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Tricolor synchronized ultrafast fiber laser for coherent anti-Stokes Raman scattering imaging
Summary
Scientists built a compact laser system that can take detailed chemical "fingerprint" images of tiny structures—like individual cells, skin layers, and even microplastic particles—without needing to add dyes or stains. This matters because it could help researchers better study how microplastics interact with human tissue (like skin) and track other health-relevant substances at the cellular level, using a more practical, all-in-one tool than previous complex laser setups.
Coherent anti-Stokes Raman scattering (CARS) microscopy requires synchronized excitation pulses with sufficient peak power, and broad pump-Stokes frequency coverage. We demonstrate a tricolor synchronized ultrafast fiber laser derived from a common 1.55 μm mode-locked Er-fiber oscillator. The master branch is frequency doubled to generate a 775 nm picosecond pulse train for fingerprint-region CARS. A second branch produces a 1063 nm picosecond pulse train through cross-phase-modulation-synchronized mode locking in a Yb-fiber oscillator, followed by fiber amplification and compression. A third branch combines large-mode-area Er/Yb Raman amplification with soliton self-frequency shift to generate high-energy femtosecond pulses at 1600-1840 nm, which are subsequently frequency doubled to provide tunable 805-920 nm femtosecond pulses. The system delivers approximately 150 mW at 775 nm, more than 1 W at 1063 nm, 1.6 W at 1840 nm with a pulse energy of 29.57 nJ, and up to 362 mW in the 805-920 nm band. The 775 nm and 805-920 nm pulse pair cover Raman shifts of approximately 480-2033 cm -1 , while the 805-920 nm and 1063 nm pulse pair cover approximately 1462-3015 cm -1 . CARS spectroscopy and imaging of ethanol, dimethyl sulfoxide, polystyrene microplastic particles, avocado pulp, onion cells, and human stratum corneum verify chemically selective imaging in both the fingerprint and high-wavenumber regions. The proposed architecture provides a compact fiber-dominant synchronized source for broadband CARS microscopy.