| 摘要 |
A free space communication receiver unit (14) comprises a photo-detector (41) for detecting the light intensity of an incoming light beam (18), and an optical lens system (42) for focusing the incoming light beam (18) onto the photo-detector (41). Variable optical power attenuation means is provided between the lens system (42) and the photo-detector (41) for the attenuation of the optical power incident on the photo-detector (41). The attenuation means comprises an aperture (46) on the optical axis (A) between the lens system (42) and the photo-detector (41), in a region where the effective diameter of the incoming light beam (18) is varying as a function of the position along the optical axis (A). The aperture (46) may be translated along the optical axis in the region, so that the effective optical power passing through the aperture (46) varies as a function of the position along the optical axis (A). |
