JPH0527286A - Optical wavelength conversion element - Google Patents

Abstract

PURPOSE:To use a substrate with which a good-quality single crystal is obtainable and to suppress the absorption of secondary harmonic waves to the substrate by specifying the compsn. ratio X of a thin-film optical waveguide consisting of ZnSxSe1-x to <=0.2 value. CONSTITUTION:The compsn. ratio X of the ZnSxSe1-x constituting the thin-film optical waveguide is set at 0.2 value. Namely, this optical wavelength conversion element is constituted by forming a clad layer 12 consisting of ZnS and the thin-film optical waveguide 13 consisting of the Zn0.2Se0. 8 which is a nonlinear optical material in this order on a GaAs substrate 11. The compsn. ratio X is 0.2 in such a case. The point at which a phase matching angle thetaattains does not exist any more even if the optical waveguide <=1 deg. is formed thick when the X exceeds 0.2 and, therefore, the X is required to be set at <=0.2. The formation of the optical waveguide into a superlattice structure is possible as well. The optical waveguide can be obtd. simply by confining the film thickness ratio of the superlattice structure of ZnS/ZnSe to 2:8, i.e. under 1:4 in such a case. This case is also included in 'specifying the compsn. ratio X to.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical wavelength conversion element for converting a fundamental wave into a second harmonic by a non-linear optical material,
More particularly, it relates to a thin film optical waveguide type optical wavelength conversion element using a semiconductor material as a nonlinear optical material.

[0002]

2. Description of the Related Art Conventionally, various attempts have been made to convert the wavelength of laser light (shorten the wavelength) by utilizing the second harmonic generation by a nonlinear optical material. As one of the optical wavelength conversion elements of this type, for example, JP-A-63-15233 and 63-152.
The optical waveguide type shown in Japanese Patent No. 34 is known. In addition, recently, for example, as shown in JP-A-2-135428, a semiconductor material is used as a nonlinear optical material,
An optical waveguide type optical wavelength conversion element in which a thin film optical waveguide is formed of this semiconductor material is also known. As the thin-film optical waveguide type optical wavelength conversion element, specifically, there is widely known one in which a clad layer made of ZnS and a thin-film optical waveguide made of ZnS _x Se _1-x are formed in this order on a substrate. ing.

On the other hand, a so-called Cherenkov radiation type is known as one type of optical wavelength conversion element of the optical waveguide type. In this optical wavelength conversion element, phase matching is achieved between the fundamental wave of the guided mode propagating through the optical waveguide and the second harmonic of the radiated mode radiated to the cladding.

[0004]

As described above, a thin film optical waveguide type optical wavelength conversion element using a semiconductor material can also be formed as this Cherenkov radiation type, in which case the second harmonic wave is generated on the substrate. The problem of being absorbed by can occur. That is, in this thin-film optical waveguide type optical wavelength conversion element, since the cladding layer and the thin-film optical waveguide are formed in this order on the substrate, the second harmonic wave radiated to the cladding layer at a predetermined phase matching angle is emitted from the element. It is possible that the substrate is reached before exiting from the end face. As the substrate material, a GaAs substrate that can obtain a good-quality single crystal is usually used. Since this GaAs substrate totally absorbs light with a wavelength of 800 nm or less, the green and blue regions (in many cases, the optical wavelength conversion element is , Which is used to obtain light in this wavelength range), is absorbed by the substrate.

The phenomenon in which the second harmonic is absorbed by the substrate as described above is naturally more remarkable as the phase matching angle is larger. In theory, this phenomenon can be suppressed by making the cladding layer very thick. However, since the cladding layer is usually formed on the substrate by epitaxial growth, the thickness can be increased to about 100 μm at most. Therefore, it is practically impossible to make the cladding layer very thick to suppress the above phenomenon.

Therefore, as disclosed in the above-mentioned Japanese Patent Laid-Open No. 2-135428, it has also been proposed to form the substrate with a material transparent to the second harmonic. More specifically, when the second harmonic is light in the green and blue regions, it is considered to use CaF ₂ or ZnS as the transparent substrate material.

However, it is difficult to obtain good quality single crystals of CaF ₂ and ZnS. Therefore, in the conventional thin film optical waveguide type optical wavelength conversion element in which the substrate is made of CaF ₂ or ZnS, a problem that the loss of the fundamental wave in the optical waveguide is large has been recognized.

The present invention has been made in view of the above circumstances, and the use of a substrate such as a GaAs substrate from which a single crystal of good quality can be obtained, and absorption of the second harmonic into the substrate is unlikely to occur. An object of the present invention is to provide a thin film optical waveguide type optical wavelength conversion element.

[0009]

As described above, the optical wavelength conversion device according to the present invention has a Z-shaped structure on a substrate such as a GaAs substrate.
In the optical wavelength conversion element for converting the fundamental wave propagating through the optical waveguide into the second harmonic wave, the cladding layer made of nS and the thin film optical waveguide made of ZnS _x Se _1-x are formed in this order. It is characterized in that the composition ratio x is 0.2 or less.

[0010]

[Operation and Effect of the Invention] As described above, the maximum growth film thickness in the epitaxial growth of semiconductor materials is generally 10
It is about 0 μm = 0.1 mm. On the other hand, as applications of blue and green laser light sources, there are many fields such as color printers, optical disks, and optical measurement, but those having an output of mW level or higher are practically desired. When this output is obtained by combining a semiconductor laser and a thin film optical waveguide type optical wavelength conversion element, the output of a single transverse mode semiconductor laser is currently about 100 mW at the maximum, and ZnS, ZnS
Considering the nonlinear optical constants of Se-based materials, an interaction length of 5 mm or more, that is, an element length L is required.

Assuming that the cladding layer thickness is D and the phase matching angle in the case of Cherenkov radiation is θ, the emitted second
The ultimate condition for the harmonics not to reach the substrate outside the cladding layer is tan θ = D / L. L = 5 mm, D =
Substituting the value of 0.1 mm into this equation gives θ = 1.14 °. That is, unless the phase matching angle θ is 1.14 ° or less,
A phenomenon may occur in which the second harmonic reaches the substrate and is absorbed.

Here, with a margin of about 10%, the phase matching angle θ
In order to make the value 1 ° or less, the composition ratio x should be 0.2
It is possible to set the following. That is, according to the study by the present inventors, the thickness of the optical waveguide and the phase matching angle θ
It was found that the relationship with and changes as shown in FIG. 2 depending on the composition ratio x. In FIG. 2, a curve a indicates x =
The relationship when 0.2, and the curve b shows the relationship when x = 0.1. This relationship changes so that the curve gradually shifts to the left while increasing the slope as x gradually decreases, and changes in the opposite manner as x gradually increases. When x exceeds 0.2, there is no point where θ becomes 1 ° or less even if the optical waveguide is thickened, as indicated by the curve c in the figure, so x ≦ 0.2 is required.

In the light wavelength conversion element of the present invention, the optical waveguide may have a superlattice structure. In that case, the film thickness ratio of the ZnS / ZnSe superlattice layer is 2: 8,
That is, it may be 1: 4 or less. In the present invention, doing this is also included in "making the composition ratio x 0.2 or less".

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below based on the embodiments shown in the drawings. FIG. 1 shows an optical wavelength conversion device 10 according to an embodiment of the present invention. This optical wavelength conversion element 10
Is a cladding layer 1 made of ZnS on a GaAs substrate 11.
2, and a thin film optical waveguide 13 made of ZnS _0.2 Se _0.8 which is a non-linear optical material is formed in this order. That is, in this embodiment, the composition ratio x is 0.2.

A laser beam 15 (wavelength: 980 nm) as a fundamental wave emitted from the semiconductor laser 14 is condensed by a condenser lens 16, enters the optical waveguide 13 from one end face thereof, and enters there. It travels to the other end surface side in the guided mode. This laser beam 15 has a wavelength of 1/2, that is, 490 n, due to ZnS _0.2 Se _0.8 constituting the optical waveguide 13.
It is converted to the second harmonic wave 15 ′ of m. This second harmonic 15 '
Radiates into the cladding layer 12 at a phase matching angle θ (so-called Cherenkov radiation), and phase matching is achieved between the second harmonic wave 15 ′ and the laser beam 15. The second harmonic wave 15 'and the fundamental wave 15 are emitted from the end face 10a of the light wavelength conversion element 10, and are passed through a filter (not shown) to produce the second harmonic wave 15'.
Only is taken out.

In this embodiment, the thickness of the optical waveguide 13 is 2.
It is set to 8 μm, and therefore, the phase matching angle θ = 0.7 ° is obtained as derived from the curve a in FIG. As described above, in order to obtain the sufficiently high intensity second harmonic wave 15 ′, if the length L of the optical wavelength conversion element 10 is 5 mm, then the thickness D of the cladding layer 12 is 61 μm from tan θ = D / L. With the above settings,
The second harmonic wave 15 'will never reach the substrate 11. The clad layer 12 having D = 61 μm can be sufficiently formed by epitaxial growth. The second harmonic wave 15 ′ having a wavelength of 490 nm is absorbed by the GaAs substrate 11 (800 n
m or less), but the element end surface before reaching the substrate 11.
Since the light is emitted from 10a, it is naturally not absorbed by the substrate 11.

Although the embodiment using the GaAs substrate 11 has been described above, the present invention can be similarly applied to the case where a substrate made of a material other than GaAs is used.

[Brief description of drawings]

FIG. 1 is a schematic side view showing an optical wavelength conversion device according to an embodiment of the present invention.

FIG. 2 is a graph showing the relationship between the thickness, the phase matching angle, and the composition ratio x of the optical waveguide made of ZnS _x Se _1-x related to the present invention.

[Explanation of symbols]

 10 Optical wavelength conversion element 11 GaAs substrate 12 Clad layer 13 Thin film optical waveguide 14 Semiconductor laser 15 Laser beam (fundamental wave) 15 'Second harmonic

Claims (1)

1. A fundamental wave propagating through the optical waveguide, which comprises a cladding layer made of ZnS and a thin film optical waveguide made of ZnS _x Se _{1 -x} formed in this order on a substrate. To
An optical wavelength conversion element for converting into a second harmonic in the absorption wavelength range of the substrate, wherein the composition ratio x is a value of 0.2 or less.