Detection technique

We make extensive use of tunable diode laser spectroscopy (TDLS). TDLS uses narrow linewidth semiconductor lasers for accurate recovery of the absolute rotational-vibrational absorption lines of gases. The rotational-vibrational absorption lines of gases have embedded in them information about the concentration, pressure and temperature of the gas. Accurate absorption line shape recovery is therefore the most direct way of estimating these quantities. TDLS with wavelength modulation, known as wavelength modulation spectroscopy (WMS) is a high resolution spectroscopic method of making simultaneous, absolute quantitative measurements of several parameters (pressure, concentration and temperature) of gases. TDLS is very widely used in a large number of harsh industrial environments such as process control, in which efficiency and safety cannot be compromised. The growth of TDLS has been greatly fuelled by the development of compact, low-power, widely tunable, narrow-linewidth and frequency-agile tunable near-infrared and mid-infrared semiconductor lasers. The wide availability of mid-infrared quantum cascade lasers has been a telling factor in the extremely rapid progress of the field of chemical sensing. WMS with 1f and 2f detection affords very high detection sensitivity (ppm, ppb, ppt), very high selectivity and the capability to multiplex several lasers to remotely and continuously monitor vast areas and sensitive installations in real-time. Recent research has led to the development of calibration-free measurement 1f and 2f WMS techniques that have tremendously increased the potency of the technique as regards field applications where one does not have the luxury of periodically re-calibrating the measurement system. Our group currently focuses on developing calibration-free 1f and 2f WMS techniques and applying these to various semiconductor lasers. TDLS systems are compact, have low power requirement, are easy to reconfigure for multiple gases, and can be readily interfaced to electronics and communications systems. The portability of these systems makes field-deployable either in the form of fixed installations or mounted on unmanned land-based or airborne vehicles. Measurements can be done in real-time with fast embedded electronics that allows compact, portable systems to be developed for use in industrial applications as well as in cross-disciplinary research that may involve field measurements.