# Steering Mirror System with Closed-Loop Feedback for Free-Space Optical Communication Terminals
## 2 Steering Mirror Design and Trade-Offs  
With the growing applications of steering mirrors, the market has seen the introduction of a diverse range of commercial products [12]. These solutions are available in a selection of actuator categories, including from electrostatic MEMS actuators, magnetic voice coils and piezo-stack micro-displacement amplifiers [13; 14; 15]. Each technology comes with its own advantages and trade-offs. MEMS mirrors, for instance, are light weight and have low power consumption. However, their lower actuating forces require the use of lower mass mirrors, thereby restricting aperture and optical power handling [16]. Voice coil actuators offer the advantage of higher actuation forces, enabling the use of larger mirrors with enhanced power handling and travel range. This comes at the expense of increased power consumption, heat generation, and limited resolution. Steering mirrors employing opposing pairs of piezo-stacks, which drive micro-displacement amplifiers, can move a larger-diameter mirror with high pointing resolution, at the expense of lower deflection angles. It is important to note that all of these actuator types require some power to maintain a fixed pointing angle, with the mirror returning to an uncontrolled position when powered off. A qualitative comparison of these primary actuator categories is shown in Table 1.  
For the receiving channel, a large mirror aperture is desirable to improve the signal-to-noise ratio in high-speed communications. Given the successful demonstration of body pointing for the coarse steering of CubeSat laser communications in previous missions [6; 17], there is flexibility in relaxing the requirement for high steering speeds on the receiving channel. This allows for a strategic trade-off, sacrificing speed for increased precision or larger scan angle. Figure 1: (**a**) Simplified transceiver terminal using a single optical train, (**b**) A simplified optical transceiver using a separated optical train design. Red corresponds to the transmitted beam data beam, green to the received data beam, and blue to the received beacon beam. The increased size of the mirror assembly allows for a broader exploration of actuator technologies and mechanical designs. Here, a simple kinematic mount was developed, featuring a spherical ball joint to facilitate tip and tilt movement. This movement is controlled by two piezoelectric inertia stick slip actuators (PIA13 VF, Thorlabs, Ely, UK). The mirror itself has a broadly rectangular aperture, measuring 130 \(\times\) 90 mm. A rendering of the mirror and mount is illustrated in Figure 2, with the positions of the spherical joint and actuator tips marked in blue and red, respectively. The placement of the actuators with respect to the pivot point results in the actuators being coupled for steering the mirror about the horizontal and vertical axes, as depicted in Figure 2. To pivot around a vertical axis, both actuators must be driven in the same direction. Conversely, to pivot the mirror around its horizontal axis, the actuators must be driven differentially. The distance of the actuator from the pivot axis can be adjusted to optimize the pointing performance, mirror stroke and angular steering speed. Positioning the actuators closer to the pivot axis will increase the slew rate and angular range of the mirror, with a trade-off of reduced pointing resolution. Inertia stick slip actuators work by using a piezo stack to turn a lead screw. These actuators provide an extended travel range of 13 mm, coupled with a fine step size of 20 nm, making them well suited for precisely steering a mirror over a long working