Picosecond-Laser-Based Fiber-Coupled Coherent Anti-Stokes Raman Scattering (CARS) Spectroscopy System

The objective of the proposed Phase-I research effort is to perform temperature and species measurements in both H2-air and C2H4-air flames using a fiber-coupled picosecond CARS system. Single-mode and step-index multi-mode fibers will be investigated for the delivery of the picosecond lasers to pump the target species and also to collect the generated CARS signal. The key element of our proposed innovation is the use of the picosecond CARS, that has the ability to suppress the nonresonant background signal from the hydrocarbon-air combustion by several orders of magnitude by tuning the delay of the probe laser after the interaction of the pump and Stokes lasers with the target. It also well-suited to the hypersonic regime for studying non-equilibrium phenomena. The unique feature of the ps-CARS technique for fiber delivery is that it requires two orders of magnitude less energy per laser pulse compared to the conventional nanosecond CARS. This effort will be based on a combination of both the experimental demonstration and a theoretical investigation of the propagation of picosecond laser pulses through various fibers. We will also explore the use of a new class of dispersion managed optical fibers such as hollow core fibers and photonic crystal fibers (PCF). Based on the success of the Phase-I effort we will build and deliver a picosecond laser-based fiber-coupled CARS system for wind-tunnel and gas turbine combustor thermometry and speciation during the Phase-II research effort.