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Comparació de la correcció del beamwander per un detector de quadrant i un detector ideal per enllaços de comunicacions quàntiques aèries.

Gómez-García, Jorge

Published Oct 15, 2015• 0 citations
OpenAlex

Abstract

We report an analysis of beam wander correction by both a quadrant and an ideal detector in a close loop configuration. The analysis is performed with the aim of finding the optimum detector for beam tracking of free space quantum communication links. The effect of beam wander will be modelled as an angle-of-arrival fluctuation at the receiver. A simulation using ray tracing software was performed to simulate the correction for different values of the angle of arrival, ranging from those typical from atmospheric turbulent regimes, with values of a few mili °, to larger values up to 1° from larger possible misalignments from the system. The simulating correcting system indicates that a quadrant detector is only successful for large values of the angle of arrival(> 0.2°),whereas for a smaller range no correction is shown due to the limiting factor imposed by aberrations of the focusing lens. We also show that the optimum spot diameter depends on the angle of arrival: if this is smaller than 0.5⁰, a spot diameter of at least Dspot/Dgap=2.5 should be used, whereas for larger values of the angle of arrival, a spot diameter of at least Dspot/Dgap=3.5 is required.

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We report an analysis of beam wander correction by both a quadrant and an ideal detector in a close loop configuration. The analysis is performed with the aim of finding the optimum detector for beam tracking of free space quantum communication links. The effect of beam wander will be modelled as an angle-of-arrival fluctuation at the receiver. A simulation using ray tracing software was performed to simulate the correction for different values of the angle of arrival, ranging from those typical from atmospheric turbulent regimes, with values of a few mili °, to larger values up to 1° from larger possible misalignments from the system. The simulating correcting system indicates that a quadrant detector is only successful for large values of the angle of arrival(> 0.2°),whereas for a smaller range no correction is shown due to the limiting factor imposed by aberrations of the focusing lens. We also show that the optimum spot diameter depends on the angle of arrival: if this is smaller than 0.5⁰, a spot diameter of at least Dspot/Dgap=2.5 should be used, whereas for larger values of the angle of arrival, a spot diameter of at least Dspot/Dgap=3.5 is required.
We report an analysis of beam wander correction by both a quadrant and an ideal detector in a close loop configuration. The analysis is performed with the aim of finding the optimum detector for beam tracking of free space quantum communication links. The effect of beam wander will be modelled as an angle-of-arrival fluctuation at the receiver. A simulation using ray tracing software was performed to simulate the correction for different values of the angle of arrival, ranging from those typical from atmospheric turbulent regimes, with values of a few mili °, to larger values up to 1° from larger possible misalignments from the system. The simulating correcting system indicates that a quadrant detector is only successful for large values of the angle of arrival(> 0.2°),whereas for a smaller range no correction is shown due to the limiting factor imposed by aberrations of the focusing lens. We also show that the optimum spot diameter depends on the angle of arrival: if this is smaller than 0.5⁰, a spot diameter of at least Dspot/Dgap=2.5 should be used, whereas for larger values of the angle of arrival, a spot diameter of at least Dspot/Dgap=3.5 is required.
Optimising the interconnection between free-space and fibre links will be necessary for future quantum communication networks. In daylight free-space quantum communication based on direct detection, the required Field Of View (FoV) of the receiver system needs to be minimised to reduce solar background noise coupling into the detectors. Reducing the FoV requires minimising beam wander effects caused by atmospheric turbulence through active optics. We implement a fine tracking system designed to correct tip and tilt wavefront aberrations, using two feedback loops; each of them consisting of a quadrant detector and a fast steering mirror for stabilising the beam in the whole optical axis of the receiver. We test the performance of the tracking system with different optical fibres in order to evaluate the reduction in the quantum bit error rate (QBER) caused by solar background noise. A reduction of 75% for single mode fibre was obtained, and 45% reduction for a 25 µm core diameter fibre, both cases for strong turbulence (Cn2~10-12 – 10-13 m-2/3) and 100 m propagating channel. These results look promising for enabling free-space Quantum Key Distribution (QKD) in wireless networks for realistic/adverse conditions such as daylight and strong turbulent regimes.
Beam wander caused by atmospheric turbulence can significantly degrade the performance of horizontal free-space quantum communication links. Classical beam wander correction techniques cannot be applied due to the stronger requirements of transmitting single photons. One strategy to overcome this limitation consists in using a separate wavelength from that of the quantum signal to analyze the beam wander and use this information for its correction. For this strategy to work adequately, both wavelengths should be affected equally by atmospheric turbulence, i.e. no chromatic effects should be originated from beam wander. In this letter, a series of experiments are performed to prove that this is the case for λ ∼ 850 nm as the quantum signal and λ ∼ 1550 nm as the tracking signal of a quantum communication system. © 2016 Wiley Periodicals, Inc. Microwave Opt Technol Lett 58:1362–1365, 2016
In practical applications to free-space quantum communications, the utilization of active beam coupling and stabilization techniques offers notable advantages, particularly when dealing with limited detecting areas or coupling into single-mode fibers(SMFs) to mitigate background noise. In this work, we introduce highly-enhanced active beam-wander-correction technique, specifically tailored to efficiently couple and stabilize beams into SMFs, particularly in scenarios where initial optical alignment with the SMF is misaligned. To achieve this objective, we implement a SMF auto-coupling algorithm and a decoupled stabilization method, effectively and reliably correcting beam wander caused by atmospheric turbulence effects. The performance of the proposed technique is thoroughly validated through quantitative measurements of the temporal variation in coupling efficiency(coincidence counts) of a laser beam(entangled photons). The results show significant improvements in both mean values and standard deviations of the coupling efficiency, even in the presence of 2.6 km atmospheric turbulence effects. When utilizing a laser source, the coupling efficiency demonstrates a remarkable mean value increase of over 50 %, accompanied by a substantial 4.4-fold improvement in the standard deviation. For the entangled photon source, a fine mean value increase of 14 % and an approximate 2-fold improvement in the standard deviation are observed. Furthermore,the proposed technique successfully restores the fidelity of the polarization-entangled state, which has been compromised by atmospheric effects in the free-space channel, to a level close to the fidelity measured directly from the source. Our work will be helpful in designing spatial light-fiber coupling system not only for free-space quantum communications but also for high-speed laser communications.
Both free-space and optical fiber quantum key distribution (QKD) links will coexist in future quantum networks, requiring efficient coupling between both types of channels. However, wavefront distortions introduced by the atmospheric channel can severely affect this efficiency. Active mechanisms such as precise beam tracking or steering can correct for some of these wavefront distortions, considerably improving the signal-to-noise ratio of the received quantum signal in many scenarios. A tracking system that uses two, instead oftypically one, controlling loops for tilt correction, stabilizes the beam in the whole optical axis of the receiver relaxing the restrictions of the receiver's optical design, and reduces the area of beam fluctuations in the receiver's focal plane a 24% more than a single-loop configuration. The tracking system was characterized in a QKD system at a 300 meter-link in moderate to strong turbulent conditions (Cn2- 10-14- 10-13m-2/3) and an improved coupling efficiency of a factor of 2.1 and 1.6 was obtained for a 9.5 μm-core diameter standard telecommunications Single Mode Fiber (SMF) and a 25 μm-core diameter multimode fiber (MMF), respectively. This reduces the quantum bit error rate (QBER) caused by solar background photons in a 52 % for the former and 39% for the latter, enabling an increase in the secret key rate ofmore than one order ofmagnitude for SMF and a factor of five for MMF. These results are promising for enabling QKD free-space links and their interconnection to fiber optic infrastructure in realistic scenarios of communication networks of high turbulence regimes and daylight conditions.
Many fundamental and applied experiments in quantum optics require transferring nonclassical states of light through large distances. In this context the free-space channels are a very promising alternative to optical fibers as they are mobile and enable to establish communications with moving objects, using satellites for global quantum links. For such channels the atmospheric turbulence is the main disturbing factor. The statistical properties of the fluctuating transmittance through the turbulent atmosphere are given by the probability distribution of transmittance (PDT). We derive the consistent PDTs for the atmospheric quantum channels by step-by-step inclusion of various atmospheric effects such as beam wandering, beam broadening and deformation of the beam into elliptic form, beam deformations into arbitrary forms. We discuss the applicability of PDT models for different propagation distances and optical turbulence strengths in the case when the receiver module has an annular aperture. We analyze the optimal detection and correction strategies which can improve the channel transmission characteristics. The obtained results are important for the design of optical experiments including postselection and adaptive strategies and for the security analysis of quantum communication protocols in freespace.