This paper describes an experimental arrangement to determine phase retardations with changing signs around zero
degree. In the experiment the phase retardation is caused by reflection from a non-periodic multilayer thin film reflector.
A prism retarder is introduced in a common polarimetric measurement to act as a compensator in order to enable the
measurement around zero degree phase retardation. Phase retardation within plus/minus a few degrees is measured in a
broad spectral range using a fiber coupled spectrometer.
During the interaction with a coating, a phase shift is introduced on the light. In general this phase shift is different for S and
P-polarized light. This means that the state of polarization may be changed during the interaction with the coating. Working with laser beams it is common to polarize the light parallel or orthogonal to the plane of incidence. In this case phase retardation does not occur. This explains why most people forget about the phase performance in their daily work. On the other hand, it is possible to utilize the phase retardation. For example it is possible to design a coating transforming a linearly polarized beam into a circularly polarized beam, and it is possible to make the phase retardation
independent of the wavelength in a certain wavelength range. Equations and a design technique are presented for first and second order optimization of the phase retardation on reflection or transmission of light from optical coatings. The optimization is performed by alternating optimization on the phase characteristics and the phase targets for S and P-polarized light. Examples of the design of laser mirrors with zero retardation and quarter wave retardation are presented.