We present a study into the gas pressure and composition within a hollow core optical fiber immediately after fabrication. Results from three different experimental techniques indicate that the ...initial absolute pressure inside of the hollow core is significantly lower than atmospheric pressure. By measuring the equilibrium height to which water ingresses into the hollow core, we estimate the absolute internal gas pressure to be 20 kPa < <inline-formula><tex-math notation="LaTeX">{{\boldsymbol{P}}_{\boldsymbol{i}}}</tex-math></inline-formula> < 29 kPa for the fibers reported here that were fabricated using standard techniques. The initial gas composition within the hollow core was studied using Raman and absorption spectroscopy and the evolution of the gas composition provides indirect information about the condition of the silica surfaces inside the fiber. The measurements indicate that these internal surfaces become saturated with atmospheric water vapor as this is drawn into the open-ended fiber, initially due to a pressure gradient post-fabrication. Our findings are an essential foundation for the study of long-term optical and mechanical performance of hollow core fibers and important for accurate characterization of these specialty fibers. The first is becoming increasingly important as commercial applications of these fibers expand.
An integrated and sensitive optical fiber vernier sensing structure is designed and demonstrated. By horizontally splicing a thin hollow fiber (THF) and a thin core fiber (TCF) between two multimode ...fibers (MMFs), two integrated parallel Mach-Zehnder interferometers (MZIs) are formed. By adjusting the lengths of the THF and the TCF reasonably, the vernier effect can be formed in air and water, respectively. As a validation, sensor probes that form vernier effect in air and water are used for bending and seawater salinity measurements, respectively. Experiments show that in the range of 0.085 m -1 to 0.19718 m -1 , the sensor exhibits a high bending sensitivity of 127.15 nm/ m -1 and a low-temperature cross-talk of 37.5 pm/℃. In the salinity range of 0‰ -9‰, the sensor exhibits a high salinity sensitivity of -17.80 nm/‰, and has good measurement repeatability and stability. The sensing structure not only behaves the merits of small size, simple preparation, and good robustness, but also behaves the advantage of ultra-high sensitivity, which demonstrates its great application potential in high-precision detection of weak signals.
A modified near field (MNF) method is proposed for measuring the refractive index profile (RIP) and geometrical parameters of multi-core fibers. The near field method is based on the near field ...intensity at the end face of optical fibers that are illuminated by a Lambertian source to get the RIP, but the methods proposed before require a leaky-mode correction factor or additional geometrical parameters to correct the measurement results. The MNF method presented in this paper offers a simplified approach, when the power of the source is constant, the distance between the source and the incident end face of optical fibers is given, and the near field intensity distribution is linear to the RIP of the de-coated optical fibers. By using SMF-28e+ as a sample to obtain the linear coefficient, the proposed method is validated by measuring the RIP of SMF-28R with an accuracy reaching 10 -4 . Finally, the RIP of 4-core fiber and 7-core fiber are measured by the MNF method accurately and effectively. Also, the geometrical parameters of optical fibers can be extracted through edge extraction and ellipse curve fitting for the distribution of near field intensity.
We assess the potential applicability and performance of multicore fibers (MCFs) against conventional single-core fibers (SCFs) in submarine transmission systems. In particular, we assume 4-core MCFs ...with nominally uncoupled cores and separate amplification of each MCF core with conventional erbium doped fiber amplifiers (EDFAs) using fan-in/fan-out (FI/FO) devices in the repeaters. We examine the effects of the number of physical fibers accommodated in the cable design, MCF and FI/FO crosstalk, FI/FO loss, span length, transceiver implementation penalty, and minimum required signal-to-noise ratio (SNR). The relative cable capacities offered by the MCF and SCF systems are evaluated under the different conditions. Then the relative cost/capacity for systems built with the two types of fiber are calculated using a system cost model for two conditions: 1) with maximum cable capacity and 2) for minimum overall cost/capacity. The potential role of multicore fibers (MCFs) in submarine cable systems is investigated in the context of electrical power limitations and physical fiber count limitations. Cable capacities are estimated based on ideal Shannon-limited capacity unless a transceiver implementation penalty, or gap-to-Shannon capacity, is applied. The systems are modeled assuming constant output power amplifiers as are commonly deployed in submarine systems. We find that MCFs offer the greatest cable capacity increase for low fiber count cables, but that the cost/capacity can be significantly higher. When minimization of cost/capacity is the design goal, MCF systems remain higher in this metric than SCF systems, while enabling only a small capacity increase at best.
Newly developed 125-μm-cladding coupled four-core fibers realized the record-low spatial mode dispersion (SMD) of 3.14 ± 0.17 ps/√km over C-band and the ultra-low attenuation of 0.158 dB/km at 1550 ...nm, both of which are the lowest ever reported among the optical fibers for the space-division multiplexed transmission. The SMD was observed to be proportional to the fiber bend curvature, i.e., inversely proportional to the bend radius, and the SMD of 3.14 ± 0.17 ps/√km was measured at the bending radius of ~31 cm. By assuming the 3.14-ps/√km SMD accumulation, the tap count of the multiple-input multiple-output digital signal processing for the crosstalk compensation is estimated to be only 63 taps for covering 99.99% power of the impulse response after 10 000-km propagation when the 25-GBaud (50-GHz sampling) is assumed. The present results demonstrate the strong applicability of the coupled multicore fiber to the ultralong-haul submarine transmission system.
Silica-based optical fibers with an ultra-high Yb concentration were systematically studied. Three the most commonly used in industry glass matrixes for active fiber core were investigated: ...aluminosilicate, phosphosilicate and aluminophosphosilicate. For all the glass hosts optical fibers doped with a record high concentration of Yb in a glass core were fabricated utilizing an all-gas-phase deposition based on MCVD technology. The factors limiting increase of Yb content in glasses and fibers were revealed. For the first time it was shown that highly Yb-doped fibers could nearly completely lose their active properties and the most probable reason for that is concentration quenching of luminescence.
The orbital-angular-momentum (OAM) modes in optical fibers have polarization mode dispersion (PMD) properties similar to those of single-mode fibers (SMFs). The +l and -l order OAM modes supported by ...the same fiber vector modes undergo random cross coupling and exhibit a frequency-dependent time delay. We name this effect "OAM-PMD" and extend the formalism developed for PMD in SMFs to describe OAM-PMD. The characteristics of the modal beat lengths, birefringence correlation lengths, and the mean value of OAM-PMD are investigated. A fixed-analyzer technique is proposed and demonstrated to characterize this phenomenon in OAM fibers. Two different types of OAM fiber are examined. The measured results are compared with the theoretical calculations.
With the increasing use of internet and mobile phones, the capacity of the backbone optical communication link has been continually growing across the world and especially in India. The total optical ...fiber cable deployed for the BharatNet initiative of Government of India is expected to increase from 3.4 million km to 5 million km in 2024-25 just for providing last-mile connectivity. Considering this deep proliferation, this article attempts to capture the diverse research carried out in India in the domain of optical communication.
With the growing demand for service access and data transmission, security issues in optical fiber systems have become increasingly important and the subject of increased research. Physical layer ...secret key generation (PL-SKG), which leverages the random but common channel properties at legitimate parties, has been shown to be a secure, low-cost, and easily deployed technique as opposed to computational-based cryptography, quantum, and chaos key methods that rely on precise equipment. However, the eavesdropper (Eve) potential for current PL-SKG in fiber communications has been overlooked by most studies to date. Unlike wireless communications, where the randomness comes from the spatial multi-paths that cannot be all captured by Eves, in fiber communications, all the randomness (from transmitted random pilots or channel randomness) is contained in the signals transmitted inside the fiber. This, therefore, enables a tapping Eve to reconstruct the common features of legitimate users from its received signals, and further decrypt the featured-based secret keys. To implement this idea, we designed two Eve schemes against polarization mode distortion (PMD) based PL-SKG and the two-way cross multiplication based PL-SKG. The simulation results show that our proposed Eves can successfully reconstruct the legitimate common feature and the secret key relied upon, leading to secret key rate (SKR) reductions of between three and four orders of magnitude in the PL-SKG schemes studied. As a result, we reveal and demonstrate a novel eavesdropping potential to provide challenges for current physical layer secret key designs. We hope to provide more insightful vision and critical evaluation on the design of new physical layer secret key schemes in optical fiber links, to provide more comprehensively secure, and intelligent optical networks.
Refractive index profile (RIP) determines the property and effectiveness of optical fibers. Accurate measurement of the RIP is essential for fiber fabrication, test and calibration. At present, ...computer tomography (CT) method is widely used in RIP measurement. However, in the measurement of the optical fiber with a complex RIP, the CT method is much time-consuming due to its dependence on small rotation angle step usually with 2° (performing 90 steps of rotation for angle range from 0° to 180°). In this paper, the total variation (TV) method is proposed to fast reconstruct the complex RIP with both two-dimension and three-dimension in transverse as well as longitude direction. The RIP of a seven-core trench-assisted optical fiber and a twisted polarization maintaining optical fiber are measured experimentally as examples. Experimental results demonstrate that the proposed TV method has higher accuracy of 4 × 10 −4 and three times faster than that of traditional CT method.
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