Recent advancements in fast steering mirror (FSM) technology are expanding the possibilities for precise beam control in various applications. Engineers are highlighting the inherent strengths of different actuator types, noting that while piezo actuators are known for their stiffness and position-holding capabilities without power, voice coil actuators offer a softer touch and require power to maintain position. The choice between these depends heavily on specific application needs.
Voice coil FSMs have seen progress, achieving closed-loop bandwidths of 750 Hz and angular resolution better than 0.1 μrad, and can be cost-effectively controlled with microcontrollers. For applications demanding high frequencies and numerous cycles, designers are emphasizing friction-free and maintenance-free designs, with flexure guiding systems and actuators becoming the state of the art. Flexures are particularly advantageous in vacuum and space environments due to their lack of need for lubrication.
Parallel kinematics and differential drives are key features in advanced dual-axis systems. A parallel-kinematic design utilizing coplanar rotational axes and a single moving platform driven by differential actuators offers benefits like preserved polarization rotation and a more compact form factor compared to traditional series configurations of single-axis mirrors. Differential actuator/sensor setups are recommended for maximum angular stability against temperature variations, especially crucial in space applications where temperature swings can be extreme, ensuring only piston motion and phase shift, not angular deviations.
Piezo steering mirrors are touted for their nanoradian resolution and rapid response times in the millisecond to microsecond range, alongside being lightweight, compact, robust, and maintenance-free. The piezoelectric effect, the basis of this technology, involves dimensional changes in certain materials when subjected to an electric field. This “inverse piezoelectric effect” allows for virtually unlimited dimensional resolution and extremely fast mechanical response, used widely in precision motion control. Modern multilayer piezo actuators, developed using techniques similar to ceramic capacitors, operate at significantly lower voltages than earlier versions, enhancing their practicality. Durability is also a key feature, with ceramic-encapsulated multilayer piezo actuators from companies like PI demonstrating the ability to withstand 100 billion cycles in life testing, as validated by tests related to the Mars rover mission.
Beyond free-space optics (FSO), laser processing is a major application area benefiting from precise beam control offered by FSMs. The parallel-kinematic FSM design, using a single mirror for both axes, presents advantages over two-mirror galvo scanners, including compactness and prevention of polarization rotation. In the medical field, ophthalmology is an expanding market for FSMs, where they are used to precisely direct laser beams in procedures aimed at reshaping the cornea, potentially reducing or eliminating the need for glasses or contact lenses.
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