JPH04368921A - Wavelength converting element and its manufacture - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wavelength conversion element using a coherent light source and used in optical information processing, optical applied measurement and control fields, and a method for manufacturing the same.

0002

[Prior Art] Polarization inversion, which forcibly inverts the polarization of a dielectric material, is possible by forming a periodic polarization inversion layer on a dielectric material, and is used to create optical frequency modulators using surface acoustic waves and polarization inversion of nonlinear polarization. It is used in wavelength conversion elements that utilize . In particular, if it becomes possible to periodically invert the nonlinear polarization of a nonlinear optical material, a second harmonic generating element with extremely high conversion efficiency can be produced. By converting light from semiconductor lasers and other sources, it is possible to create a compact short-wavelength light source, which can be applied to fields such as printing, optical information processing, and optical applied measurement and control, and is therefore being actively researched.

[0003] As a conventional wavelength conversion element using such polarization inversion, for example, Applied Physics Letter
s) 1990 No. 57 P2074 C. J. van
There is a polarization inversion type wavelength conversion element by der Poel et al. This is KTiOPO4 crystal (hereinafter referred to as “KTP”)
A wavelength conversion element is constructed by periodically ion-exchanging Rb on the surface of the substrate to form a periodic polarization inversion layer on the KTP substrate.

FIG. 4 shows the configuration of this conventional wavelength conversion element. In FIG. 4, 21 is a KTP substrate, 22 is a polarization inversion layer, 23 is a fundamental wave, and 24 is a second harmonic (hereinafter abbreviated as "SHG light"). The first period Λ of the polarization inversion layer 22 is
Effective refractive index N1 for fundamental wave (wavelength is λ) 23
and the effective refractive index N2 for SHG light (wavelength λ/2) 24, Λ=λ/2(N2 - N1)
meets the relationship. A fundamental wave 23 propagating within the waveguide is converted into SHG light 24 by periodically formed polarization inversion layers 22 . When the element length is 1 mm, the SHG conversion efficiency is reported to be 80%/W/cm2.

[0005]

[Problems to be Solved by the Invention] Conventional wavelength conversion elements using KTP have the following problems. In this wavelength conversion element, only the periodically ion-exchanged part has a higher refractive index for the fundamental wave 23 than the KTP substrate, and that part (optical waveguide) is 100 μm2 (10 μm x 10 μm
m), the power density of the fundamental wave 23 is low. The SHG light 24 is 2 for the fundamental wave 23.
Since it is proportional to the power of the power, there is a problem that the conversion efficiency to the SHG light 24 is low. Furthermore, since the SHG light 24 also exists as a radiation mode within the substrate, the near-field pattern at the output part of the element also has a radial shape, making it difficult to condense the SHG light 24 to the diffraction limit. Another problem is that it is difficult to apply to optical information systems.

In view of the above-mentioned problems, an object of the present invention is to improve the power density of the fundamental wave in the substrate, to make it possible to perform wavelength conversion with high efficiency, and to improve the efficiency of the emitted SHG light up to the diffraction limit. An object of the present invention is to provide a wavelength conversion element that enables light condensation and a method for manufacturing the same.

[0007]

[Means for Solving the Problems] A wavelength conversion element according to claim 1 includes a dielectric substrate, a low refractive index region formed in the substrate, and a region formed on the low refractive index region periodically with a period Λ. and an optical waveguide having a polarization inversion layer in which nonlinear polarization is inverted. Then, the period Λ of the polarization inversion layer is the effective refractive index N1 for the fundamental light having a wavelength λ guided through the optical waveguide.
Λ=λ/2(N2 −N1
), and the distance W between the polarization inversion layers is (Λ/
2) The relationship of <W<Λ is satisfied.

A method for manufacturing a wavelength conversion element according to a second aspect of the present invention includes the steps of: forming a low refractive index region by ion implantation in a dielectric substrate cut out in a plane perpendicular to the C-axis of a crystal; A process of forming a striped mask on the surface, a process of ion exchange inside the dielectric substrate from the non-mask part of the surface of the dielectric substrate, and a process of heat treating the dielectric substrate to form a polarization inversion layer. Contains.

[0009]

According to the present invention, by forming a low refractive index region within a dielectric substrate and providing an optical waveguide, a fundamental wave is confined within the substrate as a waveguide mode. The cross-sectional area of the optical waveguide can be reduced, the confinement of the fundamental wave is very good, and the power density within the optical waveguide does not attenuate as it propagates, resulting in very highly efficient wavelength conversion. In addition, by forming a low refractive index region inside the substrate by ion implantation, a high refractive index region is formed between the low refractive index region and the substrate surface (air), and the fundamental wave and harmonics (SH
G light) exists in a single mode state.

0010

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a configuration diagram of a wavelength conversion element according to an embodiment of the present invention. In FIG. 1, 1 is a KTP substrate, 4 is a polarization inversion layer, 5 is a low refractive index region formed by ion implantation, 6 is a fundamental wave with a wavelength of 800 nm, and 7 is a second harmonic (SHG light) with a wavelength of 400 nm. .

The direction of polarization of the polarization inversion layer 4 is reversed with respect to the KTP substrate 1. The first period Λ of this polarization inversion layer 4 is the effective refractive index N1 for the fundamental wave (wavelength is λ) 6 and the effective refractive index for SHG light (wavelength is λ/2) 7 guided through the optical waveguide. For N2, Λ=λ/
2(N2 - N1). The effective refractive index is the refractive index that light actually feels. Fundamental wave 6
The SHG light 7 whose phase can be matched with that of the polarization inversion layer 4 is generated only when the period of the polarization inversion layer 4 matches an integral multiple of the first polarization inversion period Λ. Moreover, the interval W between the polarization inversion layers 4 is (Λ
/2)<W<Λ (see Figure 2(b)).

A method of manufacturing the wavelength conversion element of this embodiment constructed as described above will be explained with reference to FIG. FIG. 2 is a process sectional view showing the method for manufacturing the wavelength conversion element of this embodiment. First, as shown in Fig. 2(a), O
Ions 2 are implanted at a current of 1 μA and a dose of 5×10 16 ions/cm 2 . Then, 1 μm from the surface
A low refractive index region 5 (KTP substrate 1) with a width of 1.2 μm is placed at a depth of
0.1 lower than the refractive index of
The area up to becomes the optical waveguide.

Next, as shown in FIG. 2(b), KT
After forming a Ti film of 1000 Å on the P substrate 1 by sputtering, the period Λ
A Ti mask 3 is formed by forming stripes with a width W (the spacing between the polarization inversion layers 4) over 10 μm in the Y propagation direction of the KTP substrate 1. After that, it was placed in a solution 8 of Ba(NO3)2 containing Rb/Tl/Ba at 360°C.
Heat treatment is performed for 90 minutes. This heat treatment is a process in which the temperature of the KTiOPO4 crystal (KTP) constituting the KTP substrate 1 is raised to near the Curie temperature, and the polarization of the crystal aligned in a certain direction is partially reversed. A possible mechanism for this polarization reversal is that first, Rb ions are exchanged with Rb ions through ion exchange, and the K concentration in the ion exchange layer is reduced. This lowers the Curie point of the ion exchange layer. Next, convert the KTP substrate 1 into KTiOP.
Curie point of O4 crystal and KX Rb1 of ion exchange layer
-X When heat-treated between the Curie points of TOPO4, only the ion exchange portion can be brought into the Curie state. Further, due to the electric field generated between the ions in the ion exchange layer and the ions in the KTP substrate 1, the polarization of the proton exchange layer in the Curie state is reversed, and a polarization inversion layer 4 is formed.

[0014] After that, the Ti mask 3 is removed with acid,
Both end faces of the waveguide were optically polished to form a wavelength conversion element. The operation of the wavelength conversion element manufactured as described above will be explained. The fundamental wave 6 incident on the optical waveguide is converted into a second harmonic in the polarization inversion layer 4, and when the fundamental wave 6 with a wavelength of 800 nm is incident, blue SHG light 7 with a wavelength of 400 nm can be generated.

Considering that the depth of the polarization inversion layer 4 of this wavelength conversion element is 1 μm, the depth of the optical waveguide is 1 μm, and the depth distribution of the TM mode of the electromagnetic field of the fundamental wave 6, there is sufficient overlap. As a result, highly efficient wavelength conversion is performed. Actually, light from a semiconductor laser with a wavelength of 800 nm and an output of 40 mW was focused by a focusing optical system, and the fundamental wave 6 was made incident on the fabricated wavelength conversion element. The fundamental wave 6 and SHG light 7 emitted from the waveguide were collimated with a lens and measured with a power meter. The results are shown in FIG. Figure 3 shows the fundamental wave 6
The figure shows the relationship between the input of and the output of the SHG light 7, where the horizontal axis is the square of the fundamental wave input and the vertical axis is the SHG light output. The output of the SHG light 7 to the wavelength conversion element with the first-order polarization inversion period is 1 mW, and the conversion efficiency at this time is 250
%/W·cm2, wavelength conversion was performed with extremely high efficiency. In addition, the emitted SHG light 7 is transmitted through a lens (NA=0.
8), it was possible to focus the light down to the diffraction limit of a spot size of 0.45 μm.

As described above, according to this embodiment, KTP
By forming an optical waveguide within the substrate 1 by ion implantation, the fundamental wave 6 is confined within the substrate as a waveguide mode. The cross-sectional area of the optical waveguide is 1 μm x 10 μm, which is different from conventional example 1.
/10, the power density of the fundamental wave is improved by ten times. Since the fundamental wave 6 propagates through the polarization inversion layer with low loss, highly efficient wavelength conversion can be obtained. The fabrication of an optical waveguide in the KTP substrate 1 has never been reported before, and a low-loss optical waveguide can be realized by high-speed ion implantation.

Furthermore, by forming the low refractive index region 5 at a depth of 1 μm or more from the surface of the KTP substrate 1 by ion implantation, a high refractive index is formed between the low refractive index region 5 and the surface (air). Since the SHG light 7 is emitted from the optical waveguide with low loss and single mode propagation, it has become easy to condense the SHG light 7 to the diffraction limit. In this example, a KTiOPO4 crystal (KTP) substrate 1 is used as the substrate.
was used, but a C plate (plane perpendicular to the C axis of the crystal) was used as the substrate.
It may be a KX Rb1-X TiOPO4 (0≦x≦1) crystal.

[0018]

[Effects of the Invention] In the wavelength conversion element and the manufacturing method thereof of the present invention, by forming a low refractive index region in the substrate and providing an optical waveguide, the confinement of the fundamental wave is very good.
Because the power density within the optical waveguide does not attenuate as it propagates, very efficient wavelength conversion can be obtained. In addition, since the optical waveguide formed by the ion implantation method emits SHG light (harmonics) with low loss and single mode propagation,
It also becomes easy to condense the G light to the diffraction limit.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram of a wavelength conversion element according to an embodiment of the present invention.

FIG. 2 is a process cross-sectional view showing a method for manufacturing the wavelength conversion element of the same example.

FIG. 3 is a diagram showing the output characteristics of the wavelength conversion element of the same example.

FIG. 4 is a configuration diagram of a conventional wavelength conversion element.

[Explanation of symbols]

1 KTP board 2 O+ ion 3 Ti mask 4 Polarization inversion layer 5 Low refractive index region 6 Fundamental wave 7 Second harmonic (SHG light) W Distance between polarization inversion layers Λ Period

Claims (2)

[Claims] 1. An optical waveguide comprising a dielectric substrate, a low refractive index region formed in the substrate, and a polarization inversion layer on the low refractive index region, the polarization of which is periodically inverted with a period Λ. and the period Λ of the polarization inversion layer is an effective refractive index N1 for fundamental light having a wavelength λ guided in the optical waveguide.
and the effective refractive index N2 for a harmonic with a wavelength of λ/2 guided in the optical waveguide, Λ=λ/2(N2 −
A wavelength conversion element that satisfies the relationship (N1) and in which the distance W between the polarization inversion layers satisfies the relationship (Λ/2)<W<Λ. 2. A step of forming a low refractive index region by ion implantation in a dielectric substrate cut out in a plane perpendicular to the C-axis of the crystal, and a step of forming a striped mask on the surface of the dielectric substrate. and a step of ion-exchanging the interior of the dielectric substrate from a non-masked portion of the surface of the dielectric substrate;
A method for manufacturing a wavelength conversion element, comprising the step of heat-treating the dielectric substrate to form a polarization inversion layer.