Branch point detection and correction using the branch point potential method

Murphy, Kevin J.; Mackey, Ruth; Dainty, Chris

doi:10.1117/12.778743

Cited by 10 publications

(8 citation statements)

References 18 publications

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“…However, under the regime of strong turbulence, Shack-Hartmann-based solutions may fail to guarantee a stable fiber coupling. For instance, in low-earth-orbit (LEO) satellite downlinks, strong turbulence may induce intense scintillation and phase wavefront singularities (branch points), thus, seriously limiting the performance of such sensor [9][10][11][12].…”

Section: Introductionmentioning

confidence: 99%

Intensity-based adaptive optics with sequential optimization for laser communications

Carrizo¹,

Calvo²,

Belmonte³

2018

Opt. Express

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Wavefront distortions of optical waves propagating through the turbulent atmosphere are responsible for phase and amplitude fluctuations, causing random fading in the signal coupled into single-mode optical fibers. Wavefront aberrations can be confronted, in principle, with adaptive optics technology that compensates the incoming optical signal by the phase conjugation principle and mitigates the likeliness of fading. However, real-time adaptive optics requires phase wavefront measurements, which are generally difficult under typical propagation conditions for communication scenarios. As an alternative to the conventional adaptive optics approach, here, we discuss a novel phase-retrieval technique that indirectly determines the unknown phase wavefront from focal-plane intensity measurements. The adaptation approach is based on sequential optimization of the speckle pattern in the focal plane and works by iteratively updating the phases of individual speckles to maximize the received power. We found in our analysis that this technique can compensate the distorted phasefront and increase the signal coupled with a significant reduction in the required number of iterations, resulting in a loop bandwidth utilization well within the capacity of commercially available deformable mirrors.

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Section: Introductionmentioning

confidence: 99%

Intensity-based adaptive optics with sequential optimization for laser communications

Carrizo¹,

Calvo²,

Belmonte³

2018

Opt. Express

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“…By convention the (i,j)th pixel is the top left of the four pixels and the vortex is placed there, a more exact position of the branch point could be established by further numerical methods but we know of no standard technique. The overall robustness of the contour sum method has been questioned under heavy noise conditions [18,21,32] making it problematic to use this with the slope discrepancy technique and therefore we have decided to use a different method [21,22]. The issue of noise with respect to the contour sum method is raised because it only does a gradient circulation around a closed loop of four phase gradients, whereas the branch point potential method described below employs all of the available slope data by using a least squares reconstruction method to calculate the potential function.…”

Section: Vortex Detectionmentioning

confidence: 99%

“…Two major difficulties in this field are the accurate and reliable detection of the vortices in the wavefront and the subsequent faithful reconstruction of the measured wavefront. Different reconstructors have been suggested [13,14] and some of these have been shown to work in laboratory conditions [15,16], while others work in theory but are impractical in reality [17,18].…”

Section: Introductionmentioning

confidence: 99%

Experimental detection of optical vortices with a Shack-Hartmann wavefront sensor

Murphy¹,

Burke²,

Devaney³

et al. 2010

Opt. Express

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Laboratory experiments are carried out to detect optical vortices in conditions typical of those experienced when a laser beam is propagated through the atmosphere. A Spatial Light Modulator (SLM) is used to mimic atmospheric turbulence and a Shack-Hartmann wavefront sensor is utilised to measure the slopes of the wavefront surface. A matched filter algorithm determines the positions of the Shack-Hartmann spot centroids more robustly than a centroiding algorithm. The slope discrepancy is then obtained by taking the slopes measured by the wavefront sensor away from the slopes calculated from a least squares reconstruction of the phase. The slope discrepancy field is used as an input to the branch point potential method to find if a vortex is present, and if so to give its position and sign. The use of the slope discrepancy technique greatly improves the detection rate of the branch point potential method. This work shows the first time the branch point potential method has been used to detect optical vortices in an experimental setup.

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“…5 Conversely, a new approach which utilizes subaperture irradiance pattern beam spread to identify branch points has recently been introduced. 32 While other approaches exist, [33][34][35][36][37] these two approaches will be the ones emphasized in this paper. Here, the circulation of phase gradients approach and beam-spread approach will be compared using a simple optical-vortex model, wave-optics simulations, as well as experimental data.…”

Section: Introductionmentioning

confidence: 99%

Branch-point detection using a Shack-Hartmann wavefront sensor

Kalensky,

Oesch,

Bukowski

et al. 2023

Unconventional Imaging, Sensing, and Adaptive Optics 2023

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In this paper, two methods for identifying branch points from Shack–Hartmann wavefront sensor (SHWFS) measurements were studied; the circulation of phase gradients approach and the beam-spread approach. These approaches were tested using a simple optical-vortex model, wave-optics simulations, and with experimental data. It was found that these two approaches are synergistic regarding their abilities to detect branch points. Specifically, the beam-spread approach works best when the branch point is located towards the center of the SHWFS’s lenslet pupil, while the circulation of phase gradients approach works best when the branch point is located towards the edge of the SHWFS’s lenslet pupil. These behavior were observed studying the simple optical-vortex model; however, they were further corroborated with the wave-optics and experimental results. The developments presented within support researchers looking to study high scintillation optical-turbulence environments as well as will inform efforts looking to develop branch-point tolerant reconstruction algorithms.

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Branch point detection and correction using the branch point potential method

Cited by 10 publications

References 18 publications

Intensity-based adaptive optics with sequential optimization for laser communications

Intensity-based adaptive optics with sequential optimization for laser communications

Experimental detection of optical vortices with a Shack-Hartmann wavefront sensor

Branch-point detection using a Shack-Hartmann wavefront sensor

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