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arXiv:1308.4592 · Optics Express

3D beam reconstruction by fluorescence imaging

N. Radwell, M. A. Boukhet, Sonja Franke‐Arnold

Published Sep 12, 2013• 4 citations
arXivHTMLPDF

Abstract

We present a technique for mapping the complete 3D spatial intensity profile of a laser beam from its fluorescence in an atomic vapour. We propagate shaped light through a rubidium vapour cell and record the resonant scattering from the side. From a single measurement we obtain a camera limited resolution of 200 × 200 transverse points and 659 longitudinal points. In constrast to invasive methods in which the camera is placed in the beam path, our method is capable of measuring patterns formed by counterpropagating laser beams. It has high resolution in all 3 dimensions, is fast and can be completely automated. The technique has applications in areas which require complex beam shapes, such as optical tweezers, atom trapping and pattern formation.

DOI
10.1364/oe.21.022215

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We present a technique for mapping the complete 3D spatial intensity profile of a laser beam from its fluorescence in an atomic vapour. We propagate shaped light through a rubidium vapour cell and record the resonant scattering from the side. From a single measurement we obtain a camera limited resolution of 200 × 200 transverse points and 659 longitudinal points. In constrast to invasive methods in which the camera is placed in the beam path, our method is capable of measuring patterns formed by counterpropagating laser beams. It has high resolution in all 3 dimensions, is fast and can be completely automated. The technique has applications in areas which require complex beam shapes, such as optical tweezers, atom trapping and pattern formation.
The interaction of spatially structured light fields with atomic media can generate spatial structures inscribed in the atomic populations and coherences, allowing for example the storage of optical images in atomic vapours. Typically, this involves coherent optical processes based on Raman or EIT transitions. Here we study the simpler situation of shaping atomic populations via spatially dependent optical depletion. Using a near resonant laser beam with a holographically controlled 3D intensity profile, we imprint 3D population structures into a thermal rubidium vapour. This 3D population structure is simultaneously read out by recording the spatially resolved fluorescence of an unshaped probe laser. We find that the reconstructed atomic population structure is largely complementary to the intensity structure of the control beam, however appears blurred due to global repopulation processes. We identify and model these mechanisms which limit the achievable resolution of the 3D atomic population. We expect this work to set design criteria for future 2D and 3D atomic memories.
Laser light can exhibit nonlinear behaviour when interacting with dense atomic vapours.\nThis can give rise to a variety of interesting phenomena such as slow light and optically induced transparency. Recently A.~J. van Lange measured nonlinear transmission of light through a Rubidium vapour and developed a physical model to describe these measurements. Van Lange also investigated the formation of different beam profiles, in a dense Rubidium vapour and from Gaussian input beams, as a function of intensity and optical frequency.\n\nHere we expand on van Lange's measurements by measuring over a large frequency range which includes multiple atomic transition lines and by employing off-axis holography which allows for simultaneous measurement of the intensity and phase. Analysis of beam profiles based on the structural similarity index (SSIM) uncovered beam profiles whose path through intensity-detuning space could be described with van Lange's generalised detuning curve. Off-axis holography measurements are in agreement with regular intensity measurements but exhibits a greater amount of noise.
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The development of new laser beam shaping methods is important in a variety of fields within optics, atomic physics and biophotonics. Spatial light modulators offer a highly versatile method of time-dependent beam shaping, based on imprinting a phase profile onto an incident laser beam that determines the intensity in the trapping plane laser field. The calculation of the required phase is a well-known inverse problem, which can be tackled with different approaches. Our method based on conjugate gradient minimisation [1] not only allows the calculation of smooth and accurate intensity profiles suitable for trapping cold atoms, but can also be used to generate multi-wavelength traps [2] and for simultaneous control over both the intensity and the phase of the light [3], with exceptionally high reconstruction fidelity. References: [1] T Harte, et al., "Conjugate gradient minimisation approach to generating holographic traps for ultracold atoms" Opt. Express 22, 26548 (2014) [2] D Bowman, et al., "Multi-wavelength holography with a single spatial light modulator for ultracold atom experiments" Opt. Express 23, 8365 (2015) [3] D Bowman, et al., "High-fidelity phase and amplitude control of phase-only computer generated holograms using conjugate gradient minimisation" Opt. Express 25, 11692 (2017)
A fluorescence detection scheme coupled to a highly sensitive nitrogen-cooled CCD camera is used to image the spatial distribution of a low-density falling rubidium atomic cloud released from an optical trap. The falling cloud passes through a thin probe laser beam tuned to resonance. The performance of the scheme is illustrated in the analysis of cold atomic clouds collimated by pinholes during their free fall under the influence of gravity. Clouds of approximately 10(4) atoms and with typically 10(6) at./cm(3) density are analyzed spatially with 24-mum resolution. This method is compared with different atomic cloud imaging techniques.
Using the unique detection properties offered by metastable helium atoms we have produced high resolution images of the transverse spatial profiles of an atom laser beam. We observe transverse fringes on the beam, resulting from quantum mechanical interference between atoms that start from rest at different transverse locations within the outcoupling surface and end up at a later time with different velocities at the same transverse position. Further, for high outcoupling rates we see the emergence of structure on the beam profile, which is not predicted by the usual Gross-Pitaevskii equation.