Many people take optical fiber for granted. My job requires focusing on finding the changes that might make a difference in the field. Some changes are radical, such as the hollow-core fiber I wrote about in “Something for Nothing” (ELECTRICAL CONTRACTOR, August 2024), or the new single fibers with multiple cores used in some new transcontinental submarine cables.
Other changes are more subtle and don’t show up in the news. I discovered a change important to many fiber optic users and installers thanks to a question from a contractor who was installing microcables and high-fiber-count cables. He knew these cables used a different type of fiber called bend-insensitive (BI) fiber. Were the fibers in his current reference test cables compatible with the BI fibers in the cables he needed to test?
Over the last few years, many fiber optic cables have become smaller and have larger numbers of fibers. Some use flexible ribbons packed tightly in the cable. Buffer coatings have been reduced to under 200 microns from the old standard of 250 microns to pack more fibers in the cable. To ensure the fiber is not overstressed in these cables, BI fibers are used.
What are bend-insensitive fibers?
Since the dawn of fiber optics 40 years ago, the standard has been G.652 single-mode fiber for the International Telecommunications Union standard. For the last decade or so, there has been G.657 fiber, a different single-mode fiber similar to G.652 but designed to have better performance under bending stress. There are several versions with increasing bend resistance.
The easy way to understand the difference between the two is that G.652 fiber has a minimum bend diameter specification of 30 mm, three times as large as G.657.A1 BI fiber at 10 mm.
Compatibility with G.652
While G.657 fiber was supposed to be a BI version of G.652 single-mode fiber, some differences have caused problems at splices and connections. The problems came from slight differences in core diameter, called mode field diameter in single-mode fiber. You could see the differences in OTDR traces taken in two directions. In one the splice loss would be high, and in the other you would see a “gainer,” the anomaly often seen when testing mismatched fibers.
To answer the compatibility question, I contacted FOA technical advisor Joe Botha of Triple Play Fiber Optic Solutions in Durban, South Africa. He has done several tests on splicing dissimilar fiber and sent me a report with some OTDR traces that showed excellent compatibility between G.652 and G.657.A1, the most widely used grade of BI fiber.
These traces showed excellent compatibility between the G.652 and G.657.A1 fibers. The differences were minuscule, more like what you would expect from two G.652 fibers. That was much less than I have seen in the past. However, seeing the OTDR traces raised some questions for me. They have a lot of information if you know what you are looking for.
Are today’s fibers different?
I wondered if the “recipe” for G.657 fiber had changed, so I contacted several technical people I know at fiber manufacturers. I got a short lesson on today’s BI fibers.
When G.657 single-mode BI fibers were introduced, they got part of their better bending characteristics from the fiber design around the core and part from reducing the mode field diameter of the core. That reduced mode field diameter is what caused the compatibility problems with G.652 fiber.
Over time, BI fiber design has changed to enable a larger core size such as G.652 fiber to meet the standard, making the two fiber types compatible.
I have been told that the fiber optic industry is moving toward most single-mode fiber being BI. It is better for the industry to have one standard, and it is good for that fiber to be more rugged.
This means that new fiber optic cables are more likely to be compatible with older fiber optic cables, no matter the design. There is less need to worry about compatibility with current patchcords and test cables. Thankfully, there is much less likelihood of getting confusing OTDR traces.
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About The Author
HAYES is a VDV writer and educator and the president of the Fiber Optic Association. Find him at www.JimHayes.com.