A carbon granule microphone with carbonaceous electrodes, which is lower in cost and more convenient for mass production than the conventional microphone with gold plated electrodes, was studied as a telephone microphone. A molded piece made from a mixture of synthetic resin and carbon powder can be used for the back electrode and paint made from it can be used to coat the dome electrode. A new microphone, whose sensitivity is as high as a conventional microphone, shows higher resistance. This means that the efficiency of the new microphone seems to be slightly less. However, the sensitivity change in the new microphone under excitation by voice is less than that of the conventional microphone. The new microphone sensitivity dependence on DC input power is almost the same as that of a conventional microphone.

This paper describes a new free-space optical switch structure based on cascaded beam shifters (each consists of a liquid-crystal polarization controller array and a birefringent plate). This structure comprises 2-input, 2-output switching elements that are locally connected by links. It is applicable to a variety of switching networks, such as a Clos network. The switching network based on this structure is an analog switch that is transparent to signal format, bit rate, and modulation 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. Experimental feasibility studies indicate that a 1024-cell beam shifter module with a 0.5-dB insertion loss and a 23-dB extinction ratio is possible at present. An alignment-free assembly technique using precise alignment guides is also confirmed. An experimental 8-stage, 1024-input 256-output concentrator shows low insertion loss characteristics (6.8dB on average) owing to the low-loss beam shifters and the alignment-free assembly technique. Practical switching networks mainly require the improvement of the extinction ratio of the beam shifter module and the development of a fiber pig-tailing technique. This switch structure is applicable to transparent switching networks such as subscriber line concentrators and inter-module connectors.