Basics of Fiber Optics
`Mark Curran/Brian Shirk
`
`
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`Fiber optics, which is the science of light transmission through very fine glass or plastic fibers,
`continues to be used in more and more applications due to its inherent advantages over copper
`conductors. The purpose of this article is to provide the non-technical reader with an overview of
`these advantages, as well as the properties and applications of fiber optics.
`
`I.
`
`Fiber optics has many advantages over copper wire (see Table 1) including:
`
`
`Advantages
`
`(cid:131)
`
`Increased bandwidth: The high signal bandwidth of optical fibers provides significantly
`greater information carrying capacity. Typical bandwidths for multimode (MM) fibers are
`between 200 and 600MHz-km and >10GHz-km for single mode (SM) fibers. Typical
`values for electrical conductors are 10 to 25MHz-km.
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`(cid:131) Electromagnetic/Radio Frequency Interference Immunity: Optical fibers are immune
`to electromagnetic interference and emit no radiation.
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`(cid:131) Decreased cost, size and weight: Compared to copper conductors of equivalent signal
`carrying capacity, fiber optic cables are easier to install, require less duct space, weigh
`10 to 15 times less and cost less than copper.
`
`(cid:131) Lower loss: Optical fiber has lower attenuation (loss of signal intensity) than copper
`conductors, allowing longer cable runs and fewer repeaters.
`
`(cid:131) No sparks or shorts: Fiber optics do not emit sparks or cause short circuits, which is
`important in explosive gas or flammable environments.
`
`(cid:131) Security: Since fiber optic systems do not emit RF signals, they are difficult to tap into
`without being detected.
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`(cid:131) Grounding: Fiber optic cables do not have any metal conductors; consequently, they do
`not pose the shock hazards inherent in copper cables.
`
`(cid:131) Electrical Isolation: Fiber optics allow transmission between two points without regard
`to the electrical potential between them.
`
`
`Representative distance
`bandwidth products
`Attenuation/km @ 1 GHz
`Cable cost ($/m)
`Cable diameter (in.)
`Data security
`EMI immunity
`
`Coaxial
`Cable
`100
`MHz km
`>45 dB
`$$$$$$$$$
`1
`Low
`OK
`
`Fiber Optic
`Cable (MM)
`500
`MHz km
`1 dB
`$
`1/8
`Excellent
`Excellent
`
`Fiber Optic
`Cable (SM)
`100,000+
`MHz km
`0.2 dB
`$
`1/8
`Excellent
`Excellent
`
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`Table 1: Advantages of Fiber Optics over Copper
`
`1
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`US Conec EX1015
`IPR2024-00115
`U.S. Patent No. 11,307,369
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`

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`Fiber Optic Link Components
`
`II.
`
`In order to comprehend how fiber optic applications work, it is important to understand the
`components of a fiber optic link. Simplistically, there are four main components in a fiber optic link
`(Figure 1).
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`(cid:131) Optical Transmitter
`(cid:131) Optical Fiber/Cable
`(cid:131) Connectors
`(cid:131) Optical Receiver
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`Figure 1: Simple Fiber Optic Link
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`Transmitter
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`II.1
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`The transmitter converts the electrical signals to optical. A transmitter contains a light source such
`as a Light Emitting Diode (LED) or a Laser (Light Amplification by Stimulated Emission of
`Radiation) diode, or a Vertical Cavity Surface Emitting Laser (VCSEL).
`
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`LED: Is used in multimode applications and has the largest spectral width that carries the
`least amount of bandwidth.
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`VCSEL: Is also used in multimode applications with a narrower spectral width that can carry
`more bandwidth than the LED.
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`LASER: Has the smallest spectral width, carries the most bandwidth, and is used in
`singlemode applications.
`
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`These sources produce light at certain wavelengths depending upon the materials from which
`they are made. Most fiber optic sources use wavelengths in the infrared band, specifically 850nm
`(1nm=10-9m), 1300nm and 1550nm. For reference, visible light operates in the 400-700nm range
`(see Figure 2).
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`Figure 2: Electromagnetic Spectrum
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`2
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`Optical Fiber/Cable
`
`
`II.2
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`In this section, we discuss the structure and properties of an optical fiber, how it guides light, and
`how it is cabled for protection.
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`An optical fiber is made of 3 concentric layers (see Figure 3):
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`(cid:131) Core: This central section, made of silica or doped silica, is the light transmitting region of
`the fiber.
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`(cid:131) Cladding: This is the first layer around the core. It is also made of silica, but not the same
`composition as the core. This creates an optical waveguide which confines the light in the
`core by total internal reflection at the core-cladding interface.
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`(cid:131) Coating: The coating is the first non-optical layer around the cladding. The coating
`typically consists of one or more layers of polymer that protect the silica structure against
`physical or environmental damage. The coating is stripped off when the fiber is
`connectorized or fusion spliced.
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`Figure 3: Optical Fiber Construction
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`• Buffer (not pictured): The buffer is an important feature of the fiber. It is 900 microns and
`helps protect the fiber from breaking during installation and termination and is located
`outside of the coating.
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`The light is "guided" down (see Figure 4) the core of the fiber by the optical "cladding" which has
`a lower refractive index (the ratio of the velocity of light in a vacuum to its velocity in a specified
`medium) that traps light in the core through "total internal reflection."
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`Figure 4: Diagram showing Total Internal Reflection
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`3
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`In fiber optic communications, single mode and multimode fiber constructions are used
`depending on the application. In multimode fiber (Figure 5), light travels through the fiber
`following different light paths called "modes." In single mode fiber, only one mode is propagated
`"straight" through the fiber (Figure 6).
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`Figure 5: Multimode Fiber Light Propagation
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`Figure 6: Single Mode Fiber Light Propagation
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`Typical multimode fibers have a core diameter/cladding diameter ratio of 50 microns/125 microns
`(10-6 meters) and 62.5/125 (although 100/140 and other sizes are sometimes used depending on
`the application). Single mode fibers have a core/cladding ratio of 9/125 at wavelengths of 1300nm
`and 1550nm.
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`Multimode Fiber
`( 62.5/125 µm )
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`Single Mode Fiber
`( 9/125 µm )
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`Core
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`Cladding
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`Multimode Fiber
`( 100/140 µm )
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`Multimode Fiber
` (50/125 µm )
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`Figure 7: Popular Optical Fiber Core/Cladding Diameter Ratios
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`4
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`Light is gradually attenuated when it travels through fiber. The attenuation value is expressed in
`dB/km (decibel per kilometer). Attenuation is a function of the wavelength (λ) of the light. Figure 8
`hows the attenuation as a function of the wavelength.
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`Figure 8: Attenuation vs. Wavelength of Optical Fiber
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`As discussed in Section II.1, the typical operating wavelengths are 850nm (nanometers) and
`1300nm in multimode, and 1300nm or 1550nm in single mode. Note that there are natural "dips"
`in the attenuation graph at these wavelengths. For example, at an 850nm operating wavelength,
`pagation (according to the graph). 3dB of attenuation
`there is 3dB attenuation after 1km pro
`eans that 50% of light has been lost.
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`m B
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`andwidth is a measure of the data-carrying capacity of an optical fiber. It is expressed as the
`product of frequency and distance. For example, a fiber with a bandwidth of 500MHz-km
`(Megahertz kilometer) can transmit data at a rate of 500MHz along one kilometer of fiber. The
`ltimode fibers. The main reason for the
`bandwidth of single mode fibers is much higher than in mu
`wer bandwidth in multimode fibers is modal dispersion.
`lo
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`In multimode fibers, information (ABC) is propagated in fiber according to N modes or paths (see
`Figure 9), as if it were "duplicated" N times (for example, in the diagram, the mode 3 path is
`longer than the mode 2 path, which are both longer than the mode 1 path). If information is too
`close, there is a risk of overlapping ("smearing") the information, and then it will not be
`e fiber. It is necessary to space the data sufficiently to avoid overlap,
`recoverable at the end of th
`i.e., to limit the bandwidth.
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`Figure 9: Modal Dispersion in Multimode Fibers
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`5
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`Modal dispersion can be alleviated to a large extent by grading the index of refraction from the
`middle of the core to the cladding (graded index fiber), thereby equalizing the paths (Figure 10).
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`In a step index fiber, the index of refraction changes abruptly from the core to the cladding. To
`help reduce modal dispersion, fiber manufacturers created graded-index fiber. Graded-index fiber
`has an index of refraction which gradually increases as it progresses to the center of the core.
`ht path propagating directly
`Light travels slower as the index of refraction increases. Thus, a lig
`d
`own the center of the fiber has the shortest path but will arrive at the receiver at the same time
`as light that took a longer path due to the graded-index of the fiber.
`
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`Figure 10: Graded Index in Multimode Fibers
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`Of course, modal dispersion is not an issue in single mode fiber because only a single mode is
`propagated (Figure 11).
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`Figure 11: Single Mode Propagation
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`Unfortunately, the optical fiber construction shown in Figure 3 is fragile. Thus, for most
`applications, the fiber must be made into a cable. There are many ways to construct a cable (tight
`buffer, loose tube, gel filled, distribution, breakout, etc). However, in our single fiber cable
`th a 900 micron buffer and built
`example (see Figure 12), the 250 micron coating is jacketed wi
`TM
`in
` ) as a strength member. As a typical
`to a 3.0mm outer sheath cable with aramid yarn (Kevlar
`example, Figure 13 portrays a cable with multiple optical fibers.
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`Figure 12: Construction of a Single Fiber Cable
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`Figure 13: Example of the Construction of a Multi- Fiber Cable
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`Connectivity
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`II.3
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`Fiber optic links require a method to connect the transmitter to the fiber optic cable and the fiber
`optic cable to the receiver. In general, there are two methods to link optical fibers together.
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`II.3.1 Fusion Splice
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`The first method is called a fusion splice. This operation consists of directly linking two fibers by
`welding with an electric arc or a fusion splicer (see Figure 14). The advantages of this approach
`are that the linking method is fast and simple and there is very little insertion loss (the loss of light
`generated by a connection is called Insertion Loss [IL]). The disadvantages are that the link is
`relatively fragile, is permanent, and the initial cost (of the fusion splicer) is high.
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`Figure 14: Fiber-optic Fusion Splicer
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`II
`.3.2 Connectors
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`The second method involves the uses of fiber optic connectors. A connector terminates the
`optical fiber inside a ceramic ferrule, using epoxy to hold the fiber in place. The connectors can
`be mated and unmated at any time. The advantages of this approach are that the connection is
`robust, the connector can be chosen according to the application, and the connector can be
`connected and disconnected hundreds or even thousands of time without damaging the
`connectors. The disadvantages of this approach are that the connectorization takes longer than
`fusion splicing, requires special tools, and the insertion loss can be higher when compared with
`fusion splicing.
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`There are two types of fiber optic connectors: physical contact and expanded beam.
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`II.3.2.1 Physical Contact Connectors
`
`e fiber in a tightly toleranced ceramic ferrule. T
`Physical contact connectors utiliz
`his allows easy
`andling of the fiber and protects it from damage. The principle of physical contact connectors
`nvolves the direct contact of polished fibers within two ceramic ferrules. The ferrules are aligned
`using a ceramic alignment sleeve (see Figure 15). Insertion loss is a function of the alignment
`accuracy and the polish quality. There are springs behind the ferrule to ensure that the two
`ferrules are in constant contact even in high vibration and shock environments.
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`hi
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`Figure 15: Physical Contact
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`Phy
`asic l contact connectors are the most common type of fiber optic connection. They are
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`rugged, repeatable, easy to clean, cost-effective, and perform well. In addition, for physical
`contact
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`pproximately 0.3dB). There are many connectors, the insertion loss is generally low (a
`ty
`pes of fiber optic connectors used in various applications. The most popular single fiber
`connectors are (see Figure 16):
`
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`(cid:131) FC-Ferrule Connector: Although the FC connector is being replaced in many applications
`(telecom and datacom) by LC and SC connectors, it is still used in measurement
`equipment. The connector has a screw threading and is keyed allowing the ferrule to be
`angle polished providing low backreflection (light is reflected back to the transmitter, most
`often at the connector interface due to an index of refraction change).
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`(cid:131) LC-Lucent Connector: LC connectors are supplanting SC connectors because of their
` panel packing density and push-pull
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`smaller size and excellent
`design. They are also
` on small form-factor pluggable transceivers.
`used extensively
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`(cid:131) SC-Subscriber Connector: SC connectors also offer a push-pull design (which reduces
`the possibility of end-face damage when connecting) and provide good packing density.
`They are still used in datacom and telecom applications.
`
`(cid:131) ST-S
`traight Tip Connector: ST connectors are engaged with a bayonet lock which is
`engaged by pushing and twisting the connector. The bayonet interlock maintains the
`spring-loaded force between the two fiber cores.
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`Figure 16: Popular Single Fiber Connectors
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` T
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`taminates (dust, mud, etc)
`hese physical contact con
`nectors perform well against particle con
`nd are usually less sensitive to liquid contaminates (water or oil). The physical contact pushes
`he way and the liquid does not degrade the connection. Physical contact connectors
`iquid out of t
`are cleaned by wiping the ferrule with a clean cloth or wipe, spraying with a cleaner or washing
`with water.
`
`It is also common to provide multiple fibers in a single connector. An example is the MPO
`(Multiple Fiber Push-On/Pull-Off-see Figure 17) connector which supports 12 fibers in a single
`ferrule. Another example is the TFOCA-II® connector which provides 4 or 12 fiber optic channels
`for harsh environment fiber-optic applications (see Figure 18).
`
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`al
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`Figure 17: MPO Multi-fiber Connector
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`Figure 18: AFSI TFOCA-II® Connector
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`II.3.2.2 Expanded Beam Technology
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`The other connector technology is expanded beam, which consists of placing a lens at the exit of
` there is an air gap between the
`ach fiber to widen and collimate the light. In this configuration,
`
`wo optical fibers/lens assemblies (see Figure 19).
`
`
`The mechanical interface between the connectors must be precise. Dust and dirt must not
`interfere with the alignment of the optical elements. Expanded beam connectors are less
`susceptible to particle contaminates such as dirt and dust but they perform poorly with liquids or
`film on the lenses. They can also be very difficult to terminate in the field.
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`The loss generated by an expanded beam connection is more than that of a physical contact
`connector due to the lenses, mechanical alignment and sometimes protective windows
`(approximately 0.8 to 2.5dB typical).
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`This type of connector performs well against particle contamination on the lens because the
`beam is expanded to a much larger size than the beam that comes directly from a fiber. However,
`any liquid or film (such as a fingerprint) on a lens creates significant loss in an expanded beam
`connector. Expanded beam connectors are also very sensitive to alignment of the lenses.
`Connectors must always be tightly coupled and kept clean on the mating surface in order to work
`p
`becau
`roperly. Cleaning an expanded beam connector must be done with care
`se any liquid
`leaner) that is trapped inside the connector m
`(water, alcohol or another c
`ay migrate to the surface
`of the lens, causing an unacceptable increase in insertion loss.
`
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`et
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`10
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`II.4
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`Receiver
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`Figure 19: Expanded Beam Technology
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`The last component of the fiber optic
`link is the optical receiver, which uses a photodiode to
`convert the optical signals into electrical. The two types of photodiodes used are: Positive Intrinsic
`Negative (PIN) and the Avalanche PhotoDiode (APD)
`
`In a similar fashion as that of the laser transmitter, the photodiode will receive wavelengths
`depending on material composition (see Figure 20).
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`Figure 20: Responsivity of a Silicon Photodiode
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`Fiber Optic Applications
`
`III.
`
`As discussed, fiber optics is used in myriad applications. Due to its low weight, high bandwidth
`capacity and immunity to electromagnetic and RF interference, fiber optics is used extensively in
`avionics on both military (see Figure 21) and commercial aircraft systems. Applications include
`radar links, video systems, sensor networks, and in-flight entertainment systems.
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`11
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`Figure 21: Military Aircraft
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`Fiber optics is also used for command, control, and telemetry in industrial applications wherever
`there are large electric motors. These motors generate large electromagnetic fields. Instead of
`using heavily shielded copper conductors, manufacturers use fiber optics to eliminate
`electromagnetic interference concerns. Examples include top drive control links for drilling rigs
`(Figure 22) and command and control for longwall mining systems.
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`Figure 22: Top Drive Control Link
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`(see Figure 23) and
`in data communications
`is also used
`Fiber optics
` longer
`telecommunications systems due to its ability to transmit high bandwidths over
`distances than copper conductors.
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`12
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`Figure 23: A Data Center using Fiber Optics
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`IV. Conclusion
`
`Fiber optics provides many advantages over copper conductors including higher bandwidth,
`transmission of signals over longer distances, lower weight and cost and immunity from
`electromagnetic interference. These attributes make it the increasingly preferred medium for
`applications such as avionics, energy, mining, broadcast, and data/telecommunications.
`
`About the Authors
`
`
`nt, Sales and Marketing: Mark has 25
`Mark Curran, Vice Preside
` ye
`ars experience in a variety
`oduct managemen
`t positions in th
`r
`e fiber optics industry,
`of sales, program management, and p
`elecommunications.
` Mark earned his Bachelors Degree in
`including military, cable television and t
`rsity of Connecticut
`Electrical Engineering from the Unive
` and a Masters Degree in Electrical
`outhern California.
`Engineering from the University of S
`
`Brian Shirk, Product Manager:
`Brian has over 15 years of
`experience in the defense and
`telecommunications industries. He is responsible for training, tools, and termination kits across
`all product lines. Prior to joining AFSI, he held a variety of engineering positions in the
`telecommunications industry. Brian graduated from DeVry Institute with a Bachelor of Science in
`Electrical Engineering Technology.
`
`Company Overview
`
`
`Amphenol Fiber Systems International (AFSI), a division of Amphenol, provides reliable and
`innovative fiber optic interconnect solutions that withstand the harsh environments of military
`(ground systems, avionics, shipboard), energy, and broadcast applications. With nearly two
`decades in business, AFSI continues to maintain its position as a global leader in fiber optic
`interconnect components and systems such as termini, M28876, 38999 assemblies, MIL-ST,
`TFOCA and the TFOCA-II® connector, which AFSI developed and patented. AFSI has delivered
`millions of fiber optic connectors in more than 34 countries. Whenever there is a need for superior
`cost-effective fiber optic systems and products that will stand up to demanding operating
`environments, you can rely on AFSI for engineering know-how, top-quality products and expert
`technical support.
`
`
`Amphenol Fiber Systems International
`1300 Central Expressway N, #100
`Allen, TX 75013
`T: (214) 547-2400
`F: (214) 547-9344
`www.fibersystems.com
`sales@fibersystems.com
`
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