 The Laser Interferometer Gravitational @-@ Wave Observatory ( LIGO ) uses two 4 @-@ km Michelson @-@ Fabry @-@ Pérot interferometers for the detection of gravitational waves . In this application , the Fabry – Pérot cavity is used to store photons for almost a millisecond while they bounce up and down between the mirrors . This increases the time a gravitational wave can interact with the light , which results in a better sensitivity at low frequencies . Smaller cavities , usually called mode cleaners , are used for spatial filtering and frequency stabilization of the main laser . The first observation of gravitational waves occurred on September 14 , 2015 . 
