Optical networking is a means of communication that uses signals encoded in light to transmit information in various types of
telecommunications network
A telecommunications network is a group of nodes interconnected by telecommunications links that are used to exchange messages between the nodes. The links may use a variety of technologies based on the methodologies of circuit switching, mes ...
s. These include limited range
local-area networks (LAN) or
wide-area networks (WAN), which cross metropolitan and regional areas as well as long-distance national, international and transoceanic networks. It is a form of
optical communication
Optical communication, also known as optical telecommunication, is communication at a distance using light to carry information. It can be performed visually or by using electronic devices. The earliest basic forms of optical communication date ...
that relies on
optical amplifiers,
laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The ...
s or
LEDs
A light-emitting diode (LED) is a semiconductor device that emits light when current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy in the form of photons. The color of the light (cor ...
and
wave division multiplexing (WDM) to transmit large quantities of data, generally across
fiber-optic cables
A fiber-optic cable, also known as an optical-fiber cable, is an assembly similar to an electrical cable, but containing one or more optical fibers that are used to carry light. The optical fiber elements are typically individually coated with ...
. Because it is capable of achieving extremely high
bandwidth
Bandwidth commonly refers to:
* Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandwidth'', or ''radio bandwidth'', a measure of the width of a frequency range
* Bandwidth (computing), the rate of data transfer, bit rate or thr ...
, it is an enabling technology for the
Internet
The Internet (or internet) is the global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a '' network of networks'' that consists of private, p ...
and
telecommunication networks that transmit the vast majority of all human and machine-to-machine information.
Types
Fiber-optic networks
The most common
fiber-optic networks are
communication networks
A telecommunications network is a group of nodes interconnected by telecommunications links that are used to exchange messages between the nodes. The links may use a variety of technologies based on the methodologies of circuit switching, mess ...
,
mesh networks
A mesh network is a local area network topology in which the infrastructure nodes (i.e. bridges, switches, and other infrastructure devices) connect directly, dynamically and non-hierarchically to as many other nodes as possible and cooperate wit ...
or
ring networks commonly used in metropolitan, regional, national and international systems. Another variant of fiber-optic networks is the
passive optical network
A passive optical network (PON) is a fiber-optic telecommunications technology for delivering broadband network access to end-customers. Its architecture implements a point-to-multipoint topology in which a single optical fiber serves multiple e ...
, which uses unpowered optical splitters to link one fiber to multiple premises for
last mile applications.
Free-space optical networks
Free-space optical networks use many of the same principles as a fiber-optic network but transmit their signals across open space without the use of fiber. Several planned
satellite constellations
A satellite or artificial satellite is an object intentionally placed into orbit in outer space. Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as solar panels or radioisotope ...
such as
SpaceX's Starlink intended for global internet provisioning will use wireless
laser communication
Free-space optical communication (FSO) is an optical communication technology that uses light propagating in free space to wirelessly transmit data for telecommunications or computer networking. "Free space" means air, outer space, vacuum, or ...
to establish optical mesh networks between satellites in outer space. Airborne optical networks between
high-altitude platforms are planned as part of
Google's Project Loon and
Facebook Aquila
The Facebook Aquila is an experimental solar-powered drone developed by Facebook for use as an atmospheric satellite, intended to act as relay stations for providing internet access to remote areas.
It first flew on 28 June 2016 with a second a ...
with the same technology.
Free-space optical networks can also be used to set up temporary terrestrial networks e.g. to link LANs on a campus.
Components
Components of a fiber-optical networking system include:
* Fiber. Multi-mode or single-mode.
* Laser or LED light source.
*
Multiplexer/demultiplexer, also called mux/demux, filter, or prism. These can include
Optical Add/Drop Multiplexer (OADM) and
Reconfigurable Optical Add/Drop Multiplexer (ROADM).
*
Optical switch, to direct light between ports without an optical-electrical-optical conversion
*
Optical splitter, to send a signal down different fiber paths.
*
Circulator
A circulator is a passive, non-reciprocal three- or four- port device that only allows a microwave or radio-frequency signal to exit through the port directly after the one it entered. Optical circulators have similar behavior. Ports are where ...
, to tie in other components, such as an OADM.
*
Optical amplifier
An optical amplifier is a device that amplifies an optical signal directly, without the need to first convert it to an electrical signal. An optical amplifier may be thought of as a laser without an optical cavity, or one in which feedback fr ...
.
*
Wave division multiplexer.
Transmission Medium
At its inception, the telecommunications network relied on
copper
Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pink ...
to carry information. But the bandwidth of copper is limited by its
physical characteristics—as the frequency of the signal increases to carry more data, more of the signal's energy is
lost as heat. Additionally, electrical signals can interfere with each other when the wires are spaced too close together, a problem known as crosstalk. In 1940, the first communication system relied on
coaxial cable
Coaxial cable, or coax (pronounced ) is a type of electrical cable consisting of an inner conductor surrounded by a concentric conducting shield, with the two separated by a dielectric ( insulating material); many coaxial cables also have a p ...
that operated at 3 MHz and could carry 300 telephone conversations or one television channel. By 1975, the most advanced coaxial system had a bit rate of 274 Mbit/s, but such high-frequency systems require a repeater approximately every kilometer to strengthen the signal, making such a network expensive to operate.
It was clear that light waves could have much higher bit rates without crosstalk. In 1957,
Gordon Gould
Gordon Gould (July 17, 1920 – September 16, 2005) was an American physicist who is sometimes credited with the invention of the laser and the optical amplifier. (Credit for the invention of the laser is disputed, since Charles Townes and A ...
first described the design of the optical amplifier and the
laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The ...
that was demonstrated in 1960 by
Theodore Maiman. The laser is a source for light waves, but a medium was needed to carry the light through a network. In 1960, glass fibers were in use to transmit light into the body for medical imaging, but they had high optical loss—light was absorbed as it passed through the glass at a rate of 1 decibel per meter, a phenomenon known as
attenuation
In physics, attenuation (in some contexts, extinction) is the gradual loss of flux intensity through a medium. For instance, dark glasses attenuate sunlight, lead attenuates X-rays, and water and air attenuate both light and sound at var ...
. In 1964,
Charles Kao
Sir Charles Kao Kuen [Charles K. Kao was elected in 1990](_blank)
as a memb ...
showed that to transmit data for long distances, a glass fiber would need loss no greater than 20 dB per kilometer. A breakthrough came in 1970, when
Donald B. Keck,
Robert D. Maurer, and
Peter C. Schultz
Peter C. Schultz, Ph.D. (born 1942), is co-inventor of the fiber optics now used worldwide for telecommunications. He is retired President (1988 to 2001) of Heraeus Tenevo Inc., a $200 million technical glass manufacturer specializing in fiber opt ...
of
Corning Incorporated
Corning Incorporated is an American multinational technology company that specializes in specialty glass, ceramics, and related materials and technologies including advanced optics, primarily for industrial and scientific applications. The c ...
designed a glass fiber, made of fused silica, with a loss of only 16 dB/km. Their fiber was able to carry 65,000 times more information than copper.
The first fiber-optic system for live telephone traffic was in 1977 in Long Beach, Calif., by
General Telephone and Electronics
GTE Corporation, formerly General Telephone & Electronics Corporation (1955–1982), was the largest independent telephone company in the United States during the days of the Bell System. The company operated from 1926, with roots tracing furth ...
, with a data rate of 6 Mbit/s. Early systems used infrared light at a wavelength of 800 nm, and could transmit at up to 45 Mbit/s with repeaters approximately 10 km apart. By the early 1980s, lasers and detectors that operated at 1300 nm, where the optical loss is 1 dB/km, had been introduced. By 1987, they were operating at 1.7 Gbit/s with repeater spacing of about 50 km.
Optical Amplification
The capacity of fiber optic networks has increased in part due to improvements in components, such as optical amplifiers and optical filters that can separate light waves into frequencies with less than 50 GHz difference, fitting more channels into a fiber. The
erbium-doped optical amplifier (EDFA) was developed by
David Payne at the
University of Southampton
, mottoeng = The Heights Yield to Endeavour
, type = Public research university
, established = 1862 – Hartley Institution1902 – Hartley University College1913 – Southampton University Coll ...
in 1986 using atoms of the rare earth erbium that are distributed through a length of optical fiber. A pump laser excites the atoms, which emit light, thus boosting the optical signal. As the paradigm shift in network design proceeded, a broad range of amplifiers emerged because most optical communication systems used optical fiber amplifiers. Erbium-doped amplifiers were the most commonly used means of supporting dense wavelength division multiplexing systems. In fact, EDFAs were so prevalent that, as WDM became the technology of choice in the optical networks, the erbium amplifier became "the optical amplifier of choice for WDM applications." Today, EDFAs and hybrid optical amplifiers are considered the most important components of wave division multiplexing systems and networks.
Wavelength Division Multiplexing
Using optical amplifiers, the capacity of fibers to carry information was dramatically increased with the introduction of
wavelength-division multiplexing (WDM) in the early 1990s. AT&T's
Bell Labs
Nokia Bell Labs, originally named Bell Telephone Laboratories (1925–1984),
then AT&T Bell Laboratories (1984–1996)
and Bell Labs Innovations (1996–2007),
is an American industrial research and scientific development company owned by mul ...
developed a WDM process in which a prism splits light into different wavelengths, which could travel through a fiber simultaneously. The peak wavelength of each beam is spaced far enough apart that the beams are distinguishable from each another, creating multiple channels within a single fiber. The earliest WDM systems had only two or four channels—AT&T, for example, deployed an oceanic 4-channel long-haul system in 1995. The erbium-doped amplifiers on which they depend, however, did not amplify signals uniformly across their spectral gain region. During signal regeneration, slight discrepancies in various frequencies introduced an intolerable level of noise, making WDM with greater than 4 channels impractical for high-capacity fiber communications.
To address this limitation,
Optelecom
Optelecom-NKF, Inc. is an American company that designs, manufactures, and markets high-bandwidth communications products, financial market data information, and business video systems.
History
The company was founded as Optelecom in 1974 by Wi ...
, Inc. and
General Instruments Corp. developed components to increase fiber bandwidth with far more channels. Optelecom and its head of Light Optics, engineer David Huber and
Kevin Kimberlin
Kevin Kimberlin is chairman of Spencer Trask & Co., an advanced technology firm. Kimberlin has distinguished himself by partnering with or backing "obsessive missionaries" including Jonas Salk, Walter Gilbert, John Wennberg and Robert Langer.
T ...
co-founded
Ciena Corp in 1992 to design and commercialize optical telecommunications systems, the objective being an expansion in the capacity of cable systems to 50,000 channels. Ciena developed the dual-stage optical amplifier capable of transmitting data at uniform gain on multiple wavelengths, and with that, in June 1996, introduced the first commercial dense WDM system. That 16-channel system, with a total capacity of 40 Gbit/s, was deployed on the
Sprint network, the world's largest carrier of internet traffic at the time. This first application of all-optical amplification in public networks was seen by analysts as a harbinger of a permanent change in network design for which Sprint and
Ciena would receive much of the credit. Advanced optical communication experts cite the introduction of WDM as the real start of optical networking.
Capacity
The density of light paths from WDM was the key to the massive expansion of
fiber optic
An optical fiber, or optical fibre in Commonwealth English, is a flexible, transparent fiber made by drawing glass (silica) or plastic to a diameter slightly thicker than that of a human hair. Optical fibers are used most often as a means t ...
capacity that enabled the growth of the Internet in the 1990s. Since the 1990s, the channel count and capacity of dense WDM systems has increased substantially, with commercial systems able to transmit close to 1 Tbit/s of traffic at 100 Gbit/s on each wavelength. In 2010, researchers at AT&T reported an experimental system with 640 channels operating at 107 Gbit/s, for a total transmission of 64 Tbit/s. In 2018, Telstra of Australia deployed a live system that enables the transmission of 30.4 Tbit/s per fiber pair over 61.5 GHz spectrum, equal to 1.2 million 4K Ultra HD videos being streamed simultaneously. As a result of this ability to transport large traffic volumes, WDM has become the common basis of nearly every global communication network and thus, a foundation of the Internet today. Demand for bandwidth is driven primarily by
Internet Protocol
The Internet Protocol (IP) is the network layer communications protocol in the Internet protocol suite for relaying datagrams across network boundaries. Its routing function enables internetworking, and essentially establishes the Internet.
...
(IP) traffic from video services, telemedicine, social networking, mobile phone use and cloud-based computing. At the same time, machine-to-machine, IoT and scientific community traffic require support for the large-scale exchange of data files. According to the Cisco Visual Networking Index, global IP traffic will be more than 150,700 Gbits per second in 2022. Of that, video content will equal 82% of all IP traffic, all transmitted by optical networking.
[Cisco Visual Networking Index: Forecast and Methodology, 2013-2018, https://www.cisco.com/c/en/us/solutions/collateral/service-provider/visual-networking-index-vni/white-paper-c11-741490.html]
Standards and protocols
Synchronous Optical Networking (SONET) and Synchronous Digital Hierarchy (SDH) have evolved as the most commonly used protocols for optical networks. The
Optical Transport Network (OTN) protocol was developed by the
International Telecommunication Union
The International Telecommunication Union is a specialized agency of the United Nations responsible for many matters related to information and communication technologies. It was established on 17 May 1865 as the International Telegraph Unio ...
as a successor and allows interoperability across the network as described by
Recommendation G.709. Both protocols allow for delivery of a variety of protocols such as
Asynchronous Transfer Mode (ATM),
Ethernet
Ethernet () is a family of wired computer networking technologies commonly used in local area networks (LAN), metropolitan area networks (MAN) and wide area networks (WAN). It was commercially introduced in 1980 and first standardized in 1 ...
,
TCP/IP
The Internet protocol suite, commonly known as TCP/IP, is a framework for organizing the set of communication protocols used in the Internet and similar computer networks according to functional criteria. The foundational protocols in the su ...
and others.
References
{{Reflist
Optical communications