Fig. 1  When the incident angle is greater than the critical angle, total internal reflection occurs.

Fig. 2  The cross section of optical fibre.

Fig. 3  Light ray undergoes total internal reflection in an optical fibre.

What is optical fibre? How does it transmit messages?

Nowadays information technology is extremely advanced and internet is very popular, so I guess many of you are accessing this website with broadband connection. To ensure that messages are transmitted rapidly and accurately, the development of optical fibre technology plays an important role. What is optical fibre? How does it transmit messages?

Let us begin with a common optical phenomenon. When an incident light ray enters medium 2 of a smaller refractive index (optically less dense medium) from medium 1 of a larger refractive index (optically denser medium), the light ray will deviate from normal after refraction. The larger the incident angle, the larger the deviation of the refracted ray would be. When the incident angle equals the critical angle c (), the light ray will be refracted along the interface. If the incident angle is larger than the critical angle, the light ray will be totally reflected into medium 1 by the law of reflection. Such a phenomenon is called total internal reflection (Fig. 1).

The centre of a simple optical fibre is made up of an optically denser medium, while an optically less dense medium forms the narrow tubes at the outer layer; the material of the medium is often glass or plastic (Fig. 2). If a light ray is directed at an end of the optical fibre, it will be transmitted in the fibre. When the light ray encounters the interface between the central layer and the outer layer, total internal reflection occurs and it is reflected back to the centre. Although light travels in a straight line, the light ray will be transmitted along the direction of the optical fibre even if it is bent (Fig. 3). Optical fibre is widely applicable, for purposes ranging from communication to the production of medical equipment such as endoscope, optical fibre sensor or optical fibre decorations.

Optical fibre communication in this era makes use of the changing intensity of light ray to represent different signals. If an incident light ray that carries a message is directed at an end of an optical fibre, the fibre will transmit the ray to the receiver at the other end. The original message can be recovered by an converter. Since not much of the light ray is lost in the course of transmission via a glass fibre, the messages can be transmitted over a very long distance without a midway transfer station. An optical fibre of several microns thick (thinner than a hair) can transmit ten millions of pulses per second. It is gradually replacing copper wire as the main communication medium because of its superiority.