A high-energy GHz-femtosecond pulse train laser amplification system and an amplification method

Abstract

The application provides a high-energy GHz femtosecond pulse train laser amplification system and an amplification method, and solves the technical problem of pulse narrowing effect caused by energy saturation effect in a fiber amplifier during high-energy pulse train amplification; after the pulse train is modulated, the amplitude of the front edge of the modulated pulse is much lower than the back edge, so the pulse train waveform will first experience the change process from exponential rise to gentle rise, then to relatively constant amplitude, and finally gradually evolve into the process of high front and low back and experience pulse narrowing, instead of pulse narrowing effect occurring at the beginning. Therefore, the pulse train modulation delays the occurrence of the pulse narrowing process, effectively suppresses the pulse narrowing effect, reduces the pulse peak power, and thus effectively avoids the accumulation of more nonlinear phase and fiber damage.

Description

Technical Field

[0001] This invention relates to an ultrashort pulse laser amplification system and amplification method, specifically to a high-energy GHz femtosecond pulse train laser amplification system and amplification method. Background Technology

[0002] Fiber femtosecond lasers, due to their inherent properties such as stability, reliability, ease of assembly, and cost-effectiveness, have been widely used in precision machining, biomedicine, and ultrafast scientific research. However, because fiber amplifiers have small core diameters, the laser mode field diameter during amplification is much smaller than that of solid-state amplifiers. Therefore, significant nonlinear effects occur during high-energy ultrashort pulse amplification, limiting their output energy and consequently restricting their application range, especially in high-efficiency femtosecond laser micromachining. Recent research on femtosecond laser material processing has found that the ablation-cooling effect of high-repetition-frequency femtosecond pulse sequences can significantly improve processing efficiency (Ablation-cooled material removal with ultrafast bursts of pulses, Nature 537, 84-88, 2016). The results show that, thanks to the energy accumulation ablation effect of high-repetition-frequency pulse trains, using GHz pulse train femtosecond lasers can effectively improve processing efficiency. Furthermore, ablation processing of materials can also be achieved when the energy of a single pulse within the pulse train is much lower than the material's damage threshold. Fiber femtosecond lasers are well-suited for generating such high-repetition-rate pulse trains. As a result, many research teams both domestically and internationally have conducted research on GHz femtosecond laser amplification systems based on fiber amplification systems.

[0003] Typically, processing materials with high damage thresholds requires pulse train lasers with high pulse train energy and single-pulse energy within the pulse train. Furthermore, when the pulse energy is sufficiently high, the spot diameter focused on the material surface can be set to a larger value, allowing the use of longer focal length focusing lenses, thereby effectively increasing the scanning speed of the spot on the material surface. Numerous studies have been reported on high-energy GHz pulse train amplification. Our work (High-Power GHzBurst-Mode All-Fiber Laser System with Sub 300 fs Pulse Duration, Photonics11, 2024) achieved the highest pulse train energy to date of 820 μJ. However, further increases in pulse train energy are again limited by nonlinear effects in the fiber amplifier and fiber damage due to the narrowing of the amplified pulse caused by energy saturation in the fiber amplifier. Pulse train mode amplification is equivalent to increasing the pulse width during the amplification process; the more pulses within the pulse train, the wider the equivalent pulse width. However, despite the ability to set a very large number of pulses, due to the energy saturation effect of the fiber amplifier, only the leading pulses of the pulse train achieve high gain, while the gain of the trailing pulses gradually decreases, causing a pulse narrowing effect. When the amplification energy reaches a certain threshold, the pulse width of the amplified pulse train no longer increases with the increase of the input signal pulse train width, but instead narrows continuously with increasing amplification energy. Narrowed pulses have higher peak power, thus generating more nonlinear effects in the fiber amplifier and even causing damage to the fiber. Summary of the Invention

[0004] The purpose of this invention is to solve the technical problem of pulse narrowing caused by energy saturation effect in fiber amplifiers during high-energy pulse train amplification, and to provide a high-energy GHz femtosecond pulse train laser amplification system and amplification method.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] A high-energy GHz femtosecond pulse train laser amplification system is characterized by comprising a fiber optic GHz pulse train laser, an acousto-optic modulator, a single-mode amplifier, a fiber optic circulator, a pulse stretcher with an embedded chirped fiber grating, a double-clad amplifier, a beam combiner, a large-mode-field double-clad gain fiber, a pump laser, and a pulse compressor.

[0007] The output end of the fiber optic GHz pulse train laser is connected to the input end of the acousto-optic modulator, and the fiber optic GHz pulse train laser and the acousto-optic modulator are controlled by a synchronous signal. The acousto-optic modulator performs exponentially increasing modulation on the pulse train envelope output by the fiber optic GHz pulse train laser.

[0008] The input terminal of the single-mode amplifier is connected to the output terminal of the acousto-optic modulator;

[0009] Port 1 of the fiber optic circulator is connected to the output of a single-mode amplifier, Port 2 is connected to a pulse stretcher, and Port 3 is connected to the input of a double-clad amplifier.

[0010] The output of the double-clad amplifier and the output of the pump laser are both connected to the input of the beam combiner.

[0011] The large-mode-field double-clad gain fiber is fused to the output pigtail of the combiner.

[0012] The pulse compressor is positioned on the optical path of the laser output from the large-mode-field double-clad gain fiber.

[0013] Furthermore, the pulse compressor includes a first reflecting mirror, a first grating, a second grating, a roof prism, and a second reflecting mirror, which are sequentially arranged on the optical path of the laser output from the large-mode-field double-clad gain fiber.

[0014] Furthermore, a collimating lens is also provided between the output end of the large-mode-field double-clad gain fiber and the first reflecting mirror.

[0015] Furthermore, the line density of both the first grating and the second grating is 1600 lines / mm.

[0016] Furthermore, the pump laser has a maximum output power of 100W.

[0017] Furthermore, the dispersion value of the chirped fiber grating within the pulse stretcher is -50 ps / nm.

[0018] Meanwhile, the present invention also provides a high-energy GHz femtosecond pulse train laser amplification method, which is characterized by comprising the following steps based on the above-mentioned high-energy GHz femtosecond pulse train laser amplification system:

[0019] Step 1: The fiber optic GHz pulse train laser outputs a pulse train laser, and the pulse train envelope is modulated exponentially by an acousto-optic modulator.

[0020] Step 2: A single-mode amplifier is used to amplify the pulse train modulated laser in one stage, and a pulse stretcher is used to stretch the amplified laser in the time domain.

[0021] Step 3: Use a double-clad amplifier to perform secondary amplification on the time-domain broadened laser, and input the secondary amplified laser into the beam combiner. At the same time, combine the pump laser input into the beam combiner.

[0022] Step 4: Input the combined laser beam into a large-mode-field double-clad gain fiber for three-stage amplification, and then input it into a pulse compressor for time-domain compression to complete the high-energy GHz femtosecond pulse train laser amplification.

[0023] Furthermore, in step 4, the step of inputting the combined laser beam into a large-mode-field double-clad gain fiber for three-stage amplification further includes: setting a collimating lens between the output end of the large-mode-field double-clad gain fiber and the first reflecting mirror to collimate the three-stage amplified laser beam and input it into the pulse compressor.

[0024] The beneficial effects of this invention are:

[0025] This invention provides a high-energy GHz femtosecond pulse train laser amplification system and method. After modulation of the pulse train, because the amplitude of the modulated pulse leading edge is much lower than that of the pulse trailing edge, the pulse train waveform undergoes a process of exponential increase, gradual increase, and then relatively constant amplitude change during amplification. Finally, it gradually evolves into a pulse narrowing process with a high leading edge and a low trailing edge, rather than experiencing pulse narrowing from the beginning. Therefore, this pulse train delays the occurrence of pulse narrowing through modulation, effectively suppressing the pulse narrowing effect, reducing the peak pulse power, and thus avoiding excessive nonlinear phase accumulation and fiber damage. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of an embodiment of a high-energy GHz femtosecond pulse train laser amplification system according to the present invention;

[0027] Figure 2 This is a waveform diagram of the modulated pulse train in an embodiment of a high-energy GHz femtosecond pulse train laser amplification system of the present invention;

[0028] Figure 3 This is a waveform diagram of the amplified pulse train at different compression powers in an embodiment of a high-energy GHz femtosecond pulse train laser amplification system of the present invention;

[0029] Figure 4 This is an autocorrelation curve of the compressed pulse in an embodiment of a high-energy GHz femtosecond pulse train laser amplification system of the present invention.

[0030] The attached figures are labeled as follows:

[0031] 1-Fiber GHz pulse train laser; 2-Acousto-optic modulator; 3-Single-mode amplifier; 4-Fiber circulator; 5-Pulse stretcher; 6-Double-clad amplifier; 7-Band combiner; 8-Collimating lens; 9-Large-mode-field double-clad gain fiber; 10-Pump laser; 11-First reflector; 12-First grating; 13-Second grating; 14-Roof prism; 15-Second reflector. Detailed Implementation

[0032] The technical solution of the present invention will be clearly and completely described below with reference to the accompanying drawings and embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0033] This invention provides a high-energy GHz femtosecond pulse train laser amplification system, such as... Figure 1 As shown, it includes a fiber optic GHz pulse train laser 1, an acousto-optic modulator 2, a single-mode amplifier 3, a fiber optic circulator 4, a pulse stretcher with a built-in chirped fiber grating 5, a double-clad amplifier 6, a beam combiner 7, a collimating lens 8, a large-mode-field double-clad gain fiber 9, a pump laser 10, a first reflector 11, a first grating 12, a second grating 13, a roof prism 14, and a second reflector 15.

[0034] In this embodiment, an acousto-optic modulator 2 with a programmable modulation signal waveform is selected, which can perform exponentially rising pre-modulation on the pulse train waveform, reduce the amplitude of the pulse leading edge of the pulse train, and thus suppress the pulse narrowing effect caused by the energy saturation effect in the amplifier. In use, the connection relationship is as follows: the input end of the acousto-optic modulator 2 is connected to the output end of the fiber optic GHz pulse train laser 1, and the fiber optic GHz pulse train laser 1 and the acousto-optic modulator 2 are controlled by a synchronous signal. The acousto-optic modulator 2 is used to perform exponentially rising modulation on the output pulse train of the fiber optic GHz pulse train laser 1.

[0035] The single-mode amplifier 3 serves as the first-stage amplifier, with its input connected to the output of the acousto-optic modulator 2. The modulated pulse train is amplified in one stage by the single-mode amplifier 3.

[0036] Port 1 of fiber optic circulator 4 is connected to the output of single-mode amplifier 3, Port 2 is connected to pulse stretcher 5, the chirped fiber grating in pulse stretcher 5 has a dispersion value of -50 ps / nm, and Port 3 is connected to the input of double-clad amplifier 6. Double-clad amplifier 6 acts as the second-stage amplifier. The pulse train amplified in the first stage is input to pulse stretcher 5 through Port 1 of fiber optic circulator 4 for time-domain stretching, and then input to double-clad amplifier 6 through Port 3 for second-stage amplification.

[0037] The output of the double-clad amplifier 6 and the output of the pump laser 10 are both connected to the input of the beam combiner 7. The pump laser 10 has a maximum output power of 100 W. The beam combiner 7 is used to combine the second-stage amplified pulse train with the laser output from the pump laser 10.

[0038] The input end of the large-mode-field double-clad gain fiber 9 is fused to the output end of the beam combiner 7, and together with the pump laser 10, they form a third-stage amplifier to amplify the laser output from the double-clad amplifier 6 in three stages.

[0039] Collimating lens 8 is set at the output end of large-mode-field double-clad gain fiber 9 to collimate the output laser of large-mode-field double-clad gain fiber 9.

[0040] The first reflector 11, the first grating 12, the second grating 13, the roof prism 14, and the second reflector 15 are sequentially arranged in the optical path of the laser output from the third-stage amplifier, forming a pulse compressor. The first grating 12 and the second grating 13 both have a line density of 1600 lines / mm, which can compress and output the input laser pulse, significantly increasing the peak power of the output pulse. This can be used to improve the processing capability and efficiency of fiber femtosecond pulse train laser systems.

[0041] This embodiment also provides a high-energy GHz femtosecond pulse train laser amplification method. The optical path is constructed according to the above-described high-energy GHz femtosecond pulse train laser amplification system, and the effect is verified, including the following steps:

[0042] Step 1: Select a fiber optic GHz pulse train laser 1 with an intrapulse repetition frequency of 1.1 GHz. The fiber optic GHz pulse train laser 1 outputs a pulse train laser. Simultaneously, an exponentially rising modulation signal with a repetition frequency of 25 kHz, synchronized with the pulse train, is input to the acousto-optic modulator 2. The pulse train is exponentially modulated using this signal. The waveform of the modulated pulse train is as follows: Figure 2 As shown.

[0043] Step 2: The laser pulse train modulated by single-mode amplifier 3 is amplified in one stage, and the average power after amplification is 22.5 mW. Then, the amplified laser is stretched in the time domain by pulse stretcher 5 (the dispersion value of the chirped fiber grating is -50 ps / nm).

[0044] Step 3: The time-domain broadened laser is amplified in two stages using a double-clad amplifier 6. The average power after amplification is 650 mW. The amplified laser is then input into the beam combiner 7, and a 100 W pump laser is input into the beam combiner 7 for beam combining.

[0045] Step 4: The combined laser beam is input into a large-mode-field double-clad gain fiber 9 for three-stage amplification. The average power after amplification is 55 W. When amplified in the large-mode-field double-clad gain fiber 9, the waveforms of the pulse trains obtained when the output powers are 10 W, 22.5 W, 35 W, and 50 W are as follows. Figure 3 As shown. According to Figure 3It is evident that the pulse envelope premodulation technique described in this application effectively suppresses the pulse narrowing effect, thereby suppressing the nonlinear effect in the main amplifier.

[0046] A laser pulse train with a third-stage amplification output power of 55 W was input into a grating-based pulse compressor with a line density of 1600 lines / mm to perform time-domain compression, obtaining a high-energy femtosecond pulse train with an average power of 50 W, a pulse train energy of 2 mJ, and a pulse width of 372 fs, which is close to the Fourier transform limit. The pulse autocorrelation curve is shown below. Figure 4 As shown.

[0047] according to Figure 4 It can be seen that the nonlinear effect was effectively suppressed, thus obtaining a compressed pulse of 372 fs, and the quality of the obtained laser pulse was good.

[0048] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions within the technical scope disclosed in the present invention should be covered within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A high-energy GHz femtosecond pulse train laser amplification system, characterized in that, It includes a fiber optic GHz pulse train laser (1), an acousto-optic modulator (2), a single-mode amplifier (3), a fiber optic circulator (4), a pulse stretcher with a built-in chirped fiber grating (5), a double-clad amplifier (6), a beam combiner (7), a large-mode-field double-clad gain fiber (9), a pump laser (10), and a pulse compressor. The output end of the fiber optic GHz pulse train laser (1) is connected to the input end of the acousto-optic modulator (2), and the fiber optic GHz pulse train laser (1) and the acousto-optic modulator (2) are controlled by a synchronous signal. The acousto-optic modulator (2) performs exponentially increasing modulation on the pulse train envelope output by the fiber optic GHz pulse train laser (1). The input terminal of the single-mode amplifier (3) is connected to the output terminal of the acousto-optic modulator (2); The Port 1 port of the fiber optic circulator (4) is connected to the output of the single-mode amplifier (3), the Port 2 port is connected to the pulse stretcher (5), and the Port 3 port is connected to the input of the double-clad amplifier (6). The output of the double-clad amplifier (6) and the output of the pump laser (10) are both connected to the input of the beam combiner (7); The large-mode-field double-clad gain fiber (9) is fused with the output pigtail of the combiner (7); The pulse compressor is set on the optical path of the laser output from the large-mode-field double-clad gain fiber (9).

2. The high-energy GHz femtosecond pulse train laser amplification system according to claim 1, characterized in that, The pulse compressor includes a first reflector (11), a first grating (12), a second grating (13), a roof prism (14), and a second reflector (15) arranged sequentially on the optical path of the laser output from the large-mode-field double-clad gain fiber (9).

3. The high-energy GHz femtosecond pulse train laser amplification system according to claim 2, characterized in that, A collimating lens (8) is also provided between the output end of the large-mode-field double-clad gain fiber (9) and the first reflector (11).

4. The high-energy GHz femtosecond pulse train laser amplification system according to claim 2, characterized in that, The line density of the first grating (12) and the second grating (13) is 1600 lines / mm.

5. The high-energy GHz femtosecond pulse train laser amplification system according to claim 1, characterized in that, The pump laser (10) has a maximum output power of 100W.

6. The high-energy GHz femtosecond pulse train laser amplification system according to claim 1, characterized in that, The dispersion value of the chirped fiber grating inside the pulse stretcher (5) is -50 ps / nm.

7. A method for amplifying high-energy GHz femtosecond pulse train lasers, characterized in that, The high-energy GHz femtosecond pulse train laser amplification system according to any one of claims 1-6 includes the following steps: Step 1: The fiber optic GHz pulse train laser (1) outputs a pulse train laser, and the pulse train envelope is modulated exponentially by an acousto-optic modulator (2); Step 2: A single-mode amplifier (3) is used to amplify the laser after pulse train modulation, and a pulse stretcher (5) is used to stretch the amplified laser in the time domain. Step 3: Use a double-clad amplifier (6) to perform secondary amplification on the time-domain broadened laser, and input the secondary amplified laser into the beam combiner (7), while simultaneously combining the pump laser input into the beam combiner (7); Step 4: Input the combined laser beam into a large-mode-field double-clad gain fiber (9) for three-stage amplification, and input it into a pulse compressor for time-domain compression to complete the high-energy GHz femtosecond pulse train laser amplification.

8. The high-energy GHz femtosecond pulse train laser amplification method according to claim 7, characterized in that, In step 4, the process of inputting the combined laser beam into the large-mode-field double-clad gain fiber (9) for three-stage amplification further includes setting a collimating lens (8) between the output end of the large-mode-field double-clad gain fiber (9) and the first reflector (11) to collimate the three-stage amplified laser beam and input it into the pulse compressor.

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