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Time Division Multiplexing

Acronym: TDM, OTDM

Definition: a multiplexing technique operating in the time domain

More general term: optical multiplexing

Opposite term: wavelength division multiplexing

Categories: lightwave communications, methods

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Time division multiplexing is a technique where several optical signals are combined, transmitted together, and separated again based on different arrival times. In an optical fiber communication system, interleaving pulse trains can carry different data channels in a single fiber [1, 3]. The use of multiple channels allows increased overall data transmission capacities without increasing the data rates of the single channels, or transmission of data of different users simultaneously. However, the time slot per bit must be reduced. Even if the bandwidth of the data modulator is limited, this can be done by using a train of ultrashort pulses (rather than a continuous optical wave) as the input of the modulator.

optical time division multiplexing
Figure 1: Schematic of optical time division multiplexing. Two interleaving pulse sequences are combined to a single pulse train. In a communications system, each pulse may represent a “1” bit (if present) or a “0” (if suppressed).

Special requirements of data transmitters for optical time division multiplexing are a short pulse duration and a low timing jitter. Also, the extinction ratio should be high, i.e. each combined channel should exhibit a very low power level between the bit slots, because such a background could otherwise interfere with other channels. For combining the signals, one typically requires some kind of optical delay lines. During signal propagation in an optical fiber, for example, the temporal spreading by effects like chromatic dispersion must be limited, or at least redone before detection with suitable dispersion compensation.

In the context of distributed fiber-optic sensors [2], optical time division multiplexing means that signals are assigned to certain locations in the sensor via their arrival times. Such systems usually operate with ultrashort pulses.

An alternative to time division multiplexing is wavelength division multiplexing, where the channels are distinguished by wavelength rather than by arrival time.

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[1]R. S. Tucker et al., “Optical time-division multiplexing for very high bit-rate transmission”, IEEE J. Lightwave Technol. 6 (11), 1737 (1988), doi:10.1109/50.9991
[2]A. D. Kersey et al., “Fiber grating sensors”, IEEE J. Lightwave Technol. 15 (8), 1442 (1997), doi:10.1109/50.618377
[3]H.-G. Weber et al., “Ultrahigh-speed OTDM-transmission technology”, J. Lightwave Technol. 24 (12), 4616 (2006), doi:10.1109/JLT.2006.885784

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See also: wavelength division multiplexing, optical data transmission, optical fiber communications, fiber-optic sensors
and other articles in the categories lightwave communications, methods


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