| The high peak power 2 μm ultrafast fiber laser system has a wide range of applications in the fields of nonlinear effect conversion,atmospheric remote sensing,material detection,military radar,and so on.Compared with solid-state lasers,fiber lasers have the advantages of simple structure,no need for collimation,good integration,and high heat dissipation efficiency.Therefore,the study of high peak power 2 μm ultrafast fiber laser system has both scientific and practical value.This article has done the following researches around this topic:1.The development of 2 μm fiber oscillators and laser amplification systems based on chirped pulse amplification(CPA)technology and nonlinear effects has been presented.The technical difficulties still existing and the direction that can be further explored have been proposed.2.The basic principles and common methods of ultra-fast oscillators have been discussed.An implementation of a high peak power 2 μm ultrafast fiber laser system have been proposed from three impacts: direct output from a fiber oscillator,the output of fiber laser amplification system based on CPA and nonlinear effect.3.The principle of Raman-induced soliton self-frequency shift(SSFS)has been expounded.In the all-silica-based thulium-doped fiber Raman soliton laser system,SSFS and power amplification have been realized simultaneously in the gain fiber,which effectively improved the SSFS efficiency and Raman soliton energy.By adjusting the pump power,a continuously tunable Raman soliton from 1.98 μm to 2.29 μm(corresponding to 310 nm range)has been achieved.A narrow pulse width of 95 fs,a maximum pulse energy of 13.3 nJ and a maximum peak power of 105 kW have been output from this system.The Raman conversion efficiency has been up to 91.5% and the Raman soliton energy has been >10 nJ at >2.22 μm.4.The physical mechanism of higher-order soliton compression has been expounded.A high-peak-power holmium-doped single-mode fiber laser amplification system has been built.Firstly,the holmium-doped fiber(HDF)oscillator has been mode-locked by nonlinear polarization evolution mode-locking technology,which can stably generate pulses at 2.08 μm.Based on soliton effect and high-order soliton compression,the pulse duration and energy after amplification have been increased by ~5 times and ~20 times,respectively.A pulse with duration of 172 fs,pulse energy of 7.2 nJ,and peak power of 24.3 kW can be obtained.This is the highest peak power that can be achieved without using spatial compression devices in a single-mode HDF laser system.5.The structure and characteristics of hollow-core photonic crystal fiber(HC-PCF)have been introduced.A HDF CPA laser system using HC-PCF for compression has been constructed.Two dispersion-compensating fibers(Nufern PM2000 D and UHNA4)were used to broaden the pulse,and the spectrums after broadening,amplifying and compressing have compared.It was found that UHNA4 had a smaller spectral modulation and was more conducive for subsequent compression.In addition,the HCPCF has a certain shaping effect on the input spectrum due to its absorption characteristics.After compression,pulses with duration of 1.8 ps could be obtained.This is the first time that HC-PCF has been used in 2 μm fiber lasers.Although the pulse performances of final output need to be further improved,the experimental results can provide some references for subsequent researches. |
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