Advanced devices based on fibers, integrated optics and liquid crystals for WDM networks

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The increment of bandwidth required for new services offered to users make necessary the use of optical fibres in data transmission. Glass optical fibres are widely used in long distance communications, and there are many devices implemented for using in these networks, but these technologies are sometimes expensive for their used in local loops. Different systems implemented over the established technology are used for increasing local loops bandwidth, but more services are demanded at home. Those applications require more bandwidth than the offered by the usual twisted copper pair. Multimode fibres (both silica and polymer) with larger core diameters and numerical aperture, allows for large tolerance on axial misalignments, which results in cheaper connectors as well as associated equipment, but with a bandwidth penalty with regards to their singlemode counterparts, mainly due to the introduction of modal dispersion. On the other hand, polymer optical fibre (POF) offers several advantages over conventional multimode optical fibre over short distances (ranging from 100m to 1000m) such as the even potential lower cost associated with its easiness of installation, splicing and connecting. This is due to the fact that POF is more flexible and ductile, making it easier to handle. Consequently, POF termination can be realized faster and cheaper than in the case of multimode silica fibre. Therefore, the number of applications that use POF is quickly increasing. POF is being used in video transmission in medical equipment, or in multimedia applications for civil aviation and high range cars, in-home and access networks, wireless LAN backbone or office LAN, and in intrinsic optical sensor networks among others. Even greater channel capacity can be available using a specific type of POF, perfluorinated Graded-Index POF (PF GIPOF), having low attenuation and large bandwidth from 650nm to 1300nm. Link lengths for in-building/home scenarios are short (less than 1 km), and thus the loss per unit length is of less importance. Transmission of 10Gbps data over 100m and transmission of 1.25Gbps Ethernet over 1 km have been experimentally demonstrated with PF GIPOF. On the other hand, combiners and multiplexers are basic elements in POF networks using Wavelength Division Multiplexing (WDM) and there are not that many already developed. It is important to have low losses devices and reconfiguration can be an additional feature in those networks. On the other hand, reconfigurable optical networks in critical applications demand devices able to have different functionalities, including switching. This work has focused in the development of different optical switches for a wide range of optical networks. Different switching technologies are available. Liquid crystals are widely used as displays, but they are also employed in telecommunications. Other common technology used in data routing is integrated optics. In this case, light propagates by means of a waveguide and the modification of its parameters makes possible switching operation. Micro-Electromechanical Mechanisms, MEMs, based in small mobile mirrors that can change the direction of the incident light when required are an important optical switching technology. The objective of the present work is the proposal of several optical switches using different technologies depending on the final application. Some of these structures have been experimentally tested whereas others have been simulated. Most of the presented switches use liquid crystals, having different functionalities and broadband operation range, so allowing wavelength division multiplexing. To these respect it has been developed an optical multiplexer/combiner and an advanced multifunctional optical switch (AMOS), both implemented with Nematic Liquid Crystal technology. It has also been developed a multiplexer/combiner based on Polymer Dispersed Liquid Crystals. The third kind of switches proposed are micro ring-resonators combined with liquid crystals. Micro ring-resonators consist of a circular waveguide attached to one or two straight waveguides acting like input and output ports. Light that passes through the structure can be filtered according to the ring resonator characteristics: ring length, coupling ratio, losses… The use of liquid crystal makes possible the tuning of the ring resonator filtering properties. The last proposed switch is made of a passive splitter and a Mach-Zehnder Interferometer. This kind of devices makes use of integrated optics and interference for switching purposes. The variation of the optical properties influencing the two light beam interference can be done in different ways: thermally, electrically… Finally, an automated optical characterization bench has been implemented in order to make easy the measurements. It is composed by a three axis translation stage with three actuators, several linear translation stages that allows the user to modify the bench structure for adapting it to his experiment, and different machinery for mounting the optics.
Optical fibers, Liquid crystals, WDM networks
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