Adaptive decision threshold for an optical multipath-interference-impaired short-reach 50-Gbps PAM4 transmission.

The short-reach optical transmission systems based on intensity modulation and direct detection (IM/DD) are gradually evolving into the networks with complex link topologies and connections, especially inside the data center. Multipath interference (MPI) introduces irregular fluctuations in the 4-level pulse amplitude modulation (PAM4) signals and therefore affects the transmission performance. This Letter proposes an adaptive decision threshold (ADT) scheme to dynamically update the decision threshold, which can track the signal fluctuations in real time and mitigate the impact of MPI noise on the transmitted PAM4 signals. Numerical simulation results present that the proposed ADT scheme can significantly improve the bit error rate (BER) performance of the MPI-impaired PAM4 transmission system considering different MPI levels and laser linewidths. In a 50-Gbps PAM4 transmission system, the ADT can improve the MPI tolerance by more than beyond 6 dB when the BER reaches the KP4-forward error correction (FEC) criterion (2.4 × 10-4), presenting a better denoising performance than the existing MPI-mitigation algorithms A1 and A2. Moreover, the ADT scheme offers a lower computation complexity compared with A1 and A2, making it more practical for implementation.

Intracavity filtering effect in a dual-output linear-cavity all-PM fiber laser mode-locked by NPE.

We have demonstrated a stable and low-noise all-polarization-maintaining (PM) ultrafast erbium-doped fiber laser mode-locked via nonlinear polarization evolution (NPE) in a linear cavity with dual outputs. A detailed design strategy is presented. The all-PM configuration enhances the capability of resistance to environmental fluctuations. Self-starting mode-locking is realized by using a non-reciprocal phase shifter. The dual-output structure offers the intracavity filtering effect, where the reflective port serves as a bandpass spectral filter, significantly improving the transmissive-port optical properties. The laser directly generates ultrashort pulses with a pulse duration of 129 fs operating at a fundamental repetition rate of 105.8 MHz. The integrated root-mean-square (RMS) relative intensity noise from 10 Hz to 10 MHz is ∼0.008%, and the integrated RMS timing jitter from 5 kHz to 10 MHz is ∼36f s. Long-term stability is confirmed in 25 h with a RMS power fluctuation of  ∼0.10%. Our high-performance fiber laser is a prospective candidate for low-noise applications.

Efficient capacity enhancement using OFDM with interleaved subcarrier number modulation in bandlimited UOWC systems.

We propose and demonstrate an efficient capacity enhancement scheme for bandlimited underwater optical wireless communication (UOWC) systems by utilizing orthogonal frequency division multiplexing with interleaved subcarrier number modulation (OFDM-ISNM). In the proposed OFDM-ISNM, joint number and constellation mapping/de-mapping is utilized to avoid error propagation and subblock interleaving is further applied to address the low-pass effect of the bandlimited UOWC system. The feasibility and superiority of the proposed OFDM-ISNM scheme for practical bandlimited UOWC systems have been verified through both simulations and experiments. The obtained results demonstrate that the proposed OFDM-ISNM scheme is capable of efficiently improving the achievable data rate of the bandlimited UOWC system. Specifically, the experimental results show a significant 28.6% capacity enhancement by OFDM-ISNM over other benchmark schemes, achieving a data rate of 3.6 Gbps through a 2-m water channel.

Machine-learning iterative optimization for all polarization-maintaining linear cavity Er:fiber laser.

All polarization-maintaining (PM) linear cavity mode-locked fiber lasers are promising ultrafast laser sources due to their compactness and environmental robustness. Here, we demonstrate a linear cavity fiber laser with all-PM configuration experimentally and investigate the mode-locking formation of the laser using a machine-learning iterative optimization method based on the Gaussian process. The optimization algorithm can converge rapidly after only 30 runs. Using the optimized parameters, we simulate the generation of mode-locked pulses from noise. The output spectrum and pulse energy are highly consistent with the experiment. Furthermore, we describe the intracavity dynamic evolution under group velocity mismatch. We then show that the pulse trapping induced by cross-phase modulation leads to the overcompensated time synchronization between the orthogonally polarized components.

Compact dual-mode waveguide crossing based on adjoint shape optimization.

We design, fabricate, and characterize a compact dual-mode waveguide crossing on a silicon-on-insulator platform. The dual-mode waveguide crossing with high performance is designed by utilizing the adjoint shape optimization. This adjoint-method-based optimization algorithm is computationally efficient and yields the optimal solution in fewer iterations compared with other iterative schemes. Our proposed dual-mode waveguide crossing exhibits low insertion loss and low crosstalk. Experimental results show that the insertion losses at the wavelength of 1550 nm are 0.83 dB and 0.50 dB for TE0 and TE1 modes, respectively. The crosstalk is less than -20 dB for the two modes over a wavelength range of 80 nm. The footprint of the whole structure is only 5 × 5 µm2.

Si/Organic Integrated Narrowband Near-Infrared Photodetector.

Spectrally selective narrowband photodetection is critical for near-infrared (NIR) imaging applications, such as for communicationand night-vision utilities. It is a long-standing challenge for detectors based on silicon, to achieve narrowband photodetection without integrating any optical filters. Here, this work demonstrates a NIR nanograting Si/organic (PBDBT-DTBT:BTP-4F) heterojunction photodetector (PD), which for the first time obtains the full-width-at-half-maximum (FWHM) of only 26 nm and fast response of 74 µs at 895 nm. The response peak can be successfully tailored from 895 to 977 nm. The sharp and narrow response NIR peak is inherently attributed to the coherent overlapping between the NIR transmission spectrum of organic layer and diffraction enhanced absorption peak of patterned nanograting Si substrates. The finite difference time domain (FDTD) physics calculation confirms the resonant enhancement peaks, which is well consistent with the experiment results. Meanwhile, the relative characterization indicates that the introduction of the organic film can promote carrier transfer and charge collection, facilitating efficient photocurrent generation. This new device design strategy opens up a new window in developing low-cost sensitive NIR narrowband detection.

Enhancing underwater VLC with spatial division transmission and pairwise coding.

In this paper, we propose and evaluate two spatial division transmission (SDT) schemes, including spatial division diversity (SDD) and spatial division multiplexing (SDM), for underwater visible light communication (UVLC) systems. Moreover, three pairwise coding (PWC) schemes, including two one-dimensional PWC (1D-PWC) schemes, i.e., subcarrier PWC (SC-PWC) and spatial channel PWC (SCH-PWC), and one two-dimensional PWC (2D-PWC) scheme are further applied for signal-to-noise ratio (SNR) imbalance mitigation in the UVLC systems using SDD and SDM with orthogonal frequency division multiplexing (OFDM) modulation. The feasibility and superiority of applying SDD and SDM with various PWC schemes in a practical bandlimited two-channel OFDM-based UVLC system have been verified through both numerical simulations and hardware experiments. The obtained results show that the performance of SDD and SDM schemes are largely determined by both the overall SNR imbalance and the system spectral efficiency. Moreover, the experimental results demonstrate the robustness of SDM with 2D-PWC against bubble turbulence. Specifically, SDM with 2D-PWC can obtain bit error rates (BERs) under the 7% forward error correction (FEC) coding limit of 3.8 × 10-3 with a probability higher than 96% for a signal bandwidth of 70 MHz and a spectral efficiency of 8 bits/s/Hz, achieving an overall data rate of 560 Mbits/s.

[Roles of the CXCR1/CXCL8 axis in abnormal proliferation of bile duct epithelial cells in primary biliary cholangitis].

Objective: To investigate the role of the CXC chemokine receptor 1 (CXCR1)/CXC chemokine ligand 8 (CXCL8) axis in the abnormal proliferation of bile duct epithelial cells in primary biliary cholangitis (PBC). Methods: 30 female C57BL/6 mice were randomly divided into the PBC model group (PBC group), reparixin intervention group (Rep group), and blank control group (Con group) in an in vivo experiment. PBC animal models were established after 12 weeks of intraperitoneal injection of 2-octanoic acid coupled to bovine serum albumin (2OA-BSA) combined with polyinosinic acid polycytidylic acid (polyI:C). After successful modelling, reparixin was injected subcutaneously into the Rep group (2.5 mg · kg(-1) · d(-1), 3 weeks). Hematoxylin-eosin staining was used to detect histological changes in the liver. An immunohistochemical method was used to detect the expression of cytokeratin 19 (CK-19). Tumor necrosis factor-α (TNF-α), γ-interferon (IFN-γ) and interleukin (IL)-6 mRNA expression were detected by qRT-PCR. Western blot was used to detect nuclear transcription factor-κB p65 (NF-κB p65), extracellularly regulated protein kinase 1/2 (ERK1/2), phosphorylated extracellularly regulated protein kinase 1/2 (p-ERK1/2), Bcl-2-related X protein (Bax), B lymphoma-2 (Bcl-2), and cysteine proteinase-3 (Caspase- 3) expression. Human intrahepatic bile duct epithelial cells were divided into an IL-8 intervention group (IL-8 group), an IL-8+Reparicin intervention group (Rep group), and a blank control group (Con group) in an in vitro experiment. The IL-8 group was cultured with 10 ng/ml human recombinant IL-8 protein, and the Rep group was cultured with 10 ng/ml human recombinant IL-8 protein, followed by 100 nmol/L Reparicin. Cell proliferation was detected by the EdU method. The expression of TNF-α, IFN-γ and IL-6 was detected by an enzyme-linked immunosorbent assay. The expression of CXCR1 mRNA was detected by qRT-PCR. The expression of NF-κB p65, ERK1/2 and p-ERK1/2 was detected by western blot. A one-way ANOVA was used for comparisons between data sets. Results: The results of in vivo experiments revealed that the proliferation of cholangiocytes, the expression of NF-κB and ERK pathway-related proteins, and the expression of inflammatory cytokines were increased in the Con group compared with the PBC group. However, reparixin intervention reversed the aforementioned outcomes (P<0.05). In vitro experiments showed that the proliferation of human intrahepatic cholangiocyte epithelial cells, the expression of CXCR1 mRNA, the expression of NF-κB and ERK pathway-related proteins, and the expression of inflammatory cytokines were increased in the IL-8 group compared with the Con group. Compared with the IL-8 group, the proliferation of human intrahepatic cholangiocyte epithelial cells, NF-κB and ERK pathway-related proteins, and inflammatory indicators were significantly reduced in the Rep group (P < 0.05). Conclusion: The CXCR1/CXCL8 axis can regulate the abnormal proliferation of bile duct epithelial cells in PBC, and its mechanism of action may be related to NF-κB and ERK pathways.

Compact hybrid five-mode multiplexer based on asymmetric directional couplers with constant bus waveguide width.

We experimentally demonstrate a hybrid mode division multiplexer (MDM) based on asymmetric directional couplers (ADCs) without transition tapers in between. The proposed MDM can couple five fundamental modes from access waveguides into the bus waveguide as the hybrid modes (TE0, TE1, TE2, TM0, and TM1). To eliminate the transition tapers between cascaded ADCs as well as to enable arbitrary add-drop to the bus waveguide, we maintain the bus waveguide width to be the same, while a partially etched subwavelength grating is introduced to reduce the effective refractive index of the bus waveguide. The experimental results demonstrate a working bandwidth of up to 140 nm.

Convolutional-neural-network-based versus vision-transformer-based SNR estimation for visible light communication networks.

Visible light communication (VLC) has emerged as a promising technology for future sixth-generation (6 G) communications. Estimating and predicting the impairments, such as turbulence and free space signal scattering, can help to construct flexible and adaptive VLC networks. However, the monitoring of impairments of VLC is still in its infancy. In this Letter, we experimentally demonstrate a deep-neural-network-based signal-to-noise ratio (SNR) estimation scheme for VLC networks. A vision transformer (ViT) is first utilized and compared with the conventional scheme based on a convolutional neural network (CNN). Experimental results show that the ViT-based scheme exhibits robust performance in SNR estimation for VLC networks compared to the CNN-based scheme. Specifically, the ViT-based scheme can achieve accuracies of 76%, 63.33%, 45.33%, and 37.67% for 2-quadrature amplitude modulation (2QAM), 4QAM, 8QAM, and 16QAM, respectively, against 65%, 57.67%, 41.67%, and 34.33% for the CNN-based scheme. Additionally, data augmentation has been employed for achieving enhanced SNR estimation accuracies of 95%, 79.67%, 58.33%, and 50.33% for 2QAM, 4QAM, 8QAM, and 16QAM, respectively. The effect of the SNR step size of a contour stellar image dataset on the SNR estimation accuracy is also studied.

4-bit DAC based 6.9Gb/s PAM-8 UOWC system using single-pixel mini-LED and digital pre-compensation.

Low-cost underwater wireless optical communication (UOWC) systems are attractive for high-speed connections among unmanned vehicles or devices in various underwater applications. Here we demonstrate a high-speed and low-cost UOWC system using a low-resolution digital to analog converter (DAC), a single-pixel mini-sized light-emitting diode (mini-LED), and digital pre-compensation (DPC). The enabled DPC scheme comprises digital pre-distortion (DPD), digital pre-emphasis (DPE), and digital resolution enhancer (DRE), which pre-compensate for mini-LED nonlinearity, the bandwidth limitation of the mini-LED and avalanche photodiode detector, and DAC resolution limitation, respectively. The simulation results show that the in-band signal-to-quantization noise ratio can be increased by 6.8 dB using DRE based on a 4-bit DAC. To further improve the system capacity, we tune the level of DPE in order to optimize the trade-off between the residual inter-symbol interference and signal-to-noise ratio. With the combination of optimized DPE and DRE, we obtain a 21.1% higher data rate compared with full DPE only and demonstrate the transmission of 6.9 Gb/s PAM-8 signal over a 2-m distance underwater based on a single-pixel mini-LED and 4-bit DAC. This paper reports a cost-effective UOWC system first using a low-resolution DAC and DPC, which offers a promising path toward low-cost underwater optical wireless networks.

LiDAR integrated IR OWC system with the abilities of user localization and high-speed data transmission.

By using narrow infrared (IR) optical beams, optical wireless communication (OWC) system can realize ultra-high capacity and high-privacy data transmission. However, due to the point-to-point connection approach, a high accuracy localization system and beam-steering antenna (BSA) are required to steer the signal beam to user terminals. In this paper, we proposed an indoor beam-steering IR OWC system with high accuracy and calibration-free localization ability by employing a coaxial frequency modulated continuous wave (FMCW) light detection and ranging (LiDAR) system. In the meantime, benefitting from the mm-level ranging accuracy of the LiDAR system, a useful approach to assess the feasibility of the link alignment between beam-steering antenna and users is first demonstrated. With the assistance of the LiDAR system, we experimentally achieved the localization of user terminals with a 0.038-degree localization accuracy and on-off keying (OOK) downlink error-free transmission of 17 Gb/s in free space at a 3-m distance is demonstrated. The highest transmission data rate under the forward error correction (FEC) criterion (Bit error rate (BER) <3.8×103) can reach 24 Gb/s.

Ultrafast dynamic switching of optical response based on nonlinear hyperbolic metamaterial platform.

The pursuit of high-speed and on-chip optical communication systems has promoted extensive exploration of all-optical control of light-matter interactions via nonlinear optical processes. Here, we have numerically investigated the ultrafast dynamic switching of optical response using tunable hyperbolic metamaterial (HMM) which consists of five pairs of alternating layers of indium tin oxide (ITO) and SiO2. The nonlinearity of the HMM is analyzed by the ultrafast dynamics of the hot electrons in the epsilon-near-zero (ENZ) ITO. Our approach allows large and broad all-optical modulation of the effective permittivity and topology of the HMM on the femtosecond time-scale. Based on the proposed HMM platform, we have shown considerable tunability in the extinction ratio and Purcell enhancement under various pump fluence. In addition, we have achieved all-optical control of the coupling strength through depositing plasmonic resonators on the HMM platform. A significant tuning of the coupled resonance is observed by changing pump fluence, which leads to a switching time within 213 fs at a specific wavelength with a relative modulation depth more than 15 dB.

Application and comparison of active and transfer learning approaches for modulation format classification in visible light communication systems.

Automatic modulation classification (AMC) is a crucial part of adaptive modulation schemes for visible light communication (VLC) systems. However, most of the deep learning (DL) based AMC methods for VLC systems require a large amount of labeled training data which is quite difficult to obtain in practical systems. In this work, we introduce active learning (AL) and transfer learning (TL) approaches for AMC in VLC systems and experimentally analyze their performances. Experimental results show that the proposed novel AlexNet-AL and AlexNet-TL methods can significantly improve the classification accuracy with small sizes of training data. To be specific, using 60 labeled samples, AlexNet-AL and AlexNet-TL increase the classification accuracy by 6.82% and 14.6% compared to the result without AL and TL, respectively. Moreover, the use of data augmentation (DA) operation along with our proposed methods helps achieve further better performances.

All-optical switching in epsilon-near-zero asymmetric directional coupler.

We propose an all-optical switch based on an asymmetric directional coupler structure with epsilon-near-zero (ENZ) layer. The nonlinear optical properties the of ENZ layer are analyzed by hot-electron dynamics process, and the all-optical operating performance of the switch on the silicon nitride platform is investigated. It is found that the pump-induced refractive index change in ENZ layer gives rise to a transfer of signal light in the optical system. We demonstrate that the proposed switch design features an insertion loss of < 2.7 dB, low crosstalk of < - 18.93 dB, and sub-pico-second response time at the communication wavelength of 1.55 µm. With ultrafast response, high performance, and simple structure, the device provides new possibilities for all-optical communication and signal processing.

Ultrafast dynamic switching of optical response based on nonlinear hyperbolic metamaterial platform: erratum.

We present an erratum to our previously published work ["Ultrafast dynamic switching of optical response based on nonlinear hyperbolic metamaterial platform," Opt. Express30(12), 21634 (2022).10.1364/OE.457875]. The corrections do not affect the results and conclusion of the original paper.

All-inorganic liquid phase quantum dots and blue laser diode-based white-light source for simultaneous high-speed visible light communication and high-efficiency solid-state lighting.

In recent years, cesium lead bromide (CsPbBr3) and cadmium selenide/zinc sulfide (CdSe/ZnS) quantum dots have been widely investigated to enhance the capacity of visible light communication (VLC) and solid-state lighting (SSL). Herein, liquid-phase color converter (LCC) glass cavities and solid-phase color converter (SCC) films with green-emitting CsPbBr3 and red-emitting CdSe/ZnS are fabricated to investigate and compare their performance. A facile high-quality LCC-based white laser diode (WLD) is fabricated by combining blue LD with LCC CsPbBr3 and CdSe/ZnS glass cavities as color conversion layers. The LCC-based WLD achieves bright white light with a color rendering index of 85, a correlated color temperature of 5520 K, and a Commission Internationale de L'Eclairage (CIE) coordinates at (0.32, 0.34). Moreover, the VLC system exhibits a modulation bandwidth of 855 MHz and the capability to transmit a real-time data rate of up to 2.1 Gbps over a transmission distance of 1.2 meters. These results indicate that the fabricated WLD is a promising lighting device for simultaneous high-speed VLC and high-efficiency SSL.

THO-OFDM scheme for visible light communication with noise suppression and dimming control.

In recent years, visible light communication (VLC) has emerged as a dual-use technique for illumination as well as communication. In VLC system, dimming control is deemed to be an essential functionality for luminous intensity adjusting and energy saving. In this Letter, a noise-suppressed triple-layer hybrid optical orthogonal frequency division multiplexing (NSTHO-OFDM) is proposed and further combined with pulse width modulation (PWM) to achieve the functionality of dimming control. The proposed NSTHO-OFDM can attain a 4.28-dB maximum power gain compared with THO-OFDM with an overall bit error rate (BER) of 1 × 10-4. Moreover, the proposed dimming scheme can achieve a wide dimming range of 89.0% while maintaining a favorable BER below 3.8 × 10-3.

A Highly Versatile Porous Core Photonic Quasicrystal Fiber Based Refractive Index Terahertz Sensor.

Miniaturized real-time fiber optic sensing systems with high sensing performance are in extreme demand. In this work, we propose a novel photonic quasicrystal fiber sensor in the terahertz region and test its sensing characteristics using the finite element method. The proposed simulated sensor numerically investigates the cancer-infected cells from the normal cells in the human cervix, blood, adrenal glands, and breast based on the difference in their refractive index changes. The effective refractive index of core-guided mode is due to the interaction of light between the refractive index of the fiber material and infiltrated normal and cancer cells, respectively. The proposed sensor exhibits a high birefringence of 0.03, a low dispersion of 0.35 ps/THz/cm, along with a high numerical aperture of 0.99. Besides, the sensor holds a less-effective material loss of 2.53 × 10-9 (dB/cm), a maximum power fraction of 88.10, a maximum relative sensitivity of 82.67%, and an effective mode area of 3.16 mm2. The results envisage that the proposed sensor displays high sensing performances with a rapid cancer detection mechanism.