The practical use of resonant optical cavities dates back to the Fabry-Perot Interferometer, devised by Charles Fabry and Alfred Perot, which is a fairly simple, but nevertheless ingenious, arrangement of two semi-transparent mirrors. See below for a great demonstration video from MIT, complete with a proper analog demo:
Today we have extremely large interferometers like LIGO, the gravitational wave detector, and microscale interferometers that can be fabricated in the thousands on a chip the size of the head of a pin.