Passively synchronized mode-locked fiber lasers for coherent anti-Stokes Raman imaging

We have proposed and implemented a polarization-maintaining passively synchronized fiber laser system, which could deliver tunable dual-color picosecond pulses by including a frequency-doubling module and a spectral broadening module. Specifically, the output from the involved Er-doped fiber laser were used to generate second-harmonic pulses at 790 nm with a quadratic nonlinear crystal. In parallel, the amplified pulses from the synchronized Yb-doped fiber laser were launched into a 150-m single mode fiber, which resulted in not only substantial spectral bandwidth broadening from 0.1 to 20.1 nm, but also a significant Raman-induced signal around 1080 nm. Consequently, narrow spectra from 1018-1051 nm and 1070-1095 nm could be continuously tuned via a tunable bandpass filter, corresponding to Raman bonds from 2835-3143 cm⁻¹ and 3312-3525 cm⁻¹. Finally, the achieved tunable synchronized pulses enabled us to microscopically examine mouse ear samples based on coherent anti-Stokes Raman and second harmonic generation imaging. Therefore, our tunable passively-synchronized fiber laser system would be promising to provide a simple and compact laser source for subsequent coherent Raman microscopy.