Anything that can be done in fiber can be done with FSO 1 Network traffic converted into pulses of invisible light representing 1’s and 0’s 2 Transmitter projects the carefully aimed light pulses into the air 5
Reverse direction data transported the same way.
3 A receiver at the other end of the link collects the light using lenses and/or mirrors 4 Received signal converted back into fiber or copper and connected to the network
FSO systems use optical wireless link heads each having:
a transceiver with a laser or LED transmitter
a lens or telescope (can have more that one)
shaping overcomes building movement
a receiver usually a semiconductor
May also employ servo motors, voice coils, mirrors, CCD arrays, and even liquid crystals and micro-electromechanical systems (MEMS) for tracking and acquisition.
FSO operates in the infrared (IR) range around 850 and 1550 nm (frequencies around 200 THz).
FSO can use Power Over Ethernet (PoE).
Beams only a few meters in diameter at a kilometer
Allows VERY close spacing of links without interference
Efficient use of energy
Ranges of 20m to more than 8km possible
Rapid installations without trenching and permitting
Direct connection to the end user
Bypasses the building owner
No roof rights
No riser rights
Easy to install
Through the window (or from the rooftop)
No trenching, no permits
Fiber-like data rates
1 ° ≈ 17 mrad -> 1 mrad ≈ 0.0573° 1 mrad 1 km 1 m Small angle approximation: Angle (in milliradians) * Range (km)= Spot Size (m) Divergence Range Spot Diameter 0.5 mrad 1.0 km ~0.5 m (~20 in) 2.0 mrad 1.0 km ~2.0 m (~6.5 ft) 4.0 mrad (~ ¼ deg) 1.0 km ~4.0 m (~13.0 ft)
A logarithmic ratio between two values
In the optical world of Power in mW,
Gain/Loss Multiplier +30 db +20 db +10 db 0 db -10 db -20 db -30 db 1000 100 10 1 .1 .01 .001
Sunlight Building Motion Alignment Window Attenuation Fog Each of these factors can “attenuate” (reduce) the signal. However, there are ways to mitigate each environmental factor. Scintillation Range Obstructions Low Clouds
Absorption or scattering of optical signals due to airborne particles
FSO wavelengths and fog droplets are close to equal in size
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.
Very similar to fog
May accompany rain and snow
Drop sizes larger than fog and wavelength of light
Extremely heavy rain (can’t see through it) can take a link down
May cause ice build-up on windows
Likely only in desert areas; rare in the urban core
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.
Challenges: Scintillation >>
Uncoated glass attenuates 4% per surface due to reflection
Tinted or insulated windows can have much greater attenuation
Possible to trade high altitude rooftop weather losses vs. window attenuation
Challenges Window Attenuation WAM
Type Cause(s) Magnitude Frequency Tip/tilt Thermal expansion High Once per day Sway Wind Medium Once every several seconds Vibration Equipment (e.g., HVAC), door slamming, etc. Low Many times per second
Results from Seattle Deployment:
15% of buildings move more than 4 mrad
5% of buildings move more than 6 mrad
1% of buildings move more than 10 mrad
High bit rates
Low bit error rates
Immunity to electromagnetic interference
Full duplex operation
Very secure due to the high directionality and narrowness of the beams
No Fresnel zone necessary
RONJA , a free implemantation of FSO utilizing High intensity LEDs
Fog (10..~100 dB/km attenuation)
Pollution / smog
If the sun goes exactly behind the transmitter, it can swamp the signal.
To those unfamiliar with FSO technology, safety can be a concern because the technology uses lasers for transmission. The two major concerns involve eye exposure to light beams and high voltages within the light systems and their power supplies. Strict international standards have been set for safety and performance.
Typically scenarios for use are:
LAN-to-LAN connections on campuses at Fast Ethernet or Gigabit Ethernet speeds.
To cross a public road or other barriers which the sender and
receiver do not own.
Speedy service delivery of high-bandwidth access to
optical fiber networks.
Temporary network installation (for events or other purposes).
For communications between spacecraft, including elements of a satellite constellation.
For inter- and intra-chip communication.
Two solar-powered satellites communicating optically in space via lasers.
LEDs and Fresnel type lenses help reduce power requirements
Wide-beam technology reduces effects of:
scintillation, and shimmer
FSO and microwave hybrid systems to overcome distance, fog, and dust.
Parallel lasers help integrity and increase the amount of data that can be transmitted.
The FSO industry shows some strength, and the FSO market is growing, though with much less speed.
In spite of this, the commercial future of free-space optical communications remains uncertain.Perhaps the best overall prospects are in space, where progress is being made in improving acquisition and tracking. Once these are perfected, the bandwidth advantages of optical free-space communications should open up a substantial market.
The FSO industry consists of mostly established vendors that manufacture equipment for various distances and speeds of transmission. The highest speed of 2.5 Gb/s promises to be increased to 10 Gb/s in future.
Free-Space Optics: Enabling Optical Connectivity in Today's Networks By Heinz Willebrand, Ph.D.,, Baksheesh S. Ghuman Sams Publishing 2001/12/21