Optical amplifier control method

Abstract

The present disclosure provides a control method for controlling an optical amplifier. A control method according to one embodiment comprises acquiring the light intensity of parametrically amplified excitation light in a first optical parametric amplifier having a first nonlinear medium, acquiring the intensity of light generated by a parametric effect in the first optical parametric amplifier and acquiring the gain of the light on the basis of the acquired light intensity, acquiring the gain of the excitation light, controlling the temperature of a second nonlinear medium that generates excitation light from fundamental wave light on the basis of the acquired light intensity of the excitation light, controlling the temperature of the second nonlinear medium on the basis of the gain of the light, and controlling the light intensity of excitation light supplied to the first optical parametric amplifier on the basis of the acquired gain of the excitation light.

Description

Optical amplification device control method 【0001】 This disclosure relates to a method for controlling an optical amplifier, and more particularly to a control method for obtaining the characteristics of an optical amplifier that amplifies light by optical parametric effects. 【0002】 Traditionally, with the recent launch of fifth-generation mobile communication systems and the spread of cloud computing, communication traffic has been increasing exponentially, necessitating a continuous increase in the capacity of optical networks. In optical fiber transmission, the transmission capacity per fiber can be improved by widening the optical transmission bandwidth, and various studies are being conducted toward the realization of ultra-broadband wavelength division multiplexing transmission. 【0003】 The wavelength band with low transmission loss in optical fibers (hereinafter also referred to as the optical transmission band) can be divided into several bands. In current long-distance optical transmission networks, the C-band and L-band bands of approximately 4-5 THz are mainly used, as they have the lowest transmission loss and allow the use of high-performance amplifiers such as erbium-doped fiber amplifiers (EDFAs). In recent years, in addition to the combined use of the C-band and L-band, research and development of ultra-broadband wavelength division multiplexing transmission systems using new optical transmission bands such as the S-band and U-band has become active. Such broadband transmission systems that use multiple transmission bands are called multiband transmission systems. Multiband transmission systems require the development of new optical transceivers to send and receive optical signals in transmission bands that have not been used conventionally. Furthermore, new optical amplifiers are needed for amplification and relaying in bands where EDFAs cannot be used. 【0004】Long-distance optical transmission systems include not only optical fibers and optical transceivers that serve as transmission paths for optical signals, but also optical nodes that apply optical signal processing, such as amplifying optical signals using optical amplifiers, and performing wavelength channel add-drop and routing. Devices that perform wavelength channel add-drop are called ROADMs (Reconfigurable optical add-drop multiplexers), and the technology for switching wavelength paths for multiple directions is called optical cross connect (OXC). Within ROADMs and OXCs, wavelength channel switching is performed using wavelength selective switches (WSS). To perform wavelength conversion of channels, it is necessary to use optical transceivers to convert optical signals into electrical signals, receive them, and then re-modulate them into light of a new wavelength. On the other hand, in multiband transmission networks, optical nodes need to be able to adapt not only to the conventional optical transmission band but also to multiple new transmission bands. Furthermore, as the number of wavelength channels increases, the number of transponders required for wavelength conversion also increases, leading to challenges such as increased equipment costs and facility scale due to increased power consumption. 【0005】Against this backdrop, broadband amplification relay technology and all-optical wavelength conversion technology using optical parametric amplifiers are attracting attention. Representative methods include using four-wave mixing, a third-order nonlinear optical effect, and using difference frequency generation, a second-order nonlinear optical effect. Highly nonlinear optical fibers are mainly used as the medium to produce the third-order nonlinear optical effect. Periodically polarized lithium niobate (PPLN) is mainly used as the medium to produce the second-order nonlinear optical effect. When signal light is input to an optical parametric amplifier, in addition to the original signal light component, idler light is generated at the output side in a frequency band symmetrical to the center frequency (degenerate frequency) of the gain band in the optical parametric amplification process. By extracting only the idler light with a wavelength filter, all-optical wavelength conversion becomes possible. Wavelength conversion by optical parametric amplification enables wavelength conversion over a wide bandwidth, making it possible to convert wavelength-division multiplexed signals from one transmission band to a different transmission band all at once. By utilizing this broadband wavelength conversion, and performing mutual conversion between conventionally used bands (e.g., C-band) and other transmission bands, multiband transmission becomes possible without the need to prepare devices such as optical transceivers, WSS, and optical amplifiers adapted to the new transmission band (see, for example, Non-Patent Document 1). Furthermore, low-latency, low-power wavelength conversion is possible without the use of optical transceivers, i.e., without optical-to-electrical conversion. 【0006】 Furthermore, optical parametric amplifiers can amplify input signal light across a wide bandwidth. They can also be applied to various wavelength bands by designing the phase matching characteristics of the medium and the frequency arrangement of the excitation light. Therefore, they can be used for broadband simultaneous amplification relay beyond the amplification bandwidth of conventional EDFAs, and as optical amplifiers for S-band and U-band frequencies not supported by EDFAs (see, for example, Non-Patent Document 2). 【0007】In optical parametric amplification utilizing difference frequency generation, a second-order nonlinear optical effect, a pump light of the second harmonic, which has a frequency twice that of the degenerate frequency, is used. High-gain optical parametric amplification requires a pump light with high optical power, but it is difficult to obtain strong second-harmonic light for signal light in the optical communication wavelength band using ordinary communication equipment. Therefore, a configuration is used in which light at the degenerate frequency is first amplified by a high-power optical amplifier such as an EDFA, and then a strong second harmonic is obtained using the second harmonic generation (SHG) process, a second-order nonlinear optical effect. To avoid unwanted interactions between wavelength channels when amplifying a WDM signal, a configuration is used in which second-harmonic generation and optical parametric amplification are performed in different second-order nonlinear media. In the following, the pump light before it is converted to the second harmonic will be called the fundamental pump light. 【0008】 The gain and bandwidth characteristics of optical parametric amplification depend on the phase matching characteristics between the signal light and the pump light in the medium. The optical parametric amplification process in a second-order nonlinear optical medium utilizes the interaction between widely separated light waves, namely the signal light and the pump light, which is a second harmonic. Therefore, satisfying the phase matching conditions between these light waves is not easy. Thus, a method is used to achieve pseudo-phase matching by introducing a periodic polarization reversal structure into the medium. A typical example is periodically poled lithium niobate (PPLN). On the other hand, the refractive index spectrum of the medium depends on temperature. Therefore, in order to achieve the phase matching designed by periodic polarization reversal, it is necessary to maintain a predetermined medium temperature. For this reason, in optical parametric media using a second-order nonlinear optical medium, a heater or Peltier device is usually attached to the medium to control and maintain a constant medium temperature (see, for example, Patent Document 1). 【0009】Because the optical parametric amplification process is polarization-dependent, a polarization diversity configuration is used to achieve polarization-independent operation with respect to the input signal light. This configuration involves splitting the input signal light into two orthogonal polarization components, applying optical parametric amplification to each, and then recombining them. When controlling the amplification gain in this polarization diversity configuration, it is conceivable to control each nonlinear medium used for amplification of each polarization component independently. Control for each nonlinear medium can be achieved by tapping a portion of the signal light at the output of each nonlinear medium, monitoring the gain and optical power, and adjusting the medium temperature and pump optical power. On the other hand, since the signal quality deteriorates if there is an amplification gain difference (polarization-dependent gain) between polarization components, the two polarization components need to be output with the same optical power at the output of the optical parametric amplifier (OPA). However, if the optical loss in the path of each polarization component and the measurement error of the monitor differ, simply making the amplification gain of each nonlinear medium the same will not make the polarization-dependent gain at the output of the OPA zero. Therefore, in order to achieve the desired amplification gain at the OPA output independently of polarization, including optical loss for each polarization component and measurement errors of the monitor, it is necessary to pre-calibrate the amplification gain to be realized for each nonlinear medium. 【0010】 Japanese Patent Publication No. 2020-86031 【0011】T. Kato, H. et al., “S+C+L-Band WDM Transmission Using 400-Gb / s Real-Time Transceivers Extended by PPLN-Based Wavelength Converter,” in Proc. Eur. Conf. Opt. Commun. (ECOC), Sept. 2022, paper We4D.4. Kobayashi, S. et al., “103-ch. 132-Gbaud PS-QAM Signal Inline-Amplified Transmission With 14.1-THz Bandwidth Lumped PPLN-Based OPAs Over 400-km G.652.D SMF,” in Proceedings of Optical Fiber Communication Conference (OFC), Th4B.6, 2023 【0012】 This disclosure provides a control method for an optical amplifier to maintain the gain spectrum in an optical parametric amplifier in an arbitrary state. 【0013】 A method for controlling an optical amplifier according to one embodiment of the present disclosure includes: acquiring the light intensity of excitation light for parametric amplification in a first optical parametric amplifier having a first nonlinear medium; acquiring the light intensity of light generated by the parametric effect in the first optical parametric amplifier from the first optical parametric amplifier and acquiring the light gain based on the acquired light intensity; acquiring the gain of the excitation light; controlling the temperature of a second nonlinear medium that generates excitation light from fundamental light based on the acquired light intensity of the excitation light; controlling the temperature of the second nonlinear medium based on the light gain; and controlling the light intensity of the excitation light supplied to the first optical parametric amplifier based on the acquired excitation light gain. 【0014】As described above, according to the control method for an optical amplifier according to one embodiment of the present disclosure, even when the ambient temperature, input signal light, set gain, etc., change, it is possible to calibrate the gain spectrum of the optical parametric amplifier of the optical amplifier to any desired state. 【0015】 This is a schematic diagram showing the general configuration of an optical amplifier according to one embodiment of the present disclosure. (a) is a diagram showing the general configuration of an optical intensity detector 180 in an optical amplifier according to various embodiments of the present disclosure, and (b) is a diagram showing the general configuration of an intensity detector 160. This is a diagram showing the configuration of the excitation light source of an optical amplifier according to one embodiment of the present disclosure. This is a diagram explaining the input and output light of an optical parametric amplifier. This is a flowchart showing a control method for an optical amplifier according to one embodiment of the present disclosure. This is a diagram showing the relationship between the temperature of a nonlinear medium and the output of the second harmonic. This is a flowchart showing a method for controlling the operating temperature of an optical parametric amplifier in the excitation light source of an optical amplifier according to one embodiment of the present disclosure. This is a diagram showing the change in gain (spectrum) due to a change in the operating temperature of a nonlinear medium in an optical parametric amplifier. This is a flowchart showing a method for controlling the operating temperature of an optical parametric amplifier that amplifies signal light in an optical amplifier according to one embodiment of the present disclosure. This shows the control flow of the optical amplifier in the excitation light source of an optical amplifier according to one embodiment of the present disclosure. This is a schematic diagram showing a modified form of This is a diagram showing a schematic configuration of a modified form of the excitation light source of an optical amplifier according to one embodiment of the present disclosure. This is a configuration diagram showing a schematic configuration of an optical amplifier according to another embodiment of the present disclosure. This is a configuration diagram showing a schematic configuration of an optical amplifier with polarization diversity according to another embodiment of the present disclosure. This is a configuration diagram showing a schematic configuration of the excitation light source in an optical amplifier with polarization diversity according to another embodiment of the present disclosure. This is a flowchart showing a control method for an optical amplifier with polarization diversity according to another embodiment of the present disclosure. 【0016】Embodiments of the present disclosure will be described in detail below with reference to the drawings. Identical or similar reference numerals indicate identical or similar elements, and repeated descriptions may be omitted. Material names and numerical values ​​in the following description are illustrative, and embodiments of the present disclosure may be carried out using other materials and numerical values ​​without departing from the spirit of the present disclosure. 【0017】 The control method for the optical amplification device according to the various embodiments described below involves calibrating the gain spectrum of the optical parametric amplifier in the optical amplification device to an arbitrary state. 【0018】 (First Embodiment) A control method for an optical amplifier according to the first embodiment of the present disclosure will be described with reference to Figures 1 to 10. Figure 1 shows a schematic configuration of an optical amplifier 10 according to the embodiment. The optical amplifier 10 includes an optical parametric amplifier 102, an excitation light source 100, a temperature regulator (TEC) 104, light intensity detectors 108 and 160, and a controller 111. The signal light is a wavelength-division multiplexed signal (WDM) in which the wavelengths of the signal light are multiplexed. 【0019】 The optical parametric amplifier 102 is an optical parametric amplifier (hereinafter also referred to as OPA-PPLN) that uses a periodically poled lithium niobate (PPLN) waveguide as a nonlinear optical medium. The OPA-PPLN 102 has a dichroic mirror type multiplexer 142, a PPLN waveguide 143, and a dichroic mirror type demultiplexer 145. The nonlinear optical medium constituting the PPLN waveguide 143 is, for example, LiNbO ３ , LiTaO ３ , LiNb(x)Ta(1-x)O ３ (0 ≤ x ≤ 1), or these may contain at least one additive selected from the group consisting of Mg, Zn, Sc, and In. 【0020】In the OPA-PPLN 102, the dichroic mirror type multiplexer 142 is configured to combine the signal light and the excitation light from the excitation light source 100. The combined signal light and excitation light are then incident on the PPLN waveguide 143. A focusing lens is positioned around the dichroic mirror type multiplexer 142. 【0021】 In the OPA-PPLN 102, the PPLN waveguide 143 is configured to generate the X-polarized signal light amplified by the optical parametric effect and the converted signal light (hereinafter also referred to as idler light). The amplified signal light and the idler light of the signal light are emitted from the PPLN waveguide 143 together with the excitation light. 【0022】 In the OPA-PPLN 102, the dichroic mirror type demultiplexer 145 is configured to separate the excitation light from the light coming from the PPLN waveguide 143. The excitation light is output from the OPA-PPLN 102 and input to the light intensity detector 160. The light from which the excitation light has been separated (amplified signal light and idler light converted from the signal light) is output from the OPA-PPLN 102 and input to the optical branching coupler 106 connected to the light intensity detector 108. 【0023】 Referring to Figure 2(a), the configuration of the light intensity detector 108 will be described. It includes an optical branching coupler 200 that branches the light from the optical branching coupler 106, a wavelength filter 201L into which one of the branched lights is incident, a wavelength filter 201H into which the other branched light is incident, a light intensity detector 202L that detects the light intensity from the wavelength filter 201L, and a light intensity detector 202H that detects the light intensity from the wavelength filter 201H. The light intensity detected by intensity detectors 202L and 202H is supplied to the controller 111. 【0024】 Wavelength filters 201L and 201H are wavelength filters configured to transmit light of predetermined wavelengths (hereinafter also referred to as monitor light). Wavelength filter 201H is configured to transmit monitor light of higher wavelengths, and wavelength filter 201L is configured to transmit monitor light of lower wavelengths. 【0025】Referring to Figure 2(b), the configuration of the light intensity detector 160 will be explained. The light intensity detector 160 consists of a light intensity detector 202 configured to detect the light intensity of the excitation light separated by the dichroic mirror type demultiplexer 145. The light intensity detected by the light intensity detector 160 is supplied to the controller 111. Instead of detecting the light intensity of the excitation light separated by the dichroic mirror type demultiplexer 145 of the OPA-PPLN 102 with the light intensity detector 160, the light intensity of the excitation light output from the PPLN waveguide 143p of the excitation light source 100, which will be described later referring to Figure 3, may be detected by the detector 160. In this case, an optical branch coupler 106 and the light intensity detector 160 will be placed between the PPLN waveguide 143p and the dichroic mirror type multiplexer 142 of the OPA-PPLN 102. When the light intensity of the excitation light separated by the dichroic mirror type demultiplexer 145 of the OPA-PPLN 102 is detected by the light intensity detector 160, there is an advantage that there is no excessive loss in the optical coupling between the output of the excitation light source 10 and the input of the OPA-PPLN 143. On the other hand, when the light intensity of the excitation light is detected between the PPLN waveguide 143p and the dichroic mirror type demultiplexer 145 of the OPA-PPLN 102, there are no factors other than the operating temperature of the optical amplifier 301p and the OPA-PPLN 102 that cause fluctuations in the output of the second harmonic generation (excitation light), so the controllability of the optical amplifier may be improved. 【0026】 The excitation light source 100 is a light source that emits excitation light which is combined with the signal light and incident on the OPA-PPLN 102. The excitation light source 100 is configured to change the intensity of the excitation light according to instructions from the controller 111. 【0027】 Figure 3 shows the configuration of the excitation light source 100. The excitation light source 100 has an amplifier 301p that amplifies the light from the fundamental wave light source 300 and a PPLN waveguide 143p as a nonlinear medium. The fundamental wave light source 300 has a degenerate frequency ω in the parametric effect in the OPA-PPLN 102. ０It is configured to generate a fundamental pump light (hereinafter also referred to as fundamental wave light) that is continuous light or pulsed light. The amplifier 301 is, for example, an erbium-doped fiber amplifier (EDFA). Since it is difficult to directly prepare excitation light with high optical power and narrow linewidth at the frequency of the second harmonic, after amplifying the continuous light of the degenerate frequency (ω ０ ) with an EDFA as a high-output optical amplifier 301p, a configuration is used in which it is converted into a second harmonic by utilizing the second harmonic generation (SHG) process in a second-order nonlinear medium. The amplifier 301p is configured to adjust the optical intensity of the output fundamental wave light according to an instruction from the controller 11. 【0028】 The PPLN waveguide 143p as a nonlinear medium generates excitation light (wavelength λ ０ , frequency ω ０ ) from the fundamental pump light (wavelength λ ), frequency ω ｐ ), frequency ω ｐ = 2ω ０ ) through the SHG process. 【0029】 The TEC 104p is configured to adjust the operating temperature of the PPLN waveguide 143p according to an instruction from the controller 111. 【0030】 Referring to FIG. 4, the input and output light of the optical parametric amplifier (OPA) will be described. FIG. 4 shows the frequency relationship of the excitation light, signal light, and idler light in the OPA. FIG. 4 shows the wavelength conversion band of the PPLN waveguide when the wavelength λ ０ (frequency ω ０ ) of the fundamental wave light is 1573 nm and the wavelength λ ｐ (frequency 2ω ０ ) of the excitation light is 786.5 nm. If the wavelength λs (frequency ω ｓ ) of the input signal light (WDM signal light) is 1536 - 1567 nm, idler light (converted light of the WDM signal) with a wavelength λc of 1579 - 1611 is generated by 2ω ０ - ω ｓ . Also, FIG. 4 shows the power of the noise (parametric fluorescence noise floor) output from the nonlinear medium. 【0031】 In this embodiment, the light intensity detector 108 splits a portion of the signal light amplified by the OPA-PPLN 102 and the idler light converted from the signal light (about 1% to 10% of the light intensity) at the optical branching coupler 106, and further filters the wavelength filter 201H and wavelength filter 201L to filter the light at a desired wavelength (λ H and λ L The desired wavelength (λ) is extracted and the light intensity is detected by intensity detectors 202L and 202H. L and λ H ) is the wavelength λ of the fundamental light among the multiple wavelengths included in the WDM signal. ０ (frequency ω ０ The wavelength closest to ) and wavelength λ ０ It can be the wavelength furthest from the target. Alternatively, the desired wavelength (λ L and λ H ) is the wavelength of the fundamental light among the multiple wavelengths of idler light converted from the WDM signal λ ０ The wavelength furthest from it λ L 'and wavelength λ ０ The closest wavelength λ H 'This is also acceptable. 【0032】 (Control method for the optical amplifier) ​​Next, with reference to Figure 5, the control method for the optical amplifier 10 of the present disclosure will be described. In step S101, the light intensity detector 160 detects the light intensity P of the excitation light. SHG (t) is detected. The controller 111 detects the light intensity P of the excitation light. SHG Get (t), where t is the number of loop iterations. 【0033】 In step S102, the light intensity detector 108 sets a predetermined frequency (ω H , ω L Light intensity Pout(ω) of idler light converted from signal light having ) L ') and light intensity Pout(ω H The controller 111 detects the light intensity Pout(ω). L ') and light intensity Pout(ω H The controller 111 then obtains the acquired light intensity Pout(ω). L ') and light intensity Pout(ω HFrom '), gain G(ω L ') and gain G(ω H The gain G(ω) is obtained. Idler light is part of parametric fluorescence (spectrum), and the gain is proportional to the intensity. The controller 111 uses the previously determined proportionality constant α to obtain the gain G(ω). L ') = αPout(ω L ') and gain G(ω H ') = Pout(ω H You can obtain '). 【0034】 In step S103, the controller 111 controls the gain G of the amplifier 301p. meas Obtain the gain G. For example, gain G meas The gain G(ω L ') and gain G(ω H The average of '), or the gain G(ω L ') and gain G(ω H The larger of the two, or gain G(ω L ') and gain G(ω H It can be the smaller of the two values ​​('). 【0035】 In step S104, the controller 111 controls the TEC 104p to adjust the operating temperature of the PPLN waveguide 143p, which is a nonlinear medium in the excitation light source 100. 【0036】 In step S105, the controller 111 controls the TEC 104 to adjust the temperature of the PPLN waveguide 143, which is the nonlinear medium in the OPA-PPLN 102. 【0037】 In step S106, the controller 111 controls the output of the optical amplifier 301p in the excitation light source 100. 【0038】 In step S107, the controller 111 increments the loop count and proceeds to step S101. 【0039】Figure 6 shows the relationship between the nonlinear medium temperature (hereinafter also referred to as SHG temperature) and the SHG output in the SHG process at the OPA-PPLN 102p of the excitation light source 100. The SHG output changes in a sinc function manner due to the change in phase matching caused by the change in the nonlinear medium temperature. The nonlinear medium temperature at which the SHG output is maximized is the optimal temperature. Since the maximum value of the SHG output at the current fundamental light intensity varies from unit to unit and changes over time, it is difficult to control it to a predetermined output value. Therefore, the nonlinear medium temperature is constantly changed, and control is performed to keep it near the maximum value using the hill climb method. In the hill climb method, the temperature is raised or lowered by a predetermined range ΔT, and it is determined whether the SHG output has improved or decreased compared to the previous value. If it has improved, the temperature is changed in the same direction in the next loop; if it has decreased, the temperature is changed in the opposite direction. 【0040】 Figure 7 shows the temperature control flow of the PPLN waveguide 143p as a linear waveguide of the OPA-PPLN 102p. Temperature control according to this flow is performed by the controller 111. In step S201, if it is the first loop (t = 0), proceed to step S202; if it is the second or subsequent loop (t ≠ 0), proceed to step S202. 【0041】 In step S202, the excitation light intensity P SHG Since only (0) exists, the controller 111 controls the TEC 104p without performing a comparison and raises the SHG temperature by ΔT. The controller 111 also sets the SHG flag to 0 (it may also lower the SHG temperature and set the SHG flag to 1). 【0042】 In step S203, the controller 111 determines whether the SHG flag is 0 or 1. This SHG flag indicates whether the temperature was raised (SHG flag = 0) or lowered (SHG flag = 1) in the previous loop. If the SHG flag is 1, the process proceeds to step S204; if the SHG flag is 0, the process proceeds to step S207. 【0043】 In step S204, the controller 111 determines the current SHG output value (excitation light intensity P SHG (t)) is the SHG output value in the previous loop (excitation light intensity PSHG It is determined whether the value is greater or less than (t-1). If the current SHG output value is greater, proceed to step S205; if the current SHG output value is less, proceed to step S206. 【0044】 In step S205, controller 111 changes the temperature in the same direction as in the previous loop. That is, controller 111 controls TEC 104p to lower the SHG temperature by ΔT. Controller 111 also keeps the SHG flag at 1. 【0045】 In step S206, controller 111 changes the temperature in the opposite direction to the previous loop. That is, controller 111 controls TEC 104p to raise the SHG temperature by ΔT. Controller 111 also sets the SHG flag to 0. 【0046】 In step S207, the controller 111 determines the current SHG output value (excitation light intensity P SHG (t)) is the SHG output value in the previous loop (excitation light intensity P SHG It is determined whether the value is greater or less than (t-1). If the current SHG output value is greater, proceed to step S208; if the current SHG output value is less, proceed to step S209. 【0047】 In step S208, controller 111 changes the temperature in the same direction as the previous loop. That is, controller 111 controls TEC 104p to lower the SHG temperature by ΔT. Controller 111 also leaves the SHG flag at 1. Increase the temperature. Controller 111 also leaves the SHG flag at 0. 【0048】 In step S209, controller 111 changes the temperature in the opposite direction to the previous loop. That is, controller 111 controls TEC 104p to lower the SHG temperature by ΔT. Controller 111 also sets the SHG flag to 1. 【0049】 The control according to the flow shown in Figure 7 is performed to produce the SHG output (excitation light intensity P SHG)(0) gradually increases, reaches the maximum value, and then is maintained near it. The temperature change range may be variable according to the magnitude of the change in the SHG output. For example, when the SHG output fluctuates greatly due to a slight temperature change, it is presumed that the deviation from the optimum temperature is large, so the temperature change range may be increased. 【0050】 Fig. 8 shows an example of the change in the gain spectrum due to the change in the operating temperature of the PPLN waveguide 143 as the nonlinear medium in the OPA-PPLN 102. T in the figure o represents the optimum temperature. Here, the degenerate frequency ω ０ The case of measuring and using the band components on the higher frequency side than will be described for control. Here, when the temperature of the nonlinear medium is high (T > T0), the frequency at which the gain peak occurs changes in the direction away from the degenerate frequency, and the gain decreases at the frequency near the degenerate frequency ω ０ . Here, the frequency at which the peak appears on the gain spectrum is the frequency at which the phase mismatch amount in the medium is 0. Conversely, when the temperature is low (T < T0), the gain at frequencies far from the degenerate frequency ω ０ decreases greatly, and the gain peak moves to the frequency near the degenerate frequency ω ０ . However, if the temperature is further decreased beyond the temperature at which the phase mismatch amount becomes 0 at the degenerate frequency ω ０ , the overall gain decreases. To achieve the maximum gain and the flatness of the gain spectrum within an arbitrary wavelength range, the gain at the frequency ω ０ closest to the degenerate frequency ω within the frequency range to be used and the gain at the frequency ω L farthest from it and the frequency ω H should be made to match. 【0051】 Fig. 9 is a flow chart of the temperature control of the PPLN waveguide 143 as the nonlinear medium in the OPA-PPLN 102. The temperature control according to this flow is executed by the controller 111. In step S301, the controller 111 obtains the frequencies ω L and ω H on the acquired gain spectrum G(ω), and the gains G(ω L ) and G(ω H) to compare their magnitudes. Based on this magnitude relationship, it is uniquely determined which way the temperature should be changed. For example, when the non-linear medium has characteristics as shown in FIG. 8, if G(ω L ) > G(ω H ), the temperature is in a low state (T < T0). In this case, control is performed to increase the temperature. Also, if G(ω L ) > G(ω H ), the temperature is in a high state (T > T0). In this case, control is performed to decrease the temperature. Note that if a conditional branch is strictly performed based on G(ω L ) > Gω H ) or G(ω L ) < G(ω H ), temperature control will be necessarily performed in every loop even though the temperature state is near the optimum value. It takes a certain amount of time for the temperature state to stabilize after changing the set value of the temperature, and there is concern that the characteristics of the output signal light will fluctuate due to this transient response. In this case, an arbitrary allowable value ΔG is set, and control may be performed only when G(ω L ) > G(ω H ) or G(ω L ) < G(ω H ) is greater than ΔG. In step S301, the controller 111 determines whether G(ω L ) - G(ω H ) > ΔG. If G(ω L ) - G(ω H ) > ΔG, it proceeds to step S302. If G(ω H ) - G(ω L ) ≧ ΔG, it proceeds to step S303. 【0052】 In step S302, the controller 111 controls the TEC 104 to increase the operating temperature of the PPLN waveguide 143 by ΔT. 【0053】 In step S303, the controller 111 determines whether G(ω H ) - G(ω L ) > ΔG. If (ω H ) - G(ω L ) > ΔG, it proceeds to step S304. If G(ω H ) - G(ω L ) = ΔG, it does nothing. 【0054】 In step S304, the controller 111 controls the TEC 104 to raise the operating temperature of the PPLN waveguide 143 by ΔT. 【0055】 Figure 10 shows the control flow of the optical amplifier 301p. When the intensity of the fundamental wave changes due to a change in the state of the optical amplifier 301p, the temperature characteristics of the PPLN waveguide 143p, which acts as a nonlinear medium, change slightly, which may cause a transient response. Also, the SHG output (excitation light intensity P SHG If the output of the optical amplifier 301p is significantly changed while the value is not near the optimal value, it becomes difficult to recognize the change in SHG output due to the temperature change of the PPLN waveguide 143p as a nonlinear medium, which may affect the control algorithm of the TEC 143p. For this reason, the control of the optical amplifier 301p may be performed every Δt loops rather than every loop. 【0056】 In step S410, the controller 111 determines the number of loop iterations t. If modulo(t, Δt) = 0, the controller 111 proceeds to step S411. If modulo(t, Δt) ≠ 0, it does nothing. 【0057】 In steps S411 to S414, the target gain value G target The measured gain G meas Compare G meas >G target If so, lower the output of the optical amplifier, G meas <G target Therefore, increase the output of the optical amplifier 301p. Here, the target gain value G target This is a value that is pre-set in the controller 111 by the operator. meas G(ω) is the average gain within an arbitrary frequency range obtained from the acquired gain spectrum. L ) and G(ω HIt is defined by one of the following or the average. The optical amplifier operates with auto-current control (control that sets and keeps the drive current value constant), auto-gain control (control that sets and keeps the gain value constant), and auto-power control (control that sets and keeps the output power value constant), and updates the set values ​​so that the output power changes by an arbitrary range ΔP. Alternatively, a variable optical attenuator may be placed in the path from the output of the optical amplifier to the signal light and mixed with it, and ΔP may be changed by the amount of attenuation. Furthermore, an arbitrary tolerance value ΔG may also be set for this control. p Set |G meas >G target |>ΔG p Control may be implemented only in that case. 【0058】 In step 411, the controller 111 |G meas >G target |>ΔG p Determine whether it is true or false. |G meas >G target |>ΔG p If so, proceed to step S412. |G meas >G target |>ΔG p Otherwise, do nothing. 【0059】 In step 412, the controller 111 controls G meas >G target Determine whether it is true. G meas >G target If so, proceed to step S413. meas >G target Otherwise, proceed to step S414. 【0060】 In step S413, the controller 111 sets the light intensity of the amplified fundamental wave light output from the optical amplifier 301p to a predetermined intensity Δ PEDFA Lower it by just that much. 【0061】 In step S414, the controller 111 sets the light intensity of the amplified fundamental wave light output from the optical amplifier 301p to a predetermined intensity Δ PEDFA Just raise it. 【0062】(Modified form 1) A modified version of the excitation light source 100 will be described with reference to Figures 11 and 12. The excitation light source 100 shown in Figure 3 was configured to obtain strong second harmonics by first amplifying the intensity of the fundamental light from the fundamental light source 300 in an optical amplifier 302 such as an EDFA, and then utilizing the SHG process in the PPLN waveguide 143p as a nonlinear medium. If the intensity of the fundamental light output by the fundamental light source 300 is sufficient, it is not necessarily required to amplify it in the optical amplifier 302. 【0063】 The excitation light source 100 shown in Figure 11 has a fundamental wave light intensity adjuster 302 instead of an optical amplifier 302. The fundamental wave light intensity adjuster 302 is configured to adjust the intensity of the fundamental wave light emitted from the fundamental wave light source 300 according to instructions from the controller 111. The fundamental wave light intensity adjuster 302 may be configured to adjust the intensity of the fundamental wave light emitted from the fundamental wave light source 300 by, for example, changing the injection current value to the fundamental wave light source 300. 【0064】 The excitation light source 100 shown in Figure 12 has a variable attenuator 303 instead of an optical amplifier 302. The variable attenuator 303 is configured to adjust the intensity of the fundamental wave light emitted from the fundamental wave light source 300 according to instructions from the controller 111. 【0065】 (Modified form 2) A modified version of the excitation light source 100 will be described with reference to Figure 13. The excitation light source 100 shown in Figure 3 was configured to obtain strong second harmonics by utilizing the SHG process in the PPLN waveguide 143p as a nonlinear medium. This configuration is not necessarily required if a light source that emits light with sufficient intensity at the frequency of the second harmonic (hereinafter also referred to as a second harmonic light source) can be prepared. 【0066】The excitation light source 100 shown in Figure 13 includes a second harmonic light source 400 and a variable attenuator 303. The variable attenuator 303 is configured to adjust the intensity of light at a frequency where the intensity is sufficient for the second harmonic emitted from the second harmonic light source 400, according to instructions from the controller 111. Instead of the variable attenuator 303, a second harmonic light intensity adjuster (not shown) may be used, which is configured to adjust the intensity of the second harmonic light emitted from the second harmonic light source 400 by changing the injection current value to the second harmonic light source 400, according to instructions from the controller 111. 【0067】 (Second Embodiment) The control method of the optical amplifier according to the second embodiment will be described with reference to Figure 14. Figure 14 shows a schematic configuration of the optical amplifier 20 according to the embodiment. The optical amplifier 20 has a wavelength filter 201s and an optical intensity detector 108 that detects a predetermined wavelength (λ) within the noise floor. H ,λ L The optical amplifier 20 differs from the optical amplifier 10 shown in Figure 1 in that it is configured to detect the light intensity of the signal light. When detecting noise (light within the noise floor of parametric fluorescence) output from the PPLN waveguide 143 as a nonlinear medium, if noise accumulates outside the bandwidth of the signal light input to the optical amplifier 20, it is not possible to accurately detect light of a predetermined wavelength within the target noise floor. Therefore, the optical amplifier 20 of this embodiment has a wavelength filter 201s installed on the input side of the OPA-PPLN 102 that is configured to extract the entire bandwidth of the signal light and remove noise components outside that bandwidth. 【0068】 Even when ambient temperature, input signal light, or set gain changes, the gain spectrum of the optical parametric amplifier can be maintained in a desired state. 【0069】 (Third Embodiment) A control method for the optical amplification device according to the third embodiment will be described with reference to Figures 15 to 17. In this embodiment, the optical amplification device has a polarization diversity configuration in which two optical amplification devices with the configuration described with reference to Figure 1 are placed between two polarizing beam splitters (PBS). 【0070】Figure 15 shows a schematic configuration of an optical amplifier 30 according to an embodiment. The optical amplifier 30 includes a polarizing beam splitter (PBS) 121 that separates the X-polarization and Y-polarization of the signal light, a PBS 122 that combines the X-polarization and Y-polarization of the signal light, and optical parametric amplifiers (OPA-PPLNs) 102x and 102y arranged between the PBS 121 and PBS 122, which use a periodically poled lithium niobate (PPLN) waveguide as a nonlinear optical medium. The OPA-PPLN 102x is configured to receive the X-polarization of the signal light, and the OPA-PPLN 102y is configured to receive the Y-polarization of the signal light. The signal light is a wavelength-division multiplexed signal (WDM) in which different wavelengths are multiplexed. The optical amplifier 30 includes an excitation light source 100x associated with OPA-PPLN 102x, a temperature controller (TEC) 104x, and light intensity detectors 108x and 160x, and an excitation light source 100y associated with OPA-PPLN 102y, a temperature controller (TEC) 104x, and light intensity detectors 108y and 160y. The controller 111 acquires detected values ​​from the intensity detectors 108x and 160x associated with OPA-PPLN 102x corresponding to X polarization and controls the excitation light source 100x and the temperature controller (TEC) 104x. Similarly, the controller 111 acquires detected values ​​from the intensity detectors 108y and 160y associated with OPA-PPLN 102y corresponding to Y polarization and controls the excitation light source 100y and the temperature controller (TEC) 104y. 【0071】 The controller 111 is configured to perform the control described in the first embodiment for both the X-polarization and Y-polarization. 【0072】Figure 16 shows the configuration of the excitation light source 100x and excitation light source 100y. The excitation light source 100x and excitation light source 100y are configured to share the light from the fundamental wave light source 300, which is split by the optical branching coupler 106, with the respective excitation light sources 100x and 100y. The configuration of the excitation light source 100 is the same as that of the excitation light source 100 shown with reference to Figure 3. Alternatively, the optical branching coupler 106 may be provided on the output side of the excitation light source 100 shown with reference to Figure 3, so that the branched excitation light is supplied to the optical parametric amplifiers (OPA-PPLN) 102x and 102y. 【0073】 Figure 17 shows a flow chart of the control method for the optical amplifier 30. Steps 501 to 506 in the control method for the optical amplifier 30 of this embodiment are controls performed with respect to X polarization and correspond to steps 101 to 106 in Figure 5. Steps 507 to 512 are controls performed with respect to Y polarization and correspond to steps 101 to 106 in Figure 5. Step 513 corresponds to step 107 in Figure 5. 【0074】 Thus, according to the control method of this embodiment, even when the ambient temperature, input signal light, set gain, etc., change, the gain spectrum of the optical parametric amplifier in the polarization diversity optical amplifier can be maintained in an arbitrary state. 【0075】 The control method disclosed herein makes it possible to calibrate the gain spectrum of the optical parametric amplifier in an optical amplifier to any desired state, even when the ambient temperature, input signal light, set gain, etc., change. 【0076】10, 20, 30 Optical Amplifier 100 Excitation Light Source 102 Optical Amplifier (OPA-PPLN) 104 Temperature Controller (TEC) 106 Optical Splitter Coupler 108 Light Intensity Detector 111 Controller 121, 122 Polarizing Beam Splitter (PBS) 142 Dichroic Mirror Multiplexer 143 PPLN Waveguide 145 Dichroic Mirror Demultiplexer 160 Light Intensity Detector 200 Optical Splitter Coupler 201 Wavelength Filter 202 Light Intensity Detector 300 Fundamental Wave Source 301 Optical Amplifier (EDFA) 302 Fundamental Wave Intensity Adjuster 303 Variable Attenuator 400 Second Harmonic Wave Source

Claims

1. A control method for controlling an optical amplifier, comprising: acquiring the light intensity of excitation light for parametric amplification in a first optical parametric amplifier having a first nonlinear medium; acquiring the light intensity of light generated by the parametric effect in the first optical parametric amplifier from the first optical parametric amplifier and acquiring the light gain based on the acquired light intensity; acquiring the gain of the excitation light; controlling the temperature of a second nonlinear medium that generates the excitation light from fundamental light based on the acquired light intensity of the excitation light; controlling the temperature of the second nonlinear medium based on the light gain; and controlling the light intensity of the excitation light supplied to the first optical parametric amplifier based on the acquired excitation light gain.

2. The control method according to claim 1, wherein controlling the temperature of a second nonlinear medium that generates the excitation light from the fundamental wave light, based on the light intensity of the acquired excitation light, includes: determining whether the temperature of the second nonlinear medium was raised or lowered at a first time; comparing the gain of the excitation light acquired at a second time after the first time with the gain of the excitation light acquired at the first time; and controlling the temperature of the second nonlinear medium based on the result of the determination and the result of the comparison.

3. The control method according to claim 1, wherein obtaining the gain of light based on the acquired light intensity includes obtaining the first light intensity and the second light intensity generated by the parametric effect in the first optical parametric amplifier from the first optical parametric amplifier, and obtaining the first light gain and the second light gain based on the acquired first light intensity and the acquired second light intensity, and controlling the temperature of the second nonlinear medium based on the light gain includes comparing the acquired first light gain and the acquired second light gain, and controlling the temperature of the second nonlinear medium based on the result of the comparison.

4. The control method according to claim 1, wherein controlling the light intensity of the excitation light supplied to the first optical parametric amplifier based on the gain of the acquired excitation light includes: comparing the gain of the acquired excitation light with a target gain value; and controlling the light intensity of the excitation light based on the result of the comparison.

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