Tunable optical filter in fiber-optic form

Light can be coupled between the two principal polarizations of birefringent fiber by using a traveling acoustic wave to produce a spatially periodic stress in the fiber. For a fixed acoustic frequency, maximum coupling occurs when the input optical wavelength is such that the beat length of the fiber equals the acoustic wavelength. By changing the acoustic frequency, the wavelength at which peak coupling occurs can be tuned. A prototype device has a passband 5 nm wide with a peak optical wavelength that can be tuned from 570 to 630 nm by changing the acoustic frequency from 2.85 to 2.55 MHz.