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 $4.5\times {10}^{-9}\mathrm{W}{\mathrm{cm}}^{-1}{\mathrm{Hz}}^{-1/2}$ is shown together with a $3\sigma$ 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.

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 π 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.

In this paper, the 2f/1f tunable diode laser wavelength modulation spectroscopy is used for the simultaneous measurement of concentration and temperature of CO ₂ in the exhaust plume of an aero engine. The suitability of the R48 spectral feature of CO ₂ at 1997.2 nm is discussed for this project and for further application of CO ₂ tomographic imaging on large-scale aero-engines. To ensure accurate recovery of gas parameters at the high exhaust temperatures, a full spectral characterization of the absorption feature is presented using a direct spectroscopy. The 2f/1f method is validated in the laboratory for controlled gas mixtures and temperatures to recover concentration and temperature. Good agreement with the actual temperature and concentration values is demonstrated. Finally, single path measurements are presented for an aero engine exhaust showing good correlation with the measured engine conditions.

A calibration-free 1f wavelength modulation spectroscopy (1f-WMS) technique for gas species parameter measurement is demonstrated. In this technique, the total magnitude of the first harmonic (1f) WMS signal is normalised by a component of the 1f residual amplitude modulation (1f-RAM) signal. This method preserves the advantages of traditional nf/1f-WMS (n×2) technique, such as immunity to the non-absorbing systematic losses and accurate recovery of gas parameters without the requirement for non-absorbing regions for normalisation at high pressure or high modulation index values (m-values). The proposed technique only requires the first harmonic signal, which has the largest magnitude of all the harmonics signals, and therefore fundamentally has a higher sensitivity to the nf/1f technique. Furthermore, since only the 1f-WMS signal is used the technique is less complex in terms of signal processing and data acquisition. The paper also shows a comparison of the proposed technique and 2f/1f for measuring CO $_{2}$ in the exhaust of a combuster. This data highlights how nonlinearities in the optical detection system as a function of frequency have a considerable effect on the recovered 2f/1f spectra causing variation in the recovered gas concentrations. This effect is not seen in the methodology proposed in this paper.

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.