Widely tunable narrow-linewidth mode-locked fiber laser via inter-cavity field interference

A widely and continuously tunable narrow-linewidth mode-locked fiber laser is reported with a record pulse-duration tuning range from 342 ps to 4.02 ns. This is realized through mode-locking a fiber laser intra-cavity coupled to a high-Q aluminum-nitride microresonator. By controlling inter-cavity fields interference, the microresonator acts as an active filter, allowing the spectral linewidth of the fiber laser to be tunable from 1.34 GHz to 122 MHz. The compressed laser linewidth of 122 MHz is three times narrower than the microresonator's 370-MHz intrinsic resonance bandwidth. This kind of mode-locked fiber lasers with large and continuous tuning ranges in both linewidth and pulse duration not only provide a new mechanism for linewidth narrowing but also have application potentials in various areas like LiDAR, spectroscopy, and quantum optics.