Wavelength Conversion Device

Abstract

Reduction of output power of light with a wavelength converted is suppressed, which is caused by a pyroelectric effect that occurs when a temperature of a wavelength conversion element including a ferroelectric substrate is changed. Provided is a wavelength conversion device that generates light different from a wavelength of a signal light when the signal light is inputted, and includes a wavelength conversion element that converts a wavelength of the signal light, and a temperature control element for controlling a temperature of the wavelength conversion element, wherein the wavelength conversion element and the temperature control element are sealed in an inside of a metal casing, the wavelength conversion element includes an optical waveguide core and a substrate having a lower refractive index to the signal light than the optical waveguide core, and the substrate is a ferroelectric substance in which directions of spontaneous polarization are random.

Claims

1. A wavelength conversion device that generates light different in wavelength from a signal light when the signal light is inputted, comprising: a wavelength conversion element that converts a wavelength of the signal light; and a temperature control element for controlling a temperature of the wavelength conversion element, wherein the wavelength conversion element and the temperature control element are sealed in an inside of a metal casing, and the inside of the metal casing is filled with dry gas including one or more selected from nitrogen, oxygen, argon or helium, the wavelength conversion element includes an optical waveguide core and a substrate having a lower refractive index to the signal light than the optical waveguide core, and the substrate is a ferroelectric substance in which directions of spontaneous polarization are random.

2. The wavelength conversion device according to claim 1, further comprising: a multiplexer that combines the signal light and control light; and a demultiplexer that separates the light different in wavelength from the signal light by 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 interposed between a first surface of an upper member and a metal casing bottom surface member disposed on a bottom surface of the metal casing, and the wavelength conversion element is placed on a second surface of the upper member, the second surface facing the first surface of the upper member, the upper member is a metal member, and values of linear expansion coefficients of the upper member and the metal casing bottom surface member are substantially equal values to a linear expansion coefficient of the Peltier element.

4. The wavelength conversion device according to claim 1, wherein the optical waveguide core has a quasi-phase-matched structure in which a quadric 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 claim 1, wherein the substrate is LiNbO.sub.3 (lithium niobate), KNbO.sub.3 (potassium niobate), LiTaO.sub.3 (lithium tantalate), LiNb.sub.(x)Ta.sub.(1-x)O.sub.3 (0≤x≤1) (lithium tantalate of a nonstoichiometric composition), or KTiOPO.sub.4 (potassium titanate phosphate), and further includes at least one selected from Mg (magnesium), Zn (zinc), Sc (scandium), or In (indium) therein as an additive.

6. The wavelength conversion device according to claim 3, wherein a linear expansion coefficient of the upper member is 10×10.sup.−6/K or more and 20×10.sup.−6/K or less.

7. The wavelength conversion device according to claim 3, wherein a linear expansion coefficient of the metal casing bottom surface member is 4×10.sup.−6/K or more and 12×10.sup.−6/K or less.

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

9. The wavelength conversion device according to claim 3, wherein a material of the metal casing bottom surface member includes one or more selected from tungsten, molybdenum, kovar, copper tungsten steel, stainless steel, or copper molybdenum steel.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0015] FIG. 1 is a view illustrating a basic configuration example of a conventional wavelength conversion device 10 that generates a difference frequency by a quasi-phase-matched technology.

[0016] FIG. 2 is a view illustrating a configuration example of a wavelength conversion device further including a metal casing 29 and a temperature control element 26 in a configuration of the wavelength conversion device 10 in FIG. 1.

[0017] FIG. 3 is a view illustrating a configuration of a wavelength conversion device 30 that is one embodiment of the present invention.

[0018] FIG. 4 is a graph showing a phase matching characteristic of the wavelength conversion device 30 in a case where a substrate is not heated for two hours at 650° C. in a dry nitrogen gas atmosphere.

[0019] FIG. 5 is a graph showing a phase matching characteristic of the wavelength conversion device 30 in a case where the substrate 32 is heated for two hours at 650° C. in the dry nitrogen gas atmosphere.

DESCRIPTION OF EMBODIMENTS

[0020] An operation principle of a wavelength conversion device of one embodiment of the present invention will be described. The wavelength conversion device of one embodiment of the present invention uses a ferroelectric crystal as a substrate that is a lower portion of a ridge optical waveguide of a wavelength conversion element. The substrate is cooled after heated at a Curie temperature or more once. By the heating and cooling treatments, polarization in directions different from a polarization direction before heating occurs at random in the substrate that is a dielectric crystal, and therefore a polarization direction of the ferroelectric substance is not single. The ferroelectric crystal having random polarization directions instead of a single polarization direction resultantly has a smaller pyroelectric effect than ferroelectric crystal having a single polarization direction, because electric fields due to polarization in different directions respectively cancel each other out inside of the ferroelectric crystal.

[0021] In other words, in the substrate before heating, macroscopically and microscopically, directions of spontaneous polarization are aligned in a fixed direction. On the other hand, in the substrate after heating, a part in a domain where the directions of spontaneous polarization are random is generated. The effect that the pyroelectric effect becomes smaller than the ferroelectric crystal of single polarization described above is realized by a structural change of the substrate.

[0022] The wavelength conversion device that is one embodiment of the present invention has a technical feature that before and after the heating and cooling treatments, values of a refractive index, a transmissivity and a linear expansion coefficient of the substrate that affect the output power of light that is converted in wavelength are almost unchanged, although piezoelectricity, electrostrictive characteristics and nonlinear optical characteristics that are the characteristics of the substrate are impaired. Since the wavelength conversion device that is one embodiment of the present invention has the technical feature, it is possible to use the wavelength conversion device while suppressing the pyroelectric effect of the substrate even in a dry gas atmosphere.

EXAMPLE

[0023] FIG. 3 is a view illustrating a configuration of a wavelength converter 30 that is one embodiment of the present invention. The wavelength converter 30 is such a wavelength conversion device that when a fundamental wave of a wavelength of 1550 nm is inputted from an input port 300 as a signal light 1a, a signal light 3c with a wavelength converted that is a second harmonic wave thereof and has a wavelength (775 nm) is outputted from an output port 301. The configuration of the wavelength conversion device 30 illustrated in FIG. 3 is same as the configuration of the wavelength conversion device 20 illustrated in FIG. 2. A difference is that a substrate 32 of a wavelength conversion element 33 adopts a ferroelectric crystal whose polarization directions are not single but random.

[0024] First, the configuration of the wavelength conversion device 30 will be described. The wavelength conversion device 30 includes a multiplexer 14 that combines the signal light 1a and control light (not illustrated), the wavelength conversion element 33 including an optical waveguide core 31 and a substrate 32, and a demultiplexer 15 that separates the signal light 3c with a wavelength converted and the control light (not illustrated). A metal casing including a first portion 39A of the metal casing and a second portion 39B of the metal casing is included to seal these elements, and inside the metal casing, a temperature control element 26, an upper member 27 and a metal casing bottom surface member 28 are further included.

[0025] The metal casing including the first portion 39A of the metal casing and the second portion 39B of the metal casing includes the input port 300 and the output port 301 for inputting and outputting light and inputting and outputting electric signals for temperature control, in side surfaces. Note that the multiplexer 14, the demultiplexer 15, and the signal light 1a are same as those in the explanation of FIG. 1, and therefore explanation will be omitted.

[0026] The optical waveguide core 31 is an optical waveguide that allows the signal light 1a to pass through an inside thereof selectively without losing intensity of the signal light 1a. A structure of the optical waveguide core 31 is not specially limited as long as the optical waveguide core has a function of outputting the signal light 3c with a wavelength converted, which has a different wavelength from the signal light 1a when the wavelength of the signal light 1a is inputted. The structure of the optical waveguide core 31 is a structure in which a quadratic nonlinear constant changes along a traveling direction of light periodically or at a period to which a predetermined modulation is given, and realizes quasi-phase matching for a single wavelength or a plurality of wavelengths, and it is also possible to adopt a multi QPM element, for example.

[0027] The substrate 32 is a substrate that is a ferroelectric and transparent to the signal light 1a, that is, does not cause light absorption. The substrate 32 is caused to function as an underclad to the optical waveguide core 31 when the ridge optical waveguide is configured, and needs to have a lower refractive index to the signal light 1a, the control light (not illustrated), and the signal light 3c with the wavelength converted, than the optical waveguide core 31.

[0028] The ferroelectric material adopted for the substrate 32 is LiNbO.sub.3, KNbO.sub.3 (potassium niobate), LiTaO.sub.3 (lithium tantalate), LiNb(x)Ta(1-x)O.sub.3 (0≤x≤1) (lithium tantalate of a nonstoichiometric composition), or KTiOPO.sub.4 (potassium titanate phosphate), and preferably further includes at least one selected from Mg (magnesium), Zn (zinc), Sc (scandium) and In (indium) as an additive therein.

[0029] The metal casing is a metal casing that joins the first portion 39A of the metal casing and the second portion 39B of the metal casing, and seals an inside so as to keep the inside airtight.

[0030] The upper member 27 is a metal member for controlling a temperature of the entire wavelength conversion element 33 including the optical waveguide core 31 and the substrate 32 uniformly.

[0031] Here, linear expansion coefficients of the upper member 27 and the metal casing bottom surface member 28 are preferably substantially equal to a linear expansion coefficient of the temperature control element 26 to suppress deformation of the upper member 27 and the metal casing bottom surface member 28 due to thermal stress that occurs due to a temperature change of the temperature control element 26. Specifically, the linear expansion coefficient of the upper member 27 is preferably 10×10.sup.−6/K or more and 20×10.sup.−6/K or less.

[0032] Further, a material of the upper member preferably includes one kind of metal or more selected from stainless steel, copper molybdenum steel, carbon steel, chrome molybdenum steel, copper, phosphorus deoxidized copper, oxygen free copper, phosphorus bronze, or brass. In the present example, oxygen free copper is used as the upper member 27.

[0033] The metal casing bottom surface member 28 is a member that has high mechanical strength, and has a high thermal conductivity to transmit heat generation and heat absorption effects by the temperature control element 26 to the wavelength conversion element 33 efficiently.

[0034] The linear expansion coefficient of the metal casing bottom surface member 28 is preferably 4×10.sup.−6/K or more and 12×10.sup.−6/K or less.

[0035] Further, the linear expansion coefficients of the upper member 27 and the metal casing bottom surface member 28 are both set to be substantially equal to the linear expansion coefficient of the temperature control element 26, and thereby it is possible to further suppress deformation of the upper member 27 and the metal casing bottom surface member 28 due to thermal stress. In other words, a range of a value of the linear expansion coefficient of the metal casing bottom surface member 28 is more preferably substantially equal to a range of a value of the linear expansion coefficient of the upper member 27.

[0036] Further, a material of the metal casing bottom surface member preferably includes one kind of metal or more selected from tungsten, molybdenum, kovar, copper tungsten steel, stainless steel, or copper molybdenum steel. In the present example, stainless steel was used as the metal casing bottom surface member 28.

[0037] The temperature control element 26 is interposed between the upper member 27 and the metal casing bottom surface member 28. The temperature control element 26 is a Peltier element. Further, in order to efficiently transmit the heat generation and heat absorption effects to the upper member 27 and the metal casing bottom surface member 28, it is possible to adopt solder joining, joining with a conductive adhesive, or other known joining methods, as a joining method with the upper member 27 and the metal casing bottom surface member 28.

[0038] Dry gas 302 is gas that fills an inside of the metal casing and does not include water. In other words, as illustrated in FIG. 3, the dry gas 302 is an atmosphere around the wavelength conversion element 33, the temperature control element 26, the upper member 27 and the metal casing bottom surface member 28 that are sealed in the metal casing.

[0039] The dry gas 302 preferably includes one kind or more selected from nitrogen, oxygen, argon or helium. In the present example, dry nitrogen gas was used as the dry gas 302.

[0040] Next, a production method of the wavelength conversion device 30 will be described. Heating and cooling treatments are performed for the substrate 32, in advance. As the substrate 32, a crystal of LiTaO.sub.3 of a thickness of 0.5 mm was put in an electric furnace, and was heated at 650° C. in a dry nitrogen gas atmosphere for two hours while the temperature was kept. After the heating, the substrate 32 was cooled until the temperature of the substrate 32 lowered to a room temperature.

[0041] By applying the heating and cooling treatments, directions of spontaneous polarization become random in the crystal of LiTaO.sub.3 that is the substrate 32. As a result, in an inside of the substrate 32, electric fields by random polarization canceled each other out, and a pyroelectric effect decreased as compared with a case of single-direction polarization, and it was possible to decrease a surface charge density of the substrate 32.

[0042] The ferroelectric substrate 32 was used as a lower portion of the ridge optical waveguide, and the optical waveguide core 31 of PPLN and of 5 μm square was formed on the substrate 32 by dry etching.

[0043] An implementer fixed the wavelength conversion element 33 produced as above onto the upper member 27, and fixed and housed the upper member 27 and the metal casing bottom surface member 28 with the temperature control element 26 interposed therebetween, to and in an inside of the first portion 39A of the metal casing with an upper surface opened. After fixing, the first portion 39A of the metal casing and the first portion 39B of the metal casing were seam-welded in the dry nitrogen gas atmosphere, and thereby hermetical sealing of the metal casing was performed. Joining spots by seam welding are denoted by 39e.

[0044] FIG. 4 and FIG. 5 are graphs showing phase matching characteristics of the wavelength conversion device 30 in a case where the substrate 32 is not heated at 650° C. in the dry nitrogen gas atmosphere for two hours in advance (FIG. 4) and a case where the substrate 32 is heated (FIG. 5) respectively. In FIG. 4 and FIG. 5, horizontal axes each shows a wavelength of the signal light 3c that is converted in wavelength and outputted from the wavelength conversion device 30, and vertical axes each shows output power thereof.

[0045] In FIG. 4, it is found that the phase matching condition is not locally satisfied in the optical waveguide core 31 due to a refraction index change of the substrate 32 due to a pyroelectric characteristic of the substrate 32, greatly deviates from a characteristic expressed by a Sinc function that is a theoretical phase matching curve, and the wavelength conversion efficiency declines. On the other hand, it is found that when the substrate 32 to which the heating treatment is performed is applied as the substrate of the wavelength conversion element as illustrated in FIG. 5, an influence of the pyroelectric effect is suppressed more favorably, a shape of a phase matching curve is close to an idealistic Sinc function and a high wavelength conversion efficiency can be obtained, as compared with a phase matching characteristic in the case where heating and cooling treatment are not performed for the substrate 32 shown in FIG. 4.

REFERENCE SIGNS LIST

[0046] 1a signal light [0047] 1b control light [0048] 1c difference frequency light [0049] 3c signal light with wavelength converted [0050] 10, 20, 30 wavelength conversion device [0051] 11, 31 optical waveguide core [0052] 12, 32 substrate [0053] 13, 33 wavelength conversion element [0054] 14 multiplexer [0055] 15 demultiplexer [0056] 26 temperature control element [0057] 27 upper member [0058] 28 metal casing bottom surface member [0059] 29 metal casing [0060] 39A first portion of metal casing [0061] 39B second portion of metal casing [0062] 39e joining portion of first portion of metal casing and second portion of metal casing [0063] 200, 300 input port [0064] 201, 301 output port [0065] 302 dry gas