From an architectural standpoint, it isclassified as,… POINT TO POINT LINKS DISTRIBUTION NETWORKS LOCAL AREA NETWORKS
• They transport information, available in the formof a digital bit stream• The link length can vary from less than akilometer to 1000’s of kilometer• They are used for high speed transmission• Optical regenerators should perform, 1. Re-amplification 2. Re-shaping 3. Re-timing
System Requirements,1. Transmission Distance2. Data Rate for a given BER
o Attenuationo Distance Bandwidth Producto Cost of the connectorso SplicingThen decide,• single or multimode fiber• step or graded index fiber
•Emissionwavelength•Spectral line width(FWHM) andnumber of modes•Output power•Stability•Emission pattern•Effective radiatingarea
Type of detector APD: High sensitivity but complex, high bias voltage (40V or more) and expensive PIN: Simpler, thermally stable, low bias voltage (5V or less) and less expensive Responsivity (that depends on the avalanchegain & quantum efficiency) Operating wavelength and spectral selectivity Speed (capacitance) and photosensitive area Sensitivity (depends on noise and gain)
Wavelength LED Systems LASER Systems.800-900 nm 150 Mb/s.km 2500 Mb/s.km(TypicallyMultimodeFiber)1300 nm (Lowest 1500 Mb/s.km 25 Gb/s.kmdispersion) (InGaAsP Laser)1550 nm (Lowest 1200 Mb/s.km Up to 500Attenuation) Gb/s.km (Best demo)
If the signal is detected by a receiver thatrequires a minimum average power at the bitrate B, the maximum transmission distance islimited The system requirements typically specified inadvance are the bit rate B and thetransmission distance L The performance criterion is specified throughthe bit-error rate (BER), a typical requirementbeing BER < 10−9.
• When the dispersion-limited transmissiondistance is shorter than the loss-limiteddistance of the system is said to be dispersionlimited. BL ≤ (4|D|σλ )−1• A solution to the dispersion problem isoffered by dispersion-shifted fibers forwhich dispersion and loss both are minimumnear 1.55 μm.
o The purpose of the power budget is to ensurethat enough power will reach the receiver tomaintain reliable performance during the entiresystem lifetimeo The minimum average power required by thereceiver is the receiver sensitivityo It is expressed in dBm
• Used to ensure that the system is able to operateproperly at the intended bit rate• Even if the bandwidth of the individual systemcomponents exceeds the bit rate, it is still possiblethat the total system may not be able to operate atthat bit rate• It is used to allocate the bandwidth amongvarious components
• The rise time Tr of a linear system is defined as thetime during which the response increases from 10 to90% of its final output value when the input ischanged abruptly.• When the input voltage across an RC circuitchanges instantaneously from 0 to V0, the outputvoltage changes as, Vout(t) =V0[1−exp(−t/RC)]
Here we focus on the factors that limit theperformance of amplified fiber links It depends on following factors, 1. Performance - limiting factor 2. Terrestrial light wave systems 3. Undersea light wave systems
The sensitivity of the optical receiver in a realisticlightwave system is affected by several physicalphenomena which, in combination with fiberdispersion, degrade the SNR at the decision circuit
Among the phenomena that degrade the receiversensitivity are,1. Modal noise2. Dispersion broadening3. Intersymbol interference4. Mode-partition noise5. Frequency chirp6. Reflection feedback.