Application Note 021 How to Choose an Optical Loss Test Set Pierre Talbot, B.Eng., Marie-Claude Michel, B.Eng., M.Sc., Product Manager, Portable and Monitoring Division Over the last few years, the cabling industry has seen the introduction of new optical test units that integrate a number of features only found previously in separate units. This trend has been beneficial to contractors and technicians because these units are usually lighter and more compact than the two or more separate devices that preceded them. Reasonable pricing for the combined units has also lowered the cost of being fully equipped for optical-fiber testing. Although the benefits of integrated equipment should be obvious, the variety of choices available for such devices can be overwhelming to a would-be buyer. This is particularly true of the most recent optical loss test sets (OLTSs), which can contain a variety of automated testing features, such as backreflection testing, visual fault location, message-sender functions and talk-set capability. Manual or Automated? The heart of the OLTS consists of a power meter and light source combination. Although there are small variations in the power output of the light sources or the dynamic range of the detectors, these components are basically similar for every OLTS on the market. Therefore, the biggest difference between units is the presence or absence of automated testing procedures. The main purpose of automated testing is to shorten testing time and reduce the possibility of operator error. Automated testing is not without cost, however, and an OLTS with automated testing capability will almost certainly be more expensive than a manual tester. To decide whether you need to invest in automated testing, devise a checklist of questions to identify the trade-offs involved. For example, quantifying the amount of time you currently spend on manual testing can help you determine how much time an automated test procedure could save you each day. Although most automated tests will save quite a bit of time, some technicians can perform optical loss tests just as quickly with a manual tester. Another factor in deciding on automation is operator error, which can be a frustrating source of lost time. Automated testing will never completely eliminate human error, but it can help to reduce it. Most automated tests coordinate source and detector wavelengths, for example, and these testers can ensure that measurements are made when the light source at the opposite end of the fiber is actually operating. If such operator errors are common for you or your technicians and frequently lead to retesting, automated testing may help you reduce testing time and can save you money. Yet another issue to consider when purchasing an OLTS is the amount of training technicians will require to operate the new unit. The simpler the tester, the faster the operator can understand and use it. Because training can be both expensive and time-consuming, an important factor in selecting a tester is determining how quickly it can be mastered. Because automated tests require fewer manipulations and key strokes, the training process is often quicker and easier. Testing Backreflection Some OLTSs can be configured to perform optical return loss (ORL), or backreflection, testing. However, if you are looking for a unit to test multimode optical fiber, this option will not only be unavailable but it would be useless if present. This is because backreflection is too high in multimode networks. However, transceivers for such networks are designed to operate properly under high-reflection conditions. www.exfo.com Application Note 021 On the other hand, an ORL testing option may be useful for a singlemode tester. This option may be desirable if your employer or your clients require ORL readings in their test documentation. Also, this is a handy function when testing older connector types because this is usually where return loss is high enough to cause problems. In addition, measuring ORL can provide necessary information about networks using transmitters that are particularly sensitive to backreflection. An optical return loss tester’s two key characteristics are accuracy and dynamic range. Accuracy determines how precisely reflection can be measured. It is generally advisable to select a tester that offers an absolute accuracy of measurement within 1 dB. Units offering less accuracy—2 dB or more, for instance—make even pass/fail testing difficult because the device can only give an approximate estimate. The dynamic range of the tester determines the types of networks that can be measured. For instance, most testers with this function offer a dynamic range of anywhere from 46 dB to 65 dB. An ORL tester with 45 dB dynamic range can measure reflections in networks using Super PC connectors; a tester with 60 dB dynamic range can measure systems with Super PC, or Ultra PC connectors; and a tester with a dynamic range of 65 dB can measure systems using Super PC, Ultra PC, or APC connectors. The higher the tester’s dynamic range, the more versatile it will be. Communicating with Your Tester Talk sets are well-known as stand-alone equipment, but message senders—a fairly recent development— perhaps merit some explanation. A message sender transmits a written message from one OLTS to another; it does so over the same fiber that is being tested. The message is usually chosen from a preconfigured list, but some units will accept custom messages typed in on the job site. Once selected, the message is sent over the fiber and displayed on the screen of a second OLTS. Sending messages in this way does have some disadvantages. Written messages are not always as rapid or as descriptive as actual speech, and it can be difficult to type a custom message in a work environment (such as when you are standing on a ladder), especially in an emergency situation. Also, since the messages are sent over the fiber being tested, communication is cut off every time you move to the next fiber. For these reasons, message senders are either moderately priced or provided free of charge. Given the wide variety of message senders and talk sets available, choosing among them—or choosing no communications capability at all—can be a difficult decision. Again, making up a checklist of questions may be helpful. A critical question, for instance, is whether you need to communicate with a partner during testing. If not, then you do not need to pay for such an option. However, this type of testing is mostly carried out by technicians working in pairs, and these technicians need to communicate with one another. In such cases, an integrated communications option in the tester would be worth investigating, since it is likely to be less expensive than buying stand-alone testing and communications equipment separately. Another question to ask is whether other methods of communication are already in use that will fulfill your needs. Cellular phones and walkie-talkies are the biggest competitors to OLTS talk sets today. Still, these devices may not be available or usable at all job sites. On the other hand, the one certainty a technician can depend on, is that both the optical fiber to be tested and the tester itself will always be available, so a talk-set option will always be on hand. The fibers being tested may be installed in underground vaults or in the basements of buildings, where thick layers of steel and concrete can impede wireless communications. In certain cases, remote work sites can also be too far removed from cellular calling areas, turning an otherwise reliable cellular phone into nothing but a noise generator. 2 Application Note 021 In considering cost, keep in mind that, depending on the amount of time you will be talking per day or per year, the cost of a cellular phone may be well above the price of a talk-set option. Take the time, therefore, to estimate how much money is currently being spent on wireless communications and use this figure as a budget guideline when evaluating the talk-set option. If you decide that a talk set or message sender is an option you need, you are still faced with a wide variety of options from which to choose, each of which will carry its own price tag. In choosing a communications option, think about practical matters first. If both your hands are normally occupied with equipment when testing, then avoid communications options that require push-to-talk operation or extensive typing. The simplest option for the technician is a full-duplex talk set; such a unit keeps you in constant communication with your partner while leaving both hands free for other work. Your talk set or message sender may not operate under all the conditions in which you will be testing. Knowing how much loss is likely to be measured between you and your partner, both on average and worst-case, will help you determine the dynamic range needed for communication. The message-sender option has the obvious advantage here, since it is generally as sensitive as the tester itself. This means that some units can function on fibers with an attenuation of up to 60 dB. Talk sets can also fare well in these circumstances because the dynamic range of these devices has improved consistently over recent years. For instance, a dynamic range of 40 dB to 45 dB is typical for high-quality talk sets today. Be sure that the dynamic range of the talk set you purchase fits your needs, and beware of full-duplex talk sets that suddenly become half-duplex as loss increases. When coupled with high loss, optical return loss can be a talk set’s downfall. Check the type of communications technology being used to be able to overcome this potential problem. Analog (FM) talk sets can be more sensitive to ORL and this can reduce their dynamic range. This may not be true of digital units. If you have a chance to field-try a unit, be sure to verify the quality of communication over worst-case fibers. User-Friendliness Once you have determined the technical characteristics you need on your unit and have identified the communication capabilities you will require, a number of companies will still remain in the race for your business. A very important consideration at this juncture is user-friendliness. How easy will the OLTS be to use? Does it require costly training? If new technicians join the team, will they be able to operate the device fairly quickly? Can the job still be completed if the operation manual is lost or damaged? Equipment user-friendliness factors are highly personal as every user has different criteria. Still, there are a few things to look for that may alleviate potential problems, and once again, they can be put into the form of a checklist. For example, the more buttons on the face of the unit, the more the device will intimidate and confuse a first-time operator. Also, besides the obvious psychological effect of a complicated faceplate, there is also the increased risk that the operator will make a mistake by pushing the wrong button. Another buttonrelated issue is the consideration of how big the buttons are. If you have small hands, try to picture someone with large fingers using the device. Could only one button be pressed at a time, or is it likely the technician would inadvertently press two or three? Buttons also need to be easy to press. Among the membrane keypads and rubber buttons available on today’s units, few are easy to use. A button should give a clear indication of when it has been successfully activated, either by a click or a change in resistance to pressure. Conversely, if a button has not been successfully activated, then it should not give any indication that it has been. Keep in mind that you may be using the tester for long periods of time on a daily basis, so any difficulty in pushing the buttons will be exaggerated and can lead to frustration. 3 Application Note 021 A good rule of thumb is to select a device with a screen that displays characters big enough to be read without reading glasses when the unit is held at arm's length. The screen should also provide sufficient contrast under all the lighting conditions in which it is likely to be used. In many cases, this means both indoor and outdoor use. Adjustable features such as backlighting and contrast control can be particularly useful for changing lighting conditions. Beware of devices that overuse abbreviations and symbols, especially when they seem to have been made up. Any reading or measurement that the unit gives should be clear to anyone using the device, particularly if the indication is associated with power status or laser activation. Of course, not all symbols or abbreviations are cryptic—VFL is a common abbreviation for visual fault locator, and the symbol for an active laser is internationally recognized. Use of such symbols and abbreviations is not only acceptable but recommended by standards bodies. To avoid having too many buttons, some OLTSs use software, dividing multiple functions into submenus. With each submenu that is activated, buttons on the faceplate of the unit control differing functions. This solution to the problem of packing many functions into a small unit with a simple user interface is very powerful and it can be user-friendly if properly implemented. However, when such a scheme is not well-planned, submenus can be more of a headache than a help compared to the traditional approach of using single-function buttons, no matter how many such buttons might be necessary. If a unit depends on submenus, make sure the device always indicates which submenu you are in and what will happen with each button you press while in that submenu. It is also useful to have a panic button that you can use to return quickly to a basic level, whether it be the main menu or a power-meter reading. Keep in mind, too, that the fewer submenu levels there are, the easier it will be to become accustomed to the unit and navigate through its various functions. An operating manual is an important adjunct to each new testing device you purchase. It typically serves as a reference during training, and it may even explain some of the theory behind the tests being performed. As you become familiar with your new unit, the operating manual will probably become less and less necessary, which is a good thing because most manuals eventually get lost or damaged. It is a good idea to avoid test equipment that leaves you feeling like you will never be able to master it without a manual at hand. If the manual is needed for daily use, the unit is probably not user-friendly enough for you. User manuals are often supplemented by a quick-reference card, which can be kept with the unit, or the device may have a help menu built into its software to aid you. Online help may be too much to ask for some OLTSs, but a field guide or quick-reference card attached to the unit can sometimes provide needed help so you do not have to search through your toolbox to find the operating manual. Such a guide or card, although probably not critical, is a definite plus when it comes to user-friendliness. 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