CN115939919B - Solid laser based on Kerr lens mode locking - Google Patents

Abstract

The invention discloses a solid laser based on Kerr lens mode locking, which is used for outputting high-stability femtosecond laser pulse capable of automatically starting Kerr lens mode locking, and belongs to the technical field of solid laser, and comprises the following steps: the laser comprises a pumping source, a laser crystal, a resonant cavity and an external cavity reflector; the pump laser output by the pump source is focused on the laser crystal through the pump light path; the laser generated by the laser crystal oscillates back and forth in the round-trip light path provided by the resonant cavity; the external cavity reflector is used for reflecting laser transmitted by the resonant cavity back to the resonant cavity, so that mode locking of the Kerr lens is started in a light disturbance mode. The invention solves the problem that the traditional Kerr lens mode locking requires a mode of applying mechanical disturbance by a worker to start mode locking, has higher stability, avoids the problem of light path deviation caused by long-term use and pushing of the resonant cavity mirror, and further can realize the high-stability and narrow-pulse-width femtosecond laser which has simple structure, easy operation and repeatable assembly.

Description

A solid-state laser based on Kerr lens mode locking

Technical Field

The present invention belongs to the technical field of solid lasers, and more specifically, relates to a solid laser based on Kerr lens mode locking.

Background technique

Ultrafast lasers have become an important research direction in the field of laser technology due to their high peak power and narrow pulse width. They have very extensive and important applications in basic scientific research, biomedicine, industrial processing, national defense and military fields. There are two main ways to generate ultrafast lasers: active mode locking and passive mode locking. The pulse width generated by active mode locking technology is generally in the order of picoseconds. To obtain ultrashort pulses in the order of femtoseconds, passive mode locking technology is usually used. The mode locking mechanism of passive mode locking is the saturable absorption effect, which can be achieved through real saturable absorbers, such as semiconductor saturable absorber mirrors (SESAM), carbon nanotubes, or graphene, or through virtual saturable absorbers, such as Kerr lens mode locking. Passively mode-locked lasers have the advantages of compact structure, stable performance and low cost. They can generate ultrashort pulses in the order of picoseconds or even femtoseconds, and can be widely used in scientific research, medical treatment and industrial production.

At present, the main methods of generating ultrashort pulses are semiconductor saturable absorber mirror (SESAM) mode locking and Kerr lens mode locking, which are relatively mature. SESAM mode locking requires the preparation of a mode locking device that combines a semiconductor saturable absorber and a reflector. It has strict selection of laser wavelength, is easily damaged when generating high-power lasers, and has high cost. Kerr lens mode locking is equivalent to forming a virtual saturable absorber, so there is no need to grow and manufacture mode locking devices specifically. Only a Kerr medium with nonlinear effects and a resonant cavity design are required to achieve mode locking. Therefore, Kerr lens mode locking has a higher damage threshold and is cheaper. At the same time, the modulation depth of Kerr lens mode locking is greater, which is conducive to the generation of ultrafast lasers with shorter pulse widths. Therefore, Kerr lens mode locking is a mode locking method suitable for generating high-power, narrow pulse width ultrafast lasers.

However, Kerr lens mode locking requires sophisticated design of the resonant cavity structure and careful adjustment, and Kerr lens mode locking usually cannot start by itself. In the experiment, the staff usually needs to apply a mechanical disturbance to start the mode locking state. The operation is relatively complicated, and the long-term use of mechanical vibration of the resonant cavity mirror can easily lead to optical path deviation and poor stability. This is undoubtedly an important issue limiting the development of Kerr lens mode-locked ultrafast lasers in actual industrial applications.

Summary of the invention

In view of the above defects or improvement needs of the prior art, the present invention provides a solid laser based on Kerr lens mode locking, so as to solve the technical problem that the existing laser based on Kerr lens mode locking cannot self-start.

In order to achieve the above object, the present invention provides a solid laser based on Kerr lens mode locking, comprising: a pump source, a laser crystal, a resonant cavity and an external cavity reflector;

Among them, the pump laser output by the pump source is focused onto the laser crystal through the pump optical path; the laser generated by the laser crystal oscillates back and forth in the round-trip optical path provided by the resonant cavity;

The external cavity mirror is used to reflect the laser transmitted from the resonant cavity back into the resonant cavity, thereby initiating Kerr lens mode locking in the form of optical perturbations.

Further preferably, when the solid-state laser is a solid-state laser other than a disk laser, the resonant cavity comprises:

The first end mirror and the second end mirror arranged at the two ends of the resonant cavity are used to reflect the oscillating laser in the resonant cavity and provide a round-trip optical path for the oscillating laser; wherein the first end mirror is a plane high-reflection mirror; and the second end mirror is an output mirror;

A first concave mirror and a second concave mirror are arranged on the round-trip optical path, and the first concave mirror and the second concave mirror form a tight focusing structure, which is used to focus the oscillating laser in the resonant cavity onto the laser crystal to achieve Kerr lens mode locking; wherein the laser crystal is located between the first concave mirror and the second concave mirror;

The high-dispersion mirror arranged on the round-trip optical path is used to compensate for the dispersion introduced by the laser crystal and the optical elements constituting the resonant cavity.

Further preferably, when the solid-state laser is a disk laser, the resonant cavity comprises:

The first end mirror and the second end mirror arranged at the two ends of the resonant cavity are used to reflect the oscillating laser in the resonant cavity and provide a round-trip optical path for the oscillating laser; wherein the first end mirror is a plane high-reflection mirror; and the second end mirror is an output mirror;

A first concave mirror and a second concave mirror are arranged on a round-trip optical path, and the first concave mirror and the second concave mirror form a tight focusing structure, which is used to focus the oscillating laser in the resonant cavity onto the Kerr medium to achieve Kerr lens mode locking; wherein the Kerr medium is located between the first concave mirror and the second concave mirror;

A high dispersion mirror arranged on the round-trip optical path is used to compensate for the dispersion introduced by the laser crystal and the optical elements constituting the resonant cavity;

The above-mentioned laser crystal is a disk-shaped laser crystal, which is used not only as a gain medium to generate laser light, but also as a reflecting mirror to reflect the oscillated laser light.

Further preferably, the above-mentioned resonant cavity further includes: an aperture arranged on the round-trip optical path, for increasing the diffraction loss in the resonant cavity.

In a preferred embodiment, the external cavity reflector is arranged outside the resonant cavity and is located on the side of the first end mirror facing away from the resonant cavity; the first end mirror is used to partially transmit and partially reflect the oscillating laser in the resonant cavity; the external cavity reflector is used to reflect the laser transmitted by the first end mirror and return the laser to the resonant cavity along the original path, thereby starting the Kerr lens mode locking in the form of optical disturbance;

The external cavity reflector is a plane mirror or a concave mirror; a side of the external cavity reflector close to the first end mirror is coated with a high reflection film for the oscillating laser, and the reflectivity is greater than 99.9%;

The first concave mirror and the second concave mirror are both coated with a high-reflection film for oscillating lasers on one side facing the resonant cavity, and the reflectivity is greater than 99.9%.

Further preferably, the external cavity reflector and the first end mirror form an external cavity, and the external cavity length is the distance between the external cavity reflector and the first end mirror; when the external cavity reflector is a plane mirror, the external cavity length is an integer multiple of the length of the resonant cavity; when the external cavity reflector is a concave mirror, the external cavity length is equal to the focal length of the external cavity reflector.

Further preferably, the side of the first end mirror away from the external cavity reflector is coated with a partially reflective and partially transparent film for the oscillating laser, and the transmittance range is 0.5% to 20%; the side of the first end mirror close to the external cavity reflector is coated with an anti-reflection film for the oscillating laser, and the transmittance is greater than 99.5%.

In a preferred embodiment 2, the external cavity reflector is arranged outside the resonant cavity and is located on the side of the second end mirror facing away from the resonant cavity; in this case, the solid-state laser further comprises a beam splitter located between the second end mirror and the external cavity reflector, which is used to transmit part of the laser light transmitted by the second end mirror and reflect part of the laser light onto the external cavity reflector, and to transmit part of the laser light reflected back by the external cavity reflector and reflect part of the laser light back to the second end mirror;

The second end mirror is used to partially transmit and partially reflect the oscillating laser in the resonant cavity; the external cavity reflector is used to reflect the laser reflected by the beam splitter and return it to the resonant cavity along the original path, thereby starting the Kerr lens mode locking in the form of light disturbance.

The external cavity reflector is a plane mirror; a side of the external cavity reflector close to the second end mirror is coated with a high reflection film for the oscillating laser, and the reflectivity is greater than 99.9%;

The first concave mirror and the second concave mirror are both coated with a high-reflection film for oscillating lasers on one side facing the resonant cavity, and the reflectivity is greater than 99.9%.

Further preferably, the external cavity reflector and the second end mirror form an external cavity, and the length of the external cavity is the distance between the external cavity reflector and the second end mirror; the length of the external cavity is an integer multiple of the length of the resonant cavity.

In a third preferred embodiment: the external cavity reflector is arranged outside the resonant cavity and is located on a side of the selected concave mirror facing away from the resonant cavity;

The selected concave mirror is used to partially transmit and partially reflect the oscillating laser in the resonant cavity; the external cavity reflector is used to reflect the laser transmitted by the selected concave mirror and return it to the resonant cavity along its original path, thereby starting the Kerr lens mode locking in the form of optical disturbance;

Wherein, the selected concave mirror is the first concave mirror or the second concave mirror;

The external cavity reflector is a concave mirror, and a high-reflection film for oscillating laser is coated on one side of the external cavity reflector close to the selected concave mirror, and the reflectivity is greater than 99.9%; the focal length of the external cavity reflector is greater than the focal length of the selected concave mirror;

The side of the first end mirror facing the resonant cavity is coated with a high reflection film for the oscillating laser, and the reflectivity is greater than 99.9%.

Further preferably, the side of the selected concave mirror facing the resonant cavity is coated with a partially reflective and partially transparent film for the oscillating laser, and the transmittance range is 0.5% to 5%; the side of the selected concave mirror facing away from the resonant cavity is coated with an anti-reflection film for the oscillating laser, and the transmittance is greater than 99.5%.

Further preferably, the external cavity reflector and the selected concave mirror form an external cavity, and the external cavity length is the distance between the external cavity reflector and the selected concave mirror; the external cavity length is the difference between the focal length of the external cavity reflector and the focal length of the selected concave mirror.

In a preferred embodiment, when the solid-state laser is a solid-state laser other than a disk laser, the external cavity reflector is arranged outside the resonant cavity and is located on the side of the laser crystal facing away from the resonant cavity; the laser crystal and the resonant cavity are placed at a Brewster angle to reflect part of the oscillating laser onto the external cavity reflector; the external cavity reflector is used to reflect the laser reflected by the laser crystal and return it to the resonant cavity along its original path, thereby starting the Kerr lens mode locking in the form of optical disturbance;

The external cavity reflector is a concave mirror; the side of the external cavity reflector close to the laser crystal is coated with a high reflection film for the oscillating laser, and the reflectivity is greater than 99.9%;

The first concave mirror and the second concave mirror are both coated with a high-reflection film for the oscillating laser on the side facing the resonant cavity, and the reflectivity is greater than 99.9%;

The side of the first end mirror facing the resonant cavity is coated with a high reflection film for the oscillating laser, and the reflectivity is greater than 99.9%.

Further preferably, the external cavity reflector and the laser crystal form an external cavity, and the external cavity length is the distance between the external cavity reflector and the laser crystal; the external cavity length is equal to the focal length of the external cavity reflector.

In a preferred embodiment 5: when the solid-state laser is a disk laser, the external cavity reflector is arranged outside the resonant cavity and is located on the side of the Kerr medium facing away from the resonant cavity; the Kerr medium and the resonant cavity are placed at a Brewster angle to reflect part of the oscillating laser onto the external cavity reflector; the external cavity reflector is used to reflect the laser reflected by the Kerr medium and return it to the resonant cavity along its original path, thereby starting the Kerr lens mode locking in the form of optical disturbance;

The external cavity reflector is a concave mirror; the side of the external cavity reflector close to the laser crystal is coated with a high reflection film for the oscillating laser, and the reflectivity is greater than 99.9%;

The first concave mirror and the second concave mirror are both coated with a high-reflection film for the oscillating laser on the side facing the resonant cavity, and the reflectivity is greater than 99.9%;

The side of the first end mirror facing the resonant cavity is coated with a high reflection film for the oscillating laser, and the reflectivity is greater than 99.9%.

Further preferably, the external cavity reflector and the Kerr medium form an external cavity, and the external cavity length is the distance between the external cavity reflector and the Kerr medium; the external cavity length is equal to the focal length of the external cavity reflector.

In general, the above technical solutions conceived by the present invention can achieve the following beneficial effects:

1. The present invention provides a solid laser based on Kerr lens mode locking. An external cavity reflector is arranged outside the resonant cavity to reflect the laser transmitted from the resonant cavity back into the resonant cavity, thereby starting the Kerr lens mode locking in the form of light disturbance, thereby realizing the self-starting of the Kerr lens mode locking, avoiding the conventional mechanical vibration mode locking, and also avoiding the optical path deviation problem caused by the mechanical vibration of the resonant cavity during long-term use, and having high stability.

2. The solid laser based on Kerr lens mode locking provided by the present invention has low cost and low price, and is suitable for large-scale industrial device fields such as industrial processing and production. On the other hand, the Kerr lens mode locking modulation depth is large, and laser pulses with narrow pulse width can be achieved. Therefore, the Kerr lens mode locking solid laser provided by the present invention has the advantages of narrow pulse width and easy industrialization, and can realize a femtosecond laser with high stability and narrow pulse width that is simple in structure, easy to operate, and can be repeatedly assembled.

3. The solid laser based on Kerr lens mode locking proposed in the present invention includes but is not limited to bulk solid lasers and disk lasers, and is applicable to laser crystals of various shapes. It can meet different usage requirements, such as bulk solid lasers are suitable for compact and miniaturized industrial needs, and disk lasers are suitable for high power and high beam quality industrial needs.

BRIEF DESCRIPTION OF THE DRAWINGS

1 is a schematic diagram of the optical path structure of a Kerr lens locked modular solid laser when the external cavity is placed outside the first end mirror and the external cavity reflector is a plane mirror, provided by an embodiment of the present invention;

2 is a schematic diagram of the optical path structure of a Kerr lens-locked modular solid laser when the external cavity is placed outside the first end mirror and the external cavity reflector is a concave mirror, provided by an embodiment of the present invention;

3 is a schematic diagram of the optical path structure of a Kerr lens mode-locked disk laser started by light disturbance when the external cavity is placed outside the first end mirror and the external cavity reflector is a plane mirror, provided by an embodiment of the present invention;

4 is a schematic diagram of the optical path structure of a Kerr lens mode-locked disk laser started by light disturbance when the external cavity is placed outside the first end mirror and the external cavity reflector is a concave mirror, provided by an embodiment of the present invention;

FIG5 is a schematic diagram of the optical path structure of a Kerr lens-locked modular solid laser started by light disturbance when the external cavity is placed outside the concave mirror provided in an embodiment of the present invention;

FIG6 is a schematic diagram of the optical path structure of a Kerr lens mode-locked disk laser started by light disturbance when the external cavity is placed outside the concave mirror provided in an embodiment of the present invention;

7 is a schematic diagram of the optical path structure of a Kerr lens locked modular solid laser started by light disturbance when the external cavity is placed outside the second end mirror provided in an embodiment of the present invention;

FIG8 is a schematic diagram of the optical path structure of a Kerr lens mode-locked disk laser started by light disturbance when the external cavity is placed outside the second end mirror provided in an embodiment of the present invention;

FIG9 is a schematic diagram of the optical path structure of a Kerr lens-locked modular solid laser started by light disturbance when the external cavity is placed outside the laser crystal according to an embodiment of the present invention;

FIG10 is a schematic diagram of the optical path structure of a Kerr lens mode-locked disk laser started by light disturbance when the external cavity is placed outside the Kerr medium provided by an embodiment of the present invention.

Detailed ways

In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

In order to achieve the above-mentioned object, the present invention provides a solid laser based on Kerr lens mode locking, which is used to achieve stable self-starting Kerr lens mode locking femtosecond pulse output, comprising: a pump source, a laser crystal, a resonant cavity and an external cavity reflector;

The pump source focuses the pump laser it generates onto the laser crystal through the pump optical path. The laser crystal is used as a gain medium to generate laser light. At the same time, the Kerr effect of the crystal is combined with the design of the resonant cavity to form a laser pulse for Kerr lens mode locking. The external cavity reflector reflects the laser transmitted from the resonant cavity back into the resonant cavity to start the Kerr lens mode locking in the form of optical disturbance.

When the solid-state laser is a solid-state laser other than a disk laser, the resonant cavity comprises:

The first end mirror and the second end mirror arranged at the two ends of the resonant cavity are used to reflect the oscillating laser in the resonant cavity and provide a round-trip optical path for the oscillating laser; wherein the first end mirror is a plane high-reflection mirror; the second end mirror is an output mirror; specifically, the second end mirror is a plane mirror, with different dielectric films coated on both sides; the side of the second end mirror facing the resonant cavity is coated with a partial reflection and partial transmission film for the oscillating laser, and the transmittance range is 0.5% to 20% (preferably 1% to 20%). The side of the second end mirror facing away from the resonant cavity is coated with an anti-reflection film for the oscillating laser, and the transmittance is greater than 99.9%;

A first concave mirror and a second concave mirror are arranged on a round-trip optical path, and the first concave mirror and the second concave mirror form a tight focusing structure, which is used to focus the oscillating laser in the resonant cavity onto the laser crystal to achieve Kerr lens mode locking; wherein the laser crystal is located between the first concave mirror and the second concave mirror;

A high dispersion mirror arranged on the round-trip optical path is used to compensate for the dispersion introduced by the laser crystal and the optical elements constituting the resonant cavity;

When the end face of the laser crystal is placed perpendicular to the resonant cavity, both sides are coated with anti-reflection films for the oscillating laser; when the end face of the laser crystal is placed at a Brewster angle with the resonant cavity, both sides are not coated; laser crystals include but are not limited to Yb:KYW, Yb:KGW, Yb:CALGO, Yb:Lu ₂ O ₃ , Yb:YAG, Yb:YGG, Ho:YAG, Cr:ZnS, Cr:ZnSe, Ti:sapphire and other crystals. Pump sources include but are not limited to semiconductor lasers, fiber lasers and solid-state lasers with output wavelengths in the range of 300nm to 5μm.

When the solid-state laser is a disk laser, the resonant cavity includes:

The first end mirror and the second end mirror arranged at the two ends of the resonant cavity are used to reflect the oscillating laser in the resonant cavity and provide a round-trip optical path for the oscillating laser; wherein the first end mirror is a plane high-reflection mirror; the second end mirror is an output mirror; specifically, the second end mirror is a plane mirror, and different dielectric films are coated on both sides; the side of the second end mirror facing the resonant cavity is coated with a partial reflection and partial transmission film for the oscillating laser, and the transmittance range is 0.5% to 20% (preferably 1% to 20%). The side of the second end mirror facing away from the resonant cavity is coated with an anti-reflection film for the oscillating laser, and the transmittance is greater than 99.9%;

A first concave mirror and a second concave mirror are arranged on a round-trip optical path, and the first concave mirror and the second concave mirror form a tight focusing structure, which is used to focus the oscillating laser in the resonant cavity onto the Kerr medium to achieve Kerr lens mode locking; wherein the Kerr medium is located between the first concave mirror and the second concave mirror;

A high dispersion mirror arranged on the round-trip optical path is used to compensate for the dispersion introduced by each optical element in the resonant cavity;

The above-mentioned laser crystal is a disk-shaped laser crystal, which is used not only as a gain medium to generate laser, but also as a folding mirror to reflect the oscillating laser; the side of the disk-shaped laser crystal facing the resonant cavity is coated with an anti-reflection film for the pump laser and the oscillating laser, and the side of the disk-shaped laser crystal facing away from the resonant cavity is coated with a high-reflection film for the pump laser and the oscillating laser; in an optional implementation manner, the disk-shaped laser crystal is disk-shaped or elliptical disk-shaped, with a thickness of 10μm to 1mm and a diameter of 3 to 30mm; the material of the disk-shaped laser crystal includes but is not limited to Yb:YAG, Ho:YAG, Tm:YAG, Ho:KYW, Yb: _CALGO , Cr:ZnSe, Yb: _LuScO3 , Yb: _Lu2O3 and the like suitable for processing into a disk-shaped laser crystal.

The Kerr medium is placed at a Brewster angle with the resonant cavity. In an optional embodiment, the Kerr medium is a material having a nonlinear Kerr effect such as sapphire or fused quartz with a thickness of 0.1 mm to 10 mm. Preferably, the Kerr medium is a sapphire sheet with a thickness of 1 mm to 3 mm.

Preferably, the resonant cavity further comprises: an aperture arranged on the round-trip optical path, for increasing the diffraction loss in the resonant cavity. Specifically, according to the mode distribution in the resonant cavity, apertures of different sizes can be arranged at different positions in the cavity, for increasing the diffraction loss in the cavity, so as to assist the Kerr lens mode locking. In an optional embodiment, the aperture of the aperture is 0.5 mm to 5 mm.

Specifically, in an optional implementation mode 1:

The external cavity reflector is arranged outside the resonant cavity and is located on the side of the first end mirror facing away from the resonant cavity; the first end mirror is used to partially transmit and partially reflect the oscillating laser in the resonant cavity; the external cavity reflector is used to reflect the laser transmitted by the first end mirror and return the laser to the resonant cavity along the original path, thereby starting the Kerr lens mode locking in the form of optical disturbance;

The external cavity reflector is a plane mirror or a concave mirror; a side of the external cavity reflector close to the first end mirror is coated with a high reflection film for the oscillating laser, and the reflectivity is greater than 99.9%;

The first concave mirror and the second concave mirror are both coated with a high-reflection film for the oscillating laser on the side facing the resonant cavity, and the reflectivity is greater than 99.9%;

Further preferably, the external cavity reflector and the first end mirror form an external cavity, and the length of the external cavity is the distance between the external cavity reflector and the first end mirror; when the external cavity reflector is a plane mirror, the length of the external cavity is an integer multiple of the length of the resonant cavity; at this time, the random pulses formed by the continuous light in the resonant cavity are enhanced in intensity under the superposition of the external synchronization pulses, and the probability of reaching the mode locking threshold among these random pulses is increased, thereby improving the ability to start mode locking;

When the external cavity reflector is a concave mirror, the length of the external cavity is equal to the focal length of the external cavity reflector. At this time, the disturbed light is at the beam waist position at the first end mirror, thereby matching the disturbed light with the laser transverse mode in the resonant cavity, thereby improving the ability to start mode locking; wherein the disturbed light is the laser reflected back to the resonant cavity by the external cavity reflector.

Preferably, the side of the first end mirror away from the external cavity reflector is coated with a partially reflective and partially transparent film for the oscillating laser, and the transmittance range is 0.5% to 20%; the side of the first end mirror close to the external cavity reflector is coated with an anti-reflection film for the oscillating laser, and the transmittance is greater than 99.5%.

In an optional implementation mode 2:

The external cavity reflector is arranged outside the resonant cavity and is located on the side of the second end mirror facing away from the resonant cavity; in this case, the solid-state laser further comprises a beam splitter located between the second end mirror and the external cavity reflector, which is used to transmit part of the laser light transmitted by the second end mirror and reflect part of it onto the external cavity reflector, and to transmit part of the laser light reflected back by the external cavity reflector and reflect part of it back onto the second end mirror;

The second end mirror is used to partially transmit and partially reflect the oscillating laser in the resonant cavity; the external cavity reflector is used to reflect the laser reflected by the beam splitter and return it to the resonant cavity along the original path, thereby starting the Kerr lens mode locking in the form of light disturbance.

The external cavity reflector is a plane mirror; a side of the external cavity reflector close to the second end mirror is coated with a high reflection film for the oscillating laser, and the reflectivity is greater than 99.9%;

The first concave mirror and the second concave mirror are both coated with a high-reflection film for oscillating lasers on one side facing the resonant cavity, and the reflectivity is greater than 99.9%.

Preferably, the external cavity reflector and the second end mirror constitute an external cavity, and the length of the external cavity is the distance between the external cavity reflector and the second end mirror; the length of the external cavity is an integer multiple of the length of the resonant cavity. At this time, the random pulses formed by the continuous light in the resonant cavity are enhanced in intensity under the superposition of the external synchronization pulses, and the probability of reaching the mode locking threshold among these random pulses increases, thereby improving the ability to start the mode locking; the disturbing light is the laser reflected back to the resonant cavity by the external cavity reflector.

In an optional implementation mode three:

The external cavity reflector is arranged outside the resonant cavity and is located on the side of the selected concave mirror facing away from the resonant cavity; the selected concave mirror is used to partially transmit and partially reflect the oscillating laser in the resonant cavity; the external cavity reflector is used to reflect the laser transmitted by the selected concave mirror and return the laser to the resonant cavity along the original path, thereby starting the Kerr lens mode locking in the form of optical disturbance;

Wherein, the selected concave mirror is the first concave mirror or the second concave mirror;

The external cavity reflector is a concave mirror, and a side of the external cavity reflector close to the selected concave mirror is coated with a high reflection film for oscillating laser, and the reflectivity is greater than 99.9%; the focal length of the external cavity reflector is greater than the focal length of the selected concave mirror;

The side of the first end mirror facing the resonant cavity is coated with a high reflection film for the oscillating laser, and the reflectivity is greater than 99.9%.

Preferably, the side of the selected concave mirror facing the resonant cavity is coated with a partially reflective and partially transparent film for the oscillating laser, and the transmittance range is 0.5% to 5%; the side of the selected concave mirror facing away from the resonant cavity is coated with an anti-reflection film for the oscillating laser, and the transmittance is greater than 99.5%.

Preferably, the external cavity reflector and the selected concave mirror constitute an external cavity, and the external cavity length is the distance between the external cavity reflector and the selected concave mirror; the external cavity length is the difference between the focal length of the external cavity reflector and the focal length of the selected concave mirror; at this time, the disturbed light is at the beam waist position at the laser crystal (Kerr medium), thereby matching the disturbed light with the laser transverse mode in the resonant cavity, thereby improving the ability to start mode locking; wherein, the disturbed light is the laser reflected back to the resonant cavity by the external cavity reflector.

In an optional implementation mode 4:

When the solid-state laser is a solid-state laser other than a disk laser, the external cavity reflector is arranged outside the resonant cavity and is located on the side of the laser crystal facing away from the resonant cavity; the laser crystal is placed at a Brewster angle with the resonant cavity to reflect part of the oscillating laser onto the external cavity reflector; the external cavity reflector is used to reflect the laser reflected by the laser crystal and return it to the resonant cavity along its original path, thereby starting the Kerr lens mode locking in the form of optical disturbance;

The external cavity reflector is a concave mirror; the side of the external cavity reflector close to the laser crystal is coated with a high reflection film for the oscillating laser, and the reflectivity is greater than 99.9%;

The first concave mirror and the second concave mirror are both coated with a high-reflection film for the oscillating laser on the side facing the resonant cavity, and the reflectivity is greater than 99.9%;

The side of the first end mirror facing the resonant cavity is coated with a high reflection film for the oscillating laser, and the reflectivity is greater than 99.9%.

Preferably, the external cavity reflector and the laser crystal form an external cavity, and the external cavity length is the distance between the external cavity reflector and the laser crystal; the external cavity length is equal to the focal length of the external cavity reflector; at this time, the disturbed light is at the beam waist position at the laser crystal, thereby matching the disturbed light with the laser transverse mode in the resonant cavity, thereby improving the ability to start mode locking; wherein the disturbed light is the laser reflected back to the resonant cavity by the external cavity reflector.

In an optional implementation mode five:

When the solid-state laser is a disk laser, the external cavity reflector is arranged outside the resonant cavity and is located on the side of the Kerr medium facing away from the resonant cavity; the Kerr medium and the resonant cavity are placed at a Brewster angle to reflect part of the oscillating laser onto the external cavity reflector; the external cavity reflector is used to reflect the laser reflected by the Kerr medium and return it to the resonant cavity along the original path, thereby starting the Kerr lens mode locking in the form of optical disturbance;

The external cavity reflector is a concave mirror; the side of the external cavity reflector close to the laser crystal is coated with a high reflection film for the oscillating laser, and the reflectivity is greater than 99.9%;

The first concave mirror and the second concave mirror are both coated with a high-reflection film for the oscillating laser on the side facing the resonant cavity, and the reflectivity is greater than 99.9%;

The side of the first end mirror facing the resonant cavity is coated with a high reflection film for the oscillating laser, and the reflectivity is greater than 99.9%.

Preferably, the external cavity reflector and the Kerr medium constitute an external cavity, and the external cavity length is the distance between the external cavity reflector and the Kerr medium; the external cavity length is equal to the focal length of the external cavity reflector; at this time, the disturbed light is at the beam waist position at the Kerr medium, thereby matching the disturbed light with the laser transverse mode in the resonant cavity, thereby improving the ability to start mode locking; wherein the disturbed light is the laser reflected back to the resonant cavity by the external cavity reflector.

In order to further illustrate the solid-state laser provided by the present invention, specific embodiments of the Kerr lens mode-locked solid-state laser started by four optical disturbances are described in detail below. It should be noted that the present invention divides solid-state lasers into disk lasers and solid-state lasers other than disk lasers; solid-state lasers other than disk lasers include: bulk solid-state lasers, semiconductor lasers, etc.

The solid lasers provided by the present invention, except for the disk laser, are all described by taking the block solid laser as an example. The laser crystal of the block solid laser in this embodiment is a block solid shape with a side length of 1 mm to 20 mm. Specifically, Figures 1 to 10 are optical path diagrams of the Kerr lens locked module solid laser and the disk laser when the external cavity is at different positions. Among them, Figure 1 is a schematic diagram of the optical path structure of the Kerr lens locked module-shaped solid laser started by light perturbation when the external cavity is placed outside the first end mirror and the external cavity reflector is a plane mirror; Figure 2 is a schematic diagram of the optical path structure of the Kerr lens locked module-shaped solid laser started by light perturbation when the external cavity is placed outside the first end mirror and the external cavity reflector is a concave mirror; Figure 3 is a schematic diagram of the optical path structure of the Kerr lens mode-locked disk laser started by light perturbation when the external cavity is placed outside the first end mirror and the external cavity reflector is a plane mirror; Figure 4 is a schematic diagram of the optical path structure of the Kerr lens mode-locked disk laser started by light perturbation when the external cavity is placed outside the first end mirror and the external cavity reflector is a concave mirror; Figure 5 is a schematic diagram of the optical path structure of the Kerr lens locked module-shaped solid laser started by light perturbation when the external cavity is placed outside the concave mirror; Figure 6 is a schematic diagram of the optical path structure of the Kerr lens mode-locked disk laser started by light perturbation when the external cavity is placed outside the concave mirror;

Figure 7 is a schematic diagram of the optical path structure of the Kerr lens locked modular solid laser started by light perturbations when the external cavity is placed outside the second end mirror; Figure 8 is a schematic diagram of the optical path structure of the Kerr lens mode-locked disk laser started by light perturbations when the external cavity is placed outside the second end mirror; Figure 9 is a schematic diagram of the optical path structure of the Kerr lens locked modular solid laser started by light perturbations when the external cavity is placed outside the laser crystal; Figure 10 is a schematic diagram of the optical path structure of the Kerr lens mode-locked disk laser started by light perturbations when the external cavity is placed outside the Kerr medium.

As can be seen from Figures 1, 2, 5, 7 and 9, the optical perturbation started Kerr lens locked modular solid laser mainly includes a pump source 1 for providing pump laser to the laser crystal, a pump optical path 2 for focusing the pump laser onto the laser crystal, a laser crystal 3 used as a gain medium, a resonant cavity 11 defined by various optical elements for providing reciprocating oscillating laser, and an external cavity 13 for providing optical perturbations.

As can be seen from Figures 3, 4, 6, 8 and 10, the optical perturbation started Kerr lens mode-locked disk laser mainly includes a pump source 1 for providing pump laser to the disk-shaped laser crystal, a pump optical path 2 for focusing the pump laser onto the disk-shaped laser crystal, a disk-shaped laser crystal 7 used as a gain medium, a Kerr medium 3 used for Kerr lens mode locking and placed at a Brewster angle with the oscillating laser, a resonant cavity 11 defined by various optical elements for providing a reciprocating oscillating laser, and an external cavity 13 for providing optical perturbations.

As can be seen from Figures 1 to 10, the resonant cavity 11 is substantially the same, and the difference lies in the design of the external cavity 13. In the embodiment provided by the present invention, the resonant cavity 11 includes: a first end mirror 9, a second end mirror 6, a high dispersion mirror 8 for providing dispersion, a first concave mirror 4 and a second concave mirror 5 for focusing the oscillating laser onto a laser crystal (Kerr medium), and an aperture 10 for increasing the diffraction loss in the cavity. For a bulk solid laser,

The resonant cavity 11 also includes a high-reflection mirror 7; and the side of the first concave mirror 4 facing the resonant cavity is coated with a high-reflection film for the oscillating laser and an anti-reflection film for the pump laser; the side of the first concave mirror 4 facing the pump light path is coated with an anti-reflection film for the pump laser.

For the disk laser, the resonant cavity 11 also includes a disk-shaped laser crystal 7 that serves as both a gain medium and a reflecting mirror; and the side of the disk-shaped laser crystal facing the resonant cavity is coated with a high-transmittance film for the oscillating laser and the pumping laser, and the side facing away from the resonant cavity is coated with a high-reflection film for the oscillating laser and the pumping laser.

In FIG. 1 and FIG. 3 , the external cavity 13 is composed of a first end mirror 9 and an external cavity plane reflecting mirror 12 .

In FIG. 2 and FIG. 4 , the external cavity 13 is composed of a first end mirror 9 and an external cavity concave reflecting mirror 12 .

In FIG. 5 and FIG. 6 , the external cavity 13 is composed of a second concave mirror 5 and an external cavity concave reflecting mirror 12 .

In FIG. 7 and FIG. 8 , the external cavity 13 is composed of a second end mirror 6 , a beam splitter 14 and an external cavity plane reflecting mirror 12 .

In FIG. 9 and FIG. 10 , the external cavity 13 is composed of a laser crystal (Kerr medium) 3 and an external cavity concave reflecting mirror 12 .

Specifically, the optical path of the optical perturbation-activated Kerr lens-locked modular solid laser and disk laser is mainly divided into a resonant cavity 11 and an external cavity 13. The resonant cavity 11 includes a first end mirror 9, a second end mirror 6, a high dispersion mirror 8, a first concave mirror 4, a second concave mirror 5, a high reflection mirror (disk laser crystal) 7, a laser crystal (Kerr medium) 3, and an aperture 10. The first end mirror 9 and the second end mirror 6 are two end mirrors of the resonant cavity 11 to provide a round-trip optical path for the oscillating laser. The high dispersion mirror 8 is used to compensate for the dispersion introduced by each optical element in the cavity. The first concave mirror 4 and the second concave mirror 5 form a tight focusing structure to focus the oscillating laser in the cavity on the laser crystal (Kerr medium) 3. The aperture 10 assists the Kerr lens locking by increasing the diffraction loss in the cavity, and the placement position is not restricted. For a block solid laser, when the end face of the laser crystal 3 is placed vertically to the resonant cavity 11, both sides are coated with an anti-reflection film for the oscillating laser. When the end face of the laser crystal 3 is placed at a Brewster angle to the resonant cavity 11, both sides are not coated. The high reflective mirror 7 is used as a folding mirror to reflect the oscillated laser. For the disk laser, the Kerr medium 3 is placed near the focus of the first concave mirror 4 and the second concave mirror 5 at the Brewster angle. The disk-shaped laser crystal 7 can be used as a gain medium to generate laser light and as a folding mirror of the resonant cavity to reflect the oscillated laser light.

The outer cavity 13 can be divided into four situations according to the placement position:

(i) When the external cavity is placed outside the first end mirror (as shown in Figures 1 to 4), the external cavity 13 includes the first end mirror 9 and the external cavity reflector 12. The side of the first end mirror 9 facing the resonant cavity 11 is coated with a partially transparent and partially reflective film for the oscillating laser, and the side of the first end mirror 9 facing the external cavity 13 is coated with an anti-reflection film for the oscillating laser. The side of the external cavity reflector 12 facing the external cavity 13 is coated with a high-reflection film for the oscillating laser. The length of the external cavity is not limited. When the external cavity reflector 12 is a plane mirror, preferably, the length of the external cavity is an integer multiple of the length of the resonant cavity; when the external cavity reflector 12 is a concave mirror, preferably, the length of the external cavity is consistent with the focal length of the concave mirror.

(ii) When the external cavity is placed outside the concave mirror (as shown in FIGS. 5 and 6), the external cavity 13 includes a second concave mirror 5 (or a first concave mirror 4) and an external cavity reflector 12. The side of the second concave mirror 5 facing the resonant cavity 11 is coated with a partially transparent and partially reflective film for the oscillating laser, and the side of the second concave mirror 5 facing the external cavity 13 is coated with an anti-reflective film for the oscillating laser. The side of the external cavity reflector 12 facing the external cavity 13 is coated with a high-reflective film for the oscillating laser. The external cavity reflector 12 is a concave mirror, and its focal length is greater than that of the second concave mirror 5. The length of the external cavity 13 is the difference between the focal length of the external cavity concave reflector 12 and the focal length of the second concave mirror 5.

(III) When the external cavity is placed outside the second end mirror (as shown in FIGS. 7-8), the external cavity 13 includes the second end mirror 6, the beam splitter 14 and the external cavity reflector 12. The side of the second end mirror 6 facing the resonant cavity 11 is coated with a film that partially reflects and partially transmits the oscillating laser, and the side of the second end mirror 6 facing the external cavity 13 is coated with an anti-reflection film for the oscillating laser. The side of the external cavity reflector 12 facing the external cavity 13 is coated with a high-reflection film for the oscillating laser, and the external cavity reflector is a plane mirror. The length of the external cavity is not limited. Preferably, the length of the external cavity is an integer multiple of the length of the resonant cavity.

(iv) When the external cavity is placed outside the laser crystal (Kerr medium) (as shown in FIGS. 9-10 ), the external cavity 13 includes a laser crystal (Kerr medium) 3 and an external cavity reflector 12. The laser crystal (Kerr medium) 3 is placed in the resonant cavity 11 at the Brewster angle to reflect part of the oscillating laser in the cavity onto the external cavity reflector 12. The side of the external cavity reflector 12 facing the external cavity 13 is coated with a high-reflection film for the oscillating laser, and the external cavity reflector is a concave mirror. The length of the external cavity is consistent with the focal length of the external cavity concave reflector 12.

In the above four embodiments of the light perturbation started Kerr lens mode-locked solid-state laser, the design of the resonant cavity 11 is basically the same, and the main difference lies in the design of the external cavity 13. Therefore, the oscillation process of the resonant cavity 11 is first introduced in conjunction with the specific examples of Figures 1 to 10, and then the light perturbation started mode locking process of the external cavity 13 is introduced respectively.

First, the oscillation process of the Kerr lens locked modular solid laser and the disk laser started by light disturbance is explained with reference to the specific examples of FIGS. 1 to 10 .

For the solid laser, the present invention uses a semiconductor laser with fiber coupling output as the pump source 1, the pump wavelength is 940nm, and is focused on the laser crystal 3 through the pump optical path 2. The laser crystal is a 5×5×5mm Yb:KGW crystal. The 1030nm laser generated by Yb:KGW is incident on the second concave mirror 5, and is reflected to the high reflection mirror 7 through the second concave mirror 5. The high reflection mirror 7 reflects the laser to the high dispersion mirror 8, and is reflected to the first end mirror 9 through the high dispersion mirror 8. The first end mirror 9 returns the laser along the original path, reaches the laser crystal 3 again, and is incident on the first concave mirror 4 through the laser crystal 3, and is finally reflected to the second end mirror 6, wherein part of the laser is returned by the second end mirror 6 along the original path and forms oscillation in the resonant cavity 11, and part of the laser passes through the second end mirror 6 and outputs a stable mode-locked pulse laser.

For the disk laser, the present invention uses a semiconductor laser with fiber-coupled output as the pump source 1, with a pump wavelength of 940nm, which is focused on the disk-shaped laser crystal 7 through the pump optical path 2. The disk crystal is a disk-shaped Yb:YAG (doping concentration is 7%) laser crystal with a diameter of 10mm and a thickness of 220μm. The side of the disk-shaped laser crystal facing away from the resonant cavity is fixed on the heat sink, and the heat generated is taken away by water cooling shock. The 1030nm laser generated by the Yb:YAG laser crystal is incident on the high dispersion mirror 8, and is reflected by the high dispersion mirror 8 to the first end mirror 9. The first end mirror 9 returns the laser along the original path, reaches the disk laser crystal 2 again and is reflected to the second concave mirror 5. The second concave mirror 5 reflects the laser and transmits it through the Kerr medium 3 to the first concave mirror 4, and is finally reflected by the first concave mirror 4 to the second end mirror 6, wherein part of the laser is reflected by the second end mirror 6 and returns along the original path to form oscillation in the resonant cavity 11, and part of the laser transmits through the second end mirror 6 to output stable high-power mode-locked pulse laser.

Then, the optical path of the optical perturbation-initiated mode locking is introduced for the outer cavity 13 in FIGS. 1 to 10 respectively.

In Figures 1 to 4, the laser reflected by the first end mirror 9 is divided into two parts. One part is returned by the original path and oscillates in the resonant cavity 11, and the other part passes through the first end mirror 9 and reaches the external cavity reflector 12, is reflected by the external cavity reflector 12 and returns to the resonant cavity 11 by the original path, starting the Kerr lens locking in the form of optical disturbance.

In Figures 5 and 6, the laser reflected on the second concave mirror 5 is divided into two parts. One part is reflected on the laser crystal (Kerr medium) 3, and the other part passes through the second concave mirror 5 and reaches the external cavity reflector 12, is reflected by the external cavity reflector 12 and returns to the resonant cavity 11 along the original path, starting the Kerr lens mode locking in the form of optical disturbance.

In Figures 7 and 8, the laser light passing through the second end mirror 6 is incident on the beam splitter 14. The laser light reaching the beam splitter 14 is divided into two parts. One part passes through the beam splitter 14 and is used for output, and the other part is reflected and reaches the external cavity reflector 12. The laser light is reflected by the external cavity reflector 12 and returns to the resonant cavity 11 along the original path, thereby starting the Kerr lens mode locking in the form of optical disturbance.

In Figures 9 and 10, the laser light reflected by the second concave mirror 5 and reaching the laser crystal (Kerr medium) 3 is divided into two parts. One part passes through the laser crystal (Kerr medium) 3 and is incident on the first concave mirror 4, oscillating in the resonant cavity 11; the other part is reflected by the laser crystal (Kerr medium) 3 onto the external cavity reflector 12, reflected by the external cavity reflector 12 and returned to the resonant cavity 11 along the original path, starting the Kerr lens mode locking in the form of optical disturbance.

The purpose of the optical perturbation-initiated Kerr lens mode-locked solid laser proposed in the present invention is to realize optical perturbation-initiated Kerr lens mode-locking. The design of the resonant cavity is not restricted, and the parameters of the specific optical elements in the cavity are selected according to the design of the resonant cavity. Here, only a disk laser with a repetition frequency of 100MHz is used as a specific embodiment to introduce the relationship between the resonant cavity and the external cavity in detail. According to the same method, the corresponding external cavity structure can be designed for any specific cavity type to realize optical perturbation-initiated Kerr lens mode-locking and obtain stable femtosecond pulse output.

According to the above embodiment, for a disk laser with a repetition frequency of 100MHz, the length of the resonant cavity, i.e., the distance between the first end mirror 9 and the second end mirror 6, is 1.5m, the curvature radius of the first concave mirror 4 and the second concave mirror 5 are both 150mm, the distance between the first concave mirror 4 and the first end mirror 9 is the long arm end of the resonant cavity, the distance between the second concave mirror 5 and the second end mirror 6 is the short arm end, and the ratio of the length of the long arm end to the short arm end is about 3.3:1. When the distance between the first concave mirror 4 and the second concave mirror 5 is between 150mm and 156mm, the resonant cavity is in the first stable zone, and when the distance between the first concave mirror 4 and the second concave mirror 5 is between 175mm and 181mm, the resonant cavity is in the second stable zone. Kerr lens mode locking can be achieved when the distance between the first concave mirror 4 and the second concave mirror 5 is near the four positions of 150.5mm, 155.5mm, 175.5mm and 180.5mm. Preferably, the distance between the first concave mirror 4 and the second concave mirror 5 is 155.5 mm, and Kerr lens locking is most easily achieved. The Kerr medium 3 is preferably a sapphire sheet with a thickness of 2 mm, and the total negative dispersion in the resonant cavity 11 is preferably -14000 fs2. The side of the second end mirror 6 facing the resonant cavity 11 is coated with a partially transparent film for the oscillating laser, and the transmittance ranges from 1% to 20%, preferably, the transmittance is 8% to 10%, and the side of the second end mirror 6 facing away from the resonant cavity 11 is coated with an anti-reflection film, and the transmittance is greater than 99.9%.

Here, only the design of the external cavity when the resonant cavity 11 is in the above embodiment is introduced. When the external cavity design is shown in Figure 3, the side of the first end mirror 9 facing the resonant cavity 11 is coated with a partially transparent film for the oscillating laser, and the transmittance range is 0.5% to 5%, preferably, the transmittance is 0.5% to 1.5%, and the side of the first end mirror 9 facing the external cavity 13 is coated with an anti-reflection film, and the transmittance is greater than 99.9%. The length of the external cavity is not limited, and is preferably a value that is an integer multiple of 1.5mm; when the external cavity design is shown in Figure 4, the side of the first end mirror 9 facing the resonant cavity 11 is coated with a partially transparent film for the oscillating laser, and the transmittance range is 0.5% to 5%, preferably, the transmittance is 0.5% to 1.5%, and the side of the first end mirror 9 facing the external cavity 13 is coated with an anti-reflection film, and the transmittance is greater than 99.9%. The length of the external cavity is not limited, and the radius of curvature of the external cavity concave reflector 12 is not limited. Preferably, when the radius of curvature of the external cavity reflector 12 is 300 mm, the length of the external cavity is 150 mm. When the external cavity is designed as shown in FIG6 , the side of the second concave mirror 5 facing the resonant cavity 11 is coated with a partially transparent film for the oscillation laser, and the transmittance range is 0.5% to 5%, preferably, the transmittance is 0.5% to 1.5%, and the side of the second concave mirror 5 facing the external cavity 13 is coated with an anti-reflection film, and the transmittance is greater than 99.9%. The radius of curvature of the external cavity concave reflector 12 is greater than the radius of curvature of the second concave mirror 5, and the length of the external cavity is the difference between the focal length of the external cavity concave reflector and the focal length of the second concave mirror. Preferably, when the radius of curvature of the external cavity concave reflector 12 is 300 mm, the length of the external cavity is 75 mm. When the external cavity is designed as shown in FIG8 , the length of the external cavity is not limited. Preferably, the length of the external cavity is an integer multiple of 1.5 m. When the external cavity is designed as shown in FIG10 , the length of the external cavity and the radius of curvature of the external cavity concave reflector are not limited. Preferably, when the radius of curvature of the external cavity reflector 12 is 300 mm, the length of the external cavity is 150 mm.

The present invention solves the problem that traditional Kerr lens mode locking requires workers to apply mechanical disturbance to start mode locking. This method of starting Kerr lens mode locking by light disturbance has high stability, avoids the problem of optical path deviation caused by pushing the resonant cavity mirror due to long-term use, and can obtain high-stability femtosecond laser pulses for starting Kerr lens mode locking by light disturbance, thereby realizing a high-stability, narrow-pulse-width femtosecond laser with simple structure, easy operation, and repeatable assembly. The light disturbance-started Kerr lens mode locking solid laser proposed by the present invention has the advantages of simple structure, good practicality and operability, batch production, repeatable assembly, etc., and can be widely used in scientific research, industrial production, biomedicine and other fields.

It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims (14)

1. A solid laser based on Kerr lens mode locking, characterized in that it comprises: a pump source, a laser crystal, a resonant cavity and an external cavity reflector; The pump laser output by the pump source is focused onto the laser crystal via the pump optical path; the laser generated by the laser crystal oscillates back and forth in the round-trip optical path provided by the resonant cavity; The external cavity reflector is used to reflect the laser transmitted from the resonant cavity back to the resonant cavity, thereby starting the Kerr lens mode locking in the form of light disturbance; When the solid-state laser is a solid-state laser other than a disk laser, the resonant cavity comprises: The first end mirror and the second end mirror arranged at the two ends of the resonant cavity are used to reflect the oscillating laser in the resonant cavity and provide a round-trip optical path for the oscillating laser; the first end mirror is a plane high-reflection mirror; the second end mirror is an output mirror; A first concave mirror and a second concave mirror are arranged on a round-trip optical path, wherein the first concave mirror and the second concave mirror form a tight focusing structure for focusing the oscillating laser in the resonant cavity onto the laser crystal to achieve Kerr lens mode locking; the laser crystal is located between the first concave mirror and the second concave mirror; A high dispersion mirror arranged on the round-trip optical path, used to compensate for the dispersion introduced by the laser crystal and the optical elements constituting the resonant cavity; When the solid-state laser is a disk laser, the resonant cavity comprises: The first end mirror and the second end mirror arranged at the two ends of the resonant cavity are used to reflect the oscillating laser in the resonant cavity and provide a round-trip optical path for the oscillating laser; the first end mirror is a plane high-reflection mirror; the second end mirror is an output mirror; A first concave mirror and a second concave mirror are arranged on a round-trip optical path, wherein the first concave mirror and the second concave mirror form a tight focusing structure for focusing the oscillating laser in the resonant cavity onto a Kerr medium to achieve Kerr lens mode locking; the Kerr medium is located between the first concave mirror and the second concave mirror; A high dispersion mirror arranged on the round-trip optical path, used to compensate for the dispersion introduced by the laser crystal and the optical elements constituting the resonant cavity; At this time, the laser crystal is a disk-shaped laser crystal, which is also used as a folding mirror to reflect the oscillated laser; The external cavity reflector is arranged outside the resonant cavity and is located on the side of the first end mirror facing away from the resonant cavity; the first end mirror is used to partially transmit and partially reflect the oscillating laser in the resonant cavity; the external cavity reflector is used to reflect the laser transmitted by the first end mirror and return the laser to the resonant cavity along its original path, thereby starting Kerr lens mode locking in the form of optical disturbance; The external cavity reflector is a plane mirror or a concave mirror; When the external cavity reflector is a plane mirror, the external cavity length is an integer multiple of the length of the resonant cavity; when the external cavity reflector is a concave mirror, the external cavity length is equal to the focal length of the external cavity reflector; wherein the external cavity length is the distance between the external cavity reflector and the first end mirror. 2. The solid-state laser according to claim 1 is characterized in that the resonant cavity further comprises: an aperture arranged on the round-trip optical path to increase the diffraction loss in the resonant cavity. 3. The solid-state laser according to claim 1 or 2, characterized in that the side of the external cavity reflector close to the first end mirror is coated with a high-reflection film for oscillating laser, and the reflectivity is greater than 99.9%; The first concave mirror and the second concave mirror are both coated with a high-reflection film for oscillating lasers on one side facing the resonant cavity, and the reflectivity is greater than 99.9%. 4. The solid-state laser according to claim 1 or 2 is characterized in that the side of the first end mirror away from the external cavity reflector is coated with a partial reflection and partial transmission film for the oscillating laser, and the transmittance range is 0.5% to 20%; the side of the first end mirror close to the external cavity reflector is coated with an anti-reflection film for the oscillating laser, and the transmittance is greater than 99.5%. 5. A solid laser based on Kerr lens mode locking, characterized in that it comprises: a pump source, a laser crystal, a resonant cavity and an external cavity reflector; The pump laser output by the pump source is focused onto the laser crystal via the pump optical path; the laser generated by the laser crystal oscillates back and forth in the round-trip optical path provided by the resonant cavity; The external cavity reflector is used to reflect the laser transmitted from the resonant cavity back to the resonant cavity, thereby starting the Kerr lens mode locking in the form of light disturbance; When the solid-state laser is a solid-state laser other than a disk laser, the resonant cavity comprises: The first end mirror and the second end mirror arranged at the two ends of the resonant cavity are used to reflect the oscillating laser in the resonant cavity and provide a round-trip optical path for the oscillating laser; the first end mirror is a plane high-reflection mirror; the second end mirror is an output mirror; A first concave mirror and a second concave mirror are arranged on a round-trip optical path, wherein the first concave mirror and the second concave mirror form a tight focusing structure for focusing the oscillating laser in the resonant cavity onto the laser crystal to achieve Kerr lens mode locking; the laser crystal is located between the first concave mirror and the second concave mirror; A high dispersion mirror arranged on the round-trip optical path, used to compensate for the dispersion introduced by the laser crystal and the optical elements constituting the resonant cavity; When the solid-state laser is a disk laser, the resonant cavity comprises: The first end mirror and the second end mirror arranged at the two ends of the resonant cavity are used to reflect the oscillating laser in the resonant cavity and provide a round-trip optical path for the oscillating laser; the first end mirror is a plane high-reflection mirror; the second end mirror is an output mirror; A first concave mirror and a second concave mirror are arranged on a round-trip optical path, wherein the first concave mirror and the second concave mirror form a tight focusing structure for focusing the oscillating laser in the resonant cavity onto a Kerr medium to achieve Kerr lens mode locking; the Kerr medium is located between the first concave mirror and the second concave mirror; A high dispersion mirror arranged on the round-trip optical path, used to compensate for the dispersion introduced by the laser crystal and the optical elements constituting the resonant cavity; At this time, the laser crystal is a disk-shaped laser crystal, which is also used as a folding mirror to reflect the oscillated laser; The external cavity reflector is arranged outside the resonant cavity and is located on the side of the second end mirror facing away from the resonant cavity; in this case, the solid-state laser further comprises a beam splitter located between the second end mirror and the external cavity reflector, which is used to transmit part of the laser light transmitted by the second end mirror and reflect part of it onto the external cavity reflector, and to transmit part of the laser light reflected back by the external cavity reflector and reflect part of it back onto the second end mirror; The second end mirror is used to partially transmit and partially reflect the oscillating laser in the resonant cavity; the external cavity reflector is used to reflect the laser reflected by the beam splitter and return it to the resonant cavity along its original path, thereby starting the Kerr lens mode locking in the form of optical disturbance; Wherein, the external cavity reflector is a plane mirror; The length of the external cavity is an integer multiple of the length of the resonant cavity; wherein the length of the external cavity is the distance between the external cavity reflector and the second end mirror. 6. The solid-state laser according to claim 5, characterized in that the resonant cavity further comprises: an aperture arranged on the round-trip optical path for increasing the diffraction loss in the resonant cavity. 7. The solid-state laser according to claim 5 or 6, characterized in that the side of the external cavity reflector close to the second end mirror is coated with a high-reflection film for oscillating laser, and the reflectivity is greater than 99.9%; The first concave mirror and the second concave mirror are both coated with a high-reflection film for oscillating laser light on one side facing the resonant cavity, and the reflectivity is greater than 99.9%. 8. A solid laser based on Kerr lens mode locking, characterized by comprising: a pump source, a laser crystal, a resonant cavity and an external cavity reflector; The pump laser output by the pump source is focused onto the laser crystal via the pump optical path; the laser generated by the laser crystal oscillates back and forth in the round-trip optical path provided by the resonant cavity; The external cavity reflector is used to reflect the laser transmitted from the resonant cavity back to the resonant cavity, thereby starting the Kerr lens mode locking in the form of light disturbance; When the solid-state laser is a solid-state laser other than a disk laser, the resonant cavity comprises: The first end mirror and the second end mirror arranged at the two ends of the resonant cavity are used to reflect the oscillating laser in the resonant cavity and provide a round-trip optical path for the oscillating laser; the first end mirror is a plane high-reflection mirror; the second end mirror is an output mirror; A first concave mirror and a second concave mirror are arranged on a round-trip optical path, wherein the first concave mirror and the second concave mirror form a tight focusing structure for focusing the oscillating laser in the resonant cavity onto the laser crystal to achieve Kerr lens mode locking; the laser crystal is located between the first concave mirror and the second concave mirror; A high dispersion mirror arranged on the round-trip optical path, used to compensate for the dispersion introduced by the laser crystal and the optical elements constituting the resonant cavity; When the solid-state laser is a disk laser, the resonant cavity comprises: The first end mirror and the second end mirror arranged at the two ends of the resonant cavity are used to reflect the oscillating laser in the resonant cavity and provide a round-trip optical path for the oscillating laser; the first end mirror is a plane high-reflection mirror; the second end mirror is an output mirror; A first concave mirror and a second concave mirror are arranged on a round-trip optical path, wherein the first concave mirror and the second concave mirror form a tight focusing structure for focusing the oscillating laser in the resonant cavity onto a Kerr medium to achieve Kerr lens mode locking; the Kerr medium is located between the first concave mirror and the second concave mirror; A high dispersion mirror arranged on the round-trip optical path, used to compensate for the dispersion introduced by the laser crystal and the optical elements constituting the resonant cavity; At this time, the laser crystal is a disk-shaped laser crystal, which is also used as a folding mirror to reflect the oscillated laser; The external cavity reflector is arranged outside the resonant cavity and is located on a side of the selected concave mirror facing away from the resonant cavity; The selected concave mirror is used to partially transmit and partially reflect the oscillating laser in the resonant cavity; the external cavity reflector is used to reflect the laser transmitted by the selected concave mirror and return it to the resonant cavity along its original path, thereby starting Kerr lens mode locking in the form of optical disturbance; Wherein, the selected concave mirror is the first concave mirror or the second concave mirror; The external cavity reflector is a concave mirror; the focal length of the external cavity reflector is greater than the focal length of the selected concave mirror; The external cavity length is the difference between the focal length of the external cavity reflector and the focal length of the selected concave mirror; wherein, the external cavity length is the distance between the external cavity reflector and the selected concave mirror. 9. The solid-state laser according to claim 8, characterized in that the resonant cavity further comprises: an aperture arranged on the round-trip optical path for increasing the diffraction loss in the resonant cavity. 10. The solid-state laser according to claim 8 or 9, characterized in that the side of the external cavity reflector close to the selected concave mirror is coated with a high-reflection film for oscillating laser, and the reflectivity is greater than 99.9%; The side of the first end mirror facing the resonant cavity is coated with a high-reflection film for oscillating laser light, and the reflectivity thereof is greater than 99.9%. 11. The solid-state laser according to claim 8 or 9 is characterized in that the side of the selected concave mirror facing the resonant cavity is coated with a partial reflection and partial transmission film for the oscillating laser, and the transmittance range is 0.5% to 5%; the side of the selected concave mirror facing away from the resonant cavity is coated with an anti-reflection film for the oscillating laser, and the transmittance is greater than 99.5%. 12. A solid laser based on Kerr lens mode locking, characterized in that it comprises: a pump source, a laser crystal, a resonant cavity and an external cavity reflector; The pump laser output by the pump source is focused onto the laser crystal via the pump optical path; the laser generated by the laser crystal oscillates back and forth in the round-trip optical path provided by the resonant cavity; The external cavity reflector is used to reflect the laser transmitted from the resonant cavity back to the resonant cavity, thereby starting the Kerr lens mode locking in the form of light disturbance; When the solid-state laser is a solid-state laser other than a disk laser: The resonant cavity comprises: The first end mirror and the second end mirror arranged at the two ends of the resonant cavity are used to reflect the oscillating laser in the resonant cavity and provide a round-trip optical path for the oscillating laser; the first end mirror is a plane high-reflection mirror; the second end mirror is an output mirror; A first concave mirror and a second concave mirror are arranged on a round-trip optical path, wherein the first concave mirror and the second concave mirror form a tight focusing structure for focusing the oscillating laser in the resonant cavity onto the laser crystal to achieve Kerr lens mode locking; the laser crystal is located between the first concave mirror and the second concave mirror; A high dispersion mirror arranged on the round-trip optical path, used to compensate for the dispersion introduced by the laser crystal and the optical elements constituting the resonant cavity; The external cavity reflector is arranged outside the resonant cavity and is located on the side of the laser crystal facing away from the resonant cavity; the laser crystal and the resonant cavity are placed at a Brewster angle, and are used to reflect part of the oscillating laser onto the external cavity reflector; the external cavity reflector is used to reflect the laser reflected by the laser crystal and return it to the resonant cavity along its original path, thereby starting Kerr lens mode locking in the form of light disturbance; at this time, the external cavity reflector is a concave mirror; The external cavity length is equal to the focal length of the external cavity reflector; the external cavity length is the distance between the external cavity reflector and the laser crystal; When the solid-state laser is a disk laser: The resonant cavity comprises: The first end mirror and the second end mirror arranged at the two ends of the resonant cavity are used to reflect the oscillating laser in the resonant cavity and provide a round-trip optical path for the oscillating laser; the first end mirror is a plane high-reflection mirror; the second end mirror is an output mirror; A first concave mirror and a second concave mirror are arranged on a round-trip optical path, wherein the first concave mirror and the second concave mirror form a tight focusing structure for focusing the oscillating laser in the resonant cavity onto a Kerr medium to achieve Kerr lens mode locking; the Kerr medium is located between the first concave mirror and the second concave mirror; A high dispersion mirror arranged on the round-trip optical path, used to compensate for the dispersion introduced by the laser crystal and the optical elements constituting the resonant cavity; At this time, the laser crystal is a disk-shaped laser crystal, which is also used as a folding mirror to reflect the oscillated laser; The external cavity reflector is arranged outside the resonant cavity and is located on the side of the Kerr medium facing away from the resonant cavity; the Kerr medium and the resonant cavity are placed at a Brewster angle, and are used to reflect part of the oscillating laser onto the external cavity reflector; the external cavity reflector is used to reflect the laser reflected by the Kerr medium and return it to the resonant cavity along its original path, thereby starting the Kerr lens mode locking in the form of light disturbance; at this time, the external cavity reflector is a concave mirror; The external cavity length is equal to the focal length of the external cavity reflector; the external cavity length is the distance between the external cavity reflector and the Kerr medium. 13. The solid-state laser according to claim 12, characterized in that the resonant cavity further comprises: an aperture arranged on the round-trip optical path for increasing the diffraction loss in the resonant cavity. 14. The solid-state laser according to claim 12 or 13, characterized in that, when the solid-state laser is a solid-state laser other than a disk laser, the side of the external cavity reflector close to the laser crystal is coated with a high-reflection film for the oscillation laser, and the reflectivity is greater than 99.9%; the sides of the first concave mirror and the second concave mirror facing the resonant cavity are both coated with a high-reflection film for the oscillation laser, and the reflectivity is greater than 99.9%; the side of the first end mirror facing the resonant cavity is coated with a high-reflection film for the oscillation laser, and the reflectivity is greater than 99.9%; When the solid-state laser is a disk laser, the side of the external cavity reflector close to the laser crystal is coated with a high-reflection film for the oscillating laser, and the reflectivity is greater than 99.9%; the sides of the first concave mirror and the second concave mirror facing the resonant cavity are both coated with a high-reflection film for the oscillating laser, and the reflectivity is greater than 99.9%; the side of the first end mirror facing the resonant cavity is coated with a high-reflection film for the oscillating laser, and the reflectivity is greater than 99.9%.