JP5945212B2 - Optical amplifier - Google Patents

Description

  The present invention relates to an optical amplifying device, and specifically to an optical amplifying device used in an optical communication system or an optical measurement system.

  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 a limited response speed of electronic components that convert an optical signal into an electrical signal, and an increase in power consumption as the speed of a transmitted signal 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, the device configuration has the advantage of being relatively simple.

  However, these laser amplifiers do not have a function of shaping a deteriorated signal light waveform. Further, in these laser amplifiers, unavoidably and randomly generated spontaneous emission light is mixed regardless of the signal component, so that the S / N ratio of the signal light is reduced by at least 3 dB before and after amplification. These lead to an increase in transmission code error rate at the time of digital signal transmission, which is a factor of reducing transmission quality.

  As means for overcoming the limitations of the conventional laser amplifier, a phase sensitive amplifier (PSA) has been studied. This phase sensitive optical amplifier has a function of shaping a signal light waveform and a phase signal which are deteriorated due to the influence of dispersion of the transmission fiber. Further, since spontaneous emission light having a quadrature phase irrelevant to the signal can be suppressed and the spontaneous emission light having the same phase can be minimized, the S / N of the signal light is not deteriorated before and after the amplification. In principle it is possible to keep.

  FIG. 1 shows a basic configuration of a conventional phase sensitive optical amplifier. As shown in FIG. 1, the PSA 100 includes a phase sensitive light amplification unit 101 using optical parametric amplification, a pumping light source 102, a pumping light phase control unit 103, and first and second light branching units 104-1. And 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 enters 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 incident signal light 110 and the phase of the excitation light 111 coincide with each other, and attenuates the signal light 110 when the two phases are shifted by 90 degrees. It has the characteristic to do. 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. Since excessive spontaneous emission light exceeding the noise of the signal light 110 is not generated, that is, the signal light 110 can be amplified without deteriorating the S / N ratio.

  In order to achieve such phase synchronization of 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 optical amplifying unit 102 realizes optical amplification without the deterioration of the S / N ratio 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. When the signal 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 the excitation light 111.

T. Umeki, O. Tadanaga, A. Takada and M. Asobe, "Phase sensitive degenerate parametric amplification using directly-bonded PPLN ridge waveguides," Optics Express, vol. 19, no. 7, pp. 6326-6332 (2011) . J. Kakande, A. Bogris, R. Slavik, F. Parmigiani, D. Syvridis, P. Petropoulos, and DJ Richardson, “First demonstration of all-optical QPSK signal regeneration in a novel multi-format phase sensitive amplifier,” Post Deadline paper, ECOC 2010, Torino, Italy, (2010).

However, the above-described prior art has the following problems. As the nonlinear optical medium for performing the above-mentioned parametric amplification, as shown in Non-Patent Document 1, a method using a second-order nonlinear optical material typified by a periodically poled LiNbO ₃ (PPLN) waveguide, and Non-Patent Document 2 As shown, there is a method using a third-order nonlinear optical material typified by quartz glass fiber.

  FIG. 2 illustrates a configuration of a conventional PSA using a PPLN waveguide disclosed in Non-Patent Document 1 and the like. 2 includes an erbium-doped fiber laser 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 PZT, 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-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 mirror 219. With.

  The first spatial optical system 211 couples light input from the input port of the first second-order nonlinear optical 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. 2, the signal light 250 incident on the PSA 200 is branched by the optical branching unit 203-1, one incident on the second second-order nonlinear optical element 204, and the other as the excitation fundamental wave light 251. The phase is controlled via the phase modulator 205 and the optical fiber expander 206 and enters the EDFA 201. In order to obtain sufficient power from the weak laser light used for optical communication to obtain a nonlinear optical effect, the EDFA 201 amplifies the incident excitation fundamental light 251, and the amplified excitation fundamental light 251 is the first secondary light. The light enters the nonlinear optical element 202-1. In the first second-order nonlinear optical element 202-1, a second harmonic (hereinafter referred to as SH light) 252 is generated from the incident excitation fundamental wave light 251, and the generated SH light 252 passes through the polarization maintaining fiber 207. The light enters the second second-order nonlinear optical element 204. 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 that is in phase with a signal, it is necessary that the phase of the signal light and the excitation light match or be shifted by π radians as described above. That is, when the second-order nonlinear optical effect is used, it is necessary that the phase φ _2ωs of the excitation light and the phase φ _{ωs of the} signal light, which has a wavelength corresponding to the SH light, satisfy the following relationship (Formula 1). .
_{Δφ = 1 / 2φ 2ωs -φ ωs} = nπ ( where, n is an integer) (Equation 1)
FIG. 3 is a graph showing the relationship between the phase difference Δφ between the input signal light and the pump light and the gain (dB) in the conventional PSA using the second-order nonlinear optical effect. As shown in FIG. 3, it can be seen that the gain is maximum when Δφ is −π, 0, or π.

  In the configuration shown in FIG. 2, in order to phase-synchronize the signal light 250 and the excitation fundamental wave light 251, the phase modulator 205 is used to perform phase modulation on the excitation fundamental wave light 251 with a weak pilot signal, and then output. A part of the signal light 253 is branched and detected by the detector 208. Since this pilot signal component is minimized when the phase difference Δφ shown in FIG. 3 is minimized, the PLL 209 is used so that the pilot signal is minimized, that is, the amplified output signal is maximized. Feedback is provided to the optical fiber stretcher 206. As a result, the phase of the excitation fundamental wave light 251 can be controlled to achieve phase synchronization between the signal light 250 and the excitation fundamental wave light 251.

  In the configuration in which the PPLN is used as a nonlinear optical medium and the signal light 250 and the SH light 252 are incident on the second second-order nonlinear optical element 204 and degenerate parametric amplification is performed, the SH light 252 is generated once and then the parametric is performed. At the time of amplification, for example, by removing the fundamental wave components using the characteristics of the dichroic mirrors 206-1 and 206-2, only the SH light 252 and the signal light 250 are changed to the second-order nonlinear optical element 204. Can be incident on a simple parametric amplification medium. 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, PPLN is used as a nonlinear optical medium and the signal light 250 is excited using the SH light 252 to perform low-noise phase-sensitive amplification 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, as shown in FIG. 3, the conventional configuration method has a characteristic of attenuating orthogonal phase components, so that a normal intensity modulation signal or a modulation signal such as BPSK or DPSK using binary phase modulation is used. However, it is not possible to amplify signals such as QPSK (four values) and 8PSK which are multi-level modulation formats.

  On the other hand, when a silica glass fiber is used as a nonlinear optical medium, as disclosed in Non-Patent Document 2 and the like, it is possible to adopt a configuration capable of phase-sensitive amplification and phase reproduction amplification of a signal such as QPSK. Are known. FIG. 4 illustrates a PSA using a quartz glass fiber that is performing QPSK phase reproduction amplification, which is four-level phase modulation.

  In FIG. 4, the first and second EDFAs 401 and 408, the first excitation light source 402, the first to fifth optical multiplexers / demultiplexers 403-1 to 403-5, the first optical fiber 404, , A demultiplexer 405, an optical circulator 406, a second pumping light source 407 made of a semiconductor laser, a second optical fiber 409, a bandpass filter 410, a PLL circuit 412, and an optical fiber stretcher 413 using PZT. A PSA 400 with is shown.

  In the example shown in FIG. 4, the signal light 415 input to the PSA 400 is amplified by the first EDFA 401 and then enters the first optical multiplexer / demultiplexer 403-1. The first excitation light 414 generated by the first excitation light source 402 is branched by the second optical multiplexer / demultiplexer 403-2, one of which is incident on the first optical multiplexer / demultiplexer 403-1 and the other is The light enters the third optical multiplexer / demultiplexer 403-3. A part of the first pumping light 414 incident on the first optical multiplexer / demultiplexer 403-1 and the amplified signal are multiplexed by the first optical multiplexer / demultiplexer 403-1, The light enters the optical fiber 404.

  FIG. 5A shows the frequency arrangement in the first optical fiber 404. When the signal combined by the first optical multiplexer / demultiplexer 403-1 enters the first optical fiber 404, as shown in FIG. 5A, four-wave mixing in the first optical fiber 404 is performed. (Hereinafter, FWM) generates a plurality of signal groups. This process is described in detail below.

  In the first optical fiber 404, first the first pumping light 414 is converted into the first idler light 416 by degenerate FWM using the signal light 415 as pumping light. Next, the signal light 415 is converted into the second idler light 417 by degenerate FWM using the first idler light 416 as excitation light. Next, the first idler light 416 is converted into the third idler light 418 by degenerate FWM using the second idler light 417 as excitation light.

  As described above, the plurality of idler lights 416 to 418 are sequentially generated in the first optical fiber 404 by the secondary FWM because the zero dispersion wavelength of the first optical fiber 404 is distributed in the length direction. It depends. That is, in the degenerate FWM, normally, the wavelength of the first optical fiber 404 becomes the zero dispersion wavelength at the wavelength that serves as the pumping light, so that the phase matching of the FWM process is achieved and wavelength conversion is performed.

However, since the structure of the optical fiber is not completely uniform in the length direction, the zero dispersion wavelength generally changes in the length direction. For this reason, as described above, the wavelengths of the plurality of idler lights 416 to 418 become wavelengths that act as excitation light, respectively, and cause secondary FWM. At this time, the phase of idler light 416 to 418 generated in each process is given by the following (Expression 2) to (Expression 4) from the phase matching condition in each FWM process.
φ _i1 = 2φ _s −φ _p1 (Formula 2)
φ _i2 = 2φ _i1 −φ _s = 3φ _s −2φ _p1 (Formula 3)
φ _i3 = 2φ _i2 −φ _i1 = 4φ _s -3φ _p1 (Formula 4)

Here, φ _s is the phase of the signal light 415, φ _p1 is the phase of the first excitation light 414, φ _i1 is the phase of the first idler light 416, φ _i2 is the phase of the second idler light 417, and φ _i3 Represents the phase of the third idler light 418.

  If attention is paid to (Equation 4) assuming that the phase of the first excitation light 414 is constant, it can be seen that the phase of the third idler light 418 is four times that of the signal light 415. Therefore, it can be understood that the phase of the third idler light 418 is constant and the phase of the carrier wave can be extracted from the QPSK signal when the signal has only an integer multiple of π / 2, such as QPSK.

  Returning to FIG. Each signal generated by the first optical fiber 404 is routed for each wavelength by the demultiplexer 405, and the third idler light 418 enters the optical circulator 406, and the second excitation is performed in the optical circulator 406. It is injection-synchronized with the laser output from the light source 407, and the remaining intensity modulation component is removed and output as the second excitation light 419.

The first excitation light 414 and the second excitation light 419 are combined by the third optical multiplexer / demultiplexer 403-3, amplified by the EDFA 408, and then the signal light 415 and the second light output from the demultiplexer 405. The idler light 417 and the fourth optical multiplexer / demultiplexer 403-4 are combined and incident on the second optical fiber 409. In the second optical fiber 409, phase sensitive amplification by FWM is performed. At this time, when attention is paid to the phases of the four lights incident on the second optical fiber 409, it is understood that the following (formula 5) and (formula 6) are established.
φ _p2 = φ _i3 = 4φ _s -3φ _p1 (Formula 5)
φ _p1 + φ _p2 = φ _s + φ _i2 = 4φ _s -2φ _p1 (Formula 6)

Here, φ _p2 is the phase of the second excitation light 419. As can be seen from (Expression 6), the sum of the phases of the first pump light 414 and the second pump light 419 and the sum of the phases of the signal light 415 and the second idler light 417 coincide. Accordingly, the phase matching condition is satisfied among the four input lights in the second optical fiber 409.

  FIG. 5B shows the frequency arrangement in the second optical fiber 409. As shown in FIG. 5B, the energy of the first excitation light 414 and the second excitation light 415 is converted into the signal light 415 and the second idler light 417, and optical parametric amplification is performed. At this time, since (Equation 6) holds only when the phase of the signal is an integral multiple of π / 2, when a QPSK signal is incident, only signals in four phase states are phase-sensitive amplified. Phase recovery amplification of the QPSK signal is achieved.

  In the actual amplification operation, since the optical phase fluctuates due to the expansion and contraction of the optical fiber components, only the signal light amplified from the output of the second optical fiber 409 is taken out by the band pass filter 410 and a part thereof is taken as the fifth. The optical multiplexer / demultiplexer 403-5 and the photodetector 410 detect and feed back to the optical fiber stretcher 413 made of PZT via the PLL circuit 412, thereby stabilizing the phase between the signal and the pumping light. Amplification is achieved.

  However, in the configuration using the above optical fiber as a nonlinear optical medium, as can be seen from the frequency arrangement in FIGS. 5A and 5B, strong excitation light in the same wavelength region as the weak signal. In addition, since there are two of them, and their pumping light is amplified by the optical fiber amplifier, it is inevitable that the spontaneous emission light generated by the optical fiber amplifier is mixed into the signal wavelength. Furthermore, in order to obtain a sufficient gain, the length of the optical fiber is as long as several hundreds of meters, which is not practical. Further, when strong CW excitation light is incident on the optical fiber, it is caused by stimulated Brillouin scattering. Since light power above a certain level cannot be incident due to backscattering, the threshold of stimulated Brillouin scattering is lowered by intentionally adding phase modulation to increase the line width of pumping light or by distributing the tension applied to the optical fiber. Therefore, there is a problem that a new noise is generated due to such an extra mechanism and the configuration is complicated.

  In view of the above-described problems of the prior art, an object of the present invention is to enable phase-sensitive amplification of multi-level phase modulation such as QPSK, and is smaller, lower noise, and simpler in structure than conventional ones. It is to provide an amplifier.

  An optical amplification device that amplifies signal light by light mixing using a second-order nonlinear optical effect, a first light source that generates first excitation light, and amplifies the input signal light and the first excitation light The first optical fiber laser amplifier to be output and the signal light amplified and output by the first optical fiber laser amplifier and the first pumping light are input, and a second harmonic is generated. A first second-order nonlinear optical element that satisfies pseudo-phase matching at a plurality of wavelengths, and a specific wavelength is separated from an output of the first second-order nonlinear optical element A branching device that bifurcates the output of the first second-order nonlinear optical element, a second excitation light source that outputs a laser that can be injection-locked to one output of the branching filter, and the branching filter Output from the second excitation light source. An optical circulator that outputs the second pumping light in synchronization with the generated laser, a second optical fiber laser amplifier that amplifies and outputs the first pumping light and the second pumping light, The first pumping light and the second pumping light that are amplified and output by the second optical fiber laser amplifier are input, and the sum frequency light of the first pumping light and the second pumping light is input. A second second-order nonlinear optical element that is generated; and a third second-order nonlinear optical element that performs non-degenerate parametric amplification by inputting the other output of the duplexer and the sum frequency light. It is characterized by.

In order to solve the above problems, an optical amplifying device according to claim 1 of the present invention is a light that amplifies multilevel phase-modulated signal light that takes M types of values by optical mixing using a second-order nonlinear optical effect. An amplification device, wherein M is an integer equal to or greater than 4 , a first light source that generates a first excitation light having a frequency ω _p , an input signal light having a frequency ω _s , and the first excitation light The first optical fiber laser amplifier that amplifies and outputs the signal light, the signal light amplified by the first optical fiber laser amplifier and output, and the first pumping light are input to generate a second harmonic A first second-order nonlinear optical element that satisfies quasi-phase matching at a plurality of wavelengths, and an output of the first second-order nonlinear optical element, Mω _s − ( and idler light having a frequency of _{M-1) ω p, (} M 1) ω _s - (By separating the idler light and the signal light having a frequency of M-2) ω _p, the Mω _s - (M-1) the idler light having a frequency of omega _p one output A duplexer that outputs the idler light having the frequency of (M−1) ω _s − (M−2) ω _p and the signal light from the other output port, and the duplexer A second excitation light source that outputs a laser that can be injection-locked to an output from one output port, and an output from the one output port of the duplexer that is output from the second excitation light source. An optical circulator that outputs the second pumping light in synchronization with the first pumping light, a second optical fiber laser amplifier that amplifies and outputs the first pumping light and the second pumping light, and the second The first amplified and output by the optical fiber laser amplifier. Of the second excitation light and the second excitation light, and generates a sum frequency light of the first excitation light and the second excitation light; and A third-order nonlinear optical element that performs non-degenerate parametric amplification by inputting the output from the other output port and the sum frequency light is provided.

An optical amplifying device according to a second aspect of the present invention is the optical amplifying device according to the first aspect of the present invention, wherein the first second-order nonlinear optical element is an optical light composed of LiNbO ₃ having a periodically poled structure. The periodic polarization inversion structure is spatially phase-modulated or periodically modulated with a period longer than the domain inversion period, and the first second-order nonlinear optical element includes at least ω _s , 2ω _The quasi phase matching wavelength coincides with the frequency of _s −ω _p and a QPSK signal is amplified.

An optical amplifying device according to a third aspect of the present invention is the optical amplifying device according to the first aspect of the present invention, wherein the first second-order nonlinear optical element is an optical waveguide made of LiNbO ₃ having a periodically poled structure. The periodic polarization reversal structure is subjected to spatial phase modulation or periodic modulation with a period longer than the polarization reversal period, and the first second-order nonlinear optical element includes at least ω _s , 2ω _The quasi phase matching wavelength matches the frequency of _s −ω _p , 4ω _s −3ω _p , and the 8PSK signal is amplified.

  According to the present invention, it is possible to generate pump light and idler light that are phase-synchronized with signal light necessary for phase sensitive amplification without causing a significant decrease in wavelength conversion efficiency. Phase-sensitive amplification of the modulation format signal of the value is possible, and separation of the excitation light is easy because the excitation light is once converted into the wavelength of the SH light and then combined with the signal light to perform parametric amplification, The degradation of the S / N ratio due to the mixing of spontaneously emitted light generated by the optical fiber amplifier used inside can be suppressed, and low-noise amplification becomes possible. Furthermore, by appropriately designing the first second-order nonlinear optical element, it is possible to cope with a multi-value modulation format such as 8PSK without reducing efficiency. In addition, since there is no problem such as stimulated Brillouin scattering compared to a phase sensitive amplifier using a conventional optical fiber, the configuration is simplified, and a nonlinear medium having an element length of about several centimeters is sufficient, so that the phase sensitive amplifier is compact as a whole. Can be configured.

  As a result, in a fiber optic communication, a signal whose phase noise has increased due to a nonlinear effect or the like in the transmission optical fiber is amplified by the phase sensitive amplifier according to the present invention, so that optical amplification with suppressed deterioration of the S / N ratio becomes possible. Since it is possible to amplify while reproducing information, it is possible to reduce phase noise and perform relay amplification.

It is explanatory drawing of the structure of the conventional PSA. It is explanatory drawing of the structure of PSA using the conventional secondary nonlinear optical effect. It is a graph which shows the relationship between phase difference (DELTA) phi between input signal light-excitation light, and a gain in PSA using the conventional secondary nonlinear optical effect. It is explanatory drawing of the structure of PSA using the conventional third-order nonlinear optical effect. It is a figure which shows the frequency arrangement | positioning of the light in the optical fiber of the conventional PSA. It is explanatory drawing of the structure of the optical amplifier which concerns on Example 1 of this invention. It is explanatory drawing of the frequency arrangement | positioning and phase matching characteristic of the light in each secondary nonlinear optical element of the optical amplifier which concerns on Example 1 of this invention. It is a figure which shows the optical spectrum of the several idler light produced | generated within the excitation light in the optical amplifier which concerns on Example 1 of this invention, signal light, and a 1st second order nonlinear optical element. It is a figure which shows the optical spectrum of the idler light of the signal amplified by the optical amplifier which concerns on Example 1 of this invention. It is a constellation map in the input / output of the signal amplified by the optical amplifier which concerns on Example 1 of this invention. It is explanatory drawing of the structure of the optical amplifier which concerns on Example 2 of this invention. It is explanatory drawing of the frequency arrangement | positioning and phase matching characteristic of the light in the 1st second order nonlinear optical element of the optical amplification apparatus which concerns on Example 2 of this invention. It is a figure explaining the design method of the PPLN circuit of the 1st second-order nonlinear optical element used for the optical amplifier which concerns on Example 2 of this invention. It is a constellation map in the input-output of the signal amplified by the optical amplifier which concerns on Example 2 of this invention.

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

Example 1
In the optical amplifying apparatus according to the first embodiment, a carrier wave is extracted from the QPSK signal, and pumping light phase-synchronized with the carrier wave is generated to configure a PSA using a PPLN waveguide. FIG. 6 shows the configuration of the optical amplifying device according to Embodiment 1 of the present invention. 6 includes first and second EDFAs 601 and 608, first and second excitation light sources 602 and 607, first to fourth optical multiplexers / demultiplexers 603-1 to 603-4, To third-order nonlinear optical elements 604, 610, and 611, a demultiplexer 605, first and second bandpass filters 609 and 612, a photodetector 613, a PLL circuit 614, and light generated by PZT. An optical amplification device 600 comprising a fiber stretcher 615 and a polarization maintaining optical fiber 616 is shown. The first second-order nonlinear optical element 604 includes a first spatial optical system 621, a first PPLN waveguide 622, a second spatial optical system 623, and a first dichroic mirror 624. The second second-order nonlinear optical element 610 includes a third spatial optical system 625, a second PPLN waveguide 626, a fourth spatial optical system 627, and a second dichroic mirror 628. The third second-order nonlinear optical element 611 includes a fifth spatial optical system 629, a third PPLN waveguide 630, a sixth spatial optical system 631, a third dichroic mirror 632, and a fourth dichroic mirror. 633.

  The first spatial optical system 621 couples light input from the input port of the first second-order nonlinear optical element 604 to the first PPLN waveguide 622. The second spatial optical system 623 couples the light output from the first PPLN waveguide 622 to the output port of the first second-order nonlinear optical element 604 via the first dichroic mirror 624. The third spatial optical system 625 couples light input from the input port of the second second-order nonlinear optical element 610 to the second PPLN waveguide 626. The fourth spatial optical system 627 couples the light output from the second PPLN waveguide 626 to the output port of the second second-order nonlinear optical element 610 via the second dichroic mirror 628. The fifth spatial optical system 629 couples light input from the input port of the third second-order nonlinear optical element 611 to the third PPLN waveguide 630 via the third dichroic mirror 632. The sixth spatial optical system 631 couples the light output from the third PPLN waveguide 630 to the output port of the third second-order nonlinear optical element 611 via the fourth dichroic mirror 632.

  In the example shown in FIG. 6, the signal light 652 that is QPSK-modulated in the 1.55 μm band and enters the optical amplifying apparatus 600 enters the first optical multiplexer / demultiplexer 603-1. The first pumping light 651 generated by the first pumping light source 602 is branched by the second optical multiplexer / demultiplexer 603-2, one of which is incident on the first optical multiplexer / demultiplexer 603-1, and the other is The light enters the third optical multiplexer / demultiplexer 603-3. A part of the first pumping light 651 and the signal light 652 incident on the first optical multiplexer / demultiplexer 603-1 are multiplexed by the first optical multiplexer / demultiplexer 603-1, and then input to the first EDFA 601. The light is incident and amplified, and is incident on the first second-order nonlinear optical element 604.

  Here, in order to extract a carrier wave from a QPSK signal in which the carrier wave disappears by using a compact PPLN waveguide, the first second-order nonlinear optical element 604 includes three pseudo-wires at a frequency interval of 100 GHz in a 1.55 μm band. A first PPLN waveguide 622 having a phase matching wavelength is fabricated.

A method for manufacturing the first PPLN waveguide 622 is illustrated below. First, a periodic electrode having a spatial phase modulation of 17 μm and a phase modulation period of 16.5 mm 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. The length of the waveguide was 49.5 mm, which is three times the periodic phase modulation period. This waveguide can be temperature-controlled by a Peltier element, and is a module capable of inputting and outputting light with a polarization maintaining fiber in a 1.55 μm band.

Here, the wavelength conversion process in the first PPLN waveguide 622 will be described. FIG. 7A shows the frequency arrangement and phase matching characteristics in the first PPLN waveguide 622. As shown in FIG. 7A, the peak on the lowest frequency side among the three phase matching peaks of the first PPLN waveguide 622 is matched with the frequency of the signal light 652. The wavelength of the first excitation light 651 is set so as to be detuned to a frequency lower by 100 GHz than the signal light 652. Under the above conditions, in the first PPLN waveguide 622, the signal light 652 is first converted into SH light, and the first excitation light 651 is generated by difference frequency generation (DFG) using the SH light as excitation light. The first idler light 653 is converted. Since the frequency of the converted first idler light 653 matches the second phase matching wavelength of the first PPLN waveguide 622, the first idler light 653 is converted into SH light, and the SH light is converted into excitation light. As a result, the signal light 652 is converted into the second idler light 654. Since the frequency of the second idler light 654 matches the third phase matching wavelength of the first PPLN waveguide 622, the second idler light 654 is converted into SH light, and the SH light is used as the excitation light. One idler light 653 is converted into a third idler light 655. At this time, the phases of the first to third idler lights 653 to 655 are given by the following equations.
φ _i1 = 2φ _s −φ _p1 (Formula 7)
φ _i2 = 2φ _i1 −φ _s = 3φ _s −2φ _p1 (Equation 8)
φ _i3 = 2φ _i2 −φ _i1 = 4φ _s -3φ _p1 (Equation 9)

Here, φ _s is the phase of the signal light 652, φ _p1 is the phase of the first pumping light 651, φ _i1 is the phase of the first idler light 653, φ _i2 is the phase of the second idler light 654, φ _i3 Represents the phase of the third idler light 655.

  Now, if it is assumed that the phase of the first excitation light 651 is constant, paying attention to (Equation 9), it can be seen that the phase of the third idler light 655 is four times the phase of the signal light 652. Therefore, it can be understood that the phase of the third idler light 655 is constant in a signal having only an integer multiple of π / 2, such as QPSK, and the phase of the carrier wave can be extracted from the QPSK signal.

  FIG. 8 shows the optical spectrum of the output of the first PPLN waveguide 622. As shown in FIG. 8, as a result of entering a 10 Gbit / sQPSK signal light 652 having a wavelength of 1550.1 nm and a first excitation light 651 having a wavelength of 1550.9 nm having a low frequency of 100 GHz into the first PPLN waveguide 622, It was confirmed that a plurality of idler lights were sequentially generated, and in the third idler light 655, the sideband due to QPSK modulation was canceled and the carrier wave could be extracted.

In the first embodiment, the first PPLN waveguide 622 has three phase matching wavelengths. However, in order to obtain the same effect, the phase matching wavelength of the first PPLN waveguide 622 is changed from the low frequency side. Two may be used. In this case, the second idler light 654 does not work as excitation light. However, when the first excitation light 651 is converted by DFG using the SH light of the first idler light 653 as excitation light, the second idler light 654 in FIG. 3 idler light having the following phase is generated at the same frequency as the idler light 655.
φ _i3 ′ = 2φ _i1 −φ _p1 = 4φ _s −3φ _p1 (Equation 10)

As is clear from the comparison of (Equation 9) and (Equation 10), it can be seen that the phases of both are the same, and the third phase matching peak is not necessarily essential. Therefore, in order to obtain the same effect as in the first embodiment, the quasi phase matching wavelength of the first PPLN waveguide 622 is set to the frequency ω _{p of} the first pumping light 651 and the frequency ω _s of the signal light 652. On the other hand, it is only necessary to match at least the frequencies of ω _s and 2ω _s −ω _p .

  Further, in the first embodiment, when there is a third phase matching peak, the third idler light 655 is accurately generated by overlapping the two types of wavelength conversion processes described above. . When attention is paid again to (Equation 9) and (Equation 10), it can be seen that the phase of the third idler light 655 is four times the phase of the signal light 652. Therefore, it can be understood that the phase of the third idler light 655 is constant in a signal having only an integer multiple of π / 2, such as QPSK, and the phase of the carrier wave can be extracted from the QPSK signal. As described above, in the first embodiment, it is possible to extract the carrier phase of the QPSK signal using the first PPLN waveguide 622 which is a little less than 50 mm.

  Referring to FIG. 6 again, the output of the first second-order nonlinear optical element 604 having the first PPLN waveguide 622 is separated into two by the wavelength demultiplexer 605, and the signal light 652 and the second light are separated. The idler light 654 enters the third second-order nonlinear optical element 611, and the third idler light 655 enters the optical circulator 606. The third idler light 655 is injection-locked to the laser output from the second excitation light source 607 (semiconductor laser) in the optical circulator 606, and the remaining intensity modulation component is removed and output as the second excitation light 656. . The first pumping light 651 and the second pumping light 656 are combined by the third optical multiplexer / demultiplexer 603-3, amplified by the second EDFA 608, and then the first pumping light 651 and the second pumping light 656. After excessive spontaneous emission light is removed by a bandpass filter 609 that transmits only the light 656, the light is incident on the second PPLN waveguide 626 of the second second-order nonlinear optical element 610.

The manufacturing method of the second PPLN waveguide 626 is substantially the same as the manufacturing method of the first PPLN waveguide 622, but the second PPLN waveguide 626 has no phase modulation applied to the domain-inverted structure having a period of 17 μm. Therefore, the phase matching wavelength when evaluated by second harmonic generation (SHG) is the frequency arrangement in the second PPLN waveguide 626 of the second second-order nonlinear optical element 610 shown in FIG. Thus, it matches the wavelength of the first idler light 653. Both the first pumping light 651 and the second pumping light 656 are separated from each other in the opposite direction by 200 GHz from the phase matching wavelength. Therefore, the phase matching condition for the sum frequency generation of the first pumping light 651 and the second pumping light 656 is satisfied, and the second PPLN waveguide 626 corresponds to the SH light of the first idler light 653. Wavelength conversion is performed on the wavelength as sum frequency light 657. At this time, the phase φ _s F of the sum frequency light 657 is given by the following (formula 11). Here, φ _p2 indicates the phase of the second excitation light 656.
φ _s F = φ _p1 + φ _p2 = −2φ _p1 (Formula 11)

  The sum frequency light 657 is separated from the first pumping light 651 and the second pumping light 656 by the second dichroic mirror 628 built in the second second-order nonlinear optical element 610, and then, the sum frequency light 657. The light enters the third PPLN waveguide 630 of the third second-order nonlinear optical element 611 through the polarization maintaining optical fiber 616 in the 78 μm band.

  On the other hand, the signal light 652 and the second idler light 654 separated by the branching filter 605 and incident on the third second-order nonlinear optical element 611 are also transmitted via the third dichroic mirror 632 to the third PPLN waveguide. 630 is incident. The third PPLN waveguide 630 has characteristics equivalent to those of the second PPLN waveguide 626, and therefore the phase matching wavelength thereof is substantially the same as that of the second PPLN waveguide 626, and the phase matching wavelengths of both are It is set to match by appropriate temperature adjustment.

  FIG. 7 (c) shows the frequency arrangement in the third PPLN waveguide 630. As shown in FIG. 7C, the frequency of the sum frequency light 657 corresponds to the SH light having the phase matching wavelength of the third PPLN waveguide 630 in the 1.55 μm band. Therefore, the phase matching condition is satisfied between the sum frequency light 657, the signal light 652, and the second idler light 654, and when the sum of the phase of the signal light 652 and the phase of the second idler light 654 is calculated, It is expressed as (Equation 12) below.

φ _s + φ _i2 = 4φ _s -2φ _p1 (Formula 12)
Comparing (Equation 12) with (Equation 11), it can be seen that the two match only when the phase of the signal is an integral multiple of π / 2, so that the QPSK signal can be phase-sensitively amplified.

  FIG. 9 shows optical spectra of the signal light 652 and the second idler light 654 amplified by the optical amplification device 600 according to the first embodiment. Unlike the conventional PSA using an optical fiber, there is no strong excitation light in the 1.55 μm band as shown in FIG. 9, and spontaneous emission light generated by the EDFA used to generate the excitation light is also mixed. Therefore, it can be confirmed that phase sensitive amplification can be performed while maintaining a high S / N ratio.

  In the actual amplification operation, in order to suppress the influence of the phase variation due to the variation in the optical path length due to the expansion and contraction of the optical fiber connecting each optical component, the first embodiment amplifies the output from the output of the third PPLN waveguide 630. Only the processed signal light is extracted using the band pass filter 612, and a part of the signal light is branched using the fourth optical multiplexer / demultiplexer 603-4. After the light is detected by the photodetector 613, the PLL circuit 614 is Thus, feedback is provided to the optical fiber stretcher 615 including the phase modulator / PZT to realize stable operation.

  In the present invention, unlike a phase sensitive amplifier using a conventional optical fiber, a mechanism such as phase modulation for avoiding stimulated Brillouin scattering is not required, and a nonlinear medium can be a waveguide of about 50 mm at most. As a whole, a compact phase sensitive amplifier can be constructed. FIG. 10 is a constellation map of an input signal and an output signal when a QPSK signal intentionally added with phase noise is input to the phase sensitive optical amplifying apparatus 600 according to the first embodiment. As shown in FIG. 10, in the phase sensitive optical amplifying apparatus 600 according to the first embodiment, amplification is performed only when the phase of the signal is an integral multiple of π / 2, and therefore phase noise observed in the input signal is detected. It can be seen that the phase reproduction type amplification is performed.

  In the first embodiment, it is arranged on the lower frequency side than the signal light 652, but it goes without saying that the same effect can be obtained even if this arrangement is arranged in the reverse order.

  The same effect as in the first embodiment can also be realized by widening the band by distributing the phase matching wavelength by chirping the polarization inversion period of the first PPLN waveguide 622, but in this case, as the excitation light Since the phase matching band is distributed to wavelengths that are not used, the conversion efficiency at the necessary wavelengths as a whole decreases. As shown in the first embodiment, wavelength conversion and carrier wave extraction are efficiently performed by appropriately designing a periodic polarization inversion structure and adopting a configuration in which phase matching peaks are periodically arranged only at necessary wavelengths. be able to.

(Example 2)
Hereinafter, an optical amplification apparatus according to Embodiment 2 of the present invention will be described. FIG. 11 shows an optical amplifying apparatus 1100 according to the second embodiment of the present invention. In the first embodiment, the phase reproduction amplification of the QPSK signal is performed using the PPLN waveguide having two to three phase matching wavelengths. However, the second embodiment further supports the 8PSK signal which is a multilevel signal format. A PPLN waveguide was configured as described above. The optical amplifying apparatus 1100 according to the second embodiment is characterized in that a carrier wave is extracted from a modulation signal having eight phases.

  11 includes first and second EDFAs 1101 and 1108, first and second excitation light sources 1102 and 1107, first to fourth optical multiplexers / demultiplexers 1103-1 to 1103-4, To third-order second-order nonlinear optical elements 1104, 1110 and 1111; demultiplexers 1105; first and second bandpass filters 1109 and 1112; photodetector 1113; PLL circuit 1114; and light from PZT. An optical amplification device 1100 including a fiber stretcher 1115 and a polarization maintaining optical fiber 1116 is shown. The first second-order nonlinear optical element 1104 includes a first spatial optical system 1121, a first PPLN waveguide 1122, a second spatial optical system 1123, and a first dichroic mirror 1124. The second second-order nonlinear optical element 1110 includes a third spatial optical system 1125, a second PPLN waveguide 1126, a fourth spatial optical system 1127, and a second dichroic mirror 1128. The third second-order nonlinear optical element 1111 includes a fifth spatial optical system 1129, a third PPLN waveguide 1130, a sixth spatial optical system 1131, a third dichroic mirror 1132, and a fourth dichroic mirror. 1133. As shown in FIG. 11, the overall configuration of the optical amplifying apparatus 1100 according to the second embodiment is substantially the same as that of the optical amplifying apparatus 600 according to the first embodiment shown in FIG. The detailed description is omitted as having the above functions. The details of the optical amplifying apparatus 1100 using the first PPLN waveguide 1122 used for carrier wave extraction according to the second embodiment will be described.

  As shown in FIG. 11, the signal light 1152 and the first excitation light 1151 are amplified by the EDFA 1101 and then enter the first PPLN waveguide 1122 having a plurality of phase matching wavelengths. The signal light 1152 and the first excitation light 1151 are set to be detuned to the low frequency side by 100 GHz. The first PPLN waveguide 1122 is designed so as to have a phase matching peak at three wavelengths, ie, a signal light wavelength, a wavelength far from the 100 GHz high frequency side, and a wavelength far from the 300 GHz high frequency side. The first PPLN waveguide 1122 is subjected to spatial phase modulation with a period of 16.92 μm and a phase modulation period of 16.5 mm.

  A wavelength conversion process in the first PPLN waveguide 1122 will be described. FIG. 12A shows the phase matching characteristics of light in the first PPLN waveguide 1122 of the first second-order nonlinear optical element 1104 related to the optical amplifying device according to the second embodiment, and FIG. Indicates the frequency arrangement of light in the first PPLN waveguide 1122. In Example 2, in order to obtain three unequally spaced peaks as shown in FIG. 12A, asymmetric phase modulation with a phase modulation period of 16.5 mm as shown in FIG. Such a phase matching curve is realized by imparting to the inversion structure. When the phase matching wavelength, the signal light 1152 and the first pumping light 1151 shown in FIG. 12B are arranged, the wavelength conversion process as described below sequentially occurs.

Under the above conditions, in the first PPLN waveguide 1122, first, the signal light 1152 is converted into SH light, and the first excitation light 1151 is generated by difference frequency generation (DFG) using the SH light as excitation light. The first idler light 1153 is converted. Since the frequency of the converted first idler light 1153 matches the second phase matching wavelength of the first PPLN waveguide 1122, the first idler light 1153 is converted into SH light, and the SH light is excited. Signal light 1152 is converted into second idler light 1154 as light. Further, when the first excitation light 1151 is converted by DFG using the SH light of the first idler light 1153 as excitation light, third idler light 1155 is generated. Since the frequency of the converted first idler light 1153 coincides with the second phase matching wavelength of the first PPLN waveguide 1122, the third idler light 1155 is converted into SH light, and the SH light is excited. As the light, the second idler light 1154 goes to the fourth idler light 1156, the first idler light 53 goes to the fifth idler light 1157, the signal light 1152 goes to the sixth idler light 1158, and the first excitation light. 1151 is converted into seventh idler light 1159, respectively. At this time, paying attention to the phases of the first idler light 1153, the third idler light 1155, the sixth idler light 1158, and the seventh idler light 1159, the phases of these idler lights are expressed by the following (formula 13). ) To (Expression 16).
φ _i1 = 2φ _s −φ _p1 (Formula 13)
φ _i3 = 2φ _i2 −φ _p1 = 4φ _s −3φ _p1 (Formula 14)
φ _i6 = 2φ _i3 −φ _s = 7φ _s −6φ _p1 (Formula 15)
φ _i7 = 2φ _i3 −φ _p1 = 8φ _s −7φ _p1 (Formula 16)

Here, φ _i6 indicates the phase of the sixth idler light 1158, and φ _i7 indicates the phase of the seventh idler light 1159. As described above, the first to seventh idler lights 1153 to 1159 can be sequentially generated only by setting three phase matching wavelengths. That is, the quasi phase matching wavelength of the first PPLN waveguide 622 is at least ω _s , 2ω _s −ω _p with respect to the frequency ω _{p of} the first pumping light 1151 and the frequency ω _s of the signal light 1152. It suffices if it matches the frequency of 4ω _s -3ω _p .

  When attention is paid to the phase of the seventh idler light 1159 expressed by (Equation 16), since the phase of the signal light 1152 is multiplied by 8, the signal taking only the phase that is an integer multiple of π / 4 of 8PSK is used. It can be seen that the phase of the seventh idler light 1159 is constant and the carrier wave can be extracted.

  Referring to FIG. 11 again, since the actual phase modulation signal is accompanied by a change in intensity, the output of the first second-order nonlinear optical element 1104 is demultiplexed by the demultiplexer 1105 and branched into two, and the signal light 1152 and the sixth The idler light 1158 enters the third second-order nonlinear optical element 1111, and the seventh idler light 1159 enters the optical circulator 1106. The seventh idler light 1159 is injection-locked to the laser output from the second excitation light source 1107 (semiconductor laser) in the optical circulator 1106, and the remaining intensity modulation component is removed and output as the second excitation light 1160. . The first pumping light 1151 and the second pumping light 1160 are combined by the third optical multiplexer / demultiplexer 1103-3, amplified by the second EDFA 1108, and then the first pumping light 1151 and the second pumping light 1160. After excessive spontaneous emission light is removed by a bandpass filter 1109 that transmits only the light 1160, the light is incident on the second PPLN waveguide 1126 of the second second-order nonlinear optical element 1110. In the second PPLN waveguide 1126, the sum frequency is generated by the first pumping light 1151 and the second pumping light 1160, whereby the sum frequency light 1161 synchronized with the signal wave phase can be generated.

Note that the phase matching wavelength of the second PPLN waveguide 1126 at this time needs to be designed so as to coincide with the third idler light 1155 shown in FIG. As a result, the first pump light 1151, the second pump light 1160, and the phase matching wavelength are all detuned to 400 GHz, which satisfies the phase matching condition for sum frequency generation. The phase φ _s F of the sum frequency light at this time is given by the following (Equation 17).
φ _s F = φ _p1 + φ _p2 = −6φ _p1 (Formula 17)

  The sum frequency light 1161 is separated from the first pumping light 1151 and the second pumping light 1160 by the second dichroic mirror 1128 built in the second second-order nonlinear optical element 1110, and then is 0. 0. The light enters the third PPLN waveguide 1130 of the third second-order nonlinear optical element 1111 via the polarization maintaining optical fiber 1116 in the 78 μm band.

  On the other hand, the signal light 1152 and the sixth idler light 1158 separated by the branching filter 1105 and incident on the third second-order nonlinear optical element 1111 also pass through the third PPLN waveguide via the third dichroic mirror 1132. 1130 is incident.

When the signal light 1152, the sixth idler light 1158, and the sum frequency light 1161 are incident on the third PPLN waveguide 1130, phase sensitive amplification is possible as described below. The sum of the phase of the signal light 1151 and the phase of the sixth idler light 1158 is given by (Equation 18) below.
φ _s + φ _i6 = 8φ _s −6φ _p1 (Formula 18)

  Comparing (Equation 18) with (Equation 17), it can be seen that since both coincide only when the phase of the signal is an integral multiple of π / 4, an 8PSK signal can be phase-sensitively amplified. FIG. 14 shows a constellation map of an input signal and an output signal when a QPSK signal intentionally added with phase noise is input to the phase sensitive amplifier according to the second embodiment. As shown in FIG. 14, in the second embodiment, amplification is performed only when the phase of the signal is an integer multiple of π / 4, so that the phase noise observed in the input signal can be reduced. It can be seen that phase reproduction type amplification is performed.

  In this way, even with a multi-value signal format such as 8PSK, it is necessary for phase-sensitive amplification without causing a significant decrease in wavelength conversion efficiency by simply changing the design of the first PPLN waveguide. It is possible to generate excitation light and idler light that are phase-synchronized with simple signal light. Further, even if the pumping light is amplified by the optical fiber amplifier as in the first embodiment, it is converted into the sum frequency light and then incident on the third PPLN waveguide to perform parametric amplification. Therefore, phase reproduction amplification can be realized without being buried in the spontaneous emission light generated and without causing the signal-to-noise ratio to deteriorate the weak signal light.

  In this embodiment, the signal light is arranged on the lower frequency side, but it goes without saying that the same effect can be obtained even if this arrangement is arranged in the reverse order.

Here, when the configuration of the present invention is applied to an optical amplifier that amplifies multilevel phase-modulated signal light that takes M (M is an integer of 4 or more) types of values, From the output of the second-order nonlinear optical element, idler light having a frequency of Mω _s − (M−1) ω _p , idler light having a frequency of (M−1) ω _s − (M−2) ω _p , and By separating the signal light, idler light having a frequency of Mω _s − (M−1) ω _p is output from one output port connected to the optical circulator, and (M−1) ω _s − (M -2) It may be configured to output idler light and signal light having a frequency of ω _p from the other output port connected to the third second-order nonlinear optical element.

100, 400 Conventional PSA
DESCRIPTION OF SYMBOLS 101 Phase sensitive light amplification part 102 Excitation light source 103 Excitation light phase control part 104-1, 203-1 1st optical branching part 104-2, 203-2 2nd optical branching part 201 EDFA
202, 604, 1104 First second-order nonlinear optical element 204, 610, 1110 Second second-order nonlinear optical element 205 Phase modulator 206, 413, 615, 1115 Optical fiber stretcher by PZT 207, 616, 1116 Polarization Holding fiber 208, 411, 613, 1113 Photodetector 209, 412, 614, 1114 Phase-locked loop circuit (PLL)
211, 621, 1121 First spatial optical system 212, 622, 1112 First PPLN waveguide 213, 623, 1123 Second spatial optical system 214, 624, 1124 First dichroic mirror 215, 625, 1125 Third Spatial optical systems 216, 626, 1126 Second PPLN waveguides 217, 627, 1127 Fourth spatial optical systems 218, 628, 1128 Second dichroic mirrors 219, 632, 1132 Third dichroic mirrors 401, 601, 1101 First EDFA
402, 602, 1102 First pump light source 403-1, 603-1, 1103-1 First optical multiplexer / demultiplexer 403-2, 603-2, 1103-2 Second optical multiplexer / demultiplexer 403-3, 603-3, 1103-3 Third optical multiplexer / demultiplexer 403-4, 603-4, 1103-4 Fourth optical multiplexer / demultiplexer 403-5 Fifth optical multiplexer / demultiplexer 404 First optical fiber 405, 605, 1105 Demultiplexer 406, 606, 1106 Optical circulator 407, 607, 1107 Second excitation light source (semiconductor laser)
408, 608, 1108 Second EDFA
409 Second optical fiber 410 Bandpass filter 600, 1100 Optical amplification device 609, 1109 First bandpass filter 611, 1111 Third second-order nonlinear optical element 612, 1112 Second bandpass filter 629, 1129 5th spatial optical system 630 3rd PPLN waveguide 631, 1131 6th spatial optical system 633, 1133 4th dichroic mirror 633

Claims (3)

An optical amplifying device that amplifies multilevel phase-modulated signal light that takes M types of values by optical mixing using a second-order nonlinear optical effect, wherein M is an integer of 4 or more,
A first light source for generating first excitation light having a frequency ω _p ;
A first optical fiber laser amplifier that amplifies and outputs the input signal light having the frequency ω _s and the first pumping light;
A first second-order nonlinear optical element that receives the signal light amplified and output by the first optical fiber laser amplifier and the first pumping light, and generates a second harmonic, A first second-order nonlinear optical element that satisfies quasi-phase matching at a wavelength;
From the output of the first second-order nonlinear optical element, it has idler light having a frequency of Mω _s − (M−1) ω _{p and} a frequency of (M−1) ω _s − (M−2) ω _p. By separating the idler light and the signal light, the idler light having the frequency of Mω _s − (M−1) ω _p is output from one output port, and the (M−1) ω _s − (M -2) a duplexer that outputs idler light having a frequency of ω _p and the signal light from the other output port;
A second excitation light source that outputs a laser capable of injection locking to an output from the one output port of the duplexer;
An optical circulator that outputs the second pumping light in an injection-synchronized manner with the laser output from the second pumping light source, the output from the one output port of the duplexer;
A second optical fiber laser amplifier that amplifies and outputs the first pumping light and the second pumping light;
The first pumping light and the second pumping light that are amplified and output by the second optical fiber laser amplifier are input, and the sum frequency light of the first pumping light and the second pumping light is input. A second second-order nonlinear optical element that generates
An optical amplifying apparatus comprising: a third second-order nonlinear optical element that performs non-degenerate parametric amplification by allowing the output from the other output port of the duplexer and the sum frequency light to enter. . The first second-order nonlinear optical element is an optical waveguide made of LiNbO ₃ having a periodically poled structure,
The periodic polarization reversal structure is subjected to spatial phase modulation or periodic modulation with a period longer than the polarization reversal period,
2. The first second-order nonlinear optical element according to claim 1, wherein the quasi-phase-matching wavelength matches at least frequencies of ω _s , 2ω _s −ω _p , and amplifies the QPSK signal. Optical amplification device. The first second-order nonlinear optical element is an optical waveguide made of LiNbO ₃ having a periodically poled structure,
The periodic polarization reversal structure is subjected to spatial phase modulation or periodic modulation with a period longer than the polarization reversal period,
The first second-order nonlinear optical element has a quasi phase matching wavelength that matches at least frequencies of ω _s , 2ω _s −ω _p , and 4ω _s −3ω _p , and amplifies an 8PSK signal. The optical amplification device according to claim 1.

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