Comparison of two photonic crystal fibers for supercontinuum-Stokes spectral-focusing-CARS hyperspectroscopy

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A popular and inexpensive experimental approach to broadband CARS microscopy involves the generation of a supercontinuum pulse from a microstructured fibre. Despite such fibres being used for CARS microscopy for over a decade, concerns about excessive spectral and polarization noise have prevented their wider adoption for this technique. In this work, the applicability of two commercially-available supercontinuum generating fibres, the NL-1.4-775-945 and the NL-PM-750 in packaged module form, are compared for spectral-focusing CARS (SF-CARS) imaging. The former has been a mainstay of SF-CARS implementations while the latter has largely found application in a related but distinct broadband CARS technique and has not been used for SF-CARS applications. We first characterize the intensity and polarization behaviour of the supercontinuum output of the fibres and demonstrate that while both can be used to produce high-quality CARS images and spectra, key operational differences between the fibres must be taken into consideration when optimizing their use for SF-CARS applications. In particular, we show via spectroscopy of a solvent sample and hyperspectral imaging of a pharmaceutical sample that the NL-1.4-775-945 is inferior for SF-CARS applications when used under fixed-power supercontinuum generation conditions. However, its operation can be significantly improved under power-tuned supercontinuum generation conditions known as spectral-surfing, an approach that does not confer the same advantage with the NL-PM-750. Furthermore, we identify a latent polarization-dependent spectral artifact in the NL-PM-750, which can degrade the CARS spectrum in time-gated techniques such as SF-CARS.