Abstract
Hollow silica waveguides (HSWs) are used to produce long path length, low-volume gas cells, and are demonstrated with quantum cascade laser spectroscopy. Absorption measurements are made using the intrapulse technique, which allows measurements to be made across a single laser pulse. Simultaneous laser light and gas coupling is achieved through the modification of commercially available gas fittings with low dead volume. Three HSW gas cell configurations with different path lengths and internal diameters are analyzed and compared with a 30 m path length astigmatic Herriott cell. Limit of detection measurements are made for the gas cells using methane at a wavelength 7.82 μm. The lowest limit of detection was provided by HSW with a bore diameter of 1000 μm and a path length of 5 m and was measured to be 0.26 ppm, with a noise equivalent absorbance of 4.1×10-4. The long-term stability of the HSW and Herriott cells is compared through analysis of the Allan-Werle variance of data collected over a 24 h period. The response times of the HSW and Herriott cells are measured to be 0.8 s and 36 s, respectively.
Highlights
Tunable diode laser spectroscopy (TDLS) [1,2] is a sensitive and highly selective technique used for the measurement of molecular concentrations of a range of gas species [3]
Hollow silica waveguides (HSWs) offer great potential for low-volume gas cells for use in the mid-IR; they can suffer from instability/vibration owing to their multimode nature if coiled [35] or if their bore diameter is more than ∼30 times the application wavelength [18]
Its noise equivalent absorbance (NEA) was calculated from the pulse data and was found to be 5.9 × 10−4, which is slightly higher than the 4.1 × 10−4 NEA measured for the 5 m path length HSW gas cell made from the 1000 μm bore diameter waveguide
Summary
Tunable diode laser spectroscopy (TDLS) [1,2] is a sensitive and highly selective technique used for the measurement of molecular concentrations of a range of gas species [3]. Thompson et al [23] used an in-house-developed, 2100 μm bore diameter hollow waveguide with an FTIR for spectroscopy of CO and measured an order of magnitude improvement in response time when compared with a 3 m multipass cell. In between HSW and PBGF is the photonic bandgap OmniGuide [30] These consist of alternating cylindrical layers of dielectric materials of different refractive indices around a central hollow core and are based on the omnidirectional mirror [31], which achieved very low loss over a spectral region of 10–15 μm. HSWs offer great potential for low-volume gas cells for use in the mid-IR; they can suffer from instability/vibration owing to their multimode nature if coiled [35] or if their bore diameter is more than ∼30 times the application wavelength [18].
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