Ultra-sensitive relative humidity sensor formed by two parallel Fabry-Perot interferometers and Vernier effect

In this paper, a relative humidity (RH) Vernier sensor based on gelatin diaphragm is proposed, in which the gelatin diaphragm is coated on the end surface of the quartz capillary by the dot-dip method, so that the prepared diaphragm is only 3 μm thick. The sensing cavity FPI1 is an air-cavity made of single-mode-fiber (SMF), quartz capillary (QC) and gelatin diaphragm. Gelatin diaphragm is an excellent humidity sensitive material, therefore FPI1 has high sensitivity to humidity. The reference cavity FPI2 is also an air-cavity spliced in the order of SMF-QC-SMF. When FPI1 and FPI2 have similar free spectral range, they form a Vernier effect in parallel, achieving maximum humidity sensitivity. The experimental results show that in the range of 1300 nm to 1650 nm, the envelope formed by two parallel FPIs is particularly regular and has a free spectral range of 95.2 nm. By tracking the variation of the envelope dip with humidity, the sensor obtained a highly fitting polynomial relationship in the range of 57 %RH − 84 %RH. However, in the range of 75 %RH − 84 %RH, the sensor achieved an excellent linear fitting relationship, with a linearity of up to 0.9997 and a sensitivity of up to 27.9726 nm/%RH. This is the most sensitive gelatin Vernier humidity sensor ever reported. And the crosstalk of temperature of the Vernier humidity sensor is only 0.0186 %RH/℃. The sensor is easy to manufacture, ultra-high sensitivity, small temperature crosstalk, good performance and low cost, and has good reversibility and stability. It is a competitive humidity candidate sensor.