hted TR 61282-4 @IEC:2013(E) 4 General 4.1 Systemtrendsleadingtonon-lineareffects The market demand for new advanced telecommunications services has driven the rapid increaseof system bandwidth,and,forsomeapplications,longer system distances. Greater bandwidth has been addressed in two ways.One way is by increasing the channel bit- rate, accomplished with optoelectronic time-division multiplexing (TDM) and various types of signal encoding. Another way is by increasing the number of channels, accomplished with wavelength division multiplexing (DWDM).Bandwidth limitations of the optical fibre cable can beovercomewithvariousdispersionmanagementtechniques. So Longer distances,defined to be the optical path lengths between 3R regenerators,can be fic) achieved bytwo methods.One method is by increasingthespanlength,where a span is defined to be the optical path between optical amplifiers (OAs).A longer span length may be nc. attainedwithfibrecableoflowerattenuationcoefficientandwithfibreopticpassive launched channel power from the output of the OA at the beginning of the span or with lower allowedpowerattheinputof theOAatthe endofthespan.Anothermethodof increasingthe optical path length is to increase the number of spans.This increases the number of OAs, but improvements can be limited by amplifier noise degradation. There are a number of interactive trade-offs in system design. For example, increasing the bit- rate reduces the span length by requiring higher received power or by requiring lower link dispersion.The latter may be addressed by dispersion compensation,but this introduces losses. Increasing the number of channels in DwDM systems aiso reduces span length due to optical multiplexing and demultiplexing losses.The loss limitations of a span can be overcome with OAs, butthese introduce noise. 4.2 Opticalamplifiersand non-linearities -27 An OA accepts a modulated signal at its input and emits an essentially identically shaped signal at its output. However, the optical power is higher (desired), and there is some additional noise (not desired). This technical report is concerned with the effects of higher power on the fibre and the implications for system design. These non-linear effects are so-called because they are not linearly proportional to launched power into the fibre or to the fibre length in either absolute units or in dB units. They are affected primarily by characteristics of the optical signal (power, optical spectrum,modulation,state of polarization),of the optical fibre (effective area, effectivelength,gaincoefficients,non-linearindex,dispersion,dispersionslope,polarization modedispersion),andofsystem aspects suchas distancebetweenregeneratorsandthe numberandspacingofchannelsinDwDMsystems.Powerlevelsaslowasseveral mWcan induce non-linear effects. One class of non-linear effects is stimulated scattering of the signal. Stimulated Brillouin scattering limits the power transmitted through the fibre by scattering some light backwards in the fibre. Stimulated Raman scattering mainly causes forward crosstalk in a DwDM system. Another class of non-linear effects is phase-shifting of the signal.This leads to self-phase modulation and modulation instability that produce distortion even on a single channel, or to cross-phase modulation and four-wave mixing that introduce interference between channels tion These interact with chromatic dispersion to degrade or enhance system performance. Soliton formation is another related effect. ted. Copyr yrighted material l - 8 - TR 61282-4IEC:2013(E) 4.3 Background and notation 4.3.1 Wavelength andfrequency These simple concepts are essential in discussing advanced optical transmission systems OnecaninterchangeablytalkaboutthevacuumwavelengthZinnmandopticalfrequencyvin modulation frequency f or the signal bit-rate B. By using the speed of light in a vacuum c, one Reuters a(nm)×v(THz)= c(nm/ps) (1) where c299,792,458 nm/ps (Scientific), The fundamental mode of a single-mode fibre has a