Free Space Optical Communication

April 2003 Picture: http://www.cablefreesolutions.com/index.htm

Mohit Garg, IIT Bombay 1

‘Wireless’ Optics?

Picture: http://www.cablefreesolutions.com/index.htm

   Fiber replaced by free space o Channel characteristics not in control o Transmitter and Receiver essentially the same Indoor and Outdoor implementations differ Three basic configurations o Line of Sight (max. bandwidth) o Directed -- Non-Line of Sight Hybrid o Non directed -- Non-Line of Sight Diffused (min. bandwidth)

Thus ‘Wireless’ need not imply Roaming

April 2003 Mohit Garg, IIT Bombay 2

Indoor links…

Picture: Optical Wireless- The Promise and Reality , Heatly and Neild    Interference o Incandescent Light (~ 2800 K) – Max. interference o Sunlight (~ 6000 K) o Fluorescent lamps Attenuation o Free Space Loss (due to beam divergence) - important o Atmospheric Loss (not much indoors) Eye Safety – Most Important o Should be class I safe (< 0.5 mW, 880 nm, LASER) o Restricts system power (though LEDs can be used at higher powers, but Bandwidth limited) April 2003 Mohit Garg, IIT Bombay 3

Outdoor links…

Picture: http://www.cablefreesolutions.com/imagelib21.htm

    Attenuation – Most Important o Atmospheric 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 o Free Space Loss (due to beam divergence) Scintillation Noise ( atmospheric turbulence induced intensity fluctuations ) – speckled pattern Alignment Issues – Line of sight Interference o Sunlight (~ 6000 K) April 2003 Mohit Garg, IIT Bombay 4

Attenuation :: Outdoor links

P

R

= P

T

P

R

~ P

T

. A

receiver

e

–σ.R

. e

–σ.R

/(Div-range)

2   Free Space losses beam divergence Atmospheric losses exponential term– dominates o Scattering + Absorption o Scattering dominates in σ

Does Attenuation depend on wavelength?

April 2003 Mohit Garg, IIT Bombay 5

Attenuation :: Scattering

Depends on particle size

 Size parameter α = 2π r/λ  ‘ r ’ varies with atmospheric composition r << λ => σ ~ λ -4 r ~ λ => σ ~ λ -1.6 to 0 r >> λ => σ ~ λ 0

Rayleigh Scattering Mie Scattering Geometric Scattering

Thus, larger λ => lower attenuation

Belief that 1550 nm is less attenuated than 785 nm in fog.

Does this apply always?

April 2003 Mohit Garg, IIT Bombay 6

Attenuation :: Scattering

…contd

785 nm 1550 nm Type Radius (μm) α (size paremeter ) α (size paremeter ) Air Molecules 0.0001

0.0008

0.0004

Haze Particle Fog Particle 0.01-1 1 to 20 0.08-8 8 to 160 0.04-4 4 to 80 Rain 100 to 10000 800 to 80000 400-400000 Snow 1000 to 5000 8000 to 400000 4000 to 20000 Hail 5000 to 50000 40000 to 800000 20000 to 400000 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 2003 Mohit Garg, IIT Bombay 7

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) = 1.6 (V>50 km) light falls off to 2% of initial value q= Size distribution of scattering particles = 1.3 (6 km

Mie Scattering calculations

April 2003 Mohit Garg, IIT Bombay 8

Scintillation Noise

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

April 2003 Mohit Garg, IIT Bombay 9

Some images

April 2003 Pictures: http://www.cablefreesolutions.com/imagelib.htm

Mohit Garg, IIT Bombay 10

Web Resources

     www.freespaceoptic.com

www.cablefreesolutions.com/casestudies.htm

www.engalco.co.uk/fso_report.htm

www.fiberwork.com.br/site/english/fso_e.htm

www.ieeexplore.ieee.org

April 2003 Mohit Garg, IIT Bombay 11