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For about four years, collaborating with its industrial and scientific partners, CNES has studied the different types of optical sensors that could help to fulfil an impactless rendez-vous and docking mission between two spatial vehicles moving on a low orbit around the Earth. As a part of that activities, a study realized by Sercel and CNES resulted in the definition of a phase measurement rangefinder able to measure distances from 2 to 500 meters with an accuracy varying from 20 millimeters to 40 millimeters at a rate of 1 Hz. The range and the field of view (+5°) of that instrument together with specific constraints due to spatial environment lead to choose high power (100 mW) current modulated continuous wave laser diode as lightning source for this application. As a matter of fact, the target satellite must keep passive during the mission. Furthermore, the sensor must work in open field to avoid the use of mechanical scanning systems and must be able to deliver a correct information even if the sun is in its field of view so that the rendez-vous scenario is not constrainted. Last, semi-conductor technologies have been preferred because their spatialisation seems to be simpler. Yet, a speckle phenomenon due to the multimode optical fibre onto which the laser diode is connected appears in the field of view. That phenomenon, associated to the dynamic behaviour of the component (spectral drift versus current intensity) prevents the sensor from delivering an homogeneous answer in different points of the field. The phenomenon has been analysed and a solution implying an adapted modulation for the laser diode is proposed. The global sensor principle is described.
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