  Chemical Species Tomography (CST) has been widely applied for imaging of
critical gas-phase parameters in industrial processes. To acquire high-fidelity
images, CST is typically implemented by line-of-sight Wavelength Modulation
Spectroscopy (WMS) measurements from multiple laser beams. The modulated
transmission signal on each laser beam needs to be a) digitised by a high-speed
analogue-to-digital converter (ADC); b) demodulated by a digital lock-in (DLI)
module; and c) transferred to high-level processor for image reconstruction.
Although a fully parallel data acquisition (DAQ) and signal processing system
can achieve these functionalities with maximised temporal response, it leads to
a highly complex, expensive and power-consuming instrumentation system with
high potential for inconsistency between the sampled beams due to the
electronics alone. In addition, the huge amount of spectral data sampled in
parallel significantly burdens the communication process in industrial
applications where in situ signal digitisation is distanced from the high-level
data processing. To address these issues, a quasi-parallel sensing technique
and electronic circuits were developed for industrial CST, in which the
digitisation and demodulation of the multi-beam transmission signals are
multiplexed over the high-frequency modulation within a wavelength scan. Our
development not only maintains the temporal response of the fully parallel
sensing scheme, but also facilitates the cost-effective implementation of
industrial CST with very low complexity and reduced load on data transfer. The
proposed technique is analytically proven, numerically examined by
noise-contaminated CST simulations, and experimentally validated using a
lab-scale CST system with 32 laser beams.
