Optical fiber pH sensor based on a multimode interference device with polymer overlay

An optical fiber pH sensor based on a multimode interference structure is presented. The sensitive element is a piece of no-core fiber (NCF) coated with a mixture of polyallylamine hydrochloride and polyacrylic acid by a modified layer-by-layer (LbL) self-assembly method. It is experimentally shown that by reducing the diameter of the NCF by chemical etching, the sensitivity is increased from −0.31nm/pH to −2nm/pH. The sensor exhibits a high linear response of 0.997 over a pH range from 5 to 11.3 with a rapid response time lower than 1 s.