JPH07270838A - Higher harmonic wave generating element - Google Patents

Abstract

PURPOSE:To obtain an element having high efficiency with a mask of only the straight parts by providing a nonlinear optical material with periodic polarization inversion parts and non-inversion parts to form optical waveguides of a discrete type and specifying the refractive index of the polarization inversion parts. CONSTITUTION:The substrate 7 of a higher harmonic generating element which makes coherent light incident on a nonlinear optical material and converts the incident light to higher harmonic waves of (n) order having a halve or smaller wavelength is formed with the rectangular polarization inversion parts 8 to invert the direction of the nonlinear polarization of the nonlinear optical material by the period of coherence length with respect to the progressing direction of the incident light, by which the phase of the incident light and the phase of the generated higher harmonics waves of the (n) order are spuriously matched. The refractive index distribution of the polarization inversion parts 8 is max. in the central part in the direction perpendicular to the optical axis of the optical waveguides and is gradually decreased toward the peripheral part. As a result, the effect of focusing light is generated and the loss by scattering of light is lessened in spite of the optical waveguides of the discrete type. The efficiency of generating and converting the second harmonic waves is thus enhanced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a harmonic generating element for obtaining a coherent light having a short wavelength by using a stable and high output coherent light source such as a laser and reducing the wavelength to half or less. .

[0002]

2. Description of the Related Art In order to obtain a harmonic generating element having a high conversion efficiency, for example, a second harmonic generating (SHG) element, it is important to match the phase between incident light and SHG light generated in the element. In the case of a bulk device, phase matching is achieved by adjusting the angle of the SHG crystal with respect to the optical axis using the refractive index anisotropy of the crystal. Further, in order to further increase the SHG generation efficiency, a method of increasing the light intensity by an optical waveguide structure is adopted. As a method of phase matching in the waveguide structure, there is a method of pseudo phase matching using a periodic structure shown in Appl.Phys.Lett.21 (1072) 140.

Among the methods of quasi phase matching in this waveguide structure, the discrete waveguide structure disclosed in Appl. Phys. Lett. 588 (1990) 1725 is simple in process and quasi phase matching. This is one of the most practical SHG elements having the structure of.

Japanese Patent Application No. 5-16401 by the inventor of the present application
The boundary between the polarization inversion part and the non-inversion part was formed into a substantially curved surface in order to improve the diffraction loss of light and the efficiency decrease due to waveguide propagation, which were problems of the discrete waveguide structure in No. 7. FIG. 2 shows a quasi-phase matching SHG element having a discrete waveguide structure having this focusing function. A domain-inverted portion (2) having a curved surface is formed on a substrate (1). Incident light (3) is focused on the end surface (4) of the SHG element. FIG. 3 is an enlarged view of the waveguide structure in the invention of FIG. Since the polarization inversion part (2) has a large refractive index with respect to the substrate (1) and the boundary (5) is a curved surface, a light focusing effect occurs at the boundary,
Although the propagating light (6) in the waveguide has a discrete structure,
Propagated with low loss. However, in the present invention, it is essential to form the domain of domain inversion with a curved surface. As described above, the SHG device in which the discrete waveguide structure has the light focusing function was proposed, and the SHG conversion efficiency was significantly improved and the scattering of the SHG output light could be reduced.

[0005]

However, in the discrete waveguide structure, the domain-inverted portion is usually formed by an exposure process using a photoresist and a photomask. In order to make the boundary between the polarization inversion part and the non-inversion part a substantially spherical surface, a photomask with many curved parts is required, and the curved part forms the part that acts as a lens inside the waveguide, so it is extremely smooth. There is a need to. Therefore, there are problems that the mask is difficult to manufacture, the accuracy is low, and the manufacture is difficult.

[0006]

In order to solve the above-mentioned problems, the present invention forms a periodic polarization inversion part and a non-inversion part in a non-linear optical material to make a pseudo phase of incident light and an nth harmonic. In a harmonic wave generation element with a structure that forms a perfect match and forms a discrete optical waveguide, the refractive index of the domain-inverted part is the maximum in the central part and the peripheral part when viewed in a direction orthogonal to the optical axis of the waveguide. It is provided with a means to gradually decrease toward the.

[0007]

According to the harmonic generating element of the present invention, light is converged, loss due to light scattering is reduced, and a high SHG conversion efficiency can be easily manufactured.

[0008]

EXAMPLE FIG. 1 shows an example of the present invention in which a rectangular domain-inverted portion (8) is formed on a substrate (7). By making the refractive index of this polarization inversion part smaller with distance from the center with respect to the optical axis of the waveguide, a light converging effect occurs, and the propagation light of the waveguide is a discrete type structure, Propagated with low loss.

FIG. 4 is an enlarged view of the structure of the polarization inversion portion in the first embodiment for showing the principle of the present invention. The distribution (9) of the refractive index of the domain-inverted portion when viewed from the direction orthogonal to the optical axis is formed so that the central portion has the maximum value and decreases as the distance from the central portion increases. In this embodiment, the refractive index decreases in proportion to the square of the distance from the optical axis, and a refractive index distribution equal to the refractive index (10) of the substrate (7) is formed at the peripheral edge. The high refractive index in the optical axis portion, for the light propagation in the medium whose refractive index decreases as it moves away from the optical axis, detailed analysis has been performed in the propagation mode of light in the so-called graded index optical fiber,
It is known to have the same light focusing effect as a lens. The propagating light (11) is subjected to a focusing action in the polarization inversion part (8) and a diverging action due to the diffraction effect in the substrate part (7), and propagates with low loss while repeating focusing and divergence as shown in FIG. To do.

FIG. 5 is a diagram showing a method of manufacturing an element which is an embodiment of the present invention. KTP (KTiOPO ₄ ) is used as the nonlinear optical material of the harmonic wave generating element. First of all,
A thin film of Ti (13) on the KTP substrate (12) shown in (a)
Are formed ((b) in the figure). After that, after exposure, development, and steps with a photomask and a photoresist, Ti is patterned (14) periodically with the width of the domain-inverted portion, and the other portion is removed by a chemical etching method (FIG. 7C). ).
Further, SiO ₂ (15) is coated on it (FIG. 3D).

FIG. 6 is an enlarged view of the patterning portion of the Ti film for showing the next step of the step of FIG. Again, photoresist exposure, development and SiO ₂ (1
5) By selective etching using HF, SiO ₂
An opening (16) is formed in (15).

FIG. 7 is a sectional structural view of FIG. 6 seen from the direction perpendicular to the optical axis. The Ti film (14) is dissolved by the selective chemical etching method using acid. In this case, the etching solution wraps around the Ti film (14) to form an opening (1
An undercut portion (18) larger than 6) is formed.

FIG. 8 is a diagram showing the ion exchange process of the step of FIG. Heat treatment was performed at about 800 ° C. using a mixed solution of RbNO ₃ and Ba (NO ₃ ) ₂ , and the KTP substrate (1
The K ion in 2) is replaced with Rb ion. The mixed solution enters the undercut portion (18) through the opening (16). The ion substitution process in the KTP substrate proceeds based on the diffusion law, but in the solution, a constant amount of Rb ions is constantly supplied to the opening (16) due to the flow and convection of the solution. On the other hand, since the solution does not easily flow in the undercut portion (18), Rb ions are supplied according to the diffusion law. Therefore, the deeper it penetrates into the KTP substrate (12) through the undercut portion (18), the more the Rb for replacement becomes.
There is a shortage of ions. As a result, Rb ions are transferred to the substrate (1
The amount (19) of Rb ions diffused into and substituted in 2) becomes maximum in the central part, and the amount of ions decreases as the distance from the central part increases.

FIG. 9 is an enlarged view of the polarization inversion portion of the finally formed element. The device is completed after removing the SiO ₂ and Ti films. Since the refractive index increases in proportion to the amount of ions replaced, the refractive index at the center of the optical axis of the polarization inversion part (20) is maximized, and a distribution where the refractive index is small at the peripheral part is formed in the element. Incidentally, in FIG. 9, this polarization inversion part (20)
The gradation of the refractive index of is shown in shades.

According to the above-described manufacturing method, the length of the polarization inversion part is 3 μm and the length of the non-inversion part is 1.
As a result of creating an SHG element having a polarization inversion part width of 5 μm and a width of 5 μm, the wavelength of the YAG laser light of 1064 nm is 530 n
converted to m, and S for the input of 100mW of the YAG laser
With an HG output of 10 mW or higher, the conversion efficiency can exceed 10%.

[0016]

As described above, according to the present invention, it is possible to manufacture a mask having a curved surface, which has been a problem in producing a harmonic generating element having a discrete polarization inversion structure having a conventional light focusing effect. The difficulty has been improved, and it has become possible to obtain a high-efficiency harmonic generating element with a mask having only a straight line portion. In addition, the width of the waveguide can be made wider than the width of light that is actually propagated, and the optical coupling efficiency at the incident end face of the harmonic generating element can be increased. As a result, by combining the harmonic generating element of the present invention with a semiconductor laser, a light source of a small blue-green wavelength, which has been difficult in the past, can be obtained.
It can be obtained at low cost.

[Brief description of drawings]

FIG. 1 is a structural diagram of a harmonic wave generating element showing an embodiment of the present invention.

FIG. 2 is a structural diagram of a conventional harmonic wave generating element.

3 is an enlarged view of the waveguide structure of FIG.

FIG. 4 is an enlarged view of the structure of the polarization inversion section according to the embodiment of the present invention.

FIG. 5 is a diagram showing a method for manufacturing an element according to an example of the present invention.

FIG. 6 is an enlarged view of a patterning portion of a Ti film in a device manufacturing process.

7 is a diagram showing a cross-sectional structure of FIG.

FIG. 8 is a diagram showing an ion exchange process.

FIG. 9 is an enlarged view of a domain-inverted portion of a finally formed element.

[Explanation of symbols]

1 Substrate in Conventional SHG Element 2 Polarization Inverted Portion in Conventional SHG Element 3 Incident Light in Conventional SHG Element 4 Conventional SHG Element End Face 5 Boundary of Polarization Inverted Portion in Conventional SHG Element 6 Propagation Light in Conventional SHG Element 7 Substrate of Example of the Present Invention 8 Polarization Reversal Portion of Example of the Present Invention 9 Refractive Index Distribution of Polarization Reversal Portion of Example of the Present Invention 10 Refractive Index of Substrate in Example of the Present Invention 11 Element of Example of the Present Invention Medium propagating light 12 KTP substrate 13 Ti thin film 14 Patterned Ti thin film 15 SiO ₂ thin film 16 Small window of SiO ₂ thin film 17 Cross section perpendicular to the optical axis 18 Undercut portion 19 Amount of ions to be replaced 20 Final Polarization inversion part

Claims (2)

[Claims] 1. A harmonic generation element for injecting coherent light into a non-linear optical material and converting the incident light into an n-th harmonic having a wavelength of half or less, in a traveling direction of the incident light. Then, by inverting the direction of the nonlinear polarization of the nonlinear optical material at a cycle of the coherence length, it has a mechanism for pseudo matching between the phase of the incident light and the phase of the generated nth harmonic. By utilizing the fact that the refractive index of the non-inverted part of the non-linear polarization is higher than that of the non-inverted part, it has a structure that constitutes a discrete optical waveguide. A harmonic generating element, wherein the distribution is maximum in the optical axis portion of the optical waveguide in the direction perpendicular to the optical axis of the optical waveguide and gradually decreases toward the peripheral portion. 2. The KTP is used as the non-linear optical material, and periodic non-linear polarization is achieved by exchanging a part of K ions in KTP with other metal ions. The harmonic generating element described.