Point and planar ultraviolet excitation/detection of hydroxyl-radical laser-induced fluorescence through long optical fibers

Abstract

We demonstrate an all-fiber-coupled, UV, laser-induced-fluorescence (LIF) detection system of the hydroxyl radical (OH) in flames. The nanosecond-pulsed excitation of the (1,0) band of the OH A(2)∑(+)-X(2)Π system at ∼283 nm is followed by fluorescence detection from the (0,0) and (1,1) bands around 310 nm. The excitation-laser beam is delivered through a 400 μm core UV-grade optical fiber of up to 10 m in length, and the fluorescence signal collected is transmitted through a 1.5 mm core 3 m long fiber onto the remote detector. Single-laser-shot planar LIF (PLIF) imaging of OH in flames is also demonstrated using fiber-based excitation. The effects of delivering intense UV beams through long optical fibers are investigated, and the system improvements for an all-fiber-coupled OH-PLIF imaging system are discussed. Development of such fiber-based diagnostics and imaging systems constitutes a major step in transitioning laser diagnostic tools from research laboratories to reacting flow facilities of practical interest.

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.…”

Fiber-Coupled High-Speed OH-PLIF Imaging in Turbulent Flames

“…10 The observed UV-laser-induced damage threshold (UV-LIDT) of the FDP fiber is approximately 6-fold higher than that of the previous state-of-the-art fiber reported by Kychakoff et al, 8 which makes it possible for delivering high UV pulse energy for OH-PLIF measurements. 9 The beam output from the laser system was then coupled into a 400-m core FDP fiber using an F=+150-mm spherical lens. The input end of the fiber was positioned behind the focal point of the lens at a location where the beam had expanded to fill ~70% of the core area; this not only reduced the laser intensity at the input surface of the fiber, avoiding damage to the fiber surface, but also allowed a margin for error in the transverse alignment of the fiber.…”

“…[8][9][10] These studies, however, have shown that the pulse energy employed for gas-phase UV-LIF is several orders of magnitude higher than that required for the condense phase, particularly at elevated flame temperatures. This high-energy requirement imposes significant constraints on fiber-coupled UV-LIF for gas-phase detection because of the intrinsic optical-damage threshold of the fibers, which limits the maximum deliverable laser energy.…”

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.…”