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Coupler LC-LC

In fiber optic communication systems, fiber optic patch cords (also known as fiber jumpers or patch cables) are the most common components for connecting optical transceivers, equipment, and fiber cables. Their performance largely depends on the polishing method of the connector ferrule end-face. The three mainstream polishing types are PC, UPC, and APC. These differences directly affect return loss, insertion loss, and suitable application scenarios. This article provides a systematic and comprehensive explanation of the differences between PC, UPC, and APC—from definitions, structures, appearance, performance parameters, application scenarios, to selection advice—to help you make the right choices in procurement and engineering projects.

I. What is End-Face Polishing of Fiber Optic Connectors?

The core diameter of an optical fiber is only 9 μm (single-mode) or 50/62.5 μm (multimode). When two fibers are mated, any air gap or surface roughness on the end-face can cause severe reflection and loss. To achieve optimal core alignment and minimize reflection, the ceramic ferrule of the fiber connector must undergo precision grinding and polishing to form different geometric shapes.

The three common polishing methods are:

• PC (Physical Contact)

• UPC (Ultra Physical Contact)

• APC (Angled Physical Contact)

The “PC” in all three names stands for “Physical Contact.” The differences lie in the shape of the contact surface and the polishing precision.

II. Detailed Comparison of the Three Polishing Types

1. PC (Physical Contact)

• End-face shape: Slightly convex spherical surface (micro-dome), with a radius of curvature typically 10–25 mm. To the naked eye, it appears flat.

• Polishing method: Standard physical polishing.

• Return loss: ≥40 dB (early standard), typically around 45 dB in practice.

• Characteristics: The earliest physical contact connector. The convex dome design allows the two fiber cores to touch at the highest point, squeezing out the air gap to achieve physical contact.

• Connector color: Usually blue or black (early multimode versions were often black).

• Applicable fiber: Primarily multimode (OM1, OM2). Now largely replaced by UPC in single-mode systems.

  
  

2. UPC (Ultra Physical Contact)

• End-face shape: Still a convex spherical surface, but with a more tightly controlled radius of curvature (typically 7–25 mm, usually 10–20 mm), resulting in a smoother, more domed appearance.

• Polishing method: Advanced machine polishing with significantly higher surface finish than PC.

• Return loss: ≥50 dB, typically 55 dB or higher.

• Characteristics: An optimized evolution of PC with better end-face polishing and surface finish. The end-face looks more “domed.” It has become the most common choice for single-mode jumpers today.

• Connector color: Usually blue.

• Applicable fiber: Primarily single-mode (OS2), also supports multimode.

  
  

3. APC (Angled Physical Contact)

• End-face shape: The end-face is polished at an 8° angle while retaining a slight convex dome.

• Polishing method: Angled precision polishing.

• Return loss: ≥60 dB, typically 65 dB or even 70 dB+.

• Characteristics: The key difference is the 8° angled face. When light hits a perpendicular end-face, reflected light travels straight back to the source (causing interference). With APC, the 8° angle reflects light into the cladding instead of back to the source, dramatically reducing return loss.

• Connector color: Usually green (the most obvious visual identifier).

• Applicable fiber: Almost exclusively single-mode (OS2).

  
  

III. Visual Comparison of End-Face Structures

(Description of common diagrams; in practice, use a microscope or interferometer)

• PC: Flat-looking with a slight dome, appears planar to the naked eye.

  
  

• UPC: Clearly domed end-face, extremely smooth surface.

• APC: 8° angled face, visibly slanted when viewed from the side, green connector.

IV. Key Performance Parameter Comparison

Note: All three can achieve very low insertion loss (<0.3 dB). The primary difference is in return loss.

V. Recommended Application Scenarios

1. PC Connectors

Gradually phased out. Still found only in some legacy multimode systems. Not recommended for new projects.

2. UPC Connectors (Current mainstream)

• Data center patching

• Enterprise LAN/WAN

• Ordinary FTTx user-side connections

• Standard optical transceivers (SFP, SFP+, QSFP, etc.)

• Surveillance, security, building cabling, and other scenarios with moderate return loss requirements

Over 90% of ordinary single-mode fiber jumpers are UPC.

3. APC Connectors (High-performance specialty)

• CATV, broadcasting and television networks

• Long-haul trunk networks, DWDM systems

• High-precision test equipment (OTDR, optical sources/power meters)

• Analog video optical transmission

• PON networks (some carriers require APC from OLT to ODN)

• Scenarios requiring extremely low return loss to avoid interference (e.g., high-power laser transmission)

Important reminder: APC must be mated only with APC, and UPC only with UPC. Never mix them! Mixing will cause extremely high insertion loss (>10 dB) and essentially block communication.

VI. How to Quickly Identify PC/UPC/APC?

The simplest way is by color:

• Blue → Almost always UPC (a few very old PC connectors are also blue)

• Green → Definitely APC

If colors are the same, check the lettering on the connector boot (PC/UPC/APC) or use a 200×+ microscope to observe the angle.

VII. Selection Advice Summary

1. Ordinary data centers, enterprise networks, and equipment room jumpers → Choose UPC (blue). Best cost-performance ratio.

2. CATV, broadcasting, PON networks, test instruments → Must use APC (green).

3. Legacy multimode systems → May need PC, but upgrade to UPC is recommended.

4. Always confirm both ends use the same polishing type (UPC-UPC or APC-APC).

5. Reputable brands (Corning, CommScope, YOFC, Huawei, etc.) offer more consistent quality and cleaner end-faces.

   
   

VIII. Conclusion

PC, UPC, and APC are all designed to achieve physical contact between fiber end-faces, but with continuously evolving polishing processes and geometries: PC → UPC → APC Return loss: 40 dB → 55 dB → 65 dB+

For the vast majority of users, UPC (blue) single-mode jumpers are perfectly sufficient for daily use. APC (green) is required only in special scenarios such as broadcasting, testing, or high-precision transmission. Understanding these differences will eliminate confusion when purchasing fiber optic patch cords in the future.