Mini review: Recent advances in long period fiber grating biological and chemical sensors

As an important member of the optical fiber sensor family, long period fiber gratings (LPFG) have attracted increased attention due to their outstanding characteristics. The LPFG is a transmission fiber grating without backward reflection, and isolation is not required in the sensing system. The resonant wavelength and transmissivity of LPFGs are very sensitive to changes in refractive index, temperature, strain and transverse load, so they are widely used. In this article, recent advances in biological and chemical applications of LPFG sensors are described. Various methods for improving the sensitivity of LPFG sensors are summarized, such as reducing the diameter of the cladding and cascading. In addition, an important method involves the coating of the LPFG sensor with a specific recognition element for biomass detection or a sensitive metal film for chemical detection. The structural parameters and principles in the literature are discussed in detail. This article analyzes and compares the progress and deficiencies in the detection process and anticipates future developments.