MEMS mirrors and controls for indoor optical wireless communication
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With the rise of the Internet of Things, more and more devices are being connected to the internet, driving up the demand for wireless connectivity. In addition to advancing the current RF technology, optical communication, specifically IR wireless communication, is a competitive option to help offload some of the wireless demand. However, one of the crucial components for an effective IR system is the ability to quickly and controllably steer the IR light around a room, providing data as a device moves. This thesis focuses on tip-tilt MEMS micromirrors for indoor optical wireless communication (OWC), specifically two key aspects, (1) design and (2) controls. We discuss two unique tip-tilt mirror designs for indoor OWC. The first is an electrothermal varifocal mirror capable of changing its shape, and therefore the shape of the beam in real time. The mirror's radius of curvature can range between -0.48 mm and 20.5 mm, focusing light from a bare fiber from a half-angle divergence of 5° to 0.18° The second mirror uses electromagnetic actuation to achieve a large quasi-static angular range capable of ±60° mechanical ±120° optical) about two rotation axes. In other words, with a laser beam focused on the mirror from the zenith, the device could direct the beam anywhere in a hemisphere. In addition to the device design, we discuss a series of control techniques that can improve the step-and-settle response time of a MEMS device by orders of magnitude compared to a traditional step input. For example, this technique is used to settle the above magnet mirror within 4.5 ms, a factor of 300 improvement over the 1.35 s settling time of a traditional step input. These techniques can be used in conjunction with pulse width modulation (PWM) to provide a fast, low-cost controls solution with no loss in performance. Designing mirrors with these types of controls in mind will not only benefit OWC but enable new applications as well.
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