LED pumped alexandrite multipass amplifier designed for acousto-optic imaging

In this paper we present an alexandrite (Cr³⁺:BeAl₂O₄) multipass amplifier pumped by blue LEDs via a luminescent concentrator (LC). This emerging pump technology provides up to 2.6 kW optical pump power during 400 µs at 10 Hz in the 530-630 nm range for low cost, simple and robust implementation. We achieve a single-pass gain of 1.38, the highest ever reported in alexandrite for a semiconductor-based pump source, and a total gain of 34 after 16 passes. The amplifier is injected by a single-frequency laser operating in a quasi-continuous wave at 761 nm. It delivers µs pulses with a peak power of 14 W. This power could significantly improve penetration depth for <i>in vivo</i> acousto-optic imaging.