Have you ever seen the optical illusion where two lines are drawn side
by side, one with arrow points and one with inverted arrow points (Figure 1)? The
line with the inverted arrow points looks longer, even though the two lines are the same
length. There is a common optical test that can also be misinterpreted if you
don’t understand what is being seen. That test is the appearance of
inaccurately high splice loss or “gainers” using an optical time domain reflectometer (OTDR).
often receive questions from splicers observing gainers and abnormally high
splice losses that they cannot correct, even after re-splicing several times. This
should sound familiar. It is the typical observed result when splicing G.652.D
glass to G.657 bend insensitive glass, but like the example in Figure 1, it is
the standard glass placed in outside plant (OSP) fiber optic cables was G.652.D
glass. The industry, however, started shifting to G.657 bend insensitive glass
as a standard OSP offering within the last five years. Splicing these different
glass types together will yield excellent and acceptable splice losses;
however, an OTDR displaying a splice from one direction or the other will often
show odd and inaccurate results.
is the difference between the two glass types that causes this effect? It is
the mode field diameter (MFD). MFD is defined as the area in the optical glass
where the mode of light travels. While most of the light energy will travel in
the core of the glass, some of the light spills over into the cladding as it
travels. That combined area is called the MFD (Figure 2).
A smaller MFD is needed to create a glass that is bend insensitive as it provides a more focused area for the light to make it through tighter bends than standard single mode
CLICK TO TWEET: In this blog CommScope's Chris Gemme explains high splice loss, the effects of Mode Field Diameter.
do standards say about MFD? Telecordia GR-20 references IEC 60793-2-50. It allows a
median MFD range of 8.8 to 9.3 microns in single-mode optical fibers. Splicing
two fibers with mode field diameters that are not the same will lead to the measurement
challenges we described above, even if both fibers are within the allowable
does OTDR present an inaccurate high splice loss or even a “gainer” in this
situation? MFD effects the backscatter of the source pulse, which is what the
OTDR is using to calculate an estimated splice loss. Changes in MFD will cause
the backscatter to appear as either an exaggerated loss or gainer to the OTDR
receiver, depending on the direction the source pulse is travelling through the
in MFD are normal under all circumstances, but there are occasions where two
fibers have MFD far enough apart that they present themselves in this fashion,
such as G.657 and G.652 glass.
3 shows how a splice with actual loss dB (+R) will display on and OTDR
when observed as two unidirectional scans. That’s why the use of unidirectional
OTDR testing may occasionally result in inaccurate loss measurement of a fiber
splice. To make an accurate measurement of splice loss, this effect must be
corrected through bidirectional averaging.
ANSI/TIA/EIA-455-8-2000 as an industry
specification states the splice loss measurements with an OTDR must be
conducted from both directions and averaged (by adding with signs) for accurate
use of unidirectional OTDR measurements for measuring system performance is
problematic in that it presents only an estimated splice loss, impacted by the
effects of backscatter variances caused by varying mode field diameters. The
greater the difference in MFD, the greater the variance in backscatter,
resulting in exaggerated and inaccurate unidirectional splice loss values. To correct
the OTDRs inaccuracy because of this effect, it is important to use a
bidirectional average which provides an actual splice loss measurement.
often do you need to correct OTDR inaccuracies in your network?