Wavelength-Selective Devices
A single optical fibre is capable of transporting multiple signals, each at its own wavelength. At the fibre entrance, the signals fed by a number of transmitters, each operating at a specific wavelength, are combined by a wavelength multiplexer and launc
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Wavelength-Selective Devices
Meint K. Smit, Ton Koonen, Harald Herrmann, and Wolfgang Sohler
7.1
Introduction
A single optical fibre is capable of transporting multiple signals, each at its own wavelength. At the fibre entrance, the signals fed by a number of transmitters, each operating at a specific wavelength, are combined by a wavelength multiplexer and launched into the transmission fibre. At the fibre exit, a wavelength demultiplexer routes these signals again according to their wavelengths to one or more receivers. If the demultiplexer selects only one wavelength channel, it is usually called a wavelength filter. When the fibres at the input and at the output of the device are identical, a wavelength multiplexer can typically be used as a wavelength demultiplexer in the opposite direction also, and vice versa. In a point-to-point link, each wavelength constitutes an independent communication path in the fibre, as illustrated in Fig. 7.1. Thus the data transport capacity of the fibre is increased by this same number of wavelengths. Besides this capacity enhancement, in an optically transparent multipointto-multipoint network the wavelength dimension can also be used as an extra degree of freedom to route signals. If the network itself does not perform any wavelength-selective routing but leaves the wavelength selection to the terminals, the signals arrive from source to destination in a single hop. Wavelengthtransmitters
receivers
fibre
Fig. 7.1. Point-to-point multi-wavelength communication link
N. Grote et al. (eds.), Fibre Optic Communication Devices © Springer-Verlag Berlin Heidelberg 2001
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Wavelength-Selective Devices
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Fig. 7.2. Single-hop multi-wavelength multipoint-to-multipoint network
routeing inside the network guides the signals from node to node in a multihop fashion. In a single-hop network, as shown in Fig. 7.2, the transmitter of each user may be set to a specific wavelength position. Also, the receiver of each user may be tuned to the wavelength desired. When different sets of users make their wavelength choices adequately, multiple communication paths may be established simultaneously in the same network without data-packet collisions. At the transmitting end as well as at the receiving end, the wavelengthselective mechanism can be fixed or tunable. When the transmitter wavelengths are fixed and the receiver wavelengths are tunable, the network operates in the 'broadcast-and-select' mode. Each user has access to the wavelength he is interested in, thus reducing the privacy of information transmitted if no extra measures are taken. On the other hand, broadcasting information from one user to all other users is inherently easy. When the transmitter wavelengths are tunable and the receiver wavelengths are fixed, the network operates in the 'wavelength addressing' mode. Provided that the user cannot manipulate the wavelength setting of his receiver, privacy is secured. Broadcasting information, however, is hampered. Making some of the transmitters as well as some of the receiver
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