Characterization of fast steering mirrors for free space communication

The low orbit of Earth is currently experiencing a surge in the number of small cube-satellites, the majority of which are utilised for communication purposes. Initially, these satellites were linked via radio frequency (RF) connections, but in recent times, optical links utilising lasers have become the norm due to their superior data transmission rates. But how can the laser on one cube-sat be consistently directed towards another, while both satellites orbit around earth? This is made possible by a fast steering mirror, which is able to redirect the laser beam with high frequency and accuracy. It enables the correction of vibrations and of micro-movements of the satellite as well as adjustments that consider the time of flight of the laser light. The distance between the sending and receiving cube-sats is typically several thousand kilometres. Any deformations of the fast steering mirror’s surface shape will directly imprint on the wavefront of the reflected laser beam, which in turn will increase the laser beam diameter due to propagation along the entire path from sending to receiving cube-sats. This will reduce the quality of the data transmission. This paper presents a discussion and comparison of two different measurement approaches for the qualification of the surface flatness of the fast steering mirror, deflectometry and wavefront sensor respectively. These two technologies are complementary in this application and both offer high accuracy at lambda/10 ptv or better for flat mirror testing. While deflectometry generally offers a higher lateral resolution, wavefront sensors measure at a higher speed and in single shot. The analysis encompasses measurements of the influence of the fast movement and corresponding acceleration on the fast steering mirror’s surface shape aberrations. Furthermore, we discuss the behaviour under normal ambient conditions as well as in vacuum conditions.