Free Space Optics

AVIKSHIT-08 FREE SPACE OPTICS

DEFINITION 





It is a line-of-sight technology in which voice, video and data are sent through the air on beams of light at speeds up to 1.25 Gbps. Like fibre FSO uses lasers to transmit data but instead of enclosing the data stream in a glass fibre, it is transmitted through air. The technology is useful where the physical connection of the transmission and receiving locations is difficult. For e.g. used in cities where the laying of fibre optics cables is difficult.

HISTORY 

 

This was first developed in 1960’s both domestically and internationally for secure communications by military. Recent advancements due to great demand for bandwidth. In late 19th century Bell’s FSO experiment converted voice sounds to telephone signals and transmitted them via air space. Although Bell’s Photophone never became a commercial success.

Why was FSO technology deployed on a much larger scale after 2000? 





Availability of Fiber Optic cables is less (i.e. it reaches only 10-15% of buildings in the world). Greater demand for bandwidth and keeping the high budgets of other networks in mind, FSO came into existence. Deployment of fiber optic cable had become a challenge.

PRINCIPLE 



FSO transmits invisible, eye-safe light beam from one “telescope” to another using low power infrared lasers in tera hertz spectrum. It provides full-duplex bi-directional capability.

MAIN COMPONENTS TRANSMITTER  Modulated laser source.  Beamwidth at receiver suitable to allow for building sway.

RECEIVER  Receives close to collimated radiation independent of transmitter pointing.  Concentrating lens.  Photodetector and amplifier are present.

HOW FSO WORKS? 2 Transmitter projects the carefully aimed light pulses into the air

3 A receiver at the other end of the link collects the light using lenses and/or mirrors

5 Reverse direction data transported the same way. • Full duplex

1 Network traffic converted into pulses of invisible light representing 1’s and 0’s

4 Received signal converted back into fiber or copper and connected to the network

ADVANTAGES- SECURITY 





FSO laser beams cannot be detected with spectrum analyzers or RF meters. FSO laser transmissions are optical, narrow & invisible which travel along a line-of-sight path that cannot be intercepted easily. Data can be transmitted over an encrypted connection adding to the degree of security in FSO transmission.

SPEED 

 

The speed of FSO transmission is comparable with the speed of optical fibre transmission. It has low error rates. Provides band widths of 100Mbps,155Mbps & 1.25Gbps.

SAFETY 



Class 1M lasers are used and exposure to the beam will not result in eye injury & therefore be considered safe. Wavelength in the range of 400 to 1400 nm which does not affect the cornea at all.

LOW COST 





It is much cost effective than other internet service. Licensing is not required. Minimization of manual labor.

INSTALLATION 

It can be installed easily i.e. on the roof tops or on the walls .

POINT-TO-POINT CONNECTIVITY OPTIONS DSL/ CABLE

T1/E1

COMMERCIAL AVAILABILITY

LIMITED

HIGH

DISTANCE

3-10 MILES

UPTIME FACTORS COST (MONTHLY)

FIBER

RF WIRELESS

OPTICAL WIRELESS

POOR

MEDIUM

HIGH

5-20 MILES

>100 MILES

<40 MILES

<3 MILES

SERVICE INTERRU -PTIONS

SERVICE INTERRUPTIONS

SERVICE INTERRUPTIONS

SPECTRUM SATURATION ,RAIN

FOG,SNOW

LOW $40-60

MEDIUM $400-600

HIGH $30005000

CAPITAL INVESTMENT ONLY

CAPITAL INVESTMEN T ONLY

DEPLOYMENT MEDIUM TIME

SLOW

SLOW

FAST

FAST

VOICE

YES

YES

YES

YES

YES

DATA

YES

YES

YES

YES

YES

VIDEO

May Be

May Be

YES

YES

YES

LICENSE

NO

YES

YES

NO

NO

SCINTILLATION WHAT IS IT?  Scintillation is the result of solar energy heating small pockets of air to slightly different temperatures, thereby creating regions of varying refractive index along the propagation path.

SCINTILLATION(ON BRIGHT SUNNY DAYS) SOLUTION 







Larger aperture receiver Widely spaced transmitter Automatic gain control characteristics Finely tuned receive filtering.

BUILDING SWAY(DUE TO THERMAL DIMENSIONAL DISTORTIONS) SOLUTION 





It is so designed that Orientation does not change even with unit movement of transmitter (as it uses a divergent beam). The combination of effective beam divergence and a well matched Field Of View will help. This is called AUTO TRACKING.

PHYSICAL OBSTRUCTIONS (e.g. birds ,construction cranes) SOLUTIONS 

Using multiple, spatially diverse beams and large receiver will eliminate interference concerns.



An object covering 98% of the receiving aperature and all but-1 transmitter will not cause the FSO LINK to drop out.



Moreover, birds are not injured.

ABSORPTION WHAT IS IT? 



It occurs when suspended water molecules in the terrestrial atmosphere extinguishes photons. It causes decrease in power density (attenuation) of FSO beam.

SOLUTION 



Use of appropriate power according to atmospheric condition and Use of spatially diverse beams helps maintain the required level of network availability.

RAIN,SNOW,FOG( Rain and snow have very little effect on FSO technology)

ATTENUATION  RAIN low attenation <9dB /km  SNOW moderate attenuation <12dB /km  FOG high attenuation upto 100dB /km 

Still, FSO installation in foggy cities like SAN FRANSISCO have achieved carrier class reliability.

SOLUTION  NETWORK PLANNING (to shorten FSO link distances and to add network redundancies). 

These are engineered such that for a large fraction of time, an acceptable power is received even in the presence of heavy fog.

Some situations of FOG

APPLICATION SPACE COMMUNICATION



Used to communicate between space crafts since outside of the atmosphere, there is little to distort the signal.

“LAST MILE” BOTTLENECK PROBLEM Wide Area Networks between major cities are extremely fast Fiber based >2.5 Gbps

Local Area Networks in buildings are also fast >100Mbps

The connections in between are typically a lot slower 0.3-1.5 Mbps

Only about 5% of commercial buildings are lit with fiber

What is “last mile”?





The last mile is the final leg of delivering connectivity from a communications provider to a customer. It is sometimes called “first mile”.

APPLICATIONS-scenarios  

 

LAN to LAN in city. To cross a public road or other barriers which the sender and receiver do not own. Mobile Wireless backhaul. High speed ,low interference WiFi /802.11 backbones.

CONCLUSION 





The markets are growing for FSO. Speed, Security and Low Cost are main advantages. Environmental factors can be mitigated to avoid attenuation.

New Areas  Space communication  Traffic and Telematics  Quantum Key