REAL-TIME DIGITAL-CONTROL OF OPTICAL INTERFEROMETERS BY THE MECHANICAL-MODULATION TECHNIQUE

We discuss the application of digital systems to the automatic control of dual-wave optical interferometers. We show that, if the mechanical -modulation technique is used for error-signal extraction, digital tec hniques can be used both for error-signal extraction and for control-s ignal generation. Therefore, apart from two front/end amplifiers that are necessary to match the dynamics of the detectors and actuators to the dynamics of the analog-to-digital converters and digital-to-analog converters, no other analog devices are required. In particular, the mechanical-modulation technique requires the synchronous demodulation of the photodiode output signal. Hence we need to implement a digital lock-in amplifier whose algorithm is described here. Finally, we descr ibe one of the possible applications of this digital control procedure , such as the control of a classic Mach-Zehnder interferometer in air.