Application Hint – Dual-Fiber Micro-OTDR Monitor for Single-Fiber “Stubs”, Links and “Dark Fiber”

Is it possible to use the Dual-Fiber Micro-OTDR SFP to monitor single-fiber optical links?  This is a question that we are often asked by our CSP and OEM Customers who want to monitor lengths of optical fiber (SMF-28 and similar), but do not need to send data.  That is, they do not want to make a data link, but they do want to monitor a span of optical fiber with our Micro-OTDR feature.  Why do they ask such a question?

Usually this asked when the optical fiber spans are relatively long, and the best performing Micro-OTDR is desired.  The figure of merit for the performance of the Micro-OTDR feature is Dynamic Range (DR).  The DR indicates the number of “dBs” available to detect an optical fiber fault over a span of optical fiber.  Recall the Micro-OTDR Rule of Thumb:

Our Single-Fiber iSFC Micro-OTDR SFPs need to share their receiver between the data transport and Micro-OTDR functions.  The receiver must be optimized for data transport, so by necessity, the receiver is suboptimal for the Micro-OTDR feature. (In general, data “wants” wide bandwidth, while the Micro-OTDR “wants” narrow bandwidth.)

However, this problem does not exist for our Dual-Fiber iDFC Micro-OTDR SFPs.  Since they have separate receivers for the two functions, each receiver is optimized for its function – data transport or Micro-OTDR.

For the iSFC Fast Ethernet Micro-OTDR SFP (which is 80 Km Class for data transport), the DR is 60 dB.  For the iDFC Fast Ethernet Micro-OTDR SFP (which is 100 Km Class for data transport), the DR is 65 dB.  So how much difference does this 5 dB make?

To illustrate the performance difference, we will compare two 1511 nm SFPs, using our simple Micro-OTDR model, which calculates the Probability of Detection of an unknown fault (unknown location and unknown reflectance) over a specified optical fiber span.

Here are the results of the model if we “force” each unit analysis to a 95% Probability of Detection:

The key result is that the iDFC Dual-Fiber model can monitor a significantly longer optical fiber span (40.5 Km versus 25.8 Km), with a Probability of Detection of an unknown fault over the span of optical fiber of 95%.  The additional 5 dB of DR does make a significant difference.

This is why we are often asked this question by our CSP and OEM Customers.