How is Light Transmitted Through Fiber Optic Cables?

In the past 20 years, fiber optic lines have taken over and changed the long-distance telephone industry. And it is also an important part of making the Internet spread all over the world. When the fiber optic patch cord replaces copper cables for long-distance telephones and Internet communications, it will greatly reduce costs.

To understand how an optical cable works, you can imagine it as a long straw or soft plastic pipe. For example, suppose that there is a pipe which is several miles long and the inner surface of the pipe has been coated with a perfect mirror, you are looking at one end of the pipe, at the other end, a friend turns on a flashlight and shine it into the pipe. Since the inside of the pipe is a perfect mirror, the light from the flashlight will be reflected from the side of the pipe (even though the pipe may be bent and twisted), and you will see it at the other end. If your friend turns on and off the flashlight in Morse code, then you can communicate with your friend through the channel. That is the essence of fiber optic patch cables.

It is feasible to make a fiber patch cable with a mirror pipe, but it will be bulky, and it is difficult to cover the inside of the pipe with a perfect mirror. Therefore, real fiber optic cables are made of glass. The glass is very pure, so even if it is several miles long, light can still penetrate it. The glass is drawn into very thin lines, its thickness is equivalent to that of human hair.

By coating the glass with plastic, you can get a mirror-like effect around the glass bundle. The mirror can produce total internal reflection, just like forming a perfect mirror coating inside a pipe. You can appreciate this reflection in a dark room with a flashlight and windows. If the flashlight passes through the window at a 90 degree angle, it will pass directly through the glass. However, if you illuminate the flashlight at a very shallow angle (almost parallel to the glass), the glass will act as a mirror, and you will see the light beam reflect off the window and hit the wall in the room. The light propagates through the fiber bounces at this shallow angle and stays completely inside the optical patch cord.

To send a telephone conversation via a multimode patch cord, the analog voice signal will be converted to a digital signal. The laser is at one end of the pipe. Modern fiber optic systems with a single laser can transmit billions of bits per second (lasers can be turned on and off billions of times per second). The latest systems use multiple lasers with different colors to put multiple signals into the same fiber optic patch.

Modern fiber optic cables can transmit signals over long distances, perhaps 60 miles (100 kilometers). On long-distance routes, there is a fiber optic distribution box every 40 to 60 miles, which can pick up the signal and forward it to the next part with maximum strength.

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