The difference between a PC, SPC, UPC and APC finish on a fiber optic connector and which one you should choose

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Whenever a connector is installed on the end of fiber, loss is 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, depending on the type of fiber and the application.
For singlemode fiber, 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. A typical hand polished connector will measure at –30dB.
To reduce the back reflection of a connector, it can be polished to SPC (Super Physical Contact) polish or UPC (Ultra Physical Contact) polish. Industry standard is a minimum of –40dB for SPC back reflection measurement and –50dB for UPC back reflection measurement.
If even less back reflection is required, an APC, or Angled Physical Contact polish, might be necessary. An APC connector has an 8º angle cut into the ferrule. These connectors are identifiable by their green color. An APC polished connector has an Industry Standard Minimum f –60dB ORL measurement. APC fiber ends have low back reflection even when disconnected.