Use a fibre optic attenuator when the optical power reaching a receiver is above its overload limit, which happens most often on short single-mode links with long-reach optics. Measure the received power, subtract the receiver's maximum input, add a margin, and pick a fixed attenuator in dB. Match the connector type, the polish (UPC or APC) and the wavelength to the link.
What a fibre attenuator does
An optical attenuator is a small passive part that removes a set amount of light from a fibre link. The loss is stated in dB. A 3 dB attenuator passes roughly half the power, a 6 dB attenuator roughly a quarter and a 10 dB attenuator roughly a tenth. It has no power supply and no moving parts in the fixed versions.
Every receiver works inside a window. Below the sensitivity limit it cannot read the signal. Above the overload limit it makes errors, and a strong enough signal can damage it. An attenuator moves the power at the receiver down into the window. This is the opposite problem to the one in a fibre loss budget, where the aim is to keep enough light arriving.
When you need one
The usual case is a short single-mode link driven by a transceiver built for a much longer reach. A laser designed to cross many kilometres can deliver more power than the receiver at the far end accepts when the fibre is only a few tens or hundreds of metres long. Typical places this shows up:
- Back-to-back links between equipment in the same room or building, using long-reach optics.
- Test and commissioning, where two transceivers are joined with a short jumper.
- Loopback testing of a transceiver, where the transmit port feeds its own receiver through a short cord.
- Links that were designed for a long route and later shortened.
The first fix is to use optics that match the real distance. If that is not possible, an attenuator is the standard answer. Read the transceiver datasheet for the maximum transmit power and the receiver overload level before you decide, because different module types differ widely.
Types of attenuator
Attenuators come in a few physical forms. The right one depends on where in the link there is space to fit it.
| Type | How it fits | Typical use |
|---|---|---|
| Plug-type (male to female) | Plugs into an adapter or a transceiver port, like a connector with a built-in loss | Fitting directly at the receiver port |
| In-line adapter type (female to female) | Sits in a panel or between two patch cords | Mid-link or at a patch panel |
| Patch cord with built-in loss | A cord made with the attenuation inside | Where a single part with one stated value is wanted |
| Variable attenuator | Adjustable by a dial or screw, usually a bench item | Laboratories and receiver testing |
Fixed values are commonly offered in steps from 1 dB up to 20 dB, though the available range depends on the maker. Inside the part, the loss comes from doped fibre, a thin coating, or a small offset or air gap. Absorbing designs generally reflect less light back than air-gap designs, which matters for the return loss of the link, covered in insertion loss vs return loss.
How to choose the dB value
Choose the value from the power at the receiver, not from the length of the link. The method has four steps.
- Find the transmitter output. Use the datasheet maximum, or measure it with a light source and power meter as described in power meter and light source testing.
- Subtract the link loss: fibre, connectors and splices.
- Compare the result with the receiver overload level.
- Pick an attenuator that brings the power below overload with a margin of a few dB, then check it stays above the receiver sensitivity.
Here is a worked example with illustrative figures, not taken from any product. A transmitter delivers +2 dBm at its maximum. The link loses 0.8 dB. The power at the receiver is +1.2 dBm. If the receiver overload is +0.5 dBm, the signal is 0.7 dB too strong. A 5 dB attenuator brings it to about -3.8 dBm, which is under the overload level with a good margin. If the receiver sensitivity is -14 dBm, there is still ample light left to read the signal.
Do not pick a value that is too large. An attenuator that pushes the power close to the sensitivity limit trades one fault for another.
Match polish, connector and wavelength
An attenuator is part of the connection, so it must match the rest of the link.
- Connector type: LC, SC, FC, ST or another style, to fit the port or adapter.
- Polish: UPC for UPC links and APC for APC links. Never mate APC to UPC, as the angled and flat faces do not seat properly. The UPC vs APC guide and the APC to UPC mating mistakes page explain the damage and the loss.
- Fibre type: single-mode attenuators are for single-mode links. Multimode links need a multimode part, and the two are not interchangeable.
- Wavelength: the stated loss applies at a given wavelength. Single-mode parts are normally rated at 1310 nm and 1550 nm, and the loss can differ a little between them. See fibre optic wavelengths.
Attenuators add a mated pair to the link, so count them in the loss budget when you check a long route.
Fitting and checking an attenuator
Clean both end faces before you fit the part. Dirt on a connector adds loss that is not part of the attenuator value and can make the result look wrong. The end face cleaning and inspection guide sets out the method.
Fit the attenuator at the receiver end where possible, so the full signal power does not run through connectors that may later be disturbed. After fitting, measure the received power again and compare it with the calculation. If the figure is outside the expected range, check polish, cleanliness and the wavelength of the meter setting before changing the value.
Label the attenuator and record its value in the link documentation. An unmarked part inside a panel is a common cause of confusion months later, when someone reads a low power level and starts searching for a fault in the fibre.
What to put on the order
A good order for attenuators names the following:
- Attenuation value: in dB, with the tolerance you accept.
- Style: plug-type, in-line adapter, or patch cord with built-in loss.
- Connector and polish: for example LC/UPC or SC/APC, with the gender of each end.
- Fibre type and wavelength: single-mode or multimode, and the wavelength the value applies to.
- Quantity and labelling: how many of each value, and the marking required.
Radix lists attenuators, MPO loopbacks and related parts under hybrid adapters, loopbacks and attenuators, and builds fibre optic patch cords to the drawing you send. Send the list through a quote request, and state the receiver power you measured if you want help choosing the value.
Related products and guides: See the attenuators and the full connectors range. Also: fibre loss budget and UPC vs APC.
Questions buyers ask
What does a fibre optic attenuator do?
It reduces the optical power passing through a link by a fixed or adjustable amount, measured in dB. A 3 dB attenuator passes about half the light and a 10 dB attenuator passes about one tenth. The aim is to keep the received power inside the range the receiver accepts.
Do multimode links need attenuators?
Rarely. Multimode transceivers use low-power sources, so overload on a short link is uncommon. Attenuators are mostly a single-mode matter, where long-reach lasers can deliver more power than a receiver accepts when the fibre is short.
Can I coil spare fibre instead of fitting an attenuator?
A tight loop does add loss, but the amount changes with the bend, the fibre type and the wavelength, and it can move over time. It is also a bend radius risk. A purpose-made attenuator gives a stated, repeatable value and is the better choice for a permanent link.
How do I choose the dB value?
Measure or calculate the power at the receiver, work out how far it is above the overload limit, then add a few dB of margin. Check that the result still sits above the receiver sensitivity with room to spare. The transceiver datasheet gives both limits.