beam 141 b 1 by 90 degrees the third polarization rotator 60 rotates the third polarized light beam 151 a 1 and the fourth polarized light beam 15 b 1 by 90 degrees the third polarization beam displacer combiner 70 combines the first polarized light beam 161 a 0 and the second polarized light beam 161 b 0 that are both zero order components into a first output light beam 170 the third polarization beam displacer combiner 70 combines the third polarized light beam 161 a 1 and the fourth polarized light beam 161 b 1 that are both first order components into a second output light beam 171 since the first output light beam is purely composed of zero order components they are not affected by acoustic waves the second output light beam 171 is purely composed of first order components they are light of a specific wavelength that went under interaction with acoustic waves with reference to fig3 the disclosed polarization independent tunable acousto optical ao filtering method that diffracts an input light beam into a first output light beam unaffected by acoustic waves and a second output light beam affected by acoustic waves is illustrated the method include the steps of first polarization separation 81 first polarization rotation 82 ao filtering 83 second polarization separation 84 second polarization rotation 85 third polarization rotation 86 and polarization combination 87 the step of first polarization separation 81 separates an input light beam into a first polarized light beam and a second polarized light beam this step can be accomplished using a birefringent crystal the step of first polarization rotation 82 rotates the polarization of the first polarized light beam by 90 degrees this.