Why is this head, which is half the size of an SC, called a “small form factor” head? Why are there so many of these heads on switches?
LC fiber optic connectors (LC Connector) are extremely common in fiber optic communications and network engineering. The fact that you mentioned it’s ‘half the size of an SC’ and its absolute prevalence on switches is determined by its physical structural design and the need for high-density network deployment.
Here are detailed answers to your questions from the perspectives of general optical engineering and network deployment:
1. Why is the LC Connector called the ‘small square head’?
- Official Origin of the Name:
The LC connector was originally developed by the renowned Lucent Technologies. Its full English name is Lucent Connector (also referred to as Local Connector in some standardization documents). - Origin of the Nickname ‘Small Square Head’:
- SC and ‘Square Head’: The traditional SC connector (Subscriber Connector) has a rectangular housing with a square cross-section. Field technicians commonly refer to it as a “square head” or “big square head”.
- Miniaturized Inheritance of LC: The LC connector inherits the rectangular square head profile of the SC in its external design, but its core physical dimensions are significantly reduced. The Zirconia ceramic ferrule used in SC connectors has an outer diameter of 2.5\ \text{mm}, while the high-precision miniature ceramic ferrule used in LC connectors has an outer diameter of only 1.25\ \text{mm}.
- Size Comparison: Since the overall cross-sectional dimensions and volume of the LC connector are only about half that of the SC, field technicians, to vividly and intuitively distinguish it from the SC, commonly call it the “small square head” (correspondingly, FC is called the “round head”, and ST is called the “bayonet round head”).
2. Why are modern switches (optical ports) almost exclusively equipped with this type of connector?
Network switches, routers, and ODFs (Optical Distribution Frames) and other equipment widely adopt LC connectors mainly due to the following three core technical advantages:
1. Meeting the Demands of Ultra-High-Density Deployment (High Density)
Rack space in modern data centers and enterprise network rooms is extremely valuable. On the panel of a standard 1\text{U} (approximately 44.45\ \text{mm} high) switch, the number of optical ports that can be accommodated is very limited if FC or SC connectors are used. Because the LC’s external dimensions are only half that of the SC, an LC connector with a duplex design occupies almost the same panel space as a single simplex SC. This allows switches to achieve double the optical port density within a limited physical space.
2. High Compatibility with Optical Modules (SFP / SFP+)
In switches, optical fibers are typically not directly plugged into the switch motherboard but are connected via pluggable optical modules (such as SFP, SFP+, SFP28, XFP, etc.) for signal conversion:
- The physical housing specifications of these modules are strictly limited by the international Multi-Source Agreement (MSA), resulting in very narrow dimensions.
- Within such a small optical module slot, two channels, transmitting (TX) and receiving (RX), must be accommodated simultaneously. Traditional connectors with 2.5\ \text{mm} ferrules (SC or FC) are too large to fit side-by-side in a single SFP module.
- Only duplex LC connectors using 1.25\ \text{mm} ferrules can perfectly fit within the housing of SFP/SFP+ modules. Therefore, LC has become the de facto standard interface for modern optical module standards.
3. Latch Mechanism Similar to RJ-45, Easy Operation and Anti-Detachment
- Anti-Detachment Locking: The LC connector uses a latching mechanism, similar to the RJ-45 (commonly known as