Generation, Optimization and Amplification of Ultrashort Infrared Pulses

Abstract

The development of few-cycle, ultra-intense laser sources in the long-wave infrared (LWIR, 8–12 μm) is of keen interest in many research fields, including molecular spectroscopy in the “fingerprint” region, laser-induced filamentation, remote sensing, and strong-field physics such as laser particle acceleration and high-harmonic generation. LWIR sources have gained their foothold due to their high transmittance in the atmosphere and their advantages granted by wavelength-scaling rules. The current absence of LWIR laser sources capable of generating ∼100-fs pulses with multi-terawatt peak power is due to the lack of broadband laser gain medium in the LWIR. The state-of-the-art CO2 lasers can produce multi-terawatt peak power, however, in a relatively long picosecond duration. Our vision for delivering such performance is through optical parametric chirped-pulse amplification (OPCPA) of ultra-broadband, carrier-envelope phase (CEP) stabilized LWIR seed. Facilitated by the coherent combining and mid-infrared Er:ZBLAN fiber technologies, the energy scaling capabilities of such architecture can be further extended.

This dissertation focuses on developing few-cycle, ultra-intense LWIR laser sources in three key aspects. First, LWIR seed pulses were generated through difference-frequency generation (DFG) in birefringent nonlinear crystals. For the first time, milliwatt-level LWIR seed pulses were produced by mixing 2.9-μm Er:ZBLAN fiber amplifier/compressor outputs with their frequency-shifted replicas. Higher pulse energy (110 nJ) and ultra-broadband spectral bandwidth (supporting two-cycle pulses) were achieved at a lower repetition rate driven by two-color Ti:sapphire laser output and assisted by genetic algorithm optimization. A single-shot infrared spectroscopy study on ammonia was performed with the LWIR seed source.

Second, parametric amplification of LWIR seed pulses was realized through noncollinear optical parametric amplification (NOPA) in a 5-mm thick GaSe crystal, pumped by nanosecond 2.7-μm pulses. As a consequence of crystal properties related to laser-induced damage threshold and the beam-pointing fluctuation, the attained parametric gain was limited.

Lastly, an active beam-pointing control system was developed to address the beam-pointing fluctuations confronted in a laser system. Using a proportional feedback loop and piezo-actuated mirror, the short-term beam-pointing fluctuations induced by ambient vibration were significantly reduced by up to an order of magnitude in a proof-of-principle study. Long-term fluctuations due to temperature drift were also eliminated.

To summarize, two broadband LWIR seed sources driven by an Er:ZBLAN fiber laser and a solid-state Ti:sapphire laser were developed, accompanied by the implementation of numerical models and optimization algorithms. Experimental and analytical efforts were presented on characterizing ultrashort LWIR pulses. Amplification of LWIR pulses was accomplished in an OPCPA apparatus with limited parametric gain. The diminished efficacy of the system was attributed to deteriorated nonlinear crystal and beam pointing. An active control system was constructed to potentially tackle the latter by compensating for the beam-pointing fluctuations using a proportional feedback-controlled piezo-actuated mirror. This dissertation research paves new paths for generating and characterizing few-cycle, CEP-stabilized ultra-intense LWIR pulses favorable by a broad spectrum of applications.