Single-frequency Er3+ doped phosphate fiber MOPA

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). 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. 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 (I13/2 + I13/2 ➔ I9/2 + I15/2). Thus, high inversion levels can reduce the pump-to-signal energy conversion efficiency and amplifier’s output capacity since ions in the I9/2 state relax non-radiatively.