Polarization Compensation Research Articles

DFT＋U calculations on [formula omitted](100) surface: thermodynamic stability and polarity compensation

Employing the DFT+U framework we investigate the thermodynamic stability of U3O8(100) surface. The most stable structure is predicted to gradually turn from the under-stoichiometric 06- to the over-stoichiometric 56-slab as the oxygen chemical potential increases, showing a continuous surface oxidation process. Oxygen atoms are found to always firstly adsorb on the lower oxidized U ions. Due to the polarity compensation, significant modification of surface charge presents on considered terminations, which is transferred via both U-5f and O-2p states, implying the nature of U3O8 as an intermediate state between the Mott–Hubbard and charge-transfer materials. It is also shown that the displacement of the outmost U6O10 layer and the surface energy are strongly related to the redistribution of surface charge. The static effect between charged layers is the major factor which influences the layer displacement. The increase in surface energy is shown to be roughly linearly dependent on the amount of the surface charge transfer.

Polarization compensation method based on the wave plate group in phase mismatch for free-space quantum key distribution

Maintaining the polarization state in communication terminals is vital for polarization-encoding free-space quantum key distribution (QKD). Wave plate group phase mismatch caused by manufacturing errors, complex environmental effects, and the working wavelength deviation can reduce the polarization compensation effect. We found in theoretical analysis, that increasing phase mismatch of wave plates leads to the compensation method failure and reduces robustness. We propose a complementary polarization compensation method, which can effectively improve the robustness. Experimental results show that this method can improve the compensation effect by 50% at a slight phase mismatch, and realize a polarization extinction ratio exceeding 250:1 at the ergodic area even if the phase deviates to 0.27π. This method is beneficial to the high-stability design of free-space QKD systems and has the potential to be applied to QKD systems operating at multiple wavelengths.

Optimization method of diffraction efficiency of cascade liquid crystal polarization gratings based on polarization compensation

Optimization method of diffraction efficiency of cascade liquid crystal polarization gratings based on polarization compensation

Atomic and Electronic Structures of the Interfaces between Perovskite La0.75Sr0.25MnO3 and Brownmillerite SrCoO2.5

Epitaxial thin films composed of perovskite and brownmillerite oxides have received increasing attention because of the novel phenomena found in these systems. Herein, the atomic and electronic structures of the La0.75Sr0.25MnO3/SrCoO2.5 interface have been studied using first‐principles calculations. From the perspective of polarity compensation, three types of possible interface models are constructed, which have different interface terminations or orientation relationship. In the model where the [100] direction of SrCoO2.5 is parallel to the interface, the interface is polar. The way to compensate polarity is atomic reconstruction. For the model with the [100] direction of SrCoO2.5 perpendicular to the interface, there are two possible interface terminations. One is nonpolar. The other is polar, but its polarity can be compensated by electronic redistribution, i.e., the valence change of Mn from +3 to +4 at the interface, as consistently revealed by the change of the bond valence sums and magnetic moments of Mn ions. For this model with electronic redistribution, accompanied by the valence change, the MnO6 octahedra near the interface have the largest distortion compared with the other two models. The density of states around Fermi level and the magnetic coupling across the interface are also discussed.

Is the polarity of the Co3O4(111) surface compensated by electron transfer from bulk donor defects?

Experimental studies have shown that the Co3O4(111) surface exists as an unreconstructed (1 × 1) truncated bulk termination. Polarity compensation of this polar surface thus has to occur by a transfer of electrons, which creates partially occupied valence states and makes the surface metallic. Here we show by Hubbard-U corrected density-functional theory calculations that such a polarity compensation mechanism would make the surface highly reactive, which is not in agreement with experimental observations. An additional compensation mechanism has to be at work, by which the partially occupied valence band states are filled by electrons without requiring a reconstruction of the surface. As such an mechanism we propose the transfer of electrons from bulk donor defects to the surface. By employing this mechanism in our calculations we recover the experimentally observed reactivity. This demonstrates that for all polar surfaces, which are fully or partially polarity compensated by the transfer of electrons, additional electronic effects have to be considered in any electronic structure calculation in order to capture the correct surface chemistry.

Fiber-based quantum secure direct communication without active polarization compensation

Fiber-based quantum secure direct communication without active polarization compensation

Phase Noise Analysis of Resonator-Enhanced Electro-Optic Comb-Based Analog Coherent Receivers

We present an architecture for wavelength-division-multiplexed coherent data center links employing resonator-enhanced electro-optic frequency comb generators as both transmitter and local oscillator (LO) sources. We show that in each frequency comb, the phase noise in all comb lines is determined completely by the optical phase noise of the seed laser and the phase noise of the microwave oscillator. We propose an analog coherent receiver that performs carrier recovery on all wavelength channels using only two phase-locked loops. We use an independent cascade of phase shifters driven by marker tone detection in each channel for polarization recovery and compensation of a static optical phase shift. We study design examples for quadrature phase-shift keying and 16-ary quadrature amplitude modulation (16-QAM), determining requirements on the transmitter and LO seed laser linewidths and the LO frequency modulation (FM) response. We find that for a dual-polarization (DP) 16-QAM link with 17 wavelength channels, each operating at 56 GBaud, if the combined linewidth of the transmitter and LO seed lasers is 100 kHz, the LO seed laser requires an FM response that is flat from d.c. to several hundred MHz to maintain a phase-error penalty below 1 dB.

Bi-Directional Self-Homodyne Transmission With MIMO-Free DSP for Next-Generation Data Center Interconnects

We propose and demonstrate bi-directional (BiDi) self-homodyne coherent detection (SHCD) system experimentally, in which the multiple input multiple output free (MIMO-free) digital signal processing (DSP) is achieved with the help of BiDi-operated adaptive polarization controller (APC). Since the polarization wandering of the local oscillator (LO) is relevant to that of the counter-propagating signal within the same fiber, the APC can realize the polarization tracking and compensation for LO and the counter-propagating signal simultaneously. The power-consuming 2x2 MIMO DSP, can be eliminated in our proposed BiDi SHCD system. The frequency offset compensation (FOC), carrier phase recovery (CPR) are also omitted thanks to the SHCD. Besides, the dedicated chromatic dispersion compensation (CDC) is no longer needed due to short-reach transmission of data center interconnect (DCI). We validate in experiment the proposed system using 23GBaud DP-32QAM signal delivering 230Gb/s over 2 km of single mode fibers (SMFs) with the laser-linewidth tolerance of 1MHz. The parameters of polarization tracking algorithm are optimized to attain the polarization rotation tolerance up to 850 rad/s under the FEC threshold of 2x10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−2</sup> . The proposed BiDi SHCD system achieves the coherent-lite DSP without MIMO, FOC, CPR and dedicated CDC algorithm in dynamic state of polarization (SOP) rotation while maintaining the full spectral efficiency (SE) of traditional coherent system.

Cost-Effective ML-Powered Polarization-Encoded Quantum Key Distribution

Secure communications have become a requirement for virtually all kind of applications. Currently, two distant parties can generate shared random secret keys by using public key cryptography. However, quantum computing represents one of the greatest threats for the finite complexity of the mathematics behind public key cryptography. In contrast, Quantum Key Distribution (QKD) relies on properties of quantum mechanics, which enables eavesdropping detection and guarantees the security of the key. Among QKD systems, polarization encoded QKD has been successfully tested in laboratory experiments and recently demonstrated in closed environments. The main drawback of QKD is its high cost, which comes, among others, from: <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</i> ) the requirements for the quantum transmitters and receivers; and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ii</i> ) the need of carefully selecting the fibers supporting the quantum channel to minimize the environmental effects that could dramatically change the polarization state of photons. In this paper, we propose a Machine Learning (ML) -based polarization tracking and compensation that is able to keep shared secret key exchange to high rates even under large fiber stressing events. Exhaustive results using both synthetic and experimental data show remarkable performance, which can simplify the design of both quantum transmitter and receiver, as well as enable the use of aerial optical cables, thus reducing total QKD system cost.

Simplified radius-directed linear Kalman filter algorithm for lightning-induced state of polarization compensation in optical ground wire

Lightning strokes can cause an ultrafast rotation of the state of polarization (RSOP) in an optical ground wire cable, which leads to communication interruptions. To address this, a simplified radius-directed linear Kalman filter (S-RD-LKF) for blind polarization demultiplexing was developed. The ultrafast RSOP under thunderstorm weather conditions was simulated based on the Heidler model, and the simulation results indicated that the proposed S-RD-LKF algorithm effectively compensated for the RSOP impairment caused by the first and subsequent lightning strokes. Moreover, the RD-LKF algorithm and the proposed S-RD-LKF algorithm exhibited a similar performance; however, the computation complexity of the proposed algorithm was lower.

“The main drawback of the book originates from its main advantage”: the evaluation strategy of polarity compensation in the reviews of German and Russian linguists

The purpose of the study is to analyze clusters of statements embedded in the evaluative strategy of polarity compensation in German and Russian academic reviews in Linguistics. Moreover, the article is aimed at identifying linguistic and cultural differences determining the usage of this evaluative strategy. The most typical feature established for the strategy under consideration roots in the balancing positive and negative assessments. For the first time, the analysis takes into account the variability of the polarity compensation strategy depending on specific patterns of the interaction between positively and negatively polarized assessments in clusters. The study carried out shows that the polarity compensation in evaluative acts accomplished in academic linguistic reviews can function as a macro-strategy of expert evaluation. Single assessments embedded in polarity compensation strategy can be implied as a recommendation, (dis)agreement with the author, proof by contradiction. The article describes the results of a comprehensive socio-communicative and discursive study of expert evaluation acts. Qualitative and quantitative analysis leads to description of the polarity compensation mechanism as well as to elicitation of lexical and grammatical means typical of this strategy. Interpretation of the statistical data processing allows us to draw conclusions about culturally specific trends in the use of the polar compensation strategy by German and Russian reviewers.

Real-time polarization compensation system for wavelength division multiplexing in low noise fiber channel based on single photon counting feedback

The physical effects such as random birefringence of fiber optic channels due to environmental influences make the optical signals transmitted in them have sensitive polarization variations, which seriously affects the performance of polarization biased code quantum key distribution systems. In this paper, a low-noise fiber channel wavelength division multiplexing real-time polarization compensation system is presented, where single photon counting is used as a feedback signal. The system can acquire the fiber channel polarization change information by detecting the photon counting of the conjugate reference light. In the system, the compensation algorithm is designed to control the electric polarization controller to calibrate the polarization state of the quantum signal light under the corresponding polarization base in real time, and the stable fiber channel polarization compensation is successfully achieved. In order to verify the effectiveness of the compensation system, a quantum key distribution test based on BB84 protocol with a transmission distance of 25.2 km is conducted, and stable test results of up to 8 hours are obtained in the laboratory environment and the simulated metropolitan area network buried fiber environment, with the average quantum bit error rate being 0.52% and 1.25%, respectively. The experimental results show that this system can guarantee the stable operation of polarization-encoded quantum key distribution in the buried fiber environment in urban areas.

Polarization Aberration Analysis and Compensation of Off-Axis Optical System with Digital Micro-Mirror Device

Polarization Aberration Analysis and Compensation of Off-Axis Optical System with Digital Micro-Mirror Device

Обобщенная нуль-эллипсометрия в схеме "поляризатор-образец-анализатор"

The null-method in generalized ellipsometry with the use of the compensator-free “polarizer ‒ sample ‒ analyzer” scheme is considered for the case of s- and p-polarized incident light on an anisotropic system. Analytical expressions are given that connect the measured angular value — the analyzer azimuth at the detected radiation intensity minimum — with the (2x2) anisotropic Jones matrix elements. To determine the optical and geometric parameters of the studied anisotropic systems, it is proposed to use this value’s dependence on the sample orientation (azimuth). The method sensitivity is estimated. It was found that it is comparable to the sensitivity of the “polarizer‒compensator‒sample‒analyzer” scheme. A comparative analysis of this method and the known photometric method of generalized ellipsometry in the “polarizer-sample-analyzer” scheme based on measuring the dependence of the reflected light intensity on the sample azimuth at the fixed polarizer and analyzer positions is carried out. It is estimated that, to obtain the same sensitivity of these two methods, the one arc minute error in the proposed method corresponds to the 0.05% relative error in determining the energy reflection coefficient in the photometric method.

Demodulation with Polarization Compensation for Weak Fiber Bragg Grating Array

Demodulation with Polarization Compensation for Weak Fiber Bragg Grating Array

EDF laser temperature sensor using double-pass cascaded-chirped long-period fiber grating in sigma-cavity configuration

We propose a novel Erbium-doped fiber laser sensor in a sigma-cavity configuration (EDFσL) for temperature sensing. In this configuration, the sigma-branch of the EDFσL, or the double-pass cascaded-chirped long-period fiber grating (C-CLPG), consists of an optical circulator, a C-CLPG, and a Faraday rotator mirror (FRM), and works as a reflective sensor device as well as a wavelength selection element with a comb-like spectrum, namely a channeled spectrum. The double-pass C-CLPG has the advantage of not only a flexible sensor arrangement, especially for remote sensing, but also the compensation effect for the polarization fluctuations in the sigma-branch caused by the measurand. In the experiment, it is shown that the FRM reflection scheme can stabilize the output polarization state of the double-pass C-CLPG during the temperature change around the C-CLPG, and it is found that the EDFσL enables a more stable and precise measurement than the previous simple EDF ring laser using the C-CLPG. Furthermore, it is also demonstrated that the oscillation is stabilized by the polarization compensation effect of the double-pass C-CLPG even if the polarization state is intentionally changed by applying the mechanical deformation near the C-CLPG.

Оптимізація поляризаційно-доплерівської селекції малорозмірних об’єктів на тлі підстильної поверхні

Using the method of maximum likelihood, the optimal algorithm of polarization-Doppler selection of objects on the background of the underlying surface, hydrometeors, urban buildings from aerospace carriers of radio electronic equipment has been synthesized. To solve the problem polarimetric properties of the scattered electromagnetic radiation of natural environments and anthropogenic objects were analyzed.The functional-deterministic mathematical model of the useful signalis determined. When solving the optimization problem, the method of maximum likelihood functionality and likelihood functionality for correlated processes were used, which contains the inverse matrix of inverse correlation functions of the observation equation. The obtained signal processing algorithm includes operations of spectral resection of passive interference. Polarization compensation of passive interference is performed by a combination of interchannel subtraction of the reflected signals of different polarizations. The quasi-optimal features of the synthesized algorithm, which has an obvious physical value and is optimal in the absence of internal noise, are considered. Based on the obtained results, a block diagram of the polarization-Doppler noise compensator and selection of useful signals against the background of noise reflections from the earth's surface has been developed. The study of the physical characteristics of the elements of the polarization covariance matrix of interferences at a high level of correlation of reflections of passive interferences of different polarizations is conducted.Based on the obtained results, it is proposed to introduce indicators of efficiency of passive interference compensation and selection of useful signals. The results of the analysis of the obtained indicators depending on the coefficient of the ratio of variances of passive interference indicate the need to use the polarization differences between signals and interference for selecting objects against the background of the underlying surface. The results obtained can be used to create ground-based and aerospace-based radar systems for detecting moving objects and objects with pronounced polarization features.

Planar diffractive grating for magneto-optical trap application: fabrication and testing.

The design, fabrication, and demonstration of a planar two-dimensional-crossed reflective diffractive grating are proposed to construct a novel optical configuration, to the best of our knowledge, potentially applied for atom cooling and trapping in a magneto-optical trap. Based on the proposed single-beam single-exposure scheme by means of an orthogonal two-axis Lloyd's mirrors interferometer, we rapidly patterned a ∼1µm period grating capable of providing a uniform intensity of the diffracted beams. The key structural parameters of the grating including the array square hole's width and depth were determined, aiming at providing a high energy of the diffracted beams to perform the atom cooling and trapping. To guarantee the diffracted beams to be overlapped possibly, we adopted a polarized beam splitter to guide the optical path of the incident and zero-order diffracted beams. Therefore, one zero-order diffracted beam with a retroreflected mode and four first-order diffracted beams with appropriate optical path constructed a three-dimensional optical configuration of three orthogonal pairs of counterpropagating beams. Finally, three pairs of the counterpropagating cooling laser beams with 9 mm diameter and >10% diffraction efficiencies were achieved, and the circular polarization chirality, purity, and compensation of the desired diffracted beams are further evaluated, which preliminarily validated a high applicability for the magneto-optical trap system.

Research on a real-time polarization compensation method for dynamic quantum communication terminals

In the experiment of quantum key distribution in free space, the polarization degradation and the propagation directions of the four polarization-encoded quantum light beams vary relative to each other and reduce the service life of the terminals, which severely limits their practical application. In order to solve the above problems, we report research on real-time polarization compensation methods for dynamic quantum communication terminals. The difference from Micius is that our proposed system emits quantum lights through the same optical fiber to ensure high-precision coaxiality, and uses a set of wave plates to compensate for both the polarization degradation caused by single-mode fiber and the polarization change caused by optical elements. We introduce our polarization compensation methods in detail and present a design for a dynamic polarization compensation scheme and experimental device. The final experimental polarization extinction ratio of our dynamic optical system is greater than 30 dB after real-time polarization compensation, which meets quantum communication requirements. The successful implementation of this method can effectively improve terminal service life and provide a foundation for the practical applications of quantum communication.

Electrostatic doping determined by band alignment in graphene on ferroelectric LiNbO3(0001) polar surfaces

Herein, first-principles density functional theory calculations are performed to investigate the interface coupling and electrostatic doping effect in the graphene/LiNbO3(0001) [denoted as Gr/LNO(0001)] heterostructure. The results show that, in addition to the coupling mechanism of the large ionic–van der Waals interaction, an unusual electrostatic doping, which is in contrast with the polarization compensation mechanism, appears in the Gr/LNO(0001) system: for graphene adsorbed on the thermodynamically preferred ferroelectric LiNbO3(0001) [denoted as LNO(0001)] positive (Z+) and negative (Z−) surfaces, carriers are of p- and n-type, respectively. Further analysis indicates that the electrostatically doped effect of the graphene layer adsorbed on the LNO(0001) polar surfaces should be determined by the particular band alignment between the LNO(0001) polar surfaces and graphene. Our studies provide theoretical evidence for designing p–n homojunctions in a graphene sheet by engineering the domain structure of the LNO(0001) ferroelectric substrates. Furthermore, the significant change in carrier density caused by the reversal of the ferroelectric polarization direction suggests that the Gr/LNO(0001) system could be a promising platform to explore ferroelectric field-effect transistors.