Search Results (88)

Terahertz wireless communication offers ultra-high bandwidth, enabling an extremely high data rate for next-generation networks. However, it faces challenges including severe propagation loss and atmospheric absorption, which limits the transmission rate and transmission distance. To address the problem, polarization division multiplexing (PDM) and antenna diversity techniques are utilized in this work to increase system capacity without changing the bandwidth of transmitted signals. Meanwhile, duobinary shaping is used to solve the problem of bandwidth limitation of components in the system, and the final duobinary signals are recovered by maximum likelihood sequence detection (MLSD). A Gaussian mixture model (GMM)-enhanced duobinary unsupervised adaptive convolutional neural network (DB-UACNN) is proposed, to further deal with channel noise. Based on the technologies above, a 2 × 2 multiple-input multiple-output (MIMO) photonic-aided terahertz wireless transmission system at 300 GHz is demonstrated. Experimental results have proved that the signal-to-noise ratio (SNR) gain of duobinary shaping is up to 1.87 dB and 1.70 dB in X-polarization and Y-polarization. The proposed GMM-enhanced DB-UACNN also shows extra SNR gain of up to 2.59 dB and 2.63 dB in X-polarization and Y-polarization, compared to the conventional duobinary filter. The high transmission rate of 100 Gbit/s over the distance of 200 m is finally realized under a 7% hard-decision forward error correction (HD-FEC) threshold. Full article

In this paper, hybrid space–time–polarization schemes for processing high-frequency (HF) radio signals transmitted through the ionospheric layers are proposed. Ionospheric radio wave propagation is characterized by several impairments, including attenuation, scintillation, dispersion, and Faraday rotation. The use of hybrid schemes combining spatial digital processing and a single-input multiple-output (SIMO) scheme based on the spatial and polarization principles is proposed. The simulation is based on a preliminary estimate of signal attenuation and spatial coordinates based on ray tracing at a distance of 1000 km between the transmitter and the receiving digital antenna array. Additionally, the bit error rates and data capacity are obtained for various configurations of hybrid spatial and polarizing types of the proposed architectures. In addition, an algorithm for modeling a broadband HF signal in the ionosphere based on the inverse discrete Fourier transform (IDFT) and the Watterson narrowband model is proposed. Schemes for processing the wideband orthogonal frequency division multiplexing (OFDM) signals after passing through the ionosphere layers are represented as well. Results indicate that the optimal configuration employs hybrid processing utilizing ordinary (O) and extraordinary (X) wave polarization, combined with spatial digital processing in a SIMO architecture. Full article

To support mode-division multiplexing with reduced inter-modal crosstalk, we propose a novel polarization-maintaining few-mode fiber design with a uniform doping profile and no air holes. The fiber employs two placed low-index inclusions to lift modal degeneracy and achieve strong birefringence while maintaining compatibility with standard MCVD and OVD fabrication processes. A genetic algorithm is used to optimize the geometrical and refractive index parameters. Finite element simulations show that the optimized design supports ten guided modes with a minimum effective index difference exceeding $3.8\times {10}^{-4}$ across the C+L band. The fiber exhibits moderate dispersion and strong modal isolation. Tolerance analysis confirms good robustness against index fluctuations and moderate sensitivity to dimensional variations. These features suggest that the proposed PM-FMF is a promising candidate for short-reach spatial-division multiplexing applications where intrinsic polarization and mode separation are desired. Full article

A nested semi-tube hollow-core anti-resonant fiber (HC-ARF) that can support the high-purity transmission of a few polarization-maintaining modes is designed in this paper. By employing bi-thickness hybrid silica/silicon anti-resonant tubes, the birefringence of the orthogonal polarized modes is significantly improved, and the weak coupling condition of the five lowest-order polarization maintaining modes, including the LP_{01_x}, LP_{01_y}, LP_{11a_x}, LP_{11b_x}, and LP_{11a_y}, can be met. The effective refractive index difference between each pair of the supported adjacent modes is larger than 1.0 × 10⁻⁴. With hybrid multi-layer nested semi-tubes, the confinement losses of the supported modes are all less than 1.50 × 10⁻¹ dB/m within a transmission band from 1.530 to 1.620 μm. The minimum confinement losses of the LP_{01_y}, LP_{01_x}, LP_{11a_y}, LP_{11a_x}, and LP_{11b_x} modes are 3.71 × 10⁻⁴ dB/m, 1.61 × 10⁻³ dB/m, 2.00 × 10⁻² dB/m, 1.30 × 10⁻¹ dB/m, and 4.20 × 10⁻² dB/m, respectively. Meanwhile, the unwanted higher-order modes are filtered out well to guarantee the modal purity. The minimum higher-order-mode extinction ratio of the lowest-loss LP₂₁ mode to the highest-loss LP₁₁ mode remains larger than 139 from 1.545 to 1.615 μm. The numerical results highlight the potential of the proposed polarization-maintaining few-mode hollow-core anti-resonant fibers in many application fields, such as short-range and high-capacity data transmission networks, fiber sensing systems, quantum communication systems, and so on. Full article

Employing commercial off-the-shelf coherent optical transmission components and methods to design a continuous variable quantum key distribution (CVQKD) system is a promising trend of achieving QKD with high security key rate (SKR) and cost-effectiveness. In this paper, we explore a CVQKD system based on the widely used polarization division multiplexed (PDM) coherent optical transmission structure and pilot-aided digital signal processing methods. A simplified pilot-aided phase noise compensation scheme based on frequency division multiplexing (FDM) is proposed, which introduces less total excess noise than classical pilot-aided schemes based on time division multiplexing (TDM). In addition, the two schemes of training symbol (TS)-aided equalization are compared to find the optimal strategy for TS insertion, where the scheme based on block insertion strategy can provide the SKR gain of around 29%, 22%, and 15% compared with the scheme based on fine-grained insertion strategy at the transmission distance of 5 km, 25 km, and 50 km, respectively. The joint optimization of pilot-aided and TS-aided methods in this work can provide a reference for achieving a CVQKD system with a high SKR and low complexity in metropolitan-scale applications. Full article

To address the issues of link mismatch and channel impairment in wireless optical communication across atmospheric-oceanic media, this paper proposes a two-hop relay transmission architecture based on the multiple-input multiple-output (MIMO)-enhanced multi-level hybrid multiplexing. The system implements decode-and-forward operations via maritime buoy/ship relays, achieving physical layer isolation between atmospheric and oceanic channels. The transmitter employs coherent orthogonal frequency division multiplexing technology with quadrature amplitude modulation to achieve frequency division multiplexing of baseband signals, combines with orthogonal polarization modulation to generate polarization-multiplexed signal beams, and finally realizes multi-dimensional signal transmission through MIMO spatial diversity. To cope with cross-medium environmental interference, a composite channel model is established, which includes atmospheric turbulence (Gamma–Gamma model), rain attenuation, and oceanic chlorophyll absorption and scattering effects. Simulation results show that the multi-level hybrid multiplexing method can significantly improve the data transmission rate of the system. Since the system adopts three channels of polarization-state data, the data transmission rate is increased by 200%; the two-hop relay method can effectively improve the communication performance of cross-medium optical communication and fundamentally solve the problem of light transmission in cross-medium planes; the use of MIMO technology has a compensating effect on the impacts of both atmospheric and marine environments, and as the number of light beams increases, the system performance can be further improved. This research provides technical implementation schemes and reference data for the design of high-capacity optical communication systems across air-sea media. Full article

Faster-than-Nyquist (FTN) tight filtering introduces serious inter-symbol interference (ISI) impairment, leading to an insufficient compensation range for conventional IQ imbalance compensation algorithms. Furthermore, receiver (Rx) IQ imbalance and ISI impairments significantly increase the convergence cost required by the squared Gardner phase detector (SGPD) timing recovery algorithm to establish a timing synchronization loop. This paper proposes a joint Rx IQ compensation and timing recovery scheme. By embedding a two-stage IQ imbalance compensation algorithm into the timing recovery feedback loop, the proposed scheme could effectively estimate and compensate for Rx IQ imbalance. Meanwhile, thanks to the innovative scheme, which equalizes Rx IQ imbalance and ISI during the timing feedback loop, the convergence cost of timing recovery could be reduced compared with the conventional blind frequency domain (BFD) scheme. The simulation results of 128 GBaud polarization multiplexing (PM) 16-quadrature amplitude modulation (QAM) FTN wavelength division multiplexing (WDM) transmission systems demonstrate that the proposed scheme could bring about 14%, 12.5%, and 16.6% improvements in the compensation range for Rx IQ amplitude imbalance, phase imbalance, and skew, respectively, compared with the conventional one. Meanwhile, the convergence cost is reduced by at least 31% with a 0.9 acceleration factor. In addition, 40 GBaud PM-16QAM FTN experiment results show that the proposed scheme could bring about a 0.8 dB improvement in the optical signal noise ratio (OSNR) compared with the conventional BFD scheme. Full article

We propose and experimentally demonstrate an Integrated Multiband-Mode Multiplexing Photonic Lantern (IM3PL) that enables the selective excitation of high-order modes and stable modal preservation across multiple wavelength bands. As a proof-of-concept configuration, the IM3PL integrates a custom-designed input fiber array composed of three 980 nm single-mode fibers (SMFs) and two few-mode fibers (FMFs) operating at 1310 nm and 1550 nm, respectively. Simulations verify that 980 nm input signals can selectively excite $L{P}_{01}$, $L{P}_{11a}$, and $L{P}_{11b}$ modes at the FMF output, while the modal integrity of high-order linear polarized modes is preserved at 1310 nm and 1550 nm. The fabricated IM3PL device is experimentally validated via near-field pattern measurements, confirming the selective excitation at 980 nm and low-loss, mode-preserving transmission at the signal bands. This work offers a scalable and reconfigurable solution for multiband high-order-mode multiplexing, with promising applications in mode-division multiplexed fiber communication systems and multiband high-mode fiber lasers. Full article

We propose and demonstrate a polarization-insensitive silicon photonic variable optical attenuator. The designed device uses a two-dimensional apodized grating coupler as a surface-normal coupling interface, which has the advantages of low-cost fiber packaging and polarization insensitivity. For optical attenuation, PIN diodes are inserted into each waveguide to act as optical absorbers. The compact device, featuring a footprint of 250 × 850 μm², exhibits a fiber-to-fiber insertion loss of 6 dB. Under a 3 V bias voltage, wavelength-dependent attenuation of 18 dB at 1295 nm and 26 dB at 1315 nm is achieved. Systematic characterization across diverse input polarization states confirms polarization-dependent loss below 0.5 dB under arbitrary polarization states, validating the device’s robust polarization insensitivity for wavelength-division multiplexing systems. Full article

This paper proposes a modified polar coding-based selected mapping (PC-SLM) scheme to reduce the peak-to-average power ratio (PAPR) in orthogonal frequency-division multiplexing (OFDM) systems. In the proposed transmitter, modulated signal vector for a subset of frozen bits, termed PAPR bits, are precomputed, enabling a single polar encoder and modulator to generate multiple modulation symbols, thereby significantly reducing the hardware complexity compared to existing PC-SLM schemes. To achieve side information (SI)-free transmission, a novel belief propagation (BP)-based receiver is introduced, incorporating a G-matrix-based early termination criterion and a frozen bit check (BP-GF) for joint detection and decoding. Simulation results show that the proposed scheme significantly reduces PAPR across various code lengths, with greater gains as the number of PAPR bits increases. Furthermore, for PC-SLM schemes employing the partially frozen bit method, the BP-GF-based receiver achieves a PAPR reduction and error correction performance comparable to that of the successive cancellation (SC)-based receiver. Additionally, the BP-GF-based receiver exhibits lower decoding latency than the successive cancellation list (SCL)-based receiver. Full article

Free space optics (FSOs) is an emerging technology offering solutions for secure and high data rate transmission in dense urban areas, back haul link in telecommunication networks, and last mile access applications. It is important to investigate the performance of the FSO link as a result of aggregate attenuation caused by different weather conditions in a region. In the present work, empirical models have been derived in terms of visibility, considering fog, haze, and cloud conditions of diverse geographical regions of Delhi, Washington, London, and Cape Town. Mean square error (MSE) and goodness of fit (R squared) have been employed as measures for estimating model performance. The dual polarization-quadrature phase shift keying (DP-QPSK) modulation technique has been employed with hybrid mode and the wave division multiplexing (MDM-WDM) scheme for analyzing the performance of the FSO link with two Laguerre Gaussian modes (LG00 and LG 01) at 5 different wavelengths from 1550 nm to 1554 nm. The performance of the system has been analyzed in terms of received power and signal to noise ratio with respect to the transmission range of the link. Minimum received power and SNR values of −52 dBm and −33 dB have been obtained over the observed transmission range as a result of multiple impairments. Random forest (RF), k-nearest neighbors (KNN), multi-layer perceptron (MLP), gradient boosting (GB), and machine learning (ML) techniques have also been employed for estimating the SNR of the received signal. The maximum R squared (0.99) and minimum MSE (0.11), MAE (0.25), and RMSE (0.33) values have been reported in the case of the GB model, compared to other ML techniques, resulting in the best fit model. Full article

Mode division multiplexing (MDM) techniques provide significant enhancement of the capacity of optical intensity modulation and direct detection (IM/DD) short-reach communication systems, like the datacenter interconnection scenarios. While the introduction of multiple modes leads to mode coupling that will extremely deteriorate the received signals, two approaches have been explored to address this issue: one involves the application of all-link weakly coupled components to suppress modal crosstalk, while the other utilizes optical multiple-input–multiple-output (MIMO) equalizers based on optical devices for signal decoupling. However, pure digital signal processing (DSP)-based schemes for mode decoupling in IM/DD MDM systems with strong mode coupling remain unexplored. In this paper, we propose to use a frequency-domain MIMO equalizer for compensating the modal interference in the strong-coupling linear-polarized (LP) MDM IM/DD system. The signal recovery capability of the proposed method is verified through numerical simulation. Finally, we successfully experimentally demonstrate the transmission of on–off-key (OOK) signals in a six-LP-mode strong-coupling MDM IM/DD system over a 10 km few-mode fiber, employing a pair of strong-coupling mode multiplexers/demultiplexers. The experimental results indicate that, with the frequency-domain MIMO equalizer, OOK signals from all modes can be recovered with an 11% hard-decision forward error correction threshold of 8.3 × ${10}^{-3}$. The proposed method facilitated by flexible DSP software offers an alternative for short-reach communication systems and has the potential to advance the practical application of MDM techniques in the future. Full article

In this paper, we have proposed a filter-assisted self-coherent detection (FASCD) scheme that reconstructs the optical field of a dual-polarization complex-valued double-sideband (DP-CV-DSB) signal. At the receiver, the carrier is extracted using an optical bandpass filter (OBPF), and a pair of orthogonal carriers is constructed to achieve polarization-division multiplexing (PDM) by a Faraday rotator mirror (FRM). To address the issue of polarization crosstalk, channel estimation is performed using the least squares (LS) method, and the estimation results are further optimized through the intra-symbol frequency-domain averaging (ISFA) method. We demonstrate the system architecture and algorithms by simulation on a 224 Gbit/s 16-ary quadrature amplitude modulation DSB-PDM-OFDM system. The system performance is improved by 1 dB using the ISFA method. Full article

This study proposes optimizing the performance of free space optic signal transmission using spatial division multiplexing. The research uses different modified duobinary modulation schemes to model and optimize three hybrid mode division multiplexing-free-space optical (MDM-FSO) channels, each operating at 40 Gb/s–40 GHz. The study also includes the parametric optimization of various components to enhance system performance. The findings are significant for achieving high data rate links for backhaul solutions and improving bandwidth for future MDM-based wireless distributed networks. The research shows that employing three linearly polarized modes as data transmission channels with direct detection can be effective. Additionally, it is discovered that adjusting the bias voltages of the two LiNbO₃ modulators can improve power sharing between the modes, thereby mitigating the power penalty. Full article

This paper proposes an accuracy enhancement method for fiber-longitudinal power profile estimation (PPE) based on convolutional neural networks (CNN). Two types of CNNs are designed. The first network treats different polarization streams identically and is denoted as CNN. The second network considers the difference between the contributions of different polarization streams to the nonlinear phase shift and is denoted as enhanced CNN (ECNN). The numerical simulation results confirm the effectiveness of the method for a 64 Gbaud/s quadrature phase-shift keying (QPSK) polarization-division-multiplexed (PDM) coherent optical communication system with a fiber length of 320 km. The effects of finite impulse response (FIR) filter length, power into the fiber, and polarization mode dispersion on the PPE accuracy are examined. Finally, the results of the proposed method are monitored in the presence of several simultaneous power attenuation anomalies in the fiber optic link. It is found that the accuracy of the PPE substantially improves after using the proposed method, achieving a relative gain of up to 71%. When the modulation format is changed from QPSK to 16-ary quadrature amplitude modulation (16-QAM), and the fiber length is increased from 360 km to 480 km, the proposed method is still effective. This work provides a feasible solution for implementing fiber-longitudinal PPE, enabling significantly improved estimation accuracy in practical applications. Full article