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In this paper, we demonstrate a simple structure, ultra-narrow linewidth, extremely high stability and high optical signal to noise ratio single-longitudinal-mode (SLM) erbium-doped fiber laser (EDFL). Nested Fiber Ring (NFR), nested in the ring cavity of EDFL, is employed to serve as a high quality mode filter to eliminate the dense longitudinal mode of the EDFL. The mode-selection capability of the RIR is investigated and discussed theoretically and experimentally. Saturable absorber, which composes of a section of unpumped polarization maintaining erbium-doped fiber and a Fabry–Perot fiber Bragg grating (FP-FBG), is employed to produce a dynamic narrowband grating to ensure the SLM laser output. Saturable absorber, consisted of a section of unpumped polarization maintaining erbium-doped fiber, produces a dynamic narrowband grating to ensure the SLM laser output. We achieve a ultra-narrow linewidth SLM laser output of 550 Hz. The laser has an optical signal-to-noise ratio (OSNR) of more than 59 dB. In addition, the stability of the wavelength and power fluctuations below 0.03 nm and 0.02 dB, respectively. The single-longitudinal-mode fiber laser has a compact structure, ultra-narrow linewidth, and excellent performance, and is expected to be used in the fields of coherent communications, precision metrology, and so on. • We propose a new structure of ultra-narrow linewidth single-longitudinal-mode erbium-doped fiber laser based on Nested Fiber Ring structure and Saturable Absorber. • The new structure has the advantages of simple structure, easy preparation and high stability. • The output performance of the proposed fiber laser is excellent, the 3-dB linewidth is 550 Hz and the optical signal-to-noise ratio is 59 dB. • The filtering function of the Nested Fiber Ring was theoretically derived and its filtering effect was analyzed. [ABSTRACT FROM AUTHOR]

• A wavelength injection locking actively Q-switched wide locking range random fiber laser (RFL) based on single-mode fiber (SMF) is proposed. • A narrow linewidth laser with tunable wavelength and power is injected into the cavity through the wavelength tunable laser (TLS), and the laser mode of RFL is selected by mode resonance. • In the C-band of 1530 to 1565 nm, different laser modes with stable center wavelength and peak power can be selected by changing the wavelength of the injected light. In this paper, a wavelength injection locking actively Q-switched wide locking range random fiber laser (RFL) based on single-mode fiber (SMF) is proposed. Within the C-band range from 1530 nm to 1565 nm, different laser modes can be selected by changing the wavelength of the injected light. Under injection locking state, the power fluctuation within one hour is less than 0.1dBm. When high nonlinear optical fiber (HNLF) is used in the RFL, the pump power is increased from 107 mW to 300 mW, the fixed sine wave modulation signal frequency is 20 kHz, the modulation voltage is 5 V, and the injection power is 15dBm. The RFL is in injection locking state and the output pulse is stable 4 pulses. When HNLF in RFL is replaced by SMF with the same length and the injection locking state is maintained, the output pulse is single pulse. Furthermore, the pump power increases from 107mW to 300mW, the pulse width increase from 1.91 μs to 3.24 μs, and the pulse repetition frequency is 20 kHz. In addition, the laser has wavelength tunable properties in the C-band range and serves as an ideal light source in the fields of laser imaging and pulse coding. [ABSTRACT FROM AUTHOR]

• This paper proposes a double-ring cavity erbium-doped random fiber laser based on bi-directional rayleigh scattering in a single-mode fiber. • The tunable laser output is realized by using the Mach-Zehnder interferometer composed of two 2 × 2 couplers as the dressing filter. • The stable and wavelength tunable random fiber laser could be realized by Mach-Zehnder interferometer filter. A wavelength-tunable double-ring cavity erbium-doped random fiber laser (RFL) based on a Mach–Zehnder interferometer (MZI) was proposed in this study. The MZI structure, which was based on two 2 × 2 optical fiber couplers, exhibits a comb-filtering effect. Rayleigh scattering (RS) was generated by a 10-km-long single-mode fiber (SMF) to provide randomly distributed feedback. A double-ring cavity structure based on forward and backward RS was built using two 2 × 2 fiber couplers. The threshold of the laser was 104 mW, and the slope efficiency of the total power output to the pump power was approximately 1.2%. When the pump power was 158 mW, a laser beam with a wavelength of 1555.3 nm was formed. By adjusting the polarization controller (PC), a single-wavelength switched RFL at 1547.9 nm and 1559.6 nm was obtained. The maximum peak power difference was less than 0.67 dB, the signal-to-noise ratio (SNR) was greater than 17.27 dB, and the peak power fluctuation was less than 0.92 dB during a scan time of 30 min at 25 °C. Regarding dual-wavelength switchable laser emission, two groups of different wavelengths with an SNR greater than 16.88 dB, power fluctuation less than 0.81 dB, and wavelength fluctuation less than 0.26 nm could be achieved under the same monitoring conditions. By further adjusting the PC, three- and four-wavelength RFLs were achieved, and the SNRs exceeded 16.31 dB and 14.64 dB, respectively. The proposed RFL has promising application prospects for distributed sensing systems and speckle imaging. [ABSTRACT FROM AUTHOR]

• A novel compound-ring resonator of an optical fiber laser configurated with two interleaved sub-ring resonators has been developed. • Single-longitudinal-mode laser has been emitted from the optical fiber laser. • A Fabry-Perot etalon has been designed as a criterion to which the frequency of the laser has been locked. • The frequency of the laser has been stabilized by the Fabry-Perot etalon and an active feedback controlling circuit. In this paper we report the development of a robust highly stable frequency single-longitudinal-mode optical fiber laser based on a Fabry-Perot etalon and an active feedback controlling system. The laser is fundamentally structured on an all-fiber compound-ring resonator which is configurated with two interleaved and optical-path-overlapped sub-ring resonators. By employing a length of erbium-doped fiber as the gain medium and a FBG as a reflector, the effective linewidth of the gain spectrum of the compound-ring resonator is restricted to be as narrow as the reflected spectrum of the FBG, which is about 0.043 nm. Because of the vernier effect between the two interleaved sub-ring resonators, the longitudinal mode spacing of the compound-ring resonator can be expanded to be as wide as the effective linewidth of the gain spectrum of it, which permits only one longitudinal mode oscillating. By employing an external Fabry-Perot etalon as a criterion and using an active feedback controlling system to tune the length of the cavity of the compound-ring resonator, the ambiance disturbances to the cavity can be compensated and the frequency of the emitted laser is locked to the offset frequency of the Fabry-Perot etalon. Therefore the frequency of the emitted laser has been stabilized to be 10−8. [ABSTRACT FROM AUTHOR]

• We propose a low threshold multi-wavelength erbium-doped fiber ring laser with NOLM and Lyot filter. • The threshold pump power of the laser is 45 mW, and can get 12 stable lasing lines, the side mode suppression ratio is greater than 25 dB. • This laser's channel spacing can be changed among of 0.08 nm, 0.34 nm and 0.89 nm by changing the PMF length. • This laser has simple structure and good stability. In this paper, the performance of a switchable multi-wavelength erbium-doped fiber laser (MWEDFL) incorporating Lyot filter and nonlinear optical loop mirror (NOLM) is described. The NOLM consisting of a polarization controller (PC2) and a single-mode fiber is effective to suppress the mode competition caused by the homogeneous broaden line in the erbium-doped fiber amplifier (EDFA). The comb filter is developed by using PC1 and polarization-maintained fiber (PMF). The wavelength interval can be switched among 0.08 nm, 0.34 nm, and 0.89 nm using different lengths of PMF via careful adjusting of the PCs. When the pump power is 45mW, the laser can emit 12 stable lasing lines within the 3 dB bandwidth, and the side mode suppression ratio (SMSR) is greater than 25 dB. The spectral power and wavelength fluctuations of 1.9 dB and 0.16 nm, respectively. Experimental results reveal that the demonstrated laser has good stability and low threshold power. [ABSTRACT FROM AUTHOR]