Thursday, October 14, 2010

What's Inside a Fiber Optic Cable? Project

When we moved to Teaneck two years ago (this post has been a long time coming) we had FiOS installed. One of the many things that went wrong that day was them bringing too short of a fiber optic cable, requiring them to cut off the one they started to install. I snagged a piece of the removed and discarded cable and this naturally led to the:

What's inside a Fiber Optic Cable Project!
A fiber optic cable consists of a few parts. Let's work our way from outside-in.

The outside of the cable consists of this stiff, hard and smooth black plastic (polyethylene) casing:
As you can see, this fiber is made by Corning. I actually found what I think is the product page here, along with a spec sheet [pdf]. The outer casing provides a strong outer protective layer.  This is what the cable looks like in cross-section (fingers add some scale):
The straight white segments you see in the first picture and in this cross-section are called "Dielectric Strength Members" in the spec sheet. That basically means "Strong Non-conducting Things."  In this case, they are Kevlar cables.  This stiffens the cable and, by sandwiching the actual fiber in between the two Kevlar things, decreases the likelihood of something damaging the optical fiber.

The next thing in is a soft opaque tube filled with a gooey gel type stuff. This is called a "buffer" and serves as an additional layer of protection for the optical fiber inside.
Here you can see the Kevlar cables and the (thicker white) buffer tube surrounding the the optical fiber.

Well, sort of. First you have the "cladding" -- the blue line in the picture.
Here's what it looks like under a microscope.  The cladding serves both as an additional protective layer, and, since it has a lower index of refraction than the "core" optical fiber, helps the light reflect back into the fiber.  Finally, then, after the plastic, the Kevlar, the buffer, the cladding [the frog, the lights, the armor], we get what's all the way inside the fiber optic cable: The optical fiber itself:
Unfortunately, I don't have a functioning micrometer (or SEM) at home, so I'm not sure how thick the fiber (or fiber+cladding) actually is, but I'd estimate on the order of 100μm [about a hair's thickness].  I could probably compare from the scale of an easier-to-measure thing, but I'm too lazy. Readers: feel free. Anyway, from the spec sheet, that seems about right, too.

And, for the record (lest you think my lack of micrometer at home lowers my scientist "cred"), the microscope images were taken on my dining room table:


  1. Great article thanks for sharing.

  2. Great help in explaining the piece I cut off today. The piece I have maybe a newer product same basic construction but materials seem different. Mine has one extra line just outside the centre between the centre and the protective Kevlar about the size of a fine white sewing thread and it looks like it is made up of fine fiber glass. Thank for putting this together.

    1. Glad you found it helpful. If you have a picture of your cable, I'd like to see it. Feel free to email me or seek me out on social media.