Free-space optical communication

Free-space optical communication (Carruthers, 2002; Heatley et al., 1998; Kahn and Barry, 2001) is suitable for low to medium bit rates. The most common application of free-space optical

communication is the remote control of consumer appliances such as stereos and television sets. Other applications are the remote control of automobile door locks and the cordless interface between computers and peripheral devices such as a mouse, keyboard, and printer.

Free-space optical communication is limited to line-of-sight applications since obstacles such as walls and floors will block the path of light. Furniture may also block the path of light. However, a light beam may be reflected from the ceiling so that communication may still be possible even if there is no direct line of sight connection between the optical transmitter and the receiver.

The wavelength of choice for free-space optical communication is the near infrared. GaAs LEDs emitting with good efficiency are readily available. Infrared light is preferred over visible light sources because the former does not provide a distraction to anyone near the optical transmitter.

Eye safety considerations limit the maximum power of optical transmitters. At a wavelength of 870 nm, the optical power is limited to typically a few mW. Other wavelengths, such as 1500 nm, allow for higher optical powers. The 1 500 nm wavelength range is termed “eye safe”, since the cornea absorbs 1 500 nm light, thus preventing light from reaching the sensitive retina. The wavelength 1 500 nm thus allows for higher optical powers than 870 nm sources.

If we restrict our considerations to small distances, the transmission medium air can be considered to be totally lossless. However, the optical signal strength decreases for uncollimated light beams due to spatial divergence. For isotropic emitters, the intensity decreases with the square of the radius, i. e.

I _ P/ (4nr2) (22.19)

where P is the optical power emitted by the source and r is the distance from the source. The decrease in intensity thus has a very different dependence compared with the intensity in fiber communication.

The rapidly decreasing intensity limits the maximum range of optical communication. Collimated light beams can overcome this problem. Transmission distances of several km are possible without significant loss provided that atmospheric conditions are good, i. e. in the absence of fog or precipitation. Semiconductor lasers are used for such collimated transmission systems due to the ability to form collimated beams with very little spatial dispersion.

Multipath distortion or multipath time delay severely limits the data rate in free-space optical communication systems. A schematic illustration of multipath distortion is shown in

Fig. 22.9. A light beam emanating from the optical transmitter may take several different paths from the transmitter to the receiver. This is especially true for rooms with high-reflectivity surfaces such as white ceilings, walls, or mirrors. As an approximate rule, the longest path is assumed to be twice as long as the shortest path between the transmitter and the receiver. This approximate rule leads to a multipath distortion time delay of

At = L / c (22.20)

where L is the transmitter-receiver distance and c is the velocity of light. The maximum data rate is then limited to

/max - 1/At. (22.21)

For a room size of 5 m, the multipath delay is about At = 17 ns. Thus the data rate will be limited to about 60 MHz.

Another limitation of free-space optical communication is the detector noise. Sunlight and incandescent light sources have strong emission in the infrared. Thus a large DC photocurrent is generated in the detector, especially under direct sunlight conditions. The detector noise can be reduced by limiting the bandwidth of the receiver system. By reducing the bandwidth of the receiver system, and thereby also the system data rate, the detector noise is reduced, since the noise spectrum is much wider than the system bandwidth.

The detector noise due to ambient light sources can also be reduced by using optical band­pass filters, long-wavelength-pass filters, or a combination of both filters. Such filters prevent unwanted ambient light from reaching the detector.

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