Generation Of High Energy Laser Pulse And Short Duration Laser Pulses By Coherent Synthesis

Femtosecond laser pulses exhibit the characteristics of short pulse width,high peak power and broadband coherent spectrum.Thus they play an important role in advanced industrial manufacturing,micro-and nano-processing,as well as scientific research.The constant development of femtosecond lasers is promoted by the booming new technologies.At present,the output parameters of ultra-short pulses obtained from single femtosecond laser amplification or nonlinear compression have already reached the limit.In order to obtain even higher pulse energy or shorter pulse width,coherent synthesis technology has become a new direction of development.At the same time,the demand for flexible transmission of femtosecond laser pulses is becoming increasingly prominent.In recent years,new type hollow core fibers provide a good solution for the transmission of high-energy pulse.In this thesis,a systematic study about the coherent pulse synthesis of femtosecond laser pulses is carried out.The coherent synthesis method enables the generation of not only high-energy pulses,but also the few-cycle pulses.A hollow core anti-resonant fiber with excellent single-mode performance is designed to meet the requirement of single-mode guidance of femtosecond laser pulses in anti-resonant fiber.The stucture of the thesis is organized as follows:1.Divided pulse amplification is investigated systematically and a method to avoid decreasing of combination efficiency is proposed.With the numerical method,the influence of systematic parameters on combination efficiency is studied,such as the birefringent crystal,the pulse chirp,the deviation of divided pulse energy and the nonlinear polarization rotation.In the experiment,a divided pulse amplification system is constructed with Yb-doped large mode area fiber.The results verify the simulative conclusion.To improve the output energy of divided pulse amplification system,a polarization-maintaining-fiber divided pulse amplification system with a pulse recombination light path is designed and built.The disadvantage of decreased combination efficiency is avoided,which provide a new method to high efficiency divided pulse amplification for short pulses.The system delivers combined pulse with a duration of 123 fs.2.Coherent synthesis based on a femtosecond Yb-doped fiber laser is systematically researched and few-cycle laser pulses are generated.The impacts of different parameters on synthesized laser pulse are studied.The relationship between the relative intensity noise and the timing jitter of pulses in nonlinear fiber is studied by simulation.With the analytical method,the influence of temporal duration and shape,central wavelength,pulse energy,chirp,etc.,on temporal quality and duration of the synthesized pulse are studied.An optimized method to generate coherent synthesized pulses with high quality is proposed.It is demonstrated that keeping similar pulse energy and duration of parent pulses leads to high temporal quality,and decreasing parent pulse duration can further lower the substrate of the synthesized pulse.In the experiment,pulses output from the femtosecond Yb-doped fiber laser system are divided into two branches and coupled into two different nonlinear fibers,respectively.Then,the output pulses of both branches show different central wavelengths while both exhibit<30 fs durations.After coherent combination,8-fs synthesized laser pulses are generated.3.The single-mode performance of single-ring antiresonant fibers and nested antiresonant fibers is investigated by finite element analysis.Several fiber structures with good single-mode performance are proposed.In the research,a method for suppressing high-order modes in antiresonant fibers is developed.When the size of the cladding tubes is properly designed,higher-order modes in the fiber core will couple with the cladding modes effectively and form high-loss supermodes.As a result,the higher-order modes are restrained.Based on this principle,a novel antiresonant fiber with hybrid cladding tubes is proposed.This structure enables suppression of the first two higher-order modes in the core simultaneously.For the nested structure,cascaded coupling is necessary to form high-loss supermodes.A nested antiresonant fiber with five nested tubes is designed to realize the cascaded coupling of higher-order modes.Moreover,a novel nested structure with hybrid extended cladding tubes is proposed.In this kind of fiber structure,higher-order mode extinction ratios of 10⁵ and even 10⁶ are obtained for the LP11 and LP21 modes,and 10⁵ for the LP02 modes.Single-mode performance is well maintained within a broad wavelength range.And the loss of the LP01 modes is as low as 3.90×10^-4 dB/m.