Km Of Fiber Research Articles

Coherent light scattering from a telecom C-band quantum dot

Quantum networks have the potential to transform secure communication via quantum key distribution and enable novel concepts in distributed quantum computing and sensing. Coherent quantum light generation at telecom wavelengths is fundamental for fibre-based network implementations, but Fourier-limited emission and subnatural linewidth photons have so far only been reported from systems operating in the visible to near-infrared wavelength range. Here, we use InAs/InP quantum dots to demonstrate photons with coherence times much longer than the Fourier limit at telecom wavelength via elastic scattering of excitation laser photons. Further, we show that even the inelastically scattered photons have coherence times within the error bars of the Fourier limit. Finally, we make direct use of the minimal attenuation in fibre for these photons by measuring two-photon interference after 25 km of fibre, demonstrating finite interference visibility for photons emitted about 100,000 excitation cycles apart.

DAS-N2N: machine learning distributed acoustic sensing (DAS) signal denoising without clean data

SUMMARY This paper presents a weakly supervised machine learning method, which we call DAS-N2N, for suppressing strong random noise in distributed acoustic sensing (DAS) recordings. DAS-N2N requires no manually produced labels (i.e. pre-determined examples of clean event signals or sections of noise) for training and aims to map random noise processes to a chosen summary statistic, such as the distribution mean, median or mode, whilst retaining the true underlying signal. This is achieved by splicing (joining together) two fibres hosted within a single optical cable, recording two noisy copies of the same underlying signal corrupted by different independent realizations of random observational noise. A deep learning model can then be trained using only these two noisy copies of the data to produce a near fully denoised copy. Once the model is trained, only noisy data from a single fibre is required. Using a data set from a DAS array deployed on the surface of the Rutford Ice Stream in Antarctica, we demonstrate that DAS-N2N greatly suppresses incoherent noise and enhances the signal-to-noise ratios (SNR) of natural microseismic icequake events. We further show that this approach is inherently more efficient and effective than standard stop/pass band and white noise (e.g. Wiener) filtering routines, as well as a comparable self-supervised learning method based on masking individual DAS channels. Our preferred model for this task is lightweight, processing 30 s of data recorded at a sampling frequency of 1000 Hz over 985 channels (approximately 1 km of fibre) in <1 s. Due to the high noise levels in DAS recordings, efficient data-driven denoising methods, such as DAS-N2N, will prove essential to time-critical DAS earthquake detection, particularly in the case of microseismic monitoring.

Node-downloadable frequency transfer system based on a mode-locked laser with over 100 km of fiber.

To meet the requirements of time-frequency networks and enable frequency downloadability for nodes along the link, we demonstrated the extraction of stable frequency signals at nodes using a mode-locked laser under the condition of 100 km laboratory fiber. The node consists of a simple structure that utilizes widely used optoelectronic devices and enables plug-and-play applications. In addition, the node can recover frequency signals with multiple frequencies, which are useful for scenarios that require different frequencies. Here, we experimentally demonstrated a short-term frequency instability of 2.83 × 10-13@1 s and a long-term frequency instability of 1.18 × 10-15@10,000 s at the node, which is similar to that at the remote site of the frequency transfer system. At the same time, frequency signals with different frequencies also achieved stable extraction with the same performance at the node. Our results can support the distributed application under large-scale time-frequency networks.

Pushing Capacity Limits With Multi-Segment SiP Modulators

The use of multiple segments on a silicon photonic modulator can increase bandwidth at the cost of greater complexity in the driving signals. We propose and demonstrate a simple driving scheme for use with dual segment modulators that involves very little additional complexity when equal length segments are used. The increased bandwidth with segmentation scales linearly with the number of segments, but the net bit rate does not; net rate depends on many factors. With an IQ modulator with two 2 mm segments, we demonstrate an improvement of 14% in net bit rate as compared to a single 4 mm segment. We examine the trade-offs in moving to three-segment modulation. We explore implementation penalties and use probabilistic shaping and optical pre-emphasis to achieve a net rate of 1.07 Tb/s with dual polarization transmission over 80 km of fiber at 116 Gbaud using 64QAM modulation.

A tale of whale and fiber: monitoring baleen whales with Distributed Acoustic Sensing (DAS)

Distributed Acoustic Sensing (DAS) converts fibers in existing underwater telecommunication fiber-optic cables into dense listening arrays of strain sensors. Recent advances in DAS interrogating technology enable increasing data quality, spatial coverage, and bandwidth, sparking interest in applied environmental sensing. Initially focused on seismic data collection, e.g., for earthquake monitoring, DAS has demonstrated abilities in detecting waterborne acoustic sources, including the low-frequency sounds of blue and fin whales (Balaenoptera musculus and B. physalus) over tens of km of fiber. DAS is instrumented from land, collects data in near-real time, and offers region-wide spatial coverage. Thus, this technology provides a unique opportunity for robust monitoring of endangered and threatened migratory baleen whales at ecologically meaningful spatial and temporal scales. This talk will introduce our research on using DAS to monitor low-frequency baleen whales, including detection and tracking, and the open-access DAS4Whales python package, developed to analyze terabytes of spatio-temporal data collected with DAS. We will also address the potential limitations of this new monitoring technology and provide a roadmap for further research and testing needed to exploit the technology's full potential.

A hybrid integrated quantum key distribution transceiver chip

Quantum photonic technologies, such as quantum key distribution, are already benefiting greatly from the rise of integrated photonics. However, the flexibility in design of these systems is often restricted by the properties of the integration material platforms. Here, we overcome this choice by using hybrid integration of ultra-low-loss silicon nitride waveguides with indium phosphide electro-optic modulators to produce high-performance quantum key distribution transceiver chips. Access to the best properties of both materials allows us to achieve active encoding and decoding of photonic qubits on-chip at GHz speeds and with sub-1% quantum bit error rates over long fibre distances. We demonstrate bidirectional secure bit rates of 1.82 Mbps over 10 dB channel attenuation and positive secure key rates out to 250 km of fibre. The results support the imminent utility of hybrid integration for quantum photonic circuits and the wider field of photonics.

Single-emitter quantum key distribution over 175 km of fibre with optimised finite key rates

Quantum key distribution with solid-state single-photon emitters is gaining traction due to their rapidly improving performance and compatibility with future quantum networks. Here we emulate a quantum key distribution scheme with quantum-dot-generated single photons frequency-converted to 1550 nm, achieving count rates of 1.6 MHz with {g}^{left(2right)}left(0right)=3.6% and asymptotic positive key rates over 175 km of telecom fibre. We show that the commonly used finite-key analysis for non-decoy state QKD drastically overestimates secure key acquisition times due to overly loose bounds on statistical fluctuations. Using the tighter multiplicative Chernoff bound to constrain the estimated finite key parameters, we reduce the required number of received signals by a factor 108. The resulting finite key rate approaches the asymptotic limit at all achievable distances in acquisition times of one hour, and at 100 km we generate finite keys at 13 kbps for one minute of acquisition. This result is an important step towards long-distance single-emitter quantum networking.

Waveform-to-Waveform End-to-End Learning Framework in a Seamless Fiber-Terahertz Integrated Communication System

Seamless fiber-terahertz integrated communication has emerged as a promising technology in the special field of 6G, including mobile fronthaul and wireless bridges. Electronic terahertz communication systems have the advantages of high-level integration, small form factors, and potentially low costs, but their drawbacks are their small bandwidths and high harmonic interference levels. In this paper, an end-to-end learning-based waveform-to-waveform automatic equalization framework (W2WAEF) is proposed to overcome the above shortcomings in a seamless fiber-terahertz integrated communication system. An attention-based three-tributary heterogeneous neural network (ATTH) is designed to simulate the channel model of a seamless fiber-wireless communication system, taking fiber optics, optical-to-electrical conversion, and electrogenerated terahertz waves into account. With the proposed method, a data rate of 80.78 Gbps is experimentally demonstrated for discrete multitone modulation (DMT) signals over 5 km of fiber and 1 m of 209-GHz terahertz signals under a 20% soft decision-forward error correction (SD-FEC) threshold of 2.4 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−2</sup> . Compared with an approach without preprocessing, a receiver sensitivity gain exceeding 1.3 dB is successfully achieved at a data rate of 60 Gbps. The proposed method is a promising scheme for meeting the high speed and low-cost demands of future seamless fiber-terahertz integrated communication systems.

Characterization of State-Preparation Uncertainty in Quantum Key Distribution

To achieve secure quantum key distribution, all imperfections in the source unit must be incorporated in a security proof and measured in the lab. Here we perform a proof-of-principle demonstration of the experimental techniques for characterising the source phase and intensity fluctuation in commercial quantum key distribution systems. When we apply the measured phase fluctuation intervals to the security proof that takes into account fluctuations in the state preparation, it predicts a key distribution distance of over 100 km of fiber. The measured intensity fluctuation intervals are however so large that the proof predicts zero key, indicating a source improvement may be needed. Our characterisation methods pave the way for a future certification standard.

Frequency Stability Requirements in Quasi-Integer-Ratio Time-Expanded Phase-Sensitive OTDR

Time-expanded phase sensitive (TE-ϕ)OTDR is a recently reported technique for distributed fiber sensing that relies on the use of an electro-optic dual frequency comb (DFC) scheme. A distinctive feature of this approach is its ability to provide high spatial resolution (on the centimeter scale) with detection bandwidths orders of magnitude lower than those of conventional ϕOTDR systems. The stringent trade-off between resolution, range and sensing bandwidth that exists in TE-ϕOTDR has demonstrated to be substantially relaxed by implementing two frequency combs with quasi-integer-ratio repetition rates. However, employing very dissimilar line separations (with a ratio between them >100) is challenging due to the need of keeping the coherence over long sequences of interferograms, which eventually limits the attainable range. In this paper, we formulate the requirements for the frequency stability of the reference clock used in a quasi-integer-ratio DFC scheme. This analysis allows us to stablish the limits on the number of comb lines (i.e., on the number of available independent sensing points) for a particular reference clock. By using a rubidium atomic clock (with a relative frequency stability of ∼10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−13</sup> ), we demonstrate up to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> sensing points along 2 km of fiber with tens of Hz sensing bandwidth.

Non-classical correlations over 1250 modes between telecom photons and 979-nm photons stored in 171Yb3+:Y2SiO5

Quantum repeaters based on heralded entanglement require quantum nodes that are able to generate multimode quantum correlations between memories and telecommunication photons. The communication rate scales linearly with the number of modes, yet highly multimode quantum storage remains challenging. In this work, we demonstrate an atomic frequency comb quantum memory with a time-domain mode capacity of 1250 modes and a bandwidth of 100 MHz. The memory is based on a Y2SiO5 crystal doped with 171Yb3+ ions, with a memory wavelength of 979 nm. The memory is interfaced with a source of non-degenerate photon pairs at 979 and 1550 nm, bandwidth-matched to the quantum memory. We obtain strong non-classical second-order cross correlations over all modes, for storage times of up to 25 μs. The telecommunication photons propagated through 5 km of fiber before the release of the memory photons, a key capability for quantum repeaters based on heralded entanglement and feed-forward operations. Building on this experiment should allow distribution of entanglement between remote quantum nodes, with enhanced rates owing to the high multimode capacity.

Closed-loop technique based on gain balancing for real-time Brillouin optical time-domain analysis

A closed-loop servo control based on balancing the gain of two probing frequencies is proposed for real-time Brillouin optical time-domain analysis (BOTDA) without post-processing. With the most basic BOTDA hardware setup, the system can perform measurement in 150 ms and track a sudden Brillouin frequency shift (BFS) change in excess of 300 MHz (corresponding to a temperature change of more than 250°C) over ∼5 km of fiber with a spatial resolution of 2 m. Moreover, the feedback loop is independent of the loss experienced by the probe and pump, with no requirement on the BFS uniformity along the fiber. All these advantages make the proposed system suitable for field applications in harsh environments.

Massively distributed fiber strain sensing using Brillouin lasing.

Brillouin based distributed fiber sensors present a unique set of characteristics amongst fiber sensing architectures. They are able to measure absolute strain and temperature over long distances, with high spatial resolution, and very large dynamic range in off-the-shelf fiber. However, Brillouin sensors traditionally provide only modest sensitivity due to the weak dependence of the Brillouin frequency on strain and the high signal to noise ratio required to identify the resonance's peak frequency to within a small fraction of its linewidth. Recently, we introduced a technique which substantially improves the precision of Brillouin fiber sensors by exciting a series of lasing modes in a fiber loop cavity that experience Brillouin amplification at discrete locations in the fiber. The narrow-linewidth and high intensity of the lasing modes enabled ultra-low noise Brillouin sensors with large dynamic range. However, our initial demonstration was only modestly distributed: measuring strain at 40, non-contiguous positions along a 400m fiber. In this work, we greatly extend this methodology to enable fully distributed sensing at 1000 contiguous locations along 3.5 km of fiber-an order of magnitude increase in sensor count and range. This highly-multiplexed Brillouin fiber laser sensor provides a strain noise as low as 34 nɛ/√Hz and we analyze the limiting factors in this approach.

Idealized four-wave mixing dynamics in a nonlinear Schrödinger equation fiber system

The observation of ideal four-wave mixing dynamics is notoriously difficult to implement experimentally due to the generation of higher-order sidebands and optical loss, which limit the potential interaction distance. Here, we overcome this problem with an experimental technique that uses programmable phase and amplitude shaping to iterate the wave mixing initial conditions injected into an optical fiber. This extends the effective propagation distance by two orders of magnitude, allowing idealized Kerr-driven dynamics to be seen over 50 km of fiber using only one short fiber segment of 500 m. Our experiments reveal the full phase space topology, showing characteristic features of multiple Fermi–Pasta–Ulam recurrence, stationary wave existence, and the system separatrix representing the boundary between two distinct regimes of spatiotemporal evolution. Experiments are in excellent quantitative agreement with numerical solutions of the differential equation system describing the wave evolution. This experimental approach can be readily adapted to study other wave mixing and nonlinear propagation phenomena in optics.

Scalable Network for Simultaneous Pairwise Quantum Key Distribution via Entanglement-Based Time-Bin Coding

A star-shaped, robust and scalable optical fiber network for simultaneous time-bin QKD is experimentally demonstrated with four participants including a field test and more than 100 km of fiber between users as well as full clock recovery.

Realization of quantum secure direct communication over 100\u2009km fiber with time-bin and phase quantum states

Rapid progress has been made in quantum secure direct communication in recent years. For practical application, it is important to improve the performances, such as the secure information rate and the communication distance. In this paper, we report an elaborate physical system design and protocol with much enhanced performance. This design increased the secrecy capacity greatly by achieving an ultra-low quantum bit error rate of <0.1%, one order of magnitude smaller than that of existing systems. Compared to previous systems, the proposed scheme uses photonic time-bin and phase states, operating at 50 MHz of repetition rate, which can be easily upgraded to over 1 GHz using current on-the-shelf technology. The results of our experimentation demonstrate that the proposed system can tolerate more channel loss, from 5.1 dB, which is about 28.3 km in fiber in the previous scheme, to 18.4 dB, which corresponds to fiber length of 102.2 km. Thus, the experiment shows that intercity quantum secure direct communication through fiber is feasible with present-day technology.

Virtual transparency in ϕ-OTDR using second order Raman amplification and pump modulation.

In distributed optical fibre sensors, distributed amplification schemes have been investigated in order to increase the measurement range while avoiding the limitation imposed by the fibre attenuation and the nonlinear effects. Recently, the use of Raman amplification with an engineered intensity modulation has been demonstrated as an efficient way to produce a virtually lossless trace employing a single-end configuration. In this paper, we propose the combination of this technique with a simultaneous second order Raman pumping scheme for increasing the measurement range. The optimal modulation profile has been numerically analyzed and we experimentally demonstrate a sensor able to detect perturbations along 70 km of fibre, with a minimal SNR penalty along the total length. Thanks to this new approach, the sensitivity in the worst point is considerably improved, and the ASD noise floor is also reduced. The measurement range is extended approximately 15 km compared with the equivalent first order pumping case.

Characterization of Radiation-Resistant Multimode Optical Fibers for Large-Scale Procurement

This article presents the results of a comprehensive characterization of special radiation-resistant multimode optical fibers, including complementary irradiation tests during mass production. Radiation-induced attenuation measurements were carried out at different wavelengths and the attenuation behavior studied with varying the dose rate. Following a preliminary qualification performed upstream the production, CERN, the European Organization for Nuclear Research, validated the procurement of 500 km of fiber for installation in the accelerator complex and experiments. A strict quality assurance plan, including quality control and irradiation tests, was implemented for monitoring the characteristics of the supplied optical fibers all along the production process. The results show resistance to radiations consistently below 60 dB/km for a total dose of 10 kGy and a dose rate of 0.2 Gy/s and stable optical performance over hundreds of fiber spools produced during a 6-year period.

Digital Filtering Techniques for Performance Improvement of Golay Coded TDM-FBG Sensor.

For almost a half-decade, the unique autocorrelation properties of Golay complementary pairs (GCP) have added a significant value to the key performance of conventional time-domain multiplexed fiber Bragg grating sensors (TDM-FBGs). However, the employment of the unipolar form of Golay coded TDM-FBG has suffered from several performance flaws, such as limited improvement of the signal-to-noise ratio (SNIR), noisy backgrounds, and distorted signals. Therefore, we propose and experimentally implement several digital filtering techniques to mitigate such limitations. Moving averages (MA), Savitzky–Golay (SG), and moving median (MM) filters were deployed to process the signals from two low reflectance FBG sensors located after around 16 km of fiber. The first part of the experiment discussed the sole deployment of Golay codes from 4 bits to 256 bits in the TDM-FBG sensor. As a result, the total SNIR of around 8.8 dB was experimentally confirmed for the longest 256-bit code. Furthermore, the individual deployment of MA, MM, and SG filters within the mentioned decoded sequences secured a further significant increase in SNIR of around 4, 3.5, and 3 dB, respectively. Thus, the deployment of the filtering technique alone resulted in at least four times faster measurement time (equivalent to 3 dB SNIR). Overall, the experimental analysis confirmed that MM outperformed the other two techniques in better signal shape, fastest signal transition time, comparable SNIR, and capability to maintain high spatial resolution.

High-Dimensional Quantum Cryptography with Hybrid Orbital-Angular-Momentum States through 25 km of Ring-Core Fiber: A Proof-of-Concept Demonstration

Quantum cryptography provides the inherent security for transmitting confidential information across free space or a fiber link. However, a high secure-key rate is still a challenge for a quantum-cryptography system. High-dimensional quantum cryptography, which can tolerate much higher channel noise, is a prospective way to share a higher secure-key rate between legitimate users, and has received substantial attention over the last decade. In particular, orbital angular momentum (OAM) can provide an abundant resource for high-dimensional quantum cryptography. Furthermore, combining spin angular momentum (SAM) with OAM can increase the encoding alphabet. Here we verify a prepare-and-measure quantum-cryptography scheme based on four-dimensional SAM-OAM hybrid states over kilometer-scale ring-core fibers. The measured quantum-bit error rates are $4.3\mathrm{%}$ for 4 km of fiber and $16.3\mathrm{%}$ for 25 km of fiber. The scheme simplifies the process of state preparation and measurement, with a compact and scalable setup.