FOA Guide

Topic: Using Attenuators With Fiber Optic Data Links

Table of Contents: The FOA Reference Guide To Fiber Optics

Using Attenuators With Fiber Optic Data Links

Most of our attention in a data link focuses on the cable plant, particularly minimizing the loss of the installed cable plant. However many links have too much power at the receiver, a consequence of having links designed for long distances being used at shorter distances. If you are unfamiliar with datalinks and there performance parameters, we suggest you refer to the FOA Guide page on datalinks.


The ability of any fiber optic system to transmit data ultimately depends on the optical power at the receiver as shown above, which shows the data link bit error rate as a function of optical power at the receiver. (BER is the inverse of signal-to-noise ratio, e.g. high BER means poor signal to noise ratio.) Either too little or too much power will cause high bit error rates.

We refer to the low power end of the operating range of the receiver as the sensitivity and the high end as overload. Too much power, and the receiver amplifier saturates, too little and noise becomes a problem as it interferes with the signal. This receiver power depends on two basic factors: how much power is launched into the fiber by the transmitter and how much is lost by the loss in the optical fiber cable plant that connects the transmitter and receiver.

The drawing below illustrates the power in a operating fiber optic data link. The transmitter output and the receiver sensitivity determine the Power Budget. The receiver sensitivity and overload levels determines the Receiver Operating Range.

receiver operating range

If the optical power at the receiver is higher than the receiver sensitivity, as shown above, the difference is the operating "margin" of the system.

If the receiver power is too high - that is greater than the upper level of the receiver operating range (see below) - as it often is in short singlemode systems with laser transmitters, you can reduce receiver power with an attenuator.
The attenuator should always be placed near the receiver to make it convenient to measure and adjust the power level at the receiver and it ensures that any reflectance will not affect the transmitter.

The attenuator should reduce the receiver power to a level near the middle of the receiver operating range, not too close to either the sensitivity limit or the overload level. The proper amount of attenuation needed can be determined during the design stage by calculating the receiver power from the transmitter output and cable plant loss budget or after installation by measuring the power at the receiver with a fiber optic power meter.

datalink iwth attenuator

Examples With Numbers

To illustrate this, consider a link with these specifications;

Transmitter Output: 0 dBm (This is the minimum power output of the transmitter. Specs may show a range of power, e.g. +3 to 0 dBm, but for calculating the power budget, the minimum power is used to be conservative.)
Receiver Operating Range: -15 to -30 dBm (That means at power levels above -15 dBm, the receiver will overload and below -30 dBm the signal to noise (S/N) ration will be low and cause a high bit-error-rate.)
Receiver Sensitivity: -30 dBm (This is the minimum power in the operating range)
Receiver Overload: -15 dBm (This is the maximum power in the operating range)

The Power Budget is calculated from the transmitter output - (0 dBm) - to the minimum power at the receiver - (- 30 dBm) - is 30 dB. That is the maximum amount of loss in the cable plant that can be tolerated. Maximum!

Since the receiver overloads at -15 dBm and the transmitter output is 0 dBm, the minimum amount of attenuation in the cable plant must be at least 15 dB or the receiver will overload. If the cable plant loss is less than 15 dB, we need an attenuator.

If we have a cable plant with total end-to-end loss is less than 15 dB, we need to add an attenuator. Actually we might say 20 dB, because we do not want the receiver to be on the edge of overload - ideally, the receiver power would be in the range of -20 dBm - and with the transmitter at 0 dBm, that means the attenuator and the cable plant need to total 20 dB.

So if the cable plant loss is 10 dB, we need a 10 dB attenuator.
So if the cable plant loss is 15 dB, we should ideally use a 5 dB attenuator.
So if the cable plant loss is 20 dB, we do not need any attenuator.
If the cable plant loss approaches 30 dB or more, we need a more powerful transmitter.

Types Of Attenuators

Attenuators can be made by introducing an end gap between two fibers (gap loss), angular or lateral misalignment, poor fusion splicing (deliberately), inserting a neutral density filter or even stressing the fiber (usually by a serpentine holder or a mandrel wrap). Attenuators are available in models with variable attenuation or with fixed values from a few dB to 20 dB or more. They may be inline, plugging into a receiver input or a patch panel near the receiver, a mating adapter for 2 patchcords or serpentine attenuators that work on adding stress loss to a patchcord.


Generally, multimode systems do not need attenuators. Multimode sources, even VCSELs, rarely have enough power output to saturate receivers. Singlemode systems, especially short links, often have too much power and need attenuators.

For a singlemode applications, especially analog CATV systems, the most important specification, after the correct loss value, is return loss or reflectance! Many types of attenuators (especially gap loss types) suffer from high reflectance, so they can adversely affect transmitters just like highly reflective connectors. 

Choose a type of attenuator with good reflectance specifications and always install the attenuator ( X in the drawing) as shown at the receiver end of the link. This is because it's more convenient to test the receiver power before and after attenuation or while adjusting it with your power meter at the receiver, plus any reflectance will be attenuated on its path back to the source. 

test with attenuator

Test the system power with the transmitter turned on and the attenuator installed at the receiver using a fiber optic power meter set to the system operating wavelength. Check to see the power is within the specified range for the receiver.

If the appropriate attenuator is not available, simply coil some patchcord around a pencil while measuring power with your fiber optic power meter, adding turns until the power is in the right range. Tape the coil and your system should work. This type of attenuator has no reflectance and is very low cost! The fiber/cable manufacturers may worry about the relaibility of a cable subjected to such a small bend radius. You should probably replace it with another type of attenuator at some point, however.

singlemode wrap attenuator
Singlemode attenuator made by wrapping fiber or simplex cable around a small mandrel. This will not work well with bend-insensitive fiber.


Table of Contents: The FOA Reference Guide To Fiber Optics


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