A multi-factor improvement strategy for heterodyne coherent anti-Stokes Raman scattering

Abstract To exploit the advantages of heterodyne coherent anti-Stokes Raman scattering (hCARS) for a wider range of applications, a multi-factor improvement strategy is presented that can be applied in its entirety or in parts, depending on the available laboratory infrastructure. One key factor of this strategy is the generation of the local oscillator in combination with the pump and Stokes pulses internally within a single light source. This mechanism allows for a high local oscillator power of more than 400 nW and a fixed phase relation of the local oscillator with respect to the pump and Stokes pulses. Another feature of the strategy is that the local oscillator adjusts automatically to the appropriate wavelength across the whole tuning range of the light source between 1300 and 3000 cm −1 , allowing for a simple setup without the need for realignment. A further key factor is an active phase stabilization scheme, enabling long-term measurements over more than 1.5 h with interferometric stability without the need for realignment at different wavenumbers. A nonresonant CARS signal provides the reference for the stabilization. For verification, the setup is applied to hCARS microscopy with 5-fold image contrast improvement due to the suppression of nonresonant CARS signal.