All About Lasers

High-power lasers are used for cutting, and for welding sheets of metal together. Lasers are being used to generate sufficient energy to initiate nuclear fusion. Lasers fire simultaneously and their beams intersect to deliver 200 kJ of energy to a fuel pellet in less than a nanosecond (10-9 s). The aim is to make the implosion so rapidly that individual nuclei in the pellet are driven into contact with one another with enough force to overcome their mutual Coulomb repulsion.

The laser beam is monochromatic and is described by an almost pure sine wave. This is because the beam, moving to and fro between the mirrors, forms a standing wave. Thus an integral number of half-wavelengths must fit exactly between the mirrors. No other wavelengths can be amplified. This makes laser light much more monochromatic than light emitted by spontaneous emission.

The fact that laser light is highly monochromatic ensures that any interference patterns produced will be very clear and easy to interpret. This has an application in compact disc (CD) players, where an interference pattern is produced between a reference laser beam and a laser beam that is reflected from steps cut in the playing surface of the disc. Another famous application of interference of laser light is the production of holograms or ‘three-dimensional photographs’.

Laser light forms a narrow, well-directed beam, which spreads at large distances by diffraction only. The spread by diffraction is through an angle of only about λ/d where d is the beam diameter. This angle is normally only a few milli-radians. The high directionality is explained by the fact that all the photons produced by stimulated emission travel in the same direction.

The fact that laser light forms a narrow beam can be used in eye surgery. It can also be used to measure distances to extreme accuracy. For example, the Apollo 14 astronauts left behind a special reflector on the Moon’s surface. By firing a laser through an Earth-bound telescope at this reflector, the time of flight of its round trip can be measured, confirming that the Moon is receding from us at the rate of a few centimetres per year.

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