Fiber-optic pressure sensor based on Fizeau receiving interferometer

Fiber-optic sensor of pressure, based on low-coherence interferometry, is presented. The sensing element is a thin stainless steel membrane that bends under the external pressure. The membrane is placed against the output fiber end of 2×1 fiber-optic coupler, forming in this way a sensing Fizeau interferometer. The receiving interferometer is a thin 0.5° Fizeau glass wedge, placed in the front of the input-side coupler arm and illuminated by the membrane-backreflected modulated light. The low-coherence interferometric pattern have been captured by 1-D CCD array and further demodulated by an algorithm based on calculation of Morlet wavelet coefficients. Any shift of the membrane, caused by pressure, produces a corresponding shift of the interferometric pattern over the linear CCD array. We have measured mechanical sensitivity of the membrane of about 200 nm/mbar. The precision/accuracy of the interferometric demodulation was about 100 nm, yielding the precision of pressure measurement of about ±0.25 mbar, in a range from 0 to 500 mbar.