For different types of optical fiber connectors, which of the following is correct?

Fiber optic connector compatibility depends on the physical geometry of the ferrule end-face. PC (Physical Contact) and UPC (Ultra Physical Contact) connectors both have a flat, non-angled polish. They are physically compatible and can be interconnected, although the connection's performance will be limited by the lower-grade (PC) connector. In contrast, APC (Angled Physical Contact) connectors have an end-face polished at an 8-degree angle. Mating an angled APC connector with a flat PC/UPC connector creates an air gap and misalignment, causing high insertion loss, high reflectance, and potential physical damage to both connector tips. Therefore, APC is not compatible with PC or UPC.

A. APC and UPC connectors are compatible: This is incorrect. The physical mismatch between the angled (APC) and flat (UPC) end-faces will cause signal degradation and potential damage.
C. PC and UPC connectors are not compatible: This is incorrect. They share the same basic physical geometry (non-angled) and can be safely interconnected.

1. The Fiber Optic Association, "FOA Guide To Fiber Optics And Premises Cabling," Chapter 7: Fiber Optic Connectors. This guide explicitly states, "APC and UPC connectors cannot be mated." and "PC and UPC... are interoperable."

2. IEC 61754-4:2013, "Fibre optic interconnecting devices and passive components - Fibre optic connector interfaces - Part 4: Type SC connector family." This standard defines the physical interface specifications for connectors, from which the incompatibility of angled and non-angled end-faces is derived.

3. Saleh, B. E. A., & Teich, M. C. (2019). Fundamentals of Photonics (3rd ed.). Wiley. Chapter 23 discusses fiber optic components, including connectors, and the principles of minimizing reflection loss, which underpins the design differences between PC/UPC and APC types.