What makes a laser a laser? A sharp threshold: pump it too weakly and it just glows, but cross the line and stimulated emission takes over.
Laser CavityLive
gain, loss & threshold
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A laser fires only when the optical gain from the pumped medium exceeds the losses of the mirror cavity. Below that threshold it merely glows like a lamp; above it, stimulated emission takes over and output rises steeply and linearly with pump power. This sharp threshold is the defining signature of laser action.
Reading this result: Well above threshold, output climbs almost linearly with pump; the slope (~0.8 here) is the slope efficiency, and lowering cavity loss shifts the whole knee left.
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How it works
Laser action requires the optical gain of the pumped medium to overcome the losses of its mirror cavity. Below threshold the device emits weak, incoherent light; above it, output climbs steeply and linearly with pump power as stimulated emission dominates. This threshold behavior distinguishes a laser from an ordinary lamp or LED.
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