Digital Phase Demodulation in Heterodyne Interferometry

Abstract

The emergence of tunable diode lasers from the communications industry creates a need in diagnostic heterodyne sensors for flexible FM demodulation techniques. By using tunable lasers, it becomes efficient to generate Heterodyne Beat Frequencies (HBF) that can be tuned to over 10 GHz with frequency stabilities to within 100 kHz. These diode lasers are small in size, rugged, reliable, and relatively cheap. These characteristics make diode lasers the ideal light sources for sensors to be integrated into industrial processes, smart structures, and on-line quality control or NDE. Since the diode lasers can easily and efficiently change the HBF, the heterodyne sensor can become a practical multi-use sensor. A high frequency ultrasonic sensor can be converted to a low frequency vibration sensor by tailoring the HBF of the heterodyne interferometer to an appropriate frequency for the desired surface motion. To make this transition possible the FM demodulation techniques must also be just as flexible. One approach, that addresses this demodulation flexibility issue, is to digitize the modulated carrier which contains the surface motion information. The modulated carrier has a center frequency at the HBF. The carrier is then digitally demodulated in the computer. This paper discusses and demonstrates Digital Phase Demodulation (DPD) techniques for obtaining displacement information from an optical heterodyne interferometer.