What is Single-mode fiber?
Single-mode fiber guides light along one path, so the signal does not spread out as it travels. Its core is roughly 8 to 10 micrometers across inside a 125 micrometer cladding, which is why it is often called 9/125 fiber. It is used for long links such as between buildings, across a campus, or between sites.
Standards classify the fiber as OS1 and OS2. Both are standard single-mode types, and OS2 has a lower maximum attenuation. OS2 is the type normally specified for new installations, and it is covered in its own glossary entry.
Single-mode differs from multimode in the transceivers it needs. Single-mode links normally use laser transceivers that match the fiber, and their optics are generally not interchangeable with multimode optics. Fiber type and transceiver must be matched, so verify both ends before ordering parts.
How it shows up in practice
- Check the fiber type on both ends of a link and match the transceivers to it. A mismatch is a common cause of failed links.
- Single-mode connectors are often color coded, but colors vary, so do not rely on color alone. Check labels and documentation.
- Clean connectors before connecting. Dirt on a fiber end face is a common cause of poor performance, and the core is small.
- Request test results, for example loss measurements, when fiber is installed or cross-connects are provided.