Turbulence mitigation of four mode-division-multiplexed QPSK channels in a pilot-assisted self-coherent free-space optical link using a photodetector array and DSP-based channel demultiplexing.

In this Letter, we demonstrate turbulence mitigation of four mode-division-multiplexing (MDM) quadrature-phase-shift-keying (QPSK) channels in a pilot-assisted self-coherent free-space optical (FSO) link using a photodetector (PD) array and digital signal processing (DSP)-based channel demultiplexing. A Gaussian pilot beam is co-transmitted with four 1-Gbaud QPSK channels carried by four orbital angular momentum (OAM) modes. The pilot beam experiences similar turbulence-induced wavefront distortion to the data beams. At the receiver, the turbulence distortion is mitigated by its conjugate during the pilot-data mixing in a PD array. Subsequently, we demultiplex the four channels by applying in DSP a fixed matrix on the signals. Results show that our approach has <3-dB turbulence-induced power penalty at a 7% forward error correction (FEC) limit under a turbulence strength of 2w0/r0 = ∼4.4. The same turbulence can cause >18-dB penalties for a local oscillator (LO)-based coherent MDM system.

Adaptive-optics-based turbulence mitigation in a 400 Gbit/s free-space optical link by multiplexing Laguerre-Gaussian modes varying both radial and azimuthal spatial indices: publisher's note.

This publisher's note contains a correction to Opt. Lett.48, 6452 (2023)10.1364/OL.506270.

Adaptive-optics-based turbulence mitigation in a 400†Git/s free-space optical link by multiplexing Laguerre-Gaussian modes varying both radial and azimuthal spatial indices.

In general, atmospheric turbulence can degrade the performance of free-space optical (FSO) communication systems by coupling light from one spatial mode to other modes. In this Letter, we experimentally demonstrate a 400 Gbit/s quadrature-phase-shift-keyed (QPSK) FSO mode-division-multiplexing (MDM) coherent communication link through emulated turbulence using four Laguerre Gaussian (LG) modes with different radial and azimuthal indices (L G 10, L G 11, L G -10, and L G -11). To mitigate turbulence-induced channel cross talk and power loss, we implement an adaptive optics (AO) system at the receiver end. A Gaussian beam at a slightly different wavelength is co-propagated with the data beams as the probe beam. We use a wavefront sensor (WFS) to measure the wavefront distortion of this probe beam, and this information is used to tune a spatial light modulator (SLM) to adaptively correct the four distorted data-beam wavefronts. Using this adaptive-optics approach, the power loss and cross talk are reduced by ∼10 and ∼18 dB, respectively.

Atmospheric turbulence strength distribution along a propagation path probed by longitudinally structured optical beams.

Atmospheric turbulence can cause critical problems in many applications. To effectively avoid or mitigate turbulence, knowledge of turbulence strength at various distances could be of immense value. Due to light-matter interaction, optical beams can probe longitudinal turbulence changes. Unfortunately, previous approaches tended to be limited to relatively short distances or large transceivers. Here, we explore turbulence probing utilizing multiple sequentially transmitted longitudinally structured beams. Each beam is composed of Bessel-Gaussian ([Formula: see text]) modes with different [Formula: see text] values such that a distance-varying beam width is produced, which results in a distance- and turbulence-dependent modal coupling to [Formula: see text] orders. Our simulation shows that this approach has relatively uniform and low errors (<0.3 dB) over a 10-km path with up to 30-dB turbulence-structure-constant variation. We experimentally demonstrate this approach for two emulated turbulence regions (~15-dB variation) with <0.8-dB errors. Compared to previous techniques, our approach can potentially probe longer distances or require smaller transceivers.

Automatic turbulence mitigation for coherent free-space optical links using crystal-based phase conjugation and fiber-coupled data modulation.

There are various performance advantages when using temporal phase-based data encoding and coherent detection with a local oscillator (LO) in free-space optical (FSO) links. However, atmospheric turbulence can cause power coupling from the Gaussian mode of the data beam to higher-order modes, resulting in significantly degraded mixing efficiency between the data beam and a Gaussian LO. Photorefractive crystal-based self-pumped phase conjugation has been previously demonstrated to "automatically" mitigate turbulence with limited-rate free-space-coupled data modulation (e.g., <1 Mbit/s). Here, we demonstrate automatic turbulence mitigation in a 2-Gbit/s quadrature-phase-shift-keying (QPSK) coherent FSO link using degenerate four-wave-mixing (DFWM)-based phase conjugation and fiber-coupled data modulation. Specifically, we counter-propagate a Gaussian probe from the receiver (Rx) to the transmitter (Tx) through turbulence. At the Tx, we generate a Gaussian beam carrying QPSK data by a fiber-coupled phase modulator. Subsequently, we create a phase conjugate data beam through a photorefractive crystal-based DFWM involving the Gaussian data beam, the turbulence-distorted probe, and a spatially filtered Gaussian copy of the probe beam. Finally, the phase conjugate beam is transmitted back to the Rx for turbulence mitigation. Compared to a coherent FSO link without mitigation, our approach shows up to ∼14-dB higher LO-data mixing efficiency and achieves error vector magnitude (EVM) performance of <16% under various turbulence realizations.

Turbulence-resilient pilot-assisted self-coherent free-space optical communications using a photodetector array for bandwidth enhancement.

We experimentally demonstrate a 4-Gbit/s 16-QAM pilot-assisted, self-coherent, and turbulence-resilient free-space optical link using a photodetector (PD) array. The turbulence resilience is enabled by the efficient optoelectronic mixing of the data and pilot beams in a free-space-coupled receiver, which can automatically compensate for turbulence-induced modal coupling to recover the data's amplitude and phase. For this approach, a sufficient PD area might be needed to collect the beams while the bandwidth of a single larger PD could be limited. In this work, we use an array of smaller PDs instead of a single larger PD to overcome the beam collection and bandwidth response trade-off. In the PD-array-based receiver, the data and pilot beams are efficiently mixed in the aggregated PD area formed by four PDs, and the four mixing outputs are electrically combined for data recovery. The results show that: (i) either with or without turbulence effects (D/r0 = ∼8.4), the 1-Gbaud 16-QAM signal recovered by the PD array has a lower error vector magnitude than that of a single larger PD; (ii) for 100 turbulence realizations, the pilot-assisted PD-array receiver recovers 1-Gbaud 16-QAM data with a bit-error rate below 7% of the forward error correction limit; and (iii) for 1000 turbulence realizations, the average electrical mixing power loss of a single smaller PD, a single larger PD, and a PD array is ∼5.5 dB, ∼1.2 dB, and ∼1.6 dB, respectively.

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.

High-Performance Organic Photodetectors by Introducing a Non-Fullerene Acceptor to Broaden Long Wavelength Detective Spectrum.

We demonstrate the broadband visible organic photodetectors (OPDs) by introducing a non-fullerene acceptor of 3,9-bis(2-methylene-(3-(1,1dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3d:2,3'-d']-s-indaceno[1,2-b:5,6-b']dithiophene (ITIC) into the bulk heterojunction (BHJ) based on a conventional system of poly(3-hexylthiophene-2,5-diyl) (P3HT):[6,6]-phenyl C71-butyric acid methyl ester (PC71BM) .The resultant OPDs exhibit a specific detectivity beyond 1012 Jones in the whole visible region ranged from 380 nm to 760 nm, and the highest detectivity reaches 2.67 × 1012 Jones at 710 nm. UV-Vis absorption spectrum, steady-state photoluminescence, atomic force microscopy, and space-charge-limited current property were applied to analyze the film characteristics of obtained OPDs. Owing to the long-wavelength absorption band of ITIC, the spectral photodetection range has been broadened effectively, and better film morphology, more effective energy transfer, and the reduced electron mobility in the active layer are responsible for the excellent photodetection capability. The proposed scheme provides a reliable strategy for implementing high-performance broadband visible OPDs.

Cumulative alcohol consumption and stroke risk in men.

BACKGROUNDS: Views on the relationship between alcohol consumption and stroke risk remain controversial. Moreover, data on cumulative alcohol intake are limited. We examined the potential impact of cumulative alcohol consumption on the risk of total stroke and its subtypes in men. METHODS: This prospective study included 23,433 men from the Kailuan Study. Cumulative alcohol consumption was taken as the primary exposure by calculating self-reported alcohol consumption from three consecutive examinations (in 2006, 2008, and 2010). The first occurrence of stroke was confirmed by reviewing medical records from 2010 to 2016. We used Cox proportional hazards regression to analyze the data. RESULTS: During the 5.9 ± 0.8 years of follow-up, 678 total strokes were identified, including 595 ischemic stroke (IS), 90 intracerebral hemorrhage and 19 subarachnoid hemorrhage cases. The adjusted hazard ratios (95% confidence intervals) of total stroke for light, moderate and heavy cumulative alcohol consumption were 1.23 (1.01-1.51), 1.49 (1.13-1.97), and 1.50 (1.21-1.86), respectively, compared with those of nondrinkers. The results were similar for IS. Cumulative alcohol consumption was not associated with intracerebral hemorrhage risk (hazard ratio 1.46; 95% confidence interval, 0.74-2.08). CONCLUSIONS: Cumulative alcohol consumption is an independent risk factor of total stroke and IS in men in a community-based cohort. Even light alcohol intake increases the risk of total stroke and IS.

[Association of serum uric acid level with amyotrophic lateral sclerosis].

OBJECTIVE: To investigate the association between the serum uric acid (UA) level and amyotrophic lateral sclerosis(ALS). METHODS: The serum UA levels were measured from 164 ALS patients and 164 gender and age matched controls. The serum creatinine (Cr) and blood urea nitrogen (BUN) levels were also measured to reduce the bias caused by possible differences in renal excretion function.The differences of the above indexes were compared between the two groups. RESULTS: The serum UA levels in ALS patients were lower than those in healthy controls ((310±88) µmol/L vs (356±78) µmol/L, P<0.01). The differences were observed in both genders. In a Logistic regression model using ALS as a dependent variable, the serum UA level was significantly associated with ALS. The differences of the serum UA levels in ALS patients stratified by the revised ALS functional rating scale (ALSFRS-R) and the progression rate of ALSFRS-R(ΔFS) were not statistically significant (P>0.05). CONCLUSIONS: ALS patients are found to have significantly lower serum UA levels than healthy individuals. The serum UA level is not involved in disease severity and progression.

Relationship between expression of matrix metalloproteinase-9 and adenylyl cyclase-associated protein 1 in chronic obstructive pulmonary disease.

OBJECTIVE: To investigate the relationship between expression of matrix metalloproteinase (MMP)-9 and expression of adenylyl cyclase-associated protein (CAP)-1 in chronic obstructive pulmonary disease (COPD). METHODS: Patients with possible respiratory disease were recruited into the study and divided into a COPD group and a non-COPD group on diagnosis. Pulmonary function tests were performed and serum concentrations of MMP-9 were measured using an enzyme-linked immunosorbent assay. MMP-9 and CAP1 expression were analysed in lung tissue and bronchoalveolar lavage fluid in all available samples using immunohistochemistry and Western blot, respectively. In addition, expression of MMP-9 and CAP1 in vitro was investigated using immunofluorescence. Expression of CAP1 in response to MMP-9 was measured in the human alveolar epithelial cell line HP-AEpiC, using Western blot. RESULTS: A total of 90 patients were included in the study: 52 were in the COPD group and 38 in the non-COPD group. Serum MMP-9 concentrations were significantly higher in the COPD than in the non-COPD group. MMP-9 serum concentrations were negatively correlated with forced expiratory volume in 1 s (FEV1), FEV1 as a percentage of the normal predicted value and the ratio of FEV1 to forced vital capacity, and were positively correlated with residual volume (RV), total lung capacity (TLC) and RV/TLC values. In lung tissue and bronchoalveolar lavage fluid samples, MMP-9 and CAP1 expression were inversely related. This relationship was confirmed in HP-AEpiC cells. High expression of MMP-9 and low expression of CAP1 was demonstrated in the COPD group compared with the non-COPD group. CONCLUSIONS: This study demonstrated an inverse relationship between CAP1 and MMP-9 expression, and high expression of MMP-9 and low expression of CAP1 in those with COPD compared with the non-COPD group. Overexpression of MMP-9 in lung tissue and its interaction with CAP1 is likely to play a major role in airway obstruction in COPD.