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.