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EE 566 Optical Communications Free Space Optics An overview Snehil Tiwari [email protected] Free Space Optics (FSO)?? FSO is a line of sight technology that uses devices such as lasers to establish connectivity for video and voice communication Currently, It can allow upto 2.5 Gbps of data rate but can be increased to 10 Gbps using WDM FSO is based on connectivity between two stations consisting of optical transceiver to achieve full duplex communication Technology: How it works? FSO is a pretty old and relatively simple technology which has been used for more than 30 years. FSO enables similar bandwidth transmission abilities as fiber optics, using similar optical transmitters and receivers It involves two FSO units (similar to BTS of wireless technology) each consisting of high power laser transmitters and receivers, a telescope is used in conjunction to guide the light and capture it at the receiving end. It is then typically interface with network switches, hub, bridge or router via multimode fiber Transceiver Design Transmitter source: Semiconductor lasers and LEDs(short range). Solid-state lasers e.g. Yd:Yag (bulky, power) Gas lasers e.g. CO2 (bulky, lifetime) Quantum Cascade (QC) lasers (cooling for higher power operation) Operation within optical windows Basic Requirements: High power operation (average power,not peak power!) High modulation speed Operation over wide temperature range Contd.. (Receivers) Detector: Basic Requirement: Compensation for lower power lasers (High gain) Capability to Reject Background noise(S/N ratio) High Acceptance Angle (NA) A typical FSO unit http://www.freespaceoptics.org/images/flightstrata_illustration.gif Typical City-Wide Broadcast FSO Deployment Technical Specification for laser Wavelength used are ~780nm to ~850 nm for short ranges and less intensity typically 20mw 1550nm (class 1M)for long range data links typically 640 mw . 1550 nm wavelength is safe for eye . 1550 nm wavelength is also being used for optical fiber communication so compatible also. Attenuation is less at longer wavelengths. FSO As A Last-Mile Solution Low investment in infrastructure typical fiber optical cable costs $ 200,000 per mile approximately for installation and support. FSO based networks can be implemented for a fraction of the cost Low delay in deployment of FSO based networks Typical deployment of a fiber based network takes well over an year, while FSO based networks may be deployed with no substantial over-heads License Free Bandwidth Unlike microwave links, optical bandwidth isn’t licensed – i.e , bandwidth is free. This amounts to lower operational costs Supports High Data Rates Technological Challenges FSO is a LOS technology where it is essential that interconnecting points must see each other without any physical obstruction. Some of the potential disturbances are:- Fog: Wavelength dependence of fog attenuation change with density Between 0.5-4um attenuation by dense fog increases with wavelength. Atmospheric Absorption: Molecular absorption (gases) Aerosol absorption (dust, smoke, water drops) Scattering: Rayleigh Scattering (gases) Mie scattering (aerosol, fog) Geometrical optics (snow, rain) Scintillation: The variation of refractive index along the propagation path caused by slight temperature variations among different air pockets. Acts like series of small lenses that deflect the beam into and out of the transmission path causes amplitude fluctuations at the receiver. Can impact BER performance ,increases with distance .therefore for long distance communication Multiple beam technology is used. Limitations •Unreliable Bandwidth availability : Variations in weather conditions affect available bandwidth •Requires Line of Sight link •Limited Range •Need sophisticated mechanism for alignment of the transmitter and receiver •Even slight mechanical disturbances may lead to loss of alignment, and may result in complete link break-down Industrial Applications Short Range Applications Long Range Applications Weather monitors; fog, snow, rain using light back-scatter Traffic counting and monitoring Optical Interconnects. Deep space probe communications; distances measured in lightyears Building to building computer data links; very high data rates. Ship to ship communications; high data rates with complete security. Telemetry transmitters from remote monitors; weather, geophysical. Electronic distance measurements; hand held units out to 1000 ft. Optical radar; shape, speed, direction and range. Remote telephone links; cheaper than microwave Wide Area Applications Campus wide computer networks City-wide information broadcasting Inter-office data links Markets For FSO The industry is expected to grow from approximately $120 million in 2000 to more than $2 billion annually by 2006, according to a study conducted by the Strategis Group, a Washington, D.C.- based telecommunications research firm. Companies working in FSO • Terabeam Corp • Lightpointe • Nortel • fSONA Communications • Crinis Networks References http://www.mrv.com/technology http://www.freespaceoptics.org/index.cfm http://www.imagineeringezine.com/ http://www.sciam.com/ http://www.wcai.com