Chapter 8:Optical Fibers and ComponentsTOPICSWDM optical networksLight transmitted through an optical fiberTypes of optical fibersImpairmentsComponents: Lasers, optical amplifiers, couplers, OXCs
WDM optical networksA point-to-point connectionIn-lineamplificationopticalfiberopticalfiberW1 TxWavelengthmultiplexerPoweramplifierW1 TxRxRxWavelengthdemultiplexerPre-amplifier
An example of an optical network
How light is transmitted through an optical fiberWaves and electrical fieldsSourceElectricfieldWave
An optical fiberCoreCladdingCladdingRadial distanceRefractive indexn1n2CoreCladdingRadial distancen2n1CladdingCoreRefractive indexa) Step-index fiber b) graded-index fiberCore and cladding
Refraction and reflection of a light rayIncident rayReflected rayRefracted rayrfn2n1
Angle of launching a ray into the fiberCoreCladdingCladdingrlCladdingCladdingCoreOpticaltransmitterCladdingCladdingCore
Multi-mode and single-mode fibersCore/diameter of a multi-mode fiber:50/125 m, 62.5/125 m, 100/140 mCore/diameter of single-mode fiber9 or 10 / 125 m
Electric field amplitudes for various fiber modesCladdingCladdingCorem=0m=1m=2
Propagation of modesCladdingCladdingCladdinga) step-index fiberb) Graded-index fiberCladding
ImpairmentsThe transmission of light through an optical fiber is subjected to optical effects, known as impairments. There are:linear impairments, andnon-linear impairments.
Linear impairmentsThese impairments are called linear because their effect is proportional to the length of the fiber.Attenuation:Attenuation is the decrease of the optical power along the length of the fiber. Dispersion Dispersion is the distortion of the shape of a pulse.
Attenuation800100012001400160018000.51.01.52.02.5Wavelength, nmAttenuation, dB
Attenuation in FiberAttenuationP(L) = 10-AL/10P(0)Where P(0) optical power at transmitter,P(L) power at distance L Km, andA = attenuation constant of the fiberReceived Power must be greater or equal to receiver sensitivity Pr
Lmax = 10/A log10(P(0)/P(r))
DispersionDispersion is due to a number of reasons, such as modal dispersion, chromatic dispersion, polarization mode dispersion.
Modal dispersionIn multi-mode fibers some modes travel a longer distance to get to the end of the fiber than othersIn view of this, the modes have different delays, which causes a spreading of the output pulse PowerPowerTimePowerTime
Chromatic dispersionIt is due to the fact that the refractive index of silica is frequency dependent. In view of this, different frequencies travel at different speeds, and as a result they experience different delays. These delays cause spreading in the duration of the output pulse.
Chromatic dispersion can be corrected using a dispersion compensating fiber. The length of this fiber is proportional to the dispersion of the transmission fiber. Approximately, a spool of 15 km of dispersion compensating fiber is placed for every 80 km of transmission fiber. Dispersion compensating fiber introduces attenuation of about 0.5 dB/km.
Polarization mode dispersion (PMD)It is due to the fact that the core of the fiber is not perfectly round. In an ideal circularly symmetric fiber the light gets polarized and it travels along two polarization planes which have the same speed. When the core of the fiber is not round, the light traveling along the two planes may travel at different speeds. This difference in speed will cause the pulse to break.
Non-linear impairmentsThey are due to the dependency of the refractive index on the intensity of the applied electrical field. The most important non-linear effects in this category are: self-phase modulation and four-wave mixing. Another category of non-linear impairments includes the stimulated Raman scattering and stimulated Brillouin scattering.
Types of fibersMulti-mode fibers: They are used in LANs and more recently in 1 Gigabit Ethernet and 10 Gigabit Ethernet.Single-mode fiber is used for long-distance telephony, CATV, and packet-switched networks. Plastic optical fibers (POF)
Single-mode fibers:Standard single-mode fiber (SSMF): Most of the installed fiber falls in this category. It was designed to support early long-haul transmission systems, and it has zero dispersion at 1310 nm.Non-zero dispersion fiber (NZDF): This fiber has zero dispersion near 1450 nm.
Negative dispersion fiber (NDF): This type of fiber has a negative dispersion in the region 1300 to 1600 nm.Low water peak fiber (LWPF): The peak in the attenuation curve at 1385 nm is known as the water peak. With this new type of fiber this peak is eliminated, which allows the use of this region.
Plastic optical fibers (POF) Single-mode and multi-mode fibers have a high cost and they require a skilled technician to install them. POFs on the other hand, are very low-cost and they can be easily installed by an untrained person. The core has a very large diameter, and it is about 96% of the diameter of the cladding. Plastic optic fibers find use in digital home appliance interfaces, home networks, and cars
ComponentsLasersPhoto-detectors and optical receiversOptical amplifiersThe 2x2 couplerOptical cross connects (OXC)
Light amplification by stimulated emission of radiation (Laser)A laser is a device that produces a very strong and concentrated beam. It consists of an energy source which is applied to a lasing material, a substance that emits light in all directions and it can be of gas, solid, or semiconducting material. The light produced by the lasing material is enhanced using a device such as the Fabry-Perot resonator cavity.
Fabry-Perot resonator cavity. It consists of two partially reflecting parallel flat mirrors, known as facets, which create an optical feedback that causes the cavity to oscillate.Light hits the right facet and part of it leaves the cavity through the right facet and part of it is reflected.
Since there are many resonant wavelengths, the resulting output consists of many wavelengths spread over a few nm, with a gap between two adjacent wavelengths of 100 to 200 GHz.A single wavelength can be selected by using a filtering mechanism that selects the desired wavelength and provides loss to the other wavelengths.
Tunable lasersTunable lasers are important to optical networksAlso, it is more convenient to manufacture and stock tunable lasers, than make different lasers for specific wavelengths. Several different types of tunable lasers exist, varying from slow tunability to fast tunability.
ModulationModulation is the addition of information on a light streamThis can be realized using the on-off keying (OOK) scheme, whereby the light stream is turned on or off depending whether we want to modulate a 1 or a 0.
WDM and dense WDM (DWDM)WDM or dense WDM (DWDM) are terms used interchangeably. DWDM refers to the wavelength spacing proposed in the ITU-T G.692 standard in the 1550 nm window (which has the smallest amount of attenuation and it also lies in the band where the Erbium-doped fiber amplifier operates.)The ITU-T grid is not always followed, since there are many proprietary solutions.
The ITU-T DWDM grid
Photo-detectors and optical receiversThe WDM optical signal is demultiplexed into the W different wavelengths, and each wavelength is directed to a receiver.Each receiver consists of a photodetector, an amplifier, and signal-processing circuit.
Optical amplifiersThe optical signal looses its power as it propagates through an optical fiber, and after some distance it becomes too weak to be detected. Optical amplification is used to restore the strength of the signal
Amplifiers: power amplifiers, in-line amplifiers, pre-amplifiersIn-lineamplificationopticalfiberopticalfiberW1 TxWavelengthmultiplexerPoweramplifierW1 TxRxRxWavelengthdemultiplexerPre-amplifier
1R, 2R, 3RPrior to optical amplifiers, the optical signal was regenerated by first converting it into an electrical signal, then apply 1R (re-amplification), or2R (re-amplification and re-shaping) or 3R (re-amplification, re-shaping, and re-timing)and then converting the regenerated signalback into the optical domain.
Amplification and Regeneration
The Erbium-doped fiber amplifier (EDFA)