Daesung Lee

Fast-scanning two-photon fluorescence imaging based on a microelectromechanical systems two- dimensional scanning mirror

Towards overcoming the size limitations of conventional two-photon fluorescence microscopy, we introduce two-photon imaging based on microelectromechanical systems (MEMS) scanners. Single crystalline silicon scanning mirrors that are 0.75 mm x 0.75 mm in size and driven in two dimensions by microfabricated vertical comb electrostatic actuators can provide optical deflection angles through a range of approximately16 degrees . Using such scanners we demonstrated two-photon microscopy and microendoscopy with fast-axis acquisition rates up to 3.52 kHz.

Self-aligned vertical electrostatic combdrives for micromirror actuation

In this paper, we analyze the effect of misalignment in electrostatic combdrives, and describe a fabrication technology that minimizes misalignment in vertical electrostatic combdrives by creating self-aligned, vertically staggered electrodes. Self-alignment of the interdigitated electrodes simplifies fabrication and minimizes failures due to electrostatic instability, thus enabling fabrication of narrow-gap, high-force actuators with high yield. The process is based on deep-reactive ion etching (DRIE) of buried-patterned silicon-on-insulator (SOI) wafers. Measurements on fabricated combdrives show relative misalignment of less than 0.05 /spl mu/m. This corresponds to less than 0.1% misalignment, which, according to our analysis, results in a travel range of 98% of that for perfectly aligned drives. The validity of the process is demonstrated by fabrication of scanning micromirrors measuring 300 /spl mu/m by 100 /spl mu/m. Optical angular deflections from 4/spl deg/ at low frequency to 40/spl deg/ at resonance were measured for an applied voltage of 75 Vpp. Resonant frequencies ranged from 5 kHz to 15 kHz for these devices, making them suitable for high-speed, high-resolution optical scanning and switching.

Fiber-optic confocal microscope using a MEMS scanner and miniature objective lens

We designed and constructed a single-fiber-optic confocal microscope (SFCM) with a microelectromechanical system (MEMS) scanner and a miniature objective lens. Axial and lateral resolution values for the system were experimentally measured to be 9.55 mum and 0.83 mum respectively, in good agreement with theoretical predictions. Reflectance images were acquired at a rate of 8 frames per second, over a 140 mum x 70 mum field-of-view. In anticipation of future applications in oral cancer detection, we imaged ex vivo and in vivo human oral tissue with the SFCM, demonstrating the ability of the system to resolve cellular detail.

Single fiber confocal microscope with a two-axis gimbaled MEMS scanner for cellular imaging

We present a single fiber reflectance confocal microscope with a two dimensional MEMS gimbaled scanner. Achieved lateral and axial resolutions are 0.82 mum and 13 mum, respectively. The field of view is 140 x 100 mum at 8 frames/second. Images and videos of cell phantoms and tissue are presented with sub-cellular resolution.

Microoptical phased arrays for spatial and spectral switching

This article describes two optical devices based on linear arrays of micromirrors. The first is a phased array of micromirrors that can be rotated as well as translated vertically to maintain coherence across the array. We demonstrate experimentally that such micromirrors are capable of high-diffraction-efficiency phased-array scanning of laser beams. The second device is a Gires-Tournois (1969) interferometer with a micromirror array that provides tunable phase modulation for the multitude of partially reflected beams within the interferometer. We demonstrate experimentally that the MEMS-GT interferometer can operate as a tunable deinterleaver for dense wavelength-division multiplexed fiber optic communication.

Chip-scale High-speed Fourier-transform Spectrometer Based on a Combination of a Michelson and a Fabry-Perot Interferometer

We report on a cascade combination of a micromachined Michelson interferometer and Fabry-Perot interferometer for compact and high-resolution Fourier-transform spectrometry. The micromirror in the Michelson interferometer is driven near its resonance frequency for large optical path length modulation and high-speed interferogram measurements. A simple bulk micromachining process is used to fabricate all optical components on a silicon optical bench platform.

Superresolution image enhancement in digital photomicrography by subpixel translation using a scanning micromirror

We report on a superresolution image enhancement technique that can improve the spatial resolution of digital photomicrography employing typical pixelated image sensors. A bulk-micromachined micromirror is used to control the image location with subpixel accuracy.

Tunable bandwidth optical filter based on MEMS Gires-Tournois interferometer

This paper demonstrates an optical filter, in which the center wavelength and 3-dB bandwidth can be tuned using MEMS technology. The proposed device has a periodic characteristic in the spectral domain with a free spectral range (FSR) matching the channel spacing of DWDM system. This spectral periodicity makes this type of device suitable for colorless narrow bandpass filters. When combined with wavelength multiplexers/demultiplexers with the channel spacing equal to the FSR of the device, for example, this optical filter can be used in any ports to tune the center wavelength and the bandwidth of individual WDM channel. Continuous tuning of the center wavelength and the FSR for these devices have been demonstrated in earlier studies. In this paper, the focus is mainly on variable optical bandwidth. Compared to filters implemented in planar lightwave circuit, the reported device offer improved insertion loss, FSR, and speed of tuning.

Micromirror-based Scan Range Enhancement in Fourier-Domain Optical Coherence Tomography

This paper reports on a micromirror-based superresolution technique that can increase the depth scan range of Fourier-domain optical coherence tomography by improving the spectral resolution of grating-based spectrometers with pixelated image sensors

Silicon masking layers for fabrication of high aspect ratio MEMS

This paper presents a fabrication technique for high-aspect ratio MEMS using a planar silicon layer bonded to a structured substrate. The bonded silicon layer provides a flat surface that can be patterned by conventional photolithography. These patterns are then transferred to the substrate through a series of etch steps. The bonded silicon layer is etched together with the substrate, so the use of additional silicon masking layers, followed by patterning and etching, can be repeated as many times as necessary. Using this approach, we have successfully demonstrated vertical mirror fabrication by a combination of KOH and DRIE etching in (110) silicon