Theoretical And Experimental Study On Coherent Pulse Stacking

High average power ultra-short,ultra-intense pulsed lasers with high pulse energy are essential tools for modern scientific discovery.Due to thermal and nonlinear effects,the development of laser pulse power has reached a plateau.Coherent pulse stacking(CPS)is a new time-domain coherent addition technique that stacks several optical pulses into a single output pulse.CPS in the fiber system can not only improve the pulse energy,but also ensure the high repetition rate,enabling the high average power ultra-short,ultra-intense pulsed laser.In this thesis,we study the CPS technique and the optical cavity phase stabilization technique,and propose theoretical models and control algorithms.We demonstrate the CPS in principle,and obtain a stable pulsed laser in the long term with high repetition rate and high stacking efficiency.The research focus of the CPS system is to optimize the amplitude and phase of input pulses,and to set the phase value of each optical cavity reasonably,so as to achieve the high pulse energy.In the thesis,we establish the Z-domain model and the state space model,and study the physical process of the CPS through analytical analysis and simulation analysis.The CPS can be scaled to multi-cavity multi-stage case.An optical system based on Herriott cells and a control system based on an FPGA(field-programmable gate array)are built to verify the theory of the CPS.A 2-stage 3-cavity CPS system,which stacks15 pulses into a single output pulse,achieves an 11.0 peak-power enhancement factor at98 kHz repetition rate,reaching 92%of its theoretical value.The stacking efficiency is76%here,close to 80%of the theoretical value.The satellite-pulse contrast ratio is 14 dB in the experiment.The optical cavity phase stabilization is the most critical technique in the CPS system.The research focus of the optical cavity phase stabilization technique is to accurately extract each optical cavity phase from limited pulse data,and to feed back and control optical cavities,so as to ensure the long-term phase stabilization of the CPS.In the thesis,three algorithms,including the direct iteration algorithm,the template vector algorithm and the stochastic parallel gradient descent algorithm,are proposed to detect the optical cavity phase.In this thesis,we discuss the noise theory of the optical cavity,and study three feedback control algorithms including CIC(cascaded integrator-comb)filter,CORDIC(coordinate rotation digital computer)and PI(proportional-integral)controller to compensate for optical cavity perturbations caused by the noise.In the experiment of optical cavity phase stabilization,we measure the jitter noise and drift noise of the optical cavity,and prove that the FPGA-based feedback control system can significantly suppress the noise.We realize a precise,high-speed and stable control of ultrafast lasers.The system bandwidth is 1.5 kHz,which is close to the theoretical limit.The long-term stabilization measurement of the 2-stage 3-cavity CPS system shows that the phases of two short cavities in Stage 1 are maintained within 0.4~?(RMS)and 0.7~?(RMS)respectively,while the phase of the long cavity in Stage 2 is stabilized at 2.1~?(RMS)phase error over 12hours.The long-term intensity stability of the stacked pulse is kept within 1.2%(RMS).The CPS system is scalable.The distributed deterministic optical cavity phase control system is the right choice for multi-cavity multi-stage CPS.The FPGA-based programmable scaling solution can simultaneously ensure high pulse energy and high repetition rate of the CPS system,leading to high average power ultrafast lasers.