Sensitivity limits of a tunable diode laser spectrometer, with application to the detection of NO_2 at the 100-ppt level

Abstract

A laser absorption spectrometer is described which uses a tunable diode laser and a 1-m multipass White cell to detect NO(2) in air with a sensitivity of better than 100 ppt. The modulation techniques employed to achieve this sensitivity are described in detail, and the noise mechanisms, which currently limit the detectable absorption coefficients to greater, similar 10(-7) m(-1), are examined.

Real-Time In Situ Measurements of Industrial Hazardous Gas Concentrations and Their Emission Gross

“…When the gas absorption is very small, i.e., kL≤0.05 (Reid, 1981;Cassidy, 1982), equation (1) can be simplified as:…”

“…C and L stand for gas concentration and total optical length. The intensity of second harmonic (2f) signal can be expressed as below (Reid, 1981;Kan, 2006):…”

“…It has been reported that a lot of reasons like wavelength drifts and etalon fringe structure change because of thermal effect can result to slow 2f signal distortion (Werle, 1996). Few technologies had been reported to eliminate those distortions like rapid background subtraction (Cassidy & Reid, 1982) and digital signal processing (Reid, 1980), but there are some limits of those ideas when the condition is changed. In fact it is inconvenient to get the background structure in real time for a in situ gas analytical system, particularly when the interference or distortion has similar frequency with the absorption signal in which the digital method could not work well.…”

Long-path monitoring of NO_2 with a 635 nm diode laser using frequency-modulation spectroscopy

“…5 The majority of diodelaser-based applications for NO 2 detection utilize multiple-pass absorption cells, mostly at reduced pressures in order to minimize line broadening. 6 -11 Such an arrangement has the disadvantage of introducing unwanted optical fringes due to etalon effects, 6 and of depending on a gas-extraction procedure, which is prone to introduce measurement errors. In the present work, long-path absorption in combination with a retroreflector is used instead, making remote sensing over several hundred meters possible, to be compared with the point-monitoring mode obtained in a multiple-pass absorption cell configuration.…”

“…At wavelengths longer than 350 nm, the absorption spectrum of NO 2 exhibits sharp spectral features, 10 that extend over the blue region into the visible wavelength region. We use a diode laser emitting at a wavelength around 635 nm in order to probe the red wing of the X Previously a sensitivity of 0.2 g͞m 3 has been demonstrated using a gas cell at low pressure and monitoring with a cryogenically cooled diode laser operating around 6.2 m. 6 Measurements using near-infrared 7 or red diode lasers 9,10 have been performed achieving sensitivities of a few g͞m 3 . The most common methods to enhance measurement sensitivity use balanced detection 9,10 or frequencymodulation techniques.…”