LED-Pumped Alexandrite Multipass Amplifier

Alexandrite (Cr3+:BeAl2O4) is a laser crystal that has been known for over 50 years and is used in many commercial dermatology lasers. It has a broad near-infrared emission band (50 nm full width at half maximum, centered at 750 nm), excellent thermo-mechanical parameters and the unique property of being more efficient at 80°C than at room temperature. Its long fluorescence lifetime (250 μs at room temperature) allows easy energy storage with interesting potential for pulse amplifiers. Surprisingly, despite its great potential, alexandrite has never been used as an industrial laser. In fact, the main problem with alexandrite lies in the pumping source: it can be pumped by flashlamps, which are not adapted for high repetition rates, or by red laser diodes, which are expensive and far less developed than the 800-980 nm laser diodes used for Nd:lasers and Yb:lasers. The absorption band of alexandrite is between 550 nm and 650 nm, right in the “yellow gap” where semiconductors remain inefficient today. We propose a novel pump source consisting of a Ce:YAG luminescent concentrator pumped by blue LEDs. The emission of this crystal perfectly matches the absorption band of alexandrite. In addition, the geometry of the luminescent concentrator (composed of a parallelepiped with large faces) allows a massive collection of LED beams for pumping [1].