An FPGA-based lock-in detection system to enable Chemical Species Tomography using TDLAS Andrea Chighine 1 , Edward Fisher 1 , David Wilson 2 , Michael Lengden 2 ,Walter Johnstone 2 and Hugh McCann 1 1 School of Engineering, The University of Edinburgh, Edinburgh, Scotland, UK 2 Department of Electronics and Electrical Engineering, University of Strathclyde, Glasgow, Scotland, UK 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 CO 2 feature in the spectrum region of 1997.2nm. The spectrum in this wavelength region shows suitably weak water absorption, enabling CO 2 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 multi- channel system to implement Chemical Species Tomography. Keywords—Field Programmable Gate Arrays (FPGA), Lock-in amplifier, Digital demodulation, Tunable Diode Laser Spectroscopy (TDLAS), Chemical Species Tomography (CST) I. INTRODUCTION Over the last two decades, tunable diode laser-absorption spectroscopy (TDLAS) has become established as a robust and widely utilized technique for sensing and measuring gas parameters in a variety of applications [1]. Over the same period, high-speed Chemical Species Tomography (CST) has become established [2], using predominantly fixed- wavelength diode-laser technology. Using continued development of narrow line-width laser technologies, TDLAS has evolved into a highly selective spectroscopic technique, which can detect the unique absorption fingerprints of various gases. Light emitted from a tunable diode laser is passed through a gas sample to a photodiode, and the detectable absorption of light can be related to the gas properties, such as concentration, pressure, and temperature. This optical absorption modality is widely utilized, as being non intrusive, fast and sensitive. For gas detection in harsh, industrial environments and at low concentration, few methods have been proposed to improve upon the detection sensitivity of TDLAS [3]. A particular method which greatly reduces the influence of laser and electronic noise and is well suited to targeting gas species with sharp absorption features, is wavelength modulation spectroscopy (WMS). The basic principle of WMS is to simultaneously modulate the laser wavelength with a sinusoidal high frequency signal (f, in the order of tens of kHz) while scanning the laser wavelength across the absorption profile of interest at low frequency (in the order of tens of Hz). The interaction between the modulated laser signal and the non-uniform absorption profile gives rise to several components in the frequency domain, which can be related to the harmonics of the laser modulation signal [4]. Lock-in amplification, using phase sensitive detection (PSD), is capable of detecting, selecting and isolating these harmonic components. By implementing a narrow band-pass filter, through heterodyne mixing electronics, the 1f and 2f harmonic signals can be detected, thus obtaining absorption information. Although important improvements in noise suppression are achieved by applying the WMS method, the harmonic components detected by the photodiode, before the lock-in stage, are not easy to analyze because of the low level gas signal being affected by noise [5]. Lock-in amplifiers, commonly used in instrumentation for their capability of detecting low-level signals affected by interference and noise, are therefore fundamental to the processing of gas signals in the TDLAS technique [6]. Continued development of Field Programmable Gate Array (FPGA) technology has allowed the implementation of low-cost and compact complex systems, while obtaining both high reliability and, in instrumentation systems, high accuracy. The progress in these technologies has prompted a number of implementations of digital lock-in amplifiers (DLIA) on FPGA platforms [7-8]. This paper demonstrates a fully digital CO 2 TDLAS system with the FPGA-based lock-in amplifier technique using phase sensitive detection. The implementation is built using a mid-level Xilinx Spartan6 FPGA with an analog to digital converter (ADC), processing the wavelength- modulated signal emitted by a diode laser tuned to scan the CO 2 absorption feature at 1997.2 nm [9]. The system discussed here is to be implemented subsequently for a 126- beam CST system for CO 2 imaging in a gas turbine exhaust plume [10-11]. The fundamentals of TDLAS and the lock-in measurement principle are described in the next section. Section III discusses the implementation and experimental setup and section IV presents preliminary measurement results comparing the FPGA-based DLIA and a conventional lock-in amplifier instrument.