Wavelength conversion device

Abstract

The object of the present invention is to suppress the reduction in the output power of wavelength-converted light caused by the thermoelectric effect generated when the temperature of a wavelength conversion element including a ferroelectric substrate is changed. The present invention provides a wavelength conversion device that generates light of a wavelength different from that of the signal light when a signal light is input, the wavelength conversion device comprising: a wavelength conversion element that converts the wavelength of the signal light; and a temperature control element that controls the temperature of the wavelength conversion element, the wavelength conversion element and the temperature control element being sealed inside a metal shell, the inside of the metal shell being filled with a dry gas containing one or more selected from nitrogen, oxygen, argon or helium, the wavelength conversion element comprising an optical waveguide core and a substrate, the substrate having a lower refractive index for the signal light than the optical waveguide core, the substrate being a ferroelectric having a random direction of spontaneous polarization.

Description

Technical Field

[0001] The invention relates to a wavelength conversion device. Background Art

[0002] An optical waveguide using periodically polarized lithium niobate (PPLN) is an element that can achieve an increase in light intensity by using a waveguide and a high wavelength conversion efficiency by using a quasi-phase matching technique. Therefore, an optical waveguide using PPLN has attracted attention as a device that plays an important role in the field of next-generation optical fiber communications and quantum computing. The optical waveguide using PPLN is used as a parametric amplifier element and an excitation light generating element that constitutes a phase-sensitive amplifier (PSA) capable of low-noise optical amplification, and can achieve high-gain, low-noise optical amplification characteristics. In addition, in the field of quantum computing, it has been reported that an optical waveguide using PPLN is inserted into a fiber ring resonator and used as a parametric oscillator element, thereby verifying large-capacity calculations at an extremely high speed compared to previous computers. In order to further improve the performance of these technologies, it is important to realize a wavelength conversion device with higher wavelength conversion efficiency.

[0003] Figure 1 1 is a diagram showing the basic structure of a conventional wavelength conversion device 10 that generates a difference frequency by using a quasi-phase matching (Quasi-Phase-Matched, hereinafter referred to as QPM) technique. Figure 1 The wavelength conversion device 10 is composed of. Figure 1 As shown, signal light 1a with low light intensity and control light 1b with high light intensity are incident on a combiner 14 for combination. The signal light 1a combined with the control light 1b travels toward a wavelength conversion element 13 including a substrate 12 and an optical waveguide core 11 arranged on the substrate 12. It is incident on one end of the optical waveguide core 11 having a periodic polarization inversion structure and exhibiting a nonlinear optical effect. When the signal light 1a passes through the optical waveguide core 11, it is converted into a difference frequency light 1c having a wavelength different from that of the signal light 1a, and is emitted 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 emitted from the optical waveguide core 11 are incident on a demultiplexer 15 and separated from each other.

[0004] In addition, several methods for producing an optical element for wavelength conversion (hereinafter referred to as a wavelength conversion element) using quasi-phase matching technology are also known. For example, after a crystal substrate exhibiting a nonlinear optical effect (hereinafter referred to as a nonlinear optical crystal) is set to a periodic polarization inversion structure, a method of using the periodic polarization inversion structure to produce a proton exchange waveguide is used. In addition, for example, after a nonlinear optical crystal substrate is also set to a periodic polarization inversion structure, a method of producing a ridge-shaped optical waveguide using a photolithography process and a dry etching process is used.

[0005] An example of manufacturing a ridge optical waveguide among these is disclosed in Patent Document 1. Patent Document 1 states that, in order to improve the light confinement effect in a ridge optical waveguide, 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 are bonded together to manufacture a wavelength conversion element. In addition, Patent Document 1 states that, in order to avoid cracks caused by deterioration of the adhesive and temperature changes, a nonlinear optical crystal of the same type as the first substrate is used as the second substrate, and the first substrate and the second substrate are heated to be diffusely bonded.

[0006] It is known that a wavelength conversion device having a wavelength conversion element is used in practice by housing the wavelength conversion element together with a combiner and a splitter in a metal housing having an input / output port capable of inputting and outputting light, so that its characteristics are not degraded due to changes in the use environment. In addition, the wavelength conversion efficiency of the wavelength conversion element is temperature-dependent, and in order to maximize its wavelength conversion efficiency, it is necessary to control the temperature of the wavelength conversion element. Therefore, the wavelength conversion device also houses a temperature control element inside the metal housing.

[0007] Figure 2 It means in Figure 1 FIG. 2 is a diagram showing an example of the configuration of a wavelength conversion device 20 that further includes a metal casing 29 and a temperature control element 26 on the basis of the configuration of the wavelength conversion device 10 . Figure 2 The wavelength conversion device shown in the figure has the following structure except Figure 1 In addition to the structure shown, it also has a metal shell 29, a temperature control element 26, an upper member 27 and a metal shell bottom member 28. The metal shell 29 has an input port 200 and an output port 201 on the side for inputting and outputting light and inputting and outputting electrical signals, and the electrical signals are used for temperature control. The upper 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 sandwiched between the upper member 27 and the metal shell bottom member 28. It should be noted that the optical waveguide core 11, the substrate 12, the wavelength conversion element 13, the combiner 14, the demultiplexer 15, the signal light 1a, the difference frequency light 1c and Figure 1 The contents are the same as those in the description, so the description is omitted.

[0008] In addition, when a wavelength conversion element using a ferroelectric crystal material is used in a wavelength conversion device, a phenomenon called light damage occurs in which the refractive index of the wavelength conversion element changes due to irradiation with light having a short wavelength, thereby reducing the characteristics. As a method for suppressing the influence of this light damage, a method of using the wavelength conversion element at a high temperature has been proposed.

[0009] Prior art literature

[0010] Patent Literature

[0011] Patent Document 1: Japanese Patent No. 3753236 Summary of the invention

[0012] Problems to be solved by the invention

[0013] In order to operate the wavelength conversion element at a high temperature, a method of heating the wavelength conversion element using a heat source such as a heater is considered. Figure 2 In the wavelength conversion device 20 shown, the wavelength conversion element 13 needs to be temperature controlled with high accuracy and speed, so a Peltier element is used as the temperature control element 26. When the wavelength conversion element 13 is set to a high temperature using the Peltier element, the following matters become a problem.

[0014] Here, the Peltier element is an element that uses the effect of absorbing heat on one surface and releasing heat on the other surface when current flows through the Peltier element. Therefore, when the temperature of the wavelength conversion element 13 is set to be higher than the temperature around the wavelength conversion element 13, it is assumed that condensation occurs on the heat-absorbing side in the atmospheric environment. When the water generated by the condensation is retained inside the Peltier element and near the wire electrode, the solder joining the Peltier element and the wire electrode is corroded by the water, and the Peltier element and the wire electrode are electrically disconnected and damaged. Or when the water generated by condensation contacts multiple electrodes, due to the potential difference between the electrodes, the components of the solder are repeatedly anodic dissolved and cathodic precipitated through the water, resulting in multiple electrodes short-circuiting and the wavelength conversion element 13 failing. Therefore, in order to realize a highly reliable wavelength conversion device 20, it is necessary to hermetically seal the wavelength conversion device 20 in a dry gas atmosphere that does not contain water.

[0015] However, when the wavelength conversion device 20 is hermetically sealed in a dry gas atmosphere, there are no atmospheric ions in the wavelength conversion device 20, so it is impossible to neutralize the surface charge on the substrate 12 generated by the thermoelectric effect of the substrate 12 as a ferroelectric when the temperature of the wavelength conversion element 13 is changed. At this time, a refractive index distribution of the substrate 12 is generated. Moreover, the change in the surface charge generated by the thermoelectric effect on the surface of the substrate 12 also affects the interior of the optical waveguide core 11, causing an uneven distribution of the charge density inside the optical waveguide core 11. Therefore, an uneven refractive index distribution is also generated inside the optical waveguide core 11. As a result, there is a problem that the wavelength that satisfies the phase matching condition in the optical waveguide core 11 changes, and the output power of the wavelength-converted difference frequency light 1c is reduced.

[0016] Solutions for solving problems

[0017] The present invention is made to solve the above-mentioned problems. One embodiment of the present invention is a wavelength conversion device, which generates light of a wavelength different from the signal light when a signal light is input, and the wavelength conversion device comprises: a wavelength conversion element for converting the wavelength of the signal light; and a temperature control element for controlling the temperature of the wavelength conversion element. Here, the wavelength conversion element and the temperature control element are enclosed in the interior of a metal shell, and the interior of the metal shell is filled with a dry gas containing one or more selected from nitrogen, oxygen, argon or helium. The wavelength conversion element includes an optical waveguide core and a substrate, and the substrate has a lower refractive index for the signal light than the optical waveguide core, and the substrate is a ferroelectric with a random direction of spontaneous polarization.

[0018] Effects of the Invention

[0019] According to the present invention, it is possible to provide a wavelength conversion device capable of suppressing a decrease in the output power of wavelength-converted light due to a pyroelectric effect generated when the temperature of a wavelength conversion element including a ferroelectric substrate is changed. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 1 is a diagram showing a basic configuration example of a conventional wavelength conversion device 10 that generates a difference frequency using a quasi-phase matching technique.

[0021] Figure 2 It means in Figure 1 FIG. 1 is a diagram showing an example of a wavelength conversion device which further includes a metal casing 29 and a temperature control element 26 in addition to the structure of the wavelength conversion device 10 .

[0022] Figure 3 It is a diagram showing the structure of a wavelength converter 30 as one embodiment of the present invention.

[0023] Figure 43 is a graph showing the phase matching characteristics of the wavelength conversion device 30 when the substrate 32 is not heated at 650° C. for two hours in a dry nitrogen atmosphere.

[0024] Figure 5 This is a graph showing the phase matching characteristics of the wavelength conversion device 30 when the substrate 32 is heated at 650° C. for two hours in a dry nitrogen atmosphere. DETAILED DESCRIPTION

[0025] The operating principle of the wavelength conversion device of one embodiment of the present invention is described. The wavelength conversion device of one embodiment of the present invention uses ferroelectric crystal as the lower part of the ridge optical waveguide of the wavelength conversion element, i.e., the substrate. The substrate is heated once above the Curie temperature and then cooled. Through the heating and cooling treatment, polarization in a direction different from the polarization direction before heating is randomly generated in the substrate as a dielectric crystal, so the polarization direction of the ferroelectric is no longer single. In the ferroelectric crystal that has a non-single polarization direction and has a random polarization direction, the electric fields generated by the polarizations in different directions cancel each other out inside the ferroelectric crystal, so as a result, the thermoelectric effect is smaller than that of the ferroelectric crystal with a single polarization direction.

[0026] That is, the direction of spontaneous polarization of the substrate before heating is consistent in a certain direction at the macroscopic and microscopic levels, and the substrate after heating produces a portion of the spontaneous polarization direction in the domain. The above-mentioned effect of a smaller thermoelectric effect than a single-polarized ferroelectric crystal is achieved by the structural change of the substrate.

[0027] Before and after the heating and cooling process, although the piezoelectricity, electrostrictive characteristics, and nonlinear optical characteristics of the substrate are impaired, the refractive index, transmittance, and linear expansion coefficient of the substrate that affect the output power of the wavelength-converted light are almost unchanged. Due to this technical feature, the wavelength conversion device as one embodiment of the present invention can be used while suppressing the pyroelectric effect of the substrate even in a dry gas atmosphere.

[0028] (Example)

[0029] Figure 3 1 is a diagram showing the structure of a wavelength converter 30 as one embodiment of the present invention. The wavelength converter 30 is a wavelength conversion device that outputs a wavelength-converted signal light 3c having a wavelength (775nm) as its second harmonic from an output port 301 when a fundamental wave having a wavelength of 1550nm is input as signal light 1a from an input port 300. Figure 3 The wavelength conversion device 30 shown in FIG. Figure 2The wavelength conversion device 20 is the same as that shown in the figure. The difference is that the substrate 32 of the wavelength conversion element 33 is made of a ferroelectric crystal having a random polarization direction instead of a single polarization direction.

[0030] First, the structure of the wavelength conversion device 30 is described. The wavelength conversion device 30 includes: a combiner 14 for combining the signal light 1a and the control light (not shown); a wavelength conversion element 33 including an optical waveguide core 31 and a substrate 32; and a demultiplexer 15 for demultiplexing the signal light 3c and the control light (not shown) after the wavelength conversion. A metal shell is provided in a manner to seal these elements, and the metal shell includes a first part 39A of the metal shell and a second part 39B of the metal shell. A temperature control element 26, an upper member 27, and a metal shell bottom member 28 are also provided inside the metal shell.

[0031] The metal shell including the first part 39A of the metal shell and the second part 39B of the metal shell has an input port 300 and an output port 301 on the side for inputting and outputting light and inputting and outputting electrical signals, and the electrical signals are used for temperature control. Figure 1 The contents are the same as those in the description, so the description is omitted.

[0032] The optical waveguide core 31 is an optical waveguide that selectively transmits the signal light 1a therein without causing a loss in the intensity of the signal light 1a. The structure of the optical waveguide core 31 is not particularly limited as long as it has the function of outputting a wavelength-converted signal light 3c having a wavelength different from that of the signal light 1a when the wavelength of the signal light 1a is input. The above structure is a structure in which the quadratic nonlinear constant changes periodically or with a period given a predetermined modulation along the traveling direction of the light, and quasi-phase matching is achieved for a single wavelength or multiple wavelengths. For example, a multi-QPM element may be used.

[0033] The substrate 32 is a ferroelectric material that is transparent to the signal light 1a, i.e., does not absorb light. When forming a ridge optical waveguide, the substrate 32 functions as a lower cladding layer for the optical waveguide core 31, and needs to have a lower refractive index than the optical waveguide core 31 for the signal light 1a, the control light (not shown), and the wavelength-converted signal light 3c.

[0034] The ferroelectric material used for the substrate 32 is preferably LiNbO3, KNbO3 (potassium niobate), LiTaO3 (lithium tantalate), LiNb(x)Ta(1-x)O3 (0≤x≤1) (lithium tantalate of indefinite ratio composition) or KTiOPO4 (potassium titanium phosphate), and they also contain at least one selected from Mg (magnesium), Zn (zinc), Sc (scandium), In (indium) as an additive.

[0035] The metal case is sealed by joining the first portion 39A of the metal case and the second portion 39B of the metal case so as to maintain airtightness inside.

[0036] The upper member 27 is a metal member for uniformly controlling the temperature of the entire wavelength conversion element 33 including the optical waveguide core 31 and the substrate 32 .

[0037] Here, in order to suppress deformation of the upper member 27 and the metal case bottom member 28 due to thermal stress generated by temperature change of the temperature control element 26, the linear expansion coefficients of the upper member 27 and the metal case bottom member 28 are preferably substantially equal to the linear expansion coefficient of the temperature control element 26. Specifically, the linear expansion coefficient of the upper member 27 is preferably 10×10 -6 / K above 20×10 -6 / K or less.

[0038] In addition, the material of the upper member is preferably selected from stainless steel, copper-molybdenum steel, carbon steel, chrome-molybdenum steel, copper, phosphorus-deoxidized copper, oxygen-free copper, phosphor bronze or brass. In this embodiment, oxygen-free copper is used as the upper member 27.

[0039] The metal case bottom surface member 28 is a member having high mechanical strength and high thermal conductivity in order to efficiently transfer the heat generation and heat absorption effect generated by the temperature control element 26 to the wavelength conversion element 33 .

[0040] The linear expansion coefficient of the metal shell bottom member 28 is preferably 4×10 -6 / K above 12×10 -6 / K or less.

[0041] Furthermore, the linear expansion coefficients of the upper member 27 and the metal case bottom member 28 are set to be substantially equal to the linear expansion coefficient of the temperature control element 26, thereby further suppressing the deformation of the upper member 27 and the metal case bottom member 28 due to thermal stress. That is, the range of the linear expansion coefficient of the metal case bottom member 28 is more preferably substantially equal to the range of the linear expansion coefficient of the upper member 27.

[0042] In addition, the material of the metal case bottom member preferably includes one or more metals selected from tungsten, molybdenum, Kovar, copper-tungsten steel, stainless steel or copper-molybdenum steel. In this embodiment, stainless steel is used as the metal case bottom member 28.

[0043] The temperature control element 26 is inserted between the upper member 27 and the metal shell bottom member 28. The temperature control element 26 is a Peltier element. In order to efficiently transfer the heat generation and heat absorption effects to the upper member 27 and the metal shell bottom member 28, soldering, bonding using a conductive adhesive, and other well-known bonding methods can be used as their bonding methods.

[0044] The dry gas 302 is a gas that does not contain water and is filled inside the metal shell. Figure 3 As shown, the dry gas 302 is the atmosphere around the wavelength conversion element 33, the temperature control element 26, the upper member 27 and the metal housing bottom member 28 sealed in the metal housing.

[0045] The dry gas 302 preferably includes one or more selected from nitrogen, oxygen, argon, or helium. In this embodiment, dry nitrogen is used as the dry gas 302 .

[0046] Next, the method for manufacturing the wavelength conversion device 30 is described. The substrate 32 is heated and cooled in advance. As the substrate 32, a crystal of LiTaO3 with a thickness of 0.5 mm is placed in an electric furnace and heated for two hours at 650° C. in a dry nitrogen atmosphere while maintaining the temperature. After the heating, the substrate 32 is cooled to reduce the temperature to room temperature.

[0047] By performing the heating and cooling treatment, the direction of spontaneous polarization becomes random in the crystal of LiTaO3 as the substrate 32. As a result, the electric fields generated by the random polarization inside the substrate 32 cancel each other, and the thermoelectric effect is smaller than that of the case of polarization in a single direction, which can reduce the surface charge density of the substrate 32.

[0048] The ferroelectric substrate 32 is used as the lower portion of the ridge optical waveguide, and an optical waveguide core 31 based on PPLN with a size of 5 μm square is formed on the substrate 32 by dry etching.

[0049] The wavelength conversion element 33 manufactured as described above is fixed to the upper member 27, and the upper member 27 and the metal shell bottom member 28 are fixedly housed inside the first part 39A of the metal shell with an upper surface opening, and the temperature control element 26 is sandwiched between them. After fixing, the first part 39A of the metal shell and the first part 39B of the metal shell are seam welded in a dry nitrogen atmosphere, thereby hermetically sealing the metal shell. The joint portion generated by the seam welding is 39e.

[0050] Figure 4 and Figure 5 They are respectively the case where the substrate 32 is not heated at 650° C. for two hours in a dry nitrogen atmosphere in advance ( Figure 4 ) and heating for two hours ( Figure 5 ) is a graph showing the phase matching characteristics of the wavelength conversion device 30. Figure 4 and Figure 5 In FIG. 1 , the horizontal axis represents the wavelength of the wavelength-converted signal light 3 c output from the wavelength conversion device 30 , and the vertical axis represents the output power thereof.

[0051] exist Figure 4 As can be seen from the figure, due to the change in the refractive index of the substrate 32 caused by the pyroelectricity of the substrate 32, the phase matching condition is not satisfied locally in the optical waveguide core 31, and the characteristic represented by the Sinc function as the theoretical phase matching curve is greatly deviated, and the wavelength conversion efficiency is reduced. Figure 5 As shown in FIG. 1 , it can be seen that when the substrate 32 subjected to the heat treatment is used as the substrate of the wavelength conversion element, Figure 4 Compared with the phase matching characteristics of the case where no heating and cooling treatment is applied to the substrate 32, the influence of the thermoelectric effect is better suppressed, and the shape of the phase matching curve is close to the ideal Sinc function, which can obtain a high wavelength conversion efficiency.

[0052] Description of reference numerals:

[0053] 1a signal light;

[0054] 1b Control light;

[0055] 1c difference frequency light;

[0056] 3c Signal light after wavelength conversion;

[0057] 10, 20, 30 wavelength conversion device;

[0058] 11, 31 optical waveguide core;

[0059] 12, 32 substrate;

[0060] 13, 33 wavelength conversion element;

[0061] 14. Combiner;

[0062] 15 splitter;

[0063] 26 Temperature control elements;

[0064] 27 upper member;

[0065] 28 Metal shell bottom member;

[0066] 29 metal shell;

[0067] 39A first portion of the metal casing;

[0068] 39B the second part of the metal casing;

[0069] 39e a joining portion of the first portion of the metal shell and the second portion of the metal shell;

[0070] 200, 300 input ports;

[0071] 201, 301 output ports;

[0072] 302 Dry gas.

Claims

1. A wavelength conversion device that generates light having a wavelength different from that of the signal light when a signal light is input, the wavelength conversion device comprising: a wavelength conversion element for converting the wavelength of the signal light; and a temperature control element, used to control the temperature of the wavelength conversion element, The wavelength conversion element and the temperature control element are sealed inside a metal housing, and the interior of the metal housing is filled with a dry gas selected from nitrogen, oxygen, argon or helium. The wavelength conversion element includes an optical waveguide core and a substrate, wherein the substrate has a lower refractive index for the signal light than the optical waveguide core, and the optical waveguide core is formed on the substrate. The substrate is a ferroelectric in which the direction of spontaneous polarization is random after being heated to or above the Curie temperature and then cooled, and functions as a cladding.

2. The wavelength conversion device according to claim 1, further comprising: A combiner for combining the signal light and the control light; and The wavelength splitter splits the light having a wavelength different from that of the signal light due to the wavelength conversion element and the control light.

3. The wavelength conversion device according to claim 1, wherein: The temperature control element is a Peltier element, The Peltier element is sandwiched between a first surface of the upper member and a metal shell bottom surface member arranged on the bottom surface of the metal shell, and the wavelength conversion element is placed on a second surface of the upper member opposite to the first surface of the upper member. The upper member is a metal member, The values ​​of the linear expansion coefficients of the upper member and the metal case bottom member are substantially equal to the linear expansion coefficient of the Peltier element.

4. The wavelength conversion device according to any one of claims 1 to 3, wherein: The optical waveguide core has a quasi-phase matching structure in which a quadratic nonlinear constant inside the optical waveguide core changes periodically or randomly along a traveling direction of the signal light.

5. The wavelength conversion device according to any one of claims 1 to 3, wherein: The substrate is LiNbO3 (lithium niobate), KNbO3 (potassium niobate), LiTaO3 (lithium tantalate), LiNb (x) Ta (1-x) O3 (0≤x≤1) (lithium tantalate of indefinite composition) or KTiOPO4 (potassium titanium phosphate), which also contain at least one selected from Mg (magnesium), Zn (zinc), Sc (scandium) or In (indium) as an additive.

6. The wavelength conversion device according to claim 3, wherein: The linear expansion coefficient of the upper member is 10×10 -6 / K above 20×10 -6 / K or less.

7. The wavelength conversion device according to claim 3, wherein: The linear expansion coefficient of the bottom surface member of the metal shell is 4×10 -6 / K above 12×10 -6 / K or less.

8. The wavelength conversion device according to claim 3 or 6, wherein: The material of the upper component includes one or more selected from stainless steel, copper-molybdenum steel, carbon steel, chrome-molybdenum steel, copper, phosphorus-deoxidized copper, oxygen-free copper, phosphor bronze or brass.

9. The wavelength conversion device according to claim 3 or 7, wherein: The material of the bottom surface component of the metal shell includes one or more selected from tungsten, molybdenum, Kovar alloy, copper-tungsten steel, stainless steel or copper-molybdenum steel.

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