Dither Locking for Acquisition, Tracking and Pointing in Free Space Optics

Abstract

With the ever-growing requirement for global infrastructure to support the global flow of information, the field of free space optics has gained significant momentum in the past decade. This Honours thesis investigated a general dither locking method for acquisition, tracking and pointing in the field of free space optics. This scheme has a number of benefits including the flexibility of its implementation, immunity from channel noise coupling in to the pointing feedback, and the ability to perform acquisition, tracking and pointing without the requirement of position sensitive detectors. The theory and potential implementations of this dither locking method were analyzed for various link topologies. A transceiver set-up using a fast steering mirror and corner cube was built and tested in the laboratory under both direct and heterodyne detection schemes, with the dither locking control program implemented digitally on a FPGA. The developed system was capable of locking the beam pointing to μrad precision, and successfully tracked vibrations in a corner cube, showing up-to a 40dB suppression of the induced jitter at low (<20Hz) frequencies. Strong turbulence was also introduced into the channel and our analysis showed that the system had a good capability of tracking the turbulence induced beam wander, even in the presence scintillation. This transceiver system was then integrated with a laser phase stabilization module developed at the University of Western Australia (UWA) and field trialed at the Mount Stromlo Observatory over horizontal channels through the atmosphere. The dither locking system successfully acquired and locked the alignment for free space links over 1km, and with the links alignment locked, the phase stabilization system developed at UWA was able to demonstrate a stabilized free space frequency transfer with a averaged fractional frequency on the order of 10⁻¹⁷ at 1s.