The Large Fiber Array Spectroscopic Telescope: fiber feed fabrication and characterization

The Large Fiber Array Spectroscopic Telescope (LFAST) project seeks to construct large arrays of small, individual fiber-fed telescopes for very high resolution spectroscopy. We are currently developing a prototype of a 20× telescope to investigate the technical requirements for LFAST. For each unit telescope, the 0.76 m primary mirror operates at f/3.5, focusing light onto our fused silica fiber with an 18 µm core, which subtends 1.4” on the sky. This receiving fiber collects and transmits light to the entrance slit of the spectrograph. We are developing a reliable fiber fabrication recipe, including fiber-end termination and polishing, to ensure consistency, efficiency, and affordability in mass manufacturing of the thousands of fibers that the future LFAST arrays require. The 18 µm core size places our optical fiber in the “few-mode” regime, which is not widely used in astronomy. Since the properties of “few-mode” fibers are not yet well characterized, extensive testing is required to gain a comprehensive understanding of their behaviors, such as focal ratio degradation, throughput and modal scrambling. We are designing optical tests to study the optical properties of the LFAST custom fibers. In this paper, we present the fiber feed design and fabrication recipe of our prototype. We also outline our optical test procedures and report results on surface flatness of our fibers.