1. Field of the Invention
The present invention relates generally to electro-optical devices for directing the propagation of light energy and, more particularly, concerns light guiding apparatus for multiplexing and demultiplexing data conveyed by propagating light streams.
2. Description of the Prior Art
Arrangements for guiding multimode optical beams within plates of electro-optical materials such as LiTaO.sub.3, LiNbO.sub.3, or the like have been discussed by the present inventor and his associates in the technical paper: "Electro-Optic Channel Waveguide Modulator for Multimode Fibers," Applied Physics Letters, Volume 28, No. 6, Mar. 15, 1976, page 321. In such devices, opposed metal strip electrodes may be affixed to both major faces of the transparent electro-optically active plate. Light is coupled into and out of the plate by glass fiber guides abutting at normal incidence the ends of the active plate adjacent opposide ends of the electrodes. Small refractive index changes produced in the electro-optic plate when suitable voltages are applied to the opposed electrodes control propagation of light from the input fiber guide with respect to an output fiber guide. Thus, modulators, hybrid couplers, and switches have been devised using various configurations of such guided wave optical systems. Also, multiplexing and demultiplexing devices have been formed by combining a plurality of electrically switchable three-port hybrid couplers. Two such switchable hybrid couplers may be combined to form a high-isolation single-pole, double throw switch, while N such couplers may be employed to generate an electrically controllable N:1 multiplexer or demultiplexer.
Electrically switchable three-port optical hybrid couplers exist in the prior art. However, with the exception of the coupler described by the present inventor and his associates in the technical paper: "Multimode Achromatic Electro-Optic Waveguide Switch for Fiber-Optic Communications," Applied Physics Letters, Volume 28, No. 12, June 15, 1976, page 716, these prior art switchable hybrid couplers generally operate only with a single mode of optical energy propagation and therefore do not have certain desirable structural features and the consequent advantages of the multimode coupler. The switchable multimode coupler of the latter technical paper is again provided with input and output fibers and with pairs of electrodes that are electrically decoupled by a gap therebetween to establish a main optical channel and a branching channel. A voltage is applied to the main channel electrodes across the thickness of the crystal in a manner to induce a refractive index increase and to form a light wave guide under the same electrodes. With no voltage or a reverse voltage applied to the branch guide electrodes, light energy does not significantly couple to the branch channel. When a voltage is applied to the branch guide electrodes, the light energy propagating in the main channel is caused to divide between the two optical channels.
These and other electro-optically switchable multimode devices may be used according to the present invention to afford efficient multiplexer or demultiplexer devices. For example, a further switchable coupler device suitable for the present purpose is presented in the A. R. Nelson U.S. patent application Ser. No. 796,103, filed May 12, 1977 for an "Electro-Optic Multiplexer with High Interchannel Isolation" and assigned to Sperry Rand Corporation. This latter device employs input and output fiber guides coupled to optical barrier wave guides formed by the application of voltages to electrodes positioned on the top and bottom surfaces of an electro-optic crystal to decrease the refractive index in the crystal between the electrodes and thus to form optical wave guides which are outlined by the electrodes. The electrodes may be positioned, for example, to form a main optical wave guide, a branch optical wave guide, and an optical gate at the intersection of the main and branch wave guides. Coupling of optical energy from the main wave guide to the branch wave guide is accomplished by removing the voltage from the gate electrode, thus eliminating the barrier existing between the two wave guides. Optical energy may be switched between the two wave guides by providing a second gate electrode in the main wave guide path and alternately applying a voltage between the two gate electrodes.
These and other switchable coupler devices, whether single or multimode, generally require relatively large voltages and power levels for excitation of the switching electrodes. These voltage levels, being of the order of 400 volts, for example, represent a principle limitation to the utility of prior electrically switchable devices with respect to, for instance, switches, multiplexers, and demultiplexers of the surface wave type. To realize a truly competitive optical multiplexer design, the low switching voltage levels available from simple conventional semiconductor driver stages are needed, levels such as about five volts. While the optical requirements for multiplexing will be reasonably well met by available prior art switchable optical couplers, the voltage and consequent power requirements have not been economically met. It is therefore the primary object of the present invention to provide a system for operating electrically switched coupler devices in optical multiplexers and demultiplexers such that relatively low excitation or switching voltages and low power may be efficiently employed.