Physics Question #738

Mouabbi Rami, a 17 year old male from Athens asks on April 2, 2002,

Why do most industrial LASER diodes emit pulsing light beams and not continuous ones? And what is the physical explanation of the coherent light beams emitted? Is it simply light that is monochromatic and whose photons are “in step” with one another?

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The answer

Simon Watkins answered on December 17, 2002

It depends on the industrial application. There are lots of high power continuous wave (cw) laser diodes available these days. For cw applications, the mirror facets need to be much more sophisticated with multi-layer optical coatings. Pulsed operation allows you to drive the laser with much higher peak currents, which allows you to use less sophistcated coatings, or none at all, since the gain is so high that the feedback provdided by the coating is not so important. On the other hand there are many applications where a continuous, stable beam is necessary. But it costs more to make them, so you better have a reason.

The coherence of the laser beam comes from the fact that lasers rely on "stimulated emission". In this process an excited atom can be stimulated to emit by another photon with exactly the same frequency. This causes the emitted photon to have exactly the same phase as the one that stimulated it. (The explanation for this comes out of quantum mechanics.) In this way a beam of photons with the identical phase can be built up inside an optical cavity.

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