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

The objective of this Phase-II research effort is to build and deliver a fully operational fiber-coupled picosecond (ps) laser-based CARS system for measuring gas-phase temperature in reacting flows. The proposed fiber-based CARS system will also be fully capable of nanosecond operation with the addition of a nanosecond pump laser provided by the customer. It will be tested in laboratory hydrocarbon-air flames followed by evaluation in an augmentor test-rig at WPAFB, OH. These measurements will allow optimizing the CARS system in preparation for measurements in the AEDC J85 test rig. During the Phase-II research effort, we will build on successful Phase I studies of fiber-optic beam propagation and focus our attention on pulsed-laser propagation through solid- and hollow-core photonic crystal fibers. The CARS system will be built in such a way so as to incorporate both multimode step-index fibers and photonic crystal fibers. 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 and affords an improvement in damage threshold. We will also design and build a special collimator to facilitate raster scanning with the fiber-based CARS system. This fiber collimator will allow two- or three-dimensional scanning of the flow field. The proposed research effort will provide new diagnostic capabilities that will enable the Air Force and gas-turbine system manufacturers to address the challenges associated with high-speed reacting and non-reacting flows. BENEFIT: The proposed research effort will provide new diagnostic capabilities that will enable the Air Force and gas-turbine system manufacturers to address the challenges associated with high-speed reacting and non-reacting flows. These tools are critical for the development and long-term health of propulsion systems for high-performance military as well as commercial systems. The proposed research will also help to advance the state of fiber technology for optical diagnostics through numerical modeling of light propagation through the optical fibers. Quantitative measurements are critical for validating numerical models of reacting and non-equilibrium phenomena affecting modern gas-turbine and hypersonic propulsion systems. Such experimental and numerical tools will prove to be extremely valuable in the analysis of military and commercial gas-turbine combustors, as well as applications with limited optical access such as internal combustion engines and stationary power generation systems.