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lasing threshold : ウィキペディア英語版
lasing threshold
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.
==Theory==
The lasing threshold is reached when the optical gain of the laser medium is exactly balanced by the sum of all the losses experienced by light in one round trip of the laser's optical cavity. This can be expressed, assuming steady-state operation, as
:R_1 R_2\exp(2g_\text\,l) \exp(-2\alpha l) = 1.
Here R_1 and R_2 are the mirror (power) reflectivities, l is the length of the gain medium, \exp(2g_\text\,l) is the round-trip threshold power gain, and \exp(-2\alpha l) is the round trip power loss. Note that \alpha>0. This equation separates the losses in a laser into localised losses due to the mirrors, over which the experimenter has control, and distributed losses such as absorption and scattering. The experimenter typically has little control over the distributed losses.
The optical loss is nearly constant for any particular laser (\alpha=\alpha_), especially close to threshold. Under this assumption the threshold condition can be rearranged as
:g_\text = \alpha_ - \frac \ln (R_1 R_2) .
Since R_1 R_2 < 1, both terms on the right side are positive, hence both terms increase the required threshold gain parameter. This means that minimising the gain parameter g_\text requires low distributed losses and high reflectivity mirrors. The appearance of l in the denominator suggests that the required threshold gain would be decreased by lengthening the gain medium, but this is not generally the case. The dependence on l is more complicated because \alpha_ generally increases with l due to diffraction losses.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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