This document provides an overview of free space optics (FSO) technology. It discusses how FSO works using light beams to transmit data through free space, as an alternative to buried fiber. The key benefits of FSO are lower costs compared to fiber due to not requiring permits for trenching. However, FSO faces challenges from atmospheric effects like fog that can attenuate the light beams. The document outlines the origins and technology of FSO, how it works, applications, and considerations like merits and demerits.

1. FSO (Free Space Optics)
By,
Vatsal Garasia

2. Outlines
• Concept
• Why Free Space Optics?
• Origin & Technology of FSO
• Working
• Challenges
• Applications
• Merits & Demerits
• References

3. Concept
• FSO - optical communication technology that
uses light propagating in free space to transfer
data.
• Line of sight technology.
• Bandwidth up to 2.5 Gbps.
• Uses LED or Laser as a light source.

4. FSO Transmitter FSO Receiver
Fig. 1 FSO Transmitter & Receiver images

5. Beam Divergence (Frequency) Effect
Fig. 2 Beam Divergence effect

6. Why Free Space Optics?
Why Not Just Bury More Fiber?
• Cost
• Rights of Way
• Permits
• Trenching
• Time
With FSO, especially through
the window, no permits, no
digging, no fees

7. Why Free Space Optics?
How FSO Works

8. 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
• Highly secure
• Efficient use of energy
• Ranges of 20m to more than 8km possible

9. Why Free Space Optics?
Deployment Behind Windows
• Rapid installations without trenching and
permitting
• Direct connection to the end user
• Bypasses the building owner
– No roof rights
– No riser rights

10. Origin
• 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
Photophone.

11. Origin (cont.)
• 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
reality.
• Though it demonstrated the basic principle of
optical transmissions.

12. Technology
• 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.

13. Working
• FSO work on simple optical transmission
system.
• Modern Laser system provide network
connectivity speed from 660 Mbps onwards.
• Two beams are kept narrow.
• The receiver detectors are either PIN diodes or
Avalanche Photodiode.
• FSO transmits invisible light beams between
two beams.

14. Working (cont.)
• It works in Tera Hertz (THz) spectrum.
• Wavelength:
• 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.

15. Working (cont.)
Fig. 3 Sub-systems used in a typical free-space optics unit

16. Sunlight
Challenges
Environmental factors

17. Challenges
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

18. Challenges
Low Clouds, Rain, Snow and Dust
• Low Clouds
– Very similar to fog
– May accompany rain and snow
• Rain
– Drop sizes larger than fog and wavelength
of light
– 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
urban core

19. Challenges
Scintillation
• Beam spreading and wandering due to propagation through air
pockets of varying temperature, density, and index of
refraction.
• Almost mutually exclusive with fog attenuation.
• Results in increased error rate but not complete outage.

20. Applications
• Metro Area Networks (MAN)
• Last Mile Access
• Enterprise connectivity
• Fiber backup
• Backhaul
• Service acceleration

21. Merits
• Flexible network solution over conventional
broadband services.
• Straight forward deployment- no licenses
required
• Low initial investment
• Ease of installation
• Re-deployability
• High bit rates and low error rates

22. Demerits
• Fog
• Physical obstructions
• Scintillation
• Solar interference
• Scattering
• Absorption
• Building sway / Seismic activity

23. References
1 Free-space optical communication - Wikipedia, http://en.wikipedia.org/wiki/Free
space_optical_communication
2 Vikrant Kaulgud, Free space optics Bridges the last mile, Electronics for U, June 2003 pp.
38-40. www.electronicsforu.com/electronicsforu/articles/hits.asp?id=822
3 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.
4 John Kaufmann, Free Space Optical Communications: An Overview ofApplications and
Technologies, Boston IEEE Communications Society Meeting, December 1, 2011.
[6] John Schuster, Free Space Optics (FSO) Technology Overview, Chief Technology Officer,
Terabeam Corporation.
www.fcc.gov/realaudio/presentations/2002/.../technology_overview.ppt
5 Andy Emmerson, Fiberless Optics, Everyday practical electronics, April 2003, pp. 248.
6 www.fsona.com
7 www.freespaceoptic.com