Free Space Optics

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Free Space Optics (FSO) communications, also called Free Space Photonics (FSP) or Optical Wireless, refers to the transmission of modulated visible or infrared (IR) beams through the atmosphere to obtain optical communications. Like fiber, Free Space Optics (FSO) uses lasers to transmit data, but instead of enclosing the data stream in a glass fiber, it is transmitted through the air. Free Space Optics (FSO) works on the same basic principle as Infrared television remote controls, wireless keyboards or wireless Palm? devices.

 

History of Free Space Optics (FSO)

??????????? The engineering maturity of Free Space Optics (FSO) is often underestimated, due to a misunderstanding of how long Free Space Optics (FSO) systems have been under development. Historically, Free Space Optics (FSO) or optical wireless communications was first demonstrated by Alexander Graham Bell in the late nineteenth century (prior to his demonstration of the telephone!). Bell?s Free Space Optics (FSO) experiment converted voice sounds into telephone signals and transmitted them between receivers through free air space along a beam of light for a distance of some 600 feet. Calling his experimental device the ?photophone,? Bell considered this optical technology ? and not the telephone ? his preeminent invention because it did not require wires for transmission.

??????????? Although Bell?s photophone never became a commercial reality, it demonstrated the basic principle of optical communications. Essentially all of the engineering of today?s Free Space Optics (FSO) or free space optical communications systems was done over the past 40 years or so, mostly for defense applications. By addressing the principal engineering challenges of Free Space Optics (FSO), this aerospace/defense activity established a strong foundation upon which today?s commercial laser-based Free Space Optics (FSO) systems are based.

 

How Free Space Optics (FSO) Works

??????????? Free Space Optics (FSO) transmits invisible, eye-safe light beams from one "telescope" to another using low power infrared lasers in the teraHertz spectrum. The beams of light in Free Space Optics (FSO) systems are transmitted by laser light focused on highly sensitive photon detector receivers. These receivers are telescopic lenses able to collect the photon stream and transmit digital data containing a mix of Internet messages, video images, radio signals or computer files.Commercially available systems offer capacities in the range of 100 Mbps to 2.5 Gbps, and demonstration systems report data rates as high as 160 Gbps.

??????????? Free Space Optics (FSO) systems can function over distances of several kilometers. As long as there is a clear line of sight between the source and the destination, and enough transmitter power, Free Space Optics (FSO) communication is possible.

Free Space optics (fso) technology

??????????? Lasers are one of the most significant inventions of the 20th century - they can be found in many modern products, from CD players to fiber-optic networks. The word laser is actually an acronym for Light Amplification by Stimulated Emiission of Radiation. Although stimulated emission was first predicted by Albert Einstein near the beginning of the 20th century, the first working laser was not demonstrated until 1960 when Theodore Maiman did so using a ruby. Maiman's laser was predated by the maser - another acronym, this time for Microwave Amplification by Stimulated Emission of Radiation. A maser is very similar to a laser except the photons generated by a maser are of a longer wavelength outside the visible and/or infrared spectrum.

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A laser generates light, either visible or infrared, through a process known as stimulated emission. To understand stimulated emission, understanding two basic concepts is necessary. The first is absorption which occurs when an atom absorbs energy or photons. The second is emission which occurs when an atom emits photons. Emission occurs when an atom is in an excited or high energy state and returns to a stable or ground state ? when this occurs naturally it is called spontaneous emission because no outside trigger is required. Stimulated emission occurs when an already excited atom is bombarded by yet another photon causing it to release that photon along with the photon which previously excited it. Photons are particles, or more properly quanta, of light and a light beam is made up of what can be thought of as a stream of photons.

A basic laser uses a mirrored chamber or cavity to reflect light waves so they reinforce each other. An excitable substance ? gas, liquid, or solid like the original ruby laser ? is contained within the cavity and determines the wavelength of the resulting laser beam. Through a process called pumping, energy is introduced to the cavity exciting the atoms within and causing a population inversion. A population inversion is when there are more excited atoms than grounded atoms which then leads to stimulated emission. The released photons oscillate back and forth between the mirrors of the cavity, building energy and causing other atoms to release more photons. One of the mirrors allows some of the released photons to escape the cavity resulting in a laser beam emitting from one end of the cavity.

Terrestrial Laser Communications Challenges

Fog

Fog substantially attenuates visible radiation, and it has a similar affect on the near-infrared wavelengths that are employed in laser communications. Similar to the case of rain attenuation with RF wireless, fog attenuation is not a ?show-stopper? for optical wireless, because the optical link can be engineered such that, for a large fraction of the time, an acceptable power will be received even in the presence of heavy fog. Laser communication systems can be enhanced to yield even greater availabilities by combining them with RF systems.

Physical Obstructions

Laser communications systems that employ multiple, spatially diverse transmitters and large receive optics will eliminate interference concerns from objects such as birds.

Pointing Stability

??????????? Pointing stability in commercial laser communications systems is achieved by one of two methods. The simpler, less costly method is to widen the beam divergence so that if either end of the link moves the receiver will still be within the beam. The second method is to employ a beam tracking system. While more costly, such systems allow for a tighter beam to be transmitted allowing for higher security and longer distance transmissions.

Scintillation

Performance of many laser communications systems is adversely affected by scintillation on bright sunny days. Through a large aperture receiver, widely spaced transmitters, finely tuned receive filtering, and automatic gain control, downtime due to scintillation can be avoided.

 

FSO: Wireless, at the Speed of Light

??????????? Unlike radio and microwave systems, Free Space Optics (FSO) is an optical technology and no spectrum licensing or frequency coordination with other users is required, interference from or to other systems or equipment is not a concern, and the point-to-point laser signal is extremely difficult to intercept, and therefore secure. Data rates comparable to optical fiber transmission can be carried by Free Space Optics (FSO) systems with very low error rates, while the extremely narrow laser beam widths ensure that there is almost no practical limit to the number of separate Free Space Optics (FSO) links that can be installed in a given location.

How Free Space Optics (FSO) can help you

??????????? FSO?s freedom from licensing and regulation translates into ease, speed and low cost of deployment. Since Free Space Optics (FSO) transceivers can transmit and receive through windows, it is possible to mount Free Space Optics (FSO) systems inside buildings, reducing the need to compete for roof space, simplifying wiring and cabling, and permitting Free Space Optics (FSO) equipment to operate in a very favorable environment. The only essential requirement for Free Space Optics (FSO) or optical wireless transmission is line of sight between the two ends of the link.

For Metro Area Network (MAN) providers the last mile or even feet can be the most daunting. Free Space Optics (FSO) networks can close this gap and allow new customers access to high-speed MAN?s. Providers also can take advantage of the reduced risk of installing an Free Space Optics (FSO) network which can later be redeployed.

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