Bend insensitive fiber - optical loss testing

The following is a Fluke Networks’ recommendation (please check with your cabling vendor for their specific test requirements):

*** You should not use multimode test reference cords made from bend insensitive fiber ***

Background:
A tight bend in a fiber can introduce additional optical loss in the link. Singlemode fiber is more vulnerable to this phenomenon, especially at the 1550 nm window. Manufacturers have now created optical fiber that can be bent without introducing additional optical loss into the link, often referred to as bend insensitive fiber (BIF). This has resulted in questions about testing multimode bend insensitive fiber links.

For multimode test reference cords with LED sources only, you must use a mandrel to strip out the leaky (unstable) modes prior to referencing the power. Only then will the optical loss measurement be accurate. This is specified in section 6.4.2.1 of ANSI/TIA-568-C.0.

Fiber core/cladding size (µm)	900 µm buffered fiber mm (in)	2.0 mm jacketed cable mm (in)	2.4 mm jacketed cable mm (in)	3.0 mm jacketed cable mm (in)
50/125	25 (0.98)	23 (0.91)	23 (0.91)	22 (0.87)
62.5/125	20 (0.79)	18 (0.71)	18 (0.71)	17 (0.67)

For example, if your fiber cords are 3 mm in diameter, you would use a 22 mm mandrel. Failure to use a mandrel can result in pessimistic readings where you could fail good links.

With test reference cords made from bend insensitive fiber, the leaky (unstable) modes are not stripped out with the 22 mm mandrel because this particular type of fiber is specifically designed to avoid that happening. To strip out the leaky (unstable) modes you would need to use a much smaller mandrel with many more turns. The disadvantage of using such a mandrel is that it will be correct for 850 nm but not for 1300 nm measurements. If you are testing for a specific application which you know will only run at 850 nm, then using a bend insensitive fiber test reference cord with a smaller mandrel could be an option.

As indicated at the beginning of this article, you should continue to use jumpers that can be mandrel wrapped for both wavelengths when testing an installed fiber link using an LED optical source, created with or without bend insensitive fiber. As always, check with the vendor offering the cabling warranty.