Combining nonlinear spectroscopy and optical fibers into a remote sensing methodology
Abstract
Fiber-optic remote sensing using two-photon excited molecular fluorescence was demonstrated as a potential in situ process measurement method for optically dense product lines. With two-photon induced fluorescence, quantitative analysis was performed at extended distances ($>$100 meters) from the laser source on both highly concentrated and highly absorbing systems. Concentration information from micromolar to near saturation levels was feasible. Optical multiplexing using fiber-optic star couplers, capable of simultaneously monitoring up to ten different sample locations with a single laser, made the method more cost effective. Corrections for source instabilities during the data collection were accomplished by installing a chemical power-squared reference solution at one of the sensing points. Pulse compression was used in an effort to gain higher peak powers and improve the detection limits. Extensions to chemical systems of interest to the fermentation industry were also made. Measurement of fluorophores in a flowing stream was performed. As a result of focussing high power, picosecond pulses of yellow light down narrow-radius glass fibers, nonlinear optical generation of blue light originating from the optical fiber and coupler was observed. A broad emission band from 400 nm to 500 nm with several sharp peaks superimposed on top of the blue background were evident in the emission spectra. Experimental results suggest that two-photon excited fluorescence of germanium dopants or defect centers inside the fiber core may be responsible for the broadband emission. Because of equipment limitations, the origins of the fine structure are not conclusively known, although wave mixing or interference effects appear to be the most likely cause. Since the fiber background followed a quadratic dependence on the incident optical power, it was used in place of the chemical reference as an internal power-squared sensor.
Degree
Ph.D.
Advisors
Lytle, Purdue University.
Subject Area
Analytical chemistry
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