Coherent Spatial and Temporal Combining of Femtosecond Fiber Lasers at the Storage Energy Limit Enabling High-Power Drivers of Laser-Plasma Accelerators and Other Secondary Radiation Sources

Abstract

This work demonstrates, for the first time, simultaneous operation of coherent beam combining (CBC) and temporal combining at the amplifier saturation limit. The techniques described and implemented in this thesis will enable fiber array size reduction and thus practical laser sources providing joule-level energies at 10-100 kHz repetition-rates, suitable for future laser-matter interaction research and applications. Through the use of coherent pulse stacking amplification (CPSA) at the storable energy limit, 9.5 mJ is extracted from a single fiber and temporally combined into a 345 fs pulse with 58% efficiency, achieving a record-high single-fiber energy in a femtosecond pulse. Through careful understanding and modeling of amplifier extraction the nonlinear phase accrual is minimized, and the effects of saturation in the 81-pulse burst are investigated and controlled. Further temporal efficiency increase will require control of individual pulse spectrum using a fast electro-optic modulator (EOM), and should increase stacked pulse energy even further while maintaining high-fidelity pulse compression. All techniques discussed are applicable to longer pulse bursts and future fibers that may store 20-50 mJ or more, energies far outside the operation regime of any other time-domain techniques.

Output energies are scaled up to 25 mJ by implementing a coherent beam combination (CBC) array of 4 spatially-combined amplifiers. The spatial combining efficiency, critical for future high-efficiency sources, exceeds 90% when the beams are phase-locked with a stochastic parallel gradient descent (SPGD) algorithm. The alignment procedures and tolerances for achieving this high efficiency are discussed, along with the future implementation of spatial and temporal auto-alignment for larger arrays. The 25 mJ burst is temporally combined with 70% efficiency while maintaining ultrashort pulses, showing that CBC does not adversely impact CPSA efficiency. Incomplete CBC phase stabilization is shown to degrade temporal combining stabilization slightly at the 7 mJ / channel amplifier set-point; rep-rate scaling and better fiber thermal management will be required in future designs to improve stabilization. This experiment achieves the highest per-channel energy in a spatio-temporal fiber-array system and provides important information for ongoing work on extending the array to 12 amplifiers capable of 100 mJ energies, and beyond, and for future power scaled systems as well.

The 4-channel CBC and CPSA system was used for the first demonstration of neutron generation from a fiber laser driver by irradiating free-flowing liquid streams of deuterated water. Late-time stream dynamics are imaged with a novel backlight probe beam that provides unique temporal resolution at delays approaching 1 ms. Neutron yield can be increased by increasing on-target energy and pulse repetition rate. In the future, high-brightness laser-driven sources applicable to medical and imaging fields may be available with this technology. Further integration of the computer-controlled probe with neutron statistics and high rep-rate operation will allow for active target and laser optimization using machine-learning, an important step for next generation high-rep rate science.