The FC connector (Ferrule Connector, or Fixed Connection) is a single-mode fiber optic connector distinguished by its threaded screw-lock coupling mechanism. Developed by NTT (Nippon Telegraph and Telephone) in Japan in the 1980s, the FC connector was the dominant single-mode connector in telecommunications and test equipment for over a decade. While LC and SC connectors have largely replaced FC in new enterprise and data center installations, FC remains the standard in optical test equipment, some telecom applications, and high-vibration environments.
FC Connector Physical Design
The FC connector uses a 2.5mm ceramic ferrule — the same diameter as SC and ST connectors — to hold and align the fiber. The defining feature is the threaded coupling nut that screws onto the mating adapter, providing a secure, vibration-resistant connection.
Key Physical Characteristics
- Ferrule diameter: 2.5mm (same as SC, ST)
- Coupling mechanism: Threaded screw-lock (key-and-slot alignment)
- Body material: Nickel-plated brass or stainless steel
- Ferrule material: Ceramic (zirconia)
- Key: Anti-rotation key ensures consistent angular alignment
- Polish types: PC, UPC, APC
The threaded coupling provides a more secure connection than the push-pull (SC) or bayonet (ST) mechanisms, making FC connectors resistant to accidental disconnection from vibration or cable tension. However, the threading requires more time to mate and unmate than push-pull connectors.
FC Connector Specifications
| Parameter | Value |
|---|---|
| Ferrule diameter | 2.5mm |
| Insertion loss (typical) | 0.2 dB (single-mode) |
| Return loss (PC) | ≥ 25 dB |
| Return loss (UPC) | ≥ 50 dB |
| Return loss (APC) | ≥ 60 dB |
| Mating cycles | 500+ |
| Operating temperature | -40°C to +75°C |
| Standard | IEC 61754-13 |
FC Polish Types
FC connectors are available in three polish types, each providing different return loss performance:
FC/PC (Physical Contact)
The fiber end face is polished to a slight convex curve, ensuring physical contact between the two fiber cores when mated. The PC polish eliminates the air gap that caused high back-reflection in older flat-polished connectors.
- Return loss: ≥ 25 dB
- Identification: No color coding (or blue)
- Use: General single-mode applications, legacy telecom
FC/UPC (Ultra Physical Contact)
An improved polish that achieves a more precise convex end face geometry, resulting in better physical contact and lower back-reflection than standard PC.
- Return loss: ≥ 50 dB
- Identification: Blue color coding (same as SC/UPC)
- Use: High-speed single-mode, digital video, CATV
FC/APC (Angled Physical Contact)
The end face is polished at an 8° angle, directing back-reflections away from the fiber core at an angle that prevents them from propagating back into the source. This provides the lowest back-reflection of any polish type.
- Return loss: ≥ 60 dB
- Identification: Green color coding (universal for APC)
- Use: WDM systems, analog CATV, high-sensitivity receivers, coherent optical systems
FC/APC is particularly important in applications where back-reflection causes noise or instability in the optical source (laser). Coherent optical systems, analog video distribution, and high-power optical amplifiers all benefit from FC/APC's very low back-reflection.
Important: FC/UPC and FC/APC connectors are not interchangeable. Mating a UPC connector with an APC connector will cause high insertion loss and potential damage to the end faces. Always verify the polish type before mating.
FC vs SC vs LC: Comparison
| Parameter | FC | SC | LC |
|---|---|---|---|
| Ferrule diameter | 2.5mm | 2.5mm | 1.25mm |
| Coupling | Threaded screw | Push-pull | Push-pull latch |
| Vibration resistance | Excellent | Good | Good |
| Panel density | Low | Medium | High |
| Mating speed | Slow (threading) | Fast | Fast |
| APC availability | Yes (green) | Yes (green) | Yes (green) |
| Current use | Test equipment, telecom | Enterprise, telecom | Data center, enterprise |
The FC connector's threaded coupling is its defining advantage and disadvantage. The screw-lock provides excellent vibration resistance and consistent angular alignment (important for APC connectors), but the threading makes FC slower to mate than SC or LC.
Where FC Connectors Are Used
Optical Test Equipment
FC connectors are the standard interface on most fiber optic test equipment:
- OTDRs (Optical Time Domain Reflectometers)
- Optical power meters
- Light sources
- Optical spectrum analyzers
- Bit error rate testers
The FC connector's consistent angular alignment (via the key-and-slot mechanism) is important for test equipment where measurement repeatability is critical. FC/APC is particularly common on test equipment used for WDM and coherent optical measurements.
Telecommunications
FC connectors were the dominant single-mode connector in telecommunications equipment through the 1990s. Many legacy telecom installations still use FC connectors in:
- Central office equipment
- DWDM (Dense Wavelength Division Multiplexing) systems
- Optical amplifiers (EDFAs)
- Submarine cable landing stations
New telecom installations typically use LC or SC, but FC remains in service in legacy equipment.
High-Vibration Environments
The threaded coupling makes FC connectors suitable for environments where vibration could cause push-pull connectors to disconnect:
- Industrial machinery
- Military and defense equipment
- Shipboard systems
- Aerospace applications
Research and Laboratory
FC connectors are common in optical research laboratories because of their availability on test equipment and their compatibility with precision optical components (isolators, circulators, modulators).
FC Connector Adapters and Hybrid Cables
FC connectors can be connected to LC or SC equipment using:
FC-to-LC adapters: Inline adapters that convert between FC and LC. Available in FC/UPC-to-LC/UPC and FC/APC-to-LC/APC configurations.
FC-to-SC adapters: Similar inline adapters for FC-to-SC conversion.
Hybrid patch cables: One end FC, the other end LC or SC. Used to connect FC-terminated equipment to LC or SC patch panels.
When using adapters or hybrid cables, ensure the polish types match (UPC-to-UPC or APC-to-APC). Never mix UPC and APC.
Terminating FC Connectors
FC connectors can be terminated using the same methods as other 2.5mm ferrule connectors:
Field polishing: Apply epoxy, insert fiber, cure, cleave, and polish. The 2.5mm ferrule uses the same polishing puck as SC and ST connectors.
Pre-polished: Pre-polished FC connectors are available from several manufacturers.
The key difference from SC and ST termination is the threading — the connector body must be oriented correctly before inserting into the adapter, and the key must align with the slot.
CZT's fiber optic connector range includes FC/UPC and FC/APC connectors for single-mode fiber, along with FC-to-LC and FC-to-SC hybrid cables for connecting legacy FC equipment to modern LC/SC infrastructure.
Summary
The FC connector is a mature, reliable single-mode fiber connector with a threaded screw-lock coupling that provides excellent vibration resistance and consistent angular alignment. While LC and SC have replaced FC in most new installations, FC remains the standard for:
- Optical test equipment (OTDRs, power meters, spectrum analyzers)
- Legacy telecom equipment
- High-vibration industrial and military applications
- Research and laboratory optical systems
When working with FC connectors, always verify the polish type (PC, UPC, or APC) and use the green color coding as a reliable indicator of APC connectors. Never mate UPC and APC connectors.
