Polarization-independent tunable optical filters using bilayer polarization gratings

Department of Electrical and Computer Engineering, North Carolina State University, 2410 Campus Shore Drive, Raleigh, North Carolina 27606, USA.
Applied Optics (Impact Factor: 1.78). 07/2010; 49(20):3900-4. DOI: 10.1364/AO.49.003900
Source: PubMed


We demonstrate a polarization-independent tunable optical filter based on switchable polarization gratings (PGs) formed using reactive and nonreactive liquid crystals (LCs). PGs are anisotropic diffraction gratings that exhibit unique properties, including a zero-order transmittance that is independent of incident polarization and that can vary from approximately 0% to approximately 100%, depending on wavelength and applied voltage. A stack of several PGs of varying thicknesses combined with an elemental angle filter yields polarization-independent bandpass tuning with minimal loss. We introduce a novel hybrid PG consisting of both reactive and nonreactive LC layers, which allows very thick gratings to be created with thin active LC layers. We demonstrate a tunable optical filter with a peak transmittance of 84% of unpolarized light, a minimum full width at half-maximum of 64 nm, and a maximum tuning range of 140 nm.

Download full-text


Available from: Michael J. Escuti, Feb 01, 2014
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We report on a broadband, diffractive, light shutter with the ability to modulate unpolarized light. This polarizer-free approach employs a conventional liquid crystal (LC) switch, combined with broadband Polarization Gratings (PGs) formed with polymer LC materials. The thin-film PGs act as diffractive polarizing beam-splitters, while the LC switch operates on both orthogonal polarization states simultaneously. As an initial experimental proof-of-concept for unpolarized light with +/- 7 degrees aperture, we utilize a commercial twisted-nematic LC switch and our own polymer PGs to achieve a peak transmittance of 80% and peak contrast ratio of 230:1. We characterize the optoelectronic performance, discuss the limitations, and evaluate its use in potential nonmechanical shutter applications (imaging and non-imaging).
    Full-text · Conference Paper · May 2011
  • [Show abstract] [Hide abstract]
    ABSTRACT: We introduce a polarization-insensitive tunable bandpass filter design having the following unique properties: (i) high peak transmittance (~ 80 - 90%) that is independent of input polarization, (ii) non-mechanical tuning over a potentially large wavelength range (> 100 nm) with a narrow passband (< 10 nm possible), (iii) low-cost, simple, and compact (thin-film) construction with a large clear aperture suitable for many simple camera systems. This is a stacked birefringent filter approach similar to Lyot and Solc fiters but with significantly less loss due to the removal of polarizers from the system. The filter is based on a stacked configuration of polymer polarization gratings (PGs) and either fixed or tunable wave plates. PGs are a class of thin film anisotropic diffraction gratings, which exhibit unique properties including zero-order transmittance that is independent of incident polarization, and practically all diffracted light appears within the zero- and first-diffraction orders with efficiency ranging from nearly 100% to 0%. In this work we explore a variety of filter stack configurations and analyze them theoretically using Jones Calculus and Poincare Sphere reasoning. Both fixed and tunable filter configurations are presented and analyzed in terms of finesse, full width at half maximum, free spectral range, and tuning range. We then present preliminary experimental data for a three stage fixed bandpass filter.
    No preview · Article · Feb 2011 · Proceedings of SPIE - The International Society for Optical Engineering
  • [Show abstract] [Hide abstract]
    ABSTRACT: We introduce a high efficiency method to control orbital angular momentum (OAM) using a novel diffractive optical element - switchable forked polarization gratings (FPGs). We successfully fabricated the element and realized electric-optical switching of the OAM state. Unlike other approaches, this OAM manipulation requires no mechanical parts or expensive instruments. It is achieved by complex and locally periodic alignment of a nematic liquid crystal (LC) layer acting on the Pancharatnam-Berry phase. We have recently introduced fixed FPG with photo-aligned liquid crystal polymer as a highly efficient OAM state controller. We now report on our experimental implementation of electrically switchable FPGs based on liquid crystal cell. The local anisotropy is obtained by photo-alignment and liquid crystal technology. The spatial patterning is achieved by polarization holography. An applied voltage field on the cell can switch the element between an OAM generating/transforming mode and a transmissive mode. The diffraction behavior and OAM conversion behavior with respect to polarization, wavelength, and external voltage are characterized. Our current samples showed diffraction efficiency of 95% and switching time of approximately 3 ms. Because they are very efficient, thin, and easily tailored via holographic fabrication, switchable FPGs are ideal elements to implement enhanced control of OAM in high capacity information applications, among others.
    No preview · Conference Paper · Sep 2011
Show more