A comparison of methods for the reduction of fiber modal noise in high-resolution spectrographs

The use of optical fibers to couple spectrographs to telescopes has been important in the search for extrasolar planets using radial velocity measurements. The ability of an optical fiber to partially scramble the input illumination enables a fiber feed to provide more uniform illumination to the spectrograph optics, but a limiting factor in fiber coupling is modal noise. Agitation of the fiber has been shown to reduce modal noise, but altering fiber transmission parameters by varying the length of the fiber may offer advantages. We report on tests comparing some of the alternative devices for reducing modal noise.