EFFPI Based on Sensitive Film
EFFPI fabricated with clean optical fiber end face can measure physical parameters such as strain, pressure, temperature, or even refractive index-based biochemical parameters. However, they are not able to perform humidity detection, gas sensing, or biosensing. Sensitive films have been widely integrated with various fiber-optic structures in order to strengthen the capability of fiber-optic sensing. There has been great progress, and considerable contribution has been made by researchers from a variety of fields. Here, we only introduce several examples.
In 2009, Fan and coworkers  developed an EFFPI gas sensor by coating a silver film and versatile vapor-sensitive polymer layer on a cleaved fiber end face. The interference occurs between the reflected light from the silver-polymer interface and the polymer-air interface. Any polymer materials, regardless of the refractive indices, can be used for the proposed microstructure. Two kinds of polymers, polyethylene glycol (PEG) 400 (RI = 1.465-1.469) and Norland Optical Adhesive (NOA) 81 (RI = 1.53-1.56), were used as examples for gas sensing and showed drastically different sensor response to hexanol, methanol, and acetone. The thickness of the polymer layer was about 30 p,m. The sensitivity for methanol vapor was 3.5 and 0.1 pm/ppm for PEG 400 and NOA 81, respectively, with a detection limit on the order of 1-10 ppm. The introduction of the silver film greatly enhanced the fringe visibility, as it has a very low refractive index, n = 0.3 + 11г. Also it enables the versatile usage of various polymer materials.
By using the SU-8 photoresist as the coating material, EFFPI was fabricated based on the film-on-fiber structure . The SU-8 film was coated directly on the cleaved fiber end face. The thickness of SU-8 film was about 30 |!m. The fringe contrast was measured from the reflective spectra as a function of the refractive index of the medium solution. The fringe visibility reached ~25 dB when the device was immersed into water. An average sensitivity of 205 dB/ RI was achieved in the range between 1.314 and 1.365. The resolution can be estimated to be 5 X 10-6 as the relative intensity resolution of the optical spectrum analyzer can reach 0.001 dB near 1550 nm. Temperature insensitivity and long-term stability for RI sensing were confirmed experimentally. In another work, the authors also investigated the wavelength shift of the interference fringe of the SU-8- coated EFFPI as a function of temperature . The sensitivity is about 200 pm/°C. The standard deviation of the peak wavelength was about ±300 pm over 60 min when the temperature was fixed at 50°C.