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 interferometrie 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. In this work, a novel single -mode Er 3+ doped (2 mol%) LMA fi ber (10/125 pm and 0.08 NA) drawn from a phosphate glass preform was studied. The phosphate host material allows Er 3+ doping concentration to be one magnitude larger than in fused silica without clustering. First, we investigated suitable fi ber processing techniques (e.g. cleaving, splicing) with focus on integration of such fi ber to fused silica fi ber components and present the corresponding process parameters. In addition to that, we demonstrate the application of the phosphate fi ber as laser active medium in a fully monolithic single -mode single -frequency core -pumped MOPA setup. We used a DFB stabilized seed module (120 mW at 1550nm) pumped by a single -mode semiconductor laser (600 mW at 976nm) (see Fig. 1). We characterized the MOPA system in terms of optical efficiency, spectral properties and GWD requirements and compare its performance to frequently used fused silica counterparts. The utilization of Er 3+ embedded phosphate glass hosts yields a promising approach for future GWD fiber laser sources.