FSO (Free Space Optics)
29 October 2013
Why Free Space Optics?
Origin & Technology of FSO
Merits & Demerits
• FSO - optical communication technology that
uses light propagating in free space to transfer
• Line of sight technology.
• Bandwidth up to 2.5 Gbps.
• Uses LED or Laser as a light source.
Why Free Space Optics? 
Very Narrow and Directional Beams
Beams only a few meters in diameter at a kilometer
Allows VERY close spacing of links without interference
No side lobes
Efficient use of energy
Ranges of 20m to more than 8km possible
Why Free Space Optics? 
Deployment Behind Windows
• Rapid installations without trenching and
• Direct connection to the end user
• Bypasses the building owner
– No roof rights
– No riser rights
• Firstly used by Greeks in 8th century.
• According to them fire as the light source, the
atmosphere as transmission medium and an
eye as a receiver.
• 19th century, Alexander Graham Bell – done
experiments - which were later called as
• Bell converted voice sounds into telephone
signals and transmitted them between
receivers through free space along a beam of
light for a distance of some 600 feet.
• But Photophone never became commercial
• Though it demonstrated the basic principle of
• Uses a directed beam of light radiation
between transmitter and receiver.
• An FSO unit consists of
1) Optical transceiver
2) Laser transmitter and receiver
• Uses lens on transmitter and receiver.
• Maximum range is about 4 kms.
• FSO work on simple optical transmission
• Modern Laser system provide network
connectivity speed from 660 Mbps onwards.
• Two beams are kept narrow.
• The receiver detectors are either PIN diodes or
• FSO transmits invisible light beams between
• It works in Tera Hertz (THz) spectrum.
• FSO can operate into two wavelengths:
1) 800 nm
2) 1550 nm
• 1550 nm wavelengths are more preferred due
to its advantages over 800 nm.
Fig. 3 Sub-systems used in a typical free-space optics unit 
Atmospheric Attenuation - FOG
• Absorption or scattering of optical
signals due to airborne particles
• Primarily FOG but can be rain, snow,
smoke, dust, etc.
• Can result in a complete outage
• FSO wavelengths and fog droplets are
close to equal in size
– (Mie Scattering)
• Typical FSO systems work 2-3X further
than the human eye can see
• High availability deployments require
short links that can operate in the fog
Low Clouds, Rain, Snow and Dust
• Low Clouds
– Very similar to fog
– May accompany rain and snow
– Drop sizes larger than fog and wavelength
– Extremely heavy rain (can’t see through it)
can take a link down
– Water sheeting on windows
• Heavy Snow
– May cause ice build-up on windows
– Whiteout conditions
• Sand Storms
– Likely only in desert areas; rare in the
• Beam spreading and wandering due to propagation through air
pockets of varying temperature, density, and index of
• Almost mutually exclusive with fog attenuation.
• Results in increased error rate but not complete outage.
Metro Area Networks (MAN)
Last Mile Access
• Flexible network solution over conventional
• Straight forward deployment- no licenses
• Low initial investment
• Ease of installation
• High bit rates and low error rates
Building sway / Seismic activity
 Free-space optical communication - Wikipedia, http://en.wikipedia.org/wiki/Free
 Vikrant Kaulgud, Free space optics Bridges the last mile, Electronics for U, June 2003 pp.
 Hemmati, H., Free-space optical communications program at JPL, Jet Propulsion Lab.,
California Inst. of Technol., Pasadena, CA, USA, IEEE Lasers and Electro-Optics Society, pp.
106 - 107, vol.1, Nov. 1999.
 John Kaufmann, Free Space Optical Communications: An Overview of Applications and
Technologies, Boston IEEE Communications Society Meeting, December 1, 2011.
 John Schuster, Free Space Optics (FSO) Technology Overview, Chief Technology Officer,
 Andy Emmerson, Fiberless Optics, Everyday practical electronics, April 2003, pp. 248.