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Using Attenuators With Fiber Optic Data Links

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

Too much power, and the receiver amplifier saturates, too little and noisebecomes a problem as it interferes with the signal. This receiver power

depends on two basic factors: how much power is launched into the fiber bythe transmitter and how much is lost by attenuation in the optical fiber cableplant that connects the transmitter and receiver.If the power is too high as it often is in short singlemode systems with lasertransmitters, you can reduce receiver power with an attenuator. Attenuatorscan be made by introducing an end gap between two fibers (gap loss),angular or lateral misalignment, poor fusion splicing (deliberately), insertinga neutral density filter or even stressing the fiber (usually by a serpentineholder 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.

Gap-loss attenuators for multimode fiber

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Serpentine attenuators for singlemode fiberGenerally, 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 andneed attenuators.For a singlemode applications, especially analog CATV systems, the mostimportant specification, after the correct loss value, is return loss or

reflectance! Many types of attenuators (especially gap loss types) sufferfrom high reflectance, so they can adversely affect transmitters just likehighly reflective connectors.

Choose a type of attenuator with good reflectance specifications andalways install the attenuator (Xin the drawing) as shown at the receiverend of the link. This is because it's more convenient to test the receiverpower before and after attenuation or while adjusting it with your powermeter at the receiver, plus any reflectance will be attenuated on its pathback to the source.

Test the system power with the transmitter turned on and the attenuatorinstalled at the receiver using a fiber optic power meter set to the system

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operating wavelength. Check to see the power is within the specified rangefor the receiver.If the appropriate attenuator is not available, simply coil some patchcordaround 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 yoursystem should work. This type of attenuator has no reflectance and is verylow cost! The fiber/cable manufacturers may worry about the relaibility of acable subjected to such a small bend radius. You should probably replace itwith another type of attenuator at some point, however.

Singlemode attenuator made by wrapping fiber or simplex cable around asmall mandrel. This will not work well withbend-insensitive fiber.

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