Multimode Fiber Optic Link Research Articles

113Gbps rainbow visible light laser communication system based on 10λ laser WDM emitting module in fiber-free space-fiber link.

With the advent of the sixth-generation mobile communication standard (6 G), the visible light communication (VLC) technology based on wavelength division multiplexing (WDM) technology can effectively solve the problem of shortage of spectrum resources and insufficient channel capacity. This paper introduces one of our technical achievements, namely the construction of a near-real-time visible light laser communication (VLLC) system based on WDM, which includes a self-designed 10-λ fully-packaged visible light laser emission module, 1 m multimode fiber - 0.175 m free space - 1 m multimode fiber optical transmission link, and receiver array. In the transmitter system, we adopt adaptive discrete multitone (DMT) modulation technique combined with Quadrature Amplitude Modulation (QAM) modulation scheme to obtain maximum spectral efficiency (SE). In the receiving system, we utilize the sparse-structured reservoir computing post-equalization algorithm to achieve superior equalization performance on the basis of the traditional post-equalization algorithm. The experimental results indicate that this quasi-real-time communication system has achieved a signal transmission rate of 113.175Gbps. To the best of our knowledge, this work has set a record in the field of high-speed visible light laser communication. Therefore, the laser communication system constructed by this work, with its flexibility in deployment and high-speed performance, demonstrates the significant potential application of visible light laser communication in data center interconnection and high-speed indoor access networks.

Augmented space signal detection method for IM/DD optical communication systems

The demand for high-speed data transmission has increased rapidly in data centers, leading to advanced optical communication technology innovation. At high modulation speed, severe linear and nonlinear signal distortions degrade the link performance. In order to increase the transmission capacity, a robust signal detection method based on quadratic discrimination in augmented space (QDAS) is proposed for high-speed intensity modulation and direct detection (IM/DD) optical communication systems. By projecting along the feature vector initially obtained by the objective function following the generalized Rayleigh quotient, signals with different features can be separated in augmented space. Subsequently, decision optimization is achieved through a quadratic discriminant model. Experimental verification is performed based on the 120 Gbps PAM8-modulated vertical-cavity surface-emitting laser (VCSEL) multimode fiber (MMF) optical link (100 m). The results show that, compared with the traditional Volterra nonlinear equalizer (VNLE), the proposed QDAS detection method shows improved and robust bit error ratio (BER) performance with the same complexity.

Deep belief network-hidden Markov model based nonlinear equalizer for VCSEL based optical interconnect

The data center has developed rapidly over the past few years, leading to the demand for high speed data transmission. Vertical cavity surface emitting lasers (VCSELs) based optical interconnect is evolving to 100 Gb/s and beyond, which makes nonlinear distortions difficult to be compensated or equalized by conventional equalizers. Moreover, the challenge becomes very complicated for conventional equalizers because of the presence of inter-symbol interference (ISI) together with the nonlinear distortions. So many neural network based DSP algorithms such as artificial neural network (ANN) have been proposed to mitigate the distortions. However, ANN has the limitations that sample’s relevant information is not considered, leading to degradation in ANN’s performance and high computational complexity. In this paper, in order to maintain an excellent capability of mitigating nonlinear distortions like other neural network based equalizers while considering the sample’s relevant information to reduce the computational complexity, we propose a deep belief network-hidden Markov model (DBN-HMM) based nonlinear equalizer which is tested in a PAM-4 modulated VCSEL and multimode fiber (MMF) optical interconnect link experimentally. The BER performance can be greatly improved compared with conventional DSP algorithms. In addition, the computational complexity of DBN-HMM based equalizer can be about 41% lower than that of ANN based method with a similar BER performance.

Design of Silica Multimode Optical Fibers with Extremely Enlarged Core Diameter for Laser-Based Multi-Gigabit Short-Range Optical Networks

This work presents an alternative fast and simple method for the design of a refractive index profile of silica multimode optical fibers (MMFs) with extremely enlarged core diameters of up to 100 µm for laser-based multi-gigabit short-range optical networks. We demonstrate some results of 100 µm core MMF graded index profile optimization performed by a proposed solution, which provides a selected mode staff differential mode delay (DMD) reduction over the “O”-band under particular launching conditions. Earlier on, a developed alternative model for a piecewise regular multimode fiber optic link operating in a few-mode regime for the computation of laser-excited optical pulse dynamics during its propagation over an irregular silica graded-index MMF with an extremely large core diameter, is utilized to estimate the potentiality of fiber optic links with the described MMFs. Here, we also present the comparison results of the simulation of 10GBase-LX optical signal transmission over 100 µm core MMFs with conventional and optimized graded-index refractive index profiles.

Nonlinear Distortion Mitigation by Machine Learning of SVM Classification for PAM-4 and PAM-8 Modulated Optical Interconnection

We demonstrated a support vector machine (SVM) based machine learning method to mitigate modulation nonlinearity distortion for PAM-4 and PAM-8 vertical cavity surface emitter laser multi-mode fiber (VCSEL-MMF) optical link. Simulations at 100 Gb/s data rate and experimental work at 60 Gb/s data rate were carried out. We achieved a significant improvement in bit error rate (BER) when complete binary tree SVMs (CBT-SVMs) are applied for both PAM-4 and PAM-8 signals. Quantitative analysis of the sensitivity gain versus modulation nonlinearity distortion is presented with experimentally verification. The results indicate that CBT-SVMs have better performance for PAM-8 compared to PAM-4. The sensitivity gain increases almost linearly with the increase of eye-linearity (increase of modulation nonlinearity distortion). Up to 2.5-dB sensitivity improvement is achieved by the proposed CBT-SVMs at eye-linearity of 1.72 for PAM-4.

Vector Intensity-Modulation and Channel State Feedback for Multimode Fiber Optic Links

Multimode fibers (MMF) are generally used in short and medium haul optical networks owing to the availability of low cost devices and inexpensive packaging solutions. However, the performance of conventional multimode fibers is limited primarily by the presence of high modal dispersion owing to large core diameters. While electronic dispersion compensation methods improve the bandwidth-distance product of MMFs, they do not utilize the fundamental diversity present in the different modes of the multimode fiber. In this paper, we draw from developments in wireless communication theory and signal processing to motivate the use of vector intensity modulation and signal processing to enable high-data rates over MMFs. Further, we discuss the implementation of a closed-loop system with limited channel state feedback to enable the use of precoding at the transmitter, and show that this technique enhances the performance in a 10 Gb/s MMF link, consisting of 3 km of conventional multimode fiber. Experimental results indicate that vector intensity modulation and direct detection with with two modulators and detectors, along with the use of limited feedback results in a 50% increase over the single laser and detector case.

End-to-End Multicore Multimode Fiber Optic Link Operating up to 120 Gb/s

A full multicore fiber optic link is demonstrated, transmitting greater than 100 Gb/s through a single strand of multimode fiber for the first time. The fiber, which consists of seven graded-index multimode cores, is used to transmit up to 120 Gb/s over 100 m using a custom multicore-fiber interfacing transmitter and receiver. 2-D arrays of vertical-cavity surface-emitting lasers (VCSELs) and vertically illuminated photodiodes (PDs) are fabricated with a geometry corresponding to the outer six cores of the seven-core fiber, which is arranged in a hexagonal pattern. Both flip-chip and wire-bonding technologies are used to package the VCSEL and PD chips with multichannel transmitter and receiver integrated circuits. Amplitude and timing margins of the end-to-end signals are analyzed through bit-error-rate (BER) measurements. The effects of electrical and optical crosstalk are shown to result in negligible degradation to the BER performance.

Self-Consistent Simulation of Opto-Electronic Circuits Using a Modified Nodal Analysis Formulation

This paper addresses the need for self-consistent simulation of mixed electrical and optical circuits and systems. Drawing on the use of modified nodal analysis (MNA) techniques ubiquitous in circuit simulation, an optical node is formulated which includes the magnitude and phase of the optical signal being simulated. This node consists of two propagating complex envelopes one for the forward direction and the other for the reverse direction. Using this formulation models are developed for a variety of devices including: lasers, photodiodes, multimode fiber, and optical connectors. The formulation allows for definition of multiple optical channels at different carrier frequencies, enables quick simulation of systems with large optical delays and optical interference effects. Several numerical examples are presented in this paper to illustrate the capability of the proposed framework and where practicable the results were compared to commercial simulators. These examples include a multimode fiber optical link, an integrated array of laser sources and a feedback controlled laser source used in a optical link with modulation achieved by the use of an electro-absorption device.

Analysis of the harmonic and intermodulation distortion in a multimode fiber optic link

We present an analytical evaluation of the harmonic and the intermodulation distortion effects produced in the transmission of an analog signal composed of various RF tones through a multimode fiber link. It is based on the electric field propagation model previously published by the authors. Results show the possibility of implementing subcarrier multiplexing techniques (SCM) with Composite Second Order (CSO) values below -50 dBc over short and middle reach multimode fiber links.

Transfer function of radio over fiber multimode fiber optic links considering third-order dispersion

Although a considerable number of multimode fiber (MMF) links operate in a wavelength region around 850 nm where chromatic dispersion of a given modal group mu is described adequately by the second derivative beta(mu) (2) of the propagation constant beta(mu)(omega), there is also an increasing interest in MMF links transmitting in the second spectral window (@1300nm) where this second derivative vanishes being thus necessary to consider the third derivative beta(mu) (3) in the evaluation of the transfer function of the multimode fiber link. We present in this paper, for the first time to our knowledge, an analytical model for the transfer function of a multimode fiber (MMF) optic link taken into account the impact of third-order dispersion. The model extends the operation of a previously reported one for second-order dispersion. Our results show that the performance of broadband radio over fiber transmission through middle-reach distances can be improved by working at the minimum-dispersion wavelength as long as low-linewidth lasers are employed.

Integrated transversal equalizers in high-speed fiber-optic systems

Intersymbol interference (ISI) caused by intermodal dispersion in multimode fibers is the major limiting factor in the achievable data rate or transmission distance in high-speed multimode fiber-optic links for local area networks applications. Compared with optical-domain and other electrical-domain dispersion compensation methods, equalization with transversal filters based on distributed circuit techniques presents a cost-effective and low-power solution. The design of integrated distributed transversal equalizers is described in detail with focus on delay lines and gain stages. This seven-tap distributed transversal equalizer prototype has been implemented in a commercial 0.18-μm SiGe BiCMOS process for 10-Gb/s multimode fiber-optic links. A seven-tap distributed transversal equalizer reduces the ISI of a 10-Gb/s signal after 800 m of 50-μm multimode fiber from 5 to 1.38 dB, and improves the bit-error rate from about 10/sup -5/ to less than 10/sup -12/.

Spatio-spectral mapping of multimode vertical cavity surface emitting lasers

This paper reports the spatial and spectral characteristics of multimode vertical cavity surface emitting laser (VCSEL) emission using near field scanning optical microscopy (NSOM). We have investigated 15 /spl mu/m diameter proton implanted 850 nm devices used in 2 Gb/s multimode fiber optic links. We have studied their near field spatial distribution of intensity and the lasing wavelengths of their transverse modes. False colored images were created to portray relative intensity, and spatial distribution information for each transverse mode. Correlation with shear force data allowed mapping of the optical distributions to topographical features of the device. Lasing filaments were observed at high drive currents. Spatially overlapping transverse modes were observed to compete for available gain while spatially isolated modes coexisted.

A highly sensitive optical detector for use in deep underwater

We have developed an optical detector module for use in deep underwater experiments that will search for high-energy neutrinos from cosmic rays and astronomical sources. This module is sensitive to single photons, is operable under high pressure, functions automatically and is remotely controlled. The optical module is based on a 15 in. diameter photomultiplier tube (PMT), with control and read-out electronics mounted inside a 17 in. diameter glass pressure housing. The photomultiplier output signal is converted into two optical pulses by the electronics, one giving the hit time and time-over-threshold of the PMT output pulse and the other giving the charge-integrated PMT signal. Information on the arrival time and intensity of light can be extracted from these pulses. The time jitter of the optical modules is ∼6 and ∼2 ns at FWHM for one photoelectron and ∼100 photoelectrons, respectively. The unit requires 10 W power delivered at 48 V DC on a line with superposed bidirectional control signals. Fast output is uni-directional on a multi-mode fiber optic link to a remote time digitizer. The module has been successfully tested and calibrated in the laboratory, and several have briefly operated in the deep ocean.