Using short helically wrapped single-mode fibers as illuminated fiducials

Abstract

The Dark Energy Spectroscopic Instrument (DESI) is a Stage IV ground-based dark energy experiment to map the large-scale structure of the universe and to probe the nature of dark energy. DESI is a massively multiplexed fiber-fed spectrograph, using a 5000-ffber-positioner focal plane assembly to image millions of galaxies. Since these fiber positioners must be positioned to 10-um accuracy, the focal plane must be mapped to micron level precision. We intend to use illuminated fiducials as point sources to accurately calibrate the focal plane surface. In this study we explored using short single-mode fibers as illuminated fiducials. However, despite the advantages of using single-mode fibers, as a near point source, optical fibers have length-dependence behavior: as shorter tend to guide core light into the cladding, which is not ideal for fiducial centroid-measurements. In this paper, we demonstrate that adding tight helical bends to the fibers eliminates unwanted flux in the cladding, improving centroid measurements by more than 50%. This technique has proven with fibers as short as 2-inches, obtaining centroid with at least 0.5-micron precision. This experiment eliminates fiber-length dependence, thus proving the viability of using short single-mode fibers as illuminated fiducials or similar applications