A plastic optical port of an optical communications module is equipped with an adapter having an inner diameter that adjusts to the outer diameter of a ferrule when the ferrule is mated with the optical port. In the mated configuration, the adapter creates an interference fit between the optical port and the ferrule that prevents or at least reduces relative movement between the optical port and the ferrule. Preventing or at least reducing relative movement between the ferrule and the optical port ensures that the distal end of the ferrule and the distal end of the fiber stub held within the port are maintained in precise optical alignment with one another. This, in turn, ensures high optical coupling efficiency for light being coupled between the distal end of the fiber stub and the distal end of the ferrule.