Laser

I

Introduction

Laser, acronym for light amplification by stimulated emission of radiation. Lasers are devices that amplify light and produce coherent light beams, ranging from infrared to ultraviolet. A light beam is coherent when its waves, or photons, propagate in step, or in phase, with one another (See Interference). Laser light, therefore, can be made extremely intense, highly directional, and very pure in colour (frequency). Laser devices now extend into the X-ray frequency range. Masers are similar devices for microwaves.

II

Principles of Operation

Lasers harness atoms to store and emit light in a coherent fashion. The electrons in the atoms of a laser medium are first pumped, or energized, to an excited state by an energy source. They are then “stimulated” by external photons to emit the stored energy in the form of photons, a process known as stimulated emission. The photons emitted have a frequency characteristic of the atoms and travel in step with the stimulating photons. These photons in turn impinge on other excited atoms to release more photons. Light amplification is achieved as the photons move back and forth between two parallel mirrors, triggering further stimulated emissions. At the same time the intense, directional, and monochromatic laser light “leaks” through one of the mirrors, which is only partially silvered.

Stimulated emission, the underlying process for laser action, was first described theoretically by Albert Einstein in 1917. The working principles of lasers were outlined by the American physicists Arthur Schawlow and Charles Hard Townes in their 1958 patent application. The patent was granted, but was later challenged by the American physicist and engineer Gordon Gould. In 1960 the American physicist Theodore Maiman observed the first laser action in solid ruby. A year later a helium-neon gas laser was built by the Iranian-born American physicist Ali Javan. Then in 1966 a liquid laser was constructed by the American physicist Peter Sorokin. The United States Patent Office court in 1977 affirmed one of Gould's claims over the working principles of the laser.

III

Types of Lasers

According to the laser medium used, lasers are generally classified as solid state, gas, semiconductor, or liquid.

A

Solid-State Lasers

The most common solid laser media are rods of ruby crystals and neodymium-doped glasses and crystals. The ends of the rod are fashioned into two parallel surfaces coated with a highly reflecting non-metallic film. Solid-state lasers offer the highest power output. They are usually operated in a pulsed manner to generate a burst of light over a short time. Bursts as short as 12 × 10^-15 sec have been achieved, which are useful in studying physical phenomena of very brief duration. Pumping is achieved with light from xenon flash tubes, arc lamps, or metal-vapour lamps. The frequency range has been expanded from infrared (IR) to ultraviolet (UV) by multiplying the original laser frequency with crystal-like potassium dihydrogen phosphate, which are even shorter, and X-ray wavelengths, which are even shorter, have been achieved by aiming laser beams at yttrium targets.