Optical Fiber and Light Emitting Diode

6 June 2017

Also it has a much higher frequency than electricity. So to make our communication more faster we invented a new guided transmission cable, the fiber-optic cable. The fiber optic communication system includes an optical transmitter which converts an electrical signal into an optical signal and sends it into the optical fiber. The information transmitted through optical fiber is typically digital information. It transmits signals in the form of light. Composition: 1 . Outer Jacket: made up of PVC or Teflon. 2. Kevlar strands inside the Jacket: to strengthen cable (also used in bulletproof ests). 3.

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Plastic coating below Kevlar: for protecting or cushioning fiber. 4. Fiber at the centre: consists of glass or plastic core surrounded by glass cladding which has a low refractive index (less density than core). Fiber-optic Cable connecters: 1 . SC (Subscriber channel connector): used for cable TV. Uses a push/pull locking system. 2. ST (Straight tip connector): It is used for connecting cable to networking devices. Uses a bayonet locking system. 3. MT-RJ: same size as RJ45. Light sources: The light source used in the fiber is light emitting diode. The light is detected by a hoto detector such as a phototransistor.

The two light sources that are widely used in fiber optics are LED (Light Emitting Diode) and ILD (Injection Laser Diode). LED is preferred for short distances and ILD for longer distances. Drawback of LED: The light provided by LED is unfocussed which hits the core boundaries and gets diffused. Conditions for reflection: 1 . The inner glass core should have a higher refractive index then the refractive index of the surrounding glass cladding. Means the glass cladding should be less dense it will help the light ray to be reflected of the cladding instead of being efracted into it. . The angle of incidence of the light ray entering in the fiber-optic cable must be greater than the critical angle. The incident light ray will be reflected inside the glass core. If the angle of incidence is less than the critical angle the light ray will be refracted that is it will enter the glass cladding and gets lost. Data transmission: The transmitter converts into light source and the receiver (photodiode) translates the light back into data bits. Modes of propagation: Mode: The number of paths followed by light rays inside the fiber-optic cable.

There are two types of modes: 1 . Single mode: light takes a single path through the core. 2. Multimode: light follows more than one paths through the core. 1. Single mode fiber: manufactured with small diameter. Lower density (less refractive index). Decrease in density results in a critical angle that is closer to 90 degrees and allows the light rays to travel almost horizontally. All light beams arrive at the destination together. Less distortion of light rays. 2. Multimode fiber: 2. 1 . Multimode step-index fiber: Density of core remains same from center to edges.

The beam of light moves at a straight line until it reaches the boundary (interface) of the core and cladding. The refractive index (low density) changes abruptly. More distortion of light rays. 2. 2. Multimode graded-index fiber: Decreases the distortion induced in step-index. Density varies. It is highest at center of core and is lowest at the edge by decreasing gradually. The light rays do not travel horizontally due to changing refractive index. Advantages: Transmission distance is greater. Uses less repeaters than other cables (can run up to 50km before requiring egeneration of signal).

Has a wide bandwidth hence higher data rates. High noise immunity (electromagnetic noise cannot affect the cable). Resistant to corrosive materials (made up of glass which is more resistant to corrosive materials). Much lighter than copper cables. Disadvantages: Cable is very expensive. Installation and maintenance requires expertise and is also costly. If broken very difficult to fix. Light propagation is unidirectional. Two fibers are needed for bidirectional communication. Losses due to absorption, dispersion and scattering. How to fix the cable if it is broken?

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