JPH03132628A - Wavelength converting element - Google Patents

Abstract

PURPOSE:To obtain the wavelength converting element having high conversion efficiency and good light condensing property by additionally diffusing magnesium oxide into a titanium-diffused region. CONSTITUTION:A polarization inversion region is formed by a titanium diffusion method in a lithium niobate crystal 5 and the addition diffusion of the magne sium oxide is so executed as to overlap on the region thereof to form the low- refractive index polarization inversion region 6 having the refractive index nearly equal to the refractive index of the region not diffused with the titanium. The optical waveguide 7 is crossed with the low-refractive index polarization inversion region 6 by the proton exchange. Since the refractive index of the titanium-diffused region is lowered by the additional diffusion of this magnesium oxide, the difference in the refractive index between the polarization inversion part and the non-inversion part is lessened. Thus, the wavelength converting element which allows condensing of light without using a special lens system and has the high conversion efficiency is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a wavelength conversion element for a laser that makes it possible to realize a coherent short wavelength light source.

[Conventional technology]

Wavelength conversion elements, especially second harmonic generation (SHG: 5e
(Harmonic Generation)
The device is extremely important industrially as a device that can obtain coherent short-wavelength light that is difficult to obtain with excimer lasers and the like.

Semiconductor lasers are used as compact, high-output coherent laser light sources in various optical communication devices and optical information processing devices. Currently, the wavelength of light obtained from this semiconductor laser is from 0.68 μm to 1.55 μm, ranging from visible red light to near-infrared light. For this reason, semiconductor laser light sources that can oscillate at shorter wavelengths such as red, green, and blue are required in order to expand the range of applications such as displays and the formation of finer light spots, but current technology does not. It is difficult to realize this type of semiconductor laser. If a wavelength conversion element capable of efficiently converting wavelength into visible light even at the output level of a semiconductor laser could be realized, the effect would be enormous.

In recent years, semiconductor laser manufacturing technology has developed, and it has become possible to obtain higher output characteristics than ever before. Therefore, by configuring a guided waveguide type SHG element, it is possible to avoid a decrease in energy density due to light diffraction, and it is possible to realize a wavelength conversion element with relatively high conversion efficiency even with a light intensity comparable to that of a semiconductor laser.

As an example of this, an optical waveguide 2 is formed in a lithium niobate crystal 1 shown in FIG. 2, near-infrared excitation light 3 is end-coupled to the optical waveguide 2, and then radiated into the crystal substrate (Cherenkov radiation). S of the method to obtain the second harmonic light 4
There is an HG element. This type of SHG element uses the fundamental wave and S
Since the phase matching of the HG waves is automatically achieved, there is no need for precise temperature control. On the other hand, the slanted wavefront in which the SHG output is the substrate radiation light is unique, and light with large aberrations comes out from the end surface of the substrate. Therefore, converting this light into a normal, easy-to-use beam with a Gaussian intensity distribution requires a sophisticated lens system, such as a cylindrical lens, to correct this aberration.

In order to solve this problem, a method has been proposed in which a region in which the spontaneous polarization is periodically reversed is provided on the optical waveguide. In this method, the polarization inversion period Δ is the coherence length Δ given by Δc=2π/(β (2ω)−2β (ω))1. By setting it to be equal to , the second harmonic can also be made into a waveguide mode, and a wavefront with good light focusing ability can be obtained. Here β(2ω)
, β(ω) are propagation constants for the second harmonic and fundamental wave of the optical waveguide, respectively.

As a method for selectively forming such spontaneous polarization inversion regions, there is a method of thermally diffusing titanium. In this method, a titanium pattern is formed on the +0 face of a lithium niobate crystal only in a region where polarization inversion does not occur, and the titanium pattern is diffused in an atmosphere of about 1100° C. for several hours.

This method is used, for example, in the R1989 Technical Digest Series Volume 4 Integrated Edition published by the Optical Society of America.
and Guided Wave Optical, published in Post Deadline Papers J PD3.
``Second Harmonics Generation'' or ``Second Harmonics Generation of Blue and Green Light Imperiody'' published in R1989 Technical Digest Series Volume 2, Nonlinear Guided-Wave Phenomena: Physics and Applications, Post-Deadline Paper J PD3, published by the Optical Society of America. Power - Bold Planer Lithium Niobate
The details are detailed in the paper entitled "Wave Guys".

[Issues to be solved by conventional technology]

The wavelength conversion elements according to the conventional techniques described above have the drawback that it is difficult to condense light in the Cerenkov radiation type as described above. Furthermore, the polarization inversion type has the following problems. In order to selectively cause polarization reversal, the titanium diffusion method has conventionally been used as described above, but this method also causes an increase in the refractive index of the diffusion portion. Therefore, a light wave propagating through an optical waveguide undergoes a change in refractive index each time it passes through the boundary of polarization inversion and non-inversion regions, and the reflection loss at the boundary becomes extremely large, giving rise to an apparent waveguide loss. This results in a large optical waveguide. As is well known, since the conversion efficiency to the second harmonic is proportional to the excitation light intensity, it is difficult to achieve high conversion efficiency with a waveguide with large waveguide loss.

An object of the present invention is to solve the above-mentioned problems and provide a wavelength conversion element with high conversion efficiency and good light focusing ability.

[Means to solve the problem]

In order to solve the above-mentioned problems of the prior art, the means used in the present invention is to provide an optical waveguide provided in a lithium niobate crystal with a spontaneous polarization region formed by selectively thermally diffusing titanium. The conversion element is characterized in that magnesium oxide is additionally diffused into the titanium diffusion region.

[Effect]

In the present invention, the second
In the harmonic generation element, a method of additionally diffusing magnesium oxide is used to compensate for the increase in refractive index of the polarization inversion portion due to titanium diffusion. The fact that the refractive index of the titanium diffused part decreases due to the additional diffusion of magnesium oxide is explained in, for example, "Fundamentals and Applications of Optical Integrated Circuits" edited by the Optics Conference of the Japan Society of Applied Physics.
(published by Asakura Shoten in 1988), pages 126 to 12
It is listed on page 7. Therefore, by patterning magnesium oxide to match the titanium diffusion portion and performing additional diffusion, it is possible to reduce the difference in refractive index between the polarization inversion portion and the non-inversion portion. Since reflection at the boundary is reduced, an optical waveguide with small propagation loss can be manufactured, and as a result, a wavelength conversion element with high conversion efficiency can be realized.

〔Example〕

The present invention will be described in detail below with reference to the drawings.

FIG. 1 is a diagram for explaining the present invention in detail. A polarization inversion region is created in the lithium niobate crystal 1 by a titanium diffusion method, and magnesium oxide is additionally diffused so as to overlap with the region, thereby forming a low refractive index polarization inversion region 6 having a refractive index almost equal to that of the non-diffusion region of titanium. It is formed. The optical waveguide 7 is fabricated so as to intersect with the low refractive index polarization inversion region 6 by proton exchange. Excitation light 8 is coupled from the end face of optical waveguide 7. The second harmonic light 9 becomes approximately the waveguide mode of the optical waveguide and is emitted from the end face of the optical waveguide. Figure 3 A, B
1 is a plan view and a sectional view showing the configuration of the present device.

The depth and width of the low refractive index polarization inversion region 6 are set to be sufficiently deeper and wider than the depth and width of the proton exchange optical waveguide 7.

FIG. 4 is a diagram for explaining the manufacturing process of an element according to the present invention. A titanium thin film 10 is prepared on the +0 face of the Z-cut lithium niobate crystal 11 (FIG. 4A), and a titanium pattern 12 corresponding to a polarization inversion pattern is formed by photoprocessing and etching (FIG. 4B). 11
A polarization inversion region 13 is formed by processing at a temperature of about 00° C. for several hours (FIG. 4C). An MgO pattern 14 is formed to match this polarization inversion region 13 (FIG. 4D). The Mg0 layer can be formed by, for example, a sputtering method. Further, the MgO pattern can be formed using a known technique such as a lift-off method or a dry etching method. The additional diffusion of MgO is several hundred degrees
The diffusion can be carried out at a diffusion temperature of C in substantially the same manner as the diffusion of titanium, thereby forming a low refractive index polarization inversion region 15 (FIG. 4E). Thereafter, the optical waveguide 16 is formed by the proton exchange method, which is a known technique (FIG. 4F).
.

〔Effect of the invention〕

According to the present invention, it is possible to provide a wavelength conversion element that is capable of condensing light without using a special lens system and has high conversion efficiency.

[Brief explanation of the drawing]

FIG. 1 is a diagram for explaining the present invention in detail, FIG. 2 is a diagram for explaining the conventional technique, and FIGS. 3 and 4 are diagrams for explaining the present invention in detail. In the figure, 1.5...lithium niobate crystal, 2,7.16...
・Optical waveguide, 3.8... Excitation light, 4.9... Second harmonic light, 6, 15... Low refractive index polarization inversion region, 10.
...Titanium thin film, 11...Z-cut lithium niobate crystal, 12...Titanium pattern, 13...Polarization inversion region, 14...Mg◯ pattern.

Claims (1)

[Claims] A wavelength conversion element having a spontaneous polarization inversion region formed by selectively thermally diffusing titanium in an optical waveguide provided in a lithium niobate crystal, characterized in that magnesium oxide is additionally diffused into the titanium diffusion region. conversion element.