Reflection analysis of absorbing film with diffractive structures for incoherent light by rigorous coupled-wave analysis.

Reflectivity is useful for evaluating the extinction coefficient; however, it is highly sensitive to the refractive index structure. In this study, we propose a novel, to the best of our knowledge, method for evaluating the influence of the structure on reflectivity using rigorous coupled-wave analysis (RCWA), and apply it to analyze the reflectivity of the dye rhodamine B. The reflection-absorption spectrum of the film was significantly affected by its surface and internal structure. We found that simulating the reflectivity of a film with an unknown internal structure, using the coherency parameter is convenient. The RCWA facilitated simultaneous treatment of the coherent diffraction by the surface structure and incoherent reflection in the film.

High accuracy cross-sectional shape analysis by coherent soft x-ray diffraction.

When the scatterer size is less than 100 wavelengths, the effect of diffraction is large. The analysis of diffraction is important for 3D shape measurement. However, in soft x rays, shapes suitable for rigorous diffraction analysis have been limited to ellipses and periodic structures. We have developed a method to expand this to any shape (isolated triangle, rectangle, etc.). Experimentally, we measured the respective widths of the cross section of a column consisting of two layers and showed that the resolution was at least a few wavelengths. For this purpose, we have also developed a fast simulation method with a small memory size.

Cross-sectional particle measurement in the resonance domain on the substrate through scatterometry.

We developed a versatile method for three-dimensional shape measurement where a specific particle can be selected on the substrate and its cross-sectional shape and size can be measured. A non-contact fast measurement is possible for the particle in the resonance domain. We applied rigorous coupled-wave analysis to the particle and calculated the diffraction patterns, comparing the patterns with the experimental results to obtain the size and shape. The shape and position of the focusing spot on the scattering particle was controlled precisely. With this method, the category of the analyzable object is extended to more shapes, such as rectangles and triangles, in addition to a conventional ellipsoid.

Precise and rapid distance measurements by scatterometry.

We found that the distances between isolated scatterers with similar columnar shapes could be measured by taking a single Fourier transform of their diffraction intensity. If the scatterers have different shapes, the distances between similar shapes can be selected from the distances between all the shapes. The distance from a specific scatterer can be measured with a resolution of 0.8 wavelengths and a precision of 0.01 wavelengths. This technique has the potential to be used in a novel optical memory that has a memory density as high as that of holographic memory, while can be fabricated by simple transfer molding. We used rigorous coupled-wave analysis to calculate the diffraction intensity. Some of the results were verified by nonstandard finite-difference time-domain simulations and experiments.

High-penetration swept source Doppler optical coherence angiography by fully numerical phase stabilization.

A high-penetration swept-source optical coherence tomography (HP-SS-OCT) system based on a 1-µm short cavity laser is developed. Doppler OCT processing is applied, along with a custom-made numerical phase stabilization algorithm; this process does not require additional calibration hardware. Thus, our phase stabilization method is simple and can be employed in a variety of SS-OCT systems. The bidirectional blood flow and vasculature in the deep choroid was successfully imaged via two Doppler modes that use different time intervals for Doppler processing. En face projection image of squared power of Doppler shift is compared to ICGA, and the utility of our method is verified.

Polarization-sensitive optical coherence tomography of necrotizing scleritis.

A polarization-sensitive swept-source optical coherence tomography system (central wavelength: 1,310 nm; A-line rate: 20 kHz) was developed to evaluate the three-dimensional structure of the anterior eye segment with the phase retardation associated with the anterior segment birefringence of the eyes. Evaluation of normal eyes and an eye with necrotizing scleritis was performed. In the sclera of the normal eyes, a striking polarization change was observed in the cumulative phase retardation images and the boundary of the sclera could be readily detected. In the eye with necrotizing scleritis, phase retardation at the sclera was low in an extensive area; this implied diffuse destruction of the collagen tissue in the sclera had occurred. Polarization-sensitive optical coherence tomography is useful as a contrast engine of the anterior eye segment and for the evaluation of pathological change in the sclera.

Diffraction pattern of triangular grating in the resonance domain.

We propose a combination of ray optics and Fraunhofer multiple-slit diffraction theory for calculating the two-dimensional triangular periodic grating in the resonance domain. The peak of the envelope pattern of angular distribution of diffraction efficiency is calculated by ray optics while the peak width is calculated using Fraunhofer theory. It was clarified, using rigorous coupled-wave analysis and a nonstandard-finite-difference time-domain method, that the envelope pattern of the diffraction of the grating could be calculated easily and understood intuitively for the design of displays and lighting.

Retardagraphy: a technique for optical recording of the retardance pattern of an optical anisotropic object on a polarization-sensitive film using a single beam.

A technique that employs a single laser beam is proposed for recording the retardance of an optical anisotropic object. The retardance pattern is converted into a polarization pattern using a quarter-wave plate and recorded on a polarization-sensitive medium. The recording medium is illuminated by homogeneous polarized light, and the light transmitted by the recording medium is analyzed to reconstruct the recorded retardance pattern.

Simultaneous depth determination of multiple objects by focus analysis in digital holography.

Focus analysis techniques from computer vision are applied to digital holography to determine the depth (range) of multiple objects and their surfaces from a single hologram capture. With this method the depths of objects can be determined from a single hologram capture without the need for manual focusing and without prior information on object location. Variance and the Laplacian of Gaussian are analyzed as focus measures, and techniques are proposed for focus plane determination from the focus measure curves. The algorithm is described in detail and demonstrated through simulation and optical experiment.

Design of a wavelength independent grating in the resonance domain.

We propose using blazed gratings in the resonance domain with period larger than the wavelength for anti-reflection and polarization selection. The inherent problem in this region is wavelength dispersion, which is solved by analyzing the total reflectivity and electric field distribution. The positional relationship between the area of strong electric field, and the side and tip of the grating is crucial to the wavelength dispersion of total reflectivity.

Ellipsometric analysis for a polarization-controlling thin film with large photoinduced chirality.

Polarization-altering properties of a unique photoinduced chiral material, which consists of a photoresponsive azobenzene (PCDY50), are investigated in detail under an ellipsometric analysis based on a circular basis. The validity of the processes is confirmed by comparing experimental results and calculations. Furthermore, various kinds of optical functions, such as an optical rotator and ellipticity modificator, are demonstrated by applying invariant ellipticity (or azimuth) state condition to the PCDY50 thin film that is less than 1 microm thick.

Antireflective grating in the resonance domain for displays.

An antireflective periodic structure different from the moth-eye structure is proposed in a resonance domain whose period is greater than the wavelength of incident light. Using rigorous coupled-wave analysis in the TE mode, a reflectivity of less than 0.2% is performed in a period larger than the wavelength, when an aspect ratio is unity. Changes in diffraction efficiency and transmissivity are small at different wavelengths. This is explained by a newly derived equation based on the vector theory.

High-speed three-dimensional human retinal imaging by line-field spectral domain optical coherence tomography.

Line-field spectral domain optical coherence tomography (LF-SDOCT) has been developed for very high-speed three-dimensional (3D) retinal imaging. By this technique, the A-line rate significantly improved to 823,200 A-lines/s for single frame imaging and 51,500 A-lines/s for continues frame imaging. The frame rate at continues frame imaging is 201 fps. This 3D acquisition speed is more than two fold higher acquisition speed than the standard flying spot SD-OCT. In this paper, the integration time of the camera was optimized for the in vivo retinal measurement and the degradation of the lateral resolution due to the ocular aberrations was suppressed by introducing the pupil stop. Owing to an optimal integration time, the motion artifact can be significantly suppressed. Also a pupil stop was employed in order to enhance the contrast of the OCT image for the effect of ocular aberrations. The in vivo 3D retinal imaging with 256 cross-sectional images (256 A-lines/image) was successfully performed in 1.3 seconds, corresponding to 0.8 volume/s. The maximum on-axis system sensitivity was measured to be 89.4 dB at a depth of 112 mum with an axial resolution of 7.4 mum in tissue. It is shown that LF-SDOCT might have a sensitivity advantage in comparison to the flying spot SD-OCT in the ultra high-speed acquisition mode.

Direct generation of high power Laguerre-Gaussian output from a diode-pumped Nd:YVO(4) 1.3-mum bounce laser.

We demonstrate direct production of a high power Laguerre-Gaussian mode from a diode-pumped Nd:YVO(4) 1.3-mum bounce amplifier with an asymmetric cavity configuration. A maximum LG output of 7.7 W is obtained.

Fast calculation method for spherical computer-generated holograms.

The synthesis of spherical computer-generated holograms is investigated. To deal with the staggering calculation times required to synthesize the hologram, a fast calculation method for approximating the hologram distribution is proposed. In this method, the diffraction integral is approximated as a convolution integral, allowing computation using the fast-Fourier-transform algorithm. The principles of the fast calculation method, the error in the approximation, and results from simulations are presented.

Simultaneous B-M-mode scanning method for real-time full-range Fourier domain optical coherence tomography.

High-speed complex full-range Fourier domain optical coherence tomography (FD-OCT) is demonstrated. In this FD-OCT, the phase modulation of a reference beam (M scan) and transversal scanning (B scan) are simultaneously performed. The Fourier transform method is applied along the direction of the B scan to reconstruct complex spectra, and the complex spectra comprise a full-range OCT image. Because of this simultaneous B-M-mode scan, the FD-OCT requires only a single A scan for each single transversal position to obtain a full-range FD-OCT image. A simple but slow version of the FD-OCT visualizes the cross section of a plastic plate. A modified fast version of this FD-OCT investigates a sweat duct in a finger pad in vivo and visualizes it with an acquisition time of 27 ms.

Three-dimensional line-field Fourier domain optical coherence tomography for in vivo dermatological investigation.

We demonstrate 3-D optical coherence tomography using only 1-D mechanical scanning. This system uses the principle of Fourier domain optical coherence tomography for depth resolution, 1-D imaging for lateral vertical resolution, and mechanical scanning by a galvanometer for lateral horizontal resolution. An in vivo human fingerpad is investigated in three dimensions with an image size of 480 points (vertical) x 300 points (horizontal) x 1024 points (depth), which corresponds to 2.1 x 1.4 x 1.3 mm. The acquisition time for a single cross section is 1 ms and that for a single volume is 10 s. The system sensitivity is 75.6 dB at a probe beam power of 1.1 mW.

Polarization contrast imaging of biological tissues by polarization-sensitive Fourier-domain optical coherence tomography.

Jones matrix imaging of biological samples by a polarization-sensitive Fourier-domain optical coherence tomography has been demonstrated using a two-dimensional CCD camera to obtain two spectra corresponding to the orthogonal polarization components simultaneously. The measurement results of a quarter-wave plate are compared between the two incident polarization sets, H-V linear and R-L circular polarization. Jones matrix imaging of the bovine tendon is demonstrated. Measured Jones matrix images are converted to equivalent Müller matrix images. Local polarization properties are obtained by longitudinal differentiation of Jones matrix components. The layered structure of the bovine tendon and birefringence are revealed.

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.

Automatic characterization and segmentation of human skin using three-dimensional optical coherence tomography.

A set of fully automated algorithms that is specialized for analyzing a three-dimensional optical coherence tomography (OCT) volume of human skin is reported. The algorithm set first determines the skin surface of the OCT volume, and a depth-oriented algorithm provides the mean epidermal thickness, distribution map of the epidermis, and a segmented volume of the epidermis. Subsequently, an en face shadowgram is produced by an algorithm to visualize the infundibula in the skin with high contrast. The population and occupation ratio of the infundibula are provided by a histogram-based thresholding algorithm and a distance mapping algorithm. En face OCT slices at constant depths from the sample surface are extracted, and the histogram-based thresholding algorithm is again applied to these slices, yielding a three-dimensional segmented volume of the infundibula. The dermal attenuation coefficient is also calculated from the OCT volume in order to evaluate the skin texture. The algorithm set examines swept-source OCT volumes of the skins of several volunteers, and the results show the high stability, portability and reproducibility of the algorithm.