Absorption spectroscopy in hollow-glass waveguides using infrared diode lasers

Hollow-glass waveguides may be a viable technology that, in some cases, may supplant heavier multi-pass cells such as White or Herriott cells for performing trace detection using tunable diode laser absorption spectroscopy. We report here a series of experiments for testing the suitability of waveguides for infrared spectroscopy. The loss characteristics of 1 mm bore diameter waveguides have been measured for straight and coiled lengths. The minimum linear loss coefficient is 0.46 (+/- 0.10) dB/m while the bending loss coefficient is 0.42 dB. Using a flow of 1 ppm nitric oxide in nitrogen mixture through a coiled 3 meter length of waveguide (coil diameter 50 cm) we could detect the fundamental R(8.5), Ω = 3/2 transition of NO with a signal-to-noise (RMS) ratio of 44:1 in direct absorption using a single mode, lead-salt diode laser with six minutes of signal averaging. Using direct absorption spectroscopy we have found that the absorption pathlength is approximately equal to the physical length of the waveguide. Broadband FM diode laser spectroscopy produces for the same transition and waveguide sample conditions a comparable signal-to-noise ratio with less than a second of signal averaging. Finally, we have also performed near-infrared spectroscopy of nitrous oxide flowing through a waveguide using a telecommunications diode laser. The RMS baseline noise for these measurements, in absorbance units, was 2 x 10-5.