Neutron optics is the branch of quantum physics devoted to the study of the optical properties of slow neutrons and their behavior as wave-particles. Slow neutrons beams (with typical energy the order of 0.025 eV, known as thermal neutrons, and also smaller) can propagate confined in guides of various transverse dimensions, longitude and geometries, under total internal reflection conditions, like in the case of classical optical waveguides. We study the properties and possible applications of neutron waveguides with small transverse dimensions. In particular, we have implemented a new algorithm to simulate neutron beams as they are confined in particular waveguides. The results, obtained from a new analytical formalism, are compared with standard numerical methods as the FDTD and, then, enhance the feasibility for recreating the beam structure as the later propagates inside the waveguide.
In this work, we review the main activities related to the International Day of Light (IDL) in Spain. We describe the actions of the Spanish Committee for IDL that has been created in Spain, reactivating the Spanish Committee for the International Year of Light created in 2015. The Spanish Committee of the IDL has been very active in promoting the realization of events, and providing tools for the IDL related information dissemination, as the specific related webpage for the IDL in Spain. The Spanish Committee also was the Spanish contact with the International Secretariat of the IDL. We describe the flagship IDL event in Spain that was organized in the Faculty of Physics Sciences of Universidad Complutense de Madrid on May 16th 2018. And we report on many other activities that have been developed in different places in Spain. These activities of the IDL in Spain are intended to make the society aware of the importance of the light and the light based technologies for the progress and well-being of the humankind. They have also been oriented to attract young talent to the studies of science and engineering.
Spatial heterodyne Fourier transform (SHFT) spectroscopy is based on simultaneous interferometric measurements implementing linearly increasing optical path differences, hence circumventing the need for mechanical components of traditional Fourier transform spectroscopy schemes. By taking advantage of the high mode confinement of the Siliconon-Insulator (SOI). platform, great interferometric lengths can be implemented in a reduced footprint, hence increasing the resolution of the device. However, as resolution increases, spectrometers become progressively more sensitive to environmental conditions, and new spectral retrieval techniques are required. In this work, we present several software techniques that enhance the operation of high-resolution SHFT micro-spectrometers. Firstly, we present two techniques for mitigating and correcting the effects of temperature drifts, based on a temperature-sensitive calibration and phase errors correction. Both techniques are demonstrated experimentally on a 32 Mach-Zehnder interferometers array fabricated in a Silicon-on-insulator chip with microphotonic spirals of linearly increasing length up to 3.779 cm. This configuration provides a resolution of 17 pm in a compact device footprint of 12 mm2. Secondly, we propose the application of compressive-sensing (CS) techniques to SHFT micro-spectrometers. By assuming spectrum sparsity, an undersampled discrete Fourier interferogram is inverted using l1-norm minimization to retrieve the input spectrum. We demonstrate this principle on a subwavelength-engineered SHFT with 32 MZIs and a 50 pm resolution. Correct retrieval of three sparse input signals was experimentally demonstrated using data from 14 or fewer MZIs and applying common CS reconstruction techniques to this data.
Since the last decades, cell phones have become increasingly popular and are nowadays ubiquitous. New generations of cell phones are now equipped with text messaging, internet, and camera features. They are now making their way into the classroom. This is creating a new teaching and learning technique, the so called m-Learning (or mobile-Learning). Because of the many benefits that cell phones offer, teachers could easily use them as a teaching and learning tool. However, an additional work from the teachers for introducing their students into the m-Learning in the classroom needs to be defined and developed. As an example, optical techniques, based upon interference and diffraction phenomena, such as holography, appear to be convenient topics for m-Learning. They can be approached with simple examples and experiments within the cell phones performances and classroom accessibility. We will present some results carried out at the Faculty of Physical Sciences in UCM to obtain very simple holographic recordings via cell phones. The activities were carried out inside the course on Optical Coherence and Laser, offered to students in the fourth course of the Grade in Physical Sciences. Some open conclusions and proposals will be presented.
We present the capability of the holographic gratings implemented in photopolymerizable glasses to be applied in spatial
and spectral manipulation of femtosecond laser pulses. For this purpose we used volume phase holographic gratings
recorded by the interference of two monochromatic and coherent beams coming from a Nd-YAG continuous laser.
We carried out experiments to determine the main features of the diffracted beams originated by the interaction of
volume holographic phase grating structures with femtosecond pulses arising from the fundamental emission of an
amplified Ti: sapphire laser system and its second harmonic. Meanwhile we have performed experiments to determine
its damage threshold indicating that this material is an excellent candidate for intense-fields and ultrashort laser pulse
applications.
KEYWORDS: Tomography, Optical tomography, Commercial off the shelf technology, Spatial light modulators, Reconstruction algorithms, Current controlled current source, Charge-coupled devices, Digital video discs, Free space optics, Optical imaging
The application of partially coherent optical beams for imaging, free space communication, random medium analysis requires controlling its mutual intensity. This task can be done using the phase-space tomography method consisting on the reconstruction of the Wigner distribution (WD), and therefore the mutual intensity, from its projections associated with the fractional power spectra. We propose two schemes that apply spatial light modulators (SLMs) for the measurements of the required WD projections in the case of one- and two-dimensional optical signals. The use of the SLMs allows rapid data acquisition and operative change of the projection number. Moreover, the measured intensity distributions do not require further rescaling, which accelerates the WD reconstruction algorithm and improves its efficiency. The developed numerical methods provide different ways for data analysis such as the reconstruction of the WD using the inverse Radon transform and its visualization for the case of one-dimensional signals; the determination of the mutual intensity for two fixed points without previous reconstruction of the entire WD for two-dimensional signals, etc. The validity of the proposed approaches has been verified experimentally for the test signals and the results are in a good agreement with the numerical simulations.
In this work we analyze the optical quality, performance, and recording mechanism of holographic diffraction gratings
recorded in photopolymerizable sol-gel glasses. These classes of holographic photomaterials have various compositions,
one of which incorporates a High Refractive Index Species (HRIS), already developed in our group GICO-UCM. The
new types of photopolymerizable glasses under study incorporate ionic liquid (IL). We present a comparative study,
showing distinctive behaviors for each photopolymerizable glass class, and determining particular features for various
ranges of applications.
The fractional Fourier transform (FT) is a powerful tool with relevant applications in optical and digital information
processing. Such applications demand a programmable and versatile optical system able to perform the
fractional FT almost at real time. We have recently developed an optical setup satisfying these requirements.
In contrast with other proposed setups, it offers the following advantages: the operation is achieved without
additional scaling and/or phase factors and a minimal number of lenses, located at fixed position, are utilized.1
In this work we present the main design features of the fractional FT processor and discuss its performance for
some relevant applications.
We present an overview of the main properties and the emerging implementations corresponding to a photopolymerizable glass modified with high refractive index species (HRIS) incorporated at molecular level. The study concerns to transmission and reflection holograms in Bragg and Raman-Nath regimens and polarization properties of
gratings with high spatial frequencies are also analyzed demonstrating a strong dependence of the refractive index
modulation with the polarization state of the reading beam. Not limited to the study of the optical properties of the
photopolymerizable glass we propose two applications of the holographic material. The first one is the fabrication of
polarizers elements with high performance at low cost such us holographic polarizers and holographic polarizers beam splitters. The second application concerns to the holographic recording of stables modes exhibiting high diffraction efficiency. Also, we have recently extended our studies to ultrashort pulse lasers in femtosecond regime. The photopolymerizable gratings are good candidates as optic elements to beam manipulation of ultrashort pulse lasers.
We report recent results obtained with volume holographic gratings recorded in a new photopolymerisable glass
modified with high refractive index species at molecular level. Various not previously observed performances have been
achieved for diffraction efficiency and angular selectivity. In particular, new overmodulation effects have been observed
for the first time experimentally in an amorphous material.
KEYWORDS: Holography, Holograms, Digital holography, Diffraction, Digital video discs, Video, Digital recording, Spatial light modulators, Physics, Multimedia
In this work we present a Virtual Holographic Laboratory for educational purposes. This project is edited on DVD support and it has been designed to be interactive: schemes, pictures, videos in order to clarify the theoretical description of the phenomena improving the understanding of its fundamental concepts. We believe that this project is helpful for undergraduate and graduate students in physics and engineering to obtain the solid knowledge about holography and to prepare for practical lessons on holography or partially substitute the lasts in the case of absence of appropriated technical base at a specific university level.
Wide application of the laser beams in science, technology and biomedicine demands the generation of the beams with desirable structure. In particular for optical trapping the beams with high transversal gradient of intensity distribution or doughnut like structures which carried out the orbital angular momentum are designed. In the recent study of the diffraction on the fractal Cantor zone plates [1-5] the possibility to generate the beams with capsule like structure (closed cylinder with the axis along the direction of beam propagation) for a certain distances has been discovered [5]. This peculiar form of the beam can be used for three dimensional particle trapping. Nevertheless the fractal zone plates are not unique gratings that generate this type of structures.
We present a short overview on the application of fractional cyclic and linear canonical transformations to optical signal processing and dedicate some of the discussions to the particular features found in the fractional Fourier transform domain.
We propose a modified arrayed waveguide grating (AWG) demultiplexer formed by an array of parallel ridge waveguides of equal length fabricated in silicon-on-insulator (SOI). Each waveguide contains two sections with different widths. The lengths of the two sections are modified from a waveguide to the adjacent waveguide, resulting in a wavelength dispersive phase array. With an appropriate demultiplexer output geometry, the light is focused in the output slab waveguide combiner. The focal position is wavelength dependent in the transverse direction, so that the optical signal is demultiplexed into separated output channels corresponding to different spectral bands. This device does not require bending waveguide sections which are essential in a conventional AWG demultiplexer, yielding very compact devices with potentially low loss. Numerical simulations have been carried out to evaluate the influence of waveguide parameters such as lengths and widths of different waveguide sections on demultiplexer performance.
We study experimentally and by numerical simulation the Fresnel diffraction on the recently introduced fractal zone plates (ZPs) associated with Cantor sets. The focusing properties of these ZPs and the evolution of the intensity patterns at the plane transversal to the propagation direction are discussed. As it follows from numerical simulation the series of conventional and doughnut-like secondary focuses are observed around the principal focus. The position, depth and the size of these focuses depends on the type of Cantor set and the level of the fractal, which are directly related to the number of the corresponding Fresnel zones. The results obtained by numerical simulations are verified in the experiments. The fractal Cantor ZPs of different levels are implemented with a liquid crystal display and with the diapositives. The experimental results obtained by both types of the dispositives are in good agreement with the theory and numerical simulations.
The main properties of n-th order power filtering in the fractional Fourier domain and its relationship to the differentiation operation are considered. The application of power filtering for solving the phase retrieval problem is proposed.
We propose a new stress-sensing technique based upon measurement of light scattering produced by an array of birefringent waveguides. When external stress is applied to the array of waveguides, their optical properties are modified via the photo-elastic effect. By monitorizing the scattered light amplitude this change can be detected and analyzed in order to assess the strength and direction of the external force.
After each laboratory session, students must answer individually three random multiple choice questions. The corresponding software has been developed by us. This self-evaluation test motivates students before and during sessions performance, and provides objective information to the teacher. The estimated students’ mean satisfaction with this system is 8.1/10.
The presence of degraded or soft wedges on a scene or digital image may cause ambiguities in depth perception. This kind of edge has been analyzed by introducing a mathematical model to digitally implement a large category of degraded edges. This model has accounted the origin of degradation, the performance of psychophysical test for measuring the degradation perception, and image characteristics currently used in artificial vision as momenta, number of gray scale level per pixels, etc. It is the subject of current researches to be applied to artificial reproduction of human visual perception.
We apply techniques of optimal control theory namely, the dynamic programming to the problem of light propagation in optical waveguides. The formulation is equivalent to the resolution of an eikonal equation. We illustrate this optimization technique for a gradient profile optical waveguide, i.e., an ideal parabolic refractive index profile distribution. We discuss the possibility of extending this procedure to other type of optical waveguides and optical media and implications in optical design.
We present a description of three methods to characterise the degraded edge imaging in difracction limited system with circular pupil of unit radius. Conditions of incoherent illumination are assumed. We present a generalisation of such a procedure by assuming the Heaviside function as a systematic generator of arbitrary soft edges.
We present a comparative study between two experimental methods to determine the modulation transfer function (MTF) of a hololens system. The two hololenses were previously recorded and tested for filtering pseudocolor. In the first method we used the classical Foucault test. The second, alternative method is based on the digital image processing of a perfect edge under incoherent illumination. From the digitized intensity line profiles we obtain the MTF and cutoff frequency of the optical system according to the reciprocity between line spread function and MTF. Comments are made on the applicability and accuracy of these two methods.
We present analytical expressions for degraded edge and thick line images under both coherent and incoherent illumination. We analyze the validity of standard resolution criteria for resolving these particular objects and apply specific criteria as the Conrady's for the particular case of the human visual system.
A systematic procedure for calculating the general transfer function of a cascaded linear optical system working under the Fresnel regime is presented. The validity of neglecting the quadratic phase factors is also discussed. For the case of inhomogeneous optical devices with Gaussian profiles, explicit expressions for the transfer functions are calculated through specific Gaussian integrals.
One of the techniques for fiber-to-fiber connection is based on a basic interferometric method to obtain a holographic grating acting as a holocoupler. Holographic couplers can be used as memory systems storing information to achieve single or multiple fiber connections using a multiplexed hologram. This device works with high efficiency in short intervals of space and time. Applications can be extended, as well, to codify the stored information in a reflection configuration as a switcher system with a laser diode. The flexibility of this technique allows coupling of alternative focusing elements such as two GRIN lenses and other configurations. In order to develop this application in optoelectronic devices, the recording process must by carefully analyzed. The recording of the holograms has been already resolved for planar and spherical waves. Nevertheless, recordings with Gaussian waves require some conditions to be accounted for due to the width waist and divergence of the output beam of the laser source.
Resolution criteria for resolving two line images under coherent and incoherent illumination are presented. This technique has been used to interpret psychophysical data obtained from hyperacuity paradigms.
The phase profile of the wave scattered by a GRIN lens has been determined using the
eikonal approximation for the scattering amplitude. This wave acts as the object wave in a
holographic register to construct a GRIN holocoupler. Different geometries in the phase profile
distribution are presented as functions of the taper shape.
We define a resolution criterion for the case of two narrow line images. For doing so, we have considered the two
lines as superposition of gradients of two Heaviside step-functions.
We present a general formulation, based on previous work, for calculating the general coherent transfer function (GCTF) of a compound optical system operating under linearity conditions in the Fresnel regime. It should be noted that only under very restrictive conditions on the system (particularly for systems obeying Herschel's condition) can the quadratic phase terms appearing in the Fresnel diffraction regime be neglected. By assuming Gaussian profiles for the refractive indices of the optical media, the GCTF is calculated. The autocorrelation function directly gives the optical transfer function and modulation transfer function distributions.
This procedure has been applied to a Tessar-type photolens system.
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