In this paper, we summarize the research on photodarkening in optical fibers. The causes of photodarkening in fiber, the influence of photodarkening on fiber laser, the experimental device of photodarkening, and the mathematical model used to study the phenomenon of photodarkening are described in detail. At the end of the paper, we summarize the means and methods to suppress photodarkening.

Mid-infrared (MIR) ultrashort laser pulses have a wide range of applications in the fields of environmental monitoring, laser medicine, food quality control, strong-field physics, attosecond science, and some other aspects. Recent years have seen great developments in MIR laser technologies. Traditional solid-state and fiber lasers focus on the research of the short-wavelength MIR region. However, due to the limitation of the gain medium, they still cannot cover the long-wavelength region from 8 to 20 µm. This paper summarizes the developments of 8–20 μm MIR ultrafast laser generation via difference frequency generation (DFG) and reviews related theoretical models. Finally, the feasibility of MIR power scaling by nonlinear-amplification DFG and methods for measuring the power of DFG-based MIR are analyzed from the author’s perspective.

Two-dimensional material nanosheets have always been a research hotspot because of their unique structure and properties. We report mode-locked operation in ytterbium-doped fiber laser (YDFL) for the first time by adopting Fe₃O₄ nanosheets as a saturable absorber (SA). The laser is capable of generating 456 ps pulses, centered around 1039 nm. Our results manifest that Fe₃O₄ nanosheets are expected to become a new type of saturable absorber, which can better promote the development of mode-locked lasers.

An ultrafast coherent long wavelength MIR source based on difference frequency generation was demonstrated. An average power of 2.5 mW at ∼18 μm was achieved. The angular distribution of the generated MIR source under the condition of tight-focusing limit shows the onset of conical emission of the MIR beam due to on-axis phase mismatching.

We systematically studied several of the most traditional hollow-core anti-resonant fiber (HC-ARF) structures, with the aim of achieving low confinement loss, single-mode performance, and high insensitivity to bending in the 2 µm band. Moreover, the propagation loss of fundamental mode (FM), higher-order mode (HOMs), and the higher-order mode extinction ratio (HOMER) under different geometric parameters were studied. Analysis showed that the confinement loss of the six-tube nodeless hollow-core anti-resonant fiber at 2 µm was 0.042 dB/km, and its higher-order mode extinction ratio was higher than 9000. At the same time, a confinement loss of 0.040 dB/km at 2 µm was achieved in the five-tube nodeless hollow-core anti-resonant fiber, and its higher-order mode extinction ratio was higher than 2700.

A “cleaving-polishing” process for end-face treatment of specialty fiber supported by thick-walled silica glass tubes is proposed. The fiber tension is determined by the fiber fracture theory for improved cleaving. This method overcomes polishing problems caused by steel or ceramic ferrules. Approximately 20 min on average is required for polishing a specialty fiber end face with a sampling roughness of 7.42 nm, confirmed by microscopy ($400\times$) and interferometry. A comparison of laser coupling efficiencies between fibers with cleaved ends and fibers with polished ends showed that the coupling efficiency of the former could be enhanced by 15% through polishing.