Femtosecond mode-locked Alexandrite crystal lasers

Alexandrite crystal is a broadband tunable laser crystal with long fluorescence lifetime and wide absorption bandwidth. It also has great mechanical strength as well as great thermo-mechanical properties. The first Alexandrite laser was developed in the 1970s, and it was the first solid-state wavelength-tunable laser that was able to operate at room temperature. It has been widely used in a variety of application fields including high-resolution spectroscopy, remote sensing, non-linear optics, radar, electro-optical countermeasure and skin treatment. Different methods have been applied to generate femtosecond pulses, including Kerr-lens mode-locking (KML), Quantum-dot (Q-dot) Saturable Absorber Mirror (SESAM), and monolayer graphene saturable absorber. The shortest pulse achieved so far was 65 fs. In this article, first, the main crystal characteristics, thermal characteristics and spectral characteristics of Alexandrite crystals are introduced. Then, the current development of ultrafast mode-locked Alexandrite lasers is reviewed. Mode-locked Alexandria crystal laser has great development and application potential in the fields of skin cosmetology and dermatology treatment.