Free-Space Laser Communication ———————— Wave Optics Simulation of Pseudo-Partially Coherent Beam Propagation Through Turbulence: Application to Laser Communications

It is known that partially (spatial) coherent beams can reduce the effects of scintillation in a free space optical link. However, practical implementations of this type of beam are still under investigation. In this work we apply a wave optics model to analyze the propagation through turbulence of a coherent beam and a particular version of a pseudo-partially coherent beam. The beam we study is created with a sequence of five Gaussian random phase screens for each atmospheric realization. We examine the average intensity profiles, the scintillation index and aperture averaging factor for a horizontal propagation scenario. We find this pseudopartially coherent beam behaves much like a Gaussian Schell-model beam. Proc. SPIE Vol. 6304, 63040L, Sep 2006. Analysis of a Ground to Satellite Optical Link with a Cooperative Satellite Beacon Santasri Basu, David Voelz ABSTRACT Establishing a free-space optical communications link between a ground station and an orbiting satellite can be aided greatly with the use of a beacon on the satellite. A tracker, or even an adaptive optics system, can use the beacon during communications activities to correct beam pointing for atmospheric turbulence and mount jitter effects. Several models have recently been developed to study the effects of turbulence on a ground-to-geostationary satellite uplink, but published results from these models have not incorporated the effects of tracking. Two effects that need to be considered with a tracking system include a pointing lead-ahead issue and an aperture mismatch issue. The lead ahead problem involves the fact that the uplink beam needs to be pointed at the position the satellite will be when the pulse arrives. This problem exists even for a geostationary satellite. The aperture mismatch involves the sensing of the beacon through an aperture that is smaller, or larger, or even displaced from the uplink transmit aperture. In both cases, the sensed tilt might not be the same as the tilt required for optimal transmission to the satellite. These two issues have been studied separately, to some extent, by researchers in the past, although their combined effects in an optical communications uplink have never been investigated. The present paper applies previously published theories in an initial assessment of the impact of these tracking issues in an optical communications uplink. The analysis considers geosynchronous Earth orbit satellites as well as low Earth orbit satellites. Proc. SPIE Vol. 6304, 63041M, Sep 2006.Establishing a free-space optical communications link between a ground station and an orbiting satellite can be aided greatly with the use of a beacon on the satellite. A tracker, or even an adaptive optics system, can use the beacon during communications activities to correct beam pointing for atmospheric turbulence and mount jitter effects. Several models have recently been developed to study the effects of turbulence on a ground-to-geostationary satellite uplink, but published results from these models have not incorporated the effects of tracking. Two effects that need to be considered with a tracking system include a pointing lead-ahead issue and an aperture mismatch issue. The lead ahead problem involves the fact that the uplink beam needs to be pointed at the position the satellite will be when the pulse arrives. This problem exists even for a geostationary satellite. The aperture mismatch involves the sensing of the beacon through an aperture that is smaller, or larger, or even displaced from the uplink transmit aperture. In both cases, the sensed tilt might not be the same as the tilt required for optimal transmission to the satellite. These two issues have been studied separately, to some extent, by researchers in the past, although their combined effects in an optical communications uplink have never been investigated. The present paper applies previously published theories in an initial assessment of the impact of these tracking issues in an optical communications uplink. The analysis considers geosynchronous Earth orbit satellites as well as low Earth orbit satellites. Proc. SPIE Vol. 6304, 63041M, Sep 2006. Wave Optics Simulation Approach for Partial Spatially Coherent Beams Xifeng Xiao and David Voelz ABSTRACT A numerical wave optics approach for simulating a partial spatially coherent beam is presented. The approach involves the application of a sequence of random phase screens to an initial beam field and the summation of the intensity results after propagation. The relationship between the screen parameters and the spatial coherence function for the beam is developed and the approach is verified by comparing results with analytic formulations for a Gaussian Schell-model beam. The approach can be used for modeling applications such as free space optical laser links that utilize partially coherent beams. Optics Express, Vol. 14, Issue 16, pp. 6986-6992, Aug 2006.A numerical wave optics approach for simulating a partial spatially coherent beam is presented. The approach involves the application of a sequence of random phase screens to an initial beam field and the summation of the intensity results after propagation. The relationship between the screen parameters and the spatial coherence function for the beam is developed and the approach is verified by comparing results with analytic formulations for a Gaussian Schell-model beam. The approach can be used for modeling applications such as free space optical laser links that utilize partially coherent beams. Optics Express, Vol. 14, Issue 16, pp. 6986-6992, Aug 2006. Wave Optics Simulation of Partially Coherent Beams X. Xiao, D. Voelz ABSTRACT We describe a numerical, wave optics approach for simulating the propagation of a partially coherent beam. The approach requires the application of a succession of phase screens at the transmit end of the link and the summation of the resulting intensity profiles at the receiving end. To test the approach, a phase screen formulation for a Gaussian Schell-model beam was derived using a Gaussian random phase screen where its parameters are related to a form of the spatial coherence function for the transmittance. The numerical model was applied to several cases of propagation through vacuum and atmospheric turbulence and the resulting average beam intensity profile is shown to be nearly identical to the profile predicted by analytical results for the Gaussian Schellmodel beam. Finally, the scintillation index of a coherent beam was investigated and the numerical result was found to generally follow the theoretical result, especially for source sizes that are relatively small or relatively larger. The model developed in this paper can be applied to analysis and design of free space optical laser links that utilize various partially coherent beams. Proc. SPIE Vol. 5892, p. 219-227, 2005.We describe a numerical, wave optics approach for simulating the propagation of a partially coherent beam. The approach requires the application of a succession of phase screens at the transmit end of the link and the summation of the resulting intensity profiles at the receiving end. To test the approach, a phase screen formulation for a Gaussian Schell-model beam was derived using a Gaussian random phase screen where its parameters are related to a form of the spatial coherence function for the transmittance. The numerical model was applied to several cases of propagation through vacuum and atmospheric turbulence and the resulting average beam intensity profile is shown to be nearly identical to the profile predicted by analytical results for the Gaussian Schellmodel beam. Finally, the scintillation index of a coherent beam was investigated and the numerical result was found to generally follow the theoretical result, especially for source sizes that are relatively small or relatively larger. The model developed in this paper can be applied to analysis and design of free space optical laser links that utilize various partially coherent beams. Proc. SPIE Vol. 5892, p. 219-227, 2005. Laser Satellite Communications with Adaptive Optics C. Ting, D. G. Voelz, M. K. Giles ABSTRACT Laser satellite communications (LCS) appear to have potential applications in the future global communication network. However, bidirectional ground-to-space link performance is degraded by atmospheric turbulence. This paper presents the simulations results of laser uplink and downlink propagation through atmospheric turbulence based on turbulence strength and link specification data obtained from the European Space Agency (ESA) ARTEMIS satellite and its associated optical ground station (OGS). The results obtained using this NMSU bidirectional link model indicate that the uplink performance can be improved significantly using tiptilt error compensation alone while the downlink performance can be improved with higher order adaptive optics compensation. Proc. SPIE Vol. 5892, p. 370-376, 2005.Laser satellite communications (LCS) appear to have potential applications in the future global communication network. However, bidirectional ground-to-space link performance is degraded by atmospheric turbulence. This paper presents the simulations results of laser uplink and downlink propagation through atmospheric turbulence based on turbulence strength and link specification data obtained from the European Space Agency (ESA) ARTEMIS satellite and its associated optical ground station (OGS). The results obtained using this NMSU bidirectional link model indicate that the uplink performance can be improved significantly using tiptilt error compensation alone while the downlink performance can be improved with higher order adaptive optics compensation. Proc. SPIE Vol. 5892, p. 370-376, 2005. Pseudo-Partially Coherent Beam for Free-Space Laser Communication D. Voelz and K. Fitzhenry ABSTRACT The use of laser beams with partial spatial coherence can reduce spatial intensity variations (scintillation) due to turbulence and therefore improve the performance of atmospheric laser communication links. For optimal performance the partial coherence needs to be managed such that there is a balance between scintillation reduction and beam spread. However, the practical creation of a partially coherent beam can be problematic. We discuss a concept where a simplified modulated phase pattern is imparted to a coherent source beam that, along with the inherent time averaging of the photodetection process, produces a beam with partially coherent characteristics. Results from a laboratory demonstration are presented that illustrate the potential application of this concept for free-space laser communications. Proc. SPIE Vol. 5550,p. 218-224, 2004.The use of laser beams with partial spatial coherence can reduce spatial intensity variations (scintillation) due to turbulence and therefore improve the performance of atmospheric laser communication links. For optimal performance the partial coherence needs to be managed such that there is a balance between scintillation reduction and beam spread. However, the practical creation of a partially coherent beam can be problematic. We discuss a concept where a simplified modulated phase pattern is imparted to a coherent source beam that, along with the inherent time averaging of the photodetection process, produces a beam with partially coherent characteristics. Results from a laboratory demonstration are presented that illustrate the potential application of this concept for free-space laser communications. Proc. SPIE Vol. 5550,p. 218-224, 2004.