PSDs with optical filters

Optical filters are often an important part of an optical measurement
system. SiTek offers the possibility to mount such filters directly onto
the package on all our PSDs.

For some applications background light, such as sun light might seriously affect the measurement results. This is often solved by using a combination of modulated light/electrical filters and optical filters. The modulated light/electrical filters are used to extract the light source signal while the optical filter reduces the level, and hence the noise contribution from the background light. In fact, by adding a relatively simple longpass filter with a cut off frequency of 830 nm, 75 % of the sun light in the PSD sensitive wavelength region will be removed. Using a narrow bandpass filter will of course give an even larger reduction.

PSD responsivity curves for a PSD, a) without any filter, b) with a longpass filter, c) with an interference bandpass filter

The best result is achieved if the optical filter replaces the standard window on the PSD, i.e. if the filter is mounted directly onto the package. This way both transmission loss and internal reflections can be minimized, something that enhances the over all performance of the measurement system. Often a smaller filter size can also be used than if the filter is mounted further away from the PSD.

SiTek can provide PSDs with filters of any kind. It might be interference band pass filters with a very narrow pass band for extreme applications or ND (Neutral density or gray) filters for an equal light reduction over the entire wavelength region. Each filter can be custom designed to provide the best PSD performance for any situation.

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