Broadband Long-wave Infrared Few-cycle Pulse Generation via Optical Parametric Chirped-pulse Amplification

Abstract

Intense ultrashort laser sources in the Long-Wave Infrared (LWIR) spectral range are needed to gain a deeper understanding of many laser-matter interaction processes in laser pulse filamentation, strong-field physics, and laser-driven particle acceleration. Ultrashort LWIR pulses are also valuable in supporting applications in molecular spectroscopy, such as the investigations of molecular dynamics. However, the lack of suitable broadband laser gain media prevents the generation of the intense ultrashort LWIR pulses directly from laser oscillators and amplifiers, with the gaseous CO2 laser being the only exception. The high cost and footprint of the advanced terawatt power level CO2 systems with relatively long picosecond pulse duration hinders their wide application.

Optical Parametric Chirped-Pulse Amplification (OPCPA) is the most promising method for intense few-cycle pulse generation in the LWIR range. OPCPA uses an instantaneous nonlinear optical process–Optical Parametric Amplification (OPA) in a Chirped-Pulse Amplification (CPA) system to boost the energy of broadband ultrashort pulses. Mid-Infrared (MIR) OPCPA has reached peak powers on the order of several gigawatts, but when extended to the LWIR regime using near-infrared pumping, less than gigawatt powers have been produced. Generating radiation in the LWIR range reduces the conversion efficiency due to unfavorably higher quantum defect and thus requires higher-energy, nanosecond MIR pump lasers. To address the needs of intense ultrashort LWIR pulses, it is proposed to develop a LWIR OPCPA source driven by MIR Er:ZBLAN fiber lasers.

This dissertation presents a numerical evaluation of GaSe and orientation-patterned GaAs performances in a nanosecond OPCPA, following a theoretical description of three-wave parametric interactions. The construction and performance of a nanosecond surrogate pump, based on optical parametric oscillation and amplification in KTiOAsO4 crystals, is then reported. It operates at a central wavelength of 2.73 μm and produces >10 mJ pulses of ∼1 ns pulse duration at a repetition rate of 10 Hz. The surrogate pump source not only enables the experimental evaluation of the OPCPA architecture but also supports the initial assessment of large-core Er:ZBLAN fiber amplifiers. The LWIR seed pulses generated via direct difference-frequency generation in an AgGaS2 crystal are centered at 10.3 μm with 1.1-μm bandwidth. The nanosecond GaSe OPCPA is further analyzed numerically in a noncollinear geometry, followed by the design and construction of a small-scale noncollinear GaSe OPCPA using the custom-built surrogate pump and seed sources for proof-of-concept studies. A design of nanosecond-long stretcher and compressor is also included, and the construction of a prototype that accommodates the current LWIR seed source with limited bandwidth is discussed. Although the broadband gain is still under testing, a preliminary study of small-signal gain in GaSe has been completed by using a narrow-band CO2 laser as the seed. Single-shot spectrometers and an autocorrelator have also been developed for characterization in the LWIR range. The development of the prototype paves the way to the first demonstration of a MIR pumped LWIR OPCPA. The prototype is aimed to reach the peak power comparable to what has been demonstrated in the MIR range to date and could be scaled to a high peak and average power in the future by pump technologies such as high-energy, coherently-combined MIR fiber laser.