This work reports comparative results of mode analysis, performed for earlier on designed silica microstructured optical fiber with ideal equiangular spiral six-ray geometry (ESSR-MOF) and for its real fabricated sample, differing from the model by weak asymmetry and deformed air holes. We discuss issues of input data preparation to specify analyzed complicated non-ideal fiber optic structure for computation by using rigorous finite-element method. Some results of the comparison between mode field patterns as well as guided mode effective refractive index spectral characteristics are represented. It is demonstrated, that potential deviations from the desired MOF geometry, occurring due to features of silica MOF fabrication technological process, should be taken into account during the design to predict real values of mode parameters for manufactured MOF.
This work reports results of laser beam profile measurements, performed for earlier on designed and successfully fabricated silica few-mode microstructured optical fiber (MOF) with hollow-GeO2-doped-ring core (HRC). We compared two drawn from the same preform HRC MOF samples without and with induced during the drawing process twisting of 790 revolutions per meter. Researched silica HRC MOF with outer diameter 65 µm contains hollow ring-core inner diameter of 30.5 µm with wall thickness of 1.7 µm and refractive index difference Δn = 0.03; 90 air holes, placed over typical hexagonal geometry in the periphery domain from the outside HRC at the distance 14 µm, with hole averaged diameter 2.5 µm and pitch 7.5 µm. According to simulation results (mode analysis, performed by rigorous finite element method via commercially available software COMSOL Multiphysics®), it supports two guided LP-modes (fundamental LP01 and the first higher-order LP11) or 4 HE/EH odd and even eigenmodes – HE11/EH11 and HE21/EH21, respectively. We present some results of laser beam profile measurements, performed under various launching conditions (different laser sources as well as excited optical fibers (both commercially available single-mode optical fiber of ITU-T Rec. G.652 and multimode optical fiber 50/125 of ISO/IEC Cat. OM2+/OM3)) at the output ends of researched HRC MOF twisted and untwisted samples as well as at the output end of large core multimode optical fiber 100/125, aligned with excited HRC MOF.
This work presents results of test series, performed for earlier on designed and successfully fabricated silica few-mode microstructured optical fibers (MOF) with six GeO2-doped cores, induced twisting 100 and 600 revolutions per meter, typical “telecommunication” outer diameter 125 µm, core diameter 8.7 µm, air hole diameter 4.6 µm, pitch 7.2 µm, and core graded refractive index profiles with height 0.0275. While Part I introduced results of differential mode delay map measurements with laser source excitation / laser-based data transmission over multimode optical fibers (MMFs) with core diameters 50 and 100 µm, combined with 6-core MOF, and Part II was concerned with researches of spectral responses, measured for fiber Bragg gratings, recorded in these MOFs, Part III reports results of far-field white light beam profile measurements, performed for weakly and strong twisted 6-core-MOFs.
This work presents results of test series, performed for earlier on designed and successfully fabricated silica few-mode microstructured optical fibers (MOF) with six GeO2-doped cores, induced twisting 50 revolutions per meter, typical “telecommunication” outer diameter 125 µm, core diameter 8.7 µm, air hole diameter 4.6 µm, pitch 7.2 µm, and core graded refractive index profiles with height 0.0275. While Part I introduced results of differential mode delay map measurements with laser source excitation / laser-based data transmission over multimode optical fibers (MMFs) with core diameters 50 and 100 µm, combined with 6-core MOF, Part II was concerned with researches of spectral responses, measured for fiber Bragg gratings, recorded in these MOFs, and Part III was focused on laser beam profile measurements, performed for weakly and strong twisted 6-core-MOFs, Part IV reports results of MMF-MOF-MMF fiber optic structure spectral response measurements under direct tension with pull load 0…300 g.
This work presents results of test series, performed for earlier on designed and successfully fabricated twisted silica fewmode microstructured optical fibers (MOF) with six GeO2-doped cores. While Part I introduces results of differential mode delay map measurements, Part II is focused on researches of spectral responses, measured for fiber Bragg gratings, recorded in these multi-core MOFs with core graded refractive index profiles and induced twisting 100 revolutions per meter. Specially setup for spectral response measurement for described complicated fiber optic element was developed, that provides selected alignment of matching singlemode optical fiber with particular single core of MOF via free space and reducing of reflection by precision 8 angle cleaving. Comparing analysis of measured spectral responses confirmed written FBGs in 2 of 6 cores, and demonstrated potentiality of fabricated complicated structure, containing multi-core MOF with FBG, for applications in multichannel fiber optic sensors with spatial division multiplexing technique.
This work presents results of test series, performed for earlier on designed and successfully fabricated silica few-mode microstructured optical fibers (MOF) with six GeO2-doped cores, induced twisting 100 revolutions per meter, typical “telecommunication” outer diameter 125 μm, core diameter 8.7 μm, air hole diameter 4.6 μm, pitch 7.2 μm, and core quasi-step / graded refractive index profiles with height 0.0360/0.0275, respectively. Part I introduces attempts for splicing of typical telecommunication optical fibers and fabricated samples of MOFs by commercially available field arc fusion splicer kits and results of differential mode delay map measurements, performed for laser excited large core (multimode) optical fibers with core diameters 50 and 100 μm, jointed via free space to described above 2 m long pilot samples of 6-GeO2-core MOFs at both receiving and transmitting ends under laser-excited gaussian optical pulse launching with precision offset conditions, while Part II is concerned with researches of spectral responses, measured for fiber Bragg gratings, recorded in these MOFs.
This work presents overview of technological issues concerned with drawing of twisted silica microstructured optical fibers. We present results of drawing tower modifications with developed and verified technological modes, that provide fabrication of silica microstructured optical fibers with induced chirality up to extremely high twisting order of 800 revolutions per meter (rpm). Thus, a work package using the original designer technical solutions for upgrade the adapter for supplying overpressure to the cane holes of the microstructured optical fiber (MOF) was carried out. Hence, the target increase in the twisting speed in the cane feed unit to 2000 rpm is ensured while simultaneously target overpressure feeding to the cane holes, which prevents the hole collapsing in the process of MOF drawing. The reliability of the adapter design and the high reproducibility of the specified cross section structure for the MOF at lengths of more than 50 meters with a twist period of 500 rpm have been experimentally confirmed. For the first time in the Russian Federation, prototypes of "stable" chiral MOF lengths (more than 50 m) of a different configuration with a maximum induced twisting of 500 rpm and MOFs prototypes with structure stability at lengths of less than 50 m with a strongly induced chirality of up to 790 rpm were fabricated. The geometric patterns of these fibers are also presented in this work.
This work introduces first time fabricated spun silica microstructured optical fiber (MOF) with inclusion of seven GeO2-doped capillaries, placed in the central part of MOF cross-section, and induced twisting up to 730 revolutions per meter. Part I discusses technological issues for manufacturing of described complicated twisted fiber optic structure, while Part II presents some results of test series, performed for successfully manufactured twisted MOF pilot samples with typical hexagonal geometry under hole radius 4.40 μm and pitch 9.80 μm, outer “telecommunication” diameter 125 μm, and center part, formed by seven hollow GeO2-doped ring cores with inner radius 2.50 μm, pitch 8.80 μm and refractive index difference Δn=0.030. Following measurements were performed: measurements of transmission spectra under various twisting order, far-field laser beam profiles, some attempts of fusion splicing of typical telecommunication optical fibers and fabricated MOF with insertion loss estimation, and spectral response measurements of both single and group WDM (Wavelength Division Multiplexing)-channels of commercially available telecom WDM-system under inclusion of 2 m length MOF into various spans of short-range lab fiber optic link.
We developed and characterized luminescent temperature sensors with a simple construction based on YAG : Ln3 + (Ln = Nd, Yb, Ce) nanocrystals and silica multimode optical fibers. Lanthanide-doped nanocrystals 40 to 60 nm in size were synthesized in the form of powders using the modified Pechini method. The obtained materials exhibited high sensitivity of luminescence intensity to temperature variations at wavelengths of 550 nm (YAG : Ce3 + ), 1030 nm (YAG : Yb3 + ), and 1064 nm (YAG : Nd3 + ) in the temperature range 50°C to 600°C. Additionally, we offered a method to eliminate influence of vibration on accuracy of temperature measurements by adding SiO2 sol to powders after their synthesis in sensitive elements.
This work reports the results of bandwidth measurements and tests, performed for earlier on designed and fabricated pilot lengths of new silica laser-optimized graded-index multimode fibers (LOMFs) with extremely enlarged core diameter up to 100 μm and "typical" "telecommunication" cladding diameter 125 μm. Presented optical fibers are targeted for harsh environment short-range multi-Gigabit onboard cable systems and industrial networks. Proposed LOMF 100/125 differs by specially optimized graded refractive index profile, that provides low differential mode delay (DMD) for selected guided modes. We present some results of tests, performed for fabricated pilot 520 m length of described LOMF 100/125, focused on researches of bandwidth features. They contain DMD map and transfer function measurement as well as 10GBase-LX/SX channels eye-diagram and bit-error-ratio reports with following direct detection of the maximal acceptable optical fiber length for guaranteed 10Gbps channel supporting.
This work presents some results of pulse and spectral responses, performed for laser-excited pilot sample of earlier on designed and fabricated chiral silica few-mode optical fiber (FMF) with induced twisting 66 revolutions per meter (rpm), typical (for telecommunication optical fibers) cladding diameter 125 μm, weakly increased core diameter up to 11 μm and numerical aperture NA=0.22, corresponding to improved height of quasi-step refractive index profile. Described FMF 11/125 provides propagation of 4 and 6 guided modes over “C” and “O”-bands, respectively. We present results of tests, focused on researches of few-mode effects, occurring under laser-excited optical signal propagation over pilot sample optical fiber, and their influence on pulse and spectral responses, including some measurements, performed for fiber Bragg grating, recorded in tested FMF 11/125.
The polymer-salt method was applied to synthesize nanoscale Gd2O3:Nd3+ phosphors in the form of thin films on the inner surfaces of capillaries which organize the structure of a silica hollow-core anti-resonant optical fiber. To obtain luminescing centers, the preform of a hollow-core anti-resonant optical fiber was impregnated with a homogeneous mixture of Gd(NO3)3 and NdCl3 dissolved in water and organic solvent (polyvinylpyrrolidone). This procedure was followed by a few post-processing steps, including drying of the impregnated preform in normal conditions and its thermal treatment at temperature 1000 °C. As a result, Gd2O3:Nd3+-based thin films were produced inside the capillaries. Finally, the modified preform was drawn into the hollow-core anti-resonant optical fiber of 120 μm in diameter at temperature 1850 °C. The analysis of crystallographic structure of the initial Gd2O3:Nd3+ nanopowder and the same nanophosphor inside the fabricated fiber revealed the absence of structural and phase transformations of synthesized nanocrystals with an average size 35 nm after drawing. The data on spectral-luminescent properties of the fabricated fiber confirmed the presence of Gd2O3:Nd3+ nanophosphors in silica glass with the main emission peak at wavelength 1064 nm. Presented method of modifying the structure of a hollow-core anti-resonant optical fiber allows formation of active silica layers without using technologically complicated and expensive CVD processes.
We propose and fabricate pilot lengths of two type microstructured optical fibers with chirality, induced during the drawing process under 10 and 66 revolutions per meter. The first one is microstructured fiber with geometry providing quasi-ring radial mode field distribution. So it imitates ring-core optical fiber properties by special formation of designed 2D-periodic structure. The second is fiber with hexagonal geometry and shifted core in relation to central axe. The work presents results of numerical analysis of fabricated samples, performed by rigorous numerical method. Here initial data were set via manufactured optical fiber end face images. We also reports some results of far field laser beam profile images, measured at the output of described fiber samples under laser source excitation at wavelength 1550 nm.
In this paper, an alternative fiber-optic method for forming vortex modes based on a chiral (twisted) microstructured fiber is proposed. This fiber can be considered as a ring-core fiber with ring core formed by capillaries. Besides, optical fibers design for transmitting optical vortexes over long distances. i.e. a vortex-maintaining fiber is also proposed. This fiber is a multimode fiber with an extremely large core. A comparison of the different types of vortex generation and vortex-maintaining fibers is also given. Both proposed fibers can be used in Radio over Fiber systems applying vortex beams.
The paper describes the polymer-salt method of neodymium-doped aluminum yttrium garnet (YAG:Nd) crystals formation inside the channels of a microstructured silica fiber preform. The crystals formation was performed through the impregnation of inner surfaces of the channels by aqueous solutions of thermally decomposable salts (yttrium nitrate, aluminum nitrate, neodymium chloride) and an organic polymer with subsequent processes of drying and thermal treatment at the temperature of 1100°C. The composite structure prepared was drawn into the fiber at the temperature of 2000°C. The X-ray diffraction analysis revealed the formation of YAG:Nd crystals from 25 nm to 37 nm in size in the silica glass matrix of the fiber. Measurement of the attenuation spectral dependence confirmed the presence of optical signal absorption bands inherent to Nd3+ ions. The shape of the nanocrystals luminescence spectrum is characteristic to the YAG:Nd with a peak at the wavelength of 1064 nm.
We present results of experimental research and comparison of differential mode delay (DMD) maps measured for silica graded index multimode optical fibers (MMFs) with strong and weak diameter variation. Preliminary for two synthesized by MCVD fiber preforms were selected by criterion of expected strong DMD due to great profile dip in the core center. Then two lengths of MFMs were drawn. The first one was manufactured according typical operations with automatic control of technological processes, while the second one was drawn under manual maintenance. Therefore two samples of MMFs of ISO/IEC Cat. OM2 with length about 1 km were manufactured with diameter variation ± 0.3 μm and ± 1.2 μm respectively. At the next stage we performed DMD map measurement of described two MMF 50/125 samples by DMD analyzer lab kit R2D2 according to ratified standards TIA-455-220-A/FOTP-220 and IEC 60793-2-10 to research and analyze influence of fiber diameter variation on mode coupling in the form of additional DMD distortions during laser-excited optical pulse propagation over MMF under a few-mode regime.
Hollow-core fibers (HCFs) which guide light by an antiresonant reflection from arrays of silica walls have been attracting much interest due to their extraordinary optical properties and potential interdisciplinary applications including highly efficient laser-matter interaction, ultra-short pulse delivery, pulse compression and low-loss mid-infrared transmission. There are several types of HCFs having either a photonic crystal cladding, Kagome lattice or a single cycle of capillaries surrounding the core. In the latter case the antiresonant guidance properties depend strongly on the core size and the shape of the core/cladding boundary.
In this work, we focus on the capabilities of two HCF designs (negative curvature of the core/cladding boundary and nodeless capillary structure) to obtain a nearly single-mode guidance from the visible to the mid-infrared spectral regions.
The first HCF (Sample A) was drawn from the stack comprising a cycle of eight touching capillaries having the wall thickness 1.5 µm which provided a negative curvature of the core/cladding boundary. The fiber was intentionally manufactured with the trapezoidal shape of the capillaries in order to minimize the interaction between the surface modes, trapped amidst the touching trapezoids, and the fundamental mode in a hollow core. The negative curvature of the boundary resulting in the octagonal shape of the core was achieved by putting an excess gas pressure inside the capillaries during the drawing process. The second HCF (Sample B) was produced from the stack comprising a cycle of six non-touching capillaries having the wall thickness 2.5 µm with a view to restrict the abovementioned interaction via breaking the surface modes coupling between the adjacent capillaries. As in the first case, the gas pressure was controlled carefully to keep all capillaries separately from each other. In both samples the core diameter was equal to 50 µm ensuring a relatively large effective mode area.
Taking into account the periodic nature of HCFs transmission windows, we simulated and measured accurately transmission spectra and modal properties of the fibers. The simulations were performed using the finite element analysis. The transmission spectra were measured by passing light from the tungsten halogen lamp through the samples of 35 cm long and registering output signal applying three optical spectrometers covering the wavelength range 600-2500 nm.
We observed a good agreement between the simulation and the experiment. The Sample A has transmission windows at the wavelengths 650, 750, 850-900, 950-1050, 1150-1300, 1450-1700, 2000-2300 nm and the Sample B – at the wavelengths 600, 650-700, 750-800, 850-950, 1000-1100, 1150-1350, 1450-1750, 1900-2400 nm. The mid-infrared window for the Sample B is larger and more pronounced in terms of relative transmission due to the larger wall thickness at the core/cladding boundary. Moreover, the Sample B is predicted to be practically single-mode in the considered spectral region, as the losses of the most competitive higher-order modes are estimated to be much above 1 dB/m. A similar regime for the Sample A is expected only when operating at the long-wavelength limit of the spectral region, due to the increase in the fundamental and higher-order modes refractive index difference.
The results of experimental study on the main technological aspects relating to a full production cycle of 50/125 μm silica multimode graded-index fibers with the central defect of the refractive index profile realized as a large dip are presented. Preform synthesis conditions for controllable implementation of the mentioned defect via MCVD method are analyzed and optimized. The effect of geometrical irregularities, induced by drawing optical fibers under the manual maintenance of the outer diameter stability, on attenuation has been explored. Applying the Weibull theory, a statistical evaluation of mechanical properties, particularly tensile strength, of the optical fibers drawn at various temperatures has been conducted.
We present modified technique for differential mode delay map measurement. Here according to well-known methods a fast laser pulse is also launched into a tested multimode fiber (MMF) via single mode fiber (SMF), which scans core of MMF under precision offset positions. However unlike known technique formalized in ratified standards, proposed modification differs by addition scanning of the output end of tested MMF by short tail of SMF. Therefore for each radial offset position at the input/output MMF ends, the shape of pulse response of launched optical signal is recorded, that provides to get more informative differential mode delay map. This work presents some results of experimental approbation of proposed modified technique for differential mode delay map measurement.
We discuss both theoretical and experimental aspects of modal discrimination phenomenon that takes place in largemode- area photonic crystal fibers. A few special fiber designs providing efficient higher-order mode filtering were implemented and investigated. First adaptation had the core comprised of 7 elements (instead of 1) with a view to reduce the pitch, since smaller pitches correspond to lower bend-induced losses. That measure aided to realize a series of fibers with a 35-75 μm core diameter propagating only the fundamental mode within a wide spectral range due to embedded leakage channels for the higher-order mode which losses were rated to be above 1 dB/m. Second variation included the fiber with circularly distributed air holes surrounding a core of 30-50 μm in diameter. Circular geometrical configuration enabled leakage losses of the higher-order mode to be 120 times larger than leakage losses of the fundamental mode. Third adaptation had the alternation of large and small air holes (C6V symmetry converted to C3V symmetry) resulting in partial or complete delocalization of the higher-order mode power outward a core region. Fourth design represented the regular triangular-lattice structure with a core of 35-60 μm in diameter shifted from its usual location in the center of the lattice. The main idea consisted in provoking an enhancement of the higher-order mode discrimination, as higher-order mode has a larger field near to the air-hole silica interfaces compared to fundamental mode. Those fibers demonstrated distinguished bending resistance properties, since could be exploited with a bending radius of 2-3 centimeters.
In this paper, we report on the design, implementation and performance issues of solid-core microstructured optical
fibers (MOFs) having two types of asymmetry introduced intentionally into the typical triangular cladding configuration.
First adaptation represents MOF with a large core shifted for the pitch value from its usual location in the center of the
lattice. Second variation includes regular structure with several peripheral air holes omitted on purpose to organize the
'incomplete cladding' design. Fiber core dimensions range from 12.5 to 35 μm. The results of investigating properties of
guided modes, transmission loss and macrobending resistance are presented. Whereas the structure with several missing
air holes in the cladding negligibly differs from the regular MOF structure, the fiber with a shifted core reveals some
essential preferences. This fiber exhibits practical fundamental mode operation with a great beam quality within the
expanded transmission spectra. The ultimate spectral widening is about 300 nm, which is possible due to a comparatively
high air filling fraction (diameter-to-pitch ratio is larger than 0.60) that helps to improve fiber bend performance. Robust
single-mode guidance originates from the enhanced higher order mode loss mechanism and consequent differential mode
attenuation factor. Minimal optical losses equal to 5 dB/km at λ = 1550 nm in the single-mode regime.
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