theresold gain coeffcient
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The lasing threshold is the lowest excitation level at which a laser's output is dominated by stimulated emission rather than by spontaneous emission. Below the threshold, the laser's output power rises slowly with increasing excitation. Above threshold, the slope of power vs. excitation is orders of magnitude greater. The linewidth of the laser's emission also becomes orders of magnitude smaller above the threshold than it is below. Above the threshold, the laser is said to be lasing. The term "lasing" is a back formation from "laser," which is an acronym, not an agent noun.
The laser threshold is the least gain which is equal to the rasonator loses. It occurs for the threshold pump-power. It causes an increasement in the optical power of the rasonator. It can be considered as the basic principle of all kind of lasers. In which region the energy of the laser is transmitted, is radiated and it results in the increasing of the optical power. It can be also considered as the amplification of the light. When the light energy is used in threshold pump then the upper layer transfers energy to the lower layer by the process of radiation and equalize the amount of energy in both the layers.