Fiber-Coupled Pulsed and High-Intensity Ultraviolet Optical Measurements for Propulsion Systems

ABSTRACT: Optical diagnostics for combustion, such as planar laser induced fluorescence (PLIF), require high power ultraviolet (UV) laser light with high spatial quality. Fiber optic delivery of such UV light would provide the robustness needed for applications in harsh environments, for example to combustors. However, the need to deliver high intensity UV light presents challenges for conventional silica fiber optics due to solarization, optical damage, and beam quality degradation due to mode coupling. Seaforth LLC"s personnel have extensive experience in high power fiber optics. We have delivered millijoule levels of nanosecond pulsed light through large core silica fibers. Through a recent innovation in fiber selection and launch, we have also achieved superior beam quality (lower M2) relative to that typically achieved, which will benefit sheet uniformity and shot-to-shot variation. In the Phase I program, we will extend this unique capability to UV light delivery. We will perform fiber characterizations under high power UV operation including parametric studies of solarization, optical damage, and non-linear effects. We will also show use of the fibers with high repetition rate (kHz) laser sources including demonstration of PLIF of the hydroxyl radical (OH) in a laboratory scale turbulent flame with burst mode imaging. BENEFIT: The ability to deliver high power nanosecond ultraviolet pulses via optical fibers with high spatial beam quality at the fiber output will benefit the entire optical combustion science research community. It will enable more versatile beam delivery for applications in harsh environments, such as combustors and engines, as well as for vacuum chambers and more typical bench-top settings. Application to combustor and engine environments is a particularly acute need owing to the lack of currently available methods. This innovation will also accelerate advances in fiber delivered laser based ignition systems for aero-turbines, where relighting at high altitudes is problematic, and for reciprocating gas engines, where conventional electric spark plug ignition systems are a bottleneck to achieving high efficiencies and low emissions. Improved fiber optic delivery of high power UV beams can also benefit areas of biomedical science such as fiber based PLIF or CARS for imaging applications.