JP2005128255A - Wavelength conversion element and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high efficiency wavelength conversion element consisting of a single mode ridge waveguide, and also to provide a method for manufacturing the conversion element in a high yield. <P>SOLUTION: In the wavelength conversion element having a core 32 consisting of a nonlinear optical crystal and the ridge optical waveguide in which the surface of the core 32 not in contact with a substrate 31 is formed of an air layer, the concentration distribution of Li on a cross section of the core 32 shows a gradual increase from the center part 33 of the core 32 to the outer peripheral part. Refractive index distribution on the cross section of the core 32 shows a gradual decrease from the center part 33 of the core 32 to the outer peripheral part. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a wavelength conversion element and a method for manufacturing the same, and more specifically, the wavelength of signal light is changed to another wavelength by using second harmonic generation, difference frequency generation, and sum frequency generation effects generated in a nonlinear optical medium. The present invention relates to a wavelength conversion element for conversion and a manufacturing method thereof.

  Conventionally, as a wavelength conversion element for converting the wavelength of light, an element using a semiconductor optical amplifier, an element using four-wave mixing, and the like are known. However, these wavelength conversion elements cannot satisfy the conditions required for the system, such as high efficiency, high speed, wide band, low noise, and polarization independence.

  On the other hand, application of wavelength conversion elements utilizing second harmonic generation, sum frequency generation, and difference frequency generation by quasi phase matching, which is a kind of second-order nonlinear optical effect, is expected. FIG. 1 shows a configuration of a conventional quasi phase matching type wavelength conversion element. The wavelength conversion element includes a multiplexer 11 that combines the signal light A having a relatively small light intensity and the control light B having a relatively large light intensity, and a waveguide made of a nonlinear optical crystal having a polarization inversion structure. 12 and a demultiplexer 13 for separating the difference frequency light C and the control light B. The signal light A is converted into the difference frequency light C having another wavelength in the waveguide 12 and emitted together with the control light B. For example, when the wavelength λ1 = 0.77 μm of the control light B, the signal light A having the wavelength λ2 = 1.55 μm is converted into the difference frequency light C having the wavelength λ3 = 1.53 μm.

  When such a wavelength conversion element is applied to a wavelength division multiplexing (WDM) transmission system that multiplexes and transmits optical signals of a plurality of wavelengths, a limited number of wavelengths can be used effectively. Functions such as wavelength group routing for batch conversion of groups can be added.

  For example, when the wavelength λ1 = 1.06 μm of the control light B and the signal light A having the wavelength λ2 = 1.55 μm are input, the mid-infrared light having the wavelength λ3 = 3.35 μm can be obtained by the difference frequency generation. . If a mid-infrared laser light source including such a wavelength conversion element is applied, a small and inexpensive gas sensor or the like can be realized. By combining control light and signal light of any wavelength, it is possible to obtain a laser light source of a new wavelength that cannot be obtained with conventional lasers, which is remarkable for increasing the sensitivity of optical instruments using visible light such as fluorescent microscopes. effective.

  A method of manufacturing such a wavelength conversion element using quasi-phase matching is that a wavelength conversion element is prepared by manufacturing a proton exchange waveguide after forming a periodically poled structure on a nonlinear optical crystal substrate such as lithium niobate. Was making.

  However, since the proton exchange waveguide forms a waveguide by proton diffusion from the surface, a high refractive index layer exists near the surface of the substrate, and the shape of the waveguide mode in the depth direction of the substrate is flat. It cannot be avoided. For this reason, the center position of the optical field of the signal light having a longer wavelength than the wavelength of the excitation light is positioned below the center position of the optical field of the excitation light. As a result, the mode overlap between the signal light and the excitation light deteriorates, and there is a difficulty in achieving highly efficient wavelength conversion.

  Therefore, the first substrate having the periodic domain-inverted structure and the second substrate are bonded together, and the thickness of the first substrate is reduced to a predetermined thickness for forming the optical waveguide. A method for manufacturing a wavelength conversion element is known (see, for example, Patent Document 1). This method is characterized in that the first substrate and the second substrate are directly bonded together by diffusion bonding by heat treatment. As shown in FIG. 2, the optical waveguide of the wavelength conversion element manufactured by this method has a ridge structure and a step-type refractive index distribution. The core 22 has three side surfaces not in contact with the substrate 21 in contact with the air layer, and a ridge-type optical waveguide (hereinafter referred to as a ridge waveguide) includes the core 22 and an air cladding.

  Thus, the ridge waveguide having the step type refractive index distribution can improve the mode overlap of the excitation light and the signal light as compared with the proton exchange waveguide having the diffusion type refractive index distribution. On the other hand, in order to achieve high-efficiency wavelength conversion, it is necessary to narrow the waveguide width so that the ridge waveguide satisfies the single mode condition.

JP 2003-140214 A

  However, the single-mode ridge waveguide shown in FIG. 2 has the following production problems. First, in order to produce a narrow ridge waveguide using a dicing saw, it is necessary to drive the dicing saw with submicron accuracy. Therefore, there is a problem that the yield is deteriorated due to unintended chipping.

Secondly, when a narrow waveguide is produced by using a dry etching method, it is necessary to produce a thick and narrow mask because the selection ratio between the LiNbO ₃ material and the mask cannot be obtained. is there. Therefore, in the lift-off process using photolithography, there is a problem that the mask shape is liable to be broken and the yield is deteriorated.

  The present invention has been made in view of such a problem, and an object of the present invention is to provide a high-efficiency wavelength conversion element formed of a single-mode ridge waveguide and a method of manufacturing a wavelength conversion element with a high yield. Is to provide.

  In order to achieve the above object, the present invention provides a ridge-type optical waveguide in which a core made of a nonlinear optical crystal and a surface of the core not contacting the substrate are made of an air layer. In the wavelength conversion element having the above, the concentration distribution of Li in the core cross section gradually increases from the central portion toward the outer peripheral portion of the core.

  A second aspect of the present invention is characterized in that the refractive index distribution of the core cross section according to the first aspect is gradually decreased from the central portion toward the outer peripheral portion of the core.

  A third aspect of the present invention is characterized in that the core according to the first or second aspect has a height and width of 7 microns or more.

  According to a fourth aspect of the present invention, the nonlinear optical crystal according to the first, second, or third aspect has a periodic domain-inverted structure.

The invention according to claim 5 is the nonlinear optical crystal according to any one of claims 1 to 4, wherein the nonlinear optical crystal is LiNbO ₃ , LiTaO ₃ , LiNb (x) Ta (1-x) O ₃ (0 ≦ x ≦ 1). Or at least one selected from the group consisting of Mg and Zn as an additive.

  The invention described in claim 6 is characterized in that a polarization-maintaining single mode optical fiber is optically coupled to the input end face of the core according to any one of claims 1 to 5.

  According to a seventh aspect of the present invention, there is provided a wavelength conversion element manufacturing method including a core made of a nonlinear optical crystal and a ridge-type optical waveguide whose surface not in contact with the substrate is an air layer. A first step of bonding the first substrate and the second substrate to each other by diffusion bonding by heat treatment, and a second step of polishing the first substrate to a predetermined thickness for forming the core And a third step of cutting the first substrate to form the core to produce a ridge-type optical waveguide, and the concentration distribution of Li in the core cross section from the central portion to the outer peripheral portion. And a fourth step of diffusing Li ions into the core so as to increase gently toward the core.

  As described above, according to the present invention, it is possible to obtain a single-mode high-efficiency wavelength conversion element by controlling the concentration distribution of Li in the core cross section of the ridge-type optical waveguide. Further, since the mode diameter of the ridge-type optical waveguide can be easily brought close to the mode diameter of the single mode optical fiber, the yield can be increased.

  Further, according to the present invention, the ridge-type optical waveguide can reduce the connection loss with the optical fiber in mounting the wavelength conversion module.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. The present invention controls the refractive index of nonlinear optical crystals such as LiNbO ₃ (hereinafter referred to as LN) and LiTaO ₃ (hereinafter referred to as LT) by changing the concentration of Li in the core cross section of the ridge waveguide by diffusion. A single mode ridge waveguide is fabricated.

  Conventionally, industrially used LN substrates or LT substrates are called congruent compositions. This composition is characterized in that the Li content is reduced as compared to the stoichiometric composition determined from the chemical quantitative ratio. This is because many Li-deficient sites are generated in the crystal in the single crystal manufacturing process due to the high vapor pressure of Li. At this time, it is known that the refractive index value of the substrate is larger in the congruent composition than in the stoichiometric composition.

  When excessive Li ions are supplied from the outside to an LN substrate or LT substrate having a congruent composition, Li diffuses into the substrate from the outside and fills the missing Li sites, so that it can be close to the stoichiometric composition. . That is, the refractive index of the outer peripheral portion of the ridge waveguide core can be lowered by diffusion of Li from the outside.

  FIG. 3 shows a configuration of a single mode ridge waveguide according to an embodiment of the present invention. When Li is diffused from the outside into the core 32 formed on the substrate 31, as described above, the refractive index of the core outer peripheral portion decreases, and a new core (a portion 33 having a high refractive index) is formed. With such a configuration, even when a ridge waveguide that is not narrowed is used, a single mode can be achieved by a simple method. In other words, the ridge waveguide has been made single mode by narrowing the width of the ridge waveguide, but it is not necessary to narrow the width of the ridge waveguide. Therefore, the yield can be improved in the production of a highly efficient wavelength conversion element.

  Compared to a conventional ridge waveguide composed of a core and an air cladding, the ridge waveguide shown in FIG. 3 includes an outer peripheral portion of the core 32, that is, a portion having a composition close to the stoichiometric composition and a central portion of the core 32. That is, the relative refractive index difference from the conventional congruent composition portion (the portion 33 having a high refractive index) is small. Therefore, the core diameter satisfying the single mode condition is larger than that of the conventional narrowed ridge waveguide, and has a mode diameter close to that of a commonly used single mode optical fiber. Such a configuration also has a mounting advantage that the coupling with the optical fiber is easy.

Here, as a means for controlling the Li concentration in the waveguide cross section, a method of diffusing Li ions in the waveguide by immersing the ridge waveguide in a lithium benzoate solution and heating it can be used. Alternatively, the ridge waveguide may be exposed to a saturated vapor of Li ₂ O in a high temperature container. As another method, a raw material powder containing at least Li ₂ O and mixed with at least one selected from the group consisting of Nb ₂ O ₅ , Ta ₂ O ₅ , MgO, and ZnO is prepared. A method may be used in which a crucible in which a ridge waveguide is disposed in this raw material powder is prepared, and the crucible is heated to a high temperature in an electric furnace to diffuse Li ions into the waveguide.

  The width of the ridge waveguide before the diffusion treatment of Li is desirably 7 microns or more. Even if a diffusion treatment is applied to a narrow waveguide having a width of less than 7 microns, the entire waveguide acts as a core because the relative refractive index difference induced by the diffusion of Li is small. Therefore, it is difficult to make a ridge waveguide of less than 7 microns into a single mode.

  EXAMPLES Hereinafter, although demonstrated using the Example of this invention, this invention is not limited to these Examples at all.

FIG. 4 shows a method for producing a single-mode ridge waveguide according to the first embodiment. The first substrate 41 is a Z-cut Zn-added LiNbO ₃ substrate in which a periodic domain-inverted structure is previously prepared, and the second substrate 42 is a Z-cut Mg-added LiNbO ₃ substrate. The substrates 41 and 42 are both 3-inch wafers whose surfaces are optically polished, and the thickness of the substrate is 300 μm. After the surfaces of the first substrate 41 and the second substrate 42 are rendered hydrophilic by normal acid cleaning or alkali cleaning, the substrates 41 and 42 are superposed in a clean atmosphere. The superposed substrates 41 and 42 are put into an electric furnace and subjected to diffusion bonding by heat treatment at 400 ° C. for 3 hours (first step). The bonded substrates 41 and 42 were void-free, and no cracks or the like occurred when returned to room temperature.

Next, using a polishing apparatus in which the flatness of the polishing surface plate is controlled, polishing is performed until the thickness of the first substrate 41 of the bonded substrates 41 and 42 becomes 15 μm. After the polishing process, a mirror-polished polishing surface is obtained by performing a polishing process (second step). A waveguide pattern is formed on the surface of a thin film substrate by a normal photolithography process, and then the substrate is set in a dry etching apparatus, and the core is formed by etching the substrate surface using CF ₄ gas as an etching gas. Was prepared (third step). In this manner, a ridge waveguide having a core having a height of 15 μm and a width of 15 μm can be produced.

  Next, a quartz container filled with lithium benzoate powder is prepared and heated to 170 ° C. The prepared ridge waveguide is immersed in molten lithium benzoate for 1 hour and then annealed in the atmosphere (340 ° C.) to diffuse Li ions into the core of the ridge waveguide (fourth step). .

  FIG. 5 shows the refractive index distribution of the cross section of the ridge waveguide subjected to the diffusion treatment. It is the refractive index measured along the broken line A-A ′. A dotted line B indicates a refractive index distribution before the diffusion process. By performing the diffusion treatment, the refractive index of the outer periphery of the core decreases, and a waveguide structure corresponding to a core having a width of about 5 microns is produced in the center of the core. The single-mode ridge waveguide thus fabricated is cut into a strip shape from a 3-inch wafer, and the wavelength conversion element having a length of 60 mm is fabricated by optically polishing the end face of the waveguide.

  When control light having a wavelength of 0.77 μm and signal light having a wavelength of 1.55 μm were incident on the prepared wavelength conversion element, converted light having a wavelength of 1.53 μm was obtained, and the efficiency was 2000% / W. Further, when the 10 waveguides thus prepared were measured, a wavelength conversion efficiency of 2000% / W or more was obtained in all 10 waveguides. According to the present embodiment, the yield is greatly improved in the production of the wavelength conversion element as compared with the case where the single mode is achieved by producing a conventional narrow waveguide.

In the second embodiment, similarly to the first embodiment, a Z-cut Zn-doped LiNbO ₃ substrate in which a periodic domain-inverted structure is prepared in advance is used as the first substrate 41, and the second substrate 42 is a Z-cut. An Mg-added LiNbO ₃ substrate is used. Each of the substrates 41 and 42 is a 3-inch wafer whose both surfaces are optically polished, and the thickness of the substrate is 500 μm. The substrate surfaces of the substrates 41 and 42 are subjected to a hydrophilic treatment by the same method as in Example 1 and are superposed in a clean atmosphere. The superposed substrates 41 and 42 are put into an electric furnace and subjected to diffusion bonding by heat treatment at 400 ° C. for 3 hours (first step). The bonded substrates 41 and 42 were void-free, and no cracks or the like occurred when returned to room temperature.

  Next, using a polishing apparatus in which the flatness of the polishing surface plate is controlled, polishing is performed until the thickness of the first substrate 41 of the bonded substrates 41 and 42 becomes 10 μm. After the polishing process, a mirror-polished polishing surface is obtained by performing a polishing process (second step). The polished thin film substrate is set on a dicing saw, and a ridge waveguide having a core having a width of 10 microns is fabricated by precision processing using a diamond blade having a particle diameter of 4 microns or less (third step).

Next, a platinum crucible filled with Li ₂ O and Nb ₂ O ₅ powder is prepared, and a ridge waveguide cut out in the platinum crucible is disposed. This is heat-treated in an electric furnace at 1100 ° C. for 1 hour to diffuse Li ions in the waveguide (fourth step). When the refractive index distribution of the cross section of the core subjected to the diffusion treatment is measured, a refractive index distribution similar to that of FIG. In this way, a waveguide structure corresponding to a core having a width of about 5 microns is produced at the center of the core. A wavelength conversion element having a length of 60 mm is produced by optically polishing the waveguide end face of the produced single-mode ridge waveguide.

  When the control light having a wavelength of 20 mW and a wavelength of 1.06 μm and the signal light having a wavelength of 1.32 μm are incident on the prepared wavelength conversion element, a yellow wavelength conversion light of 10 mW is generated at a wavelength of 0.59 μm by generating a sum frequency. The efficiency was 2500% / W high efficiency. According to the present embodiment, the yield is greatly improved in the production of the wavelength conversion element as compared with the case where the single mode is achieved by producing a conventional narrow waveguide.

  FIG. 6 shows a configuration of a wavelength conversion module using the wavelength conversion element according to the second embodiment. The wavelength conversion element 61 manufactured as described above is fixed on the carrier 62. A polarization maintaining single mode optical fiber 64 is connected to the input side of the wavelength conversion element 61. The output side of the wavelength conversion element 61 is optically coupled to the lens module 63 and emitted through the filter 65. A carrier 62 is enclosed in a package 66 to produce a wavelength conversion module. Compared with the case where a fiber is connected to a conventional narrow ridge waveguide, the connection loss is reduced and a connection loss of 0.3 dB can be achieved.

It is a perspective view which shows the structure of the conventional quasi phase matching type wavelength conversion element. It is sectional drawing which shows the structure of the conventional ridge type | mold optical waveguide made into single mode. It is sectional drawing which shows the structure of the single mode ridge waveguide concerning one Embodiment of this invention. 6 is a diagram illustrating a method for producing a single-mode ridge waveguide according to Example 1. FIG. It is a figure which shows the refractive index distribution of the ridge waveguide cross section which performed the diffusion process in Example 1. FIG. It is a block diagram which shows the wavelength conversion module using the wavelength conversion element concerning Example 2. FIG.

Explanation of symbols

11 multiplexer 12 nonlinear waveguide 13 duplexer 21, 31 substrate 22, 32 core 33 high refractive index portion 41 first substrate 42 second substrate 61 wavelength conversion element 62 carrier 63 lens module 64 polarization maintaining type Single mode optical fiber 65 Filter 66 Package

Claims (7)

In a wavelength conversion element having a core made of a nonlinear optical crystal and a ridge type optical waveguide whose surface not in contact with the substrate is made of an air layer,
2. The wavelength conversion element according to claim 1, wherein a concentration distribution of Li in the core cross section gradually increases from a central portion toward an outer peripheral portion of the core.   2. The wavelength conversion element according to claim 1, wherein the refractive index distribution of the core cross section gradually decreases from a central portion toward an outer peripheral portion of the core.   The wavelength conversion element according to claim 1 or 2, wherein the core has a height and a width of 7 microns or more.   The wavelength conversion element according to claim 1, wherein the nonlinear optical crystal has a periodic polarization inversion structure. The nonlinear optical crystal is any one of LiNbO ₃ , LiTaO ₃ , LiNb (x) Ta (1-x) O ₃ (0 ≦ x ≦ 1), or selected from the group consisting of Mg and Zn. 5. The wavelength conversion element according to claim 1, wherein at least one kind is contained as an additive.   6. The wavelength conversion element according to claim 1, wherein a polarization-maintaining single mode optical fiber is optically coupled to the input end face of the core. In a method of manufacturing a wavelength conversion element having a core made of a nonlinear optical crystal and a ridge-type optical waveguide whose surface not in contact with the substrate is made of an air layer,
A first step of bonding a first substrate made of a nonlinear optical crystal and a second substrate by diffusion bonding by heat treatment;
A second step of polishing the first substrate to a predetermined thickness for forming the core;
A third step of cutting the first substrate to form the core and producing a ridge-type optical waveguide;
A fourth step of diffusing Li ions into the core so that the concentration distribution of Li in the core cross section gradually increases from the central portion toward the outer peripheral portion of the core. A method for manufacturing a conversion element.

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