Low Angle Shift Coatings
New Technology Delivers High Performance Filters
Optical systems frequently use filters to increase signal-to-noise by isolating wavelength bands containing information from a broad background of other signals (the noise). Optical filters which limit the wavelength response of a system are frequently called Band Pass filters. These filters can have a nearly infinite variety of attributes in terms of the wavelengths of light they transmit and suppress depending on the optical system in which they are deployed.
Depending on the level of performance and wavelengths of interest, various physical mechanisms can be used to produce a band pass filter such as: materials dependent absorption, scatter, dispersion and mechanical selection, optical interference, and induced transmission. Each of these filtering mechanisms has its place in optical instrumentation. As a general rule though, the all dielectric optical interference filter is the most cost effective solution for fixed response, high throughput, robust, optical systems suitable for commercial electronic applications.
Optical interference coatings use the principle of interference to produce a variety of spectral profiles. A fundamental aspect of interference is the phase change on traversal of the film. This phase change can be thought of as the optical path difference between light waves reflected off the two interfaces that makeup an idealized optical thin film. The path through the material will change when the angle of incidence changes. The propagation angle at any interface between two materials is governed by Snell’s Law
n1 Sin(θ1) = n2 Sin(θ2)
where n is the refractive index and θ is the angle of propagation and the subscripts refer to two arbitrary materials.
This leads to many interesting optical effects. One of particular interest is that the angle of propagation for a material is inversely proportional to the real part of its refractive index. The angle of propagation determines the phase change on traversal of the layer. The shift in spectral performance is determined by the phase change. As a result, a high effective index of refraction results in lower angle sensitivity of a filter. The effective index of a film is a result of the materials used in its construction and the distribution of those materials.
Effect of Effective Refractive Index on Angle Shift
For example, the shift in angle for a Fabry-Perot Narrow Band Pass filter as a function of index of refraction of the space layer is shown in Figure 1.
Figure 1 - Change in Center wavelength of single cavity Narrow Band Pass filter (Fabry-Perot) as a function of index of refraction of the spacer layer for a 30° change in angle of incidence.
Clearly, the higher the refractive index of the spacer layer, the lower the center wavelength (CWL) shift experienced by the filter for a given angle change. Thirty degrees is a large angle but serves to illustrate the point that as required bandwidth of a filter decreases, the angular field over which it is useful drops off quickly. A filter with a 1% Bandwidth (e.g. 10 nm FWHM @ 1µm CWL) would shift by its entire width, even with refractive index of 4.5. Narrow filter width and large angular fields are inherently conflicting requirements.
The materials available for a given band pass design are determined by their region of transparency, their stability and their durability. Depending on the attributes of the optical system in question different materials can be utilized. Typical coating materials include metals, metal oxides, fluorides, nitrides and semiconductors.
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