Acousto-Optic Tunable Filter AOTF
Operation Principle
The basic operation principle is explained in the following. If a sinusoidal (fixed-frequency) RF input signal is applied to the modulator, diffraction is possible only in a narrow range of optical frequencies, where a phase matching condition involving both optical and acoustic waves is fulfilled. Looking at the diffracted light, one obtains a bandpass filter, while the non-diffracted light provides a notch filter.
With RF waves of different frequencies, one can address different regions of optical frequencies. One may, for example, use an optical input from an argon ion laser emitting on different laser lines, and with the tunable filter one can transmit just one of those lines at a time.
It is also possible to use any superposition of different RF frequencies in order to obtain diffraction for different optical frequencies. The diffraction efficiency at any wavelength can be controlled via the corresponding RF power.
Depending on the design, a AOTF may work over an optical wavelength range which is hundreds of nanometers wide. Other devices are optimized for high resolution in a narrower wavelength range. Some of them also work with ultrashort pulses.
Applications of Acousto-optic Tunable Filters
A widespread application of AOTF is in multispectral imaging, e.g. in the form of laser microscopy. The essential advantage of this technology is that very rapid scanning (e.g. compared with mechanically controlled spectrometers) is possible, allowing for a fast acquisition of microscope images with spectral information. Also, the optical setup can be quite compact.
For terrestrial observations with spectral resolution for monitoring the status of plants, for example, one may exploit the large field of view and high spatial resolution of a non-collinear filter. It is also possible to acquire additional information on the polarization of light (spectropolarimetry).
There are other applications in laser spectroscopy. For example, one may use an AOTF for selecting certain wavelengths of an excitation laser and another AOFT for spectrally filtering fluorescence light which the excitation light has caused on some sample.
Some wavelength-agile wavelength-tunable light sources use an AOTF.
The ability to rapidly select a certain wavelength region can also be used for optical fiber communications based on wavelength division multiplexing.