Using a Wavelength-Modulated Quantum Cascade Laser to Measure NO Concentrations in the Parts-per-Billion Range for Vehicle Emissions Certification

Abstract

Measurements of NO concentrations at sub-ppm levels in vehicle exhaust are needed for emissions certification of future ultra-low emission vehicles. We demonstrate a wavelength-modulation, laser-based, NO detection system suitable for this purpose. A quantum cascade distributed feedback laser (QC-DFB) operating continuous wave (cw) at ∼100 K is frequency modulated at f = 10 kHz and locked to the center of a transition at ∼1921 cm−1 in the fundamental band of NO. The demodulated signal at 2 f of the beam passing through the sample cell directly measures the NO concentration. The cell is a multipass Herriott-type with a 100-m path length. Doppler broadening, pressure broadening, and unresolved Λ doubling combine to yield a pressure for optimum sensitivity of 100 torr and a modulation amplitude of ∼600 MHz. A flowing gas system is used to avoid problems with adsorption and desorption of NO from the cell walls. The reduced pressure eliminates interference from other gas species. Detection of NO concentrations in the few parts-per-billion (ppb) range is demonstrated in diluted exhaust-gas bag samples collected in the vehicle certification process.

Supersonic free-jet quantum cascade laser measurements of ν4 for CF335Cl and CF337Cl and FTS measurements from 400 to 1260cm−1

“…Well designed distributed feedback (DFB) quantum cascade lasers (QCL) are considered excellent sources for precision IR spectroscopy, [1] for sub-Doppler techniques using saturated absorption, [2,3] as ultra-narrow stabilized sources for cavity-enhanced quantum optics applications such as NICE-OHMS [4], and for infrared based free space communications (FSC). This latter application, which has been of great interest to those interested in secure communication, typically requires the use of on-off keying (OOK) amplitude modulation techniques.…”

<I>In Situ</I> Gas Phase Measurements During Metal Alkylamide Atomic Layer Deposition

“…12 A water rovibrational transition at 7181.156 cm −1 was probed using a wavelength modulation detection scheme. 25 26 The laser current was modulated at 40 kHz and the laser transmission through the chamber was measured using a 10 MHz-bandwidth InGaAs detector, with this signal subsequently being filtered using a lock-in amplifier. The lock-in amplifier output at twice the modulation frequency, i.e., 2f (the 2f signal being proportional to the average water vapor concentration in the ALD beam path), was digitized at 200 Hz.…”

Advanced NOx Sensors for Mechatronic Applications

“…Direct absorption spectroscopy based on quantum cascade laser has been extensively studied in the last years for in situ measurements of gas components in vehicle exhaust (Weber et al, 2002;McCulloch et al, 2005;Kasyutich et al, 2009;Hara at al., 2009). QCL-based absorption analysers offer an improvement in performance, higher sensitivity and selectivity, even at low concentrations, with respect to the other spectroscopic schemes.…”