Optical transmission system and sending device

Abstract

An optical transmission system (10) according to the present invention comprises: an optical signal generation unit (111) which generates signal light; an excitation light generation unit (121) which generates excitation light; a second-order nonlinear optical element (126) to which the signal light and the excitation light are inputted, and which generates idler light by optical parametric amplification and outputs the excitation light as pilot light along with the signal light and the idler light; an optical power adjustment unit (130) which adjusts the power of the signal light, the power of the idler light, and the power of the pilot light; an optical amplification unit (140) which amplifies the signal light, the idler light, and the pilot light; an optical transmission unit (101) which transmits the signal light, the idler light, and the pilot light; and a phase sensitive amplification unit (170) which performs phase sensitive amplification of the signal light and the idler light by the optical parametric amplification by using second excitation light generated on the basis of the pilot light. 　As a result, the present invention can provide an optical transmission system capable of stably amplifying an optical signal by phase sensitive optical amplification.

Description

Optical transmission system and transmitter

[0001] The present invention relates to an optical transmission system using a phase-sensitive optical amplifier and a transmitter in the optical transmission system.

[0002] Optical communications uses a wide bandwidth of light for signal transmission, enabling high-capacity communications. It is widely used not only for terrestrial optical fiber communications, but also for a variety of other applications, such as intercontinental communications connected by undersea cables, free-space optical communications that transmit light in the space between islands, and space optical communications that connect satellites to the ground and between satellites.

[0003] In order to expand communication capacity in any application area, a transmission system that can guarantee a high signal-to-noise ratio (SNR) is required. The main cause of SNR degradation in a transmission system is spontaneously emitted optical noise (SNR) from optical amplifiers. Reducing this spontaneously emitted optical noise can improve the SNR of the transmission line. However, conventional laser amplifiers such as EDFAs have a theoretical limit to the reduction of spontaneously emitted optical noise, and the SNR of signal light decreases by at least 3 dB after amplification. As a result, the transmission bit error rate during signal transmission increases, and transmission quality deteriorates.

[0004] As a means to overcome the limitations of conventional laser amplifiers, phase-sensitive amplifiers (PSAs) have been investigated. These PSAs have the function of shaping signal light waveforms and phase signals. Furthermore, they can suppress spontaneous emission (SA) with quadrature phase unrelated to the signal, reduce in-phase SA, and suppress degradation of the S / N ratio of the signal light after amplification, maintaining transmission quality equivalent to that before amplification.

[0005] 6 shows the basic configuration of a conventional PSA 40. The PSA 40 includes a phase-sensitive amplifier 41 using optical parametric amplification, a pumping light source 42, a pumping light phase control unit 43, and first and second optical branching units 44-1 and 44-2. Signal light 45 input to the PSA 40 is branched into two by the optical branching unit 44-1, one of which enters the phase-sensitive amplifier 41 and the other of which enters the pumping light source 42. Pumping light 46 emitted from the pumping light source 42 has its phase adjusted via the pumping light phase control unit 43 and enters the phase-sensitive amplifier 41. The phase-sensitive amplifier 41 outputs output signal light 47 based on the input signal light 45 and pumping light 46.

[0006] The phase-sensitive optical amplifier 41 amplifies the signal light 45 when the phase of the incident signal light 45 matches the phase of the pump light 46. When the two phases are out of phase by 90 degrees, the phase-sensitive amplifier 41 attenuates the signal light 45. By matching the phases of the pump light 46 and the signal light 45 to maximize the amplification gain, spontaneous emission in quadrature with the signal light 45 is not generated, and spontaneous emission greater than the noise contained in the signal light is not generated in the in-phase component. In other words, the signal light 45 can be amplified without degrading the S / N ratio.

[0007] In order to achieve phase synchronization between the signal light 45 and the pumping light 46, the pumping light phase control unit 43 controls the phase of the pumping light 46 so that it is synchronized with the phase of the signal light 45 branched by the first optical branching unit 44-1. The pumping light phase control unit 43 also detects a portion of the output signal light 47 branched by the second optical branching unit 44-2 with a narrowband detector, and controls the phase of the pumping light 46 so that the amplification gain of the output signal light 47 is maximized. As a result, the phase-sensitive optical amplifier 41 achieves optical amplification that suppresses degradation of the S / N ratio based on the above principle.

[0008] Here, the pumping light phase control section 43 may be configured to control the phase of the pumping light 46 on the output side of the pumping light source 42, or may be configured to directly control the phase of the pumping light source 42. Furthermore, if the light source that generates the signal light 45 is located near the phase-sensitive amplifier section 41, a part of the light source for the signal light can be branched off and used as the pumping light.

[0009] Nonlinear optical media that perform the above-mentioned parametric amplification include second-order nonlinear optical materials, such as a periodically poled LiNbO3 (PPLN) waveguide, and third-order nonlinear optical materials, such as a silica glass fiber.

[0010] 7 shows an example of the configuration of a conventional PSA using a PPLN waveguide (Non-Patent Document 1). The PSA 50 includes an erbium-doped fiber laser amplifier (EDFA) 501, first and second second-order nonlinear optical elements 502 and 504, first and second optical branching units 503-1 and 503-2, a phase modulator 505, a PZT optical fiber stretcher 506, a polarization-maintaining fiber 507, a photodetector 508, and a phase-locked loop (PLL) circuit 509. The first second-order nonlinear optical element 502 includes a first spatial optical system 511, a first PPLN waveguide 512, a second spatial optical system 513, and a first dichroic mirror 514, and the second second-order nonlinear optical element 504 includes a third spatial optical system 515, a second PPLN waveguide 516, a fourth spatial optical system 517, a second dichroic mirror 518, and a third dichroic mirror 519.

[0011] The first spatial optical system 511 couples light input from the input port of the first second-order nonlinear optical element 502 to the first PPLN waveguide 512. The second spatial optical system 513 couples light output from the first PPLN waveguide 512 to the output port of the first second-order nonlinear optical element 502 via a first dichroic mirror 514. The third spatial optical system 515 couples light input from the input port of the second second-order nonlinear optical element 504 to the second PPLN waveguide 516 via a second dichroic mirror 518. The fourth spatial optical system 517 couples light output from the second PPLN waveguide 516 to the output port of the second second-order nonlinear optical element 504 via a third dichroic mirror 519.

[0012] Signal light 550 incident on PSA 50 is branched by optical branching unit 503-1, one of which is incident on second second-order nonlinear optical element 504, and the other is phase-controlled as pump fundamental wave light 551 via phase modulator 505 and optical fiber extender 506 and then incident on EDFA 501. In order to obtain sufficient power to obtain a nonlinear optical effect from the weak laser light used in optical communications, EDFA 501 amplifies the incident pump fundamental wave light 551 and makes it incident on first second-order nonlinear optical element 502. In first second-order nonlinear optical element 502, second harmonic wave (hereinafter, SH light) 552 is generated from the incident pump fundamental wave light 551, and the generated SH light 552 is incident on second second-order nonlinear optical element 504 via polarization-maintaining fiber 507. The second second-order nonlinear optical element 504 performs degenerate parametric amplification of the incident signal light 550 and SH light 552 to perform phase-sensitive amplification, and outputs an output signal light 553 .

[0013] In PSA, in order to amplify only light that is in phase with the signal, the signal light and pump light must be in phase or shifted by π radians. In other words, when using the second-order nonlinear optical effect, the phase φ2ωs of the pump light, which has a wavelength corresponding to the SH light, and the phase φωs of the signal light must satisfy the relationship expressed by the following equation, where n is an integer.

[0014] Δφ=1 / 2(φ2ωs−φωs)=nπ

[0015] FIG. 8 shows the relationship between the phase difference Δφ between the input signal light and the pump light and the gain (dB) in a conventional PSA utilizing the second-order nonlinear optical effect.

[0016] The gain is maximized when Δφ is −π, 0, or π.

[0017] In the configuration shown in Fig. 7, in order to phase-synchronize the signal light 550 and the pump fundamental light 551, a phase modulator 505 is used to phase-modulate the pump fundamental light 551 with a weak modulation signal, and then a portion of the output signal light 553 is branched off and detected by a detector 508. When phase synchronization is achieved such that the phase difference Δφ shown in Fig. 8 is minimized, the modulated component detected by the detector 508 is minimized, and therefore feedback is provided to the optical fiber extender 506 using a PLL circuit 509 so that the pilot signal is minimized, i.e., the amplified output signal is maximized. By controlling the phase of the pump fundamental light 551, phase synchronization between the signal light 550 and the pump fundamental light 551 can be achieved.

[0018] In a configuration in which the above-described PPLN waveguide is used as a nonlinear medium and the signal light 550 and the SH light 552 are input to the second second-order nonlinear optical element 504 to perform degenerate parametric amplification, when the SH light 552 is generated and then parametrically amplified, the pump fundamental light component can be removed using the characteristics of the dichroic mirror 514, for example, so that only the SH light 552 and the signal light 550 can be input to the parametric amplification medium such as the second second-order nonlinear optical element 504. This makes it possible to suppress noise due to the mixing of spontaneous emission light generated in the EDFA 501, thereby enabling low-noise optical amplification.

[0019] As described above, by using the PPLN waveguide as the nonlinear optical medium and exciting the nonlinear medium using the SH light 552, it is possible to perform low-noise phase-sensitive amplification by suppressing the influence of noise generated by the EDFA 501. Furthermore, the phase noise can be reduced by the characteristic of attenuating the quadrature phase component.

[0020] The phase-sensitive amplification using the degenerate parametric process described above can attenuate orthogonal phase components, as shown in Figure 8, and is therefore applicable only to the amplification of normal intensity-modulated signals and modulated signals using binary phase modulation, such as IMDD, BPSK, or DPSK. Furthermore, only signal light of one wavelength can be phase-sensitively amplified. To apply PSA to optical communication technology, a configuration capable of handling various optical signals, such as multilevel modulation formats and wavelength-multiplexed signals, is required.

[0021] Therefore, a configuration based on non-degenerate parametric amplification has been reported (Non-Patent Document 2). In this configuration, a phase conjugate light (idler light) that is paired with a signal light is input to a nonlinear medium such as a PPLN. This enables phase-sensitive amplification that can accommodate any modulation format and repeat and amplify multiple wavelengths simultaneously.

[0022] In a phase-locked technique using a non-degenerate PSA, for example, in a configuration where the PSA is located immediately after the optical signal transmitter and the light source that generates the signal light is located near the phase-sensitive optical amplifier, as shown in FIG. 7, a portion of the pump light used to generate the idler light can be branched and used.

[0023] However, when a PSA is used as a repeater amplifier or a preamplifier on the receiving side in optical transmission, the above-mentioned simple branching technique cannot be applied: unless a light source having the same phase information as the pump light used to generate the idler light on the transmitting side is used as the pump light, the PSA based on non-degenerate parametric amplification in the repeater amplifier or the preamplifier on the receiving side cannot operate.

[0024] 9 shows an example of a configuration in which a PSA is applied to a repeater amplifier in optical transmission (Non-Patent Document 3). In this configuration, pump light from a pump light generator 611 is split in a transmitter 610, and one pump light is input to a second-order nonlinear optical element (PPLN-11) 612 to generate SH light, and the SH light is input to a second-order nonlinear optical element (PPLN-12) 613 to generate idler light. The other pump light is multiplexed with the signal light, idler light, and the pump light after the idler light has been generated, and is amplified in an amplifier 614 together with the signal light and phase conjugate light (idler light) to be used as pilot light to be sent to the transmission path.

[0025] Here, the pump light requires high power to generate the idler light.

[0026] On the other hand, the pilot light is usually adjusted to a power lower than the total power of the signal light and idler light before being transmitted. This is because in spatial optical transmission, the total amount of optical power that can be transmitted is determined by the performance of the optical amplifier on the transmitting side, so when the power of the pilot light is high, the power of the signal light and idler light decreases, resulting in a deterioration of the SNR. Therefore, after branching from the pump light, a variable optical attenuator (VOA) 615 is placed before the signal light and idler light are multiplexed to reduce and adjust the power of the pilot light.

[0027] In the transmission path, pilot light is lost along with the signal light and idler light, resulting in degradation of the SNR. Therefore, if pump light with a degraded SNR is used in a PSA, the inherent low-noise characteristics of the PSA cannot be achieved. Therefore, at a repeater amplification point or a receiving device 620 on the receiving side, the pilot light is extracted using a WDM coupler 621 or the like and input to a pilot light regeneration mechanism 622, thereby restoring the pilot light intensity and SNR. Examples of the pump light regeneration mechanism 622 include an injection locking method in which pilot light is directly input to a laser of local light, and a method in which the pilot light is phase-locked with the local light using an optical phase-locked circuit. By inputting this regenerated pilot light as pump light for the PSA into a second-order nonlinear optical element (PPLN-13) 623 and inputting the SH light into a second-order nonlinear optical element (PPLN-14) 624, the signal light can be amplified by the PSA, making it possible to apply the PSA to a repeater amplifier or a preamplifier on the receiving side.

[0028] T. Umeki, O. Tadanaga, A. Takada and M. Asobe, "Phase sensitive degenerate parametric amplification using directly-bonded PPLN ridge waveguides," Optics Express, 2011, Vol.19, No.7, p.6326-6332.Takushi Kazama, Takeshi Umeki, Shimpei Shimizu, Takahiro Kashiwazaki, Koji Enbutsu, Ryoichi Kasahara, Yutaka Miyamoto, and Kei Watanabe, "Over-30-dB gain and 1-dB noise figure phase-sensitive amplification using a pump-combiner-integrated fiber I / O PPLN module," Opt. Express 29, 28824-28834 (2021). S. Shimizu et al., "Wideband PPLN-Based Phase-Sensitively Amplified Transmission of 20-Channel 96-Gbaud WDM Signal," in Journal of Lightwave Technology, vol. 40, no. 16, pp. 5467-5477, 15 Aug.15, 2022.

[0029] However, in the configuration in which the above-mentioned PSA is applied to a repeater amplifier in optical transmission (FIG. 9), there is a problem that the pilot light intensity fluctuates due to nonlinear effects in the EDFA on the transmitting side and in the transmission line fiber, making the operation of the pump light regeneration unit unstable. Details will be explained below.

[0030] FIG. 10A shows an example of a spectrum after the pilot light is multiplexed on the transmitter side in the configuration shown in FIG. 9 . The multiplexed light is subject to nonlinear optical effects when propagating through the EDFA on the transmitter side and the optical fiber of the transmission path. In this configuration, one of the nonlinear optical effects is four-wave mixing, in which a fourth light is generated by the interaction of three light beams. The idler light is generated through a parametric process from the pump light from which the pilot light is branched. As a result, the frequency arrangement of the signal light and the idler light is symmetrical around the pilot light. This results in light with the same frequency as the pilot light being generated in the four-wave mixing light generated by the interaction of the three waves of the signal light, idler light, and pilot light. Focusing on the four-wave mixing light having the same frequency component as the pilot light, the phase of this four-wave mixing light is expressed by Equation (1).

[0031]

[0032] where φ ｓ is the signal light phase, φ ｉ is the idler optical phase, φ ｐ２、ＦＨ represents the phase of the pilot light.

[0033] In the SHG process in PPLN-2, the phase of the SH light, φ Ｐ１、ＳＨ and the phase of the fundamental pump light φ ｐ１、ＦＨ is expressed by equation (2).

[0034]

[0035] where φ ｐ１、ＦＨ is the phase of the fundamental pump light for generating SH light. The fundamental pump light is emitted from the same light source as the pilot light, but the propagation path is different from that of the pilot light. Therefore, the phase φ of the fundamental pump light ｐ１、ＦＨ is the phase of the pilot light φ ｐ２、ＦＨ is different from.

[0036] The phase relationship between the signal light and the idler light is expressed by equation (3).

[0037]

[0038] φ ｐｌis the phase of the pump light used to generate the idler light. This pump light is emitted from the same light source as the pilot light, but its propagation path is different from that of the pilot light. Therefore, the phase φ of the pump light used to generate the idler light ｐ１、ＦＨ is the phase of the pilot light φ ｐ２、ＦＨ In this case, the formula (1) is expressed as the formula (4).

[0039]

[0040] From equation (4), the four-wave mixing light generated at the same frequency as the pilot light interferes with the pilot light according to the phase difference between the pump light used to generate the idler and the pilot light. Furthermore, since the difference between the path of the pump light used to generate the idler and the path of the pilot light varies depending on environmental conditions such as temperature, the phase difference varies, and the intensity of the pilot light also varies.

[0041] In the case of spatial optical communications, the transmission path is space, so nonlinear effects do not occur. Since the signal light must be transmitted over long distances in space, it is transmitted at a high output power using a high-power amplifier with an output of, for example, 5 W or more on the transmitter side. Although the fiber length inside the high-power amplifier is short, at only a few tens of meters, the optical power inside is so high that the efficiency of four-wave mixing is also high.

[0042] Figure 10B shows the intensity fluctuation of the pilot light when one wave of signal light, one wave of idler light, and pilot light pass through a 5W high-power amplifier. The intensity of the pilot light fluctuates by just over ±1 dB due to the effects of four-wave mixing within the high-power amplifier. In optical fiber communications, a high-power EDFA of the watt class is not used on the transmitter side, but the transmission fiber is long, about 80 km per span, and repeated relay amplification allows the signal to propagate even longer distances. As a result, just like in free-space optical communications, the intensity of the pilot light fluctuates due to the effects of four-wave mixing in the optical fiber.

[0043] In this way, if the pilot light fluctuates, the operation of the pilot light regeneration mechanism becomes unstable. For example, in an injection-locked regeneration mechanism, the pull-in range of injection locking is determined by the intensity of the pilot light, so the phase noise generated in the local light during pull-in changes. Also, in an optical phase-locked regeneration mechanism, the error signal required for phase locking fluctuates, making the synchronization unstable. Because the PSA is sensitive to phase, the PSA cannot operate stably even if light generated by an unstable regeneration mechanism is used as the pump light for the PSA.

[0044] As described above, in a configuration in which a PSA is applied to a relay amplifier in optical transmission, there is a problem in that the pilot light intensity fluctuates due to nonlinear effects in the EDFA on the transmitting side and nonlinear effects in the transmission path fiber, making the operation of the pump light regeneration unit unstable.

[0045] In order to solve the above-mentioned problems, the optical transmission system according to the present invention comprises an optical signal generation unit that generates signal light, a pump light generation unit that generates pump light, a second-order nonlinear optical element that receives the signal light and the pump light, generates idler light by optical parametric amplification, and outputs the pump light as pilot light together with the signal light and the idler light, an optical power adjustment unit that adjusts the power of the signal light, the power of the idler light, and the power of the pilot light, an optical amplification unit that amplifies the signal light, the idler light, and the pilot light, an optical transmission unit that transmits the signal light, the idler light, and the pilot light, and a phase-sensitive amplification unit that performs phase-sensitive amplification of the signal light and the idler light by optical parametric amplification using second pump light generated based on the pilot light.

[0046] Furthermore, a transmitting device according to the present invention is a transmitting device used in an optical transmission system that performs phase-sensitive amplification of signal light and idler light by optical parametric amplification using pump light, and includes an optical signal generating unit that generates the signal light, a pump light generating unit that generates pump light, a second-order nonlinear optical element that receives the signal light and the pump light, generates idler light by optical parametric amplification, and outputs the pump light as pilot light together with the signal light and the pump light, an optical power adjusting unit that makes the power of the signal light and the power of the idler light approximately equal and adjusts the power of the pilot light to less than half the total power of the signal light and the idler light, and an optical amplifying unit that amplifies the signal light, the idler light, and the pilot light.

[0047] According to the present invention, it is possible to provide an optical transmission system and a transmitter that can stably amplify an optical signal by phase sensitive amplification (PSA).

[0048] FIG. 1 is a schematic diagram showing the configuration of an optical transmission system and a transmitting device according to a first embodiment of the present invention. FIG. 2 is a diagram for explaining the operation of the optical transmission system and the transmitting device according to the first embodiment of the present invention. FIG. 3 is a diagram for explaining the operation of the optical transmission system and the transmitting device according to the first embodiment of the present invention. FIG. 4 is a schematic diagram showing the configuration of an optical transmission system and a transmitting device according to a second embodiment of the present invention. FIG. 5A is a diagram for explaining the operation of the optical transmission system and the transmitting device according to the second embodiment of the present invention. FIG. 5B is a diagram for explaining the operation of the optical transmission system and the transmitting device according to the second embodiment of the present invention. FIG. 5C is a diagram for explaining the operation of the optical transmission system and the transmitting device according to the second embodiment of the present invention. FIG. 6 is a diagram for explaining a conventional optical transmission system and a phase-sensitive optical amplifier. FIG. 7 is a schematic diagram showing the configuration of a conventional phase-sensitive optical amplifier. FIG. 8 is a diagram for explaining a conventional phase-sensitive optical amplifier. FIG. 9 is a schematic diagram showing the configuration of a conventional optical transmission system and a transmitting device. FIG. 10A is a diagram for explaining a conventional optical transmission system and a transmitting device. FIG. 10B is a diagram for explaining a conventional optical transmission system and a transmitting device.

[0049] First Embodiment An optical transmission system and a transmitting device according to a first embodiment of the present invention will be described with reference to FIGS. 1 to 3. FIG.

[0050] 1, an optical transmission system 10 according to this embodiment includes a transmitting device 110, a transmission line 101, and a receiving device 150. The receiving device 150 includes a repeater.

[0051] The transmitting device 110 generates idler light from input signal light and fundamental wave pump light (pump light), and transmits the pump light as pilot light together with the signal light and idler light. The signal light, idler light, and pump light (pilot light) are input to the receiving device 150 via a transmission path 101 such as an optical fiber. The receiving device 150 amplifies the signal light using the pump light and idler light with a PSA.

[0052] The signal light and fundamental wave pump light (pump light) use light in the signal wavelength band in optical transmission. For example, the wavelengths of the signal light and pump light are about 1.5 μm. The difference in frequency between the signal light and the pump light is about 200 to 400 GHz. In addition, the idler light generated from the signal light and pump light is also light in the signal wavelength band.

[0053] <Configuration of Transmitter> The transmitter 110 includes an optical signal generator 111 , an optical phase conjugator (idler light generator) 120 , an optical power adjuster 130 , and an optical amplifier 140 .

[0054] The optical signal generating unit 111 includes a semiconductor laser and generates a modulated signal (optical signal) by operating the semiconductor laser with a modulation signal. Alternatively, the modulated signal (optical signal) may be generated by modulating the oscillation light of the semiconductor laser with a modulator.

[0055] The optical phase conjugate section (idler light generating section) 120 includes a pump light generating section 121 , a multiplexing section 125 , and a second-order nonlinear optical element 126 .

[0056] The pumping light generating unit 121 includes a pumping light source (e.g., an external cavity semiconductor laser, ECLD) 122, an optical fiber amplifier (e.g., an erbium doped fiber amplifier, EDFA) 123, and a bandpass filter (BPF) 124. The pumping light generating unit 121 generates fundamental pumping light (pumping light) for generating idler light, and outputs it to a second-order nonlinear optical element 126 via a multiplexer 125.

[0057] The multiplexer 125 multiplexes the signal light from the optical signal generator 111 and the pump light from the pump light generator 121 .

[0058] The second-order nonlinear optical element (first second-order nonlinear optical element) 126 receives the signal light from the optical signal generating unit 111 and the pump light from the pump light generating unit 121, and generates idler light. The second-order nonlinear optical element 126 outputs the signal light, idler light, and pump light (pilot light) to the optical power adjusting unit 130.

[0059] The second-order nonlinear optical element 126 is a single first PPLN module (PPLN-1). In the PPLN-1 (126), a cascade process occurs: a second harmonic generation (SHG) process using fundamental pump light (pump light), and an optical parametric amplification (OPA) process occurs due to the interaction between the second harmonic (SH light) generated by the SHG process and the signal light. This generates idler light.

[0060] The PPLN-1 (126) has an optical system on the input side that couples light in the signal wavelength band (signal light, pump light) to a PPLN waveguide, and an optical system on the output side that splits the light in the signal wavelength band (signal light, idler light, pump light) and the light in the double-wave pump light band and couples them to output ports, respectively.

[0061] The optical power adjuster 130 includes a variable optical attenuator (VOA) 131 and a gain flattening filter (GFF) 132. The variable attenuator 131 adjusts the optical power of the signal light, idler light, and pump light (pilot light). The gain flattening filter 132 makes the intensities of the signal light, idler light, and pump light (pilot light) approximately equal in the 1.5 μm wavelength band. For example, the optical intensities of the signal light, idler light, and pump light (pilot light) are set within a range of plus or minus 0.3 dB.

[0062] The optical amplifier 140 includes an optical fiber amplifier (for example, an EDFA) and amplifies the signal light, idler light, and pump light (pilot light).

[0063] In the idler light generation unit 120, the fundamental wave pump light (pump light) is amplified by an EDFA in the pump light generation unit 121 and then multiplexed with the signal light. By simultaneously inputting the signal light and pump light into the PPLN-1 (126), the PPLN-1 (126) outputs the signal light, the idler light, and the fundamental wave pump light that was not consumed in the nonlinear process, and the fundamental wave pump light is used as is as pilot light.

[0064] Since the fundamental pump light input to the PPLN-1 (126) has a power of several hundred milliwatts or more to cause a nonlinear process, the power of the pilot light (fundamental pump light) output from the PPLN-1 (126) is greater than that of the signal light and the idler light. Therefore, in order to make the power of the pilot light lower than the total power of the signal light and the idler light, a variable optical attenuator (VOA) 131 and a gain flattening filter (GFF) 132 are placed after the PPLN-1 (126).

[0065] The VOA 131 adjusts the total power of the pilot light, signal light, and idler light so that it falls within the operating range of the EDFA in the subsequent stage.

[0066] The GFF 132 makes the signal light and the idler light levels approximately equal, and reduces the power of the pilot light to less than half of the total power of the signal light and the idler light. The power of the pilot light is preferably less than one-tenth of the total power of the signal light and the idler light. Furthermore, the pilot light has optical power at least strong enough to be received by the receiving device 150 and regenerated by a pump light regeneration mechanism (described below) after optical loss in the transmission path 101.

[0067] The GFF 132 may be, for example, an etalon filter, a dielectric multilayer film, a long-period fiber grating, a Mach-Zehnder type quartz waveguide, a split-beam Fourier filter, or the like. Also, a programmable filter using LCOS (Liquid Crystal on Silicon) or the like may be used.

[0068] A transmitter in a conventional optical transmission system includes two PPLN modules, namely, a PPLN module that generates SH light and a PPLN module that generates idler light by an OPA process, and uses pump light having the same frequency as the signal light.

[0069] Meanwhile, in the transmitter 110 of the optical transmission system 10, a single PPLN module 126 generates idler light by inducing SHG and OSA processes, and outputs fundamental pump light (pump light) as pilot light. To enable the generation of idler light with a single PPLN module configuration, a GFF 132 is used to make the levels of the signal light and the idler light approximately equal, and the power of the pilot light is reduced below the power of the signal light and the idler light. Furthermore, the difference in frequency between the signal light and the pump light is approximately 200 to 400 GHz, taking into account the frequency difference that can be separated using a WDM coupler or the like.

[0070] <Configuration of Receiving Device> The receiving device 150 includes an optical receiving section 160 and a phase sensitive amplifier (PSA) section 170 .

[0071] The receiver 160 receives the signal light, the idler light, and the pilot light.

[0072] The phase-sensitive amplifier (PSA) unit 170 includes a wavelength separator 171, a pump light regenerator 172, a second second-order nonlinear optical element 173, and a third second-order nonlinear optical element 174. It also includes an optical detector, a phase adjuster, and a signal light phase error detector.

[0073] The wavelength separator 171 separates the pilot light from the optical signal and the idler light. The optical signal and the idler light are sent to a third second-order nonlinear optical element 174. The pilot light is sent to a pump light regenerator 172. The wavelength separator 171 is, for example, a WDM coupler.

[0074] In the pump light regeneration unit 172, the phase of the local light from the phase-locked local oscillation light source is synchronized with the phase of the input pilot light, and is output as continuous wave local oscillation pump light (second pump light).

[0075] The second second-order nonlinear optical element 173 and the third second-order nonlinear optical element 174 are a second PPLN module (PPLN-2) and a third PPLN module (PPLN-3), respectively. The output of PPLN-2 is connected to the input of PPLN-3.

[0076] The PPLN-2 (173) generates second harmonic (SH) light by a second harmonic generation (SHG) process using the locally generated pump light from the pump light regeneration unit 172. The PPLN-3 (174) performs PSA operation by an OPA process using the SH light generated by the PPLN-2 (173).

[0077] The PPLN-2 (173) has an optical system on the input side that couples light in the signal wavelength band (locally generated pump light) from the pump light regeneration unit 172 to the PPLN waveguide, and an optical system on the output side that separates the light in the signal wavelength band from light in the double-wave pump light band (second harmonic light, SH light) and couples only the light in the double-wave pump light band to the output port.

[0078] The PPLN-3 (174) has an optical system on the input side that combines light in the signal wavelength band (signal light, idler light) with light in the double-wave excitation light band and couples it to a PPLN waveguide, and on the output side that demultiplexes light in the signal wavelength band (signal light, idler light, local excitation light) from light in the double-wave excitation light band and couples them to output ports, respectively.

[0079] A phase modulator 175, an EDFA 176, and a BPF 177 are provided on the input side of the PPLN-2 (173).

[0080] The output of the PPLN-3 (174) is branched into two paths by a branching unit 178. Signal light is output to one path. A portion of the signal light is output as detection light to the other path. The other path includes a bandpass filter (BPF) 179, a photodetector (PD) 180, and a phase adjuster. The phase adjuster includes a phase-locked loop (PLL) 181 and a fiber extender 182. In the phase adjuster, the PLL 181 feedback-controls the fiber extender 182 based on the detection light detected by the photodetector 180, thereby synchronizing the phase of the signal light with the phase of the SH light.

[0081] The signal light phase error detection unit is composed of a phase modulator 175 on the input side of PLL1 and PPLN-2 (173). A dither signal is applied to the phase modulator 175 and demodulated to generate an error signal indicating the deviation (error) of the detected signal light component (intensity) from its maximum value. Based on this error signal, a fiber extender 182 is feedback-controlled via a PLL 181 to synchronize the signal light component (intensity) to its maximum value.

[0082] In this embodiment, PPLN-1, PPLN-2, and PPLN-3 use periodically poled LiNbO3 (PPLN) waveguides with the same configuration.

[0083] <Operations of Optical Transmission System and Transmitting Apparatus> Operations of the optical transmission system and transmitting apparatus according to this embodiment will be described with reference to FIGS.

[0084] Figure 2 shows the changes in the power of the signal light, idler light, and fundamental wave pump light (output pump light) output from the PPLN-1 (126) relative to changes in the power of the fundamental wave pump light (input pump light) input to the PPLN-1 (126). The signal light, idler light, and output pump light are output from the PPLN-1 (126) via a cascade process of SHG / OPA. A PPLN with an efficiency of 600% / W is used, and the input signal light power is 0 dBm. In Figure 2, the output power of the signal light is indicated by white circles, the output power of the idler light by black triangles, and the output power of the pilot light by black circles.

[0085] As the power of the input pump light increases, the output power of the signal light and the idler light increases, and the power difference between the signal light and the idler light decreases.

[0086] On the other hand, the output power of the output pump light is larger than the output power of the signal light and the idler light. The output power of the output pump light does not increase monotonically with increasing power of the input pump light, but tends to decrease at a pump light power of about 200 mW. This is thought to be because the SHG process occurs more efficiently as the input pump light power increases, resulting in a decrease in the output power of the output pump light due to SHG.

[0087] To maximize the performance of the PSA in the receiving device (or repeater) 150, the signal light and the idler light must have approximately equal power. Therefore, when using a GFF (non-tunable GFF) that outputs a constant optical power, it is desirable to set the operating point to the pump light power at which the power of the signal light and the idler light are approximately equal, as shown in FIG. 2 . For example, when 600 mW is input, the level difference between the signal light and the idler light is approximately 1 dB, and the power of the pilot light is approximately 8 dB higher than that of the signal light and the idler light. Therefore, by adding a 1 dB loss to the light in the signal wavelength band (signal light and idler light) and a 16 dB loss to the pilot light, the output power of the signal light and the output power of the idler light become equal, and the power of the pilot light becomes less than one-tenth of the total power. To adjust the power of the pilot light, the ratio of the output power of the pilot light to the output power of the signal light and the idler light can be adjusted by increasing or decreasing the power of the input fundamental wave pump light.

[0088] In the transmitter 110, in the SHG process in the PPLN-1 (126), the phase φ of the fundamental pump light ｐ、ＦＨ and SH light φ ｐ、ＳＨ The phase relationship is expressed by equation (5).

[0089]

[0090] In addition, the phase φ of the signal light in the OPA process ｓ and the phase of the idler light φ ｉ and the phase φ of the SH light ｐ、ＳＨ The relationship is expressed by equation (6).

[0091]

[0092] The signal light, idler light, and pilot light output from the transmitter 110 generate four-wave mixing light in the EDFA 140 on the output side and in the optical fiber of the transmission line 101. The phase φ of the four-wave mixing light, which is the same frequency component as the pilot light, ＦＷＭ is the phase of the pilot light φ ｐ、ＦＨ and the phase of the pilot light is the same as the phase of the fundamental pump light. From these, equation (7) is obtained.

[0093]

[0094] According to equation (7), the frequency component that is the same as the pilot light generated by four-wave mixing has the same phase as the pilot light, so it does not interfere with the pilot light and the intensity of the pilot light does not fluctuate.

[0095] Figure 3 shows the change in the power of the pilot light over time when the signal light, idler light, and pilot light are passed through the EDFA in the transmitter 110. A 5 W output EDFA was used. The signal light, idler light, and pilot light were input to the EDFA via a cascaded SHG / OPA process in the PPLN-1 (126).

[0096] In conventional transmitting devices, the power of the pilot light fluctuates, whereas in transmitting device 110, no fluctuation in the power of the pilot light is observed.

[0097] In a system using the transmitting device 110, the power of the pilot light does not fluctuate in the PSA in the receiving device (or relay amplifier or pre-amplifier on the receiving side) 150, so the pump light regeneration unit 172 can operate stably and the intensity and SNR of the pilot light can be restored. The pilot light regeneration unit 172 may use, for example, an injection locking method in which the pilot light is directly input into a local light laser, or an optical phase locking method in which feedback is performed to a frequency control mechanism of the local light source.

[0098] This regenerated pilot light was amplified by EDFA 176, then input to PPLN-2 (173) and converted to SH light. The converted SH light, signal light, and idler light were input to PPLN-3 (174), where phase-sensitive amplification was performed via a parametric amplification process.

[0099] In this configuration, the phase and output of the recovered pilot light are stable, so the PSA unit can operate stably as a relay amplifier or a preamplifier on the receiving side.

[0100] <Method for Manufacturing PPLN Waveguide> An example of a method for manufacturing the PPLN waveguide used in this example will be described below.

[0101] First, periodic electrodes with a period of about 17 μm are formed on Zn-doped Z-cut LiNbO 3 .

[0102] Next, a polarization inversion grating corresponding to the above electrode pattern is formed in Zn:LiNbO3 by applying an electric field.

[0103] Next, the Zn:LiNbO3 substrate having this periodically poled structure is directly bonded onto the LiTaO3 cladding, and heat treatment is performed at 500°C to firmly bond the two substrates together.

[0104] Finally, the core layer is polished to a thickness of about 5 μm, and a dry etching process is used to form a ridge-type optical waveguide. This waveguide can be temperature-controlled using a Peltier element, and the length of the waveguide is, for example, 45 mm.

[0105] The second-order nonlinear optical element having the PPLN waveguide formed as described above is a module that allows light to be input and output via a fiber.

[0106] In this embodiment, an example is shown in which LiNbO3 doped with Zn is used, but other nonlinear materials such as KNbO3, LiTaO3, LiNbxTa1-xO3 (0≦x≦1) or KTiOPO4, or materials containing at least one kind selected from the group consisting of Mg, Zn, Sc and In as an additive, may also be used.

[0107] According to the present embodiment, it is possible to provide a transmitter capable of generating pilot light with reduced fluctuations in optical intensity, thereby providing an optical transmission system capable of stably amplifying an optical signal by phase-sensitive amplification (PSA).

[0108] Second Embodiment A second embodiment of the present invention will be described with reference to FIGS. 4 to 5C.

[0109] <Configuration of Optical Transmission System> In an optical transmission system 20 according to this embodiment, in a transmitting device 210, two configurations each having a second-order nonlinear optical element according to the first embodiment are arranged in parallel via a polarization splitter (PBS) 230 and a polarization combiner (PBC) 236. In a receiving device 270, two configurations each having a second-order nonlinear optical element that performs PSA as in the first embodiment are arranged in parallel via a polarization splitter (PBS) 293 and a polarization combiner (PBC) 298.

[0110] In the first embodiment, the PPLN exhibits efficient nonlinearity only in TM polarized light, and therefore can only accommodate signals with one polarization (TM polarized light).

[0111] On the other hand, the optical transmission system 20 according to this embodiment has a polarization diversity configuration and can handle signals of both polarizations.

[0112] 4, the optical transmission system 20 includes a transmitting device 210 and a receiving device 270. The receiving device 270 includes a repeater.

[0113] The transmitting device 210 generates idler light of both polarizations from input signal light and fundamental wave pump light (pump light), and transmits the pump light as pilot light together with the signal light and idler light. The signal light, idler light, and pump light (pilot light) are input to the receiving device 270 via a transmission path 201 such as an optical fiber. The receiving device 270 amplifies the signal light by PSA using the pump light and idler light for both polarizations.

[0114] <Configuration of Transmitter> The transmitter 210 includes an optical signal generator 211, an optical phase conjugator (idler light generator) 220, an optical power adjuster 250, and an optical amplifier 260. The configuration of the optical phase conjugator is different from that of the first embodiment.

[0115] The optical phase conjugator (idler light generator) 220 includes a pump light generator 221, a polarization splitter 230, a first multiplexer 231, a second multiplexer 232, a first second-order nonlinear optical element (PPLN-1) 234, a fourth second-order nonlinear optical element (PPLN-4) 235, and a polarization combiner 236. It also includes a polarized light detector 238, a polarization phase adjuster 239, and a pilot light error detector.

[0116] The pumping light generating section 221 includes a pumping light source 222, a pumping light branching section 223, one pumping light path, and the other pumping light path.

[0117] The fundamental wave pumping light (pumping light) output from the pumping light source 222 is continuous light having one polarization component (single polarization), and is split by the pumping light splitter 223, with one pumping light input to one pumping light path and the other pumping light input to the other pumping light path.

[0118] One pumping light path includes a fiber extender 224 , an optical fiber amplifier (for example, EDFA) 225 , and a bandpass filter (BPF) 226 , and outputs the pumping light to a first multiplexer 231 .

[0119] The other pumping light path includes a phase modulator 227 , an optical fiber amplifier (for example, EDFA) 228 , and a bandpass filter (BPF) 229 , and outputs the pumping light to a second multiplexer 232 .

[0120] The polarization splitter 230 splits the optical signal into two orthogonal polarization components.

[0121] The first multiplexer 231 multiplexes the signal light of one of the polarization components split by the polarization splitter 230 with one of the pump lights from the pump light generator 221. After multiplexing, the one of the pump lights has the same polarization component (one of the polarization components) as the signal light.

[0122] The second multiplexer multiplexes the signal light of the other polarization component split by the polarization splitter 230 with the other pump light from the pump light generator 221. After multiplexing, the other pump light has the same polarization component (the other polarization component) as the signal light.

[0123] The first second-order nonlinear optical element 234 converts one of the pump lights into a first second harmonic, uses the first second harmonic to generate a first idler light that is a phase conjugate of the signal light of one of the polarization components, and outputs the one of the pump lights that has not been converted into the first second harmonic as a first pilot light.

[0124] The fourth second-order nonlinear optical element 235 converts the other pump light into a second second harmonic, uses the second second harmonic to generate second idler light that is a phase conjugate of the signal light of the other polarization component, and outputs the other pump light that has not been converted into the second second harmonic as second pilot light.

[0125] In PPLN-1 (234) and PPLN-4 (235), a cascade process of OPA occurs due to the interaction between the fundamental pump light and the SH light generated by the SHG process and the signal light, which generates idler light.

[0126] PPLN-1 (234) and PPLN-4 (235) have an optical system on the input side that couples light in the signal wavelength band (signal light, idler light) to a PPLN waveguide, and an optical system on the output side that splits light in the signal wavelength band (signal light, idler light, pump light) from light in the double-wave pump light band and couples them to output ports, respectively.

[0127] The polarization combiner 236 combines the optical signals from the first second-order nonlinear optical element 234 and the fourth second-order nonlinear optical element 235 with the orthogonal polarization components of the idler light and the pilot light.

[0128] The output of the polarization combiner 236 is branched into two paths. Signal light, idler light, and pump light (pilot light) are output to one path. Signal light, idler light, and part of the pump light (pilot light) are output to the other path as detection light. The other path includes a polarized light detector 238 and a phase-locked loop (PLL) 243 that constitutes a polarization phase adjuster 239.

[0129] The polarized light detecting section 238 is composed of a band pass filter (BPF) 240 , a polarizer (POL) 241 , and a photodetector (PD) 242 .

[0130] The polarization phase adjustment unit is composed of a phase-locked loop (PLL) 243 and a fiber extender 224 arranged on one pumping light path of the pumping light generation unit 221. This allows the phases of the polarization components of the pilot light to be matched (described in detail later).

[0131] Also, the pilot light error detection unit is composed of a PLL 243 and a phase modulator 227 arranged on the other pump light path of the pump light generating unit 221. This allows adjustment so that the pilot light component (intensity) is maximized (details will be described later).

[0132] <Configuration of Receiving Device> The receiving device 270 includes an optical receiving section 280 and a phase sensitive amplifier (PSA) section 290. The configuration of the phase sensitive amplifier (PSA) section differs from that of the first embodiment.

[0133] The phase-sensitive amplifier (PSA) unit 290 includes a wavelength separation unit 291, a pump light regeneration unit 292, a pump light branching unit 299, a polarization splitting unit 293, a first polarization amplifier unit 294, a second polarization amplifier unit 296, and a polarization combining unit 298.

[0134] The wavelength demultiplexing unit 291 and the pump light regenerating unit 292 are the same as those in the first embodiment.

[0135] The pumping light branching unit 299 branches the local pumping light (second pumping light), which is continuous light from the pumping light regenerating unit 292, into one local pumping light and the other local pumping light.

[0136] The polarization splitter 293 splits the optical signal and the idler light into two orthogonal polarization components.

[0137] The first polarization amplifier 294 includes a second second-order nonlinear optical element 2941, a third second-order nonlinear optical element 2942, a first detector 2948, a first phase adjuster 2946, and a first signal light error detector. It also includes a phase modulator 2943, an EDFA 2944, and a BPF 2945 in a stage preceding the second second-order nonlinear optical element 2941. The second second-order nonlinear optical element 2941 and the third second-order nonlinear optical element 2942 are a second PPLN module (PPLN-2) and a third PPLN module (PPLN-3), respectively.

[0138] The second polarization amplifier 296 includes a fifth second-order nonlinear optical element 2961, a sixth second-order nonlinear optical element 2962, a second detector 2968, a second phase adjuster 2966, and a second signal light error detector. The second polarization amplifier 2966 also includes a phase modulator 2963, an EDFA 2964, and a BPF 2965 in a stage preceding the fifth second-order nonlinear optical element 2961. The fifth second-order nonlinear optical element 2961 and the sixth second-order nonlinear optical element 2962 are a fifth PPLN module (PPLN-5) and a sixth PPLN module (PPLN-6), respectively.

[0139] PPLN-2 (2941) and PPLN-5 (2961) have an optical system on the input side that couples light in the signal wavelength band (locally generated pump light) from the pump light regeneration mechanism to a PPLN waveguide, and an optical system on the output side that separates the light in the signal wavelength band from light in the double-wave pump light band (second harmonic light, SH light) and couples only the light in the double-wave pump light band to the output port.

[0140] PPLN-3 (2942) and PPLN-6 (2962) have an optical system on the input side that combines light in the signal wavelength band (signal light, idler light) with light in the double-wave excitation light band and couples it to a PPLN waveguide, and on the output side that separates light in the signal wavelength band (signal light, idler light, local excitation light) from light in the double-wave excitation light band and couples them to the output ports, respectively.

[0141] The first polarization amplifier 294 and the second polarization amplifier 296 each have the same configuration as in the first embodiment.

[0142] In the first polarization amplifier 294, one of the local pump light beams branched by the pump light branching unit is input to PPLN-2 (2941), the signal light beam and idler light beam of one of the polarization components split by the polarization splitting unit, and the SH light beam from PPLN-2 (2941) are input to PPLN-3 (2942), and the signal light beam of one of the polarization components is amplified by the PSA. The first phase adjuster 2946 includes a PLL 2949 and a fiber extender 2950, ​​and synchronizes the phase of the signal light beam and the phase of the SH light beam based on the error signal from the first signal light error detector so that the signal light component (intensity) is maximized.

[0143] In the second polarization amplifier 296, the other local pump light branched by the pump light branching unit is input to PPLN-5 (2961), the other polarization component signal light and idler light split by the polarization splitting unit, and the SH light from PPLN-5 (2961) are input to PPLN-6 (2962), and the other polarization component signal light is amplified by the PSA. The second phase adjuster 2966 includes a PLL 2969 and a fiber extender 2970, and synchronizes the phase of the signal light and the phase of the SH light based on the error signal from the second signal light error detector so that the signal light component (intensity) is maximized.

[0144] The polarization combiner 298 combines the optical signals from the third second-order nonlinear optical element 2942 and the sixth second-order nonlinear optical element 2962 with the orthogonal polarization components of the idler light and the local pump light.

[0145] <Operation of Optical Transmission System and Transmitting Apparatus> The operation of the optical transmission system and transmitting apparatus according to this embodiment will be described with reference to Figures 5A to 5C. The basic operation is the same as in the first embodiment.

[0146] The polarization-multiplexed signal is separated by polarization using PBS 230 in the idler generation section, and then idler light is generated in each arm via PPLN-1 (234) and PPLN-2 (2941). At this time, similar to equation (6), the reference phases of the signal light and idler light in each arm are synchronized with the phase of the fundamental pump light, as shown in equations (8) and (9) (Figure 5A).

[0147]

[0148]

[0149] The idler light having each polarization component generated by the PPLN-1 (234) and the PPLN-4 (235) is multiplexed with the signal light having each polarization component by the PBC 236 and output from the transmitter.

[0150] When these polarization-multiplexed signal light and idler light enter the PSA unit 290, they are demultiplexed into each polarization component by the PBS. Since the polarization plane of the polarization-multiplexed signal rotates randomly to some extent, the signal separated by the PBS 293 of the PSA unit 290 contains a mixture of components from each arm in the idler light generating unit 220 (FIG. 5B). When amplifying with the PSA, by matching the phase of the pump light supplied to the PPLN-3 (2942) or PPLN-6 (2962) via the PPLN-2 (2941) or PPLN-5 (2961) with the reference phase of the signal light and idler light, phase-sensitive amplification is achieved in which the phases of the signal light and idler light coherently overlap. However, as shown in FIG. 5B, the reference phase (φ ｐ、ＦＨ、ｘ and φ ｐ、ＦＨ、ｙ ) do not match, even if the phase of one polarization component matches, the phase of the other polarization component does not match.

[0151] Therefore, in the transmitter 210, the idler light generating unit 220 generates φ ｐ、ＦＨ、ｘ and φ ｐ、ＦＨ、ｙ In order to match the phases of the polarization components of the pilot light, the components of the pilot light that have passed through each arm are extracted and detected at the stage subsequent to the PBC 236, and feedback control is performed on one arm of the polarization diversity so that the phases of the polarization components of the pilot light are matched.

[0152] More specifically, a polarized light detector 238 and a polarization phase adjuster 239 are provided on a path branched from the output of the polarization combiner 236 .

[0153] The polarized light detection unit 238 is composed of a bandpass filter (BPF) 240, a polarizer (POL) 241, and a photodetector (PD) 242. The polarization phase adjustment unit 239 is composed of a phase-locked loop (PLL) 243 and a fiber extender 224 arranged on one of the pumping light paths of the pumping light generation unit 221.

[0154] In this configuration, after the PBC 236, an optical tap 237 branches the signal light, idler light, and pilot light, and a BPF 240 extracts the pilot light.

[0155] Since the pilot light is continuous wave light, if the phases of the polarized components match when the polarized components are multiplexed by the PBC 236, the pilot light becomes linearly polarized light.

[0156] On the other hand, if the phase of each polarization component of the pilot light fluctuates (difference occurs) in each arm, the pilot light will deviate (shift) from linear polarization. Therefore, by arranging a polarizer (POL) 241 that extracts only the linear polarization component after the BPF 240, it is possible to check whether the polarization state of the pilot light, i.e., whether the phase of the pilot light in each arm is consistent, can be confirmed.

[0157] Therefore, the PLL 243 transmits a feedback signal to the fiber extender 224 to perform feedback control so that the component of the pilot light that passes through the polarizer (POL) 241 and is received by the PD 242 becomes maximum.

[0158] This allows the phases of the polarization components of the pilot light to be matched.

[0159] Furthermore, the pilot light error detection unit is composed of a PLL 243 and a phase modulator 227 arranged on the other pump light path of the pump light generation unit 221. A dither signal is applied to the phase modulator 227 for demodulation, thereby generating an error signal indicating the deviation (error) from the maximum value of the pilot light component (intensity) after passing through the polarizer (POL) 241, i.e., the linearly polarized component extracted by the polarizer 241. Based on this error signal, the fiber extender 224 is feedback-controlled via the PLL 243 to synchronize the pilot light component (intensity) so that it is maximized.

[0160] This allows adjustment so that the pilot light component (intensity) is maximized.

[0161] As described above, the idler light generating unit 220 of the transmitter 210 is provided with a feed control mechanism for synchronizing the pilot light of each arm, thereby reducing the time required for φｐ、ＦＨ、ｘ and φ ｐ、ＦＨ、ｙ can be matched, and the polarization multiplexed signal can be well amplified by the PSA of the receiving device 270. Furthermore, in this configuration, the reference phases of the signal light and idler light are synchronized with the phase of the pilot light, so there is no fluctuation in the power of the pilot light due to four-wave mixing.

[0162] According to this embodiment, it is possible to provide a transmitter that can generate pilot light with reduced fluctuations in optical intensity for both polarized signals, thereby providing an optical transmission system that can stably amplify optical signals by phase-sensitive amplification (PSA).

[0163] In the embodiments of the present invention, examples of the structure, dimensions, materials, etc. of each component in the configuration and manufacturing method of the optical transmission system and transmitter are shown, but the present invention is not limited to these examples. Anything that can demonstrate the functions and effects of the optical transmission system and transmitter can be used.

[0164] It should be noted that the present invention is not limited to the above-described embodiments, and it is clear that many modifications and combinations can be made by a person having ordinary knowledge in the art within the technical concept of the present invention.

[0165] A part or all of the above-described embodiment or an example thereof can be described as, but is not limited to, the following supplementary notes.

[0166] (Supplementary Note 1) An optical transmission system comprising: an optical signal generation unit that generates signal light; a pump light generation unit that generates pump light; a second-order nonlinear optical element that receives the signal light and the pump light, generates idler light by optical parametric amplification, and outputs the pump light as pilot light together with the signal light and the idler light; an optical power adjustment unit that adjusts the power of the signal light, the power of the idler light, and the power of the pilot light; an optical amplification unit that amplifies the signal light, the idler light, and the pilot light; an optical transmission unit that transmits the signal light, the idler light, and the pilot light; and a phase-sensitive amplification unit that performs phase-sensitive amplification of the signal light and the idler light by optical parametric amplification using second pump light generated based on the pilot light.

[0167] (Supplementary Note 2) The optical transmission system according to Supplementary Note 1, wherein the optical power adjusting unit adjusts the power of the pilot light to half or less of the total power of the signal light and the idler light.

[0168] (Supplementary Note 3) The optical transmission system according to Supplementary Note 1 or Supplementary Note 2, wherein the optical power adjuster adjusts the power of the signal light and the power of the idler light to be approximately equal.

[0169] (Appendix 4) An optical transmission system described in Appendix 1 or Appendix 2, wherein the pumping light generating unit outputs the pumping light having an optical power set so that the power of the signal light and the power of the idler light are approximately equal.

[0170] (Supplementary Note 5) A polarization splitter, another second-order nonlinear optical element, and a polarization combiner, wherein the polarization splitter splits the signal light into two orthogonal polarization components, the pump light generator generates two pump lights, the signal light having one of the two polarization components and one of the two pump lights are input to the second-order nonlinear optical element, the second-order nonlinear optical element generates one idler light by optical parametric amplification, outputs the one pump light as one pilot light together with the signal light having the one polarization component and the one idler light, and outputs the signal light having the other polarization component of the two polarization components and the other pump light to the other second-order nonlinear optical element. the other second-order nonlinear optical element generates the other pump light by optical parametric amplification and outputs the other pump light as the other pilot light together with the signal light having the other polarization component and the other idler light; the polarization combiner combines the signal light of the one polarization component, the signal light of the other polarization component, the one idler light, the other idler light, the one pilot light, and the other pilot light and outputs the combined light; and the phase-sensitive amplifier performs phase-sensitive amplification of the signal light of the one polarization component and the one idler light, and phase-sensitive amplification of the signal light of the other polarization component and the other idler light.

[0171] (Supplementary Note 6) The optical transmission system according to Supplementary Note 5, comprising: a bandpass filter that receives as input a portion of the combined light branched from the output of the polarization combiner and extracts both the one pilot light and the other pilot light from the combined light; a polarizer that extracts linearly polarized components from both pilot lights; a photodetector that detects the light intensity of the linearly polarized component; a phase-locked loop that receives as input the light intensity of the linearly polarized component and outputs a feedback signal; and a fiber extender that receives as input the feedback signal, wherein the fiber extender is controlled so that the light intensity of the linearly polarized component is maximized.

[0172] (Supplementary Note 7) The optical transmission system according to Supplementary Note 6, further comprising a phase modulator that phase-modulates the one of the pump lights, and applies a dither signal to the phase modulator to demodulate the phase modulator, thereby generating an error signal that indicates a deviation of the optical intensity of the linearly polarized component from the maximum value, and feedback-controlling the fiber extender via the phase-locked loop.

[0173] (Supplementary Note 8) A transmitting device used in an optical transmission system that performs phase-sensitive amplification of signal light and idler light by optical parametric amplification using pump light, the transmitting device comprising: an optical signal generating unit that generates the signal light; a pump light generating unit that generates pump light; a second-order nonlinear optical element that receives the signal light and the pump light, generates idler light by optical parametric amplification, and outputs the pump light as pilot light together with the signal light and the pump light; an optical power adjusting unit that makes the power of the signal light and the power of the idler light approximately equal and adjusts the power of the pilot light to half or less of the total power of the signal light and the idler light; and an optical amplifying unit that amplifies the signal light, the idler light, and the pilot light.

[0174] (Supplementary Note 9) A polarization splitter, another second-order nonlinear optical element, and a polarization combiner, wherein the polarization splitter splits the signal light into two orthogonal polarization components, the pump light generator generates two pump lights, the signal light having one of the two polarization components and one of the two pump lights are input to the second-order nonlinear optical element, the second-order nonlinear optical element generates one idler light by optical parametric amplification, and outputs the one pump light as one pilot light together with the signal light having the one polarization component and the one idler light, the other second-order nonlinear optical element generates the other idler light by optical parametric amplification and outputs the other pump light together with the signal light having the other polarization component and the other idler light as the other pilot light; and the polarization combining unit combines the signal light of the one polarization component, the signal light of the other polarization component, the one pilot light, and the other pilot light.

[0175] (Supplementary Note 10) The transmitting device according to Supplementary Note 9, comprising: an optical monitoring unit that branches a portion of the one pilot light and the other pilot light to measure optical intensity; a phase error detection unit that detects a phase error between the one pilot light and the other pilot light; and a phase adjustment unit that performs feedback control based on the phase error to synchronize a phase of the one pilot light and a phase of the other pilot light.

[0176] (Supplementary Note 11) The optical transmission system described in Supplementary Note 5 or Supplementary Note 6, wherein the optical power adjustment unit adjusts the total power of the one pilot light and the other pilot light to less than half of the total power of the signal light of the one polarization component, the signal light of the other polarization component, the one idler light, and the other idler light.

[0177] (Appendix 12) An optical transmission system described in any one of Appendices 5, 6, and 11, wherein the optical power adjustment unit adjusts the total power of the signal light of the one polarization component and the signal light of the other polarization component and the total optical power of the one idler light and the other idler to be approximately equal.

[0178] (Appendix 13) An optical transmission system described in any one of Appendices 5, 6, and 11, wherein the pumping light generating unit outputs the pumping light having an optical power set so that the total power of the signal light of the one polarization component and the signal light of the other polarization component is approximately equal to the total optical power of the one idler light and the other idler.

[0179] (Supplementary Note 14) The transmitting device according to Supplementary Note 7 or Supplementary Note 8, wherein the optical power adjusting unit adjusts the power of the pilot light to half or less of the total power of the signal light and the idler light.

[0180] (Supplementary Note 15) The transmitting device according to any one of Supplementary Notes 7, 8, and 14, wherein the optical power adjusting unit adjusts the power of the signal light and the power of the idler light to be approximately equal.

[0181] (Supplementary Note 16) A transmitting device described in any one of Supplementary Notes 7, 8, and 14, wherein the pumping light generating unit outputs the pumping light having an optical power set so as to make the power of the signal light and the power of the idler light approximately equal.

[0182] (Supplementary Note 17) A transmitting device described in Supplementary Note 9 or Supplementary Note 10, wherein the optical power adjustment unit adjusts the total power of the one pilot light and the other pilot light to less than half of the total power of the signal light of the one polarization component, the signal light of the other polarization component, the one idler light, and the other idler light.

[0183] (Appendix 18) A transmitting device described in any one of Appendices 9, 10, and 17, wherein the optical power adjustment unit adjusts the total power of the signal light of the one polarization component and the signal light of the other polarization component and the total optical power of the one idler light and the other idler light to be approximately equal.

[0184] (Supplementary Note 19) A transmitting device described in any one of Supplementary Notes 9, 10, and 17, wherein the pumping light generating unit outputs the pumping light having an optical power set so that the total power of the signal light of the one polarization component and the signal light of the other polarization component is approximately equal to the total optical power of the one idler light and the other idler light.

[0185] The present invention can be applied to an optical transmission system and a transmitting device.

[0186] 10 Optical transmission system 101 Optical transmission section 111 Optical signal generation section 121 Pump light generation section 126 Second-order nonlinear optical element 130 Optical power adjustment section 140 Optical amplification section 170 Phase sensitive amplification section

Claims

1. An optical transmission system comprising: an optical signal generation unit that generates signal light; a pump light generation unit that generates pump light; a second-order nonlinear optical element that receives the signal light and the pump light, generates idler light by optical parametric amplification, and outputs the pump light as pilot light together with the signal light and the idler light; an optical power adjustment unit that adjusts the power of the signal light, the power of the idler light, and the power of the pilot light; an optical amplification unit that amplifies the signal light, the idler light, and the pilot light; an optical transmission unit that transmits the signal light, the idler light, and the pilot light; and a phase-sensitive amplification unit that performs phase-sensitive amplification of the signal light and the idler light by optical parametric amplification using second pump light generated based on the pilot light.

2. The optical transmission system according to claim 1, wherein said optical power adjusting section adjusts the power of said pilot light to half or less of the total power of said signal light and said idler light.

3. An optical transmission system according to claim 1 or 2, wherein said optical power adjusting section adjusts the power of said signal light and the power of said idler light to be approximately equal.

4. An optical transmission system according to claim 1 or 2, wherein said pumping light generating section outputs said pumping light having an optical power set so that the power of said signal light and the power of said idler light are substantially equal.

5. A polarization splitter, another second-order nonlinear optical element, and a polarization combiner, wherein the polarization splitter splits the signal light into two orthogonal polarization components, the pump light generator generates two pump lights, the signal light having one of the two polarization components and one of the two pump lights are input to the second-order nonlinear optical element, the second-order nonlinear optical element generates one idler light by optical parametric amplification, and outputs the one pump light as one pilot light together with the signal light having the one polarization component and the one idler light, 2. The optical transmission system according to claim 1, wherein the signal light having the other polarization component of the two polarization components and the other pump light of the two pump lights are input to the other second-order nonlinear optical element, the other second-order nonlinear optical element generates the other idler light by optical parametric amplification, and outputs the other pump light as the other pilot light together with the signal light having the other polarization component and the other idler light; the polarization combining unit combines the signal light of the one polarization component, the signal light of the other polarization component, the one idler light, the other idler light, the one pilot light, and the other pilot light, and outputs the combined light; and the phase-sensitive amplifier unit performs phase-sensitive amplification of the signal light of the one polarization component and the one idler light, and phase-sensitive amplification of the signal light of the other polarization component and the other idler light.

6. The optical transmission system according to claim 5, comprising: a bandpass filter that receives a portion of the combined light branched from the output of the polarization combiner and extracts both the one pilot light and the other pilot light from the combined light; a polarizer that extracts linearly polarized components from both pilot lights; a photodetector that detects the light intensity of the linearly polarized component; a phase-locked loop that receives the light intensity of the linearly polarized component and outputs a feedback signal; and a fiber extender that receives the feedback signal, wherein the fiber extender is controlled so that the light intensity of the linearly polarized component is maximized.

7. An optical transmission system according to claim 6, further comprising a phase modulator that phase-modulates the one of the pump lights, and applying a dither signal to the phase modulator to demodulate the phase modulator, thereby generating an error signal that indicates the deviation of the optical intensity of the linearly polarized component from its maximum value, and feedback-controlling the fiber stretcher via the phase-locked loop.

8. A transmitting device used in an optical transmission system that performs phase-sensitive amplification of signal light and idler light by optical parametric amplification using pump light, comprising: an optical signal generating unit that generates the signal light; a pump light generating unit that generates pump light; a second-order nonlinear optical element that receives the signal light and the pump light, generates idler light by optical parametric amplification, and outputs the pump light as pilot light together with the signal light and the pump light; an optical power adjusting unit that makes the power of the signal light and the power of the idler light approximately equal, and adjusts the power of the pilot light to less than half the total power of the signal light and the idler light; and an optical amplifying unit that amplifies the signal light, the idler light, and the pilot light.

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