Fiber Optic Network
In the telcos, singlemode fiber is used to connect long distance switches, central offices and SLCs (subscriber loop carriers, small switches in pedestals in subdivisions or office parks or in the basement of a larger building). Practically every telco's network is now fiber optics except the connection to the home. Fiber to the home is not yet cost effective - especially since most homes do not want (nor are willing to pay) for the high speed services that would justify fiber optics. CATV companies "overbuild" with fiber. They lash fiber cable onto the aerial "hardline" coax used for the rest of the network or pull it in the same conduit underground. The fiber allows them to break their network into smaller service areas that prevent large numbers of customers from being affected in an outage, making for better service and customer relations. The fiber also gives them a return path which they use for Internet and telephone connections, increasing their revenue potential.
LANs (local area networks) use fiber optics primarily in the backbone but increasingly to the desk. The LAN backbone often needs longer distance than copper cable (Cat 5/5e/6) can provide and of course, the fiber offers higher bandwidth for future expansion. Most large corporate LANs use fiber backbones with copper wire to the desktop. Fiber to the desk can be cost effective if properly designed.
Lots of other networks use fiber. CCTV is often on fiber for it's distance capability. Industrial plants use lots of fiber or distance and noise immunity. Utilities use it for network management, liking its immunity to noise also. The military uses it because it's hard to tap or jam. Airplanes use it for that reason too, but also like the lighter weight of fiber.
Designing Cable Networks
I guess this is too big a topic for a overview! But we'll pass along some hints to make life easier. First and foremost, visit the work site and check it out thoroughly. Know the "standards" but use common sense in designing the installation. Don't cut corners which may affect performance or reliability. Consider what are the possible problems and work around or prevent them. There ain't no substitute for common sense here!
Fiber's extra distance capability makes it possible to do things not possible with copper wire. For example, you can install all the electronics for a network in one communications closet for a building and run straight to the desktop with fiber. With copper, you can only go about 90 meters (less than 300 feet), so you need to keep the electronics close to the desk. With fiber, you only need passive patch panels locally to allow for moves. Upgrades are easy, since the fiber is only loafing at today's network speed!
Is Copper Really Cheaper Than Fiber'
When it comes to costs, fiber optics is always assumed to be much more expensive than copper cabling. Whatever you look at - cable, terminations or networking electronics - fiber costs more, although as copper gets faster (e.g. Cat 6) it gets more expensive, almost as much as fiber. So isn't it obvious that fiber networks are more expensive than copper? Maybe not! There is more to consider in making the decision.Why Use Fiber?
If fiber is more expensive, why have all the telephone networks been converted to fiber? And why are all the CATV systems converting to fiber too? Are their networks that different? Is there something they know we don't? Telcos use fiber to connect all their central offices and long distance switches because it has thousands of times the bandwidth of copper wire and can carry signals hundreds of times further before needing a repeater. The CATV companies use fiber because it give them greater reliability and the opportunity to offer new services, like phone service and Internet connections. Both telcos and CATV operators use fiber for economic reasons, but their cost justification requires adopting new network architectures to take advantage of fiber's strengths. A properly designed premises cabling network can also be less expensive when done in fiber instead of copper. There are several good examples of fiber being less expensive, so lets examine them.
Industrial Networks
In an industrial environment, electromagnetic interference (EMI) is often a big problem. Motors, relays, welders and other industrial equipment generate a tremendous amount of electrical noise that can cause major problems with copper cabling, especially unshielded cable like Cat 5. In order to run copper cable in an industrial environment, it is often necessary to pull it through conduit to provide adequate shielding. With fiber optics, you have complete immunity to EMI. You only need to choose a cable type that is rugged enough for the installation, with breakout cable being a good choice for it's heavy-duty construction. The fiber optic cable can be installed easily from point to point, passing right next to major sources of EMI with no effect. Conversion from copper networks is easy with media converters, gadgets that convert most types of systems to fiber optics. Even with the cost of the media converters, the fiber optic network will be less than copper run in conduit.Long Cable Runs
Most networks are designed around structured cabling installed per EIA/TIA 568 standards. This standard calls for 90 meters (295 feet) of permanently installed unshielded twisted pair (UTP) cable and 10 meters (33 feet) of patchcords. But suppose you need to connect two buildings or more? The distance often exceeds the 90 meters by the time you include the runs between the buildings plus what you need inside each building. By the time you buy special aerial or underground waterproof copper cable and repeaters, you will usually spend more than if you bought some outside plant fiber optic cable and a couple of inexpensive media converters. It's guaranteed cheaper if you go more than two links (180 meters.)Centralized Fiber LANs
When most contractors and end users look at fiber optics versus Cat 5e cabling for a LAN, they compare the same old copper LAN with fiber directly replacing the copper links. The fiber optic cable is a bit more expensive than Cat 5e and terminations are a little more too, but the big difference is the electronics which are $200 or more per link extra for fiber. However, the real difference comes if you use a centralized fiber optic network - shown on the right of the diagram above. Since fiber does not have the 90 meter distance limitation of UTP cable, you can place all electronics in one location in or near the computer room. The telecom closet is only used for passive connection of backbone fiber optic cables, so no power, UPS, ground or air conditioning is needed. These auxiliary services, necessary with Cat 5 hubs, cost a tremendous amount of money in each closet. In addition, having all the fiber optic hubs in one location means better utilization of the hardware, with fewer unused ports. Since ports in modular hubs must be added in modules of 8 or 16, it's not uncommon with a hub in a telecom closet to have many of the ports in a module empty . With a centralized fiber system, you can add modules more efficiently as you are supporting many more desktop locations but need never have more than a one module with open ports.High Speed Networking
It was over a year after Gigabit Ethernet (GbE) became available on fiber optics that it finally become available on Cat 5e. It took another couple of years before GbE on copper became significantly less expensive. In order to get GbE to work over Cat 5e, the electronics must be very complicated, and consequently as expensive as fiber. A newer version is in the wings, awaiting a Cat 6 standard, but that means the version running over Cat 5e will be obsolete before it even gets started! Finally, we went to a major distributor's seminar on advanced cabling recently and the copper marketing guy told us to go fiber for GbE.Bottom Line
So when it comes to costs, looking at the cabling component costs may not be a good way to analyze total network costs. Consider the total system and you may find fiber looks a lot more attractive.
Fiber Optic Testing
After the cables are installed and terminated, it's time for testing. For every fiber optic cable plant, you will need to test for continuity, end-to-end loss and then troubleshoot the problems. If it's a long outside plant cable with intermediate splices, you will probably want to verify the individual splices with an OTDR also, since that's the only way to make sure that each one is good. If you are the network user, you will also be interested in testing power, as power is the measurement that tells you whether the system is operating properly. You'll need a few special tools and instruments to test fiber optics. See Jargon in the beginning of Lennie's Guide to see a description of each instrument.
Getting Started
Even if you're an experienced installer, make sure you remember these things.Have The Right Tools And Test Equipment For The Job. You Will Need:
1. Source and power meter, optical loss test set or test kit with proper equipment adapters for the cable plant you are testing.2. Reference test cables that match the cables to be tested and mating adapters, including hybrids if needed.
3. Fiber Tracer or Visual Fault Locator.
4. Cleaning materials - lint free cleaning wipes and pure alcohol.
5. OTDR and launch cable for outside plant jobs.
Know How To Use Your Test Equipment
Before you start, get together all your tools and make sure they are all working properly and you and your installers know how to use them. It's hard to get the job done when you have to call the manufacturer from the job site on your cell phone to ask for help. Try all your equipment in the office before you take it into the field. Use it to test every one of your reference test jumper cables in both directions using the single-ended loss test to make sure they are all good. If your power meter has internal memory to record data be sure you know how to use this also. You can often customize these reports to your specific needs - figure all this out before you go it the field - it could save you time and on installations, time is money!Know The Network You're Testing
This is an important part of the documentation process we discussed earlier. Make sure you have cable layouts for every fiber you have to test. Prepare a spreadsheet of all the cables and fibers before you go in the field and print a copy for recording your test data. You may record all your test data either by hand or if your meter has a memory feature, it will keep test results in on-board memory that can be printed or transferred to a computer when you return to the office.
A Note On Using A Fiber Optic Source Eye Safety
Fiber optic sources, including test equipment, are generally too low in power to cause any eye damage, but it's still a good idea to check connectors with a power meter before looking into it. Some telco DWDM and CATV systems have very high power and they could be harmful, so better safe than sorry.
Fiber optic testing includes three basic tests that we will cover separately: Visual inspection for continuity or connector checking, Loss testing, and Network Testing.
Visual Inspection
Visual Tracing
Continuity checking makes certain the fibers are not broken and to trace a path of a fiber from one end to another through many connections. Use a visible light "fiber optic tracer" or "pocket visual fault locator". It looks like a flashlight or a pen-like instrument with a lightbulb or LED soure that mates to a fiber optic connector. Attach a cable to test to the visual tracer and look at the other end to see the light transmitted through the core of the fiber. If there is no light at the end, go back to intermediate connections to find the bad section of the cable.A good example of how it can save time and money is testing fiber on a reel before you pull it to make sure it hasn't been damaged during shipment. Look for visible signs of damage (like cracked or broken reels, kinks in the cable, etc.) . For testing, visual tracers help also identify the next fiber to be tested for loss with the test kit. When connecting cables at patch panels, use the visual tracer to make sure each connection is the right two fibers! And to make certain the proper fibers are connected to the transmitter and receiver, use the visual tracer in place of the transmitter and your eye instead of the receiver (remember that fiber optic links work in the infrared so you can't see anything anyway.)
Visual Fault Location
A higher power version of the tracer uses a laser that can also find faults. The red laser light is powerful enough to show breaks in fibers or high loss connectors. You can actually see the loss of the bright red light even through many yellow or orange simplex cable jackets except black or gray jackets. You can also use this gadget to optimize mechanical splices or prepolished-splice type fiber optic connectors. In fact- don't even think of doing one of those connectors without one no other method will assure you of high yield with them.Visual Connector Inspection
Fiber optic microscopes are used to inspect connectors to check the quality of the termination procedure and diagnose problems. A well made connector will have a smooth , polished, scratch free finish and the fiber will not show any signs of cracks, chips or areas where the fiber is either protruding from the end of the ferrule or pulling back into it.The magnification for viewing connectors can be 30 to 400 power but it is best to use a medium magnification. The best microscopes allow you to inspect the connector from several angles, either by tilting the connector or having angle illumination to get the best picture of what's going on. Check to make sure the microscope has an easy-to-use adapter to attach the connectors of interest to the microscope.
And remember to check that no power is present in the cable before you look at it in a microscope protect your eyes!
Optical Power - Power Or Loss? ("Absolute" Vs. "Relative")
Practically every measurement in fiber optics refers to optical power. The power output of a transmitter or the input to receiver are "absolute" optical power measurements, that is, you measure the actual value of the power. Loss is a "relative" power measurement, the difference between the power coupled into a component like a cable or a connector and the power that is transmitted through it. This difference is what we call optical loss and defines the performance of a cable, connector, splice, etc.
Measuring Power
Power in a fiber optic system is like voltage in an electrical circuit - it's what makes things happen! It's important to have enough power, but not too much. Too little power and the receiver may not be able to distinguish the signal from noise; too much power overloads the receiver and causes errors too.
Measuring power requires only a power meter (most come with a screw-on adapter that matches the connector being tested) and a little help from the network electronics to turn on the transmitter. Remember when you measure power, the meter must be set to the proper range (usually dBm, sometimes microwatts, but never "dB" that's a relative power range used only for testing loss!) and the proper wavelengths matching the source being used. Refer to the instructions that come with the test equipment for setup and measurement instructions (and don't wait until you get to the job site to try the equipment)! To measure power, attach the meter to the cable that has the output you want to measure. That can be at the receiver to measure receiver power, or to a reference test cable (tested and known to be good) that is attached to the transmitter, acting as the "source", to measure transmitter power. Turn on the transmitter/source and note the power the meter measures. Compare it to the specified power for the system and make sure it's enough power but not too much.
Testing Loss
Loss testing is the difference between the power coupled into the cable at the transmitter end and what comes out at the receiver end. Testing for loss requires measuring the optical power lost in a cable (including connectors ,splices, etc.) with a fiber optic source and power meter by mating the cable being tested to known good reference cable.
In addition to our power meter, we will need a test source. The test source should match the type of source (LED or laser) and wavelength (850, 1300, 1550 nm). Again, read the instructions that come with the unit carefully.
We also need one or two reference cables, depending on the test we wish to perform. The accuracy of the measurement we make will depend on the quality of your reference cables. Always test your reference cables by the single ended method shown below to make sure they're good before you start testing other cables!
Next we need to set our reference power for loss our "0 dB" value. Correct setting of the launch power is critical to making good loss measurements!
Clean Your Connectors And Set Up Your Equipment Like This:
Turn on the source and select the wavelength you want for the loss test. Turn on the meter, select the "dBm" or "dB" range and select the wavelength you want for the loss test. Measure the power at the meter. This is your reference power level for all loss measurements. If your meter has a "zero" function, set this as your "0" reference. Some reference books and manuals show setting the reference power for loss using both a launch and receive cable mated with a mating adapter. This method is acceptable for some tests, but will reduce the loss you measure by the amount of loss between your reference cables when you set your "0dB loss" reference. Also, if either the launch or receive cable is bad, setting the reference with both cables hides the fact. Then you could begin testing with bad launch cables making all your loss measurements wrong. EIA/TIA 568 calls for a single cable reference, while OFSTP-14 allows either method.
Testing Loss
There are two methods that are used to measure loss, which we call "single-ended loss" and "double-ended loss". Single-ended loss uses only the launch cable, while double-ended loss uses a receive cable attached to the meter also. Single-ended loss is measured by mating the cable you want to test to the reference launch cable and measuring the power out the far end with the meter. When you do this you measure 1. the loss of the connector mated to the launch cable and 2. the loss of any fiber, splices or other connectors in the cable you are testing. This method is described in FOTP-171 and is shown in the drawing. Reverse the cable to test the connector on the other end.
In a double-ended loss test, you attach the cable to test between two reference cables, one attached to the source and one to the meter. This way, you measure two connectors' loses, one on each end, plus the loss of all the cable or cables in between. This is the method specified in OFSTP-14, the test for loss in an installed cable plant.
What Loss Should You Get When Testing Cables?
While it is difficult to generalize, here are some guidelines: - For each connector, figure 0.5 dB loss (0.7 max)
- For each splice, figure 0.2 dB
- For multimode fiber, the loss is about 3 dB per km for 850 nm sources, 1 dB per km for 1300 nm. This roughly translates into a loss of 0.1 dB per 100 feet for 850 nm, 0.1 dB per 300 feet for 1300 nm.
- For singlemode fiber, the loss is about 0.5 dB per km for 1300 nm sources, 0.4 dB per km for 1550 nm.
This roughly translates into a loss of 0.1 dB per 600 feet for 1300 nm, 0.1 dB per 750 feet for 1300 nm. So for the loss of a cable plant, calculate the approximate loss as:
(0.5 dB X # connectors) + (0.2 dB x # splices) + fiber loss on the total length of cable
Troubleshooting Hints:
If you have high loss in a cable, make sure to reverse it and test in the opposite direction using the single-ended method. Since the single ended test only tests the connector on one end, you can isolate a bad connector - it's the one at the launch cable end (mated to the launch cable) on the test when you measure high loss.
High loss in the double ended test should be isolated by retesting single-ended and reversing the direction of test to see if the end connector is bad. If the loss is the same, you need to either test each segment separately to isolate the bad segment or, if it is long enough, use an OTDR.
If you see no light through the cable (very high loss - only darkness when tested with your visual tracer), it's probably one of the connectors, and you have few options. The best one is to isolate the problem cable, cut the connector of one end (flip a coin to choose) and hope it was the bad one (well, you have a 50-50 chance!)
OTDR Testing
As we mentioned earlier, OTDRs are always used on OSP cables to verify the loss of each splice. But they are also used as troubleshooting tools. Let's look at how an OTDR works and see how it can help testing and troubleshooting.
How OTDRs Work
Unlike sources and power meters which measure the loss of the fiber optic cable plant directly, the OTDR works indirectly. The source and meter duplicate the transmitter and receiver of the fiber optic transmission link, so the measurement correlates well with actual system loss.
The OTDR, however, uses backscattered light of the fiber to imply loss. The OTDR works like RADAR, sending a high power laser light pulse down the fiber and looking for return signals from backscattered light in the fiber itself or reflected light from connector or splice interfaces.
At any point in time, the light the OTDR sees is the light scattered from the pulse passing through a region of the fiber. Only a small amount of light is scattered back toward the OTDR, but with sensitive receivers and signal averaging, it is possible to make measurements over relatively long distances. Since it is possible to calibrate the speed of the pulse as it passes down the fiber, the OTDR can measure time, calculate the pulse position in the fiber and correlate what it sees in backscattered light with an actual location in the fiber. Thus it can create a display of the amount of backscattered light at any point in the fiber.
Since the pulse is attenuated in the fiber as it passes along the fiber and suffers loss in connectors and splices, the amount of power in the test pulse decreases as it passes along the fiber in the cable plant under test. Thus the portion of the light being backscattered will be reduced accordingly, producing a picture of the actual loss occurring in the fiber. Some calculations are necessary to convert this information into a display, since the process occurs twice, once going out from the OTDR and once on the return path from the scattering at the test pulse.
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| OTDR Testingt |
There is a lot of information in an OTDR display. The slope of the fiber trace shows the attenuation coefficient of the fiber and is calibrated in dB/km by the OTDR. In order to measure fiber attenuation, you need a fairly long length of fiber with no distortions on either end from the OTDR resolution or overloading due to large reflections. If the fiber looks nonlinear at either end, especially near a reflective event like a connector, avoid that section when measuring loss.
Connectors and splices are called "events" in OTDR jargon. Both should show a loss, but connectors and mechanical splices will also show a reflective peak so you can distinguish them from fusion splices. Also, the height of that peak will indicate the amount of reflection at the event, unless it is so large that it saturates the OTDR receiver. Then peak will have a flat top and tail on the far end, indicating the receiver was overloaded. The width of the peak shows the distance resolution of the OTDR, or how close it can detect events.
OTDRs can also detect problems in the cable caused during installation. If a fiber is broken, it will show up as the end of the fiber much shorter than the cable or a high loss splice at the wrong place. If excessive stress is placed on the cable due to kinking or too tight a bend radius, it will look like a splice at the wrong location.
OTDR Limitations
The limited distance resolution of the OTDR makes it very hard to use in a LAN or building environment where cables are usually only a few hundred meters long. The OTDR has a great deal of difficulty resolving features in the short cables of a LAN and is likely to show "ghosts" from reflections at connectors, more often than not simply confusing the user.
Using The OTDR
When using an OTDR, there are a few cautions that will make testing easier and more understandable. First always use a long launch cable, which allows the OTDR to settle down after the initial pulse and provides a reference cable for testing the first connector on the cable. Always start with the OTDR set for the shortest pulse width for best resolution and a range at least 2 times the length of the cable you are testing. Make an initial trace and see how you need to change the parameters to get better results.
Coming soon - our OTDR self-study course will teach you a lot more about how to use OTDRs!
Restoration
The time may come when you have to troubleshoot and fix the cable plant. If you have a critical application or lots of network cable, you should be ready to do it yourself. Smaller networks can rely on a contractor. If you plan to do it yourself, you need to have equipment ready (extra cables, mechanical splices, quick termination connectors, etc., plus test equipment.) and someone who knows how to use it.
We cannot emphasize more strongly the need to have good documentation on the cable plant. If you don't know where the cables go, how long they are or what they tested for loss, you will be spinning you wheels from the get-go. And you need tools to diagnose problems and fix them, and spares including a fusion splicer or some mechanical splices and spare cables. In fact, when you install cable, save the leftovers for restoration! And the first thing you must decide is if the problem is with the cables or the equipment using it. A simple power meter can test sources for output and receivers for input and a visual tracer will check for fiber continuity. If the problem is in the cable plant, the OTDR is the next tool needed to locate the fault.
Fiber Optic Termination
We terminate fiber optic cable two ways - with connectors that can mate two fibers to create a temporary joint and/or connect the fiber to a piece of network gear or with splices which create a permanent joint between the two fibers. These terminations must be of the right style, installed in a manner that makes them have little light loss and protected against dirt or damage in use. No area of fiber optics has been given greater attention than termination. Manufacturers have come up with over 80 styles of connectors and and about a dozen ways to install them. There are two types of splices and many ways of implementing the splice. Fortunately for me and you, only a few types are used most applications. Different connectors and splice termination procedures are used for singlemode and multimode connectors, so make sure you know what the fiber will be before you specify connectors or splices! Connectors
We'll start our section on termination by considering connectors. Since fiber optic technology was introduced in the late 70s, numerous connector styles have been developed. Each new design was meant to offer better performance (less light loss and back reflection), easier and/or termination and lower cost. Of course, the marketplace determines which connectors are ultimately successful.
Connector And Splice Loss Mechanisms
Connector and splice loss is caused by a number of factors. Loss is minimized when the two fiber cores are identical and perfectly aligned, the connectors or splices are properly finished and no dirt is present. Only the light that is coupled into the receiving fiber's core will propagate, so all the rest of the light becomes the connector or splice loss.End gaps cause two problems, insertion loss and return loss. The emerging cone of light from the connector will spill over the core of the receiving fiber and be lost. In addition, the air gap between the fibers causes a reflection when the light encounters the change n refractive index from the glass fiber to the air in the gap. This reflection (called fresnel reflection) amounts to about 5% in typical flat polished connectors, and means that no connector with an air gap can have less than 0.3 dB loss. This reflection is also referred to as back reflection or optical return loss, which can be a problem in laser based systems. Connectors use a number of polishing techniques to insure physical contact of the fiber ends to minimize back reflection. On mechanical splices, it is possible to reduce back reflection by using non-perpendicular cleaves, which cause back reflections to be absorbed in the cladding of the fiber.
The end finish of the fiber must be properly polished to minimize loss. A rough surface will scatter light and dirt can scatter and absorb light. Since the optical fiber is so small, typical airborne dirt can be a major source of loss. Whenever connectors are not terminated, they should be covered to protect the end of the ferrule from dirt. One should never touch the end of the ferrule, since the oils on one's skin causes the fiber to attract dirt. Before connection and testing, it is advisable to clean connectors with lint-free wipes moistened with isopropyl alcohol.
Two sources of loss are directional; numerical aperture (NA) and core diameter. Differences in these two will create connections that have different losses depending on the direction of light propagation. Light from a fiber with a larger NA will be more sensitive to angularity and end gap, so transmission from a fiber of larger NA to one of smaller NA will be higher loss than the reverse. Likewise, light from a larger fiber will have high loss coupled to a fiber of smaller diameter, while one can couple a small diameter fiber to a large diameter fiber with minimal loss, since it is much less sensitive to end gap or lateral offset.
These fiber mismatches occur for two reasons. The occasional need to interconnect two dissimilar fibers and production variances in fibers of the same nominal dimensions. With two multimode fibers in usage today and two others which have been used occasionally in the past and several types of singlemode fiber in use, it is possible to sometimes have to connect dissimilar fibers or use systems designed for one fiber on another. Some system manufacturers provide guidelines on using various fibers, some don't. If you connect a smaller fiber to a larger one, the coupling losses will be minimal, often only the fresnel loss (about 0.3 dB). But connecting larger fibers to smaller ones results in substantial losses, not only due to the smaller cores size, but also the smaller NA of most small core fibers.
Guide To Fiber Optic Connectors
Check out the "spotters guide" below and you will see the most common fiber optic connectors. (All the photos are to the same scale, so you can get an idea of the relative size of these connectors.)
| ST (an AT&T Trademark) is the most popular connector for multimode networks, like most buildings and campuses. It has a bayonet mount and a long cylindrical ferrule to hold the fiber. Most ferrules are ceramic, but some are metal or plastic. And because they are spring-loaded, you have to make sure they are seated properly. If you have high loss, reconnect them to see if it makes a difference. | ||
 | FC/PC has been one of the most popular singlemode connectors for many years. It screws on firmly, but make sure you have the key aligned in the slot properly before tightening. It's being replaced by SCs and LCs. | ||
 | SC is a snap-in connector that is widely used in singlemode systems for it's excellent performance. It's a snap-in connector that latches with a simple push-pull motion. It is also available in a duplex configuration. | ||
| Besides the SC Duplex, you may occasionally see the FDDI and ESCON* duplex connectors which mate to their specific networks. They are generally used to connect to the equipment from a wall outlet, but the rest of the network will have ST or SC connectors. *ESCON is an IBM trademark Below are some of the new Small Form Factor (SFF) connectors: | |||
 | LC is a new connector that uses a 1.25 mm ferrule, half the size of the ST. Otherwise, it's a standard ceramic ferrule connector, easily terminated with any adhesive. Good performance, highly favored for singlemode. | ||
 | MT-RJ is a duplex connector with both fibers in a single polymer ferrule. It uses pins for alignment and has male and female versions. Multimode only, field terminated only by prepolished/splice method. | ||
 | Opti-Jack is a neat, rugged duplex connector cleverly designed aournd two ST-type ferrules in a package the size of a RJ-45. It has male and female (plug and jack) versions. | ||
 | Volition is a slick, inexpensive duplex connector that uses no ferrule at all. It aligns fibers in a V-groove like a splice. Plug and jack versions, but field terminate jacks only. | ||
 | E2000/LX-5 is like a LC but with a shutter over the end of the fiber. | ||
 | MU looks a miniature SC with a 1.25 mm ferrule. It's more popular in Japan. | ||
 | MT is a 12 fiber connector for ribbon cable. It's main use is for preterminated cable assemblies. | ||
The ST/SC/FC/FDDI/ESON connectors have the same ferrule size - 2.5 mm or about 0.1 inch - so they can be mixed and matched to each other using hybrid mating adapters. This makes it convenient to test, since you can have a set of multimode reference test cables with ST connectors and adapt to all these connectors. Likewise, the LC, MU and E2000/LX-5 use the same ferrule but cross-mating adapters are not easy to find.
Connector Types
The ST is still the most popular multimode connector because it is cheap and easy to install. The SC connector was specified as a standard by the old EIA/TIA 568A specification, but its higher cost and difficulty of installation (until recently) has limited its popularity. However, newer SCs are much better in both cost and installation ease, so it has been growing in use. The duplex FDDI, ESCON and SC connectors are used for patchcords to equipment and can be mated to ST or SC connectors at wall outlets. Singlemode networks use FC or SC connectors in about the same proportion as ST and SC in multimode installations. There are some D4s out there too.EIA/TIA 568 B allows any fiber optic connector as long as it has a FOCIS (Fiber Optic Connector Intermateability Standard) document behind it. This opened the way to the use of several new connectors, which we call the "Small Form Factor" (SFF) connectors, including AT&T LC, the MT-RJ, the Panduit "Opti-Jack," 3M's Volition, the E2000/LX-5 and MU. The LC has been particularly successful in the US.
Connector Ferrule Shapes & Polishes
Fiber optic connectors can have several different ferrule shapes or finishes, usually referred to as polishes. early connectors, because they did not have keyed ferrules and could rotate in mating adapters, always had an air gap between the connectors to prevent them rotating and grinding scratches into the ends of the fibers.Beginning with the ST and FC which had keyed ferrules, the connectors were designed to contact tightly, what we now call physical contact (PC) connectors. Reducing the air gap reduced the loss and back reflection (very important to laser-based singlemode systems ), since light has a loss of about 5% (~0.25 dB) at each air gap and light is reflected back up the fiber. While air gap connectors usually had losses of 0.5 dB or more and return loss of 20 dB, PC connectors had typical losses of 0.3 dB and a return loss of 30 to 40 dB.
Soon thereafter, it was determined that making the connector ferrules convex would produce an even better connection. The convex ferrule guaranteed the fiber cores were in contact. Losses were under 0.3dB and return loss 40 dB or better. The final solution for singlemode systems extremely sensitive to reflections, like CATV or high bitrate telco links, was to angle the end of the ferrule 8 degrees to create what we call an APC or angled PC connector. Then any reflected light is at an angle that is absorbed in the cladding of the fiber.
Termination Procedures
Whatever you do, follow the manufacturer's termination instructions closely. Multimode connectors are usually installed in the field on the cables after pulling, while singlemode connectors are usually installed by splicing a factory-made "pigtail" onto the fiber. That is because the tolerances on singlemode terminations are much tighter and the polishing processes are more critical. You can install singlemode connectors in the field for low speed data networks, but you may not be able to get losses lower than 1 dB! Cables can be pulled with connectors already on them if, and a big if, you can deal with these two problems: First, the length must be precise. Too short and you have to pull another longer one (its not cost effective to splice), too long and you waste money and have to store the extra cable length. Secondly, the connectors must be protected. Some cable and connector manufacturers offer protective sleeves to cover the connectors, but you must still be much more careful in pulling cables. You might consider terminating one end and pulling the unterminated end to not risk the connectors. There is a growing movement to install preterminated systems but with the MT 12 multifiber connector. It's tiny not much bigger than a ST or SC, but has up to 12 fibers. Manufactures sell multifiber cables with MTs on them that connect to preterminated patch panels with STs or SCs. Works well if you have a good designer and can live with the higher loss (~1 dB) typical of these connectors.
Multimode Terminations: Several different types of terminations are available for multimode fibers. Each version has its advantages and disadvantages, so learning more about how each works helps decide which one to use.
A note on adhesives: Most connectors use epoxies or other adhesives to hold the fiber in the connector. Use only the specified epoxy, as the fiber to ferrule bond is critical for low loss and long term reliability! We've seen people use hardware store epoxies, Crazy Glue, you name it! And they regretted doing it.
Epoxy/Polish: Most connectors are the simple "epoxy/polish" type where the fiber is glued into the connector with epoxy and the end polished with special polishing film. These provide the most reliable connection, lowest losses (less than 0.5 dB) and lowest costs, especially if you are doing a lot of connectors. The epoxy can be allowed to set overnight or cured in an inexpensive oven. A "heat gun" should never be used to try to cure the epoxy faster as the uneven heat may not cure all the epoxy or may overheat some of it which will prevent it ever curing!
"Hot Melt": This is a 3M trade name for a connector that already has the epoxy (actually a heat set glue) inside the connector. You strip the cable, insert it in the connector, crimp it, and put it in a special oven. In a few minutes, the glue is melted, so you remove the connector, let it cool and it is ready to polish. Fast and easy, low loss, but not as cheap as the epoxy type, it has become the favorite of lots of contractors who install relatively small quantities of connectors.
Anaerobic Adhesives: These connectors use a quick setting adhesive to replace the epoxy. They work well if your technique is good, but often they do not have the wide temperature range of epoxies, so only use them indoors. A lot of installers are using Loctite 648, with or without the accellerator solution, that is neat and easy to use.
Crimp/Polish: Rather than glue the fiber in the connector, these connectors use a crimp on the fiber to hold it in. Early types offered "iffy" performance, but today they are pretty good, if you practice a lot. Expect to trade higher losses for the faster termination speed. And they are more costly than epoxy polish types. A good choice if you only install small quantities and your customer will accept them.
Prepolished/splice: Some manufacturers offer connectors that have a short stub fiber already epoxied into the ferrule and polished perfectly, so you just cleave a fiber and insert it like a splice. (See next section for splicing info.) While it sound like a great idea, it has several downsides. First it is very costly, five to ten times as much as an epoxy polish type. Second, you have to make a good cleave to make them low loss, and that is not as easy as you might think. Third, even if you do everything correctly, you loss will be higher, because you have a connector loss plus two splice losses at every connection! The best way to terminate them is to monitor the loss with a visual fault locator and "tweak" them.
Hints For Doing Field Terminations
Here are a few things to remember when you are terminating connectors in the field. Following these guidelines will save you time, money and frustration:
Choose the connector carefully and clear it with the customer if it is anything other than an epoxy/polish type. Some customers have strong opinions on the types or brands of connectors used in their job. Find out first, not later!
Never, never, NEVER take a new connector in the field until you have installed enough of them in the office that you can put them on in your sleep. The field is no place to experiment or learn! It'll cost you big time!
Have the right tools for the job. Make sure you have the proper tools and they are in good shape before you head out for the job. This includes all the termination tools, cable tools and test equipment. Do you know your test cables are good? Without that, you will test good terminations as bad every time. More and more installers are owning their own tools like auto mechanics, saying that is the only way to make sure the tools are properly cared for.
Dust and dirt are your enemies. It's very hard to terminate or splice in a dusty place. Try to work in the cleanest possible location. Use lint-free wipes (not cotton swaps or rags made from old T-shirts!) to clean every connector before connecting or testing it. Don't work under heating vents, as they are blowing dirt down on you continuously.
Don't overpolish. Contrary to common sense, too much polishing is just as bad as too little. The ceramic ferrule in most of today's connector is much harder than the glass fiber. Polish too much and you create a concave fiber surface, increasing the loss. A few swipes is all it takes.
Remember singlemode fiber requires different connectors and polishing techniques. Most SM fiber is terminated by splicing on a preterminated pigtail, but you can put SM connectors on in the field if you know what you are doing. Expect much higher loss, approaching 1 dB and high back reflections, so don't try it for anything but data networks, not telco or CATV.
Change polishing film regularly. Polishing builds up residue and dirt on the film that can cause problems after too many connectors and cause poor end finish. Check the manufacturers' specs.
Put covers on connectors and patch panels when not in use. Keep them covered to keep them clean.
Inspect and test, then document. It is very hard to troubleshoot cables when you don't know how long they are, where they go or how they tested originally! So keep good records, smart users require it and expect to pay extra for good records.
Splicing
Splicing is only needed if the cable runs are too long for one straight pull or you need to mix a number of different types of cables (like bringing a 48 fiber cable in and splicing it to six 8 fiber cables - could you have used a breakout cable instead?) And of course, we use splices for restoration, after the number one problem of outside plant cables, a dig-up and cut of a buried cable, usually referred to as "backhoe fade" for obvious reasons!
Splices are "permanent" connections between two fibers. There are two types of splices, fusion and mechanical, and the choice is usually based on cost or location. Most splicing is on long haul outside plant SM cables, not multimode LANs, so if you do outside plant SM jobs, you will want to learn how to fusion splice. If you do mostly MM LANs, you may never see a splice.
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| Splices |
Mechanical Splices are alignment gadgets that hold the ends of two fibers together with some index matching gel or glue between them. There are a number of types of mechanical splices, like little glass tubes or V-shaped metal clamps. The tools to make mechanical splices are cheap, but the splices themselves are expensive. Many mechanical splices are used for restoration, but they can work well with both singlemode and multimode fiber, with practice.
Which Splice?
If cost is the issue, we've given you the clues to make a choice: fusion is expensive equipment and cheap splices, while mechanical is cheap equipment and expensive splices. So if you make a lot of splices (like thousands in an big telco or CATV network) use fusion splices. If you need just a few, use mechanical splices. Fusion splices give very low back reflections and are preferred for singlemode high speed digital or CATV networks. However, they don't work too well on multimode splices, so mechanical splices are preferred for MM, unless it is an underwater or aerial application, where the greater reliability of the fusion splice is preferred.
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