Using Attenuators With Fiber Optic Data Links
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.
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 attenuation
in the optical fiber cable plant that connects the transmitter and
receiver.
If
the power is too high as it often is in short singlemode systems with
laser transmitters, you can reduce receiver power with an attenuator.
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.
Gap-loss attenuators for multimode fiber
Serpentine attenuators for singlemode fiber
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
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 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
