Other Factors Affecting FSO
When planning an FSO deployment, you must consider the application intended. Is the data traffic low-speed overnight downloads or high-speed uninterruptible video data? Is the distance between sites long? Is the location notoriously foggy? These factors all influence the selection of the most appropriate FSO system.
Low visibilities will decrease the effectiveness and availability of FSO systems. Long-term weather observations show that some cities, such as Seattle, WA, have lower average visibilities than cities such as Denver, CO. This means that for the same distance, the same FSO system in Denver will experience a higher availability than a system installed in Seattle. Low visibility can occur during a specific time period within a year or at specific times of the day (such as in the early morning hours). Especially in coastal areas, low visibility can be a localized phenomena (coastal fog). This means that for the same distance, the same FSO system in Denver will experience less downtime than in Seattle.
One solution to the negative impact of low visibility is to shorten the distance between FSO terminals to maintain a specific statistical availability figure. This provides a greater link margin to handle bad weather conditions such as dense fog. Redundant path operation can improve the availability if the visibility is limited on a local scale. Examples are fog across a river or pond or an air conditioner's exhaust stream on top of a roof. Another solution is to use a multiple beam system to maintain a higher link availability.
Low visibility and the associated high scattering coefficients are the most limiting factors for deploying FSO systems over longer distances.
Distance impacts the performance of FSO systems in three ways. First, even in clear weather conditions, the beam diverges and the detector element receives less power. For a circular beam, the geometrical path loss increases by 6 dB when the distance is increased by a factor of two. Second, the total transmission loss of the beam increases with increasing distance. Third, scintillation effects accumulate with longer distances. Therefore, the value for the scintillation fade margin in the overall power budget will increase to maintain a predefined value for the BER.
Most commercially available FSO systems are rated for operation between 255,000 m, with high-powered military and satellite systems capable of up to 2,000 km. Most systems rated for greater than 1 km incorporate three or more lasers operating in parallel to mitigate distance-related issues. It is interesting to note that in the vacuum of space, FSO can achieve distances of thousands of kilometers.
In standard O-E-O FSO systems, two elements limit the bandwidth of the overall system. These elements are the transmission source and the photo detector. When LEDs are incorporated into FSO systems, the bandwidth is typically limited to 155 Mbps. When laser sources are used, the speed can be much higher. Directly modulated lasers operating up to 2.5 Gbps are commercially available for use in FSO systems. At higher speed such as 10 Gbps or above, external modulators can be used to modulate the cw output of a laser source.
With respect to the photo detector, inexpensive Si-Pin diodes and Si-APDs supporting data rates up to 1,250 Gbps are commercially available. For operation in the 1.5 micrometer wavelength band, InGaAs detectors are used. Commercially available and off-the-shelf detectors that support a bandwidth of 10 Gbps and beyond can be used in FSO systems. However, at higher bit rates (shorter bit durations), the amount of light that can be collected by the receiver and converted into electrons is extremely low and the sensitivity of a receiver becomes a function of the bit rate. In general terms, this means the higher the bit rate, the less sensitivity. Typical sensitivity ratings are 43dBm@155Mbps and 34dBm@622Mbps. When the system reaches its sensitivity limit, the thermal (Johnson) noise impacts the bit error rate (BER) of the system.