Characterization of holographically generated beams via phase retrieval based on Wigner distribution projections

Adaptive turbulence compensation with a hybrid input-output algorithm in orbital angular momentum-based free-space optical communication

Atmospheric turbulence is a major challenge for practical orbital angular momentum (OAM)-based free-space optical (FSO) communication systems that causes intermodal crosstalk and degrades the performance of the system. Herein, we propose a hybrid input-output algorithm (HIOA)-based adaptive optics (AO) system to compensate for distorted OAM beams. The principle and parameters of the HIOA-based AO system in an OAM-based FSO system are analyzed, and the performance is discussed. The simulation results indicate that the HIOA-based AO system can effectively correct distorted OAM beams and that the HIOA improves the compensation performance and convergence speed compared to the traditional Gerchberg-Saxton algorithm. Moreover, we analyze the compensation performance based on different probe beams. Using an OAM beam with state l=1 as a probe beam can yield better correction effects than a Gaussian beam. This work verifies the feasibility of using an HIOA for adaptive turbulence compensation and provides new insights into OAM communication systems.

High efficiency, low cost holographic optical elements for ultracold atom trapping

We demonstrate a method of creating high efficiency, high fidelity, holographic optical elements for the generation of complex optical fields, in a low cost photopolymer, Bayfol HX. The desired optical field profile is generated by a spatial light modulator and written into an optically addressable photopolymer as a volume hologram. We demonstrate the utility of this approach by trapping a Bose-Einstein condensate of rubidium-87 atoms in the nodal plane of an HG_0,1 mode generated by one of these holographic optical elements. We also extend this method to the generation holograms with twice the angular momentum per photon than can be generated with a given spatial light modulator.

Programmable simulator for beam propagation in turbulent atmosphere

The study of light propagation though the atmosphere is crucial in different areas such as astronomy, free-space communications, remote sensing, etc. Since outdoors experiments are expensive and difficult to reproduce it is important to develop realistic numerical and experimental simulations. It has been demonstrated that spatial light modulators (SLMs) are well-suited for simulating different turbulent conditions in the laboratory. Here, we present a programmable experimental setup based on liquid crystal SLMs for simulation and analysis of the beam propagation through weak turbulent atmosphere. The simulator allows changing the propagation distances and atmospheric conditions without the need of moving optical elements. Its performance is tested for Gaussian and vortex beams.

Free space optical communication using beam parameters with translational and transverse rotational invariance

Two natural requirements on a measurable quantity possessed by a paraxially propagating light-field to be suitable for free space optical communication are invariance under free space propagation and invariance under transverse plane rotation. While the former invariance ensures that the measurable quantity is robust while signalling through free space, the latter invariance ensures that a detector measuring the quantity can be oriented at any angle in the transverse plane, and a measurement by the detector yields the same value for the quantity irrespective of the transverse angle, thus avoiding alignment issues. The variance matrix of a paraxially propagating light-field is analyzed from the perspective of the aforementioned invariances. That the "charge" of a paraxial light-field, which is contained in the variance matrix, and which has been previously well studied for its suitability toward free space optical communication, possesses these two invariance properties, emerges naturally in the analysis. Seven functionally independent quantities other than charge, which are derived from the variance matrix, and which share these invariances, are presented and studied for their suitability toward signalling through turbulent atmosphere using the low-order Hermite-Gaussian modes. It is found that the spot size of a Gaussian light-field can be effectively used as a switch, to communicate through short distances in a turbulent atmosphere.

Intensity-symmetric Airy beams

Theoretical, numerical, and experimental research on a novel family of Airy beams in rectangular coordinates having a symmetric transverse pattern of light intensity is presented. The intensity-symmetric Airy beams include both the symmetric Airy beam whose field amplitude is an even function of the transverse coordinates and the antisymmetric Airy beam whose field amplitude is an odd function of such coordinates. The theoretical foundations are based on the relationship of the symmetries of the spectral phase with the cosine and sine Fourier transforms. These beams are analyzed in a propagation range also including the region preceding the Fourier plane. These beams exhibit autofocusing, collapse, self-bending, and reversal propagation. Moreover, the intensity distribution is strongly asymmetric with respect to the Fourier plane. All these peculiar features were not reported for other classes of paraxial beams in a rectangular frame. The experimental generation of intensity-symmetric Airy beams is demonstrated supporting the theoretical predictions. Possible applications in planar waveguide writing and optical trapping are also discussed.

Symmetric Airy beams

In this Letter a new class of light beam arisen from the symmetrization of the spectral cubic phase of an Airy beam is presented. The symmetric Airy beam exhibits peculiar features. It propagates at initial stages with a single central lobe that autofocuses and then collapses immediately behind the autofocus. Then, the beam splits into two specular off-axis parabolic lobes like those corresponding to two Airy beams accelerating in opposite directions. Its features are analyzed and compared to other kinds of autofocusing beams; the superposition of two conventional Airy beams having opposite accelerations (in rectangular coordinates) and also to the recently demonstrated circular Airy beam (in cylindrical coordinates). The generation of a symmetric Airy beam is experimentally demonstrated as well. Besides, based on its main features, some possible applications are also discussed.

Comparing efficiency and accuracy of the kinoform and the helical axicon as Bessel-Gauss beam generators

We compare two phase optical elements that are employed to generate approximate Bessel-Gauss beams of arbitrary order. These elements are the helical axicon (HA) and the kinoform of the desired Bessel-Gauss beam. The HA generates a Bessel beam (BB) by free propagation, and the kinoform is employed in a Fourier spatial filtering optical setup. As the main result, it is obtained that the error in the BBs generated with the kinoform is smaller than the error in the beams obtained with the HA. On the other hand, it is obtained that the efficiencies of the methods are approximately 1.0 (HA) and 0.7 (kinoform).

Partially coherent stable and spiral beams

Stable and spiral coherent beams, which do not change the form of their intensity distribution apart from possible scaling and rotation during propagation and therefore possess self-healing properties, are widely applied in science and technology. On the other hand, it has been found that partially coherent light often provides better output than coherent light. Here we consider two methods for the design and experimental generation of partially coherent stable and spiral beams.

Femtosecond spatial pulse shaping at the focal plane

Spatial shaping of ultrashort laser beams at the focal plane is theoretically analyzed. The description of the pulse is performed by its expansion in terms of Laguerre-Gaussian orthonormal modes. This procedure gives both a comprehensive interpretation of the propagation dynamics and the required signal to encode onto a spatial light modulator for spatial shaping, without using iterative algorithms. As an example, pulses with top-hat and annular spatial profiles are designed and their dynamics analyzed. The interference of top-hat pulses is also investigated finding potential applications in high precision pump-probe experiments (without using delay lines) and for the creation of subwavelength ablation patterns. In addition, a novel class of ultrashort pulses possessing non-stationary orbital angular momentum is also proposed. These exotic pulses provide additional degrees of freedom that open up new perspectives in fields such as laser-matter interaction and micro-machining.

Shaping of light beams along curves in three dimensions

We present a method for efficient and versatile generation of beams whose intensity and phase are prescribed along arbitrary 3D curves. It comprises a non-iterative beam shaping technique that does not require solving inversion problems of light propagation. The generated beams have diffraction-limited focusing with high intensity and controlled phase gradients useful for applications such as laser micro-machining and optical trapping. Its performance and feasibility are experimentally demonstrated on several examples including multiple trapping of micron-sized particles.

Optical coherenscopy based on phase-space tomography

Partially coherent light provides attractive benefits in imaging, beam shaping, free-space communications, random medium monitoring, among other applications. However, the experimental characterization of the spatial coherence is a difficult problem involving second-order statistics represented by four-dimensional functions that cannot be directly measured and analyzed. In addition, real-world applications usually require quantitative characterization of the local spatial coherence of a beam in the absence of a priori information, together with fast acquisition and processing of the experimental data. Here we propose and experimentally demonstrate a technique that solves this problem. It comprises an optical setup developed for automatized video-rate measurement and a method -phase-space tomographic coherenscopy- allowing parallel data acquisition, processing, and analysis. This technique significantly simplifies the spatial coherence analysis and opens up new perspectives for the development of tools exploiting the degrees of freedom hidden into light coherence.

Full-range, complex spatial light modulator for real-time holography

We demonstrate a full-range complex and transmissive spatial light modulator (SLM) for simultaneous and independent amplitude and phase modulation of an input wave field. Arbitrary scalar complex optical fields are generated by stacking a pixelated liquid crystal display operating in phase-only (2π) modulation with passive polarization-sensitive components. The principle is based on optical combining the light fields of two neighboring phase-only modulating pixels, which were made orthogonally polarized by a structured half-wave plate, then passing through a birefringent plate to laterally shift one of the beams collinear to the other, and finally bringing to interference by a linear polarizer. Complex modulation by the proposed SLM is experimentally verified in monochrome green operation.

Coaxial holographic encoding based on pure phase modulation

We describe a simple technique for coaxial holographic image recording and reconstruction, employing a spatial light modulator (SLM) modified in pure phase mode. In the image encoding system, both the reference beam in the outside part and the signal beam in the inside part are displayed by an SLM based on the twisted nematic LCD. For a binary image, the part with amplitude of "1" is modulated with random phase, while the part with amplitude of "0" is modulated with constant phase. After blocking the dc component of the spatial frequencies, a Fourier transform (FT) hologram is recorded with a uniform intensity distribution. The amplitude image is reconstructed by illuminating the reference beam onto the hologram, which is much simpler than existing phase modulated FT holography techniques. The technique of coaxial holographic image encoding and recovering with pure phase modulation is demonstrated theoretically and experimentally in this paper. As the holograms are recorded without the high-intensity dc component, the storage density with volume medium may be increased with the increase of dynamic range. Such a simple modulation method will have potential applications in areas such as holographic encryption and high-density disk storage systems.

Wavefield imaging via iterative retrieval based on phase modulation diversity

We present a fast and robust non-interferomentric wavefield retrieval approach suitable for imaging of both amplitude and phase distributions of scalar coherent beams. It is based on the diversity of the intensity measurements obtained under controlled astigmatism and it can be easily implemented in standard imaging systems. Its application for imaging in microscopy is experimentally studied. Relevant examples illustrate the approach capabilities for image super-resolution, numerical refocusing, quantitative imaging and phase mapping.