A real-time portable tunable diode laser spectroscopy system using the TMS320F28377D digital signal processor for gas sensing applications

Tunable diode laser spectroscopy (TDLS) is a well established technique in the field of gas sensing. The features of this technique such as higher sensitivity, selectivity, negligible cross sensitivity and calibration free make this technique a reliable solution for measurement of gas parameters such as mole fraction, pressure, temperature and velocity. TDLS with direct detection and wavelength modulation spectroscopy (WMS) is being used extensively for various gas sensing applications such as greenhouse gas detection, industrial process monitoring, gas leaks detection, food quality assessment, pollution monitoring, interplanetary gas detection, high temperature measurement in the combustion environment etc. Along with the laser and photodetector, the electronics required for TDLS system are function generator, digital storage oscilloscope (DSO), lock-in amplifier and a computer. Laser and photodetector are small in size. However the electronics prevent the system to deploy for the field applications because electronics are bulky, costly and also consume more electrical power. To make the system field deployable as well as mountable on to the unmanned air vehicle (UAV), the electronics must be replaced by a single digital signal processor (DSP) board. So, this work mainly focuses to develop a compact TDLS system for airborne applications. A digital signal processor TMS320F28377D of Texas Instruments has been selected for this work. Simultaneous generation of required waveform for scanning and modulation of laser wavelength and acquisition were developed. Filtering as well as averaging were implemented to keep the functionality same as DSO. Simulation of absorption lineshape functions such as Lorentzian, Gaussian and Voigt were performed using TMS320F28377D. The simulation with 900 points of Lorentzian lineshape function requires processing time of 458 ms. An algorithm for direct detection has been designed and implemented for real time gas parameters measurement. For two parameter estimation (mole fraction and pressure), a least mean square (LMS) algorithm has been implemented to perform real time fitting of simulated gas line to experimentally acquired gas line. This LMS requires 16 seconds of processing time to adapt a change in the mole fraction of 0.8%. The sensor shows an uncertainty of 0:1 bar in the measurement of pressure whereas an uncertainty of 0:02% is shown for the measurement of mole fraction. The time resolution of 16 sec has been achieved with variable size least mean square algorithm (VS-LMS) which is good enough for slowly varying process like atmospheric gas monitoring. The developed direct detection algorithm has been performed with four different lasers in the lab. Field measurements using a 1392 nm DFB laser for ambient water vapour measurement is also performed at two different locations one at IITGN campus and second at Ramnagar, Ahmedabad. Temperature measurement using ratio of two integrated absorbance is also done using two absorption line of water vapour. However, TDLS direct detection offers a low detection sensitivity. For most of the trace gas detection applications, the sensitivity is required in parts per million (ppm) and parts per billion (ppb) range. Therefore, to increase the detection sensitivity, wavelength modulation spectroscopy (WMS) is used. TDLS-WMS requires a lock-in amplifier which is narrow band pass filter used to improve the signal to noise ratio. Therefore, a lock-in amplifier with simultaneous ramp and sinusoid generation is implemented using DSP board. It is capable of extracting different harmonic components at different phases though some improvements are required.by Neetesh Kumar SharmaM.Tech