Ringdown spectroscopy in optical waveguides

Abstract

Ringdown spectroscopy (RDS) is an absorption spectroscopic and detection technique that makes use of an optical cavity to realize a long effective pathlength through a sample and to render the measurement independent of intensity. These two features give RDS an advantage over traditional absorption techniques and allows its application in measuring concentrations of strongly absorbing analytes present in trace amounts, or in measuring weak absorptions for analytes in higher concentrations. The resonant optical cavities used here are created from optical fibre. This permits the easy construction of an inexpensive apparatus for RDS. The performance of various cavity geometries (linear, circular) in three ranges of the visible and near infrared spectrum (405, 800, and 1550 nm) was examined. Concurrent multiexponential decays arising from core modes, cladding modes, and amplified spontaneous emission were analyzed in the framework of an exponential decay model transformed into the frequency domain. The small mode field diameter of light within a fibre is well suited to probing very small liquid volumes on the order of pico- or femtolitres. This uniquely positions optical waveguide-based RDS for application in absorption detection for separation techniques such as capillary electrophoresis (CE) and high performance liquid chromatography (HPLC) where high time resolution detection is required across narrow separation channels. The experimental and theoretical work presented here was preformed with an eye towards this purpose.