Single-Beam CARS Imaging for
Reacting Flow Diagnostics
Paul J. Wrzesinski, Dmitry Pestov, Vadim V. Lozovoy,
Sukesh Roy, James R. Gord and Marcos Dantus
Coherent anti-Stokes Raman Scat- tering (CARS) is a long-standing
diagnostic technique of choice in the
combustion community for measuring
temperature and major species concentration in gas-phase-reacting flows. Additionally, the coherent excitation of Raman-active molecular vibrations offers
intrinsic chemical specificity and high
The application of traditional
nanosecond and picosecond CARS
is hindered by its cumbersome experimental implementation, requiring
multiple laser beams that are spatially
and temporally overlapped in a complex
beam geometry. However, advances in
laser and pulse-shaping technologies have
made it possible to measure the CARS
signal using a single laser beam. 1 The use
of supercontinuum sources allows for
acquisition of an entire Raman spectrum
without wavelength scanning or changes
to experimental parameters.
In single-beam CARS, researchers
use spectrally broad, ultrashort pulses
that contain the pump, Stokes, and
probe components. In our work, sub- 10
fs amplified pulses are capable of impulsively exciting Raman transitions with
wavenumbers up to 2,500 cm- 1 range. 2, 3
A narrow region on the high-frequency
side of the spectrum is selected as the
probe. Its polarization is rotated by 908
and a π-phase step is applied to minimize
the nonresonant contribution due to
the instantaneous electronic response.
Additional phase-modulation techniques
can be used to selectively excite a single
vibrational mode. More importantly,
mode-selective phase modulation allows
for the elimination of multichannel
detection. Single-channel acquisition is
desirable for imaging applications that
require high-speed beam scanning.
One proven method for mode-selective excitation is the use of pseudo-
Cars intensity [arb. units]
Distance [mm] Distance [mm]
BPS 1,200 cm- 1
BPS 1,280 cm- 1
BPS 1,330 cm- 1
BPS 1,380 cm- 1
1,000 1,200 1,400 1,600 1,800
Wavenumber [cm- 1]
Selective single-beam CARS: (a) CARS spectra of pure CO2 gas in a cell for a set of phase
functions targeting Raman transitions at various wave numbers. Excitation selectivity is accomplished using BPS. (b) Image of a CO2 jet taken without mode-selective excitation. The
CARS signal is integrated over 300-2,500 cm- 1 range. (c) Image of the same CO2 jet with the
selective excitation of the symmetric stretch ( 1,280 cm- 1 band).
random binary sequences. 4 We have
applied binary phase shaping (BPS) for
mode-selective CARS in liquids and,
more recently, in gas-phase samples. 2, 5
We have demonstrated independent
excitation of CO2 Fermi dyads and nearly
complete suppression of the nonresonant
four-wave mixing, as illustrated in (a).
To highlight the advantages of
selective Raman excitation for species
imaging, we have focused the beam of
shaped pulses into a jet of CO2 flow-ing from a rectangular nozzle mounted
on a motorized 2-D translation stage.
The spectrally integrated CARS signal
is collected at each XY position of the
stage. Two different pulse shaping
methods are used: (i) impulsive excitation for all Raman-active vibrational
modes using transform limited pulses
and (ii) selective excitation of the symmetric stretch of CO2 at 1280 cm- 1
The image obtained using impulsive
excitation (b) exhibits poor contrast due
to the resonant contribution from N2
and O2 molecules, abundant in ambient
air, and the nonresonant response that
dominates the low-frequency part of the
integrated CARS spectrum. The second
phase shaping approach (c) has the advantage of suppressing excitation at Raman
frequencies other than the target frequency as well as nearly complete elimination of the nonresonant background. It
provides high imaging contrast for the
jet of CO2 gas relative to ambient air.
Furthermore, static turbulent modulation
of the CO2 flow is clearly visible. t
Paul J. Wrzesinski ( email@example.com),
Dmitry Pestov, Vadim V.Lozovoy and Marcos Dantus are with the department of chemistry, Michigan
State University, East Lansing, Mich., U.S.A.
Sukesh Roy is with Spectral Energies, and James
R. Gord is with the Air Force Research Laboratory
1. N. Dudovich et al. Nature 418, 512-4 (2002).
2. H. Li et al. Opt. Express 16, 5499-504 (2008).
3. H. Li et al. Appl. Opt. 48, B17-22 (2009).
4. V. Lozovoy et al. Phys. Rev. A 74, 041805 (2006).
5. P. J. Wrzesinski et al. J. Raman Spectroscopy, In Press