Free-space optical (FSO) links for high-speed communications between buildings must consider detrimental environmental effects including interference from sunlight in the receiver's instantaneous field of view (IFOV). Sunlight can degrade receive sensitivity resulting in link disruptions, even with significant optical filtering. Thus it is important to characterize this environmental effect for designing and testing optical transceivers. Background light levels are highly dependent on the geometry and environmental conditions of a specific link making general statements difficult. However, we have characterized the likelihood and frequency of direct or reflected sunlight passing into or near a terminal's IFOV. We have also measured detector solar power levels under sunny and partly cloudy conditions, and measured detector sensitivity degradation as a function of background light levels. This paper presents a summary of our results.
Under favorable visibility conditions, scintillation becomes the limiting factor in estimating free space optical (FSO) link availability. In the summer of 2001, Terabeam performed several experiments to characterize the impact of scintillation on FSO products under development. In the experiment, a transmitter and a receiver were placed 1000 and 2600 meters apart and operated with the receiver at several different sized apertures and under diverse atmospheric conditions. The experimental probability of fade is then calculated and compared with two theoretical models, namely the lognormal and the gamma-gamma.
Free space optical communication systems deployed in office buildings are subject to transmission loss through windows. Window attenuation varies between 0.4 and more than 15 dB. Window attenuation values are required to calculate communications link power budget and availability. But direct measurement of window attenuation in high-rise buildings is difficult since it requires access to both sides of the window. In this paper, we present a method of measuring optical attenuation from the interior side of a window. This method is based on measuring back reflections of a laser beam propagating through a semi-transparent dielectric medium, thus eliminating the need for access to the exterior of a building. In this system, a laser beam is launched at 45 degree(s) to normal incidence in order for the user to discriminate between the various reflections from the dielectric interfaces within the medium. A photodetector is then moved through the plane of incidence and the intensities of reflections from interfaces within the medium are measured. A simple formula is used to calculate total transmission of the optical system based on the relative intensities of the incident light beam and all resulting reflections. In this approach, it is assumed that the reflectivities of the first and final interfaces are identical. The index of refraction for glass from one commercial fabricator varies little; hence the reflectivity of uncoated air-glass interfaces in a particular window is the same. The intensity of the reflection from the final interface is attenuated by the entire medium twice. By comparison of the incident, first, and final reflected intensity a transmitted intensity can be determined. The same equation is used for a medium with any number of dielectric interfaces. A measurement of optical loss through a window without access to both sides of the medium is now possible. This method has been demonstrated to be accurate (+/- 1dB) through various windows with optical losses of up to 12dB.
We have found that highly efficient waveform recall is possible in coherent transient systems in which the storage is optically thick. Coherent transients may be used in a variety of information storage and processing applications with advantages over traditional electronic methods. However, it is believed that a serious problem in application of photon echoes in practical systems is the relatively low efficiency of the process. We show in our numerical studies that waveform recall efficiencies greater than unity can be achieved in absorbing media with appropriate choice of absorption length and brief pulse area, even for very weak data pulses. We also present our preliminary experimental results in Barium vapor in which efficiencies of 50% were obtained for both the stimulated and two-pulse photon echoes.