Fringe Amplitude Modulation Method for Differential Pressure Sensor

This paper presents a new type of differential pressure (DP) sensor, which combines fiber interferometry and spectral amplitude-modulation (AM) method. The sensing system consists of two Fabry-Perot (FP) interferometric cavities with pressure movable and reflective diaphragms. The broadband lights return from the two FP type of sensor probes, and they are superimposed in the optical frequency domain. The doubled air gap size is much smaller than the coherence length inversely proportional to the spectral resolution, while the optical path difference (OPD) between two fiber channels is designed much longer than the coherence length. The superimposed intereference fringe shows a sinusoidal and amplitude modulated signal due to the air gap sizes´ difference, which is biased by the signals due to the total air gap sizes in the frequency domain. Frequency interval of the low frequency factor is converted into the DP values without any compensation for the unwanted factors, including source power fluctuation and fiber bending losses. Experimental result shows that a resolution is as high as the 0.08 kPa per THz and accuracy as high as 1.0% per full scale.