Optical Interference Filters:
The Old / New Technology

Optical Interference filters take advantage of the same principles that cause oil films on water or soap bubbles to change colors. By modifying the interface of a substance and its environment with a third material, reflectivity of the substance can be significantly altered.

Example

n1 = air
n2 = substrate
n3 = modifying material

if
n1<n3<n2 reflection at cwl will be reduced

if
n1<n3>n2 reflection at cwl will be increased

By depositing many layers of high and low index materials selective transmission and or reflection of wavelengths occurs.

The thin-film coatings produced at SpectroFilm are produced by evaporating dielectric and/or metal materials onto a substrate which is under vacuum. The thickness of the materials deposited is precisely controlled by monitoring the transmission of a monochromatic beam of light (the monitor wavelength) through the substrate. Other monitoring methods can be used but may vary in efficiency.

By altering the number and arrangement of evaporated layers, the spectral performance of the substrate can be significantly modified. Difference in the designs can yield long and short wavelength pass filters, single and multiple rejection band filters, anti-reflection coatings, and pass band filters in a variety of bandwidths and transmissions.

The fundamental element of a thin-film coating is the reflective quarter wave stack. By depositing alternating layers of high and low index materials, reflectivity at the monitor wavelength is increased. This basic element can be used to create high performance or long/short wavelength pass filters.

Figure 1. Quarter wave stack of alternating high and low index layers. Resultant spectral response shown.

When two reflective stacks are joined by a thin-film spacer layer, the reflective stacks create a FabryPerot interferometer that transmits at the monitor wavelength rather than reflects. These combined stacks are called cavities. By increasing the number of layers within a cavity and the number of cavities of a given design, performance of the coating will alter considerably.

Figure 2. Rejection created by a quarter wave reflective stack.

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