 Fabry @-@ Pérot etalons are widely used in telecommunications , lasers and spectroscopy to control and measure the wavelengths of light . Dichroic filters are multiple layer thin @-@ film etalons . In telecommunications , wavelength @-@ division multiplexing , the technology that enables the use of multiple wavelengths of light through a single optical fiber , depends on filtering devices that are thin @-@ film etalons . Single @-@ mode lasers employ etalons to suppress all optical cavity modes except the single one of interest . 
