A nonlinear compression method and apparatus for few-cycle pulse generation

Abstract

The embodiment of the application discloses a nonlinear compression method and device for few-cycle pulse generation, and solves the problem that it is difficult to obtain stable and high-quality few-cycle pulse output in the prior art. The method comprises the following steps: coupling an ultrafast laser pulse into a multi-pass cavity, and performing multiple round-trip propagation in the multi-pass cavity; a nonlinear crystal with a non-zero second-order nonlinear polarizability is arranged in the multi-pass cavity; the phase matching state and position of the nonlinear crystal are adjusted to obtain multi-dimensional coordinated adjustment parameters; the multi-dimensional coordinated adjustment parameters at least include an effective nonlinear refractive index, an optical intensity distribution and a round-trip propagation number; based on the multi-dimensional coordinated adjustment parameters, the nonlinear phase shift amount accumulated when the ultrafast laser pulse propagates in the nonlinear crystal is controlled in multiple dimensions to control the nonlinear phase shift amount to reach a target nonlinear phase shift amount; and based on the ultrafast laser pulse reaching the target nonlinear phase shift amount, spectral broadening is realized in the multi-pass cavity to obtain a few-cycle pulse.

Description

Technical Field

[0001] This application relates to the field of ultrafast laser technology, and in particular to a nonlinear compression method and apparatus for generating few-period pulses. Background Technology

[0002] Currently, ultrafast laser technology is widely used in micro-nano manufacturing, life sciences, and quantum technology, with high peak power, narrow pulse width, and large pulse energy becoming important development directions. Among them, few-period ultrafast lasers, due to their shorter pulse duration and higher peak power, show significant application prospects in attosecond sources, high-field physics, and precision machining. Chirped pulse amplification technology, as the core of industrial-grade ultrafast lasers, is limited by gain spectral bandwidth, resulting in a limited output laser spectral width. After dispersion compression, it is difficult to directly obtain few-period ultrafast laser output. Therefore, nonlinear compression technology has been proposed, which utilizes nonlinear effects such as self-phase modulation at the laser back end to broaden the spectrum, and then combines this with dispersion compensation to obtain a shorter pulse width. This has become an important technical route for realizing few-period pulses.

[0003] Common back-end nonlinear compression techniques include gas-filled hollow fiber, multi-cavity, and multi-solid-sheet methods. Multi-cavity nonlinear compression using solid media offers advantages such as compact structure, repeatable accumulation of nonlinear effects, and high transmission efficiency, making it a key technology for back-end nonlinear compression in ultrafast lasers. In existing technologies, the intensity of nonlinear effects in multi-cavity systems is primarily controlled by adjusting parameters such as the number and thickness of the nonlinear medium within the cavity, its position, and the effective propagation flux of the ultrafast laser pulse. The system's ability to adjust the nonlinear intensity depends mainly on parameters like the optical field spatial distribution and propagation flux. When the input pulse energy, pulse width, or beam parameters change, the adjustable parameters become relatively limited, and strong coupling exists between different adjustable parameters, restricting system parameter optimization and operating point matching.

[0004] Furthermore, existing multi-cavity systems commonly employ transparent media such as fused silica, sapphire, or YAG as nonlinear media, whose spectral broadening process relies on the inherent third-order nonlinear effect of the material. Since the third-order nonlinear refractive index is an intrinsic parameter of the material, its value is essentially fixed once the medium material is determined. The intensity of the nonlinear effect can only be indirectly controlled by changing the spatial distribution of the light field and the propagation flux. This limitation results in insufficient adaptive adjustment of the system under different input conditions or different target output parameters, hindering the acquisition of stable, high-quality short-cycle pulse output. Summary of the Invention

[0005] This application provides a nonlinear compression method and apparatus for generating few-period pulses, which solves the following technical problem: In the prior art, once the medium material is determined, its value is basically fixed, and the intensity of nonlinear effect can only be indirectly controlled by changing the spatial distribution of the light field and the propagation flux. This limitation makes the system's ability to adaptively adjust under different input conditions or different target output parameters insufficient, which is not conducive to obtaining stable and high-quality few-period pulse output.

[0006] The embodiments of this application adopt the following technical solutions: This application provides a nonlinear compression method for generating few-period pulses. The method includes coupling an ultrafast laser pulse into a multi-pass cavity, causing the ultrafast laser pulse to propagate multiple times within the multi-pass cavity; wherein a nonlinear crystal with a non-zero second-order nonlinear polarizability is disposed within the multi-pass cavity; adjusting the phase-matching state and position of the nonlinear crystal to obtain multidimensional synergistic adjustment parameters; wherein the multidimensional synergistic adjustment parameters include at least the effective nonlinear refractive index, light intensity distribution, and number of round-trip propagation times; based on the multidimensional synergistic adjustment parameters, performing multidimensional synergistic control on the nonlinear phase shift accumulated during the propagation of the ultrafast laser pulse in the nonlinear crystal to control the nonlinear phase shift to reach a target nonlinear phase shift; achieving spectral broadening within the multi-pass cavity based on the ultrafast laser pulse that has reached the target nonlinear phase shift; and performing dispersion compensation on the spectrally broadened ultrafast laser pulse to obtain a few-period pulse.

[0007] In one implementation of this application, the phase matching state and position of the nonlinear crystal are adjusted to obtain multi-dimensional synergistic adjustment parameters. Specifically, this includes: adjusting the phase matching state of the nonlinear crystal so that it operates under non-strict phase matching conditions; under non-strict phase matching conditions, a second-order cascaded nonlinear effect is excited inside the nonlinear crystal through continuous energy exchange between the frequency-doubled light and the fundamental light; based on the second-order cascaded nonlinear effect, an effective nonlinear refractive index related to the phase mismatch is obtained; and adjusting the position of the nonlinear crystal within the multi-pass cavity to change the beam size of the ultrafast laser pulse within the nonlinear crystal and obtain the light intensity distribution.

[0008] In one implementation of this application, an effective nonlinear refractive index related to the phase mismatch is obtained based on a second-order cascaded nonlinear effect. Specifically, this includes: determining the phase mismatch based on the difference between the fundamental frequency wave vector and the second-order frequency wave vector; determining the inherent nonlinear refractive index of the nonlinear crystal material; generating a cascaded nonlinear refractive index based on the second-order cascaded nonlinear effect; wherein the cascaded nonlinear refractive index is proportional to the square of the effective nonlinear coefficient of the nonlinear crystal; and superimposing the inherent nonlinear refractive index of the material with the cascaded nonlinear refractive index to obtain the effective nonlinear refractive index.

[0009] In one implementation of this application, obtaining an effective nonlinear refractive index related to the phase mismatch amount specifically includes: adjusting the phase matching state of the nonlinear crystal so that the phase mismatch amount changes continuously, causing the cascaded nonlinear refractive index to take a positive value on the side where the angle between the input beam of the nonlinear crystal and the optical axis is smaller than the phase matching angle, and a negative value on the other side; the effective nonlinear refractive index changes continuously with the continuous change of the phase mismatch amount.

[0010] In one implementation of this application, multidimensional collaborative control is performed on the nonlinear phase shift accumulated during the propagation of an ultrafast laser pulse in a nonlinear crystal, based on multidimensional collaborative adjustment parameters, to control the nonlinear phase shift to reach a target nonlinear phase shift. Specifically, this includes: multiplying the effective nonlinear refractive index with the light intensity distribution to obtain the nonlinear refractive index contribution at each position along the propagation direction; accumulating the nonlinear refractive index contribution at each position along the propagation path of the ultrafast laser pulse in the nonlinear crystal to obtain the cumulative nonlinear response of the ultrafast laser pulse propagating once in the nonlinear crystal; determining the single nonlinear phase shift corresponding to a single round-trip propagation based on the cumulative nonlinear response and the center wavelength of the ultrafast laser pulse; multiplying the single nonlinear phase shift with the number of round-trip propagations to obtain the total nonlinear phase shift accumulated by the ultrafast laser pulse in the multi-pass cavity, and ensuring that the total nonlinear phase shift reaches the target nonlinear phase shift.

[0011] In one implementation of this application, spectral broadening is achieved within a multi-pass cavity based on an ultrafast laser pulse that achieves a target nonlinear phase shift. Specifically, this includes: each round-trip propagation of the ultrafast laser pulse through the nonlinear crystal generates new frequency components based on the nonlinear effect corresponding to the target nonlinear phase shift; the new frequency components generated in each round-trip propagation are successively superimposed within the multi-pass cavity, causing the spectrum of the ultrafast laser pulse to gradually broaden within the multi-pass cavity; the number of round-trip propagations of the ultrafast laser pulse within the multi-pass cavity is determined according to the required spectral broadening; and when the spectral broadening reaches the target spectral width, the broadened ultrafast laser pulse is exported from the multi-pass cavity; wherein the target spectral width is determined by the degree of spectral broadening corresponding to the target nonlinear phase shift.

[0012] In one implementation of this application, dispersion compensation is performed on the spectrally broadened ultrafast laser pulse to obtain a few-period pulse. Specifically, this includes: inputting the spectrally broadened ultrafast laser pulse into a dispersion compensation module; and using the dispersion compensation module to compensate for the chirp introduced by nonlinear effects in the spectrally broadened ultrafast laser pulse, thereby compressing the time domain width of the compressed spectrally broadened ultrafast laser pulse to obtain a few-period pulse.

[0013] This application also provides a nonlinear compression device for generating few-period pulses. It includes: an ultrafast laser source for outputting ultrafast laser pulses; a mode matching module disposed at the output end of the ultrafast laser source for adjusting the beam parameters of the ultrafast laser pulses to ensure that the ultrafast laser pulses meet the stable transmission and mode matching conditions of a multi-pass cavity; a multi-pass cavity module disposed at the output end of the mode matching module, including a first concave mirror and a second concave mirror disposed opposite to each other, and at least one nonlinear crystal disposed within the cavity; the nonlinear crystal has a non-zero second-order nonlinear polarizability, used to excite a second-order entrained nonlinear effect under non-strict phase matching conditions, causing the ultrafast laser pulse to accumulate a nonlinear phase shift to achieve spectral broadening; and a dispersion compensation module disposed at the output end of the multi-pass cavity module for performing dispersion compensation on the spectrally broadened ultrafast laser pulses to obtain few-period pulses.

[0014] In one implementation of this application, the multi-cavity module further includes an inlet mirror and an outlet mirror: the inlet mirror is used to couple the ultrafast laser pulse into the multi-cavity, so that the ultrafast laser pulse propagates back and forth between the first concave mirror and the second concave mirror, and passes through a nonlinear crystal in each round-trip propagation pass, so as to achieve spectral broadening by successively accumulating nonlinear phase shift; the outlet mirror is used to export the spectrally broadened ultrafast laser pulse to the dispersion compensation module.

[0015] In one implementation of this application, the nonlinear crystal is mounted on a multidimensional adjustment frame: the multidimensional adjustment frame has at least a displacement adjustment degree of freedom along the axial direction of the multi-cavity and a rotation adjustment degree of freedom about a preset axis; the displacement adjustment degree of freedom is used to change the position of the nonlinear crystal along the axial direction to adjust the beam size and intensity distribution at the nonlinear crystal; the rotation adjustment degree of freedom is used to change the angle between the optical axis of the nonlinear crystal and the input beam to adjust the effective nonlinear refractive index of the nonlinear crystal by adjusting the phase mismatch; wherein, the displacement adjustment and rotation adjustment are set to be independent adjustments.

[0016] The above-mentioned technical solutions adopted in the embodiments of this application can achieve the following beneficial effects: In the embodiments of this application, an ultrafast laser pulse is coupled into a multi-pass cavity and propagated back and forth multiple times. Utilizing the characteristic of multiple round trips in the multi-pass cavity, the pulse passes through a nonlinear crystal in each round trip, causing the nonlinear phase shift to accumulate successively and the spectrum to broaden gradually, providing a structural basis for subsequent control. By adjusting the phase matching state and position of the nonlinear crystal, a second-order cascaded nonlinear effect is excited under non-strict phase matching conditions, obtaining an effective nonlinear refractive index. This overcomes the limitation in existing technologies where the nonlinear refractive index is an intrinsic parameter of the material and cannot be adjusted. Simultaneously, by adjusting the position, the beam size of the pulse within the crystal is changed to obtain an adjustable light intensity distribution, forming a multi-dimensional synergistic adjustment parameter combined with the number of round trips. Furthermore, the embodiments of this application control the nonlinear phase shift to reach the target value based on the multi-dimensional synergistic adjustment parameter, avoiding the problem in existing technologies where the adjustment parameters are limited and mutually coupled, leading to difficulty in accurately matching the nonlinear phase shift. Under the condition of achieving the target nonlinear phase shift, the new frequency components generated in each round trip are successively superimposed, causing the spectrum to gradually broaden to the target spectral width, avoiding the problems of insufficient spectral broadening or excessive nonlinear phase shift. The effective nonlinear refractive index control method proposed in this application has good versatility and scalability. It can flexibly adjust the nonlinear action process according to different input laser parameters, has better adaptability to different pulse energies and pulse widths, and has a wider range of applications. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. In the drawings: Figure 1 A schematic diagram of a nonlinear compression device for generating few-period pulses is provided in an embodiment of this application; Figure 2 This is a schematic diagram of a nonlinear compression method for generating few-period pulses provided in an embodiment of this application; Figure 3 The graph shows the relationship between the effective nonlinear refractive index of the BBO nonlinear crystal provided in this embodiment of the application and the angle between the optical axis and the input beam.

[0018] Figure label: 1: Ultrafast laser source, 2: Mode matching module, 3: Mirror, 4: First concave mirror, 5: Second concave mirror, 6: Inlet mirror, 7: Nonlinear crystal, 8: Outlet mirror, 9: Harmonic beam splitter, 10: First chirped mirror, 11: Second chirped mirror. Detailed Implementation

[0019] This application provides a nonlinear compression method and apparatus for generating few-period pulses.

[0020] To enable those skilled in the art to better understand the technical solutions in this application, the technical solutions in the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this specification, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of this application.

[0021] It should be noted that the term "few-cycle" in the embodiments of this application is a technical term in the field of ultrafast optics, referring to an ultrashort laser pulse with a pulse width consisting of only a few optical cycles. One optical cycle refers to the time required for the laser electric field to complete one full cycle of vibration, which is related to the center wavelength of the laser. For example, for a laser with a center wavelength of 800 nm, one optical cycle is approximately 2.67 femtoseconds; for a laser with a center wavelength of 1030 nm, one optical cycle is approximately 3.43 femtoseconds. Few-cycle pulses typically refer to laser pulses with a pulse width ranging from 1 to 10 optical cycles.

[0022] Figure 1 A schematic diagram of a nonlinear compression device for generating few-period pulses is provided in an embodiment of this application, as shown below. Figure 1 As shown, the nonlinear compression device for generating few-period pulses includes an ultrafast laser source 1, a mode matching module 2, a multi-cavity module, and a dispersion compensation module.

[0023] In one implementation of this application, an ultrafast laser source 1 is used to output ultrafast laser pulses. A mode matching module 2 is disposed at the output end of the ultrafast laser source 1 and is used to adjust the beam parameters of the ultrafast laser pulses so that the ultrafast laser pulses meet the stable transmission and mode matching conditions of the multi-pass cavity. A multi-pass cavity module is disposed at the output end of the mode matching module and includes a first concave mirror and a second concave mirror disposed opposite to each other, and at least one nonlinear crystal disposed within the cavity; the nonlinear crystal has a non-zero second-order nonlinear polarizability and is used to excite a second-order entrained nonlinear effect under non-strict phase matching conditions, so that the ultrafast laser pulse accumulates a nonlinear phase shift to achieve spectral broadening. A dispersion compensation module is disposed at the output end of the multi-pass cavity module and is used to perform dispersion compensation on the spectrally broadened ultrafast laser pulses to obtain a few-period pulse.

[0024] Specifically, the ultrafast laser source 1 is used to provide the ultrafast laser pulse to be compressed. This ultrafast laser source 1 has a wide pulse width range, including femtosecond and picosecond lasers. The wavelengths cover common near- and mid-infrared ultrafast lasers, typically including the 800nm, 1030nm, 1064nm, 1550nm, and 2000nm bands.

[0025] The mode matching module 2 is used to adjust the beam parameters of the input laser, including the beam diameter and divergence angle, to meet the focusing conditions in the nonlinear spectral broadening module. In this embodiment, the mode matching module 2 includes a first convex lens, a concave lens, and a second convex lens. The first convex lens and the concave lens are combined to adjust the diameter and divergence angle of the input beam, and the second convex lens is used to further adjust the focusing conditions and beam waist parameters of the beam, so that the input beam matches the transmission mode of the subsequent multi-cavity transmission mode.

[0026] Reflector 3 is positioned after pattern matching module 2 to change the beam propagation direction. In this embodiment, the number of reflectors can be flexibly increased or decreased depending on the available space. After pattern matching, the laser beam's propagation direction is changed by reflector 3 and coupled into the multi-cavity module.

[0027] In other embodiments, the pattern matching module may also employ other combinations of optical elements capable of beam expansion, beam contraction, collimation, focusing, or pattern matching, including but not limited to single lenses, lens groups, beam expanders, reflective optical elements, and combinations thereof.

[0028] A multi-pass cavity module is located at the output end of the reflector 3. In this embodiment, the multi-pass cavity module is used to enable the ultrafast laser pulse to form a stable multiple propagation path within the cavity, and to repeatedly pass through the nonlinear crystal located within the cavity in multiple propagation passes, thereby successively accumulating nonlinear phase shifts and achieving nonlinear spectral broadening. For example... Figure 1 As shown, the multi-channel module includes a first concave mirror 4, a second concave mirror 5, a nonlinear crystal 7, an input mirror 6, and an output mirror 8. The first concave mirror 4 and the second concave mirror 5 are arranged opposite each other, together forming a multi-channel cavity for multiple round-trip propagation of ultrafast laser pulses, and employing high-reflectivity optical elements suitable for the operating wavelength range of ultrafast lasers. Within the operating wavelength range of this embodiment, the concave mirror preferably has a reflectivity greater than 99.9% to reduce the cumulative reflection loss of the laser pulse during multiple propagation processes and improve the transmission efficiency of the multi-channel cavity. In other embodiments, a high-reflectivity mirror with appropriate reflectivity can be selected according to the center wavelength of the input laser, pulse energy, effective propagation flux, and system transmission efficiency requirements. By rationally designing the radius of curvature, spacing, and input beam pattern of the first concave mirror 4 and the second concave mirror 5, a stable multiple propagation trajectory of the laser pulse can be formed between the two concave mirrors.

[0029] The laser beam enters the multi-pass cavity through the guide mirror 6, and is subsequently reflected multiple times by the cavity mirrors. Each reflection passes through a nonlinear crystal 7 placed near the center of the cavity. This crystal has a non-zero second-order nonlinear polarizability. In the nonlinear crystal, an equivalent third-order nonlinear effect is generated through a second-order sequential nonlinear process deviating from the phase-matching condition, enabling the laser to achieve an adjustable nonlinear phase shift and spectral broadening. Furthermore, the nonlinear crystal 7 is mounted on an adjustable multidimensional mechanical structure. Each crystal can be positioned along the axial direction of the multi-channel cavity to change the beam size and intensity distribution at the crystal. Simultaneously, the angle between the optical axis of each crystal and the input beam can be adjusted to change the phase-matching state of the crystal. The laser beam after passing through the nonlinear crystal will contain some frequency-doubled light (under large phase mismatch conditions, the proportion of green light is <5%), which is reflected and filtered out by the harmonic beam splitter 9. The reflection and transmission bands of this harmonic beam splitter are determined by the center wavelength of the input laser source. The laser then passes through a lead-out mirror and enters a dispersion compensation module. This module includes a first chirped mirror 10 and a second chirped mirror 11, which compensate for the chirp introduced by the nonlinear effect, achieving pulse compression and ultimately obtaining a few-period pulse output.

[0030] In one embodiment, the nonlinear crystal 7 has a non-zero second-order nonlinear polarizability. This nonlinear crystal is used to excite a second-order cascaded nonlinear effect under non-strict phase-matching conditions, enabling the accumulation of nonlinear phase shifts in ultrafast laser pulses to achieve spectral broadening. The nonlinear crystal 7 includes, but is not limited to, barium metaborate (BBO), lithium triborate (LBO), potassium dihydrogen phosphate (KDP), potassium titanium oxyphosphate (KTP), potassium titanium oxyarsenate (KTA), bismuth metaborate (BiBO), and lithium niobate (LiNbO3), all nonlinear crystals capable of generating a second-order cascaded nonlinear effect. The number and thickness of the crystals can be selected according to the input parameter range, and combinations of different crystal structures can also be used. The nonlinear crystal 7 can be treated with different end-face methods depending on the operating wavelength, polarization state, and system structure, including but not limited to Brewster corner cutting, end-face antireflection coating, or a combination of both, to reduce interface reflection loss and improve system transmission efficiency and nonlinear compression efficiency.

[0031] In one embodiment, each nonlinear crystal 7 is mounted on a multidimensional adjustment frame, which has at least a displacement adjustment degree of freedom along the axial direction of the multi-cavity and a rotational adjustment degree of freedom about a preset axis. The displacement adjustment degree of freedom is used to change the position of the nonlinear crystal 7 along the axial direction to adjust the beam size and intensity distribution at the nonlinear crystal 7; the rotational adjustment degree of freedom is used to change the angle between the optical axis of the nonlinear crystal 7 and the input beam to adjust the effective nonlinear refractive index of the nonlinear crystal 7 by adjusting the phase mismatch; wherein, the displacement adjustment and rotational adjustment are set to be independent adjustments.

[0032] In operation, the angle between the optical axis of the nonlinear crystal 7 and the input beam is first adjusted by rotating it to operate under non-strict phase-matching conditions. At this point, the ultrafast laser pulse generates a frequency doubling process within the nonlinear crystal 7. A phase mismatch Δk exists between the frequency-doubled light and the fundamental light, where Δk is the difference between the wave vector magnitudes of the fundamental and frequency-doubled light. Under non-strict phase-matching conditions, continuous energy exchange occurs between the frequency-doubled and fundamental light, exciting a second-order cascaded nonlinear effect, producing a cascaded nonlinear refractive index that is inversely proportional to the phase mismatch and proportional to the square of the effective nonlinear coefficient. This cascaded nonlinear refractive index is added to the inherent nonlinear refractive index of the nonlinear crystal 7 material to form the effective nonlinear refractive index. By continuously adjusting the angle between the optical axis and the input beam, the phase mismatch can be continuously changed, thereby continuously changing the cascaded nonlinear refractive index. This allows the cascaded nonlinear refractive index to continuously change between positive and negative values ​​and pass through zero, thus achieving continuous control over the sign and magnitude of the effective nonlinear refractive index. This overcomes the limitation in existing technologies where the nonlinear refractive index is an intrinsic parameter of the material and cannot be adjusted. Simultaneously, by adjusting the displacement degree of freedom to move the nonlinear crystal 7 along the axial direction of the multi-pass cavity, the position of the nonlinear crystal 7 within the cavity is changed, thereby altering the beam size and intensity distribution at the nonlinear crystal 7. Combined with a settable number of round-trip propagation cycles, multi-dimensional synergistic adjustment parameters are obtained, including at least the effective nonlinear refractive index, intensity distribution, and number of round-trip propagation cycles.

[0033] When an ultrafast laser pulse passes through the nonlinear crystal 7 during each round trip, the contribution of the nonlinear phase shift in a single propagation is determined based on the product relationship between the effective nonlinear refractive index and the light intensity distribution. This contribution is accumulated according to the number of round trips, thereby precisely controlling the total nonlinear phase shift to reach the target value. This avoids the problem of limited adjustment parameters and mutual coupling leading to difficulty in accurately matching the nonlinear phase shift. The new frequency components generated in each round trip are successively superimposed within the cavity, gradually broadening the spectrum to the target spectral width, avoiding problems of insufficient spectral broadening or excessive nonlinear phase shift. The multi-channel cavity structure makes the nonlinear effects introduced each time it passes through the crystal relatively mild, which is beneficial for maintaining beam quality.

[0034] A dispersion compensation module is located at the output of the multi-cavity module and is used to compensate for the dispersion of the spectrally broadened ultrafast laser pulse. In this embodiment, the dispersion compensation module includes a first chirped mirror 10 and a second chirped mirror 11. After receiving the beam transmitted by the harmonic beam splitter 9, its dispersion is designed according to the spectral phase generated in the nonlinear crystal 7 to compensate for the chirp introduced by the nonlinear effect, thereby achieving pulse compression. Through reasonable dispersion compensation design, the broadened spectrum can be compressed to near the transform-limited pulse width, ultimately obtaining a few-period pulse. In other embodiments, the dispersion compensation module can also use structures such as grating pairs or prism pairs to achieve the same function.

[0035] Figure 2 This is a schematic diagram of a nonlinear compression method for generating few-period pulses provided in an embodiment of this application, as shown below. Figure 2 As shown, the nonlinear compression method for generating few-period pulses includes the following steps: Step 101: Couple the ultrafast laser pulse into the multi-pass cavity, so that the ultrafast laser pulse propagates back and forth multiple times within the multi-pass cavity.

[0036] In one implementation of this application, the ultrafast laser pulse output from the ultrafast laser source is adjusted by a mode matching module to adjust the beam parameters, including the beam diameter and divergence angle, so that it meets the stable transmission and mode matching conditions of the multi-channel cavity.

[0037] The pattern-matched ultrafast laser pulse is coupled into the multi-pass cavity through a guide mirror. The multi-pass cavity consists of a first concave mirror and a second concave mirror positioned opposite each other, and the ultrafast laser pulse propagates back and forth multiple times between the first and second concave mirrors.

[0038] At least one nonlinear crystal is disposed within the multi-pass cavity. The nonlinear crystal has a non-zero second-order nonlinear polarizability. The nonlinear crystal is mounted on a multidimensional adjustment frame, which has at least a displacement adjustment degree of freedom along the axial direction of the multi-pass cavity and a rotational adjustment degree of freedom about a preset axis.

[0039] In each round trip, the ultrafast laser pulse passes through a nonlinear crystal. Since the nonlinear crystal has a non-zero second-order nonlinear polarizability, the ultrafast laser pulse generates a certain nonlinear phase shift each time it passes through the nonlinear crystal. Through multiple round trips, the nonlinear phase shift is accumulated, providing conditions for subsequent spectral broadening.

[0040] Step 102: Adjust the phase matching state and position of the nonlinear crystal to obtain multidimensional synergistic adjustment parameters.

[0041] Among them, the multidimensional coordinated adjustment parameters include at least the effective nonlinear refractive index, light intensity distribution, and round-trip propagation times.

[0042] In one implementation of this application, the phase-matching state of the nonlinear crystal is adjusted so that the nonlinear crystal operates under non-strict phase-matching conditions. Under non-strict phase-matching conditions, a second-order cascaded nonlinear effect is excited inside the nonlinear crystal through continuous energy exchange between the frequency-doubled light and the fundamental light. Based on the second-order cascaded nonlinear effect, an effective nonlinear refractive index related to the phase mismatch is obtained. The position of the nonlinear crystal within the multi-pass cavity is adjusted to change the beam size of the ultrafast laser pulse within the nonlinear crystal, thereby obtaining the light intensity distribution.

[0043] Specifically, the nonlinear crystal is rotated by a multi-dimensional adjustment frame, adjusting the angle between the optical axis of the nonlinear crystal and the input beam, thus enabling the nonlinear crystal to operate under non-strict phase-matching conditions. It should be noted that for non-zero second-order nonlinear polarizability... In a nonlinear crystal, when the fundamental frequency light propagates outside the strict phase-matching condition, a frequency doubling process still occurs within the crystal. However, due to the phase mismatch, energy exchange continues between the frequency-doubled light and the fundamental frequency light, forming a second-order cascaded nonlinear effect. This process can be equivalent to a third-order nonlinear effect, generating an additional nonlinear phase on the fundamental frequency light. It is important to emphasize that the embodiments in this application utilize the equivalent third-order nonlinear phase generated by the cascaded second-order nonlinearity on the fundamental frequency light, rather than using the acquisition of high-efficiency frequency doubling output as the primary objective.

[0044] Under conditions deviating from strict phase matching, there is a continuous periodic energy exchange between the frequency-doubled light and the fundamental light. That is, the fundamental light is converted into the frequency-doubled light, and the frequency-doubled light returns to the fundamental light through the difference frequency process. This energy cycle process excites an equivalent third-order nonlinear response inside the crystal, namely the second-order cascaded nonlinear effect, thereby producing a cascaded nonlinear refractive index.

[0045] Under conditions deviating from strict phase matching, ultrafast laser pulses undergo a frequency doubling process within a nonlinear crystal, resulting in a phase mismatch between the frequency-doubled light and the fundamental light. ; in, Let be the magnitude of the wave vector of the fundamental frequency light. It represents the wave vector size of the frequency-doubled light.

[0046] The phase mismatch is continuously changed by continuously adjusting the angle between the optical axis of the nonlinear crystal and the input beam. The phase mismatch can also be controlled by adjusting the operating temperature of the nonlinear crystal. Adjustments were made.

[0047] Under non-strict phase-matched conditions, a continuous energy exchange occurs between the frequency-doubled light and the fundamental light, exciting a second-order cascaded nonlinear effect within the nonlinear crystal. The generation of this second-order cascaded nonlinear effect is related to the phase mismatch. Related cascaded nonlinear refractive index Cascaded nonlinear refractive index Phase mismatch Inversely proportional to the effective nonlinear coefficient of the nonlinear crystal. It is proportional to the square of, that is: .

[0048] Cascaded nonlinear refractive index The inherent nonlinear refractive index of nonlinear crystal materials When added together, they form the effective nonlinear refractive index: .

[0049] The phase mismatch is continuously changed by continuously adjusting the angle between the optical axis and the input beam. Continuously changing the cascaded nonlinear refractive index To achieve effective nonlinear refractive index Continuous regulation.

[0050] In this embodiment, the phase mismatch Δk can be adjusted by changing the phase matching state of the crystal, typically by changing the angle between the crystal's optical axis and the input beam or by changing the operating temperature. In this embodiment, it is preferable to adjust the phase mismatch Δk by changing the angle between the optical axis of the nonlinear crystal and the input beam, thereby achieving continuous control of the effective nonlinear refractive index corresponding to the second-order cascaded nonlinear effect.

[0051] By continuously adjusting the angle between the optical axis and the input beam, the phase mismatch Δk is continuously varied, thereby enabling the cascaded nonlinear refractive index n to... 2,casc The effective nonlinear refractive index n is positive when the angle between the input beam and the optical axis of the nonlinear crystal is less than the phase matching angle, and negative when it is less than the phase matching angle. 2,eff It changes continuously with the continuous change of phase mismatch Δk.

[0052] For example, Figure 3 This application provides a diagram showing the relationship between the effective nonlinear refractive index of a BBO nonlinear crystal and the angle between the crystal's optical axis and the input beam. Taking a BBO nonlinear crystal as an example, under a center wavelength of 1030 nm, the inherent nonlinear refractive index n of the material... 2,Kerr Cascaded nonlinear refractive index n 2,casc and effective nonlinear refractive index n 2,eff The relationship between the angle θ between the input beam and the optical axis and the change is shown in the attached figure. Figure 3 As shown. From Figure 3As can be seen, the inherent nonlinear refractive index of the material remains unchanged, while the cascaded nonlinear refractive index shows a continuous trend with the angle, and undergoes a significant change near the strict phase match. For a 1030 nm BBO crystal, effective nonlinear refractive indices of different magnitudes and signs can be obtained within appropriate angle ranges on different sides of the strict phase match angle of 23.3°. Specifically, a larger positive effective nonlinear refractive index can be obtained within an appropriate angle range greater than the strict phase match angle; a negative effective nonlinear refractive index can be obtained within an appropriate angle range on the other side of the strict phase match angle; and the effective nonlinear refractive index can be made close to zero in some transition regions. Therefore, by adjusting the angle between the crystal optical axis and the input beam, this embodiment of the application can not only change the magnitude of the effective nonlinear refractive index, but also change the sign of the effective nonlinear refractive index within a certain angle range, thereby achieving the adjustment of the nonlinear phase of the fundamental frequency light.

[0053] Compared to the traditional nonlinear compression method that uses a fixed third-order nonlinear refractive index material, the embodiments of this application use the phase matching state of the nonlinear crystal as a new adjustment parameter, so that the effective nonlinear refractive index is changed from a fixed parameter determined by the material to a parameter that can be adjusted according to the working conditions, adding a new degree of freedom to the nonlinear phase shift control of ultrafast laser pulses.

[0054] It should be noted that the cascaded nonlinear refractive index described by the above-mentioned second-order cascaded nonlinear effect, and its relationship with the effective nonlinear coefficient of the crystal and the phase mismatch of the frequency doubling process, is mainly applicable to the non-strict phase-matching operating region that satisfies the cascaded second-order nonlinear approximation. Under strict phase-matching conditions, due to the enhanced frequency doubling conversion, the energy conversion between the fundamental and harmonic light, the depletion of the fundamental light, and the group velocity mismatch, the simple cascaded approximation will be limited. Therefore, in actual operation, the effective angle range is determined based on factors such as the input pulse width and peak light intensity, and the crystal phase-matching state is adjusted within this effective operating range.

[0055] In one implementation of this application, during the round-trip propagation of the ultrafast laser pulse within the multi-pass cavity, the nonlinear crystal is moved along the axial direction of the multi-pass cavity by adjusting the displacement degree of freedom of the multidimensional adjustment frame. As the position of the nonlinear crystal changes along the axial direction, the beam cross-sectional size at the position of the nonlinear crystal within the multi-pass cavity changes, thereby altering the beam size of the ultrafast laser pulse within the nonlinear crystal and thus adjusting the light intensity distribution at the nonlinear crystal. Displacement adjustment and rotation adjustment are configured to be independent adjustments.

[0056] In this embodiment, the nonlinear crystal is moved along the axis of the multi-pass cavity to bring it closer to or away from the beam focusing position inside the multi-pass cavity, thereby increasing or decreasing the beam size at the nonlinear crystal and thus adjusting the intensity distribution at that position.

[0057] The round-trip propagation number of ultrafast laser pulses within a multi-pass cavity is a settable parameter. This allows for the acquisition of multidimensional synergistic adjustment parameters, including at least the effective nonlinear refractive index, light intensity distribution, and round-trip propagation number, providing a foundation for subsequent multidimensional synergistic control of the nonlinear phase shift.

[0058] Step 103: Based on multi-dimensional coordinated adjustment parameters, perform multi-dimensional coordinated control on the nonlinear phase shift accumulated when the ultrafast laser pulse propagates in the nonlinear crystal, so as to control the nonlinear phase shift to reach the target nonlinear phase shift.

[0059] In one implementation of this application, the effective nonlinear refractive index is multiplied by the light intensity distribution to obtain the nonlinear refractive index contribution at each position along the propagation direction. The nonlinear refractive index contributions at each position are accumulated along the propagation path of the ultrafast laser pulse within the nonlinear crystal to obtain the cumulative nonlinear response of the ultrafast laser pulse propagating once within the nonlinear crystal. Based on the cumulative nonlinear response and the center wavelength of the ultrafast laser pulse, the single nonlinear phase shift corresponding to a single round-trip propagation is determined. The single nonlinear phase shift is multiplied by the number of round-trip propagations to obtain the total nonlinear phase shift accumulated by the ultrafast laser pulse within the multi-pass cavity, and the total nonlinear phase shift is made to reach the target nonlinear phase shift.

[0060] Specifically, the nonlinear phase shift accumulated during laser propagation in a nonlinear medium is usually represented by the B integral. The magnitude of the B integral is closely related to the degree of spectral broadening and the final pulse compression ratio, and its expression is: ; Where λ is the center wavelength of the laser, I(z) is the intensity distribution of the laser along the propagation direction z, and n² is the nonlinear refractive index of the medium. In a nonlinear crystal, the nonlinear refractive index is the effective nonlinear refractive index n that includes the contribution of the second-order cascaded nonlinear effect. 2,eff .

[0061] The B-integral accumulated during laser propagation is determined by the nonlinear refractive index, light intensity distribution, and nonlinear action length. Therefore, the effective nonlinear refractive index n 2,eff Multiplying the light intensity distribution I(z) by the light intensity distribution I(z) yields the nonlinear refractive index contribution n at each position along the propagation direction. 2,eff ·I(z).

[0062] Based on the above integral expression for B, the contribution of n to the nonlinear refractive index at each location along the propagation path z of the ultrafast laser pulse within the nonlinear crystal is... 2,eff Integrating and summing I(z), we obtain the cumulative nonlinear response ∫n of the ultrafast laser pulse propagating once within the nonlinear crystal. 2,eff ·I(z)dz.

[0063] Multiplying the cumulative nonlinear response by 2π and dividing by the center wavelength λ of the ultrafast laser pulse yields the single nonlinear phase shift corresponding to a single round trip propagation, i.e., the single B integral value.

[0064] The total nonlinear phase shift accumulated by the ultrafast laser pulse in the multi-pass cavity is obtained by multiplying the single nonlinear phase shift by the number of round-trip propagation times in the multi-pass cavity, i.e., the total B integral value.

[0065] In the embodiments of this application, precise control of the total nonlinear phase shift is achieved through the aforementioned multidimensional collaborative control process. Specifically, given a fixed target total B-integral, increasing the absolute value of the effective nonlinear refractive index can appropriately reduce the light intensity at the nonlinear crystal; under the same light intensity conditions, increasing the effective nonlinear refractive index can yield a larger total B-integral.

[0066] The embodiments of this application employ a non-zero second-order nonlinear polarizability. Using a nonlinear crystal as a nonlinear medium, the phase mismatch Δk in the frequency doubling process is changed by adjusting the angle between the crystal's optical axis and the input beam, thus achieving continuous control of the effective nonlinear refractive index. By combining the adjustment of the effective nonlinear refractive index, the effective propagation flux of the multi-channel cavity, and the adjustment of the intracavity light intensity and spatial mode, the nonlinear phase shift of ultrafast laser pulses is expanded from the traditional reliance on the adjustment of a single or a few spatial parameters to a multidimensional control process involving multiple physical parameters. This further modulates the B integral accumulated during laser propagation, providing a theoretical basis for obtaining wider spectral broadening and fewer-period pulse output for ultrafast laser pulses.

[0067] Step 104: Based on the ultrafast laser pulse that achieves the target nonlinear phase shift, spectral broadening is realized in the multi-pass cavity.

[0068] In one implementation of this application, as the ultrafast laser pulse passes through the nonlinear crystal during each round trip, new frequency components are generated based on the nonlinear effect corresponding to the target nonlinear phase shift. These new frequency components generated in each round trip are successively superimposed within the multipass cavity, causing the spectrum of the ultrafast laser pulse to gradually broaden within the multipass cavity. The number of round trips of the ultrafast laser pulse within the multipass cavity is determined according to the desired spectral broadening. When the spectral broadening reaches the target spectral width, the broadened ultrafast laser pulse is exited from the multipass cavity; wherein, the target spectral width is determined by the degree of spectral broadening corresponding to the target nonlinear phase shift.

[0069] Specifically, as the ultrafast laser pulse travels through the nonlinear crystal during each round trip, a new frequency component is generated within the nonlinear crystal based on the nonlinear effect corresponding to the target nonlinear phase shift. This new frequency component is generated by the nonlinear phase shift introduced by the effective nonlinear refractive index. The ultrafast laser pulse travels through the multi-pass cavity multiple times, generating a new frequency component each time it passes through the nonlinear crystal. The new frequency components generated in each round trip are successively superimposed within the multi-pass cavity, causing the spectrum of the ultrafast laser pulse to gradually broaden. The multi-pass cavity structure ensures that the pulse passes through the nonlinear crystal with each round trip, accumulating the nonlinear phase shift successively, thereby achieving the gradual broadening of the spectrum.

[0070] The round-trip propagation number of an ultrafast laser pulse within a multi-pass cavity is determined based on the desired spectral broadening. The B-integral accumulated during laser propagation is adjusted by combining changes in the effective nonlinear refractive index, the effective propagation flux of the multi-pass cavity, and the intracavity intensity and spatial mode. With a fixed total B-integral, increasing the absolute value of the effective nonlinear refractive index can appropriately reduce the light intensity at the nonlinear crystal; under the same light intensity conditions, increasing the effective nonlinear refractive index can yield a larger total B-integral. Therefore, the round-trip propagation number can be synergistically set according to the desired spectral broadening.

[0071] The target spectral width is determined by the degree of spectral broadening corresponding to the target nonlinear phase shift. Since the magnitude of the B integral is closely related to the degree of spectral broadening and the final pulse compression ratio, when the spectral broadening reaches the target spectral width determined by the target nonlinear phase shift, the broadened ultrafast laser pulse is exported from the multi-pass cavity via the export mirror and input to the dispersion compensation module.

[0072] Step 105: Perform dispersion compensation on the spectrally broadened ultrafast laser pulse to obtain a few-period pulse.

[0073] In one implementation of this application, a spectrally broadened ultrafast laser pulse is input to a dispersion compensation module. The dispersion compensation module compensates for the chirp introduced by nonlinear effects in the spectrally broadened ultrafast laser pulse, thereby compressing the time-domain width of the spectrally broadened ultrafast laser pulse and obtaining a few-period pulse.

[0074] Specifically, after the spectral broadening reaches the target spectral width, the spectrally broadened ultrafast laser pulse is exported from the multi-pass cavity via the export mirror, and then injected into the dispersion compensation module via the harmonic beam splitter.

[0075] A dispersion compensation module is located at the output of the multi-cavity module to compensate for the spectral phase generated during nonlinear spectral broadening, thereby achieving pulse compression. In this embodiment, the dispersion compensation module includes a first chirped mirror and a second chirped mirror, the dispersion of which is designed based on the spectral phase generated in the nonlinear crystal. This compensates for the spectral phase introduced during spectral broadening, achieving pulse compression. Through a reasonable dispersion compensation design, the broadened spectrum can be compressed to near the transform-limit pulse width, achieving a short-period pulse output. In other embodiments, the dispersion compensation module can also employ structures such as grating pairs or prism pairs to achieve the same function.

[0076] The various embodiments in this application are described in a progressive manner. Similar or identical parts between embodiments can be referred to mutually. Each embodiment focuses on describing the differences from other embodiments. In particular, the embodiments for devices and non-volatile computer storage media are basically similar to the method embodiments, so the descriptions are relatively simple; relevant parts can be referred to the descriptions of the method embodiments.

[0077] The above descriptions are merely embodiments of this application and are not intended to limit the scope of this application. For those skilled in the art, various modifications and variations can be made to the embodiments of this application. These modifications or substitutions do not cause the essence of the corresponding technical solutions to depart from the spirit and scope of the technical solutions in the embodiments of this application.

Claims

1. A nonlinear compression method for generating few-period pulses, characterized in that, The method includes: An ultrafast laser pulse is coupled into a multi-pass cavity, causing the ultrafast laser pulse to propagate back and forth multiple times within the multi-pass cavity; wherein, a nonlinear crystal with a non-zero second-order nonlinear polarizability is disposed within the multi-pass cavity; The phase-matching state and position of the nonlinear crystal are adjusted to obtain multidimensional synergistic adjustment parameters; wherein, the multidimensional synergistic adjustment parameters include at least the effective nonlinear refractive index, light intensity distribution, and round-trip propagation number; Based on the aforementioned multidimensional coordinated adjustment parameters, the nonlinear phase shift accumulated by the ultrafast laser pulse during propagation in the nonlinear crystal is controlled in a multidimensional coordinated manner to control the nonlinear phase shift to reach the target nonlinear phase shift. Spectral broadening is achieved within the multi-pass cavity based on the ultrafast laser pulse that achieves the target nonlinear phase shift. Dispersion compensation is performed on ultrafast laser pulses with broadened spectra to obtain pulses with fewer periods.

2. The nonlinear compression method for generating few-period pulses according to claim 1, characterized in that, The adjustment of the phase-matching state and position of the nonlinear crystal to obtain multi-dimensional synergistic adjustment parameters specifically includes: Adjust the phase matching state of the nonlinear crystal so that the nonlinear crystal operates under non-strict phase matching conditions; Under the non-strict phase-matching conditions, a second-order cascaded nonlinear effect is excited inside the nonlinear crystal through continuous energy exchange between the frequency-doubled light and the fundamental light. Based on the aforementioned second-order cascaded nonlinear effect, the effective nonlinear refractive index related to the phase mismatch is obtained; The position of the nonlinear crystal within the multi-pass cavity is adjusted to change the beam size of the ultrafast laser pulse within the nonlinear crystal, thereby obtaining the light intensity distribution.

3. The nonlinear compression method for generating few-period pulses according to claim 2, characterized in that, The method of obtaining the effective nonlinear refractive index related to the phase mismatch based on the second-order cascaded nonlinear effect specifically includes: The phase mismatch is determined based on the difference between the fundamental frequency wave vector and the second-harmonic frequency wave vector. Determine the inherent nonlinear refractive index of nonlinear crystal materials; Based on the aforementioned second-order cascaded nonlinear effect, a cascaded nonlinear refractive index is generated; wherein, the cascaded nonlinear refractive index is proportional to the square of the effective nonlinear coefficient of the nonlinear crystal; The effective nonlinear refractive index is obtained by superimposing the inherent nonlinear refractive index of the material with the cascaded nonlinear refractive index.

4. The nonlinear compression method for generating few-period pulses according to claim 3, characterized in that, Obtaining the effective nonlinear refractive index related to the phase mismatch specifically includes: By adjusting the phase matching state of the nonlinear crystal, the phase mismatch amount is continuously changed, so that the cascade nonlinear refractive index takes a positive value on the side where the angle between the input beam of the nonlinear crystal and the optical axis is smaller than the phase matching angle, and a negative value on the other side. The effective nonlinear refractive index changes continuously with the continuous change of the phase mismatch.

5. The nonlinear compression method for generating few-period pulses according to claim 1, characterized in that, The method of performing multidimensional coordinated control on the nonlinear phase shift accumulated by the ultrafast laser pulse during propagation in the nonlinear crystal based on the multidimensional coordinated adjustment parameters, so as to control the nonlinear phase shift to reach the target nonlinear phase shift, specifically includes: The effective nonlinear refractive index is multiplied by the light intensity distribution to obtain the nonlinear refractive index contribution at each position along the propagation direction. Along the propagation path of the ultrafast laser pulse in the nonlinear crystal, the contribution of the nonlinear refractive index at each position is accumulated to obtain the cumulative nonlinear response of the ultrafast laser pulse propagating once in the nonlinear crystal. Based on the cumulative nonlinear response and the center wavelength of the ultrafast laser pulse, the single nonlinear phase shift corresponding to a single round-trip propagation is determined; The single nonlinear phase shift is multiplied by the number of round-trip propagations to obtain the total nonlinear phase shift accumulated by the ultrafast laser pulse in the multi-pass cavity, and the total nonlinear phase shift is made to reach the target nonlinear phase shift.

6. The nonlinear compression method for generating few-period pulses according to claim 1, characterized in that, The ultrafast laser pulse, based on achieving the target nonlinear phase shift, achieves spectral broadening within the multi-pass cavity, specifically including: When the ultrafast laser pulse passes through the nonlinear crystal during each round trip, it generates a new frequency component based on the nonlinear effect corresponding to the target nonlinear phase shift. The new frequency components generated during each round trip propagation are successively superimposed in the multi-pass cavity, so that the spectrum of the ultrafast laser pulse gradually broadens in the multi-pass cavity; The number of round-trip propagations of the ultrafast laser pulse within the multi-pass cavity is determined according to the required spectral broadening. When the spectral broadening reaches the target spectral width, the broadened ultrafast laser pulse is exported from the multi-pass cavity. The target spectral width is determined by the degree of spectral broadening corresponding to the target nonlinear phase shift.

7. The nonlinear compression method for generating few-period pulses according to claim 1, characterized in that, The dispersion compensation of the spectrally broadened ultrafast laser pulse to obtain a few-period pulse specifically includes: The spectrally broadened ultrafast laser pulse is input into the dispersion compensation module; The dispersion compensation module compensates for the chirp introduced by nonlinear effects in the spectrally broadened ultrafast laser pulse, thereby compressing the time domain width of the spectrally broadened ultrafast laser pulse and obtaining a short-period pulse.

8. A nonlinear compression device for generating few-period pulses, characterized in that, The device includes: An ultrafast laser source, used to output ultrafast laser pulses; A mode matching module is located at the output end of the ultrafast laser source and is used to adjust the beam parameters of the ultrafast laser pulse so that the ultrafast laser pulse meets the stable transmission and mode matching conditions of the multi-cavity laser. A multi-cavity module, located at the output of the mode matching module, includes a first concave mirror and a second concave mirror arranged opposite to each other, and at least one nonlinear crystal disposed within the cavity; the nonlinear crystal has a non-zero second-order nonlinear polarizability, used to excite a second-order entrained nonlinear effect under non-strict phase matching conditions, so that the ultrafast laser pulse accumulates a nonlinear phase shift to achieve spectral broadening. A dispersion compensation module is located at the output end of the multi-cavity module and is used to perform dispersion compensation on the ultrafast laser pulse after spectral broadening to obtain a few-period pulse.

9. A nonlinear compression device for generating few-period pulses according to claim 8, characterized in that, The multi-cavity module also includes an inlet mirror and an outlet mirror: The inlet mirror is used to couple the ultrafast laser pulse into the multi-pass cavity, so that the ultrafast laser pulse propagates back and forth between the first concave mirror and the second concave mirror, and passes through the nonlinear crystal in each round-trip propagation pass, so as to achieve spectral broadening by accumulating nonlinear phase shifts. The export mirror is used to export the spectrally broadened ultrafast laser pulse to the dispersion compensation module.

10. A nonlinear compression device for generating few-period pulses according to claim 9, characterized in that, The nonlinear crystal is mounted on a multidimensional adjustment frame: The multidimensional adjustment frame has at least a displacement adjustment degree of freedom along the axial direction of the multi-cavity and a rotation adjustment degree of freedom around a preset axis; The displacement adjustment degree of freedom is used to change the position of the nonlinear crystal along the axial direction, so as to adjust the beam size and intensity distribution at the nonlinear crystal; The rotational adjustment degree of freedom is used to change the angle between the optical axis of the nonlinear crystal and the input beam, so as to adjust the effective nonlinear refractive index of the nonlinear crystal by adjusting the phase mismatch. Displacement adjustment and rotation adjustment are set to independent adjustment.

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