Fiber optic systems transmission ability is based on the optical power at the receiver, which is reflect as the bit error rate, BER is the inverse of signal-to-noise ratio, high BER means poor signals to noise ratio. Too much power or too litter power will cause high bit error rates.
When the power is too high as it often is in short single-mode systems with laser transmitters, you can reduce receiver power with an fibre attenuator. Attenuators can be made by introducing an end gap between two fiber, angular or lateral misalignment, poor fusion splicing, inserting a neutral density filter or even stressing the fiber. Both variable and fixed attenuators are available.
Variable attenuators are usually used for margin testing, it is used to increase loss until the system has high bit error rate. Fixed attenuators may be inserted in the system cables where distances in the fiber optic link are too short and excess power at the receiver causes transmission problems.
Generally, multimode systems do not need attenuators. Multimode source, even VCSELs, rarely have enough power output to saturate receivers. Single mode system, especially short links, often have too much power and need attenuators. For a single mode application like analog CATV systems, the return loss or reflectance is very important. Many types of attenuators suffer from high reflectance, so they can adversely affect transmitters just like highly reflective connectors.
Attenuators can be made by gap loss, or a physical separation of the ends of the fibers, including bending losses or inserting calibrated optical filters. Choose one type of attenuator with good reflectance specifications and always install the attenuator at the receiver end of the link. It is very convenient to test the receiver power before and after attenuation or while adjusting it with your fiber optic meters at the receiver, plus any reflectance will be attenuated on its path back to the source.
When testing the system power, turn on the transmitter, install the attenuator a the receiver, use a fiber optic power meter set to the system operating wavelength. Check to see whether the power is within the specified range for the receiver. For accurate measurements, the fiber attenuators connector types much match the lanch and receive cables to be tested, e.g. LC fibre optic attenuators is needed to work with the LC fiber patch cable, it work in 1250-1625nm range with optional attenuation value from 1dB to 30dB.
If the appropriate attenuators is not available, simply coil some patch cord around a pencil while measuring power with your fiber optic power meter, adding turns until the power is in the right range.