Learn the basics of optical fiber technology with this free Introduction to Fiber Optics tutorial from leondumoulin.nl
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They correctly and systematically theorized the light-loss properties for optical fiber, and pointed out the right material to use for such fibers—silica glass with high purity. This discovery earned Kao the Nobel Prize in Physics in Maurer , Donald Keck , Peter C.
Initially high-quality optical fibers could only be manufactured at 2 meters per second. Chemical engineer Thomas Mensah joined Corning in and increased the speed of manufacture to over 50 meters per second, making optical fiber cables cheaper than traditional copper ones. The Italian research center CSELT worked with Corning to develop practical optical fiber cables, resulting in the first metropolitan fiber optic cable being deployed in Turin in The erbium-doped fiber amplifier , which reduced the cost of long-distance fiber systems by reducing or eliminating optical-electrical-optical repeaters, was co-developed by teams led by David N.
The emerging field of photonic crystals led to the development in of photonic-crystal fiber , [29] which guides light by diffraction from a periodic structure, rather than by total internal reflection. The first photonic crystal fibers became commercially available in Achieving a high data rate and covering a long distance simultaneously is challenging.
Optical fiber
Optical fiber is used as a medium for telecommunication and computer networking because it is flexible and can be bundled as cables. It is especially advantageous for long-distance communications, because light propagates through the fiber with much lower attenuation compared to electrical cables. This allows long distances to be spanned with few repeaters. Each fiber can carry many independent channels, each using a different wavelength of light wavelength-division multiplexing WDM.
The net data rate data rate without overhead bytes per fiber is the per-channel data rate reduced by the FEC overhead, multiplied by the number of channels usually up to 80 in commercial dense WDM systems as of [update]. For short-distance applications, such as a network in an office building see FTTO , fiber-optic cabling can save space in cable ducts. Fiber is also immune to electrical interference; there is no cross-talk between signals in different cables, and no pickup of environmental noise.
Non-armored fiber cables do not conduct electricity, which makes fiber a good solution for protecting communications equipment in high voltage environments, such as power generation facilities, or metal communication structures prone to lightning strikes. They can also be used in environments where explosive fumes are present, without danger of ignition.
Wiretapping in this case, fiber tapping is more difficult compared to electrical connections, and there are concentric dual-core fibers that are said to be tap-proof. Fibers are often also used for short-distance connections between devices. For example, most high-definition televisions offer a digital audio optical connection. Fibers have many uses in remote sensing. In some applications, the sensor is itself an optical fiber. In other cases, fiber is used to connect a non-fiberoptic sensor to a measurement system.
Depending on the application, fiber may be used because of its small size, or the fact that no electrical power is needed at the remote location, or because many sensors can be multiplexed along the length of a fiber by using different wavelengths of light for each sensor, or by sensing the time delay as light passes along the fiber through each sensor. Time delay can be determined using a device such as an optical time-domain reflectometer.
Optical fibers can be used as sensors to measure strain , temperature , pressure , and other quantities by modifying a fiber so that the property to measure modulates the intensity , phase , polarization , wavelength , or transit time of light in the fiber. Sensors that vary the intensity of light are the simplest, since only a simple source and detector are required. A particularly useful feature of such fiber optic sensors is that they can, if required, provide distributed sensing over distances of up to one meter.
In contrast, highly localized measurements can be provided by integrating miniaturized sensing elements with the tip of the fiber. Extrinsic fiber optic sensors use an optical fiber cable , normally a multi-mode one, to transmit modulated light from either a non-fiber optical sensor—or an electronic sensor connected to an optical transmitter.
A major benefit of extrinsic sensors is their ability to reach otherwise inaccessible places. An example is the measurement of temperature inside aircraft jet engines by using a fiber to transmit radiation into a radiation pyrometer outside the engine. Extrinsic sensors can be used in the same way to measure the internal temperature of electrical transformers , where the extreme electromagnetic fields present make other measurement techniques impossible.
Extrinsic sensors measure vibration, rotation, displacement, velocity, acceleration, torque, and torsion. A solid state version of the gyroscope, using the interference of light, has been developed. The fiber optic gyroscope FOG has no moving parts, and exploits the Sagnac effect to detect mechanical rotation. Common uses for fiber optic sensors includes advanced intrusion detection security systems. The light is transmitted along a fiber optic sensor cable placed on a fence, pipeline, or communication cabling, and the returned signal is monitored and analyzed for disturbances.
This return signal is digitally processed to detect disturbances and trip an alarm if an intrusion has occurred. Optical fibers are widely used as components of optical chemical sensors and optical biosensors. Optical fiber can be used to transmit power using a photovoltaic cell to convert the light into electricity. Optical fibers have a wide number of applications.
They are used as light guides in medical and other applications where bright light needs to be shone on a target without a clear line-of-sight path. In some buildings, optical fibers route sunlight from the roof to other parts of the building see nonimaging optics. Optical-fiber lamps are used for illumination in decorative applications, including signs , art , toys and artificial Christmas trees. Swarovski boutiques use optical fibers to illuminate their crystal showcases from many different angles while only employing one light source.
Optical fiber is an intrinsic part of the light-transmitting concrete building product LiTraCon. Optical fiber can also be used in structural health monitoring. This type of sensor is able to detect stresses that may have a lasting impact on structures. It is based on the principle of measuring analog attenuation. Optical fiber is also used in imaging optics. A coherent bundle of fibers is used, sometimes along with lenses, for a long, thin imaging device called an endoscope , which is used to view objects through a small hole.
Medical endoscopes are used for minimally invasive exploratory or surgical procedures. Industrial endoscopes see fiberscope or borescope are used for inspecting anything hard to reach, such as jet engine interiors. Many microscopes use fiber-optic light sources to provide intense illumination of samples being studied. In spectroscopy , optical fiber bundles transmit light from a spectrometer to a substance that cannot be placed inside the spectrometer itself, in order to analyze its composition. A spectrometer analyzes substances by bouncing light off and through them.
By using fibers, a spectrometer can be used to study objects remotely. An optical fiber doped with certain rare-earth elements such as erbium can be used as the gain medium of a laser or optical amplifier. Rare-earth-doped optical fibers can be used to provide signal amplification by splicing a short section of doped fiber into a regular undoped optical fiber line. The doped fiber is optically pumped with a second laser wavelength that is coupled into the line in addition to the signal wave.
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Both wavelengths of light are transmitted through the doped fiber, which transfers energy from the second pump wavelength to the signal wave. The process that causes the amplification is stimulated emission. Optical fiber is also widely exploited as a nonlinear medium. The glass medium supports a host of nonlinear optical interactions, and the long interaction lengths possible in fiber facilitate a variety of phenomena, which are harnessed for applications and fundamental investigation. Optical fibers doped with a wavelength shifter collect scintillation light in physics experiments.
Fiber-optic sights for handguns, rifles, and shotguns use pieces of optical fiber to improve visibility of markings on the sight. An optical fiber is a cylindrical dielectric waveguide nonconducting waveguide that transmits light along its axis, by the process of total internal reflection.
The fiber consists of a core surrounded by a cladding layer, both of which are made of dielectric materials. The boundary between the core and cladding may either be abrupt, in step-index fiber , or gradual, in graded-index fiber. The index of refraction or refractive index is a way of measuring the speed of light in a material. Light travels fastest in a vacuum , such as in outer space. The speed of light in a vacuum is about , kilometers , miles per second. The refractive index of a medium is calculated by dividing the speed of light in a vacuum by the speed of light in that medium.
The refractive index of a vacuum is therefore 1, by definition. A typical singlemode fiber used for telecommunications has a cladding made of pure silica, with an index of 1.
From this information, a simple rule of thumb is that a signal using optical fiber for communication will travel at around , kilometers per second. To put it another way, the signal will take 5 milliseconds to travel 1, kilometers in fiber. The fiber in this case will probably travel a longer route, and there will be additional delays due to communication equipment switching and the process of encoding and decoding the voice onto the fiber. When light traveling in an optically dense medium hits a boundary at a steep angle larger than the critical angle for the boundary , the light is completely reflected.
This is called total internal reflection. This effect is used in optical fibers to confine light in the core. Light travels through the fiber core, bouncing back and forth off the boundary between the core and cladding. Because the light must strike the boundary with an angle greater than the critical angle, only light that enters the fiber within a certain range of angles can travel down the fiber without leaking out.
This range of angles is called the acceptance cone of the fiber. The size of this acceptance cone is a function of the refractive index difference between the fiber's core and cladding.
Optical fiber - Wikipedia
In simpler terms, there is a maximum angle from the fiber axis at which light may enter the fiber so that it will propagate, or travel, in the core of the fiber. The sine of this maximum angle is the numerical aperture NA of the fiber. Fiber with a larger NA requires less precision to splice and work with than fiber with a smaller NA.
Single-mode fiber has a small NA. Such fiber is called multi-mode fiber , from the electromagnetic analysis see below. In a step-index multi-mode fiber, rays of light are guided along the fiber core by total internal reflection. Rays that meet the core-cladding boundary at a high angle measured relative to a line normal to the boundary , greater than the critical angle for this boundary, are completely reflected.
The critical angle minimum angle for total internal reflection is determined by the difference in index of refraction between the core and cladding materials. Rays that meet the boundary at a low angle are refracted from the core into the cladding, and do not convey light and hence information along the fiber. The critical angle determines the acceptance angle of the fiber, often reported as a numerical aperture. A high numerical aperture allows light to propagate down the fiber in rays both close to the axis and at various angles, allowing efficient coupling of light into the fiber.
However, this high numerical aperture increases the amount of dispersion as rays at different angles have different path lengths and therefore take different times to traverse the fiber. In graded-index fiber, the index of refraction in the core decreases continuously between the axis and the cladding.
This causes light rays to bend smoothly as they approach the cladding, rather than reflecting abruptly from the core-cladding boundary. The resulting curved paths reduce multi-path dispersion because high angle rays pass more through the lower-index periphery of the core, rather than the high-index center. The index profile is chosen to minimize the difference in axial propagation speeds of the various rays in the fiber. This ideal index profile is very close to a parabolic relationship between the index and the distance from the axis. Fiber with a core diameter less than about ten times the wavelength of the propagating light cannot be modeled using geometric optics.
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Instead, it must be analyzed as an electromagnetic structure, by solution of Maxwell's equations as reduced to the electromagnetic wave equation. The electromagnetic analysis may also be required to understand behaviors such as speckle that occur when coherent light propagates in multi-mode fiber. As an optical waveguide, the fiber supports one or more confined transverse modes by which light can propagate along the fiber. Fiber supporting only one mode is called single-mode or mono-mode fiber. The behavior of larger-core multi-mode fiber can also be modeled using the wave equation, which shows that such fiber supports more than one mode of propagation hence the name.
The results of such modeling of multi-mode fiber approximately agree with the predictions of geometric optics, if the fiber core is large enough to support more than a few modes. The waveguide analysis shows that the light energy in the fiber is not completely confined in the core. 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.
The mode structure depends on the wavelength of the light used, so that this fiber actually supports a small number of additional modes at visible wavelengths. Multi-mode fiber, by comparison, is manufactured with core diameters as small as 50 micrometers and as large as hundreds of micrometers. The normalized frequency V for this fiber should be less than the first zero of the Bessel function J 0 approximately 2.
These include polarization-maintaining fiber and fiber designed to suppress whispering gallery mode propagation. Polarization-maintaining fiber is a unique type of fiber that is commonly used in fiber optic sensors due to its ability to maintain the polarization of the light inserted into it. 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 intensity of the light beam or signal as it travels through the transmission medium. 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 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 Marianas Trench in the Pacific Ocean, a depth of 36, feet. 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.
Introduction to Fibre Optics
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. Each fiber in a Multi-mode cable is capable of carrying a different signal independent from those on the other fibers in the cable bundle.
These larger core sizes generally have greater bandwidth and are easier to couple and interconnect. It allows hundreds of rays to light to propagate through the fiber simultaneously. Multi-mode fiber today is used primarily in premise applications, where transmission distances are less than two kilometers. Singlemode fiber glass has a much smaller core that allows only one mode of light to propagate through the core. Singlemode fiber has a higher bandwidth and less loss than Multi-mode fiber and for this reason it is the ideal transmission medium for many applications. The standard Singlemode fiber core is approximately um in diameter.
Because of its greater information-carrying capacity, Singlemode fiber is typically used for longer distances and higher-bandwidth applications. While is might appear that Multi-mode fibers have higher information carrying capacity, this is not the case. Singlemode fibers retain the integrity of each light pulse over longer distances which allows more information to be transmitted. This is why Multi-mode fibers are used for shorter distances. POF-or Plastic Optical Fiber-is a newer plastic-based cable which promises performance similar to Singlemode Cable, but at a lower cost.
POF is still in the infancy stage although many companies are noticing its potential. Take for example, your basic telephone conversation. In the fiber optic telecommunication system, a message is sent from one end through an electric cable to an encoder which transmits a signal through a glass fiber optic cable.
It then travels through a repeater, back through the cable, into a decoder and through an electric cable into the phone line on the other end. This list is not exhaustive nor are the subjects treated in depth, have a look at our recommended text section to learn more It has an inner glass core with an outer cladding. This is covered with a protective buffer and outer jacket. This design of fibre is light and has a very low loss , making it ideal for the transmission of information over long distances.
Light in a fibre The light propagates along the fibre by the process of total internal reflection. The light is contained within the glass core and cladding by careful design of their refractive indices. The loss along the fibre is low and the signal is not subject to electromagnetic interference which plagues other methods of signal transmission, such as radio or copper wire links.