JPH0534747A - Waveguide type wavelength conversion element - Google Patents

Abstract

PURPOSE:To facilitate the control of the equiv. refractive index of a waveguide necessary for executing phase matching and to stably generate second harmonic waves. CONSTITUTION:The waveguide 2 consisting of a nonlinear medium is formed on a substrate 1. This element has a diffraction grating 3 affording a change in refractive index in the light propagation direction of this waveguide 2. The waveguide 2 is constituted of a ridge type three-dimensional waveguide. A basic wave laser beam having a prescribed wavelength is passed in the waveguide 2, by which a higher harmonic laser beam is generated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an element for generating a second harmonic. INDUSTRIAL APPLICABILITY The present invention can reduce the size of the laser light source by generating the second harmonic, and can be applied to an optical disk and a laser printer.

[0002]

2. Description of the Related Art Wavelength conversion elements, especially second harmonic generation (SH
The element G) is an element that uses an optical crystal material having a non-linear effect to convert a laser beam having a wavelength λ into a wavelength of ½ · λ. Conventionally, a high output coherent light source and a non-linear crystal bulk type element have been used for SHG elements. However, at present, there is a strong demand for miniaturization of optical disk devices, laser printer devices, etc., so that a semiconductor laser has been used as a light source instead of a gas laser. When a semiconductor laser is used as a light source, the output thereof is several mW to several tens mW, and therefore, a thin film waveguide type SHG element is used in order to obtain high conversion efficiency. Therefore, it is important to meet the phase matching condition in order to increase the conversion efficiency.
Normally, phase matching is achieved by adjusting the angle of the incident light and controlling the temperature of the nonlinear medium.

As one of the methods, Japanese Patent Laid-Open No. 63-44
There is a method disclosed in Japanese Patent No. 781. In this method, as shown in FIG. 4, the waveguide 52 is provided with a diffraction grating 54 which is a refractive index dispersion means for canceling out the refractive index difference between the fundamental wave due to the wavelength dispersion of the nonlinear medium and the harmonic. . Here, in FIG. 4, reference numeral 50 indicates a nonlinear medium, which is made of, for example, a single crystal of LiNbO ₃ and has a rectangular parallelepiped shape. A waveguide 52 formed by diffusing Ti on the substrate 50 is formed to extend in the same direction as the crystal axis of the nonlinear medium 50. Protrusions 53 are provided on the upper surface of the waveguide 52 at a predetermined pitch to form a diffraction grating 54. The pitch of the diffraction grating 54 is the same as the wavelength of the fundamental wave incident on the waveguide 52, and satisfies the black condition. In this way, the refractive index difference between the fundamental wave and the harmonic wave due to the wavelength dispersion of the nonlinear medium in the waveguide 52,
Refractive index dispersion means for canceling (in this case, the diffraction grating 5
By providing 4), phase matching can be easily achieved.

[0004]

However, the above
In the conventional method, since the three-dimensional distributed refractive index waveguide created by thermal diffusion (Ti or the like) is used, in order to achieve the phase matching required for the second harmonic generation, It is difficult to accurately control the uniformity and the distribution of the refractive index in the depth direction. For this reason, there is a problem that it is difficult to achieve accurate phase matching, and the generation efficiency of the second harmonic also greatly varies.

An object of the present invention is to provide a waveguide type wavelength conversion element having a structure in which the equivalent refractive index of the waveguide required for phase matching can be easily controlled and the second harmonic can be stably generated. To provide.

[0006]

The present invention relates to a waveguide type wavelength conversion element for generating a harmonic laser light by passing a fundamental laser light having a predetermined wavelength through a waveguide made of a non-linear medium. The waveguide has a periodic structure that changes the refractive index in the light propagation direction of the waveguide, and the waveguide is a ridge-type three-dimensional waveguide.

[0007]

In the present invention, since the waveguide has a periodic structure that changes the refractive index in the light propagation direction, it is possible to easily cancel the difference in refractive index due to wavelength dispersion and to easily perform phase matching. Can be taken. In addition, since the three-dimensional waveguide is not of the distributed index type but of the ridge type, it is possible to precisely control the refractive index, the film thickness and the width in the waveguide. The equivalent refractive index of can be precisely controlled.

[0008]

EXAMPLES Examples of the present invention will be described below. FIG. 1 is a perspective view showing the structure of a three-dimensional optical waveguide type SHG element which is an example of a waveguide type wavelength conversion element according to the present invention, and FIG. 2 is a sectional view taken along line AA of FIG. In both figures, reference numeral 1 is Li
A single crystal substrate made of TaO ₃ is shown, and the substrate orientation is a z-plate. Besides this, LiT whose substrate orientation is the y-plate
aO ₃ single crystal substrate, LiTa x Nb _1- xO ₃ (however, 0
≦ X ≦ 1), a MgO-doped LiNbO ₃ substrate or the like can also be used. LiNbO is formed on the single crystal substrate layer 1.
_A waveguide layer 2 made of ₃ is formed in a ridge shape, and a diffraction grating 3 for providing a periodic change in the transmission refractive index of the guided light is formed on the upper surface thereof.
Are formed. The waveguide 2 has a higher refractive index than the crystal substrate 1 formed on the crystal substrate 1. Waveguide layer 2
Is formed by forming a two-dimensional waveguide on the surface of the substrate by epitaxial growth or the like, and removing unnecessary portions by dry or chemical etching. The diffraction grating 3 provided on the upper surface of the waveguide layer 2 is provided to change the refractive index with a predetermined period Λ. The diffraction grating 3 is patterned by a two-beam interference method, an electron beam drawing method, or the like, or is formed by etching using a resist and then transferred by a lift-off method or an etching method. Exchange,
It is composed of a gradient index grating or a volume phase grating manufactured by performing processing such as ion implantation. The diffraction grating 3 shown in FIGS. 1 and 2.
Is a relief type grating, and the diffraction grating 3 shown in FIG. 3 is a gradient index grating.

Next, the predetermined period Λ required to change the refractive index of the diffraction grating 3 will be described. When the fundamental wave frequency of the three-dimensional optical waveguide is ω, the propagation constant for the fundamental wave is β (ω), and the propagation constant for the second harmonic of the fundamental wave is β (2ω), β (2ω) -2β (ω ) = 2π
A period Λ of refractive index change is provided in the light transmission direction of the three-dimensional optical waveguide so as to satisfy the relationship of / Λ. In the above embodiment, since the waveguide layer 2 is the non-linear medium LiNbO ₃ , when considering a light source with an oscillation wavelength of 0.83 μm, the wavelength dispersion of the equivalent refractive index of the three-dimensional optical waveguide is basically 0.83 μm. For waves, 2.22 when using the E _Y00 mode, which corresponds to ordinary light, and 2.17 when using the E _X00 mode, which corresponds to abnormal light,
The second harmonic having a wavelength of 0.415 μm is 2.39 when the E _Y00 mode corresponding to ordinary light is used and 2.28 in the E _X00 mode corresponding to extraordinary light. In the three-dimensional optical waveguide, if the equivalent refractive index for the fundamental wave is n (ω) and the equivalent refractive index for the second harmonic of the fundamental wave is n (2ω), β (2ω) -2β (ω) = 2π / Efficient second harmonic generation can be obtained if there is a refractive index change period of period Λμm that satisfies the relationship of n (2ω) -n (ω) = λ / 2Λ from Λ. The diffraction grating 3 formed on the upper surface of the waveguide layer on the crystal substrate plays this role.

In the case of the above-mentioned equivalent refractive index distribution, when the E _Y00 mode corresponding to ordinary light is used for the fundamental wave and the E _Y00 mode corresponding to ordinary light is used for the second harmonic, the period of the diffraction grating is about 3 μm. When the E _Y00 mode corresponding to ordinary light is used for the wave and the E _X00 mode corresponding to extraordinary light is used for the second harmonic, the period of the diffraction grating is about 7 μm. Further, when the E _X00 mode corresponding to the extraordinary light is used for the fundamental wave and the E _Y00 mode corresponding to the ordinary light is used for the second harmonic, the period of the diffraction grating is about 2 μm, and E _X0 corresponding to the extraordinary light is the fundamental wave. _{When the 0} mode and the _EX00 mode corresponding to ordinary light are used for the second harmonic, the period of the diffraction grating is about 4 μm, and in each case, it can be easily formed by using a normal diffraction grating manufacturing technique. is there. Further, by doping the waveguide layer 2 with MgO, it is possible to increase the threshold level of the input light that causes optical damage in the optical waveguide layer.
In addition, in the above description, X, Y, etc. representing components in the description of E _X00 , E _Y00 , etc. are written on the shoulder portion of E in the usual notation, but for convenience of display, they are referred to as the above notation. I will add a note.

[0011]

According to the present invention, by using a ridge-type three-dimensional waveguide in which the optical waveguide has a periodic structure that changes the refractive index in the light propagation direction, Also, the phase matching in the second harmonic generation can be accurately performed. Therefore, the second harmonic can be stably generated.

[Brief description of drawings]

FIG. 1 is a perspective view of a waveguide type wavelength conversion element according to an embodiment of the present invention.

FIG. 2 is a sectional view taken along line AA of FIG.

FIG. 3 is a cross-sectional view of a waveguide type wavelength conversion element composed of a diffraction grating using a gradient index grating.

FIG. 4 is a diagram for explaining a conventional second harmonic generation (SHG) element.

[Explanation of symbols]

 1 Single crystal substrate 2 Ridge-shaped waveguide layer 3 Diffraction grating

Claims (1)

Claim: What is claimed is: 1. A waveguide type wavelength conversion element for generating a harmonic laser light by passing a fundamental wave laser light having a predetermined wavelength through a waveguide made of a non-linear medium. A waveguide-type wavelength conversion element, comprising: a periodic structure that changes the refractive index in the light propagation direction; and that the waveguide is a ridge-type three-dimensional waveguide.