How to Build a Laser Death Ray

Amplified Spontaneous Emission

Any amplifying medium is unstable. It will tend to spontaneously emit whatever it amplifies. In the case of light amplifiers, you will get random, incoherent emission of light. The light which is spontaneously emitted, however, can be further amplifed by the medium. This process can be used directly, and is called self amplified spontaneous emission.

The standard arrangement is to have a very long, thin rod of lasing material. That light which is not emitted along the main axis of the rod does not have very far to get amplified, and it escapes with little energy. It is only the light which is emitted down the axis of the rod which is strongly amplified. If this beam reaches saturation, all the energy stored in the rod goes into the beam directed down the axis.

There are several drawbacks to this method. First, the beam will have a minimum divergence angle equal to the ratio of the diameter of the rod to the rod's length. If this divergence angle is larger than the divergence angle due to diffraction, the beam will be incoherant, which means that it cannot ever be focused to a diffraction limited point, but will always be spread out into an image that is larger than the diffraction limit. Further, the beam will tend to be unstable, fluctuating in intensity and generally having poor beam quality.

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