Optical Fiber - lcd.efn.unc.edu.arlcd.efn.unc.edu.ar/pdf/slides(9).pdfOptical Fiber Wavelengths Fiber optic signals belong to the infrared region which has wavelengths longer than

Optical Fiber

Jorge M. Finochietto

Cordoba – 2015

LCD EFN UNCLaboratorio de Comunicaciones DigitalesFacultad de Ciencias Exactas, Físicas y NaturalesUniversidad Nacional de Córdoba, Argentina

Outline

1 Bandwidth

2 Attenuation

3 Dispersion

4 Nonlinear Effects

Optical Fiber 2 / 36

Outline

1 Bandwidth

2 Attenuation

3 Dispersion

4 Nonlinear Effects

Optical Fiber → Bandwidth 3 / 36

Optical FiberCable

Made of glass (or plastic): core + cladding + coating

Guides optical signals (wavelengths) by total internal reflection,where 2α0 defines a cone between reflection (remains in the core)and refraction (leaves the core)

+ Inmune to electrical noise, small, light weight

− High installation cost and maintenance


Optical FiberWavelengths

Fiber optic signals belong to the infrared region which haswavelengths longer than visible lightWavelengths (λ) are related to frequencies (f ) by the speed oflight (c)

λf = c

Examples850 nm → 353 THz1300 nm → 231 THz1550 nm → 193 THz


Optical FiberBandwidth

Each wavelength occupies a portion of the spectrum which istypically expressed in nanometers (nm)

For a center wavelength λ0 with bandwidth ∆λ, the resultingfrecuency range can be obtained by differentiating previousequation

∆f =c

λ20

∆λ

Examples

1nm @ 850 nm → 415 GHz1nm @ 1300 nm → 177 GHz1nm @ 1550 nm → 125 GHz


Optical FiberWindows

Typically, 3 transmission windows are used

1st Window: 820-880 nm2nd Window: 1260 - 1360 nm (aka O-Band)3rd Window: 1530 - 1565 nm (aka C-Band)

ITU-T G.692 defines additional windows (bands)

E-Band: 1360 - 1460 nmS-Band: 1460 - 1530 nmL-Band: 1565 - 1625 nmU-Band: 1625 - 1675 nm


Optical FiberWavelength Division Multiplexing (WDM)

Optical signals (wavelengths) can be multiplexed onto a singlefiber

Coarse WDM (CWDM): up to 18 channels from 1271 to 1611 nm@ 20 nm spacing (THz!)Dense WDM (DWDM): up to 40/80 channels from C-Band orL-Band @ 0.8/0.4 spacing (100/50GHz)

Need of higher spectral efficiency resulted in a much more flexibleWDM grid (Flexgrid), which consider 12-5GHz spacings


Outline

1 Bandwidth

2 Attenuation

3 Dispersion

4 Nonlinear Effects

Optical Fiber → Attenuation 9 / 36

Optical FiberAttenuation

Low-loss fiber demonstrated in 1970 (αdB ≈ 20dB/km)By 1980, fiber attenuation reached 0,25dB/km @ 1550 nm

Theoretical limit is about 0,15dB/km @ 1550 nm

Copper (DSL) attenuation is typically > 20dB/km

Pout,mW = Pin,mW × e−αL

Pout,dBm = Pin,dBm − log(e)αL

Pout,dBm = Pin,dBm − 4,343αL

Pout,dBm = Pin,dBm − αdBL 0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1

Opt

ical

Pow

er [m

W]

Distance [km]

Theroretical Limit0.2dB/km

2dB/km20dB/km


Optical FiberLoss Mechanisms

Fiber loss is due to scattering and absorptionLight Absorption in the fiber material is transformed into heatdue to molecular resonance and wavelength impurities (hydrogenand hydroxide resonance occurs at 1244 and 1383 nm)Rayleigh Scattering causes dispersion of light in all directions,with some escaping the core and some returning down the core(backscattering)

Today, scattering is the dominant attenuation componentWater peak absorption areas have been reduced

Examples

850 nm 2.5 dB/km1300 nm 0.4 dB/km1550 nm 0.2 dB/km


Outline

1 Bandwidth

2 Attenuation

3 DispersionModal DispersionChromatic DispersionPolarization Mode Dispersion

4 Nonlinear Effects

Optical Fiber → Dispersion 12 / 36

Optical FiberTypes of Fiber

Fiber types are closely related to the diameter of the core andcladding, which defines how light travels through the fiber

Multimode Fibers (MMF) havelarge cores (50-100 um), whichallows multiple transmission paths

Easy coupling

Low-cost sources

Short distances

Singlemode Fibers (SMF) havesmall cores (8-10 um), whichallows a single transmission path

Long distances

Expensive sources

Hard coupling

Optical Fiber → Dispersion 13 / 36

Outline

1 Bandwidth

2 Attenuation


4 Nonlinear Effects

Optical Fiber → Dispersion → Modal Dispersion 14 / 36

Optical FiberModal Dispersion

Multimode fiber carries hundreds of modes, which can be thoughtof as independently propagating paths of the optical signal

Signals on different modes have different velocities, which createsmodal dispersion

Leads to broadening of signal pulses, which correspond to data bitsLeads to the overlap of pulses representing adjacent bits,distorting the signalThis phenomenon is called Inter-Symbol Interference (ISI)


Optical FiberStep-Index (SI) Multimode Fibers

Refractive index n1 is uniform in the coreFastest mode travels along the core and takes time L× n1/cSlowest mode is incident at the critical angle and takes timeL× n2

1/(c × n2), where n2 is the refractive index of the cladding

Capacity is frequently measured as the bit rate–distanceproduct (BL)

SI-MMF have a typical limit given BL < 15Mb/s − km (850 nm)and BL < 50Mb/s − km (1300 nm)


Optical FiberGraded-Index (GI) Multimode Fibers

Refractive index n1 is non-uniform in the core. decreasesgradually from the central axis to the cladding

Modes traversing the shortest path through the center of the coreencounter the highest refractive index and travel slowerModes traversing longer paths encounter regions of lowerrefractive index and travel faster

Time diff among modes in GI fibers is much smaller than in SIGI-MMF have a typical limit given BL < 160Mb/s − km (850nm) and BL < 500Mb/s − km (1300 nm)


Optical FiberMultimode Fiber Types

First MMF (1970s) based on 50 um and LED sourcesIn 1990s, 62.5 um MMF with LED sources to capture morepower and support 10Mbs over 2km

10 Gigabit Ethernet become limited to 26 m with 62.5 um andLED sources

Deployment of optimized (economical) lasers (VCSELs) @ 850nm + 50 um MMF enables 2 Gb/s - km


Outline

1 Bandwidth

2 Attenuation


4 Nonlinear Effects

Optical Fiber → Dispersion → Chromatic Dispersion 19 / 36

Optical FiberChromatic Dispersion

Chromatic dispersion (CD) refers to the phenomenon by whichdifferent spectral components of a pulse travel at differentvelocities

Material dispersion: refractive index is frequency dependent, maindispersion component.Waveguide dispersion: power distribution of a mode between thecore and cladding affects the effective refractive index. The longerthe wavelength, the more power on the cladding


Optical FiberLight Sources

Light sources have a finite non-zerowavelength spectrum whosewavelengths do not propagate at thesame velocity

Velocity at which resulting “envelopeof the wave” propagates is calledgroup velocity (GV), while its ratechange with frequency is calledgroup velocity dispersion (GVD) orsimply, chromatic dispersion (CD)

Chromatic dispersion (CD) isdescribed by the dispersion coefficentD in terms ps/(nm × km)


Optical FiberChromatic Dispersion on Singlemode Fibers

ITU-T Name D @ 1550 nm

G.652 Standard Fiber (STD) 17 ps/(nm km)G.653 Dispersion-Shited SMF (DSF) 0 ps/(nm km)G.655 Non-Zero Dispersion-Shited (NZDSF) ±[2-6] ps/(nm km)


Optical FiberMaximum Distances due to Chromatic Dispersion

Penalty for NRZ 0.5 dB 1 dB 2 dB

2.5 Gb/s 47126 18468 2973110 Gb/s 794 1193 192040 Gb/s 50 75 120

100 Gb/s 8 12 19


Outline

1 Bandwidth

2 Attenuation


4 Nonlinear Effects

Optical Fiber → Dispersion → Polarization Mode Dispersion 24 / 36

Optical FiberPolarization Mode Dispersion

Polarization Mode Dispersion (PMD) results from the differencein propagation speeds of the energy of a given wavlength, whichis split into two polarizations

Mains causes of PMD are non-circularities of the fiber andextrenal stress on the fiber

PMD typically refers to the mean value of all differential groupdelays (DGD) expressed in psSince the time efeects vary randomly, the PMD coefficient doesnot depend on the fiber length but on its square root; thus, it isexpressed as ps/

√km


Optical FiberPMD Bandwith vs. Distance

Since DGD (PMD) is a random variable, we can define a PMDoutage if the delay does exceed 3 times the average one for 20minutes a year


Optical FiberDispersion Bandwith vs. Distance


Outline

1 Bandwidth

2 Attenuation

3 Dispersion

4 Nonlinear Effects

Optical Fiber → Nonlinear Effects 28 / 36

Optical FiberNonlinear Effects

High power level and small effective fiber area, results in highfield intensity causing nonlinear effects

Nonlinear effects can be categorized as:Stimulated Scattering, which arises due to the interaction oflight waves with phonons

Stimulated Raman Scattering (SRS)Stimulated Brillouin Scattering (SBS)

Intensity-Dependent Refractive Index, which arise due to thedependence of refractive index on the intensity of the appliedelectric field, which in turn is proportional to the square of thefield amplitude.

Self-Phase Modulation (SPM)Cross-Phase Modulation (XPM)Four-Wave Mixing (FWM)


Optical FiberStimulated Scattering

In Stimulated Raman Scattering (SRS), when two or more signalat different wavelengths are injected, energy gets transferredfrom lower wavelengths to longer ones

In Stimulated Brillouin Scattering (SBS), an optical signalinduces a periodic change in the refractive index which can bedescribed as a virtual grating, resulting in scattering which ismostly reflected on the opposite direction


Optical FiberFour-Wave Mixing

Four-Wave Mixing (FWM) is an interference phenomenon thatgenerates an unwanted (ghost) signal λ123 from three signals atdifferent frequencies: λ123 = λ1 + λ1 − λ3

Ghost channels can overlap with actual signal channels, thus, notonly introducing power losses but also crosstalk

In G.653 DSF, different wavelengths @ 1550 nm travel at thesame speed and at a constant phase, thus, increasing theinterference due to FWMIn G.G53 fiber some CD is present @ 1550 nm, leading to differentwavelengths having differnet group velocities, and reducing FWMeffects


Optical FiberFour-Wave Mixing Penalty


Optical FiberFour-Wave Mixing Mitigation

Four-wave mixing is a severe problem in WDM systems usingdispersion-shifted fiber but does not usually pose a majorproblem in systems using standard fiber.

In fact, it motivated the development of NZ-DSF fiberPenalty due to four-wave mixing can be alleviated by:

Unequal channel spacing: The positions of the channels can bechosen carefully so that the beat terms do not overlap with thedata channels inside the receiver bandwidth.Increased channel spacing: This increases the group velocitymismatch between channels. This has the drawback of increasingthe overall system bandwidth, requiring the optical amplifiers to beflat over a wider bandwidth, and increases the penalty due to SRS.Reducing transmitter power and the amplifier spacing willdecrease the penaltyIf wavelengths can be demultiplexed and multiplexed in the middleof the transmission path, we can introduce different delays foreach wavelength. This randomizes the phase relationship


Optical FiberPhase Modulation Effects

Self-Phase Modulation (SPM)

High signal intensity induces local variable changes of therefractive index (aka Kerr effect)This time-varying index modulates the phase of the signal,broadening the spectrum of the transmitted pulse

Cross-Phase Modulation (XPM)

When transmitting multiple signals at tight channel spacing, theKerr effect results in modulating the phase of other signals


Optical FiberSummary of Transmission Effects


Optical Fiber

Jorge M. Finochietto

Cordoba – 2015

LCD EFN UNCLaboratorio de Comunicaciones DigitalesFacultad de Ciencias Exactas, Físicas y NaturalesUniversidad Nacional de Córdoba, Argentina

Documents

Optical Fiber - lcd.efn.unc.edu.arlcd.efn.unc.edu.ar/pdf/slides(9).pdfOptical Fiber Wavelengths Fiber optic signals belong to the infrared region which has wavelengths longer than