Investigation of a Laser-Induced Breakdown Spark as a Near Field Guide Star for Aero-Optic Measurements

High field-of-regard, aircraft-mounted laser systems typically include parts of the operating envelope in which the laser must pass through highly-turbulent flow regions such as a turbulent boundary layer or a shear layer associated with a separated flow region. At subsonic and higher flight speeds, these turbulent flow regions become optically active such that a transiting laser beam will be distorted due to index-of-refraction variations within the flow thereby degrading its on-target irradiance distribution. The study of the optical aberrations produced by compressible nearfield turbulent flows is called 'aero-optics.' The phase characteristics of the initial light beam can be restored using an adaptive-optic (AO) system that places the conjugate waveform of the aberration onto the optical wavefront of the beam prior to its transmission through the aberrating flowfield. In this case, the conjugate waveform must be determined from measurements of the aero-optic aberrations, which requires some kind of light source such as the reflected glint from the target or a natural or man-made guide star. Even for feed-forward AO correction schemes, in which flow-control techniques are used to improve the frequency bandwidth of the aberrating flow, it is anticipated that low-order optical measurements will still be used to synchronize the AO scheme with the controlled flow aberrations. This thesis deals with the measurement of aero-optic aberrations using the return light from a laser-induced air-breakdown spark. The spark is created by focusing a high-energy pulsed laser at a point outside the aircraft, thereby creating an air-breakdown spark with sufficient brightness for aero-optic measurements. The operational advantages offered by this kind of man-made guide star are significant since it can be placed at any point in space outside the aircraft. Topics addressed in the thesis include compensation of anisoplanatic differences between the spark beam and the outgoing laser beam, characterization of the optical behavior of the spark, and investigation of flow effects on the spark.