JP6774381B2 - Optical transmitter and optical transmission system using it - Google Patents

Description

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

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

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

FIG. 3 is a diagram showing a basic configuration of a PSA of the prior art. The PSA 100 includes a phase-sensitive optical amplification unit 101 using optical parametric amplification, an excitation light source 102, an excitation optical phase control unit 103, and a first optical branching unit 104-1 and a second optical branching unit 104-2. As shown in FIG. 3, the signal light 110 input to the PSA 100 is bifurcated by the optical branching unit 104-1, one incident on the phase-sensitive optical amplification unit 101 and the other incident on the excitation light source 102. .. The excitation light 111 emitted from the excitation light source 102 is phase-adjusted via the excitation light phase control unit 103, and is incident on the phase-sensitive light amplification unit 101. The phase sensitive light amplification unit 101 outputs the output signal light 112 based on the input signal light 110 and the excitation light 111.

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

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

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

FIG. 4 is a diagram showing a configuration of a PSA of the prior art using a PPLN waveguide (see Non-Patent Document 1). The PSA200 shown in FIG. 4 includes an erbium-added fiber optic laser amplifier (EDFA) 201, a first second-order nonlinear optical element 202, a second second-order nonlinear optical element 204, a first optical branching portion 203-1 and A second optical branching portion 203-2, a phase modulator 205, an optical fiber extender 206, a polarization holding fiber 207, an optical detector 208, and a phase synchronization loop (PLL) circuit 209 are provided. The first second-order nonlinear optical element 202 includes a first spatial optical system 211, a first PPLN waveguide 212, a second spatial optical system 213, and a first dichroic mirror 214. The second second-order nonlinear optical element 204 also has a similar configuration, and details thereof will be omitted.

The signal light 250 incident on the PSA 200 in FIG. 4 is branched by the optical branching portion 203-1 and one is incident on the second second-order nonlinear optical element 204. The other of the branched lights is phase-controlled and incident on the EDFA 201 via the phase modulator 205 and the optical fiber extender 206 as the excitation fundamental wave light 251. The EDFA 201 sufficiently amplifies the incident excitation fundamental wave light 251 and incidents on the first second-order nonlinear optical element 202. With the EDFA201, sufficient power can be obtained from the weak excited fundamental wave light 251 to obtain a nonlinear optical effect. In the first second-order nonlinear optical element 202, a second harmonic (hereinafter referred to as SH light) 252 is generated from the incident excitation fundamental wave light 251. The generated SH light 252 is incident on the second second-order nonlinear optical element 204 via the polarization-holding fiber 207. In the second second-order nonlinear optical element 204, phase-sensitive optical amplification is performed by performing degenerate parametric amplification with the incident signal light 250 and SH light 252, and the output signal light 253 is output.

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

FIG. 5 is a diagram showing the relationship between the phase difference Δφ between the input signal light and the excitation light and the gain in the PSA using the second-order nonlinear optical effect of the prior art. It can be seen that the gain (dB) on the vertical axis is maximum when the phase difference Δφ on the horizontal axis is −π, 0, or π.

For phase synchronization between the signal light 250 and the excitation fundamental wave light 251 first, the phase modulator 205 applies phase modulation to the excitation fundamental wave light 251 with a weak pilot signal, and a part of the output signal light 253 is branched. Detected by detector 208. This pilot signal component becomes the minimum when the phase difference Δφ shown in FIG. 5 is the minimum and the phase is synchronized. Therefore, feedback is performed to the optical fiber extender 206 by using the PLL circuit 209 so that the pilot signal is the minimum, that is, the amplified output signal is the maximum. The phase of the excitation fundamental wave light 251 can be controlled by the optical fiber extender 206 to achieve phase synchronization between the signal light 250 and the excitation fundamental wave light 251.

In response to the demand for higher speed and larger capacity of optical communication, the range of application of PSA is expanding in terms of the corresponding modulation method and multiplexing method as described below. As shown in FIG. 5, since the conventional PSA using the PPLN waveguide has a characteristic of attenuating orthogonal phase components, intensity modulation / direct using ordinary intensity modulation or binary phase modulation is used. Although it can be applied to the amplification of modulated signals such as detection (IMDD: Intensity Modulation-Direct Detection), binary phase shift keying (BPSK), and differential phase shift keying (DPSK), it is also applicable. Modulation signals such as QPSK (Quadrature Phase Shift Keying) and 8PSK, which are multi-valued modulation formats, cannot be amplified.

Non-Patent Document 2 and Non-Patent Document 3 disclose a configuration capable of phase-sensitive photoamplification of a modulated signal such as QPSK and phase reproduction amplification. Non-Patent Document 2 discloses a method using quartz glass fiber which is a third-order nonlinear optical material, and Non-Patent Document 3 discloses a method using PPLN which is a second-order nonlinear optical material.

In the multi-value modulation format, in order to extract carriers from the signal light, it is necessary to use more non-linear processes as the multi-value degree of the modulated signal increases. In that case, it is difficult to maintain the S / N of the fundamental wave light generated by carrier extraction from the signal light. Further, in the configuration of the carrier extraction method using a non-linear process a plurality of times, it is not possible to collectively amplify a WDM (Wavelength Division Multiplexing) signal in which a plurality of signals are wavelength-multiplexed.

Non-Patent Document 4 uses a pair consisting of a main signal light and its phase-conjugated light as signal light, and uses non-reduced parametric amplification to perform orthogonal amplitude modulation (QAM: Quadrature Amplitude Modulation) modulation signal or multi-level multi-value modulation format. A phase sensitive optical amplifier for a carrier signal is disclosed.

FIG. 6 is a diagram showing a configuration of PSA by non-degenerate parametric amplification of the prior art using a pair consisting of a main signal light and its phase-conjugated light as signal light. The PSA300 shown in FIG. 6 includes an optical transmitter 301 and a phase sensitive optical amplifier 302. In the optical transmitter 301, after data modulation is performed on the output light from the plurality of signal light sources 303 by using the external modulator 304, wavelength division multiplexing is performed by the combiner 305. Second harmonic (SH: Second Harmonic) light is generated by the second-order nonlinear optical element 311 from the fundamental wave light output from the local oscillation light source 307, which is different from the signal light source 303, and becomes the excitation light 312. After that, the second-order nonlinear optical element 306 generates a difference frequency light between the excitation light 312 and the wavelength-multiplexed main signal light 313, and this difference frequency light is used as phase-conjugated light with respect to the main signal light. Phase-conjugated light is also called idler light.

From the optical transmitter 301, the signal light group 316 including a plurality of pairs consisting of the multiplexed main signal light 313 and its phase-conjugated light and a part 314 of the fundamental wave light from the local oscillator 307 are separately phase-sensitive light. Output to the amplifier 302. In the phase-sensitive optical amplifier 302, the second-order nonlinear optical element 323 generates excitation light 329, which is SH light, from the fundamental wave light 314. Further, in the second-order nonlinear optical element 324, phase-sensitive light amplification is performed by using a parametric amplification process between the excitation light 329 and the signal light group 316.

FIG. 7 is a diagram schematically explaining the relationship of each part on the wavelength axis in PSA by the non-degenerate parametric amplification of the prior art of FIG. In each figure, the horizontal axis shows the wavelength and the vertical axis shows the light intensity. As shown in FIG. 7A, the optical transmitter 301 outputs a signal light group 315 composed of a plurality of pairs of the main signal light 313 and its phase-conjugated light 315, and the fundamental wave light 314. The main signal light and the phase symmetry light, which are symmetrical with respect to the basic excitation light 314, form a pair. It should be noted that the paired signal light and the phase-conjugated light have a symmetric positional relationship on the frequency axis in a strict sense. FIG. 7B shows a process (SHG: Second Harmonic Generation) of generating excitation light 329 which is SH light from fundamental wave light 314 by the second nonlinear optical element 323 of the phase sensitive optical amplifier 302. FIG. 7 (c) shows the amplified signal light group 330 obtained in the parametric amplification process (OPA: Optical Parametric Amplification) between the excitation light 329 and the signal light group 316 in the second-order nonlinear optical element 324. There is. FIG. 6 shows a configuration example using the fundamental wave light 314 branched from the optical transmitter 301, but in order to use it as a relay amplifier after long-distance fiber transmission, the carrier phase is extracted from the signal light and synchronized with the carrier phase. It is necessary to generate the locally oscillated light. In that case, as shown in Non-Patent Document 6, there is a method of transmitting the fundamental wave light as pilot light. Further, the output light from the locally oscillating light source synchronized with the carrier phase of the signal light may be used as the fundamental wave by using the optical phase synchronization (optical PLL) as shown in Non-Patent Document 7.

In recent digital coherent optical communication systems, polarized multiplex signals based on polarization division multiplexing (PDM) technology are used. Since a nonlinear optical medium such as a PPLN waveguide generally has polarization dependence, PSA of the prior art cannot amplify a polarization multiplex signal. On the other hand, Non-Patent Document 5 and Non-Patent Document 6 disclose a configuration in which phase-sensitive optical amplification is performed with respect to a polarized multiplex signal by a polarization diversity configuration using two nonlinear optical media.

FIG. 8 is a diagram showing an outline of an optical transmission system for transmitting polarized multiplex signal light of the prior art. The optical transmission system 350 includes an optical transmitter 351, an optical fiber transmission line 361, and an optical amplifier 352 having a polarization diversity configuration. The polarization diversity configuration disclosed in Non-Patent Document 5 and Non-Patent Document 6 is the same configuration as the optical amplification device 352 of the optical transmission system of FIG. In the polarization diversity configuration 352, the polarization multiplex signal transmitted through the optical fiber 361 is split by the polarization beam splitter (PBS) 362. The two separated polarized components are optically amplified by the phase-sensitive optical amplifiers 363 and 364, which are each composed of non-linear optical elements, and then re-waved by PBS 365. As will be described later, the PDM signal, which is a transmission signal of an optical transmission system that transmits polarized multiple signal light, is independently modulated based on different different data between the orthogonal polarized waves. The PDM signal can be generated by combining the optical modulation signals corresponding to the respective data with PBS.

However, the conventional optical transmission system for transmitting polarized multiple signal light has a problem due to the instability of the polarized state in the optical transmitter as described below. In order to use PSA by non-degenerate parametric amplification using a pair consisting of the main signal light and its phase-conjugated light, it is necessary to satisfy the phase synchronization condition for each of the polarization components orthogonal to all the pairs. .. In the method of generating a polarization-multiplexed signal in the optical transmitter 351 of the conventional optical transmission system as shown in FIG. 8, there is a problem that the amplification operation becomes unstable in the phase-sensitive optical amplifier 352 having the polarization diversity configuration.

Looking back at the configuration of the optical transmitter with reference to FIG. 8 again, the optical transmitter 31 is composed of two signal arms (roots) that generate two pairs of main signal light and phase-conjugated light. The carrier light from the signal light source 353 is branched by an optical coupler 354 or the like, and the carrier light is modulated by different data signals in the two light modulators 355 and 356 to generate two main signal lights. The two differential frequency generators 357 and 358 then generate their respective phase-conjugated light. The polarization of the pair of main signal light-phase-conjugated light from one of the differential frequency generators 358 is rotated by 90 ° by the polarization rotor 359, and a pair of orthogonal main signal light-phase-conjugated light 366 is obtained. Two pairs of orthogonally polarized waves are combined by PBS 360 and a PDM signal 367 is output. The polarization of the PDM signal 367 from the optical transmitter 351 rotates in the optical fiber 361 which is a transmission line. When the PDM signal 368 that has undergone polarization rotation is input to the polarization diversity configuration 352, it is branched into a photoelectric field component projected on the two polarization axes of PBS 362 in the first stage. That is, the original two orthogonal polarization components are mixed and incident on the nonlinear optical elements 363 and 364.

FIG. 9 is a diagram conceptually explaining the state of polarization in the optical transmission system of PDM light of the prior art. FIG. 9A shows two orthogonal polarizations of the PCM signal 367 at point A of the output of the optical transmitter 351. If the horizontal axis is the s-polarized (polarized) state, the vertical axis is the p-polarized (polarized) state. FIG. 9B shows the polarization state of the PDM signal 368 at point B after transmission of the optical fiber 361, and the entire optical fiber with respect to the PDM signal 367 at the output point A of the optical transmitter 351. Cumulative polarization rotation is added. FIG. 9C shows the electric field components of the respective polarization axes at points C1 and C2 after polarization separation by PBS362 in the first stage. Each polarization component of the PDM signal at point B is separated in a state where the components projected onto the two axes of PBS 362 are mixed.

In the phase-sensitive optical amplifiers 363 and 364 that perform independent phase-sensitive optical amplification, a part of the output is monitored by the phase-locked configuration as shown in FIG. 4, and the excitation light and the signal light are maximized so that the output is maximized. Give feedback to adjust the relative phase of. In the polarization diversity configuration 352, as shown in FIG. 9C, two orthogonal polarization components are incident in a mixed state. The PDM signal after optical amplification cannot be stabilized unless the phase-sensitive optical amplification conditions are stably satisfied at the same time in each phase-sensitive optical amplifier for the two orthogonal polarization components.

The state itself in which the two polarization components are mixed as shown in FIG. 9C does not cause a problem in the polarization diversity configuration 352. However, if the polarization state of the PDM signal received at point B fluctuates, the output of the phase-sensitive light increaser also becomes unstable. Here, the fluctuation of the polarization state that occurs on the optical transmission path is not targeted. The problem is the fluctuation of the polarization phase of the two orthogonal polarization components when the PDM signal is generated in the optical transmitter 351.

The optical transmitter 351 is usually composed of a light source, an optical semiconductor element, an optical component, and a polarization-holding fiber that connects them to each other. Further, a module configuration including a control circuit for controlling these element components, an interface circuit inside and outside the device, a power supply, and the like is adopted. In such a device, the internal temperature is unevenly distributed due to changes in the operating state of the device and the environmental temperature, and the distribution may change from moment to moment. In addition, depending on the environment and operating conditions outside the device, external vibration may be applied to the connecting optical fiber and optical components between the components inside the device. As described in FIG. 8, in the optical transmitter, a signal light group is generated by two signal arms by separate differential frequency generators (secondary nonlinear optical elements) 357 and 378, and polarization multiplexing is performed by PBS. , These two signal arms can be independently affected by temperature changes, vibrations, and the like. Therefore, the phases of the signal light, the phase-conjugated light and the pilot light can also vary in different ways depending on the two signal arms. Furthermore, the phases of the excitation light supplied to the two differential frequency generators can also vary in different ways. Since the phase condition fluctuates separately between the two arms that generate the polarization multiplex light, the relative phase between the two polarizations also fluctuates from moment to moment, periodically or irregularly, and fluctuates. In this way, if the PDM signal has a relative phase fluctuation between the two signal light arms in the optical transmitter, it directly affects the phase-sensitive light amplification operation in the polarization diversity configuration after transmitting the optical fiber 361. It will have an impact.

Referring to (c) of FIG. 9, in the polarization diversity configuration, the two polarization components are separated by PBS in a mixed form, and the two mixed polarization components are input to the respective phase-sensitive optical amplifiers. .. Therefore, if the relative phases of the signal light groups differ between the two polarizations in the optical transmitter in which the PDM signal is formed, the phase-sensitive light represented by FIG. 5 and the equation (1) in one phase-sensitive optical amplifier. The phase condition for amplification cannot be met. After the polarization multiplexing, the phase change received by the optical fiber undergoes the same change between the two polarizations, so the relative phase change is small. On the other hand, the fluctuation / fluctuation of the relative phase between the two polarizations generated in the optical transmitter is more direct and affects in real time between the optical transmitter 351 and the polarization diversity configuration 352. As described above, there is a problem that the fluctuation of the quality of the PDM signal output from the optical transmitter itself destabilizes the output from the phase-sensitive light increasers 363 and 364 of the polarization diversity configuration 352. With the PCM signal optical transmitter 351 as shown in FIG. 8, it was not possible to stably generate a pair of polarization-multiplexed main signal-phase-conjugated light with the configuration of the prior art.

The present invention has been made in view of such a problem, and an object of the present invention is to provide an optical transmitter that generates a PDM signal in which phase fluctuations between orthogonal polarizations are suppressed.

In order to achieve such an object, the invention according to claim 1 is polarized and multiplexed with a signal light source that outputs main signal light and a polarization multiplex modulator that modulates the main signal light. A first-order non-linear optical element that generates phase-coupled light for one polarization component of the modulated main signal light, and a polarization that rotates the output light from the first second-order nonlinear optical element. Generates phase-coupled light for the rotator and the other polarization component of the polarization-multiplexed and modulated main signal light input from the polarization rotator, and a bias containing a pair of main signal light-phase-conjugated light. Based on the second-order nonlinear optical element that outputs a wave multiplex light group, the locally oscillating light source that outputs the fundamental wave light, and the fundamental wave light, the first second-order nonlinear optical element generates phase-coupled light. One or more secondary polarized optical elements that generate a second excitation light for generating phase-coupled light with the first excitation light for and the second secondary nonlinear optical element, and one of the fundamental wave lights. This unit synchronizes the phase of the pilot signal, which is input together with the polarization-multiplexed and modulated main signal light and passes through the first secondary nonlinear optical element, with the phase of the first excitation light. The optical transmitter is provided with a phase adjusting mechanism for synchronizing the phase of the pilot signal that further passes through the second-order nonlinear optical element with the phase of the second excitation light. ..

The one or more second-order nonlinear optical elements described above are composed of a third second-order nonlinear optical element that generates a first excitation light and a fourth second-order nonlinear optical element that generates a second excitation light. (Corresponding to the first embodiment). Further, the one or more second-order nonlinear optical elements described above branch the excitation light from a single second-order nonlinear optical element, and the first and first excitation lights are used as the first excitation light and the second excitation light. It may be supplied to each of the second-order nonlinear optical elements of No. 2 (corresponding to the modification of the first embodiment).

The invention according to claim 2 is the optical transmitter according to claim 1, wherein the phase adjusting mechanism detects the level of the pilot signal on the output side of the first second-order nonlinear optical element. The level of the first feedback circuit that adjusts the phases of the main signal light and the pilot signal on the input side of the first second-order nonlinear optical element and the pilot signal on the output side of the second second-order nonlinear optical element. Is included, and the main signal light on the input side of the second second-order nonlinear optical element , the phase-conjugated light, and the second feedback circuit for adjusting the phase of the pilot signal are included.

The invention according to claim 3 is the optical transmitter according to claim 1 or 2, wherein the first second-order nonlinear optical element, the second second-order nonlinear optical element, and one or more second-order nonlinear optical elements. Each of the optical elements includes a direct junction ridge waveguide, and the direct junction ridge waveguide includes LiNbO ₃ , KNbO ₃ , LiTaO ₃ , LiNb _(x) Ta _(1-x) O ₃ (0 ≦ x ≦ 1), Alternatively, it is composed of any material of KTIOPO ₄ , or is composed of a material obtained by adding at least one selected from the group consisting of Mg, Zn, Sc, and In to any of the materials as an additive. It is characterized by.

The invention according to claim 4 generates a signal light source that outputs main signal light, a polarization multiplex modulator that modulates the main signal light, and phase-coupled light of the main signal light that is polarized and modulated. The first second non-linear optical element that outputs a group of polarized polarized light, and the first two that generates phase-coupled light with respect to one polarized component of the polarized light that is polarized and modulated. A polarizer that polarizes and rotates the second-order nonlinear waveguide, the main signal light from the first second-order nonlinear waveguide, and the phase-conjugated light with wavelength dependence, and a bias input from the polarizer. A second quadratic non-linearity that generates phase-coupled light for the other polarization component of the wave-multiplexed and modulated main signal light and outputs the polarization-multiplexed light group containing a pair of main signal light-phase-coupled light. A first secondary polarized optical element including a waveguide, a locally oscillating light source for outputting fundamental wave light, and the first secondary nonlinear optical element for generating phase-coupled light based on the fundamental wave light. The optical transmitter is provided with a second-order polarized light-emitting element that generates excitation light (corresponding to the second embodiment).

The invention according to claim 5 is the optical transmitter according to claim 4, wherein the first second-order nonlinear optical element is the first second-order nonlinear waveguide using a direct junction ridge waveguide and the above. It is characterized in that the second-order nonlinear waveguide and the polarizer have a configuration packaged in a single module.

The invention according to claim 6 is the optical transmitter according to claim 5, wherein the direct junction ridge waveguide is LiNbO ₃ , KNbO ₃ , LiTaO ₃ , LiNb _(x) Ta _(1-x) O ₃ ( It is composed of either 0 ≦ x ≦ 1) or KTIOPO ₄ , or at least one selected from the group consisting of Mg, Zn, Sc, and In is added as an additive to any of the materials. It is characterized by being composed of various materials.

The invention according to claim 7 is the optical transmitter according to any one of claims 1 to 6, wherein a plurality of the signal light sources for outputting light having different wavelengths, a plurality of corresponding polarization multiplex modulators, and a plurality of corresponding polarization multiplex modulators. A wavelength combiner that combines a plurality of the main signal lights polarized and modulated and outputs the wavelength-multiplexed light to the first secondary nonlinear optical element is provided, and the main signal lights having different wavelengths are provided. It is characterized in that phase conjugate light is generated for each.

The invention according to claim 8 comprises any of the above-mentioned optical transmitters and a locally oscillating light source that outputs fundamental wave light synchronized with the carrier phase of the polarized light multiplex light group from the optical transmitter. A polarization diversity configuration in which a wave multiplex light group is separated into two polarization components, the two polarization components are photoamplified by a nonlinear optical element, and the lightly amplified two polarization components are recombined. It is an optical transmission system characterized by having a phase-sensitive optical amplifier.

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to provide an optical transmitter that suppresses fluctuations in relative phase fluctuations between orthogonal polarizations.

FIG. 1 is a diagram showing a configuration of an optical transmitter according to a first embodiment of the present invention. FIG. 2 is a diagram showing a configuration of an optical transmitter according to a second embodiment of the present invention. FIG. 3 is a diagram showing a basic configuration of a PSA of the prior art. FIG. 4 is a diagram showing a configuration of a prior art PSA using a PPLN waveguide. FIG. 5 is a diagram showing the relationship between the phase difference between the input signal photoexcited light of PSA and the gain. FIG. 6 is a diagram showing a non-degenerate PSA configuration of the prior art using a signal light pair. FIG. 7 is a diagram showing the relationship of each part of the non-degenerate PSA on the wavelength axis of light. FIG. 8 is a diagram showing a prior art optical transmission system for polarized multiplex signals. FIG. 9 is a diagram illustrating a polarization state of each part of the PDM optical transmission system. FIG. 10 is a diagram showing a configuration of an optical transmitter according to a modification of the first embodiment.

In the optical transmitter of the present invention, a configuration is presented that stabilizes the drift fluctuation of the phase difference between orthogonal polarizations in each signal such as the main signal light, its phase-conjugated light, the excitation light, and the pilot signal. The polarization multiplex light group is generated by a polarization multiplex modulator and two subordinately connected second-order nonlinear optical elements that separately generate phase-conjugated light of the principal signal light for two orthogonal polarizations. Will be done. The phase adjustment mechanism stabilizes the relative phase drift of the principal signal light or the pilot signal between the two orthogonal polarizations in each of the second-order nonlinear optical elements that generate the phase-conjugated light of the two orthogonal polarizations. Also disclosed is an optical transmitter that includes a modular secondary nonlinear optical element that includes two secondary nonlinear waveguides and does not require a phase adjustment mechanism to adjust the relative phase between the two polarizations. The optical transmitter of the present invention also provides an optical transmission system in combination with a phase-sensitive optical amplifier having a relay-type polarization diversity configuration. Hereinafter, detailed configurations and operations of the optical transmitter of the present invention will be described together with a plurality of embodiments.

[Embodiment 1]
FIG. 1 is a diagram showing a configuration of an optical transmitter according to a first embodiment of the present invention. The optical transmitter 400 of the present embodiment shows a configuration of an optical transmitter for generating a polarization-multiplexed signal light group composed of a pair of signal light and its phase-conjugated light. Here, one main signal light and its phase-conjugated light are referred to as a pair. Modulation with one data signal for the first polarization gives a first pair of modulated main signal light and phase-conjugated light. Further, for the second polarization orthogonal to the first polarization, modulation by another data signal gives a second pair of modulated main signal light and phase-conjugated light. As will be described later, in the optical transmitter of the present invention, polarized multiplex light is first generated, and then phase-conjugated light is generated. More specifically, for one signal optical carrier, first, the main signal light polarized and multiplexed by two data signals is obtained. A plurality of polarization-multiplexed main signal lights from signal light sources having different wavelengths are wavelength-multiplexed by an optical combiner to obtain polarization-multiplexed and wavelength-multiplexed main signal light 445. Phase-conjugated light is generated for each polarization component in the two second-order nonlinear optical elements 404 and 406 with respect to the main signal light from the photosynthetic device. A polarization-multiplexed signal light group is obtained from a plurality of pairs of polarization-multiplexed and wavelength-multiplexed main signal lights and corresponding phase-conjugated lights. Pilot light having a wavelength located at the center of the wavelength of the main signal light and the wavelength of the phase-conjugated light is superimposed and transmitted at the same time as the polarization multiplex signal light group.

The optical transmitter 400 includes a plurality of signal light sources 401 that output signal lights of different wavelengths, a plurality of polarization multiplex modulators 402, a wavelength combiner 403, and a difference frequency generation (DFG). It includes a first second-order nonlinear optical element 404, a polarization rotor 405, and a second second-order nonlinear optical element 406 for DFG. The optical transmitter 400 further includes a local oscillation light source 407 that outputs a fundamental wave, an optical coupler 408 that branches the fundamental wave light, a polarizer 409 that rotates the polarization of the fundamental wave, an EDFA 409, 412, and a bandpass filter. (Band Pass Filter: BPF) 409, 413, secondary non-linear optical element 411, 414 for SH light generation (SHG), and optical fiber expansion and contraction using PZT (lead titanate) piezoelectric element It includes an instrument 418, 422, an optical coupler 416, 417, an optical detector 419, 423, and a PLL circuit 420, 424. Of these, the loop including the optical fiber stretcher 421, the optical coupler 416, the photodetector 419, and the PLL circuit 420 constitutes the first phase adjustment mechanism. Similarly, a loop including an optical fiber stretcher 425, an optical coupler 417, a photodetector 423, and a PLL circuit 424 constitutes a second phase adjustment mechanism.

In the transmitter 400, the continuous wave light (CW light) output from the local oscillation light source 407 is demultiplexed into three lights by the optical coupler 408, and the pilot light 442, the first fundamental wave light 440, and the second fundamental wave light, respectively. Used as 441. The EDFA409, 412 amplifies the first fundamental wave light 440 and the second fundamental wave light 441, respectively. The BPF 410 and 413 eliminate the noise light generated by the EDFA 409 and 412 and allow only the amplified fundamental wave light to pass through.

The second-order nonlinear optical elements 411 and 414 for SHG include a PPLN waveguide and a dichroic mirror on the output side, respectively. When the fundamental wave light transmitted through the BPF 410 is incident on the second-order nonlinear optical element 411, SH light having half the wavelength (twice the frequency) of the fundamental wave light is generated by the PPLN waveguide. The dichroic mirror separates the fundamental wave light and the SH light. Similarly, when the fundamental wave light transmitted through the BPF 413 is incident on the second-order nonlinear optical element 414, SH light having half the wavelength of the fundamental wave light is generated by the PPLN waveguide, and the fundamental wave light and the SH light are separated by the dichroic mirror. To. The output SH light is incident on the secondary nonlinear optical elements 404 and 406 for DFG as the first excitation light 443 and the second excitation light 444, respectively.

The plurality of signal light sources 401 output continuous wave light (CW light) having different wavelengths and different wavelengths from the fundamental wave light of the local oscillation light source 407. The polarization multiplexing modulator 402 performs polarization multiplexing 16QAM modulation according to independent transmission data 451 and 452 for each output light from the signal light source. The polarization-multiplexed modulated light from each of the polarization multiplexing modulator 402 is wavelength-multiplexed by the wavelength combiner 403. The polarization-multiplexed and wavelength-multiplexed main signal light is input to the first second-order nonlinear optical element 404 for DFG after the pilot light 442 is combined by the optical coupler 415. Here, the polarization direction of the pilot light 442 is tuned by the optical coupler 415 after the polarization state is set between the two orthogonal polarization components of the polarization multiplex signal. In the present embodiment, when the two orthogonal polarization components are 0 ° and 90 °, the polarization of the pilot light 442 is adjusted so as to be polarized at 45 ° by using the polarizer 409.

The first second-order nonlinear optical element 404 for DFG includes a dichroic mirror on the input side, a PPLN waveguide, and a dichroic mirror on the output side. The multiplexed main signal light and pilot light output from the optical coupler 415 and the excitation light 443 output from the second-order nonlinear optical element 411 are combined by a dichroic mirror and incident on the PPLN waveguide. In the first second-order nonlinear optical element 404, the difference frequency light between the main signal light and the excitation light 443 is generated for one of the polarization components of the main signal light. This difference frequency light becomes phase-conjugated light with respect to the main signal light. Assuming that the two orthogonal polarization components of the polarization-multiplexed main signal light are the X polarization component and the Y polarization component, respectively, in the first second-order nonlinear optical element 404 for DFG, the X of the main signal light is X. Phase-conjugated light of the polarization component is generated. On the other hand, the Y polarization component of the polarization-multiplexed main signal light passes through the second-order nonlinear optical element 404 as it is.

The output of the first second-order nonlinear optical element 404 for DFG is the second-order nonlinear optical element 406 for DFG after the polarization is rotated by the polarization rotor 405 by the λ / 2 plate. Entered. In this embodiment, a λ / 2 plate is used for polarization rotation, but a polarization rotation method such as connecting the optical fiber to the second-order nonlinear optical element 406 in a state of being twisted by 90 degrees may also be used. In this way, in the second second-order nonlinear optical element 406 for DFG, phase-conjugated light of the Y polarization component of the polarization-multiplexed main signal light is generated. On the other hand, the X polarization component passes through the second second-order nonlinear optical element 406 for DFG as it is. From the second second-order nonlinear optical element 406 for DFG, as the PDM signal output of the optical transmitter 400, a bias including a plurality of pairs consisting of polarized and wavelength-multiplexed main signal light and its phase-conjugated light. The wave multiplex signal light group 448 is output. The excitation lights 443 and 444 that have passed through the secondary nonlinear optical elements 404 and 406 for DFG are separated (449, 450) by the die clock mirrors on the respective output sides.

After the outputs of the second-order nonlinear optical elements 404 and 406 for DFG, optical couplers 416 and 417 that branch a part of the outputs are provided, respectively. The branched light from the optical coupler 416 passes through the BPF 418, and the photodetector 419 detects a change in the light intensity of the pilot light. Similarly, the branched light from the optical coupler 417 passes through the BPF 422, and the photodetector 423 detects a change in the light intensity of the pilot light. The BPF 418 and 422 separate only the pilot signal 446 combined with the main signal light 445. By feeding back to the fiber stretcher 421 by the PLL circuit 420 and feeding back to the fiber stretcher 425 by the PLL circuit 424, the output of the pilot light is maximized at each output point of the secondary nonlinear optical elements 404 and 406. Adjust the phase so that.

By adjusting the phase of the optical path with the fiber expanders 421 and 425 as described above, the phase of the main signal light, the phase-conjugated light and the pilot signal and the phase of the excitation light are adjusted in the second-order nonlinear optical element. Please note that Therefore, the loop including the PLL circuit 420 synchronizes the phase of the pilot signal transmitted through the first second-order nonlinear optical element 404 with the phase of the first excitation light. The loop including the PLL circuit 424 synchronizes the phase of the pilot signal further transmitted through the second second-order nonlinear optical element 406 with the phase of the second excitation light. These two loops function as a phase adjustment mechanism.

In the optical transmitter of the present embodiment, in a configuration in which a second-order nonlinear optical element that generates phase-conjugated light of only one polarization of two orthogonal polarizations is connected in sequence to generate a polarization multiplex signal light group. A feedback loop is formed between the input and output of each second-order nonlinear optical element. By adjusting the phase of the optical path on the input side so as to maximize the level of the pilot light in each of the two feedback loops, the relative phase drift of the signal light, phase-conjugated light, and excitation light between the two polarizations. Fluctuations can be suppressed.

As described above, feedback is provided to the fiber optic stretcher to compensate for the relative phase drift of the signal light group between the two orthogonal polarizations in the transmitter due to temperature and vibration, and thus between the two orthogonal polarizations. The polarization multiplex signal can be transmitted with the relative phase of the above stabilized. Also in the repeater, the polarization-diversified phase-sensitive optical amplifier can be stably operated.

FIG. 10 is a diagram showing a configuration of an optical transmitter according to a modification of the first embodiment. The configuration of the optical transmitter 600 is the same as that of the optical transmitter 400 of FIG. 1 except for the method of supplying the excitation light to the secondary nonlinear optical elements 404 and 406 for DFG. If the excitation light of sufficient light intensity can be supplied to the first second-order nonlinear optical element 404 and the second second-order nonlinear optical element 406 for DFG, the second-order nonlinear optical elements 411 and 414 for SHG in FIG. 1 can be used. It can also be a single second-order nonlinear optical element. As shown in FIG. 10, in the modified example optical transmitter 600, the excitation light is output from a single second-order nonlinear optical element 411 for SHG, and the excitation light is branched by the optical coupler 602 to be the first. The excitation light 443 and the second excitation light 444 are supplied to the secondary nonlinear optical elements 404 and 406 for DFG, respectively.

Therefore, the optical transmitter of the present invention has a bias of one of a signal light source 401 that outputs the main signal light, a polarization multiplex modulator 402 that modulates the main signal light, and the polarization-multiplexed and modulated main signal light. A first quadratic nonlinear optical element 404 that generates phase-coupled light for a wave component and a polarization rotator 405 that polarizes and rotates the output light from the first quadratic nonlinear optical element are polarized and modulated. A second quadratic nonlinear optical element 406 that generates phase-coupled light for the other polarization component of the main signal light and outputs a polarization-multiplexed light group including a pair of main signal light-phase-coupled light. A local oscillation light source 407 that outputs fundamental wave light, a first excitation light for generating phase-coupled light by the first second-order nonlinear optical element based on the fundamental wave light, and the second second-order nonlinear optical. One or more second-order nonlinear optical elements 411, 414 that generate a second excitation light for generating phase-conjugated light in the element, and the first second-order nonlinear optical element that is a part of the fundamental wave light. The phase of the pilot signal that transmits the above and the phase of the pilot signal that synchronizes the first excitation light phase and further transmits through the second second-order polarized light optical element, and the phase of the second excitation light. It can be carried out as if it is provided with a phase adjusting mechanism for synchronizing.

In the optical transmitter of the present embodiment, a PPLN waveguide capable of pseudo-phase matching is used as the nonlinear optical medium in each second-order nonlinear optical element. First, a periodic electrode having a period of about 17 μm was formed on LiNbO ₃ to which Zn was added. Next, a polarization reversal grating corresponding to the above electrode pattern was formed in Zn: LiNbO ₃ by the electric field application method. Next, the Zn: LiNbO ₃ substrate having this periodic polarization inversion structure was directly bonded onto the clad LiTaO ₃ , and the two substrates were firmly bonded by heat treatment at 500 ° C.

Next, the core layer was thinned to about 5 μm by polishing, and a ridge type optical waveguide was formed by using a dry etching process. The temperature of this waveguide can be adjusted by a Peltier element. The length of the waveguide was set to 50 mm. The second-order nonlinear optical element having the PPLN waveguide formed in this way has a modular configuration capable of inputting and outputting light with a polarization-holding fiber in the 1.55 μm band. In this embodiment, LiNbO ₃ to which Zn is added is used, but other non-linear materials such as KNbO ₃ , LiTaO ₃ , LiNb _x Ta _1-x O ₃ (0 ≦ x ≦ 1) or KTIOPO ₄ or A material containing at least one selected from the group consisting of Mg, Zn, Sc, and In as an additive may be used.

As described above, in the optical transmitter shown in the present embodiment, the output from the locally oscillating light source in the optical transmitter is transmitted together with the main signal light as pilot light, and the main signal light and its phase-conjugated light are polarized and multiplexed. It is possible to transmit the polarized light multiplex light group. Since the relative phase between the two polarizations is stabilized in the optical transmitter, the phase-sensitive optical amplifier having the polarization diversity configuration can be stably operated even in the repeater.

In the optical transmitter of the present embodiment, an example in which 16QAM is used as the signal modulation format has been described. However, excitation light can be generated with exactly the same configuration as the optical transmitter of the present embodiment for any format such as IMDD, BPSK, QPSK, 64QAM, 256QAM, and QAM with a higher multi-level degree. According to the optical transmitter of the present embodiment, it is possible to transmit a modulated signal in a multi-valued modulation format.

[Embodiment 2]
FIG. 2 is a diagram showing a configuration of an optical transmitter according to a second embodiment of the present invention. The optical transmitter 500 of the present embodiment shows a configuration for generating a polarization-multiplexed signal light group composed of a pair of signal light and its phase-conjugated light as in the first embodiment. In the optical transmitter of this embodiment, the pilot light is not transmitted together.

The transmitter 500 of FIG. 2 includes a plurality of signal light sources 501 that output signal lights of different wavelengths, a plurality of polarization multiplex modulators 502, a wavelength combiner 503, and a secondary for differential frequency generation (DFG). It includes a nonlinear optical element 504, a local oscillation light source 505 that outputs a fundamental wave, an EDFA506, a BPF 507, and a secondary nonlinear optical element 508 for SH light generation (SHG).

In the transmitter 500, the continuous wave light (CW light) output from the local oscillation light source 505 is used as the fundamental wave light 530. EDFA506 amplifies the fundamental wave light. The BPF 507 eliminates the noise light generated by the EDFA 506 and transmits only the amplified fundamental wave light.

The second-order nonlinear optical element 508 for SHG includes a PPLN waveguide and a dichroic mirror on the output side. When the fundamental wave light transmitted through the BPF 507 is incident on the second-order nonlinear optical element 508, SH light having half the wavelength (twice the frequency) of the fundamental wave light is generated by the PPLN waveguide. The dichroic mirror separates the fundamental wave light and the SH light. The SH light output from the second-order nonlinear optical element 508 is incident on the second-order nonlinear optical element 504 for DFG as excitation light 531.

The plurality of signal light sources 501 output continuous wave light (CW light) having a different wavelength and a wavelength different from that of the fundamental wave light of the local oscillation light source. The polarization multiplexing modulator 502 performs polarization multiplexing 16QAM modulation according to independent transmission data 535 and 536 for each output light of the signal light source 501. The polarization-multiplexed modulated light from each of the polarization multiplexing modulator 502 is wavelength-multiplexed by the wavelength combiner 503. The polarization-multiplexed and wavelength-multiplexed main signal light 532 is input to the second-order nonlinear optical element 504 for DFG.

The second-order nonlinear optical element 504 for DFG includes a dichroic mirror on the input side, a first PPLN waveguide 510 and a second PPLN waveguide 513, and a polarizer 512 arranged between the two PPLN waveguides. , Includes a dichroic mirror on the output side. The wavelength-multiplexed main signal light and the excitation light 531 output from the second-order nonlinear optical element 508 for SHG are combined by a dichroic mirror on the input side and incident on the first PPLN waveguide 510. In the P first PLN waveguide 510, a difference frequency light between the frequency of the main signal light and the frequency of the excitation light is generated for the polarization component of one of the polarization-multiplexed main signal lights 532. This difference frequency light becomes phase-conjugated light with respect to the main signal light 532. Assuming that the two orthogonal polarization components of the polarization-multiplexed main signal light 532 are the X polarization component and the Y polarization component, respectively, the phase of the X polarization component of the main signal light in the first PPLN waveguide 510. Conjugated light is generated. On the other hand, nothing acts on the Y polarization component of the polarization-multiplexed main signal light 532, and the light passes through the first PPLN waveguide 510 as it is.

The output light 533 from the first PPLN waveguide 510 passes through the wavelength-dependent polarizer 512. At this time, the polarizer 512 rotates 90 ° polarized light with respect to the main signal light and 0 ° or 180 ° polarized light with respect to the excitation light. The main signal light polarized and rotated by the polarizer 512 is input to the second PPLN waveguide 513. In the second PPLN waveguide 513, phase-conjugated light of the Y polarization component of the main signal light is generated. On the other hand, it has no effect on the X polarization component of the main signal light and passes through the second PPLN waveguide 513 as it is. A dichroic mirror for separating the excitation light is arranged after the output of the second PPLN waveguide 513. After passing through the dichroic mirror, a polarized multiplex signal light group 534 including a plurality of pairs consisting of a main signal light and corresponding phase conjugation is output to the fiber 514 as an output of the transmitter. The excitation light 531 transmitted through the second-order nonlinear optical element 504 is separated by a dichroic mirror on the output side (535).

According to the configuration of the transmitter 500 of FIG. 2, the polarization multiplex signal 532 of the main signal light output from the wavelength combiner 503 is rigid without being polarized even once by the second-order nonlinear optical element 504. Optical components can be used to generate phase-conjugated light for two orthogonal polarization components. The secondary nonlinear optical element 504 can be configured by fixing the LiNbO ₃ substrate and the polarizer 512 constituting the two PPLN waveguides 510 and 513 in the module without using a connecting optical fiber. Therefore, as compared with the configuration in which separate second-order nonlinear optical elements are interconnected by an optical fiber as in the transmitter 400 of the first embodiment, the drift and fluctuation of the relative phase between the orthogonal polarizations are negligibly small. Therefore, the optical transmitter of FIG. 2 has an advantage that, unlike the optical transmitter of the first embodiment, a control mechanism for phase synchronization between orthogonal polarizations and stabilization of relative phase is not required.

In the configuration of the optical transmitter 500 shown in the present embodiment of FIG. 2, it is possible to transmit a light wave in which the main signal light is polarized and multiplexed without transmitting the output from the local oscillation light source 505 as pilot light. Since the relative phase drift and fluctuation between the polarizations do not occur and are stabilized, the phase-sensitive optical amplifier having the polarization diversity configuration can be stably operated even in the repeater.

The material of the ridge waveguide constituting the second-order nonlinear optical element used in the first and second embodiments is only an example. That is, the ridge waveguide is constructed using any material of LiNbO ₃ , KNbO ₃ , LiTaO ₃ , LiNb _(x) Ta _(1-x) O ₃ (0 ≦ x ≦ 1), or KTIOPO _4. Alternatively, a material may be used in which at least one selected from the group consisting of Mg, Zn, Sc and In is added as an additive.

As described in detail above, the optical transmitter of the present invention stabilizes the drift fluctuation of the phase difference between orthogonal polarizations in each signal such as the main signal light, its phase-conjugated light, the excitation light, and the pilot signal. be able to. It stabilizes the relative phase between the two polarizations due to temperature fluctuations and vibrations in the optical transmitter. According to the PDM signal transmitted from the optical transmitter of the present invention, stable optical amplification operation is possible even in a phase-sensitive optical amplifier having a polarization diversity configuration in a repeater.

The present invention can generally be used in communication systems. In particular, it can be used as an optical transmitter in an optical transmission system.

100, 200, 302, 352 Phase Sensitive Optical Amplifier (PSA)
102 Excitation light source 103 Excitation light phase control unit 201, 308, 321, 409, 412, 506 EDFA
202, 204, 306, 311, 323, 324, 363, 364, 404, 406, 411, 414, 504, 508 Secondary nonlinear optical elements 206, 328, 421, 425 Optical fiber extenders 207, 361, 514 Optical fibers 208, 326, 419, 423 Optical detectors 209, 327, 420, 424 PLL circuits 301, 351, 400, 500, 600 Optical transmitters 303, 353, 401, 501 Signal light source 304, 355, 356 External modulator 402, 502 Polarization multiplex modulator 305, 403, 503 Wavelength combiner 307, 407, 505 Local oscillation light source 309, 322, 410, 413, 418, 422, 507 BPF
360, 362, 365 PBS
405 Polarized Rotor 409, 512 Polarizer 510, 513 PPLN Waveguide

Claims (8)

A signal light source that outputs the main signal light and
A polarization multiplex modulator that modulates the main signal light,
A first second-order nonlinear optical element that generates phase-conjugated light for one of the polarization components of the main signal light that has been multiplexed and modulated.
A polarized rotor that polarizes and rotates the output light from the first second-order nonlinear optical element,
A polarization-multiplexed light group that generates phase-conjugated light for the other polarization component of the polarization-multiplexed and modulated main signal light input from the polarization rotor, and includes a pair of main signal light-phase-conjugated light. The second-order nonlinear optical element that outputs
A local oscillation light source that outputs fundamental wave light,
Based on the fundamental wave light, a first excitation light for generating phase-conjugated light with the first second-order nonlinear optical element and a second for generating phase-conjugated light with the second second-order nonlinear optical element. One or more second-order nonlinear optical elements that generate two excitation lights,
The phase of the pilot signal which is a part of the fundamental wave light and is input together with the polarization-multiplexed and modulated main signal light and passes through the first second-order nonlinear optical element, and the phase of the first excitation light . It is characterized by including a phase adjusting mechanism that synchronizes the phase with the phase of the pilot signal that further passes through the second second-order nonlinear optical element and the phase of the second excitation light. Optical transmitter. The phase adjustment mechanism is
A second that detects the level of the pilot signal on the output side of the first second-order nonlinear optical element and adjusts the phases of the main signal light and the pilot signal on the input side of the first second-order nonlinear optical element. 1 feedback circuit and
The level of the pilot signal on the output side of the second second-order nonlinear optical element is detected, and the main signal light , the phase-conjugated light and the pilot signal on the input side of the second second-order nonlinear optical element are detected. The optical transmitter according to claim 1, further comprising a second feedback circuit for adjusting the phase. The first second-order nonlinear optical element, the second second-order nonlinear optical element, and the one or more second-order nonlinear optical elements each include a direct junction ridge waveguide.
Whether the direct junction ridge waveguide is composed of any of LiNbO ₃ , KNbO ₃ , LiTaO ₃ , LiNb _(x) Ta _(1-x) O ₃ (0 ≦ x ≦ 1), or KTIOPO ₄ material. The light according to claim 1 or 2, wherein the light is composed of a material obtained by adding at least one selected from the group consisting of Mg, Zn, Sc, and In to any of the materials as an additive. Transmitter. A signal light source that outputs the main signal light and
A polarization multiplex modulator that modulates the main signal light,
A first second-order nonlinear optical element that generates phase-conjugated light of the main signal light that has been polarized and modulated and outputs a group of polarized multiple light.
A first quadratic nonlinear waveguide that generates phase-conjugated light for one of the polarization components of the main signal light that has been multiplexed and modulated.
A polarizer that polarizes and rotates the main signal light and the phase-conjugated light from the first second-order nonlinear waveguide with wavelength dependence.
Generates phase-coupled light for the other polarization component of the polarization-multiplexed and modulated main signal light input from the polarizer, and produces the polarization-multiplexed light group including a pair of main signal light-phase-conjugated light. A first second-order nonlinear optical element including an output second second-order nonlinear waveguide,
A local oscillation light source that outputs fundamental wave light,
An optical transmitter including a second second-order nonlinear optical element that generates excitation light for generating phase-conjugated light in the first second-order nonlinear optical element based on the fundamental wave light. .. In the first second-order nonlinear optical element, the first second-order nonlinear waveguide using a direct junction ridge waveguide, the second second-order nonlinear waveguide, and the polarizer are packaged in a single module. The optical transmitter according to claim 4, wherein the optical transmitter has a configured structure. Whether the direct junction ridge waveguide is composed of any of LiNbO ₃ , KNbO ₃ , LiTaO ₃ , LiNb _(x) Ta _(1-x) O ₃ (0 ≦ x ≦ 1), or KTIOPO ₄ material. The optical transmitter according to claim 5, wherein the optical transmitter is composed of a material obtained by adding at least one selected from the group consisting of Mg, Zn, Sc, and In to any of the materials as an additive. .. The first two are combined by combining the plurality of signal light sources that output light having different wavelengths, the plurality of corresponding polarization multiplex modulators, and the plurality of polarization-multiplexed and modulated main signal lights. Equipped with a wavelength combiner that outputs wavelength-multiplexed light to the next-order nonlinear optical element
The optical transmitter according to any one of claims 1 to 6, wherein phase-conjugated light is generated for each of the main signal lights having different wavelengths. The optical transmitter according to any one of claims 1 to 7.
A locally oscillating light source that outputs fundamental wave light synchronized with the carrier phase of the polarized light group from the optical transmitter is provided, the polarized light group is separated into two polarization components, and the non-linear optical element is used to separate the polarized light group into two components. An optical transmission system including a phase-sensitive optical amplifier having a polarization diversity configuration in which two polarization components are optically amplified and the two polarization components photoamplified are recombined.

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