Low voltage, MEMS-based reflective and refractive optical scanners for endoscopic biomedical imaging
Imaging technologies such as optical coherence tomography (OCT), two-photon excitation fluorescence microscopy (TPEF), and second harmonic generation (SHG) microscopy require optical scanners to transversely scan a focused laser beam onto the tissue specimen being imaged. However, for in vivo early-cancer detection of internal organs the optical scanners must be integrated into slender endoscopes. The goal of this work is to develop millimeter-sized MEMS optical scanners packaged inside endoscopes to enable endoscopic biomedical imaging. This work reports MEMS micromirrors and microlens scanners fabricated using post-CMOS micromachining processes, which can provide large scan ranges at low driving voltages. Several 1-D and 2-D micromirror scanners have been designed, fabricated and characterized. Scanning micromirrors, as large as 1.3 by 1.1 mm2, have demonstrated optical scan angles greater than 40° at actuation voltages below 20Vdc. The maximum scanning speed of these devices is in the range of 200 to 500Hz, which is adequate for real-time bio-imaging. A new electrothermal microactuator design is reported which enables large vertical displacements (LVD). This LVD microactuator uses two sets of electrothermal bimorph thin-film beams to provide out-of-plane elevation to the micromirror, while keeping the mirror parallel to the substrate. LVD micromirrors have demonstrated large bi-directional scanning ability (>±40°) as well as large vertical piston motion (-0.5mm) at low driving voltages (<15V). A 1-D LVD micromirror has the ability to scan optical angles greater than 170° at its resonance frequency of 2.4kHz. Polymer microlenses integrated with the LVD microactuators have been developed for endoscopic optical coherence microscopy which requires microlenses to axially scan their focal planes by 0.5 to 2 mm. A modified fabrication process allows larger polymer lenses with better thermal isolation to be integrated. A maximum vertical displacement of 0.71 mm was obtained. These scanners have been packaged inside 5-mm diameter endoscopes to enable in vivo imaging. Endoscopic OCT with transverse and axial resolutions of 151.m and 1211m, respectively has been demonstrated at imaging speeds of 2 to 6 frames/second. TPEF and SHG imaging with imaging resolution greater than 1.5µm has been obtained. These results show the potential for the use of MEMS-based endoscopy for early-cancer detection.