Towards the LISA Backlink: Experiment design for comparing optical phase reference distribution systems

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.