Increased performance of optical coatings with ion assist during electron beam evaporator deposition (IAD) has
been well established. Over the years, the gridless Mark II end-Hall ion source has been widely used to produce
IAD optical coatings (antireflective coatings, dielectric filters and mirrors) on unheated substrates with low
losses, high precision and high environmental stability. We report on the performance of a next generation end-
Hall ion source (Mark II+) in an optical coating production environment in replacement of the earlier Mark II.
The Mark II+ was thoroughly re-designed with two major effects: (1) ion generation was enhanced resulting in a
significant increase in ion current density of about 20%, and (2) serviceability was greatly improved by
incorporating a removable and replaceable anode sub-assembly. The question arises as to how the enhanced
output of the Mark II+ might affect the properties of optical coatings when replacing a Mark II source in
established IAD coating processes. To answer this question, the spatial distribution of optical films parameters
of high and low index oxide materials were analyzed when using either ion source. The coating process and
results produced with the Mark II+ are basically compatible those of the Mark II, with slight changes in the
spatial distribution of refractive index and film thickness. Given the higher ion flux output of the improved
Mark II+, the ion source parameters for minimum absorption can be adjusted to lower, more energy saving
levels. Thus the Mark II+ is a tool to achieve comparable or marginally better results for optical coatings with
Optical thin films, especially reflection reducing coatings, are often applied to complex surfaces, e.g. small and strongly curved optical elements that are built into optical devices like object lenses for microscopes. The performance of the device depends very much on the optical quality throughout the entire surface of the optical element. In order to measure optical parameters in defined positions of a given optical surface, a fully automated microspectrophotometer for in line quality control was developed.
The microspectrophotometer consists of a Zeiss MCS 501 diode array spectrometer that is capable of fast and simultaneous data acquisition over the desired spectrum between NUV and NIR, connected via light waveguide to a Zeiss Axiostar microscope, which is operated in the reflected-light-brightfield mode, and a sample handling system that can be programmed to measure any spot on the individual surface of the optical element under examination. The size of the measured spot is in the order of about ten micrometers in diameter, allowing also characterization of defects and microscopic deviations from the desired thin film coating or optical surface. The principle and the assembly of the microspectrophotometer are presented as well as measurements of uniformity distributions of antireflection coatings on small semispheres achieved by different coating processes.
SiO2, Ta2O5 and MgF2 have been deposited by electron beam evaporation under bombardment of ions generated by three different ion or plasma sources. Multilayer systems containing 5 to 12 layers have been designed and realized. The maximum reflectance R of a glass/air interface can be reduced down to R < 0.5 percent in a spectral region of 400 nm to 700 nm with each of such AR coatings mostly exceeds that of all-oxide system in the shorter wavelength region. With scanning scratch test the scratch resistance of the coatings have been determined relative to each other.
A comparative study of different ion and plasma assisted physical vapor deposition processes at low temperature is reported. To work out a clear comparison of the different processes, the object of the study are single layers of different metal oxides like Ta2O5, TiO2, SiO2 and mixed oxides like H4 (Merck) deposited on glass and silicon substrates. Three different types of ion- (or plasma-respectively) sources are used: the cold cathode ion source from Denton (CC 104), the end hall ion source Mark II from CSC and the advanced plasma source from Leybold. Each of these processes is run under conditions concerning process parameters like bias, ion current, ion energy, beam characteristics and gas flow, which were understood to be optimized also to maintain long-term stability as realistic production conditions. The resulting metal oxide single layers are characterized by their optical properties, dispersion curves for NUV and VIS as well as absorption and scatter at discrete wavelengths. Also discussed are mechanical properties like hardness and adherence. A test method is presented which clearly shows the superior behavior of the IAD coatings.
With a modified standard coating plant simultaneous evaporation was applied to produce IR-coatings with improved performance for different applications. In one case, a single layer coating, quasi-homogeneously mixed from two different materials, achieves lower reflectance and higher laser damage thresholds at 2.78 micrometers. In another case, for a multilayer coating, 'soft' transitions between the individual layers yield a layer stack with a completely different structure with different properties, e.g. showing significantly reduced scatter.
The approved Draft International Standard 11551 on test methods for absorptance of optical laser components recently passed the international voting procedure. The utility of this standard practice document is the subject of the present round robin test. In order to cover a broad range of CO2-laser optical components, different types of metal mirrors and transmissive ZnSe-optics were included in the master sample set. After an initial inspection, this set passed through a series of optical laboratories in the United States, the United Kingdom, and Germany. The absorption of the samples was measured by calorimetric methods according to ISO DIS 11551, and the measurement results were compiled by the coordinating institute. The evaluation of the experiment was not started until all tests were completed. The results of the round robin test are discussed, compared and evaluated with respect to ageing mechanisms in optical coatings for CO2-laser systems. Although a great variety of different test facilities was employed by the round robin partners, a good agrement of the absorption values was observed for the wavelength of 10.6 micrometers . This demonstrates the versatility of the approved Draft International Standard 11551 for the calorimetric measurement of optical absorption in CO2-laser components.