JP2013182140A - Light amplification device, optical signal generator, and signal/noise ratio improvement device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve the problem that, though all of excitation light and signal light are arranged in the same communication wavelength band and an optical fiber amplifier is used for generation and amplification of excitation light in light amplification using four-wave mixing of optical fibers in prior arts, an S/N ratio in output is made lower than that in input due to mixture of amplified spontaneous emission (ASE) generated from the optical fiber amplifier and a phase sensitive light amplifier for suppression of ASE can amplify only one signal wavelength because generally employing degenerate parametric amplification and cannot simultaneously amplify a plurality of carrier waves unlike an optical comb.SOLUTION: A phase sensitive light amplifier uses a fiber laser amplifier to amplify fundamental wave light in order to obtain enough power to obtain a nonlinear optical effect from a weak laser beam for use in optical communication. The amplified fundamental wave light is made incident on a first second-order nonlinear optical element to generate a secondary harmonic. Signal light and the secondary harmonic are made incident on a second second-order nonlinear optical element to perform degenerate parametric amplification. In addition, this phase sensitive light amplifier may constitute an optical signal generator.

Description

  The present invention relates to an optical amplifying device, and more specifically to an optical amplifying device, an optical signal generator, and a signal-to-noise ratio improving device used in an optical communication system and 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 the above-mentioned problems, 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 are known. Both the fiber laser amplifier and the semiconductor laser amplifier can amplify the signal light as it is. Therefore, there is no limitation on the electrical response speed and 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. In these laser amplifiers, spontaneously emitted light that is inevitably and randomly generated is mixed regardless of the signal component. For this reason, the S / N ratio of the signal light decreases by at least 3 dB before and after amplification.

  The above-described erbium-doped optical fiber amplifier (EDFA) and semiconductor optical amplifier are laser amplifiers based on the principle of stimulated emission. In these laser amplifiers, the residual reflection of the optical components inside the device is completely suppressed, and even in the ideal case where a complete inversion distribution is obtained in the laser medium, the signal-to-noise ratio ( S / N ratio) is reduced by half (3 dB reduction). This deterioration is called the standard quantum limit in optical amplification. The standard quantum limit is not limited to laser amplification, but is also the limit common to all optical amplification (Phase Insensitive Amplification: PIA) that gives equal gain to two orthogonal optical phase components of incident light, including Raman amplification and Brilliant amplification. is there. Degradation of the S / N ratio based on the standard quantization limit leads to an increase in transmission code error rate during digital signal transmission, and is a factor that degrades transmission quality.

  As means for overcoming the limitations of the conventional laser amplifier, a phase sensitive optical amplifier (PSA) has been studied. This phase-sensitive optical amplifier has a function for shaping a signal light pulse waveform that has deteriorated due to the influence of dispersion in the transmission fiber. Further, since spontaneous emission light having a quadrature phase irrelevant to the signal can be suppressed, in principle, the S / N ratio of the signal light can be kept the same before and after amplification without deteriorating.

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. Isao Morohashi, Takahide Sakamoto, Hideyuki Tonobayashi, Tetsuya Kawanishi, Satoshi Kosako, “Mach-Zehnder Modulator-Based Optical Comb Generator and 100fs-class Pulse Generation Using Soliton Compression,” Proceedings of the 72th JSAP Meeting (Physical Society 2011 Autumn Yamagata University) 30a-P3-1 R. Tang et al., “In-line phase-sensitive amplification of multichannel CW signals based on frequency nondegenerate four-wave-mixing in fiber,” Optics Express., Vol. 16, pp. 9046-9053 (2008).

  However, the conventional optical amplifiers including the above-described phase sensitive optical amplifier have the following problems.

  FIG. 1 is a diagram showing a basic configuration of a conventional phase sensitive optical amplifier. 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.

  This optical amplifier amplifies the input signal light 110 when the phase of the signal light and the phase of the excitation light in the phase sensitive light amplification unit 101 coincide with each other. The light 110 has a characteristic to attenuate. If the phase between the pumping light and the signal light is matched so that the amplification gain is maximized using this characteristic, spontaneous emission light having a phase orthogonal to the signal light is not generated, that is, the S / N ratio is deteriorated. It is possible to amplify the signal light without doing so.

  In order to realize the phase synchronization of 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, and it is possible to realize optical amplification without degradation of the S / N 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. In addition, 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 the excitation light.

  A medium having a second-order or third-order nonlinear optical effect is used for the phase sensitive light amplification unit 101. Conventionally, these phase sensitive optical amplifiers have been mainly used in basic research fields such as squeezing for controlling the quantum state of light. In the early phase-sensitive optical amplifier research, studies using second-order nonlinear optical crystals have been reported.

  When the second-order nonlinear optical effect is used, as shown in Non-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. Phase-sensitive optical amplification is realized by making excitation light and signal light enter a nonlinear medium and performing degenerate parametric amplification (OPA) using three-wave mixing.

  FIG. 2 is a diagram for explaining the configuration of a phase sensitive optical amplifier using the second-order nonlinear optical effect of the prior art. As shown in FIG. 2, in the prior art, the laser light having a relatively high intensity from the laser light source 201 is branched. One of the branched lights is incident on an 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 light amplification is performed.

In the phase sensitive optical amplifier, in order to amplify only the light in phase with the signal, it is necessary that the phase of the signal light and the excitation light coincide with each other or be shifted by π radians as described above. That is, when using the second-order nonlinear optical effect, that the phase phi _2Omegaesu of the excitation light is a wavelength corresponding to the second harmonic, and the phases phi _.omega.s of the signal light, satisfies the following relationship (1) Necessary.
_{Δφ = φ 2ωs / 2-φ} ωs = nπ ( where, n is an integer) (1)

  FIG. 3 is a graph showing the relationship between the phase difference Δφ between the input signal light and the pumping light and the gain (dB) in the conventional phase sensitive optical amplifier using the second-order nonlinear optical effect. It can be seen that the gain is maximum when Δφ on the horizontal axis is −π, 0, or π.

  Also in the configuration of the phase sensitive optical amplifier using the second-order nonlinear optical effect shown in FIG. 2, a part of the output signal light is branched and detected by a narrow-band detector as shown in FIG. By controlling the phase of the pumping light so that the maximum is obtained, phase synchronization of the signal light and the pumping light can be realized.

  The phase-sensitive optical amplifier described with reference to FIGS. 1 and 2 has been attracting attention for its application to optical communication as the optical communication has advanced in recent years. In the field of optical communication, there is a report of a configuration using the third-order nonlinear optical effect of an optical fiber having high compatibility with optical components for communication. 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. Phase-sensitive optical amplification can be realized by making excitation light and signal light enter a nonlinear medium and performing degenerate parametric amplification using four-wave mixing.

When a third-order nonlinear medium is used and one excitation light having the same wavelength as the signal light is used, the phase φ _{ωp of the} excitation light and the phase φ _{ωs of the} signal light satisfy the relationship of the following equation (2). Necessary.
_{Δφ = φ ωp -φ ωs = nπ} ( where, n is an integer) Equation (2)

As shown in Non-Patent Document 3, instead of one pumping light having the same wavelength as the signal light, optical frequencies ω _p1 and ω _p2 satisfying the following expression (3) when the optical frequency of the signal light is ω _s. You may use two excitation light which each has.
2ω _s = ω _p1 + ω _p2 formula (3)

When a third-order nonlinear medium is used and two pump lights having wavelengths corresponding to two optical frequencies ω _p1 and ω _p2 are used, the phases φ _ωp1 and φ _ωp2 of the pump light and the phase φ _{ωs of the} signal light are It is necessary to satisfy the relationship of Formula (4).
_{Δφ = (φ ωp1 + φ ωp2} ) / 2-φ ωs = nπ ( where, n is an integer) (4)

  Even when a third-order nonlinear medium is used, as in the case of using the second-order nonlinear optical effect, a part of the output signal light is branched and detected by a narrow-band detector, and the excitation light is used so that the output signal is maximized. The phase synchronization of the signal light and the pumping light can be realized by controlling the phase.

  As described above, in the method using the optical fiber, the first method using one excitation light having the same wavelength as the signal light, or the second method using excitation light having two wavelengths different from the signal light. There is. In the case of the first method using one excitation light, it is necessary to separate the excitation light from the signal light. For this purpose, as shown in Non-Patent Document 2, a loop type fiber interferometer is used to separate the signal light and the excitation light. However, in this method, phase modulation due to GAWBS (guided acoustic wave wave Brillouin scattering) in the optical fiber is added in a form having no correlation with the light propagating through the fiber in the opposite direction. For this reason, noise characteristics are degraded. In order to avoid this problem, the second method using two excitation lights as shown in Non-Patent Document 3 has been well studied in recent years.

  FIG. 4 is a diagram for explaining the configuration of a phase sensitive optical amplifier using the third-order nonlinear optical effect of the prior art. This is a configuration using an optical fiber and two pump lights according to the second method described above. As shown in Non-Patent Document 3, first, the first excitation light 411-1 and the second excitation light synchronized with the average phase of the incident signal 410 using means such as four-wave mixing in an optical fiber. Excitation light 411-2 is generated. Next, the two pump lights (411-1, 411-2) and the signal light 410 are amplified by an erbium-doped fiber laser amplifier (EDFA) 402 and are incident on a highly nonlinear optical fiber 403. In FIG. 4, the signal light 410 and the two pump lights (411-1, 411-2) are combined and amplified by the EDFA, but only the two pump lights are amplified by the EDFA, The same effect can be obtained even if the light enters the optical fiber 403 after being multiplexed. By adjusting the phase so that the relationship expressed by the above equation (4) is established between the signal light and the two excitation lights, phase-sensitive light amplification by four-wave mixing can be realized.

  However, the above-described conventional phase sensitive optical amplifier still has the following problems. In the conventional phase sensitive optical amplifier using the second-order nonlinear optical crystal, only a configuration which can be operated 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 yet.

  In conventional phase-sensitive optical amplification using optical fibers, a configuration applicable to an optical communication system is shown, but because the four-wave mixing is used, the wavelength of the signal light and the wavelength of the excitation light are close to each other. It has become. In particular, the configuration shown in FIG. 4 shows a configuration in which necessary power is obtained by an optical fiber amplifier such as an EDFA so that the nonlinear optical effect in the optical fiber can be used. Spontaneous emission light (ASE light) is superimposed on excitation light as noise. 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 is deteriorated, and optical amplification with low noise cannot be performed. Further, a phase sensitive optical amplifier with no deterioration in S / N ratio is also demanded from the following viewpoint.

  In recent optical communication technologies, as represented by optical OFDM (orthogonal frequency division multiplexing), high-speed data is divided into a plurality of optical carriers and modulated in order to send a large-capacity signal with high frequency utilization efficiency. A data transmission / reception method called a super channel is being studied. As described above, in order to perform data modulation on a plurality of carriers in the optical domain, an optical comb composed of carriers of optical frequencies arranged at equal intervals is generated using a mode-locked laser or an optical modulator. . The generated optical comb is distributed by a demultiplexer, data modulation is performed on each carrier wave using an optical modulator, and the signals are combined again and guided to a transmission line.

  Even when used in optical communication using an optical comb, the above-described conventional phase sensitive optical amplifier has the following problems. In general, when an optical comb composed of a plurality of carrier waves is demultiplexed by a demultiplexer, modulated by a modulator, and multiplexed by a multiplexer, the insertion loss of each component is large. For this reason, the optical power of the combined optical signal is significantly attenuated compared to the original optical comb. As shown in Non-Patent Document 6, a method for generating an optical comb by a light source and a modulator having a single wavelength has also been proposed. The optical power is reduced by the conversion efficiency to the carrier wave.

  In recent optical communications, improvement in frequency use efficiency is required, and as known from Shannon's communication theory, in order to obtain high frequency use efficiency, it is required that the signal S / N is large. However, in the transmission method that modulates the optical comb described above, the loss of optical power accompanying the generation and modulation of light is large. After generating an optical signal, if the optical amplifier using a normal laser medium is amplified to the power required for transmission through the optical fiber, the signal S / N is significantly degraded due to the low input power to the optical amplifier. I will let you. Although the principle of low noise optical amplification by a phase sensitive optical amplifier is known, degenerate parametric amplification is generally used in a phase sensitive optical amplifier. For this reason, only one signal wavelength can be amplified, and a plurality of carrier waves such as an optical comb cannot be amplified simultaneously.

  As a method capable of simultaneously amplifying a plurality of wavelengths, a configuration of a phase sensitive optical amplifier using non-degenerate parametric amplification using four-wave mixing in an optical fiber is proposed as shown in Non-Patent Document 7.

  FIG. 5 is a diagram for explaining the outline of a multi-wavelength amplification method using four-wave mixing in a conventional optical fiber. In this amplification method, first, a plurality of modulated light and pump light are incident on a first optical fiber 501 called Copier, and idler light whose phase is inverted from that of input modulated light is generated by wavelength conversion using four-wave mixing. . Next, a plurality of modulated light groups and corresponding idler light groups are incident on the second optical fiber 502, and non-degenerate parametric amplification is performed. By using this configuration, it is possible to amplify phase-sensitive signal light having a plurality of wavelengths.

  However, in the optical amplification using the four-wave mixing of the optical fiber in this way, all of the excitation light and the signal light are arranged in the same communication wavelength band of 1.55 μm band. An optical fiber amplifier 503 is used for generating and amplifying pumping light. Since amplified spontaneous emission (ASE) generated from the optical fiber amplifier 503 is mixed, the S / N ratio at the output is greater than that at the input. Will also deteriorate.

  The present invention has been made in view of such problems, and an object thereof is to realize high-quality optical signal amplification with little deterioration in S / N ratio. Furthermore, a phase sensitive optical amplifier capable of amplifying a signal group having a plurality of wavelengths at once is provided. Also provided are an optical signal generator that combines a phase sensitive optical amplifier and an optical comb light source, and an optical signal S / N ratio improvement device.

  In order to achieve such an object, the present invention provides a phase-sensitive optical amplifying apparatus that amplifies signal light by optical mixing using a nonlinear optical effect. On the frequency axis, one or a plurality of signal light pairs having two wavelengths, which are symmetrically separated from the fundamental light by the same optical frequency difference and each have the same or inverted phase information, are input, and the fundamental light is An optical fiber laser amplifier to be amplified, a first second-order nonlinear optical element that is connected to the optical fiber laser amplifier and generates a second harmonic, and the fundamental light output from the first second-order nonlinear optical element A filter that separates the second harmonic from the second harmonic, a signal light that is a pair of signal light, and a multiplexer that combines the second harmonic that is excitation light. Incident wave signal light and excitation light are incident A second second-order nonlinear optical element that performs non-degenerate parametric amplification of the signal light, and a signal light amplified from the signal light and the excitation light output from the second second-order nonlinear optical element. A phase sensitive optical amplifying apparatus comprising: a filter for separation; and means for synchronizing the phase of the signal light and the phase of the excitation light. Hereinafter, the phase sensitive optical amplifying device is also referred to as a phase sensitive optical amplifier.

The invention according to claim 2 is the phase sensitive optical amplifying device according to claim 1, wherein the second harmonic wave is separated from the fundamental wave light and the second harmonic wave, and the signal light and The multiplexer for combining the second harmonic is a dichroic mirror using a dielectric film.
The invention according to claim 3 is the phase sensitive optical amplifying device according to claim 1, wherein the first second-order nonlinear optical element and the second second-order nonlinear optical element have a periodically poled structure. Each of the second-order nonlinear optical materials includes LiNbO ₃ , LNbO ₃ , LiTaO ₃ , LiNb _x Ta _x-1 O ₃ (0 ≦ x ≦ 1), and KTiOPO ₄ . One or more of them contains at least one selected from the group consisting of Mg, Zn, Fe, Sc, and In as an additive.

  The invention according to claim 4 is the phase sensitive optical amplifying device according to any one of claims 1 to 3, wherein the signal light further includes a pilot tone of continuous wave light, and the phase sensitive optical amplifying device comprises: The semiconductor laser light source further includes a filter for separating the pilot tone and a semiconductor laser light source, and the semiconductor laser light source is output from the semiconductor laser light source that is light-injected and locked by the pilot tone of the continuous wave light and phase-locked to the injected light Continuous light is used as the fundamental wave light.

  A fifth aspect of the present invention is the phase sensitive optical amplifying device according to any one of the first to third aspects, wherein the phase of the signal light between two light beams of a pair is adjusted. A dispersion compensation medium is further provided.

  The invention according to claim 6 includes the phase sensitive optical amplifying device according to any one of claims 1 to 3, to which the signal light having two wavelengths paired with spontaneous emission light noise is input. Based on the difference between the gain with respect to the signal light and the gain with respect to the spontaneous emission light noise, the signal light is selectively amplified from the signal light on which the spontaneous emission noise is superimposed, so that the spontaneous emission noise is obtained. Is a signal-to-noise ratio improving apparatus. That is, the phase sensitive optical amplifying device of the present invention also operates as a signal-to-noise ratio improving device.

According to a seventh aspect of the present invention, there is provided a light source that generates an optical comb composed of light of a plurality of wavelengths that are phase-synchronized, a wavelength of the optical comb, and a plurality of wavelengths of the optical comb. A duplexer configured to output a pair of two wavelengths of light symmetrically separated from the light of one wavelength by the same optical frequency difference to the same optical path, each of the pair of lights A plurality of optical modulators connected to one of the outputs of the duplexer for modulating the corresponding pair of light; and a multiplexer for multiplexing the outputs from the plurality of optical modulators. 4. The output of the multiplexer is input to the phase sensitive optical amplifying device according to claim 1, wherein one of the plurality of wavelengths of the optical comb is the fundamental light. This is an optical signal generator.
According to an eighth aspect of the present invention, there is provided a light source that generates an optical comb composed of light of a plurality of wavelengths synchronized in phase, a wavelength of the optical comb is separated, and a plurality of wavelengths of the optical comb are separated. A duplexer configured to output pairs of light of two wavelengths symmetrically separated from the light of one wavelength by the same optical frequency difference to different optical paths, respectively, A plurality of optical modulators for applying optical modulation having the same or inverted phase information to the light pair, and a multiplexer for combining outputs from the plurality of optical modulators 4. The output of the multiplexer is input to the phase sensitive optical amplifying apparatus according to claim 1, wherein one of the plurality of wavelengths of the optical comb is the fundamental light. The optical signal generator is characterized by the above.

  The invention according to claim 9 is a light source that generates an optical comb composed of light of a plurality of wavelengths that are phase-synchronized, and an output of the light source that generates the optical comb is input. 4. The phase sensitive optical amplifying device according to claim 1, wherein one of the wavelengths is the fundamental wave light, and the output of the phase sensitive optical amplifying device is connected to each wavelength of the optical comb. A demultiplexer for separating, a plurality of optical modulators each connected to each output of the demultiplexer, and a multiplexer for multiplexing outputs from the plurality of modulators Is an optical signal generator.

  A tenth aspect of the present invention is the optical signal generator according to any one of the seventh to ninth aspects, wherein one wavelength used for the fundamental light among a plurality of wavelengths of the optical comb is the light. It is the center wavelength of the comb.

  An eleventh aspect of the present invention is the optical signal generator according to any one of the seventh to ninth aspects, wherein the phase-locked light of the plurality of wavelengths of the optical comb modulates a single mode laser light source. The light generated by the modulation by the detector and branched from the laser light source is input as the fundamental wave light to the phase sensitive optical amplifying device.

  A twelfth aspect of the present invention is the optical signal generator according to any one of the seventh to ninth aspects, wherein the optical signal generator is any one between the light source that generates the optical comb and the phase-sensitive optical amplification device. At least one laser amplifier is inserted at the position.

  According to the present invention, in a phase sensitive optical amplifier (or a phase sensitive optical amplifying apparatus) that amplifies only a specific phase of signal light using a parametric amplification effect that is a nonlinear optical effect, a plurality of wavelengths can be collectively amplified. It becomes. While using an optical fiber amplifier to obtain sufficient power from the weak optical power used in optical communication to use parametric optical amplification, the phase of the ASE light generated by optical amplification is not superimposed on the signal light. A sensitive optical amplifier can be constructed. This enables high-quality optical signal amplification while preventing deterioration of the S / N ratio. Furthermore, noise caused by uncorrelated light such as ASE light can be suppressed by selectively amplifying signal light having a phase correlation with the excitation light by using the present invention.

  Due to the feature of the present invention that suppresses the degradation of the S / N ratio, a phase sensitive optical amplifier that can be applied to optical communication and can amplify a signal group having a plurality of wavelengths can be configured. The phase-sensitive optical amplifier capable of amplification with low noise can improve the S / N ratio of the signal in the optical fiber transmission as compared with the case of amplification with the conventional laser amplifier. It becomes possible to transmit a high-speed signal to a long distance with low power than before. In addition, since the phase noise of the incident signal light can be removed and amplified, the influence of signal degradation due to wavelength dispersion of the optical fiber is reduced, and the transmission distance of the amplified signal light can be extended.

  Furthermore, by suppressing the ASE noise, it is possible to realize a signal-to-noise ratio improving apparatus capable of improving the S / N ratio of an optical signal once deteriorated. By selectively amplifying the signal light having the phase correlation, the S / N ratio of the signal light deteriorated by the beat noise of the ASE light and the signal light can be improved.

FIG. 1 is a diagram illustrating the configuration of a conventional phase sensitive optical amplifier. FIG. 2 is a diagram for explaining the configuration of a phase sensitive optical amplifier using the second-order nonlinear optical effect of the prior art. FIG. 3 is a graph showing the relationship between the phase difference Δφ between the input signal light and the pump light and the gain in the phase sensitive optical amplifier using the second-order nonlinear optical effect of the prior art. FIG. 4 is a diagram for explaining the configuration of a phase sensitive optical amplifier using the third-order nonlinear optical effect of the prior art. FIG. 5 is a diagram for explaining the outline of a multi-wavelength amplification method using four-wave mixing in a conventional optical fiber. FIG. 6 is a diagram illustrating the configuration of the phase sensitive optical amplifier according to the first embodiment of the present invention. FIG. 7 is a diagram for explaining the operation of the phase sensitive optical amplifier according to the first embodiment of the present invention. FIG. 8 is a diagram for explaining the operation of phase-sensitive optical amplification with the configuration of the prior art. FIG. 9 is a diagram for explaining the operation of phase sensitive light amplification according to the first embodiment of the present invention. FIG. 10 is a diagram for explaining the effect of the phase sensitive optical amplifier according to the first embodiment. FIG. 10A is a diagram illustrating an input optical spectrum of a signal light group in which ASE light intentionally generated from an EDFA is mixed. (B) shows the optical spectrum of the output of an amplifier, respectively. FIG. 11 is a diagram showing beat noise levels of signal light and ASE light at the input and output when amplified by the phase sensitive optical amplifier according to the first embodiment. FIG. 12 is a diagram showing an experimental configuration for examining the effect of improving the S / N ratio using a signal obtained by performing data modulation on an optical comb. FIG. 13 is a diagram illustrating error rate data with respect to received power of an optical comb signal subjected to data modulation in the phase-sensitive optical amplifier according to the first embodiment. FIG. 14 is a diagram showing a configuration of a phase sensitive optical amplifier according to the second embodiment of the present invention. FIG. 15 is a diagram illustrating another configuration example of phase sensitive light amplification according to the second embodiment of the present invention. FIG. 16 is a diagram illustrating still another configuration example of phase sensitive light amplification according to the second embodiment of the present invention. FIG. 17 is a diagram illustrating a time waveform of a signal amplified by the phase sensitive optical amplifier according to the second embodiment. FIG. 18 is a diagram for explaining the configuration of phase-sensitive optical amplification according to the third embodiment of the present invention. FIG. 19 is a diagram showing a configuration of a phase sensitive optical amplifier according to the fourth embodiment of the present invention using a center wavelength signal as the phase synchronization means.

  The present invention provides a phase-sensitive optical amplifier (or also called a phase-sensitive optical amplifying device) that amplifies only a specific phase of signal light using a parametric amplification effect that is a nonlinear optical effect, and amplifies a plurality of wavelengths at once. Is possible. An optical fiber amplifier is used in order to obtain sufficient power from the weak optical power used in optical communication to use parametric optical amplification. A phase sensitive optical amplifier can be configured without superimposing ASE light generated along with optical amplification on signal light. This enables high-quality optical signal amplification while preventing deterioration of the S / N ratio. Furthermore, noise caused by uncorrelated light such as ASE light can be suppressed by selectively amplifying signal light having a phase correlation with the excitation light by using the present invention. An optical signal generator can be configured by combining the phase sensitive optical amplifying device of the present invention with a light source that generates an optical comb composed of light of a plurality of wavelengths synchronized in phase.

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

  6 and 7 are diagrams illustrating the configuration and operation of the phase-sensitive optical amplifier according to the first embodiment of the present invention, respectively. As shown in FIG. 6, in this embodiment, in order to obtain sufficient power for obtaining a nonlinear optical effect from weak laser light used for optical communication, a fiber laser amplifier (EDFA) 601 is used to generate fundamental light. 621 is amplified. The amplified fundamental wave light is incident on the first second-order nonlinear optical element 602-1 to generate the second harmonic 622. The signal light 620 and the second harmonic 622 are incident on the second second-order nonlinear optical element 602-2 to perform degenerate parametric amplification, thereby performing phase sensitive amplification.

  The details of the configuration shown in FIG. 6 will be described later. By adopting such a configuration, it is possible to obtain characteristic effects not found in the phase sensitive optical amplifier of the prior art as described below. In this embodiment, continuous wave light (CW light) having a wavelength of 1.54 μm is used as the fundamental wave light. As the input signal light, as shown in FIG. 7, a pair of signal light (s + 1 and s-1, s + 2) symmetrically separated by the same optical frequency difference with the optical frequency corresponding to the fundamental wavelength as the center. S-2, s + 3 and s-3, s + 4 and s-4, and so on).

  The signal light group and the fundamental light are phase-synchronized with each other. Such signal light and fundamental wave light can be generated, for example, by branching the same light source and generating one sideband by an optical modulator.

  As shown in FIG. 6, the fundamental wave light 621 passes through a phase expander 611 using a phase modulator 610 and PZT and is amplified by an erbium-doped optical fiber amplifier (EDFA) 601. After amplification, the fundamental wave light is incident on the PPLN waveguide 605-1 in the first second-order nonlinear optical element 602-1 after removing excessive spontaneous emission light generated from the EDFA 601 using the bandpass filter 604. Is done. In the PPLN waveguide 605-1, the light is converted into light 622 having a wavelength of 0.77 μm, which is the second harmonic of the fundamental light 621.

  The signal light group 620 and the second harmonic wave 622 of the fundamental light are combined by the dichroic mirror 606-2 and then incident on the PPLN waveguide 605-2 in the second second-order nonlinear optical element 602-2. Is done. The signal light group 620 is amplified by parametric amplification in the PPLN waveguide 605-2.

  Next, the amplification operation of the signal light group will be described in detail. In the present embodiment, light having the same phase at two wavelengths of the paired signal light is respectively incident. For example, it is assumed that the signals s + 1 and s-1 have the same phase information.

A non-linear amplification is performed by entering three light beams of excitation light (second harmonic wave 622 of the fundamental wave light in this embodiment), signal light, and idler light into the second-order nonlinear optical element, and performing three-way nonlinear interaction. In degenerate parametric amplification, parametric amplification of both signal light and idler light is performed when the three phases satisfy the following equation (5).
φ _SH = φ _S + φ _i + 2nπ (n is an integer) Equation (5)

Here, φ _SH , φ _S , and φ _i are the phases of the second harmonic of the fundamental light, the signal light, and the idler light, respectively. Assuming that the signal and the idler have the same phase as the pair of the signal s + 1 and the signal s-1 in the present embodiment, the following equation (6) is established with φ _i = φ _S.
φ _S = φ _SH / 2 + nπ = φ _p + nπ (n is an integer) Equation (6)

Here, φ _p is the phase of the fundamental wave light. The second harmonic phase φ _SH is expressed by 2φ _p .

  As can be seen from equation (6), when the signal light has the same phase as that of the fundamental wave light or is shifted by π, it is understood that parametric amplification occurs only when the phase light is in phase with the excitation light among two orthogonal phase components. When a signal pair having a phase orthogonal to the fundamental wave light is incident, the signal light is attenuated.

  Thus, when a pair of signal lights having the same phase information is incident as a signal, parametric amplification with phase sensitivity is performed. In the present embodiment shown in FIG. 6, since the signal light and the excitation light are connected by the fiber parts, the expansion and contraction of the fiber due to temperature fluctuation and vibration is absorbed by the PLL technology. In this embodiment, all pairs of signal light that are symmetrically separated by the optical frequency difference are phase-synchronized, so that a plurality of signal light groups can be amplified.

  FIG. 8 and FIG. 9 are diagrams schematically showing spectra of signal light and excitation light having a plurality of wavelengths used in phase sensitive light amplification. FIG. 8 shows a case where the conventional fiber laser amplifier shown in FIG. 5 and a configuration using an optical fiber as a nonlinear medium are used, and FIG. 9 is according to the first embodiment of the present invention shown in FIG. It is a figure which shows the case where a structure is used.

  As shown in FIG. 8, the phase sensitive optical amplifier using the optical fiber of the prior art uses four-wave mixing. For this reason, in order for the wavelengths of the pumping light for performing parametric light amplification and the signal light of a plurality of wavelengths to satisfy the phase matching condition, these wavelengths must be close wavelengths. As shown in FIG. 8, the signal light 801 and the pumping light 802 having a plurality of wavelengths have the same 1.55 μm wavelength band, and when the pumping light 802 is amplified by the optical fiber amplifier, the light is near The ASE light 803 is generated by the fiber amplifier.

  In order not to generate ASE light in the signal wavelength range, it is possible to adopt a configuration in which signal light of a plurality of wavelengths does not pass through the optical fiber amplifier. However, when the excitation light and the signal light of a plurality of wavelengths are combined, it is difficult to realize an optical filter with good wavelength selectivity, and the ASE light is completely cut off. I can't. As a result, the ASE light generated in the signal wavelength band is superimposed on a plurality of signal wavelengths. By mixing ASE light, the S / N ratio of the signal light having a plurality of wavelengths is deteriorated ((c) in FIG. 8).

  On the other hand, as shown in FIG. 9, in the configuration according to the present embodiment, the fundamental light 902 is amplified by an optical fiber amplifier in order to obtain sufficient power from the weak optical power used in optical communication to use parametric optical amplification. To do. At this time, the ASE light 903 is superimposed in the vicinity of the wavelength of the fundamental wave light 902 ((b) of FIG. 9). In the configuration according to this embodiment, after performing optical amplification, the fundamental wave light 902 on which the ASE light 903 is superimposed is incident on the first second-order nonlinear optical element to generate the second harmonic 904. At this time, in the wavelength band of the second harmonic 904 used as the excitation light, no broadband ASE light that causes noise other than the second harmonic of the ASE light is generated. The wavelength of the second harmonic 904 is half of the wavelength of the fundamental light 902, and the two wavelengths are sufficiently separated. Therefore, it is relatively easy to realize a filter having a high extinction ratio that separates only the second harmonic 904 from the fundamental light 902 and the second harmonic 904 of the fundamental light with a dichroic mirror or the like (FIG. 9 (c)). By connecting such a filter to the output of the first second-order nonlinear optical element, the fundamental wave light and the ASE light in the excitation light wavelength band can be completely removed. Next, the signal light 901 having a plurality of wavelengths and only the second harmonic 904 are combined and incident on the second second-order nonlinear optical element, and phase sensitive amplification by non-degenerate parametric amplification (PSA) can be realized. ((D) of FIG. 9).

  Furthermore, it has become clear that there are advantages that the following prior art does not have in performing the operation according to the present embodiment.

  In the configuration of performing phase-sensitive amplification of signal light having a plurality of wavelengths using four-wave mixing in a conventional optical fiber, as shown in Non-Patent Document 7, a signal light having a plurality of wavelengths centering on the excitation light wavelength is used. Not only the four-wave mixing between the two, but the condition for phase matching is satisfied between various wavelengths. Therefore, for example, a secondary process in which the signal light is converted into another wavelength centering on the excitation light also occurs, and the amplified signal light is successively transmitted as shown in FIG. A plurality of signals 804 are generated by copying.

  The power of the amplified signal light is dissipated due to the signal generated in the above-mentioned secondary process, and the power that can amplify the desired signal light is limited. Further, since the signals generated in a secondary manner are generated between the wavelengths of the signal light having a plurality of wavelengths, it is very difficult to remove the redundant signals generated in a secondary manner. Although a method of using an ultra-narrow band optical filter or the like for separation is conceivable, the signal loss due to the filter increases as the band of the optical filter becomes narrower. As the number of multiplexed wavelengths of signal light having a plurality of wavelengths increases, the quantity of signals that are generated secondarily increases. As a result, the secondary signal may be superimposed within the band of the original signal light. In such a case, separation by an optical filter is no longer possible, and the S / N ratio of the optical signal deteriorates.

  On the other hand, in this embodiment, since only the signal light and the second harmonic are input to the second PPLN waveguide, an unnecessary wavelength conversion process as in the prior art does not occur. In the present embodiment, even when the output power is increased to +22 dBm, output saturation is not observed, and stable amplification can be performed. In addition, unlike the case where four-wave mixing is used, a secondary extra signal is not generated.

  Here, the configuration of the phase sensitive optical amplifier according to the present embodiment will be described in detail with reference to FIGS. 6 and 7 again. In this embodiment, the fundamental wave light 621 is amplified using an erbium-doped fiber laser amplifier (EDFA) 601. The amplified fundamental wave light is input to the first second-order nonlinear optical element 602-1. In the present embodiment, in order to prevent the broadband ASE light generated from the EDFA 601 from being converted by the first second-order nonlinear optical element 602-1, between the EDFA 601 and the first second-order nonlinear optical element 602-1. A band-pass filter 604 was inserted into the filter to cut unnecessary ASE light.

The second-order nonlinear optical elements 602-1 and 602-2 of this embodiment include optical waveguides 605-1 and 605-2 made of lithium niobate (PPLN) that is periodically poled. The material of the second-order nonlinear optical element is not limited to this, and can be any one of, for example, LiNbO ₃ , LNbO ₃ , LiTaO ₃ , LiNb _x Ta _x-1 O ₃ (0 ≦ x ≦ 1), or KTiOPO _4. . Furthermore, any one of the above materials may contain at least one selected from the group consisting of Mg, Zn, Fe, Sc, and In as an additive.

  When high-intensity power is incident on the PPLN waveguide, the phase matching wavelength may change due to optical damage due to the photorefractive effect. In this embodiment, a waveguide manufactured by direct bonding shown in Non-Patent Document 4 is used so that such a problem does not occur.

  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 622 and the fundamental wave light 621 emitted from the first PPLN waveguide 605-1 were separated using a dichroic mirror 606-1.

  The second harmonic 622 having a wavelength of 0.77 μm reflected by the dichroic mirror 606-1 passes through the polarization maintaining fiber 607 having a single-mode propagation characteristic at the wavelength of 0.77 μm, and the second second-order nonlinear optical element. To 602-2. At this time, fundamental wave light and ASE light having a wavelength of about 1.54 μm that could not be completely removed by the dichroic mirror 606-1 are also incident on the polarization maintaining fiber 607. However, 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 possible to effectively transmit these unnecessary lights by propagating a length of about 1 m. Can be attenuated.

  The second harmonic guided by the polarization maintaining fiber 607 is combined with the signal light 620 having a wavelength of 1.54 μm using the dichroic mirror 606-2. The dichroic mirror 606-2 is emitted from the first PPLN waveguide 605-1 to reflect only the second harmonic, and passes through the dichroic mirror 606-1 and the polarization maintaining fiber 607 and has a wavelength of about 1.54 μm. The residual component of the fundamental wave light and the ASE light can be effectively removed.

  The signal light 620 and the second harmonic 622 are combined by the dichroic mirror 606-2 and then incident on the second PPLN waveguide 605-2. The second PPLN waveguide 605-2 has the same performance and phase matching wavelength as the first PPLN waveguide 605-1, and the signal light can be phase-sensitively amplified by non-degenerate parametric amplification. it can.

  In the present embodiment, the two PPLN waveguides (605-1, 605-2) are each controlled to have a constant temperature by individual temperature regulators. It is conceivable that the phase matching wavelengths do not match at the same temperature due to manufacturing errors of the two PPLN waveguides, but even in such a case, the phase matching wavelengths of both must be matched by individually controlling the temperatures. Can do.

  The light emitted from the second PPLN waveguide 605-2 is separated by the dichroic mirror 606-3 into the second harmonic that is the excitation light and the amplified signal light. Also at this time, since the second harmonic and the amplified signal light have completely different wavelengths, unnecessary second harmonic components in the output can be effectively removed.

  In phase sensitive light amplification, it is necessary to synchronize the phase of the excitation light and the phase of the signal light. In the present embodiment, a part of the output amplified signal light is branched by the optical branching unit 603 and received by the photodetector 608, and then phase locked by the phase locked loop circuit (PLL) 609. Using the phase modulator 610 disposed in front of the EDFA 601, weak phase modulation is applied to the fundamental wave light 621 using a sine wave. The optical detector 608 and the PLL circuit 609 detect the phase shift of the phase modulation, and feed back to the drive voltage of the optical fiber stretcher 611 and the bias voltage of the phase modulator 610 by PZT arranged in front of the EDFA 601. As a result, the fluctuation of the optical phase due to the vibration of the optical fiber component or the temperature fluctuation is absorbed, so that the phase sensitive optical amplification can be stably performed.

  When the optical comb is demultiplexed by a demultiplexer, and each signal light is modulated by a modulator and then multiplexed by a multiplexer, generally, the S / N ratio is large because loss due to modulation is large. to degrade. Also, when an optical comb is generated using a modulator, the optical power is reduced by the loss of the modulator and the conversion efficiency to a plurality of carriers, and the S / N ratio is deteriorated. Furthermore, when an optical comb whose optical power is attenuated is amplified by a laser optical amplifier such as EDFA, spontaneous emission light (ASE light) is mixed, and the S / N ratio is further deteriorated along with the amplification.

  It has been found that when the signal light group mixed with the ASE light as described above is amplified by the phase sensitive optical amplifier of the present invention, it exhibits a unique behavior that cannot be obtained at all by the conventional optical amplifier.

  FIG. 10 is a diagram for explaining the effect when the phase-sensitive optical amplifier according to this embodiment is used. FIG. 10A shows an optical spectrum of a signal light group in which ASE light generated from the EDFA is intentionally mixed. FIG. 10B shows an output optical spectrum when the signal light group in which the above-mentioned ASE light is intentionally mixed is amplified by the phase sensitive optical amplifier using the configuration according to the present embodiment.

  As can be seen from (a) and (b) of FIG. 10, the difference between the amplified signal light and the ASE light at the output, that is, the optical S / N ratio, is amplified by the phase sensitive optical amplifier according to the present embodiment. (OSNR) is surprisingly improved by about 3 dB compared to the input S / N ratio.

  As an example, attention is focused on one signal having a wavelength shorter than the center wavelength among signal pairs symmetrically separated from the center wavelength. When the input signal light was measured with a resolution of 0.01 nm, it had an OSNR of 23 dB as shown in FIG. On the other hand, as shown in FIG. 10B, the amplified output signal has an OSNR of 26 dB, and it can be seen that the optical S / N ratio is improved by about 3 dB compared to the input light. Since the amplifier according to the present embodiment has polarization dependency, a polarizer should be inserted when evaluating the input spectrum in order to evaluate the S / N ratio in a fair manner. Comparison of only the polarization components.

  The reason why this phenomenon of improving the S / N ratio occurs can be explained as follows. First, consider an operation at a non-degenerate point, excluding a degenerate point where the wavelength of the excitation light is twice the wavelength of the signal light. In the present embodiment, a signal light pair having a phase relationship with the excitation light is input. When a signal light pair having the same phase is incident at a wavelength corresponding to the signal light wavelength and the idler light wavelength as in the present embodiment, all components of the signal light can be obtained as long as the phase with the excitation light can be synchronized as described above. Is amplified.

Further, as seen in a phase-sensitive optical amplifier (PSA) using an optical fiber, anti-phase information φi = −φs + α (α is a fiber or the like) conjugate with signal light by some wavelength conversion process using an optical fiber or PPLN. Even when the signal light and the idler light are input and the phase relationship among the SH light, the signal light and the idler light satisfies the following equation (7), the parametric amplification is performed. Done.
φ _SH = φ _S + φ _i + 2nπ
= Φ _S −φ _S + α + 2nπ
= Α + 2nπ (where n is an integer) Equation (7)

  That is, when conjugate signal light and idler light are incident, all components of the signal light are amplified if the phase α determined by the optical length of the fiber or the like is combined with the excitation light. When the signal light and the excitation light whose phase relationship is determined in this way are incident, all components of the signal light are amplified by adjusting the appropriate optical length.

Next, consider the amplification of the ASE light input. When considering the relative phase from the second harmonic phase φ _SH , the ASE generates light having a random phase, and therefore includes the same phase component and quadrature phase component as the excitation light. It is thought that there is.

Considering the amplification of ASE at the same wavelength as the signal and idler light, particularly when signal light and idler light are incident, the phase of ASE generated at the signal wavelength is φ _S-ASE and the phase of ASE generated at the idler wavelength Is φi _-ASE , only the component satisfying the following equation (8) is parametrically amplified.
φ _SH = φ _S-ASE + φ _i-ASE + 2nπ (where n is an integer) Equation (8)

In the case of ASE, unlike the signal-idler in which the above-described phase relationship is determined, the phases _{S S-ASE} and _{i i-ASE} of the ASE generated at the signal wavelength and idler wavelength are random, so there is no correlation between them. . Also, φ _S-ASE and φ _i-ASE have no correlation with the second harmonic phase φ _SH . Therefore, when φ _S-ASE is fixed, out of φ _i-ASE that can take a random value, it has a phase conjugate with φ _S-ASE with reference to the phase φ _S of the second harmonic. Only the components will undergo parametric amplification.

  Considering such ASE phase randomness, it can be seen that the gain for ASE is half the gain for signal light compared to the above-mentioned correlated signal light. Therefore, amplification by the optical amplifier according to this embodiment can improve the S / N ratio when compared in the optical spectrum.

  In non-degenerate parametric amplification using an optical fiber, it is difficult to obtain the effect of improving the S / N ratio as described above. The reason is as follows. In the amplification using the four-wave mixing of the optical fiber, the pumping light, the signal light, and the idler light are all in the 1.55 μm band, and are usually close to the pumping light wavelength in order to generate the pumping light using EDFA or the like. The ASE light generated from the EDFA is mixed in the wavelength band of the signal light and idler light. In addition, since the power of the excitation light is often relatively higher than that of the signal light and idler light, the influence of noise due to ASE light mixed from outside is great. Therefore, unlike the phase sensitive optical amplifier of this embodiment, it is not possible to obtain a remarkable effect that can improve the S / N ratio with respect to input and output.

  In contrast, in the phase sensitive optical amplifier according to the present embodiment, the fundamental light is amplified by the EDFA and then converted to the second harmonic, and the ASE light in the 1.55 μm band is also removed and then incident on the parametric medium. Non-degenerate parametric amplification is performed. Therefore, mixing of ASE light generated from the EDFA used for generating the excitation light can be prevented. According to the present embodiment, it is possible to obtain an effect of improving the S / N ratio using phase sensitivity to signal light and idler light.

  Next, the operation at the degenerate point where the wavelength twice that of the excitation light and the wavelength of the signal light are the same, which is excluded in the above description, will be described.

As shown in FIG. 6, in the present embodiment, signal light having the same wavelength as twice the wavelength of the excitation light is also incident. Even at this wavelength, the optical spectrums of FIGS. 10 (a) and 10 (b) are used. As far as we can see, the S / N ratio has improved. However, as described below, when degenerate parametric amplification is performed in which the wavelength of the excitation light is twice the wavelength of the signal light, the S / N ratio is improved by comparing the input and output after photoelectric conversion. Never do. In the degenerate parametric amplification, amplification is performed when the following equation (9) is established between the signal light phase φ _S and the excitation light phase φ _p .
φ _S = φ _SH / 2 + nπ = φ _p + nπ (where n is an integer) Equation (9)

  That is, the signal light is amplified only in the component in phase with the excitation light. The same applies to the case where the ASE light is input. Considering that the phase of the ASE light is random, the gain for the ASE is higher than that for the case where the signal light phase-synchronized with the excitation light is incident. Of half. Therefore, the S / N ratio when viewed with the light power is improved by 3 dB. This is the same as the operation at the non-degenerate point.

  In the operation at the degeneracy point, in-phase components of the input ASE light are amplified, and quadrature components are attenuated. The fact that this quadrature component is not amplified will appear as a difference in gain when viewed in terms of optical power, but components that originally have quadrature phase with signal light will generate intensity noise even if it interferes with signal light. There is no. On the other hand, a component in phase with the signal light in the ASE light that interferes with the signal light and causes intensity noise is amplified by receiving the same gain as the signal. Therefore, in the phase sensitive parametric amplification at the degeneracy point, the component of the ASE light that interferes with the signal light is not reduced, and the S / N ratio after photoelectric conversion of the optical signal does not change.

Next, how the S / N ratio after photoelectric conversion behaves in the non-degenerate parametric amplification employed in the present embodiment will be described. As described above, in non-degenerate parametric amplification, the gain received by ASE light is half that of signal light. Focusing on the phase of the amplified ASE light at this time, only the component satisfying the following expression (10) is amplified among the ASE light components respectively input to the wavelengths of the signal light and the idler light as described above.
φ _SH = φ _S-ASE + φ _i-ASE + 2nπ (where n is an integer) Equation (10)

  In the operation at the degenerate point, only the component in phase with the pumping light is amplified among the input ASE light, whereas in the operation at the non-degenerate point, the ASE light at the signal light wavelength and the idler light wavelength is amplified. Only the phase is in a conjugate relationship is a necessary condition for amplification. The phase relationship among the amplified ASE light, signal light, and idler light is not particularly defined. Therefore, unlike the operation at the degeneracy point, the operation at the non-degeneration point is considered that the amplified ASE light includes a component having the same phase as that of the signal light as well as the quadrature phase component of the signal light. Since the phase of the ASE light is random for both input and output, and the gain received by the ASE light is half of the gain received by the signal, the S / N ratio determined by the beat noise with the ASE light after photoelectric conversion is amplified by 3 dB. It will be improved later.

  In comparison with the degenerate operation, the intensity of the in-phase ASE light contributing to the intensity noise in the non-degenerate operation is half the intensity of the in-phase ASE light in the degenerate operation, and the power of the entire ASE light after amplification is the degenerate operation. Same as time. Accordingly, considering that only in-phase components of the amplified ASE light cause intensity noise due to interference with the signal, the S / N ratio is improved by 3 dB in the non-degenerate operation compared to the degenerate operation.

  In order to confirm this, the beat noise levels of the signal light and the ASE light were compared between the optical comb signal input to the amplifier of the present invention and the amplified optical comb signal. Each desired carrier wave was cut out with a band-pass filter, the average power was made the same with an optical attenuator, and then measured using an electric spectrum analyzer with a built-in O / E converter.

  FIG. 11 is a diagram showing beat noise levels of signal light and ASE light at the input and output when amplified by the phase sensitive optical amplifier using the configuration according to the first embodiment. As shown in FIG. 11A, when the peak of the degeneracy point is observed, there is no difference in the noise level at the input / output of the amplifier. On the other hand, as shown in FIG. 11B, when the peak of the non-degenerate point is observed, the noise level is lowered by 3 dB due to amplification. That is, it was confirmed that the S / N ratio was improved by 3 dB. As described above, according to the phase sensitive optical amplifier of the present embodiment, the output S / N ratio can be improved more than the input by amplifying the signal whose S / N ratio is deteriorated by a laser amplifier or the like. An effect can be obtained.

  A signal subjected to actual data modulation was made incident on the phase sensitive optical amplifier according to the present embodiment, and the effect of improving the S / N ratio according to the present embodiment was examined.

  FIG. 12 is a diagram showing an experimental configuration for examining the effect of improving the S / N ratio using a signal obtained by performing data modulation on an optical comb. The optical comb generated by modulating the single wavelength light source 1201 by the optical modulator 1203 is subjected to BPSK modulation by the LN modulator 1205 and is incident on the phase sensitive optical amplifier of this embodiment shown in FIG.

  In actual optical comb modulation, a signal is amplified later using a laser amplifier such as an EDFA in order to compensate for optical comb generation and loss during data modulation. At this time, signal noise due to ASE light is added.

  In this experiment for evaluating the phase sensitive optical amplifier according to the present embodiment, in order to investigate the effect of improving the S / N ratio, an ASE noise is intentionally applied to the optical comb signal subjected to data modulation via the EDFA 1206. It is added.

  The fundamental wave light of the phase sensitive optical amplifier was branched from the single wavelength light source 1201 used to generate the optical comb. The peak at the non-degenerate point was separated from the signal before and after amplification by the demultiplexer 1210, the reception power was adjusted by the optical attenuator 1211, and the signal was received by the reception device 1212.

  FIG. 13 is a diagram illustrating error rate data with respect to measured reception power. When an optical comb whose optical power is attenuated due to generation of an optical comb and loss during data modulation is amplified by a laser optical amplifier such as EDFA, spontaneous emission light (ASE light) is mixed, and the S / N ratio is accompanied by the amplification. There was a problem that would deteriorate. From the result shown in FIG. 13, in order to obtain the same error rate as when the ASE noise is intentionally added (plot ○) and when the ASE noise is not added (plot ●), the necessary reception power is very large. I understand that

However, the data error rate (plot black square) of the output signal obtained by intentionally injecting the signal to which the ASE noise is added to the phase sensitive optical amplifier according to this embodiment is added with the original ASE noise. Compared to the input signal, there was a significant improvement in the required received power. When compared with an error rate of 10 ⁻⁹ , when the phase sensitive optical amplifier of the present invention is used, a remarkable effect of improving the power penalty due to ASE noise by 3 dB is seen.

  As can be seen from the effect of improving the S / N ratio of the optical signal with the ASE noise added and superimposed, it can be seen that the phase sensitive optical amplifier of the present invention also functions as a signal-to-noise ratio improvement device. That is, by providing the phase sensitive optical amplifier of the present invention to which the signal light having two wavelengths paired with spontaneous emission light noise is input, the gain for the signal light and the spontaneous emission light noise can be reduced. Based on the difference from the gain, it is possible to realize a signal-to-noise ratio improving apparatus in which spontaneous emission light noise is removed by selectively amplifying signal light from signal light superimposed with spontaneous emission light noise.

  FIG. 14 is a diagram showing a configuration of a phase sensitive optical amplifier according to the second embodiment of the present invention. The single wavelength light source 1401 uses 1.54 μm CW light. Using a modulator 1403, a pair of signal lights (s + 1 and s-1, s + 2 and s- are symmetrically separated by the same optical frequency difference around the optical frequency corresponding to twice the wavelength of the pumping light. 2, s + 3 and s-3, s + 4 and s-4, and so on). An optical signal generator can be constructed by combining the phase sensitive optical amplifier of the present invention with a light source that generates an optical comb composed of light of a plurality of wavelengths synchronized in phase.

  In this embodiment, an optical comb generator composed of a single wavelength light source 1401 and an optical modulator 1403 is adopted, but other methods such as a method using a mode-locked laser as a light source and a method using a nonlinear medium are used. It may be used to generate an optical comb.

  An optical comb using a duplexer 1404 designed so that two wavelengths, which are symmetrically separated from one wavelength among a plurality of wavelengths of the generated optical comb signal by the same optical frequency difference, are output to the same optical path. Each wavelength was separated. As the branching filter, a waveguide type multiplexer / demultiplexer represented by an arrayed waveguide grating (AWG) may be used, or a WSS (Wavelength Selective Switch) using MEMS is used. A multiplexer / demultiplexer using a spatial optical system may be used.

  An optical modulator 1405 is connected to each output of the demultiplexer 1404, and performs data modulation on each pair of signal lights. Next, after combining each signal pair using the multiplexer 1406, the signal is amplified by a laser amplifier 1407 such as an EDFA. In the configuration shown in FIG. 14, the data modulation signals are amplified in a lump after being combined, but the configuration is not limited to this. For example, in the case where a device in which a semiconductor modulator is used for data modulation and a semiconductor amplifier such as SOA is integrated in the modulator can be used, each pair of signals is set to a laser amplifier 1507 as in another configuration example shown in FIG. After being amplified by the above, the multiplexer 1506 may be used for multiplexing. An optical signal generator can be configured by combining the phase sensitive optical amplifying apparatus shown in FIG. 15 and a light source that generates an optical comb composed of light of a plurality of wavelengths synchronized in phase.

  14 and FIG. 15, the same data modulation is applied to each signal pair by using a duplexer that outputs two wavelengths that are symmetrically separated by the optical frequency difference to the same optical path. Yes. As another configuration example, as shown in FIG. 16, an optical demultiplexer 1604 that separates wavelengths of optical combs and an optical modulator 1605 that is connected to each output of the demultiplexer, respectively, A configuration may be used in which signal pairs of the comb that are symmetrically separated by the same optical frequency difference are respectively modulated by the same data Data1 to Dataan. Needless to say, an optical signal generator can be constructed by combining the phase sensitive optical amplifying device shown in FIG. 16 with a light source that generates an optical comb composed of light of a plurality of wavelengths synchronized in phase. .

  In the process of generating an optical comb, the optical power is reduced by the loss in the modulator and the conversion efficiency into a plurality of carriers. Also, when the optical comb is demultiplexed by a demultiplexer, data modulated by a modulator, and multiplexed by a multiplexer, the optical power is significantly higher than the original optical comb due to the insertion loss of each component. It will attenuate. When amplification is performed up to the power required for transmission through an optical fiber with an optical amplifier using a laser medium of the prior art, the signal S / N ratio is significantly deteriorated because the input power to the optical amplifier is small. Although the principle of low-noise optical amplification by a phase-sensitive optical amplifier is known, in general, a phase-sensitive optical amplifier uses degenerate parametric amplification. Therefore, the signal wavelength that can be amplified is one, and multiple carriers can be simultaneously transmitted. It could not be amplified.

  However, by using the phase sensitive optical amplifier according to the present embodiment, it is possible to amplify a multi-wavelength optical comb with low noise. Further, regarding the S / N ratio caused by the beat noise between the signal light and the ASE light, the S / N ratio can be improved more than the input by amplifying using the phase sensitive optical amplifier according to the present embodiment. Has an effect.

  The fundamental wave light of the phase sensitive optical amplifier was branched from the single wavelength light source used to generate the optical comb. The optical comb signal is incident on the phase sensitive optical amplifier according to the present embodiment. As a result of examining the optical S / N ratio (OSNR) of each input and output and the S / N ratio after photoelectric conversion, the S / N ratio of the output signal was improved by 3 dB compared to the S / N ratio of the input signal. It was. By using the configuration of the phase sensitive optical amplifier according to the present embodiment, the S / N ratio due to the beat noise between the signal light and the ASE light as intensity noise is improved.

  In addition to the S / N ratio improvement effect described above, by using the configuration of the present embodiment, a synergistic effect of the effect of suppressing the phase chirp component by attenuating the quadrature phase can be obtained. In order to confirm the amplification characteristics, the signal light after amplification was observed and the time waveform was examined.

  FIG. 17 is a diagram illustrating a time waveform of a signal amplified by the phase sensitive optical amplifier according to the present embodiment. FIG. 17A shows the output waveform of the incident signal light when no excitation light is incident, and FIG. 17B shows the output waveform when the excitation light phase and the signal light phase are set by the PLL. FIG. 17C shows output waveforms when the excitation light phase and the signal light phase are set to be shifted by 90 degrees by the PLL.

  When the excitation light phase and the signal light phase are set so as to be shifted by 90 degrees, phase sensitive amplification is realized from the state in which the ON level of the signal is attenuated as shown in FIG. I understand. A waveform in which only a transitional portion between the ON and OFF levels of the signal was amplified was observed. This indicates that phase noise is superimposed on the signal light.

  For example, when a modulator of the type using phase modulation of only one arm in the modulator is used as an optical modulator for superimposing data, chirp is generated by the data modulator. That is, the phase of the output of the modulator fluctuates when transitioning between ON and OFF, and a quadrature phase component is generated based on the ON state. For this reason, if the signal light phase and the excitation light phase are set to be orthogonal to each other, only the phase chirp component is phase-sensitively amplified. This means that in the state where the phase is matched to the ON state of the signal light, even if the input signal contains a phase chirp, the chirp component is removed, and the signal is shaped and amplified as a chirp-free signal. It shows what you can do.

  As a result of transmitting the signal generated using the configuration in the second embodiment shown in FIG. 14 through the optical fiber, the effect of removing the beat noise between the signal light and the ASE light as intensity noise and the phase chirp component Due to the suppression effect, the transmission distance could be tripled or more.

  Next, the configuration of the third embodiment of the phase sensitive optical amplifier of the present invention will be described.

  FIG. 18 is a diagram showing a configuration of a phase sensitive optical amplifier according to the third embodiment of the present invention. Using a single wavelength light source 1801 and a modulator 1803, a pair of signal lights (s + 1 and s-1, s + 1, s-1,..., Symmetrically separated by the same optical frequency difference around the optical frequency corresponding to twice the wavelength of the excitation light. An optical comb having s + 2 and s-2, s + 3 and s-3, s + 4 and s-4, and so on) is generated. In order to compensate for the loss of the modulator and the loss due to the conversion to a plurality of carriers in the generation process of the optical comb, the optical comb signal is amplified using a normal laser amplifier 1804 such as an EDFA. As the fundamental light of the phase sensitive optical amplifier, a signal branched from the single wavelength light source 1801 used to generate the optical comb is used. The optical comb signal was incident on a phase sensitive optical amplifier according to the present invention and amplified. An optical signal generator can be configured by combining the phase sensitive optical amplifier shown in FIG. 18 with a light source that generates an optical comb composed of light of a plurality of wavelengths synchronized in phase.

  Using a method similar to the method shown in the first embodiment, the optical S / N ratio (OSNR) of each input and output and the S / N ratio after photoelectric conversion are examined. Compared to the N ratio, the S / N ratio was improved by 3 dB in the output signal of the phase sensitive optical amplifier of the present embodiment. As shown in FIG. 18, an optical comb signal having a high S / N ratio can be generated by amplifying the optical comb signal using a normal laser amplifier 1804 and then using the phase sensitive optical amplifier according to the present invention.

  Each comb was individually subjected to data modulation (Data 1 to Data) using a demultiplexer 1806 for separating each wavelength of the optical comb and an optical modulator 1807 connected to each output of the demultiplexer 1806. Thereafter, an optical comb signal was incident on one optical fiber using an optical multiplexer 1808, and the signal was transmitted.

  By using the phase sensitive optical amplifier according to the present embodiment, a signal with a high S / N ratio can be generated, and the transmission distance can be increased.

Furthermore, the configuration of the fourth embodiment of the phase sensitive optical amplifier of the present invention will be described. The light source for generating signal light is close to the phase sensitive optical amplifying unit so that the phase sensitive optical amplifier is used immediately after the optical signal transmitter. Can be used as fundamental light by branching a part of the light source for signal light. However, when a phase sensitive optical amplifier is used as a repeater amplifier in optical transmission or a preamplifier at the receiving end, the phase synchronization means is used to express the pumping light phase and the signal light phase in the phase sensitive light amplifying apparatus ( It is necessary to synchronize so as to satisfy the relationship 1).

  FIG. 19 is a diagram showing a configuration of a phase sensitive optical amplifier according to the fourth embodiment of the present invention using a center wavelength signal as the phase synchronization means. In the present embodiment, a data signal using a center wavelength signal of signal light having a plurality of wavelengths as a pilot tone of CW light is used as an input signal. A pair of signal lights (s + 1 and s-1, s + 2 and s-2, s + 3 and s-3, s + 4 and s-4, symmetrically separated by the same optical frequency difference around the optical frequency. Binary phase modulation is applied to the same), and a signal having a plurality of wavelengths that can be used as a pilot tone of CW light without modulation of the center wavelength signal is used as the signal light 1930.

  Modulated signal light 1930 having a pilot tone of CW light at the center wavelength is transmitted through the transmission medium. An optical fiber was used as the transmission medium. After the polarization rotation in the optical fiber was corrected by the polarization controller 1920, only the pilot tone of the CW light was separated using a notch type filter 1921 that cuts out only the center wavelength.

  Since the light intensity of the signal is a loss of light intensity due to the transmission optical fiber, the light intensity is extremely small and the S / N ratio is degraded. After adjusting the light intensity of the branched CW light pilot tone with the attenuator 1911, the light was synchronized with the fundamental wave light source 1913 in the phase sensitive optical amplifying device through the circulator 1912. As the fundamental light source 1913, a DFB type semiconductor laser was used.

  When the light intensity input to the fundamental light source 1913 is changed using the attenuator 1911 and the state is observed with an optical spectrum analyzer, the wavelength of the semiconductor laser is drawn into the pilot tone wavelength when the light intensity is set to several tens of μW. The situation was observed. It has been found that the fundamental light source in the phase sensitive optical amplifier is phase locked to the pilot tone. As a result, it was possible to generate excitation light having a good S / N ratio from a pilot tone of signal light having a deteriorated S / N ratio.

  A signal of a plurality of wavelengths transmitted through an optical fiber has a phase shift between symmetrically separated signal light pairs due to a dispersion effect in the optical fiber. In order to compensate for this phase shift, a dispersion compensation (adjustment) medium 1922 is configured in the phase sensitive optical amplifier. As the dispersion compensation (adjustment) medium, a phase adjuster using liquid crystal such as LCOS was used. The phase may be adjusted using another means such as using a fiber having inverse dispersion. A phase adjuster (not shown) matched the phase between the signal light pair.

  When optical amplification was attempted using the phase-sensitive amplification configuration described in the first embodiment using the fundamental wave light phase-synchronized with the pilot tone of the signal light, the same characteristic results as in the first embodiment were obtained. It was. By adopting the configuration of the phase sensitive optical amplifier of the present embodiment, the phase synchronization means can be used in the relay amplification and the preamplifier at the receiving end where the light source for generating the signal light is not arranged near the phase sensitive optical amplification unit. Phase sensitive amplification could be performed by using.

  As described above in detail, according to the present invention, a plurality of wavelengths can be collectively amplified 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. It becomes. Further, an optical signal generator can be configured in combination with a light source that generates an optical comb. High-quality optical signal amplification is possible while preventing deterioration of the S / N ratio. Furthermore, noise caused by uncorrelated light such as ASE light can be suppressed by selectively amplifying signal light having a phase correlation with the excitation light by using the present invention.

  Compared with the case of amplification with a laser amplifier of the prior art, since the S / N ratio of the signal in optical fiber transmission can be improved, it is possible to transmit a signal at a higher speed than that of the prior art to a long distance with lower power. Since the phase noise of the incident signal light can be removed and amplified, the influence of signal deterioration due to the wavelength dispersion of the optical fiber can be suppressed, and the transmission distance of the amplified signal light can be extended. Furthermore, by suppressing ASE noise, the SN ratio of an optical signal once deteriorated can be improved. It is possible to improve the signal-to-noise ratio of signal light degraded by beat noise of ASE light and signal light.

  The present invention is generally applicable to communication systems. In particular, it can be used for optical communication systems and optical measurement systems.

DESCRIPTION OF SYMBOLS 101 Phase sensitive light amplification part 102 Excitation light source 103 Excitation light phase control part 104-1, 104-2, 603 Optical branching part 110, 410 Input signal light 111, 211, 411-1, 411-2 Excitation light 112, 412 Output Signal light 201 Laser light source 202 SHG crystal 203 OPA crystal 210, 620 Signal light 401 Excitation light phase synchronization means 402, 503, 601, 1206, 1407, 1607 Erbium-doped fiber laser amplifier (EDFA)
403 optical fiber 404 signal light filter 602-1, 602-2 second-order nonlinear optical element 605-1, 605-2 PPLN waveguide 606-1, 606-2 dichroic mirror 607 polarization maintaining fiber 621 fundamental wave light 1201, 1401, 1601, 1801 Single wavelength light source 1404, 1504, 1604, 1806 Multiplexer 1406, 1506, 1606, 1808 Demultiplexer

Claims (12)

In a phase-sensitive optical amplifier that amplifies signal light by optical mixing using a nonlinear optical effect,
As signal light, one or a plurality of signal light pairs having two wavelengths that are symmetrically separated from the fundamental wave light by the same optical frequency difference on the optical frequency axis and each have the same or inverted phase information are input. ,
An optical fiber laser amplifier for amplifying the fundamental light;
A first second-order nonlinear optical element connected to the optical fiber laser amplifier and generating a second harmonic;
A filter that separates the second harmonic from the fundamental light and the second harmonic output from the first second-order nonlinear optical element;
A multiplexer that multiplexes the signal light composed of a pair of signal light and the second harmonic that is excitation light;
A second second-order nonlinear optical element that receives the combined signal light and the excitation light and performs non-degenerate parametric amplification of the signal light;
A filter that separates amplified signal light from the signal light and the excitation light output from the second second-order nonlinear optical element;
A phase sensitive optical amplifying device comprising: means for synchronizing the phase of the signal light and the phase of the pumping light.   A filter that separates the second harmonic from the fundamental light and the second harmonic, and a multiplexer that combines the signal light and the second harmonic are a die using a dielectric film. 2. The phase sensitive optical amplifying device according to claim 1, wherein the phase sensitive optical amplifying device is a clock mirror. The first second-order nonlinear optical element and the second second-order nonlinear optical element each have an optical waveguide made of a second-order nonlinear optical material having a periodically poled structure,
The second-order nonlinear optical material is any one of LiNbO ₃ , LNbO ₃ , LiTaO ₃ , LiNb _x Ta _x-1 O ₃ (0 ≦ x ≦ 1), KTiOPO ₄ , or Mg, Zn, Fe, 2. The phase sensitive optical amplifying device according to claim 1, wherein at least one selected from the group consisting of Sc and In is contained as an additive. The signal light further comprises a pilot tone of continuous wave light,
The phase sensitive optical amplification device further includes a filter for separating the pilot tone, and a semiconductor laser light source,
The semiconductor laser light source is light injection locked by a pilot tone of the continuous wave light,
4. The phase sensitive optical amplifying device according to claim 1, wherein the continuous light output from the semiconductor laser light source that is phase-locked with the injected light is used as the fundamental wave light.   The phase-sensitive optical amplification according to any one of claims 1 to 3, further comprising a dispersion compensation medium for adjusting a phase between two light beams of a pair of the signal light. apparatus. The phase sensitive optical amplifying device according to any one of claims 1 to 3, wherein the signal light having two wavelengths paired with spontaneous emission light noise is input,
Based on the difference between the gain with respect to the signal light and the gain with respect to the spontaneous emission light noise, the signal light is selectively amplified from the signal light on which the spontaneous emission noise is superimposed, so that the spontaneous emission noise is reduced. An apparatus for improving a signal-to-noise ratio, characterized by being eliminated. A light source that generates an optical comb composed of light of a plurality of wavelengths synchronized in phase;
Separating each wavelength of the optical comb and making the same pair of two wavelength light pairs symmetrically separated from the light of one of the wavelengths of the optical comb by the same optical frequency difference A duplexer configured to output to the optical path;
A plurality of light modulators, each connected to an output of the duplexer corresponding to one of the light pairs, for modulating the corresponding light pair;
A multiplexer that multiplexes outputs from the plurality of optical modulators,
The output of the multiplexer is input to the phase sensitive optical amplifying device according to any one of claims 1 to 3, wherein one of the plurality of wavelengths of the optical comb is the fundamental wave light. A characteristic optical signal generator. A light source that generates an optical comb composed of light of a plurality of wavelengths synchronized in phase;
Each wavelength of the optical comb is separated, and a pair of two wavelengths of light that is symmetrically separated from the light of one of the plurality of wavelengths of the optical comb by the same optical frequency difference is different. A duplexer configured to output to the optical path;
A plurality of optical modulators, each connected to each output of the corresponding duplexer, for applying optical modulation having the same or inverted phase information to the pair of light;
A multiplexer that multiplexes outputs from the plurality of optical modulators,
The output of the multiplexer is input to the phase sensitive optical amplifying device according to any one of claims 1 to 3, wherein one of the plurality of wavelengths of the optical comb is the fundamental wave light. A characteristic optical signal generator. A light source that generates an optical comb composed of light of a plurality of wavelengths synchronized in phase;
4. The phase sensitive optical amplifying apparatus according to claim 1, wherein one of the plurality of wavelengths of the optical comb to which an output of a light source that generates the optical comb is input is the fundamental light. When,
A demultiplexer connected to the output of the phase sensitive optical amplifying device and separating each wavelength of the optical comb;
A plurality of optical modulators each connected to each output of the duplexer;
An optical signal generator comprising: a multiplexer for multiplexing outputs from the plurality of modulators.   10. The optical signal generator according to claim 7, wherein one wavelength used for the fundamental light among a plurality of wavelengths of the optical comb is a center wavelength of the optical comb.   The phase-locked light of the optical comb is generated by modulating a single-mode laser light source by an optical modulator, and the light branched from the laser light source is used as the fundamental wave light for the phase-sensitive optical amplification. The optical signal generator according to claim 7, wherein the optical signal generator is input to a device.   10. At least one laser amplifier is inserted at any position between the light source that generates the optical comb and the phase sensitive optical amplifying device. The optical signal generator described.

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