Whenever a connector is installed on the end of fiber, light loss will be incurred. Some of this light loss is reflected directly back down the fiber towards the light source that generated it. These back reflections, or optical return loss (ORL), will damage the laser light sources and also disrupt the transmitted signal. Various polish profiles are used to accommodate different types of applications. The polish types of fiber optic ferrules we’ll discuss here are the flat surface, physical contact (PC), ultra physical contact (UPC), and angled physical contact (APC).
The original fiber connector is a flat-surface connection, or a flat connector. The primary issue with flat fiber connectors is that when two of them are mated it naturally leaves a small air gap between the two ferrules; this is partly because the relatively large end face of the connector allows for numerous slight but significant imperfections to gather on the surface. A flat polish of the connector surface will result in a back reflection of about 16 dB. This is not much use for single mode fiber cables with a core size of just 8 or 9 µm, hence the necessary evolution to PC connectors.
The fiber ends are typically polished with a slight curvature, such that when the connectors are mated the fibers touch only at their cores. This is known as a “physical contact” (PC) polish. In a PC connector, the two fibers meet, as they do with the flat connector, but the end faces are polished to be slightly curved or spherical. This eliminates the air gap and forces the fibers into contact. PC type is usually required at least 40dB return loss or higher. The PC polish is the most popular connector end-face preparation, used in most applications.
An improvement to the PC connector is the UPC connector. Ultra physical contact polishing refers to the radius of the end face polishing administered to the ferrule, the precision tube used to hold a fiber in place for alignment. A UPC polish results in a dome-shaped end face that aids in optimizing the connections between two jacketed fibers. The end faces are given an extended polishing for a better surface finish. UPC type is usually required at least 50dB return loss or higher. These connectors are often used in digital, CATV, and telephony systems.
An APC connector has a curved end face which is angled at an industry-standard eight degrees. Some instances require return losses of 60dB, only APC connectors can consistently achieve such performance. This is because adding a small 8 degree angle to the end-face allows for even tighter connections and smaller end face radii. APC type is usually required at least 60dB return loss or higher. These connectors are preferred for CATV and analogue systems. Single mode fiber optic cables can be with PC, UPC or APC polished connectors, while multi-mode there has no APC ones. The picture below shows PC, UPC and APC polish types.
By polishing and curving or angling the face of the fiber ends, the light loss is reduced. The different polishes of the fiber optic connector ferrules result in different performances of the cables, mainly on the back reflection. Each connector has a “polished face” that mates with the other connector. Sometimes you will see fiber optic patch cords advertised as LC/UPC multi-mode duplex fiber optic patch cord, or FC/APC single mode simplex fiber patch cable, etc. The latter acronyms tell the buyer the type of polished face. Knowing more about these four polish types of fiber optic ferrules may be beneficial to your organization.
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