Comparison of surface- and edge-emitting LED's for use in fiber-optical communications

The performance of state-of-the-art double-heterojunction (DH) surface and edge emitters are compared with respect to their use in high-data-rate fiber-optical communication systems. Thick-window (20-25-µm) surface emitters with 2-2.5-µm thick active layers and emitting up to 15-mW optical power at 300 mA have been fabricated. For edge emitters, we use very-high-radiance-type devices with ≃ 500-Å thick active layers. For these two types of LED's we examine differences in structure and light coupling efficiency to fibers of various numerical apertures (NA). For typically good devices we compare the diodes' output power capabilities, the powers coupled into step- and graded-index fibers of various NA, and their respective frequency response. For the same drive current level, we find that edge emitters couple more power than surface emitters into fibers with NA
\lsim 0.3
. The edge emitters also have ≈ 5 times larger bandwidths. We estimate that an edge emitter can couple 5-6 times more power into low numerical aperture (NA
\lsim 0.2
) fibers than a surface emitter of the same bandwidth. We conclude that edge emitters are preferred to surface emitters for optical data rates above 20 Mbits/s.