Significance Fast steering mirror (FSM) is widely utilized in satellite laser communication, super-resolution imaging, high-precision laser aiming and other fields for its quick response, high precision and high resolution. The acquisition tracking and pointing (ATP) system relies heavily on FSM. The main challenge of satellite laser communication is the extremely precise beam stabilization and pointing for maintaining laser communication between moving satellites. In order to overcome this problem, satellite laser communication terminals must rely on the tilt mirror device called FSM to guide and stabilize these laser beams. Traditional FSM is primarily driven by piezoelectric ceramics and voice coil motors. However, thanks to the disadvantages of size and weight of traditional FSM, FSM based on micro electro mechanical system (MEMS) technology is becoming the perfect solution. There are three types of MEMS FSM with electrostatic, electromagnetic and piezoelectric driver. Besides fulfilling the deflection and speed requirements for steering the communication beams, FSM on satellite must also fulfill rigorous space qualification requirements. It is worth mentioning that the reliability of MEMS FSM as novel device applied to satellite laser communications urgently needs further research, but there is less comprehensive research work on this topic.
Progress There is no universal rule for what it means to be "space qualified", so the reliability environmental tests and standards need specify individually space qualification tests and protocols for FSM used in satellite laser communication. It includes vibration and shock test, vacuum test, radiation tolerance and thermal test. These tests address the main environmental stresses on systems during launching and operation in space. Fine aiming and advance targeting are the main roles of FSM in space laser communication terminals. Therefore, FSM not only needs to pass the above environmental tests, but also should pay attention to the changes in the optical characteristics before and after the test, such as optical surface flatness, optical reflectivity and control accuracy. Traditional FSM driven by piezoelectric ceramics (Fig.1) and voice coil motors has already withstood many performances and reliability testing for tip-tilt systems used in laser intersatellite communication. It is relatively mature and has already been working on track. However, MEMS FSM driven by electrostatic (Fig.2), electromagnetic (Fig.3) and piezoelectric (Fig.4) has advantages (Tab.1) of highly integrated, low power consumption and miniaturization, and is a very promising approach in the future. Actuators based on AlN material with self-polarizing characteristics and stable performance have been fully verified in the aerospace field, and the piezoelectric actuated MEMS FSM has been also verified in the non-space field. Finite element analysis (Fig.5) is often performed at the beginning of the design and manufacturing process to study the reliability of FSM, which can shorten the development cycle and anticipate some potential problems. Accelerated life tests with accelerating factors (Tab.2) provide a reasonable lifetime assessment of FSM in orbit, while failure criteria (Tab.2) in life tests and reliability environment tests provide verification for the qualification of FSM. The failure analysis method (Fig.6-7) is commonly used to inspect the failed FSM with or without damage, which can provide the failure mechanism of FSM and guide the reliability reinforcement and design optimization (Fig.8).
Conclusions and Prospects The working principle, environmental adaptability and the reliability requirements of FSM are introduced in this study. The failure mechanism, evaluation method and reliability reinforcement methods of the existing FSM based on different actuation principles and structures are also reviewed. Finally, the improvement of the construction and reinforcing technology of MEMS FSM is discussed and the application for satellite laser communication is proved to be viable, through comprehensive analysis and simulation. In summary, the MEMS FSM can meet most requirements of satellite laser communication, and further verification of aerospace reliability is needed in the future.
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