Optoelectronic matrix switching

Because cross talk from electromagnetic coupling increases with frequency, it is difficult to design matrix switches for high-frequency electronic signals. Signals on optical carriers are easier to isolate but more difficult to switch. A hybrid technique exploiting passive optical-signal distribution and switching by optoelectronic effects shows good promise for high isolation matrices to handle signals in the dc to 10 GHz range. The key elements for such optoelectronic switches are optical detectors that have switchable sensitivity.Initial efforts in optoelectronic switching used silicon diode detectors and forward bias to establish the off-state. Recent work has turned primarily to photoconductors fabricated in GaAs or other compound semiconductors. Such photoconductors can have very wide response bandwidths and have the advantage of a zero-bias off-state. Efforts have also been made to develop bipolar detectors that do not need forward bias for the off-state.Experimental work has progressed to the demonstration of complete matrices capable of switching up to eight incoming signals to as many outgoing lines, at bandwidths up to 500 MHz and cross-talk levels below 50 dB. Results for individual switching devices give promise of matrices for many tens of lines and bandwidths to several gigahertz. Monolithic integration of GaAs photoconductor switch arrays has been demonstrated, and work progresses to include integrated control devices and amplifiers.Matrix switches of large dimension and very large bandwidth are expected to have significant applications in areas other than communications switching. In particular, such matrices can be used to set up patterns of delay and signal recombination, using fibre delay lines, to provide rapidly selectable wide-band delay for phasing and other applications, or for wide-band tapped delay-line filters that can be reconfigured rapidly.A review of the current status in optoelectronic matrix switching is given.