Title: Towards the LISA Backlink: Experiment design for comparing optical phase reference distribution systems,
Abstract: LISA is a proposed space-based laser interferometer detecting gravitational
waves by measuring distances between free-floating test masses housed in three
satellites in a triangular constellation with laser links in-between. Each
satellite contains two optical benches that are articulated by moving optical
subassemblies for compensating the breathing angle in the constellation. The
phase reference distribution system, also known as backlink, forms an optical
bi-directional path between the intra-satellite benches.
In this work we discuss phase reference implementations with a target
non-reciprocity of at most $2\pi\,\mathrm{\mu rad/\sqrt{Hz}}$, equivalent to
$1\,\mathrm{pm/\sqrt{Hz}}$ for a wavelength of $1064\,\mathrm{nm}$ in the
frequency band from $0.1\,\mathrm{mHz}$ to $1\,\mathrm{Hz}$. One phase
reference uses a steered free beam connection, the other one a fiber together
with additional laser frequencies. The noise characteristics of these
implementations will be compared in a single interferometric set-up with a
previously successfully tested direct fiber connection. We show the design of
this interferometer created by optical simulations including ghost beam
analysis, component alignment and noise estimation. First experimental results
of a free beam laser link between two optical set-ups that are co-rotating by
$\pm 1^\circ$ are presented. This experiment demonstrates sufficient thermal
stability during rotation of less than $10^{-4}\,\mathrm{K/\sqrt{Hz}}$ at
$1\,\mathrm{mHz}$ and operation of the free beam steering mirror control over
more than 1 week.