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A single-longitudinal-mode (SLM), narrow linewidth thulium-doped fiber laser with a sampled fiber Bragg grating (SFBG), switchable among three wavelengths, with a cascade dual-coupler-ring-based compound cavity (DCR-CC) filter, is proposed and demonstrated. The coupling design, simulation analysis, and characterization of the DCR-CC filter provide the foundation for the experiment. A nonlinear polarization rotation system was injected into the cavity to suppress gain competition and achieve a laser switchable among three wavelengths. The fluctuations of the wavelength and power of the output laser are less than 0.60 nm and 0.91 dBm, respectively. For demonstration, the laser maintained in SLM operation measured by the delayed self-heterodyne method has a linewidth of < 3.7 k H z and relative intensity noise of < − 114 d B / H z .

A switchable multi-wavelength thulium-doped fiber laser (TDFL) using a four-mode fiber based Sagnac loop filter was proposed and demonstrated. The Sagnac loop, incorporating a 4.8 m long four-mode step-index fiber, acting as a comb filter was analyzed theoretically and experimentally. By adjusting two polarization controllers, eight stable single-wavelength operations and five stable dual-wavelength operations were obtained with easy switching between the different operation modes. The wavelength range covered during the single-wavelength switchable operation with a signal-to-noise ratio of up to 55 dB was ∼28.51 nm. In dual-wavelength switchable operation, the maximum and minimum wavelength spacings were ∼21.78 nm and ∼1.50 nm, respectively. Due to the abundant wavelength resources, the proposed TDFL has great potential in wavelength division multiplexing free-space optical communications, optical fiber sensing, and photonic generation of microwave signals.

For the first time, to the best of the authors’ knowledge, the transmission characteristics of sampled fiber Bragg grating (SFBG) and phase-shifted SFBG (PS-SFBG) in the 2 μ m band were investigated theoretically and experimentally in detail. Using the transmission matrix method, the dependence of the transmission spectra of the SFBG and PS-SFBG on different grating parameters was simulated; the grade-zero bandwidths of the SFBG transmission peak and the PS-SFBG phase-shifted peak were as narrow as 0.076 and 0.022 nm respectively. Based on the theoretical analysis results, we experimentally fabricated the gratings with three narrow peaks. The results showed that the grade-0 transmission peak bandwidth of the SFBG and the phase-shifted-peak bandwidth of the PS-SFBG were narrower than 0.06 and 0.09 nm, respectively, indicating that the gratings are excellent filters for 2 μ m band multi-wavelength narrow-linewidth fiber lasers.

A wavelength-interval-switchable multi-wavelength thulium-doped fiber laser (MWTDFL) is proposed and demonstrated, based on a nonlinear dual-pass Mach-Zehnder interferometer (NDP-MZI) filter for the first time. We investigate the NDP-MZI filter theoretically and experimentally, which has both the transmission characteristics of a dual-pass Mach-Zehnder interferometer (DP-MZI) for wavelength selection and a nonlinear optical loop mirror (NOLM) for suppressing wavelength competition. By tuning two polarization controllers (PCs), two 20-wavelength operations with an opposite phase and a same wavelength-interval of 0.46 nm are obtained and switched between each other flexibly. The maximum power fluctuation and wavelength drift measured are 0.612 dB and 0.04 nm, respectively. In addition, due to the power-equalizing effect resulting from the NOLM, 60 lasing wavelengths within a 3-dB bandwidth with an adjacent wavelength-interval of 0.23 nm are also obtained by adjusting the PCs carefully, and the stability of the MWTDFL is measured experimentally. Furthermore, the performance of the NDP-MZI using an 80 m highly nonlinear fiber has been studied in detail as well. The proposed MWTDFL may find great applications in optical communication and optical sensing.

A compound-cavity-based six-wavelength-switchable single-longitudinal-mode (SLM) thulium-doped fiber laser (TDFL) with a homemade polarization-maintaining sampled fiber Bragg grating (PM-SFBG) is proposed and demonstrated. The PM-SFBG in the 2 μm band is studied theoretically and experimentally, and is utilized as a polarization-dependent multi-channel reflecting filter in a multi-wavelength-switchable fiber laser, both for the first time. The SLM lasing in each channel is guaranteed by using a tri-ring sub-cavity. 6 single-wavelength operations are easily obtained and switched each other, and the maximum power and wavelength fluctuations are as low as 0.71 dB and 0.02 nm, respectively. The frequency noise of each lasing wavelength is measured through an unbalanced Michelson interferometry system, and the linewidths for all six lasing wavelengths are among 0.60–1.08 kHz, calculated by the β-separation line method on the basis of the measured frequency noise spectra. Benefiting from the enhanced polarization hole burning effect formed in the gain fiber, 9 switchable dual-wavelength operations with different wavelength intervals and orthogonal polarizations at two lasing wavelengths are obtained with the maximum power and wavelength fluctuations of only 0.95 dB and 0.03 nm, respectively. We believe the proposed TDFL will find great applications as an ideal light source in free space optical communication and optical sensing.

A continuous wave (CW) wavelength-tunable Tm3+-doped fiber laser with two sampled fiber Bragg gratings (SFBGs) has been proposed and demonstrated. This proposed laser is stable operating with a significant 14.44 nm wavelength range from 1942.09 nm to 1956.53 nm due to the Vernier effect. The laser OSNRs of different wavelengths ranged from 45 dB to 58 dB at a launched pump power of 3.67 W. The wavelength shift and power fluctuation were less than 0.03 nm and 0.46 dB, respectively. An 8.62% slope efficiency was achieved using a grating for laser extracting. To the authors’ knowledge, this is the first time that a wavelength-tunable Tm3+-doped fiber laser with two SFBGs has been demonstrated.

For the first time, to the best of our knowledge, a high stability half-open cavity multiwavelength random erbium-doped fiber laser (MW-REDFL) using distributed Rayleigh backscattering feedback in a 2 km long single-mode fiber together with a reflecting comb-filter of six-superimposed fiber-Bragg-gratings (6SI-FBG) was demonstrated experimentally. We achieved six lasing wavelength-channels at ∼2 nm spacing, with a maximum peak power difference of ≤0.930 dB for all six channels and a peak power fluctuation of ≤1.101 dB for each lasing channel in a 80 min time span, a wavelength fluctuation of ≤0.032 nm for each lasing channel in an experimental time of >1.5 h, and an output power fluctuation of ≤0.05 dB in a 4 h time span at the pump power of either 100 mW or 200 mW. At the frequencies larger than 100 kHz, the relative intensity noises of six-wavelength lasing output are −125 dB/Hz and −120 dB/Hz for pump powers of 100 mW and 200 mW, respectively. The performances threshold as low as 23.51 mW and slope efficiency as high as 10.99% of our MW-REDFL were also achieved. Better performance could be further obtained with good temperature compensation and vibration isolation packaging for practical applications.