92 CHAPTER 3. OPTICAL TRANSMITTERS
etching a well and bringing the fiber close to the emissive area. The power coupled into
the fiber depends on many parameters, such as the numerical aperture of the fiber and
the distance between fiber and LED. The addition of epoxy in the etched well tends
to increase the external quantum efficiency as it reduces the refractive-index mismatch.
Several variations of the basic design exist in the literature. In one variation, a truncated
spherical microlens fabricated inside the etched well is used to couple light into the
fiber [23]. In another variation, the fiber end is itself formed in the form of a spherical
lens [24]. With a proper design, surface-emitting LEDs can couple up to 1% of the
internally generated power into an optical fiber.
The edge-emitting LEDs employ a design commonly used for stripe-geometry
semiconductor lasers (see Section 3.3.3). In fact, a semiconductor laser is converted
into an LED by depositing an antireflection coating on its output facet to suppress lasing
action. Beam divergence of edge-emitting LEDs differs from surface-emitting LEDs
because of waveguiding in the plane perpendicular to the junction. Surface-emitting
LEDs operate as a Lambertian source with angular distributionSe(θ)=S 0 cosθin
both directions. The resulting beam divergence has a FWHM of 120◦in each direction.
In contrast, edge-emitting LEDs have a divergence of only about 30◦in the direction
perpendicular to the junction plane. Considerable light can be coupled into a fiber of
even low numerical aperture (< 0 .3) because of reduced divergence and high radiance
at the emitting facet [25]. The modulation bandwidth of edge-emitting LEDs is gen-
erally larger (∼200 MHz) than that of surface-emitting LEDs because of a reduced
carrier lifetime at the same applied current [26]. The choice between the two designs
is dictated, in practice, by a compromise between cost and performance.
In spite of a relatively low output power and a low bandwidth of LEDs compared
with those of lasers, LEDs are useful for low-cost applications requiring data transmis-
sion at a bit rate of 100 Mb/s or less over a few kilometers. For this reason, several
new LED structures were developed during the 1990s [27]–[32]. In one design, known
asresonant-cavityLED [27], two metal mirrors are fabricated around the epitaxially
grown layers, and the device is bonded to a silicon substrate. In a variant of this idea,
the bottom mirror is fabricated epitaxially by using a stack of alternating layers of two
different semiconductors, while the top mirror consists of a deformable membrane sus-
pended by an air gap [28]. The operating wavelength of such an LED can be tuned over
40 nm by changing the air-gap thickness. In another scheme, several quantum wells
with different compositions and bandgaps are grown to form a MQW structure [29].
Since each quantum well emits light at a different wavelength, such LEDs can have an
extremely broad spectrum (extending over a 500-nm wavelength range) and are useful
for local-area WDM networks.
3.3 Semiconductor Lasers
Semiconductor lasers emit light through stimulated emission. As a result of the fun-
damental differences between spontaneous and stimulated emission, they are not only
capable of emitting high powers (∼100 mW), but also have other advantages related
to the coherent nature of emitted light. A relatively narrow angular spread of the output
beam compared with LEDs permits high coupling efficiency (∼50%) into single-mode