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A modified polarization-maintaining Sagnac loop mirror is proposed, and the simulated transmission spectrum of the proposed filter is in accordance with experimental transmission spectrum. Based on the proposed filter, a multi-wavelength thulium-doped fiber laser is constructed and experimentally demonstrated. The stable multi-wavelength oscillation within 60 minutes is achieved through the nonlinear polarization rotation effect enhanced by the highly nonlinear fiber. By fine tuning the polarization state, up to 35 lasing lines are obtained within a 10-dB bandwidth with wavelength separation of ∼1.38 nm at room temperature. The wavelength shift and the output power fluctuation are less than 0.04 nm and 1.69 dB, respectively. In addition, by changing the optical coupling ratio in the Sagnac loop mirror, different outputs with 7, 8, 16 lasing lines are also obtained.

A wavelength-switchable single-longitudinal-mode thulium-doped fiber laser using a sampled fiber Bragg grating is proposed in this study. The laser can be switched between three single-wavelength and three dual-wavelength modes of operation. For the single-wavelength mode of operation, the maximum power and wavelength fluctuations in 50 min were ±0.404 dB and ±0.01 nm, and the linewidths at three wavelengths were 0.39, 1.88, and 0.89 kHz, respectively. For the dual-wavelength mode of operation, the maximum power and wavelength fluctuations in 20 min were ±0.926 dB and ±0.03 nm, respectively. The proposed thulium-doped fiber laser will find useful application in optical communication and free-space optical detection and sensing.

We propose and experimentally demonstrate a multi-wavelength thulium-doped fiber laser using two cascaded single-mode-four-mode-single-mode (SFS) fiber interferometers and a 150-m-long highly nonlinear fiber. The transmission properties of SFS fiber interferometers with two different four-mode fiber (FMF) lengths are analyzed theoretically. The interferometer with a longer FMF is used to select specific wavelengths, and the one with a shorter FMF is used to stimulate more laser lines. By adjusting the polarization controller and increasing the pump power, the number of output laser channels and the wavelength spacings can be changed and switched. By selecting the proper length of the FMFs, 31 lasing lines are achieved with covering a wavelength range of $\sim 22$ nm. The power fluctuation and the wavelength drift of the 31-wavelength operation for the proposed fiber laser are within 0.706 dB and 0.02 nm, respectively.

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.

We propose and demonstrate a high performance four-wavelength erbium-doped fiber laser (EDFL), enabled by a figure-8 compound-ring-cavity (F8-CRC) filter for single-longitudinal-mode (SLM) selection and a polarization-managed four-channel filter (PM-FCF) for defining four lasing wavelengths. We introduce a novel methodology utilizing signal-flow graph combined with Mason's rule to analyze a CRC filter in general and apply it to obtain the important design parameters for the F8-CRC filter used in this paper. By combining the functions of the F8-CRC filter and the PM-FCF filter assisted by the enhanced polarization hole-burning and polarization dependent loss, we achieve the EDFL with fifteen lasing states, including four single-, six dual-, four tri- and one quad-wavelength lasing operations. In particular, all the four single-wavelength operations are in stable SLM oscillation, typically with a linewidth of =3 MHz and an output power fluctuation of, 21 pages, 14 figures

A high-performance switchable 0.612-nm-spaced dual-wavelength erbium-doped fiber (EDF) laser is proposed and experimentally demonstrated. For the first time, we combine a compound-cavity structure (CCS) and an enhanced polarization hole burning (PHB) effect in laser cavity to obtain stable dual-wavelength lasing ( λ 1 and λ 2) and single-longitudinal-mode (SLM) operation at each wavelength without mode hopping. In the CCS, a high-quality triple-ring passive resonator is employed to achieve SLM lasing and a high-birefringence fiber Bragg grating combined with a three-loop polarization controller made with a length of EDF is used to establish a strong PHB effect. Easy switch between dual- and single-wavelength lasing modes is achieved. At a pump power of 150 mW, we obtain dual-wavelength lasing with optical signal-to-noise ratios (OSNRs) of >84 dB for both wavelengths and linewidths of 663 Hz for λ 1 and 768 Hz for λ 2, respectively; we also obtain single-wavelength lasing at λ 1 with an OSNR of >86 dB and a linewidth of 687 Hz or at λ 2 with an OSNR of >88 dB and a linewidth of 678 Hz. For both dual- and single-wavelength operations, the two lasing wavelengths are orthogonally polarized, with a degree of polarization of ∼100%, and their relative intensity noises are

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 continuous wave, switchable multi-wavelength, thulium-doped fiber laser (TDFL) with a polarization-maintaining sampled fiber Bragg grating (PM-SFBG) is proposed and demonstrated for the first time. A length of 150 m highly-nonlinear fiber (HNLF) was used to introduce the four-wave mixing effect for the efficient suppression of the wavelength competition existing in the gain medium of thulium-doped fiber. By adjusting the state of polarization (SOP) of light in the laser cavity, two six-wavelength operations at two orthogonal SOPs and a ten-wavelength operation were obtained. When the TDFL ran at the six-wavelength operation mode with a pump power of 4.11 W, there were at least four lasing wavelengths with an optical signal-to-noise ratio (OSNR) higher than 30 dB, and when it ran at the ten-wavelength operation mode with a pump power of 4.60 W, there were nine lasing wavelengths with an OSNR higher than 30 dB. Regardless of the operation mode, the TDFL exhibited high stability.

This research experimentally demonstrates a switchable, single-wavelength, thulium-doped fiber laser based on the cascading of a multimode–single-mode–multimode (MSM) fiber filter and a two-mode fiber (TMF) filter. When the MSM fiber filter suffers from bending, the blue-shift of the output spectrum can be obtained. A switchable lasing wavelength output is realized by bending the MSM fiber filter to cover different channels of the TMF filter. The output wavelength can be switched from 1982.54 to 1938.81 nm with an optical signal-to-noise ratio of higher than 40 dB. The wavelength interval of the switchable output is an integral multiple of the wavelength interval of the TMF filter. The stability of the output wavelength was tested within 60 min, and the wavelength shift and output power fluctuation were found to be less than 0.01 nm and 0.31 dB, respectively, which demonstrates a stable output performance.

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 cladding pumped thulium-doped fiber laser with tunable and switchable output was demonstrated. The single-mode-multimode-single-mode (SMS) fiber filters with different lengths of multimode fiber were fabricated and the tunability of the laser by axial strain with a sensitivity of ∼ 2.4 pm/μe was demonstrated experimentally. Single-wavelength output with an optical signal-to-noise ratio (OSNR) of up to 70 dB and a small tuning range of 2.76 nm was obtained. The fluctuations of the center wavelength and output power within 60 min were less than 0.04 nm and 0.27 dB, respectively, and the tuning sensitivity was ∼ 2.3 pm/μe. When the laser was operated in the dual-wavelength output mode, the OSNR reached 60 dB and the fluctuation of the output power was less than 1.53 dB within an observation time of 60 min. Furthermore, the output power was scaled up to 308 mW by changing the output coupling ratio and the different samples of SMS fiber filter were also applied in the fiber laser cavity to obtain different lasing channels. The proposed fiber laser is suitable for applications in sensing and communication systems.

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.

A high-performance wavelength-switchable single-longitudinal-mode (SLM) fiber laser with ultra-narrow linewidth and high stability in C-band was proposed and demonstrated, mainly benefiting from the use of a wide-band chirped Moire fiber Bragg grating (WB-CM-FBG) with 41 ultra-narrow channels. By combining the WB-CM-FBG with a manual tunable bandpass filter and using a subring cavity with a length of only 34 cm to efficiently stabilize the SLM lasing, we achieved switchable lasing operation among 41 wavelengths, with a switching-step of ~0.96 nm from 1526.592 nm to 1565.126 nm, covering the whole C-band, and obtained sub-kHz linewidth at every lasing wavelength. The high stability was validated by the measured slight fluctuations of lasing wavelength and optical power of