Real-time compensatory mode control within few-mode fibers based on adaptive spatial light modulation.

Weakly-coupled mode division multiplexing (MDM) technique is a promising candidate for capacity enhancement of short-reach optical interconnections, for which the multiple-ring-core few-mode fiber (MRC-FMF) has been proven to be an effective design method to suppress distributed modal crosstalk. Similar to low chromatic-dispersion (CD) O-band transmission based on single-mode fibers (SMF), all the mode channels in a weakly-coupled FMF for short-reach applications should achieve low CD to support intensity-modulation/direct-detection (IM/DD) transmission. In this paper, we propose, for the first time to the best of our knowledge, an index perturbation method to adjust both effective index and CD of each mode in an MRC-FMF. Firstly, an accurate modeling method to model the relationship between SiO2-GeO2 material index and the germanium concentration at different wavelengths is proposed by analyzing the index characteristics of 4 kinds of germanium-doped fused silica SMFs at the same fabrication processing, which could be utilized to calculate the CD characteristics for an MRC-FMF with perturbed index profile. Then, based on the perturbation method considering the influences on both effective index and CD, a weakly-coupled low-CD MRC-FMF supporting 4 linearly-polarized (LP) modes is designed and fabricated. The measured minimum effective index difference min|Δneff| among all modes is larger than 1.3 × 10-3, and the CD values of all the modes lie between -6 and +6 ps/km/nm ranging from 1280 to 1320 nm, which agree well with the design. The 2-km transmission experiment indicates that the fabricated MRC-FMF could support stable digital-signal-processing (DSP)-free IM/DD transmission for all the 4 LP modes. This work is beneficial to the application of short-reach weakly-coupled MDM systems.

A challenge is to measure the impairments of few-mode fiber (FMF) for mode division multiplexing (MDM) system, so as to quantify the few-mode fiber and compensate the impairments, and thus improves the transmission performance of MDM system. In this paper, the experimental setup of impairments measurement with the structure of Photonic Lantern and optical circulators is established. Not only the two kinds of mode coupling (MC) in 9.8 km 3-mode fiber and at splicing of two FMFs (0.9km and 9.8km) with an off-axis distance of 2.0μm and 2.4μm, but also the mode dependent loss (MDL) between LP 01 and LP 11 are successfully measured.

We propose a novel mode multiplexer based on phase plates followed by a Mach-Zehnder interferometer (MZI) with image inversion. After the higher-order modes are selectively converted from fundamental linear-polarized (LP) modes by the phase plates, the converted modes are coupled without fundamental loss using MZI with image inversion, in which the original spatial pattern and inverted pattern of the optical signal are interfered. Our scheme is also applicable to the coupling of degenerated LP modes such as LP(11a) and LP(11b). First, we numerically and experimentally evaluate the performance of the mode converter based on phase plates. The mode converter is suitable as long as the five LP modes such as LP(01), LP(11ab) and LP(21ab) are sustained in a few-mode fiber (FMF), although the crosstalk due to excitation of undesirable modes is unavoidable when the higher-order modes over LP(02) are sustained in FMF. Next, we develop and characterize the proposed mode multiplexers based on phase plates and MZIs with image inversion. The insertion loss is suppressed to around 3 dB for mode multiplexing of LP(11a) and LP(11b). Using a fabricated mode multiplexer for LP(31a) and LP(31b), we measure the bit-error rate performance of single-polarization mode-multiplexed quadrature-phase shift keying optical signals.

Recent progress in optical devices for mode division multiplex transmission system

Mode division multiplexing (MDM) is an emerging optical communication solution for high-capacity wired–wireless applications. Due to the presence of modal crosstalk and link impairments in MDM, this work aims to design a system that provides low complexity, an improved Shannon Capacity limit, and high spectral efficiency. In this work, a quad modal MDM system using an integrated parabolic index multimode fiber and free-space optics (PIMMF-FSO) link is presented. Four linearly polarized (LP) modes, LP01, LP22, LP03, and LP13 based on a 16 × 10 Gbps MDM system offering different sixteen channels, are realized. Results show that the system can sustain a 7.5 dB insertion loss over 100 m FSO and a 100 m fiber range for different LP modes under the impact of clear air, moderate haze, heavy rain and wet snow climates with weak turbulence. A faithful fiber range of 3000 m can be obtained successfully in the proposed system with a −10 dB link loss, −7.62 dBm received power and 10 dB noise. Compared to existing designs, the proposed design offers optimum performance in terms of high channel capacity and a high traffic rate with low complexity and high spectral efficiency. Additionally, high received power, with acceptable noise, link loss, FSO misalignments and fiber nonlinearities, is successfully obtained.

We provide differential mode delay (DMD) diagnostics for linearly polarized (LP) modes traversing a few-mode fiber (FMF). We employ a modal interferometer method with a butt coupling mechanism to measure the DMD between any two modes guided in FMFs with step-index (SI) and depressed-cladding profiles. The measurement principle is based on investigating a transmitted spectrum through temporal decomposition by means of a Fourier transform. At least four different temporal waveforms are needed for an FMF supporting four LP modes to estimate the DMD between two different LP modes. A butt coupling mechanism with an air gap between an FMF and a dispersion-shifted fiber is employed for simultaneous DMD measurement. The present diagnostics requires some knowledge of the bending loss characteristics of the FMFs and the butt coupling properties of the circular symmetry modes or circular asymmetry modes in their electric fields. The DMD diagnostics are realized taking the bending loss and butt coupling characteristics into consideration. We present experimental results for the DMD and DMD slope as a function of the wavelength in a 1450-1625 nm telecommunication band.

Design of precision spatial positioning scheme for mode division multiplexing system channels placement over optical fiber core end

New transmission techniques for short-reach optical transmission applications especially datacenter interconnections are highly expected to break current bandwidth bottleneck, among which mode division multiplexing (MDM) based on few-mode fiber (FMF) is promising by exploring different linearly-polarized (LP) modes or mode groups as spatial transmission channels. However, it has been a great challenge to deal with degenerate modes in FMFs. In this paper, a FMF design utilizing multiple ring perturbations in fiber core to suppress modal crosstalk during fiber transmission is firstly proposed and a 6-LP-mode double-ring-core (DRC) FMF is fabricated with a high minimum refractive index difference (min|neff|) of 1.4910-3 among all the modes. Secondly, a novel degenerate-mode-selective coupler (DMSC) converting each degenerate LPmn mode into the LP11 mode in a two-mode fiber is proposed. Then intensity-modulation/direct-detection (IM/DD) MDM transmission scheme without any digital signal processing (DSP) can be realized utilizing mode multiplexer/demultiplexer (MUX/DEMUX) consisting of cascaded mode-selective couplers (MSC) and DMSCs, in which each degenerate or non-degenerate LP mode is considered as an independent transmission channel. Then based on the proposed DRC-FMF and mode MUX/DEMUX, we demonstrate a 4-LP-mode prototype system using commercial single-mode 10 Gbps SFP + modules and 4K video transceivers without any hardware or software modifications. Finally, the intra-LP-mode dispersion (ILMD) effect in a weakly-coupled FMF is analyzed and a modified fixed-analyzer method is proposed for its measurement. Experimental results show that it may be one of the major impairments for IM/DD MDM transmission.

Reconfigurable all-fiber mode exchange enabled by mechanically induced LPFG for short-reach MDM networks

In recent years, different kinds of fiber-optic temperature sensors have been widely applied in various areas such as power systems and environmental monitoring, which are typically built based on temperature-induced influences to interferometers, grating structures or back-scattered light propagation in traditional single-mode fibers (SMF). In this letter, we propose a novel temperature sensing mechanism based on weakly-coupled few-mode fibers (FMF), in which signals in different linearly-polarized (LP) modes could propagate independently through the fiber, and the influence of temperature to differential mode delay (DMD) between LP modes are utilized for the sensing. The simulation model is established and the simulative results show a linearly-increased relation between DMD and temperature variations, which are then experimentally verified based on the DMD measurement for different mode pairs in a 4-LP-mode FMF utilizing white-light interferometry scheme. The proposed sensing mechanism could achieve flexible systematic structures and find high potential in various applications.

We experimentally demonstrate adaptive control of linearly polarized (LP) modes based on a fiber Bragg grating (FBG), where feedback for the adaptive algorithm is provided by the optical signal transmitted through and reflected by the FBG. In this approach, by maximizing the ratio of optical power transmitted and reflected by the FBG, we can selectively excite the LP 01 and the LP 11 modes at an optical wavelength associat ed with the appropriate FBG reflection peak. Combining this method with time-division-multiplexing, it should be possible to adaptively control guided modes at any desired location within a multimode fiber network. Keywords: Adaptive optics, mode control, few mode fiber, fiber Bragg grating 1. INTRODUCTION Traditionally, single mode fiber (SMF) are highly preferred fo r important applications such as optical commination and sensing. However, with recent developm ent of mode-division-multiplexing (MDM), there has been renewed interest in multimode fibers (MMFs). For example, in MDM, different linearly polarized (LP) modes in few mode fibers (FMFs) have been utilized as a multiplexing parameter to increase the information capacity of optical communications

Few-mode erbium-doped fiber amplifier (FM-EDFA) is a key element to realize signal gain compensation in a longdistance mode division multiplexing (MDM) system. The differential modal gain (DMG) between modes directly affects the communication quality of the MDM system. In this paper, the particle swarm optimization (PSO) method is applied to design the erbium ion doping profile for high gain and low DMG simultaneously. By adjusting the doping radius and concentration concurrently, both high signal gains and low DMG can be obtained. In the conditions of the core pump and cladding pump respectively, the erbium ion with a multi-layered doping profile is automatically optimized by the PSO for a few-mode erbium-doped fiber (FM-EDF). Results show that as a three-layered ion adjustment, the gain is higher than 20 dB and DMG is lower than 0.15 dB in a four-mode step-index fiber. PSO is easy to implement and simple to operate. Compared with the other intelligent methods, such as the genetic algorithm or gradient descent optimization algorithm, PSO has no "crossover" and "mutation". The optimization time is greatly reduced. The PSO-based fiber design provides new guidance for the improvement of fiber gain equalization.

Increasing demand for high data transmission rates and bandwidth availability driven by the rapid development of the broadband services becomes challenging in the context of constraints imposed by nowadays exploited telecommunication optical fibers. Based on single-mode fibers and multiplexing in the wavelength domain (wavelength division multiplexing, WDM), the current technology seems to have reached its fundamental limits. At present, two promising technologies are a subject of intensive research. One of them is the increase of the data transmission through multiplying the number of fiber cores, thus implementing the concept of Multi-Core Fibers (MCF), where each core is used as a separate data transmission channel. The second widely investigated technique is based on the idea of mode division multiplexing (MDM), where different transverse modes of a Few-Mode Fiber (FMF) can be used as different carriers for data channels. In this work, we demonstrate the results of R&D works and the comprehensive tests of the few-mode fibers developed within the project NMKM+. Developed few-mode fibers (both passive and active) and their commercially available counterparts have been tested with respect of transmission parameters and applicability to the real telecom systems. In particular, guided mode profiles, numerical apertures, OTDR, and dispersion characteristics for the passive fibers have been recorded and compared. Also, methods of selective excitation and detection of singular modes have been discussed and partially verified. The results of data transmission experiments have been performed and discussed with respect to the quality of transmitted signals. These have been complemented by the amplification experiments with the use of erbium-doped few-mode fibers in “classical” and microstructural geometry. The results were studied and analyzed, showing the potential for future optimization. Acknowledgement: This work has received support from the National Centre for Research and Development through project NMKM+ (TECHMATSTRATEG1/348438/16/NCBR/2018).

In this paper, link adaptation (LA) schemes based on adaptive modulation (AM), where the transmit parameters can be adapted to the varying channel conditions, are investigated for spatial modulation (SM). We propose machine learning (ML) based adaptive spatial modulation (ASM) as the optimal technique for modulation selection (MS) compared with the exhaustive search based ASM scheme. Specifically, two supervised machine learning classifiers (MLCs), K-nearest neighbors (KNN) and support vector machine (SVM), are proposed to design the MS in ASM and make the tradeoff between the system complexity and the bit error rate (BER) performance. The proposed classifiers are employed to solve the MS problem in ASM as a multi-class classification problem and obtain the best modulation selection candidate (MSC) that achieves the maximum minimum Euclidean distance. Simulation results show that, under the same spectral efficiency (SE), the proposed schemes provide considerable system complexity reduction and obtain a sub-optimal BER performance compared with the typical ASM scheme.

We demonstrate wavelength-division multiplexed (WDM) and mode-division multiplexed (MDM) transmission over a fiber recirculating loop comprising of a 25-km span of low differential mode group dispersion (DMGD) few-mode fiber carrying the LP01 and LP11 mode groups, and an inline few-mode erbium-doped fiber amplifier (FM-EDFA) providing low mode-dependent gain (MDG) per span. We successfully transmitted a 10λ × 6 × 28-Gbaud QPSK signal over a distance of 700 km.