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.

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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 coupler's output from which the absolute absorption profile is directly recovered with high accuracy using 1f detection.

Abstract-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 CO 2 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 CO 2 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.