Passive signal processing for a miniature Fabry–Perot interferometric sensor with a multimode laser-diode source

A passive signal-processing technique for addressing a miniature low-finesse fiber Fabry–Perot interferometric sensor with a multimode laser diode is reported. Two modes of a multimode laser diode separated by 3 nm are used to obtain quadrature outputs from an ~20-μm cavity. Wavelength-division demultiplexing combined with digital signal processing is used to recover the measurand-induced phase change. The technique is demonstrated for the measurement of vibration. The signal-to-noise ratio is ~70 dB at 500 Hz for ~π/2 rad displacement of the mirror, which results in a minimum detectable signal of ~200 μrad Hz−1/2. A quantitative discussion of miscalibration and systematic errors is presented.