A Variable Optical Attenuator (VOA) based on a Polymer-Dispersed Liquid Crystal (PDLC) Cell is presented. The VOA's developed prototype has been successfully tested at 660nm, 850nm and 1300nm. This ability makes it strongly recommended for networks using Perfluorinated Gradual Index Polymer Optical Fiber (PF GI-POF) technology. The prototype has revealed a better than 12dBs dynamic range and losses of <1.2dB. In addition, the VOA presents a very low power consumption and non-dependendence with polarization. In order to avoid the dependence with temperature, an electrooptical feedback is incorporated to the device, by means of a microcontroller system. Electrically controllable intermediate transmission levels can be selected this way. The response time is in the milisecond range. The present feedback prototype, developed with POF technology, includes lenses, PDLC, 1x2 POF couplers and fotodiodes in the optical part, and a microcontroller system where the feedback processing is carried out. No polarizers are required so that optical losses are minimal. Polymer-Dispersed Liquid Crystals are formed by microdroplets of liquid crystal embedded in a flexible matrix, and sandwiched between transparent electrodes. This structure scatters strongly the light. When an AC electrical field is applied to the film the material becomes transparent. A largest dynamic range could be achieved designing conveniently the radius of the microdroplets. No-dependence with polarization, high transmittance when activated, and large dynamic range within a wide range of optical wavelengths make PDLC the most appropriate liquid crystal technology for VOAs fabrication.
A 2x2 optical switch for plastic optical fibre (POF) has been developed, able to work for both 660 and 850nm simultaneous and independently of the input light's polarization, improving previous developments. The device has four bidirectional optical ports, and is able to switch from each port to any other. In this way, there are three operation modes: straight (each input connected to the corresponding output), crossed (inputs and outputs crosses) and closed (inputs connected on the one part, and output connected on the other part). As the device is bidirectional, inputs and outputs are interchangeable. The switching process is carried out by a set of Polarized Beam Splitters, Liquid Crystal cells, λ/4 plates, lens and mirrors. An electronic circuitry has been developed to control the state of the optical switch, which is shown in a Liquid Crystal Display. The system has been tested for both 660nm and 850nm, and the optical switch exhibits miliseconds switching times, an optical interchannel crosstalk better than -25 dB, and low power consumption. Applications of the switch include systems where a redundant path is needed to guarantee communication, such as safety systems in automobiles, LANs, telemedicine, heavy machinery in the industry along with coarse WDM GI (graded index) POF networks. Device size reduction is under development.