Compact, high-power, low-divergence laser transmitter beam expander for S(D)LR and optical communication

Abstract

STAR-C is a transportable station in development at DLR Institute of Technical Physics to perform satellite and space debris laser ranging (S(D)LR) with its eye-safe laser technology based on fiber-laser pumped Er:YAG laser in the spectral region of 1645 nm. Space debris with its low optical cross-section generally requires higher laser pulse energies compared to satellites equipped with retro-reflectors. Additionally, in order to maintain eye-safety laser standards, the transmitted beam diameter has to be increased. This contribution presents the opto-mechanical design of a compact, high-power, and low divergence laser transmitter beam expander intended for a transportable satellite and space debris laser ranging station (S(D)LR). The beam expander is based on a two-stage beam expander design. It comprises two off-the-shelf zoom beam expanders (Galilean beam expander) used to adjust initial beam diameter to necessary input beam diameter and to collimate the beam, and a second expander stage (also Galilean beam expander). The latter allows transmitted beam diameters of up to 160 mm (input beam diameter 10 mm). For perfect collimation and lens alignment (perpendicular to beam axis) the lens holders are movable in Z-axis and allow tilting in all directions. The mounting platform (base plate) is also designed to allow tilting for correct alignment. The opto-mechanical design of the beam expander was optimized for low thermal expansion, high-stability and low overall weight for transportable S(D)LR stations and other possible applications, e.g. in space-borne optical communication.