A theoretical study of tapered, porous clad optical fibers for detection of gases

In this paper, the detection of gases using attenuated total internal reflection (ATR) technique has been theoretically described in multimode step-index, tapered fiber with porous cladding. Using a modal approach the response time and sensitivity of the sensor have been obtained in cases of parabolic, linear and exponential–linear tapered optical fiber. It has been shown that the sensor response time and the minimum detectable concentration depend on the taper ratio and geometry of the taper profiles for given fiber parameters, the absorptivity of gas and the diffusion coefficient in the porous cladding. For a given taper profile, the higher the taper ratio, the smaller the response time and the minimum detectable concentration. Additionally, the sensor with the exponential–linear taper profile has showed faster response times and lower minimum detectable concentrations than those with the parabolic and linear profile in the case of a particular taper ratio.