James Roderic Peter Bain

Recovery of Absolute Gas Absorption Line Shapes Using Tunable Diode Laser Spectroscopy With Wavelength Modulation—Part I: Theoretical Analysis

Tunable diode laser spectroscopy is extremely important for gas detection in a wide variety of industrial, safety and environmental monitoring applications. Of particular interest is the development of calibration-free, stand-alone systems and instrumentation which can operate in high-temperature or high-pressure environments (such as in fuel cells, or gas turbine engines) where continuous and simultaneous monitoring of pressure, temperature and concentration of gases may be required. Here, in Part 1 of this two-part paper, we present the full theoretical basis and range of techniques for calibration-free line-shape recovery to allow simultaneous measurements of concentration and pressure/temperature for a wide range of potential applications. Firstly, on the basis of Fourier analysis, we present the general signal components that arise with both intensity and frequency modulation of diode lasers and identify the issues and difficulties associated with accurate line-shape recovery in conventional wavelength modulation spectroscopy (WMS). We then show how line-shape recovery may be effectively performed using first harmonic signals and, by use of a general correction function from Fourier coefficients, we extend the techniques previously reported to include arbitrary large modulation indices, different line-shape profiles and high gas concentration with non-linear absorption. Previous approximate techniques based on Taylor series expansions are included as a special case of the Fourier analysis for low modulation indices. We also show that the signal amplitudes obtained in this way can be comparable to, or even exceed, that of conventional WMS by appropriate choice of the modulation index and frequency.

Recovery of Absolute Gas Absorption Line Shapes Using Tunable Diode Laser Spectroscopy With Wavelength Modulation—Part 2: Experimental Investigation

Recovery of absolute gas absorption line shapes from first harmonic residual AM (RAM) signals in tunable diode laser spectroscopy with wavelength modulation (TDLS-WM) offers significant advantages in terms of measurement accuracy (for gas concentration and pressure), freedom from the need for calibration and resilience to errors, or drift in system parameters/scaling factors. However, the signal strength and SNR are compromised somewhat relative to conventional WM spectroscopy (WMS) by the signal dependence on the lasers intensity modulation amplitude rather than on the direct intensity, and by the need to operate at low modulation index, 0.75 in the previously reported study. In part 1 of this two-part publication, we report a more universal approach to the analysis of recovered RAM signals and absolute absorption line shapes. This new approach extends the use of RAM techniques to arbitrary m values up to 2.2. In addition, it provides the basis for a comparison of signal strength between the RAM signals recovered by the phasor decomposition approach and conventional first and second harmonic TDLS-WM signals. The experimental study reported here validates the new model and demonstrates the use of the RAM techniques for accurate recovery of absolute gas absorption line shapes to 2.2 and above. Furthermore, it demonstrates that the RAM signal strengths can be increased significantly by increasing the modulation frequency and defines regimes of operation such that the directly recovered RAM signals are comparable to or even greater than the widely used conventional second harmonic TDLS-WM signal. Finally, a critique of the RAM techniques relative to the conventional approaches is given.

Recovery of Absolute Absorption Line Shapes in Tunable Diode Laser Spectroscopy Using External Amplitude Modulation With Balanced Detection

Accurate recovery of an absorption lineshape is important in many industrial applications for simultaneous measurement of gas concentration and pressure or temperature. Here, we demonstrate a method, based on a modification to the Hobbs balanced receiver circuit, for background signal nulling when external amplitude modulation of the laser output is used. Compared with direct or non-nulled detection techniques, we demonstrate that the method significantly improves the signal-to-noise ratio to a level comparable with that of the conventional second harmonic wavelength modulation spectroscopy. Most importantly, normalisation and recovery of the lineshape is straightforward and immune to the difficulties that afflict lineshape recovery with the conventional wavelength modulation spectroscopy.