Compact gain-adjustable yb:yag high power femtosecond laser amplifier

Abstract

The application relates to the field of high-power ultrafast laser technology and discloses a compact Yb:YAG high-power femtosecond laser amplifier with adjustable gain, which comprises a double-rod cascade amplifier, a first pumping structure, a second pumping structure and a signal light amplification structure; in the application, two Yb:YAG crystal rods constitute two-stage independent amplifiers, each of which contributes to one-stage gain, and two pumping structures provide pumping for the two-stage amplifiers based on polarization beam splitting multiplexing, so that the sharing of the pumping source is realized, and the system volume and cost are greatly reduced. Meanwhile, based on the adjustable proportion of pumping light power and the differential design of the pumping light spots of the two Yb:YAG crystal rods, the two-stage gain can be flexibly adjusted, and the femtosecond seed light can be amplified to more than 300W. Meanwhile, the application improves the working stability and reliability of the amplifier through the setting of a pumping light trap and feedback adjustment based on the leakage light power.

Description

Technical Field

[0001] This invention relates to the field of high-power ultrafast laser technology, and in particular to a compact Yb:YAG high-power femtosecond laser amplifier with adjustable gain. Background Technology

[0002] High-power, high-beam-quality, and high-stability femtosecond lasers have wide applications in industrial precision machining, micro-nano manufacturing, biomedicine, and strong-field physics. Chirped pulse amplification (CPA) technology, by first broadening and amplifying the femtosecond pulse before compressing it, effectively avoids nonlinear effects and crystal damage during amplification, and is the mainstream technology for high-power femtosecond laser amplification. Yttrium-doped aluminum garnet (Yb:YAG) crystals, due to their low quantum defect rate, high thermal conductivity, and wide absorption bandwidth, have become the preferred gain medium for high-power femtosecond laser amplifiers.

[0003] Rod-shaped crystal amplifiers offer advantages such as simple structure, low cost, and good compatibility with existing lasers; however, severe thermal effects significantly limit their power output. To disperse the thermal load and achieve higher power output, a scheme using cascaded dual-crystal rods has been reported—the seed light passes sequentially through two crystal rods to obtain two stages of gain, with the thermal load shared by both rods. In terms of pumping, single-end pumping causes the pump light to decay exponentially along the crystal length, concentrating heat at the front end, while the rear end may become an absorption region due to pump intensity falling below a threshold. Dual-end pumping injects pump light simultaneously from both ends of the crystal, ensuring the entire crystal is in a net gain state along the axial direction and dispersing the heat source along the axial direction, resulting in a more uniform temperature distribution and effectively mitigating thermal effects. This is the preferred pumping method for high-power amplifiers. However, the pumping structure typically employs multiple independent LDs at each end, leading to a large system size and high cost, which contradicts the trend towards miniaturization and integration of laser amplifiers.

[0004] Therefore, there is an urgent need for a high-power Yb:YAG femtosecond laser amplifier with a compact structure, a small number of pump sources, dual-end pumping, and the ability to ensure long-term stable operation of the system. Summary of the Invention

[0005] Therefore, the technical problem to be solved by the present invention is to overcome the problem that the laser amplifier is large in size and has poor amplification stability due to the use of multiple LDs for independent pumping to achieve high power amplification in the prior art.

[0006] To address the aforementioned technical problems, this invention provides a compact Yb:YAG high-power femtosecond laser amplifier with adjustable gain, comprising: A dual-bar cascaded amplifier includes: a first Yb:YAG crystal rod and a second Yb:YAG crystal rod; The first pump structure is used to generate pump light and divide the pump light into a transmitted beam and a reflected beam with different pump powers and orthogonal polarization. The transmitted beam is connected to the first pump point of the first Yb:YAG crystal rod through its first output end, and the reflected beam is connected to the first pump point of the second Yb:YAG crystal rod through its second output end. The second pump structure is used to generate pump light and divide the pump light into a transmitted beam and a reflected beam with different pump powers and orthogonal polarization. The transmitted beam is connected to the second pump point of the second Yb:YAG crystal rod through its first output end, and the reflected beam is connected to the second pump point of the first Yb:YAG crystal rod through its second output end. The signal light amplification structure includes four dichroic mirrors, which are respectively set at the two ends of the first Yb:YAG crystal rod and the second Yb:YAG crystal rod in the incident light path, so that the input seed light passes through the first dichroic mirror, the first Yb:YAG crystal rod, the second dichroic mirror, the third dichroic mirror, the second Yb:YAG crystal rod and the fourth dichroic mirror in sequence before being output.

[0007] Preferably, both the first pump structure and the second pump structure include: Semiconductor laser diodes generate pump light; The beam splitting ratio adjustment module is set on the pump light transmission path. After collimating the pump light, it adjusts the polarization direction of the pump light before it is emitted. The polarization beam splitting module is positioned on the output path of the beam splitting ratio adjustment module. It splits the pump light into two orthogonally polarized transmitted beams and reflected beams, which are then focused and incident on one pump point of the first Yb:YAG crystal rod and the second Yb:YAG crystal rod, respectively.

[0008] Preferably, the beam splitting ratio adjustment module includes components disposed on the pump light transmission path: A collimating lens is used to collimate the pump light. Electrically controlled rotating eyeglass frames; A half-wave plate is set inside an electrically controlled rotating mirror frame. By adjusting the rotation angle of the half-wave plate, the ratio of pump light power between the transmitted beam and the reflected beam can be adjusted.

[0009] Preferably, the polarization beam splitter module includes: The first polarizing beam splitter splits the input pump light into a transmitted beam and a reflected beam with orthogonal polarization. The first focusing lens focuses the transmitted beam and then incident it onto the Yb:YAG crystal rod. The second polarizing beam splitter causes the incident reflected beam to bend and exit. The second focusing lens focuses the reflected beam after it has been deflected and then directs it onto another Yb:YAG crystal rod.

[0010] Preferably, both the first pump structure and the second pump structure further include: A photodiode is placed in the path of the transmitted beam exiting the second polarizing beam splitter to acquire the leakage light power and generate a feedback signal; The controller, which communicates with the electrically controlled rotating mirror frame and photodiode, is used to adjust the rotation angle of the half-wave plate based on feedback signals, control the pump power ratio of the transmitted beam and the reflected beam, and keep the pump power ratio of the transmitted beam and the reflected beam at a preset ratio threshold.

[0011] Preferably, the first focusing lens in the polarization beam splitting module of the first pump structure and the second pump structure has a first focal length, and the second focusing lens has a second focal length; The second focal length is greater than the first focal length so that the pump spot of the second Yb:YAG crystal rod is larger than the pump spot of the first Yb:YAG crystal rod.

[0012] Preferably, both the first pump structure and the second pump structure further include: The first pump light trap is set on the opposite path of the reflected beam output direction of the first polarizing beam splitter, and is used to absorb the remaining pump light that was not absorbed by the first Yb:YAG crystal rod and was then guided out by the first polarizing beam splitter according to the polarization state. The second pump trap is positioned on the opposite path of the transmitted beam exit direction of the second polarizing beam splitter. It is used to absorb the remaining pump light that was not absorbed by the second Yb:YAG crystal rod and was then guided out by the second polarizing beam splitter according to its polarization state.

[0013] Preferably, seed light is generated using a seed light input module and input to a first dichroic mirror; the seed light input module includes: Femtosecond seed source, used to generate femtosecond pulses; The stretcher expands the femtosecond pulse to a preset level and uses it as seed light output.

[0014] Preferably, after the input seed light passes sequentially through a first dichroic mirror, a first Yb:YAG crystal rod, a second dichroic mirror, a third dichroic mirror, a second Yb:YAG crystal rod, and a fourth dichroic mirror, the output also includes: passing through a compressor to compress to the femtosecond level, and amplifying the output power of the femtosecond pulse.

[0015] Preferably, all four dichroic mirrors are incident at 45°, exhibiting transmission characteristics for the pump light wavelength and reflection characteristics for the seed light wavelength.

[0016] Compared with the prior art, the above-described technical solution of the present invention has the following advantages: The present invention discloses a compact, gain-adjustable Yb:YAG high-power femtosecond laser amplifier based on a first pump structure and a second pump structure disposed at both ends of a dual-rod cascaded amplifier. Polarization-splitting multiplexing technology is employed in the pump structure to divide the pump light into a transmitted beam and a reflected beam, which respectively pump the first and second Yb:YAG crystal rods. This invention uses a dual-end pumping method, with pump light injected simultaneously from both ends of the Yb:YAG crystal rod. This ensures that the entire crystal segment is in a net gain state along the axial direction, avoiding the problem of the rear end potentially becoming an absorption region after single-end pumping. Furthermore, it disperses the heat source along the axial direction, resulting in a more uniform temperature distribution and effectively mitigating thermal effects. Simultaneously, the symmetrical and structurally identical first and second pump structures enable pump source sharing, and the extremely compact polarization-splitting architecture reduces the amplifier's size and cost.

[0017] The two pumping structures of this invention are both based on a semiconductor laser diode (LD), a beam splitting ratio adjustment module, and a polarization beam splitting module. Each Yb:YAG crystal rod has one pumping point at each of its left and right ends, for a total of four pumping points. This is equivalent to two independent amplifier stages sharing two LDs through polarization multiplexing. Therefore, the two semiconductor laser diodes (LDs) of this invention provide pumping for both stages simultaneously through polarization beam splitting multiplexing, achieving dual-rod dual-end pumping. The two Yb:YAG crystal rods constitute two independent amplifier stages, each contributing a stage of gain. This invention completes two stages of four-point pumping using only two LDs, significantly reducing system size and cost.

[0018] This invention incorporates a half-wave plate and an electrically controlled rotating mirror frame after the semiconductor laser diode (LD). This allows for continuous adjustment of the pump power ratio distributed to the upper and lower Yb:YAG crystal rods. The first and second paths can be flexibly configured with different pump powers according to actual gain requirements. Simultaneously, a photodiode is used as a power monitoring device, utilizing the leakage light from the second polarization beam splitter. This photodiode, in conjunction with feedback signals from the controller, adjusts the electrically controlled rotating mirror frame, thereby adjusting the rotation angle of the half-wave plate. This achieves closed-loop control of the pump power ratio and real-time correction of the LD's polarization shift. The closed-loop control of the two LDs is independent of each other, collectively ensuring the long-term stability of the pump power of both the upper and lower amplifier stages.

[0019] This invention achieves differentiated pump spot design for the two Yb:YAG crystal rods by having different focal lengths for the focusing lenses corresponding to the first and second Yb:YAG crystal rods in the two pump structures. Specifically, the second Yb:YAG crystal rod is based on a longer pump lens focal length to obtain a larger pump spot, reduce pump power density, further mitigate thermal effects, ensure the stability of the amplifier, and match the gain density requirements of the second-stage amplification, thus achieving optimized management of the two-stage gain extraction.

[0020] This invention achieves stable amplification of high-power femtosecond lasers based on a polarization-multiplexed dual-pump architecture, a dual-rod cascaded amplification structure, and an active gain management strategy. Dual-end pumping keeps the entire crystal in a net-gain state, avoiding the absorption region problems associated with single-end pumping. The dual-rod cascaded structure disperses the thermal load, and combined with a differentiated pump spot design, effectively mitigates thermal effects while optimizing the management of the two-stage gain extraction. Half-wave plate closed-loop control allows for real-time adjustment of the two-stage power distribution, improving gain configuration flexibility and ensuring the stability of the amplification system. Therefore, the amplifier of this invention, while maintaining an extremely compact polarization-multiplexed architecture, can stably amplify femtosecond seed light to levels exceeding 300W, possessing high average power, high beam quality, and long-term operational stability, providing a compact solution for industrial-grade high-power femtosecond laser amplification. Attached Figure Description

[0021] To make the content of this invention easier to understand, the invention will be further described in detail below with reference to specific embodiments and accompanying drawings, wherein: Figure 1 This is a schematic diagram of the structure of the gain-adjustable compact Yb:YAG high-power femtosecond laser amplifier of the present invention; Figure 2 This is a schematic diagram of the pump structure of the present invention; Figure 3 This is a schematic diagram of the power monitoring and feedback control module; Figure 4 This is a structural diagram of a chirped pulse amplification system based on a compact Yb:YAG high-power femtosecond laser amplifier with adjustable gain; Explanation of reference numerals in the accompanying drawings: 1. Dual-rod cascaded amplifier; 111. First Yb:YAG crystal rod; 112. Second Yb:YAG crystal rod; 2. First pump structure; 3. Second pump structure; 4. Signal light amplification structure; 41. First dichroic mirror; 42. Second dichroic mirror; 43. Third dichroic mirror; 44. Fourth dichroic mirror; 5. Semiconductor laser diode; 6. Beam splitting ratio adjustment module; 61. Collimating lens; 62. Electrically controlled rotating mirror frame; 63. Half-wave plate; 7. Polarization beam splitting module; 71. First polarization beam splitting prism; 72. First focusing lens; 73. Second polarization beam splitting prism; 74. Second focusing lens; 8. Optical fiber; 9. Photodiode; 10. Controller; 11. First pump trap; 12. Second pump trap; 13. Seed light input module; 14. Compression output module. Detailed Implementation

[0022] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, so that those skilled in the art can better understand and implement the present invention. However, the embodiments described are not intended to limit the present invention.

[0023] Reference Figure 1 As shown, the structural schematic diagram of the gain-adjustable compact Yb:YAG high-power femtosecond laser amplifier of the present invention specifically includes: The dual-rod cascaded amplifier 1 includes: a first Yb:YAG crystal rod 111 and a second Yb:YAG crystal rod 112; The first pump structure 2 is used to generate pump light and divide the pump light into a transmitted beam and a reflected beam with different pump powers and orthogonal polarization. The transmitted beam is connected to the first pump point of the first Yb:YAG crystal rod through its first output end, and the reflected beam is connected to the first pump point of the second Yb:YAG crystal rod through its second output end. The second pump structure 3 is used to generate pump light and divide the pump light into a transmitted beam and a reflected beam with different pump powers and orthogonal polarization. The transmitted beam is connected to the second pump point of the second Yb:YAG crystal rod through its first output end, and the reflected beam is connected to the second pump point of the first Yb:YAG crystal rod through its second output end. The signal light amplification structure 4 includes four dichroic mirrors, which are respectively set at the two ends of the first Yb:YAG crystal rod and the second Yb:YAG crystal rod in the incident light path, so that the input seed light passes through the first dichroic mirror 41, the first Yb:YAG crystal rod 111, the second dichroic mirror 42, the third dichroic mirror 43, the second Yb:YAG crystal rod 112 and the fourth dichroic mirror 44 in sequence before being output.

[0024] All four dichroic mirrors are incident at 45°, exhibiting transmission characteristics for the pump light wavelength and reflection characteristics for the seed light wavelength. The seed light is reflected by the first dichroic mirror and enters the first crystal rod for first-stage amplification. After being reflected by the second and third dichroic mirrors, it enters the second crystal rod for second-stage amplification. Finally, it is reflected by the fourth dichroic mirror and output, forming a U-shaped single-pass cascaded amplification optical path.

[0025] Specifically, the first pump structure and the second pump structure of the present invention are respectively disposed at both ends of the dual-bar cascaded amplifier, and their structures are completely identical; refer to Figure 2 The diagram shown is a schematic representation of the pump structure of the present invention; both the first pump structure and the second pump structure include: Semiconductor laser diode 5 generates pump light; The beam splitting ratio adjustment module 6 is set on the pump light transmission path. After collimating the pump light, it adjusts the polarization direction of the pump light before it is emitted. The polarization beam splitting module 7 is set on the output path of the beam splitting ratio adjustment module. It splits the pump light into two orthogonally polarized transmitted beams and reflected beams, and focuses them onto one pump point of the first Yb:YAG crystal rod and the second Yb:YAG crystal rod, respectively.

[0026] In this embodiment, the beam splitting ratio adjustment module 6 includes components disposed on the pump light transmission path: Collimating lens 61 collimates the pump light; 62-inch electrically controlled rotating eyeglass frame; A half-wave plate 63 is disposed within an electrically controlled rotating mirror frame 62. By adjusting the rotation angle of the half-wave plate 63, the pump light power ratio of the transmitted beam and the reflected beam can be adjusted.

[0027] In this embodiment, the polarization beam splitting module 7 includes: The first polarizing beam splitter 71 splits the input pump light into a transmitted beam and a reflected beam with orthogonal polarization. The first focusing lens 72 focuses the transmitted beam and then incident it onto the Yb:YAG crystal rod. The second polarizing beam splitter 73 causes the incident reflected beam to bend and exit. The second focusing lens 74 focuses the reflected beam that has been deflected and then incident it onto another Yb:YAG crystal rod.

[0028] In this invention, two Yb:YAG crystal rods constitute two independent amplifier stages, each contributing a stage of gain. Two semiconductor laser diodes (LDs) simultaneously pump both stages via polarization-splitting multiplexing, resulting in an extremely compact polarization-splitting architecture. Specifically, the pump light from each LD is split into upper and lower paths by a polarization-splitting prism (PBS). The upper path pumps the second Yb:YAG crystal rod (second stage), and the lower path pumps the first Yb:YAG crystal rod (first stage), achieving shared pump sources. Compared to traditional solutions that require independent pump sources for each stage (four LDs for two stages), this invention uses only two LDs to complete both stages of pumping, significantly reducing system size and cost.

[0029] In this embodiment, the first focusing lens 72 in the polarization beam splitting module 7 of the first pump structure 2 and the second pump structure 3 has a first focal length, and the second focusing lens 74 has a second focal length. The second focal length is greater than the first focal length so that the pump spot of the second Yb:YAG crystal rod 112 is larger than the pump spot of the first Yb:YAG crystal rod 111. This reduces the pump power density to mitigate the thermal effect while matching the gain density requirements of the second-stage amplification, thus achieving optimized management of the two-stage gain extraction.

[0030] Combination Figure 1 and Figure 2As can be seen, the first pump structure splits the pump light from a single semiconductor laser diode (LD) into a transmitted beam and a reflected beam with orthogonal polarization via a polarization beam splitter, which are then injected into the first pump points of the first Yb:YAG crystal rod and the second Yb:YAG crystal rod, respectively. The second pump structure splits the pump light from a single semiconductor laser diode (LD) into a transmitted beam and a reflected beam with orthogonal polarization via polarization beam splitter, which are then injected into the second pump points of the second Yb:YAG crystal rod and the first Yb:YAG crystal rod, respectively. The pump light from the two semiconductor laser diodes (LDs) reuses the same optical path space via polarization beam splitter, thus achieving dual-rod dual-end pumping using two semiconductor laser diodes (LDs).

[0031] This invention employs a dual-end pumping method, with pump light injected simultaneously from both ends of the crystal. This ensures the entire crystal segment maintains net gain along the axial direction, avoiding the potential absorption region at the rear end of single-end pumping. Furthermore, it disperses the heat source axially, resulting in a more uniform temperature distribution and effectively mitigating thermal effects. Simultaneously, the pump spots of the two Yb:YAG crystal rods can be designed differently; the second stage uses a longer pump lens focal length to obtain a larger pump spot, reducing pump power density to mitigate thermal effects while matching the gain density requirements of the second-stage amplification, thus achieving optimized management of the two-stage gain extraction. These measures enable this amplifier to amplify femtosecond seed light to levels exceeding 300W.

[0032] Based on the above embodiments, this embodiment of the invention includes a power monitoring and feedback control module, which adjusts the rotation angle of the half-wave plate by collecting the leakage light power, thereby adjusting the pump light power ratio of the pump structure. Specifically, the structure includes: The photodiode 9 is positioned on the path of the transmitted beam from the second polarizing beam splitter 73 to acquire the leakage light power and generate a feedback signal. The controller 10 is connected in communication with the electrically controlled rotating mirror frame 62 and the photodiode 9. It is used to adjust the rotation angle of the half-wave plate 63 based on the feedback signal so that the pump light power ratio of the transmitted beam and the reflected beam is maintained at a preset ratio threshold.

[0033] Reference Figure 3The diagram shows a power monitoring and feedback control module. This module includes photodiodes a and b disposed on the leakage optical path of the second polarization beam splitter in the first and second pump structures, and a controller communicatively connected to the half-wave plate, the electrically controlled rotating mirror frame, and the photodiodes in the first and second pump structures, respectively. For photodiode a in the first pump structure or photodiode b in the second pump structure, the controller executes the same control strategy. The controller adjusts the rotation angle of the half-wave plate according to the feedback signal from the photodiode, maintaining the pump light power ratio allocated to the first Yb:YAG crystal rod and the second Yb:YAG crystal rod at a preset ratio threshold, while simultaneously correcting the polarization shift of the LD in real time. For example, when the leakage light power signal detected by a photodiode increases, it indicates that the polarization state shift of the pump light path has caused a change in the beam splitting ratio. The controller then outputs a control signal to drive the corresponding electrically controlled rotating mirror frame, causing the half-wave plate to rotate by a certain angle, restoring the pump light power of that path to the set value. Conversely, when the monitoring signal decreases, the controller drives the half-wave plate to rotate in the opposite direction.

[0034] Based on dual-end pumping, a half-wave plate and an electrically controlled rotating mirror mount are installed after each LD, allowing continuous adjustment of the pump power ratio distributed to the upper and lower paths. The first and second stages can be flexibly configured with different pump powers according to actual gain requirements. This invention utilizes the leakage light from the polarization beam splitter (PBS), placing a photodiode as a power monitoring device on the leakage light path. This does not occupy space in the main optical path, does not add additional losses, and is low-cost and reliable. Simultaneously, the controller dynamically adjusts the rotation angle of the half-wave plate based on the PD feedback signal, correcting the polarization offset of the LD in real time to ensure long-term stable operation of the system.

[0035] Based on the above embodiments, both the first pump structure 2 and the second pump structure 3 of the present invention further include: The first pump trap 11 is located on the opposite path of the reflected beam exit direction of the first polarizing beam splitter 71. It is used to absorb the remaining pump light that is not absorbed by the first Yb:YAG crystal rod 111 and is then guided out by the first polarizing beam splitter 71 according to the polarization state. The second pump trap 12 is positioned on the opposite path of the transmission beam exit direction of the second polarizing beam splitter 73. It is used to absorb the remaining pump light that is not absorbed by the second Yb:YAG crystal rod 112 and is then guided out by the second polarizing beam splitter 73 according to the polarization state.

[0036] based on Figure 1 and 2As can be seen, the embodiments of the present invention include four pump traps, which are respectively connected to the outside of the two polarization beam splitters in the first pump structure and the second pump structure, and are used to absorb the remaining pump light that is not absorbed by the crystal rods. Specifically, the remaining pump light emitted from the two Yb:YAG crystal rods, based on the same polarization multiplexing optical path, passes through the corresponding dichroic mirrors, is collimated by the focusing lens, and then enters the opposite polarization beam splitter PBS. According to the polarization state, it is guided to the corresponding pump trap for complete absorption, thus completely eliminating the possibility of the remaining pump light returning to the LD from the physical optical path. No additional optical isolator is required, protecting the LD from damage caused by backlighting and significantly improving the system reliability.

[0037] Based on the above embodiments, in this embodiment, the seed light input module 13 generates and broadens femtosecond pulses to generate seed light, which is input to the first dichroic mirror. After passing through a compact Yb:YAG high-power femtosecond laser amplifier with adjustable gain, the light passes sequentially through the first dichroic mirror, the first Yb:YAG crystal rod, the second dichroic mirror, the third dichroic mirror, the second Yb:YAG crystal rod, and the fourth dichroic mirror before being amplified and output. Then, the amplified pulse is compressed by a compressor to obtain a power-amplified femtosecond pulse.

[0038] Specifically, the seed light input module 13 includes: Femtosecond seed source, used to generate femtosecond pulses; The stretcher expands the femtosecond pulse to a preset level and uses it as seed light output.

[0039] Specifically, in this embodiment, the compression output module 14 is a compressor, which outputs a power-amplified femtosecond pulse.

[0040] Specifically, both the LD in the first Yb:YAG crystal rod and the LD2 in the second Yb:YAG crystal rod utilize fiber 8 for output, with output power ranging from hundreds of watts to kilowatts. Two polarized beams with specific power ratios are obtained through a half-wave plate and a PBS. Yb:YAG is an isotropic crystal, exhibiting equal absorption efficiency for both P- and S-beams. These characteristics eliminate the need for crystal axis alignment in the polarization beam splitting scheme, resulting in a simple and reliable system.

[0041] Based on the above embodiments, the seed light is input from the seed light input module. After being broadened, the femtosecond seed light is first incident on the first dichroic mirror. After being reflected by the first dichroic mirror, it enters the first Yb:YAG crystal rod and obtains the first stage of amplification. After exiting the first crystal rod, it is reflected by the second dichroic mirror to the third dichroic mirror, and then reflected by the third dichroic mirror to enter the second Yb:YAG crystal rod and obtain the second stage of amplification. After exiting the second crystal rod, it is reflected by the fourth dichroic mirror and output into the compression output module. After compression, a high-power femtosecond pulse is output as the final amplified output light.

[0042] Based on the above embodiments, referring to Figure 4 The diagram shows the structure of a chirped pulse amplification system based on a compact, gain-adjustable Yb:YAG high-power femtosecond laser amplifier; based on Figure 4 As can be seen, the complete structure of the chirped pulse amplification (CPA) system consists of a first Yb:YAG crystal rod 111 and a second Yb:YAG crystal rod 112 arranged in parallel, a first pump structure 2, a second pump structure 3, a signal light amplification structure 4, a power monitoring and feedback control module, four pump traps, a seed light input module, and a compression output module. This embodiment applies a polarization-splitting multiplexed dual-rod dual-end pump architecture to a dual-rod cascaded single-pass amplification structure—two Yb:YAG crystal rods are placed in parallel, forming two independent amplifier stages; two LDs provide pumps to the two stages through polarization-splitting multiplexing, with one pump point at each end of each rod, totaling four pump points; a half-wave plate enables flexible power distribution between the upper and lower pump paths; a photodiode (PD) power monitoring feedback enables closed-loop control; and differentiated lens focal lengths optimize thermal management. Combined with the seed light input module and the compression output module, this constitutes a complete chirped pulse amplification system.

[0043] In this embodiment, the first Yb:YAG crystal rod 111 and the second Yb:YAG crystal rod 112 are mounted on the same water-cooled heat sink. The heat sink contains microchannels for heat dissipation via circulating cooling water, and both ends are coated with anti-reflection films for pump light and signal light. The doping concentration, length, diameter, and other parameters of the two Yb:YAG crystal rods can be independently selected according to actual needs. Those skilled in the art can optimize the design based on the target output power, two-stage gain distribution, and thermal management conditions.

[0044] Specifically, the LDs in both the first and second pump structures are optical fiber-output semiconductor lasers with an output wavelength of 969nm, which is suitable for pumping Yb:YAG, and a single-tube output power of 400W.

[0045] Specifically, the seed light input module includes a fiber femtosecond laser and a stretcher, with a center wavelength of 1030nm, outputting femtosecond-level pulses. These pulses are stretched to the picosecond to nanosecond range by the stretcher before being input to the amplifier. After being amplified by a dual-rod cascade amplification, the pulse width is compressed back to the femtosecond level by the compression output module. With a total pump power of 800W (400W each for the two LDs) and a seed light power of 30W, the amplified output light power can reach over 300W. After compression, the pulse width can be restored to the femtosecond level, with power stability better than ±1% RMS.

[0046] Specifically, the first pump structure and the second pump structure are centrally symmetrically arranged at the left and right ends of the first Yb:YAG crystal rod and the second Yb:YAG crystal rod. There is one pump point at each of the left and right ends of the first Yb:YAG crystal rod, and one pump point at each of the left and right ends of the second Yb:YAG crystal rod, for a total of four pump points.

[0047] In the first pump structure, the semiconductor laser diode LD1 is positioned below the left end of the dual-rod cascaded amplifier. The pump light output from LD1 propagates to the right, is collimated by the first collimating lens, and then its polarization direction is adjusted by a half-wave plate mounted on an electrically controlled rotating mirror frame before being incident on the first polarizing beam splitter PBS1. The half-wave plate is used to adjust the polarization direction of the incident light, thereby controlling the power ratio of the two pump lights after being split by PBS1. PBS1 splits the beam into two orthogonally polarized beams: the transmitted P-beam is horizontally to the right, focused by the first focusing lens, and injected into the left pump point (first pump point) of the first Yb:YAG crystal rod; the reflected S-beam is vertically upward and incident on the second polarizing beam splitter PBS2. PBS2 reflects and bends the S-beam to the right, focuses it by the second focusing lens, and then injects it into the left pump point (first pump point) of the second Yb:YAG crystal rod. Since PBS2 is not 100% reflective, a small amount of S-polarized light will pass through PBS2 and continue to propagate upwards. A photodiode is placed in this leaky optical path to monitor the pump light power distributed by LD1 to the second Yb:YAG crystal rod. The leaked light power is proportional to the reflected light power entering the left end of the second crystal rod. After calibration, it can accurately reflect the pump power of this path. The first pump trap is located below PBS1, and the second pump trap is located to the left of PBS2.

[0048] In the second pump structure, the semiconductor laser diode LD2 is positioned above the right end of the dual-rod cascaded amplifier. The pump light output from LD2 propagates to the left, is collimated by a collimating lens, and then its polarization direction is adjusted by a half-wave plate mounted on an electrically controlled rotating mirror mount before being incident on its first polarizing beam splitter PBS3. The half-wave plate is used to adjust the polarization direction of the incident light, thereby controlling the power ratio of the two pump lights after being split by PBS3. PBS3 splits the beam into two orthogonally polarized beams: the transmitted P-beam is horizontally to the left, focused by the first focusing lens, and injected into the right pump point (second pump point) of the second Yb:YAG crystal rod; the reflected S-beam is vertically downward and incident on its second polarizing beam splitter PBS4. PBS4 reflects and bends the S-beam to the left, focuses it by the second focusing lens, and then injects it into the right pump point (second pump point) of the first Yb:YAG crystal rod. Since PBS4 is not 100% reflective, a small amount of S-polarized light will pass through PBS4 and continue to propagate downwards. A photodiode is placed in this leaky optical path to monitor the pump light power distributed by LD2 to the first Yb:YAG crystal rod. The leaked light power is in a fixed ratio to the reflected light power entering the right end of the first Yb:YAG crystal rod. After calibration, it can accurately reflect the pump power of this path. The first pump trap is located above PBS3, and the second pump trap is located to the right of PBS4.

[0049] Specifically, a half-wave plate is installed after each LD in both the first and second pump structures, mounted on an electrically controlled rotating mirror mount, to adjust the ratio of pump power distributed to the upper and lower paths. In this embodiment, after adjustment by the half-wave plate, the pump power distributed to the first crystal rod is approximately 300W total (approximately 150W per end), and the pump power distributed to the second crystal rod is approximately 500W total (approximately 250W per end). This power distribution can be adjusted according to actual gain requirements and thermal management conditions.

[0050] In this embodiment, the pump lens corresponding to the first Yb:YAG crystal rod, i.e., the first focusing lens in the first pump structure and the first focusing lens in the second pump structure, has a first focal length; the pump lens corresponding to the second Yb:YAG crystal rod, i.e., the second focusing lens in the first pump structure and the second focusing lens in the second pump structure, has a second focal length. The second focal length is greater than the first focal length, resulting in a larger pump spot and lower pump power density for the second Yb:YAG crystal rod. This effectively mitigates the thermal effects of the second stage while matching the gain density requirements of the second-stage amplification, achieving optimized management of the two-stage gain extraction. In practical applications, the pump lens focal length can be specifically selected based on the pump power, crystal size, and thermal management conditions.

[0051] Specifically, the signal light amplification module includes a first dichroic mirror DM1, a second dichroic mirror DM2, a third dichroic mirror DM3, and a fourth dichroic mirror DM4. Seed light is incident from below the system, reflected by DM1, and enters the first Yb:YAG crystal rod to obtain the first stage of amplification. The amplified light is reflected by DM2 to DM3, and then reflected by DM3 to enter the second Yb:YAG crystal rod to obtain the second stage of amplification. Finally, the amplified light is reflected by DM4 and output. DM1, DM2, DM3, and DM4 are all dichroic mirrors with a 45° incident angle. Their film system is designed to have high transmittance for the pump light wavelength and high reflectance for the seed light wavelength (1030nm).

[0052] Specifically, the power monitoring and feedback control module includes: a photodiode positioned on the leakage optical path above PBS2, a photodiode positioned on the leakage optical path below PBS4, and a controller communicatively connected to the electrically controlled rotating mirror frame and photodiode in the first pump structure's beam splitting ratio adjustment module, and the electrically controlled rotating mirror frame and photodiode in the second pump structure's beam splitting ratio adjustment module. The photodiodes in the first and second pump structures feed back the monitored leakage light power signal to the controller. The controller adjusts the rotation angle of the half-wave plate in the first pump structure based on the feedback signal from the photodiode in the first pump structure to maintain a stable power ratio of LD1 distributed to the upper and lower pump lights. Simultaneously, the controller adjusts the rotation angle of the half-wave plate in the second pump structure based on the feedback signal from the photodiode in the second pump structure to maintain a stable power ratio of LD2 distributed to the upper and lower pump lights. The closed-loop control of the two LDs is independent of each other, jointly ensuring the long-term stability of the pump power of the upper and lower amplifier stages.

[0053] Specifically, four pump traps are connected to the outside of the two pump structures to absorb the remaining pump light that is not absorbed by the Yb:YAG crystal rod. The remaining pump light emitted from the Yb:YAG crystal rod passes through the corresponding dichroic mirrors (DM1~DM4), is collimated by the focusing lens, and is then incident on the opposite end PBS. It is guided according to its polarization state to the corresponding pump trap for complete absorption, thus physically preventing it from returning to the LD.

[0054] This invention employs polarization-splitting multiplexing technology. Two light-emitting diodes (LDs) are dual-bar, dual-end pumped via a polarization-splitting beam splitter (PBS). Each Yb:YAG crystal rod has one pump point at each end, totaling four pump points for the two Yb:YAG crystal rods. This is equivalent to two independent amplifier stages sharing two LDs through polarization multiplexing. A half-wave plate is placed after each LD and mounted on an electrically controlled mirror frame, allowing continuous adjustment of the pump light power ratio distributed to the upper and lower paths. A photodiode is used as a power monitoring device via the leakage light from the PBS, working in conjunction with the controller's feedback signal to achieve closed-loop control and real-time correction of the LD's polarization offset. The pump power and pump spot of the two Yb:YAG crystal rods are differentiated to mitigate thermal effects while optimizing the gain extraction of the two stages. This invention can constitute a complete chirped pulse amplification system, including a seed light input module, a dual-bar cascaded amplifier, and a compression output module connected in sequence. It amplifies femtosecond seed light to over 300W, offering advantages such as compact structure, excellent thermal management, flexible and adjustable power distribution, and good long-term stability.

[0055] Those skilled in the art will understand that embodiments of this application can be provided as methods, systems, or computer program products. Therefore, this application can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, this application can take the form of a computer program product embodied on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0056] This application is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of this application. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart... Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0057] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0058] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0059] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A compact Yb:YAG high-power femtosecond laser amplifier with adjustable gain, characterized in that, include: A dual-bar cascaded amplifier includes: a first Yb:YAG crystal rod and a second Yb:YAG crystal rod; The first pump structure is used to generate pump light and divide the pump light into a transmitted beam and a reflected beam with different pump powers and orthogonal polarization. The transmitted beam is connected to the first pump point of the first Yb:YAG crystal rod through its first output end, and the reflected beam is connected to the first pump point of the second Yb:YAG crystal rod through its second output end. The second pump structure is used to generate pump light and divide the pump light into a transmitted beam and a reflected beam with different pump powers and orthogonal polarization. The transmitted beam is connected to the second pump point of the second Yb:YAG crystal rod through its first output end, and the reflected beam is connected to the second pump point of the first Yb:YAG crystal rod through its second output end. The signal light amplification structure includes four dichroic mirrors, which are respectively set at the two ends of the first Yb:YAG crystal rod and the second Yb:YAG crystal rod in the incident light path, so that the input seed light passes through the first dichroic mirror, the first Yb:YAG crystal rod, the second dichroic mirror, the third dichroic mirror, the second Yb:YAG crystal rod and the fourth dichroic mirror in sequence before being output.

2. The gain-adjustable compact Yb:YAG high-power femtosecond laser amplifier according to claim 1, characterized in that, Both the first pump structure and the second pump structure include: Semiconductor laser diodes generate pump light; The beam splitting ratio adjustment module is set on the pump light transmission path. After collimating the pump light, it adjusts the polarization direction of the pump light before it is emitted. The polarization beam splitting module is positioned on the output path of the beam splitting ratio adjustment module. It splits the pump light into two orthogonally polarized transmitted beams and reflected beams, which are then focused and incident on one pump point of the first Yb:YAG crystal rod and the second Yb:YAG crystal rod, respectively.

3. The gain-adjustable compact Yb:YAG high-power femtosecond laser amplifier according to claim 2, characterized in that, The beam splitting ratio adjustment module includes components disposed on the pump light transmission path: A collimating lens is used to collimate the pump light. Electrically controlled rotating eyeglass frames; A half-wave plate is set inside an electrically controlled rotating mirror frame. By adjusting the rotation angle of the half-wave plate, the ratio of pump light power between the transmitted beam and the reflected beam is adjusted.

4. The gain-adjustable compact Yb:YAG high-power femtosecond laser amplifier according to claim 3, characterized in that, The polarization beam splitting module includes: The first polarizing beam splitter splits the input pump light into a transmitted beam and a reflected beam with orthogonal polarization. The first focusing lens focuses the transmitted beam and then incident it onto the Yb:YAG crystal rod. The second polarizing beam splitter causes the incident reflected beam to bend and exit. The second focusing lens focuses the reflected beam after it has been deflected and then directs it onto another Yb:YAG crystal rod.

5. The gain-adjustable compact Yb:YAG high-power femtosecond laser amplifier according to claim 4, characterized in that, Both the first pump structure and the second pump structure further include: A photodiode is placed in the path of the transmitted beam exiting the second polarizing beam splitter to acquire the leakage light power and generate a feedback signal; The controller, which communicates with the electrically controlled rotating mirror frame and photodiode, is used to adjust the rotation angle of the half-wave plate based on feedback signals, control the pump power ratio of the transmitted beam and the reflected beam, and keep the pump power ratio of the transmitted beam and the reflected beam at a preset ratio threshold.

6. The gain-adjustable compact Yb:YAG high-power femtosecond laser amplifier according to claim 4, characterized in that, The first focusing lens in the polarization beam splitting module of the first pump structure and the second pump structure has a first focal length, and the second focusing lens has a second focal length; The second focal length is greater than the first focal length so that the pump spot of the second Yb:YAG crystal rod is larger than the pump spot of the first Yb:YAG crystal rod.

7. The gain-adjustable compact Yb:YAG high-power femtosecond laser amplifier according to claim 4, characterized in that, Both the first pump structure and the second pump structure further include: The first pump light trap is set on the opposite path of the reflected beam output direction of the first polarizing beam splitter, and is used to absorb the remaining pump light that was not absorbed by the first Yb:YAG crystal rod and was then guided out by the first polarizing beam splitter according to the polarization state. The second pump trap is positioned on the opposite path of the transmitted beam exit direction of the second polarizing beam splitter. It is used to absorb the remaining pump light that was not absorbed by the second Yb:YAG crystal rod and was then guided out by the second polarizing beam splitter according to its polarization state.

8. The gain-adjustable compact Yb:YAG high-power femtosecond laser amplifier according to claim 1, characterized in that, Seed light is generated using the seed light input module and input to the first dichroic mirror; The seed light input module includes: Femtosecond seed source, used to generate femtosecond pulses; The stretcher expands the femtosecond pulse to a preset level and uses it as seed light output.

9. The gain-adjustable compact Yb:YAG high-power femtosecond laser amplifier according to claim 1, characterized in that, The input seed light passes sequentially through the first dichroic mirror, the first Yb:YAG crystal rod, the second dichroic mirror, the third dichroic mirror, the second Yb:YAG crystal rod, and the fourth dichroic mirror before being output. The output also includes: passing through a compressor to compress to the femtosecond level and amplifying the output power of the femtosecond pulse.

10. The gain-adjustable compact Yb:YAG high-power femtosecond laser amplifier according to claim 1, characterized in that, All four dichroic mirrors are incident at 45°, exhibiting transmission characteristics for the pump light wavelength and reflection characteristics for the seed light wavelength.

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