Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Patent
1994-09-08
1997-04-08
Chin, Wellington
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
359108, H04J 1408
Patent
active
056193602
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to optical processing in networks carrying packetised signals, and in particular to an all-optical code recognition unit for such a network.
2. Related Art
Optical fibre communication offers many advantages over conventional wire based systems, these advantages including reduced losses, increased bandwidth, immunity from electromagnetic interference (EMI), and a high level of security. The application of optical fibre technology into the local area network (LAN) is, therefore, of increasing interest. In the past, however, it has been assumed that optical networks will only penetrate small business and residential sectors if new broadband services are provided to offset the additional costs involved in the installation of the optical technology. Some of the broadband services that could be provided are alpha-numeric videotex (e.g. Prestel), photographic videotex, high definition television, interactive video on demand (video library), video telephony, interactive graphics and high-speed data services.
Although the importance of providing such services has been recognised for some time, it is difficult for telecommunications operating companies to predict their market potential and therefore justify a major investment. What is required is an entry strategy that allows optical technology to be installed economically for telephony and low-speed data services, while maintaining the potential for evolution at a marginal cost for future broadband services.
In known optical networks, routing of information is achieved at each node by electronic means, that is to say by detecting the received optical signal to give an electrical signal (plus detector noise). This electrical signal must be regenerated, after processing and switching to remove the noise, before the signal is re-transmitted optically. Regeneration is bit-rate dependent, and so restricts the information type that can be carried, thereby preventing the transmission of broadband services. The need for regeneration could be removed by coupling off, at each node, part of the received optical signal, the coupled-off signal being converted to an electronic signal which is electronically processed, the remaining uncoupled optical signal being rerouted by the electronic processor. Unfortunately, the electronic processing times severely limit the possible capacity of the optical links, so again the provision of broadband services is not practical. Thus, although the electronic processor can switch quickly (of the order of nanoseconds) it requires a relatively long time (of the order of microseconds) to process, and therefore to decide upon the necessary route of the signal. In this scheme, the uncoupled optical signal is delayed during the processing time by a long length of optical fibre, and this obviously increases the size of each switching node.
Optical routing of information at the nodes of such an optical network would increase the capacity of the network by reducing the processing time. Not only would this increase the capacity of the network, it would also decrease the vast delay lengths of optical fibre otherwise required. Optical signal processing is well known, but the particular method of optical routing in a given network will depend upon the nature of that network. A particularly advantageous type of optical network is the recently developed telephony passive optical network (TPON). This type of network has no routing mechanisms, that is to say all terminals receive all the information transmitted by the exchange. One of the main advantages offered by TPON is the ability to move transmission between customers. This is because the gross bit-rate used with TPON is 20 Mbit/s (chosen mainly to allow cheap CMOS realisation of signal processing chips), and this is divided into a basic granularity of 8 Kbit/s, that is to say 8 Kbit/s is the basic transmission unit that can be moved from customer to customer (or can be summed to provide channels of n.times.8 Kbit/s capacity). T
REFERENCES:
patent: 4922479 (1990-05-01), Su
patent: 5077727 (1991-12-01), Suzuki
Prucnal et al. "Optically-Processed Routing For Fast Packet Switching", IEEE LSC Magazine, vol. 1, No. 2, May 1990, pp. 54-66.
Hansen et al. "Optical Demultiplexing at 6 Gb/s Using a Semiconductor Laser Amplifier as an Optical Grate", Photonics Technology Letters, vol. 3, No. 11, Nov. 1991.
Marshall Ian W.
Tweddle Mark B.
British Telecommunications PLC
Chin Wellington
Negash Kinfe-Michael
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