In order to realize single longitudinal mode (SLM) output, an erbium-doped fiber (EDF) laser with fiber ring cavity is proposed which uses an un-pumped EDF saturated absorber as a SLM selector. Ring cavity length, coupler ratio and insertion loss which affect ring cavity filter performance are analyzed theoretically and simulated numerically. Un-pumped EDF length and Er3+ concentration are analyzed theoretically and simulated. Research shows that, the bigger coupling output ratio of ring cavity, the greater transmission peak; the greater mode line width; the shorter ring cavity length, the wider free spectral range; the bigger coupling loss, the smaller transmission peak; when the difference between two cavities length is big, cursor effect is not obvious. Laser threshold increases with absorber length increasing; the bigger saturated absorber concentration, the higher laser threshold.
Long cavity length composite cavity fibre laser with single longitudinal mode and narrow linewidth