Layout, Design and Integration of a Measurement Setup in the Context of the AIV of the BELA Transmitter Laser

Kurzfassung

The space probe BepiColombo will carry a laser altimeter for measuring the surface elevations of Mercury. This laser altimeter, called BELA (BepiColombo Laser Altimeter), is the first suchlike instrument developed, integrated and qualified in Europe. The instrument must fulfil high requirements. The specifications related to those requirements must be tested and verified in a special measurement setup that has been designed in scope of this work. The laser parameters to be verified are pulse energy, temporal pulse shape, wavelength, beam quality and divergence, beam profile and especially pointing stability. Pointing stability is of dominant importance as fluctuations or drifts of the beam axis can falsify the range measurements. The measurement setup for pointing stability is based on a concept published by laser altimeter verification teams from NASA. The designed concept uses a 4000 mm off-axis parabolic mirror as focusing element and a stable reference laser providing a reference signal. This reference laser uses the same optical elements as the transmitter laser and thus compensates for the disturbances introduced by them. As detector a CCD camera is used which allows determination of both spot centroids and so the relative movement of the beams to each other. To adjust the beam diameters in the focal plane a 20 x beam expander was selected for the reference laser. The balancing of the beam intensities on the CCD is realized with a wavelength dependent filter unit. Bandpass filters and baffling assure a minimal influence of background illumination effects. Stiff optomechanical elements and shrouds minimize the influence of vibration and temperature fluctuations. Considered as critical was the long focal length of the focusing mirror. Vibrations of the optical elements and the entire setup might dominate the expected small pointing stability value of the transmitter laser of 50 µrad. The mechanical stability of the setup was verified with a substitute laser and proved as feasible. Furthermore beam sizes in the focal plane were calculated and tested to be small enough for fitting into the CCD. The final tests with the transmitter laser will be performed in a thermal vacuum chamber and in a cleanroom environment. A thermal shroud for the transmitter laser is necessary therefore.