Construction and development of a picosecond UV pump/UV probe spectrometer for combustion diagnostics based on asynchronous optical sampling (ASOPS)
Abstract
This thesis deals with the construction and development of a picosecond UV pump/UV probe spectrometer for combustion diagnostics based on Asynchronous Optical Sampling (ASOPS). ASOPS avoids many of the difficulties present in conventional pump/probe spectrometers. The instrument utilizes two frequency-double dye lasers which are synchronously pumped by two frequency-doubled, mode-locked Nd:YAG lasers operating at slightly different repetition rates. The difference in repetition rates produces a repetitive relative phase walk-out of the pump and probe pulses which replaces the optical delay line used in conventional pump/probe instruments and thus reduces the data acquisition times from several minutes to microseconds. A description of this process and the instrumentation used in ASOPS is shown along with a general background of current combustion diagnostic methods. Initial combustion studies consisted of visible pump/probe measurements of atomic sodium in an atmospheric pressure flame. With the use of a LiIO$\sb3$ frequency doubling crystal, average powers of $\sim$20 mW at $\sim$300 nm could be attained with high repetition rate synchronously pumped, mode-locked dye lasers. These high UV powers were attained without the use of complicated amplification techniques normally considered essential for frequency doubling of picosecond lasers. Timing considerations concerning the best choice of beat frequency of the ASOPS technique and its effect on turbulent measurements is considered. UV pump/UV probe measurements of atomic indium in an atmospheric pressure flame are shown making the ASOPS instrument one of only three UV pump/UV probe spectrometers in the world. To improve the ASOPS signal-to-noise ratio, several noise reduction techniques have been implemented. Other noise reduction techniques have been considered and their impact on the ASOPS signal is discussed. Finally, conclusions on the ASOPS technique are presented along with future considerations.
Degree
Ph.D.
Advisors
Laurendeau, Purdue University.
Subject Area
Analytical chemistry
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