Interference fading suppression with a multi-subcarrier pulse in a distributed acoustic sensor.

A low-complexity multi-subcarrier pulse generation scheme is proposed to suppress the interference fading in a phase-sensitive optical time-domain reflectometer (Φ-OTDR) based distributed acoustic sensor (DAS) with heterodyne coherent detection. The multi-subcarrier pulse is generated in the digital domain based on the proper clipping operation of a sine signal. The localization and recovery of the disturbance signal are realized by the spectrum extraction and rotated vector sum (SERVS) method. The experimental results show that the occurrences of interference fading can be significantly reduced. The intensity fluctuation is reduced from ∼75 dB to ∼25 dB. Multiple disturbance signals are successfully demodulated to verify the effectiveness of the proposed method.

Experimental demonstration of flexible information rate PON beyond 100 Gb/s with rate-compatible LDPC codes.

The applications of rate-compatible low-density parity-check (RC-LDPC) codes are investigated for a 16 quadrature amplitude modulation (16QAM) signal and coherent detection system. With rate-compatible signals, we can provide the flexible net data rate between 135.5 Gb/s and 169.7 Gb/s in a passive optical network (PON) link. Based on the LDPC codes defined in the IEEE 802.3ca standard, we construct two sets of RC-LDPC codes with a fixed and variable information bit length. Since the puncturing operation may degrade the performance of LDPC codes, we apply the protograph-based extrinsic information transfer (PEXIT) technique to optimize the puncturing positions to mitigate the degradation. Additionally, we explore four low-complexity LDPC decoding algorithms (min sum, offset min sum, variable weight min sum, and relaxed min sum with 2nd min emulation) to investigate the relationship between the computational complexity and decoding performance. Simulation results indicate that the constructed codewords exhibit good performance in the waterfall region across a range of code rates. Finally, we conduct an experimental setup in a dual-polarization 25 GBaud 16QAM coherent PON to verify the effectiveness of the constructed LDPC codes with four decoding algorithms. The experimental results show maximal 4.8 dB receiver sensitivity differences, which demonstrate the feasibility of the method for constructing RC-LDPC codes in future high-speed flexible coherent PON.

Efficacy and safety of transanal opening of intersphincteric space in the treatment of high complex anal fistula: A meta­analysis.

The best treatment of high complex anal fistula (HCAF) is to avoid anal incontinence while improving the cure rate. On this basis, several surgical procedures for preserving the anal sphincter have been proposed. The purpose of the present study was to evaluate the efficacy and safety of transanal opening of intersphincteric space for treating HCAF. PubMed, Cochrane Library, China National Knowledge Infrastructure and the Wanfang databases were searched to collate all the articles on transanal opening of intersphincteric space for treating HCAF. A total of two researchers independently completed the whole process, from screening and inclusion to data extraction and the data was included in the RevMan 5.3 software for analysis. The main outcomes included the patients' essential characteristics, primary healing rate, management after recurrence, final healing rate, anal incontinence score before and after surgery, postoperative complication rate and types of complications. A total of six articles were included in this meta-analysis. The results showed that the weighted final healing rate of patients following transanal opening of intersphincteric space was 89% [risk differences (RD)=0.89; 95% confidence interval (CI)=0.86-0.92; I2=0%; P<0.00001]. The results of the anal incontinence score showed that there was no significant difference between the results before and after transanal opening of intersphincteric space surgery mean differences [(MD)=-0.04, Cl=-0.10-0.02, I2=0%; P=0.21]. Only 11 patients were reported to have complications, including urinary retention and bleeding following transanal opening of intersphincteric space with a complication rate of 8% (11/138) and the weighted average complication rate was 6% (RD=0.06,95% CI=0.02-0.10; I2=9%; P=0.003). Transanal opening of intersphincteric space has a high cure rate, a favorable anal incontinence score, fewer types of postoperative complications and a low complication rate; it can be used as a minimally invasive and sphincter-preserving surgical method for treating HCAF and is worthy of further promotion and research in clinical practice.

Coherent optical transmitter IQ imbalance mitigation with embedded carrier phase and frequency offset estimation.

We present a 4×4 real-valued channel equalizer with embedded phase estimator, designed for carrier phase and frequency offset estimation and compensation in coherent optical communications with in-phase/quadrature (IQ) impairments. These impairments include IQ timing skew, gain imbalance, and quadrature phase errors at the transmitter side. To achieve adaptive control of the equalizer's filter coefficients, we employ the decision-directed least mean square (DD-LMS) algorithm. This algorithm minimizes the error between the filter outputs and the desired signals in a symbol-by-symbol manner, resulting in faster channel coefficients adaptation speed. Simulation results for a 60 GBaud polarization-multiplexing 16 quadrature amplitude modulation (PM-16QAM) signal demonstrate that our proposed equalizer outperforms a 2×2 cascaded multi modulus algorithm-based (CMMA-based) equalizer, a 2×2 complex-valued method based on the DD-LMS algorithm, and the 4×4 real-valued equalizer without embedded frequency offset estimation (FOE), when the transmitter IQ impairments exist. Experimental validation is also conducted for the 60 GBaud PM-16QAM and 45 GBaud PM-64QAM signals. Similar to the simulation results, our experiments show that the proposed 4×4 real-valued equalizer with embedded FOE can achieve effective SNR penalties of less than 0.41 dB at 7 ps skew, 1.33 dB at 3.5 dB gain imbalance, and 1.64 dB at the bias voltage shift of 0.5 V for the 60 GBaud PM-16QAM signal. Finally, our experimental results confirm the effectiveness of our proposed method in carrier phase estimation as well as the frequency offset compensation, when compared with 4×4 real-valued equalizer without embedded FOE.

Binding dynamics of cavity solitons in a Kerr resonator with high order dispersion.

Cavity solitons are persistent light pulses arising from the externally driven Kerr resonators. Thanks to the passive parametric gain, cavity soliton has been endowed with the natural advantage of the chip-scaled integration since it was first experimentally generated in the fiber-based platform. Deterministic single soliton with smooth spectrum is a preferred state for numerous applications. However, multiple solitons are more common in the resonators with anomalous dispersion. In this condition, adjacent solitons are easily perturbed to attract and collide with each other. Some experimental observations deviated from the aforementioned description have recorded the stable soliton intervals that can last for a long time scale. This phenomenon is known as soliton binding and is attributed to the presence of narrow resonant sidebands in the spectrum. While the stationary configuration of two binding solitons has been investigated, the dynamical evolution remains an area for further exploration. In this paper, we discuss the binding dynamics of the cavity solitons in the presence of high-order dispersion. The proposed theoretical predictions match well with the numerical results, encompassing both the stationary stable intervals and dynamic trajectories. Our research will provide a comprehensive insight into the soliton motion induced by the internal perturbations.

Screening Plasma Proteins for the Putative Drug Targets for Carpal Tunnel Syndrome.

Carpal tunnel syndrome (CTS) is one of the most common work-related musculoskeletal disorders. The present study sought to identify putative causal proteins for CTS. We conducted a two-sample Mendelian randomization (MR) analysis to evaluate the causal association between 2859 plasma proteins (N = 35,559) and CTS (N = 1,239,680) based on the published GWAS summary statistics. Then we replicated the significant associations using an independent plasma proteome GWAS (N = 10,708). Sensitivity analyses were conducted to validate the robustness of MR results. Multivariate MR and mediation analyses were conducted to evaluate the mediation effects of body mass index (BMI), type 2 diabetes (T2D), and arm tissue composition on the association between putative causal proteins and CTS. Colocalization analysis was used to examine whether the identified proteins and CTS shared causal variant(s). Finally, we evaluated druggability of the identified proteins. Ten plasma proteins were identified as putative causal markers for CTS, including sCD14, PVR, LTOR3, CTSS, SIGIRR, IFNL3, ASPN, TM11D, ASIP, and ITIH1. Sensitivity analyses and reverse MR analysis validated the robustness of their causal effects. Arm tissue composition, BMI, and T2D may play a fully/partial mediating role in the causal relationships of ASIP, TM11D, IFNL3, PVR, and LTOR3 with CTS. The association of ASPN and sCD14 with CTS were supported by colocalization analysis. Druggability assessment demonstrated that sCD14, CTSS, TM11D, and IFNL3 were potential drug therapeutic targets. The present study identified several potential plasma proteins that were causally associated with CTS risk, providing new insights into the pathogenesis of protein-mediated CTS and offering potential targets for new therapies.

Efficacy and safety of video-assisted anal fistula treatment in anorectal fistula: a meta-analysis.

INTRODUCTION: By searching relevant literature, the recurrence rate, complication rate after video-assisted anal fistula treatment (VAAFT), and efficacy and safety of the treatment were analyzed. EVIDENCE ACQUISITION: Articles that reported the outcomes of VAAFT up to December 2020 were searched in PubMed (Medline) and Cochrane Library, in accordance with the preferred reporting items for systematic review and meta-analysis (PRISMA) screening guidelines. Two researchers independently completed the whole process from screening and inclusion to quality evaluation and bias risk assessment, and the data was included in the RevMan 5.3 software for analysis. The main outcomes were demographic data of patients, detection rate, classification of internal opening of anorectal fistula, postoperative recurrence rate, and incidence of complications. EVIDENCE SYNTHESIS: A total of 10 articles were included (779 patients). The average age of the patients was 44 years old, average operation time was 60 min, and the average follow-up time was 22 months. The ratio of male to female was 2.4:1, the ratio of high anorectal fistula to low anorectal fistula was 6.6:1, the detection rate of internal openings was 98%, the weighted recurrence rate was 24%, and the weighted complication rate was 1%. CONCLUSIONS: VAAFT is effective and safe in the treatment of anorectal fistulas.

Quantum criticality of excitonic Mott metal-insulator transitions in black phosphorus.

Quantum phase transition refers to the abrupt change of ground states of many-body systems driven by quantum fluctuations. It hosts various intriguing exotic states around its quantum critical points approaching zero temperature. Here we report the spectroscopic and transport evidences of quantum critical phenomena of an exciton Mott metal-insulator-transition in black phosphorus. Continuously tuning the interplay of electron-hole pairs by photo-excitation and using Fourier-transform photo-current spectroscopy as a probe, we measure a comprehensive phase diagram of electron-hole states in temperature and electron-hole pair density parameter space. We characterize an evolution from optical insulator with sharp excitonic transition to metallic electron-hole plasma phases featured by broad absorption and population inversion. We also observe strange metal behavior that resistivity is linear in temperature near the Mott transition boundaries. Our results exemplify an ideal platform to investigating strongly-correlated physics in semiconductors, such as crossover between superconductivity and superfluity of exciton condensation.

Cavity soliton in a cyclic polarization permutation fiber resonator.

Cavity solitons are shape-preserving waveforms infinitely revolving around a cavity, which have numerous applications from spectroscopy to telecommunications. Although the cavity solitons have been widely studied for their special time-frequency characteristics over the past decade, the spectral flatness will be a large limitation in some applications without any extra filtering process. In this paper, we report on the generation of a distinct cavity soliton in a cyclic polarization permutation fiber resonator. With the simultaneous excitation of two orthogonal polarization modes with equally opposite dispersion, vectorial cavity solitons possessing broader and flatter spectra can be generated. In order to verify the concept, a numerical model of the polarization-maintaining fiber is proposed and the soliton with a flattened spectrum can be formed. The results enrich the soliton dynamics in the vectorial dissipation system.

Atomic-Scale Confinement and Negative Refraction of Plasmons by Twisted Bilayer Graphene.

Moiré superlattices provide in-plane quantum restriction for light-matter interactions in twisted bilayer graphene (tBLG), leading to the exotic photon-Moiré physics and potential applications for light manipulation. Recently, our experiment identified a highly confined slow surface plasmons polaritons (SPPs) mode in tBLG. Here, we demonstrate that the propagation of the slow SPPs mode in tBLG is spatially tailored and steered at deep subwavelengths. Analysis by the perturbation theory indicates that the coupling between the slow SPPs mode and the Moiré system is greatly strengthened, which regulates the wavefront at the atomic scale and makes tBLG serve as a universal optical metamaterial. Consequently, the negative refraction is achieved at the interface of monolayer graphene and tBLG, by which a metalens with a controllable focal length and an extremely high resolution up to 1/150 of wavelength is devised. Our work paves the way for constructing optical metamaterial at the atomic scale and develops future photon-Moiré interaction systems.

Polarization-decoupled cavity solitons generation in Kerr resonators with flattened near-zero dispersion.

Two frequency combs emitting from a single cavity are of great potential in the field of dual-comb spectroscopy because they are mutually coherent and therefore the common mode noise can be suppressed naturally. However, it is difficult to fully and flexibly control the repetition frequency difference in most of the all-optical schemes. In this paper, a birefringence-compensation Kerr resonator is proposed for the mutual dual-comb generation. It is shown that by offset aligning the fast and slow axis with appropriate fiber length, the total birefringence of the cavity can be equalized while the local one keeps at a high level. Theoretical investigations reveal that the polarization decoupled mutual dual-comb can be generated with nearly the same power level and arbitrary repetition frequency difference. Additionally, a numerical model of polarization-maintaining fiber (PMF) with near-zero dispersion is proposed for the proof of the concept. Based on this fiber, the coherent polarization-decoupled dual-comb with 10-dB bandwidth of 33 nm can be obtained. And the repetition frequency difference can be flexibly tuned compared to the cavity without offset alignment.

Linear optical sampling enabled soliton nonlinear frequency spectrum classification.

Nonlinear Fourier transform (NFT) is a powerful tool for characterizing optical soliton dynamics, which, however, suffers from fundamental limitations that ultra-wide bandwidth photodetectors and ultra-high sampling rate analog-to-digital converters should be used when accessing the full-field information of an ultrafast optical pulse. Herein, we report on the experimental demonstration of the linear optical sampling (LOS) enabled nonlinear frequency spectrum classification of ultrashort optical pulses, which could break this limitation. Instead of traditional coherent detection, the LOS overcomes the ultra-wide bandwidth constraint of commercially available optoelectrical devices. By finely adjusting the repetition rate difference between the soliton to be characterized and the sampling pulsed source, a 55.56-TSa/s equivalent sampling rate arising in the LOS can be secured, where only 400-MHz balanced photodetectors and 5-GSa/s analog-to-digital converter are used. Meanwhile, according to the nonlinear frequency spectrum calculated from the accurate full-field information, the promising concept of soliton distillation has been experimentally verified for the first time. The LOS-enabled NFT technique provides an alternative and efficient characterization tool for ultrafast fiber lasers, which facilities comprehensive insight into soliton dynamics.

High-precision distributed detection of rail defects by tracking the acoustic propagation waves.

Nowadays, early defect detection plays a significant role for the railway safety warning. However, the existing methods cannot satisfy the requirements of real-time and high-precision detection. Here, a high-precision, distributed and on-line method for detecting rail defect is proposed and demonstrated. When a train goes through defects, the instantaneous elastic waves will be excited by the wheel-rail interaction, which will further propagate along railway tracks bidirectionally. Through mounting the backscattering enhanced optical fiber on the railway as sensors, the fiber optic distributed acoustic sensing system can record the propagation trace precisely. Further, the acoustic propagation fitting method is applied onto the propagation data to detect and locate defects along the long-distance railway. Especially, the dual-frequency joint-processing algorithm is proposed to improve the location accuracy. The field test proves that multiple defects along the railway can be successfully identified and located with a standard deviation of 0.314m. To the best of our knowledge, this work is the first report of distributed rail defect detection, which will bring a breakthrough for high-precision structural damage detection in the infrastructures such as the railway, pipeline and tunnel.

Observation of chiral and slow plasmons in twisted bilayer graphene.

Moiré superlattices have led to observations of exotic emergent electronic properties such as superconductivity and strong correlated states in small-rotation-angle twisted bilayer graphene (tBLG)1,2. Recently, these findings have inspired the search for new properties in moiré plasmons. Although plasmon propagation in the tBLG basal plane has been studied by near-field nano-imaging techniques3-7, the general electromagnetic character and properties of these plasmons remain elusive. Here we report the direct observation of two new plasmon modes in macroscopic tBLG with a highly ordered moiré superlattice. Using spiral structured nanoribbons of tBLG, we identify signatures of chiral plasmons that arise owing to the uncompensated Berry flux of the electron gas under optical pumping. The salient features of these chiral plasmons are shown through their dependence on optical pumping intensity and electron fillings, in conjunction with distinct resonance splitting and Faraday rotation coinciding with the spectral window of maximal Berry flux. Moreover, we also identify a slow plasmonic mode around 0.4 electronvolts, which stems from the interband transitions between the nested subbands in lattice-relaxed AB-stacked domains. This mode may open up opportunities for strong light-matter interactions within the highly sought after mid-wave infrared spectral window8. Our results unveil the new electromagnetic dynamics of small-angle tBLG and exemplify it as a unique quantum optical platform.

Dynamics of cavity soliton driven by chirped optical pulses in Kerr resonators.

Recent researches have demonstrated that pulsed driving is an effective method to increase the temporal overlap between cavity soliton (CS) and pump field, thereby increasing the pump-to-comb conversion efficiency. The amplitude-modulated inhomogeneity of the background wave causes the solitons to drift toward edges of the driving pulse. To eliminate the multiple temporal trapping positions, induced by the spontaneous symmetry breaking, we propose the chirped pulse driving for deterministic single soliton generation. We theoretically explain the physical mechanism of the chirp pulse driving, as the combination of amplitude and phase modulation. Our numerical simulations demonstrate the chirp is responsible for the single soliton generation. A detailed investigation for dynamics of CSs sustained by chirped pulses, shows the recovery of spontaneous symmetry breaking. In addition, the desynchronized chirped pulse driving is also considered here. Considering a weak chirp parameter, the desynchronization-dependent trapping position diagram is divided into multiple areas including two CSs, a single CS, two oscillating CSs, and no CS. With a sufficient chirp parameter considered, the trapping position curve becomes a monotonous function of the desynchronized drift velocity, which indicates deterministic single soliton generation.

Fiber structures and material science in optical fiber magnetic field sensors.

Magnetic field sensing plays an important role in many fields of scientific research and engineering applications. Benefiting from the advantages of optical fibers, the optical fiber-based magnetic field sensors demonstrate characteristics of light weight, small size, remote controllability, reliable security, and wide dynamic ranges. This paper provides an overview of the basic principles, development, and applications of optical fiber magnetic field sensors. The sensing mechanisms of fiber grating, interferometric and evanescent field fiber are discussed in detail. Magnetic fluid materials, magneto-strictive materials, and magneto-optical materials used in optical fiber sensing systems are also introduced. The applications of optical fiber magnetic field sensors as current sensors, geomagnetic monitoring, and quasi-distributed magnetic sensors are presented. In addition, challenges and future development directions are analyzed.

L-band compact Q-switched fiber laser based on a thulium-doped fiber saturable absorber.

In this paper, a compact and stable Q-switched erbium-doped fiber laser operating at around 1600 nm by employing a segment of 1-cm thulium-doped fiber saturable absorber (TDF-SA) is proposed. When the pump power is adjusted between 28 and 71 mW, the Q-switched operation can be maintained stably, and the output power increases from 74 µW to 2.6 mW. Furthermore, the peak power clamping effect is also observed when the pump power exceeds 60 mW. The structure of the cavity is greatly simplified by using all-optical and hybrid fiber components, which contributes to its long-term stability. Our results prove that the TDF can be a promising SA for all-fiber Q-switched pulses generation in the L-band.

Design and analysis of a compact subwavelength-grating-assisted 1.55/2 µm wavelength demultiplexer.

The wavelength demultiplexer (deWMUX) is an indispensable component for the wavelength diversity system. In this paper, we propose a subwavelength-grating-assisted ${{1}} \times {{2}}$ deWMUX based on the principle of multimode interference, which aims to output 1.55 and 2 µm wavelengths from two different channels. The simulation results show that the contrast of the designed deWMUX at both wavelengths is greater than 24 dB, the insertion loss is less than 0.5 dB, and the 1 dB bandwidth is wider than 100 nm. In addition, the manufacturing tolerances of the device are also studied.

Improved particle swarm optimization algorithm for high performance SPR sensor design.

The surface plasmon resonance (SPR) sensor offers high sensitivity, good stability, simple structure, and is label-free. However, optimizing a multi-layered structure is quite time-consuming within the SPR sensor design process. Moreover, it is easy to overlook optimal design when using the conventional parameter sweeping method. In this paper, the improved particle swarm optimization (IPSO) algorithm with high global optimal solution convergence speed is applied for this purpose. Based on the IPSO algorithm, the SPR sensor with transition metal dichalcogenides (TMDCs) and graphene composite is proposed and optimized. The results show that the best Ag-ITO-WS2-graphene hybrid structure can be found by the IPSO algorithm, and the maximum sensitivity is 137.4°/RIU, and the figure of merit (FOM) is 5.25RIU-1. Compared with the standard particle swarm optimization algorithm, the number of iterations can be reduced. The development of the SPR sensor provides an optimization platform, which enormously improves the development efficiency of the multi-layer SPR sensor.

Planar nonlinear metasurface optics and their applications.

Metasurfaces are artificial two-dimensional (2D) planar surfaces that consist of subwavelength 'meta-atoms' (i.e. metallic or dielectric nanostructures). They are known for their capability to achieve better and more efficient light control in comparison to their traditional optical counterparts. Abrupt and sharp changes in the electromagnetic properties can be induced by the metasurfaces rather than the conventional gradual accumulation that requires greater propagation distances. Based on this feature, planar optical components like mirrors, lenses, waveplates, isolators and even holograms with ultrasmall thicknesses have been developed. Most of the current metasurface studies have focused on tailoring the linear optical effects for applications such as cloaking, lens imaging and 3D holography. Recently, the use of metasurfaces to enhance nonlinear optical effects has attracted significant attention from the research community. Benefiting from the resulting efficient nonlinear optical processes, the fabrication of integrated all-optical nano-devices with peculiar functionalities including broadband frequency conversions and ultrafast optical switching will become achievable. Plasmonic excitation is one of the most effective approaches to increase nonlinear optical responses due to its induced strong local electromagnetic field enhancement. For instance, continuous phase control on the effective nonlinear polarizability of plasmonic metasurfaces has been demonstrated through spin-rotation light coupling. The phase of the nonlinear polarization can be continuously tuned by spatially changing the meta-atoms' orientations during second and third harmonic generation processes, while the nonlinear metasurfaces also exhibit homogeneous linear properties. In addition, an ultrahigh second-order nonlinear susceptibility of up to 104 pm V-1 has recently been reported by coupling the plasmonic modes of patterned metallic arrays with intersubband transition of multi-quantum-well layered substrate. In order to develop ultra-planar nonlinear plasmonic metasurfaces, 2D materials such as graphene and transition metal dichalcogenides (TMDCs) have been extensively studied based on their unique nonlinear optical properties. The third-order nonlinear coefficient of graphene is five times that of gold substrate, while TMDC materials also exhibit a strong second-order magnetic susceptibility. In this review, we first focus on the main principles of planar nonlinear plasmonics based on metasurfaces and 2D nonlinear materials. The advantages and challenges of incorporating 2D nonlinear materials into metasurfaces are discussed, followed by their potential applications including orbital angular momentum manipulating and quantum optics.