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

Murphy, Kevin J.; Burke, Daniel; Devaney, Nicholas; Dainty, Chris

doi:10.1364/oe.18.015448

Cited by 77 publications

(39 citation statements)

References 35 publications

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“…As a final proof we measured the wavefront of the XUV beam 21 with a Hartmann sensor. Similar experiments in the optical range have shown that the wavefront should wind up like a screw around the singularity in the centre 22 . The measured XUV wavefront together with the intensity distribution is shown in Fig.…”

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confidence: 61%

Strong-field physics with singular light beams

et al. 2012

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Light beams carrying a point singularity with a screw-type phase distribution are associated with an optical vortex. The corresponding momentum flow leads to an orbital angular momentum of the photons 1-3 . The study of optical vortices has led to applications such as particle micro-manipulation 4,5 , imaging 6 , interferometry 7 , quantum information 8 and highresolution microscopy and lithography 9 . Recent analyses showed that transitions forbidden by selection rules seem to be allowed when using optical vortex beams 10 . To exploit these intriguing new applications, it is often necessary to shorten the wavelength by nonlinear frequency conversion. However, during the conversion the optical vortices tend to break up 11-13 . Here we show that optical vortices can be generated in the extreme ultraviolet (XUV) region using high-harmonic generation 14,15 . The singularity impressed on the fundamental beam survives the highly nonlinear process. Vortices in the XUV region have the same phase distribution as the driving field, which is in contradiction to previous findings 16 , where multiplication of the momentum by the harmonic order is expected. This approach opens the way for several applications based on vortex beams in the XUV region.Places where physical quantities become infinite or change abruptly are called singularities. The presence of phase dislocations (singularities) in the wavefront of a light beam determines both the phase and intensity structure around them. As the phase becomes indeterminate at singularities, both the real and the imaginary parts of the field amplitude (that is, also the field intensity) vanish. The characteristic helical phase profiles of optical vortices are described by exp(imθ) multipliers, where θ is the azimuthal coordinate and the integer number m is their topological charge, also called dislocation strength or winding number.Recalling the fact that in free space the Poynting vector gives the momentum flow, for helical phase fronts the Poynting vector has an azimuthal component that produces an orbital angular momentum parallel to the axis of the beam. The momentum circulates around the beam axis, so such beams are said to contain an optical vortex. As has been shown 1-3 , an m-fold charged optical vortex beam carries an orbital angular momentum of mh per photon independent of the spin angular momentum (that is, the polarization state). It was shown that transitions that are forbidden by known selection rules in the electric and magnetic dipole approximation seem to be allowed when using optical vortex beams 10 . This provides a new degree of freedom in the spectroscopy of forbidden transitions. Multi-coloured optical vortices can be generated through a nonlinear frequency-conversion process such as second-harmonic generation 16 or four-wave mixing, which is an important process in the white-light vortex generation. However, as predicted in ref. 11 and observed in ref. 12, vortex breakup in self-focusing nonlinear media is an important issue for supercontinuum vortex genera...

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confidence: 61%

Strong-field physics with singular light beams

et al. 2012

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“…Previous studies used a closed path associated with 2 × 2-neighboring measurement points. [16][17][18][19][20][21][22] Instead, here we propose an alternate closed path that connects the centers of the eight nearest neighbors of measurement points, which we call an eight-connected contour in the rest of this paper (Fig. 1).…”

Section: Eight-connected Closed Contour For Optical Vortex Detectionmentioning

confidence: 99%

“…2,16 The presence and absence of phase singularities in a light beam can be determined according to the nonzero (local peak) values of the contour summations. Some improvements and modifications have been made to reduce noise; [17][18][19] however, the spatial resolution of measurements with the SH-WFS is limited by the lens size of the lenslet array.…”

Section: Introductionmentioning

confidence: 99%

Eight-connected contour method for accurate position detection of optical vortices using Shack–Hartmann wavefront sensor

et al. 2015

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Abstract. We propose a simple method of realizing an accurate position detection of phase singularities in an optical vortex (OV) beam using a Shack-Hartmann wavefront sensor (SH-WFS). The method calculates circulations which are the discrete contour integrals of phase slope vectors measured by the SH-WFS and then determines the accurate positions of the singular points by calculating the centers-of-gravity with a fixed window size around the local peak of the circulation distribution. We use closed paths that connect the centers of eightconnected, instead of 2 × 2-neighboring lenslet apertures for calculating the circulations. Both the numerical analysis and proof-of-principle experiment were performed to confirm the measurement accuracy. In experiments, the positions of singular points in OV beams generated by a liquid-crystal-on-silicon spatial light modulator were measured. The root-mean-square error of the position measurement was approximately 0.09 in units of the lens size of the lenslet array used in the SH-WFS. We also estimated the topological charges of the singular points being detected based on the peak circulations, and the results agreed well with theoretical ones. The method achieves both rapid implementation and sublens-size spatial resolution detection and is suitable for applications that require real-time control of OV beams.

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“…For Laguerre-Gaussian beams, Leach et al (2006), Murphy et al (2010) demonstrated the POAM state can be measured with a Shack-Hartmann wavefront sensor. Specifically, given an L-G beam carrying m of momentum per photon, a SH WFS measures gradients which, when summed about the optic axis, return m2π from which the OAM state can be inferred.…”

Section: Poam Detection Via Phasementioning

confidence: 99%

“…POAM can be measured using a Shack-Hartmann wavefront sensor (Leach et al 2006;Murphy et al 2010), similar to those used in conventional adaptive optical (AO) systems. Shack-Hartmann wavefront sensors (SH WFS) measure the gradient of a beam's phase, ∇ϕ.…”

Section: Introductionmentioning

confidence: 99%

The creation of photonic orbital angular momentum in electromagnetic waves propagating through turbulence

Sanchez

Oesch

Reynolds

2013

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Context.We have recently shown that the phenomenon known as "branch points" in AO are markers for photons carrying orbital angular momentum (OAM). In doing so, we have demonstrated that atmospheric turbulence creates well defined OAM states in beams propagating through it. Aims. In this paper, we extend our previous research to include any astrophysical turbulent assemblage of molecules or atoms (TAMA), demonstrating that these clouds, similar to Earth's atmosphere, also create photonic orbital angular momentum (POAM) in electromagnetic waves propagating through them. A TAMA is any gaseous cloud with a varying density and therefore variation in its index of refraction, which includes but is not limited to stellar envelopes, circumstellar disks, molecular clouds, planetary atmospheres, and the interstellar medium. Methods. We applied our previous theoretical, simulation, and laboratory results to astrophysical TAMAs. Additionally, we demonstrated how sensors designed for AO can be used to measure this POAM flux. Results. Our results apply to light propagating through any TAMA. Since TAMA are ubiquitous in the cosmos, steady, long lasting POAM fluxes will be ubiquitous as well. Conclusions. Our results, which include theory, benchtop laboratory data, and wave optic simulation, indicate that, under the right conditions, POAM fluxes can reach over 50% of the total photon flux. An initial set of on-sky experimental observations appear to corroborate the laboratory results with two of the five stars, HR 1529 and HR 1577, showing POAM fluxes of 3% ± 1% and 2% ± 1% of the total flux, and a third, HR 1895, with a PAOM flux of up to 17% ± 2% of the total flux.

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

Cited by 77 publications

References 35 publications

Strong-field physics with singular light beams

Strong-field physics with singular light beams

Eight-connected contour method for accurate position detection of optical vortices using Shack–Hartmann wavefront sensor

The creation of photonic orbital angular momentum in electromagnetic waves propagating through turbulence

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