[IMAGE]
 
[IMAGE]

[IMAGE]
 

HBBCL Free-Space Optical Communications 

What is Free Space Optical Communications?

Free space optical communications is a line-of-sight (LOS) technology that transmits a modulated beam of visible or infrared light through the atmosphere for broadband communications. In a manner similar to fiber optical communications, free space optics uses a light emitting diode (LED) or laser (light amplification by stimulated emission of radiation) point source for data transmission. However, in free space optics, an energy beam is collimated and transmitted through space rather than being guided through an optical cable. These beams of light, operating in the TeraHertz portion of the spectrum, are focused on a receiving lens connected to a high sensitivity receiver through an optical fiber.

Unlike radio and microwave systems, free space optical communications requires no spectrum licensing and interference to and from other systems is not a concern. In addition, the point-to-point laser signal is extremely difficult to intercept, making it ideal for covert communications. Free space optical communications offer data rates comparable to fiber optical communications at a fraction of the deployment cost while extremely narrow laser beam widths provide no limit to the number of free space optical links that may be installed in a given location.

The fundamental limitation of free space optical communications arises from the environment through which it propagates. Although relatively unaffected by rain and snow, free space optical communication systems can be severely affected by fog and atmospheric turbulence. The main design challenges in free space optical communications are as follows:

· Fog: Fog is vapor composed of water droplets, which are only a few hundred microns in diameter but can modify light characteristics or completely hinder the passage of light through a combination of absorption, scattering, and reflection. This can lead to a decrease in the power density of the transmitted beam, decreasing the effective distance of a free space optical link.

· Scintillation: Scintillation is the temporaland spatial variation in light intensity caused by atmospheric turbulence. Such turbulence is caused by wind and temperature gradients that create pockets of air with rapidly varying densities and, therefore, fast-changing indices of optical reflection. These air pockets act like lenses with time-varying properties and can lead to sharp increases in the bit-error-rates of free space optical communication systems, particularly in the presence of direct sunlight.

· Beam Wander: Beam wander arises when turbulent wind current (eddies) larger than the diameter of the transmitted optical beam cause a slow, but significant, displacement of the transmitted beam. Beam wander may also be the result of seismic activity that causes a relative displacement between the position of the transmitting laser and the receiving photodetector.

Free Space Optical Communications at Harvard University:

Present free space laser communication systems based on a single laser source and single detector suffer from the aforementioned problems, thus limiting their utility and reliability for many high speed data transmission applications. Although an idea originally conceived to combat the effects of small-scale fading for radio frequency communications by Alamouti and Tarokh, et. al, Space-Time Coding also provides a method for assuaging the difficulties of free space optical communications. The proposed communication system is based on the use of a Multiple Laser Multiple Detector (MLMD) architecture, which has the potential of resolving or significantly reducing the effect of these problems. Most notably, the power requirements for an MLMD system are significantly lower than those of commercial Single Laser Single Detector (SLSD) system and, thus, significantly higher data rates can be supported using equivalent transmit powers. In addition, the use of Space-Time Codes generated from orthogonal designs has been shown to effectively achieve high-rate optical communication while only requiring linear processing at the receiver.