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• This study used femtosecond laser processing technology to fabricate cascaded fiber Bragg gratings. • This study proposd an erbium-doped random fiber laser based on long-distance single mode fiber. • Random laser wavelength switching was achieved by changing the polarization state of the light in the system. This study proposed a wavelength switching system for the linear cavity erbium-doped random fiber laser (RFL) based on cascaded fiber Bragg gratings (FBGs). High reflection cascaded FBGs was fabricated by point-by-point inscription method based on femtosecond laser processing technology. The erbium-doped fiber (EDF) was used to provide effective gain for the laser system, the backward Rayleigh scattering (RS) in the long distance passive single-mode fiber (SMF) was used to provide random distributed feedback for the system, and the single-RS was changed into double-RS by the full reflection mirror to improve the feedback intensity. Combined with the frequency selection function of cascaded FBGs, the polarization state of the light in the system was changed by adjusting the polarization controller (PC) to achieve wavelength switching. The designed laser system achieved a low threshold power of 37 mW. When the pumping power was 57 mW, the output wavelength was 1549.6 nm and 1550.0 nm single-wavelength laser channel, respectively. The laser stability was observed within 50 min, and the maximum wavelength fluctuation was 0.01 nm and 0.09 nm with the maximum power fluctuation being 0.30 dB and 1.44 dB respectively. The dual-wavelength laser generation was obtained by adjusting the PC, the wavelength fluctuation was less than 0.07 nm and the power fluctuation was less than 1.01 dB. With the increase of input power, the fitting linear determination coefficient of output power was 0.979 and the correlation coefficient was 0.990. The wavelength switchable RFL proposed in this paper is uncomplicated in structure, switchable, and has excellent wavelength and power stability, which is of great significance in the fields of laser sensing, laser communication and diffuse spot imaging. [ABSTRACT FROM AUTHOR]

• This paper proposes an erbium-doped fiber laser (EDFL) system based on intrinsic Fabry-Perot interferometer (IFPI) structure for simultaneous measurement of temperature and strain. IFPI is composed of two fiber Bragg grating (FBGs) inscribed by femtosecond laser and used as the frequency selective device in fiber laser. • FBG can be inscribed on fiber core through fiber coating and cladding, its mechanical strength is greater. Moreover, FBG inscribed by femtosecond laser is more resistant to high temperature. • The changes of temperature and strain parameters can be sensed by the drifts of the laser spectrum, the most accurate spectral drifts for high-precision parameter sensitivities can be monitored due to the advantages of fiber lasers, such as narrow linewidth, high signal-to-noise ratio (SNR), high stability. In this paper, an erbium-doped fiber laser (EDFL) system based on intrinsic Fabry-Perot interferometer (IFPI) structure is proposed for simultaneous measurement of temperature and strain. IFPI with a central wavelength of 1546.2 nm, an interference fringe interval of 1.8 nm is composed of two FBGs inscribed by femtosecond laser and used as the frequency selective device in fiber laser. The signal-to-noise ratio (SNR) and 3-dB linewidth of 1546.2 nm fiber laser are 27.735 dB and 0.21 nm, respectively. For 1546.2 nm-laser stability, the maximum fluctuation of laser peak wavelength and output power is 0.08 nm and 0.655 dBm, respectively. Experimental results show that the proposed EDFL sensing system exhibits a temperature sensitivity of 9.16 pm/℃, the linearity R2 is 0.99965 within the temperature range of 100 °C-800 °C and a strain sensitivity of 7.45 pm/με, the linearity R2 is 0.99985 within the strain range of 0 με–1050 με at 800 °C. The stability of both maximum temperature and maximum strain was monitored within 30 min, and the maximum fluctuation of laser peak wavelength and output power was 0.12 nm and 0.372 dBm, 0.05 nm and 0.667 dBm, respectively. [ABSTRACT FROM AUTHOR]

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]

• Over the years 2 µm mode-locked fiber laser has been widely developed as an important light source for various significant applications, such as ultrafast mid-infrared spectroscopy. Besides, as the solutions of CGLE governed by the composite balance among dispersion, nonlinearity, gain and loss in dissipative systems, the field of solitons has become subject to extensive study. • Worth to note that, the rise of time-stretched dispersive Fourier transform (TS-DFT) technique has significantly enhanced the understanding of soliton dynamics, including bound solitons, optical rogue waves and others. However, due to the lack of appropriate dispersive components at 2 µm band with low optical transmission loss for TS-DFT measurement, direct real-time observations of ultrafast soliton dynamical phenomena in the 2 µm mode-locked fiber lasers is still limited. • In the present paper, we utilize DFT method to investigate the ultrafast nonlinear soliton dynamics in a 2 µm Tm-doped mode-locked fiber laser. The real-time observation of the nonlinear dynamics of the soliton explosions and pulsations are successively investigated. Moreover, the real-time observation of the soliton explosions associated with soliton pulsations is demonstrated in 2 µm wavelengths for the first time. In addition, soliton rains with dual-wavelength operation are also observed. Our work will benefit the development of 2 µm ultrafast light source, as well as enhance a comprehensive understanding of the soliton nonlinear dynamics in mode-locked fiber lasers. Over the years, 2 μm mode-locked fiber laser has been widely developed as an important light source for various significant applications. Worth to note that, with the help of real-time spectroscopy technique, i.e., time-stretched dispersive Fourier transformation (TS-DFT), various complex nonlinear soliton dynamics have been revealed, such as soliton molecules, soliton explosions and soliton pulsations, enhancing our understanding of the ultrafast physics. However, due to the lack of appropriate dispersive components at 2 μm band with low optical transmission loss for TS-DFT measurement, the investigation of ultrafast soliton dynamical phenomena in the 2 μm mode-locked fiber lasers is still limited. In this work, we demonstrate the direct real-time observation of soliton explosion and pulsation phenomena at 2 μm band mode-locked fiber laser, for the first time as far as we know, by using specially designed linearly chirped fiber Bragg gratings as dispersive component for TS-DFT measurement. For explosions, we observe slight explosion and drastic explosion, respectively. Particularly, the soliton pulsating explosions are also characterized in real time where the explosions occur in the process of soliton pulsation. In addition, dual-wavelength soliton rains are also observed by carefully adjusting the cavity parameters. Our work provides supplements to the current investigation of 2 μm band ultrafast science, and might enrich our understanding of soliton dynamics. [ABSTRACT FROM AUTHOR]

• In linear erbium-doped fiber laser cavity, dual-wavelength lasing performances are compared with 3 dB FLM-FBG cavity and FBG-only cavity. • In the dual-wavelength EDFL with FBG-only cavity, the lasing performances are compared when the reflectivities of the output-mirror FBGs are the same or different, and the effect of the center wavelength spacing between the two pairs of FBGs on lasing performances is studied. There are few reports in the field of linear cavity dual-wavelength fiber lasers about the laser output characteristics when 3-dB fiber loop mirror (FLM) and Fiber Bragg grating (FBG) are used as full reflective mirrors, respectively. Additionally, there is scarce research on further tuning of the FBG parameters (reflectivity and center wavelength interval) to study dual-wavelength fiber lasers. In this paper, we compare the performances of dual-wavelength lasing outputs between the 3-dB FLM-FBG cavity and the FBG-only cavity in a linear-cavity erbium-doped fiber laser (EDFL) based on the polarization hole burning effect. The results show that the optical signal-to-noise ratios (OSNRs) of the dual-wavelength lasing outputs with the FBG-only cavity are approximately 4 ∼ 5 dB higher than those with the 3-dB FLM-FBG cavity. Furthermore, the lasing output performances based on the FBG-only cavity are compared when the reflectivities of the output-mirror FBGs are the same or different. It is observed that the dual-wavelength laser with the same output reflectivity leads to higher OSNR, slope efficiency, and stability. Moreover, by varying the center wavelength interval of pairs of FBGs in the FBG-only cavity to values of 4 nm, 8 nm, and 12 nm respectively. It is shown that wider intervals result in higher OSNRs but lower laser slope efficiency. Finally, we demonstrate a dual- wavelength linear cavity EDFL operating at room temperature with two lasing wavelengths of 1550 nm and 1562 nm. The laser results in OSNRs around 50.62 dB and 50.93 dB; center wavelength variations of less than 0.030 nm and 0.035 nm; output power fluctuations of less than 0.061 mW and 0.059 mW, respectively. [ABSTRACT FROM AUTHOR]

• This paper proposes a strain sensing system based on an erbium-doped fiber laser in liquid nitrogen environment. • The FBG is inscribed through polyimide coating and cladding, realized by a femtosecond laser point-by-point inscribing method. • The strain sensitivity of the sensing system is 0.890 pm/με and the linearity is 99.72 % in liquid nitrogen environment. In this paper, a strain sensing system based on an erbium-doped fiber laser in liquid nitrogen environment is proposed and experimentally verified. A fiber Bragg grating (FBG) and a Sagnac loop are used as cavity mirrors of a fiber laser. The FBG with a central wavelength of 1532.39 nm, a 3-dB linewidth of 0.61 nm, and a signal-to-noise ratio (SNR) of 9.06 dB is inscribed through polyimide coating and cladding, realized by a femtosecond laser point-by-point inscribing method. The 3-dB linewidth and SNR of the fiber laser are 0.06 nm and 36.53 dB, respectively. The measurement results show that the linear range of the measurement is from 1,000 με to 3,000 με, the strain sensitivity of the system is 0.890 pm/με and the linearity is 99.72 % in liquid nitrogen environment. [ABSTRACT FROM AUTHOR]

In this paper, we present a thorough study on an all-fiber narrow-linewidth FBG-based laser oscillator. Having established a theoretical model, the spectrum broadening characteristics of the narrow-linewidth FBG-based oscillator were analyzed based on multiple factors. During the experiment, using optimized structural parameters according to theoretical analyses, a backward pumped laser oscillator achieved an output power of 192.4 W and a spectral 3-dB bandwidth of 33.7 pm, corresponding to a power spectral density of 5.7 W/pm. To the best of our knowledge, this is the highest PSD reported to date by an all-fiber FBG-based laser oscillator. • In this paper, we present a thorough study on an all-fiber narrow-linewidth FBG-based laser oscillator. • Having established a theoretical model, the spectrum broadening characteristics of the narrow-linewidth FBG-based oscillator were analyzed based on multiple factors. • During the experiment, using optimized structural parameters according to theoretical analyses, a backward pumped laser oscillator achieved an output power of 192.4 W and a spectral 3-dB bandwidth of 33.7 pm, corresponding to a power spectral density of 5.7 W/pm. • To the best of our knowledge, this is the highest PSD reported to date by an all-fiber FBG-based laser oscillator. [ABSTRACT FROM AUTHOR]

• Having established a simulation model, the spectrum broadening characteristics of the narrow-linewidth FBG-based amplifier were analyzed based on multiple factors. • A plum-shaped coiling design for the gain fiber was used to filter out high-order modes thus obtaining a beam quality m2 of 1.32 at the maximum output power of 3.3 kW. • A Raman suppression ratio of 26.3 dB was obtained at the maximum output power of 3.3 kW. • To the best of our knowledge, this is the highest record of narrow-linewidth fiber laser based on one stage MOPA configuration with normal commercial narrow-bandwidth fiber gratings and non-wavelength-locked laser diodes. This paper demonstrates a narrow-linewidth fiber laser with near-diffraction-limited beam quality and high Raman suppression ratio by utilizing Master Oscillator Power-Amplifier structure based on narrow-bandwidth fiber Bragg gratings. The output power reaches 3.31 kW with a slope efficiency of 85.0 %. At the maximum output power, the 3 dB, 10 dB, and 20 dB linewidths are 0.40 nm, 1.20 nm, and 3.09 nm, respectively. The beam quality M2 factor is 1.32 and the Raman suppression ratio reaches 26.3 dB. A pair of narrow-bandwidth fiber gratings were used in the oscillator to compress the linewidth of the seed laser. We used backward-pumping configuration and plum-coiled gain fiber in the amplification stage to suppress the Raman effect and select transverse mode by fiber-bending. Finally, a compact all-fiber laser was obtained with narrow-linewidth, high beam quality, and effective suppression of the Raman effect. To the best of our knowledge, this is the highest output power for a sub-nanometer-linewidth laser using narrow-bandwidth fiber gratings and none-wavelength-locked laser diodes, which achieves near-diffraction-limit beam quality. This provides a solution for high-power narrow-linewidth laser sources with high beam quality. [ABSTRACT FROM AUTHOR]