Photonic crystal fiber refractive index sensor based on surface plasmon resonance

This paper presents a photonic crystal fiber (PCF) refractive index sensor. The sensor structure is quite simple. It is composed of three segments of optical fibers spliced together. The multimode fibers with core diameter of 50 mu m are used for light input and output. The middle fiber is a short segment of PCF, ESM-12-01. Although it has some advantages such as being able to operate in single mode for a large number of light wavelengths and has great temperature stability, it also has a common drawback of the PCFs, that is, the tiny holes will collapse when they are spliced. This paper makes use of this drawback to facilitate the generation of the surface plasmon resonance. The spliced region of a PCF actually becomes a thin silica rod that is no longer a PCF or a traditional optical fiber. For this reason once the light travels into this region it diverts in all possible directions. Thus, the splice acts as a mode converter that converts the core modes of the multimode fiber into a set of the modes spreading into the PCF cladding. Among those modes some are suitable for SPR excitation. The width and the depth of the output spectrum dip depend on the length of the sensing part and the thickness and uniformity of the gold coating, and hence these parameters affect the properties of the sensor. The developed sensor is compact in size, simple to fabricate, promising in performance, and has a potential for practical applications.