A high isolation switch in LiNbO3 for coherent optical local area networks

Coherent optical local area networks employing frequency and time division multiplexing require a means of isolating each transmitter from the network during frequency acquisition and non-transmission periods. Isolation requirements depend on the intended number of transmitters within the network but for the system considered here of 100-1000 interfaces, a figure of 40-50 dB is implied. The switching speed must also be fast, of the same order as the bit period. To avoid compromising transmitter laser performance it is preferable to realise the switching function in an external integrated optic device. This also allows for the further integration of a phase modulator to provide the data modulation function. To achieve such a high isolation performance in an optical switch, without imposing excessively tight fabrication tolerances, an approach has been adopted in which two switches are integrated in series on a single substrate so that their individual isolations are additive. The author reports preliminary results at a wavelength of 1.532 μm for a switch fabricated in LiNbO₃ with 4.0 dB insertion loss (fibre-fibre), 39 dB isolation and 5 V drive voltage