  Coherent anti-Stokes Raman scattering (CARS) microscope system was built and
applied to a non-intrusive gas concentration measurement of a mixing flow in a
millimeter-scale channel. Carbon dioxide and nitrogen were chosen as test
fluids and CARS signals from the fluids were generated by adjusting the
wavelengths of the Pump and the Stokes beams. The generated CARS signals, whose
wavelengths are different from those of the Pump and the Stokes beams, were
captured by an EM-CCD camera after filtering out the excitation beams. A
calibration experiment was performed in order to confirm the applicability of
the built-up CARS system by measuring the intensity of the CARS signal from
known concentrations of the samples. After confirming that the measured CARS
intensity was proportional to the second power of the concentrations as was
theoretically predicted, the CARS intensities in the gas mixing flow channel
were measured. Ten different measurement points were set and concentrations of
both carbon dioxide and nitrogen at each point were obtained. Consequently, it
was observed that the mixing of two fluids progressed as the measurement point
moved downstream. The results show the applicability of CARS to the
non-intrusive concentration measurement of gas flows without any preprocess
such as gas absorption into liquid or solid.
