JPH043128A - Formation of partial polarization inversion region - Google Patents

Abstract

PURPOSE:To obtain the large output intensity of a second harmonic wave by shorting comb-shaped metallic patterns and the full-surface metallic films formed on the other surface of a piezoelectric substrate and heating the piezoelectric substrate to the temp. just below the Curie point, then immediately cooling the substrate. CONSTITUTION:The full-surface metallic films 20 and 3 consisting of vacuum vapor deposited films of Au are formed on both the front and rear surfaces of the substrate. A metallic film 4 for shorting which consists of the Au film shorting the full-surface metallic films 20 and 3 is formed on the side face. Further, the one full-surface metallic film of the substrate is subjected to photoetching to a comb tooth shape to form the comb-shaped metallic patterns 2. The substrate is cooled to room temp. immediately after the substrate is heated to the temp. just below the Curie point to generate polarization inversion by the electric field generated by a pyroelectric effect in the surface region between the comb teeth of the comb-shaped metallic patterns 2. The large second harmonic light output is obtd. in this way.

Description

[Detailed Description of the Invention] [Summary] Regarding a method for forming a partially polarization-inverted region, a part for creating a polarization-inverted optical waveguide for second harmonic generation with little optical damage and low loss on a piezoelectric substrate. With the aim of improving the method of forming polarization inversion regions, we developed a comb-shaped metal pattern formed on one side of a piezoelectric substrate,
The method for forming a partially polarization inverted region is configured by short-circuiting the piezoelectric substrate with a metal film formed on the other surface of the entire surface, heating the piezoelectric substrate to just below the Curie point, and then immediately cooling it. Further, a proton exchange region is formed on at least the surface of the piezoelectric substrate on which the comb-shaped metal pattern is formed, so as to further enhance the effect.

[Industrial Application Field] The present invention is directed to an optical waveguide type second waveguide which is resistant to optical damage and has high conversion efficiency.
The present invention relates to a method for forming a partially polarized region for producing 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 LiNbO5, which is a ferroelectric crystal, is cut into a block, 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, HG-P!ω/Pω) at this time is the permittivity and magnetic permeability of vacuum, respectively. ,μ. , the nonlinear optical constant of the crystal is d, and the crystal length is! , the refractive index for the fundamental wave and harmonics is nω.

When n2ω, the difference in the propagation constant between the fundamental wave and the harmonics is Δk, and the beam cross-sectional area is A, it is expressed by the following equation (1).

ηsHc, =2(μo/εo)”2(ω2d2j22
/nzω−n ω)(Pω/A) [sin”(Δkf
, /2)/(ΔkL'2)"] The second harmonic output P2ω shows the output characteristic of the 5in2 curve as seen from equation (1). In this way, the second harmonic output P2ω shows the output characteristic of the 5in2 curve. The reason for the periodic fluctuation is that nω and nlω are different. In other words, the second wave generated at each point due to the difference in refractive index
The phases of the harmonics are not aligned, and the second harmonic output fluctuates with a period of distance at which the phase shift becomes 2π. In normal crystals, the difference between nω and n2ω is large due to the wavelength dispersion of the refractive index, and therefore,! , (coherence length) are very small, and the second harmonic output P2ω is also very small. To solve this problem, a method of achieving phase matching has been proposed.

For example, the laser beam is made to enter so that 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 C are made to match. This is a method of obtaining a large second harmonic output by satisfying the so-called phase matching condition [Taiyo, Kamiya: Fundamentals of Optoelectronics, pp199-200.
1974 (published by Maruzen)].

FIG. 5 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 l. 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 i increases. This is the case. but,
In the case of the bulk crystal type described above, a crystal that satisfies the phase matching condition and also has a large nonlinear optical constant has not been obtained, so the light intensity of the fundamental wave must be increased, and low power 7 has not yet been put into practical use as a light source.

On the other hand, recently optical waveguide type elements have been used.

For example, the refractive index of extraordinary light is 0. By phase matching on the curve (thus, the refractive indices of the fundamental and second harmonic light are different), large nonlinear optical constants can be exploited, resulting in large conversion efficiencies (ηSMG).
An extremely effective method has been proposed to obtain this.

FIG. 6 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 +2 surface is optically polished so that the incident direction and polarization direction can be set to obtain the largest nonlinear optical constant of LiNb0:+, and 100 is a substrate formed on the substrate l. It is a polarization-inverted optical waveguide. As shown in the same figure (b), for example, the substrate 1 is aligned with ferroelectric polarization in the upward direction, and the polarization is arranged at equal intervals in the optical waveguide part, for example, to a certain depth that the polarization is downward in one period. Area 5
0, a polarization-inverted optical waveguide 100 is constructed in which upward polarization regions and downward polarization regions 50 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, Ara+strong, et, al, ,
Phys.

Rev, vol, 127. p1918.1962).

Δ=2πm/[β1°. (2ω)−2β1°. (ω)]
−・−(2) Here, β1°. (2ω) is the propagation constant of the second harmonic for the polarization-inverted optical waveguide 100, β1°.

Similarly, the propagation constant 2m of the fundamental wave (ω) is a positive odd number.

In addition, when the above formula is expressed using a refractive index, A=λ. m/2I n+oo(2ω) −n+oo(ω
)1-(3).

Here, λ. is the wavelength of the fundamental wave in vacuum, n1O0(2ω
) is the refractive index of the second harmonic +n1O0(ω) for the polarization-inverted optical waveguide 100, and the refractive index of the fundamental wave, 9m, is a positive odd number.

The solid line (■) shown in Fig. 5 shows an example of the ideal second harmonic output characteristic in this case, where the second harmonic output P2ω increases with the crystal length l, and also has a large nonlinear constant. The conversion efficiency has been greatly improved compared to the Norck crystal type.

In order to fabricate the poled optical waveguide 100 as described above, it is necessary to form a partially polarized region, for example, the downward polarized region 50 in FIG. 6 above. 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. 7 is a diagram showing an example of a conventional method for forming a partially polarization inverted region, and illustrates the main process order.

Step (1): A substrate 1 is prepared by optically polishing the +2 surface of a single-domain ferroelectric crystal, for example, LiNbO3.

Step (2): A 300 nm thick Ti film 30 is vacuum deposited on the treated substrate.

Step (3): The processed substrate is photoetched so that Ti remains in the portion that will become the polarization inversion region 50 to form a Ti thin film pattern 30''.

Step (4): When the treated substrate is heat-treated at about 1000°C to diffuse Ti and form a Ti diffusion region 30'',
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 liquid and any remaining Ti is removed and cleaned as necessary, partial polarization inversion will be achieved, which will become the polarization inversion region 50 of the polarization inversion type optical waveguide. A region 50 is formed.

[Problem that the invention sought to solve]

However, since the partially polarized region 50 formed by the conventional method contains Ti, as is already well known, when the partially polarized region 50 is passed through a laser beam.

Because the so-called dark value of optical damage decreases and the refractive index also changes, a polarization-inverted optical waveguide that uses this as a component is, for example, as shown by the dotted line (■) in Figure 5. The output intensity of harmonic light reaches a plateau and does not increase.
In addition, there are serious practical problems, such as increased loss due to light scattering due to refractive index fluctuations, which need to be solved.

[Means for Solving the Problem] The above problem is solved by combining the comb-shaped metal pattern 2 formed on one surface of the piezoelectric substrate 1 and the entire surface metal film 3 formed on the other surface of the piezoelectric substrate 1. This problem can be solved by a method for forming a partially polarized region, which is characterized in that the piezoelectric substrate 1 is short-circuited, heated to below the Curie point color, and then immediately cooled. Furthermore, if a proton exchange region 6 is formed on at least the surface of the piezoelectric substrate 1 on which the comb-shaped metal pattern 2 is formed, a more stable partially polarization inverted region can be formed.

[Function] Since ferroelectric materials such as LiTaO3 and LiNbO3 also have a pyroelectric effect, a bias in electric charge occurs as the temperature changes, and an electric field is generated based on this bias.

Figure 3 is a diagram explaining the polarization inversion mechanism of the present invention (Part 1)
The figure (a) is a diagram explaining the generation of charge q and electric field due to the pyroelectric effect, and the figure (b) shows a partial surface region of the upward polarization P.

That is, it shows a state in which polarization P8 reversed downward due to the electric field due to the pyroelectric effect is generated in the surface area between the comb teeth of the comb-shaped metal pattern 2.

Figure 4 is a diagram explaining the polarization inversion mechanism of the present invention (Part 2)
This figure shows the case where a proton exchange region 6 is formed on the surface of the piezoelectric substrate 1, and as is already known, polarization reversal in that region is likely to occur due to proton exchange, so it is likely that the polarization will be reversed due to the pyroelectric effect. The electric field allows the polarization inversion region 5 to be formed more stably.

That is, according to the present invention, when forming the partially polarized region 5, polarization is inverted using a pyroelectric electric field caused by the pyroelectric effect, without using means such as diffusion of Ti or diffusion of Li out of the crystal. Therefore, not only optical damage but also refractive index fluctuation in the optical waveguide does not occur.

Therefore, a large second harmonic optical output can be obtained.

Furthermore, this method has the great advantage of being applicable to crystals with low Curie points such as LiTa0,1.

〔Example〕

FIG. 1 is a diagram showing an embodiment 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 0.5mm, width 10mm
.

A single domain treated LiTa0. The piezoelectric substrate 1 is manufactured by optically polishing the +2 surface so as to obtain the largest nonlinear optical constant.

Step (2): A full-surface metal film 2 made of a vacuum-deposited Au film with a thickness of 1100 nm, for example, is applied to both the front and back surfaces of the treated substrate.
0 and 3 are formed, and the entire surface metal films 20 and 3 are short-circuited. For example, a shorting metal film 4 made of the same <Au film is formed on the side surface.

Step (3): As shown in the figure, the entire surface metal film 20 of the above-mentioned processed substrate is photoetched into a comb-shaped metal pattern 2 at equal intervals with a pattern width of 2.5 μm and a pattern gap of 2.5 μm. Form.

Step (4) Two: After heating the above-mentioned treated substrate to, for example, 600°C under curie stippling, immediately cool it to room temperature,
Polarization inversion is caused in the surface region between the comb teeth of the comb-shaped metal pattern 2 by an electric field due to the pyroelectric effect.

Step (5) Comb-shaped metal pattern left on the second treated substrate 2. Full-surface metal film 3 on the back and short-circuiting metal M4 on the side
By removing and cleaning with an appropriate treatment liquid, a piezoelectric substrate 1 on which a partially polarized region 5 for an optical waveguide is formed can be obtained.

The depth of the polarization inversion region 5 can be controlled by adjusting the maximum heating temperature and temperature change rate.

FIG. 2 is a diagram showing another embodiment of the present invention, and the main steps will be illustrated and explained in order.

Step (1): For example, thickness 0.5 mm, width 10 mm
A piezoelectric substrate 1 is produced by optically polishing the +2 surface of LiTaO3, which has been subjected to single domain treatment and has a length of 15 mm, so as to obtain the largest nonlinear optical constant.

Step (2): The above-mentioned treated substrate is immersed in a melt of benzoic acid (C6H1COOH) heated to 200 to 300'C for several tens of minutes.
Soak for several hours. By a so-called proton exchange method, the proton exchange region 6 is formed at least on the surface on which the polarization inversion region 5 is to be formed.

Step (3): A full-surface metal film 2 made of a vacuum-deposited Au film with a thickness of 1100 nm, for example, is formed on both the front and back surfaces of the treated substrate.
0 and 3 are formed, and the entire surface metal films 20 and 3 are short-circuited. For example, a shorting metal film 4 made of the same <Au film is formed on the side surface.

Step (4): As shown in the figure, the entire surface metal film 2o of the above-mentioned treated substrate is photo-etched into a comb-shaped metal pattern 2 at equal intervals with a pattern width of 2.5 μm and a pattern gap of 2.5 μm. Form.

Step (5) Two: After heating the above-mentioned treated substrate to just below the Curie point, for example, 6000C, immediately cool it to room temperature,
Polarization inversion is caused in the proton exchange region 6 between the comb teeth of the comb-shaped metal pattern 2 by an electric field due to the pyroelectric effect.

Step (6) Comb-shaped metal pattern left on the second treated substrate 2. Full-surface metal film 3 on the back surface and short-circuiting metal film 4 on the side surface
By removing and cleaning with an appropriate treatment liquid, a piezoelectric substrate 1 on which a partially polarized region 5 for an optical waveguide is formed can be obtained.

In this embodiment, a proton exchange region 6 is provided on the surface of the piezoelectric substrate 1.
is formed in advance, polarization inversion occurs at a lower temperature, and the depth of the polarization inversion region 5 closely matches the depth of the proton exchange region 6, making depth control extremely easy.

The optical waveguide type second harmonic generation element constituted by the partially polarized region 5 formed in this way has a lower optical damage value than that using the conventional polarization inversion method using Ti diffusion. The refractive index fluctuation is also small, and the conversion efficiency is greatly improved.

In the above embodiments, a thin film of Au was used as the full-surface metal films 3 and 20 and the short-circuiting metal film 4, but the invention is not limited to this, and other materials that do not affect optical damage, such as , pt, Pd, and alloys thereof may also 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.

(Effects of the Invention) As described above, according to the method of the present invention, when forming the partially domain-inverted regions 5, pyroelectric Since polarization inversion is caused by the pyroelectric field caused by the effect, neither optical damage nor refractive index fluctuation occurs in the optical waveguide. Therefore, a large second harmonic optical output can be obtained, and optical waveguide type second harmonic This greatly contributes to improving the performance and quality of the generator.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing an embodiment of the present invention, Fig. 2 is a diagram showing another embodiment of the invention, Fig. 3 is a diagram explaining the polarization inversion mechanism of the present invention (Part 1), Fig. 4 is a diagram explaining the polarization inversion mechanism of the present invention (Part 2)
, Fig. 5 is a diagram showing the second harmonic output characteristics, Fig. 6 is a diagram showing an example of an optical waveguide type second harmonic generation element, and Fig. 7 is an example of a conventional method for forming a partially polarization inverted region. FIG. In the figure, 1 is a piezoelectric substrate, 2 is a comb-shaped metal pattern, 3.20 is a metal film on the entire surface, 4 is a short-circuiting metal film, 5 is a polarization inversion region, and 6 is a proton exchange region. , I will show you the implementation fJ of the $ investigation plan! ! Figure 12 also shows 2 islands blind station 5 scale earth 7 Figure 5 Fig. 7 E shows 7 Fig. 6 explains Oai Ming's labor and reaction structure 5 Fig. 5 (one of them)

Claims (2)

[Claims] (1) short-circuiting a comb-shaped metal pattern (2) formed on one surface of the piezoelectric substrate (1) and a full-surface metal film (3) formed on the other surface of the piezoelectric substrate (1); A method for forming a partially polarized region, characterized in that the piezoelectric substrate (1) is heated to just below the Curie point and then immediately cooled. (2) A proton exchange region (6
2. The method of forming a partially polarization inverted region according to claim 1, wherein: