3D printed castle style Fabry-Perot microcavity on optical fiber tip as a highly sensitive humidity sensor

A 3D printed castle style Fabry-Perot (FP) microcavity on optical fiber tip for humidity sensing is developed and demonstrated experimentally. The castle style FP microcavity, which has periodically arranged square holes on its side wall, is directly fabricated on an optical fiber tip through two-photon polymerization. Because of the existence of square holes in the castle style FP microcavity, Polyvinyl Alcohol (PVA) can be filled into the FP microcavity and a 360-degree humidity response sensitive area is formed. The contact area between water molecules and humidity sensitive materials is significantly increased, which will result in a high uniformity and a faster response of humidity sensing. Experimental results show that, the sensitivity of proposed castle style FP microcavity humidity sensor is up to 248.9 pm/%RH with relative humidity changing from 46 %RH to 75 %RH. In addition, repeated experiments indicate that the 3D printed castle style FP microcavity humidity sensors are repeatable and stable. Such excellent sensing performance with compact structure, highly sensitive, faster response and good stability makes the proposed sensing structure a highly promising candidate for humidity sensing applications.