Abstract: A hollow-core photonic crystal fiber for sensing an oil includes a hexagonal core wall, a cladding region, and a circular layer. The hexagonal core wall defines a hollow space, where a core diameter is equal to a distance between two opposing inner vertices of the hexagonal core wall. The cladding region includes a circular cladding segment and up to four cladding arms, where the up to four cladding arms extend from the circular cladding segment to four outer vertices of the hexagonal core wall. An inner wall of the circular cladding segment, the up to four cladding arms and an outer surface of the hexagonal core wall define up to four wedge shaped air gaps. The cladding region has an inner radius with a range from 2.0 up to 2.4 times the core diameter and an outer radius with a range from 2.1 up to 2.6 times the core diameter.

Abstract: An optical sensor that includes a photonic crystal fiber (PCF) and an optical coupler. The PCF includes a hollow core with a first diameter (Dc), an inner cladding section, and an outer layer surrounding the inner cladding section. The inner cladding section includes a cladding material and a plurality of spaced apart circular-shaped channels disposed in the cladding material surrounding the hollow core. The cladding material includes at least one of glass, ceramic, and glass-ceramic. Each spaced apart circular-shaped channel has substantially the same size and an average diameter (d). A first end of the PCF is in optical communication with a light source and a second end of the PCF is in optical communication with the optical coupler.