This PDF file contains the front matter associated with SPIE Proceedings Volume 11818, including the Title Page, Copyright information, and Table of Contents

Structured light, with ability to arbitrarily tailor degrees of freedom (DoFs) of light, amplitude, phase, wavelength, etc, has recently attracted great promotion for fundamental science and advanced applications. In addition to the basic DoFs in light, there are also some complex DoFs emerged as the combination of common DoFs, such as angular momentum, vector singularity, ray-wave trajectory, spatiotemporal vortex, etc., showing their power in optical manipulation. In this talk, I summarize the advanced methods of manipulating tunable DoFs and creating new DoFs in structured light, as a roadmap guiding the development of multi-dimensional structured light. We also discuss the significance of the multi- DoF control in both fundamental physics and novel applications.

The vortex lattice and linear vortex array are network structures of multiple isolated vortices. We propose a method to generate these multi-vortex fields via superimposing two engineered edge dislocation arrays. By designing the topology of the edge dislocation array for stacking, we may obtain an arbitrary-order vortex lattice and linear vortex array. Besides, these edge dislocation array fields can be decomposed into a finite sum of harmonics, which indicates that anyorder vortex array can be created by using wave interference. It supplements the research about the vortex array generated by the interference method for the highest-order vortex lattice obtained by interferometry is 3rd at present. The feasibility of the proposed method is validated through simulations and experiments based on fourth-and fifth-order vortex lattices and linear vortex arrays.

The conservation of optical properties as a beam propagates through a scattering media is important to creating more complex optically induced structures and the transmission of high bandwidth information. By utilizing the nonlinear optical effect in the scattering bio-soft-matter, we investigate the conservation of polarization and orbital angular momentum through self-trapping and pump/probe coupled waveguides of light in sheep red blood cell suspensions at 532 nm and 780nm wavelengths. The ability to maintain these properties after multiple scattering events may lead to improvements in communication bandwidth with low loss.

There exist a variety of methods and platforms for generation of OAM beams. However, all these techniques imply that beams pass through an optical component and thus, when pulsed beam is used, the pulses can acquire dispersive broadening and distortion. The design of compact and efficient devices for OAM generation requires that these effects be characterized and quantified, and a set of parameters and techniques for their treatment should be developed. Here, the spatio-temporal properties of the ultrashort vortical pulses are analyzed numerically. Case studies will be presented. The results obtained are characterization of the effects of dispersion, geometry and discretization in the numerical modeling of ultrashort vortical pulses under various propagation conditions, and can serve as a basis for design of new optical devices.

In this work, we present a strategy for the modal decomposition of a partially coherent Ince-Gaussian (PCIG) beam. It is shown that the beam can be represented as a suitable superposition of spatially coherent modes. As soon as the beam's spatial coherence decreases, its modal content reveals the existence of transverse coherent higher order modes, with the same ellipticity. We study the dependence of the modal decomposition in terms of the ellipticity and transverse coherence length.

It is well known that entanglement is invariant to local unitary transformations, this implies the degree of entanglement or non-separability remains constant during free-space propagation, which is true for both quantum and classically-entangled modes. Here we demonstrate an exception to this rule using a carefully engineered vectorial light field, and study its non-separability dynamics upon free-space propagation. We show that the local non-separability between the spatial and polarisation degrees of freedom dramatically decays to zero, while preserving the purity of the state and hence the global non-separability. We show this by numerical simulations and corroborate it experimentally. Our results evince novel properties of classically-entangled modes, point to the need for new measures of non-separability for such vectorial fields, while paving the way to novel applications for customised structured light.

Complex vector light fields have become a topic of late due to their exotic features, such as their non-homogeneous transverse polarisation distributions and the non-separable coupling between their spatial and polarisation degrees of freedom. In general, vector beams propagate in free space along straight lines, being the Airy-vector vortex beams the only known exception. Here, we introduce a new family of vector beams that exhibit properties that have not been observed before, such as their ability to freely accelerate along parabolic trajectories. We anticipate that these novel vector beams might find applications in fields such as optical manipulation, microscopy, laser material processing, among others.

The fundamental polarization singularities of light are generally symmetric under coordinated rotations: that is, transformations which rotate the spatial dependence of the fields by an angle θ and the field polarization by a fraction of that angle, as generated by 'mixed' angular momenta of the form L + S. Generically, the coordination parameter has been thought to be restricted to integer or half-integer values. In this work we show that this constraint is an artifact of the restriction to monochromatic elds, and that a wider variety of optical singularities is available when more than one frequency is involved. We show that these new optical singularities present novel field topologies, isomorphic to torus knots, and we show how they can be characterized both analytically and experimentally. Moreover, the generator for the symmetry group of these singularities, whose algebraic structure is deeply related to the torus-knot topology of the beams, is conserved in nonlinear optical interactions.

Structured photons often serve as physical realizations of high-dimensional quantum states beneficial to quantum information. One popular way of discretizing the space is through Laguerre-Gaussian modes that can have twisted phase front and, thus, orbital angular momentum (OAM). We demonstrate that using multi-plane light-conversion techniques enables the performance of complex unitary transformations between multiple input-modes and output-modes in the quantum regime. We demonstrate various complex two-photon interferences observed in spatial modes, thereby realizing a linear optical network along a single beam path. Finally, we show that using this ability allows the generation of spatial-mode NOON-states, which offer angular super-sensitivity.

The ray-wave geometric beams (RWGBs) are fantastic structured light field with multiple degrees of freedom (DoFs). These DoFs endow countless application prospects for RWGBs in optical communication, quantum entanglement, optical tweezers. Meantime, the intricate orbital angular momentum (OAM) structures and intensity shape brought by these DoFs have caused great difficulty in its sort and limited its application. We propose a new digital holographic method to identify multi-DoFs RWGBs based on the conjugated modulation theory, thus called conjugated modulation identification (CMI). The experiment results indicate that the RWGBs were fully resolved, and the reconstructed correlation degree shows good agreement with the theoretical values. Furthermore, the 8-bit and 16-bit multi-RWGBs shift keying encoding were demonstrated. The signal is well recovered with no error, demonstrating the proposed method with good data transmission performance. Our work reveals wide potential applications of RWGBs in realizing high-speed, high-dimensional time-varying modes multiplexed encoding and high-capacity multi-channel communication..

Flat-top or super-Gaussian beams have found applications in many diverse areas, particularly in laser materials processing. The creation of at-top beams requires an infinite spatial frequency spectrum and as at-top beams are not eigenmodes of the free space wave equation only approximations of at-top beams can be created. There exists a myriad of techniques to select such beams and while the majority these approaches can be efficient, these outputs are typically at a fixed plane and evolve to non-at beams beyond that plane. Methods to increase the depth of focus of at-top beams include the use of specially designed diffractive elements. These approaches hold well for larger beams, however, they are sensitive to the input beam size and the performance is drastically reduced for smaller beams. An attractive approach is the selection of propagation invariant vector at-top beams through the superposition of a Gaussian beam and a vortex beam of orthogonal polarizations. Such beams have been selected through Spatial Light Modulators (SLM) and hold a at plateau through the target plane of a convergent lens. The poor power handling capability of the SLM, however, restricts its application at higher powers. In this work we explore the amplification, through a single stage end-pumped amplifier, of vector at-top beams as selected through a Mach Zehnder interferometer. We demonstrate stable output properties and up to a 3x increased power output as compared to each beam in the corresponding superposition. This approach presents a useful advance in at-top beams for high power applications.

Master Oscillator Power Amplifier (MOPA) systems have garnered considerable interest to power scale low power seed beams (from the Master Oscillator) to higher powers. These systems are favourable to obtain the desired laser beam performance when additional optical elements such as Bragg gratings are required. These elements do not have to endure high optical intensities inside high-power oscillators and do not affect the beam quality or power efficiency. Power amplifiers consist of two primary architectures that include end-pumping and side-pumping. The latter offers greater amplification at the expense of increased thermally-induced beam distortions such as thermal lensing, while the former offers more controllable and efficient power scaling with a limit on the amplification potential. To date, the theoretical models describing the characteristics of end-pumped systems have been limited to two-dimensional crystal architectures with an approximation of the thermal lens as a two-dimensional element. In general, the optical pumping beam is over-simplified and does not reflect realworld spatial evolution over the entirety of the crystal rod length. These approximations hold for thin crystals operating under small-signal amplification, however, they are inaccurate for high signal amplification in long crystal rod geometries. In this work, we explore three-dimensional crystal rods in end-pumped configuration, using an in Infinitesimally sliced model, to study the amplification potential and thermal lens in end-pumped power amplifiers in greater detail while using beam shaping theory to model the pump beam transformation accurately over the length of the crystal. We verify our theoretical approach experimentally using a single amplifier stage in double-pass configuration for power scaling of Gaussian beams. We demonstrate over 95% correlation between our model and the corresponding experiment and show that this correlation extends over the small and high signal amplification regions. The improved model, experimental techniques and results outlined here will provide a valuable tool for advances towards optimizing high brightness amplifiers.

Blue laser light has unique capabilities for material processing of copper gold or aluminium. The energy absorption efficiency at 450 nm in gold is orders of magnitude higher than the absorption at typical infrared wavelengths. Modern blue laser diodes enable very high energy densities. For focusing the laser beam usually fused silica optics are used. Fused silica is known for the very high solarization stability over wide range wavelengths but optical designs are limited to its specific optical position (refractive index and Abbe number). It would be beneficial for the optical design in terms of performance and flexibility to use optical glass for such applications. Only limited data is available on the solarization behavior of optical glass for high power laser radiation at 450 nm. This paper discusses the requirements on optical materials used for blue laser processing applications regarding long time stability aspects, showing recent results in the development of solarization stable glass optical glasses.

The laser stealth dicing system is a unique wafer processing system to enable high-throughput and debris-free wafer dicing. In the laser stealth dicing system, a focused pulsed laser forms a modification layer and cracks inside a silicon wafer. During pulsed laser radiation and wafer scanning, cracks formed in the previous shot interfere with a focusing laser pulse on a defocus plane. As a result, a part of interacted laser beam scatters, generating back-side splash defects on the device layer. An asymmetric beam shaping of defocused spot becomes necessary to minimize the splash defects. At the same time, the focused spot should maintain sufficiently small focused spot size to generate a modified layer inside the silicon wafer. This paper presents a concept of an innovative focusing spot shaping with an asymmetrical pupil phase filter which is individually optimized for both defocused and focused spots. We optimized the phase filter to generate a threedimensionally- asymmetrical focused spot which deforms defocused spot with a near diffraction-limited focal spot size. The through-focus spot shaping technique is enable to minimize splash defects and improve the yield of the laser wafer dicing process.

Optical phase conjugation using computer generated holograms takes advantage of the ability to tailor a wavefront for specific applications. This includes directing laser beams through obscuring or distorting media, holographic image projection, and beam tracking of various targets. These techniques employ spatial light modulators (SLM) which tend to be the limiting component in terms of resolution, response time, efficiency and cost. For beam tracking, system field of regard and target spot size depend directly on SLM resolution. We are investigating techniques to tile multiple SLMs to provide increased resolution using existing devices. Cost and complexity are a major focus. We use a ferroelectric liquid crystal SLM (Forth Dimension Displays Ltd) due to their low cost, high resolution (1024x1280) and the ability to generate phase-only binary holograms. Even with relatively inexpensive SLMs, we develop techniques to compensate for SLM non-uniformities and distortions introduced by system optics. Our approach uses a simple algorithm to measure the cumulative phase error introduced by the SLM, system optics and geometry and then to subtract that error from any ideal pattern imposed on the SLM. We present results showing the effectiveness of this technique.

Optical Coherence Tomography (OCT) is an imaging technique that performs high-resolution transverse and axial imaging of samples. We demonstrate a novel mechanical-scan free Spectral Domain OCT scheme that performs axial and transverse scans of the sample avoiding the use of any kind of mechanical platforms. The scheme makes use of a spatial light modulator in combination with some simple optical elements. This proof-of-principle demonstration can help usher in the development of alternative OCT schemes with potentially faster scan speeds than current OCT schemes by avoiding the use of any kind of mechanical platforms.

Wave branching occurs during propagation in a gently disordered medium. It appears in many different physical situations involving diverse length scales: from electron waves refracted in semiconductors to ocean waves deflected by surface eddies. Very recently, this phenomenon has been observed in light by studying the propagation of laser beams in soap films, opening an exciting field of research where the entire machinery of structured light can be brought to bear. Here, we develop computational tools to simulate and characterize the branched flow of light propagating through two-dimensional inhomogeneous media. We present the effects of varying the correlation length of the scattering medium, the influence of shaping the input beam, and the statistical features depicting the branching of light.

In recent years, the on-demand generation of structured polychromatic fields has introduced the possibility of creating more complicated polarization states. However, the study of the curves traced by the polychromatic electric field has been limited to the paraxial case, leading to Lissajous-like curves. In contrast, the new zoo of the 3-dimensional polarization curves remain almost unexplored. In this work, we propose the analysis of the 3-dimensional polarization curves generated by non-paraxial polychromatic beams. The non-paraxial regime is achieved through vector diffraction by an aplanatic lens.

We report the experimental results of the optical beam self-focusing and soliton formation in a photorefractive Fe doped lithium niobate (LN:Fe) crystal caused by a pyroelectric effect. The laser beam at 632.8 nm wavelength and with power of 5 mW and a 10 mm length LN:Fe crystal with the controlled temperature in the range of 10–45C are used. The time evolution of the soliton formation shows approximately two times decrease of the optical beam diameter to ~60 m with simultaneous bending of ~140 m opposite to the crystalline C+-axis. The physical model is developed to explain the experimental results. The generated curvilinear waveguiding channels in the crystal are long-living making them promising for applications.

Structured light with diverse degrees of freedom (DoFs) has recently attracted wide attention. Especially, the ray-wave beams that could be described by both rays and wave packet, can open multiple DoFs by the ray- wave duality. Here a generalized ray-wave beam family is proposed to unify the spatial mode evolution of the azimuthally traveling-wave (TW) and standing-wave (SW) structured light, by introducing a reconfigureable ray-split/fusion structure. We derive an elegant closed-form expression by utilizing frequency-degenerate Ince- Gaussian eigenmodes to construct a new ray-wave beams family to precisely parameterize the ray-split/fusion process, pushing structured light control in higher dimensions. We also experimentally generate these modes by dynamic control of a digital hologram system, revealing their potential applications in optical manipulation and communication.

We develop analytical expressions using the Fresnel diffraction theory of the spatial coherence function of speckle fields generated by scattering a Mathieu beam through a diffuser modelled as a delta correlated phase screen.

Conventional Shack-Hartmann sensor uses Zernike polynomials in order to approximate the wavefront of the light. Zernike approximation is well-known, well-established and widely used technique. And in most cases the quality of approximation is good enough, especially if the measured light beam has circular aperture. But when the light beam is rectangular or ringshaped (for example, if one need to measure the surface flatness of the detail that is ring-shaped), the approximation using Zernike polynomials fails. In this work we implemented the approach of the approximation of the wavefront using Bspline polynomials. We present the results of approximation of a complex simulated wavefront (Franke surface) and an experimentally measured wavefront of the ring-shaped detail using B-Spline polynomials.

Bimorph deformable mirrors can be successively used to improve focusing of a laser beam passed through a moderately scattering medium with optical density in the range of 1 ... 10. In this paper we investigate the efficiency of the stacked actuator deformable mirror with 61 piezo stacks and clear aperture of 60 mm. We demonstrate that such kind of mirrors also can be used to optimize the focal spot in the far-field. Shack-Hartmann sensor was used to measure the averaged wavefront distortions and CCD camera was used to estimate the intensity distribution of the focal spot in the far-field.