Coherent anti-Stokes Raman scattering (CARS) spectroscopy is a well established technique for making calibration-free measurements of temperature in harsh environments, such as combustors, afterburners, internal combustion engines, pool fires, etc. As shown in Fig. 1, two laser beams (green and red arrows) whose difference frequency matches the vibrational resonance of a molecule are used to generate a Raman polarization (black arrow). When a third beam (green arrow) is incident upon this Raman polarization, it coherently scatters some of its energy into a fourth beam (blue arrow) that emanates from the probe volume at a different angle and wavelength as the other beams. The geometrical arrangement of the pump, Stokes, and CARS signal beams satisfy a phase matching requirement, as shown in Fig. 2. The CARS signal beam can then be separated through spatial and spectral filtering. Because it is a coherent signal, it can propagate well away from the harsh environment of the flame zone and into a spectrometer for recording and analysis.
Fig. 1. CARS energy level diagram, showing 
how multiple laser beams (2 pumps and 
1 Stokes) can generate fourth CARS beam. 

Fig. 2.  Schematic of how the pump and Stokes 
beams are mixed in a phase matching arrangement, such as the BOXCARS configuration.