Search Results (20)

In molecular communication via diffusion (MCvD) uplinks where multiple nano-sensors report concurrently to a fusion center (FC), the long channel memory and the near–far imbalance jointly create strong multiple access interference (MAI) coupled with residual inter-symbol/inter-chip effects. This paper studies an oversampling-enabled time-domain reception for an uplink molecular code-division multiple-access (MoCDMA) system employing bipolar molecular signalling. By exploiting intra-chip oversampling at the FC, three linear detectors following the principles of maximum ratio combining (MRC), zero-forcing (ZF), and minimum mean-square error (MMSE) are developed and further enhanced through a feedback-assisted interference subtraction (FAIS) scheme that combines single-tap ISI feedback equalization with near-to-far successive MAI subtraction. Owing to the complementary structure of bipolar molecular emissions, the signal-dependent counting noise corresponding to the two molecule types can be jointly modeled in a symmetric and information-independent manner to support unified linear detection and FAIS processing. Numerical results demonstrate that oversampling effectively improves detection reliability, while increasing the molecular emission budget alone is insufficient to mitigate near–far effects. Moreover, FAIS provides significant performance gains, particularly for far NSs. Full article

To support mode-division multiplexing with reduced inter-modal crosstalk, we propose a novel polarization-maintaining few-mode fiber design with a uniform doping profile and no air holes. The fiber employs two placed low-index inclusions to lift modal degeneracy and achieve strong birefringence while maintaining compatibility with standard MCVD and OVD fabrication processes. A genetic algorithm is used to optimize the geometrical and refractive index parameters. Finite element simulations show that the optimized design supports ten guided modes with a minimum effective index difference exceeding $3.8\times {10}^{-4}$ across the C+L band. The fiber exhibits moderate dispersion and strong modal isolation. Tolerance analysis confirms good robustness against index fluctuations and moderate sensitivity to dimensional variations. These features suggest that the proposed PM-FMF is a promising candidate for short-reach spatial-division multiplexing applications where intrinsic polarization and mode separation are desired. Full article

During recent years, the optical-fiber-based simultaneous sensing of strain and temperature has attracted increased interest for different applications, e.g., in medicine, architecture, and aerospace. Specialized fiber layouts further enlarge the field of applications at much lower costs and with easier handling. Today, the performance of many sensors fabricated from conventional fibers suffers from cross-sensitivity (temperature and strain) and relatively high interrogation costs. In contrast, customized fiber architectures would make it possible to circumvent such sensor drawbacks. Here, we report on the development of a high-quality coupled-core fiber and its performance for sensors—from the initial fiber layout via elaboration of the preform and fiber up to the sensor evaluation. A compact, high-speed, and cost-effective interrogation unit using such a specialized coupled-core fiber has been designed to monitor reflectivity changes while even being able to distinguish the direction of the force or impact. Several fiber core material techniques and approaches were investigated, which made it possible to obtain a sufficient volume of material for the required fiber core number and a specialized fiber core geometry in terms of core distances and radial refractive index profile, whilst handling the non-symmetrical fiber architectures of such modeled, complex structures and balancing resources and efforts. Full article

Photodarkening (PD) effect in ytterbium-doped fiber (YDF) has a significant impact on the high-power operational stability of fiber lasers, which seriously hinders the power scaling. In this paper, the relationship between ytterbium ions uniformity and the photodarkening effect in the YDF was investigated, and the fabrication process allowing improving the ytterbium ions uniformity in the core of preforms for suppressing the photodarkening effect was developed. The Modified Chemical Vapor Deposition (MCVD) method combined with Chelate Vapor Deposition (CVD) technology was adopted for multi-layer fiber core deposition, and an all-gas-phase technical process was proposed to improve the ytterbium ions uniformity in the Al/P co-doped glass matrix. The 25/400 μm YDFs obtained by this technology achieved stable 3.5 kW laser output power for 8 h with suppressed PD and nonlinear effects. Full article

Optical sensors constitute attractive alternatives to resistive probes for the sensing and monitoring of temperature (T). In this work, we investigated, in the range from 2 to 300 K, the thermal behavior of Yb²⁺ ion photoluminescence (PL) in glass hosts for cryogenic thermometry. To that end, two kinds of Yb²⁺-doped preforms, with aluminosilicate and aluminophosphosilicate core glasses, were made using the modified chemical vapor deposition (MCVD) technique. The obtained preforms were then elongated, at about 2000 °C, to canes with an Yb²⁺-doped core of about 500 µm. Under UV excitation and independently of the core composition, all samples of preforms and their corresponding canes presented a wide visible emission band attributed to Yb²⁺ ions. Furthermore, PL kinetics measurements, recorded at two emission wavelengths (502 and 582 nm) under 355 nm pulsed excitation, showed an increase, at very low T, followed by a decrease in lifetime until room temperature (RT). A modified two-level model was proposed to interpret such a decay time dependence versus T. Based on the fit of lifetime data with this model, the absolute (S_a) and relative (S_r) sensitivities were determined for each sample. For both the preform and its corresponding cane, the aluminophosphosilicate glass composition featured the highest performances in the cryogenic domain, with values exceeding 28.3 µsK⁻¹ and 94.4% K⁻¹ at 30 K for S_a and S_r, respectively. The aluminophosphosilicate preform also exhibited the wider T operating range of 10–300 K. Our results show that Yb²⁺-doped silicate glasses are promising sensing materials for optical thermometry applications in the cryogenic domain. Full article

LMA (Large Mode Area) optical fibers are presently under active investigation to explore their potential for generating laser action or broadband emission directly within the optical fiber structure. Additionally, a wide mode profile significantly reduces the power distribution density in the fiber cross-section, minimizing the power density, photodegradation, or thermal damage. Multi-stage deposition in the MCVD-CDT system was used to obtain the structural doping profile of the LMA fiber multi-ring core doped with Tm³⁺ and Tm³⁺/Ho³⁺ layer profiles. The low alumina content (Al₂O₃: 0.03wt%) results in low refractive index modification. The maximum concentrations of the lanthanide oxides were Tm₂O₃: 0.18wt % and Ho₂O₃: 0.15wt%. The double-clad construction of optical fiber with emission spectra in the eye-safe spectral range of (1.55–2.10 µm). The calculated LP₀₁ Mode Field Diameter (MFD) was 69.7 µm (@ 2000 nm, and 1/e of maximum intensity), which confirms LMA fundamental mode guiding conditions. The FWHM and λmax vs. fiber length are presented and analyzed as a luminescence profile modification. The proposed structured optical fiber with a ring core can be used in new broadband optical radiation source designs. Full article

A highly erbium- and ytterbium-co-doped photosensitive fiber with a germanophosphosilicate glass core was fabricated by the MCVD method, utilizing an all-gas-phase deposition technique developed “in-house”. Due to doping with germanium oxide (GeO₂), this fiber revealed high-grade photosensitivity (without hydrogen loading) to UV laser radiation at a 193 nm wavelength. The short (28 mm) Fabry–Perot laser cavity was designed by inscribing two fiber Bragg gratings (highly and partially reflective FBGs) directly in the core of the fabricated fiber sample. The stable single-frequency operation regime of the designed laser was observed. The laser emission peak was centered at 1540 nm, with a linewidth of 50 kHz. The slope efficiency of the laser was 10%, and the maximal output power reached a level of 35 mW. Full article

Highly ytterbium-, aluminum- and phosphorus-co-doped silica fibers with low optical losses were fabricated by the MCVD method, utilizing an all-gas-phase deposition technique. Optical and laser properties of the active fibers with a phosphosilicate and aluminophosphosilicate glass cores doped with 1.85 mol% and 1.27 mol% Yb₂O₃ were thoroughly investigated. With the help of hydrogen loading, it was possible to induce highly reflective Bragg grating in both fiber samples using the standard phase-mask technique and 193 nm-UV laser irradiation. The ultra-short (less than 2 cm long) Fabry–Perot laser cavities were fabricated by inscribing two fiber Bragg gratings (highly and partially reflective FBGs) directly in the core of the fiber samples. The highest pump-to-signal conversion efficiency of 47% was demonstrated in such laser configuration using phosphosilicate fiber. The reasons for the low efficiency of aluminophosphosilicate fiber are discussed. Full article

The core refractive index n₂ of silica-based optical fiber preform heated to 2000 °C was determined experimentally for the first time. The measurements were carried out in the process of preform temperature reduction. It was shown that n₂ could increase up to ~1.75 in the visible spectral range at temperatures of ~2000 °C (n₂ ≈ 1.46 at room temperature). This fact suggests that pressures close to or exceeding the ultimate strength of silica glass (~20 GPa) occur in the preform core region. The local extra pressure is argued to be a possible cause of the well-known “starburst” phenomenon at the core–cladding interface of preforms with certain core compositions. The observed effect of radial cracks can be interpreted as the result of silica cladding destruction under the action of extra-high pressure in the core. Full article

In the past few decades, nanostructured carbons (NCs) have been investigated for their interesting properties, which are attractive for a wide range of applications in electronic devices, energy systems, sensors, and support materials. One approach to improving the properties of NCs is to dope them with various heteroatoms. This work describes the synthesis and study of sulfur-added carbon nanohorns (S-CNH). Synthesis of S-CNH was carried out by modified chemical vapor deposition (m-CVD) using toluene and thiophene as carbon and sulfur sources, respectively. Some parameters such as the temperature of synthesis and carrier gas flow rates were modified to determine their effect on the properties of S-CNH. High-resolution scanning and transmission electron microscopy analysis showed the presence of hollow horn-type carbon nanostructures with lengths between 1 to 3 µm and, diameters that are in the range of 50 to 200 nm. Two types of carbon layers were observed, with rough outer layers and smooth inner layers. The surface textural properties are attributed to the defects induced by the sulfur intercalated into the lattice or bonded with the carbon. The XRD patterns and X-ray microanalysis studies show that iron serves as the seed for carbon nanohorn growth and iron sulfide is formed during synthesis. Full article

In this work, a new approach to solving problems of optimal control of manufacture procedures for the production of silica optical fiber are proposed. The procedure of silica tubes alloying by the Modified Chemical Vapor Deposition (MCVD) method and optical fiber drawing from a preform are considered. The problems of optimal control are presented as problems of controlling distributed systems with objective functionals and controls of different types. Two problems are formulated and solved. The first of them is the problem of the temperature field optimizing in the silica tubes alloying process in controlling the consumption of the oxygen–hydrogen gas mixture (in the one- and two-dimensional statements), the second problem is the geometric optimization of fiber shape in controlling the drawing velocity of the finished fiber. In both problems, while using an analog to the method of Lagrange, the optimality systems in the form of differential problems in partial derivatives are obtained, as well as formulas for finding the optimal control functions in an explicit form. To acquire optimality systems, the qualities of lower semicontinuity, convexity, and objective functional coercivity are applied. The numerical realization of the obtained systems is conducted by using Comsol Multiphysics. Full article

In our research, we analyzed the energy and water consumption in diamond mining and laboratory synthesis operations. We used publicly available reports issued by two market leaders, DeBeers and ALROSA, to estimate water and energy use per carat of a rough diamond. The efficiency of the two most popular synthesis technologies for artificial diamonds—High-Pressure-High-Temperature (HPHT) and Microwave-assisted Chemical Vapor Deposition (M-CVD)—was examined. We found that the modern HPHT presses, with open cooling circuits, consume about 36 kWh/ct when producing gem-quality and average-sized (near-) colorless diamonds. ALROSA and DeBeers use about 96 kWh/ct and 150 kWh/ct, respectively, including all energy required to mine. Energy consumption of M-CVD processes can be different and depends on technological conditions. Our M-CVD machine is the least energy-efficient, requiring about 215 kWh/ct in the single-crystal regime, using 2.45-GHz magnetron for the support synthesis. The M-CVD methods of individual synthetic companies IIa Technology and Ekati Mine are different from our results and equal 77 and 143 kWh/ct, respectively. Water consumption for the HPHT and M-CVD methods was insignificant: approximately zero and 0.002 m³/ct, respectively, and below 0.077 m³/ct for ALROSA-mined diamonds. This study touches upon the impact of the diamond production methods used on the carbon footprint. Full article

Asingle-mode Yb-doped germanophosphosilicate fiber with ultra-low optical losses (less than 2 dB/km) was fabricated by means of the MCVD method utilizing an all-gas-phase deposition technique developed “in house”. The absorption and luminescent spectral properties of the fiber were thoroughly studied. The photosensitivity of the pristine (non-hydrogenated) fiber to 248 nm-laser radiation was confirmed by means of fiber Bragg grating (FBG) inscription directly during the drawing process. The random single-frequency lasing at the 1060-nm-wavelength obtained in the 21-m-long fiber with an array of weak FBG was reported. The developed laser slope efficiency in the backward-pumping scheme was measured as high as 32%. Full article

The incorporation of Ce³⁺ ions in silicate glasses is a crucial issue for luminescence-based sensing applications. In this article, we report on silica glass preforms doped with cerium ions fabricated by modified chemical vapor deposition (MCVD) under different atmospheres in order to favor the Ce³⁺ oxidation state. Structural analysis and photophysical investigations are performed on the obtained glass rods. The preform fabricated under reducing atmosphere presents the highest photoluminescence (PL) quantum yield (QY). This preform drawn into a 125 µm-optical fiber, with a Ce-doped core diameter of about 40 µm, is characterized to confirm the presence of Ce³⁺ ions inside this optical fiber core. The fiber is then tested in an all-fibered X-ray dosimeter configuration. We demonstrate that this fiber allows the remote monitoring of the X-ray dose rate (flux) through a radioluminescence (RL) signal generated around 460 nm. The response dependence of RL versus dose rate exhibits a linear behavior over five decades, at least from 330 µGy(SiO₂)/s up to 22.6 Gy(SiO₂)/s. These results attest the potentialities of the MCVD-made Ce-doped material, obtained under reducing atmosphere, for real-time remote ionizing radiation dosimetry. Full article

The technical process of the synthesis of a fluorophosphosilicate (FPS) glass core was thoroughly investigated for the first time utilizing a modified chemical vapor deposition (MCVD) all-gas-phase fabrication method. It was discovered that the limiting doping level of the silica glass simultaneously co-doped with phosphorus (P) and fluorine (F) was found to be confined by the formation of POF₃ gas. The dopants content was achieved as high as 4.7 at% of P and 1.1 at% of F in a glass core, respectively. A developed “in-house” manufacturing method makes it possible to fabricate a large mode area (LMA) purely single-mode Er–Yb co-doped optical fibers with a core diameter of 20 μm and with a lasing efficiency comparable to commercially available LMA Er–Yb optical fibers. Full article