A heavy frequency insertion amplification system

Abstract

This invention discloses a high-repetition-rate interpolation amplification system, relating to the field of high-power laser technology. It employs a three-stage architecture of time-domain separation, parallel amplification, and polarization beam combining. The core idea is to distribute the thermal load and suppress the accumulation of nonlinear effects by interleaving pulses in the time domain, and then synergistically improve the overall performance of the system through incoherent polarization beam combining. Compared with traditional coherent combining schemes, this invention completely eliminates the dependence on precise phase control, extremely high synchronization accuracy, and strict waveform matching, thus providing a highly promising technical path for synchronously achieving high repetition rate, high single-pulse energy, and high average power laser output.

Description

Technical Field

[0001] This invention relates to the field of high-power laser technology, specifically to a high-repetition-rate amplification system for achieving high-repetition-rate, high-pulse-energy laser output. Background Technology

[0002] The statements in this section are provided only as background information in connection with this disclosure and may not constitute prior art.

[0003] In recent years, the rapid development of ultrafast lasers and high-harmonic generation (HHG) technologies has opened up new avenues for the development of coherent light sources in the extreme ultraviolet (EUV) band, showing broad application prospects, especially in photonics, nanofabrication, and advanced imaging. However, HHG-based EUV light sources still face several technical bottlenecks in their journey towards high-power output and industrial application, with the core constraint being the inherent limitations of the driving laser performance.

[0004] From a physical perspective, the HHG process is essentially an extremely nonlinear optical effect with extremely low energy conversion efficiency. Therefore, to achieve practically valuable milliwatt-level EUV radiation, a high-performance femtosecond laser driver with an average power in the kilowatt range is required. This requirement places almost stringent demands on the driving laser, namely, achieving synergistic optimization across three dimensions: high repetition rate, high single-pulse energy, and high average power—a goal that constitutes the well-known "impossible triangle" problem in the field of laser technology.

[0005] The current technical bottlenecks are mainly reflected in three aspects: First, there is a significant "thermal confinement" effect in thermal management. High average power and high repetition rate operation will accumulate a large amount of heat in the laser gain medium and optical components, causing thermal lensing effect and thermal stress birefringence, resulting in beam wavefront distortion and quality degradation. Although advanced heat dissipation technologies such as microchannel cooling can be used, the mechanical vibration and fluid disturbances introduced by them will pose additional challenges to the long-term stability of the system. Second, the nonlinear effects and optical damage problems caused by high peak power are prominent. High single pulse energy combined with femtosecond pulse width will produce peak power in the gigawatt to terawatt range, which can easily induce nonlinear processes such as self-focusing and self-phase modulation, causing pulse distortion and beam degradation; at the same time, excessively high power density may exceed the laser damage threshold of optical components, limiting the system's scalability and lifespan. Third, there is an inherent contradiction in the performance of the gain medium: high single pulse energy requires a long upper level lifetime to achieve sufficient energy storage, while high repetition rate requires rapid energy extraction capability, and the two are difficult to balance in terms of physical mechanisms. This contradiction has led to a divergence in existing technological approaches. For example, Ti:sapphire lasers can achieve high single-pulse energy but have limited repetition frequency, while ytterbium-doped fiber lasers support high repetition frequency but struggle to achieve high single-pulse energy.

[0006] To address the aforementioned systemic problems, beam combining schemes such as coherent combining and spectral combining have been widely explored. While coherent beam combining helps improve output power, it also has significant limitations: it requires extremely high phase consistency among multiple laser beams; any random phase fluctuation will cause the interference effect to change from constructive to destructive, significantly reducing combining efficiency. The spatial modes of each beam (such as wavefront curvature, spot size, and intensity distribution) must be highly matched to ensure sufficient overlap in the combining region; otherwise, wavefront distortion will disrupt interference consistency and degrade beam quality. For pulsed lasers, precise time synchronization is also required; timing jitter can lead to incomplete pulse overlap, affecting peak power, and synchronization accuracy must be on the order of pulse width. Furthermore, this technology is extremely sensitive to environmental interference; temperature fluctuations and mechanical vibrations can easily cause phase mismatch, thus requiring strict vibration isolation and temperature control measures. As the number of combining paths increases, the system control difficulty increases exponentially, scalability is limited, and phase errors can cause beam quality problems such as sidelobe enhancement and spot distortion, affecting far-field performance.

[0007] In light of this, the industry urgently needs a new approach that can effectively reconcile the contradictions of the "impossible triangle." This invention proposes a femtosecond laser amplification scheme based on modular design and repetition rate interpolation synthesis. By interleaving the pulse timing of multiple laser units and efficiently synthesizing them, it aims to synergistically improve the repetition rate, single-pulse energy, and average power, providing reliable support for next-generation EUV light sources and other high-power ultrafast laser applications. Summary of the Invention

[0008] The purpose of this invention is to address the limitations of existing laser technology in simultaneously increasing high repetition rate, high single-pulse energy, and high average power by proposing a high-repetition-rate interpolation amplification system. This system employs a three-stage architecture of time-domain separation, parallel amplification, and selective polarization beam combining. Its core lies in decomposing a single high-power laser amplification task into N low-load parallel units. Specifically, by precisely controlling the emission timing of pulses in each unit, the pulses are staggered along the time axis, meaning that the pulses of each unit are sequentially delayed by a fixed interval Δt. In the beam combining stage, thin-film polarizers and Pockels cells are used for stage-by-stage beam combining, ultimately forming a high-repetition-rate pulse train that is uniformly distributed in the time domain and has the same polarization direction, thereby achieving an N-fold increase in repetition rate while maintaining high single-pulse energy characteristics. The physical basis of this method is the principle of linear superposition of laser pulses and incoherent combining technology: since the pulses do not overlap in time, the combining process only relies on energy addition and does not require strict phase matching. Therefore, this technology effectively avoids the stringent requirements of traditional coherent beam combining for phase control, synchronization accuracy, and waveform matching, and provides an innovative solution for the synchronous improvement of high repetition frequency, high single pulse energy, and high average power.

[0009] The technical solution of the present invention is as follows: A high-repetition-rate interpolation amplification system is disclosed. This system employs a three-stage architecture of time-domain separation, parallel amplification, and selective polarization beam combining. Specifically, it comprises a pulse time-domain separation module, a parallel amplification module, and a selective polarization beam combining module. The core idea is to disperse thermal load and nonlinear effects through pulse time-series interleaving, and then achieve synergistic parameter enhancement through incoherent beam combining. The physical basis of this method is the linear superposition principle of laser pulses and incoherent combining technology: since the pulses do not overlap in time, the combining process only relies on energy addition, without the need for strict phase matching; the system can linearly increase the output power and repetition frequency by increasing the number of parallel channels, thus adapting to the application requirements of different power levels; and it avoids the problem of exponentially increasing phase control complexity as the number of combining channels increases in similar coherent combining technologies.

[0010] Furthermore, the pulse time-domain separation module is configured to separate the input seed laser beam into N sub-pulses of equal power, and to generate a fixed time interval Δt between each sub-pulse, thereby arranging the sub-pulses in an alternating manner in the time domain; The parallel amplification module includes N independent amplifiers, each connected to the output of the pulse time-domain separation module, and is configured to amplify the N sub-pulses at a low repetition frequency. The selective polarization beam combining module is connected to the output of the parallel amplification module. It is configured to use a thin-film polarizer to combine the amplified sub-pulses step by step, and to use a Pockel cell to control the polarization state of the pulse sequence during the beam combining process. Finally, it outputs a repetition rate laser pulse train with uniform time distribution and consistent polarization direction.

[0011] Furthermore, the pulse time-domain separation module consists of a seed source and a beam-splitting delay network; The beam-splitting delay network includes a polarization beam splitter, a quarter-wave plate, and a mirror. It is configured to use the synergistic effect of the polarization beam splitter and the quarter-wave plate to split the seed laser beam output from the seed source into N sub-pulses of equal power, and inject them into different channels of the parallel amplification module respectively. The beam-splitting delay network also includes an auxiliary delay line configured to fine-tune the optical path of each channel, ensuring that the sub-pulses between each channel are separated according to the time interval Δt. That is, the beam-splitting delay network utilizes the synergistic effect of the polarization beam splitter and the quarter-wave plate to divide the input seed laser beam into N sub-pulses of equal power, which are then injected into different channels of the parallel amplification module. At the same time, the auxiliary delay line fine-tunes the optical path of each channel to ensure that the sub-pulses between each channel are separated according to the time interval Δt.

[0012] Furthermore, the parallel amplification module includes N independent input amplifiers; the N amplifiers are configured to operate at a low repetition frequency to decompose a single high-power laser amplification task into N low-load parallel processes, and the peak power of the system is distributed to each amplifier through the cooperative work of each amplifier; that is, these amplifiers only need to operate at a low repetition frequency to achieve efficient amplification, thereby avoiding the heat accumulation and gain saturation problems that may occur at high repetition frequencies; in addition, through the cooperative work of multiple independent amplifiers, the peak power is distributed to each unit, thereby effectively reducing the risk of nonlinear effects and optical damage.

[0013] Furthermore, the selective polarization beam combining module includes a half-wave plate, a thin-film polarizer, and a Pockels cell, and employs a hierarchical distribution beam combining principle for step-by-step beam combining; The selective polarization beam combining module is configured as follows: except for the sub-pulse output from the first channel, all sub-pulses output from the other channels are adjusted to P-polarized light through a half-wave plate; in the first-stage beam combining, the S-polarized light output from the first channel and the P-polarized light output from the second channel are incoherently combined using a first thin-film polarizer; for subsequent cascading, the beam output from the previous stage beam combining and the P-polarized sub-pulse of the next channel adjusted to P-polarization by a half-wave plate are combined using a next-stage thin-film polarizer, until the beam combining of all N channels is completed.

[0014] Furthermore, during the beam combining process of the selective polarization beam combining module, the sub-pulses of each channel do not overlap in time and maintain a separation interval Δt. Therefore, the combining process only relies on energy addition and does not require strict phase matching.

[0015] Furthermore, the system is configured to control the timing jitter of each channel sub-pulse to the order of pulse width, ensuring effective overlap of pulses within the beam combining window. The system is also configured to use an auxiliary delay line to fine-tune the optical path length to accommodate timing jitter exceeding the order of pulse width. That is, during beam combining, as long as the timing jitter generated by each channel pulse is controlled to the order of pulse width, the pulses can still effectively overlap within the beam combining window. Moreover, even if the jitter is slightly larger, it can be compensated for through fine-tuning of the delay line because the beam combining window is relatively wide.

[0016] Furthermore, during beam combining, the spatial overlap accuracy of the pulses in each channel is a core factor determining the beam quality. To achieve high-precision spatial alignment, the system also includes an active alignment subsystem for controlling the spatial overlap accuracy of the pulses in each channel. The active alignment subsystem includes a sensing subsystem and an actuator. The sensing subsystem includes a CCD camera and a wavefront sensor, configured to monitor the spatial position and wavefront quality of each channel pulse in real time. The actuator includes an electrically controlled displacement stage connected to the thin-film polarizer, configured to perform nanometer-level adjustments to the thin-film polarizer based on the monitoring results of the sensing subsystem, to ensure that the lateral alignment deviation of each channel pulse is less than half a wavelength and the angular alignment deviation is less than the diffraction limit angle.

[0017] Furthermore, the Pockel cell is configured to rapidly apply a half-wave voltage during the beam combining process within the time window of the target sub-pulse's passage, rotating the polarization state of the target sub-pulse from S-polarization to P-polarization, or from P-polarization to S-polarization, to unify the polarization state of adjacent pulses (this invention mainly relates to rotating from S-polarization to P-polarization); the pulse width of the half-wave voltage is greater than the width of the sub-pulse and less than the time interval Δt between the sub-pulses, and the voltage on the Pockel cell remains zero during the passage of non-target sub-pulses; that is, it should be noted that the voltage pulse width applied to the Pockel cell should be greater than the optical pulse width but much smaller than the pulse interval to avoid crosstalk.

[0018] Furthermore, the Pockel box is connected to a high-voltage pulse generator and an FPGA control unit; The FPGA control unit is configured to send a trigger signal to the high-voltage pulse generator before the target sub-pulse arrives at the Pockel box according to a preset sequence. The high-voltage pulse generator is configured to respond to the trigger signal and output the half-wave voltage within the time window of the target sub-pulse passing through the Pockel cell; It should be noted that the rapid change in voltage on the Pockel cell depends on a preset sequence on the FPGA, which sends a trigger signal to the high-voltage pulse generator before the target pulse arrives. The high-voltage pulse generator then outputs a half-wave voltage within the precise time window of the optical pulse passing through the Pockel cell, while the voltage remains zero during other pulses, and the polarization state remains unchanged.

[0019] Furthermore, the N independent amplifiers are regenerative amplifiers; each regenerative amplifier includes a Pockel cell, a quarter-wave plate, and a thin-film polarizer, configured to constrain the corresponding sub-pulse within the amplification cavity for multiple cyclic amplifications by controlling the voltage state of the Pockel cell, and rotate its polarization state by 90 degrees after the pulse energy is saturated so that it can be output from the thin-film polarizer.

[0020] Furthermore, the N independent amplifiers can also be slab amplifiers; each slab amplifier is configured to use a bidirectional pumping method, and its pump light is shaped to form a rectangular spot that is uniformly distributed in the horizontal direction, which is incident on the laser crystal from the end face, so that the heat distribution in the laser crystal is uniform in the horizontal direction, and the thermal lensing effect exists only in the vertical direction.

[0021] Compared with existing technologies, the advantages of this invention are: 1. This invention overcomes the key technical bottleneck of the mutual constraint between high single-pulse energy and high repetition rate. Due to an inherent contradiction in laser gain crystals—high single-pulse energy requires a long upper-level lifetime to achieve sufficient energy storage, while high repetition rate demands rapid energy extraction—the two are fundamentally conflicting in their physical mechanisms, making it difficult to simultaneously achieve high single-pulse energy and high repetition rate. Specifically, this invention uses a pulse separation module to divide the input seed laser into N power-equalized sub-pulses, and uses auxiliary delay lines to fine-tune the optical path of each channel, ensuring that the sub-pulses are separated by a time interval Δt. Subsequently, each sub-pulse is amplified in parallel, and finally, precise temporal alignment is achieved through layered polarization beam combining technology. This scheme enables the synthesized beam to retain the characteristics of high single-pulse energy while effectively increasing the repetition rate, thus overcoming the aforementioned technical bottleneck.

[0022] 2. The repetition frequency interpolation amplification technology employed in this invention, through layered polarization beam combining, avoids dependence on phase synchronization accuracy and waveform matching. This allows for a linear increase in output power and repetition frequency by increasing the number of parallel channels, thus adapting to application requirements at different power levels. Traditional coherent beam combining technology requires the phase difference between each sub-beam to be stable at the subwavelength level. Any minute vibration, temperature fluctuation, or mechanical drift can lead to phase mismatch, causing destructive interference and reducing beam combining efficiency and beam quality. In particular, as the number of beam combining channels increases, the complexity of phase control increases exponentially. Typically, a coherent beam combining system supports a maximum of several dozen channels; beyond this range, beam combining efficiency will significantly decrease due to accumulated errors (e.g., the phase error may be amplified by 1.5–2 times for each additional channel).

[0023] 3. This invention effectively solves the nonlinear effects and optical damage caused by high peak power by using multiple independent regenerative amplifiers to form a parallel amplification module. Specifically, high single-pulse energy combined with femtosecond-level pulse widths generates peak power in the gigawatt to terawatt range. This peak power easily induces nonlinear processes such as self-focusing and self-phase modulation, resulting in pulse waveform distortion and beam quality degradation. By working collaboratively with multiple independent amplifiers, the high peak power is distributed across various units, effectively mitigating nonlinear effects. Simultaneously, the independent regenerative amplifiers employ a distributed thermal management strategy, significantly reducing heat dissipation difficulties and avoiding potential thermal damage caused by heat accumulation.

[0024] 4. Compared to coherent combining laser amplification technology, the repetition rate interpolation amplification technique employed in this invention exhibits significantly reduced sensitivity to temporal jitter. This is primarily attributed to its incoherent polarization beam combining design concept. This technique utilizes temporal separation and parallel amplification mechanisms to interleave multiple high-energy pulses in the temporal domain. During the beam combining stage, the system employs thin-film polarizers and Pockels cells for step-by-step combining, ultimately generating a high-repetition-rate pulse train with uniform temporal distribution and consistent polarization direction. The physical basis of this process is the principle of linear superposition of laser pulses and incoherent combining technology: since the pulses do not overlap in time, their combining is essentially a direct addition of energy, requiring no strict phase matching conditions. Therefore, in the incoherent beam combining process, the impact of temporal jitter is limited to the timing alignment accuracy between pulses. As long as the jitter is controlled within the pulse width range, the pulses can still achieve effective overlap within the beam combining time window. Furthermore, even if the jitter is slightly larger, it can be fine-tuned and compensated for using delay lines, as incoherent beam combining itself has a relatively wide time window tolerance. Most importantly, since there is no interference effect, phase fluctuations do not affect the beam combining efficiency, which makes the system more tolerant to time jitter. Attached Figure Description

[0025] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments recorded in the embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings.

[0026] Figure 1 This is a simplified structural diagram of a frequency repetition rate interpolation amplification system according to the present invention; Figure 2 This is a schematic diagram of the time-domain pulse variation in the repetition rate interpolation amplification technique; Figure 3 This is a schematic diagram of high-speed polarization unification of pulse sequences using a Pockel cell; Figure 4This is a simplified structural diagram of an embodiment of a regenerative amplifier-based frequency interpolation amplification device; Figure 5 This is a simplified structural diagram of an embodiment of a frequency repetition rate amplification device based on a slat amplifier. Detailed Implementation

[0027] It should be noted that relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0028] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0029] Example 1 The specific scheme of the repetition rate interpolation amplification system described in this invention is as follows: Figure 1 As shown, it specifically includes: The system includes a pulse time-domain separation module 100, a parallel amplification module 300, and a selective polarization beam combining module 200. The pulse separation and delay module comprises a seed source 101 and a beam-splitting network 102–120. The beam-splitting and delay network utilizes the combined effect of a polarization beam splitter and a quarter-wave plate to divide the input seed laser beam into N sub-pulses of equal power, such as… Figure 2 As shown, the light pulses are injected into different channels of the parallel amplification module; at the same time, the optical path of each channel is finely adjusted by the auxiliary delay line to ensure that the sub-pulses between each channel are separated according to the time interval Δt.

[0030] The parallel amplification module 300 comprises N independent amplifiers that operate only at low repetition frequencies to achieve efficient amplification, thereby avoiding heat accumulation and gain saturation problems at high repetition frequencies. Furthermore, through the coordinated operation of multiple independent amplifiers, high peak power is distributed across various units, effectively mitigating nonlinear effects and the risk of optical damage. This is because the combination of high single-pulse energy and femtosecond pulse width generates peak power in the gigawatt to terawatt range. Such high peak power not only induces strong nonlinear effects (such as self-focusing and self-phase modulation), leading to pulse distortion and beam quality degradation, but also easily exceeds the laser-induced damage threshold of optical components.

[0031] The selective polarization beam combining module consists of optical elements such as a half-wave plate, a thin-film polarizer, and a Pockels cell, and performs beam combining stage by stage based on a hierarchical distribution principle. First, except for the output light from the first channel, the output light from the other channels is precisely polarized to P-polarized light by passing through a half-wave plate. In the first stage of beam combining, the thin-film polarizer 202 incoherently combines the S-polarized light output from the first channel with the P-polarized light output from the second channel. By fine-tuning the optical path using an auxiliary delay line, the synthesized light maintains its high single-pulse energy characteristics while forming a high repetition rate pulse train with uniform temporal distribution and consistent polarization, achieving repetition rate interpolation. During beam combining, time jitter is a key parameter for achieving uniform temporal distribution and repetition rate interpolation. Compared to coherent laser amplification technology, repetition rate interpolation amplification technology has significantly reduced sensitivity to time jitter, mainly due to its incoherent polarization beam combining design concept. In this incoherent beam combining, time jitter only affects the pulse overlap accuracy; as long as the time jitter is controlled within the pulse width range, the pulses can still effectively overlap within the beam combining window. Furthermore, even slightly larger jitter can be compensated for by fine-tuning the delay line due to the wider beam combining window. Since incoherent beam combining has no interference effects, phase fluctuations do not affect efficiency, thus the system is more tolerant of jitter. In contrast, traditional coherent combining techniques rely on precise phase synchronization of multiple laser beams (typically requiring phase differences to be stable on the subwavelength order). Any time jitter (caused by vibration, temperature fluctuations, or electronic delay errors) will lead to destructive interference, severely degrading beam combining efficiency and beam quality. For example, for a 1030 nm laser, even a time jitter of 0.1 picoseconds can cause a phase change of approximately 105 degrees; this phase mismatch triggers destructive interference, causing a sharp drop in beam combining efficiency (efficiency is proportional to the square of the cosine of the phase difference). Therefore, coherent combining systems must control time jitter to extremely low levels (typically below 0.1 picoseconds), requiring complex active phase feedback systems.

[0032] In the beam combining stage of repetition rate interpolation amplification technology, the spatial coincidence accuracy of the pulses in each channel is one of the core factors determining beam quality. Poor spatial coincidence leads to a series of beam quality problems, mainly including far-field spot distortion, wavefront quality degradation, and loss of combining efficiency. Specifically, when there is lateral offset or angular tilt in each sub-beam, spot splitting occurs in the far field (e.g., the focusing plane), meaning the energy is dispersed into multiple separate focal points instead of a single high-intensity spot, accompanied by sidelobe growth and an increased beam divergence angle. This distortion directly reduces the peak intensity and decreases the beam quality factor (M). 2Significant degradation occurs, far from the diffraction limit. Furthermore, spatial misalignment introduces wavefront aberrations; for example, angular deviations cause tilt aberrations, and longitudinal positional deviations lead to defocusing or astigmatism. In terms of combining efficiency, geometric overlap loss and decreased polarization coupling efficiency further weaken system performance; for example, some beam energy cannot effectively participate in beam combining due to offset, resulting in overall efficiency reduction. Therefore, in engineering, strict requirements are needed for spatial alignment accuracy to ensure beam quality approaches theoretical optimality. Based on diffraction theory, lateral alignment needs to be controlled within half-wavelength range (e.g., for a 1030 nm laser, the deviation needs to be less than 0.5 μm), and angular alignment needs to be less than the diffraction-limiting angle. These accuracy standards stem from avoiding phase errors and energy losses caused by optical path differences. To achieve high-precision spatial alignment, the system employs active alignment technology, including a sensing subsystem (CCD camera and wavefront sensor for real-time monitoring) and an actuator (electronically controlled displacement stage provides nanometer-level adjustment to the thin-film polarizer). Meanwhile, spatial alignment and time synchronization have a coupling effect. For example, spatial deviation can change the optical path and indirectly affect time alignment. Therefore, a hierarchical control strategy is required: first, coarse alignment and coarse time synchronization are performed, and then fine alignment and fine time synchronization are optimized to finally achieve real-time collaborative correction.

[0033] Furthermore, after the first-stage incoherent beam combining is completed, a half-wave voltage can be rapidly applied using the Pockel cell 204, such as... Figure 3 The pulse sequence is subjected to high-speed polarization unification to ensure that the polarization of adjacent pulses remains consistent (both are S-polarized). It should be noted that the voltage pulse width applied to the Pockels cell should be greater than the optical pulse width, but much smaller than the pulse interval, to avoid crosstalk. In the second stage, the combined beam is combined with the third channel (adjusted to P-polarization by a half-wave plate) through the second thin-film polarizer 205, and so on, until all channels are combined.

[0034] Furthermore, the Pockel cell, as an electro-optic modulator, operates on the core principle of the electro-optic effect: when a voltage is applied, the refractive index of the crystal changes linearly, thereby altering the phase delay of the passing light wave. When a half-wave voltage is applied, the Pockel cell is equivalent to a half-wave plate (λ / 2 waveplate), which can rotate the polarization direction of linearly polarized light by a specific angle (e.g., when horizontally polarized light is input, the output becomes vertically polarized light). To achieve rapid changes in the polarization direction of specific pulses in a pulse sequence, precise voltage control and timing synchronization are required. Therefore, an FPGA is used to send a trigger signal to a high-voltage pulse generator before the arrival of the target pulse, according to a preset sequence (e.g., "modulate the 3rd pulse every 10 pulses"). The high-voltage pulse generator outputs a half-wave voltage within the precise time window of the light pulse passing through the Pockel cell, while the voltage remains zero during other pulses to ensure unchanged polarization.

[0035] Example 2 This embodiment takes a 4-channel system as an example, combined with Figure 3 This section specifically demonstrates the implementation process of a recurrent frequency interpolation amplification system based on a regenerative amplifier.

[0036] The pulse separation module of this system mainly consists of a seed source 101 and a beam-splitting delay network. The seed source is a femtosecond mode-locked fiber laser with the following initial parameters: center wavelength of 1030 nm, repetition rate of 100 kHz, single pulse energy of 10 µJ, and average power of 1 W. Before entering the beam-splitting delay network, to reduce nonlinear effects during amplification, optimize energy extraction efficiency, and maintain spectral bandwidth, a pulse stretcher 200 is used to stretch the pulse; this stretcher consists of a pair of reflective gratings. In the stretched seed light, the S-polarized component is reflected by the polarization beam splitter 102 and directly enters the beam-splitting delay network; while the P-polarized component first passes through the polarization beam splitter 102, then returns via a quarter-wave plate 103 and a mirror 104, and is finally reflected by the polarization beam splitter into the beam-splitting delay network.

[0037] The beam-splitting delay network consists of numerous polarization beam splitters and quarter-wave plates. Its main function is to split the seed source output beam into four sub-pulses of equal power, which are then injected into different channels for parallel amplification. Simultaneously, auxiliary delay lines fine-tune the optical path length of each channel to ensure that the sub-pulses between channels are separated at 1µs time intervals.

[0038] The parallel amplification function is implemented by the parallel amplification module 300, which uses four independent regenerative amplifiers to process the sub-pulses of each channel. Each amplifier operates at a low repetition frequency, effectively avoiding heat accumulation and nonlinear effects. The regenerative amplifiers achieve selective energy storage and release of the pulse by precisely controlling the voltage of the Pockel cell. The operation is as follows: In the energy storage phase, a quarter-wave voltage (λ / 4 voltage) is applied to the Pockel cell, combining it with a fixed quarter-wave plate 310 within the cavity, equivalent to a half-wave plate. At this time, when the pulse circulates once within the resonant cavity, it passes through this combined optical element twice, and its cumulative effect is equivalent to passing through a full-wave plate. Therefore, the polarization state of the pulse remains unchanged and cannot exit through the thin-film polarizer 308, which acts as the output coupler, thus being confined within the cavity for multiple cycles and gain. When the pulse energy reaches saturation, the output phase begins: the voltage applied to the Pockel cell is removed. At this time, only the fixed quarter-wave plate 310 remains within the cavity. After the pulse travels back and forth once within the cavity and passes through the waveplate twice, its polarization state rotates by 90 degrees. This polarization change enables the pulse to be efficiently coupled out from the thin-film polarizer 308, thereby increasing the single-pulse energy of each channel to 10 mJ.

[0039] To maintain high single-pulse energy while improving repetition frequency and average power, a selective polarization beam combiner module is essential. The selective polarization beam combiner module employs a hierarchical incoherent polarization beam combiner scheme. Except for the first channel, the output light from the other channels is adjusted to P-polarization by a half-wave plate, and then gradually combined with the S-polarized light from the first channel in a thin-film polarizer. During beam combining, a Pockel cell rapidly applies a half-wave voltage as the target pulse passes, unifying the polarization state of adjacent pulses. It is important to note that the voltage pulse width applied to the Pockel cell should be greater than the optical pulse width but much smaller than the pulse interval to avoid crosstalk. Therefore, the rapid change in voltage on the Pockel cell relies on a preset sequence on the FPGA, sending a trigger signal to the high-voltage pulse generator before the target pulse arrives. The high-voltage pulse generator then outputs a half-wave voltage within a precise time window when the optical pulse passes through the Pockel cell, while the voltage remains zero and the polarization state unchanged during other pulses.

[0040] After sequential beam combining of four channels, the system's final output pulse repetition frequency is increased to 400 kHz, with a single pulse energy of 10 mJ. Furthermore, after compression by compressor 500, the pulse width is reduced to hundreds of femtoseconds. This result clearly demonstrates the core advantage of this technology: the output repetition frequency scales linearly with the number of channels, and it eliminates the need for complex phase synchronization required by coherent combining techniques.

[0041] However, the spatial alignment accuracy of the pulses in each channel is one of the key factors determining the final beam quality. Poor spatial alignment will lead to a series of beam quality problems such as far-field spot distortion and wavefront aberration. Therefore, strict requirements must be placed on spatial alignment accuracy during system construction to ensure that the beam quality is close to the theoretical optimal value. To achieve high-precision spatial alignment, the system adopts active alignment technology. Its sensing devices (such as CCD cameras and wavefront sensors) are responsible for real-time monitoring, while the actuator (electronically controlled displacement stage) provides nanometer-level adjustment for the optical components. In addition, through a hierarchical control strategy—first performing coarse alignment and coarse time synchronization, and then optimizing through fine alignment and fine time synchronization—real-time collaborative correction can ultimately be achieved.

[0042] Example 3 This embodiment takes a 4-channel system as an example, combined with Figure 5 The implementation process of the repetition rate insertion amplification system based on a slab amplifier is specifically demonstrated.

[0043] The overall structure of this system is similar to that of Embodiment 1, with the main difference being the replacement of the regenerative amplifier used in the parallel amplification module with a slab amplifier. The pulse separation module of this system also mainly consists of a seed source 101 and a beam-splitting delay network. However, the seed source uses a picosecond mode-locked fiber laser with the following initial parameters: center wavelength of 1030 nm, repetition frequency of 100 kHz, single-pulse energy of 100 µJ, and average power of 10 W. In the seed light, the S-polarized component is reflected by the polarization beam splitter 102 and directly enters the beam-splitting delay network; while the P-polarized component first passes through the polarization beam splitter 102, then returns via a quarter-wave plate 103 and a reflector 104, and is finally reflected by the polarization beam splitter into the beam-splitting delay network.

[0044] The beam splitting delay network divides the seed source output beam into four sub-pulses of equal power, which are then injected into different channels for parallel amplification. Simultaneously, auxiliary delay lines fine-tune the optical path length of each channel to ensure that the sub-pulses are separated at 1µs time intervals between channels.

[0045] The parallel amplification module 300 contains four independent slab amplifiers, each processing a sub-pulse from one channel. Each slab amplifier employs a bidirectional pumping method. The pump light is first shaped by two sets of cylindrical lenses, a rectangular waveguide 304, and a focusing lens 307. After shaping, the pump light forms a rectangular spot with uniform intensity distribution in the horizontal direction, approximately 0.4 mm × 10 mm in size, which is incident on the laser crystal from the end face. This spot completely covers the crystal in the horizontal direction and partially fills it in the vertical direction, resulting in uniform heat distribution in the horizontal direction and almost no thermal lensing effect, effectively avoiding wavefront distortion of the beam during amplification; the thermal lensing effect is only significant in the vertical direction. The gain crystal used has dimensions of 12 mm × 10 mm × 1 mm, cut along the a-axis, with a light transmission direction of 10 mm. The two large faces of the crystal (12 mm × 10 mm) serve as cooling surfaces, with efficient heat dissipation achieved through a copper heat sink with a microchannel water-cooling structure. Two plane mirrors are installed inside the amplification cavity as cavity mirrors, so that the seed light travels back and forth seven times in the gain medium, thereby achieving multiple amplifications.

[0046] After the amplification stage, the outputs of each channel are integrated through selective polarization beam combining. A hierarchical incoherent polarization beam combining structure is employed: the first channel maintains S-polarization output, while the remaining channels are adjusted to P-polarization via a half-wave plate. Subsequently, each beam is sequentially combined through a thin-film polarizer. During beam combining, a Pockel cell applies a half-wave voltage when the target pulse passes through, ensuring that adjacent pulses have the same polarization state. Timing control is implemented by an FPGA: it triggers a high-voltage pulse generator before the target pulse arrives according to a preset program. The generator outputs a half-wave voltage only during the precise period when the optical pulse passes through the Pockel cell, maintaining zero voltage for the rest of the time, thus ensuring that the polarization of non-target pulses is not disturbed. Finally, through sequential beam combining across four channels, the system output pulse repetition frequency reaches 400 kHz, with a single pulse energy of 10 mJ.

[0047] This embodiment successfully achieves a synergistic improvement in high repetition frequency, high single-pulse energy, and high average power through the synergistic design of time-domain separation amplification and incoherent polarization beam combining. At the same time, it effectively avoids the stringent phase synchronization requirements of traditional coherent beam combining. The system has a simple structure and high stability, and is suitable for high-end laser applications such as EUV light source driving, precision micro-nano fabrication, and ultrafast spectroscopy.

[0048] The embodiments described above merely illustrate specific implementation methods of this application, and while the descriptions are detailed and specific, they should not be construed as limiting the scope of protection of this application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the technical solution of this application, and these modifications and improvements all fall within the scope of protection of this application.

[0049] This background section is provided to generally present the context of the invention. The work of the currently named inventors, the work to the extent described in this background section, and aspects of this section that did not constitute prior art at the time of application are neither expressly nor impliedly acknowledged as prior art to the invention.

Claims

1. A frequency repetition rate interpolation amplification system, characterized in that, The system employs a three-stage architecture of time-domain separation, parallel amplification, and selective polarization beam combining, including: The pulse time-domain separation module is configured to separate the input seed laser beam into N sub-pulses of equal power and generate a fixed time interval Δt between each sub-pulse, thereby arranging the sub-pulses in an alternating manner in the time domain. The parallel amplification module includes N independent amplifiers, each connected to the output of the pulse time-domain separation module, and is configured to amplify the N sub-pulses at a low repetition frequency. The selective polarization beam combining module is connected to the output of the parallel amplification module. It is configured to use a thin-film polarizer to combine the amplified sub-pulses step by step, and to use a Pockel cell to control the polarization state of the pulse sequence during the beam combining process. Finally, it outputs a repetition rate laser pulse train with uniform time distribution and consistent polarization direction.

2. The repetition rate interpolation and amplification system according to claim 1, characterized in that, The pulse time-domain separation module consists of a seed source and a beam-splitting delay network. The beam-splitting delay network includes a polarization beam splitter, a quarter-wave plate, and a mirror. It is configured to use the synergistic effect of the polarization beam splitter and the quarter-wave plate to split the seed laser beam output from the seed source into N sub-pulses of equal power, and inject them into different channels of the parallel amplification module respectively. The beam splitting delay network also includes auxiliary delay lines configured to fine-tune the optical path of each channel, ensuring that sub-pulses between channels are separated according to the time interval Δt.

3. The frequency repetition rate interpolation and amplification system according to claim 1, characterized in that, The parallel amplification module includes N independent input amplifiers; the N amplifiers are configured to operate at a low repetition frequency to decompose a single high-power laser amplification task into N low-load parallel processes, and the peak power of the system is distributed to each amplifier through the cooperative operation of each amplifier.

4. The repetition rate interpolation and amplification system according to claim 1, characterized in that, The selective polarization beam combining module includes a half-wave plate, a thin-film polarizer, and a Pockels cell, and adopts a hierarchical distribution beam combining principle to perform step-by-step beam combining; The selective polarization beam combining module is configured as follows: except for the sub-pulse output from the first channel, all sub-pulses output from the other channels are adjusted to P-polarized light through a half-wave plate; in the first-stage beam combining, the S-polarized light output from the first channel and the P-polarized light output from the second channel are incoherently combined using a first thin-film polarizer; for subsequent cascading, the beam output from the previous stage beam combining and the P-polarized sub-pulse of the next channel adjusted to P-polarization by a half-wave plate are combined using a next-stage thin-film polarizer, until the beam combining of all N channels is completed.

5. The repetition rate interpolation and amplification system according to claim 4, characterized in that, During the beam combining process of the selective polarization beam combining module, the sub-pulses of each channel do not overlap in time and are separated by an interval Δt. The system is configured to control the time jitter of each channel sub-pulse to the order of pulse width, so that the pulses effectively overlap within the beam combining window; wherein, the system is further configured to use an auxiliary delay line to fine-tune the optical path to compensate for time jitter greater than the order of pulse width.

6. The repetition rate interpolation and amplification system according to claim 4, characterized in that, It also includes an active alignment subsystem for controlling the spatial overlap accuracy of pulses in each channel; The active alignment subsystem includes a sensing subsystem and an actuator. The sensing subsystem includes a CCD camera and a wavefront sensor, configured to monitor the spatial position and wavefront quality of each channel pulse in real time. The actuator includes an electrically controlled displacement stage connected to the thin-film polarizer, configured to perform nanometer-level adjustments to the thin-film polarizer based on the monitoring results of the sensing subsystem, to ensure that the lateral alignment deviation of each channel pulse is less than half a wavelength and the angular alignment deviation is less than the diffraction limit angle.

7. The repetition rate interpolation amplification system according to claim 4, characterized in that, The Pockel cell is configured to apply a half-wave voltage during the time window of the target sub-pulse's passage, rotating the polarization state of the target sub-pulse from S-polarization to P-polarization, or from P-polarization to S-polarization, to unify the polarization state of adjacent pulses; the pulse width of the half-wave voltage is greater than the width of the sub-pulse and less than the time interval Δt between the sub-pulses, and the voltage on the Pockel cell remains zero during the passage of non-target sub-pulses.

8. The repetition rate interpolation and amplification system according to claim 7, characterized in that, The Puker box is connected to a high-voltage pulse generator and an FPGA control unit; The FPGA control unit is configured to send a trigger signal to the high-voltage pulse generator before the target sub-pulse arrives at the Pockel box according to a preset sequence. The high-voltage pulse generator is configured to respond to the trigger signal and output the half-wave voltage within the time window of the target sub-pulse passing through the Pockel cell.

9. A frequency repetition rate interpolation and amplification system according to any one of claims 1-8, characterized in that, N independent amplifiers are regenerative amplifiers; each regenerative amplifier includes a Pockel cell, a quarter-wave plate, and a thin-film polarizer, configured to constrain the corresponding sub-pulse within the amplification cavity for multiple cyclic amplifications by controlling the voltage state of the Pockel cell, and rotate its polarization state by 90 degrees after the pulse energy is saturated so that it can be output from the thin-film polarizer.

10. A frequency repetition rate interpolation-amplification system according to any one of claims 1-8, characterized in that, N independent amplifiers are slab amplifiers; each slab amplifier is configured to use a bidirectional pumping method, and its pump light is shaped to form a rectangular spot that is uniformly distributed in the horizontal direction. It is incident on the laser crystal from the end face, so that the heat distribution in the laser crystal is uniform in the horizontal direction and the thermal lensing effect exists only in the vertical direction.

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