Device for the generation of ultraviolet light by frequency conversion of laser radiation

Abstract

The application relates to the field of laser technology and discloses a device for generating ultraviolet light through laser radiation frequency conversion, which comprises a front optical system for adjusting the light intensity distribution of an input fundamental wave beam, converting part of the incident fundamental wave beam into a second harmonic wave, or adjusting the cross-sectional light intensity distribution shape of the converted second harmonic wave, and outputting the adjusted second harmonic wave to a fourth harmonic crystal to generate a fourth harmonic wave, which is emitted from the exit interface of the fourth harmonic crystal, refracted out through the exit interface, and output as a fourth harmonic exit beam. The application adjusts the light intensity distribution shape and beam size of the fundamental wave or second harmonic wave before conversion, realizes the frequency conversion of the extracavity laser beam, effectively controls the quality of the generated fourth harmonic beam, and meets the general requirements for the characteristics of ultraviolet light, including deep ultraviolet light and vacuum ultraviolet light, or the characteristics of the beam under the condition of using a small amount of or no ultraviolet optical components.

Description

TECHNICAL FIELD

[0001] The present invention relates to the field of laser technology, and more particularly, to a device for generating ultraviolet or deep ultraviolet or vacuum ultraviolet light by frequency conversion of laser radiation. BACKGROUND

[0002] The wavelength conversion of laser radiation, usually uses the nonlinear process of transparent materials, such as nonlinear laser crystals, LBO, CLBO, BBO, KBBF, RBBF, etc., or uses gas, such as helium, neon, argon, krypton, etc., or material is bombarded by laser surface plasma, etc., by one or more effective nonlinear processes, to convert into other wavelength of coherent or incoherent radiation, to transform the output beam of a wavelength laser, to generate ultraviolet (UV), deep ultraviolet (DUV), vacuum ultraviolet (VUV), or extreme ultraviolet (EUV) beam. With the development of science and technology and industry, the application of these high-quality beams is more and more, and the research and application of these beams are more and more valued. Some applications require high power or high pulse energy, and require various ultraviolet light with high beam quality. The ultraviolet light mentioned in the present invention can also be understood as generally including ultraviolet, deep ultraviolet, vacuum ultraviolet, and extreme ultraviolet light for those skilled in the art.

[0003] However, an effective way to generate high-power or high-energy coherent ultraviolet light is to perform multiple nonlinear harmonic conversion on an infrared or near-infrared laser beam (fundamental wave), and to assist in shaping the generated beam, so that the ultraviolet beam generation system becomes very valuable. Exploration and research in these aspects can be seen in many published publications, literature, and patent or product designs. For example, Solid State Laser Engineering by Koechner, fourth edition, Springer Series in Optics Sciences V.1 (1996), and patents US5408481, 5384803, 5295143, 5047668, 5034951, 4510402, 3873825, US5,850,407, US7,016,389, JP2010064454, US7,170,911, etc.

[0004] Corresponding to these application requirements, the existing known gain media can provide the required laser radiation of various wavelengths at various power levels and operating modes by gas, solid-state material, electrically or optically pumped semiconductor medium. Gas lasers such as KrF, ArF, Ar ion can directly generate good deep ultraviolet (DUV), ultraviolet (UV), visible or infrared (ir) light. Electrically or optically pumped semiconductor lasers can generate visible or near-infrared fundamental laser. Direct semiconductor laser radiation can generate red, green or blue light, ultraviolet radiation, and has good electrical conversion efficiency. However, semiconductor diode lasers, due to their low output beam quality, are not yet available for direct frequency doubling to generate high pulse energy or high beam quality ultraviolet laser beams. For many microelectronic industry application fields such as processing of glass or flexible circuit board micro-holes, wafer cutting and packaging, and chip manufacturing processes, high beam quality ultraviolet or deep ultraviolet or vacuum ultraviolet or extreme ultraviolet wavelength light sources are required to meet the application requirements of smaller and finer scales. Nanosecond or ultrafast pulsed lasers based on intracavity or extracavity third harmonic generation or extracavity fourth harmonic generation or more frequency conversion partially solve the needs of pulsed lasers for industrial processing. However, as the fine measurement or processing of integrated circuits or microelectronics industry continues to develop, the output light quality requirements of ultraviolet lasers (including ultraviolet, deep ultraviolet, vacuum ultraviolet) are becoming higher and higher. Ultrafast pulsed lasers are generally not suitable for intracavity frequency conversion multiple frequency doubling method, and almost all high-energy picosecond or femtosecond ultraviolet conversion adopts extracavity frequency conversion method. When the pulsed beam is converted extracavity, second, third, fourth or more harmonic conversion can be directly used to generate second harmonic, third harmonic or fourth harmonic laser beams. The laser beam required for precision measurement or processing needs to meet certain requirements for the cross-sectional intensity distribution, and the beam intensity distribution is usually circular. The ultraviolet output beam generated by the general extracavity frequency conversion device also needs to be adjusted (referred to as shaping or adjusting in this patent) after the spot cross-sectional shape is generated.

[0005] Due to the high energy or high power of the ultraviolet output laser beam, the ultraviolet conversion crystal or the optical components for shaping the ultraviolet beam will be changed, causing the overall service life of the laser to be short. In practice, the service life of the optical elements for generating and adjusting the ultraviolet laser beam is difficult to meet the requirements of long-time operation of the light source. One solution is to move the ultraviolet light conversion crystal or other easily changed ultraviolet light shaping optical elements, so that the damaged crystal or optical component can be reused at a new undamaged point position, thereby avoiding or reducing the frequency of replacing components and affecting the downtime of the laser equipment.

[0006] Even so, generally speaking, the UV optical components used for shaping, in actual use, spherical mirrors (with two-dimensional curvature), cannot replace the point position by translation (in the mirror plane). In addition, the size of the UV conversion crystal or the UV optical component is limited. These factors limit the service life of the entire device.

[0007] William Grossman et al. invented a method in 1997 to extend the life of the UV light output by nanosecond pulsed UV lasers. The inventor described in US Patent 5,850,407 a clever method to separate the third harmonic beam from the fundamental and second harmonic beams by the difference in the angle of refraction caused by dispersion when the third harmonic beam exits the Brewster angle exit face of the third harmonic crystal. This method uses a single end Brewster angle cut third harmonic crystal to separate the generated third harmonic UV beam from the cavity by the dispersion of the third harmonic crystal that causes the angle of refraction to deviate at the Brewster angle. Since it is a Brewster angle exit, the third harmonic crystal exit beam interface does not need to be coated for the fundamental and UV wavelengths, and the remaining fundamental energy is returned to be re-oscillated for use. This avoids the complexity and high cost of coating the third harmonic crystal for multiple different wavelengths of anti-reflection coating. In addition, when the crystal is used in the cavity at the Brewster angle, the UV beam has a larger cross section and lower optical density at the exit interface when it exits, extending the life of the third harmonic crystal. Since the fundamental beam has a roundness determined by the cavity before it enters the Brewster angle crystal, the generated UV beam can maintain the basic shape of the fundamental beam after passing through the third harmonic crystal twice. Therefore, this patent is more suitable for intracavity third harmonic generation, while reducing the intensity of the UV light on the UV crystal UV light exit surface, thereby extending the service life of the UV crystal.

[0008] If this method is used in the cavity outside the three frequency conversion process, the patent describes the method will be the remaining fundamental directly returned to the fundamental laser source. Thus, the laser source will be very unstable, or be left strong light optical components. Therefore, so far there has been no published literature or products, directly using the method described in the patent to cavity outside the three frequency or four frequency. The patent ownership of Lightwave Electronics company in 2000 after the development of another picosecond ultraviolet, low power, laser, X-Cyte, product series, using a direct single pass Brust angle cutting of three frequency (and frequency) generation scheme. However, this product does not use the pre adjustment of the third harmonic of the fundamental or second harmonic beam cross section intensity distribution, but using the additional ultraviolet optical components of the generated ultraviolet beam conversion after shaping, also does not solve the problem of moving the point of the shaping optical components. The design of such products is complex, high cost, without cleverly solve the beam cross section intensity shaping, but using multiple ultraviolet optical elements for conversion of the beam cross section shape correction, so the product is high. Because the output power of this product is only about 200 mw, the product life is basically to meet the user's requirements, and there is no further technical improvement. In addition, so far, the solid laser device can generate deep ultraviolet or ultraviolet laser, most of which use the moving ultraviolet generated crystal material, using many (usually 2-100) points (usually referred to as the point), to extend the total life of a single deep ultraviolet conversion crystal. However, the correction of the ultraviolet beam through the ultraviolet beam optical components of the point, due to most cases, need to use the combination of cylindrical mirror and spherical mirror, and the combination cannot be moved in the cross section to extend the service life of the optical components. SUMMARY

[0009] The purpose of the present application is to solve the problems in the prior art, and the device for generating deep ultraviolet light by laser frequency conversion, by pre adjusting the beam intensity distribution shape and beam size of the fundamental or second harmonic before conversion, realizing the frequency conversion of the cavity outside the laser beam, and effectively controlling the quality of the four frequency beam, and further not needing to use or only needing to use a small amount of ultraviolet optical components, achieving the general requirements for deep ultraviolet or vacuum ultraviolet beam quality, and making the moving irradiation point operable, reducing the cost of the laser as a whole, or prolonging the service life of the conversion crystal or deep ultraviolet optical components, and simple to manufacture.

[0010] In order to achieve the above purpose, the present application adopts the following technical scheme:

[0011] The device for generating deep ultraviolet light by laser frequency conversion, comprising a front optical system and a four frequency crystal;

[0012] a front optical system for adjusting the intensity distribution of the input fundamental wave or the second harmonic wave generated by the fundamental wave or both, and converting the incident fundamental wave into the fundamental wave and the second harmonic wave, and also adjusting the cross-sectional intensity distribution shape of the converted second harmonic wave, and the adjusted fundamental wave or the second harmonic wave exits to the fourth harmonic crystal;

[0013] a fourth harmonic crystal, the beam exit surface of which is made as a Brewster angle exit interface, so that the input second harmonic wave is converted by frequency doubling in the fourth harmonic crystal to generate a fourth harmonic wave, which exits after passing through the Brewster angle exit interface, and is converted into the required fourth harmonic wave beam which has a cross-sectional intensity distribution shape substantially the same as that of the initially input fundamental wave beam.

[0014] Preferably, the front optical system includes at least one second harmonic crystal for converting the input fundamental wave into the second harmonic wave and the residual fundamental wave, so that the cross-sectional intensity distribution shape of the generated residual fundamental wave or the second harmonic wave is adjusted.

[0015] Preferably, the front optical system further includes at least one of a transition medium and a shaping module, wherein:

[0016] at least one interface of the transition medium is made as a near-Brewster incidence angle interface for shaping the intensity distribution of the input fundamental wave and exiting to the second harmonic crystal;

[0017] the second harmonic crystal partially converts the fundamental wave adjusted by the transition medium into the second harmonic wave and the residual fundamental wave, and the residual fundamental wave or the second harmonic wave exits to the shaping module or directly to the fourth harmonic crystal;

[0018] the shaping module is arranged in front of or behind the second harmonic crystal, receives the incident fundamental wave or the second harmonic wave, and performs pre-compression or expansion shaping in space or in time or both.

[0019] Preferably, the cross-sectional intensity distribution shape of the incident fundamental wave beam is circular, the cross-sectional intensity distribution shape of the residual fundamental wave, the second harmonic wave and the fourth harmonic wave in the fourth harmonic crystal is elliptical, and the shape of the fourth harmonic exit beam is circular.

[0020] Preferably, the shaping module uses one or more pieces of material transparent to the second harmonic wave, one interface of which is made at a Brewster angle.

[0021] Preferably, it further includes an optical reflection component.

[0022] The second-harmonic crystal converts the light beam adjusted by the fourth-harmonic crystal into second-harmonic waves, and the second-harmonic waves are emitted to the optical reflection component;

[0023] The optical reflection component reflects and turns the second-harmonic waves again into the second-harmonic crystal, and the second-harmonic waves are converted again into second-harmonic waves by the second-harmonic crystal to generate more second-harmonic waves, and the second-harmonic waves are emitted to the fourth-harmonic crystal. The second-harmonic waves input into the fourth-harmonic crystal are converted into fourth-harmonic waves in the fourth-harmonic crystal, and the fourth-harmonic waves are emitted after passing through the Brewster angle interface of the fourth-harmonic crystal, and are corrected into a fourth-harmonic output light beam with a cross-sectional intensity distribution shape substantially consistent with or required by the initial input fundamental light beam.

[0024] Preferably, the optical reflection component adopts a mirror surface.

[0025] Preferably, a compensation module can also be included to compensate for the parallel movement of the output light beam in space caused by the movement of the fourth-harmonic crystal. By selecting a compensation material and shape, the parallel movement of the output light beam in space caused by the movement of the fourth-harmonic crystal is pre-compensated or pre-offset to offset the final light beam variation caused by the movement of the fourth-harmonic crystal, so that the output light beam can be kept substantially unchanged in space.

[0026] The compensation module is a transparent wedge-shaped module with the same angle as the fourth-harmonic crystal, and is arranged at an appropriate angle on the device that moves the fourth-harmonic crystal. When the fourth-harmonic (FHG) crystal is translated, the second-harmonic light beam is pre-translated to partially or completely offset the translation of the output light beam caused by the movement of the fourth-harmonic (FHG) crystal required by the point switching.

[0027] The compensation module is a transparent module with parallel planes, and is arranged at an appropriate angle on the device in front of the fourth-harmonic crystal, which can be rotated. When the fourth-harmonic (FHG) crystal is translated, the second-harmonic light beam is pre-translated by rotating to partially or completely offset the translation of the output light beam caused by the movement of the fourth-harmonic (FHG) crystal required by the point switching.

[0028] The compensation module can use the following optical crystals or materials: such as, but not limited to, LBO, CLBO, KBBF, RBBF, BBO, LKP, KD*P, α-BBO, b-BBO, quartz crystal, PPLN, PPTN, PPKTP or other transparent nonlinear crystals or materials.

[0029] Preferably, a rear optical system is also included, wherein:

[0030] The four times frequency crystal converts the second times frequency light into four times frequency light. After the four times frequency light is emitted, the shape, size, or astigmatism of the four times frequency light beam is shaped by the rear optical system, and then emitted. The rear optical system comprises four one-dimensional curved surface lenses, and each two lenses form a group. One group is arranged in the same direction, and the other group is arranged in another direction.

[0031] The rear optical system comprises four one-dimensional curved surface lenses, and each two lenses form a group. One group is arranged in the same direction, and the other group is arranged in another direction.

[0032] Compared with the prior art, the present application has the following beneficial effects:

[0033] 1) The crystal shape for emitting the four times frequency light beam at the Brewster angle is used, so that the light beam area on the interface of the emitting point is large, the light intensity density is small, the damage of the crystal under the deep ultraviolet light is slow, and the service life is long.

[0034] 2) The Brewster angle is cut at the Brewster angle, and the required four times frequency ultraviolet light beam cross section intensity distribution can be easily adjusted during the four times frequency conversion process, the cross section shape of the near vision base wave light beam is maintained, the complex adjustment of the four times frequency ultraviolet light beam cross section light intensity distribution shape is reduced or omitted, the corresponding high-performance ultraviolet optical components are not required, the cost is reduced, and the reliability and service life of the product are improved.

[0035] 3) During the first frequency doubling and the second frequency doubling, a compensation material (or a compensation module) is added to pre-compensate the walk-off angle or time delay or pulse width expansion or compression of the second harmonic wave generated during the harmonic conversion process, so that the second frequency conversion efficiency is improved, and the four times harmonic (4ω) light beam is generated more efficiently.

[0036] 4) The frequency doubling crystal at the Brewster angle does not require high-difficulty optical coating of ultraviolet or deep ultraviolet wavelength on the Brewster angle cutting section, so that the crystal with the Brewster angle is simple to manufacture and low in cost.

[0037] 5) During the first frequency doubling and the second frequency doubling, one or more corresponding wedge-shaped modules are added to realize pre-compensation, so as to reduce or avoid the change of the output light in space when the four times frequency crystal moves, and the minimum change or no change of the generated four times harmonic (4ω) light beam in the moving point process is achieved. BRIEF DESCRIPTION OF DRAWINGS

[0038] Figure 1 The structure diagram of the device for generating laser radiation four times frequency according to the embodiment of the present application;

[0039] Figure 2 The structure diagram of the device for generating laser radiation four times frequency by using the second times frequency crystal or the compensation module according to the embodiment of the present application;

[0040] Figure 3 The structure diagram of the device for laser radiation four-fold frequency generation using single oblique angle (approximate Brewster angle) crystal and optical reflection component according to the embodiment of the present application;

[0041] Figure 4 The structure diagram of the device for laser radiation four-fold frequency generation using transition medium, double frequency crystal or compensation module or other optical component to pre-adjust the light beam according to the embodiment of the present application, so that the generated four-fold frequency light beam does not need to use more than one-dimensional curved surface optical component to adjust or shape the light beam shape.

[0042] Figure 5 The state diagram of the device for laser radiation four-fold frequency generation using geometrically complementary compensation module according to the embodiment of the present application, so that the spatial position of the output light beam of the four-fold frequency crystal is not changed or changed very little when the spot is moved.

[0043] Figure 6 The state diagram of the device for laser radiation four-fold frequency generation using one-dimensional curved surface optical mirror according to the embodiment of the present application, so that the lens can be moved in parallel in the direction without curvature to achieve the purpose of spot changing and the use of multiple points. DETAILED DESCRIPTION

[0044] The embodiments of the present application are described in detail below, and examples of the embodiments are shown in the accompanying drawings, wherein the same or similar reference signs represent the same or similar elements or elements having the same or similar functions throughout. The embodiments described below by referring to the accompanying drawings are exemplary and are intended to explain the present application, and cannot be understood as a limitation of the present application.

[0045] The present application provides a device for laser radiation four-fold frequency generation, by using a special input light intensity distribution pre-adjustment method, when adjusting the output ultraviolet or deep ultraviolet or vacuum ultraviolet light beam, only one or a few optical elements with one-dimensional curvature in the cross section of the laser beam, such as cylindrical mirror, but not aspherical mirror, are needed to adjust the light intensity distribution or other characteristics of the cross section of the light beam, such as, but not limited to, eliminating astigmatism or divergence, or achieving specified beam characteristics or roundness, so that the method of moving the lens in space can be used, and the same lens or multiple points on the lens can be used multiple times, to achieve the purpose of extending the service life of the lens affected by the gradually damaged ultraviolet light beam.

[0046] Suitable for both intracavity and extracavity applications, the device can generate a quadrupled frequency laser of an ultraviolet laser beam. A frequency conversion device with a medium at a Brewster angle exit interface, combined with a frequency-doubling crystal or other medium, pre-adjusts the cross-interface intensity shape of the input beam and the intermediate harmonic beam, potentially including adjustment of the walk-off angle between the interacting beams, and potentially also adjusting time stretching, compression, or delay. Ultraviolet frequency conversion is then performed, and the resulting ultraviolet output beam is emitted through a Brewster angle exit surface, thereby adjusting the cross-sectional intensity distribution of the generated ultraviolet output beam or achieving optimal conversion efficiency. Furthermore, the ultraviolet light in the embodiments includes ultraviolet, deep ultraviolet, or vacuum ultraviolet light.

[0047] The light beam quality of the secondary frequency conversion outside the cavity can be improved, the need for ultraviolet beam shaping can be eliminated or reduced, and the cost can be reduced. The area of ​​the ultraviolet light when it is emitted at the Brewster angle interface is generally increased by about 1.6 times, so that the area of ​​the light spot emitted by the quadruple frequency crystal is increased, thereby extending the service life. In addition, the nonlinear crystal generated by the quadruple frequency harmonic does not need to be coated on the output surface to achieve the minimum interface loss of Brewster angle emission, thereby achieving the generation of a deep ultraviolet beam with high-efficiency nonlinear frequency conversion and avoiding the technical difficulties of deep ultraviolet nonlinear crystal coating.

[0048] The present invention is suitable for generating quadrupled frequency or other multiple frequency multiplications, typically, but not limited to, using a 1064nm wavelength to generate a doubled frequency beam of 532nm, a tripled frequency beam of 355nm, or a quadrupled frequency beam of 266nm, or other subharmonics such as 213nm, or other mixed 193nm, deep ultraviolet, vacuum ultraviolet, or extreme ultraviolet light. It uses a simple and reliable pre-set shaped beam and multiple frequency conversions to produce ultraviolet, deep ultraviolet, or vacuum ultraviolet light, meeting the needs of manufacturing and using various ultraviolet light sources with long life, long service life, high stability, and low cost.

[0049] Example 1

[0050] like Figure 1 As shown, the device for generating the quadrupled frequency of laser radiation according to an embodiment of the present invention includes: a front optical system and a quadrupled frequency crystal.

[0051] The front optical system 1020 converts the fundamental beam into a fundamental wave or a second harmonic and adjusts the intensity distribution of the input fundamental beam or second harmonic beam to adjust the cross-sectional intensity distribution of the generated fundamental wave or second harmonic. The adjusted second harmonic is then emitted to the quadrupled frequency crystal. This front optical system 1020 can utilize a transparent incident medium, preferably with an incident interface that is equal to or approximately equal to the Brewster angle of the transparent medium.

[0052] The exit face of the fourth harmonic crystal 30 is cut at the Brewster exit angle interface, the input second harmonic is converted to fourth harmonic inside the fourth harmonic crystal, the fourth harmonic exits after passing through the Brewster exit angle interface, and is corrected to be substantially the same as the cross-sectional intensity distribution shape of the initial input fundamental beam, or the desired, fourth harmonic exit beam. The fourth harmonic crystal 30 is cut at the phase matching angle in the direction of the generated fourth harmonic, and the interface at which the fourth harmonic exits is arranged at the Brewster angle.

[0053] In particular, the appropriate front optical system 1020 is selected to achieve the cross-sectional shape of the fundamental beam or the converted second harmonic beam such that the input fundamental beam 11 is adjusted by the front optical system 1020 to efficiently generate the final desired fourth harmonic 31 after the generated second harmonic 23 and the remaining fundamental beam 12 are adjusted, and the fourth harmonic or the fourth harmonic beam 32 exits the exit interface 400 at the approximate Brewster angle after exiting the conversion device, and the cross-sectional intensity distribution or the astigmatism of the fourth harmonic beam 32 is optimized. Since the exit face is approximately at the Brewster angle, the interface 400 does not need to be coated with an anti-reflective coating and has little loss to the P-polarized fourth harmonic beam, and the beam area is increased on the interface, and the light intensity is reduced, so that the life of the interface under the irradiation of the ultraviolet beam is prolonged.

[0054] In an embodiment of the present application, a common case is that the cross-sectional intensity distribution shape of the fundamental beam is circular, the cross-sectional intensity distribution shape of the fundamental beam, the second harmonic beam, and the fourth harmonic beam inside the fourth harmonic crystal is elliptical, and the shape of the exit fourth harmonic beam is or is approximately circular. That is, the cross-sectional intensity distribution shape of the fundamental beam 11 is circular, which is a circularly symmetric beam, and the arrangement of the Brewster angle incidence of the front optical system 1020 adjusts the shape of the fundamental beam 11 such that the generated second harmonic beam 23 is elliptical, and the second harmonic beam 23 is incident on the fourth harmonic crystal 30, wherein the interface 300 of the fourth harmonic crystal 30 does not change the shape of the second harmonic beam 23, and converts the second harmonic beam 23 to generate the fourth harmonic beam 31, and the cross-sectional shape of the generated fourth harmonic beam 31 is similar or close to that of the beam 23, and after exiting the Brewster angle interface 400 of the fourth harmonic crystal 30, the final ultraviolet beam 32 is corrected to the desired circular shape, and the beam 32 can substantially maintain the shape of the fundamental beam 11. After exiting the exit interface 400, the light intensity distribution of the beam 32 reaches the desired cross-sectional intensity distribution shape.

[0055] The optical components in the device 1020 required in this embodiment can also adjust the astigmatism of the input fundamental or second harmonic beam such that the astigmatism of the exit fourth harmonic beam 32 is optimal.

[0056] The apparatus needed in this embodiment is simpler and less expensive than the apparatus needed to adjust the final output ultraviolet or deep ultraviolet beam, because the apparatus needed in this embodiment adjusts the cross-sectional intensity distribution of the incident fundamental beam or the generated second harmonic beam in advance. Because the final output ultraviolet beam does not need to be adjusted by optical components or needs to be adjusted by few optical components, the chance of damage to the components by the ultraviolet beam is reduced, and the service life of the entire system is extended. The present application includes at least one device (e.g., the fourth harmonic crystal 30) that is approximately a Brewster angle exit interface. After the ultraviolet beam exits, the cross-sectional intensity distribution of the generated harmonic output beam can reach or approximately reach the desired ideal intensity distribution shape.

[0057] It should be noted that, by appropriately selecting the front optical system 1020, this embodiment can include or not include the pre-expansion or compression of the spatial walk-off angle of the fundamental or second harmonic, or the temporal walk-off separation of the fundamental or second harmonic, the pre-expansion or compression of the pulse width, the beam translation caused by parallel movement of the fourth harmonic crystal, and full or partial compensation or adjustment.

[0058] In this embodiment, the incident interface 100 of the front optical system 1020 and the exit interface 400 of the fourth harmonic crystal are both near-Brewster interfaces, and do not necessarily need to be coated with an antireflection film.

[0059] The fourth harmonic laser device of this embodiment, which can be used in an extracorporeal or intracorporeal manner, can use Brewster angle incidence to adjust the cross-sectional intensity distribution of the input fundamental beam, a transparent medium, or a medium made at a Brewster angle, so that the fundamental beam and the fourth harmonic beam, which are incident or exit at a Brewster angle, offset all or part of the change in the beam diameter in the Brewster incidence surface after the extracorporeal beam is converted by the nonlinear crystal and exits the fourth harmonic crystal, adjust or substantially maintain the original cross-sectional shape or intensity distribution of the incident fundamental beam, so that the final exit fourth harmonic deep ultraviolet beam has a cross-sectional shape that is substantially consistent with the original incident fundamental beam shape or reaches the expected beam shape, and expensive deep ultraviolet optical components are not needed or are needed to a lesser extent for further adjustment of the beam intensity distribution shape.

[0060] Embodiment 2

[0061] In this embodiment, the front optical system uses a second harmonic crystal 20, and the incident surface of the second harmonic crystal 20 is cut at a Brewster incidence angle, so that the interface 100 does not need to be coated with an antireflection film and does not cause loss of the P-polarized incident fundamental beam at the Brewster angle.

[0062] The second-harmonic crystal 20 is used to convert the input fundamental beam into a second-harmonic wave and a residual fundamental wave, so that the cross-sectional light intensity distribution shape of the generated residual fundamental wave or second-harmonic wave is adjusted, and the adjusted second-harmonic wave is emitted to the fourth-harmonic crystal. The incidence angle interface 100 of the second-harmonic crystal is equal to or approximately equal to the Brewster angle incidence angle of the crystal, and when the Brewster surface of the crystal is used as the incidence surface 100, there is no need to coat an anti-reflection film.

[0063] The emission interface 200 of the second-harmonic crystal 20 is coated with a film that is fully transmissive to the fundamental wave or second-harmonic wave. If the fundamental wave and second-harmonic wave are of the same P-polarization, the interface 200 can be, but is not necessarily, designed to be a Brewster emission angle. It should be noted that if the polarization directions of the fundamental wave or second-harmonic wave are different, an anti-reflection film is generally needed to be coated when the incidence is directly opposite.

[0064] The emission surface 400 of the fourth-harmonic crystal 300 is cut to be a Brewster emission angle interface, the input second-harmonic wave is converted by frequency doubling in the fourth-harmonic crystal to generate a fourth-harmonic wave, and the fourth-harmonic wave is emitted after passing through the Brewster emission angle interface, and is converted into a fourth-harmonic emission beam that is substantially consistent with the cross-sectional light intensity distribution shape of the initial input fundamental beam. In the embodiment, the incidence interface of the fourth-harmonic crystal is coated with a film that is fully transmissive to the second-harmonic wave, and an anti-reflection film of a near-normal incidence geometry can be used. The manufacturing direction of the fourth-harmonic crystal 30 is selected to be the phase matching direction of the frequency doubling (2ω+2ω=4ω) of the second-harmonic beam, and the generated fourth-harmonic (4ω) beam is P-polarized and emitted along the Brewster angle when emitted from the interface 400. The fourth-harmonic crystal 30 is phase matched in the Brewster angle direction in which the fourth-harmonic wave is generated, and at least the interface from which the fourth-harmonic wave is emitted is arranged to emit the beam at the Brewster angle, so that the emission Brewster interface 400 does not necessarily need to be coated with an anti-reflection film.

[0065] The laser fourth-harmonic conversion device of the present application is used to generate a fourth-harmonic beam by frequency doubling twice outside the cavity, and the related laser beam that needs to be frequency doubled twice is cut along the Brewster angle of a transparent material, and the Brewster section that is incident and emitted at the Brewster angle is used to effectively generate and emit the fourth-harmonic laser beam, which is fully transmissive and does not need to be coated with an anti-reflection film on the Brewster interface, and is simple.

[0066] Specifically, the input fundamental beam 11 is a circular symmetric beam, after passing through the device, the beam 12 and the beam 23 are adjusted to be elliptical beam shape by the Brewster incidence interface of the second harmonic generation crystal 20. In the further frequency doubling process, the generated fourth harmonic beam 31 is elliptical in the crystal 30. After the beam 31 passes through the interface 400 and is finally output at the Brewster angle, it becomes a symmetric light intensity distribution with an approximately circular cross section, rather than a clear non-circular intensity distribution. Therefore, the need for an expensive optical system to adjust the circularity of the ultraviolet beam is eliminated.

[0067] The embodiment first uses the Brewster angle incidence of the second harmonic generation crystal 20 for the fundamental beam 11, and the Brewster angle cut of the fourth harmonic generation crystal 30, so that the fundamental and fourth harmonic waves incident or emitted at the Brewster angle offset or partially offset the change in beam diameter at the Brewster incidence surface, adjust or substantially maintain the original cross-sectional shape or intensity distribution of the fundamental beam, so that the cross-sectional shape of the final emitted fourth harmonic beam is substantially consistent or approximately consistent with the original fundamental beam shape. The present application can adjust the original beam cross-sectional shape by combining optical spherical or cylindrical mirrors, or adjust the intensity distribution of the fundamental or second harmonic wave in other forms, so that the emitted fourth harmonic beam can achieve a predetermined shape. For example, adjust the beam radius at the Brewster incidence surface of the emitted light to be significantly wider than its vertical direction, so that the cross-sectional beam size of the final generated deep ultraviolet output beam is equal or similar in the two orthogonal vertical directions, or becomes linear, to meet the required spot shape.

[0068] The device of the embodiment is generally used outside the laser cavity to generate a fourth harmonic laser beam after twice frequency doubling, along the last non-normal incidence surface for generating harmonic output, which is usually the Brewster angle output surface 400. When emitted, the spot of the p-polarized light emitted at the Brewster angle becomes larger at the output surface, the ultraviolet power density on this surface becomes smaller, and the Brewster angle surface 400 has the characteristic of full transmission of p-polarized light without the need for antireflection coating, which prolongs the service life of the device and reduces the manufacturing cost.

[0069] In addition, the front optical system 1020 can also be designed with a doubling crystal or a separate crystal medium to adjust the relative walk-off angle of the generated fundamental wave or second harmonic wave, so that the collinearity is improved when further frequency conversion, and the conversion efficiency or spot shape is improved. During the frequency conversion of the ultrashort pulse beam, a compensation medium can also be designed to pre-compensate or adjust the pulse width of the second harmonic wave, so that the time overlap degree is improved when further generating the fourth harmonic wave, the conversion efficiency is improved, or the need for further pulse compression of the ultraviolet beam is omitted, further reducing the optical elements for adjusting the ultraviolet beam that may affect the service life of the device, thereby prolonging the service life of the entire laser frequency conversion device and reducing the cost of the device.

[0070] In order to provide the above-mentioned spatial and temporal walk-off, the front optical system can further include, but is not necessarily limited to, a compensation crystal 40 based on the above-mentioned doubling crystal 20. The incident surface of the doubling crystal 20 is cut as a Brewster incident angle interface 100, which converts the input fundamental wave beam into a fundamental wave or a second harmonic wave, so that the cross-sectional intensity distribution shape of the generated second harmonic wave 22 is adjusted; the compensation crystal 40 receives the incident of the second harmonic wave 22, and realizes the pulse width compensation or pre-compensation of the second harmonic wave 22, and improves the light output efficiency or pulse width of the fourth harmonic wave 32.

[0071] The above-mentioned mode can derive a variety of special devices to further adjust the intensity distribution of the deep ultraviolet light without using multiple deep ultraviolet light resistant optical elements.

[0072] In an embodiment of the present application, the compensation module 40 can use a birefringent crystal transparent to the fundamental wave and the second harmonic wave. For example, LBO, CLBO, BBO, quartz crystal, etc. with orientation at some specific angles.

[0073] The compensation module 40 adopts one of the following forms:

[0074] (1) a combination of one or more crystals;

[0075] (2) one or more prisms or prisms at a Brewster angle;

[0076] (3) one or more chirped mirrors or pulse stretching or compression devices such as gratings, etc.;

[0077] (4) a combination of one or more lenses or mirrors.

[0078] For example, Figure 2The working principle of the shown figure is described as follows: the fundamental wave beam 11 has a conventional circularly symmetrical cross-sectional intensity distribution. After the fundamental wave beam 11 is incident on the Brewster angle interface 100 of the second harmonic generation crystal 20, the cross section is converted from a circle to an ellipse, and after refraction, the beam enters the second harmonic generation crystal 20. When the beam passes through the crystal 20, it is effectively converted into a second harmonic laser beam 22 by phase matching, and exits the crystal 20 from the interface 200. Due to the incidence at the Brewster angle, the cross-sectional size of the beam 11 becomes the fundamental wave beam 12, and the beam diameter in one direction of the beam 12 is larger than the diameter of the original beam 11 in the incident plane formed by the incident light and the normal to the end face 100. After the beam 11 passes through the crystal 20, the fundamental wave 12 and the second harmonic 22 are incident on the compensation crystal 40. After passing through the compensation crystal 40, the walk-off angle of the two beams of the fundamental wave 12 and the second harmonic 22 or the time walk-off is adjusted, and the pulse width is adjusted by adjusting the angle or temperature or other characteristics of the designed component 40 to obtain the required spatial or temporal or both adjustments, so that they are more overlapped in space or more optimized in time, pulse width or both when they enter the fourth harmonic generation crystal 30 through the interface 300 for the second conversion of the fourth harmonic. The second frequency conversion to the fourth harmonic is more effective in the crystal 30, and the fourth harmonic beam 31 is generated. After the beam 31 passes through the Brewster angle interface 400, the cross-sectional shape of the beam is adjusted from an ellipse to a circle, and becomes the fourth harmonic beam 32. The beam diameter of the fourth harmonic beam 32 in the paper is reduced after refraction by the Brewster interface, and due to the angle produced by dispersion, the remaining fundamental wave beam 16 and the remaining second harmonic beam 25 are separated at the same or different exit angles, exit the interface 400, and after a certain distance of propagation, they can be further effectively separated, and the beam cross section is substantially restored to the shape of the original fundamental wave incidence or maintains the required accuracy, and the generated fourth harmonic beam 32 is easily separated due to the refraction at the Brewster angle.

[0079] The compensation module can be a set of lenses or mirrors, which can be pre-compensated for the astigmatism of the second harmonic after exiting the second harmonic generation crystal by measurement or calculation, and can also pre-compensate for the further astigmatism between the phase matching direction of the fourth harmonic generation crystal and the direction orthogonal thereto, so that the astigmatism of the exiting fourth harmonic beam is substantially eliminated or reduced.

[0080] The Brewster angles of the interfaces 100 and 400 are substantially the same. Therefore, the beam 11 does not need to be corrected complicatedly, and can be converted into a fourth harmonic beam 32 with a nearly circular cross section.

[0081] Wherein, the rear optical component 500 is an ultraviolet optical component which further adjusts the generated ultraviolet light beam under certain conditions. The ultraviolet optical component here selects a one-dimensional curved surface lens such as a cylindrical lens or a cylindrical mirror, or a prism. Due to the use of the pre-compensation of the divergence angle or astigmatism of the second harmonic light beam or both as described in the patent, the adjustment of the ultraviolet light beam only needs to use one-dimensional curved surface optical components, so that these ultraviolet optical components can be moved by the translation device 600 without affecting the light beam adjustment, or the light beam divergence angle change caused by the change of the fourth harmonic crystal or the ultraviolet optical component with irradiation time can be corrected by moving these lenses in the light beam propagation direction, to achieve the purpose of prolonging the service life.

[0082] This way mainly has the following four advantages:

[0083] (1) The cross section of the fourth harmonic light beam on the interface 400 slope is widened, so that the fourth harmonic crystal irradiated by the light beam 32 has a long service life and does not need to be corrected subsequently to achieve the desired shape;

[0084] (2) The interface 400 does not need additional coating, which greatly reduces the cost;

[0085] (3) The fourth harmonic light beam 32, the remaining fundamental light beam 16 and the remaining second harmonic light beam 25 are separated at different exit angles and leave the interface 400 to automatically separate in space;

[0086] (4) The materials of the crystal 20 and the crystal 30 and the Brewster angle of the crystal interface 100 and the interface 400 are basically the same, so that the shape of the fundamental light beam 11 and the fourth harmonic 32 is basically the same, meeting the needs of subsequent applications.

[0087] The above embodiments provided by the present application use the second harmonic crystal 20 incident at the Brewster angle or other transparent medium adjusting the input fundamental cross-sectional light beam intensity distribution, and the fourth harmonic crystal 30 cut at the Brewster angle, so that the fundamental and fourth harmonic incident or emitted at the Brewster angle, after the cavity light beam passes through the nonlinear conversion crystal twice, the fourth harmonic light beam refracted out of the fourth harmonic crystal 30, offsets all or part of the change of the light beam diameter in the Brewster incidence plane, adjusts or basically maintains the original cross-sectional shape or intensity distribution of the fundamental, so that the final emitted fourth harmonic light beam cross-sectional shape is basically consistent or approximately consistent with the original fundamental shape. And, an additional compensation module 40 for pre-adjusting the astigmatism of the fourth harmonic light beam to be generated can be arranged or not arranged between the two light beam frequency conversion processes.

[0088] In the embodiments of the present application, the doubling crystal 20, the quadrupling crystal 30 and the compensation material 40 can be selected, such as LBO, CLBO, KBBF, RBBF, BBO, a-BBO, quartz, a combination of various nonlinear laser crystal transparent materials, cut in different directions to achieve phase matching or walk-off compensation or spatial astigmatism, or time pulse width, compensation or pre-compensation required angle. The compensation module can be a compensation crystal or other optical components, etc.

[0089] Embodiment 3

[0090] As shown in Figure 3 Embodiment 3 describes a design similar to Embodiment 2, the main difference is that the design of Embodiment 2 is reversed by the second time through the interface cut at the Brewster angle, so that the cross-sectional beam shape of the original incident beam 11 is basically maintained or similar to the shape of the output quadrupling laser beam 32. The output beam does not need to be adjusted because of the change in the beam cross-sectional roundness when the single output Brewster angle interface is used for the second frequency doubling. It should be noted that the first time through the Brewster angle component can also be a separate optical component with a pre-adjusted beam cross-sectional intensity distribution shape or function, which is separate from the quadrupling harmonic generation crystal. However, this composite arrangement has the advantages of simplicity and low cost in terms of cost and beam adjustment.

[0091] In this embodiment, the front optical system includes: a quadrupling crystal 30, a doubling crystal 20 and an optical reflection component 02, one interface of the quadrupling crystal 30 is cut as a Brewster angle interface 400, which serves as a base wave beam incident surface and a base wave and second harmonic output surface. The base wave beam is incident to the Brewster angle interface 400 of the quadrupling crystal 30, which adjusts the intensity distribution of the input base wave beam 111 to adjust the cross-sectional shape of the generated base wave, and is output to the doubling crystal 20. The doubling crystal 20 converts the base wave transmitted by the quadrupling crystal 30 into a second harmonic wave, and outputs the remaining base wave or second harmonic wave to the optical reflection component. The optical reflection component 02 reflects the base wave or the second harmonic wave after turning, and then enters the doubling crystal again. The doubling crystal converts the base wave to a second harmonic wave when the base wave passes through the doubling crystal for the second time to generate more second harmonic waves. The second harmonic wave is output to the other interface 300 of the quadrupling crystal 30. The input second harmonic wave is converted into a quadrupling harmonic wave in the quadrupling crystal 30, and the quadrupling harmonic wave is output after passing through the Brewster angle interface 400 of the quadrupling crystal 30, which restores the cross-sectional intensity distribution shape of the initial input base wave beam to be basically consistent with the quadrupling output beam 32.

[0092] In the embodiments of the present application, the optical reflection component 02 adopts one of the following forms:

[0093] (1) a reflecting surface, a retro-reflector composed of two intersecting 90-degree flat mirrors;

[0094] (2) a set of multiple reflecting surfaces;

[0095] (3) a set of reflecting surfaces, such as a retro-reflector with 180-degree turning angle;

[0096] (4) a combination including a walk-off angle adjusting crystal;

[0097] (5) a light pulse width adjusting medium, etc.

[0098] Further, the front optical system can also, but not necessarily, include a compensation module, or a compensation crystal 40, which is located between the frequency quadrupling crystal 30 and the frequency doubling crystal 20, or between the frequency doubling crystal 20 and the retro-reflector 02, for compensating the spatial walk-off angle and the temporal pulse expansion or compression delay of the fundamental wave or the second harmonic wave.

[0099] The compensation module receives the fundamental wave or the second harmonic wave, adjusts or pre-compensates the spatial walk-off angle or the temporal pulse expansion or compression delay of the fundamental wave or the second harmonic wave, so as to facilitate the generation of high-quality or high-conversion-efficiency fourth harmonic wave, and to make the beam direction of the fundamental wave or the second harmonic wave more collinear or synchronous inside the frequency conversion four times frequency crystal.

[0100] Reference Figure 3 The working principle of the embodiment is as follows: the incident light beam fundamental wave 111, which is assumed to have a circular cross-sectional shape of beam intensity distribution, enters the Brewster angle interface 400 of the frequency quadrupling crystal 30, and after entering the crystal 30 at the Brewster angle, becomes an elliptical cross-sectional shape of beam intensity distribution inside the crystal 30. The light beam 112 emitted through the normal incidence interface 300 of the crystal 30 is still elliptical, and then passes through the crystal 40 and enters the frequency doubling crystal 20 to generate the frequency doubling light beam 21. The cross-sectional shape of the frequency doubling light beam 21 is elliptical.

[0101] The optical reflection component 02 makes the fundamental (frequency ω) light beam 112 and the second harmonic (frequency 2ω) light beam 21 pass through 180 degrees or approximately 180 degrees reflection and turn again into the second harmonic crystal 20 and the compensation crystal 40, and then enter the interface 300 of the fourth harmonic crystal 30. The second harmonic crystal 20 generates the second harmonic (2ω) conversion when the fundamental 112 passes through twice. The second harmonic 22 continues to pass through the phase-matched crystal 30 to generate the second frequency conversion (2ω+2ω) in the form of an elliptical cross-section second harmonic light beam 24, and then generates the elliptical cross-section fourth harmonic (4ω) light beam, light beam 31. The elliptical light beam 31 passes through the Brewster angle interface 400, and because it is emitted at the Brewster angle, the cross-section of the light beam 31 is adjusted after exiting the interface 400, and becomes a fourth harmonic light beam 32 with or close to the original roundness of the original fundamental light beam.

[0102] In this way, the initial fundamental 111 is a circular cross-section light beam, and after passing through two Brewster angle incidence and exit sections, the generated fourth harmonic light beam 32 exits with a cross-section beam shape similar to the fundamental light beam intensity distribution. The fourth harmonic light beam 32 output by this device can basically maintain the cross-section shape of the original incident fundamental light beam, and after a certain shape input of the fundamental 111, the generated fourth harmonic output light beam 32 can achieve the design required roundness without the need for further adjustment of the beam cross-section shape. Because the adjustment of the fundamental light beam shape is much easier and cheaper than the adjustment of the fourth harmonic ultraviolet light beam, this scheme is designed ingeniously, simply and low in manufacturing cost. Because no additional optical components are used to further adjust the cross-section of the fourth harmonic ultraviolet light beam, the structure is simple and stable. Because the Brewster angle incidence crystal is used, the cross-section of the fundamental, second harmonic and fourth harmonic light beams irradiated in the crystal and on the surface is larger, and the fourth harmonic crystal life is longer. The service life of the entire device is extended.

[0103] Embodiment 4

[0104] Specifically, referring to Figure 4 The front optical system includes a transition medium 10, a second harmonic crystal 20 and a compensation crystal 40. The transition transparent medium 10 is used to pre-set the cross-section shape of the input fundamental.

[0105] The incident surface of the transition medium 10 is cut as a Brewster incident angle interface for adjusting the light intensity distribution of the input fundamental wave beam, and the adjusted fundamental wave is emitted to the second frequency conversion crystal 20; the second frequency conversion crystal 20 converts the fundamental wave emitted by the transition medium 10 into a second harmonic wave, and the second harmonic wave is emitted to the compensation module 40; the compensation module 40 receives the incident fundamental wave or second harmonic wave, and pre-sets or compensates the spatial walk-off angle or the time pulse expansion or compression delay of the second harmonic wave, so that the efficiency of the fourth harmonic wave conversion is higher, and the beam quality is better.

[0106] The transition medium has at least one interface cut at a Brewster angle, which adjusts the cross-sectional spot shape of the incident light beam, so that the adjusted fundamental wave beam has a second harmonic wave and a residual fundamental wave required for frequency doubling conversion, which is beneficial to generate the required output beam final spot shape.

[0107] The transition medium can include one or more lenses to pre-compensate the divergence angle or astigmatism of the second harmonic wave, so that the divergence angle or astigmatism or both of the generated fourth harmonic wave is compensated, thereby reducing or eliminating the need for further adjustment of the ultraviolet fourth harmonic wave, reducing the complexity or cost of the system.

[0108] It should be noted that the positions of the second frequency conversion crystal 20, the compensation module 40 and the transition medium 10 can be exchanged. That is, the front optical system includes the second frequency conversion crystal 20, the transition medium 10 and the compensation module 40.

[0109] As shown in Figure 4 The embodiment adopts an optical transparent transition medium 10 with a refractive index close to or the same as the medium 30, and one interface of the medium is made as a Brewster angle incident surface, so that when the cross section of the incident light beam enters the second frequency conversion crystal 30, the generated fourth harmonic wave is emitted through the fourth harmonic crystal 30, and the cross-sectional intensity distribution shape of the output light beam of the fourth harmonic crystal is basically maintained as the cross-sectional intensity distribution shape characteristics of the input light beam.

[0110] In the device of the present embodiment, assuming that the incident wave cross-sectional light intensity distribution shape is a circularly symmetric fundamental wave beam 11, after being refracted into the transition medium 10 at an angle of incidence close to or equal to the Brewster angle, the cross-sectional intensity distribution shape of the fundamental wave beam 11 becomes an elliptical cross-section beam 12. The beam 12 enters the second-harmonic crystal 20, and when the beam 12 undergoes harmonic conversion in the second-harmonic crystal 20, a second-harmonic (second-harmonic) beam 22 similar in cross-sectional shape to the beam 12 is generated. After the beam 12 and the second-harmonic beam 22 pass through the designed compensation module 40, the width of the pulse is broadened or color dispersed, and after appropriate adjustment of the compensation module 40, the beam 23 enters the second-converted fourth-harmonic crystal 30 to generate a more effective or high-quality fourth-harmonic beam 31. The cross-sectional light intensity distribution shape of the generated fourth-harmonic beam 31 is also elliptical and approximately has a similar elliptical shape to the second-harmonic 24 adjusted by the interface 100. The elliptical cross-section beam 31 passes through the interface 400, and when the interface Brewster angle slope and the long and short axis direction of the beam 31 are consistent, the beam 31 can be emitted from the interface 400 at the Brewster angle after passing through the Brewster angle interface, and the cross-section of the beam 31 is adjusted by the interface 400 to become a fourth-harmonic beam 32. The cross-sectional light intensity distribution of the fourth-harmonic beam 32 can reach a circularity characteristic similar to or required by the original incident second-harmonic beam 24. In this way, the cross-section of the fourth-harmonic 32 is similar to or required by the cross-section of the incident basic 111, and the fourth-harmonic has better astigmatism and divergence angle characteristics.

[0111] Thus, the present application can obtain the required cross-sectional light intensity distribution of the fourth-harmonic output beam by adjusting the symmetry of the cross-sectional light intensity distribution of the input fundamental wave. The present application can avoid or improve the complex optical device required for adjusting the shape of the fourth-harmonic, usually ultraviolet or deep ultraviolet wavelength, light beam, and overcome the difficulty of short service life of such optical device after being irradiated by ultraviolet light. Through the method of the present application, the device for obtaining the fourth-harmonic ultraviolet output light does not need to be coated with an antireflection film corresponding to the fourth-harmonic, and has the advantages of simple device, low cost, stable performance, long service life, etc.

[0112] In all the above embodiments of the present application, the crystals in the front optical system and the fourth-harmonic crystal adopt one of the following forms: LBO, CLBO, KBBF, RBBF, BBO, LKP, KD*P, BBO, quartz, PPLN, PPTN, PPKTP or other nonlinear crystals. The Brewster angle of incidence or emission of these crystals is between 10-70°.

[0113] The device for generating laser radiation four times frequency conversion according to the embodiment of the present application can realize the improvement of beam quality, cost reduction, and the extension of the life of the final conversion crystal of the out-of-cavity secondary laser frequency conversion, so that the nonlinear crystal does not need the high-technique complex output surface coating, which is a clever design and selection of nonlinear frequency conversion. The present application can generate four times frequency conversion or other multiple frequency conversion, such as the deep ultraviolet laser beam of 266nm generated by four times frequency conversion based on the fundamental wave of 1064nm, and can produce deep ultraviolet or vacuum ultraviolet, such as 193nm, 157nm, laser beam by simple and reliable multiple frequency conversion, which meets the design needs of long-life, high-stability and low-cost ultraviolet laser. The present application cleverly realizes the beam quality requirement of the out-of-cavity final laser frequency conversion by pre-setting the beam intensity distribution shape of the fundamental wave and the second harmonic wave before conversion, reduces the cost and extends the life of the conversion crystal, and is simple to manufacture.

[0114] The device for generating laser radiation four times frequency conversion according to the embodiment of the present application has the following beneficial effects:

[0115] 1) The four times frequency conversion crystal shape of the Brewster angle is used, so that the beam diameter at the exit point is large and the light intensity is small, and the life of the crystal after being irradiated by ultraviolet light is long;

[0116] 2) The Brewster angle exit or entry of the secondary or even times of the Brewster angle cutting is used, and the cross-sectional intensity distribution of the generated four times frequency conversion ultraviolet beam can be easily adjusted during the four times frequency conversion process. The cross-sectional shape of the fundamental wave beam is basically maintained, and the complex adjustment of the cross-sectional intensity distribution of the ultraviolet four times frequency conversion beam is not needed, which reduces the need for multiple ultraviolet optical components and possible damage, reduces the cost, and improves the reliability and stability of the product;

[0117] 3) The compensation module is added between the first frequency conversion and the second frequency conversion, the pre-set delay of the second harmonic pulse broadening or compression is pre-compensated, and the efficiency of the four times frequency conversion (4ω) generation can be obviously improved;

[0118] 4) The frequency conversion crystal of the Brewster angle is used, so that the Brewster angle cutting section does not need optical coating, and the crystal with the Brewster angle is simple to manufacture and low in cost.

[0119] Embodiment 5

[0120] Specifically, referring to Figure 5 , the front optical system comprises:

[0121] An apparatus, the design of the value compensation module, so that the UV output point position on the four times frequency generating crystal, can be moved to the four times frequency crystal point. Generally, only in the perpendicular to the direction of the light beam moving forward, moving the four times frequency (FHG) crystal, to achieve the use of point position change in the output surface, the output point position on the output surface of the original output beam space position may cause changes, and then, in some applications, will cause the position of the beam focus point changes. Therefore, it is necessary to eliminate or reduce the spatial position changes of the light beam caused by the translation of the four times frequency crystal. The present invention proposes a new method to reduce or avoid the parallel movement of the output UV beam caused by moving the four times frequency crystal.

[0122] A compensation method is to pre-set a compensation module, which moves together with the four times frequency crystal, to offset the movement of the light beam, adjust the light beam path, and compensate for the movement of the output beam in space caused by the movement of the four times frequency crystal, so that the overall spatial change of the four times frequency beam is small or no change.

[0123] Figure 5 A compensation design scheme is shown in 5-a, that is, by designing a compensation module that can be moved at any point of parallel movement, when the four times frequency crystal moves to cause the parallel movement of the output beam, the compensation module is moved together with the four times frequency crystal, so that the movement of the second harmonic beam caused by the compensation module can offset or approximately offset the change (movement) of the UV output beam caused by the parallel movement of the four times frequency crystal, without further movement compensation of the generated UV or deep UV beam, without the influence of the short life cycle of the compensation module under UV irradiation, and easy.

[0124] The compensation module, when moving together with the four times frequency crystal, can simultaneously cause the translation of the second harmonic beam in the opposite direction, and the translation amount is the same as or approximately the same as the parallel movement amount of the light beam caused by the translation of the four times frequency crystal, but the parallel movement directions of the two are opposite, and completely offset or mostly offset each other.

[0125] Specifically, the second harmonic beam 23, when moving the four times frequency crystal 301 (shown by a solid line) to 302 (shown by a dashed line) to change the point, the compensation module 401, due to its similar shape, but opposite symmetric placement arrangement, as Figure 5As shown in 5-a, it will also be translated to the position 402, and the converted light beam will change from the original 231 and the conversion to generate the fourth harmonics 311 and 321 to the later 232 and the conversion to generate the fourth harmonics 312 and 322, changing to the new beam path (that is, after the movement, the light beam 23 is incident on the compensation module 402 at the new position, and the light 232 passing through 402 is changed to the fourth harmonic beam 312 inside the quadruple harmonic (FHG) crystal, and becomes the beam 322 after being emitted). Figure 5 As shown in 5-a, if the refractive index and interface wedge angle of the compensation module are roughly similar to or identical to the refractive index and wedge angle of the quadrupled frequency crystal, or if the compensation module is designed through precise calculation, it can be achieved that the light output path 321 before movement and the light output path 322 after movement overlap or basically overlap, the spatial path of the output light beam does not change or changes very little, and the two can be installed on the same mobile platform and moved, which is simple in design.

[0126] The compensation module here can be an SHG crystal that generates double frequency harmonics. In this case, the SHG crystal can be used as both a frequency conversion crystal that generates double frequency and a compensation module, making the deep ultraviolet laser device compact.

[0127] In another case, Figure 5 In step 5-b, by rotating the compensation module's medium at an angle of 400°, the spatial parallel shift of beam 231 and the translational shift of the quadrupled frequency beam caused by the parallel movement of the quadrupled frequency crystal are offset or partially offset. The refractive index and shape of the compensation module are optimized through geometric calculations, resulting in minimal or no spatial positional shift of beams 321 and 322, while achieving a shifting effect on the quadrupled frequency crystal.

[0128] It should be noted that selecting a material for the compensation module with a refractive index that is the same as or similar to that of the quadrupled frequency crystal is the fundamental principle and approach to designing the compensation module. Nearly all deep-ultraviolet laser devices with extracavity quadrupled frequency generation power of approximately 1 watt or higher require the UV conversion crystal to be moved and re-pointed to maintain a sufficient lifespan (e.g., more than a month). Therefore, point shifting technology is crucial, or even essential, for devices generating light waves in the ultraviolet, deep ultraviolet, or vacuum ultraviolet bands. The method of the present invention solves the problem of parallel beam movement caused by moving the quadrupled frequency crystal, and the method is simple and easy to implement.

[0129] The compensation module mentioned here can be made of the same material as the quadruple frequency crystal, but can be made in the same or different directions so that their refractive indices are similar or the same.

[0130] Here, the commonly available four times frequency crystal can be selected from CLBO or BBO or KBBF or RBBF and the like transparent crystal, or LBO crystal or quartz crystal or calcium fluoride crystal with higher transparency. However, the selection of the material with similar or same refractive index and durability is a good selection method, for example, the FHG is selected from CLBO (the refractive index at 266 nm is about 1.6), and the compensation module material is selected from LBO or CLBO, and the refractive index of the two at 532 nm is also about 1.6.

[0131] It should be noted that in all the above embodiments of the present application, the position of the output four times frequency harmonic light spot at the exit point of the four times frequency crystal can be adjusted by parallel moving the four times frequency crystal or other media in the compensation module, which plays a role of changing the point.

[0132] By parallel moving the four times frequency crystal along the Brewster surface of the four times frequency crystal and moving the compensation module, the relative position of the output four times frequency harmonic light at the exit point on the Brewster angle surface of the four times frequency crystal is adjusted, which does not cause any change or movement of the output light beam, and plays a role of changing the point.

[0133] By appropriately moving or rotating the compensation crystal, the relative position of the output four times frequency harmonic light at the exit point on the Brewster angle surface of the four times frequency crystal is adjusted, which plays a role of changing the point.

[0134] Embodiment 6

[0135] Specifically, referring to 6-a in Figure 6 , ( Figure 6 and 6-b in Figure 6 ), the rear optical system comprises:

[0136] An apparatus for adjusting the cross-sectional shape, size, or divergence angle or astigmatism of an ultraviolet or deep ultraviolet light beam by using a one-dimensional curvature mirror, so that the position of the ultraviolet light exit point on the adjusting mirror can be changed by moving the adjusting mirror, thereby extending the service life of the mirror by using multiple positions of the mirror.

[0137] The one-dimensional curvature mirror can be, but is not limited to, a cylindrical lens, a prism, or a cylindrical mirror, but excludes the use of a spherical mirror or a spherical mirror. By using a one-dimensional curvature mirror, different curved surface positions can be selected by moving the mirror (in the direction without curvature), so as to change the point and extend the service life. Here, if a spherical mirror is used, moving the mirror in the mirror surface will cause the deflection or change of the pointing of the light beam due to the curvature in two perpendicular directions, which will cause the exit light beam to be unable to be used.

[0138] As shown in Figure 6In the illustrated example, optical components 601, 602, 603, and 604 all feature one-dimensional curvature. Components 601 and 603 are arranged as a group and have no curvature in the same direction (e.g., the Y-axis). This ensures that when they are moved along this direction, they do not cause any spatial shift in the light beam. Similarly, components 602 and 604 also feature one-dimensional curvature. Components 602 and 604 are arranged as a group and have no curvature in another direction (the X-axis). This ensures that when they are moved along this direction, they do not cause any spatial shift in the light beam.

[0139] Figure 6 6-a in the diagram shows the XZ plane. The one-dimensional curvature of the lens or lens group within the dotted line only has curvature in the X-axis. Therefore, when this lens group moves along the Y-axis, it does not cause any translation or deflection of the beam's spatial position. The lenses within the dotted line can be used multiple times by changing the point. Movement along the Z-axis can correct for the gradual change in beam divergence caused by UV beam irradiation in the X-axis.

[0140] Figure 6 Figure 6-b shows the YZ plane. The one-dimensional curvature of the lens or lens group within the dotted line is curvature only in the Y-axis direction. Therefore, when this lens group moves along the X-axis, it does not cause any translation or deflection of the beam's spatial position. The lenses within the dotted line can be used multiple times by changing positions. Movement along the Z-axis can correct for the gradual change in beam divergence caused by UV beam irradiation in the Y-axis direction.

[0141] These two sets of mutually orthogonal one-dimensional curvature lenses can adjust the beam size, divergence angle, and astigmatism of the ultraviolet light beam in two orthogonal directions to achieve the required performance requirements of the output ultraviolet light beam. At the same time, they can maintain multi-point use and extend the use cycle, so that the optical components for adjusting the ultraviolet light beam can be used multiple times, reducing costs and downtime for repairs.

[0142] The lens movement described above primarily occurs in a plane perpendicular to the direction of the beam's travel. Moving the lens in the direction of the beam's travel compensates for the gradual change in the UV light's divergence angle caused by the optical components or UV conversion crystals when the UV beam strikes them, thus ensuring that the UV beam generated by the laser maintains the same or similar beam characteristics over time.

[0143] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with the technical field, within the technical scope disclosed by the present invention, who makes equivalent replacements or changes based on the technical solution and inventive concept of the present invention, should be covered by the scope of protection of the present invention.

[0144] The term "ultraviolet light" or "deep ultraviolet light" as used herein is intended to be a generic term for ultraviolet light, deep ultraviolet light, or vacuum ultraviolet light of any wavelength in the range from 400 nm to 100 nm. For the sake of brevity, no strict distinction is made.

Claims

1. A device for generating ultraviolet light by frequency conversion of laser radiation, characterized in that: Includes front optical system and quadruple crystal; A front optical system is used to adjust the light intensity distribution of the input fundamental wave or the doubled frequency harmonic beam generated by the fundamental wave, or both, and convert the incident fundamental wave beam into the fundamental wave and the doubled frequency harmonic, and also adjust the cross-sectional light intensity distribution shape of the converted doubled frequency harmonic. The adjusted fundamental wave or doubled frequency harmonic beam is emitted to the quadrupled frequency crystal; A frequency quadrupling crystal, wherein the beam exit surface of the frequency quadrupling crystal is cut into a Brewster angle exit interface, and the input doubled frequency harmonic undergoes frequency doubling conversion within the frequency quadrupling crystal to generate a quadrupled frequency harmonic beam, and the quadrupled frequency harmonic beam is emitted after passing through the Brewster angle exit interface and is converted into a quadrupled frequency beam having a cross-sectional light intensity distribution shape substantially similar to that of the initially input fundamental wave beam or a final desired shape; The front optical system includes at least one double frequency crystal for converting the input fundamental beam into a double frequency harmonic and a residual fundamental wave, so that the cross-sectional light intensity distribution shape of the generated double frequency harmonic is adjusted; The front optical system further includes a transition medium and a shaping module, wherein: At least one interface of the transition medium is cut into a near-Brewster angle of incidence interface, which is used to shape the intensity distribution of the input fundamental wave beam and emit it onto the double frequency crystal; Alternatively, the transition medium has at least one interface that is a Brewster angle incident or exit interface, and the cross-sectional spot shape of the incident light beam is adjusted so that the adjusted fundamental wave light beam or second harmonic light beam has a final beam shape that is conducive to generating the required quadrupled frequency ultraviolet light beam; The double frequency crystal partially converts the fundamental wave adjusted by the transition medium into double frequency harmonics and the remaining fundamental wave, and emits the remaining fundamental wave or double frequency harmonics to the shaping module or directly to the quadruple frequency crystal; The shaping module is arranged in front of or behind the double frequency crystal, receives the incident fundamental wave or double frequency harmonic, and performs pre-compression or broadening adjustment of the fundamental wave or double frequency harmonic in space or time or both; Also included is a rear optical system, wherein The quadrupled frequency crystal converts the doubled frequency light into quadrupled frequency light. After the quadrupled frequency light is emitted, it is shaped into the shape, size, or astigmatism characteristics of the quadrupled frequency light beam by the rear optical system before being emitted. The rear optical system includes four one-dimensional curved lenses, each comprising two lenses in a group, wherein one group is arranged in the same direction and the other group is arranged in another direction.

2. The device for generating ultraviolet light by frequency conversion of laser radiation according to claim 1, characterized in that: The cross-sectional light intensity distribution shape of the incident fundamental wave beam is circular, the cross-sectional light intensity distribution shape of the remaining fundamental wave, the second harmonic and the fourth harmonic in the quadruple frequency crystal is elliptical, and the shape of the quadruple frequency output beam is approximately circular.

3. The device for generating ultraviolet light by frequency conversion of laser radiation according to claim 2, characterized in that: The shaping module adopts one of the following forms: 1) One or more pieces of material transparent to the second harmonic, wherein one interface is fabricated so that the outgoing light is emitted at a Brewster angle; 2) One or more devices capable of pulse length compression or stretching; 3) One or more optical components with special cutting angles, which are transparent media that compensate for the beam deviation that may occur when the quadruple frequency crystal is moved in parallel.

4. The device for generating ultraviolet light by frequency conversion of laser radiation according to claim 3, characterized in that: Also included are optical reflective components; The double frequency crystal converts the light beam adjusted by the quadruple frequency crystal into a double frequency harmonic, and emits the double frequency harmonic to the optical reflective component; The optical reflective component reflects and deflects the second harmonic, which then enters the second harmonic crystal again. When passing through the second harmonic crystal again, it undergoes second harmonic conversion to generate more second harmonics, and emits the second harmonic to the quadruple harmonic crystal. The quadruple harmonic crystal performs double frequency conversion on the input second harmonic within the quadruple harmonic crystal to generate fourth harmonics. The fourth harmonic is emitted after passing through the Brewster angle interface of the quadruple harmonic crystal and is corrected to a fourth harmonic output light beam having a cross-sectional light intensity distribution shape that is substantially consistent with the initially input fundamental light beam or a required shape.

5. The device for generating ultraviolet light by frequency conversion of laser radiation according to claim 4, characterized in that: The optical reflective component adopts: A set of components consisting of multiple reflective mirrors; Or, a set of 180-degree turning mirrors; or, a spatial walk-off angle adjustment crystal; Or, an optical pulse width modulation medium.

6. The device for generating ultraviolet light by frequency conversion of laser radiation according to claim 5, characterized in that: The quadrupled frequency crystal serves as a shaping module, and one interface of the quadrupled frequency crystal is cut into a Brewster angle interface, which has both a fundamental wave beam incident surface and an exit surface for the fundamental wave and the second harmonic. The fundamental wave beam is incident on the Brewster angle interface of the quadrupled frequency crystal, and the light intensity distribution of the input fundamental wave beam is adjusted to adjust the cross-sectional shape of the generated fundamental wave, and then emitted to the doubled frequency crystal. The doubled frequency crystal converts the fundamental wave sent by the quadrupled frequency crystal into a doubled frequency harmonic, and emits the remaining fundamental wave or doubled frequency harmonic to the optical reflective component. The optical reflective component reflects and turns the doubled frequency harmonic and then emits the doubled frequency harmonic to another interface of the quadrupled frequency crystal. The quadrupled frequency crystal converts the input doubled frequency harmonic into a doubled frequency harmonic within the quadrupled frequency crystal to generate a quadrupled frequency harmonic. The quadrupled frequency harmonic passes through the Brewster angle interface of the quadrupled frequency crystal and is emitted, restoring to a quadrupled frequency output beam having a cross-sectional light intensity distribution shape substantially consistent with that of the initially input fundamental wave beam.

7. The device for generating ultraviolet light by frequency conversion of laser radiation according to claim 1, characterized in that: The system also includes a compensation module for pre-correcting or pre-setting the offset of the spatial variation of the output beam caused by the movement of the quadruple frequency crystal by selecting a compensation material and shape, so as to offset the variation caused by the movement of the quadruple frequency crystal and keep the output beam substantially unchanged in space. The compensation module is a wedge-shaped transparent module cut at the same angle as the quadruple frequency crystal and arranged at an appropriate angle on a device that moves with the quadruple frequency crystal. When the quadruple frequency crystal translates, it pre-translates the doubled frequency light beam to partially or completely offset the translation of the output light beam caused by the movement of the quadruple frequency crystal due to the need to change the point. Alternatively, it is a transparent window with two parallel surfaces, which offsets part or all of the changes in the ultraviolet light beam caused by the movement of the quadruple frequency crystal by rotating and adjusting the refraction and translation of the double frequency light beam to be converted to produce the quadruple frequency; Alternatively, it is a transparent prism or a group of transparent prisms, which offset part or all of the changes in the ultraviolet beam caused by the movement of the quadruple frequency crystal by adjusting its spatial position relative to the double frequency beam to be converted to produce the quadruple frequency through appropriate rotation or translation.

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