2010

Using Micro OTDRs to Test Fiber Optic Networks

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

altCompared 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.

Integrated “Intelligence”

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.