Study On High Power Femtosecond Fiber Lasers And Their Coherent Beam Combining Technology

High energy density physics is one of the most important frontiers of research on fundamental physics in the world,and laser driven inertial confinement fusion and laser plasma accelerator are important research branches.At present,it is an important direction of future development to build high energy and high power laser drivers based on the concept of fiber coherent amplification network,which has great prospects of application.Although the domestic and foreign related researches are still in the initial stage of exploration,but the researchers have reached a general consensus that high average power linearly-polarized femtosecond fiber chirped pulse amplification(CPA)system and coherent beam combining(CBC)technology are the key technologies for future development.More than that,the CBC technology has potential applications in the fields of high power fiber femtosecond optical frequency comb,nonlinear pulse compression,attosecond optics and so on.The technical schemes of CBC are numerous,and coherent polarization beam combining(CPBC)technology is one of technical routes with great potential,which has not only the ability of power scaling but also excellent beam quality.Moreover,femtosecond fiber oscillator and linearly-polarized femtosecond fiber CPA system are the important prerequisite and basis for the realization of CPBC system.Based on this,we focus on the CPBC technology of femtosecond fiber lasers and systematically carry out a series of researches on all-normal dispersion passively mode-locked all-fiber oscillators and their power scaling,all-fiber linearly-polarized femtosecond CPA systems and femtosecond fiber laser CPBC systems.The main contents of this dissertation are as follows:1.The all-normal dispersion passively mode-locked all-fiber oscillators are studied.Firstly,the wave-breaking phenomenon in an all-normal dispersion passively mode-locked fiber oscillator is experimentally studied,mode locking based on dissipative soliton resonance(DSR)is successfully achieved with wave-breaking-free high energy square pulses generation,the multistability of DSR and the compressibility of these square pulses are also experimentally demonstrated,and the experimental results shows that this kind of square pulses have large nonlinear chirp and can not be dechirped by linear dispersion compensation devices,which limits their applications in CPA systems.Secondly,a low-threshold-power all-fiber cascaded Raman continuum source at 1.0–1.3 ?m is successfully achieved based on all-normal dispersion passively mode-locked fiber oscillator by optimizing the parameters of optical components.Thirdly,a linearly-polarized dissipative soliton femtosecond all-fiber oscillator at 1.06 ?m is designed and realized based on non-polarization-maintaining fiber components,the amplified spontaneous emission at 1.03 ?m is successfully suppressed and femtosecond pulses with duration of 387 fs are obtained,which is an ideal seed laser for femtosecond fiber CPA systems and their CPBC systems.Moreover,non-polarization-maintaining and polarization-maintaining fiber amplifiers are designed respectively with the established DSR all-fiber oscillator as seed laser,which have successively obtained 240 W/36 k W and 174 W/38.5 k W of high average/peak power non-linearly-polarized and linearly-polarized nanosecond square pulses.Finally,the noise-like pulses in an all-fiber oscillator are experimentally studied and high power scaling is firstly realized in the world with generating 423 W/314 k W of high average/peak power picosecond pulses.2.The all-fiber linearly-polarized femtosecond CPA systems are systematically studied in theory and experiments.Firstly,the design issues of the all-fiber femtosecond CPA systems in parameters selection of signal light,pulse stretcher,fiber amplifiers and pulse compressor are analyzed,and it is pointed out that to build all-fiber femtosecond CPA systems based on highly-doped large-mode-area double cladding Yb-doped fiber is a feasible way to enhance the average power of femtosecond fiber lasers.Secondly,a simple analytically theoretical model of the femtosecond fiber CPA system is established,the theoretical analysis shows that the positive group delay dispersion and the third-order dispersion can mutually compensate with the nonlinear phase shift,which is helpful for achieving higher power femtosecond fiber lasers by breaching the limitation of the B-integral.Lastly,all-fiber linearly-polarized femtosecond nonlinear CPA systems are established based on single mode fiber and chirped fiber Bragg grating as pulse stretchers respectively and high average power linearly-polarized(polarization extinction ratio of above 13 d B)near diffraction-limited(M2 < 1.3)femtosecond lasers are achieved with average power of 119 W/300 W,pulse width of 352 fs/ 344 fs and peak power of 4.2 MW/11 MW by appling the mutual compensation effect of dispersion and nonlinear phase shift,which is the highest average power ever reported in linearly-polarized femtosecond fiber CPA systems to date.3.The theoretical model to analytically analyze the combining efficiency of femtosecond fiber CPBC systems is established,the influences of temporal–spectral effects on combining efficiency of femtosecond fiber laser CPBC systems are analyzed emphatically and analytical equations are obtained to link the combining efficiency with specific temporal-spectral factors for the first time.The theoretical results show that the optical path difference(OPD)has the most detrimental impact on the combining efficiency,for instance,the OPD of 140? leads to the combining efficiency decrease of 5% for the femtosecond pulses with a 3 d B spectral bandwidth of 13 nm centered at 1064 nm.Besides,when the power of each channel is equal,the influence of self-phase modulation on combining efficiency has nothing to do with the spectral width and only depends on the B-integral difference((35)B),which will induce a combining efficiency dropping of 5% if the value of(35)B reachs to 0.6 radians.The theoretical results also indicate that the group delay dispersion has relatively small influence on the combining efficiency for the femtosecond pulses with limited spectral width and the group delay dispersion mismatch induced by fiber length difference of 0.2 m leads to a combining efficiency dropping of 2% for the femtosecond pulses with a 3 d B spectral bandwidth of 13 nm centered at 1064 nm.Based on the theoretical analysis,an all-fiber multilevel active control method of OPD is proposed for the first time,the different phase-locking techniques are analyzed and the control strategies of spectral phase are elaborated.4.The all-fiber adaptive control system of OPD is designed and accomplished,its performances are experimentally verified and the experimental results demonstrate that the system can completely compensate the drift of OPD in the range of ±600?.Moreover,a low power CPBC system of two femtosecond fiber lasers is established to investigate the influence of temporal-spectral effects on combining efficiency in theory and experimets and the theoretical results are in good agreement with the experimental results,which verifies the correctness of the theoretical model.Based on effective active control and optimization strategies,the established CPBC system delivers femtosecond pulses with 10 watt-level average power,pulse width of 467 fs and combining efficiency of 96%.Finally,a high power CPBC system of two femtosecond fiber lasers is also established,the combined average power reachs to 313 W and the combined femtosecond pulses have pulse width of 827 fs,which is the highest average power ever achieved in femtosecond fiber CBC systems with all-fiber structure for the moment.The combining efficiency drops to 79% due to the deterioration of laser modes,but this system still has great potential for power scaling if further enhancing the performances of single fiber amplifier.By systematic investigation in theory and experiments,the new issues in femtosecond fiber CBC systems are fully understood,the new breakthrough in control technique of OPD is successfully made,the gaps of the study on femtosecond fiber laser CBC systems with all-fiber structure is successfully filled.This dissertation is hopeful for providing some reference for the future development of high energy and high power laser drivers.