Birefringent Prisms Research Articles

Performance Analysis of the Outdoor Concentrating Photovoltaic–Thermoelectric Coupling System Under Uniform Illumination

Concentrating photovoltaic–thermoelectric (CPV–TE) coupling system is an efficient solar‐to‐electric technology, but the nonuniform illumination and temperature caused by the concentrated light have a significant impact on the power generation performance of the cell. This work improves the intensity distribution through the self‐made birefringent prism, and further studies the effect of uniform or nonuniform illumination on the power generation performance of CPV–TE system under different light intensities and cooling conditions. The results show that the output power of PV cell at uniform illumination can achieve 14.74% higher than that at nonuniform illumination due to the decline of cell’ surface temperature difference. Meanwhile, the cooling condition can further enhance the output power of PV or TE cell, and weakens the impact of the illumination nonuniformity on the power generation performance of PV cell. Through the joint optimization of uniform illumination and 10 °C cooling condition, CPV–TE coupling system increases the output power by 15.83% at 10 kW m−2. TE device can further enhance the output power by 46.09 mW through the thermoelectric conversion. Surprisingly, the environmental cost from CPV–TE system reduces carbon dioxide emission of 26471.01¥ m−2 every day. The outdoor CPV–TE coupling system has a certain assistance for the practical development.

Flexible and wide-range tuning of liquid crystal polarization grating period based on single and interference-free exposure.

We propose a period control method of liquid crystal polarization grating (LCPG) based on an nterference-free and single exposure process. By adjusting three parameters of exposure setup, including incident angle of exposure beam, wedge angle of birefringent prism and tilt angle of the sample, polarization distribution of the exposure beam is changed. The spatially variant polarization of the exposure beam is transferred to liquid crystal (LC) molecules by an azo-dye photo-sensitive layer. Consequently, the LCPG with the target period is obtained. The proposed method has high flexibility and a wide range of period adjustment covering several microns to more than thousands of microns according to calculated results. Experimental results fit well with calculations. The LCPGs with different values of period from 4.5µm to more than 200µm have been realized experimentally. The proposed interference-free method would accelerate the application of LCPGs with a robust and simple fabrication process.

Single-prism polarizer for laser emission with higher power

Subject of study. The properties of a new prismatic polarizer-analyzer of laser (optical) emission with higher power were investigated. The polarizer was designed based on a birefringent crystal in the form of a single total internal reflection prism with a triangular cross-section. Spectral dependences of the geometry of the cross-section in the refraction plane were investigated for two selected polarizer materials. Aim. The study was aimed at designing a polarizer in the form of an individual single-element device capable of functioning in the laser beams with higher power that maintains the original direction of radiation propagation with minimum optical losses and without anamorphosis of the beam cross-section. Method. The transmission of radiation through an optically anisotropic crystalline prism with an arbitrary triangular cross-section was investigated using mathematical modeling. Snell’s equations, which were developed for refraction and total internal reflection of radiation on the faces of an anisotropic prism, were used. Based on the obtained solutions, the spectral dependences of angular structural parameters of prismatic polarizers were calculated under conditions of maintaining the collinearity of the input and output rays and their refractions on the input and output faces fixed at Brewster’s angles. The energy losses and spectral ranges of operation of prismatic polarizers composed of two selected crystalline materials with a priori defined geometry were analyzed. Main results. A new prismatic polarizer was designed and investigated. Its main feature corresponds to anisotropic total internal reflection of optical radiation from one of the faces of a birefringent single prism under Brewster’s refractions on the input and output faces. We established that the device exhibits minimum optical losses, collinearity (or coaxiality) of unpolarized and polarized beams, and no anamorphotic distortion of the cross-section of the output p-polarized beam. Furthermore, we demonstrated that in the case of virtually symmetric prisms for which the difference between the structural angles and Brewster’s angles is 3°–7°, the effect of anamorphism of the beam cross-section is negligibly small. The spectral ranges of the device operation were determined for several specific designs. A weak dependence of spectral bands on the selected orientations of the polarizer crystal was observed. It manifests in the symmetry of the obtained dependences. We demonstrated that the prisms composed of YVO4 crystals are the most advantageous for obtaining the widest spectral bands in the near- and mid-infrared spectral ranges. Angular tolerances for the divergence of the incident beams were determined based on analysis of the angular deviations of the incidence angles from the Brewster’s angles. The device can also be used in observational optics as an anisotropic equivalent of the isotropic Dove prism. The difference between the proposed device and the Dove prism is the fact that the proposed device provides the potential of observing an image in ideally linearly polarized light. Practical significance. The proposed single-element design of a prismatic polarizer with Brewster’s input and output faces enables the use of the device for polarization (or polarization analysis) of laser beams with power higher than the values for which the standard multicomponent polarizers are applicable.

A common-path digital holographic microscope with a rotatable birefringent prism for synthesizing high-quality quantitative phase images from multi-angle interferograms

We propose a simple common-path off-axis interferometric system to perform high-quality quantitative phase imaging based on a rotatable birefringent prism that is placed at the output port of a conventional transmission or reflection microscope. The proposed system effectively eliminates the zero-order term of holographic data and increases the phase image reconstruction bandwidth by compounding multi-angle holograms, thereby obtaining high-quality quantitative phase images without zero-order artifacts. In addition, the proposed multi-angle phase image compounding method can eliminate the repetitive object phases caused by self-reference interferometry, thereby alleviating the problem of field-of-view reduction. Furthermore, we realize high-quality quantitative phase imaging based on multi-angle phase images without sacrificing the imaging frame rate. The concept is demonstrated by performing high-quality quantitative phase imaging of polystyrene microspheres and a freshwater alga, Pinnularia. Experimental results show that the noise level of the quantitative phase images synthesized using the proposed rotatable common-path digital holographic microscopy system is lower than that of conventional single-shot digital holographic microscope reconstruction quantitative images, and a more detailed cell internal structure can be observed.

Interference-free and single exposure to generate continuous cycloidal alignment for large-area liquid crystal devices.

An approach for generating cycloidal pattern of liquid crystal (LC) molecules based on interference-free and single exposure is illustrated. The spatial manipulation of polarization state is achieved using birefringent prism and wave plates. And then, the spatially variant polarization of exposure beam is transferred to LC molecules by azo-dye photo-sensitive layer. Consequently, the LC samples fabricated shows periodically cycloidal texture and diffraction efficiency more than 99%. The measured period Λ and diffraction angle are in good consistency with theoretical results. Thus, this exposure method provides an effective and robust way for fabricating large-area LC elements, therefore paving the way for widespread applications of high-performance diffractive LC devices.

Polarization mediated coherent and incoherent Bessel-moiré generation.

We demonstrate an efficient approach for the investigation of polarization states of a Bessel beam along the propagation direction. Furthermore, we propose a method to generate Bessel-moiré using a birefringent lens and Wollaston prism. The analysis showed that an experimentally generated incoherent moiré pattern is analogous to a theoretically simulated pattern. Moreover, we verified that inhomogeneous polarization states of a Bessel beam are still present in Bessel-moiré along its propagation direction. Our observation of Bessel-moiré due to mechanical vibration depicts the fact that incoherent moiré is stable, whereas the coherent moiré is sensitive to external vibration only along its propagation direction.

Selective Coupling Enhances Harmonic Generation of Whispering-Gallery Modes

Efficient generation of harmonics in nonlinear optical resonators, particularly in whispering-gallery-mode (WGM) resonators, has led to successes such as parametric oscillators, narrow-band single-photon sources, and versatile frequency converters. A subtle problem, though, has strongly limited efficiency and hindered precise out-coupling of the generated light. Implementing their own prior theory and a polarization-selective birefringent coupling prism, the authors demonstrate an improvement of more than an order of magnitude in the amount of out-coupled light. The results are particularly interesting for applications in quantum optics relying on WGMs.

Rapid spectro-polarimetry to probe molecular symmetry in multiplex coherent anti-Stokes Raman scattering.

We present the simultaneous detection of the spectrum and the complete polarization state of a multiplex coherent anti-Stokes Raman scattering signal with a fast division-of-amplitude spectro-polarimeter. The spectro-polarimeter is based on a commercial imaging spectrograph, a birefringent wedge prism, and a segmented polarizer. Compared to the standard rotating-retarder fixed-analyzer spectro-polarimeter, only a single measurement is required and an up to 21-fold reduced acquisition time is shown. The measured Stokes parameters allow us to differentiate between vibrational symmetries and to determine the depolarization ratio ρ by data post-processing.

The Focusing Laser Differential Interferometer, an Instrument for Localized Turbulence Measurements in Refractive Flows

The theory, design, and use of a focusing laser differential interferometer (FLDI) instrument are described. The FLDI is a relatively simple, nonimaging, common-path polarization interferometer for measuring refractive signals generated by turbulence, as well as small-amplitude acoustics and boundary-layer instabilities. It has in principle a unique ability to look through wind-tunnel windows, ignore sidewall boundary-layers and vibration, and concentrate only on the refractive signal near a pair of sharp beam foci in the core flow. The instrument's low cost and ease of implementation make it a promising alternative to traditional hot-wire anemometry (HWA) and particle-based methods for turbulence characterization. A matrix equation is written for the overall optical behavior of the FLDI, and transfer functions are developed to account for spatial filtering, f/number of the field lenses, various turbulence profiles, etc. Benchtop experiments using a turbulent sonic airjet demonstrate the focusing ability of the FLDI, its frequency response, and unwanted signal rejection. The instrument is also used to optically interrogate the flow in the Penn State Supersonic Wind Tunnel and in USAF AEDC Hypervelocity Tunnel 9, where it made preliminary measurements of freestream disturbance levels and power spectra. A central feature of the FLDI used here is the replacement of traditional fixed Wollaston birefringent prisms with variable Sanderson prisms for separation and recombination of the helium–neon laser beams, and for the accurate setting of micrometer-range beam separation distances required for successful turbulence measurements. The instrument also features phase compensation of the output, where perpendicularly polarized light signals are separately sensed by the twin photodetectors. This provides a unique ability to measure the coherence of turbulent spectra and thus to reject low-coherence noise.

Squeezed light from a diamond-turned monolithic cavity.

For some crystalline materials, a regime can be found where continuous ductile cutting is feasible. Using precision diamond turning, such materials can be cut into complex optical components with high surface quality and form accuracy. In this work we use diamond-turning to machine a monolithic, square-shaped, doubly-resonant LiNbO3 cavity with two flat and two convex facets. When additional mild polishing is implemented, the Q-factor of the resonator is found to be limited only by the material absorption loss. We show how our monolithic square resonator may be operated as an optical parametric oscillator that is evanescently coupled to free-space beams via birefringent prisms. The prism arrangement allows for independent and large tuning of the fundamental and second harmonic coupling rates. We measure 2.6 ± 0.5 dB of vacuum squeezing at 1064 nm using our system. Potential improvements to obtain higher degrees of squeezing are discussed.

Three-parameter error analysis method based on rotating coordinates in rotating birefringent polarizer system.

We propose error analysis using a rotating coordinate system with three parameters of linearly polarized light--incidence angle, azimuth angle on the front surface, and angle between the incidence and vibration planes--and demonstrate the method on a rotating birefringent prism system. The transmittance and angles are calculated plane-by-plane using a birefringence ellipsoid model and the final transmitted intensity equation is deduced. The effects of oblique incidence, light interference, beam convergence, and misalignment of the rotation and prism axes are discussed. We simulate the entire error model using MATLAB and conduct experiments based on a built polarimeter. The simulation and experimental results are consistent and demonstrate the rationality and validity of this method.

Birefringent magnetic field system for experimental checking of neutron electroneutrality by the spin interferometry technique

We describe the structure design and present the results of experimental testing of a birefringent magnetic neutron prisms for spin-echo small-angle neutron scattering (SESANS). The SESANS setup, equipped with perfect single crystals for measuring diffraction-enhanced spin interference effects, must possess unprecedented sensitivity with respect to small external actions on a neutron and is intended for use in experiments for checking the neutron electroneutrality.

Long-range polarimetric imaging through fog.

We report an experimental implementation of long-range polarimetric imaging through fog over kilometric distance in real field atmospheric conditions. An incoherent polarized light source settled on a telecommunication tower is imaged at a distance of 1.3km with a snapshot polarimetric camera including a birefringent Wollaston prism, allowing simultaneous acquisition of two images along orthogonal polarization directions. From a large number of acquisitions datasets and under various environmental conditions (clear sky/fog/haze, day/night), we compare the efficiency of using polarized light for source contrast increase with different signal representations (intensity, polarimetric difference, polarimetric contrast, etc.). With the limited-dynamics detector used, a maximum fourfold increase in contrast was demonstrated under bright background illumination using polarimetric difference image.

Direct Imaging of Transmembrane Dynamics of Single Nanoparticles with Darkfield Microscopy: Improved Orientation Tracking at Cell Sidewall

Investigation of the cellular internalization processes of individual nanoparticles (NPs) is of great scientific interest with implications to drug delivery and NP biosafety. Herein, by using dual-channel polarization darkfield microcopy (DFM) and single gold nanorods (AuNRs) as orientation probes, we developed a method that is capable of monitoring AuNR orientation dynamics during its transmembrane process. With annular oblique illumination and a birefringent prism to split AuNR plasmonic scattering into two channels of orthogonal polarizations, the AuNR azimuth and polar angles are obtained from their intensity difference and intensity sum. By placing the focal plane of the microscope objective at the elevated cell sidewall rather than at the flat cell top, interference from cellular background is reduced and the signal-to-noise ratio of AuNR orientation sensing is improved significantly, especially for AuNRs inserting into the membrane at a large out-of-plane angle. As a result, we were able to capture the complete membrane-crossing dynamics of single AuNRs. This powerful method could be utilized to obtain valuable insights on NP endocytosis mechanisms of various cells.

A new approach to polarimetric measurements based on birefringent crystals and diode lasers

A new polarimetric instrument and measurement method is described based on the use of diode lasers as radiation source (532, 650 and 1064nm) and birefringent prisms, such as Glan-Laser and Wollaston, as analyzers. The laser radiation is passed through a dichroic polarizer film for further orientation of its polarization plane at 45° in relation to the polarization plane of the analyzer. The polarized beam, oriented in that way, passes the sample cell, impinges the prism surface, and the intensities of the two emerged beams are detected by two twin silicon detectors. Ideally, in the absence of any optically active substances, the crystals produces two orthogonally polarized refracted beams of equal intensity. In the presence of an optically active substance, the arctangent of the square root of the beam intensities ratio is equal to the new polarization angle (β) of the laser beam. The rotation angle imposed for any optically active substance present in the sample cell is then given by: α=(45–β)°. Because the rotation is obtained by the ratio of the intensities of two beams, it is independent of the laser intensity, which can vary up to ±15% with no significant effect on the accuracy of the polarimetric measurement. The instrument has been evaluated for measurement of optically active substances such as sucrose and fructose. The instrument employs low cost components, is capable of attaining a repeatability of ±0.003° and can measure the rotation angle, over a ±45° range, in less than 2s. Because it does not present any moving parts it can be easily adapted for in/on-line process monitoring of optically active substances.

Spectrum reconstruction based on the constrained optimal linear inverse methods

The dispersion effect of birefringent material results in spectrally varying Nyquist frequency for the Fourier transform spectrometer based on birefringent prism. Correct spectral information cannot be retrieved from the observed interferogram if the dispersion effect is not appropriately compensated. Some methods, such as nonuniform fast Fourier transforms and compensation method, were proposed to reconstruct the spectrum. In this Letter, an alternative constrained spectrum reconstruction method is suggested for the stationary polarization interference imaging spectrometer (SPIIS) based on the Savart polariscope. In the theoretical model of the interferogram, the noise and the total measurement error are included, and the spectrum reconstruction is performed by using the constrained optimal linear inverse methods. From numerical simulation, it is found that the proposed method is much more effective and robust than the nonconstrained spectrum reconstruction method proposed by Jian, and provides a useful spectrum reconstruction approach for the SPIIS.

Birefringent prism based Fourier transform spectrometer

This paper presents the design of a rugged and compact Fourier transform spectrometer (FTS) utilizing a birefringent prism, a pair of polarizers and a linear CCD array. This design improves on existing FTS by eliminating moving parts and spreading the optical path difference (OPD) spatially (rather than temporal scanning), making the system smaller, more reliable, and dramatically reducing measurement times. Both the theoretical models for the design and experimental results of the prototype are presented. The optical performance is tested using LEDs of known wavelengths, with the fringe counting technique employed during interferogram acquisition to ensure accurate sampling of the interferogram at constant OPD intervals. Reconstructing the spectra showed that the detected wavelengths deviated from the actual wavelengths by less than 1 nm.

Switching of lasing wavelength in a sol—gel laserwith dynamic distributed feedback

A scheme of switching the lasing wavelength of active centres in a sol—gel matrix excited by external laser radiation is proposed. A distributed feedback is formed during pumping by using a right-angle prism due to the interference of the direct and reflected pump beams. The lasing wavelength is determined by the period of the interference pattern, which depends on the convergence angle of interfering beams. Control is performed by a liquid-crystal cell, which changes the pump radiation polarisation, and a birefringent prism. As a result, the convergence angle of interfering beams changes, leading to a change in the interference pattern period and the excited radiation wavelength.

Interference of polarized waves at the exit of crystal prisms and their use to control the wavefront flatness

A theoretical and experimental study was performed of the interference of polarized e- and o-waves generated by passing a laser beam (collimated, convergent or divergent) through a variable-splitting-angle birefringent prism (VSABP) with inclined optical axes in the crystalline components of the prism. The VSABP-2 modification is suitable for measuring the radius of curvature of the incident wavefront from the deflection of interference fringes. This prism was used to develop an interferometric device for controlling the degree of divergence/convergence of laser radiation and a wavefront sensor for diagnosing flat wavefronts.

Characterisation of novel magnetic materials using the USANSPOL technique

USANSPOL is a novel ultra-small-angle scattering technique with polarised neutrons for investigation of magnetic materials. It represents a polarised neutron extension to traditional USANS which works with unpolarised neutrons. The high angular resolution of this technique relies on the narrow reflection width of perfect crystal reflections and is employed in a double-crystal diffractometer. Corresponding to the μrad resolution of the set-up, micro-structures of the order of a few tenths of a micrometre up to a few tens of micrometres may be investigated. Neutron polarisation is achieved by insertion of birefringent magnetic prisms between the monochromator crystal and the sample. Rocking the analyser crystal produces a scattering pattern for both neutron spin states in a single measurement but well separated in reciprocal space. By this technique, we have recently studied various amorphous Galfenol soft-magnetic ribbons which were produced by spinning from melt at different manufacturing conditions. USANSPOL allows for a determination of domain sizes of the non-magnetised samples and a study of the growing of magnetically homogeneous regions with increasing externally applied magnetic field. The manufacturing process of the ribbons is reflected in the magnetic micro-structure of the different specimens.