JP2658381B2 - Waveguide type wavelength conversion element - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wavelength conversion element capable of realizing a small coherent short wavelength light source.

[Conventional technology]

A wavelength conversion element, particularly a second harmonic generation (SHG) element, is extremely important in industry as a device for obtaining coherent short-wavelength light which is difficult to obtain with an excimer laser or the like.

2. Description of the Related Art Semiconductor lasers are used in various optical communication devices and optical information devices as light sources that oscillate small, high-output coherent light. Currently, the wavelength of light obtained from this semiconductor laser is a wavelength in the near infrared region of 0.78 μm to 1.55 μm. In order to apply this semiconductor laser to devices such as displays more widely, light of shorter wavelength such as red, green, blue, etc. is required. However, it is difficult to realize this kind of semiconductor laser with current technology. difficult. If a wavelength conversion element capable of efficiently performing wavelength conversion can be realized even with a low input power as low as the output of a semiconductor laser, the effect is remarkable.

In recent years, semiconductor laser fabrication techniques have been developed, and higher output characteristics have been obtained than ever before. For this reason, if an optical waveguide type SHG element is configured, a decrease in energy density due to light diffraction can be avoided, and there is a possibility that a wavelength conversion element can be realized with relatively high conversion efficiency even at a light intensity equivalent to that of a semiconductor laser. As such an example, an SHG element of a type in which an optical waveguide is formed in a lithium niobate crystal, near-infrared light is transmitted through the optical waveguide, and a second harmonic radiated (Cherenkov radiation) into a crystal substrate therefrom is obtained. (JP-A-60-14222, JP-A-61-94031). This type of SHG element has the advantage that it does not require precise temperature adjustment because the phase matching condition between the fundamental wave and the SHG wave is automatically set, but the wavefront because the SHG output is substrate emission light Light with an unusual, severe aberration, and an intensity distribution as if "thin eyebrows" emerges from the end face of the substrate. Therefore, converting this light into a normal, easy-to-use beam with a Gaussian intensity distribution requires a sophisticated lens to correct this aberration. Such inconvenience does not occur if the SHG output light is channel guided light as well as the emitted light of the semiconductor laser.

[Problems to be solved by the invention]

An object of the present invention is to provide a waveguide-type wavelength conversion element having a structure in which the SHG output light becomes channel waveguide light, while eliminating the above-mentioned disadvantages of the conventional waveguide-type SHG element.

[Means for solving the problem]

The present invention provides a z-cut lithium niobate crystal plate surface having a spontaneous polarization cycle alternately reversed along the surface, having a light propagation axis coincident with the direction of the spontaneous polarization cycle, and with respect to the incident fundamental wave. A first channel optical waveguide having a width and a thickness to be a single mode waveguide is provided, and a low refractive index dielectric layer and a refractive index for a second harmonic are formed on the surface of the lithium niobate crystal. A high-dielectric-constant thin film having the same size as the refractive index of the lithium niobate crystal with respect to the fundamental wave, and a second harmonic generated from the fundamental wave in parallel with the first channel optical waveguide. Is provided, and a wave number β with respect to a fundamental wave (frequency ω) of the first channel optical waveguide is provided.
^(Ω) and the wave number β ^{(2ω) of} the second channel optical waveguide with respect to the second harmonic of the fundamental wave.
^(2ω) -2β ^(ω) = 2π / Λ The cycle of the spontaneously inverted spontaneous polarization is determined so as to substantially satisfy the relationship of 2π / Λ, and a fundamental wave is injected from one end of the first channel optical waveguide. By obtaining the second harmonic from the two-channel optical waveguide, the output light has no wavefront aberration,
In addition, a low-loss waveguide-type wavelength conversion element in which short wavelength SHG light is not absorbed by the crystal substrate can be obtained.

〔Example〕

Hereinafter, the present invention will be described in detail based on embodiments with reference to the drawings.

FIG. 1 is a view showing the structure of a waveguide type wavelength conversion element according to one embodiment of the present invention.

Reference numeral 1 denotes a lithium niobate (LiNbO ₃ ) crystal substrate, and the substrate orientation is a z-plate (that is, the normal to the substrate is the z-axis). In this crystal, a region 2 where spontaneous polarization is inverted is formed with a period on the surface of the substrate, as will be described later in a method of manufacturing the crystal. The first channel optical waveguide 3 is formed on the lithium niobate crystal substrate having the periodically inverted polarization by an ion exchange method. The output light of the semiconductor laser 9 is coupled to one end face of the first channel optical waveguide 3 as an incident fundamental wave. The fundamental wave travels through the channel optical waveguide 3 through the second-order nonlinear optical effect of the crystal.
Generates SHG light. The SHG light converted from the fundamental wave is the second
As shown in the cross-sectional views of the waveguide structure shown in the drawing, that is, FIG. 2 (a) is a cross section perpendicular to the light traveling direction, and FIG. 2 (b) is a cross sectional view along the light traveling direction. The first channel optical waveguide 3 formed by an exchange method
On top of this, a low-loss dielectric film 7 (eg, SiO ₂ or Al ₂ O ₃ ) and a dielectric film 8 (eg, Ta ₂ O ₅ or TiO ₂ ) having a relatively high refractive index , Etc.), and is guided into the air from the end face of the second channel optical waveguide 4.
The emitted SHG light is converted by the circular lens 5 into a circular collimated beam 6 having no aberration.

The above-mentioned fundamental wave is efficiently converted into SHG light, and conditions for this SHG light to be guided to an optical waveguide different from the fundamental wave,
That is, the phase matching condition is satisfied as follows, with periodic inversion polarization being the middle point.

The channel optical waveguide 3 formed by the proton ion (H ⁺ ) exchange method for propagating a fundamental wave light, which is a TM wave having an electric field component parallel to the z direction of the crystal substrate, is almost single-wave with respect to the fundamental wave (for example, wavelength 0.83 μm). The exchange depth is set to 0.5 μm and the width is set to 2 to 5 μm so that it is a one-mode waveguide, and furthermore, the confinement effect is loose and the optical waveguide has a large amount of light leaching into the substrate. ing. For example, the equivalent refractive index n ^(ω) of the waveguide for the 0.83 μm fundamental wave is close to the extraordinary optical refractive index of the substrate, 2.17, and is slightly larger due to the effect of proton ion exchange.
Set to about.

The equivalent refractive index n ^{(2ω) of} the loaded channel optical waveguide 4 provided on the surface of the lithium niobate crystal substrate for 0.415 μm SHG light has an equivalent refraction to the fundamental wave of the channel optical waveguide 3 formed by proton ion exchange. If the ratio can be set equal to n ^(ω) , efficient SHG conversion can be achieved without providing a reversal polarization period (for example, in order to satisfy the phase matching condition over a length of several mm or more, the The difference must be less than 10 ^-5 ), and it is virtually impossible to match the two with the above precision. Since there is a difference in the phase constant (β) between the two liquids, conversion from the fundamental wave to SHG light does not occur as it is.

Now, | β ^(2ω) −2β ^(ω) | = 2π / ^、, that is, a period that satisfies the relationship of | n ^(2ω) −n ^(ω) | = 0.415 / Λ. If there is, efficient SHG conversion is performed.
The reversal period of the spontaneous polarization formed on the crystal substrate plays this role. It is possible to set the equivalent refractive index near 2.18 in an optical waveguide in which Ta ₂ O ₅ is loaded on SiO ₂ .

However, if the above period is single, if there is fluctuation or temperature change in the thickness or crystal refractive index of the optical waveguide,
The equivalent refractive index of the optical waveguide changes, the above equation is no longer satisfied, and the SHG conversion becomes extremely unstable. In order to avoid this, as shown in FIG. 3, the pitch of the period of the reversal of the spontaneous polarization is gradually changed in the light transmission direction, thereby absorbing fluctuations in the setting of the equivalent refractive index and changes in temperature, thereby achieving stable operation. Na SH
G conversion can be realized. For example, the inversion cycle
The phase matching condition is satisfied if the difference in refractive index is between 0.001 and 0.1 if the light transmission direction is gradually changed from 4.15 μm to 415 μm. That is, the equivalent refractive index of the loaded channel optical waveguide 4 at a wavelength of 0.415 μm is set to 2.2.
What is necessary is just to set it between 9 and 2.08, and this can easily form the above-mentioned dielectric material having a high refractive index and a low refractive index by a sputtering method or the like.

The period of the reversal of the spontaneous polarization can be made as follows. On the + c face of the lithium niobate crystal z-plate
Provision of a periodic pattern of Ti film, 3 at high temperature (1030-1150 ℃)
When thermal diffusion is performed in air for ５5 hours, polarization reversal occurs only in the portion where Ti is diffused. This development is based on the following paper, "Surface acoustic wave reflector using polarization reversal phenomenon in Ti-diffused LiNbO ₃ ; Proceedings of the Acoustical Society of Japan, page 851, p. 3
-2-6, October 1986, authors: P. Nakamura, H. Ando and H. Shimizu. The cycle of inversion of spontaneous polarization in the above embodiment can be easily formed by using this phenomenon.

The method for determining the width, thickness, and the like of the optical waveguide is a commonly used method, and a description thereof is omitted.

〔The invention's effect〕

As described above, according to the present invention, a stable waveguide-type wavelength conversion element having no wavefront aberration in SHG output light can be obtained. Moreover, unlike the known method in which SHG light is emitted into a lithium niobate crystal having an absorption edge at a wavelength of about 0.4 μm, it is different from a known method in which the main material is SiO ₂ having excellent transmission characteristics even in a short wavelength region. Since the light is transmitted through the waveguide, the absorption loss is small. In addition, a shorter wavelength (for example, 0.
(78 μm) The wavelength of the light of the semiconductor laser can be converted.

[Brief description of the drawings]

FIG. 1 is a perspective view for explaining the structure of a waveguide type wavelength conversion element according to one embodiment of the present invention, and FIGS. 2 and 3 are sectional views. 1 ... LiNbO ₃ crystal substrate, 2 ... Polarized region, 3,4 ...
Channel optical waveguide.

Claims (1)

(57) [Claims] 1. A surface of a z-cut lithium niobate crystal plate having a spontaneous polarization cycle alternately inverted along its surface, having a light propagation axis coincident with the direction of the spontaneous polarization cycle, and A first channel optical waveguide having a width and a thickness to be a single mode waveguide, and a low refractive index dielectric layer and a second harmonic on the surface of the lithium niobate crystal. A high-dielectric-constant thin film having a refractive index substantially equal to the refractive index of the lithium niobate crystal with respect to a fundamental wave, and a second harmonic formed in parallel with the first channel optical waveguide. A waveguide-type wavelength conversion element having a second channel optical waveguide for guiding.