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.
A Mark II gridless ion source and HCES5000 hollow cathode electron source are used for ion assisted deposition (IAD) of dense coatings. It is possible to check the ion beam profile with a beam probe translated at a right angle to the beam axis. By rotating the probe it is possible to eliminate the contribution form charge exchange ions and to estimate the mean free path of the energetic ions. The beam intensity is expressed as a polynomial in cosine to the angle between the ion track and the beam axis and we derive mathematical equations to describe the result in distribution of ion current density on a flat and on an umbrella shaped substrate holder. A grid of target points is introduced immediately in front of the holder an in turn we aim the ion gun towards each of these. In each case, we calculate the mean ion current density and the variance across the rotating substrate holder. Finally, we use the obtained maps to optimize the orientation of the ion gun.
The deposition of smart-filters like the V-lambda filter demands a precise knowledge of the refractive index at every position in the multilayered thin-film structure, and the ability to control the optical thickness of each layer precisely. The smart-filters produced by us today are soft coated of Zinc-Sulphide (ZnS) and Misch-Fluoride (MiF). The deposition of the complex multilayered structures is controlled by a computer based system collecting data from a transmission measuring system. Special test runs have revealed that the packing density of the ZnS depends on the physical thickness of the undercoat whereas the packing density of the MiF depends on the substrate temperature and the packing density of the undercoat. The substrate temperature that depends on the radiated heat from the crucibles, the speed of deposition and the radiation of heat to the surroundings is predictable when the thin film design is known. The speed of deposition is servo controlled and the time consumption for the coating process stay within +/- 5 percent of the predicted value.
Errors inherent to the approximateness and incompleteness of the inverse Fourier transformation techniques can be partly compensated by using successive approximations. Especially interesting is the situation where it is possible to optimize the primary approximate solution efficiently by means of repeated iterations on unchanged conditions (closed loop optimizations). It is shown that the degree of convergence of the closed loop optimization process and the obtainable result depend on the actual choice of Q-function. The results are similar when the closed loop optimizations are performed prior to and after conversion into a two-indexed quasi-inhomogeneous solution employing double layer equivalents. The subject of the optimization of the number of layers in the two-index solution is discussed and a couple of new Q-functions that are better suited for closed loop optimizations of both converted and unconverted coatings than some of the best known existing Q-functions are presented.
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