What is an Optical Fiber Cable? How does it work?

Today we have instantaneous phone calls. Emails are fairly instantaneous. Communications are easier today than they ever have been. People’s lives are more connected today than was ever the case before.

Fiber has become the preferred method of transporting information across the world. Networks of fiber optic cables have spread all overland and on the sea bed. The US itself has 35 million miles of fiber optic cables connecting the country. Each optical fiber cable can carry up to 8 terabits of information every second. That equals 68000 hours of music, or 2000 hours of movie, every second.

The make-up of an optical fiber cable

Fibers make this information transfer possible. Each cable contains within it over a hundred fibers of purified glass, thinner than a strand of hair. The laser at one end of the fiber flashes billions of times every second. Either a 0 or a 1 represents each flash. Light travels through the cable, reflecting off the walls. Computers read the laser light emerging from the other end of the cable as binary codes.

How do fiber optics work?

Fiber optics work by the transmission of coded information through a beam of light down a glass or plastic pipe. That conducted phone calls at the speed of light during the sixties.

A cable made of hundreds of fibers can carry millions of calls.

Off the walls of the cable pipe bounces the light back and forth. Light, directed at shallow angles reflects back into the pipe. This is the ‘total internal reflection.’

The covering, or cladding, keeps the light inside the core of the cable.

All optic fiber cables are not equal. Modes help travel differentiation.

The path that a light beam tracks down the fiber are called modes. The single-mode fiber is the simplest. The ten-micron wide core sends signals straight down the middle. There is no bouncing off the cladding. Single-mode fibers, bundled together, transmit telephone, cable TV, internet signals.

We have multi-mode cables as well. Each fiber in a multi-mode cable is ten times the size of those in single-mode cables. However, multi-mode cables can carry information only over short distances. These chiefly link computer networks.

Another look at the transmission

Let’s take another look at how a fiber actually works. When a computer has information to send, it is as electrical energy that data starts out. In the computer, a laser converts the signals to photons, sending them in fast succession down the fiber.

Through the inner core of the fiber travel the photons in waves. This core has a higher refractive index. In other words, light travels more slowly. Focused within the core, the light signal is prevented from escaping the fiber. To ensure that light signals are not absorbed or scattered by impurities, fiber cores are made from highly pure materials, like silica. Large distances are traveled by keeping losses low.

The possible leaking of light in such transmission is called attenuation. This is prevented by cladding.  The low refractive index of the cladding does not permit the light signal to escape.

Another look at the physique of optical cables

During manufacture, cladding and core are made simultaneously, just after the glass has cooled sufficiently.

A plastic layer is laid for mechanical protection. This is the primary buffer.

The outer covering of the cable, called the jacket, may be made up of several layers. It depends on the industrial purpose intended.

Clarity, optical or otherwise

Points worth considering are
  • There is no hole through the middle of the optic fiber;
  • Mechanical protection is provided by the buffer and the jacket;
  • Light also travels through the cladding. So, the optical clarity of the cladding is important.

Technology is slowly edging towards the realization of the dream of carrying millions of audio and video channels through a single light beam.

Leave a Reply