Keith Ruxton

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.

Elimination of residual amplitude modulation in tunable diode laser wavelength modulation spectroscopy using an optical fiber delay line.

A new fiber-optic technique to eliminate residual amplitude modulation in tunable diode laser wavelength modulation spectroscopy is presented. The modulated laser output is split to pass in parallel through the gas measurement cell and an optical fiber delay line, with the modulation frequency / delay chosen to introduce a relative phase shift of pi between them. The two signals are balanced using a variable attenuator and recombined through a fiber coupler. In the absence of gas, the direct laser intensity modulation cancels, thereby eliminating the high background. The presence of gas induces a concentration-dependent imbalance at the couplers output from which the absolute absorption profile is directly recovered with high accuracy using 1f detection.

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.

Detection of CH

This paper demonstrates detection of methane using tunable diode laser spectroscopy (TDLS) through difference frequency generation (DFG) in order to address fundamental rotational-vibrational absorption lines, located around 3404 nm. Direct detection confirms that wavelength referencing of recovered lineshapes, developed for Near infra-red (Near-IR) systems, has been successfully transferred to the presented Middle infra-red (Mid-IR) system. Traditional 1f and 2 f TDLS with WMS detection regimes are also functionally confirmed analogous to their Near-IR equivalents.

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Recovery of the full 2f wavelength modulation spectroscopy (WMS) signal in isolation from the 2f residual amplitude modulation (RAM) due to nonlinear intensity modulation (IM) and distortion due to linear IM is demonstrated. The 2f RAM is eliminated using a fiber delay line, while the linear IM-induced distortion is eliminated by a phasor decomposition approach. This generic and robust two-pronged strategy removes the need to separately measure the 2f RAM in high-modulation-index calibration-free 2f WMS. It is also important for widely tunable 2f WMS using nontelecom diode lasers with highly nonlinear characteristics leading to high-2f RAM levels.

in the Mid-IR Using Difference Frequency Generation With Tunable Diode Laser Spectroscopy

Recently a technique to optically eliminate the background residual amplitude modulation in 1f wavelength modulation spectroscopy was demonstrated, where perfect elimination throughout the scan range was not achieved due to the wavelength-dependence of couplers and that of the laser intensity modulation. This paper theoretically analyzes the technique and experimentally demonstrates that the elimination can be perfect for one of three possible experimental configurations, making this important for potential applications with some recently-developed laser sources. For the other configurations a non-zero background slope is predicted, experimentally verified, and the anomalous nature of signals is thereby explained. A common signal normalization method is devised that is independent of the signal slope, a fact that is important for industrial deployment of such systems.

Suppression of intensity modulation contributions to signals in second harmonic wavelength modulation spectroscopy.

A limiting factor of tuneable diode laser spectroscopy (TDLS) with wavelength modulation spectroscopy (WMS) is the presence of background residual amplitude modulation (RAM) on the recovered 1st harmonic signal. The presence of this background term is due to direct modulation of the source laser power. This work presents a novel method to optically remove the unwanted background, with the major benefit being that measurement sensitivity can be increased. The recently developed phasor decomposition method1 (PDM), is a near IR (NIR) TDLS analysis technique that is used with the addition of the new RAM nulling method to recover gas absorption line-shapes. The PDM is a calibration free approach, which recovers the gas absorption line-shape and the isolated 1st derivative of the line-shape from the 1st harmonic signal. The work presented illustrates and validates the new RAM nulling procedure with measurements examining the 1650.96nm absorption line of methane (CH4) with comparisons to theory.

Influence of the wavelength-dependence of fiber couplers on the background signal in wavelength modulation spectroscopy with RAM-nulling

Tunable diode laser spectroscopy with wavelength modulation (TDLS-WMS) has long been plagued by the problem of residual amplitude modulation (RAM) [1], that limits the sensitivity in 1st harmonic detection. To circumvent the problem 2f-detection [2], has been the common electronic signal recovery approach. However, in such systems, absorption lines cannot be recovered directly, and they require calibration. To date, no method has been devised to eliminate the RAM at the optical level. We present, for the first time, a fiber-optic technique that eliminates the concentration-independent RAM component and allows 1f calibration-free recovery of gas absorption lines. The technique is stable and can be readily modified for a real-time and automated system.