Implementation of digital lock-in and concentration measurement for Wavelength Modulation Spectroscopy (WMS) based sensors using MATLAB

Wavelength Modulation Spectroscopy (WMS) using tunable diode laser is a versatile method for determination of concentration, temperature and velocity of absorbing gaseous molecules. This technique finds major application in industrial and scientific areas. An important component of sensors/systems based on these techniques is the phase sensitive detection for demodulation, in the form of hardware or software based lock-in amplifier. This paper presents the details of (a) a MATLAB based Digital Lock-in amplifier technique for extraction of harmonics (1f & 2f), from the wavelength modulated optical absorption signal, and (b) a MATLAB based program for calculating the oxygen concentration from 1f and 2f harmonics. The oxygen concentration has been measured using MATLAB implemented digital lock-in-amplifier and concentration computation formula. The deviation in the measured Oxygen concentration from the actual Oxygen concentration supplied through the mass flow controllers is less than 1%. The oxygen concentration measured using digital Lock-in amplifier has a deviation within 0.2% as compared to measurement from hardware measuring device.