Experimental investigations of fiber dynamics for the LISA backlink

Abstract

The LISA mission is a planned gravitational wave observatory in space that will use inter spacecraft laser links to measure their relative distance changes. In the current baseline implementation, each spacecraft will utilize two optical subsystems. This approach requires an optical connection between the two subsystems, planned as a fiber-based connection. These optical fibers are prone to disturbances by external factors. Thus, it was found that fiber dynamics will limit the phase performance of this connection, the "backlink." The primary contributors in the scope of LISA are fiber backscatter and phase signals induced by temperature or motion of the backlink fiber. A new transportable measurement setup was developed to obtain values for these fiber dynamics. Additional equipment was implemented to measure the temperature and motion effects: a temperature modulator and a motion simulator. The effects of ionizing radiation on the backscattered signal were investigated since backscattered light is one of the primary factors limiting the performance and not yet tested for changes in the relevant environment. Four types of fibers were tested in backscatter and temperature coupling properties: the successor of the fibers in the LISA pathfinder mission, a polarizing fiber, and later two types of fibers with larger core diameters. It was necessary to switch to these large core fiber candidates to prevent stimulated Brillouin scattering from arising. These new fiber types showed less backscatter than the previous candidates. All tested types showed no change in the backscattered power under increasing exposure to ionizing radiation within the expected levels of LISA. Therefore, no degradation of the backlink’s performance is expected over the mission duration. Temperature-to-phase coupling of the fiber candidates was measured, and it was found that the new fibers offer lower temperature coupling. This lower coupling makes the backlink less prone to phase noise induced by temperature fluctuations. The motion mock-up simulates a LISA-like fiber motion to estimate the phase coupling of this fiber motion which is less than 1 rad=°. Lastly, the measured coupling factors and the updated backscatter numbers were implemented in an existing simulation of the backlink’s performance. These simulations show that the change in fiber type is beneficial for the backlink’s performance as the noise decreases. Adding the motion into these simulations also reveals that the coupling found is low enough to be negligible and not change the performance significantly. The ongoing "Three-Backlink experiment" and the future backlink engineering model studies can be used to verify the impact of these dynamics on the performance experimentally.