Studies On Nonlinear Compression Of Ultrashort Laser Pulses In Hollow-Core Fiber

Ultrashort laser pulses with duration of only a few optical cycles are widely used tools to investigate the ultrafast dynamics in various fields such as physics,chemistry, biology, and material science. The hollow-core fiber filled with noble gas due to its high ionization potential is a popular method to obtain the high-energy few-cycle pulses based on self-phase modulation effect. However, the central wavelength of generated few-cycle pulses is usually limited to a narrow range around 800 nm. An alternative method to obtain the few-cycle pulses is the molecular gas-filled hollow-core fiber compressor. Based on molecular phase modulation, using pump-probe configuration, we can obtain the frequency-tunable few-cycle pulses, which is very promising as a tunable source.This thesis focuses on the studies of intense few-cycle pulses generation through an atomic or molecular gas-filled hollow-core fiber. The major research results can be summarized as follows:1. We theoretically studied the nonlinear compression of the 10-mJ, 62-fs, 3.6-μmlaser pulses through an argon gas-filled hollow-core fiber with large diameter of1000 μm. Using a pressure gradient, we obtain the intense 18.3-fs(~1.5 cycle)pulses at 3.6 μm only through compression with CaF2 crystal. Moreover, weshowed the relation between optimal fiber length and coupling efficiency for agiven bandwidth. These results are useful for the design of using hollow-corefiber to compress the high-energy pulses with long wavelength.2. We theoretically investigated the central wavelength tuning of probe pulse byusing ultrafast molecular phase modulation in a nitrogen gas-filled hollow-corefiber. Using an intense pump energy, increasing gas pressure, cooling gastemperature, and employing the double pump pulses can effectively enhance thewavelength tuning. With suitable pump parameters, hundreds of nanometerswavelength tuning was achieved. This method provides us an alternativeapproach to tune the central wavelength of an ultrashort laser pulse.3. We theoretically showed a pulse compression method for few-cycle laser pulsesgeneration with frequency tuning based on hollow-core fiber filled withnitrogen gas. The molecular phase modulation can impart a positive or negativechirp on probe pulse and allow for tens of nanometers wavelength tuning of thecompressed pulse, which can be used as a tunable source.Besides the above results, we also compared the two pulse compression methods for generating the few-cycle pulses and discuss the advantages or limitations of these two methods.