In 2009 the ITU (International Telecommunication Union) introduced,  ITU G.652.D  as an industry recommendation which defines a full-spectrum, low water peak, singlemode fiber - basically most single mode fiber used in the past 5 years. This improvement in fiber performance opened the door for the proliferation of Coarse Wave Division Multiplexing (CWDM Mux) and other long wavelength technologies.

An unforeseen problem with standard G.652.D compliant fiber is that they can lose much of the optical signal by being bent too tightly. This can cause potentially devastating effects on network performance. In addition, these effects are not linear across the spectrum of WDM wavelengths as shown below.

While much of today's FTTH equipment such as GPON uses long wavelengths (1490nm), next generation equipment uses even longer wavelengths (1550nm and greater). Passive and active CWDM Muxs can use 2, 4, 8 or even 16 consecutive wavelengths between 1270nm and 1610nm. 

In the FiberPlex Technologies labs, we have been able to reproduce this phenomenon in real world situations. With two WDM16 active wave division muxs connected by a G.652.D fiber, we were able to slowly introduce bend-loss while observing sequential dropout of channels starting at 1610nm while shorter wavelength channels continued to perform within spec. As the bend radius decreased, progressively shorter wavelengths would drop out. In the field, this could present itself as a really perplexing problem as most of the channels would seem to work, while others do not. This is because even though the WDM16 has an incredible optical loss budget of 24dB, at 1610nm that headroom starts to get gobbled up by bend-loss. As you can see from the chart above, when bend-loss is slowly induced, loss outside of the tolerated budget begins to affect higher wavelengths causing channels to drop out.

As long as you are aware of this phenomenon it is quite easy to mitigate. There are basically two easy options. For existing pulled fiber runs, conduct a simple physical inspection to be sure there are no bends in the fiber smaller than a 7.5mm bend radius. A more quantitative evaluation can be done by taking an optical power measurement at 1610nm and verifying that any attenuation measures within the optical loss budget of your system.

For new installations, simply use fiber that conforms to ITU G.657.A recommendation. This recommendation describes specifications for a Low Bend-Loss (Bend Tolerant) singlemode fiber. All major fiber manufacturers today have some offering of Low Bend-Loss fiber cables. It is quickly becoming the new standard and is recommended for new pulls to future proof your network.