Acousto Optic Tunable Filter basics | Acousto Optic filter

This page describes basics of Acousto Optic tunable filter. It mentions operation of Acousto Optic Tunable Filter in optical network.

This filter is widely used in WSS (Wavelength Selective Switch). This is shown in figure-1. This filter works on the principle that " Wavelength of diffracted light depends on acostic frequency". In other words, by tuning frequency of acoustic wave, desired wavelength of optical wave can be diffracted acousto-optically.

The Acousto Optic Filter can switch multiple wavelengths simultaneously. The figure-1 depicts two outputs and 1 input stream of wavelengths. The Acousto Optic Tunable Filter(AOTF) can direct wavelength channels (3,5,6) to one output and remaining channels (1,2,4) to the other output. Any arbitrary combination can be switched.

Acousto Optic Tunable Filter-AOTF fig1

There are two types of acousto optic filters viz. collinear and non-collinear. In collinear type , polarization of light which is ordinary is used. In non-collinear type, input light need not require to be polarized at all.

The figure-2 depicts structure of AOTF. Acoustic Optic tunable filter is composed of acoustic transducer, acoustic waveguide, optical waveguide and acoustic absorber.

Acousto Optic Tunable Filter-AOTF fig2

Let us understand operation of Acousto Optic Tunable Filter as shown in figure-3.
• Unpolarised light will enter into the device from left. It gets split into two orthogonal polarisations by beamsplitter prism.
• TE mode will get through AOTF and TM mode will get reflected.
• During the journey from AOTF, λ2 is resonant with acoustic wave present. As a result its polarisation will change to TM mode. The wavelength λ1 will be unaffected.
• When same light reaches second prism as shown , light with λ1 wavelength will get through. Lisht with λ2 wavelength is reflected back. This way wavelengths are separated.

Acousto Optic Tunable Filter-AOTF fig3

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