Optical Fiber Communication System Design

Zena Mohammed

Outline

Transmitter Design.

Advantages of optical fiber communication.

Enormous potential bandwidth.The optical carrier frequency in the range to Hz (generally in the near infrared around Hz or GHz) yields a far greater potential transmission bandwidth than metallic cable systems.

Small size and weight.Optical fibers have very small diameters which are often no greater than the diameter of a human hair. Hence, even when such fibers are covered with protective coatings they are far smaller and much lighter than corresponding copper cables.

Signal security.The light from optical fibers does not radiate significantly and therefore they provide a high degree of signal security.

Advantages of Optical Fiber Communication

Electrical Isolation.Optical fibers which are fabricated from glass, or sometimes a plastic polymer, are electrical insulators and therefore, unlike their metallic counterparts, they do not exhibit earth loop and interface problems.

Immunity to interference and crosstalk. Optical fibers form a dielectric waveguide and are therefore free from electromagnetic interference (EMI), radio-frequency interference (RFI), or switching transients giving electromagnetic pulses (EMPs).

Low transmission loss.Fibers have been fabricated with losses as low as 0.15 dB and this feature has become a major advantage of optical fiber communications

Ruggedness and flexibilityAlthough protective coatings are essential, optical fibers may be manufactured with very high tensile strengths

Potential low cost. The glass which generally provides the optical fiber transmission medium is made from sand – not a scarce resource.

System reliability and ease of maintenance. The reliability of the optical components is no longer a problem with predicted lifetimes of 20 to 30 years being quite common. Both these factors also tend to reduce maintenance time and costs.

Optical Transmitters (Optical System)

The role of the optical transmitter is to:oconvert the electrical signal into optical form, ando launch the resulting optical signal into the optical fiber.

Two Important Specifications of The Optical Transmitter:(i) the spectral linewidth and(ii) the extinction ratio. The values that can be achieved for these parameters depends on whether direct or external modulation is used.

Extinction Ratio.▫ Optical transmitters, no matter if directly or externally modulated. do not shut off

completely when a zero is transmitted. This undesired effect is quantified by the extinction ratio (ER), which is defined as follows:

ER= where is the optical power emitted for a zero and the power for a one.

the spectral line-width• In practice, it is difficult to build a transmitter with a linewidth

as narrow 0.08nm for . only some types of external modulators can come close to this ideal. Optical pulses that do have this narrow spectrum are known as transform limited pulses.

Incoherent emission from an LED usually displays a spectral line-width of between 20 and 50 nm when operating in the 0.8 to 0.9 μm wavelength range.

LEDs have wide spectral line-width as compare to ILDs.

Wide line width of the LEDs causes material dispersion within the fiber.

The optical transmitter consists of the following components:

Optical Source.

Electrical Pulse Generator.

Optical Modulator .

Optical Light Sources

LEDoSemiconductor deviceoMedium modulation speedoIncoherent output lightoMainly used for short range FSO systems (shorter than 1 km)

LaseroHighly directional beam profile

oUsed for long range FSO systems

oHigh modulation speed

oCoherent output light

LampoLower efficiency

compared to LED and laser

oLower costoLow modulation speedo Incoherent output lighto Provides higher power

LED Types

Fig: Dome LED Fig: Planar LED

Fig:Edge-Emitting LED

Fabry-Perot Laser

Distributed Feedback Laser

Vertical-cavity surface-emitting Laser (VCSEL)

Optical transmitter is a device that generates the signal sent through optical fibers.

The basic elements of optical fiber transmitter are shown in Figure below :

Transmitter Design

The basic elements of optical fiber transmitter:

oElectronic Interface: There is wires standard electronic connection or pins energizing the transmitter. They provide power Electronic I/P and O/P Optical signals.

oElectronic ProcessingIn some transmitters the I/P Electrical signals are electronically processed to put them into of suitable from to drive the light source.

oDrive CKTThis depends on application, requirements, data format and the light source.

oOptical Monitor It Monitors the O/P of the LASER and provides feedback to the drive CKT so that the O/P power remains stable.

oTemperature MonitorThe characteristic of semi-conductor LASER changes in temperature. The lifetime of LASER decreases with increase in operating temp and the O/P power also decrease which produce some change in O/P wavelength of the light, to keep the operating temp stable the Thermo-electric coolers are used in optical fiber transmitters these coolers control the temp of LASER.

Optical Modulator The following modulators commonly are used in optical transmitters:

o The electroabsorption modulator (EAM), which is small and can be driven with a reasonably small voltage swing. Electrically, it is a reverse-biased p-n junction.

o The Mach-Zehnder modulator (MZM), which generates the highest-quality optical pulses with a controlled amount of chirp and a high extinction ratio. Electrically, it is a (terminated) transmission line.

• The maximum transmission distance that can be achieved in an optical communication system is determined by a combination of the chromatic dispersion limit, the polarization-mode dispersion (PMD) limit, and the attenuation limit.