Fibre over the air

It uses laser operating in the infrared optical band to communicate digital data through free space. Laser is the acronym for light amplification by stimulated emission of radiation.

Compared to light emitting-diode (LED) technology, laser communication is more robust and can travel longer distances. This is because laser has high coherence and collimating characteristics, which mean that the signal is more focussed on its target and less subject to signal spreading.

FSO systems support all types of data transmission including voice and video. It is typically used for short-haul bandwidth-intensive applications where cable systems are not available or too costly to implement.

While the technology has been around for some time, its application in South Africa was limited to connecting buildings on the same site because of South Africa’s “closed sky” policy which only allowed Telkom to provide communication crossing public spaces. This changed when the then Minister of Communication, the late Dr. Ivy Matsepe-Casaburri relaxed the telecommunications policy in the late 1990s.
FSO systems are increasingly deployed in local loop applications as alternatives to radio frequency (RF)-based systems. Because SFO technology supports much higher data rate than RF-based systems, it is often used in the extension of optic fibre circuits. Systems are available to cover up to 4,4 km, but are typically deployed at distances of 100 m to 1km to provide robust high availability connectivity.

A FSO transceiver can consist of one or multiple laser transmitters and optical receivers. Once the transmitter and receiver pair is aligned, a full duplex data communication link is established. A laser beam has a Gaussian profile with most of the optical energy concentrated in the centre of the beam. With all electromagnetic signals, the beam will diverge. The beam divergence for an electromagnetic wave is defined as the increase in diameter with distance from the optical aperture from where the electromagnetic signal merges.
The laser component in the transmitter is modulated with digital data by on/off keying. For carrier class FSO, an avalanche photo diode (APD) receiver is used as the detector for the optical signals. According to Wietz Joubert, Redline’s engineering manager, using an APD receiver instead of a p-type-intrinsic-n-type (p-i-n) receiver is advantageous since the receiver sensitivity can be increased by up 1o 10dB compared with p-i-n receivers. Using this technology reduces many of the atmospheric challenges experienced with p-i-n receivers.

Atmospheric influences such as fog, snow, rain, smog, sandstorms cause infrared signal attenuation due to absorption.
Redline products operate at 808 nm and actively cooled lasers operate at 1550 nm.

As for the safety of the laser-based FSO systems, compliance with IEC standards ensures that safety and specifically eye safety standards are adhered to by FSO equipment vendors. The 1550 nm laser technology used in FSO equipment enables longer distances to be achieved as it has been found that the human eye can withstand up to 50 times more optical energy from a 1550 nm optical source compared to a laser beam operating in the 800 nm band. The reason for this is that the 1550 nm optical energy is absorbed in the cornea and the lens of the eye. The optical beam is not focused on the retina as is the case with the 800 nm optical signals.

“There are many advances using FSO communications,” said Rynier van der Watt, Redline’s MD. It requires no licensing, it is radio frequency interference-immune, quick to install and redeployed if used on a temporary basis. FSO has many applications such as enterprise connectivity, disaster recovery sites; temporary connections, campus networks, last mile connectivity to service provider and security camera connectivity.
Redline is part of the Parsec group and one of the five top international providers in the world of FSO communication equipment. The company has over 600 systems in the field of which 500 are in Africa and the Middle East. The company operates a state-of-the-art manufacturing plant in Centurion, Gauteng, where most of the FSO units are manufactured.

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First published in EngineerIT


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Fibre over the air