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This work is supported by the UK Engineering and Physical Sciences Research Council for funding through grants EP/P030017/1 and EP/M000869/1, and has received funding from the European Union's Horizon 2020 Programme through the project Advanced BiomEdical OPTICAL Imaging and Data Analysis (BE-OPTICAL) under grant agreement no. 675512. Optical approaches have broadened their impact in recent years with innovations in both wide-field and super-resolution imaging, which now underpin biological and medical sciences. Whilst these advances have been remarkable, to date, the ongoing challenge in optical imaging is to penetrate deeper. TRAFIX is an innovative approach that combines temporal focusing illumination with single-pixel detection to obtain wide-field multiphoton images of fluorescent microscopic samples deep through scattering media without correction. It has been shown that it can image through biological samples such as rat brain or human colon tissue up to a depth of seven scattering mean-free-path lengths. Comparisons of TRAFIX with standard point-scanning two-photon microscopy show that the former can yield a five-fold higher signal-to-background ratio while signifcantly reducing photo bleaching of the specimen. Here, we show the first preliminary demonstration of TRAFIX with three-photon excitation imaging dielectric beads. We discuss the advantages of the TRAFIX approach combined with compressive sensing for biomedicine. Publisher PDF

The speckle pattern produced when a laser illuminates a random medium can, with appropriate analysis, be used to uniquely identify the wavelength of the illuminating source. We have demonstrated that principal component analysis can be used as a very sensitive probe of the speckle pattern produced by random prisms [1] and integrating spheres [2]. However, to date, the state-of-the-art realisations of speckle spectrometers have been based on the use of multi-mode fibres as the scattering medium [3] and on transmission matrix analysis approaches, achieving a compact and stable device with picometre resolution. Here, we show that the speckle pattern produced by propagation through a metre-long step-index multi-mode optical fibre can be analysed with principal component analysis to achieve a femtometre-precision wavemeter, and present progress in the measurement of complete spectra, which demonstrates the applicability of our approach to many existing experiments. Moreover, we demonstrate that the speckle wavemeter can be used as part of a feedback loop to stabilise lasers to a fractional stability of 10-9. With the freedom to lock the laser at any user-desired frequency and a robust, compact setup, the method holds promise for the new generation of portable cold atom experiments currently being developed for quantum technology applications. [1] M Mazilu, et al. Opt Lett 39, 96 (2014) [2] N K Metzger, et al. Nat. Commun. 8, 15610 (2017) [3] H Cao, J. Opt. 19, 060402 (2017) Preprint

We demonstrate the transfer of orbital angular momentum (OAM) to optically levitated microparticles in vacuum. We create two-dimensional (2D) and three-dimensional (3D) optical potentials possessing OAM. In the former case the microparticle is placed within a Laguerre-Gaussian (LG) beam and orbits the annular beam profile with increasing angular velocity as the air drag coefficient is reduced. Our results reveal that there is a fundamental limit to the OAM that may be transferred to a trapped particle, dependent upon the beam parameters and inertial forces present. Whilst a LG beam scales in size with azimuthal index, recently we have created a “perfect vortex” beam whose radial intensity profile and radius are both independent of topological charge. As the Fourier transform of a “perfect vortex” yields a Bessel beam, imaging a “perfect vortex”, with its subsequent propagation thus realises a complex three-dimensional optical field. In this scenario we load individual silica microparticles into this field where the optical gradient and scattering forces interplay with the inertial and gravitational forces acting on the trapped particle. As a result the trapped microparticle exhibits a complex three-dimensional motion that includes a periodic orbital motion between the Bessel and the “perfect vortex” beam. We are able to determine the three dimensional optical potential in situ by tracking the particle. This first demonstration of trapping microparticles within a complex 3D optical potential in vacuum opens up new possibilities for fundamental studies of many-body dynamics, mesoscopic entanglement, and optical binding.

We demonstrate the transfer of orbital angular momentum to optically levitated microparticles in vacuum [1]. We prepare two-dimensional and three-dimensional optical potentials. In the former case the microparticle is placed within a Laguerre-Gaussian beam and orbits the annular beam profile with increasing angular velocity as the air drag coefficient is reduced. We explore the particle dynamics as a function of the topological charge of the levitating beam. Our results reveal that there is a fundamental limit to the orbital angular momentum that may be transferred to a trapped particle, dependent upon the beam parameters and inertial forces present. This effect was predicted theoretically [2] and can be understood considering the underlying dynamics arising from the link between the magnitude of the azimuthal index and the beam radius [3]. Whilst a Laguerre-Gaussian beam scales in size with azimuthal index ', recently we have created a "perfect" vortex beam whose radial intensity profile and radius are both independent of topological charge [4, 5]. As the Fourier transform of a perfect vortex yields a Bessel beam. Imaging a perfect vortex, with its subsequent propagation thus realises a complex three dimensional optical field. In this scenario we load individual silica microparticles into this field and observe their trajectories. The optical gradient and scattering forces interplay with the inertial and gravitational forces acting on the trapped particle, including the rotational degrees of freedom. As a result the trapped microparticle exhibits a complex three dimensional motion that includes a periodic orbital motion between the Bessel and the perfect vortex beam. We are able to determine the three dimensional optical potential in situ by tracking the particle. This first demonstration of trapping microparticles within a complex three dimensional optical potential in vacuum opens up new possibilities for fundamental studies of many-body dynamics, mesoscopic entanglement [6, 7], and optical binding [8, 9]. Publisher PDF

Moving towards label-free techniques for cell identification is essential for many clinical and research applications. Raman spectroscopy and digital holographic microscopy (DHM) are both label-free, non-destructive optical techniques capable of providing complimentary information. We demonstrate a multi-modal system which may simultaneously take Raman spectra and DHM images to provide both a molecular and a morphological description of our sample. In this study we use Raman spectroscopy and DHM to discriminate between three immune cell populations CD4+ T cells, B cells, and monocytes, which together comprise key functional immune cell subsets in immune responses to invading pathogens. Various parameters that may be used to describe the phase images are also examined such as pixel value histograms or texture analysis. Using our system it is possible to consider each technique individually or in combination. Principal component analysis is used on the data set to discriminate between cell types and leave-one-out cross-validation is used to estimate the efficiency of our method. Raman spectroscopy provides specific chemical information but requires relatively long acquisition times, combining this with a faster modality such as DHM could help achieve faster throughput rates. The combination of these two complimentary optical techniques provides a wealth of information for cell characterisation which is a step towards achieving label free technology for the identification of human immune cells.

Raman spectroscopy is a valuable tool for non-invasive and label-free identification of sample chemical composition. Recently a few miniaturized optical probes emerged driven by the need to address areas of difficult access, such as in endoscopy. However, imaging modality is still out of reach for most of them. Separately, recent advances in wavefront shaping enabled different microscopies to be applied in various complex media including multimode fibers. Here we present the first and thinnest to date Raman fiber imaging probe based on wavefront shaping through a single multimode fiber without use of any additional optics. We image agglomerates of bacteria and pharmaceuticals to demonstrate the capability of our method. This work paves the way towards compact and flexible Raman endoscopy. © (2017) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only. Publisher PDF

A photon can carry orbital angular momentum equal to an integer number of the reduced Planck’s constant. This principle expresses itself in geometrical quantization of optical vortex beams, which thus can propagate only in the form of fields having a helically wavefront characterized by an integer valued topological charge. However, one can create an optical vortex beam of an effective fractional charge by combining multiple integer vortices. Here, we investigate this apparent violation of the geometrical quantization of orbital angular momentum of light. Our approach relies on observation of the light-induced motion of a microscopic particle, which thus acts as an optomechanical probe for the optical vortex beam. A fractional topological charge corresponds to an abrupt jump in the helical phase front of the beam. This singularity expresses itself as an off-axis disturbance in the intensity profile, and thus complicates the optomechanical probing. We overcome this problem by distributing the disturbance along the vortex ring, so that a microparticle can continuously orbit due to the orbital angular momentum transfer. We demonstrate theoretically that whatever effort is put into smoothing the fractional vortex ring (as long as the net topological charge is fixed), the particle’s orbital motion cannot be as uniform as in the case of an integer vortex beam. We support this prediction by experimental proof. The experimental technique benefits from the recently introduced “perfect” vortex beams which allow an optically trapped particle to orbit along a constant trajectory irrespective of any topological charge.

The authors would like to thank the UK Engineering and Physical Sciences Research Council (EPSRC) for funding. Vortex beams with different topological charge usually have different profiles and radii of peak intensity. This introduces a degree of complexity the fair study of the nature of optical OAM (orbital angular momentum). To avoid this, we introduced a new approach by creating a perfect vortex beam using an annular illuminating beam with a fixed intensity profile on an SLM that imposes a chosen topological charge. The radial intensity profile of such an experimentally created perfect vortex beam is independent to any given integer value of its topological charge. The well-defined OAM density in such a perfect vortex beam is probed by trapping microscope particles. The rotation rate of a trapped necklace of particles is measured for both integer and non-integer topological charge. Experimental results agree with the theoretical prediction. With the flexibility of our approach, local OAM density can be corrected in situ to overcome the problem of trapping the particle in the intensity hotspots. The correction of local OAM density in the perfect vortex beam therefore enables a single trapped particle to move along the vortex ring at a constant angular velocity that is independent of the azimuthal position. Due to its particular nature, the perfect vortex beam may be applied to other studies in optical trapping of particles, atoms or quantum gases. Publisher PDF

In isotropic random optical waves, each dark area may contain optical vortices or phase singularities. In this paper, we experimentally generate a speckle pattern and observe its transition along the propagation direction. Experimental results show that the vortex density changes along the propagation direction when the continuous phase part of the speckle field is removed with a spatial light modulator. The contrast ratio of the spackle field also changes due to the transition of the field. Such a transition can be interpreted to a certain extent by the self-annihilation of vortex dipoles due to the least-squares phase removal.

We break down the equilibrium state and the diraction invariant property of a non-diracting speckle “eld byremoving its continuous part of the phase while leaving all vortices behind. During the propagation of such aphase corrected non-diracting speckle “eld, the vortex density drops down to a minimum value and then comesback to an equilibrium value which is even higher than the initial one. Before the phase corrected “eld returnsback to its new equilibrium state, another least-squares phase removal will be applied, at the position wherethere is a minimum vortex density, to further remove vort ices from the speckle “eld. Such a process of removingleast-squares phase and propagating the phase corrected “eld over a distance can be repeated to eliminate mostof optical vortices. Statistical results show that most of optical vortices can be removed from a non-diract ingspeckle “eld. Finally, a semi-plane wave without optical vortices can be obtained from an initial non-diractingspeckle “eld with multiple steps of least-square phase correction.Keywords: Non-diracting speckle “eld, Optical v ortices, Least-squares phase removal

Speckle was re-discovered after the invention of the laser in 1960. The unpublished 1963 Stanford Electronics Laboratory report by J W Goodman was the first comprehensive derivation of the first and second order statistics of the speckle intensity. This short paper describes how the senior author came to know Professor Goodman through their mutual, and lasting, interest in laser speckle. New results in speckle continue to be discovered, and we briefly describe one of these, the elimination of phase vortices using cascade adaptive optics systems.

In this paper we report on measurements of atmospheric turbulence effects arising from water air interaction. The aim of this study is to aid in the design of a free-space optical relay system to facilitate longer line-of-sight distances between relay buoys in a large expanse of water. Analysis of turbulence statistics will provide the basis for adaptive optics solutions to improve the relay signal strength affected by scintillation and beam wander. We report on experiments determining the isokinetic angle using an array of broadband incoherent sources of variable angular separation on the order of 0.1 mrad to 2.8 mrad. The experimental setup consists of a 5 inch telescope with high speed CMOS camera observing over a distance of 300 m close at a height of 1.5 m above the water surface. As part of the turbulence characterisation we experimentally estimate the relative image motion of angle-ofarrival fluctuations and perform other time series analysis. Analysis of the image motion requires new techniques due to the extended nature of the source. We explore different centroiding algorithms and surface fitting techniques.

An optical vortex, which possesses positive or negative topological charge, can be used as an information carrier in a free-space optical communication system because its special properties. By detecting the vortex with a Shack- Hartmann wavefront sensor, one can extract the information transferred by a vortex beam. However, additional optical vortices can spontaneously appear in the beam propagating over a long distance in the atmosphere or through a strongly turbulent medium. As a result, the vortex beam will contain a significant number of new vortices besides the initial one in the system receiving aperture. This may destroy the information carried by the initial vortex. In the paper, we will describe the reliability of detecting vortex with a Shack-Hartmann wavefront sensor in a scintillated vortex beam. The initial vortex can be detected even if the beam is strongly scintillated and with numerous newly emerged vortices. Numerical simulations and statistics show that the information can still be accurately interpreted to a certain extent from a vortex beam propagating through weak-to-strong atmospheric turbulence.

This paper presents a new ways-free image registration method based on adherent mark recognition method. Accurate 3D coordinates are first obtained in each unit-measurement and then all the unit-data information are amalgamated into a communal coordinate system. The stereo vision based 3D profilometer computes at least three marks' 3D coordinates first and then these coordinates are applied to calibrate the translation and orientation matrix between the two different inspection positions. This method allows any freedom of translation or orientation between every two unit-measurements without special constraints. With all of the six possible freedom transformations, the system's flexibility and adaptability are greatly enhanced, so the objects under inspection can be measured according to their inherent shapes at each optimal angle of view. As there needs no redundant ways in the measurement, the portable on- spot inspection becomes feasible. The paper analyzes two different amalgamation algorithms in detail and finally computer simulation results are given.

Binocular machine vision has been explored for so many years, but the most difficult problem and the obstacle of the system processing is the matching procedure. This thesis will give a new technique to obtain matching points of the left and right image in the binocular active vision system, without searching the whole image, or even the whole feature curve. So the time-consumed computation is reduced considerably. And the mismatching errors are reduced too. In this technique, except the epipolar constraints, we add two strong constraints into the system: Adherent-Mark, Grid (row and column). Using a new method of Grid-Coding, matching point is easy to be found.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.