Page 183 - Photonics Essentials an introduction with experiments

Source: Photonics Essentials

Chapter
8

Direct Modulation
of Laser Diodes

8.1 Introduction
Information can be carried by an optical beam only if the beam is
modulated. There are many ways to modulate the laser emission: out-
put wavelength, frequency, intensity, etc. Intensity modulation is
used most often because it is well adapted to digital communications
and relatively simple to implement. The two forms of intensity modu-
lation are external modulation and internal modulation. External
modulation can be achieved by a mechanical wheel such as a compact
disc, or by an electro-optic modulator that changes the optical density
or index of refraction of the propagation path, to mention two possibil-
ities. In some applications, the laser beam is not optically modulated
at all, internally or externally. An important example of this use is
the laser pump at 980 nm for Er 3+ optical fiber amplifiers in optical
communications.
Internal modulation is accomplished by modulating the drive cur-
rent of the laser. Current modulation has the advantage that it is
both simple and economical to implement. The disadvantages of cur-
rent modulation are related primarily to transient effects associated
with turning on or turning off the laser. Some of the main difficulties
are chirping and self-pulsations. The laser chirp refers to the change
of the laser output wavelength with time as the laser is being pulsed
on or off. The chirp may be large enough to increase the communica-
tions error rate beyond acceptable limits. Self-pulsations are the re-
sult of a resonant coupling between the population of photons and the

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