EC 2402 OPTICAL COMMUNICATION AND NETWORKING

The power generated internally by an LED may be determined by consideration of the excess
electrons and holes in the p- and n-type material respectively (i.e. the minority carriers) when it is
forward biased and carrier injection takes place at the device contacts. The excess density of electrons
n and holes p is equal since the injected carriers are created and recombined in pairs such that
charge neutrality is maintained within the structure.

3.3 LED STRUCTURES

3.3.1 Surface emitter LEDs
A method for obtaining high radiance is to restrict the emission to a small active region within
the device. The technique pioneered by Burrus and Dawson with homostructure devices was to use
an etched well in a GaAs substrate in order to prevent heavy absorption of the emitted radiation, and
physically to accommodate the fiber. These structures have a low thermal impedance in the active
region allowing high current densities and giving high-radiance emission into the optical fiber.
Furthermore, considerable advantage may be obtained by employing DH structures giving increased
efficiency from electrical and optical confinement as well as less absorption of the emitted radiation.

This type of surface emitter LED (SLED) has been widely employed within optical fiber
communications. The structure of a high-radiance etched well DH surface emitter* for the 0.8 to
0.9μm wavelength band is shown in Figure 3.2. The internal absorption in this device is very low due
to the larger bandgap-confining layers, and the reflection coefficient at the back crystal face is high
giving good forward radiance. The emission from the active layer is essentially isotropic, although
the external emission distribution may be considered Lambertian with a beam width of 120° due to
refraction from a high to a low refractive index at the GaAs–fiber interface. The power coupled Pc
into a multimode step index fiber may be estimated from the relationship:

(3.1)

where r is the Fresnel reflection coefficient at the fiber surface, A is the smaller of the fiber core cross-
section or the emission area of the source and RD is the radiance of the source.

Figure 3.2 The structure of an AlGaAs DH surface-emitting LED (Burrus type).

SCE 85 Dept. of ECE.

EC 2402 OPTICAL COMMUNICATION AND NETWORKING

However, the power coupled into the fiber is also dependent on many other factors including
the distance and alignment between the emission area and the fiber, the SLED emission pattern and
the medium between the emitting area and the fiber. For instance, the addition of epoxy resin in the
etched well tends to reduce the refractive index mismatch and increase the external power efficiency
of the device. Hence, DH surface emitters often give more coupled optical power than predicted by
Eq. (3.1).

However, for graded index fiber optimum direct coupling requires that the source diameter be
about one-half the fiber core diameter. In both cases lens coupling may give increased levels of optical
power coupled into the fiber but at the cost of additional complexity. Other factors which complicate
the LED fiber coupling are the transmission characteristics of the leaky modes or large angle skew
rays. Much of the optical power from an incoherent source is initially coupled into these large-angle
rays, which fall within the acceptance angle of the fiber but have much higher energy than meridional
rays. Energy from these rays goes into the cladding and may be lost.

Hence much of the light coupled into a multimode fiber from an LED is lost within a few
hundred meters. It must therefore be noted that the effective optical power coupled into a short length
of fiber significantly exceeds that coupled into a longer length.

The planar structure of the Burrus-type LED and other nonetched well SLEDs allows
significant lateral current spreading, particularly for contact diameters less than 25 μm. This current
spreading results in a reduced current density as well as an effective emission area substantially
greater than the contact area.

3.3.2 Edge emitter LEDs

Another basic high-radiance structure currently used in optical communications is the stripe
geometry DH edge emitter LED (ELED). This device has a similar geometry to a conventional
contact stripe injection laser, as shown in Figure 3.3.

Figure 3.3 Schematic illustration of the structure of a stripe geometry DH AlGaAs edge-emitting
LED

SCE 86 Dept. of ECE.

EC 2402 OPTICAL COMMUNICATION AND NETWORKING

It takes advantage of transparent guiding layers with a very thin active layer (50 to 100 μm)
in order that the light produced in the active layer spreads into the transparent guiding layers, reducing
self-absorption in the active layer. The consequent waveguiding narrows the beam divergence to a
half-power width of around 30° in the plane perpendicular to the junction. However, the lack of
waveguiding in the plane of the junction gives a Lambertian output with a half-power width of around
120°, as illustrated in Figure3.3. Most of the propagating light is emitted at one end face only due to
a reflector on the other end face and an antireflection coating on the emitting end face. The effective
radiance at the emitting end face can be very high giving an increased coupling efficiency into small-
NA fiber compared with the surface emitter. However, surface emitters generally radiate more power
into air (2.5 to 3 times) than edge emitters since the emitted light is less affected by reabsorption and
interfacial recombination. Comparisons have shown that edge emitters couple more optical power
into low NA (less than 0.3) than surface emitters, whereas the opposite is true for large NA (greater
than 0.3).

The enhanced waveguiding of the edge emitter enables it in theory to couple 7.5 times more
power into low-NA fiber than a comparable surface emitter. However, in practice the increased
coupling efficiency has been found to be slightly less than this (3.5 to 6 times). Similar coupling
efficiencies may be achieved into low-NA fiber with surface emitters by the use of a lens.
Furthermore, it has been found that lens coupling with edge emitters may increase the coupling
efficiencies by comparable factors (around five times).

The stripe geometry of the edge emitter allows very high carrier injection densities for given
drive currents. Thus it is possible to couple approaching a milliwatt of optical power into low-NA
(0.14) multimode step index fiber with edge-emitting LEDs operating at high drive currents (500
mA).

Edge emitters have also been found to have a substantially better modulation bandwidth of
the order of hundreds of megahertz than comparable surface-emitting structures with the same drive
level. In general it is possible to construct edge-emitting LEDs with a narrower linewidth than surface
emitters, but there are manufacturing problems with the more complicated structure (including
difficult heat-sinking geometry) which moderate the benefits of these devices.

3.4 The Semiconductor Injection Laser

The electroluminescent properties of the forward-biased p–n junction diode have been
considered in the preceding sections. Stimulated emission by the recombination of the injected
carriers is encouraged in the semiconductor injection laser (also called the injection laser diode (ILD)
or simply the injection laser) by the provision of an optical cavity in the crystal structure in order to
provide the feedback of photons. This gives the injection laser several major advantages over other
semiconductor sources (e.g. LEDs) that may be used for optical communications. These are as
follows:

1. High radiance due to the amplifying effect of stimulated emission. Injection lasers will generally
supply milliwatts of optical output power.

2. Narrow linewidth on the order of 1 nm (10 Å) or less which is useful in minimizing the effects of
material dispersion.

3. Modulation capabilities which at present extend up into the gigahertz range and will undoubtedly
be improved upon.

4. Relative temporal coherence which is considered essential to allow heterodyne (coherent) detection
in high-capacity systems, but at present is primarily of use in single-mode systems.

SCE 87 Dept. of ECE.