Temperature adjustment method and drive device for wavelength conversion element

Abstract

A wavelength conversion element module 100 includes a waveguide and a plurality of heaters Hk arranged along a traveling direction of light in the waveguide. The wavelength conversion element module 100 acquires, for each of the plurality of heaters Hk, a deviation of a phase matching curve with respect to a wavelength at a waveguide position heated by the heater Hk with its temperature for heating the waveguide made to be different from the temperatures of the other heaters Hk, and sets the temperature for heating by each of the plurality of heaters Hk so as to eliminate the acquired deviation of the phase matching curve.

Description

Wavelength conversion element driving device and temperature adjustment method

[0001] The present disclosure relates to a driving device and a temperature adjustment method for a wavelength conversion element, and more particularly to a technique for driving a wavelength conversion element using a plurality of heaters.

[0002] Wavelength conversion elements are considered to be one of the key devices for constructing high-capacity optical communication networks. Known wavelength conversion elements utilize periodically poled lithium niobate (PPLN) for second harmonic generation (SHG), difference frequency generation (DFG), and sum frequency generation (SFG). Furthermore, waveguide-type devices can effectively achieve high efficiency by using PPLN. This is because wavelength conversion efficiency is proportional to the power density of light propagating through a nonlinear medium, and the formation of a waveguide structure allows light to be confined within a limited range. For this reason, various waveguides using nonlinear materials have been researched and developed, including, for example, Ti-diffused waveguides, diffusion-type waveguides known as proton-exchanged waveguides, and ridge-type waveguides.

[0003] Although such a waveguide-type nonlinear optical medium provides high output power, the uniformity of the waveguide structure affects the wavelength conversion characteristics. Here, we explain the influence of the width and thickness distribution of the waveguide in the light wave propagation direction on a wavelength conversion element using quasi-phase matching. For example, if the length of the waveguide is 4.3 cm, an ideal phase matching curve can be realized by keeping the width and thickness of the waveguide constant at 8 μm in the light wave propagation direction z.

[0004] However, even submicron-order fluctuations in the width and thickness of the waveguide relative to the light wave propagation direction z can significantly degrade the phase matching curve from its ideal shape, necessitating high waveguide processing precision. However, it is difficult to achieve fabrication errors in the width and thickness of the waveguide within submicron limits. Therefore, Patent Documents 1 and 2 describe techniques for correcting the refractive index using multiple local heaters after waveguide fabrication. The refractive index correction is achieved by utilizing the property that the electric field amplitude of light depends on the refractive index of the material and the property that the refractive index of the waveguide materials, lithium niobate (LN) and lithium tantalate (LT), depends on temperature. In this way, the refractive index of the waveguide is determined by controlling the temperature of the local heater, thereby compensating for the reduced wavelength conversion efficiency due to fluctuations in the waveguide width and thickness.

[0005] JP 2014-211538 A JP 2014-211539 A

[0006] However, the techniques described in Patent Documents 1 and 2 require measuring the width and thickness of the waveguide to determine specific heater drive conditions. Measuring the width and thickness of the waveguide and designing a mask for forming the waveguide take time. For this reason, the techniques described in Patent Documents 1 and 2 may reduce the production efficiency of the device.

[0007] In view of the above, the present disclosure aims to provide a driving device and a temperature adjustment method for a wavelength conversion element that can optimally drive multiple heaters to obtain high wavelength conversion efficiency without the need to measure the width or thickness of the waveguide.

[0008] In order to achieve the above object, one embodiment of the wavelength conversion element driving device of the present disclosure is a wavelength conversion element driving device including a waveguide and a plurality of heaters arranged along the propagation direction of light in the waveguide, and includes: an acquisition unit that acquires, for each of the plurality of heaters, a deviation in a phase matching curve for wavelength at a position of the waveguide heated by a heater whose temperature is made different when the temperature for heating the waveguide is made different from the temperatures of the other heaters; and a setting unit that sets the heating temperature of each of the plurality of heaters so as to eliminate the deviation in the acquired phase matching curve.

[0009] A wavelength conversion method according to one embodiment of the present disclosure is a wavelength conversion method for a wavelength conversion element including a waveguide and a plurality of heaters arranged along the propagation direction of light in the waveguide, and includes the steps of: setting the temperature for heating the waveguide of each of the plurality of heaters to a different value from the temperatures of the other heaters; obtaining a deviation in a phase matching curve for wavelength at a position of the waveguide heated by the heater with the different temperature; and setting the heating temperature of each of the plurality of heaters so as to eliminate the deviation in the obtained phase matching curve.

[0010] According to the above-described embodiments, a wavelength conversion element driving device and a temperature adjustment method are provided that can optimally drive a plurality of heaters to obtain high wavelength conversion efficiency without the need to measure the width or thickness of the waveguide.

[0011] 6A, 6B, and 6C are diagrams illustrating the relationship between the uniformity of the waveguide structure and the wavelength conversion characteristics.

[0034] FIGS. 6A, 6B, and 6C are diagrams illustrating an overview of the first embodiment.

[0035] FIGS. 6A, 6B, and 6C are schematic diagrams illustrating the configuration of the wavelength conversion element of the first embodiment.

[0036] FIGS. 6A, 6B, and 6C are other schematic diagrams illustrating the configuration of the wavelength conversion element of the first embodiment.

[0037] FIGS. 6A and 6B are diagrams illustrating the response of the phase-matched wavelength to temperature changes.

[0038] FIGS. 6A and 6B are flowcharts illustrating the procedure for determining the wavelength of the peak of the SHG spectrum of the first embodiment.

[0039] FIGS. 6A, 6B, and 6C are diagrams illustrating the process in the flowchart of FIG. 6 in more detail.

[0039] FIGS. 6A, 6B, and 6C are diagrams illustrating the concept of the temperature of each heater calculated in the first embodiment.

[0039] FIGS. 6A, 6B, and 6C are diagrams illustrating the temperature setting of the heater H of the second embodiment. 1A and 1B are diagrams for explaining the first embodiment; 1A1, 1A2, 1A3, 1A4, 1A1, 1A2, 1A3, 1A4, 1A2, 1A3, 1A4, 1A2, 1A3, 1A3, 1A4, 1A2, 1A3, 1A3, 1A4, 1A2, 1A3, 1A3, 1A4, 1A2, 1A3, 1A3, 1A4, 1A2, 1A3, 1A3, 1A4, 1A2, 1A3, 1A3, 1A4, 1A3, 1A4, 1A2, 1A3, 1A3, 1A3, 1A4 ...

[0012] [Summary] First, prior to describing the embodiments of the present disclosure, the influence of the width and thickness of the waveguide on the wavelength conversion efficiency in a wavelength conversion element according to an embodiment of the present disclosure will be described. FIGS. 1(a), 1(b), and 1(c) are diagrams illustrating the relationship between the uniformity of the waveguide structure and the wavelength conversion characteristics. The horizontal axis of FIG. 1(a) indicates the light propagation direction z, and the vertical axis indicates the core width of the waveguide. The dashed line in the diagram indicates the core width design value of 8 μm. The horizontal axis of FIG. 1(b) indicates the light propagation direction z, and the vertical axis indicates the core thickness. The dashed line in the diagram indicates the thickness design value of 8 μm. As shown in FIGS. 1(a) and 1(b), the core width and thickness generally vary along the light propagation direction and are not constant values. FIG. 1(c) is a diagram illustrating the wavelength conversion characteristics of a waveguide having the width and thickness shown in FIGS. 1(a) and 1(c). The horizontal axis of FIG. 1(c) indicates wavelength, and the vertical axis indicates wavelength conversion efficiency in arbitrary units. In Fig. 1(c), the wavelength at which the wavelength conversion efficiency peaks is the phase-matching wavelength. The dashed curve in Fig. 1(c) is an ideal phase-matching curve calculated by the following equations (1) and (2). E ２ω : electric field amplitude of second harmonic wave L: length of waveguide d: nonlinear optical constant of second harmonic wave Δβ: phase mismatch amount λ ＳＨＧ : wavelength of second harmonic λ Ｆ : wavelength of the fundamental wave n ＳＨＧ : Effective refractive index of second harmonic wave n Ｆ : Effective refractive index of the fundamental wave

[0013] As shown in Figures 1(a) and 1(b), fabrication errors occur in the actual waveguide fabrication process. For example, the width of the waveguide shown in Figure 1(a) has a fluctuation of approximately ±0.15 μm. In such a case, the electric field amplitude of the second harmonic generation (SHG) obtained using the above equations (1) and (2) is shown by the solid line in Figure 1(c). In this example, the wavelength conversion efficiency is approximately 44% of the ideal value. As such, fluctuations in the waveguide width make it impossible to obtain high wavelength conversion efficiency. The same is true for the waveguide thickness. Therefore, to achieve ideal wavelength conversion efficiency, it is desirable to keep the width and thickness of the waveguide constant in the direction of light wave propagation.

[0014] In contrast, in an embodiment of the present disclosure, instead of setting the width and thickness of the waveguide with high precision, the heaters are used to correct the driving of multiple heaters arranged along the waveguide so that deviations in phase-matched wavelength caused by errors in the width of the waveguide in the portion heated by each heater are eliminated. In this way, an embodiment of the present disclosure achieves high wavelength conversion efficiency by locally adjusting the temperature of the waveguide in accordance with variations in shape along the waveguide.

[0015] [First Embodiment] Hereinafter, a first embodiment of the present disclosure will be described with reference to the drawings. The drawings of this embodiment are intended to explain the configuration, arrangement, relationship, action, effect, and technical concept of this embodiment, and do not limit the aspect ratio or specific shape.

[0016] (Driving device for wavelength conversion element) Fig. 2 is a block diagram showing a driving device for a wavelength conversion element according to the first embodiment. A wavelength conversion module 100 as a driving device of this embodiment includes a power supply 2 that supplies power to heaters H1, H2, ... Hn (hereinafter also referred to as "heater Hk" collectively) of a wavelength conversion element having a waveguide 13. The heaters Hk are heaters that are provided along the waveguide of the wavelength conversion element 1 and adjust the temperature of the waveguide at each position to set its refractive index.

[0017] The power supply 2 may be any power supply configured to supply power to the heater Hk. The power supply 2 may be, for example, a power supply that directly supplies power to the heater Hk, or a power supply that supplies power using wireless power supply technology. In wireless power supply, power is transmitted between the power supply 2 and the wavelength conversion element 1 using electromagnetic induction or electromagnetic radiation. Furthermore, the power supply 2 may be configured to supply power to the heater Hk using an external light source.

[0018] The control unit that controls the power supply to the heater Hk from the power supply 2 may be incorporated into the wavelength conversion module as the control unit of the power supply 2. Furthermore, the measurement of the peak of the second harmonic that appears when the power supply 2 is controlled and the calculation based on the measured peak may be performed consistently by a control unit incorporated in the power supply 2. Furthermore, in the present disclosure, a control device (not shown) may be connected to the power supply 2, and the power supply to the heater Hk and at least a part of the subsequent peak acquisition and calculation may be performed. Furthermore, in this embodiment, the power supply 2 may be operated manually, and the measurement and calculation of the second harmonic that appears at that time may be performed. Such control of the power supply 2 and the acquisition and calculation of the peak wavelength are performed by executing a process for setting the temperature at which the heater Hk heats the waveguide, as will be described later in FIG. 6 .

[0019] 3(a) to 3(c) and 4(a) to 4(c) are schematic diagrams for explaining configuration examples of the wavelength conversion element according to the first embodiment. 3(a) to 3(c) show an example of a ridge-type (rib-type) wavelength conversion element in which the waveguide 13 protrudes from the substrate 11, and 4(a) to 4(c) show an example of an embedded-type wavelength conversion element in which the waveguide 13 is located within the substrate 11. As the wavelength conversion element of the first embodiment, any of the examples shown in 3(a) to 4(c) can be used. In the first embodiment, in the x, y, and z coordinates shown in FIG. 2, the side toward which the arrow on the x-axis points is referred to as "up", the length along the x-axis of the waveguide 13 is referred to as the thickness, the length along the y-axis as the "width", and the length along the z-axis simply as the length.

[0020] The wavelength conversion element shown in Figures 3(a) to 4(c) includes multiple heaters Hk. The heaters Hk shown in Figures 3(a) and 4(a) are arranged in a row on the upper surface of the waveguide 13 along the longitudinal direction of the waveguide 13. The heaters Hk shown in Figures 3(b) and 4(b) are arranged in a row on the upper surface of the substrate 11 along the longitudinal direction of the waveguide 13. The heaters Hk shown in Figures 3(a), 3(b), 4(a), and 4(b) are arranged so that their longitudinal direction is along the waveguide 13. The heaters Hk shown in Figures 3(c) and 4(c) are arranged in a row on the lower surface of the substrate 11 along the longitudinal direction of ... have a longitudinal direction perpendicular to the waveguide 13. The longitudinal length of the heaters Hk shown in Figures 3(c) and 4(c) is equal to the width of the substrate 11. The temperatures set as the heater temperatures when the heaters H1, H2, . . . Hn heat the waveguides will be referred to below as T1, T2, .

[0021] The material of the heater Hk may be, for example, gold, copper, iron, titanium, nickel, chromium, etc. In this case, the heater Hk is a metal thin film, a metal wire, etc. In the first embodiment, a Peltier device may be used for the heater Hk to adjust the temperature. Note that, since direct contact between the material of the heater HK and the waveguide 13 may increase optical loss, an insulating layer may be sandwiched between the material of the heater Hk and the waveguide 13.

[0022] In this embodiment, in the wavelength conversion element described above, the temperature at which the heater Hk heats is set as follows. This makes it possible to suppress a decrease in wavelength conversion efficiency due to variations in the width and thickness of the waveguide during its manufacture. The setting of the heating temperature of the heater Hk according to the first embodiment will be described below. The temperatures T1...Tn of the heaters H1...Hn are set according to the following formulas (3.1)...(3.n) (hereinafter, formulas (3a)...(3n) will also be collectively referred to as formula (3)). T1=T-(λ1-λk) / α Equation (3.1) T2=T-(λ2-λk) / α Equation (3.2) ... Tn=T-(λn-λk) / α Equation (3.n) Where, T: reference temperature λn: peak wavelength of the phase matching curve for second harmonic generation that occurs when a temperature difference of ΔT or -ΔT is applied to any of the heaters H1...Hn λk: k is a natural number and is the peak wavelength of the phase matching curve for second harmonic generation that occurs when a temperature difference of ΔT or -ΔT is applied to one of the heaters H1...Hn, and in this embodiment is set to λ1, the peak wavelength of the phase matching curve that appears when the temperature T of the first heater is changed by ΔT. α: temperature coefficient In other words, the relationship between the constant temperature T of the heater and the temperature Tn after a change in some of the heaters is expressed using the difference between the wavelength λk of the change peak that appears when the temperature of some of the multiple heaters is changed and the wavelength λn of another change peak that appears when the temperature of some of the other heaters is changed.

[0023] 5(a) and (b) are diagrams illustrating the response of the phase matching curve to temperature changes. A method for determining the temperature parameter α in equation (3) will be described with reference to these diagrams. As shown in FIG. 5(a), for example, when the temperature is 10°C, 20°C, and 30°C, the phase matching curve exists at different wavelengths. That is, when the temperature changes, the phase matching curve shifts in terms of wavelength. For example, when the heater temperature changes to a higher value, the entire phase matching curve shifts toward longer wavelengths. The opposite is true when the heater temperature changes to a lower value.

[0024] FIG. 5(b) is a diagram showing the relationship between the peak of the phase matching curve shown in FIG. 5(a) and the temperature when the heater heats. The horizontal axis of FIG. 5(b) represents temperature, and the vertical axis represents the peak wavelength of the phase matching curve. As can be seen from FIG. 5(b), the temperature and the peak wavelength of the phase matching curve have a linear function relationship of λ = αT + β. In the first embodiment, α, which is the slope of the line (the amount of shift in the phase matching wavelength per 1°C), is set as the temperature coefficient α in equation (3). Note that α is a coefficient specific to the material of the waveguide, and its unit is nm / °C.

[0025] (Method for Adjusting Temperature of Wavelength Conversion Element) Fig. 6 is a flowchart showing a process for setting the heating temperature of the heater Hk. Fig. 7 is a diagram for explaining the process shown in Fig. 6 in more detail. Here, Fig. 7(a2), Fig. 7(b2), and Fig. 7(c2) show second harmonic (SHG) spectra (hereinafter referred to as "SH spectra") output from the optical waveguide 13. This SH spectrum is obtained by inputting light output from a wavelength-tunable light source to a wavelength conversion element and acquiring the intensity of the SH light when the input wavelength is swept using a photodetector.

[0026] In the process shown in FIG. 6, first, in step S601, a wavelength conversion element equipped with a heater (local heater) Hk is prepared. Here, in FIG. 7(a1), the horizontal axis indicates the length along the light propagation direction of the waveguide, and the vertical axis indicates the heater temperature. Also, FIG. 7(a2) shows the SH spectrum corresponding to FIG. 7(a1). In FIG. 7(a2), the horizontal axis indicates the length along the light propagation direction of the waveguide, and the vertical axis indicates the intensity of the SH spectrum. In the wavelength conversion element prepared in this process, as shown in FIG. 7(a1), when the heater temperature T is constant with respect to the light wave propagation direction z, the SH spectrum appears as a single group of spectra (single spectrum) as shown in FIG. 7(a2), although this depends on the accuracy of the waveguide processing.

[0027] Next, in step S602, the temperature of only heater H1 is changed by ΔT, as shown in FIG. 7(b1). Here, the length z of the waveguide where the temperature has increased by ΔT corresponds to the position of heater H1. By increasing the temperature of heater H1 by ΔT, the phase matching condition at the portion of the waveguide corresponding to heater H1 changes, and the SH spectrum appears as two groups, as shown in FIG. 7(b2). In step S603, the peak wavelength λ1 of the newly emerged SH spectrum (change spectrum) is detected (acquired).

[0028] Next, in step S604, as shown in FIG. 7(c1), the temperature of only heater H2 is increased by ΔT relative to the other heaters. As a result, as shown in FIG. 7(c2), an SH spectrum having a peak at wavelength λ2 is generated, similar to when only heater H1 is increased in temperature relative to the other heaters. Then, in step S605, the generated wavelength λ2 is detected in the same manner. Similarly, the temperature of each of heaters H3, H4, ..., Hn is increased by ΔT in turn (step S606), and the wavelength λn is detected (step S607). FIGS. 7(d1) and (d2) show that increasing the temperature at the position of the waveguide 13 corresponding to heater Hn generates an SH spectrum having a peak at wavelength λ2.

[0029] As described above, in the first embodiment, the temperature of heaters H1...Hn is individually changed, and the peak wavelength of the SH spectrum generated at this time is determined, thereby making it possible to determine the degree of deviation of the phase-matched wavelength at each portion along the length of the waveguide. Then, in step S608, the heating temperatures T1...Tn of heaters Hk are set so that the deviation of the phase-matched wavelength at each portion determined as described above is corrected by the heating temperature of heater Hk.

[0030] FIG. 8 is a diagram illustrating the setting of the heating temperatures T1 to Tn of Hk. The horizontal axis of FIG. 8 indicates the longitudinal position in the waveguide 13, and the vertical axis indicates the heating temperature set for Hk. As shown in FIG. 8, if the heating temperature T1 of heater H1 is the reference temperature T, then T2 is set to T-(λ2-λ1) / α according to the above settings. In this way, by correcting the difference between the reference temperature T and the heating temperature T2 of heater H2 by the temperature difference (λ2-λ1) / α, the peak wavelength of the phase matching curve can be made the same as that of heater H1 at the reference temperature (FIG. 5(b)). The same applies to heaters H2, H3, ..., Hn. As a result, the position of the phase matching curve relative to the wavelength can be aligned across the entire heater Hk, enabling the wavelength conversion element to be driven with high wavelength conversion efficiency.

[0031] 8 shows an example in which the new SH spectrum described above in FIG. 7 appears on the longer wavelength side than the SH spectrum obtained when heater Hk is set to temperature T. Therefore, the value of λn-λ1 is always positive, and the temperatures of heaters Hk, such as H1, are set lower than temperature T. In FIG. 8, the temperature T2 of heater H2 is lower than the temperature T1 of heater H1, and the temperature T3 of heater H3 is even lower than the temperature T2 of heater H2. The temperature of heater Hn is higher than the temperatures T2 and T3 of heaters H2 and H3. In this way, the temperature of each heater set in the first embodiment does not depend on the position of the waveguide whose temperature is adjusted by the heater.

[0032] As explained above, by setting the temperatures T1, T2, ... Tn of the heaters Hk according to the following formulas (3.1) ... (3.n), it is possible to realize driving of a wavelength conversion element with high wavelength conversion efficiency. T2 = T - (λ2 - λ1) / α Formula (3.1) ... Tn = T - (λn - λ1) / α Formula (3.n) In this embodiment, in formulas (3.1) ... (3.n), T is the temperature T1 of the heater H1. As explained above, in the first embodiment, a wavelength conversion element with high wavelength conversion efficiency can be manufactured by partially adjusting the temperature of each portion along the length of the waveguide with a plurality of heaters Hk.

[0033] It should be noted that the first embodiment is not limited to the embodiment described above. For example, while the above embodiment adjusts the temperature of the waveguide by increasing the temperature of the heater Hk by ΔT (ΔT is positive), the first embodiment may adjust the temperature of the waveguide by decreasing the temperature of the heater Hk by ΔT (ΔT is negative). In many cases, a new SH spectrum appears on the long-wavelength side when the temperature of the heater Hk is increased, and appears on the short-wavelength side when the temperature of the heater Hk is decreased. However, the positive or negative value of ΔT and the side on which the new SH spectrum appears differ depending on the material of the waveguide.

[0034] [Second Embodiment] Next, a second embodiment of the present disclosure will be described. A wavelength conversion element driving device and a wavelength conversion method of the second embodiment are applied to a PPLN (periodically poled lithium niobate) waveguide. In the case of a PPLN waveguide, when ΔT is positive, a new SH spectrum appears on the long wavelength side. On the other hand, when ΔT is negative, a new SH spectrum appears on the short wavelength side. This point is utilized to set the heater temperature in the PPLN.

[0035] Figures 9(a1), 9(a2), 9(b1), 9(b2), 9(c1), 9(c2), 9(d1), and 9(d2) are diagrams illustrating the temperature setting of heaters Hk, which locally adjust the temperature of a waveguide provided in a PPLN waveguide. The example shown in Figure 9 illustrates a case where 2m heaters Hk are provided. Note that m is a natural number, and 2m is an even number. Figures 9(a1) to 9(d1) show the temperatures of the heaters Hk, and Figures 9(a2) to 9(d2) show the SH spectra that appear depending on the temperature conditions. The horizontal axis of Figures 9(a1) to 9(d1) represents the position in the longitudinal direction of the waveguide, and the vertical axis represents the temperature. The horizontal axis of Figures 9(a2) to 9(d2) represents the wavelength of the SH spectrum, and the vertical axis represents the intensity of the SH spectrum.

[0036] As shown in Fig. 9(a1), when the temperature of each heater Hk is constant at T regardless of the length, a group of SH spectra S0 (single peak) appears as shown in Fig. 9(a2). Next, in the second embodiment, a temperature difference of ΔT is applied to the temperature T of heater H1, and a temperature difference of -ΔT is applied to the temperature T of the (m+1)th heater Hm+1 (Fig. 9(b1)). In this case, as shown in Fig. 9(b2), a group of SH spectra S1, an SH spectrum with a peak at wavelength λm+1 shorter than the peak of SH spectrum S1, and an SH spectrum with a peak at wavelength λ1 longer than the peak of SH spectrum S1 appear.

[0037] Next, in the second embodiment, a temperature difference of ΔT is applied to the temperature T of heater H2, and a temperature difference of −ΔT is applied to the temperature T of the (m+2)th heater Hm+2 (FIG. 9(c1)). In this case, as shown in FIG. 9(c2), a group of SH spectra S2, an SH spectrum with a peak at wavelength λm+2 on the shorter wavelength side than the peak of SH spectrum S2, and an SH spectrum with a peak at wavelength λ2 on the longer wavelength side than the peak of SH spectrum S2 appear.

[0038] Furthermore, in the second embodiment, a temperature difference is sequentially applied to two heaters in the same manner, until finally, a temperature difference of ΔT is applied to the temperature T of the mth heater Hm, and a temperature difference of -ΔT is applied to the temperature T of the 2mth heater H2m (FIG. 9(d1)). In this case, as shown in FIG. 9(d2), a group of SH spectra Sm, an SH spectrum with a peak at wavelength λ2m shorter than the peak of SH spectrum Sm, and an SH spectrum with a peak at wavelength λm longer than the peak of SH spectrum Sm appear. This process allows the wavelengths of two SH spectra to be detected at once, thereby shortening the time required to set the temperature of each heater.

[0039] Furthermore, the second embodiment can be executed in substantially the same manner as when there are an odd number of heaters, even when there are an even number. That is, when the number of heaters is 2m-1, first, a temperature difference of ΔT is applied to the temperature T of the first heater H1, and a temperature difference of -ΔT is applied to the (m+1)th heater Hm+1. This process is then repeated to apply a temperature difference of ΔT to the temperature T of the mth heater Hm. In this case, when the number of heaters is 2m-1, there is no 2mth heater, so the process of applying a temperature difference to the temperature of the 2mth heater may be omitted. Alternatively, when the number of heaters is odd, a temperature difference may be applied to the temperature of the first heater H1 as well as the heater Hm.

[0040] [Examples] Next, examples of the present disclosure will be described. (First Example) Figures 10(a) and 10(b) are diagrams for explaining the first example. The first example is an example of a wavelength conversion module and a wavelength conversion method in which there are an even number of heaters. The first example shows an example in which there are four heaters. The waveguide of the wavelength conversion element of the first example is formed under the following conditions. Furthermore, the temperature T of each heater is constant at 30°C. Length: 4.3 cm Width: 7.4 μm Thickness: 5.0 μm

[0041] 10(a) and 10(b) show the results of an experiment conducted under the above conditions. In FIG. 10(a), the horizontal axis represents the propagation direction (length) z of the light in the waveguide, and the vertical axis represents the phase mismatch amount Δβ. The solid line in FIG. 10(a) represents the case where the phase mismatch amount Δβ has a distribution, while the dashed line represents the ideal case where the phase mismatch amount Δβ has no distribution. In order to calculate the phase mismatch amount Δβ, it is necessary to measure the thickness and width of the waveguide, but this embodiment does not require this measurement.

[0042] Figure 10(b) shows the SH spectrum corresponding to Δβ in Figure 10(a). The horizontal axis of Figure 10(b) represents wavelength, and the vertical axis represents the intensity of the SH spectrum. The dashed and solid lines of the SH spectrum correspond to the dashed and solid lines in Figure 10(a). That is, the solid line SH spectrum represents the case where the phase mismatch amount Δβ has a distribution, while the dashed line SH spectrum represents the case where the phase mismatch amount Δβ does not have a distribution. When the phase mismatch amount Δβ does not have a distribution in the propagation direction of the light wave, a steep SH spectrum is obtained as shown by the dashed line, whereas when the phase mismatch amount Δβ has a distribution in the propagation direction of the light wave, the SH spectrum has a width as shown by the solid line. When the phase mismatch amount Δβ has a distribution in the propagation direction of the light wave, the wavelength conversion efficiency decreases.

[0043] Next, we will explain how the first embodiment can avoid the above-mentioned decrease in wavelength conversion efficiency. Figures 11(a1), 11(a2), 11(a3), 11(a4), 11(b1), 11(b2), 11(b3), and 11(b4) are diagrams for explaining the effects of the first embodiment. Figures 11(a1) to 11(a4) are diagrams showing the relationship between the position in the longitudinal direction of the waveguide and the temperature of the heater, with the horizontal axis representing position and the vertical axis representing temperature. Figures 11(b1) to 11(b4) are diagrams showing the wavelength and intensity of the resulting HS spectrum, with the horizontal axis representing wavelength and the vertical axis representing intensity.

[0044] In the first example, the temperature T of the four heaters was set to 30°C, and a temperature difference of ΔT = 70°C was applied to only the first heater, as shown in Figure 11(a1). Figure 11(b1) shows the SH spectrum obtained at this time. As is clear from Figure 11(b1), it was confirmed that two SH spectra appeared. The wavelength of the peak of the new SH spectrum was λ1 = 778.1 nm. Next, as shown in Figure 11(a2), a temperature difference of ΔT = 70°C was applied to only the second heater. The wavelength of the peak of the new SH spectrum was λ2 = 778.53 nm. Similarly, for the third and fourth heaters, the wavelengths of the peaks of the new SH spectrum were λ3 = 778.52 nm and λ4 = 778.295 nm, respectively, as shown in Figures 11(a3), 11(b3), 11(a4), and 11(b4).

[0045] FIG. 12 is a diagram showing the results of determining the temperature coefficient α for the first embodiment. The solid line in FIG. 12 shows the SH spectrum when all heater temperatures T are 30°C, and the dashed line shows the SH spectrum when all heater temperatures T are 50°C. The temperature coefficient α in the wavelength band of the second harmonic for the first embodiment was 0.0405 nm / °C. From the above, the temperatures T1, T2, T3, and T4 of the four heaters were determined as follows: T1 = 30°C T2 = 30 - (778.53 - 778.1) / 0.0405 to 19.38°C T3 = 30 - (778.52 - 778.1) / 0.0405 to 19.63°C T4 = 30 - (778.295 - 778.1) / 0.0405 to 25.19°C

[0046] Figures 13(a) and 13(b) show the results when the temperatures of the four heaters are set as described above. Figure 13(a) shows the heater temperature distribution, with the horizontal axis representing the position along the length of the waveguide and the vertical axis representing temperature. Figure 13(b) shows the SH spectrum output from the waveguide when the heater temperatures are set to achieve the temperature distribution shown in Figure 13(a). The horizontal axis of Figure 13(b) represents wavelength, and the vertical axis represents the intensity of the SH spectrum. The solid line in Figure 13(b) represents the SH spectrum when the temperature T of the four heaters is constant at 30°C, the dashed line represents the ideal spectrum, and the dotted line represents the SH spectrum obtained from a waveguide with the temperature gradient shown in Figure 13(a).

[0047] As described above, the first embodiment can achieve 90% of the ideal wavelength conversion efficiency. Furthermore, it was confirmed that the wavelength conversion efficiency of the first embodiment is improved by as much as 70% compared to before correction based on the temperature gradient. This first embodiment demonstrates the usefulness of the present disclosure.

[0048] (Second Example) Figures 14(a1), 14(a2), 14(a3), 14(b1), 14(b2), and 14(b3) are diagrams for explaining a second example. The second example is an example of a wavelength conversion module and a wavelength conversion method in which an odd number of heaters are provided in a PPLN waveguide. The second example shows an example in which there are five heaters. The waveguide of the wavelength conversion element of the second example is formed under the following conditions. The temperature T of each heater is constant at 30°C. Length: 4.3 cm Width: 7.4 μm Thickness: 5.0 μm

[0049] Figures 14(a1) to 14(a3) show the relationship between the position in the longitudinal direction of the waveguide and the heater temperature, with the horizontal axis representing position and the vertical axis representing temperature. Figures 14(b1) to 14(b3) show the wavelength and intensity of the resulting HS spectrum, with the horizontal axis representing wavelength and the vertical axis representing intensity. In the second example, the temperature T of the five heaters was set to 30°C, and as shown in Figure 14(a1), the first heater had a temperature difference of ΔT = 70°C, and the fourth heater had a temperature difference of ΔT = -70°C. Figure 14(b1) shows the SH spectrum obtained at this time. As is clear from Figure 14(b1), it was confirmed that three SH spectra appeared. Of the three spectra, it was confirmed that the peak that appeared when ΔT was positive appeared on the longer wavelength side than the central SH spectrum, and the peak that appeared when ΔT was negative appeared on the shorter wavelength side than the central SH spectrum. The wavelength λ1 of the peak of the SH spectrum appearing on the long wavelength side was 778.09 nm, and the wavelength λ4 of the peak of the SH spectrum appearing on the short wavelength side was 772.96 nm.

[0050] Next, as shown in FIG. 14(a2), a temperature difference of ΔT = 70°C was applied to the second heater, and a temperature difference of ΔT = -70°C was applied to the fifth heater, and it was confirmed that three SH spectra appeared. As shown in FIG. 14(b2), the wavelength of the peak that appeared on the long wavelength side was λ2 = 778.345 nm, and the wavelength of the peak that appeared on the long wavelength side was λ5 = 772.555 nm. Finally, as shown in FIG. 14(a3), a temperature difference of ΔT = -70°C was applied to the first heater, and a temperature difference of ΔT = 70°C was applied to the third heater. As shown in FIG. 14(b3), the wavelength of the peak that appeared on the long wavelength side was λ3 = 778.68 nm, and the wavelength of the peak that appeared on the short wavelength side was λ1' = 772.52 nm. Note that wavelength λ1' is the wavelength of the peak of a new SH spectrum that appears when a negative ΔT is applied to the first heater.

[0051] From the above, the temperatures T1, T2, T3, and T4 of the four heaters were determined as follows: T1 = 30°C T2 = 30 - (778.345 - 778.09) / 0.0405 to 23.7°C T3 = 30 - (778.68 - 778.09) / 0.0405 to 15.4°C T4 = 30 - (772.96 - 772.52) / 0.0405 to 19.1°C T5 = 30 - (772.555 - 772.52) / 0.0405 to 29.1°C

[0052] Figures 15(a) and 15(b) show the results when the temperatures of the three heaters are set as described above. Figure 15(a) shows the heater temperature distribution, with the horizontal axis representing the position along the length of the waveguide and the vertical axis representing temperature. Figure 15(b) shows the SH spectrum output from the waveguide when the heater temperatures are set to have the temperature distribution shown in Figure 15(a). The horizontal axis of Figure 15(b) represents wavelength, and the vertical axis represents the intensity of the SH spectrum. The solid line in Figure 15(b) represents the SH spectrum when the temperature T of the five heaters is constant at 30°C, the dashed line represents the ideal spectrum, and the dotted line represents the SH spectrum obtained from a waveguide with the temperature gradient shown in Figure 15(a).

[0053] As described above, the second embodiment can achieve 90% of the ideal wavelength conversion efficiency. Furthermore, it was confirmed that the wavelength conversion efficiency of the second embodiment is improved by as much as 70% compared to the efficiency before the temperature gradient correction. This second embodiment demonstrates the usefulness of the present disclosure.

[0054] REFERENCE SIGNS LIST 1 wavelength conversion element 2 power supply 11 substrate 13 waveguide 100 wavelength conversion module H1, . . . Hn heater

Claims

1. A driving device for a wavelength conversion element comprising a waveguide and a plurality of heaters arranged along the propagation direction of light in the waveguide, comprising: an acquisition unit that acquires, for each of the plurality of heaters, a deviation in a phase matching curve for wavelength at a position of the waveguide heated by a heater having a different temperature when the temperature for heating the waveguide is made different from the temperatures of the other heaters; and a setting unit that sets the heating temperature of each of the plurality of heaters so as to eliminate the deviation in the acquired phase matching curve.

2. A driving device for a wavelength conversion element as described in claim 1, wherein a single peak appears from the waveguide when the temperatures of all of the multiple heaters are kept constant, and when some of the multiple heaters are changed, a changing peak appears from the waveguide along with the single peak, and the relationship between the constant temperature of the heaters and the temperature after the change of some of the heaters is expressed using the difference between the wavelength of the changing peak and the wavelength of another of the changing peaks that appears when another part of the multiple heaters is changed.

3. The wavelength conversion element driving device according to claim 1, wherein the relationship between the constant temperature T of the heaters and the temperature Tn of the nth heater after the change is expressed by the following formula: Tn=T-(λn-λk) / α Tn: temperature of the nth heater T: constant temperature of the other heaters excluding the nth heater λn: wavelength of the change peak that appears when the temperature of the nth heater is changed λk: wavelength of the change peak that appears when the temperature of the representative kth heater is changed α: rate at which the peak of the phase matching wavelength of the waveguide changes depending on the heater temperature 4. The wavelength conversion element driving device according to claim 2, wherein the temperatures of some of the heaters are changed by ΔT and the temperatures of some of the other heaters are changed by −ΔT.

5. The waveguide is made of LiNbO ３ , KNbO ３ , LiTaO ３ , LiNb( ｘ )Ta( １－ｘ ) O ３ (0≦x≦1) or KTiOPO ４ 2. The wavelength conversion element driving device according to claim 1, wherein the waveguide is a ridge-type waveguide containing at least one of the above elements as a material, or containing at least one of the group consisting of Mg, Zn, Sc, and In as an additive in the material.

6. The driving device for a wavelength conversion element according to claim 1, further comprising an insulating layer between said waveguide and said heater.

7. The driving device for a wavelength conversion element according to claim 1, wherein the heater includes at least one of Au, Cu, Fe, Ti, Ni, and Cr as a material.

8. A temperature adjustment method for a wavelength conversion element comprising a waveguide and a plurality of heaters arranged along the propagation direction of light in the waveguide, comprising the steps of: making the temperature for heating the waveguide for each of the plurality of heaters different from the temperatures of the other heaters; acquiring a deviation in a phase matching curve for wavelength at a position of the waveguide heated by the heater with the different temperature; and setting the heating temperature for each of the plurality of heaters so as to eliminate the acquired deviation in the phase matching curve.

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