Development of a high power high energy ultrafast laser
Date
2021
Authors
Chi, Han, author
Rocca, Jorge, advisor
Menoni, Carmen, committee member
Marconi, Mario, committee member
Lee, Siu Au, committee member
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Abstract
This dissertation describes the development of high energy, high repetition rate laser technology based on cryogenically cooled diode-pumped Yb:YAG laser amplifiers. The key challenges of thermal management, the generation of high energy green pulses at high repetition rate, and the design of an ultrafast laser amplifier that uses the green pulses as pump are discussed in this dissertation. To aid the development of thermal management solutions, an accurate, in situ, noninvasive optical technique to generate three-dimensional (3-D) temperature maps of cryogenic amplifiers during operation at high average power was demonstrated. The temperature is determined by analyzing the fluorescence spectra of the laser material (Yb:YAG) with a neural network algorithm. The accuracy of the technique relies on a calibration that does not depend on simulations. Results are presented for a cryogenic Yb:YAG active mirror laser amplifier operating at different pump conditions, which include kW pump power level operation. Based on this temperature measurement technique, an analysis of the thermal behavior of a high-energy kilowatt-average-power diode-pumped cryogenically cooled Yb:YAG active mirror laser amplifier is presented. Maps of the temperature distribution in the laser amplifier crystal at pump powers up to 1kW were obtained for the first time by spectrally resolving the fluorescence induced by a scanning probe beam. The cryo-temperature measurement technique is applicable to other solid-state lasers materials. The wavefront distortions resulting from the front surface deformation and the overall deformation of the gain medium assembly were measured using a Mach–Zehnder interferometer. The measured deformations agree well with the results of finite element thermomechanical modeling simulations, and with the results of focal length shift measurements. The relative contributions to the optical path difference (OPD) of the mechanical deformations, refractive index changes, and electronic contribution are discussed. The pump-induced mechanical deformations of the assembly dominate the OPD changes in the kilowatt-average-pump-power cryogenically cooled Yb:YAG active mirror laser investigated. The generation of green (λ= 515 nm) Joule-level pulses at 1 kHz repetition rate was demonstrated. This was achieved by frequency doubling 1.2 J, 2 ns temporally shaped square pulses from a cryogenically cooled Yb:YAG laser in an LBO crystal. The generation of 0.94 J second-harmonic pulses at 1 kHz was demonstrated with 78% conversion efficiency. The unconverted light was sent through a second LBO crystal to generate an additional >100 mJ second-harmonic pulses to reach a total green average power of 1.04kW. A higher conversion efficiency of 89% was also achieved for 0.58 J green pulses at 1 kHz. An application of this green laser is the pumping of high average power ultrafast laser amplifiers. The design of a two-stage water-cooled Ti:Sapphire amplifier system to generate 300 mJ pulses pre-compression using this green laser as pump is discussed. The simulation of the gain and thermal distribution of the 1st and 2nd stage amplifier are presented. The first experimental results of the operation of the first amplification stage of this laser system are discussed.
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Subject
distribution
amplifier
laser system
thermal management
lasers