PROJECT ABSTRACT: Femtosecond Coherent Anti-Stokes Raman Scattering for Time-Resolved Measurements of Temperature and Species Concentration in Flames Robert P. Lucht and Xianfan Xu School of Mechanical Engineering, Purdue University, W. Lafayette, IN 47907-2088
The primary objective of the proposed work is the development and application of ultrafast-laser-based coherent anti-Stokes Raman scattering (CARS) techniques for single-shot measurements of temperature and species concentrations at data rates of 1 kHz or greater. The acquisition of fully time-resolved data from turbulent media will provide more rigorous and meaningful tests of turbulent flame models, leading eventually to combustors that are more efficient and have lower levels of pollutant emission. The 1-kHz data rate of the measurements will be a great advantage during testing of practical combustors, especially when the test duration is limited by facility considerations. The work represents a first step toward enabling femtosecond CARS based sensors for energy and environment applications. The proposed research will provide excellent training for graduate students in mechanical engineering. The femtosecond (fsec) CARS project will provide numerous opportunities for multidisciplinary activities for mechanical engineering graduate students, because operation of the fsec laser systems requires considerable expertise in optical physics and electronics. The subjects of fsec laser systems and fsec laser physics will be incorporated in a graduate class on advanced laser diagnostics. Students from underrepresented groups will be actively recruited for this research, at both the undergraduate and graduate levels. To assist in undergraduate and graduate recruitment, a fsec CARS project web site will be created. The femtosecond CARS measurements will be performed using an existing laser facility at Purdue University; laser pulses at 780 nm with a nominal pulse length of 80 fsec can be generated with pulse energies in excess of 1 mJ at a repetition rate of 1 kHz. Pump and probe pulses will be produced using an optical parametric amplifier. To measure temperature and/or species concentrations at data rates of 1 kHz or greater, a chirped-pulse probe beam with a pulse length of 2-3 psec will be used to map the temporal behavior of the Raman coherence onto the frequency spectrum of the CARS signal. The CARS signal will be dispersed using a spectrometer and detected using a back-illuminated electron multiplying CCD (EMCCD). This EMCCD camera has been developed as a frame transfer camera and CARS spectra can be extracted at data rates of 1 kHz or greater. Unique numerical methods for calculation of the fsec CARS signal generation process will be used to both explore the physics of the fsec CARS process, including the details of the interaction of the pump and Stokes beams to create the Raman coherence, and to develop computer codes for determination of temperatures and species concentrations from experimental data.
