In the US, Verizon Communications provides a FTTH service called FiOS to select high-ARPU Average Revenue Per User markets within its existing territory.

Their MSO competitors employ FTTN with coax using HFC. All of the major access networks use fiber for the bulk of the distance from the service provider's network to the customer. The globally dominant access network technology is EPON Ethernet Passive Optical Network.

In Europe, and among telcos in the United States, BPON ATM-based Broadband PON and GPON Gigabit PON had roots in the FSAN Full Service Access Network and ITU-T standards organizations under their control.

The choice between optical fiber and electrical or copper transmission for a particular system is made based on a number of trade-offs. Optical fiber is generally chosen for systems requiring higher bandwidth or spanning longer distances than electrical cabling can accommodate.

Thousands of electrical links would be required to replace a single high-bandwidth fiber cable. Another benefit of fibers is that even when run alongside each other for long distances, fiber cables experience effectively no crosstalk , in contrast to some types of electrical transmission lines.

Fiber can be installed in areas with high electromagnetic interference EMI , such as alongside utility lines, power lines, and railroad tracks. Nonmetallic all-dielectric cables are also ideal for areas of high lightning-strike incidence. For comparison, while single-line, voice-grade copper systems longer than a couple of kilometers require in-line signal repeaters for satisfactory performance, it is not unusual for optical systems to go over kilometers 62 mi , with no active or passive processing.

Single-mode fiber cables are commonly available in 12 km 7. Multi-mode fiber is available in lengths up to 4 km, although industrial standards only mandate 2 km unbroken runs. In short-distance and relatively low-bandwidth applications, electrical transmission is often preferred because of its.

Optical fibers are more difficult and expensive to splice than electrical conductors. And at higher powers, optical fibers are susceptible to fiber fuse , resulting in catastrophic destruction of the fiber core and damage to transmission components.

Because of these benefits of electrical transmission, optical communication is not common in short box-to-box, backplane , or chip-to-chip applications; however, optical systems on those scales have been demonstrated in the laboratory.

In certain situations, fiber may be used even for short-distance or low-bandwidth applications, due to other important features:. Optical fiber cables can be installed in buildings with the same equipment that is used to install copper and coaxial cables, with some modifications due to the small size and limited pull tension and bend radius of optical cables.

Optical cables can typically be installed in duct systems in spans of meters or more depending on the duct's condition, layout of the duct system, and installation technique. Longer cables can be coiled at an intermediate point and pulled farther into the duct system as necessary.

In order for various manufacturers to be able to develop components that function compatibly in fiber optic communication systems, a number of standards have been developed.

The International Telecommunication Union publishes several standards related to the characteristics and performance of fibers themselves, including. Other standards specify performance criteria for fiber, transmitters, and receivers to be used together in conforming systems.

Some of these standards are:. TOSLINK is the most common format for digital audio cable using plastic optical fiber to connect digital sources to digital receivers.

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Download as PDF Printable version. In other projects. Wikimedia Commons. Method of transmitting information. Main article: Optical amplifier. Main article: Wavelength-division multiplexing. Not to be confused with Optical spectrum. Because InGaAsP LEDs operate at a longer wavelength than GaAs LEDs 1.

Retrieved History of the World in 1, Objects. New York: DK and the Smithsonian. ISBN WCE, London UK. July 2, How Stuff Works. Retrieved 27 May September 28, Archived from the original on October 18, Optical Network Design and Implementation.

Cisco Press. Retrieved May 27, The Fiber Optics Association. Retrieved December 22, Alexander Graham Bell: Giving Voice To The World. Sterling Biographies.

New York: Sterling Publishing. American Journal of Science. Third Series. XX : — Bibcode : AmJS doi : S2CID also published as "Selenium and the Photophone" in Nature , September BJU International.

PMID In Bhat, K. Physics of semiconductor devices. New Delhi, India: Narosa Publishing House. Sendai New. Archived from the original on September 29, Retrieved April 5, Institute of Electrical and Electronics Engineers.

Laser: The Inventor, the Nobel Laureate, the Thirty-year Patent War reprint ed. Laser: The Inventor, the Nobel Laureate, and the Thirty-Year Patent War.

International Conference on Communications. Wavelength Division Multiplexing: A Practical Engineering Guide Wiley Series in Pure and Applied Optics. The New York Times. ISSN News release. September 29, Retrieved June 17, Nature Photonics.

March Bibcode : NaPho Journal of Lightwave Technology. Bibcode : JLwT S2CID — via IEEE Xplore. IEEE Photonics Technology Letters. Bibcode : IPTL Fiber-Optic Technologies.

Cisco Systems. Spurgeon Ethernet: The Definitive Guide 2nd ed. O'Reilly Media. Observer Online. Archived from the original on Advanced optical communication systems and networks. Boston: Artech House. OCLC Infinera Corp press release. New Scientist. Bibcode : NewSc. ARS Technica.

Nature Communications. arXiv : Bibcode : NatCo.. Fiber with a core diameter less than about ten times the wavelength of the propagating light cannot be modeled using geometric optics. Instead, it must be analyzed as an electromagnetic waveguide structure, according to Maxwell's equations as reduced to the electromagnetic wave equation.

Fiber supporting only one mode is called single-mode. Instead, especially in single-mode fibers, a significant fraction of the energy in the bound mode travels in the cladding as an evanescent wave.

The most common type of single-mode fiber has a core diameter of 8—10 micrometers and is designed for use in the near infrared.

Multi-mode fiber, by comparison, is manufactured with core diameters as small as 50 micrometers and as large as hundreds of micrometers. Some special-purpose optical fiber is constructed with a non-cylindrical core or cladding layer, usually with an elliptical or rectangular cross-section.

These include polarization-maintaining fiber used in fiber optic sensors and fiber designed to suppress whispering gallery mode propagation. Photonic-crystal fiber is made with a regular pattern of index variation often in the form of cylindrical holes that run along the length of the fiber.

Such fiber uses diffraction effects instead of or in addition to total internal reflection, to confine light to the fiber's core.

The properties of the fiber can be tailored to a wide variety of applications. Attenuation in fiber optics, also known as transmission loss, is the reduction in the intensity of the light signal as it travels through the transmission medium.

The medium is usually a fiber of silica glass [f] that confines the incident light beam within. Attenuation is an important factor limiting the transmission of a digital signal across large distances.

Thus, much research has gone into both limiting the attenuation and maximizing the amplification of the optical signal. The four orders of magnitude reduction in the attenuation of silica optical fibers over four decades was the result of constant improvement of manufacturing processes, raw material purity, preform, and fiber designs, which allowed for these fibers to approach the theoretical lower limit of attenuation.

Single-mode optical fibers can be made with extremely low loss. Corning's SMF fiber, a standard single-mode fiber for telecommunications wavelengths, has a loss of 0. It has been noted that if ocean water was as clear as fiber, one could see all the way to the bottom even of the Mariana Trench in the Pacific Ocean, a depth of 11, metres 36, ft.

Empirical research has shown that attenuation in optical fiber is caused primarily by both scattering and absorption.

The propagation of light through the core of an optical fiber is based on the total internal reflection of the lightwave. Rough and irregular surfaces, even at the molecular level, can cause light rays to be reflected in random directions. This is called diffuse reflection or scattering , and it is typically characterized by a wide variety of reflection angles.

Scattering depends on the wavelength of the light being scattered. Thus, limits to spatial scales of visibility arise, depending on the frequency of the incident light wave and the physical dimension or spatial scale of the scattering center, which is typically in the form of some specific micro-structural feature.

Since visible light has a wavelength of the order of one micrometer one-millionth of a meter scattering centers will have dimensions on a similar spatial scale. Thus, attenuation results from the incoherent scattering of light at internal surfaces and interfaces.

In poly crystalline materials such as metals and ceramics, in addition to pores, most of the internal surfaces or interfaces are in the form of grain boundaries that separate tiny regions of crystalline order. It has been shown that when the size of the scattering center or grain boundary is reduced below the size of the wavelength of the light being scattered, the scattering no longer occurs to any significant extent.

Similarly, the scattering of light in optical quality glass fiber is caused by molecular level irregularities compositional fluctuations in the glass structure. Indeed, one emerging school of thought is that glass is simply the limiting case of a polycrystalline solid. Within this framework, domains exhibiting various degrees of short-range order become the building blocks of metals as well as glasses and ceramics.

Distributed both between and within these domains are micro-structural defects that provide the most ideal locations for light scattering. This same phenomenon is seen as one of the limiting factors in the transparency of IR missile domes.

At high optical powers, scattering can also be caused by nonlinear optical processes in the fiber. In addition to light scattering, attenuation or signal loss can also occur due to selective absorption of specific wavelengths. Primary material considerations include both electrons and molecules as follows:.

The design of any optically transparent device requires the selection of materials based upon knowledge of its properties and limitations. The crystal structure absorption characteristics observed at the lower frequency regions mid- to far-IR wavelength range define the long-wavelength transparency limit of the material.

They are the result of the interactive coupling between the motions of thermally induced vibrations of the constituent atoms and molecules of the solid lattice and the incident light wave radiation. In other words, the selective absorption of IR light by a particular material occurs because the selected frequency of the light wave matches the frequency or an integer multiple of the frequency, i.

harmonic at which the particles of that material vibrate. Since different atoms and molecules have different natural frequencies of vibration, they will selectively absorb different frequencies or portions of the spectrum of IR light.

Reflection and transmission of light waves occur because the frequencies of the light waves do not match the natural resonant frequencies of vibration of the objects.

When IR light of these frequencies strikes an object, the energy is either reflected or transmitted. Attenuation over a cable run is significantly increased by the inclusion of connectors and splices. When computing the acceptable attenuation loss budget between a transmitter and a receiver one includes:.

Connectors typically introduce 0. Splices typically introduce less than 0. where the dB loss per kilometer is a function of the type of fiber and can be found in the manufacturer's specifications. For example, a typical nm single-mode fiber has a loss of 0. The calculated loss budget is used when testing to confirm that the measured loss is within the normal operating parameters.

Glass optical fibers are almost always made from silica , but some other materials, such as fluorozirconate , fluoroaluminate , and chalcogenide glasses as well as crystalline materials like sapphire , are used for longer-wavelength infrared or other specialized applications. Silica and fluoride glasses usually have refractive indices of about 1.

Typically the index difference between core and cladding is less than one percent. Plastic optical fibers POF are commonly step-index multi-mode fibers with a core diameter of 0. Silica exhibits fairly good optical transmission over a wide range of wavelengths.

In the near-infrared near IR portion of the spectrum, particularly around 1. Such low losses depend on using ultra-pure silica. A high transparency in the 1. Alternatively, a high OH concentration is better for transmission in the ultraviolet UV region.

Silica can be drawn into fibers at reasonably high temperatures and has a fairly broad glass transformation range. One other advantage is that fusion splicing and cleaving of silica fibers is relatively effective. Silica fiber also has high mechanical strength against both pulling and even bending, provided that the fiber is not too thick and that the surfaces have been well prepared during processing.

Even simple cleaving of the ends of the fiber can provide nicely flat surfaces with acceptable optical quality. Silica is also relatively chemically inert.

In particular, it is not hygroscopic does not absorb water. Silica glass can be doped with various materials.

One purpose of doping is to raise the refractive index e. with germanium dioxide GeO 2 or aluminium oxide Al 2 O 3 or to lower it e. with fluorine or boron trioxide B 2 O 3. Doping is also possible with laser-active ions for example, rare-earth-doped fibers in order to obtain active fibers to be used, for example, in fiber amplifiers or laser applications.

Both the fiber core and cladding are typically doped, so that the entire assembly core and cladding is effectively the same compound e. an aluminosilicate , germanosilicate, phosphosilicate or borosilicate glass. Particularly for active fibers, pure silica is usually not a very suitable host glass, because it exhibits a low solubility for rare-earth ions.

This can lead to quenching effects due to the clustering of dopant ions. Aluminosilicates are much more effective in this respect. Silica fiber also exhibits a high threshold for optical damage.

This property ensures a low tendency for laser-induced breakdown. This is important for fiber amplifiers when utilized for the amplification of short pulses. Because of these properties, silica fibers are the material of choice in many optical applications, such as communications except for very short distances with plastic optical fiber , fiber lasers, fiber amplifiers, and fiber-optic sensors.

Large efforts put forth in the development of various types of silica fibers have further increased the performance of such fibers over other materials. Fluoride glass is a class of non-oxide optical quality glasses composed of fluorides of various metals. Because of the low viscosity of these glasses, it is very difficult to completely avoid crystallization while processing it through the glass transition or drawing the fiber from the melt.

Thus, although heavy metal fluoride glasses HMFG exhibit very low optical attenuation, they are not only difficult to manufacture, but are quite fragile, and have poor resistance to moisture and other environmental attacks.

Such low losses were never realized in practice, and the fragility and high cost of fluoride fibers made them less than ideal as primary candidates. Fluoride fibers are used in mid- IR spectroscopy , fiber optic sensors , thermometry , and imaging. Fluoride fibers can be used for guided lightwave transmission in media such as YAG yttrium aluminium garnet lasers at 2.

ophthalmology and dentistry. An example of a heavy metal fluoride glass is the ZBLAN glass group, composed of zirconium , barium , lanthanum , aluminium , and sodium fluorides.

Their main technological application is as optical waveguides in both planar and fiber forms. They are advantageous especially in the mid-infrared 2,—5, nm range. Phosphate glass is a class of optical glasses composed of metaphosphates of various metals.

Instead of the SiO 4 tetrahedra observed in silicate glasses, the building block for this glass phosphorus pentoxide P 2 O 5 , which crystallizes in at least four different forms. The most familiar polymorph is the cagelike structure of P 4 O Phosphate glasses can be advantageous over silica glasses for optical fibers with a high concentration of doping rare-earth ions.

A mix of fluoride glass and phosphate glass is fluorophosphate glass. The chalcogens —the elements in group 16 of the periodic table —particularly sulfur S , selenium Se and tellurium Te —react with more electropositive elements, such as silver , to form chalcogenides.

These are extremely versatile compounds, in that they can be crystalline or amorphous, metallic or semiconducting, and conductors of ions or electrons. chalcogenide glass can be used to make fibers for far infrared transmission.

Standard optical fibers are made by first constructing a large-diameter preform with a carefully controlled refractive index profile, and then pulling the preform to form the long, thin optical fiber. The preform is commonly made by three chemical vapor deposition methods: inside vapor deposition , outside vapor deposition , and vapor axial deposition.

With inside vapor deposition , the preform starts as a hollow glass tube approximately 40 centimeters 16 in long, which is placed horizontally and rotated slowly on a lathe. Gases such as silicon tetrachloride SiCl 4 or germanium tetrachloride GeCl 4 are injected with oxygen in the end of the tube.

The gases are then heated by means of an external hydrogen burner, bringing the temperature of the gas up to 1, K 1, °C, 3, °F , where the tetrachlorides react with oxygen to produce silica or germanium dioxide particles.

When the reaction conditions are chosen to allow this reaction to occur in the gas phase throughout the tube volume, in contrast to earlier techniques where the reaction occurred only on the glass surface, this technique is called modified chemical vapor deposition.

The oxide particles then agglomerate to form large particle chains, which subsequently deposit on the walls of the tube as soot. The deposition is due to the large difference in temperature between the gas core and the wall causing the gas to push the particles outward in a process known as thermophoresis.

The torch is then traversed up and down the length of the tube to deposit the material evenly. After the torch has reached the end of the tube, it is then brought back to the beginning of the tube and the deposited particles are then melted to form a solid layer.

This process is repeated until a sufficient amount of material has been deposited. For each layer the composition can be modified by varying the gas composition, resulting in precise control of the finished fiber's optical properties.

In outside vapor deposition or vapor axial deposition, the glass is formed by flame hydrolysis , a reaction in which silicon tetrachloride and germanium tetrachloride are oxidized by reaction with water in an oxyhydrogen flame.

In outside vapor deposition, the glass is deposited onto a solid rod, which is removed before further processing. In vapor axial deposition, a short seed rod is used, and a porous preform, whose length is not limited by the size of the source rod, is built up on its end. The porous preform is consolidated into a transparent, solid preform by heating to about 1, K 1, °C, 2, °F.

Typical communications fiber uses a circular preform. For some applications such as double-clad fibers another form is preferred. Because of the surface tension, the shape is smoothed during the drawing process, and the shape of the resulting fiber does not reproduce the sharp edges of the preform.

Nevertheless, careful polishing of the preform is important, since any defects of the preform surface affect the optical and mechanical properties of the resulting fiber. The preform, regardless of construction, is placed in a device known as a drawing tower , where the preform tip is heated and the optical fiber is pulled out as a string.

The tension on the fiber can be controlled to maintain the desired fiber thickness. The light is guided down the core of the fiber by an optical cladding with a lower refractive index that traps light in the core through total internal reflection.

For some types of fiber, the cladding is made of glass and is drawn along with the core from a preform with radially varying index of refraction. For other types of fiber, the cladding made of plastic and is applied like a coating see below.

The cladding is coated by a buffer that protects it from moisture and physical damage. The coatings protect the very delicate strands of glass fiber—about the size of a human hair—and allow it to survive the rigors of manufacturing, proof testing, cabling, and installation.

The buffer coating must be stripped off the fiber for termination or splicing. An inner primary coating is designed to act as a shock absorber to minimize attenuation caused by microbending.

An outer secondary coating protects the primary coating against mechanical damage and acts as a barrier to lateral forces, and may be colored to differentiate strands in bundled cable constructions.

These fiber optic coating layers are applied during the fiber draw, at speeds approaching kilometers per hour 60 mph. Fiber optic coatings are applied using one of two methods: wet-on-dry and wet-on-wet. In wet-on-dry, the fiber passes through a primary coating application, which is then UV cured, then through the secondary coating application, which is subsequently cured.

In wet-on-wet, the fiber passes through both the primary and secondary coating applications, then goes to UV curing. The thickness of the coating is taken into account when calculating the stress that the fiber experiences under different bend configurations. In a two-point bend configuration, a coated fiber is bent in a U-shape and placed between the grooves of two faceplates, which are brought together until the fiber breaks.

where d is the distance between the faceplates. The coefficient 1. Fiber optic coatings protect the glass fibers from scratches that could lead to strength degradation. The combination of moisture and scratches accelerates the aging and deterioration of fiber strength.

When fiber is subjected to low stresses over a long period, fiber fatigue can occur. Over time or in extreme conditions, these factors combine to cause microscopic flaws in the glass fiber to propagate, which can ultimately result in fiber failure.

Three key characteristics of fiber optic waveguides can be affected by environmental conditions: strength, attenuation, and resistance to losses caused by microbending.

On the inside, coatings ensure the reliability of the signal being carried and help minimize attenuation due to microbending. In practical fibers, the cladding is usually coated with a tough resin and features an additional buffer layer, which may be further surrounded by a jacket layer, usually plastic.

These layers add strength to the fiber but do not affect its optical properties. Rigid fiber assemblies sometimes put light-absorbing glass between the fibers, to prevent light that leaks out of one fiber from entering another.

This reduces crosstalk between the fibers, or reduces flare in fiber bundle imaging applications. Modern cables come in a wide variety of sheathings and armor, designed for applications such as direct burial in trenches, high voltage isolation, dual use as power lines, [85] [ failed verification ] installation in conduit, lashing to aerial telephone poles, submarine installation , and insertion in paved streets.

Some fiber optic cable versions are reinforced with aramid yarns or glass yarns as an intermediary strength member. In commercial terms, usage of the glass yarns are more cost-effective with no loss of mechanical durability. Glass yarns also protect the cable core against rodents and termites.

Fiber cable can be very flexible, but traditional fiber's loss increases greatly if the fiber is bent with a radius smaller than around 30 mm. This creates a problem when the cable is bent around corners.

Bendable fibers , targeted toward easier installation in home environments, have been standardized as ITU-T G. This type of fiber can be bent with a radius as low as 7. Even more bendable fibers have been developed. Another important feature of cable is cable's ability to withstand tension which determines how much force can be applied to the cable during installation.

Optical fibers are connected to terminal equipment by optical fiber connectors. These connectors are usually of a standard type such as FC , SC , ST , LC , MTRJ , MPO or SMA. Optical fibers may be connected by connectors, or permanently by splicing , that is, joining two fibers together to form a continuous optical waveguide.

The generally accepted splicing method is arc fusion splicing , which melts the fiber ends together with an electric arc.

Fusion splicing is done with a specialized instrument. The fiber ends are first stripped of their protective polymer coating as well as the more sturdy outer jacket, if present.

The ends are cleaved cut with a precision cleaver to make them perpendicular, and are placed into special holders in the fusion splicer. The splice is usually inspected via a magnified viewing screen to check the cleaves before and after the splice.

The splicer uses small motors to align the end faces together, and emits a small spark between electrodes at the gap to burn off dust and moisture. Then the splicer generates a larger spark that raises the temperature above the melting point of the glass, fusing the ends permanently.

The location and energy of the spark is carefully controlled so that the molten core and cladding do not mix, and this minimizes optical loss.

A splice loss estimate is measured by the splicer, by directing light through the cladding on one side and measuring the light leaking from the cladding on the other side. A splice loss under 0. The complexity of this process makes fiber splicing much more difficult than splicing copper wire.

Mechanical fiber splices are designed to be quicker and easier to install, but there is still the need for stripping, careful cleaning, and precision cleaving. The fiber ends are aligned and held together by a precision-made sleeve, often using a clear index-matching gel that enhances the transmission of light across the joint.

Such joints typically have a higher optical loss and are less robust than fusion splices, especially if the gel is used. All splicing techniques involve installing an enclosure that protects the splice. Fibers are terminated in connectors that hold the fiber end precisely and securely.

A fiber-optic connector is a rigid cylindrical barrel surrounded by a sleeve that holds the barrel in its mating socket. The mating mechanism can be push and click , turn and latch bayonet mount , or screw-in threaded. The barrel is typically free to move within the sleeve and may have a key that prevents the barrel and fiber from rotating as the connectors are mated.

A typical connector is installed by preparing the fiber end and inserting it into the rear of the connector body. Quick-set adhesive is usually used to hold the fiber securely, and a strain relief is secured to the rear.

Once the adhesive sets, the fiber's end is polished to a mirror finish. Various polish profiles are used, depending on the type of fiber and the application. For single-mode fiber, fiber ends are typically polished with a slight curvature that makes the mated connectors touch only at their cores.

This is called a physical contact PC polish. The curved surface may be polished at an angle, to make an angled physical contact APC connection. Such connections have higher loss than PC connections but greatly reduced back reflection, because light that reflects from the angled surface leaks out of the fiber core.

Make sure your locations settings are on when you make this search. Or, you can include your city or town as a keyword in your search. You should base your decision on several factors, including the following:. During the installation, the technician will install the optical network terminal either outside or inside of the building.

When it comes to the router your business uses to connect to your fiber internet service, you have two options:. Check the label on your router for your internet network name and password. Here are some of the ways fiber internet can do this for your business.

Running a business demands fast internet speeds to stay efficient. The goal is to reduce the amount of time you spend waiting for uploads and downloads to finish, and fiber optic internet can help you achieve that goal.

With the incredible speeds that fiber internet offers, you and your employees can share data, transfer files and receive essential information faster than ever before.

So, remember the bottom line when it comes to fiber optic vs. cable internet speed — fiber is always going to win the speed test. Faster internet speeds open the door for you to use internet services to their fullest potential for your business. The cloud connects companies to their clients and even helps increase efficiency within the workplace thanks to the ease of sharing and collaborating that the cloud offers.

But full utilization of the cloud demands a strong internet connection with fast speeds, especially when using more intensive cloud programs. You can take full advantage of the cloud for your business when you have fiber internet speeds to back you up.

This will help you experience less downtime in your business. Bandwidth refers to how many people can use the internet at the same time without a reduction in speeds or internet quality. Fiber optic internet has a much wider bandwidth than regular cable internet.

Fiber internet can give you increased online security for your business. Hackers can easily access an existing cable internet network through network tapping and other methods.

This differs from fiber internet since the only way to stop a fiber internet line is to literally cut the fibers, which are buried underground. Security cameras and live-streamed footage all benefit from the high speeds.

You can get a much clearer image from your security cameras sent straight to your smartphone thanks to fiber optic internet. When other businesses have to pause operations to wait for their internet to return or handle a security breach, you can continue your work like normal.

Plus, with the ability to have many employees using the internet at the same time, your company can increase its efficiency to reach more customers and generate more sales. The cost of fiber internet will quickly pay for itself.

Fiber optic internet is the way of the future. Kirbtech is ready to provide the services you need for fiber optic network installation and setup. Contact us online today for more information on how we can help. Let us be your managed IT solution for your growing business!

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