User talk:2014HZ97018 PTP

Point to point links in optical fiber communication

The point-to-point link involves, in general, many interrelated variables: fiber, optical source and photo detector. In addition, any splices or connectors used to join individual optical fiber sections to each other and to the transmitter and the receiver are included. The links transmit in opposite directions between the equipment’s. This application may be configured using only one fiber. If configured with one fiber, fiber optic splitters are used at each end to couple the transmit signal onto the fiber and receive signal to the detector. Modulation and Source Characteristics o	The transmitting laser contributes noise to the system in a fashion that is, generally speaking, proportional to the peak transmitted laser power o	This noise is always evaluated as a fraction of the laser power and is therefore termed relative intensity noise (RIN) o	The relative intensity noise is affected rather dramatically by the environment of the diode laser o	A rather weak reflection back into the laser will both increase the magnitude of the relative intensity noise and modify its spectrum o	As the reflection increases, it can produce self-pulsations and chaos in the output of the laser, rendering it useless for communication applications Thus, the laser cannot be thought of as an isolated component in the communications system The term topology, as used here, refers to the configuration of various equipment and the fiber optic components interconnecting them. This equipment may be computers, workstations, consoles, or other equipment’s. Point-to-point links are connected to produce systems with linear bus, ring, star, or tree topologies. Point-to-point fiber optic links are the basic building block of all fiber optic systems. The following key system requirements are needed in analyzing a link: 1.	Main parameters influencing the BER 2.	Signal-attenuation 3.	Signal-dispersion 4.	Signal-Distortions in photo receiver 5.	Signal degradation by noise

To fulfill these requirements the designer has a choice of the following components and their associated characteristics: Definition Bit Error Rate (BER): Bit-Error-Rate (BER) defined as number of erroneous bits per unit time at a certain data rate B, or equivalently the probability that a bit is transmitted erroneously multiplied by the data rate B. Multimode or single mode optical fiber (a) Core size (b) Core refractive-index profile (c) Attenuation (d)Numerical aperture or mode-field diameter Coherent Fiber Links We have been looking at Intensity Modulated / Direct Detection (IM/DD) links so far where only optical power is considered (on-off modulation). -The detector detects only the power. -In coherent fibre optic links both the amplitude and phase of the light wave is considered. -Receiver should be phase locked to the transmitter. Impact of Fiber Properties on a Communications Link o	For moderate power level the fiber is a passive dispersive transmission channel o	Dispersion can limit system performance in two ways. It results in a spreading of data pulses by an amount proportional to the spectral width of the source o	This pulse spreading produces what is commonly termed ‘‘inter symbol  interference” o	This should not be confused with an optical interference effect but is simply the blurring of pulse energy into the neighbouring time slot o	In simple terms it can be thought of as a reduction in the modulation depth of the signal as a function of link length o	The effects of dispersion are often quantified in the form of a power penalty o	This is simply a measure of the additional power required to overcome the effects of the dispersion or bring the modulated power to what it would be in an identical link without dispersion Topology in PTP links A linear bus topology consists of a single transmission line that is shared by a number of equipment’s. Generally the transmission line in a fiber optic linear bus consists of two optical lines, one for each direction of communication. A ring topology consists of equipment’s attached to one another in a closed loop or ring. It also routes signals from the transmitter to the fiber connected to the next equipment. In bypass operation, the switch routes signals from the fiber connected to the previous equipment to the fiber connected to the next equipment. In each case, the connection between adjacent equipment’s on the ring is a simple point-to-point link through fiber, connectors, and switches. In the star topology, if the center hub is a passive star coupler, each equipment transmitter is connected to an input port of the coupler and an output port of the coupler is connected to each equipment receiver. The connection between any two equipment’s is a simple point-to-point link through the star coupler. If the center hub is active equipment, the connection between any two equipment’s consists of two point-to-point links. A tree topology, each branching point either a passive fiber optic splitter or an active branching device is used. In many cases both passive couplers and active branching devices are used within a particular system. Regardless of the branching method, each connection within the tree is a simple point-to-point link through splitters or multiple point-to-point links through active branching devices. External Modulation 	Currently preferred over any other form of modulation 	Done using an MZIM 	Phase difference produced in the beam

Sources Books & Internet.