Jun-ichiro Sugisaka

Non-iterative numerical method for laterally superresolving Fourier domain optical coherence tomography

A numerical deconvolution method to cancel lateral defocus in Fourier domain optical coherence tomography (FD-OCT) is presented. This method uses a depth-dependent lateral point spread function and some approximations to design a deconvolution filter for the cancellation of lateral defocus. Improved lateral resolutions are theoretically estimated; consequently, the effect of lateral superresolution in this method is derived. The superresolution is experimentally confirmed by a razor blade test, and an intuitive physical interpretation of this effect is presented. The razor blade test also confirms that this method enhances the signal-to-noise ratio of OCT. This method is applied to OCT images of medical samples, in vivo human anterior eye segments, and exhibits its potential to cancel the defocusing of practical OCT images. The validity and restrictions involved in each approximation employed to design the deconvolution filter are discussed. A chromatic and a two-dimensional extensions of this method are also described.

GaAs-based two-dimensional photonic crystal slab ring resonator consisting of a directional coupler and bent waveguides

We experimentally demonstrated a GaAs-based air-bridge-type photonic crystal ring resonator that has a triangular lattice pattern of air holes. The fabricated photonic crystal ring resonator is composed of bent waveguides and an asymmetric directional coupler that has three rows of air holes between neighboring line-defect waveguides. We successfully demonstrated ring resonant spectral response in the fabricated devices and experimentally made clear the dependence of the oscillation period on the optical path length of the ring resonator and the dependence of ring resonant spectral response on the coupling properties of the directional coupler. In addition, we theoretically and experimentally discuss the group-velocity dispersion in the photonic crystal slab waveguide.

Complex Numerical Processing for In-Focus Line-Field Spectral-Domain Optical Coherence Tomography

We propose a non-iterative deconvolution method that can improve the out-of-focus lateral resolution in line-field spectral-domain optical coherence tomography. By using an inverse filter designed from the point spread function of the system, the out-of-focus lateral resolution can be enhanced to a level comparable with the in-focus resolution over the entire axial measurement range. The resolution before and after this deconvolution has been verified experimentally. Furthermore, the method has been applied to the measurement of an ex vivo porcine eye chamber, which has demonstrated that the method was satisfactorily applicable to biomedical applications.

Resonant Characteristics in a Two-Dimensional Photonic Crystal Ring Resonator with a Triangular Lattice of Air Holes

We fabricated and characterized a GaAs-based two-dimensional photonic-crystal ring resonator consisting of a directional coupler and bent waveguides. We experimentally demonstrated the periodic transmittance variation based on ring resonance and clarified the dependence of the ring-resonant response on the coupling characteristics of the photonic-crystal directional coupler.

Short Photonic-Crystal Directional Coupling Optical Switch of Extended Optical Bandwidth Using Flat Dispersion

A directional coupling optical switch is constructed from a photonic crystal having a wide optical bandwidth. It has flat dispersion, which allows considerable shortening of its length. The switching operation of a directional coupler having flat dispersion and of an ordinary directional coupler are compared, to prove that the device length can be as short as 4.68 µm (50 times shorter than the conventional one), while maintaining the input energy and bandwidth. A 1.17 µm optical switch is fabricated, simultaneously shortening the device and extending the optical bandwidth.

Design of two-dimensional photonic crystal nanocavities with low-refractive-index material cladding

We have theoretically investigated two-dimensional photonic crystal (2D PC) nanocavities with low-refractive-index (low- n) material cladding using a 3D finite-difference time-domain method. When the refractive index of the cladding is increased from 1 to 1.535, corresponding to that of organic polymers, the Q factor markedly decreases (Q 104) by shifting the neighboring four pairs of holes inward by suitable amounts. In addition, we examined the real and wavevector space distributions of the resonant modes. We revealed significant design rules based on single and double Gaussian envelope functions, which can be used to effectively and flexibly develop and finely adjust the optical properties of 2D PC nanocavities with low- n material cladding.

Demonstration of Flat-Band Structure of Two-Dimensional Photonic Crystal Directional Coupler

Flat-band directional couplers are promising devices for the miniaturization of optical switches, slow-light generation, and other forefront applications. In the present study, the transmission spectra of fabricated directional couplers are measured. Their flat-band structures, generated by structural modulation of the photonic crystals, are explicitly verified.

Demonstration of a photonic crystal directional coupler switch with ultra short switching length

We evaluated the effect of a flat band in photonic crystal directional coupler optical switch for shortening the switching length. And we experimentally demonstrated that the flat band shortened the switching length by 29 times.

Photonic Band Engineering of Coupled Waveguide Using Geometrical Modulation

A two-dimensional photonic crystal directional coupler is a flexible optical component that can easily change optical properties such as operating frequency, wavenumber, and group velocity. We investigate the changes in the photonic bands with geometrical modulations such as radii and positions of air holes by the finite-difference time-domain method. Following the calculations, we classified the modulations by their effects on the photonic band: producing a flat band and shifting the normalized frequency of propagation modes of the coupled waveguide. We also discuss the physical implications of the relationship between the modulations and their effects on photonic bands. Finally, we applied this knowledge to the adjustment of the normalized frequency of a flat band, and then, we show that the normalized frequency of a flat band 0.291 can be decreased to 0.286. This demonstration indicates that the findings of this study can be used to obtain the desired optical properties of a directional coupler.

Demonstration of the wide control range Q factor of ring cavity with ultrashort directional coupler and curved photonic-crystal ring waveguide

We extend the control range of the Q factor of a ring cavity that consists of a photonic crystal. The control range, which determines the storage time (high Q) and efficiency of input and output (low Q) of light, is required to be wide for the random-access memory of an optical pulse train. The conventional photonic-crystal ring cavity with a directional coupler and a hexagonal-shape ring waveguide has a very narrow range. We replace these components by a directional coupler having flat dispersion and a circularly curved ring waveguide. We experimentally varied the Q factor by thermal modulation of the device and achieved a control range between 1.9×103and 1.7×104.