Acousto Optic Tunable Filter (AOTF) Basics

This page describes the basics of an Acousto Optic Tunable Filter (AOTF). It explains how it operates in an optical network. This filter is widely used in WSS (Wavelength Selective Switch), as illustrated in Figure 1.

The filter operates on the principle that the “wavelength of diffracted light depends on the acoustic frequency.” In simpler terms, by tuning the frequency of the acoustic wave, the desired wavelength of the optical wave can be diffracted acousto-optically. The Acousto Optic Filter can switch multiple wavelengths simultaneously.

Figure 1 depicts two output streams and one input stream of wavelengths.

The Acousto Optic Tunable Filter (AOTF) can direct wavelength channels (3, 5, 6) to one output and the remaining channels (1, 2, 4) to the other output. Any arbitrary combination of wavelengths can be switched.

There are two main types of acousto-optic filters: collinear and non-collinear. In the collinear type, ordinary polarization of light is used. In the non-collinear type, the input light does not necessarily need to be polarized at all.

Figure 2 depicts the structure of an AOTF. It is composed of an acoustic transducer, an acoustic waveguide, an optical waveguide, and an acoustic absorber.

Let’s understand the operation of the Acousto Optic Tunable Filter, as shown in Figure 3:

• Unpolarized light enters the device from the left. It is split into two orthogonal polarizations by a beamsplitter prism.
• The TE mode passes through the AOTF, while the TM mode is reflected.
• As light travels through the AOTF, wavelength λ2 becomes resonant with the acoustic wave present. As a result, its polarization changes to the TM mode. Wavelength λ1 remains unaffected.
• When the same light reaches the second prism, light with wavelength λ1 passes through, while light with wavelength λ2 is reflected back. This is how the wavelengths are separated.