Double-path ring cavity erbium-doped random fibre laser based on Rayleigh scattering generated by single mode fibre

This study proposes a erbium-doped random fibre laser (RFL) based on a double-path ring cavity. Random laser (RL) emission was realised via random Rayleigh scattering (RS) in a long-distance passive single-mode fibre (SMF). Double-path ring cavities were constructed using two 50:50 couplers, OC1 (optical couplers) and OC2. To achieve a stronger RFL output, Port1 and Port2 were connected to realise the construction of a double-ring cavity, which makes full use of the bidirectional RS of the SMF, enhances the RFL output, and reduces the laser output threshold. Experiments demonstrated that stable single wavelength RFL outputs of 1558.3 and 1561.8 nm can be achieved by paths 1 and 2. The laser thresholds were 46 and 14 mW, and the slope efficiencies of the total output power to pump power were 0.2 % and 0.7 %, respectively. The linearity of the pump-to-laser output power curves were 99.04 % and 99.18 %, respectively. The power displacement of path 1 was less than 0.74 dB, the wavelength fluctuation was less than 0.41 nm, and the signal-to-noise ratio (SNR) was 27.69 dB. The power displacement of path 2 was less than 0.53 dB, the wavelength fluctuation was less than 0.40 nm, and the SNR was as high as 40.20 dB. With the increase of pump power to 258 mW, the wavelength shifts of paths 1 and 2 were less than 0.46 and 0.51 nm, respectively. RFL have broad application prospects in optical fibre communications and speckle imaging.