JP2015059987A - Optical amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce a noise figure in comparison with a conventional manner in a configuration where a parametric amplifier is cascade-connected to an amplifier which amplifies signal light by inductive emission.SOLUTION: The optical amplifier includes: a first optical amplifier unit 30; an intermediate optical amplifier unit 40; and a second optical amplifier unit 60. The first optical amplifier unit generates idler light on the basis of input pump light and signal light, and outputs first amplified light including the signal light, the pump light and the idler light. The intermediate optical amplifier unit amplifies the intensity of the signal light and the idler light included in the first amplified light by inductive emission, and outputs intermediate amplified light including the signal light and the idler light. When the pump light and the intermediate amplified light are input, the second optical amplifier unit amplifies the intensity of the signal light included in the intermediate amplified light by parametric amplification, and outputs second amplified light including the signal light.

Description

  The present invention relates to an optical amplifying device, and more particularly to an optical amplifying device capable of reducing a noise figure in a wavelength band of an optical signal to be amplified.

  In an optical communication network using an optical fiber, an optical amplifying device is installed in an optical signal communication path in order to amplify the intensity of an optical signal attenuated by transmission.

  As an apparatus used for an optical amplifying device, for example, there is an EDFA (Erbium Doped Fiber Amplifier) using an optical fiber in which a core is doped with erbium ions. In the EDFA, an optical signal input to an optical fiber to which excitation light is supplied is amplified by stimulated emission.

  In EDFA, light (spontaneously emitted light) generated by spontaneous emission is added to an optical signal as noise, so that the SNR of output light is degraded as compared to the signal-to-noise ratio (SNR) of input light. . In the following description, a value obtained by dividing the SNR of the input light by the SNR of the output light is also referred to as a noise figure. Further, noise due to spontaneous emission (ASE (Amplified Spontaneous Emission) noise) cannot be reduced below the quantum limit. Therefore, in principle, optical amplification using a conventional EDFA has been impossible to reduce the noise figure to less than 2 (ie, 3 dB).

  At present, in order to cope with an increase in traffic in an optical communication network, development of a large-capacity optical transmission system to which a multi-level modulation method having excellent frequency utilization efficiency is applied. In a channel transmission capacity 400 Gbps optical transmission system that is being studied for introduction, a high-level modulation scheme such as 16-QAM (Quadrature Amplitude Modulation) is applied. As a problem in the transmission of such a multilevel modulation signal, there is a vulnerability to the above-described degradation of SNR. Therefore, an optical amplification technique with a small noise figure is required for long-distance transmission of optical signals.

  Parametric amplification is known as a technique for further reducing the noise figure when amplifying an optical signal. In parametric amplification, an optical signal to be amplified (hereinafter also referred to as signal light) and pump light are input to a nonlinear optical element. The signal light is amplified by applying the energy of the pump light to the signal light using four-wave mixing which is one of nonlinear optical effects. Nonlinear optical elements used for parametric amplification include, for example, PPLN (Periodically Poled Lithium Niobate) waveguides and highly nonlinear fibers.

  As a device using parametric amplification (hereinafter also referred to as a parametric amplifier), there are PIA (Phase Insensitive Amplifier) and PSA (Phase Sensitive Amplifier). The PIA and PSA each include a nonlinear optical element, and amplifies the optical signal by parametric amplification in the nonlinear optical element. Here, in PIA, the amplification gain does not depend on the phase of the input light. On the other hand, in PSA, the amplification gain depends on the phase of the input light. In PSA, a noise figure of less than 2 (that is, 3 dB) that cannot be realized by the above-described EDFA is achieved. Therefore, an optical amplifying device using a parametric amplifier has attracted attention as an optical amplifying device used for optical communication (for example, see Non-Patent Document 1).

  However, it is difficult for a parametric amplifier to obtain a large amplification gain as compared with an EDFA. For example, in a PIA, when a PPLN waveguide is used as a nonlinear optical element, only an amplification gain up to about 4 dB can be obtained.

  When a highly nonlinear fiber is used as the nonlinear optical element, it is necessary to limit the intensity of the pump light in order to suppress the generation of stimulated Brillouin scattering. However, in order to increase the amplification gain, it is necessary to relax the restriction on the intensity of the pump light. As this means, a method of widening the line width of the spectrum of the pump light by adding phase modulation to the pump light is generally used. However, pump light subjected to high-speed phase modulation has a problem of adding intensity fluctuation or phase fluctuation to the signal light input together with the parametric amplifier. For example, assuming a case where optical amplification is performed a plurality of times in long-distance transmission of several hundred km, the quality of signal light is significantly impaired due to accumulation of intensity fluctuations and phase fluctuations. For this reason, it is not preferable to add phase modulation to the pump light.

  Therefore, in order to compensate for the amplification gain of the parametric amplifier, there is a configuration in which EDFAs are cascade-connected after the parametric amplifier. For example, the amplification gain of the entire device can be increased by cascading EDFAs after the PIA (see, for example, Non-Patent Document 2 and Non-Patent Document 3).

Z. Tong et al. , “Nois performance of optical fiber transmission links that use non-degenerated cascaded phase-sensitive amplifiers” Optics. Express. , Vol. 18, no. 15, p. 15426, 2010 M.M. E. Marhic, “Hybrid phase-sensitive and phase-insensitive amplifiers for optical communication” in Proceedings of 17th OECC, Busan, Korea, July 12-20. M.M. E. Marhic, “Noise figure of hybrid optical parametric amplifiers”, Opt. Express 20 (27), 28578-28763 (2012).

  However, as described above, EDFA adds noise due to spontaneously emitted light. For this reason, in the configuration in which EDFAs are cascade-connected in the subsequent stage of the parametric amplifier, the amplification gain can be increased, but the noise figure also increases.

  The present invention has been made in view of the above-described problems, and an object of the present invention is to compare a conventional parametric amplifier and an amplifier that amplifies signal light by stimulated emission such as EDFA in a cascade connection. It is an object of the present invention to provide an optical amplifying device that can reduce the noise figure.

  In order to achieve the above object, an optical amplifying device according to the present invention has the following features.

  That is, the optical amplification device includes a first optical amplification unit, an intermediate optical amplification unit, and a second optical amplification unit. The first optical amplification unit generates idler light based on the input pump light and signal light, and outputs first amplified light including the signal light, pump light, and idler light. The intermediate light amplifying unit amplifies the intensity of the signal light and idler light included in the first amplified light by stimulated emission, and outputs intermediate amplified light including the signal light and idler light. Pump light and intermediate amplified light are input to the second optical amplifier. The second optical amplifying unit amplifies the intensity of the signal light included in the intermediate amplified light by parametric amplification, and outputs the second amplified light including the signal light.

  In the optical amplifying device of the present invention, the second optical amplifying unit for amplifying the signal light by parametric amplification is provided after the intermediate optical amplifying unit for amplifying the signal light by stimulated emission. Noise due to spontaneous emission light added in the intermediate light amplification unit is uncorrelated between the wavelength band of the signal light and the wavelength band of the idler light. Therefore, in the second optical amplifying unit, the amplification gain with respect to the noise caused by the spontaneous emission light is half the value of the amplification gain of the correlation noise. Therefore, in the noise added and amplified in the entire optical amplifying apparatus of the present invention, the ratio of spontaneous emission optical noise added in the intermediate optical amplifying unit can be reduced. As a result, by adjusting the respective amplification gains to the signal light in the first optical amplification unit, the intermediate optical amplification unit, and the second optical amplification unit, the final noise figure in the wavelength band of the signal light can be reduced and desired Amplification gain can be obtained.

It is the schematic block diagram which showed the 1st optical amplification apparatus typically. It is a figure which shows an example of the spectrum in a 1st optical amplifier. It is a figure which shows the result of the simulation which evaluates the characteristic of a 1st optical amplifier. It is the schematic block diagram which showed the 2nd optical amplification apparatus typically.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the shape, size, and arrangement relationship of each component are merely schematically shown to the extent that the present invention can be understood. In the following, a preferred configuration example of the present invention will be described. However, the material and numerical conditions of each component are merely preferred examples. Therefore, the present invention is not limited to the following embodiments, and many changes or modifications that can achieve the effects of the present invention can be made without departing from the scope of the configuration of the present invention.

(First optical amplification device)
With reference to FIGS. 1 and 2, the structure of an optical amplifying device (hereinafter referred to as a first optical amplifying device) according to a first embodiment of the present invention will be described.

  FIG. 1 is a schematic configuration diagram schematically showing the first optical amplifying device. In FIG. 1, each component is connected with a line, which schematically shows a transmission path through which a signal propagates. Each component may be connected by, for example, an optical fiber or an optical waveguide, or may be connected by so-called spatial coupling.

  The first optical amplifying apparatus 100 can amplify the intensity of an optical signal by installing it in an optical signal communication path in an optical communication network, for example. In that case, the optical signal to be amplified (herein also referred to as signal light) is a known optical signal modulated with data to be transmitted, such as a phase shift keying signal or an amplitude shift keying signal. .

  In addition, here, a configuration example in a state where the polarization state of the signal light is temporally stable is shown. In addition, the polarization diversity system is adopted for components having polarization dependency, such as nonlinear optical devices constituting the first optical amplification unit and the second optical amplification unit, or a polarization stabilization device is introduced. By doing so, polarization independence can be achieved.

  The first optical amplifying device 100 includes a pump light source 15, a pump optical demultiplexing unit 20, a first multiplexing unit 25, a first optical amplifying unit 30 functioning as a PIA, a filter unit 35, and intermediate optical amplification. Unit 40, second multiplexing unit 50, dispersion compensator 55, second optical amplification unit 60 functioning as a PSA, output side demultiplexing unit 65, optical phase shifter controller 75, and optical phase shifter 45. And is configured. The first optical amplifying unit 30 generates idler light based on the wavelengths of the input signal light and pump light. The intermediate light amplification unit 40 amplifies the intensity of the signal light and idler light by stimulated emission. The second optical amplifying unit 60 amplifies the intensity of the signal light by parametric amplification.

  The signal light S <b> 101 input from the input port 10 to the first optical amplification device 10 is first sent to the first multiplexing unit 25. The first multiplexing unit 25 multiplexes the signal light S101 and the pump light S103a. As the first multiplexer 25, for example, a WDM coupler / divider can be used.

  The pump light S103a is generated by the pump light source 15.

  The pump light source 15 includes, for example, an external resonator light source 16, an optical amplifier 17, and a band pass filter 18. The phase noise of the pump light is added to the signal light. For this reason, it is desirable to use the external resonator light source 16 having a sufficiently narrow line width of the frequency spectrum as the pump light source 15. For example, as the external resonator light source 16, a laser diode that generates laser diode output light whose polarization plane matches that of the signal light S101 is used.

  In addition, in the nonlinear optical device constituting the first optical amplification unit 30 and the second optical amplification unit 60, it may be necessary to increase the line width of the frequency spectrum, such as when the influence of stimulated Brillouin scattering becomes a problem. . In this case, phase modulation may be performed using an optical phase modulator driven by one or more sine waves of about 100 MHz to 2 GHz.

  The wavelength of the pump light must be set outside the wavelength band of the signal light. For example, when the wavelength band of the signal light is the C band (1530 to 1565 nm), the wavelength of the pump light may be set to a wavelength that can be separated from the C band, for example, 1525 nm or 1570 nm.

  Note that the first light amplification unit 30 and the second light amplification unit 60 require at least a pump light intensity that causes parametric amplification. For this reason, the intensity of the laser diode output light is amplified by the optical amplifier 17. For example, an EDFA is used as the optical amplifier 17.

  The laser diode output light amplified by the optical amplifier 17 is sent to the band pass filter 18. In the band pass filter 18, the transmission wavelength is set to a wavelength band that matches the wavelength band of the pump light. Then, after the ASE noise existing outside the wavelength band of the pump light is sufficiently reduced in the band pass filter 18, the laser diode output light is output from the pump light source 15 as the pump light S103.

  The pump light S103 is sent to the pump light demultiplexing unit 20 provided in the preceding stage of the first multiplexing unit 25. The pump light demultiplexing unit 20 divides the pump light S103 into two, sends one (S103a) to the first multiplexing unit 25, and sends the other (S103b) to the optical phase shifter 45. The pump light S103a input to the first multiplexing unit 25 serves as a power source for the first optical amplification unit 30 that functions as a PIA. The pump light S103b input to the optical phase shifter 45 serves as a power source for the second optical amplification unit 60 that functions as a PSA. The intensity ratio between the pump light S103a and the pump light S103b can be set to 1: 1, for example. However, in order to obtain a desired amplification gain in the first optical amplifying unit 30 and the second optical amplifying unit 60, the intensity ratio can be appropriately changed by adjusting the branching ratio of the pump light demultiplexing unit 20.

  The first multiplexing unit 25 combines the signal light S101 and the pump light S103a, and outputs a first combined light S105 including the signal light and the pump light.

  The first optical amplifying unit 30 is a parametric amplifier that is used as a PIA and amplifies signal light included in the first combined light S105 by parametric amplification. Further, idler light is generated as wavelength converted light of signal light, and first amplified light S107a including signal light, pump light, and idler light is output. The phase of the idler light is determined by the phase of the input signal light and the pump light, and the signal light and the idler light are in a correlated state. When the wavelengths of the signal light, idler light, and pump light are λs, λi, and λp, the relationship between the wavelengths is 1 / λi = 2 / λp−1 / λs.

  The first optical amplifying unit 30 needs to obtain a sufficient gain and bandwidth with an allowable pump light intensity. As a device that satisfies this requirement, a highly nonlinear fiber or a PPLN waveguide can be used. Four-wave mixing is used for highly nonlinear fibers, and SHG / DFG (second harmonic generation / difference frequency light generation) cascade wavelength conversion is the operating principle of a parametric amplifier in a PPLN waveguide. When signal light and pump light are input as input light, the parametric amplifier functions as a PIA.

  For example, in the case of using a highly nonlinear fiber, the nonlinear constant is, for example, 10 W-1 km-1 or more, the length is about 100 m to 1000 m, the zero dispersion wavelength exists near the wavelength of the pump light, and the dispersion If the slope is small, sufficient gain and bandwidth can be obtained. It is also desirable to have polarization maintaining characteristics. Further, in the case of using the PPLN waveguide, it is a condition that the quasi phase matching condition is satisfied at the wavelength of the pump light.

  The first optical amplifying unit 30 is not limited to a configuration using a highly nonlinear fiber or a PPLN waveguide. If a sufficient gain and bandwidth can be obtained, a silicon fine wire waveguide or the like may be used.

  The first amplified light S107a output from the first optical amplifying unit 30 is sent to the filter unit 35. The filter unit 35 blocks the pump light in the first amplified light S107a that is input. Then, the first amplified light S107b including the signal light and the idler light is output. Here, the first amplified light S107b output from the filter unit 35 is also referred to as processed light S107b. The processed light S107b is sent to the intermediate light amplifier 40.

  The intermediate light amplification unit 40 amplifies the intensity of the signal light and idler light included in the light to be processed S107b by stimulated emission. Then, an intermediate amplified light S109 including the amplified signal light and idler light is output.

  For example, a rare earth-doped fiber amplifier such as EDFA or a semiconductor optical amplifier can be used as the intermediate optical amplifier 40. In the intermediate light amplifying unit 40, it is necessary to input pump light in order to perform amplification by stimulated emission. Here, the intermediate light amplifying unit 40 is omitted from the drawing, assuming that it includes a light source that generates pump light.

  The intermediate amplified light S109 is input to the second multiplexing unit 50. Pump light S <b> 103 b is also input to the second multiplexing unit 50 from the pump light source 15 through the pump light demultiplexing unit 20 and the optical phase shifter 45. The first path including the first multiplexing unit 25, the first optical amplification unit 30, the filter unit 35, and the intermediate optical amplification unit 40 between the pump light demultiplexing unit 20 and the second multiplexing unit 50. The difference in propagation time with the second path including the optical phase shifter 45 is adjusted to be sufficiently shorter than the coherence time of the pump light.

  The optical phase shifter 45 adjusts the phase of the input pump light S103b so that the relative phases of the signal light, the pump light, and the idler light given by 2φp−φs−φi are constant. Note that φp, φs, and φi indicate the phases of pump light, signal light, and idler light, respectively. The relative phase may fluctuate due to mechanical vibrations of peripheral devices, environmental temperature changes, and the like. Therefore, the relative phase is made constant by adjusting the phase of the pump light S103b in the optical phase shifter 45. Here, based on a control signal S119 sent from an optical phase shifter control device 75 described later, the second optical amplifier 60 adjusts the relative phase so that the amplification gain of the signal light is maximized. As the optical phase shifter 45, for example, an optical phase shifter such as a fiber stretcher using a piezoelectric element can be used.

  The pump light S <b> 103 b whose phase is adjusted by the optical phase shifter 45 is sent to the second multiplexing unit 50.

  The second multiplexing unit 50 combines the signal light and idler light included in the intermediate amplified light S109 and the pump light S103b to generate the second combined light S111a. The second combined light S111a is output from the second combining unit 50 and sent to the dispersion compensator 55.

  The dispersion compensator 55 compensates for the chromatic dispersion of the second combined light S111a. The second combined light (hereinafter also referred to as dispersion compensation light) S111b having the chromatic dispersion compensated by the dispersion compensator 55 is input to the second optical amplification unit 60.

  In the second optical amplification unit 60 described later, the signal light is amplified with an amplification gain based on the relative phase of the signal light, the pump light, and the idler light included in the second combined light. As described above, by adjusting the phase of the pump light by the optical phase shifter 45, the relative phase is adjusted in the second optical amplification unit 60 so that the amplification gain of the signal light is maximized. However, when chromatic dispersion occurs before reaching the second optical amplification unit 60, the relative phase of the signal light, the pump light, and the idler light included in the second combined light is a value that depends on the wavelength of the signal light. It becomes. As a result, the amplification gain of the signal light in the second optical amplification unit 60 depends on the wavelength of the signal light, and a flat gain characteristic cannot be obtained. Therefore, by using the dispersion compensator 55 to compensate for the chromatic dispersion of the second combined light, the relative phase can be kept constant. Then, the amplification gain of the signal light in the second optical amplification unit 60 is stabilized.

  Note that the dispersion compensator 55 may be omitted as long as the wavelength dependency of gain does not become a problem.

  The second optical amplifying unit 60 is a parametric amplifier that is used as a PSA and amplifies the signal light included in the dispersion compensation light S111b by parametric amplification. Particularly in this embodiment, the second optical amplifying unit 60 is a non-degenerate PSA in which signal light and idler light are present at different wavelengths.

  The second optical amplification unit 60 can be configured in the same manner as the first optical amplification unit 30. When input light is signal light, idler light, and pump light, and the relative phases of the signal light, pump light, and idler light are controlled and maintained constant, the parametric amplifier functions as a PSA.

  Here, when the signal light is a wavelength division multiplexed signal, it is desirable to use the above-described PPLN waveguide as the second optical amplifying unit 60 in order to prevent interference due to four-wave mixing. However, a highly nonlinear fiber can be used if interference due to four-wave mixing does not occur.

  The second optical amplifying unit 60 outputs the second amplified light S113 including the pump light and the amplified signal light and idler light.

  The second amplified light S113 output from the second optical amplifying unit 60 is sent to the output side demultiplexing unit 65.

  The output side demultiplexing unit 65 demultiplexes the signal light, the pump light, and the idler light included in the second amplified light S113. As the output side branching unit 65, for example, a WDM multiplexer / demultiplexer can be used.

  The demultiplexed signal light S115 is sent to the output port 70 and output from the first optical amplification device 100. The idler light S117 is sent to the optical phase shifter controller 75.

  Since pump light is unnecessary after the light is output from the second optical amplifying unit 60, for example, it may be blocked by the output side demultiplexing unit 65 or may be blocked from the output side demultiplexing unit 65 by another unit not shown. It may be sent to the route and released.

  The optical phase shifter control device 75 includes, for example, intensity detection means and control signal generation means (not shown).

  The optical phase shifter controller 75 detects the intensity of the input idler light S117 in the intensity detector. The idler light is amplified by the second optical amplification unit 60 with an amplification gain corresponding to the amplification gain of the signal light. Therefore, when the amplification gain of the idler light is maximized, the amplification gain of the signal light is also maximized. Therefore, the optical phase shifter 45 is controlled so that the intensity of the idler light is maximized. Since the control of the optical phase shifter 45 is conventionally known, the description thereof is omitted here. The optical phase shifter control device 75 determines the relative phase of the second amplified light S117 when the intensity of the idler light is maximized by detecting the intensity of the idler light S117 in the intensity detection means. Then, a control signal S119 for notifying the relative phase at which the intensity of the idler light is maximized is generated by the control signal generating means, and sent to the optical phase shifter 45 described above.

(principle)
With reference to FIG. 2, the principle by which the first optical amplifying apparatus 100 described above can reduce the noise figure in the wavelength band of the signal light will be described. FIG. 2 is a diagram schematically illustrating an example of a spectrum in the first optical amplifying device 100. In FIG. 2, the horizontal axis indicates the wavelength, and the vertical axis indicates the signal intensity in arbitrary units.

  First, in the first combined light S105 output from the first combining unit 25, noise (hereinafter also referred to as signal band noise) 81a exists in a wavelength band in which signal light exists (FIG. 2A). Noise (hereinafter also referred to as idler band noise) 91a also exists in a wavelength band in which idler light, which will be described later, is generated. In FIG. 2, the noises 81a and 91a are shown localized in the wavelength bands in the vicinity of λs and λi, but in reality, the noise is not localized but exists in the entire wavelength range. The first combined light S105 is input to the first optical amplification unit 30.

  Next, the first optical amplification unit 30 amplifies the signal light included in the input first combined light S105 by parametric amplification, generates idler light, and includes first amplification including these signal light and idler light. The light S107a is output (FIG. 2B). Further, in the first amplifying unit 30, noise included in the input first combined light S105 is also amplified.

  In the first amplified light S107a, the signal band noise 180a exists in the wavelength band of the signal light, and the idler band noise 190b exists in the wavelength band of the idler light.

  The signal band noise 180a includes noise 81b obtained by amplifying the signal band noise 81a (FIG. 2A) input to the first amplification unit 30 with an amplification gain equal to the amplification gain of the signal light. Further, the signal band noise 180a includes noise 82a obtained by converting the idler band noise with a conversion efficiency equal to the wavelength conversion efficiency from the signal light to the idler light (that is, the idler light generation efficiency).

  On the other hand, the idler band noise 190a includes the noise 91b obtained by amplifying the idler band noise 91a (FIG. 2A) input to the first amplifier 30 with an amplification gain equal to the amplification gain of the signal light. Further, idler band noise 190a includes noise 92a obtained by converting signal band noise with a conversion efficiency equal to the wavelength conversion efficiency.

  As a result, the signal band noise 180a and the idler band noise 190a are correlated with each other. Such noise having a correlation with each other is hereinafter also referred to as correlation noise.

  Next, the intermediate light amplification unit 40 amplifies the signal light and idler light included in the first amplified light S107b input through the filter unit 35 by stimulated emission, and the intermediate amplification light S109 including these signal light and idler light. Is output (FIG. 2C). Further, in the intermediate light amplifying unit 40, noise included in the input first amplified light S107b is also amplified.

  In the intermediate amplified light S109, the signal band noise 280 exists in the wavelength band of the signal light, and the idler band noise 290 exists in the wavelength band of the idler light.

  The signal band noise 280 includes noise 180b obtained by amplifying the signal band noise 180a (FIG. 2B) input to the intermediate optical amplifier 40 with an amplification gain equal to the amplification gains of the signal light and idler light. The idler band noise 290 includes noise 190b obtained by amplifying the idler band noise 190a (FIG. 2B) input to the intermediate optical amplifier 40 with an amplification gain equal to the amplification gain of the signal light and idler light. . Since the noise 180b and the noise 190b are correlation noises, they are hereinafter also referred to as correlation noise 180b and correlation noise 190b.

  Further, the signal band noise 280 includes noise due to spontaneous emission light (hereinafter also referred to as spontaneous emission light noise) 83a. The idler band noise 290 includes spontaneous emission light noise 93a. These spontaneously emitted light noises 83a and 93a are not correlated with each other. Such noise having no correlation with each other is hereinafter also referred to as uncorrelated noise.

  As described above, in the intermediate amplified light S109, the signal band noise 280 includes the correlation noise 180b and the spontaneous emission light noise 83a which is uncorrelated noise. The idler band noise 290 includes correlated noise 190b and uncorrelated noise 93a.

  Next, the second multiplexing unit 50 combines the input intermediate amplified light S109 and the pump light, and outputs a second combined light S111a including the signal light, idler light, and pump light (FIG. 2 ( D)).

  Next, the second optical amplification unit 60 amplifies the signal light and idler light included in the input second combined light S111a by parametric amplification, and outputs the second amplified light S113 including these signal light and idler light. (FIG. 2E). In the first optical amplifying device 100 described with reference to FIG. 1, the dispersion compensator 55 is provided before the second optical amplifying unit 60. Therefore, as described above, in the configuration example of FIG. 1, the dispersion compensation light S <b> 111 b is input to the second optical amplification unit 60. In the second combined light S111a and the dispersion compensation light S111b, the relationships among the intensities of the signal light, idler light, signal band noise, and idler band noise are common. Here, a configuration in which the second combined light S111a is input to the second optical amplification unit 60 will be described.

  Further, in the second optical amplifying unit 60, noise included in the input second multiplexed light S111a is also amplified.

  In the second amplified light S113, signal band noise 380 exists in the wavelength band of signal light, and idler band noise 390 exists in the wavelength band of idler light.

  The signal band noise 380 is obtained by amplifying the correlation noise 180c obtained by amplifying the correlation noise 180b included in the signal band noise 280 (FIG. 2D) input to the second optical amplification unit 60 and the spontaneous emission light noise 83a. Spontaneous emission light noise 83a. The idler band noise 390 includes a correlation noise 190c obtained by amplifying the correlation noise 190b included in the idler band noise 290 (FIG. 2D) input to the second optical amplification unit 60, and a spontaneous emission light noise 93a. It includes amplified spontaneous emission noise 93b.

  Here, in the second optical amplifying unit 60 functioning as a PSA, the correlation noises 180b and 190b are amplified with an amplification gain equal to that of the signal light. Therefore, if the amplification gain of the signal light is G, the amplification gain for the correlation noises 180b and 190b is also G. On the other hand, the spontaneous emission light noises 83a and 93a are uncorrelated. Therefore, the spontaneous emission light noises 83a and 93a are amplified with an amplification gain of G / 2 with respect to the amplification gain G of the signal light. Therefore, in the second optical amplification unit 60, the amplification gain to the spontaneous emission light noise added by the intermediate optical amplification unit 40 is ½ of the amplification gain to the correlation noises 180b and 190b. Accordingly, the ratio of spontaneous emission light noise 83b in the signal band noise 380 and the ratio of spontaneous emission light noise 93b in the idler band noise 390 are reduced.

  As described above, in the first optical amplifying device 100, the intermediate optical amplifying unit 40 is disposed between the first optical amplifying unit 30 functioning as the PIA and the second optical amplifying unit 60 functioning as the PSA. The amplification gain of the spontaneous emission optical noise 83b in the final signal band noise 380 existing in the wavelength band of light can be halved. Accordingly, in the noise added and amplified by the entire first optical amplifying apparatus 100, the ratio of spontaneous emission optical noise added by the intermediate optical amplifying unit 40 can be reduced. Therefore, by adjusting the respective amplification gains to the signal light in the first optical amplification unit 30, the intermediate optical amplification unit 40, and the second optical amplification unit 60, while reducing the noise figure of the spontaneous emission light noise 83b, An amplification gain can be obtained.

(Characteristic evaluation)
The inventor performed a simulation to evaluate the characteristics of the first optical amplification device 100. In this simulation, the signal light is given by the entire first optical amplifying device 100 (that is, given by three amplifying units of the first optical amplifying unit 30, the intermediate optical amplifying unit 40, and the second optical amplifying unit 60). The relationship between (total) amplification gain and noise figure was confirmed. Here, the amplification gain of the first optical amplification unit 30 is constant at 4 dB, the amplification gain of the second optical amplification unit 60 is constant at 8 dB, and the amplification gain of the intermediate optical amplification unit 40 is changed to change the first optical amplification. The amplification gain provided in the entire apparatus 100 was changed. The intermediate light amplifying unit 40 is assumed to be an EDFA, and the spontaneous emission coefficient is set to 2.

  Here, in order to compare with the first optical amplifying apparatus 100, the same simulation was performed for the conventional optical amplifying apparatus in which the PIA and the EDFA are cascade-connected in this order. The amplification gain of PIA and the spontaneous emission light coefficient of EDFA in the conventional optical amplifying device were the same as those of the first optical amplifying device 100.

  The results are shown in FIG. FIG. 3 is a diagram showing the relationship between the amplification gain of the entire apparatus and the noise figure in the first optical amplification apparatus 100 and the conventional optical amplification apparatus. In FIG. 3, the vertical axis represents the noise figure on the dB scale, and the horizontal axis represents the amplification gain on the dB scale.

  As shown in FIG. 3, in giving the same level of amplification gain to the signal light, in the first optical amplification device 100, the noise figure of the entire device is reduced as compared with the conventional one. From this result, it was confirmed that in the first optical amplifying apparatus 100, the amplification gain can be increased by using the EDFA and the noise figure can be reduced as compared with the conventional one.

(Second optical amplification device)
With reference to FIG. 4, an optical amplifying device (hereinafter referred to as a second optical amplifying device) according to a second embodiment of the present invention will be described. FIG. 4 is a schematic configuration diagram for explaining the second optical amplifying device. In addition, the same code | symbol is attached | subjected to the same component as the 1st optical amplification apparatus mentioned above, and the overlapping description is abbreviate | omitted.

  The second optical amplifying device 200 is different from the first optical amplifying device in that the pump light S103 generated by the pump light source 15 is sent to the first multiplexing unit 25 without being branched. For this reason, in the filter unit 35, the pump light is branched from the first amplified light S107a and sent to the optical phase shifter 45. The pump light is attenuated before being sent to the optical phase shifter 45. For this reason, the pump light is amplified using, for example, EDFA as the optical amplifying unit 80, and further, ASE noise is removed using the band pass filter 85, and then sent to the second multiplexing unit 50. Even in this configuration, the same effect as the first optical amplifying device can be obtained.

10: input port 15: pump light source 16: external resonator light source 17: optical amplifier 18, 85: band pass filter 20: pump light demultiplexing unit 25: first multiplexing unit 30: first optical amplification unit 35: filter unit 40: intermediate optical amplifier 45: optical phase shifter 50: second multiplexer 55: dispersion compensator 60: second optical amplifier 65: output side demultiplexer 70: output port 75: phase shifter controller 80: light Amplifying unit 100: first optical amplifying device 200: second optical amplifying device

Claims (5)

A first light amplifying unit that generates idler light based on the input pump light and signal light and outputs first amplified light including the signal light, pump light, and idler light;
An intermediate light amplifying unit that amplifies the intensity of the signal light and idler light included in the first amplified light by stimulated emission, and outputs intermediate amplified light including the signal light and idler light;
A second light amplifying unit that receives the pump light and the intermediate amplified light, amplifies the intensity of the signal light included in the intermediate amplified light by parametric amplification, and outputs the second amplified light including the signal light; An optical amplifier characterized by the above. An optical phase shifter capable of changing a relative phase of the signal light, the pump light, and the idler light input to the second optical amplification unit;
The optical amplification device according to claim 1, wherein the second optical amplification unit amplifies the intensity of the signal light based on the relative phase. An optical phase shifter control unit;
The second amplified light includes idler light amplified in the second optical amplification unit,
The optical phase shifter control unit detects the intensity of idler light included in the second amplified light, and sends a control signal generated according to the intensity to the optical phase shifter.
The optical amplification device according to claim 2, wherein the optical phase shifter changes the relative phase in accordance with the control signal. Before the first optical amplification unit, a pump optical demultiplexing unit, a filter unit between the first optical amplification unit and the intermediate optical amplification unit, and between the intermediate optical amplification unit and the second optical amplification unit. It further includes a multiplexing unit,
The pump light demultiplexing unit bifurcates the pump light, sends one to the first optical amplification unit, and sends the other to the optical phase shifter,
The filter unit blocks the pump light,
4. The optical amplifying apparatus according to claim 2, wherein the multiplexing unit combines the intermediate amplified light and pump light that has passed through the optical phase shifter. 5. A filter unit between the first optical amplification unit and the intermediate optical amplification unit, and a multiplexing unit between the intermediate optical amplification unit and the second optical amplification unit,
The filter unit demultiplexes the first light amplification light into signal light, idler light, and pump light, sends the signal light and idler light to the intermediate light amplification unit, and sends pump light to the optical phase. To the shifter,
4. The optical amplifying apparatus according to claim 2, wherein the multiplexing unit combines the intermediate amplified light and pump light that has passed through the optical phase shifter. 5.

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