April 2003 Mohit Garg, IIT Bombay 1 Free Space Optical Communication Picture:

April 2003Mohit Garg, IIT Bombay2 ‘Wireless’ Optics?  Fiber replaced by free space oChannel characteristics not in control oTransmitter and Receiver essentially the same  Indoor and Outdoor implementations differ  Three basic configurations oLine of Sight (max. bandwidth) oDirected -- Non-Line of Sight Hybrid oNon directed -- Non-Line of Sight Diffused (min. bandwidth) Thus ‘Wireless’ need not imply Roaming Picture:

April 2003Mohit Garg, IIT Bombay3 Indoor links…  Interference oIncandescent Light (~ 2800 K) – Max. interference oSunlight (~ 6000 K) oFluorescent lamps  Attenuation oFree Space Loss (due to beam divergence) -- important oAtmospheric Loss (not much indoors)  Eye Safety – Most Important oShould be class I safe (< 0.5 mW, 880 nm, LASER) oRestricts system power (though LEDs can be used at higher powers, but Bandwidth limited) Picture: Optical Wireless- The Promise and Reality, Heatly and Neild

April 2003Mohit Garg, IIT Bombay4 Outdoor links…  Attenuation – Most Important oAtmospheric Loss (varies with weather)  0.2 dB/km in exceptionally clear weather  300 dB/km in very dense fog  Restricts the range (~500m in most commercial systems)  May need low capacity back-up RF links oFree Space Loss (due to beam divergence)  Scintillation Noise ( atmospheric turbulence induced intensity fluctuations ) – speckled pattern  Alignment Issues – Line of sight  Interference oSunlight (~ 6000 K) Picture:

April 2003Mohit Garg, IIT Bombay5 Attenuation :: Outdoor links P R = P T. A receiver. e –σ.R /(Div-range) 2 P R ~ P T e –σ.R  Free Space losses beam divergence   Atmospheric losses exponential term– dominates o oScattering + Absorption o oScattering dominates in σ Does Attenuation depend on wavelength?

April 2003Mohit Garg, IIT Bombay6 Attenuation :: Scattering Depends on particle size   Size parameter α = 2π r/λ   ‘ r ’ varies with atmospheric composition r σ ~ λ -4 Rayleigh Scattering r ~ λ => σ ~ λ -1.6 to 0 Mie Scattering r >> λ => σ ~ λ 0 Geometric Scattering Thus, larger Thus, larger λ => lower attenuation Belief that 1550 nm is less attenuated than 785 nm in fog. Does this apply always?

April 2003Mohit Garg, IIT Bombay7 Attenuation :: Scattering …contd Type Radius (μm) 785 nm α (size paremeter ) 1550 nm α (size paremeter ) Air Molecules Haze Particle Fog Particle 1 to 20 8 to to 80 Rain 100 to to Snow 1000 to to to Hail 5000 to to to Table: Comparison of beam propagation in haze and fog, Kim, McArthur and Koreevar The authors, studied the FOGGY weather conditions which were showing a discrepancy between analytical and empirical data.

April 2003Mohit Garg, IIT Bombay8 Attenuation :: Scattering …contd  The particle size distribution is difficult to obtain. so we express in terms of Visibility (V) σ= (3.91/V) x (λ/550 nm) -q V= visibility (km) light falls off to 2% of initial value V= visibility (km) light falls off to 2% of initial value q= Size distribution of scattering particles q= Size distribution of scattering particles = 1.6 (V>50 km) = 1.6 (V>50 km) = 1.3 (6 km <V< 50 km) = 1.3 (6 km <V< 50 km) = 0.16 V+0.34 (1 km <V< 6 km) Haze = 0.16 V+0.34 (1 km <V< 6 km) Haze = V (0.5 km <V< 1 km) Mist = V (0.5 km <V< 1 km) Mist = 0 ( V < 0.5 km) Fog = 0 ( V < 0.5 km) Fog The authors, proposed a new wavelength dependence through Mie Scattering calculations Earlier = V 1/3 (V < 6 km)

April 2003Mohit Garg, IIT Bombay9 Scintillation Noise Inhomogenities in Temp. and Pressure Inhomogenities in Temp. and Pressure Variations in Refractive Index along the transmission path Speckled pattern (both in time and space) at the receiver Can be removed by time and space averaging. But problems arise with restrictions on size of receiver and high bit rates.

April 2003Mohit Garg, IIT Bombay10 Some images Pictures:

April 2003Mohit Garg, IIT Bombay11 Web Resources