Instead of conventional knowledge-driven modeling, this study introduced data-driven channel modeling methods based on deep learning (DL) in a free-space optical (FSO) communication system to achieve low complexity, less modeling difficulty, high fidelity, and differentiability. The FSO channel was modeled via three DL algorithms: generative adversarial network (GAN), bidirectional long short-term memory (BiLSTM), and Bayesian neural network (BNN). Consequently, their performance was comprehensively compared in terms of optical amplitude waveforms, Kullback-Leibler (KL) divergence, 95% confidence interval, data distribution, and fitted probability density functions. The results indicated that the GAN-based channel exhibited optimal performance, implying its suitability for learning the stochastic distribution of the FSO channel. In addition, the performance of GAN-based channels was examined under different conditions: turbulence intensities (weak, moderate, and strong), transmission distances (3, 4, and 5 km), launch powers (0, 10, and 20 dBm), modulation formats (on-off keying, pulse position modulation, and pulse amplitude modulation 4), and the adaptability to channel variation. The test results were satisfactory and confirmed the ability of the GAN to learn the approximate distribution function of the FSO channel. Thus, the primary aim was to prove that for FSO channels with statistical distribution characteristics, a data-driven DL model can not only serve as an effective surrogate model for specific scenarios but also provide a potential tool for future modeling methods.

We investigate the communication performance of a few-mode EDFA based all-optical relaying system for atmospheric channels in this paper. A dual-hop free space optical communication model based on the relay with two-mode EDFA is derived. The BER performance is numerically calculated. Compared with all-optical relaying system with single-mode EDFA, the power budget is increased by 4 dB, 7.5 dB and 11.5 dB at BER $= 1\times10^{-4}$ under the refractive index structure constant $C^2_n = 2\times10^{-14}, 5\times10^{-14}$ and $1\times10^{-13}$ respectively when a few mode fiber supporting 4 modes is utilized as the receiving fiber at the destination. The optimal relay location is slightly backward from the middle of the link. The BER performance is the best when mode-dependent gain of FM-EDFA is zero.

An orbital angular momentum division multiplexing free space optical communication (OAM-DM FSO) system with channel coding can compensate atmospheric channel fading and improve system performance. An OAM-DM FSO system based on hybrid channel coding is proposed in this paper. The coding gain is improved by taking into account mode dependent channel fading difference caused by atmospheric turbulence. Simulation results show that compared with single channel coding, the coding gain is increased by 1.85 dB under Cn₂=1E-14, BER=1E-5 with non-uniform LDPC code (0.7 code rate) for an OAM₁/OAM₃ multiplexing system. In addition, for four OAM modes (+1,+3,+5,+7) multiplexing systems, the coding gain is increased by more than 3.8 dB under Cn₂=1E-14 and BER=1E-5.

The passive optical network (PON) has recently emerged as the most widely deployed and successful optical access solution. Data rate demands, driven by the growing use of broadband services, are expected to increase continuously over the next few decades. TWDM PONs and WDM PONs show the most promise for emerging broadband services involving various applications. In both TWDM and WDM PONs, a colorless light source or tunable transmitter is used on the ONU side and is indispensable for low-cost and plug-and-play ONUs. This article describes the principles of different colorless- light-source schemes and the architectures of the corresponding TWDM and WDM PON systems. Also, their transmission performances are analyzed and compared. Further, four types of wavelength control methods for light sources based on tunable laser diodes are described and analyzed. The features, characteristics, and applicability of the various colorless-light and tunablelight- source schemes are analyzed and compared.

Relay assisted transmission can compensate atmospheric turbulence, improve transmission distance, and realize communication between non-linear targets in free space optical communication (FSO) system. Space division multiplexing (SDM) greatly improves the channel capacity of FSO system. This paper focuses on the relay assisted transmission oriented to SDM wireless optical communication system. We develop a dual-hop SDM FSO communication model with two spatial mode channels. At the relay, the received optical signals are amplified by the few mode EDFA and forwarded to the destination. Simulation results show that the bit error rate (BER) of the dual-hop SDM system with a few mode EDFA based relay outperforms the single-hop SDM system. Under BER = $1 \times {10}^{ - 5}$ , the power budget is increased by 0.53 dB, 1.2 dB, and 4.88 dB, when ${\boldsymbol{C}}_{\boldsymbol{n}}^2 = \ 4 \times {10}^{ - 15}$ , $7 \times {10}^{ - 15}$ and $1 \times {10}^{ - 14}$ , respectively. Moreover, the BER performance is the best when the relay is (originally in the middle of the link) slightly closer to the source.

In this paper, a low-complexity joint compensation scheme of carrier recovery (JCSCR) for coherent free-space optical (CFSO) communication is proposed. We applied the carrier recovery joint compensation approach to a CFSO communication system in the quadrature phase shift keying (QPSK) modulation format. A signal-preprocessing stage, which effectively avoided the repetitive operations found in traditional carrier recovery schemes, was proposed. Unlike in existing carrier recovery algorithms, the modulated phase of the received signal could be accurately removed using only the sum and subtraction of real absolute values in the signal-preprocessing stage, greatly reducing the complexity of the operation. Since this algorithm avoids the traditional fourth operation, the system’s complexity is reduced while additional noise generated by fourth cross-terms would be prevented and system noise immunity would be greatly enhanced. In addition, this algorithm uses joint compensation of phase errors in the final compensation stage, further reducing the complexity of the computation of the whole algorithmic scheme. A 10 Gbps QPSK CFSO communication transmission experiment was conducted in an atmospheric turbulence channel to verify the proposed technique and improvement in receiver sensitivity.