Studies of supercontinuum generation in a photonic crystal fiber for use as Stokes beam in a single-laser femtosecond micro-CARS setup

Photonic crystal fibers (PCFs) have gained popularity thanks to the use of their broad and efficient supercontinuum (SC) emission as Stokes source in ultrafast multiplex coherent anti-Stokes Raman scattering microscopy. Here, the infrared portion of the SC emitted by a widely used PCF is studied in a basic CARS experiment with a single laser oscillator operated at various conditions of power and chirp values. The dependence of the SC and CARS signal of reference cyclohexane sample on the operative conditions of the laser is examined and illustrated by means of statistical approaches that serve the purpose of defining and quantifying the reliability of CARS measurements in relation to the characteristic of the Stokes pulse coming from the fiber. Statistical techniques based on principal component analysis, on calculation of statistical momenta of the SC spectra, and on Allan variance studies are introduced to help to identify the best working regime. Principal component analysis was used to quantify the correlation of SC spectra with PCF input power, which is found to be stronger at high laser beam chirp values. First and second momenta of SC spectra allow identification of best operating conditions with respect to peak emission wavelength and spectral width. Finally, the behavior of Allan variance shows that the time stability of Stokes and CARS signals clearly depends on the chirp of the pump pulse, evidencing that high chirp conditions correspond to lower variance values at the expense of stability for longer measurement times.