Couplers are made by fusing and tapering two fibers together so that the cores of the two fibers are close enough so that the evanescent wave from one fiber can be picked by the providing necessary coupling. The property of coupling depends on whether the fibers are multi-mode fibers or single mode fibers.
Optical Amplifiers:
Inline signal amplification is done by placing optical amplifiers along the fiber span. Erbium doped fiber amplifiers (EDFA) are generally used in WDM applications. Key performance parameters of amplifiers are gain, gain-flatness, noise level and power output.
Gains greater than 30 dB over a wide spectral width ( nm) with low noise are characteristics of EDFAs which are available in both L-band and C-band. Signal gain provided by EDFAs has reasonably flat wavelength response. However, the flatness can be improved by gain flattening optical filters. Signal can travel over 100 km between amplifiers. If longer haul is required, it will be necessary to regenerate signal. Regenerators , in addition to amplifying signals, perform what is known as 3R- operations , viz., reshaping, retiming and retransmitting.
As the total gain in EDFA is shared between different wavelength channels, the gain per channel decreases. As there are OADMs in the network, this would result in different channels being received with different power at the receiver end. The problem is addressed by equalizing filters which attenuate wavelengths that are strongly amplified.
Multiplexers (MUX)
At first sight it would seem that multiplexing different wavelengths would be a relatively simple job of simply allowing different wavelength signals to fall on an opical fiber within th latter's angle of acceptance. However, one has to take care to see that the noise associated with each channel is kept to a minimum. Channels must be isolated to ensure that noise at a different wavelength does not interfere with the signal that is being carried.
![\includegraphics[width= \textwidth]{wdm13.eps}](../Fig/math-images/math_clip_image006.gif)
nm) with low noise are characteristics of EDFAs which are available in both L-band and C-band. Signal gain provided by EDFAs has reasonably flat wavelength response. However, the flatness can be improved by gain flattening optical filters. Signal can travel over 100 km between amplifiers. If longer haul is required, it will be necessary to regenerate signal. Regenerators , in addition to amplifying signals, perform what is known as 3R- operations , viz., reshaping, retiming and retransmitting. 
