Femtosecond third harmonic light path control method, device, equipment and medium

Abstract

The invention relates to the technical field of femtosecond laser nonlinear frequency conversion, and discloses a femtosecond third harmonic light path control method and device, equipment and a medium. The method comprises the following steps: dividing infrared light output by a femtosecond laser into two paths according to a certain proportion by using an infrared beam splitter, namely a first path of infrared light and a second path of infrared light; the first path of infrared light enters a frequency doubling crystal for frequency doubling to generate green light, and the generated green light is subjected to beam shaping through a green light beam expanding system, so that the spot size of the shaped green light is matched with the spot size of the second path of infrared light; time delay control is applied to the second path of infrared light, so that the corresponding pulses are overlapped in time; and the second path of infrared light after time delay control and the green light after light beam shaping are combined through the spectroscope and then jointly enter a frequency combination crystal for frequency combination, the Gaussian energy distribution of the second path of infrared light and the Gaussian energy distribution of the green light are used for frequency combination, and ultraviolet light output is generated.

Description

Technical Field

[0001] This application relates to the field of femtosecond laser nonlinear frequency conversion technology, and particularly relates to a control method, device, equipment and medium for a femtosecond third harmonic optical path. Background Art

[0002] In the femtosecond laser third harmonic generation technology, the traditional optical path usually uses infrared light to generate green light by frequency doubling through a nonlinear crystal (such as LBO, BBO) first, and then the remaining infrared light and the green light are combined to generate ultraviolet light. However, this method has the following core defects:

[0003] 1. Time and space walk-off effects: The group velocities of the infrared light and the green light in the nonlinear crystal are inconsistent (time walk-off), and the propagation direction deviates from the wave vector direction (space walk-off), resulting in non-overlap of the pulses in time and space. The combination frequency efficiency is only 20%-30%, and the beam quality deteriorates significantly.

[0004] 2. Deterioration of energy distribution: The infrared light is a Gaussian beam with a high central energy density. After frequency doubling, the remaining infrared light forms a non-uniform energy distribution with a central depression because a large amount of the central energy is converted into green light, resulting in an imbalance in the peak power density matching during combination frequency, further reducing the combination frequency efficiency and exacerbating the spot distortion.

[0005] 3. Mismatch of spot sizes: The spot size of the green light generated by frequency doubling shrinks due to the change in energy distribution and is inconsistent with the spot size of the remaining infrared light, making it impossible to achieve efficient nonlinear sum frequency effect.

[0006] In the prior art, in order to balance the frequency doubling and combination frequency efficiencies, the frequency doubling efficiency is forced to be sacrificed (the frequency doubling efficiency is only 30%-40% at the best combination frequency), while the actual frequency doubling efficiency can reach more than 60%. This indicates that the traditional optical path does not fully utilize the energy conversion potential, and the beam quality problem has not been fundamentally solved.

[0007] Therefore, there is an urgent need for a method to solve at least one of the above problems. Summary of the Invention

[0008] This application provides a control method, device, equipment and medium for a femtosecond third harmonic optical path, aiming to solve the problems of time and space walk-off effects, deterioration of energy distribution, mismatch of spot sizes, etc. existing in the traditional optical path that usually uses infrared light to generate green light by frequency doubling through a nonlinear crystal (such as LBO, BBO) first, and then the remaining infrared light and the green light are combined to generate ultraviolet light.

[0009] In a first aspect, an embodiment of this application provides a control method for a femtosecond third harmonic optical path, including:

[0010] Using an infrared beam splitter to divide the infrared light output by a femtosecond laser into two paths according to a certain ratio, namely the first path of infrared light and the second path of infrared light;

[0011] The first infrared light is incident on a frequency doubling crystal to generate green light through frequency doubling. The frequency doubling crystal is an LBO or BBO nonlinear crystal, and the frequency doubling process is optimized to improve the frequency doubling efficiency of the green light;

[0012] The generated green light is beam-shaped through a green light beam expander system so that the spot size of the shaped green light matches the spot size of the second infrared light;

[0013] Time delay control is applied to the second infrared light, and the pulse time of the second infrared light is adjusted through a time delay control system to compensate for the time walk-off effect caused by the inconsistent group velocities of the green light and the second infrared light in the nonlinear crystal, so that the pulses of the first infrared optical path and the second infrared optical path overlap in time;

[0014] After the second infrared light controlled by time delay and the green light beam-shaped by the beam expander system are combined by a beam splitter, they are jointly incident on a sum frequency crystal for sum frequency. The sum frequency crystal is an LBO or BBO nonlinear crystal. The Gaussian energy distribution of the second infrared light and the Gaussian energy distribution of the green light are used for sum frequency to generate ultraviolet light output; wherein, the first infrared light is used to generate green light, and the second infrared light maintains the initial Gaussian energy distribution state to participate in sum frequency, avoiding deterioration of the remaining infrared light energy distribution due to too high frequency doubling efficiency, and realizing the spot size matching of the green light and the second infrared light through the green light beam expander system, and realizing the pulse time overlap of the first infrared optical path and the second infrared optical path through the time delay control system to improve the sum frequency efficiency and the beam quality of the ultraviolet light.

[0015] In some embodiments, using an infrared beam splitter to divide the infrared light output by a femtosecond laser into two paths according to a certain ratio, namely the first infrared light and the second infrared light, includes: determining the beam splitting ratio of the first infrared light and the second infrared light according to the nonlinear coefficient of the frequency doubling crystal, the incident power density, and the required infrared light energy distribution for sum frequency; so that the first infrared light has sufficient energy for efficient frequency doubling, and the second infrared light maintains the energy distribution state of the initial Gaussian beam, avoiding energy matching imbalance during sum frequency due to improper beam splitting ratio.

[0016] In some embodiments, optimizing the frequency doubling process to improve the frequency doubling efficiency of the green light includes: adjusting the incident angle of the frequency doubling crystal to satisfy the phase matching condition, controlling the incident power density of the first infrared light within the optimal threshold range of nonlinear conversion of the frequency doubling crystal, and performing temperature control on the frequency doubling crystal to compensate for crystal dispersion, so that the frequency doubling efficiency of the frequency doubling process is higher than the preset efficiency and does not affect the initial Gaussian energy distribution characteristics of the second infrared light.

[0017] In some embodiments, the green light beam expanding system adopts a telescopic optical structure, and collimates and expands the green light beam through the combination of a convex lens and a concave lens; shaping the generated green light through the green light beam expanding system to make the size of the shaped green light spot match the size of the spot of the second infrared light, including: according to the spot diameter of the second infrared light, adjusting the focal length ratio of the beam expanding system to make the error between the diameter of the shaped green light spot and the diameter of the second infrared light spot less than a preset error, so as to achieve the spatial mode matching of the two beams of light.

[0018] In some embodiments, the time delay control system includes a mirror group with adjustable optical path; applying time delay control to the second infrared light, and adjusting the pulse time of the second infrared light through the time delay control system to compensate for the time walk-off effect caused by the inconsistent group velocity of the green light and the second infrared light in the nonlinear crystal, so that the pulses of the first infrared optical path and the second infrared optical path overlap in time, including: changing the propagation optical path of the second infrared light by translating the mirror, combining the cross-correlation measurement device to monitor the pulse time overlap degree of the green light and the second infrared light in real time, and controlling the corresponding pulse time delay error within the preset range of the full width at half maximum of the femtosecond pulse, so as to achieve effective overlap in the time domain.

[0019] In some embodiments, after combining the second infrared light with time delay control and the shaped green light through a beam splitter and then jointly injecting them into a sum-frequency crystal for sum-frequency, it further includes: adjusting the incident angle of the sum-frequency crystal to meet the sum-frequency phase matching condition, controlling the polarization directions of the second infrared light and the green light to be consistent, and making the included angle between the corresponding optical axes of the beams less than the acceptance angle of the sum-frequency crystal, so as to ensure effective nonlinear sum-frequency interaction of the first infrared light and the second infrared light in the sum-frequency crystal and improve the generation efficiency of ultraviolet light.

[0020] In some embodiments, using the Gaussian energy distribution of the second infrared light and the Gaussian energy distribution of the green light for sum-frequency to generate ultraviolet light output, including: the second infrared light does not go through the frequency doubling process, and the energy distribution always maintains the uniform distribution state with high center and low periphery of the initial Gaussian beam. After shaping, the green light also forms a Gaussian energy distribution, and the center positions of the corresponding energy distributions coincide; a symmetric nonlinear action region is formed in the sum-frequency crystal to make the peak power density distributions of the infrared light and the green light match during the sum-frequency process, reduce the spatial walk-off effect caused by uneven energy distribution, so as to improve the beam quality of the ultraviolet light and suppress spot distortion.

[0021] In a second aspect, the present application provides a control device for a femtosecond third harmonic optical path, and the device includes:

[0022] An optical path division unit, configured to use an infrared beam splitter to divide the infrared light output by a femtosecond laser into two paths according to a certain ratio, namely the first path of infrared light and the second path of infrared light;

[0023] A green light generation unit, configured to make the first path of infrared light incident on a frequency doubling crystal to generate green light through frequency doubling. The frequency doubling crystal is an LBO or BBO nonlinear crystal, and the frequency doubling process is optimized to improve the frequency doubling efficiency of the green light;

[0024] A beam shaping unit, configured to shape the generated green light through a green light beam expander system so that the spot size of the shaped green light matches the spot size of the second path of infrared light;

[0025] A delay control unit, configured to apply time delay control to the second path of infrared light, and adjust the pulse time of the second path of infrared light through a time delay control system to compensate for the time walk-off effect caused by the inconsistent group velocities of the green light and the second path of infrared light in the nonlinear crystal, so that the pulses of the first infrared optical path and the second infrared optical path overlap in time;

[0026] A crystal frequency mixing unit, configured to make the second path of infrared light after time delay control and the green light after beam shaping be combined by a beam splitter and then commonly incident on a frequency mixing crystal for frequency mixing. The frequency mixing crystal is an LBO or BBO nonlinear crystal, and frequency mixing is performed using the Gaussian energy distribution of the second path of infrared light and the Gaussian energy distribution of the green light to generate ultraviolet light output; wherein, the first path of infrared light is used to generate green light, and the second path of infrared light maintains the initial Gaussian energy distribution state to participate in frequency mixing, avoiding the deterioration of the remaining infrared light energy distribution caused by too high frequency doubling efficiency, and realizing the spot size matching of the green light and the second path of infrared light through the green light beam expander system, and realizing the pulse time overlap of the first infrared optical path and the second infrared optical path through the time delay control system, so as to improve the frequency mixing efficiency and the beam quality of the ultraviolet light.

[0027] In a fourth aspect, an embodiment of the present application provides a control module, which includes a memory and a processor. The memory is used to store a computer program, and the processor is used to execute the computer program and, when executing the computer program, implement the method provided in any embodiment of the present application.

[0028] In a fifth aspect, an embodiment of the present application provides a computer-readable storage medium, which stores a computer program. When the computer program is executed by a processor, the processor is caused to implement the method provided in any embodiment of the present application.

[0029] The control method, device, equipment and medium for a femtosecond third harmonic optical path provided by an embodiment of the present application divides the infrared light into two paths. The first path is specifically used for efficient frequency doubling to generate green light, and the second path maintains the original Gaussian energy distribution and participates in frequency mixing to avoid the deterioration of the remaining infrared light energy. The time walk-off is compensated by a time delay system, and the spot size matching is achieved by a green light beam expander system to ensure that the two paths of light overlap in time and have the same spatial mode in the frequency mixing crystal. The second path of infrared light is not frequency doubled and always maintains the initial Gaussian energy distribution, forming a symmetric nonlinear interaction region with the shaped green light Gaussian distribution, improving the frequency mixing efficiency and beam quality.

[0030] Compared with the prior art, the creative advantages of the present invention are reflected in:

[0031] 1. Efficiency improvement: By splitting the beam and independently optimizing the frequency doubling and frequency mixing processes, the bottleneck of the mutual restriction between the frequency doubling and frequency mixing efficiencies in the traditional method is broken through. The frequency mixing efficiency can be significantly improved (surpassing the existing level by 20%-30%), and the energy conversion potential of the nonlinear crystal is fully utilized.

[0032] 2. Beam quality optimization: Avoid the deterioration of the remaining infrared light energy distribution and the mismatch of the spot size. Through Gaussian distribution matching and spatial walk-off compensation, the beam quality of the ultraviolet light is close to the level of the original infrared light, solving the core problem of beam distortion in the traditional method.

[0033] 3. Flexibility and controllability: Through precise adjustment of parameters such as the beam splitting ratio, time delay, and spot shaping, full-link control of the femtosecond third harmonic generation process is achieved, meeting the requirements of different nonlinear crystals and application scenarios.

[0034] In summary, through the beam splitting strategy and multi-dimensional matching control, the present invention systematically solves the contradiction between efficiency and beam quality in the existing femtosecond third harmonic generation technology, providing a key technical solution for the practical application of high-power and high-beam-quality femtosecond ultraviolet light sources.

[0035] It should be understood that the above general description and the following detailed description are only exemplary and explanatory, and cannot limit the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings required for the description of the embodiments will be briefly introduced below. Obviously, the drawings in the following description are some embodiments of the present application. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0037] Figure 1 It is a schematic flow chart of the steps of a control method for a femtosecond third harmonic optical path provided by an embodiment of the present application;

[0038] Figure 2 It is a schematic flowchart of the principle of a method for controlling a femtosecond third-harmonic optical path provided by an embodiment of the present application;

[0039] Figure 3 It is a schematic structural diagram of a device for controlling a femtosecond third-harmonic optical path provided by an embodiment of the present application;

[0040] Figure 4 It is a schematic block diagram of the structure of a control module provided by an embodiment of the present application.

[0041] It should be understood that the above general description and the following detailed description are only exemplary and explanatory, and cannot limit the present application. Detailed implementation manners

[0042] Next, the technical solutions in the embodiments of the present application will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present application.

[0043] The flowchart shown in the accompanying drawings is only an example, and does not necessarily include all the content and operations / steps, nor does it necessarily execute in the described order. For example, some operations / steps can also be decomposed, combined, or partially merged, so the actual execution order may change according to the actual situation.

[0044] It should be understood that, for the convenience of clearly describing the technical solutions in the embodiments of the present invention, in the embodiments of the present invention, terms such as "first" and "second" are used to distinguish the same items or similar items with basically the same functions and effects. Those skilled in the art can understand that the terms "first" and "second" do not limit the quantity and execution order, and the terms "first" and "second" do not necessarily mean different.

[0045] It should be understood that the terms used in the specification of the present application are only for the purpose of describing specific embodiments and are not intended to limit the present application. As used in the specification of the present application and the appended claims, unless otherwise clearly indicated by the context, the singular forms "a", "an", and "the" are intended to include the plural forms.

[0046] It should also be understood that the term " / and" used in the specification of the present application and the appended claims refers to any combination and all possible combinations of one or more of the related listed items, and includes these combinations.

[0047] The following will describe in detail some embodiments of the present application with reference to the accompanying drawings. Without conflict, the following embodiments and the features in the embodiments can be combined with each other.

[0048] In the femtosecond laser third-harmonic generation technology, the traditional optical path usually uses infrared light to generate green light by frequency doubling through a nonlinear crystal (such as LBO, BBO) first, and the remaining infrared light and the green light are then frequency combined to generate ultraviolet light. However, this method has the following core defects:

[0049] 1. Temporal and spatial walk-off effects: The group velocities of the infrared light and the green light in the nonlinear crystal are inconsistent (temporal walk-off), and the propagation direction deviates from the wave vector direction (spatial walk-off), resulting in non-overlap of the pulses in time and space. The frequency combination efficiency is only 20%-30%, and the beam quality deteriorates significantly.

[0050] 2. Degradation of energy distribution: The infrared light is a Gaussian beam with a high central energy density. After frequency doubling, the remaining infrared light forms a non-uniform energy distribution with a central depression because a large amount of the central energy is converted into green light, resulting in an imbalance in the peak power density matching during frequency combination, further reducing the frequency combination efficiency and exacerbating the spot distortion.

[0051] 3. Mismatch of spot sizes: The spot size of the green light generated by frequency doubling shrinks due to the change in energy distribution and is inconsistent with the spot size of the remaining infrared light, making it impossible to achieve efficient nonlinear frequency combination.

[0052] In the prior art, in order to balance the frequency doubling and frequency combination efficiencies, the frequency doubling efficiency is forced to be sacrificed (the frequency doubling efficiency is only 30%-40% at the best frequency combination), while the actual frequency doubling efficiency can reach more than 60%. This indicates that the traditional optical path does not fully utilize the energy conversion potential, and the beam quality problem has not been fundamentally solved.

[0053] Therefore, there is an urgent need for a method to solve at least one of the above problems.

[0054] To solve the above problems, please refer to Figure 1 and Figure 2 , the embodiments of the present application provide a control method for a femtosecond third-harmonic optical path, which is applied to a control module. Specifically, as Figure 1 shown, the provided control method for the femtosecond third-harmonic optical path includes steps S101 to S105. Details are as follows:

[0055] Step S101. Use an infrared beam splitter to divide the infrared light output by the femtosecond laser into two paths according to a certain ratio, namely the first path of infrared light and the second path of infrared light.

[0056] Specifically, use an infrared beam splitter to divide the infrared light output by the femtosecond laser into two independent beams according to a specific ratio - the first path of infrared light (used for frequency doubling to generate green light) and the second path of infrared light (directly participating in frequency combination without going through the frequency doubling process).

[0057] Beam splitter selection: A dielectric film beam splitter with high transmittance / high reflectance for infrared light is used, and the reflection / transmission ratio (such as 50:50, 30:70, etc.) is selected according to the requirements of the optical path design. Beam splitting ratio determination: According to the nonlinear coefficient of the frequency doubling crystal, the incident power density threshold, and the infrared light energy required for sum frequency, the beam splitting ratio is optimized to 30%-70%:70%-30% (the first path: the second path). The two beams of light are independently transmitted after beam splitting. The second path of infrared light does not pass through the frequency doubling crystal throughout the process, avoiding the deterioration of its energy distribution due to the central depression during the frequency doubling process.

[0058] Through the control of the beam splitting ratio, the first path of infrared light has the energy density required for efficient frequency doubling (breaking through the bottleneck of "mutual restriction between frequency doubling efficiency and sum frequency efficiency" in traditional methods), and the second path of infrared light maintains the uniform energy distribution of the initial Gaussian beam (high in the center and low in the periphery), providing a high-quality energy base for subsequent sum frequency. The second path of infrared light is not frequency doubled, completely solving the problem of "uneven energy distribution of the remaining infrared light due to frequency doubling" in traditional technologies, and eliminating the hidden danger of peak power density imbalance during sum frequency from the source.

[0059] Step S102. Incident the first path of infrared light onto the frequency doubling crystal to generate green light. The frequency doubling crystal is an LBO or BBO nonlinear crystal, and the frequency doubling process is optimized to improve the frequency doubling efficiency of the green light.

[0060] Specifically, incident the first path of infrared light onto a nonlinear crystal such as LBO or BBO, and improve the frequency doubling efficiency of the green light (such as the target ≥ 60%) by optimizing the frequency doubling process (phase matching, power density, temperature control, etc.).

[0061] Phase matching adjustment is carried out by precisely adjusting the incident angle of the crystal (error ≤ 0.1°) according to the type of the frequency doubling crystal (such as type I or type II phase matching for LBO crystal), so that the infrared light and the green light meet the phase matching condition (Δk = 0), maximizing the second-order nonlinear effect.

[0062] Power density control adjusts the incident power of the first path of infrared light through an attenuator or an aperture stop, so that the power density on the crystal surface is within the optimal nonlinear conversion threshold range of the crystal (such as usually 5×10 11 ~1.5×10 12 W / cm² for LBO crystal). The temperature of the frequency doubling crystal is controlled by a temperature control device (accuracy ±0.1°C) to compensate for the dispersion drift caused by temperature changes in the crystal, ensuring the stability and high efficiency of the frequency doubling process.

[0063] Through phase matching and power density optimization, the second harmonic generation efficiency can be increased to over 60% (significantly higher than the 30%-40% in the best sum frequency combination in traditional methods), making full use of the infrared light energy to generate high-quality green light. Since the second infrared light does not participate in second harmonic generation, the first path can focus on improving the second harmonic generation efficiency without worrying about the problem of "degradation of the remaining infrared light due to excessive second harmonic generation", realizing independent and efficient control of the second harmonic generation process.

[0064] Step S103. Shape the generated green light through a green light beam expander system so that the spot size of the shaped green light matches the spot size of the second infrared light.

[0065] Specifically, expand and shape the green light generated by second harmonic generation, and adjust its spot size through an optical system to make it precisely match the spot size of the second infrared light (such as an error < 10%).

[0066] The beam expander system is designed with a telescopic optical structure (such as a combination of a convex lens and a concave lens). The focal length f1 of the convex lens corresponds to the collimation of the green light, and the focal length f2 of the concave lens corresponds to beam expansion. The diameter of the green light spot is controlled by adjusting the focal length ratio (f2 / f1).

[0067] Size matching method: Use a beam analyzer (such as the Spiricon LBA series) to measure the spot diameter (D_IR) of the second infrared light in real time, and adjust the beam expander system so that the green light spot diameter (D_Green) satisfies D_Green = D_IR ± 10%D_IR, ensuring that the spatial modes of the two beams are consistent at the incident end of the sum frequency crystal. During the shaping process, maintain the Gaussian energy distribution of the green light to avoid introducing higher-order aberrations or energy distribution distortion due to beam expansion.

[0068] The method solves the problem in traditional technologies that "the green light spot shrinks due to the change in the second harmonic generation energy distribution". Through precise size matching, the two beams form an overlapping action area in the sum frequency crystal, reducing the loss of non-linear action efficiency caused by spot mismatch. Maintaining the Gaussian distribution and aberration control provide high-quality green light beams for subsequent sum frequency, directly improving the output beam quality of ultraviolet light.

[0069] Step S104. Apply time delay control to the second infrared light, and adjust the pulse time of the second infrared light through a time delay control system to compensate for the time walk-off effect caused by the inconsistent group velocities of the green light and the second infrared light in the non-linear crystal, so that the pulses of the first infrared light path and the second infrared light path overlap in time.

[0070] Specifically, adjust the pulse time of the second infrared light through a time delay control system to compensate for the time walk-off caused by the group velocity difference between it and the green light, so that the pulses of the two overlap in the time domain (the overlap degree ≥ 90%).

[0071] The delay system is composed of a mirror group with adjustable optical path (such as a plane mirror mounted on a translation stage). By changing the propagation path length of the second infrared light (accuracy ±1μm), the arrival time of its pulse at the sum-frequency crystal is adjusted.

[0072] Real-time monitoring and feedback uses a cross-correlation measurement device (such as SHG-FROG or autocorrelator) to detect the pulse time overlap between the green light and the second infrared light in real time. The position of the mirror is adjusted through a closed-loop control algorithm to control the time delay error within 1 / 3 of the full width at half maximum (FWHM) of the femtosecond pulse (for example, the error of a 100fs pulse ≤ 33fs).

[0073] The principle of group velocity compensation aims at the group velocity mismatch (GVM) between infrared light (~800nm) and green light (~500nm) in the LBO / BBO crystal. The cumulative effect of time walk-off inside the crystal is offset through optical path adjustment.

[0074] The method completely solves the problem of "pulse separation caused by inconsistent group velocities" in traditional technologies. Through precise time control, the two beams of light achieve time-domain synchronization in the sum-frequency crystal, greatly improving the nonlinear conversion efficiency of the sum-frequency process (the sum-frequency efficiency can be increased from 20% - 30% to more than 40%). The delay system adopts a distortion-free optical design to avoid introducing pulse broadening or chirping, ensuring that the femtosecond pulse characteristics of the second infrared light are not affected.

[0075] Step S105. After the second infrared light controlled by time delay and the green light shaped by beam shaping are combined by a beam splitter, they are jointly incident on a sum-frequency crystal for sum-frequency. The sum-frequency crystal is an LBO or BBO nonlinear crystal. The Gaussian energy distributions of the second infrared light and the green light are used for sum-frequency to generate ultraviolet light output; among them, the first infrared light is used to generate green light, and the second infrared light maintains its initial Gaussian energy distribution state to participate in sum-frequency, avoiding the deterioration of the remaining infrared light energy distribution due to too high frequency doubling efficiency. The spot size matching between the green light and the second infrared light is achieved through the green light beam expansion system, and the pulse time overlap of the first infrared optical path and the second infrared optical path is achieved through the time delay control system to improve the sum-frequency efficiency and the beam quality of the ultraviolet light.

[0076] Specifically, the second infrared light with time synchronization and spot matching and the green light are combined by a beam splitter and jointly incident on a sum-frequency crystal (LBO / BBO), and sum-frequency is performed using their Gaussian energy distributions to generate high-power and high-beam-quality ultraviolet light.

[0077] The control of the beam combination conditions includes: polarization matching: adjusting the polarization directions of the two beams of light through wave plates so that the polarization conditions required for sum frequency in the sum frequency crystal are met (for example, type-I phase matching requires the two lights to have the same polarization). Angle alignment: adjusting the incident angle of the sum frequency crystal (accuracy ±0.05°) to meet the sum frequency phase matching condition (Δk = 0), and at the same time controlling the included angle between the optical axes of the two beams of light < the acceptance angle of the crystal (for example, the acceptance angle of the LBO crystal is about 1.5° / cm) to ensure the maximization of the effective region of the nonlinear effect. The second infrared light maintains the original Gaussian distribution (high central energy density), and the green light is also Gaussian distributed after shaping. The two form a symmetric energy distribution with coincident centers in the sum frequency crystal, so that the peak power densities are matched (the peak power density of the infrared light P_IR and the green light P_Green satisfy the sum frequency efficiency formula η∝P_IR*P_Green).

[0078] Through time overlap, spot matching, and coordinated energy distribution, the sum frequency efficiency breaks through the traditional limitations. Combined with high-efficiency frequency doubling (more than 60%), the overall third-harmonic generation efficiency is significantly improved. The symmetric action of the Gaussian distributions of the two beams of light avoids the "spot distortion caused by the central depression of the remaining infrared light" in traditional technologies. The beam quality of the ultraviolet light (such as the M² factor) is close to the level of the original infrared light, solving the long-existing problem of beam degradation. By adjustable parameters (beam splitting ratio, delay amount, spot size), it adapts to different types of nonlinear crystals and femtosecond lasers, enhancing the engineering application value of the method.

[0079] The above steps, through the full-link optimization of "beam splitting and independent processing → high-efficiency frequency doubling → spatio-temporal matching control → symmetric energy sum frequency", systematically solve the three core problems of time walk-off, spatial mismatch, and energy distribution degradation in traditional femtosecond third-harmonic generation technology, realizing high-efficiency (surpassing the existing level) and high-beam-quality (close to the original infrared light) ultraviolet light output, providing key technical support for high-end applications such as precision machining and spectral analysis.

[0080] In some embodiments, using the infrared beam splitter to split the infrared light output by the femtosecond laser into two paths according to a certain ratio, namely the first infrared light and the second infrared light, includes: determining the beam splitting ratio of the first infrared light and the second infrared light according to the nonlinear coefficient of the frequency doubling crystal, the incident power density, and the required infrared light energy distribution for sum frequency; so that the first infrared light has sufficient energy for high-efficiency frequency doubling, and the second infrared light maintains the energy distribution state of the initial Gaussian beam, avoiding the energy matching imbalance during sum frequency caused by improper beam splitting ratio.

[0081] First, measure the nonlinear coefficient d of the frequency doubling crystal (such as LBO / BBO) through a spectrometer eff (the nonlinear coefficient of LBO is about 1.1 pm / V, and the nonlinear coefficient of BBO is about 1.9 pm / V), combined with the nonlinear optical theory formula PSHG∝deff 2 *I IR 2 (where PSHG is the second harmonic power and I IR is the infrared light power density), determine the optimal incident power density threshold of the second harmonic crystal (for example, the LBO crystal is usually 5×10 11 to 1.5×10 12 W / cm). Measure the total infrared light power Ptotal output by the femtosecond laser using a power meter, and calculate the energy P1 required for the first path of infrared light according to the optimal power density, P1 = Iopt * A (where A is the beam cross-sectional area), so as to determine the splitting ratio k of the first path, k = P1 / Ptotal, and the range is controlled within 30% - 70% (for example, if the target second harmonic generation efficiency is 60%, the splitting ratio of the first path is usually set to 50% - 60%).

[0082] The second path of infrared light required for sum frequency needs to maintain the initial Gaussian distribution. Therefore, a high-transmission / high-reflection dielectric film is selected for the beam splitter (for example, the reflectivity for 800nm infrared light is 30% - 70% and the transmittance is 70% - 30%), ensuring that the second path of light undergoes no nonlinear processing and its energy distribution is always:

[0083] I(r) = I0exp(-2r 2 / ω 2 ) (where ω is the beam waist radius and r is the radial distance).

[0084] Monitor the energy distribution of the second path of infrared light through a beam quality analyzer. If a central energy depression (i.e., non-Gaussian distribution) appears, fine-tune the splitting ratio (accuracy ±1%) until the standard deviation of its energy distribution is less than 5% of the initial distribution.

[0085] Through the quantitative calculation of the nonlinear coefficient and power density, avoid the blindness of "beam splitting based on experience" in traditional methods, ensure that the first path of light has the energy basis for efficient second harmonic generation (the second harmonic generation efficiency is increased to more than 60%), and the second path of light completely retains the high-energy characteristic of the central Gaussian distribution (the central energy density is increased by more than 30% compared with the traditional remaining infrared light), and solve the core problem of "peak power density mismatch" during sum frequency from the source.

[0086] Decouple the energy requirements of second harmonic generation and sum frequency, so that the two paths of light can be independently optimized (the first path pursues second harmonic generation efficiency and the second path pursues the integrity of energy distribution), break through the bottleneck of "mutual restriction between second harmonic generation efficiency and sum frequency quality" in traditional optical paths, and lay a foundation for subsequent spatio-temporal matching.

[0087] In some embodiments, optimizing the frequency doubling process to improve the frequency doubling efficiency of green light includes: adjusting the incident angle of the frequency doubling crystal to satisfy the phase matching condition, controlling the incident power density of the first infrared light within the optimal threshold range of non-linear conversion of the frequency doubling crystal, and performing temperature control on the frequency doubling crystal to compensate for crystal dispersion, so that the frequency doubling efficiency of the frequency doubling process is higher than the preset efficiency and does not affect the initial Gaussian energy distribution characteristics of the second infrared light.

[0088] Precise phase matching adjustment: The frequency doubling crystal is installed using a high-precision rotary stage (angle accuracy ±0.01°). According to the crystal refractive index formula (such as ne(λ,T) and no(λ,T) of LBO), the type-I phase matching angle θPM is calculated (for example, when doubling 800nm infrared light to 500nm green light, the type-I phase matching angle of the LBO crystal at 20°C is approximately 90°). The crystal is rotated to within the range of θPM±0.05° through an automatic alignment system to ensure that the wave vector mismatch Δk = 2kIR - kGreen≈0.

[0089] An electrically controlled attenuator (attenuation accuracy ±0.5%) is inserted into the path of the first infrared light. The power incident on the frequency doubling crystal is monitored in real time in combination with a power sensor. The attenuator is adjusted through a PID algorithm to stabilize the power density within the optimal threshold range (such as 1.2×10 12 W / cm for the LBO crystal), to avoid crystal damage caused by excessive power or a decrease in frequency doubling efficiency caused by too low power.

[0090] Temperature compensation system: A semiconductor temperature controller (accuracy ±0.1°C) is used to wrap the frequency doubling crystal. According to the crystal dispersion formula dn / dT = -1×10 -5 K -1 (typical LBO parameters), the crystal temperature is controlled at a preset value (such as 25°C) to compensate for the phase mismatch caused by temperature changes (about 0.1% fluctuation in frequency doubling efficiency per 1°C temperature change), ensuring long-term stable operation.

[0091] Through triple optimization (phase matching, power density, temperature control), the frequency doubling efficiency can be stabilized at 65% - 70% (only 30% - 40% for traditional methods), significantly improving the green light energy output and providing a stronger pump light source for sum frequency. Since the second infrared light is completely independent of the frequency doubling optical path, the adjustment of the crystal angle, power, and temperature during the frequency doubling process will not have any impact on the Gaussian distribution of the second light (such as avoiding self-phase modulation distortion caused by excessive power), ensuring the purity of the energy distribution of the second light during sum frequency.

[0092] In some embodiments, the green light beam expanding system adopts a telescopic optical structure, and collimates and expands the green light beam through the combination of a convex lens and a concave lens; shaping the generated green light through the green light beam expanding system so that the spot size of the shaped green light matches the spot size of the second infrared light, including: according to the spot diameter of the second infrared light, adjusting the focal length ratio of the beam expanding system so that the error between the diameter of the shaped green light spot and the diameter of the second infrared light spot is less than a preset error, realizing the spatial mode matching of the two beams of light.

[0093] A double-lens telescope system is adopted. The front lens is a convex lens (focal length f1 = 50 mm) for collimating the divergent green light beam, and the rear lens is a concave lens (focal length f2 = -100 mm) for beam expansion. The beam expansion ratio M = |f2 / f1| = 2. An adjustable beam expansion ratio (range 1.5 - 3.0) is realized by replacing lens groups with different focal lengths.

[0094] Spot size matching process: First, use a CCD beam analyzer to measure the waist diameter ωIR of the second infrared light (accuracy ±5 μm), and calculate its spot diameter DIR = 2ωIRexp(z / zR) at the sum-frequency crystal; z is the transmission distance, and zR is the Rayleigh length).

[0095] Adjust the position of the concave lens to change the focal length ratio so that the spot diameter DGreen of the green light after beam expansion is DIR ± 10%DIR. For example, if DIR = 2 mm, then DGreen is controlled within the range of 1.8 - 2.2 mm.

[0096] At the same time, detect the energy distribution of the green light through a beam profiler to ensure that it is still Gaussian after beam expansion (fitting degree R2 > 0.99), and avoid the introduction of a flat-top distribution or annular distortion due to lens aberration.

[0097] Through the geometric beam expansion of the telescope system, solve the problem in the traditional technology that "the green light spot shrinks due to the energy consumption of frequency doubling" (the traditional green light spot is only 60% - 70% of the infrared light), so that the spot overlap rate of the two beams of light at the incident end of the sum-frequency crystal > 95%, significantly improving the nonlinear interaction volume (the interaction volume increases by more than 40%), and directly improving the sum-frequency efficiency. Strictly control the lens tolerances (surface accuracy λ / 10) and alignment accuracy (optical axis offset < 50 μm) of the beam expansion system to ensure that the beam quality factor M2 of the green light beam < 1.1 (close to the diffraction limit), providing guarantee for the high beam quality output of ultraviolet light.

[0098] In some embodiments, the time delay control system includes a mirror group with adjustable optical path; the time delay control is applied to the second infrared light path, and the pulse time of the second infrared light is adjusted through the time delay control system to compensate for the time walk-off effect caused by the inconsistent group velocities of the green light and the second infrared light in the nonlinear crystal, so that the pulses of the first infrared light path and the second infrared light path overlap in time, including: changing the propagation optical path of the second infrared light by translating the mirror, and combining the cross-correlation measurement device to monitor the pulse time overlap degree of the green light and the second infrared light in real time, and controlling the corresponding pulse time delay error within the preset range of the full width at half maximum of the femtosecond pulse, so as to achieve effective overlap in the time domain.

[0099] A two-dimensional translation stage (accuracy ±0.1μm) is used to carry a high-reflectivity plane mirror (reflectivity for 800nm infrared light > 99.5%). After the second infrared light is reflected by the mirror, the optical path change amount ΔL = 2d (d is the moving distance of the translation stage), and the corresponding time delay Δt = ΔL / c (c is the speed of light), so that femtosecond-level time adjustment can be achieved (for example, moving 1μm corresponds to a delay of about 6.67fs).

[0100] Using a cross-correlation measurement device (such as a second-harmonic generation cross-correlator based on a BBO crystal), the green light and the second infrared light are focused on the nonlinear crystal to generate a sum-frequency signal, and the signal intensity is positively correlated with the time overlap degree of the two light paths.

[0101] A proportional-integral-derivative (PID) controller is adopted, with the cross-correlation signal intensity as the feedback signal, to automatically adjust the position of the translation stage and control the time delay error within 1 / 3 of the full width at half maximum (FWHM) of the femtosecond pulse (for example, if the pulse width is 100fs, the error ≤ 33fs), corresponding to an overlap degree > 90%.

[0102] Group velocity mismatch compensation: For the group velocity difference between the infrared light (group velocity vgIR ≈ 1.6×10 8 m / s) and the green light (vgGreen ≈ 1.7×10 8 m / s) in the LBO crystal, calculate the time walk-off Δtwalk = L(1 / vgGreen - 1 / vgIR) ≈ 20fs within the length L = 10mm of the sum-frequency crystal, and pre-compensate this walk-off amount through the delay system.

[0103] In the traditional technology, the pulse overlap degree caused by time walk-off is only 50%-60%. In this embodiment, through sub-micron optical path control and real-time feedback, the overlap degree is increased to more than 90%, the non-linear interaction time during the sum-frequency process is extended by more than 3 times, and the sum-frequency efficiency is increased from 25% to 45%. The mirror group adopts a non-chirped design (coated with an antireflection film to eliminate chromatic dispersion) to ensure that the pulse width and spectral width of the second infrared light change <5% after delay adjustment, avoiding pulse distortion caused by time control.

[0104] In some embodiments, after combining the second infrared light controlled by time delay and the green light shaped by beam shaping through a beam splitter and then jointly incident on the sum-frequency crystal for sum-frequency, it further includes: adjusting the incident angle of the sum-frequency crystal to satisfy the sum-frequency phase matching condition, controlling the polarization directions of the second infrared light and the green light to be consistent, and making the included angle between the corresponding beam optical axes less than the acceptance angle of the sum-frequency crystal to ensure effective non-linear sum-frequency interaction between the first infrared light and the second infrared light in the sum-frequency crystal and improve the generation efficiency of ultraviolet light.

[0105] Sum-frequency phase matching adjustment: Use a five-axis precision positioning stage to adjust the incident angle of the sum-frequency crystal (LBO / BBO). According to the sum-frequency phase matching condition kUV = kIR + kGreen (k is the wave vector), calculate the type-II phase matching angle (for example, when sum-frequencying 333nm from 800nm + 500nm, the phase matching angle of the BBO crystal is about 45°). Ensure that the crystal angle error <0.1° through an angle encoder (accuracy ±0.001°) to eliminate the attenuation of non-linear interaction caused by wave vector mismatch (each 1° mismatch causes about 20% efficiency drop).

[0106] Polarization direction control: Insert λ / 2 wave plates respectively in the paths of the green light and the second infrared light. Rotate the wave plates so that the polarization directions of both lights are e-light (or both are o-light, depending on the type of sum-frequency crystal) to ensure that the polarization requirements for type-I or type-II phase matching are met (polarization extinction ratio > 100:1), avoiding the loss of sum-frequency efficiency caused by polarization mismatch (the efficiency loss can reach 30% in the traditional case without controlled polarization).

[0107] Optical axis included angle control: Measure the included angle between the optical axes of the two lights using a beam aligner. By finely adjusting the angle of the beam combiner, make the included angle < the acceptance angle of the sum-frequency crystal (for example, the angle acceptance of the LBO crystal is Δθ ≈ 1.5° / cm, for a 10mm crystal, the acceptance angle is about 15°), usually controlled within 5° to ensure that the interaction regions of the two lights inside the crystal completely overlap and maximize the sum-frequency interaction length.

[0108] Through triple control of phase matching, polarization consistency, and angle alignment, the problem of "low efficiency of the sum-frequency crystal" in traditional technologies is solved. The nonlinear coupling coefficient of the sum-frequency process is increased by more than 50%, and the ultraviolet light output power is increased by 60% compared with traditional methods.

[0109] Precise angle and polarization control reduce the system's dependence on the crystal processing accuracy. Even if the crystal has a manufacturing deviation of ±0.5°, high-efficiency sum-frequency can still be achieved through dynamic adjustment, improving the reliability of engineering applications.

[0110] In some embodiments, the sum-frequency of the Gaussian energy distribution of the second infrared light and the Gaussian energy distribution of the green light to generate ultraviolet light output includes: the second infrared light does not undergo the frequency doubling process, and its energy distribution always maintains the uniform distribution state with a high center and low periphery of the initial Gaussian beam. After shaping, the green light also forms a Gaussian energy distribution, and the corresponding energy distribution center positions coincide; a symmetric nonlinear interaction region is formed in the sum-frequency crystal to match the peak power density distributions of the infrared light and the green light during the sum-frequency process, reducing the spatial walk-off effect caused by uneven energy distribution, so as to improve the beam quality of the ultraviolet light and suppress spot distortion.

[0111] No nonlinear elements are set in the path of the second infrared light (such as frequency doubling crystals, and attenuation sheets are only placed in the first path), and the transmission lenses are all made of quartz with a high damage threshold (to avoid strong light-induced nonlinear effects), ensuring that its energy distribution is always the original Gaussian type.

[0112] Among them, the infrared light power density IR(r)=I0exp(-2r 2 / ωIR 2 ), and the ratio of the central energy density to the edge energy density is maintained above 10:1 (the central energy ratio of the traditional remaining infrared light drops below 5:1 due to frequency doubling).

[0113] After beam expansion, the green light undergoes amplitude filtering through a soft aperture (an aperture-adjustable aperture) to cut off the non-Gaussian components at the edge, ensuring that its energy distribution satisfies IGreen(r)=IG0exp(-2r 2 / ωGreen 2 ), and the deviation of its energy distribution center from that of the second infrared light is <5μm (center alignment is achieved through a beam coaxial adjustment frame).

[0114] At the incident end of the sum-frequency crystal, the Gaussian distribution centers of the two beams of light coincide, forming a symmetric "double Gaussian" superposition field, and the peak power density satisfies PIR*PGreen∝exp(-4r 2 / (ωIR 2 +ωGreen 2)) so that the intensity of the non - linear effect is symmetrically distributed in the radial direction, suppressing the transverse walk - off caused by uneven energy distribution (in the traditional asymmetric distribution, the walk - off distance reaches 50 μm, and in this embodiment, it is controlled within 10 μm).

[0115] In the traditional method, the ultraviolet light spot is distorted due to uneven energy distribution (ellipticity > 1.5). In this embodiment, through double - Gaussian symmetric frequency mixing, the ellipticity of the ultraviolet light spot is < 1.1 (close to circular), and the beam quality factor M 2 decreases from the traditional 3.0 to 1.3, approaching the level of the original infrared light, meeting the stringent requirements for beam quality in precision machining (such as semiconductor lithography). The symmetric peak power density distribution makes the non - linear polarization intensity evenly distributed in space during the frequency - mixing process, avoiding the distortion effect of "strong at the edge and weak in the center" caused by the energy depression in the center, and eliminating the problems of spot splitting and energy divergence from the physical mechanism.

[0116] Please refer to Figure 3 as shown Figure 3 which is a schematic structural diagram of the control device 200 for the femtosecond third - harmonic optical path provided by an embodiment of the present application. The control device 200 for the femtosecond third - harmonic optical path is used to execute the steps of the control method for the femtosecond third - harmonic optical path shown in the above embodiments. The control device 200 for the femtosecond third - harmonic optical path can be a single server or a server cluster, or the control device 200 for the femtosecond third - harmonic optical path can be a terminal, and the terminal can be a handheld terminal, a laptop computer, a wearable device, or a robot, etc.

[0117] As Figure 3 shown, the control device 200 for the femtosecond third - harmonic optical path includes:

[0118] An optical path division unit 201, configured to divide the infrared light output by a femtosecond laser into two paths in a certain proportion by using an infrared beam splitter, namely the first - path infrared light and the second - path infrared light;

[0119] A green - light generation unit 202, configured to incident the first - path infrared light on a frequency - doubling crystal to generate green light. The frequency - doubling crystal is an LBO or BBO non - linear crystal, and the frequency - doubling process is optimized to improve the frequency - doubling efficiency of the green light;

[0120] A beam shaping unit 203, configured to shape the generated green light through a green - light beam expander system so that the size of the shaped green - light spot matches the size of the second - path infrared - light spot;

[0121] A delay control unit 204 is configured to apply time delay control to the second infrared light, and adjust the pulse time of the second infrared light through a time delay control system, so as to compensate for the time walk-off effect caused by the inconsistent group velocities of the green light and the second infrared light in the nonlinear crystal, and make the pulses of the first infrared optical path and the second infrared optical path overlap in time.

[0122] A crystal frequency mixing unit 205 is configured to combine the second infrared light after time delay control and the green light after beam shaping through a beam splitter and then jointly inject them into a frequency mixing crystal for frequency mixing. The frequency mixing crystal is an LBO or BBO nonlinear crystal. The Gaussian energy distribution of the second infrared light and the Gaussian energy distribution of the green light are used for frequency mixing to generate ultraviolet light output. Among them, the first infrared light is used to generate green light, and the second infrared light maintains the initial Gaussian energy distribution state and participates in frequency mixing, avoiding the deterioration of the remaining infrared light energy distribution due to too high frequency doubling efficiency. Moreover, the spot size matching between the green light and the second infrared light is realized through the green light beam expansion system, and the pulse time overlap between the first infrared optical path and the second infrared optical path is realized through the time delay control system, so as to improve the frequency mixing efficiency and the beam quality of the ultraviolet light.

[0123] In some embodiments, splitting the infrared light output by the femtosecond laser into two paths according to a certain ratio by using an infrared beam splitter, namely the first infrared light and the second infrared light, includes: determining the splitting ratio of the first infrared light and the second infrared light according to the nonlinear coefficient of the frequency doubling crystal, the incident power density, and the required infrared light energy distribution for frequency mixing, so that the first infrared light has sufficient energy for efficient frequency doubling, and the second infrared light maintains the energy distribution state of the initial Gaussian beam, avoiding the energy matching imbalance during frequency mixing caused by improper splitting ratio.

[0124] In some embodiments, optimizing the frequency doubling process to improve the frequency doubling efficiency of the green light includes: adjusting the incident angle of the frequency doubling crystal to satisfy the phase matching condition, controlling the incident power density of the first infrared light within the optimal threshold range of nonlinear conversion of the frequency doubling crystal, and controlling the temperature of the frequency doubling crystal to compensate for crystal dispersion, so that the frequency doubling efficiency of the frequency doubling process is higher than the preset efficiency and does not affect the initial Gaussian energy distribution characteristics of the second infrared light.

[0125] In some embodiments, the green light beam expanding system adopts a telescopic optical structure, and collimates and expands the green light beam through the combination of a convex lens and a concave lens; shaping the generated green light through the green light beam expanding system to make the size of the shaped green light spot match the size of the second infrared light spot, including: according to the spot diameter of the second infrared light, adjusting the focal length ratio of the beam expanding system to make the error between the diameter of the shaped green light spot and the diameter of the second infrared light spot less than a preset error, so as to realize the spatial mode matching of the two beams of light.

[0126] In some embodiments, the time delay control system includes a mirror group with adjustable optical path; applying time delay control to the second infrared light, and adjusting the pulse time of the second infrared light through the time delay control system to compensate for the time walk-off effect caused by the inconsistent group velocity of the green light and the second infrared light in the nonlinear crystal, so that the pulses of the first infrared optical path and the second infrared optical path overlap in time, including: changing the propagation optical path of the second infrared light by translating the mirror, and combining the cross-correlation measurement device to monitor the pulse time overlap degree of the green light and the second infrared light in real time, and controlling the corresponding pulse time delay error within the preset range of the full width at half maximum of the femtosecond pulse, so as to realize effective overlap in the time domain.

[0127] In some embodiments, after combining the second infrared light with time delay control and the shaped green light through a beam splitter and then jointly injecting them into the sum-frequency crystal for sum-frequency, it further includes: adjusting the incident angle of the sum-frequency crystal to meet the sum-frequency phase matching condition, controlling the polarization directions of the second infrared light and the green light to be consistent, and making the included angle between the corresponding beam optical axes less than the acceptance angle of the sum-frequency crystal, so as to ensure effective nonlinear sum-frequency interaction between the first infrared light and the second infrared light in the sum-frequency crystal and improve the generation efficiency of ultraviolet light.

[0128] In some embodiments, using the Gaussian energy distribution of the second infrared light and the Gaussian energy distribution of the green light for sum-frequency to generate ultraviolet light output, including: the second infrared light does not undergo the frequency doubling process, and the energy distribution always maintains the uniform distribution state with high center and low periphery of the initial Gaussian beam, and the green light also forms a Gaussian energy distribution after shaping, and the corresponding energy distribution center positions coincide; forming a symmetric nonlinear action region in the sum-frequency crystal to make the peak power density distributions of the infrared light and the green light match during the sum-frequency process, reducing the spatial walk-off effect caused by uneven energy distribution, so as to improve the beam quality of the ultraviolet light and suppress spot distortion.

[0129] It should be noted that those skilled in the art can clearly understand that for the convenience and conciseness of description, the specific working processes of the control device of the femtosecond third-harmonic optical path and each module described above can refer to the corresponding processes in the embodiments of the femtosecond third-harmonic optical path control method described in the above embodiments, and will not be elaborated here.

[0130] The above-described femtosecond third-harmonic optical path control method can be implemented in the form of a computer program, and this computer program can run on a device as shown in Figure 3 the following.

[0131] Please refer to Figure 4 , Figure 4 which is a schematic block diagram of the structure of the control module provided by the embodiments of the present application. The control module includes a processor, a memory, and a network interface connected through a device bus. Among them, the memory may include a storage medium and an internal memory.

[0132] The storage medium can store an operating device and a computer program. This computer program includes program instructions, and when the program instructions are executed, the processor can be made to execute any femtosecond third-harmonic optical path control method.

[0133] The processor is used to provide computing and control capabilities to support the operation of the entire control module.

[0134] The internal memory provides an environment for the operation of the computer program in the non-volatile storage medium. When this computer program is executed by the processor, the processor can be made to execute any femtosecond third-harmonic optical path control method.

[0135] This network interface is used for network communication, such as sending assigned tasks, etc. Those skilled in the art can understand that Figure 4 the structure shown in [figure reference] is only a block diagram of some structures related to the solution of the present application, and does not constitute a limitation on the terminal to which the solution of the present application is applied. The specific control module may include more or fewer components than those shown in the figure, or combine some components, or have different component arrangements.

[0136] It should be understood that the processor can be a Central Processing Unit (CPU), and the processor can also be other general-purpose processors, Digital Signal Processors (DSPs), Application Specific Integrated Circuits (ASICs), Field-Programmable Gate Arrays (FPGAs), or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. Among them, the general-purpose processor can be a microprocessor or the processor can also be any conventional processor, etc.

[0137] Among them, in one embodiment, the processor is used to run a computer program stored in the memory to implement the following steps:

[0138] Use an infrared beam splitter to split the infrared light output by the femtosecond laser into two paths in a certain proportion, namely the first path of infrared light and the second path of infrared light;

[0139] Incide the first path of infrared light onto a frequency doubling crystal to generate green light through frequency doubling. The frequency doubling crystal is an LBO or BBO nonlinear crystal, and optimize the frequency doubling process to improve the frequency doubling efficiency of the green light;

[0140] Shape the generated green light through a green light beam expansion system so that the spot size of the shaped green light matches the spot size of the second path of infrared light;

[0141] Apply time delay control to the second path of infrared light, and adjust the pulse time of the second path of infrared light through a time delay control system to compensate for the time walk-off effect caused by the inconsistent group velocities of the green light and the second path of infrared light in the nonlinear crystal, so that the pulses of the first infrared light path and the second infrared light path overlap in time;

[0142] After combining the second path of infrared light after time delay control and the shaped green light through a beam splitter, jointly incide them onto a sum frequency crystal for sum frequency. The sum frequency crystal is an LBO or BBO nonlinear crystal, and use the Gaussian energy distribution of the second path of infrared light and the Gaussian energy distribution of the green light for sum frequency to generate ultraviolet light output; among them, the first path of infrared light is used to generate green light, and the second path of infrared light maintains the initial Gaussian energy distribution state to participate in sum frequency, avoiding the deterioration of the remaining infrared light energy distribution due to too high frequency doubling efficiency, and realizing the spot size matching of the green light and the second path of infrared light through the green light beam expansion system, and realizing the pulse time overlap of the first infrared light path and the second infrared light path through the time delay control system to improve the sum frequency efficiency and the beam quality of the ultraviolet light.

[0143] In some embodiments, splitting the infrared light output by the femtosecond laser into two paths in a certain proportion by using an infrared beam splitter, including the first path of infrared light and the second path of infrared light, includes: determining the splitting ratio of the first path of infrared light to the second path of infrared light according to the nonlinear coefficient of the frequency doubling crystal, the incident power density, and the requirement for the energy distribution of the infrared light required for sum frequency; so that the first path of infrared light has sufficient energy for efficient frequency doubling, and the second path of infrared light maintains the energy distribution state of the initial Gaussian beam, avoiding energy matching imbalance during sum frequency due to improper splitting ratio.

[0144] In some embodiments, optimizing the frequency doubling process to improve the frequency doubling efficiency of green light includes: adjusting the incident angle of the frequency doubling crystal to satisfy the phase matching condition, controlling the incident power density of the first path of infrared light within the optimal threshold range of nonlinear conversion of the frequency doubling crystal, and performing temperature control on the frequency doubling crystal to compensate for crystal dispersion, so that the frequency doubling efficiency of the frequency doubling process is higher than the preset efficiency and does not affect the initial Gaussian energy distribution characteristics of the second path of infrared light.

[0145] In some embodiments, the green light beam expanding system adopts a telescopic optical structure to collimate and expand the green light beam through the combination of a convex lens and a concave lens; shaping the generated green light through the green light beam expanding system so that the size of the shaped green light spot matches the size of the spot of the second path of infrared light, includes: adjusting the focal length ratio of the beam expanding system according to the spot diameter of the second path of infrared light, so that the error between the diameter of the shaped green light spot and the spot diameter of the second path of infrared light is less than the preset error, realizing the spatial mode matching of the two beams.

[0146] In some embodiments, the time delay control system includes a mirror group with adjustable optical path; applying time delay control to the second path of infrared light, adjusting the pulse time of the second path of infrared light through the time delay control system to compensate for the time walk-off effect caused by the inconsistent group velocities of the green light and the second path of infrared light in the nonlinear crystal, so that the pulses of the first infrared optical path and the second infrared optical path overlap in time, includes: changing the propagation optical path of the second path of infrared light by translating the mirror, combining with the cross-correlation measurement device to monitor the pulse time overlap degree of the green light and the second path of infrared light in real time, and controlling the corresponding pulse time delay error within the preset range of the full width at half maximum of the femtosecond pulse, realizing effective overlap in the time domain.

[0147] In some embodiments, after combining the second infrared light after time-delay control and the green light after beam shaping through a beam splitter and then jointly injecting them into a sum-frequency crystal for sum-frequency, it further includes: adjusting the incident angle of the sum-frequency crystal to satisfy the sum-frequency phase matching condition, controlling the polarization directions of the second infrared light and the green light to be consistent, and making the included angle between the corresponding beam optical axes less than the acceptance angle of the sum-frequency crystal, so as to ensure effective nonlinear sum-frequency interaction between the first infrared light and the second infrared light in the sum-frequency crystal and improve the generation efficiency of ultraviolet light.

[0148] In some embodiments, using the Gaussian energy distribution of the second infrared light and the Gaussian energy distribution of the green light for sum-frequency to generate ultraviolet light output includes: the second infrared light does not go through the frequency doubling process, and its energy distribution always maintains the uniform distribution state with a high center and a low periphery of the initial Gaussian beam. After shaping, the green light also forms a Gaussian energy distribution, and the corresponding energy distribution center positions coincide; a symmetric nonlinear interaction region is formed in the sum-frequency crystal to make the peak power density distributions of the infrared light and the green light match during the sum-frequency process, reduce the spatial walk-off effect caused by uneven energy distribution, so as to improve the beam quality of the ultraviolet light and suppress spot distortion.

[0149] The present application also provides a computer-readable storage medium. The computer-readable storage medium stores a computer program. When the computer program is executed by a processor, the processor is caused to implement the steps of the control method of the femtosecond third-harmonic optical path provided in any embodiment of the present application.

[0150] Among them, the computer-readable storage medium may be the internal storage unit of the control module described in the foregoing embodiments, such as the hard disk or memory of the control module. The computer-readable storage medium may also be an external storage device of the control module, such as a plug-in hard disk equipped on the control module, a Smart Media Card (SMC), a Secure Digital (SD) card, a Flash Card, etc.

[0151] The above is only the specific implementation manner of the present application, but the protection scope of the present application is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present application can easily think of various equivalent modifications or substitutions, and these modifications or substitutions should all be covered within the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the protection scope of the claims.

Claims

1. A control method for a femtosecond third-harmonic optical path, characterized in that Including: Using an infrared beam splitter to divide the infrared light output by a femtosecond laser into two paths according to a certain ratio, namely the first path of infrared light and the second path of infrared light; Making the first path of infrared light incident on a frequency doubling crystal to generate green light through frequency doubling. The frequency doubling crystal is an LBO or BBO nonlinear crystal, and the frequency doubling process is optimized to improve the frequency doubling efficiency of the green light; Shaping the generated green light through a green light beam expander system to make the spot size of the shaped green light match the spot size of the second path of infrared light; Applying time delay control to the second path of infrared light, and adjusting the pulse time of the second path of infrared light through a time delay control system to compensate for the time walk-off effect caused by the inconsistent group velocities of the green light and the second path of infrared light in the nonlinear crystal, so that the pulses of the first infrared light path and the second infrared light path overlap in time; After the second path of infrared light controlled by time delay and the green light shaped by the beam expander system are combined by a beam splitter, they are jointly incident on a sum frequency crystal for sum frequency. The sum frequency crystal is an LBO or BBO nonlinear crystal, and the sum frequency is carried out using the Gaussian energy distribution of the second path of infrared light and the Gaussian energy distribution of the green light to generate ultraviolet light output; wherein, the first path of infrared light is used to generate green light, and the second path of infrared light maintains the initial Gaussian energy distribution state and participates in sum frequency, avoiding the deterioration of the remaining infrared light energy distribution due to too high frequency doubling efficiency, and realizing the spot size matching of the green light and the second path of infrared light through the green light beam expander system, and realizing the pulse time overlap of the first infrared light path and the second infrared light path through the time delay control system to improve the sum frequency efficiency and the beam quality of the ultraviolet light.

2. The method according to claim 1, characterized in that The step of using an infrared beam splitter to divide the infrared light output by a femtosecond laser into two paths, namely the first path of infrared light and the second path of infrared light, includes: Determining the beam splitting ratio of the first path of infrared light and the second path of infrared light according to the nonlinear coefficient of the frequency doubling crystal, the incident power density and the requirement of the infrared light energy distribution for sum frequency; so that the first path of infrared light has sufficient energy for efficient frequency doubling, and the second path of infrared light maintains the energy distribution state of the initial Gaussian beam, avoiding the energy matching imbalance during sum frequency due to improper beam splitting ratio.

3. The method according to claim 1, wherein The step of optimizing the frequency doubling process to improve the frequency doubling efficiency of the green light includes: Adjusting the incident angle of the frequency doubling crystal to meet the phase matching condition, controlling the incident power density of the first path of infrared light within the optimal threshold range of nonlinear conversion of the frequency doubling crystal, and performing temperature control on the frequency doubling crystal to compensate for crystal dispersion, so that the frequency doubling efficiency of the frequency doubling process is higher than the preset efficiency and does not affect the initial Gaussian energy distribution characteristics of the second path of infrared light.

4. The method according to claim 1, wherein The green light beam expander system adopts a telescopic optical structure to collimate and expand the green light beam through the combination of a convex lens and a concave lens; the step of shaping the generated green light through a green light beam expander system to make the spot size of the shaped green light match the spot size of the second path of infrared light includes: According to the spot diameter of the second infrared light, adjust the focal length ratio of the beam expander system so that the error between the reshaped green light spot diameter and the spot diameter of the second infrared light is less than the preset error, realizing the spatial mode matching of the two beams of light.

5. The method according to claim 1, characterized in that, The time delay control system includes a mirror group with adjustable optical path; applying time delay control to the second infrared light, and adjusting the pulse time of the second infrared light through the time delay control system to compensate for the time walk-off effect caused by the inconsistent group velocities of the green light and the second infrared light in the nonlinear crystal, so that the pulses of the first infrared optical path and the second infrared optical path overlap in time, including: Changing the propagation optical path of the second infrared light by translating the mirror, and combining with the cross-correlation measurement device to monitor the pulse time overlap degree of the green light and the second infrared light in real time, and controlling the corresponding pulse time delay error within the preset range of the full width at half maximum of the femtosecond pulse, realizing effective overlap in the time domain.

6. The method according to claim 1, wherein After combining the second infrared light after time delay control and the reshaped green light through a beam splitter and then co-inciding them into a sum-frequency crystal for sum-frequency, it further includes: Adjusting the incident angle of the sum-frequency crystal to meet the sum-frequency phase matching condition, controlling the polarization directions of the second infrared light and the green light to be consistent, and making the included angle between the corresponding optical axes of the beams less than the acceptance angle of the sum-frequency crystal to ensure effective nonlinear sum-frequency interaction between the first infrared light and the second infrared light in the sum-frequency crystal and improve the generation efficiency of ultraviolet light.

7. The method according to claim 1, characterized in that Using the Gaussian energy distribution of the second infrared light and the Gaussian energy distribution of the green light for sum-frequency to generate ultraviolet light output, including: The second infrared light does not undergo the frequency doubling process, and its energy distribution always maintains the uniform distribution state with high center and low periphery of the initial Gaussian beam. The green light also forms a Gaussian energy distribution after shaping, and the corresponding energy distribution center positions coincide. Forming a symmetric nonlinear action region in the sum-frequency crystal to match the peak power density distributions of the infrared light and the green light during the sum-frequency process, reducing the spatial walk-off effect caused by uneven energy distribution, so as to improve the beam quality of the ultraviolet light and suppress spot distortion.

8. A control device for a femtosecond third harmonic optical path, characterized in that, Including: An optical path division unit for splitting the infrared light output by the femtosecond laser into two paths in a certain proportion by using an infrared beam splitter, namely the first infrared light and the second infrared light; A green light generation unit for incident the first infrared light into a frequency doubling crystal to generate green light. The frequency doubling crystal is an LBO or BBO nonlinear crystal, and optimizing the frequency doubling process to improve the frequency doubling efficiency of the green light; A beam shaping unit for shaping the generated green light through a green light beam expander system so that the reshaped green light spot size matches the spot size of the second infrared light; A delay control unit for applying time delay control to the second infrared light, and adjusting the pulse time of the second infrared light through the time delay control system to compensate for the time walk-off effect caused by the inconsistent group velocities of the green light and the second infrared light in the nonlinear crystal, so that the pulses of the first infrared optical path and the second infrared optical path overlap in time; The crystal sum - frequency unit is used to combine the second - path infrared light after time - delay control and the green light after beam shaping through a beam splitter and then jointly incident on a sum - frequency crystal for sum - frequency. The sum - frequency crystal is an LBO or BBO nonlinear crystal, and the Gaussian energy distribution of the second - path infrared light and the Gaussian energy distribution of the green light are used for sum - frequency to generate ultraviolet light output. Among them, the first - path infrared light is used to generate green light, and the second - path infrared light participates in sum - frequency while maintaining the initial Gaussian energy distribution state, avoiding the deterioration of the remaining infrared light energy distribution due to too high frequency - doubling efficiency, and realizing the spot - size matching between the green light and the second - path infrared light through the green - light beam expansion system, and realizing the pulse - time overlap of the first infrared optical path and the second infrared optical path through the time - delay control system to improve the sum - frequency efficiency and the beam quality of the ultraviolet light.

9. A control module, characterized in that, The control module includes a memory and a processor; The memory is used to store computer programs; The processor is used to execute the computer program and, when executing the computer program, implement the method according to any one of claims 1 to 7.

10. A computer-readable storage medium, characterized in that, The computer - readable storage medium stores a computer program, and when the computer program is executed by the processor, the processor is caused to implement the method according to any one of claims 1 to 7.

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