In the year 1997, the longest optical-fiber cable will link 46 countries including India, across three continents. From Great Britain in Western Europe to Japan and South Korea in the far East, the total length of the cable will be 27 300 km. An international consortium called Fiber-optical Link Around the Globe (FLAG) is funding the project. The cable will pass underwater through the Atlantic Ocean, Mediterranean, the Red sea, the Indian Ocean and the Pacific ocean. There will be two short land stretches across Egypt and Thailand. The system is being built by two under-sea optical cable suppliers, AT&T submarine system Inc. and KDD submarine cable system, Tokyo.

An optical fiber consists of a transparent core of glass of a given refractive index surrounded by a cladding layer of material of lower index. A basic point to point circuit consists of a transmitter, a receiver, and a length of fiber. This length may be from a few meters to 50 or more kilometers long, with segments joined together with connectors or splices. The electrical signals are used as a control voltage to vary the light intensity of a light source. The output of either an LED (light emitting diode) or an ILD (Injection Laser Diode) is varied around a midbrightness intensity level. The intensity varying light is focused onto the end of the miniscule diameter glass fiber. Most of the light travels inside the core as it suffers total internal reflection at the interface of the core and the cladding layer surrounding it. At the other end of the fiber, attenuated blinking light is focussed on to a photodetector, which converts the light intensity variations back into an electrical signal.

The optical fiber technology has developed since the fifties. It has several advantages over the metallic transmission medium. The optical-fiber is less expensive, lighter in weight, equally flexible, not subject to electrical interference and more secure to interception. Most important, fiber can be made with negligible losses; as low as one decibel per kilometer.

In the FLAG system the cable consists of an inner core, containing the transmission fiber, surrounded by an interlocking array of steel wire. This protects the fiber from ocean pressure that can change the fiber characteristics. It also provides the longitudinal strength to the cable.

Additional protection comes from the water tight copper jacket. The copper jacket acts as current conductor for dc at 7500 V, 0.9 A, necessary for underwater equipment along the path. A layer of high density polyethylene plastic insulates the copper jacket.

The cable that will be laid in shallow water gets another steel tape wrapping to armor it against shark bites. In addition where the water is shallower it will have a layer of steel wire to protect it from abrasion by the ocean bottom. Near the shore the cable will be buried one meter under the sea floor.

The backbone of FLAG is third generation transoceanic optical-fiber technology. Minimum distortion in the transmission is achieved by transmitting binary digital representation of the original analog information signal, whether they are human voice, or still or motion video images. On two pairs of fibers it can support 120 000 circuits (lines). In comparison the first transatlantic telephone copper cable in 1956 carried only 36 conversations, while the first transatlantic optical-fiber cable installed in 1988 carried 800 circuits.

The cable system that will carry signals ashore is the local telephone company of the respective country. These companies will be responsible for providing the power to the undersea optical amplifiers, regenerate the signals and for the operation and the maintenance of the onshore equipment. In India, this will be done by the public sector giant Videsh Sanchar Nigam Ltd. After the FLAG's completion, transatlantic and transpacific systems will directly link up with FLAG giving the first global nonsatellite digital communication system.