Comparison of SC fibers for fs Ti:Sapphire based hyperspectral CARS microscopy

Hyperspectral coherent anti-Stokes Raman scattering (CARS) microscopy is a rapidly developing field enabling label-free, chemically selective bio-imaging based on Raman signatures [1]. A significant factor limiting its clinical application is the complexity of current laser sources. A solution immediately relevant to two-photon excited fluorescence imaging laboratories is coherently broadening a fs Ti :Sapphire laser seed in a fiber to provide the Raman wavelengths via spectral-focusing (SF) CARS. The NKT fiber with two Zero Dispersion Wavelengths (ZDWs) is one option but the spectrum exhibits low Power Spectral Density (PSD) because of the large (>800 nm) spectral broadening. Here we perform the first systematic comparison sweeping (i) input pump power, (ii) pump wavelength and (iii) fiber length comparing the coherent SC from a femtoWHITE-CARS (2 ZDWs) fiber, a fiber with one ZDW offset above the seed wavelength, and an all-normal dispersion (ANDi) fiber. Starting with the seed laser polarisation aligned to a principal fiber axis, we show the total experimentally measured spectral output and importantly the polarisation resolved spectral component on the orthogonal axis, which is a measure of the nonlinear power-dependent depolarisation[2]. This orthogonal component will degrade the efficiency of the CARS signal but still contributes to the bio-toxicity that limits the maximum power for imaging. Finally, we show representative SF-CARS microscopy images to showcase the power of this technique.