JP2009031684A - Laser device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To correct a change of a beam shape caused by variation and inequality generated in a growth condition and a working process of a nonlinear optical crystal in a laser device. <P>SOLUTION: The laser device is provided with a light source 20, a nonlinear optical crystal 2, a lens group 30 having a positive focal length as a whole in a walk-off direction of the nonlinear optical crystal 2, a cylindrical lens 6 having a positive focal length in a direction nearly orthogonal to the walk-off direction and a cylindrical lens 8 having a positive focal length in the walk-off direction. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a laser device that converts a wavelength using a nonlinear optical crystal.

Laser light is generally higher in frequency than radio waves, so it has a large capacity for information, and has the same wavelength and the same phase, so it has excellent monochromaticity and directivity, and has coherence not found in ordinary light. Yes. Furthermore, since it can be converged very finely, it has the characteristics that energy can be concentrated on a small area and high temperature and high pressure can be realized locally and instantaneously. For this reason, it is applied in various fields such as communication and information, measurement, processing technology, and medical applications.
A wavelength conversion element made of a nonlinear optical crystal capable of converting a wavelength of laser light to generate, for example, a second harmonic, is a laser light source applied to a semiconductor exposure apparatus such as a stepper used for fine processing of semiconductors, for example. And laser light generators that can oscillate stable high-power laser light.

In recent years, regarding such a laser light source and a laser light generator (for example, a light source used in a semiconductor inspection apparatus), research on increasing the output power and shortening the wavelength of laser light has been made.
Various methods are currently being used to achieve higher output and shorter wavelength of laser light. One of the most promising methods is to use output light from a solid-state laser as a nonlinear optical crystal. There is a method of obtaining a laser beam in a short wavelength region, particularly an ultraviolet light region by shortening the wavelength by the wavelength conversion. Solid lasers can be provided more efficiently and cheaply than semiconductor lasers, and are considered to be more reliable than gas lasers using KrF, ArF, or the like.

  Here, for example, the fourth harmonic of a neodymium: yttrium aluminum garnet laser (hereinafter referred to as an Nd: YAG laser) that oscillates a laser beam having a wavelength of 1064 nm, that is, a laser beam having a wavelength of 266 nm is reduced in size. Since it is possible and supplied at a relatively low cost, it is promising as a light source for a next-generation semiconductor inspection apparatus, a light source for mastering a next-generation optical recording medium using recording / reproducing light in the blue wavelength band, and the like.

  In order to actually obtain a deep ultraviolet laser beam having a wavelength of 266 nm, first, a second harmonic having a wavelength of 532 nm is obtained by SHG (Second Harmonic Generation) from a fundamental wave having a wavelength of 1064 nm, and further, by SHG, A method of obtaining a laser beam having a wavelength of 266 nm having a half wavelength, that is, a fourth harmonic is being implemented.

The process of generating the fourth harmonic will be described with reference to FIG.
As shown in FIG. 18, a laser beam La having a wavelength of 1064 nm oscillated from a light source (not shown) such as an Nd: YAG laser oscillator is a nonlinear optical crystal 101, for example, a lithium borate crystal (hereinafter referred to as an LBO crystal). ) At this time, a laser beam Lb having a wavelength of 532 nm, which is the second harmonic, is generated. Next, the laser light Lb emitted from the nonlinear optical crystal 101 passes through another nonlinear optical crystal 102, for example, a β-barium borate crystal (hereinafter referred to as a BBO crystal), and has a wavelength of 266 nm which is the fourth harmonic. Laser light Lc is generated.

  FIG. 19 is an explanatory diagram illustrating a process in which laser light having a wavelength of 532 nm is converted into laser light having a wavelength of 266 nm. In this example, laser light having a wavelength of 532 nm will be described as a fundamental wave, and laser light having a wavelength of 266 nm will be described as its second harmonic. A laser beam Lb having a wavelength of 532 nm, which is a fundamental wave, is incident on the nonlinear optical crystal 103 from the laser beam incident surface 103A and generates a second harmonic 266 nm laser beam in the nonlinear optical crystal 103 (that is, the optical axis direction (that is, It proceeds in the z direction in the figure. The generated laser beam with a wavelength of 266 nm becomes an extraordinary ray in the crystal and thus travels in a direction different from the fundamental wave, that is, in the walk-off direction. For this reason, laser light having a wavelength of 266 nm, which is the second harmonic, causes a walk-off. In FIG. 19, the traveling direction of light along the optical axis is indicated by arrows with the z direction, the walk-off direction as the x direction, and the direction orthogonal to these as the y direction.

  For this reason, the laser beam having a wavelength of 266 nm spreads in the walk-off direction (x direction in the figure) while propagating through the nonlinear optical crystal 103. On the other hand, the direction in which no walk-off occurs (that is, the y direction in the figure) does not substantially expand. Accordingly, as shown in FIG. 19, when the incident laser beam Lb is substantially circular, the laser beam Lc having a wavelength of 266 nm emitted from the exit surface 103B of the nonlinear optical crystal 103 is in the walk-off direction (ie, the laser exit surface 103B) The beam shape is flat in the x direction in the figure.

  The profile of the emitted laser beam having a wavelength of 266 nm has a Gaussian distribution in the y direction in the figure, but has a top hat shape in the x direction. FIG. 20 shows a conceptual diagram in which the beam profile becomes a top hat shape by the walk-off. In FIG. 20, the horizontal axis represents the x direction, and each beam profile at each of the positions p1 to p8 from the incident position p1 that is the incident surface of the nonlinear optical crystal to the output position p8 that is the output surface is shown. Arbitrary unit). The laser beam having a wavelength of 266 nm generated at the incident position p1 has a Gaussian distribution in the vicinity of the incident position p1. Then, a second harmonic wave having a wavelength of 266 nm is generated at each position while the fundamental wave propagates in the optical axis direction toward the emission position p8. The second harmonic laser beam generated at each position proceeds while being shifted in the x direction by walk-off one after another. For this reason, they overlap, resulting in a beam profile having a so-called top hat shape that spreads in the lateral direction as a result at the emission position p8 and becomes constant within a certain range. However, it is not a complete top hat shape, and both ends have a shape on one side of a Gaussian distribution.

The walk-off angle of the BBO crystal at a wavelength of 266 nm is 4.8 °. The length of the crystal is about 6 mm in order to obtain the necessary output of the converted laser beam having a wavelength of 266 nm.
Assuming that the walk-off angle is α and the crystal length is lz, the beam length ωx in the x direction is expressed by the following equation (1).
ωx = tan α × lz (1)
Therefore, the length of the beam in the x direction is tan 4.8 ° × 6 mm = 0.5 mm.

  For example, when a fundamental wave with a wavelength of 532 nm is incident on a BBO crystal with a length of 1z = 6 mm as a circular Gaussian beam with a beam diameter of 90 μm, the beam diameter in the y direction of the generated laser light with a wavelength of 266 nm is about 64 μm, The beam diameter is 0.5 mm as described above. The beam profile in the x direction becomes a shape having a shape of one side of the Gaussian distribution at both ends by the walk-off, that is, the top hat shape described in FIG.

  The laser beam having a wavelength of 266 nm emitted from the BBO crystal has a shape that is longer in the x direction in the near field, with a length in the x direction of 0.5 mm and a length in the y direction of 64 μm. Since the corner is large, the far field has a long shape in the y direction. When used as a light source for an apparatus or the like, it is difficult to use a beam of this shape. For this reason, it is desirable to emit as a beam having an aspect ratio of 1: 1 collimated by the beam shaping optical system.

Conventionally, in order to shape the beam in this way, a beam having an aspect ratio of 1: 1 is obtained by, for example, an anamorphic prism pair (see, for example, Patent Document 1). FIG. 21 shows a configuration diagram when an anamorphic prism pair is used. In this case, the beam emitted from the nonlinear optical crystal 104 passes through the resonator mirror 105, and the beam in the x direction and the y direction is collimated by the lens 106, and x by the anamorphic prisms 107 and 108 (anamorphic prism pair). Only the direction is magnified. Thereby, a beam having an aspect ratio of 1: 1 is obtained.
The anamorphic prism is usually composed of two or four declination prisms, and is fixed after adjusting the angle and position of each prism so as to obtain a preset enlargement or reduction ratio.

JP 2002-62555 A

In the laser apparatus that performs wavelength conversion as described above, the present inventors output even if the beam parameters of the fundamental wave incident on the nonlinear optical crystal, for example, the BBO crystal, the temperature of the nonlinear optical crystal, and the phase matching are the same conditions. It has been found that the second harmonic beam parameter varies. Here, the beam parameter indicates a beam shape, a beam intensity distribution, or a beam divergence angle. This is considered to be because the following (a) to (e) are caused by the crystal growth conditions and processing steps.
(A) Nonuniformity of internal refractive index (b) Variation of optical path length due to processing accuracy of length in optical axis direction (c) Influence of internal scatterer (d) Nonuniformity of internal absorption rate (e) Phase matching Variation in Conditions Such variation in beam parameters is particularly remarkable in the x direction.

  For example, consider the case where the length of the crystal deviates from the design value in the optical axis direction, that is, the case of (b) above. When the length of the designed crystal is 6.0 mm, the beam diameter in the x direction is 0.5 mm as described above. However, if, for example, an error of -0.3 mm occurs in the length of the crystal in the manufacturing process and the length of the crystal becomes 5.7 mm, the beam diameter in the x direction becomes 0.48 mm from Equation 1 and is reduced by 5%. To do. On the other hand, since there is no walk-off in the y direction and the beam diameter of the second harmonic generated is determined by the beam diameter of the fundamental wave in the crystal, it is hardly affected by the length of the crystal. Therefore, for example, if there is an error in the length of the crystal, the output beam deviates from 1: 1 when passing through a beam shaping optical system designed so that the aspect ratio of the beam is 1: 1. In order to correct this and obtain a beam with an aspect ratio of 1: 1, for example, it is conceivable to change the magnification of the anamorphic prism. However, since the anamorphic prism is usually bonded or fixed to a metal frame or the like at a predetermined angle and position, it is difficult to change the magnification according to the variation of the beam.

For these reasons, there is a problem that the aspect ratio cannot be adjusted due to variations in beam parameters in the x direction.
The beam diameter in the x direction also changes due to the other reasons (a), (c), (d), and (e), and the output beam deviates from an aspect ratio of 1: 1.

  In addition, the present inventors have also found that the beam parameter of the second harmonic changes with time between several minutes to several hours under certain conditions, for example, when the output is 500 mW or more. This change with time is also remarkable in the x direction. When a beam shaping optical system using an anamorphic prism is used, even if the beam aspect ratio before the change with time is adjusted to 1: 1, the beam parameter deviates from 1: 1 due to the change with time of the beam parameter. . Since the anamorphic prism cannot change the enlargement ratio as described above, this cannot be corrected.

  Furthermore, the present inventors have found that not only beam parameters but also beam pointing changes. This is because the nonlinear optical crystal absorbs the fundamental wave and the second harmonic to generate a thermal lens effect. As an example, FIG. 22 shows how the beam pointing of the second harmonic fluctuates. As described above, when the beam pointing is changed, for example, the beam is formed by the aperture installed on the optical axis, and the effective power is reduced.

  In view of the above problems, the present invention can correct changes in the beam shape caused by variations or non-uniformity in the growth conditions of the nonlinear optical crystal and processing steps when performing wavelength conversion using the nonlinear optical crystal. It is an object to provide a simple laser device.

  In order to solve the above problems, a laser apparatus according to the present invention includes a light source, a nonlinear optical crystal, a lens group having a positive focal length as a whole in the walk-off direction of the nonlinear optical crystal, and a direction substantially orthogonal to the walk-off direction. A cylindrical lens having a positive focal length and a cylindrical lens having a positive focal length in the walk-off direction.

In the laser apparatus of the present invention, it is desirable that one or more surface intervals of the lens group and the cylindrical lens are variable.
Furthermore, in the laser device of the present invention, a slit extending in a direction substantially perpendicular to the walk-off direction is provided in the vicinity of the beam waist position in the walk-off direction formed inside the laser device.
When this slit is provided, it is desirable that one or more surface intervals of the lens group, the cylindrical lens, and the slit be variable.

The second harmonic emitted from the nonlinear optical crystal in the above-described laser device of the present invention is collected by a lens group having a positive focal length in the walk-off direction as a whole to form a beam waist. Then, it is collimated by a cylindrical lens having a positive focal length in the walk-off direction. Since the profile in the walk-off direction emitted from the nonlinear optical crystal has a top hat shape, the beam formed by the lens group has a shape of a sinc ² x function that is a Fourier transform of the top hat. When this beam waist is collimated by a cylindrical lens having a positive focal length in the walk-off direction, the beam waist is again Fourier transformed to return to a top-hat shaped beam. On the other hand, the component in the direction orthogonal to the walk-off direction is collimated by a cylindrical lens having a positive focal length in the direction orthogonal to the walk-off direction.
Therefore, by adjusting the positions of the above-mentioned lens group and cylindrical lens, it is possible to suppress changes in the shape of the beam due to variations or non-uniformities in the growth conditions and processing steps of the nonlinear optical crystal, and to achieve a good beam shape. It becomes possible to emit the second harmonic.

  Further, in this configuration, when the distance between the surfaces of the lens group and the cylindrical lens is variable, that is, for example, by arranging the lens group on a stage that can be moved and adjusted in the direction along the optical axis. The beam shape can be corrected by the work.

Further, when a slit extending in the direction orthogonal to the walk-off direction is provided in the vicinity of the beam waist position, by passing through this slit, only the peak at the center of the beam is left and the side lobes on both sides can be blocked. Therefore, the shape of the obtained beam can be made close to a Gaussian beam, and a high-quality beam can be output.
In the case where the slit is provided in this way, it is possible to correct the beam shape by a very simple operation by changing the lens group, the cylindrical lens, and one or more surface intervals of the slit. .
According to the laser apparatus of the present invention, in addition to the beam shape change caused by the growth conditions and processing of the nonlinear optical crystal described above, it is also possible to correct the beam shape change caused by the change with time and the output change.

  According to the laser apparatus of the present invention, in a wavelength conversion laser using a nonlinear optical crystal, it is possible to correct a change in beam shape caused by growth conditions or processing of the nonlinear optical crystal and adjust it to a desired aspect ratio.

Examples of the best mode for carrying out the present invention will be described below, but the present invention is not limited to the following examples.
[1] First Embodiment First, the configuration and operation of a laser apparatus according to a first embodiment of the present invention will be described with reference to FIG. As shown in FIG. 1, the laser device 50 includes a light source 20 that emits a fundamental laser beam and a nonlinear optical crystal 2. In FIG. 1 and the following drawings, the walk-off direction of the nonlinear optical crystal 2 is shown as the x direction, and the direction orthogonal to this is along the y direction and along the optical axis, as in FIGS. 19 and 21 described above. The traveling direction of light is shown as the z direction. In the laser device 50, the lens group 30 having a positive focal length as a whole in the walk-off direction of the nonlinear optical crystal 2, that is, the x direction, in this example, the lenses 3 and 4 are substantially orthogonal to the walk-off direction. A cylindrical lens 6 having a positive focal length in the direction, that is, the y direction, and a cylindrical lens 8 having a positive focal length in the walk-off direction, that is, the x direction are provided.
In this example, a lens 10 that condenses the fundamental wave emitted from the light source 20 on the nonlinear optical crystal 2 is disposed between the light source 20 and the nonlinear optical crystal 2.
In this case, a slit 7 extending in the direction substantially orthogonal to the walk-off direction, that is, the y-direction, is provided in the vicinity of the beam waist position in the walk-off direction formed inside the laser device.

As the nonlinear optical crystal used in the laser apparatus of the present invention, a general nonlinear optical crystal or a nonlinear optical element obtained by subjecting the nonlinear optical crystal to processing such as periodic polarization inversion processing can be used. Examples of wavelength conversion include SHG (second harmonic generation), THG (third harmonic generation), etc., or sum frequency generation, optical parametric oscillation, and the like. The materials used include KTiOPO ₄ (KTP), β-BaB ₂ O ₄ (BBO), LiB ₃ O ₅ (LBO), LiTaO ₃ , LiNbO ₃ , their congruent (uniform melt) composition, their stoichiometric properties. Examples include a metric (stoichiometric) composition and materials to which additives such as Mg and Zn are added.

For example, C—LiNbO ₃ , C—LiTaO ₃ , S—LiNbO ₃ , S—LiTaO ₃ , MgO: C—LiNbO ₃ , MgO: C—LiTaO ₃ , ZnO: C—LiNbO ₃ , ZnO: C—LiTaO ₃ , Crystal materials such as MgO: S—LiNbO ₃ , MgO: S—LiTaO ₃ , ZnO: S—LiNbO ₃ , ZnO: S—LiTaO ₃ can be used.
They also in Hodokoshita polarization reversal _{processing, PP-C-LiNbO 3,} PP-C-LiTaO 3, PP-S-LiNbO 3, PP-S-LiTaO 3 (PPLST), PP-MgO: C-LiNbO 3 _{, PP-MgO: C-LiTaO} 3, PP-ZnO: C-LiNbO 3, PP-ZnO: C-LiTaO 3, PP-MgO: S-LiNbO 3, PP-MgO: S-LiTaO 3, PP-ZnO: _{S-LiNbO 3, PP-ZnO} : it can be mentioned crystalline element such as S-LiTaO 3, PP-KTiOPO 4.

  Here, “C” means “congruent composition” and “S” means “Stoichiometric composition”. “PP” means “Periodical Poling”, and a nonlinear optical element having a periodically poled structure can be obtained by controlling the periodically polarized light by applying voltage to the nonlinear optical crystal. These materials are processed at an appropriate angle satisfying the phase matching condition according to the wavelength used, or the (pseudo) phase matching condition is satisfied by making an appropriate periodic polarization inversion structure.

  In the laser apparatus 50 having the above-described configuration, the fundamental laser beam emitted from the light source 20 is focused on the nonlinear optical crystal 2 by the lens 10. Here, the fundamental wave is wavelength-converted to become a second harmonic, and is emitted from the exit surface of the nonlinear optical crystal 2. At this time, the second harmonic is generated in the walk-off direction at each position when traveling in the nonlinear optical crystal 2 as described above with reference to FIG. The profile has a shape close to a top hat. In FIG. 2, the beam intensity of the top hat shape is three-dimensionally represented.

The propagation of the wavelength-converted laser beam is as follows. First, the x-direction beam of the wavelength-converted beam, that is, the walk-off component beam will be described. In this case, the beam emitted from the exit surface of the nonlinear optical crystal 2 is condensed by the lens group 30 of the combination of the positive cylindrical lens 3 and the negative cylindrical lens 4 having curvature in the x direction, and the beam waist is indicated by a point W. Form in position. Then, it is collimated by a positive cylindrical lens 8 having a curvature in the x direction. Since the profile in the x direction emitted from the nonlinear optical crystal 2 has the top hat shape described with reference to FIG. 20, the beam formed by the lens group 30 by the combination of the cylindrical lenses is sinc ² which is the Fourier transform of the top hat. The shape is an x function. This beam shape is schematically shown in FIG. 3A. When this beam waist is collimated by the cylindrical lens 8, it is again Fourier transformed to return to a top hat shaped beam.

In the beam waist position W, only the peak at the center of the beam shown in FIG. 3A is left, and small peaks (side lobes) on both sides are blocked by the slits 7 extending in the y direction as shown in FIG. 3B. Thus, the shape is close to that of a Gaussian beam from which side lobes have been eliminated. By doing so, the beam ahead is propagated in the same manner as the Gaussian beam, and a high-quality beam can be obtained.
The slit may be in the vicinity of the beam waist position, that is, may be slightly deviated from the beam waist position as long as the side lobe described above can be erased.

  Next, beam propagation of a component in the direction perpendicular to the y direction, that is, the walk-off direction will be described. The beam emitted from the exit surface of the nonlinear optical crystal 2 is collimated by a positive cylindrical lens 6 having a curvature in the y direction.

FIG. 4 shows a three-dimensional representation of the beam intensity when the beam diameters in the x and y directions are equal, that is, the aspect ratio is 1: 1. The beam diameter is a beam diameter that is 1 / e ² with respect to the peak power density.
The beams in the x direction and the y direction pass through cylindrical lenses having curvatures in the y direction and the x direction, respectively, but there is no condensing or diverging effect on components orthogonal to the direction having the curvature. Is omitted.

  By the way, if the lens interval L of the lens group 30 having the positive and negative cylindrical lenses 3 and 4 having curvature in the x direction is variable, the combined focal length in the x direction can be changed. Even if the beam diameter in the x direction of the second harmonic beam emitted from the nonlinear optical crystal 2 varies, the beam waist diameter formed at the position W where the slit 7 is provided is adjusted to a certain value (design value). be able to. If the beam waist diameter in the slit 7 is constant, the beam diameter collimated by the collimating lens 8 is also constant, so that the ratio (aspect ratio) of the collimated beam diameter in the x and y directions can be easily set to a desired value. (For example, 1: 1).

For example, when the crystal has non-uniformity in the internal refractive index, the top hat shape may be disturbed, resulting in an asymmetric beam shape. An example of the beam intensity distribution in this case is shown in FIG. When a normal top hat-shaped beam is condensed, the shape of the sinc ² x function, which is the Fourier transform of the top hat, is obtained as shown in FIG. 3A, but when an asymmetric beam as shown in FIG. 5 is condensed. As shown in FIG. 6A, an asymmetric intensity distribution deviating from the sinc ² x function is obtained. Asymmetry appears mainly as a difference in intensity between the left and right small peaks called side lobes. This side lobe is shielded by a slit 7 extending in the y direction as shown in FIG. 6B, and only the peak at the center is taken out, thereby obtaining a high-quality beam with good intensity distribution symmetry as shown in FIG. 6C. be able to.
Thus, by providing the slit, it is possible to prevent the beam quality from being deteriorated due to the nonuniformity of the internal refractive index. In addition, the temporal change occurring in the range of several minutes to several hours of beam pointing of the second harmonic can be corrected by the above configuration, and the symmetry of the intensity distribution can be improved.

In addition, when the time-dependent change of the second harmonic beam pointing is large, or for a relatively large variation due to distortion of the resonator housing, the variation of the beam pointing emitted from the laser device by the following procedure Can be eliminated.
First, correction is performed by changing the temperature of the crystal. That is, a temperature control means such as a heat sink and a Peltier element for controlling the temperature of the nonlinear optical crystal 2 is provided. Thereby, a certain amount of beam pointing fluctuation can be suppressed.
Further, the beam pointing direction is detected by providing detection means such as a quadrant photodiode. Then, the detected beam pointing displacement is corrected by moving the lens constituting the beam shaping optical system. For this movement, for example, a moving mechanism for moving in a plane perpendicular to the optical axis of the lens may be provided, and the moving amount corresponding to the detected displacement may be fed back as the moving amount. That is, the lens is decentered by an appropriate amount so as to suppress fluctuations in beam pointing. By canceling the fluctuation of the beam emitted from the crystal due to the eccentricity of the lens, it is possible to avoid or sufficiently suppress the temporal change of the beam pointing finally emitted from the laser device.

  Furthermore, the divergence angle of the second harmonic may change due to the thermal lens effect, but this can be detected by monitoring the signal of the photodiode. Similar to the above-described method, for example, the change in the divergence angle can be corrected by changing the lens interval.

As described above, according to the laser device of the present embodiment, the temperature of the nonlinear optical crystal, the detection means for detecting the displacement of the beam, and the optical axis can be changed by changing the lens interval. By providing a moving means to move the lens etc. in a vertical plane and controlling the amount of movement of the moving means by feeding back the detected displacement, the beam parameter of the beam finally emitted from the laser device can be kept constant. Can do.
The distance of the lens may be adjusted by fixing each part by a detachable fixing method such as screwing without using an adhesive or the like, and may be fixed again at the time of adjustment. Also, it is possible to provide a configuration such as providing a rail having mobility in the direction along the optical axis and arranging each part thereon, or arranging each part on a motor stage that moves in the direction along the optical axis, In this case, the distance between the lenses can be easily adjusted.

[2] Second Embodiment FIG. 7 is a schematic configuration diagram of a laser apparatus according to a second embodiment of the present invention. In FIG. 7, parts corresponding to those in FIG. In this example, a case where the nonlinear optical crystal 2 is arranged in the resonator 1 having a so-called bow-tie type external resonator configuration in which four mirrors are arranged crossing each other is shown. Laser light is incident on the resonator 1 from a light source having a wavelength of 532 nm (not shown). As the output of the laser beam, for example, a continuous laser of 1 W can be used. As the nonlinear optical crystal 2, for example, a BBO crystal is used, and the length lz in the optical axis direction is set to, for example, 6.0 mm.
The resonator 1 includes an input mirror 11, an output mirror 12, and mirrors 13 and 14. The output mirror 12 is composed of a plano-convex lens having a concave surface facing the inside of the resonator 1 and a surface opposite to the concave surface. The radius of curvature is 100 mm, the thickness is 3 mm, and the material is synthetic quartz, which is installed with an inclination of 10 ° with respect to the optical axis in the x direction, that is, the walk-off direction of the nonlinear optical crystal 2. The output mirror 12 constitutes the resonator 1 and allows a wavelength-converted laser beam having a wavelength of 266 nm, for example, emitted from the nonlinear optical crystal 2 to pass outside the resonator 1.

Cylindrical lenses 4 and 6, a slit 7, and a cylindrical lens 8 are disposed on the external optical path from which the second harmonic is emitted from the output mirror 12. The cylindrical lens 4 is a plano-convex cylindrical lens having a curvature in the x direction with a convex surface directed toward the resonator side 1. For example, the curvature radius is 33 mm, the thickness is 2 mm, and the material is synthetic quartz.
The cylindrical lens 6 is a plano-convex cylindrical lens having a curvature in the y direction with the plane facing the resonator, and has a curvature radius of 71.9 mm, a thickness of 2 mm, and a synthetic quartz material.

The slit 6 is a slit that is disposed in the vicinity of the beam waist position W and extends in the y direction. The width in the x direction is 0.1 mm. A beam waist in the x direction is formed here.
The cylindrical lens 7 is a plano-convex cylindrical lens having a curvature in the x direction with the plane facing the resonator side, and has a radius of curvature of 90 mm, a thickness of 2 mm, and a material of synthetic quartz.

Here, the distance from the nonlinear optical crystal 2 to the output mirror 12 is a1, the distance from the output mirror 12 to the cylindrical lens 4 is a2, the distance from the cylindrical lens 4 to the cylindrical lens 6 is a3, and the slit from the cylindrical lens 6 to the slit Assuming that the distance to 7 is a4, the distance from the slit 7 to the cylindrical lens 8 is a5, and the refractive index of synthetic quartz at a wavelength of 266 nm is n, it is as follows.
a1: 75mm
a2: Variable a3: Variable a4: Variable a5: 179mm
n: 1.499683
Here, an example of values of the distances a2, a3, and a4 that can be taken with respect to the length lz of the nonlinear optical crystal 2 is shown in Table 1 below. When lz = 6 mm, a2 = 35 mm, a3 = 44 mm, and a4 = 38 mm.
Here, the distance is the distance between the lenses between the lenses, the distance from the nonlinear optical crystal is the distance from the exit surface, and the distance from the slit is the center point in the slit, in this case the beam waist position. The interval with W.

  The nonlinear optical crystal 2, in this case, the BBO crystal, is disposed at a position where the beam diameter of the wavelength of 532 nm is the smallest in the resonator 1, that is, at the position of the beam waist. Here, the beam shape with a wavelength of 532 nm is circular, and the beam diameter is 90 μm. The beam diameter in the x direction at a wavelength of 266 nm generated by wavelength conversion by the BBO crystal is 0.5 mm from the above formula (1), and the beam diameter in the y direction is 64 μm. The shape of the beam emitted from the nonlinear optical crystal 2 is shown in FIG. A beam having a wavelength of 266 nm passes through the output mirror 13 and is emitted to the outside of the resonator 1.

  The beam propagation of the component in the x direction of the beam emitted from the resonator will be described. The lens group of the output mirror 13 which is a spherical mirror having a negative lens action and the positive cylindrical lens 4 having a curvature in the x direction. 30, a beam waist in the x direction is formed at the position W where the slit 7 is disposed. The beam waist diameter in the x direction is 70 μm. The side lobe is removed by the slit 7 arranged here, similarly to the example described in FIG. Next, the beam is collimated by a cylindrical lens 8 into a beam having a beam diameter of 1 mm.

  On the other hand, the beam in the y direction is collimated by the cylindrical lens 6 into a beam having a beam diameter of 1 mm. Therefore, a beam with an aspect ratio of 1: 1 can be obtained. The obtained beam shape is shown in FIG.

  Here, for example, a case where a manufacturing error occurs in the length lz of the nonlinear optical crystal 2 in the optical axis direction and lz = 5.7 mm will be described. The beam of 266 nm generated by wavelength conversion by the nonlinear optical crystal 2 has a beam diameter in the x direction of 0.475 mm from the above equation (1), and the beam propagation changes. However, if the values of the distances a2, a3 and a4 are changed as shown in Table 1, the beam waist diameter in the x direction formed at the beam waist position W becomes 70 μm, as in the case where no error occurs. Thereafter, the beam propagates in the same manner, and a beam with a beam diameter of 1 mm and an aspect ratio of 1: 1 is obtained in both the x and y directions.

  On the other hand, if the distances a3, a4, and a5 are not changed, the beam diameter in the x direction is 0.95 mm, the beam diameter in the y direction is 1 mm, and the aspect ratio is 1: 0.95.

Further, a case where a manufacturing error occurs in the length of the nonlinear optical crystal 2 in the optical axis direction and lz = 6.3 mm will be described. A beam of 266 nm generated by wavelength conversion by a nonlinear optical crystal has a beam diameter in the x direction of 0.525 mm from the above equation (1), and the beam propagation changes. However, when the values of a3, a4, and a5 are changed as shown in Table 1, the beam waist diameter in the x direction formed at the position W becomes 70 μm, as in the case where no error occurs, and thereafter, the propagation propagates similarly. A beam with a beam diameter of 1 mm and an aspect ratio of 1: 1 can be obtained in both the x and y directions.
On the other hand, if a3, a4, and a5 are not changed, the beam diameter in the x direction is 1.05 mm, the beam diameter in the y direction is 1 mm, and the aspect ratio is 1: 1.05.

  Therefore, by adjusting the distance a2 between the lenses of the lens group 30 and the distances a3 and a4 between the lens 6 and the slit 7, the aspect ratio can be corrected to a desired value and a good beam shape can be obtained.

[3] Third Embodiment FIG. 10 is a schematic configuration diagram of a laser apparatus according to a third embodiment of the present invention. 10, parts corresponding to those in FIG. 1 and FIG.
This example is also an example in which the nonlinear optical crystal 2 is provided in the bow-tie resonator 1, and laser light is incident from a laser light source having a wavelength of 532 nm not shown in the drawing. As the laser light, for example, continuous light with an output of 1 W can be used. As the nonlinear optical crystal 2, for example, a BBO crystal is used, and the length lz in the optical axis direction is 6.0 mm. The output mirror 13 of the resonator 1 has a concave surface facing inward, and the opposite surface is a plane. The radius of curvature is 100 mm, the thickness is 3 mm, the material is synthetic quartz, and the x direction relative to the optical axis, that is, the walk-off direction. Installed at an angle of 10 °.

Also in this example, the output mirror 13 constitutes the resonator 1 and allows the wavelength-converted 266 nm beam emitted from the nonlinear optical crystal 2 to pass outside the resonator 1. The cylindrical lens 4 is a plano-convex cylindrical lens having a curvature in the x direction with the plane directed toward the resonator 1, and has a radius of curvature of 25 mm, a thickness of 2 mm, and a synthetic quartz material.
The cylindrical lens 5 is a plano-concave cylindrical lens having a curvature in the x direction with the concave surface facing the resonator 1, and has a radius of curvature of 25 mm, a thickness of 2 mm, and a synthetic quartz material.

The cylindrical lens 6 is a plano-convex cylindrical lens having a curvature in the y direction with the plane directed toward the resonator 1, and has a radius of curvature of 71.9 mm, a thickness of 2 mm, and a material of synthetic quartz.
The slit 7 is a slit that is disposed at the beam waist position W in the x direction and extends in the y direction. The width in the x direction is 0.1 mm. A beam waist in the x direction is formed here.
The cylindrical lens 8 is a plano-convex cylindrical lens having a curvature in the x direction with the plane facing the resonator side, and has a radius of curvature of 90 mm, a thickness of 2 mm, and a synthetic quartz material.

Here, the distance from the nonlinear optical crystal 2 to the output mirror 12 is b1, the distance from the output mirror 12 to the cylindrical lens 4 is b2, the distance from the cylindrical lens 4 to the cylindrical lens 5 is b3, and the cylindrical lens 5 to the cylindrical lens 5 is cylindrical. Assuming that the distance to the lens 6 is b4, the distance from the cylindrical lens 6 to the slit 7 is b5, the distance from the slit 7 to the cylindrical lens 8 is b6, and the refractive index of synthetic quartz at a wavelength of 266 nm is n, .
b1: 63mm
b2: 35mm
b3: variable b4: variable b5: variable b6: 179 mm
n: 1.499683
Here, an example of values of the distances b3, b4, and b5 that can be taken with respect to the length lz of the nonlinear optical crystal 2 is shown in Table 2 below. When lz = 6 mm, b3 = 45 mm, b4 = 6.5 mm, and b5 = 14 mm.

The nonlinear optical crystal 2, in this case, the BBO crystal, is disposed at the position where the beam diameter of 532 nm is the smallest in the resonator 1, that is, at the position of the beam waist. Here, the beam shape of the laser having a wavelength of 532 nm is circular, and the beam diameter is 90 μm. The beam diameter in the x direction of laser light having a wavelength of 266 nm generated by wavelength conversion by the BBO crystal is 0.5 mm from the above formula (1), and the beam diameter in the y direction is 64 μm. The shape of the beam emitted from the nonlinear optical crystal 2 is shown in FIG. A laser beam having a wavelength of 266 nm passes through the output mirror 12 and is emitted out of the resonator 1.
The beam propagation of the component in the x direction of the beam emitted from the resonator 1 will be described. The output mirror 12, which is a concave mirror having the function of a negative lens, the positive cylindrical lens 4 having a curvature in the x direction, and the x direction. A beam waist in the x direction is formed at the position W by the positive cylindrical lens 5 having a curvature. The beam waist diameter in the x direction is 70 μm. The side lobes are removed by the slits 7 disposed in the same manner as in the example described in FIG. Next, the beam is collimated by a cylindrical lens 8 into a beam having a beam diameter of 1 mm.

  On the other hand, the beam of the component in the y direction is collimated into a beam having a beam diameter of 1 mm by the cylindrical lens 6 from the beam emitted from the resonator 1. Therefore, a beam with an aspect ratio of 1: 1 can be obtained. The obtained beam shape is shown in FIG.

Here, for example, a case where a manufacturing error occurs in the length lz of the nonlinear optical crystal 2 in the optical axis direction and lz = 5.7 mm will be described. A beam with a wavelength of 266 nm generated by wavelength conversion by the nonlinear optical crystal 2 has a beam diameter in the x direction of 0.475 mm from the above equation (1), and the beam propagation changes. However, when the values of the distances b3, b4, and b5 are changed as shown in Table 2, the beam waist diameter in the x direction formed at the position W becomes 70 μm as in the case where no error occurs, and thereafter Propagates in the same way. Therefore, a beam with an aspect ratio of 1: 1 having a beam diameter of 1 mm is obtained in both the x and y directions.
On the other hand, when the distances b3, b4, and b5 are not changed, the beam diameter in the x direction is 0.95 mm, the beam diameter in the y direction is 1 mm, and the aspect ratio is 1: 0.95.

Further, a case where a manufacturing error occurs in the length lz of the nonlinear optical crystal 2 in the optical axis direction and lz = 6.3 mm will be described. The beam of 266 nm generated by wavelength conversion by the nonlinear optical crystal 2 has a beam diameter in the x direction of 0.525 mm from the above equation (1), and the beam propagation changes. However, if the values of b3, b4, and b5 are changed as shown in Table 2 above, the beam waist diameter in the x direction formed at the position W becomes 70 μm, as in the case where no error occurs, and thereafter the same Propagate. As a result, a beam with an aspect ratio of 1: 1 having a beam diameter of 1 mm is obtained in both the x and y directions.
Even in this case, if the distances b3, b4, and b5 are not changed, the beam diameter in the x direction is 1.05 mm, the beam diameter in the y direction is 1 mm, and the aspect ratio is 1: 1.05.

  Therefore, by adjusting the distance b3 between the lenses of the lens group 30 and the distances b4 and b5 between the lenses 5 and 6 and the slit 7, the aspect ratio can be corrected to a desired value and a good beam shape can be obtained. .

[3] Fourth Embodiment FIG. 13 is a schematic configuration diagram of a laser apparatus according to a fourth embodiment of the present invention. In FIG. 13, parts corresponding to those in FIGS. 1, 7, and 10 are given the same reference numerals, and redundant description is omitted.
In this example, laser light is incident on the nonlinear optical crystal 2 from a laser light source having a wavelength of 532 nm not shown in the drawing. For example, pulsed light with an average output of 1 W can be used as the laser light, and BBO crystal can be used as the nonlinear optical crystal 2. The length lz of the nonlinear optical crystal 2 in the optical axis direction is 6.0 mm. Cylindrical lenses 4 and 5, a slit 7, and cylindrical lenses 6 and 8 are disposed on the outgoing optical path of the nonlinear optical crystal 2.

  The cylindrical lens 4 is a plano-convex cylindrical lens having a curvature in the x direction with a convex surface facing the nonlinear optical crystal 2, and has a radius of curvature of 27 mm, a thickness of 2 mm, and a material of synthetic quartz. The cylindrical lens 5 is a plano-concave cylindrical lens having a curvature in the x direction with a concave surface facing the nonlinear optical crystal 2 side. The curvature radius is 25 mm, the thickness is 2 mm, and the material is synthetic quartz. The slit 7 extending in the y direction arranged at the position W has a width in the x direction of 0.12 mm. A beam waist in the x direction is formed at this position W.

  The cylindrical lens 6 is a plano-convex cylindrical lens having a curvature in the y direction with the plane directed to the nonlinear optical crystal 2 side. The curvature radius is 98.5 mm, the thickness is 2 mm, and the material is synthetic quartz. The cylindrical lens 8 is a plano-convex cylindrical lens having a curvature in the x direction with the plane directed to the nonlinear optical crystal 2 side. The curvature radius is 99 mm, the thickness is 2 mm, and the material is synthetic quartz.

The distance from the nonlinear optical crystal 2 to the cylindrical lens 4 is c1, the distance from the cylindrical lens 4 to the cylindrical lens 5 is c2, the distance from the cylindrical lens 5 to the slit 7 is c3, and the distance from the slit 7 to the cylindrical lens 6 is c4. When the distance from the cylindrical lens 6 to the cylindrical lens 8 is c5 and the refractive index of synthetic quartz at a wavelength of 266 nm is n, the following is assumed.
c1 = 85.7 mm
c2: Variable c3: Variable c4: Variable c5: Variable n = 1.499683
Here, an example of values of distances c2, c3, c4 and c5 that can be taken with respect to the length lz of the nonlinear optical crystal 2 is shown in Table 3 below. When lz = 6 mm, c2 = 34.5 mm, c3 = 27 mm, c4 = 40.5 mm, and c5 = 150 mm.

  In this case, a laser beam having a wavelength of 532 nm from a laser light source (not shown) is incident on the nonlinear optical crystal 2 to form a circular beam waist therein, and the beam diameter is 90 μm. The beam diameter in the x direction of laser light having a wavelength of 266 nm generated by wavelength conversion by the nonlinear optical crystal 2 is 0.5 mm from the above formula (1), and the beam diameter in the y direction is 64 μm. The beam shape of the second harmonic light emitted from the nonlinear optical crystal 2 is shown in FIG.

The beam propagation of the component in the x direction of the beam emitted from the nonlinear optical crystal 2 will be described. A beam waist in the x direction is formed at the position W by the positive and negative cylindrical lenses 4 and 5 having a curvature in the x direction. . The beam waist diameter in the x direction is 77 μm. The side lobe is removed by the slit having a width of 100 μm arranged in the same manner as in the example described in FIG. Next, the beam is collimated by a cylindrical lens 8 into a beam having a beam diameter of 1 mm.
On the other hand, the beam in the y-direction component is collimated by the cylindrical lens 6 into a beam having a beam diameter of 1 mm. Therefore, a beam with an aspect ratio of 1: 1 can be obtained. The obtained beam shape is shown in FIG.

Here, for example, a case where a manufacturing error occurs in the length lz of the nonlinear optical crystal 2 in the optical axis direction and lz = 5.7 mm will be described. A beam having a wavelength of 266 nm generated by wavelength conversion by the nonlinear optical crystal 2 has a beam diameter of 0.475 mm in the x direction from the above formula (1), and the beam propagation changes. However, if the values of the distances c2, c3, c4 and c5 are changed as shown in Table 3 above, the beam waist diameter in the x direction formed at the position W becomes 77 μm, as in the case where no error occurs. After that, it propagates in the same way. Therefore, a beam with an aspect ratio of 1: 1 having a beam diameter of 1 mm is obtained in both the x and y directions.
On the other hand, if the distances c2, c3, c4, and c5 are not changed, the beam diameter in the x direction is 0.95 mm, the beam diameter in the y direction is 1 mm, and the aspect ratio is 1: 0.95.

Further, a case where a manufacturing error occurs in the length lz of the nonlinear optical crystal 2 in the optical axis direction and lz = 6.3 mm will be described. The beam having a wavelength of 266 nm generated by wavelength conversion by the nonlinear optical crystal 2 has a beam diameter in the x direction of 0.525 mm from the above formula (1), and the beam propagation changes. However, when the values of the distances c2, c3, c4 and c5 are changed as shown in Table 3, the beam waist diameter in the x direction formed at the position W becomes 77 μm, as in the case where no error occurs, and thereafter Propagates in the same way. Also in this case, a beam with a beam diameter of 1 mm and an aspect ratio of 1: 1 can be obtained in both the x and y directions.
On the other hand, if the distances c2, c3, c4, and c5 are not changed, the beam diameter in the x direction is 1.05 mm, the beam diameter in the y direction is 1 mm, and the aspect ratio is 1: 1.05.

  Therefore, also in this example, by adjusting the distance c2 between the lenses of the lens group 30, the distance c3 between the lens 5 and the slit 7, the distance c4 between the slit 7 and the lens 6, and the distance c5 between the lenses 6 and 8. The aspect ratio can be corrected to a desired value to obtain a good beam shape.

In the present invention, it goes without saying that in addition to the examples described in the above embodiments, various other optical elements such as a mirror for bending the optical path may be added.
For example, in FIG. 16, as a configuration similar to that of the first embodiment described in FIG. 7, a schematic configuration diagram in the case of providing a mirror 9 that bends the optical path by approximately 90 ° between the slit 7 and the cylindrical lens 8 is illustrated. Indicates. In FIG. 16, parts corresponding to those in FIG.
Also in this example, similarly to the example shown in FIG. 7 described above, by changing the distances a <b> 2 to a <b> 4, it is possible to correct a change in the beam profile and obtain a good beam shape.

Furthermore, in the present invention, as described above, the lens or the like may be moved in a plane perpendicular to the optical axis when a relatively large change in beam pointing occurs due to distortion of the housing or the like. FIG. 17 shows a schematic configuration diagram of an example of the laser apparatus in this case. In FIG. 17, parts corresponding to those in FIGS. 6 and 16 are denoted by the same reference numerals, and redundant description is omitted.
As shown in FIG. 17, in this example, a concave lens 21 is arranged between the slit 7 and the mirror 9, and a convex lens 22 is arranged between the mirror 9 and the cylindrical lens 8, and these are in an in-plane orthogonal to the optical axis. It can be moved with Although not shown, various driving devices such as a stepping motor and a biaxial actuator can be used as the moving means.
Further, the mirror 9 is a semi-transmission mirror, for example, and a part thereof is received by the detection means 23 such as a quadrant photodiode, and the control device 24 detects the amount of movement of the lenses 21 and 22 in the directions indicated by the arrows m1 and m2, for example. It is also possible to use a configuration in which signals Sm1 and Sm2 are output to a moving means (not shown), that is, a configuration in which variations in beam pointing are fed back.
With such a configuration, the lens is decentered by the moving means, thereby making it possible to correct larger fluctuations in beam pointing.

  As described above, according to the present invention, a beam having a desired aspect ratio (for example, 1: 1) can always be obtained even if the beam diameter in the walk-off direction changes due to a manufacturing error of the nonlinear optical crystal. Of course, it is possible to cope with changes in the divergence angle in the same manner. It is possible to correct the change with time of the pointing due to the thermal lens effect by providing a slit. Furthermore, for a relatively large beam pointing shift caused by the distortion of the housing, the lens is moved in a plane perpendicular to the optical axis, so that the lens is decentered. The deviation can be eliminated, and the beam pointing can always be kept constant.

  The present invention is not limited to the configuration described in the above-described embodiment, and various modifications and changes can be made without departing from the configuration of the present invention. The converted wave converted by the nonlinear optical crystal is not limited to the second harmonic wave, but can be various wavelength conversion modes such as a third harmonic wave, sum frequency mixing, and parametric oscillation.

It is a schematic block diagram of the laser apparatus which concerns on embodiment of this invention. It is a figure which shows an example of the beam profile of a 2nd harmonic. FIGS. 8A to 8C are explanatory diagrams for explaining a beam shape correction mode in the laser apparatus according to the embodiment of the present invention. FIGS. It is a figure which shows an example of the beam profile of a 2nd harmonic. It is a figure which shows an example of the beam profile of a 2nd harmonic. FIGS. 8A to 8C are explanatory diagrams for explaining a beam shape correction mode in the laser apparatus according to the embodiment of the present invention. FIGS. It is a schematic block diagram of the laser apparatus which concerns on embodiment of this invention. It is a figure which shows an example of the beam profile of a 2nd harmonic. It is a figure which shows an example of the beam profile of a 2nd harmonic. It is a schematic block diagram of the laser apparatus which concerns on embodiment of this invention. It is a figure which shows an example of the beam profile of a 2nd harmonic. It is a figure which shows an example of the beam profile of a 2nd harmonic. It is a schematic block diagram of the laser apparatus which concerns on embodiment of this invention. It is a figure which shows an example of the beam profile of a 2nd harmonic. It is a figure which shows an example of the beam profile of a 2nd harmonic. It is a schematic block diagram of the laser apparatus which concerns on embodiment of this invention. It is a schematic block diagram of the laser apparatus which concerns on embodiment of this invention. It is a schematic block diagram of an example of the conventional laser apparatus. It is a figure which shows the propagation aspect of the conversion wave in a nonlinear optical crystal. It is a conceptual diagram of the walk-off in a nonlinear optical crystal. It is a schematic block diagram of the position English of the conventional laser apparatus. It is a figure which shows the time-dependent change of beam pointing.

Explanation of symbols

  1. Resonator, 2. 2. nonlinear optical crystal; 3. Cylindrical lens, 4. Cylindrical lens, Cylindrical lens, 6. 6. Cylindrical lens, Slit, 8. Cylindrical lens, mirror, 10. Lens, 11. Mirror, 12. Output mirror, 13. Mirror, 14. Mirror, 20. Light source, 30. Lens group

Claims (6)

A light source;
A nonlinear optical crystal;
A lens group having a positive focal length as a whole in the walk-off direction of the nonlinear optical crystal;
A cylindrical lens having a positive focal length in a direction substantially orthogonal to the walk-off direction;
And a cylindrical lens having a positive focal length in the walk-off direction.   The laser device according to claim 1, wherein one or more surface intervals of the lens group and the cylindrical lens are variable.   2. The laser device according to claim 1, wherein a slit extending in a direction substantially orthogonal to the walk-off direction is provided in the vicinity of a beam waist position in the walk-off direction formed inside the laser device.   4. The laser device according to claim 3, wherein one or more surface intervals of the lens group, the cylindrical lens, and the slit are variable.   2. The laser device according to claim 1, further comprising an adjustment mechanism that allows one or more optical elements provided in the laser device to move in a plane perpendicular to the optical axis.   A part of the converted light emitted from the nonlinear optical crystal is received to detect a change in a beam profile, and a movement amount by the adjusting mechanism is controlled in response to the change in the beam profile. Item 6. The laser device according to Item 5.

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