3.6. TRANSMITTER DESIGN 119
Figure 3.25: Transmitters employing (a) butt-coupling and (b) lens-coupling designs. (After
Ref. [97];©c1989 AT&T; reprinted with permission.)
be maximized during packaging; a splice or connector is used to join the pigtail with
the fiber cable.
Two approaches have been used for source–fiber coupling. In one approach, known
as direct orbutt coupling, the fiber is brought close to the source and held in place by
epoxy. In the other, known aslens coupling, a lens is used to maximize the coupling
efficiency. Each approach has its own merits, and the choice generally depends on
the design objectives. An important criterion is that the coupling efficiency should not
change with time; mechanical stability of the coupling scheme is therefore a necessary
requirement.
An example of butt coupling is shown in Fig. 3.25(a), where the fiber is brought in
contact with a surface-emitting LED. The coupling efficiency for a fiber of numerical
aperture NA is given by [96]
nc=( 1 −Rf)(NA)^2 , (3.6.1)whereRfis the reflectivity at the fiber front end.Rfis about 4% if an air gap exists
between the source and the fiber but can be reduced to nearly zero by placing an index-
matching liquid. The coupling efficiency is about 1% for a surface-emitting LED and
roughly 10% for an edge-emitting LED. Some improvement is possible in both cases