Waveguide imperfections

It was already pointed out that single or multilayer dielectric coatings can reduce the loss of a leaky capillary waveguide substantially, but that it was difficult to see this improvement in practice due to fabrication imperfections [11]. Advances in fabrication technology have helped improve these imperfections. In addition, a strong need for liquid-core waveguides with small cross sections of a few microns... 

Imperfections in the surface of the waveguide can also cause losses. Imperfections with dimensions of several wavelengths cause perturbation in the field, leading to coupling to higher order modes which do not have the desired transmission characteristics and also causes power loss. Losses in modem commercial optic fibers can be kept down to as little as 0.01 dB/km, but for microfabricated waveguides, losses in the order of 1-10 dB/cm for polymer and 0.1-1 dB/cm for glass cores are more common. 

The second term in Eq. (2), Ysc, is an empirical term describing other sources of optical loss within the SOA. These losses are mainly due to scattering of photons out of the guided mode due to imperfections in the semiconductor lattice, and roughness in the edges of the active-region waveguide. 

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