飞秒脉冲激光穿越大气层的特性

Abstract

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°.