Automated multi-stressor optical fiber focal ratio degradation and throughput characterization system for future massive multiplex spectroscopic survey

Future spectroscopic survey facilities such as Spec-S5 and the Wide-Field Spectroscopic Telescope (WST) are expected to contain more than 25,000 optical fibers. Characterizing each fiber's focal ratio degradation (FRD) and throughput is essential for spectroscopic calibration, sky subtraction, and overall instrument performance, but manual testing of such large fiber populations is prohibitively time-consuming.

To address this, the authors present an automated multi-stressor optical fiber characterization platform that measures both FRD and relative throughput while systematically varying optical and mechanical stress conditions. The system can characterize fibers as a function of input angle, wavelength, bend radius, twist angle, and externally applied load, and can investigate combinations of stressors simultaneously. A custom software framework automates hardware control, data acquisition, and data analysis, enabling large characterization campaigns with minimal user intervention.

Validation measurements on a multimode optical fiber showed that the platform can resolve both pronounced and subtle stress-dependent variations in FRD and relative throughput. The results exhibit the expected dependence of FRD on input angle and bend radius, reveal comparatively weak sensitivity to externally applied compressive loading under the investigated conditions, and identify a reproducible periodic dependence on fiber twist angle. In addition, relative throughput measurements indicate that the reduction in relative throughput with increasing input angle cannot be explained solely by a simple projected-area model. The automated workflow and scalability provide a versatile framework for the large-scale characterization of optical fibers required by next-generation spectroscopic survey facilities.