Passive-synchronized picosecond fiber lasers for coherent anti-Stokes Raman imaging

Coherent anti-Stokes Raman scattering (CARS) can be used to excite vibrational bonds with chemical selectivity, high spatial and spectral resolution, and high sensitivity, which has many applications in biomedical research. The common way to realize CARS imaging is illuminating sample with two synchronized ultra-short pulses. Recent development of various fiber laser schemes based on nonlinear optical effect provides compact laser source for CARS imaging. However, the nonlinear conversion in optical fiber may inevitably introduce temporal or spectral noise to the newly generated pulses. In this paper, we have proposed a polarization-maintaining (PM) passive-synchronized picosecond fiber laser system that generates dual-color picosecond pulses for CARS. An Er-doped fiber laser and a Yb-doped fiber laser were passively synchronized by cross phase modulation based on master-slave injection scheme. In the experiment, the wavelength of one branch was fixed at 791 nm, which was generated by second harmonic generation of Er-doped fiber laser. The wavelength of the other branch can be continuously tuned from 1017-1047 nm, which was obtained by adding an active spectral broadening module and an optical bandpass filter after Yb-doped fiber laser. As a result, the laser source allows to probe vibrational bonds with frequencies difference between 2809 cm^-1and 3091 cm^-1. Finally, the achieved tunable synchronized pulses enabled us to microscopically image mouse ear samples. The compact optical fiber laser proposed with PM fiber design, stable synchronization and large wavelength tunability would become a promising laser source for CARS imaging in clinical use.