COMPLEX MODEL OF FSO LINKS

Transcription

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 Δ 10 3 ν < possibility of quantum state transmission high degree of security during transmission ν

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 transceiver transmitting lenses λ 1 λ 2 : WDM λ 1, λ 2, FO Coupler λ 1, λ 2, λ 1, λ 2, λ 1, λ 2, λ 1, λ 2, receiving lense Receiving transceiver λ 1, λ 2, λ 1, λ 2, WDM λ 1 λ 2 : 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 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.

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

8 A.G. BELL and S. TAINTER, Photophone patent 235,496 granted 1880/12/14 Charles Summer Tainter Alexander 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 Prokhorov ( ) Nikolai Basov ( ) (development of laser diodes ) laser diode

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: availability of FSO link depends on the weather FSO link requires a line of site between transceivers 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 pointto-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,? 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 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) CANOBEAM DT 50 CBL (Germany) AirLaser Bit rate 25 Mb/s to 155 Mb/s 1.25 Gb/s 125 Mb/s Application: a) b) Fast Ethernet, ATM etc. Gigabit Ethernet, Fast Ethernet Range: a) b) 100 m to 2 km 1km 2km Wavelength 785 nm 850 nm Class of laser 3B! 1M IEC (eye safety) Optical dynamic range of receiver? 30 db 36 db Interface (fibre): a) b) multimode multimode singlemode Backup N Y Remote control Y Y Auto-tracking system Y N Number of beams/ number of receiving apertura 1/1 4/1

23 Producer, type LightPointe (USA) FlightStrata 155 Optical Access (USA) 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) b) 0m to 2 km db/km 1 30 db/km Wavelength 850 nm 785 nm Class of laser Optical dynamic range of receiver Interface (fibre): a) b) 1M IEC 3 B! (eye safety)?? singlemode multimode singlemode Backup N N Remote control Y Y Auto-tracking system Y N Number of beams/ number of receiving aperture 4/4 3/1

24 Producer, type SONA (Canada) SONAbeam 155-M Ideal (?) Cost-effective, reliable Bit rate 125 Mb/s to 155 Mb/s High bit rate (10 Gb/s) High secure Application: a) b) Fast Ethernet, ATM etc. Transmission of data, video and high-value data Range: a) b) 200 m to 2 km Terrestrial: 500 m Satellite: m Wavelength 1550 nm WDM Class of laser Optical dynamic range of receiver Interface (fibre): a) b) 1M IEC Eye safety (eye safety) 36 db Availability of 99.99% (?) multimode multimode singlemode Backup N Y Remote control Y Y Auto-tracking system N Y(?) Number of beams/ number of receiving aperture 4/1 4/4(?) +APC

25 Summary of availability improvement methods (Pav = 99.9%) (!) the utilization of only photonic elements the utilization of WDM and EDFA (!) multi-beam and multi-aperture transmission eye safety wavelength (1550 nm) greater aperture of transmitting system auto-tracking system (ATS) adaptive power control (APC) for exclusion of saturation optical beam shaping (OBS) for obtaining of top-hat beam the utilization of adaptive optics for reducing of power losses (!) mesh topology and less distance between transceivers (!) microwave backup

26 Modeling of FSO link Laser beam (without atmosphere) 5 Wave equation is starting point Gaussian beam (laser beam) is one of its solutions Gaussian beam is fully characterized by complex parameter Radius curvature of wavefront vs. range (,, ) (,, ) Exyz+ kexyz = jk w j kz ( ) ( z) 0 qz ϕ Exyz E w z 0 ( ) 2 2 (,, ) = e e 1 = 1 j 2 qz ( ) Rz ( ) 2 kw ( z) 6 Beam width vs. range x + y π Utilization of matrix (ABCD law) q 2 Aq = Cq B + D R/z w/w θ z/z 0 z/z 0

27 Optical wave Optical intenzity Ert (, ) Hrt (, ) = Ir ( ) = I( x, yz, ) time Fast optical changes in time Optical power P(,) z t = I(, x y,,) z t dxdy S Slow (modulation) changes in time x + y 2 w 2 0 w ( z) I( x, y, z) I0 e = wz ( ) Optical intensity distribution in Gaussian beam I/I I/I z/z x/w 0 e z/z 0

28 Laser beam Optical intensity distribution in Gaussian beam I/I e x/w laser diode beam Speckles in beam spot

29 Model of power budget The basic arrangement of the FSO link γ tot source TX TXA attenuation α tot RXA RX detector P m,txa L12 P m,rxa p(t) data (OOK modulation) P m,txa 1/2 P imp,txa P P sat,rxa dynamical range Δ t All power levels are mean value w.r.t. modulation TXA - output aperture of the transmitter; RXA - input aperture of the receiver; P m,txa - mean power radiated through TXA; P m,rxa - mean power received on RXA; α tot γ tot L 12 P 0,RXA noise floor - total attenuation; - total gain; - distance between TXA and RXA;

30 Power balance equation and power level diagram transmitter beam atmosphere receiver P [dbm] 10 P m,txa α Power level diagram 12 γ tot α ~ atm α atm 0 optical power δ P ~ sat,rxa P m, RXA saturation clear atm. -30 random Δ M -40 Power balance equation ~ Pm,TXA α 12 + γtot αatm αatm = Pm,RXA P m,rxa P 0,RXA real situation sensitivity

31 Link margin Graph of link margin M (L 12 ) M ( L12 ) = P mpd, ( 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 L 12 = 1km; Δλ = 1,5nm Areas applied

33 Atmospheric phenomena Components of α atm 1. Absorption, scattering and refraction on gas molecules and aerosols (fog, snow, rain) (slow variations) (λ = 785 nm) visibility [km] attenuation [db.km -1 ] State of the atmosphere < 0.05 > 340 Heavy fog Middle fog Weak fog or heavy rain Haze Clear

34 Atmospheric phenomena Components of α atm 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 errors 10 (7th floor, filmed from a distance of 750m) :00 06:00 12:00 18:00 00:00 29/09/2000

35 Atmospheric phenomena Components of α atm 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) Received power (P r ) c) fog

38 Results processing statistical model of installation site PDF of random atmospheric attenuation (histogram) (measured in Autumn) probability [%] Exceedance probability function of atmospheric attenuation This is probability that atmospheric attenuation exceeds given value % casu prekroceni α atm [db/km] α atm [db/km]

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) Model of installation site: probability that atmospheric attenuation exceeds given value Nedostupnost [%] ,1 λ = 850 nm 0, Koeficient útlumu [db/km]

40 Complex model of FSO link 1 M S S =90dB Brno 0.01 L 12 [km] M S =70dB M S =70dB Brno P un [%] 0.4 Milesovka Milesovka M 1 [db/km] Nomogram for unavailability of link assesment

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

42 Conclusion FSO links are a suitable technology for the last mile solution in the frame of access network 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 FSO links are flexible, simple and full-value (in terms of quality of transmission) license-free instrument of network communication technologies