Why is Erbium Doped Fiber Amplifier (EDFA) preferred over Erbium Doped Waveguide Amplifier (EDWA)? Why is Erbium Doped Fiber Amplifier (EDFA) preferred over Erbium Doped Waveguide Amplifier (EDWA)?
The question has been asked from an engineering point of view, but obviously I would also be interested in Physics of it. Is it just because of the difficulty of constructing a waveguide, or is there something more? I understand the individual benefits and limitations of a fiber amplifier and a waveguide but cannot understand the preference in this case.
http://en.wikipedia.org/wiki/Optical_amplifier#Doped_fiber_amplifiers
 A: Physics is the same, it's the question of economics. 
Erbium fiber is just that much easier to work with. It is manufactured in bulk, you cut the length you need for specific application. As a high-volume product it has much lower cost. Double clad fiber availability makes it so much cheaper to achieve high power. 
Waveguide amplifier is not something you can build that quick from off-the-shelf components. But as a finished unit it could be much more compact than EDFA.
A: The choice of either is much more than a waveguide fabrication complexity/cost.

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*Coupling: EDFA is mostly used in fiber optic communications.
Fiber-fiber coupling is easier and occurs lower connector/coupling
loss than waveguide-fiber coupling. Integrating an EDWA with network
requires precise alignment, is more complex, difficult and
time-consuming.


*Gain and noise figure: EDFAs have generally a higher gain because of long interaction length of a fiber amplifier enables a higher number of
erbium ions to be excited, resulting in a higher gain per unit
length. The same principle applies on spiral MZI over longitudnal MZI. In waveguide technology, the principal limitation arises from the compromise between the high doping levels needed to obtain gain in a short optical path and the gain and noise figure degradations these doping levels create. A good compromise is achieved for a gain of 2 to 3 dB/cm, which means that waveguides 5 to 10 cm long are needed to make an amplifier.


*Bandwidth: In EDWAs, the tightly confined and highly concentrated erbium ions doped into a solid-state waveguide, higher gain per unit length, stimulated emission based amplification, effective coupling between the ions and the optical field leads to a more narrow amplification band. In EDFAs, the cross-gain modulation based amplification where the pump photons excite the erbium ions and then the excited ions amplify the signal photons leads to a larger band width in EDFAs than EDWAs. The higher bandwidth of an EDFA enables it to amplify across various C-band wavelengths of WDM networks around 1550 nm.


*Isolator: The EDWA waveguide lack the integrated isolators to ensure against backreflections and lasing effects. Bulk isolators have to be coupled to the waveguide, makes the final product slightly bigger than it should be.
However, EDWA provides better performance than EDFA in terms of gain and noise figure for free space optical (FSO) transmission systems
Interesting Reads

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*https://www.intechopen.com/chapters/81932

*https://www.lightwaveonline.com/optical-tech/transport/article/16648816/erbiumdoped-waveguide-amplifiers-promote-opticalnetworking-evolution
