CH 4
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1 CHAPTER 4
FIBER OPTIC CONNECTIONS, SPLICING AND COUPLERS
LEARNING OBJECTIVES After you finish this chapter, you should be able to do the following:
1. Understand the inspection and cleaning process.
2. List the types of extrinsic and intrinsic losses.
3. Understand the degree to which fiber alignment and fiber mismatch problems increase extrinsic loss.
4. Detail the cleaving process for fiber- preparation.
5. Identify the types of fiber optic mechanical and fusion splices. Outline the basic splicing techniques for each type of fiber optic splice.
6. List the types of fiber optic connectors. Detail the procedure for terminating a fiber optic connector on an optical fiber.
7. Discuss the types of fiber optic passive couplers.
FIBER OPTIC CONNECTIONS Chapter 1 states that a fiber optic data link performs three basic functions. First, the data link transmitter converts an electrical input signal to an optical signal. Then, the optical fiber carries the optical signal to its destination point. Finally, the receiver converts the optical pulses back to an electrical signal identical to the original input. However, chapter 1 does not describe how optical power transitions through optical connections.
This chapter describes how the optical fiber cable plant is linked together. Starting with how the connector end faces are inspected and cleaned before making a connection. You will learn what types of losses are incurred in the cable plant and at the connection points. Also, this chapter will cover the types of connectors that are used, where they are used and how they are terminated. It also discusses the basics of fusion and mechanical splicing and how they differ from connectors.
A system connection may require either a fiber optic splice, connector, or coupler. One type of system connection is a permanent connection made by splicing optical fibers
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2 together. A fiber optic splice makes a permanent joint between two fibers or two groups of fibers. There are two types of fiber optic splices--mechanical splices and fusion splices. Even though removal of some mechanical splices is possible, they are intended to be permanent. Another type of connection that allows for system reconfiguration is a fiber optic connector. Fiber optic connectors permit easy coupling and uncoupling of optical fibers. Fiber optic connectors sometimes resemble familiar electrical plugs and sockets. Systems may also divide or combine optical signals between fibers. Fiber optic couplers distribute or combine optical signals between fibers. Couplers can distribute an optical signal from a single fiber into several fibers. Couplers may also combine optical signals from several fibers into one fiber.
In a fiber optic cable plant, minimizing loss at connection points is critical to system performance. Every connection in the plant is a possible point of failure. Extreme care must be taken when terminating and connecting the connectors. Understanding the proper building methods, what makes a quality end face, proper cleaning processes, and testing methodologies are important to the long term reliability of the fiber optic cable plant.
INSPECTION AND CLEANING
Fiber optic connection losses may affect system performance. The number one problem in maintaining a cable plant is contamination. A single contaminate particle mated in the core of an optical fiber can cause significant back reflection, insertion loss and even equipment damage. The average dust particle is 2 to 5 microns in diameter (.000002 to .000005um), which is not visible to the human eye. The typical filter size used in HVAC systems both commercially (in buildings) and onboard naval vessels is 5 microns. Most dust particles flow right through them. That is why you still have to dust your house! The filters were not designed to remove dust, just the big stuff. It would be like expecting a screen door to keep out the wind. Standard filters are not designed with optics in mind. See Figure 4-1 below.
Figure 4-1. - Connector Contamination Dirt/Contamination
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3 Therefore, a 4 micron dust particle sitting on the core of an 8 micron single mode fiber has the potential to create at least a 50 percent loss of power in the system. If a class 3a Laser is the light source, it has enough power to burn the dust particle into the glass core causing permanent damage. Also, if it is a single mode system, the back reflection caused by this particle can create bit errors and lower system performance to the point where the Laser will shut itself down in order to protect itself. Good cleaning processes are critical to system performance.
Before any optical connection is made you should inspect the end face using at least a 200X microscope (the Navy requires a 400X microscope) before mating the connector up. Also, since a dust particle has a negative charge and the glass and ferrule on the end face are dielectrics, the dust particle is attracted to the end face of the connector. Once the dust particle makes contact with the end face an ionic bond is made. In order to break the ionic bond a fiber optic preparation fluid with static dissipating properties should be used to clean the end face.
Cross contamination is also a major problem. If one side of the connection is dirty and is connected to a clean end face. The dirty one will cross contaminate the clean end face. That means you have to clean both end faces of the connectors being mated together before the connection can be made.
Microscopes are used to inspect single terminus connectors in the cable plant. Video inspection probes are used when inspecting multi-terminus connectors or at the ports of the electronics. See Figure 4-2 below. Every time a connection is disconnected or separated the end faces have to be inspected and cleaned prior to hooking them back together. See Figure 4-3.
Figure 4-2. - Video Inspection Probe
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Figure 4-3. - Inspect and Clean Process
Q1. What is the number one problem in maintaining a fiber optic cable plant? Q2. What magnification of microscope does the Navy require? Q3.What type of inspection equipment is used to view multiple terminus connectors?
TYPES OF OPTICAL FIBER LOSS Ideally, optical signals coupled between fiber optic components are transmitted with no loss of light. However, there is always some type of imperfection present at fiber optic connections that causes some loss of light. Minimizing that loss is very important to system operation and performance.
Where does the loss come from in the fiber optic cable plant? It is the sum of three components; 1) cable loss 2) connector loss and 3) splice loss. The cable loss is set by industry standards at 3.5dB/km @ 850nm and 1.5dB/km@1300 nm for multimode fiber. Single mode fiber is 1dB @ 1310nm, 1550nm and 1625nm for indoor cable and @ 0.5dB/km for outdoor cable at all three single mode wavelengths.
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5 Cable loss for the N avy is different from commercial standards due to the fire retardant process that those cables are required to have. They are 4.5dB/km @ 850nm and 2.0dB/km @ 1300 for multi-mode cables and 1.5dB/km @ 1310nm and 1550nm for single mode.
Insertion loss for all connectors is 0.75dB per mated pair and commercial splice loss is 0.3dB for both mechanical and fusion splices. Splice losses in the Navy are set at 0.2dB for fusion and mechanical splices. The sum of these losses (cable, splice, and connector) is called the Maximum Allowable Loss or MAL. The MAL determines the pass/fail criteria of the fiber optic cable plant and the links that make it up.
Let’s review some properties first discussed in chapter 2. Intrinsic losses are caused by the manufacturing process and are inherent in the glass. Extrinsic losses are caused by external forces being applied to the cables and connections.
Intrinsic Attenuation Intrinsic attenuation results from materials inherent to the fiber and from the manufacturing process. As precise as manufacturing is, there is no way to eliminate all impurities. When a light signal hits an impurity in the fiber, one of two things occurs: It scatters or it is absorbed. Intrinsic loss can be further characterized by two components: • Material absorption • Rayleigh scattering Material Absorption
Material absorption occurs as a result of the imperfection and impurities in the fiber. The most common impurity is the hydroxyl (OH-) molecule, which remains as a residue despite stringent manufacturing techniques.
Attenuation Versus Wavelength
The OH- symbols indicate that at the 950nm, 1380nm, and 2730nm wavelengths, the presence of hydroxyl radicals in the cable material causes an increase in attenuation. These radicals result from the presence of water remnants that enter the fiber-optic cable material through either a chemical reaction in the manufacturing process or as humidity in the environment. The variation of attenuation with wavelength due to the water peak for standard, single-mode fiber-optic cable occurs mainly around 1380 nm.
Absorption
Absorption accounts for three percent to five percent of fiber attenuation. This phenomenon causes a light signal to be absorbed by natural impurities in the glass and converted to vibration energy or some other form of energy such as heat. Unlike scattering, absorption can be limited by controlling the amount of impurities during the manufacturing process. Because most fiber is extremely pure, the fiber does not heat up because of absorption.
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6 Rayleigh Scattering
As light travels in the core, it interacts with the silica molecules in the core. Rayleigh scattering is the result of these collisions between the light wave and the silica molecules in the fiber. Rayleigh scattering accounts for about 96 percent of attenuation in optical fiber. If the scattered light maintains an angle that supports forward travel within the core, no attenuation occurs. If the light is scattered at an angle that does not support continued forward travel, however, the light is diverted out of the core and attenuation occurs. Depending on the incident angle, some portion of the light propagates forward and the other part deviates out of the propagation path and escapes from the fiber core. Some scattered light is reflected back toward the light source. This is a property that is used in an Optical Time Domain Reflectometer (OTDR) to test fibers. The same principle applies to analyzing loss associated with localized events in the fiber, such as splices.
Short wavelengths are scattered more than longer wavelengths. Any wavelength that is below 800 nm is unusable for optical communication because attenuation due to Rayleigh scattering is high. At the same time, propagation above 1700 nm is not possible due to high losses resulting from infrared absorption.
Extrinsic Attenuation Extrinsic attenuation can be caused by two external mechanisms: macro bending or micro bending. Both cause a reduction of optical power. If a bend is imposed on an optical fiber, strain is placed on the fiber along the region that is bent. The bending strain affects the refractive index and the critical angle of the light ray in that specific area. As a result, light traveling in the core can refract out, and loss occurs.
A macro bend is a large-scale bend that is visible, and the loss is generally reversible after bends are corrected. See Figure 4-4. To prevent macro bends, all optical fiber has a minimum bend radius specification that should not be exceeded. This is a restriction on how much bend a fiber can withstand before experiencing problems in optical performance or mechanical reliability. Longer wavelengths are more susceptible to bending losses
The second extrinsic cause of attenuation is a micro bend. Micro bending is caused by imperfections in the cylindrical geometry of fiber during the manufacturing process. Micro bending might be related to temperature, tensile stress, or crushing force. Like macro bending, micro bending causes a reduction of optical power in the glass.
Optical fibers would not be practical transmission media if their ability to guide light required them to be kept perfectly straight. It must be realized that any deviation from perfect straightness causes some light to scatter into the cladding and be lost. Such deviations can occur in two ways; via large bends that can be seen by the human eye, called macro bends, and by microscopically small deviations in the fiber axis, called micro bends. See Figure 4-4.
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7 Attenuation varies with the wavelength of light. Windows are low-loss regions, where fibers carry light with little attenuation. Refer back to Chapter 1. The first generation of optical fibers operated in the first window around 820nm to 850nm. The second window is the zero dispersion region of 1300m, and the third window is the 1550nm region.
The decibel is the unit of measurement used in optics to describe loss. A decibel is a ratio of the output power compared to the input power. 3dB is considered the half power ratio. For example; a transmitter has an output of 10 watts of power and it feeds that power into a link where a 3dB loss is incurred in the cable plant. The amount of power available at the receiver is now 5 watts.3dB = a 50% power loss or half the power. If the loss in the plant increased to 10dB the amount of power now available at the receiver would be 1 watt or a 90 percent drop of power. From this brief example it should be obvious that minimizing loss in the cable plant is extremely important to the performance and reliability of the network.
One important feature of attenuation in an optical fiber is that it is constant at all modulation frequencies within the bandwidth. In copper cables, attenuation increases with the frequency of the signal; the higher the frequency the higher the attenuation. As a result, signal frequency limits the distance a signal can be sent before a repeater is needed to regenerate the signal.
Bending Loss
For bending radii larger than a couple of inches, macro bending losses are small and imperceptible. For bending radii less than a few inches, loss increases rapidly and becomes prohibitively large at a certain critical radii.
Micro bends can cause high -order modes to reflect at angles that will not allow further reflection. The light is lost. Micro bends can occur during the manufacture of the fiber or can be caused by the technician. Manufacturing and cabling techniques have advanced to minimize micro bends and their effects. Micro bends are typically caused by the technicians installing the cables. Cable bands and wire ties being cinched down too tightly are the primary problem areas.
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Figure 4-4. - Loss and Bends
Q4. Where does loss come from in a fiber optic cable plant? Q5. What factors cause intrinsic and extrinsic losses? Q6.What unit of measurement is loss expressed in fiber optics? Fiber Alignment A main source of extrinsic loss in fiber connections is poor fiber alignment. The three basic coupling errors that occur at connection points are fiber separation (longitudinal misalignment), lateral misalignment, and angular misalignment. Most alignment errors are the result of mechanical imperfections introduced by fiber jointing techniques. However, alignment errors do result from installers not following proper connection procedures.
With fiber separation, a small air gap remains between fiber-end faces after completing the fiber connection. Figure 4-5 illustrates this separation of the fiber-end faces.
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Figure 4-5. - End Separation
Lateral, or axial, misalignment occurs when the axes of the two fibers are offset in a perpendicular direction. Figure 4-6 shows this perpendicular offset of the axes of two connecting fibers.
Figure 4-6. - Lateral Misalignment
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10 Angular misalignment occurs when the axes of two connected fibers are no longer parallel. The axes of each fiber intersect at some angle (Θ). Figure 4-7 illustrates the angular misalignment between the core axes.
Figure 4-7. - Angular Misalignment
L oss caused by lateral and angular misalignment typically is greater than the loss caused by fiber separation. Loss, caused by fiber separation, is less critical because of the relative ease in limiting the distance of fiber separation. However, in some cases, fiber optic connectors prevent fibers from actual contact. These fiber optic connectors separate the fibers by a small air gap. This air gap eliminates damage to fiber-end faces during connection and in high vibration applications
Most connectors are designed so that the connector ferrule end faces contact when the connector is mated. These are called Physical Contact (PC) connectors. The physical contact (PC) polish technique was developed for most connectors so that the fibers would touch when mated and lower insertion loss and back reflection.
Losses due to fiber alignment depend on fiber type, core diameter, and the distribution of optical power among propagating modes. Fibers with large NAs reduce loss from angular misalignment and increase loss from fiber separation. Single mode fibers are more sensitive to alignment errors than multimode fibers because of their small core size. However, alignment errors in multimode fiber connections may disturb the distribution of optical power in the propagating modes, increasing coupling loss.
Q7. List the three basic errors that occur during fiber alignment. Q8. When the axes of two connected fibers are no longer in parallel, the two connected fibers are in what kind of misalignment?
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11 Q9. What does PC stand for? Q10. Which are more sensitive to alignment errors, single mode or multimode fibers? FIBER END PREPARATION Fiber-end preparation begins by removing the fiber buffer and coating material from the end of the optical fiber. Removal of these materials involves the use of mechanical strippers or chemical solvents. When using chemical solvents, the removal process must be performed in a well-ventilated area. For this reason mechanical strippers are used for buffer and coating removal in the shipboard environment. After removing the buffer and coating material, the surface of the bare fiber is wiped clean using a wiping tissue. The wiping tissue must be wet with isopropyl alcohol before wiping.
The next step in fiber-end preparation involves cleaving the fiber end to produce a smooth, flat fiber end face. The score-and- cleave, or scribe-and-cleave, method is the basic fiber cleaving technique for preparing optical fibers for termination. The score-and- cleave method consists of lightly scoring (scratching) the outer surface of the optical fiber and then pulling the glass straight off. A carbide or diamond blade is used to score the glass. See Figure 4-8.
Figure 4-8. - Hand Held Fiber Cleaver
Q11. What buffer stripping method is used on naval ships?
Q12. What is the basic fiber cleaving technique for preparing optical fibers for termination? FIBER MISMATCHES Fiber mismatches are a source of intrinsic loss. As stated before, intrinsic loss results from inherent fiber characteristics of the two connecting fibers. Fiber mismatches occur when manufacturers fail to maintain optical or structural (geometrical) tolerances during fiber fabrication.
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12 Types of fiber mismatches include fiber geometry mismatches, NA mismatch, and refractive index profile difference. Fiber geometry mismatches include core diameter, cladding diameter, core ellipticity, and core-cladding concentricity differences. Figure 4- 9 illustrates each type of optical and geometrical fiber mismatch. Navy fiber specifications tightly specify these parameters to minimize coupling losses from fiber mismatches.
Figure 4-9. - Types of optical and geometrical fiber mismatches that cause intrinsic loss.
Core diameter and NA mismatch have a greater effect on intrinsic loss than the other types of fiber mismatches. In multimode fiber connections, the loss resulting from core diameter mismatch, NA mismatch, and refractive index profile difference depends on the characteristics of the launching fiber. Loss from core diameter mismatch results only if the launching fiber has a larger core radius (a) than the receiving fiber. Loss from NA mismatch results only if the launching fiber has a higher NA than the receiving fiber. Loss from refractive index profile difference results only if the launching fiber has a larger profile parameter (α) than the receiving fiber.
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13 Q13. List six types of fiber mismatches.
Q14. Does loss from refractive index profile difference result when the receiving fiber has a larger profile parameter ( α) than the transmitting fiber? FIBER OPTIC SPLICES History of Fusion Splicing
Much of the fundamental research concerning fusion splicing was conducted in the 1970’s. The first documented optical fiber fusion splices were performed by Dan L. Bisbee at Bell Laboratories. Bisbee proposed numerous techniques that are now standard practice in optical fiber fusion splicing: Proper preparation of fiber ends, obtaining planar end faces, and utilization of mirrors to permit orthogonal views of fiber end faces.
Initial optical losses of pioneering multimode fusion splices were measured to be as low as 0.5dB. Researchers in the UK performed the first fusion splice of single-mode fibers. This group recognized the extreme importance of fiber alignment for achieving low-loss single mode fibers.
In 1973, Bell Labs documented the first optical fiber cleaver designed to take advantage of brittle fracture to obtain extremely flat fiber end faces. With this utility, losses as low as 0.04 dB were achieved with multimode fibers. Initially, fusion splicing used nichrome wire as the heating element to melt or fuse fibers together. However in 1976, researchers at Hitachi in Japan turned to carbon dioxide (CO 2) lasers. At the same time, researchers at Corning Inc. suggested the use of an electric arc discharge to splice silica fibers.
Bisbee documented multimode fiber splice losses as low as 0.03 dB with the electric arc discharge technique. The first mass fusion splices of optical fiber ribbon cable was performed with a CO 2 laser by Kinoshita and Kobayashi in 1979. However the splices could only be spliced one at a time. In 1981, Tachikura followed the lead of Kohanzadeh by simultaneously splicing five multimode fibers with an electric arc. In 1985, Krause and Kurkjian of Bell Labs reported fabricating single-mode fiber splices whose failure strengths were just as good as the unspliced fiber. During the 1980’s and 1990’s, a variety of commercial fusion splicing equipment was introduced (electric heat source splicers) and a wide variety of specialty fibers was introduced (erbium-doped gain fiber, dispersion -compensating fiber, etc.).
Today, fusion splicers c an splice single fibers or up to 24 fiber count ribbon fibers at the same time. The small size of the fusion splice and the development of automated fusion-splicing machines have made electric arc fusion one of the most popular splicing techniques in commercial applications. The splices offer sophisticated, computer -controlled alignment of optical fibers to achieve low optical losses (0.02 dB).
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14 Basics of Fusion Splicing An optical fiber splice is a permanent fiber joint whose purpose is to establish an optical connection between two individual optical fibers. Connecting two optical fibers requires precise alignment of the mated fiber. This is required so that nearly all the light is coupled from one optical fiber across a junction to the other optical fiber.
Fusion splicing is performed with a specialized instrument (splicer) to fuse the fibers together. Fusion splicers use an electric arc to weld two optical fibers together. The process of fusion splicing involves using localized heat to melt or fuse the ends of two optical fibers together. The splicing process begins by preparing each fiber end for fusion. Fusion splicing requires that all protective coatings be removed from the ends of each fiber. The fiber is then cleaved (cut) with a precision cleaver to make it perpendicular.
The fibers are then placed into special holders (some splicing systems use removable holders, so fiber can be loaded into the holders and then fiber preparation commences) in the splicer. The splice is usually inspected via a magnified viewing screen to check the fiber end cleaves before and after the splice. The splicer uses small motors to align the end faces together, and emits a small spark between electrodes at the gap to burn off dust and moisture.
Then the splicer generates a larger spark that raises the temperature above the melting point of the glass, fusing the ends together permanently. The location and energy of the spark is carefully controlled so that the molten core and cladding don't mix, and this minimizes optical loss.
Finally, the splice loss is estimated by the splicer. Common techniques for loss estimation are: directing light through the cladding on one side and measuring the light leaking from the cladding on the other side or by taking measurements of the physical properties of the completed splice from the fiber image and using an algorithm to determine splice loss.
Purpose of Fusion Splicing Since the purpose of using fiber as a transmission medium is to transmit light, the fiber joint must transmit as much light power as possible with as little loss and back reflection as can be designed into the joint. Generally, fiber connections fall into two categories: the permanent or fixed joint (using a splice), and the non-fixed joint (using a connector).
Splices offer a lower return loss, lower attenuation, and are physically stronger than connectors. They're usually less expensive, require less labor, constitute a smaller joint for inclusion into splice closures, offer a better hermetic seal, and allow either individual or mass splicing. Splicing usually occurs in the middle of a long distance cable or where a permanent connection is required.
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Fusion splicing uses an electric arc to ionize the space between prepared fibers to eliminate air and heat the fibers to proper temperature (2,000C). The electrodes should be replaced on the typical splicer every 1000 arcs. The fibers then feed in as semi-liquids and meld together. A plastic sleeve with a strength member is used as a protective device that replaces the previously removed plastic coating. This process generally requires a controlled environment, such as a splicing van or trailer, to reduce the possibility of dust and other contamination.
Due to the "welding" process, it's sometimes necessary to modify the fusion parameters to suit particular types of fibers, especially if you have to fuse two different fibers (from two different manufacturers or with different core/cladding structures).
Fusion splicers are easily used with multimode fiber but single mode fiber adds a degree of difficulty due to the small size of the core. With multimode fibers, it is sufficient to align the claddings to achieve a low splice loss. Single mode fibers, however, demand that the cores be aligned. Achieving the best fusion is a complex matter involving not only precise alignment of fibers, but careful application of the correct arc discharge power and timing. An incorrect discharge, for example, can deform the cores. Splicers have adjustable power and time to accommodate different fibers and application needs.
The earliest splicers required a great degree of operator skill to align the fibers, often visually through a microscope. Often power through the fibers was measured as the fibers were being aligned. At the point of maximum power at the junction, the fibers were locked in place and fused. Such power monitoring could be done remotely with an OTDR or locally by techniques that injected a small amount of light into cladding at a bend in the fiber. Micropositioners hold the fibers and allow them to be precisely aligned three dimensionally along the x, y, and z axes, either manually or automatically.
Figure 4-10. - Fusion Splicer
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16 Today’s splicers have a high degree of computer control and analysis to automate the fusion process. See Figure 4-10. The profile alignment system uses collimated light reflected off a mirror, through the fiber at right angles to the axis, and into a video camera. The video camera connects both to a screen for the operator and to a computer that analyzes the images.
Figure 4-11. - Estimated Loss
Because of the way light is refracted by different refractive indices of the fiber, the cladding will appear dark and the core will appear light. The splicer’s computer analyzes the images to locate the centerlines of the cores. The computer then moves the fibers into alignment. The camera can move to analyze the fiber on two perpendicular planes. Once the fibers are aligned, the computer will estimate the loss of the splice. See Figures 4-11. If the value is not acceptable, the operator can clean the fibers or recleave them. Once the loss is within acceptable limits, the operator initiates the fuse cycle. After the splice is complete, the operator can view the alignment of the cores.
The biggest danger in fusion splicing is the fibers shifting along the axis during fusion due to the surface tension of the molten glass. The splicer, by knowing the parameters of the fiber and the arc discharge, can compensate automatically for axis shift. Another danger is that fusion splices have had their mechanical strength members removed as part of the process. This leaves them open to failing under stress. Typically a
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17 heat shrinkable sleeve is applied over the splice to protect it from stress. A fusion splice generally will fail close to but not exactly at the splice point.
Optical fibers might have to be spliced for a number of reasons. In the commercial industry, optical fibers may need to be fusion spliced due to longer lengths of optical fiber needed for network installs or fiber damage which requires emergency repair. In addition, splices might be required at building entrances to transition to properly rated cable, wiring closets or potentially any point between a transmitter and receiver. Optical fiber splices also permit repair of optical fibers damaged during installation, accident, or stress. System designers generally require fiber splicing whenever repeated connection or disconnection is unnecessary or unwanted.
In the military environment, there are a number of reasons why fusion splicing is preferred if repeated connection or disconnection is unnecessary. One reason splicing is preferred is to reduce the amount of time it takes to complete the installation of fiber connections in a Fiber Optic Interconnection Box (FOICB). This reduction in time is possible because the alternative to fusion splicing is to terminate and polish optical connectors and time associated with installing and testing fusion splices is reduced as compared to connectors.
System design may require that fiber connections or links have specific optical properties (low loss) that are met only by splicing fibers. Fusion splicing allows for less optical loss within a given optical link. For optical power budgeting purposes, fusion splicing increases the amount of power in a given link since coupled connector loss is out of the picture. Fusion splicing has the potential to save money for the Navy with regards to the time involved for installing and testing Fiber Optic Cable Plants (FOCPs).
Another example is to possibly eliminate the amount of FOICBs throughout Navy vessels. This is possible due to potentially allowing more connections within each FOICB. The FOCP is designed to be as maintenance free as possible. Therefore, with fusion spliced optical fibers within the FOICBs, there is no need for cleaning and/or repairing optical connectors.
Advantages to Fusion Splicing Fusion splice joints are compact, exhibiting an area no larger than the original optical fiber. This does not include when optical splices are protected by a protective sleeve. Optical loss and reflectance of a fusion splice are typically much lower than alternative fiber connecting technologies. Fusion splices are permanent and they exhibit mechanical strength and long-term reliability that approaches the original fiber.
Fusion splices are very stable to their alignment. They can withstand extremely high temperatures or extremely high optical power densities. They do not allow dust or contaminants to enter the optical path and reduction in optical testing.
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18 Disadvantages to Fusion Splicing Splicing requires special training to operate the fusion splicing equipment. However, what doesn’t require special training nowadays! Upfront costs for equipment and materials have the potential to be costly ($18,000 for a splicer). Generally, the splicer is the more expensive, while the splicing materials are the cheaper. Routine maintenance is required for the fusion splicing equipment. The preventative/routine maintenance schedule is based on the number of splices completed not a period of time.
Lack of understanding of fusion splice technology can be detrimental to the outcome of the fusion splices if not corrected. Most fusion splicers make it difficult to make a bad splice, however the potential is still there. That is why special training is needed.
Q15. Define a fiber optic splice. Q16. Fiber splicing is divided into two broad categories that describe the techniques used for fiber splicing. What are they? Q17. How often should the electrodes be replaced on a fusion splicer?
Q18. What are the advantages to fusion splicing over conventional terminations?
MECHANICAL SPLICES
Figure 4–12. - Splices, from left, Fusion Splice, Elastomeric, Ultrasplice, Camlock, FiberLok, AT&T Rotary Splice
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19 Mechanical splices, see figure 4- 12, are used to create permanent joints between two fibers by holding the fibers in an alignment fixture and reducing loss and reflectance with a transparent index matching gel or optical adhesive between the fibers that matches the optical properties of the glass. Mechanical splices generally have higher loss and greater reflectance than fusion splices, and because the fibers are crimped to hold them in place, do not have a strong fiber retention or pull-out strength. The splice component itself, which includes a precision alignment mechanism, is more expensive than the simple protection sleeve needed by a fusion splice.
Mechanical splices are most popular for fast, temporary restoration or for splicing multimode fibers in a premises installation. They are also used - without crimping the fibers - as temporary splices for testing bare fibers with OTDRs or OLTSs. Of course most prepolished splice connectors (the Navy refers to these as quick connect connectors or QCCs) use an internal mechanical splice (several actually have fusion splices) so the mechanisms and techniques described here apply to those also.
The advantage of mechanical splices is they do not need an expensive machine to make the splices. A relatively simple cleaver and some cable preparation tools are all that's needed, although a visual fault locator (VFL) is useful to optimize some types of splices.
Alignment Mechanisms
The biggest difference between mechanical splices is the way the fibers are aligned. Here are some typical methods.
Figure 4-13. - Capillary Tube
The simplest method of making a mechanical splice is to align two fibers in a small glass tube with a hole just slightly larger than the outside diameter of the fibers and butt the ends together. This type of splice works well with UV-curable adhesive as well as an index-matching gel located between the fibers. The Ultrasplice is a capillary splice see figure 4-13.
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V-Groove
Figure 4-14. - Fixed V-Groove Splices
Fixed V-groove splices are quite simple and work well. They work for single fibers or even for fiber ribbons as shown here. The grooved alignment plates can be made of many types of materials and are quite inexpensive, see figure 4-14.
Figure 4-15. - 3M Fiberlok
The 3M Fiberlok, see figure 4-15, is a version of a V- groove splice that uses a metal stamping inside a plastic case to both align fibers and crimp them. It's elegant design and good performance has made it one of the most popular mechanical splices.
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Figure 4-16. - Fused Glass Array
The fused glass array, see figure 4-16, method has a more complex alignment mechanism, made from four small glass rods fused together with a bend in the middle. The fibers follow the grooves made by the joint of two rods. The complexity and expense of this, especially compared to a simple V-groove, limited its use.
Figure 4-17. - GTE Elastomeric Splice
The GTE Elastomeric splice (still available from Corning) see figure 4-17, uses soft elastomers to hold the fibers in position. It's similar to a fixed v-groove, but the grooves are soft so they accommodate slight variations in fiber diameter easily.
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Figure 4-18. - Rotary Mechanical Splice
The AT&T Rotary splice, see figure 4-18, is more like a connector. The fibers are glued into glass ferrules and polished. They are then inserted into an alignment sleeve and rotated until the lowest loss is obtained, this is called tuning the splice. Again, complexity and cost, plus labor required, limited their popularity. NOTE: This splice is approved by the Navy and is currently installed in all the AGEIS platforms. The installation methods are found in MIL-STD 2042() Part 5 Method C.
The Tyco LightCrimp* Splices (P/N 1985368-1 or A-A-59917) see Figure 4-19, are used in the commercial industry for new installations and repair of telephone company central offices, CATV head ends, inter-building backbones and customer premise applications. The Navy has approved this splice only for use as a damage control repair replacement for an existing fusion splice. This splice has been approved (FEB 2013) and is listed on the current Qualified Parts List (QPL). The completed splices are intended to be stored in a qualified MIL-DTL-24728/8 spice tray and splice tray holder mounted inside a qualified MIL-I-24728 fiber optic interconnection box.
Figure 4-19. - Tyco LightCrimp Mechanical Splice
Cleaving Is Important
The most important step in mechanical splicing is cleaving the fiber properly. Most mechanical splicing kits come with an inexpensive cleaver that looks like a stapler or a beaver’s tail and hence is commonly called a beaver cleaver, see figure 4-20.
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Figure 4-20. - Fitel S-315A Single Fiber Cleaver (Nick Named - Beaver Tail Cleaver)
While this cleaver can produce quality results, its operation requires some practice and consistent use. The same can be said of all inexpensive hand-held cleavers. A better choice is one of the more expensive cleavers used for fusion splicers. It’s cost is offset quickly by producing higher yields. See figure 4-21 showing examples of good and bad cleaves.
Figure 4-21. - Examples of Cleaves
Mechanical Splicing Process
Cable and fiber preparation is practically the same as for fusion splicing.
Prepare the cables to be spliced
Ensure all jacketing material, Kevlar and fillers are removed leaving only the OFCCs. Remove the OFCC 2mm jacket about 8 to 10 inches exposing the 900um tight buffer for splicing. Ensure that there is enough tight buffer left for dressing the buffer tubes and fibers in the splice closure. Leave the proper amount of strength members to
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24 attach the cable to the closure. Refer to the direction and methods for installing a mechanical splice found in the Emergent Repair Kit. (If not found onboard contact the ISEA at NSWC Dahlgren Va.) Remove the tight buffer about 30-40mm exposing the glass fibers for splicing. Generally splice closures will require ~1 m buffer tubes inside the closure to and ~ 1 m fiber inside the splice tray.
Prepare the fibers to be spliced
The process is the same for all splice types: strip, clean & cleave. Each fiber must be cleaned thoroughly before stripping for splicing. When ready to splice a fiber, strip off the buffer coating(s) to expose the proper length of bare fiber. Clean the fiber with appropriate wipes. Cleave the fiber using the process appropriate to the cleaver being used.
Splicing
Most splices are designed to limit the depth of the fiber insertion by the buffer coating on the fiber. Some splices clamp both fibers at once. Insert the first fiber until it bottoms out. Then inset the second until you see the first fiber move. That tells you the two butt ends are touching. Once the two ends are making contact clamp the fibers in place. If they are clamped separately inset the ends of the fibers until they bottom out and then clamp both individually in place.
Optimizing Splices Using A Visual Fault Locator
You can improve the loss of a mechanical splice by gently withdrawing one of the fibers a slight amount, rotating it slightly and reinserting it. It works best with a VFL (visual fault locator) if the fiber ends that are being spliced are visible.
Figure 4-22. - Checking a Splice with a VFL
Shine a visual fault locator into the fiber and note the light loss at the splice, see Figure 4-22 (Left in photo). Pull one fiber out by 1-2 mm (about 1/16 inch.). Rotate the fiber slightly and reinsert fully. Keep trying and watch for minimal light (Right in photo.) Crimp fiber in place.
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25 19. What are some of the advantages of using a mechanical splice? Q20. The Navy recommends using what two mechanical splices? Q 21. What piece of test equipment makes it easy to locate a bad splice?
FIBER OPTIC CONNECTORS
A connector is a disconnectable device used to connect a fiber to a source, detector, or another fiber. It is designed to be mated and demated many times. A splice is a device used to connect one fiber to another permanently.
The key to a fiber -optic interconnection is precise alignment of the mated fiber cores (or the mode field diameter in single-mode fibers) so that nearly all the light is coupled from one fiber across the junction into the other fiber. Contact between the fibers is not even mandatory. The demands of precise alignment on small fibers create a challenge to the designer of the connector or splice. TIA 568-B states the maximum allowable loss per splice is .3 dB and the maximum allowable loss per mated pair is .75 dB. The following is a list of desirable features for a fiber-optic connector or splice:
Low loss- the connector or splice should cause little loss of optical power across the junction.
Easy installation- the connector or splice should be easily and rapidly installed without need for extensive special tools or training.
Repeatability- a connector should be able to be connected and disconnected many times without changes in loss.
Consistency- there should be no variation in loss; loss should be consistent whenever a connector is applied to a fiber.
Economical- the connector or splice should be inexpensive, both in itself and in special application tooling.
Light- duty connectors and heavy-duty connectors are two ways that the Navy classifies fiber optic connectors. The light-duty connector in shipboard applications has to be housed in areas that protect it from the environment such as in an interconnection box or equipment enclosure. The single terminus connector has a heavy duty spring and is keyed. It has a 2.5mm ferrule and is pre-radiused for dome end polishes for either
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26 multimode or single mode applications. They are described in specification sheets 16, 17, and 18 of MIL-C-83522. Figure 4-23 shows a sample of the ST light-duty connector.
Figure 4-23. - ST Light Duty Connector
Figure 4-24 shows one type of heavy-duty connector designed for use in harsh Navy environments. This connector is described by the military specification MIL-C- 28876. This connector comes in various sizes capable of 4, 8 or 31 channels. Each channel is terminated using a MIL-PRF-29504 pin or socket termini. The pin is housed in the plug end and the socket is housed in the receptacle. The pin and socket are joined together in an alignment sleeve, which attaches to the ceramic ferrule. Fiber alignment occurs when the pin terminus slides into the alignment sleeve of the socket terminus. The termini are held within an insert, called the nest, in the connector shell. When the connector halves are mated, the connector inserts align the mating termini, which then align the mating fibers. The connector shell and back shell protect the termini from the surrounding environment and provide strain relief for the multifiber cable.
Figure 4-24. - MIL-C-28876 Heavy-Duty Connector
Another type of multi-terminus connector is the Hermaphroditic that comes in numerous configurations. The Navy uses the 4 and 12 channel configurations. See Figure 25. The 12 channel is used for pier side connectivity for all communications while in port stateside.
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27
Figure 4-25. 12. - Channel and 4 Channel Hermaphroditic Connector
Figure 4-26 shows how an expanded-beam connector uses two lenses to expand and then refocus the light from the transmitting fiber into the receiving fiber. Expanded- beam connectors are normally plug adapter- plug type connections. Fiber separation and lateral misalignment are less critical in expanded beam coupling than in butt-jointing. The same amount of fiber separation and lateral misalignment in expanded beam coupling produces a lower coupling loss than in butt-jointing. However, angular misalignment is more critical. The same amount of angular misalignment in expanded- beam coupling produces a higher loss than in butt-jointing. Expanded-beam connectors are also much harder to produce. Present applications for expanded-beam connectors include multifiber connections, edge connections for printed circuit boards, and other applications.
Figure 4-16. - Expanded-beam connector operation
Q 22. What qualities are desirable features of a connector?
Q 23. What two ways does the Navy classify connectors?
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28 Causes of Loss in an Interconnection
There are three different types of factors that can cause loss in fiber-optic interconnections:
1. Intrinsic or fiber-related factors are those caused by variations in the fiber itself. 2. Extrinsic or connector-related factors are those contributed by the connector itself. 3. System factors are those contributed by the system.
Intrinsic factors are loss factors caused in the manufacturing process. NA- mismatch loss occurs when the NA of the transmitting fiber is larger than that of the receiving fiber. Core-diameter-mismatch loss occurs when the core or diameter of the transmitting fiber is larger than that of the receiving fiber. Cladding-diameter-mismatch loss occurs when the claddings of the two fibers differ, since the cores will no longer align.
Concentricity loss occur s because the core may not be perfectly centered in the cladding. The geometric axes of the core and cladding should coincide.
Ellipticity (or ovality) loss occurs because the core or cladding may be elliptical rather than circular. The ellipticity or ovality tolerance of the core and cladding equals the minimum diameter divided by the maximum diameter.
These variations exist in any fiber. The manufacturer controls these variations by manufacturing fiber to tight tolerances. In the past few years, manufacturing techniques have improved significantly that fiber tolerances are much tighter, typically +/- 0.2 um, resulting in a range of 123 to 127 um and a maximum loss of 0.28 dB.
Extrinsic factors contributing to loss occur when two fibers are not perfectly aligned on their center axes, even if there is no intrinsic variation in the fibers. The loss results from the difficulty of manufacturing a device to the exacting tolerances required. The four main causes of loss that a connector or splice must control are:
1. Lateral displacement 2. End separation 3. Angular misalignment 4. Surface roughness
Lateral displacement is when one fiber’s axis does not coincide with that of the other. Loss is determined by the ratio of the lateral offset to the fiber diameter. The acceptable offset becomes less as the fiber diameter becomes smaller. Connector manufacturers attempt to limit displacement to less than 5% of the core diameter.
End separation between two fibers will result in two types of loss. The first is Fresnel reflection loss, which is caused by the difference in refractive indices of the two fibers and the air gap.
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29 Connectors and Installation
After fiber optic cables have been installed, connectors shall be used to interconnect two optical cable components (OFCC's) inside an interconnection box or equipment. It should be apparent by now that optical fibers cannot be terminated using the same tools and procedures used to terminate metallic conductors in conventional communication cables.
Where termination of copper conductors is dependent upon a solid metal-to-metal contact, the termination of optical fibers is dependent upon alignment of the fiber cores. The fibers must be configured in such a way as to provide maximum light transfer between them. Connectors are designed to provide this alignment, and your job is to mount the fibers in the connectors in a way that will facilitate maximum alignment and hold the fibers firmly in place.
This topic will address the issues related to connector inst allation. First, we will discuss the purpose of connectors and the configurations in which they are used. Next the many styles of connectors available and how they may be used. The type and use of the basic tools and materials required and also take a look at basic procedures required for installing all connectors. Specific tools and procedures for the connectors used will be addressed in detail during the tool kit and connector demonstrations.
Purpose of Connectors The termination of optical fibers is normally accomplished through the use of connectors. A connector is defined as a device that allows an optical fiber to be repeatedly connected or disconnected to cables, patch panels, transmitters (source) or receivers (detector). Please do not confuse a connector with a splice. A Fiber Optic splice is a permanent junction between two fibers not designed to be repeatedly connected.
Patch Cords - Patch cords are relatively short sections of jacketed or buffered fiber cable that are generally used to connect items of end equipment. A patch cord will have a connector at each end.
Pigtails - Pigtails are sections of cable with a connector only at one end, which is the end to be connected at the equipment. The other end of a pigtail will be spliced to the cable that enters from the field. Pigtails are often configured in an arrangement called "breakout" cable. The breakout cable is nothing more than multiple pigtails contained within a sheath. Pigtails or breakout cables have become less prevalent in recent years because installers are terminating the field cable directly to many equipment items. This higher standard has been adopted by the military. This eliminates a splice location at each end of the cable, which results in lower attenuation.
Furcation - Blown Optical Fibers (BOF) are made with 500 µm acrylate coating and most connectors are made to accommodate 2 or 3mm jacketed fibers with 900 µm buffer.
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30 Blown Optical Fibers (BOF) with the 500 µm acrylate coating can be terminated using a furcation unit. An outer jacket with strength member is placed around the small buffer to build up the fiber for a connector fit. Blown Optical Fiber cable furcation terminate just like conventional fiber. See Figure 4-27.
Figure 4-27. - Furcation Unit
Descriptions of Connectors
Connectors may be described in many different ways, depending on the criteria used. There are basic styles, each of which could include variables such as finishing technique or ferrule type. Connector quality can be based on bore quality, thermal properties, and mechanical properties.
Styles of Connectors Standardization is a serious problem throughout industry and the military. Connector selection is one of the most affected areas. Only use those connectors listed in the contract or specification for the job or statement of work. Let’s look at some of the most widely used connectors. As you become familiar with the connectors, keep three basic considerations in mind: (1) threaded connectors can cause big problems when excessive torque is used, (2) keyed connectors allow for consistent, repeatable alignment, (3) as more fiber is installed, termination space will become more valuable.
SMA CONNECTORS. SMA (Sub-Miniature, Type A) Connectors, originally designed by Amphenol, use a threaded coupling nut without a keying device. The two basic types are the 905 style and 906 style. The 905 uses a straight ferrule and the 906 has a step-down nose to allow use of plastic alignment bushings for maximum alignment. Originally designed with a steel ferrule for multimode applications, they are now available with ceramic ferrules for single-mode applications. The primary problems that
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31 arise with the use of SMA connectors are crushing due to over-tightening of the threads, and repeatability of alignment because of the lack of a keying device.
BICONIC CONNECTORS. Named for their conical shape, many items of equipment installed during the 1980s still require interfacing using Biconic connectors. These were the first connectors used on single-mode fibers, although, they are available for single-mode or multimode applications. Biconic connectors are not keyed and early problems developed with repeatability and crushing due to over tightening. Later versions of Biconic connectors were available with a keying feature.
ST CONNECTORS. ST (Single Terminus) Connectors were designed by AT&T Bell Laboratories for use with single-mode or multimode fibers. They use quick-release keyed bayonet couplings that are preferred in situations where severe vibrations are not expected. The ST is probably the most popular and widely used connector in local area networks, test equipment and other applications. The keying feature ensures that the fiber is always inserted to the mating bushing with the same orientation. The bayonet coupling prevents crushing due to over-tightening.
FC CONNECTORS. Named FC for “field connector,” it was originally devised by Nippon Telephone and Telegraph (NTT) for telecommunications. It was used by MCI in its fiber optic telephone network in the 1980s. The connector has a threaded coupling feature similar to the SMA for use in high-vibration environments. The threads would be difficult to over tighten because stops have been installed to obtain repeatable torque. It also offers a keying feature similar to the ST, except that some FC connectors are “tunable.” The term “tunable” means the keying slot can be rotated to find optimal alignment and will remain in that alignment until moved again. The FC connector is available for single-mode and multimode applications.
SC CONNECTORS. Named SC from “subscriber connector,” it was also developed by NTT and gained popularity throughout the 1990s for both single-mode and multimode applications. They use a push-pull engagement for mating and are designed to be pull-proof so a slight pull on the cable will not disengage the connection. The SC connector is a strong competitor to the FC and ST connectors due to the ease in constructing multifiber connectors for duplex configurations. Connectors such as the FC, ST, and SMA that require twisting are not readily adaptable to multifiber connections in high-density applications because of the space required to allow rotation. AMP has already developed a mini SC for even higher density applications.
Connector End Finish In addition to styles, the type of finish may categorize connectors. The three most common types of finishes are the flat finish, the physical contact (PC) or Angled Physical Contact (APC).
FLAT END FINISH. This type of finish is typically used on some of the older styles of connectors, such as the SMA, ST (non pre-radiused) and Biconic. The tip is
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32 polished down flat in order to achieve perfect mating with another connector. It is nearly impossible to get two perfectly flat, perpendicular surfaces. This often prevents the fiber cores from touching. This air space between connectors creates a Fresnel reflection increasing loss. See Figure 4-28.
Figure 4-28. Connector Air Gap
PHYSICAL CONTACT (PC) OR DOMED FINISH. The PC is a finish technique applied to the fiber end face. Many manufacturers produce different styles of connectors with PC finishes. The PC ferrule end is domed with a high precision convex spherical end surface. PC finishes provide consistently lower losses than flat end face connectors, it reduces back reflection to 30 dB and PC virtually eliminates air spaces between mated connectors because the fibers will always touch near the core. ST, FC and SC connectors are available with PC finishes.
NOTE: For ST connectors, this procedure only works for connector ferrules that have been pre-radiused by the connector manufacturer. Some multimode optical fiber connectors may not have pre-radiused ferrules. When implementing this finish on multimode optical fiber connectors, verify with the manufacturer that the connectors have a pre-radiused ferrules. See Figure 4-29.
Figure 4-29. - Flat Finish and PC End Finish
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33 Connector Ferrules
Another connector variable is the type of ferrule used to hold the fiber and provide alignment.
CERAMIC FERRULES. Ceramic ferrules offer the best performance and are preferred for use on both multimode and single-mode fibers. Ceramics are strong and have precision, machined fiber bores. They also have superior thermal and mechanical properties so performance does not vary due to temperature or environmental fluctuations. They are also the most expensive. MIL-SPEC connectors only use ceramic ferrules. There are two types of ceramic ferrules Zirconia and Alumina. Alumina ceramic produces a rough polish due to its chemical makeup whereas Zirconia ceramic produces highly polished connectors and is preferred in industry. Used on all MIL-SPEC connectors.
STAINLESS STEEL FERRULES. These are middle-grade ferrules that will have quality nearly equal to ceramic, but thermal performance may be considerably less reliable than ceramic. These connectors may be best suited in applications where temperature in the operating environment is stable. On the other hand, stainless steel ferrules are stronger than ceramic and less susceptible to breakage or cracking.
PLASTIC FERRULES. Plastic ferrules will be least expensive, and will also be less reliable than ceramic or stainless steel. These are also known as polymer or polymer composite.
Q 24. What factors cause loss in an interconnection?
Q 25. What four main factors must a connector or splice control?
Q 26. What is the purpose of a connector?
Q 27. Blown Optical Fiber uses what size acrylate coating?
Q 28. What does APC stand for?
Q 29. When can a dome polish be used on multimode fiber?
Q 30. What type of ferrule do MIL-SPEC connectors use?
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34 FIBER OPTIC COUPLERS
Some fiber optic data links require more than simple point- to-point connections. These data links may be of a much more complex design that requires multi-port or other types of connections. Figure 4-30 shows some example system architectures that use more complex link designs. In many cases these types of systems require fiber optic components that can redistribute (combine or split) optical signals throughout the system.
One type of fiber optic component that allows for the redistribution of optical signals is a fiber optic coupler. A fiber optic coupler is a device that can distribute the optical signal (power) from one fiber among two or more fibers. A fiber optic coupler can also combine the optical signal from two or more fibers into a single fiber. Fiber optic couplers attenuate the signal much more than a connector or splice because the input signal is divided among the output ports. For example, with a 1 × 2 fiber optic coupler, each output is less than one-half the power of the input signal (over a 3 dB loss). See Figure 4-31.
Figure 4-31. - 1 X 2 Fiber Optic Splitter Figure 4-30. - Examples of Complex System Architectures
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35 Fiber optic couplers can be either active or passive devices. The difference between active and passive couplers is that a passive coupler redistributes the optical signal without optical-to-electrical conversion. Active couplers are electronic devices that split or combine the signal electrically and use fiber optic detectors and sources for input and output.
Figure 4-32 illustrates the design of a basic fiber optic coupler. A basic fiber optic coupler has N input ports and M output ports. N and M typically range from 1 to 64. The number of input ports and output ports vary depending on the intended application for the coupler. Types of fiber optic couplers include optical splitters, optical combiners, X couplers, star couplers, and tree couplers.
An optical splitter is a passive device that splits the optical power carried by a single input fiber into two output fibers. Figure 4-33 illustrates the transfer of optical power in an optical splitter. The input optical power is normally split evenly between the two output fibers. This type of optical splitter is known as a Y-coupler. However, an optical splitter may distribute the optical power carried by input power in an uneven manner. An optical splitter may split most of the power from the input fiber to one of the output fibers. Only a small amount of the power is coupled into the secondary output fiber. This type of optical splitter is known as a T-coupler, or an optical tap.
Figure 4-32. - Basic Passive fiber Optic Coupler Design Figure 4-33. - Optical Splitter
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36 An optical combiner is a passive device that combines the optical power carried by two input fibers into a single output fiber. Figure 4-34 illustrates the transfer of optical power in an optical combiner.
An X coupler combines the functions of the optical splitter and combiner. The X coupler combines and divides the optical power from the two input fibers between the two output fibers. Another name for the X coupler is the 2 × 2 coupler.
Star and tree couplers are multiport couplers that have more than two input or two output ports. A star coupler is a passive device that distributes optical power from more than two input ports among several output ports. Figure 4-35 shows the multiple input and output ports of a star coupler. A tree coupler is a passive device that splits the optical power from one input fiber to more than two output fibers. A tree coupler may also be used to combine the optical power from more than two input fibers into a single output fiber. Figure 4-36 illustrates each type of tree coupler. Star and tree couplers distribute the input power uniformly among the output fibers.
Figure 4-34. - Optical Combiner Figure 4-35. - Star Coupler
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37
Fiber optic couplers should prevent the transfer of optical power from one input fiber to another input fiber. Directional couplers are fiber optic couplers that prevent this transfer of power between input fibers. Many fiber optic couplers are also symmetrical. A symmetrical coupler transmits the same amount of power through the coupler when the input and output fibers are reversed.
Passive fiber optic coupler fabrication techniques can be complex and difficult to understand. Some fiber optic coupler fabrication involves beam splitting using micro lenses or graded-refractive-index (GRIN) rods and beam splitters or optical mixers. These beam splitter devices divide the optical beam into two or more separated beams. Fabrication of fiber optic couplers may also involve twisting, fusing, and tapering together two or more optical fibers. This type of fiber optic coupler is a fused biconical taper coupler. Fused biconical taper couplers use the radiative coupling of light from the input fiber to the output fibers in the tapered region to accomplish beam splitting. Figure 4-37 illustrates the fabrication process of a fused biconical taper coupler.
Figure 4-36. - (X M) and (N X 1) Tree Coupler Designs.
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38
Q31. What is the difference between passive and active fiber optic couplers? Q32. Which type of optical splitter (Y-coupler or T-coupler) splits only a small amount of power from the input fiber to one of the output fibers? Q33. Describe a directional coupler
. Figure 4-37. - Fabrication of a fused biconical taper coupler (star coupler)
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39 SUMMARY Now that you have completed this chapter, let's review some of the new terms, concepts, and ideas you have learned. You should have a thorough understanding of these principles before moving on to chapter 5.
FIBER OPTIC CONNECTIONS transfer optical power from one component to another. Fiber optic connections also permit fiber optic systems to be more than just a point-to-point data link.
A FIBER OPTIC SPLICE is a permanent joint between two fibers or two groups of fibers.
FIBER OPTIC CONNECTORS permit easy coupling and uncoupling of optical fibers.
FIBER OPTIC COUPLERS distribute or combine optical signals between fibers.
POOR FIBER END PREPARATION and POOR FIBER ALIGNMENT are the main causes of coupling loss.
FIBER-TO-FIBER COUPLING LOSS is affected by intrinsic and extrinsic coupling losses. INTRINSIC COUPLING LOSSES are caused by inherent fiber characteristics. EXTRINSIC COUPLING LOSSES are caused by jointing techniques.
A FIBER PIGTAIL is a short length of optical fiber (usually 1 meter or less) permanently fixed to a fiber optic component, such as an optical source or detector.
FRESNEL REFLECTION occurs twice in a fiber-to-fiber connection. A portion of the optical power is reflected when the light first exits the source fiber. Light is then reflected as the optical signal enters the receiving fiber.
INDEX MATCHING GEL eliminates or reduces the step change in the refractive index at the fiber interface, reducing Fresnel reflection.
INTRINSIC ATTENUATION is the loss that occurs due to manufacturing processes.
EXTRINSIC ATTENUATION is the loss caused by external forces being applied to the cables and connections.
MAXIMUM ALLOWABLE LOSS is the sum of the cable loss, connector loss and splice occurring in the optical cable plant. The MAL determines the pass/fail criteria of the optical link and cable plant.
POOR FIBER ALIGNMENT is a main source of coupling loss in fiber-to-fiber connections. The three basic coupling errors that occur during fiber alignment are fiber separation (longitudinal misalignment), lateral misalignment, and angular misalignment.
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40
In FIBER SEPARATION a small gap remains between fiber-end faces after completing the fiber connection. LATERAL, or AXIAL, MISALIGNMENT is when the axes of the two fibers are offset in a perpendicular direction. ANGULAR MISALIGNMENT is when the axes of the two fibers are no longer parallel.
SINGLE MODE FIBERS are more sensitive to alignment errors than multimode fibers because of their small core diameters and low numerical apertures.
The MODE POWER DISTRIBUTION (MPD) is the distribution of radiant power among the various modes propagating along the optical fiber.
Poor FIBER END PREPARATION is another source of extrinsic coupling loss. An optical fiber end face must be flat, smooth, and perpendicular to the fiber's axis to ensure proper fiber connection.
The SCORE-AND-SCRIBE method is the basic fiber cleaving technique for preparing optical fibers for termination.
POLISHING the fiber ends removes most surface imperfections introduced by the fiber cleaving process. Fiber polishing involves a step-down method. The first step is to air polish the connector to remove any rough shards left behind during the cleave. The next step involves polishing the relatively large epoxy bead down to a light haze. Next a medium grid polishing paper is used to remove all the adhesive left on the end face. The final step is polishing with a fine grit of paper that will produce a highly fine polished end face.
FIBER MISMATCHES are a source of intrinsic coupling loss. Types of fiber mismatches include fiber geometry mismatches, NA mismatch, and refractive index profile difference.
FIBER GEOMETRY MISMATCHES include core diameter, cladding diameter, core ellipticity, and core-cladding concentricity differences.
CORE DIAMETER MISMATCH causes coupling loss only if the launching fiber has a larger core radius than the receiving fiber.
NA MISMATCH causes coupling loss only if the launching fiber has a higher NA than the receiving fiber.
A REFRACTIVE INDEX PROFILE DIFFERENCE causes coupling loss only if the launching fiber has a larger profile parameter than the receiving fiber.
MECHANICAL and FUSION SPLICING are two broad categories that describe the techniques used for fiber splicing. A mechanical splice is a fiber splice where mechanical
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41 fixtures perform fiber alignment and connection. A fusion splice is a fiber splice where localized heat fuses or melts the ends of two lengths of optical fiber together.
In MECHANICAL SPLICING, mechanical fixtures hold the two optical fibers in alignment for an indefinite period of time without movement. The amount of splice loss is stable over time and unaffected by changes in environmental or mechanical conditions.
ARC FUSION involves the discharge of electric current across a gap between two electrodes. By placing the fiber end between the electrodes, the electric discharge melts or fuses the ends of the fibers.
PREFUSION involves a short discharge of electric current across the gap between the electrodes. In prefusion the fiber ends are cleaned and rounded to eliminate any surface defects that remain from fiber cleaving.
A FIBER OPTIC CONNECTOR is a demateable device that permits the coupling of optical power between two optical fibers or two groups of fibers.
FIBER ALIGNMENT in a fiber optic connector is the critical parameter in maintaining total insertion loss below the required level.
FIBER OPTIC CONNECTORS can affect system performance by poor end face geometry and contamination issues. These problems will create high insertion loss and back reflection.
OPTICAL BACK REFLECTION is reduced by index matching gels in mechanical splices and crimp-on connectors and physical contact polishes.
BUTT-JOINTED and EXPANDED BEAM CONNECTORS are two ways to classify fiber optic connectors. Butt-jointed connectors bring the prepared ends of two fibers into physical contact with each other. Expanded beam connectors use two lenses to first expand and then refocus the light from the transmitting fiber into the receiving fiber.
LIGHT-DUTY and HEAVY-DUTY CONNECTORS are two ways that the Navy classifies fiber optic connectors. Light-duty connector shipboard applications include locations that protect the connectors from the environment such as in an interconnection box. Heavy-duty applications require a very rugged, stand-alone, sealed connector.
A PASSIVE COUPLER redistributes an optical signal without optical to electrical conversion.
An OPTICAL SPLITTER is a passive device that splits the optical power carried by a single input fiber into two output fibers.
An OPTICAL COMBINER is a passive device that combines the optical power from two input fibers into a single output fiber.
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42
A STAR COUPLER is a passive device that distributes optical power from more than two input ports among several output ports.
A TREE COUPLER is a passive device that splits the optical power from one input fiber to more than two output fibers. A tree coupler may also be used to combine the optical power from more than two input fibers into a single output fiber.
DIRECTIONAL COUPLERS are fiber optic couplers that prevent the transfer of optical power from one input fiber to another input fiber.
A SYMMETRICAL COUPLER transmits the same amount of power through the coupler when the input and output fibers are reversed.
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43 ANSWERS TO QUESTIONS Q1. THROUGH Q33. A1. Contamination.
A2. 400X microscope.
A3. Video Inspection Probe
A4. Cable loss, Splice loss and Connector loss.
A5. Intrinsic losses are caused by manufacturing processes and Extrinsic losses are caused by external forces being applied to the cables and connections.
A6. Decibels or dB.
A7. Fiber separation (longitudinal misalignment), lateral misalignment, and angular misalignment.
A8. Angular misalignment.
A9. Physical Contact
A10. Single mode.
A11. Mechanical stripping.
A12. Score-and- cleave.
A13. Core diameter mismatch, cladding diameter mismatch, core ellipticity, core and cladding concentricity differences, NA mismatch, and refractive index profile differences.
A14. Yes
A15. Is a permanent joint whose purpose is to establish an optical connection between two individual optical fibers..
A16. Mechanical and fusion splicing.
A17. Every 1000 arcs.
A18. Optical loss and reflectance are much lower, are permanent, stable, and do not allow dust to enter the optical path.
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44 A19. Temporary restoration or for splicing multimode fibers in a premises installation. They do not need expensive machines, just a simple cleaver and some preparation tools.
A20. The AT&T Rotary splice and the Tyco LightCrimp.
A21. Visual Fault Locator ( V FL).
A22. Low loss, easy installation, repeatability, consistency, economical.
A23. Light duty and heavy duty connectors.
A24. Intrinsic, extrinsic and system losses.
A25. Lateral displacement, end separation, angular misalignment, surface roughness
A26. A connector is defined as a device that allows an optical fiber to be repeatedly connected or disconnected to cables, patch panels, transmitters or receivers..
A27. 500um.
A28. Angled Physical Contact.
A29. When the connector has a pre-radiused ferrule..
A30. Zirconia Ceramic.
A31. Passive couplers redistribute optical signals without optical- to-electrical conversion.
A32. T-coupler.
A33. A fiber optic coupler that prevents the transfer of power between input fibers.