Fiber-based UV laser-diode fluorescence sensor for commercial gasolines

We report on an optical fiber probe, coupled to a 404-nm laser diode, as a fluorescence sensor for monitoring of commercial gasolines. The principle of operation of the sensor is based on quantifying the intensity of the Stokes-shifted fluorescence from some of the heavier polycyclic aromatic hydrocarbons C/sub x/H/sub y/,(x,y) /spl ges/ (14,10) present in gasolines as minor constituents. The normalized efficiency of the optical fiber probe, as a function of its geometry, is calculated in the cases of single-fiber and parallel dual-fiber designs. The spatial and temporal resolutions achievable by the sensor are discussed as a function of design parameters The performance of the sensor is investigated experimentally for commercial gasolines in the liquid and gas phase. The optimal excitation wavelength for such sensors is investigated in the range of 350-400 nm. The linear sensitivity to vapor concentrations of retail gasoline fuel is demonstrated in the range of 4%-125% of combustion stoichiometry at 10 bar and 180/spl deg/C. Statistical processing of the data from the sensor allows distinction to be made between different forecourt gasoline suppliers, as well as fuel varieties (unleaded, low sulfur, etc.).     

Toward in-cylinder absorption tomography in a production engine

Design requirements for an 8000 frame/s dual-wavelength ratiometric chemical species tomography system, intended for hydrocarbon vapor imaging in one cylinder of a standard automobile engine, are examined. The design process is guided by spectroscopic measurements on iso-octane and by comprehensive results from laboratory phantoms and research engines, including results on temporal resolution performance. Novel image reconstruction techniques, necessary for this application, are presented. Recent progress toward implementation, including details of the optical access arrangement employed and signal-to-noise issues, is described. We present first cross-cylinder IR absorption measurements from a reduced channel-count (nontomographic) system and discuss the prospects for imaging.     

Generic-type hierarchical multi digital signal processor system for hard-field tomography

This article introduces the design and implementation of a hierarchical multi digital signal processor system aimed to perform parallel multichannel measurements and data processing of the type widely used in hard-field tomography. Details are presented of a complete tomography system with modular and expandable architecture, capable of accommodating a variety of data processing modalities, configured by software. The configuration of the acquisition and processing circuits and the management of the data flow allow a data frame rate of up to 250 kHz. Results of a case study, guided path tomography for temperature mapping, are shown as a direct demonstration of the system's capabilities. Digital lock-in detection is employed for data processing to extract the information from ac measurements of the temperature-induced resistance changes in an array of 32 noninteracting transducers, which is further exported for visualization.     

Efficiency and data correction for OFAPT sensors with fiber receivers

This paper addresses the problem of calculations of the efficiency, as well as the necessary raw data correction, essential for the design of an optically induced fluorescence auto-projection tomography (OFAPT) sensor head. Instead of discretizing an analytically derived equation, relevant to a certain symmetry, discretization is implemented on the algorithmic level, thus developing a universal, fully numerical approach to sensitivity and efficiency calculations. The algorithm is built around the calculation of the efficiency of the fluorescence collection from a small discrete voxel. Further, the contributions from all voxels, excited by the OFAPT laser beam at a given distance from the receiver's tip, are added together, to yield the efficiency of the sensor head. OFAPT measurements are simulated on a 64/spl times/64 phantom and the proposed algorithm for correction of the raw data is implemented and assessed. The effect of the magnitude of the absorption coefficient on the data, collected under certain sensor head geometry, is studied in detail. On this basis, optimal excitation beam and receiver configurations are discussed.     

Design of high-performance photodiode receivers for optical tomography

The design of instrumentation hardware for tomographic systems must take careful account of measurement noise. This is especially true in near-infrared absorption tomography, where the signal of interest is typically only a few percent of the total signal at the detector, and the available optical power may have to be shared among many measurement channels. In this paper, the monitoring of photodiodes in near-IR absorption tomography is examined in detail, but much of the material is applicable at wavelengths ranging from the UV to beyond 2.5 /spl mu/m. The authors' application involves the frequency region 50 kHz to 2 MHz, which lies above that utilized in the majority of radiometric sensing systems, yet substantially below telecoms bit rates. The problem is further distinguished by the use of phase-sensitive detection schemes, which make local noise density more relevant than wideband noise performance and relax the requirement for dc precision. Alternative transimpedance circuit configurations, including both single-ended and differential topologies, are analyzed with a view to optimization of the signal-to-noise ratio. Typical values of photodiode capacitance and shunt resistance are shown to result in significant noise gain, greatly increasing the importance of amplifier voltage noise relative to other intrinsic noise sources. It is shown that for applications of this type, viable alternatives to the traditionally dominant FET amplifier do exist. The relative susceptibility to coupled interference is also considered. The results of practical tests, involving class-leading operational amplifiers, are presented to support the analyses. These results also underline the need for careful circuit layout and shielding if the capabilities of these devices are to be fully exploited.     

Digitally balanced detection for optical tomography

Analog balanced Photodetection has found extensive usage for sensing of a weak absorption signal buried in laser intensity noise. This paper proposes schemes for compact, affordable, and flexible digital implementation of the already established analog balanced detection, as part of a multichannel digital tomography system. Variants of digitally balanced detection (DBD) schemes, suitable for weak signals on a largely varying background or weakly varying envelopes of high frequency carrier waves, are introduced analytically and elaborated in terms of algorithmic and hardware flow. The DBD algorithms are implemented on a low-cost general purpose reconfigurable hardware (field-programmable gate array), utilizing less than half of its resources. The performance of the DBD schemes compare favorably with their analog counterpart: A common mode rejection ratio of 50 dB was observed over a bandwidth of 300 kHz, limited mainly by the host digital hardware. The close relationship between the DBD outputs and those of known analog balancing circuits is discussed in principle and shown experimentally in the example case of propane gas detection.     

Guided-path tomography sensors for nonplanar mapping

This paper introduces guided-path tomography (GPT) as a method for imaging on nonplanar surfaces by taking measurements at their periphery. The theory and practical implementation (hardware and software) of GPT is illustrated in the case of temperature mapping. The temperature distribution is obtained from dc measurements of the temperature-induced resistance changes (accuracy /spl plusmn/0.02%) in a grid of a small number of noninteracting transducers forming the GPT sensor. Images of the temperature distribution around a heated tip and across a thermally nonhomogenous flow are reconstructed using the additive algebraic reconstruction technique (AART). Results from applying several strategies for the design of the sensor are shown and discussed. Problems concerning the general applicability of GPT in its variants, as well as the improvement of its current implementation are discussed in detail and some industrial applications of GPT temperature mapping are suggested. It is shown that using a GPT temperature imaging sensor it is possible to generate Radon-transformed "phantom" data from electrical measurements.     

Applicability of blue/UV laser diodes for the measurement of vaporized fuel fluorescence around stoichiometric concentrations

A fluorescence-based fiber-optic probe is implemented for use with a blue/UV laser diode to form a rugged, nonintrusive fuel sensor. Measurements are obtained from a test vessel controlled to replicate the temperature and pressure environment in a typical combustion engine. A fuel containing a specific polycyclic aromatic hydrocarbon (perylene, C20H12) is used to obtain wavelength resolved information and concentration variation characteristics. Emission intensity is examined in atmospheres with and without oxygen at pressures from 1 to 10 bar. The signals from a retail gasoline fuel present a greater challenge as they result from a complex mixture of hydrocarbon components. Detection of gasoline vapor concentrations in the range of 4% to 125% of stoichiometry is demonstrated.     

Optical Absorption Measurements in Particle-Containing Ambients Using Gated Ratiometric Detection

We demonstrate a ldquotrading time for signalrdquo approach for the measurement of species-specific optical transmission, in ambients rich in particulates, using the example case of a turbulent fluidized bed reactor. The method is introduced on the basis of Monte Carlo simulations, as well as analysis of experimental data, of beam propagation for a range of path lengths, and particle concentrations. In a particular embodiment for relative propane-concentration measurements, data is acquired only during gated time intervals of partial beam occlusion detected in real time. The digitized data streams from the signal and background beams is processed by algorithms performing gating, digital balanced detection, and dual wavelength ratiometry. This combination, implemented on inexpensive programmable hardware, results in at least an order of magnitude improvement in sensitivity.