Electromagnetic random beams with non-uniform source correlations have been recently shown to develop, on propagation in free space, the regions in transverse cross-sections where the degree and the state of polarization can significantly differ from those beyond that region. The size of the region and the values of polarimetric properties in it can be fully controlled from the source plane. In this paper the influence of a random isotropic medium on such beams is shown to suppress the effect in several ways, in particular by shifting the location of the region back to the axis.
©2012 Optical Society of America
The possibility of prescribing the free-space evolution of random beams generated by spatially non-uniformly correlated sources has been recently illustrated in  on the basis of the general theory developed in . Unlike the classic Gaussian Schell-model sources, for which the typical correlation size, say δ, is the same everywhere in the source plane, the sources in  can have a region where δ is greater than elsewhere. In this latter case the weaker diffraction of light from the region with larger δ and stronger diffraction from regions with smaller δ lead, on propagation at some distance, to redistribution of intensity benefiting the region with larger δ. Hence non-uniform source correlations can result in peculiar self-focusing effects, both on and off the beam axis. Moreover, if the intensity spike is set to be off the axis it is shown to remain at the same transverse location as the propagation distance increases. The intensity enhancement effect is generally suppressed if the beam propagates in the presence of a random isotropic medium, for example, the turbulent atmosphere . It still can occur but both its strength and its displacement from the axis are reduced.
Recently the electromagnetic [EM] generalization of model in  was introduced . Just like the intensity distribution in the scalar model, the polarization properties of electromagnetic non-uniformly correlated beams have been shown to form regions with much larger or much smaller values of polarimetric parameters compared to the values outside of that region. In this article we explore the behavior of EM non-uniformly correlated beams in greater detail and determine the effect of the isotropic and homogeneous atmospheric turbulence on polarimetric modulation.
2. Electromagnetic non-uniformly correlated beam propagation in atmosphere
Let us consider an electromagnetic non-uniformly correlated field propagating from the source plane into the half-space containing turbulent atmosphere. The cross- spectral density matrix of such a beam in the source plane has form 2]Eq. (1) to be written in the form of Eq. (2) we may set4].
The paraxial form of the extended Huygens-Fresnel principle which describes the interaction of waves with random medium implies that the elements of the cross-spectral density matrix at two positions and in the same transverse plane of the half-space are related to those in the source plane as [5, 6]Eq. (5) becomes 
The elements of the matrix (7) can then be found on combination of Eqs. (9) and (10) with formula (3) and applying numerical integration. This article will be only concerned with evaluation of the distribution of spectral density (intensity at fixed frequency) and the degree of polarization which are related to the cross spectral density matrix by expressions
We will assume here that the turbulent atmosphere is isotropic, homogeneous and governed by statistics described by a generalized model for the power spectrum Ф(к) , in which the slope 11/3 of the conventional van Karman spectrum is generalized to an arbitrary parameter α, i.e.Eq. (13) is a generalized refractive-index structure parameter with units m3-α. With the power spectrum in Eq. (13) we have
3. Numerical examples
We will now illustrate the behavior of the spectral density and the degree of polarization of typical beams generated by source (1) on interaction with atmospheric turbulence (13) by a number of numerical examples. Unless specified in the captions, the source and the medium parameters are chosen as follows: , .
The 2D and 3D transverse distribution of the beam’s spectral density at two selected distances z = 30 m and z = 1 km are shown in Figs. 1 , 2 , and 3 . One sees from Fig. 1 that, unlike the uniformly distributed beam, the non-uniformly distributed beam may generate two different spectral density peaks depending on the choices of the values of shifts at z = 30 m. Such trend diminishes at a larger distance, z = 1 km.
This effect can be more clearly seen from the 3D version in Figs. 2 and 3. Hence the control of the peak or even the distribution of the spectral density at certain transverse plane along the propagating path may be fulfilled by a careful selection of the shift values.
In Figs. 4 and 5 we show the shift of the intensity center of the beam at certain transverse plane on propagation and its spectral degree of polarization at the intensity center for selected source and turbulence parameters. One sees that although the spectral degree of polarization varies slightly with α for non-Kolmogorov turbulence, it alters significantly with the change in source correlation length δxy.
The number of turning points of the degree of polarization, which is a very special property of the non-uniformly distributed beams, increases and becomes more evident with the increase of the coherence length δxy. Unlike in the case of free space propagation , where the value of the degree of polarization saturates to certain level after propagating at long distances, it returns to its value at the intensity center at the source plane gradually resulting in one more turning point (compared to free space propagation) due to the uniformly correlated turbulence.
One more interesting presentation of the distribution of the spectral degree of polarization at z = 30m and z = 1 km in the three-dimensional version is shown in Figs. 6 and 7 . The maximum of the polarization profile is obvious in free space and depresses somewhat in non-Kolmogorov turbulence, with the largest depression corresponding to α = 3.05.
Thus we have illustrated that the EM non-uniformly correlated beams propagating in random isotropic media, such as atmosphere, at relatively small distances, can still form the regions in transverse cross-sections where the polarization properties are modulated differently from the rest of the cross-section. However such modulation is gradually suppressed by medium fluctuations, both in strength and in location, with respect to the optical axis. In the current paper we have also explored the new possibility of the EM non-uniformly correlated beams to form a controllable combination of the off-axis intensity and degree of polarization maxima, due to its vectorial nature.
Random EM beams carry huge potential for atmospheric applications since the combination of partial coherence and partial polarization, being two means of diversity, lead to significant decrease in scintillation and, hence, improve the receiver’s signal-to noise ratio. If, additionally, the strong self-interference effects leading to drastic intensity and polarization modulation are possible such beams become a unique class for atmospheric communication and sensing systems.
Z. Mei’s research is supported by Zhejiang Provincial Natural Science Foundation of China (Y6100605). O. Korotkova’s research is supported by grants N00189-12-T-0136 and FA9550-12-1-0449.
References and links
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