Superluminescent light emitting diodes (SLEDs) are unique semiconductor light emitters in the sense that they combine the spatial coherence properties of lasers with the temporal incoherence of a LED. They are sometimes referred to as a "diode lasers operated below threshold", "high power LEDs" or "super LEDs".

The high spatial coherence translates into a small divergence angle of the light beam emitted by the SLED chip. This enables a high coupling efficiency into fiber pigtails. Typically, 50% of the power emitted from the SLED chip is coupled into a single-mode fiber.

The temporal incoherence (short coherence time) is equivalent to a short coherence length. This means that when light from an SLED is split into two beams and one of the beams is delayed with respect to the other by more than the coherence length, the two beams will not interfere with each other any longer when recombined. In the spectral domain, this property manifests itself as a broadband spectrum, i.e. light is emitted over a wide range of wavelengths.

In contrast, the spectrum of a laser is monochromatic, i.e. light is emitted only within a very narrow wavelength band, resulting in a long coherence length.