JP6110547B1 - Optical amplifier - Google Patents

Abstract

Amplifying two orthogonal polarization components to achieve both reduction in the number of optical components and reduction in the influence of reflected light. A phase sensitive optical amplifier includes a deflecting beam splitter (1), a second-order nonlinear optical element (8, 9), and a polarization rotator (10, 11). The deflection beam splitter (1) separates the signal light and the fundamental light into orthogonal polarization components, and combines the TM polarization component of the signal light from the fiber (24) and the TE polarization component of the signal light from the fiber (17). Wave and output to the fiber (19). The second-order nonlinear optical element (8) generates the second harmonic light of the TM polarization component of the fundamental light from the fiber (20) and amplifies the TM polarization component of the signal light from the fiber (21). The second-order nonlinear optical element (9) generates the second harmonic light of the TM polarization component of the fundamental light from the fiber (24) and amplifies the TM polarization component of the signal light from the fiber (23). [Selection] Figure 1

Description

  The present invention relates to an optical amplifying apparatus 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 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. 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 the phase sensitive optical amplifier can suppress spontaneous emission light having a quadrature phase irrelevant to the signal and the in-phase spontaneous emission light can be minimized, the S / N of the signal light before and after amplification is sufficient. In principle, it is possible to maintain the same without deteriorating.

  FIG. 3 is a block diagram showing a basic configuration of a conventional phase sensitive optical amplifier. As shown in FIG. 3, the phase-sensitive optical amplifier 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 branches. Parts 104-1 and 104-2. As shown in FIG. 3, the signal light 110 input to the phase sensitive optical amplifier 100 is branched into two by an optical branching unit 104-1, one of which is incident on the phase sensitive optical amplifying unit 101, and the other is an excitation light source. 102 is incident. 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 phase sensitive optical amplifier using a PPLN waveguide disclosed in Non-Patent Document 1 and the like. The phase-sensitive optical amplifier 200 shown in FIG. 4 includes an erbium-doped fiber amplifier (EDFA) 201, first and second second-order nonlinear optical elements 202 and 204, first and second Optical branching sections 203-1 and 203-2, a phase modulator 205, an optical fiber expander 206 using a PZT (lead zirconate titanate) piezoelectric element, a polarization maintaining fiber 207, and a photodetector 208. And a phase locked loop (PLL) circuit 209. The first second-order nonlinear optical element 202 includes a first spatial optical system 211, a first PPLN waveguide 212, a second spatial optical system 213, and a first dichroic mirror 214. The second second-order nonlinear optical element 204 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 phase sensitive optical amplifier 200 is branched by the light 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 the phase sensitive optical amplifier, in order to amplify only the light in phase with the signal, it is necessary that the phase of the signal light and the excitation light coincide with each other 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 pump light and the gain (dB) in a conventional phase sensitive optical amplifier 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.

  As shown in FIG. 5, a conventional phase sensitive optical amplifier using a PPLN waveguide has a characteristic of attenuating orthogonal phase components. Therefore, an IMDD (normal intensity modulation or binary phase modulation) is used. Although applicable to amplification of modulation signals such as Intensity Modulation Direct Detection (BPSK), Binary Phase Shift Keying (BPSK), and Differential Phase Shift Keying (DPSK), QPSK (Quadrature Phase Shift Keying), which is a multi-level modulation format, 8PSK, etc. The modulated signal 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.

However, the above-described prior art has the following problems. Since the conventional phase sensitive optical amplifier shown in FIG. 4 greatly depends on the polarization of the input signal light, there is a problem in applying it to an actual optical communication system. A second-order nonlinear optical medium having a crystal anisotropy represented by LiNbO ₃ has different refractive indices and nonlinear constants for two orthogonal polarizations.

Usually, the PPLN waveguide has a configuration in which d33, which is the highest nonlinear optical constant, can be used, and a periodically poled structure is provided so that phase matching can be achieved in the configuration. For example, when a Z-cut LiNbO ₃ substrate is used, the condition of phase matching is satisfied for TM (Transverse Magnetic) polarized light, while the refractive index for TE (Transverse Electric) polarized light that is orthogonally polarized is Because of the difference, phase matching cannot be satisfied at the same wavelength. Even if the phase matching can be satisfied by any method, the nonlinear constant for the TE polarized light uses a component smaller than d33, so that the efficiency is greatly reduced.

  That is, an optical parametric process using a second-order nonlinear medium typified by PPLN greatly depends on the polarization direction of incident light, and can be amplified only for a single polarization. That is, in the conventional configuration shown in FIG. 4, when the polarization state of the light incident on the PPLN waveguide is not constant, the amplification characteristic as the phase sensitive optical amplifier is not constant.

  In actual optical transmission, it is possible to configure the optical transmitter so that the polarization directions of the optical signals from the optical transmitter are always the same. However, an optical fiber normally used as a transmission line, represented by a single mode fiber (SMF), does not maintain polarization. For this reason, the polarization direction of the optical signal after transmission is not constant. Furthermore, the polarization of the optical signal changes gradually due to a disturbance such as a temperature change applied to the optical fiber. When amplifying such an optical signal, in an optical amplifier whose gain changes with respect to the polarization direction of the optical signal, the intensity of the amplified optical signal changes from time to time. Unsuitable for use.

  Furthermore, in recent optical communications, a polarization division multiplexing (PDM) system is used in which separate signals are superimposed on two orthogonal polarizations to improve frequency utilization efficiency. In other words, the signal light transmitted through the optical fiber has two polarization components, and the polarization is rotated in a state where the two polarizations are orthogonal to each other, and the direction of rotation of this polarization always changes. To do. When amplifying such an optical signal, it is not sufficient to be independent of changes in the polarization direction, and it is necessary to amplify two polarization components. However, a configuration capable of satisfying these conditions in a conventional phase sensitive optical amplifier has not been shown.

  As a method for making polarization independent of optical amplifiers, polarization diversity is performed by separating the polarization with a polarization separation element, combining the two separated polarizations with TM polarization, and then phase-amplifying each. Configuration is conceivable. In order to perform polarization diversity by separating the optical path for the two polarized waves, at least the time between separating the polarized waves and amplifying the light and combining the polarized waves is separated into two. It is essential that the delays coincide in the optical path and that there is no phase variation between the two separated optical paths.

  In Non-Patent Document 3, two independent signals having orthogonal polarizations are independently phase-sensitive amplified and then combined using a polarizing beam splitter (PBS) to generate a PDM signal. The configuration to be shown is shown. However, in this configuration, since the phase sensitive optical amplifier is composed of fiber parts, phase fluctuations occur between two independent optical paths. Therefore, with the configuration disclosed in Non-Patent Document 3, diversity with respect to signal light having an arbitrary polarization cannot be realized. For this reason, Non-Patent Document 3 assumes application as an optical amplifier for a transmitter that can handle signals having two polarizations independently.

  Although different from the polarization independence in the phase sensitive optical amplifier, several configurations for eliminating the polarization dependence of the PPLN waveguide have been proposed for the wavelength converter using the PPLN waveguide. Non-Patent Document 4 shows a configuration in which signal light is reciprocated in a PPLN waveguide. In this configuration, first, signal light including two orthogonal polarization components and unipolar excitation light are incident on a PPLN waveguide, and two orthogonal polarizations are generated by a difference frequency generation process in the PPLN waveguide. Wavelength converted light is obtained from the light of one of the components.

  Subsequently, after the polarization of the signal light and the wavelength converted light is rotated using the wave plate, the signal light is incident on the PPLN waveguide from the direction opposite to the initial incident direction together with the wavelength converted light and the excitation light. At this time, the wavelength-converted light can be obtained from only one polarization component. However, the polarization component of the signal light is orthogonal to the polarization component obtained in the forward path because it is incident after rotating the polarization using the wave plate. The wavelength converted light can be obtained. Finally, since a combined component of the wavelength-converted light obtained in the forward path and the wavelength-converted light obtained in the return path is obtained as output converted light, wavelength-independent wavelength conversion can be realized.

  Non-Patent Document 5 shows a configuration in which signal light including two orthogonal polarization components is divided into respective polarization components by a polarization beam splitter (PBS) and then incident from both sides of the same PPLN waveguide. Has been. In this configuration, after the polarization is separated by PBS, the separated polarization is made incident from both sides of the PPLN waveguide, and the polarization is separated again by synthesizing the polarization by the same PBS. Since the optical paths can be made equal for two light components, the phase variation that occurs when polarization separation is performed can be minimized.

  Thus, in the wavelength converter using a PPLN waveguide, the structure which eliminates polarization dependence by using the same PPLN waveguide bidirectionally is shown. However, such a configuration cannot be directly applied to a configuration for obtaining a gain of light, such as a phase sensitive optical amplifier using parametric optical amplification. This is because, in an optical amplifier, the effect of light reflection from components increases as the gain increases. Moreover, in the phase sensitive optical amplifier, it is necessary to synchronize the phase of the interacting signal light and the pumping light. When the light reflection increases, the mechanism for performing the phase synchronization is also affected, so that the stable It is difficult to realize the amplification operation.

  Hereinafter, problems due to the influence of light reflection when excited in both directions of the PPLN waveguide will be described with reference to schematic drawings. FIG. 6 shows a conceptual diagram of the influence of reflection when excitation light is incident on a waveguide device made of a second-order nonlinear optical material such as PPLN from both directions.

  In the configuration example shown in FIG. 6, it is assumed that the Z-cut PPLN waveguide 300 is used to amplify signal light in a wavelength band of 1560 nm. In this case, excitation light having a wavelength of 780 nm band is used. This combination of wavelengths can be amplified for signal light of any wavelength if the polarization inversion period of the PPLN waveguide 300 is appropriately selected. Each polarization component of the signal light is separated using a PBS (not shown) or the like to obtain signal light Inputa and signal light Inputb including different polarization components.

  The signal light Inputa and the excitation light Pumpa are combined using a dichroic mirror 301 having a dielectric multilayer film, and then incident on the PPLN waveguide 300 from the left side of FIG. At this time, both the signal light Inputa and the pumping light Pumpa are adjusted to TM polarization and then incident on the dichroic mirror 301. The amplified signal light OutputA is obtained by the parametric amplification process in the PPLN waveguide 300. At this time, a part of the signal light OutputA is branched by the duplexer 303 and received by the optical receiver 304. The phase of the signal light Inputa and the pumping light Pumpa is synchronized using a phase locked loop (PLL) circuit (not shown) based on the electrical signal obtained by the optical receiver 304.

  Similarly, the signal light Inputb and the excitation light Pumpb are combined by using the dichroic mirror 302 having a dielectric multilayer film, and then incident on the PPLN waveguide 300 from the right side of FIG. At this time, both the signal light Inputb and the pumping light Pumpb are adjusted to TM polarization and then incident on the dichroic mirror 302. The amplified signal light Output B is obtained by the parametric amplification process in the PPLN waveguide 300. A part of the signal light OutputB is branched by the demultiplexer 305 and received by the optical receiver 306. Based on the electrical signal obtained by the optical receiver 306, the phase of the signal light Inputb and the pumping light Pumpb is synchronized using a phase-locked loop circuit (not shown).

In FIG. 6, the influence of light reflection is shown above and below the PPLN waveguide 300 in the drawing. Reflection components (GRb, G ² Rb, G ² R) of the signal light Inputb are reflected on the PPLN waveguide 300 by reflecting the reflection components (GRa, G ² Ra, G ² R ² a, G ³ R ² a) of the signal light Inputa. ² b, G ³ R ² b) are shown below the PPLN waveguide 300.

  In FIG. 6, only the reflection of signal light at the output end of the PPLN waveguide 300 is considered in order to briefly explain the influence of light reflection. In an actual configuration, there is also reflection at the input end of the PPLN waveguide 300 and reflected light from the dichroic mirrors 301 and 302. Although not shown in FIG. 6, since an optical component such as a lens is used for input / output of the PPLN waveguide 300 in the actual configuration, reflected light from these optical components also exists. Furthermore, although it is actually necessary to take into account the interaction between the reflected excitation light and signal light, it is not shown in FIG. Further, the number of reflections of the signal light indicates only the number of times necessary for explaining the influence (generally two times), but in actuality, it is reflected in multiple.

  The state of the influence of the reflection of the signal light Inputa shown in FIG. 6 will be described. The signal light Inputa incident from the left side is output by being multiplied by G in the PPLN waveguide 300. The amplified signal light is Ga. The signal light Inputa is reflected at the reflectance R at the end face EdgeB (the output end of the signal light Inputa) of the PPLN waveguide 300. This reflected light is designated as GRa.

The reflected light GRa at the end face EdgeB is similarly multiplied by G by the interaction with the excitation light Pumpb while propagating through the PPLN waveguide 300. Actually, the gain of amplification by the pumping light Pumpa may be slightly different from the gain of amplification by the pumping light Pumpb, but here it is approximated to the same value G. That is, since a gain of G times can be obtained even in the return path, there is reflected light output from the end face EdgeA (input end of the signal light Inputa) of the PPLN waveguide 300. A component that is reflected once by the end face EdgeB and is emitted from the end face EdgeA is defined as G ² Ra.

A part of the light reflected once by the end face EdgeB of the PPLN waveguide 300 is reflected by the end face EdgeA with the reflectance R. Let this reflected light be G ² R ² a. While propagating in the PPLN waveguide 300, the reflected light G ² R ² a at the end face EdgeA is multiplied by G by the interaction with the excitation light Pumpa and emitted from the end face EdgeB. The light this emits and G ³ R ² a.

  Similarly, the signal light Inputb incident from the right side is multiplied by G in the PPLN waveguide 300 and output. The amplified signal light is assumed to be Gb. The signal light Inputb is reflected at the reflectance R at the end surface EdgeA of the PPLN waveguide 300 (the output end of the signal light Inputb). This reflected light is designated as GRb.

The reflected light GRb at the end face EdgeA is similarly multiplied by G by the interaction with the excitation light Pumpa while propagating through the PPLN waveguide 300. That is, since a gain of G times is obtained even in the return path, reflected light output from the end face EdgeB (input end of the signal light inputb) of the PPLN waveguide 300 exists. A component that is reflected once by the end face EdgeA and emitted from the end face EdgeB is defined as G ² Rb.

A part of the light reflected once by the end face EdgeA of the PPLN waveguide 300 is reflected by the end face EdgeB with a reflectance R. This reflected light is G ² R ² b. The reflected light G ² R ² b at the end face EdgeB is multiplied by G by the interaction with the excitation light Pumpb while propagating through the PPLN waveguide 300, and is emitted from the end face EdgeA. The light this emits and G ³ R ² b.

Therefore, when the signal lights Inputa and Inputb and the pumping lights Pumpa and Pumpb are incident on the same PPLN waveguide 300 in both directions, if the output lights of the signal lights Inputa and Inputb are OutputA and OutputB, respectively, at the end face of the waveguide Even if the reflection is performed twice, the output light OutputA and OutputB include a reflection component as shown in the following equation.
Output A = Ga + G ² Rb + G ³ R ² a + (2)
Output B = Gb + G ² Ra + G ³ R ² b + (3)

As the signal light OutputA, it is originally desirable to output only Ga which is the signal light Inputa multiplied by G. However, as shown in FIG. 6 and Equation (2), it can be seen that the reflected light composed of G ² Rb + G ³ R ² a is also output at the same time. This reflected light interferes with the amplified signal light Ga and becomes noise that degrades the signal quality.

Here, suppose that the intensity of each of the signal lights Inputa and Inputb is 1, the amplification gain of the PPLN waveguide 300 is 20 dB (G = 100 times), and the reflectance at the end face of the PPLN waveguide 300 is −30 dB (R = 0.0). 001), since the signal lights Inputa and Inputb are multiplied by 100, the intensity of each of the amplified signal lights Ga and Gb becomes 100. However, the intensity of the reflected light G ² Rb, G ² Ra that becomes a noise component is 10, and the intensity of the reflected light G ³ R ² a, G ³ R ² b is 1, and only these two components are considered. However, noise with an intensity of 10% or more is generated with respect to the amplified main signals Ga and Gb.

  It is clear that the influence of reflected light becomes larger due to multiple reflection in practice. As is clear from FIG. 6 and Expression (2), when the same waveguide is simply excited from both directions, the light is amplified by both reflection and reciprocation, so that the influence of the reflected light increases. I understand that. That is, even when the reflectivity R is smaller than the gain G, the multiplier applied to the reflectivity R and the multiplier applied to the gain G are different, so that the intensity of the reflected light increases so as to greatly affect the amplified main signal component. There is a problem that it ends up. In addition, when the gain G is increased, there is a problem that oscillation occurs depending on circumstances.

  Furthermore, in order to apply the configuration in which the signal light reciprocates in the PPLN waveguide to phase sensitive optical amplification, it is necessary to synchronize the phase of the signal light and the excitation light. However, since the reflected light, which is a noise component, has a phase different from that of the amplified main signal, stable phase synchronization is achieved when phase synchronization control is performed by receiving a part of the output light. There was also a problem that it was not possible. For the above reasons, the polarization-independent phase-sensitive optical amplifier cannot be realized with the configurations disclosed in Non-Patent Document 4 and Non-Patent Document 5.

  To deal with such a problem, Non-Patent Document 6 shows a configuration in which two signal lights separated by polarization are amplified using different PPLN waveguides. A schematic diagram of this configuration is shown in FIG. Each polarization component in the signal light is separated using a PBS (not shown) to obtain a signal light Inputa having a longitudinal polarization component and a signal light Inputb having a transverse polarization component, respectively.

  The signal light Inputa and the excitation light Pumpa are combined using a dichroic mirror 402 made of a dielectric multilayer film, and then incident on the PPLN waveguide 400 from the left side of FIG. At this time, both the signal light Inputa and the pumping light Pumpa are adjusted to TM polarization and then incident on the dichroic mirror 402. The amplified signal light OutputA is obtained by the parametric amplification process in the PPLN waveguide 400. At this time, part of the signal light OutputA is branched by the duplexer 404 and received by the optical receiver 406 via the polarizer 405. The phase of the signal light Inputa and the pumping light Pumpa is synchronized using a phase locked loop circuit (not shown) based on the electrical signal obtained by the optical receiver 406.

  Similarly, the signal light Inputb and the excitation light Pumpb are combined using a dichroic mirror 403 made of a dielectric multilayer film, and then incident on the PPLN waveguide 401 from the right side of FIG. At this time, both the signal light Inputb and the pumping light Pumpb are adjusted to TM polarization and then incident on the dichroic mirror 403. The amplified signal light OutputB is obtained by the parametric amplification process in the PPLN waveguide 401. A part of the signal light OutputB is branched by the demultiplexer 407 and received by the optical receiver 409 via the polarizer 408. Based on the electric signal obtained by the optical receiver 409, the phase of the signal light Inputb and the pumping light Pumpb is synchronized using a phase locked loop circuit (not shown).

  In FIG. 7, the influence of light reflection is shown above and below the PPLN waveguides 400 and 401 in the drawing. The reflection component of the signal light Inputa of the longitudinal polarization component is shown below the PPLN waveguides 400 and 401, and the reflection component of the signal light Inputb of the transverse polarization component is shown below the PPLN waveguides 400 and 401.

  In FIG. 7, only the reflection of the signal light at the output ends of the PPLN waveguides 400 and 401 is considered in order to briefly explain the influence of the light reflection. In an actual configuration, there is also reflection at the input ends of the PPLN waveguides 400 and 401 and reflected light from the dichroic mirrors 402 and 403. Although not shown in FIG. 7, since optical components such as lenses are used for input and output of the PPLN waveguides 400 and 401 in the actual configuration, reflected light from these optical components also exists. Furthermore, although it is actually necessary to take into account the interaction between the reflected excitation light and signal light, it is not shown in FIG. Further, the number of reflections of the signal light indicates only the number of times necessary for explaining the influence (generally two times), but in actuality, it is reflected in multiple.

The influence state of the reflection of the signal light Inputa of the longitudinal polarization component will be described. The signal light Inputa incident from the left side is output by being multiplied by G in the PPLN waveguide 400. This amplified signal light is referred to as Ga _(TM) . At the end face EdgeB of the PPLN waveguide 400, the signal light Inputa is reflected with the reflectance R. This reflected light is designated as GRa _(TM) . Since no excitation light exists in the reverse direction, the reflected light GRa _(TM) at the end face EdgeB is not amplified while propagating through the PPLN waveguide 400.

The longitudinally polarized component signal light Ga _(TM) output from the end face EdgeB after being multiplied by G passes through the polarization rotator 410 and is converted into TE polarized light, and then enters the PPLN waveguide 401. At this time, since no excitation light exists in the same direction as the propagation direction, the signal light Ga _(TM) is not amplified while propagating through the PPLN waveguide 401. Not only the excitation light does not exist, but also the polarized light is TE-polarized light that does not cause an interaction, so that the signal light Ga _(TM) is not amplified even if there is reflected excitation light or the like. As a result, the light propagates through the PPLN waveguide 401, and after receiving the linear loss L, the light is emitted from the end face EdgeD of the PPLN waveguide 401. The signal light emitted from this end face EdgeD is defined as GLa _(TE) .

The end surface EdgeD of the PPLN waveguide 401 reflects TE-polarized signal light with a reflectance R. This reflected light is designated as GLRa _(TE) . The excitation light in the PPLN waveguide 401 exists in the same direction as the reflected TE-polarized signal light, but does not cause an interaction because the polarization is orthogonal, and the reflected light GLRa _(TE) is also amplified here. There is no.

The signal light GLRa _(TE) output from the end face EdgeC of the PPLN waveguide 401 after being reflected by the end face EdgeD and propagating through the PPLN waveguide 401 passes through the polarization rotator 410 and is converted to TM polarized light. The light enters the PPLN waveguide 400. Again, since there is no excitation light in the same direction, the amplification process does not occur, and after receiving the linear loss L, the light exits from the end face EdgeA of the PPLN waveguide 400. The signal light emitted from this end face EdgeA is GL ² Ra _(TM) .

Further, the reflected light of the signal light GL ² Ra _{(TM) at} the end face EdgeA is GL ² R ² a _(TM) . The reflected light GL ² R ² a _(TM) is amplified with a gain G and emitted from the end face EdgeB of the PPLN waveguide 400. The signal light emitted from the end face EdgeB is assumed to be G ² L ² R ² a _(TM) . The signal light G ² L ² R ² a _(TM) passes through the polarization rotator 410 and is converted to TE polarized light, and then enters the PPLN waveguide 401. After receiving the linear loss L, the PPLN waveguide 401 The light exits from the end face EdgeD. The signal light emitted from this end face EdgeD is defined as G ² L ³ R ² a _(TE) .

Further, the reflected light GRa _(TM) at the end face EdgeB of the PPLN waveguide 400 propagates through the PPLN waveguide 400 and then reflects at the end face EdgeA. This reflected light is designated as GR ² a _(TM) . The reflected light GR ² a _(TM) is amplified with a gain G and emitted from the end face EdgeB of the PPLN waveguide 400. The signal light emitted from this end face EdgeB is assumed to be G ² R ² a _(TM) . The signal light G ² R ² a _(TM) passes through the polarization rotator 410 and is converted to TE polarized light, and then enters the PPLN waveguide 401. After receiving the linear loss L, the end face of the PPLN waveguide 401 Ejected from EdgeD. The signal light emitted from the end face EdgeD is assumed to be G ² LR ² a _(TE) .

More light _{Ga (TM), GRa (TM} ), GLa (TE), GLRa (TE), GL 2 Ra (TM), GL 2 R 2 a (TM), G 2 L 2 R 2 a (TM), When G ² L ³ R ² a _(TE) , GR ² a _(TM) , G ² R ² a _(TM) , and G ² LR ² a _(TE) are defined in the case of the signal light inputb, Gb _(TM) , GRb _(TM) , GLb _(TE) , GLRb _(TE) , GL ² Rb _(TM) , GL ² R ² b _(TM) , G ² L ² R ² b _(TM) , G ² L ³ R ² b _(TE) , GR ² b _(TM) , G ² R ² b _(TM) , G ² LR ² b _(TE) .

Accordingly, if the output lights of the signal lights Inputa and Inputb are OutputA and OutputB, the output lights OutputA and OutputB include a reflection component as shown in the following equation.
Output A = GLa _(TE) + GRb _(TM) + GL ² Rb _(TM) + G ² LR ² a _(TE)
+ G ² L ³ R ² a _(TE) + GLR ² A _(TE) (4)
Output B = GLb _(TE) + GRa _(TM) + GL ² Ra _(TM) + G ² LR ² b _(TE)
+ G ² L ³ R ² b _(TE) + GLR ² b _(TE) (5)

As the signal light OutputA, it is originally desirable that only GLa which is the signal light Inputa multiplied by G is output. However, as shown in FIG. 7 and formula (4), it can be seen that the reflected light composed of GRb + GL ² Rb + G ² LR ² a + G ² L ³ R ² a + GLR ² A is also output at the same time. However, these reflected lights can be significantly reduced by using the configuration shown in FIG. 7 as compared to the case where excitation is simply bidirectional as shown in FIG.

  Here, it is assumed that the intensity of the signal light Inputa is 1, the amplification gain of the PPLN waveguides 400 and 401 is 20 dB (G = 100 times), and the reflectance at the end faces of the PPLN waveguides 400 and 401 is −30 dB (R = 0.0). 001) When the linear loss in the PPLN waveguides 400 and 401 is considered as 4 dB (L = 0.4), the signal light Inputa is multiplied by 40, so that the intensity of the amplified signal light GLa becomes 40.

The intensity of the reflected light GRb _(TM) that becomes a noise component is 0.1, the intensity of the reflected light GL ² Rb _(TM) is 0.016, and the intensity of the reflected light G ² LR ² a _(TE) is 0.004. The intensity of the reflected light G ² L ³ R ² a _(TE) is 0.00064, and the intensity of the reflected light GLR ² a _(TE) is 0.00004. Only noise having an intensity of about 0.3% with respect to the signal GLa is generated. The reason is that, as can be seen from the equation (4), unlike the bidirectional excitation configuration shown in FIG. 6, the multiplier applied to the gain G is not larger than the multiplier applied to the reflectance R. This is because the reflected light is not greatly amplified. In the configuration of FIG. 7, reflection is considered about twice. Actually, there is multiple reflection, but unlike the bidirectional excitation configuration, the intensity of reflected light decreases each time reflection is repeated. The influence of reflection is very small.

As can be seen from Equation (4), the second term GRb _(TM) on the right side and the third term GL ² Rb _(TM) on the right side, which are relatively large reflected light components among the small noise components, are amplified main signal GLa. _(TE) has orthogonal polarization components. In the configuration of FIG. 7, both polarization components of the input signal light are separated using PBS, and the output light is multiplexed by PBS. For this reason, the reflected light component having a polarization orthogonal to the main signal light is not superimposed on the main signal light as noise and is filtered by the PBS.

Therefore, since the second term GRb _(TM) on the right side and the third term GL ² Rb _(TM) on the right side, which are the reflected light components of Expression (4), are removed, the leakage of the reflected light with respect to the main signal light is Only the same polarization component as the main signal light such as the fourth term G ² LR ² a _(TE) and the fifth term G ² L ³ R ² a _{(TE) on} the right side is obtained. The total intensity of the same polarization components as those of the main signal light is about 0.01% of the intensity of the main signal light and is extremely small.

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, p.6326-6332, 2011 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 Communications Conference (OECC 2013, Kyoto, Japan) PDP paper PD2-3, 2013 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, Vol. 22, No. 3, p.2473-2482, 2014 Masaki Asobe, Hiroshi Miyazawa, Osamu Tadanaga, Yoshiki Nishida, and Hiroyuki Suzuki, “A Highly Damage-Resistant Zn: LiNbO3 Ridge Waveguide and its Application to a Polarization-Independent Wavelength Converter”, IEEE Journal of Quantum Electronics, Vol.39, No .10, p.1327-1333, 2003 Hongbin Song, Osamu Tadanaga, Takeshi Umeki, Isao Tomita, Masaki Asobe, Shuto Yamamoto, Kunihiko Mori, and Kazushige Yonenaga, “Phase-transparent flexible waveband conversion of 43 Gb / s RZ-DQPSK signals using multiple-QPM-LN waveguides”, Optics Express, Vol.18, No.15, p.15332-15337, 2010 T.Umeki, T.Kazama, O.Tadanaga, K.Enbutsu, M.Asobe, Y.Miyamoto, H.Takenouchi, “PDM signal amplification using PPLN-based polarization-independent phase-sensitive amplifier”, Journal Lightwave Technology, Vol. .33, No.7, p.1326-1332, 2015

As described above, the configuration shown in FIG. 4 has a problem that the gain changes with respect to the polarization direction of the incident optical signal, and the two polarization components cannot be amplified. was there.
Further, the technique disclosed in Non-Patent Document 3 has a problem that the phase-sensitive optical amplifier cannot be made polarization independent.
In addition, when a configuration that eliminates polarization dependence by using a PPLN waveguide bidirectionally disclosed in Non-Patent Document 4 and Non-Patent Document 5 is applied to a phase-sensitive optical amplifier, the effect of reflected light is reduced. There is a problem that it is difficult to realize a stable amplification operation because of an increase in size.

  On the other hand, as in Non-Patent Document 6, it is possible to suppress reflected light noise by adopting a configuration in which two polarization-separated signal lights are amplified by different PPLN waveguides. However, the configuration shown in Non-Patent Document 6 has a problem that the number of optical components such as a second-order nonlinear optical element, an optical amplifier for generating pumping light, and an optical filter increases. In addition, in a configuration in which excitation light is supplied from the outside of the loop, a circuit that synchronizes the phase of each polarization component of the signal light and the excitation light is required, which complicates the configuration and makes phase synchronization control difficult. There was a problem of becoming.

  In view of the above problems, the object of the present invention is to be able to amplify two orthogonal polarization components, respectively, and to reduce both the number of optical components and the influence of reflected light. It is to provide a phase sensitive optical amplifier of the type.

  The optical amplifying device of the present invention includes a first optical path through which signal light propagates, a second optical path through which fundamental wave excitation light propagates, a third optical path formed in a loop shape, and the first optical path. The incident signal light and the fundamental wave excitation light incident from the second optical path are separated into two orthogonal polarization components for output, and the signal light incident from the first circulation direction of the third optical path Of the TM polarization component and the TE polarization component of the signal light incident from the second rotation direction opposite to the first rotation direction of the third optical path, and output to the second optical path The polarization direction of the TM polarization component of the signal light and the TE polarization component of the fundamental wave excitation light, which are separated by the separation multiplexing element and output by the polarization separation multiplexing element and output in the first circulation direction of the third optical path, is 90 respectively. A signal that is rotated and incident from the second direction of the third optical path A first polarization rotator for rotating the polarization directions of the TM polarization component and the TE polarization component of the fundamental wave excitation light by 90 °, and the first polarization rotator passing through the first polarization rotator. Generating second harmonic light of the TM polarization component of the fundamental excitation light incident from the first circular direction and parametrically amplifying the TM polarization component of the signal light incident from the second circular direction of the third optical path A second-order nonlinear optical element including a first optical waveguide; and a second harmonic light of a TM polarization component of the fundamental excitation light incident from the second circulation direction of the third optical path. A second second-order nonlinear optical element including a second optical waveguide for parametrically amplifying the TM polarization component of the signal light incident from the first rotation direction of the third optical path; and the first second-order nonlinear optical TE of signal light emitted from optical element The polarization direction of the TM polarization component of the light component and the fundamental wave excitation light is rotated by 90 ° to enter the second secondary nonlinear optical element from the first circumferential direction, and the second secondary nonlinear optical A second polarization direction of the TE polarization component of the signal light emitted from the element and the TM polarization component of the fundamental excitation light is rotated by 90 ° and is incident on the first second-order nonlinear optical element from the second circulation direction. And the second harmonic light generated by the first second-order nonlinear optical element as the harmonic excitation light for parametric amplification in the second second-order nonlinear optical element. For entering the second second-order nonlinear optical element from the second harmonic light generated by the second second-order nonlinear optical element for parametric amplification in the first second-order nonlinear optical element It is characterized in further comprising a fourth optical path to be incident from the second rotating direction to said first second-order nonlinear optical element as harmonic excitation light.

Further, in one configuration example of the optical amplifying device of the present invention, the phase of the signal light and the harmonic excitation light is further controlled by controlling the phase of the fundamental excitation light propagating through the second optical path. Phase synchronization means for synchronizing in the first and second optical waveguides is provided.
Further, in one configuration example of the optical amplifying device of the present invention, the phase synchronization means propagates the phase adjusting means for adjusting the phase of the fundamental pumping light propagating in the second optical path and the third optical path. Detection means for extracting a part of the signal light and converting it into an electrical signal, and based on the output signal of the detection means, the phase adjustment means is supplied to the phase adjustment means so that the phase of the signal light and the harmonic excitation light is synchronized. And a control means for generating a control signal.
In addition, in one configuration example of the optical amplifying device of the present invention, the basic polarization component in the direction opposite to the TM polarization component of the signal light and the TE polarization component of the signal light emitted from the polarization demultiplexing element to the second optical path is further provided. A circulator is provided for adding wave excitation light to the second optical path and causing the fundamental wave excitation light to enter the polarization demultiplexing multiplex element.
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.

  According to the present invention, by providing the first to fourth optical paths, the polarization demultiplexing element, the first and second polarization rotators, and the first and second second-order nonlinear optical elements, Each orthogonal polarization component can be parametrically amplified, and both the reduction in the number of optical components and the reduction in the influence of reflected light can be achieved.

It is a block diagram which shows the structure of the phase sensitive optical amplifier which concerns on embodiment of this invention. It is a figure explaining the influence of the reflected light in embodiment of this invention. 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 figure explaining the subject at the time of using the structure of the conventional bidirectional | two-way excitation. It is a figure explaining the structure of the conventional phase sensitive optical amplifier which suppresses the influence of reflected light noise.

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

  The phase sensitive optical amplifier which is an optical amplifying device according to the present embodiment shows a configuration of a phase sensitive optical amplifier using a PPLN waveguide that can be amplified independent of a signal light input having an arbitrary polarization. FIG. 1 is a block diagram showing a configuration of a phase sensitive optical amplifier according to an embodiment of the present invention. In this embodiment, input signal light polarized in an arbitrary polarization direction is divided into two polarization components using a deflecting beam splitter (hereinafter referred to as PBS), and each polarization component is rotated clockwise and counterclockwise. The fundamental wave light that propagates in a loop in both directions and generates excitation light for signal light amplification is input to the PBS from the opposite direction to the signal light, and the two polarization components are divided clockwise and counterclockwise. Use the configuration.

  The phase sensitive optical amplifier includes a PBS 1 that is a polarization demultiplexing multiplex element, a phase modulator (PM) 2, an optical fiber expander 3 using a PZT piezoelectric element, a polarization controller 4, an EDFA 5, The band-pass filter 6, the circulator 7, the second-order nonlinear optical elements 8 and 9, the polarization rotators 10 and 11, the duplexer 12, the polarizer 13, the photodetector 14, and the control unit. A PLL circuit 15 and polarization maintaining optical fibers 16 to 25 are provided.

  The polarization maintaining optical fiber 16 constitutes a first optical path, the polarization maintaining optical fiber 19 constitutes a second optical path, and the polarization maintaining optical fibers 17, 20, 21, 23, and 24 are loop-shaped thirds. The polarization maintaining optical fiber 22 constitutes a fourth optical path. The phase modulator 2 and the optical fiber expander 3 constitute phase adjusting means. Further, the branching filter 12, the polarizer 13, and the photodetector 14 constitute detection means for extracting a part of the signal light and converting it into an electrical signal.

  The second-order nonlinear optical element 8 includes a spatial optical system 80, a dichroic mirror 81, a PPLN waveguide 82, a spatial optical system 83, and a dichroic mirror 84. The secondary nonlinear optical element 9 includes a spatial optical system 90, a dichroic mirror 91, a PPLN waveguide 92, a spatial optical system 93, and a dichroic mirror 94.

  The spatial optical system 80 couples the port of the edge surface EdgeA of the secondary nonlinear optical element 8 and the PPLN waveguide 82 via a dichroic mirror 81. The spatial optical system 90 couples the port of the end face EdgeD of the second-order nonlinear optical element 9 and the PPLN waveguide 92 via a dichroic mirror 91. The spatial optical system 83 couples the port of the end face EdgeB of the second-order nonlinear optical element 8 and the PPLN waveguide 82 via the dichroic mirror 84. The spatial optical system 93 couples a port of the end face EdgeC of the second-order nonlinear optical element 9 and the PPLN waveguide 92 via a dichroic mirror 94.

  In the example shown in FIG. 1, the excitation light used for optical amplification is generated from the fundamental wave light having the same wavelength band as the signal light by second harmonic generation. Here, the signal wavelength is described as a 1560 nm band used in optical fiber communication. In the parametric amplification process using the second-order nonlinear optical effect, light having a wavelength approximately half the signal wavelength (780 nm band) is required as excitation light. Excitation light is generated from the fundamental wave light in the same wavelength band that can be phase-synchronized with the signal light, using the sum frequency generation process or second harmonic generation process in the PPLN waveguides 82 and 92.

  For example, when a phase sensitive optical amplifier can be installed near a signal light source (not shown), the light branched from the signal light source may be used as the fundamental light, or the output from the signal light source cannot be used. May generate the fundamental light in such a manner that a part of the signal light is synchronized with a local light source (not shown) in the phase sensitive optical amplifier by light injection locking or the like.

  When signal light in the 1560 nm band tilted in an arbitrary polarization direction is incident from the polarization maintaining optical fiber 16, the PBS 1 separates the incident signal light into two orthogonal polarization components. The TM polarization component of the signal light is reflected by the PBS 1 and output to the polarization maintaining optical fiber 17 as the signal light of the longitudinal polarization component, and enters the loop 30 in the counterclockwise direction (first circulation direction) in FIG. be introduced.

  On the other hand, the EDFA 5 amplifies the fundamental wave light. The bandpass filter 6 removes unnecessary spontaneous emission (ASE) light generated by the EDFA 5 and transmits only the fundamental light. The fundamental wave light output from the bandpass filter 6 is input to the PBS 1 from the side opposite to the signal light via the circulator 7 and the polarization maintaining optical fiber 19. The PBS 1 separates the incident fundamental wave light into two orthogonal polarization components. At this time, the polarization of the fundamental light is adjusted by the polarization controller 4 so that the powers of the two orthogonal polarization components of the fundamental light are equalized.

  Of the fundamental light, the TE polarization component is transmitted through the PBS 1 and output to the polarization maintaining optical fiber 17 and is introduced into the loop 30 in the counterclockwise direction of FIG. The polarization rotator 10 rotates the polarization directions of the signal light and the fundamental wave light propagating counterclockwise through the polarization maintaining optical fiber 17 by 90 °. As the polarization rotator 10, an optical element that rotates linearly polarized light by 90 °, such as a λ / 2 wavelength plate, may be used, or a mechanism that connects the polarization maintaining fiber by rotating it by 90 ° physically. But you can.

  The TE-polarized signal light and the TM-polarized fundamental light that have passed through the polarization rotator 10 propagate through the polarization-maintaining optical fiber 20 and enter the second-order nonlinear optical element 8. When the TM-polarized fundamental light is incident on the PPLN waveguide 82 of the second-order nonlinear optical element 8, the second harmonic light having a half wavelength (780 nm band) of the fundamental light is generated by the second harmonic generation process in the PPLN waveguide 82. Occur. This second harmonic light becomes the excitation light (harmonic excitation light) of the second-order nonlinear optical element 9. The dichroic mirror 84 of the second-order nonlinear optical element 8 transmits the fundamental light remaining without being converted and reflects the excitation light, thereby separating the light of these two wavelengths. The fundamental light is output to the polarization maintaining optical fiber 21, and the excitation light is output to the polarization maintaining optical fiber 22.

  In the PPLN waveguide 82, the signal light of the longitudinal polarization component propagates as TE polarized light, so that it is not subjected to nonlinear processes such as parametric amplification, and is transmitted to the polarization maintaining optical fiber 21 through the dichroic mirror 84. Is done. That is, the second-order nonlinear optical element 8 simply becomes a linear loss medium for the signal light of the longitudinally polarized component amplified by the TE polarization.

  The polarization rotator 11 rotates the polarization directions of the TE-polarized signal light and the TM-polarized fundamental light output from the PPLN waveguide 82 and propagated through the polarization-maintaining optical fiber 90 by 90 °. The TM-polarized signal light and the TE-polarized fundamental light that have passed through the polarization rotator 11 propagate through the polarization-maintaining optical fiber 23 and enter the second-order nonlinear optical element 9.

  The dichroic mirror 94 of the second-order nonlinear optical element 9 is output from the TM-polarized signal light and the TE-polarized fundamental light and the PPLN waveguide 82 that have passed through the polarization rotator 11 and propagated through the polarization-maintaining optical fiber 23. The pumping light propagated through the holding optical fiber 22 is multiplexed. Due to the parametric amplification process in the PPLN waveguide 92 of the second-order nonlinear optical element 9, energy transfer from the excitation light occurs, and TM-polarized signal light is amplified. In the PPLN waveguide 92, the fundamental wave light propagates with TE polarized light that does not cause an interaction, and therefore, does not receive a nonlinear process such as parametric amplification.

  The dichroic mirror 91 of the second-order nonlinear optical element 9 transmits TM-polarized signal light and TE-polarized fundamental wave light, and reflects the excitation light to separate these lights. The TM polarized signal light and the TE polarized fundamental light are output to the polarization maintaining optical fiber 24.

  The duplexer 12 branches a part of the TM-polarized signal light and the TE-polarized fundamental wave light propagating through the polarization maintaining optical fiber 24. The polarizer 13 transmits only TM-polarized signal light and makes it incident on the photodetector 14. The photodetector 14 converts the incident signal light into an electrical signal. The PLL circuit 15 provides feedback to the phase modulator 2 and the optical fiber expander 3 so that the output signal detected by the photodetector 14 becomes maximum. The phase modulator 2 performs phase modulation on the fundamental light according to the output of the PLL circuit 15. The optical fiber expander 3 expands and contracts the polarization maintaining optical fiber through which the fundamental wave light propagates according to the control signal output of the PLL circuit 15. In this way, by controlling the phase of the fundamental wave light, the phase synchronization between the fundamental wave light and the signal light of the longitudinal polarization component, that is, the phase between the excitation light generated from the fundamental wave light and the signal light of the longitudinal polarization component Synchronization can be achieved.

  In an actual phase-sensitive amplification operation, it is necessary to suppress the influence of phase fluctuations due to fluctuations in the optical path length due to the expansion and contraction of the optical fibers connecting the optical components. In the present embodiment, stable operation can be realized by performing the phase synchronization control as described above.

  The PBS 1 reflects the TM-polarized signal light among the TM-polarized signal light and the TE-polarized fundamental light propagated through the polarization-maintaining optical fiber 24 and outputs the TM-polarized signal light to the polarization-maintaining optical fiber 19 from the output port. Are transmitted through the input port to the polarization maintaining optical fiber 16 to separate the two light beams. The TM polarized signal light propagates through the polarization maintaining optical fiber 19, passes through the circulator 7, and is output to the polarization maintaining optical fiber 25.

  On the other hand, the PBS 1 transmits the TE polarization component of the signal light incident from the polarization maintaining optical fiber 16. This TE polarization component is output to the polarization maintaining optical fiber 24 as signal light of a transverse polarization component, and is introduced into the loop 30 in the clockwise direction (second circulation direction) in FIG. In addition, among the fundamental wave light propagated through the polarization maintaining optical fiber 19, the TM polarization component is reflected by the PBS 1, and is output to the polarization maintaining optical fiber 24 as the fundamental wave light of the longitudinal polarization component. Is introduced into the loop 30.

  When the TM-polarized fundamental wave light is incident on the PPLN waveguide 92 of the second-order nonlinear optical element 9, the second harmonic light having a half wavelength (780 nm band) of the fundamental light is generated by the second harmonic generation process in the PPLN waveguide 92. Occur. This second harmonic light becomes the excitation light (harmonic excitation light) of the second-order nonlinear optical element 8. The dichroic mirror 94 of the second-order nonlinear optical element 9 separates the light of these two wavelengths by transmitting the fundamental wave light that remains without being converted and reflecting the excitation light. The fundamental light is output to the polarization maintaining optical fiber 23, and the excitation light is output to the polarization maintaining optical fiber 22.

  In the PPLN waveguide 92, the signal light of the transverse polarization component propagates with TE polarization, so that it does not receive a nonlinear process such as parametric amplification, and is transmitted to the polarization maintaining optical fiber 23 through the dichroic mirror 94. Is done. That is, the second-order nonlinear optical element 9 is simply a linear loss medium for TE-polarized signal light.

  The polarization rotator 11 rotates the polarization directions of the TE-polarized signal light and the TM-polarized fundamental light output from the PPLN waveguide 92 and propagated through the polarization-maintaining optical fiber 90 by 90 °. The TM-polarized signal light and the TE-polarized fundamental light that have passed through the polarization rotator 11 propagate through the polarization-maintaining optical fiber 21 and enter the second-order nonlinear optical element 8.

  The dichroic mirror 84 of the second-order nonlinear optical element 8 is output from the TM-polarized signal light and the TE-polarized fundamental wave light and the PPLN waveguide 92 that have passed through the polarization rotator 11 and propagated through the polarization-maintaining optical fiber 21. The pumping light propagated through the holding optical fiber 22 is multiplexed. Due to the parametric amplification process in the PPLN waveguide 82 of the second-order nonlinear optical element 8, energy transfer from the excitation light occurs, and TM-polarized signal light is amplified. In the PPLN waveguide 82, the fundamental wave light propagates with TE-polarized light that does not cause an interaction, so that it does not receive a nonlinear process such as parametric amplification.

  The dichroic mirror 81 of the second-order nonlinear optical element 8 transmits TM-polarized signal light and TE-polarized fundamental wave light, and reflects the excitation light to separate these lights. The TM polarized signal light and the TE polarized fundamental light are output to the polarization maintaining optical fiber 20.

  The polarization rotator 10 rotates the polarization directions of TM-polarized signal light and TE-polarized fundamental light propagating clockwise through the polarization-maintaining optical fiber 20 by 90 °. The TE-polarized signal light and the TM-polarized fundamental light that have passed through the polarization rotator 10 propagate through the polarization-maintaining optical fiber 17 and enter the PBS 1.

  The PBS 1 transmits the TE-polarized signal light out of the incident TE-polarized signal light and TM-polarized fundamental light, and outputs the TE-polarized signal light from the output port to the polarization maintaining optical fiber 19, and reflects the TM-polarized fundamental light. By outputting to the polarization maintaining optical fiber 16 from the input port, these two lights are separated. The TE-polarized signal light propagates through the polarization maintaining optical fiber 19, passes through the circulator 7, and is output to the polarization maintaining optical fiber 25.

  With respect to the clockwise signal light component, it is not necessary to provide a configuration for phase synchronization unlike the counterclockwise signal light. In the configuration of the present embodiment, the propagation paths of the TE polarization component and the TM polarization component of the signal light and the excitation light are common. For this reason, if either TE polarization or TM polarization of the signal light is phase-synchronized with the excitation light, both TE polarization and TM polarization are phase-synchronized. If there is the composition of.

Here, a method for manufacturing the PPLN waveguides 82 and 92 used in the present embodiment will be exemplified 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. The length of the waveguide was 50 mm. This waveguide can be temperature-controlled by a Peltier element. The second-order nonlinear optical elements 8 and 9 having the PPLN waveguides 82 and 92 formed as described above have a module structure capable of inputting / outputting light with a polarization maintaining optical fiber in a 1.5 μm band.

In the present embodiment, LiNbO ₃ to which Zn is added is used as the material of the PPLN waveguides 82 and 92. However, other nonlinear materials such as KNbO ₃ , LiTaO ₃ , and LiNb _x Ta _(1-x) O 2 are used. 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. .

  In FIG. 2, the conceptual diagram of the effect of suppression of the influence of reflected light in the structure of this Embodiment is shown. Here, the spatial optical systems 80, 83, 90, 93, etc. of the second-order nonlinear optical elements 8, 9 are not shown. As described above, the signal light propagated through the polarization-maintaining optical fiber 16 is separated into two orthogonal polarization components using the PBS 1, and the signal light Inputa (TM-polarized signal light) of the longitudinal polarization component and the lateral polarization are respectively separated. A wave component signal light Inputb (TE-polarized signal light) is obtained.

  After the TM polarized signal light Inputa is adjusted to TE polarized light by the polarization rotator 10, it passes through the PPLN waveguide 82 of the second-order nonlinear optical element 8, and is converted to TM polarized light by the polarization rotator 11. The light enters the PPLN waveguide 92 of the second-order nonlinear optical element 9. In the PPLN waveguide 82, the excitation light Pumpa is generated by the second harmonic generation by the fundamental light, and the signal light Inputa and the excitation light Pumpa are combined by the dichroic mirror 94 of the second-order nonlinear optical element 9, and the second-order nonlinearity is generated. In the PPLN waveguide 92 of the optical element 9, the amplified signal light OutputA is obtained by parametric amplification using the signal light Inputa and the excitation light Pumpa.

  At this time, a part of the signal light OutputA is branched by the branching filter 12 and received by the optical receiver 14 through the polarizer 13. The phase of the signal light Inputa and the pumping light Pumpa is synchronized using the PLL circuit 15 based on the electric signal obtained by the optical receiver 14.

  Similarly, the TE-polarized signal light Inputb passes through the PPLN waveguide 92 of the second-order nonlinear optical element 9, is converted into TM-polarized light by the polarization rotator 11, and then the PPLN waveguide 82 of the second-order nonlinear optical element 8. Is incident on. In the PPLN waveguide 92, the excitation light Pumpb is generated by the second harmonic generation by the fundamental light, and the signal light Inputb and the excitation light Pumpb are combined by the dichroic mirror 84 of the second-order nonlinear optical element 8, and the second-order nonlinearity is obtained. In the PPLN waveguide 82 of the optical element 8, the amplified signal light Output B is obtained by parametric amplification using the signal light Inputb and the pumping light Pumpb.

  In FIG. 2, the influence of light reflection is shown above and below the PPLN waveguides 8 and 9 in the drawing. The reflection component of the signal light of the longitudinal polarization component is shown on the PPLN waveguides 8 and 9, and the reflection component of the signal light of the transverse polarization component is shown below the PPLN waveguides 8 and 9.

  In FIG. 2, only the reflection of the signal light at the output ends of the PPLN waveguides 8 and 9 is considered in order to briefly explain the influence of the light reflection. In an actual configuration, there is also reflection at the input ends of the PPLN waveguides 8 and 9 and reflected light from the dichroic mirrors 81, 84, 91 and 94. Although not shown in FIG. 2, since optical components such as lenses are used for input and output of the PPLN waveguides 8 and 9 in the actual configuration, reflected light from these optical components also exists. Furthermore, although it is actually necessary to take into account the interaction between the reflected excitation light and signal light, it is not shown in FIG. Further, the number of reflections of the signal light indicates only the number of times necessary for explaining the influence (generally two times), but in actuality, it is reflected in multiple.

The influence of the reflection of the signal light of the longitudinal polarization component will be described. The TE-polarized signal light Inputa incident from the left side in FIG. 2 receives the linear loss L in the PPLN waveguide 82 and is emitted from the end face EdgeB of the PPLN waveguide 82. Let this emitted light be La _(TE) . Further, the signal light Inputa is reflected at the reflectance R at the end face EdgeB of the PPLN waveguide 82. Let this reflected light be LRa _(TE) . Excitation light from the right side to the left direction in FIG. 2 exists, but the reflected light LRa _(TE) is TE-polarized light, so that non-linear interaction does not occur, and the reflected light LRa _(TE) propagates in the PPLN waveguide 82. It is not amplified during the process. As a result, the light propagates through the PPLN waveguide 82 and simply receives the linear loss L, and then exits from the end face EdgeA of the PPLN waveguide 82. The light emitted from the end face EdgeA is L ² Ra _(TE) .

The signal light La _(TE) emitted from the end face EdgeB of the PPLN waveguide 82 passes through the polarization rotator 11 and is converted to TM polarized light, and then enters the PPLN waveguide 92. The signal light La _(TM) in the PPLN waveguide 92 undergoes parametric amplification with a gain G by the pumping light Pumpa propagating in the same direction, and is emitted from the end face EdgeD of the PPLN waveguide 92. This emitted light is referred to as GLa _(TM) . Further, at the end face EdgeD of the PPLN waveguide 92, the signal light GLa _(TM) of the longitudinal polarization component is reflected with the reflectance R. This reflected light is designated as GLRa _(TM) .

Since the pumping light Pumpb exists in the same direction as the reflected longitudinally polarized signal light GLRa _(TM) , the reflected light GLRa _(TM) is also subjected to parametric amplification. However, the optical power of the excitation light Pumpb in the PPLN waveguide 92 increases from the end face EdgeD toward the end face EdgeC. The reason is that the pump light Pumpb is generated by the second harmonic generation in the PPLN waveguide 92, and the efficiency of the pump light generation is proportional to the square of the nonlinear interaction length. Power is at its maximum.

  In the process of simultaneously performing pump light generation and parametric amplification, the parametric gain is about 1/4 as compared with the case of always pumping with the pump light having the maximum power (see “MHChou, I .Brener, MMFejer, EEChaban, and SBChristman, “1.5-μm-band wavelength conversion based on cascaded second-order nonlinearity in LiNbO3waveguides”, IEEE Photonics Technology Letters, Vol. 11, No. 6, p. 653-655 , 1999 ”).

The reflected light that has undergone the process in which excitation light generation and parametric amplification are performed simultaneously is defined as 1/4 G ² LRa _(TM) . The light 1 / 4G ² LRa _(TM) is emitted from the end face EdgeC of the PPLN waveguide 92, passes through the polarization rotator 11, is converted into TE polarized light, and then enters the PPLN waveguide 82. Here too, there is pumping light propagating in the same direction, but the reflected light 1 / 4G ² LRa _{(TE) that} has passed through the polarization rotator 11 is TE-polarized light, so no nonlinear interaction occurs, and the amplification process is After the linear loss L is not generated, the light exits from the end face EdgeA of the PPLN waveguide 82. This emitted light is set to 1/4 G ² L ² Ra _(TE) .

At the end surface EdgeA of the PPLN waveguide 82, TE polarized light 1 / 4G ² L ² Ra _(TE) is reflected with a reflectance R. This reflected light is set to 1/4 G ² L ² R ² a _(TE) . The reflected light 1 / 4G ² L ² R ² a _(TE) is emitted from the end face EdgeB of the PPLN waveguide 82 after receiving the linear loss L. This emitted light is set to 1/4 G ² L ³ R ² a _(TE) . The light ¼ G ² L ³ R ² a _(TE) passes through the polarization rotator 11 and is converted into TM polarized light, and then enters the PPLN waveguide 92. The signal light ¼ G ² L ³ R ² a _(TM) in the PPLN waveguide 92 undergoes parametric amplification with a gain G by the pumping light Pumpa propagating in the same direction, and is emitted from the end face EdgeD of the PPLN waveguide 92. This emitted light is set to 1/4 G ³ L ³ R ² a _(TM) .

Further, the TE polarized light L ² Ra _(TE) is reflected with the reflectance R at the end face EdgeA of the PPLN waveguide 82. This reflected light is assumed to be L ² R ² a _(TE) . The reflected light L ² R ² a _(TE) is emitted from the end face EdgeB of the PPLN waveguide 82 after receiving the linear loss L. This emitted light is L ³ R ² a _(TE) . The light L ³ R ² a _(TE) passes through the polarization rotator 11 and is converted to TM polarized light, and then enters the PPLN waveguide 92. The signal light L ³ R ² a _(TM) in the PPLN waveguide 92 undergoes parametric amplification with a gain G by the pumping light Pumpa propagating in the same direction, and is emitted from the end face EdgeD of the PPLN waveguide 92. This emitted light is assumed to be GL ³ R ² a _(TM) .

The above optical La _(TE) , LRa _(TE) , L ² Ra _(TE) , La _(TM) , GLa _(TM) , GLRa _(TM) , 1/4 G ² LRa _(TM) , 1/4 G ² LRa _{( ^{TE), 1 / 4G 2 L}} 2 Ra (TE), 1 / 4G 2 L 2 R 2 a (TE), 1 / 4G 2 L 3 R 2 a (TE), 1 / 4G 2 L 3 R 2 a ( _TM) , 1 / 4G ³ L ³ R ² a _(TM) , L ² R ² a _(TE) , L ³ R ² a _(TE) , L ³ R ² a _(TM) , GL ³ R ² a _{(TM ) For} signal light inputb, Lb _(TE) , LRb _(TE) , L ² Rb _(TE) , Lb _(TM) , GLb _(TM) , GLRb _(TM) , 1/4 G ² LRb _{( TM)} , 1 / 4G ² LRb _(TE) , 1 / 4G ² L ² Rb _(TE) , 1/4 G ² L ² R ² b _(TE) , 1/4 G ² L ³ R ² b _(TE) , 1 / 4G ² L ³ R ² b _(TM) , 1/4 G ³ L ³ R ² b _(TM) , L ² R ² b _(TE) , L ³ R ² b _(TE) , L ³ R ² b _{(TM ),} GL ³ R ² To become _(TM).

Accordingly, if the output lights of the signal lights Inputa and Inputb are OutputA and OutputB, the output lights OutputA and OutputB include a reflection component as shown in the following equation.
Output A = GLa _(TM) + L ² Rb _(TE) + 1 / 4G ³ LR ² a _(TM)
+ 1 / 4G ³ L ³ R ² a _(TM) + 1 / 4G ² L ² Rb _(TE)
+ GL ³ R ² a _(TM) (6)
Output B = GLb _(TM) + L ² Ra _(TE) + 1 / 4G ³ LR ² b _(TM)
+ 1 / 4G ³ L ³ R ² b _(TM) + 1 / 4G ³ L ² Ra _(TE)
+ GL ³ R ² b _(TM) (7)

As the signal light OutputA, it is desirable that only GLa, which is G-times signal light, is output. As shown in FIG. 2 and formula (6), L ² Rb _(TE) + 1 / 4G ³ LR ² a _(TM) + 1 / 4G ³ L ³ R ² a _(TM) + 1 / 4G ² L ² Rb _(TE) Reflected light consisting of + GL ³ R ² a _{(TM) is} also output at the same time, but this reflected light uses the configuration of the present embodiment, so that compared to the case where it is simply excited bidirectionally as shown in FIG. Can be greatly reduced.

  Here, it is assumed that the intensity of the signal light Inputa is 1, the amplification gain of the PPLN waveguides 8 and 9 is 20 dB (G = 100 times), and the reflectance at the end faces of the PPLN waveguides 8 and 9 is −30 dB (R = 0.0). 001) When the linear loss in the PPLN waveguides 8 and 9 is considered as 4 dB (L = 0.4), the signal light Inputa is multiplied by 40, so that the intensity of the amplified signal light GLa becomes 40.

The intensity of the reflected light L ² Rb _(TE) that becomes a noise component is 0.00016, the intensity of the reflected light 1 / 4G ³ LR ² a _(TM) is 0.1, and the reflected light 1 / 4G ³ L ³ R ² The intensity of a _(TM) is 0.04, the intensity of reflected light 1 / 4G ² L ² Rb _(TE) is 0.004, and the intensity of reflected light GL ³ R ² a _(TM) is 0.0000064. Even if all of the two components are considered, only noise having an intensity of about 0.35% is generated with respect to the amplified main signal GLa. This is because, in this embodiment, it is possible to adopt a configuration in which the amplification process for each polarization component is performed independently. In FIG. 2, reflection is considered twice, and multiple reflections are actually present. However, unlike the conventional bidirectional excitation configuration, reflection is performed within a realistic gain range (around 20 dB). Since the intensity of the reflected light decreases each time, the influence of multiple reflection is extremely small.

Furthermore, the present embodiment is configured so that the influence of reflected light can be reduced using polarization dependency. As can be seen from Equation (6), among the noise components, L ² Rb _{(TE) in} the second term on the right side and ¼ G ² L ² Rb _{(TE) in} the fifth term on the right side are the amplified main signal GLa _(TM) and Have orthogonal polarizations. That is, the TM-polarized main signal is reflected by the PBS 1 and output, while the TE-polarized reflected light passes through the PBS 1 and does not enter the output port. Therefore, these TE-polarized reflected light is not superimposed as noise on the amplified signal light, and is filtered by the PBS 1. Thereby, L ² Rb _{(TE) in} the second term on the right side and ¼ G ² L ² Rb _{(TE) in} the fifth term on the right side, which are reflected light components, are removed. Very small.

  Even in the photodetector 14 for performing phase synchronization, if there is reflected light, it will be affected as noise, but here as well, with respect to components having different polarizations in the reflected light, a single polarization such as the polarizer 13 is also applied. If filtering is performed using an optical device that transmits a wave component and does not transmit a polarization component having a quadrature phase, the influence of reflected light can be extremely reduced.

  As described above, in the configuration shown in FIG. 1, two orthogonal polarization components can be amplified by the same path, the influence of phase fluctuation during separation and amplification can be suppressed, and the signal By setting the incident directions of light and fundamental light as in this embodiment, it is possible to realize parametric amplification of each polarization component while reducing the number of optical components, and to reduce the influence of reflected light. It becomes possible.

  First, linearly polarized CW (Continuous Wave) signal light having a polarization of 45 ° was input to the phase sensitive optical amplifier of the present embodiment shown in FIG. The intensity of the excitation light output from the second-order nonlinear optical element 8 and the intensity of the excitation light output from the second-order nonlinear optical element 9 are adjusted so that the same gain is obtained clockwise and counterclockwise in FIG. We tried phase sensitive amplification. As a result, a 45 ° linearly polarized signal amplified as an output was obtained.

  Next, after the signal light was modulated in a binary phase modulation (BPSK) format, polarization scrambling for randomly rotating the polarization was performed and input to the phase sensitive optical amplifier of the present embodiment. When the bit error rate of the input and output was measured, it was confirmed that the bit error rate for the output signal was the same as the bit error rate for the input signal and was amplified without a power penalty, and the phase sensitive optical amplifier of this embodiment Was able to amplify the input of any polarization, and was confirmed to be able to operate independently of the polarization.

  Next, a polarization multiplexed signal using a modulation signal for both two orthogonally polarized lights was input to the phase sensitive optical amplifier of the present embodiment. For the polarization multiplexed signal, light generated from the same light source was used for each polarized light. In this case, the light passing through the PBS 1 and the light reflected by the PBS 1 have a shape in which two polarization components are mixed. However, since multiple signals are generated from the same light source, phase synchronization with the pumping light is possible, and both polarization components can be amplified.

  In the present embodiment, an example is shown in which a phase-sensitive optical amplifier that requires phase synchronization is configured. However, the effect of the present invention is not lost even if phase-insensitive optical parametric amplification is used. By using the present invention, a phase insensitive optical parametric amplifier can be operated in a polarization independent state.

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

  DESCRIPTION OF SYMBOLS 1 ... Polarizing beam splitter, 2 ... Phase modulator, 3 ... Optical fiber expander, 4 ... Polarization controller, 5 ... EDFA, 6 ... Band pass filter, 7 ... Circulator, 8, 9 ... Secondary nonlinear optical element, 10 , 11 ... Polarization rotator, 12 ... Demultiplexer, 13 ... Polarizer, 14 ... Photo detector, 15 ... PLL circuit, 16 to 25 ... Polarization maintaining optical fiber, 30 ... Loop, 80, 83, 90, 93: Spatial optical system, 81, 84, 91, 94 ... Dichroic mirror, 82, 92 ... PPLN waveguide.

Claims (5)

A first optical path through which signal light propagates;
A second optical path through which the fundamental excitation light propagates;
A third optical path formed in a loop;
The signal light incident from the first optical path and the fundamental wave excitation light incident from the second optical path are each separated into two orthogonal polarization components and output, and the first round of the third optical path is output. The TM polarization component of the signal light incident from the direction and the TE polarization component of the signal light incident from the second rotation direction opposite to the first rotation direction of the third optical path are combined to generate the second light beam. A polarization demultiplexing element that outputs to the optical path;
The polarization direction of the TM polarization component of the signal light and the TE polarization component of the fundamental wave excitation light, which are separated by the polarization demultiplexing element and output in the first circulation direction of the third optical path, is rotated by 90 °, respectively. A first polarization rotator that rotates the polarization directions of the TM polarization component of the signal light and the TE polarization component of the fundamental wave excitation light incident from the second rotation direction of the third optical path by 90 °, respectively;
Second harmonic light of the TM-polarized component of the fundamental excitation light that has passed through the first polarization rotator and entered from the first circulation direction of the third optical path is generated, and the second harmonic light of the third optical path is generated. A first second-order nonlinear optical element including a first optical waveguide for parametrically amplifying the TM polarization component of the signal light incident from the two circulation directions;
The second harmonic light of the TM polarization component of the fundamental wave excitation light incident from the second circulation direction of the third optical path is generated and the TM polarization of the signal light incident from the first circulation direction of the third optical path is generated. A second second-order nonlinear optical element comprising a second optical waveguide for parametric amplification of the component;
The polarization direction of the TE polarization component of the signal light emitted from the first second-order nonlinear optical element and the TM polarization component of the fundamental wave excitation light are each rotated by 90 °, and the second secondary from the first circulation direction. While making it enter into the nonlinear optical element, the polarization direction of the TE polarization component of the signal light emitted from the second secondary nonlinear optical element and the TM polarization component of the fundamental wave excitation light are respectively rotated by 90 °, and the second circulation A second polarization rotator incident on the first second-order nonlinear optical element from a direction;
The second harmonic light generated from the first second-order nonlinear optical element is used as harmonic excitation light for parametric amplification in the second second-order nonlinear optical element from the first circumferential direction. A second harmonic light generated by the second second-order nonlinear optical element is made incident on the nonlinear optical element and is used as a harmonic excitation light for parametric amplification in the first second-order nonlinear optical element. And a fourth optical path incident on the first second-order nonlinear optical element from the direction. The optical amplification device according to claim 1,
Further, by controlling the phase of the fundamental pumping light propagating in the second optical path, the phase synchronization for synchronizing the signal light and the phase of the harmonic pumping light in the first and second optical waveguides. An optical amplification device comprising means. The optical amplification device according to claim 2,
The phase synchronization means includes
Phase adjusting means for adjusting the phase of the fundamental wave excitation light propagating in the second optical path;
Detecting means for taking out a part of the signal light propagating in the third optical path and converting it into an electrical signal;
And a control means for generating a control signal to the phase adjustment means so that the signal light and the phase of the harmonic excitation light are synchronized based on an output signal of the detection means. apparatus. In the optical amplification device according to any one of claims 1 to 3,
Further, the fundamental excitation light in the direction opposite to the TM polarization component of the signal light and the TE polarization component of the signal light emitted from the polarization demultiplexing element to the second optical path is added to the second optical path. An optical amplifying apparatus comprising: a circulator that makes fundamental wave excitation light incident on the polarization splitting multiplexer. In the optical amplification device according to any one of claims 1 to 4,
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.

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