Fiber Basics

Fiber Basics

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Fiber Basics A fiber is about the width of a human hair and is measure in either microns or nano-meters. Different manufacturers make different fiber structures, although they all have a core and they all have "cladding", which is the reflective material that surrounds the the core. In the early days of development, fibers were large, brittle, rods of glass called "waveguides" (they guided the light waves through the tube), and they were only experimental, and only capable of supporting low frequencies. Modern fiber is extremely thin, flexible, and is capable of transmitting extremely high frequencies.

Fiber Exposed - the outer PVC jacket, the Cladding, and the Core

Here is a detailed cross-section of a single strand of fiber.

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1-way Simplex Fiber

2-way Duplex Fiber (Pair)

Fiber-Optic Cables

The actual cables may contain one, or many fibers, along with support strands and an outer sheath. There are two types of optic cables: Loose-Tube Cable  - in a loose-tube cable design, color-coded plastic buffer tubes house and protect optical fibers. A gel filling compound impedes water p enetration. Excess fiber length (relative to  buffer tube length) insulates fibers from stresses stresses of installation and environmental loading. Buffer tubes are stranded around a dielectric or steel central member, which serves as an anti-buckling element. The cable core, typically uses aramid aramid yarn, as the primary tensile tensile strength member. The outer polyethylene jacket is extruded over the core. If armoring is required, a corrugated steel tape is

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formed around a single jacketed cable with an additional jacket extruded over the armor. Loose-tube cables typically are used for outside-plant installation in aerial, duct and direct-buried applications. Tight-Buffered Cable  - the buffering material is in direct contact with the fiber. This design is suited for "jumper cables" which connect outside plant cables to terminal equipment, and also for linking various devices in a premises network. Multi-fiber, tight-buffered cables often are used for intra building, risers, general building and plenum applications. The tight-buffered design provides a rugged cable structure to protect individual fibers during h andling, routing and connectorization. Yarn strength members keep the tensile load away from the fiber. As with loose-tube cables, optical specifications for tight-buffered cables also should include the maximum performance of a ll fibers over the operating temperature range and life of the cable. Averages should not be acceptable.

Tightly Buffered Cable

Layman's Description of Fiber - a Mirrored Tube To understand how a fiber optic cable works, imagine an immensely long drinking straw or flexible  plastic pipe. For example, imagine a pipe that is several miles long, and has twists and turns in it. If a friend shined a flashlight into one end, you would see nothing, because of the turns. But imagine that the inside surface of the pipe has been coated with a perfect mirror. Now , if several miles away at the other end, your friend turns on the flashlight and shines it into the pipe, you will see the light !! Since the interior of the pipe is a perfect mirror, the flashlight's light will reflect off the sides of the pipe (even though the pipe may curve and twist) and you will see it at the other end. If your friend were to turn the flashlight on and off in a morse code fashion, your friend could communicate with you through the pipe. That is the essence of a fiber optic cable!

the cladding is a mirror-like coating that allows the light to reflect and bounce off the inner walls - enabling it to make it around tight turns and corners !! Making a cable out of a mirrored tube would work, but it would be bulky and it would also be hard to coat the interior of the tube with a perfect mirror. A real fiber optic cable is therefore made out of glass. The glass is incredibly pure so that, ev en though it is several miles long, light can still make it through (imagine glass so transparent that a window several miles thick still looks clear). The glass is drawn into a very thin strand, with a thickness comparable to that of a human hair. The glass strand is then coated in two layers of plastic.

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By coating the glass in plastic, you get the equivalent of a mirror around the glass strand. This mirror creates total internal reflection, just like a perfect mirror coating on the inside of a tube does. You can experience this sort of reflection with a flashlight and a window in a dark room. If you direct the flashlight through the window at a 90 degree angle, it passes straight through the glass. However, if you shine the flashlight at a very shallow angle (nearly parallel to the glass), the glass will act as a mirror and you will see the beam reflect off the window and hit the wall inside the room. Light traveling through the fiber bounces at shallow angles like this and stays completely within the fiber.

Amplification and Regeneration As light travels down a fiber, it loses power, an d the sharp transitions (representing binary data - or 1's and 0's) of the digital signal become smoothed out and loses power. This is rectified by placing amplifiers and regenerators into series with the fiber cable. Fiber can carry light pulses much farther than copper, and therefore the amp/regens can be spaced farther apart. A typical Singlemode system is shown below, with ILA's (In-Line Amplifiers) placed every 100 km:

ILA's every 100 km - used with Intercity (long haul) SingleMode Fiber

An optic amplifier (or repeater), merely increases the power of the signal (i.e. makes the light  brighter). A regeneration station (called "regen") will reshape the digital signal into sharp, welldefined 1's and 0's. In general, with metro fiber routes, there are about 4 or 5 amps for every regen, as shown below:

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How Far can the Light Go before Requiring an Amp or Regen ?? The distance capability of an optical system varies greatly !! It is dependant on the data rate, the type of transmitter & receiver, and the type of fiber. It also depends on whether the system uses WDM (Wave Division Multiplexing), where, instead of the traditional, single white light, a number of different "colors" of light are transmitted, and therefore this allows multiple data channels. Today,  because of the huge expense of running fiber - most optical fiber networks use WDM, so that the  provider can squeeze as much bandwidth out of each cable, as possible.

Distance Limitations and Controlling Cost through Concatenation Very high data rates cannot travel far without regeneration. For example, the new 10 Gigabit Ethernet (10 Gbps is approx OC-192, and 1 Gbps is approx OC-24) can only travel a few hundred meters max. The newest fiber is called "laser-optimized fiber", and it can support 550 meters for 10Gbps, or 1100 meters for 1Gbps. Of course, it requires excellent transmitter/receiver equipment as well to achieve those distances. Think about that - only 550 meters !! That means if a provider wants to build a nationwide, 10 Gbps backbone, they need to install thousands of expensive repeaters and regens !! This gives you an idea, why fiber networks are so expensive. Well, actually there is a way to reduce the cost dramatically. Instead of installing pure OC-48 (2.5 Gbps) or OC-192 (10 Gbps) systems, the provider can install OC-48c or OC-192c systems. The "c" stands for "concatenation", and this means that several fibers or several wavelengths on one fiber are used at lower speeds, and then concatenated at the endpoints. For example Metro fiber systems (within a city) typically use CWDM (Course Wave-Division Multiplexing), and can only travel 5 km or so before requiring amplification and/or regeneration of the signal. Longhaul, intercity fiber uses DWDM (Dense Wave Division Multiplexing), which is much more expensive because it is tightly controlled and high-quality and carries more channels (more colors of light) - and it can carry a signal quite a distance -- perhaps 42 to 60 miles (70 to 100 km). Undersea fiber systems use very expensive transmitters and cabling, and can travel up to 400 miles before requiring amplification/regeneration. On a long distance line, there is an equipment hut every 40 to 60 miles. The hut contains equipment that picks up and retransmits the signal down the next segment at full strength.

"Launching" - the Light Source at the Transmitter and Receiving it With fiber-optics, there are two types of light sources that launch   light (shine light) into one end of the fiber: LED (Light Emmitting Diode) - these are going away, because their light power is rather weak and not concentrated into a small enough beam

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Laser - this is now the light source of choice for fiber-optic systems.

For the receiving end, Photo-detectors  are used.

Speed of Light (299,792,458 m/s) *** also see the "Electro-Magnetic Spectrum " page *** for a details look, view the U.S. Full Spectrum Chart  (Acrobat format - make sure to set zoom to 100%) The base speed of light is defined with the assumption that it is traveling in a vacuum. In a vacuum, light travels at exactly 299,792,458 m/s (metres per second), which is often rounded off to 300,000 km/sec or 186,000 mi/sec. NOTE: all electromagnetic radiation waves propagate at the speed of light

Refraction and the Refractive Index Of course, here on earth, light travels through air, or in the case of fiber optic cables - through glass and the speed of light is always slowed somewhat by the material that is moves through. Light moves slower in water, glass and through the atmosphere than in a vacuum.

Refraction of a Beam of Light through a Glass Cube

This slowing effect has been measured for each substance, and the measurement has been recorded as an index, for calculations of light speed through that medium. This is called therefractive index.

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The Refractive Index, n is the ratio whereby light is slowed down by a particular medium. Here are the most common values for n : Medium

Refractive Index Speed of Light in Medium "n " (299,792,458/ n )

vacuum

1.0

299,792,458 m/s

air

1.0003

299,702,547 m/s

ice

1.31

228,849,204 m/s

water

1.33

225,407,863 m/s

glass

1.5

199,861,638 m/s

*** click Here for a Table listing all Refractive Indices The speed of all electromagnetic radiation in vacuum is the same, approximately 3*108 meters per second, and is denoted by c. So if v is the phase velocity of radiation of a specific frequency in a specific material, then the refractive index is given by: n = c/v (Refractive index of Medium, n = speed of light in a vacuum/speed of light in that medium) Therefore, if we rearrange it:

Speed of light in a Medium = 299,792,458 / Refractive Index of that Medium Example:

Speed of light in Glass = 299,792,458 / 1.5 = 199,861,638

Critical Angle (Acceptable Angle) The critical angle is the key to the successful operation of a fiber-optic cable !!! The core fiber and the cladding are both made of glass silica, but are doped differently to allow for different critical angles. The idea is to allow light to pass through the core (i.e. the angle of the light exceeds the critical angle, and passes through), and to NOT allow it to pass through the cladding, but instead reflect off (i.e. the angle of the light is less than the critical angle, and is reflected o ff the walls). Total internal reflection  - this is a function of the refraction indices of both the cladding and the fiber core. It causes the light to be guided down the fiber. Total internal reflection is directly related to the fact that the refractive index of the cladding is less than the refractive index of the core.

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When a beam of light strikes a refractive material, the amount and direction of refraction depen ds on the angle that it intersects the material at. If the angle is very slight, the light is instead reflected off, like striking a mirror. Numerical Aperture  - total internal reflection is what causes light to be guided along the length of an optical fiber. First, however, light must fall inside an acceptable angle so that it can enter into the fiber’s core. As you can see below - light at a sharp angle is lost because the refraction is not enough to redirect it within the core. The angle at which light will be redirected to within the core is called the "Acceptance angle" or "Critical Angle". The numerical aperture (NA) measures the range of acceptance of light into a fiber. The angle over which a fiber accepts light depends on the refractive indices of the core and cladding glass. Refraction bends a ray of light entering a fiber so that it is at a smaller angle to the axis of the fiber than it was in air.

A good analogy to this is skipping stones. They will skip off the water if they hit it at a slight angle. But if you throw the rock too steeply down, it will simply enter the water.

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Similarly, the light beam will be reflected (refracted) the wall of the object if the angle is small. But if the angle is steep, or even perpendicular (90 degrees), then the light passes through the material. The exact point where the angle becomes too large to be refracted, is called the criti cal angle. The critical angle is different for different materials, and depends on the refractive index of each material.

Fiber Core and the Critical Angle  - we want the light to pass through the core (the glass) and therefore want to shine the light source into the core at a perpendicular angle. This allows the light to  pass through the core, with the least amount of refraction. Fiber Cladding and the Critical Angle - in this case we do NOT want the light to pass through (in which case it will escape the glass core and go into the cladding), and instead want it to reflect. The walls of the glass core are glazed by a gaseous material in the manufacturing process, to make them act as a mirror to reflect the light beam and allow it to bounce off and continue down the cable. So for this case, the ideal is to stay below the critical angle of the material deposited o nto the outer  portion of the glass core. The Cone of Acceptance -  if the light tries to enter the core at too much of an angle, it will be reflected back out. The angle at which light can enter the fiber without escaping into the cladding is called the "cone of acceptance".

Light must enter within this "Cone of Acceptance" to be Reflected inside the Walls

As shown below, most of the light enters within the cone of acceptance - it enters the fiber core, and is reflected off the cladding wall. However, there is always some light that enters the fiber outside the cone of acceptance will not be transmitted through, and instead will be refracted through the cladding, and is lost.

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Light within Cone is Reflected - Light outside Cone passes through the inner Walls

Wavelengths Light is defined by its wavelength. It is a member of the frequency spectrum, and each frequency (sometimes called color ) of light has a wavelength associated with it.. Light is energy that travels (propagates) forward with a sinusoidal pattern, just as AC power does, and just as Radio Frequencies do. All channels of the spectrum have both a frequency, and a wavelength. You rarely hear RF channels referred to by their wavelengths, because it is easy enough to classify them by their frequencies. However, the frequencies of light are so high that the numbers become cumbersome. Therefore, they are identified by their wave lengths.

Common Wavelengths in fiber-optic communication systems  - wavelengths typically range from 800 to 1600 nm (nano-meters, where 1 nm = 1 billionth of a meter = 10-9 meters. You may see 633 and 780 nm from time to time, but by far the most common wavelengths actually used are 850 & 1300 for MultiMode fiber, and 1310 and 1550 nm for SingleMode fiber

There are many complex reasons for the selection of those particular wavelengths. But the main reason is that the Scattering/Absorption is low at those frequencies/wavelengths. In the chart below, it becomes evident that there are three low-lying areas of absorption, and an ever-decreasing amount of scattering as wavelengths increase. As you can see, all three popular wavelengths have almost zero absorption. However, 1550 nm waves suffer from much less scattering than 850 nm. We will discuss the high absorption areas, or "water peaks", later.

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One Wavelength Defined -  a wavelength is the distance between the peaks of two consecutive cycles of a sine wave, or between any two identical positions of the recurring pattern, as shown below:

Frequency and Wavelength are inversely proportional, and are a function of the speed of  propagation.. For example, light and radio waves are waves of electric and magnetic energy that travel through space at the speed of light, which is approximately 300 million meters per second. The distance a signal travels during one complete cycle is a wavelength. The speed or velocity of a wave (meters/sec) is equal to its wavelength (meters) times its frequency (Hz):

v = wf  w = v/f = 300,000,000/f  f = v/w = 300,000,000/w  Now, to make this simpler, since light wavelengths are normally given in nm (nano-meters), which is .000000001 meter, we can rewrite the equations in terms of w in nm :

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w (nm) = If we measure a radio wave’s frequency in megahertz (MHz), and its wavelength in meters, wavelength X frequency = 300 Using this formula, we can determine the wavelength if we know the frequency, or the frequency if we know the wavelength.To find the frequency, divide the wavelength into 300. The frequency of a 6 meter wave is 300 / 6 = 50 MHz The most common wavelengths for fiber-optic light is 1310 and 1550 nm (nm = nano-meter, which is 1 billionth of a meter). 1 nm = 10-9 meters.

Effects of Refraction on Different Wavelengths Different wavelengths respond differently to a given refractive index. For example, white light is composed of many wavelengths (it is the sum of all colors). As we all know, when white light passes through a refractive material such as a water particles in air, it is separated into it's constituent colors, and an array colors results (a rainbow). A similar effect occurs with a glass prism, as shown below. The refractive index bands shorter wavelengths more than longer ones. This can be seen below  blue and violet are bent more than green and red :

A simple Prism with white light Injected

This is why some frequencies of light travel through a multi-mode fiber more efficiently than others they refract just the right amount, and bounce off the wall of the core without refracted too sharply.

Multiple Fibers in one Sheath It is counter-productive for a company to spend millions on trenching and conduit installation for fiber, only to run out of bandwidth a few years down the road. Today's companies lay hundreds of fibers at a time, either is bundled groups, or all in one sheath. Here is a sample, Sieman's XGLO, 72fiber cable:

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Multi-Mode vs Single-Mode Fiber

A mode is a defined path in which light travels. A light signal can propagate through the core of the optical fiber on a single path (single-mode fiber) or on many paths (multimode fiber). The mode in which light travels depends on geometry, the index profile of the fiber, and the wavelength of the light. Fundamental, or Axial Mode  - the light that travels directly down the fiber core. It encounters the least resistance because it does not bounce off the walls. Single-mode fiber has only one mode - the axial mode. Higher Order Modes  - light that is at an angle, and bounces off the wall of the cladding. Only multimode fiber has higher order modes.

Multimode fiber uses multiple spectrums (colors) of light, while Single Mode uses a single spectrum of light.

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Multi-mode vs Single-mode Fiber (note the differences in Diameter of the Core) Although not shown - Single-mode will bounce off the wall if there is a turn in the cable !!

MultiMode Fiber multimode fiber is more expensive than singlemode fiber - but the equipment is less expensive (and equipment comprises 90% of the cost !!! ) MultiMode fiber uses numerous spectrums (colors) of light . . . each takes a different path through the fiber, bouncing off the reflective walls (note from the diagram - that that some take the direct route, straight through). It has the advantage of less expensive interfaces and electronics cost. However, distance is limited and the fiber itself is more pricey than single-mode.

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Multimode cable is made of of glass fibers, with a common diameters in the 50-to-100 micron range for the light carry component (the most common size is 62.5). Multimode fiber gives you high bandwidth at high speeds over medium distances. Light wav es are dispersed into numerous paths, or modes, as the y travel through the cable's core typically 850 or 1300nm. Typical multimode fiber core diameters are 50 and 62.5 microns, or um (um is micrometers, or microns - which is 1-millionth of a meter) for data c ommunications - and 100 and 200 micrometers for low-speed industrial usages.

Typical Diameters of MM Cables

However, in long cable runs (greater than 3000 feet [914.4 ml), multiple paths of light can cause signal distortion at the receiving end, resulting in an unclear and incomplete data transmission. There are two main types of MultiMode fiber construction - Step Index, and Graded Index. Step just means that the change in the refractive index is sudden (a "step"), while graded means the change is gradual:

Step-Index MultiMode - has a large core, up to 100 microns in diameter. As a result, 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 alternative pathways cause the different groupings of light rays, referred to as modes, to arrive separately at a receiving  point. The pulse, an aggregate of different modes, begins to spread out, losing its welldefined shape. The need to leave spacing between pulses to prevent overlapping limits  bandwidth that is, the amount of information that can be sent. Consequently, this type of fiber is best suited for transmission over short distances, in an endoscope, for instance

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Graded-Index MultiMode  - 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. Also, rather than zigzagging off the cladding, light in the core curves helically because of the graded index, 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: a digital pulse suffers less dispersion.

Single-Mode Fiber Single-mode fiber is for long distances, and therefore is not as common as multi-mode. It uses one  beam of light that shoots straight through. It has the advantage of high information-carrying capacity, low attenuation, long distance, and low fiber cost. Telephone and cable television networks install millions of kilometers of this fiber every year. Single-mode is a single strand of glass fiber with an extremely narrow diameter of only 8.3 to 10 microns (typically 9 microns is used) that has one mode of transmission. Wavelengths used for singlemode fiber are typically 1310 or 1550 nm. It carries higher bandwidth than multimode fiber, but requires a light source with a narrow spectral width. It is also called mono-mode optical fiber, singlemode fiber, single-mode optical waveguide, uni-mode fiber. Single-mode fiber gives you a higher transmission rate and up to 50 times more distance than multimode, but it also costs more. Single-mode fiber has a much smaller core than multimode. The small core and single light-wave virtually eliminate any distortion that could result from overlapping light pulses, providing the least signal attenuation and the highest transmission speeds of any fiber cable type. Mode Field Diameter -  like multimode, singlemode uses a step-index construction of the core. The mode-field diameter is the actual width of the light propagating through the core - not the core diameter. If you measure the intensity of the light beam going across the diameter of the core - it forms a gaussian distribution (a bell curve) as shown below. A typical SMF (Single-Mode Fiber) has approximately 80% of the light within the core, and 20% of light expands beyond the core and into the cladding. So the diameter of the entire mode includes the light within the cladding.

In otherwords, the Mode Field is described a s the entire light beam, and is larger than the core. Therefore the Mode Field Diameter is larger than the core diameter. Many fiber spec sheets will list  both the core diamater and the mode field diamater:

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Mode Field Diameter - typically 80% of the Core Diameter

The larger the mode field diameter - the easier it is to splice and connect, but the more sensitive it  becomes to bending losses. Cutoff Wavelength  - this is the minimum wavelength at which the fiber will support only one mode. Wavelengths that are shorter than the cutoff wave length, can actually allow higher-order modes to  propagate. This turns a singlemode fiber into a multimode fiber, and modal dispersion becomes a  problem.

Top fiber transmitting a wavelength longer than the cutoff length (results is a smooth movement through the waveguide, without bouncing off the walls) Bottom Fiber transmitting a Wavelength below the Cutoff length (results in creation of multiple Higher-Order modes, which bounce off the walls)

POF (Plastic Optical Fiber) POF (Plastic Optical Fiber) is a newer cable with a plastic core. Typical plastic fiber is 1000 microns in diameter with a 980 micron core, and is used in lighting, signs, automotives, and small local area networks. It promises performance similar to glass cable on very short runs, but at a lower cost. As you can probably guess, plastic is not quite as "pure" or "clear" as glass. Although it looks perfectly clear to the eye - it has a slightly more foggy interior as compared to glass, and therefore a greater resistance (attenuation) to light.

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Plastic Fiber Local Area Network 

POF is also much thicker than glass fiber, and cannot support the longer wavelengths of glass (the shorter the wavelength, the less distance it can travel). There are also plastic/glass composite fibers available.

Mode-Field Diameter  Not all light travels through the core of the fiber, but is distributed through both the core and the cladding. The "mode field" is the distribution of light through the core and cladd ing of a particular fiber. Mode-Field Diameter (MFD) defines the size of the power distribution. When coupling light into or out of a fiber, MFD is important in understanding light loss.

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The two Primary Factors that Cause Loss of Power Light has power !! It is not the same as electrical power, but nevertheless, it does contain power. If you don't believe that - just look at the farms of solar panel that collect sunlight and convert it ti electrical power. Like copper cables, optical cables also have power loss factors. The two main factors that cause power loss in fiber cables are: • Attenuation • Dispersion (Scattering) These are the two primary factors that answer the question - "why can the light travel through fiber forever?". Attenuation is resistance to the light. Dispersion is the spreading out of the light. In a  perfect vacuum in space - light will travel forever, and since there are no particles, and no substances, the light will encounter no attenuation. But let's look at the real worlkd - specifically, light traveling through a fiber medium: Lasers are used as a source for fiber-optic data communications. They refine and intensify light, but there is no such thing as a perfect laser with no dispersion !!!

Attenuation Unlike a perfect vacuum - glass is not a pure medium. It has imperfections, and tiny particles in it. Therefore it does resist light somewhat. The reduction in signal strength is measured as attenuation. Attenuation measurements are made in decibels (dB). Th e decibel is a logarithmic unit that indicates the ratio of output power to input power. Each optical fiber has a characteristic attenuation that is normally measured in decibels per kilometer (dB/km). Optical fibers are distinctive in that they allow high-speed transmission with low attenuation. Each type of fiber has a different attenuation versus wavelength curve, none of which is linear or exponential, but rather a complicated series of peaks and valleys. The attenuation of the light through

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a glass optical waveguide has been decreased over the last 25 years to less than 0.02 db/km and scientists are trying to decrease the attenuation even more. The following graph depicts the attenuation for a specific type of plastic optical fiber. As the graph shows, certain wavelengths are ideal for transmission because they fall in the valleys of the curve. For this type of plastic fiber, the best suited wavelengths a re 570 nm and 650 nm. For most glass fibers there are valleys around 850 nm, 1300 nm, and 1550 nm. For other types of fiber, such as HPCS (Hard Plastic Clad Silica) fiber, the attenuation curve is relatively flat over a wide spectral range.

Measuring Attenuation Indirect Measurements Cutback Method - the attenuation of a fiber can be measured by transmitting a signal through it and measuring the power at the opposite end. The fiber is then cut near the input end without changing the launch conditions and the power is measured again. The loss per unit length can be calculated from the difference  between these two values. Substitution Method  - a short length of reference fiber is compared with the attenuation of the fiber being tested.

Both the cutback and substitution methods have drawbacks. The attenuation measured by the cutback method varies according to the numerical aperture and the spectral bandwidth of the source. In the substitution method, the coupling losses of the fiber being tested and the reference fiber may be different, so the value derived may not be correct.

Direct Measurement OTDR (Optical Time-Domain Reflectometry)  - this is a more reliable technique for measuring attenuation in fiber. In this method, a brief signal (a

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short burst of optical power) is introduced into the fiber. When this signal encounters imperfections or discontinuities, some of the light is reflected by a  beamsplitter (a glass plate at a 45 degree angle to the incident light or two  prisms cemented together with metal-dielectric film between their faces) onto a detector, where it is amplified and displayed 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 o f OTDR is that it yields more accurate results.

Dispersion or Scattering (pulse spreading) If you turn on a light bulb, as you move away from the bulb, the light will be weaker and weaker. Dispersion merely spreads the light out, more and more, as it travels forward. If you shine a laser into space, it would disperse much less than a light bulb, and the light would be stronger (along the line where you aimed it). However, it will still disperse gradually. In a fiber, as light travels through it, the same thing happens only worse - because fiber is not a pure medium like air. It has impurities, couplers, and even the ends that can cause scattering in all directions. Lasers are used in fiber-optics. Light is sent down the fiber in the form of a series of pulses. As  pulses travel down the fiber they spread out. This spreading is known as dispersion. Dispersion is undesirable because it can cause bit errors when the signal reaches the receiver. To avoid bit errors, it is necessary to condition the signal using dispersion compen sation or to regenerate the signal using a repeater. The signal must be regenerated prior to the occurrence of any errors.

Scattering and OTDR's - actually, a ray of light is  partially scattered into many directions by microscopic variations in the core-cladding interface - in cluding back  !!. This effect is actually a key for the use toward th e sour ce  of OTDR's (Optical Time Domain Reflectometers). They depend on light being scattered back to the source, and they

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measure the intensity of this reflected light and compa re it with the intensity of the transmitted light and the characteristics of the fiber to measure the distance to a break or problem area of the fiber. Scattering can be caused by impurities in the glass, fiber ends, breaks, or splices.

Fibers with imperfections causing back-scattering Top fiber has impurities Middle Fiber has a crack in it Bottom fiber has a break 

The angle of some of the light rays impinging at these variations is changed enough so that they are refracted onto different paths and do not experience TIR (see Snell’s Law). For this reason, some light energy is lost. Scattering is responsible for up to 90 percent of the total attenuation. Rayleigh scattering causes about a 2.5 dB/km loss at 820 nm but less than 1 dB/km in wavelengths over 1000 nm. Modal Dispersion (also called Polarization Mode Dispersion) - AFFECTS MULTIMODE ONLY !! is where the two orthogonal polarization states of the mode separate, resulting in pulse spreading.

The "axial mode" (fundamental mode) arrives first, since it does not bounce off the walls of the cladding, and has a shorter overall path. The higher order   modes arrive later. Remember, the higher

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the order of a mode is - the more angle it has, and the farther it has to travel. Since the light modes have differing distances to travel, and also encounter differing amounts of resistance from refraction the result is that the data pulses may actually meld together and be unrecognizable at the far end !! The effects of modal dispersion increase with both data rate and distance. For this reason, singlemode fiber is mandatory for intercity (long distance) runs of fiber, an d multimode is used only for short distances (metropolitan areas):

Modal Dispersion - different modes (angles) traveling at different Speeds (problematic with MultiMode fiber) Chromatic Dispersion - affects single-mode fiber only !! Different "colors" of light (different wavelengths) propagate through fiber at different rates of speed. This effect only interferes with transmissions of extremely high bit rates. These rates are much too fast for multimode fiber, but can wreak havoc with single-mode. This is why single-mode fiber is used with wavelengths that encounter the least dispersion (i.e 1310 nm). For 1550 nm wavelengths,dispersion shifted  fiber and non-zero dispersion fiber is often used.

Chromatic Dispersion - different wavelengths (implies different frequencies) traveling at different Speeds (problematic with SingleMode fiber) Effect of Transmitter Line-Width on Dispersion - of the two types of optical transmitters, lasers suffer from much less dispersion than LED's, because they only transmit one wavelength (they have a narrow "line-width). :

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The Line-width of Laser vs LED

Reducing Modal Dispersion by using Graded Index Fiber With step-index, the refractive indices of the core and cladding change abruptly at their interface. This is the least expensive manufacturing process, has the most modal dispersion, and is used for short distance, low-speed applications. With graded-index fiber, layers of glass with smoothly varying refractive indices comprises the core. Because the core has the highest refractive index, the axial mode is slowed down, while the higher-order modes travel faster. This causes all modes to arrive at the far end of the fiber, closer together. This reduces modal dispersion, allowing longer fiber runs and higher transmission speeds.

Graded Index fiber on the bottom - causes the normally fast Axial mode to Slow down

Other Power Loss Factors Absorption Impurities in the glass, such as metal and hydroxyl ions. The ions absorb light energy and emit it as heat. The absorbed light by impurities turn photons into phonons (heat), include ions of copper, iron, cobalt, chromium and the hydroxyl (OH-) ions of water. The OH-ions of water and the molecular resonance of SiO2 are the principal reasons that light energy is absorbed by the fiber.

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Impurities absorb light and Emit Heat

Shorter wavelengths are absorbed more easily by impurities in the glass. This is another reason why infrared light is used instead of visible light - it minimizes absorption through it's longer wav elengths:

Reduction of Absorption through the use of Shorter Wavelengths (higher frequencies)

Water Peaks But what about the areas where absorption is high? They are also referred to as "water peaks".

Do we simply lose those frequencies? Are they unusable? Previously they were unusable, but recent advances in fiber manufacturing processes have made great strides. “Water Peaks” form during the fiber-fabrication process. Special precautions are taken to ensure that a low level of impurities contaminates the fiber, but in standard fiber, the OH ion impurities are not

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completely eliminated and they lead to the two absorption peaks shown above, near 1230nm and 1380nm range. However, specially prepared fibers, like the Lucent All-Wave, have been created - that cause these water peaks to virtually disappear !! This has opened up the entire window from 1280nm to 1600nm.

Bending Two types of bending exist: micro-bending and macro-bending. Micro-bending  is microscopic imperfections in the fiber geometry, such as, rotational asymmetry, variations of the core diameter, and course interfaces between the core and cladding caused by  pressure, tension and twist.

Fiber crimp allows light to escape Macro-bending  is the more commonly know type, and is when the fiber cable is bent in certain areas to accommodate the path it takes. For example, upon entering a splice box, the technician may have to bend the fiber 90-degrees to reach its termination port. All fiber has a listed bend limit, given as a minimum radius. You can buy plastic bend guides to do this, that prevent bending below the specified minimum radius. In general, macrobending can cause less than total reflection at the core-tocladding boundary. Bending loss is usually unnoticeable if the diameter of the b end is larger than 10 cm.

Bend exceeds this fiber's max radial Bend rating - light escapes

 In multi-mode fibers, a large number of modes is always present; and each mode in a fiber is attenuated differently (differential-mode attenuation). In single-mode fibers this effect is not as much of a problem.

Fresnel Reflection Fresnel Reflection is optical power loss in a fiber cau sed by light reflected off the ends of the fiber. This effect occurs due to differences in the refractive indices of the core and the air at the interfaces.

Poor Fiber Quality Loss Issues

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Poor finishing of the fiber ends scatters a portion of the light, as does variations in the diameters and cross sections of the core and cladding along the length of the fiber. Cheap fiber and connectors can also have a dramatic effect on transmission throughput. In particular, singlemode fiber must be manufactured with very tight tolerances (plus/minus 1 micron accuracy).

Fiber Splice and Connector Mismatches When splicing or connecting two fibers, the cladding and core must EXACTLY match. This is extremely ciritical, and even a few microns of mismatch can cause tremendous losses to occu r.

Two Fibers Conncted poorly - causes loss of light power and reflects light (back scattering)

Concentricity If the core is not centered properly within the fiber cable, it generally drifts throughout the length of the cable. This causes poor throughput of the light beam:

Fiber Curl Fiber is stored in rolls, and develops a curvature, or curl - over time. When two fiber segments are spliced or connected, any remaining curl can cause poor connections. This problem can usually be defeated by holding and reversing the curve manually for a minute or so, which has the effect of flattening out the fiber.