How do optical fibres work?
Fibre optics work similarly to how telephones and cellphones transmit data which send and receive information using cables or waves. With fibre optics, it sends information coded in a beam of light down a glass pipe.
The glass of optical fibres absorb very little light. Light getting in at one end goes through repeated reflection and emerges at the other end. Optical fibres carry light signals down called a mode - the path that a light beam follows down the fibre.
Within an optical fibre, dense glass is used for the interior, whereas a less dense glass is used on the exterior layers. With this, light is refracted in a way that the angle is greater than the critical angle, causing total internal reflection. This phenomena ensures that the transfer of data exerts a minimum loss of energy.
The glass of optical fibres absorb very little light. Light getting in at one end goes through repeated reflection and emerges at the other end. Optical fibres carry light signals down called a mode - the path that a light beam follows down the fibre.
Within an optical fibre, dense glass is used for the interior, whereas a less dense glass is used on the exterior layers. With this, light is refracted in a way that the angle is greater than the critical angle, causing total internal reflection. This phenomena ensures that the transfer of data exerts a minimum loss of energy.
Total Internal Reflection
In a fibre optic cable, every tiny photon (particle of light) travels by reflecting (bouncing repeatedly) off the glass walls. Light does not just travel down the glass pipe, but it reflects against the glass at an angle of less than 42 degrees. This phenomenon is called total internal reflection - one of the things that keeps light inside the pipe.
For example, if you have a flashlight and shine it nearly parallel to a glass window, the latter will act as a mirror and the beam of light will reflect off the window and hit a wall inside the room. Light travelling in a fibre reflects at shallow angles like above, and stays completely within the fibre. Here is a video explaining total internal reflection in a fibre optics cable: |
In short:
Total internal reflection describes the phenomena in which photons (particles of light) is reflected greater than the critical angle. This causes the light to reflecting (bouncing back) off walls whilst travelling forward.
Structure of the cable
Another thing that keeps light in the pipe is the structure of the cable, made up of three separate parts:
1. The main part of the cable is called the core and which is where the light travels through. 2. The cladding is another layer of glass wrapped around the core. The cladding helps to keep the light signals inside the core. This is possible because it is made of a different type of glass to the core. Between the two mediums, the cladding has a lower refractive index. This means that the cladding is less dense than the core. As a result, the cladding causes total internal reflection that stops the light from escaping and keeps it bouncing down the core. The cladding does not absorb any light from the core, so the light wave can travel great distances. However, some of the light signal degrades within the fibre, due to impurities in the glass. This depends upon the purity of the glass and the wavelength of the transmitted light. For example: 850 nm = 60 to 75 percent/km 1,300 nm = 50 to 60 percent/km High quality optical fibres show much less signal degradation - less than 10 percent/km at 1,550 nm. 3. The plastic coating that protects the fibre from damage and moisture is called the buffer coating. The bundles are protected by the jacket, which is the cable’s outer covering. This makes it easy to use the fibre without breaking it. |
Fibre Optic Communication
Fibre-optic communication is a method of conveying information from one place to another by sending beams of light through an optical fibre.
In order to send telephone conversations through a fibre optic cable, analogue voice signals are transformed into digital signals. On one end of the pipe, there is a laser that switches on and off several billion times per second to send information.
A fibre optic communications system is composed of three parts:
The Transmitter – This is located inside the device that need to send or receive data. This is made up of a light emitter and the electronics that control the light to send information. This converts the electric signal to a light signal and sends it through the fibre to the receiver. The emitter is a Light Emitting Diode, more commonly known as an LED.
The fibre Medium (cable) – The medium is the fibre optic cable and the connectors and splices carry the light from the transmitter to the receiver.
The Receiver – Like the transmitter, this is located inside the device that need to send or receive data. This is a light detector that converts the received light signals back to electrical signals and an amplifier that sets the signal for use in the station. The detector is a light detecting diode. The latest systems however, use multiple lasers with different colours to fit multiple signals into the same fibre.
The end devices are called Stations. The Repeater is a station. On a long distance line, there is a repeater every 50 to 100km. The repeater picks up and boost the signal to the next segment.
Simplex communication is when signals are transmitted over a fibre optic cable in only one direction.
A Duplex communication is the two-way communication needed between stations. The Repeater requires duplex communications.
In order to send telephone conversations through a fibre optic cable, analogue voice signals are transformed into digital signals. On one end of the pipe, there is a laser that switches on and off several billion times per second to send information.
A fibre optic communications system is composed of three parts:
The Transmitter – This is located inside the device that need to send or receive data. This is made up of a light emitter and the electronics that control the light to send information. This converts the electric signal to a light signal and sends it through the fibre to the receiver. The emitter is a Light Emitting Diode, more commonly known as an LED.
The fibre Medium (cable) – The medium is the fibre optic cable and the connectors and splices carry the light from the transmitter to the receiver.
The Receiver – Like the transmitter, this is located inside the device that need to send or receive data. This is a light detector that converts the received light signals back to electrical signals and an amplifier that sets the signal for use in the station. The detector is a light detecting diode. The latest systems however, use multiple lasers with different colours to fit multiple signals into the same fibre.
The end devices are called Stations. The Repeater is a station. On a long distance line, there is a repeater every 50 to 100km. The repeater picks up and boost the signal to the next segment.
Simplex communication is when signals are transmitted over a fibre optic cable in only one direction.
A Duplex communication is the two-way communication needed between stations. The Repeater requires duplex communications.