Signal recovery in optical wireless communication using photonic convolutional processor.

Guest Editorial

In optical wireless communication (OWC) systems, channel estimation and detection of the transmitted symbols have been conventionally performed using analytical methods assuming that the optical channel follows a certain model, e.g., free-space model, input-dependent noise model, or Poisson model. In practical OWC systems, channels do not necessarily follow a specific model. Hence, it is difficult, if not impossible, to derive analytical models that provide optimal performance in realistic optical channels. Motivated by the success of neural networks in estimation and classification in various fields, we propose a neural network-based methodology for detection and estimation for OWC that does not rely on a channel model. Simulation results show that the proposed learning-based estimation and detection schemes achieve the optimal performance of the maximum likelihood detector under different channel state information assumptions.

In optical communication systems, fibre nonlinearity is the major obstacle in increasing the transmission capacity. Typically, digital signal processing techniques and hardware are used to deal with optical communication signals, but increasing speed and computational complexity create challenges for such approaches. Highly parallel, ultrafast neural networks using photonic devices have the potential to ease the requirements placed on the digital signal processing circuits by processing the optical signals in the analogue domain. Here we report a silicon photonice-lectronic neural network for solving fibre nonlinearity compensation of submarine optical fibre transmission systems. Our approach uses a photonic neural network based on wavelength-division multiplexing built on a CMOS-compatible silicon photonic platform. We show that the platform can be used to compensate optical fibre nonlinearities and improve the signal quality (Q)-factor in a 10,080 km submarine fibre communication system. The Q-factor improvement is comparable to that of a software-based neural network implemented on a 32-bit graphic processing unit-assisted workstation. Our reconfigurable photonic-electronic integrated neural network promises to address pressing challenges in high-speed intelligent signal processing.

We propose and experimentally demonstrate a full-duplex integrated passive optical network (PON) and indoor optical wireless communication (OWC) system. For the PON part, an external cavity laser (ECL) and a directly modulated laser (DML) are used as optical sources for downlink and uplink, respectively. The OWC subsystem utilizes a visible light-emitting diode (LED) for downlink and an infrared LED (IR-LED) for uplink transmission. To enable high spectral efficiency (SE), high-speed transmission, and flexible multiple access with simplified optical network unit (ONU)-side digital signal processing (DSP), multilevel, multiband quadrature amplitude modulation (QAM) and hybrid 128/64/16 Nyquist single-carrier frequency domain equalization (N-SC-FDE) modulation are implemented in this paper. In terms of 62.5 MHz OWC N-SC-FDE signals, the system successfully delivered a further 140 cm indoor distance after transmitting over a span of 25 km standard single mode fiber (SSMF) together with three subbands of 250 MHz PON signals. As a proof of concept, an integrated system of symmetrical full-duplex 5 Gb/s PON and 250 Mb/s OWC at each direction for three wired users and one wireless user is successfully achieved. The measured bit error ratios (BERs) for each user are all smaller than the 7% pre-forward-error-correction (pre-FEC) limit of 3.8 × 10−3, which shows the promising potential and feasibility of this proposal to extend multiple services from metro areas to suburban areas.

Nowadays, the coherent optical communication system plays an important role in communication field because of large capability and bandwidth. A coherent optical communication, based on high-order modulation and digital signal processing technologies, consists of optical transmitters, optical fiber lines, optical amplifiers and optical receivers. In the high-speed coherent optical communication system, the phase noise from the transmitter laser and the local oscillator laser can significantly degrade the performance of the signal transmission and detection, especially for the systems using high-order modulation format, such as m-ary phase shift keying (mPSK) and m-ary quadrature amplitude modulation (m-QAM). Therefore, investigations on laser phase noise compensation algorithm based on digital signal processing technologies has become more and more significant. In this work, a multi-ring carrier phase recovery algorithm is developed for compensating the laser phase noise in optical fiber communication systems using high-order modulation formats. Degradations on the performance of communication systems due to the laser phase noise have been investigated. The system performance using the proposed algorithm and the conventional Viterbi-Viterbi algorithm were also evaluated in 9-channel and 15- channel, 32-Gbaud, Nyquist-spaced QPSK, 16-QAM, 64-QAM and 256-QAM coherent transmission systems with considering the impact of the laser phase noise. It is found that the phase noise leads to stricter constraints on the linewidths of transmitter-side and receiver-side lasers, and it can greatly degrade the achievable information rates in communication systems. Besides, compared to the conventional Viterbi-Viterbi algorithm, which is usually applied in the QPSK system, our proposed algorithm can also well mitigate the laser phase noise in 16-QAM, 64-QAM and 256-QAM optical communication systems.
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The design, fabrication, optical and mechanical characteristics, and reliability of several kinds of the latching-type single-mode-fiber (SMF) switch are reported. Practical applications of these switches to optical fiber communication systems and optical testing and sensing systems are also demonstrated. In the developed switches, a movable fiber is actuated by the magnetic attraction force produced between permanent magnets and a micromagnetic-alloy pipe fixed around the fiber. The ultralight micropipes (0.13 (i.d.)-0.18 (o.d.)/spl times/5/sup L/ mm, 400 /spl mu/g) were successfully massproduced by a fiber-micromachining technique based on metallizing and etching of sacrificial fibers. We used a refractive-index-matching oil in the switches to reduce insertion loss and reflected light. The several hundred fabricated 1/spl times/2 standard SMF switches exhibited excellent optical characteristics, such as typical insertion loss of 0.36 dB, return loss of 49 dB and crosstalk of -70 dB. They exhibited stable latching-type operation with loss changes of below 0.1 dB during 10/sup 5/ switching operations with minimum driving power of 9 mW and switching time of 2 ms. The switches were subjected to endurance (10/sup 8/ switching operations), heat cycles, shock and vibration, and long-term (6.5 years) intermittent operation tests, and revealed no significant optical or mechanical degradation. A 1/spl times/8 latching-type SMF switch was constructed using seven 1/spl times/2 switches cascaded in three stages and uniform insertion losses of below 0.7 dB and return losses of over 47 dB at each of eight ports were obtained. The 1/spl times/2 and 1/spl times/8 switches have been applied to NTT's optical communication systems. We fabricated a compact and cost-effective wavelength switch by installing microfilter chips in the 1/spl times/2 switch and confirmed stable latching-type wavelength switching between 1.3- and 1.55-/spl mu/m optical signals. We also designed and fabricated a polarization-maintaining switch using polarization maintaining and absorption reducing (PANDA) fibers and confirmed stable polarization maintaining switching operations. These switches are applicable not only to optical fiber communication systems, but also to optical testing and sensing systems.

This study presents a scalable inter-satellite optical wireless communication (IS-OWC) system based on coarse wavelength division multiplexing (CWDM) and high-power erbium-doped fiber amplifiers (EDFAs) to support high-bandwidth data transmission to address 6G and non-terrestrial network (NTN) backhaul needs. An 18-channel CWDM arrangement supports an aggregated data rate of 180 Gbps through 5500 km optical wireless link. Every channel takes up a unique wavelength (1270–1610 nm with 20 nm separation), designed to provide maximum interference protection and high spectral efficiency. Signal amplification is ensured by EDFAs to provide data continuity over long distances. Performance parameters like bit error ratio (BER) and Q-factor exhibit reliable transmission up to 4500 km and degradation beyond this distance to warrant error correction and sophisticated modulation to support far-range use cases. The study demonstrates the promise of CWDM-based IS-OWC with EDFAs to be an economical high-speed option for 6G and NTN backhaul to offer mass-scale access in remote and underserved regions.

光纤通信已经成为目前通信网络发展的主流方向。但是随着光纤通信系统向着超远距离、超大容量、超高速率的方向发展，光纤色散已经成为其巨大的阻碍。如何有效地控制光通信系统中产生的色散，使光信号在大容量、高速率、长距离传输中无误码地到达接收端，已经成为人们迫切需要解决的问题。对光通信系统中的色散进行监测并进行动态补偿的研究，使得超大容量、超长距离、超高速率的光纤通信系统成为可能，有助于最终实现全光智能自动交换网络。在众多测量光纤色散的方法中，调制相移法具有结构简单，可靠性强等优点。本文中，我们对使用调制相移法测量光纤色散进行了研究和仿真，讨论了在不同光纤长度和不同调制频率下，影响调制相移法测量结果精度的因素。通过适当地选取调制频率，我们可以使用调制相移法测得不同光纤长度下色散的精确值。另外在本文中，我们通过仿真验证了调制相移法可以测量具有任意色散–波长曲线的光纤。因此在实际应用中，调制相移法是可以应用于高精度光纤色散的测量。 Optical fiber communication is the mainstream of communication development at present. However, with the development of optical fiber communication system which trending towards ultra-long length, ultra-large capacity and ultra-high speed, fiber dispersion turned to be a great obstacle. How to control the dispersion created in the optical communication system effectively while force optical signal to reach the receiving end in high-capacity, high-rate and error-free long-distance transmission has increasingly become a research focus of optical fiber communication systems. It is a great significance to research about monitor and dynamic compensate dispersion in the optical communication systems, so that the ultra-large capacity, ultra-long length and ultra-high speed optical fiber communication systems would not be impossible, all-optical intelligent network with automatic switching functions would come true. Among the varieties of methods to measure fiber dispersion, the phase-modulation shift method is the unique one, which has the advantages of simple structure, high reliability. In this paper, we make a research and simulate to use the phase-modulation shift method, meanwhile we also discuss about the factors which affect the accuracy of the results in the different fiber lengths and modulation frequencies. By the means that selecting appropriate modulation frequency, we can use modulation phase shift method to measure the exact value of dispersion comes out from different lengths of fiber. Moreover, this paper also proves that the phase-modulation shift method is able to measure the optical fiber with arbitrary dispersion-wavelength curve, which is already verified by our simulation. Therefore, the phase-modulation shift method is well to be applied to take practice in the high-precision fiber dispersion measurement.

This review concentrates on the state-of-the-art hardware-oriented receiver aspects for optical wireless communication (OWC), and points to the importance of BER performance analysis and modeling in presence of non-perpendicular light incidence. Receivers in OWC networks for 6G applications have to work for strongly different light incidence angles, to allow the formation of connections to locally separated transceivers without the need for rotation units and accurate adjustment. In turn, and in combination with fully integrated optical receivers, reduction of cost and increased comfort can be achieved. Fully integrated [bipolar] complementary metal-oxide-semiconductor ([Bi]CMOS) receivers with on-chip avalanche photodiodes (APDs) and single-photon avalanche diodes (SPADs) are presented and their performance in optical wireless communication is summarized. Impressive data rates up to 2 Gbit/s and free-space transmission distances up to 27 m at bit error ratios (BER) below 10−9 are reached with linear-mode APD receivers. The importance of optical interference in the isolation and passivation stack on top of the integrated photodiodes is illuminated. To be able to predict the dependence of the BER of single-photon avalanche diode (SPAD) receivers on the light incidence angle, a model, which includes a model for the photon detection probability and a standing-wave model for the isolation and passivation stack, is extended. The dependence of the BER on the light incidence angle onto the photodiodes is investigated by electromagnetic simulation for optical transmission of the layers on top of the photodiode, device simulation for the avalanche triggering probability and BER modeling with MATLAB. It is found that incidence angles up to 30° have moderate influence on the BER and that the BER degrades significantly for incidence angles larger than 50°.

With the volumes of transmitted information growth, the task of increasing the noise immunity and reliability of data transmission networks arises. The presence of interference such as nonlinear loss, chromatic dispersion, polarization mode dispersion, and inter-channel interference decreases the performance of fiber optical communication systems. Forward Error Correction (FEC) codes are vital in optical communication systems to achieve highly efficient and highly reliable communication. The paper presents research on a simulation model where FEC channel-coding schemes based on the Bose-Chaudhuri-Hocquenghem (BCH) cycling block code are integrated with non-return-to-zero on-off-keying (NRZ-OOK) modulated wavelength division multiplexed (WDM) fiber optical communication systems. The results highlight the selection of coding parameters in the adopted FEC approach. In our paper, the BCH (255, 231), BCH (255, 223), and BCH (255,191) FEC channel-coding schemes are chosen for integration in the NRZ-OOK modulated WDM optical transmission system. The paper shows the main advantages and drawbacks of the selected FEC coding scheme and provides a comparison of the proposed schemes based on their main features, such as bit error ratio (BER), which were obtained through the simulations of 25 Gbit/s NRZ-OOK WDM fiber optical system. The dynamics of changes in BER was investigated using the Monte Carlo (MC) method for precise results. The paper shows that integration of FEC channel-coding schemes based on the BCH codes improves the noise immunity and reliability of the WDM Fiber Optical Communication Systems.

The metasurface based beam-steering devices with the advantages of large steering angles, arbitrary channels and ultra-compactness have played an important role for data allocation and exchange in the optical wireless communication. However, the current metasurface based optical wireless communication systems are mainly on intensity modulation and direct detection (IM/DD), which shows a relative lower transmission capacity, lower received optical signal-to-noise ratio (OSNR) and complexity of system. In this study, a bidirectional multichannel optical wireless system enabled by a polarization independent metasurface with coherent modulation and reception is designed and experimentally demonstrated, which exhibits exclusive 100 Gbps coherent optical signals to multiusers with their own wavelengths, 2 m free space distance and field of view of 20° × 20°. In addition, the proposed system can support optical broadcasting system with capacity of 900 Gbps. The demonstrated metasurface assisted optical wireless communication system merges the optical coherent communication techniques and emerging concept of metasurface, which reduces the complexity and cost of the system while contributing a high transmission capacity, opening a new avenue for high performance optical wireless communications.

The efficient and accurate evaluation of the transmission performance of high-capacity optical communication systems has always attracted significant research attentions. The enhanced Gaussian noise (EGN) model is considered as an excellent solution to predict the system performance taking into account linear and nonlinear transmission impairments. Since the conventional form of the EGN model is complicated and intractable for a fast computation, the closed-form simplification has been regarded as a direction to significantly reduce the computational complexity. However, the accuracy of such a closed-form EGN model becomes a main concern in the application of ultra-wideband optical communication systems. In this work, we have investigated the accuracy of the closed-form EGN model for ultra-wideband optical fiber communication systems, where the performance of the system using electronic dispersion compensation, multi-channel nonlinearity compensation and full-field nonlinearity compensation has been evaluated in terms of symbol rate, number of channels and signal power. Our work will provide an insight on the application of the EGN model in next-generation ultra-wideband long-haul optical fiber communication networks.

Optical wireless technology has been considered as a promising candidate to provide high-speed wireless connectivity in personal work/living spaces. Previously we proposed and demonstrated a high-speed indoor communication system by incorporating limited mobility with localization function. Furthermore, we showed that by using a novel single channel imaging receiver, better system performance can be achieved and the demonstration was based on single-mode fiber due to its low transmission loss feature. However, in most already installed inbuilding optical fiber distribution networks, multi-mode fiber was deployed since it is low-cost and easy to handle. In this paper, we experimentally demonstrate the feasibility of the proposed optical wireless personal area communication system with single channel imaging receiver based on multi-mode fiber. In addition, the wavelength-division-multiplexing (WDM) technology is also incorporated to realize an ultra-broadband connectivity in wireless personal area networks. The results show that a simultaneous operation of a 4 × 10 Gbps down-link and a 400 Mbps up-link can be realized simultaneously and the beam footprint is larger than 1 m even when a channel selecting optical bandpass filter with ~77% transmission efficiency was used.

In this paper, the performance of current digital pulse modulation schemes for optical wireless communications was examined by simulation, and we concluded that PPM is suitable for long distance optical wireless communication systems especially for military use due to its best power efficiency and also suitable for optical wireless communication systems working near sea surface; DPIM and DH-PIM are suitable for normal outdoor applications, for example, being used in city or suburb, for its moderate performance and no need of symbol-level synchronization; MPPM is especially suitable for indoor optical wireless communication systems with diffused channel due to its best bandwidth efficiency. (4 pages)

The development of optical communications systems from long fibers to compact wireless networks has led to the possibility of linking two satellites using a wireless optical link. The optical communication systems have been upgraded to use of wireless systems. The two satellites can be connected using optical wireless link. The optical wireless communication allows the exchange of information between two satellites at a higher speed. To act as a source of light, laser can be used and as an optical link, and a high-frequency light signal can be used. In this paper, we are analyzing and optimizing the different aspects of an optical communication link by selecting different modulation scheme. The IS-OWC link has been analyzed using PSK, DPSK, and OQPSK. The bit error rate (BER) and Q-factor have been used as the main determinant to analyze the performance of optical link between two satellites.