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absorption
The propagation of light through the core of an optical fiber is based on 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 wide variety of reflection angles.

Light 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 recently 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. This phenomenon has given rise to the production of transparent ceramic materials.

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 a 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 both metals and alloys, 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.[60]

At high optical powers, scattering can also be caused by nonlinear optical processes in the fiber.

Scattering

In addition to light scattering, attenuation or signal loss can also occur due to selective absorption of specific wavelengths, in a manner similar to that responsible for the appearance of color. Primary material considerations include both electrons and molecules as follows:

At the electronic level, it depends on whether the electron orbitals are spaced (or "quantized") such that they can absorb a quantum of light (or photon) of a specific wavelength or frequency in the ultraviolet (UV) or visible ranges. This is what gives rise to color.

At the atomic or molecular level, it depends on the frequencies of atomic or molecular vibrations or chemical bonds, how close-packed its atoms or molecules are, and whether or not the atoms or molecules exhibit long-range order. These factors will determine the capacity of the material transmitting longer wavelengths in the infrared (IR), far IR, radio and microwave ranges.

The design of any optically transparent device requires the selection of materials based upon knowledge of its properties and limitations. The Lattice absorption characteristics observed at the lower frequency regions (mid IR to far-infrared 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. Hence, all materials are bounded by limiting regions of absorption caused by atomic and molecular vibrations (bond-stretching)in the far-infrared (>10 µm).

Thus, multi-phonon absorption occurs when two or more phonons simultaneously interact to produce electric dipole moments with which the incident radiation may couple. These dipoles can absorb energy from the incident radiation, reaching a maximum coupling with the radiation when the frequency is equal to the fundamental vibrational mode of the molecular dipole (e.g. Si-O bond) in the far-infrared, or one of its harmonics.

The selective absorption of infrared (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) 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 infrared (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.

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Attenuation in fiber optics, also known as transmission loss, is the reduction in intensity of the light beam (or signal) as it travels through the transmission medium. Attenuation coefficients in fiber optics usually use units of dB/km through the medium due to the relatively high quality of transparency of modern optical transmission media. The medium is usually a fiber of silica glass that confines the incident light beam to the inside. 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. Empirical research has shown that attenuation in optical fiber is caused primarily by both scattering and absorption. Single-mode optical fibers can be made with extremely low loss. Corning's SMF-28 fiber, a standard single-mode fiber for telecommunications wavelengths, has a loss of 0.17 dB/km at 1550 nm.[58] For example, an 8 km length of SMF-28 transmits nearly 75% of light at 1,550 nm. 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 Marianas Trench in the Pacific Ocean, a depth of 36,000 feet.[59]

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Single Mode fiber optic cable has a small diametral core that allows only one mode of light to propagate. Because of this, the number of light reflections created as the light passes through the core decreases, lowering attenuation and creating the ability for the signal to travel further. This application is typically used in long distance, higher bandwidth runs by Telcos, CATV companies, and Colleges and Universities.

Left: Single Mode fiber is usually 9/125 in construction. This means that the core to cladding diameter ratio is 9 microns to 125 microns.

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Multimode fiber optic cable has a large diametral core that allows multiple modes of light to propagate. Because of this, the number of light reflections created as the light passes through the core increases, creating the ability for more data to pass through at a given time. Because of the high dispersion and attenuation rate with this type of fiber, the quality of the signal is reduced over long distances. This application is typically used for short distance, data and audio/video applications in LANs. RF broadband signals, such as what cable companies commonly use, cannot be transmitted over multimode fiber.

Above: Multimode fiber is usually 50/125 and 62.5/125 in construction. This means that the core to cladding diameter ratio is 50 microns to 125 microns and 62.5 microns to 125 microns.

What’s Happening Inside The Multimode Fiber

Step-Index Multimode Fiber

Due to its large core, some of the light rays that make up the digital pulse may travel a direct route, whereas others zigzag as they bounce off the cladding. These alternate paths cause the different groups of light rays, referred to as modes, to arrive separately at the receiving point. The pulse, an aggregate of different modes, begins to spread out, losing its well-defined shape. The need to leave spacing between pulses to prevent overlapping limits the amount of information that can be sent. This type of fiber is best suited for transmission over short distances.

Graded-Index Multimode Fiber

Contains a core in which the refractive index diminishes gradually from the center axis out toward the cladding. The higher refractive index at the center makes the light rays moving down the axis advance more slowly than those near the cladding. Due to the graded index, light in the core curves helically rather than zigzag off the cladding, reducing its travel distance. The shortened path and the higher speed allow light at the periphery to arrive at a receiver at about the same time as the slow but straight rays in the core axis. The result: digital pulse suffers less dispersion. This type of fiber is best suited for local-area networks.

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Fiber optic cables transmit data through very small cores at the speed of light. Significantly different from copper cables, fiber optic cables offer high bandwidths and low losses, which allow high data-transmission rates over long distances. Light propagates throughout the fiber cables according to the principle of total internal reflection.

There are three common types of fiber optic cables: single-mode, multimode, and graded-index. Each has its advantages and disadvantages. There also are several different designs of fiber optic cables, each made for different applications. In addition, new fiber optic cables with different core and cladding designs have been emerging; these are faster and can carry more modes. While fiber optic cable are used mostly in communication systems, they also have established medical, military, scanning, imaging, and sensing applications. They are also used in optical fiber devices and fiber optic lighting.

Fiber optic cable is a filament of transparent material used to transmit light, as shown in Figure 1.2. Virtually all fiber optic cables share the same fundamental structure. The centre of the cable is referred to as the core. It has a highter refractive index than the cladding, which surrounds the core. The contact surface between the core and the cladding creates an interface surface that guides the light; the difference between the refractive index of the core and cladding is what causes the mirror like interface surface, which guides light along the core. Light bounces through the core from one end to the other according to the principle of total internal reflection, as explained by the laws of light. The cladding is then covered with a protective plastic or PVC jacket. The diameters of the core,cladding, and jacket can vary widely; for a single fiber optic cable can have core, cladding, and jacket diameters of 9, 125, and 250 um, respectively.

Figure 1.3 shows the structure of a typical fiber optic cable. The cores of most fiber optic cables are made from pure glass, while the cladding are made from less pure glass. Glass fiber optic cable has the lowest attenuation over long distances but comes at the highest cost. A pure glass fiber optic cable has a glass cladding. Fiber optic cable cores and claddings may be made from plastic, which is not as clear as glass but is more flexible and easier to handle. Compared with other fiber cables, Plastic Optical Fiber Cable is limited in power loss and bandwidth. However, they are more affordable, easy to use, and attractive in applications where high bandwidth or low loss is not a concern. A few glass fiber cable cores are clad with plastic. Their performance, though not as good as all-glass fiber cables, is quite respectable.

The jacket is made from polymmer (PVC, plastic, etc.) to protect the core and the cladding from mechanical damage. The jackets has several major attributes, including bending ability, abrasion resistance, static fatigue protection, toughness, moisture resistance, and the ability to be stripped. Fiber optic cable jackets are made in different colours for colour-coding identification. Some optical fibers are coated with a copper-based alloy that allows operation at up to 700 and 500℃ for short and long periods, respectively.

Fiberstore is a leading supplier of Bulk Fiber Optic Cable and components into the umbilical and towed array products for the oil & gas sector. The key technology for these products is Fiberstore’s patented stainless steel fiber optic tube technology which packages the optical fiber in the best possible way resulting in a robust, compact product that is suitable for the high pressure of the subsea environment. Fiberstore will customize the design to meet your needs to include different fiber counts, fiber types, metal types, tube sizes, belting materials, armor type, armor size, armor count, encapsulation types, color, print, packaging and length.

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Copper communication works by sending electrical pulses through a copper wire. The strength of the signal determines how much of it will be retained by the time it reaches its destination. At the destination (e.g. the router), the wire’s electromagnetic field is constantly monitored for changes. As the field gets stronger, the destination registers a ‘1’  (logic high). If it dips below a certain measurement, a ‘0’ (logic low) is registered.
BANDWIDTH
Fiber provides more bandwidth than copper and has standardized performance up to 10 Gbps and beyond, something that it is impossible to achieve when using copper. More bandwidth means that fiber can carry more information with far greater efficiency than copper wire.
RANGE OF TRANSMISSION
Since data travels in the form of light (in total internal reflections, the loss of quality is negligible) in fiber-optic cables, very little signal loss occurs during transmission and data can move at higher speeds and greater distances.
SUSCEPTIBLE OF INTERFERENCE

Fiber-optic cable is also much less susceptible to noise and electromagnetic interference than copper wire. For example, over a distance of two kilometers, copper wire would experience a great deal of degradation in quality, while there would be virtually none over the same distance using fiber-optic cable. It is so efficient,  in fact, that roughly 99.7% of the signal reaches the router in most cases.

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As the name suggests, fibre optic technology uses pulses of light to carry data along strands of glass or plastic.

It's the technology of choice for the government's National Broadband Network (NBN), which promises to deliver speeds of at least 100Mbps.

When we're talking about 'speed' were actually talking about throughput (or capacity) — the amount of data you can transfer per unit time, says Associate Professor Robert Malaney from the University of New South Wales, School of Electrical Engineering and Telecommunications.

And fibre optics can definitely transfer more data at higher throughput over longer distances than copper wire. For example, a local area network using modern copper lines can carry 3000 telephone calls all at once, while a similar system using fibre optics can carry over 31,000.

So what gives it the technical edge over copper wires?

Traditional copper wires transmit electrical currents, while fibre optic technology sends pulses of light generated by a light emitting diode or laser along optical fibres.

"In both cases you're detecting changes in energy, and that's how you encode data.

"With copper wires you're looking at changes in the electromagnetic field, the intensity of that field and perhaps the phase of the wave being sent down a wire.

"With fibre optics, a transmitter converts electronic information into pulses of light — a pulse equates to a one, while no pulse is zero. When the signal reaches the other end, an optical receiver converts the light signal back into electronic information," explains Malaney.

The throughput of the data is determined by the frequency range that a cable will carry — the higher the frequency range, the greater the bandwidth and the more data that can be put through per unit time.

And this is the key difference — fibre optic cables have much higher bandwidths than copper cables.

"Optical fibre can carry much higher frequency ranges — note that light is a very high frequency signal — while copper wire attenuates or loses signal strength at higher frequencies," says Malaney.

Also, fibre optic technology is far less susceptible to noise and electromagnetic interference than electricity along a copper wire.

"You can send the signal for over 200 kilometres without any real loss of quality while a copper cable signal suffers a lot of degradation over that distance," says Malaney.

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Bend losses are a frequently encountered problem in fiber optics: optical fibers exhibit additional propagation losses when they are bent. Typically, these losses rise very quickly once a certain critical bend radius is reached. This critical radius can be very small (a few millimeters) for fibers with robust guiding characteristics (high numerical aperture), whereas it is much larger (often tens of centimeters) for single-mode fibers with large mode areas.

Generally, bend losses increase strongly for longer wavelengths, although the wavelength dependence is often strongly oscillatory due to interference with light reflected at the cladding/coating boundary, and/or at the outer coating surface. The increasing bend losses at longer wavelengths often limit the usable wavelength range of a single-mode fiber. For example, a fiber with a single-mode cut-off wavelength of 800 nm, as is suitable for operation in the 1-μm region, may not be usable at 1500 nm, because they would exhibit excessive bend losses. Note that even without macroscopic bending of a fiber, bend losses can occur as a result of microbends, i.e., microscopic disturbances in the fiber, which can be caused by imperfect fabrication conditions.

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Refraction is the bending of the path of a light wave as it passes across the boundary separating two media. Refraction is caused by the change in speed experienced by a wave when it changes medium. A comparison of the angle of refraction to the angle of incidence provides a good measure of the refractive ability of any given boundary. The more that light refracts, the bigger the difference between these two angles. This relationship between the angles of incidence and refraction and the indices of refraction of the two media is known as Snell's Law. Snell's law applies to the refraction of light in any situation, regardless of what the two media are. This study of the refraction of light as it crosses from one material into a second material yields a general relationship between the sines of the angle of incidence and the angle of refraction. This general relationship is expressed by the following equation:

where

Θ i ("theta i") = angle of incidence
Θr ("theta r") = angle of refraction
n i = index of refraction of the incident medium
n r = index of refraction of the refractive medium

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The acceptance angle of an optical fiber is defined based on a purely geometrical consideration (ray optics): it is the maximum angle of a ray (against the fiber axis) hitting the fiber core which allows the incident light to be guided by the core. The sine of that acceptable angle is called the numerical aperture, and it is essentially determined by the refractive index contrast between core and cladding of the fiber, assuming that the incident beam comes from air or vacuum

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Fiber optics refers to the technology of transmitting light down thin strands of highly transparent material, usually glass but sometimes plastic. Fiber optics is used in communications, lighting, medicine, optical inspections etc. sensors. Fiber Optics in communications works by sending light signals down hair thin strands of glass fiber (and sometimes plastic fiber.) An optical fiber is a long thin strand of glass. A fiber-optic cable is made up of incredibly thin strands of glass known as optical fibers; one cable can have as few as two strands or as many as several hundred. Light from a laser enters at one end of the fiber, striking the surface of the glass at an angle greater than the critical angle. An optical fiber is a long thin strand of glass. A fiber-optic cable is made up of incredibly thin strands of glass known as optical fibers; one cable can have as few as two strands or as many as several hundred. Light from a laser enters at one end of the fiber, striking the surface of the glass at an angle greater than the critical angle.

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Total internal reflection is a phenomenon that happens when a propagating wave strikes a medium boundary at an angle larger than a particular critical angle with respect to the normal to the surface . If the refractive index is lower on the other side of the boundary and the incident angle is greater than the critical angle, the wave cannot pass through and is entirely reflected. The critical angle is the angle of incidence above which the total internal reflection occurs.

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Reduces loss of light from the core into the surrounding air

Reduces scattering loss at the surface of the core

Protects the fiber from absorbing surface contaminants

Adds mechanical strength

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Core and cladding are typically made of glass or plastic. Most important specification of the core is the index of refraction which is the value for light bending passing through the material and for the speed of that light could travel through material with. Cladding is having lower refractive index than the core. It allows light to stay inside the fiber and not escape into cladding, since it will be reflected.

Coating is simply a protective layer that is protecting core and cladding from the fracture.

Whether the fiber is single mode or multi-mode is defined by the thickness of the fiber optic stand. Thin core would support only single pathway for the light. Thicker core means more angles for input signal, thus being able to transmit data in multiple paths and modes.

Single mode fiber have some additional limitations due to nature of the cable. The concentrated laser is required, which is able to send signal precisely through such thin media. Every connection require precision to connect small diameter fibers, and hold them in position which affects cost of installation. Single fibers are mostly used for backbone and other long-distance parts of the application.

On the other hand, multi-mode fiber is having larger diameter core, which allows using cheaper lasers and LEDs as a source. Having larger cores simplifies the task of connecting fibers. All these points facilitate manufacturing process and reduce production costs. However, cheaper components have negative effect on both transmission distance and bandwidth. That makes multi-mode solutions better suited for the short connections in the network.

The materials used for fibers vary depending on the type of fiber. For smallest diameter single-mode cables the step-index glass is used. It is typically narrow core (8.3 microns) with the 125 microns cladding, which makes cable size manageable. For the multi-mode fiber there are two options for material for the core. First, is the graded index glass of 50 or 62.5 microns core. It is designed for the laser-driven 1-100 Gbps applications. Second, is the multi-mode plastic with diameter from 50 microns and up. This is the lowest quality fiber and typically for short distances or visible light decorations.

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http://www.ad-net.com.tw/structure-of-fiber-optics-cable/
https://en.wikipedia.org/wiki/Optical_fiber#Mechanisms_of_attenuation
http://www.tpub.com/neets/tm/106-3.htm
https://www.boundless.com/physics/textbooks/boundless-physics-textbook/geometric-optics-24/reflection-refraction-and-dispersion-169/total-internal-reflection-and-fiber-optics-609-6258/
https://www.rp-photonics.com/acceptance_angle_in_fiber_optics.html
https://www.scienceabc.com/innovation/fibre-optic-copper-faster-better-signal-transmission-bandwidth-speed-cost-fast.html
http://www.cables-solutions.com/the-core-and-cladding-of-fiber-optic-cable.html
http://www.multicominc.com/training/technical-resources/single-mode-vs-multi-mode-fiber-optic-cable/

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The same way as copper cables, fiber cables vary in specifications, mainly depending on the applications they are designed for. Variations might affect core diameter, materials used, and environment cable could be used in. In order to better understand  how fiber cable works, one should have a look at its components (see Figure 1). When stripping optical fiber cable of its layers, following components can be observed:

Outer jacket;

Optional protection from physical damage;

Strength members;

Buffer;

Optical fiber stand.

Each optical fiber stand has three components inside:

Core;

Cladding;

Coating.

Size of the core is varying between 8 and 63 microns. Pieces of the strand are so tiny they can easily penetrate skin and in some cases travel through human body with blood vessels. This is one more reason why installation should be done by professionals using specially intended gear.