The measurement of gases associated with industrial processing/emissions monitoring has become increasingly important as the need to improve efficiencies in process control has increased, and legislation governing emissions has come into force. Gases including NOx, SOx, CO2, CO, NH3, and H2O commonly are used to assess processes such as combustion and quenching, while many fall under emissions legislation resulting from the Kyoto agreement.

Figure 1. The Micro Sensor is a compact QCL-based gas sensor developed for industrial applications.
The measurement of these gases traditionally has relied on a suite of different optical technologies including Non-Dispersive Infra Red (NDIR) and Fourier Transform Infra Red (FTIR). However, poor sensitivity and selectivity, combined with concerns over cross-interference and measurement accuracy, has prevented the widespread adoption of these technologies in industry. Cascade Technologies determined that many of these concerns are fundamentally associated with the inherent low resolution of the infrared (IR) source, prompting exploration of recent advances in Quantum Cascade Lasers (QCLs) as sources for industrial gas monitoring systems.

The QCL

Figure 2. Recorded spectra of industrially relevant gases. These spectra show the high S/N and selectivity that can be achieved with the QCL system.
Since they were first demonstrated at Bell Laboratories in 1994, quantum cascade lasers have been gaining acceptance as the mid-IR source of choice because of their spectral resolution, noise performance, and power output compared to conventional IR sources.

Conventional semiconductor lasers such as the lead-salt devices commonly used in the mid-IR rely on electron hole recombination across the doped semiconductor bandgap to emit photons. The QCL, which is about the size of a pinhead, operates on a fundamentally different principle whereby electrons cascade down a series of quantum wells, which result from the growth of very thin layers of semiconductor material. Whereas a single electron- hole recombination produces a single photon, the QCL electron cascades down between 20 and 100 quantum wells, producing a photon at each step. This electronic waterfall means QC lasers can emit several watts of peak power in pulsed operation and tens of milliwatts CW, compared to a fraction of a milliwatt for most lead-salt lasers.

The wavelength of the QCL is determined not by the choice of semiconductor material as with conventional lasers, but by adjusting the physical thickness of the semiconductor layers themselves. This removes the material barriers commonly associated with conventional semiconductor laser technology and opens up the possibility of near-infrared through to THz spectral coverage. For the first time, an infrared spectroscopic source, which has no need for cryogenic cooling, provides high-output power, large spectral coverage, excellent spectral quality, good tunability, and high spectral resolution.

Sensor Performance

Cascade Technologies has developed the Micro Sensor, a compact and ruggedized gas sensor developed specifically for industrial applications (see Figure 1). Typical outputs from the gas sensor for industrial gases such as SO2, NO2, N2O, H2S, CH4, CH2O, and H2O are shown in Figure 2. All measurements were concluded within 1 microsecond, including multiple absorption features for each gas with greater dynamic range than results available from field-deployable industrial gas measuring systems.

Figure 3. Tests performed recently on the QCL gas sensing technology included assessing the instrument’s linearity of response, susceptibility to cross-interference, reading drift, instrument noise, and inter-comparison between recorded and theoretically predicted data.
QCL gas sensing capabilities were demonstrated recently during a Department of Trade and Industry (DTI)-funded consultancy at the UK standards lab, the National Physical Laboratory (NPL), MCERTS facility. The tests included assessing the instruments’ linearity of response, susceptibility to cross interference, reading drift, instrument noise, and inter-comparison between recorded and theoretically predicted data.

These initial sensor tests, measuring N2O and CH4, proved very encouraging. For example, instrument response with known gas concentrations showed an excellent linear response (correlation coefficient r2 effectively equal to one) with a maximum deviation of 0.2%, 0.7%, and 0.1% observed, respectively, for N2O/air, CH4/air, and N2O/N2 compared to the MCERTS requirement of less than 2%. In addition, for the ranges tested, the maximum effect observed for cross sensitivity due to interfering substances was only 0.4% for N2O and 0.3% for CH4, comparing favorably to a 4% MCERTS requirement.

Sensor drift was assessed by passing a fixed concentration of determined gas (N2O) through the sensor for a period of four hours. The reading drift during that period was less than 2 ppb. Finally, the recorded spectral features for both N2O and CH4 were compared with theoretical predictions. RMS residuals were equivalent to less than 600 ppt in a 1-Hz bandwidth, providing strong evidence of the ultimate noise performance of the QCL gas sensor technology.

Cascade Technologies has successfully demonstrated that the inherent properties of the QCL — high-output powers, large-spectral coverage, excellent spectral quality, good tenability, and high resolution — can be utilized to overcome many of the concerns associated with incumbent technologies such as NDIR and FTIR in the industrial gas monitoring market.

This article was written by Iain Howieson, Chief Technology Officer at Cascade Technologies, Stirling, Scotland. For more information, contact Mr. Howieson at This email address is being protected from spambots. You need JavaScript enabled to view it..


Photonics Tech Briefs Magazine

This article first appeared in the September, 2006 issue of Photonics Tech Briefs Magazine.

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