Section 2.1: Brief history of lasers
Laser is the acronym of Light Amplification by Stimulated Emission of Radiation. Laser is light of special properties, light is electromagnetic (EM) wave in visible range. It is a long history for human beings to realize that light is both wavelike and particle like. In 1704, Newton characterized light as a stream of particles. The Young’s interference experiment in 1803 and the discovery of the polarity of light convinced scientists of that time that light is wave. Maxwell’s electromagnetic theory explained light as rapid vibrations of EM field due to the oscillation of charged particles. At the turn of the 20th century, the black body radiation phenomena challenged the wavelike light theory. According to Maxwell’s EM theory, the energy intensity of EM emissions with frequency f is proportional to the square of this frequency, integrate the intensity from zero to infinity frequencies over the limited black body volume will result in infinite energy, which is of course impossible! It was until Plank introduced the "quantum" concept in 1900 when this was explained. Thus energy is not continuous, it is discrete and can only be the multiples of a small unit. Einstein proposed the concept of "photon", we can say light is composed of individual particles called photons which posses a discrete amount of energy or quanta. Einstein also predicted in 1917 that when there exist the population inversion between the upper and lower energy levels among the atom systems, it was possible to realize amplified stimulated radiation, i.e., laser light. Quantum Mechanics was developed to explain these new phenomena since 1920. Now we think that light is composed of "particles" whose motion probability is determined by its wavelike behavior.
The stimulated emission has the same frequency and phase with the incident radiation, this can be explained by quantum mechanics. So laser light would be very different from normal light in that it is coherent. Coherent light sources have numerous applications. Many people tried to find methods for amplified stimulated emission, but it was not realized until 1960, about half a century after Einstein’s prediction.
The first laser was invented by Maiman in May, 1960. It was a solid ruby laser. Many kinds of laser were invented soon after the solid ruby laser—first Uranium Laser by IBM labs(in Nov.,1960), first Helium-Neon Laser by Bell Laboratories in 1961, first semiconductor laser by Robert Hall at General Electric Labs in 1962, first working Nd:YAG laser and CO2 laser by Bell Laboratories in 1964, argon ion laser in 1964, chemical laser in 1965, metal vapor laser in 1966, …, it seems lots of things can be made to lase! When we know the principles of laser, this won’t be too big a surprise. But the wide and continuously expanding applications of lasers are indeed miracles.
The laser world is really rich and interesting, we will limit our discussions on topics that have close relations with laser machining processes. To be used in material processing, lasers must have sufficient power, this reduces the eligible lasers to the following—CO2 lasers, Nd-YAG or Nd-glass lasers and Excimer lasers, etc. Before we study individual lasers, let’s first examine the properties of laser beams.