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A guided-mode resonance mirror was designed for reflecting a vertically incident Gaussian beam of 3.6-µm beam waist to a backpropagating Gaussian beam. A grating coupler (GC) is integrated in a waveguide resonance cavity consisting of a pair of distributed Bragg reflectors (DBRs) on a reflection substrate. An incident free-space wave is coupled by the GC into the waveguide, and the guided wave is resonated in the waveguide cavity and coupled out by the same GC to a free-space wave simultaneously in resonance condition. The reflection phase can vary by 2π rad, according to wavelength in a wavelength band of resonance. The grating fill factors of the GC were apodized to have a Gaussian profile in its coupling strength and resultantly maximize a Gaussian reflectance defined by the power ratio of backpropagating Gaussian beam to the incident Gaussian beam. The fill factors of the DBR were also apodized in the boundary zone to the GC in order to avoid discontinuity in equivalent refractive index distribution and resultant scattering loss. Guided-mode resonance mirrors were fabricated and characterized. The Gaussian reflectance of the mirror with the grating apodization was measured to be 90%, higher by 10% than that of the mirror without apodization. It is also demonstrated that the reflection phase changes more than π rad within wavelength band of 1 nm. The fill factor apodization narrows the resonance band.
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A narrowband retroreflector consisting of a grating coupler and a waveguide cavity integrated on a highly reflective substrate is proposed. A theoretical model based on coupled-mode theories is discussed to provide analytical expression of the reflection and transmission coefficients under oblique incidence. The retroreflector was designed with a 20-µm aperture for 1540-nm-wavelength operation and 8-deg-angle incidence. Finite-difference time-domain simulation showed a retroreflection spectrum with a bandwidth of 2 nm and a maximum retroreflectance of 85% and a minimum specular reflectance of 5%.
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The far-field pattern of an optical wave radiating from a grating coupler integrated in a directional coupler is discussed based on the coupled mode theory. It is shown that there are two types of intensity distributions with respect to the radiation angle. One type shows a simple single lobe, while the other has a deformed lobe or two lobes. Far-field patterns are discussed along with near-field patterns. They are determined by directional coupling strength as well as radiation decay strength. A parameter is introduced to indicate the type. The indicator is useful as a key parameter for device design.
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Cavity-resonator-integrated guided-mode resonance filters (CRIGF) consisting of a grating coupler in a waveguide resonator formed by two distributed Bragg reflectors of different reflectances can act as a wavelength-selective reflector and an input waveguide coupler for an incident free-space wave. Integration of CRIGFs in a waveguide is proposed to give an array of external mirrors and a wavelength division multiplexer for constructing a compact multi-wavelength light source. Four CRIGFs of 10-µm-size aperture with a wavelength spacing of 15 nm were designed and fabricated. The reflectance of 62% and output efficiency of higher than 18% were theoretically predicted. Multiplexing of four wavelengths was confirmed experimentally.
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Resonant nanogratings and periodic metasurfaces express diverse spectral and polarization properties on broadside illumination by incident light. Cooperative resonance interactions may yield shaped spectra for particular applications, in contrast to a multilayer dielectric mirror. Here, we provide guided-mode resonance filters with flat-top spectra suitable for wavelength division multiplexing systems. Applying a single one-dimensional grating layer sandwiched by two waveguides, we theoretically achieve high-efficiency flat-top spectra in the near-infrared region. This result is obtained by inducing simultaneous nearly degenerate resonant modes. The resonance separation under this condition controls the width of the flat-top spectrum. This means we can implement spectral widths ranging from a sub-nanometer to several nanometers applying fundamentally the same device architecture.
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A cavity-resonator-integrated guided-mode resonance filter (CRIGF) consists of a grating coupler inside a pair of distributed Bragg reflectors. A combination of a CRIGF with a high-reflection substrate can provide a new type of a band-stop reflector with a small aperture for a vertically incident wave from air. A narrow stopband was theoretically predicted and experimentally demonstrated. It was quantitatively shown that reflection spectra depended on optical-buffer-layer thickness. The reflector of 10-µm aperture was fabricated and characterized. The extinction ratio in reflectance was measured to be lower than -20 dB at a resonance wavelength. The bandwidth at -3 dB was 0.15 nm.
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A cavity-resonator-integrated guided-mode resonance filter is a kind of narrowband filters, which uses a resonance effect of a waveguide cavity. Two experimental methods for determining the cavity length were investigated in order to estimate the response time of the filter. SiO(2)-based filters for operation at 1540-nm wavelength were fabricated and their cavity lengths were determined from measured resonance wavelengths. In the both of methods, the cavity length determined to be 65 µm and the response time was estimated to be 4 psec.
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A guided-mode resonance mirror (GMRM) consists of a waveguide grating integrated on an optical buffer layer on a high-reflection substrate. An incident free-space wave at the resonance wavelength is once coupled by the grating to a guided mode and coupled again by the same grating back to free space. The reflection characteristics of a GMRM are numerically calculated and theoretically analyzed. It is predicted that notch filtering or flat reflection spectra are obtained depending on the optical buffer layer thickness. Design of short cavities using a GMRM is discussed for potential application in surface-mount packaging of diode lasers onto a photonic circuit board.
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A new type of mirror, based on guided-mode resonance, was proposed and discussed to provide a mirror having high reflectance and large wavelength dependence of reflection phase variation. The proposed mirror consists of a surface grating integrated in a channel waveguide on a high-reflection layer. A SiO2-based device was fabricated for 0.85-µm wavelength operation, and reflection phase variation of almost π, with wavelength change of sub-nanometers, was confirmed experimentally.
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We propose a single-shot phase-unwrapping method using two wavelengths in parallel phase-shifting digital holography (PPSDH). The proposed method enables one to solve the phase ambiguity problem in PPSDH. We conducted an experiment of the proposed method using two lasers whose wavelengths are 473 and 532 nm. An object having about 1.9 µm step, which is 7.1 times larger than the half wavelength of one of the lasers (266 nm), was fabricated by using vapor deposition of aluminum. Single-shot measurement of the height of the object was successfully demonstrated, and the validity of the proposed method was verified.
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A superresolution method for interference fringes obtained by parallel four-step phase-shifting digital holography is proposed. A complex amplitude distribution of an object wave is derived from a recorded hologram by parallel phase-shifting interferometry using two pixels without any interpolation procedures. Multiple distributions are derived by changing one of the two pixels when conducting phase-shifting interferometry. The angular spectrum distribution of the object wave is obtained by both the Fourier transforms and synthesis of the spectrum distribution from the Fourier-transformed images in the spatial frequency domain. Available space bandwidth is extended to half of that of an image sensor.
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We propose a parallel phase-shifting digital holography using a spectral estimation technique, which enables the instantaneous acquisition of spectral information and three-dimensional (3D) information of a moving object. In this technique, an interference fringe image that contains six holograms with two phase shifts for three laser lines, such as red, green, and blue, is recorded by a space-division multiplexing method with single-shot exposure. The 3D monochrome images of these three laser lines are numerically reconstructed by a computer and used to estimate the spectral reflectance distribution of object using a spectral estimation technique. Preliminary experiments demonstrate the validity of the proposed technique.
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We propose a method for extending the space bandwidth (SBW) available for recording an object wave in parallel phase-shifting digital holography using a four-channel polarization-imaging camera. A linear spatial carrier of the reference wave is introduced to an optical setup of parallel four-step phase-shifting interferometry using a commercially available polarization-imaging camera that has four polarization-detection channels. Then a hologram required for parallel two-step phase shifting, which is a technique capable of recording the widest SBW in parallel phase shifting, can be obtained. The effectiveness of the proposed method was numerically and experimentally verified.
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We propose a single-shot digital holography for recording multiwavelength and complex amplitude information by using a single monochromatic image sensor. The zeroth-order wave and conjugate image in each wavelength are removed from a recorded single hologram by applying parallel phase-shifting interferometry. Angular multiplexing is utilized to record the complex amplitude of an object wave in each wavelength separately, and no color filter is required. The effectiveness of the proposed technique was experimentally verified.
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We propose an image reconstruction algorithm for recovering high-frequency information in parallel phase-shifting digital holography. The proposed algorithm applies three kinds of interpolations and generates three different kinds of object waves. A Fourier transform is applied to each object wave, and the spatial-frequency domain is divided into 3×3 segments for each Fourier-transformed object wave. After that the segment in which interpolation error is the least among the segments having the same address of the segment in the spatial-frequency domain is extracted. The extracted segments are combined to generate an information-enhanced spatial-frequency spectrum of the object wave, and after that the formed spatial-frequency spectrum is inversely Fourier transformed. Then the high-frequency information of the reconstructed image is recovered. The effectiveness of the proposed algorithm was verified by a numerical simulation and an experiment.
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A guided-mode resonance filter integrated in a waveguide cavity resonator constructed by two distributed Bragg reflectors is designed and fabricated for miniaturization of aperture size. Reflection efficiency of >90% and wavelength selectivity of 0.4 nm are predicted in the designed SiO(2)-based filter with 50-µm aperture by a numerical calculation using the finite-difference time-domain method. A maximum reflectance of 67% with 0.5-nm bandwidth is experimentally demonstrated by the fabricated device at around 850-nm wavelength.
Assuntos
Miniaturização/instrumentação , Miniaturização/métodos , Óptica e Fotônica/instrumentação , Óptica e Fotônica/métodos , Desenho de Equipamento , Germânio/química , Dióxido de Silício/químicaRESUMO
We propose an image-reconstruction algorithm of parallel phase-shifting digital holography (PPSDH) which is a technique of single-shot phase-shifting interferometry. In the conventional algorithms in PPSDH, the residual 0th-order diffraction wave and the conjugate images cannot be removed completely and a part of space-bandwidth information is discarded. The proposed algorithm can remove these residual images by modifying the calculation of phase-shifting interferometry and by using Fourier transform technique, respectively. Then, several types of complex amplitudes are derived from a recorded hologram according to the directions in which the neighboring pixels used for carrying out the spatial phase-shifting interferometry are aligned. Several distributions are Fourier-transformed and wide space-bandwidth information of the object wave is obtained by selecting the spectrum among the Fourier-transformed images in each region of the spatial frequency domain and synthesizing a Fourier-transformed image from the spectrum.
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Algoritmos , Holografia/métodos , Interpretação de Imagem Assistida por Computador/métodos , Imageamento Tridimensional/métodos , Interferometria/métodos , Processamento de Sinais Assistido por Computador , Aumento da Imagem/métodosRESUMO
Parallel phase-shifting digital holography is capable of three-dimensional measurement of a dynamically moving object with a single-shot recording. In this letter, we demonstrated a parallel phase-shifting digital holography using a single femtosecond light pulse whose central wavelength and temporal duration were 800 nm and 96 fs, respectively. As an object, we set spark discharge in atmospheric pressure air induced by applying a high voltage to between two electrodes. The instantaneous change in phase caused by the spark discharge was clearly reconstructed. The reconstructed phase image shows the change of refractive index of air was -3.7 × 10(-4).
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Holografia/instrumentação , Aumento da Imagem/instrumentação , Interpretação de Imagem Assistida por Computador/instrumentação , Imageamento Tridimensional/instrumentação , Refratometria/instrumentação , Processamento de Sinais Assistido por Computador/instrumentação , Desenho de Equipamento , Análise de Falha de EquipamentoRESUMO
A cavity-resonator-integrated guided-mode resonance filter (CRIGF) has been proposed and investigated in order to realize high-efficiency narrowband reflection with a small aperture. The CRIGF consists of a grating coupler integrated in a cavity resonator constructed by a pair of distributed Bragg reflectors on a thin-film waveguide. This time, orthogonally crossed integration of two CRIGFs was demonstrated in order to obtain polarization-independent reflection spectrum. An SiO2-based device with 10 µm aperture was designed and fabricated for around 850 nm wavelength operation, and narrowband polarization-independent reflection was confirmed experimentally.
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We propose a single-shot digital holography in which the complex amplitude distribution is obtained by spatial-carrier phase-shifting (SCPS) interferometry and the correction of the inherent phase-shift error occurred in this interferometry. The 0th order diffraction wave and the conjugate image are removed by phase-shifting interferometry and Fourier transform technique, respectively. The inherent error is corrected in the spatial frequency domain. The proposed technique does not require an iteration process to remove the unwanted images and has an advantage in the field of view in comparison to a conventional SCPS technique.