Progress in very high bit-rate light wave systems has? been stimulated by consistent demands for expanded transmission capacity. These demands have led to increased interest in multi giga bit per-second pulse-code modulated (PCM) systems, and have put an emphasis on the need for high-speed and wide-band electronics in light wave transmitters and receivers. To date, it has generally been possible to meet these needs with high-speed Si and GaAs circuits, but at gigabit-per-second bit rates it becomes increasingly difficult to develop the necessary digital electronic circuits. One method to relieve this electronic speed bottleneck is to extend the well-known techniques of electrical multiplexing into the optical domain. The two main approaches to optical multiplexing are optical wavelength-division? (or frequency-division) multiplexing and optical time-division multiplexing. This presentation ?concentrates on optical time division multiplexing. In optical time-division multiplexing (OTDM), a high bit-rate data stream is constructed directly by time-multiplexing several lower bit-rate optical streams. Similarly, at the receiver end of the system, the very high bit-rate optical signal is demultiplexed to several lower bit-rate optical signals before detection and conversion to the electrical domain. This approach to optical time-division multiplexing and demultiplexing moves the demand for high-speed performance away from electronic devices such as transistors,? and places it on? optical? and optoelectronic? devices? such? as? pulsed? semiconductor? lasers? and optical? switches.? The? time-division? multiplexing? approach is? a? purely? digital? technique and? is? therefore? compatible? with? the? concept? of? an? all-digital? network? that combines switching and transmission.? In addition,? optical time-division? multiplexing? offers? system? design? flexibility,? including? the possibility? of adjustable? bandwidth? allocation in different baseband channels and the possibility of simple? system? hardware? in? which? only? a? single? transmitter laser is required for all channels. The potential of optical? time division? multiplexing? and demultiplexing? for? very? high? bit-rate? PCM? systems? has been? recognized? for? more? than? two? decades? ?but until? recently? ??there have been few system-level demonstrations? of? the? technique? at? multi gigabit-per- second bit rates. The implementation of very high bit-rate. OTDM systems has been slow because electronic multiplexing has usually served adequately and because the necessary hardware, such as high-speed optical switches and compact pulsed semiconductor lasers, has only recently reached a sufficient state of refinement. This paper describes recent ??experiments in optical time-division multiplexing and demultiplexing that have been made possible by improvements in lasers and switch /modulators. Our emphasis is on very high bit-rate point-to-point transmission systems but many of the concepts are also relevant to multiuser systems and time-multiplexed photonic switching networks. We review system architectures, describe the requirements on individual system components, and give examples of transmission system experiments operating at bit rates up to 16 Gbit /s. One of the key factors affecting the performance of multiplexed systems is crosstalk between baseband channel . In presentation explore sources of crosstalk in optical time-division multiplexing and demultiplexing, and describe how the main system components affect the overall crosstalk performance. Under the simplest conditions, a medium can carry only one signal at any moment in time.?????? For multiple signals to share one medium, the medium must somehow be divided, giving each signal a portion of the total bandwidth.The current techniques that can accomplish this include
1.1 Different types multiplexing techniques
1.frequency division multiplexing (FDM) ?
2.Time division multiplexing (FDM)
3.wavelength division multiplexing (WDM)
4.code division multiplexing (CDM)
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