Page 195 - Photonics Essentials an introduction with experiments

Direct Modulation of Laser Diodes

Direct Modulation of Laser Diodes 189

plicated than the transient response of LEDs because of the interac-
tion between the carrier density and the photon density. Models of the
modulation rate lead directly to coupled differential equations. Sim-
plifying assumptions can be use to decouple these equations, but the
result leads to an underestimation of the laser modulation rate. This
approach allows qualitative appreciation of the important physical
parameters that affect the transient response. Numerical simulation
is probably a better approach if quantitative prediction is sought.
When the laser is pulsed from “off” to “on,” there is a time delay
that occurs before any laser light appears. This delay is proportional
to the difference between the “off”-state current and the threshold
current. As the laser turns “on,” there is oscillation of the light output
that occurs in time. The frequency of this oscillation increases as the
difference between the threshold current and the final steady-state
current increases. These oscillations eventually die out with a time
proportional to the excess carrier lifetime.
These parameters limit the bandwidth that is achievable using cur-
rent modulation of laser output power. Note that the small-signal ac
bandwidth of the laser may by much greater than the bit rate for digi-
tal communications. In a similar vein, the laser transient properties
cannot be correctly deduced from a simple experiment in which a
small ac modulation is applied to the laser, and the modulated output
power is measured as a function of signal frequency.

Bibliography

1. M. Ming-Kang Liu, Principles and Applications of Optical Communica-
tions, Irwin, New York, 1996.
2. A. Yariv, Optical Electronics in Modern Communications, 5th Edition Ox-
ford University Press, New York, 1997.

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