JPH05249520A - Optical second higher harmonic generator - Google Patents

Abstract

PURPOSE:To provide the optical second higher harmonic generator, the output of which fluctuates less with a deviation in incident light wavelength and temp. change. CONSTITUTION:The phase matching conditions of optical second higher harmonic generation are gradually changed in a light transmission direction by periodic changes of nonlinear constant or phase constants, i.e. by a method of forming stripped optical wavelengths 13 on a substrate installed with periodic domain inversion regions 2 and gradually changing the angle theta in the propagation direction of the optical wavelengths or installing thin films on the optical wavelengths and gradually changing the thicknesses of the thin films in the propagation direction., by which the optical second higher harmonic generation is executed in a wide wavelength range.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical second harmonic wave generator utilizing a non-linear optical effect for obtaining a laser light source of short wavelength used in the field of optical information processing.

[0002]

2. Description of the Related Art An optical disk memory device has a feature that it can have a larger capacity and a smaller size than a magnetic memory device, and is used in various fields such as a computer terminal and a CD. Further densification of optical discs is an important theme for realization of large-capacity memory, and is being studied. The most practical method for increasing the density is to reduce the size of the recording pit, and for that purpose, the writing and reading light beams are narrowed down as much as possible. The convergent beam diameter of the light beam is determined by the lens aperture, the focal length, and the light wavelength, and it is necessary to shorten the wavelength of light as much as possible when the parts such as the lens are the same.
The development of a short-wavelength semiconductor laser can be considered first for shortening the wavelength of light, but when a currently used III-V group compound is used, oscillation in the wavelength band of 0.6 μm is the limit, and further The realization of short-wavelength green and blue laser light sources requires the realization of other, for example, II-VI group compound semiconductor lasers, and there are many problems in achieving the practical level at present.

On the other hand, optical second harmonic generation (hereinafter referred to as SHG) using the non-linear optical effect makes it possible to relatively easily adjust the wavelength to 1
Since it can be set to / 2, it is being studied as a practical method for obtaining a short wavelength. In order to realize SHG, it is necessary to pass a light beam through a non-linear medium and match the incident pumping light with the phase velocity of the second harmonic.

Conventionally, the phase matching method of this kind of optical second harmonic generator is to pass a light beam through a bulk of lithium niobate (LiNbO ₃ ) crystal, potassium niobate (KNbO ₃ ) crystal or the like to control the temperature. A method of adjusting the phase matching, a method of providing an optical waveguide on a crystal substrate and propagating incident light in the optical waveguide to obtain a second harmonic wave radiated by Cherenkov radiation, and a method of entering the same into the optical waveguide. A method of propagating light and providing a periodic structure of a nonlinear optical constant or a propagation constant in an optical waveguide to obtain phase matching has been reported. Of these methods, it is possible to concentrate the incident light energy most to improve the light efficiency, and to obtain a minute focused spot, which is suitable for obtaining a small light source because the periodic structure of the optical waveguide is used. Is the way to do it.

An example of a conventional optical second harmonic generator based on this phase matching method is shown in the perspective view of FIG. In FIG.
Lithium niobate (LiNbO ₃₎ are arranged domain inversion region 2 + width of the x-axis direction on the Z plane is almost Omega _0/2 in the y-axis direction striped crystal substrate 1 is the period Omega ₀ in the x-axis direction There is. Further, a stripe-shaped optical waveguide 3 which is formed on the surface of the substrate by proton exchange and propagates in the x-axis direction
Are formed. The domain inversion region 2 and the stripe-shaped optical waveguide 3 have almost the same depth. Where wavelength λ
The incident light 4 of ₁ is incident on the optical waveguide 3 and is converted into a second harmonic wave 5 having a half wavelength. The period Ω ₀ of the domain inversion region 2 is set to a value that satisfies the phase matching condition of the following equation so that SHG is added and the operation is performed efficiently.

2π / Ω ₀ = | k ₂ −k ₁ | (1) where k ₁ and k ₂ are the incident light 4 and the second harmonic wave 5, respectively.
Is a propagation constant in the optical waveguide 3.

[0007]

The conventional second harmonic generator of FIG. 3 is very sensitive to the phase matching condition of the equation (1), and it may be caused by the wavelength shift or the shift of the propagation characteristic due to the temperature change. If the equation (1) is not satisfied, the efficiency will drop sharply. In particular, when the oscillation wavelength of the semiconductor laser, which is incident light, changes due to temperature or the like, there is a practical problem that the second harmonic output also decreases.

It is an object of the present invention to provide a second harmonic wave generator which has a small fluctuation with respect to the shift of the incident light wavelength and the temperature change.

[0009]

An optical second harmonic generator of the present invention comprises a plurality of band-shaped domain inversion regions periodically formed in the Y-axis direction on a substrate having a nonlinear optical effect, and the domains. In a second optical harmonic generator comprising a striped optical waveguide formed in the X-axis direction crossing the inversion region, the striped optical waveguide has a light propagation axis in the X direction.
It is formed so as to change at a changing rate that gradually changes in the axial direction. Further, the stripe-shaped optical waveguide may be formed so that the rate of change is abruptly changed at the entrance and exit ends.

And Y on a substrate having a nonlinear optical effect
An optical second harmonic generator comprising a plurality of groove-shaped domain inversion regions periodically formed in the axial direction and a stripe-shaped optical waveguide formed in the X-axis direction crossing the domain inversion regions, A thin film whose thickness gradually increases in the light traveling direction is formed on the striped optical waveguide. The thin film formed on the striped optical waveguide may be formed by abruptly changing the thickness in the vicinity of the input / output end of the striped optical waveguide.

[0011]

The optical second harmonic generator of the present invention is constructed so that the phase matching condition that is satisfied gradually changes as the incident light propagates through the optical waveguide. That is, in the first invention in which the traveling direction of the striped optical waveguide is gradually changed, the period Ω ₀ of the domain inversion region sensed by the incident light in the optical waveguide is gradually changed and, for example, the phase is changed by changing the wavelength of the incident light. Even if the matching condition (1) is not satisfied in one region, the matching condition is set to be satisfied in another region in the optical waveguide.

In the second invention in which a thin film having small light absorption is provided on the optical waveguide, the fact that the propagation constant of light propagating in the optical waveguide changes depending on the thickness of the thin film is utilized, and By forming a thin film on the striped optical waveguide and increasing its thickness with respect to the light traveling direction, the propagation constant changes in the light traveling direction so that the phase matching condition of equation (1) is satisfied in any region. To

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a perspective view showing an embodiment of a second harmonic generator according to the first invention of the present invention. In FIG. 1, a conventional second substrate shown in FIG. 3 is formed on a LiNbO ₃ crystal substrate 1.
A domain inversion region 2 having the same shape as the harmonic generator is formed. Here, the period Ω ₀ has an incident light wavelength of 0.7 to
When it is 1.0 μm, the value is about several to several tens of μm. The domain inversion region 2 may be formed by, for example, installing a SiO ₂ film on the region where the domain is to be inverted, and
There is a method of performing thermal annealing treatment at 0 to 1100 ° C. for 1 to several hours.

A striped optical waveguide 13 is further formed on the LiNbO ₃ crystal substrate 1, and its propagation direction is inclined by θ with respect to the x axis, and the value of θ is from the entrance to the exit of the optical waveguide. It gradually changes from 0 ° to θ _m . The effective domain inversion period Ω with respect to the incident light propagating through the stripe-shaped optical waveguide 13 is Ω = Ω ₀ / cos θ (2), so that if θ _m = 2.5 °, then Ω≈1.0
It becomes 01Ω _{0 and the} 0.1% cycle can be changed,
It is possible to satisfy the phase matching condition even if the incident light wavelength varies to the same degree.

The change of the domain inversion period as described above can be obtained by directly gradually changing the position of the domain inversion region, but in this case, it is 0.1%.
For example, if Ω ₀ = 10 μm, the change width of the period becomes 0.01 μm to obtain such a change, and such high precision micromachining is difficult. S is inversely proportional to the change width of the above cycle
Since the effective length of HG is determined, the conversion efficiency will be significantly reduced if the variation width of the cycle is too large by the conventional processing method. On the other hand, in the present embodiment, the variation width of the cycle can be easily adjusted with high accuracy.

Incidentally, the stripe-shaped optical waveguide 1 of this embodiment
The manufacturing method of 3 is to install a metal plate mask having an optical waveguide-shaped opening on a substrate, and place it in a solution of benzoic acid or the like at 200 ° C.
Soak at 300 ℃ for several minutes to tens of minutes, then 200-300 ℃
A proton exchange method or the like in which annealing treatment is performed is used.

FIG. 2 is a perspective view showing an embodiment of the second harmonic generator which is the second invention of the present invention. In FIG. 2, a domain inversion region 2 having the same shape as that of the conventional second harmonic generator shown in FIG.
It is formed on the iNbO ₃ crystal substrate 1. Further, a silicon dioxide (SiO ₂ ) thin film 10 having a taper shape in the light transmitting direction is formed on the striped optical waveguide 3.

The propagation constant of the optical waveguide is different when the air is above the optical waveguide and when it is coated with another substance, and its value also depends on the thickness of the coated substance. Furthermore, the rate of change greatly depends on the wavelength of the propagating light. Here, by changing the thickness of the SiO ₂ thin film 10 in the range of 0 to several thousand angstroms in the light transmission direction, the values of k ₁ and k _{2 in} the above equation (1) are changed, and the change with respect to normal k _{1 is} changed. Since it is larger, | k ₁ −
The value of k ₂ | changes according to the light transmission direction, the wavelength satisfying the phase matching condition changes in the light transmission direction, and the fluctuation of the incident light wavelength can be covered. Further, similarly to the embodiment of FIG. 1, since the change with respect to the propagation constant can be set small, highly accurate adjustment is possible.

In the present invention, the conversion efficiency with respect to wavelength and temperature is changed by performing weighting so that the conversion efficiency gradually decreases in the incident end of the stripe-shaped optical path 3 or 13 and in the region near the incident end. It can be further reduced. The weighting is performed, for example, by weakening the confinement of light into the optical waveguide at the entrance / exit end to lower the energy density, or by rapidly changing the value of θ in FIG. 1 or the thickness of the SiO ₂ film in FIG. 2 at the entrance / exit end. It is obtained by a method such as changing.

[0020]

As described above, the present invention has an effect of obtaining the second harmonic generator having a small output fluctuation with respect to the deviation of the incident light wavelength and the temperature change.

[Brief description of drawings]

FIG. 1 is a perspective view of a first embodiment according to the first invention.

FIG. 2 is a perspective view of a first embodiment according to the second invention.

FIG. 3 is a perspective view of a conventional example.

[Explanation of symbols]

1 LiNbO ₃ crystal substrate 2 domain inversion region 3 striped optical waveguide 10 SiO ₂ thin film

Claims (4)

[Claims] 1. A plurality of strip-shaped domain inversion regions periodically formed in the Y-axis direction on a substrate having a non-linear optical effect, and a striped optical waveguide formed in the X-axis direction intersecting the domain inversion regions. In the optical second harmonic generator, the stripe-shaped optical waveguide is formed such that the direction of the light propagation axis changes at a rate of change that gradually changes in the X-axis direction. 2 harmonic generator. 2. The optical second harmonic wave generator according to claim 1, wherein the striped optical waveguide is formed by abruptly changing the rate of change at the entrance and exit ends. 3. A plurality of groove-shaped domain inversion regions periodically formed in the Y-axis direction on a substrate having a nonlinear optical effect, and stripe-shaped optical waveguides formed in the X-axis direction intersecting the domain inversion regions. An optical second harmonic generator including a waveguide, wherein a thin film having a thickness gradually increasing in a light traveling direction is formed on the stripe-shaped optical waveguide. 4. The optical film according to claim 3, wherein the thin film formed on the striped optical waveguide is formed by abruptly changing the thickness in the vicinity of the input / output end of the striped optical waveguide. 2 harmonic generator.