A multi-stage cascaded laser frequency multiplication device

Abstract

This invention discloses a multi-stage cascaded laser frequency doubling device, belonging to the field of laser frequency doubling technology. It includes a front-end frequency doubling module and at least one cascaded frequency doubling module arranged sequentially along the optical path. The cascaded frequency doubling module includes a second-harmonic crystal, a third-harmonic crystal, a beam splitter, a delay adjustment device, and a beam combiner. The front-end frequency doubling module, beam splitter, second-harmonic crystal, beam combiner, and third-harmonic crystal are connected sequentially. By extending the multi-stage continuous cascaded crystal architecture, the long crystal is split into a sequence of thin frequency doubling crystals, suppressing time drift and pulse broadening. Each stage is equipped with a multi-band fully transparent film system to reduce dispersion. Furthermore, a beam splitting and beam combining bypass and dynamic time delay compensation are designed to achieve precise spatiotemporal beam combining of ultraviolet light, enabling femtosecond lasers to possess both high efficiency, low broadening, and excellent pulse quality.

Description

Technical Field

[0001] This invention belongs to the field of laser frequency doubling technology, specifically relating to a multi-stage cascaded laser frequency doubling device. Background Technology

[0002] Femtosecond ultraviolet lasers, with their extremely high temporal resolution and peak power, have crucial applications in ultrafast scientific research and precision industrial processing. Currently, using nonlinear crystals to generate second and third harmonics is the mainstream technique for obtaining femtosecond ultraviolet pulses.

[0003] However, there is an inherent physical bottleneck in frequency conversion using nonlinear crystals: group velocity mismatch. Inside the crystal, the fundamental and harmonic light propagate at different speeds due to the difference in refractive index, leading to a time walk-off effect. On the femtosecond pulse timescale, this walk-off, at the level of hundreds of femtoseconds and millimeters, rapidly causes the fundamental and harmonic pulse envelopes to physically separate in the time domain, disrupting the effective overlap of the nonlinear interaction. Consequently, the frequency doubling conversion efficiency decreases sharply with increasing crystal length, while the output ultraviolet pulse exhibits significant time-domain broadening.

[0004] To address the aforementioned issues, existing technologies typically employ strategies such as optimizing crystal thickness or introducing external optical path delay compensation. In engineering practice, while reducing crystal thickness can suppress time drift, it directly sacrifices the effective length of nonlinear interactions, leading to low conversion efficiency. Conversely, increasing crystal length can partially improve conversion efficiency, but the increased drift effect degrades the pulse waveform. Furthermore, some existing solutions attempt to compensate for drift by constructing complex delay systems containing multiple mirrors outside the crystal. However, at the femtosecond wavelength, the alignment of such external structures is extremely demanding, and the additional transmission elements and air path introduced into the optical path introduce high-order dispersion that is difficult to eliminate, further worsening the time-domain waveform of the ultrashort pulse and causing a significant drop in peak power.

[0005] Therefore, existing solutions cannot simultaneously achieve high conversion efficiency and low pulse broadening, resulting in low overall nonlinear conversion efficiency. Summary of the Invention

[0006] The purpose of this invention is to solve the problem that it is impossible to achieve both high conversion efficiency and low pulse broadening at the same time, resulting in low overall nonlinear conversion efficiency, and to propose a multi-stage cascaded laser frequency doubling device.

[0007] The present invention proposes a multi-stage cascaded laser frequency doubling device, including a front-end frequency doubling module composed of a first second-frequency doubling crystal and a first third-frequency doubling crystal, and at least one first-stage cascaded frequency doubling module is connected in series along the optical path after the front-end frequency doubling module. The first cascaded frequency doubling module includes a second second-harmonic crystal, a second third-harmonic crystal, a first beam splitter, a delay adjustment device, and a beam combiner. The light-incident end of the first beam splitter is connected to the light-outcident end of the front-end frequency doubling module; the first light-outcident end of the first beam splitter is connected to the light-incident end of the second frequency doubling crystal, and the second light-outcident end is connected to the light-incident end of the delay adjustment device. The light-emitting end of the second second-harmonic crystal is connected to the first light-input end of the beam combining element; The light-emitting end of the delay adjustment device is connected to the second light-input end of the beam combining element; The light-emitting end of the beam combining element is connected to the light-input end of the second third harmonic crystal.

[0008] Optionally, a second cascaded frequency multiplier module can be connected in series along the optical path after the front-end frequency multiplier module; The second-cascaded frequency doubling module includes a second second-harmonic crystal, a second third-harmonic crystal, a first beam splitter, a delay adjustment device, and a beam combiner. The light-incident end of the first beam splitter is connected to the light-outcident end of the front-end frequency doubling module; the first light-outcident end of the first beam splitter is connected to the light-incident end of the second frequency doubling crystal, and the second light-outcident end is connected to the light-incident end of the delay adjustment device. The light-emitting end of the second second-harmonic crystal is connected to the light-input end of the second third-harmonic crystal; The light-emitting end of the second and third harmonic crystal is connected to the first light-input end of the beam combining element; The light-emitting end of the delay adjustment device is connected to the second light-input end of the beam combining element.

[0009] Optionally, the delay adjustment device includes a first reflector and a second reflector; The light-incident end of the first reflector is connected to the second light-outcident end of the first beam-splitting element; The light-emitting end of the first reflector is connected to the light-incident end of the second reflector; The light-emitting end of the second reflector is connected to the second light-incident end of the beam combiner.

[0010] Optionally, the light-emitting end of the first second-harmonic crystal of the front-end frequency doubling module is connected to the light-input end of the first third-harmonic crystal; the light-emitting end of the first third-harmonic crystal is connected to the light-input end of the first beam splitter.

[0011] Optionally, the first second harmonic crystal is used to receive the fundamental frequency light and output the first fused light; the first fused light includes the fundamental frequency light and the second harmonic light; The first third harmonic crystal is used to receive the first fused light and output the second fused light, the second fused light comprising fundamental frequency light, second harmonic light and third harmonic light; The first beam splitter is used to receive the second fused light and split the optical path to obtain the upper main optical path and the lower bypass optical path; The upper main optical path passes through the second second-frequency doubling crystal to obtain the first branch light; The lower branch bypass optical path passes through the delay adjustment device to obtain the second branch optical; The beam combining element is used to receive the first branch light and the second branch light and spatially combine them to obtain a combined light. The second and third harmonic crystals are used to receive the combined light and perform high-order nonlinear frequency transformation to output multi-level harmonic light.

[0012] Optionally, by adjusting the positions of the first and second reflectors in the delay adjustment device, the optical path length passing through the delay adjustment device can be adjusted, thereby achieving time-domain synchronization of the first branch light and the second branch light at the beam combining element.

[0013] Optionally, at least one third-cascaded frequency multiplier module may be connected in series along the optical path after the front-end frequency multiplier module. The third cascaded frequency doubling module includes a second second harmonic crystal and a second third harmonic crystal; The light-incident end of the second second-harmonic crystal is connected to the light-outcident end of the front-end frequency doubling module; the light-outcident end of the second second-harmonic crystal is connected to the light-incident end of the second third-harmonic crystal. The output end of the third cascaded frequency doubling module is connected to the input end of the second beam splitter.

[0014] Optionally, the light-emitting end of the first second-harmonic crystal of the front-end frequency doubling module is connected to the light-input end of the first third-harmonic crystal; the light-emitting end of the first third-harmonic crystal is connected to the light-input end of the second second-harmonic crystal.

[0015] Optionally, the light-emitting surface of the first second-harmonic crystal and the light-incident surface of the first third-harmonic crystal are coated with a high-transmission film that includes at least the fundamental frequency light and the second-harmonic light band; the light-emitting surface of the first third-harmonic crystal, as well as all the light-emitting surfaces of the second second-harmonic crystal and the second third-harmonic crystal, are coated with a high-transmission film that includes at least the fundamental frequency light, the second-harmonic light and the third-harmonic light band.

[0016] The beneficial effects of this invention are: This invention proposes a multi-stage cascaded laser frequency doubling device. By defining a specific closed-loop connection topology between the front-end frequency doubling module and at least one first-stage cascaded frequency doubling module, including the beam splitter, second-harmonic crystal, delay adjustment device, beam combiner, and third-harmonic crystal, the traditional single long crystal is forcibly split into a multi-stage thin crystal series structure. This physically suppresses pulse broadening caused by time walk-off within a single crystal. Simultaneously, a residual light multiplexing path is constructed by using the branch connections between the beam splitter, the main second-harmonic crystal, and the bypass delay adjustment device. The fundamental and second-harmonic light that are not fully converted in the previous stage are extracted in a distributed, step-by-step manner to accumulate and improve the overall conversion efficiency. Furthermore, the precise connection between the delay adjustment device, the beam combiner, and the subsequent third-harmonic crystal provides an independent optical path compensation path. Precise spatiotemporal beam combining of multi-stage third-harmonic light is achieved without introducing additional high-order dispersion, thus overcoming the physical bottleneck in existing technologies where high conversion efficiency and low pulse broadening cannot be simultaneously achieved. Attached Figure Description

[0017] The invention will now be further described with reference to the accompanying drawings.

[0018] Figure 1 A schematic diagram of a multi-stage cascaded laser frequency doubling device provided in an embodiment of the present invention; Figure 2 A schematic diagram of another multi-stage cascaded laser frequency doubling device provided in an embodiment of the present invention; Figure 3 A schematic diagram of another multi-stage cascaded laser frequency doubling device provided in an embodiment of the present invention; In the diagram: 100, front-end frequency doubling module; 101, first second frequency doubling crystal; 102, first third frequency doubling crystal; 1, first cascaded frequency doubling module; 11, second cascaded frequency doubling module; 103, second second frequency doubling crystal; 104, second third frequency doubling crystal; 105, first beam splitter; 10, delay adjustment device; 106, first reflector; 107, second reflector; 108, beam combiner; 2, third cascaded frequency doubling module; 201, second beam splitter. Detailed Implementation

[0019] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0020] Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] It should be noted that in this article, the first and second second harmonic crystals are the same second harmonic crystal model; the first and second third harmonic crystals are the same third harmonic crystal model; the first and second reflectors are the same model; and the first and second beam splitters are the same model. Example 1 This embodiment 1 provides a multi-stage cascaded laser frequency doubling device. See also... Figure 1 This multi-stage cascaded laser frequency doubling device is a multi-stage cascaded beam-splitting frequency doubling device. The device includes the following: A front-end frequency multiplier module 100 is provided, and at least one first-stage frequency multiplier module 1 (splitter type) is connected in series along the optical path direction after the front-end frequency multiplier module 100. The first cascaded frequency doubling module 1 includes a second second frequency doubling crystal 103, a second third frequency doubling crystal 104, a first beam splitter 105, a delay adjustment device 10, and a beam combiner 108; The light input end of the first beam splitter 105 is connected to the light output end of the front-end frequency doubling module 100; the first light output end of the first beam splitter 105 is connected to the light input end of the second frequency doubling crystal 103; and the second light output end of the first beam splitter 105 is connected to the light input end of the delay adjustment device 10. The light-emitting end of the second second-harmonic crystal 103 is connected to the first light-incident end of the beam combiner 108; the light-emitting end of the delay adjustment device 10 is connected to the second light-incident end of the beam combiner 108; and the light-emitting end of the beam combiner 108 is connected to the light-incident end of the second third-harmonic crystal 104. The light-emitting surfaces of the first second-harmonic crystal 101 and the light-incident surfaces of the first third-harmonic crystal 102 are coated with high-transmission films that include at least the fundamental frequency and second-harmonic light bands; the light-emitting surfaces of the first third-harmonic crystal 102, as well as all light-emitting surfaces of the second second-harmonic crystal 103 and the second third-harmonic crystal 104, are coated with high-transmission films that include at least the fundamental frequency, second-harmonic light and third-harmonic light bands.

[0022] The front-end frequency doubling module 100 includes a first second frequency doubling crystal 101 and a first third frequency doubling crystal 102; the light-emitting end of the first second frequency doubling crystal 101 is connected to the light-input end of the first third frequency doubling crystal 102; the light-emitting end of the first third frequency doubling crystal 102 is connected to the light-input end of the first beam splitter 105.

[0023] The delay adjustment device 10 includes a first reflector 106 and a second reflector 107. The light-incident end of the first reflector 106 is connected to the second light-outcident end of the first beam splitter 105; The light-emitting end of the first reflector 106 is connected to the light-incident end of the second reflector 107; The light-emitting end of the second reflector 107 is connected to the second light-incident end of the beam combiner 108.

[0024] The first second harmonic crystal 101 is used to receive the fundamental frequency light and output the first fused light; the first fused light includes the fundamental frequency light and the second harmonic light. The first third harmonic crystal 102 is used to receive the first fused light and output the second fused light, the second fused light including the fundamental frequency light, the second harmonic light and the third harmonic light; The first beam splitter 105 is used to receive the second fused light and split the optical path to obtain the upper main optical path and the lower bypass optical path. The upper main optical path passes through the second second-harmonic crystal 103 to obtain the first branch light; The lower bypass optical path passes through the delay adjustment device 10 to obtain the second branch optical; The beam combiner 108 is used to receive the first branch light and the second branch light and spatially combine them to obtain a combined light. The second and third harmonic crystal 104 is used to receive the combined light beam, perform high-order nonlinear frequency conversion, and output multi-level harmonic light.

[0025] Specifically, by adjusting the positions of the first reflector 106 and the second reflector 107 in the delay adjustment device 10, the optical path of the lower branch bypass optical path is adjusted, thereby achieving time-domain synchronization of the first branch light and the second branch light at the beam combining element 108.

[0026] Specifically, for the first second-harmonic crystal 101, the phase matching angle is adjusted by changing the spatial orientation of the first second-harmonic crystal 101 relative to the incident beam, thereby obtaining the maximum second-harmonic light output power.

[0027] After the front-end frequency doubling module generates a mixture of fundamental, second, and third harmonic light, the optical path is split into two paths using a first beam splitter. One path further generates higher-order harmonics via a second second-harmonic and a second third-harmonic crystal, while the other path precisely compensates for time-domain drift between different frequency components through a delay adjustment device. Finally, a beam combiner achieves time-domain synchronization of the two pulses. Compared to the traditional single-long crystal scheme, this design avoids a sharp drop in conversion efficiency and significant pulse broadening caused by drift effects. At the same time, it eliminates the need for a complex external optical delay system, fundamentally eliminating the drawbacks of high-order dispersion introduction and stringent optical path alignment. Through the cascading effect of the second set of crystals, the device effectively extends the overall length of nonlinear interactions, making full use of the unconverted fundamental and harmonic light, thus achieving both high frequency doubling conversion efficiency and excellent time-domain waveform preservation in femtosecond ultraviolet pulse generation.

[0028] In one implementation, the phase matching angle is adjusted by changing the spatial orientation of the first second harmonic crystal 101 relative to the incident beam, the specific method of which is determined by the technician.

[0029] In one implementation, see [link to implementation details]. Figure 1The system consists of an infinite number of cascaded frequency doubling regions with identical internal structures. As shown in the figure, the dashed box represents the first cascaded frequency doubling module 1 and the infinity symbol indicates that the input of the first cascaded frequency doubling module is through the first beam splitter 105, and the output is through the second third frequency doubling crystal 104. Taking the first cascaded frequency doubling module 1 region as an example, the beam is first split into two branches by the first beam splitter 105: the upper main beam passes through the second second frequency doubling crystal 103; the lower bypass beam is constructed by the first reflector 106 and the second reflector 107. In order to realize the dynamic adjustment of the bypass optical path, the first reflector 106 and the second reflector 107 can be selectively assembled as a whole in the delay adjustment device 10. Before entering the second third frequency doubling crystal 104, the two beams are spatially combined by the beam combiner 108. After being combined, the two laser beams enter the second third frequency doubling crystal 104 together for higher-order transformation.

[0030] In one implementation, existing technologies, to improve the conversion efficiency of ultraviolet light (i.e., third harmonic conversion), can only make the nonlinear crystal longer or thicker. However, in femtosecond lasers, the longer the crystal, the greater the difference in the transmission speed of different wavelengths of light within it, leading to group velocity mismatch. This causes the pulses to stagger in time, ultimately resulting in severe pulse widening and a decrease in peak power. The core advantage of this connection method is that it splits a long crystal into multiple thin crystals. The thin crystals themselves have minimal time travel, ensuring that the pulse width does not broaden. Then, the unused fundamental frequency light and residual second harmonic light from the previous stage enter the next stage for further conversion. In this way, through multi-stage superposition, both the ultra-short pulse width of the femtosecond pulse is guaranteed, and the overall conversion efficiency is multiplied.

[0031] In one implementation, the beam splitter and beam combiner in the scheme are both dichroic mirrors or harmonic separators, and their end faces are coated with optical thin films of specific wavelengths. Their optical characteristics are: high reflectivity for third harmonic light and high transmittance for fundamental and second harmonic light, thereby realizing the spatial separation of beams of different wavelengths.

[0032] In one implementation, a multi-stage cascaded beam splitter frequency multiplier can be connected in series with multiple first-stage frequency multiplier modules. However, as the number of stages increases, the residual fundamental frequency light energy will continuously decay. When the energy density is lower than the conversion threshold of the nonlinear crystal, subsequent stages will no longer be able to generate new frequency-doubled light. Therefore, in actual engineering, it is generally necessary to connect 2 to 4 in series to meet the requirements. The specific number is determined by the technical personnel.

[0033] In one implementation, traditional nonlinear crystals typically only have conventional single-band or dual-band antireflection coatings applied to the operating wavelength. In this scheme, the fundamental infrared light, second-harmonic green light, and third-harmonic ultraviolet light are mixed together and transmitted coaxially in the cascaded direct main optical path. If a traditional scheme is used, a large number of additional dichroic mirrors are needed to separate them, process them separately, and then merge them. This would introduce a large group velocity dispersion, which would destroy the time-domain waveform of the femtosecond pulse. Through the specially designed multi-band and three-band inclusive high-transmittance coatings, these three wavelengths of light are allowed to pass directly through the same crystal without loss, which greatly simplifies the optical path structure, eliminates unnecessary intracavity dispersive elements, and fundamentally protects the time-domain fidelity of the femtosecond pulse.

[0034] In one implementation, after passing through the first third harmonic crystal 102, the light beam is a mixture of fundamental frequency light, second harmonic light, and third harmonic light. After passing through the first beam splitter 105, the lower bypass is the light entering the delay adjustment device 10: only the third harmonic light, i.e., ultraviolet light / violet light, is present. The first beam splitter 105 preemptively strips out the ultraviolet light just generated in the first stage. The upper main light path is the light entering the second second harmonic crystal 103: the remaining incompletely converted residual fundamental frequency infrared light and residual second harmonic green light enter the next stage crystal to generate new third harmonic light.

[0035] In one implementation, the fundamental frequency light is infrared light, the second harmonic light is green light, and the third harmonic light is violet light. Infrared light enters the first second harmonic crystal 101, generating green light. Infrared and green light enter the first third harmonic crystal 102, generating the first batch of violet light. This violet light reaches the first beam splitter 105 and is split: the first batch of violet light is separated and travels along a bypass path to avoid the crystal; the remaining infrared and green light continue along the main path, entering the second second harmonic crystal 103 and the second third harmonic crystal 104, exciting and generating… The second batch of violet light, at the beam combiner (beam combiner element 108), combines with the first batch of violet light that was just generated, and the two batches of violet light are superimposed and output, thus obtaining an extremely high-power ultraviolet femtosecond laser. Because the photon energy of violet light is very high, if it is forced to pass through the subsequent second second-harmonic crystal 103 and second third-harmonic crystal 104, it will be strongly absorbed by the crystal material, resulting in severe energy loss and causing the crystal to heat up and generate a thermal lensing effect, destroying the beam quality.

[0036] In one implementation, the beam splitter is a dichroic mirror or a two-color beam splitter, with the following optical characteristics: high reflection or high transmission for the ultraviolet band of third harmonic light, depending on the specific placement angle; and high transmission or high reflection for the infrared and green bands of fundamental and second harmonic light, thereby achieving spatial separation of wavelengths. The beam combining element 108 is an ultraviolet polarization beam combiner, which utilizes the polarization characteristics of two ultraviolet beams to achieve efficient spatial and temporal superposition output.

[0037] In one implementation, the duration of the femtosecond pulse is extremely short, one trillionth of a second. The first batch of violet light travels through the air bypass and the second batch travels through the crystal main path. Because the refractive indices of the media they pass through are different, the time it takes for them to reach the beam combiner 108 will have an extremely small difference. If they are not strictly synchronized, the two pulses will be out of sync in time, and the pulse width after beam combining will become wider. The delay adjustment device 10 is actually an optical mirror assembly with a precision translation stage. The first mirror 106 and the second mirror 107 are fixed on a linearly sliding mechanical platform. By finely adjusting this platform forward or backward, the total physical length of the lower bypass optical path can be directly changed. According to the principle of the constancy of the speed of light, changing the physical path length can change the flight time of the first batch of violet light to reach the beam combiner 108 with extreme precision until it perfectly overlaps with the second batch of violet light on the femtosecond scale, thereby achieving time-domain synchronization.

[0038] In one implementation, for a beam splitting architecture supporting unlimited cascade expansion, in each cascaded region, the mixed beam is first split by a beam splitter. The residual main optical path passes through the second-harmonic crystal of that region and is then spatially combined with the third-harmonic light transmitted from the bypass path before entering the third-harmonic crystal of that region via a beam combiner. If there is a need to adjust the optical path difference in the bypass path of the beam splitting architecture, the spatial position of the mirror group in the bypass path is changed to adjust the optical path of the next branch, thereby controlling the time point at which the two branch beams arrive at the beam combiner. During this stage conversion and beam combining process, the third-harmonic light parameters of the output terminal are monitored in real time, and the phase matching angle of each crystal and the optical path difference of the corresponding bypass path are optimized in a coordinated manner to accurately compensate for the time drift accumulated in the multi-stage optical path transmission. After the final output third-harmonic light parameters reach the optimal index, the spatial positions of all optical components in the optical path are fixed.

[0039] Example 2

[0040] In Example 2, see Figure 2 The present invention provides a schematic diagram of another multi-stage cascaded laser frequency doubling device, which is a two-stage beam splitting and polarization combining frequency doubling device, including a front-end frequency doubling module 100, and a second cascaded frequency doubling module 11 (beam splitting and polarization combining type) is connected in series along the optical path direction in the front-end frequency doubling module 100. The second cascaded frequency doubling module 11 includes a second second frequency doubling crystal 103, a second third frequency doubling crystal 104, a first beam splitter 105, a delay adjustment device 10, and a beam combiner 108. The light input end of the first beam splitter 105 is connected to the light output end of the front-end frequency doubling module 100; the first light output end of the first beam splitter 105 is connected to the light input end of the second frequency doubling crystal 103; and the second light output end of the first beam splitter 105 is connected to the light input end of the delay adjustment device 10. The light-emitting end of the second second-harmonic crystal 103 is connected to the light-incident end of the second third-harmonic crystal 104; the light-emitting end of the second third-harmonic crystal 104 is connected to the first light-incident end of the beam combiner 108; the light-emitting end of the delay adjustment device 10 is connected to the second light-incident end of the beam combiner 108. The light-emitting surfaces of the first second-harmonic crystal 101 and the light-incident surfaces of the first third-harmonic crystal 102 are coated with high-transmission films that include at least the fundamental frequency and second-harmonic light bands; the light-emitting surfaces of the first third-harmonic crystal 102, as well as all light-emitting surfaces of the second second-harmonic crystal 103 and the second third-harmonic crystal 104, are coated with high-transmission films that include at least the fundamental frequency, second-harmonic light and third-harmonic light bands.

[0041] The front-end frequency doubling module 100 includes a first second frequency doubling crystal 101 and a first third frequency doubling crystal 102; the light-emitting end of the first second frequency doubling crystal 101 is connected to the light-incident end of the first third frequency doubling crystal 102; the light-emitting end of the first third frequency doubling crystal 102 is connected to the light-incident end of the first beam splitter 105. The delay adjustment device 10 includes a first reflector 106 and a second reflector 107; The light-incident end of the first reflector 106 is connected to the second light-outcident end of the first beam splitter 105; The light-emitting end of the first reflector 106 is connected to the light-incident end of the second reflector 107; The light-emitting end of the second reflector 107 is connected to the second light-incident end of the beam combiner 108.

[0042] In one implementation, see [link to implementation details]. Figure 2 The device has a first second-harmonic crystal 101 and a first third-harmonic crystal 102 arranged sequentially at the front end. After the beam passes through the first third-harmonic crystal 102, it is split into two branches by the first beam splitter 105: the upper main optical path passes through the second second-harmonic crystal 103 and the second third-harmonic crystal 104 in sequence, and generates secondary third-harmonic ultraviolet light by using the residual fundamental frequency light and the second-harmonic light; the lower bypass optical path is constructed by the first reflector 106 and the second reflector 107, which is specifically used to transmit the third-harmonic ultraviolet light generated by the first stage. The first reflector 106 and the second reflector 107 can be selectively assembled as a whole in the delay adjustable device. Unlike the infinite cascade architecture, the beam combining element 108 of this basic two-stage architecture is set after the second third-harmonic crystal 104. The secondary third-harmonic ultraviolet light generated by the upper main optical path and the primary third-harmonic ultraviolet light transmitted by the lower bypass path spatially converge at the beam combining element 108.

[0043] In one implementation, for a basic two-stage beam splitting and polarization combining architecture, the mixed beam is first split into two branches by a beam splitter. The main optical path enters the secondary crystal for higher-order transformation, while the bypass path transmits the first-stage third-harmonic light. Subsequently, the optical path is finely adjusted so that the third-harmonic light generated by the two paths spatially converges after the secondary third-harmonic crystal via a polarization combiner.

[0044] Example 3

[0045] In Example 3, see Figure 3 The present invention provides a schematic diagram of another multi-stage cascaded laser frequency doubling device. This device is a multi-stage cascaded direct-through frequency doubling device, including a front-end frequency doubling module 100. At least one third-stage cascaded frequency doubling module 2 and a second beam splitter 201 are connected in series after the front-end frequency doubling module 100 along the optical path direction. The third cascaded frequency doubling module 2 (straight-through type) includes a second second frequency doubling crystal 103 and a second third frequency doubling crystal 104; the light input end of the second second frequency doubling crystal 103 is connected to the light output end of the front-end frequency doubling module 100; the light output end of the second second frequency doubling crystal 103 is connected to the light input end of the second third frequency doubling crystal 104. The output end of the third cascaded frequency doubling module 2 is used to connect with the input end of the third cascaded frequency doubling module 2 connected in series along the optical path direction, or to connect with the input end of the second beam splitter 201. The light-emitting surfaces of the first second-harmonic crystal 101 and the light-incident surfaces of the first third-harmonic crystal 102 are coated with high-transmission films that include at least the fundamental frequency and second-harmonic light bands; the light-emitting surfaces of the first third-harmonic crystal 102, as well as all light-emitting surfaces of the second second-harmonic crystal 103 and the second third-harmonic crystal 104, are coated with high-transmission films that include at least the fundamental frequency, second-harmonic light and third-harmonic light bands.

[0046] In one implementation, see [link to implementation details]. Figure 2 A multi-stage cascaded direct-through frequency doubling device for laser frequency doubling also adopts a series architecture that supports infinite level expansion. In each direct-through cascaded region, such as the second second harmonic crystal 103 and the second third harmonic crystal 104 in the third cascaded frequency doubling module 2, the mixed beam output after continuous nonlinear frequency transformation with infinite level expansion is finally separated into residual fundamental frequency light, second harmonic light and generated third harmonic light by the second beam splitter 201 set at the end of the optical path.

[0047] In one implementation, the first second-harmonic crystal 101, the first third-harmonic crystal 102, and the second-harmonic and third-harmonic crystals in subsequent cascaded regions are arranged sequentially on preset optical path nodes according to the inclusive film system configuration rule of high transmission in the front-end dual-band and high transmission in the rear-end tri-band. Each crystal is controlled at its theoretical phase-matching temperature to complete the initialization preparation of the optomechanical system. The fundamental frequency light is introduced into the first second-harmonic crystal 101, and the phase-matching angle is adjusted by changing the spatial attitude of the crystal relative to the incident beam to obtain the maximum output power of the second-harmonic light. Subsequently, the mixed beam of the fundamental frequency light and the second-harmonic light enters the first third-harmonic crystal 102, and the spatial attitude of the crystal is adjusted synchronously to complete the first-stage third-harmonic conversion. The mixed beam after the first-stage nonlinear transformation enters the subsequent cascaded optical path for assembly and adjustment according to the selected device architecture: for a multi-stage cascaded direct-through architecture, the mixed beam directly passes through the second-harmonic and third-harmonic crystals in each subsequent cascaded region sequentially, and the spatial attitude of each crystal is finely adjusted step by step to perform continuous nonlinear frequency transformation.

[0048] In summary, a multi-stage cascaded laser frequency doubling device is proposed. By supporting a multi-stage continuous cascaded crystal architecture that allows for infinite order expansion, the traditional monolithic long crystal is split into a spatially combined sequence of second and third harmonic thin crystals, physically suppressing pulse broadening caused by time walk-off within the single crystal. Simultaneously, by utilizing the residual fundamental and second harmonic light from the incompletely converted previous stages to directly participate in the nonlinear transformation of subsequent stages, a residual light multiplexing mechanism is achieved, enabling distributed, step-by-step energy extraction. This allows the overall conversion efficiency to continuously accumulate with the increase in the number of cascade stages, breaking through the efficiency ceiling of traditional architectures. The light-emitting end faces of each crystal stage employ a fully transmissive multi-band multiplexing film system with specific band combinations, ensuring efficient beam sharing across multiple frequency bands. While transmitting in lines, the need for additional beam splitters and birefringent compensation media is eliminated, fundamentally reducing the system's accumulated high-order dispersion. For the separation and re-superposition of ultraviolet light, a beam splitting and beam combining bypass configuration is designed, which includes beam splitting elements, independent transmission bypasses, main conversion optical paths, and spatial topological connections. This configuration can pre-strip ultraviolet light to avoid material absorption loss and thermal distortion. Furthermore, through the optical path dynamic time delay compensation mechanism configured in the bypass, independent and precise time-domain compensation of multiple optical paths can be achieved by translating the physical position of the reflector group as a whole. This ensures that the third harmonic light generated at each stage is accurately superimposed in the spatiotemporal dimension during beam combining, thus comprehensively taking into account high conversion efficiency, low pulse broadening, and excellent pulse temporal quality in the femtosecond band.

[0049] The foregoing has described one embodiment of the present invention in detail, but this is merely a preferred embodiment and should not be considered as limiting the scope of the invention. All equivalent variations and modifications made within the scope of the claims of this invention should still fall within the scope of the claims.

Claims

1. A multi-stage cascaded laser frequency doubling device, characterized in that, It includes a front-end frequency doubling module (100) composed of a first second frequency doubling crystal (101) and a first third frequency doubling crystal (102), and at least one first cascaded frequency doubling module (1) is connected in series along the optical path after the front-end frequency doubling module (100). The first cascaded frequency doubling module (1) includes a second second frequency doubling crystal (103), a second third frequency doubling crystal (104), a first beam splitter (105), a delay adjustment device (10), and a beam combining element (108). The light-incident end of the first beam splitter (105) is connected to the light-outcident end of the front-end frequency doubling module (100); the first light-outcident end of the first beam splitter (105) is connected to the light-incident end of the second frequency doubling crystal (103), and the second light-outcident end is connected to the light-incident end of the delay adjustment device (10). The light-emitting end of the second second-frequency doubling crystal (103) is connected to the first light-incident end of the beam combining element (108); The light-emitting end of the delay adjustment device (10) is connected to the second light-incident end of the beam combining element (108); The light-emitting end of the beam combining element (108) is connected to the light-input end of the second third harmonic crystal (104).

2. The multi-stage cascaded laser frequency doubling device according to claim 1, characterized in that, A second-stage frequency multiplier module (11) is connected in series along the optical path after the front-end frequency multiplier module (100). The second cascaded frequency doubling module (11) includes a second second frequency doubling crystal (103), a second third frequency doubling crystal (104), a first beam splitter (105), a delay adjustment device (10), and a beam combiner (108). The light-incident end of the first beam splitter (105) is connected to the light-outcident end of the front-end frequency doubling module (100); the first light-outcident end of the first beam splitter (105) is connected to the light-incident end of the second frequency doubling crystal (103), and the second light-outcident end is connected to the light-incident end of the delay adjustment device (10). The light-emitting end of the second second-harmonic crystal (103) is connected to the light-incident end of the second third-harmonic crystal (104); The light-emitting end of the second third harmonic crystal (104) is connected to the first light-incident end of the beam combining element (108); The light-emitting end of the delay adjustment device (10) is connected to the second light-input end of the beam combining element (108).

3. A multi-stage cascaded laser frequency doubling device according to claim 1 or 2, characterized in that, The delay adjustment device (10) includes a first reflector (106) and a second reflector (107). The light-incident end of the first reflector (106) is connected to the second light-outcident end of the first beam splitter (105); The light-emitting end of the first reflector (106) is connected to the light-incident end of the second reflector (107); The light-emitting end of the second reflector (107) is connected to the second light-incident end of the beam combining element (108).

4. The multi-stage cascaded laser frequency doubling device according to claim 3, characterized in that, The light-emitting end of the first second-harmonic crystal (101) of the front-end frequency doubling module (100) is connected to the light-input end of the first third-harmonic crystal (102); the light-emitting end of the first third-harmonic crystal (102) is connected to the light-input end of the first beam splitter (105).

5. A multi-stage cascaded laser frequency doubling device according to claim 4, characterized in that, The first second harmonic crystal (101) is used to receive the fundamental frequency light and output the first fused light; the first fused light includes the fundamental frequency light and the second harmonic light; The first third harmonic crystal (102) is used to receive the first fused light and output the second fused light, the second fused light comprising fundamental light, second harmonic light and third harmonic light; The first beam splitter (105) is used to receive the second fused light and split the optical path to obtain the upper main optical path and the lower bypass optical path; The upper main optical path passes through the second second-frequency doubling crystal (103) to obtain the first branch light; The lower branch bypass optical path passes through the delay adjustment device (10) to obtain the second branch light; The beam combining element (108) is used to receive the first branch light and the second branch light and spatially combine them to obtain a beam combined light. The second and third frequency-harmonic crystal (104) is used to receive the combined light and perform high-order nonlinear frequency transformation to output multi-level frequency-harmonic light.

6. A multi-stage cascaded laser frequency doubling device according to claim 5, characterized in that, By adjusting the positions of the first reflector (106) and the second reflector (107) in the delay adjustment device (10), the optical path through the delay adjustment device (10) is adjusted, thereby achieving time-domain synchronization of the first branch light and the second branch light at the beam combining element (108).

7. A multi-stage cascaded laser frequency doubling device according to claim 1, characterized in that, At least one third-level frequency multiplier module (2) is connected in series along the optical path after the front-end frequency multiplier module (100). The third cascaded frequency doubling module (2) includes a second second frequency doubling crystal (103) and a second third frequency doubling crystal (104). The light-input end of the second second-harmonic crystal (103) is connected to the light-output end of the front-end frequency doubling module (100); the light-output end of the second second-harmonic crystal (103) is connected to the light-input end of the second third-harmonic crystal (104); The output end of the third cascaded frequency doubling module (2) is connected to the input end of the second beam splitter (201).

8. A multi-stage cascaded laser frequency doubling device according to claim 7, characterized in that, The light-emitting end of the first second-harmonic crystal (101) of the front-end frequency doubling module (100) is connected to the light-input end of the first third-harmonic crystal (102); the light-emitting end of the first third-harmonic crystal (102) is connected to the light-input end of the second second-harmonic crystal (103).

9. A multi-stage cascaded laser frequency doubling device according to claim 6 or 8, characterized in that, The light-emitting surface of the first second-harmonic crystal (101) and the light-incident surface of the first third-harmonic crystal (102) are coated with a high-transmission film that includes at least the fundamental frequency light and the second harmonic light band; the light-emitting surface of the first third-harmonic crystal (102), and all light-emitting surfaces of the second second-harmonic crystal (103) and the second third-harmonic crystal (104) are coated with a high-transmission film that includes at least the fundamental frequency light, the second harmonic light and the third harmonic light band.

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