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. This review offers a comprehensive analysis of recent advances in optical. An optical pH sensor basically comprises two essential parts: A pH sensitive sensor layer and a read-out device (pH meter).