Variations of the induced retardance in the aperture of liquid-crystal variable retarders LCVR and spatial light modulators LC-SLM are measured. It is found that the induced retardance, in LCVR, shows variations over the aperture which depend on the wavelength of the light used and the voltage applied. Larger retardance variations appear near to the edges of the cell. There are also larger variations in the maximum and minimum unwrapped retardances in the retardance versus voltage curves. These voltage values and the edges positions of the cells should be avoided in applications of these devices. On the other hand, the induced retardance is calculated pixel by pixel in LC-SLM. For the same gray level over the entire LC-SLM screen, there are differences in the induced retardance between pixels located in a central area and pixels located in the edges. For correct calibration of the LC-SLM, the retardance value as a function of gray level was determined, pixel by pixel.
We present numerical results of the simulation of the polarimetric response of microscopic, semitransparent specimens immersed in water. The polarimetric response observed in our simulations comes from the shape and structure of the observed specimens, rather than from intrinsic polarization properties of the material out of which they are made. We discuss the computational methods used to perform the simulations, which consist of four stages (scattering, propagation from the specimen to the microscope objective, propagation through a polarimeter, and focusing onto the detector), as well as the feasibility of measuring the simulated polarimetric response of the specimens using a well-calibrated polarimetric microscope. As part of our analysis, we discuss some technical limitations of the equipment available in our group to build a polarimetric microscope and some limitations of the calibration method that we use, namely: the eigenvalue calibration method.
We propose two different in-line optical schemes for the implementation of Biaxial Crystal (BC) based polarimeters. Unlike already existing BC polarimeters prototypes, our proposed architectures only require of a single BC and only one CCD camera, this leading to more feasible and cheaper prototypes. The first scheme is restricted to linear metrology and we provide its interest to be applied under low-intensity conditions. The second architecture is suitable for complete polarimetry, this being achieved by including an optical module to properly split and steer the input light. The BC polarimeters were implemented and tested by measuring different known input polarizations and we obtained excellent results in terms of accuracy and repeatability.
Nowadays, digital holographic systems are based on two main optical schemes: off-axis (OA) and inline (IL) holographic systems. In OA set-ups, the reference and the object beams present a relative angle at the registration plane. Thus, a real image of the object can be obtained without the influence of conjugated images by performing a spatial filtering at the reconstructed plane. IL configurations are less sensitive to vibrations and air flows than OA configurations, but the undesired influence of conjugated images in the final hologram is not avoided. To overcome this limitation, a number of IL based methods have been proposed. One interesting approach is the phase-shifting technique, which leads to efficient holograms for IL applications. However, due to the time-sequential nature of this technique, it is somewhat inappropriate for dynamic processes. We present a new method, for IL digital holography, based on a doublesideband (DSB) filter. This method not only removes the conjugate images in the reconstruction process but also reduces the distortions that usually appear when using single-sideband filters. Moreover, it is only time-limited by the acquisition time of the CCD camera. The appropriateness of the technique to be applied in dynamic processes was tested for the tracking of micro-particles. To this aim, particle holographic images were obtained by using the DSB method and afterwards processed with digital picture recognition methods, this allowing us to accurately track the spatial position of the particles. By using this approach, the instantaneous trajectory and velocity described by glass microspheres in movement were experimentally determined
We present the continuous scan operation of the ALBA-NOM as a working mode that allows obtaining low noise in
short time, as well as high accuracy measurements. In the traditional step-scan operation, the position of the probe beam
is kept fixed while many data points of autocollimator are averaged for noise reduction. This operation mode is very
safe, as one has a perfect correspondence between mirror position and measured angle, but it is time inefficient, as it
disregards all the data values acquired during motion, and basically averages data values taken under identical
conditions. On the other hand, continuous scan is less safe in terms of correspondence between mirror position and slope,
especially for NOM systems for which the autocollimator does not accept an electronic trigger. Nevertheless, it is
possible to perform independent acquisitions of the autocollimator and of the linear stage data during a scan, and
synchronize signals a posteriori. This solves the main problem of continuous scan with a NOM. Continuous scan
operation for performing measurements is very efficient for noise reduction per unit time, as it allows integrating every
single data value taken by the autocollimator. In addition, it opens the possibility of introducing pitch variations of the
mirror between scans. This allows obtaining many independent datasets that can be combined using error suppression
techniques to reduce not just noise but systematic errors too. In this paper we report the methods and the main results.
On the 20th of December 2013, The United Nations (UN) General Assembly 68th Session proclaimed 2015 as the International Year of Light and Light-based Technologies (IYL 2015). The proclamation of an International Year focusing on the light science and applications recognizes the importance of light in the society, which plays a vital role in our daily lives, being visible in a widespread number of different areas, as for instance, in technology, education, energy, art, agriculture, health, among many others. In this work, the members of the Image Processing Laboratory from the Universitat Autònoma de Barcelona (UAB), analyze the concept of readapting some experiments in optics -usually conducted in different courses at the UAB physics degree- into the artistic context of the MACBA (Museu d’Art Contemporani de Barcelona). This project, called SummerLight, takes place within the framework of the IYL, as part of the activities devised to promote the visibility of light. The readapted experiments are expected to teach and improve the knowledge of high school students with respect to different important physical phenomena related to the wave nature of light as polarization, interferences and diffraction. This study analyzes the suitability of the proposed experiments in terms of student optical skills improvement. In addition, its contextualization into an artistic scenario is also discussed.
We study and test a new technique for in-line digital holography which avoids the formation of the conjugate images. Inline digital holography is based in a common path configuration. In this case, the hologram is produced by the interference between the reference wave front and the diffracted wave front by an almost transparent object. Twin images are obtained with obscured rings that difficult the determination of the best focusing plane. To avoid the conjugated image, the information of the magnitude and phase of the wave front are needed.
In a recent work a new in-line digital holography technique was proposed. In this method the object is illuminated with a collimated wave front. A plane, close to the particles distribution is imaged onto a CCD by means of a convergent lens and at the same time, a knife edge is placed in the focal plane of the lens in order to block half of spatial frequency spectrum. In this way, by means of a numerical processing performed on the Fourier plane, it is possible to eliminate one of the components (real or conjugate) of the reconstructed images nevertheless it is observed a tiny deformation of the resulting hologram image.
To compensate this effect, we propose a new configuration in which we implement the knife edge technique on both parts of the spectrum at the same time. Finally in the computer, we process the holograms to build one complete without deformation. This hologram is used to recover the wave front at different planes without the influence of the conjugate image.
Liquid Crystals on Silicon (LCOS) displays are a type of LCDs that work in reflection. Such devices, due to the double pass that the light beam performs through the LC cells, lead to larger phase modulation than transmissive LCDs with the same thickness. By taking advantage of this modulation capability exhibited by LCOS displays, we propose a new experimental set-up which is able to provide customized state of polarization spatial distributions just by means of a single LCOS display. To this aim, a double reflection on different halves of the display is properly performed. This fact is achieved by including a compact optical system that steers the light and performs a proper polarization plane rotation.
The set-up has been experimentally implemented and some experimental concerns are discussed. The suitability of the system is provided by generating different experimental spatial distributions of polarization. In this regard, well-known polarization distributions, as axial, azimuthal or spiral linear polarization patterns are here provided. Based on the excellent results obtained, the suitability of the system to generate different spatially variant distributions of polarization is validated.
A method to obtain a digital holographic movie by means of a point diffraction interferometer is described. By using a parallel aligned liquid crystal wave plate of size 3×3 cm2, a phase shifting interferometric technique is implemented. The optical setup is able to operate at 10 frames/s. Each frame can be digitally refocused to reconstruct different planes of the three-dimensional (3-D) object. By changing the frame, the object can be refocused in a different time. As an example, we show a movie which contains two objects that are separated by 160 mm. The movie shows how, for a given time (a fixed hologram frame), we can focus at different planes (separated 0.1 mm) of the 3-D object, and by changing the frame the movement of the object can be observed.
The technology of liquid crystal panels (LCP) is very mature and can be applied in many different areas in which the modulation of an input light beam is needed. LCP can be used to modulate amplitude, phase or polarization state of the input beam, being the phase modulation the most important. For this reason parallel aligned LCP are the most recommended for most applications. In this paper we review a method for the characterization of these devices, especially when temporal fluctuations are present. Next we revise several applications in different fields: a) A point diffraction interferometer with phase shifting capabilities is demonstrated and applied to the recording of digital holograms that can focus, digitally, the different planes of a 3D object; b) Different techniques to obtain superresolution imaging beyond the diffraction limit of the aperture, or the geometrical limitation imposed by the pixilation of the CCD device are shown, and c) the construction of Stokes and Mueller polarimeters with different types of LCP are also shown.
We present a ray trace through the sphere in support of sub-aperture testing of the shape of a transparent sphere,
used as a volume (density) standard. The radius of curvature for a sub-aperture may be determined after finding
zero for the defocus aberration, localizing the back focal plane.
We examine the viability of implementing the Vectorial Shearing Interferometer (VSI) in order to determine the
sphericity of a glass standard patron. We use the patron sphere as an optical component to collimate the expanded wave
front of a laser. The collimated wave front impinges on the VSI. The resulting fringe pattern carries the phase gradient
information of the original wave front, in the sheared direction. We estimate qualitatively the asphericity of the element
under test, comparing the phase gradients in different sections of the sphere. When the gradients of two or more
directions are identical, we consider that the standard patron has uniform sphericity. Here, we report on the experimental
setup to test the asphericity, the experimental results, and perform comparison with theoretical calculations.
The shearing interferometry is an alternative technique for assessment and measurement of wave-front aberrations. The vectorial shearing interferometer uses a displacement system to shear a wave front in vectorial form. We describe the wave-front displacement system incorporating a novel configuration of a pair of wedge prisms. Its main advantage is that the shearing is constant from the displacement system to image plane. The shear direction depends of the relative orientation of the prisms. The magnitude of the shearing is proportional to the distance between prisms. The proportionality constant depends of the wedge angle and the material of the prisms. This constant gives the sensibility to the displacement of the system. We describe and analyze the wave-front propagation through the displacement system using the exact ray trace. The deviation of the shearing by the difference in the wedge angles is larger than the produced by the oblique incidence of the wave front. Finally, we compare experimental results with exact ray trace simulations. We observe that the theoretical data corresponds to the experimental results. There is a maximum rms error of ≈ 10 μm.
We use the modulation transfer function (MTF) to evaluate the performance of several multi-aperture interferometric configurations for the detection of a faint planet in the vicinity of its bright star. We design non-redundant interferometric layouts that provide satisfactory coverage of the spatial frequencies of interest. We propose a design incorporating a rotating, rotationally shearing interferometer in a gravity-free environment. The side peak of its MTF may be centered on the spatial frequency associated with likely planet coordinates, resulting in the planet signal enhancement and isolation.