Low complexity equalization in coherent optical communication based on Fermat number transform.

In order to reduce the power consumption of digital signal processing (DSP) in a coherent optical communication system, a low complexity equalization scheme in DSP flow of a 400 Gb/s DP-16QAM system has been proposed. This scheme is based on Fermat number transform (FNT), which sequentially performs static equalization (SE) and dynamic equalization (DE) in the transform domain. For different distances, the proposed scheme finds the optimal solution under the condition that transform length and data bit width are mutually restricted under different transmission distances while achieving low complexity and optimal performance. The experimental results show that the adopted transform-domain equalization (TrDE) scheme has much lower computational complexity than the traditional frequency-domain equalization (FDE) and time-domain equalization (TDE) nearly without any performance loss. In the 80, 160, and 240 km scenarios, the number of multiplier is reduced by more than 72%, and the advantage becomes more obvious as the transmission capacity increases.

Bias-aided DD-LMS equalizer for optical multipath-interference-impaired high-speed PAM4 transmission systems.

Multipath interference (MPI) noise induces drastic fluctuations in high-speed 4-level pulse amplitude modulation (PAM4) intensity modulation direct detection (IMDD) systems, severely degrading the transmission performance. Here, we propose a bias-aided decision-directed least mean square (DD-LMS) equalizer to eliminate the MPI noise. In the simulation, the proposed bias-aided DD-LMS equalizer could adeptly track and compensate the MPI-impaired PAM4 signal, markedly improving the bit error rate (BER) performance under different MPI levels and laser linewidths. Within a 112 Gbps PAM4 transmission system, the proposed equalizer achieves a MPI tolerance over 4 dB at the KP4-forward error correction (FEC) threshold (2.4 × 10-4), outperforming existing MPI suppression techniques.

Enabling endogenous distributed acoustic sensing in a digital subcarrier coherent transmission system.

To monitor the health of the fiber network and its ambient environment in densely populated access/metro network areas, in this Letter, an endogenous distributed acoustic sensing (DAS) has been proposed and achieved in a coherent digital subcarrier multiplexing (DSCM) system. Rather than specially allocating a sensing probe in general integrated communication and sensing schemes, the fractional Fourier transformed (FrFT) training sequence (TS) designated for time/frequency synchronization in DSCM coherent communications has been repurposed for sensing. While achieving excellent synchronization performance of communication, the FrFT-based TS can also be concurrently utilized to perform distributed vibration sensing. Experimental results demonstrate that the FrFT-based timing/frequency synchronization sequence is repurposed to achieve a DAS sensitivity of 70 p ε/Hz at a spatial resolution of 5 m, along with 100-Gb/s 16 quadrature amplitude modulation (QAM) DSCM transmission, without a loss of spectral efficiency.

High-security constellation shaped self-homodyne coherent system with 4-D joint encryption.

In recent years, the self-homodyne coherent (SHC) system and the constellation shaping (CS) technique have drawn considerable attention due to their abilities to further improve the transmission capacity for various scenarios. From the security point of view, the CS technique and the SHC infrastructure also provide new dimensions for encryption. We propose a high-security and reliable SHC system based on the CS technique and the digital chaos. With a four-dimensional hyperchaotic system, chaotic sequences are generated and used for the exclusive or operation, chaotic constant composition distribution matching, phase disturbance, and optical-layer time-delay disturbance. Moreover, 64-ary circular quadrature amplitude modulation (64CQAM) format is adopted for transmission due to its advantages of sensitivity to phase noise, immunity to conventional digital signal processing, and ability of time-mismatch masking, which is verified by simulation in a SHC system. Last, we conduct an experimental verification in a 20GBaud probabilistically shaped 64CQAM SHC system. Consequently, with a large-linewidth laser source, optical-layer security can be protected by time mismatches of tens of picoseconds. And the digital-layer security is protected by an enormous key space of 10127. The proposed scheme can provide reliable real-time encryption for the optical fiber transmission, serving as a potential candidate for the future high-capacity inter/intra-datacenter security interconnect.

Enabling cost-effective high-performance vibration sensing in digital subcarrier multiplexing systems.

Enabling communication networks with sensing functionality has attracted significant interest lately. The digital subcarrier multiplexing (DSCM) technology is widely promoted in short-reach scenarios for its inherent flexibility of fine-tuning the spectrum. Its compatibility with large-scale as-deployed coherent architectures makes it particularly suited for cost-sensitive integrated sensing and communication applications. In this paper, we propose a scheme of spectrally integrating the digital linear frequency modulated sensing signal into DSCM signals to achieve simultaneous sensing and communication through shared transmitter. Consequently, this cost-effective scheme has been demonstrated to achieve 100-Gb/s dual-polarization quadrature phase-shift keying (DP-QPSK) and 200-Gb/s dual-polarization 16-ary quadrature amplitude modulation (DP-16QAM) transmission with a distributed acoustic sensing sensitivity of 69 pε/Hz and 88 pε/Hz respectively, at a spatial resolution of 4 m.

Blind SOP recovery of eigenvalue communication system based on a nonlinear Fourier transform.

Owing to the random birefringence of optical fibers, the recovery of the state of polarization (SOP) is urgently needed, especially in the nonlinear spectrum division multiplexing transmissions. Based on the variance of the polarization power ratio among symbols as the cost function, we propose a novel algorithm for the blind SOP recovery of eigenvalue communications. In the single eigenvalue transmissions with phase-shift keying or 16-ary amplitude and phase-shift keying constellations, at least 25.3 dB polarization extinction ratio can be achieved by using a block length of 30, even under 7 dB OSNR condition. It also shows that the proposed algorithm can be employed in multi-eigenvalue NFDM transmissions and full-spectrum modulated NFDM system. In the experiment, our proposed algorithm performs the same as the training symbol based method in back-to-back and less than 3000 km fiber link conditions; a maximum performance gain of 1.6 dB was obtained in ultra-long-haul condition (4300 km). It also shows that the impact of the polarization mode dispersion of a single-mode fiber on the algorithm is negligible.

Adaptive bias control of optical IQ modulator with low LFM dither and strong fluctuation resistance.

An innovative approach has been proposed for adaptive bias control in optical IQ modulators. In contrast to traditional approaches that utilize sine dither, this method employs a linear frequency modulated (LFM) signal as the dither, associated with the fractional Fourier Transform (FrFT) to extract the bias point drift. The LFM signal, after undergoing FrFT, transforms into a compressed signal (CS) with energy concentration in the fractional domain. Utilizing this signal for bias point monitoring, the proposed method demonstrates robust bias control even in the presence of substantial interferences, as substantiated by comprehensive simulations and experimental investigations. Remarkably, in a 20-Gbaud 16QAM signal transmission, the proposed approach achieves stable control of the bias point for over 4 hours, even in the presence of voltage fluctuations, while effectively reducing the dither amplitude by half. Furthermore, it maintains a low bit error rate (BER) below 10-5 even under intentional external interference.

PDM-SHD NFDM transmission with envelope-detection feedback polarization tracking and optical injection locking.

Nonlinear frequency division multiplexing (NFDM) systems, especially the eigenvalue communications have the potential to overcome the nonlinear Shannon capacity limit. However, the baud rate of eigenvalue communications is typically restricted to a few GBaud, making it challenging to mitigate laser frequency impairments such as the phase noise and frequency offset (FO) using digital signal processing (DSP) algorithms in intradyne detections (IDs). Therefore, we introduce the polarization division multiplexing-self-homodyne detection (PDM-SHD) into the NFDM link, which could overcome the impact of phase noise and FO by transmitting a pilot carrier originating from the transmitter laser to the receiver through the orthogonal polarization state of signal. To separate the signal from the carrier at the receiver, a carrier to signal power ratio (CSPR) unrestricted adaptive polarization controlling strategy is proposed and implemented by exploiting the optical intensity fluctuation of the light in a particular polarization rather than its direct optical power as the feedback. Optical injection locking (OIL) is used subsequently to amplify optical power of pilot carrier and mitigate the impact of signal-signal beat interference (SSBI). Additionally, the effects of cross-polarization modulation (XPolM) and modulation instability (MI) in long haul transmission are explored and inhibited. The results show that the tolerable FO range is about 3.5 GHz, which is 17 times larger than the ID one. When 16-amplitude phase shift keying (APSK) or 64-APSK constellations are used, identical Q-factor performance can be obtained by using distributed feedback (DFB, ∼10 MHz) laser, external cavity laser (ECL, ∼100kHz), or fiber laser (FL, ∼100 Hz), respectively, which demonstrates that our proposed PDM-SHD eigenvalue communication structure is insensitive to the laser linewidth. Under the impact of cycle slip, the Q-factor difference of 16-APSK signal between the ECL-ID system and ECL-SHD system can be up to 8.73 dB after 1500 km transmission.

Health benefits of saponins and its mechanisms: perspectives from absorption, metabolism, and interaction with gut.

Saponins, consisting of sapogenins as their aglycones and carbohydrate chains, are widely found in plants and some marine organisms. Due to the complexity of the structure of saponins, involving different types of sapogenins and sugar moieties, investigation of their absorption and metabolism is limited, which further hinders the explanation of their bioactivities. Large molecular weight and complex structures limit the direct absorption of saponins rendering their low bioavailability. As such, their major modes of action may be due to interaction with the gastrointestinal environment, such as enzymes and nutrients, and interaction with the gut microbiota. Many studies have reported the interaction between saponins and gut microbiota, that is, the effects of saponins on changing the composition of gut microbiota, and gut microbiota playing an indispensable role in the biotransformation of saponins into sapogenins. However, the metabolic routes of saponins by gut microbiota and their mutual interactions are still sparse. Thus, this review summarizes the chemistry, absorption, and metabolic pathways of saponins, as well as their interactions with gut microbiota and impacts on gut health, to better understand how saponins exert their health-promoting functions.

Modeling and mitigation of polarization crosstalk-induced nonlinearity for the polarization-multiplexed carrier self-homodyne system.

In this Letter, we analytically model the impact of polarization crosstalk in the polarization-multiplexed carrier self-homodyne (PMC-SH) system with adaptive polarization control technology. When the optical paths of the signal and local oscillator (LO) are matched well, it is found that the polarization crosstalk results in a nonlinear shift on the constellation. Thus, we further propose a compensation scheme based on a low-complexity polynomial nonlinear equalizer (PNLE). Both simulation and experimental results validate our theoretical analysis. Moreover, the proposed PNLE-based compensation scheme achieves up to 1.23 dB tolerance improvement with respect to polarization crosstalk for 20 Gbaud 64QAM in the experiment.

3D chaos lidar system with a pulsed master oscillator power amplifier scheme.

We investigated the characteristics of chaos-modulated pulses amplified by a pulsed master oscillator power amplifier (MOPA) for application in a new chaos lidar system in this study. Compared with the loss modulation applied in a continuous-wave (CW) time-gating scheme, the pulsed MOPA scheme could generate chaos-modulated pulses with much higher peak power, resulting in an improved peak-to-standard deviation of sidelobe level (PSLstd) in correlation-based lidar detection. When the pulsed MOPA scheme was applied at a duty cycle of 0.1% and pulse repetition frequency of 20 kHz, which correspond to specifications compliant with eye safety regulations, it outperformed the CW time-gating scheme with respect to PSLstd by 15 dB. For the first time, we applied the chaos lidar system with the pulsed MOPA scheme to execute high-resolution, high-precision three-dimensional (3D) face profiling from a distance of 5 m. We also added the corresponding PSLstd value to each pixel in the point clouds to generate false-color images; thus, we obtained 3D images of a scene with multiple objects at a range of up to 20 m.

All-optical polarization split of the signal and LO for a bi-directional self-homodyne coherent system.

In this Letter, we propose a cost-efficient bi-directional (BiDi) polarization-multiplexed self-homodyne coherent detection (SHCD) system, in which only one fiber link and one adaptive polarization controller (APC) are required. By employing the correlation of the state of polarization (SOP) between the upstream and downstream light, one APC is capable of stabilizing SOPs of the counterpropagating waves at the same time. The signal and local oscillator (LO) can be optically split by a polarization beam splitter (PBS), relaxing pressure of the digital signal processing (DSP) and simplifying the coherent receiver. The impact induced by polarization cross talk and delay mismatch is collectively investigated by theoretical analysis and simulation. Finally, the proposed scheme is experimentally verified through 120 Gbit/s 16-quadrature amplitude modulation (16-QAM) transmission, achieving a satisfying laser-linewidth tolerance of 10 MHz and a polarization rotation tolerance of up to 45 rad/s.

Geometric shaping optimization of 64-APSK constellation in discrete nonlinear frequency division multiplexing systems.

We experimentally demonstrated a geometric shaped (GS) 64-ary amplitude phase shift keying (64-APSK) eigenvalue transmission. The signal is modulated on the scatter coefficient of a single eigenvalue and linear minimum mean square error (LMMSE) estimator is used to reduce the noise. The channel response is estimated by transmitting a normally distributed 64-APSK constellation through a communication link. Based on the polar coordinates distribution of the received constellation, the diameter distributions for each circle can be obtained so that circles with larger noise can obtain larger judgment width. After optimization, the experimental results show that the Q-factor gain is 1.13 dB under 22 dB received optical signal to noise ratio (OSNR) configuration and 0.88 dB after 900 km transmission compared with normally distributed APSK configuration.

Predictors of successful discontinuation from renal replacement therapy during AKI: A meta-analysis.

The predictors of weaning time of renal replacement therapy (RRT) remain controversial for special patients suffering from acute kidney injury (AKI). The present work aims to perform a meta-analysis to evaluate proper predictors of RRT weaning in AKI patients. We systematically searched EMBASE, PubMed, and Cochrane Central Register of Controlled trials for literatures between 1984 and June 2019. Studies evaluating predictors of weaning success of RRT in patients of AKI were included. Random-effects model or fixed-effects model meta-analyses were performed to compute a standard mean difference (SMD). Newcastle-Ottawa Scale was employed to assess the risk of bias. We included 10 observational trials including 1453 patients. Twelve predictors including urine output, serum creatinine, serum urea, mean arterial pressure, central venous pressure, lactate, serum potassium, serum bicarbonate, pH value, SOFA score, urinary urea, and urinary creatinine were identified, showing urine output (p = 0.0000), serum creatinine (p = 0.008), serum potassium (p = 0.02), serum bicarbonate (p = 0.01), pH value (p = 0.03), urinary urea (p = 0.002), and urinary creatinine (p = 0.02) were significantly associated with weaning success. With the limited evidence, we speculate that urine output, serum creatinine, serum potassium, serum bicarbonate, pH value, urinary urea, and urinary creatinine might be associated with successful weaning.

Generations of chaos-modulated pulses based on a gain-switched semiconductor laser subject to delay-synchronized optical feedback for pulsed chaos lidar applications.

We generate and analyze chaos-modulated pulses based on a gain-switched semiconductor laser subject to delay-synchronized optical feedback for pulsed chaos lidar applications. Benefited by the aperiodic and uncorrelated chaos waveforms, chaos lidar possesses the advantages of no range ambiguity and immunity to interference and jamming. To improve the detection range while in compliance with the eye-safe regulation, generating chaos-modulated pulses with higher peak power rather than chaos in its CW form is desired. While using an acousto-optic modulator to time-gate the CW chaos into pulses could be lossy and energy inefficient, in this paper, we study the generation of chaos-modulated pulses using a gain-switched laser subject to delay-synchronized optical feedback. Under different feedback strengths and modulation currents of gain-switching, we investigate the quality of the chaos-modulated pulses generated by analyzing their ratio of chaos oscillations, peak sidelobe levels (PSLs), and cross-correlation peaks under different mismatching conditions between the pulse repetition interval (PRI) and the feedback time delay τ. With proper feedback strengths and modulation currents, we find that synchronizing the gain-switching modulation with the delayed feedback (PRI = τ) is essential in generating the chaos-modulated pulses suitable for the pulsed chaos lidar applications. When mismatching occurs, we identify sequences of dynamical periods including stable, periodic, and chaos oscillations evolved within a pulse.

Receiving sensitivity improvement by wide-spectrum OAM with OTDM and polarization multiplexing in weak atmospheric turbulence.

In this paper, a wide-spectrum orbital angular momentum (OAM) system with a polarization and optical time division multiplexing (OTDM) free-space transmission system is experimentally demonstrated. To enhance the system transmission performance in atmospheric turbulent channel, a wide-spectrum laser and an OAM beam are used. The wide-spectrum laser can be generated by utilizing pumped laser to pump nonlinear fiber, and OAM can be generated with a special light modulator. Furthermore, OTDM and polarization multiplexing methods are used to enhance the communication rate from 4 Gbit/s to 32 Gbit/s. With the use of the wide-spectrum laser and the OAM beam, the receiving scintillation index (SI) can be reduced, and detection sensitivity can be improved. It is the first time a wide-spectrum OAM communication system performance has been studied. It is shown that under weak atmospheric turbulence condition, the SI can be reduced by 38% and the receiving sensitivity can be improved by 3.18 dB via wide-spectrum OAM beams.

Free-space transmission system in a tunable simulated atmospheric turbulence channel using a high-repetition-rate broadband fiber laser.

We demonstrate free-space transmission based on a broadband fiber laser at 16 Gbit/s over a simulated atmospheric turbulence channel. The broadband laser pulse is part of a supercontinuum generated by a homemade picosecond laser based on Raman gain soliton compression pumping a segment of highly nonlinear fiber. The scintillation indices, eye patterns, and bit error rates of transmission based on the broadband laser and a narrow-linewidth laser are compared. The results show a 29.5% reduction in the scintillation index and sensitivity of -28.6 dBm at the forward error correction limit, which has a 2.9 dB improvement compared with the narrow-linewidth system. It is feasible to use broadband lasers as carriers combining optical time division multiplexing as a multiplexing method to improve the communication performance under weak atmospheric turbulent conditions.

3D pulsed chaos lidar system.

We develop an unprecedented 3D pulsed chaos lidar system for potential intelligent machinery applications. Benefited from the random nature of the chaos, conventional CW chaos lidars already possess excellent anti-jamming and anti-interference capabilities and have no range ambiguity. In our system, we further employ self-homodyning and time gating to generate a pulsed homodyned chaos to boost the energy-utilization efficiency. Compared to the original chaos, we show that the pulsed homodyned chaos improves the detection SNR by more than 20 dB. With a sampling rate of just 1.25 GS/s that has a native sampling spacing of 12 cm, we successfully achieve millimeter-level accuracy and precision in ranging. Compared with two commercial lidars tested side-by-side, namely the pulsed Spectroscan and the random-modulation continuous-wave Lidar-lite, the pulsed chaos lidar that is in compliance with the class-1 eye-safe regulation shows significantly better precision and a much longer detection range up to 100 m. Moreover, by employing a 2-axis MEMS mirror for active laser scanning, we also demonstrate real-time 3D imaging with errors of less than 4 mm in depth.

Scintillation index reducing based on wide-spectral mode-locking fiber laser carriers in a simulated atmospheric turbulent channel.

We report on a communication link demonstration in a 1 km simulated atmospheric turbulent channel with a wide-spectral mode-locking fiber laser. Wide-spectral beams are part of the supercontinuum, which is generated from pumping a dispersion-shifted fiber by an active mode-locked fiber laser. In addition, the propagation effects of wide-spectral beams were investigated experimentally in a simulated atmosphere channel. The characteristics of bit error rate and eye pattern before and after turbulence were analyzed, respectively. The experimental results showed that the sensitivity of the whole link reaches -40 dBm. The scintillation index of free-space optical communication between wide-spectral partially coherent and narrow-spectral coherent beams was compared, which indicates that the wide-spectral partially coherent optical communication link is more resistant to atmospheric turbulence. We experimentally verified that the scintillation index of wide-spectral carriers is dependent on coherent degree rather than spectral width.

Defects of Lipid Synthesis Are Linked to the Age-Dependent Demyelination Caused by Lamin B1 Overexpression.

Lamin B1 is a component of the nuclear lamina and plays a critical role in maintaining nuclear architecture, regulating gene expression and modulating chromatin positioning. We have previously shown that LMNB1 gene duplications cause autosomal dominant leukodystrophy (ADLD), a fatal adult onset demyelinating disease. The mechanisms by which increased LMNB1 levels cause ADLD are unclear. To address this, we used a transgenic mouse model where Lamin B1 overexpression is targeted to oligodendrocytes. These mice showed severe vacuolar degeneration of the spinal cord white matter together with marked astrogliosis, microglial infiltration, and secondary axonal damage. Oligodendrocytes in the transgenic mice revealed alterations in histone modifications favoring a transcriptionally repressed state. Chromatin changes were accompanied by reduced expression of genes involved in lipid synthesis pathways, many of which are known to play important roles in myelin regulation and are preferentially expressed in oligodendrocytes. Decreased lipogenic gene expression resulted in a significant reduction in multiple classes of lipids involved in myelin formation. Many of these gene expression changes and lipid alterations were observed even before the onset of the phenotype, suggesting a causal role. Our findings establish, for the first time, a link between LMNB1 and lipid synthesis in oligodendrocytes, and provide a mechanistic framework to explain the age dependence and white matter involvement of the disease phenotype. These results have implications for disease pathogenesis and may also shed light on the regulation of lipid synthesis pathways in myelin maintenance and turnover. SIGNIFICANCE STATEMENT: Autosomal dominant leukodystrophy (ADLD) is fatal neurological disorder caused by increased levels of the nuclear protein, Lamin B1. The disease is characterized by an age-dependent loss of myelin, the fatty sheath that covers nerve fibers. We have studied a mouse model where Lamin B1 level are increased in oligodendrocytes, the cell type that produces myelin in the CNS. We demonstrate that destruction of myelin in the spinal cord is responsible for the degenerative phenotype in our mouse model. We show that this degeneration is mediated by reduced expression of lipid synthesis genes and the subsequent reduction in myelin enriched lipids. These findings provide a mechanistic framework to explain the age dependence and tissue specificity of the ADLD disease phenotype.