Walter Scott, Jr. College of Engineering

Graduate Exam Abstract

Han Chi
Ph.D. Final
Aug 20, 2021, 2:30 pm - 5:00 pm
ERC Classroom A210
Development of a High Power High Energy Ultrafast Laser
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. Temperature map results are presented for a kW-level cryogenic Yb:YAG active mirror laser amplifier operating at different pump conditions. The wavefront distortions resulting from the gain medium assembly of the kW-level Yb:YAG laser amplifier were measured. The measured temperature distributions and gain medium deformations agree well with the finite element thermomechanical modeling simulations. The relative contributions to the optical path difference (OPD) are discussed.

The generation of a green (λ = 515 nm) Joule-level pulses at 1 kHz repetition rate was demonstrated, which was achieved by frequency doubling Joule-level, 2 ns temporally shaped square pulses from a cryogenically cooled Yb:YAG laser in an LBO crystal. 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 hundreds of millijoule pulses using this green laser as pump is presented. The simulation of the gain and thermal distribution of the amplifiers are discussed. The first experimental results of this laser system are presented.
Adviser: Prof. Jorge Rocca
Co-Adviser: N/A
Non-ECE Member: Prof. Siu Au Lee
Member 3: Prof. Carmen Menoni
Addional Members: Prof. Mario Marconi
H. Chi1, Y. Wang1, et al., “Demonstration of a Kilowatt Average Power, 1 Joule, Green Laser,” Opt. Lett. 45, 6803-6806 (2020).

Y. Wang1, H. Chi1, et al., “1.1 J, 1 kHz Repetition Rate, Yb:YAG Picosecond Laser,” Opt. Lett. 45, 6615-6618 (2020).

H. Chi, C. M. Baumgarten, et al., “Thermal Behavior Characterization of a kW-Power Level Cryogenically-Cooled Yb:YAG Active Mirror Laser Amplifier,” J. Opt. Soc. Am. B 36, 1084-1090 (2019).

H. Chi, K. A. Dehne, et al., “In situ 3-D temperature mapping of high average power cryogenic laser amplifiers,” Opt. Express 26, 5240-5252 (2018).

H. Chi, K. A. Dehne, et al., “In Situ 3-D Temperature Mapping of High Average Power Cryogenic Laser Amplifiers,” in Conference on Lasers and Electro-Optics, OSA Technical Digest (Optical Society of America, 2018), paper SM1N.5.

B. A. Reagan, C. Baumgarten, E. Jankowska, H. Chi, et al., “Scaling diode-pumped, high energy picosecond lasers to kilowatt average powers,” High Power Laser Science and Engineering 6, E11. 2018.

H. Chi, K. Dehne, et al.,“In situ temperature mapping of high energy, high average power cryogenic amplifiers,” Bulletin of the American Physical Society 62, 2017.
Program of Study:
PH 571
GRAD 544
ECE 799
ECE 795
ECE 673
ECE 604
ECE 574
ECE 506