A femtosecond pulse width precision adjustment compensation device and method

Abstract

The application provides a kind of femtosecond pulse width precision adjustment compensation device and method, it is related to laser processing technical field, device includes: mode-locked seed source, stretcher, amplifier, first compressor and second compressor;Second compressor includes movable mirror, climbing mirror, grating and grating base material;Movable mirror is configured to adjust the diffraction number of femtosecond laser on the grating by changing the distance between it and grating base material;Grating base material is made of nonlinear material, for generating self-phase modulation effect under the action of femtosecond laser, realizing spectrum widening and pulse compression;By adjusting the diffraction number, the precise adjustment compensation of femtosecond pulse width is realized.The application provides a kind of technical scheme that can withstand high power, high energy laser, and realize the on-line precision adjustment compensation of pulse width.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of laser processing, in particular to a femtosecond pulse width precision adjustment compensation device and method. BACKGROUND

[0002] In the field of femtosecond laser technology, the chirped pulse amplification (CPA) system is a classic scheme to obtain high peak power femtosecond laser. It expands the femtosecond pulse to picosecond or nanosecond level in time domain through the stretcher, amplifies the energy through the amplifier, and then compresses the pulse back to femtosecond level through the compressor (such as spatial grating pair or bulk grating), and finally outputs femtosecond laser with 300fs to 500fs.

[0003] However, with the deepening of application fields such as ultrafast spectroscopy and microscopic imaging, the laser pulse width is required to be within hundreds of femtoseconds, while having high energy and high peak power. In order to break through the pulse width limit of traditional CPA technology, nonlinear compression technology emerges as the times require. This technology uses the nonlinear effect (such as self-phase modulation) in the optical medium to further expand the spectrum, and combines dispersion compensation to compress the pulse width to one half to one tenth of the original width. For example, the prior art uses a multi-cavity composed of a pair of concave mirrors and sets a quartz plate in it to generate nonlinear effect, which can achieve pulse compression, but the multi-cavity structure is complex and difficult to adjust, and the compression amount of the system is usually fixed at the time of construction, lacking the ability of online precise adjustment.

[0004] In addition, in the compression link, the conventional scheme usually uses a chirped mirror to compensate for dispersion. However, the chirped mirror has a relatively thick coating layer and a low damage threshold, and it is difficult to withstand the long-term impact of high-power femtosecond laser. Although there are schemes (such as patent CN202410784672.X) that use bulk Bragg gratings to replace chirped mirrors to improve the damage threshold, such schemes also face the problem of fixed compression ratio and inability to dynamically and precisely adjust the pulse width according to application requirements.

[0005] Therefore, there is an urgent need in the art for a technical scheme that can withstand high power and high energy laser and realize online precise adjustment and compensation of pulse width. SUMMARY

[0006] In view of the above-mentioned defects or deficiencies in the prior art, the present application aims to provide a femtosecond pulse width precision adjustment compensation device and method.

[0007] In a first aspect, the present application provides a femtosecond pulse width precision adjustment compensation device, comprising: a mode-locked seed source, a stretcher, an amplifier, a first compressor and a second compressor.

[0008] The second compressor comprises a movable mirror, a climbing mirror, a grating and a grating substrate.

[0009] The movable mirror is configured to adjust the diffraction number of the femtosecond laser on the grating by changing its distance from the grating substrate;

[0010] The grating substrate is made of a nonlinear material and is used to generate a self-phase modulation effect under the action of a femtosecond laser, thereby achieving spectral broadening and pulse compression.

[0011] By adjusting the diffraction number, precise adjustment and compensation of the femtosecond pulse width can be achieved.

[0012] According to the technical solution provided in the embodiments of this application, the grating substrate is made of ultraviolet fused silica.

[0013] According to the technical solution provided in the embodiments of this application, by moving the movable reflector, the diffraction number of the femtosecond laser on the grating is adjusted to 6 to 10 times, so as to compress the pulse width from 300fs to the range of 152fs to 98fs.

[0014] According to the technical solution provided in the embodiments of this application, the surface of the grating substrate is coated with a high-reflectivity film layer, which is used to reflect the femtosecond laser back to the grating to increase the diffraction number.

[0015] According to the technical solution provided in the embodiments of this application, the thickness of the grating substrate of the ultraviolet fused silica is 2mm.

[0016] According to the technical solution provided in the embodiments of this application, the grating has a line density of 500 lines / mm.

[0017] Secondly, this application proposes a femtosecond pulse width precision adjustment and compensation method, which employs the femtosecond pulse width precision adjustment and compensation device as described above, and includes the following steps:

[0018] A femtosecond laser pulse sequence is generated from the mode-locked seed source, the femtosecond laser pulse sequence comprising multiple femtosecond laser pulses;

[0019] The femtosecond laser pulse is stretched in the time domain using the stretcher to obtain a stretched laser pulse.

[0020] The power of the broadened laser pulse is amplified using the amplifier to obtain an amplified laser pulse.

[0021] The amplified laser pulse is initially compressed using the first compressor to obtain an intermediate femtosecond laser with an intermediate pulse width.

[0022] The intermediate femtosecond laser is incident on the second compressor for nonlinear compression.

[0023] The nonlinear compression is achieved by moving the movable mirror in the second compressor to change the diffraction number of the intermediate femtosecond laser on the grating, while simultaneously using the self-phase modulation effect in the grating substrate made of nonlinear material to perform spectral broadening and pulse compression, so as to output a compensated femtosecond laser pulse after precise adjustment and compensation.

[0024] The technical solution provided in the embodiments of this application further includes the following steps:

[0025] The real-time pulse width of the compensated femtosecond laser pulse is monitored in real time, and the real-time pulse width is compared with the preset pulse width.

[0026] Based on the comparison results, a control signal is generated and fed back to the movable reflector to automatically adjust its position so as to stabilize the real-time pulse width at the preset pulse width.

[0027] According to the technical solution provided in the embodiments of this application, the step of incidenting the intermediate femtosecond laser onto the second compressor for nonlinear compression includes the following steps:

[0028] The intermediate femtosecond laser with a pulse width of 300 fs and a power of 20 W is incident on the second compressor;

[0029] By moving the movable reflector, the diffraction number of the intermediate femtosecond laser on the grating is precisely adjusted to 6, 8, or 10 times, so as to compress the pulse width of the intermediate femtosecond laser to 152 fs, 128 fs, or 98 fs, respectively.

[0030] According to the technical solution provided in the embodiments of this application, before the step of incidenting the intermediate femtosecond laser onto the second compressor for nonlinear compression, the following steps are further included:

[0031] The real-time parameters of the intermediate femtosecond laser about to enter the second compressor are obtained, including pulse energy, pulse width and beam diameter;

[0032] Based on the real-time parameters, the intensity factor of the expected nonlinear effect generated by the intermediate femtosecond laser in the grating substrate is calculated;

[0033] The nonlinear effect intensity factor is compared with a preset threshold range, and based on the comparison result, one of the following operations is performed:

[0034] If the nonlinear effect intensity factor is within the preset threshold range, then the subsequent nonlinear compression step is executed;

[0035] If the nonlinear effect intensity factor is lower than the minimum value of the preset threshold range, a first adjustment command is generated and fed back to the amplifier and / or the first compressor to increase the pulse energy of the intermediate femtosecond laser and / or compress the pulse width until the nonlinear effect intensity factor reaches the preset threshold range.

[0036] If the nonlinear effect intensity factor is higher than the maximum value of the preset threshold range, a second adjustment command is generated and fed back to the amplifier to reduce the pulse energy of the intermediate femtosecond laser and prevent the grating substrate from being damaged due to excessive nonlinear effect.

[0037] Compared with the prior art, the beneficial effects of this application are as follows:

[0038] I. Achieved precise online adjustment of pulse width: By introducing a movable mirror and utilizing its distance from the grating substrate to directly control the diffraction number of the femtosecond laser on the grating, continuous and precise control of the nonlinear compression is achieved. Users can dynamically and precisely adjust the output pulse width to the desired value (e.g., from 300 fs to a range of 98 fs-152 fs) without changing the core optical structure, overcoming the inherent defects of fixed compression and inconvenient adjustment in existing technologies.

[0039] II. Combining High Damage Threshold and High Compression Efficiency: This design abandons chirped mirrors with low damage thresholds, employing a structure combining a grating with a grating substrate exhibiting nonlinear effects. The grating itself possesses a high damage threshold, enabling it to withstand high-energy lasers. Simultaneously, it cleverly utilizes the self-phase modulation effect of the grating substrate (such as ultraviolet fused silica) to achieve spectral broadening, integrating the nonlinear compression process within the compressor. This results in a compact structure that eliminates the need for additional complex multi-channel cavities and discrete nonlinear media, significantly improving system stability and reliability while ensuring high compression efficiency.

[0040] III. Simplified Structure and Optimized Performance: This device efficiently integrates nonlinear generation and dispersion compensation functions into the second compressor. Key parameters can be controlled by adjusting the position of a single reflector. The optical path structure is straightforward, avoiding the assembly and adjustment difficulties and stability issues caused by complex structures such as multi-cavity structures. While achieving high performance, it is also conducive to system integration and productization. Attached Figure Description

[0041] Figure 1 This is a schematic diagram of the femtosecond pulse width precision adjustment and compensation device provided in the embodiments of this application;

[0042] Figure 2 This is a schematic diagram of the structure of the climbing mirror provided in an embodiment of this application;

[0043] Figure 3A flowchart illustrating the steps of the femtosecond pulse width precision adjustment and compensation method provided in this application embodiment.

[0044] The text labels in the image represent:

[0045] 1. Mold-locked seed source; 2. Stretcher; 3. Amplifier; 4. First compressor; 5. Second compressor; 5-1. Movable mirror; 5-2. Climbing mirror; 5-3. Grating; 5-4. Grating substrate; 6. Beam splitter. Detailed Implementation

[0046] The present application will now be described in further detail with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and not intended to limit it. Furthermore, it should be noted that, for ease of description, only the parts relevant to the invention are shown in the accompanying drawings.

[0047] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. This application will now be described in detail with reference to the accompanying drawings and embodiments.

[0048] Example 1

[0049] As mentioned in the background section, in view of the problems in the prior art, this application proposes a femtosecond pulse width precision adjustment and compensation device, such as... Figures 1-2 As shown, it includes: a mode-locked seed source 1, a stretcher 2, an amplifier 3, a first compressor 4, and a second compressor 5;

[0050] The second compressor 5 includes a movable reflector 5-1, a climbing mirror 5-2, a grating 5-3, and a grating substrate 5-4;

[0051] The movable reflector 5-1 is configured to adjust the number of diffractions of the femtosecond laser on the grating 5-3 by changing the distance between it and the grating substrate 5-4;

[0052] The grating substrate 5-4 is made of a nonlinear material and is used to generate a self-phase modulation effect under the action of femtosecond laser to achieve spectral broadening and pulse compression.

[0053] By adjusting the diffraction number, precise adjustment and compensation of the femtosecond pulse width can be achieved.

[0054] Specifically, the intermediate femtosecond laser obtained after initial compression by the first compressor 4 passes through a beam splitter 6 before entering the second compressor 5. The function of the beam splitter 6 is as follows: Transmission main optical path: The beam splitter 6 allows the vast majority (e.g., over 95%) of the intermediate femtosecond laser to pass through and be collimated along its original propagation direction to the movable reflector 5-1 of the second compressor 5 for subsequent nonlinear compression and precision adjustment. Reflection monitoring optical path: The beam splitter 6 reflects a small portion (e.g., 1%~5%) of the intermediate femtosecond laser to a beam perpendicular to the main optical path (e.g., ...). Figure 1 In the monitoring optical path (as shown).

[0055] Specifically, mode-locked seed source 1 is the starting point of the entire system, used to generate ultrashort, ultrafast laser pulse sequences. In this embodiment, it is typically a laser based on a semiconductor saturable absorber mirror or a nonlinear fiber loop, capable of outputting a stable femtosecond pulse train with a center wavelength of 1030 nm (corresponding to the gain band of Yb-doped fiber amplifier 3), a repetition rate of 30 MHz, and an average power of 6 mW. Its function is to provide an initial light source with an extremely narrow time domain and a pure spectrum. Stretcher 2: This module is responsible for stretching the femtosecond pulses output from mode-locked seed source 1 in time. This is a key step in chirped pulse amplification technology, aiming to reduce the peak power of the pulse and prepare for subsequent amplification. In practice, a pair of transmissive or reflective gratings can be used, utilizing the positive dispersion introduced by the gratings to stretch the pulse from tens of femtoseconds to approximately 1 nanosecond. At this point, the peak power of the pulse drops from the kilowatt level to the watt level (approximately 0.2W), thus avoiding nonlinear effects caused by high peak power in amplifier 3, such as self-focusing and spectral distortion. Amplifier 3: This module is used to increase the energy of the broadened pulse. It typically employs a multi-stage fiber amplifier or solid-state amplifier structure, using doped ions (such as Yb). 3+ Stimulated emission amplifies the average power of nanosecond pulses from the milliwatt range to tens of watts or even higher. In this embodiment, it amplifies 6mW of power to over 20W. The amplification process is performed at low peak power, thus ensuring both efficiency and safety.

[0056] Specifically, the first compressor 4 functions to initially compensate for the dispersion accumulated in the pulse stretcher 2 and amplifier 3, compressing the pulse back to the femtosecond range. In practice, it can be a grating pair with the opposite dispersion to stretcher 2, or a volume Bragg grating. In this device, it successfully compressed a 1-nanosecond pulse to approximately 300 femtoseconds, restoring the high peak power characteristics and preparing for subsequent nonlinear compression. The second compressor 5 is not a simple compressor, but a composite module integrating nonlinear spectral broadening and dispersion compensation, with adjustable compression. It contains four key components:

[0057] Movable mirror 5-1: This is a planar mirror controlled by a precision mechanical structure (such as an electric translation stage). Its position can be precisely controlled and moved. Its core function is to directly determine the round-trip path of the laser beam in subsequent optical elements by changing the optical path between it and the grating substrate 5-4, thereby controlling the number of diffractions of the beam on the grating 5-3.

[0058] Climbing mirror 5-2: This is a reflector with a specific angle and position. Its function is to "climb" or "shift" the laser beam after it has been diffracted once by grating 5-3 in space, and then precisely guide it back to grating 5-3 for the next diffraction. It works in conjunction with movable reflector 5-1 to form a stable multi-pass structure.

[0059] Grating 5-3: This is a diffractive optical element with parallel gratings, with a grating density of, for example, 500 lines / mm. It diffracts the incident femtosecond laser, and its core function is to provide a large negative dispersion to compensate for the positive chirp caused by nonlinear effects, thereby compressing the spectrally broadened pulse into a narrower pulse in the time domain.

[0060] Grating Substrate 5-4: This is the substrate material on which the grating 5-3 is attached. The key to this invention is that this substrate is made of a nonlinear material, such as ultraviolet fused silica. When a high-peak-power femtosecond laser (e.g., 300 fs, 20 W) penetrates it, it induces a strong self-phase modulation (SPM) effect. SPM is a third-order nonlinear optical effect. Its principle is that changes in the intensity of the light pulse itself cause an instantaneous change in the refractive index of the medium, resulting in new frequency components (i.e., redshift and blueshift) at the leading and trailing edges of the pulse, ultimately leading to a significant broadening of the pulse's spectrum.

[0061] The working principle is explained below: This implementation is based on the Fourier transform limit relationship that "time-domain pulse width is inversely proportional to spectral width". The second compressor 5 cleverly integrates the nonlinear process (SPM causing spectral broadening) and the linear process (diffraction compensation by grating 5-3) into the same compact space. The number of diffractions (N) is precisely controlled by moving the movable mirror 5-1, which is essentially controlling the number of times the pulse undergoes the "SPM spectral broadening - grating dispersion compression" cycle (N / 2 times). The more diffractions, the stronger the cumulative nonlinear spectral broadening effect, and the narrower the pulse after compression by grating 5-3. This achieves a leap from "fixed compression" to "programmable precision-adjustable compression". It solves the problem of low damage threshold of chirped mirrors (because grating 5-3 itself is high-power resistant) and overcomes the defect of fixed compression amount, realizing continuous, online precision adjustment of output pulse width for the first time on a single high-power femtosecond laser device.

[0062] In a preferred embodiment, the grating substrate 5-4 is made of ultraviolet fused silica.

[0063] Specifically, in the manufacturing of the second compressor 5, ultraviolet fused silica is selected as the grating substrate 5-4. Ultraviolet fused silica is an amorphous silica glass. Its implementation incorporates the following considerations:

[0064] Excellent nonlinear properties: Ultraviolet fused silica possesses a high third-order nonlinear refractive index, which means that at a given laser intensity, it can produce a more significant self-phase modulation effect, thereby more effectively broadening the spectrum and providing a basis for pulse compression. This is the core physical property for realizing the nonlinear compression function of this invention.

[0065] Extremely high laser damage threshold: Compared with the multilayer dielectric film of the chirped mirror, the bulk ultraviolet fused silica itself has an extremely high laser damage threshold, which can withstand long-term irradiation by high-energy (such as 50μJ) and high-peak-power femtosecond laser after being amplified by amplifier 3, ensuring the reliability of the device to operate stably at high power.

[0066] Wide transmission range and low dispersion: Ultraviolet fused silica exhibits high transmittance across a broad wavelength range from deep ultraviolet to near-infrared, making it particularly suitable for the Yb laser system with a center wavelength of 1030 nm as described in this invention. Simultaneously, its material dispersion in the near-infrared band is relatively small and smooth, avoiding the introduction of uncontrollable additional higher-order dispersion, which is beneficial for obtaining high-quality, substrate-free compressed pulses.

[0067] Excellent optical uniformity and mechanochemical stability: As the grating substrate 5-4, it must be free of bubbles and streaks, possessing extremely high optical uniformity to ensure wavefront quality and avoid laser beam distortion. Furthermore, it exhibits good thermal stability and chemical inertness, guaranteeing the long-term stability of the device under various environments.

[0068] In a preferred embodiment, by moving the movable reflector 5-1, the diffraction number of the femtosecond laser on the grating 5-3 is adjusted to 6 to 10 times, so as to compress the pulse width from 300 fs to the range of 152 fs to 98 fs.

[0069] Specifically, the operator or automatic control system precisely moves the movable mirror 5-1 by controlling the electric translation stage. Each specific position of the movable mirror 5-1 uniquely corresponds to an optical path structure, thereby determining a fixed diffraction number (N).

[0070] When the diffraction order N=6, the pulse undergoes approximately three complete "nonlinear broadening-linear compression" cycles in the grating substrate 5-4. Under this condition, the initial 300 femtosecond, 20-watt pulse has its spectrum broadened to a specific width. According to the Fourier transform limit principle, this broadened spectrum supports a narrower pulse. After dispersion compensation in grating 5-3, the final output pulse is compressed to 152 femtoseconds. This represents a good balance between compression efficiency and system complexity.

[0071] When the diffraction order N=8, the movable mirror 5-1 is moved to a position that extends the optical path, and the pulse undergoes approximately 4 cycles. The longer interaction length and the greater number of cycles result in a stronger cumulative nonlinear effect, further broadening the spectrum. Consequently, the final output pulse width is compressed to a narrower 128 femtoseconds. This order typically achieves very high compression efficiency and good pulse quality.

[0072] When the diffraction order N=10: the movable mirror 5-1 is moved to the position where the optical path is longest, the pulse goes through about 5 cycles, the nonlinear effect is maximized, and the spectral broadening reaches its limit. At this time, the device outputs the narrowest pulse width achievable under this configuration—98 femtoseconds, entering the high-performance range within 100 femtoseconds.

[0073] The essence of this implementation is the quantitative mapping relationship between "diffraction number - pulse width". The technical principle is that the diffraction number is directly proportional to the effective interaction length of the pulse in the nonlinear medium, thus determining the spectral broadening caused by SPM. A larger broadening, after compression by grating 5-3, corresponds to a shorter pulse width. Limiting the diffraction number to 6-10 has extremely high engineering value: it clearly indicates the specific and effective operating range of the core variable that needs to be controlled to achieve the goal of "compressing from 300 fs to within 100 femtoseconds". This avoids blind trial and error by those skilled in the art in diffraction numbers from 0 to infinity, directly providing a verified formula that can stably produce the expected results, greatly improving the operability and repeatability of the method, and is a concrete manifestation of achieving precise adjustment.

[0074] In a preferred embodiment, the surface of the grating substrate 5-4 is coated with a high-reflectivity film layer to reflect the femtosecond laser back to the grating 5-3 to increase the diffraction number.

[0075] Specifically, when manufacturing the grating 5-3, we not only create periodic grating lines on one side of the substrate through processes such as photolithography and etching, but also deposit a high-reflectivity dielectric film on the other surface of the substrate (i.e. the surface opposite to the photolithographic surface).

[0076] This film is designed for the center wavelength of femtosecond lasers (e.g., 1030nm) and has extremely high reflectivity (typically >99.5%). Its function is as follows: when the femtosecond laser pulse first penetrates the grating substrate 5-4, is diffracted by the grating 5-3 to the climbing mirror 5-2, and then reflected back to the grating 5-3 for a second diffraction, the beam that should have left the compression module will now be incident on this surface coated with the high-reflectivity film.

[0077] The high-reflectivity coating reflects almost the entire beam of light back, causing it to penetrate the grating substrate 5-4 again and continue to propagate in the optical path formed by the movable reflector 5-1 and the climbing mirror 5-2. This design ingeniously upgrades the original "single-way transmission" system into a "two-way or even multi-way" system.

[0078] This implementation achieves a significant increase in nonlinear interaction efficiency: each time the beam penetrates the grating substrate 5-4, it triggers the SPM effect. The high-reflectivity coating forces the beam to "return," ensuring that the pulse penetrates the nonlinear medium twice with each external optical path loop. This means that, with the same number of diffractions (i.e., the range of movement of the movable mirror 5-1), the effective nonlinear interaction length is almost doubled. This either achieves a stronger compression effect without increasing the device size, or achieves the same amount of compression with fewer diffractions, improving the system's compactness and efficiency. It also enables finer adjustment precision: the diffraction number (N) is now a more "sparse" control variable (because each diffraction involves two nonlinear interactions), and the continuous variation of the pulse width is mainly achieved by fine-tuning the position of the movable mirror 5-1 to change the precise incident angle and interaction area of ​​the beam in the medium. This makes the adjustment curve smoother, facilitating more precise control. Optimized optical path and improved stability: This "returning" optical path design makes the system more compact, reduces the number of optical components and alignment difficulty, and improves the overall mechanical stability and environmental robustness of the module.

[0079] In a preferred embodiment, the thickness of the grating substrate 5-4 of the ultraviolet fused silica is 2 mm.

[0080] Implementation Considerations: Nonlinear Accumulation: The substrate needs sufficient thickness to ensure that a significant self-phase modulation (SPM) effect accumulates during the propagation of the femtosecond laser pulse, thereby effectively broadening the spectrum. If the thickness is too thin (e.g., <1 mm), the nonlinear interaction length is insufficient, the SPM effect is weak, and it is difficult to achieve the expected spectral broadening and pulse compression effects.

[0081] Dispersion and transmission loss: However, the thickness should not be too large. An excessively thick substrate (e.g., >3mm) will introduce excessive material dispersion and linear transmission loss, which will not only increase the burden of grating 5-3 compensation, but may also reduce the overall efficiency of the system and even affect the quality of the final compressed pulse.

[0082] Mechanical strength and optical processing: The 2mm thickness provides good mechanical stability to the substrate, which facilitates high-precision grating 5-3 etching and high-reflectivity film deposition, while ensuring the flatness and surface accuracy of the optical components.

[0083] Specifically, limiting the thickness to 2 mm essentially determines the optimal operating point between nonlinear effects and negative factors. This thickness is sufficient to allow the pulse to generate adequate self-phase modulation within the substrate, while avoiding excessive dispersion and energy loss due to excessive material length. This is an experimentally verified preferred size that can stably and efficiently achieve the nonlinear compression function described in this invention. It ensures reliable pulse compression to the femtosecond level under input pulse parameters (e.g., 300 fs, 20 W).

[0084] In a preferred embodiment, the grating 5-3 has a line density of 500 lines / mm.

[0085] Specifically, dispersion: the higher the line density, the greater the negative dispersion provided by grating 5-3, meaning a shorter physical optical path is required to compress a pulse per unit bandwidth. This is crucial for achieving a compact second compressor 5 structure. A line density of 500 lines / mm provides sufficiently large negative dispersion for femtosecond lasers with a center wavelength around 1030 nm to effectively compensate for the positive chirp generated by SPM and compress the broadened spectrum back to a transform-limited pulse.

[0086] Diffraction efficiency: The line density needs to be matched to the operating wavelength band. 500 lines / mm is a classic and efficient choice for near-infrared femtosecond lasers, providing the required dispersion while ensuring high diffraction efficiency (typically >95%). Excessively high line density may lead to decreased diffraction efficiency and increased manufacturing difficulty, while too low a density will not provide sufficient compression capability.

[0087] Laser damage threshold: Compared with chirped mirrors, metal-coated or dielectric film gratings 5-3 have a higher damage threshold. The manufacturing process of gratings 5-3 with 500 lines / mm is mature and can withstand the high-power femtosecond laser involved in this invention.

[0088] Example 2

[0089] This application proposes a method for precise adjustment and compensation of femtosecond pulse width, employing the femtosecond pulse width precise adjustment and compensation device as described above. Figure 3 As shown, it includes the following steps:

[0090] S1. A femtosecond laser pulse sequence is generated by the mode-locked seed source 1, the femtosecond laser pulse sequence comprising multiple femtosecond laser pulses;

[0091] S2. The femtosecond laser pulse is stretched in the time domain using the stretcher 2 to obtain the stretched laser pulse;

[0092] S3. The power of the broadened laser pulse is amplified using the amplifier 3 to obtain an amplified laser pulse;

[0093] S4. The amplified laser pulse is initially compressed using the first compressor 4 to obtain an intermediate femtosecond laser with an intermediate pulse width.

[0094] S5. The intermediate femtosecond laser is incident onto the second compressor 5 for nonlinear compression;

[0095] The nonlinear compression is achieved by moving the movable mirror 5-1 in the second compressor 5 to change the diffraction number of the intermediate femtosecond laser on the grating 5-3, while simultaneously using the self-phase modulation effect in the grating substrate 5-4 made of nonlinear material to perform spectral broadening and pulse compression, so as to output a compensated femtosecond laser pulse after precise adjustment and compensation.

[0096] Specifically, a femtosecond laser pulse sequence is generated:

[0097] Implementation: The mode-locked seed source 1 is started and run. This seed source acts as a master oscillator, using active or passive mode-locking technology to maintain a fixed phase relationship between different longitudinal modes within the laser, resulting in coherent superposition and generating a sequence of laser pulses with equal temporal intervals and extremely narrow pulse widths (typically tens of femtoseconds). In this embodiment, it outputs a stable pulse train with a center wavelength of 1030 nm, a repetition frequency of 30 MHz, and an average power of 6 mW.

[0098] Temporal expansion:

[0099] Implementation: The femtosecond pulse sequence output from the seed source is guided and incident onto the stretcher 2 (e.g., a grating pair consisting of a transmission grating or a reflection grating). Different frequency components in the pulse experience different optical path lengths within the grating pair; high-frequency (blue-shifted) components lag behind, while low-frequency (red-shifted) components lead, thus being stretched in the time domain. This step applies the core idea of ​​chirped pulse amplification technology. By introducing positive dispersion, the pulse is stretched from the femtosecond level to the nanosecond level (e.g., 1 ns), and the peak power decreases sharply (e.g., from the kilowatt level to the watt level). This lays the foundation for safe and efficient amplification in the next step, effectively suppressing nonlinear effects during the amplification process.

[0100] Power amplification:

[0101] Implementation: The broadened nanosecond pulse is injected into amplifier 3. Amplifier 3 is typically a multi-stage structure, such as a preamplifier system and a main amplifier 3. It utilizes doped fiber (e.g., Yb-doped fiber) or a gain medium, and is excited by pump light, causing the signal light to undergo stimulated emission and gain energy. The purpose of this step is to increase the pulse energy. Amplification at low peak power avoids optical damage and nonlinear effects, thereby achieving stable extraction of high average power (e.g., 20W) and high pulse energy (e.g., 50μJ).

[0102] Preliminary compression:

[0103] Implementation: The amplified pulse is fed into the first compressor 4. This compressor (e.g., another set of grating pairs or volume gratings) is designed to provide negative dispersion, opposite to that of the stretcher 2, precisely compensating for the total dispersion accumulated during the stretching and amplification of the pulse. This step aims to restore the pulse to the femtosecond range. Through dispersion compensation, the nanosecond pulse is recompressed to hundreds of femtoseconds (e.g., 300 fs), reconstructing the peak power and preparing suitable input conditions for the subsequent nonlinear compression stage.

[0104] Nonlinear compression:

[0105] Implementation: The pre-compressed femtosecond laser (intermediate femtosecond laser) is incident onto the second compressor 5. This step includes two simultaneous and interrelated sub-processes:

[0106] Adjusting the diffraction number: By moving the movable reflector 5-1, its position in the optical path is changed. A slight change in the position of the movable reflector 5-1 will precisely change the incident point of the laser beam before it reaches the grating 5-3, thereby changing the round-trip path of the beam between the grating 5-3 and the climbing mirror 5-2, and finally achieving continuous and precise control over the diffraction number of the beam on the grating 5-3.

[0107] Utilizing the self-phase modulation effect: When a high-peak-power femtosecond laser penetrates a grating substrate 5-4 made of a nonlinear material (such as ultraviolet fused silica), the intensity change causes an instantaneous change in the refractive index of the medium, resulting in new frequency components in the pulse and achieving spectral broadening. Simultaneously, the grating 5-3 performs dispersion compensation on the broadened pulse, achieving time-domain compression. It uses "adjusting the diffraction number" as a control variable, dynamically linking it to the two physical processes of "nonlinear spectral broadening" and "linear dispersion compensation." The technical principle is that the diffraction number (N) directly determines the number of times the pulse undergoes the core cycle of "nonlinear broadening-linear compression" (approximately N / 2). More cycles mean a stronger cumulative nonlinear effect and a wider spectrum, which, according to the Fourier transform limit, supports a narrower pulse width. The technical effect is to achieve online, continuous, and precisely adjustable output pulse width. Users can simply move the movable mirror 5-1 to obtain high-quality pulse outputs ranging from 152 fs to 98 fs as needed, perfectly solving the problem of fixed compression in existing technologies.

[0108] In a preferred embodiment, the following steps are also included:

[0109] The real-time pulse width of the compensated femtosecond laser pulse is monitored in real time, and the real-time pulse width is compared with the preset pulse width.

[0110] Based on the comparison results, a control signal is generated and fed back to the movable reflector 5-1 to automatically adjust its position so as to stabilize the real-time pulse width at the preset pulse width.

[0111] Specifically, a beam splitter is inserted into the final output optical path of the system to couple out a small portion (e.g., ~1%) of the compensated femtosecond laser pulses and guide them to a real-time pulse width measurement device. This device can be a commercially available or self-developed instrument based on technologies such as autocorrelators, frequency-resolved optical switches, or streak cameras. This instrument continuously measures the real-time pulse width of each pulse or batch of pulses. Through physical detection and mathematical calculation, the most critical output parameter—pulse width—is directly obtained as a sensing signal for the feedback system. The measurement device transmits the measured real-time pulse width data to a central processing unit. This unit stores the user-preset target pulse width value. The CPU performs high-speed, continuous comparison calculations between the real-time data and the preset value, calculating the deviation value (Δτ). Based on the calculated deviation value (Δτ), the CPU generates a control signal according to a preset control algorithm (such as a PID control algorithm). This signal is sent to a precision electric translation stage that drives the movable reflector 5-1. For example, if the real-time pulse width is greater than the target value (Δτ>0), the control signal instructs the translation stage to move to increase the number of diffractions, thereby enhancing the compression effect; conversely, it reduces the number of diffractions.

[0112] This implementation constitutes a complete negative feedback closed loop. Its technical principle is: the deviation in the output is fed back to adjust the control quantity (the position of the movable reflector 5-1), thereby reducing the deviation in the output. The resulting technical effects are: high precision and stability: it can actively compensate for pulse width drift caused by factors such as ambient temperature fluctuations, mechanical vibrations, or fluctuations in the laser's own power, keeping the output pulse width stable at a preset value over a long period, with precision reaching the femtosecond level. Improved reliability: it ensures that laser output conforming to specifications can be delivered at all times, greatly enhancing the practical value and reliability of the entire system in industrial applications and precision scientific research.

[0113] In a preferred embodiment, the step of incidenting the intermediate femtosecond laser onto the second compressor 5 for nonlinear compression includes the following steps:

[0114] The intermediate femtosecond laser with a pulse width of 300 fs and a power of 20 W is incident on the second compressor 5;

[0115] By moving the movable reflector 5-1, the diffraction number of the intermediate femtosecond laser on the grating 5-3 is precisely adjusted to 6, 8 or 10 times, so as to compress the pulse width of the intermediate femtosecond laser to 152fs, 128fs or 98fs respectively.

[0116] In a preferred embodiment, prior to the step of incidenting the intermediate femtosecond laser onto the second compressor 5 for nonlinear compression, the following step is further included:

[0117] The real-time parameters of the intermediate femtosecond laser about to enter the second compressor 5 are obtained, including pulse energy, pulse width and beam diameter;

[0118] Based on the real-time parameters, the intensity factor of the nonlinear effect expected to be generated by the intermediate femtosecond laser in the grating substrate 5-4 is calculated;

[0119] The nonlinear effect intensity factor is compared with a preset threshold range, and based on the comparison result, one of the following operations is performed:

[0120] If the nonlinear effect intensity factor is within the preset threshold range, then the subsequent nonlinear compression step is executed;

[0121] If the nonlinear effect intensity factor is lower than the minimum value of the preset threshold range, a first adjustment command is generated and fed back to the amplifier 3 and / or the first compressor 4 to increase the pulse energy of the intermediate femtosecond laser and / or compress the pulse width until the nonlinear effect intensity factor reaches the preset threshold range.

[0122] If the nonlinear effect intensity factor is higher than the maximum value of the preset threshold range, a second adjustment command is generated and fed back to the amplifier 3 to reduce the pulse energy of the intermediate femtosecond laser and prevent the grating substrate 5-4 from being damaged due to excessive nonlinear effect.

[0123] Specifically, at the entrance of the second compressor 5, key parameters of the incoming intermediate femtosecond laser are measured in real time using a beam splitter and a set of diagnostic equipment. These parameters include at least: Pulse energy (E): measured using a photoelectric energy meter; Pulse width (Δt): measured using a fast photodetector and autocorrelation meter; Beam diameter (D): measured using a CCD camera or knife-edge method to measure the beam waist radius; Calculation of the nonlinear effect intensity factor (γ): The acquired real-time parameters (E, Δt, D) are fed into the processor and calculated according to the formula γ ∝ E / (Δt * D). 2 The calculation is performed. This factor γ is a dimensionless quantity or a quantity with specific units, which comprehensively reflects the intensity of the SPM effect produced by the pulse in the nonlinear medium. The higher the pulse energy, the narrower the pulse width, and the smaller the beam diameter, the larger the γ value and the stronger the nonlinear effect. Comparison with preset threshold range: The calculated γ value is compared with a preset safe and effective threshold range [γ_min, γ_max] in the system.

[0124] Specifically, the preset safe and effective threshold range [γ_min, γ_max] is a reference value range corresponding to the proportional relationship γ ∝ E / (Δt * D²) of the nonlinear effect intensity factor γ, determined through the following experimental calibration steps. The calibration steps are as follows:

[0125] Establishing a benchmark: After the femtosecond pulse width precision adjustment and compensation device is built, keep its core optical components (especially the grating substrate and laser wavelength) unchanged. At this time, the proportionality coefficient between γ and E / (Δt * D²) is a fixed constant.

[0126] Experiments determine the range boundaries:

[0127] Determine the lower limit reference value (γ_min): Gradually increase the pulse energy E of the intermediate femtosecond laser input to the second compressor (or reduce Δt by adjusting the first compressor, or reduce the beam diameter D), and measure the compression effect of the output pulse. The calculated value of E / (Δt * D²) that produces significant nonlinear spectral broadening and stabilizes the pulse width compression rate at over 30% is determined as the lower limit reference value.

[0128] Determine the upper limit reference value (γ_max): Continue to increase the intensity of the input laser (by increasing E, decreasing Δt, or D) until a critical state is reached where damage to the grating substrate is imminent but has not yet occurred (e.g., determined by monitoring scattered light or surface morphology). The calculated value of E / (Δt * D²) corresponding to this critical state is determined as the upper limit reference value.

[0129] Setting the threshold range: The resulting interval [γ_min, γ_max] is used as the preset threshold range. During subsequent automatic control, the system monitors the pulse parameters in real time and calculates E / (Δt * D²), compares the calculation result with [γ_min, γ_max], and executes corresponding control commands based on the comparison result.

[0130] For example, using an ultraviolet fused silica grating substrate, for an intermediate femtosecond laser with a pulse width of 300 fs and a power of 20 W, the reference value range determined through the above calibration steps is γ_min = 0.05 and γ_max = 0.12 (unit: μJ·fs). -1 ·mm -2 The system sets this [0.05, 0.12] as a preset threshold range.

[0131] Specifically, based on the comparison results, a three-option decision is made:

[0132] If γ ∈ [γ_min, γ_max]: This indicates that the input conditions are ideal, and the system issues a command to allow the laser to enter the second compressor 5 to perform the nonlinear compression step.

[0133] If γ < γ_min: This indicates that the nonlinear effect is too weak to be effectively compressed. The system generates a first adjustment command, which is fed back to the front-end amplifier 3 (instructing it to increase the pump power to increase the pulse energy E) and / or the first compressor 4 (instructing it to fine-tune to further compress the pulse width Δt) until γ is increased to the effective range.

[0134] If γ > γ_max: This indicates that the nonlinear effect is too strong, which may cause optical damage to the grating substrate 5-4 (such as self-focusing filamentation). The system generates a second adjustment command, which is fed back to amplifier 3, instructing it to reduce the pump power to reduce the pulse energy E until γ drops to a safe range.

[0135] This implementation ensures compression effect and consistency: it fundamentally guarantees that the starting point of each nonlinear compression is optimal, thus making the compression effect stable and repeatable, and the output pulse quality highly consistent. It actively prevents optical damage: by monitoring the upper limit of γ and actively reducing power, it greatly improves the system, especially the safety and lifespan of the expensive grating 5-3 and grating substrate 5-4. It achieves system-level intelligent collaboration: it transmits the requirements of the second compressor 5 back to the front-end amplifier 3 and compressor, realizing cross-module adaptive linkage and global optimization of the entire laser system. This represents an advanced form of intelligent laser control, greatly enhancing the technical barriers, advancement, and practicality of this patented method.

[0136] This document uses specific examples to illustrate the principles and implementation methods of this application. The descriptions of the above embodiments are only for the purpose of helping to understand the methods and core ideas of this application. The above descriptions are only preferred embodiments of this application. It should be noted that due to the limitations of written expression, while there are objectively infinite specific structures, those skilled in the art can make several improvements, modifications, or changes without departing from the principles of this invention, and can also combine the above technical features in an appropriate manner. These improvements, modifications, changes, or combinations, or the direct application of the inventive concept and technical solution to other situations without modification, should all be considered within the scope of protection of this application.

Claims

1. A femtosecond pulse width precision adjustment and compensation device, characterized in that, include: Model-locked seed source (1), stretcher (2), amplifier (3), first compressor (4), and second compressor (5); The second compressor (5) includes a movable reflector (5-1), a climbing mirror (5-2), a grating (5-3), and a grating substrate (5-4); the grating substrate (5-4) is the substrate material on which the grating (5-3) is attached; The movable mirror (5-1) is configured to adjust the number of diffractions of the femtosecond laser on the grating (5-3) by changing the distance between it and the grating substrate (5-4); The grating substrate (5-4) is made of a nonlinear material and is used to generate a self-phase modulation effect under the action of femtosecond laser to achieve spectral broadening and pulse compression; By adjusting the diffraction number, precise adjustment and compensation of the femtosecond pulse width can be achieved.

2. The femtosecond pulse width precision adjustment and compensation device according to claim 1, characterized in that: The grating substrate (5-4) is made of ultraviolet fused silica.

3. The femtosecond pulse width precision adjustment and compensation device according to claim 1, characterized in that: By moving the movable mirror (5-1), the diffraction number of the femtosecond laser on the grating (5-3) is adjusted to 6 to 10 times, so as to compress the pulse width from 300 fs to the range of 152 fs to 98 fs.

4. The femtosecond pulse width precision adjustment and compensation device according to claim 1, characterized in that: The grating substrate (5-4) is coated with a high-reflectivity film to reflect the femtosecond laser back to the grating (5-3) to increase the diffraction number.

5. The femtosecond pulse width precision adjustment and compensation device according to claim 2, characterized in that: The thickness of the grating substrate (5-4) of the ultraviolet fused silica is 2 mm.

6. The femtosecond pulse width precision adjustment and compensation device according to claim 1, characterized in that: The grating (5-3) has a line density of 500 lines / mm.

7. A method for precise adjustment and compensation of femtosecond pulse width, characterized in that, The femtosecond pulse width precision adjustment and compensation device as described in any one of claims 1-6 includes the following steps: A femtosecond laser pulse sequence is generated from the mode-locked seed source (1), the femtosecond laser pulse sequence comprising multiple femtosecond laser pulses; The femtosecond laser pulse is stretched in the time domain using the stretcher (2) to obtain a stretched laser pulse; The power of the broadened laser pulse is amplified using the amplifier (3) to obtain an amplified laser pulse; The amplified laser pulse is initially compressed using the first compressor (4) to obtain an intermediate femtosecond laser with an intermediate pulse width. The intermediate femtosecond laser is incident on the second compressor (5) for nonlinear compression; The nonlinear compression is as follows: by moving the movable mirror (5-1) in the second compressor (5), the diffraction number of the intermediate femtosecond laser on the grating (5-3) is changed, and at the same time, the spectral broadening and pulse compression are performed in the grating substrate (5-4) made of nonlinear material using the self-phase modulation effect, so as to output a compensated femtosecond laser pulse after precise adjustment and compensation.

8. The femtosecond pulse width precision adjustment and compensation method according to claim 7, characterized in that: It also includes the following steps: The real-time pulse width of the compensated femtosecond laser pulse is monitored in real time, and the real-time pulse width is compared with the preset pulse width. Based on the comparison results, a control signal is generated and fed back to the movable reflector (5-1) to automatically adjust its position so as to stabilize the real-time pulse width at the preset pulse width.

9. The femtosecond pulse width precision adjustment and compensation method according to claim 7, characterized in that: The step of incidenting the intermediate femtosecond laser onto the second compressor (5) for nonlinear compression includes the following steps: The intermediate femtosecond laser with a pulse width of 300 fs and a power of 20 W is incident on the second compressor (5); By moving the movable mirror (5-1), the diffraction number of the intermediate femtosecond laser on the grating (5-3) is precisely adjusted to 6, 8 or 10 times, so as to compress the pulse width of the intermediate femtosecond laser to 152fs, 128fs or 98fs respectively.

10. The femtosecond pulse width precision adjustment and compensation method according to claim 8, characterized in that: Before the step of incidenting the intermediate femtosecond laser onto the second compressor (5) for nonlinear compression, the following steps are also included: The real-time parameters of the intermediate femtosecond laser that is about to enter the second compressor (5) are obtained, including pulse energy, pulse width and beam diameter; Based on the real-time parameters, the intensity factor of the nonlinear effect expected to be generated by the intermediate femtosecond laser in the grating substrate (5-4) is calculated; The nonlinear effect intensity factor is compared with a preset threshold range, and based on the comparison result, one of the following operations is performed: If the nonlinear effect intensity factor is within the preset threshold range, then the subsequent nonlinear compression step is executed; If the nonlinear effect intensity factor is lower than the minimum value of the preset threshold range, a first adjustment command is generated and fed back to the amplifier (3) and / or the first compressor (4) to increase the pulse energy of the intermediate femtosecond laser and / or compress the pulse width until the nonlinear effect intensity factor reaches the preset threshold range; If the nonlinear effect intensity factor is higher than the maximum value of the preset threshold range, a second adjustment command is generated and fed back to the amplifier (3) to reduce the pulse energy of the intermediate femtosecond laser and prevent the grating substrate (5-4) from being damaged due to excessive nonlinear effect.

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