Fook Siong Chau

Tunable liquid-filled lens integrated with aspherical surface for spherical aberration compensation.

A novel liquid-filled lens design is presented. During fabrication, high precision single point diamond turning (SPDT) is introduced into standard soft lithography process to fabricate an aspherical surface constituting one end of lens. This enables the spherical aberration associated with the operation of the conventional liquid-filled lenses to be compensated for. Through flexibly optimizing this surface contour, it can be designed to work within particular working regions with improved optical quality. At the same time, the deformable elastic membrane is still adopted at the other end of the lens, thus preserving the high focal length tunability. This proof of concept and the performance of the proposed lens have been demonstrated using the lateral shearing interferometry experiment..

Dynamic tuning of an optical resonator through MEMS-driven coupled photonic crystal nanocavities

We present dynamic tuning of optical resonance using microelectromechanical systems (MEMS)-driven coupled photonic crystal (PhC) nanocavities. The device consists of an air-suspended one-dimensional PhC nanocavity coupled to input and output waveguides and a perturbing nanocavity attached to a submicrometer MEMS comb drive. Resonance tuning is achieved through varying the gap between the two coupled cavities. We demonstrate experimentally that resonance can be tuned up to 8nm with no significant deterioration in the Q factor. The proposed mechanism potentially enables a new platform of on-chip photonic devices that can achieve a large tuning range with low power and small footprint and may find useful applications in tunable optical/photonic devices.

An in-plane nano-mechanics approach to achieve reversible resonance control of photonic crystal nanocavities

Control of photonic crystal resonances in conjunction with large spectral shifting is critical in achieving reconfigurable photonic crystal devices. We propose a simple approach to achieve nano-mechanical control of photonic crystal resonances within a compact integrated on-chip approach. Three different tip designs utilizing an in-plane nano-mechanical tuning approach are shown to achieve reversible and low-loss resonance control on a one-dimensional photonic crystal nanocavity. The proposed nano-mechanical approach driven by a sub-micron micro-electromechanical system integrated on low loss suspended feeding nanowire waveguide, achieved relatively large resonance spectral shifts of up to 18 nm at a driving voltage of 25 V. Such designs may potentially be used as tunable optical filters or switches.

Flaw detection in composites using time-average shearography

Abstract Results on flaw detection of glass fibre reinforced plastic beams using time- average shearography are presented here. Detection and sizing of flaws such as debonds or delaminations are successfully carried out using this technique. For easy detection of flaws, the component has to be excited at the resonance frequencies of the flaws. As the flaw gets smaller, a higher frequency is required. For flaws of the same exterior size, a deeper one will also require a higher frequency.

Liquid tunable diffractive/refractive hybrid lens

We present a liquid tunable diffractive/refractive hybrid lens fabricated through what we believe to be a novel process that combines single-point diamond turning with soft lithography techniques. The hybrid lens achieves focal length tunability by changing its shape and, at the same time, utilizes the unique dispersion property of diffractive surfaces to enhance its spectral performance within a wide tuning range.

A real-time digital shearing speckle interferometer

Shearography is an interferometric method which measures surface displacement derivatives. Although useful for strain analysis and non-destructive testing, the conventional technique is cumbersome and not very suitable for an industrial environment. The authors present the development of an electronic/digital technique which eliminates most of the shortcomings. It enables real-time shearographic fringes to be obtained.

Influence of Boundary Conditions on the Dynamic Characteristics of Squeeze Films in MEMS Devices

Micromechanical structures that have squeeze-film damping as the dominant energy dissipation mechanism are of interest in this paper. For such structures with narrow air gap, the Reynolds equation is used for calculating squeeze-film damping, which is generally solved with trivial pressure boundary conditions on the side walls. This procedure, however, fails to give satisfactory results for structures under two important conditions: 1) for an air gap thickness comparable to the lateral dimensions of the microstructure and 2) for nontrivial pressure boundary conditions such as fully open boundaries on an extended substrate or partially blocked boundaries that provide side clearance to the fluid flow. Several formulas exist to account for simple boundary conditions. In practice, however, there are many micromechanical structures such as torsional microelectromechanical system (MEMS) structures that have nontrivial boundary conditions arising from partially blocked boundaries. Such boundaries usually have clearance parameters that can vary due to fabrication. These parameters, however, can also be used as design parameters if we understand their role on the dynamics of the structure. We take a MEMS torsion mirror as an example device that has large air gap and partially blocked boundaries due to static frames. We actuate the device and experimentally determine the quality factor Q from the response measurements. Next, we model the same structure in ANSYS and carry out computational fluid dynamics analysis to evaluate the stiffness constant K, the damping constant D, and the quality factor Q due to the squeeze film. We compare the computational results with experimental results and show that without taking care of the partially blocked boundaries properly in the computational model, we get unacceptably large errors.

Locating and Sizing Disbonds in Glassfibre-Reinforced Plastic Plates Using Shearography

In this paper, disbonds of known shapes and sizes are deliberately created at different layers of glassfibre-reinforced plastic (GRP) laminates which are subsequently «vaccum-stressed» during the test. Experimental results show that both the shape and size of disbonds can be deduced easily; the depth of disbonds can also be estimated

Micromachined in-plane vibrating diffraction grating laser scanner

In this letter, we present a novel micromachined vibrating diffraction-grating laser scanner that utilizes in-plane angular vibration of a planar diffraction grating causing the diffracted laser beam to scan. The proposed micromachined diffraction-grating laser scanner can operate with low voltage and has the potential to scan at high frequencies without the optical performance degradation due to dynamic nonrigid-body deformation, which is prevalent in conventional high-speed out-of-plane torsional micromirror scanners. A prototype grating scanner has been developed using micromachining technology to demonstrate this new microscanner principle. The tested device is capable of scanning an optical angle of 15.9/spl deg/ with virtually bow-free scan-line at a resonant frequency of 8.34 kHz for a 635-nm wavelength incident laser beam, electrostatically driven by 15-V dc bias and 15-V ac voltages.

High-speed, high-optical-efficiency laser scanning using a MEMS-based in-plane vibratory sub-wavelength diffraction grating

In this paper, we report the modeling, fabrication and characterization of a microelectromechanical systems (MEMS)-based sub-wavelength diffraction grating under in-plane motion for high-optical-efficiency high-speed laser-scanning applications. The scanner utilizes in-plane rotational vibration of a planar microstructure to change the orientation of the diffraction grating, hence causing a diffracted laser beam to scan with less dynamic wavefront deformation as compared with conventional scanning micromirrors. An optical efficiency of more than 75% is experimentally achieved with a simple gold-coated binary sub-wavelength grating. When operated in air and electrostatically driven by 45 V dc bias and 84 V peak-to-peak ac voltages, the 1 mm diameter grating is capable of scanning an optical scan angle of 13.7° with a 632.8 nm wavelength incident laser beam at a resonant frequency of 20.35 kHz. The measured optical resolution is around 310 pixels per unidirectional scan.