This PDF file contains the front matter associated with SPIE Proceedings Volume 10744, including the TItle Page, Copyright information, Table of Contents, Author and Conference Committee lists.

The ultimate goal of dark ray optics" is to control and manipulate the trajectories of phase singularities -i.e., day rays- in an analogous way as we deal with ordinary rays. The development of these analogies requires an adequate mathematical description. We will describe the fundamental concepts of this mathematical approach by emphasizing the essential role played by the symmetry properties of the optical elements used to manipulate highly charged or multi-singular Gaussian beams. We will unveil the nontrivial connections between discrete symmetry, OAM and the topological properties of these beams embedding nontrivial dark ray structures.

In this work, we present the implementation of an experimental setup for controlling the phase gradient of arbitrary light beams, using a spatial light modulator. Simple arbitrary shapes are initially proposed based on their parametric equations, and the desired beam shape, as well as its phase behavior are interpreted through an algorithm. The analysis of the electric field distribution and its manipulation through the topological charge and the normal direction respect every position of an arbitrary shape, allow to encode in a Spatial Light Modulator the behavior of the phase of a light beam. The far field intensity profiles are captured, studied and compared to those designed. The phase of a set of generated beams is tested using a linear polarizer as an analyzer and by an optical trapping setup.

Invited Talk

Tailored optical fields whose polarization and phase are coupled in a non-separable fashion usually referred to as vector beams, have become ubiquitous in a wide variety of research fields. These complex light fields can be generated through a combination of physical optical elements such as spiral phase plates, q-plates and quarter wave plates. However, such "hard-coded" optical devices present some restrictions such as their inability to generate multiple structured light fields simultaneously. This has stimulated a wide range of generation techniques based on computer controlled devices such as spatial light modulators (SLMs). SLMs provide on demand, fast and flexible holographic means to create multiple tailored optical fields simultaneously (multiplexing). Here we present a novel technique that utilizes the superposition principle, enabling digital generation of many vector beams on a single hologram. Unlike other techniques, this method is purely digital and allows the generation of a wide variety of vector beams by simply changing the encoded digital hologram. As a proof of concept we demonstrate simultaneous generation of vector beams with different polarization states on the High Order Poincaré Sphere (HOPS). This novel technique will be of impact in the context of quantum and classical communication, optical micro-manipulation and super resolution microscopy.

Commercial non-tunable q-plates have become popular optical elements to generate vector beams at the design wavelength where the device exhibits half-wave (HW) retardance. However, their use is restricted since both the topological charge and the operating wavelength are set in fabrication. In this work, we report how to make such commercial q-plates more versatile for generating vector beams of higher topological charge and in different wavelength ranges. First, we show how to add, subtract or change the sign of the charge, by combining q-plates with HW plates. Second, we perform a broadband spectral characterization of the q-plate retardance, and identify the wavelengths with retardance values relevant for vector beam generation π, ±π/2). The wavelength is then used as a tuning parameter to change the device performance from a HW q-plate to a QW q-plate. The vector beams expected at these QW wavelengths are obtained as a superposition of the input polarization state and the output state of a HW q-plate. Experimental results are shown using the red and blue lines of an Ar-Kr laser. For input linearly polarized light of 488 nm the device generates hybrid vector beams (where the ellipticity varies with the azimuthal angle), while for 647 nm pure radial vector beams with constant ellipticity are obtained. These results could extend the use of commercial q-plates for multicolor vector beam applications.

In this work, we discuss the generation of spatially inhomogeneous polarization beams (SIPBs), by using two liquid crystal polarization holograms (PHs). The first PH generates two circularly opposed polarized scalar beams, which are collinearly recombined by the second PH. Owing to the tunability of the liquid crystal birefringence, we demonstrate experimentally the generation of SIPBs with efficiency near to 100%. By taking advantage of the diffraction properties, the high efficiency, and the intrinsic achromaticity of the polarization holograms, the method aims to overcome the limitations related to stability and efficiency, making it attractive for applications.

We report here acquisition of Pancharatnam Phase by higher order C-point singularities. This geometric phase is gained when these polarization singularities undergo discrete cyclic transformations over the closed geodesical trajectories on the fundamental Poincaré Sphere. It is shown that the amount of Pancharatnam Phase acquired by higher order C-points is determined by the solid angle subtended by the closed trajectory at the centre of the Poincaré Sphere and the index of the higher order C-point. We use index inversion hopping mechanism to demonstrate this Pancharatnam excursions of higher order C-points.

We present polymer-based holograms with varying structure angle for each individual micrometer-sized pixel. The holograms are numerically calculated utilizing an iterative optimization procedure including a first-order Taylor series expansion to determine the slope of each pixel. The holograms are fabricated by 3D laser lithography. Due to the small pixel sizes and the individual slope of each pixel aliases can be avoided and the overall intensity of the desired projection is strongly increased compared to holograms consisting of larger and uniform pixels. Furthermore, the discrepancy between calculated and measured intensity distribution is strongly reduced.

In the field of micro- and nanostructuring multibeam ultra short pulsed (USP) laser processing attracts increasing attention due to its ability to generate periodic pattern with high throughput. For the generated structures, there is a wide range of applications including e.g. functional or design structures for the automotive industry, consumer electronics or filtration technology.

Compared to state-of-the-art single-beam laser drilling by means of pulsed femto- and picosecond laser radiation, the multibeam approach allows for the application of high pulse energies maintaining the surface quality of a typical USP laser process. On the one hand the homogenous distribution of the applied pulse energy across the surface of the processed workpiece by a multitude of beamlets results in a reduced process time and a more economic laser process. On the other hand fundamental aspects of process strategies and thermal management have to be reconsidered.

Due to the number (>100) of beamlets focused onto a relatively small scan field area of less than 4x4 mm, the relevance of contamination, microscopic and macroscopic heat accumulation become increasingly relevant. Therefore, specially designed scanning strategies, suction units and additional surface treatments have to be applied to generate hole pattern with packing density of more than 30 percent and reduced surface contamination.

Engraving a holographic image directly on metal surface has a great potential application. Common laser engraving devices can produce only limited grayscale image. Some of devices can create colors by effect of discoloration.

To make effective and controllable diffractive gratings the optical system was developed combining high power laser source and interferometric dot-matrix approach. The paper presents considerations and some experimental data.

The direct interference gratings were formed on flat metal surfaces like Ni, Cr, brass and stainless steel like AISI 305. Stable structures with adequate diffraction efficiency were obtained. The experimental setup used 1064 nm high-frequency pulsed fiber laser. The samples shows a threshold character of the interaction that requires beam-shaping devices to convert Gaussian distribution to flattop with minimum power losses. The resulting diffraction gratings had 275 lines per mm and nearly sine profile. Pi-shaper 6_6_1064 allowed most effective filling of metal surface thus giving good grating spatial uniformity and diffraction efficiency.

Direct holographic gratings on flat metal surfaces were obtained and prototype dot matrix machine suggested. Spatial frequency needs to be increased to get better color separation and the range of materials expanded. We expect the new approach to improve quality and security features of holographic security marking.

Most automated holographic origination systems work in “stop-and-repeat” mode thus making framed fringe structure. It means that the final holographic image consists of “bricks”. So, origination systems giving non-fragmented, continuous fringe structure is useful for expert verification. “Light pen” origination systems (LP) make continuous rainbow lines scanning a photoresist plate with a focused laser spot. Intensity distribution across the spot effects greatly on diffraction efficiency and image quality. Beam shaping, not exactly gauss-to-flat, has a critical value for such systems.

Positive photoresists are in wide use for rainbow hologram origination. The media have high contrast and very narrow linear part of an exposure curve. The Gaussian intensity profile of a scanning spot gives over-exposure in the center of rainbow line and under-exposure next to its periphery. A rainbow lines lose brightness and edge sharpness. The device developed was equipped with 6_6_VIS pi-shaper for 405 nm. The device optical scheme and a number of practical aspects are considered. A circular spot even with flattop gives non-uniform exposure across a rainbow line. Two solutions were studied: M-shape spot and a flattop square spot. Both can be done with Pi-shapers.

M-spot gives better results for smooth lines, while square aperture is better for sharp tracks.

Pi-shaper required special attention for the alignment of the system components especially to tilts and shifts. The system developed allows high-speed hologram origination, adequate security features for expert authentication and prevents the possibility of being counterfeited by most recording systems.

We present an idea to use optical vortex as a spatial light modulator (SLM) phase modulation depth marker. It can open a way to fast and efficient SLM calibration, which is required especially for SLMs that are able to change between introduced phase shift. Additionally, it may be used to control optical system aberrations and proper SLM phase shift, simultaneously. In this paper, the typical calibration method is discussed in brief. Next the new method based on vortex quality inspection is presented. The idea of the new method is preliminary verified in experiment, which is followed by a discussion of further research, that need to be done.

We have invented an efficient phase-only light shaping modality that can simultaneously control the distribution of multiple beams and shaping of these beams individually in a volume. It is coined Holo-GPC and extends the capabilities of both Generalized Phase Contrast (GPC) and Holography. Holo-GPC can be considered as a hybrid combination of holography that can create extended 2D or 3D beam distributions and GPC that forms noise-free sculpting of the individual beams.

Measurement systems for metrology incorporating laser-triangulation methods have the problem of speckle noise. This noise is an effect of the coherence of the laser light in combination with the projection onto rough object surfaces. In this contribution, we show results for using spatial light modulators within a simple and effective method in order to reduce the speckle noise in laser-based triangulation.

In the last decades, spatial light modulators have been intensively used for different applications in optical measurement systems. Today, the elements have high enough resolutions to be used even for simple holographic applications. We generate dynamic holograms with a pixelated spatial light modulator by inscribing multiple holograms. The laser-illuminated holograms microscopically translate the measuring point in the object plane. Due to the minimal different spot positions, the speckle patterns are also subject of change.

By averaging of the intensity field in the camera plane the speckle noise can be reduced and the accuracy of the spot's position measurement is increased. Furthermore, experimental measurements show features of correcting spot deformations due to optical system aberrations like defocus, astigmatism and coma.

The controllable axial long depth of focus (DOF) is required in many optical applications, and the axial DOF of a light beam can be controlled by diffractive optical elements (DOEs). However, the trade-off between the sidelobe and controllable DOF often be ignored, which is difficult to implement their desired effect in many applications. In this paper, a modified Gerchberg—Saxton (GS) algorithm is presented for generating DOEs to realize suppressed sidelobe and controllable axial DOF in an acceptance variation of normalized intensity and sidelobe. In the simulation, we optimized the DOEs to control the length of DOF, suppress sidelobe and calculate spot size, and the corresponding length of DOF is within the range of 60λ to 600λ with a different portion (1-7) multi-pure phase DOEs at the numerical aperture is 0.035. Experimental results are also shown to demonstrate the effectiveness of the proposed algorithm. We anticipate that this optical system can be used in laser fabrications, including optical manufacture, highly dynamic cutting and high-quality cutting.

A novel optical architecture is designed and fabricated in order to overcome the limits of the traditional sorter based on log-pol optical transformation for the demultiplexing of optical beams carrying orbital angular momentum (OAM) of light. The proposed configuration simplifies the alignment procedure and significantly improves the compactness of the optical device. In addition, the integration of an optical fan-out provides higher resolution on OAM modes separation. Since the miniaturization level and the optical configuration require to operate beyond the paraxial approximation, a rigorous formulation of transformation optics in the non-paraxial regime is developed and applied for the first time. Samples have been fabricated as 256-level phase-only diffractive optics with high-resolution electron-beam lithography and tested for the demultiplexing of optical vortex superposition in a 8-bit free-space optical link. The results confirm the expected high efficiency and resolution in OAM sorting.

Multi-plane light conversion is a method of performing spatial basis transformations using cascaded phase plates separated by Fourier transforms or free-space propagation. In general, the number of phase plates required scales with the dimensionality (total number of modes) in the transformation. This is a practical limitation of the technique as it relates to scaling to large mode counts. Firstly, requiring many planes increases the complexity of the optical system itself making it difficult to implement, but also because even a very small loss per plane will grow exponentially as more and more planes are added, causing a theoretically lossless optical system, to be far from lossless in practice. Spatial basis transformations of particular interest are those which take a set of spatial modes which exist in the same or similar space, and transform them into an array of spatially separated spots. Analogous to the operation performed by a diffraction grating in the wavelength domain, or a polarizing beamsplitting in the polarization domain. Decomposing the Laguerre-Gaussian, Hermite-Gaussian or related bases to an array of spots are examples of this and are relevant to many areas of light propagation in free-space and optical fibre. In this paper we present our work on designing multi-plane light conversion devices capable or operating on large numbers of spatial modes in a scalable fashion.

Higher-order modes are controllably excited in water-filled kagomè-, bandgap-style, and simplified hollow-core photonic crystal fibers (HC-PCF). A spatial light modulator is used to create amplitude and phase distributions that closely match those of the fiber modes, resulting in typical launch efficiencies of 10–20% into the liquid-filled core. Modes, excited across the visible wavelength range, closely resemble those observed in air-filled kagomè HC-PCF and match numerical simulations. These results provide a framework for spatially-resolved sensing in HC-PCF microreactors and fiber-based optical manipulation.

In this paper we consider numerical model of an adaptive coherent fiber array system that is utilized for remote transmission of laser power to an active photovoltaic cell (PVC)-based receiver array. The PVC array performs optical-to-electrical power conversion, and provides a feedback signal that is sent to the laser transmitter via optical and/or RF link. The feedback signal is utilized for real-time adaptive shaping of laser power density distribution at the PVC array for achieving the following objectives: (a) Minimization of laser power losses caused by mismatch between size and shape of the transmitter beam footprint and the PVC array. For optimal performance, the projected laser beam footprint should be adaptively changed to fit the PVC area under continuously changing turbulence strength, distance to the target, system field of view, platform jitter, etc. and (b) Reduction of laser beam power fluctuations inside the PVC caused by errors in target/load tracking, and laser beam aimpointing and aimpoint stabilization. In the numerical simulations the optical power with adaptive beam shaping was performed over 3 km and 7 km distances in turbulent atmosphere. The results demonstrate ability of the adaptive fiber array systems with 21 sub-apertures considered, for efficient adaptive beam shaping resulting in significant power beaming efficiency improvement.

Acousto-optic Bragg diffraction is an electronically controllable angular-selective type of laser beam scattering. A variety of transfer functions of acousto-optic Bragg diffraction in crystals is observed from the point of view of laser beam shaping. Those include one-dimensional isotropic filtering, and two-dimensional anisotropic X- and O-type transfer functions. Those types of transfer functions are useful for obtaining flat-top laser beam shaping, focusing, and wavefront sensing. Dynamic generation of ultrasonic waveforms is discussed as a method for adaptive control of the transfer function shape.

The correction of the laser beam aberrations and the formation of the laser beam intensity is very important scientific task. This problem widely is being decided at this moment. The employment of the bimorph deformable mirrors for this kind of applications is very promising. But this type of the wavefront correctors has one reasonable shortcoming – low spatial resolution of the control electrodes, it doesn’t allow to compensate for the high-order wavefront aberrations. This kind of aberrations is valuable for imaging applications, mainly if needs to reconstruct specific details. Therefore, we have to use the wavefront correctors with high spatial resolution of the electrodes. In this work we present two types of the bimorph deformable mirrors for solving this problem – multilayer bimorph (multimorph) mirrors and bimorph mirrors with high density of the control electrodes. To place high number electrodes on the piezodisk the laser engraving technology was used, and ultrasonic welding technology used to make the wire connection to these electrodes. We developed the powerful numerical model to simulate bimorph mirrors.

The latest results on intensity distribution transformation from Gaussian to a flattop and doughnut are presented in the paper. The wavefront was modified with bimorph deformable mirror to reach the desired intensity distribution in the farfield. LC phase modulator was also considered as an alternative device for laser beam shaping. The theoretical calculations and experimental results of the efficiency of different types of wavefront correctors are given.

This paper focuses on femtosecond laser cutting and drilling using a patented technology for suppressing the conicity generated by the ablation saturation. We will show that it is now possible to use a high power femtosecond laser, of several hundreds of watts, for zero taper cutting by considering a precession movement on the beam like a donut beam shaping and by applying a beam splitting on this engineered beam to feed several standard scanners.

We have proposed and demonstrated the generation of high power, wide band square-wave pulse in a figure-8 Yb-doped mode-locked fiber laser. The fiber laser operates at 1083.65 nm with 3 dBbandwidth of 6.5 nm, which can emit nanosecond square pulses with a maximum average output power of 1.31 W and peak power 40 W, respectively. The bandwidth, to the best of our knowledge, is the widest output bandwidth in a square-wave pulse fiber laser operating at 1 μm band.

This paper presents our latest experimental results of welding and cutting of steel and aluminum using a High Power, High Brightness Direct Diode Laser (DDL) upgraded with a Laser-Integrated Dynamic Beam Shaper (DBS). TeraDiode’s DDLDBS laser system transforms the beam distribution from Gaussian-like to Doughnut shape, and a beam parameter product from 4 to 25mm·mrad and doing so in a continuous, real-time manner with very low latency below 10ms. These capabilities unlock a wide range of processing parameters and an overall improved quality for metal cutting and welding while using simplified processing heads. This capability can be applied to DDL from 500W to 8000W power range.

Laser welding by means of multi-kilowatt solid state lasers can be considerably improved if the focused welding spot is embedded in a pre-heating spot generated e.g. by an additional laser. To improve the compactness of the optical system, the same functionality can be effectively achieved by means of diffractive diffusers. Because such a diffractive optical solution may suffer from the presence of speckles, a comprehensive characterization of the laser source is performed. The paper includes the design, the compensation of the difference in the intensity levels, the fabrication and the optical performances of the fabricated DOEs. We furthermore present the functionality of the DOEs in the welding process.

Older wire-based interconnection technology has a way of finding new applications. In many cases traditional wire provides a means for overcoming the shortfalls of the latest and most sophisticated micro flex circuit techniques. Use of wire seems somewhat antiquated. However small gauge coated wire, such as 38-gauge (0.004” diameter) stainless steel, offers benefits when applied to newer medical and automotive sensor applications where coiling or excessive vibration requires a wire connection with greater flexibility. Years ago, wire conductors of this size were used in precision applications such as read/write heads for disk drives, only to be replaced by more efficient assembly methods that leveraged the use of micro flex circuits. Even with these changes in the marketplace, newer applications emerge that benefit from such small wire, coated with a dielectric such as Teflon or polyimide. This trend has led to a resurgence for precision wire stripping or precision laser micromachining technology. UV laser micromachining, utilizing imaging of a laser beam, such as an Excimer or UV DPSS laser, provides a minimally thermal ablative process for the removal of dielectric coatings while minimizing the heat affected zone on the underlying exposed wire. These laser beams are first shaped to illuminate an aperture that is then imaged to perform a high finesse ablation process to remove away the coating in precise locations along the wire. This paper outlines several laser beam shaping techniques used to ablate such micro-sized wires and wire harnesses with an emphasis on laser beam shaping methods that are commonly used as well as techniques that allow for maximum throughput. Techniques are explored such as using single sided and retroreflective optics in conjunction with the laser imaging and beam shaping system. Evaluation of a retroreflective wire stripping technique with performance results showing the sophistication of such simple optical designs are presented.

Laser beams structured to have a uniform peak intensity profile ( at-top) have become ubiquitous and a topic of research interest in many fields. These beams can be digitally generated using spatial light modulators (SLMs) where an input Gaussian beam is transformed into a at-top at the focal plane of a focusing lens. However, the resulting uniform peak intensity distribution degrades while the beam propagates in space. Here, we present theoretical simulations and demonstrate experimentally the creation of propagation invariant vector at-top beams. We achieve this by coaxial superposition of a Gaussian and a donut beam with orthogonal polarization states. The polarization state of the generated vector at-top beam is characterized using spatially resolved stokes measurements and compared to theoretical simulations. Such beams will be of great impact in the fields of material processing and optical trapping.

The interesting propagation properties of periodical gratings have been widely studied in the last decades. A special characteristic involving periodical structures is the self-imaging, in free propagation. Commonly the Talbot effect only consider the scalar nature of the fields, however the vectorial nature of the fields also plays an important role in free propagation. In this work, we study theoretically and experimentally the free propagation at fractional Talbot distances of inhomogeneous polarization periodical gratings, which are characterized by orthogonal polarization states. We found that the polarization states change in a peculiar way at different planes. We also generate some particular polarization gratings by means of a spatial light modulator and demonstrate that the theoretical and experimental results are in good agreement.

We study the spatial coherence properties of partially coherent vortex beams. The mathematical model employed for the generation and control of the spatially partially coherent beam is based on a statistical model, in which the field is constructed from the incoherent superposition of an ensemble of individual vortices carrying topological charges of different values and handedness. Results show that if the ensemble from which the partially coherent vortex is generated consists of a mixed composition of individual vortices it is not always possible to determine its topological charge unambiguously.

Current communication systems make use of polarization and wavelength multiplexing to increase transmission rates. To offer a further improvement, Orbital Angular Momentum (OAM) modes (or Laguerre-Gaussian modes) provide an infinite dimensional space. In this work, Laguerre-Gaussian modes are multiplexed and de-multiplexed by means of two Spatial Light Modulators (SLMs), making use of both their azimuthal and radial degrees of freedom. Due to the orthogonality of these modes, a modal decomposition technique is employed to detect the transmitted modes. Here we demonstrate this concept by transmitting an image over a 150 meter free-space link. The free-space link is also characterized in terms of its optical turbulence.