Research On High Power Mid-infrared Picosecond Pulse Bunch Laser Sources Based On Difference Frequency Generation

Mid-infrared(MIR)laser sources operating at spectral region around 3 ?m are among the most attractive ablation lasers for both soft and densely calcified tissue,owing to the high fundamental vibrational absorption for liquid water.To date,picosecond MIR pulse lasers have been used as high-precision surgery scalpel to realize efficient ablation.Compared with these traditional single pulse lasers,the ultrafast pulse bunch lasers used for ablation-cooled material removal have been demonstrated faster ablation efficiency with less laser pulse energies,as well as less thermal damage to the bulk.Thus the similar benefits to the biological tissue ablation seem to be feasible and significant when using mid-IR pulse bunch lasers.Non-linear frequency conversion,as a common technique for generating mid-infrared lasers,can achieve wide spectral range and ultra-short pulse laser output.Among all the nonlinear materials for parametric conversion,the quasi-phase-matched(QPM)periodically poled magnesium oxide doped lithium niobate(PPMgLN)crystals are the most promising candidates due to their wide spectral range,large effective nonlinear coefficient and high damage threshold.This dissertation focuses on the main oscillation-power amplification(MOPA)structure of ytterbium fiber laser and its pumped difference frequency system(DFG)to generate high-power mid-infrared ultra-short pulse bunch laser sources.The specific work includes:1.Passive mode-locked fiber lasers based on nonlinear amplification loop mirror(NALM)and two-dimensional material saturable absorbers(MoO3/CNTs)were built respectively.The former was chosen as the seed source for generating ultrashort pulse bunch due to its higher output power as well as long-term stability,and a reflection type nonlinear phase shifter was introduced to realize self-starting of the structure.The laser outputs pulse with average power of 15 mW,repetition rate of 16.32 MHz and pulse width of 8.5 ps.Its spectrum centered at 1030 nm with the 3dB bandwidth of 0.57 nm.2.After being output from isolator,the mode-locked seed source passed through a pulse multiplier,which consisted of five cascaded 50:50 fiber couplers,to transform each laser pulse to pulse bunch with width of 4.8 ns.Within each bunch there were 16 equally spaced pulses with time intervals around 300 ps.Subsequently,a two-stage polarization maintaining fiber MOPA system was introduced to increase the average power of the pulse bunch to 27.8 W.The high beam quality(MX2?1.52,MY2?1.79),high power,linear polarization,ultrashort pulse bunch laser can be used as a high-quality pump source for DFG systems.3.In the DFG system,it is very difficult to generate signal pulse punch accurately synchronized with the pump laser since its narrow sub-pulse width and short interval time.Thus this dissertation proposed the use of nanosecond pulse as signal,synchronous pumping DFG scheme with the pump pulse bunch for the first time.The signal laser was composed of a FP-type semiconductor laser and a two-stage fiber MOPA system,delivering an average power of 4 W with a single pulse duration of 5.1 ns.By choosing the suitable PPMgLN crystal period and optimizing its working temperature,high average power ps pulse bunch output of 3.1 W at 3.07?m was obtained,corresponding to the slope efficiencies of 12%.The idler center wavelength was located at 3.07 ?m,and its beam quality was MX2?2.03 and MY2?1.61.4.A linearly polarized ps pulse bunch Yb fiber MOPA at 1030 nm was employed as pump while a synchronized broadband nanosecond pulse Er fiber MOPA served as signal to build a wavelength-tuned DFG system for the first time.The signal consisted of a SLED semiconductor laser and a three-stage cascaded MOPA system,delivering an output power of 5 W and spectral bandwidth of 50 nm around 1580 nm.When continuously adjusting the crystal temperature from 25 to 95?,the tunable idler wavelength in the range of 3.03 ?m to 2.84 ?m was achieved,reaching the highest average idler power of 4.6 W around 3.0 ?m with conversion efficiency of 16.5%.It was found that greater than 3.5 W of achieved idler output power was insensitive to crystal temperature and fluctuated just in a moderate amount over the whole tuning range,indicating its potentiality for different biological tissues ablation.