JPH0713210A - Wavelength conversion element - Google Patents

Abstract

PURPOSE:To make it possible to enhance conversion efficiency and to enhance second higher harmonic waves having high intensity by diffusing silver onto a KTiOPO4 crystal at a period of integer times the coherence length, thereby forming ion-diffused layers. CONSTITUTION:The silver ion exchange layers 2 which are the regions formed by periodically diffusing silver ions at the period of twice the coherence length of the KTiOPO4 crystal 1 are formed by depositing aluminum by evaporation on the KTiOPO4 crystal 1, applying a resist and subjecting this resist to exposure, developing and etching to form a mask made of the aluminum of 4mum period, then immersing the mask into a silver nitrate melt. A laser beam emitted as a basic wave 6 from a semiconductor laser 4 of 852nm wavelength and 500mw output is condensed by a lens 5 to the KTiOPO4 crystal 1 and is propagated into the KTiOPO4 crystal. Consequently, the output intensity of the second higher harmonic wave 7 of 426nm wavelength attains 2mw.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wavelength conversion element used in the field of optical information processing utilizing coherent light and the field of optical application measurement.

[0002]

2. Description of the Related Art FIG. 3 shows a block diagram of a conventional wavelength conversion device using an optical waveguide type wavelength conversion element. 1 is KTiO
PO4 crystals and 3 are discontinuous optical waveguides in which rubidium ions are diffused periodically, the refractive index of the diffused portion is increased, and the ferroelectric polarization is inverted. The optical waveguide 3 is K
After depositing aluminum or titanium on the TiOPO4 crystal 1, a pattern mask of the optical waveguide 3 is periodically formed by photolithography and etching, and then put in a melt of rubidium nitrate and barium nitrate at about 300 ° C. Are periodically diffused into the KTiOPO4 crystal.

The operating principle of wavelength conversion using this wavelength conversion element will be described below. Semiconductor laser 4 with wavelength 850 nm
The laser light emitted from is condensed on the end face of the optical waveguide 3 by the lens 5 and guided by the optical waveguide 3 as the fundamental wave 6. The fundamental wave 6 propagating through the optical waveguide 3 is converted into the second harmonic wave 7 having a wavelength of 425 nm propagating through the optical waveguide 3 by the second-order nonlinear effect in all regions of the optical waveguide 3. At this time, if the fundamental wave 6 and the second harmonic wave 7 are not phase-matched, the second wave generated at each point of the optical waveguide 3 is generated.
The second harmonics 7 do not strengthen each other. As a result, as shown in FIG. 4, the intensity of the second-order harmonic wave 7 varies depending on the coherence length even if the distance by which the fundamental wave 6 is guided through the optical waveguide 3 increases.
The second and higher harmonics 7 cannot be efficiently extracted because the integration is not performed by simply repeating the maximum and the minimum. Therefore,
It has been proposed to prevent the second harmonics from canceling each other out by inverting the ferroelectric polarization at a cycle that is an integral multiple of the coherence length. For example, if the ferroelectric polarization is inverted to the coherence length, the second harmonic wave 7 increases with the waveguide distance as shown in FIG. In lithium niobate crystals and lithium tantalate crystals,
It is possible to invert the ferroelectric polarization by diffusing hydrogen ions with a benzoic acid or pyrophosphoric acid melt, then heating to 600 ° C. to 1100 ° C. and then rapidly cooling.

[0004]

The KTiOPO4 crystal has an advantage of excellent resistance to light damage in the wavelength region of the blue laser as compared with the lithium niobate crystal and the lithium tantalate crystal, and the wavelength conversion element using KTiOPO4. Is expected to realize a high-intensity blue laser that is impossible with the above-mentioned two crystals. However, in a lithium niobate crystal or a lithium tantalate crystal, ferroelectric polarization inversion can be realized relatively easily, while in contrast to KTi.
In the OPO4 crystal, it is not easy to reliably reverse the ferroelectric polarization. Compared to lithium niobate crystals and lithium tantalate crystals, it is KTiO
This is because the PO4 crystal has high electrical conductivity, and it is difficult to form a local electric field that induces inversion, and therefore, the ferroelectric polarization does not easily invert.

Optical waveguide 3 of KTiOPO4 crystal 1 of FIG.
Is a form in which potassium ion portion of KTiOPO4 crystal is replaced with rubidium ion, that is, RbTiOPO
It is almost close to 4 crystals. The magnitude of the second-order nonlinear constant of the rubidium ion diffusion part is 2 times that of the KTiOPO4 crystal.
It is almost the same as the nonlinear coefficient. Now, in FIG. 3, the second harmonic wave 7 generated at each point of the optical waveguide 3 is generated with substantially the same intensity regardless of the ion diffusion portion or the non-diffusion portion. Therefore, when the ferroelectric polarization is not inverted in the rubidium ion diffusion portion of the optical waveguide 3, the second harmonic wave 7 generated at each point of the optical waveguide 3 and the second harmonic wave guided up to that point. Reinforce each other in region a,
In the region b, on the contrary, they weaken each other, and as shown in FIG.
It does not increase by simply repeating the maximum and minimum for each coherence length. As a result, the wavelength conversion efficiency is remarkably lowered, and there is a problem that the high-intensity second harmonic wave 7 cannot be extracted.

Further, in order to invert the ferroelectric polarization of KTiOPO4, rapid cooling from about 340 ° C. is required, but the KTiOPO4 crystal is distorted in the crystal by the rapid cooling as compared with the lithium niobate crystal and the lithium tantalate crystal. Is likely to occur and the crystal often breaks.
An object of the present invention is to provide a wavelength conversion element using a KTiOPO4 crystal, which has a high conversion efficiency and is capable of obtaining a second harmonic having a high light intensity.

[0007]

The present invention is characterized in that an ion diffusion layer is formed by diffusing silver on a KTiOPO4 crystal at a cycle of an integral multiple of the coherence length. A linear optical waveguide orthogonal to the layer is formed.

[0008]

The second-order nonlinear constant of the KTiOPO4 crystal in which the potassium ion portion is replaced with the silver ion, that is, the second-order nonlinear constant of the AgTiOPO4 crystal is less than 1/1000 compared with the second-order nonlinear constant of the KTiOPO4 crystal. In addition, silver easily diffuses into the KTiOPO4 crystal, the silver diffusion region has no absorption in the visible region, and hardly changes the refractive index. In the present invention, the silver ions are periodically diffused on the KTiOPO4 crystal at a cycle that is an integer multiple of the coherence length, because the diffusion portion is made of AgTiOPO4 or a composition close to that, and the second-order nonlinearity of the silver ion diffusion portion This is to reduce the effect and suppress the generation of second harmonics from the diffused portion.

Now, when silver ions are periodically diffused into a KTiOPO4 crystal at a period twice the coherence length,
In the region a in which silver ions are not diffused, the second harmonics generated at each point reinforce each other, and the guided second harmonics are integrated with the waveguide distance. On the other hand, in the region b where silver ions are diffused, the second harmonic is hardly generated. Therefore, there is almost no interference between the second harmonic generated in the region b and the second harmonic guided up to that point, and the second harmonic does not decrease so much.
Therefore, the second harmonic is generated only from the region a, and as shown in FIG. 4, the second harmonic is integrated one after another along with the waveguide distance, and finally a large second harmonic can be extracted. .

Further, as shown in FIG. 2, if an optical waveguide is formed by diffusing rubidium ions at right angles to the diffused portion, the quadratic nonlinear constant lowered by the effect of silver ions is:
Although it is recovered to some extent by the effect of the rubidium ion, it is slight, but rather, the light density is increased by the confinement effect of the optical waveguide, and more efficient wavelength conversion can be realized.

[0011]

【Example】

(Embodiment 1) FIG. 1 shows a wavelength conversion device using the wavelength conversion element of the present invention. 1 is 15 mm in length with the main surface on the -C surface,
It is a KTiOPO4 crystal having a width of 10 mm and a thickness of 0.5 mm. 2 is a region in which silver ions are diffused periodically. For the silver ion exchange layer, aluminum was vapor-deposited on the KTiOPO4 crystal, and the steps of resist coating, exposure, development, and etching were performed to prepare an aluminum mask with a period of 4 μm, and then a silver nitrate melt at 280 ° C. was formed. It was formed by immersion for 15 hours. As a result of measuring the silver ion diffusion region, the period is 4 μm,
A silver ion diffusion layer having a depth of 200 μm was observed. Wavelength 8
Laser light emitted as a fundamental wave 6 from a semiconductor laser 4 having a wavelength of 52 nm and an output of 500 mW is subjected to KT by a lens 5.
Focus on the iOPO4 crystal 1 to obtain the KTiOPO4 crystal 1
Propagated inside. The output intensity of the second harmonic having a wavelength of 426 nm was measured with an optical power meter and found to be 2 mW.

(Embodiment 2) FIG. 2 shows a wavelength conversion device using a wavelength conversion element according to an embodiment of the present invention. 1 is a K having a -C surface as a main surface and having a length of 15 mm, a width of 10 mm, and a thickness of 0.5 mm.
It is a TiOPO4 crystal. Similar to Example 1, an aluminum mask having a pattern formed periodically was formed on the KTiOPO4 crystal, and then immersed in a silver nitrate melt for 50 minutes to form a silver ion diffusion layer having a period of 4 μm and a depth of 5 μm. did. Furthermore, after removing the aluminum, the aluminum is reused for KTiOPO.
4 crystal was vapor-deposited, and resist coating, photolithography, development, exposure and etching were performed to form a linear optical waveguide mask pattern orthogonal to the silver ion diffusion region. This was formed with 90 mol% rubidium nitrate and 10 mol%.
% Barium nitrate mixed melt for 40 minutes, width 4μ
A linear optical waveguide having a m and a depth of 4 μm was manufactured. Wavelength 850
laser light emitted as a fundamental wave 6 from the semiconductor laser 4 by the lens 5 is end-polished with KTiO.
The light was condensed on the end face of the optical waveguide 3 of the PO4 crystal 1, and the 200 mW waveguide was propagated in the optical waveguide 3. The output intensity of the second harmonic wave having a wavelength of 425 nm emitted from the optical waveguide 3 was measured by an optical power meter and found to be 1.2 mW.

(Comparative Example) Similar to Example 2, K having a -C plane as a main surface and having a length of 15 mm, a width of 10 mm and a thickness of 0.5 mm.
After preparing a mask made of aluminum on which TiOPO4 crystals were periodically formed, the mask was immersed in a mixed melt of 97 mol% rubidium nitrate and 3 mol% barium nitrate at 340 ° C. for 45 minutes and rapidly cooled. Similar to 3,
An intermittently inverted optical waveguide having a width of 4 μm, a period of 4 μm, and a depth of 4 μm was produced. Wavelength 852nm,
Laser light emitted as a fundamental wave 6 from the semiconductor laser 4 is subjected to edge polishing by a lens 5 to form KTiOPO4.
The light is condensed on the end face of the optical waveguide 3 of the crystal 1 and is collected in the optical waveguide 3.
A 0 mW waveguide was propagated. The output intensity of the second harmonic having a wavelength of 426 nm emitted from the optical waveguide 3 was measured by an optical power meter and found to be 0.6 mW.

[0014]

As described above, according to the present invention,
It is possible to realize reliable and efficient wavelength conversion without the generated second harmonic wave being canceled by incomplete ferroelectric polarization inversion. As a result, it is possible to obtain a high-intensity laser beam that has never been obtained, and its practical effect is great.

[Brief description of drawings]

FIG. 1 is a perspective view of a first embodiment showing a configuration of a wavelength conversion device using a wavelength conversion element according to the present invention.

FIG. 2 is a perspective view of a second embodiment showing a configuration of a wavelength conversion device using a wavelength conversion element according to the present invention.

FIG. 3 is a perspective view showing a configuration of a wavelength conversion device using a conventional wavelength conversion element.

FIG. 4 is a diagram showing the relationship between the distance that the fundamental wave 6 propagates in the crystal and the output of the extracted second harmonic.

[Explanation of symbols]

 1 ... KTiOPO4 crystal 2 ... Silver ion diffusion layer 3 ... Optical waveguide 4 ... Semiconductor laser 5 ... Lens 6 ... Fundamental wave 7 ... Second harmonic wave

Claims (2)

[Claims] 1. A KT with a cycle that is an integral multiple of a coherence length.
A wavelength conversion element, wherein silver ions are periodically diffused on an iOPO4 crystal to form an ion diffusion layer. 2. The wavelength conversion element according to claim 1, wherein a linear waveguide orthogonal to the ion diffusion layer is formed.