Multiple-beam system for fast Raman spectrometry

NASA Tech Briefs, Jul 2002

A complete vibrational spectrum can be obtained from one laser pulse.

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 C^sub 2^ decreased while that of CO^sub 2^ increased.

John H. Glenn Research Center, Cleveland, Ohio

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/tsp 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.