COMMUNICATION TECHNOLOGIES

Archive for the ‘Fiber Communication’ Category

The active medium is created in fiber during fabrication itself by adding rare earth elements such as Erbium, Yttrium, neodymium or Praseodymium. The host fiber material can be standard silica, a fluoride – based glass, or a multicompositional glass. The operating regions of these devices depend on the host material and doping elements. The most popular material for long telecommunication applications is a silica fiber doped with erbium, which is known as erbium-doped fiber amplifier. In some cases, yttrium is added to increase the pumping efficiency and the amplifier gain. The operation of an EFDA by itself normally is limited to the 1530 to 1560 nm region. When combined with a Raman fiber amplifier that boosts the gain at higher wavelength.

To know how an EDFA works we need to understand the energy level structure of erbium. The erbium atoms in silica are actual erbium ions which are erbium atoms that have lost three of the outer electrons. The transition of the outer electron in this ions to higher energy states is known as the rising the ions to higher energy level. Rising ions to higher energy level is achieved by two pump level namely metastable and higher level. Get info about airsoft guns here. The metastable means that the life times for the transition from this state to ground state are very long compared with the life times to the states that led to this level. The metastable state, the pump, the ground state are actually bands of closely spaced energy level that form the manifold which is known as star splitting.

Scattering losses in glass arise due to following factors namely microscopic variations in the material density, compositional fluctuations, structural inhomogeneities and structural defects occurring during fiber fabrication. As glass is composed by randomly connected network of molecules and several oxides, these are the major cause of compositional structure fluctuation. The first two gives rise to refractive-index variations within the glass over distance that are small compared with wavelength. These index variations cause scattering of light which is named as Raleigh scattering. Now let us discuss about the different types of scattering losses. There are two types of scattering losses namely linear scattering and non linear scattering.

Linear scattering consist of two types of scattering namely Rayleigh scattering and Mie scattering. Linear scattering transfers linearly the optical power in one propagation mode to different mode. These losses will occur in leaky mode or radiation mode. It will not continue to propagate within the core of the fiber. Scattering loses will be more in multimode fibers due to higher dopent concentration and greater compositional fluctuations. All linear processes there is no change of frequency on scattering. Now let us discuss about nonlinear scattering. This nonlinear scattering causes the optical power from one mode to be transferred in either the forward or backward direction to the same, or other modes, at a different frequency. It depends critically upon the optical power density within the fiber and hence only becomes significant above threshold power levels.

The cable television system is used for distributing high quality TV signals to a very large number of users. This system feeds increased TV programs to subscribers who pay a fee for this service. The cable system may have many more active channels than a receiver can directly select. This required use of a special type active converter in the head end. The main signal source of cable TV is from various satellites. High gain parabolic dish antennas are used for receiving satellite signals. The down-link signals from most communication satellites are in C band frequency range. The signals received by dish antennas are first converted into a lower frequency by using low noise block converter. The VHF and UHF terrestrial broadcast signals are received conventional antennas, mounted on a high rise building.

Events like local sports and cultural programs can be distributed by using cable television network. Programs like popular movies, plays, songs earlier recorded are also distributed through cable network. The signals from, various TV channels are applied to the combining network. The signal received by the dish antenna is converted into low frequency signals by using LNB converter. Similarly the signal received by UHF antennas is converted into low frequency signals by using translator. The combining network combines all the signals, and is allotted separate carrier frequency for each channel. The outputs from combining network are fed to a number of trunk cables through a broadband distribution amplifier.

The optical fibers are made up of silica glass or silicon dioxide. This material is very easily available from the earth. Hence optical fibers are very low cost and cheap compared to metallic wires. At many places electrical and electromagnetic interference is the major problem. Fiber optic communication is most suitable at such places since it does not involve an electric voltage or current. Since optical fibers have very large bandwidth, large number of signals can be transmitted through very small size of cables. Thus fibre optic cables offer small size and weight. Basically light waves are also electromagnetic waves with very high frequencies in the range of 3 x 10/6 GHZ. Hence very large bandwidths are possible in fiber optic communication.

This increases number of channels and frequency bands. Thus the information carrying capacity of optical fibers is much higher than conventional microwave radio systems. The transmission losses in optical fibers are very small. The optical fibers offer ruggedness and flexibility. Presently optical fibers are replacing metallic wires in many applications because of all the above advantages. When alight enters in the fiber, major part of the light propagates through it. Some portion of the fiber escapes out of the fiber and some part s lost due to internal absorption. The light which is contained within the fiber is guided to the far end. This is possible because the light is totally reflected again within the fiber.

Wider Bandwidth and Greater Information Capacity:

Optical fibers have greater information capacity than metallic cables because of the inherently wider bandwidths available with optical frequencies. Optical fibers are available with bandwidths up to a several thousand gigahertz. The primary electrical constants in metallic cables cause them to act like low-pass filters, which limit their transmission frequencies, bandwidth, bit rate, and information-carrying capacity. Modern optical fiber communications systems are capable of transmitting several gigabits per second over hundreds of miles, allowing literally millions of individual voice and data channels to be combined propagated over one optical fiber cable.

Immunity to Crosstalk and Static Interference:

Optical fiber cables are immune to crosstalk because glass and plastic fibers are nonconductors of electrical current. Therefore, fiber cables are not surrounded by a changing magnetic field, which is the primary cause of cross between metallic conductors located physically close to each other. Because optical fiber cables are nonconductors of electrical current, they are immune to static noise due to electromagnetic interference caused by lightning, electric motors, relays, fluorescent lights, and other electrical noise sources. Fiber cables do not radiate electromagnetic energy.

Environmental Immunity and Safety Convenience:

Optical fiber cables are most resistant to environmental extremes than metallic cables. Optical cables also operate over a eider temperature range and are less affected by corrosive liquids and gases. Optical fiber cables are safer and easier to install and maintain than metallic cables. Because glass and plastic fibers are nonconductors, there are no electrical currents or voltages associated with them. Optical fibers can be used around volatile liquids and gasses with out worrying about their causing explosions or fires.

Lower Transmission Loss and Security:

Optical fibers have considerably less signal loss than their metallic counterparts. Optical fibers are currently being manufactured with as little as a few-tenths-of-a-decibel loss per kilometer. Consequently, optical regenerators and amplifiers can be spaced considerably farther apart than with metallic transmission lines. Optical fiber cables are more secure than metallic cables. It is virtually impossible to tap into a fiber cable without the user’s knowledge; optical cables cannot be detected with metal doctors unless they are reinforced with steel for strength.

Absorption Losses:

Absorption losses in optical fibers analogous to power dissipation in copper cables; impurities in the fiber absorb the light and convert it to heat. The ultrapure glass used to manufacture optical fibers is approximately 99.9999 %pure. Still absorption losses between 1 dB/km and 1000 dB/km are typical. Essentially, there are three factors that contribute to the absorption losses in optical fibers: ultraviolet absorption, infrared absorption, and ion resonance absorption.

Material, or Rayleigh, Scattering Losses:

During manufacturing, glass is drawn into long fibers of very small diameter. During this process, the glass is in a plastic state. The tension applied to the glasses causes the cooling glass to develop permanent submicroscopic irregularities. When light rays propagating down a fiber strike one of these impurities, they are diffracted. Diffraction causes the light to disperse or spread out in many directions. Some of the diffracted light continues down the fiber, and some of it escapes through the cladding. The light rays’ escapes represent loss in light power.

Chromatic, or Wavelength, Dispersion:

Light-emitting diodes emit light containing many wavelengths. Each wavelength within the composite light signal travels at a different velocity when propagating through glass. Consequently, light rays that are simultaneously emitted from an LED and propagated down an optical fiber do not arrive at the far end of the fiber at the same time, resulting in an impairment called chromatic distortion. Chromatic distortion can be eliminated by using a monochromic light source such as an injection laser diode. Chromatic distortion occurs only in fibers with a single mode of transmission.

Radiation Losses:

Radiation losses are caused mainly by small bends and kinks in the fiber. Essentially, there are two types of bends: microbends and constant-radius bends. Microbending occurs as a result of differences in thermal contraction rates between the core and the cladding material. A microbend is a miniature bend or geometric imperfection along the axis of the fiber and represents a discontinuity in the fiber where Rayleigh scattering can occur. Microbending losses generally contribute less than 20% of the total attenuation in a fiber constant-radius bends are caused by excessive pressure and tension and generally occur when fibers are bent during handling or installation.

Single-mode Step-index Optical Fiber:

Single-mode step-index fibers are the dominant fibers used in today’s telecommunications and data networking industries. A single-mode step-index fiber has a central core that is significantly smaller in diameter than any of the multimode cables. In fact, the diameter is sufficiently small that there is essentially only one path that light may take as it propagates down the cable. In the simplest form of single-mode step-index fiber, the outside cladding is simply air. Consequently, a single-mode step-index fiber has a wide external acceptance angle, which makes it relatively easy to couple light into the cable from an external source. However, this type of fiber is very weak and difficult to splice or terminate.

Multi-mode Step-index Optical Fiber:

Multimode step-index fibers are similar to the single-mode step-index fibers except the center core is much larger with the multimode configuration. This type of fiber has large light-to fiber aperture and, consequently, allows more external light to enter the cable. The light rays that strike the core/ cladding interface at an angle greater than the critical angle are propagated down the core in zigzag fashion, continuously reflecting off the interface boundary. Light rays that strike the core/ cladding interface at an angle less than the critical angle enter the cladding and are lost. As a result, all light rays do not follow the same path and, consequently, o not take the same amount of time to travel the length of the cable.

Multimode Graded-index Optical Fiber:

Multimode graded-index fibers are characterized by a central core with a non uniform refractive index. Thus, the cable’s density is maximum the center and decreases gradually toward the outer edge. Light rays propagate down this type of fiber through refraction rather than reflection. As a light propagates diagonally across the core toward the center, it is continually intersecting a less dense to more dense interface. Consequently, the light rays are constantly being refracted, which results in a continuous bending of the light rays. Light enters the fiber at many different angles. As the light rays propagate down the fiber, the rays traveling in the outermost area of the fiber travel a greater distance than the rays traveling near the center.