The 330A variable single-mode attenuator allows for quick optical power adjustments when performing fiber optic tests and measurements. By serving as a fiber optic potentiometer, the 330A makes receiver sensitivity testing simple and convenient. The 330A can also be used to simulate long lengths of single-mode fiber when developing transmission links. The 330A is an essential tool for performing bit error rate (BER) tests and system margin analysis. The low back-reflections induced by the instrument ensure that the transmitter output is not adversely affected during testing.