Michael Lengden

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.

3D-printed miniature gas cell for photoacoustic spectroscopy of trace gases.

A new methodology for the development of miniature photoacoustic trace gas sensors using 3D printing is presented. A near-infrared distributed feedback (DFB) laser is used together with a polymer-based gas cell, off-the-shelf fiber optic collimators, and a microelectromechanical system (MEMS) microphone to measure acetylene at 1532.83 nm. The resonance behavior of the miniature gas cell is analyzed using a theoretical and experimental approach, with a measured resonance frequency of 15.25 kHz and a Q-factor of 15. A minimum normalized noise equivalent absorption of 4.5×10(-9)  W cm(-1) Hz(-1/2) is shown together with a 3σ detection limit of 750 parts per billion (ppb) for signal averaging times of 35 s. The fiber-coupled delivery and miniature cost-effective cell design allows for use in multipoint and remote detection applications.

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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We report on the development of three systems intended to provide fast, non-intrusive measurement of cross-sectional distributions of pollutant species within gas turbine exhaust flows, during ground-based testing. This research is motivated by the need for measurement systems to support the introduction of technologies for reducing the environmental impact of civil aviation. Tomographic techniques will allow estimation of the distributions of CO2, unburnt hydrocarbons (UHC), and soot, without obstruction of the exhaust, bypass or entrained flows, from measurements made in a plane immediately aft of the engine.

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

This paper presents the development of a Raman fiber amplifier optical source with a maximum output power of 1.1 W centered around 1651 nm, and its application in miniaturized 3D printed photoacoustic spectroscopy (PAS) trace gas sensing of methane. The Raman amplifier has been constructed using 4.5 km of dispersion shifted fiber, a 1651 nm DFB seed laser, and a commercial 4 W EDFA pump. The suppression of stimulated Brillouin scattering (SBS) using a high-frequency modulation of the seed laser is investigated for a range of frequencies, leading to an increase in the optical output power of the amplifier and reduction of its noise content. The amplifier output was used as the source for a miniature PAS sensor by applying a second modulation to the seed laser at the resonant frequency of 15.2 kHz of the miniature 3D printed gas cell. For the targeted methane absorption line at 6057 cm-1, the sensor system performance and influence of the SBS suppression is characterized, leading to a detection limit (1σ) of 17 ppb methane for a signal acquisition time of 130 s, with a normalized noise equivalent absorption coefficient of 4.1 · 10-9 cm-1 W Hz-1/2 for the system.

Progress towards non-intrusive optical measurement of gas turbine exhaust species distributions

We report on the installation and commissioning of two systems for the measurement of cross-sectional distributions of pollutant species in jet exhaust, within the engine ground test facility at INTA, Madrid. These systems use optical tomography techniques to estimate the cross-sectional distributions of CO2 and soot immediately behind the engine. The systems are designed to accommodate the largest civil aviation engines currently in service, without obstruction of the exhaust or bypass flows and with negligible effect upon the entrained flow behavior. We describe the physical construction and installation status of each system. In the case of the CO2 system, we examine the challenges of achieving the structural rigidity necessary for adequate suppression of pointing error within 126 laser-based transmittance measurements, each utilizing a 7 m overall path length. We describe methods developed for efficient implementation of co-planarity and 4-degree-of-freedom alignment of individual paths within this beam array. We also present laboratory performance data for three alternative optical designs that differ in their approach to the management of pointing error and turbulence-induced beam wander and spread. The FLITES soot monitoring capability is based on laser induced incandescence (LII) and uses a short-pulse fiber laser and two CCD cameras, in an autoprojection arrangement. We describe the measurement geometry currently being implemented in the test cell and discuss optical design issues, including once again the effect of the plume itself.

Miniaturized Photoacoustic Trace Gas Sensing Using a Raman Fiber Amplifier

This paper presents concentration measurements of water vapour and methane, taken in-situ on an operational solid oxide fuel cell (SOFC) test rig using tunable diode laser spectroscopy (TDLS). Methane concentration measurements are presented for the TDLS system and are compared with concentration measurements taken using gas chromatography (GC). Furthermore, purge times for the SOFC gas-analysis system have been calculated using TDLS, which are measurements that cannot be obtained directly using GC. Finally, water vapour concentration measurements in the SOFC cathode are shown for different system operating conditions: a dry cathode cycle and during the introduction of water vapour. As GC cannot be used to measure water vapour in the SOFC cathode stream, a direct comparison cannot be made with the TDLS measurements.

Implementation of non-intrusive jet exhaust species distribution measurements within a test facility

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.

Tunable Diode Laser Based Concentration Measurements of Water Vapour and Methane on a Solid Oxide Fuel Cell

Tunable diode laser spectroscopy combined with wavelength modulation spectroscopy (WMS) is an important technique for noninvasive measurements of gas parameters such as pressure, concentration, and temperature in high-noise harsh environments. A variety of laser types are used for these applications, and the modulation characteristics can have significant effects on line shape recovery. Here, we identify important characteristics of distributed feedback (DFB) lasers that need to be taken into account in the context of WMS and illustrate the effects with a 2-μm wavelength multiquantum-well DFB laser used for CO2 detection. The modulation response of the laser is measured, and we demonstrate how the phasor decomposition method (PDM) may be used to obtain accurate line shapes from the first harmonic WMS signals by correcting for phase variation across the lasers low-frequency current sweep. We also demonstrate how the PDM approach can be improved by removing the need to preset the orientation of the lock-in axis to isolate the residual amplitude modulation component, making it more suitable for field applications.