TDLAS using FPGA-based lock-in detection for multi-channel Chemical Species Tomography

Abstract

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 CO2 feature in the spectrum region of 1997.2nm. The spectrum in this wavelength region shows suitably weak water absorption, enabling CO2 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.

A Custom, High-Channel Count Data Acquisition System for Chemical Species Tomography of Aero-Jet Engine Exhaust Plumes

“…By applying a current ramp (10-100 Hz sawtooth) to a narrowwavelength laser, an optical beam is repeatedly wavelength-scanned across a previously measured absorption feature [2,3,29,30]. For WMS, a secondary 10-500 kHz sinusoidal dither is applied [31] allowing phase sensitive detection [32] and digital lock-in amplification (DLIA) [19,20,21,22]. The gas parameters [2,3,11,29,30,33] and ultimately the path concentration integral (PCI) of the target gas along the beam [3,7,8,25], can be inferred by using a spectroscopic curve-fitting model based upon the Beer-Lambert law and the known properties of the gas.…”

“…Likewise, equipment must not interfere with the jet-engine nor modify the aerodynamic properties of the cell. Distributing the DAQ allows reduction in system-level costs and at-site digitization affords advantages for digital signal processing (DSP) [19,20] and data communications [11]. For the intended 126-beam imaging system, the 12 nodes yield a requirement of 10.5 channels/node.…”

“…Likewise, data acquisition has been achieved using modular, parallel channels with in-situ FPGA-based lock-in amplification specifically for low-cost and high signal-to-noise ratios (SNRs) [13]. This latter technique is employed elsewhere [19,20,21,22], where the common theme is measurement of a modulated carrier giving the near-total rejection of out-of-band noise. The disadvantage however, is often the signal-processing, where a high purity homodyne reference is required alongside analog or digital multiplication.…”

Fully Integrated Photoacoustic NO2 Sensor for Sub-ppb Level Measurement

“…Another part of the signal processing circuit is a microcontroller-based digital lock-in amplifier (DLIA), as shown in Figure 2. Recently, several kinds of DLIAs have been studied, which, however, either use expensive and hard-to-develop integrated chips like Digital Signal Processor (DSP) [22] or Field-Programmable Gate Array (FPGA) [23], or employ the microcontrollers (MCU) [24] for only low frequency signal processing, with low precisions acquired, due to the limited computing power and lack of floating processing ability. As the rapid development of integrated circuits, a new MCU with a floating processing unit and digital signal processing instructions now is available for the DLIA as the sampling controller and computing core.…”

Measurement of CO2 Concentration and Temperature in an Aero Engine Exhaust Plume Using Wavelength Modulation Spectroscopy

“…As engines typically operate for over six hours per day this implies that the required data transmission and storage rates are far higher than realistically achievable. In the final tomography campaign the transmission rates and the total recorded data will be reduced by demodulating the 126 2f/1f signals at source using custom FPGA designs [18]. The in-phase and quadrature first and second harmonic signal measurements will then be transferred to a PC and recorded, reducing the data rate to 10 kSs −1 per channel.…”