We see two types of reflections in our daily lives – the sky is reflected in a pond or a lake, and we see a dim reflection of ourselves when we look through a window that is closed.
Figure 1: Reflection in nature
These two types of reflections enable communication in optical links, but can cause signal inaccuracy. The sky reflecting in a lake is as a result of the speed of light, or Rl, changing at the air / water boundary. This change results in a total reflection. Total reflection occurs as long as the angle of reflection is ‘proper’.
In terms of physics, we will see the sky reflecting as long as the rays of light strike the water within a ‘critical angle’ (Snell’s Law). As we look at the water closer and closer to our feet, we will stop seeing the sky at some point, but we will instead see into the water. For now, let’s call the maximum angle at which the reflection occurs the ‘critical angle.’ This reflection within a critical angle enables transmission through an optical fiber.
Figure 2: Reflection and Refraction at surface
Although it is tempting to use the term ‘critical angle’, the correct technical term
for this critical angle is ‘numerical aperture’ (NA). The NA is a measure of the critical angle and is defined in the equation below.
NA = n sinA,
where n is the index of refraction of the medium, and A is the maximum half angle of the cone of light that can enter or exit the medium.
Since the sine of an angle in degrees is a dimensionless number, the numerical aperture has no dimension.
The dim reflection on the surface of a closed window is known as a partial, or ‘Fresnel’ reflection. A Fresnel reflection can occur at any time when light moves from one medium to another. To be more precise, a Fresnel reflection will occur if the Rl in the two media, in this case air and glass, are different. These reflections are important in fiber communication systems because connectors and splices create conditions at which the speed of light can change. When excessive connector and splice reflections occur, this results in signal transmission errors.
Refraction, or the bending of light is something we often see. When looking at a pen in a glass of water from the side, the pen appears to be bent. This refraction, or bending, occurs whenever light moves from a material with one Rl to a material with a different Rl. Understanding refraction is important when dealing with graded index multimode fibers.
Refraction obeys one simple rule: the bending will be towards the region with the lower speed. Stated differently, the bending will be towards the region with the higher Rl.
You may also read more about absorption and scattering in fiber cables in this article.