Abstract: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.

We have theoretically investigated two-dimensional photonic crystal (2D PC) L1–L21 cavities with low-refractive-index (low-n) material cladding using the 3D finite-difference time domain method assisted by group theory in the time domain. We investigated various optical properties of 2D PC L-type cavities including resonant frequency, modal symmetry, Q factor, modal volume, real-space distribution, wavevector-space distribution and resonant wavevector condition. The resonant modes in 2D PC L-type cavities are Bloch-like resonant modes. As the size of the cavity increases, the fundamental resonant mode changes from the Fabry–Perot (FP)-like resonant mode to the distributed-feedback (DFB)-like resonant mode with the envelope of the half-cycle sine window. The DFB-like resonant mode can suppress the radiation loss in the vertical direction. The DFB-like resonant mode can achieve a high Q factor (Q > 105) in a small cavity (L < 10 µm). The DFB-like resonant mode is the intrinsic resonant mode in 2D PC L-type cavities, and it is naturally formed without fine tuning. These results show the high potential of 2D PC L-type cavities with low-n material cladding.

This paper presents a method based on the use of an image sensor for obtaining the complex amplitudes of beams diffracted from an object at two different wavelengths. The complex amplitude for each wavelength is extracted by the Doppler phase-shifting method. The principle underlying the proposed method is experimentally verified by using the method with two lasers having different wavelengths to measure the surface shape of a concave mirror.

We extend topology optimization method with function-expansion-based refractive index distribution to optimization for three-dimensional optical circuits, in which a refractive index distribution in a design region is expressed by an expansion with some analytical functions. Three-branch optical waveguides have been optimized as numerical examples. Equally branching three-branch waveguides are achieved using our method. A limitation of topology optimization in two dimensions and dependency of initial structure are also shown.

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<103). We finely adjusted the positions of the holes at both edges of the 2D PC nanocavities. The Q factor can be increased by a factor of up to 21 (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.