Metal/dielectric transmission interference filters with low reflectance. 2. Experimental results

The successful fabrication of metal/dielectric multilayer filters requires not only accurate control of the individual layer thicknesses, but also a good knowledge of the optical constants of the materials used in the filters. In the case of metal films, it is also essential to know whether any transition layers are formed at the interfaces and, if so, how their thicknesses and optical constants depend on the deposition conditions. An automatic, real-time process control, magnetron sputtering deposition system was modified to permit the manufacture of metal/dielectric filters using optical monitoring techniques. To illustrate the performance of this system, two bandpass filters, a short-wavelength pass filter, and a neutral density filter were produced, all having a low reflectance for light incident on one side. The metal layers used in these filters consisted of either Ni or Ag. TheAg films could be protected from the O(2) plasma using thin Ni or Si films. Good agreement was obtained between the calculated and measured spectral transmittance and reflectance curves.     

Global sensitivity analysis of bandpass and antireflection coating manufacturing by numerical space filling designs

We present the effectiveness of global sensitivity analyses of optical coatings manufacturing to assess the robustness of filters by computer experiments. The most critical interactions of layers are determined for a 29 quarter-wave layer bandpass filter and for an antireflection coating with eight non-quarter-wave layers. Two monitoring techniques with the associated production performances are considered, and their influence on the interactions classification is discussed. Global sensitivity analyses by numerical space filling designs give clues to improve filter manufacturing against error effects and to assess the potential robustness of the coatings.     

Solid-spaced filters: an alternative for narrow-bandpass applications

Solid-spaced filters are composed of one or several thin wafers of excellent optical quality acting as Fabry-Perot spacer layers. We study the different steps of the design and the manufacture of filters following dense-wavelength-division-multiplexing specifications. The design method of such filters requires a tight synergy between numerical simulations and experimental characterizations to correct possible thickness errors. Experimental results of the manufacture and characterization of a three-cavity narrow-bandpass filter and of an interleaver filter are given.     

Application of in situ ellipsometry in the fabrication of thin-film optical coatings on semiconductors

Thin-film interference filters, suitable for use on GaAs- and InP-based lasers, have been fabricated by use of the electron-cyclotron resonance plasma-enhanced chemical vapor deposition technique. Multilayer film structures composed of silicon oxynitride material have been deposited at low temperatures with an in situ rotating compensator ellipsometer for monitoring the index of refraction and thickness of the deposited layers. Individual layers with an index of refraction from 3.3 to 1.46 at 633 nm have been produced with a run-to-run reproducibility of 0.005 and a thickness control of 10 A. Several filter designs have been implemented, including high-reflection filters, one- and two-layer anitreflection filters, and narrow-band high-reflection filters. It is shown that an accurate measurement of the filter optical properties during deposition is possible and that controlled reflectance spectra can be obtained.     

Application of optical broadband monitoring to quasi-rugate filters by ion-beam sputtering

Methods for the manufacture of rugate filters by the ion-beam-sputtering process are presented. The first approach gives an example of a digitized version of a continuous-layer notch filter. This method allows the comparison of the basic theory of interference coatings containing thin layers with practical results. For the other methods, a movable zone target is employed to fabricate graded and gradual rugate filters. The examples demonstrate the potential of broadband optical monitoring in conjunction with the ion-beam-sputtering process. First-characterization results indicate that these types of filter may exhibit higher laser-induced damage-threshold values than those of classical filters.     

Implementation of long-wavelength cut-off filters based on critical angle

Our first attempts at the fabrication of long-wavelength infrared cut-off filters with extended transmission and rejection regions that are based on the use of the critical angle, the dispersion of refractive indices, and on thin-film interference were not very successful. The design of the filter consisted of layers placed at the interface between two high-index prisms. Using the available deposition equipment, the layers produced were porous and very rough. The pores adsorbed water vapor, which resulted in absorption. The roughness made the process of optical contacting very difficult. In this paper we describe the adjustments in the design and deposition processes that allowed us to obtain filters with a better and more stable performance.     

Reliable deposition of induced transmission filters with a single metal layer

The sensitivity of an induced transmission filter (ITF) design to deposition errors is analyzed for the case of a single metal layer ITF. Theoretical knowledge of the least and most sensitive layers within the ITF design improves deposition reliability when using broadband optical monitoring of only the dielectric part of such metal-dielectric filters. Linearly variable ITFs have been successfully fabricated using this developed approach for error compensation.     

Robust optical coating design with evolutionary strategies

The optical performance of interference filters depends on systematic and statistical variations of the thicknesses and indices of refraction of the layers that occur during production and use. Assuming that their distributions are known, the expected performance can be optimized as a function of the nominal layer thicknesses with the help of strategies that mimic biological evolution. This results in filter designs that are easier to manufacture and more robust to use. The method is illustrated for color shifts that are rather sensitive to layer thickness variations. Its scope is entirely general, and it could be applied to other tolerancing problems that arise in optical design.     

Step method: a new synthesis method for the design of optical filters with intermediate refractive indices

We propose a new synthesis method for the design of multilayer optical filters with intermediate refractive indices, the step method. This method consists in adding infinitesimally small index steps in the index profile at optimal positions and then reoptimizing the thickness and the refractive index of the layers. Application of the method to the design of an antireflective coating, a low-pass edge filter, and an immersed polarizing beam splitter shows that it provides interesting solutions, even in the absence of a proper starting design. The formalism developed for the method also serves to demonstrate that the optimal filter consists of either homogeneous layers that maximize the effective refractive index contrast, or of graded-index layers.     

Stepwise graded refractive-index profiles for design of a narrow-bandpass filter

A combination of stepwise graded refractive-index profiles and a cavity structure is used for designing narrow-bandpass filters of TiO(2)/SiO(2) multilayer films upon BK7 glass substrates. Symmetrical profiles of stepwise graded refractive indices result in high transmittance of passbands for the designed filters. The bandwidth of the narrow-bandpass filter is controlled by adjustment of parameters such as the thickness and the number of layers in the multilayer stack. This design is proposed as a new and simple method for coating synthesis of optical filters.     

High-performance and highly stable 0.3-nm-full-width-at-half-maximum interference optical filters

Fabrication of very-narrow-bandpass optical tunable filters [(0.3 nm full width at half-maximum) (FWHM)] is reported. To improve the film densities, the O(2) ion-assisted deposition-method is used in the fabrication. In the succession of high- and low-refractive-index layers, the commonly used TiO(2) material is replaced by Ta(2)O(5), which suits the ion-assisted fabrication technique. The relative thicknesses of the filter multilayer structure of 1/2/1 are modified to 0.998/2.007/0.998, which reduces the shift difference in the central wavelengths with regard to the p and s polarizations when the filter is tilted. These improvements enabled fabrication of 0.3-nm-FWHM optical tunable filters with improved stability characteristics.     

Nonpolarizing and polarizing filter design

We describe a nonpolarizing filter design at oblique incidence and a polarizing filter design at normal incidence that use a uniaxially anisotropic layer. The phase thicknesses and the optical admittances of the layers are compensated for by the birefringent properties of a thin film at oblique incidence. This concept can be applied to the design of nonpolarizing bandpass and edge filters at oblique incidence and of polarizing beam splitters at normal incidence. Besides, the dependence of narrow-bandpass filters on normal incidence is discussed.     

R.F. sputtering system for the preparation of multilayer optical filters

Thin film multilayer optical filters are usually prepared by vacuum evaporation of successive layers. The deposition of thin films by sputtering has many advantages over the vacuum evaporation technique and hence it is employed for the fabrication of better quality optical filters. However, the filters may be more time consuming to prepare if a single-target sputtering system is used because two different materials are required for alternate layers. For the preparation of multilayer optical filters, a special r.f. sputtering jig arrangement with two target holders is fabricated. Using this the successive layers can be deposited in one pumpdown. A simple coupling arrangement used to match the r.f. generator output impedance to the load is explained. With all the sputtering parameters such as the pressure, the temperature and the power maintained constant except the time, control of the thickness of the multilayers is achieved. A calibration chart of sputtering time versus thickness of the materials to be deposited is prepared for TiO₂ and SiO₂ for the above purpose. Using this calibration chart, the deposition time of different layers of any optical filters based on these dielectric materials can be fixed to obtain optical filters of acceptable quality and reproducibility.     

On-line reoptimization of filter designs for multivariate optical elements

An automated method for producing multivariate optical element (MOE) interference filters that are robust to errors in the reactive magnetron sputtering process is described. Reactive magnetron sputtering produces films of excellent thickness and uniformity. However, small changes in the thickness of individual layers can have severe adverse effects on the predictive ability of the MOE. Adaptive reoptimization of the filter design during the deposition process can maintain the predictive ability of the final filter by changing the thickness of the undeposited layers to compensate for the errors in deposition. The merit function used, the standard error of calibration, is fundamentally different from the standard spectrum matching. This new merit function allows large changes in the transmission spectrum of the filter to maintain performance.     

Tunable phase-only optical filters with a uniaxial crystal

A novel method of fabricating phase-only optical filters that is based on the properties of a uniaxial crystal is proposed. With these optical filters, the phase differences are tunable among the different filter zones. Many focal patterns can be obtained if these optical filters are placed in front of a lens; furthermore, these optical filters can also be used to make up for the distortions in fabrications in which they were used only as untunable optical filters.     

Narrowband multichannel filters and integrated optical filter arrays

We propose and demonstrate three approaches to achieve narrowband multichannel filters. These are multiple heterostructures with defects, guided-mode resonance (GMR) Brewster filters with multiple channels, and integrated narrow bandpass filter arrays. Transmission studies for multiple heterostructures with defects are presented. We show that the enlargement of the forbidden band and multiple-channel filtering can be reached simultaneously with these configurations. GMR Brewster filters with multiple channels can be obtained with a single-layer grating. The same properties can be obtained by use of double-layer structures that consist of a homogeneous layer and a grating with equal refractive index. We developed a combinatorial etching technique that has 32 elements on a single substrate with which to fabricate integrated narrow bandpass filters. Single- and double-chamber integrated optical filter arrays were fabricated by use of this etching technique. These narrowband multichannel filters and narrow bandpass filter arrays show good filtering features and can be utilized in many optical applications.     

Design of adjustable superresolving filters based on birefringent crystals

A two-mode adjustable superresolving filter based on a birefringent filter is proposed. This kind of filter has superresolution in two modes of adjustment. One is rotation of the binary pupil filter on the optical axis of the system and the other is the tilt of the filter away from the pupil plane on axis parallel or perpendicular to the optical axis of the crystal. The filters act as complex amplitude filters in the former mode, and as pure phase filters in the latter. By analyzing two superresolving parameters, we obtain the optimal design parameters that ensure a large field of view, a large superresolving range, and a high setting accuracy. This kind of filter can provide more flexibility in practical applications.     

Influence of Temperature on the Optical Properties of Narrow-band Interference Filters

For narrow-band interference filters, we generally observe a small variation of peak wavelength with time; this variation of filter characteristics depends to a large degree on the materials of which these filters are made, and on the manufacturing process. Several explanations have been given to take account of this effect, which is particularly dramatic in the case of very narrow-band filters, so it is of the utmost importance to study in detail the influence of parameters such as the shift of temperature within the plant during deposition. The aim of this work is to know how to produce filters whose optical characteristics are steady with time. A significant advance in the study of this problem has been obtained by simulating the monitoring process on a computer. However, it is necessary, for each material used, to determine the dilatation coefficients of both refractive index and thickness. In this work we show how these coefficients may be deduced from measurements of shifts with temperature of the peak wavelength of complete narrow-band filters of two different designs. A theoretical study of the effect of temperature on the position of the passband of a dielectric narrow-band filter is performed, and from experimental measurements on complete filters we estimate the dilatation coefficients of the optical thicknesses of the component materials. The results obtained are slightly different from those given by other authors; this can be easily explained because the approximations made in their calculations of the dilatation coefficients are not accurate enough. Good agreement is obtained if an accurate check is performed from their experimental results.     

Active metameric security devices using an electrochromic material

In order to increase the anticounterfeiting performance of interference security image structures, we propose to implement an active component using an electrochromic material. This novel device, based on metamerism, offers the possibility of creating various surprising optical effects, it is more challenging to duplicate due to its complexity, and it adds a second level of authentication. By designing optical filters that match the bleached and colored states of the electrochromic device, one can obtain two hidden images-one appearing when the device is tilted, and the other one disappearing when the device is colored under an applied potential. Specifically, we present an example of a filter that is metameric with the colored state of the electrochromic device, demonstrate how the dynamic nature of the device offers more fabrication flexibility, and discuss its performance. We also describe a design methodology for metameric filters based on the luminous efficiency curve of the human eye: this approach results in filters with a lower number of layers and hence lower fabrication costs, and with a lower color difference sensitivity under various illuminants and for nonstandard observers.     

Optische Interferenzfilter auf Polymerfolien

Optical interference filter on polymer films - Spectral channel separation in 3-D projections by wavelength division multiplexing In this work the preparation of optical interference filters on polymer foils for spectral channel separation for 3D projections is presented. The suitability of various polymer substrates for this purpose was investigated by measurements of the layer adhesion and the optical haze. Optimization of the energy input during the coating process and of the optical layer design offered a possibility to coat optical interference layer systems for spectral channel separation on PET polymer foil. It was possible to build working 3D glasses. Furthermore the application of gradient filters for the compensation of the spectral shift due to different incident angles and the processing of the coated filter foils were discussed.