The material properties of the cornea are important determinants of corneal shape and refractive power. Corneal ectatic diseases, such as keratoconus, are characterized by material property abnormalities, are associated with progressive thinning and distortion of the cornea, and represent a leading indication for corneal transplantation. We describe a corneal elastography technique based on optical coherence tomography (OCT) imaging, in which displacement of intracorneal optical features is tracked with a 2-D cross-correlation algorithm as a step toward nondestructive estimation of local and directional corneal material properties. Phantom experiments are performed to measure the effects of image noise and out-of-plane displacement on effectiveness of displacement tracking and demonstrated accuracy within the tolerance of a micromechanical translation stage. Tissue experiments demonstrate the ability to produce 2-D maps of heterogeneous intracorneal displacement with OCT. The ability of a nondestructive optical method to assess tissue under in situ mechanical conditions with physiologic-range stress levels provides a framework for in vivo quantification of 3-D corneal elastic and viscoelastic resistance, including analogs of shear deformation and Poisson's ratio that may be relevant in the early diagnosis of corneal ectatic disease.
Colonic crypt morphological patterns have shown a close correlation with histopathological diagnosis. Imaging technologies such as high-magnification chromoendoscopy and endoscopic optical coherence tomography (OCT) are capable of visualizing crypt morphology in vivo. We have imaged colonic tissue in vitro to simulate high-magnification chromoendoscopy and endoscopic OCT and demonstrate quantification of morphological features of colonic crypts using automated image analysis. 2-D microscopic images with methylene blue staining and correlated 3-D OCT volumes were segmented using marker-based watershed segmentation. 2-D and 3-D crypt morphological features were quantified. The accuracy of segmentation was validated, and measured features are in agreement with known crypt morphology. This work can enable studies to determine the clinical utility of high-magnification chromoendoscopy and endoscopic OCT, as well as studies to evaluate crypt morphology as a biomarker for colonic disease progression.
Fourier-domain optical coherence tomography (FDOCT) has attained popularity due to its static parts, high imaging
speed, and high sensitivity. FDOCT makes use of spectral interferometry and collects data in the spectral domain,
either using a spectrometer with a detector array or by a single point detector with a wavelength-swept light source.
The axial resolution depends on the bandwidth of the spectrum. The spectral response of the spectrometer is always
desired to be flat in order to have the best axial resolution corresponding to the light source spectrum. Unfortunately, the
optics consisting of the spectrometer usually shape the spectrum. The optimum optics design and alignment will
minimize the spectral shaping. The frequency response simulation by advanced optical design software displays a clear
picture for our design and system alignment.
The axial imaging range of FDOCT according to the Fourier transform relationship is ultimately limited by a fringe
visibility degrading curve with increasing imaging depth due to the spectral sampling spacing called the fall-off .
This limitation is significant for applications of spectrometer-based FDOCT where a long imaging range is desirable
(e.g the anterior segment of the eye), especially when imaging uses 1.3 &mgr;m light because large pixel-count arrays are not
currently commercially available. Although resolving complex-conjugate ambiguity and Sub-pixel shifting have
extended the image range, the imaging range of FDOCT is still limited by the fall-off, which is a primary concern in the
design of a spectrometer-based FDOCT system. A mathematical model of spectrometer-based FDOCT can aid in
understanding of signal formation, including fall-off [ref OL].
This report presents an advanced analysis of the optics of optical coherence tomography (OCT) systems. Study indicates that spectral filtering functions are contributed by the system optics, the coupling efficiency of both sample and reference arms, and the position of the sample relative to the optics. Simulations by advanced optical software, experiments on two existing OCT systems and the modified theoretical formula demonstrate that the different optics give different filtering functions. These filtering function narrow down the spectrum, shift the center of the spectrum and drop the amplitude of the intensity. Defining a composite standard deviation and a composite center wavelength of the two filtered spectra, we developed a new formula to describe the interference term of OCT, which clearly indicates the detail of the filtering function. An experiment in an extreme condition has been investigated in one of our existing OCT systems. We placed a narrow bandwidth filter into the reference arm. The filter has a center wavelength of 1313.6 nm with a full bandwidth of 6.4nm. The light source has a center wavelength of 1292nm with a bandwidth of 34nm. Experimental resolutions are 19 microns and 85 microns, with and without filter respectively. The calculations are 21.6 and 85.5 microns, respectively, using our new formula.
This paper reports new progress of the Wuhan lidar system. At the present time, our lidar works both at nighttime, to measure the sodium layer in menopause region, and at daytime to measure the aerosol in lower atmosphere region. The daytime working lidar system is equipped with a Faraday Anomalous Dispersion Optical Filter (FADOF), working at the Na resonance line (589 nm) and having an ultra-narrow bandwidth of 2 GHz. The daytime system uses this FADOF to obtain the lidar signal from an altitude of 20 km in our primary experiment. We will also report a comparison of the rms velocity measured by MF radar and Na lidar. A 90% confidence in rms velocity has been achieved.
A variety of experiments are underway in the Quantum Optics Group at Caltech which investigate the quantum nature of atom-field interactions at the level of individual atoms and quanta. A recent technical advance in support of this research is the observation of cooling and trapping of single neutral cesium atoms in magneto-optical trap. Discrete steps are recorded in the fluorescence signal from the trap and are associated with the arrival and departure of individual trapped atoms. Such a spatially localized sample of a single atom with small kinetic energy is an enabling advance for diverse studies in quantum optics, including the possibility of spectroscopy with squeezed and other forms of nonclassical light and cavity quantum electrodynamics with strong coupling of an atom to the field of an optical cavity.