Evanescent Field Absorption Sensor Using a Pure-Silica Defected-Core Photonic Crystal Fiber

An evanescent field absorption sensing technique in liquid solutions is demonstrated using a microstructured photonic crystal fiber (PCF). Aqueous solution of cobalt chloride was infiltrated into all air holes of the PCF. The defected-core with a central hole in the PCF together with an increase in the surface volume with the evanescent field penetration into the air holes enhanced the liquid absorption sensitivity. The effects of solution concentration, detection directions, and PCF length on the absorption sensitivity are investigated experimentally. We have obtained a linear relationship between the evanescent field absorption and liquid concentration and also between the evanescent field absorption and PCF length. The absorption sensitivity using the longitudinal detection method is increased by more than 60 times compared with that using perpendicular measurement technique.