High performance multimodal CARS microscopy using a single femtosecond source

摘要

We demonstrate high performance coherent anti-Stokes Raman scattering (CARS) microscopy using a single femtosecond Ti:Sapphire laser source combined with a photonic crystal fiber (PCF). By adjusting the chirp of the femtosecond pump and Stokes laser pulses, we achieve high quality multimodal imaging (simultaneous CARS, two-photon fluorescence, and second harmonic generation) of live cells and tissues. The tuneable Ti:sapphire output provides the pump beam directly, while part of this is converted to the red-shifted Stokes pulse using a PCF having two close-lying zero dispersion wavelengths. This PCF gives good power and stability over Stokes shifts ranging from below 2300 cm~(-1) to over 4000 cm~(-1). This tuning range can be accessed by simply controlling the time delay between the input pulses. This allows fast, continuous computer-controlled tuning of the Stokes shift over a broad range, without involving any adjustment of either the femtosecond laser or the PCF. The simultaneous optimization of CARS, two-photon fluorescence and second harmonic generation is achieved by controlling the degree of chirp and involves a trade-off between spectral resolution of the CARS process and signal strength. This is illustrated by showing applications of the multimodal CARS imaging and optimization technique to biomedical problems involving both live cells and tissues.