The acousto-optic tunable filter (AOTF) is an all-solid-state optical filter that operates on the principle of acousto-optic dif fraction in an anisotropic medium. The center wavelength of the filter passband can be rapidly tuned across a wide spectral range by changing the frequency of the applied radio frequency (RF) signal. In addition to the electronic tunability, other outstanding features of the AOTF include: large angular aperture while maintaining high spectral resolution, inherent intensity, and wavelength modulation capability.
Acousto-optic tunable filter find many applications:
- Rapid-scan Spectrometers. The rapid, random-access tuning of the AOTF makes it well-suited to rapid-scan spectrometer applications, such as time-resolved spectra analysis.
- Multispectral Imaging. Use of noncollinear AOTF imaging in astronomy has been demonstrated by spectropolarimetry of stars and planets
- Fiber Optic Communication. One important application where the AOTF has shown great potential is in the area of fiber-optic communication. Due to its capability of selecting a narrow optical band over a wide spectral range within a time duration of microseconds, the AOTF appears to be well-suited to perform wavelength division multiplexing (WDM) networks.
- Spectropolarimeter and fluorescence spectrometer. The great advantage using an AOTF is to be capable of measuring spatial, spectral and polarization characteristics of a target, in real time, without moving parts, only with a single instrument.
AOTFs use the slow-shear acousto-optic interaction. This is polarization sensitive. For an optical input along the z-axis (θi=0) the polarization should be circular for 100% (relative) efficiency, or along the t-axis (θi=90º) linearly polarized. For directions in-between, the input should be some elliptical state for maximum diffraction efficiency. In practice, this will approximate to a linear state for θi>10º i.e. far enough away from the optic axis to ignore optical activity.