Rami Arieli: "The Laser Adventure" Chapter 3 page 1
Chapter 3: The Laser System

The laser is a system that is similar to an electronic oscillator.

An Oscillator is a system that produces oscillations without an external driving mechanism.

 To demonstrate an oscillator, we can use the familiar acoustic analog:

A sound amplification system has a microphone, amplifier and speaker.

When the microphone is placed in front of the speaker, a closed circuit is formed, and a whistle is heard out of the speaker.

The whistle is created spontaneously, without any external source.

Explanation: The speaker's internal noise is detected by the microphone, amplified and the amplified signal is again collected by the microphone. This positive feedback continues until a loud whistle is heard.