An experimental study has been made on the contribution to the effective scintillation index due to aperture averaging and interference effects when using multiple retroreflectors in free-space optical links. These studies are of relevance to asymmetric data links where modulating retroreflectors are used at the remote end of the free-space optical link. For closely spaced retroreflectors the effect of coherent interference at the receiver is seen to increase the effective variance of the received signal, whereas spatial averaging is apparent for more widely spaced retroreflectors. The scintillation index, probability density functions, and fade rates are all affected by the interference. Experiments were conducted over both a short-range (500 to 800 m) and a long-range (16 km) link. The long-range link used three retroreflectors having a spacing that was large enough that interference effects contributed minimally to the observed variance. However, the variance clearly depended on the number of retroreflectors. In addition to measuring the scintillation index, images of the returned light were recorded using a fast framing camera. These experiments were conducted over land and mixed water-land terrains in the case of the short-range links, and exclusively over water in the case of the 16-km link from Chesapeake Beach to Tilghman Island.
This paper presents the results of a successful bidirectional free-space optical link across 16 km to a modulated retroreflector array. The link was implemented at the Naval Research Laboratory's Chesapeake Bay Detachment laser test range. A 6-W cw 1550-nm class 1 M interrogation beam was used to illuminate an array of three modulated cat's-eye retroreflectors located on a tower across the Chesapeake Bay on Tilghman Island. The modulated retroreflectors had a diameter of 16 mm and were arranged in a triangular pattern with a spacing of 30 cm. The interrogating terminal employed a 100-µrad divergence and a high-speed pointing and tracking system to maintain link alignment. Link testing occurred over 12 days in the months of September, October, and November of 2006. Topics presented in this paper include the link scenario for the 16-km free-space optical link, the link budget, and terminal designs, as well as link acquisition and performance. Link performance results presented include data transmission throughput, scintillation data, and pointing and tracking results.
Free-space optical links for high-speed network communications between buildings must consider the detrimental environmental effects of terminal base disturbances. Terminal base motion results in tracking and pointing losses, which cause link outages if the base motion is sufficiently large (beyond terminal field of regard) or fast (too fast for the tracking system to reject). Thus it is important to characterize this environmental effect for design and test of optical terminals. Base motion is highly dependent on the installation environment of a specific link making general statements difficult. We have characterized terminal base disturbance levels through a combination of vibration measurements in numerous buildings, data gathered from operating links, and review of building stiffness and wind statistics in various cities. This paper presents a summary of our results.
In this paper, we discuss the results of two experiments for monitoring the performance of optical wireless links. The first experiment is the measurement of the 10-second Bit Error Rate using a commercial transceiver system; the second experiment, which involves hardware assembled at our laboratory, is the measurement of optical power at the receiver at a 1 kHz sampling rate and the subsequent computation of relevant statistics. Both experiments were performed in an urban environment but over different paths with data being collected for several months. The results are compared with visibility measurements taken at local airports and with measurements of the path averaged atmospheric structure constant, Cn2.
In this paper, we present experimental results of dynamic aberration correction based on gradient descend algorithms. The experimental setup included a 37-actuators piezoelectric deformable mirror to distort dynamically an input laser beam and a 37-element micromachined deformable membrane mirror to correct the resulting wavefront distortions. We generated time-varying aberrations using the first mirror and used the light power focused onto a pinhole as our optimization matrix. We programmed a computer to maximize this metric and control the shape of the micromachined deformable membrane mirror for wavefront correction. We implemented in this computer a simple gradient descend algorithm and a stochastic perturbation gradient descent algorithm. We present experimental data on the convergence and stability of this adaptive system for various conditions of dynamic turbulence.
In this paper we address the problem of measuring the performance of optical wireless communication links limited by atmospheric turbulence. Because atmospheric turbulence occurs on a short time scale but depends on meteorological parameters, complete link characterization would require measurement over several days with a 10 ms or less time resolution. We discuss two techniques to characterize the performance of optical wireless communication links. The first technique consists in the measurement versus time of OWC system bit error rate. The second technique consists in the statistical characterization of the optical channel. Finally, we analyze some of the experimental data and compare them to measured C2n.
Adaptive optics can be used to improve the performance of optical wireless communications links degraded by atmospheric turbulence. Accurate wavefront sensing is necessary for some adaptive optics systems to compensate for the effects of atmospheric turbulence. Although the Shack- Hartmann sensor can provide accurate wavefront sensing under controlled conditions, scintillation can restrict the performance of Shack-Hartmann wavefront sensing by creating large intensity fluctuations. These intensity fluctuations can create errors in the wavefront measurement if the intensity dynamic range of the Shack-Hartmann sensor is exceeded. The result of computer simulations which model the performance of the Shack-Hartmann wavefront sensor are presented. Specifically it is shown that the intensity dynamic range of the Shack-Hartmann wavefront sensor can be increased by operating with saturated pixels without an increase in error in the measured wavefront. Operating conditions that maximize the intensity dynamic range of the Shack-Hartmann senor are presented. Experimental results are presented which support the results of the computer simulation.
Adaptive optics can be used to improve the performance of a near-ground laser communication link by reducing intensity scintillation of an optical carrier collected by a down- range receiver. Such an adaptive optics system may include a Shack-Hartmann wavefront sensor to measure the wavefront of a beacon laser, and a deformable mirror to apply the appropriate conjugate wavefront to a transmitted communication beam. In this paper, we investigate the performance of such an adaptive optics system using multiple phase screen Monte Carlo computer simulations. Specifically, we consider the effects of intensity dynamic range of the wavefront sensor and of the aperture size of the adaptive optics on the mean and variance of the received intensity.
The development of high performance line-of-sight optical communication links through the turbulent atmosphere is facilitated by laboratory tests of schemes involving adaptive optics, beam tracking, modulation and coding, aperture averaging, fading statistics, and transmitter/receiver diversity. A water-filled turbulence tube has been implemented to simulate, in some respects, the effects produced on a laser beam when it propagates several kilometers through the air. This tube is being used to investigate on a laboratory scale: aperture averaging, fluctuation statistics, optical path difference, high data rate modulation, and various coding schemes. The liquid- filled turbulence tube causes fluctuations on a slower time scale than does the atmosphere. At low turbulence levels it produces log-normal fluctuation statistics, causes tip-tilt errors similar to those previously observed for atmospheric paths, and has already allowed evaluation of aperture averaging and fade statistics. It also allows the testing of various technological schemes to deal with atmospheric turbulence effects without any specific assumptions, such as weak Kolmogorov turbulence, being built into the model.
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