Ali Derakhshandeh, Peter A. Hoeher, Stephan Pachnicke
A novel approach to underwater optical wireless coherent communications using liquid crystal spatial light modulators (LC-SLMs) and an aperture averaging lens, in combination with optical phased-array (OPA) antennas, is presented. A comprehensive channel model that includes a wide range of underwater properties, including absorption, scattering, and turbulence effects, is employed to simulate the underwater optical wireless communication (UOWC) system in a realistic manner. The proposed system concept utilizes aperture averaging and adaptive optics techniques to mitigate the degrading effects of turbulence. Additionally, OPA antennas are integrated into the system to provide electronic beam steering capabilities, facilitating precise pointing, acquisition, and tracking (PAT) between mobile underwater vehicles. This integration enables high-speed and reliable communication links by maintaining optimal alignment. The numerical results show that under strong turbulence, our combined turbulence-compensation approach (LC-SLM plus aperture averaging) can extend the communication range by approximately threefold compared to a baseline system without compensation. For instance, at a soft-decision FEC threshold of <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mn>1.25</mn><mo>×</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>2</mn></mrow></msup></mrow></semantics></math></inline-formula>, the maximum achievable link distance increases from around <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mn>10</mn><mspace width="0.166667em"></mspace><mi mathvariant="normal">m</mi></mrow></semantics></math></inline-formula> to over <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mn>30</mn><mspace width="0.166667em"></mspace><mi mathvariant="normal">m</mi></mrow></semantics></math></inline-formula>. Moreover, the scintillation index is reduced by more than <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mn>90</mn><mo>%</mo></mrow></semantics></math></inline-formula>, and the bit error rate (BER) improves.
Organic crystals offer promising potential for THz generation, but face limitations in wavelength tunability and damage threshold. By applying tilted pulse‐front pumping to organic crystals an additional degree of freedom is introduced into the pumping conditions enabling a wider range of pumping wavelengths without compromising phase matching. Additionally, the lifespan of organic materials can be extended by using longer pumping wavelength and eliminate lower‐order multi‐photon absorption, allowing for higher pumping intensity without significant free‐carrier absorption, thus increasing the damage threshold. Simulations predict significant improvement for four out of six investigated crystals when tilted pulse‐front pumping is applied. By using volume phase holographic grating one can achieve pulse‐front tilt in organic crystals in collinear geometry with high diffraction efficiency. Design parameters are also presented.
In this paper we study the transmission of higher-order modes, including optical vortices (OVs), through a bus fiber evanescently coupled with a vertical array of ring resonators (VAR), which form a vertically stacked multi-ring resonator. It is shown that the OV transmission curves have a characteristic structure that we explain by the manifestation of the band structure of an infinite stack of coupled ring resonators. We demonstrate a fundamental possibility of using VARs as elements of delay lines for fiber-optic communications using orbital angular momentum. It is shown that the VAR is capable of serving as a delay line element for even and odd Laguerre–Gauss modes.
A Single-Photon Gate A long-standing goal in optics is to produce an all-optical transistor, in which the transmission of a light beam can be controlled by a single photon. Using a system in which a cloud of cesium atoms is coupled to an optical cavity, Chen et al. (p. 768, published online 4 July; see the Perspective by Volz and Rauschenbeutel) were able to control transmission through the optical cavity by exciting the atomic ensemble using a “gate” laser pulse. Just one gate photon stored was sufficient to detune the system and switch the transmission of source photons through the cavity. Optical transmission through a cesium-filled cavity can be controlled by a single stored photon. [Also see Perspective by Volz and Rauschenbeutel] The realization of an all-optical transistor, in which one “gate” photon controls a “source” light beam, is a long-standing goal in optics. By stopping a light pulse in an atomic ensemble contained inside an optical resonator, we realized a device in which one stored gate photon controls the resonator transmission of subsequently applied source photons. A weak gate pulse induces bimodal transmission distribution, corresponding to zero and one gate photons. One stored gate photon produces fivefold source attenuation and can be retrieved from the atomic ensemble after switching more than one source photon. Without retrieval, one stored gate photon can switch several hundred source photons. With improved storage and retrieval efficiency, our work may enable various new applications, including photonic quantum gates and deterministic multiphoton entanglement.
Fernanda Hediger Borges, Douglas Silva da Hora Oliveira, Giulia Paulino Hernandes
et al.
We report high incorporation of rare earth ions (RE3+) into hafnia nanoparticles prepared by the sol–gel method and investigate how these dopants affect hafnia structure and phase transformation. An ethanolic suspension containing 5-nm hafnia nanoparticles was obtained from HfOCl2.8H2O in ethanol. Pure and 0.1–7 mol% Eu3+-doped materials afforded HfO2 monoclinic phase, whereas hafnia nanoparticles added with 10 and 20 mol% Eu3+ were stabilized in the tetragonal phase. Structural evolution of the nanoparticles was analyzed by Eu3+ luminescence spectroscopy and excited level lifetimes. The emission spectra in the visible region showed an increase of the Eu3+ site symmetry due to hafnia phase transformation from monoclinic to tetragonal upon increasing Eu3+ concentration. Concentration quenching, followed by lifetime measurements, occurred at high Eu3+ concentration (20 mol %). The hafnia tetragonal phase was stabilized with non-optically active La3+ (a fixed concentration of 10 mol %), co-doped with a lower concentration of Eu3+ ions (from 0.1 to 3 mol %). This strategy ensured that Eu3+ luminescence in tetragonal hafnia was intense and prevented quenching by the high Eu3+ concentration. In this sense, the hafnia structure and emission properties can be tailored by the RE3+ concentration, so that an interesting material for applications in photonics and biophotonics can be achieved.
Bistable liquid crystal (LC) shutters have attracted much interest due to their low energy consumption and fast response time. In this paper, we demonstrate an electrically tunable/switchable biostable LC light shutter in biological optics through a three–step easy–assembly, inexpensive, multi–channel shutter. The liquid crystal exhibits tunable transparency (100% to 10% compared to the initial light intensity) under different voltages (0 V to 90 V), indicating its tunable potential. By using biomedical images, the response time, resolution, and light intensity changes of the LC under different voltages in three common fluorescence wavelengths are displayed intuitively. Particularly, the shutter’s performance in tumor images under the near–infrared band shows its application potential in biomedical imaging fields.
Adrian Krohn, Andrej Harlakin, Steffen Arms
et al.
In visible light communication systems, the ability to suppress interference caused by other light sources is a major benefit towards performance improvements. Especially for large transmitter arrays or even multi-cell arrangements, the interference problem needs to be handled. In previous work, we have presented a liquid crystal display (LCD) used as an adaptive interference-suppression filter mounted in front of each photodetector. The display elements are switched on and off in such a way that light emitted by unwanted light sources ideally is blocked, but light emitted by desired light sources reaches the detector. The pattern generated by the LC display has strong impact on the system performance. In this paper, we propose combined precoding in conjunction with LCD-based interference suppression in order to increase the signal-to-interference-plus-noise ratio and to ensure user fairness in massive MIMO scenarios. The suggested precoding strategy uses a new heuristic optimization approach based on the Santa Claus problem on unrelated machines known from computer sciences, and employs only binary entries in the weighting matrix. Corresponding results are compared with a genetic evolutionary optimization strategy and with conventional zero-forcing precoding. Regarding performance evaluation, we perform numerical ray-tracing simulations and present a room-scale VLC testbed for experimental verification.
Romala Sattibabu, Pradip Kumar Dey, B.N. Shivakiran Bhaktha
et al.
Low-crosstalk zero-gap directional coupler (ZDC) based TE-TM polarization splitter/combiner was designed at 1.55 µm transmitting wavelength using titanium indiffused LiNbO3 technology and effective-index based matrix method (EIMM). Single mode waveguides and ZDC were designed from its fabrication parameters. The coupling length of ZDC was optimized to 1826 µm to achieve a splitter crosstalk around −60 dB for both polarizations. At input and output ends of ZDC low-loss cosine-generated S-bent waveguides were used. For proof-of-concept, the device was fabricated and tested. Details of fabrication technology of the device are discussed. DC sputtering process was adopted for titanium film deposition, and wet-oxygen ambient was used during high-temperature diffusion of titanium into LiNbO3 to suppress lithium out-diffusion. Our first experiment with the splitter yields crosstalk values −10.79 dB for TE and −10.21 dB for TM mode. The measured fiber-device total insertion losses were 3.13 dB and 5.15 dB for TE and TM polarizations. The higher crosstalk values of the fabricated polarization-splitter can be significantly lowered by using optical detector of higher efficiency, and coupling length compensation during mask-data generation.
In dual-carrier (DC)-paired fiber optic long-reach transmission, which extends a point-to-multipoint coherent system to a cascaded reconfigurable optical add-drop multiplexing (ROADM) system, a narrow passband causes severe performance degradation of the edge subcarriers. In this study, we proposed two-dimensional frequency and polarization domain coding and multicarrier-frequency control for cascaded ROADM filtering of robust multicarrier transmissions at a single wavelength. The proposed coding scheme reduced fiber nonlinearity, polarization-dependent loss (PDL), and passband narrowing (PBN) simultaneously. The multicarrier frequency control algorithm, which feeds back the received signal quality at multiple points, reduced PBN owing to long-term laser frequency drifts. With the proposed coding, the Q value of 56 Gbit/s × DC-paired dual-polarized (DP)-4QAM and 112 Gbit/s × DC-paired DP-16QAM signals improved by 1 dB and 0.2 dB, respectively, compared to that when no coding was used. Furthermore, the long-term frequency control emulated through the emulation model of laser frequency drifts and the performance degradation of average Q value were less than 0.4 dB and 0.1 dB, respectively. To the best of our knowledge, no other study has evaluated point-to-multipoint transmission over ROADM-induced optical filtering systems that apply frequency and polarization domain coding and multicarrier frequency control for DC-paired DP-<italic>M</italic>-QAM signals. The proposed methods enable stable operation while suppressing the effects of PDL and PBN, which are problems in multicarrier transmission, in long-reach optical fiber transmission systems that integrate access and metro networks.
In the field of glass science and technology, as well as for historical glasses, a remarkable importance is devoted to the understanding of the interaction between the glass surfaces and the surrounding environment. Glass fabrication and preservation are very important issues in several research fields, involving both industrial and scientific problems. In general, a multi-technique approach should be used in order to achieve a better understanding of the complex phenomena involving reactions among glass surface atoms and environmental ones. In this frame, one of the most promising investigation technique is the X-ray photoelectron spectroscopy, XPS (also known as electron spectroscopy for chemical analysis, ESCA) mainly because of its ability to give information about the chemical bonds of the investigated atoms. In this paper the first part is devoted to the description of the basics of the technique, while in the second part several applications to the analysis of oxide glass surfaces are reported and discussed. The aim of this paper is to provide valuable help to all those who want to start or deepen the study of glass surfaces by this technique.
In this study, nanosecond-pulse lasing was achieved using a vertical-external-cavity surface-emitting laser (VECSEL) with only a single gain chip. A simple linear-cavity setup was used for the VECSEL, and pulsed lasing with a pulse width of ∼9 ns and a repetition frequency of ∼54 MHz was achieved. Dual-wavelength lasing at 974 and 978 nm was observed. The average output power during pulsed operation exceeded 100 mW. As the pumping power was increased, the laser output power could be switched off. However, stable dual-wavelength emission was maintained. The maximum output power under dual-wavelength operation could reach 455 mW.
S.V. Eremeev, A.V. Abakumov, D.E. Andrianov
et al.
A new method for decomposing an image into separate objects of interest is proposed in the article. The developed method is based on the use of persistent homology. A process of direct and reverse image transformation is shown. Following direct transformation, the original image is represented as a set of matrices that can be divided into basic and detailing ones. The basic matrices contain information about the basic structure of objects in the images, and the detailing ones include data about the details of these objects, about small objects or the noise component. It is shown that there is a certain analogy with the Wavelet transformation, but the proposed method is based on a fundamentally different theoretical basis. A numerical example reflecting the basic essence of the method is described in detail. Properties of the decomposition and the possibility of using standard algebraic operations on decomposition matrices are described. The reverse transformation allows us to take into account the changed properties of individual objects and synthesize a new image. Prospects of using the proposed decomposition for solving practical problems are demonstrated. Algorithms have been developed for binarization of images and removal of text on a non-uniform background. Data analysis and processing is carried out using a unified approach in the space of decomposition matrices. The results of binarization have shown that, when compared with analogues, the developed algorithm will perform better when the binarization is used to isolate a multitude of individual objects. The obtained results of the algorithm for deleting text on a non-uniform background confirm that the information is completely deleted without affecting the rest image areas.
The influence of the nonlocality effect on the optical characteristics of the near field of a plasmonic nanolaser resonator is considered. A computer model based on the Discrete Sources method has been developed for the analysis of the near-field characteristics of a layered nanoparticle located on a transparent substrate in an active medium. In this case, the nonlocality of the plasmon metal is taken into account within the framework of a Generalized Nonlocal Optical Response model. Excitation of a particle by both propagating and evanescent waves is investigated. "Optimal" directions of external excitation have been established. It is found that excitation by an evanescent wave leads to a higher intensity of the near field. It is demonstrated that accounting for the nonlocal effect in the plasmonic metal significantly reduces the field amplification factor.
Hai-Long Wang, En-Ming You, Rajapandiyan Panneerselvam
et al.
This review focuses on the advance of optical design from nano/micro to macro in SERS and SEIRA, especially on the optical coupling between nano/micro and macro scales.
As flexible wearable sensors and imagers are receiving attention from diverse social sectors, the freely attachable photothermoelectric (PTE) conversion technique should be evaluated to develop a highly usable safety sensor network. Although carbon nanotube (CNT)‐related materials should be effective, the key parameters/structures that maximize PTE conversion have not been clarified, thus hindering optimum device design and practical use. Herein, the flexible, sensitive broadband photodetection operation based on a coupling configuration between the CNT film photo/heat/electron channel and metal electrode is evaluated. Experimental PTE measurements and steady‐state thermal distribution simulations reveal that a series coupling of a p‐type CNT film channel and a highly negative Seebeck coefficient counter metal electrode facilitate superior photodetection performances than those of a parallel coupling configuration. Furthermore, subsequent device designs provide sensitive broadband photodetection from the millimeter‐wave to visible light wavelength regions with a minimum noise equivalent power of 5 pWHz−1/2 in an uncooled nonvacuum condition. Simultaneously, the mechanical flexibility of the proposed photodetector allows for its use in freely attachable sheet imager applications on curvilinear objects, and the nondestructive 3D photomonitoring of a defective intricately bent sample is demonstrated.
Applying the quadrature amplitude modulation (QAM) format, quantum-noise randomized cipher (QNRC) systems hide the signal states in quantum phase noise and amplitude noise to prevent eavesdropping. In this paper, based on the traditional wire-tap channel model analysis method, the physical-layer security of QAM-QNRC system is investigated quantitatively under the metric of secrecy rate. The general expressions of secrecy rates of the data and key are derived separately. Furthermore, the maximum reachable secrecy rate of a QAM-QNRC system is put forward, under which the data and key are both safe in the view of mutual information evaluation. Finally, the variation trend of secrecy rate with various system parameters is discussed in detail. The simulation results show that we can obtain a higher secrecy rate by setting reasonable parameters, such as the level of ciphertext, mesoscopic signal power, and inner gain at the transmitter. Meanwhile, the security of the key is the main constraint of the maximum reachable secrecy rate.
Stripe rust caused by Puccinia striiformis f. sp. tritici (Pst) is a devastating wheat disease worldwide. Potential application of near-infrared spectroscopy (NIRS) in detection of pathogen amounts in latently Pst-infected wheat leaves was investigated for disease prediction and control. A total of 300 near-infrared spectra were acquired from the Pst-infected leaf samples in an incubation period, and relative contents of Pst DNA in the samples were obtained using duplex TaqMan real-time PCR arrays. Determination models of the relative contents of Pst DNA in the samples were built using quantitative partial least squares (QPLS), support vector regression (SVR), and a method integrated with QPLS and SVR. The results showed that the kQPLS-SVR model built with a ratio of training set to testing set equal to 3 : 1 based on the original spectra, when the number of the randomly selected wavelength points was 700, the number of principal components was 8, and the number of the built QPLS models was 5, was the best. The results indicated that quantitative detection of Pst DNA in leaves in the incubation period could be implemented using NIRS. A novel method for determination of latent infection levels of Pst and early detection of stripe rust was provided.