JPH0361931A - Second harmonic wave generating element - Google Patents

Abstract

PURPOSE:To efficiently generate second harmonic waves by setting an ordinary light refractive index and an extraordinary light refractive index in certain specific ranges. CONSTITUTION:An LiNbO3 thin-film waveguide layer is formed on an LiTaO3 substrate to form the second harmonic wave generating element (SHG element). The wavelength lambdamum of a basic wave laser beam, the film thickness Tmum of the LiNbO3 thin-film waveguide layer, the ordinary light refractive index nos1 of the LiTaO3 substrate at the wavelength lambdamum of the basic wave laser beam, the ordinary light refractive index noF1 of the LiNbO3 thin-film waveguide layer at the wavelength lambdamum of the basic wave laser beam, the extraordinary light refractive index neS2 of the LiTaO3 substrate at the second harmonic wave wavelength lambdamum/2, and the extraordinary light refractive index neF2 of the LiNbO3 thin-film waveguide layer at the second harmonic wave wavelength lambdamum/2 are respectively set within the ranges of the numerical values expressed by equation I. The SHG element having the higher conversion efficiency is obtd. in this way.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a second harmonic generation element having an extremely high conversion efficiency to second harmonic light.
○, relates to a second harmonic generation element (hereinafter, the second harmonic generation element is referred to as an SHG element) in which a L i N b O 3 thin film waveguide layer is formed on a substrate.

(Prior art) An SHG element is an element that converts an incident laser beam of wavelength λ into a wavelength of λ/2 and outputs it by utilizing the nonlinear optical effect of an optical crystal material having a nonlinear optical effect. Since the wavelength of light is converted to 1/2, the recording density can be quadrupled by applying it to optical disk memories, CD players, etc., and it can also be applied to laser printers, photolithography, etc. This allows high resolution to be obtained.

Conventionally, a bulk single crystal of a nonlinear optical crystal using a high-power gas laser as a light source has been used as a 5LIG element. However, in recent years there has been a strong demand for downsizing of entire devices such as optical disk devices and laser printers, and gas lasers require an external modulator for optical modulation, making them unsuitable for downsizing. There is a need for an SHG element that can be directly modulated and that can use a semiconductor laser, which is cheaper and easier to handle than a gas laser.

Note that when a semiconductor laser is used as a light source, since the output of the semiconductor laser is low at -JI2 of several mW to several tens of mW, it is necessary to use an SHG element that can obtain particularly high conversion efficiency.

By the way, optical crystal materials that have a nonlinear optical effect and can be used for SHO elements include lithium niobate (LiNbO3) and lithium tantalate (L1Ta).
oi ), KTiOPO4, K N b Ox , B
a t N a N b s 013, Ba, LiNb
, 0.5, etc. Among them, L-iNbo3 is the most suitable because it has a high nonlinear optical constant and a small optical loss.

(Problems with the Prior Art) Conventionally, as a method for forming an optical waveguide using LiNbO3 material, LiNb0. It is known to form a layer with a changed refractive index by subjecting a bulk single crystal to Ti diffusion, proton exchange, out-diffusion, etc., but the waveguides obtained by these methods are For laser light sources, especially laser light sources with a wavelength of 1 μm or less, it is difficult to increase the difference in refractive index with the bulk, and the second
Phase matching for generating harmonics, that is, the refractive index (effective refractive index) of the incident laser light in the waveguide and the second
It is extremely difficult to match the effective refractive index of harmonic light [Yamada, Miyazaki; IEICE technical research report, MW8
7-113 (1988)], and since the boundary between the waveguide and the bulk is not clear, the light wave easily spreads from the waveguide and it is difficult to concentrate the optical energy, making it difficult to obtain particularly high conversion efficiency. There was a problem.

For this reason, the inventors used LjNbO3 material,
Using a 0.8 μm band semiconductor laser light source, we investigated various combinations that could achieve phase matching between the fundamental wave light and the second harmonic light, and developed a LiTaO3 single crystal with a specific refractive index and a specific refractive index on its surface. They discovered the conditions under which phase matching could be achieved with a structure in which a LiNbO5 thin film was combined as a waveguide layer, and filed an application for Japanese Patent Application Laid-Open No. 160804/1983.

However, the above invention has an extremely narrow scope of application as it can only use a 0.8 μm band semiconductor laser as the thin film waveguide layer source, and furthermore, the refractive index of the material is generally Because it varies depending on the wavelength,
It has been difficult to apply this method to laser light sources with different wavelengths.

Therefore, the present inventors developed an SHG using LiNbO3 material that enables phase matching with laser light sources of various wavelengths.
As a result of intensive research on the device conditions, we found that the SH
The G element is: )3, the thin film waveguide wavelength (λμm), the film thickness of the thin film waveguide layer (TtIm), )3, the ordinary refractive index of the substrate at the thin film waveguide wavelength (λμm) (no□) ,)3
, 'gt film waveguide layer wave layer optical refractive index (no□) at the thin film waveguide layer wave length λμm〉, second harmonic wavelength (λμm/
The second The present invention was completed based on the new discovery that harmonics can be generated extremely efficiently.

(Means for Solving the Problem) The present invention is a second harmonic generation element in which a LiNb0° thin film waveguide layer is formed on a LiTaO3 substrate,
, the thin film waveguide layer wavelength (λμm), the film 1'\(Tum) of the LiNbO3 thin film waveguide layer, )3, the ordinary refractive index of the LiTaO3 substrate at the thin film waveguide layer wavelength (λμm) (n
o□), )3, the ordinary refractive index (nov+) of the LiNb01 thin film waveguide layer at the thin film waveguide wavelength (λzm), and the extraordinary refractive index (nov+) of the LiTaO3 substrate at the second harmonic wavelength (λμm/2) .〉 and second harmonic wavelength (λμ
m/2) in LiNb0. The second harmonic generation element is characterized in that the extraordinary refractive index (n1□) of the thin film waveguide layer is within the range of the following numerical values.

λ-0,68~0. 94μm T-0,3~16μm n,, -2,22~2.38 1os+ -2,10~2.25 nav! =2. 24-2. 42 n,,,=2. 22-2.38 (Function) The structure of the SHG element of the present invention is that LiTa○, L on the substrate
It is necessary that an iNb0z thin film waveguide layer be formed.

The reason why it is necessary for the SHG element to have a structure in which a thin film waveguide layer is formed on a substrate is that the generation of second harmonic light in a SHO element in which a thin film waveguide layer is formed on a substrate is as follows. Not only does it have the advantage of being able to utilize the energy of light concentrated in a thin film, and because the light waves are confined within the thin film and do not spread, they can interact over long distances, but they also This is because SHO elements using bulk single crystals have the advantage of being able to achieve phase matching even with substances that cannot be phase matched by utilizing the mode dispersion of thin films. The reason why it is necessary to use LiNbO5 is the above-mentioned L i N b Oz
LiT has a large nonlinear optical constant, low optical loss, and can form a uniform film.
This is because aO3 has a similar crystal structure to the above-mentioned LINbO, and it is easy to form a thin film of the above-mentioned LiNbO5, and high-quality and inexpensive crystals are easily available.

Note that the LiTaO3 substrate is advantageously a single crystal substrate.

The SHG element of the present invention comprises: )3, the wavelength of the thin film waveguide layer (λ
μm) is required to be 0.68 to 0.94 μm.

The reason for this is 3) above.Although it is advantageous for the thin film waveguide layer (λμm) to have a wavelength as short as possible, it is not possible to generate laser light with a wavelength shorter than 0.68μm using a semiconductor laser. On the other hand, when using the thin film waveguide layer with a wavelength longer than 0.94 μm, the wavelength of the second harmonic obtained is Since it is l/2 of the layer,
This is because the wavelength region can be relatively easily generated directly by a semiconductor laser, and there is no advantage to using an SHC element. 3) The wavelength (S) of the thin film waveguide layer is preferably 0.78 to 0.86 μm, which is relatively easy to obtain a semiconductor laser light source;
．． 82 to 0.84 μm is practically suitable.

The thickness (T) of the LiNbOs thin film waveguide layer of the SHC device of the present invention is required to be in the range of 0.3 to 16 μm.

The reason is that the film thickness (T) of the '9J film waveguide layer is 0.
This is because if it is thinner than 3 μm, it is difficult to make the fundamental laser beam incident, and the incidence efficiency is low, making it difficult to obtain substantially high SHG conversion efficiency.On the other hand, if it is thicker than 16 μm, the optical power density is low, This is because the SHG conversion efficiency becomes low, making it difficult to use as an SHG element in either case. The thickness of the thin film waveguide layer is preferably 0.5 to 10 IIm, particularly 1
~8 μm is practically suitable.

The SHG element of the present invention comprises: )3, the wavelength of the thin film waveguide layer (λ
) of the Li T a Ox substrate (n
os+) is 2.10 to 2.25, )3, the ordinary refractive index (no□) of the LiNbO5 thin film waveguide layer at the thin film waveguide wavelength (λ) is 2.22 to 2.38, and the second harmonic Extraordinary refractive index of LiTa0+ substrate at wavelength (λ/2) (
n@sg ) is in the range of 2.22 to 2°38 and the extraordinary optical refractive index (n1lFt) of the LiN b Ox thin film waveguide layer at the second harmonic wavelength (λ/2) is in the range of 2.24 to 2.42. It is necessary to be within

The reason is that if it is not within the above range, the conversion efficiency to second harmonic light is low and it is not practical.

LiNb0. Thin film waveguide layer, and LiTaO3 group (anti-Na, Cr.

It is advantageous to use a material whose refractive index is adjusted by containing different elements such as Mg, Nd, Ti, and V.

The L i N b O3 thin film waveguide layer and LiTa
By containing Na, Cr, Nd, Ti, etc. in the 0+ substrate, the LiNbO3 thin film waveguide layer and Li
The refractive index of the TaO3 substrate can be increased, and Mg
, V, etc., the L i N b
O3 thin film waveguide layer and LiTa○ can lower the refractive index of the substrate.

The content of Na is preferably 0.1 to 10%. The reason for this is that the Na content is 10 mo
If it exceeds 1%, use LiNbO3 thin film waveguide layer or LiTa○.

This is because crystals with different structures are precipitated in the substrate, which deteriorates the optical properties. If it is lower than %, the refractive index will hardly change, and in any case, it will not be a practical SHG element. The content of Na is preferably 0.8 mo1% to 2 mo1%.

Further, the content of Cr is preferably 0.02 to 20m01%. The reason for this is that if the C content exceeds 20 mo1%, crystals with different structures will precipitate in the LiNbO5 thin film waveguide layer or LiTaOs substrate, resulting in a decrease in optical properties. This is because if Q is lower than 1 mo1%, the refractive index will hardly change, and in either case, it will not be a practical SHG element.
Mo1% to 10mo1% is suitable.

Further, the content of Mg is preferably 0.1 to 20m01%. The reason for this is that when the Mg content exceeds 20 mo1%, the LiNb0tfl film waveguide layer or LiTa○ is used.

This is because crystals with different structures will precipitate in the substrate, deteriorating the optical properties, and if it is lower than 0.1 mo1%, there will be almost no effect in preventing the optical tJ1m, so in either case, it is not practical. This is because it does not become an SHG element. Among others, the content of Mg is 2. Omo1%~10mo
1% is preferred.

The content of Ti is preferably 0.2 to 30m01%, the reason for this is that the content of Ti is 30m01%.
If it exceeds 1%, use L iN b 03 thin film waveguide layer or L i T
This is because crystals with different structures will precipitate in the a03 substrate, resulting in a decrease in optical properties.Also, if it is lower than 0.2 mo1%, the refractive index will hardly change, so in either case, it cannot be used as a practical SHG element. This is because it is not. The Ti
Among them, the content of 1. Omo1% to 15mo1% is suitable.

The Nd content is 0. The reason for this is that if the Nd content exceeds lomo 1%, crystals with different structures will precipitate in the LiNbO5 thin film waveguide layer or the LiTa0° substrate. This is because the optical properties deteriorate, and 0,0
This is because if it is lower than 2 mo1%, the refractive index will hardly change, and in any case, it will not be a practical SHG element. The Nd content is particularly 065 mo
1% to 5mo1% is suitable.

The content of V is preferably 0.05 to 30 mo 1%. The reason for this is that the V content is 30 mo
If it exceeds 1%, crystals with different structures will precipitate in the LiNb0° thin film waveguide layer or LiTa0 = substrate.
This is because the optical properties deteriorate, and 0.05mol
If it is lower than %, the refractive index will hardly change, and in any case, it will not be a practical SHG element.

The content of the above (■) is particularly 2.0 mo1% to 10 mo
1% is preferred.

Note that the above content is LiNbC1+ or LiT
It is expressed as mo1% of the different element with respect to a○.

The above Na, Cr, MBNd, Ti on the LiTaO5 substrate
, L i N b O3 containing different elements such as V
The method for forming the thin film waveguide layer is to mix raw materials and different elements or different element compounds in advance, and then deposit L on the L i T a 03 substrate using liquid phase epitaxial growth.
Method for forming iNbOx thin film waveguide layer, sputtering method, organometallic chemical salt Jfl (MOCVD) method,
Molecular beam epitaxy (MBE) can be applied, and LiTa() + substrate or LiNb
O5 thin film waveguide layer contains Na, Cr, Mg, Nd, Ti, V
Various methods such as a diffusion method and an ion implantation method can be used as a method for incorporating a different kind of element such as.

The SHG element of the present invention has LiNb0. Thickness (Tum) of the thin film waveguide layer, )3, ordinary refractive index of the LiNb03FjE film waveguide layer at the wavelength (zμm) of the thin film waveguide layer (no□
), LiTaO at the second harmonic wavelength (λμm/2)
5 The extraordinary refractive index of the substrate (n,.

8) and Li at the second harmonic wavelength (λμm/2)
The extraordinary refractive index (n,) of the Nb0*Fl film waveguide layer.

2) but It is preferable that the following relational expression be satisfied.

The reason for this is that LiTa○ and SHG in which a LiNb0zFjt film waveguide layer is formed on the substrate.
This is because unless the element has a structure that satisfies the above relational expression, the conversion efficiency to second harmonic light will be low.

In order to obtain particularly high conversion efficiency, it is advantageous to satisfy the following, and it is particularly preferable to satisfy the following.

In the SHG element of the present invention, the optical axis (Z-axis) of the thin-film waveguide layer is 0±
It is preferably within the range of 15@ or 90±15@.

The reason for this is that in 3) above, when the optical axis (Z-axis) of the thin film waveguide layer is within the above range, the conversion efficiency to the second harmonic is
This is because it is extremely expensive. 3) The angle of incidence of the thin film waveguide layer is preferably within the range of 0±5@ or 90±5°.

Furthermore, the SHO element of the present invention is advantageously of the channel type with a width of 1 to 10 μm.

The reason why a channel type SHO element is advantageous is that the optical power density can be increased compared to the slab type, and the reason why a width of 1 to 10 μm is advantageous is as follows.
If the width is smaller than 1 μm, it is difficult to introduce the incident light into the waveguide, and the incidence efficiency is low, resulting in a low SHG conversion efficiency. This is because if it is larger, the optical power density decreases, and the SHG conversion efficiency decreases.

As a method for manufacturing the channel type SHG element, for example, a thin film waveguide layer is formed on a substrate by a method such as sputtering or liquid phase epitaxial growth, and then photolithography and RF are performed on the thin film waveguide layer. A method such as forming a Ti waveguide pattern by sputtering and performing ion beam etching using this as an etching mask to create a channel type SHG element can be used.

Examples of the present invention will be described in detail below.

Example 1-1 Various thicknesses of Mg (5 m
o 1%) solid solution LiNbO5 single crystal yi film was grown.

The surface was mirror-polished and the end faces on both sides were mirror-polished to allow light to enter and exit from the end faces, thereby forming an SHG element.

When the fundamental laser light wavelength λ is 0.83 μm, Li
The ordinary refractive index (no□) of TaO3 single crystal substrate is 2.14
1. Ordinary refractive index of LiNbO5 single crystal thin film waveguide layer (16
F1) is 2.271, L at the second harmonic wavelength λ/2
The extraordinary refractive index (nas2> of the iTa01 single crystal substrate is 2
The extraordinary refractive index (n*rt) of the ゜261 LiNbOi single crystal thin film waveguide layer was 2.26B.

In the SHG element, Mg solid solution LiNb0. The thickness of the thin film was 2.92 μm.

This SHG element has a wavelength of 0.83 μm and a wavelength of 40 m.
When the SHG conversion efficiency was measured when a W semiconductor laser was incident at an incident angle of 90° to the crystal axis (Z axis) of a single crystal thin film, it was 13.4%, which was a very high efficiency. .

Example 1-2 Basically the same as Example 1-1, but with Mg solid? 8L
An SHG element was manufactured in which the thickness of the iNbOs thin film was 12.2 μm. This SHG element has a wavelength of 0.83.17 m for this SHO element,
A 40mW semiconductor laser is aligned with the crystal axis (Z axis) of a single crystal thin film.
When the SHG conversion efficiency was measured when the light was incident at an incident angle of 90°, it was 1.0%, and a sufficiently high efficiency was obtained as an SHG element.

Example 1-3 Basically the same as Example 1-1, but with Mg solid solution Lt
An SHG element was manufactured in which the thickness of the NbOs thin film was 0.7 μm.

This SHG element has a wavelength of 0.83 μm and a power of 40 mW.
When the SHG conversion efficiency was measured when a semiconductor laser of It was done.

Example 2-1 X-cut L with thickness 0.5 +wm by RF sputtering
Nd (neodymium) (1mol1%) solid solution LiNbO5 single crystal thin films of various thicknesses were grown on an iTa0z single crystal substrate.The 0 surface was mirror polished, the end faces on both sides were mirror polished, and light was input and output from the end faces. This made it possible to create an SHG element. When the fundamental laser light wavelength λ is 0.9 μm, LiTa
The ordinary refractive index (n1lff+) of the 03 single crystal substrate is 2.
141. The ordinary refractive index (no□) of the Nd solid-fused LiNbO3 thin film waveguide layer is 2.275, and the extraordinary refractive index (na32) of the LiTaO2 single crystal substrate at the second harmonic wavelength λ/2 is 2.275.
) is 2,245, Nd solid solution LiNb0. The extraordinary refractive index (neF!) of the thin film waveguide layer was 2.268.

In the SHG element, Nd solid solution! , the thickness of the 1Nb03 thin film was 5.92 μm.

This SHO element is corresponds to

For this SHG element, a 40 mW semiconductor laser with a wavelength of 0.9 μm is directed toward the crystal axis (Z axis) of the single crystal thin film.
When the 5I(C conversion efficiency) was measured at an incident angle of Oo, it was 11.0%, and a very high efficiency was obtained.

Example 2-2 Basically the same as Example 2-1, but with Nt5 hard 99
SHG with LiNb03 thin film thickness of 14.8μm
The device was fabricated.

This S HG element has a wavelength of 0.9.T (nOFl nest ) for this S HG element. When the SHG conversion efficiency was measured when a semiconductor laser of um, 40 mW was incident at an incident angle of Oo with respect to the crystal axis (Z axis) of a single crystal Yl film, it was 0.5%, which is sufficient for an SHG element. High efficiency was obtained.

Example 2-3 Basically the same as Example 2-1, but with Nd solid solution Li
A SHO element was manufactured in which the thickness of the Nb01 thin film was 0.7 μm.

This SHG element has T (no□ -n□!) Wavelength 0.9#m, 40mW for this SHG element
When the SHG conversion efficiency was measured when a semiconductor laser of Ta.

Example 3-1 Various thicknesses (1 m) were deposited on a LiTaO5 single crystal substrate with a thickness of 0.5 m by the liquid phase eviaxial growth method.

1XNa, 5nolO) Solid solution LiNb0. The surface on which the single-crystal Fi film was grown was mirror-polished, and the end faces on both sides were mirror-polished to allow light to enter and exit from the end faces, producing an SHG element. When the fundamental laser light wavelength λ is 0.78 μm, the ordinary refractive index (Dos+
) is 2.153, (laolχNa+ 5+mol
χMg0) Solid volume L i N b Os FIN
The ordinary refractive index (nor+) of the H waveguide layer is 2.281, and the extraordinary refractive index (nas) of the LiTaO5 single crystal substrate at the second harmonic wavelength λ/2 is 2.272.
Na, 5molXMgO) solid solution LiNb0. Extraordinary refractive index (nl) of the thin film waveguide layer.

) was 2.276.

In the SHG element, (1mol'1Na, S+
golZMgo) solid? YongLiNb0! The thickness of fi1m was set to 2.10 μm.

This SHG element is Corresponds to T (nor+rLst).

Regarding this SHO element, the wavelength is 0.78 μm, 40 mW
When the SHG conversion efficiency was measured when the semiconductor laser was incident at an incident angle of 90° with respect to the crystal axis (Z axis) of the single crystal thin film, it was 110%, and a very high efficiency was obtained.

Example 3-2 Basically the same as Example 3-1, but (Na.

An SHG element was manufactured in which the thickness of the Mg0) solid solution LiNb0z thin film was 10.4 μm.

This SHG element is T(n, □ -n.!) corresponds to

Regarding this SHG element, the wavelength is 0.78 μm, 40 mW
The 5f (G conversion efficiency) when a semiconductor laser of Sufficiently high efficiency was obtained.

Example 3-3 Basically the same as Example 3-1, but (Na.

A 5r (G element) was prepared in which the thickness of the Mg0) solid solution LiNbO3 thin film was 0.5 μm.

This SHO element is T (n, □ -n1□) corresponds to

Regarding this SHO element, the wavelength is 0.78μm, 40m
When we measured the SHG conversion efficiency when a W semiconductor laser was incident at an incident angle of 90° to the crystal axis (Z axis) of a single crystal thin film, it was 1.2%, which is a sufficiently high efficiency for an SHG element. Obtained.

(Effects of the Invention) As described above, according to the present invention, extremely high SHG
Li on a LiTaO3 single crystal substrate with conversion efficiency
It is possible to provide an SHG element in which a NbOs FJm waveguide layer is formed.

Claims (1)

[Claims] 1. A second harmonic generation element in which a LiNbO_3 thin film waveguide layer is formed on a LiTaO_3 substrate, the fundamental laser light wavelength (λμm), the film thickness of the LiNbO_3 thin film waveguide layer ( Tμm), the ordinary refractive index of the LiTaO_3 substrate at the fundamental laser light wavelength (λμm) (n_0_3_1), the ordinary refractive index of the LiNbO_3 thin film waveguide layer at the fundamental laser light wavelength (λμm) (n_0_F_
1), extraordinary refractive index (n_a_3_2) of LiTaO_3 substrate at second harmonic wavelength (λμm/2)
and the extraordinary optical refractive index (n_a_F
A second harmonic generation element characterized in that _2) are each within the range of the following numerical values. λ=0.68-0.94μm T=0.3-16μm n_a_F_1=2.22-2.38 n_0_3_1=2.10-2.25 n_a_F_2=2.24-2.42 n_a_s_2=2.22-2 .38 2. Thickness (Tμm) of LiNbO_3 thin film waveguide layer, ordinary refractive index of LiNbO_3 thin film waveguide layer (n_0_F_
1), extraordinary refractive index (n_a_3_2) of LiTaO_3 substrate at second harmonic wavelength (λμm/2)
and the extraordinary optical refractive index (n_a_F
The second harmonic generating element according to claim 1, wherein _2) is expressed by the following relational expression (α). 0.01≦(n_a_F_2−n_a_s_2)/T(
n_0_F_1-n_a_s_2)≦1.1...(α)3
, the angle of incidence (θ) of the fundamental laser beam with respect to the optical axis (Z axis) of the thin film waveguide layer is 0±15° or 90±15°.
2. The second harmonic generating element according to claim 1, wherein the second harmonic generating element is .degree.