The phase-matching condition required for efficient acousto-optic polarization coupling limits the operation of some types of fiber-optic frequency shifters to relatively low frequencies (1–2 MHz). Collinear phase matching can occur at higher frequencies if the acoustically induced coupling is made spatially periodic. Using this technique, a frequency shifter operating at 5.6 MHz (2.8 times the usual collinear phase-matching frequency) has been demonstrated. A maximum coupling efficiency of 26% was achieved using 32 W of electrical input power. A 3-dB modulation bandwidth of 110 kHz resulted for the design described here. Unwanted spectral components were suppressed by 24 dB.
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