JP2006098716A - Optical waveguide and manufacturing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To avoid failures due to thermal bonding in the manufacturing method of a stuck type optical waveguide made of a ferroelectric and other materials. <P>SOLUTION: In the manufacturing method of the optical waveguide, in which a plurality of base materials are jointed and at least the base materials on the light-propagating side are composed of ferroelectric materials, jointing is made possible at the room temperature, by selecting the base materials or forming thin film layers so that the main components of the jointing face are all silicon dioxide and by dropping a hydrogen fluoride contained solution or an alkaline solution on the jointing face to fuse the base materials. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for producing an optical waveguide and an optical waveguide produced by this method, and is particularly suitably used for a ridge type optical waveguide using a ferroelectric material produced by bonding substrates.

Various optical devices are used in the fields of optical information communication systems, measuring devices such as interferometers, pickup lasers for optical disks, and printing devices.
One of these optical devices is an optical waveguide. An optical waveguide is generally composed of a portion called a core and a clad, and light is confined and propagated in the core by designing the core so that the refractive index of the core is larger than that of the clad.
Alternatively, by forming such an optical waveguide in a ferroelectric material, it is used as an optical switch or a wavelength conversion element. The quasi phase matching element used as a wavelength conversion element forms a periodic polarization inversion structure inside a ferroelectric bulk crystal such as LiNbO ₃ or LiTaO ₃ and further forms an optical waveguide so as to cross the periodic polarization inversion structure. Thus, phase matching is performed on the light passing through the optical waveguide. In the quasi phase matching element, a phase matching element corresponding to a desired wavelength is created by adjusting the period of the polarization inversion layer.

  Thus, when an optical waveguide is produced as a device such as an optical waveguide or a wavelength conversion element, the difference in refractive index between the optical waveguide layer (core) and the surrounding layers is important. That is, if the refractive index difference at the boundary surface between the optical waveguide layer and the surrounding layer is steep, light can be completely confined in the waveguide, but if the refractive index changes slowly at the boundary, Light passing through the waveguide oozes out and attenuates.

  At the same time, due to the demand from the application to the device, the shape of the light passing through the optical waveguide (guided light mode shape) is required to be as good as possible in the vertical and horizontal symmetry as much as possible. It is necessary to make the shape of the boundary surface with the surrounding layer (the boundary surface where the refractive index difference occurs) symmetric vertically and horizontally. Ideally, it is desirable that the cross-sectional shape of the optical waveguide layer is circular, such as the core layer and the cladding layer of the optical fiber, but the optical waveguide formed on the ferroelectric crystal substrate is on a flat substrate. The cross-sectional shape of the optical waveguide layer cannot be made circular because it must be manufactured by a process such as pattern etching used in a semiconductor process. Therefore, normally, as described in Patent Document 1, a ridge-type structure in which the cross section of the optical waveguide layer is processed into a chevron is used. In the ridge-type optical waveguide, the substrate layer is sandwiched between the air layers in the left-right direction, and a structure that combines both the refractive index difference and the left-right symmetry is realized, but is difficult to realize in the up-down direction.

  As a conventional example of a method for forming a refractive index difference in the vertical direction, as in Patent Document 1, a proton exchange treatment is performed from the processed surface side of the substrate to form an exchange treatment portion, and a ridge is formed in a part of the exchange treatment portion. It is known to form an optical waveguide by forming a mold structure and then subjecting the proton exchange treatment part to thermal diffusion treatment. However, in this method, the upper surface is the boundary between the air layer and the proton exchange layer, whereas the lower surface is the boundary between the proton exchange layer and the proton non-exchange layer, and is not a clear boundary. Therefore, the refractive index difference at the top surface is steep and the refractive index difference is large, whereas the refractive index change is not steep at the lower end surface compared to the upper end surface and the refractive index difference is small, and the shape of the boundary surface in the vertical direction However, the vertical symmetry as an optical waveguide was not good.

Therefore, in order to improve vertical symmetry, an optical waveguide has been devised in which different substrates having different refractive indexes are bonded in the vertical direction (for example, Patent Documents 2 and 3).
JP 2002-365680 A Japanese Patent No. 2574594 JP 2004-145261 A

The bonded ridge type optical waveguide device described in Patent Document 2 is heat-treated by hydroxyl group or oxygen through SiO ₂ , but uses a ferroelectric material that forms the optical waveguide, particularly polarization inversion. In the quasi phase matching wavelength conversion element, the polarization is disturbed by the heat treatment, and adverse effects such as a decrease in conversion efficiency occur.
In addition, in the bonded ridge type optical waveguide device described in Patent Document 3, there is proposed a technique in which bonding is performed through a thin film layer such as Ta ₂ O ₅ and a layer having a low refractive index is sandwiched. It is assumed that heat treatment is performed, and an adverse effect due to the heat treatment is unavoidable.

  An object of the present invention is to provide a method for producing an optical waveguide suitable for a quasi phase matching wavelength conversion element, in which bonded optical waveguides are produced without performing heat treatment, and adverse effects due to heat do not occur.

  The optical waveguide manufacturing method of the present invention made to solve the above problems is a method of manufacturing an optical waveguide in which a plurality of base materials are joined and at least the base material on which light is propagated is made of a ferroelectric material. The main component of the bonding surface is silicon dioxide, and bonding is performed by interposing a solution for melting the base material on the bonding surface.

  According to this method, the substrate can be bonded at room temperature without going through a heat treatment step, and it is easy to produce and does not require equipment for heat treatment. In the production of an optical waveguide based on a ferroelectric material, There is no polarization non-uniformity caused by heat treatment. In particular, in an optical waveguide using periodic polarization reversal using quasi-phase matching, disturbance of periodic polarization reversal due to heat treatment does not occur, so that an optical waveguide with excellent conversion efficiency can be provided.

  Hereinafter, an optical waveguide and a manufacturing method thereof according to the present invention will be described with reference to FIGS. In the embodiment described below, a method for producing a ridge-type optical waveguide used as a harmonic wavelength conversion element using a ferroelectric material as a base material will be described as an example. However, the present invention is not limited to this. Instead, the present invention can be applied without departing from the spirit of the invention.

(A)
A ferroelectric material A serving as a first substrate on which an optical waveguide is formed is prepared. As materials, lithium niobate (LiNbO ₃ ) and lithium tantalate (LiTaO ₃ ), as well as KNbO ₃ -based crystals having a nonlinear crystal optical effect can be used. However, the material is not limited to these, and any ferroelectric material applicable as an optical waveguide device can be used.
Moreover, in order to make the first substrate function as a harmonic wavelength conversion element, periodic polarization inversion is performed in advance so as to match a desired wavelength by a known method such as voltage application. The periodically poled structure is obtained, for example, by performing electrode patterning on the substrate surface and applying a voltage to form a periodically poled layer. Since the method of forming the periodically poled layer on the ferroelectric material is a well-known technique, the description thereof is omitted.

(B)
Next, a thin film B mainly composed of SiO ₂ or SiO ₂ is formed on at least the bonding surface side (the lower side in the figure) of the first base A. The thickness to be formed may be a thickness that can be confined by light (for example, about 0.5 μm). As a method for forming the thin film, a known film forming method such as a sputter film forming device or a CVD device may be used.

(C)
A second substrate C is prepared. The second substrate C is a member used to support the first substrate A, and may be any material having a fastness capable of supporting the first substrate A. Examples of the second substrate include glass substrates such as quartz glass, soda lime glass, lead glass, and borosilicate glass, as well as other materials having a silicon oxide film applied to the bonding surface in the same manner as the first substrate, such as a plastic substrate. Metal substrates, semiconductor substrates, etc. can be used.
If the second substrate C is made of the same material as the first substrate A, and the thin film B mainly composed of SiO ₂ or SiO ₂ is formed on the bonding surface side as in (b), the expansion coefficient with respect to temperature change is It becomes equal and can be set as the structure which is hard to produce peeling and distortion.

(D)
The first substrate A and the second substrate C are joined. It is desirable that the bonding surface be cleaned prior to bonding. For cleaning, organic cleaning with acetone and methanol, cleaning using a mixed solution of H ₂ SO ₄ and H ₂ O ₂ , cleaning using a mixed solution of NH ₃ , H ₂ O ₂ and water, and pure water were used. This is done by combining washing.
Next, a solution containing hydrofluoric acid or an alkaline solution is dropped onto the bonding surfaces of both substrates. The dropped solution diffuses along the joint surfaces of both substrates. In addition, the solution containing hydrofluoric acid used here is a mixture of hydrofluoric acid and water, for example, having a mixing ratio of 1%, or buffered hydrofluoric acid (ammonium fluoride and hydrofluoric acid). Or a commercially available hydrofluoric acid stock solution (46%), and an alkaline solution is an inorganic alkaline solution such as KOH, NaOH, NH ₄ OH, TMAH (tetramethyl hydroxide). An organic alkali solution such as ammonium) can be used.
The bonding surfaces to which the solution is applied are appropriately aligned at room temperature and overlapped. Preferably, a load, for example 31 gf / cm ² , is applied to the substrate and left for an appropriate time, for example 24 hours, to complete the bonding.

(E)
Of the substrates that have been joined, the first substrate A side on which the optical waveguide is formed is polished and thinned. Since the thickness after polishing is the final thickness of the optical waveguide, it is set to a constant thickness determined by the wavelength according to the purpose of use.

(F)
The mask material D is patterned using a photolithography technique to a width that becomes the width of the waveguide. Ferroelectric materials generally have a slow etching rate in methods such as dry etching, so it is better to pattern a metal material such as Cr or Ni with a resist and use it as a mask material for a ferroelectric material substrate. .

(G)
The first substrate A is etched so as to become a known ridge structure by a technique such as dry etching. As a processing method, of course, a known processing method such as direct machining, ion milling, or laser ablation may be used in addition to dry etching.

(H)
The mask material D is removed. The light introduced into the ridge-shaped part (optical waveguide E) is totally reflected at the boundary between the left and right ridge-shaped air layers and the upper air layer and the lower layer B (having a lower refractive index than the first substrate A). Light is guided. Although the optical waveguide has been completed with this, since the symmetry is ensured and the performance is improved by making the upper and lower refractive indexes the same, the SiO ₂ layer F is formed as a refractive index adjustment layer on the completed ridge shape. Was deposited. Thereby, it is possible to obtain an optical waveguide having good symmetry in the vertical and horizontal directions. Here, a periodically poled structure is formed in the optical waveguide portion, and functions as a quasi-phase matching element.

(Modified Example)
The shape of the optical waveguide is not limited to that of the above embodiment. In order to set the refractive indexes at the upper and lower interfaces of the optical waveguide to be the same, in the above example, the SiO ₂ layer F is formed on the upper surface of the waveguide. On the contrary, as shown in FIG. A gap G may be formed on the side, and the target may be secured by forming an air layer above and below the optical waveguide. However, an approximate value may be sufficiently acceptable depending on the application and required performance. The shape of the optical waveguide is not limited to the ridge type, and may be a known groove type.

  The present invention relates to an optical information communication system, a measuring device such as an interferometer, a pickup laser for an optical disk, an optical switch using an optical waveguide used in the field of a printing device, and a wavelength conversion in which a periodic structure is formed in a ferroelectric. It can be used to fabricate optical waveguides such as elements, second harmonic generation elements, various optical devices, and optical waveguide devices.

It is a figure explaining the manufacturing method of the optical waveguide which concerns on this invention. It is a figure which shows another structural example of the optical waveguide which concerns on this invention.

Explanation of symbols

A First substrate B Thin film C Second substrate D Mask material E Optical waveguide (ridge structure)
F Refractive index adjustment layer G Void

Claims (4)

  A method of manufacturing an optical waveguide in which a plurality of base materials are joined and at least the base material on which light is propagated is made of a ferroelectric material. A method for producing an optical waveguide, comprising joining with a solution for melting a material interposed.   The method for producing an optical waveguide according to claim 1, wherein the solution for melting the base material is a solution containing hydrofluoric acid or an alkaline solution.   3. The method of manufacturing an optical waveguide according to claim 1, wherein the ferroelectric material is one whose polarization is periodically reversed.   An optical waveguide manufactured by the method according to claim 1.

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