Free Space Optics
Free Space Optics
FSO may sound new and experimental but in fact it predates optical fiber and has its roots in wartime efforts to develop secure communication systems that did not require cable and could withstand radio jamming. FSO has been around for more than a decade, but it is only recently that interest in this technology has started to grow. 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.
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 laser 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. [pic]
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. 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. [pic]
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
FogFog 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? 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 customer’s 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.
The Market. Why FSO? Breaking the Bandwidth Bottleneck
Why FSO? The global telecommunications network has seen massive expansion over the last few years. First came the tremendous growth of the optical fiber long-haul, wide-area network (WAN), followed by a more recent emphasis on metropolitan area networks (MANs). Meanwhile, local area networks (LANs) and gigabit ethernet ports are being deployed with a comparable growth rate. In order for this tremendous network capacity to be exploited, and for the users to be able to utilize the broad array of new services becoming available, network designers must provide a flexible and cost-effective means for the users to access the telecommunications network. Presently, however, most local loop network connections are limited to 1.5 Mbps (a T1 line). As a consequence, there is a strong need for a high-bandwidth bridge (the “last mile” or “first mile”) between the LANs and the MANs or WANs.
A recent New York Times article reported that more than 100 million miles of optical fiber was laid around the world in the last two years, as carriers reacted to the Internet phenomenon and end users’ insatiable demand for bandwidth. The sheer scale of connecting whole communities, cities and regions to that fiber optic cable or “backbone” is something not many players understood well. Despite the huge investment in trenching and optical cable, most of the fiber remains unlit, 80 to 90% of office, commercial and industrial buildings are not connected to fiber, and transport prices are dropping dramatically.
Free Space Optics (FSO) systems represent one of the most promising approaches for addressing the emerging broadband access market and its “last mile” bottleneck. Free Space Optics (FSO) systems offer many features, principal among them being low start-up and operational costs, rapid deployment, and high fiber-like bandwidths due to the optical nature of the technology.
Broadband Bandwidth Alternatives
Access technologies in general use today include telco-provisioned copper wire, wireless Internet access, broadband RF/microwave, coaxial cable and direct optical fiber connections (fiber to the building; fiber to the home). Telco/PTT telephone networks are still trapped in the old Time Division Multiplex (TDM) based network infrastructure that rations bandwidth to the customer in increments of 1.5 Mbps (T-1) or 2.024 Mbps (E-1). DSL penetration rates have been throttled by slow deployment and the pricing strategies of the PTTs. Cable modem access has had more success in residential markets, but suffers from security and capacity problems, and is generally conditional on the user subscribing to a package of cable TV channels. Wireless Internet access is still slow, and the tiny screen renders it of little appeal for web browsing.
Broadband RF/microwave systems have severe limitations and are losing favor. The radio spectrum is a scarce and expensive licensed commodity, sold or leased to the highest bidder, or on a first-come first-served basis, and all too often, simply unavailable due to congestion. As building owners have realized the value of their roof space, the price of roof rights has risen sharply. Furthermore, radio equipment is not inexpensive, the maximum data rates achievable with RF systems are low compared to optical fiber, and communications channels are insecure and subject to interference from and to other systems (a major constraint on the use of radio systems).
A free space optical (FSO) system offers a flexible networking solution that delivers on the promise of broadband. Only free space optics or Free Space Optics (FSO) provides the essential combination of qualities required to bring the traffic to the optical fiber backbone – virtually unlimited bandwidth, low cost, ease and speed of deployment. Freedom from licensing and regulation translates into ease, speed and low cost of deployment. Since Free Space Optics (FSO) optical wireless 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 the equipment to operate in a very favorable environment.
The only essential for Free Space Optics (FSO) is line of sight between the two ends of the link. ➢ Freedom from licensing and regulation leads to ease, speed and low cost of deployment. ➢ Since FSO units can receive and transmit through windows it reduces the need to compete for roof space, simplifying wiring and cabling. ➢ Only need is the line of sight between the two ends of the link. ➢ Providers take advantage of the reduced risk in installing FSO equipment, which can even be re-deployed. ➢ Zero chances of network failure.
➢ Virtually unlimited bandwidth.
➢ Metro network extensions – FSO is used to extend existing metropolitan area fiberings to connect new networks from outside. ➢ Last mile access – FSO can be used in high-speed links to connect end users with ISPs. ➢ Enterprise connectivity – The ease in which FSO can be installed makes them a solution for interconnecting LAN segments, housed in buildings separated by public streets. ➢ Fiber backup – FSO may be deployed in redundant links to backup fiber in place of a second fiber link. ➢ Backhaul – Used to carry cellular telephone traffic from antenna towers back to facilities into the public switched telephone networks.
Free Space Optics (FSO) Challenges
The advantages of free space optical wireless or Free Space Optics (FSO) do not come without some cost. When light is transmitted through optical fiber, transmission integrity is quite predictable – barring unforeseen events such as backhoes or animal interference. When light is transmitted through the air, as with Free Space Optics (FSO) optical wireless systems, it must contend with a complex and not always quantifiable subject – the atmosphere.
➢ Fog and free space optics (fso)
➢ Physical obstructions and free space optics (fso)
➢ Free space optics (fso) pointing stability – building sway, tower movemen
t ➢ Scintillation and free space optics (fso)
➢ Solar interference and free space optics
FSO enables optical transmission of voice video and data through air at very high rates. It has key roles to play as primary access medium and backup technology. Driven by the need for high speed local loop connectivity and the cost and the difficulties of deploying fiber, the interest in FSO has certainly picked up dramatically among service providers worldwide. Instead of fiber coaxial systems, fiber laser systems may turn out to be the best way to deliver high data rates to your home. FSO continues to accelerate the vision of all optical networks cost effectively, reliably and quickly with freedom and flexibility of deployment.
University/College: University of California
Type of paper: Thesis/Dissertation Chapter
Date: 10 November 2016
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