Gas cells for tunable diode laser absorption spectroscopy employing optical diffusers. Part 2: Integrating spheres

Abstract

New designs for gas cells are presented that incorporate transmissive or reflective optical diffusers. These components offer simple alignment and can disrupt the formation of optical etalons. We analyse the performance-limiting effects in these cells of random laser speckle (both objective and subjective speckle), interferometric speckle and self-mixing interference, and show how designs can be optimised. A simple, single pass transmissive gas cell has been studied using wavelength modulation spectroscopy to measure methane at 1651nm. We have demonstrated a short-term noise equivalent absorbance (NEA, 1σ) of 2×10 -5 , but longer term drift of up to 3×10 -4 over 22 hours. PACS codes 07

Using integrating spheres with wavelength modulation spectroscopy: effect of pathlength distribution on 2nd harmonic signals

“…In practice at high levels of reflectivity, the difference between Eqs. (12) and (13) is negligible (less than 1% difference for ρ > 96%).…”

“…This geometry disrupts the formation of etalons within the cell, which in conventional cells would be performance -limiting [11] . However it also creates a random interference phenomenonlaser specklethat must be minimised in order to realise the SNR benefits of the longer pathlength [12] . For the measurement geometry described in this paper, we have previously estimated the noise equivalent absorbance (NEA) in direct spectral scans to be 5×10 -5 (short term) and 4×10 -4 (drift over a 21-hour period) [12] .…”

Gas in scattering media absorption spectroscopy – from basic studies to biomedical applications

“…This opens up for the possibility of sensitive gas analysis without using, e.g., multi‐pass cells, avoiding cost and alignment issues. A related approach utilizing a gas‐filled integrating sphere with strongly reflecting/scattering surface has been reported .…”

An Investigation into the Effects of Pressure on Gas Detection Using an Integrating Sphere as Multipass Gas Absorption Cell: Analysis and Discussion

“…A significant number of works launched by Hawe of the University of Limerick has resulted in a broadband spectroscopic sensor for CO 2 in 2005 (at 1590 nm) 10 and in 2007 (at 1570 nm and 2000 nm) 11 with a detection limit of 200 ppm as well as NO 2 , SO 2 and O 3 (in the UV-visible) in 2006, 12 2000 13 and 2008 14 with limits of detection of 5 ppm, 10 ppm and 500 ppm, respectively. More recent work performed by Hodgkinson et al 15,16 has enabled the realisation of spectroscopic sensor for CH 4 (at 1.65 µm) with limits of detection of 75 ppm and 0.4 ppm for a sphere diameter of 50 mm using direct scan and 100 mm using wavelength modulation spectroscopy, respectively. All of these studies confirm an important advantage of using an integrating sphere compared with standard multipass optical cells.…”