Multimode Fiber Links Research Articles

Modal noise improvement in 10-Gb/s offset launch in multimode fiber link with multimode fiber taper

Modal noise improvement in 10-Gb/s offset launch in multimode fiber link with multimode fiber taper

ACO–OFDM with Improved Bandwidth Efficiency over Long Haul and MIMO Optical Fiber Communication Systems

Authors propose an improved orthogonal frequency division multiplexing (OFDM) scheme in which the information samples are transmitted in otherwise discarded negative frame of asymmetrically clipped OFDM (ACO–OFDM). ACO–OFDM has two similar positive and negative OFDM envelopes so that clipping does not result in loss of signal information. The flipped polarity negative samples are inserted at the zeros of positive frame or back to back with positive frame, in time-compressed frame such that composite signal has the same frame duration as ACO–OFDM but improved transmission power, bandwidth efficiency. Simulation results for bit error rate with respect to number of subcarriers and optical launch power show that the proposed time-compressed OFDM frames perform better in high-dispersion condition over long distance, high data rate, and high transmission power over single-mode fiber. It has been demonstrated that the proposed time-compressed frame structure also performs well with the 2*2 MIMO setup for multimode fiber link.

Adapting Mach–Zehnder Mesh Equalizers in Direct-Detection Mode-Division-Multiplexed Links

A Mach–Zehnder mesh (MZM), which is comprised of a network of tunable $2\times 2$ Mach–Zehnder interferometers and embedded photodetectors (PDs), can be used to perform arbitrary unitary matrix multiplications in the optical domain and compensate modal crosstalk in short-reach mode-division-multiplexed (MDM) links that use direct detection (DD). MZMs can be adapted using a self-configuration method, proposed by Miller, where multiple low-speed and low-power code sequences are superimposed on parallel high-speed information streams. We show that self-configuration in its original form is a sub-optimal equalization method for high-speed data transmission because adaptation based on detected code strengths is adversely impacted by low measurement signal-to-noise ratios and interference from the high-speed information streams. These impairments prevent the method from accurately tracking the millisecond-timescale modal dynamics of short-reach DD-MDM channels. We propose small modifications to the self-configuration method that can enable the MZM to track up to $10^8$ -fold faster channel dynamics. In particular, we show that replacing continuous equalization of low-power code sequences by periodic equalization of full-power training signals and using special optimization methods can yield faster MZM tuning. We also discuss the tradeoffs between MZM architectures that embed PDs inside the mesh and those that have PDs at the output ports only. Our results indicate that optimally designed MZMs and their associated control methods can increase the information capacity of short-reach multimode optical fiber links.

A high speed 100 Gbps MDM-SAC-OCDMA multimode transmission system for short haul communication

In this work, a cost-effective spectrum amplitude coding optical code division multiple access (SAC-OCDMA) multimode transmission system is designed with the integration of mode division multiplexing (MDM) scheme in order to transmit 100 Gbps of data over 11 km multimode link. OCDMA scheme is carried out by using random diagonal (RD) code whereas MDM scheme is carried out by using modal multiplexing of Hermite Gaussian (HG) modes 00 and 01. The reported results show the successful transmission of 100 Gbps data over multimode link. Furthermore, the performance of multimode fiber and few mode fiber is investigated in terms of bit error rate which shows that few mode fiber performs better than multimode fiber link for the proposed MDM-OCDMA system. Moreover, the decomposition of HG modes into linearly polarized (LP) modes at receiver side is presented.

Intelligent 2-Dimensional Soft Decision Enabled by K-Means Clustering for VCSEL-Based 112-Gbps PAM-4 and PAM-8 Optical Interconnection

In this work, we proposed an intelligent 2-dimensional soft decision (2D SD) enabled by k-means clustering, for vertical-cavity surface-emitting laser (VCSEL) based 112-Gbps PAM-4 and PAM-8 optical interconnection. At high modulation speed, VCSEL based link suffers from severe level nonlinearity, level-dependent noise and inter-symbol interference (ISI). For characterizing the above-mentioned three distortions, 2D signaling is performed through time-slotted mapping of PAM. Without extra requirement of Monte Carlo approach, channel conditional probability density function (PDF) can be intelligently estimated using inline data, thanks to 2D k-means machine learning. Thus, improved precision of log likelihood ratio (LLR) can be realized by additional consideration of nonlinearity, level-dependent noise and ISI. Both simulations and experiments have been carried out for proof-of-concept investigations on VCSEL and multimode fiber (MMF) links. 112-Gbps PAM-4 and PAM-8 signaling have been experimentally realized using a commercial-product-level VCSEL with 100-m MMF transmission. The results indicate significant improvement of the proposed k-means 2D SD without training using prior-known sequences.

Nonlinear Distortion Reduction Effect of Graded-Index Plastic Optical Fiber

We experimentally demonstrate that a graded-index plastic optical fiber (GI POF) can significantly reduce transmitted signal distortion caused by the nonlinear response of a multimode fiber (MMF) link based on a vertical-cavity surface-emitting laser (VCSEL). The low distortion of the signal transmission is attributed to the strong mode coupling in the GI POF, which suppresses the increment in signal distortion due to the external optical feedback, by reducing the self-coupling of problematic back-reflected light into the VCSEL cavity. The strong mode coupling is closely associated with polymer-specific microscopic heterogeneous structures in the fiber core materials, suggesting that the signal distortion can be further reduced by controlling the microscopic properties. The nonlinear distortion reduction effect of our developed GI POF will be invaluable for next-generation MMF links based on multilevel modulation in the ultrahigh-definition era.

Unconventional plastic optical fiber design for very short multimode fiber link.

We introduce a graded-index plastic optical fiber (GI POF) design for very short-distance household applications, in which the transmission quality is predominantly determined by system noise rather than the loss and bandwidth. The developed GI POF has strong mode coupling with low accompanying scattering loss, which is closely related to the specific microscopic heterogeneities in the core material. Such characteristic mode coupling significantly decreases reflection noise, improving the transmission quality compared with silica GI multimode fiber (MMF) for lengths below 30 m. Moreover, in the GI POF link, the transmission quality tends to improve with increasing fiber length, despite the increased loss and decreased bandwidth. This feature suggests that the system noise can be controlled by the microscopic heterogeneous properties of the GI POF for a very short MMF link, where the fiber loss and bandwidth are sufficiently low and high, respectively. This unconventional concept for optical-fiber design can advance fiber-optic communication in emerging applications in households located near optical network terminals.

Dyadic Probabilistic Shaping of PAM-4 and PAM-8 for Cost-Effective VCSEL-MMF Optical Interconnection

In this paper, dyadic probabilistic shaping (PS) for pulse amplitude modulation (PAM) has been investigated and experimentally demonstrated for intensity-modulation and direct-detection optical interconnection systems. Improved achievable information rate can be obtained under the condition of limited bandwidth and signal-to-noise ratio (SNR). Theoretical investigations of generalized mutual information have been performed. 0.61- and 1.74-dB SNR gains by dyadic PS can be obtained for PAM-4 and PAM-8, respectively, to achieve error free assuming optimal 20% FEC. Meanwhile, such fixed distributions can offer a considerably broad SNR range (10.3-16.7 dB for PAM-4 and 13.4-24.6 dB for PAM-8) with positive gain. Moreover, experimental investigations have been carried out over optical multimode fiber (MMF) links at 850 nm using a commercial-product-level vertical-cavity surface-emitting laser (VCSEL) chip. PAM-8 signaling by PS at the net rate of 75 Gb/s has been realized with 100-m OM3 fiber transmission. Energy efficient signaling can be achieved with up to 46% theoretical power reduction by PS. Experimentally, we obtained 16% reduction of optical power for the VCSEL-MMF optical link. The dyadic distribution of PS, due to its simplicity of coding, as well as considerable shaping gain and energy efficiency, is expected to be an opportune solution for the cost-sensitive short-reach scenarios.

Nonlinear Kerr and intermodal four-wave mixing effect in mode-division multiplexed multimode fiber link

We represent the effect of nonlinear propagation in mode-division multiplexed multimode fiber (MMF) link. The effect of intermodal four-wave mixing (IMFWM) and intramodal FWM for waves with different spatial modes (LP01, LP02, LP11, and LP12) over MMF has been analyzed with the help of an optical spectrum analyzer using VPI Photonics/VPI transmission Maker™ version 9.7 simulator. It can be concluded that high launch power and low dispersion generated sidebands in the cases of both intermodal and intramodal FWM. These FWM products grow in number with variation in wavelength of probe as well as these products grow toward the central pump frequency of that particular mode. It mathematically represents the multimode nonlinear propagation of different mode groups (containing N spatial modes in each group) in MMF due to Kerr effect. Thus, nonlinear interaction between modes results in interference that degrades the performance of multichannel mode-division multiplexed systems. To achieve full-phase matching and reduce the effect of IMFWM, chromatic dispersion has been increased in each spatial mode throughout the link.

Intrinsically Stabilized Plastic Optical Fiber Link Subject to External Optical Feedback

We develop a graded-index plastic optical fiber (GI POF) that can significantly stabilize a multimode fiber (MMF) link with a vertical-cavity surface-emitting laser (VCSEL) subject to external optical feedback. It is demonstrated that the dominant mechanism for this stabilization effect is the strong mode coupling of the GI POF, which is closely related to the polymer-specific microscopic heterogeneities in the GI POF core material. Such mode coupling decreases correlation between the problematic reflected light field and VCSEL cavity field, increasing tolerance of the MMF link for external optical feedback. Our developed GI POF with specific microscopic heterogeneities prevents the external-optical-feedback-induced critical destabilization observed for silica GI MMFs. These results suggest that the stabilization effect can be controlled by microscopic properties regardless of the fiber attenuations and core refractive indices.

Efficient Quantification and Simulation of Modal Dynamics in Multimode Fiber Links

Sudden environmental effects, such as mechanical vibration, wind, and lightning, impart microsecond-timescale changes to the transmission matrix of multimode optical fibers. We introduce a timescale parameter to characterize the rate of channel changes in mode-division-multiplexed (MDM) links. We show how to efficiently generate continuous unitary matrices that form the basis for a simplified, yet general, dynamic channel model incorporating the effects of modal dispersion, mode-dependent loss, and time-varying mode coupling. We show that fast environmental perturbations can be modeled by deriving the unitary matrices from a linear ramp model, where the ramp slopes depend on the timescale parameter. We discuss the implications of channel dynamics on adaptive multiple-input-multiple-output (MIMO) equalization at the receiver in long-haul MDM links using coherent detection and in short-reach MDM links using direct detection. We show that the channel timescale and the rotation rate of the generalized Stokes vector on the generalized Poincare sphere are both predictive of adaptive MIMO equalizer misadjustment. Our simplified channel model can be applied to optimize adaptive MIMO equalization algorithms for tracking modal dynamics.

40 Gbps Laguerre-Gaussian and Hermite-Gaussian Optical Mode Division Multiplexing for Radio over Fiber System

Abstract Radio over Fiber technology is utilizing the high capacity of optical fiber system and wireless mobility network. In this work, we proposed two different optical propagation modes namely Laguerre-Gaussian (LG) and Hermite-Gaussian (HG) setting to (0.1), (0.2) mode types generated by a special CW laser source at varying wavelength from 800 nm to 1150 nm with 50 nm spacing channel Mode division Multiplexing (MDM) technique. These wavelengths transmitted×5Gb/s data modulated on PSK modulator having 100 GHz radio frequency subcarrier over 5 km Parabolic-Index Multimode Fiber link. Q factor value and eye diagram used to evaluate system performance which showed the maximum Q factor value found at 800 nm channel 1 equal to 12.0113 for both LG 0 1 and HG 0.1. While the maximum Q factor appears at 800 nm channel 1 equal to 8.49756 for LG 0,2 and 3.56785 at 1000 nm channel 5 for HG 0.2.

Weakly-coupled mode division multiplexing over conventional multi-mode fiber with intensity modulation and direct detection

Multi-mode fiber (MMF) links are expected to greatly enhance capacity to cope with rapidly increasing data traffic in optical short-reach systems and networks. Recently, mode division multiplexing (MDM) over MMF has been proposed, in which different modes in MMF are utilized as spatial channels for data transmission. Strongly-coupled MDM techniques utilizing coherent detection and multiplex-input-multiplex-output (MIMO) digital signal processing (DSP) are complex and expensive for short-reach transmission. So the weakly-coupled approach by significantly suppressing mode coupling in the fiber and optical components has been proposed. In this way, the signals in each mode can be independently transmitted and received using conventional intensity modulation and direct detection (IM-DD). In this paper, we elaborate the key technologies to realize weakly-coupled MDM transmission over conventional MMF, including mode characteristic in MMF and weakly-coupled mode multiplexer/demultiplexer (MUX/DEMUX). We also present the up-to-date experimental results for weakly-coupled MDM transmission over conventional OM3 MMF. We show that weakly-coupled MDM scheme is promising for high-speed optical interconnections and bandwidth upgrade of already-deployed MMF links.

High-Speed Bi-Directional Transmission Over Multimode Fiber Link in IM/DD Systems

We demonstrate a 14.5-Tb/s bi-directional transmission over 2.2 km of OM2 fiber using the selective excitation of four mode groups based on multiplane light conversion technology, wavelength division multiplexing, and direct detection. Forty wavelengths per mode group are used; each wavelength is modulated with a discrete multitone scheme with a fast Fourier transform length of 1024 subcarriers. An 88-Gs/s digital-to-analog converter is used to drive an external lithium niobate Mach-Zehnder modulator. The impact of each component in the line is studied experimentally. Moreover, Chow's rate-adaptive algorithm for bit and power allocation is optimized and adapted to the region where the bit error rate (BER) increases. In this paper, we are considering a hard-decision forward error correction (FEC) limit corresponding to a BER of 5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> and assume a 7% overhead. To the best of our knowledge, the achieved bitrate in this paper is the highest throughput transmitted over multimode fibers using direct detection.

Staggered CAP—A New Spectrally Efficient Modulation Format for Optical Communications

In this letter, staggered carrierless amplitude phase (sCAP), a new spectrally efficient modulation format, is proposed for optical transmission. By staggering the in-phase and quadrature components of the CAP modulation, simultaneous transmission of two additional pulse amplitude modulation signals is possible: the first one being baseband and the second one lower sideband. The new modulation format has several advantages: 100% bandwidth efficiency, rectangular-shaped spectrum, minimum tap number shaping filters, minimum requirements for analog-to-digital conversion sampling rate, low peak-to-average power ratio, and capability of bit and power loading. The experimental results in vertical cavity surface emitting laser multimode fiber link indicate that the new format by far outperforms regular CAP.

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.

Extended Reach 10 Gb/s Transmission with an Optical I/Q Modulator Using VCSELs over OFDM-Based Multimode Fiber Link

This paper investigates the possibilities of extending the reach of a 10 Gb/s orthogonal frequency division multiplexing- (OFDM-) based multimode fiber (MMF) link with an optical I/Q modulation technique through vertical cavity surface emitting lasers (VCSELs). The proposed I/Q modulation technique comprises two VCSELs in a 90° hybrid combination to produce I/Q modulated OFDM signal. The results show the excellent performance (in terms of BER and Q factor) of the direct-detection optical- (DDO-) OFDM system with the proposed I/Q modulation in comparison to the direct modulation case. The proposed system is able to achieve worst-case BER of about 1.8616 × 10−3 and Q factor of about 10.9949 dB over a 5 km MMF link. The I/Q modulation technique in the DDO-OFDM system has further been investigated for extending the transmission reach of the MMF link using multispan configuration.

Comparative Analysis of the Effect of Distance on Signal Strength using Selected Communication Media Comparative Analysis of the Effect of Distance on Signal Strength using Selected Communication Media

This paper presents an investigation of the effect of distance on communication via optical fibre single mode and multimode structure. It also gives a comparative analysis of transmission over radio wave and fibre optic links. First, data were collected at different distances in the multimode and single mode optic fibre link, recorded and analyzed. Secondly, data was taken and compared to multimode fibre optic and microwave (18GHz) links. It was observed that communication through multimode optic fibre link degraded noticeably with the increase in distance in comparison with the single mode optic fibre link. In comparison with the 18GHhz microwave link, the multimode fibre optic link performed better as a communication medium. This information is especially useful for making critical decisions for the deployment of future communication infrastructure.

Fundamental-mode MMF transmission enabled by mode conversion

Modal dispersion in conventional multi-mode fiber (MMF) will cause serious signal degradation and an effective solution is to restrict the signal transmission in the fundamental mode of MMF. In this paper, unlike previous methods by filtering out higher-order modes, we propose to adopt low-modal-crosstalk mode converters to realize fundamental-mode MMF transmission. We design and fabricate all-fiber mode-selective couplers (MSC), which perform mode conversion between the fundamental mode in single-mode fiber (SMF) and fundamental mode in MMF. The proposed scheme is experimentally compared with center launching method under different MMF links and then its wavelength division multiplexing (WDM) transmission performance is investigated. Experimental results indicate that the proposed mode conversion scheme could achieve better transmission performance and works well for the whole C-band.

Transmission Over SSMF at 850 nm: Bimodal Propagation and Equalization

The combination of 850 nm vertical-cavity surface-emitting laser (VCSEL) with standard single-mode fiber (SSMF) presents an effective and low-cost interface to increase the reach provided by multi-mode fiber links. At 850 nm, SSMF propagates two modes, and in this study, it has been experimentally shown that the different commercially available SSMF's present dissimilar values of differential mode delay. To cope with this unequal behavior of modal dispersion, we propose a scheme based on bidirectional decision feedback equalization (BiDFE) to overcome limited performance of other solutions as mode filtering or classical equalizers. A single span SSMF cabling model, including a measurement-derived statistical characterization of optical connectors, is simulated to evaluate the reach provided by the equalizer attending to both the conditions of the fiber excitation and the characteristics of the VCSEL. A minimum 1.45 km link length at 10 Gb/s is achieved if a linear combining BiDFE (LC-BiDFE) equalizer is included in the receiver, whatever laser launching condition and employing a single-transverse mode VCSEL. If a multi-transverse mode VCSEL is used, the reach provided by LC-BiDFE is slightly reduced but assuring a minimum coverage of 1.15 km.