doping concentration of the active

doping concentration of the active layer, injection carrier density, and activelayer thickness. We shall see the effects of these parameters in the following
sections.

The two basic LED configurations being used for fiber optics are surface emitters (also called Bunrts or from emitters) and edge emitter-t'.2' In the surface emitter the plane of the active light—emitting region is oriented perpendicularly to the axis of the fiber, as shown in Fig. 4-9. in this configuration a well is etched through the substrate of the device, into which a fiber is then cemented in order to accept the emitted light. The circular active area in practical surface emitters is nominally 50 um irt diameter and up to 2.5 um thick. The emission pattcrn is essentially isotropic with a 120° half-power beam width.

This isotropic pattern from a surface emitter is called a tambertirm pattern. In this pattern the source is equally bright when viewed from any direction, but the power diminishes as cos 8, where 8 is the angle between the viewing direction and the normal to the surface (this is because the projected area one sees decreases as cos 8). Thus the power is down to Sll percent of its peak when 5' = 6U“. so that the total halt-power beam width is 120“.

The edge cmitter depicted in Fig. 4-10 consists of an active junction region, which is the source of the incoherent light, and two guiding layers. The guiding layers both have a refractive index which is lower than that of the active region but higher than the index of the surrounding material. This structure forms a waveguide channel that directs the optical radiation toward the fiber core. To match the typical fiber core diameters [50 to 100 pm), the contact