Tunable multi-wavelength random fiber laser based on a hybrid filter composed of Mach–Zehnder interferometer and Sagnac loop

A tunable and switchable multi-wavelength linear half-open cavity erbium-doped random fiber laser (RFL) based on a hybrid filter composed of tunable Mach-Zehnder interferometer (MZI) and Sagnac loop was proposed. The MZI incorporating optical delay line (ODL) was embedded within the Sagnac loop to form a hybrid filter. Random distributed feedback (RDFB) was provided by backward Rayleigh scattering generated by single-mode fiber (SMF). The laser showed a threshold of 61 mW and a slope efficiency of approximately 2.15 %. With the ODL delay fixed and pump power set at 248 mW, three distinct lasing states were achieved by adjusting the polarization controller (PC), corresponding to 9-, 6-, and 3-channel multi-wavelength random outputs. When the ODL delay time was increased, the free spectral range (FSR) of the MZI decreased, compressing the spectral bandwidth and reducing the number of output channels to 5, 4, and 2, respectively. Additional lasing states were realized by further adjusting the ODL and PC. The laser output remained stable, with an intensity standard deviation–to–mean ratio of 0.276 % and a maximum power fluctuation of 0.141 mW. The proposed RFL featured a compact structure, stable multi-wavelength output, and flexible tunable, showing promise for applications in distributed sensing and imaging systems.