Bryan A. Tozer, Lasermet Ltd.
The rapid development and implementation of optical telecommunication technology have significantly aided the growth of Information Technology in the 1990s. Ready access to the Internet and the decreasing cost of telephone calls, both nationally and internationally are both the result of the low capital cost and high capacity of optical fiber telecommunication links.
Light-emitting diode and laser optical links that operate in "free space" are increasingly used for short-range data transmission between computers within the same office and between offices where a direct visual contact is available. Free-space ground-to-air and ground-to-satellite communication can also benefit from the use of lasers.
Whenever lasers are mentioned, questions of safety are never far away. The International Electrotechnical Commission is responsible for the most widely accepted laser safety standard, IEC 60825-1, which provides advice on maximum permissible exposures to laser light and classifies laser systems based on a hazard assessment. ("Hazard" is the potential to cause harm. It must be distinguished from "risk" which is the probability that harm will actually be caused.)
Except for wavelengths longer than 1400 nm, the possibility of injury from exposure to the radiation arises because the intense laser radiation can be focused to a very small point on the retina, causing local heating and damaging the photosensitive receptors. In most cases where excessive exposure occurs, the damage is permanent and, if the radiation is focused on the macula, there will be serious loss of central vision.
At 1550 nm the anterior portions of the eye absorb the radiation, causing excessive heating and hence corneal burning or cataract. Because the radiation is not focused in this case, the threshold for biological damage is greater than at the shorter wavelengths.