Abstract

Single-frequency Er³⁺ doped fiber amplifiers in MOPA configuration are promising candidates to fulfil the challenging requirements of laser sources at 1.5 μm for the third generation of interferometric gravitational wave detectors (GWDs) [1]. An all-fiber design ensures excellent beam quality, -stability and reliability compared to bulk concepts. However, in monolithic systems high power operation gives rise to non-linear effects such as stimulated Brillouin scattering (SBS), which eventually limit the output capacity of such fiber MOPAs [2]. Since the threshold power for SBS scales inversely with the length and cross-section of the fiber, researchers placed much attention to enhance the Er³⁺ doping levels and therefore decrease necessary fiber length for sufficient signal amplification. However, already at moderately low doping concentrations Er³⁺ ions tend to form clusters in industry-standard fused silica fibers. Consequently, the short intra-cluster distances between adjacent Er³⁺ ions introduce quenching effects in terms of homogenous up-conversion processes (I_13/2 + I_13/2 ➔ I_9/2 + I_15/2). Thus, high inversion levels can reduce the pump-to-signal energy conversion efficiency and amplifier’s output capacity since ions in the I_9/2 state relax non-radiatively.

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