Savart Plate Research Articles

Guide to the design of broadband full polarization imager based on dispersion-compensation Savartplates.

We provide a broadband channeled, modulated full polarization imaging technology based on dispersion-compensation Savart plates in 2020. It has the advantages of being compact, using the snapshot method, and having a bandwidth of 0.132µm. It is thus invaluable for applications in diverse fields, including remote sensing, biomedicine, and military science. However, there are a lot of angle restrictions in the system. In practice, these angles cannot achieve such high machining precision, and we use the tolerance or compensation method of errors to analyze the influence of the angle deviation. This analysis will help the system achieve better compactness and stability and provide analysis methods for systems that use crystals as its key elements.

White-light channeled imaging polarimeter using Savart plates and a polarization Sagnac interferometer.

A Stokes white-light channeled imaging polarimeter using Savart plates and a polarization Sagnac interferometer (IPSPPSI) is presented, which provides an effective solution to the problem of channel aliasing in broadband polarimeters. The expression for the light intensity distribution and a method to reconstruct polarization information are derived, and an example design for an IPSPPSI is given. The results reveal that a complete measurement of the Stokes parameters in broad band can be achieved with a snapshot on a single detector. The use of dispersive elements like gratings suppresses broadband carrier frequency dispersion so the channels in the frequency domain do not affect each other, ensuring the integrity of information coupled across the channels. Furthermore, the IPSPPSI has a compact structure and does not employ moving parts or require image registration. It shows great application potential in remote sensing, biological detection, and other fields.

Toward practical weak measurement wavefront sensing: spatial resolution and achromatism.

The weak measurement wavefront sensor detects the phase gradient of light like the Shack-Hartmann sensor does. However, the use of one thin birefringent crystal to displace light beams results in a wavelength-dependent phase difference between the two polarization components, which limits the practical application. Use of a Savart plate, which consists of two such crystals, can compensate for the phase difference and realize achromatic wavefront sensing when combined with an achromatic retarder. We discuss the spatial resolution of the sensor and experimentally reconstruct a wavefront modulated by a pattern. Then we obtain the Zernike coefficients with three different wavelengths before and after modulation. Our work makes this new wavefront sensor more applicable to actual tasks like biomedical imaging.

Modified Septenary System-based logic gates using SLM and savart plate

Optical computing is a significant improvement over conventional electronic circuits due to much higher computing speeds and better data handling capabilities. A multivalued logic (MVL) system incorporates more than two logical states to enable higher data density, reduce or eliminate ripple-through carries in arithmetic operations. Optical multivalued logic may be one of the best solutions in modern computational system to resolve the recent problems associated with the existing one. The singed-bit states in a multivalued logic system also permits simplified carry or borrow free operations. In this paper an optical multivalued logic system with signed-bit is introduced, called Modified Septenary System (MSS). The seven logical states of the system are introduced and different arithmetic and logical operations are also implemented accordingly. The different logic gates of this system are designed and realized with SLM and savart plate using Optical Tree Architectures (OTA).

Design and implementation of a portable colposcope Mueller matrix polarimeter.

.Significance: Mueller matrix polarimetry can provide useful information about the function and structure of the extracellular matrix. A portable and low-cost system could facilitate the clinical assessment of cervical anomalies in low-resource settings.Aim: We introduce a low-cost snapshot Mueller matrix polarimeter that does not require external power, has no moving parts, and can acquire a full Mueller matrix in , to conduct a feasibility study for cervical imaging in the low-resource setting.Approach: A snapshot system based on two sets of Savart plates, a ring illuminator with polarizing elements (generating four polarization states), and one camera is introduced. Stokes vectors are formulated to recover the polarization properties of the sample. Then, using Mueller matrix decomposition, the depolarization and retardance information is extracted.Results: We report the results on 16 healthy individuals (out of 22 patients imaged), whose Pap smear showed no malignant findings from mobile clinics in rural region of Mysore, India. The depolarization and retardance information was in agreement with previous reports.Conclusions: We introduce an imaging system and conducted a feasibility study on healthy individuals. This work could futurely translate into diagnostic applications to provide a quantitative platform in the clinical environment (e.g., cervical cancer screening).

Newly designed modified trinary-valued logic gates using SLM-based Savart plate

Multivalued optical logic shows immense promise in modern computing systems with several advantages like ultrahigh computational speed, large amount of data handling capability and high data density. The logic gates are building blocks of computational systems, and all the logic circuits can be realized by cascading different logic gates. This paper proposes a new set of designs for existing logic gates in the Modified Trinary Number system with more efficiency and less complexity. The newly designed gates enable direct cascading of multiple gates and require lesser number of SLMs and Savart plates for realization, hence greatly reducing the circuit complexity of larger circuits. The Optical Tree Architecture of these new logic gates is realized using Spatial Light Modulators (SLM) and Savart plates. Moreover, a comparative study between the newly designed gates and existing gates is also incorporated to demonstrate the reduction in SLMs and Savart plates in the newly developed designs.

Multislit spectro-polarimeter: a facility instrument for the Multiapplication Solar Telescope

The multislit spectro-polarimeter (MSSP) is a grating-based littrow spectrograph with five slits at the entrance aperture. The polarimeter consists of a nematic liquid crystal variable retarder as the modulator and a Savart plate as an analyzer. It is one of the facility instruments on the Multiapplication Solar Telescope at the Udaipur Solar Observatory, developed to measure the magnetic fields of the Sun in the photosphere and chromosphere. MSSP currently operates only at 630.2 nm (FeI), but will be upgraded to cover CaII at 854.2 nm, HeI at 1083.2 nm, and FeI at 1565.3 nm. The spectrograph has a spectral dispersion of 15.8 mÅ ± 1.2 mÅ at 630.2 nm. The polarimeter has a sensitivity of the order of 10 − 2 and the root mean squared noise in the Stokes spectrum (continuum wavelength points of Stokes Q, U, and V) is 0.015I. To obtain an estimate of the instrument induced polarization, an analytical model is developed to determine the polarization introduced by the telescope. A polarimetric calibration (PolCal) unit is used to calibrate the downstream optical path from the telescope exit pupil up to the detector in MSSP. A residual polarization cross talk of 10% is measured in the data after applying PolCal corrections. The polarimetric data obtained from the engineering run (first-light) are inverted using NICOLE, to extract the magnetic field parameters. The field strength derived from MSSP observations is compared with the data obtained from helioseismic and magnetic imager and is found to lie within ±70 G in the umbral region and ±200 G in the penumbral region.

Trinary magnitude comparator using SLM based Savart plate

The demand of present technology inviting the popularity of multivalued optical computation system to coup up with the latest scenario of ultrahigh processing speed and handling large amount of data The magnitude comparator is the heart of the arithmetic and logic unit (ALU) in any logical processing and computing system. In this paper, a trinary magnitude comparator circuit has been proposed and implemented with modified trinary number (MTN) system. Optical tree architecture (OTA) of the proposed circuit has been realized reasonably using Savart plate and spatial light modulators (SLM). A simulation algorithm has also been developed and implemented to prove the authenticity of the proposed circuit through the simulation.

Broadband snapshot polarimetric imaging based on dispersion-compensated Savart plates

We present a dispersion-compensated Savart plate (DCSP) by combining two kinds of birefringent crystals with specific dispersion characteristics. It can generate dispersive shearing distance that is approximately proportional to the wavelength in a spectral range. The DCSP can substitute the conventional Savart plate in the channeled modulated polarimetric imaging (CMPI) to generate an achromatic fringe field, which can significantly increase the contrast ratio of the fringes. Simulation results based on MgF2 and YVO4 are provided to demonstrate the acquirement of a broadband full polarization image by snapshot covering spectrum domain from 446 nm to 578 nm.

Quadruple half adder and half subtractor design using SLM and Savart plate

This paper proposes quadruple half adder and half subtractor circuit design using spatial light modulator (SLM) and Savart plate. Over the last 2 decades, advanced technology at high-speed computation has generated large volumes of data. For some novel approaches in data processing techniques to deal with this large number of data, the ultra-high speed data processing system is required. Now optical multiple-valued logic functions are of greatest interest in recent years towards optical logic and information processing system. In order to overcome the electronic bottlenecks and fully exploit the advantages of optics, polarization properties of light realization come from nonlinear optics. It increases the scope of all-optical signals processing in high-speed photonic networks. To this aim, the designs need the use of nonlinear optics capable of controlling multi-valued signals. And to achieve this goal, the polarization properties of light in optical domain are used to the design of quadruple half adder half subtractor circuits.

Technique for generating periodic structured light beams using birefringent elements.

We put forward a simple, scalable and robust technique for generating periodically structured light beams with intensity patterns, e.g. of the form cos2n(kxx) cos2m(kyy), where kx and ky are real numbers that can be tailored and n and m are integers. The technique combines the use of Gaussian beams with curved wavefronts, birefringent crystals (Savart plates) and linear polarizers. Applications range from photolithography to fabrication of micro-lens array and fiber Bragg gratings, 3D printing and tailoring of optical lattices for trapping atoms and molecules.

Implementation of quadruple valued flip-flops using CMOS and spatial light modulator-based Savart plate

The designing and simulation of multi-valued logic (MVL) is a very interesting and important task in the present scenario. The memory devices in MVL system are the most important area of modern research. The MVL system consists of a number of intermediate states between the true and false instead of the conventional binary logic consists of only two states - 'true' and 'false'. Hence, the MVL enables more information to be handled in a much compact manner, especially suitable for handling big data. In quadruple valued logic (QVL) system, the additional two intermediate states are denoted as 'partially known' and 'partially unknown' which can be explored significantly to sequential logics also, e.g., flip-flops, etc. The present paper deals with the design and simulation of different kind of flip-flops in QVL system, using complementary metal oxide semiconductor (CMOS) and its optical implementation using spatial light modulator (SLM) and Savart plate.

Snapshot imaging spectrometry with a heterodyned Savart plate interferometer

Imaging spectrometers are frequently used in remote sensing for their increased target discrimination capabilities over conventional imaging. Increasing the spectral resolution of these sensors further enables the system’s ability to discriminate certain targets and adds the potential for monitoring narrow-line spectral features. We describe a high spectral resolution (Δλ=1.1 nm full-width at half maximum) snapshot imaging spectrometer capable of distinguishing two narrowly separated bands in the red-visible spectrum. A theoretical model is provided to detail the first polarization grating-based spatial heterodyning of a Savart plate interferometer. Following this discussion, the experimental conditions of the narrow-line imaging spectrometer (NLIS) are provided. Finally, calibration and target identification methods are applied and quantified. Ultimately it is demonstrated that in a full spectral acquisition the NLIS sensor is capable of less than 3.5% error in reconstruction. Additionally, it is demonstrated that neural networks provide greater than 99% reduction in crosstalk when compared to pseudoinversion and expectation maximization in single target identification.

Optical-rotatory-dispersion measurement approach using the nonlinear behavior of the geometric phase.

We propose a method for high-sensitivity optical rotatory dispersion (ORD) measurement of optically active samples that takes advantage of the nonlinear behavior of the geometric phase (GP). To measure the GP as a function of wavelength, we use a multichannel Fourier transform spectrometer (MC-FTS) that is based on Savart plate birefringent-polarization interference, into which we newly insert a zeroth-order quarter-wave plate (QWP). The modified MC-FTS allows us to measure the wavelength dependence of the GP and thus that of the optical rotation angle due to the sample. In this paper, we describe the proposed approach and demonstrate proof-of-principle experiments.

Compact three-dimensional head-mounted display system with Savart plate.

We propose three-dimensional (3D) head-mounted display (HMD) providing multi-focal and wearable functions by using polarization-dependent optical path switching in Savart plate. The multi-focal function is implemented as micro display with high pixel density of 1666 pixels per inches is optically duplicated in longitudinal direction according to the polarization state. The combination of micro display, fast switching polarization rotator and Savart plate retains small form factor suitable for wearable function. The optical aberrations of duplicated panels are investigated by ray tracing according to both wavelength and polarization state. Astigmatism and lateral chromatic aberration of extraordinary wave are compensated by modification of the Savart plate and sub-pixel shifting method, respectively. To verify the feasibility of the proposed system, a prototype of the HMD module for monocular eye is implemented. The module has the compact size of 40 mm by 90 mm by 40 mm and the weight of 131 g with wearable function. The micro display and polarization rotator are synchronized in real-time as 30 Hz and two focal planes are formed at 640 and 900 mm away from eye box, respectively. In experiments, the prototype also provides augmented reality function by combining the optically duplicated panels with a beam splitter. The multi-focal function of the optically duplicated panels without astigmatism and color dispersion compensation is verified. When light field optimization for two additive layers is performed, perspective images are observed, and the integration of real world scene and high quality 3D images is confirmed.

A METHOD TO CALIBRATE THE HIGH-RESOLUTION CATANIA ASTROPHYSICAL OBSERVATORY SPECTROPOLARIMETER

ABSTRACT The Catania Astrophysical Observatory Spectropolarimeter (CAOS) is a white-pupil cross-dispersed échelle spectrograph with a spectral resolution of up to R = 55,000 in the 375–1100 nm range in a single exposure, with complete coverage up to 856 nm. CAOS is linked to the 36-inch telescope, at Mount Etna Observatory, with a couple of 100 μm optical fibers and it achieves a signal-to-noise ratio better than 60 for a V = 10 mag star in one hour. CAOS is thermally stabilized in temperature within a 0.01 K rms, so that radial velocities are measured with a precision better than 100 m s−1 from a single spectral line. Linear and circular spectropolarimetric observations are possible by means of a Savart plate working in series with a half-wave and a quarter-wave retarder plate in the 376–850 nm range. As is usual for high-resolution spectropolarimeters, CAOS is suitable to measure all Stokes parameters across spectral lines and it cannot measure the absolute degree of polarization. Observations of unpolarized standard stars show that instrumental polarization is generally zero at 550 nm and can increase up to 3% at the other wavelengths. Since polarized and unpolarized standard stars are useless, we suggest a method to calibrate a high-resolution spectropolarimeter on the basis of the polarimetric properties of spectral lines formed in the presence of a magnetic field. As applied to CAOS, observations of magnetic chemically peculiar stars of the main sequence show that the cross-talk from linear to circular polarization is smaller than 0.4% and that conversion from circular to linear is less than 2.7%. Strength and wavelength dependences of cross-talk can be entirely ascribed, via numerical simulations, to the incorrect retardance of achromatic wave plates.

Alignment error analysis of the snapshot imaging polarimeter.

A snapshot imaging polarimeter (SIP) system is able to reconstruct two-dimensional spatial polarization information through a single interferogram. In this system, the alignment errors of the half-wave plate (HWP) and the analyzer have a predominant impact on the accuracies of reconstructed complete Stokes parameters. A theoretical model for analyzing the alignment errors in the SIP system is presented in this paper. Based on this model, the accuracy of the reconstructed Stokes parameters has been evaluated by using different incident states of polarization. An optimum thickness of the Savart plate for alleviating the perturbation introduced by the alignment error of the HWP is found by using the condition number of the system measurement matrix as an objective function in a minimization procedure. The result shows that when the thickness of a Savart plate is 23 mm, corresponding to the condition number 2.06, the precision of the SIP system can reach to 0.21% at 1° alignment tolerance of the HWP.

Imaging spectral bandwidth criterion equation of channeled modulated polarization imaging system

Channeled modulated polarimetry imaging (CMPI) is a novel detection technology which can acquire full-Stokes parameters of each pixel of the sensor. Compared with the other imaging polarimetric technologies, CMPI has advantages in compact, high spatial resolution and acquiring full-Stokes information simultaneously. It has been widely used in remote sensing, military reconnaissance and biomedical diagnosis. However CMPI can only be used for quasi-monochromatic light during full-Stokes imaging, which leads to low signal-to-noise ratio in many cases especially under the condition of low light. Expanding the imaging spectral bandwidth of the CMPI is of great urgency. In order to expand the bandwidth, the limitation factors and conditions of the imaging bandwidth should be clearly understood first. So an imaging bandwidth criterion is deduced in this paper for the researchers to estimate the limitation bandwidth of the CMPI. We analyze the factors which might affect the fringe visibility based on a Savart plate (SP) CMPI and obtain the conclusion that carry frequency (CF) is the main factor which restricts the bandwidth. Then, according to the definition of CF, = /(f), in which is the shearing distance of SP, is the imaging wavelength, and f the focal length of imaging lens, we investigate how these factors influence the CF. It turns out that is the main factor which causes the fringe to arise in a certain CPI system while would add an error to CF within 5% in visible light domain. To investigate how the wavelength influences the imaging spectral bandwidth, we deduce the total irradiance on the image plane under broadband light and use Fourier transform for it to obtain the distribution of the spatial frequency of the image plane. And the conclusion is obtained that the CF bandwidth be expressed as (20-1/(2L), 20 + 1/(2L)) referred to as the Rayleigh criterion, in which 0 is the central CF and L is the range of the imaging plane. After substituting the relevant parameters into the CF bandwidth, we can obtain the imaging spectral bandwidth criterion equation as = 2D02/(4D2-02) , in which is the maximum imaging bandwidth, D is the maximum optical path difference, and 0 is the central wavelength of the CMPI system. To validate the accuracy of the spectral bandwidth criterion, some simulations are conducted to generate a maximum imaging spectral bandwidth while the visibility of the fringes decreases to 0.5 for the fringes which cannot be distinguished when the visibility is less than 0.5. The results show that the error between the simulated spectral bandwidth and the calculated spectral bandwidth is less than 1 nm. This criterion value fits the test well for the SP CMPI system. In addition, it can also be used for estimating the maximum imaging bandwidth of the other CMPI system whose shearing distance is independent or quasi-independent of wavelength.

Effect of half wave plate angle mismatch on channel modulating imaging result and its compensation

Polarization imaging technology is a powerful tool in remote sensing, bioscience, or other scientific areas science, and it can extract the polarization information of target effectively. As a novel polarization imaging technology, the channel modulating polarization imaging has been widely investigated in recent years, owing to its prominent advantages of compact, snapshot and full-Stokes acquirable. In the polarization imaging system based on Savart Plate, a half wave plate with its optical axis at an angle of 22.5 is used to rotate the vibration direction of each incident light to 45. Thus, the amplitude of the light could be equally divided by the second Savart Plate polariscope. Finally, the different components of light will interfere with each other on the focal plane and the target polarization information will be modulated in the interference pattern. Since the intensity distribution of interference pattern is sensitive to the orientation of the half wave plate, a small mismatch angle of half wave plate will lead to a wrong polarization image. In order to solve this problem, we investigate the relationship between the mismatch angle and the image intensity grabbed by focal plane array (FPA) and propose an error eliminating method to improve the accuracy of polarization imaging. We analyze how the mismatch angle affects the light intensity and deduce the expression of the image obtained by the FPA. According to the expression, the raw image we grabbed directly by the FPA is a superposition of the modulated Stokes images with different carrier frequencies. Compared with the ideal expression, the expression we obtained shows that the channels of Stokes parameter S1 and S2,3 each contain a constant factor which is related to the mismatch angle . On the basis of this expression, we propose a method to measure the mismatch angle by imaging a target twice, one is behind a polarizer that is oriented at 0 and the other at 45. Then we can calibrate the system by calculating the mismatch angle through the demodulated images. To image a target with the calibrated system, we just demodulate the raw image obtained by the FPA and then divide the reconstructed stokes images by the constant factor which is determined by . For a mismatch angle of 0.5, a computer simulation is conducted. The result shows that through the compensation method, the accuracies of S1, S2 and S3 can be increased by 0.06%, 3.49% and 3.49%, respectively.

Analysis of the clear aperture of Savart plates in polarization interference imaging spectrometer

The principle of beam splitting of interference imaging spectrometer based on Savart plates is presented. The propagation track of light wave in crystal and the exit aperture is analysed by combining wave normal-tracing method with ray-tracing method at random incidence angle and azimuth angle. The relationship among exit aperture, incident angle, incident position and azimuth angle is deduced in detail. The requirements that the propagation track of light remains in Savart plates and would not exit from the top surface, bottom surface and side are discussed in detail. The area and the position of exit aperture are simulated by computer, which proves the correctness of the deduction for normal incidence of light. It is shown that the lateral shear of single Savart plate restricts the boundary of clear aperture, and the area of the clear aperture is only 85.73% against the total incident surface. The parameter of experimental prototype is introduced and the clear aperture is in detail analysed and discussed by the method mentioned above. The results show that the accurate and the approximate values of exit aperture area of experimental prototype are greatly different, and the position of exit aperture are shifted into the lower right, which may reduce the image quality and even cannot generate the double-beam interference fringes in some specific areas. The effective clear aperture as a function of azimuth angle is also presented. It shows that the used clear aperture area is between 0.8005 and 0.8547 while changes from 0 to 2, in order to match the conditions that the value of area availability decreases to 0.6976 when the light always propagates inside the Savart plates. The article shows that the change of clear aperture caused by crystal birefringence phenomenon cannot be ignored when selecting the instrument aperture stop and parameter of interference imaging spectrometer. The clear aperture of the two beams, o-light and e-light, which propagate in Savart plates should be calculated respectively and then they are used to determine the last clear apertures of plates. This study provides a theoretical and practical guidance for study, design, modulation, experiment and engineering of interference imaging spectrometers.