Einstein worked out stimulated emission in 1917, more than forty years before the first laser actually fired in 1960. As long as the upper level holds fewer atoms than the lower one, the device is merely a lamp; only when the pump power is strong enough to push the population inversion ΔN past threshold does the photon count in the cavity surge into a beam. The reflectivity of the output mirror is the interesting trade-off: the tighter the mirror, the stronger the gain inside the cavity but the less light escapes, and a mirror that is too perfect leaves no usable beam at all.
Einstein worked out stimulated emission in 1917, more than forty years before the first laser actually fired in 1960. As long as the upper level holds fewer atoms than the lower one, the device is merely a lamp; only when the pump power is strong enough to push the population inversion ΔN past threshold does the photon count in the cavity surge into a beam. The reflectivity of the output mirror is the interesting trade-off: the tighter the mirror, the stronger the gain inside the cavity but the less light escapes, and a mirror that is too perfect leaves no usable beam at all.
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