1. Field of the Invention
The invention pertains to electro-optic devices for switching unpolarized or polarized optical signals and more particularly to two-state optical switches which couple preselected pairs of terminals in each switch state.
2. Description of the Prior Art
Large information transmission capacity, immunity to electromagnetic interference, and freedom from ground loop problems ideally suit optical transmission systems for linking distributed computers and computer controlled industrial system components. These optical transmission systems utilize optical fibers to serially link a multiplicity of optical repeater stations. Power failure at one of the serially linked stations, however, may interrupt the data chain and cause the entire system to fail. To prevent such a catastrophe, a fail safe optical switch is employed at each repeater and component station which operates to bypass that station when local power is lost. These fail safe switches must possess low insertion loss properties, provide high isolation between the input and the output optical fibers during the "Power On" mode, and must function effectively with unpolarized light coupled thereto from a multi-mode fiber.
One switch design to accomplish this bypass function employs mirrors positioned on a metallic bar that is held in place between input and output optical fibers by an electromagnet. Light entering the input fiber is coupled to the station receiver by reflection from a mirror to an optical fiber coupled to the receiver+s detector. The optical signal from the station transmitter is coupled to an optical fiber wherefrom it illuminates a second mirror and deflected therefrom to an output fiber to propagate therealong to the next station. When the power fails, the electromagnet is deenergized and a spring, attached to the metallic bar, pulls the metal bar-mirror combination from the path between the input and output optical fibers, thus allowing light to be directly coupled therebetween. This system provides adequate switching performance, but does not possess the reliability inherent in an electro-optic switch. Electro-optic switches of the prior art, though possessing sufficiently low insertion losses, do not exhibit sufficiently high isolation between the input and output optical fibers during the energized switch mode. One such switch is described in the technical paper "Electrically Controlled Optical Switch for Multi-Mode Fiber Applications" by R. E. Wagner and J. Cheng, published in Applied Optics Sept. 1, 1980, Volume 19, Number 17. While this switch possesses sufficiently low insertion loss properties, it requires a plurality of costly polarizing beam splitters and extremely tight component tolerances to achieve crosstalk ratios in the energized state of -20 dB at best, a crosstalk level that is not sufficiently low to meet system requirements.