Gas Concentration Measurement by Coherent Raman Anti-Stokes Scattering

A novel method for gas concentration measurement is described. This method is based on a four wave mixing process: two collinear light beams of frequencies o>1 and a>2 generate a collinear (anti-Stokes) wave at frequency 2(Dl—co2 when traversing a gas containing a Raman active molecular species with vibrational frequency wv = coj — co2. The intensity of the new wave is proportional to the square of the number density of resonant molecules. This scattering is much more intense than spontaneous Raman scattering. Possible experimental set-ups are reviewed. Typical results obtained with H2 gas are also presented: the concentration in a hydrocarbon flame was mapped, and small jets through the atmosphere were photographed in anti-Stokes light. The limitations of the method are discussed, and its field of application is assessed.