Optical attenuator instruments are used to flexibly control optical signal power levels in test setups. A primary application is determining optical receiver sensitivity by measuring the BER vs. input signal power. The N7766A and N7768A provide 2 or 4 attenuator channels respectively for multimode fiber setups and are available with either 50 µm, 62.5 µm or 80 µm fiber ports. The output power is monitored, especially to allow accurate and stable power setting independent of the input power. Convenient offset parameters allow the set power to correspond to the signal after other optical components in the setup path.