M.S. Thesis Presentation by Charles F. Bergh
Thrusday, April 15, 1999

(Dr. Tom Kurfess, advisor)

"Development of an Interferometric System for Process Monitoring"


Within manufacturing there is a continuing need for the development of sensors to monitor critical process variables in increasingly hostile environments.  Accurate and timely measurements allow for on-line, closed-loop control of the process, thereby reducing defective or sub-standard product.  Traditional methods often fail when exposed to excessive temperature, radiation, or vibration.  Complex geometries or irregular surface finished are problematic for contact and near-contact sensors.  Additionally, bandwidth is often limited due to sensor resonance.  Laser-based interferometers offer a means by which high-bandwidth inspections can be made without physical contact.  However, interferometric measurements are greatly affected by the optical "noise" introduced by surface finish and work-piece vibration.  These problems have been addressed by using the optical compensation or by using complex signal tracking and averaging techniques.  Both increase the complexity, physical size, and cost of the system.  Commercial interferometers are typically designed for use in laboratories and are not durable enough for use in manufacturing environments.  For these reasons, it would be advantageous to develop an interferometer which is compact and robust, has lower sensitivity to surface finish or work-piece vibration, and is sufficiently flexible to allow integration into an automated system.

This research will develop a reference-beam interferometer with an optical-fiber probe.  The interferometer will consist of a compact diode laser source, optics to form and recombine reference and probe beams, and a detector.  The fiber probe will consist of several meters of optical fiber and distal-end focusing optics so that measurements can be made in varying locations while the interferometer optics and detector remain in a single, fixed position.  The CLUE (Compensated Laser Ultrasound Evaluation) sensor, developed by Hughes Research Labs, will be utilized as the detector.  This detector is robust, compact, and essentially combines optical detection and compensation with electronic post-processing hardware into a single semiconductor element.

The interferometer will be compared to a commercial heterodyne interferometer for sensitivity to surface finishes.  The signal-to-noise ratios of each interferometer will be compared.  The resulting system will produce a rugged, compact interferometric system designed to meet the requirements of a manufacturing environment.  One immediate application of this research is real-time weld quality control.