A controllable attenuator includes a pair of collimators respectively connected to input and output fibers. A pair of reflection devices are respectively positioned behind the pair of collimators opposite to the corresponding input and output fibers. A U-like light path is defined among the pair of collimators and the pair of reflection devices. A neutral density filter is moveably positioned between the pair of reflection devices wherein a moving direction of the filter is preferably parallel to a longitudinal direction of the pair of collimators. An ND filter position indicator such as a potentiometer, is used to dynamically monitor attenuation setting.