The FLITES project (Fiber-Laser Imaging of gas Turbine Exhaust Species) aims to provide a videorate imaging (100 fps) diagnostic tool for application to the exhaust plumes of the largest civil aero-jet engines. This remit, enabled by chemical species tomography (CST) currently targeting Carbon Dioxide (CO2), requires system design that facilitates expansion of multiple parameters. Scalability is needed in order to increase imaging speeds and spatial resolutions and extend the system towards other pertinent gases such as the oxides of Nitrogen and Sulphur and unburnt hydrocarbons. This paper presents a fully-scalable, non-invasive instrument for installation in a commercial engine testing facility, technical challenges having been tackled iteratively through bespoke optical and mechanical design, and it specifically presents the high-speed data acquisition system (DAQ) required. Measurement of gas species concentration is implemented by tunable diode laser absorption with wavelength modulation spectroscopy (TDLAS-WMS) using a custom, high-speed 10-40 MS/s/channel 14-bit DAQ. For CO2 tomography, the system uses 6 angular projections of 21 beams each. However, the presented DAQ has capacity for 192 fully-parallel 10-Hz to 3-MHz differential inputs, achieving a best-case SNR of 56.5 dB prior to filtering. Twelve Ethernet-connected digitization nodes based on field-programmable gate array (FPGA) technology with software control, are distributed around a 7-m diameter mounting "ring". Hence, the high data rates of 8.96-Gb/s per printed circuit board (PCB) and 107.52 Gb/s for the whole system, can be reduced using local digital lock-in amplifiers (DLIAs). We believe this DAQ system is unique in both the TDLAS and CST literature.

We consider the inverse problem of concentration imaging in optical absorption tomography with limited data sets. The measurement setup involves simultaneous acquisition of near-infrared wavelength-modulated spectroscopic measurements from a small number of pencil beams equally distributed among six projection angles surrounding the plume. We develop an approach for image reconstruction that involves constraining the value of the image to the conventional concentration bounds and a projection into low-dimensional subspaces to reduce the degrees of freedom in the inverse problem. Effectively, by reparameterizing the forward model, we impose, simultaneously, spatial smoothness and a choice among three types of inequality constraints, namely, positivity, boundedness, and logarithmic boundedness in a simple way that yields an unconstrained optimization problem in a new set of surrogate parameters. Testing this numerical scheme with simulated and experimental phantom data indicates that the combination of affine inequality constraints and subspace projection leads to images that are qualitatively and quantitatively superior to unconstrained regularized reconstructions. This improvement is more profound in targeting concentration profiles of small spatial variation. We present images and convergence graphs from solving these inverse problems using Gauss-Newton's algorithm to demonstrate the performance and convergence of our method.

Abstract-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.We describe a CO2 imaging system that performs wavelength modulation spectroscopy (WMS) simultaneously on 126 beam paths. Its novel architecture uses a Tm-doped fiber amplifier to generate sufficient optical power for the entire beam array (> 3 W) from a single 1997.2 nm diode-laser seed, reducing cost and enabling fully parallel detection and signal recovery. Various optical propagation issues are considered, including those arising from the varying degrees of interaction with the exhaust flow that exist within the beam array, as well as pointing errors arising from the limited rigidity of the measurement system's structure.We also report first steps towards a similar UHC measurement system, operating in the mid-infrared (MIR) region and targeting partially decomposed or oxidized fuel constituents, including formaldehyde and propene. Progress towards the chalcogenide glasses and fibers, needed for light delivery and/or amplification at these wavelengths is described. Finally, we report on the development status of a tomographic soot imaging system, based on laser induced incandescence (LII). We have demonstrated both long (192 ns) and short (17 ns) pulse variants of LII using fiber laser sources. Single path tests on a laboratory soot generator and, in the long pulse case, on a jet engine have confirmed that the energy and beam quality available from the fiber lasers is sufficient to enable an autoprojection approach, using just two intensified CCD cameras having 'near-orthogonal' views, with respect to the excitation laser.

This paper presents the design, implementation and test of a compact, low-cost and fully digital signal recovery system for tunable diode laser absorption spectroscopy (TDLAS) in narrow line-width gas sensing applications. An FPGA-based digital lock-in amplifier (DLIA), in conjunction with TDLAS using the wavelength modulation spectroscopy (WMS) technique, is utilized to demodulate and extract first (1f) and second (2f) harmonic signals for a narrow CO 2 feature in the spectrum region of 1997.2nm. The spectrum in this wavelength region shows suitably weak water absorption, enabling CO 2 detection with high resolution. Gas-cell experiments were carried out using the DLIA and a conventional rack-mounted commercial lock-in amplifier. The comparison between the two systems shows good agreement, validating the feasibility of this approach and demonstrating the prospect for extension to a massively multichannel system to implement Chemical Species Tomography.