076 - Low SWAP optical clock control unit

076 - Low SWAP optical clock control unit


The need for high-accuracy, resilient holdover techniques to be available in the event of GNSS signal denial has been widely emphasised. Whilst current microwave GNSS satellite clocks, and best-in-class commercial microwave clocks, provide holdover for relatively short periods in situations where GNSS system disciplining is lost, it is clear that future resilient PNT systems will need to maintain improved accuracies over longer fly-wheeling intervals. One solution for this is to develop robust low-SWaP optical oscillators and clocks with better accuracies than microwave systems. Significantly higher accuracies (by factors of up to x100) have already been demonstrated by the best national metrology laboratory-based optical clocks. 

Solutions focussed on future optically-augmented PNT applications include the use of high TRL optical reference cavities capable of operating either in space or in terrestrial mobile environments. Optical cavity-stabilised lasers, where the cavity material is ultra-low-expansion (ULE) glass, have demonstrated very low and well-characterised frequency drift at the level of 1 part in 1015, where the frequency drift can be removed by opto-electronic feed-forward techniques. 

Compared to other approaches (e.g. Horizon) where one single laser wavelength, or none, is foreseen, this Optical Control Unit is based on the requirement for the control of multiple laser sources: wavelength knowledge, wavelength drift control and duty cycle. For Strontium optical frequency standard/clock there are 6 wavelengths. With multiple wavelength requirements, additional complexity comes, but also vastly enhanced performance.

Objective: to develop a dual-axis cavity clock control unit to TRL 6/7. In particular, developing this key component of an optical atomic clock, such that a fully integrated unit is available with appropriate size, weight and power consumption for terrestrial mobile applications. The unit should also be developed to have robustness to vibration, shock and radiation conditions consistent with launch and relevant orbit conditions for use in space applications.

Description of innovation/Tasks:  ULE glass dual-axis cubic cavity is the leading vibration- and force-insensitive optical reference cavity technology for operation in any orientation, in normal gravity or micro-gravity in space. It can provide complete frequency stabilisation control of the optical clock laser (clock transition) and auxiliary lasers needed for an optical clock via dual axes of the cube, which will offer high accuracy, robustness, resilience and extended holdover times for PNT systems. 

The main tasks to be performed will be:

  • characterisation of ULE glass material growth orthogonal axes, optimal mounting arrangements and mirror finesses across the dual axes (very high finesse low thermal noise coating on clock axis, medium broadband finesse on auxiliary axis), 
  • performance testing (vibration insensitivity, frequency stability and long-term drift) and environmental testing (vibration/shock, thermal vacuum and radiation hardness), digital opto-electronic servo system with components that have space-compatible equivalents. 

Expected output: Fully functional and environmentally-tested cubic cavity in vacuum chamber with thermal and frequency stabilisation at world-leading performance for a space-deployable cavity, which is a critical component for ultra-high-stability lasers and high accuracy optical atomic clocks (the other components being the physics package, the laser and the frequency comb).