Alignment-free filter array: Snapshot multispectral polarization imaging based on a Voronoi-like random photonic crystal filter.

We develop a photonic crystal filter with a new structure and propose a method to realize a snapshot multispectral polarization camera by mounting the filter on a monochrome imager with no requirement for a specific alignment. The developed filter is based on the Voronoi structure, which forms multilayered photonic crystals with random wave-like structures in each of the Voronoi cells. Because the transmission characteristics of the multilayered photonic crystal can be controlled simply by changing the microstructure, there is no need to change the manufacturing process and materials for each Voronoi cell. Furthermore, the Voronoi cell is randomly distributed so that the filter can be junctioned with the imager at arbitrary positions and angles without the need to position the filter during mounting, although it requires measurement of the camera characteristics and an image restoration process after filter mounting. In this experiment, we evaluated to reconstruct spectra as well as linearly polarized components and RGB images in the visible wavelength range from a single exposure image.

Direct estimation of NIR reflection spectra utilizing a snapshot-type spectrometer with photonic crystal multi-spectral filter arrays.

We fabricated 16-, 25-, 36-, and 64-channel distributed passband-type multi-spectral filter arrays by utilizing a multilayer-type photonic crystal and integrated them onto a CCD to form a snapshot-type spectroscopic sensor. Reflection spectra from target objects (fruits) under broadband light illumination were estimated directly using the Wiener estimation method. A root mean square error of the reflectivity on the order of 2∼5% was obtained under optical shot noise with 6×6 pixel binning. A number of constituent filters of 36 was sufficient for this type of fruit spectral measurement. We also visualized reflection images at specified wavelengths by applying the estimation method to a multiple filter region on the sensor.

NIR spectrum estimation utilizing a photonic crystal distributed passband-type multiple filter array.

A near-infrared (NIR) spectral sensor consisting of a 25-channel dielectric multi-patterned filter array (MFA) and CCD is proposed and fabricated. The MFA consists of a wavy dielectric multilayer with a gradient layer profile on a silica substrate with surface grating. The incoming NIR spectrum is predicted by Wiener estimation utilizing the MFA's spectral responses and a set of training spectral data. Estimation performance is evaluated under various optical shot-noise conditions.

Snapshot multispectral polarization imaging using a photonic crystal filter array.

A new filter array and a demosaicking method for snapshot multispectral polarization imaging are proposed in this paper. The proposed filter array is a thin-film wavy multilayer structure regarded as a photonic crystal that can be fabricated using the autocloning method. The multispectral polarization filter array is developed by altering the wave structure of the photonic crystal at each pixel. In addition, we propose a demosaicking method for multispectral polarization images by considering snapshot imaging as a linear model. In the experiments, we evaluated the recovered spectrum error in some color charts and showed various demosaicked images such as multispectral polarization images, specific-band degree of linear polarization images, polarized RGB images, and non-polarized RGB images.

Near-infrared imaging of liquid mixtures utilizing multi-channel photonic crystal wavelength filters.

We demonstrate a near-infrared (NIR) spectroscopy-based real-time imaging system of aqueous alcohol solutions (ethanol containing water). The system obtains a set of NIR images by the combination of an InGaAs area sensor and four-channel patterned photonic crystal wavelength filters. Acquired pictures were decomposed into a set of NIR images and processed by principal component analysis (PCA). According to a preliminary experiment for water/ethanol mixture samples, we confirmed that the system was capable of identifying the mixture ratio with accuracy of the order of a few percentage points at a frame rate of approximately 24 frames per second (fps).

Resonantly guided modes in microstructured optical fibers with a circular array of high-index rods.

A microstructured optical fiber with a new type of waveguiding mechanism is proposed. The fiber consists of a circular rod array of high index material (n=3.48) embedded in a low index background (n=1.44). The rod array exhibits guided-mode resonance (GMR) for cylindrical waves arriving from inside the array, and thus functions as a highly reflective circular wall. Through finite-difference time-domain (FDTD) simulations, we confirmed light confinement and guidance near the GMR wavelength. Basic optical characteristics such as dispersion relations, loss spectra, and mode field profiles were calculated.

Multichannel bandpass filters utilizing multilayer photonic crystal.

We demonstrate multichannel optical bandpass filters utilizing alternating dielectric multilayers stacked on a patterned substrate. A microstructured Si/SiO(2) multilayer was formed by rf biased magnetron sputtering process on a fused silica substrate with surface gratings. The bandwidths of each filter channel were measured to be 50~175 nm in the near infrared region. Control of center wavelength and bandwidth by the grating pitch was verified.

Guided-mode resonance in curved grating structures.

The guided-mode resonance phenomenon in curved grating structures was studied. By using finite-difference time-domain simulation in the cylindrical coordinate system, we investigated the dependence of the peak reflectivity and bandwidth of the resonance upon the curvature radius. We clarified that the reflectivity and bandwidth were similar to those of flat grating structures for a finite range of curvature. We also discussed the key factor that determines the performance of reflection.

Design and fabrication of multichannel Si/SiO2 autocloned photonic crystal edge filters.

An Si/SiO2 multilayer with zigzag layer interfaces is fabricated on a patterned silica substrate using the autocloning method. The multilayer is designed to function as multichannel long-wave pass type edge filters with various cutoff wavelengths. A cutoff wavelength shift of the order of 190 nm in the near infrared region (1300-1500 nm) is experimentally demonstrated.

Antireflection coatings for multilayer-type photonic crystals.

The possibility of antireflection (AR) coatings on a dielectric multilayer having sub-wavelength deep structural modification is investigated. We numerically surveyed the effect of reflectivity reduction attained by double-layer AR coatings for a wavy multilayer on a patterned substrate. It was clarified that double-layer AR coatings for wavy multilayer is possible with a similar performance level as conventional flat multilayer. Also, it was demonstrated that a pair of AR layers effectively works for a wide range of the horizontal pitch.

Sharp edge wavelength filters utilizing multilayer photonic crystals.

A novel configuration of dielectric multilayer wavelength filters for enabling sharp cut-off characteristics is proposed. By applying perturbations to the multilayer structures such as corrugation or lateral film isolation, deep optical stopbands can be created as a result of the coupling between the obliquely and horizontally propagating light waves. Numerical simulation by FDTD revealed that the proposed structure had approximately two to three times larger decay constants than that of unmodulated flat multilayer.