Ultrafast optical beamsteering for space applications

Prior to the UFOS project, initial proof-of-concept demonstrations had to be carried out at very high temperatures (108°C) over a very narrow operating temperature range (e.g., 5°C). The workpackages (WP) of the UFOS project were therefore carefully designed to address some of the limitations that had been identified in the preliminary proof-of-concept demonstrations. Specifically, the broad aims of the project were to develop chiral nematic LC mixtures that could be operated at temperatures below 100°C (ideally <50°C) and that still exhibited strong flexoelectric coupling (i.e., they can still result in a tilt of the optic axis of 45°). This relatively large tilt of the optic axis is a mandatory requirement to ensure that a full 2 phase modulation is observed when the LC layer is sandwiched between two quarter waveplates as per the UFOS modulator configuration. In accordance with WP2 conclusions, later mixtures were tailored for speed over temperature requirements.

The project targeted the following performance metrics:
(i) lower operational temperature (<100°C);
(ii) reduced transmission losses (<25%);
(iii) achieve a more uniform alignment;
(iv) lower drive voltage (<50V);
(v) ensure mechanical stability and recovery of the alignment.

The key achievement was the development of a state-of-the-art mixture that meets the following requirements:
(i) Operational temperatures from 30°C - 45°C;
(ii) Reduced transmission losses to <15% using new alignment process;
(iii) Achieved a more uniform alignment using appropriate electric field conditions;
(iv) State-of-the-art mixture exhibits voltages >50V due to presence of polymer, non-polymer mixtures exhibit lower voltages;
(v) Recovery of the alignment and stability to laser powers of 2W

This mixture combines speed with lower temperature operation and full 2Pi phase. It can be polymerized to make the device rugged, withstands thermal cycling and exposure to UV light. Tests with the non-polymerizable mixture demonstrated resilience to high laser powers and exhibits <15% transmission losses (8% of which come from Fresnel reflection losses from the glass substrates). This could be reduced further using a reflective device geometry combined with anti-reflection coatings. For the state-of-the-art mixture, the drive voltages have exceeded the 50 V targeted, which is due to the presence of the polymer network.