Fiber¶
Danger
Active fiber uses infrared light, which is invisible to the human eye but can permanently damage your retina. Never look directly into a fiber cable or port!
When splicing, cutting, or breaking fiber, be aware that fiber shards are tiny glass slivers that can penetrate skin and enter the bloodstream. Handle with care and dispose of shards properly.
Light goes in, light comes out. Fiber optic cables transmit data using pulses of light through thin glass or plastic strands.
Unlike copper, fiber is immune to electromagnetic interference, supports much longer distances, and carries significantly more bandwidth.
Overview / Terminology¶
Single-mode vs Multi-mode is about the fiber core size.
Single-mode (9 µm core) allows one light path. Multi-mode (62.5 or 50 µm core) allows multiple light paths.
Yellow jacket = single-mode. Orange/aqua/violet/green jacket = multi-mode.
Simplex vs Duplex is about the number of fiber strands used.
Simplex = one strand. Duplex = two strands (TX and RX).
BiDi uses one simplex fiber for both directions by using different wavelengths for TX and RX.
OS1, OS2, OM1, OM2, OM3 etc. stands for Optical Single or Multi mode.
To make it simple, see the numbers as a performance level. These are standardized by ISO/IEC 11801.
In an emergency, on short runs, you can use an SM transceiver on an OM fiber.
| Type | Jacket | Core | Source | Max Distance |
|---|---|---|---|---|
| OS1 | Yellow | 9 µm | Laser | ~10 km (attenuation-limited) |
| OS2 | Yellow | 9 µm | Laser | ~200 km (attenuation-limited) |
| Type | Jacket | Core | Source | 1G | 10G | 25G | 40G | 100G | 400G |
|---|---|---|---|---|---|---|---|---|---|
| OM1 | Orange | 62.5 µm | LED | 275 m | 33 m | — | — | — | — |
| OM2 | Orange | 50 µm | LED | 550 m | 82 m | — | — | — | — |
| OM3 | Aqua | 50 µm | VCSEL | 1,000 m | 300 m | 70 m | 100 m | 70 m | — |
| OM4 | Violet | 50 µm | VCSEL | 1,000 m | 550 m | 100 m | 150 m | 150 m | — |
| OM5 | Lime Green | 50 µm | VCSEL | 1,000 m | 550 m | 100 m | 150 m | 150 m | 150 m |
LC, SC, MTP, FC, ST are the connector type.
MPO and MTP look the same, but cannot be mixed. MPO is the standard, MTP is invented by companies. MTP are stronger and with less loss.
| Connector | Description | Common Use |
|---|---|---|
| LC | Small, push-pull latch | SFP/QSFP transceivers, data centers |
| SC | Square, push-pull | FTTH ONTs, telecom |
| MTP/MPO | Multi-fiber (8, 12, 24, or 32 fibers) | 40G-800G+ parallel optics |
| ST | Round, bayonet twist-lock | Legacy, industrial, broadcasting |
| FC | Threaded screw-on | Test equipment |
UPC or APC are the angle of the connector tip.
A blue connector is UPC, it has a flat end face. A green connector is APC, it has an 8 degree angle to minimize reflections in sensitive video and GPON networks.
Never mix them because the flat UPC and angled APC endfaces won't align, causing permanent physical damage and massive signal loss.
Transceivers/Optics are hot swappable modules that convert electrical signals to optical and vice versa. Stuck module? Lift the locking tab past the cage with a firm needle.
LC:
SFP 1G · SFP+ 10G · SFP28 25G · SFP56 50G.
LC or MTP/MPO:
QSFP+ 40G · QSFP28 100G · QSFP56 200G · QSFP-DD 400G.
MTP/MPO only:
OSFP 400G–800G
Some vendors (Cisco, Juniper, HPE) lock their ports to accept only their branded optics. Some equipment allows disabling the check with service unsupported-transceiver or similar.
DDM / DOM / DDMI Three names for the same feature, a real time health reporting built into a transceiver, standardized under SFF-8472.
Exposes at minimum five parameters: Temperature · Supply Voltage - TX Bias Current - TX Power - RX Power.
Each has alarm thresholds, letting you catch a dying laser or dirty connector before it becomes an outage. Not all transceivers support it.
Tight Buffer cables protect each fiber individually with a 900 µm plastic coating, making them thicker and easier to handle than other types. They can contain anywhere from 1 to 144+ fibers and are most commonly used for indoor patch cables, pigtails, and building distribution. They're straightforward to terminate but offer less resistance to temperature swings and moisture compared to loose tube designs.
Unitized variants bundle fibers into sub-groups, each with its own inner jacket, which makes routing and managing multiple fibers during installation much easier.
Non-unitized (Distribution) variants run all fibers under a single outer jacket with no sub-grouping - more flexible and space-efficient, but requires more care to organize during termination.
Loose Tube cables house bare 250 µm fibers loosely inside protective buffer tubes that run along a central strength member. Because the fibers aren't fixed in place, they can flex and shift independently, giving them excellent resistance to temperature changes and mechanical stress.
Gel-filled tubes are packed with a water-blocking gel that provides strong moisture protection, though the gel makes splicing messier and more time-consuming. Commonly used for direct burial applications.
Dry (Air-blown) designs replace gel with water-blocking tape or powder, keeping things clean and allowing the cable to be blown into microducts using compressed air. This is the dominant method used in modern FTTH deployments.
Armored cables add one or more protective layers around the fiber bundle, making them suitable for harsh environments where mechanical damage, moisture, or interference is a concern.
Steel Wire Armored (SWA) wraps the cable in steel wires, providing strong resistance to both rodents
What to Match When Buying Optics¶
Both ends of a fiber link need compatible transceivers. Here's what must match:
Speed Both ends must run at the same speed obviously. A 1G SFP can't talk to a 10G SFP+. Some multi-rate modules exist, but both ends still need to agree.
Code Use the same code on both ends. SR with SR, LR with LR, ER with ER. The code defines the wavelength, so mixing them won't work.
Fiber Type Multi-mode fiber (orange, aqua, or green cables) needs SR or SX optics. Single-mode fiber (yellow cables) needs LR, ER, BX, or other long-distance optics.
Connector Most optics use LC duplex connectors. BiDi/BX uses LC simplex. High-speed parallel optics (40G+) use MTP or MPO.
BiDi/BX Exception These need complementary pairs, not identical modules. One transmits 1310nm and receives 1490nm, the other does the reverse. Two identical BiDi modules won't work together.
Common Optic Designations¶
This section was changed 80% by LLM due to having to look up many things and I had some things wrong. Verified by multiple LLMs
IEEE 802.3 Ethernet Optical Transceiver Standards¶
IEEE 802.3 Ethernet standards use letter codes to indicate the type of optical transceiver. The code describes the fiber type, wavelength, and maximum distance.
| Code | Speed(s) | Fiber (core) | Wavelength | Simplex/Duplex | Distance |
|---|---|---|---|---|---|
| SX | 1G | Multi-mode (50 µm) | 850 nm | Duplex | Up to 550 m |
| SR | 10G, 25G, 40G, 100G, 200G, 400G | Multi-mode (50 µm) | 850 nm | Duplex | 26-400 m (varies by speed/fiber) |
| LR | 10G, 25G, 40G, 100G, 200G, 400G | Single-mode (9 µm) | 1310 nm | Duplex | Up to 10 km |
| ER | 10G, 25G, 40G, 100G, 200G, 400G | Single-mode (9 µm) | 1550 nm | Duplex | Up to 40 km |
| LX | 1G | Single-mode (9 µm) Multi-mode (50/62.5 µm) |
1310 nm | Duplex | Up to 5 km Up to 550 m |
| BX10 | 1G, 10G, 25G, 100G | Single-mode (9 µm) | 1270/1330 or 1310/1490 nm | Simplex | Up to 10 km |
| BX20 | 10G | Single-mode (9 µm) | 1270/1330 nm | Simplex | Up to 20 km |
| BX40 | 1G, 10G, 25G, 100G | Single-mode (9 µm) | 1270/1330 or 1310/1490 nm | Simplex | Up to 40 km |
| DR | 100G, 200G, 400G, 800G | Single-mode (9 µm) | 1310 nm | Duplex | Up to 500 m - 2 km |
| FR | 100G, 200G, 400G, 800G | Single-mode (9 µm) | 1310 nm | Duplex | Up to 2 km |
| CWDM | 10G, 40G, 100G, 400G | Single-mode (9 µm) | 1270–1610 nm | Duplex | Varies (typically 40-80 km) |
| DWDM | 10G, 40G, 100G, 400G | Single-mode (9 µm) | C-band (1530–1565 nm) | Duplex | Varies (typically 40-80+ km) |
Note Distance specifications vary based on fiber quality (OM1/OM2/OM3/OM4/OM5 for multi-mode) and speed. Values shown are typical minimums from IEEE specifications.
BX/BiDi transceivers These require complementary pairs - one module transmits on one wavelength while receiving on another, and vice versa. Two identical BX modules cannot communicate.
BiDi terminology "BiDi" (Bidirectional) is industry shorthand for single-fiber operation, not an IEEE standard designation. The actual IEEE standards are the BX designations (1000BASE-BX10, 10GBASE-BX40, etc.).
Non-standard designations ZR and ZR+ are vendor-specific extensions (MSA specifications), not IEEE 802.3 standards. They typically extend reach to 80-120 km using 1550nm wavelength.
PON (Passive Optical Network) Standards¶
PON technologies use point-to-multipoint architecture for fiber-to-the-home (FTTH) and enterprise applications.
| Standard | Organization | Download Speed | Upload Speed | Wavelengths | Max Distance | Max Split Ratio |
|---|---|---|---|---|---|---|
| 1G-EPON | IEEE 802.3ah | 1 Gbps | 1 Gbps | 1490nm ↓ / 1310nm ↑ | 20 km | 1:32 |
| 10G-EPON | IEEE 802.3av | 10 Gbps | 10 Gbps | 1577nm ↓ / 1270nm ↑ | 20 km | 1:32 |
| 25G-EPON | IEEE 802.3ca | 25 Gbps | 25 Gbps | 1577nm ↓ / 1270nm ↑ | 20 km | 1:32 |
| 50G-EPON | IEEE 802.3cr | 50 Gbps | 50 Gbps | 1577nm ↓ / 1270nm ↑ | 20 km | 1:64 |
| GPON | ITU-T G.984 | 2.488 Gbps | 1.244 Gbps | 1490nm ↓ / 1310nm ↑ | 20 km | 1:64 or 1:128 |
| XG-PON | ITU-T G.987 | 10 Gbps | 2.5 Gbps | 1577nm ↓ / 1270nm ↑ | 20-60 km | 1:64 or 1:256 |
| XGS-PON | ITU-T G.9807.1 | 10 Gbps | 10 Gbps | 1577nm ↓ / 1270nm ↑ | 20-40 km | 1:64 or 1:256 |
| NG-PON2 | ITU-T G.989 | 40 Gbps+ | 10 Gbps+ | TWDM: 4-8 wavelengths | 40-60 km | 1:64 to 1:256 |
Note PON systems also commonly use 1550nm for RF video overlay. The ↓ symbol indicates downstream (OLT to ONU), and ↑ indicates upstream (ONU to OLT).
IEEE vs ITU-T EPON (Ethernet PON) is IEEE-based and carries native Ethernet frames, while GPON/XG-PON/XGS-PON are ITU-T standards using GEM (GPON Encapsulation Method). Both are widely deployed globally.
Cleaning and Handling¶
A single dust particle can kill a fiber link if in the way. I've also heard they can on the long links, be burned into the corrector/optic causing permanent damage.
Inspect connectors before plugging in (don't look into lit fibers)
Cap unused ports and cables
Clean with lint free wipes + isopropyl alcohol, or use fiber cleaning pens
Never touch the ferrule end, that makes cleaner a bit harder
A fiber inspection scope is worth it if you work with fiber regularly
Practical Tips¶
Minimum bend radius, think tennis ball size, don't go tighter (even though most are fine with ping ball size)
Label both ends of every cable
Keep spare patch cables and cleaning supplies handy
Test with a light source and power meter, or OTDR for longer runs
Need to run a bunch of fibers to the same location? MPO/MTP lets you easily run 12 core to somewhere, and split it out using a splitter box to LC or whatever you need :D