It is well known that atmospheric turbulence diminishes the performance of laser communications systems. Among the multiple degradations caused by turbulence is fading and surging of the received signal, usually referred to as scintillation. If a minimum probability of error receiver is employed for on-off keying (OOK), it is necessary to understand the two conditional probability densities (pdfs) corresponding to the transmission of ones and zeros. These probability densities are the distributions of signals received when the laser is on when sending binary ones and when the laser is off sending binary zeros. Many theoretical studies have determined the expected forms of the pdfs. An ongoing experimental study operating a low-power, low data rate link over a range of 9.3 Km has been started at Colorado State University-Pueblo to carefully examine the effects of atmospheric turbulence on laser communications. Experimental models of actual, true and typical pdfs have been obtained. The results do not always match theoretical predictions. The non-stationary nature of these pdfs is also a problem that must be addressed. This paper summarizes the experimental testing and shares a number of its conclusions.
It is well known that atmospheric turbulence causes degradations in the performance of atmospheric links. The focus of this study is the effect of scintillation on the performance of optical On-Off keying (OOK) links in horizontal atmospheric paths. A new approach to improving the performance of OOK links in the atmosphere is presented in this paper: optimum adaptive thresholding for anticipated, future scintillation values. We look first at the non-time varying irradiance case. The theory of minimum probability of error receivers for this case is well established. When the link operates in a turbulent atmosphere, however, this theory will not necessarily hold. A new adaptive predictive control (APC) approach is described, and to test its effectiveness, a specific link is assumed and the expected BER performance with and without APC was determined by computer simulation. We see that all measures of the BER are better for the APC receiver than the fixed threshold receiver. APC is shown to illustrate a simple way to improve the effectiveness of atmospheric links. Other areas of laser communications that APC might potentially benefit are all processes that satisfy the two criteria of slow time variation and sample-to-sample correlation.
By combining an aspherical reflector and a thick lens at its center, it is possible to produce an ultra wide-angle optic called the GEM. The device was originally conceived as means for broadcasting optical signals over nearly a full angular sphere. Since the fabrication and testing of several GEMs, wide angle receiving and imaging functions have also been evaluated.
Atmospheric turbulence effects on multigigabit/s data rate, pulse-position modulation (PPM) laser downlinks are examined for M = 4 or less using laser transmitter power of 100 mW to 1 W. From the impulse response curves, it is shown that the pulse spreading can be about 5 ps for strong turbulence and about 0.05 ps for weak turbulence. Thus in clear air turbulence without clouds and fog, the contribution from the stretching of a short pulse of 50 ps width will be negligible in both strong and weak turbulence. The high-data rate communication system performance is analyzed in terms of a BER using a thermalized equation derived here. It is concluded that, for the weak turbulence case, the required laser transmitter power to achieve a BER of 10 to the -8th is about 700 mW of power. For the strong turbulence case, a peak laser transmitter power of 1 kW is required to achieve a BER of 10 to the -5th.