JP2009025555A - Manufacturing method of optical waveguide - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To manufacture an optical waveguide wherein propagation loss is reduced by recovering oxygen deficiency in vacuum plasma. <P>SOLUTION: This manufacturing method of the optical waveguide for forming the optical waveguide in an oxide compound substrate includes first process of forming the optical waveguide in the oxide compound substrate by dry etching process, and a second process of re-coupling the oxygen damaged by the dry etching process by applying heat treatment to the oxide compound substrate having the optical waveguide in an oxygen atmosphere. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a method for manufacturing an optical waveguide, and more particularly, to a method for processing an oxide compound substrate and a method for manufacturing an optical waveguide, which are widely used for wavelength conversion elements using nonlinear optical effects and electro-optical effects.

  In recent years, in order to increase the communication capacity of an optical communication system, a wavelength division multiplexing (WDM) communication system that multiplexes and transmits a plurality of lights having different wavelengths has been actively introduced. In such a WDM communication system, in order to effectively use a limited number of wavelengths, there is a demand for practical use of a wavelength conversion device that converts a signal wavelength into an arbitrary signal wavelength.

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

  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 an optical waveguide composed of a multiplexer 11 that multiplexes signal light A having relatively small light intensity and control light B having relatively large light intensity, and 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 optical 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.

A method of manufacturing such a wavelength conversion element using quasi-phase matching is described in that a periodic polarization reversal structure is formed on a nonlinear optical crystal substrate such as lithium niobate (LiNbO ₃ , hereinafter referred to as LN), The wavelength conversion element was produced by producing. However, since the proton exchange waveguide forms an optical 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.

  On the other hand, in order to improve optical confinement in the waveguide and realize highly efficient wavelength conversion using a nonlinear optical material with a bulk structure, a wavelength conversion element having a ridge type optical waveguide structure has been proposed. Yes. In the method of manufacturing a ridge-type optical waveguide, a periodically poled structure in which a nonlinear constant is periodically modulated is manufactured on an Mg-added LN substrate, and then bonded to an LN substrate prepared separately using an adhesive. After the substrate thickness of the Mg-added LN substrate is reduced by surface grinding, a ridge-type optical waveguide is manufactured by precision grinding using a dicing saw (for example, see Non-Patent Document 1).

M.Iwai, et al., “High-power blue generation from a periodically poled MgO: LiNbO3 ridge-type waveguide by frequency doubling of a diode end-pumped Nd: Y3Al5O12 laser”, Applied Physics Letters, Vol.83, No. 18, p.3659-3661, 3 Nov. 2003 Makoto Minakata, "LiNbO3 Optical Waveguide Device", IEICE Theory, Vol.J77-CI, No.5, p.194-205, 1994.5

  However, in the conventional processing method using a dicing saw, the size of the waveguide core is limited by the processing accuracy. For example, in the above-described precision grinding of the Mg-added LN substrate, the core width is 10 μm, and it is difficult to form an optical waveguide having a small core diameter. There is also a problem that it is difficult to form an optical waveguide other than a straight line in the longitudinal direction of the waveguide. In order to solve these problems, a method for forming an optical waveguide using a dry etching process is considered promising.

  However, it is known that an oxide compound such as LN takes a very long time for processing by a dry etching process. Further, when the oxide compound substrate is exposed to vacuum plasma for a long time, there is a problem in that a compound defect occurs. For example, it is known that when an LN substrate is exposed to vacuum plasma for a long time, Li and oxygen are lost (see, for example, Non-Patent Document 2). When oxygen is deficient from the substrate, the substrate is darkened, and there is a problem that propagation loss of the optical waveguide increases.

  The present invention has been made in view of such problems, and an object of the present invention is to provide a method of manufacturing an optical waveguide that recovers oxygen vacancies in vacuum plasma and has low propagation loss.

  In order to achieve the above object, the present invention provides a method for manufacturing an optical waveguide in which an optical waveguide is formed on an oxide compound substrate, wherein the oxide compound substrate is formed by a dry etching process. And a second step of recombining oxygen deficient in the dry etching process by heat-treating the oxide compound substrate on which the optical waveguide is formed in an oxygen atmosphere. The process is provided.

  The oxide compound substrate is formed by bonding a first substrate on which the optical waveguide core is formed and a second substrate, and the refractive index of the first substrate is higher than the refractive index of the second substrate. Is also expensive. An oxide compound substrate in which the first substrate and the second substrate are directly bonded to each other by diffusion bonding by heat treatment may be used. An oxide compound substrate made of the first substrate on which the optical waveguide core is formed by crystal growth on the second substrate by a liquid phase epitaxial method can also be used.

  The heat treatment in the second step is performed at a temperature not lower than 200 degrees and not higher than the Curie temperature of the oxide compound substrate. More preferably, it is performed at a temperature of 350 ° C. or higher and 800 ° C. or lower. The heat treatment is preferably performed for 3 hours or more.

The first substrate may have a periodic polarization reversal structure having a second-order nonlinear effect and having a nonlinear constant periodically modulated. The oxide compound substrate includes LiTaO ₃ , KTaO ₃ , KNb _(x) Ta _(1-x) O ₃ (0 <x <1), K _(y) Li _(1-y) Nb _(x) Ta _(1-x) O ₃ (0 <x <1, 0 <y <1), KNbO ₃ , KTiOPO ₄ , or at least one selected from the group consisting of Mg, Zn, Sc, In It can also be set as an oxide compound.

  As described above, according to the present invention, oxygen deficient in the dry etching process is recombined by performing heat treatment in an oxygen atmosphere after the dry etching process, so that an optical waveguide with less propagation loss is manufactured. It becomes possible.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Conventionally, it has been known that when an oxide compound substrate is exposed to vacuum plasma for a long time, a compound deficiency occurs, and oxygen enters and exits the crystal depending on the atmosphere (see Non-Patent Document 2). However, a method for recovering deficient oxygen, that is, a method of recombining oxygen to a deficient oxide compound substrate has not been known. The present invention is based on the finding that deficient oxygen can be recovered by heat treatment in an oxygen atmosphere, and an increase in propagation loss of the optical waveguide can be suppressed.

FIG. 2 shows a method of manufacturing a thin film substrate for a wavelength conversion element according to Example 1 of the present invention. The first substrate 21 is a Z-cut Zn-added LiNbO ₃ substrate, and a periodically domain-inverted structure is prepared in advance so that the phase matching condition is satisfied in the 1.5 μm band. The second substrate 22 is a Z-cut Mg-added LiNbO ₃ substrate. Each of the first substrate 21 and the second substrate 22 is obtained by adding an additive to LiNbO ₃ , and the thermal expansion coefficients are almost the same. Further, the refractive index of the first substrate 21 is higher than the refractive index of the second substrate 22 by changing the kind of the additive. Each of the first substrate 21 and the second substrate 22 is a 3-inch wafer (FIG. 2 shows a rectangular substrate for convenience) having both surfaces optically polished, and the thickness of the substrate is 300 μm.

  After the surfaces of the first substrate 21 and the second substrate 22 are made hydrophilic by normal acid cleaning or alkali cleaning, the substrates 21 and 22 are superposed in a clean atmosphere in which microparticles are not present as much as possible. The superposed substrates 21 and 22 are placed in an electric furnace and subjected to diffusion bonding by heat treatment at 400 degrees for 3 hours (first step). The bonded substrates 21 and 22 are void-free with no interposition of microparticles or the like on the bonding surfaces, and no cracks or the like occurred when the temperature was 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 21 of the bonded substrates 21 and 22 becomes 20 μm. After polishing, polishing is performed to obtain a mirror-polished surface (second step). When the parallelism, which is the difference between the maximum height and the minimum height of the substrate, was measured using an optical parallelism measuring machine, submicron parallelism was obtained almost entirely except for the periphery of a 3-inch wafer. . The thin film substrate 23 for the wavelength conversion element has a uniform composition over the entire area of the 3-inch wafer because the first substrate 21 and the second substrate 22 are directly bonded together by diffusion bonding by heat treatment without using an adhesive. The substrate has a film thickness.

FIG. 3 shows a method of manufacturing the wavelength conversion element according to Example 1 of the present invention. In the third step, an optical waveguide is formed on the thin film substrate 23 using a dry etching process. First, masks 24a and 24b corresponding to the waveguide pattern are formed on the surface of the thin film substrate 23, that is, the surface of the first substrate 21 by an ordinary photolithography process. Next, the thin film substrate 23 is set in a dry etching apparatus, and the surface of the thin film substrate 23 is etched using CF ₄ gas as an etching gas. In this manner, the ridge-type optical waveguides 25a to 25c having a core having a height of 8 μm and a width of 8 μm, which are difficult to manufacture by precision grinding using a dicing saw, can be manufactured with high accuracy. As shown in the drawing, in the dry etching process, the optical waveguide 25 has a mesa shape because the etching selectivity between the mask and the thin film is not large.

  FIG. 4 shows a step of cutting out the wavelength conversion element from the substrate. Here, a plurality of ridge-type optical waveguides were produced in parallel on a thin-film substrate 23 which is a 3-inch wafer. The thin film substrate 23 is cut into a strip shape for each optical waveguide, and both end surfaces 26a and 26b of the optical waveguide 25 are optically polished to produce a wavelength conversion element having a length of 60 mm. Measurement light of 0.98 μm is incident on one end face of the optical waveguide of these wavelength conversion elements. When the outgoing light obtained from the other end face is measured, the propagation loss is a very large loss of -15 dB. The etching rate of LN with respect to dry plasma was very slow, and exposure of the substrate to the plasma for 20 hours caused oxygen to escape from the substrate, increasing propagation loss.

  Therefore, in the fourth step, the substrate on which the optical waveguide 25 has been formed in the third step is placed in an electric furnace, and heat treatment is performed at 400 ° C. for 1 hour in an oxygen atmosphere (oxygen concentration 50%). Thereby, oxygen deficient by plasma etching can be recovered. After the oxygen annealing treatment, a wavelength conversion element is manufactured by the method shown in FIG. 4, and the propagation loss is measured. When measurement light of 0.98 μm was incident on one end face, the propagation loss was measured as −8 dB. Thereafter, the substrate on which the optical waveguide 25 was formed was again placed in an electric furnace and further subjected to heat treatment for 2 hours. When the propagation loss was measured, it was improved to -2 dB.

  When this propagation loss value is converted to the propagation loss of the optical waveguide per unit length, it becomes a very low loss of 0.3 dB / cm. When a control light having a wavelength of 0.77 μm and a signal light having a wavelength of 1.55 μm are incident on the wavelength conversion element that has been subjected to the oxygen annealing treatment for 3 hours in total, a wavelength conversion light having a wavelength of 1.53 μm is obtained with high efficiency Wavelength conversion was realized.

  A conventional wavelength conversion element using a proton exchange waveguide and a wavelength conversion element obtained by bonding a substrate serving as a waveguide using an adhesive cannot be subjected to heat treatment at a high temperature (200 degrees or more) again. This is because the former causes re-diffusion of protons and the latter causes melting of the adhesive. Since the wavelength conversion element in which the substrates are bonded by direct bonding as in Example 1 can be subjected to heat treatment at a high temperature again, oxygen is recombined to the oxide compound substrate in which defects are generated, and propagation loss is small. Suitable for production of optical waveguide.

  The heat treatment in the fourth step is preferably 200 ° C. or more, and 350 to 800 ° C. is appropriate in consideration of the feasible processing time. Moreover, in order to prevent the crystal | crystallization of a board | substrate material from changing, it is desirable to carry out below the Curie temperature of an oxide compound board | substrate. The heat treatment time depends on the temperature, but is preferably 1 hour or longer at 400 ° C., and is preferably 3 hours or longer. At 600 degrees, 30 minutes or longer is desirable, and at 800 degrees, 15 minutes or longer is desirable. The higher the temperature, the shorter the processing time. Further, an oxygen concentration of 40% or more is suitable.

FIG. 5 shows a method for producing a thin film substrate for a wavelength conversion element according to Example 2 of the present invention. The second substrate 42 is a Z-cut Mg-added LiNbO ₃ substrate. A 10 μm Zn-doped LiNbO ₃ crystal is grown on the second substrate 42 by a liquid phase epitaxial method. The grown crystal becomes the first substrate 41. After polishing the surface of the first substrate 41, a polishing surface is obtained by polishing.

Next, an optical waveguide similar to that of Example 1 is manufactured using a dry etching process. That is, as a first step, masks 44a and 44b corresponding to the waveguide pattern are formed on the surface of the thin film substrate 43, that is, the surface of the first substrate 41 by an ordinary photolithography process. Next, the thin film substrate 43 is set in a dry etching apparatus, and the surface of the thin film substrate 43 is etched using CF ₄ gas as an etching gas. In this manner, ridge-type optical waveguides 45a to 45c having a core having a height of 8 μm and a width of 8 μm can be manufactured.

  Similar to the process shown in FIG. 4, the thin film substrate 43 is cut into a strip shape for each optical waveguide, and both the end faces of the optical waveguide 45 are optically polished, thereby producing a wavelength conversion element having a length of 10 mm. Measurement light of 0.98 μm is incident on one end face of the optical waveguide of these wavelength conversion elements. When the outgoing light obtained from the other end face is measured, the propagation loss is a very large loss of -3 dB.

  Therefore, in the second step, the substrate on which the optical waveguide 45 is formed in the first step is put in an electric furnace, and heat treatment is performed at 400 degrees for 3 hours in an oxygen atmosphere (oxygen concentration 50%). Thereby, oxygen deficient by plasma etching can be recovered. After the oxygen annealing treatment, a wavelength conversion element is manufactured by the method shown in FIG. 4, and the propagation loss is measured. When measurement light of 0.98 μm is incident on one end face, the propagation loss is measured as a very small value of −0.5 dB.

  Since the wavelength conversion element in which the substrate is produced by crystal growth as in Example 2 can be subjected to heat treatment at a high temperature again, as in Example 1, oxygen is recombined with the deficient substrate, Suitable for fabrication of optical waveguides with low propagation loss.

In the present embodiment, LN is described as the oxide compound of the substrate material, but LiTaO ₃ , KTaO ₃ , KNb _(x) Ta _(1-x) O ₃ (0 <x <1), K _{( y)} Li _(1-y) Nb _(x) Ta _(1-x) O ₃ (0 <x <1, 0 <y <1), KNbO ₃ , KTiOPO ₄ , or Mg, Zn, Sc, An oxide compound containing at least one selected from the group consisting of In as an additive can also be used.

It is a figure which shows the structure of the conventional quasi phase matching type | mold wavelength conversion element. It is a figure which shows the method to produce the thin film substrate for wavelength conversion elements concerning Example 1 of this invention. It is a figure which shows the method of producing the wavelength conversion element concerning Example 1 of this invention. It is a figure which shows the process of cutting out a wavelength conversion element from a board | substrate. It is a figure which shows the method to produce the thin film substrate for wavelength conversion elements concerning Example 2 of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 Multiplexer 12 Optical waveguide 13 Divider 21,41 1st board | substrate 22,23 2nd board | substrate 23,43 Thin film substrate 24,44 Mask 25,45 Optical waveguide 26 End surface

Claims (9)

In the method of manufacturing an optical waveguide for forming an optical waveguide on an oxide compound substrate,
A first step of forming an optical waveguide on the oxide compound substrate by a dry etching process;
And a second step of recombining oxygen deficient in the dry etching process by heat-treating the oxide compound substrate on which the optical waveguide is formed in an oxygen atmosphere. A method for manufacturing a waveguide.   The oxide compound substrate is formed by bonding a first substrate on which the optical waveguide core is formed and a second substrate, and the refractive index of the first substrate is higher than the refractive index of the second substrate. The method of manufacturing an optical waveguide according to claim 1, wherein   3. The optical waveguide according to claim 2, wherein the oxide compound substrate is formed by directly bonding a first substrate on which the optical waveguide core is formed and a second substrate by diffusion bonding by heat treatment. Manufacturing method.   The oxide compound substrate includes a first substrate on which a crystal is grown on a second substrate by a liquid phase epitaxial method to form the optical waveguide core, and the refractive index of the first substrate is The method for manufacturing an optical waveguide according to claim 1, wherein the refractive index of the substrate is higher than that of the second substrate.   5. The method of manufacturing an optical waveguide according to claim 1, wherein the heat treatment in the second step is performed at a temperature of 200 ° C. or more and a Curie temperature or less of the oxide compound substrate.   The method for manufacturing an optical waveguide according to claim 5, wherein the heat treatment in the second step is performed at a temperature of 350 degrees to 800 degrees.   The method for manufacturing an optical waveguide according to claim 1, wherein the heat treatment in the second step is performed for 3 hours or more.   8. The method of manufacturing an optical waveguide according to claim 1, wherein the first substrate has a second-order nonlinear effect and has a periodically poled structure in which a nonlinear constant is periodically modulated. The oxide compound substrate includes LiTaO ₃ , KTaO ₃ , KNb _(x) Ta _(1-x) O ₃ (0 <x <1), K _(y) Li _(1-y) Nb _(x) Ta _{(1 -x)} O ₃ (0 <x <1, 0 <y <1), KNbO ₃ , KTiOPO ₄ , or at least one selected from the group consisting of Mg, Zn, Sc, and In as an additive 9. The method for manufacturing an optical waveguide according to claim 1, wherein the optical waveguide is made of an oxide compound.

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