JPH05341341A - Optical waveguide type second higher harmonic generating element - Google Patents

Abstract

PURPOSE:To form the structure of an element which more efficiently generates second higher harmonics by using titanate potassium phosphate. CONSTITUTION:A conversion efficiency is improved by providing a region 2 where the directions of spontaneous polarizations are periodically inverted and a ridge type optical waveguide 5 guiding the second higher harmonics perpendicular to the region 2 on the Z surface of a titanate potassium phosphate single crystal substrate 1. Otherwise the conversion efficiency is improved by forming a channel waveguide 4 formed in the direction of the period in the region 2 where the directions of spontaneous polarizations are periodically inversed on the Z surface of the single crystal substrate and the ridge type part 5 guiding the second higher harmonics thereon.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical waveguide type second harmonic generation element (SHG element) for realizing a compact laser light source of short wavelength in the fields of optical information processing, optical measurement and medical field.
It is about.

[0002]

At present, the wavelength of a semiconductor laser capable of oscillating at a practical level is in the infrared-red range, and it is difficult to realize a semiconductor laser capable of oscillating shorter wavelengths of green and blue with high output. Therefore, a method of obtaining a short wavelength laser by generating a second harmonic from light of a semiconductor laser or the like that oscillates infrared light has been proposed.

When the second harmonic wave is generated, the power of the fundamental wave (frequency ω) and the power of the second harmonic wave (frequency 2ω) have a relationship expressed by the following equation. P (2ω) ∝ {S (n, n, m)} ² × {P (ω)} ² / w (1) where P (ω) and P (2ω) are the fundamental wave and the second harmonic, respectively. Power, w is the width of the optical waveguide, S (n, n, m)
Is a spatial coupling coefficient that represents the overlap of the electromagnetic field distributions of the fundamental wave and the second harmonic. {S (n, n, m )} = ∫ S f 2 (n, ω) · f (m, 2ω) dS (2) where, f (n, ω) is the fundamental wave, f (m, 2ω ) Indicates the field distribution of the second harmonic, and S indicates the cross section.

The SHG elements proposed so far are manufactured by using a single crystal of lithium niobate (LiNbO ₃ ). As an example, SH using quasi phase matching
G element (EJ Lim, MM Fejer, R.M.
L. Byer: Electron. Lett. , 25,
174 (1990)). In the case of a device made of lithium niobate, since lithium niobate is weak against optical damage, the refractive index of the optical waveguide changes due to the generated P (2ω), and the term of {P (ω)} ^{2 in} the equation (1). If P is increased, P (2ω) does not increase proportionally and tends to be saturated. Further, since the refractive index of the optical waveguide changes with time, it is difficult to obtain P (2ω) stably for a long time.

In order to improve the drawbacks of the SHG element using lithium niobate, SHG using KTiOPO ₄ which is said to be more resistant to optical damage than lithium niobate by two digits or more.
An element having the structure shown in FIG. 2 has been proposed (C.
J. van der Poel, J.M. D. Bierle
in and J. B. Brown, Appl. Phy
s. Lett. 57, 2074 (1990)). In FIG. 2, 51 is a Z plate of KTiOPO ₄ single crystal, and 52 is an Rb ion exchange waveguide. When the Ba ion is added to the Rb ion-exchanged waveguide of No. 52, the direction of spontaneous polarization of the waveguide portion is inverted with respect to the bulk when Ba ions are added, and SHG is obtained by quasi-phase matching. However, since this structure has a structure in which the optical waveguide is interrupted, while the light is propagated from the part where the waveguide is formed to the part where the next waveguide is formed, the formation of the waveguide is formed. It will pass through the part that is not done. In the bulk portion, the effect of confining light disappears, so that both the incident beam and the generated second harmonic beam spread. That is, P (ω) in the equation (1)
Is attenuated as the light travels, and w is actually larger than the waveguide width indicated by 52. Therefore, there is a problem that it is not possible to obtain the power expected from the calculated value as P (2ω).

Therefore, the present inventor has found that a region in which the direction of spontaneous polarization is periodically inverted on the Z plane of a single crystal substrate of KTiOPO ₄ and a continuous region in which light propagates in the direction of the period. The optical confinement effect is increased by the waveguide type wavelength conversion element characterized by having the channel waveguide of (Japanese Patent Application No. 4-34506).

[0007]

SUMMARY OF THE INVENTION It is an object of the present invention to fabricate a structure of an element which produces a second harmonic more efficiently.

[0008]

According to the present invention, an optical waveguide portion that guides a second harmonic wave at right angles to a region in which spontaneous polarization is inverted is formed on a Z plane of a KTiOPO ₄ single crystal substrate which is resistant to optical damage. It has a ridge-type optical waveguide with a small refractive index and a larger refractive index than the substrate, or it has a region in which the direction of its own polarization is periodically inverted and a continuous first region on which the light propagates in the direction of the period. The SHG element is characterized by having a ridge-type optical waveguide having a channel waveguide of, and a refractive index smaller than that of the optical waveguide portion that guides the second harmonic in parallel to the region, and a refractive index larger than that of the substrate. ..

[0009]

In the present invention, the conversion efficiency is improved by loading the ridge type portion to increase the integral value {S (n, n, m)} of the equation (2). is there. By making the refractive index change as shown in FIG. 4, the field distribution of the fundamental wave fω is almost confined in the substrate by the ridge type portion as shown in FIG. Because it propagates across layers,
The field cancellation becomes small, S (n, n, m) becomes large, and the conversion efficiency increases.

[0010]

Embodiment An embodiment according to the present invention will be described with reference to FIG.
In FIG. 1, reference numeral 1 is a Z plate which is a single crystal of KTiOPO ₄ . On the Z-plane of this single crystal, a region in which the spontaneous polarization shown in 2 is inverted is formed. The KTiOPO ₄ single crystal is connected so that the output light of the semiconductor laser 3 enters the optical waveguide. When the output light from the semiconductor laser is coupled so as to have an electric field component parallel to the Z direction in the optical waveguide formed on the KTiOPO ₄ substrate, the maximum nonlinear optical constant r ₃₃ = 14 × 10 ^{− of} KTiOPO ₄ is obtained. ¹² m / V is available. Now, assuming that light having a wavelength of 850 nm is introduced as a fundamental wave from the semiconductor laser into the optical waveguide, the inversion period is 3-5 μm in the case of the first-order quasi-phase junction.
The second harmonic generated in the optical waveguide is radiated from the end face of the crystal and is condensed and used by a lens or the like.

The region 2 in which the spontaneous polarization is inverted can be manufactured as follows. First, a region other than the region in which the direction of spontaneous polarization is not desired to be reversed is covered with a Ti film by using a photolithography technique. After that, Ba (NO ₃ ) ₂ and RbN
When KTiOPO ₄ is heat-treated in a mixed melt of O ₃ , Ti
K ⁺ ions and Rb ⁺ ions are exchanged in the part without the film, and a region in which the spontaneous polarization is inverted is formed. After the ion exchange, the Ti film is removed by etching.

When forming a ridge type optical waveguide, the ridge layer is formed with a hole in the resist at a right angle to a line which is spontaneously polarized by photolithography, and then S is formed by RF sputtering or the like.
A thin film whose refractive index is adjusted with a mixture of iO ₂ and Ta ₂ O ₅ or the like is attached, the resist is removed, and the structure is formed to form a structure as shown in FIG.

When the optical waveguide is formed in the substrate, the spontaneous polarization is inverted as described above, and then the following process is performed. The optical waveguide is produced by ion exchange, like the region in which the direction of spontaneous polarization is reversed. First, the Ti film is covered except for the region where the optical waveguide is formed. Next, RbNO ₃ and Tl
Heat treatment is performed in a mixed melt of NO ₃ . A channel waveguide is formed by this heat treatment. Further, a thin film having a refractive index adjusted with a mixture of SiO ₂ and Ta ₂ O ₅ or the like is attached thereto by RF sputtering or the like, and the Ti film is removed by etching. As a result, a structure as shown in FIG. 1 (b) is manufactured.

[0014]

As described above, according to the element of the present invention, a second harmonic wave is generated from a light source such as a semiconductor laser that oscillates light having an infrared-red wavelength, thereby stabilizing the output at a high output. It is possible to obtain a laser beam having a short wavelength.

[Brief description of drawings]

1A is a schematic view of an SHG element in which a ridge layer is loaded in an optical waveguide width according to an embodiment of the present invention, FIG. 1B is a channel waveguide according to an embodiment of the present invention, and a ridge layer is further provided. It is the schematic of the SHG element which loaded.

FIG. 2 is a schematic view of a conventional SHG device using a KTiOPO ₄ crystal.

FIG. 3 is a diagram showing a light field distribution in the device of the present invention.

FIG. 4 is a diagram showing a change in refractive index from a substrate to a ridge layer.

[Explanation of symbols]

1 Z-plate of KTiOPO ₄ 2 Region where spontaneous polarization is inverted 3 Semiconductor laser 4 Channel waveguide 5 Ridge layer 81 Ridge layer 82 Optical waveguide

Claims (2)

[Claims] 1. A region in which the direction of spontaneous polarization is periodically inverted and a ridge-type optical waveguide that guides a second harmonic to the region on the Z plane of a KTiOPO ₄ single crystal substrate. An optical waveguide type second harmonic generation element characterized by: 2. A region in which the direction of spontaneous polarization is periodically inverted on the Z plane of a KTiOPO ₄ single crystal substrate, and a continuous first channel on that region in which light propagates in the direction of the period. An optical waveguide type second harmonic generation element having a waveguide and having a ridge type optical waveguide for guiding a second harmonic in parallel to the region.