Light Amplification by Stimulated Emission of Radiation LASER

created by Kenneth Howard

laser

Device for producing a narrow beam of light, capable of travelling over vast distances without dispersion, and of being focused to give enormous power densities (10⁸ watts per cm² for high-energy lasers). The laser operates on a principle similar to that of the maser (a high-frequency microwave amplifier or oscillator). The uses of lasers include communications (a laser beam can carry much more information than can radio waves), cutting, drilling, welding, satellite tracking, medical and biological research, and surgery. Sound wave vibrations from the window glass of a room can be picked up by a reflected laser beam. Lasers are also used as entertainment in theatres, concerts, and light shows.

Recording a transmission hologram. Light from a laser is divided into two beams. One beam goes directly to the photographic plate. The other beam reflects off the object before hitting the photographic plate. The two beams combine to produce a pattern on the plate which contains information about the 3-D shape of the object. If the exposed and developed plate is illuminated by laser light, the pattern can be seen as a 3-D picture of the object.

Laser material

Any substance in which the majority of atoms or molecules can be put into an excited energy state can be used as laser material. Many solid, liquid, and gaseous substances have been used, including synthetic ruby crystal (used for the first extraction of laser light in 1960, and giving a high-power pulsed output) and a helium–neon gas mixture, capable of continuous operation, but at a lower power. A silicon-based laser was created in 2004, using the natural atomic vibrations of silicon nanocrystals to generate the light.

In a gas laser, electrons moving between the electrodes pass energy to gas atoms. An energized atom emits a ray of light. The ray hits another energized atom causing it to emit a further light ray. The rays bounce between mirrors at each end causing a build-up of light. Eventually it becomes strong enough to pass through the half-silvered mirror at one end, producing a laser beam.

Helium–neon lasers

The helium–neon laser is the commonest and cheapest kind; it consists of a sealed glass tube containing a mixture of helium and neon gases at low pressure and with mirrors sealed onto the tube at either end. An electrical discharge is passed through the tube from two sealed-in electrodes. The energy of the discharge ‘pumps’ the neon atoms, and photons of wavelength 0.6328 × 10⁻⁶ m are emitted (red light). The light bounces between the mirrors and is amplified at each pass. One of the mirrors is slightly transparent to allow the pencil beam of red light to emerge.