Fabry-Pérot cavity based on polyimide cold-spliced and Vernier effect for relative humidity sensing application

Abstract

Utilizing humidity sensitive material polyimide (PI) and optical Vernier effect, a humidity sensor with high sensitivity is fabricated and verified by experiments. The sensor consists of two Fabry-Perot interferometers (FPIs) in parallel. The Fabry-Perot (F-P) cavity of FPI1 is composed of two single mode fibers (SMF) cold spliced through PI, which is a PI cavity. When the ambient humidity rises, the expansion of PI after absorbing water molecules directly changes the F-P cavity length of FPI1, so FPI1 is particularly sensitive to humidity. The relative humidity (RH) sensitivity of a single FPI1 is measured to be 0.5361 nm/%RH. The F-P cavity of FPI2 is composed of "SMF - quartz capillary - SMF" in turn hot splicing, which is an air cavity. FPI2 with air cavity is insensitive to RH. The FSRs of FPI1 and FPI2 are controlled to approach, and the Vernier effect is generated after they are connected in parallel, which further amplifies the RH sensitivity of FPI1. When using Vernier effect to measure RH, the RH sensitivity of the sensor is as high as − 7.6221 nm/%RH, which increases the RH sensitivity of a single FPI1 by nearly 14.2 times. Moreover, the temperature cross sensitivity of the sensor is about 0.067%RH/°C, which is very small. This sensor has the advantages of easy fabrication, low cost, high sensitivity, good repeatability and stability, and fast response.