Single-pixel imaging employs structured illumination to record images with very simple light detectors. It can be an alternative to conventional imaging in certain applications such as imaging with radiation in exotic spectral regions, multidimensional imaging, imaging with low light levels, 3D imaging or imaging through scattering media. In most cases, the measurement process is just a basis transformation which depends on the functions used to codify the light patterns. Sampling the object with a different basis of functions allows us to transform the object directly onto a different space. The more common functions used in single-pixel imaging belong to the Hadamard basis or the Fourier basis, although random patterns are also frequently used, particularly in ghost imaging techniques. In this work we compare the performance of different alternative sampling functions for single pixel imaging, all of them codified with a digital micromirror device (DMD). In particular, we analyze the performance of the system with Hadamard, cosine, Fourier and noiselet patterns. Some of these functions are binary, some others real and other complex functions. However, all of them are codified with the same DMD by using different approaches. We perform both numerical and experimental tests with the different sampling functions and we compare the performance in terms of the efficiency and the signal-to-noise ratio (SNR) of the final images.
The complete phase and amplitude information of biological specimens can be easily determined by phase-shifting digital holography. Spatial light modulators (SLMs) based on liquid crystal technology, with a frame-rate around 60 Hz, have been employed in digital holography. In contrast, digital micro-mirror devices (DMDs) can reach frame rates up to 22 kHz. A method proposed by Lee to design computer generated holograms (CGHs) permits the use of such binary amplitude modulators as phase-modulation devices. Single-pixel imaging techniques record images by sampling the object with a sequence of micro-structured light patterns and using a simple photodetector. Our group has reported some approaches combining single-pixel imaging and phase-shifting digital holography. In this communication, we review these techniques and present the possibility of a high-speed single-pixel phase-shifting digital holography system with phase-encoded illumination. This system is based on a Mach-Zehnder interferometer, with a DMD acting as the modulator for projecting the sampling patterns on the object and also being used for phase-shifting. The proposed sampling functions are phaseencoded Hadamard patterns generated through a Lee hologram approach. The method allows the recording of the complex amplitude distribution of an object at high speed on account of the high frame rates of the DMD. Reconstruction may take just a few seconds. Besides, the optical setup is envisaged as a true adaptive system, which is able to measure the aberration induced by the optical system in the absence of a sample object, and then to compensate the wavefront in the phasemodulation stage.
We have applied an active methodology to pre-service teacher training courses and to active teacher workshops on Optics. As a practical resource, a set of demonstrations has been used to learn how to perform classroom demonstrations. The set includes experiments about polarization and birefringence, optical information transmission, diffraction, fluorescence or scattering. It had been prepared for Science popularization activities and has been employed in several settings with a variety of audiences. In the teacher training sessions, simple but clarifying experiments have been performed by all the participants. Moreover, in these workshops, devices or basic set-ups, like the ones included in our demonstration set, have been built. The practical approach has allowed the enthusiastic sharing of teaching and learning experiences among the workshop participants. We believe that such an active orientation in teacher training courses promotes the active and collaborative teaching and learning of Optics in different levels of Education.
Optical vortices are employed in optical trapping applications for their ability to set microparticles into rotation. Devil’s Vortex Lenses have high diffraction efficiency and it is possible to take advantage of their particular volumetric focal structure to design versatile and efficient optical tweezers. In this communication, we report a simple design procedure, involving arrays of Devil’s Vortex-Lenses implemented in a programmable Spatial Light Modulator, for generating spatial distributions of optical vortices. In our approach, the preferred position and topological charge value can be assigned to each vortex in the structure, tuning the desired angular momentum. We have demonstrated the generation of 3D optical vortex distributions through arrays of Devil’s Vortex-Lenses, including configurations with charges and momenta of opposite sign. Our experimental results present an excellent agreement with the simulations we have developed.
Resolution in digital holography microscopy can be improved by enlarging the hologram aperture. We review different
techniques for resolution enhancement in digital holography, and present a system for reconstructing single-exposure online
(SEOL) digital holograms with improved resolution using a synthetic aperture. In our method, several recordings are
made in order to compose a synthetic aperture, shifting the camera within the hologram plane. After processing the
synthetic hologram, an inverse Fresnel transformation provides an enhanced resolution reconstruction. The method
employs a simple set-up, including no microscope objective.
In this contribution we describe a method for achieving a phase-only modulation regime with an off-the-shelf twisted
nematic liquid crystal display (TNLCD). The keystone of this procedure involves illumination of an addressed TNLCD
with circularly polarized light. The analysis of the distribution of the output polarization states in the S1-S2 plane as the
applied voltage is changed suggests a simple way to optimize the liquid crystal phase response. For this purpose, a
properly oriented quarter-wave plate followed by an analyzer is used behind the TNLCD. Laboratory results for a
commercial display are presented. Our experiments show a phase modulation depth of 240º for a wavelength of 514 nm
with a residual intensity variation lower than 4%.
An optical system based on short-coherence digital holography suitable for three-dimensional microscopic investigations is described. An in-line arrangement is used, in which the light source is a short-coherence laser and the holograms are recorded on a CCD sensor. The interference (hologram) occurs only when the path lengths of the reference and the object beam are matched within the coherence length of the laser. The phase of the object wave front in the plane of the CCD detector is determined by phase shifting. The image of the part of the sample that matches the reference beam is reconstructed by numerical evaluation of the hologram. The three-dimensional scanning of the sample is easily obtained by shifting the position of a mirror in the reference arm of the interferometer. Reconstruction of different layers of the sample is possible up to a depth of several hundred micrometers. However, in tissues and other biological samples, while the surface of the sample is properly reconstructed, the multiple scattering and the inhomogeneties in the refractive index reduce the imaging quality of the deepest layers. The possibility to correct numerically this sample-induced aberrations has been studied. Simulations and experimental results are presented.
A fiber-optic voltage sensor for high-voltage lines is presented. The sensor relies on the piezoelectric effect of PZT ceramic tubes. A lengths of single-mode fiber is wound onto the tubes and mounted in the arms of a Mach-Zehnder interferometer.