RECENT FREE-SPACE PHOTONIC SWITCHES

This paper briefly reviews recent studies on free-space photonic switc hes, and discusses classifications, applications and technical issues to be solved. The free-space photonic switch is a switch that uses lig ht beam interconnections based on free-space optics instead of guided- wave optics. A feature of the free-space switch is its high-density th ree-dimensional structure that enables compact large-scale switches to be created. In this paper, the free-space switches are classified by their various attributes such as logical network configuration, path-e stablishment method, number of physical stages, signal-waveform transm ission form. interconnection optics and so on. The logical network con figuration (topological geometry or topology) is strongly related to t he advantages of the free-space switches over the guided-wave switches . The path-establishment method (path-shifting/branching-and-gating) a nd the number of physical stages (single-stage/multistage) are related to physical switching characteristics. Signal-waveform transmission f orm (analog/digital) is related to switch application. Interconnection optics (imaging system/micro-beam system) is related to the density a nd volume of the switching fabric. Examples of the free-space switches (single-stage, analog multistage, digital multistage and photonic ATM switches) are described. Possible applications for analog switches ar e subscriber-line concentrators, inter-module connectors, and switchin g networks for parallel or distributed computer systems. Those for dig ital switches include multistage space-division switches in time-divis ion circuit-switching or packet switching systems (including asynchron ous transfer mode [ATM] switching system) for both communications swit ching systems and parallel/distributed computer systems. Technical iss ues of the free-space switches (system, device, assembly technique) mu st be solved before creating practical systems. In particular, the ass embly technique is a key issue of the free-space switches.