Optical waveguide, wavelength changing element and short-wavelength laser light source

Description: TECHNICAL FIELD [Detailed description of the invention]

Industrial application fields The present invention relates to an optical waveguide device, a wavelength conversion element, and a short wavelength laser light source used in the fields of optical information processing and optical application measurement control to which a coherent light source is applied. Conventional technology The conventional optical waveguide configuration is LiNbO.₃A striped Ti film is formed on a substrate by sputtering and photolithography, heat-treated at a high temperature to form a waveguide, and then Mg is thermally diffused on the waveguide to determine the refractive index on the substrate side of the waveguide. There was a method of lowering the shape so that the waveguide was embedded in the substrate and making the electromagnetic field distribution of the propagating waveguide mode symmetrical with respect to the propagation axis. Figure 5 shows the configuration of a conventional optical waveguide. 1'is the substrate, 2'is the waveguide, 3'is the Mg diffusion layer, and 4'is the electromagnetic field distribution in the waveguide mode propagating through the waveguide. As a result, the electromagnetic field distribution in the waveguide mode propagating in the waveguide and the electromagnetic field distribution in the conductor mode propagating in the optical fiber are matched, and a high coupling efficiency of the waveguide-fiber is obtained. In addition, as conventional wavelength conversion elements, E.Lim, MMFejer.RLByer, Second harm onic generation of green light in a periodically-poled LiNbO₃ waveguide "Submitted to CLEO '89, Baltimore, Md. Etc. Fig. 6 shows the basic configuration diagram of the conventional wavelength conversion element. 5'is the substrate, 6' Is a waveguide made of a non-linear material, and 7'is a part made of a non-linear material and the non-linear polarization is inverted with respect to the non-linear polarization of the waveguide. The manufacturing method is that after forming Ti stripes by sputtering and photolithography, Ti is diffused at 100 ° C to form a polarization reversal grating layer. Next, in order to form a proton exchange waveguide, a waveguide mask is formed of aluminum, then heat-treated in benzoic acid at 300 ° C., the mask is removed, and annealing is performed to form a waveguide. By the polarization reversal grating formed by such a method, layers having different nonlinear polarizations are periodically formed in a waveguide made of a nonlinear substance, the phase of the second harmonic is matched, and a highly efficient wavelength conversion element is constructed. it can. Problems to be solved by the invention In the optical waveguide formed by the above method, since the electromagnetic field distribution of the waveguide light exists inside the substrate, the efficiency of the surface integrated element formed for the purpose of controlling the waveguide light is lowered. In addition, it is difficult to control the diffusion of the low refractive index layer formed on the surface, and it is difficult to make the electromagnetic field distribution completely axisymmetric. Furthermore, in the ion exchange waveguide formed at a relatively low temperature (200 ° C to 500 ° C), the method of lowering the refractive index of the waveguide surface by Mg thermal diffusion treated near 1000 ° C cannot be used. There are problems such as difficulty in forming an embedded waveguide. In view of the above points, it is an object of the present invention to provide an optical waveguide device having high diffraction efficiency by bringing the waveguides and gratings formed on the two substrates into close contact with each other so as to face each other. Further, in the wavelength conversion element having the above configuration, since the polarization inversion layer is formed by Ti diffusion, diffusion occurs not only in the depth direction but also in the lateral direction. Therefore, it is difficult to form a polarization inversion layer sufficiently deep with respect to the waveguide in view of the grating period (3 μm), and therefore it is difficult to improve the conversion efficiency of the wavelength conversion element. .. In view of the above points, the present invention determines the depth of the polarization inversion layer formed in the waveguide and the waveguide by bringing the waveguides having the polarization inversion layers formed on the two substrates into close contact with each other so as to face each other. It is an object of the present invention to provide a highly efficient wavelength conversion element, which makes it possible to double the conventional value. Means to solve problems In order to solve the above problems, the present invention comprises a first substrate, an optical waveguide formed on the surface of the first substrate, a portion formed on the surface of the optical waveguide having a different refractive index periodically, and the like. 2 substrates, an optical waveguide formed on the surface of the second substrate, and periodic refraction with the same period as the portions having different refractive indexes formed on the surface of the optical waveguide and formed on the first substrate. With different rates The optical waveguide formed on the first substrate and the optical waveguide formed on the second substrate are in close contact with each other with respect to the traveling direction, and the refractive indexes formed on the first substrate are different periodically. The portion and the portion formed on the second substrate having a different refractive index periodically Close to each other so as to face each other Or The optical waveguide formed on the first substrate and the optical waveguide formed on the second substrate are in close contact with each other with respect to the traveling direction, and the refractive indexes formed on the first substrate are different periodically. The part having a different refractive index at each time of the part and the part having a different refractive index formed on the second substrate They are in close contact with each other so that they are offset by 1/2 cycle from each other. An optical waveguide device having any of the above configurations. Further, the first substrate, an optical waveguide made of a non-linear material formed on the surface of the first substrate, a periodically inverted portion of polarization formed in the optical waveguide, a second substrate, and the above-mentioned An optical waveguide made of a non-linear material formed on the surface of a second substrate, an optical waveguide formed in the optical waveguide and a portion whose polarization is periodically inverted, and an optical waveguide formed on the first substrate and the second substrate. The optical waveguides formed on the substrate of No. 1 are in close contact with each other with respect to the traveling direction, and the portion formed on the first substrate whose polarization is inverted periodically and the portion formed on the second substrate periodically polarized. The optical waveguide conversion element is in close contact with each other so that the polarization inversion portions of the inversion portions are opposed to each other. Further, a short wavelength laser light source including a coherent light source, the light wavelength conversion element, and a condensing optical system in which light from the coherent light source is incident on the optical waveguide of the light wavelength conversion element is used as a semiconductor laser. A short-wavelength laser light source comprising the above-mentioned light wavelength conversion element and directly connecting the light emitting portion of the semiconductor laser and the end face of the optical waveguide of the light wavelength conversion element. Action According to the present invention, a waveguide having a symmetrical structure can be constructed, whereby the electric field distribution in the waveguide mode propagating the waveguide can be made symmetrical with respect to the waveguide propagation axis, so that the coupling efficiency with the optical fiber is increased. be able to. Further, when the gratings formed on the optical waveguide are brought into close contact with each other so as to face each other, the overlap between the waveguide light and the grating becomes large, so that the diffraction efficiency can be significantly increased. Further, by shifting the periods of the gratings produced on the waveguide by 1/2 and bringing them into close contact with each other, the period of the grating can be effectively halved, so that short wavelength light can be diffracted. .. By adhering optical waveguides having periodic polarization inversion layers formed on each substrate so as to face each other, the electromagnetic field distribution in the waveguide mode propagating through the optical waveguide is symmetrical with respect to the waveguide propagation axis. Since the polarization inversion portion can be formed in the central portion of the waveguide having a high electromagnetic field density, wavelength conversion with high efficiency can be realized. Example FIG. 1 shows a configuration that is similar to the embodiment of the present invention described below and is a prerequisite for the embodiment of the present invention. An optical waveguide provided with the second substrate and an optical waveguide formed on the surface of the second substrate, and the optical waveguide formed on the surface of the first substrate and the optical waveguide formed on the surface of the second substrate are in close contact with each other. Is. In Fig. 1, 1 and 2 are LiNbO.₃Board, 3 is LiNbO₃Proton exchange waveguide formed on substrate 1, 4 is LiNbO₃The proton exchange waveguide formed on the substrate 2 and 5 are the incident parts produced by polishing. The production method is LiNbO after washing.₃Ta on substrates 1 and 2 by sputtering method₂O₅To form. Ta by photolithography and dry etching₂O₅After preparing the striped mask of, heat-treat in pyrophosphoric acid at 230 ° C for 5 minutes to form a proton exchange waveguide. The prepared proton exchange waveguide was annealed in air at 350 ° C. for 1 hour to prepare optical waveguides 3 and 4 having a width of 5 μm and a depth of 2 μm. After cleaning these two waveguides, they were aligned with a mask aligner to bring the waveguides 3 and 4 into close contact with each other, and then both end surfaces were polished to form an incident portion. When the near-field pattern of the waveguide light emitted from the end face of the waveguide was observed by exciting the light of a semiconductor laser having a wavelength of 0.8 μm on the waveguide formed in close contact with each other, it was almost circular, and the electromagnetic field distribution of the waveguide light was observed. Was found to be symmetric with respect to the propagation axis. Next, coupling with a core-based 6 μm single-mode fiber was performed. After aligning the fiber and the waveguide under a microscope, it was fixed with a UV curable resin. When the light of a semiconductor laser having a wavelength of 0.8 μm was excited into the waveguide and the loss at the junction of the fiber waveguide was measured, it was 0.8 dB. This value is smaller than the coupling loss of 1 dB between the conventional embedded waveguide and the fiber. In addition, gel-like LiNbO having a refractive index of 2.2 between two substrates to be brought into close contact with each other.₃Was able to reduce the waveguide loss from 0.8 dB / cm to 0.5 dB / cm. As described above, by using a substance having a refractive index similar to that of the substrate such as organic or gel-like glass as the matching material, the loss of the waveguide can be reduced. Further, by applying a non-linear substance to the above-mentioned substrates 1 and 2, a wavelength conversion element can be configured. In the above-mentioned premise example, LiNbO is used as an optical waveguide.₃A proton exchange waveguide on a substrate was applied, but the optical waveguide is not limited to this, for example, an ion exchange waveguide on a glass substrate, LiNbO.₃Similar effects can be obtained with any thin film waveguide that can be formed on a substrate, such as a Ti diffusion waveguide on a substrate or a waveguide on an organic material. Based on the above premise example, an embodiment of the present invention in which the premise example is further improved will be described next. (Example 1) FIG. 2 shows the configuration of the optical waveguide in the first embodiment of the present invention, and is the configuration of the optical waveguide provided with a reflective grating. 1 and 2 are LiNbO₃Board, 3 is LiNbO₃Proton exchange waveguide formed on substrate 1, 4 is LiNbO₃A proton exchange waveguide formed on the substrate 2, 5 is an incident portion formed by polishing, and 6 is a reflection type grating formed on the waveguide 3. On the proton exchange waveguide prepared in the above-mentioned premise example, SiO₂Was deposited by 500 Å sputtering, and then a grating with a period of 0.4 μm and a width of 0.1 μm was formed by a photolithography method, and then the two waveguides were brought into close contact with each other to make the grating period 0.2 μm. Finally, the end face was polished to form an incident portion. By laminating the formed gratings on the two waveguides, the grating period could be halved from the created period. Further, by forming the grating inside the waveguide, it is possible to adopt a configuration in which the perturbation exists inside the waveguide by the grating, and it is possible to improve the efficiency of the grating. This point was confirmed by preparing 4 samples (L = 100,200,500,1000) with the grating length as a parameter and measuring the reflection efficiency. As a result, a reflection of 98% or more was obtained at a grating length of 200 μm. Since this was 98% at a grating length of 500 μm in the past, the grating of this example is very efficient. It should be noted that the control of the waveguide light by the electrodes can also be performed with high efficiency because the electro-optic effect can be similarly exerted inside the waveguide. (Example 2) FIG. 3 shows the configuration of the wavelength conversion element in the second embodiment of the present invention, in the optical waveguide made of a first substrate, a non-linear material formed on the surface of the first substrate, and the optical waveguide. A portion having a periodic structure in which the formed nonlinear polarization is inverted with respect to the nonlinear polarization of the optical waveguide, and an optical waveguide composed of a second substrate and a nonlinear substance formed on the surface of the second substrate. The optical waveguide formed on the surface of the first substrate and the second optical waveguide are provided with a portion made of a non-linear material in which the non-linear polarization formed in the optical waveguide is inverted with respect to the non-linear polarization of the optical waveguide and having a periodic structure. It is a wavelength conversion element in which the optical waveguide formed on the surface of the substrate and the portion where the non-linear polarization is inverted are brought into close contact with each other. In Fig. 3, 11 and 12 are LiNbO.₃Board, 13 is LiNbO₃A proton exchange waveguide formed on the substrate 11, 14 is a polarization inversion layer formed by Ti diffusion, and 15 is LiNbO.₃A proton exchange waveguide formed on the substrate 12, 16 is a polarization inversion layer formed by Ti diffusion. Next, the manufacturing method will be described. LiNbO, a non-linear substance₃After forming Ti stripes with a period of 2 μm, a width of 0.4 μm, and a grating length of 1 mm on the substrates 11 and 12, Ti is diffused at 1000 ° C for 1 hour to form a Ti diffusion layer having a width of 1 μm and a depth of 0.4 μm. Made. Ti diffusion layer is LiNbO₃The polarization is reversed with respect to the non-linear polarization of the substrate. Next, after forming a waveguide mask with aluminum to form a proton exchange waveguide, heat treatment with benzoic acid at 300 ° C is performed, the mask is removed, annealing is performed, and a waveguide 13 having a width of 4 μm and a depth of 0.5 μm is performed. , 15 to form. The waveguide maintains the non-linear polarization of the substrate. In this way, a grating made of the polarization inversion layers 14 and 16 made of the non-linear material whose polarization is inverted with respect to the non-linear polarization of the waveguide can be formed in the waveguide made of the non-linear material. After aligning the two prepared waveguides with a mask analyzer, they were brought into close contact with each other and the end faces were polished and formed. Since the Ti diffusion depth is half that of the conventional wavelength conversion element, a grating with a period of 2 μm could be formed. The manufactured wavelength conversion element was excited with the light of a semiconductor laser having a wavelength of 0.8 μm and 10 mW, and emitted light having a wavelength of 0.4 μm, which is the second harmonic of the excitation light. The generated second harmonic is 5nW, and the conversion efficiency is 10% / W · cm.²Met. The conversion efficiency of a wavelength conversion element in which a polarization inversion grating is formed on a conventional waveguide is 2.4% / W · cm.²Therefore, a very high conversion efficiency was obtained by the wavelength conversion element of this embodiment. In this embodiment, LiNbO is used as a substrate made of a nonlinear optical material.₃Was applied, but other MgO-doped LiNbO₃, LiTaO₃, KNbO₃Any substance with high non-linearity such as a ferroelectric substance such as MNA, an organic substance such as MNA, and a compound semiconductor such as ZnS can be applied as a substrate. Further, in this embodiment, the polarization inversion layer is formed in the waveguide, but by injecting ions such as Si into the waveguide using a metal mask such as Ti instead of the polarization inversion layer, the inside of the waveguide is formed. Even if a wavelength conversion element is produced by periodically forming a layer of a linear substance, a wavelength conversion element having a conversion efficiency about half that of the wavelength conversion element of the present embodiment can be produced. Even if the conversion efficiency is about half that of this embodiment, the conversion efficiency is higher than that of the wavelength conversion element having the conventional configuration. (Example 3) FIG. 4 shows the configuration of the short-wavelength laser light source according to the third embodiment of the present invention, and includes a coherent light source, an optical waveguide composed of a first substrate and a non-linear material formed on the surface of the first substrate. A portion composed of a non-linear material whose non-linear polarization formed in the optical waveguide is inverted with respect to the non-linear polarization of the optical waveguide and having a periodic structure, and a non-linear material formed on the second substrate and the surface of the second substrate. An optical waveguide made of the same and a portion made of a non-linear material in which the nonlinear polarization formed in the optical waveguide is inverted with respect to the nonlinear polarization of the optical waveguide and having a periodic structure, and formed on the surface of the first substrate. The optical waveguide, the optical waveguide formed on the surface of the second substrate, the portion where the non-linear polarization is inverted, and the wavelength conversion element provided with the incident portion formed by polishing the surface perpendicular to the waveguide, and the above. It is a short-waveguide laser light source provided with a condensing optical system that excites light from a coherent light source to an incident portion of the wavelength conversion element. In Fig. 4, 11 and 12 are LiNbO.₃Board, 13 is LiNbO₃A proton exchange waveguide formed on the substrate 11, 14 is a polarization inversion layer formed by Ti diffusion, and 15 is LiNbO.₃A proton exchange waveguide formed on the substrate 12, 16 is a polarization inversion layer formed by Ti diffusion, 17 is an incident part formed by polishing the end face, 18 is a condensing optical system, and 19 is a semiconductor laser having a wavelength of 0.8 μm. is there. In the wavelength conversion element produced in Example 2, the grating length was set to 1 cm, and the focusing optical system and the semiconductor laser were modularized to form a short wavelength laser light source of 14 × 14 × 28 mm. The output of the semiconductor laser was 40 mW, and a second harmonic output of 5 mW was obtained. This made it possible to construct a compact, high-power short-wavelength laser light source. If the semiconductor laser is directly connected to the waveguide incident portion, the module can be further miniaturized. Effect of the invention As described above, the present invention constitutes a waveguide having a symmetrical structure, and based on the configuration, the electric field distribution in the waveguide mode propagating the waveguide can be made symmetrical with respect to the waveguide propagation axis. The coupling efficiency with the fiber can be increased. Further, since the gratings formed on the optical waveguide are brought into close contact with each other so as to face each other, the overlap between the waveguide light and the grating becomes large, so that the diffraction efficiency can be significantly increased. Furthermore, by adopting a configuration in which the grating periods produced on the waveguide are shifted by 1/2 period from each other and closely attached to each other, the grating period can be effectively halved, and short-wavelength diffraction can be enabled. At the same time, the wavelength conversion efficiency can be improved. In addition, the depth of polarization inversion can be doubled by the configuration in which the optical waveguides having periodic polarization inversion formed on each substrate are in close contact with each other so as to face each other. The overlap is increased, and the conversion efficiency from the fundamental wave to the harmonic can be greatly improved. By applying these optical wavelength conversion elements and a collimating light source such as a semiconductor laser, a compact, high-power short-wavelength laser light source can be configured.

[Simple explanation of drawings]

FIG. 1 is a structural perspective view of an optical waveguide which is a premise of an embodiment of the present invention, FIG. 2 is a structural perspective view of a reflective optical waveguide using a waveguide according to an embodiment of the present invention, and FIG. 3 is a present. FIG. 4 is a structural perspective view of a wavelength conversion element according to another embodiment of the present invention, FIG. 4 is a structural perspective view of a short wavelength laser light source according to another embodiment of the present invention, and FIG. 5 is a basic configuration diagram of a conventional optical waveguide. Figure 6 is a basic configuration diagram of a conventional wavelength conversion element. 1,2 ... board, 3,4 ... waveguide, 5 ... incident part.