Fiber Span Research Articles

Transmitter and receiver impairment monitoring using adaptive multi-layer linear and widely linear filter coefficients controlled by stochastic gradient descent.

We propose a monitoring method for individual impairments in a transmitter (Tx) and receiver (Rx) by using filter coefficients of multi-layer strictly linear (SL) and widely linear (WL) filters to compensate for relevant impairments where the filter coefficients are adaptively controlled by stochastic gradient descent with back propagation from the last layer outputs. Considering the order of impairments occurring in a Tx or Rx of coherent optical transmission systems and their non-commutativity, we derive a model relating in-phase (I) and quadrature (Q) skew, IQ gain imbalance, and IQ phase deviation in a Tx or Rx to the WL filter responses in our multi-layer filter architecture. We evaluated the proposed method through simulations using polarization-division multiplexed (PDM)-quadrature phase shift keying and a transmission experiment of 32-Gbaud PDM 64-quadrature amplitude modulation over a 100-km single-mode fiber span. The results indicate that both Tx and Rx impairments could be individually monitored by using the filter coefficients of adaptively controlled multi-layer SL and WL filters precisely and simultaneously, decoupled by chromatic dispersion and frequency offset, even when multiple impairments existed.

Demonstration of a fiber optical communication system employing a silica microsphere-based OFC source.

The fabrication of microsphere resonators and the generation of optical frequency combs (OFC) have achieved a significant breakthrough in the past decade. Despite these advances, no studies have reported the experimental implementation and demonstration of silica microsphere OFCs for data transmission. In this work, to the best of our knowledge, we experimentally for the first time present a designed silica microsphere whispering-gallery-mode microresonator (WGMR) OFC as a C-band light source where 400 GHz spaced carriers provide data transmission of up to 10 Gbps NRZ-OOK modulated signals over the standard ITU-T G.652 telecom fiber span of 20 km in length. A proof-of-concept experiment is performed with two newly generated carriers (from 7-carrier OFC) having the highest peak power. The experimental realization is also strengthened by the modeling and simulations of the proposed system showing a strong match of the results. The demonstrated setup serves as a platform for the future experimental implementation of silica microsphere WGMR-OFC in more complex WDM transmission system realizations with advanced modulation schemes.

Revisiting the nonlinear Gaussian noise model for hybrid fiber spans

We rederive from first principles and generalize the theoretical framework of the nonlinear Gaussian noise model to the case of coherent optical systems with multiple fiber types per span and ideal Nyquist spectra. We focus on the accurate numerical evaluation of the integral for the nonlinear noise variance for hybrid fiber spans. This task consists in addressing four computational aspects: (1) Adopting a novel transformation of variables (other than using hyperbolic coordinates) that changes the integrand to a more appropriate form for numerical quadrature; (2) Evaluating analytically the integral at its lower limit, where the integrand presents a singularity; (3) Dividing the interval of integration into subintervals of size π and approximating the integral over each subinterval by using various algorithms; and (4) Deriving an upper bound for the relative error when the interval of integration is truncated in orderto accelerate computation. We apply the proposed analytical model to the performance evaluation of coherent optical communications systems with hybrid fiber spans composed of quasi-single-mode and single-mode fiber segments. More specifically, the model is used to optimize the lengths of the optical fiber segments that compose each span in order to maximize the system performance. We check the validity of the optimal fiber segment lengths per span provided by the analytical model by using Monte Carlo simulation, where the Manakov equation is solved numerically using the split-step Fourier method. We show that the analytical model predicts the lengths of the optical fiber segments per span with satisfactory accuracy so that the system performance, in terms of the Q-factor, is within 0.1 dB from the maximum given by Monte Carlo simulation.

Ethernet fronthaul transport of an upper-physical layer functional split using time-aware shaping

An Upper Physical layer functional Split (UPS) for an Ethernet fronthaul network is modelled, and the frame delay and Frame Delay Variation (FDV) limitations are investigated. The results show that contention in Ethernet switch ports can cause an increase in the delay and FDV beyond proposed specifications for the UPS and other time-sensitive traffic types such as I/Q-type traffic. Time Aware Shaping (TAS) can significantly reduce or even remove FDV for UPS traffic and I/Q-type traffic, but it is shown that TAS design aspects have to carefully consider the transmission pattern of the contending traffic in the Ethernet fronthaul network switches. Taking into account the transmission pattern of the UPS traffic, different time allocations within TAS window sections are proposed in conjunction with both receiver and transmitterside buffering. Further, it is proven that using TAS with higher link rates for example, 10 Gbps link rates or beyond makes it possible to transport the UPS and time sensitive traffic within its specification over fronthaul fiber spans, more than 10 km length, and/or more hops as TAS can potentially eliminate any increase in the FDV of the UPS traffic.

Assessment on the in-field lightpath QoT computation including connector loss uncertainties

Reliable and conservative computation of the quality of transmission (QoT) of transparent lightpaths (LPs) is a crucial need for software-defined control and management of the wavelength division multiplexing optical transport. The LP QoT is summarized by the generalized SNR (GSNR) that can be computed by a QoT estimator (QoT-E). Within the context of network automation, the QoT-E must rely only on data from the network controller or provided by network elements through common control protocols and data structures. Therefore, given the theoretical accuracy of the QoT-E, the in-field accuracy in the GSNR computation is also determined by the level of knowledge of input parameters. Among these, a fundamental value is the connector loss at the input of each fiber span, which defines the actual power levels triggering the nonlinear effects in the fiber, and so defining the amount of nonlinear interference and spectra tilt due to the stimulated Raman scattering introduced by the fiber span. This value cannot be easily measured and may vary in time because of equipment update or maintenance. In this paper, we consider a lab measurement campaign in which the GSNR has been computed by means of the open source project Gaussian noise model in Python (GNPy) and analyze the computation error distribution. We show how the assumption on the value for the connector loss modifies the GSNR computation error and how the GSNR computation is more conservative while accurate at the lower values for the connector loss. Using the outcome of the measurement campaign carried out in the laboratory, we present results on the error of GSNR computation in a production network, specifically, over two paths of the Microsoft core network. Using GNPy with the assumption of a connector loss of 0.25 dB as derived from the measurement campaign carried out in the laboratory, and using the physical layer description from the network controller, we show that GNPy is not conservative by overestimating the GSNR in only 5% of cases, while in conservative predictions, the underestimation error exceeds 1 dB only for a few outliers.

Managing Self-Phase Modulation in Pseudo-Linear Multimodal and Monomodal Systems

We propose a new semi-analytical model, describing the bandwidth evolution of pulses propagating in dispersion managed (DM) transmission systems using multimodal graded-index fibers (GRIN) with parabolic index. The model also applies to monomodal fiber DM systems, representing the limit case where beam self-imaging vanishes. The model is successfully compared with the direct integration of the (1+1)D nonlinear Schr\"odinger equation for parabolic GRIN fibers, and to experimental results performed by using the transmission of femtosecond pulses over a 5 m span of GRIN fiber. At the high pulse powers that are possible in multimodal fibers, the pulse bandwidth variations produced by the interplay of cumulated dispersion and self-phase modulation can become the most detrimental effect, if not properly managed. The analytical model, numerical and experimental results all point to the existence of an optimal amount of chromatic dispersion, that must be provided to the input pulse, for obtaining a periodic evolution of its bandwidth. Results are promising for the generation of spatio-temporal DM solitons in parabolic GRIN fibers, where the stable, periodic time-bandwidth behaviour that was already observed in monomodal systems is added to the characteristic spatial beam self-imaging.

Forward Transmission Based Ultra-Long Distributed Vibration Sensing With Wide Frequency Response

A novel ultra-long distributed vibration sensor using forward transmission, coherent detection, and a frequency-shifted optical delay line is proposed and experimentally demonstrated. In the proposed scheme, a pair of multi-span optical fibers are deployed for sensing. A frequency-shifted optical delay line consisting of an acousto-optic modulator and a time delay fiber is used at the far end of these two fibers. Coherent detection is used to retrieve the vibration-induced phase fluctuations of the base-band signals as well as the intermediate frequency signals generated by the frequency-shifted optical delay line. Two differential phase signals can be calculated from the obtained phase fluctuations, which can then be used to localize the vibration events by correlation operations. Localization of a few hundred Hz, around 1 kHz and tens of kHz vibrations has been experimentally demonstrated over a total length of 1230 km sensing fiber. Less than 125 m spatial resolution can be obtained over the 615 km sensing range for vibrations with larger than 1 kHz frequency by using averaging of 30 times tests due to the nature of asynchronous operation. Frequencies from infrasound to ultrasound can be detected. The proposed scheme has advantages of ultrabroad frequency response, ultra-long sensing range and simple sensing structure due to the nature of forward transmission and coherent detection. The sensing length can be further extended using more fiber spans and erbium-doped fiber amplifiers, making it a promising candidate for vibration event detection and localization in long-haul and ultra-long-haul fiber links.

High Capacity Converged Passive Optical Network and RoF-Based 5G+ Fronthaul Using 4-PAM and NOMA-CAP Signals

Network architectural changes to satisfy all the 5G+ mobile network specifications and requirements are necessary due to the popularization of streaming and cloud applications on omnipresent portable devices. The combination of massive installation of micro-cell antenna sites with the cloud access radio network (C-RAN) architecture has recently been nominated as a promising technology for high-capacity mobile fronthaul links, albeit at a high cost. An alternative approach for next-generation fronthaul networks is to utilize the already deployed passive optical networks (PONs) where wireless and wired services may coexist in a converged manner. Non-orthogonal multiple access (NOMA) modulation with multi-band carrierless amplitude and phase modulation (NOMA-CAP) has recently been investigated as a promising 5G+ modulation format candidate to increase the capacity and flexibility of future mobile networks. Here, we experimentally demonstrate the convergence of a NOMA-CAP wireless waveform with a single-carrier wired signal in a PON scenario using radio-over-fiber (RoF) technology. Specifically, fifteen NOMA-CAP bands, with two NOMA power levels to double the capacity, transmit 15 Gb/s multiplexed with a digital 10 Gb/s four-level pulse amplitude modulation (PAM-4) signal for downlink application. Two converged system implementations have been considered, first using electrical frequency division multiplexing (EFDM) and secondly using the hybrid EFDM-wavelength division multiplexing (EFDM-WDM). Successful transmission through a 25 km span of standard single-mode fiber is achieved with negligible transmission penalty for both proposed converged solutions.

Symmetric-type dispersion maps in dispersion-managed optical link with mid-span spectral inversion

In long-haul optical communication systems consisting of single-mode fiber spans and fiber amplifiers, such as an erbium-doped fiber amplifier, signal distortion causes performance to deteriorate because of group velocity dispersion and fiber nonlinearity. A combination of dispersion management and optical phase conjugation is an effective technique of compensating for the distortion. In an optical link configured with this combination, the dispersion map mainly affects the compensation for the distorted optical signals. Improvements in system performance have been reported for various types of dispersion maps. In this study, a symmetric type of dispersion map with respect to the midway optical phase conjugator is proposed. The effect of the proposed dispersion maps on the compensation for the distorted 24 channel × 40 Gbps wavelength-division-multiplexed signals was assessed through numerical simulation. It was confirmed that antipodal-type dispersion maps are most appropriate for the compensation, as well as for the flexibility of the link configuration.

Temperature monitoring of tumor hyperthermal treatments with optical fibers: comparison of distributed and quasi-distributed techniques

Abstract Optical fiber based sensors capable of measuring temperature distribution over a given length are particularly attractive in the biomedical field, especially in the case of real-time monitoring of minimally invasive hyperthermal treatments of tumors, such as laser ablation, given their intrinsic multiplexing along a single fiber span and the unique properties of optical fibers in terms of flexibility, size and electromagnetic compatibility. The paper compares two sensing approaches: one is based on an array of Bragg gratings inscribed in the core of a single-mode fiber, the other detects Rayleigh backscattering from an unmodified single-mode fiber using coherent optical frequency domain reflectometry. Following an introduction, which describes the most distinctive features of the two approaches, the paper presents three comparative experiments, each devised to highlight a peculiar aspect of the sensors. In the first experiment the sensors are exposed to various constant temperature distributions to evaluate the uniformity of the readings along the fiber length. In the second experiment, a known temperature profile is generated through an ad hoc setup to evaluate the actual capability of the two approaches to reconstruct the temperature distribution. Finally, in the third experiment the two sensors are employed in a simulated tumor laser ablation procedure to measure the temperature distribution in the irradiated area. Both sensors provide results whose error, mainly due to cross sensitivity between temperature and strain, is acceptable for the considered biomedical application.

Highly coherent multi-octave polarization-maintained supercontinuum generation solely based on ZBLAN fibers.

We present a highly stable polarization-maintained supercontinuum (SC) using a setup solely based on ZBLAN (ZrF4-BaF2-LaF3-AlF3-NaF) fibers. The pumping source consists of a femtosecond master oscillator fiber amplifier based on thulium-doped ZBLAN fibers. It provides multi-watts of output power with the center wavelength of 1920 nm at 1 MHz repetition rate. The SC generated by pumping an elliptical core passive single-mode ZBLAN fiber spans from 350 nm to 4.5 µm and exhibits high stability. We characterized the SC pulse using sum-frequency cross-correlation frequency resolved optical gating.

Transmission method of improved fiber nonlinearity tolerance for probabilistic amplitude shaping.

For probabilistic amplitude shaping (PAS), we propose a super-symbol transmission method that improves fiber nonlinearity tolerance. A simple fiber nonlinearity low-pass filtering model, as well as its interaction with the spectral dip of signal's intensity waveform, is provided to explain the origin of this nonlinear benefit. With 25-GHz-spaced, 26 × 22.5 GBaud dual-polarized PAS-64 quadrature amplitude modulation (QAM) signals transmitted over 12 spans of 80-km standard single mode fiber (SSMF), the proposed method is found to provide ∼0.15-dB gain over the previous finite-blocklength method with intra-DM pairing, ∼0.26-dB gain over finite-blocklength method with inter-DM pairing, and ∼0.44-dB benefit over the traditional method, all with a feasible blocklength at 200.

Adaptive equalization of transmitter and receiver IQ skew by multi-layer linear and widely linear filters with deep unfolding.

We propose a multi-layer cascaded filter architecture consisting of differently sized strictly linear (SL) and widely linear (WL) filters to compensate for the relevant linear impairments in optical fiber communications including in-phase/quadrature (IQ) skew in both transmitter and receiver by using deep unfolding. To control the filter coefficients adaptively, we adopt a gradient calculation with back propagation from machine learning with neural networks to minimize the magnitude of deviation of the filter outputs of the last layer from the desired state in a stochastic gradient descent (SGD) manner. We derive a filter coefficient update algorithm for multi-layer SL and WL multi-input multi-output finite-impulse response filters. The results of a transmission experiment on 32-Gbaud polarization-division multiplexed 64-quadrature amplitude modulation over a 100-km single-mode fiber span showed that the proposed multi-layer SL and WL filters with SGD control could compensate for IQ skew in both transmitter and receiver under the accumulation of chromatic dispersion, polarization rotation, and frequency offset of a local oscillator laser source.

High security OFDM-PON with a physical layer encryption based on 4D-hyperchaos and dimension coordination optimization.

In this article we have enhanced the security of an orthogonal frequency division multiplexed passive optical network (OFDM-PON) based on four dimensional (4D) encryption, including constellation, subcarrier, symbol and time, which is proposed for the first time in this paper. 4D-hyperchaotic mapping is used to generate four masking factors to achieve ultra-high security encryption in four different dimensions. During the encryption, dimension coordination optimization is adopted, which effectively reduces the time cost of the system and improves the encryption efficiency by 3 times. At the same time, probabilistic shaping (PS) technology is used to further optimize the system that has effectively improved the bit error performance by about 1 dB. The proposed encryption technique for OFDM-PON has been demonstrated successfully with the help of experiments. The generated OFDM signal is modulated by the quadrature amplitude modulation (QAM) technique, which transmitted 16 Gb/s data rate across a 25 km fiber span of standard single-mode fiber. The values of bit error rate (BER) and peak-to-average-power ratio (PAPR) are analyzed during the experiments, and the obtained results show that the proposed security-enhanced OFDM-PON has high sensitivity and security and can be well compatible with PS and OFDM technologies. The proposed scheme has very reliable security performance and also has excellent benefit improvement, which is very promising in the future PS-OFDM-PON.

Mode-division multiplexed transmission of wavelength-division multiplexing signals over a 100-km single-span orbital angular momentum fiber

We experimentally demonstrate mode-division multiplexed (MDM) transmission using eight orbital angular momentum (OAM) modes over a single span of 100-km low-attenuation and low-crosstalk ring-core fiber (RCF). Each OAM mode channel carries 10 wavelength-division multiplexing (WDM) signal channels in the C band, with each WDM channel in turn transmitting 16-GBaud quadrature phase-shift keying signal. An aggregate capacity of 2.56 Tbit/s and an overall spectral efficiency of 10.24 bit/(s · Hz) are realized. The capacity-distance product of 256 (Tbit/s) · km is the largest reported so far for OAM fiber communications systems to the best of our knowledge. Exploiting the low crosstalk between the OAM mode groups in the RCF, the scheme only requires the use of modular 4 × 4 multiple-input multiple-output processing, and it can therefore be scaled up in the number of MDM channels without increasing the complexity of signal processing.

Trends in Optical Span Loss Detected Using the Time Series Decomposition Method

Optical fiber cables deployed in the last 30 years have been considered to be long-lived components due to the generally high level of reliability of glass and the robustness of cable construction. Like all other components however, optical cables can change with age as a result of their construction or deployment characteristics as well as operational and environmental stress factors. In this article, we apply a time-series decomposition method on span loss data collected during 12 months in four bidirectional spans of a production network in order to detect long-term degradation of the fiber plant. After extracting the trend component, a Mann–Kendall test is applied to the span loss curves and a Sen's slope estimator test is used for determining the magnitude of span loss change. The method allows detecting a 1.3% increase in span loss over the 12-month period of observation in one of the fiber spans. This trend is confirmed using a linear regression model computed according to Theil Sen method applied to additional one-year data. The proposed method can allow the early detection of fiber plant degradation and the proactive planning of fiber replacement.

Dual Polarization Full-Field Signal Waveform Reconstruction Using Intensity Only Measurements for Coherent Communications

Conventional optical coherent receivers capture the full electrical field, including amplitude and phase, of a signal waveform by measuring its interference against a stable continuous-wave local oscillator (LO). In optical coherent communications, powerful digital signal processing (DSP) techniques operating on the full electrical field can effectively undo transmission impairments such as chromatic dispersion (CD), and polarization mode dispersion (PMD). Simpler direct detection techniques do not have access to the full electrical field and therefore lack the ability to compensate for these impairments. We present a full-field measurement technique using only direct detection that does not require any beating with a strong carrier LO. Rather, phase retrieval algorithms based on alternating projections that makes use of dispersive elements are discussed, allowing to recover the optical phase from intensity-only measurements. In this demonstration, the phase retrieval algorithm is a modified Gerchberg Saxton (GS) algorithm that achieves a simulated optical signal-to-noise ratio (OSNR) penalty of less than 4dB compared to theory at a bit-error rate of 2 times 10-2. Based on the proposed phase retrieval scheme, we experimentally demonstrate signal detection and subsequent standard 2x2 multiple-input-multiple-output (MIMO) equalization of a polarization-multiplexed 30-Gbaud QPSK transmitted over a 520-km standard single-mode fiber (SMF) span.

Physical layer impairments aware routing and spectrum allocation algorithm for transparent flexible-grid optical networks

In this paper, we discuss physical layer impairments (PLIs) aware routing and spectrum allocation (RSA) algorithm for the transparent flexible-grid optical networks. The flexible-grid optical networks have the capability of delivering data with multi-rate, multi-reach, high spectrum efficiency, high flexibility, and low bit error rates (BER). However, in transparent flexible-grid optical networks, as the transmission distance increases, optical signals are subject to traverse over many bandwidth-variable wavelength cross connects (BV-WXCs) and multiple fiber spans. As a result, the PLIs get accumulated and are added to the optical signal. These accumulated impairments degrade the signal quality to an unacceptable level at the receiver. Hence, the quality of transmission (QoT) falls below the acceptable threshold value and making it hard for the receiver to detect the signal properly. This problem has not received enough attention in the literature yet. Therefore, our objective is to develop an impairments aware RSA algorithm, which establishes the QoT satisfied flexpath based on the available resources and the quality of the signal available at the receiver. We formulate the PLIs-aware RSA problem as an integer linear program (ILP) that provides an optimal solution. We propose a PLIs-aware heuristic algorithm for large-size networks where ILP is not efficient. Using extensive simulation results, we compute the total blocking probability and the spectrum efficiency. The results show that our algorithm outperforms in terms of blocking probability and spectrum utilization.

Analysis of Disturbance-Induced “Virtual” Perturbations in Chirped Pulse $\phi$ -OTDR

When a disturbance acts on a fiber it induces a change in the local refractive index that influences the fiber backscattering trace. If a chirped pulse φ-OTDR setup is used to interrogate the fiber, this refractive index change appears as a local shift of the received trace, linear to the acting perturbation. However, the refractive index change influences the round trip time of all the backscattering components generated by further fiber sections as well. Due to the high sensitivity of chirped pulse φ-OTDR, the change in the round trip time of the backscattering components, which is usually negligible, may appear as a virtual perturbation in certain conditions. In this letter we derive a mathematical model for the virtual perturbation induced by a disturbance acting on the fiber, when the measurement is performed by a chirped pulse φ-OTDR. We experimentally validate the model by inducing a temperature change on a known span of fiber while monitoring its effects in a further fiber section kept at rest. The experimental results are then analyzed and compared with the theoretical ones.

Recent Advances in 100+nm Ultra-Wideband Fiber-Optic Transmission Systems Using Semiconductor Optical Amplifiers

We report on the use of semiconductor optical amplifiers (SOAs) to extend the optical bandwidth of next generation optical systems to 100 nm and beyond. After discussing the technological progress and the motivation for rekindling the interest in SOAs for line amplification, we describe the innovative approach developed for the realization of ultra-wideband (UWB) SOAs. Leveraging custom design of singly polarized SOAs to provide gain over 100+ nm bandwidth, we developed a polarization diversity architecture to realize UWB-SOA modules. Embedded in a compact package, the UWB amplifier modules have been successfully used to demonstrate 100+ Tb/s transmissions. We subsequently review recent experimental transmission results based on such novel 100+ nm wide semiconductor optical amplifiers, including our first demonstration of 100+Tb/s transmission over 100 km distance, our field trial using real-time traffic, and finally the transmission of 107 Tb/s throughput over three spans of standard single mode fiber (SSMF) using hybrid UWB Raman/SOA amplification technique.