CN116526275A - Ultra-short pulse generating device and method based on concave spherical lens and multi-pass cavity - Google Patents

Abstract

The invention discloses an ultra-short pulse generating device and method based on a concave spherical lens and a multi-pass cavity, so as to obtain ultra-short pulse laser with pulse width smaller than forty femtoseconds and improve the stability of a pulse laser beam mode. The device comprises a femtosecond fiber laser, a pre-chirp regulation and control module, a beam mode matching module, a nonlinear spectrum widening module and a dispersion compensation module; the nonlinear spectrum widening module widens the spectrum of the laser pulse through the light beam passing through a pair of self-phase modulation media in the multi-pass cavity for multiple times; the self-phase modulation medium reduces the nonlinear pattern matching requirement; the method is provided with a pre-chirp regulation and control module, and the nonlinear phase displacement in the multi-pass cavity is regulated through the self-phase modulation medium displacement, and the self-focusing effect of the pulse laser beam is weakened; the invention has high generation efficiency and can be suitable for input pulses with peak power in a set range.

Description

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

technical field

The invention relates to the field of ultrafast laser technology, 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 technique

Ultrashort pulse lasers have broad application prospects in material processing, medical technology, and ultrafast science. Important ultrafast scientific research results are very dependent on advanced ultrashort pulse laser technology, and thus also promote the development of ultrafast laser technology. Traditional Ti:Sapphire lasers have been widely used in the past 30 years due to their advantages of high peak power and short pulse width. In recent years, Ti:Sapphire lasers are gradually being replaced by high repetition rate, high average power Ytterbium-doped fiber femtosecond lasers in related applications of extreme ultraviolet light and photoelectron spectroscopy systems. However, the narrow bandwidth of ytterbium-doped fiber femtosecond lasers and the corresponding long pulse width bring challenges for post-pulse compression techniques.

The laser pulse width output by existing femtosecond lasers is in the range of forty to hundreds of femtoseconds. In order to further obtain ultrashort pulses with shorter pulse widths, the laser pulses are input to a separate post-pulse compression device. The two basic technical methods commonly used in pulse compression devices are to use gas-filled hollow-core fibers or solid fused silica plates to broaden the spectrum under the nonlinear optical Kerr effect and then perform dispersion compensation. In contrast, the technical method of solid fused silica sheet can be applied to lasers with an average power higher than kilowatts, and has lower requirements on the stability of the input laser beam. However, at high light intensities, solid fused silica plates exhibit cone emission, which degrades the beam spatial mode and reduces output efficiency. On the other hand, the laser beam needs to pass through the fused silica plate multiple times, causing temporal pulse splitting in the process of continuously broadening the spectrum. Moreover, the existing multi-pass cavity is placed with a flat plate material, which can only target a specific pulse power, and the mode matching only matches the eigenmode of the multi-pass cavity without considering the nonlinear mode matching of the material in the cavity.

Contents of the invention

In order to solve the above problems in the prior art, the present invention proposes an ultrashort pulse generating device based on a concave spherical lens and a multi-pass cavity and a generating method thereof, so as to reduce the pulse width of the ultrashort pulse laser and improve the pulse width of the pulse laser. The stability of the beam mode improves the femtosecond laser with high power and long pulse width to an ultrashort pulse laser with a pulse time width of less than 40 femtoseconds.

An object of the present invention is to propose an ultrashort pulse generating device based on a concave spherical lens and a multi-pass cavity.

The ultrashort pulse generation device based on the concave spherical lens and the multi-pass cavity 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; wherein, the nonlinear The spectral 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. A fixed distance constitutes a multi-pass cavity. A 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 body. The thickness at the center is the thinnest and the thickness is thicker as it is farther away from its center. The material is a transparent nonlinear material with optical Kerr effect. A pair of self-phase modulation media is symmetrically placed on both sides of the center of the multi-pass cavity. The center of the through cavity is symmetrical, and they are respectively placed on the translation stage, and the translation stage can drive a pair of self-phase modulation media away from or close to the center symmetrically;

The femtosecond laser sends out the 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 in the horizontal plane, and introduces negative dispersion to the laser pulse beam, so that after the The spectral broadening effect of the phase modulation medium is better, and the pre-chirped pulse beam is input to the beam mode matching module; the beam mode matching module matches the pre-chirped pulse beam waist to the eigenbeam of the multi-pass cavity under nonlinear conditions Waist mode, and make the transformed beam waist position in 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 cavity under stable cavity conditions A circular multi-pass optical path; a pair of self-phase modulation media as a nonlinear medium, the laser pulse spectrum is broadened under the self-phase modulation of the optical Kerr effect, and the shape is a concave spherical body that can offset the self-focusing effect of the optical Kerr effect; in multiple In the cavity, the spot size of the beam gradually increases from the center to the outside. The optical path of each optical path through the self-phase modulation medium is related to the position of the self-phase modulation medium, and the distance of each optical path through the self-phase modulation medium The position is related to the position of the self-phase modulation medium. If the self-phase modulation medium is far away from the center of the multi-pass cavity, the optical path of each optical path through the self-phase modulation medium increases, and the increase of the optical path through the self-phase modulation medium makes the nonlinear phase The displacement increases, and the nonlinear phase shift is related to the size of the output beam spatial mode. Therefore, by adjusting the relative position of a pair of self-phase modulation media, while symmetrically approaching or away from the center, the position of the beam entering the self-phase modulation medium is different. , to adjust the optical path of the 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 in the spatial mode of the beam output by the multi-pass cavity relative to the set incident pulse energy; 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 Fourier-limited short pulses within the pulse spectral width range.

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

The beam mode matching module includes: lens group, delay line and mirror; the lens group includes concave lens and 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 through the mirror, and the lens group is connected and the laser pulse beam of the delay line; the delay line includes a right-angle reflector, a translation stage and a right-angle retroreflector or two reflectors placed perpendicularly to each other, and the right-angle retroreflector or two reflectors placed perpendicularly to each other are placed on On the translation stage, use the delay line to adjust the optical path, so that the position of the transformed beam waist is located in the center of the multi-pass cavity.

The front surface of the multi-pass cavity coupling mirror of the nonlinear spectrum broadening module is coated with a high-reflection film, and the reflectivity is greater than 99%. The multi-pass cavity mirror is two concave spherical reflectors, the reflection surfaces of the two concave spherical reflectors are facing each other, according to the curvature of the concave spherical surface of the multi-pass cavity mirror, it is placed according to the distance set by the conditions for forming a stable cavity to form a stable cavity ; The multi-pass cavity mirror has a wide spectrum and high reflectivity, the spectral range is from 900 nanometers to 1200 nanometers, and the reflectivity is greater than 99%. The material of the self-phase modulation medium is transparent crystal or glass.

The dispersion compensation module includes a half-wave plate and two chirped mirrors; the two chirped mirrors are placed in parallel on the rotary table, and the chirped mirror is driven to rotate by the rotary table to adjust the angle at which the incident light enters the chirped mirror; in the dispersion compensation module In this method, a lens is used to collimate the beam; the collimated beam is incident on the half-wave plate; by adjusting the half-wave plate, the polarization direction of the beam is in the horizontal plane; the beam emitted from the half-wave plate enters a pair of chirped mirrors at a set angle ; The chirped mirror is used to reflect the laser pulse beam multiple times to compensate the dispersion, and obtain the Fourier limit short pulse in the pulse spectrum range.

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

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

1) The femtosecond laser emits a laser pulse beam, and the laser pulse is a collimated beam;

2) Obtain the pre-chirped pulse beam:

The collimated beam emitted by the femtosecond laser is introduced into the pre-chirp control module. The pre-chirp control module makes the polarization direction of the laser pulse in the horizontal plane, and introduces negative dispersion to the laser pulse beam, so that the spectrum of the self-phase modulation medium is broadened The effect is better, forming a pre-chirped pulse beam;

3) Realize the mode matching of the laser pulse beam to the multi-pass cavity:

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

4) Obtain the nonlinear spectral broadening of the laser pulse:

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 the condition of a stable cavity; a pair of self-phase modulation media is used as a nonlinear medium, and the self-phase modulation of the optical Kerr effect The laser pulse spectrum is broadened downward, and the shape is a concave spherical body, which 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 pass passes through the self-phase modulation medium The optical path is related to the position through the self-phase modulation medium, and the position of each light path through the self-phase modulation medium is related to the position of the self-phase modulation medium, and the self-phase modulation medium is far away from the center of the multi-pass cavity, and each pass light The optical path through the self-phase modulation medium increases, and the increase of 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 space mode, so by adjusting a pair of self- The relative position of the phase modulation medium is symmetrically close to or away from the center at the same time, so that the position of the beam entering the self-phase modulation medium is different, so as to adjust the optical path of the beam passing through the self-phase modulation medium, thereby changing the nonlinear phase shift in the multi-pass cavity, Make the laser pulse beam relatively stable under the set incident pulse energy, the beam spatial mode output by the multi-pass cavity;

5) Dispersion compensation for 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 the Fourier within the pulse spectrum range. Leaves extreme short pulses.

Advantages of the present invention:

(1) The generation efficiency is high, and the laser pulse beam spatial mode obtained is stable: there is a gentle zone between the laser pulse beam spatial mode and the nonlinear phase in the multi-pass cavity. According to the present invention, a pair of self-phase modulation media can be moved The relative position adjusts the nonlinear phase in the multi-pass cavity, so that the device works in a stable mode; the shape and thickness selection of a pair of self-phase modulation media can also avoid the cone emission of the laser beam, so that the laser pulse energy can be kept in the target mode, and a higher High ultrashort pulse generation efficiency;

(2) It is suitable for 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;

(3) The mode matching difficulty between the laser pulse beam and the multi-pass cavity is reduced: the device of the present invention adopts a pair of self-phase modulation media whose shape is a concave spherical body, which is equivalent to a concave lens, has the effect of diverging beams, and can offset the laser beam The focusing effect when passing through the nonlinear medium can reduce the peak power fluctuation of the laser pulse beam in the nonlinear medium without performing nonlinear mode matching processing on the multi-pass cavity;

(4) Enhance the nonlinear spectral broadening effect and improve the laser pulse compression ratio: the present invention sets a pre-chirp module in the nonlinear spectral broadening module of the multi-pass cavity, and introduces negative dispersion, which is beneficial to prevent the laser pulse beam from splitting in time, Get a wider spectrum.

Description of drawings

Fig. 1 is the schematic diagram of an embodiment of the ultrashort pulse generation device based on concave spherical lens and multi-pass cavity of the present invention;

Fig. 2 is a spectrum diagram before and after the laser pulse is incident on the nonlinear spectrum broadening module 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.

Detailed ways

The present invention will be further elaborated below through specific embodiments in conjunction with the accompanying drawings.

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

The pre-chirp control module 2 includes in turn: a first half-wave plate 6, a first mirror 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 installed on the first translation platform, and a precision adjustable translation platform is used on the first translation platform;

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; it is reflected to the delay line by the second reflector 13; the delay line includes a rectangular reflector 14, translation platform and right-angle retroreflector 15, and right-angle retroreflector 15 is placed on the second translation platform;

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, and the multi-pass cavity coupling mirror is positioned at the multi-pass cavity In front of the cavity mirror, the concave surfaces of 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 is made of solid concave spherical nonlinear material, and the center of the self-phase modulation medium is located at the center of the multi-pass cavity. On the axis, the thickness of the self-phase modulation medium at its center is the thinnest and the thickness farther away from its center is thicker. A transparent nonlinear material with optical Kerr effect is adopted. The first and second self-phase modulation medium 18 and 19 are symmetrically placed on both sides of the center of the multi-pass cavity, and are symmetrical to the center of the multi-pass cavity, and are respectively placed on the third translation stage, and the translation stage can drive a pair of self-phase modulation media away from or close to the center symmetrically;

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 in parallel on the rotary table, driven The chirped mirror rotates to adjust the angle at which the incident light enters the chirped mirror.

The femtosecond laser 1 sends 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, thereby passing through the self-phase The spectral broadening effect of the modulation medium is better. It is incident on the first grating 8 and the second grating 9 through the top of the first reflector 7, and the position of the second grating 9 is adjusted by the translation of the first translation stage to control the distance between the grating pairs and introduce negative Dispersion, changing the waveform of the pre-chirped laser pulse to form a pre-chirped pulse beam; the laser pulse beam after passing through the first grating 8 and the second grating 9 decreases in height after passing through the retroreflector 10, turns back under the original beam and passes through the second grating again A grating 8 and a second grating 9 are reflected by the first mirror 7 and input to the beam pattern matching module 3; the concave lens 11 and the convex lens 12 match the size of the beam waist to the intrinsic beam waist of the multi-pass cavity under nonlinear conditions Mode, reflected to the delay line by the second reflector 13, and 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 the third reflector 16, Then it is introduced into the multi-pass cavity of the nonlinear spectrum broadening module 4 through the multi-pass cavity coupling mirror 17; the distance between the first and second multi-pass cavity mirrors 20 and 21 satisfies the stable cavity condition, and is formed under the stable cavity condition A closed-loop multi-pass optical path; a pair of self-phase modulation media as a nonlinear medium broadens the laser pulse spectrum under the self-phase modulation of the optical Kerr effect, and the shape is a concave spherical body that can offset the self-focusing effect of the optical Kerr effect. The pulse beam is reflected back and forth between the multi-pass cavity mirrors, 16 back and forth, 32 times through the first and second self-phase modulation media 18 and 19, and the laser pulse beam passes through the first and second self-phase modulation media 18 multiple times and 19 obtained spectral broadening; in the multi-pass cavity, the spot size of the 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 through the self-phase modulation medium, while The position of each optical path passing through the self-phase modulation medium is related to the position of the self-phase modulation medium. If the self-phase modulation medium is far away from the center of the multi-pass cavity, the optical path of each optical path through the self-phase modulation medium increases. Through the self-phase modulation The increase of the optical path of the modulation medium increases the nonlinear phase shift, and the nonlinear phase shift is related to the size of the output beam spatial mode, so by adjusting the relative positions of the first and second self-phase modulation media 18 and 19, the symmetry Close to or away from the center, so that the position of the beam entering the self-phase modulation medium is different, to adjust the optical path of the 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 relative to the set Under the incident pulse energy, the spatial mode of the beam output by the multi-pass cavity is stable; the beam is reflected by the fourth mirror 22 to the dispersion compensation module 5, and the beam is collimated by the collimating lens 23, and the collimated beam is incident on the second half-wave plate 24. By adjusting the second half-wave plate 24 so that the polarization direction of the beam is in the horizontal plane, the beam emitted from the second half-wave plate enters the first and second chirped mirrors 25 and 26 at a set angle, and the chirped mirrors are used to The sub-reflected laser pulse beam compensates for dispersion and obtains Fourier-limited short pulses in the spectral range of the pulse.

In this embodiment, the laser pulse parameters are pulse width 300fs, pulse energy 300μJ, center wavelength 1030nm, and repetition frequency 333KHz; the grating is selected as 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-channel cavity mirror is 50.8mm, the radius of curvature is 300mm, and the distance between the multi-channel cavity mirrors is 480mm. The diameter of the self-phase modulation medium of the concave spherical body material is 30mm, and the radius of curvature is -400mm. It is made of fused silica material and coated with a 1030nm anti-reflection coating. The spectrum shown in Figure 2 is generated by the nonlinear spectrum broadening module.

Finally, it should be noted that the purpose of the disclosed embodiments is to help further understand the present invention, but those skilled in the art can understand that various replacements and modifications can be made without departing from the spirit and scope of the present invention and the appended claims. It is possible. Therefore, the present invention should not be limited to the content disclosed in the embodiments, and the protection scope of the present invention is subject to the scope defined in the claims.

Claims (8)

1. A device for generating ultrashort pulses based on a concave spherical lens and a multi-pass cavity, characterized in that the device for generating ultrashort pulses includes: a femtosecond fiber laser, a pre-chirp control module, a beam pattern matching module, a A linear spectrum broadening module and a dispersion compensation module; wherein, the nonlinear spectral broadening module includes a multi-pass cavity coupling mirror, two multi-pass cavity mirrors and a pair of self-phase modulation media, and the multi-pass cavity coupling mirror is located in front of the multi-pass cavity mirror , the concave surfaces of 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 adopts a solid concave spherical nonlinear material, and 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 body, and the thickness is the thinnest at the center of itself and the thickness is thicker the farther away from the center of itself. The material is a transparent nonlinear material with optical Kerr effect. A pair of self-phase modulation medium Symmetrically placed on both sides of the center of the multi-pass cavity, and symmetrical to the center of the multi-pass cavity, and respectively placed on the translation stage, the translation stage can drive a pair of self-phase modulation media away from or close to the center symmetrically; The femtosecond laser sends out the 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 in the horizontal plane, and introduces negative dispersion to the laser pulse beam, so that after the The spectral broadening effect of the phase modulation medium is better, and the pre-chirped pulse beam is input to the beam mode matching module; the beam mode matching module matches the pre-chirped pulse beam waist to the eigenbeam of the multi-pass cavity under nonlinear conditions Waist mode, and make the transformed beam waist position in 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 cavity under stable cavity conditions A circular multi-pass optical path; a pair of self-phase modulation media as a nonlinear medium, the laser pulse spectrum is broadened under the self-phase modulation of the optical Kerr effect, and the shape is a concave spherical body that can offset the self-focusing effect of the optical Kerr effect; in multiple In the cavity, the spot size of the beam gradually increases from the center to the outside. The optical path of each optical path through the self-phase modulation medium is related to the position of the self-phase modulation medium, and the distance of each optical path through the self-phase modulation medium The position is related to the position of the self-phase modulation medium. If the self-phase modulation medium is far away from the center of the multi-pass cavity, the optical path of each optical path through the self-phase modulation medium increases, and the increase of the optical path through the self-phase modulation medium makes the nonlinear phase The displacement increases, and the nonlinear phase shift is related to the size of the output beam spatial mode. Therefore, by adjusting the relative position of a pair of self-phase modulation media, while symmetrically approaching or away from the center, the position of the beam entering the self-phase modulation medium is different. , to adjust the optical path of the 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 in the spatial mode of the beam output by the multi-pass cavity relative to the set incident pulse energy; 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 Fourier-limited short pulses within the pulse spectral width range. 2. The ultrashort pulse generating device according to claim 1, wherein the pre-chirp control module adopts an acousto-optic programmable dispersion filter; or, the pre-chirp control module comprises successively: a first half-wave plate 1. A pulse chirp control device and a retroreflector, the pulse chirp control device is placed on the translation stage, and the pulse chirp control device adopts a pair of gratings or a pair of dispersion prisms. 3. The ultrashort pulse generating device as claimed in claim 1, wherein the beam pattern matching module comprises: a lens group, a delay line and a reflector; the lens group includes a concave lens and a convex lens, and the mode under nonlinear conditions Matched to the intrinsic beam waist mode of the multi-pass cavity; the optical path is changed by the mirror, and the laser pulse beam is connected to the lens group and the delay line; the delay line includes a right-angle mirror, a translation stage, and a right-angle retroreflector or are placed perpendicular to each other The two mirrors, right-angled retroreflectors or two mirrors perpendicular to each other are placed on the translation stage, and the optical path is adjusted using the delay line, so that the position of the transformed beam waist is located in the center of the multi-pass cavity. 4. The ultrashort pulse generating device according to claim 1, wherein the front surface of the multi-pass cavity coupling mirror of the nonlinear spectrum broadening module is coated with a high reflection film, and the reflectivity is greater than 99%. 5. ultrashort pulse generator as claimed in claim 1, is characterized in that, described multi-pass cavity cavity mirror is two concave spherical reflectors, and the reflective surfaces of two concave spherical reflectors are towards opposite, according to multi-pass cavity The concave spherical curvature of the cavity mirror is placed according to the distance set by the conditions for forming a stable cavity to form a stable cavity. 6. The ultrashort pulse generating device according to claim 1, characterized in that, the multi-pass cavity mirror has a wide spectrum and high reflectivity, the spectral range is from 900nm to 1200nm, and the reflectivity is greater than 99%. 7. The ultrashort pulse generating device according to claim 1, wherein the dispersion compensation module comprises a half-wave plate and two chirped mirrors; the two chirped mirrors are placed in parallel on the rotary table, driven by the rotary table The chirped mirror rotates to adjust the angle at which the incident light enters the chirped mirror; in the dispersion compensation module, the beam is collimated with a lens; the collimated beam enters the half-wave plate; by adjusting the half-wave plate, the polarization direction of the beam is Located on the horizontal plane; the beam exiting 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. 8. A method for producing an ultrashort pulse generating device based on a concave spherical lens and a multi-pass cavity as claimed in claim 1, wherein the method for producing comprises the following steps: 1) The femtosecond laser emits a laser pulse beam, and the laser pulse is a collimated beam; 2) Obtain the pre-chirped pulse beam: The collimated beam emitted by the femtosecond laser is introduced into the pre-chirp control module. The pre-chirp control module makes the polarization direction of the laser pulse in the horizontal plane, and introduces negative dispersion to the laser pulse beam, so that the spectrum of the self-phase modulation medium is broadened The effect is better, forming a pre-chirped pulse beam; 3) Realize the mode matching of the laser pulse beam to the multi-pass cavity: The pre-chirped pulse beam is input to the beam mode matching module, and the beam mode matching module matches the pre-chirped pulse beam waist to the intrinsic beam waist mode of the multi-pass cavity under nonlinear conditions, and makes the transformed beam beam The position of the waist is located in the center of the multi-pass cavity; 4) Obtain the nonlinear spectral broadening of the laser pulse: 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 the condition of a stable cavity; a pair of self-phase modulation media is used as a nonlinear medium, and the self-phase modulation of the optical Kerr effect The laser pulse spectrum is broadened downward, and the shape is a concave spherical body, which 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 pass passes through the self-phase modulation medium The optical path is related to the position through the self-phase modulation medium, and the position of each light path through the self-phase modulation medium is related to the position of the self-phase modulation medium, and the self-phase modulation medium is far away from the center of the multi-pass cavity, and each pass light The optical path through the self-phase modulation medium increases, and the increase of 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 space mode, so by adjusting a pair of self- The relative position of the phase modulation medium is symmetrically close to or away from the center at the same time, so that the position of the beam entering the self-phase modulation medium is different, so as to adjust the optical path of the beam passing through the self-phase modulation medium, thereby changing the nonlinear phase shift in the multi-pass cavity, Make the laser pulse beam relatively stable under the set incident pulse energy, the beam spatial mode output by the multi-pass cavity; 5) Dispersion compensation for 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 the Fourier within the pulse spectrum range. Leaves extreme short pulses.