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
TXTX θ ≈ 10−3 rad
Laser diode
high monochromatic wave – high concentration of information g(ν) Δν Δν −3 ν < 10 ν 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 FO λ1, λ2, … λ λ1, λ2, … λ , λ , … 2 WDM Coupler 1 2 : λ , λ , … : 1 2 λ1, λ2, …
receiving lense Receiving transceiver
λ1 λ , λ , … 1 2 λ2 λ , λ , … WDM 1 2 : :
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“ – 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.“ 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. 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 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) 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) 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 Network element FSO FSO 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/s 125 Mb/s Application: a) Fast Ethernet, ATM etc. Gigabit Ethernet, b) Fast Ethernet Range: a) 100 m to 2 km 1km b) 2km Wavelength 785 nm 850 nm
Class of laser 3B ! 1M 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 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) 0m to 2 km 2.2 km @ 10 dB/km b) 1 km @ 30 dB/km Wavelength 850 nm 785 nm
Class of laser 1M IEC 3 B ! (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 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 1M 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 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 22 starting point ∇ Exyz(, ,)+= kExyz (, ,) 0
22 xy+ ⎛⎞π − jk − jkz⎜⎟+−ϕ() z Gaussian beam (laser beam) is w0 2qz() ⎝⎠2 one of its solutions Exyz(, ,)= E0 e e wz() Utilization of matrix (ABCD law) 11 2 Gaussian beam is fully characterized =−j AqB + by complex parameter 2 1 qz() Rz () kw() z q2 = Cq1 + D Radius curvature of wavefront vs. range Beam width vs. range 5 6 4.5 4 5 3.5 4 3 w/w R/z0 2.5 0 3 2 2 1.5 1 1 θ 0.5 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 Optical wave Optical intenzity Optical power G G G GG Ert(,)×==Hrt(,) Ir () I(x,,)yz P(,) z t= ∫ I (, x y ,,) z t dxdy time S Fast optical changes in time Slow (modulation) changes in time
xy22+ 2 −2 ⎡⎤w0 wz2 () Optical intensity distribution Ixyz(, ,)= I0 ⎢⎥ e ⎣⎦wz() in Gaussian beam
1 1
0.9 0.9
0.8 0.8
0.7 0.7
0.6 0.6
0.5 I/I 0.5 I/I0 0 z/z0 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
z/z0 x/w0 Laser beam 1
0.9
0.8
0.7 Optical intensity distribution 0.6 0.5 I/I0 in Gaussian beam 0.4
0.3
0.2 -2 0.1 e
0 -3 -2 -1 0 1 2 3 x/w
laser diode
beam
Speckles in beam spot Model of power budget
The basic arrangement of the FSO link γ tot TX TXA RXA RX attenuation αtot source detector
Pm,TXA Pm,RXA L12
Psat,RXA p(t) data (OOK modulation) P dynamical
Pm,TXA ≈ 1/2 Pimp,TXA range Δ P t 0,RXA 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; Power balance equation and power level diagram
atmosphere beam transmitter receiver
Power level diagram P [dBm] α Pm,TXA 12 ~ α 10 αatm atm −γ tot 0 optical -10 power P saturation δ ~sat,RXA -20 Pm,RXA “clear” atm.
-30 Pm,RXA real situation random Δ M -40 P0,RXA sensitivity Power balance equation ~ Pm,TXA – α12 + γtot – αatm – αatm = Pm,RXA Link margin
Graph of link margin M ()LPLP=− () M (L12) 12mPD , 12 0,PD
StationaryStationary modelmodel ofof thethe linklink byby itselfitself
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 errors 10
(7th floor, filmed from a distance of 750m) 1
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 100 attenuation (histogram) 10 (measured in Autumn)
1 probability [%]
0.1
0 -5 0 5 10 15 20 25 30 35 [dB/km] αatm 2 10 Exceedance probability function of atmospheric attenuation
1 This is probability that atmospheric 10 attenuation exceeds given value % casu prekroceni
0 10 0 5 10 15 20 25 30 35 [dB/km] αatm 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
λ = 850 nm atmospheric attenuation 1
exceeds given value Nedostupnost [%]
0,1
0,01 0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300
Koeficient útlumu [dB/km] CComplexomplex modelmodel ofof FSOFSO linklink
1 0.01 MMS=90dBS=90dB Brno
0.8 Brno 0.1 MS=70dB MS=70dB [%] [km] un 12 P L 0.6 1
MilesovkaMilesovka 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 MonitoringMonitoring ofof atmosphericatmospheric phenomenaphenomena inin selectedselected sitessites
Selected Czech Republic sites: 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