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Multiple-Beam System
for Fast Raman Spectrometry
A complete vibrational spectrum can be obtained from one
laser pulse.
John H. Glenn Research Center, Cleveland, Ohio
A developmental instrumentation system rapidly acquires
full Raman spectra of gas molecules. The system is based on
the principle of multiplex coherent antistokes Raman spectroscopy
(CARS) and incorporates improvements over prior multiplex
CARS systems. Among the potential applications for systems
like this one are (1) imaging (including microscopy), (2)
detection of molecular species of interest for diagnosis of
flames and other possibly rapidly changing systems, and (3)
detection of molecular species of interest for gas chromatography.
Heretofore, multiplex CARS systems have been capable of
obtaining Raman spectra rapidly (as fast as one spectrum per
laser pulse). The spectra in question are associated with
vibrations of the molecules of interest. The bandwidths of
the dye lasers that provide the excitation in such systems
have typically been limited, such that in a typical case,
the measured spectral range covers no more than about one-third
of the vibrational wavelength range of the molecular species
of interest. This developmental system is capable of covering
the full vibrational wavelength ranges of typical molecules
of interest.
The developmental system includes a neodymium: yttrium aluminum
garnet (Nd:YAG) pump laser, a hydrogen Raman cell, a degenerate
 -barium
borate optical parametric oscillator (BBO OPO), associated
optics for manipulating multiple beams of light so that the
beams overlap, detection optics, a monochromator, and an intensified
charge-coupled device. In operation, a broadband beam and
a narrowband beam are overlapped in space and time at a sampling
point. The multiplex CARS process, based upon the nonlinear
optical effect, generates a new broadband beam that is blue-shifted
with respect to the narrowband beam. This new beam is dispersed
and detected by use of the monochromator and the intensified
charge-coupled device. For each pulse of the pump laser, a
complete vibrational spectrum can be recorded.
In a test, the system was used to detect various molecular
species at different positions in a sooty flame. By observing
the heights of the peaks in the vibrational spectra as the
sampling point was moved outward from the center of the flame,
it was possible to determine that the concentration of C2
decreased while that of CO2 increased.
This work was done by Peter C. Chen's research group
at Spelman College for Glenn Research Center. For further
information, access the Technical Support Package (TSP) free
on-line at www.nasatech.com
under the Physical Sciences category.
Inquiries concerning rights for the commercial use of
this invention should be addressed to NASA Glenn Research
Center, Commercial Technology Office, Attn: Steve Fedor, Mail
Stop 4—8, 21000 Brookpark Road, Cleveland, Ohio 44135.
Refer to LEW-17194.
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