JPH0262522A - Waveguide type wavelength converting element - Google Patents

Abstract

PURPOSE:To offer the high-efficiency waveguide type wavelength converting element by providing a waveguide layer in single-layered or multi-layered structure containing a material with nonlinear optical effect on an optical crystal substrate and specifying the relation between the refractive index between the nonlinear material and optical crystal substrate. CONSTITUTION:An optical waveguide layer 2 consisting of an organic material single-crystal film 3 with nonlinear optical effect, a high-refractive-index thin film 4 like TiO2, and a low-refractive-index layer 5 like SiO2 is provided on the substrate 1 like LiNbO3 crystal. The refractive index nS (2omega) of the optical crystal substrate 1 to a 2nd high frequency (frequency 2omega) and the refractive index nf(omega) of the organic nonlinear material 3 to a basic wave (frequency omega) are so specified that nS(2omega)>nf(omega). The double layer which is constituted as mentioned above is given total reflection condition to basic wave light by light interference effect and becomes an excellent waveguide, but given transmission condition to a 2nd higher harmonic to form a emission waveguide for emitted light 7 in the substrate crystal.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a wavelength conversion element that makes it possible to realize a coherent short wavelength compact light source.

[Conventional technology]

Wavelength conversion element, especially second harmonic generation (SHG: 5
eeond Harmonic Generation
) devices are extremely important in industry as devices that can obtain coherent short-wavelength light that is difficult to obtain with excimer lasers and the like.

Semiconductor lasers are used as compact, high-power coherent laser light sources in various optical communication devices and optical information devices. Currently, the wavelength of light obtained from this semiconductor laser is 0.7
The wavelength is in the near-infrared region of 8 μm to 1.55 μm. In order to apply this semiconductor laser more widely to laser-applied equipment such as displays, optical disks, and laser printers,
There is a demand for light with shorter wavelengths such as red, green, and blue, but with current technology it is difficult to suddenly realize this type of semiconductor laser. If a wavelength conversion element capable of efficiently converting wavelength even with the output of a semiconductor laser could be realized, the effect would be enormous.

In recent years, semiconductor laser device technology has developed, and it has become possible to obtain higher output oscillation characteristics than ever before.

Therefore, by configuring an optical waveguide type SHG element, it is possible to avoid a decrease in energy density due to light diffraction, and it is possible to realize a wavelength conversion element with relatively high conversion efficiency even with a light intensity comparable to that of a semiconductor laser. An example of this is a method in which an optical waveguide is formed in a lithium niobate crystal, near-infrared light is transmitted through the optical waveguide, and the second harmonic is radiated (Cherenkov radiation) into the crystal substrate. The SHG element was invented (Japanese Patent Application Laid-Open No. 61-94031>. This type of SHG element has the advantage that it does not require precise temperature control because the phase matching conditions between the fundamental wave and the SHG wave are automatically established. However, the drawback of this known example is that efficient SHG excitation has not been successful due to the small nonlinear optical effect of the waveguide itself.In the above known example, the substrate is LiNb0. Ion (H4)
Conversion is used to form optical waveguides. As is well known, L
Performing H+ exchange on an iNbO3 crystal increases the symmetry of the crystal and reduces various effects resulting from the asymmetry of the crystal, such as piezoelectric effects, electro-optic effects, and nonlinear optical effects. For this reason, an efficiency of only about 1% is obtained with a fundamental wave input of 40 mW.

[Problem to be solved by the invention]

SUMMARY OF THE INVENTION An object of the present invention is to provide a waveguide type wavelength conversion element having a structure that eliminates the drawbacks of the above-mentioned conventional waveguide type SHG element and achieves high efficiency.

[Means to solve the problem]

The present invention is a waveguide-type wavelength conversion element in which the second harmonic of light whose substrate wave is waveguide light is extracted as radiation light to the substrate, and in which the material constituting the waveguide layer of the waveguide uses nonlinear optical The second harmonic (frequency 2ω
) with respect to the fundamental wave (frequency ω) of the organic nonlinear material, and the refractive index nf (ω) with respect to the fundamental wave (frequency ω) of the organic nonlinear material. It has become.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be explained in detail using the accompanying drawings.

(Example 1) Fig. 1 is a diagram showing the cross-sectional structure of a waveguide type wavelength conversion element which is the first example of the present invention, where 1 is an optical crystal, 2 is an organic material 3 with high nonlinear optical effect. It is a leading wave layer with More specifically, the optical crystal 1 is, for example, a LiNbO3 crystal plate with a refractive index ns larger than about 1.8, and the substrate orientation is, in this embodiment, a y-plate (that is, the normal to the substrate is the y-axis). The light transmission direction is chosen to be the Z axis. The optical waveguide layer 2 provided on the surface of the crystal includes an organic material single crystal film 3 having a nonlinear optical effect, a thin layer 4 with a high refractive index such as TiO□, and a layer 4 with a low refractive index such as SiO3. 5.

In this example, the single crystal film of an organic material with a high nonlinear optical effect is, for example, MNA (methyl aniline, 2-
methyl-4-nit, roaniline) is used.

The optical waveguide layer 2 having the above configuration has the following optical properties. The fundamental wave (frequency ω) of the TE wave, which is incident light and has an electric field component parallel to the X-axis of the LiNbO3 crystal substrate 1, is propagated with extremely low loss, but it is twice as harmonic (frequency 2ω). ), it is well emitted as synchrotron radiation to the L i N b OS crystal substrate. For example, when light with a wavelength of 0.8 μm is the fundamental wave, the refractive index for this light is 1.8
Even if MNA with a size of about 2.52 is placed directly on a LiNbO3 crystal with a size of about 2.52, L i N b
Since the refractive index of the 03 crystal substrate is greater than the refractive index of the MNA, the MNA does not function as an optical waveguide.

The low refractive index SiO□ (refractive index of about 1.45) layer 5 is L
i N b O3 crystal substrate 1 is provided with a thickness of about 1 μm, and a high refractive index thin film (Ti'02, refractive index 2
．． 5. Thickness 0.056. czm) 4. On this double layer, MAN is provided to a thickness of about 2 μm. Due to the optical interference effect, the double layer thus provided provides total reflection conditions for fundamental wave light having a wavelength of 0.8 μm, while impairing transmission conditions for twice the harmonics. That is, the MNA on this double layer serves as a good waveguide for fundamental wave light with a wavelength of 0.8 μm, and on the other hand, serves as a radiation waveguide for twice the harmonics with a wavelength of 0.4 μm.

As the fundamental wave 6 of the TE wave, which has an electric field component parallel to the X-axis of the LiNbO3 crystal substrate 1 and is injected from the end face of the organic material single crystal film 3 having a nonlinear optical effect, advances through the organic material single crystal film 3, Through the second-order nonlinear optical constant dll, twice the harmonic of the TE wave having the same electric field component as the fundamental wave is excited. Since the optical characteristics of the optical waveguide layer 2 are as described above, this double harmonic is emitted into the substrate crystal as radiated light 7, but it is efficiently emitted at an angle θ under the following conditions. That is, the angle θ is such that the refractive index of the L i N b O3 crystal substrate 1 is ns, and the refractive index of the organic material single crystal film 3 is n
When f is given, nf=nscO5θ. Refractive index of LiNbO3 crystal substrate 1 for wavelength 0.4 μm (
In the crystal orientation of this example, ns (ordinary refractive index) is 2.41
The -angle θ is about 41 degrees.

If n r > n s, the angle θ that satisfies the above equation
does not exist. In order to extract the radiation light 7 into the substrate crystal, n r < ns is an essential condition.

Here, the light transmission direction is set to a different direction of the L i N b 03 crystal substrate, for example, the same y plate (i.e., the normal line erected to the substrate is the y axis), the light transmission direction is the X axis, and the electric field vibration direction is the Z axis. If selected, ns will be the extraordinary refractive index (about -2.31) and the angle θ will be about 38 degrees. At this time, LiNb
The maximum nonlinear optical constant d33 of the O3 crystal also contributes effectively.

(Example 2) In the first example described above, the waveguide 2 has a two-dimensional planar structure. In order to increase the power density of the fundamental wave and further improve the conversion efficiency of SHG, it is desirable that the fundamental wave waveguide has a channel structure. To make this possible, the second
The structure of the waveguide shown in the vertical cross section in the light transmission direction in the figure may be used. In other words, other than the area that should be the channel,
A thin high refractive index layer 8 may be provided at a mid-thickness position of the waveguide. The region having the high refractive index intermediate layer 8 is not at the fundamental wave and becomes a cutoff. Therefore, the fundamental wave is guided in a concentrated manner in a region where there is no intermediate layer 8.

〔Effect of the invention〕

As explained above, according to the present invention, it is possible to use an organic nonlinear material with an extremely large nonlinear optical constant, resulting in high efficiency, and since the phase matching condition is satisfied by Cerenkov radiation, the film thickness of the waveguide layer can be varied. A waveguide type wavelength conversion element that is stable even against temperature fluctuations in refractive index and the like can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a sectional view illustrating the structure of a waveguide type wavelength conversion element according to the first embodiment of the present invention, and FIG. 2 is a sectional view illustrating the structure of the waveguide type wavelength conversion element according to the second embodiment. FIG. 1...L i NbO3, 2... Optical waveguide layer, 3...
・Nonlinear organic material single crystal film, 4...high refractive index thin layer, 5
...Low refractive index thin layer, 8...High refractive index intermediate layer. 11i2]

Claims (1)

[Claims] A waveguide layer having a single layer structure or a multilayer structure containing a material exhibiting a nonlinear optical effect is provided on an optical crystal substrate, and the refractive index n_s(2
ω) and the refractive index n_f(ω) for the fundamental wave (frequency ω) of the organic nonlinear material, n_s(2ω)>n_f.
A waveguide type wavelength conversion element characterized by having a condition (ω).