Showing papers in "Journal of Lightwave Technology in 2022"

Neuromorphic Silicon Photonics and Hardware-Aware Deep Learning for High-Speed Inference

Abstract: The relentless growth of Artificial Intelligence (AI) workloads has fueled the drive towards non-Von Neuman architectures and custom computing hardware. Neuromorphic photonic engines aspire to synergize the low-power and high-bandwidth credentials of light-based deployments with novel architectures, towards surpassing the computing performance of their electronic counterparts. In this paper, we review recent progress in integrated photonic neuromorphic architectures and analyze the architectural and photonic hardware-based factors that limit their performance. Subsequently, we present our approach towards transforming silicon coherent neuromorphic layouts into high-speed and high-accuracy Deep Learning (DL) engines by combining robust architectures with hardware-aware DL training. Circuit robustness is ensured through a crossbar layout that circumvents insertion loss and fidelity constraints of state-of-the-art linear optical designs. Concurrently, we employ DL training models adapted to the underlying photonic hardware, incorporating noise- and bandwidth-limitations together with the supported activation function directly into Neural Network (NN) training. We validate experimentally the high-speed and high-accuracy advantages of hardware-aware DL models when combined with robust architectures through a SiPho prototype implementing a single column of a 4:4 photonic crossbar. This was utilized as the pen-ultimate hidden layer of a NN, revealing up to 5.93% accuracy improvement at 5GMAC/sec/axon when noise-aware training is enforced and allowing accuracies of 99.15% and 79.8% for the MNIST and CIFAR-10 classification tasks. Channel-aware training was then demonstrated by integrating the frequency response of the photonic hardware in NN training, with its experimental validation with the MNIST dataset revealing an accuracy increase of 12.93% at a record-high rate of 25GMAC/sec/axon.

A Review of Distributed Fiber–Optic Sensing in the Oil and Gas Industry

Abstract: Fiber–optic sensors have been widely deployed in various applications, and their use has gradually increased since the 1980 s. Distributed fiber–optic sensors, which enable continuous and real–time measurements along the entire length of an optical fiber cable, have undergone significant improvements in underlying industries. In the oil and gas industry, distributed fiber–optic sensors can provide significantly valuable information throughout the life cycle of a well and can monitor pipelines transporting hydrocarbons over great distances. Here, we review the deployment of fiber–optic Rayleigh–based distributed acoustic sensing (DAS), Raman–based distributed temperature sensing (DTS), and Brillouin–based distributed temperature and strain sensing (DTSS) in the oil and gas industry. In particular, we describe the operation principle and basic experimental setups of the DAS, DTS, and DTSS, highlighting their applications in the upstream, midstream, and downstream sectors of the oil and gas industry. We further developed a prototype of a fiber–optic hybrid DAS–DTS system that simultaneously measures vibration and temperature along a multimode fiber (MMF). The reported hybrid sensing system was tested in an operational oil well. This work also discusses the challenges that might hinder the growth of the distributed fiber–optic sensing market in the petroleum industry, and we further point out the future directions of related research.

Digital Longitudinal Monitoring of Optical Fiber Communication Link

Abstract: Optical transmission links are generally composed of optical fibers, optical amplifiers, and optical filters. In this paper, we present a channel reconstruction method (CRM) that extracts physical characteristics of multiple link components such as longitudinal fiber losses, chromatic dispersion (CD), multiple amplifiers’ gain spectra, and multiple filters’ responses, only from receiver-side (Rx) digital signal processing (DSP) of data-carrying signals. The concept is to reconstruct a virtual copy of an actual transmission channel in the digital domain, where optical fibers and amplifiers are modeled as the split-step Fourier method for the Manakov equation while optical filters are emulated as complex-valued finite impulse response filters. We estimate the model parameters such as losses, CD, gains, and filter responses from boundary conditions, i.e., transmitted and received signals. Experimental results show that, unlike traditional analog testing devices such as optical time-domain reflectometers and optical spectrum analyzers, CRM visualizes multi-span characteristics of fibers, amplifiers, and filters in Rx DSP, and thus localizes anomaly components among multiple ones without direct measurement.

Bridging the Terahertz Gap: Photonics-Assisted Free-Space Communications From the Submillimeter-Wave to the Mid-Infrared

Abstract: Since about one and half centuries ago, at the dawn of modern communications, the radio and the optics have been two separate electromagnetic spectrum regions to carry data. Differentiated by their generation/detection methods and propagation properties, the two paths have evolved almost independently until today. The optical technologies dominate the long-distance and high-speed terrestrial wireline communications through fiber-optic telecom systems, whereas the radio technologies have mainly dominated the short- to medium-range wireless scenarios. Now, these two separate counterparts are both facing a sign of saturation in their respective roadmap horizons, particularly in the segment of free-space communications. The optical technologies are extending into the mid-wave and long-wave infrared (MWIR and LWIR) regimes to achieve better propagation performance through the dynamic atmospheric channels. Radio technologies strive for higher frequencies like the millimeter-wave (MMW) and sub-terahertz (sub-THz) to gain broader bandwidth. The boundary between the two is becoming blurred and intercrossed. During the past few years, we witnessed technological breakthroughs in free-space transmission supporting very high data rates, many achieved with the assistance of photonics. This paper focuses on such photonics-assisted free-space communication technologies in both the lower and upper sides of the THz gap and provides a detailed review of recent research and development activities on some of the key enabling technologies. Our recent experimental demonstrations of high-speed free-space transmissions in both frequency regions are also presented as examples to show the system requirements for device characteristics and digital signal processing (DSP) performance.

Challenges and Enabling Technologies for Multi-Band WDM Optical Networks

Abstract: The ever increasing capacity demand on optical networks and the slowdown of improving spectral efficiency lead to the solution of utilizing more wavelength band in existing optical fibers. More and more operators will extend from C band to C+L bands, or plan to deploy C+L systems, and in future they may utilize more of other bands. This paper firstly discusses different optical layer architectures for multi-band system. Using optical components that can natively support multi-band is attractive to us, because in this way the multi-band system can be planned and operated similarly with the single-band system, which is customer friendly and potentially more economical. We address mainly three technical challenges in this paper. The first one is the wide-band fiber optical amplifier (OA). After a short review of recent research progress, we propose a hybrid erbium-doped fiber (EDF) and bismuth-doped fiber (BDF) based OA approach, and demonstrated single-stage amplification over 100 nm in extended C+L bands. The second challenge is the optical cross connect based on wavelength selective switch (WSS). It is not trivial to realize a C+L-band WSS while preventing the key optical parameters (filtering bandwidth, loss, channel isolation, etc.) to be degraded. We analyze different technical schemes, and propose an optical design based on an LCoS with a large panel size and a high-dispersion diffraction grating. The optical design and simulation on a 2×35 WSS reveals this approach can support 100-nm extended C+L spectrum and is promising for commercialization. The third challenge is the SRS induced WDM channel power inequality, which is more severe for multi-band system. We propose a solution by using OAs with a novel optical spectrum processor (OSP). Simulation and experiment showed that the power of the WDM channels can be equalized on a per span basis, which can prevent accumulation of stimulated Raman scattering (SRS) induced wavelength division multiplexing (WDM) channel power transfer and can meanwhile keep the optical signal to noise ratio (OSNR) of WDM channels well equalized.

Optical Amplifiers for Multi–Band Optical Transmission Systems

Abstract: Opening new wavelength bands is the most economic step for further increasing the capacity of optical transmission links. Characteristics of different amplifier technologies for signal amplification in different wavelength bands are detailed. In particular, the suitability of these technologies for short–term and mid–term implementation is considered. An important criterion is the availability of qualified components, notably the required pump laser diodes. On this basis, solutions for the near–term and the mid–term are discussed.

Constellation Shaping Chaotic Encryption Scheme With Controllable Statistical Distribution for OFDM-PON

Abstract: In order to improve the signal transmission performance and physical layer security in orthogonal frequency division multiplexing-based passive optical network (OFDM-PON), we propose a constellation shaping chaotic encryption (CSCEn) scheme with controllable statistical distribution. In this scheme, a q uadrature amplitude modulation (QAM) symbol sequence is divided into several sub-sequences, and a further probabilistic shaping (PS) is performed by constellation region replacement in terms of the corresponding statistical information ( SI ). Then the sequence SI is encoded and encrypted into chaotic signal phases by employing our key distribution algorithm. At the receiver, the SI is extracted first to restore the original signal . To verify the effectiveness of this scheme, we successfully demonstrate an encrypted PS-16-QAM signal transmission over a 25-km standard single mode fiber (SSMF). The results show that the proposed scheme can flexibly improve bit error rate (BER) performance with low deployment complexity and provide sufficient security to resist attacks from illegal optical network units ( ONUs), thus it displays a promising application prospect for future secure PON.

Enabling Optical Network Technologies for 5G and Beyond

Abstract: We review a series of innovative optical network technologies for 5G and beyond mobile networks, enabling high-throughput mobile any-haul (x-haul) via wavelength-division multiplexing, bandwidth-efficient mobile front-haul via hybrid digital-analog radio-over-fiber, Shannon-limit-approaching long-haul transmission for core networks via advanced coding and probabilistic constellation shaping, low-latency 50-Gb/s passive optical network for cost-effective x-haul traffic aggregation, and service-enabling optical transport network capable of bandwidth-guaranteed network slicing with fine granularity. The vision and main application scenarios of the 5^th generation fixed network (F5G) are also discussed. With its capability to support enhanced fixed broadband, guaranteed reliable experience, full fiber connection, energy-efficient broadband communication, real-time broadband communication, and harmonized communication and sensing, F5G is well positioned to not only support mobile networks, but also complement them to jointly meet the ever-increasing communication demands in the era of 5G and 6G.

Thermal Modelling of Silicon Photonic Ring Modulator with Substrate Undercut

Abstract: Ring modulators for silicon photonic (SiPho) optical transceivers are extremely thermally sensitive and require thermal tuning for stable operation. This tuning is achieved with integrated heaters and in this work two aspects of the heaters are investigated: firstly, how to thermally model the heater-waveguide system in a SiPho ring modulator. Secondly, how to improve the energy efficiency by adapting the design in order to minimize the thermal tuning power consumed by the heater. Silicon substrate undercut (UCUT) is done in order to achieve out-of-plane thermal isolation and prevent heat flow directly into the Si substrate below the device. Introducing the UCUT increases the heater efficiency ($\eta _{h}$) with a factor X3.08. It is shown that the influence of the UCUT cross section on the heater efficiency can be neglected, only the total area and metals significantly influence the thermal behaviour. Comparison between electro-thermal and pure thermal simulation shows very similar behaviour. Finally, the UCUT causes an increase in thermal time constant with factor X3.68.

Ultrahigh-Net-Bitrate 363 Gbit/s PAM-8 and 279 Gbit/s Polybinary Optical Transmission Using Plasmonic Mach-Zehnder Modulator

Abstract: We summarize our experimental exploration of the capabilities of an ultrabroad-bandwidth plasmonic Mach-Zehnder modulator (MZM), in an intensity modulation and direct detection (IM/DD) system for short-reach optical transmission up to 10 km. We study modulation, transmission, and reception of ultrahigh-symbol-rate (up to 304 GBd) multi-level optical signals with two different signaling schemes: pulse amplitude modulation (PAM), with up to 8 amplitude levels and partial-response-encoded binary (polybinary) modulation with memory length up to 4. By mapping the performance to a concatenated soft-decision (SD) and hard-decision (HD) forward error correction (FEC) coding scheme, a net bitrate of 363.4 Gbit/s is possible with PAM-8 signaling and 279.0 Gbit/s with tetrabinary (polybinary) signaling after 10 km standard single-mode fiber transmission. Considering an HD-only coding scheme, a net bitrate of 318.0 Gbit/s is possible with PAM-6 and 277.1 Gbit/s with tetrabinary.

Coherent Free-Space Optical Communications: Opportunities and Challenges

Abstract: The ever-increasing data rate demand for wireless systems is pushing the physical limits of standalone radio-frequency communications, thus fostering the blooming of novel high-capacity optical wireless solutions. This imminent penetration of optical communication technologies into the wireless domain opens up a set of novel opportunities for the development of a new generation of wireless systems providing unprecedented capacity. Unlocking the full potential of free-space optics (FSO) transmission can only be achieved through a seamless convergence between the optical fiber and optical wireless domains. This will allow taking advantage of the staggering progress that has been made on fiber-based communications during the last decades, namely leveraging on the latest generation of Terabit-capable coherent optical transceivers. On the other hand, the development of these high-capacity optical wireless systems still faces a set of critical challenges, namely regarding the impact of atmospheric turbulence and pointing errors. In this work, we provide an in-depth experimental analysis of the main potentialities and criticalities associated with the development of ultra-high-capacity FSO communications, ultimately leading to the long-term (48-hours) demonstration of a coherent FSO transmission system delivering more than 800 Gbps over $\sim$42 m link length, in an outdoor deployment exposed to time-varying turbulence and meteorological conditions.

An SNR-improved Transmitter of Delta-sigma Modulation Supported Ultra-High-Order QAM Signal for Fronthaul/WiFi Applications

Abstract: In this paper, we proposed an improved mobile fronthaul architecture employing the SNR-improved delta-sigma digitization scheme and 4-level pulse amplitude modulation (PAM-4) format. Different from traditional 2-bit quantization, this scheme deploys two-fold delta-sigma quantization, which uses 1-bit delta-sigma modulator to quantize the signal, and another 1-bit delta-sigma modulator to quantize the in-band noise. A significant reduction of the in-band noise can be achieved only applying a differentiator, bringing about a much better noise shaping performance. Meanwhile, the two 1-bit streams can be combined and transmitted in a PAM-4 manner through an intensity modulation direct detection (IM-DD) channel. We have successfully experimentally demonstrated the digitization and transmission of 65536 quadrature amplitude modulation (QAM) baseband orthogonal frequency division multiplexing (OFDM) signal with sampling rate of 1.25 GSa/s over 20-km fiber with standard 10-Gbaud PAM-4 signal, with a signal to noise ratio (SNR) of 57.7 dB. Compared to the conventional 2-bit quantized OFDM scheme, experimental results show that a significant SNR improvement of 17.7 dB has been achieved by the proposed scheme. In addition, we also realized a transmission of 16384-QAM intermediate frequency (IF) signal at the center frequency of 3.5 GHz (with an SNR of 49.6 dB). An improvement of 15.9 dB can also be maintained.

Transparent Optical-THz-Optical Link at 240/192 Gbit/s Over 5/115 m Enabled by Plasmonics

Abstract: A transparentOptical-subTHz-Optical link providing record-high single line rates of 240 Gbit/s and 192 Gbit/s on a single optical carrier over distances from 5 to 115 m is demonstrated. Besides a direct mapping of the optical to a 230 GHz subTHz-carrier frequency by means of a uni-traveling carrier (UTC) photodiode, we demonstrate direct conversion of data from the subTHz domain back to the optical domain by a plasmonic modulator. It is shown that the subTHz-to-optical upconversion can even be performed at good quality without any electrical amplifiers. Finally, at the receiver, the local oscillator is employed to directly map the optical signal back to the electrical baseband within a coherent receiver.

Underwater and Water-Air Optical Wireless Communication

Abstract: Optical communication has been employed in a wide range of applications, including terrestrial, submarine, inter-satellite, and even space communication. It is particularly successful in fiber-based communication networks that vastly reshape modern life through the Internet. With the intensified activities such as undersea resource exploration, ecosystem monitoring, and recreation, underwater is an exciting new arena for optical wireless communication (OWC). In this paper, we review the recent progress of underwater and water-air OWC systems. Channel characterization, communication system design, and performance investigations are given. Critical limitations and effective mitigation methods to overcome the influence of bubbles and waves are presented. We also address the current issues for proper performance comparison under different wave conditions. With further research in channel modeling, device innovation, and system design optimization, practical and robust underwater and water-air OWC systems can be realized.

Hybrid Fiber-Optic Sensor for Seawater Temperature and Salinity Simultaneous Measurements

Abstract: In this letter, we propose a compact hybrid fiber-optic sensor for seawater temperature and salinity simultaneous measurements. The device consists of a hollow-core fiber (HCF)-based Fabry-Perot interferometer (FPI) and no-core fiber (NCF)-based anti-resonance (AR) structure. A U-shaped groove is inscribed in the HCF by femtosecond laser for liquid inflow and salinity sensing. A section of the polymer coating is retained on the NCF to excite the AR effect for temperature measurement. The incident light is partially reflected at the HCF fusion end-faces and forms the FPI reflection spectrum, while the transmission light in the NCF cladding is coupled to the polymer coating and forms the AR phenomenon. In theoretical modeling, the temperature and salinity responses of the hybrid sensor are analyzed separately, and the corresponding sensitivities are calculated theoretically. In the experiment, the results show that the temperature and salinity sensitivities are -4.948 nm/ and 0.235 nm/, respectively. In addition, the calibration test, time stability, and repeatability of the sensor are also evaluated by the experiments. The construction method of this hybrid sensing structure is flexible and accurate, so it is expected to be applied in the seawater parameter measurements.

Low Complexity Neural Network Equalization Based on Multi-Symbol Output Technique for 200+ Gbps IM/DD Short Reach Optical System

Abstract: Nowadays, Neural network (NN) has been proved to be an effective solution for nonlinear equalization in short reach optical systems. However, recent research has mainly focused on implementing more powerful NNs for equalization, while ignoring their adaptability to equalization tasks. In this paper, we propose Multi-Symbol Output (MSO)-Neural Networks (NN) for nonlinear equalization in high-speed short reach optical interconnects. The results show that the proposed MSO design works well on Deep Neural Networks (DNN), Long Short-Term Memory neural networks (LSTM) and Gate Recurrent Unit (GRU), which are the recent NN-based equalization structures. By increasing output symbols of the NNs, the number of slide windows in equalization can be sharply reduced, and so the complexity is reduced. By the same time, more information is brought to the MSO-NNs in back-propagations, therefore performance gain achieved. A 212-Gb/s 1-km Pulse Amplitude Modulation (PAM)-4 optical link is experimentally demonstrated as the target system, and the proposed MSO-NN equalizers are used to compare with traditional equalization algorithms including Volterra Nonlinear Equalizer (VNE) and single-symbol output NNs. Experimental results show that MSO design could help reduce the complexity of NN required for nonlinear equalization in the target system by around 2/3, and the proposed MSO-LSTM performs much better than VNE and 1 dB better than SSO-LSTM at the same time. Based on the proposed MSO-LSTM, transmission with BER under HD-FEC over 1 km NZDSF is achieved with a ROP at -2 dBm. Our work is well expandable and the proposed MSO design can be extended to other NN-based equalizers, which can help reduce complexity and learn more info from the training data and gain performance. The proposed MSO-NNs further enhance the performance and reduces the complexity of NN equalizers, provides assurance for future real-time high-speed short-range optical systems, and brings new ideas to NN-based equalizer design.

Simultaneous Transmission of 5G MMW and Sub-THz Signals Through a Fiber-FSO-5G NR Converged System

Abstract: Simultaneous transmission of fifth-generation (5G) millimeter-wave (MMW) and sub-THz signals through a fiber-free-space optical (FSO)-5G new radio (NR) converged system is successfully achieved. It is an inventive demonstration that utilizes a cross medium of optical fiber, optical wireless, and RF wireless to enhance the aggregate transmission rate to 60 Gb/s. We transmitted 20 Gb/s in the 50-GHz MMW, 100-GHz sub-THz, and 150-GHz sub-THz bands at the same time through the seamless fiber-FSO-5G NR convergence, comprising 20-km single-mode fiber (SMF), 500-m optical wireless, and 2-m (50-GHz MMW)/1-m (100-GHz sub-THz)/0.5-m (150-GHz sub-THz) 5G wireless transmissions. Sufficiently low bit error rate (< forward error correction criterion of 3.8×10⁻³) and error vector magnitude (

Seismic Monitoring With Distributed Acoustic Sensing From the Near-Surface to the Deep Oceans

Abstract: Distributed acoustic sensing (DAS) delivers real-time observation ofphysical perturbations such as vibrations or strain variations in conventional optical fibers with high sensitivity. The high density of sensing points and large network footprint provided by a single DAS system, along with the availability of a vast optical fiber network already deployed both in land and in oceanic regions, contrast with the high deployment and maintenance cost of conventional instrumentation networks for seismology. This situation has triggered a rapid growth of DAS deployments for seismic monitoring in recent years. Photonic engineers and geophysicists have joined efforts to prove the value of optical fibers as distributed seismometers, which has resulted in a wide panoply of tests demonstrating diverse applicability across the geosciences. For example, DAS has been successfully applied recording local to teleseismic earthquakes, monitoring glacial icequakes, and observing oceanographic phenomena at the sea floor. Most of the realized tests have been performed using commercially available optical fiber interrogators based on phase-sensitive optical time-domain reflectometry. Among them, DAS based on chirped pulse distributed acoustic sensing have provided optimized performance in terms of both range and sensitivity, particularly at low frequencies. In this communication, we provide a comprehensive review of the current situation of DAS for seismology applications, focusing on near surface monitoring, where already deployed optical fibers can be repurposed as sensor networks.

Co-Packaged Photonics For High Performance Computing: Status, Challenges And Opportunities

Abstract: Photonics die or integrated photonics modules co-packaged with compute engines have the potential to deliver significant improvements in power, bandwidth and reach needed to meet the computing and communication demands of data centers and other high-performance computing (HPC) systems. The challenges and solutions in co-packaging photonics modules are described through two case studies; one of a network-switch die co-packaged with socketable photonics modules and another of a Field Programmable Gate Array (FPGA) co-packaged with optical dies (tiles). The technical requirements to deliver the promise of co-packaged photonics in high volume are outlined.

Secret-Key Provisioning With Collaborative Routing in Partially-Trusted-Relay-based Quantum-Key-Distribution-Secured Optical Networks

Abstract: Quantum Key Distribution (QKD) is a promising technology that provides proven unconditional security based on fundamentals of quantum physics, especially for point-to-point communications. It could be applied in large-scale optical networks for long-distance key provisioning mainly by three relaying methods: quantum-repeater-based QKD, trusted-relay-based QKD, and measurement-device-independent QKD (MDI-QKD). However, quantum-repeater-based QKD is still under study because of the immature technologies such as the preliminary quantum memory. The trusted-relay-based QKD is vulnerable since insecurity of non-ideal single-photon sources and detectors cannot be ignored in practical applications. On the other hand, for MDI-QKD, there exists a limitation on its key rate under long-distance communications. According to these limitations, embedding protocols like MDI-QKD into the existing trusted-relay-based QKD secured optical networks (QKD-ON) with usage of untrusted relays is a promising key-provisioning scheme. In this paper, we study such partially-trusted relay scenarios and focus on its routing of keys in different kinds of typical network topologies. A partially-trusted-relay-based QKD method is described, which can allow a pair of optical nodes sharing secret keys under the coexistence of trusted relays and untrusted relays. The secret-key provisioning with collaborative routing (SKP-CR) algorithm is proposed to search for the optimal key-relay routing path. We perform the simulations with different proportion of trusted relays versus untrusted relays, initial secret keys in the quantum key pools (QKPs), and traffic load. The simulations verify that the SKP-CR algorithm can significantly outperform the conventional trusted-relay-based scheme in terms of key-distribution success rate with an improvement of up to 62% with a mesh topology.

Experimental Study on Dual-Parameter Sensing Based on Cascaded Sagnac Interferometers With Two PANDA Fibers

Abstract: A sensitivity-amplifying optical fiber sensor using cascaded Sagnac interferometers (CSIs) was designed and experimentally demonstrated for the dual-parameter measurement of strain and temperature. The proposed CSIs sensor consisted of two Sagnac interferometers (SIs) in which two identical PANDA polarization-maintaining fibers (PMFs) for just slightly different lengths were inserted. An envelope with a period of 123.6 nm, which was amplified more than 10 times than the single SI, was observed in the interference spectrum. Experimental results showed that the strain sensitivity increased from 32.91 pm/μϵ for the single SI to 336 pm/μϵ for the CSIs, while the temperature sensitivity increased from −1.38 nm/°C for the single SI to −14.86 nm/°C for the CSIs, correspondingly. We also found that the envelope of the CSIs shifted in opposite directions when the longer or shorter PMF was selected as the sensing arm. Higher sensitivity could be realized by selecting longer PMF as the sensing arm. Our proposed CSIs sensor possesses the merits of simple structure, high sensitivity, low cost, and low hysteresis effect, which make it a competitive candidate for strain and temperature monitoring.

Deep-Learning-Based Earthquake Detection for Fiber-Optic Distributed Acoustic Sensing

Abstract: In this paper, deep learning models trained with real seismic data are proposed and proven to detect earthquakes in fiber-optic distributed acoustic sensor (DAS) measurements. The proposed neural network architectures cover the three classical deep learning paradigms: fully connected artificial neural networks (FC-ANNs), convolutional neural networks (CNNs) and recurrent neural networks (RNNs). Results demonstrate that training these networks with seismic waveforms measured by traditional broadband seismometers can extract and learn relevant features of earthquakes, enabling the reliable detection of seismic waves in DAS measurements. The intrinsic differences between DAS and seismograph waveforms, and eventual errors in the labelling of the DAS data, slightly reduce the performance of the models when tested with the distributed acoustic measurements. Despites of that, trained models can still reach up to 96.94% accuracy in the case of CNN and 93.86% in the case of CNN+RNN. The method and results here reported could represent an important contribution to the development of an early warning earthquake system based on DAS technology.

Simultaneous Measurement of Temperature and Relative Humidity Using Cascaded C-shaped Fabry-Perot interferometers

Abstract: In this paper, a novel platform for simultaneous measurement of relative humidity (RH) and temperature using dual Fabry–Perot interferometers (FPIs) based on C-shaped fiber was demonstrated. The sensor was composed by splicing two sections of C-shaped fiber between single mode fiber (SMF). Polydimethylsiloxane (PDMS) and polyvinyl alcohol (PVA) are filled in the two sections of C-shaped fibers to increase sensitivity to temperature and RH. This is the first demonstration that solid polymer materials can be added to the C-shaped fiber interferometers for sensing. In our experiment, RH sensitivities of -0.128 nm/%RH and 0.038 nm/%RH in the range of 20%RH to 45%RH, and temperature sensitivities of 0.022 nm/°C and -0.722 nm/°C in the temperature range of 15 °C to 45°C, were acquired for dual FPIs, respectively. We verified that it is possible to use the sensitivity matrix method to measure two parameters simultaneously. In addition, it has the benefits of simple structure, excellent stability and high sensitivity, and has a broad application prospect in agriculture, food processing and environmental measurement.

Convolutional Neural Network-Aided DP-64 QAM Coherent Optical Communication Systems

Abstract: Optical nonlinearity impairments have been a major obstacle for high-speed, long-haul and large-capacity optical transmission. In this paper, we propose a novel convolutional neural network (CNN)-based perturbative nonlinearity compensation approach in which we reconstruct a feature map with two channels that rely on first-order perturbation theory and build a classifier and a regressor as a nonlinear equalizer. We experimentally demonstrate the CNN equalizer in 375 km 120-Gbit/s dual-polarization 64-quadrature-amplitude modulation (64-QAM) coherent optical communication systems. We studied the influence of the dropout value and nonlinear activation function on the convergence of the CNN equalizer. We measured the bit-error-ratio (BER) performance with different launched optical powers. When the channel size is 11, the optimum BER for the CNN classifier is 0.0012 with 1 dBm, and for the CNN regressor, it is 0.0020 with 0 dBm; the BER can be lower than the 7$\%$ hard decision-forward threshold of 0.0038 from −3 dBm to 3 dBm. When the channel size is 15, the BERs at −4 dBm, 4 dBm and 5 dBm can be lower than 0.0020. The network complexity is also analyzed in this paper. Compared with perturbative nonlinearity compensation using a fully connected neural network (2392-64-64), we can verify that the time complexity is reduced by about 25$\%$, while the space complexity is reduced by about 50$\%$.

Vibration Detection and Localization Using Modified Digital Coherent Telecom Transponders

Abstract: We demonstrate a vibration detection and localization scheme based on bidirectional transmission of telecom signals with digital coherent detection at the receivers. Optical phase is extracted from the digital signal processing blocks of the coherent receiver, from which the vibration component is extracted by bandpass filtering, and the position along the cable closest to the vibration's epicenter is recovered by correlation. We demonstrate our scheme first using offline experiment with 200-Gb/s DP-16QAM, and we report field trial results over installed fiber to detect real-world vibration events.

Opportunities and Challenges for Long-Distance Transmission in Hollow-Core Fibres

Abstract: The loss of hollow-core Nested Antiresonant Nodeless Fiber (NANF) has been steadily decreasing lately, approaching that of standard Single-Mode Fiber (SMF). As for non-linear effects, they are already three to four orders of magnitude lower than in SMF. Theoretical predictions and experimental evidence also hint at a much wider usable bandwidth than SMF, potentially several tens of THz. Propagation speed is 50%faster, a key feature in certain contexts. We investigate the potential impact of possible future high-performance NANF on optical communication systems, assuming that NANF continues on its current path towards better performance. We look at system throughput in different scenarios, addressing links from 100 km to 4,000 km, assuming different NANF optical bandwidths and loss. We found that NANF might enable throughput gains, vs. a benchmark SMF Raman-amplified C+L system, on the order of 1.5x to 5x, at reasonable system parameter values, including launch power. We also consider NANF Inter-Modal-Interference (IMI) and show that the value required for negligible system impact is about –60 dB/km, close to the currently best reported values. We finally look at more long-term scenarios in which NANF loss might get below that of SMF and we show that in this context repeaterless or even completely amplifierless systems might be possible, delivering 300400 Tb/s per NANF, over 200 to 300 km distances. While several technological hurdles remain before NANF systems become practical, NANF appears to have the potential to become an attractive and possibly disruptive alternative to conventional solid-core silica fibers.

Spectral Demodulation of Fiber Bragg Grating Sensor Based on Deep Convolutional Neural Networks

Abstract: This paper presents a new method of demodulating the spectrum of fiber Bragg grating (FBG) based sensors by employing deep convolutional neural networks (DCNN). As a proof of demonstration, FBG-based temperature sensor was utilized to conduct temperature measurement and over 1700 samples of the spectral raw data were recorded to train and validate the DCNN model. Using such method, the temperature information can be directly extracted from the experimentally obtained FBG spectra without any peak tracking algorithms. Since it makes full use of the information containing the full spectrum rather than only the central wavelength, it overcomes the limit of traditional fitting method and could improve the measurement accuracy of FBG effectively, which can reach 99.95% and its mean square error (MSE) is just 0.1080 °C, an order of magnitude less than that achieved by the traditional maximum peak method. The proposed method could reduce the need of high-performance hardware of equipment, whose accuracy can still maintain a high level when the sampling rate is reduced. Additionally, the universality of the method was experimentally demonstrated through the accurate demodulation of tilted FBG spectrum, and the relevant measurand can be retrieved directly from the entire spectrum instead of detecting the change of particular peaks. The proposed approach provides a cost-effective solution for the FBG based sensing system, and is promising for establishing sensing networks to implement smart monitoring.

High-Q Toroidal Dipole Metasurfaces Driven By Bound States in the Continuum for Ultrasensitive Terahertz Sensing

Abstract: A novel metallic toroidal dipole (TD) metasurface driven by Friedrich-Wintgen bound states in the continuum (FW-BIC) is theoretically proposed for terahertz (THz) sensing. By tuning the middle gap distance without breaking the symmetry of unit cell, the FW-BIC and quasi-BIC mode can be excited via resonance coupling between dipole modes. Based on the cyclic distribution of anti-aligned magnetic dipoles and the calculated scattering powers, TD resonance is demonstrated qualitatively and quantitatively; also FW-BIC is verified by far-field transmission spectrum and near-field enhancement spectrum. More importantly, it is the first time to exploit this quasi-BIC TD resonance for THz sensing to the best of our knowledge. For micron film sensing with frequency shift (FS) method, numerical results show the sensitivity, the Q-factor and the corresponding figure of merit (FoM) can simultaneously reach 775.7 GHz/RIU, 1016, and 284, respectively. Moreover, for nano film sensing where FS method is inapplicable, the amplitude difference method is utilized and the simulated results show it has superior sensing capability. Our proposed structure opens up an avenue to develop multifunctional and ultrasensitive THz sensors.

Enabling S-C-L-Band Systems With Standard C-Band Modulator and Coherent Receiver Using Coherent System Identification and Nonlinear Predistortion

Abstract: One promising and competitive solution to keep up with the rapid growth in data traffic while at the same time addressing increasing network cost, is the efficient reuse of legacy optical fiber infrastructure. This is highly desirable as deployed single mode fibers represent a valuable asset in the network while new installations would require high additional investments. Multiband (MB) or ultra-wideband (UWB) systems, combined with high symbol rates and higher-order modulation formats, are promising solutions to capitalize the already existing fiber plants. In this contribution, we experimentally demonstrate S-C-L-band reception with 64 GBd dual-polarization (DP) 64-ary and 32-ary quadrature-amplitude modulation (QAM) while using C-band components off-the-shelf (COTS) such as DP-IQ modulators and coherent receivers. To achieve such broadband operation with components that are not optimized for an out-of-band use, mitigation of the associated penalties is decisive. To this end, we apply an end-to-end electro-optical Volterra-based coherent system identification followed by nonlinear digital predistortion of the transmitter. We achieve 150-nm operation bandwidth of the transmission system by performing only a single identification and predistortion at a reference wavelength of 1500 nm.

Progresses of Pilot Tone Based Optical Performance Monitoring in Coherent Systems

Abstract: Pilot tone (PT) is a low frequency, small intensity modulation applied to high speed optical channel. Traditionally PT is used for channel identification and channel power monitoring. A pilot tone scheme suitable for optical performance monitoring (OPM) in coherent optical communication systems is described. Two effects, Stimulated Raman scattering (SRS) and dispersion fading, are discussed. An important enhancement, multiband pilot tone, is introduced, which provides sub-channel monitoring capability. With the advanced PT technologies, signal spectrum, and relative frequency offset between signal and optical filter can be monitored with sub-GHz resolution. PT also enables direct OSNR and fiber nonlinear noise monitoring with high accuracy and sensitivity.