Fast analytical approximation for arbitrary geometries in diffuse optical tomography

Diffuse optical tomography is a novel imaging technique that resolves and quantifies the optical properties of objects buried in turbid media. Typically, numerical solutions of the diffusion equation are employed to construct the tomographic problem when media of complex geometries are investigated. Numerical methods offer implementation simplicity but also significant computation burden, especially when large three-dimensional reconstructions are involved. We present an alternative method of performing tomography of diffuse media of arbitrary geometries by means of an analytical approach, the Kirchhoff approximation. We show that the method is extremely efficient in computation times and consider its potential as a real-time three-dimensional imaging tool.     

Noncontact optical tomography of turbid media

Optical tomography of turbid media has so far been limited by systems that require fixed geometries or measurements employing fibers. We present a system that records noncontact optical measurements from diffuse media of arbitrary shapes and retrieves the three-dimensional surface information of the diffuse medium. We further present a novel method of combining this composite data set and obtain accurate fluorescence reconstructions. This approach offers significant experimental simplicity and yields high-information-content datasets. The performance of this novel tomographic approach is demonstrated with experimental reconstructions of phantoms.     

Improving the diffuse optical imaging spatial resolution of the cerebral hemodynamic response to brain activation in humans

We compare two geometries of sources and detectors for optimizing the diffuse optical imaging resolution of brain activation in humans. Because of limitations in the instruments' dynamic range, most diffuse optical brain activation images have used only nonoverlapping measurements. We demonstrate theoretically and with a human experiment that a simple geometry of sources and detectors can provide overlapping measurements within the limitation of instrumentation dynamic range and produce an image resolution and localization accuracy that is twofold better.     

Birefringence imaging of human skin by polarization-sensitive spectral interferometric optical coherence tomography

We have developed a spectral interferometric optical coherence tomography (OCT) system with polarization sensitivity that is able to measure a two-dimensional tomographic image by means of one-dimensional mechanical scanning. Our system, which has an axial resolution of 32 mum , calculates the distribution of each element of the Müller matrix of a measured object from 16 OCT images. The OCT system successfully reveals the birefringent nature of human skin tissue.     

In vivo whole-body imaging of optical agent dynamics using full angle fluorescence diffuse optical tomography

Dynamic fluorescence diffuse optical tomography （FDOT） is important in drug deliver research. In this letter, we first image the metabolic processes of micelles indocyanine green throughout the whole body of a nude mouse using the full-angle FDOT system with line illumination （L-FDOT）. The resolution of L-FDOT is evaluated using phantom experiment. Next, in vivo dynamic tomographic images （100 frames; approximately 170 min） of mouse liver and abdomen are shown and cross-validated by planar fluorescence reflectance imaging in vitro. Results provide evidence on applicability of the tomographic image wholebody biological activities in vivo on minute timescale （approximately 1.7 min） using L-FDOT.     

Spectral characterization of fiber gratings with high resolution

We demonstrate a high-resolution technique to measure the optical magnitude and phase responses of fiber gratings. The technique employs single-sideband modulation of an optical source and has spectral resolution in the hertz regime. The technique is demonstrated by measurement of the phase ripples of unapodized and apodized chirped gratings as well as the transmission spectrum of a pi-phase-shifted grating. Although it is demonstrated on fiber gratings, the technique is applicable to any optical device.     

Tomographic imaging of a suspending single live cell using optical tweezer-combined full-field optical coherence tomography

We propose a label-free depth-resolved tomographic scheme for imaging a single live cell in fluid. This approach utilizes a modified time-domain full-field optical coherence tomography (FF-OCT) system combined with an optical tweezer technique. The optical trap for holding a moving specimen is made by tightly focusing a 1064 nm Q-switching pulsed laser beam with a 1.0 NA microscope objective in the sample arm of the FF-OCT part. By cosharing the probe for both systems, the optical actions of trapping and cellular resolution tomographic imaging could be achieved simultaneously. Feasibility of the combined system is demonstrated by imaging micron-sized polystyrene beads and a living suspension cell in medium.     

Ultrahigh-resolution optical coherence tomography by broadband continuum generation from a photonic crystal fiber

We have developed an ultrahigh-resolution optical coherence tomographic system in which broadband continuum generation from a photonic crystal fiber is used to produce high longitudinal resolution. Longitudinal resolution of 1.3-microm has been achieved in a biological tissue by use of continuum light from 800 to 1400 nm. The system employed a dynamic-focusing tracking method to maintain high lateral resolution over a large imaging depth. Subcellular imaging is demonstrated.     

近红外漫射光层析成像实验研究

实验验证了所建立的时间分辨光学层析成像测量系统的光学参量预测能力、多个测量通道的一致性,表明了本测量系统具有层析成像的潜力.研究了采用特征数据量（平均飞行时间、广义脉冲谱技术为基础的用数据类型R）及全时间分辨数据的图像重建算法在光学参量重建准确度、重建出的对象尺寸、空间分辨率等方面的各自优势.     

Two-dimensional optical tomography of hemodynamic changes in a preterm infant brain

Our preliminary results on two-dimensional (2D) optical tomographic imaging of hemodynamic changes in a preterm infant brain are reported. We use the established 16-channel time-correlated single photon counting system for the detection and generalized pulse spectrum technique based algorithm for the image reconstruction. The experiments demonstrate that diffuse optical tomography may be a potent means for investigating brain functions and neural development of infant brains in the perinatal period.     

Resonator-aided single-atom detection on a microfabricated chip

We use an optical cavity to detect single atoms magnetically trapped on an atom chip. We implement the detection using both fluorescence into the cavity and atom-induced reduction in cavity transmission. In fluorescence, we register 2.0(2) photon counts per atom, which allows us to detect single atoms with 75% efficiency in 250 micros. In absorption, we measure transmission attenuation of 3.3(3)% per atom, which allows us to count small numbers of atoms with a resolution of about 1 atom.     

Optical coherence tomography using a frequency-tunable optical source

We have developed a simple, wide-optical-bandwidth, high-resolution system for performing rapid optical frequency domain reflectometry measurements and applied it to multidimensional tomographic imaging. The source is a grating-tuned external cavity semiconductor laser with a tuning capability of 25 nm in 100 ms. We discuss system performance and show a two-dimensional optical coherence tomography image of a thin glass sandwich structure as a preliminary demonstration of the systems depth and resolution capabilities.     

Two-dimensional coherent detection imaging in multiple scattering media based on the directional resolution capability of the optical heterodyne method

This paper describes a new application of optical heterodyne detection using a laser beam for two-dimensional imaging of the internal structure of strongly scattering media in which the structure is completely obstructed from normal visual observation. The directional resolution capability for image formation due to the excellent antenna properties of the heterodyne technique is verified experimentally using a ground glass to cause strong scattering of the signal beam. Successful image detection of a test target placed in a highly scattering absorptive medium, with spatial resolution better than 400 m in the case of our experiments, demonstrates that this Coherent Detection Imaging (CDI) method can overcome the diffuse nature of images in media such as those of biomedical interest and others to achieve scanning and tomographic imaging.     

Visualized numerical assessment for near infrared diffuse optical tomography with contrast-and-size detail analysis

The purpose of this study is to propose an objective contrast-and-size detail (CSD) analysis for near infrared diffuse optical tomography (NIR DOT), of which the concept is derived from the subjective contrast detail (CD) analysis. We define a measure for numerical CSD analysis based on the resolution estimation of contrast and size. Following that, the contrast-and-size map of resolution can be calculated and displayed for each corresponding image in the map; furthermore, a CSD resolution curve can be plotted by calculating the average value of the projection corresponding to the physical quantity/axis (size or contrast). To provide some worked examples about the proposed CSD analysis evaluating the imaging performance of different reconstruction methods, Tikhonov regularization and edge-preserving regularization with different weighting functions were employed. Results suggested that using edge-preserving regularization with the generalized Lorentzian weighting function is the most attractive for the estimation of absorption-coefficient images.     

Evaluation of the improved three‐dimensional resolution of a synchrotron radiation computed tomograph using a micro‐fabricated test pattern

A micro test pattern prepared by focused ion beam milling was used to evaluate the three‐dimensional resolution of a microtomograph at the BL20B2 beamline of SPring‐8. The resolutions along the direction within the tomographic slice plane and perpendicular to it were determined from the modulation transfer functions. The through‐plane resolution perpendicular to the tomographic slice was evaluated to be 8 µm, which corresponds to the spatial resolution of two‐dimensional radiographs. In contrast, the in‐plane resolution within the slice was evaluated to be 12 µm. Real‐space interpolation was performed prior to the tomographic reconstruction, giving an improved in‐plane resolution of 8.5 µm. However, the 8 µm pitch pattern was resolved in the interpolated slice image. To reflect this result, another resolution measure from the peak‐to‐valley difference plot was introduced. This resolution measure gave resolution limits of 7.4 µm for the in‐plane direction and 6.1 µm for the through‐plane direction. The three‐dimensional test pattern along with the interpolated reconstruction enables the quantitative evaluation of the spatial resolution of microtomographs.     

Effect of asymmetric stress relaxation on the polarization-dependent transmission characteristics of a CO2 laser-written long-period fiber grating

The effect of stress-induced birefringence on polarization-dependent transmission characteristics was thoroughly investigated for a long-period fiber grating (LPFG) that was fabricated by use of a stress relaxation method with a CO2 laser. A series of two-dimensional axial stress profiles for one complete period of a LPFG was measured with an optical tomographic measurement technique. We found that the asymmetry in the stress distribution of the cladding was much larger than in the core of the LPFG. The splitting of polarization-dependent loss peaks in the optical transmission spectrum was calculated based on the measured asymmetric stress profiles and was compared with an experimental result.     

Signal-to-noise analysis for propagation of laser radiation through a tissue-like medium by diffuse photon-density waves

Biomedical optical imaging in the near-infrared (NIR) region provides the possibility to detect and determine pathological and functional changes in human tissue without the drawback of ionizing radiation. Of special promise is the application of this technology for the detection of joint diseases, such as rheumatoid arthritis (RA). It has been shown that optical changes in the synovial fluid and the vasculature surrounding the joints can be detected with optical methods. Applying optical tomographic methods one should be able to localize and quantify these changes for detection of the onset of RA. The first studies have been limited to continuous wave imaging. However, it is well known that enhanced resolution and better separation between absorption and scattering properties of tissue can be achieved using intensity modulated light sources. Intensity modulation of laser light in the MHz region leads to propagation of so-called diffuse photon density waves (PDW) through the tissue In this study we report on basic experimental results to determine performance and sensitivity of PDW-transillumination of tissue like phantoms. We used a vector network analyzer to generate and analyze intensity modulation from 100 MHz up to 1 GHz via a diode laser and an avalanche photo diode. Scans were performed across phantoms containing a layer with different absorbing and scattering properties bounded by an edge. The thickness of the phantoms was chosen similar to human fingers to gain information for optimization of tomographic imaging of finger joints. We experimentally determined the signal-to-noise ratio (SNR) of the system and compared the results to theoretical predictions. Noise and SNR of amplitude and phase depend on frequency of modulation. While the amplitude SNR decreases with frequency, phase SNR increases to assume a maximum value. We found that the inserted layer can be better characterized using phase information, which becomes more valuable as the source modulation frequency is increased. On the other hand, the sensitivity to perturbations is highest in the amplitude data obtained at lower frequencies. Thus, for tomographic imaging, optimal modulation frequencies should be found depending on the tissue type and nature of tissue inhomogeneities.     

Picosecond linear optical pulse shapers based on integrated waveguide Bragg gratings

We demonstrate a general linear pulse-shaping technique based on integrated III-V Bragg gratings (BGs). Such a technique allows for the synthesizing of complex waveforms with picosecond resolution using a compact single-waveguide design. This approach is experimentally demonstrated by fabricating and testing a series of integrated ultrafast optical pulse shapers based on BG geometries acting as time-domain code generators operating at 500 Gbits/s.     

Acousto-optic-assisted diffuse optical tomography

We introduce and experimentally demonstrate acousto-optic-assisted diffuse optical tomography (DOT) using a holography-based acousto-optic setup. The method is based on probing a scattering medium with a localized acoustical modulation of the phase of the scattered light. The optical properties of the scattering medium are recovered with ultrasound-limited resolution by applying DOT reconstruction methods on a set of the measured intensities of light, modulated at different locations throughout the medium.     

Optical Measurement Techniques for Optical Fiber and Waveguide Devices

We describe three major optical characterization methods for fiber and fiber devices. A simple servo controlled scanning fiber-optic confocal microscope is proposed for determining the refractive index profile of an optical fiber. To measure the chromatic dispersion of a short length fiber a Mach-Zehnder fiber interferometer with a novel interferometric distance meter is introduced. At the end, a tomographic method is demonstrated for determining the 2-D stress profile of a fiber.