Skip to main content

Fiber Optics, Part 3

I recall a statement from one Fiber optic tutorial. It said, “Never, never, NEVER take a new connector in the field until you have installed enough of them in the office that you can put them on in your sleep. The field is no place to experiment or learn! It'll cost you big time!”

For me that pretty much summarizes the difficulty in splicing or installing connectors on FO cables. It may not be rocket science, but it’s pretty close.

It’s not coax

This is sometimes difficult for video engineers to get their head around the difficulty in splicing fiber optic cable or installing FO connectors. After all, video engineers have been putting on BNC connectors for eons. What could be so hard about splicing fiber or installing connectors? Well, the truth is, fiber represents a whole new world when it comes to splicing or installing connectors. Let’s run through some basics.

First, an FO cable core can be smaller than a human hair. Try this experiment. Pull two lengths of hair from your head (assuming you have hair up there). Now put one in each hand and try to line up the ends. Bet you can’t.

A single mode fiber cable core may have a diameter as small as 8μm. A human hair is twice as large. Of course splicing FO or installing a connector isn’t done with a vice and soldering iron. One uses an FO splicer, which includes a microscope. This illustrates part of the difficulty in splicing or putting connectors on fiber. We’re dealing with extremely small technology.

Second, the primarily reason for using fiber is to get either longer distance connectivity or transport higher bandwidth signals—or both. Without proper techniques, the right tools and lots of practice, the result can be worse than trying to shove 3Gb/s down a piece of RG/6 with phono plugs.

Consider the options

There are more than 80 different types of FO connectors. Fortunately, in the United States we have standardized on three types; SC, ST and LC. The nomenclature means; Subscriber Connector, Straight Tip, and Lucent Connector. They are illustrated in Figure 1. The SC and LC connectors use snap fittings. The ST connector resembles a standard BNC bayonet fitting, but is smaller, 2.4mm.

Most of the equipment you’ll encounter will use one of these connectors. All provide similar performance in terms of insertion loss and return loss. Each uses a physical-contact (PC) ferrule finish. The SC and LC connectors are available with an angle-polished ferrule that minimizes reflectance therefore supporting extremely high-speed data or analog video over singlemode fiber. These configurations are called SC/APC and LC/APC. These connectors can be field installed by attaching them to the fiber with adhesives and polishing the end, or using pre-polished connectors that are spliced onto the fibers.

To be clear, your connector options may depend greatly on the vendor of your FO equipment. Some vendors supply their gear with a choice of connector. Other vendors may not provide the option. Choices are good, but once you pick a connector type stay with it. Adapters are available, but why make your life more difficult. Adapters just complicate things.

Unlike copper fittings, you need to consider the loss that accompanies FO connectors. And, these losses will accumulate. It doesn’t take more than a few connectors, add a patch bay and soon, you may have accumulated more loss in just the FO connectors than in the entire length of FO cable.

Next week’s article will continue the review of connectors, splicing, in other words, terminations.

Additional resources:

Fiber Optics, Part 1

Fiber Optics, Part 2

Broadcast Engineering web seminar cast: Best practices for using fiber in broadcast facilities

Harris Optical power budget calculator

Extron: A Fiber Optic Connector Primer