Six-Wavelength-Switchable SLM Thulium-Doped Fiber Laser Enabled by Sampled FBGs and 3 × 3 Coupler Based Dual-Ring Compound Cavity Filter

A switchable and tunable six-wavelength single-longitudinal mode (SLM) thulium-doped fiber laser in the 2050 nm band is proposed and experimentally demonstrated. Two cascaded sampled fiber Bragg gratings (SFBGs), acted as an integrated six-channel reflector, are to define the lasing wavelength channels in a multi-wavelength switchable fiber laser for the first time. The switchability of the fiber laser was realized by polarization-dependent loss modulation. SLM lasing in each channel was guaranteed by a novel passive dual-ring compound cavity (DRCC), composed of two symmetric 3 × 3 optical couplers (OCs). Six single-wavelength operations are easily obtained and switched, with the high optical signal-to-noise ratios of >71 dB, >72 dB, >62 dB for
λ
₁₁
–
λ
₁₃
and > 65 dB, >61 dB, >59 dB for
λ
₂₁
–
λ
_23,
respectively. The maximum power and wavelength fluctuations are as low as 0.56 dB and 0.01 nm, respectively. The measured laser linewidths ranged from 7.5 to 9.2 kHz, calculated by the β-separation line method based on the measured frequency noise spectra. Nine switchable dual-wavelength operations with different wavelength intervals and two triple-wavelength operations were also obtained. Our proposed fiber laser has great potential in free-space optical communication.