Generation and Atmospheric Propagation of High Energy Ultrashort Pulses at High Repetition Rate

Defense applications in directed energy, such as channeling of intense laser and microwave beams and guiding of electrical discharges in the atmosphere, x-ray and gamma ray generation, remote sensing, and others will greatly benefit from the availability of efficient high energy ultrashort pulse lasers capable of delivering Joule and multi-Joule-level pulses at kHz repetition rates (kW average powers). Presently, Joule-level picosecond and femtosecond lasers are mostly limited to repetition rates of 10 Hz. Moreover, since these lasers are typically pumped by flashlamps, these lasers are large in size, inefficient, and non-portable. Consequently, studies of Joule-level energy laser pulse propagation in the atmosphere have been mostly limited to low repetition rates. A new high average power, high energy, picosecond diode-pumped Yb:YAG laser technology was demonstrated at Colorado State University achieving for example 1.1 J of a few picosecond duration train of pulses at 1000 Hz or 1,100 W average power (Optics Letters 2020), that is scalable to multi-kW average power. A goal of this project was to gain understanding of the propagation of high energy (Joule-level) picosecond laser pulses generated at repetition rates up to 1 kHz in air and kW level average powers. This includes the study of laser filamentation is this new regime, in which the short time separation between contiguous laser pulses causes the filamentation of a pulse to be affected by the atmospheric density depression caused by the preceding pulses. These studies include experiments that use these laser filaments to guide electrical discharge channels in the atmosphere at repetition rates up to 1 kHz. Another goal of the project is to extend high energy kHz repetition rate laser capability to the femtosecond range by using our diode-pumped cryogenic Yb:YAG laser technology to pump a high average power Ti:Sapphire laser.