|
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 :. Here and are the mirror (power) reflectivities, is the length of the gain medium, is the round-trip threshold power gain, and is the round trip power loss. Note that . 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 (), especially close to threshold. Under this assumption the threshold condition can be rearranged as :. Since , both terms on the right side are positive, hence both terms increase the required threshold gain parameter. This means that minimising the gain parameter requires low distributed losses and high reflectivity mirrors. The appearance of 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 is more complicated because generally increases with 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）』 ■ウィキペディアで「lasing threshold」の詳細全文を読む スポンサード リンク
|