Integrated-Optical Channel-Waveguide Frequency Shifter*

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Abstract
Single-sideband optical frequency shifters compatible with optical fibers would be useful in a variety of applications including fiber-optic gyros and optical heterodyne communciations. Acoustooptic frequency shifters, which are now commonly used, are not compatible with optical fiber systems since they are broad-beam devices and require angular tuning to utilize their full electrical bandwidth. Here we present experimental results on a channel-waveguide frequency shifter being developed independently by us and others1 which utilizes TE/TM mode conversion produced by a traveling refractive-index-wave perturbation. In our approach, an interdigitated electrode structure is driven with a three-phase electrical signal to generate the index perturbation so that phase matching and therefore high conversion efficiency can be obtained from arbitrarily low frequencies to several gigahertz. The use of a three-phase-drive interdigitated electrode array is similar in concept to the wide-band slab-waveguide Bragg-diffraction frequency translator reported earlier.2 In the initial device reported here, which was fabricated in Ti:LiNbO3 for operation at λ = 1.15 μm, we obtained 10% conversion efficiency at 90 MHz. Much higher conversion efficiencies should be obtainable with improved designs. Here we report on the principles of operation of these devices, their fabrication, and the experimental results.