| Laser speckle monitoring is a way to measure the motion of an optically rough surface without making physical contact to that surface. Laser speckle is inherently sensitive to motion in the plane of the reflecting surface, which gives speckle methods a distinct advantage over classical interferometric techniques, such as homodyne, heterodyne, and Doppler interferometry. The speckle monitoring technique used in this current investigation is unique among speckle methods for its ability to detect displacement with sub-speckle precision, in real time at ultrasonic bandwidths, all without the use of any special interferometric arrangement. Combined with optical ultrasonic generation, laser speckle monitoring provides a viable, inexpensive, and simple alternative system for completely non-contact structural and material analysis.;This thesis details the exploration of a unique detector design specifically engineered with a speckle-monitoring application in mind. Small arrays of these detectors have been fabricated from silicon-on-insulator (SOI) technology and put to work measuring surface vibration in laboratory experiments. The important characteristics of these photodetectors are their small physical dimensions on the order of 10 mum, ultrasonic bandwidth extending into the megahertz regime, and high internal gain - the ability to generate multiple electrons of photocurrent for every photon captured.;A prototype vibration detection system has been tested together with several all-optical vibration generation and detection schemes. Both high-powered pulsed lasers and lowpower modulated CW diode lasers are used to set up vibrations in simple structures and the surface motion is captured by a speckle monitoring arrangement with SOI detectors. These tests are used to evaluate the speckle monitoring system itself and the capabilities of the detector prototypes. With the appropriate detector arrays and appropriate signal processing, direct laser speckle monitoring, combined with optical generation of ultrasound, shows promise as an alternative fully non-contact diagnostic technique for the physical evaluation of certain structures and materials. |
