Numerical investigation of a depressed-index core photonic crystal fiber for gas sensing

A depressed-index core photonic crystal fiber (PCF) was numerically analyzed for gas sensing application. The dependence of relative sensitivity, effective mode area, confinement loss, and bend loss on the fiber parameters as well as wavelength are investigated. Lowering the refractive index of core reduces the index contrast between the core and the cladding weakens the index-guidance of the fiber, and enhances the relative sensitivity and the mode area simultaneously. Relative sensitivity and effective mode area increase with the cladding background index, especially in the short wavelength region, which are opposite to that in a conventional PCF. The enhancements of the relative sensitivity and the normalized effective mode area are more than 3700% (from 0.0065 to 0.25) and 680% (from 3.85 to 30), respectively, as compared with that of the non-depressed-index core PCF.