Highly sensitive humidity measurement with temperature compensation based on a special dual-modal interferometric sensor and varied period Vernier effect

This paper proposes a novel highly sensitive humidity sensor with temperature compensation, mainly consisting of a special modal interferometer bonded with thick polyimide (PI) solid films. The modal interferometer is constructed by splicing a special dual-mode fiber (DMF) with two sections of regular single mode fiber (SMF), forming an SDS (SMF-DMF-SMF) structure. Due to the particular design of the refractive index of the DMF, only LP 01 and LP 02 modes are transmitted, which causes a unique wavelength (UWL) to appear in the spectrum. The UWL breaks the periodicity of the conventional interferometric spectrum. More importantly, both sides of UWL respond differently to various measurands, which enables the interferometer to measure dual parameters simultaneously. To achieve a higher humidity sensitivity, the thick PI solid films are used for the first time to manufacture the interferometric humidity sensors. Their thickness is at least one order of magnitude greater than that of the PI coating used in the traditional humidity sensors, which enhances the sensor's response to humidity variation significantly. The performance of the proposed sensor is further improved with the aid of the varied period Vernier effect by paralleling a bare dual modal interferometer based on the SDS structure as a reference. Similar to the sensing modal interferometric spectrum, the output spectrum of the varied period Vernier effect scheme owns the UWL as well, which makes it easy to distinguish the wavelength shifts of the specific peaks to the measurands. The validity of the proposed scheme is verified by an experimental case, where the humidity sensitivity and the temperature sensitivity are achieved as 1044.26 pm/%RH and 1467.99 pm/°C, respectively.