Free Space Optics: Current Applications and Future Challenges

ATMOSPHERIC EFFECTS ON FSO LINK 

The actual transmission rates are weather dependent as the atmospheric attenuation is variable and hard to predict. The link may not work well in locations where the atmosphere is affected by conditions such as fog, storm, scintillation etc. FSO and Fog: Fog substantially attenuates visible radiation and it has a similar effect on the near infrared wavelengths that are employed in the FSO system (1550nm). Note that the effect of fog on the free space optical wireless radiation is entirely analogous to the attenuation and fades suffered by the RF wireless system due to rain fall. Similar to the case of rain attenuation with RF wireless fog attenuation is not a ‘showstopper’ for FSO (signals block out on the television during heavy rain) because the optical link can be engineered such that for a large fraction of time an acceptable power is received even in presence of heavy fog. FSO optical wireless based communication system can be enhanced to yield even greater availabilities [6]. FSO and Scintillation: Atmospheric scintillation can be thought of as changing light intensities in time and space at the plane of a receiver detecting the signal from a transmitter at a distance. When the beam is scintillated, photons of light are temporally steered by pockets of air in random direction. This is the same atmospheric effect what causes stars to appear to twinkle at night. The received signal level at the detector fluctuates due to thermally induced changes in the index of refraction of the air along the transmit path. The index changes cause the atmosphere to act like a collection of small prisms and lenses that deflect the light beam into and out of the transmit path. The time scale of these fluctuations is about the time it takes a volume of air the size of the beam to move across the path and therefore is related to wind speed. For the case of free-space optics, which implies horizontal path propagation and therefore stronger scintillation, the distribution tends to be more exponential. One parameter that is often used as a measure of the scintillation strength is the atmospheric structure parameter. This parameter, which is directly related to wind speed, roughly measures how turbulent the atmosphere is [8]. To overcome the scintillation effects automatic gain control mechanism is used and also clock recovery phase lock loop time constant eliminates the effects of scintillation and jitter transference.

CHALLENGES AND FUTURE OF FSO 

Free Space Optics (FSO) has become a viable, high-bandwidth wireless alternative to fiber optic cabling. The primary advantages of FSO over fiber are its rapid deployment time and significant cost savings. The disadvantage of FSO over fiber is that laser power attenuation through the atmosphere is variable and difficult to predict, since it is weather airports, the link availability as a function of distance can be predicted for any FSO system. These availability curves provide a good indication of the reasonable link distances for FSO systems in a particular geographical area. The carriers and ISPs are another potential large user of FSO systems, especially for last-mile metro access applications. If FSO systems are to be used in telecommunication applications, they will need to meet much higher availability requirements. Carrier-class availability is generally considered to be 99.999% . An analysis of link budgets and visibility-limiting weather conditions indicates that to meet carrier-class availability, FSO links should normally be less than 140m (there are cities like Phoenix and Las Vegas where this 99.999% distance limitation increases significantly). This calculation is based on a 53 dB link budget. This concept is extended to the best possible FSO system, which would have a 10 W transmitter and a photo counting detector with a sensitivity of 1 nW. This FSO system would have a 100 dB link margin, which would only increase the 99.999% link distance to 286 m. A more practical solution to extending the high availability range would be to back up the FSO link with a lower data rate radio frequency (RF) link. This hybrid FSO/RF system would extend the 99.999% link range to longer distances and open up a much larger metro/access market to the carriers. It is important to realize that as the link range increases, there will be a slight decrease in overall bandwidth. To show the geographical dependence of FSO performance, the first map of FSO availabilities contoured over area is need to be presented. This map is the first step to developing an attenuation map for predicting FSO performance, which could be used in similar fashion to the International Telecommunication Union (ITU)/Crane maps for predicting microwave performance.

CONCLUSIONS In our survey, we observed that most of the applications of FSO are for short range communication. The FSO communication system using NRZ line code with 1550 nm rather than that of 850 nm operating wavelength utilizing APD receiver in different weather conditions achieves a remarkable performance in order to keep an acceptable received signal power and BER levels. In the rain environment the normal FSO communication system distorts the signal. However with effective reduction in atmospheric turbulences using different modulation techniques the distance may be extended up to a larger extent.