Optofluidic Tomography on a Chip

Using lensfree holography we demonstrate optofluidic tomography on a chip. A partially coherent light source is utilized to illuminate the objects flowing within a microfluidic channel placed directly on a digital sensor array. The light source is rotated to record lensfree holograms of the objects at different viewing directions. By capturing multiple frames at each illumination angle, pixel super-resolution techniques are utilized to reconstruct high-resolution transmission images at each angle. Tomograms of flowing objects are then computed through filtered back-projection of these reconstructed lensfree images, thereby enabling optical sectioning on-a-chip. The proof-of-concept is demonstrated by lensfree tomographic imaging of C. elegans.     

Fluorescent Image Formation in the Fibre-optical Confocal Scanning Microscope

Abstract

Reported in this paper are theoretical studies on two-dimensional and three-dimensional image formation with fluorescent objects in a fibre-optical confocal scanning microscope. An effective point spread function is introduced to derive the two- and three-dimensional optical transfer functions. It is found that, unlike confocal fluorescence microscopes with a finite circular detector, there is no missing cone of spatial frequencies, and no negative tail in the transfer function. The effect of the system parameters on the optical sectioning property is also investigated.     

Signal dependence and noise source in ultrasound-modulated optical tomography

A Monte Carlo modeling technique was used to simulate ultrasound-modulated optical tomography in inhomogeneous scattering media. The contributions from two different modulation mechanisms were included in the simulation. Results indicate that ultrasound-modulated optical signals are much more sensitive to small embedded objects than unmodulated intensity signals. The differences between embedded absorption and scattering objects in the ultrasound-modulated optical signals were compared. The effects of neighboring inhomogeneity and background optical properties on the ultrasound-modulated optical signals were also studied. We analyzed the signal-to-noise ratio in the experiment and found that the major noise source is the speckle noise caused by small particle movement within the biological tissue sample. We studied this effect by incorporating a Brownian motion factor in the simulation.     

Applications of short-coherence digital holography in microscopy

We present an optical system based on short-coherence digital holography suitable for the imaging of three-dimensional microscopic objects. The short temporal coherence properties of the light source allow optical sectioning of the sample. Proper reconstruction of different layers within biological samples is possible up to a depth of a few hundred micrometers, but multiple scattering and inhomogeneities in the refractive index reduce the imaging quality for deeper layers. We have studied the possibility of numerically correcting sample-induced aberrations, and we now propose a method of improving image quality. Numerical simulations and preliminary experimental results show that compensation of these aberrations is possible to some extent.     

Three-dimensional Imaging in Confocal Fluorescent Microscopy with Annular Lenses

Abstract

Three-dimensional imaging properties in confocal fluorescent microscope systems with annular lenses are investigated in terms of the three-dimensional optical transfer function (OTF). Starting from the OTF, we also consider the optical sectioning property. The dependence of the OTF and the optical sectioning strength on the radius of the central obstruction of the lens is revealed. For the case of a finite-sized circular pinhole in front of the detector, both the OTF and the optical sectioning strength, together with the signal level, are calculated as a function of the radius of the pinhole. Investigations show that for a given finite size of the pinhole, the optical sectioning strength can be increased by altering the radius of the central obstruction of the lens to an optimum value, and that a higher signal level may be maintained by using an annular objective only.     

Partially coherent lensfree tomographic microscopy [Invited

Optical sectioning of biological specimens provides detailed volumetric information regarding their internal structure. To provide a complementary approach to existing three-dimensional (3D) microscopy modalities, we have recently demonstrated lensfree optical tomography that offers high-throughput imaging within a compact and simple platform. In this approach, in-line holograms of objects at different angles of partially coherent illumination are recorded using a digital sensor-array, which enables computing pixel super-resolved tomographic images of the specimen. This imaging modality, which forms the focus of this review, offers micrometer-scale 3D resolution over large imaging volumes of, for example, 10-15 mm(3), and can be assembled in light weight and compact architectures. Therefore, lensfree optical tomography might be particularly useful for lab-on-a-chip applications as well as for microscopy needs in resource-limited settings.     

组织精细结构的光学高分辨数据获取与图像重建

光学弱相干层析成像(OCT)技术可以对生物组织高分辨率无损成像。依据获得的样品光学截面数据,再通过三维重建,可以得到生物组织精细结构的再现。在生物组织结构再现的研究中,对于目前常用的生物组织切削照相、X-ray、超声这些方法,OCT作为一种无损高分辨率的3D光学显微技术是一种很好的补充。     

Simultaneous three-dimensional step-height measurement and high-resolution tomographic imaging with a spectral interferometric microscope

A noncontact, nonmechanical scanning, wide-field spectral interference microscope is developed for simultaneous measurement of three-dimensional step-height of discontinuous objects and tomographic imaging. A superluminescent diode (SLD) is used as a broadband light source and a liquid-crystal Fabry-Perot interferometer (LC-FPI) as a frequency-scanning device. By means of changing the injection current to the SLD, the spectral profile of the SLD is equalized, and a constant light input to the interferometer is achieved over the entire frequency-scan range. The Fourier-transform technique is used to determine both the amplitude and the phase of spectral fringe signals. Three-dimensional height distribution of a discontinuous object is obtained from the phase information, whereas optically sectioned images of the object are obtained either from the amplitude information alone or from the combination of both the amplitude and phase information. Experimental results with submicrometer resolution are presented for both step-height measurement and tomographic sectioning.     

Triple-sensor multiplexed frequency-modulated continuous-wave interferometric fiber-optic displacement sensor

A triple-sensor multiplexed fiber-optic displacement sensor, which can measure the displacements of three different objects or the three-dimensional displacement of a single object, is introduced. The sensor is based on the principles of optical frequency-modulated continuous-wave interference and frequency-division multiplexing. The beat signals from the individual sensors are assigned in the frequency domain and separated with different electrical bandpass filters. The displacements of objects can be determined simultaneously by detecting the phase shifts of the corresponding signals. The cross talk between the individual sensors is evaluated, and an accuracy of 0.08 microm in a dynamic range of 1000 microm is achieved.     

A system of recording the motion parameters of large and small objects under the conditions of outer space

Detection of dust particles and large objects by means of a stereoscopic optical system is described. The interaction of light with the study objects, transformation of the light flux into the signals of a CCD array, and processing of information from the arrays are considered. Calculations of the precision of the system are presented. Sensors with a novel design and prediction algorithms for calculation of the position of an object are used to reduce the load on the computer.     

Transparent thin-film characterization by using differential optical sectioning interference microscopy

We propose an optical thin-film characterization technique, differential optical sectioning interference microscopy (DOSIM), for simultaneously measuring the refractive indices and thicknesses of transparent thin films with submicrometer lateral resolution. DOSIM obtains the depth and optical phase information of a thin film by using a dual-scan concept in differential optical sectioning microscopy combined with the Fabry-Perot interferometric effect and allows the solution of refractive index and thickness without the 2pi phase-wrapping ambiguity. Because DOSIM uses a microscope objective as the probe, its lateral resolution achieves the diffraction limit. As a demonstration, we measure the refractive indices and thicknesses of SiO2 thin films grown on Si substrate and indium-tin-oxide thin films grown on a glass substrate. We also compare the measurement results of DOSIM with those of a conventional ellipsometer and an atomic force microscope.     

Dual-spatial integration for longitudinal localization of inclusions in turbid media

We introduce a technique called dual-spatial integration (DSI) that is used to isolate and enhance inclusions that differ only by their longitudinal placement within a scattering medium. DSI uses three different source-detector configurations to section a scattering medium into three longitudinal zones. This sectioning permits the extraction of structures close to surfaces and the enhancement of those structures located in the central part of the medium. Both the simulation and the experimental results indicate that DSI has potential interest for applications in biomedical imaging such as optical mammography.     

Single-lens single-image incoherent passive-ranging systems

We introduce a new system for single-lens single-image incoherent passive ranging. The only a priori object information this system requires is that the objects to be ranged must possess a low-pass spatial frequency spectrum. Physically, this system for passive ranging is a standard optical imaging system that is customized with a special-purpose optical mask or filter. Analytically, this optical mask customizes the transfer function of the optical system in such a way that objects form images that contain range-dependent information. This range-dependent information lies in the spatial spectrum nulls or zeros of the image.     

Meetings Calendar

The optical properties of periodic objects are used to obtain contrast reversal, i.e. the negative is obtained in a simple optical arrangement, in real-time, without chemical processing. Two methods of contrast reversal are presented. Method I makes use of the self-imaging phenomenon and Method II, the discrete form of the Fourier spectrum of periodic signals. Experimental results are included, illustrating the effectiveness of the methods.     

High-speed 3D imaging using photorefractive holography with novel low-coherence interferometers

The paper reports recent progress in developing high speed 3D imaging systems based on low coherence photorefractive holography with high-speed depth-sectioned imaging at 476 frames per second. It is demonstrated that photorefractive holography can utilize a wide variety of sources of differing spatial and temporal coherence, including a novel all-solid-state broadband laser. Also presented is a novel real-time optical sectioning technique based on structured illumination combined with photorefractive holography that provides real-time optical sectioning when imaging with reflected light or with fluorescence.     

Sorting of micron-sized particles using holographic optical Raman tweezers in aqueous medium

We have constructed a holographic optical tweezers system combined with Raman spectroscopy to sort trapped particles. Our software automatically moves the trapped objects to the measurement positions to obtain individual Raman signals from multiple trapped particles. We performed the sorting by comparing their spectra with the previously measured training dataset using the correlation coefficients. We used yeast cells and polystyrene beads as test particles. This study aims to show that biological particles can be separated using single cell analysis with combined holographic optical tweezers and Raman spectroscopy system.     

A compact,automated and long working distance optical tweezer system

We demonstrate a compact, automated, long working distance optical tweezer system using a novel mechanism for controlling the position of the optical trap. Our system uses a single focusing lens with a working distance of 4.5?mm and the trapping beam is steered by moving the lens with a miniature coil-magnet assembly. The sample is imaged through a 100×?microscope objective and a CCD camera captures the magnified image. A custom image processing software detects the position of the laser beam and identifies the sample objects. This information is used to generate appropriate electrical signals to drive the coils which move the focusing lens along the desired path. The system is fairly simple and power efficient due to minimal usage of optical elements in the laser path; hence our setup is simple, low-cost and requires low optical power. Computer-generated arbitrary trapping paths and time-shared trapping patterns are successfully demonstrated. Efficient trapping of micron size spheres with laser powers as low as 1.5?mW is observed.     

Quantitative 3D mapping of fluidic temperatures within microchannel networks using fluorescence lifetime imaging

We describe a novel method for quantitatively mapping fluidic temperature with high spatial resolution within microchannels using fluorescence lifetime imaging in an optically sectioning microscope. Unlike intensity-based measurements, this approach is independent of experimental parameters, such as dye concentration and excitation/detection efficiency, thereby facilitating quantitative temperature mapping. Micrometer spatial resolution of 3D temperature distributions is readily achieved with an optical sectioning approach based on two-photon excitation. We demonstrate this technique for mapping of temperature variations across a microfluidic chip under different heating profiles and for mapping of the 3D temperature distribution across a single microchannel under applied flow conditions. This technique allows optimization of the chip design for miniaturized processes, such as on-chip PCR, for which precise temperature control is important.     

Acousto-Ultrasonic Optical Fiber Sensors: Overview and State-of-the-Art

This paper gives a review of acoustic and ultrasonic optical fiber sensors (OFSs). The review covers optical fiber sensing methods for detecting dynamic strain signals, including general sound and acoustic signals, high-frequency signals, i.e., ultrasonic/ultrasound, and other signals such as acoustic emissions, and impact induced dynamic strain. Several optical fiber sensing methods are included, in an attempted to summarize the majority of optical fiber sensing methods used to date. The OFS include single fiber sensors and optical fiber devices, fiber-optic interferometers, and fiber Bragg gratings (FBGs). The single fiber and fiber device sensors include optical fiber couplers, microbend sensors, refraction-based sensors, and other extrinsic intensity sensors. The optical fiber interferometers include Michelson, Mach-Zehnder, Fabry-Perot, Sagnac interferometers, as well as polarization and model interference. The specific applications addressed in this review include optical fiber hydrophones, biomedical sensors, and sensors for nondestructive evaluation and structural health monitoring. Future directions are outlined and proposed for acousto-ultrasonic OFS.     

Microscopic origins of band broadening in chromatography. Polarity distribution in C(18) stationary phase probed by confocal ratiometric imaging of Nile red

Band broadening is a major factor that influences the efficiency and resolution of chromatographic separations. Studies of microscopic origins of band broadening, such as the micropolarity distribution of chromatographic stationary phase, can provide a better understanding of many chromatographic phenomena and retention behavior. In this work, we probe the chemical environments of C18 chromatographic stationary phase with quantitative confocal fluorescence microscopy under real reversed-phase liquid chromatography conditions. Ratiometric imaging of C18 interface is achieved by loading the stationary phase with a polarity-sensitive dye, Nile red, and optical sectioning with confocal microscopy. The results reveal that there are uniform micropolarity distributions inside individual chromatographic beads, but the polarity may differ between stationary-phase particles. The homogeneity of micropolarity of individual beads suggests that there are not any spatially large exposed silica sites beyond the optical resolution in C18 stationary phase. The strong adsorption sites are smaller in size than the optical resolution of a few hundred nanometers. The heterogeneity between chromatographic beads indicates that the interactions of Nile red with C18 bonded phase are different between beads. This contributes to the broad overall polarity distribution of the C18 stationary phase and can be one of the factors that cause band broadening in separations. With its high spatial resolution and optical sectioning capabilities, confocal fluorescence imaging is shown to be an ideal method to probe the chromatographic stationary phase. The distribution of micropolarity sheds light on the microscopic heterogeneity in chromatographic processes and its influence on chemical separations.