ANALOG FREE-SPACE OPTICAL SWITCH STRUCTURE-BASED ON CASCADED BEAM SHIFTERS

This paper describes a new free-space optical switch structure based o n cascaded beam shifters (each consists of a liquid-crystal polarizati on controller array and a birefringent plate). This structure comprise s 2-input, 2-output switching elements that are locally connected by l inks. It is applicable to a variety of switching networks, such as a C los network. The switching network based on this structure is an analo g switch that is transparent to signal format, bit rate, and modulatio n type, so it can handle various types of optical signals. Theoretical feasibility studies indicate that compact large-scale switches (i.e., 100-1000 ports) with relay lens systems can be implemented using beam shifters with a 0.4-dB insertion loss and a 30-dB extinction ratio. E xperimental feasibility studies indicate that a 1024-cell beam shifter module with a 0.5-dB insertion loss and a 23-dB extinction ratio is p ossible at present. An alignment-free assembly technique using precise alignment guides is also confirmed. An experimental 8-stage, 1024-inp ut 256-output concentrator shows low insertion loss characteristics (6 .8 dB on average) owing to the low-loss beam shifters and the alignmen t-free assembly technique. Practical switching networks mainly require the improvement of the extinction ratio of the beam shifter module an d the development of a fiber pig-tailing technique. This switch struct ure is applicable to transparent switching networks such as subscriber line concentrators and inter-module connectors.