Introduction

To give a little more practical emphasis to some of the ideas we have dealt with so far, let us consider some of the details of the two laser systems in which population inversion and laser oscillation were first demonstrated. One of these lasers uses an amplifying medium that is a crystalline solid – the ruby laser; in the other the amplifying medium is a gas – a mixture of helium and neon. In each case, the amplifying medium is pumped into a state of population inversion by feeding energy into it in an appropriate way. Laser oscillation occurs when the amplifying medium is placed between a pair of suitable aligned mirrors that provide the necessary optical feedback to cause oscillation to occur. The ruby laser was the first operational laser, being demonstrated on May 16, 1960 by Theodore Maiman of the Hughes Aircraft Company in Malibu, California [1].

That the ruby laser was the first laser to be demonstrated surprised many in the scientific community. This is because the ruby laser is a three-level laser, which was expected to be much more difficult to operate than a four-level laser. This is an important distinction, which we will examine before describing the first two lasers in detail.

Three- and four-level lasers

The distinction between three- and four-level lasers can be illustrtated with the aid of Fig. 3.2. Energy is fed into the system to move particles from the ground state, level 0, to a pumping energy level, level 3.