Hyeon-Cheol Park

Forward imaging OCT endoscopic catheter based on MEMS lens scanning

This Letter reports a fully packaged microelectromechanical system (MEMS) endoscopic catheter for forward imaging optical coherence tomography (OCT). Two-dimensional optical scanning of Lissajous patterns was realized by the orthogonal movement of two commercial aspherical glass lenses laterally mounted on two resonating electrostatic MEMS microstages at low operating voltages. The MEMS lens scanner was integrated on a printed circuit board and packaged with an aluminum housing, a gradient index fiber collimator, and an objective lens. A home-built spectral-domain OCT system with 60 kHz A-line acquisition rate was combined with the endoscopic MEMS catheter. Three-dimensional images of 256×256×995 voxels were directly reconstructed by mapping the A-line datasets along the Lissajous patterns. The endoscopic catheter can provide a new direction for forward endoscopic OCT imaging.

Micromachined lens microstages for two-dimensional forward optical scanning

This work presents a novel approach for a miniaturized optical scanning module based on lateral and piston motion of two commercial lenses by MEMS actuation. Two aspheric glass lenses of 1 mm diameter are assembled on two electrostatically actuated microstages moving along perpendicular axes to tilt optical path. The compact integration secures the effective beam aperture of 0.6 mm within the device width of 2 mm. The lens mass provides high-Q motions at low operating voltages of DC 5 V and AC 10 V, i.e., the lateral angle of 4.6 degrees at 277 Hz and the vertical angle of 5.3 degrees at 204 Hz. The device can provide a new direction for miniaturizing laser scanning based endoscopes or handheld projectors.

Lissajous fiber scanning for forward viewing optical endomicroscopy using asymmetric stiffness modulation

We report a fully packaged and compact forward viewing endomicroscope by using a resonant fiber scanner with two dimensional Lissajous trajectories. The fiber scanner comprises a single mode fiber with additional microstructures mounted inside a piezoelectric tube with quartered electrodes. The mechanical cross-coupling between the transverse axes of a resonant fiber with a circular cross-section was completely eliminated by asymmetrically modulating the stiffness of the fiber cantilever with silicon microstructures and an off-set fiber fragment. The Lissajous fiber scanner was fully packaged as endomicroscopic catheter passing through the accessory channel of a clinical endoscope and combined with spectral domain optical coherence tomography (SD-OCT). Ex-vivo 3D OCT images were successfully reconstructed along Lissajous trajectory. The preview imaging capability of the Lissajous scanning enables rapid 3D imaging with high temporal resolution. This endoscopic catheter provides many opportunities for on-demand and non-invasive optical biopsy inside a gastrointestinal endoscope.

Electrothermal MEMS fiber scanner for optical endomicroscopy.

We report a novel MEMS fiber scanner with an electrothermal silicon microactuator and a directly mounted optical fiber. The microactuator comprises double hot arm and cold arm structures with a linking bridge and an optical fiber is aligned along a silicon fiber groove. The unique feature induces separation of resonant scanning frequencies of a single optical fiber in lateral and vertical directions, which realizes Lissajous scanning during the resonant motion. The footprint dimension of microactuator is 1.28 x 7 x 0.44 mm3. The resonant scanning frequencies of a 20 mm long optical fiber are 239.4 Hz and 218.4 Hz in lateral and vertical directions, respectively. The full scanned area indicates 451 μm x 558 μm under a 16 Vpp pulse train. This novel laser scanner can provide many opportunities for laser scanning endomicroscopic applications.

Electrothermal MEMS parallel plate rotation for single-imager stereoscopic endoscopes

This work reports electrothermal MEMS parallel plate-rotation (PPR) for a single-imager based stereoscopic endoscope. A thin optical plate was directly connected to an electrothermal MEMS microactuator with bimorph structures of thin silicon and aluminum layers. The fabricated MEMS PPR device precisely rotates an transparent optical plate up to 37° prior to an endoscopic camera and creates the binocular disparities, comparable to those from binocular cameras with a baseline distance over 100 μm. The anaglyph 3D images and disparity maps were successfully achieved by extracting the local binocular disparities from two optical images captured at the relative positions. The physical volume of MEMS PPR is well fit in 3.4 mm x 3.3 mm x 1 mm. This method provides a new direction for compact stereoscopic 3D endoscopic imaging systems.

Forward-viewing endoscopic OCT catheter using asymmetrically resonant fiber scanner

This work presents a forward viewing endoscopic OCT catheter based on a resonant fiber scanning. Two-dimensional optical scanning in a Lissajous pattern was realized by a piezoelectric tube with quartered electrodes and asymmetrically resonant fiber cantilever. Asymmetrically resonant fiber cantilever was assembled by additional fiber fragment and silicon supporting structure to avoid mechanical coupling effect at resonance. The endoscopic catheter of 3.2 mm diameter was assembled and combined with SD-OCT system. Three dimensional images were directly reconstructed by mapping the A-line data sets along non-repeating Lissajous trajectories with high temporal resolution.

Millimeter scale electrostatic mirror with sub-wavelength holes for terahertz wave scanninga)

This work reports the design, microfabrication, and characterization of highly reflective electrostatic mirrors with sub-wavelength holes for scanning terahertz (THz) waves. The mirror consists of an aluminum coated silicon mirror plate precisely assembled on the top of two axis torsional microactuators. The mirror plate with sub-wavelength microholes not only provides high reflectivity over 98% at THz waves by decoupling the surface plasmon resonance but also reduces air damping by allowing air to flow through the mirror plate during the mirror scanning. The device can provide many opportunities for miniaturized THz time domain spectroscopic imaging systems.

Electrothermal MEMS fiber scanner with lissajous patterns for endomicroscopic applications

We report an electrothermal MEMS fiber scanner with Lissajous patterns for endomicroscopic applications. The MEMS fiber scanner consists of a double hot arm structure, and a directly mounted optical fiber. The mounted single mode optical fiber moves to angled direction due to both lateral and vertical force of MEMS fiber scanner due to Joule heating. Besides, top silicon structure makes resonance separation of the single mode fiber in lateral, and vertical directions. Resonance frequency separation allows two-dimensional Lissajous scanning without any additional structures, and Lissajous patterns were successfully obtained within 16 Vpp. This scanner can give a new direction for compact and cost-effective endomicroscopic catheter.

Millimeter-scale scanning MEMS mirror with sub-wavelength micro-hole arrays for terahertz wave scanning

Millimeter scale scanning MEMS mirror with sub-wavelength microhole arrays has been successfully demonstrated for terahertz wave scanning. Sub-wavelength microhole arrays provide both high reflectivity at THz waves and low air damping during the mirror scanning.

Micromachined two dimensional lens scanner with large aperture beam

This work present a novel approach for miniaturized optical scanning using the two-axis MEMS lens scanning system with large beam diameter of 0.6mm. A millimeter aspheric glass lenses are integrated on electrostatic MEMS actuators to fold optical path. By integrating optical components perpendicularly on MEMS actuators, more compact device size along the optical axis is accomplished within 2mm. The scanning angle of 4.6° and 5.3° at the scanning speed of 276.5Hz and 294.4Hz for x-scanning and y-scanning respectively are achieved, when actuated by only DC 5V and AC peak to peak 10V biased resonance excitation.