Performance Enhancement in Multicore Fiber Using Trench Assisted Technique

In this paper, the theoretical unification of linear and nonlinear inter-core crosstalk (IC-XT) in step-index single-mode multicore fiber (MCF) media is reported and validated experimentally. In order to estimate the IC-XT when operating in linear and nonlinear regimes, the coupled-mode theory (CMT) and the coupled-power theory (CPT) have been unified in both power regimes. The theoretical analysis of the CMT indicates that in coupled MCFs with reduced core-to-core distance (core pitch) the nonlinear self-coupling and cross-coupling effects should be considered when operating with high optical powers. However, considering a core pitch value higher than three times the core radius only the self-coupling effect should be taken into account for estimating the nonlinear IC-XT. Considering these results, the CPT is also extended to nonlinear regime including the dominant nonlinear coupling effect. Using both CMT and CPT, the statistical model of nonlinear IC-XT is completed with the closed-form expressions for estimating the cumulative distribution function, the probability density function and the crosstalk mean and variance as a function of the power level launched into a single-core of the MCF. The crosstalk model presented is additionally extended when multiple cores are simultaneously excited. Finally, the theoretical model is experimentally validated in a homogeneous four-core fiber considering different bending radius configurations.

Semi-analytical approach for calculation of intercore crosstalk in multicore fibers

The discrete changes model (DCM) of inter-core crosstalk (ICXT) in weakly-coupled homogeneous multi-core fibers (MCFs) is generalized to a dual polarization (DP) scheme. This model provides theoretical expressions for the two polarization fields of the ICXT at the MCF output. Therefore, it may be of particular interest in the design of direct-detection MCF systems, where the photodetected ICXT results mainly from the beating between the ICXT field at the MCF output and the carrier of the interfered core. The DP-DCM is validated by comparison of the mean ICXT power and ICXT field amplitude estimates with the ones obtained with a rigorous, yet much more computationally demanding, model based on the coupled local mode theory (CLMT). Good agreement between the mean ICXT power estimates obtained with the DP-DCM and CLMT is observed when the inter-core coupling coefficient variation along the MCF is small. Good agreement is also observed when comparing the probability density functions of the ICXT field amplitude.

The numerical analysis of inter-core crosstalk in multicore fibers (MCFs) based on a novel theoretical approach that includes stimulated Raman scattering (SRS) and four-wave mixing (FWM) nonlinearities in the presence of random perturbations is presented. The coupled propagation equations can be employed to numerically analyze wavelength-division multiplexed systems with homogenous and heterogeneous cores. Propagation of ten optical channels, 200-GHz spaced, along 10 km of a homogeneous two-core fiber shows that the wavelength dependence of the relative average inter-core crosstalk induced by SRS and FWM is comparable with the one produced by only considering the wavelength dependence of the linear coupling coefficient, thus allowing tuning of the overall effect. Our formalism represents a valuable tool in the design of future multi-channel MCF systems.

Crosstalk in multi-core fibers with α-index single-mode cores is investigated theoretically. It is clarified that the index parameter α of the core has a great effect on the crosstalk of multi-core fibers.

We report a numerical analysis of inter-core crosstalk in multicore fibers (MCFs) based on a dual-core-fiber model that includes Stimulated Raman Scattering (SRS) and Four Wave Mixing (FWM) nonlinearities in the presence of random perturbations.

This paper proposes and investigates the use of multicore fiber (MCF) media performing space-division multiplexed transmission for next-generation optical fronthaul systems. We report the experimental demonstration of combined radio-over-fiber (RoF) transmission of full-standard LTE-Advanced (LTE-A) and WiMAX signals providing fronthaul connectivity in 150 m of 4-core fiber (4CF), transmitting simultaneously fully independent wireless services. Operating in linear and nonlinear optical power regimes, the experimental evaluation verifies that the error vector magnitude (EVM) is not degraded when intercore and intracore Kerr nonlinearities are excited in MCF with high input power levels. As a result, nonlinear regime is proposed as a key factor to reduce the temporal EVM fluctuation induced by the random nature of the intercore crosstalk in MCF. In addition, MCF fronthaul applied to converged fiber-wireless polarization multiplexed passive optical networks is demonstrated to transmit LTE-A and WiMAX signals over two orthogonal optical polarizations. The polarization-multiplexed signal is transmitted in RoF over 25.2 km of standard single-mode fiber and then demultiplexed and injected in different cores of the 4CF to provide fronthaul connectivity. Finally, the extension of multicore optical fronthaul capacity is proposed using MIMO LTE-A signals. The tolerance of the MIMO LTE-A RoF transmissions to in-band crosstalk is reported and compared to single-input single-output (SISO) configuration. The experimental results indicate that MIMO configuration is more tolerant than SISO to in-band crosstalk considering both internal and external interferences. MIMO and SISO configurations are compared when transmitted in RoF over a 4CF operating in linear and nonlinear regimes and core interleaving nonlinear stimulation is proposed to reduce the temporal and spectral EVM fluctuation when the same wireless standard is propagated in each core.

We present the fabrication of fiber Bragg gratings (FBGs) in a trench-assisted heterogeneous multicore fiber (MCF). Two obviously different Bragg reflection peaks are obtained due to the slight difference of refractive indices between the center core and the outer cores. To investigate the reflections of the two FBGs simultaneously, only a segment of multimode fiber is inserted between the lead-in single mode fiber and the MCF. The experimental results confirm that the curvature sensitivity of the FBG in the outer core is a sinusoidal function of the bending orientation angle. The maximum linear curvature sensitivity is about 0.128 nm/m−1. The cross sensitivity to temperature or externally applied axial strain can be eliminated by discriminating the different responses of FBGs inscribed in outer cores and the center core. Thus this MCF with FBGs can be utilized as a directional bending sensor. Moreover, the proposed sensor offers several advantages, such as low cost and flexibility in fabrication.

The complete analytical solutions of the mode-coupling dynamics for seven-core multicore fibers (MCFs) with identical cores is proposed. All the coupling coefficients C(mn) of adjacent cores and nonadjacent cores as a function of the structural parameters are investigated. It is shown that the coupling coefficients can decrease by adjusting the structural parameters. In addition, the trench-assisted structure could be used for suppressing cross talk in MCF, and the effective area A(eff) can be enlarged without degrading the crosstalk properties. Simulations suggest that low cross talk and/or large A(eff) could be achieved by adjusting the trench parameters. Large mode area could be obtained by utilizing small trench (small trench width c, small refractive index difference Δ(trench)) in the trench-assisted MCF (TA-MCF), whereas low cross talk could be achieved by utilizing larger trench (large trench width c, large Δ(trench)) in the TA-MCF.

In order to realize fast and accurate estimation of intercore crosstalk in bent multicore fibers (MCFs), an analytical expression of the average power-coupling coefficient (PCC) based on an exponential autocorrelation function is, for the first time, derived, resulting in no need for heavy numerical computations. It is revealed that, when the bending radius is large and the correlation length is large, the average PCC is inversely proportional to the correlation length and to the square of the propagation constant difference Δβ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">mn</sub> between core m and core n, and when the bending radius is small and the correlation length is large, the average PCC is proportional to the bending radius and is independent of the correlation length. When the correlation length is small, on the other hand, the average PCC is proportional to the correlation length and is independent of the bending radius. For homogeneous MCFs (Δβ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">mn</sub> = 0) with small bending radius, the average PCC coincides with the mean crosstalk increase per unit length derived from the coupled-mode theory of Hayashi et al. that is proportional to the bending radius. Average crosstalk values calculated by using the analytical expression derived here are in excellent agreement with those of numerical solutions of coupled-power equations, irrespective of the values of bending radius and correlation length.

Effects of increasing core count in limited cladding diameter and cladding thickness constraints and their arrangements, i.e., core layouts on the crosstalk variation in heterogeneous and trench-assisted (TA) heterogeneous MCFs are investigated using the expression of crosstalk derived from the coupled mode equations and the finite element method (FEM) based FemSIM software. Only two kinds of heterogeneous cores having different core radius and refractive index are used in the present analysis. For 12-Core TA MCF, approx. 35 dB of crosstalk level can be optimized by proper selection of core layout, and whereas for high core count such as 24-Core and 30-Core TA MCFs, this value reduces to 30 dB and 20 dB, respectively, due to the decrease in the core pitch and the increase in the number of surrounding cores. This result shows that core count and layout have a significant impact on the crosstalk variations in MCFs.

We performed intensive simulation work to find out key parameters which have a great influence on crosstalk in multi-core fibers. The design of core pitch, core/trench profile were optimized simultaneously to achieve optical properties suitable for the future large capacity transmission with low crosstalk less than −45dB after 100km and large effective areas larger than 130μm2.

Monitoring the geometry of an moving element is a crucial task for example in robotics. The robots equipped with fiber bend sensor integrated in their arms can be a promising solution for medicine, physiotherapy and also for application in computer games. We report an all-fiber intensity bend sensor, which is based on microstructured multicore optical fiber. It allows to perform a measurement of the bending radius as well as the bending orientation. The reported solution has a special airhole structure which makes the sensor only bend-sensitive. Our solution is an intensity based sensor, which measures power transmitted along the fiber, influenced by bend. The sensor is based on a multicore fiber with the special air-hole structure that allows detection of bending orientation in range of 360°. Each core in the multicore fiber is sensitive to bend in specified direction. The principle behind sensor operation is to differentiate the confinement loss of fundamental mode propagating in each core. Thanks to received power differences one can distinguish not only bend direction but also its amplitude. Multicore fiber is designed to utilize most common light sources that operate at 1.55 μm thus ensuring high stability of operation. The sensitivity of the proposed solution is equal 29,4 dB/cm and the accuracy of bend direction for the fiber end point is up to 5 degrees for 15 cm fiber length. Such sensitivity allows to perform end point detection with millimeter precision.

Multi-core fiber has aroused considerable interest as one of potential candidates for space division multiplexing that provides an additional freedom degree to increase optical fiber capacity to overcome the transmission bottleneck of current single-mode fiber optical networks. Few-mode fiber is also under intense study as a means to achieve space division multiplexing. We propose a novel dual-mode large-mode-area multi-core fiber (DMLMAMCF), which uses multi-core structure to realize few-mode condition when pursuing large mode-area. The proposed fiber consists of 5 conventional silica-based cores in the center region and 14 air hole cores surrounding the center cores. The outer circle with 12 air hole cores, which function similarly to the fluorine doping region in the bend-insensitive fiber, can mitigate the bending loss when keeping large mode area. The symmetrically distributed two cores on both sides of the center core in central region can reduce the half second-order LP11 mode consisting of two degenerate HE11 modes, TE01 mode, two degenerate HE21 modes and TM01 mode, thus leading to the remaining four vector modes, i.e. two degenerate HE11 modes and two degenerate HE21 modes. That is the reason why we call it strict dual-mode. We focus on large-mode-area properties and bending characteristics of the dual-mode. The influence of structural parameters that include corepitch Λ, refractive index difference between core and cladding Δn, and fiber core radius a, on mode characteristics and mode area of HE11 mode and HE21 mode is investigated in detail. The results reveal that it is helpful to increase the effective area of fundamental mode when we increase the value of corepitch, reduce the refractive index and fiber core radius. The effective mode area of HE11 is about 285.10 μm2 under the strict dual-mode condition. In addition, the relationship between bending loss and bending radius, and the relationship between effective mode area and bending radius of two modes are both investigated. For the HE11 mode, the least bending loss is about 5×10-5 dB/m while the least effective mode area with bending radius larger than 0.6 m is about 285.10 μm2. The HE21 mode is more sensitive to bend effect. The least bending loss is about 0.028 dB/m and the effective mode area is larger than 280.00 μm2 except for resonant coupling points. Large effective areas of both modes with low bending loss can be realized. Larger effective mode area with larger corepitch, appropriate refractive index difference and fiber core radius can be achieved. This fiber may find its usage in high power fiber lasers and amplifiers.

Imaging endoscopes have enabled the development of minimally invasive procedures in a wide range of medical applications. Flexible endoscopes have additional advantages over rigid endoscopes. Remarkably, they have an enhanced capability of being guided through the internal conduits of the human body. The development of imaging endoscopes based on coherent optical fiber bundles have made high resolution fiber endoscopy possible. In the last years, multicore fibers have been proposed as an alternative to coherent fiber bundles for high resolution applications. Both types of structures entail several limiting factors. Among them, the most critical one is the optical crosstalk that takes place between the parallel contiguous fibers of the device, which provokes a worsening in the contrast of the images. Therefore it imposes a limit to the quality of the endoscopic system that must be avoided. In this work, we present a theoretical model for the study of optical coupling in multicore fibers, which is based on an electromagnetic optics approach. This model is applied to the analysis of crosstalk within fiber imaging endoscopes. It includes the effect of core non-homogeneities and bendings. The essential equations of the model will be shown. These equations provide us with a theoretical basis that is subsequently applied to fiber endoscopes design. Therefore, we present a robust method for the adjustment of opto-geometrical parameters of the fiber in order to fulfil the quality requirements for a certain application. The key role of core diameter variations in the quality of the image will be specially highlighted.