Light waves interaction with an analyte in fiber-optic sensors for mid-IR spectroscopy

Determining chemical composition of substances remotely in real time is a current challenge. We address this issue by developing the fiber-optic sensors for mid-IR spectroscopy. In this paper, we take a glance at interaction of fiber modes with a liquid analyte through an accurate theoretical analysis based on wave optics concepts briefly described herein for a multidisciplinary audience. As an example, a multimode core-only chalcogenide fiber immersed into an aqueous acetone solution has been considered as a sensing element. The power decay length of the fiber mode along the fiber due to light absorption in the mid-IR at vibrational transitions of the analyte molecules has been shown to be determined by the penetration depth of the mode into the analyte and varies from nanometers to hundreds of meters for the modes, respectively, from the highest to the lowest order. A decisive parameter has been found that is the ratio of the sensing element length to the power decay length of a fiber mode. The highest sensitivity of the sensor is achieved when the ratio magnitude is approximately between 0.5 and 2. Light delivering in the higher-order modes having small power decay lengths allows for creating highly sensitive compact devices.