A continuous wave deep ultraviolet laser and method

Abstract

The application belongs to the technical field of laser, and proposes a continuous wave deep ultraviolet laser and method, which comprises a single-frequency seed laser, a multi-stage optical fiber amplification module, a single-pass second harmonic conversion module, a resonant enhancement fourth harmonic conversion module and a beam shaping module, the single-frequency seed laser is used for outputting fundamental frequency laser, the multi-stage optical fiber amplification module is used for power amplifying the fundamental frequency laser, the single-pass second harmonic conversion module directly uses a periodical polarization nonlinear crystal to convert the amplified fundamental frequency laser into second harmonic laser, the resonant enhancement fourth harmonic conversion module is used for converting the second harmonic laser into deep ultraviolet laser through a resonant cavity, and the beam shaping module is used for shaping the deformed spot of the deep ultraviolet laser into a Gaussian spot, under the premise of ensuring efficient output of the continuous wave deep ultraviolet laser, the application reduces the number of resonant enhancement cavities, and reduces the system complexity and debugging difficulty.

Description

Technical Field

[0001] This invention relates to the field of laser technology, specifically to a continuous-wave deep ultraviolet laser and method. Background Technology

[0002] The statements in this section are merely background information related to the present invention and do not necessarily constitute prior art.

[0003] Continuous-wave deep ultraviolet (DUV) laser sources, especially those with wavelengths in the 200nm-300nm range, have been widely applied in industrial and scientific research fields such as semiconductor wafer defect detection and flow cytometry analysis due to their short wavelength and high photon energy. Compared to visible and near-ultraviolet light, DUV lasers have a significant advantage in Rayleigh scattering intensity, as their scattering intensity is inversely proportional to the fourth power of the wavelength, significantly improving detection sensitivity. Simultaneously, the DUV band can effectively suppress the autofluorescence background of samples, improving the signal-to-noise ratio. Furthermore, continuous-wave operation offers advantages such as low peak power and minimal optical damage to the sample. Therefore, developing stable, high-power, and high-beam-quality continuous-wave deep ultraviolet laser sources is of great engineering significance.

[0004] Converting infrared lasers to the deep ultraviolet band through multi-level frequency doubling (such as second harmonic and fourth harmonic generation) using nonlinear optics remains the most mature, flexible, and scalable technology. Because the nonlinear frequency conversion efficiency is highly sensitive to light intensity, and the instantaneous light intensity of continuous-wave lasers is much lower than that of pulsed lasers, generating continuous-wave deep ultraviolet lasers is significantly more difficult than generating pulsed lasers. To improve conversion efficiency, existing technologies typically employ resonant enhancement cavity structures to increase intracavity optical power. Early research has reported the use of solid-state lasers such as Nd:YAG and Nd:YVO4 as the fundamental frequency source, combined with intracavity second harmonic generation and resonant enhancement fourth harmonic generation, to achieve deep ultraviolet continuous-wave output. For example, systems using CLBO crystals can achieve an output power of 5 watts.

[0005] With technological advancements, fundamental frequency laser sources are gradually evolving from traditional solid-state lasers to single-frequency lasers, such as non-planar ring oscillators (NPROs), external cavity semiconductor lasers (ECDLs), and fiber lasers, often used in conjunction with multi-stage fiber amplification systems. However, due to the low single-pass second harmonic conversion efficiency, existing high-power systems generally still require two resonant enhancement cavities, resulting in complex system structures and high requirements for cavity length locking and servo control stability, increasing the difficulty of system implementation and operation and maintenance costs. To simplify the system structure, some research has proposed using kilowatt-level high-power fiber lasers combined with single-pass LBO crystals to generate second harmonics. However, this approach typically requires the introduction of an additional phase modulator to broaden the laser linewidth, thereby increasing the stimulated Brillouin scattering threshold, leading to an output laser linewidth reaching tens of gigahertz, which is unsuitable for applications with high requirements for linewidth and noise performance.

[0006] Periodically polarized nonlinear crystals (such as PPLN and PPsLT) are considered an effective way to improve single-pass frequency doubling efficiency under low to medium fundamental frequency power conditions due to their high effective nonlinear coefficient and high single-pass frequency doubling efficiency. Previous studies have attempted to use periodically polarized crystals in continuous-wave deep ultraviolet lasers, but the second harmonic output power in existing schemes remains limited, thus restricting the output capability of subsequent fourth harmonics. In contrast, at the 532nm band, the single-pass second harmonic output power based on PPsLT crystals has generally reached the tens of watts, with performance comparable to resonant cavity solutions. Meanwhile, single-pass frequency doubling structures have potential advantages in system compactness, frequency noise, and relative intensity noise, and are unaffected by cavity-locked systems, thus being considered a competitive alternative.

[0007] Despite significant advancements in output power for continuous-wave deep ultraviolet lasers, beam quality has long been a relatively neglected issue. For fourth harmonic generation, BBO and CLBO crystals remain the most commonly used nonlinear crystal materials, regardless of the method used to achieve the second harmonic. BBO crystals possess a large effective nonlinear coefficient, but their large walk-off angle in the visible light band easily introduces severe spatial walk-off effects, leading to beam distortion in the walk-off direction and a decrease in beam quality. To mitigate this problem, existing technologies often employ paired rotating crystals for walk-off compensation. However, this method is extremely sensitive to angle adjustment, difficult to assemble and adjust, and unsuitable for intracavity frequency doubling structures. Even with CLBO crystals, which typically require longer crystal lengths to improve conversion efficiency, significant beam distortion still occurs. Summary of the Invention

[0008] To address the common problems of existing continuous-wave deep ultraviolet lasers, such as complex system structure, numerous resonant enhancement cavities, difficult servo control, low single-pass frequency doubling efficiency, and degraded deep ultraviolet output beam quality, this invention provides a continuous-wave deep ultraviolet laser and method. While ensuring high-efficiency continuous-wave deep ultraviolet laser output, it reduces the number of resonant enhancement cavities, thereby lowering system complexity and debugging difficulty. Simultaneously, through an effective beam shaping method, it achieves near-diffraction-limited deep ultraviolet laser output, thus expanding its applications in high-end industrial inspection and precision scientific experiments.

[0009] To achieve the above objectives, the present invention adopts the following technical solution:

[0010] In a first aspect, the present invention provides a continuous wave deep ultraviolet laser.

[0011] A continuous-wave deep ultraviolet laser includes: a single-frequency seed laser, a multi-stage fiber amplification module, a single-pass second harmonic conversion module, a resonant-enhanced fourth harmonic conversion module, and a beam shaping module. The single-frequency seed laser is used to output a fundamental frequency laser, the multi-stage fiber amplification module is used to amplify the power of the fundamental frequency laser, the single-pass second harmonic conversion module directly uses a periodically polarized nonlinear crystal to convert the amplified fundamental frequency laser into a second harmonic laser, and the resonant-enhanced fourth harmonic conversion module is used to convert the second harmonic laser into a deep ultraviolet laser through a resonant cavity.

[0012] The beam shaping module is used to shape the distorted spot of the deep ultraviolet laser into a Gaussian spot. The beam shaping module includes a second collimating lens, a concave cylindrical lens, a convex cylindrical lens, and a slit arranged sequentially along the optical path. The second collimating lens is used to collimate the spot in the non-departing direction, the concave cylindrical lens is used to provide phase compensation for the spot in the departing direction to make it diverge, and when the spot sizes in the non-departing direction and the departing direction are equal, the convex cylindrical lens collimates the departing spot, and the slit is used to shield the spot at the edge.

[0013] In one implementation of the first aspect of the present invention, the single-frequency seed laser is an external cavity semiconductor laser, a distributed feedback semiconductor laser, or a fiber laser.

[0014] In one implementation of the first aspect of the present invention, the multi-stage fiber amplification module adopts an all-fiber structure and is used to amplify the fundamental frequency laser output by the single-frequency seed laser to the level of hundreds of watts.

[0015] In one implementation of the first aspect of the present invention, the single-pass second harmonic conversion module includes a focusing lens, a periodically polarized nonlinear crystal and a first collimating lens arranged sequentially along the optical path. The single-pass second harmonic conversion module is also equipped with a temperature control device to achieve stable frequency doubling output under quasi-phase matching conditions.

[0016] In one implementation of the first aspect of the present invention, the resonant enhancement fourth harmonic conversion module adopts a bowtie-type resonant cavity, and a nonlinear crystal is placed in the bowtie-type resonant cavity to realize the fourth harmonic conversion.

[0017] As a further limitation of the first aspect of the present invention, the resonant enhancement fourth harmonic conversion module includes: a nonlinear crystal, a piezoelectric ceramic, an input flat mirror, a high-reflection flat mirror, a concave mirror, a two-color concave mirror, a quarter-wave plate, a polarizing beam splitter, a photodetector, a subtractor, a PI controller, a high-voltage driver, and a beam sampler.

[0018] An input flat mirror, a high-reflection flat mirror, a concave mirror, and a bicolor concave mirror constitute a bowtie-shaped resonant cavity. A nonlinear crystal is arranged in the optical path between the concave mirror and the bicolor concave mirror. The 532nm second harmonic laser output from the single-pass second harmonic conversion module is transmitted through the input flat mirror into the bowtie-shaped resonant cavity. The 532nm second harmonic laser is reflected by the high-reflection flat mirror and the concave mirror in sequence before entering the nonlinear crystal. The nonlinear crystal performs nonlinear frequency doubling under the action of the 532nm second harmonic laser.

[0019] Part of the 532nm second harmonic laser is converted into a 266nm deep ultraviolet fourth harmonic laser. A nonlinear crystal outputs a mixed beam of the 532nm second harmonic laser and the 266nm deep ultraviolet fourth harmonic laser. The 266nm deep ultraviolet fourth harmonic laser in the mixed beam is output through a bowtie-shaped resonant cavity via a two-color concave mirror. The 532nm second harmonic laser is reflected to the input flat mirror, part of the 532nm second harmonic laser is reflected to a high-reflection flat mirror, and part of the 532nm second harmonic laser is transmitted to the beam sampler.

[0020] The beam sampler transmits the 532nm second harmonic laser to the polarization beam splitter via a quarter-wave plate. The polarization beam splitter splits the received light into two orthogonal linearly polarized beams, which are directed to two corresponding photodetectors. The photodetectors convert the light intensity signal into an electrical signal. The two electrical signals are transmitted synchronously to a subtractor to calculate the difference. The difference is input to a PI controller. The PI controller is amplified and corrected by a proportional-integral algorithm, and then outputs a control signal to the piezoelectric ceramic via a high-voltage driver.

[0021] The piezoelectric ceramic is fixedly connected to the high-reflection mirror. The expansion and contraction of the piezoelectric ceramic drives the translation of the high-reflection mirror, correcting the cavity length of the bowtie-type resonant cavity until the error signal returns to zero and the cavity length returns to the resonant state, thus achieving stable locking.

[0022] As a further limitation of the first aspect of the invention, the nonlinear crystal is a BBO or CLBO crystal.

[0023] In one implementation of the first aspect of the present invention, the continuous wave output power of the deep ultraviolet laser is in the watt range.

[0024] In one implementation of the first aspect of the present invention, the beam shaping module compensates for the phase delay of the walk-off effect in the orthogonal direction of the beam spot and outputs a circular Gaussian beam spot.

[0025] Secondly, the present invention provides a method for generating continuous-wave deep ultraviolet laser.

[0026] A method for generating continuous-wave deep ultraviolet laser, utilizing the continuous-wave deep ultraviolet laser of the first aspect of this invention, includes the following process:

[0027] Single-frequency seed lasers provide single-frequency, narrow-linewidth, and frequency-stable fundamental continuous-wave lasers;

[0028] A multi-stage fiber amplification module amplifies the fundamental frequency continuous wave laser in multiple stages to obtain a high-power fundamental frequency continuous wave laser.

[0029] The single-pass second harmonic conversion module uses a periodically polarized nonlinear crystal to perform single-pass second harmonic conversion on high-power fundamental frequency continuous wave laser in a quasi-phase-matched manner to obtain second harmonic laser.

[0030] The resonant-enhanced fourth harmonic conversion module performs fourth harmonic conversion on the second harmonic laser through the resonant-enhanced cavity structure, and uses cavity length locking technology to maintain the stability of the resonant-enhanced cavity, outputting deep ultraviolet continuous wave laser.

[0031] The beam shaping module shapes the deep ultraviolet continuous wave laser beam, compensating for beam intensity and phase distortion caused by the nonlinear crystal walk-off effect, and finally obtains a watt-level power continuous wave deep ultraviolet laser with a near-circular symmetric Gaussian spot.

[0032] Compared with the prior art, the beneficial effects of the present invention are:

[0033] This invention leverages the unique properties of periodically polarized nonlinear crystals to successfully achieve efficient single-pass second harmonic conversion, breaking away from the traditional approach where both second and fourth harmonics rely on resonant enhancement cavities. Instead, it uses only one resonant enhancement cavity in the fourth harmonic stage, completely eliminating the need for complex dual-cavity designs. Traditional dual-cavity structures require two independent resonant cavity components and high-precision locking and servo control of the two cavity lengths, significantly increasing system complexity and debugging difficulty. The single-cavity design of this invention drastically reduces the number of optical components related to the resonant cavity, simplifies the optical path layout, and makes the overall system more compact. Furthermore, the locking operation of a single resonant cavity eliminates the need for complex coordinated adjustments between the two cavities, reducing the stringent requirements for locking technology and decreasing the workload and time costs during debugging. This makes system setup and subsequent maintenance more convenient and efficient, significantly improving the system's operability and practicality.

[0034] This invention successfully achieves high-power continuous-wave deep ultraviolet laser output. Single-frequency characteristics are crucial for many high-end applications, but traditional technologies often struggle to achieve high-power output while maintaining this characteristic. This invention optimizes the collaborative operation of a multi-stage fiber amplification module and a single-pass second harmonic conversion module. During the amplification of the fundamental frequency laser power, it effectively maintains the laser's single-frequency narrow linewidth characteristic, providing a high-quality light source foundation for subsequent harmonic conversion. The efficient single-pass frequency doubling capability of the periodically polarized nonlinear crystal ensures high conversion efficiency for the second harmonic, thus providing sufficient energy input for the generation of the fourth harmonic. The resonant-enhanced fourth harmonic conversion module further improves the energy conversion efficiency, resulting in a high overall optical-to-optical conversion efficiency. This efficient energy conversion mechanism not only achieves high-power output but also possesses excellent engineering scalability. By rationally adjusting the parameters of each module according to actual application needs, the output power can be further increased, meeting the diverse requirements for high-power continuous-wave deep ultraviolet lasers in different scenarios.

[0035] To address the walk-off effect that easily occurs in nonlinear crystals during harmonic conversion, this invention designs a dedicated beam shaping module to precisely and specifically compensate for the resulting spatial phase distortion. During deep ultraviolet laser generation, the walk-off effect of nonlinear crystals causes beam propagation deviation in specific directions, leading to beam distortion and reduced beam quality. This severely limits the use of lasers in high-precision applications. The beam shaping module of this invention, through its unique optical design, can accurately identify and compensate for the phase delay caused by the walk-off effect, effectively correcting the beam propagation characteristics in orthogonal directions. After shaping, the originally distorted beam is corrected to a near-circular symmetrical Gaussian distribution. This ideal beam shape has high spatial consistency and stability, resulting in a more uniform energy distribution and significantly improved focusing performance. The significantly improved beam quality allows the laser to exhibit superior performance in applications requiring high-precision beam control, expanding its application boundaries.

[0036] This invention abandons the traditional approach of increasing the stimulated Brillouin scattering threshold by widening the linewidth, fundamentally ensuring the low-frequency noise and low relative intensity noise characteristics of the output laser. In traditional solutions, widening the linewidth leads to decreased laser coherence and poorer frequency stability, making it difficult to meet the stringent performance requirements of applications. This invention, however, optimizes the structure and operating parameters of the multi-stage fiber amplification module, effectively suppressing stimulated Brillouin scattering while increasing laser power, without sacrificing linewidth. This ensures the output laser maintains high frequency stability and coherence, and the low noise characteristics guarantee stable performance during long-term operation, with minimal fluctuations in various parameters of the output beam. Whether it's the stringent requirements for laser stability in precision scientific experiments or the high standards for beam quality consistency in high-end industrial testing, this invention is perfectly suited, demonstrating strong applicability and stability, and can operate stably and continuously in complex and harsh working environments.

[0037] In terms of material selection, this invention prioritizes nonlinear crystal materials with low hygroscopicity and relatively controllable cost. This choice reduces the engineering implementation cost of the system from the source, while significantly improving the long-term operational reliability of the system. Crystal materials with low hygroscopicity can effectively avoid performance degradation and damage caused by absorbing moisture from the air during use, reducing system failures caused by changes in material properties, extending the lifespan of the crystal and the maintenance cycle of the system. The cost-controllable material selection makes the manufacturing cost of the entire laser more competitive in the market, achieving high-performance laser output without excessive investment. In addition, the simple system structure design reduces the number of optical components and complex assembly processes, reducing the probability of errors during system production and debugging, further improving the reliability of the system. The combination of stable material properties and simple system structure ensures that the system can maintain stable performance output during long-term operation, with low maintenance costs, making it very suitable for industrial promotion and enabling rapid large-scale production and widespread application.

[0038] Advantages of additional aspects of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0039] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0040] Figure 1 A schematic diagram of a continuous deep ultraviolet laser provided as an exemplary embodiment of the present invention;

[0041] Figure 2A specific optical path diagram of a continuous deep ultraviolet laser provided as an exemplary embodiment of the present invention;

[0042] Figure 3 The wavelength of the deep ultraviolet laser is measured as an exemplary embodiment of the present invention.

[0043] Figure 4 The variation of deep ultraviolet output power with second harmonic power and fourth harmonic conversion efficiency is provided as an exemplary embodiment of the present invention.

[0044] Figure 5 A shaped spot pattern provided as an exemplary embodiment of the present invention;

[0045] The components include: 1. Single-frequency seed laser module; 2. Multi-stage fiber amplification module; 201. Connecting fiber; 3. Single-pass second harmonic conversion module; 301. Focusing lens; 302. Periodically polarized nonlinear crystal; 303. First collimating lens; 304. Heating device; 4. Resonance-enhanced fourth harmonic conversion module; 401. BBO crystal; 402. Piezoelectric ceramic; 403. Input flat mirror; 404. High-reflection flat mirror; 405. Concave mirror; 406. Two-color concave mirror; 407. Quarter-wave plate; 408. Polarizing beam splitter prism; 409. Photodetector; 410. Subtractor; 411. PI controller; 412. High-voltage driver; 413. Beam sampler; 5. Beam shaping module; 501. Second collimating lens; 502. Concave cylindrical lens; 503. Convex cylindrical lens; 504. Slit. Detailed Implementation

[0046] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0047] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0048] This implementation proposes a continuous deep ultraviolet laser, such as... Figure 1 As shown, it includes a single-frequency seed laser module 1, a multi-stage fiber amplification module 2, a single-pass second harmonic conversion module 3, a resonant-enhanced fourth harmonic conversion module 4, and a beam shaping module 5.

[0049] In this implementation, the single-frequency seed laser module 1 uses an external cavity semiconductor laser (ECDL) with a working wavelength of 1064nm. The output is a continuous wave single longitudinal mode laser with an output power of 10mW and a linewidth of less than 15kH, which is used as the fundamental frequency light source for the entire system. The external cavity semiconductor laser (ECDL) achieves stable output through temperature control and current control to ensure the frequency stability of the subsequent frequency doubling process.

[0050] Understandably, in some other implementations, the single-frequency seed laser module 1 can also use a distributed feedback semiconductor laser (DFB) or a fiber laser with a linewidth <1MHz, which will not be described in detail here.

[0051] In this implementation, such as Figure 2 As shown, the multi-stage fiber amplification module 2 (adopting an all-fiber structure) is connected to the single-frequency seed laser module 1 through the connecting fiber 201. It includes a three-stage ytterbium-doped fiber amplifier, which is used to amplify the power of the light emitted by the single-frequency seed laser module 1 step by step (to amplify the fundamental frequency laser output by the single-frequency seed laser module 1 to the level of hundreds of watts).

[0052] In this implementation, the fundamental frequency laser output power after fiber amplification is above 100W, the output beam remains near the diffraction limit, the M² factor is less than 1.2, and the laser linewidth does not show significant broadening.

[0053] In this implementation, the single-pass second harmonic conversion module 3 includes a periodically polarized nonlinear crystal 302, which is used to convert the 1064nm fundamental frequency laser into a 532nm second harmonic laser in a single pass.

[0054] In this implementation, the periodically polarized nonlinear crystal 302 is a periodically polarized lithium tantalate crystal (PPsLT) with a crystal length of 30 mm and a polarization period of 8 mm. A quasi-phase-matched method is used to generate the second harmonic. The focusing lens 301 converges the fundamental frequency light onto the center of the periodically polarized nonlinear crystal 302. The heating device 304 controls the temperature of the periodically polarized nonlinear crystal 302 at its most efficient temperature (to achieve stable frequency doubling output under quasi-phase-matched conditions). The first collimating lens 303 collimates the generated second harmonic before outputting it. Testing shows that, without the need for a resonant enhancement cavity, the single-pass second harmonic output power can reach over 15 W, with a conversion efficiency significantly higher than traditional single-pass LBO or BBO frequency doubling schemes.

[0055] In some other implementations, the periodically polarized nonlinear crystal 302 can also be a periodically polarized lithium niobate crystal (PPLN) to achieve high-efficiency single-pass second harmonic conversion of the fundamental frequency laser.

[0056] In this implementation, the resonant enhancement fourth harmonic conversion module 4 (using Hänsch–Couillaud locking technology or Pound–Drever–Hall locking technology to achieve cavity length stabilization) includes a bowtie-shaped resonant cavity composed of an input flat mirror 403, a high-reflection flat mirror 404, a concave mirror 405, and a two-color concave mirror 406, and a BBO crystal 401 (using type I phase matching) disposed in the cavity. The BBO crystal 401 is arranged in the optical path between the concave mirror 405 and the two-color concave mirror 406. The 532nm second harmonic laser output from the single-pass second harmonic conversion module 3 is transmitted into the bowtie-shaped resonant cavity through the input flat mirror 403. The 532nm second harmonic laser is reflected by the high-reflection flat mirror 404 and the concave mirror 405 in sequence before entering the BBO crystal 401. The BBO crystal 401 performs nonlinear frequency doubling under the action of the 532nm second harmonic laser.

[0057] A portion of the 532nm second harmonic laser is converted into a 266nm deep ultraviolet fourth harmonic laser. A mixed beam of the 532nm second harmonic laser and the 266nm deep ultraviolet fourth harmonic laser is emitted from the BBO crystal 401. The 266nm deep ultraviolet fourth harmonic laser in the mixed beam passes through a bicolor concave mirror 406 and outputs to a bowtie-type resonant cavity. The 532nm second harmonic laser is reflected to the input flat mirror 403, a portion of the 532nm second harmonic laser is reflected to a high-reflection flat mirror 404, and a portion of the 532nm second harmonic laser is transmitted to a beam sampler 413. The beam sampler 413 collects the reflected signal, which, after passing through a quarter-wave plate 407, is polarized... The beam splitter prism 408 splits the beam into two beams, which strike two photodetectors 409 respectively. The electrical signals generated by the photodetectors 409 are used by a subtractor 410 to generate error signals. After being processed by a PI controller 411, these signals are sent to a high-voltage driver 412, which then feeds back to the piezoelectric ceramic 402 to control the cavity length and lock the resonant enhancement cavity (the piezoelectric ceramic is fixedly connected to a high-reflection mirror; the expansion and contraction of the piezoelectric ceramic drives the translation of the high-reflection mirror, correcting the cavity length of the bowtie-shaped resonant cavity until the error signal returns to zero and the cavity length returns to the resonant state, achieving stable locking). This further converts the 532nm second harmonic laser into a 266nm deep ultraviolet continuous wave laser. Under the above configuration of the present invention, the system can stably output 266nm continuous wave deep ultraviolet laser, and wavelength testing is as follows. Figure 3 As shown, the maximum output power is approximately 3W or more, corresponding to a fourth harmonic conversion efficiency of over 30%. Figure 4 As shown. Figure 5 The image shows the shaped light spot, which can be seen to exhibit a circular Gaussian light spot distribution.

[0058] Optionally, in some other implementations, BBO crystal 401 (i.e., barium metaborate crystal) can also be replaced with CLBO crystal (i.e., lithium cesium borate crystal).

[0059] Optionally, in some other implementations, the bowtie-shaped resonator can be replaced with a ring or shaped resonator to enhance the second harmonic optical power within the cavity and improve the fourth harmonic conversion efficiency.

[0060] In this implementation, the beam shaping module 5 (used to modulate the spatial phase of the deep ultraviolet laser in the walk-off direction to compensate for the phase distortion introduced by the nonlinear crystal walk-off effect, so that the output beam is restored to a near-circular symmetrical Gaussian distribution) includes a second collimating lens 501, a concave cylindrical lens 502, a convex cylindrical lens 503, and a slit 504. By reasonably selecting the focal length and spacing of the cylindrical lenses, the spatial phase in the walk-off direction is compensated, so that the wavefront curvature of the deep ultraviolet laser in the two orthogonal directions tends to be consistent, thereby restoring the near-circular symmetrical Gaussian beam distribution.

[0061] The specific beam shaping method is as follows: the second collimating lens 501 collimates the spot in the direction that has not strayed, the concave cylindrical lens 502 provides phase compensation to the spot in the direction that has strayed, causing it to diverge. When the spot sizes in both directions are equal, the convex cylindrical lens 503 collimates the strayed spot, and the slit 504 is used to shield small spots at the edges. Testing shows that the 266nm laser beam after beam shaping exhibits a near-Gaussian distribution, with an M² factor preferably less than 1.2, approaching the diffraction limit. Figure 4 As shown.

[0062] Through the above technical solution, the present invention achieves high-power, high-efficiency and high-beam-quality continuous-wave deep ultraviolet laser output while reducing the number of resonant enhancement cavities.

[0063] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A continuous wave deep ultraviolet laser, characterized in that, comprising: a single-frequency seed laser, a multi-stage fiber amplifier module, a single-pass second harmonic conversion module, a resonant-enhanced fourth harmonic conversion module and a beam shaping module, the single-frequency seed laser is used to output a fundamental frequency laser, the multi-stage fiber amplifier module is used to power amplify the fundamental frequency laser, the single-pass second harmonic conversion module directly uses a periodically poled nonlinear crystal to convert the amplified fundamental frequency laser into a second harmonic laser, and the resonant-enhanced fourth harmonic conversion module is used to convert the second harmonic laser into a deep ultraviolet laser through a resonant cavity; the beam shaping module is used to shape a deformed spot of the deep ultraviolet laser into a Gaussian spot, and the beam shaping module comprises, in sequence along an optical path, a second collimating lens, a concave cylindrical lens, a convex cylindrical lens and a slit, the second collimating lens is used to collimate a spot in a non-walk-off direction, the concave cylindrical lens is used to phase compensate and diverge a spot in a walk-off direction, when the spot sizes in the non-walk-off direction and the walk-off direction are equal, the convex cylindrical lens collimates the walk-off spot, and the slit is used to shield the edge spot; the resonant-enhanced fourth harmonic conversion module adopts a bow-tie resonant cavity, and a nonlinear crystal is placed in the bow-tie resonant cavity to realize fourth harmonic conversion; the resonant-enhanced fourth harmonic conversion module comprises a nonlinear crystal, a piezoelectric ceramic, an input flat mirror, a high-reflection flat mirror, a concave mirror, a bichromatic concave mirror, a quarter-wave plate, a polarization beam splitter prism, a photodetector, a subtracter, a PI controller, a high-voltage driver and a beam sampler; the input flat mirror, the high-reflection flat mirror, the concave mirror and the bichromatic concave mirror constitute the bow-tie resonant cavity, the nonlinear crystal is arranged on an optical path between the concave mirror and the bichromatic concave mirror, 532 nm second harmonic laser output by the single-pass second harmonic conversion module is transmitted into the bow-tie resonant cavity through the input flat mirror, the 532 nm second harmonic laser enters the nonlinear crystal after being reflected by the high-reflection flat mirror and the concave mirror in sequence, and the nonlinear crystal performs nonlinear frequency doubling under the action of the 532 nm second harmonic laser; part of the 532 nm second harmonic laser is converted into 266 nm deep ultraviolet fourth harmonic laser, a mixed beam of the 532 nm second harmonic laser and the 266 nm deep ultraviolet fourth harmonic laser is emitted from the nonlinear crystal, the 266 nm deep ultraviolet fourth harmonic laser in the mixed beam is output from the bow-tie resonant cavity through the bichromatic concave mirror, the 532 nm second harmonic laser is reflected to the input flat mirror, part of the 532 nm second harmonic laser is reflected to the high-reflection flat mirror, and part of the 532 nm second harmonic laser is transmitted to the beam sampler; the beam sampler transmits the 532 nm second harmonic laser to the polarization beam splitter prism through the quarter-wave plate, the polarization beam splitter prism divides the received light into two beams of orthogonal linearly polarized light, and the two beams of light are respectively directed to two corresponding photodetectors, the photodetectors convert the light intensity signals into electrical signals, the two electrical signals are synchronously transmitted to the subtracter to be subtracted, the difference value is input to the PI controller, the PI controller is amplified and corrected through a proportional-integral algorithm, and a control signal is output to the piezoelectric ceramic through the high-voltage driver. The piezoelectric ceramic is fixedly connected with the high-reflectivity mirror, and the high-reflectivity mirror is driven to translate by the expansion and contraction of the piezoelectric ceramic, so that the cavity length of the bow-tie resonant cavity is corrected until the error signal returns to zero, the cavity length of the resonant cavity returns to the resonant state, and stable locking is realized.

2. The continuous wave deep ultraviolet laser of claim 1, wherein the single frequency seed laser is an external cavity semiconductor laser, a distributed feedback semiconductor laser, or a fiber laser.

3. The continuous wave deep ultraviolet laser of claim 1, wherein the multi-stage fiber amplification module is a full fiber structure, and the multi-stage fiber amplification module is configured to amplify the fundamental frequency laser output by the single frequency seed laser to a hundred watt level.

4. The continuous wave deep ultraviolet laser of claim 1, wherein the single-pass second harmonic conversion module comprises a focusing lens, a periodically poled nonlinear crystal, and a first collimating lens arranged in sequence along an optical path, and the single-pass second harmonic conversion module is further configured with a temperature control device for realizing stable frequency doubling output under quasi-phase matching conditions.

5. The continuous wave deep ultraviolet laser of claim 1, wherein the nonlinear crystal is a BBO or CLBO crystal.

6. The continuous wave deep ultraviolet laser of claim 1, wherein the continuous wave output power of the deep ultraviolet laser is a watt level.

7. The continuous wave deep ultraviolet laser of claim 1, wherein the beam shaping module compensates for the phase delay of the walk-off effect in the orthogonal direction of the spot, and outputs a circular Gaussian spot. The continuous wave deep ultraviolet laser of any one of claims 1-7, comprising the following processes: The single frequency seed laser provides a single frequency, narrow linewidth, and frequency stable fundamental continuous wave laser; The multi-stage fiber amplification module multi-stage power amplifies the fundamental continuous wave laser to obtain a high-power fundamental continuous wave laser; The single-pass second harmonic conversion module uses a periodically poled nonlinear crystal to perform single-pass second harmonic conversion on the high-power fundamental continuous wave laser in a quasi-phase matching manner to obtain a second harmonic laser; The resonant enhanced fourth harmonic conversion module performs fourth harmonic conversion on the second harmonic laser through a resonant enhanced cavity structure, and uses cavity length locking technology to maintain the stability of the resonant enhanced cavity to output a deep ultraviolet continuous wave laser; The beam shaping module performs beam shaping on the deep ultraviolet continuous wave laser to compensate for the beam intensity and phase distortion caused by the walk-off effect of the nonlinear crystal, and finally obtains a near-circular symmetric Gaussian spot of a watt-level power continuous wave deep ultraviolet laser.

8. A method of generating a continuous wave deep ultraviolet laser, characterized by, ​ ​ ​ ​ ​ ​

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