SCRAMBLING PROPERTIES OF OPTICAL FIBERS AND THE PERFORMANCE OF A DOUBLE SCRAMBLER

Optical fibers have become increasingly commonplace in astronomical instrumentation during the past decade, especially in coupling telescopes to spectrographs. In addition to the astrophysical results there is also a growing literature on the properties of optical fibers as they pertain to astronomical observations but this has tended to emphasize transmission and focal-ratio degradation. This paper looks at the scrambling properties of optical fibers which are particularly important in precision radial-velocity observations. We find that a single multimode step-index fiber, such as is commonly used in astronomical applications, is deficient as a scrambler; that is the output beam has some memory of the input object position and incident angle. We go on to characterize a solution to this problem which is a prototype double scrambler using two fibers coupled by an afocal lens system.