Mid-infrared laser absorption spectroscopy in coiled hollow optical waveguides

A new nitric oxide (NO) sensor is intended for use in assessment of airway inflammation with applications in asthma diagnosis and management as well as in other health care applications involving inflammation in the gastrointestinal tract and the urogenital organs. The sensor was designed to measure trace quantities of NO in air using the combination of hollow optical waveguides and quantum cascade lasers. The primary application intended is analysis of exhaled breath. The unique marriage of the components and the novel design provides for rapid response to concentration changes while maintaining sensitive measurement capabilities. We achieved a lower detectable limit of 58.8 ppb of NO in N-2 with a 0-90% response time of 0.48 s. The QC laser was operated at room temperature in pulsed current mode near 5.4mum. The hollow waveguide used to make these measurements was 9m in length and the inside diameter was 1000mum. The waveguide was coiled with a 15cm radius of curvature and perforated on the interior walls of the coils to allow gas to flow into and out of the waveguide. The sensor can easily be converted to measure other gases in the mid-infrared by selecting a QC laser whose output is coincident with the absorption line of interest.