Novel structure of an ultra-narrow-bandwidth fibre laser based on cascade filters: PGFBG and SA

An ultra-narrow-linewidth single-longitudinal-mode (SLM) erbium-doped fibre (EDF) ring laser based on cascade filters, which include a phase-gap fibre Bragg grating (PGFBG) and a saturable absorber (SA) is proposed and demonstrated. A PGFBG is used as the first filter to coarsely select the longitudinal mode, the second filter is an SA, which selects the SLM from a few existing longitudinal modes after the first filter by creating a self-writing filter. We measure the linewidth of the fibre ring laser using the delayed self-heterodyne method. The result is approximately 1.2 KHz with a 3 dB linewidth. This fibre laser also has high laser pump power efficiency: the optical to optical efficiency is approximately 15% for a sufficiently long EDF. The stable single linear polarization SLM output of this fibre laser is ensured using polarization-maintaining (PM) devices and the SA.