CARS microscopy with folded BoxCARS phasematching

Three-dimensional microscopy based on coherent anti-Stokes Raman scattering (CARS) is a powerful new imaging technique, in which the contrast arises f rom molecular vibrations, Based on a simple numerical model, it is shown ho w the CARS interaction volume depends on the focusing parameters and the ty pe of phasematching used. Collinear phasematching yields an ellipsoidal int eraction volume, with lateral dimensions that readily cause vignetting of t he CARS signal emission at the collection microscope objective. A folded Bo xCARS phasematching geometry on the other hand, results in an almost cylind rical interaction volume at the cost of a reduced resolution, for which the possible vignetting of the CARS emission is much reduced. In addition, thi s type of phasematching provides spatial separation of the signal from the input laser beams, permitting simple signal detection of low frequency vibr ational modes. Calculations show that when CARS is performed in a microscop ic volume, the phasematching restraint on tuning over the vibrational band is strongly relaxed, A first example of CARS imaging using a folded BoxCARS imaging geometry is shown.