CARS is usually compared to the induced Raman scattering, and also to spontaneous Raman scattering. CARS combines the advantages of a strong signal of the induced scattering with a wide application area of Raman spectroscopy. The intensity of the induced Raman scattering is a few orders higher than the intensity of spontaneous Raman scattering, however it is observed only when the intensity is higher than the threshold one, which depends on a media absorbance and the derivative of polarizability. The observance of the induced scattering is limited only by the strongest in Raman scattering lines of high-density materials.
In contrast to Raman scattering when the light is scattered in all directions, anti-Stokes signal keeps the direction, set by the incident waves and at special conditions stimulates other photons to scatter in the same direction accumulating the signal coherently. Like laser radiation the photons of anti-Stokes signal are phased with each other, scatter in a phase-matched direction and are therefore easy – detectable. Unlike the microscopy based on linear processes of radiation and imaging, where signal intensities are linearly connected with the laser power, CARS signal is generated on the basis of the 3rd-order nonlinear processes and has more complicated nonlinear dependencies on the intensity of incident radiation. CARS signal is proportional to the squared pumping wave intensity and directly proportional to the Stokes wave intensity as well as to all squared contributions to tensor X(3), so that X(3) includes summed responses of all molecules in a zone of interaction in focal waist of laser radiation, and correspondingly CARS is proportional to the squared concentration of molecules contributing to X(3). It allows using CARS (at certain conditions, along with the selective and non-invasive method) for quantitative measurements of chemical substance concentration in a sample.
It seems that the Raman spectroscopy and CARS should have the same sensitivity, as the same molecular transitions are used by them. However a CARS signal is much more intensive (~105) than a spontaneous Raman signal, cubically depends on excitation power, possesses an accumulation effect in the direction of phase synchronism and its properties are close to the properties of laser signals. It allows the substantial time reduction of signal accumulation (down to units of microsecond per pixel) and nondestructive real-time analysis, what is almost impossible in other types of imaging – confocal fluorescent microscopy or Raman microscopy.