Amplified Spontaneous Emission (ASE).

Even when the pump power is below the lasing threshold, a laser usually emits a low level of light. This emission originates from spontaneous emission in the gain medium and is subsequently amplified. Although amplified spontaneous emission (ASE) can be extremely weak in lasers with low cavity losses, it often limits the maximum achievable gain in laser amplifiers. Once the gain reaches several tens of decibels, ASE begins to extract a significant amount of power. As illustrated in the figure below, ASE affects fiber amplifiers by causing gain saturation when the pump power exceeds approximately 0.6 W.

Although fluorescence is emitted in all directions, ASE can exhibit strong directionality in gain media with a high aspect ratio—such as optical fibers or long laser rods—because these structures provide a much longer amplification path along a specific direction.

ASE is not always detrimental. It can be utilized to generate light sources with a smooth broadband spectrum, high output power, and excellent spatial coherence, making them ideal for applications such as optical coherence tomography (OCT), fiber optic gyroscopes, and fiber optic sensing. ASE light sources, also known as superluminescent sources (SLSs), are essentially lasers without resonator mirror feedback and are typically based on laser diodes.