飞秒脉冲激光穿越大气层的特性

Propagation characteristics of femtosecond laser pulses through atmosphere

对不同脉宽、不同中心波长、不同入射天顶角情况下飞秒激光脉冲在大气层中的传输特性进行了数值模拟.理论计算结果表明,在50 km高空大气的二阶色散值为3.44×10-6 ps2/km,与大气在海平面的二阶色散值(2.09×10-2 ps2/km)相比,约小4个数量级;在激光脉冲非垂直发射情况下,当入射天顶角小于60°时,谱宽为40 nm的飞秒激光脉冲的蒙气差值、大气角色散值以及时域色散导致的脉冲展宽量均会随入射天顶角缓慢增加,当入射天顶角大于60°时,这种增长会急剧变大.大气的角色散作用还会使传输光束的横向尺寸和光谱空间分布发生改变,但其改变量要远小于直径为200 mm的光束的衍射效应产生的相应变化.在垂直发射条件下,对于厚度为50 km的大气层而言,800,1064,1550 nm三个不同中心波长的瑞利脉冲宽度分别为700,605,495 fs.由于入射天顶角的增加直接导致有效传输光程增加,随着入射天顶角由0°增至90°,瑞利脉宽单调增加,最大可增至垂直发射时的3倍左右.

The propagation characteristics of femtosecond laser pulses of various temporal durations and central wavelengths are numerically investigated. These characteristics are required when transmitting pulses through the atmosphere at different incident zenith angles. The second-order calculations show that at altitudes of 50 km the dispersion is about 3.44×10-6 ps2/km, significantly less than the corresponding value of 2.09×10-2 ps2/km at sea level. For incident zenith angles below 60°, the difference in the refraction of the various spectral components of femtosecond laser pulses, as well as the pulse duration expansion caused by dispersion, rises gradually with increasing incident zenith angle; for zenith angles above 60°, the rise is much higher. Whereas atmospheric angular dispersion can lead to changes in the spatial distribution of the spectrum and also the transverse dimension of the transmitted beam, such changes in angular dispersion for a 200-mm diameter beam are negligible when compared with the change in diameter from diffraction. Numerical calculations predict that the Rayleigh pulse width for 50 km thick atmosphere at the 0° incident zenith angle are 700, 605, and 495 fs respectively for wavelengths centered at 800, 1064, and 1550 nm, respectively. Because an increasing incident zenith angle will inevitably cause an increase in the effective optical path length, the Rayleigh pulse width at different wavelengths will rise monotonically for angles from 0° to 90°. Near the zenith angle of 90°, this width can be more than three times larger than that at the zenith angle of 0°.