JPH04296829A - Manufactuer of optical wave guide type optical element generatingf second harmonics - Google Patents

Abstract

PURPOSE:To manufacture an optical wave guide type optical device generating second harmonics, provided with a pseudo-phase adjusting means, for example, due to polarization reversal structure, at a stable characteristic. CONSTITUTION:After a cyclic polarization reversal structure 2 is formed on the surface of a LiNbO3 base 1 by Ti diffusion or Li2O external diffusion, a mirror surface polishing is carried out including mechanical polishing so that the surface layer is removed, and an optical wave guide 3 is then formed, so as to manufacture an aimed optical wave guide type optical device generating second harmonics.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to a method for manufacturing an optical waveguide-type optical second harmonic generation device, and in particular an optical waveguide-type optical second harmonic generation device having a polarization inversion structure within the optical waveguide or in contact with the optical waveguide. It is related to the manufacturing method of the element.

0002

2. Description of the Related Art The need for short wavelength light is increasing for purposes such as high-density recording and high-resolution reproduction of optical discs, magneto-optical discs, etc.

As a device for extracting this short wavelength light, there has been remarkable research and development toward the practical application of optical waveguide type optical second harmonic generation elements (optical waveguide type SHG elements) which have high second harmonic generation efficiency.

This type of optical waveguide type SHG device is shown in FIG. 2C.
As shown in its schematic cross-sectional view, LiNbO3 is a nonlinear optical crystal with a large nonlinear optical constant and excellent heat resistance.
Pseudo-phase matching is achieved by a polarization inversion structure 2 in which the polarization (domain) is periodically inverted in order to achieve phase matching between the fundamental wave and the second harmonic on the substrate 1 and to increase the second harmonic generation efficiency, so-called SHG efficiency. A configuration is adopted in which the means is formed within the optical waveguide 3.

[0005] Furthermore, a linear waveguide was formed on a nonlinear optical single crystal substrate of LiNbO3, a fundamental wave was input to this waveguide, and a second harmonic component due to Cerenkov radiation was extracted from the nonlinear optical crystal substrate side. In the Cerenkov radiation type SHG as well, it has been proposed to reduce the Cerenkov radiation angle α and form a periodic polarization inversion structure.

As a method for forming such a polarization-inverted structure, for example, a Ti diffusion method in which Ti is diffused from the crystal surface (Hiromasa Ito, Hidekai Zhang, Fumio Inaba, Proceedings of the 49th Japan Society of Applied Physics Conference 919 (1988)) is used. ), or Li2
Out-diffusion method of O (Jonas Webjoern et al.
．． al, IEEE. PHOTONICS TEC
HNOL. LETT. 1, 1989, PP31
6-318) has been proposed.

As shown in FIG. 2A, the Ti diffusion method uses LiNb whose polarization direction is made into a single domain as indicated by the arrow a.
O3 (referred to as LN) in a direction perpendicular to the plane of paper in FIG. 2 with a pitch corresponding to the wavelength Λ of a pattern corresponding to a pattern in which polarization period inversion is to be formed on the main surface 1a on the +z (+c) side of the substrate 1, for example. A Ti metal layer 4 in the form of parallel stripes extending over the area is formed to a thickness of several tens of angstroms by vapor deposition or sputtering, and is patterned by selective etching using photolithography.

1000-1100 in an oxygen atmosphere
By performing thermal diffusion at a temperature of .degree. C., as shown in FIG. 2B, the polarization is reversed as indicated by arrow b at the point where Ti is diffused.

[0009] Thereafter, for example, a striped waveguide 3 is formed on the surface 1a side of the substrate 1 by a proton exchange method or the like along the direction of periodic reversal of polarization, as shown in FIG. 2C.

[0010] According to such a configuration, a periodic polarization inversion structure portion 2 in which polarization is repeatedly inverted in the waveguide direction is formed in the waveguide 3, thereby forming a quasi-phase matching means. As for the Li2O outdiffusion method, the Ti
Instead of the metal layer 4, a mask layer of a material such as SiO2 that does not interdiffuse with LN, such as SiO2, is formed on the surface of the LiNbO3 (LN) substrate 1 by, for example, full-surface vapor deposition or selective etching by photolithography after sputtering, as described above. Similarly, a pattern having a pitch corresponding to the period of periodic polarization inversion to be finally obtained is formed.

[0011] Thereafter, similar heat treatment is performed to remove Li2O in the LN substrate 1 in the areas where the mask layer is not formed.
In this method, the Li2O is diffused out, and polarization inversion is formed by the portion where the Li2O escapes.

[0012] Thereafter, a striped waveguide 3 is formed similarly by the proton exchange method.

In the case of the Ti diffusion method described above, almost no Ti metal layer 4 formed on the LN substrate 1 remains on the surface due to the diffusion, and in the case of the Li2O out-diffusion method, the SiO2 mask layer also remains. Since this is sufficiently thin, it does not affect the formation of the waveguide 3, and the waveguide 3 can be formed by simply chemically etching or cleaning the surface, for example.

However, in the waveguide type SHG obtained by these methods, as shown in the plan view and cross-sectional view of FIGS. 3A and 3B, when light of L is introduced into the input end of the waveguide 3, the output end As shown in the patterns on the screen S in FIGS. 3B and 4, the spots of the emitted light on the screen are a spot Lg due to the waveguide mode and a radiation mode indicated by C in FIG. 3B from the waveguide 3. The spot Lc becomes, for example, elliptical or crescent-shaped, and the second harmonic generated by quasi-phase matching also has many radiation modes, making it impossible to efficiently obtain the second harmonic in the guided mode.

For example, the second radiation emitted from the end face of the SHG element
The power of harmonics is 1 including guided mode and radiation mode.
mW, but when only the waveguide mode is measured, it is 0.45mW
Met.

The generation of such a radiation mode is caused by the fact that when the above-mentioned Ti diffusion method is used, Ti that remains undiffused on the main surface 1a side of the LN substrate 1 or Ti that has been diffused in large quantities on the surface is then During the high-temperature heat diffusion treatment in an oxygen atmosphere, it is oxidized and remains as TiO2, as shown by the broken lines in FIGS. 2B and 2C, or as Li2.
SiO2, which acts as a mask for O out-diffusion, produces a so-called mask effect in the subsequent proton exchange process of the optical waveguide 3, or forms irregularities on the surface.
In addition, as shown in FIG. 5, the cross section of the optical waveguide 3 is further diffused and unevenness is generated in the optical waveguide 3, discontinuous portions are formed, and unevenness in the refractive index is caused. Furthermore, we have determined that this is the cause of increased propagation loss.

[0017]

Problems to be Solved by the Invention In the present invention, it is possible to effectively reduce the propagation loss of the fundamental wave, increase the waveguide power, improve the SHG conversion efficiency, and further reduce the generated second harmonic (
The present invention provides a method for manufacturing an optical waveguide type optical second harmonic generation element that can reduce the propagation loss of SH waves.

[0018]

[Means for solving the problem] In the present invention, FIG.
As shown in the schematic cross-sectional views of each process in A to C, the surface of the LiNbO3 substrate 1 (principal surface 1
a) a step of forming the domain-inverted structure 2 by Ti diffusion or Li2O out-diffusion method (FIG. 1B); and a mirror polishing step of removing the surface layer of the domain-inverted structure 2 by mechanical polishing. After that, the optical waveguide 3 is formed by, for example, proton exchange (FIG. 1C).

[0019]

[Operation] In the above-mentioned manufacturing method of the present invention, the surface layer of the polarization inversion structure 2 formed by Ti diffusion or Li2O out-diffusion method is mechanically removed and polished to a mirror surface. Stable Ti or SiO2
The optical waveguide 3, which is formed by proton exchange from this mirror side, is formed by removing foreign substances such as oxides generated during Ti diffusion or Li2O out-diffusion, and by removing the uneven surface due to the presence of these particles. It can be formed uniformly and reliably to have a uniform thickness over the entire length without creating discontinuities.

Furthermore, since the portion of the surface layer of the substrate 1 with a high Ti concentration is removed, changes in the refractive index due to residual Ti, for example, are eliminated, so that the polarization inversion structure portion 2 is formed with almost no change in the refractive index. is formed.

[0021] Therefore, the second harmonic obtained thereby can also have a good spot shape with less radiation mode and less distortion.

[0022]

EMBODIMENTS Referring to FIG. 1, a case will be described in which the manufacturing method of the present invention is formed by a Ti diffusion method to form a polarization inversion structure.

First, as shown in FIG. 1A, Ti is deposited on the +z (+c) main surface 1a of the single-domain LN substrate 1.
A Ti metal layer 4 is formed in a parallel stripe pattern with a period corresponding to the pattern of polarization period inversion to be finally obtained by photolithography.

[0024] Then, in an oxygen atmosphere, 1000 to 110
Thermal diffusion treatment is performed by heat treatment at 0°C, and the deepest part depth d is, for example, 1 μm or more and 10 μm in the part where Ti is diffused.
The polarization inversion structure portion 2 is formed to have a thickness of approximately 3 μm or more and 10 μm or less, preferably 3 μm or more and 10 μm or less.

Thereafter, as shown in FIG. 1B, the LN substrate 1
The polarization-inverted structure is mechanically polished from the main surface 1a side of the main surface 1a to the position shown by the dashed line f in FIG. Polishing is performed to a depth shallower than the depth d of the deepest part of part 2. In this case, mirror polishing is performed so that the remaining inverted structure portion 2 has a depth dp of 1 μm or more, for example, 1 to 2 μm. And this newly created smooth mirror surface 1A
The optical waveguide 3 is formed from the side by a well-known proton exchange method, for example, by immersion in hot phosphoric acid and subsequent heat treatment.

Example 1 A periodic polarization inversion structure 2 is inverted by Ti diffusion facing the main surface 1a consisting of the +z plane (+c plane) of a single-domain LN substrate 1. It was formed at a distance of 4 μm from the main surface 1a of the LN substrate 1.

[0027] After that, mechanical polishing is performed using a diamond slurry made of 1/4 μm diamond powder, and then polishing is performed using a diamond slurry with a particle size of 1/10 μm, so that the entire surface is polished to a depth of 3 μm from the main surface 1a. After removal, a smooth mirror surface 1A was formed.

Thereafter, a channel-type, ie, striped, optical waveguide 3 was formed by the proton exchange method. By guiding a near-infrared laser beam of 866 nm through this waveguide, a blue laser beam of 0.5 mW was obtained in the waveguide mode. In this case, the emission mode of the generated blue light was hardly observed.

In the first embodiment described above, the polarization inversion structure 2 is formed by the Ti diffusion method, but for example, Li2O is diffused out by forming a SiO2 mask layer in a predetermined pattern and performing heat treatment. It is also possible to form a periodic polarization inversion structure and perform the same polishing and optical waveguide formation as in Example 1. In the above example, an optical waveguide type SHG element is obtained in which an optical waveguide is formed in a nonlinear optical crystal by a proton exchange method, but when obtaining a Cerenkov radiation type optical waveguide type SHG element, The present invention can also be applied.

In the above example, the surface of the polarization inversion structure is removed by mechanical polishing using diamond slurry or colloidal silica. The surface layer of the polarization inversion structure can also be removed by a mechanochemical polishing method.

[0031]

Effects of the Invention As mentioned above, in the present invention, Ti
By mechanically removing the surface layer of the polarization-inverted structure formed by the diffusion method or the Li2O out-diffusion method and making it mirror-finished, the surface properties are made uniform, so that the optical waveguide formed by subsequent proton exchange can be 3 can be formed as a homogeneous optical waveguide over its entire length.

[0032] As a result, fundamental wave propagation loss is significantly reduced, waveguide power is increased, SHG conversion efficiency is improved,
It is possible to reliably obtain an optical waveguide type SHG element having practically excellent characteristics such as improved radiation mode and improved spot shape.

[Brief explanation of drawings]

FIG. 1 is a process diagram of one embodiment of the production method of the present invention.

FIG. 2 is a process diagram of a conventional polarization inversion method.

FIG. 3 is a plan view and a cross-sectional view of an optical path of an SHG element.

FIG. 4 is a pattern diagram of a radiation mode of a conventional SHG element.

FIG. 5 is a cross-sectional view of an optical waveguide type SHG element obtained by a conventional manufacturing method.

[Explanation of symbols]

1 LiNbO3 base 2 Polarization inversion structure part 3 Optical waveguide

Claims (1)

[Claims] 1. A step of forming a polarization inversion structure on the surface of a LiNbO3 substrate by Ti diffusion or Li2O out-diffusion, and a mirror polishing step that includes mechanical polishing to remove the surface layer of the polarization inversion structure. A method for manufacturing an optical waveguide type optical second harmonic generating element, which comprises: and then forming an optical waveguide 3 on the mirror surface side.