Different Types Of Optical Fibers Research Articles

Damage characterization of FRP embedded with different optical fibers under tensile load using acoustic emission and micrographic real-time inspection

Embedding optical fibers into fiber-reinforced plastics (FRPs) for composite health monitoring has attracted the interests of academia and industry recently though the impact of embedded optical fiber on FRP damage has not been clarified. In this study, the damage to the FRP structure caused by the embedded optical fibers was investigated. Five FRP samples incorporated with different types of optical fibers were subjected to tensile testing, and the crack initiation and propagation were in situ monitored using acoustic emission (AE) and micrographic inspection. After the AE signals were classified in terms of peak amplitude and peak frequency using K-means++ clustering, the damage progression from matrix cracking, fiber/matrix debonding, delamination, and fiber break were characterized. In-situ micrographic inspection clearly revealed the damage evolution around the embedded optical fiber. By correlating the stress-strain curves with the AE clustering and micrographic inspection results, it was verified that larger-diameter optical fibers reduce the modulus and ultimate strength of the FRP, while a good optical fiber/matrix interface mitigates these negative effects. The embedded optical fiber accelerated the crack propagation around it, leading to changes in the damage pattern. This phenomenon is the primary cause of FRP failure. Notably, polyimide-coated optical fibers with an outside diameter of 50 μm are ideal for embedded sensing as their incorporation into the matrix reduces the ultimate strength of FRP by only 1.8% with minimal impact on crack propagation.

Power over fiber in radio access networks: 5G and beyond

This paper introduces the concept of power over fiber (PoF) and potential applications envisioned in radio access networks with optical fronthauling using different types of optical fibers. It is an open discussion on exploring PoF technology with current experiments integrating analog radio over fiber using 5G-NR signals in compliance with 3GPP, PoF, and monitoring techniques and general requirements in future deployments along with its potential to be part of the energy efficiency strategy in beyond 5G networks.

A comprehensive review of optical fiber technologies in optogenetics and their prospective developments in future clinical therapies

As the incidence of neurological disorders rises, the need for innovative and minimally invasive treatment methods is paramount. This review comprehensively explores the state and the application of optical fiber technologies in optogenetics, highlighting the transformative potential of these advancements for future clinical therapies. Central to our discussion is the exploration of different types of optical fibers in in vivo optogenetics, including their design, fabrication techniques, and applications in neuromodulation. We explore the spectrum of optical fiber varieties, ranging from traditional to biocompatible and biodegradable optical fibers, as well as multifunctional and multimaterial designs. Fabrication methods are discussed, encompassing techniques such as thermal drawing, extrusion, extrusion-based 3D printing, and casting. The review further highlights the vast application landscape of optical fibers, encompassing light transmission and stimulation, fluid delivery, and electrical signal transmission. We delve into the unique properties and functionalities of these optical fibers, highlighting their respective strengths and limitations. By synthesizing current research and outlining key advancements, optical fibers in optogenetics are positioned to make significant contributions to the ongoing efforts in neuroscience to develop more effective and precise treatment modalities for neurological disorders.

Cascaded vernier effect optic fiber temperature sensor with DSHF-Based MZI and FPI

This paper introduces a optical fiber temperature sensor based on vernier effect. The sensor's design consists of two integrated interferometric structures: the Mach-Zehnder interferometer (MZI) and the Fabry-Perot interferometer (FPI). Both interferometric structures exhibit symmetry, but they employ different types of optical fibers. Specifically, the MZI employs dual side hole fiber (DSHF), while the FPI is constructed using hollow-core optical fibers.Given the MZI's superior sensitivity to temperature changes compared to the FPI, it is chosen as the sensing arm. The paper conducts theoretical research and experimental validation of the optical fiber sensor's sensing principles. The findings reveal a sensor sensitivity of −305.42 pm/°C when measuring temperatures within the 32–50 °C range. Stability testing, repeatability testing, and response time testing are carried out using a constant temperature chamber and a water bath, affirming the sensor's excellent repeatability and stability, along with a response time faster than that of conventional thermometers.

Wearable Optical Fiber Sensors for Biomechanical Measurement in Medical Rehabilitation: A Review

With the increasing demand for rehabilitation of the elderly population and patients with limb injuries, it is imperative to develop new wearable health monitoring system and assistive rehabilitation equipment. Optical fiber sensing has the advantages of light-weight, anti-electromagnetic interference, higher strain and elastic limit, safety and dependableness. It has been significantly utilized in engineering prospecting, building structure monitoring, and medical and health fields. At present, the application of optical fiber sensing in medical rehabilitation has been developed rapidly, however, the discussion and summary of its key theories and technologies are insufficient. This review details different types of optical fiber sensing mechanisms and devices, as well as the measurement of physical parameters using optical fiber sensors and application examples in the field of medical rehabilitation. This review systematically analyzes the advance of the sensitivity, flexibility, accuracy, and wearability of the optical fiber sensing in the monitoring of human body sign information. This review also discusses the challenges of making a new generation of optical fiber sensors that fit human joints, muscles, and rehabilitation equipment. Finally, this review provides a research direction for the realization of flexible, comfortable, safe, stable monitoring of human biomechanical parameters and real-time precise control of rehabilitation equipment in the future.

Improvement in fusion performance between G652.D fiber and Ultra-low-loss fiber by controlling discharge current of fusion splicer

Due to factors such as external environment, splicing tools and differences in the fiber material itself, there are still many problems with the fusion performance of different kinds of optical fibers hybrid splicing. Up to now, the theoretical studies and experimental results reported on the loss and strength of fusion joint under harsh conditions have been too few and incomplete. Therefore, we have investigated the key issues to be solved in fiber fusion process, and firstly analyzed thermal characteristics in melt region, comprising fiber dopants diffusion effect and fiber core expansion, and then discussed the effects of fusion current and fusion time on fusion temperature, splice loss and fusion strength. Besides, a calibration method to improve discharge current of fusion splicer is proposed and splice loss model and fusion strength model for different types of optical fibers are established. Furthermore, fiber fusion-splicing experiments have been conducted under adverse circumstances, obtaining a large amount of experimental data. The simulation results are basically consistent with the theoretical analysis. Ultimately, the method has been directly applied to the field fiber fusion engineering with good results.

Temperature Dependence of Radiation-Induced Attenuation of a Fluorine-Doped Single-Mode Optical Fiber at Infrared Wavelengths

Harsh environments can combine radiations and extreme temperature constraints, which can both degrade the optical performances of silica-based optical fibers. Among the different types of optical fibers, the ones having a core in pure-silica or doped with Fluorine are known to present, generally, the lowest steady state radiation-induced attenuation (RIA) to high cumulated doses (> 10 kGy) at room temperature. In this work, we investigate how the RIA levels and kinetics of a radiation hardened F-doped single-mode optical fiber depend on the irradiation temperature. To achieve this, we performed a systematic study on the combined temperature (from -80°C to 80°C) and steady state X-ray radiation (up to 100 kGy) effects on a F-doped single-mode fiber with a high temperature acrylate coating in the infrared domain. We then discuss the basic mechanisms at the origin of the RIA and its temperature dependence.

Polymer tapered pillar on a fiber end fabricated by UV irradiation using a high-NA fiber

The increasing need for single-mode fibers (SMFs) and advances in silicon photonics (SiPh) devices have led to the need for an efficient method of optical coupling between them. To achieve a higher coupling between them, a polymer tapered pillar was fabricated on the end face of the SMF by applying the optical diffraction effect and a self-written waveguide technology using a high numerical aperture (HiNA) fiber. The initial 10.4 µm spot size was reduced to 4.17 µm at 1.55 µm wavelength, and the greatest coupling efficiency of –1.01 dB was reached between an SMF with a tapered pillar and uncured resin cladding and a HiNA fiber. Full Text: PDF ReferencesR. Marchetti, C. Lacava, L. Carroll, K. Gradkowski, P. Minzioni, "Coupling strategies for silicon photonics integrated chips [Invited]", Photonics Res. 7, 201 (2019). CrossRef R. Bachelot, C. Ecoffet, D. Deloeil, P. Royer, D.-J. Lougnot, "Integration of micrometer-sized polymer elements at the end of optical fibers by free-radical photopolymerization", Appl. Opt. 40, 5860 (2001). CrossRef P. Pura, M. Szymanski, M. Dudek, L.R. Jaroszewicz, P. Marc, M. Kujawinska, "Polymer Microtips at Different Types of Optical Fibers as Functional Elements for Sensing Applications", J. Lightwave Techn. 33, 2398 (2015). CrossRef O. Mikami, R. Sato, S. Suzuki, C. Fujikawa, "Polymer Microlens on Pillar Grown From Single-Mode Fiber Core for Silicon Photonics", IEEE Photonics. Technol. Lett. 32, 399 (2020). CrossRef Y. Kamiura, T. Kurisawa, C. Fujikawa, O. Mikami, "High optical coupling efficiency of polymer microlens and pillar on single mode fiber for silicon photonics", Jpn. J. Appl. Phys. 61, SK1009 (2022). CrossRef F. Tan, H. Terasawa, O. Sugihara, A. Kawasaki, T. Yamashita, D. Inoue, M. Kagami, C. Andraud, "Two-Photon Absorption Light-Induced Self-Written Waveguide for Single-Mode Optical Interconnection", J. Lightwave Tech. 36, 2478 (2018). CrossRef H. Terasawa, O. Sugihara, "Near-Infrared Self-Written Optical Waveguides for Fiber-to-Chip Self-Coupling", J. Lightwave Technol. 39, 7472 (2021). CrossRef K. Vanmol, K. Saurav, V. Panapakkam, H. Thienpont, N. Vermeulen, J. Watté, J. Van Erps, "Mode-field Matching Down-Tapers on Single-Mode Optical Fibers for Edge Coupling Towards Generic Photonic Integrated Circuit Platforms", J. Lightwave Tech. 38, 4834 (2020). CrossRef Y. Saito, K. Shikama, T. Tsuchizawa, H. Nishi, A. Aratake, N. Sato, "Tapered Self-Written Waveguide between Silicon Photonics Chip and Standard Single-Mode Fiber", Opt. Fiber Communication Conference (OFC2020), paper W1A.2, (2020). CrossRef Y. Saito, K. Shikama, T. Tsuchizawa, N. Sato, "Tapered self-written waveguide for a silicon photonic chip I/O", Opt. Lett. 47(12), 2971 (2022). CrossRef N.A. Baharudin, C. Fujikawa, O. Mitomi, A. Suzuki, S. Taguchi, O. Mikami, S. Ambran, "Tapered Spot Size Converter by Mask-Transfer Self-Written Technology for Optical Interconnection", Photon. Technol. Lett. 29, 949 (2017). CrossRef H. Nawata, K. Ohmori, Proc. International Conference on Electronics Packaging (ICEP), paper 23 (2014) DirectLink S. J. Frisken, "Light-induced optical waveguide uptapers", Opt. Lett. 18, 1035 (1993). CrossRef Y. Obata, Y. Oyama, H. Ozawa, T. Ito, O. Mikami, T. Uchida, "Multi-array Self-written Waveguides using Photo-mask for Optical Surface Mount Technology", International Conference on Electronics Packaging (ICEP), paper 225 (2005). DirectLink P. Yin, J.R. Serafini, Z. Su, R. Shiue, E. Timurdogan, M.L. Fanto, S. Preble, "Low connector-to-connector loss through silicon photonic chips using ultra-low loss splicing of SMF-28 to high numerical aperture fibers", Opt. Expr. 27, 24188 (2019). CrossRef https://coherentinc.force.com/Coherent/UHNA3?cclcl=en_US, (18/09/2022). DirectLink

Investigation of an Information Leakage Channel in the Area of Optical Fiber Bending

An information leakage channel created on the basis of fiber bending when transmitting information over an optical fiber is studied. An increase in the bending diameter of an optical fiber is shown to lead to a decrease in the power of optical radiation proceeding from it. In a bending diameter range from 5 to 20 mm, the highest radiation power was observed for the optical fiber G.655, while the lowest corresponding figure was recorded for G.657. The information leakage channel bandwidth with changed bending diameter is determined for different types of optical fiber. The bandwidth of the information leakage channel is shown to depend on the location of the photodetector used to register optical radiation taken from the bend of the optical fiber.

Morphological changes of veins and perivenous tissues during endovenous laser coagulation using 2-μm laser radiation and various types of optical fibers

ObjectiveTo determine the morphological changes in veins and perivenous tissues after endovenous laser coagulation (EVLC) using laser radiation with a wavelength of 1910 nm and different types of fibers (bare tip and radial). MethodsThe EVLC procedure was carried out on 22 surface veins of six sheep. The radiation source was a diode-pumped solid-state laser, which was based on a LiYF4:Tm crystal and had an emission wavelength of 1910 nm and a maximum output power of 10 W. Two types of optical fibers were used: (1) bare tip and (2) radial or radial with two rings. Histological and morphometric methods were used, and the statistical digital data were analyzed. ResultsThe use of a linear endovenous energy density of 20 J/cm and optical bare fibers for veins with diameters of 3-4 mm resulted in a slit-shaped or wide venous wall perforation. A thermal effect was observed on the perivenous connective tissue (PVCT), which caused damage to its structures. Wide perforations were accompanied by complete destruction of the PVCT in the projection of the formed defect. The distance between the remaining vein wall fragment, located opposite to the perforation, and injured small vessels was 257.7 ± 23.6 μm. The radius of thermal damage increased to 2073.5 ± 8.0 μm near the vessel perforation. Using optical radial fibers for veins with diameters of 3.9 ± 0.5 mm did not lead to perforations. The destructive effect of the laser on small vessels of the PVCT extended to a distance of 425.7 ± 22.0 μm. ConclusionsAnalysis of thermal vessel damage in perivenous tissue after EVLC with bare-tip fiber shows that in the projection of a wide perforation, the damaged vessels of the PVCT are located at a large distance from the coagulated vein wall. On the opposite side of the perforation, the distance from the coagulated vein wall to the damaged vessels of the PVCT is significantly reduced because of the minimal output of laser radiation energy through the poorly damaged part of the wall. Using an optical radial fiber facilitates the application of a uniform distribution of thermal energy to the vein wall and damage to all its layers; at the same time, it minimizes the thermal energy that extends beyond the vein wall and damages the surrounding tissue. Clinical RelevanceThe use of radiation with a wavelength of 1910 nm will make it possible to carry out endovenous laser coagulation of varicose veins at lower power values compared with radiation in the micron and one and a half micron regions of the spectrum. Understanding of morphological changes of veins and perivenous tissues after endovenous laser coagulation with 1910-nm laser radiation and different types of optical fibers (bare-tip, radial, radial 2ring) help predict possible complications and reduce their rate.

Research on Smart Mattress Based on Fiber Unbalanced Sagnac Loop

Because the contact vital sign detection system is monitored through skin contact, it affects the user’s comfort and causes inconvenience. And most of the non-contact equipment is not high precision, or the cost is too high. To solve the above problems, this article is based on the physiological activity of slight vibration in multimode optical fiber micro bending deformation which result in the change of light intensity principle, considering the structure of the optical fiber Sagnac loop sensor has simple structure, low cost, but the signal to noise ratio is poor and is not sensitive to low frequency signal, this paper designed a based on intelligent mattress of non-equilibrium sagnac loop structure. The mattress can obtain the heart impact map signal in real time by changing the small light intensity, and obtain the heart rate, respiration rate and other information through the algorithm processing the signal. Mattress in the design of the sensing optical fiber module will be connected to two different types of optical fiber of Unbalanced Sagnac loop structure, solve the balance Sagnac loop structure of optical fiber access signal noise is bigger, poor sensitivity were weakness, go through the serpentine line way of embedded mattress, and using the mattress cover material for optical fiber micro bending cycle critical mechanical adjustment cycle, the mattress is of high sensibility. Cardiac and respiratory signals obtained in the laboratory were compared with those obtained from the balanced Sagnac loop, and through the fast Fourier transform (FFT) in frequency domain signal is analyzed, the vital signs of signal more clearly this mattress stable and less noise, change the tester for many times after tests, get the signal of high accuracy, the error is very small. The experiment shows that the mattress can realize the non-sensory vital signs signal detection, convenient for users to monitor their physical conditions in real time in daily work and life, and can timely notify the family members and medical staff in emergency, to avoid the occurrence of accidents.

ЧИСЛЕННО-АНАЛИТИЧЕСКИЙ МЕТОД СИНТЕЗА ОПТИМАЛЬНЫХ НЕРАВНОМЕРНЫХ ЧАСТОТНЫХ ПЛАНОВ DWDM

The possibilities of increasing the throughput of fiber-optic transmission systems by using an uneven frequency grid are investigated. In this case, the bandwidth of each channel is selected so that the transmission rate is the same for all channels. In this work, both linear and some nonlinear effects are taken into account, leading to the distortion of the optical pulse. Simulation of nonlinear effects is based on a model in the form of a generalized nonlinear Schrödinger equation. The developed program provides modeling of linear and nonlinear distortions for the DWDM range (from 1460 to 1625 nm). The characteristics of different types of optical fiber are also provided. Non-linear effects are investigated for NZ DSF-type dispersion-shifted fiber. Differential equations are solved by the method of splitting according to physical factors. It is shown that for this type of fiber at distances of 100 km and more, a soliton transmission mode appears. In this case, the frequency band of the soliton regime can reach significant values (up to 5 THz) at typical lengths of the regeneration sections of the order of 100-300 km. A method for calculating the bandwidth of uneven frequency plans is proposed. This method has been tested for a 15 THz band. A specific example of calculations is given for the comparison base in the form of a uniform frequency plan with a single channel bandwidth of 50 GHz. It is shown that optimal non-uniform frequency plans can significantly increase the throughput of DWDM systems: in the given example, approximately 3 times. At the same time, the complexity of the equipment increases slightly.

Best selected optical fibers with wavelength multiplexing techniques for minimum bit error rates

Abstract In this study, six different types of optical fibers (SMF, HNLF, HNL-DFF, NZDSF, LEAF (step core) and LEAF (ring core)) can be used for different number of channels (16, 32, 64, and 80) at different bit rates to calculate the quality factor for these types of optical fibers at the optimum length for max BER × 10−9 and min BER × 10−12. By using the software Optisystem version 13, the work has been demonstrated and compared with previous study to find the best one under the same operating conditions.

Fiber optic sensing of concrete cracking and rebar deformation using several types of cable

Distributed fiber optic sensing (DFOS) is increasingly being applied in civil engineering. One recently emerging type of DFOS, Optical Frequency Domain Reflectometry (OFDR), offers improved accuracy and resolution. This paper presents an experimental study that investigates the ability of OFDR, implemented using several types of fiber optic cable, to detect concrete cracking and large strain steel deformation. More specifically, the experimental investigation involved six reinforced concrete specimens with different types of optical fiber embedded in both the concrete and rebar. OFDR successfully detected continuous rebar deformation and bond stress through the post yielding range, as well as strain localization indicative of concrete cracking. The type of optical fiber is shown to have a substantial influence on the measured strain response after the onset of nonlinear behavior (i.e., cracking and yielding) and into the large deformation regime where measurement survivability becomes an issue. The higher performing fibers reveal unprecedented detail regarding the interaction between concrete and rebar and demonstrate potential for application in RC structures. Based on these findings, the paper gives practical recommendations for cables with different size and structure.

Research and Comparison of Brillouin Characteristics of Single-Mode Fiber, Few-Mode Fiber and Multi-Mode Fiber

This article systematically studies and compares the Brillouin characteristics of single-mode fiber (SMF), few-mode fiber (FMF) and multi-mode fiber (MMF). The Brillouin frequency shift (BFS), Brillouin linewidth, gain peak, and Brillouin gain spectrum of different types of optical fibers in the case of single mode and mode superposition are investigated. The results show that in single mode case, the BFS, Brillouin linewidth, and gain peak of the FMF and MMF decrease as the mode order increases. Considering the mode superposition case, the SMF has the largest BFS, followed by the MMF, and the smallest is FMF. The MMF has the largest Brillouin linewidth, followed by the FMF, and the smallest is SMF. The SMF has the largest gain peak, followed by FMF, and the smallest for MMF. This work provides an important reference value for the further study of Brillouin distributed fiber optical sensors.

Characterization of supercontinuum process pumped by amplified dissipative solitons

We report an experimental study of the supercontinuum (SC) generated by molecules of solitons (MS) and noise-like pulses (NLP) and their dependence on pump power in two different types of optical fibers: 500 m of standard fiber (SMF-28, Corning) and 100 m of high-nonlinearity fiber with a zero-dispersion slope (Furukawa HNLF-ZS). Our results show a more complex dependence of the SC spectra on pump power for the MS than for the NLP used for pump. We attribute such differences to soliton fission of MS whereas this process is already done within the NLP. We use a nonlinear optical loop mirror (NOLM) connected to the output of the fiber under test to evaluate the amplitude of ultrashort pulses, which compose SC and found that amplitude is changed strongly throughout the spectrum. The NOLM suppresses pulses with low peak power, this is especially pronounced for wavelengths longer than ∼1750 nm for both fibers, and in the region between 1450 nm–1640 nm for the high-nonlinearity fiber.

A tunable Mach–Zehnder interferometer based on dual micro-cavity photonic crystal fiber for load measurement

Optical fiber technologies are widely engaged in variety of industries, machines and production lines due to their precise results and low cost. Optical sensors are fabricated using different types of optical fibers. In this paper, a new fabrication approach of, in-fiber, tunable Mach–Zehnder interferometer with dual micro-cavities using a photonic crystal fiber (PCF) has been proposed for load measurement application. A large mode area (LMA-10) PCF is used to splice between two equal lengths of single-mode fibers using fusion splicing technique. Different parameters such as arc power, length of the PCF and the overlap gap between samples have been considered to control the fabrication process. Ellipsoidal shape micro-cavities were experimentally achieved parallel to the propagation axis having dimensions of (24.92–62.32) µm of width and (3.82–18.2) µm of length. Results showed that higher sensitivity values of 0.15 nm/N and 0.32 nm/N were achieved with elliptical width of 18.2 µm. The simplicity of sensor fabrication process, controlled parameters of cavity creation, small and compact size and high sensitivity of large mode area that the PCFs exhibit, add more advantage for load measurement applications.

6-MeV Electron Exposure Effects on OFDR-Based Distributed Fiber-Based Sensors

The impact of exposing an optical fiber to 6-MeV electrons on the performances of optical frequency domain reflectometry (OFDR) distributed sensors is investigated. Six different types of optical fibers with different core compositions and coatings have been tested: four fibers are metal coated (copper, gold, or aluminum) for high-temperature >300 °C) operations while the two others have telecom-grade acrylate coatings for operation below 80 °C. The fiber Rayleigh signature used to perform the OFDR sensing remains almost unaffected after an electron exposure. Indeed, the measured radiation-induced temperature errors are lower than about 3 °C, close to the setup uncertainties, when the OFDR operates as a temperature sensor.

Application of distributed fiber optic sensing technique in land subsidence monitoring

Distributed fiber optic sensing (DFOS), a newly developed structure health monitoring technique, has been proved to be a very suitable and useful technique for the monitoring and the early warning of structural engineering. Its application in geotechnical engineering, especially land subsidence resulted by groundwater withdraw, was limited by the complex characteristics of geotechnical materials in the field. In this paper, Brillouin optical time domain reflectometer (BOTDR) and fiber Bragg grating (FBG) techniques were used to monitor the deformation of soil layers and pore water pressure for nearly 2 years in a 200-m borehole with four different types of optical fibers planted directly in the borehole. The result demonstrated that cables with better sheath protection and higher tension strength are more suitable for field monitoring. The main compaction occurs at two thick aquitards which are adjacent to the pumping confined aquifer. The deformation of soil layers shows the conformance with the change of groundwater level over time and the deformation of sand layer had instantaneity while that of clay layer had hysteresis. Rebound amount caused by the rise of ground water level was small, and the re-compression rate significantly decreases after rebounding. This paper demonstrates that the DFOS technique is a very advanced monitoring method for the investigation on the mechanism of land subsidence and the evaluation of soil compression deformation potential.

1 × N Fiber Optic Coupler Based on a Polyhedral Gradient-Index Lens

A $1 \times N$ fiber coupler based on a single-polyhedral gradient-index lens (p-GRIN) is proposed. The entrance end of the p-GRIN has polyhedral convex cone shape with N surfaces and the exit end has a flat surface. When light from a single-core optical fiber illuminates the input end of the p-GRIN, N spatially separated light beams will be focalized at the output end and coupled, respectively, into N different fibers cores. With ray-tracing analysis, the coupling parameters design model was established. Calculation and simulation with ZEMAX showed that the output beam positions can be optimized by adjusting the input cone angle and the distance between the input fiber end and the cone tip. The light beam power distribution is simulated and mode field evolution is investigated. The results indicate that high coupling efficiency could be achieved with optimized parameters design. This method may be adapted for different types of optical fibers and provide a practical solution for light coupling between single- and multicore fibers.