Spectro-temporal evolution of mode-locked lasing in fiber frequency-shifted feedback laser

As a type of mode-locked fiber laser, fiber frequency-shifted feedback lasers (FSFLs) have been rarely studied regarding the buildup process of mode locking. We carried out an experimental investigation to show the unique characteristics of the process, such as the direct evolution of the Q switched mode-locked state to the mode-locked state, the unique phenomenon of spectral center frequency oscillation, evolution to a double-pulse state in harmonic mode locking, millisecond stabilization time, and exponentially decreasing change of relative phase per round trip. For the first time, to the best of our knowledge, the double-beam heterodyne method is applied to measure the evolution of laser spectra with pulse width in the nanosecond range. Understanding the unique buildup mechanism may aid in the engineering and application of FSFLs.