Free Space Optical Polarization De-multiplexing and Multiplexing by means of Conical Refraction

Polarization de-multiplexing and multiplexing by means of conical refraction is proposed to increase the channel capacity for free space optical communication applications. The proposed technique is based on the forward-backward optical transform occurring when a light beam propagates consecutively along the optic axes of two identical biaxial crystals with opposite orientations of their conical refraction characteristic vectors. We present experimental proof of usefulness of the conical refraction de-multiplexing and multiplexing technique by increasing in one order of magnitude the channel capacity at optical frequencies in a propagation distance of 4m

Optical Nonlinear Correlations in Disordered Media

Imaging through scattering and random media is an outstanding problem that to date has been tackled by either measuring the medium transmission matrix or exploiting linear correlations in the transmitted speckle patterns. However, transmission matrix techniques require interferometric stability and linear correlations such as the memory effect, can be exploited only in thin scattering media. Here we uncover an unexpected nonlinear correlation in randomly scattered fields that connects different realisations of the scattering medium and exists in the absence of the speckle memory effect. Besides the novelty of the nonlinear relationship itself, these results provide a route to imaging through dynamic and thick scattering media with applications for deep-tissue imaging or imaging through smoke or fog

Conical refraction healing after partially blocking the input beam

In conical refraction, when a focused Gaussian beam passes along one of the optic axes of a biaxial crystal it is transformed into a pair of concentric bright rings at the focal plane. We demonstrate both theoretically and experimentally that this transformation is hardly affected by partially blocking the Gaussian input beam with an obstacle. We analyze the influence of the size of the obstruction both on the transverse intensity pattern of the beam and on its state of polarization, which is shown to be very robust

Variational inference for computational imaging inverse problems

Machine learning methods for computational imaging require uncertainty estimation to be reliable in real settings. While Bayesian models offer a computationally tractable way of recovering uncertainty, they need large data volumes to be trained, which in imaging applications implicates prohibitively expensive collections with specific imaging instruments. This paper introduces a novel framework to train variational inference for inverse problems exploiting in combination few experimentally collected data, domain expertise and existing image data sets. In such a way, Bayesian machine learning models can solve imaging inverse problems with minimal data collection efforts. Extensive simulated experiments show the advantages of the proposed framework. The approach is then applied to two real experimental optics settings: holographic image reconstruction and imaging through highly scattering media. In both settings, state of the art reconstructions are achieved with little collection of training data

Extreme ultraviolet fractional orbital angular momentum beams from high harmonic generation

We investigate theoretically the generation of extreme-ultraviolet (EUV) beams carrying fractional orbital angular momentum. To this end, we drive high-order harmonic generation with infrared conical refraction (CR) beams. We show that the high-order harmonic beams emitted in the EUV/soft x-ray regime preserve the characteristic signatures of the driving beam, namely ringlike transverse intensity profile and CR-like polarization distribution. As a result, through orbital and spin angular momentum conservation, harmonic beams are emitted with fractional orbital angular momentum, and they can be synthesized into structured attosecond helical beams –or “structured attosecond light springs”– with rotating linear polarization along the azimuth. Our proposal overcomes the state of the art limitations for the generation of light beams far from the visible domain carrying non-integer orbital angular momentum and could be applied in fields such as diffraction imaging, EUV lithography, particle trapping, and super-resolution imaging

Laser beams with conical refraction patterns

Laser beams with cone-refracted output from the plane mirror is demonstrated for the first time in lasers based on intracavity conical refraction (CR) phenomenon. Transverse profile of such lasers comprises a crescent ring of CR-like distribution, where any opposite points are of orthogonal linear polarizations. We confirm the existence of such mode of CR lasers by two observations. First, cascaded CR in reflection geometry has been demonstrated for first time and it provides experimental prove that a light beam passed along optic axis of a biaxial crystal, reflected back from a plane mirror and passed again through the crystal is restored. Second, CR cavity mode with CR-like pattern through the plane mirror is experimentally and theoretically demonstrated for the first time

Microparticle manipulation and imaging through a self-calibrated liquid crystal on silicon display

We present in this paper a revision of three different methods we conceived in the framework of liquid crystal on silicon (LCoS) display optimization and application. We preliminarily demonstrate an LCoS self-calibration technique, from which we can perform a complete LCoS characterization. In particular, two important characteristics of LCoS displays are retrieved by using self-addressed digital holograms. On the one hand, we determine its phase-voltage curve by using the interference pattern generated by a digital two-sectorial split-lens configuration. On the other hand, the LCoS surface profile is also determined by using a self-addressed dynamic micro-lens array pattern. Second, the implementation of microparticle manipulation through optical traps created by an LCoS display is demonstrated. Finally, an LCoS display based inline (IL) holographic imaging system is described. By using the LCoS display to implement a double-sideband filter configuration, this inline architecture demonstrates the advantage of obtaining dynamic holographic imaging of microparticles independently of their spatial positions by avoiding the non-desired conjugate images

Type I and type II second harmonic generation of conically refracted beams

Type I and type II second harmonic generation (SHG) of a beam transformed by the conical refraction phenomenon are presented. We show that, for type I, the second harmonic intensity pattern is a light ring with a point of null intensity while, for type II, the light ring possesses two dark regions. Taking into account the different two-photon processes involved in SHG, we have derived analytical expressions for the resulting transverse intensity patterns that are in good agreement with the experimental data. Finally, we have investigated the spatial evolution of the second harmonic signals, showing that they behave as conically refracted beams.Peer ReviewedPostprint (published version

Imaging from temporal data via spiking convolutional neural networks

A new approach for imaging that is solely based on the time of flight of photons coming from the entire imaged scene, combined with a novel machine learning algorithm for image reconstruction: a spiking convolutional neural network (SCNN) named Spike-SPI (Spiking - Single Pixel Imager). The approach uses a single point detector and the corresponding time-counting electronics, which provide the arrival time of photons in the form of spikes distributed over time. This data is transformed into a temporal histogram containing the number of photons per arrival time. A SCNN that converts the 1D temporal histograms into a 3D image (2D image with depth map) by exploiting the feature extraction capabilities of convolutional neural networks (CNNs), the high dimensional compressed latent space representations of a variational encoder-decoder network structure, and the asynchronous processing capabilities of a spiking neural network (SNN). The performance of the proposed SCNN is analysed to demonstrate the state-of-the-art feature extraction capabilities of CNNs and the low latency asynchronous processing of SNNs that offer both higher throughput and higher accuracy in image reconstruction from the ToF data, when compared to standard ANNs. The results of Spike-SPI show an increase in spatial accuracy of 15% over then ANN, using the Intersection of Union (IoU) for the objects in the scene. While also delivering a 100% increase over then ANN in object reconstruction signal to noise ratio (RSNR) from ~3dB to ~6dB. These results are also consistent across a range of IRF (Instrument Response Functions) values and photo counts, highlighting the robust nature of the new network structure. Moreover, the asynchronous processing nature of the spiking neurons allow for a faster throughput and less computational overhead, benefiting from the operational sparsity in the single point sensor

Interferometric characterization of the structured polarized light beam produced by the conical refraction phenomenon

The interest on the conical refraction (CR) phenomenon in biaxial crystals has revived in the last years due to its prospective for generating structured polarized light beams, i.e. vector beams. While the intensity and the polarization structure of the CR beams are well known, an accurate experimental study of their phase structure has not been yet carried out. We investigate the phase structure of the CR rings by means of a Mach-Zehnder interferometer while applying the phase-shifting interferometric technique to measure the phase at the focal plane. In general the two beams interfering correspond to different states of polarization (SOP) which locally vary. To distinguish if there is an additional phase added to the geometrical one we have derived the appropriate theoretical expressions using the Jones matrix formalism. We demonstrate that the phase of the CR rings is equivalent to that one introduced by an azimuthally segmented polarizer with CR-like polarization distribution. Additionally, we obtain direct evidence that the Poggendorff dark ring is an annular singularity, with a π phase change between the inner and outer bright rings.We acknowledge financial support from the Spanish MINECO and Fondos FEDER (FIS2012-39158-C02-01, FIS2011-23719, BES-2010-031696, and AP2010-2310), and the Catalan Government (2014 SGR 1639). C. Iemmi appreciates the support from UBACyT 20020100100689, CONICET PIP 112-200801-03047, and ANPCYT PICT 2010-02179 (Argentina)