Fiber¶
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.
This is still a draft and needs cleanup. Claude Opus 4.5 & Gemini 3 Pro claim the content is accurate. LLM were used for grammar and spellchecking.
Terminology¶
These terms sound similar for some, but mean completely different things.
Single-mode vs Multi-mode is about the fiber core size.
Single-mode (9 µm core) allows one light path. Multi-mode (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.
Simplex = one strand. Duplex = two strands (TX and RX).
Most links use duplex.
BiDi uses one simplex fiber for both directions by using different wavelengths for TX and RX.
Fiber Types¶
Single-mode (9 µm core)¶
Jacket color: Yellow
Grades: OS1 (indoor), OS2 (indoor/outdoor)
The tiny core allows only one light path, enabling transmission over tens to hundreds of kilometers. Used for anything beyond a few hundred meters. In practice, OS2 is what you'll find everywhere.
Multi-mode (50 µm core)¶
Jacket color: Orange, Aqua, or Lime green depending on grade
The larger core allows multiple light paths, which limits distance to a few hundred meters. Cheaper transceivers make it popular for short data center runs.
| Grade | Jacket | 10G | 40G | 100G | 400G |
|---|---|---|---|---|---|
| OM3 | Aqua | 300 m | 100 m | 70 m | - |
| OM4 | Aqua/Violet | 400 m | 150 m | 100 m | 100 m |
| OM5 | Lime green | 400 m | 150 m | 150 m | 150 m |
OM5 advantages are best realized with SWDM optics
OM1 (62.5 µm) and OM2 are legacy and rarely used in new installations.
Don't mix single-mode and multi-mode. Different core sizes, different optics, different wavelengths.
Single-mode optics on multi-mode fiber might work over very short distances with lots of signal loss. Multi-mode optics on single-mode fiber generally won't work at all. If it works in your lab over 5 meters, don't expect it to work in production.
Cable Construction¶
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.
Tight Buffer¶
Fibers coated directly with 900 µm buffer, making them thicker and easier to handle. Can contain 1 to 144+ fibers. Mostly used for indoor patch cables, pigtails, and building distribution. Easy to terminate but less protected against temperature and moisture.
Tight-buffered cables come in two styles
Unitized cables have groups of fibers bundled together, each group surrounded by its own jacket. This makes them easier to handle, install, and manage, especially when pulling or routing multiple fibers at once.
Non-unitized/Distribution cables have all fibers under a single outer jacket without sub-grouping, making them more flexible and easier to route in tight spaces, but a bit trickier to organize during installation.
Loose Tube¶
Bare 250 µm fibers placed inside protective tubes around a central strength member. Fibers can move freely, providing better protection against temperature changes and mechanical stress.
Gel-filled Tubes filled with water-blocking gel. Great moisture protection, messy to splice. Used for direct burial.
Dry/Air-blown Uses water blocking tape or fittings instead of gel. Blown into microducts with compressed air. This is what most FTTH deployments use now.
Armored¶
Additional protective layers for harsh environments.
Steel Wire Armored (SWA) Rodent and crush resistant. Common for direct burial.
Corrugated Steel Tape Lighter than SWA, still rodent resistant.
Aluminum Lighter than steel. Used for aerial installations.
Dielectric No metal, uses aramid (Kevlar). Required near high voltage.
Connectors¶
Common Types¶
| 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 |
| FC | Threaded screw-on | Test equipment |
LC is the standard for networking equipment.
SC is what you'll see on your FTTH ONT.
MTP/MPO is for high-speed parallel optics (40G and above).
Polish Types (UPC vs APC)¶
| Type | Color | Use Case |
|---|---|---|
| UPC | Blue | Data networks, Ethernet, enterprise |
| APC | Green | FTTH, PON, CATV, WDM |
Never mix blue and green connectors!
APC is angled, UPC is flat. Connecting them damages both and causes permanent signal loss.
FTTH uses APC (green) because PON is sensitive to reflections.
SFP Modules¶
Small Form-factor Pluggable modules are hot-swappable transceivers that convert electrical signals to optical and vice versa. When buying SFP modules, you need to match several parameters.
Tip
Remember to unplug the fiber cable before removing the transceiver. Most SFPs have a locking mechanism that only disengages when the bail (the little metal wire or plastic handle) is pulled down. If you pull while the fiber is plugged in, you will break the latch.
Stuck transceiver? Poke the center part of the locking mechanism with a strong needle. You need need to lift it past the "cage"
Form Factors¶
| Type | Speed | Connector |
|---|---|---|
| SFP | 1G | LC duplex |
| SFP+ | 10G | LC duplex |
| SFP28 | 25G | LC duplex |
| SFP56 | 50G | LC duplex |
| QSFP+ | 40G | MTP/MPO or LC |
| QSFP28 | 100G | MTP/MPO or LC |
| QSFP56 | 200G | MTP/MPO or LC |
| QSFP-DD | 400G | MTP/MPO or LC |
| OSFP | 400G-800G | MTP/MPO |
Higher speeds increasingly use parallel optics (MTP/MPO) with multiple fiber pairs, though single-fiber options exist for most.
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) | 1310 nm | Duplex | Up to 5 km |
| LX | 1G | Multi-mode (50/62.5 µm) | 1310 nm | Duplex | 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.
Compatibility¶
Some vendors (Cisco, Juniper, HPE) lock their ports to accept only their branded optics.
Buy original (expensive) or third-party modules coded for your vendor (cheaper, usually works fine).
Some equipment allows disabling the check service unsupported-transceiver or similar.
DOM (Digital Optical Monitoring)¶
Most optics report TX power, RX power, temperature, voltage, and bias current in real-time.
If RX power is too low, check patch cables and clean connectors.
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