Handbook of Radio and Optical Networks Convergence

Optical transmission systems and networks are key elements of the worldwide communications infrastructure. Optical fibers span the globe across land and oceans, underpinning the digital economy and data services that play an increasing role in the daily lives of billions of people. In this section of the Handbook of Radio and Optical Networks Convergence, we focus on optical communications and the technologies that carry more than 99% of transmitted data worldwide. In this introductory chapter, we give a brief overview of the evolution of optical transmission systems and the current technologies under active research for the systems of the future. Finally, we outline the chapters in this section of the handbook and introduce the authors sharing their knowledge on topics ranging from the information theory of the fiber channel to real-world networking technologies and current research for the technologies of the optical network of tomorrow.

The enormous potential of the fiber-optic channel to transmit data over long distances at high rates has been gradually unlocked by means of a number of key technological innovations underpinned by the mature understanding of lightwave propagation in optical fibers. This chapter reviews the main properties of the fiber-optic channel, starting from the structure of ideal linear optical fibers and proceeding to the derivation of the equations governing signal propagation in single-mode fibers in the presence of various physical phenomena. These are chromatic dispersion, attenuation and amplification, polarization-mode dispersion, polarization-dependent loss, and nonlinear distortions. In the context of nonlinear propagation, the chapter also provides an introduction to the modeling of the nonlinear interference noise in coherent systems that are used today. The chapter targets readers who are familiar with the fundamental concepts involved in the study of electromagnetic fields and signal theory but have no specific background on fiber-optic transmission. It consists of five sections that are largely self-contained but with references used to avoid long derivations and maintain brevity.

Since the development of the first semiconductor laser in the 1960s, much R&D effort has been concentrated on designing and developing optical transceivers (TRxs) for reliable optical data transmission. In short, an optical TRx is tasked with converting electrical signals to optical signals (E/O conversion) at its input and with converting optical signals to electrical signals (O/E conversion) at its output. In this chapter, the operating principles of the main opto-electronic components comprising an optical TRx, namely, the laser, the modulator, and the photodetector, are studied. In addition, modern aspects on integrated photonics are discussed, due to the significant growth and importance of these technologies in recent years. The chapter aims to provide an up-to-date overview of the aforementioned topics, focusing on technologies that are currently being deployed or soon-to-be deployed in modern optical networks and transmission systems.

Optical amplifiers based on the stimulated emission of radiation paved the way for the global expansion of the high-speed internet, revolutionizing the world of telecommunications. They have further enabled highly efficient, high-power laser tools contributing to many areas of industry, medicine, and other domains. This chapter starts with a description of the erbium-doped fiber amplifier (EDFA) for optical fiber communications in the 1550 nm wavelength region. The structure, components, theory, and optimization issues of EDFAs are discussed and used as an example to describe the basics of numerical modeling of optical amplifiers. We then give a brief description of other rare-earth-doped amplifiers, including praseodymium-doped amplifiers for the 1.3 micrometer band and thulium-doped fiber amplifiers for wavelength ranges below and above the band covered by the EDFAs. Next, technologies of cladding-pumped, high-power fiber amplifiers and lasers, mainly based on ytterbium-doped double-clad fibers, are briefly reviewed before an overview of principles of operation of Raman fiber amplifiers, semiconductor optical amplifiers (SOA), parametric fiber amplifiers, and bismuth-doped fiber amplifiers. Finally, selected applications of optical amplifiers are discussed, namely the maximization of the transmission distance without in-line optical amplifiers, EDFAs for spatial mode division multiplexing in multicore optical fibers, and bidirectional EDFA for precise time and frequency transmission.

This chapter discusses the basic concepts of digital optical transmission systems. In particular, the key components and structure of digital transmitters and receivers for coherent optical fiber transmission are addressed. This includes modulation formats, digital pulse shaping, and optical modulation at the transmitter side. At the receiver side, digital dispersion compensation, including the use of overlap and save techniques, is first described. Timing and carrier recovery techniques for optical signals are also reviewed, along with polarization demultiplexing through the use of multiple-input and multiple-output subsystems. The description includes the use of the constant modulus algorithm, as well as decision-directed and data-aided least mean squares algorithms.

The rapid evolution of communication technologies, such as 5G and beyond, relies on optical networks to support the challenging and ambitious requirements that include both capacity and reliability. This chapter begins by giving an overview of the evolution of optical access networks, focusing on Passive Optical Networks (PONs). The development of the different PON standards and requirements aiming at longer reach, higher client count, and delivered bandwidth are presented. PON virtualization is also introduced as the flexibility enabler. Triggered by the increase of bandwidth supported by access and aggregation network segments, core networks have also evolved, as presented in the second part of the chapter. Scaling the physical infrastructure requires high investment and hence, operators are considering alternatives to optimize the use of the existing capacity. This chapter introduces different planning problems such as routing and spectrum assignment problems, placement problems for regenerators and wavelength converters, and how to offer resilience to different failures. An overview of control and management is also provided. Moreover, motivated by the increasing importance of data storage and data processing, this chapter also addresses different aspects of optical data center interconnects. Data centers have become critical infrastructure to operate any service. They are also forced to take advantage of optical technology in order to keep up with the growing capacity demand and power consumption. This chapter gives an overview of different optical data center network architectures as well as some expected directions to improve the resource utilization and increase the network capacity.

Driven by future (beyond 5G and 6G) services and applications, optical networks are required to support an ever increasing and more dynamic data traffic in a flexible and efficient manner. In this evolutionary scenario, an optical bandwidth allocation based on a fixed-grid spectrum is no longer suitable for addressing the envisioned multi-Tb/s capacity links and dynamic connectivity, while ensuring an optimal usage of the network resources and the available infrastructure. Thus, optical switching systems require advanced technologies to support this novel flexible paradigm.This chapter presents the fundamental building blocks, the network elements and technologies enabling elastic optical networks, flex-grid channel allocation and routing, as well as multiple and advanced switching operations. This includes the bandwidth-variable optical cross-connect, wavelength selective switch, and reconfigurable optical add/drop multiplexer, as well as flexible transceivers, such as the (sliceable) bandwidth/bitrate variable transceiver, and software defined networking-based control and orchestration. Commercial technologies as well as latest research solutions currently under investigation are presented and discussed highlighting a path for optical switching systems enabling flexible operation and dynamic high-capacity connectivity for both optical and data center networks.

Optical fiber communications systems have experienced a tremendous development over the past decades, enabling a steady exponential increase of data rates over short and long distances. Over the last 10 years, it became clear that using current fibers and/or spectral transmission bands won’t support a significant further increase. After introducing single-mode fiber-based optical transmission systems and the technological evolution that enabled current transmission systems, this chapter gives an overview on two distinct ongoing research directions for a drastic increase of data rates: (1) to further push the data rates limits in current single-mode fibers and (2) to explore a new multiplexing dimension, the spatial dimension of optical fibers to drastically increase the per-fiber data rates. The former topic focuses on approaches to increase the spectral efficiency and hence the amount of data that can be transported in a given bandwidth as well as adding further spectral bands where single-mode fibers guide at low loss. This approach is particularly interesting when maximizing data rates in existent fiber infrastructure. The second part describes novel optical fibers such as few-mode and multi-core fibers and related space-division multiplexing technologies that have been demonstrated to offer a strong per-fiber capacity increase of 2 orders of magnitude.

As 5G rollout accelerates globally, the mobile network operators are facing more heavy lifting over the fixed transport infrastructure, carrying not only broadband but also time-critical traffic between the data center and antenna sites. With optical fiber being the predominant transport medium for 5G radio access networks, existing and emerging optical transport technologies can be leveraged to provide a cost-saving and future-proof RAN architecture. This chapter covers optical 5G transport challenges and feasible solutions, and how the optical underlay and the transport protocol can be used to consolidate future front- and backhaul traffic on an optical network infrastructure.

Today wireless communication is an indispensable necessity. Over the past three decades, we have seen the transformational impact of wireless connectivity on society. The evolution from a simple 2G network through to current 5G and beyond network deployments has no doubt transformed the world into a highly networked society. This transformation has led to a demand for massive performance gains across key wireless network specifications (connectivity, data throughput, latency, energy consumption). Even with a modest adoption of 5G-like services, it has become a massive challenge to meet these demands, which inevitably places a heavy burden on the transport network (fronthaul, backhaul, and midhaul) infrastructure. This is an on-going challenge, which is further compounded by the overcrowding of the wireless spectrum in the lower microwave region.The chapter explores the different technologies and the challenges that have driven the evolution of radio access networks from the perspective of physical layer and medium access control (MAC) layer implementation.

Coherent optical communication is a game-changer optical fiber transport technology for high-speed data transmissions in long-haul networks and data center interconnects, enabling a widespread upgrade and new deployment of DWDM networks to speeds of 100 Gbps, 200 Gbps, and 400 Gbps per wavelength. Recently, the potential of using coherent optics in access networks incurs a lot of discussions in both industry and academia. Following the continuation of growing bandwidth demands in ultra-high-definition video streaming, cloud computing, immersive gaming, 5G, VR, and remote health care, we are pushing really hard on traditional intensity-modulation direct-detection systems and approaching their performance limit. On the other hand, with the progress of design simplification, silicon photonics, and semiconductor manufacturing process, the cost of coherent devices continues to reduce, which may enable coherent optics to replace direct-detection links in mobile fronthaul, aggregation nodes, and access networks. It is anticipated that, with the greatly improved receiver sensitivity and stronger robustness under chromatic dispersion, coherent optics could significantly enhance the transmission distance and number of connected users for next-generation passive optical networks (PONs). This migration model has been demonstrated in the optical industry before the DWDM system technology started in long haul and subsequently migrating to metro and edge access; forward error correction (FEC) encoding and decoding followed the same pattern. Benefiting from initial long-haul technology development, coherent optics for access networks is poised to follow the same natural progression. In this chapter, we reviewed the development, operation principles, and digital signal processing techniques of coherent optical systems. The key technologies to enable the future point-to-multipoint coherent optical access network have also been discussed.

This chapter presents a literature survey of data security in optical fiber networks. The evolution of data security in optical fibers starts from PHY-layer encryption based on ITU-T G.709 optical transport networks (OTN) standards, where the security of classical cryptography is based on computational complexities of intractable mathematical problems. The emerging quantum computers make threats to classical cryptography. Two strategies are developed to address these threats, i.e., post-quantum cryptography (PQC) and quantum key distribution (QKD). PQC algorithms are still based on computational complexities, whereas QKD provides information-theoretic security guaranteed by quantum mechanics. A comprehensive literature review of QKD is presented with a focus on quantum networks in optical fibers. Three issues hindering QKD technologies from wide commercial deployment include distance limit, noise interference, and QKD in passive optical networks (PONs). The key rate of a QKD link scales linearly with the channel transmittance. In optical fibers, the channel transmittance decays exponentially with fiber distance due to the absorption of photons. This limits the distances of fiber-based QKD links to hundreds of kilometers. To extend distances, trusted and untrusted relay technologies were developed, depending on key exposure at the relay node. Since QKD links are vulnerable to noise and interference from classical data traffic, most reported QKD systems need dedicated/dark fibers. But this is cost-prohibitive and impractical in real deployments. The only chance for the commercial success of QKD technologies is integration into existing fiber networks and sharing the same fibers with classical data traffic, which in turn brings the second issue, spontaneous Raman scattering (SpRS) noise from classical channels. Several noise mitigation strategies are discussed based on wavelength/time-division multiplexing (WDM/TDM) techniques. Finally, as the last mile of optical fiber networks, PONs are not only the bottleneck of data throughput but also the most vulnerable segment of data security. Most eavesdropping occurs in PONs leveraging the point-to-multipoint (P2MP) topology, where the downstream data are broadcast to all users. In PONs, the imbalanced attenuation to classical downstream data and the upstream QKD channel makes the downstream data in the feeder fiber become the dominant source of SpRS noise. A dual-feeder fiber method is introduced to mitigate SpRS noise by isolating QKD links in a dedicated feeder fiber.

Millimeter-wave (MMW) and terahertz (THz) wireless links have recently attracted a significant interest because of their ultrahigh data rate. However, the application of MMW/THz wireless links is limited because of its propagation characteristics. The absorption of electromagnetic waves at MMW/THz bands by air and rain is considerably large, which limits the transmission distance of the wireless links for the MMW/THz bands. Moreover, the diffraction loss for the MMW/THz bands is significantly large, and a line-of-sight (LOS) environment is necessary for the MMW/THz wireless links.Based on the propagation characteristics, various use cases, such as a wireless backhaul/fronthaul link, mobile wireless access, wireless LAN, kiosk download, and intra- and inter-chip connections, have been proposed. It is important to construct propagation models applicable to this frequency band for using the MMW/THz band wireless systems practically, because these models are used to design the link budget of wireless systems and estimate interferences between the radio stations located in proximity.This chapter describes the features and subjects of MMW and THz band propagation for the major use cases of MMW/THz wireless links.

Radio channel measurements and modeling to describe radio propagation characteristics are essential for the design and evaluation of new wireless systems. In particular, for fifth-generation (5G) and beyond 5G high-frequency radio systems in the millimeter-wave and terahertz bands, angle scanning via highly directive antennas or phased arrays is the most popular method for obtaining angular channel characteristics. This section describes the multidimensional channel sounding measurement and multipath parameter estimation techniques.

This chapter describes millimeter-wave experiment trials for 5G. 5G is the first mobile communication system that exploits millimeter-wave bands such as 28 GHz, and for overcoming a larger path loss in the millimeter-wave bands, massive MIMO technologies were introduced for 5G to obtain beamforming gain. 5G experiment trials to verify the massive MIMO technologies were conducted for realizing 5G radio access, and additional experiment trials on advanced massive MIMO technologies for further enhancement of 5G, so-called 5G Evolution/5G-Advanced are being performed. This chapter briefly introduces an overview and results of DOCOMO’s millimeter-wave experiment trials for 5G and 5G Evolution. For beyond 5G and 6G, extremely high data rate exceeding 100 Gbps is required, and hence so-called THz (sub-THz) wave targeting the frequency from 100 GHz to 300 GHz has attracted much attention. This chapter also shows over-100 Gbps throughput and coverage performances by link-level simulations using a 100 GHz-band massive MIMO technology. In addition, 100 GHz-band system-level performances are shown by developed 6G simulator assuming multiple base stations and mobile stations in a future factory environment.

Terahertz (THz)-waves offer the possibility for wireless transmissions of high data rates, due to the available broader bandwidths at higher carrier frequencies. Applications of broadband wireless access over short ranges and fixed wireless links based on THz-waves are favorable. They can be incorporated as wireless outdoor links in difficult environments, for a last mile access, or as indoor links for broadband connectivity, between servers in data centers, for instance.Especially the frequency range between 300 GHz and 900 GHz is very promising for ultra-broadband applications. However, in contrast to wireless links in the lower GHz bands, the free-space path-loss is quite high, and the attenuation due to molecules in the air or water droplets can significantly decrease the transmittable data rates in the envisaged THz frequency range.In this chapter, the requirements for wireless THz links are reviewed for a frequency range up to 900 GHz by the help of an atmospheric attenuation profile, including a clear atmosphere, fog, and rain. For a 1 km long, fixed wireless THz link, the requirements for transmittable data rates, spectral efficiencies, antenna gain, stability, carrier linewidth, and phase noise for various modulation formats are addressed. For ultra-high data rates and a stable operation of an up to 1 km outdoor THz link, antenna gains of up to 70 dBi have to be used for the transmitter and receiver antennas. Due to the low wavelength of THz waves, even high-gain antennas are quite small. However, the resulting higher directivity requires an adaptive control for the compensation of mechanical fluctuations in the link.

Wireless communication with the carrier frequency at THz spectrum ranges brings many advantages such as higher volume of data traffics. The wave propagation at higher frequency ranges reveals, however, the unique characteristic, which is distinguished from the wave propagation at sub-6G frequency domains. As an example for that, wave propagation at higher frequency (e.g., sub-mm frequency) is susceptible to the atmospheric conditions such as humidity and rain. This implies to the importance of a new propagation simulator for THz frequency ranges, which covers the significant wave propagation behavior of THz frequency. This book chapter thus introduces a THz propagation simulator including both system level and link level simulator, with which the feasibility and the scalability of the wireless communication at sub-mm frequency domain can be identified. For that, this book chapter contains such topics across the board of THz simulator: the requirements of THz propagation for the simulator, the brief introduction of the software framework, the operation process of the THz propagation simulator with respect to system level simulation, as well as link level simulation and the corresponding simulation results. This chapter will give readers an overview of the THz simulator and its assessment possibility of the propagation channel with respect to the system level as well as the link level.

In this chapter, an example of experimental and analytical investigation of line-of-sight (LOS) reflections in the frequency range of 220–330 GHz at up to 2-m distance for the short-range THz communications systems is presented. To characterize the general behavior of the THz LOS reflections, power delay profiles are measured for various reflector sizes, reflectivities, and link distances with a frequency-domain measurement setup and analyzed with simple formulas. In this example, from the experimental and analytical investigation, it was revealed that, even with small reflectors such as the packaging or housing bodies of the transmitter and receiver, the interference due to the LOS reflection that used to be ignored in the THz channel analysis so far is not ignorable and the direct signal to the reflected interference ratio is nearly constant at link distances shorter than a certain threshold. As the distance increases above the threshold, the LOS reflection signal abruptly disappears and finally becomes ignorable. In addition, the dependency of the threshold distance on the antenna gain, and dimensions and reflectivity of the reflecting bodies, will be analytically derived. The observations and results in this example can be used for simulations of signal propagation in future short-range THz wireless communication systems. Further complicated measurement and analysis can be developed based on this example.

Fixed wireless system (FWS) is mainly used in mobile network systems. As an RF band of the FWS, the microwave bands (6–42 GHz) have been used so far; however, the millimeter-wave band (60–175 GHz), which can realize a large capacity by widening the bandwidth, is now being commercialized. In this chapter, elementary modem technologies in the FWS are introduced. In particular, phase noise suppression, which is a key to realizing high-order modulation scheme and millimeter-wave bands, is explained. Regarding the millimeter-wave bands, regulations for the FWS and the propagation characteristics such as the rainfall attenuation indicated by the ITU-R recommendations are introduced. Moreover, multiplexing transmission technologies for realizing further high-capacity, especially spatial multiplexing transmission technologies suitable for the millimeter-wave bands, which has been attracting attention in recent years, are explained, and as a target of the near future, specifications for realizing a 100 Gbps transmission system using the multiplexing technology are described.

Millimeter-wave transmission systems based on radio over fiber techniques are discussed for application to fronthaul and backhaul links in advanced radio access networks. The seamless convergence between millimeter-wave and optical signals is realized by analog radio over fiber techniques in both optical-to-electrical and electrical-to-optical signal conversions. The configuration of radio over fiber transceivers is introduced along with their advantages for the realization of millimeter-wave signals, followed by a discussion of example systems for millimeter-wave wireless fronthaul and backhaul connections.

This chapter describes a variety of millimeter-wave wireless standards based on wireless local area network (LAN) and personal area network (PAN) technologies. As same as microwave technologies, IEEE Standards Association (SA) has developed some standards, such as IEEE 802.11ad and IEEE 802.15.3c/e. Some international and industry standards are also introduced.

Nowadays, the threat of collisions caused by foreign object and debris is commonly recognized to impact airport safety since the Concorde accident. This chapter describes the development of the FOD detection system using radio over fiber in order to widen the coverage of millimeter wave radars. We introduced the development of a FOD detection system that uses a hybrid sensor millimeter-wave radar network and a high-sensitivity camera. In order to extend the coverage of millimeter-wave radar, the radio over fiber system is installed in the radar network. The optical two-tone RoF system provides the doubled RF frequency and stable reception of the optical signal in the field. The results of a field test using the prototype conducted at Narita International Airport showed that it was able to detect a piece of metal determined by AC of FAA and achieve the ability to take photographs clearly even at night. In the laboratory tests, we also develop a multistatic radar system to improve the performance of the probability of the detection and mapping without any scanning of the antennas. The prototype system makes the radar image by switching the transmission, and plural mono/bistatic measurements increase the precision of the positioning of the FODs.

This chapter is summarizing a set of recent progress on uni-traveling carrier photodiode (UTC-PD)-based THz emitters and their application to THz communications: signal generation, 100 Gbit/s in the 300 GHz range using a linear UTC-PD, antenna testing with modulated data signals, and Tx/Rx system using a UTC-PD as Tx, coupled with III-V based GaAs receivers. Then, recent examples of testing devices based on photonics techniques are given, leveraging the photonics-based approach. In particular, the testing of non-linear behavior of active THz devices (like amplifiers) is presented, for intermodulation measurements and datacom testing of a set of passive devices as flexible waveguides.

Beyond 5G (B5G), which is envisioned to have a user bit rate of 100 Gbps, will require a seamless backhaul to support it. The High-Altitude Platform System (hereafter referred to as HAPS), low earth orbit satellites, and NTN (Non-Terrestrial Network) communication platforms using aircraft are considered to be promising candidates as one of the use cases.NTN is expected to enable not only three-dimensional expansion of communication areas, but also various use cases with high data rates on a global scale. To realize such use cases, a high-speed, high-capacity link between the backhaul on the ground and the sky is indispensable. In particular, it is important to utilize HAPS that covers the stratosphere with stable climatic conditions.In this chapter, we discuss a communication scheme using the THz band (100 GHz band) as a new feeder link from the viewpoint of ensuring link performance even in relatively bad weather and achieving high speed and high capacity, taking into account free-space optical (FSO) communication and High-Altitude-Platform-Stations (HAPS) using existing frequencies, as well as the weather conditions required for a feeder link connecting ground stations to the stratosphere; the feasibility of 10 Gbps class THz-feeder link throughput in light rainfall will be discussed in terms of propagation characteristics, link budget, and main transmission signal processing.

This chapter strives to give a complete picture of designing THz systems in CMOS technologies. First, it motivates the usage of such technologies as a necessary step toward the high-volume production of the THz systems. Second, it states the CMOS design challenges when operating at such high frequencies, mainly lack of power gain, low supply voltage, and increased loss of passive structures. Then, it gives a survey of commonly used techniques to address those challenges, both in the transmitter and the receiver case. To illustrate the potential of the presented design approaches, four CMOS chip examples are described. In the transmitter case, a 670 GHz 4 × 2 oscillator-radiator array and a 420 GHz multiplier-based transmitter, both implemented in a 40 nm CMOS technology, are presented. In the receiver case, a 28 nm CMOS 605 GHz harmonic injection-locked receiver and a 40 nm CMOS 420 GHz superheterodyne IQ receiver are depicted. Although targeting different applications and operating with different principles, those chips use some common, standard design techniques that can be identified as the foundation for most THz CMOS designs. The chapter is concluded with a description of a 420 GHz phase-imaging system, consisting of 420 GHz transmitter and receiver CMOS chips. Finally, the phase-imaging results obtained by using the designed system are demonstrated as illustrations of the potential of the THz system.

Since historical artworks are unique, nondestructive and noncontact measurements are desired in the field of heritage science. Thus, electromagnetic techniques, such as radar remote sensing and X-ray tomography, are suitable for observing the internal structure of heritage objects. THz time-domain imaging allows the observation of the internal structure of a painting composed of preparation and support layers, which conservators desire to see even if they must take a small sample. The most important advantage of THz time-domain imaging is the ability to observe a cross-sectional image without cutting, as well as to obtain areal information at any depth from the surface. About 15 years ago, a portable, turnkey-type THz time-domain imaging system became commercially available, and the first on-site observation in a museum was carried out at the Uffizi Gallery in Florence, Italy, in 2008. Since then, THz time-domain imaging has been applied to historical objects and artworks, including Egyptian mummies, tempera panel paintings from the Renaissance, Japanese panel screens, and contemporary paintings on canvas. Case studies introduced in this chapter proved that THz time-domain imaging contributes to conservation planning by practical evaluation of the physical condition of artworks.

Use cases and requirements for sixth generation (6G) mobile communication systems have been discussed in some organizations. The peak data rate, minimum latency, and maximum connectivity are expected to be hundreds of Gbit/s, 100 μs, and tens of millions of devices/km2, respectively. Analog radio-over-fiber (A-RoF) technologies including intermediate frequency-over-fiber (IFoF) technologies have been studied for mobile fronthaul (MFH) links in the fifth generation (5G) and 6G mobile systems. This is mainly because, in the A-RoF/IFoF transmission systems, large-capacity and highly-spectral-efficiency optical transmission can be achieved by simple analog intensity modulation and direct detection (IMDD) scheme, and configuration of optical and mobile equipment in antenna sites can be simplified. In the future, these features will gain more importance as they can effectively reduce optical transmission costs and rapidly expand service areas. In this chapter, the overview of mobile communication systems is firstly explained. Radio access network (RAN) architecture, requirements for 6G mobile services, and key wireless communication technologies for 5G and 6G are included. Next, A-RoF and IFoF transmission systems for MFH application are explained as the main part of this chapter. Technical issues in such transmission systems and some of the solutions are introduced. Point-to-multipoint (PtMP) IFoF transmission systems for efficiently accommodating many antennas are also shown. Additionally, various frequency conversion technologies for IFoF systems, and analog beamforming technologies by controlling optical signals, especially for A-RoF systems are described.

In this chapter, the concept and necessity of radio over fiber-based indoor networks are reviewed, along with the classification and structure of indoor networks based on radio over fiber technology. Several important design considerations and some important demonstrations for radio over fiber-based indoor networks are also examined. As mobile communication technology evolves, the increasing bandwidth of baseband signals and the growing data throughput requirements of users will drive a rising interest in radio over fiber-based indoor networks, showcasing efficient performance from both a Capital Expenditure and Operational Expenditure perspective.

The provision of broadband Internet connections and advanced services to moving users on high-speed trains is gaining a lot of interest owing to the rapid deployment of high-speed railway worldwide and the emergence of broadband services. Nevertheless, high mobility, frequent handover, and diverse transmission environments remain as bottlenecks for a high-speed communication network for high-speed rails. In this chapter, a radio-over-fiber-based linear-cell network using backhaul links in the millimeter-wave band is present to overcome the challenges. Using the system, the radio cells along the railway track can be significantly simplified, rendering the system a cost-effective solution. In addition, by combination with optical switching technology, a moving backhaul network together with the train can be constructed. Thus, a handover-free network over a long coverage area can be realized. The system concept and operating principle of the system are described. The proof-of-concept demonstration in the laboratory was conducted. In addition, a field trial on a high-speed rail system in Japan was also demonstrated.

Free space optical (FSO) communication is a promising alternative in terms of meeting high data rates, providing a secure, and reliable communication. However, optical beam is prone to environmental conditions which affects FSO communication performance severely. This chapter deals with the basic concepts of FSO communication, including the transmission of data through free space using modulated optical beam. The components of FSO communication systems such as transmitters and receivers, different modulation schemes, the impact of atmospheric conditions on optical beam propagation, and the mitigation techniques for reducing the channel distortion effects are explored.

Atmospheric turbulence that affects propagating light waves is described. A model of the atmosphere that distorts the wavefront of light waves is outlined, and the energy cascade theory of turbulence and the structure function of wind velocity are presented along with definitions of the outer scale and the inner scale. Structure functions are introduced to treat random quantities, where the relationship between the structure function of refractive index and the structure function of temperature is denoted. The Kolmogorov spatial power spectral density of refractive index fluctuations is expressed with the structure parameter of the refractive index. Representative models of the refractive index structure parameters as a function of altitude are shown with examples of calculations. Assuming that the atmosphere is a plane perpendicular to the direction of light wave propagation, the coherence length of the atmosphere, or Fried’s parameter, is derived from the wave structure function. The isoplanatic angle is presented as an indicator to show that two wavefronts with different propagation angles can be considered approximately equivalent, and the Greenwood frequencies at which two wavefronts measured can be considered approximately equivalent are introduced. For the phenomena observed in light waves propagating in atmospheric turbulence, theoretical expressions of intensity fluctuation, focal point variation, and movement of the irradiated area of the transmitted light waves are shown.

Free-space optical (FSO) communication has emerged as a transformative technology with immense potential for redefining the landscape of future broadband access networks. This chapter explores the principles, advantages, challenges, and diverse application scenarios of FSO in the context of evolving communication requirements for broadband connectivity. With its high data rates, low latency, and license-free spectrum operation, FSO is a promising solution for last-mile connectivity, urban and suburban deployment, wireless backhaul in mobile networks, rural connectivity, and disaster recovery. The chapter also delves into the challenges inherent in FSO technology, such as atmospheric interference and alignment issues, proposing mitigation strategies to enhance its robustness. FSO’s unique advantages, including security features and ease of deployment, position it as a key enabler for providing high-speed, reliable, and flexible broadband access for future networks. As ongoing research continues to refine FSO technology, its role in enhancing network redundancy and diversity is highlighted, with the potential to revolutionize connectivity in diverse and dynamic environments. Also, as the development of FSO progresses, its integration into broadband access networks offers a glimpse into the future of wireless optical communication, ushering in an era of connectivity that is both innovative and resilient.

A paradigm-changing revolution on the area of high-speed wireless communication systems is currently being forced by the unstoppable increase on bandwidth demand. Due to spectrum exhaustion, traditional radio frequency (RF) communications can hardly cope with the foreseen requirements for next-generation cellular and satellite communications. This is motivating the pursuit of alternative wireless communication technologies, exploiting new spectral bands and transmission paradigms. In this sense, optical wireless communications, also widely known as free-space optics (FSO), are nowadays perceived as the most promising technology to ensure a future-proof solution for the upcoming generations of wireless networks. Among their multiple advantages, FSO systems are particularly appealing for their virtually unlimited bandwidth, potentiating ultrahigh-capacity communications. Following this premise, this chapter starts by reviewing the most relevant application scenarios and enabling technologies that are fueling the development of novel high-capacity FSO systems. Special attention will be dedicated to the role of digital coherent optics in enabling the recent demonstration of record-breaking multi-terabit optical wireless communications. Subsequently, the issue of time-varying power fadings generated by atmospheric turbulence is discussed, including methodologies for its statistical modeling and experimental procedures for controlled turbulence emulation. Motivated by this challenge, the following sections cover the development of tailored channel modeling and estimation techniques, which can be employed for the optimization of advanced modulation schemes, enabling time-adaptive data-rate provisioning in FSO systems. Finally, this chapter is concluded with the discussion of an ultrahigh-capacity FSO field trial, exploiting the use of digital coherent optics and wavelength-division multiplexing (WDM) technology to achieve up to 5 Tbps data-rate transmission over 1.8 km.

Since the early days of space communications, radio waves have served as the carriers of information. It is only very recently that laser links light up the sky, tapping from the vast amount of bandwidth in the terahertz regime of the electromagnetic (EM) spectrum. In this chapter, we dive into the trade-offs between radio frequency (RF) and free space optics (FSO) connections. Thanks to the much higher carrier frequency, FSO brings clear advantages for high data rate, large distance, and/or ultra-secure communications. However, there are some challenges associated with it challenges including demanding pointing requirements, turbulence- and pointing-induced fading, and the need for line-of-sight operations. We shortly highlight various use cases and discuss the design trade-offs to arrive at high data rate, robust, low-latency, and high-availability connections. The chapter concludes with an outlook to the next generation of FSO terminals and addresses topics including software-defined, adaptive FSO as well as multiuser multi-beam communications and concludes on the convergence of RF and FSO technologies, holding the promise to unlock the best of both worlds.

This chapter explores the innovative developments in free-space optical (FSO) communication and its integration within optical networks. The chapter begins with an overview of FSO technology, detailing its principles and applications. The seamless integration of FSO and fiber networks is also discussed, emphasizing fault-protection structure to ensure robust and reliable communication, improve the network’s flexibility, address the last-mile connectivity challenge, provide high-speed data services to last-mile users, and cost-effective solutions for urban and remote areas. This chapter also explores the combination of optical sensor systems and optical communication systems using a shared integration of fiber and FSO channel to reduce cost, improve flexibility, and enhance the transmission link distance of optical networks. Additionally, presents techniques to enhance the performance and coverage range of FSO links such as bidirectional fiber/FSO, point-to-multipoint connection, multipoint-to-multipoint connection, unmanned aerial vehicles (UAV)-based FSO systems, and satellite-based FSO systems. An investigation of the challenges faced by FSO systems in optical networks is conducted, along with innovative solutions to overcome these challenges. Furthermore, the application of machine learning (ML) techniques for improving the performance of optical networks is discussed. It discusses how ML contributes to mitigating challenges, and improving overall reliability.

This chapter delves into the potential of relay-assisted transmission to alleviate fading in free-space optical (FSO) systems. We focus on evaluating the efficacy of a dual-hop relay system for RF (millimeter-wave and terahertz)/FSO (RF/FSO) communications in the presence of pointing errors. We construct an integrated channel model that accounts for various factors, encompassing pointing errors, channel fading, and deterministic path loss. More specifically, the RF link undergoes α − μ fading with pointing errors, while the FSO link is subject to ΓΓ fading and pointing errors. Employing amplify-and-forward relaying, we derive mathematical expressions for the probability density function and cumulative distribution function of the end-to-end signal-to-noise ratio, considering the collective effect of pointing errors and channel fading. Utilizing the derived statistical expressions, we assess critical performance metrics, including the outage probability, average bit error rate, and ergodic capacity for the relaying schemes under consideration. To validate our analytical conclusions, we perform Monte Carlo simulations. The results highlight a robust correlation between the system’s performance, specific pointing errors, and fading parameters associated with the RF/FSO connections.

The rising demand for underwater communication systems stems from the expanding range of human activities in underwater realms, including scientific data collection, environmental monitoring, military and national security uses, and oil field exploration. While wire-line systems, especially optical fibers, offer real-time communication, their inflexibility, high costs, and operational drawbacks limit their practical use, fueling the need for underwater wireless communication. Acoustic signaling is a common choice, enabling transmission over kilometers but with quite limited data rates, thus hindering high-bandwidth applications. For shorter ranges, optical communication emerges as a high-capacity alternative, utilizing the transparency of water to transmit blue or green light. Modeling the underwater optical channel is particularly important for designing effective Underwater Visible Light Communication (UVLC) systems. The traditional Beer-Lambert law faces limitations, leading to exploration of alternative models for accurate predictions. Evaluating UVLC performance reveals that the propagation distance is limited to tens of meters in non-turbid water under ideal conditions. However, this distance can decrease due to other factors, including turbulence-induced fading, which affects link fidelity. Therefore, solutions are needed to improve link fidelity and overcome the limitations.

Visible light communication (VLC) has emerged as a transformative technology that leverages light as a carrier for wireless data transmission. This chapter delves into the fundamentals of VLC, beginning with an insightful exploration of its background and subsequently addressing the advantages and challenges inherent in VLC systems. The discussion extends to diverse application areas where VLC proves to be a promising solution. Notably, the chapter highlights the motivation for integrating VLC into the 6G and beyond communication landscape. The heart of VLC lies in its principles, comprehensively outlined in the second section. Here, the basics of light propagation and the key components of VLC systems are explained, providing a solid foundation for understanding the intricacies of this communication paradigm. Channel modelling in VLC is explored in detail, with a special focus on indoor line-of-sight (LOS) VLC channel model. The fourth section discusses the diverse modulation techniques employed in VLC, ranging from pulse-based methods to optical orthogonal frequency division multiplexing (OFDM) and VLC-specific modulation techniques. This chapter encapsulates the essence of VLC, offering a comprehensive overview of its principles, channel modelling, and modulation techniques.

Information communication technology is becoming increasingly wireless, and significant changes in society by making the power supply wireless, i.e., the wiring that remains for devices. Optical wireless power transmission has advantages over wireless power transmission using electromagnetic waves in that it is smaller, can provide power over longer distances, and is free of electromagnetic noise. Therefore, it is expected to be used in internal implantable devices, many small IoT terminals, information devices, home appliances, mobile objects such as drones, robots and EVs, underwater applications, and space applications. On the other hand, although it is possible to construct them using the established technologies of laser light sources and solar cells, this is a field that has only recently begun to attract attention, and the research and development record is still insufficient. It is believed that this method will rapidly become more active. In this chapter, the advantages and challenges of optical wireless power transmission, technologies, and the latest trends are explained.

Driven by the growing market of wireless backhaul and fronthaul transport, serving wireless systems with passive optical networks (PONs) represents a significant set of work in the international standardization bodies. This chapter reviews PON features and the requirements of using PON for wireless backhaul and fronthaul transport. Solutions specified in ITU-T and IEEE standards are discussed in detail to resolve challenges such as PON link asymmetry, registration delay, and upstream scheduling latency. Application examples in 5G wireless backhaul and fronthaul transport are investigated based upon the mainstream PON technologies. New and on-going standardization efforts in the area are discussed as the last part of this chapter to highlight the directions and future work.

Spectrum aspects and standardization efforts for wireless communications in the frequency range 252–450 GHz have been observed since 2008. These activities have yielded already one wireless standard and a revision at around 300 GHz published by IEEE 802 (Institute of Electrical and Electronics Engineers) in 2017 and 2023 as well as the identification of additional 137 GHz of spectrum for terahertz (THz) communications by the World Radio Communication Conference (WRC) in 2019. Moreover, THz communications are seen as one candidate for the standardization of the upcoming sixth generation of mobile wireless system (6G) at 3GPP (Third Generation Partnership Project). This chapter provides an overview of the current status and ongoing activities in spectrum aspects and standardization for THz communications. Furthermore, the chapter includes a detailed description of the regulatory situation in the 252–450 GHz frequency range. Finally, a summary of the already existing IEEE standard and ongoing further efforts toward new standards is provided.

Standardization activities related to radio-over-fiber (RoF) technologies and systems have been performed in some standardization organizations, such as the International Electrotechnical Commission (IEC), the Asia-Pacific Telecommunity (APT), and the International Telecommunication Union (ITU). In this chapter, at first, major terminologies, which are used in the standardization documents related to RoF technologies and systems, are described. Next, general requirements for RoF systems and standardized RoF systems are addressed. Then, standardized device measurement methods of E/O and O/E conversion devices are explained.

Correct visualization of electromagnetic waves requires a sensor capable of detecting the actual EM field non-intrusively. This chapter introduces electric and magnetic field sensing technologies that utilize nonintrusive E-field and B-field probes based on the Pockels electro-optic (EO) effect and the magneto-optic (MO) Faraday effect. In contrast to conventional field sensors, such as antennas and diode sensors, which contain metallic components, these field probes are made of entirely dielectric materials. Hence, they negligibly perturb the electric and magnetic fields that they measure. The detected electric and magnetic fields are “true” and nearly distortion-free, and the waveform of the field is exact even in a complex electromagnetic environment, where conventional field sensors fail to produce reliable and reproducible results. The EO and MO field sensors have an extreme frequency bandwidth. This extensive bandwidth enables the EO and MO probes to detect all Fourier frequency components for a complex waveform. Hence, in principle, the sensors can measure the exact waveform for the EO probe, from ELF up to terahertz frequency. They are compact and also have an exceptional dynamic range. In addition, the sensors cause negligible reflection and perturbation of the electromagnetic field, enabling them to detect the near-field and far-field without disturbing the signal source – the capability a conventional probe cannot provide. Also, they are vector sensors capable of detecting the direction of the E- and B-fields. So, the field probes are suitable for various applications, which are otherwise impossible or very difficult to perform with conventional field probes.

Electro-optic (EO) sensing is a mature technique where visualization of electromagnetic waves must be minimally invasive. The fully dielectric nature of EO materials associated with photonics provides numerous advantages over conventional electrical methods. EO probes can measure fast electric fields up to the terahertz frequency associated with ultrafast optical sources. The probes measure extreme levels of an intense electric field up to the MV/m scale without damage or saturation. They can also be implemented with a ceramic ferrule pigtailed with polyimide optical fibers so as to take measurements even in boiling oil. Such robustness of these probes against high power and temperature levels can facilitate field measurements in harsh environments with, for instance, plasma or electrostatic discharges. The probes can be miniaturized on the optical-fiber scale, thus allowing extremely fine resolutions down to the mode-field diameter of a fiber. Among these unique features of EO probes, low invasiveness is the most distinguished merit for microwave sensing. In this chapter, we present a minimally invasive EO probing system that is specialized for millimeter-wave 5G antenna measurements. Two types of EO probes are presented, as well as a commercial-grade control unit. Practical techniques to enhance the stability of the field measurement system and the visualization of the measurement are also discussed. Finally, visualized electric field images of horn, open-ended waveguide, and phased array antennas operating on the millimeter-wave frequency for 5G communication are presented.

This chapter describes the principles of a self-heterodyne and nonpolarimetric frequency down-conversion techniques, and introduces several visualization systems based on these techniques. The visualization technique described here is based on photonics technology, and it demonstrates visualization in the range of 1–600 GHz with the identical system configuration and electro-optic (EO) probes. In addition, using frequency/phase noise cancellation technique together with the nonpolarimetric frequency down-conversion technique, it is possible to visualize the field distribution of the electric field emitted from a self-oscillating device and frequency-modulated continuous wave (FMCW) signal. This chapter also shows various visualization examples in the microwave, millimeter-wave, and terahertz (THz)-wave regions. The visualization example in the microwave region (1–10 GHz) shows the spatial distribution of the electric field between the signal line and the ground plane of the microstrip line. As the examples in the millimeter-wave region, the visualizations of the electric field scattered by a rough metal surface, 77 GHz electric field transmitted through a car bumper, and FMCW signals are presented. Finally, the demonstrations in the THz band includes the visualizations of the radiation field from a broadband optical-to-electrical (O/E) converter (120–600 GHz), wavefront transformation by metal hole array (MHA), modification of the beam shape by a self-healing beam during propagation, and propagation of a pulse train with a center frequency of 120 GHz and a repetition rate of 10 GHz.

Introducing a device that can measure high-frequency magnetic fields with high spatial resolution. This measuring device utilizes a magnetic garnet material and a laser. Because it uses light for measurement, it is less invasive and can measure high frequencies up to 5 GHz. In addition, since it can measure not only amplitude but also phase information, it is also possible to measure the state of wave propagation.

This chapter describes the frequency domain and time-domain antenna measurement techniques and their application with visualization. First, we show that our developed far-field antenna gain and far-field antenna factor estimation equation using the amplitude center distance applied Friis transmission formula for two antennas. Then, to show the advantage of using the proposed formula, we have demonstrated the far-field antenna factor estimation for three types of antennas, such as a biconical antenna, a log-periodic dipole array antenna, and a hybrid antenna (the log-periodic dipole array antenna with a bowtie antenna). In addition, we show that our developed unwanted wave suppression method from the antenna measurement results using a time-domain and a pulse compression technique on the open area test site. Next, we show that our developed an optical fiber link port extender of a vector network analyzer for antenna measurement. Next, we show our developed antenna near-field measurement system, which consists of an optical fiber link system and an arm vertical articulated robot. Finally, we show our developed optical fiber link millimeter-wave generation system and a measurement result. In addition, we have demonstrated a millimeter-wave antenna gain measurement method using an automotive FMCW radar as a transmission antenna.

In this chapter, we discuss the concepts of monolithic photonic integrated circuits and heterogeneous integration. Furthermore, we discuss the monolithic and heterogeneous integration and implementation technologies for optical and radio wave convergence device fabrication using high-precision, ultracompact, and broadband wavelength-tunable lasers as examples of heterogeneous photonic integrated circuits. Section “Introduction” reviews the importance of photonic integrated circuits in communications networks in light of the social background. Section “Photonic Integrated Circuit and Photonic Integration Technology” reviews photonic integrated circuits and photonic integration technology with a focus on silicon photonics-based photonic integrated circuits. Section “Monolithic Photonic Integration Circuits” outlines the monolithic integration technology and mainly introduces the monolithic integration technology by the intermixing of the quantum structure and its application for the optical device. In section “Hybrid/Heterogeneous Photonic Integrated Circuits,” hybrid/heterogeneous photonic integrated circuits are discussed with a focus on photonic integrated circuits with a structure in which each chip of different materials are joined at their end facets, and the importance of heterogeneous material integration technology is mentioned. Section “Future Prospects for Photonic Integrations” concludes this chapter with a review of the future prospects of photonic integration.

Silicon is widely regarded as the most significant material in semiconductor technology. It has been used as the primary material for CMOS transistors because of the excellent oxide-semiconductor interface that could be obtained by oxidizing the substrates. Since the commercialization of silicon-on-insulator (SOI) wafers, silicon has also been considered one of the best materials for photonics. The material itself is transparent to telecom wavelengths, and very compact optical waveguides could be realized from the large refractive index contrast between silicon and oxides. Although silicon has the potential to be used as a material for photonics purpose, it cannot emit light. To add light emitter on silicon, integration of III-V materials, which have direct bandgap structures, on silicon has been widely studied. Integration of III-V materials could solve one of the fundamental problems, i.e., light-emitting problem of silicon. This chapter provides an overview of research and development activities related to III-V/silicon integration. In the first half, the chapter explains current technical details regarding the integration methodologies of III-V/Si integration, and then the chapter reviews recent advancements of various heterogeneous integration technologies by each research institute.

The concept of quantum dot (QD) lasers and their advantages were predicted in the 1980s, and considerable efforts have been devoted to developing high-performance QD lasers. The current QD lasers exhibit low-threshold currents, high-temperature operations, and wide spectra. In addition, QD lasers are expected to act as optical sources in integrated photonic circuits, such as Si photonics. This chapter introduces and discusses the research and development history of QD lasers.

This chapter contains the basic rules for designing, fabricating, and using semiconductor optical amplifiers. The objective is to explain the influence of SOA design on its main static and dynamic characteristics. In particular, the key role of optical confinement is described, since it determines the level of interaction between photons and carriers. These interactions are then detailed, and the resulting carrier dynamics is studied in details, as it governs most of the performances of SOA, and determines its applicability for practical applications. These applications are then presented, either for linear amplification or for nonlinear operations such as data generation, processing, or routing. Linear applications of SOA require low noise figure, large saturation output power, and large carrier lifetime to limit the addition of linear and nonlinear noise to the amplified signal. On the contrary, SOA should present low saturation output power and carrier lifetime for nonlinear applications. The role of amplified spontaneous emission is evidenced and explained to achieve this.

This chapter focuses on high-speed photodetectors (PDs) and their integration into advanced driver/control circuits, particularly analog radio-over-fiber communications configured for point-to-point or point-to-multipoint access in 5G/beyond-5G networks. Compared with digital-based radio-over-fiber, analog radio-over-fiber technology provides high-data-rate and low-latency wireless communications. High-speed PDs are essential components of radio-over-fiber technologies.In section “Introduction,” several types of PDs are described. Sections “Photodetector’s Basic” and “Surface/Side Illuminated Photodetector” describe the fundamental PIN-PD structure and theories based on surface and edge illumination. In addition to the fundamental PD, an avalanche photodetector (APD) is introduced to increase responsivity in section “Avalanche Photodetector.” In section “Uni-travelling Carrier Photodetector,” the fundamental theories for high-speed designing using a uni-traveling carrier photodetector (UTC-PD), including the major factors of carrier traveling, carrier drifting, and the CR time constant, are explained. To improve the output photocurrent and output radio frequency (RF) power, section “High Photocurrent and High RF Output Power Photodetector” reviews the electric field screening and thermal impedance effects at the junction. State-of-the-art high-speed and high-power PDs in the 100 GHz range from previous reports are reviewed in section “State of the Art of High-Power Photodetector.” In section “Fabrication of UTC-photodetector,” UTC-PD fabrication and characteristics are discussed with a nonbias UTC-PD as an example. Advanced driver/control circuits, optical-to-electrical (O/E) package designs, implementations for hybrid integrations, and their applications to fiber wireless communication are discussed in section “Advanced Driver/Control Circuits” for a 100 GHz optical-to-radio converter module. A demonstration using high-data-rate fiber wireless communication using an optical-to-radio converter module is also described.

A high-speed electro-optic (EO) modulator is a key device for the conversion from radio signals to optical signals in modern optical communication networks and microwave photonics systems. Commercially available high-speed EO modulators using traveling-wave electrodes and optical waveguides composed of LiNbO3 crystal or semiconductor (GaAs, InP, Si, etc.) can be operated in the frequency ranges from DC to ~40 GHz and are widely used for long-haul optical fiber communication systems, data centers, and optical signal processing systems. However, higher frequency operations over 40 GHz are still challenging owing to significant loss effects at feeding cables and connectors, and to coupling difficulty of modulation signals to electrodes. One promising solution to overcome these problems is to feed high-frequency (millimeter-/THz-wave) signals to optical modulators via free space with a planar antenna integrated on the modulator substrate surface. Therefore, antenna-integration on the substrate of EO modulators are rather attractive for advanced microwave photonic applications. In particular, signal conversions from wireless millimeter-/THz-wave signals to optical signals are to be indispensable for Beyond-5G/6G mobile wireless communication systems with the radio-over-fiber (RoF) technology. In this chapter, the basis of high-speed EO modulation and the recent study for antenna-integrated EO modulators are discussed.

Since the global data traffic increases every year, capacity enhancement is required for anticipating the large data traffic and bottleneck in the future. It can be solved by using high microwave frequency operation. However, they have large transmission losses in the air. It can be solved by connecting pico/femto cell through optical fiber cables. Thus, broadband wireless communication can be realized by utilizing the radio and optical networks through radio-over-fiber (ROF) technology. In the ROF technology, converters between wireless microwave and lightwave signals are required as the key devices. On of them is the converters from wireless microwave to lightwave signals. It can be composed of an antenna and optical modulator. The challenges in high-frequency microwave operation are microwave losses, substrate resonant mode, precise tuning requirement, and so on. Electro-optic modulators (EOMs) are promising for the conversion with their advantages such as high-speed operation, large bandwidth, and good linearity. Optical modulation through the Pockels effect can be obtained by applying microwave electric fields. This chapter presents and discusses in detail relating gap-embedded patch-antenna EOMs for wireless microwave-lightwave signal conversion. The devices fabricated on an EO crystal consist of patch antennas with narrow gaps at the center and optical waveguides located close to the narrow gaps. Several device structures are presented in this chapter including a gap-embedded patch-antenna EOM, an array of gap-embedded patch-antenna EOM, and meandered gap-embedded patch-antenna EOM. Additionally, performance of the devices for communication and sensing applications are also presented.

In optoelectronic devices based on their phase modulation, a large electric field–induced change in refractive index with a small absorption loss is required. So far, a large linear electro-optic effect (Pockels effect) in ferroelectric materials such as lithium niobate, bulk semiconductors, and nonlinear optical polymers, and the carrier plasma effect in silicon are mainly used for high-speed Mach–Zehnder modulators (MZM) and other photonic devices. The electro-optic effect in III–V semiconductor multiple quantum wells is also a promising candidate for a large electric field–induced index change because the change in index caused by the quantum-confined Stark effect (QCSE) can be utilized in addition to the Pockels effect.In this chapter, a five-layer asymmetric coupled quantum well (FACQW) for large electric field–induced index change as so-called a potential-tailored quantum well is discussed, and their applications to photonic devices, such as an MZM, microring resonator (MRR)-based devices, and antenna-integrated modulators for radio-over-fiber systems, are presented. The FACQW is expected to exhibit a giant electric field–induced index change in the transparent wavelength region, which is over one order of magnitude larger than that of a conventional rectangular quantum well.

Implementing advanced optical modulation formats with coherent detection, enabling both amplitude and phase modulations of lightwave, is one of the methods to enhance the spectral efficiency in optical transmission. Modulation and demodulation of complex vector modulation of lightwave carrier in the same manner as that of the radio frequency carrier necessitates precise control over much higher lightwave frequencies. This chapter delves into methodologies for realizing optical vector modulation through dual modulation of a directly modulated laser and an electro-absorption modulator, serving as phase and amplitude modulators, respectively. Such modulation can be performed using either discrete or integrated devices. To achieve complex modulation by this method, it involves precise component characterization, the modulating signal generation, and signal processing at both transmitter and receiver. The compact nature of integrated laser and electro-absorption modulator components offers advantages for transmitting both baseband and analog radio frequency signals over short distances.