Editorial Feature

How Do Fiber Optic Cables Work?

Fiber Optic, Cables

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Fiber to the Home (FTTH) might be a common word today but over twenty years ago, it was only a distant milestone on a road-map. The first all-optical fiber cable was laid across the Atlantic in 1996. It extended 3500 miles from Tuckerton, New Jersey before branching off to Widemouth in England and Penmarch in France. Today, all of our deep, under-sea cables are all-optical fibers. It is an incredible invention that has revolutionized the information age.

Fiber optic cables are functionally analogous to copper cables; the century-old technology they have replaced, however, they are fundamentally different from each other in the nature of the medium in which they carry information. Fiber optic cables consist of one or many optical fibers that carry information encoded in a carrier light wave. A copper cable can only transmit a fraction of that same information over a modulated electrical signal, at a much lower speed, and with greater losses.

Light in an optical fiber is contained within the core which is usually made of silicon dioxide (glass) or plastic. This core is covered with a cladding which is often the same silicon dioxide that is either doped with boron or germanium in order to decrease its refractive index below that of the core. This difference in refractive indices between the core and cladding is crucial to the functioning of the optical fiber.

Fiber optic cables operate on the principle of total internal reflection (TIR). This phenomenon of TIR occurs in situations where light traveling in a dense medium of higher refractive index is incident upon an interface with a less dense medium of a lower refractive index. In such cases, if the angle of incidence of the ray of light is greater than the critical angle of the interface, the light is reflected back into the dense medium.

It was in 1840 that Swiss physicist Daniel Colladon first discovered that water could carry light by total internal reflection. Thirty years later, Irish physicist John Tyndall demonstrated this by pouring water out of a jug, where the light curved around following the path of the water. Endoscopes that applied this principle of TIR were invented in the 1950s and by 1970, scientists had invented the first fiber optic cable.

Information in an optical fiber is modulated onto a carrier light wave. For example, if a computer is to transmit information using pulse modulation, electronic data originating from the system in the form of 1s and 0s is converted into light pulses before being subsequently transmitted along optical fibers.

On receiving this signal, the receiver computer extracts the information by applying the shared decoding algorithm. However, if this signal is transmitted over long distances, it will be attenuated, largely due to imperfections and impurities in the glass core of the optical fiber. Engineers have circumvented this problem by building signal re-amplifiers into the fiber optic cables.

The signal carried by an optical fiber can either be of a single wavelength or be split over different wavelengths. The single-mode fibers that are used for single wavelength applications are virtually distortion free due to the lack of any secondary light-wave that would have interacted with the primary signal and distorted it. These fibers have a narrow core of under 10 μm and are a commercially superior choice for long-range transmission applications where multi-mode fibers are not comparable in efficiency.

Multi-mode optical fibers are cheaper to manufacture owing to the larger sizes of their cores (greater than 60 μm) and are consequently a better choice for short-distance, high-speed transmission applications under distances of 10 miles for which they do not require re-amplification.

Current fiber optic cables provide unsurpassed transmission bandwidths compared to copper wires, coaxial cables and microwave links, however, the speed at which they propagate light is still only seventy percent of the speed of light in vacuum. With the latest research in hollow-core optical fibers, speeds of up to 99.7 % of the speed of light in vacuum have been experimentally demonstrated.

Researchers are also focused on reducing fiber losses and a team at University College London have claimed to have doubled the distance across which single-mode fibers can transmit signals with losses comparable to the current industry standard. Until we find a carrier faster than light, fiber optic cables will be our information highways for the foreseeable future.

Sources:

Poletti, F., et al. “Towards High-Capacity Fibre-Optic Communications at the Speed of Light in Vacuum.”, vol. 7, no. 4, 2013, pp. 279–284. https://doi.org/10.1038/nphoton.2013.45.

Maher, Robert, et al. “Spectrally Shaped DP-16QAM Super-Channel Transmission with Multi-Channel Digital Back-Propagation.”, vol. 5, no. 1, Mar. 2015. https://doi.org/10.1038/srep08214.

Corning. “How It Works: Optical Fiber”

Christopher C. Davis. "Fiber Optic Technology and Its Role in The Information Revolution"

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