All-polarization-maintaining figure-eight Er-fiber ultrafast laser with a bidirectional output coupler in the loss-imbalanced nonlinear optical loop mirror

In this Letter, we report on a novel architecture for a self-starting mode-locked figure-eight erbium-doped fiber laser using a loss-imbalanced nonlinear optical loop mirror (NOLM) with a bidirectional output coupler. An all-polarization-maintaining structure is adopted. A 2 × 2 optical coupler with a splitting ratio of 50:50 is used at the junction to form an NOLM. Another coupler with a splitting ratio of 10:90 is introduced at one end of the fiber loop. The 10:90 coupler plays two roles: power attenuator and bidirectional output coupler. This architecture can achieve both large modulation depth and good self-starting ability simultaneously. With this architecture, the self-starting mode-locking operation is achieved easily with pump power above the threshold. The clockwise and counter-clockwise mode-locked output powers are 10.1 and 10.3 mW, respectively, with the repetition rate of 3.63 MHz. The spectral bandwidths of the clockwise and counter-clockwise mode-locked output pulses are 7.4 and 2.9 nm, and the corresponding pulse widths of the direct outputs are 530.6 fs and 1.55 ps, respectively.