Chia-Wei Sun

Polarized light propagation through scattering media: time-resolved Monte Carlo simulations and experiments.

A study of polarized light transmitted through randomly scattering media of a polystyrene-microsphere solution is described. Temporal profiles of the Stokes vectors and the degree of polarization are measured experimentally and calculated theoretically based on a Monte Carlo technique. The experimental results match the theoretical results well, which demonstrates that the time-resolved Monte Carlo technique is a powerful tool that can contribute to the understanding of polarization propagation in biological tissue. Analysis based on the Stokes-Mueller formalism and the Mie theory shows that the first scattering event determines the major spatial patterns of the transmitted Stokes vectors. When an area detected at the output surface of a turbid medium is circularly symmetrical about the incident beam, the temporal profile of the transmitted light is independent of the incident polarization state. A linear relationship between the average order of the scatters and the light propagation time can be used to explain the exponential decay of the degree of polarization of transmitted light.

Dental optical coherence tomography.

This review paper describes the applications of dental optical coherence tomography (OCT) in oral tissue images, caries, periodontal disease and oral cancer. The background of OCT, including basic theory, system setup, light sources, spatial resolution and system limitations, is provided. The comparisons between OCT and other clinical oral diagnostic methods are also discussed.

A Brain-Wave-Actuated Small Robot Car Using Ensemble Empirical Mode Decomposition-Based Approach

An ensemble empirical mode decomposition (EEMD)-based approach was developed to extract steady-state visual evoked potentials (SSVEPs) for wireless handling of a small robot car. Three visual stimuli, flickering at 13, 14, and 15 Hz, were displayed on a liquid crystal display monitor to induce users SSVEPs. The induced SSVEPs were used to control three movement functions (forward, left, and right) of the small robot car. Users gazed at one chosen visual stimulus at one time, and the induced SSVEP was recognized to activate the desired movement function. In this paper, all subjects were requested to handle the small robot car to complete an S-shaped course four times. The proposed system utilized only one electroencephalography (EEG) channel placed at the Oz position. The acquired EEG signals were first segmented into 1-s epochs, and each epoch was then decomposed by EEMD into a series of oscillation components, denoted as intrinsic oscillatory functions (IOFs), representing multiscale features of the recorded signal. The SSVEP-related IOFs were then recognized using a matched filter detector (MFD), including a matched filter demodulator and an amplitude detector. The visual stimulus, which contributed maximum power to the MFD, was recognized as the gazed target. In this paper, all subjects could actuate the small robot car using the proposed EEMD-based brain computer interface system to complete an S-shaped course four times; the mean execution time, number of valid detections, and command transfer interval over the 11 subjects were 84.5 s, 51.13 commands, and 1.65 s/command, respectively.

Study of photon migration with various source-detector separations in near-infrared spectroscopic brain imaging based on three-dimensional Monte Carlo modeling.

We have simulated photon migration with various sourcedetector separations based on a three-dimensional Monte Carlo code. Whole brain MRI structure images are introduced in the simulation, and the brain model is more accurate than in previous studies. The brain model consists of the scalp, skull, CSF layer, gray matter, and white matter. We demonstrate dynamic propagating movies under different source-detector separations. The multiple backscattered intensity from every layer of the brain model is obtained by marking the deepest layer that every photon can reach. Also, the influences of an absorption target on the brain cortex are revealed.

Myocardial tissue characterization based on a polarization-sensitive optical coherence tomography system with an ultrashort pulsed laser

A polarization-sensitive optical coherence tomography (PSOCT) system using a femtosecond-laser as the broadband light source is implemented with the axial resolution of 5 microm in free space. Through the design of path-length difference between the two polarization inputs and the modulation of one of the polarization inputs, the PSOCT images of various input and output polarization combinations can be distinguished and simultaneously collected. The PSOCT system is then used for in vitro scanning of the myocardium tissues of normal and infarcted rat hearts. The destruction of the birefringence nature of the fiber muscle in the infarcted heart can be clearly observed.

Subgingival calculus imaging based on swept-source optical coherence tomography

We characterized and imaged dental calculus using swept-source optical coherence tomography (SS-OCT). The refractive indices of enamel, dentin, cementum, and calculus were measured as 1.625 ± 0.024, 1.534 ± 0.029, 1.570 ± 0.021, and 2.097 ± 0.094, respectively. Dental calculus leads strong scattering properties, and thus, the region can be identified from enamel with SS-OCT imaging. An extracted human tooth with calculus is covered with gingiva tissue as an in vitro sample for tomographic imaging.

Two-axis MEMS scanners with radial vertical combdrive actuators : design, theoretical analysis, and fabrication

We propose the employment of radial vertical combdrive actuators to implement two-axis micro-electro-mechanical systems (MEMS) scanners. The devices are designed based on a five-layer polysilicon surface micromachining process. A cross-bar spring structure consisting of lower and upper torsion springs is incorporated to achieve two rotational degrees of freedom, enabling the dual-axis rotation. Both the vertical combdrive actuators and the torsion springs are hidden underneath the mirror to achieve a small form factor. Theoretical analysis is performed for comparison of various designs. Preliminary experimental results are also obtained.

Linearization of a two-axis MEMS scanner driven by vertical comb-drive actuators

A driving scheme using a pair of differential voltages (Vx, Vy) over a bias voltage is proposed to linearize the dc characteristic (angle versus voltage) of a two-axis MEMS scanner. The micromirror has a gimbal-less structure and is driven by vertical comb-drive actuators in conjunction with a leverage mechanism. At an optimal bias voltage of 53 V, a linear optical scan range of ±3.2° is achieved experimentally in both the x and y directions with the differential voltages ranging from −10 V to + 10 V.

Resolution improvement with dispersion manipulation and a retrieval algorithm in optical coherence tomography

We propose and demonstrate what is to our knowledge a novel technique of improving the spatial resolution of an optical coherence tomography (OCT) system given a non-Gaussian light source spectrum. By using dispersive materials in the reference arm of the OCT system, the resultant dispersion variation led to a full-width at half maximum (FWHM) of the interference fringe envelope smaller than the Fourier transform-limited value of a Gaussian spectral shape with the same spectral FWHM, at the expense of significant tails. The effects of the tails, which would blur the OCT images, were tremendously reduced with a retrieval algorithm. Simulation results and processed OCT scanning images have shown the capability of the proposed technique.

Myocardial tissue characterization based on the time-resolved Stokes-Mueller formalism.

Time-resolved Stokes vectors of transmitted optical signals were measured to differentiate normal and stunned myocardium tissues. The corresponding Mueller matrices were calculated based on the Stokes-Mueller formalism. Our experimental results indicated that the time-resolved Mueller matrices could provide information about myocardial architectural alteration in stunned myocardium. Thus, the Stokes-Mueller measurement can be a useful method in cardiovascular research.