For understanding the working principle of optical fiber cable clearly, we first further learn the components of an optical fiber cable.
Components of Fiber Cable
A fiber optic cable can be divided into three parts. It is a coaxial cable, and the center most part is called the core which is made out of a very clear glass tube and carries the information. The plastic covering above it which causes the reflection of light signals is called the cladding and the sheathing that protects the optical fiber is called the coating. In a single mode fiber, the core is about 5-10 microns in diameter. A dimension of 5 to 10 microns is equivalent to the millionth of a meter. That small is the core of an optical fiber.
The reason why core is made out of glass is that, glass is incredibly pure so that, even though it is several miles long, light can still make it through. The glass is drawn into a very thin strand, with a thickness comparable to that of a human hair. The glass strand is then coated in two layers of plastic. So fiber optic cable is also called glass fiber cable.
In order for the finished cable to transmit data signals, it needs to be connected to the three other main components of a fiber optic system, named optical transmitter, optical receiver and optical regenerator.
Optical transmitter, a device which converts electrical and analog signals into either On-Off or Linear modulating light signals, then releases that data into the fiber optic cable. The pattern of light waves forms a code that carries a message. The cable then relays the data emitted by the optical transmitter to the optical receiver, which accepts the light signal and reformats the data into its original form.
The receiver is essentially performing the opposite function of the transmitter. Optical receivers receive the light signal from the fiber optic able and turn it back into information that a computer or television know how to understand and use. It then sends the decoded signal to the computer or television.
Sometimes a light signal must travel through a fiber optic cable over a very long distance. Although signal degradation is minimal in a fiber optic patch cable, some degradation does occur. When a cable covers a long distance, optical regenerators are placed at certain intervals along the cable. Optical regenerators are fibers that have been treated with a laser. The molecules in the fiber allow the signal traveling through the fiber optic cable to take on laser properties themselves, which strengthens the light signal. Optical regenerators essentially strengthen the light signal that is traveling through a fiber optic cable.
Working Principle of Fiber Optic Cable
The light travels through the glass strands and continuously reflects off of the inside of the mirrored plastic coatings in a process known as “Total Internal Reflection”.
When light travels from a medium of lower refractive index to that of a higher refractive index, it bends towards the normal. The normal is a line that is perpendicular to the interface of two mediums. However, when light travels from a medium of higher refractive index to that of a lower refractive index, it bends away from the normal.
The angle made by the incident ray to the normal is called the angle of incidence and the angle made by the refracted ray with the normal at the point of incidence in the other medium is called the angle of refraction. Now, consider that light is traveling from a medium with higher refractive index to that of a lower refractive index. As the angle of incidence increases, the angle of refraction also increases. Now, if the angle of incidence is increased to a point that the angle of refraction becomes perpendicular to the angle of refraction, almost parallel to the interface of the two media, then this particular of angle of incidence is called the critical angle. If the angle of incidence is further increased beyond this angle of incidence, then the refracted light will be returned to the same medium, reflected. This is the process of total internal refraction.
What happens in fiber optics is that the light is sent at such an angle almost parallel to the optical fiber, it goes through the process of total internal reflection and travels through hundreds of kilometers. Besides, as we all know that the highest speed of any form of energy is that of light energy. So, it has to be the fastest way of communication. Light gets reflected at the walls of the pure glass and hence travels through hundreds of kilometers.
In order for optical fibers to transmit data over long distances, they need to be highly reflective. On their way to being spooled, newly-pulled glass fibers pass through coating cups and ultraviolet ovens, which respectively apply and then cure the thin plastic buffer coating that creates a mirror effect within the fiber.
The typical three types of fiber optic cables, including multimode fiber optic cable, single mode cable and plastic optical fiber cable (POF), are all adaptable to the basic structure and working principle.