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Tuneable and stable multi-wavelength thulium-doped ring-cavity fibre laser based on Sagnac loop and Mach–Zehnder filter utilizing thin-core fibre

A multi-wavelength thulium-doped fibre laser based on a Sagnac loop and a Mach–Zehnder filter was designed and experimentally demonstrated. In this ring-cavity fibre laser, the Sagnac loop was composed of 2 m-long polarization-maintaining fibres, and the pigtail fibre core/cladding diameter of all components was 10/125 µm. A 4 m-long thin-core fibre with a 5/130 µm core/cladding diameter was spliced into the cavity to obtain the Mach–Zehnder structure. The wavelength spacings of the comb spectra of the Sagnac loop and the Mach–Zehnder filter were 3.2 nm and 20.3 nm, respectively. When the pump power was 150 mW, single-wavelength lasing occurred with 3 nm-wavelength spacing within the 1857–1897 nm range; the maximum power difference was less than 1.27 dB; the side-mode suppression ratio was greater than 32.68 dB; and the peak power fluctuations, measured over a 10 min interval, were less than 0.2 dB. A tuneable dual-wavelength lasing mode was also realized by adjusting the polarization controllers, with emission occurring simultaneously at 1874 and 1891 nm. In this case, the power fluctuations were less than 1.36 dB and 0.53 dB, respectively. A tuneable triple-wavelength lasing mode was also demonstrated, with a side-mode suppression ratio greater than 34.51 dB.