Optical vortices, beams with spiral wavefronts and screw phase dislocations have been explored in applications in optical manipulation, quantum optics, and the next generation of optical communications. In traditional methods, optical vortices are generated using space light modulators or spiral phase plates, which would sharply decrease the integration of optical systems. Different from previous transverse mode conversion outside the cavity, here we experimentally demonstrate a direct generation of ultrafast vortex beam from a Tm:CaYAlO4 oscillator by pattern matching of a six-mirror-folded-cavity resonator. By accurately adjusted the angle of the end mirror and the distance L between the M4 and the SESAMs to control the beam diameter of laser incidence on the gain medium in the sagittal and tangential planes, a stable 2 µm ultrafast vortex laser emission of annular Laguerre–Gaussian (LG) mode was obtained with a maximum output power of 327 mW and pulse duration of 2.1 ps. A simple YAG crystal plate was used as handedness selector and a homemade Mach–Zehnder (MZ) interferometer has verified the vortical property of the LG01 mode. By furtherly controlling the cavity mode pattern matching, other stable transverse-mode operations for TEM00, high-order Hermite-Gaussian (HG) transverse mode and doughnut-shaped beams were successfully realized. This work provides a flexible and reliable way to generate mid-infrared ultrafast vortex beams and is of special significance for applications in the areas of molecular spectroscopy and organic material processing amongst others.

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The emission wavelength of an ultrafast laser generated by thulium (Tm)-doped fiber laser (TDFL) ranges from 1.7 to 2.1 μm, covering the water-absorbing band and atmospheric transmission window. In this study, an intelligent Tm-doped mode-locked fiber laser was experimentally demonstrated by combining a genetic algorithm (GA) with an adaptive mutation rate and a nonlinear polarization rotation mode-locked fiber oscillator. A closed-loop feedback system was set up in the experiment, including an oscilloscope, a laptop computer, an electric polarization controller, and a mode-locked fiber oscillator. Based on the aforementioned intelligent design of manual-operation-free, a stable femtosecond level noise-like-mode-locked pulse with an output power of 57.7 mW and a central wavelength of 1973 nm was automatically generated. The evolutionary dynamics of the different parameter structures of the GA-controlled ultrafast TDFL with varying mutation rates were also investigated. This study will pave the way for generating robust ultrafast lasers in the short-wave infrared region.

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We report on a novel type of h-shaped pulse with femtosecond substructures from a linear-cavity semiconductor saturable-absorber mirror mode-locked thulium-doped fiber laser. It yields a stable nanosecond h-shaped pulse-train with a pulse envelope width ranging from 880 ps to 1.41 ns with a typical substructure pulse duration around 440 fs. The obtained h-shaped pulse is a noise-like pulse with strong robustness to pump power changes and environmental perturbation. It can also operate in the harmonic mode-locking regime. The scheme makes the structure compact and stable, and can obtain a MHz repetition rate, which is advantageous for real-life applications