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

A method of interrogating a free-space Fabry–Perot sensing interferometer (FPI) by angle tuning the incident beam in order to vary the optical pathlength is described. A system based on a PC controlled precision galvanometer and a 1–1 beam expander that enabled the angle of incidence to be rapidly and continuously varied without translation is described. This system was used to demonstrate that the transfer functions (ITF) of low finesse FPIs of optical thicknesses in the range 80–200 μm could be accurately recovered by tuning the angle of incidence over less than 9◦. Two sensor interrogation schemes are described. One involves scanning through the ITF in constant phase steps by applying an arcosine voltage waveform to the galvanometer and then returning to the angle at which the ITF phase derivative is a maximum. The other uses a mirror continuously rotating through 360◦ to repetitively scan through the ITF in constant angular steps. The signal is then recovered at the instant in time that the maximum value of the ITF derivative occurs. It is considered that angle tuning offers a flexible and inexpensive alternative to interrogating free-space FP sensors by wavelength tuning.