兰姆凹陷消失.ppt

三 输出功率与能量 In section 4.1 we derived an expression for the threshold population inversion at which the laser gain becomes equal to the losses. We would expect that as the pumping intensity is increased beyond the point at which N2-N1=Nt the laser will break into oscillation and emit power. In this section we obtain the expression relating the laser power output to the pumping intensity. We also treat the problem of optimum coupling-that is, of the mirror transmission that results in the maximum power output. 激光器稳态工作的建立 连续或长脉冲激光器的输出功率(output power) 短脉冲激光器的输出能量(output energy) In chapter 3 we found that an atomic medium with an inverted population (N2N1) is capable of amplifying an electromagnetic wave if the latter’s frequency falls within the transition lineshape. Consider next the case in which the laser medium is placed inside an optical resonator. As the electromagnetic wave bounces back and forth between the two reflectors it passed trough the laser medium and is amplified. If the amplification exceeds the losses caused by imperfect reflection in the mirrors and scattering in the laser medium, the energy stored in the resonator will increase with time. This causes the amplification constant to decrease as a result of gain saturation. The oscillation level will keep increasing until the saturated gain per pass just equals the losses. At this point the net gain per pass is unity and no further increase in the radiation intensity is possible-that is, steady state oscillation obtains. （一）激光器稳态工作的建立 当外界激发作用增强时，小信号增益系数g0(?)增大，此时I?q必须增大到一个更大的值才能使g(?q,I?q) 降低到gt并建立起稳定工作状态，因此激光器的输出功率增加。 稳态工作时的大信号增益系数总是等于gt 。 结论：在一定的激发速率下，即当g0(?)一定时，激光器的输出功率保持恒定；当外界激发增强时，输出功率随之上升，但在一个新的水平上保持恒定。 （二）连续或长脉冲激光器的输出功率 1、均匀加宽单模激光器 ??腔内平均光强I?q 在驻波型激光器中，腔内存在着沿腔轴方向传播的光I+和反方向传播的光I-，二者同时参与饱和作用 如果T1，稳态工作时增益系数也很小，近似认为I+=I-，腔内平均光强I?q=2 I+。 稳态情况下， ??输出功率 设激光束的有效截面面积为A，则激光器的输出功率为 The total loss encountered by the oscillating laser mode can conveniently be attributed to two different sources: (a) the ine