Showing papers by "Ignacio Moreno published in 2009"

Vortex sensing diffraction gratings

Abstract: We demonstrate a specially designed vortex sensing diffraction grating that generates multiple vortex patterns in the different diffracted orders. When this grating is illuminated with a separate vortex beam, the sign and order of the topological charge of the incident beam can be easily detected. Experimental results are shown for a variety of vortex beams including fractional values of the topological charge, and where both the diffraction grating and incident vortex illumination beam are generated by two different liquid crystal displays (LCDs). The programmability offered by the LCDs offers extremely convenient flexibility.

Influence of the incident angle in the performance of Liquid Crystal on Silicon displays

Abstract: In this paper we experimentally analyze the performance of a twisted nematic liquid crystal on silicon (LCoS) display as a function of the angle of incidence of the incoming beam. These are reflective displays that can be configured to produce amplitude or phase modulation by properly aligning external polarization elements. But we demonstrate that the incident angle plays an important role in the selection of the polarization configuration. We performed a Mueller matrix polarimetric analysis of the display that demonstrates that the recently reported depolarization effect observed in this type of displays is also dependant on the incident angle.

Wavelength-compensated color Fourier diffractive optical elements using a ferroelectric liquid crystal on silicon display and a color-filter wheel

Abstract: In this work we describe the experimental realization of a simple scheme capable of implementing RGB improved dynamic color binary-phase Fourier computer-generated holograms (CGHs) by means of a single ferroelectric liquid crystal on silicon (FLCOS) display and an electronically controlled color-filter wheel. Tricolor multiwavelength illumination is achieved by aligning an Ar-Kr laser (wavelengths lambda(B)=488 nm and lambda(G)=568 nm) and a He-Ne laser ((R)=633 nm). Chromatic compensation is achieved by synchronizing a time sequence of properly scaled CGHs displayed on the FLCOS display with the corresponding filter from the color wheel. Quality CGHs are designed for each color component by using an optimized iterative Fourier transform algorithm applied to a phase-only modulation display. As a result, we present excellent experimental results on the reconstruction of these time-multiplexed wavelength-compensated diffractive optical elements and color CGHs.

Influence of the temporal fluctuations phenomena on the ECB LCoS performance

Abstract: Liquid Crystal Displays (LCD) technology has shown to be very useful in numerous optical applications because of its capability to work as Spatial Light Modulators (SLM). The Liquid Crystal on Silicon (LCoS) displays are reflective LCDs that, because of the double pass of the incident beam through the device, give higher phase modulation than transmission LCDs with the same thickness of the LC layer. In recent works, we have thoroughly analyzed the response of a twisted nematic LCoS display working in normal incidence and we have detected that the kind of electrical signal addressed to our display produces fluctuations as a function of time of the LCoS display molecules optical axis orientation. Moreover, the temporal fluctuations produce two different physical effects that can adversely affect in optical applications. On one hand, the molecules fluctuations may produce changes at the reflected state of polarization as a function of time, introducing certain amount of effective depolarization. On the other hand, it has been demonstrated that the molecules fluctuations produce phase fluctuations that may affect the efficiency of diffractive optics elements. Here, we have characterized an Electrically Controlled Birrefrigence (ECB) LCoS display that allows us to manipulate the electrical signal addressed to it. Then, by selecting the appropriate electrical signal we can increase or decrease the fluctuation phenomena. Therefore, we have performed a polarimetric study of our (ECB) LCoS, analyzing its response dependence with different electrical addressed signals and taking special attention to the depolarization and retardance response of our device.

Operational modes of a ferroelectric LCoS modulator for displaying binary polarization, amplitude, and phase diffraction gratings

Abstract: We analyze the performance of a ferroelectric liquid crystal on silicon display (FLCoS) as a binary polarization diffraction grating. We analyze the correspondence between the two polarization states emerging from the displayed grating and the polarization and intensity of the diffracted orders generated at the Fourier diffraction plane. This polarization-diffraction analysis leads, in a simple manner, to configurations yielding binary amplitude or binary phase modulation by incorporating an analyzer on the reflected beam. Based on this analysis, we present two useful variations of the polarization configuration. The first is a simplification using a single polarizer, which provides equivalent results for amplitude or phase modulation as the more general operational mode involving two polarizers. The second variation is proposed to compensate the reduction of the diffraction efficiency when the operating wavelength differs from the design one (for which the FLCoS liquid-crystal layer acts as a half-wave plate). In this situation we show how the ideal grating performance can be recovered in spite of the phase-shift mismatch originated by chromatic dispersion. In all cases, we provide experimental results that verify the theoretical analyses.

Teaching diffraction gratings by means of a phasor analysis

Abstract: Diffraction gratings play a very important educational role in describing wave properties of light In this work we present a simple phasor technique to fully describe binary amplitude diffraction gratings with different slit widths relative to the grating period, as well as binary phase gratings with different phase shift This analysis, which is directly derived from the Huygens principle, introduces a slit phasor to account for the diffracted orders relative intensity, and a grating phasor that accounts for the grating’s resolving power The proposed phasor technique is mathematically equivalent to the Fourier transform calculation of the diffraction orders amplitude, and it can be useful to explain binary diffraction gratings in a simple manner in introductory physics courses Experimental results probing this theoretical analysis are included with the use of a liquid crystal display

Characterization and analysis of LCoS displays: application to diffractive optics

Abstract: This work shows a characterization and a polarimetric analysis of a Liquid Crystal on Silicon (LCoS) display, device that works with reflection of the light. We have observed that the optical axis of the LCoS display molecules fluctuates as a function of the time as a consequence of the type of electrical signal addressed to the device. These time fluctuations lead to two different physical phenomena that may decrease the efficiency when addressing diffractive elements to the display: the effective depolarization and the phase-fluctuations phenomena. We have developed a study of these two phenomena and its influence on diffractive elements. In particular, two different characterization methodologies suitable to obtain the Mueller matrix of the LCoS display are shown. The obtained results are provided and processed to perform a polarimetric study. Next, an intensity or a phase optimization of the LCoS display response is done, in order to obtain configurations of external polarizers and waveplates that allow us to improve the use of this device in optical applications. This study is done as a function of the incident angle and as a function of the wavelength, in order to detect the influence of these parameters on the effective depolarization origin and of the phase-fluctuations. In addition, we have analyzed the influence of the phase-fluctuations phenomena on the efficiency of a phase grating and a digital hologram addressed to the LCoS display.

Poincaré sphere analysis of a ferroelectric liquid crystal optical modulator: application to optimize the contrast ratio

Abstract: The Poincare sphere representation is used to analyze the polarization transformation achieved with a ferroelectric liquid crystal (FLC) optical modulator. This device acts as a switchable wave-plate, in which the orientation of the principal axes rotates under the action of an applied bipolar voltage. In the standard operational mode for intensity switching, the rotation angle of the principal axes is �θ = π/4 and the phase shift is φ = π (half-wave-plate). However, for wavelengths different from the design one, the FLC deviates from the half-wave-plate performance and the optical contrast is diminished. We use the Poincare sphere representation to perform a theoretical analysis of the intensity switch performance of the FLC modulator as a function of the phase shift φ. Using spherical trigonometric relations we derive analytical expressions for the intensity contrast as a function of φ and we show how to compensate its decrease when φ � π by using appropriate elliptically polarized illumination. These situations are experimentally demonstrated using a commercially available FLC cell.

Circular-polarization-splitting common-path interferometer based on a zero-twist liquid-crystal display

Abstract: We present a compact optical circular-polarization-splitting common-path interferometer based on a zero-twist liquid-crystal display (LCD). A blazed diffraction grating is encoded onto the LCD. The optical system produces a reference beam that has one sense of circularly polarized light, while the diffracted beam has the opposite sense of circularly polarized light. Using a linear polarizer, these two beams form an interferogram that can be used to analyze optically active media. Experimental results are provided showing the detection of left-handed-rotary and right-handed-rotary media.

Circular polarization-splitting common-path interferometer based on a zero-twist liquid crystal display

Abstract: It is present a unique compact optical circular-polarization-splitting common-path interferometer, described based on a zero-twist liquid crystal display (LCD) [1]. A blazed diffraction grating is encoded onto the LCD. The optical train produces two beams, a reference beam with a sense of circular polarized light and a diffracted one with the opposite sense of circular polarized light. Using a linear polarizer, these two beams form an interferogram that can be used to analyze optically active media. The detail of the light behaviour through the optical train is present using Jones matrix's.

Variable waveplate-based polarimeter for polarimetric metrology

Abstract: Polarimetry is an optical technique currently used in many research fields as biomedicine, polarimetric metrology or material characterization, where the knowledge of the state of polarization of light beams and the polarizing properties of polarizing samples is required. As a consequence, in such as applications it is necessary to use polarimeters which by means of radiomentric measurements, lead to the obtaining of some important polarimetric information. As is known, polarimeters include a state of polarization detector (PSD), which is typically formed by combinations of waveplates and polarizers. Then, intensity measurements corresponding to the projection of the analyzed state of polarization upon different configurations of the PSD used, leads to the determination of the polarimetric properties of light beams. Here, we have studied and optimized a polarimeter based on PSD system containing two electronically variable retardance waveplates. The variable waveplates are based on the Liquid Crystal Display technology, allowing the implementation of a complete polarimeter without mechanical movements.

Indexed optical composite filters

Abstract: A new technique called indexed composite filters (ICFs) for multiple object pattern recognition is proposed. Contrary to the usual so-called composite filters, the proposed ICF offers the possibility to identify which object of the learning class is detected at the output. The major advantage of ICF is that the output gives for every object detected two correlation peaks encoding the position and identity of the detected object. Computer simulation results are given using alphabetic input scenes that illustrates the efficiency of the proposed method.