Using Micro OTDRs to Test Fiber Optic Networks
- Tuesday, 26 January 2010
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.