JPH11326969A - Laser light wavelength converter - Google Patents

Abstract

(57) [Problem] To provide a wavelength converter for converting a laser beam into a high-quality harmonic with high conversion efficiency with a small number of parts. SOLUTION: This wavelength conversion device condenses light with a condenser lens, converts the wavelength of the laser light as a fundamental wave by a nonlinear optical crystal a plurality of times to generate a harmonic, and converts the wavelength of the laser light into a harmonic. Device. One condensing lens 5
And a second harmonic generation crystal 1 and a third harmonic generation crystal 2 respectively formed of a nonlinear optical crystal,
The optical axes of the second harmonic generation crystal and the third harmonic generation crystal are mutually inverted. Further, the second harmonic generation crystal is located at the focal position of the fundamental wave, and the third harmonic generation crystal is located behind the focal position of the fundamental wave and are arranged close to each other.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wavelength conversion device for laser light, and more particularly, to a device requiring two or more wavelength conversions. The present invention relates to a wavelength conversion device for converting a laser beam into a harmonic with high efficiency.

[0002]

2. Description of the Related Art Using a laser beam as a fundamental wave, a nonlinear optical crystal performs wavelength conversion a plurality of times to generate harmonics,
Devices that convert the wavelength of laser light into higher harmonics are being put into practical use, and have attracted attention in laser application devices that use laser light. By the way, in order to put the laser light wavelength converter into practical use, it is essential to increase the conversion efficiency of the laser light.

[0003]

However, in the conventional laser light wavelength converter, the effective working length contributing to wavelength conversion cannot be increased due to the effect of the walk-off angle in the nonlinear optical crystal, and the conversion efficiency is reduced. It was difficult to get higher.
Therefore, there has been an attempt to increase the effective working length by alternately inverting the optical axis of a large number of non-linear optical crystal pieces that convert to the same wavelength, thereby increasing the effective working length. Attempts have been made to improve the spatial overlap of each wavelength by using a correction lens between multiple nonlinear optical crystals, but stable high conversion due to the effects of refraction at the end face of the nonlinear optical crystal and chromatic aberration of the optical system, etc. It is practically difficult to obtain the efficiency, and the number of parts of the apparatus increases, the parts cost increases, the manufacturing process becomes complicated, the manufacturing cost increases, and it is not economically acceptable.

Accordingly, an object of the present invention is to provide a wavelength conversion device for increasing the conversion efficiency of the second and subsequent non-linear optical crystals with a small number of parts, and for converting the wavelength of laser light into higher harmonics with high conversion efficiency. That is.

[0005]

In order to achieve the above object, a wavelength conversion device for laser light according to the present invention generates a harmonic by performing wavelength conversion a plurality of times with a nonlinear optical crystal using a laser light as a fundamental wave. An apparatus for converting the wavelength of a laser beam into a harmonic, comprising: a condensing optical system for condensing a fundamental wave; and two or more nonlinear optical crystals having mutually different generated wavelengths, and at least one nonlinear optical crystal. Are rotated 180 ° with respect to the fundamental wave propagation direction, and all adjacent crystals are arranged close to each other without an optical system interposed therebetween. According to the present invention, in the second and subsequent nonlinear optical crystals in which each wavelength whose phase is matched rotates the optical axis by 180 °, the propagation shift angle due to the walk-off angle is shifted in the opposite direction to the nonlinear optical crystal up to the previous stage. I do. Therefore, the spatial overlap of each phase matching wavelength shifted by the influence of refraction or walk-off angle at the nonlinear optical crystal end face can be corrected to some extent by the nonlinear optical crystal itself without using a correction optical system between the nonlinear optical crystals. Conversion efficiency is improved. Further, in the present invention, since the beam cross-sectional areas of the second and subsequent nonlinear optical crystals can be minimized by the close arrangement, the conversion efficiency is improved.

In a preferred embodiment, in the device for converting to the third harmonic, the second harmonic generation crystal is located at the focal position of the fundamental wave, and the third harmonic generation crystal is located at the focal position of the fundamental wave. It is located further back. Further, the focal position of the fundamental wave may be arranged before the second harmonic generation crystal. In addition, the crystal spacing is set so that the beam cross-sectional area of the fundamental laser light condensed by the condensing optical system and the high-order harmonic laser light generated by each harmonic generation crystal becomes as small as possible in the next-stage harmonic crystal. Is getting narrower. That is, the crystal spacing is reduced so that the beam cross-sectional area of the focused fundamental wave laser light and the second harmonic laser light generated in the first crystal becomes as small as possible in the second and subsequent crystals, and The propagation angle is corrected by the walk-off angle in order to improve the spatial overlap of each incident light in the subsequent crystals. The condensing optical system used in the present invention is a condensing mechanism system using a single lens, a cylindrical lens, and a plurality of other lenses, and the configuration does not matter. In a preferred embodiment, the effect can be confirmed with a single lens having a simple structure.

In the present invention, a nonlinear optical crystal refers to a non-linear optical crystal such as a uniaxial crystal or a biaxial crystal having optical anisotropy (non-linear optical effect). It does not matter, but preferably, the harmonic generation crystal is LB
_{It is made of 3} O ₅ or β-BaB ₂ O ₄ . Especially,
LB ₃ O ₅ crystals are preferred because they have a small walk-off angle and can have a long in-crystal conversion action length.

[0008] A wavelength conversion device for laser light according to the present invention is a device for converting a laser beam to a harmonic by generating a harmonic by performing wavelength conversion a plurality of times with a nonlinear optical crystal using the laser beam as a fundamental wave. , A single condensing lens, a second harmonic generation crystal, a fourth harmonic generation crystal, and a fifth harmonic generation crystal formed of a nonlinear optical crystal, respectively. The generation crystal, the fourth harmonic generation crystal, and the fifth harmonic generation crystal are arranged close to each other, and the second harmonic generation crystal, the fourth harmonic generation crystal, and the fifth harmonic It is characterized in that any two of the generation crystals have the same optic axis in the same direction and the other one has the opposite optic axis. In the present invention, preferably, the second harmonic generation crystal, the fourth harmonic generation crystal, and the fifth harmonic generation crystal are each LB
_{It is made of 3} O ₅ or β-BaB ₂ O ₄ .

In the case where the third harmonic is generated by the second-order nonlinear effect, the fundamental light is condensed by a lens in the second harmonic generation crystal to obtain a high light-condensing density, and the focal position is arranged in the crystal. . In the third harmonic generation crystal, the second
By providing a close arrangement in which the distance from the third harmonic generation crystal is narrowed, a high light collection density can be obtained without using a light collection system for the third harmonic generation crystal again. The propagation directions of the second harmonic light and the fundamental light generated in the second harmonic generation crystal are shifted by the walk-off angle, but the shift due to the walk-off angle in the third harmonic generation crystal is opposite to that in the opposite direction. By doing so, the spatial overlap between the fundamental wave light and the second harmonic light in the third harmonic generation crystal can be corrected to some extent, and high-efficiency wavelength conversion can be realized.

As described above, according to the present invention, high conversion efficiency can be realized with a small number of components by utilizing the close arrangement between crystals and the walk-off angle of each wavelength by condensing a single lens. I have. By using a walk-off angle to correct the proximity arrangement of multiple crystals and the spatial overlap of laser light of each wavelength to some extent, a laser light wavelength conversion device with a small number of parts and high conversion efficiency is realized. can do. The present invention is not limited by the wavelength of the fundamental wave, the type of nonlinear crystal, the type of light-collecting optical system, the number of crystals, and the order of harmonics, and the present invention can be applied to achieve the effects of the present invention. Can be. For example, as a fundamental wave, a laser beam of 1064 nm and an oscillation wavelength of 1 emitted by a Nd: YLiF ₄ laser are used.
This is possible in all wavelength ranges where the wavelength can be converted by the nonlinear optical crystal, such as laser light of 047 and 1053 nm. Also, 3
More than one crystal is also possible.

[0011]

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings. Embodiment 1 This embodiment is an example of an embodiment of a laser beam wavelength converter according to the present invention, and FIG. 1 is a schematic diagram showing a configuration of a laser beam wavelength converter of this embodiment. is there. The wavelength conversion device for laser light of the present embodiment uses LB ₃ O ₅ (LBO) as the nonlinear optical crystals 1 and 2 for generating harmonics, and the third harmonic 355 nm of the fundamental laser light having a wavelength of 1064 nm. A second harmonic generation crystal 1 and a third harmonic generation crystal 2, each formed of a nonlinear optical crystal, and a condenser lens 5, as shown in FIG. I have. In the present wavelength conversion device, the second harmonic generation crystal 1 is of type I angular phase matching and is arranged at the focal position of the fundamental wave. The third harmonic generation crystal 2 is of type II angular phase matching, and is disposed behind the focal position of the fundamental wave. Since the LBO crystal is a biaxial crystal, the walk-off angle is triangular, but only one is large.

The polarization of the fundamental wave light 6 is an ordinary ray of the second harmonic generation crystal 1, and the polarization direction of the second harmonic light 7 generated by type I angle phase matching becomes an extraordinary ray.
Since the LBO crystal is a negative crystal in which the refractive index of the extraordinary ray is smaller than that of the normal ray, the propagation direction of the second harmonic light is shifted in the direction opposite to the optical axis 3 with respect to the fundamental light.
At this time, the phase matching angle θ of the second harmonic generation crystal 1 is
θ = 11.6 ° and the walk-off angle is 0.403 °. Compared with other crystals, for example, β-BaB ₂ O ₄ (BBO), the LBO crystal has a smaller walk-off angle and a longer conversion length within the crystal, but in order to obtain a practically high conversion efficiency, a lens must be used. 5 focuses the laser light.

The third harmonic generation crystal 2 generates a third harmonic light 8 by angular phase matching between the fundamental light 6 having a divergence angle and the second harmonic light 7. At this time, the phase matching angle θ of the crystal 2 is θ = 42.19 °, and the walk-off angle is 0.533 °. By arranging the crystal 2 close to the crystal 1, the wavelength can be converted in a region where the beam sectional area is as small as possible, and the conversion efficiency can be increased. In the present embodiment, the optical axis of the crystal 2 is opposite to the optical axis of the crystal 1 with respect to the fundamental wave propagation direction. Although there is a difference in the walk-off angle between the crystals 1 and 2 because the phase matching angles are different, the fundamental light 6 deviated by the walk-off angle in the crystal 1 and the refraction of each wavelength between the crystals 1 and 2. And the second harmonic light 7
Is corrected to some extent at the time of angular phase matching in the direction of the axis 9 and phase matching is performed, so that the wavelength conversion operation length can be lengthened and high wavelength conversion efficiency can be achieved.

FIG. 2 shows the average output operation repetition dependence of the third harmonic of the pulse laser light when the wavelength converter of the first embodiment is used. In the present embodiment, 1.25 W is obtained by 4 kHz operation by inverting the optical axis of each crystal and disposing them close to each other. The power is 0.35 W in the same direction of the optical axis, and a high conversion efficiency of about 3.5 times has been achieved. In FIG. 2, the operation repetition is changed from 1 to 40 kHz, but the effect of the present embodiment can be seen without depending on the repetition. FIG. 3 shows the effect of the present embodiment when the length of the third harmonic generation crystal is changed. From FIG. 3, it can be seen that the average power of the present embodiment is about 3 to 4 times the average output without depending on the crystal length and the peak intensity of the fundamental wave. FIG. 4 shows an average output in which each crystal interval is changed. For a crystal spacing of 2.4 mm, 17 for 0.2 mm
% Average output is high, and the effect of minimizing the beam cross-sectional area in the third harmonic generation crystal by the close arrangement can be confirmed.

In the present embodiment, the proximity arrangement of a plurality of crystals and the spatial overlap of the laser beams of each wavelength are corrected to some extent with a walk-off angle, so that the number of parts is small, the cost is low, the conversion efficiency is high, and the beam Quality can be realized.

The reasons for the high conversion efficiency achieved by the above characteristics can be summarized as follows: the effect of the close arrangement and the fundamental in the third harmonic crystal due to the difference between the refraction angle and the walk-off angle between the crystals at each wavelength. This is due to the effect of correcting the shift of the spatial overlap between the wave and the second harmonic by inverting the optical axis. Although the average output value changes, there is an effect of the present embodiment without depending on the crystal length and the focal length of the condenser lens. Further, a BBO crystal having a larger walk-off angle than an LBO crystal has a larger shift angle, but has the same effect, and is possible with a birefringent crystal. Further, although a single lens is used as the condenser lens, a cylindrical lens or another condenser optical system is also possible.

Although the third harmonic having a wavelength of 1064 nm has been described as an example, the present invention has the effect of correcting the overlap between other wavelengths. It is possible to apply. As a fundamental wave, 1064
In addition to nm, laser light having an oscillation wavelength of 1047 nm or 1053 nm oscillated by an Nd: YLiF ₄ laser can be used in any wavelength region where the wavelength can be converted by a nonlinear optical crystal. Further, three or more nonlinear optical crystals may be used as the harmonic generation crystal.

Second Embodiment In the first embodiment, an example has been described in which the focal position is disposed in the second harmonic generation crystal. However, this arrangement is because the nonlinear constant of the third harmonic generation crystal is small. This is suitable for the case where the conversion efficiency is increased by reducing the beam cross-sectional area and increasing the power density in the third harmonic crystal. As shown in FIG. 5, the same effect can be obtained even if the focal position is arranged in the third harmonic generation crystal. Further, although the average output of the third harmonic light is lowered, the effect of the present invention can be exerted even if the focal position is shifted and the crystal is damaged as shown in FIG.

Embodiment 3 This embodiment is an example of an apparatus for generating a fifth harmonic using three second, fourth and fifth harmonic generation crystals. FIG. 7 is a flow sheet showing the configuration of the wavelength converter for generating the fifth harmonic of the third embodiment. The wavelength conversion device for laser light of this embodiment is
, A second harmonic generation crystal 1 arranged at a focal position of a fundamental wave, a fourth harmonic generation crystal 11 and a fifth harmonic generation crystal 12 sequentially arranged rearward and rearward. And The optical axes 3 and 13 of the second harmonic generation crystal 1 and the fourth harmonic generation crystal 11 are set in the same direction, and the spatial overlap of the fundamental wave and the fourth harmonic in the fifth harmonic crystal 12 is reduced. The displacement is corrected by setting the optical axis 14 in the opposite direction, thereby realizing high efficiency of wavelength conversion. Further, depending on the magnitude of the walk-off angle, the optical axes 3 and 13 may be set in opposite directions to each other and the optical axis 14 may be aligned in either direction to obtain an optimum spatial overlap.

[0020]

According to the present invention, the close arrangement of a plurality of harmonic generation crystals and the spatial overlap of laser beams of each wavelength are corrected to some extent by a walk-off angle, so that the number of parts is small and the cost is low. In addition, a wavelength conversion device capable of converting a laser beam into a harmonic having high conversion efficiency and high beam quality is realized.

[Brief description of the drawings]

FIG. 1 is a schematic diagram illustrating a configuration of a wavelength conversion device according to a first embodiment.

FIG. 2 is a graph showing a relationship between a repetition frequency and an average output.

FIG. 3 is a graph showing a relationship between a fundamental wave peak intensity and a THG output intensity ratio.

FIG. 4 is a graph showing the relationship between the crystal spacing and the THG average output.

FIG. 5 is a schematic diagram illustrating a configuration of a wavelength conversion device for laser light according to a second embodiment.

FIG. 6 is a schematic diagram illustrating a configuration of another laser light wavelength converter according to the second embodiment.

FIG. 7 is a schematic diagram illustrating a configuration of a wavelength conversion device for laser light according to a third embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 2nd harmonic generation crystal 2 3rd harmonic generation crystal 3 Optical axis of 2nd harmonic generation crystal 4 Optical axis of 3rd harmonic generation crystal 5 Condensing lens 6 Fundamental wave light 7 2nd harmonic Wave light 8 Third harmonic light 9 Adjustment axis 10 Fifth harmonic light 11 Fourth harmonic generation crystal 12 Fifth harmonic generation crystal 13 Optical axis of fourth harmonic generation crystal 14 Fifth harmonic generation Optical axis of crystal 15 4th harmonic light

Claims (9)

[Claims] An apparatus for wavelength-converting a laser beam as a fundamental wave beam two or more times by a non-linear optical crystal and converting the laser beam into a higher harmonic beam, comprising: a condensing optical system for condensing a fundamental wave; And two or more nonlinear optical crystals having different generation wavelengths, and the optical axis of at least one nonlinear optical crystal is set to be 1 to the fundamental wave propagation direction.
A wavelength conversion device for laser light, wherein the device is rotated by 80 ° and all adjacent crystals are arranged close to each other without an optical system interposed therebetween. 2. The nonlinear optical crystal for converting the wavelength of the fundamental wave for the first time is disposed at the focal position of the fundamental wave to be collected, and the nonlinear optical crystal for performing the wavelength conversion for the second and subsequent times is disposed behind the focal position. 2. The wavelength conversion device for laser light according to claim 1, wherein: 3. The laser beam wavelength converter according to claim 1, wherein the focal position of the fundamental wave is located before the plane of incidence of the nonlinear optical crystal for wavelength conversion for the first time. 4. The laser light of each wavelength to be phase-matched in the nonlinear optical crystal for wavelength conversion for the second time and thereafter is spatially overlapped by the walk-off angle of the nonlinear optical crystal so that the phase is matched. Claim 1 characterized by the following:
The wavelength conversion device for laser light according to any one of claims 1 to 3. 5. The non-linear optical crystal which performs wavelength conversion for the second time and thereafter, the interval between adjacent crystals is 5 mm or less so that the laser beam cross-sectional area of each wavelength to be phase-matched becomes as small as possible. The laser beam wavelength converter according to any one of claims 1 to 4. 6. The method according to claim 1, wherein the second harmonic generation crystal and the third harmonic generation crystal convert the wavelength of the laser light into the third harmonic by using the fundamental light as the laser light. The wavelength conversion device for laser light according to any one of the above. 7. A wavelength conversion to a fourth harmonic by a second harmonic generation crystal, a third harmonic generation crystal, and a fourth harmonic generation crystal using a laser beam as a fundamental wave light. The laser light wavelength converter according to any one of claims 1 to 5. 8. A wavelength conversion to a fifth harmonic by a second harmonic generation crystal, a fourth harmonic generation crystal, and a fifth harmonic generation crystal using laser light as fundamental light. The laser light wavelength converter according to any one of claims 1 to 5. 9. The non-linear optical crystal is LB ₃ O ₅ or β-B
2. The semiconductor device according to claim 1, wherein said substrate is made of aB ₂ O _4.
9. The wavelength conversion device for laser light according to any one of items 1 to 8.