Over the past decade, single-pixel imaging (SPI) has established as a viable tool in scenarios where traditional imaging techniques struggle to provide images with acceptable quality in practicable times and reasonable costs. However, SPI still has several limitations inherent to the technique, such as working with spurious light and in real time. Here we present a novel approach, using complementary measurements and a single balanced detector. By using balanced detection, we improve the frame rate of the complementary measurement architectures by a factor of two. Furthermore, the use of a balanced detector provides environmental light immunity to the method.
In the last years, single-pixel imaging (SPI) was established as a suitable tool for non-invasive imaging of an absorbing object completely embedded in an inhomogeneous medium. One of the main characteristics of the technique is that it uses very simple sensors (bucket detectors such as photodiodes or photomultiplier tubes) combined with structured illumination and mathematical algorithms to recover the image. This reduction in complexity of the sensing device gives these systems the opportunity to obtain images at shallow depth overcoming the scattering problem. Nonetheless, some challenges, such as the need for improved signal-to-noise or the frame rate, remain to be tackled before extensive use in practical systems. Also, for intact or live optically thick tissues, epi-detection is commonly used, while present implementations of SPI are limited to transillumination geometries.
In this work we present new features and some recent advances in SPI that involve either the use of computationally efficient algorithms for adaptive sensing or a balanced detection mechanism. Additionally, SPI has been adapted to handle reflected light to create a double pass optical system. Such developments represent a significant step towards the use of SPI in more realistic scenarios, especially in biophotonics applications. In particular, we show the design of a single-pixel ophtalmoscope as a novel way of imaging the retina in real time.
We describe a method to image objects through scattering media based on single-pixel detection and microstructured illumination. Spatial light modulators are used to project a set of microstructured light patterns onto the sample. The image is retrieved computationally from the photocurrent fluctuations provided by a single-pixel detector. This technique does not require coherent light, raster scanning, time-gated detection or a-priori calibration process. We review several optical setups developed by our research group in the last years with particular emphasis in a new optical system based on a double-pass configuration and in the combination of single-pixel imaging with Fourier filtering.
One challenge that has long held the attention of scientists is that of clearly seeing objects hidden by turbid media, as smoke, fog or biological tissue, which has major implications in fields such as remote sensing or early diagnosis of diseases. Here, we combine structured incoherent illumination and bucket detection for imaging an absorbing object completely embedded in a scattering medium. A sequence of low-intensity microstructured light patterns is launched onto the object, whose image is accurately reconstructed through the light fluctuations measured by a single-pixel detector. Our technique is noninvasive, does not require coherent sources, raster scanning nor time-gated detection and benefits from the compressive sensing strategy. As a proof of concept, we experimentally retrieve the image of a transilluminated target both sandwiched between two holographic diffusers and embedded in a 6mm-thick sample of chicken breast.
Precise control of light propagation through highly scattering media is a much desired goal with major technological implications. Since interaction of light with turbid media results in partial or complete depletion of ballistic photons, it is in principle impossible to transmit images through distances longer than the extinction length. In biomedical optics, scattering is the dominant light extinction process accounting almost exclusively for the limited imaging depth range. In addition, most scattering media of interest are dynamic in the sense that the scatter centers continuously change their positions with time. In our work, we employ single-pixel systems, which can overcome the fundamental limitations imposed by multiple scattering even in the dynamically varying case. A sequence of microstructured light patterns codiﬁed onto a programmable spatial light modulator are used to sample an object and measurements are captured with a single-pixel detector. Acquisition time is reduced by using compressive sensing techniques. The patterns are used as generalized measurement modes where the object information is expressed. Contrary to the techniques based on the transmission matrix, our approach does not require any a-priori calibration process. The presence of a scattering medium between the object and the detector scrambles the light and mixes the information from all the regions of the sample. However, the object information that can be retrieved from the generalized modes is not destroyed. Furthermore, by using these techniques we have been able to tackle the general problem of imaging objects completely embedded in a scattering medium.
We demonstrate the utilization of Dammann lenses encoded onto a spatial light modulator (SLM) for triggering nonlinear effects. For continuous illumination Dammann lenses generate a multifocal pattern characterized by a set of N foci diffraction orders, all with the same intensity. We theoretically show that for pulses shorter than 100 femtosecond (fs) the effects of chromatic aberrations influence the uniformity of the generated pattern. Multifocal second harmonic generation (SHG) and on-axis multiple filamentation are produced and actively controlled in β-BaB<sub>2</sub>O<sub>4</sub> (BBO) and fused silica samples, respectively, with an amplified Ti:Sapphire femtosecond laser (30 fs at FWHM). Our proposal allows us to dynamically control both the quantity of foci and the distance among them. The output diffraction pattern is in good agreement with theoretical calculations. The measured spectra at the rear face of the supercontinuum sample for different separation among foci are also provided. The potential of this technique is very promising in different fields of nonlinear optics or in applications of in-depth materials microprocessing.
As digital holographic microscopy (DHM) uses microscope objectives (MO) for enlarging the sample, some associated effects that are not present in optical microscopy have to be considered as quantitative phase imaging (QPI) is regarded. The remaining phase curvature introduced by the MO, which does not affect the optical microscopes, plays a determinant role in the performance of the DHM. In this contribution a thorough analysis of the physical parameters that control the appropriate utilization of MOs in DHM is presented. The analysis is carried for QPI. We study the sample phase perturbations that the MO phase curvature introduces. An analysis of the regular ways as these phase anomalies are removed is presented. The study is supported by means of numerical modeling and experimental results.
We recognize some photonic-crystal-fiber structures, made up of soft glass, that generate ultrawide (over an octave),
very smooth and highly coherent supercontinuum spectrum when illuminated with femtosecond pulsed light around
1.55 μm. The design of soft-glass microstructured fiber geometry with nearly ultraflattened, positive and low dispersion
is crucial to accomplish the above goals.
We examine quasiperiodic multilayers arranged according to <i>m</i>-bonacci sequences that combine ordinary positive index materials and dispersive metamaterials with negative index in certain frequency ranges. When the averaged refractive index, in volume, of the multilayer equals zero, the structure does not propagate light waves and exhibits a forbidden band. In this contribution we recognize some approximated analytical expressions for the determination of the upper and lower limits of the above mentioned zero-average refractive index band gap.
We investigate the appearence of non-Bragg band gaps in 1D fractal photonic structures, specifically the Cantor-like lattices combining ordinary positive index materials and dispersive metamaterials. It is shown that these structures can exibit two new type of photonic band gaps with self-similarity properties around the frequencies where either the magnetic permeability or the electric permittivity of the metamaterial is zero. In constrast with the usual Bragg gaps, these band gaps are not based on any interference mechanisms. Accordingly, they remain invariant to scaling or disorder. Some other particular features of these polarization-selective gaps are outline and the impact on the light spectrum produced by the level of generation of the fractal structure is analyzed.
We describe the application of nearly wavelength-independent optical processors to develop several broadband security techniques. Our achromatic optical configurations, based in the appropriate combination of a small number of diffractive and refractive lenses, are designed to work under temporally incoherent illumination. In this way, we are able to develop a method to reconstruct color Fourier holograms, an optical system to perform color pattern recognition and a technique to encrypt and decrypt color input objects, in all cases under white-light illumination. Moreover, we extend these ideas to work under both spatially and temporally incoherent illumination. This allows us to perform color pattern recognition and optical encryption techniques under natural light.
We describe some optical combinations of refractive and diffractive lenses to compensate for the inherent wavelength dispersion shown by interference and diffraction patterns under white-light point-source illumination. In a second phase, this achromatic behavior is also applied to the case of spatially incoherent polychromatic light, i.e., to totally incoherent illumination. Finally, the above results are extended to the correction of chromatic distortion associated with diffraction of femtosecond pulse light. Several experimental results are shown.
We describe several methods to extend security techniques based on optical processing to work under broadband illumination. The key question of our procedures is the design of dispersion-compensated optical processors by combining a small number of diffractive and refractive lenses. Our optical configurations provide, in a first-order approximation, the Fraunhofer diffraction pattern of the input signal in a single plane and with the same scale for all the wavelengths of the incident light. In this way, our achromatic hybrid systems allow us to reconstruct color holograms with white light. These achromatic hybrid (diffractive-refractive) systems are applied, in a second stage, for implementing color processing operations with white light, such as color pattern recognition. In this direction, we design also a technique to encrypt color input objects into computer generated color holograms, which are decrypted optically with an achromatic joint transform correlator architecture under white-light illumination. Finally, we describe a totally-incoherent optical processor that is able to perform color processing operations under natural illumination (both spatially and temporally incoherent). This system is applied to perform color pattern recognition and optical encryption operations under natural light. Numerical and experimental results are shown.
A novel optical set-up that allows a totally incoherent Lau effect is demonstrated. It is based on dispersion-compensated techniques that employ strong dispersive elements. In this way, three commercially available diffractive lenses and a refractive objective are used for achromatic Lau fringes production with spatial and temporally incoherent illumination.
Confocal scanning microscopy is an imaging technique meanly featured by its unique optical sectioning capacity when imaging three-dimensional objects. In this work we improve the axial resolution of these setups by introducing pupil filters with specified amplitude transmittance. Our filter-design procedure, which of course is valid for the nonparaxial regime, is applied to both conventional and 4Pi confocal microscopes.
We report on a hybrid (diffractive-refractive) wavelength-independent imaging setup with an intermediate achromatic filtering plane in the Fresnel domain. Therefore, the system acts as a chromatically-compensated Fresnel processor able to perform space-variant color pattern recognition operations in a single step.
We study the group-velocity dispersion properties of a novel class of Bragg fibers. They are radially-symmetric microstructured fibers having a high-index core (silica in our case) surrounded by a cylindrical multilayer omnidirectional mirror as cladding, which is formed by a set of alternating layers of silica and a lower refractive-index dielectric. The interplay between the unusual geometric dispersion shown by the multilayer cladding of the fiber and the material dispersion corresponding to the silica core allows us to achieve an achromatic flattened dispersion behavior in the 0.8 μ<i>m</i> wavelength window and even an ultraflattened behavior in the 1.5 μ<i>m</i> range for some specific designs.
Diffraction-based optical correlators working under broadband illumination, in contrast to their coherent counterparts, allow us to exploit color information. However, the use of the wavelength as an additional parameter requires to take into account the chromatic dispersion inherent to the diffraction process. In this contribution, we describe a novel family of dispersion-compensated broadband optical correlators that operate some in the Fourier and some in the Fresnel region. In both cases, the chromatic compensation is achieved by a proper combination of a small number of commercially available optical elements, conventional diffractive and refractive lenses. In all cases, and with a single matched filter, the chromatic content of the correlation peak provides the spectral composition of the detecting color signal. On top of that, some of our optical solutions work with point-source illumination and others with spatially-incoherent light. In this way, on the one hand, our spatially coherent optical designs permit to perform the color correlation in amplitude for each spectral channel. Accordingly, working in the Fresnel domain, we achieve a space-variant color pattern recognition setup. On the other hand, totally incoherent optical correlators, which are linear in irradiance, provide important practical advantages as they employ natural light and allow us to deal with diffuse, reflecting or self-luminous color objects.
This paper gives the theoretical basis for the development of a novel modal method to describe 3D dielectric structure modes. To this end, the vector wave equation, which determines the magnetic field, is written in terms of a linear operator, whose eigenvectors satisfy orthonormality relation. The key of our method is to obtain a matrix representation of the wave equation in a basis that is defined by the modes of an auxiliary system. Our proposed technique can be applied to systems with arbitrary 3D real or complex refractive-index distributions. In this work we have focused on thin-film photonic crystal waveguides with an asymmetrical core.
The scope of this work is the compensation of the chromatic dispersion inherent to free-space light propagation, both in the Fraunhofer and in the Fresnel diffraction region. The cornerstone of our procedure lies in achieving, in a first- order approximation, the incoherent superposition of the monochromatic versions of the selected diffraction pattern in a single plane and with the same scale for all the wavelengths of the incident light. Our novel optical configurations with achromatic properties for the field diffracted by a screen are formed by a proper combination of a small number of conventional diffractive and refractive lenses, providing an achromatic real image of the diffraction pattern of interest. The residual chromatic aberrations in every case are low even when the spectrum of the incident light spreads over the whole visible region. The resulting achromatic hybrid (diffractive- refractive) systems are applied, in a second stage, for implementing several achromatic diffraction-based applications with white light, like wavelength-independent spatial- frequency filtering, achromatic pattern recognition, white- light array generation, and to designing a totally-incoherent optical processor that is able to perform color processing operations under natural illumination (both spatially and temporally incoherent).
Different optical architectures designed for compensating the chromatic dispersion inherent to free-space broadband light diffraction are presented. These devices are formed by a small number of conventional refractive objectives and diffractive lenses. In a second stage, several achromatic diffraction-based information processing techniques working with spatially-coherent or spatially-incoherent white-light illumination are also discussed.
A novel analysis of specially designed photonic crystal fibers accounts for the existence of endlessly single-mode structures with flattened dispersion. Our approach permits to control the fiber dispersion in terms of its geometrical parameters.
Proc. SPIE. 3749, 18th Congress of the International Commission for Optics
KEYWORDS: Optical components, Diffraction, Optical signal processing, Signal attenuation, Laser processing, Signal processing, Near field diffraction, Cylindrical lenses, Modes of laser operation, Spherical lenses
We report on a general analytical procedure to analyze the axial focusing properties of uniform cylindrical waves truncated by a rectangular window. The resulting on axis diffraction pattern explicitly depends on the square of the window height-to-width ratio. Depending on the value of this parameter, different kinds of axial behavior are observed. In particular, it is found that for low values of this parameter and low Fresnel number, an inverse focal-shift phenomenon can appear.
We report herein an hybrid (diffractive-refractive) lens triplet showing quasi-wavelength-independent optical Fourier transform capabilities. The wavelength compensation carried out by our novel optical design is exact for the axial position of the Fourier transform of the input. Nevertheless, a very low residual transversal chromatic aberration remains. Results of laboratory experiments will be shown.
We have combined a fusion-pulling technique and a standard metal-coating technique to fabricate in-line single-mode fiber filters, polarizers and sensors. The reduction of the core and cladding diameters in a tapered fiber causes the evanescent field to extend beyond the outer boundary. Thus, when a metal film is evaporated on a tapered fiber, the surface plasma modes cause coupling to the fiber mode.
We obtain that the irradiance impulse response along certain axes of an optical imaging system, that suffers from primary spherical aberration and longitudinal chromatic aberration, can be calculated from 1D integrations in a single 2D phase- space representation, the Wigner distribution function associated with a certain azimuthally-averaged version of the pupil of the system. This result is applied to study the response along the optical axis of unstable laser resonators and optical systems working under polychromatic illumination.
The scope of this work is the compensation of the chromatic dispersion inherent to free-space light propagation, both in the Fraunhofer and in the Fresnel diffraction region. The cornerstone of our procedure lies in achieving, in a first-order approximation, the incoherent superposition of the monochromatic versions of the selected diffraction pattern in a single plane and with the same scale for all the wavelengths of the incident light. Our novel optical configurations with achromatic properties for the field diffracted by a screen are formed by a proper combination of a small number of conventional diffractive and refractive lenses, providing an achromatic real image of the diffraction pattern of interest. The residual chromatic aberrations in every case are low even when the spectrum of the incident light spreads over the whole visible region. The resulting achromatic hybrid (diffractive- refractive) systems are applied, in a second stage, for implementing several achromatic diffraction-based applications with white light, like wavelength-independent spatial-frequency filtering, achromatic pattern recognition, white-light array generation, and to designing a totally-incoherent optical processor that is able to perform color processing operations under natural illumination (both spatially and temporally incoherent).
We report a new achromatic Fourier processor constituted basically by a quasi wavelength- independent imaging forming system whose first half performs an achromatic Fourier transform of a color input object. Consequently, this optical architecture, formed by a small number of diffractive and refractive lenses, provides an intermediate achromatic real Fraunhofer plane and a final color image with a high signal-to-noise ratio. In this way, our optical processor can perform simultaneously the same spatial filtering operation for all the spectral components of the broadband illumination.
The boundary perturbation method is applied to investigate the changes in the lowest modes of optical fibers sustaining super-Gaussian beams due to the cladding with a constant refractive index. This contribution illustrates the exploitation of quantum-mechanical analogies and methods in the study of optical phenomena.
A technique is presented for improving 3-D resolution in confocal scanning microscopy. The technique is based on the equal contribution to the image of the illuminating and the collecting lenses. It is proposed, then, to apodize such lenses with complementary filters. The combined action of both filters produces a narrowness of the point spread function of the system both in the image plane, and along the optical axis.
We discuss the formation of self-images in an optical fiber with quadratic refractive index variations, and in its lens tandem equivalent setup. We identify two family sets of self-images. The first family set has geometrical magnification less or greater than unity. The other family set is obtained by longitudinal replication, with unit magnification of the first family set.
A method for the calculation of the axial illuminance and chromaticity as polychromatic merit functions of optical imaging systems is presented. We show that the Wigner distribution function of the pupil of the system allows us to obtain all the monochromatic components needed for the calculation of these parameters. From this single phase-space representation, the merit functions can be obtained in a polar fashion for a variable spherical and longitudinal chromatic aberrations. Numerical examples for an axially apodizing filter are shown.
Two novel algorithms for the binarization of continuous rotationally symmetric real positive pupil filters are presented. Both algorithms are based on 1-D error diffusion concept. The original gray-tone apodizer is substituted by a set of transparent and opaque concentric annular zones. Depending on the algorithm the resulting binary mask consists of either equal width or equal area zones. The diffractive behavior of binary filters is evaluated. It is shown that the pupils with equal width zones give Fraunhofer diffraction pattern more similar to that of the original continuous-tone pupil than those with equal area zones, assuming in both cases the same resolution limit of printing device.
We describe a novel approach that leads to an analytical formula for shaping the power spectrum of 1-D gratings and zone plates with different opening ratios by using apodizers expressable as Legendre series. Some applications of this formulation are presented. 1.
We describe an optical method for obtaining inregister incoherent superposition of 2D diffraction patterns in all the planes parallels to the exit pupil. This means that the consonance is independent of the plane of detection. 1 . DISCUSSION If we illuminate with a point source a screen with an aplitud transmittance propor tional to the Montgomery rings'' then its Fraunhofer diffraction pattern will appear pen dicaly along the optical axis2 . A second incoherent point source illuminating the same screen on axis but in a different position will produce another set of self images. Is easy to show that if its position fits with one of the self images of the first point source then both sets are in consonance. Exactly the same procedure can be applied to a third source a fourth source an so on. More over not only the self images are in con sonance but all the diffraction patterns associated with the screen. This means that there are different position for a point source along the axis that produces exactly the same diffraction field of these screen. If we multiply the Montgomery ring by a second amplitud screen then the resultant pat tern will exhibits the same behavior. We call Montgomery patterns to the diffraction pat terns produced in both cases. In this sense if we choose one of those screens (Montgome ry rings x amplitud screen)