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Vortex beams with orbital angular momentum (OAM) have infinite mode orthogonality, which can greatly boost communication capacity through mode-division multiplexing. However, current exploration of OAM beam transmission primarily focuses on free-space or specialty fibers limited by the OAM spiral phase wavefront characteristics. Due to the mode field phase sensitivity, these methods are constrained by turbulence interference and fabrication error tolerance, restricting transmission distance of OAM beam at practical application. Herein, we propose an OAM transmission scheme using commercial multimode fiber (MMF) exploiting the eigenmodes superposition theory. Leveraging linear superposition of HE and EH vector modes with ± (π/2) phase shift over MMF, OAM excitation and transmission can be supported with low interference. Simultaneously, due to the decreased demand for fabrication precision in the gradient core structure, the tolerance for machining errors is enhanced. As a proof-of-concept, we have demonstrated a multidimensional OAM multiplexing communication with 640-channel (4 OAM modes, 80 wavelengths and 2 polarization states) over a 5 km MMF, successfully transmitting QPSK-OFDM signals at a total rate of 15 Tbit/s. The system demonstrated OAM mode purity exceeding 65% and bit error rate (BER) below the forward error correction (FEC) threshold (3.8 × 10−3). Multiplexed OAM signals can be effectively transmitted in MMF over short to medium distances with acceptable mode crosstalk, while remaining compatible with wavelength and polarization dimensions, enabling the construction of multi-dimensional multiplexing local area networks. • Exploiting the eigenmodes superposition theory, an OAM transmission scheme using commercial multimode fiber was proposed. • A 640-channel multi-dimensional OAM multiplexing system is constructed over 5-km MMF. • 15 Tbit/s QPSK-OFDM signals are successfully transmitted withBERs below the FEC threshold and pattern purity above 65%. • The BERs are reduced by about 2 orders of magnitude after LDPC encoding for channel equalization. [ABSTRACT FROM AUTHOR]

We propose a cost-effective THz wireless communication scheme for future femtocell networks that leverages either direct gain modulation (DGM) or cross gain modulation (XGM) of a semiconductor optical amplifier (SOA) to convert coaxially or optically carried data into intensity-modulated THz waves, respectively. This architecture, based on an SOA and a uni-traveling-carrier photodiode (UTC-PD), has advantages over typical THz modulation schemes involving a resonant tunneling diode or an electro-optic modulator combined with a UTC-PD. Consequently, it facilitates the seamless convergence of hybrid fiber-coaxial networks to THz radio access networks with low cost, straightforward monolithic integration, and a more compact footprint. Furthermore, we experimentally verified the possibility of non-return-to-zero on–off keying wireless communication over 0.5 m in the 300 GHz band, achieving real-time transmission of 4 Gbit/s for coaxial access via an SOA's DGM and 10 Gbit/s for fiber access via the SOA's XGM. Notably, these data rates were attained by simple non-coherent THz intensity demodulation based on a Fermi-level managed barrier diode while meeting the 7% hard-decision forward error correction (HD-FEC) threshold (3. 4 × 1 0 − 3 ). Finally, we conducted an analysis of the merits and drawbacks of the proposed THz communication scheme based on the combination of an SOA and a UTC-PD, discussed bottlenecks that hinder further rate improvement, and explored possibilities for optimization. In conclusion, this scheme is crucial for realizing future ultra-high-speed wireless networks that seamlessly connect to fiber-coaxial networks. • Presented a photonic THz wireless communication system using DGM and XGM in an SOA. • Demonstrated 300 GHz OOK over 0.5 m with 4 Gbps coaxial and 10 Gbps fiber access. • Exhibited cost-effective and compact features that suitable for femtocell networks. [ABSTRACT FROM AUTHOR]

Nonlinear frequency division multiplexing (NFDM) is a promising technique for optical fiber communication that can overcome the Kerr nonlinear effects of optical fibers. Hermite–Gaussian (HG)-carriers are a potential candidate for constructing the nonlinear spectrum in NFDM systems, as they have lower time–bandwidth product and higher tolerance to truncation noise than conventional sinc-carriers. However, HG-carriers are affected by interference and truncation error between subcarriers, which degrade the system performance. In this paper, we propose a HG-carriers reconstruction method based on Gram–Schmidt Orthogonalization, which can effectively restore the orthogonality between subcarriers and reduce the distortion. Through a numerical simulation, we explore the use of HG-carriers and sinc-carriers to construct the nonlinear spectrum, and analyze their performance differences under different system bandwidth parameters. We also implement HG-carriers in a narrowband (6.8 GHz) b-modulation dual-polarization NFDM system and optimize them to achieve a maximum spectral efficiency (SE) of 8.89 bits/s/Hz (4.44 bits/s/Hz per polarization) over 960 km with bit error rate (BER) below the hard decision forward error correction (HD-FEC) threshold. Our results demonstrate that HG-carriers outperform sinc-carriers in narrowband NFDM systems, and that our proposed HG-carriers reconstruction method can significantly improve the Q-factor and SE. • Hermite–Gaussian carrier is superior to sinc carrier in narrowband nonlinear frequency-division multiplexing systems. • Nonlinear Fourier transform is used to encode and decode signals in optical fibers. • The maximum spectral efficiency of 8.89 bits/s/Hz is achieved over a transmission distance of 960 km. • The Hermite–Gaussian carrier is reconstructed using the Gram–Schmidt orthogonalization method. • The proposed system can be used for high-speed optical communication with low power consumption. [ABSTRACT FROM AUTHOR]

In dual-polarization continuous spectrum nonlinear frequency division multiplexing (DP-CS NFDM) systems, the presence of noise destroys the independence between subcarriers in the nonlinear spectral domain (NSD). Additionally, as the modulation order and the number of subcarriers increase, the signal power increases and the effect of noise on the performance of the system becomes more severe. In this paper, we propose a noise equalization scheme based on probabilistic shaping (PS) and complex-valued neural network (c-ANN) for dual-polarization transmission CS NFDM system with high-power signals. The scheme utilizes PS to reduce the distribution probability of high-power constellation points of the signal with high-order modulation format, to decrease the peak-to-average power ratio of the continuous spectrum subcarriers and the average power of the signal, which can achieve the suppression and equalization of the amplifier spontaneous emission (ASE) noise and the processing noise. In addition, the scheme uses the c-ANN to effectively reduce the correlation between subcarriers caused by the noise and improve the system performance. The effectiveness of the proposed scheme is verified in the DP-CS NFDM system, and the simulation results show that the Q-factor gain is enhanced by roughly 1.1 dB and 0.8 dB for the number of subcarriers of 128 and 256, respectively, compared to the classic nonlinear Fourier transform (NFT) scheme. Among them, with different noise figure (NF) and satisfying the 7% forward error correction (FEC) threshold, the 512QAM signal with an entropy of 8 after equalization increase the transmission distance by 80 km longer than that of uniform 256QAM signals for the number of subcarriers of 128. Compared with the joint scheme based on PS and real-valued neural network (r-ANN), this scheme has better equalization performance at the same complexity, and the complexity can be reduced by 50% under the same performance. • For the DP-CS NFDM system, a novel noise equalization scheme based on PS and c-ANN is proposed. • The order of the signals that can be processed by this scheme is the highest at present. • This scheme has a good benefit to noise equalization. • Compared with the joint scheme of PS and r-ANN, this scheme has better equalization performance at the same complexity. [ABSTRACT FROM AUTHOR]

Optical encryption has received increasing attention recently as information security is playing a crucial role in the modern information society. Particularly, the intensive investigation and rapid development of metasurfaces have significantly facilitated the research of optical encryption with enhanced security and increased capacity for information. Nevertheless, most metasurface encryption schemes still suffer from the challenge of dynamic modulation. Here, we demonstrate a novel dynamic optical encryption scheme in which information can be encrypted in light fields in a reconfigurable manner. Specifically, the spatial frequencies of optical images are utilized as information carriers and dynamically modulated for encrypting information. With the developed reconfigurable Fourier optics technique, various kinds of messages can be directly embedded in the spatial frequency domain of the ciphertext images and maintained with high fidelity. Importantly, information security and capacity can benefit from high-speed dynamic modulation with advanced coding techniques, an example of which has been demonstrated via exploiting the position, profile, and time degrees of freedom of spatial frequency. The proposed novel optical encryption scheme may pave the way for realizing high-performance information encryption systems. • Using the reconfigurable Fourier optics technique can enable dynamic optical encryption to increase efficiency. • The security can be enhanced by encrypting messages at the Fourier domains of optical images. • Information security and capacity can also benefit from high-speed dynamic modulation with advanced coding techniques. [ABSTRACT FROM AUTHOR]

Free space optical communication has become increasingly popular in the past decade due to its terahertz bandwidth, unlicensed spectrum, and enhanced security features. This technology has been attracting significant interest from the network community due to its broad applications in future military and civilian protocols. Recently, the use of orthogonal spatial states of the light field, including those with nonzero orbital angular momentum, has increased communication system capacities and bit-transfer rates per photon. However, atmospheric turbulence disrupts the orthogonal spatial modes over propagation. We present a novel deep learning architecture, cGULnet, to mitigate the effects of atmospheric turbulence on spatial multiplexing of Laguerre–Gaussian structured light modes. The model predicts the phase correction patterns required for an adaptive optical system. It is a U-shaped architecture trained adversarially to a patch discriminator similar to conditional generative adversarial neural-networks. The network is trained for exact phase recovery systems without approximating Zernike polynomials, enabling its use in highly phase-sensitive but high-speed spatial multiplexed communication systems. The proposed architecture can predict the successive three frames in a single shot using the previous ten frames. The method was evaluated for a wide range of beam waist sizes in a simulated Laguerre–Gaussian spatial mode multiplexed 1 km free space optical communication link. We have considered 20 dB optical signal-to-noise ratio, with turbulence conditions parameterized by the refractive index structure parameter of C n 2 ≤ 5 × 1 0 − 15 m − 2 3 . It was found to reduce the bit error rate to the International Telecommunication Union recommended forward error correction limit of 3. 8 × 1 0 − 3 under high turbulence conditions. [Display omitted] • Conditional adversarial neural network compensates time delay in adaptive optics for atmospheric correction. • Study predicts complete Zernike order for phase-sensitive spatially multiplexed free space optical communication, outperforming existing literature. • Results showcase bit error rate in simulated 1km 4-Laguerre Gaussian mode 4-QAM multiplexed 15 Gbps link under ITU recommendations. [ABSTRACT FROM AUTHOR]

Coherent optical orthogonal frequency division multiplexing (CO-OFDM) systems suffer from the high peak-to-average ratio (PAPR) of orthogonal frequency division multiplexing (OFDM) signals, which leads to the back-off of laser power and fiber nonlinear effects, thereby deteriorating the bit error rate (BER) performance and limiting the transmission distance. Iterative clipping and filtering (ICF) scheme, as a direct, simple and effective PAPR reduction scheme that only processes signals at the transmitter, but requires multiple fast Fourier transform (FFT)/inverse fast Fourier transform (IFFT), resulting in high computational complexity. We propose a novel scheme using the machine learning (ML) technique, which is based on nonlinear real-valued support vector regression (NRSVR) and trained on ICF scheme. The simulation results show that compared with ICF scheme, the proposed scheme has a similar P A P R 0 for 1 0 − 4 complementary cumulative distribution function (C C D F), an additional 15 km of single mode fiber (SMF) transmission for 16 dB optical signal-to-noise ratio (O S N R) and 1 0 − 3 B E R , and a significant time complexity reduction. Compared with other ML-based schemes, the proposed scheme also achieves better performance and lower computational complexity. • A novel peak to average power ratio reduction scheme is proposed. • The support vector regression is trained on clipping and filtering scheme. • The complexity of proposed scheme is significantly reduced. [ABSTRACT FROM AUTHOR]

a 100Gbit/s novel full duplex free space optical (FSO) access system scheme is proposed and demonstrated. Using the downlink hybrid 16phase shift keying (PSK)/256quadrature amplitude modulation (QAM)-orthogonal frequency division multiplexing (OFDM) and duobinary uplink signals. In downlink branch, in-phase/quadrature-phase (I/Q) modulation based on Mach-Zehnder modulator (MZM) and coherent detection technology are adopted. Meanwhile, 0.1Terahertz (THz) wave signal generated by two independent optical sidebands beating frequency at positive intrinsic-negative photodiodes (PIN PDs) for hybrid downlink 16PSK/256QM-OFDM signals. In uplink branch, intensity modulation and direct detection (IM/DD) can achieve reduced the communication budget cost and simplified the system configuration Simulation results show that the value of bit error ratio (BER) less than hardware decision forward error correction (HD-FEC) threshold after 300 m FSO channel transmission under the different weather conditions (such as sunny, rainy and snow). [ABSTRACT FROM AUTHOR]

In this paper, an improved deep learning-based end-to-end autoencoder is proposed for unmanned aerial vehicle (UAV) to ground free space optical communication to mitigate atmospheric turbulence. Deep neural network (DNN) is applied to the intensity modulation/direct detection (IM/DD) autoencoder, including transmitter, receiver as well as channel model. The performance is improved by two-stage deep learning training because the minimum Hamming distance between the codewords is increased through pre-training. Simulation results show that the bit error rate of our proposed scheme can reach the 7% hard-decision forward error correction (HD-FEC) threshold at signal-to-noise ratio of approximately 22 dB and in strong atmospheric turbulence where the maximum Rytov variance is 3.5. Our proposed scheme can outperform the state-of-the-art IM/DD system with PPM transmitter and maximum likelihood receiver by achieving approximately 12 dB improvement and reducing ∼51.3% of the decoder's running time without the need for accurate channel state information. • A data-driven deep learning-based autoencoder is proposed for UAV-to-Ground free space optical communication which can mitigate atmospheric turbulence. • The proposed scheme can reach the HD-FEC threshold at 22 dB with maximum Rytov variance 3.5. • The proposed scheme shows approximately 12 dB improvement compared with PPM transmitter and ∼51.3% reduction in decoder's running time compared with ML receiver without the need for perfect channel state information. [ABSTRACT FROM AUTHOR]

Based on a developed analytical model, we demonstrate the feasibility of intensity-modulation direct-detection (IM/DD) optical orthogonal frequency division multiplexing (OOFDM) system over 1 km of multimode fiber (MMF) at data rates up to 64 Gb/s, at 1550 nm. The effect of data rates, number of subcarriers and fiber length are investigated for different number of excited mode groups that can be generated using the offset launch technique. Indeed, we show that when we increase the efficiency of the Volterra equalizer, by raising its input taps from 7 to 9, we can reduce the system impairments of 5.45 dB in terms of Q factor at 800 m. Furthermore, we prove that by exciting more mode groups the MMF bandwidth is reduced potentially. Finally, thanks to the efficiency of Volterra equalizer, we achieve a bit error rate (BER) below the forward error correction (FEC) limit, even in the case of the full excitation technique. • Develop an analytical model for IM/DD OOFDM MMF system operating at 64 Gb/s. • Study the effect of data rates, number of subcarriers and fiber length. • Show that the Volterra equalizer can reduce the system impairments. • Adopt the Volterra equalization in order to overcome the dispersion effects of the MMF fiber. [ABSTRACT FROM AUTHOR]

Abstract Presently, most of the long-reach underwater wireless optical communication (UWOC) systems employ laser diodes rather than light-emitting diodes (LEDs) as the transmitters. Due to lasers' smaller divergence angles, those UWOC systems could realize longer communication links. Although LED-based long-reach UWOC has distinguished advantages, it suffers from very low received optical power. Fortunately, highly sensitive multi-pixel photon counter (MPPC) and energy efficient pulse position modulation (PPM) pave a way towards LED-based long-reach UWOC. In this paper, we investigated the underlying working principle of an MPPC and proposed a UWOC system based on single LED as well as a lens-free MPPC with digital output. The relationship between the MPPC p.e. threshold and its received optical power was theoretically studied and further proved by experimental results. Single 2.3-MHz, 3-W blue LED was used as the transmitter to generate 8-PPM to 64-PPM signals with a 5-MHz slot frequency. After a 46-m underwater transmission, the measured BERs were all below the forward error correction (FEC) limit, which were achieved with less than 100 incident photons during each pulse slot. [ABSTRACT FROM AUTHOR]

This paper presents improved underwater wireless optical communication (UWOC) based on a 490 nm passively mode-locked vertical-external-cavity surface-emitting laser (VECSEL) with the repetition rate of 1.46 GHz and the pulse width of 2.25 ps. After conducting a comprehensive evaluation of the performance characteristics of quasi-cyclic low-density parity-check (QC-LDPC) coding and pulse-position modulation (PPM) in relation to bit-error-rate (BER) and communication capacity across different signal-to-noise ratios (SNRs), a passively mode-locked VECSEL based UWOC system is designed and experimentally performed. The UWOC system employs the QC-LDPC encoding technique and PPM modulation scheme that are most suitable for the system. Compared with the case without QC-LDPC, the minimum received optical power with coding is improved by 3.5 dBm, and the SNR with coding is improved by 0.4 dB under condition of 256-PPM and forward error correction (FEC) threshold of 3.8×10−3. Meanwhile, compared with the UWOC system using a continuous-wave (CW) VECSEL as the light source, the attenuation coefficient of the UWOC system based on the mode-locked VECSEL is decreased by 0.04 m-1, and the BER of the mode-locked VECSEL based UWOC system is decreased by 0.3 dB. • A passively mode-locked VECSEL is employed to improve the performance of UWOC. • A UWOC system combining a mode-locked VECSEL and QC-LDPC is bult and characterized. • Pulsed laser has obviously smaller attenuation coefficient underwater than CW light. • BER of mode-locked VECSEL is significantly better than that of CW VECSEL in UWOC. [ABSTRACT FROM AUTHOR]

In the letter, we propose to use delta-sigma modulator (DSM) technology to transmit OFDM high-order QAM signals in a single-carrier radio-over-fiber (RoF) system, and conduct experimental verification. The experiment demonstrates the transmission of OFDM-256/1024/4096 QAM signals using one-bit DSM in the W band in a 20 km SSMF and 1 m wireless RoF system, and successfully realized that the BER of OFDM-256 and 1024 QAM after system transmission is lower than the hard decision forward error correction (HD-FEC) threshold of 3.8 × 10 − 3 , and the BER after 4096 QAM transmission is lower than the soft decision forward error correction (SD-FEC) threshold of 4.0 × 10 − 2. The combination of DSM and the photonics-aided RoF solution can effectively solve the problem that the nonlinear damage caused by the bottleneck of the optoelectronic device in the RoF system limits the transmission of high-order QAM signals, and it is expected to be applied in the future 6G optical wireless network. • We propose to use delta-sigma modulator (DSM) technology to transmit OFDM high-order QAM signals in a single-carrier radio-over-fiber (RoF) system. • We have successfully realized the transmission of OFDM-256/1024/4096QAM signals on 20 km fiber and 1 m wireless through experiments. • The combination of DSM and photonics-assisted RoF scheme can effectively solve the problem that the nonlinear damage caused by the bottleneck of optoelectronic devices in the RoF system limits the transmission of high-order QAM signals. [ABSTRACT FROM AUTHOR]

In this paper, an enhanced two-dimension (2-D) color space diversity technique using red, green, and blue light emitting diode (RGB LED) overlapping is proposed and investigated experimentally to improve the system reliability for optical camera communication (OCC). The transmitter adopts an 8-color shift keying (8-CSK) modulation scheme with two RGB LED overlapping. The receiver uses a camera to form a multi-input single-output system to improve capacity. Different mapping methods are proposed and compared in 2-D YCbCr and CIElab color space to avoid the influence of brightness in conventional 3-D RGB space. Besides, clustering-based demapping is applied in the OCC system to transfer the color information into a data stream. The experimental results show that the mapping scheme with the maximum Euclidean distance in 2-D YCbCr color space has the best bit error rate (BER) performance. Compared with the conventional 3-D RGB color space, the proposed enhanced 2-D YCbCr color space diversity has a higher resolution to signal, which can realize 1.46 dB Q-factor improvement at an illumination of 660 lux. In addition, it can achieve a net data rate of 11.52 kb/s and maintain a 7% forward error correction (FEC) requirement even under low illumination. It is verified that the proposed scheme is a potential application for capacity and performance improvement. [ABSTRACT FROM AUTHOR]

A comprehensive comparison among different post-equalizers with relatively low complexity is studied for C-band 100-km dispersion-uncompensated standard single-mode fiber (SSMF) intensity modulation and direct detection (IM/DD) transmission systems with data rate beyond 100 Gb/s/ λ. These representative post-equalizers include feed-forward equalizer (FFE), polynomial-based nonlinear FFE (P-NFFE), absolute-term based nonlinear FFE (AT-NFFE), FFE combined with decision-feedback equalizer (FFE-DFE), polynomial based nonlinear FFE-DFE (P-NLE), absolute-term based nonlinear FFE-DFE (AT-NLE), and their lower-complexity versions of FFE-DFE-WS, P-NLE-WS, and AT-NLE-WS which use weight sharing (WS) to eliminate weight redundancy. We demonstrate 100-Gb/s/ λ and 112-Gb/s/ λ on-off keying (OOK) transmissions over a 100-km SSMF. For 112-Gb/s transmission, BERs of FFE-DFE(-WS), P-NLE(-WS), and AT-NLE(-WS) can reach 7% hard-decision forward error correction (HD-FEC) limit at 3.8 × 10 − 3. Under 7% HD-FEC limit, AT-NLE-WS with 32 real-valued multiplications shows similar BER performance as P-NLE and AT-NLE with a small power penalty of < 0.4 dB, but outperforms P-NLE-WS, AT-NLE, and P-NLE in huge complexity reduction of approximately 83.2%, 92.4%, and 92.7%, respectively. Therefore, taking both the BER performance and the operation complexity into account, AT-NLE-WS is the optimal equalizer and is a strong candidate to be applied in low-cost C-band IM/DD interconnections with data rates beyond 100 Gb/s/ λ and transmission distance up to 100 km. • The demonstrations of ≥ 50-Gb/s and ≥ 100-km IM/DD transmissions in terms of DSP, data rate, distance, and FEC threshold are summarized • A comprehensive comparison among different receiver-side equalizers is conducted for C-band 100-km SSMF based IM/DD systems. • The AT-NLEWS is a strong candidate to be applied in low-cost C-band IM/DD interconnections with transmission distance up to 100 km. [ABSTRACT FROM AUTHOR]

We investigate how to extend the air-space distance for Terahertz (THz) signal by using optimized lens layout. After a delivery over 129.6 cm air-space we realize the BER of 10 Gb/s QPSK signal at 450 GHz smaller than 1 × 10 −4 with this optimized lens layout. If only two lenses are employed, the BER is higher than forward error correction (FEC) threshold at the input power of 15 dBm into the photodiode. [ABSTRACT FROM AUTHOR]

In this paper, we propose an orthogonal circulant matrix transform (OCT)-based adaptive frame-level-forward error correction (FEC) scheme for fiber-visible laser light communication (VLLC) system and experimentally demonstrate by Reed–Solomon (RS) Code. In this method, no extra bits are spent for adaptive message, except training sequence (TS), which is simultaneously used for synchronization and channel estimation. Therefore, RS-coding can be adaptively performed frames by frames via the last received codeword-error-rate (CER) feedback estimated by the TSs of the previous few OFDM frames. In addition, the experimental results exhibit that over 20 km standard single-mode fiber (SSMF) and 8 m visible light transmission, the costs of RS codewords are at most 14.12% lower than those of conventional adaptive subcarrier-RS-code based 16-QAM OFDM at bit error rate (BER) of 1 0 − 5 . [ABSTRACT FROM AUTHOR]

Convergence of sensing and communication in the same architecture is being considered for implementation in the future sixth-generation (6G) networks due to spectrum congestion and cost efficiency. This paper presents an integrated architecture for time-of-flight (ToF) depth sensing and optical communication simultaneously based on Hamiltonian coding function. The feasibility and performance of this framework are verified by theoretical simulation and experiments. Compared with sinusoid coding scheme, the proposed system based on single-frequency Hamiltonian coding modulation achieves a depth measurement accuracy of tens of millimeters and zero error code in communication, and the data rate 20 Mbps. By developing a multi-frequency Hamiltonian coding fusion algorithm, not only the maximum unambiguous range is increased from 15 m to 60 m, but the average ranging precision is improved to 4.46 mm as well. Experimentally, information of 1000 bits carried by single-frequency is demonstrated. Multi-frequency fusion system can accommodate three times that of single frequency. Within the maximum measurable experimental scope, the value of communication bit error rate (BER) always tends to be zero, under 7% pre-forward error correction (pre-FEC) limit of 3. 8 × 1 0 − 3 . • Integrated framework of time-of-flight depth sensing and optical communication based on Hamiltonian coding is proposed. • The framework enables both high precision ranging (7.5mm@2 ∼ 4m&K=5) and 20Mbps data rate based on single-frequency modulation. • A multi-frequency Hamiltonian coding fusion algorithm is proposed, the maximum unambiguous range is extended from 15m to 60m • Experimentally, the ranging precision is beyond 2 times and the communication capacity is 3 times that of single frequency. [ABSTRACT FROM AUTHOR]

For discrete spectrum modulated nonlinear frequency division multiplexing (DS-NFDM) transmission, the combined effect of laser frequency offset (FO) and amplifier spontaneous emission (ASE) noise would cause a perturbative shift of eigenvalues and corresponding spectral distortion. Moreover, the classical 4th-power feedforward frequency offset estimation (FOE) scheme is not suitable for DS-NFDM system with low baud rate. In this paper, we firstly deduce an FO impairment model with ASE noise in nonlinear Fourier domain (NFD). Meanwhile, we have proposed a training sequence-assisted FOE (TS-FOE) scheme, which effectively reduces the impact of ASE noise. The 2 GBaud DS-NFDM 16QAM simulation results illustrate that this scheme achieves an FOE error within 1 MHz using a training sequence (TS) with length of 64. Besides, its FOE range achieves [ − R s / 2 , + R s / 2 ], where R s denotes the baud rate of signal. Furthermore, the effectiveness of TS-FOE scheme has been verified by 2 GBaud NFDM QPSK experimental system. The results demonstrate that when optical signal-to-noise ratio (OSNR) is set 13 dB, the TS-FOE scheme presents stable FO compensation performance for different FOs within 1 GHz, and the corresponding bit error rates (BERs) are all below the 7% hard decision forward error correction (HD-FEC) threshold. Meanwhile, its complexity is on the order of O L and related with the length L of TS, which is as one thousandth as that of angle search FOE scheme. • We firstly have analyzed the combined effects of FO and ASE noise on the perturbation of eigenvalue shift, and established an NFD-FO model. • The NFD-FO model reveals that the eigenvalue perturbations caused by ASE noise are transformed into amplitude noise and phase noise. • We propose a low-complexity TS-FOE scheme, which effectively reduces the impact of ASE noise on FOE accuracy. • The complexity of this scheme is reduced to the order of O(L). [ABSTRACT FROM AUTHOR]

In this paper, we propose a performance improvement and cost-efficient discrete Hartley transform (DHT)-based asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) scheme for visible light communications (VLC). The simple one-dimensional modulation constellation and simplified encoding structure reduce the complexity of system considerably. The DHT-spreading technique is employed to reduce peak-to-average power ratio (PAPR) of ACO-OFDM signals. Moreover, the intra-symbol frequency-domain averaging (ISFA) technique is used to increase the accuracy of channel estimation by removing the effect of ambient noise in the VLC channel effectively. To verify the feasibility of the proposed scheme, we study its performance via simulation. This scheme reduces the requirement to the resolution of DAC and increases the tolerance to the nonlinear characteristics of LED, both of which are cost-efficient. At forward error correction (FEC) limit ( B E R = 1 × 1 0 − 3 ), simulation results illustrate that compared with DHT-based ACO-OFDM without the ISFA technique, our scheme has 3.2 dB and 2.7 dB improvement of the required E b ∕ N 0 when BPSK and 4-PAM are modulated, respectively. [ABSTRACT FROM AUTHOR]

This paper proposes a realistic and global simulation to predict the behavior of a Radio over Fiber (RoF) system before its realization. In this work we consider a 2 × 2 Multiple-Input Multiple-Output (MIMO) Orthogonal Frequency Division Multiplexing (OFDM) RoF system at 60 GHz. This system is based on Spatial Diversity (SD) which increases reliability (decreases probability of error) and Spatial Multiplexing (SMX) which increases data rate, but not necessarily reliability. The 60 GHz MIMO channel model employed in this work based on a lot of measured data and statistical analysis named Triple-S and Valenzuela (TSV) model. To the authors best knowledge; it is the first time that this type of TSV channel model has been employed for 60 GHz MIMO-RoF system. We have evaluated and compared the performance of this system according to the diversity technique, modulation schemes, and channel coding rate for Line-Of-Sight (LOS) desktop environment. The SMX coded is proposed as an intermediate system to improve the Signal to Noise Ratio (SNR) and the data rate. The resulting 2 × 2 MIMO-OFDM SMX system achieves a higher data rate up to 70 Gb/s with 64QAM and Forward Error Correction (FEC) limit of 10 −3 over 25-km fiber transmission followed by 3-m wireless transmission using 7 GHz bandwidth of millimeter wave band. [ABSTRACT FROM AUTHOR]

Taking optical frequency combs as WDM laser sources for coherent optical communication systems has great potential in low-cost, low-complexity, and large-capacity communications. Master–slave carrier phase recovery (MS-CPR) utilizes the phase coherence of received signals to reduce the computational complexity (CC) in frequency comb-based communication systems. In long-haul systems, chromatic dispersion (CD) weakens the phase coherence and makes MS-CPR ineffective. Besides, nonlinear effects greatly degrade the quality of received signals. A master–slave carrier phase recovery method using optical phase conjugation (OPC) is proposed for frequency comb-based long-haul coherent optical communication systems. By placing the OPC module in the middle of the link, not only the weakening of phase coherence caused by CD is solved, but also nonlinear effects are mitigated. The simulation results show that the proposed method has better performance than conventional MS-CPR and independent carrier phase recovery (ICPR). In the 1000 km long system with 16 quadrature amplitude modulation (QAM) signals, the BER of proposed method is 9.3% of conventional MS-CPR and even 40% lower than ICPR, and the BER of all channels is below the threshold for 7% overhead hard-decision forward error correction (7% HD-FEC). • A new master–slave carrier phase recovery method using optical phase conjugation is proposed. • A low-complexity carrier phase recovery method is realized. • The performance of proposed method is better than the independent carrier phase recovery. [ABSTRACT FROM AUTHOR]

A fifth-generation (5G) new radio (NR) fiber-free-space-optical (FSO)-wireless system with dual-polarization and single"–carrier optical modulation schemes is proposed and realized for the first time. Compared with the single-polarization scheme, adopting the dual-polarization scheme is attractive because it can significantly enhance the transmission capacity of the system. Compared with the multi-carrier optical modulation scheme, using single-carrier optical modulation is attractive because it can effectually suppress the dispersion-induced limitations caused by fiber transmission and distortions produced by beating from multiple optical carries. Through 40-km single-mode fiber, 500-m FSO, and 4-m/3-m/2-m/1-m/0.5-m 5G wireless cascaded-medium, 5G NR millimeter-wave (MMW) and sub-THz signals are transmitted with low bit error rates (<3.8 × 10 − 3 forward error correction limit) and clear constellations in x - and y -polarizations. This newly-built 5G NR fiber-FSO-wireless system shows promise to enable 5G NR evolution toward MMW and sub-THz bands, which has a substantial impact on the convergence of fiber optics, optical wireless, and 5G wireless communications. • A 5G NR fiber-FSO-wireless systems with dual-polarization and single-carrier optical modulation schemes are implemented. • The system's capacity is considerably increased with dual-polarization scheme. • The system's performance is significantly enhanced with single-carrier optical modulation scheme. • It shows promise to enable 5G NR evolution towards MMW and sub-THz bands. • It has a huge impact on fiber-FSO-5G wireless convergence. [ABSTRACT FROM AUTHOR]

We propose a heterogeneous thirteen-core few-mode fiber, which has the excellent characteristics of low crosstalk (XT), large effective mode field area (A e f f) and low dispersion slope (D s l o p e). The double high refractive index ring can precisely control the effective refractive index difference between the LP mode in the core and reduce intra-core XT. The relative spatial efficiency (RSE) of the 13-core 6-mode fiber reaches 33.84, and the number of spatial channels is as high as 130. The inter-core crosstalk is less than −60 dB/km at 1550 nm. The results show that the proposed fiber with low crosstalk and large A e f f is sufficient to satisfy the requirements of long-distance transmission in the future. • A heterogeneous 13-core 6-mode fiber is proposed to reduce inter-core crosstalk while enhancing bending resistance. • Heterogeneous core arrangement can increase the propagation constant difference between adjacent cores. • The maximum differential mode delay for the highest order mode (LP 12) and LP 01 mode is 28 ps/m. • The number of spatial channels of 13-core 6-mode fiber based on space division multiplexing is as high as 130. [ABSTRACT FROM AUTHOR]

We propose and experimentally illustrate the use of Long Short Term Memory Neural Network (LSTMNN) and inter-symbol feature extraction (IFE) to recover 4-level pulse amplitude modulation (PAM4) data transmitted from a silicon micro-ring modulator (SiMRM) operated at 160 Gbit/s. Pre-hard-decision (HD) forward error correction (FEC) requirement (i.e. bit-error-rate, BER < 3.8 × 10−3) is achieved in the 160 Gbit/s PAM4 signal produced using a 47-GHz bandwidth SiMRM after 1 km single-mode-fiber (SMF) transmission. • We propose and demonstrate a compact 160 Gbit/s silicon micro-ring modulator (SiMRM) using Long Short Term Memory Neural Network (LSTMNN). • The LSTMN and IFE are used to mitigate the nonlinearity in 4-level pulse amplitude modulation (PAM4) signal produced by the SiMRM. • Detail discussion about the implementation of the LSTMNN and the IFE algorithm are presented. • Pre-hard-decision (HD) forward-error-correction (FEC) requirement is achieved in the 160 Gbit/s PAM4 after 1 km single-mode-fiber (SMF) transmission. [ABSTRACT FROM AUTHOR]

We propose a scheme of all-optical clock recovery (AOCR) based on ultrahigh-order mode locking by a semiconductor optical amplifier (SOA), which can directly extract the optical clock from non-return-to-zero (NRZ) pseudo-random-binary-sequence (PRBS) data. Firstly we demonstrate that the active mode locking system can generate stable high repetition frequency ultrashort pulse train, and adjusting the modulator's bias voltage and polarization states one can realize doubling the repetition frequency. Due to robust mode locking configuration, we successfully realize the AOCR up to 40 Gbits/s from the PRBS (231-1) NRZ optical signals without NRZ to RZ conversion, while the signal to noise ratio of the recovered optical clock is more than 20 dB with high stability, and the pulse width is short as 1.85 ps. Owing to the ultrafast nonlinear polarization rotation in the SOA, our scheme can execute AOCR for data rate of 100 Gbits/s and more. • Propose and demonstrate a novel scheme of all-optical clock recovery (AOCR) based on ultrahigh-order (>2000th) mode-locked laser. • Using actively mode-locking technology we generate high-stable high-frequency (11 GHz) optical pulse train. • Successfully carry out the all-optical clock recovery from 40 Gbits/s PRBS NRZ optical signals. • The scheme has the ability to recover optical clock signals from 100 Gbits/s data. [ABSTRACT FROM AUTHOR]

An arrayed waveguide grating (AWG) configuration can simultaneously perform the optical discrete Fourier transform and multiplex and demultiplex (MUX/DeMUX) two optical modes, to optically generate/process orthogonal frequency division multiplexing (OFDM) signals in future scalable, converged two-mode optical communication systems. We use a single AWG-based MUX/DeMUX, inter-mode and intra-optical-subcarrier symbol interleaving to mitigate the mode/subcarrier crosstalk. We analyze the performance of a 2-mode × 8 optical-subcarrier × 10 Gbit/s, on-off keying (OOK), differential phase shift keying (DPSK), and differential quadrature phase shift keying (DQPSK) mode division multiplexing (MDM)-OFDM system, that can be used in high-speed, large-capacity short-range optical link, using only passive all-optical signal processing. We numerically demonstrate that it possible to achieve error-free transmission for an 8 subcarrier × 10 Gbit/s × 2 mode system, using OOK and DPSK modulation and for a 4 subcarrier × 10 Gbit/s × 2 mode system, using DQPSK modulation, using intra-optical-subcarrier symbol interleaving. [ABSTRACT FROM AUTHOR]

A 10 Gb/s bidirectional wavelength division multiplexing passive optical network (WDM-PON) with reflective semiconductor optical amplifier (RSOA) based colorless optical network unit (ONU) is proposed and analyzed for next generation gigabit class optical access network. Differential phase shift keying (DPSK) modulated signal is used in downstream and further reused as a seeding wavelength for upstream data modulation. By exploiting the constant envelope property of DPSK seed signal, the re-modulation noise in upstream receiver is effectively minimized without employing any constraint on extinction ratio of downstream signal. Orthogonal frequency division multiplexing (OFDM) signal is used in upstream transmission to overcome the limited bandwidth (∼1 GHz) response of RSOA remodulation. The results show that the proposed 10 Gb/s symmetric WDM-PON can achieve good performance over 25 km fiber transmission with error free operation in downstream and bit error rate (BER) lower than forward error correction (FEC) limit in upstream. [ABSTRACT FROM AUTHOR]

A highly reliable optical orthogonal frequency division multiplexing (OFDM) system based on systematic polar encoded prefix-free codebook distribution matching (PCDM) is proposed. The PCDM is used to convert the uniform binary data into the unipolar coded amplitude symbols with the target probability distribution, which are then encoded by systematic polar code to maintain non-uniform probability distribution. The redundant parity bits generated by polar encoding are used as signs, map to the bipolar amplitude symbols as the real or imaginary part of the final constellation symbols when combined with the unipolar amplitude symbols. The proposed scheme generates a polar encoded probability-shaped 16 quadrature amplitude modulation (PS-Polar-16QAM) OFDM signal, which improves the bit error rate (BER) performance of the optical transmission system. The transmission of a 70 Gb/s polar encoded PS-Polar-16QAM-OFDM signal over a 2 km seven-core fiber is demonstrated experimentally. In comparison to normal-16QAM-OFDM without polar code and PCDM, the polar encoded PS-Polar-16QAM-OFDM achieves about 3.65 dB gain in receiver sensitivity at a BER of 10−4. The proposed polar encoded PS-Polar-16QAM-OFDM based on PCDM has a broad application prospect in a low-cost and reliable optical access system due to its good BER performance. [ABSTRACT FROM AUTHOR]

In this paper, we experimentally demonstrated a pilot-aided and linear interpolated channel estimation technique in the real-time direct-detection optical orthogonal frequency division multiplexing (DDO-OFDM) system using a cost-effective directly modulated laser (DML). It has been verified that the pilot-aided and linear interpolated channel estimation technique can help to compensate the sampling frequency offset (SFO) effect. The experimental results show that, based on the pilot-aided and linear interpolated channel estimation technique, even at a SFO of 170 ppm, a 16-QAM-OFDM signal can be successfully transmitted over 100-km SSMF under the hard-decision forward-error-correction (HD-FEC) threshold with a bit error rate of 3.8×10 − 3 . And the effect of up to ~25 ppm SFO can be negligible. [ABSTRACT FROM AUTHOR]

Sampling frequency offset (SFO) between digital-to-analog converter (DAC) and analog-to-digital converter (ADC) will severely degrade the transmission performance of orthogonal frequency division multiplexing (OFDM) based visible light communication (VLC) system. In this paper, a novel SFO mitigation scheme based on rotated K-means clustering is proposed, which differs from traditional SFO compensation based on phase estimation and feedback. When SFO causes the constellation points to deviate from their original positions, this scheme can adaptively update the clustering centroids to achieve the correct symbol demapping without phase estimation and compensation. Experimental results indicate that after 3.5 m free-space transmission, the SFO tolerance of quadrature phase shift keying (QPSK) and 16-ary quadrature amplitude modulation (16-QAM) can reach ± 130 and ± 65 ppm with a bit error rate (BER) less than the hard-decision forward error correction (HD-FEC) threshold of 3.8e−3, respectively. • A novel scheme for SFO mitigation is proposed with clustering algorithm of machine learning. • Phase estimation and compensation is not required compared to conventional SFO mitigation schemes. • The proposed scheme is relatively robust to imperfect channel estimation. [ABSTRACT FROM AUTHOR]

Generally, only binary modulation formats, such as on–off keying (OOK) and pulse position modulation (PPM), can be detected by a single photon avalanche diode (SPAD). In this paper, we propose to use a single SPAD receiver to achieve underwater wireless optical communication (UWOC) with high-order modulation formats. The feasibility is theoretically analyzed, and firstly demonstrated in a proof-of-concept orthogonal frequency division multiplexing (OFDM) experiment. The proposed OFDM system using quadrature phase shift keying (QPSK) and 16 quadrature amplitude modulation (16-QAM) can work with less than 68 and 390 photons per symbol, and with receiver sensitivities below the forward error correction (FEC) limit as high as −84.5 dBm and −77.8 dBm, respectively. These extraordinarily sensitive results indicate that the proposed scheme is a promising solution to long-distance UWOC systems or internet of underwater things (IoUT). • It is theoretically analyzed and experimentally proved that a single SPAD could work linearly. • The concept of using a single SPAD receiver to achieve OFDM UWOC is firstly proposed and demonstrated. • Less than 68/390 photons per symbol are needed for the proposed QPSK/16-QAM OFDM system, which is quite sensitive. [ABSTRACT FROM AUTHOR]

With the development of high-speed and large-capacity optical fiber communication technology, nonlinear damage has been one great obstacle to the development of optical fiber communication. Transmission distance and signal energy are two main factors that affect the nonlinear damage. The nonlinear compensation algorithm based on perturbation theory can compensate nonlinear damage. However, the coefficients of perturbation matrix cannot be calculated easily. In this paper, we make use of machine learning as a tool, propose an equalizer with support vector regression based on perturbation theory. For a 120 Gb/s, 375 km transmission distance, dual-polarization 64 quadrature amplitude modulation communication system, bit error ratio (BER) is lower than hard-decision forward error correction threshold 3. 8 × 1 0 − 3 in the launched optical power (LOP) from -4 dBm to 3 dBm and when LOP is 1 dBm, BER is lower than forward error correction threshold 1. 0 × 1 0 − 3 . Moreover, it is shown that the optimum LOP is increased by 2 dB. [ABSTRACT FROM AUTHOR]