Enhanced Optical Frequency References for Next Generations of Gravity Missions

Aboard GRACE-FO, the first and up to now only intersatellite laser ranging interferometer (LRI) was launched in 2018. Developed as a technology demonstrator, it exceeds the performance of the actual main instrument, which is based on microwave ranging (MWI), by several orders of magnitude. Therefore, the LRI will most likely be the prime technology for next generations of gravity missions, opening up further optimization opportunities, but also new challenges.

In a future mission, the central axis will not be occupied by a microwave system, allowing new interferometer layouts as e.g. an on-axis design. Beside the layout, the frequency reference of the laser plays a crucial role. Currently, optical cavities that provide high frequency stability are used, while at the same time only small portion of the available size- mass- and power-budget is required for the laser stabilization unit. However, relative frequency references also bring some disadvantages that make absolute references a possible alternative, especially when the MWI is no longer available. Relative references are dependent on external environmental influences, which can be suppressed on short timescales by a well-designed setup, while on long timescales they become apparent and influence the so-called scale-factor. The scale-factor corresponds to the absolute frequency of the laser and affects the conversion of the phase measurement into a length change and therefore the performance of the LRI. Absolute frequency references based on molecular iodine or hybrid references consisting of a cavity and a molecular iodine spectroscopy unit would suppress these dependencies and will simultaneously ease up the initial acquisition process of the LRI.

We will discuss possible alternatives to optical cavities for next generation gravity missions and will outline the advantages and disadvantages of the different technologies. In addition, we analyze proposed technologies that would allow a readout of the absolute frequency of optical cavities in space and underline these with first measurement results from the laboratory.