A group of researchers demonstrated that accurate polarization-based measurements can be achieved when the probe beam passes through an optical fiber of varying birefringence
3-D projection of the polarization state of the light exiting the fiber on the Poincaré sphere.
Dutch Institute for Fundamental Energy Research
Traditional ellipsometry, a method for measuring the dielectric properties of thin films, requires precise knowledge of the light beam polarization that probes the sample. A group led by Feng Liu and Chris J. Lee of the Dutch Institute for Fundamental Energy Research (formerly FOM Institute for Plasma Physics Rijnhuizen in the Netherlands) demonstrated that accurate polarization-based measurements can be achieved even when the probe beam passes through an optical fiber of varying birefringence (Opt. Express 20, 870).
The researchers directed a linearly polarized light beam from a He-Ne laser through a polarization-maintaining fiber, part of which was immersed in a heated water bath to simulate environmental changes that would stress the fiber material. The team needed to change the fiber’s temperature by only three degrees Celsius to explore the complete range of polarization states. Except for the presence of the fiber and the water bath, this is the same setup used in standard ellipsometry experiments.
The scientists plotted the output from the fiber on a Poincaré sphere. Despite the stresses on the fiber, the polarization states of the emerging light traced out a 2-D orbit on the sphere. Small deviations from this orbit yield details about the thin film being investigated.
By studying the orientation of the orbit, researchers can make polarization-based measurements without knowing the precise details of the polarization of the probe beam. The important detail is the change of the polarization during the measurement.
To demonstrate that this technique works, the team performed fiber-based ellipsometry on three multilayer Bragg mirrors, two of which were coated with carbon slightly less than 1-nm thick. The fiber-based experiment detected the carbon coatings via measurements that compared well with those from a conventional ellipsometry setup.
Ellipsometry is useful in the monitoring of epitaxial fabrication, contamination monitoring of optical coatings and other areas of surface science and engineering.
Patricia Daukantas is a freelance science writers who specialize in optics and photonics.