Mode-locked fibre lasers generating ultrashort pulses with the advantages of robustness, compactness and excellent beam quality are of tremendous interest in applications such as laser material processing, medicine, precision measurement, biological photonics, ultrafast spectroscopy, optical communication and scientific research. In recent decades, mode-locked ultrafast fibre lasers operating in the near-infrared and mid-infrared spectral regions have been well developed, but ultrafast laser sources in the visible spectral region (380-760 nm) still heavily rely on Ti:sapphire mode-locked oscillator and optical parametric amplification systems (or frequency doubling of near-infrared ultrafast lasers), suffering from a large footprint and an extremely high cost. Researchers desire an alternative ultrafast visible laser solution that is compact, low cost, user friendly and maintenance free. Passive mode locking in all-fibre format could satisfy all these demands, and therefore, there is strong research motivation to develop passively mode-locked fibre lasers in the visible region.