Instead it focuses on the practical aspects of designing, installing, testing and troubleshooting fiber optic cable plants and networks. This book has been edited from the training programs of the Fiber U fiber optic training conferences and those developed by a number of professional instructors in fiber optics.
The Fiber U conference was developed as an annual training program which offered a combination of classroom seminars and multivendor hands-on training. Fiber U training also kept pace with the changes in technology, with each year's presentations and products being the latest state of the art.
The Fiber U program and this book was developed by a number of instructors, all of whom are involved in teaching fiber optics courses regularly. Maximum recommended installation load, installation load, or installa- tion force in kg-force or pounds-force, or N 2. Minimum recommended installation bend radius, installation bend radius, short-term bend radius, or loaded bend radius in in. Diameter of subcable and buffer tubes 5. Recommended temperature range for installation in degrees centigrade 6.
Recommended temperature range for storage in degrees centigrade Maximum Recommended Installation Load The maximum recommended installation load is the maximum tensile load that can be applied to a cable without causing a permanent change in attenuation or breakage of fibers. This characteristic must always be specified. It is particularly important in installations that are long, outdoors, or in conduits; it is of lesser importance when cables are laid in cable trays or installed above suspended ceil- ings.
We present typical and generally accepted values of installation loads in Table Choose the value that best fits your application. If you believe that your application will require a strength higher than those typically specified, then you will want to specify a strength higher than those in Table The cost increase of specifying such a higher strength is a small per- centage, typically 5 to 10 percent, of the cost of the cable.
Minimum Recommended Installation Bend Radius The minimum recommended installation bend radius is the minimum radius to which cable can be bent while loaded to the maximum recommended installation load. This radius is limited more by the cabling materials than by the bend radius of the fiber. This bending can be done without causing a permanent change in attenuation, breakage of fibers, or breakage of any portion of the cable structure.
This bend radius is usually, but not always, specified as being no less than 20 times the diameter of the cable being bent. Specifying the bend radius is impor- tant when pulling by machine or hand through conduit, or in any long pulls. Conversely, you can choose the cable and specify the conduits or ducts in which you are to install the cable so that you do not violate this radius.
Diameter of the Cable, Subcable, and Buffer Tubes The cable must fit in the location in which it is to be installed. This is especially true if the cable is to be installed in a partially filled conduit. It will not be impor- tant if the cable is directly buried, installed above suspended ceilings, or in cable trays. If the diameter is limited by the space available, the diameter limits may be the only factor that determines which of the five designs of the cable you must choose.
If cable diameter must be limited, the ribbon designs will be the smallest. The diameter of the subcable and the buffer tube of the cable can also become a limiting factor. In addition, the diameter of the element must be less than the maximum diameter that the back shell of the connector will accept.
Recommended Temperature Ranges for Installation and Storage All cables have a temperature range within which they can be installed without damage to either the cable materials or the fibers.
It is more important for out- door installations or in extreme arctic or desert environments and not impor- tant for indoor installations. In general, the materials of the cable restrict the temperature range of installation more than do the fibers. Note that not all cable manufacturers include the temperature range of installation in their data sheets. In this case, the more conservative temperature range of operation can be used.
In severe climates, such as those in deserts and the arctic, you will need to specify a recommended temperature range for storage in degrees Centigrade. This range will strongly influence the materials used in the cable.
There are 21 such specifications. Temperature range of operation 2. Minimum recommended long-term bend radius 3. Long-term use load 5. Vertical rise distance 6.
Flame resistance 7. UV stability or UV resistance 8. Resistance to damage from rodents 9. Resistance to damage from water Crush loads Resistance to conduction under high voltage fields Toxicity Abrasion resistance Resistance to solvents, petrochemicals, and other chemicals Hermetically sealed fiber Radiation resistance Impact resistance Gas permeability Stability of filling compounds Vibration Temperature Range of Operation The temperature range of operation is the temperature range within which the attenuation remains less than the specified value.
Typical ranges of operation are given in Table for various types of applications. In general, there are very few applications in which fiber optic transmission cannot be used solely for reasons of temperature range of operation.
For operation at such high temperatures, fibers are usually, but not always, incorporated into a cable structure consisting of a metal tube.
There are two reasons for considering the temperature range of operation: the physical survival of the cable and the increase of attenuation of the fiber when the cable is exposed to temperature extremes. All cables are composed of plastic materials. These plastic materials have temperatures above and below which they will not retain their mechanical prop- erties.
After long exposure to high temperatures, plastics deteriorate, become soft, and, in some materials, crack. Under exposure to low temperatures, plastics become brittle and crack when flexed or moved.
Obviously, under these condi- tions, the cable would cease to provide protection to the fiber s. The second reason for considering the temperature range of operation is the increase in attenuation that occurs when cables are exposed to extremes of tem- perature.
Optical fibers have a sensitivity to being handled. This sensitivity is seen when the fibers are bent. This contracting and expanding results in the fiber being bent on a micro- scopic level. Either the fiber is forced against the inside of the plastic tube as the plastic contracts, or the fiber is stretched against the inside of the tube as the plas- tic expands. In either case, the fiber is forced to conform to the microscopically uneven surface of the plastic.
On a microscopic level, this is similar to placing the fiber against sandpaper. This microscopic bending results in light escaping from the core of the fiber. This escaping light results in an increase in attenuation. This type of behavior means that the user must determine the temperature range of operation in order to ensure that there will be enough light for the system to func- tion properly. Minimum Long-Term Bend Radius The minimum recommended long-term bend radius is the minimum bend radius to which the cable can be bent for its entire lifetime.
It is important for cables installed in conduits designed for electrical cables. It is usually, but not always, specified as being no less than 10 times the diameter of the cable. Compliance with Electrical Codes Fiber optic cables used in indoor applications must meet the requirements of the NEC and applicable local electric codes, some of which are more stringent than the NEC. Article of the NEC addresses optical cables. Article addresses cables that combine copper and fiber.
The NEC specifies six ratings. The fourth letter, if any, indicates the rating. Such cables must pass the UL shaft test, which is more stringent than the UL test. Use of plenum-rated cables allows you to reduce the total installed cost of the cables by eliminating the cost for the installation of metal conduit.
The specifica- tion concerned with the requirements for plenum cables both copper and fiber is the NEC, Section These products have lower cost, easier instal- lation, and better appearance than the original fluorocarbon cables.
Long-Term Use Load Most fiber optic cables are designed for unloaded use, not for use with any sub- stantial load. In these cases, the cables are subjected to loads, either self-loads or loads from the environment, such as wind, snow, and ice loads on aerial cables. All of these factors depend on the spacing between poles. Care in specifying the long-term use load characteristic is required to ensure that the strain the cable allows to be applied to the fiber s does not exceed a crit- ical value.
If this critical value is exceeded, it is likely that the fiber s will sponta- neously, and for no apparent reason or cause, break. This value depends on the design and construction of the cable, but typically runs 10 to 30 percent of the maximum recommended installation load.
If the cable will experience a significant long-term use load, this specification will be more important than the maximum recommended installation load. In these cases, the maximum span length is specified instead of the long-term use load.
Typical long-term use loads are presented in Table When cables are installed in a riser within a building or in a long vertical length outdoors , the self-weight of the cable imposes a load on the cable. This load must be less than the maximum use load. Typical vertical rise distances are presented in Table Flame Resistance Flame resistance is required for applications other than building applications, including shipboard and aircraft installations.
In these applications, you will want to specify that the cables be constructed of flame-resistant materials. Many commonly used materials are either flame resistant in their most commonly used formulations, or can be made flame resistant through the use of additives. When you specify flame resistance, you will need to reference a specification, such as the UL specification 94, and specify the level of flame resistance required i.
UV-resistant polyurethanes and polyvinyl chlorides PVCs are also available. However, the expected life of these two materials is much less than the more than year life exhibited by polyethylene-jacketed telephone cables. Before choosing any jacket material other than black polyethylene for outdoor use, check its expected life span.
Resistance to Damage from Rodents In environments containing active rodents, you will want to protect buried cable from damage caused by gnawing. There has been a trend away from the use of armored cables. Instead, buried inner ducts are used to provide the rodent resis- tance previously met by armored designs. This type of cable has an additional layer of material that acts to give the cable significant resistance to crushing and being bitten through.
There are penalties to these additional layers. First, armored cables are more expensive than nonarmored cables. Second, these cables are usually much less flexible than unarmored cables. There are four basic types of armored cable products: galvanized steel armor with or without plastic coating on the armor , copper tape armor, braided stainless steel or bronze armor, and dielectric armor. The armor most com- monly used on fiber optic cables is galvanized steel.
It is effective and has the lowest cost of the armoring materials. However, it is the stiffest of the metallic armoring materials. Copper tape armor is helically wrapped around the cable with some spacing between the successive wraps. This type of product is rarely used on fiber optic cables. Because of its relatively flexible nature, braided armor is used in sit- uations if rodent resistance and flexibility are required.
Dielectric armoring is only available from a single source in the United States. This type of armoring is rarely needed and rarely used. It is the stiffest and most expensive of all types of armoring. The addition of a dielectric armor often doubles the cost of the cable.
A filled and blocked cable has a filling material inside each of the loose buffer tubes and a blocking material that fills all empty space between the tubes. These cables are not as water- resistant as filled and blocked cables. Breakout cables are not filled and blocked. Before using any design that is not filled and blocked, request test data to support the water resistance claimed. Crush Loads The crush load is the maximum load that can be applied perpendicular to the axis of a cable without causing a permanent increase in attenuation or breakage of fibers.
There are two crush loads: short-term and long term. Short-term can mean during installation or during use. The long-term crush load is that load that can be applied during the entire life of the cable.
Before you can determine the crushing requirements for your cable, you have to answer two basic questions. First, is the occurrence of crushing likely? If it is not a likely occurrence, then you will not need to be concerned with the crush performance of the cable you need. It has been the experience of the author that most of the cable products available today have crush performance sufficient to meet the needs of the typical user.
This is so because most of the applications involve installation in relatively benign locations in which the occurrence of crushing is not likely. Examples of these benign locations include conduits, trays, cable troughs, plenums, and aerial locations. Examples of locations in which crushing performance is of importance are field tactical cables in which the cable is likely to be run over by trucks and tanks , electronic news gathering ENG , and temporary cable placement for sporting broadcast applications, shipboard use in which the cable has a reasonable possibility of being crushed between bulkhead doors , and direct burial of fiber optic cable.
If you determine that crushing is of concern, then you need ask the second question: Is the application of a crush load likely to be a short-term or a long- term condition?
Typical performances of commercial cables are given in Table Resistance to Conduction under High Voltage Fields In a number of typical applications under high voltage fields, fiber optic cables need to be nonconducting. Some fiber optic cables in use are exposed to voltages as high as 1,, volts. In other applications, fiber optic cables need to be unattractive to lightning.
These cables contain no halogens, which burn to produce acidic gases that attack lungs and corrode electronic equipment. These cables are 10 to 15 percent more expensive than PVC cables. In addition to toxicity requirements, some municipalities require registration of all cables installed in order to keep track of the material content.
In the United States, New York is the first state to require such registration. Cables manu- factured for use in Japanese and European buildings are required to be halogen free.
In these applications, the cables need to meet a flexibility requirement. Flexibility requirements must be met by both cable materials and by fibers. Polyurethane jacketing materials are commonly used to meet this require- ment.
These materials will result in an increase in the cost of the cable, but will increase the flexibility to 10, cycles from the 1,cycle level available with the lower cost PVC and polyethylene jacketing materials.
Fibers can be made to meet the requirements of high flexibility and dynamic applications through the inclusion of a proof stress level.
In such situations, as in elevator cables and in optical power ground wire OPGW , some users have adopted a policy of requiring that the fibers be proof tested to at least kpsi. Failures have been observed with dynamic loading of cables containing fibers proof stressed to only 50 kpsi.
Need for this resistance will determine the material used as the jacket. Resistance to Solvents, Petrochemicals, and Other Chemicals In some situations, you need to specify that the cables be resistant to deteriora- tion from exposure to certain chemicals.
Examples to which cables are occasion- ally exposed are gasoline, aircraft fuel, fuel oil, greases, and crude oil. To ensure such resistance, an immersion test is required. Hermetic sealing is required because contact with moisture or other chemicals results in significant reduction in the strength of the fiber, and absorption of hydrogen from water results in a significant increase in attenuation. This hermetic sealing can be done in one of two methods.
In the first method, the fiber is sealed inside of a welded steel tube. In the second method, the fiber is coated with a proprietary hermetic coating by the manufacturer. With both methods, the fiber is protected from degradation of its performance. Radiation Resistance When you intend to use a fiber optic cable in an environment subjected to ioniz- ing radiation—such as in the core of a nuclear power plant, outer space, or an x- ray chamber—you must specify that both the cable materials and the fiber be radiation resistant.
The cable materials must be radiation resistant in order to retain acceptable mechanical properties, since these properties tend to be degraded by exposure to ionizing radiation. The fiber must also be radiation resistant, since the attenuation of a fiber can be increased by such exposure. Radiation-resistant fibers are available from a number of suppliers. Such fibers have smaller increases in attenuation with increasing radiation dosage than other more commonly used commercial fibers. In addition, these fibers have shorter recovery times and lower total residual increases in attenuation after such exposure.
Impact Resistance In certain situations, you may want to specify the resistance of your fiber optic cable to impact forces. In these situations, you will specify impact resistance. As a practical matter, we have found most fiber optic cables to be highly resistant to damage from impacts. Unless impact is a likely occurrence in the envi- ronment in which the cable must survive, specification of impact resistance is not needed. Gas Permeability Some environments require that the cable not allow gases or moisture to travel through the cable.
Examples of such environments are cables carrying signals from underground nuclear tests to equipment on the surface and underground cables leading to equipment located in underground vaults. In this case, gas or moisture permeability tests and limits must be specified. Stability of Filling Compounds Some environments subject the cable to frequent temperature and strain cycling. The pumping of filling compounds can cause problems to equipment at the ends of the cable.
In this case, stability or flow tests and limits must be specified. Vibration In some situations, vibration may cause loose-tube cables to experience changes in attenuation.
There is insufficient data available to recommend against loose- tube designs. However, in such situations, a tight-tube design may be preferable. Installing spare fibers offers two major advantages. First, you can use spare fibers in the event of a cable or connector problem. Second, spare fibers provide for future growth of fiber applications.
Fiber is very inexpensive relative to installed cable cost and there is no cost for installing spare fibers as part of a cable being installed.
If you need to install additional fibers in the future, you will incur two installation charges. Include Singlemode Fibers in Multimode Cables As bandwidths and bit rates increase, multimode fibers will eventually run out of capacity. Singlemode fibers provide essentially unlimited bandwidth.
Include Fibers in Any Copper Cables Include fiber in any copper cable, as the cost of installing the fiber is free. You need not install connectors, although doing so is advised. Use Dual Wavelength Install Some other countries and some U. Singlemode Choose the nm singlemode fiber. Systems designed to operate at this wave- length have lower cost than nm systems. Do not choose fiber designed for both and nm unless you expect to use wavelength division multiplex- ing or optical amplifiers in the future.
Cable Design Choice Indoor 1. For longer distances, use premise-type cable. If your environment is rugged, use breakout design; it is more rugged than the premise. The price premium is insurance against future mainte- nance cost. Use all-dielectric design. If plenum cables are required, look for plenum-rated PVC products. Use one of three water-blocked and gel-filled loose-tube designs. If midspan access is important, use the stranded loose-tube design.
This design has an easily removable outdoor jacket over an inner structure that meets NEC requirements. Or, use a blocked cable that meets the appropriate NEC requirements. Fiber Performance Multimode Choose dual wavelength specifications. Singlemode Choose single wavelength specifications. Two specifications must be considered when specifying optical cable. They are: 1. Match items on the right with cables on left. Also, to keep from com- plicating procedures too greatly, all references are to glass fiber unless plastic fiber is specifically mentioned.
Although there are many reasons for fiber joints, the four most common are: 1. Fibers and cables are not endless and therefore must eventually be joined. Fiber may also be joined to distribution cables and splitters.
At both transmit and receive termination points, fibers must be joined to that equipment. The last and scariest reason is cable cuts and their subsequent restoration. Since we have established a need for fiber joints, we should now make that task worthwhile.
To that end, all fiber joints must be mechanically strong and optically sound with low loss. Fiber joints must be capable of withstanding mod- erate to severe pulling and bending tests. Fiber joints fall generally into two categories: the permanent or fixed joint that uses a fiber splice, and the terminating nonfixed joint that uses a fiber optic connector. Let us examine these individual types of joints. Splices are used as permanent fixtures on outside and inside plant cables.
Typical uses include reel ends, pigtail vault splices, and distribution breakouts. In addition to the benefits of low loss and high mechanical strength, additional con- siderations are expense per splice and possible reusability of the splice itself.
Fiber optic connectors are used as terminating fixtures for inside plant cables, outside plant cables as they terminate in a central office, interfaces between ter- minals on LANs, patch panels, and terminations into transmitters and receivers. Whether one joins fibers using splices or connectors, one negative aspect is always common to both methods—signal loss. This loss of light power at fiber joints is called attenuation. Sometimes the losses occur in the fiber itself and other times at fiber joints.
Measurement of attenuation loss is made in decibels dB. The decibel is a mathematical logarithmic unit describing the ratio of output power to input power in any system fiber or copper. Attenuation in the optical fiber itself usually occurs as a result of absorption, reflection, diffusion, scattering, or dispersion of the photon packets within the fiber.
However, losses also occur at splices and connections. The factors that cause attenuation in connectors or splices Figure fall into two categories: intrinsic and extrinsic losses. Intrinsic losses occur from factors over which the craftsperson has very little control and are generally caused by engineering design or manufacturing flaws in the fiber itself. The more prominent intrinsic losses include: 1. Core eccentricity 2. Features chapter-end review questions that allow for knowledge assessment and targeted review of topics needing reinforcement.
Serves as an excellent preparation manual for the Certified Fiber Optic Technician certification exam. Provides your students with more hands-on exercises with a separate lab manual. Fiber Optics Technician's Manual 4th Ed. Be the first to review this product. The optical medium permits the control signals to be carried error-free through the electrically noisy environment.
Tapping into a fi- ber cable without being detected is extremely difficult. High Bandwidth Optical fiber has a relatively high bandwidth in comparison to other transmission media. This permits much longer transmission distances and much higher signal rates than most media. For example, all undersea long-haul telecommunications cable is fiber-optic. This technology is allowing worldwide communications voice, video and data to be available mainstream.
With new technologies, such as VCSEL transmitters, parallel optics and wavelength division multiplexing WDM , services such as video-on-demand and video conferencing will be available to most homes in the near future. Voltage Isolation Fiber isolates two different voltage potentials.
For example, it will Voltage Insulation eliminate errors due to ground-loop potential differences and is ideal for data transmission in areas subject to frequent electrical storms, and presents no hazard to the field installer. The transmitter contains a emit light. As electrons pass through an active light-emitting diode LED , laser diode LD , or region of semiconductor material, they cause pho- vertical cavity surface-emitting laser VCSEL that tons to exit.
The light produced by an LED has a converts an electrical current into an optical signal. The LED can be the light back into an electrical signal and an am- turned on and off at speeds which make data com- plifier that makes the signal easier to detect. The munications possible. The most common wavelengths for LED emitters are nm for plastic fiber; nm and nm LED for multi-mode glass fiber.
LEDs can also be made to emit Transmitter Receiver light from the edge, producing a more narrow, col- limated beam that can launch into single-mode fi- Emitters ber. The two major groups of optical emitters for com- munication links are light-emitting diodes LEDs Lasers and lasers. Photons produced by a semi- conductor laser are initially trapped in a cavity and Emitters are made from semiconductor com- bounce off reflective surfaces at each end.
They pounds. Eventually the power in the example emit particles of light photons when cavity increases enough so that a very bright, co- subjected to an electric current.
Using the right combination of elements, and the right processing parameters, engineers can produce The gain medium determines the properties of the a variety of different wavelengths, brightness, and output light, including wavelength, output power, modulation speeds how fast the emitter can be and whether the light exits in pulses or in a con- turned on and off of the emitted light.
By optimizing its wavelength, the light can be effi- Semiconductor lasers are devices with a unique ciently coupled into the appropriate type of fiber structure that makes them more efficient. The wavelengths span the 1. Laser Diodes visible and infrared spectrums up to nm. In Laser diodes, the active gain material is a III- Typically, wavelengths nm and above are V semiconductor compound.
Because the ac- considered to be long wavelength LWL and those tive region is very small and the input current is below nm are considered to be short wave- relatively low, laser diodes output optical length SWL. By contrast, some medical lasers and industrial lasers output power in megawatts. These devices from the ultraviolet to the infrared. Very few, however, must be cleaved, and mounted on their sides.
At the present time, most commercially available LEDs are made from combina- Furthermore, it is more difficult to align the la- tions of gallium Ga , arsenic As , phosphorus P , ser to the connector ferrule than it is to align an Aluminum Al , and Indium In. LED to the connector ferrule. For these rea- sons, lasers are more expensive than LEDs. Certain materials can be made to produce a range of Laser diodes are commonly found in nm wavelengths by adjusting the relative proportions of the and nm wavelengths for communications, constituent elements.
It should be noted that LEDs actu- and nm and nm for pumping lasers. For example, an nm 2. This laser behaves ample, plastic optical fiber has a minimum loss of as a laser diode, but can be manufactured similarly transmitted light at wavelengths around nm.
There- to an LED, because it emits light from the surface. VCSELs for use with glass optical fiber. Glass fiber loses a are being produced at nm and nm, with minimum of light at wave-lengths around nm, which is within the spectral band of an InGaAsP LED.
The laser begins with spon- made from silicon or germanium: they allow current to taneous output at low forward currents, increased to a flow in the forward direction and block current from point known as the Threshold Current level. Biasing flowing in the reverse direction. Current flowing above the threshold current causes the device to reach through the diode in the forward direction generates a the stimulated emission region of operation.
The forward current optical fiber is typically far higher than that of an LED. Typically the laser also has a narrower output optical spectra, thus providing better performance at high bit The light output of an LED is nearly proportional to the rates over long distances.
The disadvantage is that the magnitude of its forward current. Therefore, controlling threshold current is temperature dependent and circuit the forward current controls the light output of the LED. The added circuit complexity increases the cost of the laser transmitter. The semiconductor optical ampli- LED current is de- termined by the dif- fier is formed by applying current to a lower-bandgap ference between the active layer surrounded by higher-bandgap materials.
Light forward voltage of R the LED, divided by radiates from the laser in a narrow-angled cone. It is the current through the LED As the current increases, the optical amplifier gain in- that controls the light output. FP lasers can produce tions of a fiber-optic link: distance and data rate.
The VCSEL uses a Bragg reflector, a multi-layer have to work closely with the customer to deter- dielectric mirror composed of alternate layers of high mine the details. For data com- munication applications, the diameter of the laser is Fiber as an Optical Waveguide made large enough to support multiple modes. This Reflected light causes the central lasing peak to spectrally broaden.
The wider spectral width is often designed into VCSELs to The most basic optical waveguide is the step-index avoid mode-selective loss in multi-mode fiber applica- fiber, which consists of two layers, a core and a tions. These layers are made of materials that have different optical transmission properties. One The major advantage of the VCSEL design is ease of important property of these materials is the index manufacture, simplified on-wafer testing, and simple of refraction n , a material constant that deter- packaging, resulting in lower costs.
The device has mines the direction of the light through the mate- higher output power and higher modulation rates than rial. The detector must be able to detect the same wavelength emitted by the transmitter, or the link will not operate. The detector is always The index of refraction of the core must be larger followed by a pre-amplifier preamp that converts than that of the cladding for the light to travel the current coming from the diode to a voltage, through the waveguide: which is used by most amplification schemes.
For some glass fibers, the cladding is SiO2 glass Other detectors include avalanche photo-diodes, and the core has dopants such as germanium to which are more complex to operate due to their increase the index of refraction. The light travels through the waveguide in paths called modes. Each ray of light has a different One important property of a receiver is its sensi- mode, which creates a crowded waveguide. Not tivity. This property defines how small a signal all of the modes that exit a light source are trans- the receiver is able to detect.
The receiver must be mitted through the waveguide. A specific angle, able to detect a signal over the top of any electro- the acceptance angle, serves as a cutoff for the magnetic noise that may be interfering with the light modes.
A careful design of the preamp and fol- lowing circuitry, plus metal shielding around the receiver, can improve the sensitivity. Transmitted Cladding Chromatic dispersion results from the spectral width of Lost the emitter.
The spectral width determines the number of different wavelengths that are emitted from the LED All light rays that enter the fiber at an angle greater or laser. Because longer and are lost. This angle can be calculated based on wavelengths travel faster than shorter wavelengths the value, n, of the core and cladding, and is usu- higher frequencies these longer wavelengths will arrive ally represented as the numerical aperture, NA, a at the end of the fiber ahead of the shorter ones, spread- ing out the signal.
The NA is de- fined in the following illustration. One way to decrease chromatic dispersion is to narrow the spectral width of the transmitter. Lasers, for ex- Cladding ample, have a more narrow spectral width than LEDs. Modal dispersion deals with the path mode of each Cladding light ray.
As mentioned above, most transmitters emit many different modes. Bandwidth Bandwidth determines how much information can High-order Mode longer path Cladding be transmitted through a waveguide at one time. This is due to the properties of order modes. These modes take much longer to travel light versus electricity. One way to reduce The bandwidth of a transmission medium is lim- modal dispersion is to use graded-index fiber.
Unlike ited by an effect called dispersion. The more dispersion shown at the top of the following page. If they get close enough to each other, they can overlap, causing errors in the signal, as illustrated at the top of the next column. The high-order modes spend most of the Distance time traveling in the lower-index cladding layers near m the outside of the fiber. These lower-index core layers allow the light to travel faster than in the higher-index The flat line is where the fiber does not contribute to any High-order Modes faster limiting effects.
The sloping line is where the data rate Cladding decreases as the distance increases. Finally, there is a cut-off point, the vertical line, at which the receiver can no longer detect the signal. Different types of fiber and different emitter wave- Low-order Cladding Modes lengths and properties will change the shape of this Graded-index Fiber slower curve, so you must take all components of the link into account when determining a solution.
Generally, dis- Note: Shading is used only to make the illustration clearer tance versus data rate graphs and tables are available for most Agilent general-purpose fiber-optic components. Therefore, their higher velocity compen- sates for the longer paths of these high-order modes.
A The following table compares the major specifications good waveguide design appreciably reduces modal dis- of fiber-optic cable. As its name implies, single mode Specification Plastic Plastic Glass fiber transmits only one mode of light so there is no Bandwidth-Length- 3 20 - Product MHz-km spreading of the signal due to modal dispersion.
Numerical Aperture 0. For a step-index multi- Attenuation mode fiber, distortion effects tend to limit the band- As light pulses travel through the fiber, they lose width-distance product to about 20 MHz-km. In graded- some of their photons, which decreases their ampl- index fibers, pulse broadening is minimized at a specific itude. This is know as attenuation, the other major operating wavelength. The dis- fiber. At- the data rate. At- Propagation Delay tenuation varies over the light spectrum.
Propagation delay is the length of time it takes the out- put of a device to go from one state to another state, either low to high or high to low, after having been stimulated to change at the input. It is also a measure of Rayleigh Scattering the time required for a signal or pulse to travel through a A ray of light is partially scattered into many directions medium.
The following table lists the typical propaga- by microscopic variations in the core-cladding interface. For this reason, some light energy is lost. Glass Fiber 5. Rayleigh scattering causes about a 2. Impurities in the glass, which absorb light energy and turn photons into phonons heat , include ions of copper, When there are parallel logic paths using individual par- iron, cobalt, chromium and the hydroxyl OH- ions of allel links, the signals can arrive at the other end of the water.
The OH-ions of water and the molecular reso- links at different times because the transit time for each nance of SiO2 are the principal reasons that light energy link is different.
If this happens, individual link delays is absorbed by the fiber. The strobe edge should Bending be timed to just follow the longest individual transition Two types of bending exist: micro-bending and macro- delay and to precede the shortest individual transition bending. Micro-bending is microscopic imperfections in delay. Macro-bending is due to fibers curved around tween the delay of a low-to-high edge and the delay of a diameters of about a centimeter, causing less than total high-to-low edge of the output signal waveform.
Ideal- reflection at the core-to-cladding boundary. Bending ly, low-to-high and high-to-low transitions should have loss is usually unnoticeable if the diameter of the bend is exactly the same time delay.
The time difference be- larger than 10 cm. In multi-mode fibers, a large number of modes is always present; and each mode in a fiber is attenuated different- Wavelength Division Multiplexing WDM ly differential-mode attenuation. In single-mode fibers The data rate for a single optical channel eventually this effect is not as much of a problem.
The available bandwidth can be Optical power loss in a fiber can also be caused by increased by adding multiple wavelength channels. A wavelength multiplexer combines the laser wavelength - light reflected off the ends of the fiber by differences channels onto a single fiber with low loss, then later in the refractive indices of the core and air at the in- separates them at the receiver end of the link.
The loss per unit length can be calcu- cific location, the optical signal was detected and routed lated from the difference between these two values. WDM systems allow the optics to route the optical signal. These systems can also add and drop Substitution Method certain wavelength channels from a fiber while simulta- A short length of reference fiber is compared with the neously sending on other channels.
Each type of fiber has a different attenuation ver- Both the cutback and substitution methods have draw- sus wavelength curve, none of which is linear or backs. The attenuation measured by the cutback method exponential, but rather a complicated series of varies according to the numerical aperture and the spec- peaks and valleys.
In the substitution method, the coupling losses of the fiber being tested and the ref- The attenuation of the light through a glass optical erence fiber may be different, so the value derived may waveguide has been decreased over the last 25 not be correct. Direct Measurement A more reliable technique for measuring attenuation in a fiber is optical time-domain reflectometry OTDR.
In this method, a brief signal a short burst of optical The following graph depicts the attenuation for a speci- power is introduced into the fiber. When this signal fic type of plastic optical fiber. As the graph shows, cer- encounters imperfections or discontinuities, some of the tain wavelengths are ideal for transmission because they light is reflected by a beamsplitter a glass plate at a 45 fall in the valleys of the curve.
For this type of plastic degree angle to the incident light or two prisms cement- fiber, the best suited wavelengths are nm and ed together with metal-dielectric film between their nm.
For most glass fibers there are valleys around faces onto a detector, where it is amplified and dis- nm, nm, and nm. For other types of fiber, played on an oscilloscope. The ODTR display is a smooth curve for a continuous length of fiber and an irregular curve if discontinuities or splices exist.
The advantage of OTDR is that it yields more accurate results. The number of these corrupted bits divided by the total number of re- Wavelength nm ceived bits within an arbitrary time period is the bit This feature makes these types of fibers useful for a va- riety of applications. The lower the BER value, the fewer the number of errors in the transmission. This means that on average, one error occurs for Cutback Method every million pulses sent.
Typical error ratios for opti- cal fiber systems range from to local-area The attenuation of a fiber can be measured by transmit- networks require a BER as low as The BER can ting a signal through it and measuring the power at the be measured by repeatedly transmitting and receiving a opposite end. The fiber is then cut near the input end suitable length of a pseudo-random bit sequence PRBS without changing the launch conditions and the power is data.
BER - receiver sensitivity--the minimum optical power that 16 the receiver requires to operate at the specified data rate and bit error ratio P R dBm avg. So, the most obvious way to These two sets of parameters allow a designer to de- reduce the probability of error is to increase the ampli- termine the distance and power budgeting for the fi- tude of the optical signal applied to the receiver.
If optical function as efficient antennas. A second method, for power losses are greater than the worst-case power extremely noisy applications, is to use PIN diode pre- budget, the BER increases and the reliability of the amps in electrically conductive plast5ic or all-metal link decreases.
Simple and inexpensive tenuation and other possible losses in the link. Optical Power Ratio - System Losses. Note: For the fiber-optic cable to transmit and receive, Receiver sensitivity is specified at a BER; however, in the flux margin must be positive. For this reason, the transceiver is Each of these losses is expressed in decibels dB as usually tested during an artificially shorter time at a Pout much higher BER. Note: dBm is a unit used to describe optical power with Optical budgeting is a design technique that takes respect to one milliwatt.
The optical power budget is PT - PR, where PT is the Maximum Distortion at power launched by the transmitter and PR is the power signal voltage Best sampling time sampling times required by the receiver for proper operation. These V2 values are usually expressed in dBm, so the difference is Slope increases sensitivity to expressed in dB. If the losses between transmitter and timing errors Noise margin V1 receiver exceed the power budget, then the power at the Threshold receiver will not be adequate to assure proper operation.
If the signal is meas- An oscilloscope display of a long stream of ran- ured at its midpoint, then this time-width indicates dom bits light pulses is used to determine trans- the amount of jitter. Due to distortion the more that the eye closes and the greater the in- of the waveform by a variety of factors , the dis- crease in timing errors.
As the slope approaches the played light pulses do not align, but instead over- horizontal, the possibility of error increases. Any non-linearities will produce asymme- try in the eye pattern. Parameters such as rise- and fall-times, extinction ratio, overshoot, undershoot, and pulse-width distortion can be observed. Much information about system performance can be Most serial communications standards now define a gathered from the eye pattern: minimum condition on eye-pattern opening template for the serial data stream.
The area from which the - The width of the eye defines the time interval during waveform is excluded is the mask. The mask limits that specify the eye-pattern opening are - Amplitude distortion in the data signal reduces the an improvement over the older technique of specifying height of the eye opening.
The smaller this dimen- pulse parameters individually. It is compact, conveying sion, the more difficult it is to detect the signal with- important information without placing undue restrictions out error. Limiting rise and fall times, pulse- - The height of the eye opening at the specified sam- width distortion, overshoot, damping, undershoot, and so pling time shows the noise margin or immunity to forth, is unwieldy and insufficient by comparison.
In this ap- Mask region 1 plication, parallel data is serialized into a high- bandwidth PECL data stream. At the other end of 1. Absolute Amplitude V This system of translating parallel to serial then Relative Amplitude Mask region 2 serial to parallel minimizes the number of trans- mission lines required for interconnecting the sub- systems. Mask region 3 0. The Host Protocol IC, which uses sig- waveform is not permitted to appear. It is con- - definitions of the time and amplitude scales.
ECL uses a negative power supply. Busses in- dors.
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