Laser

A laser at NASA/Ames Research Center

A Laser is a machine that makes an amplified, single-colour source of light. This source of light acts like light from a tiny point (or from very far away). If you put special lenses in front of it, you can make laser light into a very narrow beam. A laser uses mirrors to turn the light into a single colour, and make the light act as if it is from a tiny point (or from very far away). It uses gases to make the light amplified, like in a fluorescent light bulb. LASER is actually an acronym that stands for Light Amplification by Stimulated Emission of Radiation.

Mechanism

A laser creates light by a very specific mechanism involving an optical gain medium. Basically the mechanism is to take advantage of the amount of time an optical gain medium will spend in an excited state. First, the medium is excited by a separate pumping source, raising the energy in the medium to the level of an excited state, for example as in Figure 1. However, the energy is not emitted all at once, but rather a fast transition, due to the excited state having a shorter lifetime, from an excited state to the metastable state occurs. Next, a slow transition, because the metastable state has a longer lifetime, from metastable state back down to the ground state is observed, which is the actual laser radiation. This amplification is due to the fast pace of putting the medium into the metastable state, which releases its energy at a slower rate than the faster process of putting the medium into the metastable state. Thus, there is spends most of its time in the metastable state ready to emit radiation. This process of maintaining an energy level is called saturation and creates a very intense continuous beam of light at a very narrow wavelength, which we would identify as a laser beam.¹

Design

The flash light provides the pumping energy to the optical medium, in this case a crystal of ruby. As described above, the radiation saturates the medium in a higher energy level, which emits light when it returns to a lower energy level. This light now moves through the medium between the two mirrors that reflect the light back and forth between them. One of the mirrors, however, only partially reflects the light, allowing some to escape. The escaping light makes up the laser beam. ¹

This is a simple design; the type of medium used usually defines the type of laser, which is not always a crystal. Crystals used for lasers include ruby as well as a garnet crystal made of yttrium and aluminum with the rare earth metal mixed in. Gases can be used for laser by using helium, nitrogen, carbon dioxide, neon or others. Finally, the smallest lasers using semiconductor diodes to produce the light. ¹

History

Albert Einstein was the first to put forward the idea of stimulated emission that could produce a laser. From that point many years were spent confirming the presence of the phenomenon, but it was not until 1959 that the term laser was coined by Gordon Gould in a research paper. The first working laser was put together and operated by Theodore Maiman at the Hughes Research Laboratories in 1960, using a solid ruby laser such as the one in Figure 2. ²

Applications

Lasers have found many uses in everyday life as well as in industry. Lasers are found in CD and DVD players, where they read the code from the disk that gets translated into a song or movie. A laser is also used to read the bar code on items being purchased, allowing quick identification of a product. Lasers are used in the medicine, particularly in LASIK eye surgery, where the laser is used to repair the shape of the cornea. A very important application of lasers is as a light source of spectroscopic studies in chemistry. A laser is even used to measure the distance of the Moon from Earth by reflecting off a large reflector left by the Apollo missions and measuring the time it takes for the light to travel to the Moon and back again.

References

1. Skoog, Douglas A., Holler, F. James, Crouch, Stanley R. Principles of Instrumental Analysis. Toronto, ON. Thomson Brooks/Cole, 2007.

2. Gribbin. J, In Search of Schroedinger's Cat New York, Bantam, 1984