Free-space switching for optical fibre networks

Abstract

The problems of point-to-point communication have essentially been solved by advances in semiconductor laser and optical fibre technologies. Direct optical signal amplification has also become possible in recent years using Erbium-doped fibres [1], and uninterrupted glass fibre lengths of over 6000km are now being installed [2]. But the technologies used for signal routeing have yet to catch up with the huge potential bandwidths that optical fibres offer. The research outlined in this dissertation relates to the optical implementation of freespace routeing architectures for communication networks. These systems potentially allow hundreds or even thousands of signals to be optically routed through each other in a compact 3-dimensional volume, incurring very little crosstalk. This is achieved by the use of programmable computer-generated holograms (CGHs) displayed on a ferroelectric liquid crystal (FLC) spatial light modulator (SLM). The SLM provides fast 2-dimensional binary modulation of coherent light and acts as a dynamically reconfigurable diffraction pattern. The dissertation outlines the optical design and implementation of three such free-space switching architectures, where the SLM is used to deflect input signals to the required outpuJs in a bandwidth independent 'optically-transparent' manner. Experimental investigations of a broadband I-to-I5 fibre selector switch, a tuneable wavelength filter, and the initial experimental characterisation of a full N-to-M fibre crossbar switch are presented. These transparent switches offer a more compatible approach for routeing high bandwidth optical signals than existing electronic switch designs. The target applications for this FLCSLM technology are reconfigurationally unintensive operations involving very high optical modulation frequencies, where switch failures would be expensive. Applications include circuit-switching of broadband or wavelength-multiplexed data, and network management or restoration. The diffractive nature of CGHs makes them extremely robust in their operation and therefore well suited to the above applications. The dynamic CGH patterns used to perform the routeing are shift invariant and do not fail catastrophically if individual SLM pixels stop working. The optical efficiencies and crosstalk noise rejection are only limited by the diffraction efficiencies of the CGH-SLM. Relevant theoretical efficiency limits are derived and compared with experimental measurements. Better than -30dB crosstalk rejection is demonstrated by the I-to-I5 switch, and measurements suggest that less than 6dB insertion loss will be possible in future iterations, even with considerably more fibre ports.