Guided-mode based Faraday rotation spectroscopy within a photonic bandgap fiber

Abstract

Microstructured optical fibers provide a unique environment for new compact sensing of gases as they offer advantages including long optical pathlengths, strong confinement of high power light and extremely small sample volumes compared to free-space gas sensing architectures. Here we investigate the interaction of a modulated magnetic field with guided light to detect a paramagnetic active gaseous medium within a hollow-core photonic bandgap fiber (HC-PCF). This novel fiber-optic approach to Faraday Rotation Spectroscopy (FRS) demonstrates the detection of molecular oxygen at 762.309 nm with nano-liter detection volume. By using a differential detection scheme for improved sensitivity, guided-mode FRS spectra were recorded for different coupling conditions of the light (i.e., different light polarization angles) and various gas sample pressures. The observed FRS signal amplitudes and shapes are influenced by the structural properties of the fiber, and magneto-optical properties of the gas sample including the magnetic circular birefringence (MCB) and the magnetic circular dichroism (MCD). A theoretical model has been developed to simulate such FRS signals, which are in good agreement with the observed experimental results and provide a first understanding of guided-mode FRS signals and dynamics of the magneto-optical effects inside the optical fiber. The results show that microstructured optical fibers can offer a unique platform for studies concerning the propagation of light in linearly and circularly birefringent media. © 2013 SPIE.