Walking-off compensation device for laser frequency tripling

Abstract

The invention discloses a walk-off compensation device for laser frequency tripling, and relates to the technical field of laser, and the device comprises a fundamental frequency light source which is used for generating fundamental frequency laser; the frequency doubling unit is used for converting the fundamental frequency laser into frequency doubling laser; the frequency tripling unit comprises a frequency tripling nonlinear crystal and is used for converting the frequency-doubled laser and the residual fundamental frequency laser into frequency tripled laser; the walk-off pre-compensation unit is arranged between the frequency doubling unit and the frequency tripling unit, the walk-off pre-compensation unit comprises a spherical lens which is eccentrically arranged, the spherical lens is used for enabling the frequency doubling laser and the residual fundamental frequency laser which is not completely converted to generate a preset included angle, and the preset included angle is matched with a walk-off angle generated in the frequency tripling nonlinear crystal; and the light beam shaping unit is used for outputting ultraviolet laser. According to the invention, a complex pre-compensation optical element does not need to be added in front of the frequency tripling unit, walk-off compensation can be realized only through the eccentrically placed spherical lens, and the structure of the optical system is greatly simplified.

Description

Technical Field

[0001] The invention relates to the field of laser technology, and in particular to a walk-off compensation device for laser frequency tripling. Background Art

[0002] The triple frequency system is mainly used to output ultraviolet laser, which is usually obtained from infrared laser through a series of nonlinear frequency conversion technologies. For example, 1064nm laser generates second harmonic 532nm laser through frequency doubling, and 532nm laser and residual 1064nm laser that does not participate in frequency doubling generate 355nm laser. Continuous or quasi-continuous 355nm laser can be widely used in semiconductor wafer slitting and wafer defect detection.

[0003] High repetition rate quasi-continuous 355nm laser has high repetition frequency, low single pulse energy and low harmonic conversion efficiency. In order to improve the harmonic efficiency, the spot size of the interactive laser in the crystal can only be reduced while other parameters remain unchanged. However, a small spot will make the walk-off effect more obvious, resulting in serious spot degradation, which will bring great pressure to subsequent plastic surgery.

[0004] Existing technologies are mainly divided into two types of solutions: the first type is to compensate for the walk-off effect during the frequency tripling process by various means; the second type is to directly add complex beam shaping elements after the frequency tripling. The second type of compensation method will face the problems of short life and poor stability of the laser.

[0005] The first type of compensation method mainly includes: adding a beam separation optical element in front of the tripled frequency crystal, such as a tilted flat plate of fused quartz glass; or introducing a certain amount of walk-off during the doubled frequency process to separate the light beam entering the tripled frequency crystal; there is also the use of tripled frequency crystals placed in series; or cutting the incident end face of the tripled frequency crystal at a certain angle to compensate for the walk-off effect.

[0006] However, the existing technical solutions have increased the difficulty of implementation to varying degrees. They require the introduction of too many pre-compensation optical elements, which brings certain difficulties to engineering and makes the optical system more complicated and inconvenient to install and debug. At the same time, the solution of cutting crystals at special angles requires very high calculation accuracy. Nonlinear crystals cut at special angles and special coatings will increase the cost of trial and error and are more prone to errors.

[0007] Therefore, a simple and effective laser system is urgently needed to compensate for the spatial walk-off effect in the frequency tripling process to obtain a 355nm ultraviolet laser output with high conversion efficiency and high beam quality. Summary of the invention

[0008] The present invention discloses a walk-off compensation device for laser frequency tripling, aiming to solve the technical problems existing in the prior art. The present invention adopts the following technical solutions: On the one hand, in one embodiment of the present invention, a walk-off compensation device for laser frequency tripling is provided, comprising: A fundamental frequency light source, used to generate fundamental frequency laser; A frequency doubling unit, used for converting fundamental frequency laser into frequency doubling laser; A frequency tripling unit, including a frequency tripling nonlinear crystal, used for converting the frequency-doubled laser and the residual fundamental frequency laser into a frequency-tripling laser; A walk-off pre-compensation unit is arranged between the double frequency unit and the triple frequency unit, and the walk-off pre-compensation unit includes an eccentrically arranged spherical lens, and the spherical lens is used to make the double frequency laser and the residual fundamental frequency laser that is not completely converted generate a preset angle, and the preset angle is adapted to the walk-off angle generated in the triple frequency nonlinear crystal; Beam shaping unit, used to output ultraviolet laser.

[0009] As a preferred technical solution, the eccentricity of the spherical lens is positively correlated with the preset angle.

[0010] As a preferred technical solution, the spherical lens is a plano-convex spherical lens.

[0011] As a preferred technical solution, the focal length of the spherical lens is 20~150mm.

[0012] As a preferred technical solution, the fundamental frequency light source includes at least a picosecond laser light source or a femtosecond laser light source, the wavelength of the fundamental frequency laser includes at least 1064nm, 1030nm, and 1550nm, and the repetition frequency can be configured to at least 10MHz-1GHz.

[0013] As a preferred technical solution, the frequency doubling unit includes a focusing lens and a frequency doubling nonlinear crystal.

[0014] As a preferred technical solution, the frequency doubling nonlinear crystal is configured in a non-critical phase matching mode, and the phase matching angle θ m =90°.

[0015] As a preferred technical solution, the frequency tripling nonlinear crystal adopts a type II phase matching method.

[0016] As a preferred technical solution, the polarization directions of the residual fundamental frequency laser and the doubled frequency laser are perpendicular to each other.

[0017] As a preferred technical solution, the beam shaping unit is configured as a Kepler-type beam expander lens group, including a positive lens and a negative lens, wherein the focal length ratio of the positive lens and the negative lens is used to determine the magnification of the tripled frequency laser, and the optical axis distance between the positive lens and the negative lens determines the beam waist position of the tripled frequency laser.

[0018] One embodiment of the above invention has the following advantages or beneficial effects: The present invention mainly provides a walk-off compensation device for laser frequency tripling. By arranging an eccentrically placed spherical lens between a double frequency unit and a triple frequency unit, the spatial walk-off effect in the triple frequency nonlinear crystal is pre-compensated, thereby effectively improving the nonlinear frequency conversion efficiency and improving the beam quality of the output laser.

[0019] Compared with the prior art, the present invention does not need to add complex pre-compensation optical elements in front of the frequency tripling unit, and can achieve walk-off compensation only through an eccentrically placed spherical lens, which greatly simplifies the structure of the optical system and facilitates installation and debugging; secondly, the present invention does not need to adopt a special cutting angle for the frequency tripling nonlinear crystal, avoiding the high cost and high risk brought by special angle crystal processing and special coating; thirdly, the walk-off compensation amount of the present invention can be accurately controlled by adjusting the eccentricity of the spherical lens, and the operation is simple and the adjustment is convenient.

[0020] In addition, the present invention adopts a non-critical phase matching method for double frequency conversion, avoiding the spatial walk-off effect in the double frequency process and ensuring the quality of the double frequency laser beam; at the same time, through the reasonable design of the beam shaping unit, high-quality ultraviolet laser output with specific parameters can be obtained to meet the needs of practical applications.

[0021] The present invention is suitable for ultraviolet laser systems with high repetition rate and low pulse energy, and is particularly suitable for application scenarios such as semiconductor wafer slitting and wafer defect detection, and has important practical value in this field. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for describing the embodiments, which constitute a part of the present invention. The exemplary embodiments of the present invention and their descriptions explain the present invention and do not constitute improper limitations on the present invention. In the drawings: Figure 1 A structural block diagram of a walk-off compensation device for laser frequency tripling in a preferred implementation manner disclosed in an embodiment of the present invention; Figure 2 A schematic diagram of a partial optical path structure of a walk-off compensation device for laser frequency tripling in a preferred implementation manner disclosed in an embodiment of the present invention; Figure 3A schematic diagram of the principal axis coordinate system of a frequency tripling nonlinear crystal in a preferred implementation manner disclosed in an embodiment of the present invention; Figure 4A A schematic diagram of the distribution of the unprecompensated walk-off 532nm laser and the residual 1064nm laser in a frequency tripling nonlinear crystal in a preferred implementation manner disclosed in an embodiment of the present invention; Figure 4B This is a schematic diagram of the distribution of 532nm laser and residual 1064nm laser in a frequency tripling nonlinear crystal under pre-compensation in a preferred implementation manner disclosed in an embodiment of the present invention.

[0023] Description of reference numerals: A fundamental frequency light source 10 , a frequency doubling unit 20 , a first plano-convex spherical lens 21 , a frequency doubling nonlinear crystal 22 , a walk-off pre-compensation unit 30 , a second plano-convex spherical lens 31 , a frequency tripling unit 40 , a frequency tripling nonlinear crystal 41 , and a beam shaping unit 50 . DETAILED DESCRIPTION

[0024] In order to make the purpose, technical solution and advantages of the present invention clearer, the technical solution of the present invention will be clearly and completely described below in conjunction with the specific embodiments of the present invention and the corresponding drawings. In the description of the present invention, it should be noted that the term "or" is usually used in the sense of including "and / or", unless the content clearly indicates otherwise.

[0025] In the description of the present invention, the terms "first", "second", etc. are only used for distinguishing descriptions and cannot be understood as indicating or implying relative importance. In the following embodiments, "fundamental frequency light" refers to "fundamental frequency laser light".

[0026] Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0027] The 532nm laser is generated by the 1064nm laser through frequency doubling. The phase matching condition satisfied by the frequency doubling is non-critical phase matching. No walk-off effect will occur in the process of generating the 532nm laser. The 1064nm laser and the 532nm laser generated by it will maintain coaxial transmission in the nonlinear crystal.

[0028] 355nm laser is generated by sum frequency method. The sum frequency conversion efficiency of 355nm laser is expressed as follows:

[0029] in, is the sum frequency conversion efficiency, is the peak power density of the incident light, is the crystal length, is the beam area of ​​the incident light, is the phase mismatch. The sum frequency conversion efficiency and the walk-off angle are inversely proportional. The larger the walk-off angle, the lower the conversion efficiency. The walk-off effect will inevitably occur during the frequency tripling process. The existence of the walk-off effect will reduce the conversion efficiency and the ellipticity of the light spot.

[0030] In order to produce high-power, high-repetition-rate 355nm ultraviolet lasers, it is necessary to increase the peak power density of the incident light and the length of the crystal. Under the premise that the fundamental frequency parameters do not change, the most effective way to increase the peak power density of the incident light is to reduce the incident light spot size. The smaller the incident light spot, the shorter the horizontal overlap length of the 1064nm laser and the 532nm laser in the crystal. The longer the crystal length, the more obvious the reduction in the spot ellipticity. Both of these methods of increasing the power of the 355nm laser will lead to a more obvious impact of the walk-off effect.

[0031] Furthermore, increasing the peak power density of the incident light can increase the output power of the 355nm laser to a certain extent, but the small incident light spot can easily damage the film layer of the nonlinear crystal. At the same time, the single photon energy of the 355nm laser generated by the sum frequency is higher and the light spot is smaller, which can easily cause irreversible damage to the film layer on the rear end of the crystal and subsequent optical components.

[0032] The walk-off effect will have a significant impact on the ellipticity and beam quality of the 355nm laser. In order to improve this problem, it is necessary to introduce a shaping element in the subsequent optical path to shape the beam of the 355nm laser. However, during the long-term operation of the laser, the life of the shaping element will be greatly reduced.

[0033] In the prior art, the compensation for the triple frequency walk-off effect mainly adopts the solution of adding complex optical elements or using crystals with special cutting angles, which not only increases the complexity of the optical path system, but also makes installation and debugging difficult. At the same time, the crystals with special cutting angles are expensive and prone to processing errors. In addition, the back-end beam shaping solution adopted in the prior art often reduces the service life of the laser and the system stability. In view of the above problems, the present invention provides a walk-off compensation device for laser triple frequency, and the specific implementation methods of the present invention will be described in detail in conjunction with the accompanying drawings.

[0034] refer to Figure 1In a preferred embodiment of the present invention, a walk-off compensation device for laser frequency tripling comprises a fundamental frequency light source 10, a double frequency unit 20, a walk-off pre-compensation unit 30, a triple frequency unit 40 and a beam shaping unit 50 which are arranged in sequence, wherein the fundamental frequency light source 10 is used to generate fundamental frequency laser, the double frequency unit 20 is used to convert the fundamental frequency laser into double frequency laser, the triple frequency unit 40 is used to convert the double frequency laser and the residual fundamental frequency laser into triple frequency laser, the walk-off pre-compensation unit 30 is used to make the double frequency laser and the residual fundamental frequency laser that is not completely converted generate a preset angle, and the preset angle is adapted to the walk-off angle generated in the triple frequency unit 40, and the beam shaping unit 50 is used to finally output ultraviolet laser.

[0035] In a preferred embodiment, the fundamental frequency light source 10 can be a solid laser or a fiber laser, and the fundamental frequency laser includes but is not limited to a picosecond laser light source or a femtosecond laser light source, and the wavelength of the fundamental frequency laser includes but is not limited to 1064nm, 1030nm, and 1550nm. The characteristic parameters of the fundamental frequency laser, including average power, peak power, pulse width, repetition frequency, beam quality factor, line width, etc., directly affect the corresponding characteristic parameters of the output triple frequency laser.

[0036] In a preferred embodiment, the fundamental frequency laser is a picosecond laser with a wavelength of 1064 nm, and the tripled frequency laser finally outputted is a 355 nm laser.

[0037] In a preferred embodiment, the laser repetition frequency can be configured according to the actual application requirements: in high-speed application scenarios such as semiconductor wafer slicing and defect detection, a repetition frequency of the order of hundreds of MHz can be selected to obtain higher processing speed and detection efficiency; in application scenarios with higher requirements for single pulse energy, it can also be configured to a lower repetition frequency, such as the order of kHz, to obtain a higher single pulse energy output. The walk-off compensation scheme of the present invention has strong applicability to repetition frequency and can meet the needs of different application scenarios. The applicable repetition frequency range can at least extend from 10MHz to 1GHz. The specific repetition frequency can be selected and configured according to actual needs.

[0038] Preferably, the polarization state of the fundamental frequency laser is horizontal polarization, so that it can be adapted to the phase matching condition of the subsequent nonlinear crystal. Specifically, this polarization configuration can make the double and triple frequency processes adopt type I phase matching and type II phase matching respectively, thereby obtaining the optimal frequency conversion efficiency.

[0039] like Figure 2 In a preferred embodiment, the double frequency unit 20 includes a focusing lens and a double frequency nonlinear crystal 22. The focusing lens is preferably configured as a first plano-convex spherical lens 21. The double frequency nonlinear crystal 22 preferably adopts a non-critical phase matching method. The phase matching angle θ m =90°, when θm =90°, the wave vector direction K of the fundamental frequency laser (e-light) completely coincides with the energy flow direction S. At this time, the walk-off angle of the fundamental frequency light wave in the doubled frequency nonlinear crystal 22 is 0°, and the doubled frequency laser will obtain high harmonic efficiency and the same beam quality as the fundamental frequency laser. If the parameters of the focusing lens or the beam characteristic parameters of the fundamental frequency laser are changed, the optimal ratio of the residual fundamental frequency laser and the doubled frequency laser power after doubled frequency can be obtained.

[0040] In a preferred embodiment, the focal length of the first plano-convex spherical lens 21 is preferably 50-200 mm, which can ensure high conversion efficiency while matching the focusing parameters with the crystal length to avoid thermal effects and damage caused by over-tight focusing; the doubled frequency nonlinear crystal 22 is preferably an LBO crystal, which has a higher damage threshold, a larger acceptance angle, a lower thermal effect, and exhibits excellent long-term stability in a high repetition rate picosecond laser system. Of course, the doubled frequency nonlinear crystal 22 can also select other suitable nonlinear crystals, as long as they can meet the corresponding phase matching conditions and damage threshold requirements, such as BBO crystals, BIBO crystals or CLBO crystals.

[0041] Preferably, the LBO crystal is plated with 1064nm and 532nm anti-reflection films in its light transmission direction; the temperature of the LBO crystal heating furnace is 150°C, at which the thermal expansion coefficient of the crystal is small, and the influence of temperature fluctuation on phase matching is also small, which is conducive to obtaining stable output. It is preferred to use PTC temperature control method, and the LBO crystal cutting angle is θ1=90°. =0°, θ1 is the angle between the wave vector and the optical axis, is the azimuth of the wave vector. The phase matching mode of LBO crystal is non-critical phase matching. After frequency doubling, second harmonic 532nm laser will be generated.

[0042] In a preferred embodiment, the polarization property of the double frequency unit 20 is 1064(e)+1064(e)→532(o). This double frequency process not only avoids the walk-off effect, but also obtains a higher nonlinear coefficient. Under the first plano-convex spherical lens 21 with a focal length of 150mm, the double frequency process will obtain the maximum second harmonic conversion efficiency.

[0043] In a preferred embodiment, by adjusting the focusing parameters and the fundamental frequency laser power, the fundamental frequency laser will generate vertically polarized second harmonic 532nm laser and residual 1064nm laser after passing through the double frequency unit 20, and the power ratio of the 532nm laser and the residual 1064nm laser is close to 1:1, so as to optimize the conversion efficiency of the subsequent triple frequency process.

[0044] In a preferred embodiment, the walk-off precompensation unit 30 is configured as an eccentrically arranged spherical lens, which has two important functions: one is to serve as a frequency tripling focusing lens, and the other is to achieve walk-off precompensation by adjusting the eccentricity. Preferably, the walk-off precompensation unit 30 is configured as a second plano-convex spherical lens 31.

[0045] The eccentric setting of the second plano-convex spherical lens 31 will cause the incident doubled frequency laser (e-light) and the residual fundamental frequency laser (o-light) to produce a preset angle. This preset angle is positively correlated with the eccentricity of the second plano-convex spherical lens 31. The eccentricity can be adjusted to accurately match the walk-off angle generated in the tripled frequency nonlinear crystal 41, thereby achieving walk-off compensation. The more eccentric the light beam passing through the second plano-convex spherical lens 31 is, the greater the walk-off amount that can be pre-compensated is.

[0046] like Figure 4B After passing through the eccentric second plano-convex spherical lens 31, the two laser beams will be separated in space: the position of the residual fundamental frequency laser (o light) is lower, and the position of the doubled frequency laser (e light) is higher. This spatial distribution is caused by the eccentricity of the second plano-convex spherical lens 31, and its separation angle needs to match the walk-off angle in the triple frequency process to achieve the best compensation effect.

[0047] Furthermore, as a focusing lens for triple frequency, the focal length of the second plano-convex spherical lens 31 directly affects the triple frequency conversion process. By adjusting the focal length, the beam size at the triple frequency crystal can be optimized, thereby affecting the beam waist size and conversion efficiency of the triple frequency output laser. This dual-function design can simplify the optical path structure and improve the system integration.

[0048] Preferably, the focal length of the second plano-convex spherical lens 31 is preferably 20-150 mm. If the focal length is too short, the light beam will be focused too tightly, which may easily cause thermal effects and damage to the nonlinear crystal; if the focal length is too long, the light beam size will be too large, reducing the conversion efficiency of the triple frequency, and will also increase the spatial size of the entire optical path. More preferably, the focal length of the second plano-convex spherical lens 31 is 50-150 mm. On the one hand, this focal length can form a suitable light beam size at the triple frequency crystal, which is conducive to obtaining the best conversion efficiency. On the other hand, it can also make the eccentricity adjustment of the second plano-convex spherical lens 31 more flexible, which is convenient for accurately controlling the walk-off pre-compensation angle.

[0049] In a preferred embodiment, the triple frequency unit 40 includes a triple frequency nonlinear crystal 41. The triple frequency nonlinear crystal 41 is preferably an LBO crystal, supports one-dimensional and two-dimensional crystal point switching functions, and can significantly improve the crystal service life by changing the laser irradiation position. In addition, the triple frequency nonlinear crystal 41 can also select other suitable nonlinear crystals, such as BBO crystals, BIBO crystals, CLBO crystals, etc., as long as the triple frequency phase matching and damage threshold requirements can be met.

[0050] In a preferred embodiment, the LBO crystal cutting angle is θ2=44°, =90°, using the type II phase matching method, the temperature of the LBO crystal is 50°C, the phase matching occurs on the yoz principal axis section of the LBO crystal, and the walk-off direction is perpendicular to the yoz plane (n x direction), the phase matching plane is the plane formed by the light wave vector and the z-axis, θ2 is the angle between the wave vector and the optical axis, which is determined by the requirements of the phase matching condition, is the azimuth of the wave vector. The polarization property is: 1064(o)+532(e)→355(o). The walk-off effect generated by this process will be compensated by placing an eccentric second plano-convex spherical lens 31 at its front end.

[0051] Preferably, the front end face of the LBO crystal is coated with 1064nm and 532nm anti-reflection films to reduce the incident loss, and the rear end face of the crystal can be polished or coated with a 355nm protective film to increase the service life; the polarization directions of the residual 1064nm fundamental frequency laser (o light) and the 532nm laser (e light) generated by the second harmonic are perpendicular to each other to achieve an efficient triple frequency conversion process.

[0052] Specifically, LBO crystal has a high damage threshold, a suitable nonlinear coefficient, and a wide angle and temperature acceptance bandwidth. These characteristics make it particularly suitable for achieving stable triple frequency conversion in high repetition rate picosecond laser systems; through optimized cutting angles and phase matching methods, good output beam quality can be obtained while ensuring conversion efficiency; the focal length selection of the second plano-convex spherical lens 31 needs to balance the conversion efficiency and thermal effects, avoiding crystal damage while ensuring sufficient light intensity.

[0053] In the nonlinear frequency conversion without walk-off and satisfying the phase matching condition, the harmonic conversion efficiency generated is the largest, but when the phase matching condition is satisfied, the walk-off angle is not necessarily zero, and the walk-off angle depends on the relative relationship between the polarization and wave vector direction of the e-light in the crystal and the direction of the crystal optical axis. Specifically, in the triple frequency nonlinear crystal 41, the wave vector direction K of the double frequency laser (e light) does not coincide with the energy flow direction S. If the first laser beam and the second laser beam are incident into the nonlinear crystal in a colinear manner, the energy flow direction of the residual fundamental frequency laser (o light) does not coincide with the energy flow direction of the double frequency laser (e light), so the beam directions of the two laser beams in the triple frequency nonlinear crystal 41 will be separated, and the angle between the two laser beams is the walk-off angle, which will significantly reduce the harmonic conversion efficiency in the triple frequency crystal, and the emitted triple frequency laser spot will be deformed by walk-off, and the beam will produce a poor caustic curve, which will bring certain difficulties to the subsequent beam shaping.

[0054] In order to overcome the above-mentioned problem, the present embodiment adopts the aforementioned walk-off pre-compensation design. When two laser beams are incident on the tripled frequency nonlinear crystal 41, the wave vector direction of the downward residual fundamental frequency laser (o light) is perpendicular to the front end face of the crystal, while the upward doubled frequency laser (e light) forms a specific angle with the front end face of the crystal. The two laser beams do not overlap at the front end face of the crystal. Through this pre-compensation configuration, the e light will gradually shift toward the o light direction inside the crystal. When the e light overlaps with the o light due to the walk-off effect in the nonlinear crystal, a larger interaction area can be formed, thereby obtaining higher harmonic efficiency and better beam quality.

[0055] Specifically, by matching the walk-off pre-compensation with the walk-off effect in the tripled frequency nonlinear crystal 41, the effective interaction length of the two light beams can be extended to the greatest extent. Since the effective spatial overlap ensures the uniformity of the nonlinear interaction process and avoids the beam distortion caused by the spatial separation of the beams, it can not only improve the conversion efficiency of the tripled frequency, but also ensure that the output tripled frequency laser has good beam quality.

[0056] In a preferred embodiment, when the focal length of the second plano-convex spherical lens 31 is 50-150 mm, in the collinear phase matching mode, the wave vector K of the 532 nm laser (e light) in the triple frequency nonlinear crystal 41 and the direction of the energy flow S do not overlap, when the 1064 nm laser (o light) and the 532 nm laser are incident in the triple frequency nonlinear crystal 41 in a collinear manner, the 1064 nm laser and the 532 nm laser will generate a walk-off angle of 9.49 mrad, which is the angle between the energy flow directions of the 1064 nm laser and the 532 nm laser, and the energy flow direction of the 355 nm laser in the crystal will be consistent with the energy flow direction of the 1064 nm laser. The second plano-convex spherical lens 31 placed eccentrically in front of the triple frequency nonlinear crystal 41 can pre-compensate the walk-off angle generated by the 1064 nm laser and the 532 nm laser in the triple frequency nonlinear crystal 41 by changing its eccentricity.

[0057] In a preferred embodiment, the beam shaping unit 50 is configured as a Kepler beam expander lens group, including a positive lens and a negative lens, and the lens parameters of the beam expander lens group are adjusted to obtain a triple frequency laser with specific parameters, such as divergence angle, beam waist size, and beam waist position. Specifically, the focal length ratio of the positive lens and the negative lens determines the magnification of the triple frequency laser, and the divergence angle and spot size of the output beam can be accurately controlled; the spacing adjustment between the two lenses affects the convergence of the output beam, thereby controlling the beam waist position.

[0058] In a preferred embodiment, the working process of each component in the walk-off pre-compensation device for laser frequency tripling is as follows: First, the fundamental frequency light source 10 outputs a fundamental frequency laser with a wavelength of 1064nm and horizontal polarization. The fundamental frequency laser is injected into the double frequency unit 20, passes through the first plano-convex spherical lens 21 with a focal length of 150mm, and then generates a vertically polarized 532nm double frequency laser and a residual 1064nm fundamental frequency laser in the double frequency nonlinear crystal 22. The power ratio of the two laser beams is close to 1:1. Subsequently, the 532nm double frequency laser and the residual 1064nm fundamental frequency laser enter the second plano-convex spherical lens 31, whose focal length is configured to be 100m. m, by eccentrically placing the second plano-convex spherical lens 31, the walk-off amount that will be generated in the tripled frequency nonlinear crystal 41 is pre-compensated; after passing through the tripled frequency nonlinear crystal 41, 355nm tripled frequency laser is generated through nonlinear frequency conversion, while some 1064nm and 532nm lasers are still not converted. Therefore, before entering the beam shaping unit 50, a dichroic mirror can be used to separate the three wavelengths of laser light, and only the 355nm tripled frequency laser light is retained to enter the beam shaping system to obtain the desired beam parameter output.

[0059] Specifically, Figure 3, is a schematic diagram of the principal axis coordinates of the frequency tripling nonlinear crystal 41, where n x 、n y 、n z represents the three principal axis refractive indices of the biaxial crystal, which correspond to the refractive index values ​​of the crystal in different principal axis directions. K is the wave vector direction. The triple frequency phase matching occurs on the yoz principal axis section of the triple frequency nonlinear crystal 41, that is, in the vertical direction. The corresponding triple frequency nonlinear crystal 41 angle is θ2=44°. =90°, the walking direction occurs at the main axis coordinate n x Direction, that is, horizontal direction.

[0060] refer to Figure 4A , is the distribution of the unprecompensated walk-off 532nm laser and the residual 1064nm laser in the tripled frequency nonlinear crystal 41, refer to Figure 4B , is the distribution of 532nm laser and residual 1064nm laser in the tripled frequency nonlinear crystal 41 under pre-compensation condition.

[0061] Compared with the prior art, the embodiment of the present invention pre-compensates for the spatial walk-off effect in the tripled frequency nonlinear crystal 41 by arranging an eccentrically placed second plano-convex spherical lens 31 between the doubled frequency unit 20 and the tripled frequency unit 40, thereby effectively improving the nonlinear frequency conversion efficiency and improving the beam quality of the output laser.

[0062] Compared with the prior art, the present invention does not need to add complex pre-compensation optical elements in front of the triple frequency unit 40, and can achieve walk-off compensation only through an eccentrically placed second plano-convex spherical lens 31, which greatly simplifies the structure of the optical system and facilitates installation and debugging; secondly, the present invention does not need to adopt a special cutting angle for the triple frequency nonlinear crystal 41, avoiding the high cost and high risk brought by special angle crystal processing and special coating; thirdly, the walk-off compensation amount of the present invention can be accurately controlled by adjusting the eccentricity of the second plano-convex spherical lens 31, which is simple to operate and easy to adjust.

[0063] In addition, the present invention adopts a non-critical phase matching method for double frequency conversion, avoiding the spatial walk-off effect in the double frequency process and ensuring the quality of the double frequency laser beam; at the same time, by reasonably designing the beam shaping unit 50, high-quality ultraviolet laser output with specific parameters can be obtained to meet practical application needs.

[0064] The present invention is suitable for ultraviolet laser systems with high repetition rate and low pulse energy, and is particularly suitable for application scenarios such as semiconductor wafer slitting and wafer defect detection, and has important practical value in this field.

[0065] Although example embodiments have been described herein with reference to the accompanying drawings, it should be understood that the above example embodiments are merely exemplary and are not intended to limit the scope of the present application to this. Those of ordinary skill in the art may make various changes and modifications therein without departing from the scope and spirit of the present application. All these changes and modifications are intended to be included within the scope of the present application as required by the appended claims.

[0066] In the description provided herein, a large number of specific details are described. However, it is understood that the embodiments of the present application can be practiced without these specific details. In some instances, well-known methods, structures and techniques are not shown in detail so as not to obscure the understanding of this description.

[0067] Similarly, it should be understood that in order to streamline the present application and help understand one or more of the various inventive aspects, in the description of the exemplary embodiments of the present application, the various features of the present application are sometimes grouped together into a single embodiment, figure, or description thereof. However, the method of the present application should not be interpreted as reflecting the following intention: the claimed application requires more features than the features clearly stated in each claim. More specifically, as reflected in the corresponding claims, the inventive point is that the corresponding technical problem can be solved with features less than all the features of a single disclosed embodiment. Therefore, the claims following the specific embodiment are hereby explicitly incorporated into the specific embodiment, wherein each claim itself serves as a separate embodiment of the present application.

[0068] Those skilled in the art will understand that, except for mutually exclusive features, all features disclosed in this specification (including the accompanying claims, abstract and drawings) and all processes or units of any method or device disclosed in this specification may be combined in any combination. Unless otherwise explicitly stated, each feature disclosed in this specification (including the accompanying claims, abstract and drawings) may be replaced by an alternative feature that provides the same, equivalent or similar purpose.

Claims

1. A walk-off compensation device for laser frequency tripling, characterized in that: include: A fundamental frequency light source, used to generate fundamental frequency laser; A frequency doubling unit, used for converting the fundamental frequency laser into a frequency doubling laser; A frequency tripling unit, comprising a frequency tripling nonlinear crystal, for converting the frequency doubled laser and the residual fundamental frequency laser into a frequency tripled laser; a walk-off pre-compensation unit, arranged between the double frequency unit and the triple frequency unit, the walk-off pre-compensation unit comprising an eccentrically arranged spherical lens, the spherical lens being used to make the double frequency laser and the residual fundamental frequency laser that is not completely converted generate a preset angle, the preset angle being adapted to the walk-off angle generated in the triple frequency nonlinear crystal; Beam shaping unit, used to output ultraviolet laser.

2. The walk-off compensation device for laser frequency tripling according to claim 1, characterized in that: The eccentricity of the spherical lens is positively correlated with the preset angle.

3. The walk-off compensation device for laser frequency tripling according to claim 2, characterized in that: The spherical lens is a plano-convex spherical lens.

4. The walk-off compensation device for laser frequency tripling according to claim 3, characterized in that: The focal length of the spherical lens is 20-150 mm.

5. The walk-off compensation device for laser frequency tripling according to claim 1, characterized in that: The fundamental frequency light source includes at least a picosecond laser light source or a femtosecond laser light source, the wavelength of the fundamental frequency laser includes at least 1064nm, 1030nm, and 1550nm, and the repetition frequency can be configured to be at least 10MHz-1GHz.

6. The walk-off compensation device for laser frequency tripling according to claim 1, characterized in that: The frequency doubling unit comprises a focusing lens and a frequency doubling nonlinear crystal.

7. The walk-off compensation device for laser frequency tripling according to claim 6, characterized in that: The frequency doubling nonlinear crystal is configured in a non-critical phase matching mode, and the phase matching angle θ m =90°.

8. The walk-off compensation device for laser frequency tripling according to claim 1, characterized in that: The triple frequency nonlinear crystal adopts a type II phase matching method.

9. The walk-off compensation device for laser frequency tripling according to claim 8, characterized in that: The polarization directions of the residual fundamental frequency laser and the doubled frequency laser are perpendicular to each other.

10. The walk-off compensation device for laser frequency tripling according to claim 1, characterized in that: The beam shaping unit is configured as a Kepler beam expander lens group, including a positive lens and a negative lens, wherein the focal length ratio of the positive lens to the negative lens is used to determine the magnification of the tripled frequency laser, and the optical axis distance between the positive lens and the negative lens determines the beam waist position of the tripled frequency laser.

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