Electrothermal micromirror with dual-reflective surfaces for circumferential scanning endoscopic imaging

摘要

Optical coherence tomography (OCT) has emerged as a powerful imaging technology due to its cellular or even subcellular resolution (1 to 10 μm) that is, for instance, suitable for early cancer detection.1 However, slow speeds and large sizes of most conventional optical scanning mechanisms limit OCT applications in real-time in vivo endoscopic imaging of internal organs. Furthermore, for intravascular and internal organ applications such as in heart arteries and lung bronchi, full-circumferential scanning (FCS) is desired. Early FCS research efforts focused on spinning the entire optical fiber and its assembly,2 which has the limitations of scanning speed and optical coupling stability. Other FCS–endoscopic OCT (FCS-EOCT) imaging probes also have been developed, such as employing commercially available micromotors to spin a mirror or prism to obtain 360-deg scanning3,4 or using a microelectromechamical systems (MEMS) scratch drive array to generate a 360-deg rotation.5nRecently, scanning MEMS micromirrors have been extensively investigated for biomedical imaging applications due to their advantages of small size, fast scanning speed, and low cost. Micromirrors based on various actuation methods including electrostatic, electromagnetic, and electrothermal actuations have been developed.6,7,8,9,10,11,12,13,14,15 Electrostatic micromirrors are limited by their small scanning ranges. For instance, Piyawattanametha et al.6 demonstrated an electrostatic micromirror with an angular comb drive design and achieved mechanical scan angle (MSA) ±6.2 deg at 55 V. Relatively larger deflection ranges have also been demonstrated, such as an electrostatic micromirror with ±10 deg MSA by Hsu et al.7 and a gimbal-less electrostatic actuator by Milanovic et al. with a maximum MSA of about 25 deg at resonance.8nHowever, to achieve FCS by a single micromirror with dual-reflective surfaces, a larger deflection angle of at least 90 deg MSA is required. An MEMS scratch drive array was proposed in Ref. 5, but the device structure and fabrication are rather complicated. Electromagnetic micromirrors normally provide larger deflection range.9,10,11 For example, up to 65-deg optical scan angle (OSA) was demonstrated by Yalcinkaya et al.,9 and similar designs were used by Microvision, Inc., for high-resolution displays.10 Ji et al. even demonstrated an electromagnetic micromirror with MSA larger than 90 deg.11 However, electromagnetic micromirrors usually require bulky external coils or magnets to generate the electromagnetic field, which limits the miniaturization ability and complicates the endoscope assembly.11,12 On the other hand, electrothermal actuation generally offers large actuation forces and scanning ranges.13,14,15 For example, an electrothermally actuated bimorph micromirror with large OSA over 100 deg has been used for barcode scanner in Ref. 15. In addition, small driving voltages make electrothermal micromirrors preferable for biomedical imaging applications,16,17 and MSAs as large as 124 deg at only 12.5 Vdc have been demonstrated.18 This rotation angle can be increased even further, but the maximum OSA is limited to 180 deg.nIn order to overcome this limitation, we propose to use a micromirror that has both surfaces being highly reflective. The achievable OSA thus can be doubled by utilizing both surfaces. The simple structure of the electrothermal micromirrors makes the fabrication of dual-reflective surfaces relatively easier than that for the electromagnetic micromirrors, which requires magnetic materials to be integrated on the back side of the mirror plate.11 Also there is no external component needed for electrothermal micromirrors. Therefore, it facilitates the assembly and further miniaturization of the imaging endoscopes. In this paper, the dual-reflective micromirror design, fabrication method, and device testing results are presented. A novel FCS-EOCT imaging probe design based on this dual-reflective micromirror is also proposed.