Ultrashort pulse generation device and method based on concave spherical lens and multi-pass cavity

Abstract

The present invention discloses an ultrashort pulse generation device and method based on a concave spherical lens and a multi-pass cavity, which are used to obtain ultrashort pulse lasers with a pulse width of less than 40 femtoseconds and improve the stability of the pulse laser beam mode. The device of the present invention includes a femtosecond fiber laser, a pre-chirp control module, a beam mode matching module, a nonlinear spectrum broadening module, and a dispersion compensation module. The nonlinear spectrum broadening module broadens the laser pulse spectrum by passing the light beam through a pair of self-phase modulation media multiple times in the multi-pass cavity. The self-phase modulation medium reduces the nonlinear mode matching requirements. The method of the present invention is provided with a pre-chirp control module, and the self-phase modulation medium is used to adjust the nonlinear phase shift in the multi-pass cavity and reduce the self-focusing effect of the pulsed laser beam. The present invention has high generation efficiency and is applicable to input pulses with peak powers within a set range.

Description

Technical Field

[0001] The present invention relates to the field of ultrafast laser technology, and in particular to an ultrashort pulse generating device based on a concave spherical lens and a multi-pass cavity and a generating method thereof. Background Art

[0002] Ultrashort pulse lasers have broad application prospects in materials processing, medical technology, and ultrafast science. Important ultrafast scientific research results are highly dependent on advanced ultrashort pulse laser technology, which has also promoted the development of ultrafast laser technology. Traditional titanium sapphire lasers have been widely used in the past three decades due to their advantages of high peak power and short pulse width. In recent years, in applications related to extreme ultraviolet light and photoelectron spectroscopy systems, titanium sapphire lasers are gradually being replaced by high repetition rate, high average power ytterbium-doped fiber femtosecond lasers. However, ytterbium-doped fiber femtosecond lasers have narrow bandwidth and correspondingly long pulse width, which brings challenges to post-pulse compression technology.

[0003] Existing femtosecond lasers output pulse widths ranging from forty to a hundred femtoseconds. To achieve even shorter ultrashort pulses, the laser pulses are then fed into a separate post-pulse compression device. Two basic techniques commonly employed in pulse compression devices are to use either a gas-filled hollow-core fiber or a solid-state fused silica slab to broaden the spectrum using the nonlinear optical Kerr effect and then perform dispersion compensation. In contrast, the solid-state fused silica slab approach can be applied to lasers with average powers exceeding kilowatts and requires less stable input laser beams. However, at high light intensities, solid-state fused silica slabs exhibit cone emission, which degrades the spatial mode of the beam and reduces output efficiency. Furthermore, the laser beam must pass through the fused silica slab multiple times, and this continuous spectral broadening process can also cause temporal pulse splitting. Furthermore, existing multi-pass cavities contain a flat plate of material, which can only be used for a specific pulse power. Mode matching only addresses the intrinsic modes of the multi-pass cavity and fails to account for the nonlinearities of the cavity material. Summary of the Invention

[0004] In order to solve the problems existing in the above-mentioned prior art, the present invention proposes an ultrashort pulse generating device and a generating method based on a concave spherical lens and a multi-pass cavity, so as to reduce the pulse width of the ultrashort pulse laser, improve the stability of the pulse laser beam mode, and upgrade the high-power, long-pulse femtosecond laser to an ultrashort pulse laser with a pulse time width of less than forty femtoseconds.

[0005] One object of the present invention is to provide an ultrashort pulse generating device based on a concave spherical lens and a multi-pass cavity.

[0006] The ultrashort pulse generating device based on a concave spherical lens and a multi-pass cavity of the present invention comprises: a femtosecond fiber laser, a pre-chirp control module, a beam mode matching module, a nonlinear spectrum broadening module and a dispersion compensation module; wherein the nonlinear spectrum broadening module comprises a multi-pass cavity coupling mirror, two multi-pass cavity mirrors and a pair of self-phase modulation media, the multi-pass cavity coupling mirror is located in front of the multi-pass cavity mirror, the concave surfaces of the two multi-pass cavity mirrors face each other and are separated by a set distance to form a multi-pass cavity, the pair of self-phase modulation media adopts a solid concave spherical nonlinear material, the center of the self-phase modulation medium is located on the central axis of the multi-pass cavity, the shape of the self-phase modulation medium is a concave spherical surface, the thickness is thinnest at its center and the thickness increases as it moves away from its center, and the material adopts a transparent nonlinear material with an optical Kerr effect, the pair of self-phase modulation media are symmetrically placed on both sides of the center of the multi-pass cavity and symmetrically about the center of the multi-pass cavity, and are respectively placed on a translation stage, and the translation stage can drive the pair of self-phase modulation media symmetrically away from or close to the center;

[0007] The femtosecond laser emits a laser pulse beam to the pre-chirp control module, and the laser pulse beam is a collimated beam; the pre-chirp control module makes the polarization direction of the laser pulse beam located in the horizontal plane, and introduces negative dispersion into the laser pulse beam, so that the spectrum broadening effect after passing through the self-phase modulation medium is better, forming a pre-chirped pulse beam input to the beam mode matching module; the beam mode matching module matches the waist of the pre-chirped pulse beam to the intrinsic waist mode of the multi-pass cavity under nonlinear conditions, and makes the position of the transformed beam waist located at the center of the multi-pass cavity; the pre-chirped pulse beam is introduced into the multi-pass cavity through the multi-pass cavity coupling mirror; the multi-pass cavity mirror forms a closed-loop multi-pass optical path under stable cavity conditions; a pair of self-phase modulation media act as nonlinear media to broaden the laser pulse spectrum under the self-phase modulation of the optical Kerr effect, and the shape of the concave spherical surface can offset the self-focusing effect of the optical Kerr effect; in the multi-pass cavity, the spot size of the beam gradually increases from the center to the outside, and each optical path passes through the self-phase modulation medium The optical path length is related to the position of the self-phase modulation medium through which the optical path passes, and the position of each optical path through the self-phase modulation medium is related to the position of the self-phase modulation medium. As the self-phase modulation medium moves away from the center of the multi-pass cavity, the optical path length of each optical path through the self-phase modulation medium increases. The increase in the optical path length through the self-phase modulation medium increases the nonlinear phase shift, and the nonlinear phase shift is related to the size of the output beam spatial mode. Therefore, by adjusting the relative positions of a pair of self-phase modulation media, symmetrically approaching or moving away from the center, the position where the light beam enters the self-phase modulation medium is different, thereby adjusting the optical path length of the light beam through the self-phase modulation medium, thereby changing the nonlinear phase shift in the multi-pass cavity, so that the spatial mode of the laser pulse beam output from the multi-pass cavity is stable relative to the set incident pulse energy; the light beam is collimated by the dispersion compensation module, the polarization direction of the beam is adjusted to be in the horizontal plane, and the laser pulse beam is dispersion compensated to obtain a Fourier-limited short pulse within the pulse spectrum width range.

[0008] The pre-chirp control module adopts an acousto-optic programmable dispersion filter; or, the pre-chirp control module includes in sequence: a first half-wave plate, a pulse chirp control device and a retroreflective mirror, the pulse chirp control device is placed on a translation stage, and the pulse chirp control device adopts a grating pair or a dispersion prism pair.

[0009] The beam mode matching module includes: a lens group, a delay line and a reflector; the lens group includes a concave lens and a convex lens, and the mode matching is the intrinsic beam waist mode of the multi-pass cavity under nonlinear conditions; the optical path is changed by the reflector to connect the laser pulse beam of the lens group and the delay line; the delay line includes a right-angle surface reflector, a translation stage and a right-angle retroreflector or two reflectors placed perpendicular to each other, the right-angle retroreflector or the two reflectors placed perpendicular to each other are placed on the translation stage, and the delay line is used to adjust the optical path so that the position of the transformed beam waist is located at the center of the multi-pass cavity.

[0010] The front surface of the multi-pass cavity coupling mirror in the nonlinear spectral broadening module is coated with a high-reflectivity coating, achieving a reflectivity exceeding 99%. The multi-pass cavity mirrors consist of two concave spherical reflectors, with their reflective surfaces facing each other. They are positioned at a distance determined by the curvature of their concave spherical surfaces to form a stable cavity. The multi-pass cavity mirrors exhibit high reflectivity across a wide spectrum, exceeding 99% from 900 to 1200 nanometers. The self-phase modulation medium is made of transparent crystal or glass.

[0011] The dispersion compensation module includes a half-wave plate and two chirped mirrors. The two chirped mirrors are placed in parallel on a rotating table, which drives the chirped mirrors to rotate to adjust the angle at which the incident light enters the chirped mirrors. In the dispersion compensation module, a lens is used to collimate the light beam. The collimated light beam is incident on the half-wave plate. The half-wave plate is adjusted so that the polarization direction of the light beam is in the horizontal plane. The light beam emitted from the half-wave plate enters a pair of chirped mirrors at a set angle. The chirped mirrors are used to reflect the laser pulse beam multiple times to compensate for dispersion and obtain Fourier-limited short pulses within the pulse spectrum range.

[0012] Another object of the present invention is to provide a method for generating ultrashort pulses based on a concave spherical lens and a multi-pass cavity.

[0013] The ultrashort pulse generation method based on a concave spherical lens and a multi-pass cavity of the present invention comprises the following steps:

[0014] 1) The femtosecond laser emits a laser pulse beam, which is a collimated beam;

[0015] 2) Obtaining a pre-chirped pulse beam:

[0016] The collimated beam emitted by the femtosecond laser is introduced into a pre-chirp control module. The pre-chirp control module makes the polarization direction of the laser pulse lie in the horizontal plane and introduces negative dispersion into the laser pulse beam, thereby achieving better spectral broadening effect after passing through the self-phase modulation medium, thus forming a pre-chirped pulse beam.

[0017] 3) Realize mode matching of laser pulse beam to multi-pass cavity:

[0018] The pre-chirped pulse beam is input to a beam mode matching module, which matches the pre-chirped pulse beam waist to the intrinsic beam waist mode of the multi-pass cavity under nonlinear conditions, and makes the position of the transformed beam waist located at the center of the multi-pass cavity;

[0019] 4) Obtain nonlinear spectral broadening of laser pulses:

[0020] The light beam is introduced into the multi-pass cavity through the multi-pass cavity coupling mirror, and the multi-pass cavity mirror forms a closed-loop multi-pass optical path under stable cavity conditions; a pair of self-phase modulation media acts as a nonlinear medium to broaden the laser pulse spectrum under the self-phase modulation of the optical Kerr effect, and the shape of the concave spherical body can offset the self-focusing effect of the optical Kerr effect; in the multi-pass cavity, the spot size of the light beam gradually increases from the center to the outside, and the optical path of each light path passing through the self-phase modulation medium is related to the position of the self-phase modulation medium, and the position of each light path passing through the self-phase modulation medium is related to the position of the self-phase modulation medium. As the distance from the center of the multi-pass cavity increases, the optical path of each optical path through the self-phase modulation medium increases. The increase in the optical path through the self-phase modulation medium increases the nonlinear phase shift, and the nonlinear phase shift is related to the size of the output beam spatial mode. Therefore, by adjusting the relative positions of a pair of self-phase modulation media, symmetrically moving closer to or farther away from the center, the position where the light beam enters the self-phase modulation medium is changed, thereby adjusting the optical path of the light beam through the self-phase modulation medium, thereby changing the nonlinear phase shift in the multi-pass cavity, so that the laser pulse beam has a stable spatial mode relative to the set incident pulse energy.

[0021] 5) Dispersion compensation of laser pulses:

[0022] The laser pulse beam output by the nonlinear spectrum broadening module is incident on the dispersion compensation module. The dispersion compensation module collimates the beam, adjusts the polarization direction of the beam to be in the horizontal plane, and performs dispersion compensation on the laser pulse beam to obtain a Fourier-limited short pulse within the pulse spectrum range.

[0023] Advantages of the present invention:

[0024] (1) High generation efficiency and stable spatial mode of the laser pulse beam: There is a flat region between the spatial mode of the laser pulse beam and the nonlinear phase in the multi-pass cavity. According to the present invention, the nonlinear phase in the multi-pass cavity can be adjusted by moving the relative positions of a pair of self-phase modulation media, so that the device operates in a stable mode. The shape and thickness of the pair of self-phase modulation media can also avoid cone emission of the laser beam, so that the laser pulse energy remains within the target mode, and a high ultrashort pulse generation efficiency is obtained.

[0025] (2) Applicable to femtosecond laser pulse beams with different pulse peak powers: According to the present invention, since a pair of self-phase modulation media can be moved and adjusted, the present invention can optimize and debug femtosecond lasers with different pulse energies;

[0026] (3) The difficulty of mode matching between the laser pulse beam and the multi-pass cavity is reduced: the device of the present invention uses a pair of self-phase modulation media in the shape of concave spheres, which are equivalent to concave lenses and have a diverging beam effect, which can offset the focusing effect of the laser beam when it passes through the nonlinear medium. It is not necessary to perform nonlinear mode matching processing on the multi-pass cavity and can also reduce the peak power fluctuation of the laser pulse beam in the nonlinear medium;

[0027] (4) Enhance the nonlinear spectrum broadening effect and improve the laser pulse compression ratio: The present invention sets a pre-chirp module in the nonlinear spectrum broadening module of the multi-pass cavity to introduce negative dispersion, which is beneficial to prevent the laser pulse beam from splitting in time and obtain a wider spectrum. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 Schematic diagram of an embodiment of an ultrashort pulse generating device based on a concave spherical lens and a multi-pass cavity according to the present invention;

[0029] Figure 2 This is a spectrum diagram of the laser pulse before and after it is incident on the nonlinear spectrum broadening module according to an embodiment of the ultrashort pulse generation device based on a concave spherical lens and a multi-pass cavity of the present invention. DETAILED DESCRIPTION

[0030] The present invention will be further described below through specific embodiments in conjunction with the accompanying drawings.

[0031] like Figure 1 As shown, the ultrashort pulse generating device based on the concave spherical lens and the multi-pass cavity of this embodiment includes: a femtosecond fiber laser 1, a pre-chirp control module 2, a beam mode matching module 3, a nonlinear spectrum broadening module 4 and a dispersion compensation module 5; wherein,

[0032] The pre-chirp control module 2 includes: a first half-wave plate 6, a first reflector 7, a first grating 8, a second grating 9 and a retroreflector 10. The first grating 8 and the second grating 9 are placed in parallel, and the second grating 9 is mounted on a first translation stage. The first translation stage adopts a precision adjustable translation stage.

[0033] The beam pattern matching module 3 includes a lens group, a delay line and a second reflector 13; the lens group includes a concave lens 11 and a convex lens 12; the beam is reflected to the delay line by the second reflector 13; the delay line includes a right-angle surface reflector 14, a translation stage and a right-angle retroreflector 15, and the right-angle retroreflector 15 is placed on the second translation stage;

[0034] The nonlinear spectrum broadening module 4 includes a multi-pass cavity coupling mirror 17, first and second multi-pass cavity mirrors 20 and 21, and first and second self-phase modulation media 18 and 19. The multi-pass cavity coupling mirror is located in front of the multi-pass cavity mirror. The concave surfaces of the two multi-pass cavity mirrors face each other and are separated by a set distance to form a multi-pass cavity. A pair of self-phase modulation media uses a solid concave spherical nonlinear material. The center of the self-phase modulation medium is located on the central axis of the multi-pass cavity. The thickness of the self-phase modulation medium is thinnest at its center and increases as it moves away from its center. A transparent nonlinear material with an optical Kerr effect is used. The first and second self-phase modulation media 18 and 19 are symmetrically placed on both sides of the center of the multi-pass cavity and are symmetrical about the center of the multi-pass cavity. They are respectively placed on a third translation stage. The translation stage can drive the pair of self-phase modulation media symmetrically away from or close to the center.

[0035] The dispersion compensation module 5 includes a collimating lens 23, a second half-wave plate 24, and first and second chirped mirrors 25 and 26. The first and second chirped mirrors 25 and 26 are placed parallel to each other on a rotating table, and the chirped mirrors are rotated by the rotating table to adjust the angle of the incident light entering the chirped mirrors.

[0036] The femtosecond laser 1 emits a laser pulse beam to the pre-chirp control module 2, and the laser pulse beam is a collimated beam; the laser pulse beam passes through the first half-wave plate 6, so that the polarization direction of the laser pulse beam is located in the horizontal plane, so that the spectrum broadening effect after passing through the self-phase modulation medium is better, and is incident on the first grating 8 and the second grating 9 above the first reflector 7. The position of the second grating 9 is adjusted by the first translation stage to control the spacing of the grating pair, introduce negative dispersion, change the waveform of the pre-chirped laser pulse, and form a pre-chirped pulse beam; after passing through the first grating 8 and the second grating 9, the laser pulse beam is lowered after passing through the retroreflector 10, and returns below the original beam and passes through the first reflector again. A grating 8 and a second grating 9 are reflected by a first reflector 7 and input to a beam mode matching module 3; a concave lens 11 and a convex lens 12 match the beam waist size mode to the intrinsic beam waist mode of the multi-pass cavity under nonlinear conditions, and reflect it to the delay line through a second reflector 13. The delay line adjusts the optical path so that the position of the transformed beam waist is located at the center of the multi-pass cavity; the pre-chirped pulse beam is reflected by a third reflector 16, and then introduced into the multi-pass cavity of the nonlinear spectrum broadening module 4 through a multi-pass cavity coupling mirror 17; the distance between the first and second multi-pass cavity mirrors 20 and 21 meets the stable cavity condition, and a closed loop multi-pass optical path is formed under the stable cavity condition; a pair of self-phase modulation media are used as The nonlinear medium broadens the laser pulse spectrum under the self-phase modulation of the optical Kerr effect, and the shape of the concave spherical body can offset the self-focusing effect of the optical Kerr effect. The laser pulse beam is reflected back and forth between the mirrors of the multi-pass cavity, 16 times back and forth, and passes through the first and second self-phase modulation media 18 and 19 32 times. The laser pulse beam obtains spectrum broadening when passing through the first and second self-phase modulation media 18 and 19 multiple times; in the multi-pass cavity, the spot size of the light beam gradually increases from the center to the outside, and the optical path of each optical path passing through the self-phase modulation medium is related to the position of the self-phase modulation medium, and the position of each optical path passing through the self-phase modulation medium is related to the position of the self-phase modulation medium. The position of the self-phase modulation medium is related to the distance from the center of the multi-pass cavity. The optical path of each optical path passing through the self-phase modulation medium increases. The increase in the optical path passing through the self-phase modulation medium increases the nonlinear phase shift, and the nonlinear phase shift is related to the size of the output light beam spatial mode. Therefore, by adjusting the relative positions of the first and second self-phase modulation media 18 and 19, and symmetrically approaching or moving away from the center, the position where the light beam enters the self-phase modulation medium is different, thereby adjusting the optical path of the light beam passing through the self-phase modulation medium, thereby changing the nonlinear phase shift in the multi-pass cavity, so that the laser pulse beam is stable relative to the set incident pulse energy.The light beam is reflected by the fourth reflector 22 to the dispersion compensation module 5. It is collimated by the collimating lens 23 and then incident on the second half-wave plate 24. The second half-wave plate 24 is adjusted to keep the polarization direction of the light beam in the horizontal plane. The light beam emerging from the second half-wave plate enters the first and second chirped mirrors 25 and 26 at a set angle. The chirped mirrors are used to reflect the laser pulse beam multiple times to compensate for dispersion, producing a Fourier-limited short pulse within the pulse's spectral range.

[0037] In this embodiment, the laser pulse parameters are pulse width 300fs, pulse energy 300μJ, central wavelength 1030nm, and repetition frequency 333KHz; the grating selected is a near-infrared grating with 300 lines and a blaze angle of 24.8 degrees. The mode matching lens group is selected as a -500mm concave lens and a 400mm convex lens, and the distance between them is adjusted by a translation stage. The diameter of the multi-pass cavity mirror is 50.8mm, the radius of curvature is 300mm, and the spacing between the multi-pass cavity mirrors is 480mm. The self-phase modulation medium of the concave spherical material has a diameter of 30mm and a radius of curvature of -400mm. It is made of fused quartz material and coated with a 1030nm anti-reflection film. The nonlinear spectral broadening module generates Figure 2 Spectra shown.

[0038] Finally, it should be noted that the purpose of disclosing the embodiments is to facilitate a further understanding of the present invention. However, those skilled in the art will appreciate that various substitutions and modifications are possible without departing from the spirit and scope of the present invention and the appended claims. Therefore, the present invention should not be limited to the contents disclosed in the embodiments; the scope of protection claimed by the present invention shall be determined by the scope defined in the claims.

Claims

1. An ultrashort pulse generating device based on a concave spherical lens and a multi-pass cavity, characterized in that: The ultrashort pulse generating device includes: a femtosecond fiber laser, a pre-chirp control module, a beam mode matching module, a nonlinear spectrum broadening module and a dispersion compensation module; wherein the nonlinear spectrum broadening module includes a multi-pass cavity coupling mirror, two multi-pass cavity mirrors and a pair of self-phase modulation media, the multi-pass cavity coupling mirror is located in front of the multi-pass cavity mirror, the concave surfaces of the two multi-pass cavity mirrors face each other and are separated by a set distance to form a multi-pass cavity, the pair of self-phase modulation media adopts a solid concave spherical nonlinear material, the center of the self-phase modulation medium is located on the central axis of the multi-pass cavity, the shape of the self-phase modulation medium is a concave spherical surface, the thickness is thinnest at its center and the thickness increases as it is farther away from its center, and the material adopts a transparent nonlinear material with an optical Kerr effect, the pair of self-phase modulation media are symmetrically placed on both sides of the center of the multi-pass cavity, and are symmetrical about the center of the multi-pass cavity, and are respectively placed on a translation stage, and the translation stage can drive the pair of self-phase modulation media symmetrically away from or close to the center; The femtosecond laser emits a laser pulse beam to the pre-chirp control module, and the laser pulse beam is a collimated beam; the pre-chirp control module makes the polarization direction of the laser pulse beam located in the horizontal plane, and introduces negative dispersion into the laser pulse beam, so that the spectrum broadening effect after passing through the self-phase modulation medium is better, forming a pre-chirped pulse beam input to the beam mode matching module; the beam mode matching module matches the waist of the pre-chirped pulse beam to the intrinsic waist mode of the multi-pass cavity under nonlinear conditions, and makes the position of the transformed beam waist located at the center of the multi-pass cavity; the pre-chirped pulse beam is introduced into the multi-pass cavity through the multi-pass cavity coupling mirror; the multi-pass cavity mirror forms a closed-loop multi-pass optical path under stable cavity conditions; a pair of self-phase modulation media act as nonlinear media to broaden the laser pulse spectrum under the self-phase modulation of the optical Kerr effect, and the shape of the concave spherical surface can offset the self-focusing effect of the optical Kerr effect; in the multi-pass cavity, the spot size of the beam gradually increases from the center to the outside, and each optical path passes through the self-phase modulation medium The optical path length is related to the position of the self-phase modulation medium through which the optical path passes, and the position of each optical path through the self-phase modulation medium is related to the position of the self-phase modulation medium. As the self-phase modulation medium moves away from the center of the multi-pass cavity, the optical path length of each optical path through the self-phase modulation medium increases. The increase in the optical path length through the self-phase modulation medium increases the nonlinear phase shift, and the nonlinear phase shift is related to the size of the output beam spatial mode. Therefore, by adjusting the relative positions of a pair of self-phase modulation media, symmetrically approaching or moving away from the center, the position where the light beam enters the self-phase modulation medium is different, thereby adjusting the optical path length of the light beam through the self-phase modulation medium, thereby changing the nonlinear phase shift in the multi-pass cavity, so that the spatial mode of the laser pulse beam output from the multi-pass cavity is stable relative to the set incident pulse energy; the light beam is collimated by the dispersion compensation module, the polarization direction of the beam is adjusted to be in the horizontal plane, and the laser pulse beam is dispersion compensated to obtain a Fourier-limited short pulse within the pulse spectrum width range.

2. The ultrashort pulse generator according to claim 1, wherein: The pre-chirp control module adopts an acousto-optic programmable dispersion filter; Alternatively, the pre-chirp control module includes in sequence: a first half-wave plate, a pulse chirp control device and a retroreflective mirror, the pulse chirp control device is placed on a translation stage, and the pulse chirp control device adopts a grating pair or a dispersion prism pair.

3. The ultrashort pulse generator according to claim 1, wherein: The beam mode matching module includes: a lens group, a delay line and a reflector; the lens group includes a concave lens and a convex lens, and the mode matching is the intrinsic beam waist mode of the multi-pass cavity under nonlinear conditions; the optical path is changed by the reflector to connect the laser pulse beam of the lens group and the delay line; the delay line includes a right-angle surface reflector, a translation stage and a right-angle retroreflector or two reflectors placed perpendicular to each other, the right-angle retroreflector or the two reflectors placed perpendicular to each other are placed on the translation stage, and the delay line is used to adjust the optical path so that the position of the transformed beam waist is located at the center of the multi-pass cavity.

4. The ultrashort pulse generator according to claim 1, wherein: The front surface of the multi-pass cavity coupling mirror of the nonlinear spectrum broadening module is plated with a high reflection film, and the reflectivity is greater than 99%.

5. The ultrashort pulse generator according to claim 1, wherein: The multi-pass cavity mirrors are two concave spherical reflectors, the reflective surfaces of the two concave spherical reflectors face each other, and are placed at a distance set according to the concave spherical curvature of the multi-pass cavity mirrors and the conditions for forming a stable cavity to form a stable cavity.

6. The ultrashort pulse generator according to claim 1, wherein: The multi-pass cavity mirror has a wide spectrum and high reflectivity, the spectrum range is from 900nm to 1200nm, and the reflectivity is greater than 99%.

7. The ultrashort pulse generator according to claim 1, wherein: The dispersion compensation module includes a half-wave plate and two chirped mirrors; the two chirped mirrors are placed in parallel on a rotating table, and the chirped mirrors are driven to rotate by the rotating table to adjust the angle at which the incident light enters the chirped mirrors; in the dispersion compensation module, a lens is used to collimate the light beam; the collimated light beam is incident on the half-wave plate; the half-wave plate is adjusted so that the polarization direction of the light beam is located in the horizontal plane; the light beam emitted from the half-wave plate enters a pair of chirped mirrors at a set angle; the chirped mirrors are used to reflect the laser pulse beam multiple times to compensate for dispersion, thereby obtaining a Fourier-limited short pulse within the pulse spectrum range.

8. A method for generating an ultrashort pulse based on a concave spherical lens and a multi-pass cavity as claimed in claim 1, characterized in that: The production method comprises the following steps: 1) The femtosecond laser emits a laser pulse beam, which is a collimated beam; 2) Obtaining a pre-chirped pulse beam: The collimated beam emitted by the femtosecond laser is introduced into a pre-chirp control module. The pre-chirp control module makes the polarization direction of the laser pulse lie in the horizontal plane and introduces negative dispersion into the laser pulse beam, thereby achieving better spectral broadening effect after passing through the self-phase modulation medium, thus forming a pre-chirped pulse beam. 3) Realize mode matching of laser pulse beam to multi-pass cavity: The pre-chirped pulse beam is input to a beam mode matching module, which matches the pre-chirped pulse beam waist to the intrinsic beam waist mode of the multi-pass cavity under nonlinear conditions, and makes the position of the transformed beam waist located at the center of the multi-pass cavity; 4) Obtain nonlinear spectral broadening of laser pulses: The light beam is introduced into the multi-pass cavity through the multi-pass cavity coupling mirror, and the multi-pass cavity mirror forms a closed-loop multi-pass optical path under stable cavity conditions; a pair of self-phase modulation media acts as a nonlinear medium to broaden the laser pulse spectrum under the self-phase modulation of the optical Kerr effect, and the shape of the concave spherical body can offset the self-focusing effect of the optical Kerr effect; in the multi-pass cavity, the spot size of the light beam gradually increases from the center to the outside, and the optical path of each light path passing through the self-phase modulation medium is related to the position of the self-phase modulation medium, and the position of each light path passing through the self-phase modulation medium is related to the position of the self-phase modulation medium. As the distance from the center of the multi-pass cavity increases, the optical path of each optical path through the self-phase modulation medium increases. The increase in the optical path through the self-phase modulation medium increases the nonlinear phase shift, and the nonlinear phase shift is related to the size of the output beam spatial mode. Therefore, by adjusting the relative positions of a pair of self-phase modulation media, symmetrically moving closer to or farther away from the center, the position where the light beam enters the self-phase modulation medium is changed, thereby adjusting the optical path of the light beam through the self-phase modulation medium, thereby changing the nonlinear phase shift in the multi-pass cavity, so that the laser pulse beam has a stable spatial mode relative to the set incident pulse energy. 5) Dispersion compensation of laser pulses: The laser pulse beam output by the nonlinear spectrum broadening module is incident on the dispersion compensation module. The dispersion compensation module collimates the beam, adjusts the polarization direction of the beam to be in the horizontal plane, and performs dispersion compensation on the laser pulse beam to obtain a Fourier-limited short pulse within the pulse spectrum range.

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