Yb³⁺-Doped Non-π-Phase-Shifted Distributed FeedbackFibre Lasers

Distributed feedback (DFB) fibre lasers usually have two longitudinal modes symmetrically located at each side of the Bragg wavelength because they have the same lowest gain threshold. In order to obtain single longitudinal output, π-phase-shift, DFB fibre lasers were often adopted. However, we found that, even with no π-phase-shift, the single frequency output can be acquired from Yb³⁺-doped fibre DFB lasers. The polarization beat frequency was measured to be 366 MHz.