A wavelength encoded optical fiber sensor based on multimode interference in a coreless silica fiber

A wavelength encoded optical fiber sensor using a three-segmented fiber structure is proposed. The device consists of a coreless silica fiber (CSF) which is coated with a thin film and spliced between two standard single-mode fibers (SMFs), forming a SMF-CSF-SMF (SCS) structure. When light is transmitted from the SMF into the CSF, the LP01 mode in the SMF is coupled to the LP0n modes, and a multimode interference occurs in the CSF. These modes interact with the thin film, hence the thickness and refractive index of the thin film can affect the modal interference. We analyze the transmission spectra of the SCS structure to obtain the characteristics of the sensor including sensing sensitivity. Numerical simulations are carried out by using the Beam Propagation Method (BPM) to investigate the multimode interference in the SCS. Two different conditions are considered in our studies: 1) changing the refractive index of a fixed-thickness film, and 2) varying the film thickness with certain refractive index. It has been found that the wavelength corresponding to the minimum output power increases 0.33509 nm when the refractive index changes every 0.01 from 1.33 up to 1.40, and 6.760 nm when the thickness enhances form 0 to 1000 nm. The trend of the raise is mostly linear for the former simulation, but gets slower and slower for the latter. The SCS structure can serve as a fiber platform for non-labeling bio-sensing when a bio-film is coated to the CSF.