Comparison of the scrambling properties of bare optical fibers with microlens coupled fibers

摘要

Abstract: In addition to linking telescopes with grating spectrographs, optical fibers are now commonly used to smooth the photometric barycenter of the slit thus making possible the measurements of very small amplitude Doppler-Fizeau wavelength shifts of the spectral lines. However, optical fibers still transmit a fractional amount of guiding and seeing errors. To better the precision on the measurement of the barycenter of the spectral lines it has been suggested that using microlenses at both ends of the fiber link could further increase the level of scrambling of the input image on the fiber. Optical fibers coupled with microlenses take advantage of the large plate scale factor present on slow foci of large aperture telescope as for example, the multiobject high resolution spectrograph FUEGOS for the European very large telescope. This experiment compares the coupling of a conventional 'bare' optical fiber link with a fiber cable with microlenses at both ends. Light is coupled into both fibers in the optimum condition for focal ratio degradation reduction and to avoid fiber dependence, the same fiber has been used for both links. The set-up used for this feature is described. A description is given of the test bench set up to measure the residual photometric barycenter shifts through both links and of the experimental condition to achieve subpixel resolution down to a thousandth of a pixel. The measurements are done both on the slit and on a spectral line through a spectrograph. The spectral stability obtained with those two fiber links are discussed and have been finally compared with an optical fiber double scrambler. !13