Theoretical And Experimental Study On Solid-state Ultrafast Laser Amplifier

With the continuous implementation of "Made in China 2025" and the advent of 5G era,there are more and more market demands for ultrafast laser precision processing in consumer electronics,medical and health care,aerospace,precision machinery and other fields.As the most important light source,high-repetition-frequency(above hundreds of kHz)and high-power(above tens of Watts)ultrafast lasers undertake the important task of improving processing efficiency and precision,and controlling cost at the same time.In order to meet the above industrial application requirements,focusing on the shortcomings of ignoring the influence of pulse repetition frequency,lack of description of the inclined folded optical path,and ignoring the influence of pulse overlap on spectral narrowing in the existing theoretical model,this paper systematically carried out theoretical and experimental study on 1 μm band high-repetition-frequency and high-power solid-state ultrafast laser amplification technology,which provides a powerful tool for the design and optimization of such amplifiers.The main contents and innovations of this paper are summarized as follows:1.For the first time,the Frantz-Nodvik equation is modified by analogy Q-switching theory,and the expression of the output pulse energy varying with pulse repetition frequency is derived.Based on this theory,a one-dimensional finite element slice model of bulk-crystal picosecond pulse amplification is established.Then,a picosecond amplifier experiment with adjustable pulse repetition frequency is set up to verify the above theory and model,and it is found that the output pulse energy decreased exponentially with the increase of pulse repetition frequency.Based on this experiment,a picosecond amplifier prototype with adjustable pulse repetition frequencies of 25 kHz-25 MHz is successfully developed.2.The thermal effects of Nd:YVO4 bulk crystals with different doping concentrations,different end cap lengths,and LD end-pumping with different output fiber core diameters are simulated by finite element software.Meanwhile,the comprehensive effects of Nd:YVO4 crystal’s doping concentration and pump laser’s M2 factor on the small-signal gain of the amplifier are studied theoretically and experimentally for the first time.It is found that high-gain and high-brightness picosecond laser amplification can be obtained when the high-beam-quality LD is used to pump the long composite crystal with a low doping concentration.Then,through the off-axis amplification experiment,the influence of off-axis angle on the output power and beam quality of the amplifier is analyzed,and it is found that it has the best value.Based on the above research results,a two-stage off-axis dual-pass picosecond laser amplifier with a maximum gain of 28 dB and beam quality M2 of about 1.3 is finally built.3.According to the operation principle of linear cavity regenerative amplifiers,the design criteria of the regenerative cavity for cavity length,seed optical mode reproduction and component damage are proposed.Based on these criteria,by using Nd:YVO4 crystal with high damage threshold coating and large aperture BBO electro-optic switch,a regenerative resonator with a cavity length of 1.8 m and its intracavity telescope directly composed of the crystal thermal lens and a long-focal-length plano-convex lens is built.At the same time,the period-doubling bifurcation phenomenon is suppressed by reducing the amplification times.Finally,a near-diffraction-limit stable picosecond laser output with adjustable repetition frequencies of 100-500 kHz and maximum average power of 12.2 W is obtained.4.The theoretical model of Innoslab picosecond pulse amplification is established for the first time by unfolding the multi-pass folded optical path and considering the anisotropy of the stimulated emission cross-section in the crystal.Then,the model is verified by 1-,4-,6-and 8-passes Innoslab picosecond laser amplification experiments.Meanwhile,the effects of the incident angle of seed light and the number of amplification passes are also studied.It is found that the high pass-number amplification structure with a small incident angle can obtain higher extraction efficiency and slope efficiency.Based on this conclusion,an 8-+4-passes two-stage Innoslab picosecond amplifier is built.Finally,a 1 MHz,13.6 ps pulsed laser output with an average power of 123 W is obtained.5.The spectrum of the chirped pulse is shaped by prism pair and birefringent filter respectively,and their modulation effects are compared.It is found that using a birefringent filter for direct spectral modulation can simultaneously obtain larger depression depth and lower light intensity attenuation.Finally,by building a Yb:YAG double-pass chirped-pulse amplifier,the width of the output spectrum with or without spectral shaping is compared,and the suppression of the saddle-shaped shaping spectrum on the gain narrowing effect is verified.6.By combining the pulse sequence model in the time domain with the amplifier slice in the space domain,and introducing the crystal’s stimulated emission cross-section and self-absorption loss varying with frequency,a theoretical model of broadband chirped pulse double-pass amplification considering the influence of pulse overlap is established for the first time.Then,by using the model for numerical simulation and building corresponding experiments,it is found that the narrowing degree of the amplification spectrum is positively correlated with the degree of pulse overlap.Therefore,it is necessary to increase the distance from the 0°mirror to the crystal in the double-pass amplifier to minimize the pulse overlap.According to this conclusion,a three-stage Yb:YAG bulk-crystal/single-crystal-fiber chirped-pulse double-pass amplification system is built.By comparing the amplification ability of the bulk crystal and single-crystal fiber,it is found that using single-crystal fiber as the gain medium can obtain higher extraction efficiency and slope efficiency.Finally,the output pulse is compressed by a single-transmission-grating compressor,and thus a 1 MHz,914 fs ultrafast laser with an average power of more than 50 W is obtained.