An active optical cable (AOC) is a fibre cable with an optical transceiver permanently built into each end, so the port sees an electrical interface while the signal travels as light. AOCs reach further than copper DAC cables and are thinner and lighter, but they draw power and have a fixed length. They are used for switch and server links and for long HDMI runs.
What an active optical cable is
An active optical cable, often called an AOC cable, is a length of fibre with a transceiver sealed into each plug. The plug presents an electrical interface to the port, exactly like a pluggable module of the same form factor. Inside, a driver and laser turn the electrical signal into light, and at the far end a photodiode and receiver turn it back. Most data-centre AOCs use 850 nm VCSEL lasers over multimode fibre.
The fibre is never exposed, so there are no optical connectors to inspect or clean. Because the optics and fibre are matched at the factory, the cable is sold as one part with a fixed length, and the host port powers it. In data centres AOCs are made in the same form factors as transceivers. In audio-visual work they are made as HDMI and DisplayPort cables that replace long copper runs.
AOC vs DAC vs transceivers and patch cords
| Option | Reach | Weight and bend | Power | Cost drivers | When to choose |
|---|---|---|---|---|---|
| Passive DAC (twinax copper) | A few metres, shorter as the data rate rises | Heaviest and stiffest; thicker gauge for longer lengths | Almost none | Copper gauge, length, form factor | Server to top-of-rack switch inside one rack |
| Active copper cable | Somewhat longer than passive DAC | Thinner than passive DAC for the same reach, still copper weight | Low, for the signal conditioning in the plugs | Electronics in both plugs plus copper | Adjacent racks where passive DAC is too short |
| AOC | Tens of metres; multimode AOCs commonly to about 100 m | Light and thin, with a tighter bend radius than copper | Higher than copper, drawn at both ends | Optics at both ends, length, form factor, coding | Row-to-row runs, crowded trays, fixed point-to-point links |
| Transceivers plus patch cords | Set by the optic: tens to hundreds of metres on multimode, kilometres on single-mode | Patch cords are light and route through panels | Similar to AOC per port | Two transceivers plus cabling; cabling reused at upgrades | Structured cabling, links through patch panels, longer reaches |
The AOC's strength is simplicity: one part, no connectors to clean, and no loss budget to manage. Its limits follow from the same design. If one end fails, the whole cable is replaced. It cannot pass through a patch panel, so it suits direct point-to-point links only. It must be pulled with both plugs attached, and its length cannot be changed on site.
Separate transceivers with patch cords cost more per link at short lengths, but each part can be replaced on its own, and a good fibre plant stays in place when the optics are upgraded to a higher speed.
Form factors and data rates
| Form factor | Lanes and rate | Common use |
|---|---|---|
| SFP+ | 1 x 10G | 10G server and switch access ports |
| SFP28 | 1 x 25G | 25G server ports |
| QSFP+ | 4 x 10G (40G) | 40G switch ports and 4 x 10G breakout |
| QSFP28 | 4 x 25G (100G) | 100G switch ports and 4 x 25G breakout |
| QSFP56 | 4 x 50G (200G) | 200G links |
| QSFP-DD and OSFP | 8 lanes (400G and 800G) | Spine and AI cluster links |
A 40G QSFP cable can be a DAC or an AOC. Both use the QSFP+ form factor and carry four lanes of 10G, so the choice comes down to length, weight and power. Whichever you use, the plug at each end must match the port, and the cable must be accepted by the switch. Many switches read the cable's EEPROM coding and may refuse or warn about cables not coded for that platform, so state the host equipment when ordering.
Breakout AOCs
A breakout AOC has one high-speed plug at one end and several lower-speed plugs at the other. Common versions are QSFP+ to four SFP+ (40G to 4 x 10G) and QSFP28 to four SFP28 (100G to 4 x 25G). They connect one switch port to four servers, or a newer switch to older equipment.
The switch port must support breakout mode and be configured for it. The fan-out legs are a fixed length, so check that all four reach their ports. For breakouts that need to be patched or rerouted later, the alternative is a parallel optic transceiver with an MPO to LC breakout cable.
HDMI and DisplayPort AOCs
An HDMI AOC carries the high-speed video lanes on fibre. Many designs are hybrid, with copper conductors for power and the low-speed control signals such as DDC and CEC. The cable is usually powered from the source port, though some designs add a separate power input for sources that supply too little.
Most HDMI and DisplayPort AOCs are directional. One plug is marked for the source (player, PC, camera or switcher) and the other for the display. Installed backwards, the cable gives no picture. Plan the pull so each end arrives where it should, check the plug fits through the conduit, and pull the cable with the plug protected rather than by the plug itself.
Radix Fibrotech makes an 8K fibre optic HDMI active optical cable in 10 m to 150 m lengths for venues, meeting rooms and production work. See broadcast and live production for related fibre video products.
Data-centre links without AOCs
For switch and server links, Radix supplies the cabling side of the transceiver-plus-cabling approach rather than data-centre AOCs. Parallel optics such as 40G and 100G SR4 use MPO connectors, served by MTP and MPO trunk and patch assemblies built to the polarity and length of the link. Duplex optics such as SFP+ and SFP28 short-reach modules use LC to LC duplex patch cords.
This route makes sense where links cross patch panels, where the fibre plant should outlast several generations of optics, or where lengths change often. The 400G and 800G parallel optics cabling guide covers fibre counts for the newer speeds.
What to specify when ordering an AOC
- Form factor and data rate at each end, including breakout configuration and leg lengths.
- Length. The measured route plus slack at each end. AOCs cannot be shortened or joined, so order the next length up.
- Host equipment. Switch or server make and model, so the cable can be coded to suit.
- Jacket rating. Riser, plenum or LSZH, as the building requires.
- For HDMI and DisplayPort. Resolution, refresh rate and interface version, which end is the source, conduit size, and fixed or portable use.
- Quantity and spares. A failed AOC is replaced whole, so hold spares of each length.
For the Radix HDMI AOC or for MPO and LC cabling, send these details through the request a quote page.
Questions buyers ask
What is the difference between AOC and DAC?
A DAC is a copper twinax cable with the plugs attached, while an AOC carries the signal as light over fibre with a transceiver in each plug. DACs are cheaper and draw almost no power but reach only a few metres. AOCs reach tens of metres and are lighter and thinner.
Is a 40G QSFP cable an AOC or a DAC?
It can be either. A 40G QSFP+ cable can be copper DAC for short in-rack runs or an AOC for longer runs. Both plug into the same QSFP+ port and carry four 10G lanes. Choose by length, weight and power, and check the switch accepts the cable's coding.
Can an HDMI AOC be installed in either direction?
Usually not. Most HDMI active optical cables are directional, with one plug marked for the source and the other for the display. Installed backwards, the cable gives no picture. Check the markings before pulling it through conduit, because reversing it later means pulling it again.
Can I cut or extend an active optical cable?
No. The fibre ends inside sealed plugs, so the length is fixed and the cable cannot be cut and reterminated. Measure the route, add slack at each end and order the next length up. For runs that will be patched or changed, use transceivers with patch cords.
Does Radix Fibrotech make QSFP active optical cables?
Radix makes the 8K HDMI active optical cable. For switch and server links it supplies the passive cabling used with separate transceivers: MTP and MPO assemblies for parallel optics, and LC duplex patch cords for duplex optics, made to the length and polarity of the link.