Research On High Power Mid-infrared Laser Sources Pumped By Femtosecond Fiber Lasers

Mid-infrared laser source,or namely,laser radiation in the spectral range of 3-5?m,becomes research hotspots in the strong laser fields ever since long time ago,as it covers the high transparency atmospheric window and the intrinsic absorption bands of a large number of molecules and chemicals.Meanwhile,due to recent rapid developments of passively mode-locked fiber laser and the technique of ultrafast fiber laser amplification,femtosecond fiber lasers,which benefit from shorter pulse width and higher peak power in comparison with traditional long-pulse lasers,attract many researchers' and technicals' attention as pumping lasers of nonlinear frequency conversion.Among all the nonlinear materials or crystals for parametric conversion,the quasi-phase-matched(QPM)periodically poled magnesium oxide doped lithium niobate(PPMgLN)crystals are one of the most promising candidates due to their large effective nonlinear coefficient,collinear interaction scheme,high damage threshold,wavelength tunability and design flexibility.For this purpose,the goal of this dissertation was the design and optimization of the generation of femtosecond source in the mid-infrared spectral range with high repetition rates together with its pump laser.Three parts of work were mainly included.Firstly,MOPA-structured,linearly-polarized,femtosecond Yb-doped fiber laser with pre-chirped management amplification(PCMA)scheme seeded by a mode-locked fiber laser,were both theoretically and experimentally investigated.Secondly,fiber-laser-pumped PPMgLN-based optical parametric amplification(OPA)with commercial Er-doped fiber ASE source was theoretically and experimentally studied.Thirdly,self-seeded femtosecond difference frequency generation(DFG)together with supercontinuum generation was theoretically and experimentally studied.First of all,nonlinear polarization rotation(NPR)mechanism was employed to fabricate a stable all-normal-dispersion fiber laser seed,which outputs near linear-chirped pulses with repetition rates of 84MHz,pulse width of 3.8ps and pulse energy of 1.2nJ.Its average power was boosted to 500mW in the pre-amplifier consisted of non-polarization-maintaining fiber after a polarization sensitive isolator,while the output polarization extinction ratio was kept over 10dB with the help of a polarization controller.The pulse width after pre-amplifier can be dechirped to near 143fs after compression,indicating excellent pulse quality.Pre-chirp management fiber amplifier was presented in the final power amplification stage.By modifying the initial chirp of the mode-locked seed laser,the Yb fiber pump laser exhibits a final output power of 14 W with sub 100 fs pulse duration after power amplification and compression.A high power linear-polarized Yb laser with pulse bunch output was also demonstrated.After the pulse repetition rates was increased to 1.344GHz with a pulse multiplier,pulse bunches with repetition rates of 2MHz was selected by a acousto-optics modulator.The performance under different duty ratio was also experimentally tested.Later in this dissertation,two kinds of single-pass OP A/DFG seeded by different type of seed signal were introduced on the basis of home-made PPMgLN crystal and the femtosecond fiber laser mentioned above.The key parameters including laser thresholds,optimal focus diameter,idler spectral width were also numerical studied in our simulation model.One of them was a high power PPMgLN based femtosecond OPA featuring a spectral seamless broadband mid-IR output seeded by incoherent sources.When the OPA was seeded with a broadband amplified spontaneous emission(ASE)source,a damage-limited 0.6 W broadband mid-infirared radiation was experimentally obtained under the pump power of 10.15 W at 82MHz repetition rate,corresponding to an overall OPA conversion efficiency of 32.7%.The 3-dB bandwidth of the mid-IR idler was 291.9 nm,centering at 3.34?m.Apart from that,supercontinuum generation in the photonic crystal fiber pumped by femtosecond fiber laser was demonstrated to construct the DFG seed signal,the spectral energy of which was designed to cover the demanded spectral ranges at most.Then an over 1 W average power at?3550 nm wavelength via self-seeded DFG through a 5 cm-long PPMgLN crystal was demonstrated.The pump and signal sources are derived from the identical pre-chirp managed femtosecond Yb-fiber amplifier with sub-100-fs pulse duration and 84MHz repetition rate for simple synchronization.This result is believed to be among the highest-average-power,femtosecond mid-infrared radiation obtained via DFG,with measured our-hour root-mean-square deviation of less than 1%.The measured M2 factor were 2.08 and 2.42 for horizontal and vertical axis,respectively.