JPH07294970A - Second harmonic wave generating element and its production - Google Patents

Abstract

PURPOSE:To produce a second harmonic wave generating element which lessens scattering loss of light generated in case of respective segments of a square shape, is free from leaking light and has high conversion efficiency by forming the respective segments to a lens shape. CONSTITUTION:The scattering loss of the light generated in the case of the respective segments of the square shape is lessened by forming the respective segments to the lens shape even if a single crystal of lithium niobate(LN) or lithium tantalum (LT) is used. More specifically, non-polarization inversion regions and polarization inversion regions having a higher refractive index than the refractive index of the bulk of the lens molds or the polarization inversion regions or non-polarization inversion regions having the higher refractive index than the refractive index of the bulk of the lens molds are alternately formed to the lens shape in the segments shape on the single crystal z plate 1 consisting of the LN or the LT. There are no differences in the refractive index between the polarization inversion regions and the non- polarization inversion regions at the time the polarization inversion is executed by impression of the voltage and, therefore, optical waveguides 2 are newly set. In such a case, the lens type optical waveguides 2 may be arranged at proper periods.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the field of optical information processing using laser light, the field of optical applied measurement control, the field of printing / platemaking,
The present invention relates to an optical wavelength conversion element used in the medical field or the like, particularly a second harmonic generation element.

[0002]

2. Description of the Related Art In order to efficiently convert the wavelength of light into a second harmonic, it is necessary to satisfy the phase matching condition in the wavelength conversion element. As this method, angle phase matching, temperature phase matching, a method using a waveguide, etc. have been proposed and used conventionally. Recently, a method called quasi-phase matching using a periodic structure has been attracting attention as a phase matching method. For example, Phys. Rev., Vol. 127, p. 1918 (1962)
At JA Armstrong et al. In this method, the polarization direction in the crystal is periodically inverted to compensate for the phase mismatch between the fundamental wave and the harmonic.

Here, the polarization reversal means reversing the direction of polarization of the single crystal dielectric material polarized in a fixed direction to be used. A polarization inversion type SHG element is obtained by applying this method to a second harmonic generation (SHG) element.

Various methods have been used so far for causing this polarization inversion. For example, Lim EJ, Fejer
MM, Beyer RL and Kozolovsky WJ: Electron. Le
As in tt., 25, 11, p. 731 (1989), thermal diffusion of Ti metal is used in lithium niobate (LN) single crystals. For example, K. Mizuuchi et al .: Appl. Phys. Let
As in t., 58, p. 2732 (1991), a method of rapid heating after proton exchange is used in lithium tantalate (LT) single crystal. As shown in FIG. 1, LN or LT
In the crystal substrate 1, the domain-inverted regions 3 and the non-domain-inverted regions 4 are formed in a comb shape, and the channel waveguide 2 is formed in the direction orthogonal to the pattern to form a domain-inverted SHG element. FIG. 1 shows the structure of this SHG element.

Further, as shown in FIG. 2, in the KTP substrate 5, a segment type channel waveguide 2 is manufactured by an ion exchange method by immersing it in a molten salt of rubidium nitrate, and at the same time, barium nitrate is added to the molten salt. A polarization-inverted SHG element is manufactured by adding and reversing the polarization of that portion (Appl. Phys.
Lett., Vol.57, No.20, p.2074 (1990)). This SH in Figure 2
The structure of a G element is shown. Here, in FIG. 2, 3 is a polarization inversion region, and 4 is a non-polarization inversion region.

So far, we have been able to solve the problem of leaked light, which has been a problem in segment type waveguides in KTP, by forming each segment into a lens shape (JP-A-5-164017).

[0007]

However, even in the case of using LN or LT crystal, if each segment is a square type, the incident fundamental wave and the generated fundamental wave are generated in principle due to scattering in the Fresnel region. Leaked light appears on both second harmonics. As a result, the conversion efficiency was lowered. In addition, since the second harmonic is also scattered, the phenomenon that the output beam cannot be narrowed down to the diffraction limit is also caused.

An object of the present invention is LN, LT or KTP.
A second harmonic generating element using the above is provided, which is a segment type second harmonic generating element with no leakage light, and a method for manufacturing the same.

[0009]

In order to solve the above problems, even when LN and LT single crystals are used, the shape of each segment is made into a lens shape so that each segment is rectangular. It was discovered that the scattering loss of light generated in the can be reduced. Specifically, as shown in FIG. 3, in the LN / LT single crystal z-plate 1, a non-polarization inversion region and a polarization inversion region having a higher refractive index than the lens type bulk in a segment shape, or higher than the lens type bulk. By alternately creating polarization inversion regions and non-polarization inversion regions of the refractive index, the laser light emitted from one ion-exchanged segment is focused by the refractive index distribution of that segment, resulting in light scattering loss. It was discovered that it can be reduced. Also,
Since the refractive index distribution due to this ion exchange is a graded type in the depth direction, it was discovered that the confinement of light in the depth direction also becomes a lens type and has a structure with very little leakage light.

The shape of the segment can be various shapes such as an ellipse and a circle as shown in FIG. However, when the segment period and the polarization inversion period match, the rectangular shape of FIG. 4 shows a conventional example, which is omitted. The segment interval may be set to a value for the fundamental wave calculated by quasi phase matching. About the width of the segment,
Within 20 μm, the place where the required value of light conversion efficiency appears can be selected. Ion exchange depth is 30 from the surface layer
Up to about μm is sufficient. For the ion exchange method, a proton exchange source such as benzoic acid, phosphoric acid or pyrophosphoric acid can be used.

The polarization reversal method can be achieved by annealing within 5 minutes after ion exchange to a Curie temperature of about 400 to 630 ° C. for the LT crystal and about 600 to 1100 ° C. for the LN crystal. Alternatively, it can be achieved by applying a high direct current electric field in a vacuum or oil between the conductive metals attached to both sides of the single crystal. The distance between the non-polarization-inverted region and the polarization-inverted region may be set to a value for the fundamental wave calculated by quasi-phase matching.

When polarization inversion is performed by applying a voltage, there is no difference in the refractive index between the polarization inversion portion and the non-polarization inversion portion, so a new optical waveguide is set. For this purpose, lens type waveguides may be arranged at an appropriate period, and an example thereof is shown in FIG. Alternatively, the various shapes of FIG. 4 can be used. This arrangement interval can be determined by the focal length determined by the refractive index difference between the waveguide portion and the bulk portion. If the arrangement interval can be made long, the ratio occupied by the ion exchange portion can be reduced with respect to the entire device length.

Further, the conductive metal deposited on both sides of the single crystal should have a suitable T depending on the ion exchange source used.
A mask pattern forming metal such as i, Al, Ta, Ni, Cr, or an alloy thereof can be used.

[0014]

When the above method is used, even if LN or LT single crystals are used, by making the shape of each segment a lens, the scattering loss of light that occurs when each segment is prismatic can be reduced. You can

Further, even in the case of a single crystal of LN, LT, KTP having a deep domain inversion region by voltage application, it is possible to manufacture an element having a good light-collecting property by using the lens-shaped optical waveguide.

[0016]

EXAMPLES Examples according to the present invention will be described below. (First Embodiment) FIG. 3 shows a perspective view of an SHG element according to a first embodiment of the present invention. The substrate 1 uses LN. In order to manufacture this SHG element, first, a resist is spin-coated on the plus z plane of the LN substrate 1, and the resist is patterned into a segment lens shape by photolithography. The period of the segment is 4 microns.
After Ti was sputtered to a thickness of 150 μm, the resist was removed to form a lens pattern of Ti. The device is completed by placing it in a heating furnace and holding it at 1050 ° C. for 10 minutes to diffuse titanium and invert the polarization of the diffused portion. Then, after polishing the end face, laser light was introduced. A second harmonic wave of 5 μW was obtained with respect to the laser light of 873 nm of 10 mW.

(Second Embodiment) FIG. 3 shows a perspective view of an SHG element according to a second embodiment of the present invention. The substrate 1 uses LN. In order to manufacture this SHG element, first, a resist is spin-coated on the plus z plane of the LN substrate 1, and the resist is patterned into a segment lens shape by photolithography. The period of the segment is 4 microns. After sputtering Ti to a thickness of 500 μm,
After lift-off, a segment-shaped lens-shaped hole pattern was formed in the Ti film. Dip in benzoic acid at 250 ° C. for 1 to 3 hours to perform proton exchange on the window-opened portion. further,
The device is completed by rapidly raising the temperature to 880 ° C., holding it for 30 seconds to 10 minutes, and then rapidly cooling it to a constant temperature. Then, after polishing the end face, laser light was introduced. A second harmonic of 3 μW was obtained for 10 mW of 872 nm laser light.

(Third Embodiment) FIG. 3 is a perspective view of an SHG device according to a third embodiment of the present invention. The substrate 1 uses LT. In order to manufacture this SHG element, first, a resist is spin-coated on the minus z surface of the LT substrate 1, and a segment type lens-shaped patterning of the resist is performed by photolithography. The period of the segment is 4 microns. Ti was sputtered to a thickness of 500 μm and then lifted off to form a segment-shaped lens-shaped perforation pattern on the Ti film. 1 to 250 ℃ benzoic acid
Dip for 3 hours and exchange the protons in the window-opened part. Further, the device is completed by rapidly raising the temperature to 550 ° C., holding it for 10 seconds to 10 minutes, and then rapidly cooling it to a constant temperature. Then, the end face was polished and then introduced into a laser beam. 875 nm
The second harmonic of 4 μW was obtained with respect to the laser light of 10 mW.

(Fourth Embodiment) FIG. 3 is a perspective view of an SHG device according to a fourth embodiment of the present invention. In order to manufacture this SHG element, first, a resist was spin-coated on the plus z surface of the LT substrate 1, and a segment type lens-shaped patterning of the resist was performed by photolithography. The period of the segment is 4 microns. 500 μm Ti
After being sputtered to a thickness of 1, the film was lifted off to form a segment-shaped lens-shaped hole pattern on the Ti film. A titanium film having a thickness of 500 μm was evenly attached to the minus z surface. A direct current voltage of 25 kV / mm is applied in a vacuum or in oil, with the plus z plane as plus and the minus z plane as minus. After that, 1 to benzoic acid at 250 ℃
Immerse for 3 hours and perform ion exchange of the window opening, 350
The device is completed by annealing at 15 ° C. for 15 minutes to form the waveguide 2. Then, after polishing the end face, laser light was introduced. For 10 mW of 875 nm laser light,
A second harmonic of 4 μW was obtained.

(Fifth Embodiment) FIG. 3 shows a perspective view of an SHG device according to a fifth embodiment of the present invention. In order to manufacture this SHG element, first, a resist was spin-coated on the minus z surface of the LN substrate 1, and a segment type lens-shaped patterning of the resist was performed by photolithography. The period of the segment is 4 microns. 500μ for Ti
After sputtering to a thickness of m, lift-off was performed to form a segment-shaped lens-shaped drilling pattern on the Ti film. A titanium film having a thickness of 500 μm was uniformly deposited on the plus z surface. A direct current voltage of 25 kV / mm is applied in a vacuum or in oil, with the plus z plane as plus and the minus z plane as minus. After that, 1 to benzoic acid at 250 ℃
Immerse for 3 hours and perform ion exchange of the window opening, 350
The device is completed by annealing at 15 ° C. for 15 minutes to form the waveguide 2. Then, after polishing the end face, laser light was introduced. For 10 mW laser light of 870 nm,
A second harmonic of 2 μW was obtained.

(Sixth Embodiment) FIG. 5 shows a perspective view of an SHG device according to a sixth embodiment of the present invention. In order to manufacture this SHG element, first, a resist was spin-coated on the plus z surface of the LT substrate 1, and the resist was comb-shaped patterned by photolithography. This period is 4 microns. After Ti was sputtered to a thickness of 500 μm, the resist was removed and a Ti comb pattern was formed. A titanium film having a thickness of 500 μm was evenly attached to the minus z surface. A direct current voltage of 25 kV / mm is applied in a vacuum or in oil, with the plus z plane as plus and the minus z plane as minus. Here, in FIG. 5, 3 is a polarization inversion region, and 4 is a non-polarization inversion region. Next, the titanium film is removed, and titanium is newly attached to the plus z surface. Lenticular holes are formed with a cycle of 10 μm, and then 2
The element is completed by immersing it in benzoic acid at 50 ° C. for 1 to 3 hours to perform ion exchange on the windowed portion, and annealing at 350 ° C. for 15 minutes to form the waveguide 2. And
Laser light was introduced after polishing the end faces. A second harmonic of 4 μW was obtained for 10 mW of 875 nm laser light.

The present invention is not limited to the above embodiments, and various modifications can be made within the scope of the invention. For example, in the above fourth and fifth embodiments, the LT and LN substrates are used, and the case where the waveguide is formed by ion exchange after the polarization inversion is performed by the voltage application has been described. However, instead of the LT and LN substrates, To KT
A P substrate may be used. Even when the KTP substrate is used, the same operation and effect as those of the fourth and fifth embodiments can be obtained.

In the sixth embodiment, the case where the LT substrate is used has been described, but instead of the LT substrate,
LN or KTP substrates may be used. Even when an LN or KTP substrate is used, the same action and effect as those of the sixth embodiment can be obtained.

[0024]

As described above, according to the present invention, L
Even when N or LT single crystal is used, by making the shape of each segment a lens, it is possible to reduce light scattering loss that occurs when each segment is rectangular,
As a result, it is possible to fabricate a second harmonic generation element having no light leakage and high conversion efficiency. In addition, it is possible to obtain an element having a good light-condensing characteristic despite the segment type.

Further, even in the case of a single crystal of LN, LT or KTP having a deep domain inversion region by applying a voltage, it is possible to manufacture an element having a good light collecting property by using the lens-shaped optical waveguide. Similarly, despite being a segment type,
It is possible to obtain an element with good light condensing characteristics.

[Brief description of drawings]

FIG. 1 is a schematic diagram of a conventional domain-inverted SHG device in lithium niobate (LN).

FIG. 2 is a schematic diagram of a conventional polarization inversion SHG element in KTP.

FIG. 3 is a schematic view of a domain-inverted SHG element according to the present invention.

FIG. 4 is a shape diagram of masks of various segments.

FIG. 5 is a schematic view of a domain-inverted SHG element according to the present invention.

[Explanation of symbols]

 1 LN or LT substrate 2 Optical waveguide 3 Polarization inversion region 4 Non-polarization inversion region 5 KTP substrate

Claims (6)

[Claims] 1. An optical waveguide is formed on an LN or LT single crystal z plate so that the shape of each segment of the single crystal of each segment is a lens type when viewed from above, and the optical waveguide is formed in a direction not forming the optical waveguide. A quasi-phase-matching second harmonic generation element characterized in that domain-inverted regions and domain-inverted regions are alternately formed. 2. The method for manufacturing the second harmonic generation element according to claim 1, wherein a Ti film is formed in a thickness of 50 to 500 μm on the LN single crystal z plate when polarization inversion is performed, and the diffusion is performed by heating. Then, the polarization of the diffused portion is inverted to form an optical waveguide at the same time, which is a method for producing a second harmonic generation element. 3. The method for producing the second harmonic generating element according to claim 1, wherein in the LN or LT single crystal z-plate, a heat treatment after proton exchange is performed when polarization inversion is performed, and thus the ion exchange portion is formed. A method of manufacturing a second harmonic generation element, which comprises reversing the polarization of. 4. A LN, LT or KTP single crystal z plate is formed with a mask pattern in which one surface is segmented with a conductive metal into a lens shape to form a lens pattern, and the other surface is coated with a conductive metal. Applying a high DC voltage to both sides to invert the polarization of the metal-covered part of the holed surface, and further forming an optical waveguide by ion-exchange or ion-exchange and annealing the holed part. A method for producing a quasi phase matching type second harmonic generation device characterized. 5. A LN, LT or KTP single crystal z plate is prepared by forming a mask pattern in a comb shape with a conductive metal on one surface, and depositing the conductive metal on one surface on the other surface, and applying a high DC voltage to both surfaces. Then, the polarization of the part covered with metal is reversed, and after removing the conductive metal, a mask pattern is created by segmenting each segment to form a lens-type optical waveguide and forming a mask pattern. A method of manufacturing a quasi-phase-matching second harmonic generation element, characterized in that an optical waveguide is formed by exchange or ion exchange and annealing. 6. The method for producing a second harmonic generation element according to claim 5, wherein the period of the lens type optical waveguide is set to a length according to the degree of increase or decrease of the refractive index due to ion exchange. .