fiber optics lecture

7/28/2019 Fiber Optics Lecture

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Fiber Optics Defined as that branch of optics that deals with the transmission of

light through ultrapure fibers of glass, plastic, or some other form of

transparent media.

History of Fiber Optics

The use of light as a mechanized means of communicating has longbeen a part of our history. Paul Revere, as we know, used light back in

1775 to give warning of the approach of British troops from Boston.

In 1880, Alexander Graham Bell invented a device called thephotophone. The photophone used sunlight reflected off a moving di-

aphragm to communicate voice information to a receiver. Although

the device worked, it was slightly ahead of its time.

One of the first noted experiments that demonstrated the transmissionof light through a dielectric, medium has been credited to a British

natural philosopher and scientist named John Tyndall. In 1854,

Tyndall demonstrated before the British Royal.


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Milestones in Fiber Optics

1950s Brian O'Brien, Sr., Harry Hopkins, and Naringer Kapanydeveloped the two-layer fiber consisting of an inner core in which

light propagates and an outer layer surrounding the core called the

cladding, which is used to confine the light. The fiber was later usedby the same scientists to develop the flexible fiberscope, a device

capable of transmitting an image from one end of the fiber to its

opposite end. Its flexibility allows peering into areas that are normally

not accessible. The fiberscope, to this day, is still widely used,

particularly in the medical profession to peer into the human body.

Invention of the laser(light amplification by stimulated emissions ofradiation) by Charles H. Townes allowed intense and concentrated

light sources to be coupled into fiber.

Charles K. Kao and George Hockham of StandardTelecommunications Laboratories of England performed several

experiments to prove that if glass could be made more transparent by

reducing its impurities, light loss could be minimized. Their research

led to a publication in which they predicted that optical fiber could be

made pure enough to transmit light several kilometers. The global

race to produce the optimum fiber began.

1967 Losses in optical fiber were reported at 1000 dB/km.

1970 Losses in optical fiber were reported at 20 dB/km.

1976 Losses in optical fiber were reported at 0.5 dB/km.



1988 NEC Corporation sets a new long-haul record of 10 Gbits/s over80.1 km of dispersion-shifted fiber using a distributed feedback laser.

1988 The Synchronous Optical Network (SONET) was published bytheAmerican National Standards Institute (ANSI).

Multimedia applications for business have become the major impetus

for increased use of optical fiber within the LAN, MAN, and WAN

environment. These applications include employee training, desktop

conferencing, and news.


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Advantages of Fiber Optic Systems Bandwidth.

Less Loss.

Noise Immunity and Safety.

Less Weight and Volume. Security.

Flexibility.

Economics.

Reliability.

Disadvantages of Fiber Optic Systems Interfacing Costs.

Strength.

Remote Powering of Devices.

A Typical Fiber-Optic Telecommunications System

Theory of Light

Nature of Light:

Sir Isaac Newton and his followers believed that light consistedof rapidly movingparticles (orcorpuscles),

Dutch physicist Christian Huygens regarded light as being aseries ofwaves.


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Thomas Young, student at Cambridge University, Young'sinterests included the study of sound waves and their effects onthe human ear. Noting the similarities between the interaction of

sound waves and waves produced by water, Young wasinspired to investigate the hypothesis that light was also aseries of waves.

In 1860, established by James Clerk Maxwell theorized thatelectromagnetic radiation consists of a series of oscillatingwaves made up of an electric field and a magnetic fieldpropagating at right angles to each other.

By 1905, however, quantum theory, introduced by AlbertEinstein and Max Planck, showed that when light is emitted orabsorbed it behaves not only as a wave, but also as an

electromagnetic particle called a photon. A photon is said to possess energy that is proportional to its

frequency (or inversely proportional to its wavelength).

This is known as Planck's Law,

E=h x fwhereE - photon's energy (J)h - Planck's constant, 6.63 X 10 34 J-s

f - frequency of the photon (Hz)

Example:Compute the energy of a single photon radiating from a standard lightbulb. Assume that the average frequency of the emitted light is 1014Hz (which has a wavelength equal to 300 nm).

Electromagnetic Spectrum

Frequency and wavelength are directly related to the speed of light.For most practical purposes, they are governed by the equation


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= c/f

where:c = speed of light, 3 X 10s m/s

f = frequency (Hz) = wavelength (m)

TABLEUnits Typically Used to Designatethe Wavelength of Light

Unit Meters Inches

Nanometer

Micrometer

Angstrom

10-9

10-6

10-

10

39.4 X 10~9

39.4 X 10~6

3. 94 X 10~

9

That portion of the electromagnetic spectrum regarded as light hasbeen expanded the three basic categories of light:

1. Infrared: that portion of the electromagnetic spectrum having awavelength ranging from 770 to 106 nm. Fiber-optic systemsoperate in this range.

2. Visible: that portion of the electromagnetic spectrum having a

wavelength ranging from 390 to 770 nm. The human eye,responding to these wavelengths, allows us to see the colorsranging from violet to red, respectively.

3. Ultraviolet: that portion of the electromagnetic spectrum havinga wavelength ranging from 10 to 390 nm.

Speed of Light

The speed of light is at its maximum velocity when traveling in a

vacuum or free space. It is equal to

(2.997925 0.000001) X 108 m/s = 3 X 108 m/s


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Snells Law

Properties of wave:

Reflection Refraction

Diffraction

The amount of bending that light undergoes when entering a differentmedium is determined by the medium's index of refraction,generally denoted by the lettern. Index of refraction is the ratio of thespeed of light in a vacuum, c, to the speed of light in the givenmedium, v. This relationship is given by the equation

= c/vwhere:

refractive index, unitless

c velocity of light in free space

v velocity of light in any given medium

In 1621, the Dutch mathematician Willebrord Snell established that

rays of light can be traced as they propagate from one medium to

another based on their indices of refraction.

Snell's Lawis stated by the equation.


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Snell's Law:

n 1sin 1 = n 2 sin 2

where

n1

= refractive index of material 1

1 = angle of incidence

n 2 = refractive index of material 2

2= angle of refraction

Example

Refer to figure, using Snell's Law, compute the angle refraction for a

light ray traveling in air and incident on the surface of water at an

angle of 52 with respect to the normal.

TABLE

Index of Refraction for Various Mediums

Medium Index of Refraction

Vacuum 1.0

Air 1.0003

Water 1.33Ethyl alcohol 1.36

Fused quartz 1.46

Optical fiber 1.6 (nominal)

Diamond 2.2 (nominal)


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Total Internal Reflection

EXAMPLE: The direction of the light ray in the first example isreversed so that the light ray emerges from the water into air.

Compute the angle of refraction in air for a ray with the same incidentangle of 52.

Critical angle over which total internal reflection occurs

Light entering a medium whose index of refraction is less than themedium from which it exits will refract away from the normal unless

the angle of incidence exceeds the critical angle. When this occurs, atotal internal reflection occurs.


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Snell's Law is used to compute the refracted angle of the light ray asit exits the water and enters the air. In our solution, we can see that itis impossible to have an angle whose sine is greater than 1. Whenthis happens, a phenomenon known as total internal reflectionoccurs. Instead of the light ray refracting away from the normal as itenters the air, the ray is reflected off the interface and bounces backinto the water.

Critical Angle

EXAMPLE: Compute the critical angle above which total internal

reflection occurs for light traveling in a glass slab surrounded by air.

The glass slab has an index of refraction equal to 1.5, and for air, it is

equal to 1.

Parts of Fiber OpticsTwo key elements that permit light guiding through optical fibers are

its core and its cladding.

Fiber's core is manufactured of ultrapure glass (silicon dioxide)or plastic. The core of the fiber therefore guides the light andthe cladding contains the light.

Surrounding the core is a material called the cladding. A fiber'scladding is also made of glass or plastic. Its index of refraction,however, is typically 1 % less than that of its core. This permitstotal internal reflection of rays entering the fiber and striking thecore-cladding interface above the critical angle of approximately82 [sin(1/1.01)]. The cladding material is much lesstransparent than the glass making up the core of the fiber. This


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causes light rays to be absorbed if they strike the core-claddinginterface at an angle less than the critical angle.

Propagation Modes and Classifications of FiberA fiber's mode describes the propagation characteristics of an

electromagnetic wave as it travels through a particular type of fiber.

Two modes of transmission in a fiber

single-mode fiberwhen a ray of light is made to propagate in

one direction only, that is, along the center axis of the fiber.

multimode fiberIf there are a number of paths in which the light

ray may travel.

m = {[ d/sqt (12 - 2

2)] 2/2}where:m = number of mode propagationd = core diameter, m = wavelength (m)n 1 = refractive index of core

n 2 = refractive index of cladding

EXAMPLE: Compute the number of transmission modes for a lightray transmitted into a multimode step-index fiber having a corediameter of 50 m, a core index of 1.60, and a cladding index of1.584. The wavelength of the light ray is 1300 nm.

Refractive Index Prof ile of fiber core

Step index

Graded index


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Different Types of Fiber Optics

Single mode step-index fiber

The core of the fiber is manufactured substantially smallerrelative to multimode fiber.

In addition, the index of refraction of the core is furtherreduced, thus increasing the critical angle or decreasingthe angle at which the ray must penetrate the core withrespect to its central axis.

The intent is to permit light rays to propagate in one modeonly: through the central axis of the fiber.

The advantage to this is that all light rays travel the samepath and therefore take the same length of time to prop-agate to the end of the fiber.

Modal dispersion is minimized, and higher transmission

speeds are attained. Today's single-mode fiber dominates the

telecommunications industry. Although it is costly, it offersthe best performance in terms of information capacity(bandwidth) and transmission distance.

The disadvantage, however, is that the small corediameter tightens the requirements for coupling lightenergy into the fiber.

A laser is typically used as the light source.

Another disadvantage with single-mode fiber is the task ofsplicing and terminating the fiber. Alignment of the fiberrequires precise control.

Multimode step-index fiber

Useful in local applications that do not require enormoustransmission speed.

Core diameters range from 50 to 1000 m, this relativelylarge core size.


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It supports many propagation modes and permits the useof simple and inexpensive LED transmitters and PINdiode receivers.

The index of refraction is manufactured uniformlythroughout the core of multi-mode step-index fiber.

Light rays entering the fiber and exceeding the criticalangle will bounce back and forth to the end of the fiber.

This abrupt change in index of refraction between thecore and the cladding has resulted in the name step-indexormultimode step-index fiber.

Each ray will obey the law stating that the angle ofincidence is equal to the angle of reflection.

Good attenuation and bandwidth performance areobtained in the 820-nm region. Even better performance

is obtained in the 1300-nm range for multimode step-index fiber.

Multimode graded-index fiber To reduce the amount of pulse spreading or modal

dispersion arising from various propagation delay timesassociated with multimode step-index fiber, multimodegraded-index fiberis used.

This type of fiber serves as an intermediary betweensingle-mode and multimode step-index fiber in terms ofcost and performance.

Multimode graded-index fiber is characterized by its core

having an index of refraction that is gradedfrom its centerout to the cladding interface:that is, the index of refractionis highest at the center and gradually tapers off towardthe perimeter of the core. A bending effect is produced onlight rays as they deviate from the central axis of the fiber.

Modal dispersion is considerably reduced.


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Fiber Optics Construction Glass cladding/ glass core

Plastic cladding/ glass core

Plastic cladding/plastic core

Coating

Surrounding the fiber's cladding

is typically applied to seal and preserve the fiber's

strength and attenuation characteristics.

Coating materials include lacquer, silicones, and

acrylates.

Some fibers have more than one protective coating sys-

tem to ensure that there are no changes in characteristicsif the fiber is exposed to extreme temperature variations.

Sealing the fiber with a coating system also protects the

fiber from moisture.


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A phenomenon called stress corrosion orstatic fatigue may occur if

the glass fiber is exposed to humidity. Silicon dioxide crystals will

interact with the moisture and cause bonds to break down.

buffer jacket The coating is further surrounded

The buffer provides additional protection from abrasionand shock.

Some fibers have tightly buffered jackets, and some haveloosely buffered jackets.

strength member

It gives the fiber strength in terms of pulling.

Various methods used to strengthen a fiber-optic cableare considered next

outer jacket

Used to contain the fiber and its surrounding layers.

The outer jacket is usually made of polyurethane material.

Attenuation Losses In Optical

Attenuation is the term used by fiber manufacturers to denote thedecrease in optical power from one point to another.

= 10 log ( Po/Pin) Absorption loss

Rayleigh Scattering

Radiation Losses

Numerical Aperture, NA

It characterizes a fibers light gathering capability. Defined as the sine of the half the angle of a fibers light

acceptance cone.

NA = sin A

NA = sqt (12 - 2

2)


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Where:

NA= numerical aperture

A= acceptance angle of fiber core

1 = refractive index of the core

2 = refractive index of the cladding

Insertion Losses1. Intrinsic losses

Type of losses are due to factors beyond the control of the

user.

Core diameter

Numerical aperture

Index profile

Core/cladding eccentricity

Core concentricity

2. Extrinsic losses

Type of losses results from splicing and connectors assemble

process

Mechanical offsets between the fiber ends

Contaminants between fiber ends

Improper fusion, bending and crimping method

End finishes

Mechanical Offsets

Angular misalignment

Lateral misalignment

End separation

End face roughness

Losses due to reflection


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LIGHT-EMITTING DEVICES

Several devices are emitters of light, both natural and artificial. Few ofthese devices, however, are suitable for fiber-optic transmitters. What

we are interested in is a light source that meets the followingrequirements:

The light source must be able to turn on and off several tens ofmillions, and even billions, of times per second.

The light source must be able to emit a wavelength that istransparent to the fiber.

The light source must be efficient in terms of coupling lightenergy into the fiber.

The optical power emitted must be sufficient enough to transmitthrough optical fibers.

Temperature variations should not affect the performance of thelight source.

The cost of manufacturing the light source must be relativelyinexpensive.

Two commonly used devices that satisfy the above requirementsare:

the LED (light-emitting diode)

the ILD (injection laser diode).

LED Versus ILD

The major difference between the LED and the ILD is the manner inwhich light is emitted from each source.

The LED is an incoherentlight source that emits light in a disor-derlyway as compared to the ILD, which is characterized as acoherentlight source that emits light in a very orderlyway.

The ILD can therefore launch a much greater percentage of its

light into a fiber than an LED. Figure illustrates the differencesin radiation patterns.

Both devices are extremely rugged, reliable, and small in size.

In terms of spectral purity, the LED's half-power spectral widthis approximately 50 nm, whereas the ILD's spectral width isonly a few nanometers, shown in the figure. Ideally a singlespectral line is desirable. As the spectral width of the emitter


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increases, attenuation and pulse dispersion increase. Thespectral purity for the ILD and its ability to couple much morepower into a fiber make it better suited for long-distancetelecommunications links. In addition, the injection laser can beturned on and off at much higher rates than an LED. Thedrawback, however, is its cost, which may approach severalhundreds of dollars as compared to a few dollars for LEDs inlarge quantities. Table 18-4 lists the differences in operatingcharacteristics between the LED and the ILD.

Incoherent radiation Coherent radiation

Radiation patterns for the (a) LED and (b) ILD.

Light-Emitting Diode

Injection laser half-power spectral widthis only a few nanometers

LED half-power spectral widthapproximately 50 nm.

Comparison of special widths between the fiber-optic LED and ILD

Because of its simplicity and cost, the LED is by far the most widelyused light-emitting device in the fiber-optic industry. Most of us arefamiliar with LEDs capable of emitting visible light. They are used incalculators, watches, and a multitude of other visible displays. TheLEDs used in fiber optics operate on the same principle. By passingcurrent through the LED's PN junction, recombination occurs


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between holes and electrons. This causes particles of light energycalled photons to be released or emitted. The wavelength of thesephotons is a function of the crystal structure and composition of thematerial.Extensive research in material science, physics, and chemistry hasmade it possible to grow highly reliable crystals used in themanufacturing of fiber-optic LEDs. These LEDs are designed to emitlight in the infrared region for reasons explained earlier. Varioussemiconductor materials are used to achieve this. Pure gallium-arsenide (GaAs) emits light at a wavelength of about 900 nm. Byadding a mixture of 10% aluminum (Al) to 90% GaAs, gallium-aluminum-arsenide (GaAlAs) is formed, which emits light at awavelength of 820 nm. Recall that this is one of the optimumwavelengths for fiberoptic transmission. By tailoring the amount of

aluminum mixed with GaAs, wavelengths ranging from 800 to 900 nmcan be obtained To take advantage of the reduced attenuation lossesat longer wavelengths, it is necessary to include even more exoticmaterials. For wavelengths in the range 1000 to 1550 nm, acombination of four elements is typically used: indium, gallium,arsenic, andphosphorus. These devices are commonly referred to asquaternary devices. Combining these four elements produces thecompound indium-gallium-arsenide-phosphide (InGaAsP). Bytailoring the mixture of these elements, 1300- and 1550-nmemissions are possible.

Table:

Type OutputPower, W

Peakwavelength, nm

Spectralwidth, nm

Rise time,ns

LED 250700

1500

820820820

353535

1266

Laser 40006000

8201300

42

11


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Injection Laser Diode

Laseris an acronym forlight amplification by stimulated emissions ofradiation.

There are many types of lasers on the market. They are constructedof gases, liquids, and solids. For many of us, when we think of a laserwhat comes to mind is a relatively large and sophisticated device thatoutputs a highly intense beam of visible light. Although this is in parttrue, the laser industry is currently devoting a great deal of efforttoward the manufacture of miniature semiconductor laser diodes.Laser diodes are also called injection laser diodes (ILDs) becausewhen current is injected across the PN junction, light is emitted.Illustrates the construction of an ILD. ILDs are ideally suited for use

within the fiber-optic industry due to their small size, reliability, andruggedness. Various ILD package designs are shown in the figure.

Construction of the injection laser diode.

The ILD is basically an oscillator. A cavity built into its layeredsemiconductor structure serves as a feedback mechanism to sustainoscillation. The amplification necessary for oscillation is produced by

creating a condition in the cavity called apopulation inversion. A largedensity of holes and electrons in the cavity of the ILD is waiting torecombine. By injecting a large density of current (holes andelectrons) into the cavity, some holes and electrons in the cavityrecombine and release photons of light. A population inversion occurswhen a greater percentage of these holes and electrons combine torelease photons of light instead of creating additional holes and


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electrons. The released photons stimulate other holes and electronsto recombine and release even more photons, thus producing gain oramplification. To achieve oscillation, the ends of the crystal structureare polished to a mirror finish. This provides a feedbackmechanismthat causes light to reflect back and forth in the cavity,stimulating other electrons and holes to recombine and release morephotons. Losingis said to occur.

LIGHT-RECEIVING DEVICES

The function of the fiber-optic receiving device is to convert lightenergy into electrical energy so that it may be amplified andprocessed back to its original state. A device commonly used for thispurpose is thephoto diode orphotodetector.

Photodetectors

made of semiconductor materials

use the reverse mechanism of transmitting devices. Instead ofstimulated emissions of radiation, absorption ofphotons occur.When photons are absorbed, nholes and electrons are created,thus producing current. This is known as the photoelectriceffect.

Receiver sensitivity can be characterized by one of two parameters: quantum

efficiency, orresponsivity.

Both parameters are essentially the same.

Both are a measure of an optical receiver's sensitivity to aparticular wavelength.

If each particle of light (photon) illuminating the surface of aphotodetector were converted to a useful electron-hole pair, thequantum efficiency would be equal to 1, or unity. If 10% of thelight were reflected off of its surface, the quantum efficiencywould be equal to 90%, and so forth.

From a system designer's point of view, a more practical way ofinterpreting a receiver's sensitivity is to consider the amount ofoptical power that is directly converted to current at a specificwavelength. Most manufacturers specify their receivers in thismanner, which is called responsivity. Responsivity has the units


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of amperes per watt (A/W) or microamperes per microwatt(A/W). Typical values may range from 0.2 A/W to as high as100 A/W.

PIN Photodiode The most common photodetector used in fiber optics is the PIN

photodiode.

PIN is an acronym for P-type, intrinsic (I), N-type semiconductormaterial.

The P and N regions are heavily doped. The I region (shown asthe depletion region) is a lightly doped (near intrinsic) N-typematerial. Its purpose is to increase the depletion and absorptionregions when a reverse-biased potential is applied to the diode.In comparison to the LED, which emits light in the forward-

biased condition, the PIN photodiode is used in the reverse-biased condition. Current does not flow unless light (photons)penetrates the depletion region. When this occurs, electronsare raised from their valence band to the conduction band, thusleaving excess holes. These current carriers will begin to flowdue to the attractive force of the applied potential. Themagnitude of the current is proportional to the intensity of melight. Typical values for responsivity are in the order of 0.5 A/W.Bias voltages may range from 5 to 20 V.

The degree of absorption in a PIN photodiode depends on thewavelength and material of which the device is made. Forexample, if the diode is made of silicon, wavelengths of light inthe range 800 to 900 nm will penetrate. PIN diodes made ofindium-gallium-arsenide absorb light in the 1300-nm range.Sensitivity to longer wavelengths is achieved with indium-gallium-arsenide-phosphide. It gives the dimensions and across-sectional view of a typical silicon PIN photodiodesensitive to light in the 800 nm range.


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Avalanche Photodiode

The PIN photodiode is extremely well suited for most fiber-optic

applications, its sensitivity to light (responsivity) is not as great

as the avalanche photodiode (APD). Due to their inherent gain, typical values of responsivity for

APDs may range from 5 A/W to as high as 100 A/W.

This is considerably higher than the PIN photodiode, which

makes it extremely attractive for fiber-optic communications

receivers.

The APD functions in the same manner as the PIN photodiode,

except that a larger reverse-biased potential is necessary. This

is usually on the order of over 100 V. The APD is constructed ina manner that causes an avalanche condition to occur if a suffi-

ciently large reverse-biased potential is applied to it. As the

reverse bias increases, electron-hole pairs gain sufficient

energy to create additional electron-hole pairs, thus creating

additional ions (positive and negative charged particles). A

multiplication or avalanche of carriers occurs. This effect is

called impact ionization, which is considered to be an internal

gain advantage over the PIN photodiode.

The APD is extremely responsive to light, it is not without its

drawbacks. Unfortunately temperature and bias stabilization are

necessary with the APD, as both of these parameters influence

its performance. Also, costs are considerably higher than for

PIN photodiodes. In terms of size, the APD is similar to the PIN

photodiode.


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Splicing

Splice Connection

Two methods are used for splicing fiber ends together: the

mechanical splice, and the fusion splice. In a mechanical splice, twoends of fibers are brought together, aligned with a mechanical fixture,and glued or crimped together. In a fusion splice, alignment isperformed under a microscope. An electric arc is drawn that melts thetwo glass ends together and forms a strong bond. In general, thefusion splice offers better performance specifications in terms ofsplice loss. Both methods are commonly used in the field, but onemethod may not necessarily be better than the other. That is, sometechnicians feel more comfortable with one method over the other.The more sophisticated mechanical and fusion splices often includeprealignment under a microscope and final precision alignmentperformed automatically.

Fiber preparation

Cable preparation is necessary prior to splicing. The cablemanufacturer's procedures should be carefully followed for eachcable design.

Fiber ends are typically stripped of their plastic jacket and strengthmember and cleavedto a 90 angle. It may be necessary to removeany coating material. A number of methods are used to do this, suchas mechanical stripping tools, thermal stripping equipment, andchemical strippers.

The goal of cleaving is to produce a flat, smooth, perpendicular fiberend face.* The scribe-and-break method is generally used to cleavefibers. Both manual and automated tools are available. The quality ofthe cleave is one of the most important factors in producing high-

quality, low-loss fusion or mechanical splices. Whatever the methodused, the cleave must be a clean break with no burrs or chips. Thefiber end angle should be less than 1. The ends are then polishedand cleaned with a cleaning agent such as isopropyi alcohol or freon.


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Mechanical spliceDozens of mechanical splicing techniques are used to join bothsingle-mode and multimode fiber ends. Each has its advantages anddisadvantages. To achieve performances comparable to fusion

splicing, large, bulky, and expensive equipment is often used. Thismay include fixtures with built-in microscopes, micrometers, digitalalignment test circuitry, and ultraviolet curing lamps, a considerableamount of operator training and judgment is required. The currenttrend by many manufacturers is to produce inexpensive mechanicalsplicing kits that make splicing efficient and easy to perform with littletraining. Two of the more popular types of mechanical splices are theV-groove splice and the tube splice.

V-groove spliceThe V-groove splice shown in the figure, uses a block or substratewith a precisely cut groove through its center. For multiple splices, inthe case of ribbon cables, several V-grooves are manufactured intothe substrate for alignment. The fibers are placed in the grooves andbutted up against each other. Index matching glue is often used tobond the ends together and reduce reflection losses. Assuming thecore and cladding of the fibers are the same diameter, the groove willcause them to align with each other on the same axis. A top matchingplate is placed over the spliced fiber and secured. The entire splice is

placed into a box called a splice enclosure, which is used forenvironmental protection and strain relief.

Tube spliceFibers can also be aligned and spliced together through the use ofcapillary tubes. The tubes are machined to the diameter of the fiberand tunneled at each end to allow the fibers to be easily inserted. Forsingle-mode fiber, the capillary is typically round-holed, whereas formultimode fiber, the capillary is triangular-holed. Figure 18-21

illustrates various types of tube splices. Fiber ends are initiallyprepared in the manner described earlier. Index matching fluid is theninserted into the tube, and the fibers are pushed to the center of thetube against each other. The splice is exposed to ultraviolet light,which is used to polymerize or cure the epoxy or index matching fluid,thus forming a strong bond.


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(a) V-groove splice; (b) multiple V-groove splice for ribbon cable.

Some of the latest tube splice designs have become extremelypopular due to their simplicity. Many do not require epoxy, thuseliminating the need for ultraviolet curing equipment. Instead, thefibers are held in place-by a mechanical fixture or a crimp, as shownin the figure Losses tend to be slightly higher for these types ofdesigns (0.1 to 0.2 dB); however, they are extremely reliable,economical, and easy to install.

Fusion Splice

The fusion splice uses an electric arc to fuse or weld two fiber endstogether. The resulting splice is usually stronger than that ofunspliced fiber. Losses under 0.01 dB can be achieved with a fusionsplice. The trade-off, however, is that operator training and judgmentare relatively high compared to that required for the mechanicalsplice. In addition, equipment is bulky and often very expensive.Sophisticated microprocessor controllers are used to eliminateoperator judgment by automating most of the process.

fiber optics lecture

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