Choosing the right attenuator

Single-mode fiber optic transceivers are designed for long-distance fiber optic links.

They have laser transmitters with power to cover distances that usually range from 10 to 160 kilometers, and photoreceptors with the appropriate sensitivity to receive the laser beam from the remote equipment despite the attenuation of the optical fiber and the interconnection elements that may occur. along the way, connectors, fusion splices, adapters, etc.

When single-mode fiber optic transceivers are interconnected with short fibers, the receiver may stop working due to light saturation or, in the worst case, damage the photoreceptor.

To avoid this, attenuators are used that limit the amount of light to appropriate levels.

The appropriate amount of attenuation needed can be determined during the design stage by calculating the receiver power from the transmitter output and the cable plant loss budget or after installation by measuring the power at the receiver with a meter. of fiber optic power.

We must consult the technical sheet of the transceivers that we are going to interconnect to calculate the minimum and maximum quantity that the installation supports.

Let's look at the technical sheet of the compatible Ericsson RDH90120/41422 transceiver from cables24.com as an example.

It has an output power range of 0 to +5dB and a reception sensitivity of -23dB

The power budget is calculated using the minimum output value of the transmitter (0 dBm) and subtracting the minimum power at the receiver which is -23 dB. Therefore 23 dB is the maximum amount of loss that can be tolerated in cable installation. Maximum!

On the sheet we also see that the receiver is overloaded at -3 dB and the maximum output of the transmitter is 5 dB, the minimum amount of attenuation in the cable plant must be at least 8 dB or the receiver will be overloaded. If the loss of the cable installation is less than 8 dB, we need an attenuator.

The attenuator should reduce the receiver power to a level close to the middle of the receiver's operating range, not too close to either the sensitivity limit or the overload level.

The attenuator should always be placed close to the receiver to make it convenient to measure and adjust the power level at the receiver and to ensure that any reflectance does not affect the transmitter.