ip-admat, 19.05.2005light guides1 konrad mertens, lab for optoelectronics and sensors department of...

IP-ADMAT, 19.05.2005 Light Guides1

Konrad Mertens,

Lab for Optoelectronics and Sensors

Department of Electrical Engineering and Computer Science

Münster University of Applied Sciences

Basics and Applications of Light Guides

What the hell the data highway is made of?


1) Historical review

2) Why fibers for communication?

3) Principles of light guidance

4) Attenuation

5) Fiber types

6) Components of fiber communication systems

7) Real fiber systems

Outline


- before time:

- end of 18th cent.: bar telegraph

- 1960: first laser (rubine)

1962: first semiconductor laser (heavily cooled)

1970: first semiconductor laser (room temp.)

- 1970ies: low loss glass fibers (Corning Glas)

- 1980ies: fibers in wide distance networks

- 1990ies: fibers in city networks

- since 1995: penetration of fibers in local area networks

1) Historical Review


Bartelegraph of Monsieur Chappé

- 96 different signs

- e.g. Paris-Lille: 2 min


2) Why fibers for communication?

- low attenuation

- high bandwidth

- thin and flexible

- no electromagnetic emmissions (sidetalk)

- no electromagnetic immissions (e.g. from generators etc.)

- isolation of potentials between transmitter and receiver

Advantages:


Comparison of attenation: copper cable fiber

copper: attenuation rises dramatically at higher data rates

fibers: attenuation is independent of data rate

fibers

coaxial cable

twisted pair

copper cable:

Mbit/s

R L

C

data rate


Comparison: copper cable fiber cable

Example: Wide distance cable between Münster and Hamburg (300 km)

Capacity: 100.000 phone calls (6.4 Gbit/s)

copper

cable

fiber

cable

5 kg/m

300 g/m

Ø 8 cm

Ø 2 cm

100 amplifiers necessary

no amplifiers necessary


- often still more expensive

- contacting is “special”

- special components are necessary (optical transmitters and receivers)

- special devices (measurement etc.) are necessary

- lack of knowledge of engineers and technicians

Disadvantages of fibers versus copper caples


3) Principle of light guidance

Principle of total reflection:

critical angle of total reflection:

e.g. glas/air: n = 1,5 / 1 -> 1t = 42

n1 sin 1 = n2 sin 2

1t = arcsin (n2 /n1)

Law of refraction:

n2

n1

2

1

n1 > n2


Light guidance in fibers

n2

n1

- refractive index n1 must be larger than n2

- light ist guided in the core through repeated total reflection

- light „sees“ an „inner mirrored pipe“

- if the incidence angle is too large, light will be radiated

Coating (SiO2)

Core (SiO2 with doped Ge)


4) Attenuation in fibers

in dB/km

Rayleigh-scattering

attenuation coefficient

0

2

4

6

OH - Absorption

in nm800 1000 1200 1400 1600850 nm

1. window1300 nm

2. window1550 nm

3. window

5

3

1

14


Example

Power P1 P2

L = 10 km

Fiber with attenuation coefficient dB/km

Attenuation A = · L = 0,3 dB/km · 10 km = 3 dB

This means: half of the Power is still there!

Compare this to copper cable: L50% = 30 m

Compare this to window glass: L50% = 3 cm


5) Fiber Types I: Multimode - Fibers

a) Step index fiber: refractive index n(r) has a step

n(r)

n2

n1

radius n2

n1

incoming impulse: outcoming impulse:

- Broadening of the pulse „Dispersion“:

only low data rates are possible

d = 50 m

this fiber is rarely used


b) Graded index fiber: n(r) changes smoothly

n(r)

n2

n1

r n2

n1max


- The outer light rays run fasterFree space velocity c

Refractive index n(velocity v = )

much less pulse broadening!


Fiber Types II: Singlemode - Fibers

Idea: Lets build a fiber where only one „mode“ matches in

1.: small core ( d = 10 m)

2.: small refractive index step

Measures:

n2

n(r)

r n2

n1


d = 10 m

That´s it!

n1


Singlemode versus Multimode - Fiber

Monomode-fibers offer: - large bandwidths (e.g. 100 Gbit/s)

- low attenuation

but: - light injection is difficult

- laser diodes are necessary

We use them for high data rates and long distances

In local area regime multimode fibers dominate (still…)


6) Optical Communication Systems

Principle:

electricalsignal

electricalsignal

opticalsignal

fiber

opticaltransmitter

opticalreceiver

We need: - optical transmitter: LED or laserdiode

- optical receiver: photodiode

- connectors, etc.


Optical Transmitters

Light emitting diodes (LED)

n

p

+

-

construction: example:

+ cheap

- small power

- only for multimode fibers

properties:

Laser diodes (LD)

construction: example: properties:

n

pi

-

+

+ high power

+ suitable for single mode fibers

- expensive

- easily damaged


Optical Receivers

Photo diodes

1. optical Window: (850 nm): Silicon (cheap)

2. + 3. optical window (1300, 1550 nm): Ge or InP (expensive)

construction: example:

p+

n

-+

Licht

Materials:


Connecting fibers

electrode

light arc

cladding

fiber coating

fiber core

automatic position control

a) fiber connectors:

b) fiber splicing:

connectormating adaptor connectormating adaptor

electrode


7) Real Fiber Systemsa) Wide Area Networks (WAN)

Technology: Wavelength division multiplexing (WDM)

Prism, n= f()

Receiv. 2

fiber

Principle:

Laser 1

Laser 2

Spectrum:

P()

Receiv. 1

1300 1400 1500 1600 1700

0,2

0,4

0,6

100 nm

attenuation

nm

e.g.: channel separation of 0,8 nm: more than 100 channels possible

dB/km


Example for Wide Area Connections: transatlantic cable TAT-14

- two cables in separate routes

- each cable contains 8 fibers

- every fiber transports 16 wavelength channels (wavelength division multiplexing)

- every of the 16 lasers has a data rate of 10 Gbit/s

Total capacity: 640 Gbit/s 10 Mill. simultaneous phone calls!!


b) Metropolitan Area Networks (MAN)

e.g. Fiber Double Ring (10 Gbit/s)

155 Mbit/sPhone call

swiching

cross-connector

10 Gbit/s

company

network

to next city

10 Gbit/s

+ flexible

+ fail-safe

cross-connector

cross-connector

cross-connector

cross-connector

cross-connector

„last mile“


c) Local Area Networks (LAN)

classic: 10 Mbit/s

Fast Ethernet: 100 Mbit/s

Gigabit Ethernet: 1000 Mbit/s = 1 Gbit/s

10G Ethernet: 10.000 Mbit/s = 10 Gbit/s

e.g.: Ethernet:

Hub

SwitchHost

Host

Host HostPC

PC

PC

PC

Work-

station

fibers

copper

In the next five years fibers will reach the end user!


Conclusions

Fibers have significant advantages against copper cables

Technologies are there to use them in wide, mid and local range

Fibers will penetrate even the “last mile”

Only fibers make the “data highway” possible


Production of fibers:

1) Preform production (e.g. OVD: Outside Vapour Deposition):

2) Pulling of the fiber:

Substrate rod

a) Growing on: b) Tempering:

SiCl4 GeCl4

Rußschicht

c) Collapse

1500 °C 2000 °C

Preform:Length: ca. 1 m

Diam.: ca. 2 cm

regulation

Take-up reel

Pulling velocity: ca. 300 m/min


“normal” Repeater:

Erbium-doped Fiber

Pump-Laser

Advantages:

no opt./electr. and electr./opt. conversion necessary

multiple bitstreams on different wavelenghts can be amplified

Optical Fiber Amplifiers

Optical Amplifier:

= 1550 nm = 1550 nm

= 1480 nm

Prinziple:Excitiation:

E2

E1

Stimulated Emission:

E2

E1

Amplification Curve:

= 1480 nm

= 1550 nm

25 dB

15501540 15601530 nm


BT 1

BT 2

BT 3

BT 4

BT 5

Gigabit/Switch100 Mbit/s

50 Mbit/s

Wohnheim

Laserzentrum BT19 / Mensa BT11 BT12

STM-1(155 Mbit/s)

MS

BT 8

BT 6/7 BT 6/7

Bürgerkamp HGI

Multimodefaser mit 1 Gbit/s

Singlemodefaser mit 1 Gbit/s

Twisted Pair mit 100 Mbit/s

• An jedem Switch: 24 oder 48 Kupferanschlüsse (100 Mbit/s) zu den Laboren

• Parallel zu jeder MMF eine zweite Backup-MMF mit 100 Mbit/s

zu den Laboren

3

2

10

4 2

9

226

54 6


Ethernet-Standards mit LWL

Standard-Ethernet: 10 Base F: - 10 Mbit/s, Lmax = 2000 m

- Multimode-Fasern, LEDs, = 850 nm,

Fast-Ethernet: 100 Base FX - 100 Mbit/s, Lmax = 400 m

- Multimode-Fasern, LEDs, = 1300 nm

Gigabit-Ethernet: a) 1000 BaseSX: -1000 Mbit/s, Lmax = 550 m,

- Multimode-Fasern, Laserdioden, = 850 nm,

b) 1000 BaseLX: - 1000 Mbit/s, Lmax = 5000 m,

- Monomode-Fasern, Laserdioden, = 1300 nm

10G-Ethernet: 10G Base E (z.B.): - 10 Gbit/s, Lmax = 40 km

- Monomode-Fasern, Laserdioden, = 1550 nm


Übersicht: Einsatz von Glasfasern

Distanz Datenrate Fasertyp

bis 50 m

bis 2 km

bis 20 km

> 20 km

bis 100 kBit/s

bis 10 Mbit/s

bis 100 Mbit/s

> 100 Mbit/s

Kunststoffaser

Multimode-Gradientenfaser

Multimode-Gradientenfaseroder Monomodefaser

Monomodefaser

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