Theory of quarter-wave-stack dielectric mirrors used in a thin fabry-perot filter

I present a new derivation of the analytic form for the phase shift near resonance and the optical penetration length upon reflection from a distributed dielectric mirror consisting of a quarter-wave stack. The requirement of proper termination to achieve high reflectivity is suspended to investigate large optical penetration depths. Separate equations, derived for N and N + 1/2 layer pairs, are convenient for the design of tunable Fabry-Perot filters with a specified tuning range. The analysis is also applicable to distributed Bragg reflectors, vertical-cavity surface-emitting lasers, and resonant photodiodes. I show that the penetration length can sharply reduce the overly broad free spectral range of an ultrathin Fabry-Perot filter that might be useful in applications such as tunable wavelength filters for wavelength division multiplexing applications. The results also demonstrate regimes of zero dispersion and of superluminal reflection in the dielectric mirrors, which are of particular interest in photonic bandgap structures.     

Multilayer Systems: Antireflecting Systems,High-Reflecting Stacks and Interference Filters

The properties of three multilayer systems which are of potential importance in scientific photography are examined. Antireflecting systems of one and two layers are considered and the conditions under which single layer blooming is likely to be inadequate are mentioned. The use of multilayer stacks of quarter-wave layers as an alternative to the semi-transparent metal filmis discussed. Finally the properties of interference filters are compared with those of dye filters. Both silver + dielectric and all-dielectric interference filters are considered.     

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.     

Tolerances For Layer Thicknesses in Dielectric Multilayer Coatings and Interference Filters

A theory is developed for dielectric multilayer coatings in which the layers depart from calculated thickness. The theory is applied to alternating systems of quarter wave layers of ZnS and MgF₂. The effects of thickness errors are: (1) A shift of the wavelength at which maximum reflectance occurs; and (2) a change in phase shift upon reflection. The magnitude of these effects, and also their dependence on various parameters, are determined. Statistical tolerances for layer thicknesses are computed for given tolerances on the multilayer performance. The accuracy required for producing dielectric interference filters is up to about 40 times higher than the accuracy sufficient for the production of dielectric mirrors and beam splitters. Various techniques of experimentally controlling film thicknesses, and their accuracies, are discussed. The production of mirrors and beam splitters deviating from theoretical maximum reflectance by only 1 percent seems to be possible with Dufour’s simple single photocell method of monitoring film thicknesses. With more precise methods, such as those developed by Giacomo and Jacquinot, or Traub, the production of interference filters appears to be possible to within plus or minus one half their half widths.     

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.     

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.     

Design and manufacture of metal/dielectric long-wavelength cutoff filters

Thin films of high reflecting metal, such as Ag, have a high reflectance in the long-wavelength region. When they are combined with dielectric layers, it is possible, through thin film interference effects, to induce transmission in certain shorter wavelength regions. Thus, they are useful components for the design of long-wavelength cutoff filters with a broad rejection region. In this paper, metal/dielectric multilayer designs based on this principle are numerically investigated. Three designs with different cutoff wavelengths and with very broad transmission regions in the visible or near-IR spectral ranges are presented. An excellent rejection on the long-wavelength side extends beyond 20 μm. Experimental results for one of the designs produced in our magnetron sputtering system are given.     

Inductive cross-shaped metal meshes and dielectrics

The Micro-Stripes program has been used to calculate resonance wavelengths and the bandwidth of inductive cross-shaped metal meshes in contact with dielectric layers. The shift of the resonance wavelength, depending on the thickness of the dielectric layers, has been studied for two refractive indices. The transmittance of two mesh filters with dielectric spacers or embedded in a dielectric has been calculated for specific alignment of the crosses of one mesh with respect to the other. Transmission line theory has been used to calculate the transmittance of two mesh filters with nonaligned crosses and dielectric layers. A coupled oscillator model has been used for interpretation of the interaction of resonance and Fabry-Perot modes.     

Laser-Induced Damage of a 1064-nm ZnS/MgF(2) Narrow-Band Interference Filter

The laser-induced damage thresholds, absorptances, and damage morphologies of ZnS/MgF(2) interference filters that were designed to allow radiation at wavelengths near 1064 nm to pass through them have been examined. The damage morphologies as well as their laser behaviors suggest that the initial damage is located not at the surface layers but near the interface of the spacer layer where ZnS is sublimed to form many little bubbles. The electric field distribution and the temperature rise in the multilayer was calculated to model this interesting phenomenon. Various explanations for the thermodynamic coupling are presented.     

Measurements of the phase shift on reflection for low-order infrared Fabry-Perot interferometer dielectric stack mirrors

A simple technique based on a Fizeau interferometer to measure the absolute phase shift on reflection for a Fabry-Perot interferometer dielectric stack mirror is described. Excellent agreement between the measured and predicted phase shift on reflection was found. Also described are the salient features of low-order Fabry-Perot interferometers and the demonstration of a near ideal low-order (1-10) Fabry-Perot interferometer through minimizing the phase dispersion on reflection of the dielectric stack. This near ideal performance of a low-order Fabry-Perot interferometer should enable several applications such as compact spectral imagers for solid and gas detection. The large free spectral range of such systems combined with an active control system will also allow simple interactive tuning of wavelength agile laser sources such as CO(2) lasers, external cavity diode lasers, and optical parametric oscillators.     

亚波长波导光栅导模共振研究

弱调制介质光栅可等效为平板波导,经其衍射的高级次子波与波导模式耦合时,形成导模 共振。由高级子波在介质光栅中的光程及菲涅耳相移,导出了垂直入射时弱调制介质光栅共振位置的解析表达式,其预测结果和严格耦合波理论所得值一致。导模共振对入射波参数和光栅参数极为敏感,具有窄带效应,可用来制作窄带滤波片。     

High-extinction-ratio resonant cavity polarizer for quantum-optics measurements

The use of a high-finesse Fabry-Perot ring cavity with an odd number of reflections as a high-extinction-ratio resonant polarizer is shown. Experimental results from quantum-noise measurements using resonant cavities as spatial and spectral filters and precision polarizers are presented.     

On the Limiting Bandwidth of Interference Filters : Influence of Temperature during Production

Computer simulation of thin-film multilayer production has shown that the effects of errors in the layers on the performance of complete coatings depends strongly on the particular monitoring process used. Deposition of dielectric quarter-wave multilayers is usually controlled by a method which involves the detection of the maxima and minima of transmission for the centre wavelength of the filter, the turning value method, and we know that this technique profits from an exceptionally efficient error compensation. The true tolerances in stack production are much larger than we would calculate neglecting this compensation. However, the analysis of experimental results shows that we must consider other effects which have so far been neglected in the computer simulation and which affect the monitoring process enough to prevent the fabrication of interference filters with very narrow transmission bands. Here we describe how the computer simulation is modified to take account of the influence of temperature changes during filter manufacture. First, it is necessary, for each material used, to determine the dilatation coefficients of both refractive index and thickness; they may be deduced from measurements of shifts with temperature of the centre wavelength of narrow-band filters of two different designs. Then, as a first approximation, we assume in the simulation a gradual temperature change which is always proportional to the deposited thickness and which reaches a total of 60°C for the complete filter. The calculations show that the effects of such a temperature change cannot be neglected even if the actual monitoring is perfect, that is if the deposition is always terminated exactly at a turning value of transmittance. The optical characteristics of very narrow-band filters, and especially of double half-wave filters, are seriously degraded. If, in addition, errors in determination of the turning values are included, then the effects are considerably worse. This can explain why it is so difficult to make filters with passband width less than 8 Å. The results of this study can be used to choose optimal conditions for the production of narrow-band filters.     

Strong coupling between nanoscale metamaterials and phonons

We use split ring resonators (SRRs) at optical frequencies to study strong coupling between planar metamaterials and phonon vibrations in nanometer-scale dielectric layers. A series of SRR metamaterials were fabricated on a semiconductor wafer with a thin intervening SiO(2) dielectric layer. The dimensions of the SRRs were varied to tune the fundamental metamaterial resonance across the infrared (IR) active phonon band of SiO(2) at 130 meV (31 THz). Strong anticrossing of these resonances was observed, indicative of strong coupling between metamaterial and phonon excitations. This coupling is very general and can occur with any electrically polarizable resonance including phonon vibrations in other thin film materials and semiconductor band-to-band transitions in the near to far IR. These effects may be exploited to reduce loss and to create unique spectral features that are not possible with metamaterials alone.     

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.     

Controlling the spectral response in guided-mode resonance filter design

Techniques for controlling spectral width are used in conjunction with thin-film techniques in the design of guided-mode resonance (GMR) filters to provide simultaneous control over line-shape symmetry, sideband levels, and spectral width. Several factors that could limit the minimum spectral width are discussed. We used interference effects for passband shaping by stacking multiple GMR filters on top of one another. A design is presented for a 200-GHz telecommunications filter along with a tolerance analysis. Compared with a conventional thin-film filter, the GMR filter has fewer layers and looser thickness tolerances. Grating fabrication tolerances are also 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.     

Microwave detection and depth determination of disbonds in low-permittivity and low-loss thick sandwich composites

Nondestructive evaluation (NDE) of disbonded low-permittivity and low-loss dielectric multilayered composite media is of considerable interest in many applications. The ability of microwaves to penetrate inside dielectric materials makes microwave NDE techniques very suitable for interrogating structures made of multilayered dielectric composites. Additionally, the sensitivity of microwaves to the presence of dissimilar layers in such materials allows for accurate detection of a disbonded layer. In a multilayered composite, a disbond may occur between any two (or more) layers. The potential of utilizing microwave NDE techniques for the detection and depth estimation of disbonds in a thick sandwich composite is investigated. This study utilizes a theoretical model developed for investigating the interaction of microwave radiation from an open-ended rectangular waveguide sensor with ann-layer dielectric composite medium. The influence of the standoff distance between the sensor and the medium and the operating frequency on the sensitivity of disbond detection and depth estimation is studied to obtain an optimum set of parameters for enhanced detection sensitivity. Results of the theoretical study are presented with a discussion on the optimization process for a thick sandwich composite composed of 13 dielectric layers.     

The impact of the structural dimensions and optical properties of bulk material on the optical performance of diffraction microstructures: crystalline silicon gratings and applications

This paper examines the impact of the structural dimensions and optical properties of the bulk material on the optical performance of gratings and microstructures. We present a brief summary of a rigorous electromagnetic diffraction analysis, and the S-matrix propagation algorithm we have developed. Infrared spectral transmission measurements in the range 2.5–25 μm, along with a rigorous full-vector simulation, are presented. Good agreement between theory and experiment is obtained. The optical performances of various optical devices incorporating grating structures are presented. Such devices include narrow-band filters and polarizers, and three-dimensional photonics band-gap structures.