Optical Amplifiers……….The Payback!!
Optical amplifiers alleviate that problem by amplifying all the channels together completely in the optical domain; therefore, optical amplifiers can enhance the transmission distance. So, does that mean that optical amplifiers can increase the amplifying distance as much as they wants? Not really! Amplifiers come at a price and induct a trade off; they enhance the signal power level, but at the same time, they add their own complement of noise. This noise is amplified spontaneous emission (ASE).
The noise is random in nature, and it is accumulated at each amplification stage.
Amplifier noise is a severe problem in system design. A figure of merit here is the optical signal-to-noise ratio (OSNR) requirement of the system. The OSNR specifies the ratio of the net signal power to the net noise power. It is a ratio of two powers; therefore, if a signal and noise are both amplified, system OSNR still tells the quality of the signal by calculating this ratio. System design based on OSNR is an important fundamental design tool.
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OSNR is not just limited to optical amplifier-based networks. Other active and passive devices can also add noise and create an OSNR-limited system design problem. Active devices such as lasers and amplifiers add noise. Passive devices such as taps and the fiber can add components of noise. In the calculation of system design, optical amplifier noise is considered the predominant source for OSNR penalty and degradation. That does not imply unimportance to other sources of OSNR penalty.
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