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.
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.
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.
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.
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 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.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print format on
SPIE.org.