Pupil-engineered coherent anti-Stokes Raman scattering spectroscopy

Coherent anti-Stokes Raman scattering (CARS) spectroscopy suffers from non-resonant background (NB) that is several orders of magnitude stronger than CARS signals. NB occupies the camera sensor dynamic range and hampers quantitative detection of weak Raman signals at low molecular concentrations. Here, we propose a novel technique to reduce NB for heterodyne CARS spectroscopy, by independently controlling the parities of the amplitude point spread functions of CARS and NB. We realize the parity control by applying the pupil engineering of the excitation and detection optical systems. A proof-of-concept experiment shows that the signal-to -background ratio (SBR) of the CARS signal to the NB can be improved as designed.