A couple of years ago, several fiber manufacturers introduced optical fibers that were much less sensitive to bending. Normally, bending or stressing an optical fiber causes loss. In normal use, optical fibers had to be handled carefully and not be subjected to tight bends, kinks or pinching, which is a problem around patch panels and transmission equipment.

Bend-insensitive (BI) fibers were introduced using radical demonstrations. Fiber was kinked and knotted or even stapled to wood with a staple gun, in violation of every rule that techs have been taught about handling optical fiber for the entire history of the industry. Some of us were disturbed by the demos, as we were afraid poorly trained techs would use these techniques on regular fiber with disastrous results. We also worried that the higher stress on BI fiber would cause long-term fiber failures.

How was bend insensitivity achieved?

Making fibers less sensitive to bending was not difficult. Since the lost light was coming out of the core and into the cladding, modifying the cladding to simply reflect the light back into the core greatly reduced the bending losses (see diagram). The reflection is accomplished using an additional layer of glass in the cladding, a simple and inexpensive process during fiber manufacture. The method is not new. In fact it was used in the first single-mode (SM) fibers more than 25 years ago.

Both SM and multimode (MM) BI fibers are available. Since there is no advantage of BI fibers in long-distance outside plant systems, BI SM fiber is only used for patchcords. However, BI SM fibers are compatible with regular SM fibers and can be spliced or connected without worry, although it may require special fusion splicer programming and may not work with local injection and detection fiber alignment systems used on some splicers.

Bend-insensitive multimode fiber (BIMMF), however, was reputed to have some compatibility problems, both with regular MM fiber and among different brands of BIMMF. As you are probably aware, the loss of MM fiber is very sensitive to mode fill, and the light carried in the higher order modes—the light that travels near the outside edge of the core—is the issue. Since BIMMF reflects the light lost in bends back into the core, BIMMF can have considerably different mode fill than regular fiber, especially in the higher order modes. The greater amount of light in the higher order modes in BIMMF was reported to cause higher loss when joined to regular fiber.

A heated controversy over the compatibility and even necessity of BIMMF swept the industry and aired publicly. One manufacturer of BIMMF claimed to overcome the incompatibility issues with regular fibers by slightly changing the makeup of the fiber’s core, but the company’s design patents led to questions about whether other manufacturers would be able to use the same techniques.

Recent additional testing by several fiber manufacturers appears to show that the controversy was “much ado about nothing,” and BIMMF compatibility is not an issue-—or at least, it’s no longer an issue if BIMMF designs have been updated to the latest designs.

While the controversy over BIMMF compatibility may be over, BIMMF still creates some problems for installers testing MM cable plants. Modal distribution in MM fiber affects testing since the control of light in the higher order modes is an important issue, potentially causing big differences in the loss measured.

To produce a standard test condition, MM fiber is usually subjected to a mandrel wrap to control the higher order modes. The mandrel wrap method does not work with BIMMF, since it is specifically designed to be unaffected by bending. Several manufacturers have provided guidelines for mode control with BIMMF, but the methods used are radical, to say the least, involving extremely small diameter mandrels.

A better solution is controlling the fiber optic test source. A new measurement method, which can determine if the source is appropriate for testing, has been developed, but that method is not without controversy either. I’ll discuss it next month.

In the meantime, if you have any questions about fiber compatibility, consult with your fiber supplier. Be sure you get the latest technical information, not marketing claims from the previous controversy. And, if you and your installers read or watch promotional materials for BI fibers, please be warned and warn them about abusing fiber optic cables. This especially means to keep all of them away from staple guns when installing fiber.


HAYES is a VDV writer and educator and the President of The Fiber Optic Association. Find him at www.jimhayes.com.