Mumbai University > Electronics and Telecommunication > Sem 7 > Optical Fibre Communication

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Optical Time Division Multiplexing (OTDM)

A strategy for increasing the bit rate of digital optical fiber systems beyond the bandwidth capabilities of the drive electronics is known as optical time division multiplexing (OTDM).

OTDM can provide users with better throughput delay performance, faster single-channel access times for high-data-rate end users.

Principle:

The principle of this technique is to extend ETDM by optically combining a number of lower speed electronic baseband digital channels.

The optical multiplexing and de-multiplexing ratio is 1: 4, with a baseband channel rate of 40 Gbit/s. Hence the system can be referred to as a four-channel OTDM system.

Features:

1.Single wavelength

2.High bit rate (up to 640Gb/s)

3.Immature technology

4.Chromatic dispersion & timing jitter

A block schematic of an OTDM system which has demonstrated 160 Gbit /s transmission over 100 km is shown in Figure

Working

1.The four optical transmitters are driven by a common 40 GHz clock using quarter bit period time delays

2.Mode-locked semiconductor laser sources which produced short optical pulses were utilized at the transmitters to provide low duty cycle pulse streams for subsequent time multiplexing

3.Data was encoded onto these pulse streams using integrated optical intensity modulators which gave return-to-zero transmitter outputs at 40 Gbit/s.

4.These IO devices were employed to eliminate the laser chirp (see Section 6.7.3) which would result in dispersion of the transmitted pulses as they propagated within the single-mode fiber, thus limiting the achievable transmission distance.

5.The four 40 Gbit /s data signals were combined using an OTDM multiplexer. Although four optical sources were employed, they all emitted at the same optical wavelength.

6.The 40 Gbit /s data streams were bit interleaved to produce the 160 Gbit/ s signal.

7.At the receive terminal the incoming signal was decomposed into the 40 Gbit /s baseband components in a de-multiplexer.

8.Hence single-wavelength 160 Gbit /s optical transmission was obtained with electronics which only required a maximum bandwidth of about 40 GHz as return-to-zero pulses were employed.

9.The transmitter and receiver employed electro absorption modulators to provide for operation at the high transmission rate and furthermore negative dispersion fibers were also incorporated to compensate for the positive dispersion of the standard single-mode fiber (SSMF).

10.Moreover, a field trial employing such transmitters and receivers at a transmission rate of 160 Gbit /s over deployed SSMF has been successfully carried out.

Advantages

1. The user gets full bandwidth of the channel in a particular time slot

2. For busty signals such as voice TDM gives maximum utilization of the channel

3. Most suitable technique for digital transmission

Disadvantages

1.It is not much suitable for continues signal

2.Extra guard is necessary

3.Synchronization is necessary