Page 185 - Photonics Essentials an introduction with experiments

Direct Modulation of Laser Diodes

Direct Modulation of Laser Diodes 179

ters on the modulation rate could be appreciated and used in design.
Finding closed-form solutions was an important consideration be-
cause few people had access to supercomputers that might better
model the situation. But now that we all have supercomputers sitting
on our desktops, the door is wide open for development of a new mod-
el that is both more accurate and more useful.

8.2 Time-Dependent Behavior of Laser Diodes
during Current Modulation

When you turn on a laser by a pulse of current, there are three things
that happen. First there is a time delay while the population inver-
sion builds up to the threshold level. Next the laser begins stimulated
emission of light at energy E 1 . As time goes on, this energy decreases,
and the wavelength of emission increases. The emission of light de-
pletes the level of carrier inversion, and causes the light intensity to
decrease. When the recombination decreases, the level of inversion in-
creases, completing the cycle. These events are diagrammed schemat-
ically in Fig. 8.1.
To put these events in perspective, consider the current systems
specification for optical fiber telecommunications. In order to carry
the maximum amount of information in an optical fiber, communica-
tion channels are assigned on the basis of wavelength. This is called
wavelength-division multiplexing, or WDM. The useful amplification
band of Er 3+ amplifiers is 30 nm. The current specification calls for
100 channels in this band. This means that the spacing in wave-
length between each channel is 0.3 nm. This is called dense wave-
length division multiplexing, or DWDM. If the wavelength of a laser
changes by more that 0.2 nm during modulation, clearly there will
be a problem.
In Fig. 8.2, we show a flow diagram for laser emission. This figure is
somewhat more complicated than the corresponding diagram for
LEDs shown in Fig. 6.13. The photon density is increased by both in-
creases in the carrier density and the optical gain. We will use this
schematic diagram to build a model of the time dependence of laser
action.
The laser modulation properties are based on
dN
= change in the electron–hole concentration
dt
and

dN
= change in the photon population.
dt

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