In-line acousto-optic frequency shifter in two-mode fiber

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Abstract
An in-line two-mode fiber frequency shifter that uses double conversion in two independent acousto-optic interaction regions is proposed and successfully implemented. Light remains guided in the first-order mode after being shifted in frequency, which provides for easy splicing to single-mode fibers. Net shifts can be up or down in frequency and can be made very small (a fraction of a hertz) by operating in the difference mode. The high acoustic bandwidth (tens of kilohertz) of the single-conversion approach is maintained while image sideband and carrier suppression are potentially improved.