Compact Multipass Alexandrite Amplifier Pumped by 2850 LEDs, on the Road to 100 Hz Operation

Alexandrite (Cr ³⁺ :BeAl 2 O 3 ) is a well-known laser material used in dermatology [1], and for LiDAR applications [2]. It is usually pumped by flashlamp [3], red laser diodes [2] and more recently by LED-pumped luminescent concentrators (LC) [4]. Alexandrite is generally used in laser oscillators but rarely in amplifiers because of its low emission cross section. Whereas a small signal gain of several units can be reached under flashlamp pumping, much lower values have been reported under semiconductor pumping [2], [4]. Multipass amplification is then necessary to reach a significant gain. A gain of 2 in a four passes configuration has been previously reported [2] and a gain of 4 for an eight-passes one [4]. In this work we report a significant gain enhancement by the increase of two parameters: the pump power which provides up to 2.5 kW, and the number of passes that can reach 28.