Wavelength conversion device

Abstract

A wavelength conversion device (4) into which signal light (1a) is input and which outputs output light (1f) of a different wavelength from the signal light (1a) comprises: a wavelength conversion element (130) that includes an optical waveguide core (11) and a substrate (12), a first main surface (121) of which contacts the optical waveguide core (11); and a temperature control element (26) that controls the temperature of the wavelength conversion element (130), wherein at least one recessed grove (16) is provided to a second main surface (122) of the substrate (12), which corresponds to the reverse surface from the first main surface (121).

Description

Wavelength conversion device

[0001] The present disclosure relates to wavelength conversion devices, and more particularly to wavelength conversion devices that operate under temperature control.

[0002] Periodically poled lithium niobate (PPLN) is a nonlinear optical material with a structure in which the direction of polarization is periodically reversed within a lithium niobate (LiNbO3) crystal. PPLN is also suitably used as a material for known wavelength converters that generate difference frequencies using quasi-phase-matched (QPM) technology. A wavelength converter having a ridge-type optical waveguide with a QPM structure is described, for example, in Patent Document 1.

[0003] The wavelength conversion element described in Patent Document 1 includes a substrate and an optical waveguide core formed on the substrate. It is known that when a ferroelectric material is used for the waveguide core of such a wavelength conversion element, the waveguide core is subject to optical damage, in which the refractive index decreases when irradiated with short-wavelength light. To suppress the effects of optical damage, it is effective to use the wavelength conversion device at a high temperature. To maintain the wavelength conversion device at this high temperature, temperature control is performed using, for example, a Peltier element.

[0004] Patent No. 3753236

[0005] However, when signal light and control light are multiplexed and input into an optical waveguide core, and then output and demultiplexed, if the control light with high optical intensity is input into the optical waveguide core, the optical waveguide core is heated by light absorption. The Peltier element is controlled to cool the optical waveguide core and maintain a constant temperature. As a result, a temperature gradient occurs at the interface between the optical waveguide core and the Peltier element, and large thermal stress is generated in the substrate, the support member, and their interfaces. When this thermal stress reaches the optical waveguide core, the photoelastic effect of the core stress causes fluctuations in the effective refractive index of the optical waveguide core. This causes local fluctuations in the optical phase of the signal light propagating through the optical waveguide core, resulting in fluctuations in the wavelength conversion efficiency and, as a result, fluctuations in the output light intensity of the wavelength conversion device.

[0006] The present disclosure has been made in consideration of the above points, and aims to provide a wavelength conversion device that can relieve local stress on an optical waveguide and stabilize the light intensity of wavelength-converted light.

[0007] A wavelength conversion device according to one aspect of the present disclosure is a wavelength conversion device that receives signal light and outputs output light having a wavelength different from that of the signal light, and includes a wavelength conversion element including an optical waveguide core and a substrate whose first main surface is in contact with the optical waveguide core, and a temperature control element that controls the temperature of the wavelength conversion element, wherein the substrate has at least one groove on a second main surface corresponding to the back surface of the first main surface.

[0008] According to the above-described embodiment, it is possible to provide a wavelength conversion device that is advantageous in easing local stress on the optical waveguide and stabilizing the light intensity of wavelength-converted light.

[0009] 1 is a cross-sectional view illustrating a wavelength conversion element included in a basic wavelength conversion device; FIG. 2 is a diagram illustrating a wavelength conversion device using the wavelength conversion element shown in FIG. 1; FIG. 3 is a diagram illustrating temperature distortion occurring in the wavelength conversion device shown in FIG. 2; FIG. 4 is a diagram illustrating a wavelength conversion device of a first embodiment; (a) is a view of a substrate on which grooves are formed, viewed from the back side; (b) is a cross-sectional view of the substrate shown in (a); (a) is a cross-sectional view of a wavelength conversion element; (b) is a partially enlarged view of (a); (b) to (f) are views illustrating other top views of grooves; (a) is a cross-sectional view common to (b) to (f); (a) is a cross-sectional view of a wavelength conversion device of a second embodiment; (b) is a graph showing the relationship between the position on the substrate of the wavelength conversion device of the second embodiment and the depth of the groove; (c) is a graph showing the relationship between the position on the substrate (depth of the groove) of the wavelength conversion device of the second embodiment and the optical characteristics; (a) is a cross-sectional view of a wavelength conversion device of a third embodiment. 10A is a graph showing the relationship between the position on the substrate of the wavelength conversion device of the third embodiment and the density of the grooves. 10B is a graph showing the relationship between the position on the substrate of the wavelength conversion device of the third embodiment (the density of the grooves) and the optical characteristics. 10C is a graph showing the relationship between the position on the substrate of the wavelength conversion device of the third embodiment (the density of the grooves). 10D is a graph showing the relationship between the position on the substrate of the wavelength conversion device of the third embodiment (the density of the grooves) and the optical characteristics. 10E is a graph showing the relationship between the position on the substrate of the wavelength conversion device of the third embodiment (the density of the grooves) and the optical characteristics. 10F ...

[0010] [Basic Configuration of Wavelength Conversion Element] Prior to describing the first, second, and third embodiments (hereinafter also collectively referred to as "the present embodiment") of the present disclosure, a basic wavelength conversion device will first be described. FIG. 1 is a cross-sectional view for explaining a wavelength conversion element included in a wavelength conversion device. The wavelength conversion element shown in FIG. 1 is a wavelength conversion device including a wavelength conversion element 13 that outputs a difference frequency signal by QPM technology. The wavelength conversion element 13 includes a substrate 12 and an optical waveguide core 11 formed on the substrate 12. The wavelength conversion device further includes a multiplexer 14 and a demultiplexer 15.

[0011] As shown in FIG. 1, the multiplexer 14 multiplexes the signal light 1a and the control light 1b. The optical intensity of the control light 1b is higher than that of the signal light 1a. The multiplexed light 1e is incident on one end of the optical waveguide core 11. The optical waveguide core 11 has a periodically poled structure and exhibits a nonlinear optical effect. As the signal light 1a passes through the optical waveguide core 11, it is converted into difference frequency light 1c, which has a wavelength different from that of the signal light 1a, and is emitted as output light 1f from the other end of the optical waveguide core 11 together with the control light 1b. The difference frequency light 1c and control light 1b emitted from the optical waveguide core 11 are incident on the demultiplexer 15 and separated from each other.

[0012] 2 is a diagram illustrating a wavelength conversion device including the wavelength conversion element 13 shown in FIG. 1 , a temperature control element 26, a metal housing bottom surface 28 to which the temperature control element 26 is fixed, a bonding agent 23, and a support member 27. The bonding agent 23 fixes the support member 27 to the temperature control element 26 and the temperature control element 26 to the metal housing bottom surface 28. The support member 27 is a metal member for uniformly controlling the temperature of the entire wavelength conversion element 13. The temperature control element 26 is interposed between the support member 27 and the metal housing bottom surface 28, and heat is conducted between the support member 27 and the metal housing bottom surface 28. Note that, for example, a Peltier element is used as the temperature control element 26.

[0013] FIG. 3 is a diagram illustrating thermal distortion occurring in the wavelength conversion device shown in FIG. 2. When the wavelength conversion device is in use, the temperature of the wavelength conversion element 13 rises significantly due to optical absorption caused by the incidence of high-intensity control light, etc. The temperature control element 26 cools the wavelength conversion element 13 to a predetermined appropriate temperature. At this time, the optical waveguide core 11 is hot, while the temperature control element 26 is cold, resulting in a temperature gradient between the optical waveguide core 11, the substrate 12, the support member 27, and the surface of the temperature control element 26. Therefore, thermal expansion distortion proportional to the product of the temperature difference between the operating temperature and the mounting temperature (bonding temperature) of each component and the thermal expansion coefficient (linear expansion coefficient) of each component occurs in each component. However, in reality, because each component is bonded and fixed with a bonding agent 23 or the like, thermal stress corresponding to the product of the difference in thermal expansion coefficient and the difference in elastic modulus between the bonded components occurs at the interface between each component. As shown in FIG. 3, in the wavelength conversion device, the wavelength conversion element 13 expands in the optical waveguiding direction, and the temperature control element 26 contracts.

[0014] Here, examples of linear expansion coefficients are given. The optical waveguide core 11 is made of a second-order nonlinear optical crystal, and is known to have a relatively large thermal expansion coefficient, although this depends on the crystal orientation. Examples of materials used for the optical waveguide core 11 include LiNbO3 (lithium niobate), KNbO3 (potassium niobate), LiTaO3 (lithium tantalate), and KTiOPO4 (potassium titanyl phosphate). For example, periodically poled lithium niobate (PPLN) has a relatively large thermal expansion coefficient similar to that of lithium niobate crystal (LiNbO3). At around room temperature (25°C), it is approximately 7.5 ppm / °C in the Z-axis direction and approximately 15.4 ppm / °C in the X-axis and Y-axis directions. At around 300°C, it exhibits an even higher thermal expansion coefficient.

[0015] For example, when a nonlinear optical crystal is used for the optical waveguide core 11 or the substrate 12 and a metal material is used for the support member 27, copper (Cu) has a thermal expansion coefficient of approximately 16.8 ppm / °C, which is quite close to the above, but zinc (Zn) has a thermal expansion coefficient of approximately 30.2 ppm / °C, tin (Sn) has a thermal expansion coefficient of approximately 26.9 ppm / °C, and aluminum (Al) has a thermal expansion coefficient of approximately 23 ppm / °C, which is larger than that of lithium niobate. Gold (Au) has a thermal conductivity of approximately 14.3 ppm / °C, iron (Fe) has a thermal conductivity of approximately 12.1 ppm / °C, titanium (Ti) has a thermal conductivity of approximately 8.5 ppm / °C, nickel (Ni) has a thermal conductivity of approximately 13.3 ppm / °C, and silicon (Si) has a thermal conductivity of approximately 2.8 to 7.3 ppm / °C, which are smaller than lithium niobate. Even with a uniform temperature distribution, the temperature difference between the temperature at the time of mounting and joining (adhesive fixing) and the operating temperature will cause thermal stress in either the optical waveguide core 11 or between the substrate 12 and the support member 27.

[0016] Stress generated at the interfaces between the components of the wavelength conversion element affects the optical waveguide core 11. At this time, even if the amount of strain is so small that it does not affect the optical coupling position, internal stress due to thermal expansion strain is generated in the optical waveguide core 11. In such a case, the effective refractive index of the optical waveguide core fluctuates due to the photoelastic effect of the stress on the optical waveguide core 11. As a result, the (effective) refractive index of the optical waveguide core fluctuates locally, causing local fluctuations in the optical phase of the propagating signal light, which in turn fluctuates the wavelength conversion efficiency, resulting in fluctuations in the output light intensity of the wavelength conversion device, which has been a problem.

[0017] Next, the effect of the bonding agent 23 on stress will be described. The bonding agent 23 is a silver paste, a thermally conductive paste, or the like. Silver paste is produced by adding a silver microparticle filler to a thermosetting resin such as a thermosetting epoxy resin or a thermosetting acrylic resin. A thermally conductive paste can be produced by mixing a metal or inorganic filler with high thermal conductivity, or by using a thermosetting resin with high thermal conductivity after hardening. Regardless of the bonding agent 23, bonding strength is exerted by the curing reaction of the thermosetting resin, and therefore, even a slight curing shrinkage occurs during the curing reaction of the cured resin.

[0018] For the above reasons, even if the linear expansion coefficients of the substrate 12, the support member 27, and the temperature control element 26 are all the same, interfacial stress is generated at the interface between each member due to the cure shrinkage reaction of the bonding agent 23. Therefore, stress is generated at the interface between the substrate 12 and the support member 27 due to the cure shrinkage of the bonding agent 23.

[0019] [First embodiment] A first embodiment of the present disclosure will be described below with reference to the drawings. The drawings used in the first embodiment are intended to explain the configuration, arrangement of each part, effects, functions, and technical concepts of the present disclosure, and do not limit the specific shape, aspect ratio, etc.

[0020] Fig. 4 is a diagram for explaining the wavelength conversion device 4 of the first embodiment, and is a cross-sectional view taken along the optical axis of the optical waveguide core 11. Of the components shown in Fig. 4, the same components as those shown in Fig. 1 and Fig. 2 are denoted by the same reference numerals.

[0021] The wavelength conversion device 4 is a wavelength conversion device that receives signal light 1a and outputs output light 1f having a wavelength different from that of the signal light 1a. The wavelength conversion device 4 is equipped with a wavelength conversion element 130 including an optical waveguide core 11 and a substrate 12 whose first main surface 121 is in contact with the optical waveguide core 11. Note that the "main surface" here may be any main surface of the substrate 12, and the position, area, etc. of the main surface are not specified.

[0022] The wavelength conversion device 4 further includes a multiplexer 14 that multiplexes the signal light 1a and the control light 1b, and a demultiplexer 15 that demultiplexes the output light 1f from the wavelength conversion element 130 into difference frequency light 1c having a wavelength different from that of the signal light 1a and the control light 1b. In the first embodiment, the wavelength conversion element 130 and the demultiplexer 15 are separate elements, and optical input and output are performed by spatial optical coupling. With this configuration, it is possible to separate the wavelength conversion element 130 from the optical demultiplexing function, so the wavelength conversion element 130 can be realized using only an optical waveguide core with a relatively simple shape, such as a linear optical waveguide. Therefore, the local stress distribution generated in the wavelength conversion element 130 can be controlled by a relatively simple method.

[0023] The wavelength conversion device 4 also includes a temperature control element 26 that controls the temperature of the wavelength conversion element 130. The temperature control element 26 in the first embodiment is a Peltier element. The Peltier element is an element that utilizes the phenomenon of creating a temperature difference from voltage due to the Peltier effect, which is a type of thermoelectric effect, and utilizes the effect that when a current is passed through the Peltier element, heat is absorbed on one side and simultaneously heat is generated on the other side. Cooling and heating can be switched depending on the direction of the current passed. Note that, as the temperature control element, if the operating temperature of the wavelength conversion element 130 is sufficiently higher than room temperature, the operating temperature can be maintained by providing auxiliary heating, in which case a heater such as a ceramic heater can also be used.

[0024] The wavelength conversion device 4 further includes a support member 27 for supporting the wavelength conversion element 130 between the main surface 121 of the substrate 12 and the temperature control element 26. It is desirable to use a material for the support member that has the same linear thermal expansion coefficient as the optical waveguide core 11 and the substrate 12, but it is difficult to achieve perfect matching. Therefore, by using a support member with a higher thermal conductivity than the materials of the optical waveguide core 11 and the substrate 12, it is possible to minimize the difference in temperature distribution on the back surface of the substrate.

[0025] In addition, the wavelength conversion device 4 is housed in a metal housing (not shown), the temperature control element 26 is provided between the main surface 272 (third surface) of the support member 27 and the bottom surface 28 of the metal housing, and the wavelength conversion element 130 is provided on the main surface 271 (fourth surface) corresponding to the back surface of the main surface 272 of the support member.

[0026] The substrate 12 is made of a material having a lower refractive index for the signal light 1a than the optical waveguide core 11. Specifically, LiNbO3 (lithium niobate), KNbO3 (potassium niobate), LiTaO3 (lithium tantalate), LiNb (x) Ta (1-x) O3 (0≦x≦1) (non-stoichiometric lithium tantalate), KNb (x) Ta (1-x)The material contains at least one of O3 (0≦x≦1) (potassium tantalate of non-stoichiometric composition) and KTiOPO4 (potassium titanyl phosphate), and contains at least one selected from Mg (magnesium), Zn (zinc), Sc (scandium), and In (indium) as an additive.

[0027] On the other hand, the optical waveguide core 11 is made of LiNbO3 (lithium niobate), KNbO3 (potassium niobate), LiTaO3 (lithium tantalate), LiNb (x) Ta (1-x) O3 (0≦x≦1) (non-stoichiometric lithium tantalate), KNb (x) Ta (1-x) It is desirable that the substrate be a nonlinear optical crystal having a nonlinear optical effect, such as O3 (0≦x≦1) (potassium tantalate of nonstoichiometric composition) or KTiOPO4 (potassium titanyl phosphate). Therefore, it is desirable that the substrate material has a lower refractive index in the optical wavelength band to be used than the optical waveguide core, and a linear expansion coefficient close to that of the optical waveguide core.

[0028] The substrate 12 also has at least one groove 16 on a main surface 122, which is a second main surface corresponding to the back surface of the main surface 121. The groove 16 will be described below.

[0029] (Groove) The groove 16 relieves stress applied to the optical waveguide core 11 of the first embodiment. The principle behind this is explained below. To suppress changes in the optical characteristics of the optical waveguide core 11 due to temperature increases, the temperature control element 26 is cooled and absorbs heat generated in the optical waveguide core 11. At this time, a temperature difference occurs between the main surface 121 and the main surface 122 of the substrate 12. At the interface between the optical waveguide core 11, which is an optical nonlinear crystal, and the substrate 12, stress occurs at the bonding interface due to the difference in linear expansion coefficients. To reduce the generated stress, a material with a linear expansion coefficient close to that of the optical waveguide core 11 is used for the substrate 12. In this case, a relatively large internal stress is generated in the substrate 12 due to the temperature difference between the optical waveguide core 11 side of the substrate 12 and the temperature control element 26 side, and the linear expansion coefficient of the substrate 12. When the internal stress is transmitted to the optical waveguide core 11, the optical characteristics of the optical waveguide core 11 fluctuate.

[0030] In contrast, a wavelength conversion device 4 according to an embodiment of the present invention has a plurality of grooves 16 on a main surface 122 of a substrate 12. Fig. 5(a) is a view of the substrate 12 on which a plurality of grooves 16 are formed, viewed from the main surface 122 side, showing a top view of the grooves 16. In Fig. 5(a), the optical waveguide cores 11 formed on the main surface 121 are indicated by dashed lines. Fig. 5(b) is a cross-sectional view of the substrate 12 shown in Fig. 5(a) taken along the arrows B, B.

[0031] As shown in FIG. 5A, multiple grooves 16 are arranged along the light waveguiding direction indicated by the optical axis 111. As shown in FIG. 5A, the shape of the grooves 16 in a top view is symmetrical with respect to a line parallel to the optical axis 111 of the optical waveguide core 11. Also, the shape of the grooves 16 in a top view shown in FIG. 5B is rectangular. Rectangular grooves are not limited to having a structure having a length equal to the width of the main surface 122 of the substrate 12 (crossing the main surface 122). However, grooves 16 crossing the main surface 122 are desirable because they can be easily fabricated by cutting processing such as a dicing saw. For example, even if the substrate thickness is not uniform, it is possible to measure the thickness of the substrate 12 after processing the optical waveguide core, and change the spacing (density) and depth of the dicing saw blade cutting grooves to match the local thickness, thereby correcting and processing the substrate 12 to achieve consistent local stress relaxation.

[0032] 6(a) and 6(b) are diagrams illustrating how the grooves 16 relieve stress. FIG. 6(a) is a cross-sectional view of the wavelength conversion element 130, and FIG. 6(b) is an enlarged view of the area indicated by VIb in FIG. 6(a). P1, P2, P3, and P4 in FIG. 6(b) are locations where stress concentrates. According to the first embodiment in which the grooves 16 are formed, when distortion occurs on the main surface 122 of the substrate 12 due to expansion or contraction caused by a temperature difference, the shapes of the grooves 16 and the convex portions 17 present on both sides of the grooves 16 change. The stress acting on the substrate 12 is relieved by the change in shape of the grooves 16 and the convex portions 17, thereby reducing the stress itself transmitted from the substrate 12 to the optical waveguide core 11.

[0033] 6(b), when a member bonded to the rear surface of the substrate expands or contracts, the convex portions 17 deform themselves, thereby absorbing the distortion that accompanies the expansion or contraction. In the stress concentration portion and the convex portions 17 in FIG. 6(b), local stress concentration occurs, which makes it easier for the refractive index to fluctuate due to the photoelastic effect, but in other portions, the generation of internal stress is suppressed, making it difficult for local fluctuations in the effective refractive index of the optical waveguide to occur.

[0034] 6(b), when a member bonded to the main surface 122 of the substrate 12 expands or contracts, the convex portions 17 of the recessed grooves 16 deform themselves, thereby absorbing the strain that accompanies the expansion or contraction. Localized stress concentration occurs in the convex portions 17 at the stress concentration portions P1 and the like, which makes the refractive index more susceptible to fluctuations due to the photoelastic effect, but the generation of internal stress is suppressed in other portions. This makes it difficult for localized fluctuations in the effective refractive index of the optical waveguide core 11 to occur.

[0035] As described above, in the first embodiment, by forming a groove in the main surface 122 of the substrate 12, it is possible to reduce stress on the optical waveguide core 11 of the wavelength conversion element 130 and stabilize its optical characteristics. However, the first embodiment is not limited to the configuration described above. For example, the shape of the groove 16 is not limited to a rectangular groove in top view having a length equal to the width of the substrate 12 as shown in FIG. 6A, but may be any shape as long as it is line-symmetrical with respect to the optical axis 111 in top view.

[0036] 7(a) to 7(e) are diagrams illustrating other examples of grooves. FIG. 7(f) is a diagram illustrating an example of a groove that is not symmetrical with respect to the optical axis 111 when viewed from above. FIGS. 7(b) to 7(f) are top views of the substrate 12 as viewed from the main surface 121. FIG. 7(a) is a cross-sectional view common to FIGS. 7(b) to 7(f), including a cross-sectional view taken along arrows A1 and A1 in FIG. 7(b), a cross-sectional view taken along arrows A2 and A2 in FIG. 7(c), a cross-sectional view taken along arrows A3 and A3 in FIG. 7(d), a cross-sectional view taken along arrows A4 and A4 in FIG. 7(e), and a cross-sectional view taken along arrows A5 and A5 in FIG. 7(f). The reference numeral for the groove 16 in FIG. 7(a) corresponds to the groove 16 in FIG. 7(e).

[0037] As shown in Figures 7(a) to 7(e), a plurality of grooves may be arranged along the light waveguiding direction, and may be line-symmetrical with respect to a line parallel to the optical axis 111 (Figure 5(a)). Such a groove configuration can realize symmetric distortion or stress relaxation with respect to the light propagation direction of the optical waveguide core, and can realize a structure that does not generate twisting distortion or residual stress with respect to the light propagation direction of the optical waveguide core. Specifically, the following shapes are possible.

[0038] The groove 161 in Fig. 7(b) has a rectangular shape whose length in the waveguiding direction of the optical waveguide core 11 is longer than the direction perpendicular to the waveguiding direction. The groove 162 in Fig. 7(c) has an elliptical shape whose length is longer in the waveguiding direction of the optical waveguide core 11. The groove 162 in Fig. 7(d) has a rectangular shape that is longer in the width direction of the substrate 12, similar to the grooves 16 in Figs. 5(a) and 7(e), but is shorter than the groove 16. The grooves 161 to 163 are arranged in three rows along a straight line parallel to the optical axis of the optical waveguide core 11, and each row of the grooves 161, 162, and 163 satisfies the conditions that a plurality of grooves are arranged along the light waveguiding direction and are line-symmetrical with respect to a straight line parallel to the optical axis.

[0039] On the other hand, although the grooves 174 shown in Figure 7(f) are arranged in multiple rows along the waveguide direction, none of them are line-symmetrical along the optical axis when viewed from above. When the main surface 122 of the substrate 12 expands or contracts, the grooves 174 lose balance in the vertical strain relaxation shown in Figure 5(f), causing torsional strain, residual stress, or bending strain stress. When torsional strain, residual stress, or bending strain or stress occurs, even if the off-diagonal components of the refractive index tensor of the optical waveguide core are originally zero, the off-diagonal components of the refractive index tensor of the optical waveguide core become large, causing TE-TM mode conversion of the propagating light and local fluctuations (variations) in the effective refractive index of the optical waveguide, resulting in degradation of the optical characteristics of the wavelength conversion element.

[0040] As described above, the first embodiment satisfies the condition that a plurality of grooves are arranged along the light guide direction and are line-symmetrical with respect to a line parallel to the optical axis. The shape and size of each groove may be arbitrary. Furthermore, if residual stress does not affect the optical waveguide core, there is no need to form grooves on the entire back surface of the substrate.

[0041] [Second embodiment] A second wavelength conversion device is provided with a plurality of grooves 16, and the lengths (widths) of the grooves 16 along the waveguiding direction of the wavelength conversion element are the same, and the depths of the grooves 16 are different in at least some of the grooves.

[0042] (Dependence of each characteristic on groove depth) The depth of the groove in the second embodiment will be considered. The depth of the sidewall of the groove may be deep enough that it is not affected by the seepage (evanescent light) of the optical field of the signal light and control light propagating through the optical waveguide core. If the seepage depth at which the electric field strength of the evanescent light is 1 / e is d, the optical wavelength is λ, the refractive index of the medium on the incident side (core) is n1, the refractive index on the transmission side (substrate) is n2, and the incident angle is θ1, then this can generally be expressed by equation (1): d=λ / (2π((n1·sinθ1) 2 -n2 2 ) 1 / 2 )...Formula (1)

[0043] Since the optical intensity of evanescent light attenuates to 1 / e or less when the distance is approximately equal to the wavelength of the light propagating through the core, it is sufficient to leave a thickness approximately twice the wavelength of the signal light or control light. Therefore, if the substrate thickness is Lsub, the wavelength of the signal light is λs, and the wavelength of the control light is λp, then since λs > λp, it is possible to form grooves with a depth of up to Lsub - 2 × λp. In particular, in order to relieve stress caused by the temperature difference between the optical waveguide core and the temperature control element, it is desirable for the groove to be deeper. Furthermore, if the groove depth is shallower, the degree to which the groove itself can relieve stress by changing its shape decreases, while conversely, if the groove depth is deeper, the degree to which the groove itself can relieve stress by changing its shape increases.

[0044] FIG. 8( a) is a diagram illustrating the dependence of the characteristics of a wavelength conversion device on the depth of the grooves, showing a cross section along the optical axis of a wavelength conversion element 131 having multiple grooves 16 with different depths d. The wavelength conversion element 131 in FIG. 8( a) has grooves 16 on the main surface 122 of a substrate 12, the depth d of which gradually decreases along the light waveguide direction. The main surface 122 of the substrate 12 is adhesively fixed to a support member 27 with an adhesive material (not shown). The multiple grooves 16 with different depths all have a uniform width. Furthermore, the spacing between adjacent grooves 16 is also uniform. FIG. 8( b) is a diagram showing the relationship between the depth of the grooves 16 in FIG. 8( a) and their position on the substrate 12 (position on a straight line along the waveguide direction). The horizontal axis of FIG. 8( b) represents the position of the grooves 16 on the substrate, and the vertical axis represents the depth of the grooves 16.

[0045] FIG. 8(c) shows the difference in elastic modulus between the substrate 12 and the adhesive material on the horizontal axis, with the position of the groove 16 on the substrate on the vertical axis. The local difference in elastic modulus between the substrate 12 and the adhesive material is averaged out by thermal stress, and as shown in FIG. 8(c), tends to decrease as the depth of the groove 16 increases. That is, in the second embodiment, by adjusting the depth of the groove 16, the elastic modulus of the substrate 12 can be made closer to that of the bonding agent. Furthermore, the inventors have found that the differences in not only the elastic modulus but also the thermal conductivity, linear expansion coefficient, and other material property values ​​between the adhesive material and the substrate 12 have the same depth dependency as in FIG. 8(c). Therefore, in the second embodiment, even if the grooves have the same width, the degree of stress relaxation can be adjusted locally by locally adjusting the depth of the grooves 16. Therefore, according to the second embodiment, a wavelength conversion element can be provided in which the depth of the grooves 16 is increased in areas where strong local stress is applied.

[0046] [Third embodiment] The wavelength conversion device of the third embodiment is provided with a plurality of grooves 16, and the lengths (widths) of the plurality of grooves 16 along the wave-guiding direction of the wavelength conversion element 130 are the same, and the intervals between adjacent grooves among the plurality of grooves are different in at least some of the plurality of grooves.

[0047] (Dependence of Characteristics on Groove Density) FIG. 9( a) is a diagram for explaining the dependence of the characteristics of a wavelength conversion device on the groove density, showing a cross section along the optical axis of a wavelength conversion element 132 having multiple grooves 16 with different spacings w between adjacent grooves 16. Note that in the third embodiment, w is expressed, for example, as the distance from the descending edge of one groove 16 to the descending edge of an adjacent groove 16. The wavelength conversion element 132 in FIG. 9( a) has multiple grooves 16 on the main surface 122 of the substrate 12, the spacing w of which increases stepwise along the light waveguide direction. The main surface 122 of the substrate 12 is adhesively fixed to the support member 27 with an adhesive material (not shown). The multiple grooves 16 with different densities have a constant width, but may have a constant or different depth. FIG. 9( b) is a diagram showing the relationship between the spacing w of the grooves 16 in FIG. 9( a) and their position on the substrate 12 (position on a straight line along the waveguide direction). In FIG. 9B, the horizontal axis represents the position of the grooves 16 on the substrate, and the vertical axis represents the interval w between the grooves 16 .

[0048] In Figure 9(c), the horizontal axis represents the position of the grooves 16 on the substrate, and the vertical axis represents the difference in elastic modulus between the substrate 12 and the adhesive material. The local difference in elastic modulus between the substrate 12 and the adhesive material is averaged out by thermal stress, and as shown in Figure 9(c), the difference tends to decrease as the spacing between the grooves 16 decreases, i.e., as the density of the grooves 16 increases. In other words, in the third embodiment, the elastic modulus of the substrate 12 can be made closer to that of the bonding agent by adjusting the density of the grooves 16. Furthermore, the inventors have found that the differences in the material properties between the adhesive material and the substrate 12, such as the thermal conductivity and linear expansion coefficient, as well as the elastic modulus, have the same density dependency as in Figure 9(c). Therefore, in the third embodiment, even for grooves of the same width, the degree of stress relaxation can be adjusted locally by locally adjusting the density of the grooves.

[0049] 10 is a diagram illustrating stresses associated with a wavelength conversion device including a wavelength conversion element 133 based on the technical concept of the third embodiment. The stresses corresponding to the temperature difference between the optical waveguide core 11 and the temperature control element 26 and the strains caused by the difference in the linear expansion coefficients of the components mainly cause strain in the horizontal direction parallel to the optical waveguide core 11. Therefore, as shown in FIG. 10 , in order to alleviate the stress caused by the strain in the horizontal direction, it is preferable to disperse and absorb the strain by deforming the convex portions 17 on both sides of more grooves than to disperse and absorb the stress caused by a single groove 16 formed in the substrate 12. In this way, the stress relaxation by the grooves 16 is also dispersed, thereby alleviating local stress concentrations and reducing the impact of stress on the optical waveguide core 11.

[0050] That is, as shown in Fig. 10, it is desirable to form a concave structure with high density in the substrate portion where distortion occurs due to the difference in linear expansion coefficient of each member and the resulting large local stress occurs due to the mounting structure, the structure of the heating portion, the structure of the cooling portion, etc. In the example shown in Fig. 10, the concave grooves 16 are formed with high density near the center of the substrate 12 in the drawing, that is, in the portion where the wavelength conversion element 133 is fixed to the temperature control element 26.

[0051] Next, a wavelength converter according to an embodiment of the present disclosure will be described. The configuration of the wavelength converter according to this embodiment is similar to that of the wavelength converter 4 shown in FIG. 4, and therefore will not be illustrated or described here.

[0052] In this example, a fundamental wave with a wavelength of 1550 nm is input to the wavelength conversion element as signal light 1a, and difference frequency light 1c having a wavelength (775 nm) which is the second harmonic of signal light 1a is output. In this example, an alignment device is used to control the spatial position at which signal light 1a and control light 1b output from the multiplexer 14 are optically coupled to the optical waveguide core 11, thereby maximizing the optical coupling rate. In addition, an alignment device is used to control output light 1f output from the optical waveguide core 11 so that it is optically coupled to the demultiplexer 15. The alignment device may be an optical fiber, or may be a spatial optical system alignment device that controls the position of a reflecting mirror or a focusing lens.

[0053] Typically, to use it as a wavelength conversion device, an alignment device is set so that the light transmittance is maximized at the intended temperature, and the device is fixed by YAG welding, adhesive bonding, screw fastening, etc. In this embodiment, the wavelength conversion element 130 is formed by laminating a LiNbO3 substrate with a comb-shaped electrode structure formed in advance using Au and a periodic polarization structure formed at a high voltage of approximately 1000 V, with the Z axis perpendicular to the substrate, on a LiTaO3 substrate. After thin film polishing, a ridge-shaped optical waveguide core 11 is fabricated on the surface of the substrate 12 by dry etching using Ar plasma. The end faces of the optical waveguide core 11 are cut with a dicing saw, and anti-reflective coatings for the signal light 1a and the control light 1b are formed on the end faces using a metal multilayer film or the like. Then, a groove 16 of a certain depth is cut on the back surface of the substrate 12 in a direction perpendicular to the propagation direction of the optical waveguide core 11 using a dicing saw, thereby forming the wavelength conversion element 130.

[0054] The wavelength conversion element 130 fabricated as described above was fixed onto a copper support member 27. Furthermore, a temperature control element 26 was placed on the surface of the bottom surface 28 of a brass metal housing, and the support member 27 and the temperature control element 26, and the temperature control element 26 and the bottom surface were fixed together with a bonding agent 23. The bonding agent 23 was a silver paste resin made of a thermosetting epoxy resin filled with silver filler. The effect of the bonding agent 23 was achieved by heating and curing at 110°C.

[0055] For comparison, a flat wavelength conversion element without grooves 16 formed on the back surface of the LiTaO3 substrate was fabricated and designated module number 1. In addition, a wavelength conversion element was fabricated in which grooves 16 with a width of approximately 200 μm, a depth of 200 μm, and intervals of 1 mm were formed by cutting using a dicing saw in the direction perpendicular to the linear optical waveguide on the entire back surface of the LiTaO3 substrate and designated module number 2.

[0056] Thereafter, for module 1 and module 2, a contact thermometer or a radiation thermometer was used to confirm that the wavelength conversion element 130 had reached the intended temperature of 65°C. The alignment device was then controlled to fix the position so that the light transmittance was maximized. Then, the current of the temperature control element 26 for both module 1 and module 2 was adjusted, and the set temperature of the Peltier element serving as the temperature control element 26 was changed to 35°C. As a result, for both module 1 and module 2, the wavelength of the second harmonic wave shifted toward shorter wavelengths as the temperature decreased. However, the decrease in the optical output intensity of the second harmonic wave of the difference frequency light 1c for module 1 was approximately 1% greater than the optical intensity for module 2. Furthermore, when the set temperature of the Peltier element was changed to 95°C, the wavelength of the second harmonic wave shifted toward shorter wavelengths as the temperature decreased for both module 1 and module 2. It was also confirmed that the optical output intensity of the second harmonic wave of the difference frequency light 1c of module number 1 was reduced by about 1% compared to the optical intensity of module number 2.

[0057] REFERENCE SIGNS LIST 1a signal light 1b control light 1c difference frequency light 1e combined light 1f output light 4 wavelength conversion device 11 optical waveguide core 12 substrate 13, 130, 131, 132, 133 wavelength conversion element 14 multiplexer 15 demultiplexer 16, 161, 162, 163, 174 groove 17 protrusion 23 bonding agent 26 temperature control element 27 support member 28 metal housing bottom surface 111 optical axis 121, 122 principal surface

Claims

1. A wavelength conversion device that receives signal light as input and outputs output light having a wavelength different from that of the signal light, comprising: a wavelength conversion element including an optical waveguide core and a substrate whose first main surface is in contact with the optical waveguide core; and a temperature control element that controls the temperature of the wavelength conversion element, wherein the substrate has at least one groove on a second main surface corresponding to the back surface of the first main surface.

2. The wavelength conversion device according to claim 1, wherein a plurality of said recessed grooves are arranged along the optical waveguiding direction of said optical waveguide core when viewed from above.

3. A wavelength conversion device according to claim 1, wherein the shape of the groove as viewed from above is line-symmetric with respect to a line parallel to the optical axis of the optical waveguide core.

4. The wavelength conversion device according to claim 1, further comprising a support member for supporting said wavelength conversion element between said second main surface of said substrate and said temperature control element.

5. A wavelength conversion device according to claim 1, further comprising: a multiplexer for multiplexing said signal light and control light; and a demultiplexer for separating said output light into light having a wavelength different from that of said signal light and said control light.

6. A wavelength conversion device as described in claim 4, which is housed in a metal housing, the temperature control element is provided between the third surface of the support member and the bottom surface of the metal housing, and the wavelength conversion element is provided on a fourth surface corresponding to the back side of the third surface of the support member.

7. The wavelength conversion device according to claim 1, wherein the temperature control element is a Peltier element.

8. The substrate has a lower refractive index for the signal light than the optical waveguide core, and is made of LiNbO3 (lithium niobate), KNbO3 (potassium niobate), LiTaO3 (lithium tantalate), LiNb (x) Ta (1-x) O3 (0≦x≦1) (non-stoichiometric lithium tantalate), KNb (x) Ta (1-x) 2. The wavelength conversion device according to claim 1, comprising at least one of O3 (0≦x≦1) (non-stoichiometric potassium tantalate) and KTiOPO4 (potassium titanate phosphate), and containing at least one selected from Mg (magnesium), Zn (zinc), Sc (scandium), and In (indium) as an additive.

9. A wavelength conversion device according to claim 1, wherein a plurality of the grooves are provided, the lengths of the plurality of grooves along the wave-guiding direction of the wavelength conversion element are the same, and the depths of the plurality of grooves are different in at least some of the plurality of grooves.

10. A wavelength conversion device as described in claim 1, wherein a plurality of the grooves are provided, the lengths of the plurality of grooves along the wave-guiding direction of the wavelength conversion element are the same, and the intervals between adjacent grooves among the plurality of grooves are different in at least some of the plurality of grooves.

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