Biophotonics_Concepts_to_Applications

Theedge emitterdepicted in Fig.4.7consists of an active junction region, which
is the source of the incoherent light, and two guiding layers. This structure forms a
waveguide channel that directs the optical radiation toward the edge of the device
where it can be coupled to an opticalfiber core. The emission pattern of the edge
emitter is more directional than that of the surface emitter, as is illustrated in
Fig.4.7. In the plane parallel to the junction, where there is no waveguide effect,
the emitted beam is lambertian with a half-power beam width ofθ||= 120°. In the
plane perpendicular to the junction, the half-power beam can be on the order
θ⊥≈ 25 – 35°, thereby forming an elliptical output beam.

Double

heterojunction

layers

Confinement

layers

Circular

active region

Circular

metal contact

Substrate

SiO 2 isolation

layer

Metal contact

Bonding

material

Optical fiber

Fig. 4.6 Schematic (not to scale) of a high-radiance surface-emitting LED

Double

heterojunction

layers

Light-guiding

confinement

layers

Active region

(as a stripe

along the device)

Metal contact

SiO 2 isolation

layer

Metal stripe contact that defines the active area

Incoherent optical

120° output beam

25-35°

Fig. 4.7 Schematic (not to scale) of an edge-emitting double-heterojunction LED

100 4 Fundamentals of Light Sources