attenuators used to regulate optical signals are disclosed . according to one aspect of the present invention , an assembly including an ir source and an ir sensor is provided to sense blockage of optical signals transmitted between two collimators via a fixed common light blocker . the movement of the light blocker is sensed by a sensing assembly including an ir source and ir sensor . by detecting the photocurrent from the ir sensor and a feedback circuit , the attenuation of the optical signals can be well controlled .