Sang-hun Lee

Micro-optical system based 3D imaging for full HD depth image capturing

20 Mega-Hertz-switching high speed image shutter device for 3D image capturing and its application to system prototype are presented. For 3D image capturing, the system utilizes Time-of-Flight (TOF) principle by means of 20MHz high-speed micro-optical image modulator, so called optical shutter. The high speed image modulation is obtained using the electro-optic operation of the multi-layer stacked structure having diffractive mirrors and optical resonance cavity which maximizes the magnitude of optical modulation. The optical shutter device is specially designed and fabricated realizing low resistance-capacitance cell structures having small RC-time constant. The optical shutter is positioned in front of a standard high resolution CMOS image sensor and modulates the IR image reflected from the object to capture a depth image. Suggested novel optical shutter device enables capturing of a full HD depth image with depth accuracy of mm-scale, which is the largest depth image resolution among the-state-of-the-arts, which have been limited up to VGA. The 3D camera prototype realizes color/depth concurrent sensing optical architecture to capture 14Mp color and full HD depth images, simultaneously. The resulting high definition color/depth image and its capturing device have crucial impact on 3D business eco-system in IT industry especially as 3D image sensing means in the fields of 3D camera, gesture recognition, user interface, and 3D display. This paper presents MEMS-based optical shutter design, fabrication, characterization, 3D camera system prototype and image test results.

Three-dimensional imaging using fast micromachined electro-absorptive shutter

Abstract. A 20-MHz switching high-speed light-modulating device for three-dimensional (3-D) image capturing and its system prototype are presented. For 3-D image capturing, the system utilizes a time-of-flight (TOF) principle by means of a 20-MHz high-speed micromachined electro-absorptive modulator, the so-called optical shutter. The high-speed modulation is obtained by utilizing the electro-absorption mechanism of the multilayer structure, which has an optical resonance cavity and light-absorption epilayers grown by metal organic chemical vapor deposition process. The optical shutter device is specially designed to have small resistor–capacitor–time constant to get the high-speed modulation. The optical shutter is positioned in front of a standard high-resolution complementary metal oxide semiconductor image sensor. The optical shutter modulates the incoming infrared image to acquire the depth image. The suggested novel optical shutter device enables capturing of a full high resolution-depth image, which has been limited to video graphics array (VGA) by previous depth-capturing technologies. The suggested 3-D image sensing device can have a crucial impact on 3-D–related business such as 3-D cameras, gesture recognition, user interfaces, and 3-D displays. This paper presents micro-opto-electro-mechanical systems-based optical shutter design, fabrication, characterization, 3-D camera system prototype, and image evaluation.

Experimental observation of sub-terahertz backward-wave amplification in a multi-level microfabricated slow-wave circuit

In our earlier paper dealing with dispersion retrieval from ultra-deep, reactive-ion-etched, slow-wave circuits on silicon substrates, it was proposed that splitting high-aspect-ratio circuits into multilevels enabled precise characterization in sub-terahertz frequency regime. This achievement prompted us to investigate beam-wave interaction through a vacuum-sealed integration with a 15-kV, 85-mA, thermionic, electron gun. Our experimental study demonstrates sub-terahertz, backward-wave amplification driven by an external oscillator. The measured output shows a frequency downshift, as well as power amplification, from beam loading even with low beam perveance. This offers a promising opportunity for the development of terahertz radiation sources, based on silicon technologies.

A robust design and fabrication of micromachined electro-absorptive optical modulator for 3D imaging

A time-of-flight (TOF) based three dimensional (3D) image capturing system and its enhanced optical modulating device are presented. The 3D image capturing system includes 850nm IR emitter (typically compact Laser diodes) and high speed image modulator, so called optical shutter. The optical shutter consists of multi-layered optical resonance cavity and electro-absorptive layers. The optical shutter is a solid-state controllable filter which modulates the IR image to extract the phase delay due to TOF of the emitting IR light. This presentation especially addresses robustness issues and solutions when operated under practical environments such as ambient temperature variation and existence of strong ambient light (e.g. outdoors). The wavelength of laser diode varies substantially depending on the ambient temperature, which degrades the modulation efficiency. To get a robust operation, the bandwidth of transmittance of the optical shutter is drastically improved with a novel coupled Fabry-Perot resonance cavity design to come up with the wavelength variation of the laser diode. Also, to suppress the interference of solar irradiance to IR source signal, a novel driving scheme is applied, in which IR light and optical shutter modulation duties are timely localized, i. e. ‘bursted’. Suggested novel optical shutter design and burst driving scheme enable capturing of a full HD resolution of depth image under the realistic usage environments, which so far tackle the commercialization of TOF cameras. Design, fabrication, and evaluation of the optical shutter; and, 3D capturing system prototype, image test results are presented.

Dispersion retrieval from multi-level ultra-deep reactive-ion-etched microstructures for terahertz slow-wave circuits

A multi-level microstructure is proposed for terahertz slow-wave circuits, with dispersion relation retrieved by scattering parameter measurements. The measured return loss shows strong resonances above the cutoff with negligible phase shifts compared with finite element analysis. Splitting the circuit into multi levels enables a low aspect ratio configuration that alleviates the loading effect of deep-reactive-ion etching on silicon wafers. This makes it easier to achieve flat-etched bottom and smooth sidewall profiles. The dispersion retrieved from the measurement, therefore, corresponds well to the theoretical estimation. The result provides a straightforward way to the precise determination of dispersions in terahertz vacuum electronics.

MOEMS-based time-of-flight camera for 3D video capturing

We suggest a Time-of-Flight (TOF) video camera capturing real-time depth images (a.k.a depth map), which are generated from the fast-modulated IR images utilizing a novel MOEMS modulator having switching speed of 20 MHz. In general, 3 or 4 independent IR (e.g. 850nm) images are required to generate a single frame of depth image. Captured video image of a moving object frequently shows motion drag between sequentially captured IR images, which results in so called ‘motion blur’ problem even when the frame rate of depth image is fast (e.g. 30 to 60 Hz). We propose a novel ‘single shot’ TOF 3D camera architecture generating a single depth image out of synchronized captured IR images. The imaging system constitutes of 2x2 imaging lens array, MOEMS optical shutters (modulator) placed on each lens aperture and a standard CMOS image sensor. The IR light reflected from object is modulated by optical shutters on the apertures of 2x2 lens array and then transmitted images are captured on the image sensor resulting in 2x2 sub-IR images. As a result, the depth image is generated with those simultaneously captured 4 independent sub-IR images, hence the motion blur problem is canceled. The resulting performance is very useful in the applications of 3D camera to a human-machine interaction device such as user interface of TV, monitor, or hand held devices and motion capturing of human body. In addition, we show that the presented 3D camera can be modified to capture color together with depth image simultaneously on ‘single shot’ frame rate.