The Fiber Optic Association (FOA) has added a section on bend-insensitive fiber to its Reference Guide to Fiber Optics. The section covers both bend-insensitive singlemode and bend-insensitive multimode fiber (BIMMF).
The topic of BIMMF has been a point of debate among optical-fiber and fiber-cable producers in recent months. The new section of the reference guide begins by describing what makes bend-insensitive fiber different - both structurally and performance-wise - from traditional fiber.
The reference guide explains, "Bend-insensitive fiber adds a layer of glass around the core of the fiber which has a lower index of refraction that literally 'reflects' the weakly guided modes back into the core when stress normally causes them to be coupled into the cladding." It later notes, "When you look at the end of a bend-insensitive fiber in a microscope with angled lighting, you can sometimes actually see the trench as a gray ring around the core."
Later, the section brings up the issue of bend-insenstive fiber's compatibility with traditional fibers. It says there is no simple answer to the question of how the inclusion of higher-order modes affects bandwidth.
The reference guide then explains the issue of testing BIMMF can also be considered unresolved: "The mandrel wrap specified in most standards doesn't affect BI fibers the same way as conventional fibers, so either special sources or very small mandrels are required. The encircled flux standard for modal fill which is being adopted by many new standards will address some of these concerns. Check with your fiber vendor for their recommendations."