Uses for traditional optical time domain reflectometers, or “OTDRs” include the certification and troubleshooting of very long-haul fiber optic networks such as campus and metro networks. In many LAN, WAN or FTTH environments, however, fiber links are relatively short and, therefore, usually operate over multimode fiber cables at wavelengths of 850nm and 1300nm. A new generation of “Micro OTDR” is ideally suited for this application, which typically has a maximum range of about 25 miles.
Micro OTDRs are, by definition, more compact, user friendly, and less expensive than standard OTDRs. Better quality micro OTDRs also have the option of measuring both multimode and singlemode fibers through different test ports on the instrument. This is important because for longer fiber runs, including metro networks, singlemode fiber is usually deployed operating at wavelengths of 1300nm and 1550nm.
Shedding Light On Events
Compared with simple optical loss test sets, micro OTDRs are capable of far more than merely indicating the numeric value of attenuation and length of a fiber optic link. On the contrary, these devices identify all “events” that might interfere with the normal propagation of light through the fiber such as splices and connectors, crushing through too narrow a bend radii, breaks, mechanical damage, and the effects of aging. As it is not sufficient to merely identify these events, micro OTDRs also measure the distance to these events, as well as calculate their attenuation and reflection values. When armed with this type of information, a technician knows exactly where along the cable to look for a fault. Like all OTDRs, micro OTDRs work by sending a laser pulse down the fiber and then measuring the light that is reflected back towards the OTDR from any anomaly along the fiber. By knowing the refractive index of the particular fiber and the time it takes for this “back scattered” light to reach the OTDR, the instrument can calculate the distance to each event in addition to its attenuation value.
The micro OTDR display shows all events detected on the link in a graphical format (OTDR trace) in which the xaxis indicates distance from the start of the fiber and the y-axis indicates the relative light power received due to reflections (Figure 1). The events identified on the OTDR trace can usually also be represented in the form of an event table (Figure 2). Correct interpretation of events is normally no problem for old hands. However, things can get tricky even for experienced personnel when small reflective events cannot be clearly distinguished from the background noise. In these situations a lesser-quality micro OTDR may register events that don’t exist, confusing the user. Usually, these errors are quickly identified. More advanced instruments are equipped with powerful analysis software that results in an accurate graphical representation of the link, helping to eliminate incorrect interpretation of OTDR traces. This software uses the concept of the “integrated measurement technician”, meaning that these OTDRs demand very little expertise on the part of the user and can be used productively and without error after only a short practice period.
A clear example of this “integrated measurement technician” concept is the “autotest” feature that these devices offer. The user is only required to set the wavelength and measurement duration. The OTDR then automatically sets all other parameters such as pulse width and measurement range (link length) to optimum values. Once these settings are stored, subsequent measurements can be made with a single press of a button. In fact, when using autotest, not one but several measurements can be made in quick succession. For instance, if the OTDR has a refresh rate of four measurements per second and a measurement duration of 5 seconds is set, the link being tested is subjected to 20 measurements and the average value of these is subsequently calculated and indicated as the result. If the OTDR is equipped with a real-time mode, this averaging process is eliminated and the OTDR trace displays any changes to the link as they occur. Such live measurements permit the observation of a force effect, such as when unavoidable bending of the fiber occurs. The real-time mode is also a good method for identifying intermittent faults. Finally, there is the expert mode through which the OTDR permits users to access and change all measurement parameters and tests, so that application specific tasks can be performed.
Apart from the more standard modes of operation described above, some micro OTDRs offer one or two additional features, usually as an option, which save time when performing frequent operations. An example might be the ability of the unit to identify the end of the fiber with simultaneous measurement of fiber length. Your specific application needs will decide whether these options are worth the added expense.
Certification Without Expertise
The certification functions of a micro OTDR are more widely used in installation applications than in maintenance operations. In fact, the verification of total loss together with an indication of the link length using an optical power meter and light source is sufficient for acceptance of networks according to ISO 11801 (Tier 1), but customers often require more. In this instance, more means certification according to US Standard TSB140 (Tier 2). Acceptance according to Tier 2 builds on the requirements of Tier 1 by prescribing OTDR trace documentation for each link in addition to loss and length measurements. These traces provide a rigorous open summary of the condition of the network and the influence of splices and connectors. However, manual evaluation of these OTDR traces demands both expertise and a considerable amount of time in order to determine a pass or fail for every fiber tested. Should one of the two prerequisites be lacking, more advanced OTDRs offer a feature that automatically provides a pass/fail indication after each autotest to give a clear and instant statement on link quality (Figure 3). Generally this feature is available as a firmware option.
Micro OTDRs usually offer abundant storage capacity for measurement records that include OTDR traces and event table data. Nonetheless, even OTDRs that can store 500 data records may run out of memory sooner than one would expect. At some point, users will want to upload test data to a PC for archiving and report generation. Most manufacturers provide PCbased analysis and documentation software for this. Uploading measurement data to a PC is performed most conveniently via an integrated USB interface, using either a USB memory stick for intermediate storage or direct via a USB cable.
The performance of different micro OTDRs and the service provided varies widely from one manufacturer to another. Standard functions on one instrument are charged options on another. In these situations careful comparison of products, options and suppliers is certainly worthwhile. One such option is the so-called bidirectional viewer. This viewer permits two OTDR traces which where shot from different ends of the fiber to be overlaid and displayed on screen for ease of comparison. This feature is essential when trying to identify situations where two fibers with different refractive indexes may have been incorrectly spliced together. In this case, the two OTDR traces would show this obscure fault merely as an attenuation at the splice point in one direction and again in the other. Overlaying the two traces in the bidirectional viewer reveals the actual situation.
In order to spare users the risk of eye damage caused through using conventional microscopic inspection techniques on end faces — peering into a laser-loaded fiber can quickly blind you — it is recommended that the micro OTDR should offer a video microscope as an option. A pen-shaped video camera with adapter tips for connection to standard connector types provides a greatly enlarged view of the connector end face on the OTDR display. Plus, there is no hazard to eyesight. An additional advantage is that the picture can be stored so that photos of contaminated surfaces “before and after” cleaning can be added to documentation as evidence of the efficiency of a maintenance operation.