ASE stands for amplified spontaneous emission. When a gain medium is pumped by external energy and achieves population inversion, the spontaneous emission is amplified by the gain medium. If there is wavelength selective reflective elements (such as mirrors) to form an oscillation cavity, a laser is then produced. If no oscillation cavity is formed, the output is ASE light with low coherence and wide spectrum. The spectrum of the ASE source reflects the gain/loss spectra of the active medium and other components in the optical system.
ASE is not desirable in a laser system since the ASE power consumes the pump energy and hence reduces the efficiency of the laser. However, ASE light are useful as a broadband light source. It is widely used to characterize the spectrum of fiber optical components, devices or systems. ASE light are also used in optical signal processing applications for its low coherence feature.
Gain flattening filters, usually based on fiber Bragg gratings, are usually designed to flatten the output spectrum of the ASE source, to produce flat wide band light source.