Optical amplifier

Abstract

An optical amplifier according to the present disclosure uses a utilization rate of excitation light in an OPA to cause the OPA to maximally and stably exhibit performance without extracting a part of signal light on a path of the signal light. The optical amplifier according to the present disclosure comprises two blocks, which are an excitation light generation unit and an optical parametric amplification unit. Further, a stabilization mechanism for excitation light output and a stabilization mechanism for the optical parametric amplifier are provided to each of the two blocks. A variable splitting-ratio coupler is provided between the two blocks and a splitting-ratio characteristic of the variable splitting-ratio coupler is used to control the two stabilization mechanisms mentioned above. Each stabilization mechanism includes control at a low speed for a temperature adjuster and control at a higher speed for a control target other than the temperature adjuster.

Description

[Technical Field]

[0001] The present invention relates to an optical parametric amplifier used in an optical communication system or an optical control system. [Background technology]

[0002] Nonlinear optical devices and electro-optical devices are being developed in a wide range of fields, including optical signal wavelength conversion and optical modulation in optical communications, optical measurement, optical processing, medicine, and bioengineering. The optical frequencies used by these devices range from ultraviolet to visible to infrared to terahertz. In optical communications, they are also used to generate and modulate coherent light, and for wavelength conversion operations using difference frequency generation and amplification operations using the parametric effect.

[0003] As the nonlinear optical and electro-optical media used in these devices, oxide-based compound substrates such as lithium niobate (LiNbO3) are known as promising materials due to their extremely high second-order nonlinear optical and electro-optic constants. One example of an optical device using lithium niobate is a periodically poled lithium niobate (PPLN) waveguide element, which is widely used in light sources due to its high wavelength conversion efficiency. Wavelength conversion elements using PPLN waveguide elements and other elements utilize the processes of second harmonic generation (SHG), difference frequency generation (DFG), and sum frequency generation (SFG).

[0004] By using a wavelength conversion element with high wavelength conversion efficiency, it is possible to realize an amplifier of signal light through the optical parametric amplification process by transferring energy from the pump light power to the signal light.In optical parametric amplifiers (OPAs), phase-sensitive amplifiers, 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] Furthermore, the degenerate optical parametric amplification process can generate quantum-correlated photon pairs, which can generate non-classical states such as squeezed light and heralded single-photon states. These non-classical states of light are expected to be important resources for optical quantum computers and quantum optical sensing technologies. [Prior art documents] [Non-patent literature]

[0006] [Non-Patent Document 1] M. Asobe et al., “Broadband optical parametric amplification using PPLN waveguide pumped by detuned second harmonic”, Vol. 30, No. 6 / 14 Mar 2022 / Optics Express 9473 Summary of the Invention [Problem to be solved by the invention]

[0007] In order for an OPA to achieve its maximum performance, it is necessary to maintain phase matching conditions. In particular, in the case of a waveguide-type OPA that uses a PPLN waveguide element, it is important to maximize the wavelength conversion efficiency by controlling the temperature of the waveguide itself. In addition, in order to stabilize the gain of the OPA, it is also important to stabilize the intensity of the pump light input to the OPA.

[0008] In conventional OPAs that handle classical signal light, a portion of the signal light is extracted and used for the above-mentioned control. However, when generating quantum light, such as squeezed light generation, the extraction of a portion of the signal light causes contamination with a vacuum field, which degrades the quality of the optical quantum signal. In addition, more stable control is needed to deal with fluctuations in wavelength conversion efficiency due to fluctuations in environmental temperature, etc. [Means for solving the problem]

[0009] One aspect of the present invention is an optical parametric amplifier including an optical amplifier for fundamental light, a frequency doubler consisting of a first second-order nonlinear optical element and generating pump light from the amplified fundamental light, a pump light generation unit including a first temperature regulator for adjusting the temperature of the first second-order nonlinear optical element, a multiplexer for multiplexing signal light and pump light, an optical parametric amplifier consisting of a second second-order nonlinear optical element and amplifying the signal light from the multiplexer, a demultiplexer for demultiplexing the amplified signal light from the optical parametric amplifier and pump light, and a second temperature regulator for adjusting the temperature of the second second-order nonlinear optical element, a coupler that branches the pump light from the pump light generation unit to a first output port that supplies pump light to the multiplexer and a second output port connected to a first photodetector, the coupler being able to adjust the branching ratio by a control voltage V, and a coupler that determines the intensity of the pump light from the pump light generation unit based on an electrical signal from the first photodetector and the control voltage V. a frequency doubler control unit that generates a control signal to the optical amplifier and a control signal to the first temperature regulator based on the first error signal and controls the intensity of the pump light from the pump light generation unit to a constant value; a second pump light calculation unit that calculates the intensity of the input pump light to the multiplexer based on the electrical signal from the first photodetector and the control voltage V; a second photodetector that detects the intensity of the output pump light demultiplexed from the demultiplexer; a utilization rate calculation unit that calculates the utilization rate of the pump light in the optical parametric amplifier from the intensity of the input pump light and the intensity of the output pump light; a pump light intensity control unit that adjusts the control voltage V based on the utilization rate to control the intensity of the input pump light to a constant value; and an optimal temperature control unit that generates a control signal to the second temperature regulator based on the adjusted control signal V and controls the utilization rate to a constant value. [Effects of the Invention]

[0010] The present invention enables the optical parametric amplifier to operate stably at maximum performance. [Brief explanation of the drawings]

[0011] [Figure 1] 1A and 1B are diagrams illustrating the configuration and operation of an optical amplifier according to the present disclosure. [Figure 2] 10A and 10B are diagrams illustrating the operation of a variable branching ratio coupler and calculation of an excitation light level. [Figure 3] 10A and 10B are diagrams illustrating the configuration and operation of a more specific example of an optical amplifier according to the present disclosure. [Figure 4] 10A and 10B are diagrams illustrating a stabilization control operation of the conversion efficiency in the optical amplifier of the present disclosure. [Figure 5] FIG. 1 is a diagram showing a circuit for optimizing a phase matching condition in a conventional OPA. DETAILED DESCRIPTION OF THE INVENTION

[0012] The optical amplifier disclosed herein maximizes and stably realizes OPA performance by utilizing the utilization rate of pump light in the OPA without extracting a portion of the signal light from the signal light path. The optical amplifier disclosed herein comprises two blocks: a pump light generation unit and an optical parametric amplifier unit. Furthermore, each of the two blocks is equipped with a pump light output stabilization mechanism and an optical parametric amplifier stabilization mechanism. A variable branching ratio coupler is provided between the two blocks, and the branching ratio characteristics of this variable branching ratio coupler are used to control the two stabilization mechanisms. The following description first outlines the problems encountered when handling optical quantum signals with conventional OPAs, and then describes the configuration and operation of the optical amplifier disclosed herein.

[0013] FIG. 5 shows a control circuit for optimizing the phase matching condition in a conventional OPA. The OPA 300 includes a multiplexing circuit 3 for multiplexing signal light and pump light on the input side of the OPA unit 4, and a demultiplexing circuit 5 for demultiplexing signal light and pump light on the output side. A portion of the amplified signal light is branched by an optical branching circuit 51, for example, at a ratio of 99:1. The branched signal light 55 is converted into an electrical signal by a photodetector 52. The electrical signal from the photodetector 52 indicates the level of the amplified signal light, and the conversion efficiency of the OPA unit is calculated based on this electrical signal. Furthermore, an optimal temperature adjustment circuit 54 controls a thermoelectric cooler (TEC) 16. The TEC 16 adjusts the temperature of, for example, a PPLN waveguide element in the OPA unit 4 to maximize the conversion efficiency of the OPA unit 4. When the phase matching condition is optimal, the conversion efficiency is maximized, and the OPA 300 produces the maximum signal light level.

[0014] In the configuration shown in Figure 5, a portion of the amplified signal light is branched by optical branching circuit 51. Therefore, a loss corresponding to branched light 55 occurs in the output signal light. Although we will not go into details here, for quantum light such as squeezed light generation, i.e., for photon signals, the occurrence of loss corresponds to the addition of noise from a vacuum field. When dealing with non-classical light, the inclusion of noise from a vacuum field means a degradation of signal quality. For OPAs that handle not only conventional signal light but also non-classical photon signals, a mechanism is needed to optimally and stably control the phase matching condition without degrading signal quality.

[0015] 1 is a diagram illustrating the basic configuration and operation of an optical amplifier according to the present disclosure. The optical amplifier 100 comprises a pump light generating unit 101 and an optical parametric amplifier unit 102. As will be described later, the optical amplifier 100 further comprises control units for the pump light generating unit 101 and the optical parametric amplifier unit 102. Between the pump light generating unit 101 and the optical parametric amplifier unit 102, a variable branching ratio coupler 7 is provided, which is capable of adjusting the pump light level to the optical parametric amplifier unit 102.

[0016] The variable branching ratio coupler 7 controls the pump light output (level PSHG ) is input to the optical parametric amplifier 102 as pump light (level P in ) and monitor light (level P Monitor The branching ratio can be varied by controlling the control voltage V. One of the outputs from the variable branching ratio coupler 7 is input to the optical parametric amplifier 102, and the other is converted into an electrical signal by a photodetector 8 that is sensitive to the pump light wavelength. Monitor is used to control the optical parametric amplifier 102, the details of which will be described later.

[0017] The pump light generating unit 101 amplifies the input fundamental light using the optical amplifier 1 to generate high-intensity fundamental light. The optical amplifier 1 can be an optical fiber amplifier, such as an erbium-doped fiber amplifier (EDFA). The optical fiber amplifier used has an externally controllable injection current value. The frequency doubler 2 generates second harmonics (SH) from the fundamental light and outputs pump light (SHG light) for optical parametric amplification. Like the optical parametric amplifier 4, the frequency doubler 2 can be one that uses a PPLN waveguide element, which is a second-order nonlinear optical element. However, it does not have to be a waveguide type as long as it can generate pump light (SHG light). The frequency doubler 2 and the optical parametric amplifier (OPA) 4 are each equipped with temperature-controlled oscillators (TECs) 15 and 16 and a temperature measuring device (not shown) to control the temperature of the nonlinear waveguides located inside.

[0018] The optical parametric amplifier 102 includes a multiplexer 3 for the signal light and pump light, and a demultiplexer 5 for the signal light and pump light, located before and after the OPA 4. The demultiplexer 3 and the multiplexer 5 can each be realized by a spatial optical component that combines a lens, a dichroic mirror that demultiplexes the pump light and the signal light, etc. Alternatively, a wavelength multiplexer / demultiplexer of the optical fiber fused directional coupler type can also be used.

[0019] The difference from the conventional OPA shown in Figure 5 is that the pump light from the demultiplexer 5 is used to control the optical parametric amplifier 102, rather than the signal light. The demultiplexer 5 is equipped with a filter that separates the signal light and the pump light into different ports, and 100% of the signal light is guided to the output port on the signal light side. Therefore, there is no loss in the signal light, and no noise is mixed in from the vacuum field. Unlike the configuration shown in Figure 5, which extracts a portion of the signal light and incurs loss, there is no degradation in the quality of the optical quantum signal.

[0020] The variable branching ratio coupler 7 adjusts the branching ratio of the input pump light (level P in ) is used to amplify the signal light by the optical parametric amplification process in the OPA 4, and the output pump light from the demultiplexer 5 decreases by the amount used (level P out In the optical amplifier 100 of the present disclosure, the input pump light (P in ), and the output pump light (P out ) and the electrical signal obtained by detecting the excitation light ratio (P out / P in ) is calculated. This pump light ratio value is used to control the optical parametric amplifier 102, as will be described later.

[0021] The configuration in Figure 1 excluding the pump light generating unit 101, variable branching ratio coupler 7, and optical parametric amplifier unit 102 is the control circuit for the optical amplifier 100, which operates based on electrical signals from the two photodetectors 6 and 8. This control circuit is implemented by hardware processing using an electrical circuit, software calculation processing using a computer, or a combination of these. Below, we will explain the control operations of the pump light generating unit 101 and the optical parametric amplifier unit 102. First, we will explain the configuration of the variable branching ratio coupler 7 and its role in the optical amplifier 1.

[0022] 2A and 2B are diagrams for explaining the operation of the variable branching ratio coupler and the calculation of the pump light level. FIG. 2A shows the configuration of the variable branching ratio coupler 7, which has three optical ports and a control port. The level P SHG The pump light (SHG light) having a level P in and a second branched light for control at level P Monitor The branching ratio to the two output ports can be externally controlled by a control voltage V. The branching ratio variable coupler can be realized, for example, by an Mach-Zehnder interferometer (MZI) type variable coupler in which a phase modulator is inserted into an MZI, or a variable coupler that combines a polarization rotator and a polarization splitting coupler.

[0023] FIG. 2(b) shows the relationship between the control voltage V and the branching ratio in the variable branching ratio coupler. The horizontal axis shows the control voltage V, and the vertical axis shows the transmittance T from the input port to each output port. A solid curve 38 shows the transmittance (P in / P SHG ) and the dashed-dotted curve 39 indicates the transmittance (P Monitor / P SHG ) where each transmittance is expressed by the following formula: P Monitor / P SHG =T(V) Equation (1) P in / P SHG =1-T(V) Equation (2) That is, let T(V) be the transmittance curve 39 from the input port to the second output port. The relationship between equations (1) and (2) is uniquely determined by the transmittance characteristics of the variable branching ratio coupler shown in Figure 2(b). Equations (1) and (2) can be further modified as follows: P SHG =P Monitor / T(V) Equation (3) P in =PMonitor (1-1 / T(V)) Equation (4) From the above equations (3) and (4), P can be determined from the electrical signal of the photodetector. Monitor and the transmittance curve T(V), the output pumping light level P from the pumping light generating unit 101 is calculated. SHG and the input pump light level P in Therefore, if the relationship data between the control voltage V and the transmittance characteristic T(V) of the variable branching ratio coupler is acquired in advance, the output pump light level P can be calculated from the electrical signal from the photodetector 8 in FIG. SHG and the input pump light level P in You can understand the following.

[0024] In the pump light generating unit 101, the level P of the output pump light SHG In order to keep the level P of the pump light generated from the frequency doubler 2 constant, feedback control is performed on the TEC 15 in the optical amplifier 1 and the frequency doubler 2. The target control operation is to SHG This control is performed by first calculating the level P of the excitation light generated by the wave number doubler 2 using the electric signal from the photodetector 8 by the first excitation light calculation unit 13 according to the formula (3). SHG The calculation of the first pump light calculation unit 13 can be performed as a calculation process by a processor of a computer (not shown). For this calculation, previously obtained data on the relationship between the control voltage V of the variable branching ratio coupler and the transmission characteristic T(V) can be stored in the memory of the computer (not shown). The calculation of the first pump light calculation unit 13 can be performed using the stored data. Furthermore, the data on the relationship between the control voltage V and the transmission characteristic T(V) can also be received from a storage means external to the optical amplifier.

[0025] Calculated excitation light level P SHG Then, the frequency doubler control unit 14 adjusts the level P of the pumping light. SHG The control signal S to the TEC15 is set to be constant. TEC1 and the control signal S to the optical amplifier 1 AMP Generates a control signal SAMP The gain of the optical amplifier 1 can be changed by changing the gain of the optical amplifier 1. When the gain of the optical amplifier 1 changes, the level of the fundamental wave light changes, and the level P SHG The respective control signals are used to control two objects, namely, feedback to the optical amplifier 1 and feedback to the TEC 15. More specific control will be described later as an example.

[0026] In the optical amplifier 100 of the present disclosure, control for stabilizing the operation of the optical amplifier 100, which will be described below, is also performed on the optical parametric amplifier section 102 using an electrical signal obtained from the monitor light of the variable branching ratio coupler 7 shown in FIG.

[0027] Referring back to FIG. 1, the second pump light calculation unit 9 performs the calculation of equation (4) to calculate the level P of the input pump light to the optical parametric amplifier unit 102 from the electrical signal from the photodetector 8. in As described above, the output pump light separated from the OPA 4 is input from the demultiplexer 5 of the optical parametric amplifier unit 102 to the photodetector 6. The pump light not used for parametric conversion in the OPA 4 is output from the demultiplexer 5, and its level P out An electrical signal corresponding to the light emitted from the photodetector 6 is obtained.

[0028] In the optical amplifier 100 of the present disclosure, the utilization factor calculation unit 10 calculates the ratio P between the pump light level on the input side and the pump light level on the output side of the OPA 4. out / P in This ratio represents the proportion of unused pump light relative to the pump light input to the OPA 4. Therefore, α in the following equation represents the proportion of pump light input to the OPA that is used. α=1-(P out / P in ) Formula (5) The state in which an optical parametric amplifier operates with the highest conversion efficiency corresponds to the state in which the pump light is most utilized. The pump light utilization rate α defined in equation (5) corresponds to the OPA conversion efficiency. Optimal control of the OPA means maximizing the pump light utilization rate α and stabilizing the utilization rate α. Conventional optical amplifiers extract a portion of the signal light to maximize the signal light level. In contrast to conventional technologies, the optical amplifier disclosed herein uses the pump light level ratio instead of the signal light level as the control variable. This eliminates loss in the signal light, and as mentioned above, does not degrade the quality of the optical quantum signal. This configuration is suitable for quantum optics applications because it can stabilize characteristics without inserting a lossy medium into the signal light line.

[0029] The second pump light calculation unit 9 calculates the input pump light level P in is required, and the output excitation light level P out Utilizing this, the utilization rate calculation unit 10 calculates the current ratio P out / P in Based on these values, the pumping light intensity control unit 11 calculates the ratio P out / P in Alternatively, the utilization rate α is controlled to be constant at a predetermined target value.

[0030] The calculated excitation light intensity ratio P out / P in Alternatively, based on the utilization rate α, the pump light intensity control unit 11 performs feedback to two control targets, namely, the variable branching ratio coupler 7 and the TEC 16. The pump light intensity control unit 11 generates a control voltage V, which is a control signal, and adjusts the branching ratio to control the input pump light level P in The TEC 16 is controlled to be constant through the optimum temperature control unit 12. TEC2 is generated so that the conversion efficiency of the OPA4 is constant, i.e., the ratio of pump light P out / P in Alternatively, temperature control is performed to stabilize the utilization rate α at a predetermined target value.

[0031] Therefore, the present invention provides an optical parametric amplifier 102 including an optical amplifier 1 for fundamental light, a frequency doubler 2 consisting of a first second-order nonlinear optical element and generating pump light from the amplified fundamental light, and a first temperature regulator 15 for adjusting the temperature of the first second-order nonlinear optical element, an optical parametric amplifier 102 including a multiplexer 3 for multiplexing signal light and pump light, an optical parametric amplifier 4 consisting of a second second-order nonlinear optical element and amplifying the signal light from the multiplexer, a demultiplexer 5 for demultiplexing the amplified signal light from the optical parametric amplifier and pump light, and a second temperature regulator 16 for adjusting the temperature of the second second-order nonlinear optical element, a coupler 7 for branching the pump light from the pump light generator to a first output port for supplying pump light to the multiplexer and a second output port connected to a first photodetector, the branching ratio of which can be adjusted by a control voltage V, and a power amplifier 7 for detecting the intensity of the pump light from the pump light generator based on an electrical signal from the first photodetector and the control voltage V. a frequency doubler control unit 14 that generates a control signal to the optical amplifier and a control signal to the first temperature regulator based on the first error signal and controls the intensity of the pump light from the pump light generation unit to a constant value; a second pump light calculation unit 9 that calculates the intensity of the input pump light to the multiplexer based on the electrical signal from the first photodetector and the control voltage V; a second photodetector 6 that detects the intensity of the output pump light demultiplexed from the demultiplexer; a utilization rate calculation unit 10 that calculates the utilization rate of the pump light in the optical parametric amplifier from the intensity of the input pump light and the intensity of the output pump light; a pump light intensity control unit 11 that adjusts the control voltage V based on the utilization rate to control the intensity of the input pump light to a constant value; and an optimal temperature control unit 12 that generates a control signal to the second temperature regulator based on the adjusted control signal V and controls the utilization rate to a constant value.

[0032] The control of the optical parametric amplifier 102 is performed by controlling one control target (ratio P out / P in or utilization factor α), the input pump light level P inand the temperature of the TEC 16. This feature, which enables more stable temperature control of the OPA 4, will be described further below. In the following optical amplifier example, we will present more specific configurations and operations of the control circuits in the stabilization mechanism for the pump light output of the optical amplifier 1 and the stabilization mechanism for the optical parametric amplifier shown in FIG.

[0033] 3 is a diagram illustrating the configuration and operation of a specific embodiment of the optical amplifier of the present disclosure. Optical amplifier 200 is an optical amplifier 100 with the basic configuration shown in FIG. 1, in which the control circuit that processes electrical signals is replaced with a more specific configuration. Therefore, the lines that handle optical signals in optical amplifier 200 have the same configurations as pump light generation unit 101, optical parametric amplifier unit 102, and variable branching ratio coupler 7 of optical amplifier 100 in FIG. 1. Description of the configuration and operation of these units will be omitted, and only the control circuit with a specific configuration will be described.

[0034] In the control circuit for the pump light generating unit, the pump light output level P from the frequency doubler 2 SHG In order to make constant the P obtained by the calculation of the first excitation light calculation unit 13, SHG Based on this information, a proportional-integral-derivative (PID) controller 28 generates a control signal S Amp is generated and fed back to the optical amplifier 1. Amp There is no limitation on the signal form such as voltage or current for the optical amplifier 1. If the optical amplifier 1 is an optical fiber amplifier whose output level (gain) can be controlled by the injection current, feedback may be applied to the injection current. In this case, a control signal S Amp In some cases, a current drive source driven by a current source may be added.

[0035] Pump light output level P SHGIn order to keep the temperature of the TEC 15 constant, PID control is used together with a dither signal. The reference frequency signal generated by the signal generator 20 is input to the frequency mixer 30 together with a portion of the control signal from the PID controller 28. The frequency mixer 30 is used to extract an error signal for PID control from the portion of the control signal from the PID controller 28 and the reference frequency signal from the signal generator 29. In other words, the error signal required for the PID controller 32 is obtained by cutting the high frequency components of the demodulated output of the frequency mixer 30 with an LPF 31. In order to set the temperature adjustment of the TEC 15 in the frequency doubler 2 to an optimum condition, a slow frequency dither signal is input from the signal generator 29 to the adder 33, and the adder 33 outputs the control signal S to the TEC 15. TEC1 Apply to.

[0036] In the control method described above, a dither signal is generated within the system, and the response signal is demodulated with the dither signal to obtain a function obtained by differentiating the response function. This differential function is then used as an error function to determine the point at which the differential function becomes zero, i.e., the point at which the response function reaches its maximum value.

[0037] In the optical amplifier 200 of the present disclosure, the slow fluctuation of the conversion efficiency of the frequency doubler 2 due to the addition of a dither signal is controlled by the control signal S TEC1 This is offset by the control by the relatively fast control signal S while keeping the TEC15 at the peak of wavelength conversion efficiency and reducing power consumption. TEC1 The excitation light intensity can be stabilized by controlling the

[0038] The OPA 4 of the optical parametric amplifier section is also stabilized by the following control circuit. As explained in FIG. 1, the second pump light calculation section 9 calculates the input pump light level P in is calculated, and the output excitation light level P obtained by the photodetector 6 is out , and the division calculation unit 10 calculates the current ratio P out / P in The division calculation unit 10 can also calculate the utilization rate α.

[0039] The current ratio P of the excitation light obtained by the division calculation unit 10 out / P in Alternatively, based on the information on the utilization rate α, a control signal is fed back to the variable branching ratio coupler 7 by the PID controller 22. This control signal is the control voltage V of the variable branching ratio coupler 7, and the ratio P out / P in Alternatively, the utilization rate α is maintained constant.

[0040] Excitation light ratio P out / P in Alternatively, to keep the utilization factor α constant, the temperature of the OPA 4 is set to an optimum condition, similar to the temperature control in the frequency doubler 2. The adder 23 converts a slow frequency dither signal from the signal generator 24 into a control signal S TEC2 The reference frequency signal generated by the signal generator 24 is input to the frequency mixer 23 together with the control signal from the PID controller 22. The frequency mixer 23 extracts an error signal for PID control from the control signal from the PID controller 22 and the reference frequency signal from the signal generator 24. That is, the high-frequency components of the demodulated output of the frequency mixer 23 are filtered out by the LPF 25, thereby obtaining the error signal required for the PID controller 26. The slow fluctuations in the conversion efficiency of the OPA 4 due to the addition of the dither signal are offset by the control signal V to the variable branching ratio coupler 7. The OPA conversion efficiency can be stabilized by controlling the relatively fast control signal V while maintaining the temperature of the TEC 16 at the peak of wavelength conversion efficiency and reducing power consumption. In the control of the optical amplifier 200 described above, the frequency of the dither signal applied to the TECs 15 and 16 depends on the performance of the TEC itself and the temperature control target, but generally, a frequency of 10 Hz or less is sufficient.

[0041] Fig. 4 is a diagram illustrating the operation of stabilization control of conversion efficiency in an optical amplifier according to the present disclosure. Fig. 4(a) shows the relationship between wavelength λ and wavelength conversion efficiency in a wavelength conversion element. The wavelength conversion efficiency has a peak value at a certain wavelength, and this peak wavelength varies depending on the temperature T of the element. As the element temperature varies from T1 to T4, the wavelength conversion characteristics vary as shown by curves 40-1 to 40-4, with the peak position moving. Therefore, it can be understood that the temperature-dependent characteristics (temperature-conversion efficiency characteristics) of the wavelength conversion element also have a peak for a certain operating wavelength.

[0042] FIG. 4(b) illustrates the stabilization of conversion efficiency in the optical amplifier of the present disclosure. The graph on the left side of FIG. 4(b) shows the relationship between the temperature of the wavelength conversion element and the conversion efficiency at the operating wavelength. For example, considering the wavelength conversion element of the OPA 4 in FIG. 1, curve 41-1 indicates that the required conversion efficiency is obtained at the peak position of the optimal temperature. In reality, due to fluctuations in the ambient temperature of the optical amplifier, the current element temperature may be lower than the optimal temperature, at operating point 42-1. In such a situation, conventional technology uses a dither signal to control the temperature using a TEC to bring the operating point 42-1 to the peak position. Note that adding a dither signal to the control signal is equivalent to adding a small fluctuation to the temperature T on the horizontal axis on curve 41-1. The dither signal added to the TEC control signal controls the temperature of the wavelength conversion element relatively slowly toward the peak position of the wavelength conversion efficiency of curve 41-1.

[0043] In the optical amplifier of the present disclosure, in addition to the temperature control of the TEC 16 described above, high-speed control is performed by the control voltage V applied to the variable branching ratio coupler 7. The level P of the input pump light is controlled by the control voltage V. inBy increasing the input pump light level from P1 to P2, the operating point changes from curve 41-1 to curve 41-2 in Figure 4(b). By moving the operating point from point 42-1 to point 42-2, the conversion efficiency at the current temperature can be maintained at the required conversion efficiency. This operation can also be explained by the relationship between the input pump light level and conversion efficiency in the OPA, shown in the graph on the right side of Figure 4(b). By changing the input pump light level from P1 to P2, the operating point can be immediately controlled from point 42-1 to point 42-2, toward the required conversion efficiency at the operating wavelength.

[0044] Referring again to FIG. 3, the stabilization control of the conversion efficiency of the OPA 4 is performed by controlling one control object (the ratio P out / P in or utilization factor α), the input pump light level P in and the temperature of TEC 16. Control of TEC 16 using a dither signal from signal generator 24 in Fig. 3 corresponds to control of slowly moving point 42-1 in Fig. 4(b) to the peak position of curve 41-1. Control by control voltage V to variable branching ratio coupler 7 in Fig. 3 corresponds to quickly controlling the operating point from point 42-1 to point 42-2 in Fig. 4(b).

[0045] The temperature fluctuation caused by the addition of the dither signal is offset by the control voltage V applied to the variable branching ratio coupler 7. Therefore, the OPA 4 maintains a constant required conversion efficiency, i.e., a constant pump light ratio P out / P in Even if the ambient temperature fluctuates significantly, the conversion efficiency (pump light utilization rate α) is always maintained at a constant value by high-speed control of the variable branching ratio coupler 7, so that even when handling photon signals, the signal quality does not deteriorate in the optical amplifier.

[0046] The same operation as the stabilization control of the conversion efficiency of the OPA 4 described above is also performed in the stabilization control of the conversion efficiency of the frequency doubler 2. That is, the pump light output level P SHGThe control of the TEC 15 using the dither signal from the signal generator 29 in FIG. 3 corresponds to the control of slowly moving the operating point of the temperature-conversion efficiency curve to the peak position of the curve. The control signal S to the optical amplifier 1 in FIG. 3 Amp The control by the pump light output level P SHG 4(b), the pumping light output level P SHG is always kept constant.

[0047] As described above, in the optical amplifier of the present disclosure, the conversion efficiency of each nonlinear element is stabilized by a corresponding stabilization mechanism in each block of the pump light generation unit 101 and the optical parametric amplification unit 102. Common control information (P SHG , P in The two stabilization mechanisms stabilize the output of each block without causing any degradation in the quality of the optical quantum signal.

[0048] 1 and 3, the circuit for processing the electrical signals may be hardware processing using an analog circuit or a digital circuit, as described above. The electrical signals from the two photodetectors 6 and 8 are input to a computer using an A / D converter, and the control signals (S Amp , S TEC1 , S TEC2, V) of the variable branching ratio coupler 7. A memory storing relationship data between the control voltage V of the variable branching ratio coupler 7 and the branching ratios to the first output port 36-1 and the second output port 36-2 can also be used. Therefore, all functional blocks from the output points of the two photodetectors 6 and 8 to the respective control signals can also be implemented by software processing using a processor and memory. A D / A converter may be included in the generation of the control signals.

[0049] According to the present invention, an optical amplifier with stable conversion efficiency can be provided without causing a deterioration in the quality of the optical quantum signal. [Industrial Applicability]

[0050] The present invention can be used in optical communication systems.

Claims

1. an optical amplifier for fundamental light; a frequency doubler including a first second-order nonlinear optical element for generating pump light from the amplified fundamental wave light; and a first temperature regulator for regulating the temperature of the first second-order nonlinear optical element; an excitation light generating unit including: a multiplexer that multiplexes the signal light and the pump light; an optical parametric amplifier comprising a second second-order nonlinear optical element, which amplifies the signal light from the multiplexer; a demultiplexer for demultiplexing the amplified signal light from the optical parametric amplifier and the pump light; and a second temperature regulator for regulating the temperature of the second second-order nonlinear optical element; an optical parametric amplifier unit including: a coupler that branches the pumping light from the pumping light generating unit into a first output port that supplies the pumping light to the multiplexer and a second output port that is connected to a first photodetector, and that can adjust a branching ratio by a control voltage V; a first pumping light calculation unit that calculates the intensity of the pumping light from the pumping light generation unit based on the electrical signal from the first photodetector and the control voltage V, and generates a first error signal; a frequency doubler control unit that generates a control signal to the optical amplifier and a control signal to the first temperature regulator based on the first error signal, and controls the intensity of the pumping light from the pumping light generation unit to a constant value; a second pump light calculation unit that calculates the intensity of the pump light input to the multiplexer based on the electrical signal from the first photodetector and the control voltage V; a second photodetector for detecting the intensity of the output pump light demultiplexed from the demultiplexer; a utilization rate calculation unit that calculates a utilization rate of pump light in the optical parametric amplifier from the intensity of the input pump light and the intensity of the output pump light; an excitation light intensity control unit that adjusts the control voltage V based on the utilization rate to control the intensity of the input excitation light to a constant value; an optimum temperature control unit that generates a control signal to the second temperature regulator based on the adjusted control signal V and controls the utilization rate to be constant; An optical amplifier comprising:

2. The frequency doubler control unit a first proportional-integral-derivative (PID) controller that generates the control signal to the optical amplifier; a first signal generator that generates a reference frequency signal; a first mixer that multiplies the control signal from the first PID controller to the optical amplifier by the reference frequency signal; a low-pass filter for the demodulated signal from the first mixer; a second PID controller that generates the first error signal from an error signal from the low-pass filter; a first adder that adds a dither signal from the signal generator to the first error signal; Including, The excitation light intensity control unit and the optimum temperature control unit are a third PID controller that generates the control signal V to the coupler; a second signal generator that generates a reference frequency signal; a second mixer that multiplies the control signal V from the third PID controller by the reference frequency signal; a second low-pass filter for the demodulated signal from the second mixer; a fourth PID controller that generates a second error signal from the output of the second low-pass filter; a second adder that adds a dither signal from the signal generator to the second error signal; 2. The optical amplifier of claim 1, comprising:

3. 3. The optical amplifier according to claim 2, wherein the frequencies of the dither signals from the first signal generator and the second signal generator are each 10 Hz or less.

4. a processor; a memory storing data relating to the relationship between the control voltage V of the coupler and the branching ratios to the first output port and the second output port; Furthermore, 2. An optical amplifier according to claim 1, wherein the operations of the first pump light calculation unit, the frequency doubler control unit, the second pump light calculation unit, the utilization rate calculation unit, the pump light intensity control unit, and the optimal temperature control unit are performed by calculation processing by the processor based on the relationship data.

Images