BI-SMF TestingAfter its introduction to the market in 2009, bend insensitive multimode fiber (BIMMF) received plenty of attention. But some might not realize that bend insensitive singlemode fiber (BI-SMF) actually came first.

Due to smaller cabinets and longer wavelengths used in outside plant (OSP) and passive optical network (PON) applications, BI-SMF was introduced in 2007 under the International Telecommunications Union (ITU) G.657 standard.

However, splicing ITU G.652D standard singlemode fiber to newer G.657 BI-SMF fiber can cause some confusion.

Watch Out for Gainers

While much of the debate surround BIMMF has focused on its compatibility with non-BIMMF, that hasn’t been as much of an issue with singlemode fiber since only one mode is guided in the core. But there is a consideration when it comes to testing.

G.657 BI-SMF has a core refraction index profile that looks quite different than a standard G.652D singlemode fiber. The mode field diameter (MFD) of BI-SMF is smaller than standard fibers. While the mismatch of MFD between bend-insensitive and standard singlemode fiber has little effect on actual splice loss, exaggerated losses or “gainers” can occur during Tier 2 OTDR testing. Why?

OTDRs use backscattered light to measure the loss of a fiber channel, launching light into the fiber and measuring the amount of backscattered light and how long it takes, which determines the location of where the loss occurs in the channel. Because it uses backscattered light to determine loss, it can sometimes be fooled when light goes from a smaller MFD into a larger MFD, or vice versa.

If the light travels from a smaller MFD into a larger MFD, the loss appears greater. If light travels from a larger MFD into a smaller MFD, the OTDR will indicate that power was actually gained at that splice point. Hence the term “gainer.” The solution is typically an easy one.

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Always Test Both Ways

As recommended in TIA standards for measuring fiber attenuation using an OTDR, bidirectional (2-way) testing is the only way to achieve a truly accurate measurement. With bidirectional OTDR testing, the actual loss of the splice is the average of the two measurements.

Unfortunately, some are tempted to only conduct 1-way OTDR testing as the ability to do so is one of the benefits of an OTDR. However, 1-way testing can be very misleading and potentially lead to having to retest eventually anyway. Most OTDR on the market today actually do the averaging for you so it’s not like you need to manually figure out the splice loss.

Belden offers both standard and bend-insensitive FiberExpress singlemode fiber. While Belden only requires Tier 1 testing of insertion loss using an optical loss test set (OLTS), if Tier 2 testing is required per the specification or for troubleshooting purposes, we recommend always using bidirectional testing.

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