Optical fiber operates with very different physics than copper cabling. In this video, you’ll learn about multimode fiber, single-mode fiber, and the difference between UPC and APC connections.
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Communication over optic fiber is communicating using wavelengths of light. This is very different than communicating using copper cables where an electrical signal will create radio frequency interference that could be heard from outside of that cable. With a fiber optic communication, there is no radio frequency or any type of signal that can be tapped from the outside.
Copper cables also communicate over hundreds of meters of distance, whereas something sent over optic fiber can communicate over kilometers of distance. And since it’s just light that’s going through these optic fibers, any external interference with radio frequencies will have no impact on the light that’s going through optic fiber.
Here’s an image of optic fiber and the connectors that you would have on the end of it, and you can see that it is very narrow strands of optic fiber that are sent from one end to the other. And usually there’s a larger coating around the outside to help protect it. At the end of the optic fiber where we’re plugging into our networking equipment, there’s often ceramic ferrule around the end to help protect this very delicate fiber optic that’s on the inside.
Here’s a cross-section of some optic fiber. The core is where we will send that light from one end of the fiber to the other. Around the core is a cladding, and the cladding has a lower index of refraction, which means the light is going to reflect off of the cladding and stay inside the core. And to protect all of this, a coating is added around the outside of both the core and the cladding.
Here’s a close up picture of the end of an SC connector. This is the connector that will be plugging into your networking equipment. And the fiber core is this very small dot that’s right in the middle. It’s almost difficult to see in this picture. That core is protected with a ceramic ferrule around the outside. This allows you to connect and disconnect this fiber from your networking equipment without damaging the fiber that’s on the inside.
When you start working in fiber optics, you’ll generally work with multi-mode fiber or single mode fiber. Multi fiber is commonly used for relatively short range communication. In the world of fiber optics, that’s usually two kilometers or less in length. The type of light that’s being used to send the signal through the fiber is usually something that’s also relatively inexpensive, something like an LCD. And we call this multi-node fiber, because the core of the fiber is larger than the wavelength of the light that we’re sending through the fiber.
As the signal propagates down the fiber, it will begin to disperse and you will have multiple modes of signal arrive at the other end. Single mode fiber is commonly used for much longer distances. With some single mode fiber configurations, you can go up to 100 kilometers without having to reprocess that signal. However, this comes at a financial cost, because we’re using much more expensive light sources to be able to send that all the way down the single mode fiber. The core of the single mode fiber is much narrower than the multi-node fiber allowing us to send that single mode from one end to the other.
As you can already tell, the physics of dealing with optic fiber are very different than the physics we had with copper cables. We have to worry about light that we’re sending out. We also have to worry about the light that may be reflected back to the original source.
There are two different kinds of connectors that can help us control this reflected light. One of these is a UPC connector. That stands for ultra polished connectors, and there is a zero degree angle between the end of the ferrule and the device that we’re plugging into. There’s usually a very high return loss when you’re using a UPC or ultra polished connector. The other type of connector you may see is an APC. This is an angle polished connector where the ferrules connect to each other at a slight angle, and so the reflection is a little bit different when you’re using an APC.
There will commonly be a lower amount of light returned to the source, but you’re also losing a little bit more light on the connector itself. Here’s what two UPC connectors look like if you were to push them onto each other. You can see there is a zero degree angle difference between these two ends of that UPC. And you can see as we’re sending signal down this connection, there will be reflections that come back from this zero degree angle connector.
This is very similar to taking a flashlight and shining it through a window. If you’re directly in front of that window, some of that light will be reflected back to you and make it difficult for you to see. One way to avoid that is to stand at an angle to the window, and that’s effectively what we do with an APC or angle polish connector. You can see there is an 80 degree difference here in these connectors, so as this light passes through, it will be reflected off at an angle rather than being reflected directly back to the source.