Wavelength converter and control method thereof

Abstract

The present disclosure provides, for a wavelength converter, a temperature control method and a temperature control system for realizing, compared to conventional technologies, a higher suppressing effect against decreases in wavelength conversion efficiency occurring due to temperature changes caused by driving of a nonlinear optical element. A temperature control system (40) according to the present disclosure comprises: a first temperature control unit (41) that controls the temperature of a first nonlinear optical element (13); a second temperature control unit (42) that controls the temperature of a second nonlinear optical element (14); and a signal generator (43) that generates a fluctuation signal that causes fluctuation in the characteristics of a second harmonic outputted from the first nonlinear optical element (13) and of output light outputted from the second nonlinear optical element (14), and that transmits the fluctuation signal to the first temperature control unit (41) and the second temperature control unit (42).

Description

[Technical Field]

[0001] The present disclosure relates to a wavelength converter and a control method thereof, and more particularly to a wavelength converter capable of temperature control for maintaining a quasi-phase matching condition of a nonlinear optical element and a temperature control method thereof. [Background technology]

[0002] Many nonlinear optical and electro-optical elements are being developed to generate and modulate coherent light across the ultraviolet, visible, infrared, and terahertz ranges for applications such as optical signal wavelength conversion and optical modulation in optical communications, optical measurement, optical processing, medicine, and bioengineering. Among these, optical parametric amplifiers and wavelength converters have achieved various performance improvements, including improved conversion efficiency and wavelength conversion of high-power light, thanks to the development of ridge-type waveguides in which oxide crystals with periodically poled structures are directly bonded to a substrate. Furthermore, elements using lithium niobate (LiNbO3) as the core of these ridge-type waveguides have been put to practical use due to their high conversion efficiency, and further widespread use is expected in the future.

[0003] One possible application of wavelength converters and optical parametric amplifiers is a light source in the mid-infrared wavelength range. Strong absorption lines, such as those from the normal vibrations of various environmental gases, exist in the mid-infrared wavelength range of approximately 2 to 5 μm, and so the development of a compact mid-infrared light source is desired. For such a light source, it is considered promising to combine a technologically mature pump light source in the 1 μm range with a nonlinear optical element using difference frequency generation (DFG), which can use signal light in the communication wavelength band.

[0004] Furthermore, in the visible light wavelength range around 0.5 μm, there is a wavelength range that is difficult to achieve with semiconductor lasers. As a light source that outputs laser light of such wavelengths, nonlinear optical elements that use pump light sources around 1 μm and employ second harmonic generation (hereinafter referred to as SHG) or sum frequency generation (hereinafter referred to as SFG) are considered promising.

[0005] FIG. 1 illustrates an exemplary configuration of a wavelength converter 10 using a second-order nonlinear optical medium. The wavelength converter 10 includes a laser light source 11 that emits fundamental light; an amplifier 12 that provides sufficient power to the fundamental light emitted from the laser light source 11 to achieve a nonlinear optical effect; a first nonlinear optical element 13 that generates a second harmonic from the amplified fundamental light by SHG; and a second nonlinear optical element 14 that uses the second harmonic as pump light and separately input signal light and performs nondegenerate parametric amplification and wavelength conversion of the pump light and signal light by DFG. The amplifier 12 may be, for example, an erbium-doped fiber amplifier (EDFA). The first nonlinear optical element 13 and the second nonlinear optical element 14 may be, for example, ridge-type waveguides using periodically poled lithium niobate (PPLN).

[0006] In the wavelength converter 10 configured as described above, the first nonlinear optical element 13 receives the fundamental light as input and performs wavelength conversion of this fundamental light by SHG to generate a second harmonic (i.e., outputting a light wave with half the wavelength of the fundamental light). The second nonlinear optical element 14 uses the second harmonic generated by the first nonlinear optical element 13 as pump light and performs nondegenerate parametric amplification and wavelength conversion of this pump light and separately input signal light by DFG. As a result, amplified signal light generated by the optical parametric effect and wavelength-converted light (idler light) corresponding to the frequency difference between the signal light and the second harmonic are simultaneously output from the second nonlinear optical element 14. Extracting only the amplified light functions as an optical amplifier, while extracting only the idler light functions as a wavelength converter. In this specification, a device that functions as both an optical parametric amplifier and a wavelength modulator is referred to as a "wavelength converter."

[0007] The wavelength conversion process in the second nonlinear optical element is explained below. Here, wavelength conversion by DFG is explained as an example, but the principle is the same for optical parametric amplification.

[0008] 2A and 2B are diagrams conceptually illustrating the spectrum of light waves during each wavelength conversion process in a wavelength converter. FIG. 2A shows the spectrum of the fundamental light output from the laser light source 11, FIG. 2B shows the phase matching curve for SHG in the first nonlinear optical element 13, and FIG. 2C shows the phase matching curve for DFG in the second nonlinear optical element 14. The fundamental light is a single laser beam and exhibits a spectrum of a single wavelength as shown in FIG. 2A. On the other hand, the phase matching band for SHG in the first nonlinear optical element 13 is sufficiently wider than the spectrum of the fundamental light as shown in FIG. 2B. In contrast, the phase matching band for DFG in the second nonlinear optical element 14 is a wider phase matching band as shown in FIG. 2C (i.e., it has high conversion efficiency for a wide range of wavelengths of input signal light).

[0009] Typically, in nonlinear optical elements such as the first nonlinear optical element 13 and the second nonlinear optical element 14 shown in FIG. 1, a quasi-phase matching condition is satisfied between interacting light waves in the core of the optical waveguide in order to perform wavelength conversion with high efficiency. One method for satisfying the quasi-phase matching condition is to apply a periodically poled structure, such as the PPLN described above. In elements with a periodically poled structure, the crystal orientation (domain orientation) of the core material (LiNbO3 in the case of PPLN) is periodically inverted with respect to the optical axis direction, resulting in a structure in which the spontaneous polarization of the element material is periodically inverted with respect to the optical axis direction. When the inversion period satisfies Equation 1, the quasi-phase matching condition is satisfied.

[0010]

number

[0011] where n p , n s , n c are the effective refractive indices of the pump light (second harmonic), signal light, and converted light (idler light), respectively, and λ p , λ s , λ c are the wavelengths of the pump light (second harmonic), signal light, and converted light (idler light), respectively, and Λ is the reversal period of the spontaneous polarization.

[0012] At this time, the wavelength λ of the signal light s Even if the wavelength of the signal light is changed, the same conversion efficiency can be obtained as long as the formula (1) is satisfied between the converted light and the pump light. s and the effective refractive index of the converted light, n c also changes, but due to the dispersion of the material, n s As the number of c By reducing the λ / 2, the formula (1) can be satisfied even if the signal light wavelength is changed. As a result, the phase matching curve for the DFG in the second nonlinear optical element has a wide wavelength conversion band, as shown in Fig. 2(c), and the first nonlinear optical element 13 and the second nonlinear optical element 14 exhibit different phase matching characteristics.

[0013] On the other hand, it is known that the refractive index of a material depends on temperature. That is, in wavelength converter 10 as shown in Figure 1, the refractive index of the cores included in first nonlinear optical element 13 and second nonlinear optical element 14 changes due to changes in the ambient temperature or heating caused by element operation, and this can change the quasi-phase matching condition. In particular, when functioning as an optical parametric amplifier, strong pump light must be injected into the core, which increases the light absorption within the core and the resulting heat generation, and this can significantly change the quasi-phase matching condition.

[0014] Figure 3 shows the change in the phase matching characteristics of a nonlinear optical element with changes in operating temperature. Figure 3(a) shows the change in the phase matching curve for SHG, and Figure 3(b) shows the change in the phase matching curve for DFG. The dashed lines in the figures indicate the wavelength of the pump light. As shown in Figure 3(a), the phase matching curve of SHG at a given temperature T shifts toward longer wavelengths as the temperature of the nonlinear optical element increases (e.g., to T+0.5°C). Conversely, it shifts toward shorter wavelengths as the temperature of the nonlinear optical element decreases (e.g., to T-0.5°C). Therefore, assuming a constant wavelength of the input pump light, a change in temperature results in a decrease in wavelength conversion efficiency. On the other hand, as shown in Figure 3(b), the phase matching characteristics of DFG change their overall wavelength characteristics with temperature.

[0015] Therefore, in a wavelength converter that uses a core with a periodically poled structure as a nonlinear optical element, it is important to maintain the core temperature constant so as to maintain the quasi-phase matching condition. A conventional control method for maintaining the core temperature constant is to install a temperature control mechanism such as a heater or Peltier element in the first nonlinear optical element 13 and the second nonlinear optical element 14. In this method, a temperature sensor such as a thermistor or thermocouple is installed in or near the first nonlinear optical element 13 and the second nonlinear optical element 14, and the core temperature is maintained constant by controlling the heater or Peltier element according to the temperature monitored by the sensor.

[0016] However, such a conventional temperature control method is based on the average temperature of the first nonlinear optical element 13 and the second nonlinear optical element 14. Therefore, since the temperature of the core itself is not monitored, the temperature is not necessarily controlled to the optimum temperature from the viewpoint of maintaining the quasi-phase matching condition. In particular, in the above-mentioned optical parametric amplifier, the heat generated by the pump light is localized in the core, so that control based on the average temperature as in the conventional technology may be insufficient from the viewpoint of obtaining high wavelength conversion efficiency. [Prior art documents] [Non-patent literature]

[0017] [Non-Patent Document 1] T. Umeki, O. Tadanaga, A. Takada, and M. Asobe, “Phase sensitive degenerate parametric amplification using directly-bonded PPLN ridge waveguides,” Opt. Express, Vol. 19 No. 7, pp. 6326-6332 (2011) Summary of the Invention

[0018] The present disclosure has been made in consideration of the above-mentioned problems, and its purpose is to achieve a wavelength converter that is more effective than conventional in suppressing the decrease in wavelength conversion efficiency that accompanies temperature changes.

[0019] In response to the above-described problems, the present disclosure provides a wavelength converter using a nonlinear optical effect, comprising: a first nonlinear optical element that performs wavelength conversion by second harmonic generation; a second nonlinear optical element that receives as input light the second harmonic generated by the first nonlinear optical element and separately input signal light and performs wavelength conversion by optical parametric amplification and difference frequency generation; a first temperature controller that performs temperature control for the first nonlinear optical element; a second temperature controller that performs temperature control for the second nonlinear optical element; and a temperature controller that is communicably connected to the first temperature controller and the second temperature controller, generates a first fluctuation signal that fluctuates a first characteristic of the second harmonic output from the first nonlinear optical element and a second fluctuation signal that fluctuates a second characteristic of output light output from the second nonlinear optical element, and outputs the first fluctuation signal to the first temperature controller and the second temperature controller. a signal generator that transmits a second fluctuation signal to the first temperature control unit; a first temperature adjustment device that is installed near the first nonlinear optical element and adjusts the temperature of the first nonlinear optical element; a first multiplier that is installed on the output side of the first nonlinear optical element and generates a first error signal based on the first fluctuation signal and fluctuations in the first characteristic; a second temperature control unit that is installed near the second nonlinear optical element and adjusts the temperature of the second nonlinear optical element; and a second multiplier that is installed on the output side of the second nonlinear optical element and generates a second error signal based on the second fluctuation signal and fluctuations in the second characteristic, and the first temperature adjustment device and the second temperature adjustment device are feedback controlled based on the first error signal and the second error signal.

[0020] Further, the present disclosure provides a method for controlling a wavelength converter using a nonlinear optical effect, comprising the steps of controlling the temperature of a first nonlinear optical element and controlling the temperature of a second nonlinear optical element, wherein the step of controlling the temperature of the first nonlinear optical element comprises the steps of: a signal generator generating a first fluctuation signal and periodically fluctuating a first characteristic of a second harmonic output from the first nonlinear optical element based on the first fluctuation signal; detecting the first characteristic of the second harmonic output from the first nonlinear optical element; a first multiplier generating a first error signal based on the first fluctuation signal transmitted from the signal generator and the detected first characteristic of the second harmonic; and controlling a set temperature of a first temperature adjustment device based on the first error signal, wherein the step of controlling the temperature of the second nonlinear optical element comprises: Fundamental wave light input to the first nonlinear optical element detecting a second characteristic of the light wave or the second harmonic having the same wavelength as the first fluctuation signal or the second fluctuation signal transmitted from the signal generator and the detected fundamental wave light and controlling the set temperature of the second temperature adjustment device based on the second error signal. [Brief explanation of the drawings]

[0021] [Figure 1] FIG. 1 is a diagram showing an exemplary configuration of a wavelength modulator using a second-order nonlinear optical medium. [Figure 2] 2(a) and 2(c) are diagrams conceptually illustrating the spectrum of light waves during each wavelength conversion process in a wavelength converter. FIG. 2(a) shows the spectrum of the fundamental wave light output from a laser light source, FIG. 2(b) shows the phase matching curve for SHG in the first nonlinear optical element, and FIG. 2(c) shows the phase matching condition for DFG in the second nonlinear optical element. [Figure 3] 3A and 3B are diagrams showing changes in the phase matching characteristics of a nonlinear optical element with respect to changes in the operating temperature, where FIG. 3A shows the change in the phase matching curve in SHG, and FIG. 3B shows the change in the phase matching curve in DFG. [Figure 4]FIG. 1 illustrates a temperature control system for a wavelength converter according to an embodiment of the present disclosure. [Figure 5] 1 is a flowchart illustrating a method for controlling the temperature of a wavelength converter according to an embodiment of the present disclosure. [Figure 6] 10A and 10B are diagrams conceptually illustrating behavior of changes in the light intensity of converted light when the set temperature of a temperature adjustment device is subject to fluctuations. [Figure 7] FIG. 1 illustrates a temperature control system for a wavelength converter according to an embodiment of the present disclosure. [Figure 8] 1 is a flowchart illustrating a method for controlling the temperature of a wavelength converter according to an embodiment of the present disclosure. [Figure 9] 10 is a diagram conceptually showing the behavior of the power change of the second harmonic when the set temperature of the temperature adjustment device is subjected to a change. FIG. [Figure 10] FIG. 1 illustrates a temperature control system for a wavelength converter according to an embodiment of the present disclosure. [Figure 11] 1 is a flowchart illustrating a method for controlling the temperature of a wavelength converter according to an embodiment of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0022] Various embodiments of the present disclosure will be described in detail below with reference to the drawings. The same or similar reference numerals indicate the same or similar elements, and redundant description may be omitted. The following description is an example, and some configurations may be omitted or modified, or additional configurations may be added, as long as they do not deviate from the gist of one embodiment of the present disclosure.

[0023] The wavelength converter and its control method according to the present disclosure are characterized by applying minute fluctuations to the set temperatures of temperature adjustment devices installed near the first nonlinear optical element 13 and the second nonlinear optical element 14 shown in FIG. 1 or to the frequency of the fundamental light input to the first nonlinear optical element 13, and controlling the temperature based on the resulting fluctuations in the characteristics (intensity, power, etc.) of the output light. The characteristics (intensity, power, etc.) of the output light correspond to the direct changes in the core temperature and the corresponding effective refractive index. Therefore, compared to temperature control using conventional techniques, the wavelength converter can more effectively suppress the decline in wavelength conversion efficiency.

[0024] Furthermore, in the wavelength converter and the control method thereof according to the present disclosure, the fluctuation of the output light from the second nonlinear optical element 14 may utilize output light near a degenerate wavelength that is not normally used, which can have the advantage of having little effect on other output light.

[0025] In the following description of various embodiments, as described above, the first nonlinear optical element 13 performs wavelength conversion by SHG, and the second nonlinear optical element 14 performs wavelength conversion by non-degenerate parametric amplification and DFG. In addition, the cores of the first nonlinear optical element 13 and the second nonlinear optical element 14 are described as ridge-type waveguides to which PPLN is applied.

[0026] (First embodiment) A first embodiment of the present disclosure will be described in detail below with reference to the drawings. In this embodiment, wavelength converter 10 shown in FIG. 1 performs feedback control on a temperature adjustment device installed near first nonlinear optical element 13 and second nonlinear optical element 14 based on the optical intensity detection results of the output light waves.

[0027] 4 illustrates a temperature control system 40 for a wavelength converter according to an embodiment of the present disclosure. The wavelength converter 10 shown in FIG. 1 is also illustrated in the figure. The temperature control system 40 for a wavelength converter according to an embodiment of the present disclosure includes a first temperature controller 41 that controls the temperature of the first nonlinear optical element 13, a second temperature controller 42 that controls the temperature of the second nonlinear optical element 14, and a signal generator 43 that is communicatively connected to the first temperature controller 41 and the second temperature controller 42, generates a first fluctuation signal and a second fluctuation signal that fluctuate the set temperatures for controlling the temperatures of the first nonlinear optical element 13 and the second nonlinear optical element 14, and transmits the first fluctuation signal to the first temperature controller 41 and the second fluctuation signal to the second temperature controller 42, respectively.

[0028] Furthermore, the first temperature control unit 41 includes a first temperature adjustment device 411 that is installed near the first nonlinear optical element 13 and controls the temperature of the first nonlinear optical element 13, an optical branching coupler 412 that branches the second harmonic output from the first nonlinear optical element 13, a first optical intensity detector 413 that performs optical intensity detection for one of the second harmonics branched by the optical branching coupler 412, a first multiplier 414 that is connected to the signal generator 43 and the first optical intensity detector 413 and generates a temperature error signal based on the first fluctuation signal output by the signal generator 43 and the optical intensity detection result output by the first optical intensity detector 413, and a first feedback control unit 415 that generates a feedback signal based on the error signal generated by the first multiplier 414 and sends it to the first temperature adjustment device 411.

[0029] On the other hand, the second temperature control unit 42 is installed near the second nonlinear optical element 14 and includes a second temperature adjustment device 421 that controls the temperature of the second nonlinear optical element 14, and a second temperature adjustment device 422 that adjusts the temperature of the second nonlinear optical element 14. fundamental wave light and an optical wavelength branching coupler 422 that branches only the optical wave having the same wavelength as the optical wavelength branching coupler 422. fundamental wave lighta second light intensity detector 423 that performs light intensity detection for a light wave having the same wavelength as the signal generator 43; a second multiplier 424 that is connected to the signal generator 43 and the second light intensity detector 423 and generates an error signal based on the second fluctuation signal output by the signal generator 43 and the light intensity detection result output by the second light intensity detector 423; and a second feedback control unit 425 that generates a feedback signal based on the error signal generated by the second multiplier 424 and transmits the feedback signal to the second temperature adjustment device 421.

[0030] In the first feedback control section 415 and the second feedback control section 425, for example, control can be performed by a proportional-integral-derivative (hereinafter referred to as PID) method.

[0031] In this embodiment, the first nonlinear optical element 13 further includes a first demultiplexer 131 that demultiplexes the second harmonic (a light wave having half the wavelength of the fundamental light) from the output light output from the core. Additionally, the second nonlinear optical element includes a first multiplexer 141 that is installed on the input side of the core and multiplexes the second harmonic (pump light) with a separately input signal light, and a second demultiplexer 142 that is installed on the output side of the core and transmits the signal light component and reflects the second harmonic from the output light output from the core. The first demultiplexer 131, the first multiplexer 141, and the second demultiplexer 142 may be, for example, dichroic mirror types. In this embodiment, the signal light component lightwave that has passed through the second demultiplexer 142 is input to the optical wavelength branching coupler 422.

[0032] In the wavelength conversion element temperature control system 40 according to an embodiment of the present disclosure configured as above, it is possible to monitor the optical intensity of the second harmonic wave output from the first nonlinear optical element 13 and perform feedback control of the set temperature of the first temperature adjustment device 411 based on the detection result. It is also possible to monitor the optical intensity of the excitation light output from the second nonlinear optical element and perform feedback control of the set temperature of the second temperature adjustment device 421 based on the detection result. A temperature control method using the temperature control system of a wavelength converter according to an embodiment of the present disclosure will be described in detail below.

[0033] 5 is a flowchart illustrating a temperature control method 50 for a wavelength converter according to one embodiment of the present disclosure. The temperature control method 50 for a wavelength converter according to this embodiment includes step 51 of controlling the temperature of first nonlinear optical element 13 and step 52 of controlling the temperature of second nonlinear optical element 14. Note that it is preferable that temperature control is performed continuously in steps 51 and 52 while wavelength converter 10 is operating.

[0034] Further, step 51 includes step 511 of transmitting a first fluctuation signal from the signal generator 43 to the first temperature adjustment device 411 to periodically fluctuate the set temperature; step 512 of detecting the optical intensity of the second harmonic output from the first nonlinear optical element 13 with a first optical intensity detector 413; step 513 of generating an error signal with a first multiplier 414 based on the first fluctuation signal transmitted from the signal generator 43 and the optical intensity of the second harmonic detected by the first optical intensity detector 413; step 514 of transmitting the error signal from the first multiplier 414 to a first feedback control unit 415, which then generates a feedback signal; and step 515 of transmitting the feedback signal from the first feedback control unit 415 to the first temperature adjustment device 411 to appropriately change the set temperature.

[0035] On the other hand, step 52 includes step 521 of transmitting a second fluctuation signal from the signal generator 43 to the second temperature adjustment device 421 to periodically fluctuate the set temperature; and step 522 of periodically fluctuating the set temperature of the light waves output from the second nonlinear optical element 14. fundamental wave lightand a step 522 in which a second light intensity detector 423 detects the light intensity of a light wave having the same wavelength as the second fluctuation signal transmitted from the signal generator 43 and the light intensity detected by the second light intensity detector 423. fundamental wave light the second multiplier 424 transmitting the error signal to the second feedback control unit 425, which then generates a feedback signal; and the second feedback control unit 425 transmitting the feedback signal to the second temperature adjustment device 421, which then appropriately changes the set temperature.

[0036] As described above, the first nonlinear optical element 13 performs wavelength conversion by SHG. As shown in FIG. 3(a), in wavelength conversion by SHG, the wavelength conversion efficiency is maximized by controlling the temperature so that the wavelength of the pump light coincides with the peak wavelength (the wavelength at which the conversion efficiency is maximized) on the phase matching curve. Therefore, it is believed that the highest wavelength conversion efficiency can be maintained by detecting the optical intensity of the output converted light (second harmonic) and controlling the set temperature of the first temperature adjustment device 411 so that the wavelength of the pump light coincides with the peak wavelength on the phase matching curve. However, simply detecting the optical intensity value of the second harmonic wave does not determine where that value is located on the phase matching curve, making it impossible to determine whether the temperature should be increased or decreased. Therefore, in the present disclosure, in step 511, the set temperature of the first temperature adjustment device 411 is periodically and minutely changed to determine where the optical intensity of the currently output converted light coincides on the phase matching curve. The period of the fluctuation must be set to a period that can track the temperature change of the PPLN, taking into account the time constant of the temperature of the core (here, PPLN is used) included in the first nonlinear optical element 13. Then, by monitoring the second harmonic wave affected by this temperature fluctuation with the optical branching coupler 412 and the first optical intensity detector 413, it is possible to detect the optical intensity fluctuation of the converted light, including the increase or decrease, in response to the temperature fluctuation.

[0037] Figure 6 conceptually illustrates the behavior of the intensity change of the converted light when the set temperature of a temperature control device is fluctuated. When the set temperature is not fluctuating, the intensity of the converted light (output second harmonic) remains constant over time at any value on the phase matching curve. However, when the set temperature is fluctuated, the intensity of the converted light fluctuates over time according to the period of the temperature fluctuation (corresponding to 61 in the figure). Next, when the average temperature of the temperature fluctuation is increased or decreased, the average intensity fluctuation of the converted light also increases or decreases accordingly (corresponding to 62 and 63 in the figure). Note that the phase matching curve has a distribution shape that is symmetrical with respect to wavelength, centered on the peak wavelength. Therefore, there are two average temperatures that exhibit the same light intensity, making it difficult to distinguish between them (corresponding to 61 and 63 in the figure). However, although the average light intensity of both is the same, the phases of the light intensity fluctuation waveforms are different, making it possible to distinguish between them using the detection method described above. In this way, by varying the average temperature while slightly varying the set temperature of the temperature adjustment device, it is possible to determine where the current converted light is located on the phase matching curve from the error in the optical intensity and waveform of the converted light. As a result, it is possible to determine the temperature required to shift the converted light to the peak wavelength, and to control the temperature to achieve the highest wavelength conversion efficiency.

[0038] On the other hand, the second nonlinear optical element 14 performs wavelength conversion using optical parametric amplification and DFG. As shown in FIGS. 2 and 3, the phase matching curve for wavelength conversion using DFG (the optical parametric effect is based on a similar principle) differs from that for wavelength conversion using SHG in terms of the distribution shape and the behavior of the distribution change with temperature. Therefore, even if a method similar to that of step 51 described above is applied to the second nonlinear optical element 14, it is not possible to control the temperature so as to achieve the highest wavelength conversion efficiency. However, in this disclosure, by utilizing the bandwidth characteristics unique to wavelength conversion using PPLN in the core and optical parametric amplification, it is possible to control the temperature so as to achieve the highest wavelength conversion efficiency using a procedure similar to step 51, even in wavelength conversion using DFG (including optical parametric amplification). To achieve this, the optical wavelength branching coupler 422 separates only the lightwaves having the same wavelength as the signal light from the lightwaves output from the second nonlinear optical element 14. The reason for this is as follows. First, when wavelength conversion or optical parametric amplification is performed using a PPLN core, the input signal light cannot have the fundamental wavelength (degenerate wavelength) of the pump light (in this case, the second harmonic) or wavelengths nearby. This is because the converted light appears as a reflection around the pump light wavelength. If a signal light with a wavelength near this wavelength is input, the converted light will also be output at the same wavelength, resulting in interference and making separation impossible. Therefore, lightwaves near the degenerate wavelength are essentially unused, which has the advantage of minimizing the impact on other signal lights even when used as monitoring light. Second, near the degenerate wavelength, the variation in conversion efficiency with temperature follows a distribution shape that peaks at the optimal temperature, similar to the behavior of the optical intensity change of converted light due to SHG shown in Figure 6. Therefore, a decrease in wavelength conversion efficiency can be suppressed using a procedure similar to that of step 51.

[0039] In this embodiment, the core of the second nonlinear optical element is PPLN, but lithium tantalate (LiTaO3), lithium niobium tantalate (LiNb (x) Ta (1-x) O3 (0≦x≦1)) or at least one of Mg, Zn, Sc, and In is added to these. The same effect can be achieved by applying a modified oxide material.

[0040] As described above, a wavelength conversion element and a control method thereof according to an embodiment of the present disclosure utilize the phase matching characteristics of a nonlinear optical element using PPLN to suppress a decrease in wavelength conversion efficiency due to SHG and DFG (including optical parametric amplification). In this embodiment of the present disclosure, the optical intensity of the converted light output from the nonlinear optical element is monitored by directly monitoring the refractive index and temperature changes of the core. Therefore, compared to conventional techniques that control the average temperature of the entire element, it can be said that this is more effective in suppressing a decrease in wavelength conversion efficiency due to temperature changes.

[0041] (Second embodiment) A second embodiment of the present disclosure will be described in detail below with reference to the drawings. In this embodiment, the second optical intensity detector 423 in the first embodiment is replaced with a detector that monitors the power of the second harmonic wave reflected by the second demultiplexer 142.

[0042] 7 illustrates a temperature control system 70 for a wavelength converter according to one embodiment of the present disclosure. As shown in the figure, temperature control system 70 for a wavelength converter according to this embodiment has a configuration in which second temperature control unit 42 in temperature control system 40 for a wavelength converter shown in FIG. 4 is replaced with a second temperature control unit 71.

[0043] Furthermore, the second temperature control section 71 is provided near the second nonlinear optical element 14 and includes a second temperature adjustment device 421 for controlling the temperature of the second nonlinear optical element 14, a power meter 711 for detecting the power of the second harmonic wave output from the second nonlinear optical element 14, and a second temperature control device 421 connected to the signal generator 43 and the power meter 711 for detecting the power of the second harmonic wave output from the signal generator 43. 2and a second feedback control unit 425 that generates a feedback signal based on the error signal generated by the second multiplier 424 and transmits the feedback signal to the second temperature adjustment device 421. As shown in Fig. 7, the second temperature control unit 71 in this embodiment does not include the optical wavelength branching coupler 422 in the first embodiment, because it monitors the second harmonic reflected by the second demultiplexer 142. Also, a power meter 711 is provided instead of the second optical intensity detector 423.

[0044] 8 is a flowchart illustrating a temperature control method 80 for a wavelength converter according to an embodiment of the present disclosure. The temperature control method 80 for a wavelength converter according to this embodiment includes step 51 for controlling the temperature of first nonlinear optical element 13 and step 81 for controlling the temperature of second nonlinear optical element 14. The temperature control method 80 for a wavelength converter is a variation of the temperature control method 50 for a wavelength converter according to the first embodiment, in which step 52 for controlling the temperature of second nonlinear optical element 14 is replaced by step 81. As with the first embodiment, it is preferable that temperature control be performed continuously in steps 51 and 81 while wavelength converter 10 is operating.

[0045] Step 81 includes step 811 of transmitting a second fluctuation signal from the signal generator 43 to the second temperature adjustment device 421 to periodically fluctuate the set temperature; step 812 of detecting the power of the second harmonic output from the second nonlinear optical element 14 with a power meter 711; step 813 of generating an error signal with a second multiplier 424 based on the second fluctuation signal transmitted from the signal generator 43 and the power of the second harmonic detected by the power meter 711; step 814 of transmitting the error signal from the second multiplier 424 to a second feedback control unit 425, which then generates a feedback signal; and step 815 of transmitting the feedback signal from the second feedback control unit 425 to the second temperature adjustment device 421 to appropriately change the set temperature.

[0046] 9 is a diagram conceptually illustrating the behavior of changes in the power of the second harmonic when the set temperature of the temperature adjustment device is subject to fluctuations. In wavelength conversion by DFG, wavelength conversion and parametric amplification are performed by energy transfer from the second harmonic (pump light) to signal light or converted light. Therefore, the more efficiently wavelength conversion by DFG occurs, the lower the power of the second harmonic itself becomes (the wavelength at which the power shows a minimum value corresponds to the peak wavelength of the phase matching curve shown in FIG. 6). In this embodiment, this phenomenon is utilized to control the set temperature of the second temperature adjustment device 421 in step 81 so that the temperature at which efficient conversion occurs, i.e., the power of the output second harmonic, is always minimized.

[0047] As described above, even if the temperature of the second nonlinear optical element 14 is controlled by monitoring the power of the second harmonic wave, the same effects as those of the first embodiment can be obtained.

[0048] (Third embodiment) A second embodiment of the present disclosure will be described in detail below with reference to the drawings. In this embodiment, the frequency of the input fundamental light is modulated in the temperature control of the first nonlinear optical element 13.

[0049] 10 is a diagram illustrating a temperature control system 100 for a wavelength converter according to an embodiment of the present disclosure. As shown in the figure, the temperature control system 100 for a wavelength converter according to this embodiment is the temperature control system 40 of the first embodiment or the temperature control system 70 of the second embodiment, and further includes a first temperature controller 41 disposed between the laser light source 11 and the amplifier 12 and a frequency modulator 101 connected to a signal generator 43 so as to receive a first fluctuation signal. In the figure, as an example, the temperature control system 70 of the second embodiment includes the frequency modulator 101, but the temperature control system 40 of the first embodiment may also include the frequency modulator 101.

[0050] The frequency modulator 101 can be configured to drive, for example, an LN modulator using LiNbO3 or a modulator using the acousto-optic effect by voltage-controlled oscillation.

[0051] FIG. 11 is a flowchart showing a temperature control method 110 for a wavelength converter according to an embodiment of the present disclosure. The temperature control method 110 for a wavelength converter according to this embodiment includes step 111 for controlling the temperature of the first nonlinear optical element 13 and step 52 for controlling the temperature of the second nonlinear optical element 14. The temperature control method 110 for a wavelength converter is similar to the temperature control method 50 for a wavelength converter according to the first embodiment, except that step 51 for controlling the temperature of the first nonlinear optical element 13 is replaced with step 111, or the temperature control method 80 for a wavelength converter according to the second embodiment, except that step 51 for controlling the temperature of the first nonlinear optical element 13 is replaced with step 111. However, in step 52 (or step 81) of this embodiment, the second harmonic wave input to the second nonlinear optical element 14 is already in a frequency-modulated state, as will be described later. Therefore, the step of imparting a minute fluctuation to the light wave input to the second nonlinear optical element 14 (step 521 in FIG. 5 and step 811 in FIG. 8) is not necessary. Also, in the figure, as an example, step 51 for controlling the temperature of first nonlinear optical element 13 in temperature control method 50 of a wavelength converter in the first embodiment is shown replaced with step 111, but step 51 in FIG. 8 described in the second embodiment may also be replaced with step 111. Note that, as in the first and second embodiments, it is preferable that temperature control is always performed in steps 111 and 52 (or step 81) while wavelength converter 10 is operating.

[0052] Step 111 includes step 1111 of transmitting a first fluctuation signal from the signal generator 43 to the frequency modulator 101 to fluctuate the frequency of the fundamental light; step 1112 of detecting the optical intensity of the second harmonic output from the first nonlinear optical element 13 with a first optical intensity detector 413; step 1113 of generating an error signal related to the frequency with a first multiplier 414 based on the first fluctuation signal transmitted from the signal generator 43 and the optical intensity of the second harmonic detected by the first optical intensity detector 413; step 1114 of transmitting the error signal related to the frequency to a first feedback control unit 415, which generates a feedback signal; and step 1115 of transmitting the feedback signal from the first feedback control unit 415 to the first temperature adjustment device 411 to appropriately change the set temperature.

[0053] In the temperature control method 110 of the wavelength converter according to this embodiment configured as described above, fundamental light whose frequency has been modulated by frequency modulator 101 is input to first nonlinear optical element 13, which outputs a second harmonic whose frequency has been similarly modulated. Since there is a unique relationship between the frequency error and the temperature error, the same effects as those of the first and second embodiments can be achieved even if the frequency error is monitored.

[0054] Since the frequency modulation is transferred to the second harmonic through wavelength conversion by SHG in the first nonlinear optical element 13, the second harmonic (pump light) input to the second nonlinear optical element 14 is also necessarily frequency modulated. Therefore, it is sufficient to install one frequency modulator between the laser light source 11 and the amplifier 12, as shown in Fig. 10. [Industrial Applicability]

[0055] The wavelength converter and its control method according to the present disclosure have higher core temperature controllability and are effective in suppressing a decrease in wavelength conversion efficiency compared to conventional techniques. Therefore, they are expected to be used as wavelength converters for light sources in the mid-infrared wavelength range or light sources in wavelength ranges that are difficult to achieve with semiconductor lasers.

Claims

1. A wavelength converter using a nonlinear optical effect, a first nonlinear optical element that performs wavelength conversion by second harmonic generation; a second nonlinear optical element that performs wavelength conversion by optical parametric amplification and difference frequency generation using the second harmonic generated by the first nonlinear optical element and separately input signal light as input light; a first temperature control unit that controls the temperature of the first nonlinear optical element; a second temperature control unit that controls the temperature of the second nonlinear optical element; a signal generator communicatively connected to the first temperature control unit and the second temperature control unit, generating a first fluctuation signal that fluctuates a first characteristic of the second harmonic output from the first nonlinear optical element and a second fluctuation signal that fluctuates a second characteristic of output light output from the second nonlinear optical element, and transmitting the first fluctuation signal to the first temperature control unit and the second fluctuation signal to the second temperature control unit; Equipped with The first temperature control unit, a first temperature adjustment device disposed near the first nonlinear optical element and adjusting the temperature of the first nonlinear optical element; a first multiplier disposed on the output side of the first nonlinear optical element, for generating a first error signal based on the first fluctuation signal and the fluctuation of the first characteristic; Equipped with The second temperature control unit, a second temperature adjustment device disposed near the second nonlinear optical element and configured to adjust the temperature of the second nonlinear optical element; a second multiplier disposed on the output side of the second nonlinear optical element, for generating a second error signal based on the second fluctuation signal and the fluctuation of the second characteristic; Equipped with the first temperature adjustment device and the second temperature adjustment device are feedback-controlled based on the first error signal and the second error signal; The wavelength converter further comprises an optical wavelength branching coupler, the second temperature control unit being installed on the output side of the second nonlinear optical element, which branches off only a light wave having the same wavelength as the fundamental wave light input to the first nonlinear optical element from the output light output from the second nonlinear optical element.

2. the first characteristic is the optical intensity of the second harmonic wave output from the first nonlinear optical element, 2. The wavelength converter according to claim 1, wherein the second characteristic is the optical intensity of the light wave having the same wavelength as the fundamental wave light input to the first nonlinear optical element branched by the optical wavelength branching coupler.

3. The first temperature control unit, The first nonlinear optical element is provided on an input side thereof, communicatively coupled to the signal generator; 3. The wavelength converter according to claim 1, further comprising a frequency modulator that modulates the frequency of the fundamental light wave input to the first nonlinear optical element.

4. The first nonlinear optical element and the second nonlinear optical element are ridge-type waveguides, and the core material for wavelength conversion is LiNbO 3 , LiTaO 3 , LiNb (x) Ta (1-x) O 3 4. The wavelength converter according to claim 1, wherein the material is selected from the group consisting of oxide materials in which 0≦x≦1 is satisfied and at least one of Mg, Zn, Sc, and In is added to the oxide materials.

5. A method for controlling a wavelength converter using a nonlinear optical effect, comprising: controlling the temperature of the first nonlinear optical element; controlling the temperature of the second nonlinear optical element; Equipped with the step of controlling the temperature of the first nonlinear optical element a step of generating a first variation signal by a signal generator and periodically varying a first characteristic of the second harmonic wave output from the first nonlinear optical element based on the first variation signal; detecting the first characteristic of the second harmonic wave output from the first nonlinear optical element; a first multiplier generating a first error signal based on the first fluctuation signal transmitted from the signal generator and the first characteristic of the detected second harmonic; controlling a set temperature of a first temperature adjusting device based on the first error signal; Equipped with the step of controlling the temperature of the second nonlinear optical element detecting a second characteristic of a light wave output from the second nonlinear optical element and having the same wavelength as the fundamental wave light input to the first nonlinear optical element; a second multiplier generating a second error signal based on the second fluctuation signal transmitted from the signal generator and the second characteristic of the detected light wave having the same wavelength as the fundamental light input to the first nonlinear optical element; controlling a set temperature of a second temperature adjusting device based on the second error signal; A temperature control method comprising:

6. the step of controlling the temperature of the second nonlinear optical element the signal generator generates a second fluctuation signal, and periodically fluctuates the second characteristic of the light wave output from the second nonlinear optical element and having the same wavelength as the fundamental wave light input to the first nonlinear optical element based on the second fluctuation signal; the step of periodically varying the first characteristic is a step of transmitting the first variation signal to the first temperature adjustment device to vary a set temperature of the first temperature adjustment device; 6. The temperature control method according to claim 5, wherein the step of periodically varying the second characteristic is a step of transmitting the second variation signal to the second temperature adjustment device and varying the set temperature of the second temperature adjustment device.

7. 6. The temperature control method according to claim 5, wherein the step of periodically varying the first characteristic is a step of transmitting the first variation signal to a frequency modulator and varying the frequency of the fundamental light input to the first nonlinear optical element.

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