Few-mode Fiber Link Research Articles

Dead Zone Fault Detection Optimization Method for Few-Mode Fiber Links Based on Unexcited Coupled Higher-Order Modes

Dead Zone Fault Detection Optimization Method for Few-Mode Fiber Links Based on Unexcited Coupled Higher-Order Modes

In-line mode-dependent loss equalizer with femtosecond laser induced refractive index modification.

We demonstrate an in-line all-fiber mode-dependent loss (MDL) equalizer with femtosecond laser induced refractive index (RI) modification. By inscribing an RI-modified structure into the core of a few-mode fiber (FMF), a differential mode attenuation (DMA) can be achieved for LP01 and LP11 modes. The DMA can serve as an in-line MDL equalizer for the long-haul mode-division multiplexing transmission system. Through numerical simulations, we identify that the LP01 mode has a larger attenuation than that of higher-order modes, where the sign of DMA is contrary to that of the conventional FMF links and devices. Finally, a proof-of-concept experiment is implemented by inscribing an RI modified region with a width of 4 µm, a height of 13 µm, and a length of 200 µm into the FMF core. An average additional attenuation of 8.4 dB and 3 dB can be applied to the LP01 and LP11 modes over the C-band, respectively, leading to an MDL equalization range of 5.4 dB. Meanwhile, the average polarization dependent loss (PDL) of the LP01 and LP11 modes induced by the in-line MDL equalizer is approximately 0.3 dB over the C-band. Power matrix measurement indicates that the in-line MDL equalizer has a negligible mode coupling. The proposed in-line MDL equalizer with a wider range and low insertion loss is feasible by precise manipulation of femtosecond laser inscription.

High-gain integrated in-line few-mode amplifier enabling 3840-km long-haul transmission

An integrated few-mode erbium-doped fiber amplifier (FM-EDFA) with high modal gain is suitable for the in-line amplification in mode-division multiplexing transmission (MDM) systems. We first experimentally demonstrate a dual-stage integrated FM-EDFA supporting three linear polarization modes. Consisting of integrated passive components with low insertion losses, the FM-EDFA has a similar structure and performance to widely used commercial single-mode EDFAs. The averaged modal gain of 25 dB, the differential modal gain (DMG) of < 1.1 dB , and noise figures of 5–7 dB are simultaneously achieved. In addition, the DMG of the 3M-EDF itself is ∼ 0.3 dB . Moreover, an MDM transmission experiment with the in-line few-mode amplification by our proposed FM-EDFA over a 3840-km few-mode fiber link for a 28-Gbaud quadrature phase-shift keying (QPSK) signal is demonstrated.

Influence of birefringence- and core ellipticity-induced mode coupling on the gain statistics of forward-pumped Raman amplified few-mode fiber links.

The equations describing light propagation in a few-mode fiber for space-division multiplexing are derived under the presence of linear mode coupling and both Kerr- and Raman-induced nonlinearity. By considering physical models of stress birefringence and core ellipticity, the effect of such fiber imperfections on the gain of a forward-pumped Raman-amplified link is assessed through numerical simulations. The average gain and the variation of signal power at the output of the amplified fiber span is numerically evaluated for different levels of coupling strength in fibers supporting 2 and 4 groups of LP modes, identifying three main propagation regimes and assessing the effect of coupling between different groups of degenerate modes.

Fault detection sensitivity analysis and optimization of the few-mode fiber link under a dynamic spatial mode crosstalk cumulative effect.

Due to the cumulative effect of dynamic spatial mode crosstalk, it is difficult to obtain the amplitude distribution of backscattering with high purity. Hence, the fault detection sensitivity (FDS) of the few-mode fiber (FMF) link is deteriorated, and the fault location accuracy is limited. This work investigates the characteristics and optimization method of FDS for the FMF link under the dynamic spatial mode crosstalk cumulative effect. We establish a mathematical model and analyze the influence of dynamic spatial mode crosstalk on the FDS of LP01, LP11a, and LP11b modes. The crosstalk mentioned above is caused by different splice misalignments and rotation angles during FMF fusion splicing. The results show that the dynamic spatial mode crosstalk has a great influence on the detection sensitivity of the high-order spatial mode. To solve this problem, the FDS optimization method is proposed based on high-order mode waveform reconstruction. The coupling efficiency matrix at the fusion splice point is estimated by the Rayleigh backscattering waveform of each mode, and the Rayleigh backscattering waveform of high-order spatial modes is reconstructed. The simulation experiment is carried out based on the above theoretical model. The results show that the proposed method can effectively eliminate the influence of the cascading dynamic crosstalk accumulation on the fault loss of the high-order spatial mode, and then optimize the FDS of the FMF link. This study offers new opportunities to develop FMF fault detection devices with high detection sensitivity performance.

Reconfigurable Few-Mode Fiber-Based Microwave Photonic Filter

We experimentally demonstrate, for the first-time to our knowledge, tunable true-time delay line operation over a few-mode fiber link, in which the time delay can be continuously and widely tuned, from 46.3 to 105.6 ps, by simply sweeping the operating optical wavelength over the 35-nm range of the C-band. This has become possible thanks to the particular modal dispersion properties of the developed double-clad step-index few-mode fiber, as it features relatively evenly-spaced incremental chromatic dispersion values among 5 spatial modes. To date, it is the first time a dispersion-diversity FMF with this property has been experimentally reported. We assess the performance of this true-time delay line when applied to microwave signal filtering in both space and wavelength diversities, where a variety of reconfigurable 5, 7 and 10-tap microwave filters with free spectral ranges ranging from 7.7 to 27.1 GHz are experimentally demonstrated.

Fault detection performance of a multi-mode transmission reflection analysis for a few-mode fiber link.

We propose a novel monitoring technique based on multi-mode transmission reflection analysis for a long-reach few-mode fiber (FMF) based mode division multiplexing system. By launching unmodulated continuous-wave optical light modes into the corresponding spatial modes of the FMF, the transmitted and reflected or backscattered optical powers can be measured and quantitatively analyzed to accurately characterize and locate the fault. The influences of the capture fraction, attenuation coefficient, and Rayleigh backscattering coefficient are discussed, and simulation results show that the proposed method can realize the fault location of the FMF link. Moreover, considering the influence of mode crosstalk on localization accuracy, it is clear that using the monitoring combination modes LP01 and LP21 gives a high precision of 3.58 m.

Efficient multiplexing using delayed CSI in few-mode fiber links

The constant demand for high data rates has resulted in few-mode optical fiber (FMF) based multiplexed systems an attractive solution. While the separability of FMF modes permits effective multiplexing, modal dispersion limits data rates. Conventional signal processing based approaches to compensate for modal dispersion incur significant computational complexity. On the other hand of principal modes (PMs) can mitigate dispersion with very little compensation requirements at the receiver. However, in the presence of high mode-dependent losses (MDL), the PMs’ performance degrades significantly. In this paper, we propose the use of a polar decomposition based scheme, wherein the impairments of the fiber can be viewed as a cascade of a dispersive system and a lossy (MDL) system. Compensating for the MDL using polar decomposition permits the use of PMs with near optimal performance. Further, we discuss effective quantization techniques to track the variation of PMs and propose effective low-rate feedback mechanisms for PM feedback to the transmitter for optimized transmission, with interpolation across WDM channels to reduce the feedback burden. Finally, we also discuss the impact of delay in feedback on achievable rate when the fiber channel undergoes temporal variation. Simulations reveal that the polar decomposition and quantized PM approach yields effective data rates for various fiber conditions, even with temporal channel variations, thereby indicating that this would be a promising solution for low complexity multiplexing based FMF systems.

Fiber Vector Eigenmode Multiplexing Based High Capacity Transmission Over 5-km FMF With Kramers-Kronig Receiver

Vector modes (VMs) with spatially nonhomogeneous polarization patterns, combining polarization and spatial-mode degrees of freedom together, are regarded as the eigenmodes of optical fiber. Such eigenmodes have potential to be used to implement the high capacity spatial division multiplexing (SDM)/mode division multiplexing (MDM) based optical fiber communication systems. Here, we respectively demonstrate 2 VMs of the same order (l = +2, EH11o and EH11e modes) and 2 VMs of different orders (l = 0, HE11o mode and l = +2, EH11e mode) multiplexing transmissions over 5-km few-mode fiber (FMF). The mode crosstalk after FMF transmission has been minimized to prepare high capacity signal transmission without multiple-input-multiple-output (MIMO) digital signal processing (DSP). To increase the transmission capacity and decrease the system cost, the Kramers-Kronig (KK) receiver has been employed and the single-wavelength two sets of eigenmodes multiplexed 360 Gbit/s and 400 Gbit/s signal transmissions over 5-km FMF link are realized, respectively. Then, to further burst the transmission capacity, 2 vector-mode-division-multiplexing channels (HE11o and EH11e modes) in combination of 5 wavelength-division-multiplexing (WDM) channels with a total of 10 parallel channels have been demonstrated. A total data rate of 1.12 Tbit/s with 28 GBaud 16-state quadrature amplitude modulation (QAM) signal has been realized over the 5-km FMF. In all data-carrying experimental demonstrations, the bit-error rate (BER) performance of each VM channel is below the 7% hard decision forward error correction threshold (FEC) at BER of 3.8 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> . The experimental results highlight that the VMDM-based optical transmission technology could find enormous potential in huge-capacity short-reach optical interconnects.

Fault detection sensitivity enhancement based on high-order spatial mode trend filtering for few-mode fiber link.

In this paper, we propose and experimentally verify a method for optimizing the fault detection sensitivity of few mode fiber (FMF) link based on high-order spatial mode trend filtering. The employment of high-order mode trend filtering as a signal processing tool identifies meaningful level shifts from FMF optical time-domain reflectometer (FMF-OTDR) profile, which is associated with the problem of the minimization of the intrinsic random noise and modal crosstalk impact on the acquired data. A FMF link fault detection system is built, and the proposed method is utilized to detect the fault loss characteristics of 7.2 km 6-mode fiber with three fusion splice points with different fusion quality, and the detection results of each mode are compared with the results obtained by FMF-OTDR. The experimental results show that our proposed method can effectively improve the low fault detection sensitivity of high-order spatial mode caused by random noise and mode crosstalk.

Loss measurement of each mode in few-mode fiber links with crosstalk-inducing points based on optical time domain reflectometry.

We propose a novel technique based on optical time domain reflectometry for characterizing the losses of transmission modes along few-mode fiber links. The technique estimates the transfer matrix at a crosstalk-inducing point from the measured backscattered waveforms and then eliminates the crosstalk influence of the point on the waveforms. Waveforms that are free from the crosstalk influence enable us to obtain the true losses independently of the crosstalk-inducing point. A proof-of-concept demonstration is performed on an optical fiber with a crosstalk-inducing splice and a bend, and the results show the usefulness of the proposed technique.

Ring-core few-mode fiber for tunable true time delay line operation.

We propose, for the first time to our knowledge, tunable true time delay line operation for radiofrequency signals on a few-mode fiber link. In particular, the custom design of a 7-LP-mode ring-core few-mode fiber together with a set of 5 broadband long period gratings inscribed at the proper positions along the fiber allows 4-sample true time delay line tunability over a 20-nm optical wavelength range. We study the performance of the designed true time delay line in the context of reconfigurable microwave photonics signal processing by theoretically evaluating microwave signal filtering and optical beamforming networks for phased array antennas.

Mode Multiplexing Using Principal Modes in Few-Mode Fiber Links With Delayed Feedback

Mode division multiplexing (MDM) in few-mode fibers significantly enhances data rates in fiber links. However, cross-talk due to modal dispersion degrades performance, and this is predominantly compensated through complex MIMO-DSP equalization. “Principal modes” (PMs) of the fiber enable nearly modal dispersion-free communication, though this requires the PMs to be fed back to the transmitter. We propose an effective quantization and feedback method for a PM-based FMF-MDM link that enables equalization-free multiplexing, even with delayed feedback. Simulations reveal that the performance of the proposed approach is close to the ideal PM performance, even with limited quantization and feedback delay.

Sampled true time delay line operation by inscription of long period gratings in few-mode fibers.

We propose and experimentally demonstrate distributed microwave photonics signal processing over a few-mode fiber link by implementing 4-sample true time delay line operation. The inscription of a set of long period gratings at specific locations along the few-mode fiber allows the excitation of the higher-order modes while adjusting the individual sample group delays and amplitudes that are required for sampled true time delay line behavior. Since solely the injection of the fundamental mode at the few-mode fiber input is required, we render this signal processing system independent of any preceding fiber link that may be required in addition to distribute the signal. We experimentally validate the performance of the implemented true time delay line when applied to radiofrequency signal filtering.

Compensation of Mode Crosstalk in MDM System Using Digital Optical Phase Conjugation

We propose to utilize the digital optical phase conjugation (D-OPC) technique for the compensation of mode crosstalk in the mode-division-multiplexed (MDM) transmission system. To realize the D-OPC technique, we send the CW light from the receiver side, measure its wavefront at the transmitter side, and then, transmit the phase-conjugated signal to the receiver. By using this technique, we demonstrate the transmission of four LP modes, each carrying 10-Gb/s ON–OFF keying signals, over a 100-m long few-mode fiber link. These signals are detected by using direct-detection receivers with no electrical equalizer. The results show that, by applying the proposed D-OPC technique, we can achieve the bit-error rate of < 10−3 for all four modes.

Nonlinear Performance of Few-Mode Fiber Links With Intermediate Coupling

This paper reviews and extends the study of nonlinear performance of few-mode fiber links operating in all different linear coupling regimes for different mode delay maps. Nonlinear performance was estimated using the four-wave-mixing theory and the split-step Fourier method including semi-analytical solutions for linear mode coupling of arbitrary strength. The optimum link configurations minimizing the nonlinear penalty at practical levels of training sequence length are presented, including the required coupling strength to give nonlinear distortion below that of the single-mode propagation without mode coupling, and the impact of mode delay maps. Finally, different approximate solution methods of the multimode nonlinear Schrodinger equation are compared, highlighting the accuracy of a stochastic solution method including distributed linear mode coupling.

Impact of modulation bandwidth on multiplexing using principal modes in MMF links.

Multimode fibers (MMFs) are widely used for short fiber links. However, the data rates through MMFs is limited owing to modal dispersion. The so-called "principal modes" (PMs) permit transmission and multiplexing through the MMFs without modal dispersion for small modulation bandwidths. For larger modulation bandwidths, however, they lose their dispersion-free nature. In this paper, we model the impact of modulation bandwidth and mode coupling strength on the performance of PMs. We develop a simulator that characterizes the dispersion and cross-talk of the PMs of few-mode and large-core graded-index MMFs with mode-dependent losses (MDL). Simulations reveal that for fibers without MDL, for modulation frequencies beyond 10 GHz diminishes the PMs' frequency response by more than 1 dB for 100 m in large-core MMF links and 10 km few-mode fiber links. With MDL, simulations reveal that for modulation bandwidths beyond 2 GHz diminishes the frequency response by 3 dB for a 1 km few-mode fiber and by more than 4 dB for a 1 km large-core multimode fiber. While multiplexing using PMs in large-core MMFs with MDL, we find that for modulation bandwidths beyond 3 GHz, the cross-talk is 20 dB in 1 km large-core MMF links, thereby limiting system performance.

Semi-Analytical Modelling of Linear Mode Coupling in Few-Mode Fibers

This paper reviews and extends a method for the semi-analytical solution of the coupled linear differential equations that describe the linear mode coupling arising in few-mode fibers due to waveguide imperfections. The semi-analytical solutions obtained proved to be accurate when compared to numerical solution methods. These solutions were integrated into a multisection model with split-steps for mode dispersion and mode coupling. Simulations using this model matched the analytical predictions for the statistics of group-delays in few-mode fiber links, considering different coupling regimes with and without mode delay management.

Characterization of distributed mode crosstalk in few-mode fiber links with low MIMO complexity

We theoretically model and numerically analyze the linear behavior of distributed mode crosstalk in a step-index weakly-coupled 7-mode fiber. This fiber is assumed to be used for one-polarization uncoupled mode-division multiplexing (MDM) systems with: (1) sparse MIMO equalizers which are performed over only degenerate LP modes, or (2) one single differential mode delay-independent MIMO equalizer which is performed over all non-degenerate LP modes. For the above two low-complexity MIMO configuration schemes, the impacts of distributed mode coupling on the multi-path interference-dependent achievable distance and system quality are empirically evaluated, through the numerical simulations for uncoupled MDM transmissions of a single-channel 28 GBaud QPSK signal over the fiber.

Statistics of accumulated splice-cross talk in weakly coupled few-mode fiber links

Abstract For uncoupled mode-division-multiplexed systems in weakly coupled (WC) few-mode fibers (FMFs),we numerically investigate the effect of splices on the statistical regularities of WC property preservation alongthe transmission link. At each splice, the probability density functions of intermodal cross talk (XT) for thetransmittedlinearlypolarizedmode(s)arenumericallyanalyzedbyusingGaussianfitting.Undertheassessmentcriterion of accumulated intermodal XT affected by discrete splices, we give an empirical evaluation on theuncoupled probability value versus transmission distance, for WC FMF links with different splicing qualities. © 2016 Society of Photo-Optical Instrumentation Engineers (SPIE) [DOI: 10.1117/1.OE.55.7.076118] Keywords: few-mode fibers; mode cross talk; mode-division multiplexing; fiber optics communications.Paper 160703 received May 7, 2016; accepted for publication Jul. 12, 2016; published online Jul. 28, 2016. 1 IntroductionFew-mode fibers (FMF) systems have been considered as apromising solution to overcome the fundamental capacitylimit of single-mode (SM) systems. In an FMF system, inwhich the separation between nondegenerate linearly polarized(LP) modes is preserved [i.e., to use the weakly coupled (WC)approach aimed at minimizing distributed mode coupling],each LP mode(s) can be separately detected using simple 2×2 (nondegenerate LP