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Optical Networks - _8_7 Optical Safety_100

Optical Networks - _8_7 Optical Safety_100 - 8.7 Optical...

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8.7 Optical Safety 501 Fiber Wavelength interfaces Compliant O/E/O 1541 nm Noncompliant 1310 nm O/E/O Noncompliant 1310 nm Noncompliant WDM mux/demux O/E/O TDM 10 Gb/s 1552.524 nm 1553.329 nm 1554.134 nm 1551.721 nm ITU l 4 2.5 Gb/s + Figure 8.10 Different types of interfaces between a WDM optical network and its clients. equipment prior to transmission. For example, the WDM equipment could in- clude multiplexing of SONET OC-48 streams into OC-192 streams. This could reduce costs by eliminating the separate equipment that would normally be needed to perform this function. The level of transparency offered by the network also affects the type of adap- tation performed at the edges of the network. The network needs to be capable of transporting multiple bit rates. In general, the optical path can be engineered to support signals up to a specified maximum bit rate. The adaptation devices and re- generators used within the network need to be capable of supporting a variety of bit rates as well. An important enabler for this purpose is a programmable clock data recovery chip that can be set to work at a variety of bit rates. The chips available today are capable of handling integral multiples of bit rates. They are also capable of handling a narrow range of bit rates around a mean value. For example, a single chip could deal with SONET OC-24 signals or with Gigabit Ethernet signals, which are both around 1.25 Gb/s but not exactly at the same rate. 8.7 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 wave- length 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 emis- sion levels, and the relevant classes for communication systems are described next.
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502 Control and Management In some cases, these safety issues can limit the allowable optical power used in the system. A
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