A molecule absorbs the laser light at a certain wavelength and the resulting fluorescence emission shifts to a longer wavelength. Therefore a dichroïc filter is able to separate the fluorescence wavelength from the laser emission wavelength.
Combining this principle with fiber optics is particularly interesting for remote sensing, high sensitivity and compactness. The proprietary technology of SEDI-ATI multimode Wavelength Division Multiplexer (WDM) is based on a direct deposition of a dichroïc filter on optical fiber end surface by electron beam evaporation. This simple and robust construction makes an ideal component for a two channels bidirectional transmission with a high isolation into a single optical fiber. The dichroïc filter can be calculated to fit a specific application for instance injection of UV laser light into one port for absorption by the targeted material and detection of the fluorescence from the material in the second port of the WDM.