Matched Pair of AOTFs with Net Zero Frequency-shift

Abstract

Acousto-Optic Tunable Filters are used in many applications where their fast agile tuning characteristics can be used to advantage. In addition, their ability to conserve spatial-coherence is useful in specialist applications, for example with single-mode optical fibres. Since, as is common with most AO devices, AOTFs generally use a travelling acoustic wave, the diffracted beam is frequency-shifted by an amount that is essentially equal to the RF drive frequency. This can be inconvenient, especially in applications where the AOTF is deployed in a multi pass configuration such as a recirculating-ring or laser-cavity. Often in such configurations a second AO device (usually similar to the first) is deployed in such a way so as to compensate for the frequency-shift. For an AO device using the isotropic AO interaction this is relatively straightforward since there is a symmetry about the device and interaction. However, an AOTF uses the anisotropic interaction which is asymmetric, and this is frequently further complicated by the inclusion of pointing-stability compensation. This compensation is usually achieved by cutting an appropriate angled wedge on the back (exit) face of the AOTF.We describe and characterise a pair of matched AOTFs operating in opposite quadrants of the “K-space” diagram that when combined have true reciprocity and give a net zero frequency shift whilst maintaining excellent pointing-stability. Any minor deviations in manufacture are self-compensated making these components ideal for use within a single-mode fibre network, or laser cavity. Furthermore, small controlled frequency-shifts (up to about 10kHz) may be introduced with little or no detriment to the alignment of the system, or indeed the throughput when pigtailed between two single mode optical fibres.