Generation,Amplification And Nonlinear Frequency Conversion Of Ultrafast Laser At 1.0μM

Ultrafast lasers with pulse durations on the orders of picosecond and femtosecond are widely used in various fields,such as supercontinuum generation,photoelectron microscopy,and material micromachining.In addition,there is a great demand for ultrafast lasers with high power and high repetition rate in the field of industrial processing.For the above reasons,the research on ultrafast lasers with high power and high beam quality is of great significance.During the laser amplification process,the thermal effect generated by the crystal seriously affects the output power and beam quality.In order to obtain ultrafast laser with high beam quality and high power,a series of research work has been carried out on the generation,amplification and frequency conversion of high-repetition femtosecond pulsed lasers at 1.0 μm.The femtosecond fiber laser is used as a seed source,and the fiber preamplifier and the singlecrystal thin-rod main amplifier are used for power amplification.The thermal effect in the crystal is analyzed from three aspects:theory,simulation and experiment.By changing the wavelength of the pump source and crystal parameters,as well as optimizing the beam profiles of the pump,the output power is improved while ensuring high beam quality.The nonlinear transformation process of the final output laser of the fiber amplifier is explored.The main contents are as follows:1.The nonlinear amplification loop mirror and nonlinear polarization evolution mode locking technology are studied from both theoretical and experimental aspects.Based on the theory,the relationship between transmittance and nonlinear phase shift in the fiber loop is investigated.According to the theory,the figure-9 fiber laser is built,which can achieve the pulse output with a repetition rate of 65 MHz and the pulse width of 641 fs at 1040 nm,and can realize self-starting.The principle of nonlinear polarization evolution mod-locking was described.Based on the theory,the NPE fiber laser was built to achieve the pulse output at 1030 nm with the repetition rate of 59 MHz and the pulse width of 489 fs.2.The research on amplification and compression of femtosecond laser at 1.0 μm was carried out.The fiber amplifier system is simulated theoretically,and the relationship between the fiber amplifier and the gain fiber length is explored.Based on the theory,the amplification system is simulated.The three-stage fiber pre-amplifier was constructed with a figure-9 fiber laser as the seed,with an output power of 7.8 W and a pulse width of 9 ps.On this basis,the pulse width is optimized,the transmission gratings are used as the compressor,the femtosecond pulse output with an average power of 4.1 W and a pulse width of 225 fs is finally realized,and the compression efficiency is 74.5%.3.Theoretical and experimental research on Yb:YAG single-crystal thin-rod amplifier is carried out.The thermal effect in crystal is explored.Based on the steady-state heat conduction equation and boundary conditions,the temperature field distribution of Yb:YAG single-crystal thin-rod with different parameters was simulated,and the effects of crystal diameter and doping concentration on temperature distribution in crystal was obtained.The Yb:YAG single-crystal thin-rod main amplifier is built,and the amplifier is optimized according to the simulation results.Firstly,the effects of crystal diameter and doping concentration on the output of the amplifier are studied.The 940 nm LD was used as pump.The maximum output power of Φ1 mm-2 at.%doping single-crystal thin-rod double-pass amplifier is 40 W for 130 W of pump power,and the beam profile is complete.The maximum output power of Φ2 mm-1 at.%doping crystal double-pass amplifier is 35 W for 130 W of pump power,and the characterization of the beam profile reveals a good beam quality of Mx2=1.52 and My2=1.18.Then,the effect of changing the pump wavelength on the output of the amplifier is investigated.The pump power was replaced by the 969 nm LD.The maximum output power of Φ2 mm-1 at.%doping crystal double-pass amplifier is 34 W for 150 W of pump power.The characterization of the beam profile reveals a good beam quality of Mx2=1.56 and My2=1.13 at 150 W of pump power.4.Based on the optical fiber amplification system,the non-linear frequency conversion technology is used to realize the femtosecond second harmonic output.The near-infrared laser with an average power of 4 W and a pulse width of 225 fs was used as the fundamental frequency.The green laser with an average power of 0.88 W was obtained by frequency doubling of the BBO crystal,and the frequency-doubling conversion efficiency was 22%.The green laser with an average power of 0.6 W was obtained from PPLN crystal,and the frequencydoubling conversion efficiency was 15%.