Optical heterodyne detection requires frequency shift of an optical beam, which can be realized using an acousto-optic modulator (AOM) that shifts the frequency of the diffracted beam with the acoustic frequency (from several tens of MHz, typically). Low frequency shift can be achieved using two, consecutively placed, modulators driven with slightly different frequencies. This configuration, however, results in lower efficiency and higher complexity. To avoid these problems and extremely low frequency, TeO2 based, single cell AOM have been developed. The AOM performance has carefully been investigated and design considerations maintaining the high diffraction efficiency are given. The suppression of the unwanted effects arisen from low frequency operation, such as presence of multiple diffraction orders, elliptical distortion of the diffracted beam, has been examined. The special anisotropic diffraction geometry allows a frequency shift as low as 5 MHz with diffraction efficiency greater than 85% (> 90% at 8 MHz), simultaneously, as it has been demonstrated.