Milonni P W, Thode L E
Appl Opt. 1992 Feb 20;31(6):785-800. doi: 10.1364/AO.31.000785.
We describe a two-level model for the mesospheric sodium resonance fluorescence excited by a train of pulses. For pulse durations that are short compared with the 16-ns natural lifetime, population-rate equations are inadequate and must be replaced by density-matrix (Bloch) equations. We briefly contrast these two approaches and discuss several issues associated with pulse-train excitation. Analytical approximations to averages over atomic velocities and the transverse spatial profile of the laser are described. Estimates of return photon numbers for trains of pulses, ranging in duration from tens of picoseconds to half a nanosecond, are made and these estimates are compared with those of more conventional long-pulse lidar schemes. Roughly similar photon numbers are predicted for both long and short pulses whenever their average intensities are comparable. For average intensities, less than ~ 10 W/cm(2), trains of asymptotically equal to 0.5-ns pulses yield greater photon returns than trains of asymptotically equal to 30-ps pulses. For larger intensities the reverse can be true.
我们描述了一种用于由一系列脉冲激发的中层钠共振荧光的两级模型。对于与16纳秒自然寿命相比很短的脉冲持续时间，速率方程并不适用，必须用密度矩阵（布洛赫）方程来代替。我们简要对比了这两种方法，并讨论了与脉冲序列激发相关的几个问题。描述了对原子速度和激光横向空间分布的平均值的解析近似。对持续时间从几十皮秒到半纳秒不等的脉冲序列的返回光子数进行了估计，并将这些估计值与更传统的长脉冲激光雷达方案的估计值进行了比较。只要长脉冲和短脉冲的平均强度相当，预测的光子数大致相似。对于平均强度小于约10 W/cm² 的情况，渐近等于0.5纳秒的脉冲序列产生的光子返回比渐近等于30皮秒的脉冲序列更多。对于更大的强度，情况可能相反。
