High Gain and PER in Double-Pass Rod Fiber Amplifier Module

In the past few years single pass amplification using rod-type fibers has become an interesting solution for high power laser sources. In the meantime, double-pass configuration has not been extendedly studied [1,2]. After theoretical [1] and experimental work [3] on mode degradation occurring in double pass, we present here new results obtained with an upgraded setup using state of the art rod-type fiber based on a distributed mode filter design (NKT Photonics aeroGAIN-ROD-PM85) [4]. We obtain up to 112 W output power, i.e. 35 dB of amplification when seeding with 30 mW of 20 ps/20 MHz/1030 nm pulses, in a preliminary unoptimized setup, pump power limited. We observe excellent beam quality and a polarization extinction ratio (PER) above 20 dB.