A multiple-input single-output configuration is used to study the effectiveness of spatial diversity in reducing single-photon losses caused by atmospheric turbulence in quantum communications over free-space. The system consists of two transmitters and a single receiver, where two parallel beams are transmitted, each comprising a 660-nm beacon and a 515-nm collinear single-photon signal. The beacon is intended to compare the turbulence conditions on each path to determine the most suitable transmitter for the single-photon signal. An outdoor experimental campaign with a propagation distance of 500 meters was conducted to obtain results under real turbulence conditions. First, the correlation between the 515-nm and 660-nm collinear signals was measured at 0.82, showing that the beacon signal accurately represents the state of turbulence affecting the single-photon signal most of the time. The spatial correlation was measured at separation distances between the transmitters of 12 and 20 centimeters, yielding correlation coefficients of 0.35 and 0.17, respectively. As expected, the spatial correlation decreases with increasing separation distance. Finally, a slight improvement in the detection of the single-photon signal was observed when using spatial diversity, but further improvements to the system and longer propagation distances are needed to obtain more promising results.
Laser communication link performance in free space depends heavily on atmospheric conditions present on the propagation path. Distortions due to atmospheric turbulence, such as scintillation and beam wander, greatly diminish signal detection and performance. With the final goal of improving the communication link, experimental measurements and analysis of turbulence strength are presented as metrics for determining the system’s detection limits. Experimental optical trials were recorded over a 1-km horizontal path in order to study the intensity fluctuations, beam dance, and the spatial spectrum properties under different regimes. By taking into account wind speed and vibrations of the building where the measurements were carried on, correlations between variables are shown with the use of a photodetector, and a 2D lateral effect position sensor.
Accurate measurement of laser light phase after propagation through underwater optical turbulence is crucial for defense and commercial applications like underwater communications and sensing. Traditional phase-measuring methods, like Shack-Hartmann wavefront sensors, have limited effectiveness in strong optical turbulence. The Gerchberg-Saxton (GS) method utilizes synchronized intensity images in the image and Fourier planes and retrieves the phase through an iterative algorithm. We evaluate the Gerchberg-Saxton algorithm's accuracy for laser light propagation through simulated Kolmogorov turbulence and experimentally generated Rayleigh-Bénard (RB) natural convection. The results of the phase retrieved from the experimental data recorded in pupil and focal planes are compared with the phase measurements from a Shack-Hartmann sensor. We tested the efficacy of the Gerchberg-Saxton algorithm to estimate the phase of laser light upon propagation through underwater optical turbulence.
An experimental campaign for the propagation of laser modes carrying orbital angular momentum (OAM) over 1 kilometer has been established at UANDES university campus. We describe our method for estimating OAM spectra and the average topological charge values from the images delivered by Shack-Hartmann sensor. For OAM beams transmitted with a single topological charge we analyze the average departure of the measured charge with respect to the intended one and the spread of these values as a function of turbulence strength.
In order to improve laser communication link performance, turbulence strength is an important parameter to characterize the system’s correcting limits and detection availability. By setting an experimental optical link through free space horizontal propagation in a 1-km path, we study the strength of different turbulence scenarios through the Rytov approximation and scintillation of the beam, and compare methods of experimental detection of the refractive-index structure constant of the turbulence, C2n . Results show that, under low and medium turbulence regimes, both methods behave similiarly as a way to predict C2n ; however, with larger turbulence strength, the beam’s displacements in the focal plane are more sensitive than the intensity fluctuactions.
Spatial diversity is a technique widely used in wireless communications to enhance the signal quality at the receiver. We propose a multiple-input single-output system that utilizes this technique to enhance a free-space optics quantum communication link by reducing the amount of photon losses caused by atmospheric turbulence, thus increasing the capacity of the quantum channel. The system consists of two transmitters with uncorrelated optical paths, and a single receiver. A 515-nm quantum signal is transmitted through the transmission path with the highest gain, dynamically chosen by comparing the signal distortions of a 660-nm classical signal. Preliminary experiments with a single transmitter have been conducted in a laboratory environment with atmospheric turbulence generated via heat guns. We observed that the single-photon channel is highly correlated with the fluctuations of the 660-nm classical signal, so that an improvement in the former is expected when selecting the path with highest gain. The number of photon counts received was compared with the turbulence-free scenario, revealing that the mean number of counts decreased, and its standard deviation increased when turbulence is present, as expected.
We present an experimental campaign consisting on the propagation of laser modes with orbital angular momentum (OAM), carried out in our campus, for a total propagation distance of 1 kilometer. In this proceedings article we describe our experiment and describe some preliminary results, using topological charges up to |ℓ| = 45. We also demonstrate that a Shack-Hartmann sensor may be used for OAM sensing in the presence of weak to intermediate turbulence without the help from adaptive optics.
An Adaptive Optics (AO) system may offer an alternative to compensate and correct for beam degradation by reducing turbulence distortions that affect signal detection over horizontal propagation. Based on an experimental testbed placed in the laboratory, we simultaneously study the effects of the communication signal detection, beam wavefront and image quality using a continuous membrane-type deformable mirror and Shack- Hartmann wavefront sensor. By inducing distorting effects on the beam with a Spatial Light Modulator and turbulence masks that are Rytov variance-equivalent to that of actual atmospheric scenarios, and by employing a Zernike polynomials decomposition, beam correction was achieved and signal detection improved. Our results show that both beam-spreading and beam-wandering were reduced after correction, but more significantly, the beam's intensity percentage over detector surface increased in 164%. Future improvements are discussed as an experimental campaign is being prepared to evaluate a closed-loop AO setup for an FSO communication link over a 400-m range at the university campus to evaluate the effectiveness of such approach at different hours of the day and weather conditions.
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