There are many challenges in characterizing open-air laser propagation. Varying measurement methodologies with diverse instrumentation can provide contradictory results for both optical turbulence and extinction. This paper assesses instrumentation accuracy w.r.t. a propagating laser and explores the optimal experimental setup. Accordingly, an open-air experiment was conducted to characterize the atmosphere and high-energy laser (HEL) propagation with commonly deployed instrumentation. Presented here is a comparative evaluation of the instrumentation results to far-field HEL measurements. The differential image motion monitor (DIMM) and the wide angle tele-radiometric transmissometer (WATT) provided measurements with the lowest mean percent error for optical turbulence and extinction, respectively. This suggests that path-integrated dual-sided instrumentation outperforms nodal measurements. However, it is found that nodal measurements perform best near and at the laser’s aperture height if dual-sided instrumentation is not available. Additionally, a quality-control routine is outlined for all deployed instruments considered in the evaluation. The experimental results in this paper yield instrumentation performance for characterizing static laser propagation over land. Further research is recommended to assess instrumentation over longer optical paths both static and dynamic.
Free Space Optical links suffer from atmospheric effects where turbulence causes the beam to break up (scintillation), which could increase the variance in the signal at the receiver and ultimately worsening the optical link. Various techniques to reduce scintillation exist to alter how the atmosphere will affect the beam, one of them being wavelength diversity of the optical source. Diversifying the wavelength can reduce the scintillation of the optical beam due to the wavelength dependence on the refractive index of the atmosphere. An experiment was conducted comparing a broadband laser source and a monochromatic source over an instrumented 13.5km path. Beam profile and scintillation measurements were conducted along with BLS-2000 Cn2 measurements. This experiment investigates the effects of a short-coherence length / broad-band nature of a source and its ability to reduce scintillation in turbulent atmosphere. This paper will discuss the experimental setup, analysis, and conclusions of this novel experiment.
We explore the impacts of adverse weather on the propagation of a pulsed 1.5um laser source over a 1km maritime channel. The propagation path along this channel is well-instrumented with sensors to measure standard weather conditions (wind, temperature, humidity and rainfall), visibility and atmospheric turbulence. Data collected to characterize the propagation path are used to initialize channel modeling and predict performance of 1.5um propagation. A high-speed detector and a camera located at the target board recorded temporal and spatial effects of rain on the propagated laser beam. The data are analyzed for pulse width, beam profile, beam wander as a function of rainfall and compared to the channel model.
Optical link prediction capabilities are important for planning and operating a free-space, optical communication link outdoors. The impact of the atmospheric channel on the optical link is dependent on factors like geographic location and local atmospheric conditions. Currently, link predication and analysis tools, like the Navy Atmospheric Vertical Surface Layer Model (NAVSLAM) and the Path Characterization System (PCS), are used to predict the atmospheric characteristics and received power. Additionally, the Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS) is a model that has recently been integrated into PCS and needs to have its prediction of refractive index structure parameter (Cn2) validated from outdoor range data. The bit error rate is a measure of error probability and when combined with baud rate, can be used to predicted the data transmission rate. In this experiment, the characterization of a one-way laser communication link was conducted outdoors on a 1 km range in a littoral environment. During communication, the bit error rate and received power were measured on the receiver side of the link. Additionally, the local atmospheric conditions were measured and utilized by link analysis tools, including the NAVSLaM, COAMPS, and PCS. These tools have been used to quantify the turbulence and path losses and evaluated against range measurements. The bit error rate and received power were also computed and compared with the measurements on the range to evaluate link predictive capabilities.
Propagation of laser beams through a turbulent atmosphere over extended ranges can cause significant beam scintillation and wander which can degrade the effectiveness of a Free Space Optical (FSO) link. The use of a spectrally broadband laser light source, with a high spatial coherence and short temporal coherence, could lead to improved performance in one or both of these areas. This experiment investigates the effect of temporal coherence on the far-field turbulence induced effects on the beam. Narrow linewidth coherent sources were compared against a broadband source over a 13.5 km slant-path. The path was instrumented with a path averaged turbulence monitoring device during data collection along with a range of other meteorological parameters to predict atmospheric parameters. Target board beam profile data was collected to measure the spatial statistics due to atmospheric turbulence along with silicon detectors to measure the temporal statistics of the atmospheric turbulence effects. This data is analyzed and compared to full diffraction wave propagation simulation results. Our analysis shows the benefit that the broadband source does not suffer as many scintillation effects as the narrow-linewidth sources.
Free space optical communication utilizing modulating retro-reflectors (MRR) can greatly reduce the complexity of a system in both pointing requirements as well as the necessity for a laser transmitter at both ends of the link. Retroreflectors are susceptible to the same atmospheric turbulence effects of scintillation and beam wander of any laser communication system. An MRR link using an array (N>1) of retroreflectors is affected by self-interference of the return beams. This self-interference can create additional fluctuation that compound to increase the apparent scintillation of the received signal. Data were collected over a 1km outdoor path on the interference pattern returned from a pair of 7mm and 12.5mm retro-reflectors, with multiple spacing distances, in varying turbulence regimes with a 1550nm and 1070nm laser. The interference data of the retroreflectors were correlated with Cn2 data collected simultaneously over the same 1km horizontal path. Under weak turbulence, the self-interference fringes matched diffraction theory, under stronger turbulence regimes the self-interference fringes were either visibly reduced or completely destroyed. We also analyze the contrast of the interference fringes as a function of wavelength for varying turbulence regimes as well as the ability to measure Fried’s parameter from the retroreflector spacing and the returned self-interference pattern.
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