Differences Between PC, UPC, and APC Interface Fiber Patch Cords

In fiber optic communication systems, the polishing type of fiber patch cord interfaces directly affects key performance metrics such as insertion loss (signal attenuation during transmission) and return loss (unwanted signal reflection). Among the most common polishing standards—PC, СКП, и БТР—each differs in structural design, performance characteristics, and application scenarios. This article details their core differences to guide proper selection for specific use cases.

1. PC Interface: Physical Contact

Definition & Structural Features

PC (Physical Contact) is the most basic fiber interface polishing standard, designed to minimize air gaps between mating ceramic ferrules (the core component that aligns fiber cores). Its key structural traits include:

• Micro-spherical polishing: The end-face of the ceramic ferrule is polished into a slight micro-spherical convex shape. This design ensures the fiber core (typically 9μm for single-mode or 50/62.5μm for multimode) sits at the “peak” of the convex surface.
• Air gap reduction: When two PC interfaces are mated, the micro-spherical convexity presses the ferrules tightly together, eliminating air gaps between the end-faces. Air gaps are a major cause of insertion loss and signal reflection, so this design directly improves transmission efficiency.

Performance Characteristics

• Insertion Loss (IL): Typically ≤ 0.25 dB. This low attenuation makes PC suitable for most general-purpose fiber links.
• Return Loss (RL): Generally ≥ 35 dB. While effective at reducing reflection, it is not optimized for scenarios requiring ultra-low reflection.

2. UPC Interface: Ultra Physical Contact

Definition & Structural Features

UPC (Ultra Physical Contact) is an enhanced version of the PC standard, developed to further improve signal integrity for high-speed or high-bandwidth applications. Its structural improvements include:

• Refined polishing precision: Building on PC’s micro-spherical design, UPC uses advanced polishing techniques to create a more uniform, smoother end-face. The convexity of the ferrule is optimized to ensure even tighter contact between mating interfaces.
• Superior surface finish: UPC interfaces have a higher surface 光洁度 (smoothness) than PC. This reduces micro-scratches or irregularities on the ferrule end-face, which can cause additional insertion loss or reflection.
• Dome-shaped end-face: Visually, UPC ferrules appear more “rounded” at the tip compared to PC, ensuring the fiber core aligns perfectly with the mating core.

Performance Characteristics

• Insertion Loss (IL): Typically ≤ 0.15 dB. This is 40% lower than PC, making UPC ideal for high-speed networks (e.g., 100G/400G Ethernet) where minimal signal attenuation is critical.
• Return Loss (RL): Generally ≥ 40 dB. The improved surface smoothness and tighter contact reduce reflection significantly, making UPC suitable for sensitive optical systems.

3. APC Interface: Angled Physical Contact

Definition & Structural Features

APC (Angled Physical Contact) is a specialized polishing standard designed to eliminate unwanted signal reflection—an essential requirement for analog or high-sensitivity digital systems. Its unique design includes:

• 8° angled end-face: Unlike PC/UPC’s flat/micro-spherical end-faces, APC ferrules are polished at an 8° angle relative to the fiber axis. This angle redirects reflected light into the fiber’s cladding (instead of back to the light source), where the cladding absorbs the stray light.
• Tight physical contact: Despite the angle, APC interfaces still require precise alignment and tight contact between ferrules to maintain low insertion loss. The angled design does not compromise physical connection quality.

Performance Characteristics

• Insertion Loss (IL): Typically ≤ 0.3 dB. Slightly higher than PC/UPC due to the angled end-face (which increases alignment complexity), but still within acceptable limits for most professional applications.
• Return Loss (RL): ≥ 50 dB (often up to 60 dB). This is the lowest reflection rate among the three standards, making APC irreplaceable for systems sensitive to reflection (e.g., CATV, FTTH PON, and analog optical transmission).

Comparative Table: PC vs. UPC vs. APC

Сценарии применения

Choosing the right standard depends on the system’s performance requirements:

• PC: Suitable for general-purpose, low-cost scenarios where reflection is not a critical concern. Examples include:
  • Legacy enterprise LANs (1G/10G Ethernet).
  • Basic telecom access networks.
  • Short-distance indoor fiber links (e.g., between switches in a small data center).
• СКП: Ideal for high-speed, high-bandwidth digital systems requiring minimal attenuation. Examples include:
  • Data center backbone links (100G/400G/800G Ethernet).
  • Cloud computing and big data transmission networks.
  • High-performance storage area networks (SANs).
• БТР: Mandatory for systems sensitive to signal reflection. Examples include:
  • CATV (cable TV) networks (analog signals are highly reflection-sensitive).
  • FTTH PON (Passive Optical Network) systems (e.g., GPON/EPON).
  • Analog optical sensors and medical imaging equipment.

Identification Methods

To avoid mismatching interfaces (which can cause high loss or damage), industry standards use color-coding for patch cord boots (the protective sleeve around the ferrule):

• PC: Usually синий or бесцветный boots (no standardized color, but blue is common for single-mode).
• СКП: Typically синий or белый boots (consistent with PC for compatibility in some systems, but verified via performance specs).
• БТР: Universally green boots—this is the most strict color standard to prevent accidental use in non-reflection-sensitive systems.

Заключение

PC, UPC, and APC interfaces are not interchangeable—their design and performance are tailored to specific use cases. For cost-effective general use, PC is sufficient; for high-speed digital networks, UPC’s low attenuation is preferred; and for reflection-critical systems like CATV or FTTH, APC is indispensable. Understanding these differences ensures optimal signal quality, reduces network downtime, and extends the lifespan of fiber optic equipment.