Class B lasers naturally exhibit damped relaxation oscillations and, as for any nonlinear oscillator, their responses to a time-periodic modulation of a parameter are rich and varied. The study of forced oscillators itself has a long history. Systematic studies started with Edward Appleton (1922) and Balthasar van der Pol (1927) who showed that the frequency of a triode generator can be entrained by a weak external signal with a slightly different frequency. These studies were of high practical importance because such generators became basic elements of radio communication systems. The next impact on the development of the theory of forced oscillators came from the Russian school when control engineering became an emerging discipline. Alexandr Aleksandrovich Andronov (1901–1952) was a key figure in the development of mathematical techniques for driven oscillators, yet his name, and his contributions to control theory and nonlinear dynamics, are much less well known in the West than they deserve to be. As we shall demonstrate later in this chapter, these analytical techniques are totally appropriate for our laser problems.

Today, lasers and fiber optic cables have replaced the electronic amplifying tubes and cables. Light signals are modulated with the information to be sent into fiber optic cables by lasers. Telephone fiber drivers may be solid state lasers the size of a grain of sand and consume a power of only half a milliwatt. Yet they can send 50 million pulses per second into an attached telephone fiber and encode over 600 simultaneous telephone conversations.