A Novel Optical Fiber Mach–Zehnder Interferometer Based on the Calcium Alginate Hydrogel Film for Humidity Sensing

A novel optical fiber humidity sensor based on a Mach-Zehnder Interferometer (MZI) is proposed and experimentally demonstrated. The MZI is fabricated by three sections of single mode fiber (SMF) and two sections of multimode fiber (MMF), where the two MMFs are spliced among the three SMFs. To obtain an excellent sensor, two sections of MMF are etched with hydrofluoric acid, then sodium alginate is coated on the corroded areas and immersed in calcium chloride solution to form calcium alginate (CaAlg) hydrogel by fully crosslinking sodium alginate with calcium ions. While humidity is between 15%~80%, the sensitivity of the sensor can reach 0.48346 dB/%RH. In addition, the proposed sensor shows good performances in the repeatability, stability, response and recovery time, which can be potentially used in the medical storage field.