Otterbein Miscellany May 1965 combined

The source of an intense beam of photons of the proper frequency is self generated by the optical cavity containing the inverted population atoms. Once the excitation of the long lived state has begun, some of the excited atoms will begin to emit photons of energy hfj^Q they spontaneously undergo the transition from state E]^ to Eq. Photons from these spontaneous transitions are emitted in random directions. Those moving towards the side of the cavity pass through the edge and are lost. Those moving along the long dimension of the cavity begin to encounter other atoms still in the excited state, stimulating them to emit coherent photons in the same direction thus multi­ plying the intensity of the beam as they move toward the end of the cavity. Upon reaching the mirrors, 99 of every 100 photons incident on the mirrors are reflected back down the axis of the cavity to stimulate still more excited atoms to emit. Thus, as long as there remain atoms in the Ej energy state, there will be a beam of photons moving either way in a single direction to stimulate them to add coherent photons to this beam. If the number of available excited atoms should fall below the limit whereby they can maintain more available photons than are lost through the end mirrors, the photons in the stimulating beam will begin to be absorbed by ground state atoms and the light amplification will cease.

For this reason, in lasers which are excited by radiation (where f20 is called the pumping frequency) once the lasing action has started, the long lived level is depopulated at a much greater rate than the pumping radiation can repopulate it through the spontaneous emission from level E2. This comes about because the rate of stimulated emission depends upon the intensity of the radiation at the critical frequency, and this intensity is increased tremendously by the photon amplification in the laser beam. This type of laser therefore emits a relatively short pulse of very high intensity coherent radiation. Such pulsed lasers can produce very large energy concentrations into very small physical spaces.

A continuous lasing action may be obtained through a some­ what different method of excitation. In a mixture of helium and neon gas, pumping action is attained by a system of electrical excitation by either a radio frequency or a direct current dis­ charge.

The neon atoms possess a scheme of energy levels which may be represented as in Figure 2.

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