Optimizing fibre connectors for high throughput and low FRD for next-generation multi-fibre astronomical instruments

The demand for optical fibre connectors, which offer high throughput, outstanding reliability, and minimal Focal Ratio Degradation (FRD) has become critical in the era of large telescopes where science productivity could be enhanced if focal plane fibres and fibre bundles had the flexibility to be reconnected to feed to any of the instrument suite or for instrument changes. At Astralis we are investigating if it will be physically possible to develop a fibre connector with the ability to connect any fibre pairs without a difference in throughput at a 1% (aim, 2% requirement) level, which has not previously been possible, and is very challenging. The initial stages of this work have been focused on analysis of the best-performing existing connector with the goal of identifying areas where connector (and mating adapters) improvements are most needed. From that analysis, the main causes of FRD and throughput loss have been identified. New connectors are being prototyped to improve the fibre alignment, FRD from both the connector holes, expansion of materials and fibre pressures. In addition, we investigate alternative coupling methods, including dry coupling and the application of index matching gels. We present our findings on the performance of the existing connectors in terms of fibre throughput and FRD in both the optical (370-1000 nm) and near-infrared (1000-2200 nm) wavelength regions. The key design considerations for the next prototype connectors are discussed.