By: Ruth Lindley, Product Manager, Process Products, Cascade Technologies; and Paul Black, Development Manager, Cascade Technologies
Hi, this is Ruth Lindley and Paul Black and today we’d like to discuss a number of challenges in process, detection, and environmental applications that can be addressed using quantum cascade laser technology. Quantum cascade lasers (QCLs) offer significant benefits for gas analysis in a range of applications including industrial process control and continuous emissions monitoring. These benefits were covered in a paper that was presented at ISA last year titled, “Multiple Component Real Time Impurity and Process Composition Analysis Using MID IR Process Analytical Tunable Laser Spectroscopy (PATLS),” and are summarised below in Table 1.
Table 1: Benefits of QCL gas analysers
SCIENCE QCLs operate in the mid infra-red: the so-called fingerprint region where most molecules exhibit strong, sharp absorptions. This information-rich area is ideal for making spectroscopic measurements.
QCLs are semiconductor devices in which electrons transition through a series of quantum wells, emitting a photon at each step. The wavelength of the photons is governed by a grating which is etched into a laser waveguide. As the laser is switched on it heats up which causes the grating to expand and this in turns causes the photons to scan through a range of wavelengths. This wavelength sweep is known as a frequency chirp: it is a core technology which is patented by Cascade Technologies Ltd.
The frequency chirp lasts a few hundred nanoseconds and sweeps through a 1 to 4 cm-1 (wavenumber) spectral region. This wavelength range and the high resolution achieved by a QCL make it particularly suitable for the spectroscopic analysis of narrow rotational structures, such as those exhibited by small molecules in the gas phase (Figure 1). The lasers are pulsed very rapidly, with repetition frequencies up to 100 kHz, which allows for rapid and real-time measurements. In addition, the rapid accumulation of data allows multiple spectra to be co-averaged, improving the signal to noise ratio of the resultant spectrum and lowering the detection limit.
Figure 1: An information rich region of 3 wavenumbers in the mid-IR, showing the strong, sharp absorptions of many components.
TECHNOLOGY Cascade Technologies analysers typically feature between 4 and 6 lasers, each designed to measure one or two components. The lasers are chirped rapidly in sequence giving the capability of monitoring multiple species simultaneously and in real time.
Each laser is housed in a factory-aligned and collimated module which makes field servicing a plug and play activity. The individual beams are co-aligned using an optical manifold and directed into a sample cell.
To achieve a long path length in a low volume sample cell, mirrors are used to fold the light beam giving an actual path length of up to 30 m within a 320 mL volume. This is achieved with 150 passes of light across a 20 cm base path length. This long path length enables measurements to be made at concentrations as low as single ppb. In addition, using a beam splitter, light may be directed along a second path which crosses the short axis of the cell, hence allowing higher concentration measurements to be made (Figure 2). By selecting the amount of folding in each light path, the analyser can be designed to measure a suite of components with a very wide dynamic range.
On exiting the cell, the laser radiation is directed onto a Mercury Cadmium Telluride detector and the detected signal is fed into a custom developed dual-channel digitiser for conversion into real-time data. A comprehensive fitting routine is used to rapidly analyse the resultant spectra to accurately calculate the concentrations of the gas species that are present.
Figure 2 – Simulation of the laser light path through a dual path length measurement cell. This particular example shows a combination of 30 m and 0.8 m paths in a perpendicular arrangement.