Interrogation of free-space Fabry–Perot sensing interferometers by angle tuning

Abstract

A method of interrogating a free-space Fabry–Perot sensing interferometer (FPI) by angletuning the incident beam in order to vary the optical pathlength is described. Asystem based on a PC controlled precision galvanometer and a 1–1 beam expanderthat enabled the angle of incidence to be rapidly and continuously variedwithout translation is described. This system was used to demonstrate that thetransfer functions (ITF) of low finesse FPIs of optical thicknesses in the range80–200 µm could be accurately recovered by tuning the angle of incidence over less than9°.Two sensor interrogation schemes are described. One involves scanning throughthe ITF in constant phase steps by applying an arcosine voltage waveform tothe galvanometer and then returning to the angle at which the ITF phasederivative is a maximum. The other uses a mirror continuously rotating through360° to repetitively scan through the ITF in constant angular steps. The signal is then recoveredat the instant in time that the maximum value of the ITF derivative occurs. It isconsidered that angle tuning offers a flexible and inexpensive alternative to interrogatingfree-space FP sensors by wavelength tuning.