Establishment Of A Preliminary Prototype For The Spaceborne Laser-Frequency-Stabilization System

This dissertation mainly describes the development of a preliminary spaceborne laser frequency stabilization prototype for the application of inter-satellite laser ranging missions. These missions require the laser system to be space-qualified,being able to withstand the launching process, and has the frequency noise lower than 30 Hz/?Hz from 0.01 Hz to 10 Hz. The preliminary prototype described in this thesis is one of the possible solutions for such a spaceborne stabled laser project. In this prototype, the frequency of an Nd:YAG solid-state laser is actively stabilized to the resonance of an ultra-stable Fabry-Perot (FP) cavity by the Pound-Drever-Hall (PDH) laser stabilization method. In order to meet the compact, robust and stability requirements of the optical system, the FP cavity and its corresponding coupling and mode-matching optical layout are integrated into a quasi-monolithic ultra-stable optical bench by the space-qualified hydroxide-catalysis bonding technique, meanwhile whole-fiber devices are applied to the PDH opical links instead of the usual free-space devices. For the automation and stability requirements, digital controllers are more suitable in spaceborne missions, and in this prototype a multi-functional digital controller with self-analyzing abilities and inner-outer loop control scheme is designed, developed and applied to the laser frequency locking with enough gain in low-frequency range and a control bandwidth of higher than 30 kHz to fully suppress the laser frequency noise. The controller also allows automatically locking and re-locking the laser frequency when the system is on orbit. For the tests of the prototype on ground,a specially designed vacuume system and an active temperature stabilization system are developed to suppress the environmental air flow, pressure and temperature fluctuations; a commercial active vibration isolation platform is applied to suppress the influence of seismic vibrations. The on-ground test of this preliminary prototype shows feasibility for the research of engineering model with improved properties in the future.The research throughout this thesis is mainly about the designing, establishment, experiments and charaterations about the prototype, including:1) precision optical layout design for the mode-matching and coupling between laser and the FP cavity, optimization of the geometry parameters and mounting positions of the quasi-monolithic optical bench by finite-element analysis (FEA), as well as the experimental fabrication of the optical bench that is made of ultra-low expansion (ULE) glass by hydroxide-catalysis bonding; 2) design, establishment and tests of the corresponding vacuume system and temperature stabilization system with the vacuum level of 2×10-7 mbar and the in-loop temperature fluctuation of less than 0.7 mK. as well as the analysis of the noise isolation system with the acoustic isolation of 40 dB and the residual seismic vibration noise at a level of 10-7 g/?Hz; 3) establishment of the whole-fiber PDH laser frequency stabilization optical system, as well as the design and experimental setup of the multi-functional FPGA (Field Programmable Gate Array) digital laser feedback controller:4) experiments of laser frequency stabilization and the corresponding optimizations and testings. In order to characterize the laser frequency stability, the stabilized laser from our prototype is sent to another lab through a 25-m long polarization maintaining fiber for the beat note analysis with an ultra-stable clock laser with sub-Hz frequency stability. The 25-m fiber induces an additional frequency noise to laser at the fiber output by up to 10 Hz/?Hz, thus an active fiber-noise compensation system is developed to compensate the fiber-induce frequency noise by an optical phase-lock-loop (PLL) between the local laser and the remote laser, with the additional fiber noise suppressed to a negligible level of less than 0.015 Hz/VHz after the compensation. Finally, the preliminary results with the linewidth of about 50 Hz in shorterm and a frequency noise spectrum density of less than 30 Hz/VHz from 0.7 Hz to 10 Hz have been observed during the beat note analysis.My Ph.D study also involves a previous research on the experimental setup and characterization of two ultra-stable laser systems as the clock laser and shelving laser for the 27Al+ optical frequency standard. Two semi-conductor lasers were frequency stabilized to two independent commercial ultra-stable cavities with ultra-high Finesse by PDH locking, and the preliminary result with a linewidth of less than 2 Hz and the Allan deviation of 2×10-14 at Is was observed by the beat note between those two stabled lasers. Besides, a 30-cm long ultra-stable re-entrant cavity with multiple materials and low thermal noise limit is newly designed and optimized by FEA simulations, which has a flexible thermal expansion property and shows a solution for both requirements of a low thermal noise limit and a zero coefficient of thermal expansion (CTE) of cavity at room temperature among various batches of ULE glasses.