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.
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