Propagation properties of cross beam in turbulent medium are studied. A cross beam is constructed by the sum of two
highly asymmetric Gaussian beams placed along transverse axes. It is known that such beams, when propagating in free
space, will exhibit contrasting diffraction behaviours; they expand widely in one axis, while they are almost non-diffracting
in the other axis within useful link lengths. This behaviour allows detecting the two components and a sum
component if desired separately with a practical multiaperture receiver. Bearing in mind that this property can be
exploited for a diversity scheme, our present work focuses on the propagation of such beams in turbulent atmosphere. To
this end, starting with a source field expression of the cross beam, the second order mutual coherence function is
formulated at the receiver plane. Intensity plots describing the dependence on the source and propagation parameters on
the receiver plane are provided. The results tend to confirm the applicability of the concept provided the design
parameters are appropriately chosen. For a decisive assessment, however, turbulence-induced beam wander must also
be examined.
Propagation of higher order annular Gaussian (HOAG) laser beams in free space is examined. HOAG beams are defined as the difference of two Hermite-Gaussian (HG) beams; thus, they can be produced by subtracting a smaller beam from a larger beam, that are cocentered and both possess HG mode field distributions. Such beams can be considered as a generalization of the well-known annular Gaussian beams. We formulate the source and receiver plane characteristics and kurtosis parameter of HOAG beams propagating in free space and evaluate them numerically. In comparison to HG beams, HOAG beams have a broader beam size with outer lobes of kidney shape. The amount of received power within the same receiver aperture size, that is, power in bucket, is generally lower for higher order beams. The convergence of the kurtosis parameter to an asymptotic value for higher order beams takes much longer propagation distances compared to zero-order beams.
The design of any optical system operating in the atmosphere requires previous investigations of atmospheric turbulence conditions at the system's location. In addition, if outdoor optical measurements are to be useful in assessing the performance of the optical system, or in checking propagation theory, they must be supported by simultaneous direct measurements of the atmospheric turbulence. Stellar observations provide a useful and convenient means for these purposes in the case of ground-satellite optical communications. We first review the refractive index structure parameter profiling techniques based on stellar observations. For implementation we have selected the technique based on spatiotemporal analysis of captured speckle patterns. There is ample evidence that turbulence in the free atmosphere confined to thin, horizontal layers separated by nonturbulent regions. The lifetime of such layers are of the order of several hours. The technique allows us to detect these layers, giving simultaneously the altitude, horizontal mean wind velocity and integrated refractive index value for each 1 km interval. We briefly describe the experimental setup and the measurement technique. We then present some sample results. Initial results show that the current state of the experimental setup and the processing algorithm can detect only the most prominent layers, and altitude accuracy is less than expected. We are currently looking for ways to improve the technique to allow detection of more layers.
Atmospheric turbulence severely degrades the performance of uplinks. Employment of adaptive optics to enhance ground-space laser communication systems has recently been considered and possible benefits have been shown. Uplink scintillation reduction by using multiple transmitters is also being considered. What appears to be currently missing in these works is the realization that transmitter beam-size is a crucial design parameter and its optimum value changes continuously according to changing turbulence conditions along the propagation path. In this paper we consider a configuration where uplink transmitter beam-size is controlled in real-time in response to measured turbulence parameters to maximize mean intensity and minimize fluctuations on the satellite receiver. Controlling the beam-size, especially by a factor of two or so, in relatively long time scales (seconds) should not be a problem. Although it is obvious that this will greatly improve the performance of especially a variable data rate communication system, we find that the current state of knowledge is fragmentary and insufficient to quantify the improvement in any general and sufficiently accurate way. Semi-analytic simulations can be useful in exploring the improvement for different scenarios and site conditions
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