High concentration Er-doped phosphate glass optical fibers for single-frequency fiber amplifiers

The continuous improvement of interferometric gravitational-wave detectors (GWDs) and the preparations for next generation of GWDs set highly demanding requirements on their laser sources. A promising candidate to fulfill the challenging requirements of GWD laser sources is the hybrid master-oscillator power fiber amplifier (MOPFA) configuration. The implementation of a MOPFA relies principally on commercial silica glass-based optical fiber technology, which has been key in the successful development of high-power fiber amplifiers but that poses also a limitation to power scaling of these devices. It is well known that erbium (Er) ions tend to cluster in silica glass leading to ion-ion interactions and degradation of performance. The limited concentration of RE ions per unit length implies a limited optical gain per unit length and thus the requirement for long amplifying fiber lengths that enforce deleterious nonlinear effects, foremost stimulated Brillouin scattering (SBS). Numerous SBS suppression techniques have been proposed, alongside investigation of specialty optical fibers. One of the most promising solutions is the use of highly doped optical fibers based on multicomponent phosphate glass that allows the fabrication of ultra-compact active devices with minimized nonlinearities. To realize compact optical fiber amplifiers operating at 1.5 µm, a series of highly Er3+-doped custom phosphate glass compositions was designed and fabricated to be used as active materials for the core of the fiber amplifiers. Suitable cladding compositions were explored. Core and cladding glasses were synthesized by melt-quenching method. The core glass was cast into a cylindrical mold to form a rod, whereas the cladding tube was fabricated by extrusion technique. Phosphate fibers were then manufactured by drawing the preform assembled by rod-in-tube technique. Preliminary results of the application of the Er3+-doped phosphate fiber as laser active medium in a fully monolithic single-mode single-frequency core-pumped MOPFA setup resonantly pumped at around 1480 nm are presented.