Page 194 - Photonics Essentials an introduction with experiments

Direct Modulation of Laser Diodes

188 Advanced Topics

3. The principal assumption that we used to obtain results in closed
form was to presume that the radiative recombination time re-
mains constant. as a function of carrier density. The results in Eq.
8.8 and 8.19 should not be used to make quantitative calculations.
They can be used to understand the systematic behavior of the
turn-on delay and the relaxation oscillations. Prebiasing the laser
close to threshold and driving it on well beyond threshold will min-
imize the turn-on delay. This action will also increase the average
photon density with the result that the relaxation oscillation fre-
quency will go up, and the decay rate of the oscillations will become
shorter. Current semiconductor laser engineering is following this
path.

An alternative approach to controlling relaxation oscillations is to
illuminate the active region with an independent constant light
source. The intensity of this light source is not coupled to the electron
density and its presence interferes with the resonant oscillation be-
tween the electron density and the photon density, diminishing the
amplitude of the relaxation oscillations, but probably not having a
strong an effect on the decay time.
These considerations show that the current model for laser modula-
tion is incomplete. Development of a more realistic model could be
made possible by allowing the important parameters to vary with the
electrical pumping rate and the photon density. These changes will
generate differential equations that cannot be solved in closed form.
This is an inconvenience, the importance of which will continue to di-
minish as computer power continues to increase.
A decade ago, the current model was used to predict that the limit
to direct modulation of semiconductor lasers was about 5 GHz. Many
experiments showed this to be incorrect, and 10 GHz lasers are now
commercially available. Today’s systems engineers would like to have
lasers that can be driven at 40 GHz. There is debate about whether or
not this is possible. Having a better model for predicting the time re-
sponse would be a big help in designing these components. An alter-
native solution is to run the laser at dc and use an external modula-
tor. This is a more costly solution, but one that can provide the
required performance.

8.3 Summary
Semiconductor lasers are used in communication systems where the
light output is modulated directly by changes in drive current. The
transient response of laser to a change in drive current is more com-

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