Training_Transmission_GB Page 1 Transmission September 2013

Training_Transmission_GB Page 1

Transmission

September 2013


Principle of optical communication

Transmissionchannel

Tx EO

RxOE

Receiver

Converter

Transmitter

Converter

Optical transmission length is restricted by the

attenuation or dispersion.


The electromagnetic wave

Electric wave

Magnetic wave

Propagationdirection

[Meter]

Wavelength

Time scale[Seconds]

Period Frequency = 1 /

Light is an electromagnetic wave and can be described with Maxwell’s equations.


c = f * c: speed of light

f: frequency

: wavelength

n = c0 / cc0: v in Vakuum

c: v in Medium

What is light

speed of light: Refraction index:in vacuum 300‘000 km/s 1.0in water 220‘000 km/s 1.3in glass 200‘000 km/s 1.5

Light = electromagnetic wave (Wave-particle dualism)

Visible spectrum for human eyes: Ultraviolett 380-780 nm Infrared

Wavelengths for data transmission in Glass Optical Fiber = invisible infrared: 850 nm, 1300 nm, 1550 nm, …


(Speed of light in vacuum)

Speed of light C (electromagnetic radiation) is:

C0 = f * (Wellenlänge x Frequenz) C0 = 299793 km/s

Velocity of electromagnetic wave

Remarks: An x-ray-beam (l = 0.3 nm), a radar-beam (l = 10 cm ~ 3 GHz) or an infrared-beam (l = 840 nm) have the same velocity in vacuum


Gamma x-ray UV Infrared Microwave Rundfunk

Visible light - electromagnetic radiation

[www.wikipedia]


Wavelength range - optical transmission

Infrared range Visible range

GOF Multimode

(850 – 1300nm)

POF (520 – 650nm)

PCF (650 – 850nm)

1. opticalwindow

1800 1600 1400 1200 1000 800 600 400

Wavelength [nm]

2. opticalwindow

3. opticalwindow

Singlemode

(1310 – 1650nm)


When light or other electromagnetic waves hit a medium (e.g. air) a part of the light bounces back (reflection) the other strikes trough (refraction).

Refraction & reflection of light


Sun light

[www.wikipedia]

Refraction & reflection of light

electromagnetic spectrum


e.g. nair= 1,0003, ncore= 1,5000 oder nsugar water= 1,8300

Change of velocity of light in matter

Velocity of light (electromagnetic radiation) is: always smaller than in vacuum, it is

Cn (Velocity of Light in Matter)

n = C0 / Cn

n is defined as refractive index (n = 1 in Vacuum)

n is dependent on density of matter and wavelength

Refractive index


Transmission


Principle fiber optic transmission

Optical fiber uses the effect of total reflection.

RefractionTotal

refraction Reflection

Glass material with slightly

lower density

n1 (cladding)

n0 (core) α2

α1

α2

90°

αin αout

Glass material with slightly

higher density


Fiber structure

Fiber optic is working with total reflection and therefore 2 materials with different density/refractive index are necessary:

• pure• homogeneous refractive index within core

Ø

yy µ

m

Ø

xx µ

m

n2 (Cladding)

n1 (Core)

Reflection

αin αout

Primary coating


Glass fiber index profile

Core = continuous

refraction index profile

Core= several layers with unequal

refraction index profiles

Core = parabolic

refraction index profile

Step index Multi-step index Graded index


Type of fiber

Optical fiber

Step Index (SI) Graded Index (GI)

Single mode (SM) Multi mode (MM) Multi mode (MM)

9/125µm (GOF)Low water peakDispersion shiftedNon zero dispersion shifted

980/1000 µm (POF)500/750 µm (POF)200/230 µm (PCF)

50/125 µm (GOF)62.5/125 µm (GOF) 120/490 µm (POF) D

iam

ete

rR

efr

actio

nIn

de

xP

rofil

e


n1 n2

Refractive indexprofile

(Step index)

~ 680 Modes at NA = 0.2, d = 50 m & = 850 nm~ 292 Modes at NA = 0.2, d = 50 m & = 1300 nm

Number of modes M = 0.5x(pxdxNA/l)2

n1

n2

n1

Same core density makes modes’ speed different (every mode travels for a different length)

Multimode fiber (Step index)

Input Output


Multimode fiber (Graded index)

Refractiv index profile

(Graded index)

~150 Modes at NA = 0.2, d = 50 m & = 1300 nm

Number of modes M = 0.25x(pxdxNA/l)2

n1

n1n1

Input Output

n1 n2

n2

Different core density makes modes’ speed same(every mode travels for about same length)


Single mode fiber

Refractive indexprofile

(Step Index)Example: n1 =1.4570 and n2 = 1.4625Remarks: One mode (2 polarizations)

n1 n2

n1

n1

n2

Input Output

if core is small enough only 1 mode can get transmitted.


Optical characteristics


Optical characteristics influences

1

2

3

Attenuation[dB]

Attenuation[dB]

DispersionDispersion

Numerical Aperture (NA)

[-]


[-]

Power loss alongthe optical link

Power loss alongthe optical link

Pulse broadeningand

signal weakening

Pulse broadeningand

signal weakening

Coupling lossLED/Laser fiber

fiber fiberfiber e.g. APD*

Coupling lossLED/Laser fiber

fiber fiberfiber e.g. APD*

Transmission distance

Transmission distance

Signal bandwidth &

transmission distance

Signal bandwidth &

transmission distance

Couplingcapacitance

Couplingcapacitance

Term Effect Limitation

* Avalanche photodiode


Attenuation


Attenuation is the reduction of the optical power due to

Attenuation

Fiber

Bending

Connection

Caused by absorption, scattering or a coupling. Value indication in decibel (dB)

Pin Pout


P0

L = 10log ------- [dB] P1

L: Pegel

P0: power in

P1: power out

1 dB = 80 % Power 3 dB = 50 % Power10 dB = 10 % Power

Decibel

1/2 1/23 dB 6 dB0 dB

100% 50% 25%Pin Pout


Attenuation

Attenuation in DB

remaining power in %

0.1 97.70.2 95.50.3 93.30.4 91.20.5 89.10.6 87.70.7 85.10.8 83.20.9 81.11.0 79.42 63.13 50.1

Attenuation in DB

Verbleibende power in %

4 39.85 31.66 25.19 12.6

10 1020 1.030 0.140 0.0150 0.00160 0.000170 0.0000180 0.000001


Attenuation contributors

Fiber (material) AbsorptionScattering

Connection (fiber end to fiber end)intrinsic extrinsic

Bending (fiber and cable)Micro bendingMacro bending

Kann nicht durch Installateur beeinflusst werden


Fiber - attenuation (intrinsisch)

Material absorption3 to 5% of attenuationdue to chemical doping process impurity

Residual OH (water peak)

absorb energy and transform it in heat/vibration

greater at shorter wavelength n1 (cladding)

n0 (Core)Rayleigh Streuung96 % of attenuationdue to glass impurity

reflects light in other direction depending on size of particles

depends on wavelength (>800nm)

n1 (cladding)

n0 (Core)


Insertion loss - intrinsic

Differences in

Core diameter

Numerical aperture

Refractive index profile


Insertion loss - extrinsic

Due to

Lateral offset

Axial separation

Axial tilt


Insertion loss - extrinsic

Due to:

Fresnel reflection

Surface roughness


Dämpfung (extrinsisch)

Micro-bending

(can not be influenced by installer)

Cable production process caused by

imperfections in the core/cladding interface

Macro-bending

(can be influenced by installer)

Bending diameter < 15x cable Ø

Macro-bending is not only increasing the attenuation it also shortens lifetime of a fiber (micro cracks)

n 1 (cla

dding

)

n 0 (Cor

e)


800 1000 1200 1400 1600

wavelength [nm]

3.5

2.5

1.5

Att

enu

atio

n [

dB

/km

]

3.Window1550 nm

SiOH-absorptions

Rayleigh-scattering (~ 1/

950 1240 1440

5. Window 4.

Window1625 nm

Attenuation spectrum GOF

1.window850 nm

2.window1310 nm


Dispersion


Dispersion (fiber length dependency)

Dispersion are all effects that considerably influence pulse „widening“ and pulse „flattening“.

Input pulse

L1

L2 + L2

L1 + L2 + L3

Output pulse after Lx

The dispersion increases with longer fiber length and/or higher bit rate.


Dispersion

Moden DispersionProfil Dispersion

Moden DispersionProfil Dispersion

ChromatischeDispersion

[ps/km * nm]

ChromatischeDispersion

[ps/km * nm]

Polarisations Moden Dispersion

PMD[ps/(km)]

Polarisations Moden Dispersion

PMD[ps/(km)]

Multimode Faser Single-mode Faser

Dispersion is the widening and overlapping of the light pulses in a optical fiber due to time delay differences.


Modal dispersion

• Step index profile• Delay of modes in the fiber• Lowest-order mode propagates along the optical axis.• Highest-order mode extended length lowest speed

cladding

core

limit

MM Fiber with step index (SI) profile

V = constant refractive indexLarge propagation delay →

low bandwidthe.g. PMMA SI-POF, DS-POF


Modal dispersion

• Parabolic index profile• Increase speed of rays near margin• Time differences between low and high order modes is

minimizes

cladding

core

limit

MM Fiber with graded index (GI) profileV2>V1 parabolic index

“no” propagation delay → high bandwidthe.g. GI-GOF, GI-POF


Chromatic dispersion

Singlemode chromatic dispersion Dominant type of dispersion in SM fibers and is caused by wavelength dependent effects. Chromatic dispersion is the cumulative effect of material and waveguide dispersion

Multimode chromatic dispersionAs waveguide dispersion is very low compared to material dispersion it can be disregarded.


Delay (PMD)n2 (Mantel)

n1 (Kern)

Polarisationsmoden-Dispersion (PMD)

„slow axis" ny

„fast axis“ nx< n y

y

x

PMD occurs in SM fibers• high bit rate systems >40Gb/s• systems with a very small chromatic

dispersion

A mode in SM fiber has two orthogonal polarizations

nx: magnetisches Feld, ny: elektrisches Feld


Bandwidth length definition

= Kapazität der Datenübertragung Längenabhängig aufgrund Dispersion

• Pulse widening limits bandwidth B and the maximum transmission rate Mbps

• Pulse widening is approx. proportional to the fiber length L

Approximation for bandwidth-length product:B x L = BLP

500MHz x 1km = 500MHz*km

BLPL

500MHz*km0.5kmB = = = 1000MHz


Numerical aperture



Light rays outside acceptance angle leak out of

core

NA = (n20 – n2

1) = sin Standard SI-POF = NA 0.5 → 30°Low NA SI-POF = NA 0.3 → 17.5°

Light rays guided

in core


Waveguide dispersion

2w0 Beam waste

Numerical Aperture: NA = sin = (n2

2 - n12)0.5 = w0

Example: NA = 0.17 and Q = 9.8°

80% of light in core

20% of the light in cladding

2w0 Verlust

Akzeptanzwinkel

Mode field diameter

2w0

Waveguide dispersion occurs when the mode filed is entering into the cladding. It is wavelength and fiber size dependent.

Documents

Training_Transmission_GB Page 1 Transmission September 2013