Abstract: In an optical amplifier evaluating system of this invention, a pulse method using optical modulators and a probe method are combined to obtain a gain G and noise figure NF of an optical fiber amplifier. A first optical modulator turns on and off signal light output from a first light source. An optical multiplexer multiplexes the optical signal turned on and off by the first optical modulator and probe light output from a second light source. This optical signal multiplexed by the optical multiplexer is selectively fed into an optical fiber amplifier.

Abstract: To know an optical path switching error, a stepping motor is rotated at a predetermined angular step so as to couple an optical fiber collimator attached to its rotating shaft to an optical fiber collimator of optical fiber collimators that is designated by a designation signal. The direction of the optical axis of the optical fiber collimator is detected by a slit disk attached to the rotating shaft, and sensors for detecting slit arrays formed in the slit disk. Detection signals from the sensors are determined to correspond to the designation signal.

Abstract: A first optical attenuation filter is arranged in a predetermined optical path and attenuates light to be attenuated with a first variable attenuation amount. A second optical attenuation filter is arranged in the predetermined optical path and attenuates, with a second variable attenuation amount, the light which has been transmitted through the first optical attenuation filter. The first and second optical attenuation filters can be driven in the forward and reverse directions and each has an attenuation amount characteristic which changes monotonously in both the forward and reverse directions. An attenuation amount setting section sets a desired attenuation amount to the light to be attenuated. First and second drivers are coupled to drive the first and second optical attenuation filters in the forward and reverse directions, respectively.