Automation and Control of Fabry–Pérot Interferometers

Fabry-Peacuterot interferometry (FPI), which was originally invented for spectroscopy, is now evolving as a basic technology for ultrafine dimensional stabilization and measurement. To this end, the light path length of an optical cavity and the wavelength of a laser source injected into the cavity have to be tuned to each other through a set of frequency and/or displacement actuators driven by a sharp and narrow signal-encoding total-cavity detuning. Digital control is essential in facilitating and automating FPI use in view of space applications and routine instrumentation. This paper shows how embedded model control (EMC) technology, which was developed by one of the authors, allows to systematically proceed from fine dynamics and requirements to the EM, which is the core of control design and algorithms. In this framework, all critical control issues have a coordinated solution: disturbance estimation and rejection, command constraints and multiplicity, hybrid dynamics, constraints due to unmodeled dynamics, and performance analysis. Several experimental results are illustrated and discussed in the light of the methodology