COMPLEX MODEL OF FSO LINKS

Otakar Wilfert

Brno University of Technology

Pforzheim, July 2007

Outline

1 Introduction (definition and history)

2 Design of FSO links and their parameters

3 Steady model of the FSO link

4 Statistical model of installation site

5 Complex model

6 Conclusion

Basic characteristics of laser radiation

� high directivity - high concentration of optical power

TX θ ≈ 10 −3 rad

Laserdiode

� high monochromatic wave - high concentration ofinformation g(ν)

Δν Δν ν

ν

<10 −3

� possibility of quantum state transmission - high degree of security during transmission

Definition Free-Space optical link (FSO link)

transmits an optical signal through the atmosphere.

Optical power is concentrated to one or more narrow beams

and optical wave can be divided into several optical channels.

(Their application is suitable in situations where the use of optical cable is impossible and desired bit rate is too high for a microwave link).

Wave and space division of optical signal

transmitting lenses

Transmitting transceiver

λ1

λ2 WDM

: :

FO λ1, λ2, …

Coupler

receiving lense

λ1, λ2, …

λ1, λ2, … λ1, λ2, … λ1, λ2, …

Receiving transceiver

λ1

λ1, λ2, … λ1, λ2, … λ2

WDM : :

4 beams N-optical channel 2.5 Gb/s in each channel Fully: N x 2.5 Gb/s

History of optical communication

Bell’s „photophone“ -The first device in the history which transmits message by optical beam

Washington, Franklin Park

”FROM THE TOP FLOOR OF THIS BUILDING WAS SENT ON JUNE 3, 1880 OVER A BEAM OF LIGHT TO 1325 L

STREET THE FIRST WIRELESS TELEPHONE MESSAGE IN THE HISTORY OF THE WORLD. THE APPARATUS

USED IN SENDING THE MESSAGE WAS THE PHOTOPHONE INVENTED BY ALEXANDER GRAHAM BELL

INVENTOR OF THE TELEPHONE.“

Bell regarded his photophone as: “the greatest invention I have ever made; greater than the telephone”.

source (Sun)

modulator mirror

Bell’s „photophone“ publication:

Principle of Bell’s „photophone“

receiver

Alexander Graham BELL, Ph.D., "On the Production and Reproduction of Sound by Light", American Journal of Sciences, Third Series, vol. XX, n°118, Oct. 1880, pp. 305- 324.

A.G. BELL and S. TAINTER, Photophone patent 235,496 granted 1880/12/14

Charles AlexanderSummer Tainter Graham Bell

Authentic drawing of „photophone“ details

However, the radio communications demonstrated by Marconi (in 1895) had got bigger progress.

Development of optical communications in free space was made possible by achievements of semiconductor optoelectronics, fiber optics and laser technology.

Theodor Harold Maiman (invention of laser 1960)

fotodiodes

Aleksandr Mikhailovich Nikolai Basov laser diodeProkhorov (1916 - 2002) (1922 - 2001)

(development of laser diodes - 1962)

1966 Kao and Hockham pointed out thatlong-distance communication by fiber is possible

Charles K. Kao (born in 1927)

Kao a Fleming in 2004 (Princeton University)

Today: 0,1dB/km (in spectral window 1550nm)

Bell’s laboratory “today”:

Scientists and engineers from Bell Labs demonstrated (New Jersey) optical link working in free space:

Range 4,4 km, Bit rate 10 Gb/s, Wavelength 1550 nm

Link design includes fiber elements (EDFA, WDM, fiber couplers etc.)

Prototype of multichannels FSO link (demo picture of progress)

From history to present day

Photophone

Advantages:

the narrow beams guarantee high spatial selectivity so there is no interference with other links

high bit rate of communication (of 10 Gbit/s) enables them to be applied in all types of networks

optical band lies outside the area of telecommunication offices, therefore, a license is not needed for operation

the utilization of quantum state transmission promises long-term security for high-value data

Disadvantages: 9 availability of FSO link depends on the weather

9 FSO link requires a line of site between transceivers

9 birds and scintillation cause beam interruptions

For reliability improvement number of new methods is

applied:

1. Photonic technology 2. Multi beam transmission 3. Wavelength and space division 4. Beam shaping 5. Auto-tracking system 6. Microwave backup 7. Adaptive optics 8. Polygonal (mesh) topology

Simplified drawing of the FSO transceiver (example)

FSO link network integration

Network element

FSO FSO Network element

Transceivers of FSO link are generally protokol transparent

FSO link substitutes optical fiber

FSO links arrangement into ”mesh“ topology

Unprofessional activity in area of FSO link

Ronja = Reasonable Optical Near Joint Access

“Ronja an User Controlled Technology (like Free Software) project of optical point-to-point data link. The device has 1.4km range and has stable 10Mbps full duplex data rate. Ronja is an optoelectronic device you can mount on your house and connect your PC, home or office network with other networks.“

? BER, ? availability,

http://ronja.twibright.com ? reliability, ? dynamic,

? power margin, …

Some realizations Laser transmitter (3 beams)

Transmitter with LED (1 svazek)

Receiver with

PIN photodiode

Czech professional activity in the area of FSO links

ORCAVE - FSO link of the Czech company Miracle Group

� 2 laser beams � auto-tracking system � range 2.0 km @ BER = 10-9

� wavelength 1550 nm � management system � monitoring system etc.

ORCAVE - structural design

� 2 laser beams � auto-tracking system � installation

� receiver optical system � management system

Commercially obtainable FSO links Basic characteristics

Examples of commercially obtainable FSO:

Canon (Japan): CANOBEAM DT 50 CBL (Germany): Air Laser Light Pointe (USA): Flight Spectrum 155/2000 Optical Access (USA): TereScope-OptiLink TS155/DST/CD SONA Optical Wireless (Canada): SONA beam 155-M Light Pointe (USA): Flight Strata

(Parameters of selected FSO follow)

Producer, type Canon (Japan) CBL (Germany)CANOBEAM AirLaser

DT 50

Bit rate 25 Mb/s to 155 Mb/s 1.25 Gb/s125 Mb/s

Application: a) Fast Ethernet, ATM etc. Gigabit Ethernet,b) Fast Ethernet

Range: a) 100 m to 2 km 1 kmb) 2 km

Wavelength 785 nm 850 nm

Class of laser 3 B ! 1 M IEC(eye safety)

Optical dynamic range ? 30 dBof receiver 36 dB

Interface (fibre): a) multimode multimodeb) singlemode

Backup N Y

Remote control Y Y

Auto-tracking system Y N

Number of beams/ 1/1 4/1number of receiving apertura

Producer, type LightPointe (USA) Optical Access (USA)FlightStrata 155 TereScope-OptiLink

TS155/DST/CD

Bit rate 1.5 Mb/s to 155 Mb/s 10 Mb/s to 155 Mb/s

Application: a) Fast Ethernet, ATM etc. Ethernet, ATM etc.b)

Range: a) 0 m to 2 km 2.2 km @ 10 dB/kmb) 1 km @ 30 dB/km

Wavelength 850 nm 785 nm

Class of laser 1 M IEC 3 B !(eye safety)

Optical dynamic range ? ?of receiver

Interface (fibre): a) singlemode multimodeb) singlemode

Backup N N

Remote control Y Y

Auto-tracking system Y N

Number of beams/ 4/4 3/1number of receiving aperture

Producer, type SONA (Canada) Ideal (?)SONAbeam 155-M Cost-effective, reliable

Bit rate 125 Mb/s to 155 Mb/s High bit rate (10 Gb/s)High secure

Application: a) Fast Ethernet, ATM etc. Transmission of data,

b) video and high-value data

Range: a) 200 m to 2 km Terrestrial: 500 mb) Satellite: 30 000 m

Wavelength 1550 nm WDM

Class of laser 1 M IEC Eye safety(eye safety)

Optical dynamic range 36 dB Availability of 99.99%of receiver (?)

Interface (fibre): a) multimode multimodeb) singlemode

Backup N Y

Remote control Y Y

Auto-tracking system N Y(?)

Number of beams/ 4/1 4/4(?)number of receiving aperture +APC

Summary of availability improvement methods (Pav = 99.9%)

9 (!) the utilization of only photonic elements

9 the utilization of WDM and EDFA

9 (!) multi-beam and multi-aperture transmission

9 “eye safety“ wavelength (1550 nm)

9 greater aperture of transmitting system

9 “auto-tracking“ system (ATS)

9 adaptive power control (APC) for exclusion of saturation

9 optical beam shaping (OBS) for obtaining of top-hat beam

9 the utilization of adaptive optics for reducing of power losses

9 (!) “mesh“ topology and less distance between transceivers

9 (!) microwave backup

Modeling of FSO link Laser beam (without atmosphere)

Wave equation is starting point ∇ 2 E(x, y, z)+ k 2 E(x, y, z) = 0

2 2 x +y ⎛ π ⎞

Gaussian beam (laser beam) is E(x, y,z) = − jk w 0 E e

− j kz⎜ +ϕ ( z )− ⎟ 2q( z ) ⎝ 2 ⎠e one of its solutions

Gaussian beam is fully characterized 1 =

0 w ( z )

1 2 − j

Utilization of matrix (ABCD law)

Aq + B

5

4.5

4

3.5

3

R/z0 2.5

2

1.5

1

0.5

0

by complex parameter

Radius curvature of wavefront vs. range

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

z/z0

q(z)

6

5

4

w/w0 3

2

1

0 5

2 R(z) kw (z)

Beam width vs. range

θ

0 0.5 1 1.5 2

1 q 2 = Cq 1

2.5 3 3.5 4

z/z0

+ D

4.5 5

Optical wave

G G G G

E(r,t)×H(r,t) time

Fast optical changes in time

Optical intenzity

G =I(r) =I(x, y,z)

2 2 2 x +y

Optical power

P(z,t) =∫ I(x, y,z,t)dxdy S

Slow (modulation) changes in time

I( x, y,z) = I

1

0.9

0.8

0.7

0.6

I/I0 0.5

0.4

0.3

0.2

0.1

0

⎡ 0 ⎢

⎣

w 0

w(z)

−2 ⎤ e ⎥

⎦

2 w (z)

1

0.9

0.8

0.7

0.6

I/I0 0.5

0.4

0.3

0.2

e -2 0.1

0

Optical intensity distribution in Gaussian beam

-3 -2 -1 0 1 2 3

x/w0

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

z/z0

Laser beam 1

0.9

0.8

0.7

Optical intensity distribution in Gaussian beam

laser diode

0.6

I/I0 0.5

0.4

0.3

0.2

0.1 e-2

0 -3 -2 -1 0 1 2 3

x/w

beam

Speckles in beam spot

Model of power budget The basic arrangement of the FSO link

γ tot

TX

source

TXA

P m,TXA

attenuation αtot

L 12

RXA RX

detector

P m,RXA

Psat,RXA

p(t) data (OOK modulation) Pdynamical

Pm,TXA ≈ 1/2 Pimp,TXA range Δ

P0,RXAt

All power levels are “mean” value w.r.t. modulation noise floor

Pm,TXA - mean power radiated through TXA;TXA - output aperture

of the transmitter; RXA - input aperture

of the receiver;

Pm,RXA - mean power received on RXA;

αtot - total attenuation;

γtot - total gain;

L12 - distance between TXA and RXA;

Power balance equation and power level diagram

transmitter

P [dBm]

beam atmosphere

Power level diagram

α

receiver

10 Pm,TXA 12

−γ α~ atm αatm

tot

0

optical power

-10

-20

-30

-40 random

δ

Δ M

Psat,RXA saturation ~ P m,RXA “clear” atm.

Pm,RXA real situation

P0,RXA sensitivity Power balance equation

Pm,TXA - α12 + γtot - ~atm - αatm = Pm,RXA

Link margin

Graph of link margin M (L12) M(L ) =P 12 m,PD (L ) −P 12 0,PD

Stationary model of the link by itself

Link margin is possible to utilize for: increasing of range, increasing of link immunity against weather

Atmospheric phenomena

Transmission of „clear“ atmosphere

measured at sea level L12 = 1km; Δλ = 1,5nm

Areas applied

Atmospheric phenomena

Components of αatm

¾ 1. Absorption, scattering and refraction

on gas molecules and aerosols (fog, snow, rain)

(slow variations)

(λ = 785 nm) visibility attenuation State of the atmosphere[km] [dB.km-1]

< 0.05 > 340 Heavy fog

0.2 - 0.5 85 - 34 Middle fog

1.0 - 2.0 14 - 7.0 Weak fog or heavy rain

2.0 - 4.0 7.0 - 3.0 Haze

10 - 23 1.0 - 0.5 Clear

Atmospheric phenomena

Components of αatm

¾ 2. Beam deflection (diurnal variations)

(temperature or mechanical deformation of

consoles)

¾ 3. Short-term interruptions of the beam (short pulses)

caused by birds, insect,

1e4

1e3

1e2

10

1 (7th floor, filmed from a distance of 750m) 0 00:00 06:00 12:00 18:00 00:00

29/09/2000

Atmospheric phenomena

Components of αatm

¾ 4. Fluctuation of optical intensity (noise-like)

caused by air turbulence

f [Hz]

time of day

¾ 5. Background radiation

FSO testing link

Bit rate: 155 Mb/s Range : 750m Single beam

On-line monitoring: - BER - power levels - meteorological data

In operation since 1999

Measurements on testing link

turbulence, birds, …

Bit error rate (BER) a)

% Error free sec. (EFS) b)

c)Received power (Pr)

fog

Results processing -

statistical model of installation site

PDF of random atmospheric attenuation (histogram) (measured in Autumn)

Exceedance probability function of atmospheric attenuation

100

10

1

0.1

0 -5 0 5 10

102

15 20 25 30

35

αatm [dB/km]

This is probability that atmospheric attenuation exceeds given value

101

100 0 5 10 15 20 25 30 35 α atm [dB/km]

Synthesis of stationary model of the link and statistical model of installation site

Model of the link: link margin vs. range

Availability of the link - complex model (model of the given link in selected installation site)

100

Model of installation site: probability that atmospheric attenuation exceeds given value

10

1 λ = 850 nm

0,1

0,01

0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300

Koeficient útlumu [dB/k m]

Complex model of FSO link

1 0.01

MS=90dB Brno

0.8 0.1

MS=70dB

0.6 1

Milesovka0.4 10

0.2 1000 20 40 60 80 100 120 140 160 180

M1 [dB/km]

Nomogram for unavailability of link assesment

Monitoring of atmospheric phenomena in selected sites

Selected sites:

Czech Republic

Prague (750m)

Brno (950m)

FSI

Milesovka hill (Donnersberg)

FEKT

- Long-term monitoring of optical power and BER - Meteorological sensors

Conclusion

9 FSO links are a suitable technology for the ”last mile”

solution in the frame of access network

9 The utilization of the FSO links is requested namely in

situations where the use of an optical cable is

impossible and desired bit rate is too high for a

microwave links

9 FSO links are flexible, simple and full-value

(in terms of quality of transmission) license-free instrument

of network communication technologies