JPH036540A - Optical second harmonic wave generating element - Google Patents

Abstract

PURPOSE:To enhance optical second harmonic wave generating (SHG) efficiency to >=10% by providing a basic wave waveguide consisting of a thin film of Ta2O3 doped with TiO2 on an LiNbO3 nonlinear optical system substrate formed with periodic domain inversion regions on the surface and selecting the periodic domain inversion regions in a specific range. CONSTITUTION:The basic wave waveguide 12 consisting of the thin film of the Ta2O3 doped with the TiO2 is provided on the LiNbO3 nonlinear optical crystal substrate 11 formed with the periodic domain inversion regions 10 on the surface. The relation between the domain inversion period LAMBDA of the periodic domain inversion regions 10 and the thickness (h) of the basic wave waveguide is selected within the range where the point (a) of (2.38, 0.5), the point (b) of (3.5, 0.5), the point (c) of (4.46, 0.16), the point (d) of (2.38, 0.16) and further the point (a) in respective (x, y) coordinate points are successively and linearly connected in the x-y orthogonal coordinate system in which the inversion period, LAMBDA(mum) is taken at the x-axis and the thickness (h) (mum) at the y axis. A large refractive index difference is attained in this way and the SHG efficiency is enhanced to >=10%.

Description

Detailed Description of the Invention [Industrial Application Field] The present invention uses a semiconductor laser in the 0.8 μm band (0.78 to 0.88 μm) as an optical second harmonic generating element, particularly as a fundamental wave light source. , and relates to an optical second harmonic generation element that extracts light of a wavelength outside of that.

[Summary of the invention]

The present invention relates to an optical second harmonic generation device, which has a fundamental waveguide made of Tie, doped Ta, O, and thin films on a LiNb 0° nonlinear optical crystal substrate with periodic domain inversion regions formed on the surface. The relationship between the domain inversion period of the periodic domain inversion region and the thickness h of the fundamental waveguide is expressed by taking the domain inversion period Δ (μm) on the y-axis and the thickness h (μm) on the y-axis. In the X-y orthogonal coordinate system, each (x, y) coordinate point is (2,38,0,5), (3,
5,0,5), (4,46,0,16), (2,3
8゜0, 16) and (2, 38, 0, 5) are selected within the range connected by a straight line, and in this way 0
．． To make it possible to extract light having a wavelength of % of that of semiconductor laser light in the 8 μm band with high efficiency.

[Conventional technology]

The optical second harmonic generation element (hereinafter referred to as SHG element) is ω
When light with a frequency of It is possible to optimize the use of laser light in each technical field. For example, by shortening the wavelength of laser light, it is possible to improve the recording density in optical recording/reproduction, magneto-optical recording/reproduction, etc. using laser light.

A Cerenkov radiation type SHG that forms a linear leading wave path on a nonlinear single crystal substrate, inputs the fundamental wave of near-infrared light, and extracts second harmonics, such as green and blue light, from the substrate side as radiation modes. For example, applied physics
Letters (＾ppliedPhysics Lette)
rs)ll, 447 (1970). This SHG has a ZnS polycrystalline guiding waveguide formed on a ZnO nonlinear single crystal substrate, and has a thickness of 1.06μ.
m using a Nd:YAG laser, 0.53, l/
The second harmonic of Il+ is obtained. However, in this case, since the waveguide is polycrystalline, the propagation loss is large, and the d constant of the ZnS substrate is small, so the efficiency is quite low. In the SHO device disclosed in 19p-ZG-1, 2, 3, 4, a LiNbO3 substrate (1) is used as shown in Fig. 8, and an optical waveguide ( 2). in this case,
A 0.84 μm semiconductor laser is used as the fundamental wave light source, and the input power is about 100 mW, and the SHG of 1 to 2 mW is used.
Light is being obtained. This type of SHO element has a major feature of automatic phase matching.

However, the SHG efficiency of all of these is 1 to 2% or less for an input of 100 mW, and does not reach the practically required efficiency of, for example, 10%.

To improve this SHG efficiency, fundamental waveguide mode and SHG
It is important to improve the overlap of the field distributions of radiation modes. However, when using ordinary single-domained (poled) LiNbO3, there is a limit to improving the overlapping of this field distribution, and there is a limit to improving efficiency.

Further, as shown in FIG. 8, the radiated SHG wave has a lateral spread angle θ of 10° or more, and the spot S also has a crescent shape, which poses a problem in terms of light collection characteristics.

[Problem to be solved by the invention]

In order to solve the above-mentioned problems, the present invention aims to 5) increase the IG efficiency to 10% or more and improve the spread of SHG light when a semiconductor laser light in the 0.8 μm band is used as the light source, that is, the fundamental wave. Excellent light-gathering characteristics with small corners 5)
(The purpose is to obtain a SHG element that can obtain G light.

[Means to solve the problem]

For example, as shown in FIG. 1, the present invention provides TiOz-doped Ta, O,
It has a fundamental waveguide (12) made of a thin film.

As described above, the structure of providing a fundamental waveguide (12) made of a TiOx-doped Ta, O, thin film on a LiNbO5 nonlinear crystal substrate (11) is disclosed in Japanese Patent Application No. 63-265787 filed by the present applicant. This was proposed in the application “Optical second harmonic generation device”, and in this case, the waveguide (1
2) and the nonlinear substrate (11) can be made large and varied by selecting the amount of Ti doping.

Domain inversion period Δ of periodic domain inversion region (10)
is within the Cerenkov radiation condition, that is, where n, zs are the refractive indices of the substrate at the wavelength of the SHG destination, and k
p is the wave number of the SHG destination, β9. is the propagation constant of the fundamental wave). In particular, in the present invention, in the above configuration, the relationship between the domain inversion period of the periodic domain inversion region (10) and the thickness h of the fundamental waveguide is shown in FIG. In the x-y orthogonal coordinate system with the inversion period Δ (μm) on the y-axis and the thickness h (μm) on the y-axis, at each (x, y) coordinate point,
Point a of (2.38, 0.5), point b of (3,5,0,5), point 0 of (4,46°0.16), and (2.38
, 0.16) and the above-mentioned point a are selected within a range that linearly connects them one after another.

(13) shows the SHG device according to the present invention as a whole.

[Function] In the configuration of the present invention, T is particularly formed on the LiNbOx substrate (11).
Fundamental waveguide (1
2), it is possible to realize a large refractive index difference. In addition, by forming periodic domain inversion regions (10) on the surface, the field distribution of the fundamental waveguide mode and the SHG radiation mode is This causes the surface to exhibit properties different from those in which the surface has uniform domains. Further, under this high difference in refractive index, in the coordinate system of Δ-h in FIG. 2 mentioned above, point a
The domain inversion period Δ(μ
m) and the thickness h (μm) of the fundamental waveguide (12), high SHG efficiency of 10% or more for 0.8 μm band semiconductor laser light, or as high as 30% in some cases, can be achieved. You can gain efficiency.

〔Example〕

An embodiment of the present invention will be described with reference to FIG.

In this example, a fundamental waveguide is constructed as a carriage type.

A nonlinear single-crystal substrate (11) made of a LiNbO3 plate is prepared, and periodic domain inversion regions (1
0) is formed. This periodic domain inversion region (10
), for example, after forming a vapor deposited Ti film in stripes on the main surface of the substrate (11) so as to arrange them in parallel with a predetermined width and pitch, and then heating the film at temperatures of 1100°C to 1100°C.
C1 can be formed by heat treatment at, for example, 1050°C for several hours. Moreover, this periodic domain inversion region (10) has a relationship between the thickness h and the period shown in FIG. A period Δ (μm) corresponding to the range of 0.16 to 0.5 μm is selected.

Then, on the surface of the LiNbO3 substrate (11) on which the periodic domain inversion region (10) is formed, a fundamental waveguide (12 ) to form. Here, the reason why the width W of the waveguide (12) is set to 2 μm or less is that when doing so, single mode SHG radiation light can be easily obtained. The reason why is set to 5 mm or less is to make it as small as possible. and,
This waveguide (12) is formed on a LiNbO3 substrate, for example.
An amorphous thin film in which a205 is doped with TiO□ is formed by CVD (chemical vapor deposition), and this is formed by photolithography, for example, by RIE (reactive ion etching).

C of this TiO□-doped Taz05 amorphous thin film
VD is, for example, tankurpentaethoxide Ta (OC
zHs) s and titanium tetraisopropoxide Ti,
(0-1-CJ7) 4 can be used as a raw material gas on the substrate (11) at a substrate temperature of 600°C.

Alternatively, when lowering the substrate temperature by applying photo-CVD, for example, the amount of Ti doped in the amorphous film can be increased.

TiO□-doped TatOs forming the waveguide (12)
The thin film has a ratio of Ti atoms to the sum of Ta and Ti atoms (Ti/Ta+Tt (atomic %)) of 15 to 6.
Select so that it is 0 atomic %. In other words, for example, wavelength 0
．． When we measured the relationship between the composition Ti/Ta+Ti (atomic %) and the refractive index n at 84 μm, we obtained the results shown in Figure 4. According to this, the refractive index n increases in proportion to the amount of Ti; Ti amount (Ti+Ta) >60 (atomic%
), crystallization occurs and the transmission tltM increases rapidly. That is, in the range of 0 less Ti amount (Ti + Ta)≦60 (atomic %), and more preferably in the range of 15<Ti amount (Ti
+Ta) <60 (atomic %), a good amorphous film is produced, and its refractive index n is 2.165 to 2.36.
I was able to select 5. Here, since the refractive index ne (refractive index for extraordinary light) of the LiNb0i substrate (11) is about 2,165, the difference Δn in the side refractive index is 0, Δn<0.
It can be selected within the range of 2 depending on the amount of Ti doped. Furthermore, it was confirmed that even in the 0.4 μm band, it can be varied within Δn≦0.3.

In the present invention, a 0.8 μm band semiconductor laser is used as the fundamental wave light source.

Example 1 A SHG element having the configuration described in FIG. 3 above is constructed. In this case, the fundamental wave wavelength is 0.84 μm, and by selecting the TiO□ doping of the fundamental waveguide (12), the refractive index difference Δn' with respect to the fundamental wave between the waveguide (12) and the LiNb0a substrate (11) is 0.15. The same refractive index difference Δn5NG with respect to the wavelength of 0.42 μm of the second harmonic SHO light was set as 0.22. And in this case 1
Waveguide (12) that can obtain SHG light of 10IllW or more for 00mW semiconductor laser light (fundamental wave input)
When measuring the relationship between the thickness h (μm) and the domain inversion period A (μm), the solid line (51
) is the range surrounded by. And now this solid line (51
), h = 0.3 μm.

Looking at the field distribution of the fundamental waveguide mode of the SHG element according to the present invention and the SHG radiation mode at Δ=3.27 μm, the 6th
The result will be as shown in the figure.

Comparative example 1 Although the configuration was the same as in Example 1, h = 0.80 μm.

A=15 μm.

The field distributions of the fundamental waveguide mode and the SHG radiation mode in Comparative Example 1 are as shown in FIG.

In FIGS. 6 and 7, curves (61) and (71) represent the field distribution of the fundamental waveguide mode, respectively, and curves (62) and (12) represent the field distribution of the SHG radiation mode, respectively. In each figure, the vertical axis indicates the position in the thickness direction of the SHG element (13), where the 0 point indicates the interface between the waveguide (12) and the substrate (11), and each division is 0.1 μm. Corresponds to thickness. The horizontal axis indicates the magnitude of the amplitude of the magnetic field, and the square of the amplitude peak of the fundamental waveguide mode is normalized to 1, and the peak of the amplitude of the SHG radiation mode is normalized to 1. As is clear from FIG. 6, in the SHG element according to the present invention of Example 1, the substrate (11
), the square of the amplitude of the fundamental waveguide mode and the field amplitude of the SHG radiation mode overlap well near the interface with the waveguide (12). In other words, the interaction is large and high S
HO efficiency is obtained. And the SHG in this case is about 3
Accordingly, the interaction length, that is, the length l of the waveguide (12) can be reduced, and as mentioned above, the length 2 can be made 6 mm or less. can. In addition, the Cerenkov radiation angle in this case is approximately 1″
It is. On the other hand, in the case of Comparative Example 1, the Cherenkov radiation angle is as small as about 2@, but as is clear from FIG. 7, near the interface between the substrate (11) and the waveguide (12), , the SHG efficiency is as small as about 104% because the above-mentioned overlap of field distributions is not observed.

The following Examples 2 to 7 are based on the TiO
□ Doped Ta, 0. In the thin film waveguide (12), T
By changing the doping amount of i0z, the above Δnt (fundamental wave 0.8
4 a m) and Δn'' (S HG wave 0.42
This is an example of changing pm).

Example 2 Same configuration as Example 1, but Δnf=0.10°Δ
n'''=0.15.

Example 3 Same configuration as Example 1, but Δn'=0.075
+Δn! NG=0.13.

Example 4 Same configuration as Example 1, but Δn'=0.06°Δ
n"'-0.11.

Example 5 Same configuration as Example 1, but Δnf=0.04°Δ
n'''=0.09.

Example 6 Same configuration as Example 1, but Δn'=0.020
+Δn·s' (0=0.30.

In these Examples 2 to 6, the thickness h (μm) of the waveguide (2)−domain inversion period A (μm) is the range surrounded by solid lines (52) to (57) in FIG. 5, respectively. At this time, the SHG efficiency was achieved at 10% or more.

In each of the above-mentioned sides, the cladding layer on the surface of the waveguide (12) has n=1.0 (air), or the cladding layer has nf=1.9. When n■0=2.2, the range of hΔ in which the SHO efficiency of Example 1 is 10% or more is the range surrounded by the solid line (57) in FIG. In addition, in the above embodiment, the h-Δ range in which the SHO efficiency is 10% or more is the case where the wavelength of the 0.8 μm band semiconductor laser, that is, the wavelength of the fundamental wave is 0.84 μm. This is the area (2) surrounded by the solid line (21) in Figure 2. However, if the wavelength of a 0.8 μm semiconductor laser is longer than this, for example 0.88 μm, the area surrounded by the solid line (22) in Figure 2 will be the area (2). (2), and when the wavelength is 0.78 μm, the region (2) is shown surrounded by a solid line (23) in FIG. However, within the range surrounded by points a-d in FIG. 2, the waveguide thickness h (μm) and the domain inversion circumference! tl
If IA (μm) is selected, the SHG efficiency can be made at least 10% or more by setting the cladding layer and other conditions. Also, at this time, the Cherenkov radiation angle was kept below 5°.

[Effects of the Invention] In the configuration of the present invention, T is particularly formed on the LiNbO3 substrate (11).
ie.

In addition to realizing a large refractive index difference by providing a fundamental waveguide (12) made of a doped TazOs thin film, a periodic domain inversion region (10) is formed on the surface, and the waveguide (12) The thickness h(μ
m) and the domain inversion period (μm), it is possible to improve the overlap of the field distributions of the fundamental waveguide mode and the SHG radiation mode, and SHO
It is possible to keep the efficiency at 10% or more and the interaction length, and thus the waveguide length i, at 6 mm or less.

In addition, the overlap of the field distributions is calculated between the substrate (11) and the waveguide (1).
2), and the Cerenkov angle can be reduced, the thickness of the substrate (11) can be reduced, and along with the improvement of miniaturization, the radiation angle of the SMC tip is reduced, and along with this, the Spread angle θ in the plane direction explained in the figure
Since it can be made smaller, it brings about many benefits such as improved light condensing characteristics, that is, simplification of the optical system in actual use of this SHG element.

[Brief explanation of the drawing]

1 and 3 are respectively enlarged perspective views of the optical second harmonic generating device (SHO device) according to the present invention, and FIGS. 2 and 5 are respectively the waveguide thickness h and the domain inversion period. Figure 4 shows the selection range of waveguide refractive index and Ti
6 and 7 are field distribution curve diagrams, respectively, and FIG. 8 is a perspective view of a conventional SHO element. (10) is a periodic domain inversion region, (11) is a substrate,
(12) is a fundamental waveguide.

Claims (1)

[Claims] LiNbO with periodic domain inversion regions formed on the surface
_3 TiO_2-doped Ta_ on a nonlinear optical crystal substrate
It has a fundamental waveguide made of a 2O_5 thin film, and the relationship between the domain inversion period Λ of the periodic domain inversion region and the thickness h of the fundamental waveguide is such that the domain inversion period Λ (μm) is taken as the x-axis, The above thickness h (μm) is y
In the x-y orthogonal coordinate system, each (x, y) coordinate point is (2.38, 0.5), (3.5, 0.5), (
4.46, 0.16), (2.38, 0.16), (2
．． 38, 0.5) within a range sequentially connected by a straight line.