JP2014089254A - Phase sensitive optical amplification apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a phase sensitive optical amplification apparatus that is applicable to optical communication, allows amplification with low noise, and has higher amplification factor than the conventional art.SOLUTION: The phase sensitive optical amplification apparatus comprises: an optical fiber laser amplifier; a first secondary nonlinear optical element including an optical waveguide for generating excitation light from fundamental wave light; a filter for separating only the excitation light from the fundamental wave light and the excitation light; a multiplexer for multiplexing signal light and the excitation light; a second secondary nonlinear optical element including an optical waveguide for performing parametric amplification of the signal light using the excitation light; a filter for separating the signal light and the excitation light; phase synchronization means for synchronizing phases of the signal light and the excitation light; means for branching a part of the excitation light; and a balanced photodetector for detecting the excitation light outputted from the second secondary nonlinear optical element and the part of the branched excitation light. The phase synchronization means includes: phase modulation means; and a phase synchronization loop circuit for performing feedback based on a signal strength change detected by the balanced photodetector.

Description

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

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

  As amplifying means for solving this problem, there are fiber laser amplifiers and semiconductor laser amplifiers that amplify signal light by making excitation light incident on an optical fiber doped with rare earth elements such as erbium and praseodymium. Since the fiber laser amplifier and the semiconductor laser amplifier can amplify the signal light as it is, there is no limitation on the electrical processing speed that has been a problem in the identification reproduction optical repeater. In addition, there is an advantage that the device configuration is relatively simple. However, these laser amplifiers do not have a function of shaping a deteriorated signal light pulse waveform. Further, in these laser amplifiers, unavoidably and randomly generated spontaneous emission light is mixed regardless of the signal component, so that the S / N ratio of the signal light is reduced by at least 3 dB before and after amplification. These lead to an increase in transmission code error rate at the time of digital signal transmission, which is a factor of reducing transmission quality.

  As means for overcoming the limitations of the conventional laser amplifier, a phase sensitive amplifier (Phase Sensitive Amplifier: PSA) has been studied. This phase sensitive optical amplifying device has a function for shaping a signal light pulse waveform deteriorated due to the influence of dispersion of the transmission fiber. In addition, since spontaneous emission light having a quadrature phase irrelevant to the signal can be suppressed, it is possible in principle to keep the signal light SN ratio before and after amplification without deterioration.

J. A. Levenson, I. Abram, T. Rivera, and P. Grainger, “Reduction of quantum noise in optical parametric amplification,” J. Opt. Soc. Am. B, vol. 10, pp. 2233-2238 (1993). W. Imajuku, and A. Takada, “Gain characteristics of coherent optical amplifiers using a Mach-Zehnder interferometer with Kerr Media,” IEEE J. Quantum Electron., Vol. 35, no. 11, pp. 1657-1665 (1999) . R. Slavik et al., “All-optical phase and amplitude regenerator for next-generation telecommunications system,” Nature Photonics., Vol. 4, pp. 690-695 (2010). T. Umeki, O. Tadanaga, and M. Asobe, “Highly efficient wavelength converter using direct-bonded PPZnLN ridge waveguide,” IEEE J. Quantum Electron., Vol. 46, no. 8, pp. 1206-1213 (2010) . R. Slavik et al., “All-optical phase-regenerative multicasting of 40 Gbit / s DPSK signal in a degenerate phase sensitive amplifier,” In Proceedings of the European Conference and Exhibition on Optical Communication (ECOC 2010, Torino, Italy) MO .1.A.2.

Japanese Patent Application No. 2011-009894

  However, the above-described prior art has the following problems.

  FIG. 1 shows a basic configuration of a conventional phase sensitive optical amplifying device. This optical amplifier includes a phase sensitive light amplification unit 101, a pumping light source 102, a pumping light phase control unit 103, and two light branching units 104-1 and 104-2. In this optical amplifier, the input signal light 110 is amplified when the phase of the signal light and the pumping light in the phase sensitive light amplification unit 101 satisfies a specific relationship described later, and the phase of both is shifted by 90 degrees from the specific relationship described later. When the quadrature phase relationship is established, the input signal light 110 has a characteristic of attenuating. Using this characteristic, the phase between the pumping light and the signal light is controlled and synchronized so that the amplification gain is maximized. When the phase is synchronized, spontaneous emission light that is orthogonal to the signal light is not generated, that is, the SN ratio is degraded. It is possible to amplify the signal light without doing so.

  In order to achieve phase synchronization between the signal light and the excitation light, the phase of the excitation light 111 is controlled so as to be synchronized with the phase of the input signal light 110 branched by the optical branching unit 104-1. The pumping light phase control unit 103 detects a part of the output signal light 112 branched by the light branching unit 104-2 with a narrow-band detector, and controls the phase of the pumping light 111 so that the output signal becomes maximum. To do. As a result, the phase sensitive light amplifying unit 101 is controlled so that the phase of the signal light and the phase of the pumping light are synchronized with each other, and it is possible to realize optical amplification without degradation of the SN ratio. The pumping light phase control unit 103 may be configured to directly control the phase of the pumping light source 102 in addition to the configuration of controlling the phase of the pumping light on the output side of the pumping light source 102 as shown in FIG. When the light source that generates the signal light is arranged near the phase sensitive light amplification unit, a part of the light source for signal light can be branched and used as excitation light.

  A medium having a second-order or third-order nonlinear optical effect is used for the phase sensitive amplification unit. Conventionally, these phase-sensitive optical amplifiers have been used mainly in basic research fields such as squeezing for controlling the quantum state of light. However, with the recent advancement of optical communication, the application of phase sensitive optical amplifying devices to optical communication is attracting attention.

  When the third-order nonlinear optical effect is used, an optical fiber or the like is used as the nonlinear medium, and as shown in Non-Patent Document 2, one excitation light having the same wavelength as the signal light is used, and the excitation light and the signal light are combined. Phase-sensitive amplification can be achieved by entering a nonlinear medium and performing degenerate parametric amplification using four-wave mixing. However, in a phase sensitive optical amplifier using an optical fiber, although a configuration applicable to an optical communication system is shown, a configuration in which the wavelengths of signal light and pumping light are close to each other because four-wave mixing is used. It has become. As shown in Non-Patent Document 3, in many cases, a configuration for obtaining necessary power by an optical fiber laser amplifier such as an erbium-doped fiber laser amplifier (EDFA) is selected so that the nonlinear optical effect in the optical fiber can be used. However, when optical amplification is performed with the EDFA, amplified spontaneous emission light (ASE light) is superimposed on excitation light as noise. Here, since the wavelength of the excitation light and the wavelength of the signal light are close to each other, it is difficult to remove the ASE light, and the ASE light generated from the EDFA is also superimposed on the signal light wavelength. As a result, the S / N ratio of the signal light deteriorates, and there is a disadvantage that optical amplification with low noise cannot be performed.

  In the conventional phase-sensitive optical amplifying device using the second-order nonlinear optical effect, only a configuration in which operation is performed using a pulse laser light source having a high output enough to cause SHG or parametric amplification is shown. A configuration that can be applied to an optical communication system that generally handles weak light has not been proposed until recently.

  However, recently, Patent Document 1 discloses a phase sensitive optical amplifying apparatus having a configuration that can be applied to an optical communication system that handles generally weak light using a second-order nonlinear optical effect. was suggested. In the invention proposed in Patent Document 1, the phase sensitive optical amplifying device can be configured without superimposing the ASE light generated along with the optical amplification on the signal light, thereby preventing the SN ratio from deteriorating. However, there is an advantage that high-quality optical signal amplification becomes possible.

  When using the second-order nonlinear optical effect, as shown in Non-Patent Document 1 and Patent Document 1, an optical crystal or the like is used as a nonlinear medium, and a wavelength corresponding to the second harmonic of the signal light is used as excitation light. In this case, pumping light and signal light are incident on a nonlinear medium, and phase sensitive amplification is achieved by performing degenerate parametric amplification (OPA) using three-wave mixing. As shown in FIG. 2, the most basic configuration in this case branches the laser light from the laser light source 201, one is incident on a SHG (Second Harmonic Generation) crystal 202, and the other is used as the signal light 210. The excitation light 211 and the signal light 210 converted to the second harmonic are incident on the nonlinear optical crystal 203 capable of degenerate optical parametric amplification, and phase sensitive amplification is performed.

In the phase sensitive optical amplifying device, in order to amplify only the light in phase with the signal, it is necessary that the phase of the signal light and the excitation light coincide with each other or be shifted by π radians as described above. That is, when the second-order nonlinear optical effect is used, it is necessary that the phase φ2ωs of the excitation light, which is a wavelength corresponding to the second harmonic, and the phase φωs of the signal light satisfy the following relationship (Equation 1). Become.
Δφ = 1 / 2φ2ωs−φωs = nπ (where n is an integer) (Formula 1)
FIG. 2 is a diagram illustrating a configuration of a phase sensitive optical amplifying device using a second-order nonlinear optical effect. FIG. 3 is a graph showing the relationship between the phase difference Δφ between the signal light and the pump light and the signal light gain (dB) in the conventional phase sensitive optical amplifying device using the second-order nonlinear optical effect. It can be seen that the gain is maximum when Δφ is −π, 0, or π.

  Also in the configuration shown in FIG. 2, as shown in FIG. 1, a part of the output signal light is branched and detected by a narrow-band detector, and the phase of the excitation light is controlled so that the output signal becomes maximum. Thus, phase synchronization between the signal light and the pumping light can be achieved.

  However, the problem with the configuration shown in FIG. 1 is that a part of the signal light 112 output in order to control the phase of the pumping light must be branched and detected. For this reason, all of the amplified light cannot be used, and as a result, there is a problem that the amplification efficiency of the optical amplifying device is lowered. As a solution to this, it is desired to realize means for performing phase synchronization without using amplified signal light, but a specific configuration and method for stably performing phase synchronization have not yet been disclosed.

  In view of the above-described problems of the prior art, the object of the present invention is a phase-sensitive amplification that is applicable to optical communication and that can be amplified with low noise and has a higher amplification factor than before. Is to provide a device.

  The present invention relates to a phase-sensitive optical amplifying device that amplifies signal light by optical mixing using a nonlinear optical effect, and includes an optical fiber laser amplifier that amplifies fundamental light and a second-order nonlinear optical that is periodically poled. A first second-order nonlinear optical element including an optical waveguide made of a material for generating excitation light from the fundamental light, a filter for separating only the excitation light from the fundamental light and the excitation light, and An optical waveguide for performing parametric amplification of the signal light using the excitation light, comprising a multiplexer that multiplexes the signal light and the excitation light, and a second-order nonlinear optical material that is periodically poled. A second second-order nonlinear optical element provided, a filter for separating the amplified signal light and the excitation light, phase synchronization means for synchronizing the phase of the signal light and the phase of the excitation light, Only the excitation light is separated from the fundamental wave light and the excitation light. Means for branching a part of the excitation light outputted from the filter, and a balance type for detecting the excitation light outputted from the second second-order nonlinear optical element and a part of the branched excitation light And a phase locked loop circuit for performing feedback based on a change in signal intensity detected by the balanced photodetector. .

  In one embodiment of the present invention, the phase-locking means includes a phase modulator that is the phase-modulating means, the phase-locked loop circuit, and an optical length expander, and the phase-locked loop circuit includes the phase-modulating circuit. And a return to the optical length expander.

  In one embodiment of the present invention, the phase synchronization means includes the phase-locked loop circuit and a semiconductor laser that generates light that is phase-synchronized with the pump light and modulates the phase by controlling the laser drive current. The phase-locked loop circuit is configured to perform feedback to the drive current of the semiconductor laser.

  In one embodiment of the present invention, the signal processing apparatus further includes means for inverting a signal detected by the balanced photodetector or a feedback signal of the phase locked loop circuit.

  In one embodiment of the present invention, the filter that separates the amplified signal light and the excitation light includes a dichroic mirror using a dielectric film, an optical demultiplexing device using multimode interference, or a directional coupling. The optical demultiplexing element is used.

  According to the present invention, in a phase sensitive optical amplifier that amplifies only a specific phase of signal light using a parametric amplification effect that is a nonlinear optical effect, parametric light amplification is used from weak optical power used in optical communication. The phase sensitive optical amplifier can be configured without superimposing the ASE light generated along with the optical amplification on the signal light while using the optical fiber amplifier to obtain sufficient power for the SNR. It is possible to amplify high-quality optical signals while preventing deterioration.

  Furthermore, as a particularly excellent effect, phase control is performed using the second harmonic, which is excitation light that has not been used in conventional signal processing, instead of amplified signal light. There is no need to split a part of the signal light, and all the amplified signal light can be used, and the amplification factor of the optical amplifying device can be improved.

  As a result, the signal-to-noise ratio of the signal in the optical fiber can be improved by the phase-sensitive optical amplifier that can be applied to optical communication and can be amplified with low noise. It is possible to transmit up to a distance.

It is a figure which shows the structure of the conventional phase sensitive type | mold optical amplifier. It is a figure which shows the structure of the phase sensitive type | mold optical amplification apparatus using a 2nd order nonlinear optical effect. 5 is a graph showing a relationship between a signal light-excitation light phase difference Δφ and a signal light gain in a phase-sensitive optical amplification device using a second-order nonlinear optical effect. It is a figure which shows the basic composition of the phase sensitive type optical amplifier in this invention. 6 is a graph showing the relationship between the phase difference Δφ between signal light and pumping light and the second harmonic intensity in the phase sensitive optical amplifier according to the present invention. 5 is a graph showing a relationship between a phase difference Δφ between signal light and pumping light and the intensity of a phase modulation frequency component in the signal light in a phase sensitive optical amplifying apparatus that uses signal light for phase synchronization. The phase difference Δφ between the signal light and the excitation light and the phase modulation frequency at the second harmonic in the phase-sensitive optical intensifier that uses the second harmonic as the excitation light for phase synchronization and performs phase modulation on the fundamental light. It is a graph which shows the relationship with the intensity | strength of a 2nd harmonic component. The phase difference Δφ between the signal light and the excitation light and the phase modulation frequency at the second harmonic in the phase-sensitive optical intensifier that uses the second harmonic that is the excitation light for phase synchronization and performs phase modulation on the excitation light. It is a graph which shows the relationship with the intensity | strength of a component. It is a figure which shows the structure of the phase sensitive type optical amplifier which concerns on the 1st Embodiment of this invention. 1 is a diagram illustrating an applied configuration of a phase sensitive optical amplifying device according to a first embodiment of the present invention. It is a figure which shows the structure of the phase sensitive type | mold optical amplifier which concerns on the 2nd Embodiment of this invention. It is a figure which shows the structure of the phase sensitive type | mold optical amplifier which concerns on the 3rd Embodiment of this invention.

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

[Basic configuration]
FIG. 4 shows a basic configuration of the phase sensitive optical amplifying device of the present invention. This optical amplifying device includes a phase sensitive light amplifying unit 401, a pumping light source 402, a pumping light phase control unit 403, and two optical branching units (such as optical couplers) 404-1, 404-2. In this optical amplifier, when the phase of the signal light and the pumping light in the phase sensitive light amplification unit 401 satisfies the relationship of (Expression 1), the input signal light 410 is amplified, and the phase of both is 90 degrees from the relationship of (Expression 1). When the phase relationship is shifted, the input signal light 410 has a characteristic of attenuating. If the phase between the pumping light and the signal light is synchronized so that the amplification gain is maximized by using this characteristic, the spontaneous emission light having the quadrature phase with the signal light is not generated, that is, the SN ratio is not deteriorated. Signal light can be amplified. The difference between the present invention and the invention described in Patent Document 1 is a method of mainly achieving phase synchronization as described below.

  In order to achieve phase synchronization between the signal light and the pumping light, the phase of the input signal light 410 branched by the optical branching unit 404-1 and the phase of the pumping light 411 are synchronized while satisfying the relationship of (Equation 1). To control. Instead of partially branching the output signal light 412 at the optical branching unit 404-2, the second harmonic wave 413, which is the excitation light, is detected by a narrow-band detector so that the output signal of the second harmonic wave 413 is minimized. The pumping light phase controller 403 controls the phase of the pumping light 411. As a result, in the phase sensitive light amplification unit 401, the phase of the signal light and the phase of the pumping light are controlled so as to satisfy the relationship of (Equation 1) and are synchronized, thereby realizing optical amplification without degradation of the SN ratio. be able to. The pumping light phase control unit 403 may be configured to directly control the phase of the pumping light source 402 in addition to the configuration of controlling the phase of the pumping light on the output side of the pumping light source 402 as shown in FIG. When the light source that generates the signal light is arranged near the phase sensitive light amplification unit, a part of the light source for signal light can be branched and used as excitation light.

  FIG. 5 is a graph showing the relationship between the phase difference Δφ between the signal light and the excitation light and the light intensity of the second harmonic that is the excitation light in the phase sensitive optical amplifying device of the present invention. It can be seen that when Δφ is −π, 0, or π, the gain of the signal light by the parametric amplification is maximized, so that the light intensity of the second harmonic used for amplification is minimized.

  The above is the basic configuration of the phase sensitive optical amplifying device of the present invention. However, there are the following problems to realize this idea.

  The first problem is that the change in the light intensity of the second harmonic, which is the excitation light shown in FIG. 5, is very small. In the present invention, it is necessary to perform phase synchronization between the signal light and the second harmonic so that the light intensity of the second harmonic is minimized, but if this light intensity change is small, it is easily affected by noise, Difficult to maintain minimum strength. Therefore, phase synchronization can be obtained only for a very short time even if the second harmonic is simply received by a normal photodetector and the change in the light intensity is used. In order to increase the intensity change, the received light intensity can be amplified by an amplifier, but the received second harmonic has a large average output intensity as a baseline, so it will try to amplify until stable phase synchronization is obtained. Then, the input of a phase locked loop (PLL) circuit that receives and processes the amplified signal is saturated.

  The second problem is the change in gain of the fiber laser amplifier that amplifies the fundamental light. As described above, the second harmonic light intensity change is amplified to the extent that the input of the PLL circuit is not saturated. However, since it is difficult to obtain a sufficient light intensity change, it is amplified to near the saturation point. Fiber laser amplifiers are designed to maintain temperature and gain, but some gain changes due to the influence of the external environment such as room temperature. If you adjust to the limit of the saturation point, the input saturation occurs when the gain increases.On the other hand, if there is a margin from the saturation point, the light intensity change of the second harmonic will change more than desired when the gain decreases. It becomes considerably small and stable phase synchronization cannot be obtained.

  Therefore, in the present invention, in order to solve the problem that the light intensity change of the second harmonic is small and the average output intensity is large and the problem that the gain of the fiber laser amplifier fluctuates, the balance for detecting the difference between the two inputs is used. A second-order nonlinearity that uses optical detectors (Balanced Photodetector) to detect changes in the light intensity of the second harmonic, which is the excitation light, and performs optical parametric amplification as the other input for taking the difference A part of the second harmonic used for input to the optical element is branched and used. In this configuration, the intensity of the pumping light input to the second-order nonlinear optical element that performs optical parametric amplification decreases, but the fiber that amplifies the fundamental light so that the actual input intensity to the element becomes the value before branching. Since the problem can be solved by slightly increasing the gain of the laser amplifier or the optical amplifier for pumping light, there is almost no effect in practical use. In this configuration, since the balanced photodetector is a detector that takes a difference between two inputs, it is possible to subtract the average output intensity that is the baseline of the second harmonic. This difference detection converts the light intensity change of the second harmonic used to obtain the phase synchronization into an intensity change near the zero point, so that the weak light intensity change is sufficient until stable phase synchronization is obtained by the amplifier. Even if amplified, the input of the PLL circuit does not saturate. Since there is a margin until the input is saturated, even if the gain of the fiber laser amplifier changes, there is no significant effect. By using this configuration in this way, the above-described problems are solved, and stable phase synchronization can be obtained using the second harmonic.

  In phase sensitive amplification, it is necessary to synchronize the phases of the excitation light and the signal light. The characteristics of phase synchronization will be described in detail below. In phase synchronization, first, weak phase modulation is performed on the fundamental wave light or the excitation light by a phase modulator such as a phase modulator or laser drive current control. When phase modulation is performed on the fundamental wave light and a part of the amplified signal light is used for phase synchronization, when the phase modulation frequency component is detected by the phase comparator of the PLL circuit, the amplification gain of the signal light is as shown in FIG. 6 has a Δφ dependence, so that the graph of FIG. 6 is obtained. When Δφ is 0, the intensity of the modulation component is 0, and the intensity increases as the phase difference increases. In addition, since the sign is different due to the positive / negative phase difference, it can be determined on which side the phase is shifted. Based on this information, feedback is performed so that Δφ becomes 0 to realize phase synchronization. On the other hand, when phase modulation is applied to the fundamental wave light and the second harmonic wave used for amplification is used for phase synchronization, if the harmonic component of the phase modulation frequency is detected by the phase comparator of the PLL circuit, the graph of FIG. It becomes like this. As shown in FIG. 5, in the second harmonic, the Δφ dependency is opposite to that of the fundamental wave light. Therefore, the Δφ dependency of the harmonic component of the phase modulation frequency is opposite to that in the case of using the fundamental wave light. Become. When phase modulation is applied to the second harmonic and the second harmonic used for amplification is used for phase synchronization, it is necessary to detect the phase modulation frequency component instead of the harmonic, but the graph of FIG. As described above, the Δφ dependency has an opposite sign to that in the case of using the fundamental light.

[First Embodiment]
FIG. 9 shows the configuration of the phase sensitive optical amplifying apparatus 900 according to the first embodiment of the present invention. In the phase sensitive optical amplifying apparatus 900 of this embodiment, an amplification characteristic was evaluated when a 10 Gb / s NRZ signal was input using an LN Mach-Zehnder modulator as the data signal intensity modulator 901. In this embodiment, in order to obtain sufficient power for obtaining a nonlinear optical effect from weak laser light used for optical communication, a fundamental light is amplified using a fiber laser amplifier. The amplified fundamental wave light is incident on the first second-order nonlinear optical element to generate a second harmonic that is excitation light. Phase sensitive amplification is performed by making signal light and a second harmonic incident on the second second-order nonlinear optical element and performing degenerate parametric amplification.

  In this embodiment, in order to amplify 1.54 μm signal light, a part of the input signal light 920 is branched by an optical branching unit (such as an optical coupler) 921-1 and used as the fundamental wave light 920-1. The fundamental light 920-1 is amplified using an erbium-doped fiber laser amplifier (EDFA) 902. The amplified fundamental wave light 920-1 is input to the first second-order nonlinear optical element 903-1. In this embodiment, in order to prevent the broadband ASE light generated from the EDFA 902 from being converted by the first second-order nonlinear optical element 903-1, it is between the EDFA 902 and the first second-order nonlinear optical element 903-1. A band-pass filter 904 was inserted into the filter to cut unnecessary ASE light. The second-order nonlinear optical element 903 of this embodiment includes an optical waveguide (PPLN waveguide) 905 made of lithium niobate (PPLN) that is periodically poled. The PPLN waveguide 905 can utilize the highest nonlinear optical constant d33 of lithium niobate by quasi-phase matching and can obtain a high optical power density by the optical waveguide structure. By doing so, high wavelength conversion efficiency can be obtained. When high-intensity power is incident on the PPLN waveguide 905, the phase matching wavelength may change due to optical damage due to the photorefractive effect. In this embodiment, in order to prevent such a problem, Non-Patent Document 4 A waveguide manufactured by direct bonding shown in FIG.

  In this embodiment, the fluctuation of the phase matching wavelength is suppressed by using a direct junction waveguide using, as a core, lithium niobate doped with Zn having excellent optical damage resistance. Moreover, high wavelength conversion efficiency was realized by reducing the core diameter to about 4 μm by dry etching. The second harmonic wave 920-2 and the fundamental wave light 920-3, which are the excitation light, emitted from the first PPLN waveguide 905-1 are the first dichroic arranged in the first nonlinear optical element 903-1. Separated using mirror 906-1. The 0.77 μm second harmonic 920-2 reflected by the first dichroic mirror 906-1 passes through the polarization maintaining fiber 907 having a single mode propagation characteristic at this wavelength of 0.77 μm. The second nonlinear optical element 903-2 is led. At this time, excitation light and ASE light in the vicinity of a wavelength of 1.54 μm that could not be completely removed by the first dichroic mirror 906-1 are also incident on the polarization maintaining fiber 907. Since this fiber is weakly confined with respect to light having a wavelength of 1.54 μm, the unnecessary light can be effectively attenuated by propagating a length of about 1 m. The second harmonic 920-2 guided by the polarization maintaining fiber 907 is a signal light having a wavelength of 1.54 μm using the second dichroic mirror 906-2 disposed in the second nonlinear optical element 903-2. Is combined with 920-4. The second dichroic mirror 906-2 is emitted from the first PPLN waveguide 905-1 to reflect only the second harmonic 920-2, and the first dichroic mirror 906-1 and the polarization maintaining fiber 907 are reflected. It is possible to effectively remove the residual components of the fundamental wave light having a wavelength of about 1.54 μm and the ASE light passing through. The input signal light 920-4 and the second harmonic wave 920-2 that is the excitation light are combined and are incident on the second PPLN waveguide 905-2. The second PPLN waveguide 905-2 has the same performance and phase matching wavelength as the first PPLN waveguide 905-1, and phase-sensitively amplifies the input signal light 920-4 by degenerate parametric amplification. be able to. In the present embodiment, the two PPLN waveguides 905-1 and 905-2 are each controlled to have a constant temperature by an individual temperature controller. Due to manufacturing errors of the two PPLN waveguides 905-1 and 905-2, the phase matching wavelength may not match at the same temperature, but even in such a case, by controlling the temperatures of both separately, The phase matching wavelength can be matched. The light emitted from the second PPLN waveguide 905-2 is second harmonic wave 920-5 which is excitation light by the third dichroic mirror 906-3 disposed in the second nonlinear optical element 903-2. And amplified output signal light 920-6. At this time, since the second harmonic wave 920-5 and the amplified output signal light 920-6 have completely different wavelengths, the amplified output signal light 920-6 and the second harmonic wave 920-5 at the output are Effectively separated.

  In the present embodiment, unlike the invention described in Patent Document 1 in which a part of the amplified output signal light 920-6 is branched and used for phase synchronization, the second harmonic 920-5 that is excitation light is used. Phase synchronization. Using the phase modulator 908 arranged in front of the EDFA 902, weak phase modulation is applied to the fundamental wave light 920-1 by a sin wave. The second harmonic 920-5, which is the excitation light separated by the third dichroic mirror 906-3, was received by the balanced photodetector 909. Of the signals received by the balanced photodetector 909, one is the second harmonic 920-5 separated by the third dichroic mirror 906-3, and the other is multiplexed using the second dichroic mirror. The second harmonic 920-2 is branched by an optical branch (such as an optical coupler) 921-2. The input to the second nonlinear optical element 903-2, which was reduced by the optical branch 921-2, was compensated by increasing the gain of the EDFA 902. By operating the balanced photodetector 909 with appropriate settings, the average output intensity that is the baseline of the second harmonic 920-5 separated by the third dichroic mirror 906-3 is subtracted, and the second harmonic 920 is obtained. The weak light intensity change of −5 was converted into an intensity change near the zero point. The converted weak intensity change was amplified to an appropriate level by the voltage amplifier 910 and input to the PLL circuit 911. By detecting a phase shift with a PLL circuit and performing feedback (feedback) to the drive voltage of the PZT optical fiber stretcher 912 and the bias voltage of the phase modulator 908 arranged in front of the EDFA 901, vibrations of the optical fiber components Absorption of fluctuations in optical phase due to temperature fluctuations enables stable phase-sensitive amplification. As described above, the optical signal intensity corresponding to phase modulation depends on Δφ depending on whether the amplified output signal light 920-6 or the pumping light 920-2 used for amplification is used for phase synchronization. The operation of the PLL circuit 911 is different because of different characteristics. Therefore, the PLL circuit 911 may be adjusted depending on which is used for phase synchronization. However, if the signal received by the balanced photodetector 909 or the feedback signal of the PLL circuit 911 is inverted by the inverting circuit 913, the operation is the same as the other method, and the PLL circuit 911 needs to be changed. Absent. For this reason, if the structure which arrange | positions the inversion circuit 913 is employ | adopted, there exists a merit that the change from the method using the amplified output signal light 920-6 becomes easy. In the present embodiment, the inverting circuit 913 is disposed after the voltage amplifier 910. In the present embodiment, all of the amplified output signal light 920-6 can be utilized by synchronizing the phase so as to satisfy the relationship of (Equation 1) using the second harmonic 920-2 used for amplification. As a result, the gain of the phase sensitive optical amplifying device 900 is increased by about 0.7 dB as compared with the case where the amplified output signal light 920-6 is used as in the invention described in Patent Document 1. .

  Similarly to the invention described in Patent Document 1, in the state in which the phase is matched with the ON state of the signal light, even if the input signal includes the phase chirp, the chirp component is removed and there is no chirp. It can be shaped and amplified as a signal.

  In this embodiment, instead of the third dichroic mirror 1006-3, a filter that separates the second harmonic wave 1020-5 that is excitation light and the amplified output signal light 1020-6 is shown in FIG. As shown, an optical demultiplexing element (MMI optical demultiplexing element) 1006 using multi-mode interference (MMI) disposed downstream of the second PPLN waveguide 1005-2 can also be used.

  By integrating the MMI optical demultiplexing element 1006 designed to separate the second harmonic 1020-5 and the amplified signal light 1020-6 on the same substrate, a smaller phase-sensitive optical amplification device 1000 can be obtained. It is possible to obtain. A similar small phase sensitive optical amplifying device can be obtained by using an optical demultiplexing element using directional coupling instead of the MMI optical demultiplexing element 1006. If the 0.77 μm band light used for phase synchronization includes 1.54 μm band light, it may become a noise component when performing phase synchronization. 1014 may be inserted to cut light in the 1.54 μm band. Further, by adjusting the operation of the PLL circuit, it is possible to adopt a configuration in which an inverting circuit is not disposed as shown in FIG.

  The phase-sensitive optical amplifying apparatus 1000 shown in FIG. 10 is the same as that shown in FIG. 9 except that the third dichroic mirror 906-3 is changed to an optical demultiplexing element 1006 and a high-pass filter 1014 is inserted. The configuration is the same as that of the phase sensitive method of amplifying light 900 according to the first embodiment shown in FIG. Specifically, input / output signal light 1020, 1020-4, 1020-6, fundamental light wave 1020-1, second harmonic waves 1020-2, 1020-5, optical branching units 1021-1, 1021-2, data Signal intensity modulator 1001, EDFA 1002, nonlinear optical elements 1003-1 and 1003-2, bandpass filter 1004, PPLN waveguides 1005-1 and 1005-2, dichroic mirrors 1006-1 and 1006-2, polarization maintaining filter 1007, phase modulator 1008, balanced photodetector 1009, voltage amplifier 1010, PLL circuit 1011, PZT optical fiber stretcher 1012, and inverting circuit 1013 are input / outputs of the phase sensitive optical amplifier 900 shown in FIG. Output signal light 920, 920-4, 920-6, fundamental light wave 920-1, second harmonic wave 920- 920-5, optical branching units 921-1 and 921-2, data signal intensity modulator 901, EDFA 902, nonlinear optical elements 903-1 and 903-2, bandpass filter 904, PPLN waveguides 905-1 and 905 -2, dichroic mirrors 906-1 and 906-2, polarization maintaining filter 907, phase modulator 908, balanced photodetector 909, voltage amplifier 910, PLL circuit 911, PZT optical fiber stretcher 912, and inverting circuit 913 Correspond.

[Second Embodiment]
Next, a phase sensitive optical amplification device 1100 according to a second embodiment of the present invention will be described. FIG. 11 shows the configuration of the phase sensitive optical amplifying apparatus 1100 of this embodiment. In the present embodiment, the apparatus is configured to amplify a 1.54 μm signal as in the first embodiment. The point that the second harmonic is generated and the degenerate parametric amplification is performed using the two PPLN waveguides 1105-1 and 1105-2 is the same as that of the first embodiment. The difference is a method of separating the second harmonic 1120-2 from the fundamental light 1120-1 and a method of multiplexing the second harmonic 1120-2 and the signal light 1120-4. According to the present invention, phase sensitive amplification can be performed while suppressing deterioration of the SN ratio of signal light caused by ASE light generated from an optical fiber laser amplifier. However, in this embodiment, the effect can be used effectively. I made it. Also in this embodiment, dichroic mirrors 1106-1 and 1106- are used for separating the second harmonic 1120-2 from the fundamental light 1120-1 and for combining the second harmonic 1120-2 and the signal light 1120-4. 2 is used. In general, a dichroic mirror that reflects light of one wavelength and transmits light of the other wavelength is often used to separate or multiplex two lights having different wavelengths. In the case of an application for cutting light, it is desirable to use a configuration in which a specific wavelength light to be cut is reflected and used. On the other hand, in the case of a configuration in which light having a specific wavelength to be cut is transmitted and necessary light is reflected and extracted, it is necessary to make the reflectance of the mirror at an unnecessary wavelength very small. Compared to making the mirror reflectivity at an unnecessary wavelength very small, it is relatively easy to reduce the transmittance of light of a specific wavelength to be cut. It is possible to effectively suppress unnecessary light. In the present embodiment, the apparatus is configured based on such a concept.

  In the optical branching unit 1121-1, the fundamental wave light 1120-1 having a wavelength of 1.54 μm is branched from the input signal light 1120 and used as an optical fiber by the LN phase modulator 1108 for phase synchronization and the PZT optical fiber stretcher 1112. Through the EDFA 1102. The amplified fundamental wave light 1120-1 is incident on the first PPLN waveguide 1105-1 in the first second-order nonlinear optical element 1103-1 to generate the second harmonic 1120-2. In the present embodiment, only the second harmonic 1120-3 is effectively extracted from the fundamental light 1120-1 emitted from the first PPLN waveguide 1105-1, and the ASE light generated from the EDFA 1102 is effectively removed. For this purpose, a first dichroic mirror 1106-1 that reflects the 1.55 μm band and transmits the 0.77 μm band is provided after the first PPLN waveguide 1105-1. The second harmonic 1120-2 having a wavelength of 0.77 μm is guided to the second second-order nonlinear optical element 1103-2 via the polarization-maintaining fiber 1107 having single-mode propagation characteristics at this wavelength. Yes. As in the first embodiment, this fiber, which is a single mode at 0.77 μm, has a weak light confinement with respect to light having a wavelength of 1.54 μm. Therefore, it is unnecessary to propagate a length of about 1 m. It is possible to effectively attenuate fundamental wave light and ASE light in the vicinity of a wavelength of 1.54 μm. The second harmonic 1120-2 guided by the polarization maintaining fiber 1107 is combined with the input signal light 1120-4 having a wavelength of 1.54 μm using the second dichroic mirror 1106-2. In the present embodiment, the 1.54 μm band is reflected so that the residual components of the fundamental wave light and the ASE light in the vicinity of the wavelength of 1.54 μm passing through the polarization maintaining fiber 1107 can be effectively removed. A dichroic mirror that transmits the 77 μm band was used. After the input signal light 1120-4 and the second harmonic 1120-2 are combined, they are incident on the second PPLN waveguide 1105-2, and the signal light can be phase-sensitive amplified by degenerate parametric amplification. . The light emitted from the second PPLN waveguide 1105-2 is separated into the second harmonic 1120-5 and the amplified output signal light 1120-6 by the third dichroic mirror 1106-3. In the present embodiment, a dichroic mirror that reflects the 0.77 μm band and transmits the 1.54 μm band is used as the third dichroic mirror 1106-3. Also in the present embodiment, the second harmonic 1120-5 separated by the third dichroic mirror 1106-3 and the light branch 1121-2 branch before being combined using the second dichroic mirror 1120-6. The second harmonic 1120-2 is received by the balanced photodetector 1109 and then phase-synchronized by the PLL circuit 1111 so that the phase-sensitive amplification can be stably performed. In this embodiment, the inverting circuit 1113 is operated on the feedback signal of the PLL circuit 1111. In addition, since a balanced photodetector incorporating an amplifier is employed, no voltage amplifier is disposed after the photodetector 1109. Also in this embodiment, since all the amplified output signal light 1120-6 can be used, the gain of the phase sensitive optical amplifying apparatus 1100 is approximately as compared with the case where the amplified output signal light 1120-6 is used. It increased by 0.7 dB.

  In this embodiment, the dichroic mirrors 1106-1, 1106-2, and 1106-3 having different characteristics are separated from the second harmonic 1120-2 from the fundamental light 1120-1 and the second harmonic 1120-2. A phase sensitive optical amplifier that can obtain high signal quality without mixing ASE light from the EDFA 1102 that adversely affects the signal-to-noise ratio of the signal into the signal light because it is used for multiplexing with the input signal light 1120-4. Could be configured.

[Third Embodiment]
Next, a phase sensitive optical amplification device 1200 according to a third embodiment of the present invention will be described. FIG. 12 shows the configuration of the phase sensitive optical amplifying apparatus 1200 of this embodiment.

  In the above-described first and second embodiments, the light 1220-1 branched from the input signal light 1220 is used as the fundamental light. That is, the fundamental wave light 1220-1 is obtained by amplifying the same light source as the input signal light 1220. For example, when used for a transmitter in optical communication, it is considered that the same light source as described above is used for signal light and fundamental light, and that the necessary light is added to the signal light after branching the fundamental light. It is done. On the other hand, in the third embodiment, the apparatus is configured as shown in FIG. 12 so that the signal light modulated in advance can be amplified. The apparatus 1200 according to the present embodiment can amplify a signal such as a binary phase modulation (BPSK) or binary differential phase modulation (DPSK) signal or a normal intensity modulation without adding noise.

  In this embodiment, in order to obtain the fundamental wave light 1220-1, the input signal light 1220 is branched by the optical branching unit 1221-1, and the branched fundamental wave light 1220-1 is amplified by the EDFA 1202. The amplified fundamental wave light 1220-1 is incident on the first PPLN waveguide 1205-1 in the first second-order nonlinear optical element 1203-1, and the second harmonic wave 1220-2 of the signal light is generated. The first dichroic mirror 1206-1 is used to separate only the second harmonic 1220-2 from the light emitted from the first PPLN waveguide 1205-1. The separated second harmonic 1220-1 oscillates at a wavelength of 0.77 μm, and enters the semiconductor laser 1207-1 that can be phase-modulated by controlling the drive current, thereby performing injection locking. The output of the semiconductor laser 1207-1 is amplified by a semiconductor optical amplifier 1207-2 having a gain in the same wavelength band as that of the semiconductor laser, and the second harmonic 1220-7, which is pumping light, is used as the second dichroic mirror 1206- 2 is combined with the input signal light 1220-4 having a wavelength of 1.54 μm. The input signal light 1220-4 and the second harmonic 1220-7 having a wavelength of 0.77 μm are combined and then incident on the second PPLN waveguide 1205-2, and the signal light is phase-sensitive by degenerate parametric amplification. Can be amplified.

In order to perform phase sensitive amplification, it is necessary to generate excitation light synchronized with the average phase of the signal light incident on the semiconductor optical amplifier 1207-2. In this embodiment, even when a signal that has been subjected to binary phase modulation is used, it is possible to generate pumping light that is synchronized with the average phase. The operation principle will be briefly described below. In binary phase modulation, the signal phase is modulated to two values of 0 and π radians, and the signal is transmitted. When such a signal is amplified by the EDFA 1202 and then incident on the first PPLN waveguide 1205-1 to generate the second harmonic 1220-2, the second harmonic phase φ2ω is expressed as 2).
φ2ω = 2φωs (Formula 2)
Here, φωs is the phase of the signal light. Therefore, the phase of the second harmonic with respect to the signal whose phase is modulated into binary values of 0 and π is binary of 0 and 2π, and is output as light in which the phase fluctuation due to phase modulation is canceled. In an actual phase modulation signal, it is difficult to ideally modulate only the phase, and the signal is accompanied by intensity modulation. Therefore, in order to obtain excitation light having no intensity modulation component, the second harmonic from which the phase modulation component is removed is synchronized with the average phase of the signal light using injection locking as in this embodiment, It is desirable to use excitation light.

  In the present embodiment, excitation light without intensity modulation synchronized with the average phase is generated from the signal light subjected to phase modulation using injection locking. As a result, even if phase noise is added to the signal light, the phase component orthogonal to the original signal can be attenuated by phase sensitive amplification, so that the signal phase and quadrature phase noise components are removed. Such signal reproduction can be performed.

  In this embodiment, weak phase modulation is performed by controlling the drive current of the semiconductor laser 1207-1 used for injection locking. The second harmonic 1220-5 separated by the third dichroic mirror 1206-3 and the second harmonic branched by the optical branch 1221-2 before being combined using the second dichroic mirror 1206-2. After receiving 1220-7 by the balanced photodetector 1209, the PLL circuit 1211 detects the phase shift and performs feedback to the drive current of 0.77 μm so as to satisfy the relationship of (Equation 1) and synchronize. It compensates for phase fluctuations caused by vibrations and temperature fluctuations of optical components, and enables stable phase-sensitive amplification. In this embodiment, since the balanced photodetector 1209 having an inverting output function is used, no inverting circuit is arranged. In the present embodiment, since the second harmonic is phase-modulated, the phase synchronization frequency component is detected and phase synchronization is performed unlike the above-described embodiment. The input to the second nonlinear optical element 1203-2, which was decreased by the optical branching, was compensated by increasing the gain of the semiconductor optical amplifier 1207-2. Also in this embodiment, since all the amplified signal light 1220-6 can be used, the gain of the phase sensitive optical amplifying apparatus 1200 is about 0. 0 as compared with the case where the amplified signal light 1220-6 is used. It increased by 7 dB. In this embodiment, the EDFA 1202 is used to obtain power that enables the second harmonic generation 1220-2 in the first PPLN waveguide 1205-1. However, the ASE light generated from the EDFA 1202 is phase-sensitive amplification. Since the light does not enter the second PPLN waveguide 1205-2, the signal-to-noise ratio degradation of the signal light due to the ASE light of the optical amplifier 1207-2 can be prevented. The ASE light is also generated from the semiconductor optical amplifier 1207-2 operating at a wavelength of 0.77 μm. However, since this light is completely different in wavelength from the signal light, the second and third dichroic mirrors 1206-2 and 1206 are generated. 3 can be removed almost completely, and phase sensitive amplification can be performed without degrading the signal-to-noise ratio of the signal light.

101, 401 Phase sensitive light amplification units 102, 402 Excitation light source 103, 403 Excitation light phase control units 104-1, 104-2, 404-1, 404-2, 921-1, 921-2, 1021-1, 1021 -2, 1121-1, 1121-2 Optical splitter 110, 410, 920, 920-4, 1020, 1020-41120, 1120-4, 1220, 1220-4 Input signal light 111, 211, 411 Excitation light 112, 412, 920-6, 1020-6, 1120-6, 1120-6 Output signal light 201 Laser light source 202 SHG crystal 203 OPA crystal 210 Signal light 413, 920-2, 920-5, 1020-2, 1020-5, 1120-2, 1120-5, 1220-2, 1120-5, 1120-7 Second harmonic 900, 1000, 110 , 1200 phase sensitive optical amplifier 901 and 1001 data signal intensity modulator 902,1002,1102,1202 EDFA
903-1, 1003-1, 1103-1, 1203-1 First nonlinear optical element 903-2, 1003-2, 1103-2, 1203-2 Second nonlinear optical element 904, 1004 Band pass filter 905- 1, 1005-1, 1105-1, 1205-1 1st PPLN waveguide 905-2, 1005-2, 105-2, 1205-2 2nd PPLN waveguide 906-1, 1006-1, 1106- 1, 1206-1 First dichroic mirrors 906-2, 1006-2, 1106-2, 1206-2 Second dichroic mirrors 906-3, 1106-3, 1206-3 Third dichroic mirrors 907, 1007, 1107 Polarization maintaining filters 908, 1008, 1108 Phase modulators 909, 1009, 1109, 1209 Type optical detectors 910, 1010, 1110, 1210 Voltage amplifiers 911, 1011, 1111, 1211 PLL circuits 912, 1012, 1112 PZT optical fiber stretchers 913, 1013, 1113 Inversion circuits 920-1, 920-3, 1020- 1, 1020-3, 1120-2, 1120-5, 1220-1 Fundamental light wave 1006 Optical demultiplexing element 1014 High-pass filter 1207-1 Semiconductor laser 1207-2 Semiconductor optical amplifier

Claims (5)

A phase-sensitive optical amplifier that amplifies signal light by optical mixing using a nonlinear optical effect,
An optical fiber laser amplifier for amplifying the fundamental light;
A first second-order nonlinear optical element comprising an optical waveguide for generating excitation light from the fundamental light, which is made of a second-order nonlinear optical material that is periodically poled;
A filter that separates only the excitation light from the fundamental light and the excitation light;
A multiplexer for multiplexing the signal light and the excitation light;
A second second-order nonlinear optical element comprising an optical waveguide for performing parametric amplification of the signal light using the excitation light, the second-order nonlinear optical material comprising a periodically-polarized second-order nonlinear optical material;
A filter for separating the amplified signal light and the excitation light;
Phase synchronization means for synchronizing the phase of the signal light and the phase of the excitation light;
Means for branching a part of the excitation light output from a filter that separates only the excitation light from the fundamental wave light and the excitation light;
A balanced photodetector that detects the excitation light output from the second second-order nonlinear optical element and a part of the branched excitation light;
The phase-locked optical amplifier apparatus comprising: a phase-locked loop circuit that performs feedback based on a change in signal intensity detected by the balanced photodetector.   The phase-locking means includes a phase modulator that is the phase-modulating means, the phase-locked loop circuit, and an optical length expander, and the phase-locked loop circuit includes the phase modulator and the optical length expander. The phase sensitive optical amplifying device according to claim 1, wherein feedback is performed.   The phase-locked loop unit includes the phase-locked loop circuit and a semiconductor laser that generates light phase-synchronized with the pumping light and modulates the phase by controlling the driving current of the laser. 2. The phase sensitive optical amplifying device according to claim 1, wherein feedback is performed on the driving current of the semiconductor laser.   4. The phase sensitive optical amplifying apparatus according to claim 1, further comprising means for inverting a signal detected by the balanced photodetector or a feedback signal of the phase locked loop circuit.   The filter that separates the amplified signal light and the excitation light is a dichroic mirror using a dielectric film, an optical demultiplexing element using multimode interference, or an optical demultiplexing element using directional coupling. 5. The phase sensitive optical amplifying device according to claim 1, wherein

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