Cladding Size Research Articles

Investigation on the randomly-coupled unit grouping multi-core fibers with different unit arrangement

Randomly-coupled unit grouping multi-core fiber (RC-UG-MCF) is a new fiber structure that combines weakly and strongly coupled multi-core fiber technologies. Compared to the weakly-coupled MCF with a large cladding size, the RC-UG-MCF can shrink the cladding size by grouping the cores. On the other hand, the RC-UG-MCF only maintains a few cores in the strongly-coupled condition, which can reduce the multiple-input multiple- output digital signal processing (MIMO-DSP) complexity compared to the coupled MCFs with full MIMO-DSP for all the cores. In this paper, we intro- duce three types of randomly-coupled unit grouping 12-core fiber, which are randomly-coupled 2-core unit grouping MCF (RC-2CUG-MCF), RC-3CUG- MCF and RC-4CUG-MCF. Subsequently, we compare these three RC-UG- MCFs in terms of group delay spread, inter-unit crosstalk (XT), and cladding size. To achieve an inter-unit XT of −40 dB/100 km, RC-3CUG-MCF can provide a solution to maintain MIMO-DSP scale at the level of 6 × 6 with 180-µm cladding radius. However, the RC-2CUG-MCF has worse inter-unit XT, and the RC-4CUG-MCF has larger MIMO complexity. Based on the analysis, there is a trade-off relationship between the MIMO-DSP scale and the fiber cladding size, assuming that the target inter-unit XT is fixed.

Mitigating the Strong Inter-Core Crosstalk during MCF Transmissions by Neural-Network-Equalizer-Based MIMO-DSP and Approaches for Its Simplification

In this paper, we report our recent progress related to advanced digital signal processing (DSP) technologies to address the strong inter-core crosstalk (IC-XT) during multicore fiber (MCF) optical transmissions. MCF transmission technology has significant potential to break through the capacity crunch of single-mode fiber (SMF) transmissions. However, strong coupling among cores, namely, inter-core crosstalk (IC-XT), is unavoidable for high-density space-division multiplexing (SDM) transmissions using MCFs with the standard cladding size. To deal with this issue, we propose some novel DSP structures to eliminate IC-XT with considerable simplicity, based on the neural network equalizer (NNE)-based multiple-input and multiple-output digital signal processing (MIMO-DSP). The traditional NNE-based MIMO-DSP method has the ability to process the coupled SDM tributaries transmitted over MCFs; however, it exhibits complexity limitations for practical implementations. The implementation complexity of the NNE-based method is mainly attributed to the time-consumption of the training process and the large weight (neurons) numbers of the equalizers. Thus, we propose two main approaches to simplify NNE-based MIMO-DSP for the practical implementation of MCF transmissions: (1) To reduce the time-consumption of the training process in NNE-based MIMO-DSP, the idea of transfer learning (TL) is employed for initializing the weights, resulting in the faster convergence of the equalizers. (2) IC-XT cancellation is performed along with MIMO-DSP; thus, the dimensionality of MIMO-DSP could be reduced. To validate the performance improvement of the proposed machine learning DSP methods, both simulations and experiments related to transmissions and reception over MCFs were conducted. The results indicate that the proposed novel DSP structures possess the advantages of reduced complexity and improved robustness to IC-XT, which is beneficial for the next-generation high-density SDM transmissions.

Fabrication and laser performance of cladding uniform core tapered fiber

Compared with traditional uniform fiber, the long tapered fiber has the advantage of suppressing nonlinear effects and maintaining excellent beam quality. Both the cladding size and core size of the long tapered fiber reported so far vary simultaneously in a constant core-to-cladding diameter ratio (CCDR) along the length. In this work, we propose a technique for preparing cladding uniform core tapered fiber (CUCTF) with a varying CCDR, and have successfully fabricated CUCTF. The core diameters of narrow end and wide end are 31.2 μm and 52.5 μm, respectively. The cladding diameter is always 400 μm along the length. The laser performance of CUCTFs with different structures (wide to narrow, narrow to wide) are investigated based on a co-pumped fiber amplifier. Between them, the CUCTF with the wide to narrow structure has a better M2 factor of 1.455 due to mode filtering.

Microstructure and properties of a laser cladded NiCrBSi alloy coating

Abstract Laser cladded NiCrBSi alloy coating was fabricated on the surface of a 42CrMo roll using a 6 kW fiber laser. The effects of the laser power, scanning speed and feeding rate on the cladding layer form, size, dilution rate, microstructure, and hardness of coating were studied by using optical microscopy (OM), scanning electron microscopy (SEM) and a microhardness tester. The results show that the microstructure, size, dilution rate and hardness of the cladding layer had changed with an increase in laser power, powder feeding rate and scanning speed. The appropriate parameters of the laser cladding experiment are as follows: the laser power is 2000 W, powder feed rate 20 g × min-1, the scanning speed 4 mm × s-1. The cladding layer and the substrate exhibit good metallurgical bonding under the above processing parameters. The microstructure of the cladding layer is fine, the dilution rate is 9.8 wt.-%, and the microhardness of the cladding layer is 710.7 HV.

Microstructure and properties of a laser cladded NiCrBSi alloy coating

Abstract Laser cladded NiCrBSi alloy coating was fabricated on the surface of a 42CrMo roll using a 6 kW fiber laser. The effects of the laser power, scanning speed and feeding rate on the cladding layer form, size, dilution rate, microstructure, and hardness of coating were studied by using optical microscopy (OM), scanning electron microscopy (SEM) and a microhardness tester. The results show that the microstructure, size, dilution rate and hardness of the cladding layer had changed with an increase in laser power, powder feeding rate and scanning speed. The appropriate parameters of the laser cladding experiment are as follows: the laser power is 2000 W, powder feed rate 20 g × min-1, the scanning speed 4 mm × s-1. The cladding layer and the substrate exhibit good metallurgical bonding under the above processing parameters. The microstructure of the cladding layer is fine, the dilution rate is 9.8 wt.-%, and the microhardness of the cladding layer is 710.7 HV.

Effect of adhesive joint stiffness on optimal size of large-format cladding comparison of artificial and real environment

The efficiency and lifespan of bonded façade joints can be increased and extended by designing the optimal size of the façade cladding. In the case of large-format cladding, the maximal possible size is always desired. The authors of this paper have scrutinised a relationship between the size of the façade cladding and the stiffness of the support, i.e. the bonded joint. In general, the higher the stiffness of the joint, the smaller the size of the cladding can be used in the design. After the experimental analysis of the material properties, very simple numerical approach with consideration of a linear stress-strain behaviour was chosen to determine the optimal size of the façade cladding. This paper investigates the material properties of selected adhesive systems, two of which are polyurethane-based and two are based on silyl modified polymers. The effect of the stiffness of the adhesive joint was assessed in combination with four different façade claddings (Cetris Basic, multilayer solid wood panel, solid timber plank and wood plastic composite) with a relatively high thermal and moisture expansion. It was, therefore, necessary for the bonded joint to be flexible and to adapt to these dimensional changes without creating greater internal tension. Moreover, the experimental part included artificial as well as real weathering conditions which allowed to compare the reliability of commonly used laboratory methods. While in combination with Cetris Basic the recorded data did not show any significant differences and the weathering method was less important than the chemical composition of the adhesive system, the results obtained in combination with solid wood panel demonstrated exactly the opposite. The joint stiffness with PU-I recorded after weathering in the real environment had dramatically increased, which led to the strength and adhesive elongation reduction by more than 50%, this also caused a premature failure of the bonded joint, and it greatly affected the cladding size that had to be reduced by 46%. Similar phenomenon was observed in combination with both MS-based adhesive systems after artificial weathering. Moreover, the size of the cladding had to be reduced by up to 56% in combination with MS-I after real weathering. On the other hand, the rigidity of PU-II and MS-II tested in the real environment had considerably decreased which allowed an enlargement of the cladding size by more than 30%.

Hybrid Grating in Reduced-Diameter Fiber for Temperature-Calibrated High-Sensitivity Refractive Index Sensing

We propose and experimentally demonstrate a hybrid grating, in which an excessively tilted fiber grating (Ex-TFG) and a fiber Bragg grating (FBG) were co-inscribed in a reduced-diameter fiber (RDF). The hybrid grating showed strong resonances due to coupling among core mode and a set of polarization-dependent cladding modes. This coupling showed enhanced evanescent fields by the reduced cladding size, thus allowing stronger interaction with the surrounding medium. Moreover, the FBG’s Bragg resonance confined by the thick cladding was exempt from the change of the surrounding medium’s refractive index (RI), and then the FBG can work as a temperature compensator. As a result, the Ex-TFG in RDF promised a highly sensitive RI measurement, with a sensitivity up to ~1224 nm/RIU near the RI of 1.38. Through simultaneous measurement of temperature and RI, the temperature dependence of water’s RI is then determined. Therefore, the proposed hybrid grating with a spectrum of multi-peaks embedded with a sharp Bragg resonance is a promising alternative for the simultaneous measurement of multi-parameters for many RI-based sensing applications.

Functionalized fiber-optic long-period grating with reduced cladding size for humidity sensing

Relative humidity (RH) is an important factor in the field of structural health monitoring, especially during the early stage of corrosion. Many methods have been proposed for humidity sensing, and one of the attractive sensors is fiber-optic long-period grating (LPG) sensors. Unfortunately, the current sensing sensitivity of this kind of sensors is limited. A thin fiber-optic LPG sensor with a self-assembled thin film of PAH + / PAA − is proposed and demonstrated for humidity measurements. The LPG sensor is inscribed in a single-mode fiber using a CO2 laser, and the cladding size is reduced to about 27 μm. It shows that an LPG sensor with a reduced cladding size has an enhanced refractive index sensing characteristic, compared to those with normal cladding size due to the enhancement of the evanescent field. In a next step, selectivity is added to the LPG sensor coated with a film of PAH + / PAA − for functionalization to be sensitive to humidity. The resonance spectral responses of LPGs are experimentally investigated with respect to its sensitivity to a change in humidity that modifies the index of the nanolayer and the cladding, leading to a resonant wavelength shift. The experimental results show that the coated thin LPG has a highly sensitive resonance wavelength shift of −220.75 pm / % RH for an RH variation from 25% to 80%, of which the sensitivity is enhanced thrice compared to those with a normal cladding size. The proposed sensing setup opens LPG structures for a variety of sensing and detection applications.

Measurement of resonant bend loss in anti-resonant hollow core optical fiber.

We present the results of measurements of resonant spectral bend loss using a novel apparatus in a series of hollow core anti-resonant optical fibers, important for their applications in the delivery of industrial power ultra-short laser pulses. The measured bend losses exhibit clear wavelength-bend diameter resonances. We demonstrate, in good agreement with theoretical analysis, that the sensitivity to bend diameter (in terms of minimum bend radii) is dependent on the ratio between cladding and core structure size. By decreasing the cladding capillary diameter: core diameter ratio from 0.70 to 0.43 the minimum bend diameter is decreased from >160 mm to ~15 mm at a wavelength of 800 nm. Furthermore it is demonstrated that the exact position of the loss bands is highly dependent on the orientation of the fiber structure with the bend plane.

Numerical and Experimental Analysis of Sensitivity-Enhanced RI Sensor Based on Ex-TFG in Thin Cladding Fiber

We report a highly sensitive refractive index (RI) sensor in the aqueous solution, which is based on an 81°-tilted fiber grating structure inscribed into a thin cladding fiber with 40 μm cladding radius. The numerical analysis has indicated that the RI sensitivity of cladding resonance mode of the grating can be significantly enhanced with reducing cladding size. This has been proved by the experimental results as the RI sensitivities of TM and TE resonance peaks in the index region of 1.345 have been increased to 1180 nm/RIU and 1150 nm/RIU, respectively, from only 200 and 170 nm/RIU for the same grating structure inscribed in standard telecom fiber with 62.5-μm cladding radius. Although the temperature sensitivity has also increased, the change in temperature sensitivity is still insignificant in comparison with RI sensitivity enhancement.

Large mode area fibers with asymmetric bend compensation

Fibers with asymmetrical bend compensation offer to completely remove the tradeoff between mode area and single-modedness, with potentially huge impact on high-power amplification. These fibers would be difficult to fabricate, but are the only fundamental-mode strategy that can remove the bend-distortion limitations on mode-area scaling. Here, we show that even imperfect fibers can achieve essentially complete HOM suppression for areas of 2000 square microns or larger. Ultimate performance limits due to finite cladding size and fabrication imperfections are calculated.

Analysis of optical properties of fundamental-mode in waveguide tapered fibers

An analysis of optical properties of fundamental-mode in waveguide tapered fibers is theoretically investigated and realized in this paper. The waveguide device is tapering an SMF-28 fiber to few tens of micrometers of diameter. For discussing the cladding size of waveguide structure affects the fundamental-mode cutoff (FMC) and the optical characteristics of the devices, to etch outer cladding to reduce the pure-silica cladding diameter proceeded and to compare optical properties of FMC with non-etched cladding tapered fibers. Numerical results show the cutoff wavelength of FMC is mainly dominated by the size of squeezed core and slope of FMC could be influenced by the size of pure-silica cladding.

Improved air–silica photonic crystal with a triangular airhole arrangement for hollow-core photonic bandgap fiber design

We propose an improved photonic crystal (PC) cladding design for existing air-guiding photonic bandgap (PBG) fibers whose cladding airholes are arranged in a triangular lattice pattern. By increasing the sizes of concentrated silica regions in the cladding PC, we can have a larger degree of freedom in controlling the cladding bandgap regions. We predict that a fiber made from this type of cladding would perform better in terms of the PBG-guiding wavelength range, radiation loss owing to finite cladding size, and the ability to avoid surface mode problems.

Optical fiber with novel geometry for evanescent-wave sensing

Abstract First results in the preparation and analysis of an optical fiber with a novel geometry which facilitates the access of chemical species to the evanescent field for sensing purposes are presented. This ‘s-fiber’ is of approximately sectorial cross section with the core located in the carefully rounded vertex of the sector. Using a perturbation method, the dependence of the attenuation coefficient of the fundamental mode in a weakly-guiding, step-index s-fiber on the fiber normalized frequency, vertex angle and cladding thickness are determined. Attenuation coefficients several times higher than in D-fibers are theoretically attainable. Preforms for drawing s-fibers are prepared from standard MCVD preforms by accurate grinding and polishing the preforms to a desired sectorial shape. Multimode s-fibers with core dimension of about 30 μm and cladding size of about 170 μm and exhibiting satisfactory strength have been drawn. Resulting shapes of the fiber and core depend on the shape, structure and composition of the preform, drawing temperature and drawing velocity. Results have proved the feasibility of the chosen approach to the laboratory preparation of s-fibers. In preliminary experiments the sensing ability of the drawn fibers has been examined.