Detection of Chemical Compounds Using Amplified Fiber Loop Ring-Down Spectroscopy

Abstract

Cavity ring-down spectroscopy (CRDS) is an absorption spectroscopic technique. In CRDS the concentration of an analyte is determined by measuring the reduction in finesse of an optical cavity made from two highly reflective (R>99.9%) mirrors once a sample is introduced. Optical loss is traditionally determined from the exponential intensity decay of a short laser pulse that was injected into the cavity. This decay is the longest for an empty, high finesse cavity and is reduced when the sample absorbs or scatters light. In this project, the optical cavity is made from fiber optic waveguides and the light source is a continuous wave (cw) diode laser. It is used to detect analytes such as acetylene, ammonia and other amines through their overtone absorption in the telecom region at 1500 nm. The experiment is done by increasing the ratio of desired loss (extinction caused by the sample), to undesirable loss (from the waveguide or solvents) through amplification of the ringdown signal using an erbium doped fiber amplifier (EDFA). The EDFA is inserted into a fiber-optic loop and its gain is increased above the lasing threshold. The gain of the, now lasing, fiber loop is "clamped" to a high and constant value, thereby removing unwanted gain fluctuations, and all losses in the loop are compensated for. If one now inserts a laser light pulse at the lasing wavelength of the loop it would circulate through the loop indefinitely, whereas a light pulse at a wavelength that is being absorbed by an analyte would experience a decrease with time at a rate that depends only on the magnitude of the sample absorption. By enclosing the sample gap with a gas cell both acetylene and ammonia have been detected down to ~25 ppm and ~5.9 Torr respectively. Subsequently, a 1% solution of aminotoluene was detected in an interrogation volume of 5.65 pL by having inserted a fiber with a hole drilled in it as the sample gap. At present, the drilled fiber has been replaced with photonic crystal fiber such that small volumes of gases may be detected with a longer effective path length.