Schematic of the rapid-scanning FT-CARS system.
A pulse from the laser is first sent into the Michelson interferometer with a rapid-scanning retro-reflective optical path length scanner in one of its arms. The scanner is arranged in such a way that the returned pulse from the scanner travels back along the same path as the incident pulse except for the time delay produced by the scanner. A train of dual collinear pulses with a variable time delay with respect to each other exits the interferometer and focused onto the sample. The resulting filtered anti-Stokes signal is detected by the avalanche photodetector and digitized by the digitizer with an external clock from the continuous-wave interferograms. This measurement can be repeated with trains of dual collinear pulses with an increasing time delay with respect to each other in order to obtain the interferogram in which the CARS signal is encoded. The CARS spectrum can be obtained by taking the Fourier transform of the interferogram. The acquisition of the CARS spectrum can be repeated at the rate equivalent to the scan rate of the path length scanner multiplied by a factor of two (from the round trip of the scanner). DM: dichroic mirror, PBS: polarizing beamsplitter, QWP: quarter-wave plate, APD: avalanche photodetector.