Choosing the right optical-loss test set
Here are a few questions to ask when determining the proper diagnostic system for your application.
Lynn Mason exfo E.O. Engineering
he past few years have seen the development of new optical-test units that integrate a number of previously distinct pieces of equipment. This trend has been beneficial to operators because the new units are lighter and more compact than the two or more separate pieces of equipment that preceded them. The reasonable pricing of these new units has resulted in lower costs for optical testing.
Although the benefits of integrated equipment are obvious, the variety of choices can be overwhelming (see photo). This is particularly true for the newest optical-loss test sets (oltss), which can contain any combination of automated testing features, including backreflection testers, talk sets, message-sender functions, and visual fault locators (vfls). This article is designed to help sift through the multiple possibilities to find the ideal olts for any given application.
Manual or automated loss testing?
The heart of each olts consists of a power meter and light source combination. Although there are some small variations in the power output of the sources or the dynamic range of the detectors, these components remain basically similar for every olts on the market.
What differentiates them is the presence or absence of automated testing procedures. The principal purpose of an automated test is to shorten testing time and reduce the possibility of human error during measurements. Unfortunately, automated testing is not free, and an automated test set will almost certainly be more expensive than a manual tester. The following list of questions can help you decide whether automated testing is worth the investment.
How much time do you spend testing a single fiber? How long would you take to test a single fiber with the automated test? How many fibers can you test in an average day? These questions will help determine how much time an automated test can save each day. Although most automated tests will help save a lot of time, some technicians can perform the tests just as quickly with a manual tester.
How much time do you lose retesting fibers after a human error? Wasting time due to human error can be frustrating. Automated testing will never eliminate human error, but it can help reduce it. Most automated tests coordinate source and detector wavelengths and ensure that the stored measurements are taken when the source at the opposite end is actually on. If this sort of error occurs in your day, automated testing may be very helpful to avoid losing time.
How much training will this unit require? The simpler the unit, the faster you can learn it, understand it, and use it. Because training can be expensive and time-consuming, it is important to look at how quickly a unit can be mastered. Automated tests require fewer manipulations and key strokes, so training is often quicker and easier.
Do you need backreflection testing? Some oltss can be configured to perform optical-return loss (orl), or backreflection tests. However, if you are looking for a multimode fiber testing unit, not only will this option be unavailable but it would also be completely useless if it were available. Reflections are simply too high in multimode systems, and transmission equipment is made to work well in these conditions.
If you are looking for a singlemode olts, then an orl option may be useful. For example, if your employer or client requires orl readings in test reports, then obviously you must have this function to comply. Also, if you are working on systems with older connector types, you may find the function handy because this is usually where orl can be high enough to cause problems. Measuring orl can also prove useful when testing systems with transmission equipment that is particularly sensitive to backreflection.
An orl tester should possess two key characteristics: accuracy and dynamic range. Accuracy will determine how precisely the reflection can be measured; it is generally advisable to select a tester that offers an absolute accuracy within 1 dB. Units that offer poor accuracy (۬ dB or more) make testing--even pass/fail tests--very difficult because the unit can provide only an estimate.
The higher the dynamic range, the more versatile the orl tester will be. Most testers offer anywhere from 40 to 65 dB of dynamic range. For example, an orl tester with 45 dB of range would be able to measure reflections in systems containing Super PC connectors; a tester with 60 dB of dynamic range can measure systems with Super PC or Ultra PC connectors; and a tester with 65 dB of range can measure systems with Super PC, Ultra PC, or apc connectors.
Message-senders
Talk sets are well-known as stand-alone equipment, but message-senders--a recent development--merit a short explanation. A message-sender sends a written message from one olts to another over the fiber that is being tested. The message is usually chosen from a preconfigured list, but some units will accept custom messages that are typed in. Once selected, the message will be sent over the fiber and will appear on the screen of the second olts.
Message-sending comes with a few caveats. Messages are not always as rapid or as descriptive as actual speech. It can be very difficult to type a custom message, especially in an emergency situation. Also, because the messages are sent over the fiber being tested, communication is cut off every time you go to the next fiber. For these reasons, message-senders are either moderately priced or provided at no cost.
Choosing between message senders and the wide variety of talk sets (or choosing nothing at all) can be a tough decision. The following list of questions will help you choose the right option.
Do you need to communicate with the operator of the other olts? If the answer is "no," then you do not need to pay for such an option. If, however, you are part of the majority of operators who do need to communicate with an olts partner, then an integrated option will be worth investigating because it should be less expensive than buying stand-alone units.
Are other methods of communication available? Cellular phones and walkie-talkies are the biggest competitors to olts talk sets. However, it is important to look at the availability of these items at work sites. The one certainty for a fiber technician is that fiber will always be at a work site and a fiber talk set will always be available. The fibers under test often are installed in underground vaults or in building basements, and the extra steel and concrete above them can prevent wireless communications. Certain remote work sites may also be too far away from cellular calling areas, turning an otherwise reliable cellular phone into a noise generator.
As for cost, depending on the amount of time you will be talking per day or per year, the cellular phone may cost much more than a talk-set option. Determine how much would be spent on wireless communications and use this as a budget guideline for a talk-set option.
Do you have a free hand when testing? Even if you decide that a talk set or message-sender option will be part of your purchase, there is still a wide variety of options left to choose from, each carrying its own price tag. Look at practicality first: If both your hands are full of equipment while you are testing, avoid devices that require push-to-talk operation or extensive message typing. The simplest option will likely be the full-duplex talk set, which will keep you in constant communication while freeing your hands.
How much loss--average and worst case--is measured over the fibers under test? Knowing how much loss is between you and your olts partner will help determine what kind of dynamic range will be required for communication. Message options have the obvious advantage here, because they are generally as sensitive as the loss tester on the olts. This means that some units can function on fibers with up to 60 dB of attenuation. Talk sets also can fare well in such applications, because dynamic ranges have consistently improved (40 to 45 dB of range is typical for a high-quality talk set). Be sure that the dynamic range of the talk set fits your needs, and beware of full-duplex talk sets that suddenly become half-duplex as the loss increases.
How much orl--average and worst case--is present over the fibers under test? When coupled with high loss, orl can be the last nail in a talk set`s coffin. Check whether the set uses analog (FM) or digital communications. Analog talk sets can be more sensitive to orl, which can reduce their dynamic range. If you have a chance to try a unit in the field, be sure to verify the quality of the communication on some of your worst fibers.
User-friendliness
Once you have decided which pieces of equipment you need and can afford to buy, an important factor to consider is user-friendliness. How easy will the selected unit be to use? Does it require extensive or costly training? If new people join the work team, will they be able to figure things out? Can you still operate the tester if the operation manual is lost or damaged?
These factors are highly personal; not every individual will feel the same way about a piece of equipment. Still, there are a few things to look for that may indicate potential problems.
Does the unit have more buttons than you have fingers? The more buttons there are on the front of a unit, the more this unit will intimidate and confuse operators the first time they see it. Also, multiple buttons increase the risk of making mistakes.
Are the buttons big enough and easy to press? Considering the size of the buttons is also important. If your hands are small, try to picture users with very large fingers using the same unit. Will it be easy for them to press only one button at a time, or is it likely they will inadvertently press two or three simultaneously?
The buttons also need to be easy to press; among the membrane keypads and rubber buttons available on units today, not all are easy to use. A button also should give a clear indication of when it has been activated by either a click or a change in the button`s resistance to pressure. Conversely, if the button has not been activated, then it should not give any indication. Also, make sure that the buttons are not too difficult to activate, as this can become a source of frustration in the long run.
Is the screen easy to read? A good rule of thumb is to choose a screen on which the main characters are big enough to be read, without reading glasses, when held at arm`s length. The screen should also offer good contrast in all anticipated lighting conditions. In many cases this means both indoors and outdoors. Features like backlighting and adjustable contrasts can be particularly useful for changing lighting conditions.
Is the information on the screen easy to understand at all times? Beware of units that overuse abbreviations and symbols, especially when they seem to have been made up just for use on the unit in question. Any reading or indication should be clear to anyone who sees it, particularly if it is associated with power status or laser activation.
Of course, not all symbols and abbreviations are cryptic: vfl is commonly accepted to describe visual fault locators, and the symbol for an active laser is known internationally. Use of these symbols is both acceptable and often recommended by standards bodies.
Does the unit contain a maze of submenus? To avoid having multiple buttons, some olts have divided all available functions into submenus. With each submenu, a number of buttons on the unit will give access to new functions. This solution is very powerful and can be very user-friendly if well-implemented. When poorly implemented, submenus can be a nuisance compared to traditional one-function buttons, no matter how many are necessary.
Make sure that the unit always lets you know which menu is active and what will happen each time you press a key. Also be sure that there is a "panic button" that can quickly get you back to a state that you are comfortable with, be it a power meter reading or a main menu. Also keep in mind that the fewer levels of submenus there are, the sooner you will become accustomed to using the unit.
What will happen if the operation manual is lost or damaged? It is important to have a good operation manual with every new unit. It serves as a reference through formal training and can even explain some of the theory behind the tests the unit can perform. As you get familiar with the unit, the manual should become less important. This is good because most manuals do eventually get lost or damaged. Avoid units that leave you feeling like you will never be able to get by without a manual. If the manual becomes a crutch, the unit is probably not user-friendly enough for you.
Is there any help menu or quick-reference card attached to the unit? There will always be situations in which the information on the screen is simply not enough. Online help can sometimes be a lot to ask from an olts, but a field guide or quick-reference card attached to the unit can often give just the right information without having to root around for the unit`s operation manual. Although this is not a critical factor to user-friendliness, it is a definite plus.
Finally, keep in mind how the unit will be used. The device must meet minimum requirements. However, the number of extra options that should be purchased is sometimes difficult to determine. The proper configuration will often differ from one application to another, and buyers must evaluate their own needs. u
Lynn Mason is outside-plant product manager at exfo E.O. Engineering, in Vanier, QC, Canada.