JPH06214277A - Second harmonic wave generating element and its production - Google Patents

Abstract

PURPOSE:To alternately produce non-polarization inversion regions and polarization inversion regions having no refractive index difference from a substrate in order to facilitate element design in the production of the pseudo phase matching type second harmonic wave generating element for which the minus (z) face of KTP is utilized. CONSTITUTION:This pseudo phase matching type second harmonic wave generating element is produced by forming parallel grating-like master patterns consisting of a Ti metal, etc., on the minus face of the KTP, then alternately producing the non-polarization inversion regions 4 and the polarization inversion regions 3 having no refractive index difference from the substrate and producing a channel waveguide 2 orthogonal therewith.

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 used in the fields of optical information processing utilizing laser light, optical applied measurement control field, printing / plate making field, and medical field.

[0002]

2. Description of the Related Art In order to efficiently convert the wavelength of light into a second harmonic, it is necessary to satisfy the phase matching condition in the wavelength conversion element. As this method, angle phase matching, temperature phase matching, a method using a waveguide, etc. have been proposed and used conventionally. Recently, a method called quasi-phase matching using a periodic structure has been attracting attention as a phase matching method. For example, Phys. Rev., Vol. 127, p, 1918, 196
2., as shown by JA Armstrong et al. In this method, the polarization direction in the crystal is periodically inverted to compensate for the phase mismatch between the fundamental wave and the harmonic.

Here, the polarization reversal means reversing the direction of polarization of the single crystal dielectric material polarized in a fixed direction to be used. A polarization inversion type SHG element is obtained by applying this method to an SHG element. Various methods have been used to cause this polarization inversion. For example, thermal diffusion of Ti metal is used in lithium niobate (LN) single crystals. For lithium tantalate (LT) single crystal, a method of rapid heating after proton exchange is used. In both LN and LT crystals, polarization inversion regions and non-polarization inversion regions are formed in a lattice pattern, and channel waveguides are formed in a direction orthogonal to the pattern to form a device.
FIG. 2 shows the structure of the device.

Further, in KTP, a segment type channel waveguide is produced by an ion exchange method by immersing in a molten salt of rubidium nitrate, and at the same time barium nitrate is added to the molten salt to invert the polarization. Has produced a domain-inverted SHG device (Appl. Phys. Lett., Vol. 57, No. 20, p, 207).
4, 1990). FIG. 3 shows the structure of the device.

[0005]

As a method of reversing the polarization of KTP as described above, only the method of adding Ba salt to Rb salt has been reported so far. In this method, a difference in refractive index occurs between the ion-exchanged portion and the non-ion-exchanged portion. When designing and manufacturing the SHG element, there is a problem in that it is necessary to calculate the width of polarization inversion in consideration of this slight difference in refractive index.

[0006]

In order to solve the above problems, it is necessary to prevent a difference in refractive index after polarization inversion. Therefore, in the present invention, in order to invert the polarization in KTP, ion exchange is performed by using a molten salt of a mixed salt of potassium and barium so that only polarization inversion is performed and no difference in refractive index occurs. I discovered that I can do it. In addition, first, polarization inversion is performed with a molten salt of a mixed salt of rubidium (or thallium) and barium, and rubidium is returned to potassium with a molten salt of potassium. I found that I can prevent it. As a result, in the KTP single crystal, a quasi-phase matching type second harmonic generating element characterized by alternately manufacturing the non-polarization inversion region and the polarization inversion region having no refractive index difference with the substrate can be produced. .

The salt used may be any salt as long as it is below the temperature at which the KTP crystal decomposes. Regarding the molar ratio of the potassium salt to the barium salt, the larger the ratio of the barium salt, the shorter the time for polarization reversal. Also,
Instead of barium salt, magnesium salt, calcium salt,
Strontium salts may be used. As a mask, T
i was used, but depending on the type of salt used, Al, T
It is possible to use a mask pattern forming metal such as a, Ni, Cr or the like, or an alloy thereof.

[0008]

According to the present invention, the domain inversion region having no difference in refractive index can be produced by the above-mentioned constitution, the device design including the mask pattern transfer error at the time of device production can be simplified, and a more accurate device can be produced. Furthermore, if a channel waveguide is formed at right angles to the grating-type domain-inverted region, an SHG element having no scattering loss and high conversion efficiency can be manufactured. Figure 1
Shows the structure of the device.

[0009]

EXAMPLES Examples according to the present invention will be described below. 1. FIG. 1 shows a perspective view of an optical wavelength conversion device which is a first embodiment of the present invention. The substrate uses KTP. This board 5
A resist was spin-coated on the minus z surface of the above, and the resist was patterned into a grating shape by photolithography. The grating period is 4 microns. After Ti sputtering, the resist was removed and a Ti pattern was formed. Molten salt of 300 to 80 mixture of potassium nitrate and barium nitrate (other salts may be used) (300-).
By immersing it in 450 ° C.) for 10 minutes to 4 hours, non-polarization inversion regions 4 and polarization inversion regions 3 having no difference in refractive index from the substrate were alternately produced. After removing Ti, the channel waveguide 2 is formed in the direction orthogonal to this stripe pattern by the same method as described above.
An optical wavelength conversion element can be prepared by immersing in a molten salt of a salt such as rubidium, cesium, and thallium using a mask.

Table 1 shows a comparison of conversion efficiency between the device according to the present invention and the conventional segment type device. It shows almost the same characteristics. Both element lengths are 5 mm. Table 2 shows the equivalent refractive index when immersed in a molten salt (350 ° C.) of a mixture of 80:20 potassium nitrate and barium nitrate for 10 minutes.

[0011]

[Table 1]

[0012]

[Table 2]

2. FIG. 1 is a perspective view of a light wavelength conversion device which is a second embodiment of the present invention. The substrate uses KTP. A resist was spin-coated on the minus z surface of the substrate 5, and the resist was patterned into a grating by photolithography. The grating period is 4 microns. After Ti sputtering, the resist was removed and a Ti pattern was formed. 8 of rubidium nitrate and barium nitrate
Molten salt (30) of a mixture of 0 to 20 (other salts may be used)
By immersing it in 0 to 450 ° C. for 10 minutes to 4 hours, non-polarization inversion regions 4 and polarization inversion regions 3 having no difference in refractive index from the substrate were produced alternately. Then, a molten salt of potassium nitrate (300-4
It is immersed in 50 ° C.) for 10 minutes to 4 hours to ion-exchange rubidium with potassium. By removing Ti, in a direction orthogonal to this stripe pattern, a channel waveguide is prepared by immersing it in a molten salt of rubidium, cesium, thallium, etc. using a Ti mask in the same manner as above,
An optical wavelength conversion element can be made.

Table 1 shows a comparison of conversion efficiency between the device according to the present invention and the conventional segment type device. It shows almost the same characteristics. Both element lengths are 5 mm. Table 2 shows the equivalent refractive index when immersed in a molten salt (350 ° C.) of a mixture of 80:20 potassium nitrate and barium nitrate for 10 minutes and when ion exchange was carried out by returning to potassium.

[0015]

As described above, according to the present invention, K
After forming a grating-shaped mask pattern of Ti metal or the like on the minus z plane of TP, ion exchange is performed to alternately form non-polarization inversion regions and polarization inversion regions having no refractive index difference with the substrate. Then, consideration of the refractive index difference between the non-polarization-inverted region and the polarization-inverted region at the time of manufacturing the device and the device design including the transfer error of the mask pattern are simplified, and a more accurate device can be manufactured.

[Brief description of drawings]

FIG. 1 is a schematic diagram of a polarization inversion type wavelength conversion element according to an embodiment of the present invention.

FIG. 2 is a schematic diagram of a polarization inversion type wavelength conversion element in lithium niobate (LN).

FIG. 3 is a schematic view of a conventional polarization inversion wavelength conversion element in KTP.

[Explanation of symbols]

 1 LN substrate 2 Optical waveguide 3 Polarization inversion region 4 Non-polarization inversion region 5 KTP substrate

Claims (3)

[Claims] 1. A quasi-phase matching type second harmonic wave generating element, wherein in a KTP single crystal, the non-polarization inversion region and the substrate have the same refractive index. 2. A non-polarization inversion region is formed by ion exchange with a molten salt of a mixed salt of potassium and barium after forming a parallel mask pattern of a mask pattern forming metal on the minus z plane of a KTP single crystal. A method of manufacturing a quasi-phase-matching second harmonic generation element, characterized in that a substrate and a domain-inverted region having no difference in refractive index are alternately manufactured, and a channel waveguide perpendicular to the substrate is manufactured. 3. A non-polarization inversion region is formed by ion-exchange with a molten salt of a mixed salt of rubidium and barium after forming a parallel grating-shaped mask pattern with a metal for forming a mask pattern on the minus z plane of a KTP single crystal. The domain-inverted regions are alternately produced, and rubidium is returned to potassium by a molten salt containing the same part to eliminate the difference in the refractive index between the non-domain-inverted part and the domain-inverted part, and a channel waveguide orthogonal to it is prepared. A method of manufacturing a quasi-phase-matching second harmonic generation device characterized by the above.