No abstract

MEMS mirrors have a wide range of applications, most of which require large field-of-view (FOV). Immersing MEMS mirrors in liquid is an effective way to improve the FOV. However, the increased viscosity, convective heat transfer and thermal conductivity in liquid greatly affect the dynamic behaviors of electrothermally actuated micromirrors. In this paper, the complex interactions among the multiple energy domains, including electrical, thermal, mechanical and fluidic, are studied in an immersed electrothermally actuated MEMS mirror. A damping model of the immersed MEMS mirror is built and dimensional analysis is applied to reduce the number of variables and thus significantly simplify the model. The solution of the fluid damping model is solved by using regression analysis. The dynamic response of the MEMS mirror can be calculated easily by using the damping model. The experimental results verify the effectiveness and accuracy of these models. The difference between the model prediction and the measurement is within 4%. The FOV scanned in a liquid is also increased by a factor of 1.6. The model developed in this work can be applied to study the dynamic behaviors of various immersed MEMS actuators.

We present a systematical study on comparison between water and dry coupling in photoacoustic tomography of the human finger joints. Compared to the direct water immersion of the finger for water coupling, the dry coupling is realized through a transparent PDMS film-based water bag, which ensures water-free contact with the skin. The results obtained suggest that the dry coupling provides image quality comparable to that by water coupling while eliminating the wrinkling of the finger joint caused by the water immersion. In addition, the dry coupling offers more stable hemodynamic images than the water coupling as the water immersion of the finger joint causes reduction in blood vessel size.

Electrothermal bimorph-based scanning micromirrors typically employ widely-used silicon dioxide (SiO2) as the electrical and thermal isolation material. However, due to the brittle nature of SiO2, such micromirrors may not be able to even survive a slight collision, which greatly limits their application range. To improve the robustness of electrothermal micromirrors, a polymer material is incorporated to partially replace SiO2 as the electrical and thermal isolation material as well as the anchor material. In particular, photosensitive polyimide (PSPI) is used to simplify the fabrication process. In this work, PSPI-based electrothermal micromirrors have been designed, fabricated and tested. The PSPI-type micromirrors achieved a maximum optical scan angle of ± 19.6 ° and a maximum vertical displacement of 370 µm both at only 4 Vdc. With a mirror aperture size of 1mm × 1 mm, the PSPI-type micromirrors withstood over 200 g accelerations from either vertical or lateral directions in the impact experiment. In the drop test, the PSPI-type micromirrors survived falls to a hard floor at heights up to 21 cm. In the standard frequency sweeping vibration test, the PSPI-type micromirrors withstood 21 g and 29 g acceleration in the vertical and lateral vibration, respectively. In all these experimental tests, the PSPI-type micromirrors demonstrated at least 4 times better robustness compared to the SiO2-type micromirrors fabricated in the same batch.

Scanning MEMS mirrors can extend confocal laser microscopy into endoscopic applications, but the practical use of MEMS mirror-based confocal endomicroscopy is hindered partially by various image distortions such as barrel, fan-shaped and nonlinear distortions. In this work, the nonlinear scanning behaviors of an electrothermal MEMS mirror are analyzed and incorporated into an optical scanning model that takes all these three types of distortions into account. The model generates a 2D spatial mapping that can be applied to correct all of the image distortions in one step without a calibration board. To experimentally validate this method, a confocal laser endomicroscope employing a two-axis scanning electrothermal MEMS micromirror is designed and constructed, and confocal fluorescence images of a patterned micro-structure are obtained with the MEMS endomicroscope. The results show that the overall image distortion is reduced by at least one order of magnitude in the length direction.