1

Outline:

Fibre and fibercharacteristics Transmitters Modulation ReceiversPassive couplersFiltersTransmission systems and optical networks

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Optical fibre, characteristic

• Large bandwidth (theoretical 50 THZ)• Low attenuation (0,2 dB/km at 1550nm).• Physical size beneficial, light and thin, simplifies

installation • Splicing and mounting connectors more complex• Immune to electromagnetic interference• Environmentally friendly material (sand!).

3

Propagation through fibre

• Lightpulses are reflected in the core when hitting the cladding => approximately zero loss

Andreas Kimsås, Optiske Nett

4

Snells law• Snells law:

– θkappe= 90° (for total refraction)

• Refractive index:

• Critical angle for total reflection:

kappekappekjernekjerne nn sinsin

kjerne

kappekritisk n

n1sin

1 luftmateriale

vakummateriale n

c

cn

5

Andreas Kimsås, Optiske Nett

Multi-Mode vs. Single Mode Fibre

Multi mode•Core > 50 um. •Light being reflected with different angle travels different distances •Pulse spreading

Single Mode• Core < 10 um => single mode• Less pulse spreading

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Fibermodes

• Multimode:– Core diameter typical 50-100μm.

– NA = Numerical Aperture

– Number of modes (m) depends on normalized frequency (V), a = core-diameter, NA:

• Singlemode– Core-diameter typically 10μm.

– Criteria for single-mode is V < 2.4048

– No mode-dispersion gives better transmission properties than multimode more difficult to couple to the lightsource.

NAaV 2

210

2VmV

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Coupling light into the fibre

• Single modus– Coupling into the tiny 10 micrometer core is demanding

– Lining up the light-source is a significant part of the production cost

– Laser is preferred light-source

– LED has too large beam

• Multimode– Larger core diameter simplifies coupling

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Attenuation in the fibre• Rayleigh-scattering:

– Dominant

– Inhomogenities in the fibre and the structure of the glass.

– Occurs when the lightbeam hits the inhomogenities in the glass

– Sets the theoretical lower limit of fibre attenuation L≈1/λ^4

• Absorption:– Metal-ions, especially hydroksyliones (OH¯) at approx. 1400nm.

– Pollution from production, or doped material for achieving the optical properties desired.

• Radiation loss:– E.g variations in core-diameter and inhomogenities between the

core and the cladding, e.g. Microbends or airbubbles.

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Attenuation in the fibre

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Transmission window and applied wavelength bands

Figur fra “Fiber Optic Communication Systems”, G. Agrawal, Wiley

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Dispersion• Pulse spreading when propagating through the fibre.• To much spreading results in intersymbol- interference• Limits the maximum transmissionrate through the fibre. • Three types of dispersion:

– Modi-dispersion: Light travelling in different modi undergoes different delays through the fibre. Not present in SM!

– Material-dispersion (chromatic): Refractive index is function of wavelength– Waveguide-dispersion: Propagation of different wavelengths depends on

the characteristic of the waveguide, e.g. Index, geometry of core and cladding.

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Zero dispersion

• At 1300 nm in standard fibre– Material (chromatic) dispersion is close to zero at 1300 nm

– Not minimum loss

• ~ 1500 nm in dispersion shifted fibre– Manufactured for zero dispersion in 1500 nm region

– Design core and cladding to give negative waveguide dispersion

– At a specific wavelength, material and waveguide dispersion will result in zero total dispersion.

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Chromatic dispersion

Figure: S. Bigo, Alcatel: Talk at Norwegian electro-optics meeting2004

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Point to point fibre-optical system

Transmitter(Laser+

modulator)

Receiver(fotodiode +

amplifier)Fibre

Important limitation:

Attenuation:Some light being absorbed in fibre

Dispersion:

Time Time

Pulse Spreading

Illustration: Lucent Technologies

Speed of light depends on wavelength

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Optical transmitters - LASER

1. Active laser medium2. Laser pumping energy3. Mirror (100%)4. Mirror (99%)5. Laser beam

Constructive interference

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Semiconductor laser• Most common transmitter in optical communication

– Compact design

• Material give frequency ranges (Fermi-Dirac distr.)– Population inversion: Electrons in n-region and holes in P-region

– Electrons in n-region (conduction band) combine with holes (valence band) in p-region

• Cavity length decides frequency1) Forward biasing create population Inversion2) Electrons combine with holes, releases photons3) Stimulated emission

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Stimulated emission

Ei

Ef

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Stimulated emission

A chain reaction!

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Optical transmitters - LED

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Light Emitting Diodes (LEDs)

• Not sufficient in long distance fibre transmission– Wideband source => dispersion

– Power is lower than for a laser

• Employed at shorter distances– Maximum a few hundred meters, depends on bitrate

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Optical receiversPhotodiode:

Avalanche diode = Higher sensitivity

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Modulation

• OOK modulation (on-off-keying) – NRZ (No Return Zero) most often used

– RZ (Return Zero), some use

– More advanced modulation formats being launched for 40 and 100 Gb/s pr. Channel systems.

• Employ phase and/or polarisation

– Phase and polarisation modulation not employed in systems for < = 10 Gb/s bitrate.

• External modulation, e.g. Employing external modulator: MZ interferometer

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Modulation II

• Direct modulation of laser – Switch laser on and off– Difficult to fabric laser that can be switched at high speed,

simultaneously having proper transmission characteristics. – Undesirable frequency variations (chirp) and Limited extinction ratio

• External modulation – Mach-Zehnder interferometer– External component being fed electrically – May be Integrated with laser– High extinction ratio prolongs transmission distance

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fibre-optical transmission at longer distances

Transmitter(Laser+

modulator)

Receiver(photodiode +

aplifierFibre

Must be compensated:

Attenuation:Some light being absorbed

Dispersion:

Time Time

Pulse Spreading

Illustration: Lucent Technologies

Light of speed wavelength dependent

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What is a long distance?

• 100 m?• 10 Km?• 1000 Km?

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What is a long distance?

• 100 m?– LAN

• 10 Km?– Access network

• 1000 Km?– Transport network

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Long distance optical system

• Attenuation must be compensated– Regeneration

– Attenuation

• Dispersion must be compensated– Dispersion compensation employing fibre

– Electronic compensation

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Regeneration• 1R regeneration = Amplification (Reamplification)

– Usually an optical amplifier– Amplifies the signal without conversion to electrical – Typically transparent for signal (shape, format and modulation)

• 2R Reamplification & Reshaping:– Reshapes the flanks of the pulse as well as the floor and roof of the pulse, removes

noise. – Usually electronic– Optical solutions still subject to research

• 3R Reamplification & Reshaping & Retiming:– Synchronisation to original bit-timing. (regeneration of clock)– Usually involves electro-optic conversion – Optical techniques in the research lab.

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Optical amplifier characteristics

• Amplifier parameters: – Gain

– Bandwidth of gain

– Saturation level

– Polarisation sensitivity

– Amplifier noise

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Optical fibre amplifier

• Doped-fiber amplifier: – Doping = Inserting small amounts of one material into a second

material

– An Erbium doped silica fibre is fed with a pump-signal together with the original signal.

– Doped atoms are being excited to a higher energy level

– The pumping signal is a high power signal with a wavelength lower than the wavelength to be amplified (typically 980 nm or 1480 nm fore EDFA).

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Erbium Doped Fiber Amplifier (EDFA)• Widely deployed in optical networks

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Optical amplifiers overview

• Semiconductor-laser amplifier: – Signal is sendt through the active region of the semiconductor

– Stimulated emission results in a stronger signal

– May be integrated with other components (e.g. Output of a switch or a transmitting laser)

– Widely employed in research projects on all-optical switches.

– Recently employed in commercially available compact tunable laser-modules

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Available wavelength range depends on amplifier technology

0

0,1

0,2

0,3

0,4

0,5

1200 1300 1400 1500 1600

Wavelength (nm)

Lo

ss (

dB

/km

)

EDFAEDFAC - bandC - band

1530-15621530-1562

EDFAEDFAL - bandL - band

1570-16001570-1600

ALTERNATIVE AMPLIFIER TECHNLOGIES: RAMAN AND SOA

PDFAPDFA1300 nm1300 nm

Commercially available Still subject to research

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Long distance fibre-optical transmission

Transmitter(Laser+

modulator)

Receiver(photodiode +

amplifier)Fibre

To be compensated:

Dispersion:

Time Time

Pulse Spreading

Illustration: Lucent Technologies

Speed of light is wavelength dependent

EDFA

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Dispersion in transmission fibre

• Dispersion depends on fibretype • G652, “Standard fibre” -17 Ps/nm*km @ 1550 nm• Dispersion shifted fibre: 0 dispersion @ 1550 nm• Non – Zero (NZ) dispersion shifted fibre: -3 to -6

Ps/nm*km

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Dispersion Compensating Fibre (DCF)• Negative dispersion compared to transmission fibre• Much higher dispersion/km => Shorter fibre than

transmission fibre required for achieving zero dispersion

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Long distance fibre-optical transmission

Transmitter(Laser+

modulator)

Receiver(fotodiode +

amplifier)Long Fibre

Compensation of amplitude and dispersion

EDFADCF

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Noise from optical amplifiers

• Amplified Spontaneous Emission (ASE)– Photons are being emitted without stimulation

• Noise distributed through the entire amplification band

• May be limited through filtering out the wavelengths where amplification is desirable

• Optical filter needed

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Interference between two light sources

• Constructive– Light in phase results in addition and

increased intensity

• Destructive– Light out of phase (180 degrees) results in

extinguished pulse

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Mach-Zehnder interferometer

• At given frequencies the delay equals duration of a wavelength => constructive interference

• At given frequencies the delay equals duration of half a wavelength => destructive interference

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Mach-Zehnder based modulator

• Modulates phase of one or both paths– E.g voltage on => phase being changed => extinguished pulse

Electronic modulation

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Series of Mach-Zehnder

• Applicable as an optical filter• Adjustable delay enables adjustable frequency

– A chain of filters helps sharpening up the filter characteristic

– Very fast adjustment-time: As low as 100 ns

– High attenuation (multiple stages)

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Etalon based adjustable filter• Cavity with parallell mirrors in each end • Free spectral range (FSR)

– Periode between repetition of pass-band

• Finesse– FSR/width of channel

• Fabry-Perot– Mechanical, large range adjustable, slow adjustment - 10 ms.

Adjustable to n wavelengths

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Acusto-optical filter• RF waves converted to sound-waves in a piezo electrical

crystal (transducer)• Soundwaves results in mechanical movements • Mechanical movements in crystal alters refractive index • The crystal then works as a grating • Adjustment within 10 Micro-seconds• Possible to filter out several frequencies simultaneously by

sending several RF waves with different frequency to a transducer

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Filters with fixed wavelength

• Gratingbased filters e.g. Diffraction gratings– Flat layer of transparent material, constructive interference in

bumps for a given wavelength, destructive for other wavelengths

• Arrayed Waveguide Grating (explained later)

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Optical couplers

• One or more fibers in, several fibres out – Divides the optical signal on several fibres.

• Signal power is divided on the output-fibres• Splitting ratio is varying

– 50/50, 50 % on each of two fibres– 10/90, 10 % in one, 90 % in a second.

• Attenuation from input to output depends on splitting ratio– 50/50 splitter results in 3 dB attenuation (halving the power)

Combiner

Splitter

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Optical couplers

• Coupler employed as splitter:– One input divided on two or more outputs– Splitting ratio (α) indicates share of power to each output – 1x2 splitter is typical 50:50, however some power is being reflected (40-50

dB weaker than payload signal). This is called return-loss.– Connection-loss between fibre and coupler also attenuates the signal

• Coupler employed as combiner:– Opposite use as a splitter; several inputs, single output.– Returnloss and connection-loss as for the splitter

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Arrayed waveguide Grating• 1 X N or N X N coupler divides the light on N

waveguides of different length• Waveguides is then coupled together, resulting in

interference • On each of the N outputs, constructive interference is

achieved for a specific wavelength and destructive interference for the other wavelengths

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Multiplexing/Demultiplexing

• Optical multiplexing: Couple several waveguides together into a fibre.

• Optical demultiplexing: Separate wavelengths from an input fibre into several output fibres with a single wavelength in each.

• Is this useful?

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Transmission systems and aspects for optical networks

By: Steinar Bjørnstad

As part of the training course ”optical networks”

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Overview transmission, transmission effects and limitations

• Wavelength Division Multiplexed (WDM) systems• Give a brief introduction to limiting effects in optical

transmission systems– Polarization Mode Dispersion

– Non-linear effects

– Limiting factors

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WDM and bandwidth utilization• Optical fibre has unique transmission properties

– 25 THz bandwidth available in low loss region

– Another 75 THz available (higher attenuation)

• How can we utilize the bandwidth?– Electronic components can not process signals beyond ~100 GHz

0

0,1

0,2

0,3

0,4

0,5

1200 1300 1400 1500 1600

Wavelength (nm)

Lo

ss (

dB

/km

)

25 THz

Optical fibre loss spectre

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Fibre optical transmission system

ReceiverOptical fibreSingle modus

Electric input data

Laser & modulator

Amplifieror

regeneratorOptical fibreSingle modus

Electric output data

Wavelength Division Multiplexing (WDM) transmission system:Add lasers & modulators + receivers

Time Division Multiplexing = TDM

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Fibre optical transmission system

ReceiverOptical fibreSingle modus

Electric input data

Laser & modulator

Amplifieror

regeneratorOptical fibreSingle modus

Electric output data

Polarisation multiplexing:Doubles capacity

PBS

PBSCombine! At Telenor 32 WDM X 2.5 Gb/s TDM

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From regenerator to optical amplifier

11 11

11 1111 1144 44

11

22

33

44

11

22

33

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Tidligere utbygging

RegeneratorTerminalFiber

Før: 1 kanal pr fiber

Optiskforsterker

MultiplekserDemultiplekser

2,5 Gb/s =30000

Opptil20 000 000

WDM: 4-128 kanaler

pr fiberNåværende utbygging

Wavelength Division Multiplexing(WDM), mangedobler kapasitet i fiber

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Polarisation Mode Dispersion (PMD)

• Fibre has two principle states of polarization – Light travels with different velocity in the two states

– Difference in arrival time: Differential Group Delay (DGD)

• Caused by elliptic fibre core – Bad fabrication process

– Optical components may also cause PMD

• PMD is frequency dependent• It varies with time

– Statistical process, Maxwellian probability density

– PMD: Mean value over time of DGD (expressed in picoseconds)

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PMD impact• Maximum tolerable PMD

– 10 % - 20 % of bit period (Depends on modulation format)

– = 10 ps for 10 Gb/s, = 2.5 ps for 40 Gb/s

• PMD new fibre <= 0.1 ps/ (Alcatel standard fibre)• Distance limits of new fibre

TDM Bitrate

2.5 Gb/s 10 Gb/s 40 Gb/s 160 Gb/s 640 Gb/s

PMD Max length

1.6*105 Km 10,000 Km 625 Km 40 Km 2.5 Km

= DPMD/

km

L

- Can be compensated, currently expensive- PMD on installed fibre can be as high as e.g. 2 ps/- Distance limit: 1.5 km at 40 Gb/s

km

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The good, The bad, The Ugly

Non-linear effects:

Impact & Applications

Superhero! Enables signal processing components

Ugly pulses!Nightmare! For system designers

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Non-linear effects

• Scattering effects in fibre medium– Stimulated Brillouin Scattering (SBS): Backward scattering from

acoustic waves

– Stimulated Raman Scattering (SRS): Interaction of light waves with phonons (molecular vibrations)

• Fibre refractive index dependence on optical power– Four Wave Mixing (FWM)

– Self-Phase Modulation (SPM)

– Cross-Phase Modulation (XPM)

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Frequencyff11 ff22-- ff22 ff2222 -- ff1122 ff11

Original Wavelengths

New Wavelengths

Distortion by non-linear effects• Four-Wave Mixing (FWM)

– Intermodulation products

– In WDM and as intrachannel products for high TDM rates

Number of new wavelengths = N2(N-1)/2where N = number of original wavelengths

N

248

FWM Products

224

224

Especially a problem in WDM!

Lucent-97

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Distortion by non-linear effects• Suppress by

– Moderate channel powers

– Avoid zero dispersion fibre

– Polarisation interleaving of channels

Lucent-97

0

Dispersion-Shifted Fiber (25 km)

Wavelength (1 nm/div.)

1546.55

Signals

Especially a problem when D = 0!

N

248

FWM Products

224

224

Mixing products

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Raman amplifiers principle• Using the transmission fibre as a gain medium• Pumping the fibre forwards and/or backwards• Coupled in through couplers or multiplexers

Sender(Laser+

modulator)

Mottaker(fotodiode + forsterker)Fiber

Laser Pump

forward

Laser Pump

Backward

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Stimulated Raman Scattering (SRS)• SRS in high channel count WDM systems

– Higher wavelengths experience gain

– Lower wavelengths attenuation

Output spectrum

Input spectrum

150 nm10 THz 20

Ram

an G

ain

Frequency shift

Power is shifted to upper from lower channels

Stolen-79

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Raman amplifiers

• Decreases noise• Increases bandwidth• High pump powers needed

– High demands for installation

• Expensive (very)• Not widely deployed

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Raman amplification benefits