JP3332497B2 - Wavelength converter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wavelength converter for shortening the wavelength of light.

[0002]

2. Description of the Related Art In recent years, with the increase in information density, researches for shortening the wavelength of light used in an optical data processing device or an optical communication device have been advanced. In various fields such as the medical and chemical industries, the demand for short-wavelength coherent light is increasing.

One of the methods for obtaining short wavelength coherent light is as follows.
Then, using a nonlinear optical material, the second harmonic generation (SH
G:SecondHarmonicGeneration)
There is a wavelength conversion device that realizes this.
Research is being actively conducted. Such different wavelengths
In a nonlinear interaction between
Is important. Condition that this condition is not satisfied
In the state, the nonlinear polarization wave and light wave generated by the interaction
Interference occurs between them, and large conversion efficiency cannot be obtained.

[0004] As one of the wavelength converters described above, Japanese Patent Application Laid-Open No. 61-18934 discloses that a fundamental wave is incident from one end face of a buried type channel waveguide produced by proton exchange on a LiNbO ₃ substrate. Then, a device in which the SH wave is emitted obliquely to the substrate side is disclosed. The phase matching method in this wavelength converter is called a Cherenkov radiation type, and has an advantage that the rate of change in the conversion efficiency with respect to the change in the wavelength of the fundamental wave is extremely small.

[0005] Japanese Patent Application Laid-Open No. 2-12135 discloses LiTa.
A wavelength conversion device has been disclosed in which when a fundamental wave is incident from one end face of a LiNbO ₃ single crystal thin film crystal-grown on an O ₃ substrate so as to be guided in a TM mode, SH wave guided light in a TE mode is generated. The phase matching method at this time is called a mode dispersion phase matching method, and has an advantage that a large conversion efficiency and a good beam shape can be obtained.

[0006]

However, the conventional Cherenkov radiation type device as described above has a problem that the conversion efficiency is as low as about 1% at the highest, and the beam shape is poor because it is radiated into the birefringent substrate. is there. On the other hand, the conventional device using the mode dispersion phase matching method has a problem that it is necessary to strictly control the shape of the waveguide and the wavelength of the fundamental wave in order to satisfy the phase matching. The present invention has been made in view of such circumstances, and the fundamental wave is a guided mode that propagates through the nonlinear optical medium layer, and the second harmonic is a radiation mode that cannot propagate through the nonlinear optical medium layer. It is an object of the present invention to provide a wavelength converter capable of obtaining a high-quality beam with a higher conversion efficiency than the conventional one.

[0007]

According to a first aspect of the present invention, there is provided a wavelength conversion apparatus for receiving a fundamental wave from one end face and generating a second harmonic from the other end face by a nonlinear optical effect. In addition, a non-linear optical medium layer and a periodic multilayer film layer formed by repeatedly laminating a plurality of types of layers laminated in a certain order are laminated, and the fundamental wave incident from the one end face is The second harmonic generated in the nonlinear optical medium layer propagates in the nonlinear optical medium layer in a Bragg reflection guided mode, and is radiated from the nonlinear optical medium layer to the periodic multilayer layer. It propagates through the inside and exits from the other end face. Such invention in claim 2, incident fundamental wave from one end face, by a nonlinear optical effect, in the wavelength conversion device for generating a second harmonic from the other end face, on a substrate, and a non-linear optical medium layer, 2 A periodic multilayer film layer is formed by repeatedly laminating various types of layers, and the two types of layers constituting the periodic multilayer film have a refractive index of each material of n _a ,
When n _b (n _a <n _b ) and film thicknesses a and b, the fundamental wave
do it,

(Equation 4) The filled, for the second harmonic wave generated in the nonlinear optical medium layer,

(Equation 5) And A and D in the above equations (1) and (2) are

(Equation 6) It is characterized by being represented by

[0008] A wavelength converter according to a second invention is characterized in that, in the first invention, the nonlinear optical medium layer is made of a single crystal of LiNbO ₃ .

[0009]

In the present invention, the fundamental wave becomes a guided mode that propagates through the nonlinear optical medium layer under Bragg reflection conditions .
Harmonics are radiated from the nonlinear optical medium layer to the periodic multilayer layer.
With periodic multilayer film such that the mode, the second harmonic is radiated efficiently to the periodic multilayer film side by variation rate is small Cherenkov radiation type phase matching of the conversion efficiency with respect to the variation of the fundamental wave wavelengths In addition, the generated second harmonic is obtained by laminating a plurality of types of layers in a certain order.
Since the light is emitted to the outside after being propagated through the multilayered multilayer film layer formed by the repeated stacking , the beam shape is improved .

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be specifically described below with reference to the drawings showing the embodiments. FIG. 1 is a schematic perspective view showing a wavelength converter according to the present invention. 1 in the figure is Z-cut Li
The substrate is made of TaO ₃ and has a thickness of 2.7μ on the substrate 1.
m, a non-linear optical medium layer 2 made of LiNbO ₃ is formed thereon, and a periodic multilayer film layer 3 is further formed by stacking 10 pairs of TiO ₂ and CeO ₂ in this order with a thickness of 0.285 μm.
Is deposited.

When a fundamental wave is incident on the nonlinear optical medium layer 2 on one end face of the wavelength converter, the fundamental wave propagates in the nonlinear optical medium layer 2 in a Bragg reflection guided mode, and the generated S
The H wave is radiated toward the periodic multilayer film layer 3, propagates through the periodic multilayer film layer 3, and is emitted from the other end surface. At this time, since the SH wave is less likely to propagate and bend through the periodic multilayer film layer 3 having the thin film structure, the beam shape can be made more circular. In addition, the phase matching at this time can be controlled by the thickness of each of the materials constituting the nonlinear optical medium layer 2 and the periodic multilayer film layer 3, so that it is possible to manufacture the device more easily than before.

A method for determining the film thickness of the nonlinear optical medium layer 2 and the periodic multilayer film layer 3 will be described. As a result of the study by the inventors, it has been found that the above-described propagation is realized when one or both of the materials constituting the periodic multilayer film layer 3 have a refractive index higher than that of the nonlinear optical medium layer 2. ing. Therefore, periodic multilayer film 3 constituting the two multilayer material A, the refractive index each n _a of _{B, n b (n a <} n
_b ), when the film thicknesses are a and b,

[0013]

(Equation 7)

For SH waves,

[0015]

(Equation 8)

If the above condition is satisfied, the fundamental wave becomes a Bragg reflection guided mode and the SH wave becomes a radiation mode with respect to the nonlinear optical medium layer 2. Where A and D are given by the following equations.

[0017]

(Equation 9)

In this embodiment, as a material satisfying the expressions (1) and (2), the multilayer film material A is made of CeO ₂ and the multilayer film material B is made of CeO _2.
Uses TiO ₂ . FIG. 2 is a graph in which the range in which the fundamental wave satisfies the expression (1) is indicated by shading, and the range in which the SH wave does not satisfy the expression (2) is indicated by hatching in each film thickness when these materials are used. It is. In this graph, the fundamental wave wavelength is 0.84 μm, the effective refractive index of the fundamental wave is 2.25,
The case where the effective refractive index of the SH wave is 2.45 is shown. In this embodiment, the point P in the graph shown in FIG. 2 is adopted, and a = b = 0.285 μm. It should be noted that for the SH wave, a result was obtained in which the equation (2) was satisfied even when the effective refractive index value was changed to another value under this condition. For the fundamental wave, the effective refractive index could be fixed at 2.25 by setting the thickness of the nonlinear optical medium layer 2 to 2.7 μm.

Next, a manufacturing method based on the above set values is as follows. That is, on a Z-cut LiTaO ₃ substrate, a 2.7 μm thick LiNb
O ₃ is crystal-grown, and TiO ₂ of the multilayer material B and CeO _{2 of the} multilayer material A are mixed at 350 ° C. by EB evaporation.
In this order, ten pairs of layers each having a thickness of 0.285 μm are laminated to form the periodic multilayer film layer 3. Then, cutting is performed so that the element length becomes 10 mm, and both end surfaces are polished. A 0.84 μm wavelength semiconductor laser beam (red) was applied to the wavelength converter manufactured in this manner by TM.
In the mode, about 40 mW incident, blue S of about 1.2 mW
The H wave was emitted from the periodic multilayer film 3 in a good beam shape.

FIG. 4 is a perspective view showing another embodiment of the apparatus of the present invention. In the present embodiment, the waveguide is a nonlinear optical medium layer 12 whose upper layer is channeled. This channelization is performed by injecting ions (He ⁺ ions) to a certain depth from the surface on both sides of the waveguide portion,
The ion implantation part 6 is formed by lowering the refractive index.
With such a configuration, the conversion efficiency can be increased as compared with the embodiment shown in FIG.

FIG. 5 is a perspective view showing still another embodiment of the apparatus of the present invention. In the present embodiment, the nonlinear optical medium layer 2
A periodic multilayer film layer 13 formed as a waveguide is formed thereon as a waveguide. With such a configuration, the conversion efficiency can be increased as compared with the embodiment shown in FIG. In order to further improve the conversion efficiency, for example, when a structure obtained by combining the structures shown in FIGS. 4 and 5, that is, a structure in which the nonlinear optical medium layer and the periodic multilayer film are channeled, higher conversion efficiency is expected. Can be.

The number of pairs of the periodic multilayer film layer 3 is as follows.
The number of pairs at which the Bragg reflectance becomes appropriate may be appropriately selected. In this embodiment, the nonlinear optical medium layer 2 is made of LiTa.
O ₃ was formed by LiNbO ₃ was grown on the substrate may be formed by a LiNbO ₃ substrate and the proton exchange, to form a buffer layer at a deep position from the LiNbO ₃ substrate surface by high-energy ion implantation, than A shallow layer may be used as the waveguide. Non-linear optical materials other than LiNbO ₃ , such as LiTaO ₃ , KNbO ₃ , KTP,
Alternatively, even if an organic nonlinear optical material is used, the wavelength converter according to the present invention can be realized. Further, the periodic multilayer film layer 3
Is not limited to CeO _2, TiO ₂ material, other inorganic dielectric material such as SiN, SiC, amorphous semiconductor materials such as ZnS, or organic dielectric materials may be also a film forming method EB
Not only the vapor deposition but also other methods such as a sputtering method, a CVD method, and a sol-gel method depending on a material can be used.

[0023]

As described above, in the wavelength converter according to the present invention, the fundamental wave becomes a guided mode that propagates through the nonlinear optical medium layer under the Bragg reflection condition, and the second harmonic wave is generated in the nonlinear optical medium layer. with periodic multilayer film such that the mode emitted periodically multilayer film from the inner, second harmonic efficiently by variation rate is small Cherenkov radiation type phase matching of the conversion efficiency with respect to the variation of the fundamental wave wavelengths The second harmonic emitted and emitted to the periodic multilayer film side is emitted to the outside after propagating through the periodic multilayer film in which thin films are stacked in multiple cycles, so that a high-quality beam can be obtained. Has an excellent effect.

[Brief description of the drawings]

FIG. 1 is a perspective view showing a wavelength converter according to the present invention.

FIG. 2 is a graph showing a settable film thickness of a multilayer material.

FIG. 3 is a diagram schematically showing electromagnetic field distributions of a fundamental wave and an SH wave.

FIG. 4 is a perspective view showing another embodiment of the wavelength converter according to the present invention.

FIG. 5 is a perspective view showing still another embodiment of the wavelength converter according to the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Substrate 2, 12 Nonlinear optical medium layer 3, 13 Periodic multilayer film layer 4 Fundamental wave 5 SH wave (2nd harmonic)

Continuation of front page (56) References JP-A-2-113227 (JP, A) JP-A-1-310902 (JP, A) JP-A-1-105220 (JP, A) JP-A-64-90427 (JP) , A) G. LEO, et. al. 28, No. 28, No. 28, No. 4, pp.,, Chernkov Second-Harmonic Generation in Multilayer Waveguide de Structures, IEEE JOURNAL OF QUANTUM ELECTRONICS, Vol. 2, pp. 534-546P. M. LAMBKIN, et. a l. , Nonlinear Semiconductor Conductor Bragg Reflection Waveguide Structures, IEEE JORNAL OF QUANTUM ELECTRONICS, Vol. 27, No. 3, pp. 824-829 p. YEH, et. al. I., Electromagnetic propagation in periodic stratified media. General theory, Journal of the Optic Society of America, Vol. 67, no. 4, pp. 423-438 (58) Field surveyed (Int. Cl. ⁷ , DB name) G02F 1/35-1/39 JICST file (JOIS)

Claims (2)

(57) [Claims] 1. A non-linear optical system in which a fundamental wave is incident from one end face.
In a wavelength conversion device that generates a second harmonic from the other end face due to a chemical effect, a layer in which a non-linear optical medium layer and a plurality of types of layers are laminated in a certain order on a substrate is repeated.
The periodic multilayer film layer that has been multilayered by laminating
The fundamental wave incident from the one end face is the nonlinear optical medium.
In the Bragg reflection guided mode in the
The second harmonic generated in the optical medium layer is the second harmonic.
Radiated from the layer to the periodic multilayer layer,
, And exits from the other end face . 2. A wavelength conversion device for injecting a fundamental wave from one end face and generating a second harmonic from the other end face by a non-linear optical effect, wherein a nonlinear optical medium layer and two types of layers are provided on a substrate. and laminating the multi-layered periodic multilayer film by repeatedly laminating the two types of layers constituting the periodic multilayer film, each of the material
The refractive index each _{n a, n b (n a} <n b), and the thickness a, a b
In the case, for the fundamental wave, The filled, for the second harmonic wave generated in the nonlinear optical medium layer, Equation 2] And A and D in the above equations (1) and (2) are given by: A wavelength converter characterized by the following.