Optical Amplifier

Abstract

A configuration of an excitation light generation device for providing an excitation light having a good SN ratio to a PSA is disclosed. Further, a configuration of a relay amplifier of the PSA including the excitation light generation device is also shown. The following disclosure includes the excitation light generation device, an optical amplification device including the excitation light generation device, and an optical transmission system. More specifically, the excitation light generation device for maintaining the SN ratio of the excitation light in a high state by utilizing an optical sensitive amplification function with respect to the excitation light generated by an optical phase lock loop is disclosed. The excitation light generation device of the present disclosure generates a local oscillation excitation light using the OPLL and having a sufficiently high SN ratio, which makes an inherent low noise operation of the PSA possible even to a signal light having a high SN ratio.

Claims

1. A device that generates an excitation light for optical phase sensitive amplification to amplify a signal pair of a signal light and an idler light of the signal light, comprising: an optical phase lock unit to generate a plurality of sideband lights in synchronization with a phase of the signal pair by an optical phase lock loop (OPLL) with respect to the plurality of sideband lights produced by modulating a local oscillation light; and an excitation light cut out unit to extract, as an excitation light, one sideband light of the plurality of synchronized sideband lights, wherein the excitation light cut out unit includes: a first second-order nonlinear optical element to generate a second harmonic of the local oscillation light; a phase adjuster to adjust a phase for each sideband light with respect to the plurality of synchronized sideband lights; a second second-order nonlinear optical element to perform parametric amplification to the phased-adjusted sideband light; a means to synchronize a phase of the second harmonic and a phase of the one sideband light amplified by the second second-order nonlinear optical element; and an optical filter to extract only the one sideband light.

2. The device according to claim 1, wherein the phase adjuster is configured to: set the phase between the one sideband light and the second harmonic such that an amplification operation is performed in the second second-order nonlinear optical element; and set the phases between other sideband lights excluding the one sideband light as well as the local oscillation light and the second harmonic such that an attenuation operation is performed in the second second-order nonlinear optical element.

3. The device according to claim 1, wherein the optical phase lock unit includes: a third second-order nonlinear optical element to generate a sum frequency light from the signal pair; a modulator to produce the plurality of sideband lights by modulating the local oscillation light; a fourth second-order nonlinear optical element to generate a second harmonic of the sideband light from the modulator; a phase lock means to detect a phase difference between the one sideband light of the plurality of sideband lights and the sum frequency light and to provide a feedback to the modulator according to the phase difference; a first splitter to split the local oscillation light at a preceding stage side of the modulator; and a second splitter to split the plurality of synchronized sideband lights at a subsequent stage side of the modulator.

4. The device according to claim 1, wherein the one sideband light is a primary sideband light on a high frequency side of the local oscillation light.

5. The device according to claim 1, wherein an optical waveguide included in the second-order nonlinear optical element is a directly bonded ridge waveguide, wherein the directly bonded ridge waveguide is made of any material from among LiNbO.sub.3, KNbO.sub.3, LiTaO.sub.3, LiNb.sub.(x)Ta.sub.(1-x)O.sub.3(0≤x≤1), and KTiOPO.sub.4, or a material in which at least one kind selected from a group consisting of Mg, Zn, Sc, and In is added as an additive to any of these materials.

6. A relay type optical amplification device, comprising: the device according to claim 1; and a phase sensitive amplifier including: a fifth second-order nonlinear optical element to generate a second harmonic from the excitation light generated by the excitation light cut out unit; a sixth second-order nonlinear optical element to perform non-degenerate parametric amplification of the signal pair; and a phase lock means to synchronize the phase of the signal pair and the phase of the excitation light.

7. The device according to claim 2, wherein the optical phase lock unit includes: a third second-order nonlinear optical element to generate a sum frequency light from the signal pair; a modulator to produce the plurality of sideband lights by modulating the local oscillation light; a fourth second-order nonlinear optical element to generate a second harmonic of the sideband light from the modulator; a phase lock means to detect a phase difference between the one sideband light of the plurality of sideband lights and the sum frequency light and to provide a feedback to the modulator according to the phase difference; a first splitter to split the local oscillation light at a preceding stage side of the modulator; and a second splitter to split the plurality of synchronized sideband lights at a subsequent stage side of the modulator.

8. The device according to claim 2, wherein the one sideband light is a primary sideband light on a high frequency side of the local oscillation light.

9. The device according to claim 3, wherein the one sideband light is a primary sideband light on a high frequency side of the local oscillation light.

10. The device according to claim 2, wherein an optical waveguide included in the second-order nonlinear optical element is a directly bonded ridge waveguide, wherein the directly bonded ridge waveguide is made of any material from among LiNbO.sub.3, KNbO.sub.3, LiTaO.sub.3, LiNb.sub.(x)Ta.sub.(1-x)O.sub.3(0≤x≤1), and KTiOPO.sub.4, or a material in which at least one kind selected from a group consisting of Mg, Zn, Sc, and In is added as an additive to any of these materials.

11. The device according to claim 3, wherein an optical waveguide included in the second-order nonlinear optical element is a directly bonded ridge waveguide, wherein the directly bonded ridge waveguide is made of any material from among LiNbO.sub.3, KNbO.sub.3, LiTaO.sub.3, LiNb.sub.(x)Ta.sub.(1-x)O.sub.3(0≤x≤1), and KTiOPO.sub.4, or a material in which at least one kind selected from a group consisting of Mg, Zn, Sc, and In is added as an additive to any of these materials.

12. The device according to claim 4, wherein an optical waveguide included in the second-order nonlinear optical element is a directly bonded ridge waveguide, wherein the directly bonded ridge waveguide is made of any material from among LiNbO.sub.3, KNbO.sub.3, LiTaO.sub.3, LiNb.sub.(x)Ta.sub.(1-x)O.sub.3(0≤x≤1), and KTiOPO.sub.4, or a material in which at least one kind selected from a group consisting of Mg, Zn, Sc, and In is added as an additive to any of these materials.

13. A relay type optical amplification device, comprising: the device according to claim 2; and a phase sensitive amplifier including: a fifth second-order nonlinear optical element to generate a second harmonic from the excitation light generated by the excitation light cut out unit; a sixth second-order nonlinear optical element to perform non-degenerate parametric amplification of the signal pair; and a phase lock means to synchronize the phase of the signal pair and the phase of the excitation light.

14. A relay type optical amplification device, comprising: the device according to claim 3; and a phase sensitive amplifier including: a fifth second-order nonlinear optical element to generate a second harmonic from the excitation light generated by the excitation light cut out unit; a sixth second-order nonlinear optical element to perform non-degenerate parametric amplification of the signal pair; and a phase lock means to synchronize the phase of the signal pair and the phase of the excitation light.

15. A relay type optical amplification device, comprising: the device according to claim 4; and a phase sensitive amplifier including: a fifth second-order nonlinear optical element to generate a second harmonic from the excitation light generated by the excitation light cut out unit; a sixth second-order nonlinear optical element to perform non-degenerate parametric amplification of the signal pair; and a phase lock means to synchronize the phase of the signal pair and the phase of the excitation light.

16. A relay type optical amplification device, comprising: the device according to claim 5; and a phase sensitive amplifier including: a fifth second-order nonlinear optical element to generate a second harmonic from the excitation light generated by the excitation light cut out unit; a sixth second-order nonlinear optical element to perform non-degenerate parametric amplification of the signal pair; and a phase lock means to synchronize the phase of the signal pair and the phase of the excitation light.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0048] FIG. 1 is a diagram for explaining the configuration of the phase sensitive optical amplifier according to the conventional technique.

[0049] FIG. 2 is a diagram of the configuration of the phase sensitive optical amplifier using a second-order nonlinear optical effect.

[0050] FIG. 3 is a graph illustrating the relationship of the phase difference Δϕ of the input signal light and the excitation light with the gain.

[0051] FIG. 4 is a configuration diagram of the relay type PSA using the optical phase lock loop according to the conventional technique.

[0052] FIG. 5 shows diagrams for schematically describing spectra of a signal light and the like in each part of the OPLL.

[0053] FIG. 6 is a diagram showing the relationship between the excitation light intensity and the PSA gain.

[0054] FIG. 7 is a diagram showing the configuration of the optical amplification device using the OPLL according to the present disclosure.

[0055] FIG. 8 shows diagrams for describing an action to each sideband light in the excitation light generation device.

[0056] FIG. 9 is a diagram for describing a gain saturation characteristic in a PPLN waveguide module.

[0057] FIG. 10 is a diagram showing a relationship between an SN ratio of an input signal light and a noise factor of the relay type PSA.

DESCRIPTION OF EMBODIMENTS

[0058] In the following description, a configuration of an excitation light generation device in which an excitation light having a good SN ratio is provided to a PSA is disclosed. Further, a configuration of a relay amplifier of the PSA that includes the excitation light generation device is also shown. The following disclosures include the excitation light generation device, and an optical amplification device and an optical transmission system that include the excitation light generation device. More specifically, the excitation light generation device is disclosed that maintains an SN ratio of the excitation light in a high state by using an optical sensitive amplification function, with respect to the excitation light generated by the OPLL. An operation as a relay type PSA, which uses an excitation light with a low noise that is supplied from this excitation light generation device, is disclosed.

[0059] FIG. 7 is a diagram showing the configuration of an optical amplification device 500 that uses the OPLL according to the present disclosure. As main components thereof, the optical amplification device 500 is provided with a PSA 502, an optical phase lock unit 501 for generating an excitation light synchronized with a signal light by the OPLL, and an excitation light cut out unit 600. The configurations and operations of the PSA 502 and the optical phase lock unit 501 are generally the same as the configuration and the operation of the conventional technique showed in FIG. 4. The excitation light cut out unit 600 maintains an excitation light, which is obtained from the optical phase lock unit 501 and phase-locked by the OPLL, in a high SN ratio and supply the excitation light with a low noise to the PSA 501. The excitation light cut out unit 600 maintains an excitation light that is phase-locked by the OPLL obtained from the optical phase lock unit 501 in a high SN ratio, so as to supply the excitation light of low noise to the PSA 501. The excitation light cut out unit 600 has a function of the PSA and a function of a bandpass filter, and a BPF 316 in FIG. 4 is replaced with the excitation light cut out unit 600. The optical phase lock unit 501 and the excitation light cut out unit 600 are to operate as excitation light generation devices.

[0060] Hereinafter, a configuration and an operation of each component of the optical amplification device 500 will be described with reference to FIG. 7. As described above, the configuration of the optical phase lock unit 501 is generally the same as the configuration of the local oscillation phase lock circuit 301 in the OPLL configuration of the conventional technique of FIG. 4. Therefore, differences between them will be described. A signal light 504 is tapped by an optical coupler 506 and inputted into a third second-order nonlinear optical element (PPLN-3) 509 via a BPF 507 and an EDFA 508. A local oscillation light 525 from a local oscillation light source 503 is inputted into an LN modulator 514 via an EDFA 515. An excitation light modulated by the LN modulator is input into a fourth second-order nonlinear optical element (PPLN-4) 510.

[0061] Here, the configuration is different from that of FIG. 4 in that before and after the LN modulator 514, optical couplers 516 and 517 are provided. The optical coupler 516 at the preceding stage splits a zeroth component of the local oscillation light, that is, the excitation light to supply a zeroth component light 526 to the excitation light cut out unit 600. The optical coupler 517 at the subsequent stage splits the local oscillation light including a primary sideband light and subjected to modulation to supply the modulated local oscillation light 527 to the excitation cut out unit 600. These split signals will be further described below together with the operation of the excitation light cut out unit 600.

[0062] A detection output 522 is obtained from a balanced detector 511, and a low-speed error signal 523 is further obtained from the detection output 522 by a loop filter 512. The error signal 523 is inputted as a control signal of the VCO 513. An oscillation output 524 from the VCO 513 is supplied to the above-mentioned LN modulator 514 as a modulation signal for generating a sideband signal. The operation of the OPLL is the same as in the case of FIG. 4. Therefore, the description thereof will be omitted.

[0063] The excitation light cut out unit 600 is provided with a first second-order nonlinear optical element (PPLN-1) 602 and a second second-order nonlinear optical element (PPLN-2) 604. Both of them are, for example, PPLN waveguide modules that operate to maintain the SN ratio of the excitation light produced by the primary sideband light from the optical phase lock unit 501 as will be described later. The zeroth component light 526 split at the preceding stage of the LN modulator 514 described above is inputted, via a EDFA 601 and a BPF 614, into the first second-order nonlinear optical element (PPLN-1) 602 that generates an excitation light of the SH band by the SHG process. In the first second-order nonlinear optical element 602, an SH light 610 of the zeroth component light 526 is generated by the SHG process.

[0064] The modulated local oscillation light 527 split at the subsequent stage of the LN modulator 514 described above is inputted into the second second-order nonlinear optical element (PPLN-2) 603 via a piezoelectric (PZT) type optical fiber expander 605 and a phase adjuster 606. The second second-order nonlinear optical element 603 performs a phase sensitive amplification operation to the phase-adjusted primary sideband light 611 by an optical parametric amplification (OPA) process. In the amplified primary sideband light 612, only the primary sideband light is cut out by a BPF 608 to be inputted into an EDFA 518 as an excitation light.

[0065] The amplified primary sideband light 612 is split by an optical coupler 607, and a detection signal is obtained by a photodetector 609. The detection signal is fed back to the phase lock loop (PLL) circuit 604. A path from the photodetector 609 that detects an output to which the optical sensitive amplification is performed, the PLL 604, and to the PZT 605 has the same configuration as that of the phase lock circuit described in FIG. 2.

[0066] The excitation light cut out unit 600 uses the zeroth component light 526 of the excitation light, that is, a carrier component of the excitation light, that has been split at the preceding stage of the LN modulator 514, as an excitation light of the parametric amplification by the second second-order nonlinear optical element 603. Therefore, phase sensitive amplification can be performed in one time to all components of the modulated local oscillation light 527 that has been split at the subsequent stage of the LN modulator 514. In other words, in the second second-order nonlinear optical element 603, the degenerate phase sensitive amplification to the zeroth component of the local oscillation light 527 and the non-degenerate phase sensitive amplification to the components other than the zeroth component of the local oscillation light 527 are used at the same time. Though the primary sideband light that is eventually used as an excitation light 613 is the one obtained by the LN modulator 514, it is supplied to the PAS 502 in a state in which the SN ratio deterioration is suppressed to a minimum by the parametric amplification operation in the second second-order nonlinear optical element 603.

[0067] As described above, in the excitation light generation device of the present disclosure, the optical phase lock unit 501 and the excitation light cut out unit 600 use four second-order nonlinear optical elements (PPLN waveguide modules). Of these, the third second-order nonlinear optical element 509 (PPLN-3), the fourth second-order nonlinear optical element 510 (PPLN-4), and the first second-order nonlinear optical element 602 (PPLN-1) are used to produce the SH light. Only the second second-order nonlinear optical element 603 (PPLN-2) is used for the parametric amplification. The three second-order nonlinear optical elements (PPLN-1, PPLN-3, and PPLN-4) for producing the SH light are each provided with the PPLN waveguide, as well as a first space optical system and a second space optical system before and after the PPLN waveguide. The first space optical system couples a light inputted into the PPLN waveguide module to the PPLN waveguide, and the second space optical system couples a light outputted from the PPLN waveguide to an output port of the PPLN waveguide module.

[0068] The second-order nonlinear optical element (PPLN-2) for the parametric amplification is provided with a PPLN waveguide, as well as a third space optical system and a first dichroic mirror on one end of the PPLN waveguide and a fourth space optical system and a second dichroic mirror on the other end of the PPLN waveguide. The third space optical system couples a light inputted into the PPLN waveguide module to the PPLN waveguide via the first dichroic mirror, and the fourth space optical system couples a light outputted from the PPLN waveguide to an output port of the PPLN waveguide module via the second dichroic mirror.

[0069] Hereinafter, fabrication method of the PPLN waveguide used in the excitation light generation device of the present disclosure will be described in an exemplarily manner. First, a periodic electrode having a period of approximately 17 μm is formed on LiNbO.sub.3 added with Zn. Then, a polarization inversion grating according to an electrode pattern is formed in Zn:LiNbO.sub.3 by an electric field application method. Next, the Zn:LiNbO.sub.3 substrate having this periodical polarization inversion structure is directly bonded on LiTaO.sub.3 serving as a clad, and both substrates are firmly joined by heat treatment of 500 C°. Subsequently, a core layer is thinned to around 5 m by polishing, and an optical waveguide of the ridge type is formed by using a dry etching process. A temperature of this optical waveguide can be adjusted with a Peltier element, and the length of the optical waveguide is set to 50 mm. The second-order nonlinear optical element having the PPLN waveguide formed in this manner is configured as a mode of a module that allows input and output of the light by a polarization maintaining fiber of the 1.5 μm zone. In the present disclosure, LiNbO.sub.3 added with Zn is used, but other nonlinear materials such as KNbO.sub.3, LiTaO.sub.3, LiNb.sub.xTa.sub.1-xO.sub.3(0≤x≤1), and KTiOPO.sub.4, or a material containing at least one kind selected from a group consisting of Mg, Zn, Sc, and In added to them as an additive, may be used.

[0070] Next, an operation of the optical amplification device 500 including the excitation light generation device shown in FIG. 7 will be described in more detail. The operation of the optical phase lock unit 501 is the same as the operation of the local oscillation phase lock circuit 301 in the OPLL configuration of the conventional technique shown in FIG. 4. To be more specific about the operating condition, modulation is performed to the local oscillation light 525 by a sine wave-like electric signal of approximately 20 GHz with respect to the LN modulator 514. In other words, the VCO 513 outputs an electric signal 524 of approximately 20 GHz in the vicinity of the medium value of the input error voltage (the VCO control voltage) 523.

[0071] The LN modulator 514 is an optical modulator that utilizes refractive index change caused by the Pockels effect of LiNbO.sub.3 crystal, and widely used as an external modulator that modulates a CW light such as a DFB laser. In the present disclosure, an intensity modulator is used as the LN modulator 514, but a phase modulator may be used. By way of examples of optical frequencies of the respective units of the optical amplification device 500, the optical frequency of the signal light subjected to data modulation may be 193.1 THz, the optical frequency of the idler light may be 192.9 THz, and the optical frequency of the local oscillation light may be 193 THz.

[0072] In the configuration of the conventional technique shown in FIG. 4, the primary sideband light ϕ.sub.L+1 is cut out by the BPF 316 to be used as the phase-locked excitation light. The intensity of each sideband light obtained after modulation is lowered due to a modulator loss. Further, in order to use only the primary sideband light ϕ.sub.L+1 of the sideband lights as the excitation light, a filter 316 is used for cutting out. In order to obtain sufficient attenuation of unnecessary lights, loss in a transmission region including the sideband light ϕ.sub.L+1 is increased, which lowers the intensity of the excitation light. Since the amplification is performed by the EDFA 17 to compensate the intensity of the excitation light, the final SN ratio of the excitation light 327 is significantly deteriorated.

[0073] In contrast with this, in the configuration of the excitation light generation device of the present disclosure of FIG. 7, excessive SN ratio deterioration can be avoided by performing the phase sensitive amplification to the excitation light (the primary sideband light) generated via the LN modulator 304, with the second second-order nonlinear optical element 603 of the excitation light cut out unit 600. A local oscillation light, that is, a zeroth component light that is a carrier component of the excitation light is split at the preceding stage side of the LN modulator 514, and the split zeroth component light 526 is used as an excitation light of the parametric amplification. Thereby, phase sensitive amplification is performed in one time to all components of the modulated local oscillation light 527 split from the subsequent stage side of the LN modulator 514. There are two significances in causing the second second-order nonlinear optical element 603 to perform the phase sensitive amplification to the excitation light.

[0074] The first significance is that by using the amplification operation and the attenuation operation of the phase sensitive amplification, it is possible to have the second-order nonlinear optical element 604 to serve both functions as am amplifier and a filter. Referring to FIG. 5, for example, in the sideband light generated via the LN modulator 314, the sideband lights that are to be paired such as ϕ.sub.L−1 and ϕ.sub.L+1, ϕ.sub.L−2 and ϕ.sub.L+2 are phase-locked with each other. For this reason, the phase sensitive amplification is possible to both a carrier component and a sideband component. Here, in the excitation light generation device of FIG. 7, the phase adjuster 606 is provided at the preceding stage side of the second second-order nonlinear optical element 603 (PPLN-2) that performs the phase sensitive amplification.

[0075] FIG. 8 shows diagrams for schematically describing effects to the respective sideband lights in the excitation light generation device of the present disclosure. FIG. 8 (a) shows a spectrum of a modulated excitation light immediately before the phase adjuster 606 of the excitation light cut out unit 600. A fundamental wave component of the excitation light indicated as 0 has the maximum level, primary sideband lights (+1, −1) and secondary sideband lights (+2, −2) are present on both sides thereof. Note that the number in the parenthesis shows an order of the sideband. Here, by the phase adjuster 606, the phase of the primary sideband lights (+1, −1) is adjusted in relation with the SH light 610 which is an excitation light such that the gain in the second second-order nonlinear optical element 603 is maximum. The relationship between the gain and the phase in the PSA of FIG. 3 should be referred to. On the other hand, the phases of the fundamental wave component and the secondary sideband light (+2, −2) are adjusted in relation with the SH light 610 such that the gain is minimum, that is, the attenuation is maximum in the second second-order nonlinear optical element 603.

[0076] Various items can be used as the phase adjuster 606, and by way of example, a filter with a wavelength selectivity using LCOS (Liquid Crystal On Silicon) can be used. With a filter made by the LCOS, an attenuation amount and a phase rotation amount can be adjusted for each wavelength. Additionally, as the phase adjuster, a combination of a wavelength multiplexer/demultiplexer and a phase modulator can be used.

[0077] FIG. 8 (b) shows a spectrum in an output of the second second-order nonlinear optical element 603. In the present disclosure, since the primary sideband light is used as an excitation light of the PSA 502 for relay amplification, the excitation light cut out unit 600 operates to amplify only the primary sideband lights (+1, −1) to be cut out as the excitation lights. The phase adjuster 606 is used to adjust a phase for each component of the sideband light of the modulate local oscillation light 527 such that only the sideband light that is desired to be cut out by the second second-order nonlinear optical element 603 is operated for amplification, and the remaining sideband lights and the like are operated for attenuation. Thereby, a large intensity difference can be obtained between the desired sideband light and the other components without producing an excessive optical loss.

[0078] Specifically, an amplification gain of the phase sensitive amplification by the second second-order nonlinear optical element 603 is 20 dB. On the other hand, at the time of the attenuation operation, an attenuation of −15 dB can be obtained in the second second-order nonlinear optical element 603. Therefore, the intensity difference (contrast) of approximately 35 dB or more can be obtained between the desired primary sideband light and other unnecessary sideband components. In order to obtain a further larger contrast with an optical power, a bandpass filter 608 is installed at the subsequent stage of the second second-order nonlinear optical element 603. As a result, as shown in FIG. 8 (c), the difference in level between the optical intensity of the desired excitation light and the optical intensity of the unnecessary sideband components is 50 dB in the entire excitation light cut out unit 600.

[0079] The second significance in performing the phase sensitive amplification to the excitation light by the second-order nonlinear optical element is that a gain saturation phenomenon of the parametric amplification can be used. In the parametric amplification, an amplified output higher than the optical intensity of the excitation light that serves as an energy source for amplification cannot be obtained. For this reason, gain saturation is caused when the light to be amplified approaches the optical intensity of the excitation light.

[0080] FIG. 9 is a diagram for describing the gain saturation characteristics in the PPLN waveguide module. The input/output characteristics with respect to a light having the optical frequency of 193.1 THz in a phase matching state is shown of the second second-order nonlinear optical element 603 in FIG. 7. Increase of the output power stops in the vicinity of 0 dBm of the input power of the light to be amplified, where the gain is saturated. Since the optical power to be outputted is constant with respect to the input optical power in the gain saturation region, the time fluctuation of the pump light described in FIG. 6 can be significantly reduced. Generally, the time variation of the laser beam output is also known as an intensity noise. In the excitation light cut out unit 600 of FIG. 7, by amplifying the primary sideband light in the gain saturation region, the intensity noise is compressed to improve the SN ratio of the amplified primary sideband light 612, that is, the SN ratio of the excitation light in the second second-order nonlinear optical element output. In other words, in the gain saturation region, since the optical power to be outputted is constant with respect to the input optical power, the intensity fluctuation is compressed to improve the quality of the excitation light. In order to use this gain saturation region, the output power of the local oscillation light is adjusted in the EDFA 515 immediately after the local oscillation light source 503 such that the power of the excitation light to be inputted into the second second-order nonlinear optical element 603 is 0 dBm or more.

[0081] As described above, the excitation light cut out unit 600 that performs the phase sensitive amplification to the excitation light with the second-order nonlinear optical element can cut out the primary sideband light of the excitation light without an excessive loss by using the two actions, namely, the amplification operation and the attenuation operation of the phase sensitive amplification. It is possible to suppress the SN ratio of the excitation light due to a decrease in intensity (decrease in S) caused by the modulator 514 and an increase in noise (increase in N) caused by the EDFA. Further, by using the gain saturation region of the phase sensitive amplification, it is possible to compress the time variation of the excitation light intensity and improve the SN ratio and the quality of the excitation light.

[0082] In order to stabilize the phase sensitive amplification operation by the second second-order nonlinear optical element 603 in the excitation light cut out unit 600, the optical coupler 607 is installed at the subsequent stage side of the second second-order nonlinear optical element 603 to take out a part of the output light. From the viewpoint of the parametric amplification of the second second-order nonlinear optical element, the SH light 610 is an excitation light, and the phase-adjusted primary sideband light 611 is a light targeted for amplification. A change of the optical intensity is detected by the photodetector 609, and then, using the PLL circuit 604, a feedback is performed to the PZT 605 such that the phase of the SH light 610, which is an excitation light, and the phase of the primary sideband light 611 targeted for amplification are synchronized.

[0083] FIG. 10 is a diagram for showing the relationship between the SN ratio of the input signal light and the noise factor of the relay type PSA. The cases in which the excitation light according to the configuration of the conventional technique shown in FIG. 4 is supplied to the PSA are indicated by white dots, and the cases in which the excitation light is supplied to the PSA by the excitation light generation device of the present disclosure shown in FIG. 7 are indicated by black dots. The horizontal axis represents the SN ratio of the input signal lights 304 and 504, and the horizontal axis represents the noise factor of the relay type SAEs 302 and 502. It is shown that, when the excitation light is supplied to the relay type PSA configured by the conventional configuration, the noise factor gradually starts to deteriorate around the point where the SN ratio of the input signal light exceeds 30 dB. This means that a noise is occurring in the PSA though the quality of the input signal light into the relay type PSA is improving. This is resulted from the SN ratio of the excitation light not being sufficiently good in comparison with the SN ratio of the signal light. In other words, it means that the characteristics of the low noise property of the PSA cannot be sufficiently obtained unless the SN ratio of the excitation light for causing the PAS to operate is constantly in a better state than the SN ratio of the signal light to be amplified.

[0084] On the other hand, when the excitation light is supplied to the PSA by the excitation light generation device of the present disclosure, the noise factor maintains a constant value of 1 dB or more regardless of the value of the SN ratio, until the SN ratio of the input signal light reaches 38 dB. Even if the quality of the input signal light is good, the optical sensitive amplification while maintaining the quality is possible, thus making it possible to confirm that the noise characteristic is significantly improved when the PSA is used as a relay amplifier.

[0085] In the above-described disclosure, the example has been described in which the primary sideband light on the high frequency side of the local oscillation light is used to generate the excitation light in the LN modulator. This is because the generation intensity of the primary sideband light is large, which makes it easier to handle. However, as a sideband light, the primary sideband light on the low frequency side may be used, and two or more sideband lights may be used. In addition, a central oscillation frequency of the VCO that supplies the modulation signal to the LN modulator in the OPLL is set to 20 GHz, but the present disclosure is not limited to this.

[0086] As described above in detail, when the local oscillation excitation light having a sufficiently high SN ratio using the OPLL is generated by the excitation light generation device of the present disclosure, the inherent low noise operation of the PSA is made possible in the relay type PSA even with respect to the signal light having the high SN ratio. By the excitation light generation device of the present disclosure, it is possible to broaden an application range of the PSA, which is a key to improving the SN ratio necessary for large-capacity optical transmission.

INDUSTRIAL APPLICABILITY

[0087] The present invention can be used for communications. More specifically, it can be used for an optical communication system.