Optical Networking

Optical networking is a means of communication that uses signals encoded in light to transmit information in various types of telecommunications networks. 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 that relies on optical amplifiers, lasers or LEDs and wave division multiplexing (WDM) to transmit large quantities of data, generally across fiber-optic cables. Because it is capable of achieving extremely high bandwidth, it is an enabling technology for the Internet 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, mesh networks or ring networks commonly used in metropolitan, regional, national and international systems. Another variant of fiber-optic networks is the passive optical network, 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 such as SpaceX's Starlink intended for global internet provisioning will use wireless laser communication 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 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, to tie in other components, such as an OADM.
*Optical amplifier.
* Wave division multiplexer.

Transmission Medium

At its inception, the telecommunications network relied on copper 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 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 first described the design of the optical amplifier and the laser 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 1964, Charles Kao 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 of Corning Incorporated 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, 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 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 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, 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 co-founded Ciena Corp in 1992