阅读说明：本技术 一种侧面泵浦Yb:YAG超短脉冲激光放大器 (YAG ultra-short pulse laser amplifier with side pump ) 是由 刘兆军 王上 赵智刚 丛振华 张行愚 于 2021-09-13 设计创作，主要内容包括：本发明涉及一种基于侧面泵浦结构的Yb:YAG超短脉冲激光放大器,包括沿光路依次设置的种子光源、光隔离器、第一聚焦透镜、第一Yb:YAG侧面泵浦模块、法拉第旋光器、第二聚焦透镜、第三聚焦透镜、第二Yb:YAG侧面泵浦模块。综合技术补偿Yb:YAG准三能级系统泵浦阈值高以及侧面泵浦强度低的不足之后,侧面泵浦结构可以利用闪光灯、半导体激光巴条等低亮度光源轻松实现千瓦量级以上的泵浦功率,利用其高功率弥补亮度不足的缺点。同时侧面泵浦可以使用更大尺寸的Yb:YAG晶体作为增益介质,既为泵浦光提供足够大的泵浦面积,又为信号光提供足够高的增益。(The invention relates to a Yb: YAG ultrashort pulse laser amplifier based on a side pumping structure, which comprises a seed light source, an optical isolator, a first focusing lens, a first Yb: YAG side pumping module, a Faraday optical rotator, a second focusing lens, a third focusing lens and a second Yb: YAG side pumping module which are sequentially arranged along a light path. After the defects of high pumping threshold and low side pumping intensity of a Yb-YAG quasi-three-level system are compensated by the comprehensive technology, the side pumping structure can easily realize pumping power of more than kilowatt magnitude by using low-brightness light sources such as a flash lamp and a semiconductor laser bar, and the defect of insufficient brightness is overcome by using high power of the side pumping structure. Meanwhile, the Yb/YAG crystal with larger size can be used as a gain medium for side pumping, so that a sufficiently large pumping area is provided for pumping light, and sufficiently high gain is provided for signal light.)

1. A Yb: YAG ultrashort pulse laser amplifier based on a side pumping structure is characterized by comprising a seed light source, an optical isolator, a first focusing lens, a first Yb: YAG side pumping module, a Faraday optical rotator, a second focusing lens, a third focusing lens and a second Yb: YAG side pumping module which are sequentially arranged along a light path;

YAG side pumping module, amplifying, polarization direction adjusting, and clockwise rotating by 90 degrees; then, signal light is subjected to beam adjustment through a 4f imaging system consisting of a second focusing lens and a third focusing lens, and the adjusted signal light enters a second Yb, YAG side pumping module for amplification; finally, the signal light is output from the second Yb: YAG side pumping module.

2. The Yb: YAG ultrashort pulse laser amplifier based on the side pumping structure of claim 1, wherein the first Yb: YAG side pumping module comprises a first Yb: YAG crystal, a first 940nm semiconductor laser bar, a first cooling water input port, a first cooling water output port; the first 940nm semiconductor laser bars are distributed at equal angle intervals by taking a first Yb: YAG crystal as a center, a cooling water channel is arranged between the first 940nm semiconductor laser bars and the first Yb: YAG crystal, cooling water enters a first Yb: YAG side pumping module from a first cooling water inlet, and after contacting with the first Yb: YAG crystal, the cooling water takes away part of heat generated by the first Yb: YAG crystal and flows out from a first cooling water outlet to dissipate heat of the first Yb: YAG crystal;

the second Yb: YAG side pumping module comprises a second Yb: YAG crystal, a second 940nm semiconductor laser bar, a second cooling water inlet and a second cooling water outlet; the second 940nm semiconductor laser bars are distributed at equal angle intervals by taking the second Yb: YAG crystal as a center, a cooling water channel is arranged between the second 940nm semiconductor laser bars and the second Yb: YAG crystal, cooling water enters the second Yb: YAG side pumping module from a second cooling water inlet, and after the cooling water is contacted with the second Yb: YAG crystal, part of heat generated by the cooling water is taken away and flows out from a second cooling water outlet to dissipate heat of the second Yb: YAG crystal.

3. The Yb: YAG ultrashort pulse laser amplifier based on the side-pumped structure as claimed in claim 1, wherein the diameter of the first Yb: YAG crystal and the diameter of the second Yb: YAG crystal are 4-10 mm, and the total length of the first Yb: YAG crystal and the second Yb: YAG crystal is 80-120 mm.

4. The Yb: YAG ultrashort pulse laser amplifier based on the side-pumped structure as claimed in claim 1, wherein the diameter of the first Yb: YAG crystal and the second Yb: YAG crystal is 5mm, and the total length of the first Yb: YAG crystal and the second Yb: YAG crystal is 100 mm.

5. The Yb: YAG ultrashort pulse laser amplifier based on side-pumped structure as claimed in claim 2, wherein the number of the first 940nm semiconductor laser bars and the second 940nm semiconductor laser bars is 20-30, and the total output power is 1200-1800W;

more preferably, the number of the first 940nm semiconductor laser bars and the second 940nm semiconductor laser bars is 20, and the total output power is 1200W.

6. The Yb: YAG ultrashort pulse laser amplifier based on the side-pumped structure as claimed in claim 2, wherein the first Yb: YAG crystal and the second Yb: YAG crystal both adopt a bonded structure, and each of the two ends of the first Yb: YAG crystal and the second Yb: YAG crystal is provided with a section of undoped pure YAG crystal, and the middle part of the Yb: YAG crystal is a Yb: YAG crystal, and Yb of the Yb: YAG crystal is Yb³⁺The doping concentration range of the ions is 0.5-3.0 at.%;

further preferably, Yb of the first Yb: YAG crystal and the second Yb: YAG crystal³⁺The ion doping concentration was 1 at.%.

7. The Yb: YAG ultrashort pulse laser amplifier based on the side pumping structure as claimed in claim 2, wherein the temperature of the cooling water is set to be 20-25 ℃, and the flow rate of the cooling water is greater than 10L/min.

8. The Yb: YAG ultrashort pulse laser amplifier based on the side pumping structure as claimed in claim 1, wherein the signal light output by the seed light source is linearly polarized light, the central wavelength is 1030nm, and the pulse width is in picosecond or femtosecond order.

9. The Yb: YAG ultrashort pulse laser amplifier based on the side-pumped structure as claimed in claim 1, wherein the transmission spectrum range of the optical isolator is 1020-1040 nm, the transmittance is greater than 95%, the isolation is greater than 30dB, and the clear aperture is 8 mm.

10. The Yb: YAG ultrashort pulse laser amplifier based on side-pumped structure as claimed in any of claims 1-9, wherein the first focusing lens is a plano-convex lens, the coating range is 1020-1060 nm, and the focal length is 200 mm; the second focusing lens is a plano-convex lens, the coating range is 1020-1060 nm, and the focal length is 100 mm; the third focusing lens is a plano-convex lens, the coating range is 1020-1060 nm, and the focal length is 75 mm.

Technical Field

The invention relates to a Yb-YAG ultrashort pulse laser amplifier based on a side pumping structure, and belongs to the technical field of laser amplifiers.

Background

Ultrashort pulse laser in generalMeaning that the pulse width is in picoseconds (10)^-12s) or femtosecond (10)^-15s) magnitude pulse laser, which has the characteristics of extremely narrow pulse, extremely wide spectrum, extremely high peak power and the like. Due to the cold processing characteristic of ultrashort pulse laser, ultrashort pulse laser has been widely and deeply applied in the fields of scientific research, industrial production, biomedical treatment, semiconductor material processing and the like, and the market demand for ultrashort pulse laser with higher power and higher energy is increasing day by day.

The mode-locked laser is an effective technical means for obtaining ultrashort pulse laser, however, the power and energy of the pulse laser directly obtained from the mode-locked resonant cavity are generally low, and a laser amplifier is required to amplify the pulse laser to meet application requirements. Among them, ytterbium-doped yttrium aluminum garnet (Yb: YAG) crystals are widely used as gain media for ultrashort pulse laser amplifiers due to their excellent physical and optical characteristics. From the viewpoint of pumping structure, current Yb: YAG amplifiers are mainly based on end-pumped structures such as a thin-rod Yb: YAG amplifier, a Yb: YAG single crystal fiber amplifier, a Yb: YAG slab amplifier, and a Yb: YAG thin-plate amplifier.

The end-pumped structure is simple, but the effective gain that the signal light can obtain in the crystal is sensitive to the brightness of the pump light. The pumping light source of the Yb: YAG amplifier mainly takes a semiconductor laser coupled and output by an optical fiber as a main component, and one of the obvious characteristics is low brightness. Taking 940nm semiconductor laser with 150W power as an example, the numerical aperture of the fiber core of the output optical fiber is 0.15, the diameter of the fiber core is 105 μ M, and the beam quality factor M of the output laser²Is close to 26 and is the highest brightness of the current high-power fiber-coupled-out semiconductor lasers. Low brightness means that the rayleigh length of the semiconductor output laser is limited, leading to a short length of the crystal that can provide effective gain for the signal light. For example, an end-pumped fine-rod Yb: YAG amplifier, the effective crystal length is typically less than 50 mm. In addition, the output power of the fiber-coupled semiconductor laser is limited, and the output power of a single module of a 940nm or 969nm commercially available semiconductor laser is generally below 1kW at present, which greatly limits the output power of a Yb: YAG laser amplifier.

Compared with end pumping, the side pumping structure can easily realize pumping power of more than kilowatt level by using low-brightness light sources such as flash lamps and semiconductor laser bars, and the defect of insufficient brightness is overcome by using high power of the side pumping structure. In addition, the side-pumped structure can use a larger size crystal as the gain medium, providing a large pumping area while also providing sufficient space for the crystal to dissipate heat.

However, side-pumped structures are currently widely used in Nd: YAG crystal or ceramic oscillators and amplifiers, and although there has been little early research on Yb: YAG side-pumped structures, they are limited to continuous laser oscillators. By energy level structural analysis, Nd: YAG belongs to a four-level system, while Yb: YAG belongs to a quasi-three-level system. In contrast, the quasi-three-level system requires a higher pumping threshold for population inversion, and requires higher pump brightness. The side-pumped structure has the pump light filled in the whole crystal, and the pump power density per unit volume is relatively small because the size of the crystal is generally relatively large. In contrast, the pumping spot in the end-pumped structure is usually small, and the diameter of the pumping spot is usually about 200-500 μm, so that the pumping power density of the two is close to an order of magnitude difference under the same power. Therefore, the side-pumped structure has not been widely used in Yb: YAG laser amplifiers.

Since Nd: YAG and Yb: YAG are two different energy level structure mechanisms, the side pumping scheme of the Nd: YAG four-level system cannot be simply applied to the Yb: YAG crystal. The problem of high Yb: YAG pumping threshold value is solved by technical improvement, so that the advantage of high side pumping power is fully utilized, such as optimizing a side pumping structure, controlling the crystal thermal effect by adopting a low-temperature refrigeration technology, increasing the Yb: YAG crystal length, improving the Yb: YAG crystal ion doping concentration, and changing the Yb: YAG crystal ion doping distribution, such as adopting a radial gradient doping technology, namely, the crystal center position is highly doped and the doping concentration is gradually reduced along the radial direction.

In addition, the ultrashort pulse laser amplifier not only considers the power characteristic of the pump laser, but also the gain characteristic of the signal light, such as the spectral gain bandwidth of the signal light, is important. It is known that the spectral gain bandwidth of Yb: YAG is close to 9nm, compared to the gain bandwidth of Nd: YAG of about 0.6nm, and thus the Yb: YAG laser amplifier can support femtosecond pulse laser output, while the Nd: YAG can be used only for picosecond pulse laser amplifiers. Moreover, the pump absorption bandwidth (8 nm) of Yb: YAG is more than twice that of Nd: YAG crystal (<4nm), which is more advantageous for the absorption of pump light.

In comprehensive consideration, compared with end pumping, the side pumping structure can easily realize pumping power of kilowatt magnitude or above by using a low-brightness pumping light source. At the same time. After the defects of high pumping threshold and low side pumping intensity of a Yb-YAG quasi-three-level system are compensated through the technology, the advantage of high side pumping power can be further fully utilized. Therefore, the side-pumped structure provides an effective technical scheme for further improving the output power of the Yb: YAG ultrashort pulse laser amplifier.

Disclosure of Invention

Aiming at the defects of the prior art and comprehensively considering the advantages of a side-pumped structure and the research defects of a side-pumped Yb: YAG amplifier, the invention provides a Yb: YAG ultrashort pulse laser amplifier based on the side-pumped structure.

YAG laser amplifier adopts semiconductor laser bars as pumping light source, and can easily realize pumping power above kilowatt level. After the defects of high pumping threshold and low side pumping intensity of a Yb-YAG quasi-three-level system are compensated by the comprehensive technology, the advantage of high side pumping power can be further fully utilized. In addition, compared with an end-pumped structure, the amplifier adopts Yb: YAG crystal with larger size as a gain medium, which can provide enough pumping area for pumping light, enough high gain for signal light and enough space for crystal heat dissipation. Therefore, the amplifier provides an effective technical scheme for further improving the output power of the ultrashort pulse laser amplifier taking Yb: YAG as a gain medium.

Interpretation of terms:

1. yb: YAG: ytterbium-doped yttrium aluminum garnet;

2. YAG: yttrium aluminum garnet;

the technical scheme of the invention is as follows:

a Yb: YAG ultrashort pulse laser amplifier based on a side pumping structure comprises a seed light source, an optical isolator, a first focusing lens, a first Yb: YAG side pumping module, a Faraday optical rotator, a second focusing lens, a third focusing lens and a second Yb: YAG side pumping module, wherein the seed light source, the optical isolator, the first focusing lens, the first Yb: YAG side pumping module, the Faraday optical rotator, the second focusing lens, the third focusing lens and the second Yb: YAG side pumping module are sequentially arranged along a light path;

YAG side pumping module, amplifying, polarization direction adjusting, and clockwise rotating by 90 degrees; then, signal light is subjected to beam adjustment through a 4f imaging system consisting of a second focusing lens and a third focusing lens, and the adjusted signal light enters a second Yb, YAG side pumping module for amplification; finally, the signal light is output from a second Yb: YAG side pumping module;

the optical isolator only allows signal light to pass through in the forward direction and isolates laser transmitted in the reverse direction, so that the purpose of protecting the seed light source is achieved; the Faraday rotator is used for adjusting the polarization direction of signal light and compensating the thermal depolarization effect of the two-stage Yb, namely the YAG side pumping amplifier during high-power work; the second focusing lens and the third focusing lens form a 4f imaging system which is used for adjusting the light beam of the signal light amplified and output by the first Yb: YAG side pumping module so as to realize the effect of compensating the thermal lens effect of the second Yb: YAG side pumping module.

According to the invention, the first Yb: YAG side pumping module comprises a first Yb: YAG crystal, a first 940nm semiconductor laser bar, a first cooling water inlet and a first cooling water outlet; the first 940nm semiconductor laser bars are distributed at regular intervals in a pentagonal shape by taking a first Yb: YAG crystal as a center, a cooling water channel is arranged between the first 940nm semiconductor laser bars and the first Yb: YAG crystal, cooling water enters a first Yb: YAG side pumping module from a first cooling water inlet, and partial heat generated by the cooling water is taken away after the cooling water is contacted with the first Yb: YAG crystal and flows out from a first cooling water outlet to dissipate heat of the first Yb: YAG crystal;

the second Yb: YAG side pumping module comprises a second Yb: YAG crystal, a second 940nm semiconductor laser bar, a second cooling water inlet and a second cooling water outlet; the second 940nm semiconductor laser bars are distributed at regular intervals in a pentagonal shape with a second Yb: YAG crystal as a center, a cooling water channel is arranged between the second 940nm semiconductor laser bars and the second Yb: YAG crystal, cooling water enters the second Yb: YAG side pumping module from a second cooling water inlet, and after the cooling water is contacted with the second Yb: YAG crystal, part of heat generated by the cooling water is taken away and flows out from a second cooling water outlet to dissipate heat of the second Yb: YAG crystal.

The first 940nm semiconductor laser bars provide pumping energy for a first Yb YAG crystal, and the second 940nm semiconductor laser bars provide pumping energy for a second Yb YAG crystal;

according to the invention, the first Yb: YAG crystal and the second Yb: YAG crystal both adopt a bonding structure, an undoped pure YAG crystal is respectively arranged at two ends of the first Yb: YAG crystal and the second Yb: YAG crystal, and the middle part of the Yb: YAG crystal is the Yb: YAG crystal³⁺The doping concentration range of the ions is 0.5-3.0 at.%;

further preferably, Yb of the first Yb: YAG crystal and the second Yb: YAG crystal³⁺The ion doping concentration was 1 at.%.

According to the invention, the diameters of the first Yb to YAG crystal and the second Yb to YAG crystal are 4-10 mm, and the total length of the first Yb to YAG crystal and the second Yb to YAG crystal is 80-120 mm;

further preferably, the diameters of the first Yb: YAG crystal and the second Yb: YAG crystal are 5mm, and the total length of the first Yb: YAG crystal and the second Yb: YAG crystal is 100 mm.

According to the invention, preferably, the number of the first 940nm semiconductor laser bars and the second 940nm semiconductor laser bars is 20-30, and the total output power is 1200-1800W;

more preferably, the number of the first 940nm semiconductor laser bars and the second 940nm semiconductor laser bars is 20, and the total output power is 1200W.

According to the invention, the temperature of the cooling water is preferably set within the range of 20-25 ℃, and the flow rate of the cooling water is more than 10L/min.

According to the invention, the signal light output by the seed light source is linearly polarized light, the central wavelength is 1030nm, and the pulse width is picoseconds (10)^-12s) or femtosecond (10)^-15s) magnitude.

According to the invention, the transmission spectral range of the optical isolator is 1020-1040 nm, the transmittance is greater than 95%, the isolation is greater than 30dB, and the clear aperture is 8 mm.

According to a preferred embodiment of the present invention, the first focusing lens is used for spatial mode adjustment of the signal light;

YAG crystal can produce thermal lens effect under high pump power, and can produce obvious focusing action when signal light passes through the crystal, so as to prevent the excessive focusing of the signal light from causing optical damage to the interior or output end face of the crystal and ensure that the signal light has enough large spot diameter in the whole crystal to extract energy, the first focusing lens is utilized to adjust the signal light, so that the signal light enters the crystal at a certain divergence angle, and the effect of pre-compensating the thermal lens is realized.

According to the invention, the first focusing lens is preferably a plano-convex lens, the coating range is 1020-1060 nm, and the focal length is 200 mm; the second focusing lens is a plano-convex lens, the coating range is 1020-1060 nm, and the focal length is 100 mm; the third focusing lens is a plano-convex lens, the coating range is 1020-1060 nm, and the focal length is 75 mm.

The invention has the beneficial effects that:

1. compared with an end-face pumping structure, the side-face pumping does not need to consider the brightness problem of pumping laser particularly, a low-brightness light source can be used as pumping light, and pumping power above kilowatt level can be realized easily.

2. Compared with an end-pumped structure, the Yb/YAG crystal with larger size can be used as a gain medium for side pumping, so that a sufficient pumping area can be provided for pumping light, sufficient gain is provided for signal light, a sufficient space is provided for crystal heat dissipation, and good crystal heat dissipation efficiency under high power is ensured.

3. Compared with an end-pumped structure, the side-pumped laser enters the Yb: YAG crystal from the side uniformly, the pump light is distributed uniformly along the axial direction of the whole crystal and has almost the same radial thermal distribution gradient at any position along the axial direction, and the signal light can be amplified uniformly when being transmitted along the Yb: YAG crystal, so that the beam distortion caused by the non-uniform thermal effect can be relieved to a greater extent.

Drawings

FIG. 1 is a schematic diagram of a side-pumped Yb: YAG ultrashort pulse laser amplifier according to the present invention;

1. the device comprises a seed light source, 2, an optical isolator, 3, a first focusing lens, 4, a first Yb: YAG crystal, 5, a first 940nm semiconductor laser bar, 6, a first cooling water inlet, 7, a first cooling water outlet, 8, a Faraday optical rotator, 9, a second focusing lens, 10, a third focusing lens, 11, a second Yb: YAG crystal, 12, a second 940nm semiconductor laser bar, 13, a second cooling water inlet, 14 and a second cooling water outlet.

Detailed Description

The present invention will be further described by way of examples, but not limited thereto, with reference to the accompanying drawings.

Examples

A Yb: YAG ultrashort pulse laser amplifier based on a side pumping structure is shown in figure 1 and comprises a seed light source 1, an optical isolator 2, a first focusing lens 3, a first Yb: YAG side pumping module, a Faraday optical rotator 8, a second focusing lens 9, a third focusing lens 10 and a second Yb: YAG side pumping module which are sequentially arranged along a light path;

the signal light output by the seed light source 1 firstly passes through an optical isolator 2 and then passes through a first focusing lens 3 to carry out beam adjustment, the adjusted signal light enters a first Yb, YAG side pumping module to be amplified, and the polarization direction of the amplified signal light is adjusted by a Faraday optical rotator 8 to enable the amplified signal light to rotate 90 degrees clockwise; then, signal light is subjected to beam adjustment through a 4f imaging system consisting of a second focusing lens 9 and a third focusing lens 10, and the adjusted signal light enters a second Yb, YAG side pumping module for amplification; finally, the signal light is output from a second Yb: YAG side pumping module;

the optical isolator 2 only allows the signal light to pass through in the forward direction and isolates the laser transmitted in the reverse direction, so that the purpose of protecting the seed light source is achieved; the Faraday rotator 8 is used for adjusting the polarization direction of signal light and compensating the thermal depolarization effect of the two-stage Yb, namely the YAG side pumping amplifier during high-power work; the second focusing lens 9 and the third focusing lens 10 form a 4f imaging system, and are used for performing beam adjustment on the signal light amplified and output by the first Yb: YAG side pumping module to realize the effect of compensating the thermal lens effect of the second Yb: YAG side pumping module.

The first Yb: YAG side pumping module comprises a first Yb: YAG crystal 4, a first 940nm semiconductor laser bar 5, a first cooling water inlet 6 and a first cooling water outlet 7; the first 940nm semiconductor laser bars 5 are distributed at regular intervals in a pentagonal shape by taking the first Yb: YAG crystal 4 as the center, a cooling water channel is arranged between the first 940nm semiconductor laser bars 5 and the first Yb: YAG crystal 4, cooling water enters the first Yb: YAG side pumping module from a first cooling water inlet 6, and partial heat generated by the cooling water is taken away after the cooling water is contacted with the first Yb: YAG crystal 4 and flows out from a first cooling water outlet 7 to dissipate heat of the first Yb: YAG crystal 4;

the second Yb: YAG side pumping module comprises a second Yb: YAG crystal 11, a second 940nm semiconductor laser bar 12, a second cooling water inlet 13 and a second cooling water outlet 14; the second 940nm semiconductor laser bars 12 are distributed at regular intervals in a pentagonal shape with the second Yb: YAG crystal 11 as the center, a cooling water channel is arranged between the second 940nm semiconductor laser bars 12 and the second Yb: YAG crystal 11, cooling water enters the second Yb: YAG side pumping module from a second cooling water inlet 13, and partial heat generated by the cooling water is taken away after the cooling water is contacted with the second Yb: YAG crystal 11 and flows out from a second cooling water outlet 14 to dissipate heat of the second Yb: YAG crystal 11.

The first 940nm semiconductor laser bar 5 provides pumping energy for the first Yb YAG crystal 4, and the second 940nm semiconductor laser bar 12 provides pumping energy for the second Yb YAG crystal 11;

the first Yb: YAG crystal 4 and the second Yb: YAG crystal 11 both adopt a bonding structure, two ends of the first Yb: YAG crystal 4 and the second Yb: YAG crystal 11 are respectively provided with a section of undoped pure YAG crystal, the middle part is the Yb: YAG crystal, and the Yb thereof is³⁺The doping concentration of the ions was 1 at.%;

the diameters of the first Yb: YAG crystal 4 and the second Yb: YAG crystal 11 are 5mm, and the total length of the first Yb: YAG crystal 4 and the second Yb: YAG crystal 11 is 100 mm;

the number of the first 940nm semiconductor laser bars 5 and the second 940nm semiconductor laser bars 12 is 20, and the total output power is 1200W;

the temperature of the cooling water was set to 25 ℃ and the flow rate of the cooling water was set to 12L/min.

The signal light output by the seed light source 1 is linearly polarized light, the central wavelength is 1030nm, and the pulse width is picoseconds (10)^{- 12}s) or femtosecond (10)^-15s) magnitude.

The transmission spectrum range of the optical isolator 2 is 1020-1040 nm, the transmittance is greater than 95%, the isolation is greater than 30dB, and the clear aperture is 8 mm.

The first focusing lens 3 is a plano-convex lens, the coating range is 1020-1060 nm, and the focal length is 200 mm;

YAG crystal can produce the thermal lens effect under high pump power, can produce obvious focusing effect when the signal light passes through the crystal, in order to prevent the signal light from excessively focusing and causing optical damage to the crystal inside or output terminal surface, and guarantee that the signal light has the spot diameter big enough in the whole crystal to extract energy, so utilize first focusing lens 3 to adjust the signal light, make the signal light enter the crystal with certain divergence angle, thus realize the effect to the precompensation of thermal lens.

The second focusing lens 9 is a plano-convex lens, the coating range is 1020-1060 nm, and the focal length is 100 mm; the third focusing lens 10 is a plano-convex lens, the coating range is 1020-1060 nm, and the focal length is 75 mm;

the second focusing lens 9 and the third focusing lens 10 form a 4f imaging system, and are used for performing beam adjustment on the signal light amplified and output by the first Yb: YAG side pumping module to realize the effect of compensating the thermal lens effect of the second Yb: YAG side pumping module.

The working method of the Yb: YAG ultrashort pulse laser amplifier based on the side pumping structure comprises the following steps:

the signal light output by the seed light source 1 firstly passes through the optical isolator 2, and the optical isolator 2 only allows the signal light to pass through in the forward direction and isolates the laser transmitted in the reverse direction, so that the purpose of protecting the seed light source is achieved; then the signal light is subjected to beam adjustment through a first focusing lens 3, so that the signal light enters a Yb: YAG crystal at a certain divergence angle, and the effect of thermal lens precompensation is realized; the adjusted signal light enters a first Yb: YAG side pumping module for amplification; the polarization direction of the amplified signal light is adjusted by a Faraday optical rotator 8 to enable the amplified signal light to rotate 90 degrees clockwise, and the amplified signal light is used for compensating the thermal depolarization effect of a YAG side pumping amplifier in two stages when the YAG side pumping amplifier works at high power; then, the signal light is subjected to beam adjustment by a 4f imaging system consisting of a second focusing lens 9 and a third focusing lens 10, so that the signal light enters a second Yb: YAG crystal 11 at a certain divergence angle, and the effect of thermal lens precompensation is realized; the signal light after the light beam adjustment enters a second Yb: YAG side pumping module for amplification; and finally, outputting the amplified signal light from a second Yb: YAG side pumping module.