Ms. Elena Nicolescu Profile

Elena Nicolescu

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Proceedings Article | 23 February 2011 Paper

Polarization-insensitive, stacked, liquid crystal polarization grating bandpass filters

Elena Nicolescu, Michael Escuti

Proceedings Volume 7934, 79340V (2011) https://doi.org/10.1117/12.875473

KEYWORDS: Optical filters, Wave plates, Transmittance, Polarization, Tunable filters, Polymers, Liquid crystals, Polarizers, Bandpass filters, Diffraction gratings

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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.

Proceedings Article | 27 August 2008 Paper

Polarization-independent tunable optical filters based on bilayer polarization gratings

Elena Nicolescu, Michael Escuti

Proceedings Volume 7050, 705018 (2008) https://doi.org/10.1117/12.795679

KEYWORDS: Optical filters, Polarization, Liquid crystals, Transmittance, Polymers, Diffraction, Linear filtering, Diffraction gratings, Tunable filters, Birefringence

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We report enhanced polarization-independent tunable optical filters based on liquid crystal (LC) and reactive mesogen (RM) polarization gratings (PGs). This new design achieves tunable passbands with significantly smaller bandwidth and allows for potential applications in spectroscopy and beyond. Analogous to Lyot and Solc filters, our filter is constructed of multiple bilayer polarization gratings (BPGs) of varying thicknesses, with the potential for highly compact implementation. BPGs are a hybrid of a switchable/tunable liquid crystal (LC) PG and a reactive mesogen (RM) PG. By adding the RM layer, the BPG provides a significant advantage over LCPGs for the filter application in that it allows very thick gratings to be created with thin active LC layers. As such, the difficulty in fabricating LCPGs with arbirtrarily large thicknesses is much less of a concern. BPGs exhibit the unique properties of PGs, including polarization independent zero-order transmittance, as well as diffraction at visible and infrared wavelengths. Our unique design enables a high peak transmittance (~ 90%) as well as a significantly improved full-width-at-half-maximum (FWHM). Here, we present preliminary data, discuss the unique capabilities and compelling advantages of our filter. We analyze performance in terms of finesse, 3dB bandwidth (FWHM), and free-spectral-range by comparison to theoretical simulation.

Proceedings Article | 12 September 2007 Paper

Polarization-independent tunable optical filters based on liquid crystal polarization gratings

Elena Nicolescu, Michael Escuti

Proceedings Volume 6654, 665405 (2007) https://doi.org/10.1117/12.735305

KEYWORDS: Optical filters, Polarization, Liquid crystals, Tunable filters, Transmittance, Diffraction gratings, Diffraction, Polarizers, Fabry–Perot interferometers, Birefringence

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We introduce and demonstrate a novel tunable optical filter that is insensitive to input polarization. While the most obvious application of this novel filter is in compact spectroscopy, all technologies that are dependent on tunable passband filters can benefit from it. Analogous to Lyot and Solc filters, this filter is constructed of multiple liquid crystal polarization gratings (LCPGs) of different thicknesses. LCPGs are switchable, anisotropic, thin diffraction gratings which exhibit unique properties including diffraction at visible and infrared wavelengths that can be coupled between only the zero- and first-orders, with nearly 100% and 0% experimentally verified efficiencies. Most relevant to the filter concept introduced in this work, the transmittance of the LCPG zeroth order is independent of the incident polarization. When combined with an elemental spatial filter, polarization-independent bandpass tuning can be achieved with minimum loss. The unique filter design enables a high peak transmittance (~90%) that is difficult in competing polarizer-based technologies. In this work we derive the core principles of the tunable filter, present preliminary experimental data, and discuss the capabilities of the filter in terms of finesse, 3dB bandwidth (full-width at half-maximum), and free-spectral-range. We will also evaluate the most likely practical limitations imposed by material properties and fabrication.

Proceedings Article | 12 September 2007 Paper

Compact spectrophotometer using polarization-independent liquid crystal tunable optical filters

Elena Nicolescu, Michael Escuti

Proceedings Volume 6661, 666105 (2007) https://doi.org/10.1117/12.735152

KEYWORDS: Optical filters, Spectrophotometry, Polarization, Tunable filters, Liquid crystals, Photodetectors, Spectroscopy, Diffraction gratings, Diffraction, Polarizers

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We introduce and demonstrate a simple spectrophotometer system insensitive to input polarization and with strong potential for compact and low-cost implementation. This technology has a wide variety of potential applications ranging from astronomy to medicine and even the cosmetics industry. To enable more powerful and portable microspectrometers we employ a novel design based on a tunable liquid crystal filter with polarization-independence, which is constructed of stacked liquid crystal polarization gratings (LCPGs). These switchable, anisotropic, thin diffraction gratings exhibit unique properties that include diffraction at visible and infrared wavelengths that can be coupled between only the zero- and first-orders (with nearly 100% and 0% experimentally verified efficiencies), depending on the applied voltage and wavelength of incident light. When combined with an elemental spatial filter, polarization-independent bandpass tuning can be achieved with minimum loss. Analogous to Lyot and Solc filters, several LCPGs are layered and introduced into a temporally resolved system using a single photodetector. The unique filter design enables improvement in terms of resolution and sensitivity by eliminating the polarization dependence present in all competing birefringence-based technologies. Also, the temporal detection system has a potential for improved miniaturization compared to any competing relevant approach and decreased cost by avoiding highly sensitive alignment, reflective diffraction components, Fabry- Perot cavities, and expensive detectors. In this work we describe the core principles of the tunable filter, present a representative spectrometer system design, report preliminary experimental data, and discuss the capabilities of the system in terms of spectral range, resolution, and sensitivity.

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