WO2023084596A1

WO2023084596A1 - Optical parametric amplifier

Info

Publication number: WO2023084596A1
Application number: PCT/JP2021/041173
Authority: WO
Inventors: 貴大柏崎; 毅伺梅木; 拓志風間
Original assignee: 日本電信電話株式会社
Priority date: 2021-11-09
Filing date: 2021-11-09
Publication date: 2023-05-19
Also published as: JPWO2023084596A1

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

Optical parametric amplifier

The present invention relates to an optical system using nonlinear optical effects, and more specifically to an optical parametric amplifier.

Devices made of nonlinear materials such as nonlinear optical materials and electrooptic materials are used in a wide range of applications such as optical signal wavelength conversion and optical modulation in optical communication, optical measurement, optical processing, medical care, and biotechnology. The wavelengths used also span the ultraviolet, visible, infrared, and terahertz regions, and various types of devices are being developed mainly for the generation and modulation of coherent light.

As a nonlinear optical medium and an electrooptic medium, for example, an oxide compound substrate such as lithium niobate (LiNbO ₃ ) is a promising material having extremely high secondary nonlinear optical constants and electrooptic constants. As an example of an optical device using high nonlinearity of lithium niobate, a wavelength conversion element using periodically poled lithium niobate (PPLN) is known. The wavelength conversion element utilizes mechanisms of second harmonic generation (SHG), differential frequency generation (DFG), and sum frequency generation (SFG) by PPLN.

If a wavelength conversion element with high wavelength conversion efficiency is used, optical parametric amplification occurs due to energy transfer from pumping light power to signal light, and an optical amplifier for signal light can be configured. Among optical parametric amplifiers, a phase sensitive amplifier (PSA), which has amplification characteristics according to the phase relationship between pump light and signal light, is expected as a technology capable of low-noise optical amplification.

When the decay operation is used in optical parametric amplification, photon pairs with quantum correlation are generated by the degenerate optical parametric amplification process, and non-classical states such as squeezed light and heralded single photon states are generated. It is possible. These lights are also expected to be important resources for optical quantum computers and sensing technology using quantum light.

Optical parametric amplification is sensitive to the optical phases of the signal light and pump light, regardless of whether amplification or attenuation is used. For application to an actual system, it is necessary to keep the optical phase relationship between the excitation light and the signal light entering the optical parametric amplifier in a predetermined state at all times. In an apparatus using optical parametric amplification, pumping light and signal light pass through different optical paths, are spatially multiplexed by a multiplexer or the like, and enter an optical parametric amplifier. Since the excitation light and the signal light pass through different paths, they are affected by changes in the optical path lengths of various optical components due to disturbances, and it is difficult to always keep the phase relationship between the two lights constant.

Therefore, optical phase feedback control is performed so as to reduce the influence of disturbance on the excitation light and signal light, stabilize the phase relationship, and keep the target characteristics of the device constant.

JP 2015-225127 specification

However, when controlling the optical phase of an optical parametric amplification system using feedback control, many electrical circuits such as signal generators, frequency mixers, and low-pass filters were required, complicating the system. The configuration required for optical phase synchronization has been an obstacle to simplification and cost reduction of systems and devices using optical parametric amplification.

One aspect of the present invention is an optical multiplexer for one or more signal light and pump light, a phase modulator on the input side of the optical multiplexer and arranged in the signal light path or the pump light path, an optical parametric amplifier for optically amplifying the signal light; and a control light within a frequency band of the signal light, or a phase conjugation with respect to the control light, arranged in the signal light path on the input side of the optical parametric amplifier. a monitor light separator for separating at least one of the control light and the control idler light as monitor light; and converting the light intensity of the monitor light into an electrical signal. and a feedback gain adjuster that receives the electrical signal as an error signal, sends a control signal to the phase modulator, and has at least an integrating circuit.

To provide a low-cost optical parametric amplifier with a simpler configuration.

1 is a diagram showing a schematic configuration of a conventional optical parametric amplifier; FIG. It is a figure explaining the basic operation|movement of an optical parametric amplifier. 1 is a diagram showing a schematic configuration of an optical parametric amplification device of the present disclosure; FIG. FIG. 4 is a diagram for explaining an error signal for optical phase synchronization in comparison with a conventional technique; 1 is a diagram showing the configuration of an optical parametric amplifier of Example 1; FIG. FIG. 10 is a diagram showing the configuration of an optical parametric amplifier of Example 2; FIG. 10 is a diagram showing the configuration of an optical parametric amplifier of Example 3; FIG. 10 is a diagram showing the configuration of an optical parametric amplifier of Example 4;

The optical parametric amplification device of the present disclosure provides a novel scheme for optical phase synchronization between signal light and pump light. In the optical state, a new control light phase-shifted with respect to the signal light is introduced. By using the optical intensity of the control light after being optically parametrically amplified as an error signal for feedback control, many of the components required in the conventional technology are eliminated, simplifying the configuration of the optical parametric amplifier. can. The optical parametric amplifier can set the phase relationship between the signal light and the pump light to either amplifying operation or attenuating operation. Therefore, the optical parametric amplification device includes, for example, a PSA that uses an amplification operation, and also includes a squeezed light generation device that uses an attenuation operation. In the following, the problems in the conventional optical phase synchronization will be described first, and then the configuration and operation for optical phase synchronization of the optical parametric amplifier of the present disclosure will be described.

FIG. 1 is a diagram showing a schematic configuration of a conventional optical parametric amplifier. In optical parametric amplifier 100 , signal light 111 and pumping light 112 are input to phase modulator 101 . The phase modulator 101 adjusts the optical phase difference between the signal light 111 and the pumping light 112 using a control signal 120 from a feedback control section, which will be described later. The signal light 111 and pump light 112 whose optical phase difference, ie relative phase, has been adjusted are combined by a combiner 102 and input to an optical parametric amplifier 103 including a nonlinear optical device. The amplified signal light is separated into output signal light 113 and monitor light 114 by monitor light separation section 104 . The monitor light 114 is converted by a photodetector (PD: Photo Detector) 105 into a light intensity signal 115 which is an electrical signal.

The optical intensity signal 115 is input to a feedback controller that implements optical phase synchronization between the signal light 111 and the pump light 112 . In FIG. 1, for comparison with the optical parametric amplifier of the present disclosure, which will be described later, the feedback control unit is divided into the modulation/demodulation circuit unit 121 shown within the dotted line and the feedback gain control unit 109.

The modulation/demodulation circuit section 121 has a signal generator 108 that generates a modulation signal for feedback control, and one modulation signal 118 a is supplied to the mixer 106 . From the signal generator 108 , the other modulated signal 118 b is added with the feedback signal 119 via the adder 110 and supplied to the phase modulator 101 as the control signal 120 . The modulated signal 118 b gives 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 controls the optical phase difference between the signal light 111 and the pump light 112 to be in-phase (0) or anti-phase (π), thereby converting the signal light 111 into light with the maximum gain. Amplify. Phase modulation by the modulating signal 118b causes the optical parametric amplifier 103 to slightly deviate from the maximum gain state. The phase modulation by the phase modulator 101 is converted into fluctuations 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.

If there is no disturbance or noise in each optical phase of the signal light and excitation light, the optical intensity signal 115 of the monitor light and the modulated signal 118a from the signal generator 108 will be the same, and the mixer output 116 will have only a DC component. On the other hand, if disturbance and noise are added to each optical phase of the signal light and pump light, the mixer output 116 becomes an AC signal containing various frequency components, and the output band-limited by the LPF 107 is the target of the optical phase difference. An error signal 117 (deviation between the target value and the output value) reflecting the deviation/deviation from the value is obtained. A path from the PD 105 to the LPF 107 operates as a demodulation circuit.

Based on the error signal 117 , the feedback gain control section 109 performs feedback control to generate the control signal 119 . Feedback gain control section 109 can use, for example, PID (Proportional-Integral-Differential) control widely used in automatic control.

As in the optical phase-locked feedback control shown in FIG. 1, the method of modulating the optical phase difference between the signal light and pump light to be controlled, demodulating the modulated components again, and synchronizing the optical phase is based on the laser frequency This is the same as the Pound-Drever-Hall method (PDH method) used for stabilization and the like. The error signal 117 is generated as shown in FIG. 1 because, in order to effectively utilize the optical parametric amplification characteristics, it is necessary to control the optical parametric amplification device so that the amplification or attenuation is maximized. .

FIG. 2 is a diagram explaining the basic operation of an optical parametric amplifier. Here, a non-degenerate phase sensitive optical amplifier (PSA) is considered as an example. FIG. 2(a) shows the frequency relationship among signal light, idler light, and excitation light in a non-degenerate PSA. The non-degenerate PSA simultaneously optically amplifies the signal light 201a and the corresponding idler light (phase conjugate light) 202a. The signal light 201a and the phase conjugate light 202a are positioned symmetrically about the reference frequency 203 (f/2) on the frequency axis, and the reference frequency 203 has half the frequency f of the excitation light 204. FIG. The term "idler light" refers to light that is symmetrical and phase conjugate with the signal light with respect to a reference frequency, and may be used interchangeably with phase conjugate light in the following description.

(b) of FIG. 2 explains the amplification conditions and attenuation conditions in the non-degenerate PSA in a complex plane. (b) of FIG. 2 shows the optical signal on the complex plane. The phase relationship of light 202b is shown. In the signal diagram expressed in the complex plane, the signal light and its idler light are represented as having their phases rotated in opposite directions as their frequencies depart from the reference frequency. When 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 departs from the reference frequency, and the combined phase of the two lights is zero. or -π. In the non-degenerate PSA, amplification occurs when this synthesized phase 205 is in phase with the phase of the pumping light 204 (amplification phase: 0 or π). On the other hand, if the combined phase 205 of the signal light 201b and the idler light 202b is orthogonal to the phase of the excitation light (attenuation phase: π/2 or −π/2), attenuation occurs. Since the phase of the excitation light 204 is generated in direct relation to the reference frequency light on the signal light generation side (transmitting side), it has the same phase as the reference frequency light and is in the state of phase 0 on the complex plane. be.

(c) of FIG. 2 shows the relationship between the difference between the synthesis phase and the amplification phase, and the amplification operation and the attenuation operation. The horizontal axis indicates the phase difference between the combined phase of the signal light 201b and the idler light 202b and the amplification phase (0°), and the vertical axis conceptually indicates the light intensity at the output of the non-degenerate PSA. . When the phase difference is 0 and π, the amplification operation becomes maximum and peaks 210-1 and 210-2 are generated. -1, resulting in 211-2.

The relationship between the combined phase of the signal light and idler light and the amplification phase or attenuation phase described above is the same even in a degenerate PSA in which the signal light and idler light are arranged at the same frequency. Therefore, in order 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 intended operating state.

In the prior art optical parametric amplifier 100 of FIG. 1, a control signal 120 is applied to the phase modulator 101 for optical phase-synchronous feedback control to bring the combined phase into 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 combined phase from the target value. The error signal in feedback control must change monotonically before and after the target value of the physical quantity to be controlled and the measured value. Referring again to FIG. 2(c), for optical parametric amplification, the intensity signal is near a peak or dip in both the intended operation of maximum amplification and maximum attenuation. The light intensity signal 115 from the PD 105 shifts in the same direction regardless of which direction the combined phase of the signal light and the idler light, which are objects to be controlled, shifts. For example, if the predetermined operation is maximum amplification, the light intensity signal 115 will decrease for either shift of the composite phase. Also, if the predetermined operation is maximum attenuation, the light intensity signal 115 increases regardless of the synthetic phase shift. Therefore, the optical intensity signal after optical parametric amplification cannot be used as it is as an error signal.

In order to obtain an error signal that can be used for feedback control, the excitation light or signal light is phase-modulated, and the modulation/demodulation circuit section 121 shown in FIG. ing. The modulation/demodulation circuit section 121 requires electric circuits such as the signal generator 108 for modulation, the mixer 106 for demodulation, and the LPF 107, which greatly complicates the system configuration.

The optical parametric amplification device of the present disclosure provides a novel scheme for optical phase synchronization between signal light and pump light. In the new mechanism, a control light having a combined phase shifted from the combined phase of the signal light is newly introduced. By using the optical intensity of the control light after being optically parametrically amplified as an error signal for feedback control, many electric circuit components are unnecessary, and the optical parametric amplification device is simplified and reduced in cost. .

The optical parametric amplification device of the present disclosure includes an optical phase shifter that generates control light having a different combined phase with respect to signal light to be amplified or attenuated.

[Basic configuration]
FIG. 3 is a diagram showing a schematic configuration of the optical parametric amplification device of the present disclosure. The optical parametric amplifier 10 includes a phase modulator 1 to which signal light 11 and pumping light 12 are input, a multiplexer 2 , an optical parametric amplifier 3 , a monitor light separator 4 and a photodetector (PD) 5 . Amplified (or attenuated) signal light 13 is output from the monitor light separator 4, and the monitor light 14 is also separated and output. The operation of these elements that implement optical parametric amplification is exactly the same as the prior art optical parametric amplification device 100 shown in FIG. 1, and detailed description thereof is omitted here.

The first difference between the optical parametric amplifier 10 shown in FIG. 3 and the conventional optical parametric amplifier 100 shown in FIG. It is provided with an optical phase shifter 6 for generating shifted control light. The second difference is that the optical intensity signal 15 of the control light (monitor light) from the PD 5 can be used as it is as the error signal 16, so the modulation/demodulation circuit section 121 required in the configuration of the prior art is no longer necessary. It is a point. As an electric circuit on the output side of the PD 5, for example, a PID controller that functions as a feedback gain control section 7 that generates a control signal 17 for controlling the phase modulator 1 is sufficient. The electrical circuitry required for optical phase-locked feedback control can be greatly simplified compared to prior art configurations.

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 the process of generating an error signal in the prior art optical parametric amplifier shown in FIG. Taking the 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 positioned 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 signal light. As described with reference to FIG. 2, when the non-degenerate PSA is used in an amplified state, the phase difference between the synthesized phase of the signal light and idler light and the amplified phase is controlled to be 0 or π. In this target state of controlling the phase difference to 0 or π, the light 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, the monotonic error signal 117 is obtained by providing the modulation/demodulation circuit section 121 . This error signal 117 has a sign that is inverted with respect to the reference voltage of the feedback control circuit when the phase difference between the target values is 0 or π, and has good linearity and can be used for feedback control.

(b) of FIG. 4 shows an error signal in the optical parametric amplifier of the present disclosure shown in FIG. In the optical parametric amplifier 10 of FIG. 3, part of the light that could also be used as signal light is phase-shifted in advance and used as control light, and this control light is used as monitor light 14 after optical amplification. The signal light group 221 shown in (b) of FIG. 4 is the signal light closest to the reference frequency indicated by the dotted arrow on the frequency axis from the signal light group 220 shown in (a) of FIG. It excludes the idler light. Light that can be used as signal light and its idler light in the prior art is used as control light 213a and control idler light 214a in the optical parametric amplifier 10 of the present disclosure. Referring to the control light 213b and control idler light 214b shown on the complex plane in FIG. is phase-shifted.

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

As shown in FIG. 4(b), if the phase shift amount of the control light with respect to the signal light is .pi./4, the light intensity signal 15 output from the PD 5 deviates from the peak shown in FIG. 4(b). corresponds to the slanted portion 212 . 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 whose composite phase 215 is shifted by π/4 is a monotonic function linearity is good. Therefore, the light intensity signal 15 obtained from the PD 5 can be used as it is as the error signal 16 for the phase synchronization feedback control without processing.

Therefore, the optical parametric amplifying device 10 of the present disclosure includes one or more optical multiplexers 2 for signal light and pump light, and a phase shifter arranged on the input side of the optical multiplexer and in the signal light path or the pump light path. a modulator 1, an optical parametric amplifier 3 for optically amplifying the signal light, and a control light within the frequency band of the signal light, arranged in the signal light path on the input side of the optical parametric amplifier, or the control light an optical phase shifter 6 that changes the optical phase of the control idler light that is phase conjugate with respect to the light; a monitor light separator 4 that separates at least one of the control light and the control idler light as monitor light 14; A photodetector 5 for converting the optical intensity of light into an electrical signal 15 and a feedback gain adjuster 7 which receives the electrical signal as an error signal, sends a control signal 17 to the phase modulator, and has at least an integrating circuit. It can be implemented as provided.

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

The reference frequency described above corresponds to the excitation light wavelength of a third-order nonlinear optical medium. In the case of a second-order nonlinear optical medium, the frequency is half the excitation light frequency. The optical phase shifter 6 may shift the phase of only the control light, or may shift the phases of both the control light and the control idler light. As explained in FIG. 2, in optical parametric amplification, if the combined phase of the signal light and the idler light of the signal light is in phase (0 phase) or opposite phase (π phase) to the amplification phase determined by the phase of the pumping light, Optical amplification occurs, and in quadrature (π/2-phase or 3π/2-phase) optical attenuation occurs. An optical parametric amplifier is generally required to collectively amplify all signal channels arranged within a frequency band in which signal light can be optically amplified. Therefore, all signal channels must be optically parametrically amplified with the combined phases of the signal light and the corresponding idler light matching the respective amplification phases.

Now, let us consider the phase fluctuation that occurs in the signal light in the optical transmission line. In general, when there is only first-order refractive index dispersion in the transmission line, even if there are multiple signal lights (multiple channels) to be processed within the optical amplification band, each signal light and the corresponding idler light are The composite phases are aligned with the amplification phases (0 or π) shown in FIG. 2(b). In both the prior art and the optical parametric amplifier of the present disclosure, a phase-locking field is used to suppress fluctuations in the phase difference between the pumping light and the signal light due to the effects of disturbance and noise occurring in each path of the pumping light and the signal light. Back control is performed.

In the optical parametric amplifier 10 of the present disclosure, prior to optical parametric amplification, the combined phase of the control light and its idler light is shifted with respect to the combined phase of the signal light and its idler light. The obtained intensity signal can be used as an error signal as it is.

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 section 4 and taken out as the monitor light 14 . The light intensity of the monitor light 24 is converted into the light intensity signal 15 by the PD 5 and used as the error signal 16 as it is. As already explained in FIG. 4, at the target operating state (eg maximum amplification) of the signal light, the optical intensity signal 15 of the phase-shifted control light varies monotonically. In particular, when the combined phase of the control light is set to a phase shifted by π/4+nπ/2 (where n is an integer), the linear slope of the error signal around the target value becomes steep, and optical phase synchronization with good performance can be achieved.

Since the light used as the control light can utilize the frequency band that is not used for transmission, the transmission band is not substantially limited by the optical parametric amplifier of the present disclosure. For example, a reference frequency may be used to set the control light and the control idler light with light of the same frequency. Alternatively, the frequency channel 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 have a configuration in which only the control light is extracted, delayed, and returned to the original optical path (Embodiment 1). A configuration (embodiment 2) in which control light is obtained by being incident on a medium may also be used. The optical fiber itself, which is a transmission medium, can be used as the secondary dispersion medium, and the control light is preferably positioned away from the reference frequency so as to be greatly affected by the secondary refractive index dispersion. A more specific configuration and operation of the optical parametric amplifier will be described below.

FIG. 5 is a diagram showing the configuration of the optical parametric amplifier of Example 1. FIG. The optical parametric amplifier 20 of FIG. 5 has the same configuration as the optical parametric amplifier 10 shown in FIG. 3, and has a more specific configuration of the optical phase shifter 21 for generating the control light. Therefore, description of the overall configuration and basic operation of the optical parametric amplifier 20 is omitted.

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 optical parametric amplification for signal light is 1545.32 nm. The wavelength of the excitation light was set to around 780 nm, which corresponds to the double wave of the signal light. The wavelength of the control light was set to 1530.00 nm within the wavelength band of the signal light. 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.

The optical phase shifter 21 applies a phase shift 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 at a position folded around the reference frequency on the frequency axis. The control light shifter 21 of this embodiment comprises a wavelength separator 22 for cutting out only the control light from the signal light path, a delay line 23 and a wavelength multiplexer 24 . As the wavelength separator 22, an arrayed waveguide grating (AWG: Arrayed Waveguide Grating) was used. The demultiplexing AWG 22 inputs the signal light as it is to the multiplexing AWG 24 . The control light extracted by the demultiplexing AWG 22 is phase-shifted by the delay line 23 . Using a multiplexing AWG as the wavelength multiplexer 24 , the signal light and the phase-shifted control light are multiplexed again and input to the phase modulator 1 .

The method for shifting the phase of the control light with the optical phase shifter 21 includes "Method 1" in which only the control light is phase-shifted and "Method 2" in which both the control light and the control idler light are shifted. What is important in any method is to shift the combined phase of the control light and the control idler light with respect to the signal light.

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, the phase shift amount of the control light is π/2 + nπ (n is an integer). It is preferable to At this time, since the phase of the control idler light is not shifted, the combined 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 the best linearity and the largest slope in the vicinity of the π/4 shift from the amplification phase, and the feedback gain high feedback control becomes possible.

In the monitor light separation unit 4 of FIG. 5 as well, the AWG was used to extract the control light as the monitor light 14 . The optical intensity signal 15 of this monitor light was input to the feedback gain control unit 7 and the feedback control signal 17 was returned to the phase modulator 1 inserted in the pumping light path on the upstream side of the optical parametric amplifier 3 . The phase modulator 1 used an LN modulator that phase-modulates the excitation light. With the configuration of FIG. 5, an optical phase-locked feedback circuit that is simpler than that of the prior art realizes a stable signal light amplification operation in the maximum gain operation state. It was also confirmed that by reversing the polarity of the feedback gain, the phase of the optical parametric amplifier 20 can be synchronized with the feedback control signal having the opposite phase and the attenuation operation state.

The above description is an example in which the optical phase shifter phase-shifts only the control light, but "Method 2" in which both the control light and the control idler light are cut out and their respective optical phases are shifted is also possible. In the case of method 2, it is desirable that the sum of the phase shift amount of the control light and the phase shift amount of the control idler light is π/2+nπ (where n is an integer). When the sum of the phase shift amounts of the two lights is π/2, the combined phase on the complex plane is always near π/4, and the light intensity signal 15 of the monitor light 14 has good linearity and the largest slope. becomes.

As an example of setting the sum of the phase shift amounts of the above two lights to π/2 in "Method 2", for example, the shift amounts of the control light and the control idler light may be the same 45° (π/4). Alternatively, the shift amount may be unbalanced, and one of the control light and the control idler light may be shifted by 20 degrees and the other by 70 degrees. Further, if the shift amount of the control light is 90° and the shift amount of the control idler light is 0°, it will be understood that this corresponds to the above-described "Method 1" in which only the control light is extracted and phase-shifted.

The wavelength separator 22 for cutting out control light and control idler light from signal light is not limited to AWG, and simpler directional couplers, diffraction gratings, etc. may be used. Also, the wavelength separator 4 for cutting out the monitor light is not limited to AWG, but may be a directional coupler, a diffraction grating, or the like. Furthermore, even if the light taken out as the monitor light is not the control light but the control idler light, the optical phase-locked feedback control is realized without any problem.

The wavelength of the control light may be placed anywhere within the wavelength band of the signal light to be optically parametrically amplified. As described in (b) of FIG. 4, the control light and the control idler light may be arranged at or near the center wavelength (corresponding to the reference frequency) of the wavelength band of the signal light. If the control light is set to have the central wavelength, the control light and the control idler light have the same wavelength and cannot be distinguished from each other (corresponding to degenerate phase sensitive amplification). However, from the viewpoint of optical phase-locked feedback control, the signal light was operated in the maximum gain state, and a stable amplification operation of the signal light was realized. Alternatively, the wavelength of the control light can be set at the edge of the wavelength band of the signal light, and the control light and the control idler light can be arranged at both ends of the wavelength band of the signal light. Since the optical phase shifter 21 cuts out only one of the control light and the control idler light, it is possible to use a directional coupler with a lower cost and a simpler configuration than the AWG.

The feedback gain controller 7 used in the optical parametric amplifier 20 can perform a phase synchronization function if it includes an integrator. Therefore, the general-purpose PID controller shown in FIG. 5 is just an example, and does not need to include all elements of the deviation between the output value and the target value, its integration, and differentiation. Feedback gain controller 7 may further include a linear amplifier and a differentiator. By increasing the gain of the linear amplifier to widen the loop band, it is possible to cope with sudden characteristic deterioration due to disturbance.

What is important in the optical parametric amplification device of the present disclosure is to make the synthetic phase of the signal light and the synthetic phase of the control light different. Therefore, the amount of shift of the composite phase is not limited to π/4 (45°). Note that a phase shift of the order of 20-70°, for example, will allow the linear portion of the intensity signal to be utilized. Therefore, the precision of the shift amount in the optical phase shifter 21 can be loose if the phase fluctuations assumed for optical phase synchronization are small.

Also, in the above description, it is assumed that the signal light and its idler light are input to the optical parametric amplifier. Depending on the optical transmission system, pilot light may be transmitted together with signal light, or only signal light may be transmitted to regenerate pump light and idler light. For example, some optical repeater amplifiers regenerate excitation light and idler light from the signal light itself and perform optical parametric amplification for repeater amplification. In such a case, various variations are possible for the method of realizing the optical phase shift of the control light, and the method is not limited to the configuration shown in FIG.

In the optical phase shifter 21 of the first embodiment, the control light or the control idler light is cut out and phase-shifted by giving a delay. can also

FIG. 6 is a diagram showing the configuration of an optical parametric amplifier according to the second embodiment. The optical parametric amplifier 30 shown in (a) of FIG. 6 has the same configuration as the optical parametric amplifier 20 shown in FIG. For example. Therefore, description of the overall configuration and basic operation of the optical parametric amplifier 30 is omitted.
As the optical parametric amplifier 3, as in the first embodiment, a PPLN waveguide, which is a second-order nonlinear element, is used. The reference frequency is set to 194 THz, and the center of the wavelength band of optical parametric amplification for signal light is 1545.32 nm. The wavelength of the excitation light was set to around 780 nm, which corresponds to the double wave of the signal light. The wavelength of the control light was set to 1530.00 nm within the wavelength band of the signal light. At this time, the wavelength of the control idler light is 1560.95 nm. In this embodiment, the optical phase shifter 31 does not use an optical component, but uses an optical fiber as a transmission line.

(b) of FIG. 6 illustrates the arrangement configuration of the control light in the second embodiment. In general, materials constituting an optical fiber have wavelength dispersion of refractive index. When the frequency of the signal light 217a is close to the center frequency of the optical parametric amplification band, the frequency of the idler light 218a, which is in a conjugate relationship 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.

(c) of FIG. 6 explains the phase shift caused by the refractive index dispersion of the optical fiber on the complex plane. Since 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, the phase difference between the composite phase 219 and the amplification phase is kept constant even when propagating through the transmission line. However, as the signal channel becomes farther from the center frequency, the influence of the secondary refractive index dispersion of the optical fiber on the signal light and its idler light increases. Second-order refractive index dispersion changes the orientation of the combined phase of the signal light and idler light on the complex plane. In this embodiment, as shown in FIG. 6B, the pair of control light 213a and control idler light 214a is arranged at a position distant from the center frequency. In an arrangement such as that of FIG. 6(b), the signal light, including the control light and the control idler light, propagates through an optical phase shifter 31, which is a length of optical fiber with the required second-order dispersion. As a result, as shown in (c) of FIG. 6, the combined phase 215 of the control light 213b and the control idler light 214b is shifted from the amplification phase (0°).

As described above, an optical fiber, which is a transmission line with second-order refractive index dispersion, can perform the phase shifting function of an optical phase shifter. Therefore, compared with the optical phase shifter 21 including the demultiplexer and the multiplexer of the first embodiment, the optical parametric amplification device similar to that of the first embodiment can be realized with the optical phase shifter 31 having a simpler configuration. As an optical fiber that realizes the optical phase shifter 31, a high-dispersion fiber such as a photonic crystal fiber can be used in addition to an ordinary optical fiber.

In the monitor light separation unit 4 in FIG. 6(a), the control light was taken out 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 the feedback control signal 17 was returned to the phase modulator 1 inserted in the pumping light path on the upstream side of the optical parametric amplifier 3 . The phase modulator 1 used an LN modulator that phase-modulates the excitation light. With the configuration of FIG. 6(a), an optical phase-locked feedback circuit that is simpler than the prior art achieves stable signal light amplification operation in the maximum gain operation state. As in the first embodiment, it was also confirmed that phase synchronization was achieved with 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 separator 4 was used as the control idler light, the optical phase-locked feedback control was realized without any problem.

In both the first and second embodiments described above, the optical phase shifter is placed on the front stage side of the phase modulator 1 . However, as long as only the synthetic phase of the control light can be shifted with respect to the synthetic phase of the signal light, the position of the optical phase shifter is not limited, and other variations are possible.

FIG. 7 is a diagram showing the configuration of the optical parametric amplifier of Example 3. FIG. The optical parametric amplifier 40 of Example 3 has substantially the same configuration as the optical parametric amplifiers of Examples 1 and 2 described above. A 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 the same error signal 16 as in

Embodiments

1 and 2 can be obtained if a predetermined phase shift can be given to the signal light only to the combined phase of the control light and the control idler light. .

In the configuration of an actual optical amplifier such as a PSA, a plurality of PPLNs are often used for purposes other than optical parametric amplification, such as pump light regeneration circuits and pump light stabilization. In such a case, an optical circuit (not shown) exists on the front stage side of the phase modulator 1 and the optical multiplexer 2 in FIG. Therefore, in an actual device using optical parametric amplification, the location of the optical phase shifter 41 is also flexible.

FIG. 8 is a diagram showing the configuration of the optical parametric amplifier of the fourth embodiment. This embodiment is another variation of the position of the optical phase shifter. The optical parametric amplifier 50 of Example 4 also has substantially the same configuration as each of the optical parametric amplifiers of Examples 1-3. A 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 an error signal 16 similar to that of the first to third embodiments can be obtained if a predetermined phase shift can be given to the signal light only to the combined phase of the control light and the control idler light.

In each of the above-described embodiments, the phase modulator 1 is depicted as passing through both the signal light path and the excitation light path in order to show the general configuration, but at least one of the signal light path and the excitation light path should go through The phase modulator 1 only needs to be able to vary the phase difference between the signal light and the excitation light by means of the control signal 17 for optical phase synchronization feedback control. Therefore, either the signal light path or the excitation light path may be arranged, or both paths may be used depending on the form of the phase modulator. can.

In addition, each of the above embodiments has been described as an optical parametric amplification device, but as long as the optical parametric amplification mechanism is used, it can be used as an optical signal attenuation device or as a wavelength conversion device. Also, depending on the setting conditions of the phase relationship between the pumping light and the signal light, it can 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 amplification device".

As described in detail above, in the optical parametric amplifier of the present disclosure, the combined phase of the control light and the control idler light is shifted with respect to the combined phase of the signal light and its idler light, and the control light or the control idler light is shifted. is used as monitor light. This makes it possible to omit the modulation/demodulation circuit in the prior art, simplify the configuration of the optical parametric amplifier, and realize cost reduction.

The present invention can be used for optical signal processing devices such as optical amplification.

Claims

one or more signal light and pump light optical multiplexers;
a phase modulator arranged on the input side of the optical multiplexer and in the signal light path or the pumping light path;
an optical parametric amplifier that optically amplifies the signal light;
Light that is arranged in the signal light path on the input side of the optical parametric amplifier and that changes the optical phase of control light within the frequency band of the signal light or control idler light that is phase conjugate with respect to the control light. a phase shifter;
a monitor light separator that separates at least one of the control light and the control idler light as monitor light;
a photodetector that converts the optical intensity of the monitor light into an electrical signal; and a feedback gain adjuster that receives the electrical signal as an error signal, sends a control signal to the phase modulator, and has at least an integration circuit. Optical parametric amplifier.
2. The optical parametric amplifier according to claim 1, wherein a combined phase of said control light and said control idler light is set to be different from a combined phase of said signal light and said idler light of said signal light.
the one or more signal lights include a plurality of signal lights each containing information and a plurality of idler lights corresponding to the plurality of signal lights;
3. The optical parametric amplifier according to claim 1, wherein said optical parametric amplifier includes a second-order nonlinear optical element or a third-order nonlinear optical element.
4. The optical parametric according to claim 1, wherein said control light and said control idler light are arranged at both ends of said frequency band of said signal light on a frequency axis, and said optical phase shifter is an optical fiber. amplifier.
The optical parametric amplifier according to any one of claims 1 to 4, wherein said optical phase shifter shifts optical phases of both said control light and said control idler light in the same direction.
6. The optical parametric amplifier according to claim 5, wherein the sum of the phase shift amount of said control light and the phase shift amount of said control idler light is π/2+nπ (n is an integer).
5. The optical parametric amplifying device according to claim 1, wherein said optical phase shifter shifts the optical phase of either said control light or said control idler light by .pi./2+n.pi. (where n is an integer).