Optical parametric amplifier

Abstract

This optical parametric amplifier: is configured so as to propose a novel contrivance for optical phase synchronization between the signal beam and the pump beam; and provides an optical phase-synchronization feedback circuit of simplified configuration. In the optical state, a control beam phase-shifted with respect to the signal beam is newly introduced. An optical-intensity signal from the control beam after having been optical-parametrically amplified is employed as-is as an error signal for feedback control. Rendering many of the configurational components that have been required with conventional technology unnecessary allows the configuration of the optical parametric amplifier to be simplified. The optical parametric amplifier makes it possible to set the phase relationship between the signal beam and the pump beam for either amplification operation or attenuation operation. The control beam may be situated in any position within the optical amplification band of the signal beam.

Description

[Technical field]

[0001] The present invention relates to an optical system using a nonlinear optical effect, and more particularly to an optical parametric amplifier. [Background technology]

[0002] Devices using nonlinear materials such as nonlinear optical materials and electro-optical materials are used in a wide range of applications, including optical signal wavelength conversion and optical modulation in optical communications, optical measurement, optical processing, medicine, bioengineering, etc. The wavelengths used span the ultraviolet, visible, infrared, and terahertz ranges, and various types of devices are being developed primarily for the generation and modulation of coherent light.

[0003] As a nonlinear optical medium and an electro-optical medium, for example, an oxide-based compound substrate such as lithium niobate (LiNbO3) is a promising material with very high second-order nonlinear optical constant and electro-optic constant. As an example of an optical device using the high nonlinearity of lithium niobate, a wavelength conversion element using periodically poled lithium niobate (PPLN) is known. In the wavelength conversion element, the mechanisms of second harmonic generation (SHG), difference frequency generation (DFG), and sum frequency generation (SFG) by PPLN are used.

[0004] By using a wavelength conversion element with high wavelength conversion efficiency, optical parametric amplification occurs due to the transfer of energy from the pump light power to the signal light, making it possible to configure an optical amplifier for the signal light. In optical parametric amplifiers, phase sensitive amplifiers (PSAs), which have amplification characteristics according to the phase relationship between the pump light and the signal light, are expected to be a technology that enables low-noise optical amplification.

[0005] By utilizing the damping action in optical parametric amplification, photon pairs with quantum correlation can be generated by the degenerate optical parametric amplification process, making it possible to generate squeezed light and non-classical light such as a heralded single-photon state. This light is also expected to be an important resource for optical quantum computers and quantum light-based sensing technology.

[0006] Optical parametric amplification is sensitive to the optical phases of the signal light and pump light, whether it is used in amplification or attenuation. To apply it to an actual system, the optical phase relationship between the pump light and the signal light entering the optical parametric amplifier must always be kept in a specified state. In a device using optical parametric amplification, the pump light and the signal light pass through separate optical paths, and then are spatially multiplexed by a multiplexer or the like before being entered into the optical parametric amplifier. Because the pump light and the signal light pass through separate paths, they are affected by changes in the optical path length of various optical components due to external disturbances, and it is difficult to always keep the phase relationship between the two lights constant.

[0007] Therefore, optical phase feedback control is performed to reduce the influence of disturbances on the pump light and signal light, stabilize the phase relationship, and keep the target characteristics of the device constant. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] JP 2015-225127 A Summary of the Invention [Problem to be solved by the invention]

[0009] However, when controlling the optical phase of an optical parametric amplification system using feedback control, many electrical circuits such as a signal generator, frequency mixer, and low-pass filter are required, making the system complicated. The configuration required for synchronizing the optical phase has been an obstacle to simplifying and reducing the cost of systems and devices that use optical parametric amplification. [Means for solving the problem]

[0010] One aspect of the present invention is an optical parametric amplification device comprising: an optical multiplexer for one or more signal lights and pump lights; a phase modulator on the input side of the optical multiplexer and arranged in a signal light path or a pump light path; an optical parametric amplifier for optically amplifying the signal light; an optical phase shifter arranged in the signal light path on the input side of the optical parametric amplifier and changing the optical phase of control light within a frequency band of the signal light or a control idler light that is phase conjugate to the control light; a monitor light separator for separating at least one of the control light or the control idler light as a monitor light; a photodetector for converting the optical intensity of the monitor light into an electrical signal; and a feedback gain adjuster having at least an integrating circuit, which receives the electrical signal as an error signal, sends a control signal to the phase modulator, and Effect of the Invention

[0011] To provide a low-cost optical parametric amplifier with a simpler configuration. [Brief description of the drawings]

[0012] [Figure 1] FIG. 1 is a diagram showing a schematic configuration of a conventional optical parametric amplifier device. [Diagram 2] FIG. 1 is a diagram illustrating a basic operation of an optical parametric amplifier. [Diagram 3] 1 is a diagram showing a schematic configuration of an optical parametric amplifier device according to the present disclosure. [Figure 4] FIG. 1 is a diagram for explaining an error signal for optical phase synchronization in comparison with the prior art. [Diagram 5]1 is a diagram showing a configuration of an optical parametric amplifier according to a first embodiment. [Figure 6] FIG. 1 is a diagram showing a configuration of an optical parametric amplifier according to a second embodiment. [Figure 7] FIG. 13 is a diagram showing the configuration of an optical parametric amplifier according to a third embodiment. [Figure 8] FIG. 13 is a diagram showing the configuration of an optical parametric amplifier according to a fourth embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] The optical parametric amplifier of the present disclosure provides a new mechanism for optical phase synchronization between signal light and pump light. In the optical state, a control light that is phase-shifted with respect to the signal light is newly introduced. By using the optical intensity of the control light after optical parametric amplification as an error signal for feedback control, many components required in the conventional technology are unnecessary, and the configuration of the optical parametric amplifier can be simplified. The optical parametric amplifier can set the phase relationship between the signal light and the pump light to either an amplification operation or an attenuation operation. Therefore, the optical parametric amplifier includes, for example, a PSA that uses an amplification operation, and also includes a squeezed light generation device that uses an attenuation operation. Below, first, the problems in optical phase synchronization of the conventional technology are described in detail, and then the configuration and operation for optical phase synchronization of the optical parametric amplifier of the present disclosure are described.

[0014] FIG. 1 is a diagram showing a schematic configuration of a conventional optical parametric amplifier. In an optical parametric amplifier 100, a signal light 111 and a pump light 112 are input to a phase modulator 101. The phase modulator 101 adjusts the optical phase difference between the signal light 111 and the pump light 112 by a control signal 120 from a feedback control unit described later. The signal light 111 and the pump light 112 whose optical phase difference, i.e., relative phase, has been adjusted are multiplexed by a multiplexer 102 and input to an optical parametric amplifier 103 including a nonlinear optical device. The amplified signal light is separated by a monitor light separation unit 104 into an output signal light 113 and a monitor light 114. The monitor light 114 is converted by a photodetector (PD) 105 into an optical intensity signal 115, which is an electrical signal.

[0015] The optical intensity signal 115 is input to a feedback control section that performs optical phase synchronization between the signal light 111 and the pump light 112. In order to make a comparison with an optical parametric amplifier device of the present disclosure described later, in Fig. 1, the feedback control section is divided into a modulation / demodulation circuit section 121 shown within a dotted line, and a feedback gain control section 109.

[0016] The modulation / demodulation circuit section 121 includes a signal generator 108 that generates a modulation signal for feedback control, and one of the modulation signals, 118a, is supplied to the mixer 106. The other modulation signal, 118b, is added to a feedback signal 119 via an adder 110 from the signal generator 108, and is supplied to the phase modulator 101 as a control signal 120. The modulation signal 118b provides phase modulation to the optical phase difference between the signal light 111 and the pump light 112 by the phase modulator 101. The optical parametric amplifier 103 optically amplifies the signal light 111 in a maximum gain state by controlling the optical phase difference between the signal light 111 and the pump light 112 to be in phase (0) or out of phase (π). By applying phase modulation by the above-mentioned modulation signal 118b, the optical parametric amplifier 103 deviates slightly from the maximum gain state. The phase modulation by the phase modulator 101 is converted into a fluctuation in the optical amplification gain, and the optical intensity of the output light from the optical parametric amplifier 103 is amplitude modulated by the modulation signal 118b.

[0017] If there is no disturbance or noise in the optical phases of the signal light and the pump light, the optical intensity signal 115 of the monitor light and the modulated signal 118a from the signal generator 108 will be identical, and the mixer output 116 will have only a DC component. On the other hand, if disturbance or noise is added to the optical phases of the signal light and the pump light, the mixer output 116 will be an AC signal containing various frequency components, and the output band-limited by the LPF 107 will be an error signal 117 (deviation between the target value and the output value) that reflects the deviation or shift of the optical phase difference from the target value. The path from the PD 105 to the LPF 107 operates as a demodulation circuit.

[0018] Based on the error signal 117, feedback gain control section 109 performs feedback control to generate control signal 119. For example, feedback gain control section 109 can utilize PID (Proportional-Integral-Differential) control, which is widely used in automatic control.

[0019] The method of optical phase synchronization by modulating the optical phase difference between the signal light and pump light to be controlled and demodulating the modulated component again to achieve optical phase synchronization, as in the optical phase-locked feedback control shown in Fig. 1, is similar to the Pound-Drever-Hall method (PDH method) used for laser frequency stabilization, etc. The reason for generating the error signal 117 as shown in Fig. 1 is that it is necessary to control the optical parametric amplifier so that the amplification or attenuation is maximized in order to effectively utilize the optical parametric amplification characteristics.

[0020] FIG. 2 is a diagram for explaining the basic operation of an optical parametric amplifier. Here, a non-degenerate phase-sensitive amplifier (PSA) is taken as an example. FIG. 2(a) shows the frequency relationship between signal light, idler light, and pump light in a non-degenerate PSA. In a non-degenerate PSA, signal light 201a and corresponding idler light (phase conjugate light) 202a are optically amplified simultaneously. The signal light 201a and the phase conjugate light 202a are located symmetrically on the frequency axis with respect to a reference frequency 203 (f / 2), and the reference frequency 203 has a frequency that is half the frequency f of the pump light 204. The term "idler light" refers to light that is located symmetrically with the signal light with respect to the reference frequency and has a phase conjugate relationship, and in the following description, it is used interchangeably with phase conjugate light.

[0021] FIG. 2B illustrates the amplification and attenuation conditions in a non-degenerate PSA in the complex plane. FIG. 2B illustrates an optical signal on the complex plane, showing the phase relationship between the signal light 201b and the corresponding idler light 202b when the phase of the light of the reference frequency (center frequency) is taken as the reference (0°). In the signal diagram shown in the complex plane, the signal light and its idler light are represented as having phases that rotate in opposite directions as the frequency moves away from the reference frequency. In a state where there is no chromatic dispersion of the refractive index in the transmission line, the phases of the signal light 201b and the idler light 202b rotate in opposite directions at the same speed as the frequency moves away from the reference frequency, and the composite phase of the two lights becomes 0 or -π. In a non-degenerate PSA, amplification occurs when this composite phase 205 becomes in phase with the phase of the pump light 204 (amplification phase: 0 or π). On the other hand, if the composite phase 205 of the signal light 201b and the idler light 202b becomes orthogonal to the phase of the pump light (attenuation phase: π / 2 or -π / 2), attenuation occurs. The phase of the pump light 204 is generated in direct relation to the light of the reference frequency on the signal light generation side (transmission side), so it has the same phase as the light of the reference frequency and is in a state of phase 0 on the complex plane.

[0022] 2(c) shows the relationship between the difference between the composite phase and the amplified phase and the amplification and attenuation operations. The horizontal axis shows the phase difference between the composite phase of the signal light 201b and the idler light 202b and the amplified phase (0°), and the vertical axis conceptually shows the optical intensity at the output of the non-degenerate PSA. When the phase difference is 0 or π, the amplification operation is at its maximum, generating peaks 210-1 or 210-2, and when the phase difference is π / 2 or -π / 2, the attenuation operation is at its maximum, generating dips 211-1 or 211-2.

[0023] The above-mentioned relationship between the composite phase of the signal light and the idler light and the amplification phase or attenuation phase is the same for a degenerate PSA in which the signal light and the idler light are arranged at the same frequency. Therefore, to use the PSA in a desired state such as maximum amplification or maximum attenuation, it is necessary to control the composite phase of the signal light and the idler light to a predetermined state according to the target operating state.

[0024] In the conventional optical parametric amplifier 100 of FIG. 1, a control signal 120 is applied to the phase modulator 101 for optical phase-locked feedback control to set the composite phase to a predetermined operating state. In the optical parametric amplifier 100 of FIG. 1, the optical intensity signal 115 obtained from the PD 105 cannot be used as an error signal indicating deviation of the composite phase from a target value. The error signal in feedback control needs to change monotonically around the target value of the physical quantity or measured value to be controlled. Referring again to FIG. 2(c), in the case of optical parametric amplification, the intensity signal is near a peak or a dip in both the maximum amplification and maximum attenuation states, which are the target operations. Regardless of which direction the composite phase of the signal light and the idler light, which are the control objects, shifts, the optical intensity signal 115 from the PD 105 shifts in the same direction. For example, when the predetermined operation is maximum amplification, the optical intensity signal 115 decreases regardless of which direction the composite phase shifts. Also, when the predetermined operation is maximum attenuation, the optical intensity signal 115 increases regardless of which direction the composite phase shifts. Therefore, the optical intensity signal after optical parametric amplification cannot be used as an error signal as it is.

[0025] In order to obtain an error signal that can be used for feedback control, the pump light or signal light is phase-modulated, and the modulation / demodulation circuit section 121 shown in Fig. 1 is provided to generate an error signal from the optical intensity signal 115. The modulation / demodulation circuit section 121 requires electrical circuits such as a signal generator 108 for modulation, a mixer 106 for demodulation, and an LPF 107, which makes the system configuration significantly more complicated.

[0026] The optical parametric amplifier disclosed herein provides a new mechanism for optical phase synchronization between signal light and pump light. In this new mechanism, a control light having a composite phase shifted from the composite phase of the signal light is newly introduced. By using the optical intensity of the control light after optical parametric amplification as an error signal for feedback control, many electrical circuit components are unnecessary, simplifying the optical parametric amplifier and reducing its cost.

[0027] The optical parametric amplifier of the present disclosure includes an optical phase shifter that generates control light having a different composite phase with respect to the signal light to be amplified or attenuated.

[0028] [Basic configuration] Fig. 3 is a diagram showing a schematic configuration of an optical parametric amplifier according to the present disclosure. The optical parametric amplifier 10 comprises a phase modulator 1 to which signal light 11 and pump light 12 are input, a multiplexer 2, an optical parametric amplifier 3, a monitor light separation unit 4, and a photodetector (PD) 5. Amplified (or attenuated) signal light 13 is output from the monitor light separation unit 4, and monitor light 14 is also separated and output. The operation of these elements that perform optical parametric amplification is exactly the same as that of the conventional optical parametric amplifier 100 shown in Fig. 1, and detailed description thereof will be omitted here.

[0029] The first difference between the optical parametric amplifier 10 in FIG. 3 and the conventional optical parametric amplifier 100 in FIG. 1 is that the optical parametric amplifier 10 includes an optical phase shifter 6 that generates control light by shifting the phase of a portion of the input signal light by a predetermined value in the optical state. The second difference is that the optical intensity signal 15 of the control light (monitor light) from the PD 5 can be used as the error signal 16 as it is, so that the modulation / demodulation circuit unit 121 required in the conventional configuration is no longer necessary. As the electrical circuit on the output side of the PD 5, only a PID controller, for example, that functions as the feedback gain control unit 7 that generates the control signal 17 for controlling the phase modulator 1 is required. Compared to the conventional configuration, the electrical circuit required for optical phase-locked feedback control can be significantly simplified.

[0030] FIG. 4 is a diagram for explaining an error signal for optical phase synchronization in the optical parametric amplifier 10 of the present disclosure in comparison with the conventional technology. FIG. 4(a) shows a process of generating an error signal in the conventional optical parametric amplifier shown in FIG. 1. Taking a non-degenerate PSA as an example, a signal light group 220 including a plurality of signal lights and an idler light is optically amplified. The idler light is located symmetrically to the signal light with respect to the reference frequency indicated by the dotted arrow on the frequency axis. The signal light group may be a single signal light and its idler light, or may be a plurality of signal lights and a plurality of idler lights corresponding to each of the signal lights. As described in FIG. 2, when the non-degenerate PSA is used in an amplified state, the phase difference between the composite phase of the signal light and the idler light and the amplified phase is controlled to be 0 or π. In this target state in which the phase difference is controlled to 0 or π, the optical intensity signal 115 is at peak positions 210-1 and 20-2. Since the error signal at the peak position cannot be used for feedback control, a monotonic error signal 117 is obtained by providing a modulation / demodulation circuit section 121. This error signal 117 has good linearity, as its sign is inverted with respect to the reference voltage of the feedback control circuit when the phase difference of the target value is 0 or π, and can be used for feedback control.

[0031] FIG. 4(b) shows an error signal in the optical parametric amplifier of the present disclosure shown in FIG. 3. In the optical parametric amplifier 10 of FIG. 3, a part of the light that can also be used as the signal light is phase-shifted in advance to become the control light, and this control light is used as the monitor light 14 after optical amplification. The signal light group 221 shown in FIG. 4(b) is obtained by removing the signal light and its idler light that are closest to the reference frequency indicated by the dotted arrow on the frequency axis from the signal light group 220 shown in FIG. 4(a). The light that can also be used as the signal light and its idler light in the conventional technology is used as the control light 213a and the control idler light 214a in the optical parametric amplifier 10 of the present disclosure. Referring to the control light 213b and the control idler light 214b shown on the complex plane in FIG. 4(b), the composite phase 215 of these two lights 213b and 214b is shifted by π / 4 from the amplification phase (0).

[0032] If a part of the signal light is coupled by a coupler or the like and detected as a monitor light by a PD to obtain an intensity signal as in the conventional technology, the signal light is controlled to a maximum amplification state. Therefore, as shown in (a) of FIG. 4, the intensity signal is measured as peak voltages 210-1 and 210-2 with a phase difference of 0. On the other hand, in the optical parametric amplifier 10 disclosed herein, only the control light that is phase-shifted with respect to the signal light is separated and detected by the monitor light separation unit 4, and the optical intensity signal 15 is directly used as the error signal 16.

[0033] As shown in Fig. 4(b), if the amount of phase shift of the control light relative to the signal light is π / 4, the optical intensity signal 15 output from the PD 5 corresponds to the slope portion 212 off the peak shown in Fig. 4(b). The intensity signal of the signal light controlled to the amplified phase (phase difference 0) is near the peak 210, whereas the slope portion 212 of the intensity signal of the control light with a composite phase 215 shifted by π / 4 varies monotonically and has good linearity. Therefore, the optical intensity signal 15 obtained from the PD 5 can be used as it is, without processing, as the error signal 16 for phase-locked feedback control.

[0034] Therefore, the optical parametric amplification device 10 disclosed herein can be implemented as comprising: an optical multiplexer 2 for one or more signal light and pump light; a phase modulator 1 arranged on the input side of the optical multiplexer in the signal light path or the pump light path; an optical parametric amplifier 3 for optically amplifying the signal light; an optical phase shifter 6 arranged on the signal light path on the input side of the optical parametric amplifier for changing the optical phase of control light within the frequency band of the signal light or a control idler light that is phase conjugate with respect to the control light; a monitor light separator 4 for separating at least one of the control light or the control idler light as monitor light 14; a photodetector 5 for converting the optical intensity of the monitor light into an electrical signal 15; and a feedback gain adjuster 7 having at least an integration circuit, which receives the electrical signal as an error signal, sends a control signal 17 to the phase modulator, and

[0035] The optical phase shifter 6 in the optical parametric amplifier 10 in FIG. 3 gives a predetermined phase shift to the control light arranged in a band in which the signal light can be optically amplified, based on the phase of the signal light. As described above, the control light is used as monitor light for optical phase-locked feedback control of the signal light to be controlled. The phase-shifted control light is optically parametrically amplified in the optical parametric amplifier while interfering with a control idler light located at a position where the frequency is folded back around a certain reference frequency. However, since the control light is optically amplified in a state where it is shifted from the maximum amplification state and the amplification gain is suppressed compared to the signal light, the linear portion of the intensity signal shifted from the peak position is used as an error signal as it is. Since the control light is in a state where the amplification gain is insufficient in the optical parametric amplification, it is preferable that the control light does not contain transmission information, unlike the signal light.

[0036] The above-mentioned reference frequency corresponds to the pump light wavelength in the case of a third-order nonlinear optical medium. In the case of a second-order nonlinear optical medium, the frequency is half the pump light frequency. The optical phase shifter 6 may shift the phase of only the control light, or may shift the phase of both the control light and the control idler light. As explained in FIG. 2, in optical parametric amplification, if the composite phase of the signal light and the idler light of the signal light is in phase (0 phase) or out of phase (π phase) with the amplification phase determined by the phase of the pump light, optical amplification occurs, and if it is orthogonal (π / 2 phase or 3π / 2 phase), optical attenuation occurs. In an optical parametric amplifier, it is usually required to collectively amplify all signal channels arranged within a frequency band in which the signal light can be optically amplified. Therefore, for all signal channels, the composite phase of the signal light and the corresponding idler light must be matched with the amplification phase.

[0037] Here, consider the phase fluctuation that occurs in the signal light in the optical transmission line. In general, when only first-order refractive index dispersion exists in the transmission line, even if there are multiple target signal lights (multiple channels) within the optical amplification band, the composite phase of each signal light and the corresponding idler light will be aligned to the amplification phase (0 or π) shown in Figure 2(b). In both the conventional technology and the optical parametric amplifier disclosed herein, phase-locked feedback control is performed to suppress fluctuations in the phase difference between the pump light and the signal light due to the effects of disturbances and noise occurring in each path of the pump light and the signal light.

[0038] In the optical parametric amplifier device 10 disclosed herein, prior to optical parametric amplification, the composite phase of the control light and its idler light is shifted relative to the composite phase of the signal light and its idler light, so that the intensity signal obtained from the control light can be used as an error signal as is.

[0039] After passing through the optical parametric amplifier 3 in Fig. 3, one or both of the control light and the control idler light are separated by the monitor light separation unit 4 and extracted as monitor light 14. The optical intensity of the monitor light 24 is converted to an optical intensity signal 15 by the PD 5 and used as is as the error signal 16. As already explained in Fig. 4, in the target operating state of the signal light (e.g., maximum amplification), the optical intensity signal 15 of the phase-shifted control light changes as a monotonous function. In particular, when the combined phase of the control light is set to a phase shifted by π / 4+nπ / 2 (n is an integer), the slope of the straight line around the target value of the error signal becomes steep, and optical phase synchronization with good performance can be achieved.

[0040] Since the light used as the control light can utilize a frequency band not used for transmission, the optical parametric amplifier of the present disclosure does not substantially limit the transmission band. For example, the control light and the control idler light may be set to the same frequency light using a reference frequency. Also, the control light and the control idler light may be set to the frequency channel farthest from the reference frequency in the frequency band of the signal light. The optical phase shifter 6 may be configured to extract only the control light, delay it, and return it to the original optical path (Example 1), or to obtain the control light by inputting the entire signal light into a secondary dispersion medium without extracting the control light (Example 2). The optical fiber itself, which is the transmission medium, can be used as the secondary dispersion medium, and it is preferable that the control light is located away from the reference frequency so that it is significantly affected by secondary refractive index dispersion. The configuration and operation of the optical parametric amplifier will be described below in more detail. EXAMPLES

[0041] Fig. 5 is a diagram showing the configuration of an optical parametric amplifier according to the first embodiment. The optical parametric amplifier 20 in Fig. 5 has the same configuration as the optical parametric amplifier 10 shown in Fig. 3, and has a more specific configuration of an optical phase shifter 21 for generating control light. Therefore, a description of the overall configuration and basic operation of the optical parametric amplifier 20 will be omitted.

[0042] In this embodiment, a PPLN waveguide, which is a second-order nonlinear element, is used as the optical parametric amplifier 3. The reference frequency is set to 194 THz, and the center of the wavelength band of the optical parametric amplification for the signal light is 1545.32 nm. The wavelength of the pump light is set to about 780 nm, which corresponds to the double wave of the signal light. Within the wavelength band of the signal light, the wavelength of the control light is set to 1530.00 nm. At this time, the wavelength of the control idler light is 1560.95 nm. The optical parametric amplifier 3 is not limited to one using a PPLN waveguide, and may be one using a highly nonlinear fiber.

[0043] In the optical phase shifter 21, a phase shift is applied to the control light arranged within the optical amplification band of the signal light. The control light is optically parametrically amplified while interfering with the control idler light located at a position folded back around the reference frequency on the frequency axis. The control light shifter 21 of this embodiment is composed of a wavelength separator 22 that extracts only the control light from the signal light path, a delay line 23, and a wavelength multiplexer 24. A demultiplexing arrayed waveguide grating (AWG) is used as the wavelength separator 22. The demultiplexing AWG 22 inputs the signal light directly to the multiplexing AWG 24. The control light extracted by the demultiplexing AWG 22 is phase shifted by the delay line 23. The multiplexing AWG is used as the wavelength multiplexer 24, and the signal light and the phase-shifted control light are multiplexed again and input to the phase modulator 1.

[0044] There are two methods for shifting the phase of the control light by the optical phase shifter 21: "Method 1" which shifts the phase of only the control light, and "Method 2" which shifts both the control light and the control idler light. In either method, the important thing is to shift the combined phase of the control light and the control idler light relative to the signal light.

[0045] In the case of "Method 1" in which the optical parametric amplifier 20 is used at maximum gain and only the control light is extracted and phase-shifted by the demultiplexing AWG 22, it is preferable to set the phase shift amount of the control light to π / 2 + nπ (n is an integer). At this time, since the phase of the control idler light is not shifted, the composite phase of the phase-shifted control light and the control idler light is π / 4. Therefore, as shown in Fig. 4(b), the optical intensity signal 15 of the monitor light 14 uses the portion 212 with good linearity and the steepest slope near a phase shift of π / 4 from the amplified phase, enabling feedback control with high feedback gain.

[0046] For the monitor light separator 4 in Fig. 5, the control light was extracted as the monitor light 14 using an AWG. The optical intensity signal 15 of this monitor light was input to the feedback gain controller 7, and a feedback control signal 17 was returned to the phase modulator 1 inserted in the pump light path upstream of the optical parametric amplifier 3. The phase modulator 1 used an LN modulator that adds phase modulation to the pump light. The configuration in Fig. 5 realized stable amplification of the signal light in the maximum gain operation state with an optical phase-locked feedback circuit that is simpler than the conventional technology. It was also confirmed that by reversing the positive and negative of the feedback gain, the feedback control signal could be made to have an inverse phase, and the optical parametric amplifier 20 could be phase-locked in the attenuation operation state.

[0047] The above explanation was an example in which the optical phase shifter shifts the phase of only the control light, but "Method 2" may also be used in which both the control light and the control idler light are extracted and their optical phases are shifted. In Method 2, it is desirable that the sum of the phase shift amounts of the control light and the control idler light is π / 2 + nπ (n is an integer). When the sum of the phase shift amounts of the two lights is π / 2, the composite phase is always close to π / 4 on the complex plane, and the optical intensity signal 15 of the monitor light 14 has good linearity and is at the steepest slope.

[0048] As an example of setting the sum of the phase shift amounts of the two light beams described above in "Method 2" to π / 2, the shift amounts of the control light and the controlled idler light may each be the same 45° (π / 4). The shift amounts may also be unbalanced, with one of the control light and the controlled idler light being shifted by 20° and the other by 70°. Furthermore, if the shift amount of the control light is set to 90° and the shift amount of the controlled idler light to 0°, it will be understood that this corresponds to "Method 1" in which only the control light is extracted and phase-shifted.

[0049] The wavelength separator 22 for extracting the control light and the control idler light from the signal light is not limited to AWG, and may be a simpler directional coupler, diffraction grating, etc. Furthermore, the wavelength separator 4 for extracting the monitor light is not limited to AWG, and may be a directional coupler, diffraction grating, etc. Furthermore, even if the light extracted as the monitor light is the control idler light instead of the control light, optical phase-locked feedback control is realized without any problem.

[0050] The wavelength of the control light may be placed anywhere within the wavelength band of the signal light to be optically parametrically amplified. As explained in FIG. 4(b), the control light and the control idler light may be placed at or near the center wavelength (corresponding to the reference frequency) of the wavelength band of the signal light. When the control light is set to the center wavelength, the control light and the control idler light become the same wavelength light, and it becomes impossible to distinguish between them (corresponding to degenerate phase-sensitive amplification). However, from the viewpoint of optical phase-locked feedback control, the signal light is operated in the maximum gain state, and the signal light is amplified stably. The wavelength of the control light may be set to the edge position of the wavelength band of the signal light, and the control light and the control idler light may be placed at both ends of the wavelength band of the signal light. In order to extract only one of the control light or the control idler light by the optical phase shifter 21, it is also possible to use a directional coupler with a lower cost and simpler configuration than the AWG.

[0051] The feedback gain controller 7 used in the optical parametric amplifier 20 can perform the phase synchronization function if it includes an integrator. Therefore, the general-purpose PID controller shown in Fig. 5 is only an example, and does not need to include all of the elements of the deviation between the output value and the target value, its integral, and its derivative. The feedback gain controller 7 may further include a linear amplifier and a differentiator. By increasing the gain of the linear amplifier and widening the loop band, it becomes possible to deal with sudden deterioration of characteristics due to disturbances.

[0052] What is important in the optical parametric amplifier of the present disclosure is to make the composite phase of the signal light and the composite phase of the control light different. Therefore, the shift amount of the composite phase is not limited to π / 4 (45°). For example, it should be noted that if the phase shift is about 20 to 70°, the linear portion of the intensity signal can be used. Therefore, the accuracy of the shift amount in the optical phase shifter 21 can be loose if the phase fluctuation expected for optical phase synchronization is small.

[0053] In the above description, the signal light and its idler light are input to the optical parametric amplifier. Depending on the optical transmission system, pilot light may be sent together with the signal light, or only the signal light may be transmitted, thereby self-regenerating the pump light and idler light. For example, some optical repeater amplifiers regenerate the pump light and idler light from the signal light itself, and perform optical parametric amplification for repeater amplification. In such cases, the method of realizing the optical phase shift of the control light can be varied in many ways and is not limited to the configuration of FIG. 5.

[0054] In the optical phase shifter 21 of the first embodiment, a configuration has been shown in which the control light or the control idler light is extracted and a delay is applied to shift the phase, but the refractive index dispersion of the optical transmission line can also be used as the optical phase shifter. EXAMPLES

[0055] Figure 6 is a diagram showing the configuration of an optical parametric amplifier according to embodiment 2. An optical parametric amplifier 30 shown in Figure 6(a) has the same configuration as the optical parametric amplifier 20 shown in Figure 5, and is an example in which an optical phase shifter 31 for generating control light is realized by an optical fiber. Therefore, a description of the overall configuration and basic operation of the optical parametric amplifier 30 will be omitted. As the optical parametric amplifier 3, a PPLN waveguide, which is a second-order nonlinear element, was used, as in the first embodiment. The reference frequency was set to 194 THz, and the center of the wavelength band of the optical parametric amplification for the signal light was 1545.32 nm. The wavelength of the pumping light was set to about 780 nm, which corresponds to the double wave of the signal light. Within the wavelength band of the signal light, the wavelength of the control light was set to 1530.00 nm. At this time, the wavelength of the control idler light was 1560.95 nm. In this embodiment, no optical components were used as the optical phase shifter 31, and an optical fiber, which is a transmission line, was used.

[0056] Fig. 6(b) illustrates the arrangement of the control light in the second embodiment. In general, the material constituting the optical fiber has a chromatic dispersion of the refractive index. When the frequency of the signal light 217a is close to the center frequency of the band of the optical parametric amplification, the frequency of the idler light 218a, which is conjugate with the signal light, is also close to the center frequency. Therefore, the influence of the second-order refractive index dispersion of the optical fiber becomes small, and only the influence of the first-order refractive index dispersion becomes dominant.

[0057] FIG. 6(c) illustrates the phase shift caused by the refractive index dispersion of the optical fiber on the complex plane. The first-order refractive index dispersion does not change the direction of the composite phase 219 of the signal light 217b and its idler light 218b, so the phase difference between the composite phase 219 and the amplified phase is kept constant even when propagating through the transmission line. However, the influence of the second-order refractive index dispersion of the optical fiber on the signal light and its idler light increases as the signal channel becomes farther from the center frequency. The second-order refractive index dispersion changes the direction of the composite phase of the signal light and the idler light on the complex plane. In this embodiment, as shown in FIG. 6(b), a pair of the control light 213a and the control idler light 214a is arranged at a position away from the center frequency. In the arrangement shown in FIG. 6(b), the signal light including the control light and the control idler light propagates through the optical phase shifter 31, which is an optical fiber of a length having the required second-order dispersion. As a result, as shown in FIG. 6(c), a composite phase 215 of the control light 213b and the control idler light 214b is shifted from the amplified phase (0°).

[0058] As described above, an optical fiber, which is a transmission line having a second-order refractive index dispersion, can perform the phase shifting function of an optical phase shifter. Therefore, an optical parametric amplifier similar to that of the first embodiment can be realized with an optical phase shifter 31 having a simpler configuration than the optical phase shifter 21 including the demultiplexer and multiplexer of the first embodiment. As the optical fiber for realizing the optical phase shifter 31, in addition to a normal optical fiber, a high-dispersion fiber such as a photonic crystal fiber can also be used.

[0059] As for the monitor light separation unit 4 in FIG. 6(a), the control light was extracted as the monitor light 14 using the AWG as in the first embodiment. The optical intensity signal 15 of this monitor light was input to the feedback gain control unit 7, and a feedback control signal 17 was returned to the phase modulator 1 inserted in the pump light path on the upstream side of the optical parametric amplifier 3. The phase modulator 1 used an LN modulator that adds phase modulation to the pump light. The configuration in FIG. 6(a) realized a stable amplification operation of the signal light in the maximum gain operation state by the optical phase-locked feedback circuit that is simpler than the conventional technology. As in the first embodiment, it was also confirmed that the phase was locked to the attenuation operation by reversing the positive and negative of the feedback gain. Even if the light extracted as the monitor light by the monitor light separation unit 4 was used as the control idler light, the optical phase-locked feedback control was realized without any problem. EXAMPLES

[0060] In both of the above-mentioned first and second embodiments, the optical phase shifter is placed in the front stage of the phase modulator 1. However, as long as it is possible to shift only the combined phase of the control light with respect to the combined phase of the signal light, the position of the optical phase shifter is not limited, and other variations are possible.

[0061] 7 is a diagram showing the configuration of an optical parametric amplifier according to the third embodiment. An optical parametric amplifier 40 according to the third embodiment is generally the same as each of the configurations of the optical parametric amplifiers according to the first and second embodiments. The difference is that an optical phase shifter 41 is provided in the signal light path between the phase modulator 1 and the multiplexer 2. It is clear that if a predetermined phase shift can be given to only the combined phase of the control light and the control idler light with respect to the signal light, an error signal 16 similar to that of the first and second embodiments can be obtained.

[0062] In the configuration of an actual optical amplifier such as a PSA, multiple PPLNs are often used for purposes other than optical parametric amplification, such as pump light regeneration circuits and pump light stabilization. In such cases, optical circuits (not shown) exist in front of the phase modulator 1 and optical multiplexer 2 in Fig. 7. Therefore, in an actual device that uses optical parametric amplification, there is flexibility in the location of the optical phase shifter 41. EXAMPLES

[0063] FIG. 8 is a diagram showing the configuration of an optical parametric amplifier according to a fourth embodiment. This embodiment is another variation in the position of the optical phase shifter. An optical parametric amplifier 50 according to the fourth embodiment has substantially the same configuration as each of the optical parametric amplifiers according to the first to third embodiments. The difference is that an optical phase shifter 51 is provided in the signal light path between the multiplexer 2 and the optical parametric amplifier 3. It is clear that if a predetermined phase shift can be given to only the combined phase of the control light and the control idler light with respect to the signal light, an error signal 16 similar to that of the first to third embodiments can be obtained.

[0064] In the above-mentioned embodiments, the phase modulator 1 is illustrated as passing through both the signal light path and the pump light path in order to show a general configuration, but it is sufficient if it passes through at least one of the signal light path or the pump light path. The phase modulator 1 is only required to be able to cause a fluctuation in the phase difference between the signal light and the pump light by the control signal 17 for optical phase synchronization feedback control. Therefore, it may be arranged so as to pass through only one of the signal light path or the pump light path, or it may be configured so as to pass through both paths as necessary depending on the form of the phase modulator.

[0065] In addition, although the above-mentioned embodiments have been described as optical parametric amplifiers, as long as the optical parametric amplification mechanism is used, the device can also be used as an optical signal attenuator or wavelength converter. In addition, depending on the setting conditions for the phase relationship between the pump light and the signal light, the device can also be used as an application device that handles non-classical light, and it should be noted that the application is not limited to the "optical amplifier".

[0066] As described above in detail, in the optical parametric amplifier of the present disclosure, the composite phase of the control light and the control idler light is shifted relative to the composite phase of the signal light and its idler light, and the control light or the control idler light is used as the monitor light. This makes it possible to omit the modulation / demodulation circuit used in the conventional technology, simplify the configuration of the optical parametric amplifier, and reduce costs. [Industrial Applicability]

[0067] The present invention can be used in optical signal processing devices such as optical amplifiers.

Claims

1. one or more optical multiplexers for signal light and pump light; a phase modulator disposed on the input side of the optical multiplexer, the phase modulator being disposed on the signal light path or the pumping light path; an optical parametric amplifier that optically amplifies the signal light; an optical phase shifter that is disposed on the signal light path on the input side of the optical parametric amplifier and that changes an optical phase of a control light set in a frequency band of the signal light and phase-shifted with respect to the signal light, or a control idler light that is phase conjugate with respect to the control light; a monitor light separator that separates at least one of the control light and the control idler light as a monitor light; a photodetector for converting the optical intensity of the monitor light into an electrical signal; a feedback gain adjuster having at least an integrating circuit, the feedback gain adjuster receiving the electrical signal as an error signal and sending a control signal to the phase modulator; An optical parametric amplifier comprising:

2. 2. The optical parametric amplifier according to claim 1, wherein a combined phase of the control light and the control idler light is set to be different from a combined phase of the signal light and the idler light of the signal light.

3. the one or more signal lights include a plurality of signal lights each including information and a plurality of idler lights corresponding to the plurality of signal lights; 3. The optical parametric amplifier according to claim 1, wherein the optical parametric amplifier includes a second-order nonlinear optical element or a third-order nonlinear optical element.

4. 4. The optical parametric amplifier according to claim 1, wherein the control light and the control idler light are arranged at both ends of the frequency band of the signal light on a frequency axis, and the optical phase shifter is an optical fiber.

5. 5. The optical parametric amplifier according to claim 1, wherein the optical phase shifter shifts the optical phases of both the control light and the control idler light in the same direction.

6. 6. The optical parametric amplifier according to claim 5, wherein a sum of an amount of phase shift of the control light and an amount of phase shift of the control idler light is π / 2+nπ (n is an integer).

7. 5. The optical parametric amplifier according to claim 1, wherein the optical phase shifter shifts the optical phase of either the control light or the control idler light by π / 2+nπ (n is an integer).

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