Gelatin-Coated Michelson Interferometric Humidity Sensor Based on a Multicore Fiber With Helical Structure

A Michelson interferometer with gelatin coated for relative humidity (RH) measurement is proposed and demonstrated. It is fabricated by connecting the lead in/out single mode fiber, multimode fiber, triangular four core fiber (FCF) in a certain order. A gold film is deposited on the end face of the FCF as a reflector. A helical structure is fabricated by twisting the FCF under continuous arc discharging. Gelatin is coated onto the whole sensor to sense RH changes. When the RH of the external environment changes, both the refractive index (RI) and the size of the gelatin film changes, so the effective RI of the FCF cladding and the strain of the sensor changes accordingly, resulting in the resonance dip wavelength change in the interference signal. Such a probe is applied for RH measurement with a sensitivity of about -0.185 nm/%RH in the range from 45.0%RH to 81.7%RH with the response/recovery time of approximately 5.24/7.14 s, respectively.