Photonics

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11 pages, 2375 KiB

Open AccessArticle

Wide and Deep Learning-Aided Nonlinear Equalizer for Coherent Optical Communication Systems

by Ziyun Jiang, Xinyu Liu and Liuchao Zhang

Photonics 2024, 11(2), 141; https://doi.org/10.3390/photonics11020141 - 01 Feb 2024

Viewed by 707

In this study, we developed a wide and deep network-based nonlinear equalizer to compensate for nonlinear impairment in coherent optical communication systems. In our proposed equalizer, the power feature factor and inter-symbol feature sequence in the received signal are analyzed by two combined [...] Read more.

In this study, we developed a wide and deep network-based nonlinear equalizer to compensate for nonlinear impairment in coherent optical communication systems. In our proposed equalizer, the power feature factor and inter-symbol feature sequence in the received signal are analyzed by two combined networks, wide and deep, respectively, so that the information contained in the signal can be fully utilized. We designed an experiment using a 120 Gbit/s 64-quadrature amplitude modulation (64-QAM) coherent optical communication system over a 375 km standard single-mode fiber (SSMF) to verify the performance of the proposed wide and deep network-based nonlinear equalizer. The experimental results showed that the proposed wide and deep network-based nonlinear equalizer achieved better performance at lower complexity compared with the traditional neural network-based nonlinear equalizer. The proposed equalizer significantly improved the equalization effect at a cost of a 0.3% increase in parameters, which indicates the potential of the proposed method for application in coherent optical communication systems. Full article

(This article belongs to the Special Issue Novel Advances in Optical Communications)

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18 pages, 6179 KiB

Open AccessArticle

Geometrically Shaped Odd-Bit QAM Constellations Suitable for Principal Component-Based Phase Estimation

by Xishuo Wang, Kai Lv, Qi Zhang, Lei Zhu and Xiangjun Xin

Photonics 2024, 11(2), 140; https://doi.org/10.3390/photonics11020140 - 01 Feb 2024

Viewed by 696

Abstract

For high-speed optical communication systems, laser phase noise (LPN) stands as a pivotal factor influencing the quality of the received signal. Therefore, the employment of a highly accurate carrier phase recovery (CPR) algorithm at the receiving end is indispensable to ensure the reliability [...] Read more.

For high-speed optical communication systems, laser phase noise (LPN) stands as a pivotal factor influencing the quality of the received signal. Therefore, the employment of a highly accurate carrier phase recovery (CPR) algorithm at the receiving end is indispensable to ensure the reliability of transmission. While a CPR algorithm called principal component-based phase estimation (PCPE) has been proven to be capable of achieving low-complexity and high-performance phase recovery for even-bit quadrature amplitude modulation (QAM) (i.e., square QAM) signals, it is not compatible with traditional cross-shaped odd-bit QAM signals. To circumvent this problem, a signal constellation design scheme based on geometric shaping (GS) is proposed. The pair-wise optimization (PO) algorithm is used to optimize the constellation structure of 32QAM and 128QAM signals in order to obtain results that are compatible with the PCPE algorithm. Monte Carlo simulation results reveal that for odd-bit QAM signals utilizing PCPE for phase recovery, the proposed GS constellations enhance the mutual information (MI) performance across the entire measured signal-to-noise (SNR) range. Moreover, compared with regular 32QAM and 128QAM constellations using the well-known blind phase search (BPS) algorithm, the proposed GS and PCPE scheme can achieve SNR gains of 1.10 dB and 2.59 dB, respectively, when considering the 20% soft-decision forward error correction (SD-FEC) overhead. Verification through commercial simulation software corroborates these findings, demonstrating that the proposed GS constellations are particularly suitable for the PCPE algorithm, especially under conditions of high optical signal-to-noise ratio (OSNR). To the best of our knowledge, this is the first time that the incompatibility between the PCPE algorithm and odd-bit QAM signals has been investigated, and the proposed GS scheme has broadened the application scope of the low-complexity CPR algorithm. Full article

(This article belongs to the Special Issue Novel Advances in Optical Communications)

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12 pages, 5422 KiB

Open AccessArticle

Automatic Power Optimization of a 44 Tbit/s Real-Time Transmission System over 1900 km G.654.E Fiber and Widened C+L Erbium-Doped Fiber Amplifiers Utilizing 400 Gbit/s Transponders

by Anxu Zhang, Yuyang Liu, Lipeng Feng, Huan Chen, Yuting Du, Jun Wu, Kai Lv, Hao Liu, Xia Sheng and Xiaoli Huo

Photonics 2024, 11(1), 88; https://doi.org/10.3390/photonics11010088 - 17 Jan 2024

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Abstract

Power unevenness, mainly induced by stimulated Raman scattering, has been a major problem in multi-band transmission systems, especially in the upcoming field-deployed 400 Gbit/s widened C band plus L band system for backbone long-haul and ultra-long-haul scenarios. To reduce the impact of power [...] Read more.

Power unevenness, mainly induced by stimulated Raman scattering, has been a major problem in multi-band transmission systems, especially in the upcoming field-deployed 400 Gbit/s widened C band plus L band system for backbone long-haul and ultra-long-haul scenarios. To reduce the impact of power unevenness, we propose an automatic power optimization (APO) algorithm to guarantee reliable transmission for all channels, especially the channels at short wavelengths. The simulation results show that the power unevenness of output power after 5-span transmission in the C band is up to 11 dB before APO, while after APO is applied, it is greatly improved to less than 1.6 dB. To further investigate the performance of the APO scheme, we conduct a real-time 44 Tbit/s C+L transmission system over 1900 km G.654.E fiber utilizing 400 Gbit/s transponders. The experimental results show that the power unevenness has been effectively compensated from 12 dB to 4 dB in the entire 11 THz range, making the received optical signal-to-noise ratio relatively flat (3.4 dB). Moreover, the capacity and distance product of this system is 83.6 Pbit/s·km (44 Tbit/s × 1900 km), and to the best of our knowledge, this is a record capacity and distance product in the real-time single-mode fiber transmission system. Full article

(This article belongs to the Special Issue Novel Advances in Optical Communications)

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15 pages, 2773 KiB

Open AccessArticle

Terahertz Replica Generation of Ultra-High Data Rate Transmission in an Electro-Optical Semiconductor Optical Amplifier Mach–Zehnder Interferometer System

by Hassan Termos and Ali Mansour

Photonics 2024, 11(1), 83; https://doi.org/10.3390/photonics11010083 - 17 Jan 2024

Viewed by 847

Abstract

This article presents an analysis of an electro-optical up-converter relying on a semiconductor optical amplifier Mach–Zehnder interferometer (SOA-MZI). The pulsed control signal is generated by an optical pulse clock (OPC) with a repetition rate of

f_{s} =

19.5 GHz. The intermediate frequency [...] Read more.

This article presents an analysis of an electro-optical up-converter relying on a semiconductor optical amplifier Mach–Zehnder interferometer (SOA-MZI). The pulsed control signal is generated by an optical pulse clock (OPC) with a repetition rate of

f_{s} =

19.5 GHz. The intermediate frequency (IF) signal, which carries the modulation format known as quadratic phase shift keying (QPSK) at a frequency

f_{I F}

, is shifted at the output of the SOA-MZI to high outlet mixing frequencies

{n f}_{s} \pm f_{I F}

, where

n

represents the harmonic order of the OPC. To examine the characteristics of the sampled QPSK signals, we employ the Virtual Photonics Inc. (VPI) emulator and evaluate them using significate metrics like error vector magnitudes (EVMs), conversion gains, and bit error rates (BERs). The up-mixing process is mainly achieved through the cross-phase modulation (XPM) effect in the SOA-MZI, which operates within a 195.5 GHz ultrahigh frequency (UHF). The electro-optical SOA-MZI up-converter demonstrates consistent uplifting conversion gains across the scope of the output mixing frequencies. The simulated conversion gain deteriorates from 38 dB at 20 GHz to 13 dB at 195.5 GHz. The operational efficiency of the electro-optical SOA-MZI design, employing the standard modulation approach, is also evaluated by measuring the EVM values. The EVM attains a 24% performance level at a data rate of 5 Gbit/s in conjunction with the UHF of 195.5 GHz. To corroborate our results, we compare them with real-world experiments conducted with the UHF of 59 GHz. The maximum frequency range of 1 THz is attained by increasing the OPC repetition rate. Ultimately, through elevating the control frequency to 100 GHz, the generation of terahertz replicas of the 4096-QAM (quadrature amplitude modulation) compound signal becomes achievable at heightened UHF, extending 1 THz, while maintaining a data transmission rate of 120 Gbit/s and upholding exceptional performance characteristics. Full article

(This article belongs to the Special Issue Novel Advances in Optical Communications)

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12 pages, 3656 KiB

Open AccessArticle

A High-Security Probabilistic Constellation Shaping Transmission Scheme Based on Recurrent Neural Networks

by Shuyu Zhou, Bo Liu, Jianxin Ren, Yaya Mao, Xiangyu Wu, Zeqian Guo, Xu Zhu, Zhongwen Ding, Mengjie Wu, Feng Wang, Rahat Ullah, Yongfeng Wu, Lilong Zhao and Ying Li

Photonics 2023, 10(10), 1078; https://doi.org/10.3390/photonics10101078 - 25 Sep 2023

Viewed by 740

Abstract

In this paper, a high-security probabilistic constellation shaping transmission scheme based on recurrent neural networks (RNNs) is proposed, in which the constellation point probabilistic distribution is generated based on recurrent neural network training. A 4D biplane fractional-order chaotic system is introduced to ensure [...] Read more.

In this paper, a high-security probabilistic constellation shaping transmission scheme based on recurrent neural networks (RNNs) is proposed, in which the constellation point probabilistic distribution is generated based on recurrent neural network training. A 4D biplane fractional-order chaotic system is introduced to ensure the security performance of the system. The performance of the proposed scheme is verified in a 2 km seven-core optical transmission system. The RNN-trained probabilistic shaping scheme achieves a transmission gain of 1.23 dB compared to the standard 16QAM signal, 0.39 dB compared to the standard Maxwell-Boltzmann (M-B) distribution signal, and a higher net bit rate. The proposed encryption scheme has higher randomness and security than the conventional integer-order chaotic system, with a key space of 10,163. This scheme will have a promising future fiber optic transmission scheme because it combines the efficient transmission and security of fiber optic transmission systems. Full article

(This article belongs to the Special Issue Novel Advances in Optical Communications)

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16 pages, 4295 KiB

Open AccessArticle

High-Capacity Free Space Optics-Based Passive Optical Network for 5G Front-Haul Deployment

by Rahat Ullah, Sibghat Ullah, Waqas A. Imtiaz, Jahangir Khan, Peer Meher Ali Shah, Muhammad Kamran, Jianxin Ren and Shuaidong Chen

Photonics 2023, 10(10), 1073; https://doi.org/10.3390/photonics10101073 - 24 Sep 2023

Cited by 3 | Viewed by 882

Abstract

With the expansion of Information and Communication Technology, it is important to develop a communication network that can provide high-capacity ubiquitous connectivity. This work proposes an energy-efficient passive optical network (PON) using orthogonal frequency division multiple access (OFDMA) and wavelength division multiplexing (WDM) [...] Read more.

With the expansion of Information and Communication Technology, it is important to develop a communication network that can provide high-capacity ubiquitous connectivity. This work proposes an energy-efficient passive optical network (PON) using orthogonal frequency division multiple access (OFDMA) and wavelength division multiplexing (WDM) to facilitate the dense deployment of radio units (RUs) in a beyond 5G (B5G) communication network. High-speed connectivity is ensured by employing a hybrid PON architecture that includes a combination of free space optics (FSO) links and optical fiber (OF) media to carry OFDM and WDM multiplexed traffic. Furthermore, an optical frequency comb generator (OFCG) is utilized at the transmitter module to generate and leverage the spectrum for transmitting information from baseband units (BBUs) to the RUs situated near the end users. The proposed system is analyzed through (i) simulation analysis using Optisystem for transmission capacity computations and (ii) mathematical analysis to determine the total savings in energy. The simulation analysis shows that the given architecture can carry data across 3 km of FSO medium using 512 subcarriers per BBU transmitting at 10 Gbps of data with QPSK-modulated bit sequence. Additionally, energy efficiency shows that the use of an OFCG cuts the total energy usage by 22% at the transmitter module without negatively impacting the system’s high cardinality and transmission capacity. Full article

(This article belongs to the Special Issue Novel Advances in Optical Communications)

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15 pages, 4735 KiB

Open AccessArticle

Enhancing the Anti-Dispersion Capability of the AO-OFDM System via a Well-Designed Optical Filter at the Transmitter

by Kai Lv, Chao Yu, Hao Liu, Anxu Zhang, Lipeng Feng, Xia Sheng, Yuyang Liu and Xishuo Wang

Photonics 2023, 10(9), 1053; https://doi.org/10.3390/photonics10091053 - 15 Sep 2023

Cited by 2 | Viewed by 715

Abstract

This paper proposes a novel method to improve the anti-dispersion ability of the all-optical orthogonal frequency division multiplexing (AO-OFDM) system. By replacing the Sinc-shaped filter with a Gauss-shaped filter for sub-carrier generation and inserting a cyclic prefix (CP), the impact of dispersion on [...] Read more.

This paper proposes a novel method to improve the anti-dispersion ability of the all-optical orthogonal frequency division multiplexing (AO-OFDM) system. By replacing the Sinc-shaped filter with a Gauss-shaped filter for sub-carrier generation and inserting a cyclic prefix (CP), the impact of dispersion on the system can be significantly mitigated. Formula derivation and numerical analysis of the pulse-shaping function of the AO-OFDM system in the time domain for each cycle indicated that the pulse-shaping function generated by the Gauss-shaped filter was less affected by the dispersion effect than that of the Sinc-shaped filter. Meanwhile, less inter-carrier crosstalk between carriers was also observed. After carrying out system transmission simulations employing these two different filters, we found that the AO-OFDM system based on the Gauss-shaped filter could greatly improve the anti-dispersion ability compared with the system based on a Sinc-shaped filter. When the parameter settings in both schemes were identical, that is, the number of subcarriers was 32 and the power of a single subcarrier was −13 dBm, the bit error rate (BER) of the system based on the proposed Gauss-shaped filter after 60 km SMF transmission was only 1.596 × 10⁻³, while the BER of the traditional Sinc-shaped filter based system scheme was as high as 8.545 × 10⁻². Full article

(This article belongs to the Special Issue Novel Advances in Optical Communications)

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13 pages, 4374 KiB

Open AccessArticle

Hybrid Constellation Shaping 64QAM Based on Hexagonal Lattice of Constellation Subset

by Xiangyu Liu, Qi Zhang, Xiangjun Xin, Yongjun Wang, Feng Tian, Qinghua Tian, Leijing Yang and Yi Zhao

Photonics 2023, 10(9), 1008; https://doi.org/10.3390/photonics10091008 - 04 Sep 2023

Cited by 1 | Viewed by 958

Abstract

Increasing demand for higher-speed and large-capacity data communications has driven the development of constellation shaping technology. This paper proposes a hybrid constellation shaping scheme for 64-quadrature amplitude modulation (64QAM) based on hexagonal lattice of a constellation subset. The proposed scheme aims to enhance [...] Read more.

Increasing demand for higher-speed and large-capacity data communications has driven the development of constellation shaping technology. This paper proposes a hybrid constellation shaping scheme for 64-quadrature amplitude modulation (64QAM) based on hexagonal lattice of a constellation subset. The proposed scheme aims to enhance the nonlinear tolerance of higher-order modulated signals and further improve the constellation shaping gain. The initial quantitative characterization of the constellation is firstly performed based on the hexagonal lattice structure. Then, the objective function of maximizing constellation figure of merits (CFM) is utilized to determine the position distribution of constellation points, resulting in the generation of the geometric shaping-64QAM (GS-64QAM) signal. Finally, according to concentric hexagonal layers, all constellation points are divided into multiple subsets where points within the same subset are assigned the same probability, and the hybrid shaping-64QAM (HS-64QAM) signal is generated. To validate the effectiveness of the proposed scheme, the experimental verification was demonstrated in a 120 Gbit/s multi-span coherent optical communication system. Experimental results indicate that, at the soft-decision forward error correction threshold, HS-64QAM achieves an optical signal-to-noise ratio (OSNR) gain of 1.9 dB and 4.1 dB over uniform GS-64QAM in back-to-back and 375 km transmission scenarios, respectively. Furthermore, HS-64QAM achieves an OSNR gain of 2.7 dB and 7.6 dB over uniform Square-64QAM in back-to-back and 375 km transmission scenarios, respectively. Full article

(This article belongs to the Special Issue Novel Advances in Optical Communications)

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