Free-Space Optical Communication

Free space optical (FSO) communication is the wireless transmission of data via a modulated optical beam directed through free space, without fiber optics or other optical systems guiding the light.

From: Encyclopedia of Modern Optics, 2005

Fundamentals of Free-Space Optical Communications Systems, Optical Channels, Characterization, and Network/Access Technology

Arun K. Majumdar, in Optical Wireless Communications for Broadband Global Internet Connectivity, 2019

4.2 Free-Space Optical Communication Systems: Various Types for Different Optical Network Architecture

FSO communication systems are where free space acts as a communication channel between transceivers that are line-of-sight (LOS) for successful transmission of optical signals. The channel can be atmosphere, space, or vacuum, whose characteristics determine the transmission and reception of optical signals for designing reliable and efficient communication systems. Using FSO technology data is transmitted by propagation of light through atmospheric or space communication channels, allowing optical connectivity. FSO communication offers a high data rate to meet the tremendous increasing demand of broadband traffic mostly driven by Internet access and HDTV broadcasting services. Compared to fiber optics technology, FSO offers much more flexibility in designing optical network architectures at very high speeds, at tens and hundreds of Gbit/s rates. However, FSO communication is affected by atmospheric effects, which limits sensitivity and achievable data rates with acceptable BER. Some of these degradations are turbulence, absorption, and scattering, and various mitigation techniques exist for reliable and efficient data transmission [1] and to increase the communication performance. Both point-to-point, point-to-multipoint, multipoint-to-point, and multipoint-to-multipoint FSO communications are possible, depending on the different scenarios of establishing optical links. FSO communication is the most practical alternative to solve the bottleneck broadband connectivity problem. The data rates provided by FSO links continue to increase in both long- and short-range applications. FSO will be one of the most unique and powerful tools to address connectivity bottlenecks that have been created in high-speed networks during the past decade due to the tremendous success and continued acceptance of the Internet. The next generation of Internet connectivity will push the limits of existing infrastructure with high-bandwidth applications such as videoconferencing, streaming multimedia content, and network-enabled portable devices. Clearing these bottlenecks is crucial for the future growth and success of the contemporary Internet society. The bandwidth of optical communications access and edge networks will be needed to satisfy these demands. Communication systems are concerned with the transmission of information from a source to a user. The purpose of a communication system is therefore to transfer information. A very basic block diagram of any communication system (optical or radiofrequency (RF)) is shown in Fig. 4.1.

Figure 4.1. Basic communication system.

Fig. 4.1 shows a single point-to-point system, whereas in a multiplexed system there may be multiple input and output message sources and users (also called destinations). Fig. 4.2 shows other possible configurations and links for multipoint connections.

Figure 4.2. Point-to-point and multipoint connections and links: (A) point-to-point architecture, (B) mesh architecture, and (C) point-to-multipoint architecture.

OWC is the next frontier for high-speed broadband connection and offers the following unique features and advantages: high bandwidth/capacity, ease of deployment, compact size, low power, and improved channel security. OWC can transmit and exchange voice and video communication data through the atmosphere/free-space at the rates of tens of Gbit/s and much more.

FSO communications can provide high data rates in Gbits/s ranges through the atmosphere for ranges from a few hundreds of meters to a few kilometers. FSO links include the following: (1) chip-to-chip communication, (2) indoor infrared (IR) or VLC, (3) interbuilding communication, and (4) free-space laser communications including airborne, space borne, and deep space missions.

Fig. 4.3 shows various FSO communication link applications and scenarios for different categories of transmission ranges. For very short distances such as chip-to-chip or board-to-board interconnections FSO links are also used to address the complex communication requirement in high-speed, compact optoelectronic devices and lower power consumption to make them compatible with other interconnected network devices. Other communication links are discussed next.

Figure 4.3. Wireless optical communications: (A) chip-to-chip, (B) indoor infrared or visible light communication, (C) interbuilding communication, and (D) airborne/space platform free-space laser communications.

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Basics of Worldwide Broadband Wireless Access Independent of Terrestrial Limitations

Arun K. Majumdar, in Optical Wireless Communications for Broadband Global Internet Connectivity, 2019

2.2 Optical Wireless Communication Technologies

FSO communication [2] is considered to be one of the key technologies for realizing very-high speed multi-Gb/s large-capacity aerospace communications. FSO lasercom uses lasers as signal carriers and can provide a line-of-sight, wireless, high-bandwidth, communication link between remote sites. The wireless aspect of FSO lasercom has an advantage particularly in metropolitan area networks (MANs) where in cities, laying broadband fiber optic cables is expensive. FSO lasercom offers substantial advantages over conventional radio frequency (RF) wireless communications technology such as much higher bandwidth (data rates), low probability of intercept for higher security, low power requirements, and much smaller packaging to make them portable. As will be discussed later in this book, even high-speed optical/photonic devices, which can include transmitter, modulator, necessary optics to transmit and receive, as well as fast signal processing chips, Microelectromechanical systems (MEMS), and nanotechnology, can all be integrated in a very small device that eventually will be incorporated with existing smart phones, laptops, or tablets. This approach gives tremendous freedom to operate all the modern gadgets needed for Internet connectivity among various users on the fly to transmit and receive digital information anytime, anywhere without signal drop.

FSO is a practical solution for creating a three-dimensional global broadband communication grid. The main drivers behind the emergence of advanced FSO [3] are recent high demands in bandwidth, thanks to the recent growth of Internet usage for Internet protocol television (IPTV), voice over Internet protocol (VoIP), and YouTube and Twitter, and the immediate future and coming applications in Internet of Things (IoT). With the rapid growth of data-centric devices and the general deployment of broadband access networks, high data rates from 10 Gbps to more spectrally efficient 40 Gbps or 100 Gbps/channel and beyond is a recent trend of development. To provide the most flexibility, communication channels must be wireless, so there numerous challenges exist today to accomplish higher transmission rates due to degradation in the signal quality as a result of atmospheric channel impairments. Atmospheric effects such as turbulence and scattering that are important for optical communication channels from the ground to an altitude of a few km are the most dominant effects relevant to optical communications.

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OPTICAL COMMUNICATION SYSTEMS | Free Space Optical Communications

R. Martini, in Encyclopedia of Modern Optics, 2005

Introduction

Free space optical (FSO) communication is the wireless transmission of data via a modulated optical beam directed through free space, without fiber optics or other optical systems guiding the light. The fundamental idea goes back to ancient times, as light (or smoke) signals were used to transmit information. From a more modern point of view, Graham Bell's patent on the photophone may mark the onset of modern FSO techniques, as it transmitted audio signals (i.e., voice) via the modulation of sunlight. A renaissance of FSO systems started with the availability of lasers, light sources with high output power and high coherence, which allowed the accurate direction of the light beams over long distances. During the 1970s and 1980s the main proposed application of FSO systems was for secure and long distance (50–1000 km) communication, mainly targeted for ground–satellite or satellite–satellite communication. This focus changed drastically over the last decade as a new market for FSO grew in the establishment of high bandwidth data link and their integration over a locally restricted area.

The main competitors in this market are the fiber-based optical network, the RF communication system, as well as the low bandwidth copper cable-based system. In comparison to the closely related wireless radio frequency transmission, the higher frequency of the optical carrier (∼1014–1015 Hz) thereby allows for much higher transmission rates, comparable to those of typical fiber optic networks. On the other hand, the use of an optical carrier also results in much more directed beam propagation, which requires an undisturbed line-of-sight between emitter and detector. This restricts the application of most FSO systems to a range between a few hundred meters up to several kilometers, which are still favorable for distribution of high bandwidth networks over a locally restricted area. This makes FSO a highly attractive candidate for the ‘last-mile’ distribution of high-bandwidth Ethernet to the individual homes.

The simplicity of setup of FSO links, as well as their modularity, is thereby their biggest advantage, compared to fiber-based networks. It makes them not only highly cost efficient, but it eases the maintenance or allows for fast and easy upgrades, as it does not require any extensive and time-consuming installation – in contrast to the installation of fiber optic cables. This advantage makes FSO highly attractive for temporary installations (emergency or short-time high broadcast situations, such as the Olympic Games), as well as to overcome geometrical restraints (river, seas, etc.). As most detector and emitter systems are typically based on the same electro-optical components used for fiber optic networks, they seamlessly integrate and expand an existing network without complications in data handling.

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Last-Mile Problem Revisited With Potential Solution for Broadband Connectivity

Arun K. Majumdar, in Optical Wireless Communications for Broadband Global Internet Connectivity, 2019

3.2.1 Atmospheric Effects on Free-Space Optical Communication Relevant to Last-Mile Problem: Analysis of All-Weather Broadband Free-Space Optical 99.999% Availability

Fog interference can be significant at FSO wavelengths because of the scattering process. FSO communications can still be achieved for low to medium fog conditions for a few km (∼1–4 km) distance without appreciable signal loss as long as some light (photons) reach the detector receiver. For heavy (thick) fog the signal can be reduced to a point where connectivity will be lost. The link availability is the percentage of time over a year that an FSO link will be operational and five-nines is defined as:

5Nines=99.999%=Down5min/year

The atmospheric effects on FSO communications is discussed in detail in another chapter of this book. FSO parameters to analyze communication performance relevant to all-weather effects are:

Attenuation: Caused by two main factors: (1) absorption and (2) scattering. Atmospheric conditions responsible for attenuation relevant to FSO communications are fog, snow, rain, and turbulence.

Link Margin: The performance of a lasercom system is generally quantified by the power received and link margin (LM). The LM calculation is therefore essential to design an acceptable system and is defined as

LM=AvailablereceivedpowerReceiverpowerrequiredtoachieveaspecifiedBitErrorRate(BER)atagivenrate

Note that the link margin value shows how much margin an FSO system has at a given range to compensate for scattering, absorption (due to fog, snow, and rain), and scintillation (turbulence). Table 3.1 shows the all-weather broadband availability showing the visibility, attenuation in dB/km at 785 nm, effective link range for an FSO system under various weather conditions [1,2].

Table 3.1. All-Weather Broadband Availability [1]

A typical value of attenuation caused by turbulence (not shown in the table) for a range of 850 m is 3.2 dB. Fog has the worst effect on FSO communication because of scattering effects and can be even 350 dB/km. Rain and snow have less adverse effects for optical links and can be in the ranges of 45 and 150 dB/km in heavy rain or snow [1]. For a transmitter power of 30 mW and using a detector of sensitivity 25 nW, the researchers showed that the link margin is 54 dB, and the range of 140 m is still possible even in the presence of worst-case 350 dB/km fog (corresponding to 99.999% link availability) [1]. Other results of availability and reliability of FSO links from the estimation from measured fog parameters [3] show from a visibility experiment on October 31, 2011 for 312 m range on a foggy day that the link was down only 90 times per day (0.104% of the day). Their calculated availability of the FSO link was 99.896% for a 312 m path [3]. For a 100 m path, it was estimated that the availability should reach 99.999%. The results from the visibility curves on November 15, 2011 showed that all values of visibility were higher than the thresholds for the FSO system and therefore the availability of the FSO system was 100%.

3.2.1.1 Impulse Response Function in the Case of Multiple Scattering

In calculating the FSO communications system performance operating in the presence of multiple scattering media such as dense rain, light fog, fog and cloud using transmitting laser pulses, it is necessary to evaluate the impulse response function of the scattering medium. The theoretical model and analysis is discussed in [4], showing impulse response functions for various atmospheric conditions of fog and rains, and therefore is not repeated here. The analysis is important to determine the possibility of using FSO communication as a potential solution for the last-mile problem. In [4] bit-error-rate (BER) versus required transmitter power for multiple scattering media (fog and cloud) for a short input laser pulse is calculated, which is helpful in designing an FSO communication system for the last-mile.

Last-Mile Problem Fiber backbone already has the availability of multigigabit connectivity. Basically, the last-mile problem is the problem of utilizing the multigigabit (or even more) connectivity point to the buildings, campus, and industries within about a mile without any existing fiber links for establishing high speed Internet connection. This is still a challenging issue to address the network connectivity problems between the buildings using advanced transceivers and fast, compact optical devices that can handle multigigabits or higher data rates.

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Principles and Implementation of Secure Free-Space Optical Wireless Communications

Arun K. Majumdar, in Optical Wireless Communications for Broadband Global Internet Connectivity, 2019

7.2.2.1 Basics and Mathematical Representations of Generating Chaos

Chaos-based FSO communication is discussed in detail in Ref. [6]. Chaotic behavior can be expected from any dynamic system that shows sensitivity toward initial conditions; that is, a very high relationship with the previous values exists so that the value of the system at any point of time depends on the previous values. Small differences in initial conditions yield widely diverging outcomes for chaotic systems. Chaotic systems possess the ideal characters to be employed in crypto systems and by using chaotic methods we can prevent all kinds of intrusions. The message can only be retrieved at the receiver's end by generating the same chaos. Chaos can be generated mathematically. A recursive algorithm scan is used to calculate the values. Any Xith value depends immediately on Xi-1st value so that the value can be recursively calculated. Mathematical equations can be used to calculate in a simpler way. Consider the following function:

(7.7)f(x)=px(1x)

This second order function can be used to generate mathematical chaos. Eq. (7.7) is bounded for the limits 0 < p < 4. The equation can be written as:

(7.8)xn1=pxn(1xn)

The starting value is x0 and in this iterative form every nth value depends on all other previous values. The plot of such functions is also called chaotic maps. For 0 < p < 3, the function converges to a particular value after some number of iterations. As p is increased to just greater than 3, the curve splits into two branches. This splitting is termed bifurcation. Mathematically this tends to chaos. As the parameter p is further increased, the curves bifurcate again. With further increased value of p the bifurcation becomes faster and beyond a certain value of p known as the “point of accumulation,” where periodicity gives way to complete chaos. This happens for p > 3.57 whereas for p = 4, chaos values are generated in the complete range of 0–1. This is the point we are interested in. During 3.6 < p < 4, complete randomness and chaotic behavior is observed.

Chaotic signals generated in nonlinear electrical circuits [7–9] and lasers [10] can potentially be used as carriers for information transmission in a communication system. The advantage of a broadband information carrier is that it can enhance the robustness of communication channels to interferences with narrow-band disturbances. The broadband coding signal in a chaos-based communication is generated at the hardware level where chaotic carriers offer a certain degree of privacy in the data transmission. Thus, a new type of high data rate communication system can be designed using waveforms generated by a deterministic chaotic system to carry information in a robust manner. Chaotic communication systems are based on chaos synchronization where synchronized chaotic emitters and receiver lasers are employed to encode and decode information at the hardware level. The generated chaotic signal at the emitter hides the message, which can be recovered when using the appropriate receiver. Messages (information) are embedded within a chaotic carrier in the emitter, and recovered after transmission by a receiver that is synchronized with the emitter. A nonlinear filtering process is performed at the receiver where a message-free chaotic signal is generated locally, which is then subtracted from the encoded transmitted signal to recover the message (information). Chaotic optical communication is possible when the broadband chaotic emissions from two spatially separated emitters (lasers) are synchronized to each other. In order to satisfy the requirement for synchronization of the two lasers, the irregular time evolution of the emitter laser optical power must be perfectly reproduced at the receiver laser. Decoding the message from the chaotic carrier is based on the nonlinear phenomenon of chaos synchronization between the emitter and the receiver so that the message can be extracted by subtracting the chaotic carrier from the input (chaotic carrier + message).

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All-Optical Broadband Global Communications for Internet Connectivity

Arun K. Majumdar, in Optical Wireless Communications for Broadband Global Internet Connectivity, 2019

5.5.1 High-Altitude Platform-Based Optical Wireless Communications

FSO technology can be used very effectively for establishing high capacity backhaul links from HAPs or satellite-based networks. FSO communication has been shown to provide gigabit capacity backhaul links due to its low cost and rapid deployment speed in comparison with conventional backhaul technologies like RF or optical fibers. This section focuses on optical backhaul links between HAPs, satellite, and ground, which can serve as future broadband backhaul communication channels. HAPs are quasi-stationary vehicles like helium-filled airships or aircrafts located at an altitude of 17–25 km in the stratosphere. HAPs are placed far from the atmospheric region so they can provide better channel conditions than satellites and better LOS conditions in almost all coverage areas, thus offering much more network flexibility. At this altitude, the effects of atmosphere on optical beam propagation are less severe than the turbulence effects close to the ground. HAP altitudes are cloud free so that more reliable communication links are possible between HAPs and satellites. Besides broadband capability, HAPs offer large coverage area (3–7 km) and flexible capacity increase through spot beam resizing and quick deployment. HAPs can be used to relay the high capacity optical data through the atmosphere to the ground locations and therefore their backhaul optical link can easily be connected to the core through terrestrial gateway stations. In other words, HAP-based optical networks and communication architectures can be developed for a complete combined optical backhaul for connecting very high data rate connectivity for both terrestrial and space systems simultaneously.

Using HAPs for completing the satellite-to-ground optical link on-board regenerative HAPs payload can perform the task in two parts: (1) optical link from satellite-to-HAP the atmosphere can be assumed to behave like almost free-space with not much atmospheric effects, and (2) HAP-to-ground (terrestrial) optical links, which will be somewhat affected by the lower atmospheric turbulence and scattering effects, but can be effectively mitigated for establishing high data rate optical communications. On-board satellite data processing time can also be reduced to make it more efficient, and with efficient optical networks design to channel and distribute enormous data at extremely high data rate. Fig. 5.14 depicts a concept architecture where HAPs can stand alone as well as integrated with both satellite and terrestrial systems. Broadcasting and broadband services over a large coverage area including suburban and remote areas can be provided by the integrated ground-HAP-satellite system at a low cost of deployment. Large solid-state in terabyte-size memories can be used as multiple optical payloads to store data from the satellite and transmit to the ground terminals, optimizing the satellite visibility time [26].

Figure 5.14. Concept architecture showing high-altitude platforms integrated with satellite and terrestrial systems.

Fig. 5.15 depicts an integrated system architecture for satellite-HAP-UAV-terrestrial terminals for FSO communications emphasizing to handle site diversity and redundancy.

Figure 5.15. Integrated system architecture for satellite-UAV-HAP-terrestrial terminals showing site diversity and redundancy.

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Specialty Fiber Optic Cables

Casimer DeCusatis, John Fox, in Fiber Optic Data Communication, 2002

4.9.2. FREE SPACE OPTICAL LINKS

For some applications, optical fiber may not be necessary at all; in cases where it is difficult or expensive to install fibers, free space optical communication may provide an alternative. For example, free space optics have been proposed for communication between buildings in areas where fiber cannot be readily installed underground or between tall office buildings in metropolitan areas [41], between ships at dock and their mooring station, or to bridge the last mile between private homes and a service provider network [42]. Most of these systems consist of some form of telescope optics to collimate a tight beam for long-distance transmission, or to receive the dispersed light at a remote location. As one might expect, these approaches must deal with pointing accuracy and do not work as well under adverse weather conditions, since the infrared light signal is strongly absorbed by moisture in the atmosphere. On a smaller scale, free space optical interconnects have also been proposed for intra-machine communications, using micro-optics and lenses to collimate and focus light beams within a computer system or between adjacent equipment racks. While many such systems have been proposed, commercial products for use within a computer system are not yet available.

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Conclusions and Discussion

Arun K. Majumdar, in Optical Wireless Communications for Broadband Global Internet Connectivity, 2019

All-optical concepts to establish global Internet connectivity and access at high speed are described in this book, covering all aspects of fundamentals of free-space optical communications and high bandwidth fiber-optics backbones to design optical links suitable for various scenarios in a number of applications. The Internet is now really part of our social fabric and is essential to how we connect, communicate, share, and collaborate. Digital technologies, information and communication technologies, and broadband global connectivity, when combined, offer various opportunities to improve people's lives in worldwide societies by exchanging and sharing information. Basics of achieving worldwide broadband Internet connectivity independent of terrestrial limitations are explained in detail with technology development in mind. As the world becomes increasingly digital, the concepts developed in a number of chapters provide potential technology solutions for affordable broadband within reach of people worldwide. Real-time accessing of information is helping to transform various sectors ranging from healthcare, education, business, and many others. Global Internet connectivity will make it possible to collect and analyze larger and more complex kinds of data. The fundamentals of all-optical Internet connectivity and the associated communication technologies described in this book with an emphasis on developing innovative architectures and design of optical communication link systems are based on the concepts and demonstrations of some of the following areas: demonstration of low earth orbit satellite-to-ground laser communications; optical antenna; all-optical small-cell solution; constellation of satellites, microsatellites, and CubeSat for establishing Internet access to remote locations via drones, balloons, and high-altitude platforms; Internet of Things to integrate with 5G technology; Li-Fi–based hotspots to replace Wi-Fi; chip-based compact optical devices such as optical routers and optical switching to be integrated with optical network systems for Internet access by smart phone; and quantum communications using quantum entanglement for achieving the most secure quantum Internet; just to name a few.

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Advances in Semiconductor Lasers

Joseph P. Donnelly, ... George W. Turner, in Semiconductors and Semimetals, 2012

1 Introduction

High-power high-brightness single-mode semiconductor lasers and amplifiers are of interest for a variety of applications including pumping fiber amplifiers, free-space optical communications, and laser radars. Arrays of such lasers are also attractive for beam combining to achieve even higher single-mode power. Power and brightness in single-mode semiconductor lasers are generally limited by catastrophic optical damage (COD), thermal effects, or two-photon absorption (TPA) (Ahmad et al., 2008; Juodawlkis et al., 2008; Motamedi et al., 2008). In order to overcome these power limitations generally encountered with conventional ridge or buried heterostructure single-mode devices, other structures that increase the mode size in the lateral direction while keeping the device single-moded, such as tapered structures (Delepine et al., 2001; Donnelly et al., 1998; Walpole et al., 1992, 1996, 2000) and angled-grating distributed feedback structures (Bewley et al., 2001; Lang et al., 1998; Pezeshki et al., 1999; Sarangan et al., 1999) have been developed. Tapered lasers are capable of high power but suffer from beam instabilities, and the astigmatic output beam is difficult to couple efficiently into a fiber. Angled-grating distributed feedback structures are difficult to fabricate, and the large-aspect-ratio beam is also difficult to couple efficiently into a fiber. High-power broad-stripe laser structures with broadened waveguides were developed both to increase the mode size in the transverse direction and to decrease modal loss (Buda et al., 1997; Garbuzov et al., 1996, 1999; Lee et al., 2002; Mawst et al., 1996; Petrescu-Prahova et al., 1994). For high power, the output aspect ratio is large, as the transverse waveguide thickness is generally limited to ∼ 1 μm. Some increase in the vertical direction can be obtained by using a two-step lower cladding in which the mode exponentially decays only slowly in the first lower cladding. Large-diameter vertical-cavity lasers are also being developed for high power but require an external cavity for single-mode operation (Kuznetsov et al., 1997). Coherent arrays of lasers have also been investigated for increasing output power and Watt-class, stable-beam anti-guide arrays have been reported (Botez et al., 1991).

In this chapter, we describe a new class of semiconductor diode emitters, the slab-coupled optical waveguide laser (SCOWL) and amplifier (SCOWA) that offer several advantages for single-spatial mode, high-output power devices. Mode filtering due to slab coupling of higher-order modes allows a much larger cross-sectional area and a lower loss in a single mode than is possible in conventional ridge lasers. The large size of the single mode results in a relatively low power density at the facets, and the low loss permits the construction of long devices that enhance heat removal from the device at high-power operation. In addition, SCOW devices can be designed such that the spatial mode profile is nearly circular, allowing coupling (without the use of lenses) with high-coupling efficiency to single-mode fibers.

Section 2 provides an overview of the SCOW concept and describes the initial SCOWL demonstration in the InGaAsP/InP material system at a wavelength of 1.3 μm. Sections 3 and 4 summarize the variety of work that we have performed to develop SCOW-based devices in the AlGaAs/GaAs and InGaAsP/InP material systems, respectively. Details of the device design, fabrication, packaging, and performance are presented. Specific SCOW devices that are described include single-element Fabry-Perot lasers and amplifiers, laser and amplifier arrays, wavelength-beam-combined arrays, coherent-beam-combined arrays, single-frequency external-cavity lasers, and monolithic and external-cavity mode-locked lasers. For each of these devices, the SCOW concept has enabled higher power and lower noise than has been previously obtained from single-mode semiconductor waveguide gain media.

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Current Recent Research and Future Directions

Arun K. Majumdar, in Optical Wireless Communications for Broadband Global Internet Connectivity, 2019

10.1 Introduction

This book has attempted to develop and analyze the unlimited possibilities in advancing the all-optical concept technology for creating global Internet connectivity access for all. The technology is based on optical communications: free-space optical (FSO) and fiber-optics–based communications both take advantage of high bandwidth, high-speed data rates and high channel capacity, which are essentials to satisfy the demands of today's requirements for Internet access. A complete system with all-optical technology for establishing global Internet connectivity is still a future potential; however, based on a number of experimental demonstrations worldwide using optical communications are paving the way to finally accomplish this goal. There are many individual segments of these successful demonstrations such as indoor, terrestrial, ground-to-satellite, and intersatellites optical communications, all of which play important roles in establishing this overall all-optical global Internet connectivity. Although FSO communications have been studied for years, FSO networking is still in its infant stage when we apply it to handle global networking. Many global players are working on solutions to make the idea of global access to the Internet for all a reality, but not necessarily using our approach of all-optical technology. Some of the global players include Google and their project Loon, Facebook with intrenet.org, and Microsoft using TV whitespaces. This book provides some technology solutions where we can complement many proposed and demonstrated solutions of global players with the optical technology developed in this book. This is where this book differs, with its emphasis entirely on optical technology to accomplish this goal. Based on demonstrations worldwide by many researchers, the future of global access to the Internet for all looks very promising. Hopefully someday soon it will not be an idea anymore, but reality.

The area of all-optic technology for global Internet access is extremely interdisciplinary and requires knowledge of physics; electrical, optical, and communication engineering; satellite and information technology and communication software; and related subjects. Many research challenges and open FSO networks require research efforts for practical and realistic solutions. This chapter discusses some of the recent research efforts and points out some future directions. The research areas relevant to the goal of this book are organized in three general areas: (1) advanced electrooptic or optoelectronic components and devices needed as part of the optical network to transmit, route, or receive the information; (2) laser or optical communication subsystems/systems including indoor, terrestrial, or satellite-based; (3) innovative optical communications architectures, concepts, and networks; (4) information communication software-based new solutions; and (5) totally new concepts useful for global Internet access, for example, quantum communications and entanglement for secure communications and quantum computation.

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