Full-fiber ellipsometry: the (im)possibility of eliminating line-of-sight

Abstract

Ellipsometry is a non-contact, extremely accurate technique for characterising thin films, which over the past decades have become vital in a host of applications. However, ellipsometry suffers from a number of disadvantages, the most inconvenient of which are that it requires line-of-sight access to the sample, and demands careful calibra- tion of any optical components. Succeeding in building an ellipsometer that does not require line-of-sight could severely reduce especially in situ measurement complexity, as well as allowing a single ellipsometer to be timeshared over multiple setups, forming a significant cost re- duction. In this work, an ellipsometry setup that uses optical fibers in lieu of direct line-of-sight is investigated. We demonstrate that the output polarisation states from such a full-fiber ellipsometer form a well-defined band on the Poincaré sphere, which is preserved and can be seen as a generalised polarisation state. Changes to this generalised polarisation state can then be used to obtain the ellipsometric angles Ψ and ∆ from a given sample. Provided the setup is well-characterised, the accuracy is comparable to that of conventional ellipsometry.