The self-heterodyne method is an interferometric technique which can be used to measure the linewidth of a laser, particularly a single-frequency laser. One portion of the Laser Beam is sent through a long optical Fiber which provides some time delay. Another portion is sent through an acousto-optic modulator, which shifts all the optical Frequency components by some tens of megahertz. Both beams are finally superimposed on a beamsplitter, and the resulting Beat Note is recorded with a photodetector. From this signal, the laser linewidth can then be calculated.