High performance all metal telescope for satellite based laser communication terminals

A beam expander telescope is a critical key component in satellite based laser communication terminals (LCTs), since it should maintain its optomechanical stability and imaging performance under extreme environmental conditions and launch stresses. In this report we describe the design, development and functional verification of an afocal off-axis catadioptric metal telescope made of AlSi alloy, that is able to satisfy all performance requirements for operation within a LCT. The telescope has an aperture size of 70 mm and a magnification of 5.6x. The diffraction limited performance is accomplished within a tight wave front error (WFE) budget of ≤ 50 nm including defocus. Polarization duplexer functionality is created with a high polarization extinction ratio (PER) of ≥ 20 dB by utilizing a quarter waveplate assembly. The telescope design is optimized to ensure very low stray and false light to the detector while simultaneously providing high transmission at the operating laser wavelength of 1064 nm. The dimensional and thermo-mechanical stability of the telescope is verified by performing thermal-vacuum and vibration tests. All optomechanical structure including light-weighted mirrors are made of a same aluminum alloy (AlSi) with a coefficient of thermal expansion (CTE) matched electroless nickel coated layer. Athermalization of the telescope is obtained by an ocular lens mounted on an aluminum compensator. In addition, we present a calibration unit (CU) design which is separately integrated into the telescope with a beam sampling mirror, allowing the in orbit co-alignment of the transmitter (TX) and receiver (RX) beams. The functional verification of the complete telescope system is successfully demonstrated under thermal-vacuum environment.