Understanding Fiber Optics [Part 2] – Fiber Optic ConnectorsOptace
Fiber optic connectors are the terminations at the end of fiber optic cables to provide attachment to a transmitter, receiver or other cable and allow for re-mateable connections.
Fiber optic cables carry information between two places using entirely optical (light-based) technology. For the light pulses to transmit effectively, fiber optic connectors must mechanically couple and align the cores of the fibers perfectly. Whether you are installing a brand-new fiber optic network or adding a connection, it is important for the connection to be highly precise in order to facilitate high speed fiber optic networking. That being the case, let’s dive in to fiber optic connectors!
Before we get to that…
There are different types of fiber optic connectors and each has its own design, depending on the implementation. To better understand these design differences, lets have a look at the major components of a fiber optic connector;
Ferrule — this is a thin structure (often cylindrical), usually made from ceramic, metal, or high-quality plastic, that forms a tight grip on the fiber.
Connector body — this is a plastic or metal structure that holds the ferrule and attaches to the jacket and strengthens members of the fiber cable itself.
Coupling mechanism — this is a part of the connector body that holds the connector in place when it gets attached to another device. It may be a latch clip, a bayonet-style nut, or similar device.
Types of Fiber Optic Connectors
There are more than 100 types of connector but we are only going to have a look at the 4 most commonly used connectors, i.e., SC, ST, LC and FC.
The SC connector was developed in Japan by NTT (the Japanese telecommunications company), and is believed to stand for ‘Subscriber Connector’ or ‘Standard Connector’. SC connectors use a round 2.5mm ferrule and come with a locking tab that enables push on / pull off mating mechanism to offer quick insertion and removal. The SC connector can be utilized with single-mode and multi-mode fiber optic cables.
The connector body of an SC connector is square shaped. Two SC connectors are commonly bound together with a plastic clip, creating a duplex connection.
Developed by Lucent Technologies, the LC connector otherwise known as a ‘Lucent Connector’ measures about half the size of an SC connector. Available in simplex or duplex versions, LC connectors can be used with both single-mode and multi-mode cables. The LC connector uses a 1.25mm ferrule with a retaining tab mechanism.
ST connectors were one of the first connector types widely implemented in fibre optic networking applications. Originally developed by AT&T, ST stands for ‘Straight Tip’ connector. The ST connector utilizes a 2.5mm ferrule with a round plastic or metal body. The connector stays in place with the help of a “twist-on/twist-off” bayonet-style lock mechanism.
FC is an acronym for ‘ferrule connector” or ‘fiber channel’. The connectors have a threaded body and a position locatable notch to achieve exact locating of the SMF in relation to the receiver and the optical source. Once the connector is installed, its position is maintained with total precision. The FC is designed for durable connections, and can be used in high-vibration environments.
All these connectors feature an end face at the ferrule that is either polished at an angle, or curved; a design feature that is dependent on implementation. The two most commonly used polish styles are APC (Angled Physical Contact) and UPC (Ultra Physical Contact).
The main difference between APC and UPC connectors is the fiber end face.
An APC connector usually has a green body with a curved end face, angled at an industry-standard 8 degrees which allows for even tight connections and smaller end-face radii. Thus, any light that is redirected back towards the source is actually reflected out into the fiber cladding, again by virtue of the 8 degree angled end-face. APC ferrules offer return losses of -65dB (the higher the value, the better).
A UPC fiber connector which usually has a blue-colored body, has a slightly curved end face for better core alignment. With UPC connectors, any reflected light is reflected straight back towards the light source. The back reflection of UPC connector is about -55 dB.
APC or UPC?
Generally, the APC connector has a better performance than the UPC type. APC is best for high bandwidth applications and long haul links e.g. FTTx (fiber to the x), passive optical network (PON) and wavelength-division multiplexing (WDM). These applications are more sensitive to return loss, thus APC is a better solution to offer the lowest return loss.
However, massive employment of APC connectors will cause higher cost. If your project budget is of importance to you, UPC might be a better choice.