2. Semiconductor optical amplifiers and Fabry-Perot lasers

Semiconductor Optical Amplifiers, or SOAs, consist of an active guide delimited by two anti-reflective facets. Their role is to amplify the photons injected by one facet by stimulated emission, then to collect them by the other facet.

The main challenge is to reduce reflections as much as possible, so that amplified photons make only one pass, at least for the vast majority of them.

To achieve this, thin films are deposited on the facets to obtain power reflection coefficients of less than 0.1%. The guide is generally non-perpendicular to the output facet to further reduce reflections. Finally, mode adapters called tapers are introduced at the ends of the SOA to improve the coupling of the optical mode to a standard fiber. These tapers also reduce reflections at the facets by increasing the mode size, thereby reducing the probability of...