In this work, we present the design of a refractive Axicon lens to be used in refractive index optical sensing. The lens is designed to generate a Bessel – Gauss beam at the wavelength of 3.3 microns. At that wavelength, the absorption of the CH4 is maximum and thus a maximum change in the refractive index due to the CH4 gas is also expected. The intensity profile of the generated beam is quite sensitive to the index of refraction of the surrounding medium. Placing the optical detector at the point of maximum change in the intensity with refractive index allows the measurement of the refractive index change and hence the gas percentage with very high sensitivity. Our design shows sensitivity greater than 970 % per RIU. We also develop an analytical formulation for the intensity variation with the refractive index. The results obtained analytically are confirmed by the finite difference time domain FDTD calculation. From the analysis and the derived expressions, we demonstrate the effect of the Axicon base angle on the sensitivity and hence allow for the lens optimization to achieve maximum sensitivity for a target application.

Date Published: 3 March 2020
PDF: 8 pages
Proc. SPIE 11276, Optical Components and Materials XVII, 112760Z (3 March 2020); doi: 10.1117/12.2545988

Published in SPIE Proceedings Vol. 11276:
Optical Components and Materials XVII
Shibin Jiang; Michel J. F. Digonnet, Editor(s)