Development of a low cost laser diode modulation circuit for direct and 2f spectroscopic measurements of water

This thesis documents the development of a modulation circuit to detect Near-IR water absorption lines using two different wavelength modulation spectroscopy techniques. The modulation circuit was designed using operational amplifiers and zero crossover function generators to operate at different voltages and frequencies. This low cost circuit overcame the limitations of modulation circuits used by others and was very versatile when measuring water absorption using both direct and indirect spectroscopic techniques. This research systematically determined the circuit performance during the detection of two water transition lines at 1.3987 mum and 1.3997 microm using wavelength modulation spectroscopy. Indirect spectroscopy using the 2f Voigt line shape of water, a high speed lock-in amplifier with a single InGaAs photodetector was demonstrated with this circuit. This ultra-fast spectroscopic technique collected 1 million data points capturing the complete water transition spectrum in 1/2000 of a second.