The Nd:YAG laser operates in a four-state cycle as shown in the figure, and the ruby laser operates in a three-state cycle (compare with Fig. 28.24b ). In which laser is it easier to maintain a population inversion? Why? Explain why the Nd:YAG laser can produce a continuous beam, but the ruby laser can produce only brief pulses of laser light.

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Spontaneous transition Metastable state Optcal Stimulated emission pumping (absorption) Spontaneous transition Ground state