There are two generic types of light sources used in fiber optic circuits for telecommunications: LEDs and LDs. LED stands for Light-Emitting Diode and LD stands for Laser Diode.
The LEDs used in telecommunications emit light in the near-infrared region. They are cheaper when compared to most lasers. LEDs are mostly used with multimode fiber because they emit light in a broad cone that can only be captured efficiently by the large numerical aperture of multimode fibers.
In its simplest form, an LED is a forward-biased p-n homojunction. Radiative recombination of electron-hole pairs in the depletion region generates light; some of it escapes from the device and can be coupled into an optical fiber. The emitted light is incoherent with a relatively wide spectral width (30 ~ 60 nm) and a relatively large angular spread.
An LED structure can be classified as surface-emitting or edge-emitting, depending on whether the LED emits light from a surface that is parallel to the junction plane or from the edge of the junction plane. This figure shows the design of the two types of LEDs.
Both types can be made using either a p-n homojunction or a heterostructure design in which the active region is surrounded by p- and n-type cladding layers. The heterostructure design leads to superior performance. It provides a control over the emissive area and eliminates internal absorption because of the transparent cladding layers.
The LED is notoriously inefficient. With proper design, surface-emitting LEDs can couple 1% of the internally generated power into an optical fiber.