EVALUATING A FEMTOSECOND LASER-INDUCED BREAKDOWN SPARK AS AN AERO-OPTIC ILLUMINATION SOURCE

Abstract

Currently, limitations exist on collecting aero-optic wavefront data in a wind tunnel dueto the types of sources available to illuminate the flow-field for the sensing optics. Collimated laser sources are commonly used, but are limited by the ability to place hardware to steer the beam towards the sensing optics. Laser-Induced Breakdown (LIB) sparks have also been tested, but create additional measurement errors due to variations in their size and position with each laser pulse. In this work, a new approach using a Femtosecond Laser-Induced Breakdown (FSLIB) spark is evaluated as a possible solution to the problems faced by nanosecond LIB sparks, namely the significant amount of spark size and position variation present in the latter. The FSLIB spark was imaged with a camera in order to study the amount of pulse-to-pulse position and size change present in its generation. Additionally, the FS-LIB spark was used to collect aero-optic data in conjunction with a Shack-Hartmann style wavefront sensor on a Mach 2.8 flowfield. The results of this analysis indicated that the FS-LIB spark experiences significantly less pulse-to-pulse variation in its size and position than a nanosecond LIB spark. In addition, the wavefront data collected with the FS-LIB spark compared favorably to data collected with a more conventional collimated laser beam for illumination. This indicates that the FS-LIB spark is a promising alternative to the use of collimated sources in aero-optic data collection.