Humidity sensing characteristics of compact coupled optical fiber structure by microsphere and tapered capillary tube

In this paper, an optical fiber humidity sensor has been proposed based on the coupling structure using BaTiO 3 microsphere and conical cavity of capillary pigtail. The sensitive material polyvinyl alcohol (PVA) was coated near the coupling region of microsphere and conical cavity to improve its sensing performance. The humidity sensitivity has been demonstrated to be 0.09 dBm/%RH based on intensity demodulation technique. The morphological parameters of the coupling structure are further optimized to obtain the maximum sensitivity of -0.297 dBm/%RH. Compared with other microsphere based optical fiber sensors, the sensor has the advantages of compact structure, simple manufacture, low production cost, high sensitivity and good stability.