JPH07253603A - Second harmonic generating element - Google Patents

Abstract

PURPOSE:To enhance the incident efficiency at the time of admission of light from the end face of a second harmonic generating(SHG) element of a pseudo phase matching type discretely arrayed with regions of a round type which are inverted in polarization and have the refractive index higher than the refractive index of a substrate at a constant period. CONSTITUTION:A nonlinear optical crystal is internally provided discretely at specified periods with the round type regions 43 which are inverted in polarization and have the refractive index higher than the refractive index of the substrate. This regions function as discrete optical wave guides. The incident end side thereof is provided with a channel type optical waveguide 42. The front end of this channel type optical waveguide 42 is formed to a semicircular shape or the distance between the channel type optical waveguide 42 and the discrete waveguides is set at <=1.5micron, by which the light of the channel type optical waveguide 42 is admitted into the discrete optical waveguides.

Description

Detailed Description of the Invention

[0001]

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

[0002]

2. Description of the Related Art At present, it is difficult to realize a semiconductor laser capable of oscillating short wavelength green and blue with high output. For this reason, a method has been proposed in which infrared light is made incident into a nonlinear optical crystal to generate a second harmonic having a wavelength half that of the incident wave in the crystal. To obtain the second harmonic efficiently,
It is necessary to align the phase of the second harmonic generated at each point in the nonlinear optical crystal. As one of the methods for achieving phase matching, there is a method called quasi phase matching using polarization inversion of a nonlinear optical crystal.

The structure of the SHG element utilizing quasi phase matching can be roughly classified into two types. One is composed of a periodically poled structure and a channel waveguide orthogonal to it. As an example, an SHG element (EJLim, MMFejer, manufactured on a single crystal substrate of lithium niobate (LiNbO ₃ ))
RLByer: Electron. Lett., 25 , 731 (1989)). This type of device can propagate both incident waves and second harmonics through the channel waveguide with low loss, but it requires a two-step manufacturing process of manufacturing the polarization inversion structure and the channel waveguide. The manufacturing cost is high.

Another structure of the quasi-phase matching SHG element is one in which regions having the functions of polarization inversion and optical waveguide are arranged discretely at a constant period. For example, Appl.
Phys. Lett 57, (1990) 2074 has polarization inversion on the KTP single crystal substrate, and light spreads from the high refractive index region to the next rectangular high refractive index region, resulting in propagation loss. There is a problem of getting bigger. In the SHG element, the SH in which the polarization is inverted and the prismatic region having a high refractive index is provided
G element is announced. The element having this structure has an advantage that it is easy to manufacture because it can be manufactured in a single process, but since 100 or more regions of the square type having a high refractive index (square type) are discretely provided, The propagation loss is cumulative and becomes a large value.

In order to reduce the propagation loss due to the rectangular high refractive index region, the shape of each high refractive index region may be rounded (Japanese Patent Application No. 5-164017). At this time, the light propagates while converging in the array of convex lenses, so the propagation loss is reduced. However, in the structure of the optical waveguide in which the discrete high refractive index regions are arranged in a constant cycle, there is a problem that the coupling efficiency is poor when the light enters the optical waveguide from the end face. The reason for this is as follows.

As shown in FIG. 5A, a circular high refractive index area 1
When the end surface is polished at the center of 2, the incident light 11 is likely to enter the optical waveguide. When polishing is performed at the position of the substrate as shown in FIG. 5B, it becomes very difficult for incident light to enter. However, since the diameter of the round high refractive index region is as small as several μm, it is very difficult to carry out polishing at the target position. Therefore, it was not possible to produce an optical waveguide with good coupling efficiency with good reproducibility.

[0007]

DISCLOSURE OF THE INVENTION The present invention is directed to incident light into an optical waveguide in which polarization-inverted and circular regions having a higher refractive index than the substrate are periodically arranged from the end face. It seeks to increase efficiency.

[0008]

In order to solve the above-mentioned problems, in a nonlinear optical crystal, a circular region in which polarization is inverted and whose refractive index is higher than that of the substrate is discrete at a constant period. And a channel type optical waveguide for introducing light into the discrete optical waveguide. In order to put light into the discrete optical waveguide without loss, the end of the channel type optical waveguide should be a semi-circular shape, or the shape of the end should be rectangular and the distance from the first circular optical waveguide. Is 1.5 μm or less.

First, the case where the end of the channel type optical waveguide has a semicircular shape will be described with reference to FIG. Incident light 2
1 enters the channel type optical waveguide 22 whose end surface is polished.
The boundary 23 between the channel waveguide and the substrate 25 is a semicircle having the same radius of curvature as the circular region 24 having a polarization inversion and a high refractive index. Since the channel type optical waveguide 22 has a higher refractive index than the substrate 25, by making the boundary portion 23 convex with respect to the substrate, 23 acts as a lens and has an effect of converging the incident light 21.

The period Λ is given by Λ = λ / 2 (n _SH −n _F ) which is a condition for establishing the quasi-phase junction. Here, λ is the wavelength of incident light, n _F is the refractive index of the nonlinear optical crystal with respect to the fundamental wave, and n _SH is the refractive index of the nonlinear optical crystal with respect to the second harmonic. Usually Λ is 3 μm
To 10 μm. The second harmonic is most efficiently generated when the diameter of the circular region having the polarization inversion and the high refractive index is half of Λ. However, when Λ is small, the diameter is too small to allow light to enter.
In such a case, the diameter is made larger than half Λ.
Usually, the diameter of the rounded region is between 2 μm and 5 μm.

The width of the channel type optical waveguide is equal to the diameter of the circular region and is 2 μm to 5 μm.

It is desirable that the length 1 of the channel type optical waveguide 22 be 1 mm or more so that the end surface can be surely polished at this portion. Further, in order to prevent the SHG element from becoming unnecessarily large, it is desirable that the thickness be 5 mm or less.

Next, a case where the distance between the channel type optical waveguide and the first circular region which is polarization-inverted and has a high refractive index is 1.5 μm or less will be described with reference to FIG. .

The incident light 31 is a channel type optical waveguide 32.
To 34 and spread out into the substrate. Since the divergence angle at this time is small, the incident light spreads in the substrate if the distance L between the channel type optical waveguide 32 and the circular region 33 which is polarization-inverted and has a high refractive index is reduced. The loss can be extremely small. L to 1.
If the thickness is larger than 5 μm, the spread into the substrate becomes too large to be ignored, which is not desirable.

In FIG. 2, the length and width of the channel type optical waveguide and the period Λ between the circular regions are the same as those in FIG.

[0016]

【Example】

(Embodiment 1) An embodiment according to the present invention will be described with reference to FIG. 41 is a Z plate of KTP (KTiOPO ₄ ). Reference numeral 42 is a channel type optical waveguide, and 43 is a circular region in which polarization is inverted and which has a high refractive index. An optical waveguide consisting of 42 and 43 was prepared by immersing a KTP substrate in a mixed salt of rubidium nitrate and barium nitrate at 350 ° C. for 15 minutes. When heat treatment is performed in a mixed salt of rubidium nitrate and barium nitrate, the treated portion has a high refractive index and the polarization direction is reversed at the same time.

The surfaces 44 and 45 are end-polished.

The circular regions are lined up with a period of 4 μm. The diameter of the circular region is 2.5 μm. The width of the channel type optical waveguide 42 is 2.5 μm, and the tip end is a semicircular shape with a diameter of 2.5 μm. 850 nm incident light 10
When 0 mW was put in, 850 nm light was 50 mW and 42
A 5 nm second harmonic of 1 mW was emitted from the device.

(Embodiment 2) An embodiment according to the present invention will be described with reference to FIG. 51 is a Z plate of lithium tantalate, 52
Is a channel-type optical waveguide, and 53 is a circular region in which polarization is inverted and which has a high refractive index. The optical waveguide consisting of 52 and 53 is produced by exchanging a lithium tantalate substrate with a proton. By controlling the conditions of the proton exchange, it is possible to increase the refractive index of the proton exchanged region and at the same time reverse the polarization direction of the region.

The surfaces 54 and 55 are end-polished. The circular regions are arranged at a period of 7.4 μm. The diameter of the circular region is 4 μm. The width of the channel type optical waveguide 52 is 4 μm. The distance between the channel type optical waveguide and the first circular region is 1 μm.

200 nm of incident light of 1064 nm is applied to this device.
When entering mW, 1064nm light 120mW and 532
The second harmonic of 20 mW of nm emitted from the device.

[0022]

As described above, according to the device of the present invention, incident infrared light can be efficiently introduced into a discrete round optical waveguide, and the efficiency of second harmonic generation can be improved. You can

[Brief description of drawings]

FIG. 1 is a diagram simply showing the principle of an element of the present invention.

FIG. 2 is a diagram simply showing the principle of the device of the present invention.

FIG. 3 is a schematic diagram of an SHG device according to an embodiment of the present invention.

FIG. 4 is a schematic diagram of an SHG device according to an embodiment of the present invention.

FIG. 5 is a schematic view showing a state of polishing an end surface of a conventional element.

[Explanation of symbols]

11 incident light 12 circular high refractive index region 13 substrate 21 incident light 22 channel type optical waveguide 23 boundary between channel type optical waveguide and substrate 24 circular region having polarization inversion and high refractive index 25 substrate 26 Line showing the way of propagation of incident light 31 Incident light 32 Channel type optical waveguide 33 Circular region with polarization inversion and high refractive index 34 Line showing how incident light spreads 35 Substrate 41 KTP Z Plate 42 Channel Type Optical Waveguide 43 Polarization Inverted and Highly Round Regions 44, 45 End Polished Surface 51 Lithium Tantalate Substrate 52 Channel Type Optical Waveguide 53 Polarization Inverted, and High refractive index circular area 54,55 End surface polished

Claims (2)

[Claims] 1. An optical waveguide in which circular regions having polarization inversion and a refractive index higher than that of a substrate are discretely arranged at a constant period in a nonlinear optical crystal, and the discrete optical waveguide is incident. And a channel type optical waveguide whose end on the side of the discrete type optical waveguide is a semicircular shape that can function as a lens. 2. An optical waveguide in which polarization-inverted circular regions having a higher refractive index than the substrate are discretely arranged at a constant period in a nonlinear optical crystal, and the discrete optical waveguide is incident. And a channel type optical waveguide arranged so that the distance to the first circular region which is on the side and has a high refractive index is 1.5 μm or less. Second harmonic generation element.