JPH03260635A - Formation of partial polarization inversion region - Google Patents

Abstract

PURPOSE:To form partial polarization inversion regions to prevent the generation of an optical damage or a fluctuation in the refractive index in optical waveguides by partially heating the heat absorbing film on a substrate to a Curie point or above, then lowering the temp. of the film to room temp. to generate a polarization inversion. CONSTITUTION:The heat absorbing film 2 or the pattern part of the heat absorb ing film is provided on the substrate consisting of a ferroelectric material in at least the range where the partial polarization inversion regions 4 are to be formed. This substrate is heated up to the temp. right below the Curie point and further, the regions of the heat absorbing film 2 of the range where the partial polarization inversion regions 4 are to be formed, i.e., the partial heating regions 3, are heated partially to the Curie point or above; thereafter, the temp. of the substrate 1 is lowered. Since the ferroelectric material has a pyroelectric effect, the regions partially heated by a laser beam at the time of the temp. fall are subjected to the polrization inversion. The partial polarization inversion regions 4 which do not exert adverse influence are formed as the optical waveguides if the heat absorbing film 2 is removed and washed by suitable treating liquids.

Description

[Detailed Description of the Invention] [Summary] Regarding a method for forming a partially polarization inverted region,
The purpose of this project is to improve the method for forming partially polarized regions used to create a polarization-inverted leading waveguide with small optical damage and low loss on a ferroelectric material in order to generate harmonics. a step of providing a heat absorbing film or a heat absorbing film pattern portion in a range forming at least a partially polarization inverted region on a substrate, a step of heating the substrate to just below the Curie point, and a step of heating the substrate to a point just below the Curie point; A step of irradiating a desired region of the absorption film or a region including at least the heat absorption film pattern portion with a laser beam to heat the formation range of the partially polarization inverted region to a temperature higher than the Curie point, and lowering the temperature of the substrate. Afterwards, the method for forming a partially polarization inverted region is configured to include at least the step of removing the heat absorbing film or the heat absorbing film pattern portion.

[Industrial application field]

The present invention provides an optical waveguide type second waveguide that is resistant to optical damage and has high conversion efficiency.
The present invention relates to a method for forming a partially polarized region for creating a polarized optical waveguide for a harmonic generating element.

In recent years, lasers, particularly semiconductor lasers (LDs), have come to be widely used as light sources for laser printers, laser scanners, optical discs, and the like. However, on the other hand, there is a growing demand for shorter wavelengths (for example, from infrared light to visible light) to increase storage capacity and make handling easier. Although the development of short wavelength oscillation in semiconductor lasers is progressing, it is quite difficult to reduce the oscillation wavelength to 600 nm or less at the current technological level, and other technologies, such as second harmonic generation (SHG), are currently being developed. )
There is a strong demand for the development of devices that can obtain short-wavelength coherent light, especially techniques for creating polarization-inverted optical waveguides for use in such devices.

[Conventional technology]

Conventionally, as a second harmonic generation element for laser light, one that passes laser light through a bulk nonlinear optical crystal is well known.

For example, if LiNbO3, which is a ferroelectric crystal, is cut into a bronch, both sides are optically polished to form a laser beam exit surface, and a laser beam with an angular frequency ω is input from one side.
A second harmonic with twice the angular frequency 2ω is generated from the opposite side. The conversion efficiency (η, H6-P2ω/Pω) at this time is the dielectric constant and magnetic permeability of the vacuum, respectively. ,μ. , the nonlinear optical constant of the crystal is d, the crystal length is l, the refractive index for the fundamental wave and harmonics is nωn2ω, and the difference in propagation constant between the fundamental wave and harmonics is Δ
k and the beam cross-sectional area is 8, it is expressed by the following equation (1).

ηS 11 G −2 (ε0/μo) 3/Z (ω2
d212/nzω°nω) (Pω/8) [5in2(Δ
k12)/(Δ kj2/2)21(1) The second harmonic output P2ω is (1) 5i as shown from 2
The output characteristics of the n2 curve are shown. In this way, the periodic fluctuation of the second harmonic output P2ω with the 5in2 curve characteristic is n
This is because ω and n2ω are different. That is, due to the refractive index difference, the phases of the second harmonics generated at each point are not aligned, and the second harmonic output fluctuates with a period of distance at which the phase shift is 2π. In normal crystals, the difference between nω and n2ω is large due to the wavelength dispersion of the refractive index, and therefore,! c (coherence length) is very small. That is, the second harmonic output P2ω also becomes very small. To solve this problem, a method of achieving phase matching has been proposed.

For example, the refractive index n of the extraordinary light of the second harmonic light λ2
, and the refractive index n0 of the ordinary light of the fundamental wave light λ1 are made to coincide with each other. This is a method of obtaining a large second harmonic output by satisfying the so-called phase matching condition [Massyo, Kamiya: Fundamentals of Optoelectronics, pp199-2
00.1974 (published by Maruzen)].

Figure 3 is a diagram showing the second harmonic output characteristics (when phase matching is achieved), where the vertical axis is the second harmonic output P2ω and the horizontal axis is the crystal length! It is. In the figure, the broken line (■) indicates the refractive index of the second harmonic light λ2 and the refractive index of the fundamental wave light λ1 explained above, so that the second harmonic output P2ω increases as the crystal length l increases. This is the case. but,
In the case of the bulk crystal type described above, the phase matching condition is satisfied.

In addition, since a crystal with a large nonlinear optical constant has not been obtained, the light intensity of the fundamental wave must be increased, and semiconductor lasers do not have sufficient light source intensity and have not been put into practical use.

On the other hand, recently, optical waveguide type elements have been used to, for example,
Phase matching is performed on the refractive index n8 curve of the extraordinary light (therefore, the refractive index of the fundamental wave and the second harmonic light are different)
An extremely effective method has been proposed in which a large nonlinear optical constant can be utilized and, as a result, a large conversion efficiency (η, H0) can be obtained.

FIG. 4 is a diagram showing an example of an optical waveguide type second harmonic generation element, in which (a) is a perspective view and (b) is a cross-sectional view of XX.

In the figure, 1 is a substrate, for example, a substrate whose +7 surface is optically polished so that the incident direction and polarization direction can be set to obtain the largest nonlinear optical constant of LiNbO3, and 100 is a substrate l
This is a polarization-inverted optical waveguide formed above. Same figure (b)
As shown in , for example, if the ferroelectric polarization of the substrate l is aligned downward, and regions with a certain depth where the polarization is directed upward are formed at regular intervals in the optical waveguide portion with one period Δ, A polarization-inverted optical waveguide 100 is configured in which downward polarization regions and upward polarization regions B are arranged at equal intervals.

Now, for example, if a laser beam with an angular frequency ω is made to enter the polarization-inverted optical waveguide 100 from the left side and exit from the right side, the phase matching condition will be satisfied when the following equation is satisfied, and a large second harmonic output will be obtained. It is known that (J,
A. Armstrong, et al., Phys.

Rev, vol, 127. p1918.1962).

Δ−2πm1IB ,,,(2ω)−2β1o o (
ω) ] −−<1) Here, β, , (2ω) is the propagation constant of the second harmonic for the polarization-inverted optical waveguide 100,
β,. . Similarly, the propagation constant 1m of the fundamental wave (ω) is a positive odd number.

In addition, when the above equation is expressed using a refractive index, Δ−λ. m/2I n ,, (2ω) −n 1
0°(ω)] −(2).

Here, λ. is the wavelength of the fundamental wave in vacuum + nl. .

(2ω) is the refractive index +nl of the second harmonic for the polarization-inverted optical waveguide 100. . (ω) is also the refractive index of the fundamental wave 1
m is a positive odd number.

The solid line marked ■ in Fig. 3 shows an example of the ideal second harmonic output characteristic in this case, in which the second harmonic output pgω increases with the crystal length l, and moreover, using a large nonlinear constant, Because of this, the conversion efficiency is greatly improved compared to the bulk crystal type.

In order to fabricate the polarization-inverted guide waveguide 100 as described above, a partially polarization-inverted region, for example, the upward polarization region B in FIG. 4 must be formed. Two such methods are well known.

The first method uses a phenomenon in which when a suitable mask is placed on the substrate 100 and the substrate is heated to a high temperature, Li on the exposed surface is diffused to the outside and the polarization of that portion is reversed. However, in this case, the depth of the polarization inversion region is only about 1 μm at most, and the refractive index of that portion also changes, making it difficult to put it to practical use as an optical waveguide.

The second method uses the phenomenon that when Ti is deposited on the part that will become the polarization inversion region and then heat-treated, the polarization in that part is reversed, and the depth can reach several μm, making it practical. It is expected that optical waveguides will be fabricated.

FIG. 5 shows an example of a conventional method for forming a partially polarization inverted region, and illustrates the order of the main steps.

Step (1): A substrate 1 is prepared by optically polishing the +7 plane of a single-domain ferroelectric crystal, for example, LiNbO3.

Step (2): A 300 nm thick Ti layer with a weight of 0 is vacuum deposited on the treated substrate.

Step (3): The treated substrate is photo-etched so that Ti remains in the portion that will become the polarization inversion region B to form a Ti thin film pattern 20'.

Step (4): When the treated substrate is heat-treated at about 10,006 C to diffuse Ti and form a Ti diffusion region 20'',
The polarization of that portion of the surface of the substrate 1 is reversed.

Step (5): If the treated substrate is treated with an appropriate treatment solution and any remaining Ti is removed and cleaned as necessary, partial polarization becomes the polarization inversion region B of the polarization inversion type guide waveguide. A reversal region (B) 40 is formed.

[Problem to be solved by the invention]

However, since the partially polarized region (1'1) 40 formed by the above-mentioned conventional method contains Ti, when laser light is passed through it, as is already well known, the so-called light Since the damage value decreases and the refractive index also changes, a polarization-inverted optical waveguide that uses it as a component will, for example, absorb second harmonic light, as shown by the dotted line (■) in Figure 3. There are serious practical problems that need to be solved, such as not increasing the output intensity and also increasing loss due to light scattering due to changes in the refractive index.

[Means to solve the problem]

The above-mentioned problem consists of a step of providing a heat absorbing film 2 or a heat absorbing film pattern portion 2'' on a substrate 1 made of a ferroelectric material in an area where at least a partially polarization inverted region 4 is to be formed;
A step of heating the substrate to just below the Curie point, and irradiating a desired region 1 of the heat absorption film 2 on the heated substrate 1 or a region including at least 2° of the heat absorption film pattern with a laser beam to achieve partial polarization. Partial decentralization inversion including at least a step of heating the formation range of the inversion region 4 to a temperature higher than the Curie point, and a step of removing the heat absorption 11g2 or the heat absorption film pattern portion 2° after lowering the temperature of the substrate 1. This problem can be solved by the method of forming the regions.

[Effect]

According to the present invention, when forming the partially polarized region 4, the heat absorbing film 2 or the heat absorbing film 2, which does not have a bad influence on the optical waveguide, is formed without using means such as diffusion of Ti or diffusion of Li out of the crystal. The absorbing film pattern portion 2 is partially heated above the Curie point and then lowered to room temperature to cause polarization inversion in the pyroelectric field caused by the pyroelectric effect. If the polarization-inverted optical waveguide 100 is manufactured, not only optical damage but also refractive index fluctuation in the optical waveguide will not occur, and therefore a large second harmonic optical output can be obtained. Furthermore, LiTa0. This method has the great advantage of being applicable to crystals with low Curie points, such as crystals with low Curie points.

[Example] FIG. 1 is a diagram showing an example of the present invention, showing the main steps in order. The explanation will be given below according to the figures.

Step (1): For example, thickness 0.5 mm, 1111
0mm.

Substrate 1 is prepared by optically polishing the +2 surface so as to obtain the largest nonlinear optical constant of LiTaO3 treated in a single domain having a length of 15 mm.

Step (2) Applying a heat absorbing film 2 on the above-mentioned treated substrate. for example,
Au is vacuum-deposited to a thickness of 1100 nm.

Step (3) Heat the above-mentioned treated substrate to a temperature that does not exceed the cue point, for example, 6008C, and further heat the area of the heat absorption film 2 that will become the partially polarization inverted area 4.

That is, the partially heated region 3 is partially heated to a temperature above the Curie point by scanning with a YAG laser beam, for example, and then cooled to room temperature.

LiTa0. Since ferroelectric materials such as ferroelectric materials also have a pyroelectric effect, the region partially heated by the YAG laser beam during this temperature drop is polarized by the pyroelectric field based on the pyroelectric charges in the other non-irradiated regions. be reversed.

Step (4) The heat absorbing film 2 left on the second treated substrate is
For example, if it is removed and cleaned with an appropriate treatment liquid, a partially polarized region 4 that will not have any adverse effect as a leading waveguide will be formed.

The depth of the partially polarized region 4 can be controlled by adjusting the power and irradiation time of the irradiated laser beam.

Including this partially polarization inverted region 4, a well-known method is used, for example, to form a mask made of SiO□ sputtering, leaving a portion that will become an optical waveguide, and using benzoic acid (C6) heated to 200 to 300°C. C00) l) melt for several tens of minutes
Soak for several hours. If the refractive index is increased by about 1% compared to the surrounding substrate portion by a so-called proton exchange method, the polarization-inverted guiding waveguide 100 can be formed. The optical waveguide-type second harmonic generation device fabricated in this way has better optical damage values and smaller refractive index fluctuations than those using the conventional polarization inversion method using Ti diffusion. Conversion efficiency is greatly improved.

FIG. 2 is a diagram showing another embodiment of the present invention.

In the figure, 2'' is a heat-absorbing film pattern section. Note that the same reference numerals are given to the same parts as those explained in the above-mentioned countries, and the explanation of the same parts will be omitted.

The main steps will be explained step by step below.

Step (1): For example, thickness 0.5 mm, medium 10 mm
Substrate 1 is prepared by optically polishing the +2 surface so that the largest nonlinear optical constant of 1 TaO is obtained.

Step (2) Applying a heat absorbing film 2 on the above-mentioned treated substrate. for example,
Au is vacuum-deposited to a thickness of 1100 nm.

Step (3) The above-treated substrate is photoetched so that Au remains in the portion that will become the polarization inversion region B to form a heat absorbing film pattern portion 2''.

Step (4) Two, the above-mentioned treated substrate is heated to a temperature not exceeding the cue point, for example, 600°C, and the region including at least the heat-absorbing film pattern portion 2 is heated with, for example, YA.
G By scanning the laser beam, it is partially heated to above the Curie point and then cooled to room temperature. In this case, Li
Since the substrate 1 made of TaO3 is transparent and has low absorption of YAC laser light, the temperature rises above the Curie point only in the region where the heat absorbing film pattern portion 2' is present.

Then, as in the case of the above embodiment, the heat absorbing film pattern portion 2 whose temperature is raised above the Curie point when the temperature drops.
The polarization of the crystal part at the bottom of the crystal is inverted at a predetermined depth by a pyroelectric field based on pyroelectric charges.

Step (5) Partially polarized region of the heat absorbing film pattern portion 2° left on the second treated substrate, for example, where Au is removed and cleaned with an appropriate treatment liquid so that it will not have any adverse effects as an optical waveguide. 4 is formed.

In this case, there is no need to strictly control the position of the YAG laser beam irradiation area, so there is a great practical advantage.

The known method includes this partially poled region 4.

For example, if the refractive index is increased by about 1% compared to the surrounding substrate portion using the proton exchange method as described above, the polarization-inverted optical waveguide 100 can be formed. A generating element can be manufactured.

In the above embodiments, a thin film of 80 was used as the heat absorption film 2 or the heat absorption pattern portion 2, but other materials that do not affect optical damage, such as PT, Pd, and alloys thereof, may also be used. It's okay.

Further, partial heating is not limited to YAG laser irradiation, and it goes without saying that other laser beams or heating means may be used.

The embodiments described above are merely examples, and it goes without saying that other preferred materials and configurations, or combinations thereof, may be used as long as they comply with the spirit of the present invention.

〔Effect of the invention〕

As described above, according to the method of the present invention, when forming the partially domain-inverted region 4, no means such as diffusion of Ti or diffusion of Li out of the crystal is used, so that the optical waveguide is not adversely affected. The heat-absorbing film 2 or the heat-absorbing film pattern 2' is partially heated above the Curie point and then lowered to room temperature to cause polarization reversal in the pyroelectric field caused by the pyroelectric effect. If the polarization-inverted optical waveguide 100 is manufactured using the region 4 as a constituent part, not only optical damage but also refractive index fluctuation in the optical waveguide will not occur. Furthermore, LiTa
This method has the great advantage that it can be applied to crystals with low cue points such as 01, and it greatly contributes to improving the performance and quality of optical waveguide type second harmonic generation elements.

FIG. 4 is a diagram showing an example of an optical waveguide type second harmonic generation element, and FIG. 5 is a diagram showing an example of a conventional method for forming a partially polarization inverted region.

In the figure, 1 is the board, 2 is a heat absorption film; 2” is the heat absorption film pattern part, 3 is a partial heating area;

[Brief explanation of drawings]

Fig. 1 is a diagram showing an embodiment of the present invention, Fig. 2 is a diagram showing another embodiment of the invention, and Fig. 3 is a diagram showing the second harmonic output characteristic. Main dish≦The back of the sun and moon is still there. I'm here! Maximum 32*m carved stems IshiD envoy] magazine z
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Claims (1)

[Claims] A step of providing a heat absorbing film (2) or a heat absorbing film pattern portion (2') on a substrate (1) made of ferroelectric material in an area where at least a partially polarized region (4) is formed. heating the substrate (1) to just below the Curie point; At least a region containing at least the partially polarized region (
4) heating the formation area above the Curie point, and after lowering the temperature of the substrate (1), heating the heat absorbing film (2).
) or removing the heat absorbing film pattern portion (2').