Optical filters on lenses for compact optical systems

Plano-convex lense coated with a bandpass filter (at 670 nm). The coating has a special gradient, which ensures a constant spectral position of the central wavelength on the lense regardless of the position. The light in the remaining spectral range (340–1100 nm) is heavily blocked. Total coating thickness 22 micrometer, number of coatings.
© Fraunhofer IST, Falko Oldenburg
Plano-convex lense coated with a bandpass filter (at 670 nm). The coating has a special gradient, which ensures a constant spectral position of the central wavelength on the lense regardless of the position. The light in the remaining spectral range (340–1100 nm) is heavily blocked. Total coating thickness 22 micrometer, number of coatings.

Challenge 

Optical filters are usually applied to planar surfaces. In optical systems, intermediate imaging is then necessary, so the light may have to be expanded and planarized in order to be focused again thereafter. This leads to a larger and heavier optical system, resulting in a poorer payload. 

Solution 

When filters are applied to 3D surfaces, this usually results in an inhomogeneous coating with a decrease in coating thickness towards the edge. This causes a spectral shift of the spectrum. In addition, the oblique angle of incidence of the light on the lens causes a further shift. This was compensated for by an appropriate gradient coating on the lens. A mask was used during the coating process, the shape of which was automatically determined by simulation with the aid of a digital twin of the EOSS® coating system.

Added value 

As a result, a lens was successfully coated with a bandpass filter. The central wavelength (constant from the position on the lens) is 670 nm with a half-width of 10 nm. The filter also has broadband blocking (200 - 1100 nm) of OD2. The solution can be easily implemented for sputtering systems, especially of the EOSS® type, and transferred for other types of coatings. 

Further information

Schematic view of the bandpass filter.
© Fraunhofer IST, Falko Oldenburg
Schematic view of the bandpass filter.
Diagram of the lens to be coated and the expected spectra.
© Fraunhofer IST
Diagram of the lens to be coated and the expected spectra.
Spectra of the bandpass filter.
© Fraunhofer IST
Spectra of the bandpass filter.

Funding reference

The work was supported by the ESA Contract No. 4000117857/16/NL/PS. 

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