Vladimir V. Demidov

Observation of cladding modes spatio-spectral distribution in large mode area photonic crystal fiber

We report the observation of spatio-spectral distribution in cladding modes of a single-mode large mode area photonic crystal fiber. The cladding modes excitation was achieved without any external fiber exposure. The optical field patterns of the cladding modes within different pump wavelength are investigated. To the best of knowledge the spatio- spectral distribution in cladding modes of large mode photonic crystal fiber is demonstrated for the first time. The results are of immediate interest in applications demanding devices based on core and cladding mode coupling in photonic crystal fibers.

Novel bend-resistant design of single-mode microstructured fibers

We designed and manufactured two types of a silica-based microstructured fiber (also referred to as holey fiber) having a large core size: first type which contains a circular core and correspondingly possesses the circular symmetry in the waveguide cladding, and second one representing non-typical triangular structure with periodically placed large and small holes. Our primary aim of investigation remained the same as at the previous stages [1], i.e. to create favourable conditions for the strong attenuation of the higher-order mode. The main technological method we applied was variation of air content in the area surrounding the core, especially in the first air-hole ring.

Results of differential mode delay map measurements performed for multimode optical fibers with strong and weak diameter variation

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.

2-Fold inner cladding symmetry design for actualizing 100μm-core class double-clad large-pitch fibers with the preferential fundamental mode amplification property

Silica/air large-pitch photonic crystal fibers, known simply as large-pitch fibers (LPFs) due to their hole-to-hole spacing 10 times larger than the wavelength, performing mode-field diameters (MFDs) about 50 μm along with nearly diffraction-limited output beam quality provide a versatile platform for realizing compact and stable multi-kW average power and multi-mJ pulse energy ultrashort pulse fiber laser systems [1]. Major efforts have been devoted to improving mode delocalization phenomena in Yb-doped double-cladding LPFs with a view to reduce the spatial overlap of higher-order modes (HOMs) with the doped area [2]. However, the thermo-optic effect which changes the refractive index profile and reconfines a group of the most competitive HOMs (TE01, HE21, TM01) in the gain region jeopardizes the acceptable conditions for the practical fundamental mode (FM) operation and consequently power scaling capabilities of such fibers [3]. For this reason, development of new double-clad LPF designs providing extended HOMs delocalization properties under the extreme heat load is an urgent challenge in the field of fiber laser technologies.

Simulation and analysis of mode staff excitation during “O”-band optical signal launching to graded multimode fiber with large central defect of refractive index profile via standard singlemode fiber

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.

Methods and technique of manufacturing silica graded-index fibers with a large central defect of the refractive index profile for fiber-optic sensors based on few-mode effects

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.

New possibilities of higher-order mode filtering in large-mode-area photonic crystal fibers

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.

Large-Core Photonic Crystal Fibers: Efficient Cladding Designs for Strong Single-Mode Propagation with Low Optical Losses

We provide theoretical and experimental analysis of the effect that core location (central, shifted), cladding symmetry (C6V, C3V, C12V) and amount of air-hole rings has on higher-order mode discrimination toward single-mode propagation in large-core structures.

Design and Characterization of Single-Mode Microstructured Fibers with Improved Bend Performance

Over the last few years, clear progress has been made in research and development of single-mode optical fibers with a large core (when core diameter exceeds 10 μm). Such advances were stimulated essentially by growing requirements for means of high power laser radiation transmission. The urgent problem of laser beam delivery lies in the necessity of the primary Gaussian power distribution of light inherent to many laser sources to be maintained without both temporal and spatial distortions. So optical fibers that support only a single transverse mode prove to be the most appropriate technique for efficient light transfer in production areas of complex or compact architecture. But there are still a number of limitations to cope with. For instance, as the power density of generated laser beams increases, the fiber core has to be expanded adequately in order to minimize the impact of undesirable nonlinear effects such as Raman scattering, Brillouin scattering and self-phase modulation. Moreover, fiber material will exhibit irreversible breakdown if the power level equals or exceeds the critical damage threshold. Conventional single-mode fibers with step-index or graded-index refractive index profile can be acceptably adapted for the realization of large cores. However, the core dimensions enlargement permanently results in the reduction of the refractive index difference between the core and the cladding (∆n). This, in turn, affects adversely the numerical aperture of the fiber (NA), which then has to be reduced twice from its standard values of larger than 0.1 to achieve core diameters of approximately 15 μm at a wavelength around 1 μm (Tunnermann et al., 2005). Such NA lowering weakens considerably the fiber waveguiding so the optical fiber becomes very sensitive to various perturbations, especially to bending effects. Further decrease of NA will require keeping the uniformity of the core refractive index in the vicinity of 10-4 – 10-5. It is technologically unattainable when using chemical vapor-phase deposition methods for the fiber preform fabrication. An alternative flexible approach to solve this challenge is based on exploiting unique wave guiding properties of microstructured optical fibers (MOFs), also known as photonic crystal fibers or holey fibers. MOF design can relatively easily provide extended cores and hence large effective mode areas that nowadays reach values of even thousands of μm2. This phenomenon perfectly coordinates with the ability to manage accurately the effective ∆n value at a level of as low as 0.0001 or less. Furthermore, MOFs, as opposed to single-mode