We describe the origin of spectral modal noise in a fiber-coupled spectrograph. Prior investigations of this phenomenon have considered optical fibers with large cores that support thousands of optical modes and used a statistical treatment to analyze spectral noise that arises from interference between the modes. We examine spectral modal noise for a fiber-coupled spectrograph with a smaller-diameter fiber core, optimized for adaptively corrected telescopes, that supports only a few propagating modes for visible and near-infrared light. Simulations to predict the minimum amount of fiber stretch required to shift spectral oscillations by a half-period are confirmed by experiment.
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