We report on environmentally stable long-cavity ultrashort erbium-doped fiber lasers, which self-start mode-locking at quite low thresholds by using spectrally filtered and phase-biased nonlinear amplifying long-loop mirrors. By employing 100-m polarization-maintaining fiber (PMF) in the nonlinear loop, the fundamental repetition rate reaches 1.84 MHz and no practical limitation is found to further decrease the repetition rate. The filter used in the long loop not only suppresses Kelly sidebands of the solitons, but also eliminates the amplified spontaneous emission which exists widely in low-repetition-rate ultrafast fiber lasers. The bandwidth of the filter is optimized by using a numerical model. The laser emits approximately 3-ps pulses with an energy of 17.4 pJ, which is further boosted to $1.5~\unicode[STIX]{x03BC}\text{J}$ by using a fiber amplifier.

In this paper, we reported both the experimental demonstration and theoretical analysis of a Raman fiber laser based on a master oscillator–power amplifier configuration. The Raman fiber laser adopted the dual-wavelength bidirectional pumping configuration, utilizing 976 nm laser diodes and 1018 nm fiber lasers as the pump sources. A 60-m-long $25/400~\unicode[STIX]{x03BC}\text{m}$ ytterbium-doped fiber was used to convert the power from 1070 to 1124 nm, realizing a maximum power output of 3.7 kW with a 3 dB spectral width of 6.8 nm. Moreover, we developed a multi-frequency model taking into consideration the Raman gain spectrum and amplified spontaneous emission. The calculated spectral broadening of both the forward and backward laser was in good agreement with the experimental results. Finally, a 1.5 kW, 1183 nm second-order Raman fiber laser was further experimentally demonstrated by the addition of a 70-m-long germanium-doped passive fiber.

A few-mode erbium-doped fiber (FM-EDF) is fabricated using modified chemical vapor deposition in combination with liquid solution. The core and cladding diameters of the fiber are approximately 19.44 and 124.12 μm, respectively. The refractive index difference is 0.98%, numerical aperture (NA) is 0.17, and normalized cut-off frequency at 1550 nm is 6.81. Therefore, it is a five-mode fiber, and can be used as a higher-order mode gain medium. Furthermore, a long period fiber grating (LPFG) is fabricated, which can convert LP₀₁ mode to LP₁₁ mode, and its conversion efficiency is up to 99%. The first-order orbital angular momentum (OAM) is also generated by combining the LPFG and polarization controller (PC). Then, an all-fiber amplification system based on the FM-EDF and LPFG, for LP₁₁ mode and first-order OAM beams, is built up. Its on-off gain of the LP₁₁ mode beam is 37.2 dB at 1521.2 nm. The variation, whose transverse mode field intensity of first-order OAM is increased with the increase of pumping power, is obvious. These show that both the LP₁₁ mode and first-order OAM beams are amplified in the all-fiber amplification system. This is a novel all-fiber amplification scheme, which can be used in the optical communication fields.

A spectrally flat mid-infrared supercontinuum (MIR-SC) spanning 2.8–3.9 μm with a maximum output power of 411 mW was generated in a holmium-doped ZBLAN fiber amplifier (HDZFA). A broadband fiber-based SC covering the 2.4–3.2 μm region was designed to seed the amplifier. Benefiting from the broadband seed laser, the obtained SC had a high spectral flatness of 3 dB over the range of 2.93–3.70 μm (770 nm). A spectral integral showed that the SC power beyond 3 μm was 372 mW, i.e., a power ratio of 90.6% of the total power. This paper, to the best of our knowledge, not only demonstrates the first spectrally flat MIR-SC directly generated in fluoride fiber amplifiers, but also reports the highest power ratio beyond 3 μm obtained in rare-earth-doped fluoride fiber until now.

We demonstrate a nonlinearity optimization method by altering distribution of passive fibers in a dissipative-soliton mode-locked fiber laser to level up output parameters. In the numerical simulation, we found that the passive fiber segment after gain fiber characterizes the highest average B-integral among fiber segments. By reducing the length of this fiber section and keeping the total passive fiber length as constant, the output pulse energy can be effectively scaled up while maintaining a short dechirped pulse duration, resulting in boosting peak power. With this method, 37-nJ pulses are generated from a dissipative-soliton mode-locked cladding pumped ytterbium-doped single-mode fiber laser in the experiment. The pulse can be dechirped to 66 fs with 350 kW peak power. Moreover, the pulse pedestal is suppressed by a vector-dispersion compressor.

In this paper, we propose and experimentally investigate a linearly polarized narrow-linewidth random fiber laser (RFL) operating at 1080 nm and boost the output power to kilowatt level with near-diffraction-limited beam quality using a master oscillation power amplifier. The RFL based on a half-opened cavity, which is composed of a linearly polarized narrow-linewidth fiber Bragg grating and a 500 m piece of polarization-maintained Ge-doped fiber, generates a 0.71 W seed laser with an 88 pm full width at half-maximum (FWHM) linewidth and a 22.5 dB polarization extinction ratio (PER) for power scaling. A two-stage fiber amplifier enhances the seed laser to the maximal 1.01 kW with a PER value of 17 dB and a beam quality of Mx2=1.15 and My2=1.13. No stimulated Brillouin scattering effect is observed at the ultimate power level, and the FWHM linewidth of the amplified random laser broadens linearly as a function of the output power with a coefficient of about 0.1237pm/W. To the best of our knowledge, this is the first demonstration of a linearly polarized narrow-linewidth RFL with even kilowatt-level near-diffraction-limited output, and further performance scaling is ongoing.

Transverse mode instability (TMI) has become the major limitation for power scaling of fiber lasers with nearly diffraction-limited beam quality. Compared with a co-pumped fiber laser, a counter-pumped fiber laser reveals TMI threshold enhancement through a semi-analytical model calculation. We demonstrated a 2 kW high-power counter-pumped all-fiberized laser without observation of TMI. Compared with the co-pumped scheme, the TMI threshold is enhanced at least 50% in counter-pumped scheme, moreover, stimulated Raman scattering and four-wave mixing are suppressed simultaneously.

To design a compact erbium-doped fiber laser, a high-concentration erbium-doped fiber (EDF) is needed. However, increasing the erbium ion (Er3+) concentration can reduce the EDF performance via the Er3+-Er3+ interaction. In this Letter, we investigate the Er3+-Er3+ interaction effect by designing a tunable erbium-doped fiber-ring laser (EDFRL). This is the first time (to the best of our knowledge) that someone has considered different numbers of ions per cluster and simulated the EDFRL output power degradation due to ion–ion interaction. If the number of ions in the cluster is increased, the lasing output power will decrease accordingly. The most dominant effect is seen in the 1530 nm wavelength region, where the EDF shows a higher signal absorption compared to the other wavelength region. Moreover, a comparison has been done for lasing performance analysis with different dopant ion concentrations. The comparison results show that a higher dopant concentration is advantageous for longer-wavelength lasing.

A frequency-tunable wireless access scheme based on optoelectronic oscillating technology is proposed and experimentally demonstrated. By using this scheme, the frequency of the transmitted wireless signals can be tuned by adjusting the wavelength of the input light. The 1.25 Gb/s on-off keying signals with the carrier frequency of 8–14.5 GHz are generated and transmitted through a radio over fiber link. The envelope detecting technique is employed in the receiver to support the down-conversion and demodulation. Electrical local oscillators are not required in the transmitter and receiver end, which simplifies the system structure and lowers the cost.

A model that is based on the propagation equation and coupled mode theory is introduced in order to describe stimulated Raman scattering (SRS) effects in long tapered fiber amplifiers. Based on the presented model, fiber amplifiers with uniform and long tapered fibers are theoretically and numerically simulated. It can be drawn from the results of our simulations that the long tapered fiber has the advantage in suppressing SRS when applied in fiber laser amplifiers. Our results can provide guidance in the designing of system configuration in long tapered-fiber-based fiber laser systems.