Intracavity birefringence-controlled GHz-tuning range passively harmonic mode-locked fiber laser based on NPR

We experimentally demonstrate a harmonic-order controllable L-band Er-doped passively mode-locked fiber laser based on nonlinear polarization rotation (NPR). Distinct from all previous reports, we find that the intracavity birefringence is able to control the harmonic order of a passively mode-locked fiber laser. Experimentally, under a constant pump power of 704 mW, the harmonic order can be tuned from 113th to 39th monotonically by adjusting the polarization controller orientation only. The corresponding repetition rate changes from 2.21 to 0.77 GHz simultaneously. Remarkably, the super-mode suppression ratio of each harmonic order we observed is higher than 29 dB with a maximum of 36.5 dB. Simulated transmission spectra of NPR prove that the changed transmission plays an important role in controlling the harmonic order.