JP6114442B1 - Optical amplification device and optical transmission system - Google Patents

Abstract

Amplification with low noise is realized while ensuring a high S / N of a pilot tone. A phase sensitive optical amplifier (8) includes a pilot tone, a signal light group and an idler light group symmetrically detuned to a short wavelength side and a long wavelength side with a wavelength of the pilot tone as a center wavelength. A demultiplexer (80) for taking a part of these lights as input, a bandpass filter (81) for extracting a pilot tone, a local light source (83), and the phase of the local light from the pilot tone. A circulator (84) synchronized with the phase, a second-order nonlinear optical element (88) that generates second harmonic light of local light, and second-harmonic light output from the second-order nonlinear optical element (88) as excitation light A second-order nonlinear optical element (91) that performs non-degenerate parametric amplification of the signal light group and idler light group and performs degenerate parametric amplification of the pilot tone. [Selection] Figure 1

Description

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

  In a conventional optical transmission system, an identification regenerative 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 signal attenuated by propagating through the optical fiber. It was done. However, this identification / reproduction optical repeater has problems such as limited response speed of electronic components that convert optical signals into electrical signals, and increased power consumption as the speed of transmitted signals increases. .

  As optical amplification means for solving this problem, there are fiber laser amplifiers and semiconductor laser amplifiers that amplify signal light by making excitation light incident on an optical fiber doped with rare earth elements such as erbium and praseodymium. Such fiber laser amplifiers and semiconductor laser amplifiers can amplify signal light as it is, so that there is no limitation on the electrical processing speed that has been a problem in the identification reproduction optical repeater. In addition, there is an advantage that the configuration of the device is relatively simple.

  However, these laser amplifiers do not have a function of shaping a deteriorated signal light waveform. In these laser amplifiers, spontaneously emitted light that is inevitably and randomly generated is mixed regardless of the signal component, so that the S / N (Signal to Noise ratio) of the signal light is reduced by at least 3 dB before and after amplification. To do. The lack of the waveform shaping function and the decrease in S / N lead to an increase in the transmission code error rate during digital signal transmission, which is a factor of reducing the transmission quality.

  As means for overcoming the limitations of the conventional laser amplifier, a phase sensitive amplifier (PSA) has 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. In addition, PSA can suppress spontaneous emission light having a quadrature phase irrelevant to the signal and can minimize in-phase spontaneous emission light. In principle, it is possible to keep them the same.

  FIG. 3 is a block diagram showing a basic configuration of a conventional PSA. As shown in FIG. 3, the PSA 100 includes a phase-sensitive optical amplification unit 101 using optical parametric amplification, a pumping light source 102, a pumping light phase control unit 103, and first and second optical branching units 104-1. And 104-2. As shown in FIG. 3, the signal light 110 input to the PSA 100 is branched into two by the optical branching unit 104-1, one incident on the phase sensitive light amplification unit 101, and the other incident on 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 enters the phase sensitive light amplification unit 101. The phase sensitive amplification 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 incident signal light 110 and the phase of the excitation light 111 coincide with each other. It has a characteristic to attenuate. 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. Excess spontaneous emission light beyond the noise of the signal light 110 is not generated. Therefore, the signal light 110 can be amplified without degrading the S / N.

  In order to achieve such 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. The phase of the excitation light 111 is controlled. In addition, 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, and the amplification gain of the output signal light 112 is maximized. The phase of the excitation light 111 is controlled so that As a result, the phase sensitive light amplification unit 101 realizes light amplification without S / N degradation based on the above principle.

  The pumping light phase control unit 103 may be configured to directly control the phase of the pumping light source 102 in addition to the configuration of controlling the phase of the pumping light 111 on the output side of the pumping light source 102. Further, when the light source that generates the signal light 110 is disposed near the phase-sensitive light amplification unit 101, a part of the signal light source can be branched and used as excitation light.

As the nonlinear optical medium for performing the parametric amplification described above, a method using a second-order nonlinear optical material typified by a periodically poled LiNbO ₃ (Period-ically Poled Lithium Niobate: PPLN) waveguide and a quartz glass fiber are typical. A method using a third-order nonlinear optical material is known.

  FIG. 4 is a block diagram showing a configuration of a conventional PSA using a PPLN waveguide disclosed in Non-Patent Document 1 and the like. A PSA 200 shown in FIG. 4 includes an erbium-doped fiber amplifier (EDFA) 201, first and second second-order nonlinear optical elements 202 and 204, and first and second optical branching sections. 203-1 and 203-2, a phase modulator 205, an optical fiber stretcher 206 using a PZT (lead zirconate titanate) piezoelectric element, a polarization maintaining fiber 207, a photodetector 208, and phase synchronization And a loop (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 includes a third spatial optical system 215, a second PPLN waveguide 216, a fourth spatial optical system 217, a second dichroic mirror 218, and a third dichroic. And a mirror 219.

  The first spatial optical system 211 couples light input from the input port of the first second-order nonlinear element 202 to the first PPLN waveguide 212. The second spatial optical system 213 couples the light output from the first PPLN waveguide 212 to the output port of the first second-order nonlinear optical element 202 via the first dichroic mirror 214. The third spatial optical system 215 couples light input from the input port of the second second-order nonlinear optical element 204 to the second PPLN waveguide 216 via the second dichroic mirror 218. The fourth spatial optical system 217 couples the light output from the second PPLN waveguide 216 to the output port of the second second-order nonlinear optical element 204 via the third dichroic mirror 219.

  In the example shown in FIG. 4, the signal light 250 incident on the PSA 200 is branched by the optical branching unit 203-1, and one light enters the second second-order nonlinear optical element 204. The other light branched by the optical branching unit 203-1 is incident on the EDFA 201 after being phase-controlled by the phase modulator 205 and the optical fiber expander 206 as the excitation fundamental wave light 251. In order to obtain sufficient power to obtain the nonlinear optical effect from the weak laser beam used for optical communication, the EDFA 201 amplifies the incident excitation fundamental wave light 251, and the amplified excitation fundamental wave light 251 is converted into the first secondary light. The light is incident on the nonlinear optical element 202.

  In the first second-order nonlinear optical element 202, second harmonic light (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 via the polarization maintaining fiber 207. The second second-order nonlinear optical element 204 performs phase-sensitive amplification by performing degenerate parametric amplification with the incident signal light 250 and the SH light 252, and outputs the output signal light 253.

In PSA, in order to amplify only light in phase with a signal, it is necessary that the signal light and the pumping light have the same phase or the phase is shifted by π radians as described above. That is, when using the second-order nonlinear optical effect, and a phase phi _.omega.s of the excitation light phase phi _2Omegaesu the signal light is light of a wavelength corresponding to the SH light satisfy the relationship of Equation (1) below Necessary. Here, n is an integer.
_{Δφ = 1/2 (φ 2ωs} -φ ωs) = nπ ··· (1)

  FIG. 5 is a graph showing the relationship between the phase difference Δφ between the input signal light and the pumping light and the gain (dB) in the conventional PSA using the second-order nonlinear optical effect. As can be seen from FIG. 5, the gain is maximized when the phase difference Δφ between the input signal light and the pump light is −π, 0, or π.

  In the configuration shown in FIG. 4, the phase modulator 205 performs phase modulation on the excitation fundamental wave light 251 in accordance with a weak pilot signal. The second optical branching unit 203-2 splits a part of the amplified output signal light 253 so as to enter the photodetector 208. The photodetector 208 converts the incident signal light into an electrical signal. The pilot signal component becomes minimum when the phase difference Δφ shown in FIG.

  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 detected by the photodetector 208 is maximized. The optical fiber expander 206 expands and contracts the optical fiber through which the excitation fundamental wave light 251 propagates according to the output of the PLL circuit 209. In this way, the phase of the excitation fundamental light 251 can be controlled to achieve phase synchronization between the signal light 250 and the excitation fundamental light 251.

  In the configuration in which the PPLN waveguide is used as a nonlinear medium and the signal light 250 and the SH light 252 are incident on the second second-order nonlinear optical element 204 to perform degenerate parametric amplification, the SH light 252 is generated once. When performing parametric amplification, for example, the components of the excitation fundamental light are removed by using the characteristics of the dichroic mirror 214, so that only the SH light 252 and the signal light 250 are converted into a parametric amplification medium such as the second second-order nonlinear optical element 204. Can be made incident. Therefore, noise due to mixing of spontaneous emission light generated by the EDFA 201 can be prevented, so that low-noise optical amplification can be performed.

  As described above, the PPLN waveguide is used as a nonlinear optical medium, and the nonlinear medium is excited using the SH light 252 so that low-noise phase-sensitive amplification can be achieved without being affected by noise generated by the EDFA 201. In addition, the phase noise can be reduced by utilizing the characteristic of attenuating the quadrature component.

  However, the above-described prior art has the following problems. As shown in FIG. 5, a conventional PSA using a PPLN waveguide has a characteristic of attenuating orthogonal phase components. Therefore, IMDD (Intensity Modulation Direct) using normal intensity modulation or binary phase modulation is used. Detection), BPSK (Binary Phase Shift Keying), DPSK (Differential Phase Shift Keying), etc., which can be applied to the amplification of modulated signals, but also QPSK (Quadrature Phase Shift Keying) and 8PSK etc. There is a problem that it cannot be amplified.

  On the other hand, by using a configuration based on non-degenerate parametric amplification as disclosed in Non-Patent Document 2 and Non-Patent Document 3, multi-level phase modulation signals such as QPSK and QAM (Quadrature Amplitude Modulation) It is known that phase sensitive amplification and phase reproduction amplification can be adopted.

  Here, attention is focused on the phase synchronization method when the phase sensitive optical amplifier is applied to the optical communication technology more specifically. For example, when the light source that generates the signal light is disposed near the phase sensitive optical amplifier as in the case where the phase sensitive optical amplifier is used immediately after the optical signal transmitter, the light emitted from the signal light source A part of the light can be branched and used as excitation light for a phase sensitive optical amplifier. However, when a phase sensitive optical amplifier is used as a repeater amplifier in optical transmission, it is necessary to extract the average phase from the optically modulated signal light and generate pump light that is synchronized with the carrier phase of the signal light. There is. When the phase sensitive optical amplifier is used as a relay amplifier in actual optical transmission, it is important to configure the phase sensitive optical amplifier including this carrier phase extraction method.

  In general, when second harmonic light is generated from signal light using a medium having a second-order nonlinear optical effect, the wavelength of second harmonic light, which is light used as excitation light, becomes half of the wavelength of signal light. As a result, it is necessary to use an optical component having a wavelength different from the communication wavelength band for an optical device for performing carrier phase extraction or the like. However, when optical components having wavelengths other than the communication wavelength band are used, various obstacles occur. For example, because the maturity of optical components varies depending on the wavelength, the specifications for realizing phase-sensitive optical amplifiers such as the characteristics and specifications of optical components cannot be satisfied, or very expensive components are required to meet the specifications. Problems such as having to use occur. More specifically, it is difficult to obtain a high-quality semiconductor laser or the like, and the light intensity, light beam width, usable wavelength, and the like are limited.

  There are also major challenges for optical amplifiers. In a region where the wavelength is shorter than the communication wavelength, such as second harmonic light, an optical fiber laser amplifier or the like cannot be used. Some semiconductor amplifiers have been put to practical use, but due to problems such as amplification factor and saturation intensity, even if these semiconductor amplifiers are used to amplify the second harmonic light, they will become phase sensitive optical amplifiers. There is a problem that sufficient light intensity cannot be obtained as the excitation light to be used, and the S / N of the excitation light used in the phase sensitive optical amplifier is deteriorated due to the noise figure (NF) of the semiconductor amplifier. .

  Furthermore, optical components for light having a shorter wavelength than the communication wavelength such as second harmonic light often have problems from the viewpoint of reliability depending on the components, and phase sensitive optical amplifiers using these optical components are actually used. This is an obstacle to use in the optical communication system. For this reason, it is preferable that the carrier wave extraction from the signal light and the excitation light generation be realized in the communication wavelength band.

  As shown in Non-Patent Document 4 and Non-Patent Document 2 in the study of conventional phase-sensitive optical amplifiers using a medium having a second-order nonlinear optical effect, binary phase shift keying (Binary Phase Shift Keying: BPSK) A phase sensitive amplification method including a method of extracting a carrier phase from a quaternary phase modulation signal (Quadrature Phase Shift Keying: QPSK) signal is shown. Further, as shown in Non-Patent Document 5, a carrier phase extraction method using four-wave mixing in an optical fiber having a third-order nonlinear effect is also shown.

  However, since these methods cancel the modulation using a non-linear process, there is a problem that it is difficult to extract a carrier wave from a multi-valued signal such as a QAM (Quadrature Amplitude Modulation) signal. As a solution to this problem, there is a method in which a continuous wave (CW) pilot tone signal having the same phase as the carrier phase of the modulation signal is transmitted together with the signal light.

  Here, a method of transmitting a pilot tone signal for generating a fundamental wave for phase sensitive amplification will be described.

  The modulated signal light subjected to data modulation propagates through a transmission medium such as an optical fiber, and a signal is sent. FIG. 6 shows a configuration example of an optical transmission system in which a phase sensitive optical amplifier is used as a relay amplifier that performs optical amplification to compensate for light intensity loss in the transmission medium. The optical transmission system is provided in the middle of the optical fiber 3, a transmitter (Tx) 1, a phase sensitive optical amplifier (PSA) 2, an optical fiber 3 that connects the transmitter 1 and the phase sensitive optical amplifier 2. And a dispersion / polarization compensator 4.

  The transmitter 1 includes an array waveguide grating (AWG) 10, a light source (External Cavity Laser Diode: ECLD) 11, a duplexer 12, an EDFA (Erbium-Doped Fiber Amplifier). ) 13, a band pass filter (BPF) 14, a second-order nonlinear optical element 15 for second harmonic generation (SHG), and a difference frequency generation (DFG) The second-order nonlinear optical element 16, a multiplexer 17, and a wavelength selective optical switch (WSS) 18 are provided.

  The phase sensitive optical amplifier 2 includes a wavelength demultiplexer 20, a bandpass filter 21, a variable optical attenuator (VOA) 22, a local light source 23, a circulator 24, a demultiplexer 25, A phase modulator (PM) 26, an EDFA 27, a bandpass filter 28, a second-order nonlinear optical element 29 for generating a second harmonic, a fixed delay line 30, and PZT (lead zirconate titanate) An optical fiber expander 31 using a piezoelectric element, a second-order nonlinear optical element 32 for optical parametric amplification (OPA), a duplexer 33, a bandpass filter 34, a photodetector 35, A PLL (Phase Locked Loop) circuit 36 and a wavelength multiplexer 37 are provided.

  As described above, when the light source that generates the signal light is disposed near the phase-sensitive optical amplifier, a part of the signal light source can be branched and used as the excitation light. However, when a phase sensitive optical amplifier is used as a repeater amplifier in optical transmission, it is necessary to synchronize the phases of the pumping light and the signal light in the phase sensitive optical amplifier using, for example, the phase synchronization means described below.

  In the configuration shown in FIG. 6, the AWG 10 multiplexes a plurality of signal lights subjected to data modulation. The light source 11 outputs unmodulated CW light. The EDFA 13 amplifies the CW light output from the light source 11. The band pass filter 14 removes noise light generated by the EDFA 13 and transmits only CW light. The second-order nonlinear optical element 15 generates second harmonic light from the CW light output from the bandpass filter 14 and uses it as excitation light. Thereafter, the second-order nonlinear optical element 16 generates difference frequency light between the excitation light and the wavelength-multiplexed signal light, and this difference frequency light is used as idler light for the signal light.

  On the other hand, the branching filter 12 branches a part of the CW light output from the light source 11. The multiplexer 17 combines the signal light and idler light output from the second-order nonlinear optical element 16 and the CW light output from the duplexer 12. The wavelength selection type optical switch 18 outputs different intensities for each wavelength of the signal light, idler light, and CW light.

  In this way, the transmitter 1 transmits unmodulated CW light as a pilot tone signal in addition to data-modulated signal light and idler light. FIG. 7A shows the spectrum of the signal light group 300 output from the transmitter 1. 7A to 7E, the vertical axis represents the light intensity, and the horizontal axis represents the wavelength. 6 and 7A, 301 is signal light, 302 is idler light, and 303 is a pilot tone of CW light.

  The signal light group 300 output from the transmitter 1 is transmitted through the optical fiber 3. A part of the signal light group 300 output from the transmitter 1 is branched by the wavelength demultiplexer 20 of the phase sensitive optical amplifier 2 and passed through the bandpass filter 21, so that the CW light transmitted together with the signal light group 300 is transmitted. The pilot tone 303 is extracted. FIG. 7B shows the extracted pilot tone 303, and FIG. 7C shows the signal light 301 and idler light 302 that have passed through the wavelength demultiplexer 20.

  The variable optical attenuator 22 adjusts the light intensity of the pilot tone 303. The light intensity adjusted pilot tone 303 is synchronized with the local light source 23 through the circulator 24. As the local light source 23, a Fabry-Perot type semiconductor laser was used. The phase of the semiconductor laser is drawn into the phase of the pilot tone 303, whereby the local light source 23 in the phase sensitive optical amplifier 2 is phase-synchronized with the pilot tone 303.

  The local light that is phase-synchronized with the pilot tone 303 is phase-controlled by the phase modulator 26 and enters the EDFA 27. The EDFA 27 amplifies the local light output from the phase modulator 26. The band pass filter 28 removes noise light generated by the EDFA 27 and transmits only local light. The second-order nonlinear optical element 29 generates second harmonic light from the local light output from the bandpass filter 28 and makes the second harmonic light incident on the second-order nonlinear optical element 32 as excitation light.

  The fixed delay line 30 delays the signal light 301 and the idler light 302 transmitted through the wavelength demultiplexer 20 by a predetermined time in order to adjust the delay time with the excitation light. The signal light 301 and idler light 302 that have passed through the fixed delay line 30 are phase-controlled by the optical fiber expander 31 and enter the second-order nonlinear optical element 32. The second-order nonlinear optical element 32 amplifies the signal light 301 and the idler light 302 by a parametric amplification effect.

  The demultiplexer 33 branches a part of the amplified signal light and idler light. The bandpass filter 34 allows only the signal light and idler light to pass through and enter the photodetector 35. The photodetector 35 converts incident light into an electrical signal. The PLL circuit 36 provides feedback to the optical fiber expander 31 so that the signal detected by the photodetector 35 is maximized. The optical fiber expander 31 expands and contracts the optical fiber through which the signal light 301 and the idler light 302 propagate according to the output of the PLL circuit 36. Thus, the phase of the signal light 301 and the idler light 302 can be controlled to achieve phase synchronization between the signal light 301 and the idler light 302 and the excitation light.

  By adopting the above configuration, it is possible to perform phase sensitive amplification of signal light even in a relay amplifier in which a light source that generates signal light is not arranged near the phase sensitive optical amplifier. In this configuration, in consideration of using a relay amplifier in multiple stages, a new pilot tone is sent to the next relay amplifier together with the signal light in order to operate the next relay amplifier. Specifically, the duplexer 25 of the phase sensitive optical amplifier 2 branches a part of the local light that is phase-synchronized with the pilot tone 303 (FIG. 7D). Reference numeral 304 in FIGS. 6 and 7D denotes local light that is branched by the duplexer 25.

  The wavelength multiplexer 37 combines the signal light and idler light amplified by the second-order nonlinear optical element 32 and the local light output from the demultiplexer 25 (FIG. 7E). In FIG. 6 and FIG. 7E, 305 is a signal light group output from the phase sensitive optical amplifier 2, 306 is signal light, 307 is idler light, and 308 is a pilot tone.

  However, the conventional method of transmitting pilot tones has the following problems. When the pilot tones are separated using the wavelength demultiplexer 20 as in the configuration shown in FIG. 6, it is difficult to completely extract only the pilot tones of the center wavelengths of the signal light and idler light. Since the light within the finite band is branched, it is necessary to sufficiently separate the wavelengths of the signal light and the idler light. For this reason, it is necessary to provide a band that cannot transmit a signal called a guard band between the signal light and the idler light, and the wavelength band of the signal light and the idler light is limited, and within a finite band. There was a problem that the signal could not be packed closely.

  In the configuration shown in FIG. 6, the wavelength demultiplexer 20 and the wavelength multiplexer 37 are used for demultiplexing and multiplexing of the pilot tone, the signal light, and the idler light. In this configuration, the wavelength demultiplexer is used. In accordance with the extinction characteristics of the optical multiplexer 20 and the wavelength multiplexer 37, there is a problem that residual light from the original pilot tone is mixed into the new pilot tone. Most of the light of the pilot tone is branched by the wavelength demultiplexer 20 that extracts only the pilot tone, and does not enter the second-order nonlinear optical element 32. However, since the actual extinction ratio of the wavelength demultiplexer 20 (the ratio of the light output from the original wavelength demultiplexing port to the residual light output from the port that should be extinguished) is finite, Even after passing through the waver 20, the remaining pilot tone enters the second-order nonlinear optical element 32.

  The residual pilot tone is degenerate parametric amplified by the second-order nonlinear optical element 32 (the pilot tone is a degenerate wavelength) and then enters the wavelength multiplexer 37. Although the residual pilot tone is also extinguished by the wavelength multiplexer 37, the extinction ratio is finite, so that a new pilot tone (local light from the multiplexer 25) is multiplexed by the wavelength multiplexer 37. Residual pilot tone is mixed. As a result, noise is generated due to interference between the new pilot tone and the residual pilot tone. When relay amplifiers are used in multiple stages, this noise accumulates every time relay amplification is performed, so that it becomes difficult to ensure a high S / N of the pilot tone used as pumping light.

  In order to realize a low-noise phase-sensitive optical amplifier, it is necessary to perform phase synchronization with high accuracy by using excitation light having a sufficiently high S / N with respect to the S / N of signal light. The reason for this is that when pump light with a low S / N is used, the S / N of the signal light deteriorates due to energy transfer from the noise component of the pump light, and the phase error between the pump light and signal light causes phase noise. It is because it becomes a cause. For this reason, high quality is required for the excitation light. That is, a high S / N is also required for the local light for generating the excitation light, that is, the pilot tone for synchronizing the phase of the local light and the signal light.

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 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 R. Slavik et al., “All-optical phase and amplitude regenerator for next-generation telecommunications system”, Nature Photonics, vol.4, pp.690-695, 2010.

As described above, the technique disclosed in Non-Patent Document 1 has a problem in that it cannot amplify a modulation signal of a multilevel modulation format.
In the techniques disclosed in Non-Patent Document 2, Non-Patent Document 4 and Non-Patent Document 5, there is a problem that it is difficult to generate a pumping light by extracting a carrier wave phase from a multilevel signal.
In the configuration shown in FIG. 6, signals cannot be densely arranged in a finite band, and it is difficult to ensure a high S / N of a pilot tone used as excitation light. There was a problem that it was difficult to realize an amplifier.

  An object of the present invention is to improve the band utilization efficiency in view of the problems of the prior art as described above, to amplify a modulation signal of a multi-level modulation format, and to have a high pilot tone S / N. Is to provide a phase sensitive optical amplifier capable of amplification with low noise and an optical transmission system using the phase sensitive optical amplifier. In particular, an object of the present invention is to provide a phase sensitive optical amplifier applicable as a relay amplifier in optical transmission including a method for extracting a carrier phase of a signal.

  The optical amplifying device of the present invention receives light including a pilot tone and a signal light group and an idler light group symmetrically detuned to a short wavelength side and a long wavelength side with the wavelength of the pilot tone as a center wavelength, A first demultiplexer for branching part of the signal light group, idler light group and pilot tone; and a bandpass filter for extracting only light of the pilot tone wavelength from the output light of the first demultiplexer; A local light source that outputs local light, first phase synchronization means for synchronizing the phase of the local light with the phase of the pilot tone extracted by the band-pass filter, and local light that is synchronized with the phase of the pilot tone. And a first second-order nonlinear optical element including a first optical waveguide for generating the second harmonic light of the local light, and an output from the first second-order nonlinear optical element. The second harmonic light is used as excitation light to perform non-degenerate parametric amplification of the signal light group and idler light group input from the outside, and the pilot tone input from the outside together with the signal light group and idler light group. By controlling the phase of the second secondary nonlinear optical element having the second optical waveguide for performing degenerate parametric amplification, the signal light group input from the outside, the idler light group, and the pilot tone, And a second phase synchronization means for synchronizing the phase of the signal light group, the idler light group, the pilot tone, and the phase of the excitation light in the second optical waveguide.

In one configuration example of the optical amplifying device of the present invention, the first and second optical waveguides are LiNbO ₃ , KNbO ₃ , LiTaO ₃ , LiNb _x Ta _(1-x) O ₃ (0 ≦ x ≦ 1). ) Or KTiOPO ₄ , or a material obtained by adding at least one selected from the group consisting of Mg, Zn, Sc, and In as an additive to any of these materials It is characterized by this.
Further, in one configuration example of the optical amplifying device of the present invention, the local light source is a semiconductor laser, and the first phase synchronization means applies a pilot tone extracted by the bandpass filter to the semiconductor laser. By synchronizing the injection, the phase of the local light is synchronized with the phase of the pilot tone.
In one configuration example of the optical amplifying device of the present invention, the second phase synchronization means propagates the signal light group, the idler light group, and the pilot tone before entering the second second-order nonlinear optical element. A first optical fiber expander that mechanically expands and contracts an optical fiber to be expanded, and a first detection that extracts a part of the amplified light output from the second second-order nonlinear optical element and converts it into an electrical signal And the first optical fiber expander so that the phase of the signal light group, idler light group, pilot tone, and phase of the excitation light are synchronized with each other based on the output signal of the first detection means. And at least first control means for generating a control signal.

  The optical transmission system of the present invention connects the optical amplifier, a transmitter that outputs light including the signal light group, idler light group, and pilot tone, and the transmitter and the optical amplifier. The transmitter includes a fundamental light source that outputs fundamental light, a second duplexer that branches a part of the fundamental light, and output light from the second duplexer. A multiplexer for multiplexing with a signal light group as a pilot tone, and a third optical waveguide for generating the second harmonic light of the fundamental wave light by using the fundamental wave light output from the fundamental light source as an input. The difference between the third second-order nonlinear optical element and the second harmonic light output from the third second-order nonlinear optical element as excitation light and the signal light group output from the multiplexer Non-degenerate parametric of idler light group which is frequency light And a fourth second-order nonlinear optical element having a fourth optical waveguide for performing degenerate parametric amplification of the pilot tone output from the multiplexer and the fourth-order nonlinear optics. Third phase synchronization means for synchronizing the phase of the pilot tone and the phase of the fundamental light in the fourth optical waveguide by controlling the phase of the pilot tone before entering the element; It is a feature.

One configuration example of the optical transmission system of the present invention is characterized in that the optical amplifying devices are connected in multiple stages.
In one configuration example of the optical transmission system of the present invention, the third and fourth optical waveguides are LiNbO ₃ , KNbO ₃ , LiTaO ₃ , LiNb _x Ta _(1-x) O ₃ (0 ≦ x ≦ 1). ) Or KTiOPO ₄ , or a material obtained by adding at least one selected from the group consisting of Mg, Zn, Sc, and In as an additive to any of these materials It is characterized by this.
In the configuration example of the optical transmission system of the present invention, the third phase synchronization means mechanically expands and contracts the optical fiber through which the pilot tone before entering the fourth second-order nonlinear optical element propagates. A second optical fiber expander, a 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, and an output signal of the second detection means And at least second control means for generating a control signal to the second optical fiber expander so that the phase of the pilot tone and the phase of the fundamental wave light are synchronized with each other. Is.

  According to the present invention, it is possible to suppress the S / N deterioration of a pilot tone used as pumping light by generating a pilot tone that can be phase-sensitively amplified together with a signal light group and an idler light group, and has low noise characteristics. An optical amplifying device capable of excellent phase sensitive amplification can be realized. In the present invention, since it is not necessary to provide a guard band in the vicinity of the center wavelength, it is possible to pack signals more densely than in the past, and to improve band utilization efficiency. In the present invention, a modulation signal of a multi-level modulation format can be amplified. Further, in the present invention, since the S / N deterioration of the pilot tone can be suppressed, even when a plurality of relay amplifiers are connected in multiple stages when using an optical amplifier as a relay amplifier, optical transmission with excellent low noise characteristics Is possible.

It is a block diagram which shows the structure of the optical transmission system which concerns on embodiment of this invention. It is a figure which shows the spectrum of the light of each part in the optical transmission system of FIG. It is a block diagram which shows the structure of the conventional phase sensitive optical amplifier. It is a block diagram which shows the structure of the phase sensitive optical amplifier using the conventional secondary nonlinear optical effect. It is a graph which shows the relationship between the phase difference between input signal light and excitation light, and a gain in the phase sensitive optical amplifier using the conventional secondary nonlinear optical effect. It is a block diagram which shows an example of a structure of the conventional optical transmission system. It is a figure which shows the spectrum of the light of each part in the optical transmission system of FIG.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  In the phase sensitive optical amplifier and the optical transmission system which are the optical amplifying devices according to the present embodiment, the configuration of the phase sensitive optical amplifier using the PPLN waveguide is shown. In this embodiment, the pilot tone for realizing the relay amplification operation of the phase sensitive optical amplifier can be phase sensitive amplified together with the signal light, and the pilot tone is cut out by the wavelength multiplexer / demultiplexer and the signal light. It is the composition which does not need re-combining with. Similarly to the signal light, the pilot tone is propagated while being relayed and amplified by a low-noise phase-sensitive optical amplifier, and a part of the pilot tone is branched and used as excitation light at the time of phase-sensitive amplification. An optical transmission system with very little degradation of / N can be realized, and the conventional problems can be solved.

  FIG. 1 is a block diagram showing the configuration of the optical transmission system according to the present embodiment. The optical transmission system includes a transmitter (Tx) 5, an optical fiber 6, a dispersion / polarization compensator 7, and a phase sensitive optical amplifier (PSA) 8.

  The transmitter 5 includes an AWG 50, a wavelength tunable light source 51 serving as a fundamental light source, a demultiplexer 52, a phase modulator (PM) 53, an optical fiber expander 54 using a PZT piezoelectric element, and a multiplexer. 55, an EDFA 56, a bandpass filter 57, a second-order nonlinear optical element 58 for generating a second harmonic, a second-order nonlinear optical element 59 for generating a difference frequency, a duplexer 60, and a bandpass filter 61 And a photodetector 62, a PLL circuit 63 serving as control means, and a wavelength selective optical switch 64. The duplexer 60, the bandpass filter 61, and the photodetector 62 constitute detection means that extracts a part of the light output from the second-order nonlinear optical element 59 and converts it into an electrical signal.

  The phase sensitive optical amplifier 8 which is an optical amplifying device includes a demultiplexer 80, a band pass filter 81, a variable optical attenuator 82, a local light source 83, a circulator 84 serving as a phase synchronization means, and a phase modulator 85. An EDFA 86, a bandpass filter 87, a second-order nonlinear optical element 88 for generating a second harmonic, a fixed delay line 89, an optical fiber stretcher 90 using a PZT piezoelectric element, and an optical parametric amplification A second-order nonlinear optical element 91, a duplexer 92, a bandpass filter 93, a photodetector 94, and a PLL circuit 95 serving as control means are provided. The duplexer 92, the bandpass filter 93, and the photodetector 94 constitute detection means for taking out part of the amplified light output from the second-order nonlinear optical element 91 and converting it into an electrical signal.

  FIG. 1 shows a configuration for amplifying an optical signal transmitted from a transmitter 5 through an optical fiber 6 by a phase sensitive optical amplifier 8 of a repeater, and then outputting the amplified optical signal to an optical fiber 9 in the subsequent stage. Is shown. In this example, the configuration of one relay amplifier is shown, but a relay amplifier using the phase sensitive optical amplifier 8 of FIG. 1 can be connected in multiple stages.

  The operation of the optical transmission system according to this embodiment will be described below. The AWG 50 of the transmitter 5 combines a plurality of signal lights subjected to data modulation. The wavelength tunable light source 51 generates continuous wave (CW) fundamental light having a wavelength of 1550 nm for generating excitation light used in the DFG process. The EDFA 56 amplifies the fundamental light output from the wavelength variable light source 51 to an optical power of about 3W. The band pass filter 57 removes noise light (spontaneously emitted light) generated by the EDFA 56 and transmits only the fundamental light.

  The second-order nonlinear optical element 58 includes a spatial optical system 580, a PPLN waveguide 581, a spatial optical system 582, and a dichroic mirror 583. The spatial optical system 580 couples the fundamental light output from the bandpass filter 57 to the PPLN waveguide 581. When the fundamental light enters the PPLN waveguide 581, second harmonic light having a wavelength half that of the fundamental light is generated by the PPLN waveguide 581. The spatial optical system 582 couples the light output from the PPLN waveguide 581 to the output port of the second-order nonlinear optical element 58 via the dichroic mirror 583. At this time, the fundamental wave light and the second harmonic light are separated by the dichroic mirror 583, and the second harmonic light enters the second-order nonlinear optical element 59 as the excitation light 504.

Here, a method for manufacturing the PPLN waveguide 581 used in this embodiment will be described below. 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. Next, the Zn: LiNbO ₃ substrate having this periodic domain-inverted structure was directly bonded onto the LiTaO _{3 serving} as the cladding, and heat treatment was performed at 500 ° C. to firmly bond both substrates.

  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. This waveguide can be temperature-controlled by a Peltier element, and the length of the waveguide is 50 mm. The second-order nonlinear optical element 58 having the PPLN waveguide 581 formed as described above has a module structure capable of inputting and outputting light using a 1.5 μm band polarization maintaining fiber.

Here, LiNbO ₃ doped with Zn is used as the material of the PPLN waveguide 581, but other nonlinear materials such as KNbO ₃ , LiTaO ₃ , LiNb _x Ta _(1-x) O ₃ (0 ≦ x ≦ 1) or KTiOPO ₄ may be used, and at least one selected from the group consisting of Mg, Zn, Sc, and In may be added thereto as an additive. Note that a PPLN waveguide, which will be described later, constituting the second-order nonlinear optical elements 59, 88, 91 can also be manufactured in the same manner as the PPLN waveguide 581.

  On the other hand, the demultiplexer 52 of the transmitter 5 branches a part of the fundamental light output from the wavelength variable light source 52. The fundamental light branched by the demultiplexer 52 becomes a pilot tone 503. In order to synchronize the phase with the signal light, the phase modulator 53 performs predetermined phase modulation on the pilot tone 503 output from the duplexer 52. The multiplexer 55 multiplexes the signal light group 501 output from the AWG 50 and the pilot tone 503 light output from the phase modulator 53.

  The second-order nonlinear optical element 59 includes a spatial optical system 590, a dichroic mirror 591, a PPLN waveguide 592, a spatial optical system 593, and a dichroic mirror 594. The spatial optical system 590 transmits the light output from the multiplexer 55 (the combined light of the signal light group 501 and the pilot tone 503) and the excitation light 504 output from the secondary nonlinear optical element 58 via the dichroic mirror 591. To the PPLN waveguide 592. An idler light group that is a difference frequency light between the excitation light 504 and the signal light group 501 is generated by the PPLN waveguide 592.

  The spatial optical system 593 couples light from the PPLN waveguide 592 to the output port of the second-order nonlinear optical element 59 via the dichroic mirror 594. At this time, the signal light group 501, the idler light group and pilot tone 503 and the excitation light 504 are separated by the dichroic mirror 594, and the signal light group 501, idler light group and pilot tone 503 are demultiplexed from the second-order nonlinear optical element 59. Is output to the optical device 60 and the wavelength selective optical switch 64.

  The wavelength selective optical switch 64 outputs the signal light group 501, idler light group, and pilot tone 503 with different intensities for each wavelength. FIG. 2A shows the spectrum of the signal 500 output from the transmitter 5. 2A to 2D, the vertical axis represents light intensity, and the horizontal axis represents wavelength. Reference numeral 502 in FIG. 2A denotes an idler light group. Thus, the signal light group 501 and the idler light group 502 are symmetrically arranged on the short wavelength side and the long wavelength side with the wavelength of the pilot tone 503 as the center wavelength.

  In the present embodiment, the wavelength of pilot tone 503 is 1550 nm. In the transmitter 5, the data-modulated signal light groups 501 are arranged at equal intervals on the short wave side from 1550 nm. As described above, the wavelength tunable light source 51 was used to generate fundamental light having a wavelength of 1550 nm for generating excitation light used in the DFG process. A part of the fundamental light is extracted as a pilot tone 503 by the demultiplexer 52 and is combined with the signal light group 501 by the multiplexer 55. By inputting the fundamental wave light amplified to 3 W by the EDFA 56 to the second-order nonlinear optical element 58, the excitation light 504 having a wavelength of 775 nm, which is a second harmonic of 1550 nm, was generated. An excitation light intensity of 1 W was obtained by 3 W fundamental wave light.

  By inputting the excitation light 504 and the signal light group 501 including the pilot tone 503 to the second-order nonlinear optical element 59, an idler light group 502 is generated at a position symmetrical to the signal light group 501 with a center wavelength of 1550 nm as the center. (FIG. 2A). The transmission band of the present embodiment is C band 1535 nm to 1565 nm. That is, the modulated optical signal is densely arranged in a band of ± 15 nm with the center wavelength of 1550 nm as the center.

  At this time, since only the pilot tone 503 has the same wavelength as the center wavelength 1550 nm, the wavelength of idler light generated by the second-order nonlinear optical element 59 is also the same as that of the pilot tone 503. That is, although the difference frequency light between the excitation light 504 and the signal light group 501 is generated by the non-degenerate parametric process in the second-order nonlinear optical element 59, the degenerate parametric process is caused for the pilot tone 503. In the degenerate parametric process, the pilot tone 503 and its idler light interfere coherently, so that the light intensity fluctuates due to the phase relationship between them. For this reason, the transmitter 5 is always phase-synchronized so that the phases of the excitation light 504 and the pilot tone (fundamental wave light) 503 as the reference phase are matched.

  Originally, if the pilot tone 503 is combined with the signal light group 501 after the second-order nonlinear optical element 59, a degenerate parametric process does not occur. Therefore, the phase synchronization between the excitation light 504 and the pilot tone 503 is achieved. There is no need to call. However, in order to realize the collective phase sensitive amplification of the signal light group 501 and the pilot tone 503 by the phase sensitive optical amplifier 8, the phase synchronization of the pilot tone 503 and the pump light 504 in the transmitter 5 is necessary. The reason for this will be described below.

  In order to realize non-degenerate phase-sensitive amplification, the transmitter 5 collectively generates idler light groups 502 to be paired with signal light groups 501 by the DFG process. For this reason, the reference phase of all pairs of the signal light and the corresponding idler light coincides with the phase of the excitation light 504 used in the DFG process. If the phase of the pumping light used in the phase sensitive optical amplifier 8 can be synchronized with the reference phases of the signal light group 501 and the idler light group 502 generated in this way, phase sensitive amplification for all wavelengths can be realized. .

  However, in order for the pilot tone 503 to simultaneously perform phase sensitive amplification simultaneously with the signal light group 501, the phase of the pilot tone 503 itself must be synchronized with the reference phase. In the transmitter 5, when the pilot tone 503, the signal light group 501, and the idler light group 502 are combined after the second-order nonlinear optical element 59, the pilot tone 503 uses the fundamental light and the light source used in the DFG process. However, since the propagation paths are different, the phases thereof are shifted from the reference phases of the signal light group 501 and the idler light group 502.

  When the phase of the pilot tone 503 is out of phase with the reference phase, the phase sensitive optical amplifier 8 cannot collectively amplify the phase of the signal light group 501 and the pilot tone 503, resulting in a phase shift from the reference phase. Accordingly, the degenerate parametric gain varies. Since this gain fluctuation becomes intensity noise of the pilot tone 503, the pilot tone 503 having a good S / N cannot be transmitted. If the phase of pilot tone 503 can be synchronized with the reference phase, the output light of degenerate parametric amplified pilot tone 503 can always receive the maximum phase sensitive gain. For this reason, in order to transmit the pilot tone 503 having a high S / N, it is necessary to generate the pilot tone 503 synchronized with the reference phase of the signal light group 501 and the idler light group 502 in the transmitter 5. Become.

  Therefore, in the present embodiment, in the second-order nonlinear optical element 59, the signal light group 501 is a DFG process, and only the pilot tone 503 is a degenerate parametric process, and the degenerate parametric amplified pilot tone 503 has a maximum output. Thus, the phase of the pilot tone 503 is synchronized with the reference phase of the excitation light 504. As a result, it is possible to create a state in which all wavelengths of the signal light group 501, idler light group 502 and pilot tone 503 are phase-synchronized.

  In order to synchronize the pilot tone 503 with the reference phase, as described above, the phase modulator 53 is used to perform phase modulation on the pilot tone 503 with a weak signal, and then the output light that has passed through the second-order nonlinear optical element 59 A part was branched by the duplexer 60. The band-pass filter 61 allows only the light of the pilot tone wavelength out of the output of the duplexer 60 to pass through and enter the photodetector 62. The photodetector 62 converts the incident light into an electrical signal.

  The weak modulation signal component of the pilot tone is minimum when the phase difference Δφ shown in FIG. 5 is minimum, that is, when phase synchronization is achieved. Therefore, the PLL circuit 63 provides feedback to the optical fiber expander 54 so that the modulation signal component of the pilot tone is minimized, that is, the amplified output signal detected by the photodetector 62 is maximized. The optical fiber expander 54 expands and contracts the optical fiber through which the pilot tone 503 propagates according to the output of the PLL circuit 63. In this way, the phase of the pilot tone 503 can be controlled to achieve phase synchronization between the excitation light 504 and the pilot tone 503.

  The signal light group 501, idler light group 502, and pilot tone 503 propagated through the optical fiber 6 are compensated for dispersion and polarization in the optical fiber 6 by the dispersion / polarization compensator 7, and then to the phase sensitive optical amplifier 8. Is entered. The duplexer 80 branches a part of the input light. The band pass filter 81 extracts only light having a pilot tone wavelength from the output light of the duplexer 80. Reference numeral 505 in FIGS. 1 and 2B denotes a pilot tone extracted by the band pass filter 81.

  The variable optical attenuator 82 adjusts the light intensity of the pilot tone 505. An optical amplifier may be used instead of the variable optical attenuator 82. The pilot tone 505 having the adjusted light intensity is synchronized with the local light source 83 through the circulator 84. The local light source 83 is composed of a semiconductor laser. The wavelength of local light is 1550 nm. The local light having the same wavelength and the same phase information as the pilot tone 503 of the transmitter 5 was generated by the light injection locking. 506 in FIGS. 1 and 2C indicates local light emission.

  In order to synchronize the phase between the signal light group 501 and the idler light group 502, the phase modulator 85 performs predetermined phase modulation on the local light 506. The EDFA 86 amplifies the local light 506 output from the phase modulator 85 to an optical power of about 5 W. The band pass filter 87 removes noise light (spontaneously emitted light) generated by the EDFA 86 and transmits only the local light 506.

  The second-order nonlinear optical element 88 includes a spatial optical system 880, a PPLN waveguide 881, a spatial optical system 882, and a dichroic mirror 883. The spatial optical system 880 couples the local light 506 output from the bandpass filter 87 to the PPLN waveguide 881. When the local light 506 enters the PPLN waveguide 881, the PPLN waveguide 881 generates second harmonic light having a half wavelength of the local light 506.

  The spatial optical system 882 couples the light output from the PPLN waveguide 881 to the output port of the second-order nonlinear optical element 88 via the dichroic mirror 883. At this time, local light and second harmonic light are separated by the dichroic mirror 883, and the second harmonic light is incident on the second-order nonlinear optical element 91 as excitation light 507. In the present embodiment, the PPLN waveguide 881 generates excitation light 507 having a wavelength of 775 nm, which is a double wave of 1550 nm. An excitation light intensity of 2 W was obtained by local light 506 of 5 W.

  Next, the fixed delay line 89 delays the signal light group 501, the idler light group 502 and the pilot tone 503 transmitted through the duplexer 80 by a predetermined time in order to adjust the delay time with respect to the excitation light 507. The signal light group 501, idler light group 502, and pilot tone 503 that have passed through the fixed delay line 89 are incident on the second-order nonlinear optical element 91.

  The second-order nonlinear optical element 91 includes a spatial optical system 910, a dichroic mirror 911, a PPLN waveguide 912, a spatial optical system 913, and a dichroic mirror 914. The spatial optical system 910 passes the signal light group 501, idler light group 502, pilot tone 503, and excitation light 507 output from the second-order nonlinear optical element 88 through the fixed delay line 89 through the dichroic mirror 911 to PPLN. Coupled to waveguide 912.

  The PPLN waveguide 912 collectively amplifies the phase of the signal light group 501, idler light group 502, and pilot tone 503 by a parametric amplification effect. At this time, in the PPLN waveguide 912, a non-degenerate parametric process is caused in the signal light group 501 and the idler light group 502, and a degenerate parametric process is caused in the pilot tone 503.

  The pilot tone 503 is synchronized with the reference phase (phase of the fundamental wave light) of the pumping light 504 of the transmitter 5 by the phase control in the transmitter 5, and the pumping light 507 is also pumped by the pumping light 504 (pilot) by the light injection synchronization by the circulator 84. The phase of the signal light group 501, idler light group 502, and pilot tone 503 transmitted through the duplexer 80 is synchronized with the excitation light 507 in the PPLN waveguide 912. Since the control is performed, it is possible to realize phase-sensitive amplification for all wavelengths at once.

  In order to synchronize the signal light group 501, idler light group 502, and pilot tone 503 with the excitation light 507, as described above, phase modulation is applied to the local light 506 using the phase modulator 85 with a weak signal. The demultiplexer 92 branches a part of the output light that has passed through the second-order nonlinear optical element 91. The band-pass filter 93 allows only the light of the pilot tone wavelength out of the output of the duplexer 92 to pass through and enter the photodetector 94. The photodetector 94 converts the incident light into an electrical signal.

  By combining the signal light group 501, idler light group 502, pilot tone 503, and excitation light 507 by the dichroic mirror 911 of the second-order nonlinear optical element 91, the weak modulated signal component of the local light 506 is converted into the pilot tone. 503 is superimposed. The weak modulated signal component of the pilot tone that appears at the output of the photodetector 94 becomes minimum when the phase difference Δφ shown in FIG. 5 is minimum, that is, when phase synchronization is achieved.

  The PLL circuit 95 provides feedback to the optical fiber expander 90 so that the modulation signal component of the pilot tone is minimum, that is, the amplified output signal detected by the photodetector 94 is maximum. The optical fiber expander 90 expands and contracts the optical fiber through which the signal light group 501, idler light group 502 and pilot tone 503 propagate according to the output of the PLL circuit 95. In this way, by controlling the phases of the signal light group 501, the idler light group 502 and the pilot tone 503, the signal light group 501, the idler light group 502 and the pilot tone 503 are within the PPLN waveguide 912 of the second-order nonlinear optical element 91. The phase and the phase of the excitation light 507 can be synchronized.

  FIG. 2D shows the spectrum of the signal 508 output from the phase sensitive optical amplifier 8. 509 is an amplified signal light group, 510 is an amplified idler light group, and 511 is an amplified pilot tone.

  In the present embodiment, in order to realize phase synchronization, only the pilot tone is extracted by the bandpass filter 93, but the present invention is not limited to this, and light of one wavelength in the signal light group or idler Even if light of one wavelength in the light group is extracted, phase sensitive amplification of all wavelengths can be performed.

  In this embodiment, the PPLN waveguide, which is a second-order nonlinear optical element, is used as a parametric amplification medium. However, a third-order nonlinear optical element such as a highly nonlinear fiber or a semiconductor laser may be used. In that case, the local light output from the local light source may be used as the excitation light 507 to perform a parametric amplification operation of the signal light.

  In this embodiment, light injection locking is used for phase synchronization of pilot tone and local light, but other methods may be used. For example, phase synchronization may be performed by feeding back a high-speed electric signal represented by a Costas loop to a local light source.

  Further, in the present embodiment, a configuration in which relay transmission is performed to the optical fiber 9 at the subsequent stage after optical amplification is shown, but it may be input to a receiver or the like after amplification. In other words, the output light after being amplified by the second-order nonlinear optical element 91 is input to the receiver, so that the phase sensitive optical amplifier 8 of the present embodiment can be used as a preamplifier in front of the receiver.

  In this embodiment, the signal light group 501 is arranged on the short wavelength side and the idler light group 502 is arranged on the long wavelength side with the wavelength of the pilot tone 503 as the center wavelength, but the signal light group 501 is long. It may be arranged on the wavelength side, and the idler light group 502 may be arranged on the short wavelength side.

  The effect of this embodiment will be described. The present embodiment provides a configuration of a phase sensitive optical amplifier characterized by generating a pilot tone capable of performing phase sensitive amplification together with a signal light group and an idler light group. Since this is different from the conventional configuration for extracting the tone, there is no interference due to the residual pilot tone, which is a problem in the conventional method, and it is possible to suppress the S / N deterioration of the pilot tone used as the excitation light. Since local light with a high S / N can be generated even when the number of relay stages is increased as compared with the conventional method, a phase sensitive optical amplifier excellent in low noise can be applied as a multistage relay amplifier. Further, since it is not necessary to cut out a pilot tone by a WDM filter when generating pumping light for phase sensitive amplification, it is not necessary to provide an unnecessary guard band near the center wavelength. Thereby, the utilization efficiency of a band can be improved.

  The present invention can be applied to a technique for amplifying an optical signal.

  DESCRIPTION OF SYMBOLS 5 ... Transmitter, 6 and 9 ... Optical fiber, 7 ... Dispersion and polarization compensator, 8 ... Phase sensitive optical amplifier, 50 ... Array waveguide grating, 51 ... Wavelength variable light source, 52, 60, 80, 92 ... minute 55, 85 ... phase modulator, 54, 90 ... optical fiber expander, 55 ... multiplexer, 56, 86 ... erbium-doped fiber laser amplifier, 57, 61, 81, 87, 93 ... band pass filter, 58, 59, 88, 91 ... second-order nonlinear optical element, 62 ... photodetector, 63, 95 ... PLL circuit, 64 ... wavelength selective optical switch, 82 ... variable optical attenuator, 83 ... local light source, 84 ... Circulator 89 ... Fixed delay line 94 ... Photo detector 580, 582, 590, 593, 880, 882, 910, 913 ... Spatial optical system, 581, 592, 881, 912 ... PPLN waveguide, 583, 5 1,594,883,911,914 ... dichroic mirror.

Claims (8)

The light including the pilot tone and the signal light group and the idler light group symmetrically detuned to the short wavelength side and the long wavelength side with the pilot tone wavelength as the center wavelength is input, and the signal light group and the idler light group are input. And a first duplexer for branching a part of the pilot tone,
A bandpass filter for extracting only light of the wavelength of the pilot tone from the output light of the first duplexer;
A local light source that outputs the local light,
First phase synchronization means for synchronizing the phase of the local light with the phase of the pilot tone extracted by the bandpass filter;
A first second-order nonlinear optical element including a first optical waveguide for receiving local light synchronized with the phase of the pilot tone and generating second harmonic light of the local light;
The second harmonic light output from the first second-order nonlinear optical element is used as excitation light to perform non-degenerate parametric amplification of the signal light group and idler light group input from the outside, and the signal light group and idler A second second-order nonlinear optical element including a second optical waveguide for performing degenerate parametric amplification of the pilot tone input from the outside together with a light group;
By controlling the phase of the signal light group, idler light group, and pilot tone input from the outside, the phase of the signal light group, idler light group, pilot tone, and phase of the excitation light are changed to the second phase. An optical amplifying device comprising: second phase synchronization means for synchronizing in the optical waveguide. The optical amplification device according to claim 1,
The first and second optical waveguides are made of any one of LiNbO ₃ , KNbO ₃ , LiTaO ₃ , LiNb _x Ta _(1-x) O ₃ (0 ≦ x ≦ 1), or KTiOPO _4. An optical amplifying apparatus comprising: a material obtained by adding at least one selected from the group consisting of Mg, Zn, Sc, and In as an additive to any of these materials. The optical amplification device according to claim 1 or 2,
The local light source is a semiconductor laser;
The first phase synchronization means synchronizes the phase of the local light with the phase of the pilot tone by performing light injection synchronization of the pilot tone extracted by the bandpass filter with the semiconductor laser. Optical amplification device. In the optical amplification device according to any one of claims 1 to 3,
The second phase synchronization means includes
A first optical fiber expander that mechanically expands and contracts an optical fiber through which a signal light group, an idler light group, and a pilot tone that are not incident on the second second-order nonlinear optical element propagate;
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;
Based on the output signal of the first detection means, the control to the first optical fiber expander is performed so that the phase of the signal light group, idler light group, pilot tone and the phase of the excitation light are synchronized. An optical amplifying apparatus comprising at least first control means for generating a signal. An optical amplification device according to any one of claims 1 to 4,
A transmitter that outputs light including the signal light group, the idler light group, and a pilot tone;
A transmission fiber connecting the transmitter and the optical amplification device;
The transmitter is
A fundamental light source that outputs fundamental light;
A second duplexer for branching a part of the fundamental light;
A multiplexer that combines the output light of the second duplexer with the signal light group as a pilot tone;
A third second-order nonlinear optical element including a third optical waveguide for receiving the fundamental light output from the fundamental light source and generating second harmonic light of the fundamental light;
The second harmonic light output from the third second-order nonlinear optical element is used as excitation light, and the idler light group that is the difference frequency light between the excitation light and the signal light group output from the multiplexer is non-degenerate. A fourth second-order nonlinear optical element comprising a fourth optical waveguide for performing degenerate parametric amplification of a pilot tone generated by a parametric process and output from the multiplexer;
By controlling the phase of the pilot tone before entering the fourth second-order nonlinear optical element, a phase of the pilot tone and the phase of the fundamental light are synchronized in the fourth optical waveguide. An optical transmission system comprising phase synchronization means. The optical transmission system according to claim 5, wherein
An optical transmission system, wherein the optical amplifying devices are connected in multiple stages. The optical transmission system according to claim 5 or 6,
The third and fourth optical waveguides are made of any one of LiNbO ₃ , KNbO ₃ , LiTaO ₃ , LiNb _x Ta _(1-x) O ₃ (0 ≦ x ≦ 1), or KTiOPO _4. An optical transmission system comprising: a material obtained by adding at least one selected from the group consisting of Mg, Zn, Sc, and In as an additive to any of these materials. The optical transmission system according to any one of claims 5 to 7,
The third phase synchronization means includes
A second optical fiber expander that mechanically expands and contracts an optical fiber through which a pilot tone before entering the fourth second-order nonlinear optical element propagates;
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;
Second control means for generating a control signal to the second optical fiber expander based on the output signal of the second detection means so that the phase of the pilot tone and the phase of the fundamental wave light are synchronized. An optical transmission system comprising at least: