Tunable triple-wavelength mode-locked ytterbium fiber laser with birefringence filter

A mode-locked ytterbium fiber laser based on all normal dispersion (ANDi) technique and nonlinear polarization rotation (NPR) is investigated with birefringent plate as a spectral filter in this paper. Tunable triple-wavelength simultaneous operation is realized at central wavelengths of 1031.4, 1046.8 and 1066.5 nm, respectively. A maximum output power up to 161.4 mW is obtained at 1031.4 nm wavelength. By only changing the direction of the optical axis of the uniaxial birefringent filter, tunable range as wide as 19 nm is demonstrated. To the best of our knowledge, this is the widest tuning range and highest power reported so far for ANDi mode-locked ytterbium fiber lasers with triple-wavelength operation. Our analysis shows that the mechanism of the triple-wavelength mode-locked operation results mainly from the four-wave mixing in optical fibers; in addition, the wide gain bandwidth of Yb also plays a critical role for the stable oscillation of the third mode-locked wavelength. The energy conversion process governed by a set of four coupled equations is realized experimentally through the adjustment of the polarization controller, which is related to the phase matching.