High Transverse Resolution Research Articles

Rigorous electromagnetic analysis of metallic cylindrical focusing micromirrors with high diffraction efficiency, achromatic aberration and long focal depth

In this paper, we propose a kind of metallic cylindrical focusing micromirrors (MCFMs) with parabolic profiles, and the focal performance is investigated based on rigorous electromagnetic theory and the boundary element method (BEM). Numerical results by the BEM show that the proposed MCFM possesses a high diffraction efficiency and axially achromatic aberration. In addition, through designing a modulating phase function, the MCFM holds long focal depth and high transverse resolution simultaneously. It is believed that the MCFM should have wide application prospects in micro-optical systems, such as micro solar concentrators.

Edge enhancement for in-phase focal modulation microscope

In-phase focal modulation microscopy (IPFMM) with single photon excited fluorescence is presented. Optical transfer functions and images of thin and thick fluorescent edges in IPFMM are investigated. The results show that, compared with confocal microscopy, using IPFMM can result in a sharper image of the edge, and the edge gradient can be increased up to 75.4% and 58.9% for a thick edge and a thin edge, respectively. Signal level is also discussed, and the results show that, to obtain high transverse resolution with IPFMM, the normalized detector pinhole radius should not exceed 2.8.

Generation of an extended depth of focus using diffractive micro-lenses with binary structures in the non-paraxial domain

Diffractive micro-lenses (DMLs) with binary structures are analyzed and designed to generate an extended depth of focus (DOF) based on rigorous electromagnetic theory in the non-paraxial domain. The DOF can be extended by setting a positive preset DOF and by setting a lower height for the binary profile of the DML, respectively. Compared with the paraxial approximation (scalar theory), the focal characteristics of the designed DMLs, such as the actual DOF, the ratio between the DOF of the designed DML and the DOF of the conventional DML, and the spot size of the central lobe at the actual focal plane, for different f-numbers and for different heights of the binary profile, have been studied by using the rigorous vector method (FDTD method). Numerical and graphic results show that the designed DMLs indeed have an extended DOF while maintaining high transverse resolution by modulating the distribution and height of the binary structures of a DML's profile. The phenomenon that the variation of the height of the patterns can improve the DOF is also given an initial interpretation.

Binary sub-wavelength diffractive lenses with long focal depth and high transverse resolution

This study explores two-dimensional binary sub-wavelength diffractive lenses (BSDLs) for implementing long focal depth and high transverse resolution based on the rigorous electromagnetic theory and the finite-difference time-domain method. Focusing performances, such as the actual focal depth, the ratio between the focal depth of the designed BSDL and the focal depth of the conventional sub-wavelength lens and the spot size of the central lobe at the actual focal plane, for different f-numbers, have been studied in the case of TE incidence polarization wave. The rigorous numerical results indicate that the designed BSDLs indeed have long focal depth and high transverse resolution by modulating the binary sub-wavelength characteristic sizes. Because BSDLs have the ability for monolithic integration and can require only single step fabrication, the investigations may provide useful information for BSDLs' application in micro-optical systems.

Rigorous electromagnetic analysis of the common focusing characteristics of a cylindrical microlens with long focal depth and under multiwavelength illumination

The common focusing characteristics of a cylindrical microlens with a long focal depth and under a given multiple-wavelength illumination are analyzed based on the boundary element method (BEM). The surface-relief profile of a finite-substrate-thickness microlens with a long focal depth is presented. Its focusing performances, such as the common extended focal depth (CEFD), the spot size, and the diffraction efficiency, are numerically studied in the case of TE polarization. The results show that the CEFD of the microlens increases initially, reaches a peak value, and then decreases with increasing preset focal depth. Two modified profiles of a finite-substrate-thickness cylindrical microlens are proposed for enlarging the CEFD. The rigorous numerical results indicate that the modified surface-relief structures of a cylindrical microlens can successfully modulate the optical field distribution to achieve longer CEFD, higher transverse resolution, and higher diffraction efficiency simultaneously, compared with the prototypical microlens. These investigations may provide useful information for the design and application of micro-optical elements in various multiwavelength optical systems.

Diffractive infrared lens with extended depth of focus

An infrared imaging lens that produces beams with long focal depth while preserving the entire aperture for full light capturing and high transverse resolution is presented. Based on the conjugate-gradient algorithm, a diffractive optical lens with four times depth of focus improvement at the 10.6-µm wavelength has been designed and fabricated. ZEMAX simulation has confirmed the designed lens performance showing legible images over an extended image range. Through laser direct-write photoresist patterning and subsequent reactive ion etching on a germanium substrate, we have realized a diffractive lens with four times depth of focus improvement over a conventional infrared lens of the same numerical aperture. The extended depth of focus lens should yield a variety of applications in focus-free imaging without reducing lens aperture.

Extended focus depth for Fourier domain optical coherence microscopy

We report on a new detection scheme for Fourier domain optical coherence microscopy that exhibits high transverse resolution along an axially extended focal range. Nearly constant transverse resolution of approximately 1.5 microm along a focal range of 200 microm is experimentally verified with a maximum sensitivity of 105 dB. A broad-bandwidth Ti:sapphire laser allowed for an axial resolution of 3 microm in air.

Statistical Mapping of Speckle Autocorrelation for Visualization of Hyperaemic Responses to Cortical Stimulation

Statistically mapped speckle autocorrelation images (SAR) were used to track the hemodynamically active perfusion regions in the rat cortex during and following DC current stimulation with high transverse spatial resolution (38 um). The SAR images provided a spatio-temporal information about the net activation patterns of Cerebral Blood Flow (CBF) changes over a period of time as against those changes for each frame interval estimated using spatial contrasts derived from the first order spatial statistics. Thus the information about the relative maxima of perfusion during a Transient Hyperaemic Episode (THE) across different regions in the imaging window could be identified without the need for actually having to estimate the spatial contrast maps of the imaged region for each frame contained in the time window of observation. With the application of DC stimulation, the regions with a high correlation in the temporal fluctuations were representative of the areas that underwent least changes in activation. By varying the intensity of stimulation, THEs were observed for stimulation current densities in the range 0.1-3.8 mA/mm2 using both the derived speckle contrast maps and concurrently on a Laser Doppler Flow meter, with its probe positioned 1 mm from the site of stimulation. For current densities below the lower threshold of stimulation, the SAR images revealed an unprecedented reduction in the surge amplitude at sites distal to the region of stimulation. This was accompanied by an increase in pixel areas representing minimally active regions of perfusion ("perfusion islets") with no identifiable peak in the hemodynamic responses estimated from speckle contrast variations. The SAR images can be a useful tool for visualization of slow wave perfusion dynamics during cortical stimulation.

Dynamic focus in optical coherence tomography for retinal imaging

Imaging the human retina still represents the main field of application of optical coherence tomography (OCT). A major advantage of OCT is the decoupling of depth resolution (given by the coherence length of the light source) and transverse resolution (given by the focal spot size). This enables images of the retina with high depth resolution. On the other hand, in most OCT systems, a limited transverse resolution has been accepted to provide a sufficiently large (approximately 1 mm) depth of focus. However, to obtain images with high transverse resolution throughout the whole depth of the retina (especially in the nerve head region) a tracking of coherence gate and focus position (dynamic focus) is essential. This study realizes a dynamic focus in a time domain transversal (en face) scanning system for retinal imaging. We show that maintenance of a transverse resolution of approximately 4.4 microm can be achieved over an optical depth of 1 mm in a model eye and apply our method to imaging the human retina in vivo.

Hilbert phase microscopy for investigating fast dynamics in transparent systems

We introduce Hilbert phase microscopy (HPM) as a novel optical technique for measuring high transverse resolution quantitative phase images associated with optically transparent objects. Because of its single-shot nature, HPM is suitable for investigating rapid phenomena that take place in transparent structures such as biological cells. The potential of this technique for studying biological systems is demonstrated with measurements of red blood cells, and its ability to quantify dynamic processes on a millisecond scale is exemplified with measurements of evaporating micrometer-sized water droplets.

Multiple-wave lateral shearing interferometry for wave-front sensing

Multiple-wave achromatic interferometric techniques are used to measure, with high accuracy and high transverse resolution, wave fronts of polychromatic light sources. The wave fronts to be measured are replicated by a diffraction grating into several copies interfering together, leading to an interference pattern. A CCD detector located in the vicinity of the grating records this interference pattern. Some of these wave-front sensors are able to resolve wave-front spatial frequencies 3 to 4 times higher than a conventional Shack-Hartmann technique using an equivalent CCD detector. Its dynamic is also much higher, 2 to 3 orders of magnitude.

Achromatic hybrid refractive-diffractive lens with extended depth of focus

A method for designing achromatic hybrid refractive-diffractive elements that can produce beams with long focal depths while they preserve the entire aperture for capture of light and high transverse resolution is presented. Its working principle is based on the combination of a diffractive optical element that generates a long range of pseudonondiffractive rays and a refractive lens of opposite dispersion to form an achromatic hybrid lens. A hybrid lens with a fast f-number (f/1) that works in the entire visible wave band (400-700 nm) was designed and fabricated. Simulation results demonstrate a factor-of-10 improvement in depth of focus compared with that of a conventional f/1 lens, with matching 1-microm lateral resolution. Experimental results confirm the effectiveness of the proposed method through demonstration of an achromatic hybrid lens with better than a factor-of-7 improvement in depth of focus and 1-microm transverse resolution.

Analysis of a cylindrical microlens array with long focal depth by a rigorous boundary-element method and scalar approximations.

We investigated the focal characteristics of open-regional cylindrical microlens arrays with long focal depth by using a rigorous boundary-element method (BEM) and three scalar methods, i.e., a Kirchhoff and two Rayleigh-Sommerfeld diffraction integral forms. Numerical analysis clearly shows that the model cylindrical microlens arrays with different f-numbers can generate focusing beams with both long focal depth and high transverse resolution. The performance of the cylindrical microlens arrays, such as extended focal depth, relative extended focal depth, diffraction efficiency, and focal spot size, is appraised and analyzed. From a comparison of the results obtained by the rigorous BEM and by scalar approximations, we found that the results are quite similar when the f-number equals f/1.6; however, they are quite different for f/0.8. We conclude that the BEM should be adopted to analyze the performance of a microlens array system whose f-number is less than f/1.0.

Dynamic focus control in high-speed optical coherence tomography based on a microelectromechanical mirror

In high-resolution optical coherence tomography (OCT), a high numerical aperture sample lens is required to achieve high lateral resolution. This situation leads to a limited depth of focus, which can yield sub-optimal lateral resolution outside the focal zone for deep-imaging OCT systems. To maintain high transverse resolution over the entire depth scan, we designed a high-speed dynamic focus control system based on a microelectromechanical mirror. The silicon nitride deformable mirror shifted the focus of the sample beam of the OCT interferometer in synchrony with the coherence-gate scan in the reference arm. As a result, the coherence gate remained at the beam focus during the whole imaging process. The prototype focus control setup was integrated into a high-speed (8 kHz depth scan frequency) OCT system. Improvement to the image quality was demonstrated with a standard resolution target.

Dual-axis confocal microscope for high-resolution in vivo imaging.

We describe a novel confocal microscope that uses separate low-numerical-aperture objectives with the illumination and collection axes crossed at angle theta from the midline. This architecture collects images in scattering media with high transverse and axial resolution, long working distance, large field of view, and reduced noise from scattered light. We measured transverse and axial (FWHM) resolution of 1.3 and 2.1 microm, respectively, in free space, and confirm subcellular resolution in excised esophageal mucosa. The optics may be scaled to millimeter dimensions and fiber coupled for collection of high-resolution images in vivo.

Analysis of a closed-boundary axilens with long focal depth and high transverse resolution based on rigorous electromagnetic theory.

We find that a microcylindrical axilens with a closed boundary and with an f-number less than 1 still can achieve the properties of long focal depth and high transverse resolution, unlike a microcylindrical axilens with an open boundary, which fails to maintain those properties for low f-numbers. The focusing characteristics of the closed-boundary axilens and the open-boundary axilens are numerically investigated based on the boundary integral method. The numerical results show that the ratio of the extended focal depth of the closed-boundary axilens to the focal depth of the conventional microlens can reach up to 1.26 and 2.12 for the preset focal depths 3 and 5 microm, respectively, even though the f-number is reduced to 1/3.

En face imaging of single cell layers by differential phase-contrast optical coherence microscopy

Optical coherence microscopy (OCM) is capable of imaging the backscattering potential of a sample with high transversal and axial resolution. We report on a combination of OCM with a differential phase-contrast technique that permits imaging of the subwavelength optical path differences that occur between a narrow beam probing a sample and its surrounding. This technique allows small transversal refractive-index variations close to a selected interface to be seen. We report on the method and present first images of a test sample and a single cell layer. The cells act as phase objects; imaging the phase properties improves the contrast compared with that of intensity images.

Rigorous electromagnetic analysis of a microcylindrical axilens with long focal depth and high transverse resolution.

We first present nonparaxial designs for a microcylindrical axilens with different long focal depths and rigorously analyze electromagnetic field distributions of the axilens using integral equations and the boundary-element method. Numerical results show that the designed axilenses indeed have the special feature of attaining a long focal depth while keeping high transverse resolution for numerical apertures of 2.4, 2.0, and 1.0. The ratio between the extended focal depth of the designed axilens and the focal depth of the conventional focal lens is 1.41, the corresponding maximal extended focal depth of the axilens can reach 28 microm, and the spot size of the focal beam is approximately 10 microm over the focal range.

In vivo ultrahigh-resolution optical coherence tomography

Ultrahigh-resolution optical coherence tomography (OCT) by use of state of the art broad-bandwidth femtosecond laser technology is demonstrated and applied to in vivo subcellular imaging. Imaging is performed with a Kerr-lens mode-locked Ti:sapphire laser with double-chirped mirrors that emits sub-two-cycle pulses with bandwidths of up to 350 nm, centered at 800 nm. Longitudinal resolutions of ~1mum and transverse resolution of 3mum, with a 110-dB dynamic range, are achieved in biological tissue. To overcome depth-of-field limitations we perform zone focusing and image fusion to construct a tomogram with high transverse resolution throughout the image depth. To our knowledge this is the highest longitudinal resolution demonstrated to date for in vivo OCT imaging.

Diffractive phase elements that synthesize color pseudo-nondiffracting beams

The design of diffractive phase elements (DPE's) for generating color pseudo-nondiffracting beams (PNDB's) in a multiple-wave illuminating system by the conjugate-gradient method is described. The axial-intensity distributions for dual-color PNDB's generated by the DPE's are shown. The color PNDB's behave as segmented axial-intensity distributions; in each of segments only one color persists. The sequence of wavelength components in the color PNDB's can be arbitrarily preset. Three-dimensional plots of a dual-color PNDB indicate the characteristics of a beam with high transverse resolution.