Wavelength conversion device

Abstract

Provided is a wavelength conversion device that prevents variation in optical characteristics of an element due to an electric field, by suppressing charges which are generated on the surface of the element. A wavelength conversion element according to one embodiment of the present disclosure generates light of a wavelength different from the wavelength of signal light when such signal light is inputted, the device comprising an optical waveguide core, a substrate having a refractive index lower than that of the optical waveguide core with respect to signal light, and a conductive layer having electrical conduction properties and arranged in contact with at least a part of the surface of the optical waveguide core.

Description

[Technical field]

[0001] The present disclosure relates to a wavelength conversion device, and more particularly to an optical waveguide structure of a wavelength conversion element of a wavelength conversion device. [Background technology]

[0002] Optical waveguides using periodically poled structures such as periodically poled lithium niobate (hereinafter referred to as PPLN) have been attracting attention as waveguides that play an important role in the fields of next-generation optical fiber communications and quantum computing. PPLN waveguides are elements that can realize high wavelength conversion efficiency by increasing the optical intensity and using quasi-phase matching technology. For example, it is known to fabricate ridge-type optical waveguides using nonlinear optical crystals with periodically poled structures (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 3753236 Summary of the Invention

[0004] Generally, ferroelectric crystals or crystals lacking a center of symmetry have properties such as piezoelectricity and pyroelectricity, and thus spontaneous polarization occurs in elements containing the crystals when they are influenced by the external environment. At this time, ions of the opposite polarity to the spontaneous polarization that exist around the element are collected, and an electric charge is generated on the surface of the element, generating a demagnetizing field opposite to the spontaneous polarization. A large demagnetizing field fluctuates the optical characteristics of the element, such as the wavelength and light intensity. It is desirable to prevent the generation of electric charges on the surface of the element, thereby preventing the optical characteristics of the element from fluctuating due to the electric field.

[0005] In order to achieve this objective, a wavelength conversion device according to one embodiment of the present disclosure is a wavelength conversion device that generates light of a wavelength different from the wavelength of a signal light when the signal light is input, and includes an optical waveguide core, a substrate having a refractive index for the signal light lower than the refractive index of the optical waveguide core, and a conductive layer having electrical conductivity arranged in contact with at least a portion of the surface of the optical waveguide core. [Brief description of the drawings]

[0006] [Figure 1] FIG. 1 is a diagram showing a basic configuration of a wavelength converter. [Diagram 2] FIG. 2 is a diagram showing the basic structure of a wavelength converter housed in a metal housing. [Diagram 3] FIG. 3 is a diagram showing a basic structure of a wavelength converter housed in a metal housing. [Figure 4] FIG. 4 is a diagram showing a cross-sectional structure of a modified example of the first embodiment of the present disclosure. [Diagram 5] FIG. 5 is a diagram showing a cross-sectional structure of a wavelength conversion element according to the second embodiment of the present disclosure. [Figure 6] FIG. 6 is a diagram showing a cross-sectional structure of a modified example of the second embodiment of the present disclosure. [Figure 7] FIG. 7 is a diagram showing a cross-sectional structure of a wavelength conversion element according to the third embodiment of the present disclosure. [Figure 8] FIG. 8 is a diagram showing a cross-sectional structure of a modified example of the third embodiment of the present disclosure. [Figure 9] FIG. 9 is a diagram showing a cross-sectional structure of a wavelength conversion element according to the fourth embodiment of the present disclosure. [Figure 10] FIG. 10 is a diagram showing a cross-sectional structure of a modified example of the fourth embodiment of the present disclosure. [Figure 11] FIG. 11 is a diagram illustrating a configuration of a wavelength converter according to the first embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0007] Hereinafter, the embodiments of the present disclosure will be described in detail with reference to the drawings. The same or similar reference symbols indicate the same or similar elements, and repeated explanations may be omitted. In addition, the material names, compositions, or numerical values ​​are merely examples, and it goes without saying that other material names, compositions, or numerical values ​​may be used without departing from the gist of the present disclosure.

[0008] Prior to describing the embodiments, an optical waveguide using PPLN will be described, and then various embodiments will be described.

[0009] (Optical waveguide using PPLN) Optical waveguides using PPLN are used as parametric amplifiers and pump light generators in phase sensitive amplifiers (PSAs) that can amplify light with low noise, and have achieved high gain and low noise optical amplification characteristics. In the field of quantum computing, it has been reported that an optical coherent Ising machine device has been realized by inserting an optical waveguide using PPLN into a fiber ring resonator and using it as a parametric oscillator, and that it has been demonstrated to perform large-capacity calculations at extremely high speeds compared to conventional computers. In order to further improve the performance of these technologies, it is important to realize wavelength conversion devices with higher wavelength conversion efficiency.

[0010] (Quasi-phase matching explanation) FIG. 1 is a diagram showing the basic configuration of a conventional wavelength converter that generates a difference frequency by a quasi-phase matching (hereinafter referred to as QPM) technique. The configuration of a wavelength converter 10 in FIG. 1 is disclosed in Patent Document 1. As shown in FIG. 1, in the wavelength converter 10, a signal light 1a having a low optical intensity and a control light 1b having a high optical intensity are input to a multiplexer 14 and multiplexed. The signal light 1a multiplexed with the control light 1b proceeds toward a wavelength conversion element 13 including a substrate 12 and an optical waveguide core 11 arranged on the substrate 12. The signal light 1a is input to one end of the optical waveguide core 11 that has a periodically poled structure and exhibits a nonlinear optical effect. When passing through the optical waveguide core 11, the signal light 1a is converted into a difference frequency light 1c having a wavelength different from that of the signal light 1a, and is output from the other end of the optical waveguide core 11 together with the control light 1b. The difference frequency light 1c and the control light 1b outputted from the optical waveguide core 11 enter the demultiplexer 15 and are separated from each other.

[0011] In this case, by using a wavelength conversion element 13 having a periodic polarization inversion structure in which the polarization direction of a ferroelectric crystal or a crystal lacking a center of symmetry is periodically inverted by 180° and realizing QPM using the periodic polarization inversion structure, it is possible to generate second harmonic generation (hereinafter referred to as SHG) or optical parametric oscillation in the wavelength conversion device 10.

[0012] Generally, the refractive index of nonlinear optical crystals has wavelength dispersion, so the speed of the fundamental wave and the second harmonic are not equal, resulting in a phase difference. For this reason, the composite wave of second harmonics generated along the optical path within the crystal is a periodic function. The second harmonics generated at each point within the crystal propagate with a phase shift between each harmonic, and the phase difference between the generated second harmonic and the second harmonic generated at a distance called the coherent length Lc is π. The intensity of the composite harmonic decreases when the coherent length Lc is exceeded. The intensity of the composite harmonic increases and decreases repeatedly during this period. Quasi-phase matching (QPM) is a method of inverting the phase of the polarization wave generated from the optical nonlinear material every period, i.e., inverting the sign of the nonlinear optical constant d.

[0013] In this case, if the period of periodic polarization inversion, called the QPM condition, is doubled to the coherent length Lc, the phase of the second harmonic generated at a distance of the coherent length Lc is inverted, and the phase of the composite second harmonic from the coherent length Lc is corrected. As a result, the optical intensity of the second harmonic generated in the nonlinear optical crystal is added without decreasing, and the amplitude (intensity) of the second harmonic increases, resulting in the generation of second harmonic light. This feature allows the maximum component of the nonlinear optical constant to be used, and can also be used in crystals with small birefringence.

[0014] In optical difference frequency generation (DFG), the refractive index of the wavelength conversion element at wavelength λ3 is n3, the refractive index at wavelength λ2 is n2, the refractive index at wavelength λ1 is n1, the polarization inversion period is Λ, and the coherent length is Lc. When formulas 1 and 2 hold, the polarization waves generated from the nonlinear optical crystal are amplified. In other words, formulas 1 and 2 are the phase matching conditions for quasi-phase matching (QPM). Λ = 2 Lc (Eq. 1) n3 / λ3-n2 / λ2-n1 / λ1-1 / Λ=0 (Formula 2)

[0015] Unlike the angle matching method, this quasi-phase matching method can use the material orientation that produces the maximum component of the nonlinear susceptibility of second-order nonlinear crystals, etc., and has the advantage that the operating wavelength range can be set by selecting the inversion period.In addition, by making it into an optical waveguide, light can be confined densely in a narrow area and propagated over long distances, so highly efficient wavelength conversion has been achieved to date.

[0016] (QPM optical waveguide creation method) There are also several known methods for fabricating optical elements (hereinafter referred to as wavelength conversion elements) that perform wavelength conversion using quasi-phase matching technology. For example, a method is to make a crystal (hereinafter referred to as nonlinear optical crystal) substrate that exhibits a nonlinear optical effect into a periodically poled structure, and then to fabricate a proton exchange waveguide using the periodically poled structure. Similarly, for example, a method is to make a nonlinear optical crystal substrate into a periodically poled structure, and then to fabricate a ridge-type optical waveguide using a photolithography process and a dry etching process.

[0017] The above-mentioned Patent Document 1 discloses an example of fabricating a ridge-type optical waveguide. Patent Document 1 describes fabricating a wavelength conversion element by bonding a first substrate of a nonlinear optical crystal having a periodic polarization inversion structure and a second substrate having a refractive index smaller than that of the first substrate in order to improve the light confinement effect in the ridge-type optical waveguide. Patent Document 1 also describes using the same type of nonlinear optical crystal as the first substrate as the second substrate, and applying heat to the first substrate and the second substrate to perform diffusion bonding in order to avoid cracks due to deterioration of the adhesive or temperature changes.

[0018] (Wavelength conversion device mounting structure) In practical terms, it is known that wavelength conversion devices equipped with wavelength conversion elements are used by storing the wavelength conversion element together with a multiplexer and a demultiplexer in a metal case equipped with an input / output port capable of inputting and outputting light, so that the characteristics do not deteriorate due to changes in the usage environment. Furthermore, the wavelength conversion efficiency of the wavelength conversion element has temperature dependency, and it is necessary to control the temperature of the wavelength conversion element in order to maximize the wavelength conversion efficiency.

[0019] FIG. 2 is a diagram showing an example of the configuration of a wavelength converter 20 further including a metal housing bottom member 28 and a temperature control element 26 in addition to the configuration of the wavelength converter in FIG. 1. The configuration of the wavelength converter shown in FIG. 2 further includes a metal housing bottom member 28, a temperature control element 26, and a support member 27 for fixing the temperature control element 26, in addition to the configuration shown in FIG. 1. The support member 27 is a metal member for uniformly controlling the temperature of the entire wavelength conversion element 13 including the optical waveguide core 11 and the substrate 12. The temperature control element 26 is interposed between the support member 27 and the metal housing bottom member 28, and is bonded and fixed by the support member 27 that allows heat conduction between the temperature control element 26, the support member 27, and the metal housing bottom member 28, and is difficult to move in the fixed position. Note that the optical waveguide core 11, the substrate 12, the wavelength conversion element 13, the multiplexer 14, the demultiplexer 15, the signal light 1a, and the difference frequency light 1c are the same as those in the description of FIG. 1 above, and therefore will not be described. The wavelength converter 20 may include a metal housing 29 with an input port 200 and an output port 201 on the side thereof.

[0020] In addition, when a wavelength conversion element using a ferroelectric crystal material is used in a wavelength conversion device, a phenomenon called optical damage occurs in which the refractive index of the wavelength conversion element changes due to irradiation with light having a short wavelength, causing the characteristics to deteriorate. As a method of suppressing the effects of this optical damage, it has been proposed to use the wavelength conversion element at high temperatures.

[0021] (Issues of spontaneous polarization and the counter electric field caused by the generation of ions) However, the wavelength conversion element described above generally uses a ferroelectric crystal or a crystal lacking a center of symmetry, and thus has properties such as piezoelectricity and pyroelectricity, and generates spontaneous polarization in the wavelength conversion element itself due to the influence of the external environment, etc. At this time, ions of the opposite polarity to the spontaneous polarization that exist around the wavelength conversion element are collected, and an electric charge is charged on the surface of the wavelength conversion element, generating an anti-electric field opposite to the spontaneous polarization.

[0022] Although the effect of a small demagnetizing field is small, a large demagnetizing field affects adjacent polarization domains in the polarization inversion structure of the wavelength conversion element and changes the phase and intensity of the polarization wave depending on the field strength. Such a large demagnetizing field changes the wavelength and light intensity of the generated difference frequency light and second harmonic wave.

[0023] For example, in optical modulators using optical nonlinear crystals, the electric field generated by the counter electric field can cause fluctuations in the applied voltage value corresponding to the optical modulation period (commonly known as the DC drift phenomenon).

[0024] Furthermore, when the temperature of the wavelength conversion element is changed to change the magnitude of the spontaneous polarization, the reversal electric field once generated affects the wavelength conversion element so as to resist the change.

[0025] For example, even if the temperature of the wavelength conversion element is changed, due to the influence of the counter electric field, the change in the magnitude of the spontaneous polarization does not quickly follow the change in temperature of the wavelength conversion element itself, and the wavelength conversion element exhibits hysteresis-like behavior in which the element characteristics change slightly after the change in temperature.

[0026] The occurrence (change) of the spontaneous polarization as described above and the accompanying generation of an anti-electric field occur in the following situations.

[0027] (Issue 1: Changes in the wavelength of wavelength-converted light due to surface charges generated on the optical waveguide core, which has pyroelectric properties, caused by temperature changes such as changes in the operating temperature) When changing or fine-tuning the wavelength of light generated by a wavelength conversion element, the temperature of the wavelength conversion element is changed to vary the refractive index by utilizing the temperature change in the refractive index, thereby changing the (quasi-)phase matching condition associated with the relationship in (Equation 2), and changing and adjusting the wavelength of light generated.

[0028] However, when the temperature is changed as described above, since wavelength conversion elements generally use ferroelectric crystals or crystals lacking a center of symmetry, pyroelectricity generates a surface potential due to a change in spontaneous polarization, which in turn generates an anti-electric field.

[0029] This occurs not only due to optical wavelength adjustment but also due to fluctuations in environmental temperature, such as temperature changes due to daytime or seasons. Therefore, a large temperature change in the wavelength conversion element generates a correspondingly large surface potential and counter electric field.

[0030] For example, if the operating temperature of the wavelength conversion element is higher than room temperature, the wavelength conversion element may be affected by the reversal electric field associated with the surface charge unless the temperature is raised slowly enough to allow the charge of ions, etc. that causes the reversal electric field to discharge naturally.

[0031] (Issue 2: A sudden change in the intensity of the incident light causes light absorption in the optical waveguide, which generates a surface charge on the optical waveguide core that has pyroelectricity due to temperature absorption, resulting in a change in the wavelength of the wavelength-converted light.) Furthermore, when wavelength conversion of signal light is realized, high-intensity control light is input to effectively convert the wavelength, but if there is even a slight amount of light absorption of the high-intensity control light, a part of the high-intensity control light is absorbed and converted into heat. Then, similar to the generation of a demagnetizing field caused by pyroelectricity due to temperature change, a high-intensity light input also generates a large demagnetizing field in the wavelength conversion element itself.

[0032] (Issue 3: Changes in optical characteristics due to electric charges generated in the optical waveguide core due to piezoelectricity caused by thermal stress from the mounted components of the wavelength conversion device, caused by changes in the wavelength conversion element or environmental temperature, or an increase in the input light intensity) Another issue is that second-order nonlinear optical crystals, such as LiNbO3 (lithium niobate), KNbO3 (potassium niobate), LiTaO3 (lithium tantalate), and KTiOPO4 (potassium titanyl phosphate), are known to have a relatively large thermal expansion coefficient, although this depends on the crystal orientation.

[0033] For example, periodically poled lithium niobate (PPLN), like lithium niobate crystal (LiNbO3), has a relatively large thermal expansion coefficient, with a coefficient of approximately 7.5 ppm / °C in the Z-axis (a-axis) direction and approximately 15.4 ppm / °C in the X-axis and Y-axis (c-axis) directions at room temperature (25°C), and an even higher coefficient of thermal expansion at around 300°C.

[0034] For example, assume that a nonlinear optical crystal is used for the optical waveguide core 11 or the substrate 12 of the wavelength conversion element 13 shown in FIG. 2, and a metal material is used for the support member 27. The thermal expansion coefficient value of copper (Cu) as a metal material is about 16.8 ppm / °C, which is quite close to the thermal expansion coefficient value of the above-mentioned nonlinear optical crystal. However, the thermal expansion coefficient value of about 30.2 ppm / °C of zinc (Zn), about 26.9 ppm / °C of tin (Sn), about 23 ppm / °C of aluminum (Al) as a metal material is larger than that of lithium niobate, and the thermal expansion coefficient value of about 14.3 ppm / °C of gold (Au), about 12.1 ppm / °C of iron (Fe), about 8.5 ppm / °C of titanium (Ti), about 13.3 ppm / °C of nickel (Ni), about 2.8 to 7.3 ppm / °C of silicon (Si) as a metal material is smaller than that of lithium niobate. Thus, even with a uniform temperature distribution, thermal stress will occur between the optical waveguide core 11 or substrate 12 and the support member 27 due to the temperature difference between the temperature during mounting and bonding (adhesive fixing) and the operating temperature.

[0035] Furthermore, various materials with different thermal expansion coefficients are used for the mounting components of the wavelength conversion device 20. Therefore, when a temperature change occurs due to a change in the wavelength conversion element 13 or the environmental temperature, or an increase in the input light intensity, thermal stress due to the temperature change occurs naturally between the mounting components of the wavelength conversion device 20. Of course, thermal stress also occurs between the wavelength conversion element 13 and the support member 27, so the spontaneous polarization fluctuates due to the thermal stress on the wavelength conversion element 13 and the piezoelectricity of the wavelength conversion element itself, and an anti-electric field is generated accordingly.

[0036] (Issue 4: Effect of induced charges due to electric field distribution from outside the optical waveguide core of wavelength conversion elements such as Peltier elements) As shown in FIG. 2, the wavelength conversion element is often used simultaneously with a temperature control element 16 such as a Peltier element or a heater, and an electronic thermometer such as a thermistor, in order to stabilize the optical characteristics. Here, the Peltier element is an element that utilizes the phenomenon of creating a temperature difference from the voltage of 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 absorption occurs on one side and heat is generated on the other side at the same time. However, on the other hand, due to the influence of the leakage electric field generated by the temperature control element, etc., a potential is generated on the surface of the wavelength conversion element 13, and induced charge ions are generated on the surface due to this. This may cause a potential similar to the counter electric field to occur. In addition, if the wavelength conversion element 13 is covered with a conductive metal housing 29, etc., and the potential of the metal housing 29 is not the ground potential, a potential similar to the counter electric field may also be generated due to the influence of static electricity, etc.

[0037] (Issue 5: Attraction of dust and other particles due to electrostatic charge, and its effect on localized heating and photo-induced burnout) When an electric charge is generated on the surface of the wavelength conversion element 13 and an anti-electric field is generated, foreign matter such as dust is attracted to the charged electric charge, and the foreign matter such as dirt accumulates on the surface of the optical waveguide core 11 of the wavelength conversion element 13. Therefore, when a control light with a high optical intensity is propagated through the optical waveguide core 11, the leaking light of the control light with a high optical intensity is absorbed by the accumulated foreign matter such as dirt, causing problems such as localized heat generation and burning of the wavelength conversion element itself.

[0038] Hereinafter, an embodiment of a wavelength conversion element that prevents the generation of charges on the surface of the element, thereby preventing the optical characteristics of the element from fluctuating due to an electric field, will be described. The wavelength conversion element according to various embodiments of the present disclosure described below generally includes a substrate, an optical waveguide core formed on the substrate, and an electrically conductive layer having electrical conductivity arranged in contact with at least a part of the surface of the optical waveguide core. The wavelength conversion element according to various embodiments can be used by being adhesively fixed to a support member 27 in place of the wavelength conversion element 13 in the configuration of the wavelength conversion device 20 shown in FIG. 2.

[0039] (First embodiment) (Structure in which the optical waveguide core and the surface of the substrate are covered with a conductive layer) Fig. 3 is a cross-sectional schematic diagram showing a wavelength conversion element according to a first embodiment of the present disclosure. The wavelength conversion element shown in Fig. 3 includes a substrate, an optical waveguide core formed on the substrate, and a conductive layer having electrical conductivity formed adjacent to the surfaces of the substrate and the optical waveguide core. The substrate has a refractive index lower than that of the optical waveguide core for the signal light.

[0040] In the configuration of a wavelength conversion element 313a shown in FIG. 3, an optical waveguide core formed on a substrate 312 is shown as a PPLN optical waveguide core 311, and corresponds to the optical waveguide core 11 formed on the substrate 12 in FIG.

[0041] 3, the conductive layer having electrical conductivity is shown as a conductive transparent film 350. In this embodiment, the conductive transparent film 350 is provided adjacent to both sides of the PPLN optical waveguide core 311 on the surfaces of the PPLN optical waveguide core 311 and the substrate 312, that is, on both the upper surface and two side surfaces of the PPLN optical waveguide core 311, and on the upper surface of the substrate 312 facing the lower surface of the PPLN optical waveguide core 311. The conductive transparent film 350 is not provided on the lower surface (the surface facing the upper surface of the substrate 312) of the surfaces of the PPLN optical waveguide core 311. The conductive transparent film 350 may also be provided between the lower surface of the PPLN optical waveguide core 311 and the upper surface of the substrate 312. The conductive transparent film 350 may not be provided on the upper surface of the PPLN optical waveguide core 311.

[0042] In this embodiment, the conductive transparent film 350 is provided on the upper surface of the surface of the substrate 312. More specifically, the conductive transparent film 350 is provided adjacent to both sides of the PPLN optical waveguide core 311 on the upper surface of the substrate 312. The conductive transparent film 350 is not provided on a portion of the upper surface of the substrate 312 that corresponds to the lower surface of the PPLN optical waveguide core 311. The conductive transparent film 350 covering the upper and side surfaces of the PPLN optical waveguide core 311 and the conductive transparent film 350 covering the upper surface of the substrate 312 are continuous so that the electric charge generated on the surface of the PPLN optical waveguide core 311 can be moved to the outside.

[0043] The electrically conductive layer (conductive transparent film 350) does not need to cover the entire surface of the PPLN optical waveguide core 311 formed on the substrate 312, but only needs to be able to remove the anti-electric field (electrostatic charge) generated on the surface of the PPLN optical waveguide core 311.

[0044] As described above, when an optical waveguide core is constructed using a ferroelectric crystal or a crystal lacking a center of symmetry, the polarization direction of light due to difference frequency generation or second harmonic generation in the wavelength conversion element is generally perpendicular or parallel to the substrate. In this case, the polarization inversion direction of quasi-phase matching is perpendicular or parallel to the substrate. Therefore, in order to reduce the charging and depolarization field generated in the polarization direction of the wavelength conversion element 313a, it is sufficient to remove the depolarization field due to charging generated on the surface of the PPLN optical waveguide core 311 by using a conductive layer (conductive transparent film 350) to move charges of both polarities of the polarization to the outside, or by using a conductive layer to move charges between adjacent polarization domains with opposite polarities.

[0045] FIG. 4 is a cross-sectional schematic diagram for explaining a modified example of the first embodiment of the present disclosure. For example, the wavelength conversion element 313b shown in FIG. 4 includes a conductive transparent film 350 provided adjacent to the upper surface and one side surface of the PPLN optical waveguide core 311. The wavelength conversion element 313b also includes a conductive transparent film 350 provided adjacent to both sides of the PPLN optical waveguide core 311 on the upper surface of the substrate 312. The conductive transparent film 350 covering the right side of the upper surface of the substrate 312 is continuous with the conductive transparent film 350 covering the upper surface and side surface of the PPLN optical waveguide core 311. The conductive transparent film 350 covering the left upper surface of the substrate 312 is in contact with the left side surface of the PPLN optical waveguide core 311. Even if the conductive transparent film 350 covers a part of the side surface and the upper surface of the PPLN optical waveguide core 311 in this way, the same effect as the effect of the structure of FIG. 3 can be obtained.

[0046] However, if the conductive transparent film 350 is formed only on adjacent polarization domains, the removal of the charge and the counter electric field may be localized and may not be sufficient. As a result, the charge may remain locally on the surface of the PPLN optical waveguide core 311. Therefore, it is desirable to provide a conductive layer with a large area that can release the charge on the surface of the PPLN optical waveguide core 311 over as much of the wavelength conversion element as possible. For example, it is desirable to form the conductive transparent film 350 on the entire surface of the PPLN optical waveguide core 311 or a larger portion of the surface.

[0047] The wavelength conversion element 313a shown in FIG. 3 and the wavelength conversion element 313b shown in FIG. 4, together with the temperature control element 26, constitute a wavelength conversion device, as described with reference to FIG.

[0048] (Conductive layer material) The conductive layer (conductive transparent film 350) only needs to have electrical conductivity that can conduct electrons or ions and cancel the reversal electric field generated in the wavelength conversion element. Therefore, the material of the conductive layer does not need to be a material with excellent electrical conductivity, and may be an inorganic oxide having semiconductor properties, a metal material, a conductive organic material, or the like, which is lightly doped into an insulator to provide electrical conductivity.

[0049] (Metal materials as conductive layers) Materials with good electrical conductivity that can be used for the conductive layer include metal materials with metallic bonds. Materials that can be used for the conductive layer include the following materials. In addition to precious metals such as Au, Pt, Au-Pd, and Pt-Pd, various metal materials such as Al, Ag, and Cu can be used. However, if a material that oxidizes in the air and loses electrical conductivity or a material that has high light absorption in the wavelength band of the light used is used, the light absorption will be large and the optical properties of the wavelength conversion element may be significantly lost.

[0050] (Conductive inorganic oxide materials as conductive layers) Furthermore, a conductive inorganic oxide can be used as the conductive layer (conductive transparent film 350) in one embodiment of the present disclosure. For example, In2O3-based materials such as ITO (Indium Tin Oxide: tin-doped indium oxide) and transparent conductive inorganic materials such as ZnO, SnO2, CdO, TiO2, CdIn2O4, Cd2SnO2, Zn2SnO4, TiO2Nb, LaCuOS, CuAlO2, MgInO4, CdGaO4, TiN, ZrN, HfN, and LaB6-based materials can be used. However, even if a material is relatively transparent in the visible light band, it is not necessarily transparent in all wavelength bands of light used in the wavelength conversion element, so the type of electrically conductive material and the compounding concentration must be adjusted according to the light wavelength used.

[0051] (Organic conductive materials as conductive layers) In addition, a conductive organic material can be used as the conductive layer (conductive transparent film 350) in one embodiment of the present disclosure. As the organic material, a conductive polymer or a surfactant can be used. It is possible to impart electrical conductivity to an insulating material by using a conductive polymer represented by polyacetylene, or a conductive polymer material containing aromatics such as polypyrrole, polyaniline, polythiophene, polyethylenedioxythiophene (PEDOT), poly(p-phenylene), polyfluorene, poly(p-phenylenevinylene), and polythienylenevinylene, or by mixing conductive particles such as carbon powder and silver nanowires into an insulating organic material using graphite, carbon, or carbon nanotubes. In addition, by applying a surfactant-based antistatic agent or a silica-based antistatic agent to the surface of the wavelength conversion element, moisture in the environment can be adsorbed on the surface, reducing the surface resistance and generating conductivity, thereby obtaining a similar effect.

[0052] (Method of forming conductive layer) In the case of the above-mentioned metal materials and conductive inorganic oxide materials, it is possible to form a conductive layer having electrical conductivity on the surface of the wavelength conversion element mainly by a sputtering method, a vacuum deposition method, or a CVD method (chemical vapor deposition method). In this case, for example, when using a vacuum deposition method, it is possible to form a conductive layer only in a desired part in a desired shape by using a photoresist or the like to cover the part where the conductive layer is not to be formed, and removing it by lift-off after the film formation.

[0053] In addition, in the case of the organic materials described above, in addition to the vacuum deposition method, it is also possible to disperse them in a solvent, etc., and form a conductive layer on the surface of the wavelength conversion element by a spin coating method, a dipping method, a spray method, etc.

[0054] (Range of electrical conductivity (resistivity) of conductive layer) As described above, the conductive layer of the wavelength conversion element of one embodiment only needs to have electrical conductivity that conducts electrons or ions and cancels the counter electric field generated in the wavelength conversion element, and it is desirable that the light absorption is low enough not to reduce the light transmittance of the wavelength conversion element.

[0055] 2, in the case of a wavelength conversion element 13 mounted in a conductive metal housing 29 and subject to small temperature changes, the generation of a reversible electric field due to sudden charging in the optical waveguide core 11 is unlikely to occur. Therefore, in such a case, the surface resistivity (sheet resistance) of the conductive layer (conductive transparent film 350) is set to 1×10 12 It is desirable that the value be less than [Ω / sq.].

[0056] Furthermore, when the temperature of the wavelength conversion element changes rapidly, the surface resistivity (sheet resistance) of the conductive layer is 1×10 10 It is desirable that the value be less than [Ω / sq.].

[0057] It is also desirable for the conductive layer to have as low an optical absorptance as possible at the wavelength of light being used.

[0058] (Second embodiment) (A conductive layer is laminated between the surface of the optical waveguide core and the surface of the substrate) Fig. 5 is a cross-sectional schematic diagram showing a wavelength conversion element according to a second embodiment of the present disclosure. The wavelength conversion element 313c shown in Fig. 5 includes a conductive transparent film 350, which is a conductive layer arranged between the upper surface of the substrate 312 and the lower surface of the PPLN optical waveguide core 311 so as to contact the lower surface of the PPLN optical waveguide core 311. With the structure shown in Fig. 5, the conductive transparent film 350 (conductive layer) is formed between the PPLN optical waveguide core 311 and the substrate 312 or the support member 27, so that not only are the charges and anti-electric fields generated on the surface of the PPLN optical waveguide core 311 itself suppressed and attenuated, but also the effect of blocking the electromagnetic field by the conductive transparent film 350 is exhibited, so that the influence of the electric field from the lower part of the PPLN optical waveguide core 311 can be reduced.

[0059] Fig. 6 is a schematic cross-sectional view for explaining a modified example of the second embodiment of the present disclosure. The wavelength conversion element 313d shown in Fig. 6 includes a conductive transparent film 350 disposed between the upper surface of the substrate 12 and the lower surface of the PPLN optical waveguide core 311, similar to the configuration of Fig. 5, and further includes a conductive transparent film 350 disposed on the upper surface of the PPLN optical waveguide core 311. By forming the conductive transparent film 350 on the upper surface of the PPLN optical waveguide core 311, it is possible to suppress and attenuate the electric charge and the anti-electric field generated on the surface of the PPLN optical waveguide core 311 itself.

[0060] The wavelength conversion element 313c shown in FIG. 5 and the wavelength conversion element 313d shown in FIG. 6, together with the temperature control element 26, constitute a wavelength conversion device, as described with reference to FIG.

[0061] 5 and 6, a conductive layer is formed on at least one of the upper and lower surfaces of the PPLN optical waveguide core 311 (the surface of the PPLN optical waveguide core 311 parallel to the upper surface of the substrate 312), and therefore, for example, a movement of charge in a direction parallel to the upper surface of the substrate 312 effectively removes a reversal electric field caused by an electric charge such as an electrified charge generated on the surface of the PPLN optical waveguide core 311. In addition, by forming a conductive layer (conductive transparent film 350) on the upper surface of the substrate 312, an effect of removing excess electric charge generated on the upper surface of the substrate 312 is also achieved. Therefore, for example, the effect of removing the electrification and the reversal electric field for a wavelength conversion element called TYPE II in which a control light with a polarization plane rotated 90 degrees with respect to a signal light in a direction perpendicular to the upper surface of the substrate 312 generates a wavelength-converted light in the PPLN optical waveguide core 311 is increased.

[0062] (Third embodiment) (The substrate surface and / or the sidewall of the optical waveguide core are covered with an electrically conductive layer.) Fig. 7 is a cross-sectional schematic diagram showing a wavelength conversion element according to a third embodiment of the present disclosure. The wavelength conversion element 313e shown in Fig. 7 includes a conductive transparent film 350, which is a conductive layer arranged on one of the two side surfaces (the right side in the example of Fig. 7) of the PPLN optical waveguide core 311 and on the upper surface of the adjacent substrate 312. The conductive transparent film 350 provided on the side surface of the PPLN optical waveguide core 311 and the conductive transparent film 350 provided on the upper surface of the substrate 312 are in contact (continuous).

[0063] 7, since a conductive layer is formed on at least one side surface of PPLN optical waveguide core 311, the reversal electric field of charges such as static electricity is effectively removed by the movement of charges in a direction perpendicular to the upper surface of substrate 312, for example. Furthermore, by forming conductive transparent film 350 on the upper surface of substrate 312, an effect of removing excess static electricity generated on the upper surface of substrate 312 is also achieved. Therefore, the effect of removing static electricity and reversal electric field for a wavelength conversion element called TYPE I in which signal light or control light generates wavelength-converted light in PPLN optical waveguide core 311 in a direction perpendicular to the upper surface of substrate 312 is enhanced.

[0064] 8 is a schematic cross-sectional view for explaining a modified example of the third embodiment of the present disclosure. The wavelength conversion element 313f shown in FIG. 8 includes conductive transparent films 350 arranged on two side surfaces of the PPLN optical waveguide core 311 and on both sides of the PPLN optical waveguide core 311 on the upper surface of the substrate 312. The conductive transparent films 350 provided on the side surfaces of the PPLN optical waveguide core 311 are in contact (continuous) with the conductive transparent film 350 provided on the upper surface of the substrate 312. According to the configuration of FIG. 8, it is possible to remove charging and anti-electric field more effectively than the configuration in which the conductive transparent film 350 is formed on one side of the PPLN optical waveguide core 311 shown in FIG. 7.

[0065] As described with reference to FIG. 2, the wavelength conversion element 313e shown in FIG. 7 and the wavelength conversion element 313f shown in FIG. 8 constitute a wavelength conversion device together with the temperature control element 26.

[0066] (Fourth embodiment) (The substrate surface adjacent to the optical waveguide core is covered with an electrically conductive layer.) Fig. 9 is a schematic cross-sectional view showing a wavelength conversion element according to a fourth embodiment of the present disclosure. A wavelength conversion element 313g shown in Fig. 9 includes a conductive transparent film 350 disposed on one side of a PPLN optical waveguide core 311 on the upper surface of a substrate 312. The conductive transparent film 350 contacts the side surface of the PPLN optical waveguide core 311.

[0067] Fig. 10 is a schematic cross-sectional view for explaining a modified example of the fourth embodiment of the present disclosure. A wavelength conversion element 313h shown in Fig. 10 includes conductive transparent films 350 arranged on both sides of a PPLN optical waveguide core 311 on the upper surface of a substrate 312. The conductive transparent films 350 are in contact with the side surfaces of the PPLN optical waveguide core 311.

[0068] As described with reference to FIG. 2, the wavelength conversion element 313g shown in FIG. 9 and the wavelength conversion element 313h shown in FIG. 10 constitute a wavelength conversion device together with the temperature control element 26.

[0069] In the structure of the conductive layer (conductive transparent film 350) shown in FIGS. 9 and 10, the cross-sectional area where the surface of the PPLN optical waveguide core 311 and the conductive layer come into contact with the electrostatic charge and the demagnetizing electric field generated on the surface of the PPLN optical waveguide core 311 is small. Therefore, the effect of quickly and effectively removing the electrostatic charge and the demagnetizing electric field is small. On the other hand, since the area where the electric field of the light seeping out from the PPLN optical waveguide core 311 comes into contact with the conductive layer (conductive transparent film 350) is small, the optical propagation loss of the PPLN optical waveguide core 311 can be kept low. According to the structure of the conductive layer (conductive transparent film 350) shown in FIGS. 9 and 10, the electrostatic charge and the demagnetizing electric field can be removed to a certain extent while keeping the optical propagation loss of the PPLN optical waveguide core 311 low.

[0070] Hereinafter, examples of the wavelength conversion device according to the present disclosure will be described, but the present disclosure is not limited to these examples.

[0071] Example 1 Fig. 11 shows a schematic configuration diagram of a wavelength converter 1120 of Example 1. The wavelength converter 1120 shown in Fig. 11 is configured to input a fundamental wave having a wavelength of 1550 nm as the signal light 1a, and output a wavelength-converted difference frequency light 1c having a wavelength (775 nm) which is the second harmonic of the signal light 1a.

[0072] In this Example 1, the temperature of the wavelength conversion element 313 in the wavelength conversion device 1120 placed on the optical table 1128 was rapidly changed in a short period of time, and the time change in the fluctuation in the wavelength of the second harmonic wave resulting from this was measured by a measuring instrument 1138, thereby evaluating the effects of charging and reverse electric field generated in the PPLN optical waveguide core 311 on the substrate 312.

[0073] The configuration of the wavelength converter 1120 will be described. The wavelength converter 1120 includes a multiplexer 14, a wavelength conversion element 313, and a demultiplexer 15. The wavelength converter 1120 also includes an alignment device 1134 disposed between the multiplexer 14 and the wavelength conversion element 313, and an alignment device 1135 disposed between the wavelength conversion element 313 and the demultiplexer 15.

[0074] The wavelength conversion element 313 includes a PPLN optical waveguide core 311, a substrate 312, and a conductive transparent film 350 (not shown) provided on the surfaces of the PPLN optical waveguide core 311 and the substrate 312. In this embodiment, a LiTaO3 substrate is used as the substrate 312.

[0075] The multiplexer 14 multiplexes the signal light 1 a from the laser light source 1131 and the control light 1 b from the light source 1133 , which have propagated through the optical fiber 1132 .

[0076] The alignment device 1134 controls the spatial position at which the light multiplexed by the multiplexer 14 is optically coupled to the PPLN optical waveguide core 311. Using the alignment device 1134, it is possible to perform control so as to maximize the optical coupling rate.

[0077] The alignment device 1135 controls the spatial position at which the difference frequency light 1 c and the control light 1 b outputted from the PPLN optical waveguide core 311 are optically coupled to the demultiplexer 15 .

[0078] Although alignment devices 1134 and 1135 are shown, the same applies to alignment devices of a spatial optical system that control the positions of a reflecting mirror and a condenser lens.

[0079] The demultiplexer 15 demultiplexes the difference frequency light 1c and the control light 1b. The difference frequency light 1c is guided through an optical fiber 1137 and input to a measuring device 1138. The control light 1b is input to a measuring device 1136.

[0080] On the optical surface plate 1128, there are provided a temperature variable adsorption stage 1126 for adjusting the temperature of the wavelength conversion element 313, and a support member 27 for supporting the wavelength conversion element 313 on the temperature variable adsorption stage 1126.

[0081] The temperature variable adsorption stage 1126 corresponds to the temperature control element 26 and the metal housing bottom member 28 described with reference to FIG.

[0082] Typically, to use it as a wavelength conversion device, the alignment devices 1134 and 1135 are set so that the light transmittance is maximized at the intended temperature of use, and it is possible to fix it by YAG welding, adhesive fixing, screw fastening, etc.

[0083] In this embodiment, the PPLN optical waveguide core 311 is formed by laminating LiNbO3, on which a periodic polarization structure has been formed in advance at a high voltage of about 1000 V using a comb-shaped electrode structure formed from Au, onto a LiTaO3 substrate (substrate 312) so that the arrangement direction of the polarization domains is perpendicular to one of the main surfaces (top surface) of the substrate, and after thin film polishing, it is processed into a ridge shape by dry etching using Ar plasma.

[0084] Thereafter, the end faces of the LiTaO3 substrate (substrate 312) and the PPLN optical waveguide core 311 were cut using a dicing saw, and anti-reflective coatings for the optical wavelengths of the signal light 1a and the control light 1b were formed on the end faces using a metal multilayer film (not shown) or the like for the signal light 1a and the control light 1b, and the end faces were used as wavelength conversion element 313.

[0085] The wavelength conversion element 313 fabricated as described above was fixed onto the support member 27 .

[0086] A temperature variable suction stage 1126, which combines a Peltier element and a brass metal member having a vacuum suction hole, was placed on an optical table 1128, and the wavelength conversion element 313 was fixed on the temperature variable suction stage 1126, and the optical characteristics were evaluated.

[0087] First, a wavelength conversion element 313 (corresponding to the wavelength conversion element 13 shown in Figure 2) not having a conductive transparent film 350 which is a conductive layer was fixed on a temperature variable adsorption stage 1126, and the change in wavelength of the second harmonic generated from the wavelength conversion element 313 not having a conductive transparent film 350 was measured when the temperature of the temperature variable adsorption stage was suddenly heated from 55°C to 85°C and when it was suddenly cooled from 85°C to 55°C.

[0088] Thereafter, the end face of the PPLN optical waveguide core 311 was protected with a heat-resistant polyimide film tape or aluminum foil, and then a Pt-Pd film of about 50 Å was formed as the conductive transparent film 350, which was a conductive layer, by a sputtering method, to produce a wavelength conversion element 313a having the conductive transparent film 350 of the form described with reference to Fig. 3. The produced wavelength conversion element 313a was placed on the temperature variable adsorption stage 1126, and the optical characteristics were evaluated while changing the temperature variable adsorption stage.

[0089] As a result, in both cases where the temperature of the temperature-variable adsorption stage was rapidly heated from 55° to 85°C, and where it was rapidly cooled from 85°C to 55°C, the wavelength conversion element 313 having the conductive transparent film 350 was able to reduce the time it took for the second harmonic wavelength to change by approximately 10% or more compared to the wavelength conversion element 313 not having the conductive transparent film 350.

[0090] However, possibly due to the influence of light absorption by the conductive layer (conductive transparent film 350), a decrease in the light transmittance of the signal light and a decrease in the light intensity of the second harmonic were observed.

[0091] Example 2 Next, a second embodiment of the present disclosure will be described in detail below. The configuration of a wavelength converter of the second embodiment is similar to that of Fig. 11. A wavelength converter 1120 of the second embodiment is configured to input a fundamental wave having a wavelength of 1550 nm as a signal light 1a, and output a wavelength-converted difference frequency light 1c having a wavelength (780 nm) that is the second harmonic of the signal light 1a.

[0092] The wavelength converter 1120 of this second embodiment was evaluated using the optical characteristic evaluation measurement system of FIG. 11 which is exactly the same as that of the first embodiment described above, and therefore a detailed description thereof will be omitted.

[0093] In the case of the wavelength conversion device 1120 of this embodiment 2, the optical intensity of the control light 1b was suddenly changed in a short period of time while keeping the wavelength and optical intensity of the signal light 1a incident on the wavelength conversion element 313 in the wavelength conversion device 1120 placed on the optical table 1128 unchanged, and the effects of electrostatic charging and reverse electric field generated in the PPLN optical waveguide core 311 were evaluated by measuring the time changes in the fluctuations in the wavelength and optical intensity of the difference frequency light 1c that were changed as a result.

[0094] First, using a wavelength conversion element 313 (corresponding to wavelength conversion element 13 in Figure 2) that does not have a conductive layer (conductive transparent film 350), the temperature of the temperature-variable adsorption stage 1126 was fixed at 55°C, and the optical wavelength and optical intensity of the signal light 1a were kept fixed. The changes in the wavelength and optical intensity of the generated difference frequency light 1c over time were measured when the optical intensity of the control light 1b was suddenly doubled from its initial value, and when the optical intensity was suddenly halved from twice the initial value to the initial value.

[0095] Then, the end face of the PPLN optical waveguide core 311 of the wavelength conversion element 313 not having the conductive transparent film 350 was protected with a heat-resistant polyimide film tape or aluminum foil, and then a SiO2-In2O3 film of about 1 μm was formed as a conductive layer by a sputtering method using a sputtering target having a SiO2-In2O3 (composition ratio = 1:0.1) composition, thereby producing a wavelength conversion element 313a having the conductive transparent film 350 of the embodiment described with reference to Figure 3. The produced wavelength conversion element 313a was placed on the temperature variable adsorption stage 1126, and the optical characteristics of the wavelength conversion element 313 having the conductive transparent film 350 were evaluated.

[0096] As a result, the temperature of the temperature variable adsorption stage 1126 was fixed at 55°C, and the optical intensity of the signal light 1a was fixed while the optical intensity of the control light 1b was suddenly doubled from the initial value, and the optical intensity was suddenly halved from twice the initial value to the initial value. The time to change the wavelength and optical intensity of the generated difference frequency light 1c in the wavelength conversion element 313 having the conductive transparent film 350 was shortened by about 10% or more compared to the wavelength conversion element 313 without the conductive transparent film 350. [Industrial Applicability]

[0097] The present disclosure makes it possible to suppress fluctuations in wavelength conversion characteristics due to changes in temperature of the optical waveguide core of a wavelength conversion element and changes in the intensity of incident light. This reduces the external influence on the wavelength conversion element, thereby providing a wavelength conversion device with stable optical characteristics. [Explanation of symbols]

[0098] 1a signal light 1b Control light 1c difference frequency light 10 Wavelength conversion device 11 Optical waveguide core 12 Substrate 13 Wavelength conversion element 14 Multiplexer 15 Duplexer 20 Wavelength conversion device 26 Temperature control element 27 Support member 28 Metal case bottom part 29 Metal Case 200 input ports 201 Output port 311 PPLN optical waveguide core 312 Substrate 313a-h Wavelength conversion element 350 Conductive transparent film 1120 Wavelength conversion device 1126 Variable Temperature Adsorption Stage 1128 Optical Table 1131 Laser light source 1132 Optical Fiber 1133 light source 1134 Alignment device 1135 Alignment device 1136 Measuring instruments 1138 Measuring instrument

Claims

1. A wavelength conversion device that generates light having a wavelength different from that of a signal light when the signal light is input, comprising: An optical waveguide core; a substrate having a refractive index lower than the refractive index of the optical waveguide core with respect to the signal light; a conductive layer having electrical conductivity and disposed in contact with at least a portion of a surface of the optical waveguide core; Equipped with the conductive layer is disposed in contact with an upper surface of the optical waveguide core, at least one of two side surfaces of the optical waveguide core, and at least one of both sides of the optical waveguide core on an upper surface of the substrate opposite to the lower surface of the optical waveguide core; the conductive layer disposed on the top surface of the optical waveguide core is in contact with the conductive layer disposed on the side surface of the optical waveguide core, A wavelength conversion device, in which the conductive layer disposed on the side surface of the optical waveguide core and the conductive layer disposed on the upper surface of the substrate are in contact with each other.

2. A wavelength conversion device that generates light having a wavelength different from that of a signal light when the signal light is input, comprising: An optical waveguide core; a substrate having a refractive index lower than the refractive index of the optical waveguide core with respect to the signal light; a conductive layer having electrical conductivity and disposed in contact with at least a portion of a surface of the optical waveguide core; Equipped with The conductive layer is disposed between an upper surface of the substrate facing the lower surface of the optical waveguide core and the lower surface of the optical waveguide core, and on at least one of both sides of the optical waveguide core on the upper surface of the substrate facing the lower surface of the optical waveguide core; a conductive layer arranged between the upper surface of the substrate and the lower surface of the optical waveguide core in contact with the lower surface of the optical waveguide core and the upper surface of the substrate, and a conductive layer arranged on at least one of both sides of the optical waveguide core on the upper surface of the substrate in contact with the conductive layer.

3. The wavelength conversion device according to claim 2 , wherein the conductive layer is disposed on an upper surface of the optical waveguide core in contact with the optical waveguide core.

4. A wavelength conversion device that generates light having a wavelength different from that of a signal light when the signal light is input, comprising: An optical waveguide core; a substrate having a refractive index lower than the refractive index of the optical waveguide core with respect to the signal light; a conductive layer having electrical conductivity and disposed in contact with at least a portion of a surface of the optical waveguide core; Equipped with the conductive layer is disposed on at least one of two side surfaces of the optical waveguide core and on at least one of both sides of the optical waveguide core on an upper surface of the substrate opposite to a lower surface of the optical waveguide core; A wavelength conversion device, wherein the conductive layer arranged in contact with a side surface of the optical waveguide core is in contact with the conductive layer arranged on at least one of both sides of the optical waveguide core on the upper surface of the substrate.

5. A wavelength conversion device that generates light having a wavelength different from that of a signal light when the signal light is input, comprising: An optical waveguide core; a substrate having a refractive index lower than the refractive index of the optical waveguide core with respect to the signal light; a conductive layer having electrical conductivity and disposed in contact with at least a portion of a surface of the optical waveguide core; Equipped with A wavelength conversion device, wherein the conductive layer is disposed on at least one of both sides of the optical waveguide core on an upper surface of the substrate opposite a lower surface of the optical waveguide core, and in contact with a side surface of the optical waveguide core.

6. The surface resistivity of the electrically conductive layer is 1×10 12 6. The wavelength conversion device according to claim 1, wherein the refractive index is 0.01 to 0.5 [Ω / sq.] or less.

7. The wavelength conversion device according to claim 1 , further comprising a temperature control element for controlling a temperature.

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