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Experimental results of an adiabatic mode multiplexer (AMM) based novel 2×2 optical switch/variable optical attenuator that consists of a symmetric Y-branch interferometer (mode converter) and adiabatic mode multiplexer/demultiplexer pair are presented. It is shown that both normally cross and bar designs have identical optical performance. Extinction ratio without thermal activation is wavelength independent. Over C-band the attenuation at -20 dB varies by less than 0.3 dB and polarisation dependent loss is less than 1 dB. It is proved that the mode converter plays a dominant role in polarisation state sensitivity at high attenuation values and can be optimised by varying interferometer arm length. Since an AMM-based switch does not require process optimisation steps it can be fabricated within a process optimised for other optical devices.