Optical TDM system

Submitted byAmandeep kaur

1243844D4 ECE

• Optical communication• Multiplexing and why• Types of multiplexing• Basic requirement

WHAT IS ??

Types of Multiplexing

• Wavelength division multiplexing(WDM)• Optical time division multiplexing(OTDM)

• Hybrid WDM-OTDM

PROBLEMS WITH OTDM

• Nonlinearity associated with fibre, eg. Stimulated Raman Scattering results in SNR degradation as the number of channel increases

• Four wave mixing: limits the channel spacing• Cross phase modulation: limits the number of channels• High gain flat amplifiers• Packet switched service by means of light paths: an extremely

inefficient way of utilizing network resources

OTDM – What does it offer?• Flexible bandwidth on demand at burst rates of 100 Gb/s perwavelength (in the longer term).• The total capacity of single-channel network = DWDM ,but OTDM provide: potential improvements in network performance in terms of useraccess time, delay and throughput, depending on the user rates andstatistics.• Less complex end node equipment (single-channel Vs.

multichannels)

• Can operate at both: 1500 nm (like WDM) due to EDFA 1300• Offers both broadcast and switched based network.

Key Components for OTDM 1. Transmitter • Ultra short optical pulse generation • Modulation format (OOK, DQPSK, DPSK, M-ARY QAM) 2. Transmission channel • Dispersion-managed fiber for ultra-short pulse propagation 3. Receiver 8 • Optical clock extraction • OTDM demultiplexing

• Higher bit rate multiplexed data streams

System Composition • light emitting part,• transmission • receiving part.

Transmitting Partconsists of • ultra-narrow pulse light source high repetition frequency ultra-short optical pulse source species includes erbium doped fiber ring laser, active modelockingSemiconductor laser, semiconductor ultra-short pulse source, ultrashort optical pulse sources.

• the optical time division multiplexer

The Receiving PartIncludes• demultiplexer,• optical clock extraction • low-speed optical receiver.

DemultiplexerOptical demultiplexer includes• optical Kerr switch,• optical modulator switch, • four wave mixing (FWM) switch, • nonlinear optical loop mirror (NOLM), • cross phase modulation (XPM) switching etc

Demultiplexing schemes for OTDM signals based on (a) cascaded LiNbO3 modulators,(b) XPM in a nonlinear optical-loop mirror, and (c) FWM in a nonlinear medium.

cascaded LiNbO3 modulators

• modulator halves the bit rate by rejecting alternate bits in the incoming signal.• Different channels can be selected by changing the phase of the clock

signal.• Advantage-uses commercially available components.• Disadvantages-limited by the speed of modulators. The electro-optic

technique also requires a large number of expensive components, some of which need high drive voltage.

nonlinear optical-loop mirror (NOLM)

• constructed using a fiber loop whose ends are connected to the two output ports of a 3-dB fiber coupler• Sagnac interferometer• mirror because it reflects its input entirely when the

counterpropagating waves experience the same phase shift over one round trip.

FWM in a nonlinear medium

• OTDM signal is launched together with the clock signal (at a different wavelength) into a nonlinear medium.• the pulse train at this new wavelength is an exact replica of the

channel that needs to be demultiplexed.• A polarization-preserving fiber is often used as the nonlinear medium

for FWM