Effect of Repetition Rate on Ultrashort Pulse Laser Propagation and Energy Deposition

Modern USPL (ultra short pulse laser) development is trending towards higher repetition rates and higher average power systems. High peak power, low repetition rate USPLs have long been used to generate laser filaments, which consist of a plasma channel and region of focused high intensity propagation. Filamentation leads to heat deposition in the air from linear and nonlinear effects, producing a gas density depression that persists over hydrodynamic timescales (milliseconds). This is long after the femtosecond pulse has passed. In the "single-shot" (similar to 10 Hz) regime of filamentation, the time between pulses allows the air density to return to equilibrium before the next pulse arrives. Prior work has experimentally measured the single shot gas density depression via interferometry(1) and demonstrated that high repetition rate filamentation leads to deflection of subsequent pulses due to residual heating from the prior pulses(2,3). This work experimentally examines USPL thermal blooming as a function of laser repetition rate. Residual heating effects between pulses are demonstrated through measurements of the energy deposition by the laser filament. The temporally and spatially resolved energy deposition is extracted from interferometric measurements of the phase shift due to the gas density depression. Comparison is made between experimentation and modeling, as well as verification of past results. This work demonstrates how atmospheric propagation of modern high average power, high repetition rate USPL pulses differ from traditional single shot USPL systems.