This document discusses free-space optical (FSO) links, which transmit optical signals through the atmosphere without fiber. It provides a history of optical communication beginning with Bell's photophone in 1880. The document outlines the basic characteristics and advantages/disadvantages of FSO links. It also presents models to characterize laser beams, power budgets, and link margins for FSO systems. Overall, the document provides an overview of FSO link technology and modeling.

1. COMPLEX MODEL OF
FSO LINKS
Otakar Wilfert
Brno University of Technology
Pforzheim, July 2007

2. 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

3. Basic characteristics of laser radiation
 high directivity - high concentration of optical power
TX
θ ≈ 10 −3 rad
Laser
diode
 high monochromatic wave - high concentration of
information g(ν)
Δν Δν <10 −3
ν
ν
 possibility of quantum state transmission - high degree of
security during transmission

4. 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).

5. Wave and space division
of optical signal
transmitting
lenses
Transmitting transceiver
λ1 FO λ1, λ2, …
λ2 λ1, λ2, … λ1, λ2, …
WDM Coupler
: λ1, λ2, …
: λ1, λ2, …
receiving lense
Receiving transceiver
λ1
λ1, λ2, … λ2
λ1, λ2, … WDM
:
:
4 beams
N-optical channel
2.5 Gb/s in each channel
Fully: N x 2.5 Gb/s

6. History of optical communication
Bell’s „photophone“ - Washington, Franklin Park
The first device in the history which
transmits message by optical beam
”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.“

7. Bell regarded his photophone as:
“the greatest invention I have ever made; greater than the telephone”.
source (Sun)
Principle of Bell’s „photophone“
modulator
mirror receiver
Bell’s „photophone“ publication:
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.

8. A.G. BELL and S. TAINTER, Photophone patent 235,496
granted 1880/12/14
Charles Alexander
Summer Tainter Graham Bell
Authentic drawing of „photophone“ details

9. 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 diode
Prokhorov (1916 - 2002) (1922 - 2001)
(development of laser diodes - 1962)

10. 1966 Kao and Hockham pointed out that
long-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)

11. 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

12. 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

13. 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

14. Simplified drawing of the FSO transceiver (example)

15. FSO link network integration
Network
element
FSO FSO Network
element
Transceivers of FSO link are generally protokol transparent
FSO link substitutes optical fiber

16. FSO links arrangement
into ”mesh“ topology

17. 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,
? reliability,
http://ronja.twibright.com
? dynamic,
? power margin, …

18. Some realizations Laser transmitter
(3 beams)
Transmitter
with LED
(1 svazek)
Receiver with
PIN photodiode

19. 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.

20. ORCAVE - structural design
 2 laser beams  auto-tracking system  installation
 receiver optical system  management system

21. 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)

22. Producer, type Canon (Japan) CBL (Germany)
CANOBEAM AirLaser
DT 50
Bit rate 25 Mb/s to 155 Mb/s 1.25 Gb/s
125 Mb/s
Application: a) Fast Ethernet, ATM etc. Gigabit Ethernet,
b) Fast Ethernet
Range: a) 100 m to 2 km 1 km
b) 2 km
Wavelength 785 nm 850 nm
Class of laser 3B! 1 M IEC
(eye safety)
Optical dynamic range ? 30 dB
of receiver 36 dB
Interface (fibre): a) multimode multimode
b) singlemode
Backup N Y
Remote control Y Y
Auto-tracking system Y N
Number of beams/ 1/1 4/1
number of receiving apertura

23. 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/km
b) 1 km @ 30 dB/km
Wavelength 850 nm 785 nm
Class of laser 1 M IEC 3B!
(eye safety)
Optical dynamic range ? ?
of receiver
Interface (fibre): a) singlemode multimode
b) singlemode
Backup N N
Remote control Y Y
Auto-tracking system Y N
Number of beams/ 4/4 3/1
number of receiving aperture

24. 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 m
b) 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 multimode
b) 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

25. 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

26. Modeling of FSO link
Laser beam (without atmosphere)
Wave equation is 2
starting point
∇ E(x, y, z)+ k 2 E(x, y, z) = 0
2 2
x +y ⎛ π⎞
w0 − jk − j⎜kz+ϕ ( z )−
Gaussian beam (laser beam) is
one of its solutions E(x, y,z) = E0 e 2q( z )
e ⎝ 2⎠
w(z)
Utilization of matrix
(ABCD law)
Gaussian beam is fully characterized 1 = 1 −j 2
by complex parameter Aq1+ B
q(z) R(z) kw 2 (z) q 2=
Cq 1+ D
Radius curvature of wavefront vs. range Beam width vs. range
5 6
4.5
4 5
3.5
4
3
R/z0 2.5 w/w0 3
2
2
1.5
0.5
1
1 θ
0 0
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
z/z0 z/z0

27. Optical wave Optical intenzity Optical power
GG GG G
P(z,t) =∫ I(x, y,z,t)dxdy
E(r,t)×H(r,t) =I(r) =I(x, y,z)
time S
Fast optical changes in time
Slow (modulation) changes in time
2 2
⎡ ⎤
w0
2 −2x 2+y Optical intensity distribution
w (z)
in Gaussian beam
w(z) ⎥
I( x, y,z) = I 0 ⎢ e
⎣ ⎦
1 1
0.9 0.9
0.8 0.8
0.7 0.7
0.6 0.6
I/I0 0.5 I/I0 0.5
0.4 0.4
0.3 0.3
0.2 0.2
e -2 0.1
0.1
0 0
-3 -2 -1 0 1 2 3 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
x/w0 z/z0

28. Laser beam 1
0.9
0.8
0.7
Optical intensity distribution 0.6
in Gaussian beam I/I0 0.5
0.4
0.3
0.2
0.1 e-2
0
-3 -2 -1 0 1 2 3
x/w
laser diode
beam
Speckles in beam spot

29. Model of power budget
The basic arrangement of the FSO link
γ tot
TX TXA RXA
attenuation αtot RX
source detector
P m,TXA L 12 P m,RXA
Psat,RXA
p(t) data (OOK modulation) P
dynamical
Pm,TXA ≈ 1/2 Pimp,TXA range Δ
P0,RXA
t
All power levels are “mean” value w.r.t. modulation noise floor
Pm,TXA - mean power radiated through TXA;
TXA - output aperture Pm,RXA - mean power received on RXA;
of the transmitter;
αtot - total attenuation;
RXA - input aperture
γtot - total gain;
of the receiver; L12 - distance between TXA and RXA;

30. Power balance equation and power level diagram
atmosphere
beam
transmitter receiver
Power level diagram
P
[dBm] Pm,TXA α 12
10 α~ atm αatm
−γtot
0
optical -10
power Psat,RXA saturation
δ ~
-20 P m,RXA “clear” atm.
-30 Pm,RXA real situation
random Δ M
-40
P0,RXA sensitivity
Power balance equation
Pm,TXA - α12 + γtot - ~atm - αatm = Pm,RXA

31. Link margin
Graph of link margin
M (L12) M(L 12 ) =Pm,PD (L12 ) −P0,PD
Stationary model of
the link by itself
Link margin is possible to utilize for:
increasing of range,
increasing of link immunity against weather

32. Atmospheric phenomena
Transmission of „clear“
atmosphere
measured at sea level L12 = 1km; Δλ = 1,5nm
Areas
applied

33. Atmospheric phenomena
Components of αatm
 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

34. Atmospheric phenomena
Components of αatm
 2. Beam deflection (diurnal variations)
(temperature or mechanical deformation of consoles)
 3. Short-term interruptions of the beam (short pulses)
caused by birds, insect,
1e4
1e3
1e2
10
(7th floor, filmed from a distance of 750m) 1
0
00:00 06:00 12:00 18:00 00:00
29/09/2000

35. Atmospheric phenomena
Components of αatm
 4. Fluctuation of optical intensity (noise-like)
caused by air turbulence
f [Hz]
time of day
 5. Background radiation

36. FSO testing link
Bit rate: 155 Mb/s
Range : 750m
Single beam
On-line monitoring:
- BER
- power levels
- meteorological data
In operation since 1999

37. Measurements on testing link
turbulence, birds, …
Bit error rate (BER) a)
% Error free sec. (EFS) b)
c)
Received power (Pr)
fog

38. Results processing -
statistical model of installation site
100
PDF of random atmospheric
attenuation (histogram) 10
(measured in Autumn)
1
0.1
0
-5 0 5 10 15 20 25 30 35
αatm [dB/km]
102
Exceedance probability function of
atmospheric attenuation
This is probability that atmospheric 101
attenuation exceeds given value
100
0 5 10
α
15 20
atm [dB/km]
25 30 35

39. 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 10
atmospheric attenuation 1 λ = 850 nm
exceeds given value
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]

40. Complex model of FSO link
1 0.01
MS=90dB Brno
0.8 0.1
MS=70dB
0.6 1
Milesovka
0.4 10
0.2 100
0 20 40 60 80 100 120 140 160 180
M1 [dB/km]
Nomogram for unavailability of link assesment

41. Monitoring of atmospheric phenomena
in selected sites
Selected Czech Republic
sites:
Prague
(750m)
Brno (950m)
FSI
Milesovka hill
(Donnersberg)
FEKT
- Long-term monitoring of
optical power and BER
- Meteorological sensors

42. 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