Ph.D. Dissertation Defense
Friday, August 23th, 2013 – 12:00 pm
ETAS, Room 575
“An Investigation of Electro-Optical 1/f Noise Reduction in an Open–Path Tunable Diode Laser Spectrometer”
Samira Adnan Mahdi
Applied Science Graduate Program, Applied Physics, UALR
Advisor: Dr. Gary Anderson, PhD, Professor of Systems Engineering, UALR
Abstract: A mobile spectrometer system has been designed to scan the near-surface atmosphere for ammonia gas over a wide range of distances (10 m to 1 Km). Since the system is designed for space applications, it needs to be small, lightweight, and low power, which dictates the use of relatively low frequency measurement scans. The spectrometer uses a diode laser, which is subject to a large of electro-optical 1/f noise component at these low frequencies. In this work, digital signal processing techniques are developed to reduce the effects of 1/f noise in gas measurements. A Double Fast Fourier Transform-Based Filter (DFFT-BF) method is developed to maximize the measurement sensitivity of a low frequency Tunable Diode Laser Spectrometer (TDLS) system. The 1/f noise spectrum and the uncertainty in the wavelength of the diode laser emission due to 1/f frequency noise is investigated in both simulations and experiments. Also, measurements of ammonia gas using its 1543 nm absorption peak were performed using a sinusoidal waveform to drive the diode laser. A signal processing method that is proposed reduces the effects of electro-optical 1/f noise while keeping the measurement signal relatively constant. Measurements with different modulated waveforms are also examined, including a sawtooth with superimposed sine wave and a sinusoidal wave with a multiple step offset to drive the diode laser. In addition, the system is tested in outdoor measurements of ammonia gas concentrations. The results show that the DFFT-BF method reduces the effects of the 1/f noise in the measurement.