PMMA Microsphere-Based Fabry-Perot Fiber-Optic Humidity Sensor

Polymethyl methacrylate (PMMA) microsphere-based Fabry-Perot (F-P) fiber-optic humidity sensor is proposed. A hollow-core fiber (HCF) is spliced with a single-mode fiber (SMF), and a PMMA microsphere is inserted into the HCF. Three sensors with different PMMA microsphere sizes (110, 120, and $130~\mu $ m) were fabricated by using PMMA microsphere as a moisture-sensitive material, and the moisture-sensing characteristics of these sensors with different cavity lengths (45, 60, 75, and $90~\mu $ m) were measured in the range of relative humidity (RH) from 35%RH to 95%RH. The results show that the sensor with a PMMA microsphere size of $110~\mu $ m and a cavity length of $45~\mu $ m has the best sensitivity to humidity of 0.074 nm/%RH. Its repeatability, stability, response time, and temperature effect are also investigated. This is the first fiber-optic humidity sensor with an F-P structure using PMMA microsphere as filler instead of moisture-sensitive film or gel. Compared to moisture-sensitive film or gel that are prone to aging and hardening leading to a short service life and are prone to deliquescence under high humidity, the sensor proposed in this article has the advantages of wide measurement range that can be applied to high humidity measurement, fast response time, good long-term stability, and long service life.