Acousto-Optic Birefringent Fiber Frequency Shifters

Single-mode birefringent fiber guides two orthogonally polarized waves. These waves travel at slightly different speeds, giving rise to a beat pattern as the net polarization state evolves with propagation down the guide. A periodic stress pattern, with spatial period equal to the beat length of the fiber, can produce coupling between these two modes1. If the stress pattern is itself a travelling wave, light coupled from one polarization to the other will be shifted in frequency by the frequency of the acoustic wave.