In the propagation of laser radiation over large distances, the influence of the atmosphere due to air turbulence is no longer negligible. This effect is particularly noticeable in propagation near the bottom of the atmospheric boundary layer. These disturbances lead primarily to statistical fluctuations of beam deflections and to a broadening of the beam cross-section. Next to the increasing demand that a laser beam must hit a specific, possibly fast-moving target the average intensity of the laser radiation is decreased.
The typical time constants for changes in atmospheric turbulence cells are in the range of 10 to 1000 milliseconds. Depending on the prevailing meteorological conditions, deflection angles of up to 100 microradians occur. Tracking systems incorporating beam deflection devices enable precise beam direction control onto the desired target through adaptation of the rotation and angle of deflection. Other disturbances in the beam direction can be caused by the laser source itself or vibration of the transmitter optics mounting.
High-precision detection and fast tracking in real-time enables beam direction towards fast-flying, remote objects, such as space debris and unmanned aerial vehicles (UAVs). This requires a tracking accuracy in the submicroradian range and a bandwidth of up to several kilohertz. Such real-time systems are being developed in the Active Optical Systems department of the Institute of Technical Physics and tested under real atmospheric conditions. The systems are based on tilting mirrors with various actuator technologies. CCD or CMOS image sensors or high-sensitivity quadrant diodes with in-house developed measuring amplifier electronics are used as position sensors.
To verify the suitability of these systems under real conditions, several experiments have been conducted on the Institute of Technical Physics optical test range in Lampoldshausen. In these studies, turbulence-induced beam direction instability of six microradians (standard deviation) could be reduced to less than 300 nanoradians over a distance of 130 metres. This corresponds to a directional deviation of approximately one millimetre over a distance of three kilometres.
The data obtained in the atmospheric tracking experiments enable the scaling of the system requirements to the relevant distances of several kilometres. Typical scenarios, such as passive and active optical tracking of fast flying objects, are compared with the data acquired in such a manner.
In addition, this lays the foundations for visually tracking satellites and space debris very precisely in low Earth orbit (LEO) and geostationary orbit (GEO), as well as being able to determine the corresponding orbital data.