Investigation of optical fibers for gas-phase, ultraviolet laser-induced-fluorescence (UV-LIF) spectroscopy

Abstract

We investigate the feasibility of transmitting high-power, ultraviolet (UV) laser pulses through long optical fibers for laser-induced-fluorescence (LIF) spectroscopy of the hydroxyl radical (OH) and nitric oxide (NO) in reacting and non-reacting flows. The fundamental transmission characteristics of nanosecond (ns)-duration laser pulses are studied at wavelengths of 283 nm (OH excitation) and 226 nm (NO excitation) for state-of-the-art, commercial UV-grade fibers. It is verified experimentally that selected fibers are capable of transmitting sufficient UV pulse energy for single-laser-shot LIF measurements. The homogeneous output-beam profile resulting from propagation through a long multimode fiber is ideal for two-dimensional planar-LIF (PLIF) imaging. A fiber-coupled UV-LIF system employing a 6 m long launch fiber is developed for probing OH and NO. Single-laser-shot OH- and NO-PLIF images are obtained in a premixed flame and in a room-temperature NO-seeded N(2) jet, respectively. Effects on LIF excitation lineshapes resulting from delivering intense UV laser pulses through long fibers are also investigated. Proof-of-concept measurements demonstrated in the current work show significant promise for fiber-coupled UV-LIF spectroscopy in harsh diagnostic environments such as gas-turbine test beds.

Modeling of fiber-optic fluorescence probes for strongly absorbing samples

“…(iii) The dead volume can be reduced by using either claddingless fibers for fluorescence excitation and detection or, as described by Booksh and coworkers, a coaxial capillary collection waveguide with a claddingless excitation fiber [14]. (iv) With regard to dynamic range, the common method of detecting fluorescence normal to the excitation axis [18,19,26] is inferior to any of the fiber-coupled methods when the thickness of the sample cuvette is larger than the approximate decay length of the secondary absorption. (v) Collecting fluorescence in transmission-as is still common in microscopy ("trans-fluorescence detection") [27,28]is similarly (in-) efficient compared with collection at right angles.…”

“…These studies were therefore not designed to quantify the effect of primary absorption of the excitation light and secondary absorption of the emitted fluorescence. The (re-) absorption of light may be neglected when the sample is very dilute or in the gas phase [18,19] but cannot be ignored in many undiluted liquid samples. An experimental study by Ozanyan et al [20] was supplemented with numerical modeling of the light emission from a single-fiber probe and a bifurcated probe to illustrate the effect of the dead volume on strongly attenuating samples.…”

Development of fiber-coupled high-speed particle image velocimetry (PIV) for flow fields measurements in gas turbine engine hardware

“…[22]. The fiber end surface had been polished by the vendor, and no marks were observed under a microscope at 100x magnification.…”

Application of Optical Measurement Techniques in Combustion Test-Cell Environments

“…For line-of-sight fiber-coupled LAS applications in practical high-pressure gas turbine combustor test article, a time-division-multiplexed (TDM) system based on fiber Bragg gratings has been developed and demonstrated for monitoring gas temperature and concentration of H 2 O and CH 4 at the repetition frequency of 50 kHz [1]. Fiber-coupled high-speed PLIF/PIV detection systems (up to 10 kHz) employing a 20-foot-long multimode fiber have been developed for probing two-dimensional OH and NO concentrations and flow velocity in turbulent flames [2,3,4]. Currently, efforts are under way to explore advanced fiber designs to generate larger laser sheets with sufficient laser pulse energy delivery at the test section for technology transition into the test cell.…”