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Orbital angular momentum (OAM), inherent to light beams characterized by helical phase fronts, represents an alternative spatial degree of freedom for photons. In recent years, OAM-based communication has gained significant attention as a potential solution to increase the capacity and spectral efficiency of optical fiber communication systems. In optical fibers, OAM modes can be generated and manipulated by utilizing specialized optical components and techniques. One approach involves using specially designed optical fibers, such as ring–core fibers, which have the capability to support multiple OAM modes. In the manufacturing process, the ring fiber is hardly made perfectly, especially the doped-cores. The introduced defect breaks the symmetry of the ring–core, which has a serious impact on OAM mode transmission. Using a material with a slightly higher refractive index than the cladding to wrap the ring–core is effective in reducing impairment of defects. After numerical simulation, the proposed method improves the walk-off length by a factor of 2–40. [ABSTRACT FROM AUTHOR]

Mode division multiplexing technology can regard each eigenmode in optical fibers as an independent channel to carry different data information, helpful for improving the transmission capacity of optical fiber communication systems. The spatial distribution, state of polarization and propagation constant of guided optical eigenmodes in the magneto-optical fiber are derived theoretically by the analysis of linear polarization and exact modes. The transverse distribution of the eigenmodes in the MO fiber can be constructed by those of exact modes in the conventional fiber. The effect of applied magnetic field on the eigenmodes and their effective refractive indices is studied. Simulation results show that, the eigenmodes in the MO fiber may be represented by the pseudo-modes of circular polarization ( CP ℓ n R / L ) or the true modes of orbital angular momentum ( OAM ± ℓ , n R / L ) when applied magnetic field is sufficiently large. The measurement method for weak magnetic field based on Stokes parameters is proposed. The magnetic tunability of OAM mode selective couplers is also analyzed. [ABSTRACT FROM AUTHOR]

Photonic lantern is a key device in space division multiplexing (SDM) system. The key challenge of a photonic lantern is mode scalability, which requires the taper length to increase nonlinearly as the mode number scales up. The traditional photonic lantern fabrication method requires stacking the input fibers into the hollow, low-index outer cladding before tapering. It implicitly sets geometric constraints on the input fibers' core positioning. We propose a photonic lantern design with drilling preform and reduced cladding fibers to lift these constraints and make photonic lanterns more adiabatic. By analyzing the effects of loosening the constraints on the adiabatic requirement of a three-mode photonic lantern, we find further progress could be made to alleviate this adiabatic requirement. The optimal structure for our design is proposed and demonstrated through the beam propagation method (BPM). Our findings could help further improve the mode scalability of photonic lanterns. [ABSTRACT FROM AUTHOR]

Extended L-band erbium-doped fiber amplifiers (EDFAs) have attracted much attention in recent years despite their relatively low gain levels. In this paper, a dual-stage extended L-band EDFA with improved gain level is demonstrated by using an Er/Yb/P co-doped fiber-based double-pass structure assisted by a low noise pre-amplifier. High gain levels of up to 48.79 dB at 1566 nm and 20.05 dB at 1621.4 nm are achieved with saturated output power at 1605 nm of 20.58 dBm under a total pump power of only 400 mW. Bandwidths with the gain of more than 20 and 30 dB are reached up to 66 nm (1555.4–1621.4 nm) and 58.4 nm (1557.5–1615.9 nm), respectively. The noise figure benefited by using the low noise pre-amplifier is 5.40 ± 1.55 dB in the 1565–1610 nm range. The wide gain bandwidth, high gain level and relatively low pump power give it great potential for future high-capacity optical fiber communication systems. [ABSTRACT FROM AUTHOR]

The increasing demand for transmission capacity in fiber-optic communications makes multimode fibers (MMFs) attractive by enabling simultaneous multi-channel data transmission. However, inherent mode crosstalk among transmission channels limits its applicability. In this Letter, we propose to overcome this problem via wavefront shaping. By actively modulating the incident light with a special wavefront, not only the transmitted power for a specific channel can be enhanced but also crosstalk among different transmission channels can be mitigated. As a proof of concept, we experimentally demonstrated wavefront shaping assisted dual-channel optical communications through a single MMF. 10 Gbaud four-level pulse amplitude modulation signals were sent through both channels simultaneously, enabling a total bit rate of 40 Gbps. Both channels were analyzed with bit error rates below the hard-decision forward error correction limit, confirming the validity of the proposed scheme. [ABSTRACT FROM AUTHOR]

In this paper, we experimentally demonstrate a net bit rate of 261.7 Gbit/s in a seven-core transmission system with a Kramers–Kronig (KK) receiver. The 10 GBaud 16-level quadrature amplitude modulation (QAM) signal is transmitted over a 2.5 km seven-core fiber, and the relationship between carrier-to-signal power ratio, signal power, frequency spacing, and optical power is analyzed. Moreover, a multiple-input multiple-output (MIMO) Volterra equalization algorithm with crosstalk-dependent steps is proposed to compensate for inter-core crosstalk and impairments induced by other devices. Compared to the single-input single-output (SISO) Volterra equalizer, the CSPR can be reduced by 1.3 dB, and the received power gain can reach up to 0.7 dB. [ABSTRACT FROM AUTHOR]

We built the cross‐spectral density matrix elements analytical expression of random electromagnetic Gaussian vortex (REGV) beam propagating in gradient‐index (GRIN) fiber, which is used to study the polarization properties of the beam. The distribution of spectral degree of polarization (SDOP) and state of polarization (SOP) show the same periodicity and symmetry. The distribution law of SDOP and SOP is unchanging with the topological charge m changing and is changed by adjusting the spatial self‐correlation length σxx. The research results have guiding significance for the propagation of random electromagnetic vortex beams in optical fibers, which could be considerable value in high‐speed and large‐capacity optical fiber secure communication. [ABSTRACT FROM AUTHOR]

This paper presents a transition taper for coupling light between optical fibers with different geometries and refractive index profiles used in Power over Fiber (PoF) systems. Global energy efficiency and costs are critical parameters when delivering high power to remote areas. High-power lasers have maximum coupling for large core fibers, while widespread multimode optical (OM1) fibers used in optical communications are cheaper. We study the optical losses between large core fibers (200 µm) and OM1 fibers (62.5 µm) theoretically and experimentally. We demonstrate that improvements of 2 dB can be obtained by adding the new tapered structure to the system, compared to the direct splice between both fibers. There is good agreement between measured and calculated loss values using a new Gaussian loss model to describe splices between tapered and straight fibers. The fabrication of the transition taper is also described. We also measure the numerical aperture (NA) changes in the downtaper zone and demonstrate that the lower the NA of the input light, the higher the efficiency improvement. [ABSTRACT FROM AUTHOR]

Few-mode fibers have been used in contemporary communication with mode multiplexing and space-division-multiplexing techniques to enhance the capacity crunch in recent communication links. The design parameters of the proposed fiber are predicted through machine learning-based inverse design approach, using regression model. The proposed few-mode fibre profile parameters are predicted with an accuracy of 99.95% to guide five to ten modes with weak coupling among the guided modes. In second phase of this work, the authors used a finite difference method-based solver to obtain the modal characteristics of the proposed fibre with predicted parameters for six guided modes, namely, LP01, LP11, LP21, LP31, LP41, and LP51. The numerical simulation results show that the predicted profile parameter maximizes effective mode-area and minimizes the inter-channel crosstalk for mode division multiplexing transmission over C-band. Besides this, the proposed ring-core few-mode fiber also exhibits nearly zero-dispersion for LP01 mode at 1550 nm along with low dispersion for other higher-order modes. Finally, an intensity-modulation and direct-detection mode multiplexed transmission link without erbium-doped fiber amplifier is established with six-spatial channels over 50 km and an attenuation of 0.18 dB/km to achieve minimum bit-error-rate of 4.45×10-9. [ABSTRACT FROM AUTHOR]

A single-polarization solid-core anti-resonant fiber is proposed, and the influence of the fiber core material anisotropy of the solid-core anti-resonant fiber on polarization characteristics is investigated using the finite element method. Single-polarization guidance is achieved by using the anisotropy of optical fiber materials, which also ensures high birefringence. The numerical simulation results indicate that there are two single-polarization intervals (1210–1440 nm and 1490–1560 nm), with a maximum bandwidth of up to 230 nm, when the confinement loss difference between the two orthogonal polarizations exceeds two orders of magnitude. Specifically, when the work wavelength is 1550 nm, a polarization extinction ratio (PER) of 108 is obtained by optimizing the structure parameters. Additionally, the y-polarization fundamental mode (YPFM) can be well confined in the fiber center with a low confinement loss of 0.04 dB/m, while the x-polarization fundamental mode (XPFM) has a huge confinement loss larger than 4.65 dB/m due to the coupling with the tube mode. The proposed single-polarization solid-core anti-resonant fiber has a huge potential in applications such as laser systems, fiber-optic gyroscopes, and optical fiber communications. [ABSTRACT FROM AUTHOR]

Copyright of Study on Optical Communications / Guangtongxin Yanjiu is the property of Study on Optical Communications Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

This paper extends previous studies work on the application of Machine Learning (ML) to distortion compensation in optical communication systems with optical fibers nonlinear coefficients (γ) that exceed current industry standards e.g. γ > 1.4 W - 1 k m - 1 . To quantify the improvement afforded by ML methods under different transmission conditions the procedures are applied to a model of a typical single-frequency optical communication system with, a 3200 km fiber length, double polarization, and a 16-QAM modulation format. The performance of both transmitters and receivers that incorporate Neural Networks (NNs) are then examined over a wide range of γ values. In all cases considered, the system Q-factor is improved although the degree of enhancement is dependent on the signal to noise ratio. The ML structures that were investigated include Siamese neural networks (SNN) implemented at the receiver end as well as two-stage architectures that employ NNs at the transmitter together with a classifier at the receiver side. Classifiers ranging from simple decision tree structures to boosting, forests, extra trees, and Multi-layer perceptron (MLP) were further examined and found to provide significant enhancement for γ > 4 W - 1 k m - 1 . The optimal performance for highly nonlinear systems was achieved for two-stage systems with random forest or extra tree classifiers. Finally, empirical equations for each ML technique were derived that relate the Q-factor enhancement and the value of γ , and the number of triplet terms that are input into the neural network. These results could potentially be employed to relax manufacturing constraints and accordingly reduce system costs. [ABSTRACT FROM AUTHOR]

Mode-division multiplexing (MDM) using few-mode fibers (FMFs) appears as a promising technology to increase fiber capacity by a few orders of magnitude and sustain the traffic demand for the decades to come. The potential of MDM lies in its ability to exploit multiple modes within a single optical fiber strand. Since 2011 and the first promising demonstrations, impressive progress has been made. In this article, we will show how the most recent advances in design and fabrication have improved the performance of FMFs and MDM systems, and allowed to turn research demonstrations into practical deployments. [ABSTRACT FROM AUTHOR]

Randomly-coupled multi-core fiber (MCF) technology has come to attract lots of attention because of its strong applicability to long-haul transmission systems. Compared with weakly-coupled MCFs with independent cores, it can simultaneously realize higher spatial channel density and ultralow transmission loss using existing ultralow-loss single-mode fiber (SMF) core designs. The strong mode coupling characteristics of randomly-coupled MCFs can provide favorable optical properties, such as suppressed accumulation of modal dispersion (MD), mode-dependent loss (MDL), and nonlinear impairments. This article gives an overview of randomly-coupled MCF technology advancements. First, we describe the classification and design of randomly-coupled MCFs and explain what the randomly-coupled MCFs are and how they are designed. State-of-the-art randomly-coupled MCFs can accommodate four, seven, or 12 cores in a standard 125- $\mu \text{m}$ cladding while achieving ultralow transmission loss and/or small MD, which are very promising for long-haul transmission media. Next, we present the methods to characterize the optical properties of randomly-coupled MCFs and the difference compared to conventional SMF measurements. We also show the low-loss low-MDL connectivity of this type of MCF and the cabling that can suppress MD. A field-deployed randomly-coupled MCF cable testbed is also presented, which confirmed the favorable optical properties of randomly-coupled MCFs after deployment. Then, multi-core amplifier technologies are briefly summarized, and finally, we discuss the performance improvements in the transmissions over randomly-coupled MCFs and suitable application areas. [ABSTRACT FROM AUTHOR]

Mode-division multiplexing (MDM) can achieve ultra-high data capacity in optical fiber communication. Several impressive works on multicore fiber (MCF), multi-mode fiber, and few-mode multicore fiber have made significant achievements in MDM. However, none of the previous works can simultaneously maintain the transmission loss, chromatic dispersion (CD), and differential group delay (DGD) at a relatively low level. A nested multiple hollow-core anti-resonant fiber (NMH-ARF) has significant potential for applications in MDM. This study proposes a novel NMH-ARF with its structural design based on the traditional single-core nested anti-resonant fiber. We increased the number of nodes between capillaries. By changing the position of the nested tubes, several interconnected areas form when a single core is separated. We investigated the mode-coupling theory and transmission characteristics of this fiber. This fiber structure showed a low sensitivity to bending and achieved a super-low DGD and a super-low confinement loss (CL) at a wavelength of 1.55 µm while keeping CD relatively low. [ABSTRACT FROM AUTHOR]

For enhancing channel capacity and spectrum efficiency, vortex beam carrying orbital angular momentum (OAM) has been ardently investigated in optical communication. However, the crosstalk among OAM modes induced by mode coupling has seriously hindered the development of vortex beam communication (OBC). A novel concept of mitigating crosstalk within an OAM-mode group for a short-haul optical fiber communication (OFC) is proposed by reversing the transmission matrix. To support more OAM modes propagation, a step-index OAM fiber with ring-shaped profile is also designed and fabricated, which can support 6 vector/OAM modes propagation. A proof-of-concept experiment is performed for verifying the feasibility of the proposed concept, where a quadrature-phase-shift-keying (QPSK) signal is utilized as an excitation signal. Bit-error-ratio (BER) and constellation diagram (CD) are employed to evaluate the performance of the proposed system. The captured intensity profiles and interference diagrams demonstrate that the propagated four OAM beams within the 2rd OAM-mode group are successfully received. The measured BER and CD illustrate that the crosstalk within the mode group can be significantly mitigated. Further, although the data speed or fiber length is increased, the system performance using the proposed concept is still superior to the case without increasing fiber length or data speed when the proposed concept is missed. Therefore, the proposed concept is effective for mitigating the crosstalk and can be then used for a short-haul OAM-based OFC. [ABSTRACT FROM AUTHOR]

• A fabrication method of a multi-core fiber coupler without a central core is proposed, and the coupling efficiency of the coupler reaches 96.41%. • The thermally diffused coupler fabricated by thermal diffusion is analyzed by simulation and experiment. • The coupling efficiency of the hydrogen-loaded fiber is 4.9 times higher than that of the original fiber in the same heating tim. An efficient method for fabricating multi-core fiber couplers based on the thermal diffusion technique is proposed to realize the connection of single-mode fibers to multi-core fibers without a central core. The fabrication efficiency of the multi-core fiber coupler is improved by fiber-loaded hydrogen. The simulation results show that the single-mode fiber and the multi-core fiber can be coupled stably and efficiently by using the designed double-clad fiber to connect the single-mode fiber and the multi-core fiber. Double-clad fiber and multi-core fiber were loaded with hydrogen to enhance the dopant diffusion rate. Three-dimensional refractive index measurements of the multi-core fiber coupler show that the dopant diffusion rate and the diffusion zone length of the hydrogen-loaded fiber are significantly increased compared to the original fiber. These enhancements contributed to the shorter fabrication time of multi-core fiber couplers. The multi-core fiber couplers fabricated by thermal diffusion technology have the advantages of high coupling efficiency, high integration, simple manufacture, and high mechanical strength. Enhancing the diffusion rate of optical fibers by using the hydrogen loading method will increase the application potential of multi-core optical fiber couplers prepared by thermal diffusion technology, and accelerate the translation of multi-core fibers toward optical communication and optical fiber sensing. [ABSTRACT FROM AUTHOR]

As the fundamental building block of optical fiber communication technology, thermally drawn optical fibers have fueled the development and prosperity of modern information society. However, the conventional step‐index configured silica optical fibers have scarcely altered since their invention. In recent years, thermally drawn multifunctional fibers have emerged as a new yet promising route to enable unprecedented development in information technology. By adopting the well‐developed preform‐to‐fiber manufacturing technique, a broad range of functional materials can be seamlessly integrated into a single fiber on a kilometer length scale to deliver sophisticated functions. Functions such as photodetection, imaging, acoustoelectric detection, chemical sensing, tactile sensing, biological probing, energy harvesting and storage, data storage, program operation, and information processing on fiber devices. In addition to the original light‐guiding function, these flexible fibers can be woven into fabrics to achieve large‐scale personal health monitoring and interpersonal communication. Thermally drawn multifunctional fibers have opened up a new stage for the next generation of information technology. This review article summarizes an overview of the basic concepts, fabrication processes, and developments of multifunctional fibers. It also highlights the significant progress and future development in information applications. [ABSTRACT FROM AUTHOR]

Besides, the transmittivity/reflectivity of grating spectrum is reached to approximation of 97.5 % at central Bragg wavelength value of 1.55 m. In the other side, the reflection level is reached to -59 dB, average group delay ripple is reached to 0.63 and output pulse intensity approximation is reached to 94.8 % at approximation zero delay time. In OptiGrating simulation, the grating shape is the sine index modulation profile within one grating period is given by: HT ht Graph (8) Constant grating period with no chirp is estimated by the following mathematical formula: HT ht Graph (9) In order to estimate the average group delay ripple in this study, which is given by the following formula based on Ref [[12]]: HT ht Graph (10) where I A SB apod sb i and I A SB unapod sb i are the apodized and unapodized peak to peak group delay ripple amplitude respectively, and I i is the sharpness parameter. [Extracted from the article]

We investigated offline the evolution of radiation-induced attenuation (RIA) in polymer perfluorinated cyclic transparent optical polymer (CYTOP) optical fiber irradiated with different gamma radiation doses of 1–50 kGy. We considered the wavelength range varying from 800 up to 1750 nm. The RIA spectra measured at different times after irradiation have qualitatively similar shapes. However, quantitatively the RIA evolution significantly depends on the spectral range: in the short-wavelength part, the transmission partly recovers, while in the infrared, at wavelengths above 1300 nm the RIA grows and then saturates becoming permanent. The effects of temperature and photobleaching on RIA dynamics are examined. The ability of the fiber transmission recovery is investigated in the 600–1750-nm range with the use of climatic chamber. [ABSTRACT FROM AUTHOR]

Mode-division multiplexing based on few-mode fibers is expected to overcome capacity limits of current single-mode fiber communications. Mode permutation as an approach for extending transmission reach has attracted great attention. In this paper, we propose a mirror-flipped mode permutation (MFMP) scheme based on newly developed fibers with symmetric differential mode delay (DMD). We demonstrate a 28-GBaud transmission over a 1542-km 6-mode fiber link, which suppresses modal-dispersion impact by 36% and mitigates mode-dependent-loss effect with 3.1-dB Q-factor improvement, compared with the traditional cyclic mode permutation at 1233 km. Then, we experimentally verify that the proposed MFMP works well even using a fiber with a not exactly symmetric group delay distribution. Benefiting from a low DMD in this fiber, we demonstrate a 6-mode PM-QPSK transmission at 25 GBaud with 16-λ channels, extending the transmission distance to 3000 km. • We propose a new type of 6-mode fibers with MFMP, enabling greatly reduced DMD and MDL in transmission. • Benefiting from the scheme, we demonstrate a 6-mode PM-QPSK transmission at 25 GBaud with 16-wavelength channels over 3000 km. • Considering fiber's group delay distribution would build a link between fiber properties and system configuration. • We believe that this work highlights the strong potential of high-mode-count FMFs for long-haul MDM transmission systems. [ABSTRACT FROM AUTHOR]

In this paper, a pseudo-elliptical-ring-core polarization-maintaining few-mode fiber structure assisted by concentric circular stress-applying region is proposed. Numerical simulation results show that the structure can stably support 14 transmission modes. By introducing two elliptical stress regions with low refractive index inside the core, and a circular stress region with concentric core between the fiber core and cladding, the effective refractive index difference between higher-order modes is significantly improved. In the C + L band, the minimum refractive index difference between adjacent modes is not less than 2 × 10−4. At 1550 nm, the effective refractive index difference between the two fundamental modes reaches 4.8 × 10−4. The mode dispersions are less than |-33| ps/nm/km, and the maximum bending loss is in the order of 10−7 dB/m (bending radius ≥2.5 cm). This work may pave the way for the development of short-distance large-capacity optical fibers. • The min Δ n eff between adjacent modes is not limited by the separation degree of the two fundamental modes. • The Δ n eff between two fundamental modes reaches 4.8 × 10−4 at 1550 nm. • Concentric circular stress region equalizes the separation degree of the lower and higher order degenerate modes. • In the whole C + L band, the Δ n eff between each guided mode is not less than 2 × 10−4. [ABSTRACT FROM AUTHOR]

As the data-rate of the optical fiber communication system evolves to 400 Gb/s or even 1 Tb/s, higher requirements are put forward for transmission rate and repeater distance. To meet the requirements, further decreasing the optical fiber attenuation coefficient is one of the effective methods. In this paper, we did research on the relationship between the distribution of elements and frozen-in stress in the core layer and the cladding. Subsequently, their effect on optical fiber attenuation is also investigated. The results show that the co-doping of germanium, fluorine, and chlorine in the core layer can improve the stress matching between the core layer and the optical cladding. In addition, based on our research, the outer cladding with a high softening point can restrain the stress mismatch between the core layer and the optical cladding. Furthermore, the thickness of the second cladding will also affect the stress mismatch. Eventually, ultra-low loss fibers with the optimal attenuation of 0.156 dB/km at 1550 nm were prepared successfully by analyzing the stress distribution inside the optical fiber and adjusting the doping amount of germanium, fluorine, chlorine and other elements in the core layer, optical cladding, and outer cladding to maintain that the core layer and the optical cladding share the same stress. • Researched the effect of frozen-in stress on optical fiber attenuation and proposed a method to get lower attenuation. • Propose a scheme of Ge,F and Cl CO-dropped,suitable for mass production of ultra-low loss fiber core rods by VAD. • Reducing the stress difference between the fiber core layer and the optical cladding can reduce fiber attenuation. • Providing fiber support for the evolution of a single channel to a higher rate of optical fiber communication system. [ABSTRACT FROM AUTHOR]

Multiplexing multiple orbital angular momentum (OAM) modes of light has proven to be an effective way to increase data capacity in fiber-optic communications. However, existing techniques for distributing the OAM modes rely on specially designed fibers or couplers. Direct transmission of multiplexed OAM modes through a long standard multimode fiber remains challenging because the strong mode coupling in fibers disables OAM demultiplexing. Here, we propose a deep-learning-based approach to recover the scattered data from multiplexed OAM channels without measuring any phase information. Over a 1-km-long standard multimode fiber, our method is able to identify different OAM modes with an accuracy of more than 99.9% in the parallel demultiplexing of 24 scattered OAM channels. To demonstrate the transmission quality, color images are encoded in multiplexed twisted light and our method achieves decoding the transmitted data with an error rate of 0.13%. Our work shows that the artificial intelligence algorithm could benefit the use of OAM multiplexing in commercial fiber networks and high-performance optical communication in turbulent environments. [ABSTRACT FROM AUTHOR]

Copyright of Informacije MIDEM: Journal of Microelectronics, Electronic Components & Materials is the property of MIDEM Society and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Further attenuation of Rayleigh scattering of silica glass is required to extend the capacity of long‐distance optical fiber communication. To theoretically examine the effect of aluminum and fluorine co‐doping on the density fluctuations of silica glass, which is related to Rayleigh scattering, a set of force‐matching potentials (FMP) for simulating F‐doped aluminosilicates was developed using Bayesian optimization based on density functional theory calculations. Molecular dynamics (MD) simulations with the new FMP could evaluate the densities of silica glasses to which a small amount of fluorine was doped and those of aluminosilicate glasses with a wide range of aluminum contents within reasonable accuracy. The FMP successfully represents the changing role of aluminum from a network former without a compensating cation (threefold coordination) to that with a compensating cation and a charge compensator in the aluminosilicates. Indeed, relative concentrations of four, five, and sixfold‐coordinated aluminum observed by NMR measurements were reproduced better than the original Teter potential at a high aluminum content. At an aluminum content lower than 1 mol%, threefold‐coordinated aluminum was observed, which is consistent with ESR measurements. After careful validations of the FMP, the effect of the co‐doping of alumina and fluorine on the density fluctuations of silica glass was computationally examined. Consequently, it was expected that the co‐doping might not sufficiently attenuate the Rayleigh scattering, even though 1 wt% fluorine would be able to reduce the density fluctuations of aluminosilicate glasses for some extent. This is because more alumina‐doping increases density fluctuations of silica glass if the drawing temperature and procedure are the same with those for silica glass fiber. Thereby, a possible fabrication process to sufficiently attenuate the Rayleigh scattering of the F‐doped aluminosilicate glass was proposed, according to the density fluctuation analysis at high temperature. [ABSTRACT FROM AUTHOR]

Orbital angular momentum (OAM) modes, having unique properties of a helical phase structure and doughnut intensity profile, have been widely studied in fiber-optic communications, in terms of OAM modulation and OAM multiplexing. In general, different types of specialty fibers with a ring-shape structure are preferred for more stable OAM transmission, which, however, may face greater manufacturing challenge and larger fiber loss compared to standard multi-mode fibers (MMFs). Therefore, the widely deployed and commercially available standard MMFs that can support hundreds of OAM modes have recently attracted great attention. In this paper, we review recent research progress in OAM communications based on standard MMFs. First, the basic concept of OAM and different types of specially designed OAM fibers are briefly introduced. Then, the OAM mode properties in MMFs and recent works, including OAM mode modulation, multiple-input multiple-output (MIMO)-free OAM mode group multiplexing, small-scale partial MIMO assisted OAM mode multiplexing, and OAM-based heterogeneous fiber-optic networks, are presented. The OAM communications using other widely deployed standard single-mode fibers are also briefly introduced as supplementary. Finally, key challenges and perspectives of OAM communications based on standard MMF are discussed and summarized. [ABSTRACT FROM AUTHOR]

• A system has been developed to analyze mode evolution in fiber fusion splicing. • New method to characterize mode variation based on 3D refractive index profile. • The mode evolution process can be obtained by the proposed method. Fiber fusion splicing tremendously affects the modal stability, insertion loss and transmission quality of the optical fiber communication systems. Especially, in the few-mode fibers, a tiny dissatisfactory fiber splicing operation may lead to a big modal perturbation or unexpected mode coupling. In this paper, we propose a 3D refractive index profile visualization method to demonstrate the mode activation and evolution in fiber fusion splicing. A refractive index profile measuring instrument is established and applied to detect the 3D refractive index profile deformation caused by the fiber splicing. Then, based on the measured values of 3D refractive index profile, the mode activation is analyzed for fiber splicing between a single-mode fiber and a two-mode fiber with different core-offset distance. Results show that the mode begins to tremble with a core-offset of 4 μm and the high-order modes are effectively activated at the core-offset of 7 μm. The basic theoretical model and experiment results provide a profound support in almost all kinds of fiber splicing operation, mode interference construction and mode coupling modulation. Additionally, the method can be expanded to the research on fiber cleaving and bending. [ABSTRACT FROM AUTHOR]

Mode exchange (ME) can effectively improve the utilization of network resources in short-distance mode division multiplexing (MDM) communication systems. In this paper, a mechanical long-period fiber grating (LPFG) mode converter is used to realize the channel exchange of LP 01 - and LP 11 -mode dual-polarization (DP) signals, with the channel crosstalk of −14 dB or less. We experimentally tested the receiver sensitivities of two DP channels, which are dependent on channel crosstalk and polarization-dependent loss (PDL), and demonstrated the practicability of the real-time data transmission and exchange for 100 Gb/s dual-polarization quadrature phase shift keying (DP-QPSK) in multiple-input multiple-output free (MIMO-free) MDM system. The experimental results show that the ME system can provide the optical power margin of about 25 dB, capable of effectively supporting the metropolitan transmission and exchange between the data centers. • The exchange between LP01 and LP11 modes is achieved through a mechanical long-period fiber grating (LPFG) mode converter, each of which carries 100 Gb/s dual-polarization quadrature phase shift keying (DP-QPSK) signal. • The modal crosstalk of two DP channels is less than -14 dB, and the mode conversion efficiencies of the LPFG are 61.1% and 84.7%, respectively. • The real-time data transmission and exchange for 100 Gb/s DP-QPSK signals in multiple-input multiple-output free (MIMO-free) mode division multiplexing (MDM) system is demonstrated, with the optical power margin of about 25 dB. [ABSTRACT FROM AUTHOR]

This study introduces a novel 9-core, 4-mode fiber characterized by its bend resistance and minimal crosstalk. Utilizing a hetero-core scheme, the cores are effectively isolated through trenches and air holes, significantly reducing inter-core crosstalk. The cores are strategically arranged in a square array to facilitate easier splicing. Through comprehensive simulation calculations, it is demonstrated that four LP modes can be consistently transmitted within the C+L band. The effective refractive index difference between adjacent modes exceeds 10−3, ensuring negligible intra-core crosstalk. Concurrently, the inter-core crosstalk of the LP02 mode is maintained below − 40 dB/100 km. The fiber's bending radius can be minimized to 60 mm without compromising performance, with the corresponding bending loss for the LP02 mode in the two outer cores measured at 1.6 × 10−5 dB/m and 3.95 × 10−6 dB/m, respectively. Each of the nine cores exhibits an effective mode area exceeding 95 µm2, and the fiber's relative core multiplexing factor at 1550 nm is 27.62. The proposed fiber's superior performance positions it as a promising solution for practical applications in optical fiber communication networks, potentially addressing the burgeoning capacity crisis in communication systems. [ABSTRACT FROM AUTHOR]

In this paper, the gain engineering of a semiconductor quantum dots fiber amplifier (SQDFA) is studied. The optical amplification is realized by exciting the PbS quantum dots coated around the tapered single-mode fiber coupler with an evanescent wave. The gain spectra and saturation properties of the SQDFA are obtained via solving the rate and coupled power equations based on the two-level model. Then, the effect of the signal wavelength, pump power and amplifier length on the saturation characteristics is numerically simulated. The obtained results show that the proposed model is in a good agreement with the previous experimental results. Unique characteristics of the SQDFA such as the broadband optical gain and smooth bandwidth are of great interest for dense wavelength-division multiplexing systems. Therefore, the proposed model show promising applications in the future fiber-optic communications. [ABSTRACT FROM AUTHOR]

We propose a practical scheme to demonstrate the combination and subsequent self-similar compression of two pulses with the same or different central wavelengths while propagating through a nonlinear fiber with exponentially decreasing dispersion. To initiate these processes, two raised cosine pulses with the same or different wavelengths is modulated using a phase modulator to acquire the same chirp at the input of the fiber. While propagating through the nonlinear fiber, these chirped pulses first coalesce into a single pulse and during further propagation get compressed into a single ultrashort high-power pulse. The output pulse possesses a large compression factor, high proportion of energy and peak power compared to a single input pulse. We also report the combination and compression of five raised cosine pulses with different wavelengths to achieve an appreciable compression effect, indicating that this system works well even with a small number of input pulses. The proposed scheme provides a simple way to generate high power ultrashort pulse with high energy and good quality in a short length of fiber. [ABSTRACT FROM AUTHOR]

The fabrication of long period fiber gratings (LPFGs) based on thin-cladding fiber (TCF) has been demonstrated by adopting electric-arc discharge (EAD) technique. In order to analyze the sensing characteristics of arc-induced TCF LPFGs, the refractive index (RI), temperature and strain sensing characteristics were also experimentally investigated, respectively. Compared with conventional LPFGs, the excellent performance of the LPFGs based on TCF is that it has a higher RI sensitivity for the same order cladding mode due to the stronger evanescent field of thinner cladding. The proposed LPFGs based on this simple fabrication method have a narrow full width at half maximum (FWHM), which provides the possibility for using as filters or sensors in the applications of optical communications or fiber-optic sensing. [ABSTRACT FROM AUTHOR]

The main purpose of this study was to investigate how leadership skills and stakeholder management acting together to influence the execution of fibre optic infrastructure. The study adopted the pragmatism research paradigm, with a cross-sectional survey design. Census was used to select 187 respondents from a target population of 187 functional members of staff in fibre optic infrastructure departments of two mobile telecommunication and four internet service companies in Nairobi County, Kenya. A self-administered structured questionnaire was used to collect quantitative data while an interview guide and document review guide were used to collect qualitative data. Inferential statistical analysis was performed using multiple regression. It was demonstrated that leadership skills and stakeholder management act together to have a significant positive influence on the execution of fibre optic infrastructure. Therefore, there is a need for companies to ensure that stakeholders are involved in all phases of a project from inception to closure. Mobile telecommunication and internet service providing companies should also develop training programs to improve the leadership skills of project leaders and make use of conflict management strategies and communication skills to ensure appropriate management of change. It was suggested that similar and comparable studies should be conducted in other countries across the world. [ABSTRACT FROM AUTHOR]

In this paper we propose a novel type of highly-birefringent and ultra-wideband low-loss photonic crystal fiber (PCF) incorporating elliptical holes in the cladding and rhombic holes located along the transversal axes. The PCF is simulated and analyzed with finite-difference time-domain method. The properties of PCFs were investigated for various distribution, shape and size of air holes. For the optimized PCF design the birefringence reaches 8 × 10−3 at 1550 nm and the confinement loss is as low as 5 × 1 0 − 4 dB/km with single mode operation in an ultra-broad transmission band (500 nm–1800 nm). Furthermore, the single mode and single polarization transmission could be achieved in this PCF at longer wavelengths beyond 1600 nm. This kind of PCF has potentials in optical long haul communication and optical fiber sensing. [ABSTRACT FROM AUTHOR]

The transmission capacity of optical fiber communication can be effectively improved by the combination of dual-polarization (DP) high-order modulation format with mode division multiplexing (MDM) technology. We build up an experimental system for amplification and transmission of DP-MDM signals with orthogonal mode combination (OMC) and identical mode combination (IMC). The MDM transceiver units are composed of mode-selective photonic lanterns (MSPLs) and few-mode polarization controllers (FMPCs). The few-mode erbium-doped fiber amplifier (FM-EDFA) used here is constructed by homemade isolation and wavelength division multiplexers (IWDMs). The amplification and transmission experiments for 100 Gb/s dual-polarization quadrature phase shift keying (DP-QPSK) MDM signals are carried out by the commercial optical transport network (OTN) platform. An effective method of optimally receiving DP-MDM signals are presented by appropriately controlling the FMPC in the MDM receiver unit. The experimental results show that, (1) OMC DP-MDM signals have a better performance than IMC cases in differential modal gain (DMG), which provides a new means to reduce the DMG; and (2) under the optimization conditions, OMC and IMC DP-MDM systems possess a close receiver sensitivity, but the former has a larger optical power margin than the latter by 2.2 dB. • To the best of our knowledge, the combination scheme of dual-polarization (DP) with spatial modes has not been investigated deeply until now. • Firstly, a simple and feasible experimental device for generating two types of mode combinations including orthogonal mode combination (OMC) and identical mode combination (IMC) are proposed and verified. • Next, the modal gain and differential modal gain (DMG) of DP-MDM signals under the two mode combinations are compared. It is shown that OMC DP-MDM signals can offer a new approach to modal gain equalization. • Finally, an effective method of optimally receiving DP-MDM signals is presented to improve the transmission performance. [ABSTRACT FROM AUTHOR]

We propose and demonstrate a symmetric step-apodized distributed feedback (DFB) fiber laser, which has an improved efficiency and a narrow linewidth. This special DFB laser cavity is based on a unique symmetric step-apodized π-phase-shifted fiber Bragg grating (FBG) directly inscribed in a heavily erbium-doped fiber. The symmetric step-apodized phase-shifted grating is equivalent to a weak uniform phase-shifted FBG embedded in a pair of strong FBGs, and this design could increase the effective cavity length of a DFB fiber laser. Moreover, the symmetric step-apodized DFB laser cavity proposed in this work has a phase shift located in the cavity center. It could simplify the design process significantly due to the symmetry in the laser cavity. Experimental results show that the slope efficiency of a symmetric step-apodized DFB fiber laser could be increased to 1.06%, which is significantly higher than that of a uniform DFB fiber laser (i.e., 0.45%) under the same cavity length and pump conditions. In addition, the proposed DFB fiber laser exhibits a high stability. The fluctuations in lasing wavelength and output power were less than 12 pm (corresponding to a frequency shift of 1.49 GHz) and 0.13 dB within 24 h, respectively. Moreover, the full-width at half-maximum linewidth of the symmetric step-apodized DFB fiber laser was ∼2.6 kHz, measured by the delayed self-heterodyne method with a 50-km fiber delayed line. As such, the proposed symmetric step-apodized DFB fiber laser could potentially be used as high-performance light source for fiber-optic sensors or coherent optical communication systems. [ABSTRACT FROM AUTHOR]

A microwave phase-control scheme is proposed and experimentally demonstrated. Two lasers are combined in an optical fiber coupler to generate a beat signal. The beat frequency is tuned by controlling the frequency of one laser. Using the phase shift of the beat waves with different frequencies during the propagation in an optical fiber, the phase of the radio-frequency (RF) signal generated by a photodetector (PD) can be controlled. Using the phase shift during the propagation of beat waves in an optical fiber with different beat frequencies, the phase of the RF signal generated by a PD connected to the fiber can be controlled. A tunable phase shift ranging from 0 deg to 1400 deg is obtained for frequencies from 6 to 10 GHz. This scheme offers the advantages of fast tuning and precise phase control of an RF signal. [ABSTRACT FROM AUTHOR]

The fabrication and characterization of IR chalcogenide polarization-maintaining (PM) step-index optical fibers with elliptical-core and 1-in-line-core have been reported for the first time. An improved isolated co-extrusion method was used to fabricate these core-shaped PM fibers. The elliptical core had a horizontal radius of a = 3.66 μm, vertical radius of b = 1.83 μm and the 1-in-line core of a = 4.83 μm, b = 1.42 μm, respectively. Single-mode PM beam spots were observed for the elliptical-core and 1-in-line-core fibers in the near-field energy distributions. The highest values of birefringence of the elliptical-core and 1-in-line-core fibers are 2.09 × 10 −4 at 2.7 μm and 3.272 × 10 −4 at 2.8 μm, respectively. The extinction ratios of −3.7 dB and −2 dB were achieved in fibers of 0.5 m long with elliptical-core and 1-in-line-core, respectively. [ABSTRACT FROM AUTHOR]

We investigated numerically the possibility of all normal dispersion fiber design for near-infrared supercontinuum generation based on a standard air-silica microstructure. The design procedure includes finding of target dispersion profile and subsequent finding of appropriate geometrical fiber design by inverse dispersion engineering. It was shown that the tailoring of dispersion profile could increase the spectral width of generated supercontinuum while maintaining perfect spectral flatness. Conditions necessary for wide and flat supercontinuum generation as well as restrictions imposed by chosen materials were discussed. As a result of design and optimization procedure, an air-silica design was found providing normal dispersion up to \text3\;\mu \textm. Simulation results with 10 nJ, 100 fs pulses demonstrate supercontinuum generation up to 1.3 octave; whereas pumping with 30 nJ, 100 fs pulses could provide 1.8 octavse supercontinuum. [ABSTRACT FROM PUBLISHER]

Anti-resonant fibers featured with multiple cladding tubes are promising light guiding media with a broadband low-loss transmission window in the optical spectrum. Herein, we numerically demonstrate anti-resonant fibers with ultra-low confinement loss and good single mode property, which is automatically designed by swarm intelligence. The basic idea of the particle swarm optimization algorithm is to find the optimal solution by the cooperation and information sharing among particles in the group. It is demonstrated that the particle swarm optimization algorithm can be utilized to design such anti-resonant fibers with multiple geometry parameters. By breaking the mirror symmetry of the fiber's cross section, the coupling between the fundamental mode and cladding mode is further suppressed than the mirror symmetry one. As a result, ultra-low confinement loss as small as 2.2 × 10−8 dB/m can be achieved in simulation. Our results might facilitate the advanced applications of nested anti-resonant fibers in optical communication and high energy pulsed laser, etc. • Ultralow loss anti-resonant fiber designed by swarm intelligence is demonstrated. • The confinement loss is 10-8 dB/m and the higher order mode extinction ratio is ∼ 500. • The symmetry breaking effect can substantially suppress the modes coupling. [ABSTRACT FROM AUTHOR]

The intensity modulator is a key component in optical fiber communication. In this paper, an ultra-short graphene light intensity modulator based on hybrid plasmonic waveguide (HPW) array is proposed. Al 2 O 3 embedded with double-layer graphene at a certain interval is placed on the side polishing surface of the microfiber Mach–Zehnder modulator (MZM), and the silver-semicircular array is placed on the surface of the transition material as hybrid plasmonic waveguide for the first time in all-fiber modulator. Analyzing by building a 3D model in finite difference time domain software, the hybrid surface plasmon polariton (HSPP) between silver-semicircles significantly enhances the subwavelength confinement and the light–graphene interaction. The modulation range of the real part of effective refractive index (N eff) can reach to 0.01984 by adjusting the Fermi level of graphene. Therefore, π phase change can be achieved only in a modulation length of 45 μ m , and the insert loss can be controlled below 0.0159 dB/ μ m through the analysis of the imaginary part of the N eff , which also leads to a low total loss of 0.7155 dB. Owing to the excellent phase modulation efficiency, high extinction ratios of 39.39 dB and 47.77 dB can be reached for port 1 and port 2, respectively. Double-layer graphene forms the flat capacitor, which brings high modulation efficiency while achieving a high modulation bandwidth of 59.945 GHz. The application of HPW in graphene optical fiber modulator greatly improves the working efficiency, reduces the difficulty of integration, overcomes high insert loss, improves the extinction ratio, this modulator can be widely used in the future optical fiber communication system. • An ultra-short bilayer graphene twin-core modulator based on hybrid plasmonic waveguide (HPW) array is proposed to achieve light intensity modulation by adjusting the phase of the light transmitted in fiber core. • The effect of the geometrical parameters and shape of HPW on the intensity of the excited hybrid surface plasmon polariton and on the performance of the modulator has been investigated. • The proposed modulator can achieve an intensity modulation range of over 99% at both ports, with extinction ratios of 39.39 dB and 47.77 dB at port 1 and port 2 respectively. • The bandwidth of proposed modulator is 59.945 GHz while the insert loss is lower than 0.0159 dB/ μ m, and the energy consumption is only 29.43 pJ/bit. [ABSTRACT FROM AUTHOR]

Light-based technologies are strongly supported by various glass materials in general. This article focuses on glasses containing rare-earth elements, doped fibers, as well as their photonic functions such as optical amplifications in fiber telecommunication. Scientific progress and applications of rare-earth spectroscopy and technologies of doped glasses and fibers in recent 60 years have been more than dramatic. Development of various laser diodes has been strongly linked with realization of novel devices of rare-earth-doped glasses. In addition to many milestones listed in the resolution of International Year of Light, we should notice that 2015 is also the 50th anniversary of the erbium-doped glass laser, which is later transformed into the optical fiber amplifier, the enabler of today's information society. [ABSTRACT FROM AUTHOR]

• A frequency-domain method is developed to measure the DMD of multimode fibers. • Vector network analyzer is used to measure the complex transfer functions. • The time-domain output pulses can be calculated from CTF with proper procedures. • Frequency-domain method yields DMD results equivalent to the time domain method. • The frequency domain method can conduct DMD measurements at very low optical power. We report a frequency-domain method for measuring the differential mode delay (DMD) and bandwidth of multimode fibers (MMFs). Using a frequency domain instrument, vector network analyzer (VNA), the method measures the complex transfer functions (CTFs) of multimode fibers for a given set of launch conditions. As a result of the fiber length, which is long relative to sampling points used in VNA, the frequency sampling is under-sampled and the measured CTF is aliased. Aliased CTF behaves very differently from fully sampled CTF with dependence on the sampling frequency step. With proper procedures, the CTF can be transformed to local time frame centered around DMD into a modified CTF. The time-domain output pulses can be further recovered through an inverse Fourier transform. By recovering output pulses using complex transfer functions of a multimode fiber at controlled launch offsets, one can subsequently follow the same procedures of existing time domain methods to determine the effective modal bandwidth of the multimode fiber under test (FUT). As a result of the method, DMD and modal bandwidth measurements can be undertaken using frequency domain instruments rather than time domain instruments in a mathematically equivalent manner for multimode fibers. The measurement method utilizes CW light sources that are readily available in a wide range of wavelengths, thus providing greater flexibility for measuring the modal bandwidth of MMFs at different wavelengths, including WDM applications other than 850 nm. The very high sensitivity of the VNA allows the measurement at much lower optical power compared to the time domain instrument. Several measurements at 850 nm have been conducted using the frequency domain method and validated by comparison with the results from the time domain method. The method is suitable for measuring MMFs from a short length of a few hundred meters to a very long length close to 20 km. We have also measured an MMF at 910 nm using the method to demonstrate its flexibility in adopting different wavelengths. [ABSTRACT FROM AUTHOR]

• This paper explores the design of a 1 × 4 linear array multicore fiber with 125-μm cladding for O-band transmission. • Fiber design for a balanced performance between inter-core crosstalk and the tunneling loss of outer cores near the cladding edge was identified. • We demonstrated error-free 400G LR4 and 100G LWDM transmission over 6.3 km of an MCF with a total data rate of 1 Tb/s. This paper explores in detail the design of a 1 × 4 linear array multicore fiber with 125-μm cladding for O-band transmission. Through modeling we identified fiber design parameters for a balanced performance between inter-core crosstalk and the tunneling loss of the outer cores near the cladding edge. Guided by the modeling results, we have fabricated a 1 × 4 multicore fiber with improved performance. Using such fiber, we were able to demonstrate error-free 400G LR4 and 100G LWDM transmission over 6.3 km with a total data rate of 1 Tb/s. [ABSTRACT FROM AUTHOR]