Mumbai University > Electronics and telecommunication > Sem 7 > optical communication and networks

Marks: 07

Years: MAY 2016

Optical Safety: The semiconductor lasers used in optical communication systems are relatively low-power devices; nevertheless, their emissions can cause serious damage to the human eye, including permanent blindness and burns. The closer the laser wavelength is to the visible range, the more damage it can do, since the cornea is more transparent to these wavelengths. For this reason, systems with lasers must obey certain safety standards. Systems with lasers are classified according to their emission levels, and the relevant classes for communication systems . In some cases, these safety issues can limit the allowable optical power used in the system.

• A Class I system cannot emit damaging radiation. The laser itself may be a high-power laser, but it is prevented from causing damage by enclosing it in a suitably interlocking enclosure. The maximum power limit in a fiber for a Class I system is about 10 mW (10 dBm) at 1.55 μ m and 1 mW(0 dBm) at 1.3 μ m. Moreover, the power must not exceed this level eve n under a single failure condition within the equipment. A typical home CD player, for example, is a Class I system.

• A Class IIIa system allows higher emission powers—up to 17 dBmin the 1.55 μ m wavelength range—but access must be restricted to trained service personnel. Class IIIa laser emissions are generally safe unless the laser beam is collected or focused onto the human eye.

• A Class IIIb system permits even higher emission powers, and the radiation can cause eye damage even if not focused or collected. Under normal operation, optical communication systems are completely “enclosed” systems—laser radiation is confined to within the system and is not seen outside. The problem arises during servicing or installation, or when there is a fiber cut, in which case the system is no longer completely enclosed and emission powers must be kept below the levels recommended for that particular system class.

• Communication systems deployed in the enterprise world must generally conform to Class I standards since untrained users are likely to be using them. Systems deployed within carrier networks, on the other hand, may likely be Class IIIa systems, since access to these systems is typically restricted to trained service personnel. The safety issue thus limits the maximum power that can be launched into a fiber.

For single-channel systems without optical power amplifiers using semiconductor lasers, the emission levels are small enough (− 3 to 0 dBm typically) that we do not have to worry much about laser safety. However, with WDM systems, or with systems using optical power amplifiers, we must be careful to regulate the total power into the fiber at all times.