  The Laser Ranging Interferometer onboard the Gravity Recovery and Climate
Experiment Follow-On mission proved the feasibility of an interferometric
sensor for inter-satellite length tracking with sub-nanometer precision,
establishing an important milestone for space laser interferometry and the
general expectation that future gravity missions will employ heterodyne laser
interferometry for satellite-to-satellite ranging. In this paper we present the
design of an on-axis optical bench for next-generation laser ranging which
enhances the received optical power and the transmit beam divergence, enabling
longer interferometer arms and relaxing the optical power requirement of the
laser assembly. All design functionalities and requirements are verified by
means of computer simulations. A thermal analysis is carried out to investigate
the robustness of the proposed optical bench to the temperature fluctuations
found in orbit.
