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Compact spectrophotometer using polarization-independent liquid crystal tunable optical filters
Abstract and Figures
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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... A new fabrication process that separates holographic recording and liquid crystal alignment via the photoalignment surface layer was proposed to overcome the complexity of volume holographic recording (usually limited by recording efficiency and low induced birefringence of the recording medium) and the difficulty of liquid crystal alignment in such a complex spatial pattern using rubbing techniques [119,17]. With benefits of development of new liquid crystal materials and photoalignment techniques [18,96], more practical applications arose in displays [10,9,98,120,121], imaging/nonimaging polarimeters [96,19,122,123], imaging/nonimaging spectrometers [124,125,126], beam steering [100,127], and optical switching/imaging [128]. ...
Efficient control of light polarization is essential in any optical systems where polarized light is used or polarization information is of interest. In addition to intensity and wavelength, polarization of light gives a very useful/powerful tool to control light itself and observe many interesting optical phenomena in nature and applications. Most available light sources, however, produce unpolarized or weakly polarized light except some of fancy lasers. Therefore, efficient polarization control/generation is important to improve/advance existing or emerging technologies utilizing polarized light. It is also true that polarization can be used to control another properties of light (i.e., intensity, direction). We have introduced and demonstrated achromatic polarization gratings (PGs) as broadband polarizing beam splitters performing ˜100% theoretical efficiency over a wide spectral range. The novel design of achromatic PGs and their effective fabrication method will be presented. Experimental demonstration will show that practically 100% efficient diffraction is achieved by achromatic PGs embodied as thin liquid crystal (LC) layers patterned by holographic photoalignment techniques. Non-ideal diffraction behaviors of the PGs also have been investigated beyond the paraxial limitations via numerical analysis based on the finite-difference time-domain method. We, first, study the effect of the grating regime for this special type of anisotropic diffraction gratings with the minimum assumptions. Optical properties of the PGs at oblique incidence angles and in a finite pixel are numerically predicted and confirmed by experiments. Design and fabrication of small-period PGs are discussed to show how to achieve high diffraction efficiency and large diffraction angles at the same time. Three key innovative technologies utilizing the unique diffraction properties of the PGs have been introduced and experimentally demonstrated. The first application for light-efficient LC displays is the polymer-PG display, which allows an immediate brightness improvement (up to a factor of two) of conventional LC displays by replacing absorbing polarizers with achromatic PGs as thin, transmissive polymer films. We demonstrate the first proof-of-concept prototype projector based on the polymer-PG display and we also discuss optical design considerations and challenges toward a viable solution for our ultrabright pico-projector applications of the polymer-PG display. Second, two novel beam steering concepts based on the PG diffraction have been proposed. The polarization-sensitive diffraction of the PGs provides very attractive beam steering operations with ultra-high efficiency over wide steering angles by all-thin-plate electro-optical systems. We developed a non-mechanical, wide-angle beam steering system using stacked PGs and LC waveplates, and we also demonstrated a continuous beam steering using two rotating PGs, named the Risley grating as a thin-plate version of the Risley prism. The third PG application is in imaging and non-imaging spectropolarimetry. We have shown a snapshot, hyperspectral, full-Stokes polarimeter using inline PGs and quarter-waveplates. The use of PGs as a new polarimetric element for astronomical instruments in the mid-wave IR wavelengths also has been proposed to overcome current limitations of existing IR polarimeters. In the last part of this Dissertation, we introduce a polarization-type Fresnel zone plates (P-FZPs), comprising of spatially distributed linear birefringence or concentric PG (CPG) patterns. Effective fabrication methods of P-FZPs have been developed using polarization holography based on the Michelson interferometer and photoalignment of LC materials. We demonstrated high-quality P-FZPs, which exhibit ideal Fresnel-type lens effects, formed as both LC polymer films and electro-optical LC devices. We also discuss the polarization-selective lens properties of the P-FZPs as well as their electro-optical switching. In summary, we have explored the fundamental diffraction behavior of the polarization gratings and their applications in advanced optics and photonics. The achromatic designs of the PGs allow their broadband diffraction operation over a wide range of spectrum, which increases the applicability of the PGs with a great extent. Three novel technologies that directly benefit from the distinct diffraction properties of the PGs have been developed. In addition, a new diffractive lens element operating solely on light polarization has been introduced and experimentally demonstrated. We conclude this Dissertation with our suggestions of a number of potential innovations and advances in technologies that can be enabled by polarization gratings and related technologies.
... Unlike amplitude and phase gratings, PGs operate by locally modulating the polarization state of light passing through, and the wavelength separation occurs by the grating equation since the PGs are merely birefringent gratings. PGs have been studied for many applications, including microdisplays, 15 tunable filters, 16 spectrophotometers, 17 and beam steerers. 18 Recently, we demonstrated an achromatic PG 1 that accomplishes both chromatic and polarization separation with ∼100% efficiency across the entire visible wavelength range. ...
Measurements of complete polarization and spectral content across a broad wavelength range of a scene are used in various fields including astronomy, remote sensing, and target detection. Most current methods to acquire spectral and polarimetric information need moving parts or modulation processes which lead to significant complexity or reduce sampling resolution. Here we present a novel snapshot imaging spectropolarimeter based on anisotropic diffraction gratings known as polarization gratings (PGs). Using multiple PGs and waveplates, we can acquire both spectrally dispersed and highly polarized diffractions of a scene on a single focal plane array, simultaneously. PGs uniquely produce only three diffracted orders (0 and ±1), polarization independent zeroth-order, polarization sensitive first-orders that depend linearly with the Stokes parameters, and easily fabricated as polymer films suitable for visible to infrared wavelength operation. The most significant advantage of our spectropolarimeter over other snapshot imaging systems is its capability to provide simultaneous acquisition of both spectral and polarization information at a higher resolution and in a simpler and more compact way. Here we report our preliminary data and discuss the cogent design of our imaging spectropolarimeter.
... We suggest a different LC approach, virtually insensitive to polarization, which uses a single diffractive LC element and combines the low cost and simple fabrication of LC devices with the high performance obtained by MEMS devices. The design is based on Liquid Crystal Polarization Gratings (LCPG) [12]- [15], which have been extensively studied for applications including microdisplays [16], tunable optical filters [17], spectrophotometers [18], and beamsteering applications [19], [20]. These are a class of thin-film diffraction gratings which operate by periodically modulating the polarization of light across the wavefront, as opposed to the conventional phase or amplitude. ...
We demonstrate a variable optical attenuator (VOA) based on liquid crystal polarization gratings (LCPGs), which eliminates the need for complex polarization management found in competing LC technologies. We then configure the VOA as a multi-channel wavelength blocker resulting in a simple, compact architecture with high performance and low cost. Together with a dual fiber collimator, relay lenses, a diffraction grating, a quarter wave plate, and a mirror we achieve optical attenuation of 50 dB with minimal polarization dependent loss (0 3 dB) and insertion loss (2 5 dB). The device also manifests competitive wavelength flatness (0 35 dB variation), response times (40 ms), and temperature dependent loss (47 dB maximum attenuation up to 85 C). We describe the principle of operation, explain the fab-rication process and optimization challenges, and finally present the system design and experimental results for a four-channel, 100 GHz wavelength blocker in the C-band.
... These characteristics make the filter highly suitable for integration into a compact spectrometer system, which we have designed, implemented, and analyzed in a parallel study. 15 ...
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.
Thin circular polarization gratings, characterized by high diffraction efficiency and large, up to 42°, diffraction angles were created by polarization holography for the first time. The high efficiency of the gratings is the result of the specific properties of a photo-crosslinkable liquid crystalline polymer and a two-step photochemical/thermal processing procedure. A diffraction efficiency of up to 98% at 532 nm has been achieved for gratings with periods of 700 nm. In contrast to polarization gratings with larger periods these gratings exhibit Bragg properties. So one beam is either transmitted or diffracted depending on the direction of the circular polarization of the incident light, whereas the maximal diffraction efficiency is achieved only at the proper incident angle. The fabrication procedure consists of holographic exposure of the film at room temperature which provides the photo-selective cycloaddition of cinnamic ester groups. Upon subsequent thermal annealing above Tg bulk photo-alignment of the LC polymer film occurs enhancing the optical anisotropy within the grating. The holographic patterning provides high spatial resolution, the arbitrary orientation of the LC director as well as high optical quality, thermal and chemical stability of the final gratings. Highly efficient symmetric and slanted circular polarization gratings were fabricated with the proposed technique.
This chapter introduces the fundamental requirements for measuring radiometric quantities with arbitrary sensors and gives an overview of the basic measuring principles. The radiometric quantities of optical materials, components and systems generally have to be measured dependent on of the wavelength. Instruments which are used to measure in this way are called spectrometers. The main component of all types of spectrometer is the monochromator. An optical filter can be considered as a simple monochromator. The chapter describes the basic functions and inter-relationships of monochromators. Different types of spectrometer, which are able to measure spectrally resolved optical characteristics such as transmission and reflection of small test probes, are commercially available. The chapter gives a short overview of the basic configurations and concepts. It describes a suitable technique based on the method of light addition to calibrate spectrophotometers without the need to introduce calibrated filter sets.
Spectrometer plays an important role, and is already an indispensable analytical instrumentation especially in biomedical, chemical and other fields. Instrument miniaturization will expand its application areas. And miniaturization design for all parts of optical path is the key to the whole task. This paper focuses on the research of the luminous source system design used in broad-band (200nm~1100nm) spectrometer. Considering the source wave band, a combination of deuterium lamp and tungsten lamp are chosen. According to the structure of the source, the main work concerns a collecting agglutinate lens and an elliptical reflector which acts as a light coupler. The optical design and the overall structure's optical path are simulated. On the basis of optical design and the selection as well as manufacture of the key components, the light source optical system of the project was analyzed experimentally and the results are satisfactory.
We introduce and demonstrate a compact, nonmechanical beam steering device based on liquid Crystal (LC) Polarization Gratings (PGs). Directional control of collimated light is essential for free-space optical communi-cations, remote sensing, and related technologies. However, current beam steering methods often require moving parts, or are limited to small angle operation, offer low optical throughput, and are constrained by size and weight. We employ multiple layers of LCPGs to achieve wide-angle (> ±40 •), coarse beam steering of 1550 nm light in a remarkably thin package. LCPGs can be made in switchable or polymer materials, and possess a continuous periodic birefringence profile, that renders several compelling properties (experimentally realized): ∼ 100% experimental diffraction efficiency into a single order, high polarization sensitivity, and very low scat-tering. Light may be controlled within and between the zero-and first-diffraction orders by the handedness of the incident light and potentially by voltage applied to the PG itself. We implement a coarse steering device with several LCPGs matched with active halfwave LC variable retarders. Here, we present the preliminary experimental results and discuss the unique capability of this wide-angle steering.
We report our recent experimental results on a new polarization-independent, liquid crystal (LC) spatial light modulator (SLM). Based on a periodic nematic director profile, the modulator acts as a switchable diffraction grating with only 0th- and +/-1st-orders at efficiencies of >= 99%, manifests contrast ratios ~600:1 (for laser light), switching times of ~2ms, and threshold voltages of < 1V/mum. Results of modulating broadband, unpolarized light from light-emitting-diodes (LEDs) indicates that contrast ratios are ~100:1 so far. Note that incoherent scattering for visible light is very low, and that samples are typically completely defect-free over large areas. An important feature of this diffractive polarization-independent SLM compared to its predecessors is its potential to achieve much larger diffraction angles, which enables a larger aperture (and etendue). In addition to describing the fabrication and characteristics of this SLM in general, we report on our initial progress in implementing a projection display system. All of the surprising and useful results from this grating arise from its continuous nematic director, which is most properly classed as a switchable polarization grating (PG). The SLM described here offers the inherent advantages polarization-independence at the pixel-level and fairly fast switching with nematic LCs, while maintaining similar switching voltages, cell thickness, contrast ratios, and materials.
We report on our first experimental comparison of polarization- independent projection schemes using the liquid crystal polarization grating (LCPG) modulator as the active element. This recently-demonstrated LC grating has unique diffraction properties, and we evaluate its performance using the bright- and dark-field Schlieren configurations and consider its use with a TIR-prism assembly. Each scheme shows promise for high- brightness and good-contrast projection for different applications.
The measurement of complete polarimetric parameters for a broad spectrum of wavelengths is challenging because of the multi-dimensional nature of the data and the need to chromatically separate the light under test. As a result, current methods for spectropolarimetry and imaging polarimetry are limited because they tend to be complex and/or relatively slow. Here we experimentally demonstrate an approach to measure all four Stokes parameters using three polarization gratings and four simultaneous intensity measurements, with potential to dramatically impact the varied fields of air/space-borne remote sensing, target detection, biomedical imaging/diagnosis, and telecommunications. We have developed reactive mesogen polarization gratings using simple spin-casting and holography techniques, and used them to implement a potentially revolutionary detector capable of simultaneous measurement of full polarization information at many wavelengths with no moving or tunable elements. This polarimeter design not only enables measurements over a likely bandwidth of up to 70% of the center wavelength, it is also capable of measurements at relatively high speed (MHz or more) limited only by the choice of photo-detectors and processing power of the system. The polarization gratings themselves manifest nearly ideal behavior, including diffraction efficiencies of greater than 99%, strong polarization sensitivity of the first diffraction orders, very low incoherent scattering, and suitability for visible and infrared light. Due to its simple and compact design, simultaneous measurement process, and potential for preserving image registration, this spectropolarimeter should prove an attractive alternative to current polarization detection and imaging systems.
We have developed a high resolution near-infrared temperature tunable
cryogenic spectrometer with solid Fabry-Perot etalons. It is designed
and built for diffuse emission of ionized hydrogen Brackett-gamma line
studies, although with the appropriate pre-filter it can be configured
for any near infrared lines. The etalons made from silicon and germanium
operate near 77K. The high refractive index of these etalons allows for
the construction of a very compact spectrometer. Germanium etalon with
20mm clear aperture is equivalent to a gas spaced Fabry-Perot
interferometer of about 80 mm in diameter. A strong temperature
dependence of the refractive index for these two materials makes it easy
to tune etalons. Combination of these factors allowed to build a
compact, high resolution (R=12000) high throughput instrument.
A new birefringent filter composed of a series of retardation plates between a single pair of polarizers has been described by Solc. The filter is analyzed and compared with the Lyot filter. It is found that the Solc filter is inferior to the Lyot filter in the suppression of parasitic light in secondary maxima near the primary transmission bands, although its band width is slightly less, and its transparency is very much better if both filters use Polaroid film for polarizers.
Birefringent chain filters were first described 10 years ago. Since then such filters have undergone considerable development, and experience has been gained with their use. This article includes only a brief review of previous work, which is supplemented with new information.
Optical birefringent niters, which depend for their action on the interference of polarized light, can be designed to transmit very sharp bands (down to a fraction of an angstrom in width). The elementary theory necessary for their design is given.Three forms of wide field filters designed by Lyot are described in detail. A more recently developed split element, wide field filter requires only half as many polarizers as the earlier types, which may be an advantage for some applications.Various methods of adjusting the transmission bands of a birefringent filter, including the use of elements of variable thickness, and phase shifters are discussed. For most purposes the electro-optical phase shifters are probably the most promising. For special purposes, such as spectrophotometry, phase shifters composed of rotating fractional wave plates may be more advantageous. Two such phase shifters and their application in simple and split element filters are described.A few crystalline materials which have been used or might be used to advantage in birefringent filters are mentioned.Finally, the possibility of using polarizing interferometers in combination with birefringent elements for filters with extremely sharp transmission bands (in the range of hundredths or thousandths of an angstrom) is very briefly discussed.
A Lyot-type liquid crystal tunable filter (LCTF) suitable for high-definition Raman chemical imaging has been developed. The LCTF has been incorporated into an efficient Raman imaging system that provides significant performance advantages relative to any previous approach to Raman microscopy. The LCTF and associated optical path is physically compact, which accommodates integration of the LCTF within an infinity-corrected optical microscope. The LCTF simultaneously provides diffraction-limited spatial resolution and 7.6-cm-1 spectral bandpass across the full free spectral range of the imaging spectrometer. The LCTF Raman microscope successfully integrates, in a facile manner, the utility of optical microscopy and the analytical capabilities of Raman spectroscopy. In this paper the LCTF Raman imaging system is described in detail, as well as results of initial studies of polymer and corrosion product model systems.
A two-element tunable Lyot filter operating in the terahertz (THz) frequency range is demonstrated. The central bandpass frequency of the filter can be continuously tuned from 0.388 to 0.564 THz (a fractional tuning range of 40%) using magnetically controlled birefringence in nematic liquid crystals. The transmission bandwidth is 0.1 THz and the insertion loss of the present device is 8 dB due to the scattering of LC molecules in the thick LC cells. This filter can be operated at room temperature.