Picosecond Laser Pulse Distortion by Propagation through a Turbulent Atmosphere

Abstract

We have been investigating the influence of atmospheric turbulence on the propagation of a picosecond laser pulse. The figure of merit of presented results is the time of propagation, its absolute delay and jitter. Phase wavefront deformation or beam profile changes were not studied. The correlation of the atmospheric turbulence with the propagation delay fluctuation was measured. The research was motivated by the needs of highly precise laser ranging of ground, air, and space objects; and highly precise and accurate time transfer ground-to-space and ground-to-ground by means of picosecond optical laser pulse. Firstly for comparison, lets briefly summarize the effects of a turbulent atmosphere to continuous laser beam. The total effect of atmospheric turbulences on a continuous laser beam propagation is a highly complex subject. Atmospheric turbulences can be defined as random spatial variations in the refraction index of the atmosphere resulting in a distortion of the spatial phase fronts of the propagating signal. Spatial phase front distortion induces the variable path of light energy and thus all effects described later on. Variations of the refraction index are caused by the turbulent motion of the atmosphere due to the variations in temperature and gradients in the water vapour. Following (Degnan, 1993), the optically turbulent atmosphere produces three effects on low power laser beams: 1) beam wander, 2) beam spread and 3) scintillations. Severe optical turbulence can result in a total beam breakup. Beam wander refers to the random translation of the spatial centroid of the beam and is generally caused by the larger turbulent eddies through which the beam passes. In astronomical community it is usually referred as seeing. Beam spread is a short term growth in the effective divergence of the beam produced by smaller eddies in the beam path. The two effects are often discussed together in terms of a “long term” and “short term” beam spread. The “long term” beam spread includes the effects of beam wander, whereas the “short term” beam spread does not. For more details, see (Degnan, 1993). Maximum turbulence occurs at mid-day in the desert (low moisture) under clear weather conditions. For the usual laser wavelength of 532 nm one can expect 2.4-4.6 cm for the coherence length at zenith angles of 0° and 70° respectively. At the tripled Nd:YAG wavelength (355 nm) the corresponding values are 3.1 and 1.6 cm (Degnan, 1993). Turbulence induced beam spreading will only have a significant impact on beam divergence (and hence signal level) if the coherence length is on the order of, or smaller than, the original effective beam waist radius. Since a typical 150 μrad beam implies an effective waist radius of 2.26 mm, the effect of beam spread on signal level for such systems is relatively small, i.e. a few percent.