler light generation of pilot signal light are simultaneously performed using a PPLN waveguide. In the optical transmission system 600 of the present invention, it is assumed that the optical amplifying apparatus 603 which is the relay type PSA of FIG. 6 is used in a multistage configuration. For this reason, the signal light group including the pair of signal light and pilot tone is collectively amplified by the relay type PSA, and is always sent in the transmission fiber. The transmitter 700 uses the light branched from the signal light source 701 as the fundamental wave excitation light 725 for the parametric process. Therefore, the pilot signal light 723 and its idler light 724 have the same reference phase as that of the fundamental wave excitation light 725. By synchronizing the pumping light used in the relay type PSA to this reference phase, it is possible to amplify the pilot signal light and idler light in a non-degenerate phase sensitive manner. In the second PPLN module 708, the transmitter 700 is configured to simultaneously perform degenerate phase sensitive optical amplification for signal light and non-degenerate parametric conversion for pilot signal light. A signal light group including a pair of signal light and pilot tone is transmitted from the transmitter 700, and the phase of the signal light and pilot tone pair is made the same to synchronize the phases, so that the optical amplifying apparatus 603 is configured as a relay type PSA. It is also possible to have a multi-stage configuration.

  Referring again to FIG. 6, like the transmitter 700 of FIG. 7, the parametric process in the optical amplifier 603 is operated using two PPLN waveguide modules. A third PPLN waveguide module 615 for generating SH light (SHG) serving as excitation light and a fourth PPLN waveguide module 616 for phase sensitive optical amplification (OPA) are used. The configurations of the two PPLN waveguide modules 615 and 616 are the same as the two PPLN waveguide modules 707 and 708 of the transmitter 700, respectively.

  The signal light group 722 including the pair of signal light and pilot tone transmitted from the transmitter 700 through the transmission fiber 601 is compensated for dispersion and polarization in the optical fiber by the dispersion and polarization compensator 602. Thereafter, the signal is input to the fourth PPLN waveguide module 616 of the optical amplification device 603. A part of the input signal light group 722 is demultiplexed by the optical cover 604. The branched signal light group 722 is demultiplexed by cutting a pair of pilot tones into pilot signal light 723 and idler light 724 using an optical demultiplexer 605. The demultiplexed pilot signal light 622 is input to a local oscillation light source (Lo2) 609 using an optical circulator 607, and light injection synchronization is performed. The pilot signal light 723 output from the local oscillation light source of the transmitter 700 is propagated along the path where the local oscillation light source 609 exists. Similarly, the demultiplexed idler light 623 is input to the local oscillation light source (Lo3) 610 using the optical circulator 606, and light injection synchronization is performed. The idler light 724 from the transmitter 700 is propagated along the path where the local oscillation light source (Lo3) 610 exists. The local oscillation light sources 609 and 610 can be configured by a semiconductor laser, for example.

  The local oscillation light source (Lo2) 609 outputs light having the same wavelength as the pilot signal light 723 in the transmitter 700, and the local oscillation light source (Lo3) 610 outputs light having the same wavelength as the idler light. The local oscillation light 624 corresponding to the pilot signal light having the same wavelength and the same phase information as the pilot signal light 723 is generated by light injection locking. Similarly, local oscillation light 625 corresponding to idler light 724 having the same wavelength and the same phase information as idler light 724 was generated. Two local oscillation lights 624 and 625 are combined by the optical coupler 608, and a combined local oscillation light 611 is obtained. The local oscillation light 611 was amplified to 3 W by the EDFA 613 and the spontaneous emission light of the EDFA 613 was removed by the BPF 614 and then incident on the third PPLN waveguide module 615.

  The sum of pilot signal light and its idler light symmetrically detuned from the signal light (wavelength 1550 nm) by the sum frequency generation (SFG) operation in the PPLN waveguide in the third PPLN waveguide module 615 Excitation light 626 having a frequency is generated. That is, the relationship of pumping light frequency = pilot signal light frequency + idler light frequency is established. This excitation light has a frequency twice that of the signal light and a wavelength of 775 nm, which is half the wavelength of the signal light. By generating the sum frequency of pilot signal light 723 and its idler light 724 that are in phase conjugate relation, excitation light 626 having the same phase information as the signal light can be generated. In the third PPLN waveguide module 615, the pumping light 626 having a power of 1 W was obtained from the combined local oscillation light 611 having a total power of 3W. This excitation light 626 is incident on the fourth PPLN waveguide module 616.

  In the PPLN waveguide in the fourth PPLN waveguide module 616, a degenerate parametric process is caused for signal light and a non-degenerate parametric process is caused for a pair of pilot tones. As shown in FIG. 7, the signal light 721 and the pilot tone pair are each synchronized with the reference phase of the basic excitation light 725 in the transmitter 700. A signal light group 722 including a pair of signal light 721 and pilot tone transmitted from the transmitter 700 is input to the optical amplifying device 603 in FIG. Also in the optical amplifying device 603 of FIG. 6, by generating and using the pumping light 626 synchronized with the reference phase of the transmitter 700 from the input signal light group, the signal light group in the optical amplifying device 603 that is a relay type PSA. Phase-sensitive optical amplification that combines all wavelengths can be realized.

  In order to perform phase synchronization in the optical amplifying device 603, the phase 612 is applied to the local oscillation light 611 obtained by combining the phase modulation with a weak signal, and the output light 728 passed through the fourth PPLN module 616. The part was branched by an optical coupler 617. Only the wavelength of the signal light is cut out by the BPF 618, and the change in the light intensity is detected by the photodetector 619. Using the PLL circuit 620, feedback was applied to the optical fiber stretcher 621 so that the phase of the pumping light 626 and the phase of the signal light 721 were synchronized. Here, only the signal light 721 is cut out in order to perform phase synchronization. However, even if one of the other pilot tone pairs is cut out, phase synchronization is performed in the same manner, and the signal light 721 and the pilot tone pair are separated. The collective PSA of the signal light group 722 including it is possible.

  As described above, in the optical amplifying device 603 of the present invention, the excitation light 626 is not generated from the modulated signal light, but is generated from the unmodulated pilot tone pair transmitted together with the signal light. 626 is generated. Therefore, the carrier phase can be extracted even for a signal that has been multi-valued in the intensity direction, regardless of the modulation format.

  In the present embodiment, a configuration is shown in which optical amplification is performed by the optical amplifying apparatus 603 that is a relay-type PAS, and then relay transmission is performed to the optical fiber 622 in the subsequent stage. However, the optical amplification may be input to a receiver or the like after optical amplification. . The optical amplification device 603 in FIG. 6 can also be used as a preamplifier in the preceding stage of the receiver by inputting the output light after being optically amplified by the fourth PPLN waveguide module 616 to the receiver.

  With the optical amplifying device of the present invention, an optical transmission system can be realized that simultaneously transmits a pair of signal tones and a pilot tone capable of collective phase sensitive optical amplification. Degradation of the SNR of the excitation light used for phase sensitive light amplification is suppressed. When used as a relay-type PSA, it generates high-quality excitation light even for multilevel intensity-modulated signals such as PAM4, compared to the prior art method of extracting the carrier phase from data-modulated signal light In addition, phase-sensitive optical amplification with low noise becomes possible. Compared with the optical transmission system of the prior art, even if the number of relay stages is increased, local oscillation light having a high SNR can be regenerated, and multistage relay amplification can be realized by low-noise phase-sensitive optical amplification.

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

100, 200, 400 Phase sensitive optical amplifier 102 Excitation light source 103 Excitation light phase controller 201, 403, 414, 613, 705 EDFA
202, 204, 404, 405, 416, 417, 615, 616, 707, 708 PPLN waveguide module 203-1, 203-2, 402, 408, 418, 604, 608, 617, 702, 709, 715 Optical coupler (Light splitting part)
205, 409, 602 Phase modulator 206, 207, 421, 621, 713 Optical fiber stretcher 209, 420, 620, 712 PLL circuit 406, 415, 614, 618, 706, 710 BPF
407, 714 Local oscillation light source 412, 609, 610 Semiconductor laser (local oscillation light source)
413, 606, 607 Optical circulator 600 Optical transmission system 601, 628 Transmission fiber 602 Dispersion and polarization compensator 603 Optical amplifier 605 Optical demultiplexer 700 Transmitter 701 Signal light source 703 Modulator

Claims (8)

A signal light and a pilot tone pair including pilot signal light detuned to the short wavelength side centered on the wavelength of the signal light and idler light of the pilot signal light symmetrically detuned to the long wavelength side An optical amplifying device for amplifying a phase-sensitive optical signal group comprising:
An optical splitter for branching a part of the signal light group;
An optical demultiplexer for extracting and demultiplexing the pilot signal light and the idler light for each wavelength from the output light of the optical splitter;
A first local oscillation light source that outputs a first local oscillation light that is phase-synchronized with the pilot signal light;
A second local oscillation light source that outputs a second local oscillation light that is phase-synchronized with the idler light;
A first second-order nonlinear optical element that generates a sum frequency light of the first local oscillation light and the second local oscillation light;
A second degenerate parametric amplification is performed on the signal light and a non-degenerate parametric amplification is performed on the pair of pilot tones using the sum frequency light output from the first second-order nonlinear optical element as excitation light. A second-order nonlinear optical element of
An optical amplifying apparatus comprising: phase synchronizing means for synchronizing the phase of the signal light group and the phase of the excitation light in the second second-order nonlinear optical element.   The first local oscillation light source is a semiconductor laser that oscillates in synchronization with the pilot signal and is synchronized with the phase of the pilot signal, and the second local oscillation light source is in injection synchronization with the idler light. 2. The optical amplifying device according to claim 1, wherein the optical amplifying device is a semiconductor laser that oscillates and synchronizes with a phase of the idler light. The phase synchronization means includes
A first optical fiber expander that expands and contracts an optical fiber through which the signal light group propagates before entering the second second-order nonlinear optical element;
First detection means for extracting a part of the amplified light output from the second second-order nonlinear optical element and converting it into an electrical signal;
Control means for generating a control signal to the first optical fiber expander so that the reference phase and the phase of the pumping light are synchronized based on an output signal of the first detection means. The optical amplifying device according to claim 1 or 2. Each of the first second-order nonlinear optical element and the second second-order nonlinear optical element includes an optical waveguide, and the optical waveguide includes LiNbO ₃ , KNbO ₃ , LiTaO ₃ , LiNb _x Ta _(1-x) O ₃ (0 ≦ x ≦ 1) or any one of KTiOPO ₄ , or at least one selected from the group consisting of Mg, Zn, Sc, and In was added as an additive to any of these materials 4. The optical amplifying device according to claim 1, wherein the optical amplifying device is made of a material. A transmitter for outputting the signal light group;
An optical amplifying device according to any one of claims 1 to 4,
An optical transmission system composed of a transmission fiber connecting the transmitter and the optical amplification device,
The transmitter is
A signal light source that outputs the carrier light of the signal light;
A pilot signal light source for outputting the pilot signal;
A third-order nonlinear optical element that receives the partially branched carrier wave light as a fundamental excitation light and generates a second harmonic light of the fundamental excitation light;
Using the second harmonic light output from the third second-order nonlinear optical element as excitation light, the signal light obtained by modulating the carrier wave is amplified, and the difference frequency light between the pilot signal light and the second harmonic light A fourth second-order nonlinear optical element that generates the idler light,
By controlling the phase of the signal light group before entering the fourth second-order nonlinear optical element, the phase of the signal light group and the phase of the fundamental excitation light are changed to the fourth second-order nonlinear optics. An optical transmission system comprising: second phase synchronization means for synchronizing in the element. The second phase synchronization means includes
A second optical fiber expander that expands and contracts an optical fiber through which the signal light group propagates before entering the fourth second-order nonlinear optical element;
Second detection means for extracting a part of the light output from the fourth second-order nonlinear optical element and converting it into an electrical signal;
Based on the output signal of the second detection means, a second control signal is generated for the second optical fiber expander so that the phase of the signal light group and the phase of the fundamental pumping light are synchronized. The optical transmission system according to claim 5, further comprising:   6. The optical transmission system according to claim 5, wherein the fourth-order nonlinear optical element degenerates parametric amplification of the signal light and generates the idler light by a non-degenerate parametric process.   8. The optical transmission system according to claim 5, wherein the optical amplifying devices are connected in multiple stages.

Images