By Alexis Bohlin and Christopher Kliewer
Coherent anti-Stokes Raman spectroscopy (CARS), one of the genuinely optical diagnostic techniques in combustion research, has emerged over the years by detailing chemically reacting flows and has been vitally important in validating combustion model grid data. As valuable as it is, however, CARS presents drawbacks. For example, the conventional setup, which includes multiple laser beams, imposes a high level of complexity to perform a successful experiment. Further, as typically used, CARS provides single-point measurements—which fall short of the spatially correlated information needed to fully characterize multi-dimensional reacting flows.
Following a recent string of advances,1–3 CRF researchers Alexis Bohlin and Christopher Kliewer have developed a new coherent Raman imaging spectrometer that acquires spectral information rendered in two spatial dimensions (2D) to generate a planar array of thousands of CARS spectra—all within the instantaneous timeframe of a single laser shot.4 In new work, the researchers extended their previously reported simplified two-beam CARS setup to simultaneously measure Raman transitions up to 4200 cm–1, allowing, in principle, detection of all Raman active vibrational or rotational manifold distributions for major flame species.5 This unique spatially correlated assessment method, incorporating all CARS benefits, enables new cutting-edge diagnostics for many fast, dynamical gas-phase systems.
Figure 1 outlines the method for generating and detecting a 2D CARS signal using the two-beam CARS phase matching scheme.2 Specifically, a femtosecond (fs) combined pump/Stokes beam is focused to a thin sheet, impulsively exciting the molecules across a planar spatial region. A narrowband picosecond probe beam intersects this region, generating a CARS signal from each spatial location within the plane. This signal-generation plane is relay-imaged through the grating onto the face of a charge-coupled device (CCD) camera. In this way, each gas-phase Raman transition frequency gets mapped to a different position on the detector.
Diagnostic Imaging in Flames
The technique was first demonstrated in a flame by imaging the pure-rotational manifold of N2 and O2. Because most gas-phase rotational transitions have very narrow isolated spectral lines, developers designed the spectrometer to detect each Raman transition within a single pixel on the CCD. As a result, no convolution exists between space and frequency within the CARS image, allowing for a simple pixel-to-pixel extraction method to recover the CARS intensity spectra. As an example, 2b within Figure 2 (which provides spatial maps of specific S-transitions) clearly shows that the high rotational states of the N2 molecule become populated by the progress and energy release of the combustion reaction.
Ultrabroadband Multiplex Imaging
Because the pump and Stokes fields are automatically overlapped in time and space in the two-beam CARS phase-matching arrangement, robust use of ultrashort pulses (< 8 fs) becomes feasible. These pulses in turn create ultrabroadband excitation of CARS resonances, allowing detection of not only multiple manifolds within the same molecule (e.g., the S-branch and the Q-branch of N2), but also many different molecules simultaneously.
In fact, Bohlin and Kliewer recently demonstrated the 2D-CARS hyperspectral spectrometer over a bandwidth of 4200 cm–1. This coherent Raman planar imaging of quantum state-resolved spectra of many molecules opens many new directions in combustion research. For instance, Figure 3, which displays an image taken within a single laser shot of the O2 Q-branch overlaid with the H2 S(3) transition acquired in a H2-jet diffusion flame, shows oxidizer and fuel spatial distributions as they are consumed in the combustion process. Such complementary multiplex detection should enable direct determination of the instantaneous mixture fraction, a key turbulent-combustion modeling parameter, with no need to add tracers that can change the combustion chemistry.
1. Bohlin, A. and C. J. Kliewer. Communication: Two-dimensional gas-phase coherent anti-Stokes Raman spectroscopy (2D-CARS): Simultaneous planar imaging and multiplex spectroscopy in a single laser shot. J. Chem. Phys. 2013, 138 (22), 221101.
2. Bohlin, A., B. D.Patterson, and C. J. Kliewer. Communication: Simplified two-beam rotational CARS signal generation demonstrated in 1D. J. Chem. Phys. 2013, 138 (8), 081102.
3. Bohlin, A. and C. J. Kliewer. Two-beam ultrabroadband coherent anti-Stokes Raman spectroscopy for high resolution gas-phase multiplex imaging. Appl. Phys. Lett. 2014, 104 (3), 031107.
4. Bohlin, A. and C. J. Kliewer. Diagnostic Imaging in Flames with Instantaneous Planar Coherent Raman Spectroscopy. J. Phys. Chem. Lett. 2014, 5 (7), 1243-1248.
5. Bohlin, A. and C. J. Kliewer. Single-shot hyperspectral coherent Raman planar imaging in the range 0–4200 cm–1. Appl. Phys. Lett. 2014, 105 (16), 161111.