Page 150 - Photonics Essentials an introduction with experiments

Lasers

144 Photonic Devices

ductor laser is by far the industry leader in terms both of the number
of units sold and the volume of revenues. Semiconductor laser diodes
are the key component in a number of common devices such as a CD-
players, scanners, printers, and DVD readers. They are also the key
components in optical fiber telecommunications for generating the
light waves that travel down the fiber.
In this chapter, we will discuss the principles of laser action, start-
ing first with a short example of an electronic amplifier that you can
build in a few minutes in the lab. The physical principles that cause
laser action to occur are analogous to those that cause oscillation to
occur in electronic circuits. We will build on this analogy to develop a
model for the level of current required to induce lasing in a p-n diode,
and how this current depends on the parameters of the laser struc-
ture. The laser is a special kind of LED. You already know about
many of the measurements, such as the I–V characteristic and the
light–current characteristic of these devices.

7.1 Amplifiers and Feedback
If you were to imagine the simplest amplifier circuit you could think
of, it might resemble the circuit in Fig. 7.1. This shows an NPN tran-
sistor with an input on the base, a bias on the collector, and the emit-
ter shorted to ground. This amplifier works as follows. When the po-
tential of the gate is close to common, the transistor is turned off, and

Figure 7.1. A circuit diagram of a simple class A transistor amplifier.

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