Hirofumi Kadono

Novel functional imaging technique from brain surface with optical coherence tomography enabling visualization of depth resolved functional structure in vivo

Mapping of the activity of brain by optical intrinsic signal imaging (OISI) provides a two-dimensional activation pattern of visual cortical areas at a resolution of a few hundred microns. However, integration of the intrinsic signal over depth results in loss of finer information about functional organization across the depth. Here, we report the first successful implementation of optical coherence tomography (OCT) at around 30 microm depth resolution to investigate cortical functions of a cat brain in vivo. This technique, named functional OCT (fOCT) provided visually evoked changes in the OCT signal. The fOCT signal shows stimulus specificity that correlates well with that of the intrinsic signals and provides depth resolved layer specific functional information.

Phase shifting common path interferometer using a liquid-crystal phase modulator

Abstract A common path phase shifting interferometer is proposed using a liquid-crystal cell as a phase modulator. In the proposed common path interferometer, the specular component of the object light, separated in the Fourier transform plane, plays the role of a reference wave for the diffracted component of the object. A homogeneous-type nematic liquid-crystal cell is used as a phase only modulator, to introduce an arbitrary phase modulation between the specular and the diffracted components to achieve phase shifting interferometry. This makes quantitative measurements possible as well as qualitative observations for phase objects like in a Zernikes phase contrast microscope. Finally, experiments were carried out to show the validity of the proposed method. Experimental results show that the present method is quite stable against mechanical shocks and air fluctuation owing to its common path arrangement. The accuracy of the present method was experimentally verified to be better than 1 30 of wavelength by comparing the results with those obtained by the heterodyne interferometer.

Dynamic electronic speckle pattern interferometry (DESPI) phase analyses with temporal Hilbert transform

In this study, we propose the Hilbert transform (HT) method for phase analysis of a Dynamic ESPI signal. The data processing is performed in the temporal domain, using the temporal history of the interference signal at every single pixel. The final results give a temporal development of the two-dimensional deformation field. To reduce the influence of the fluctuations of bias intensity on the calculated phase, it was removed prior to performing the HT. This method was demonstrated for defects distinction and the determination of the sign change in the deformation field in two different experiments. The range of measurement lies between submicrons and tens of microns and the spatial resolution is better when compared to the fringe analysis method and the spatial carrier method.

Implementation of optical coherence tomography (OCT) in visualization of functional structures of cat visual cortex

We report the application of optical coherence tomography (OCT) for visualizing a 1D depth resolved functional structure of cat brain in vivo. The OCT system is based on the known fact that neural activation induces structural changes such as capillary dilation and cellular swelling. Detecting these changes as an amplitude change of the scattered light, an OCT signal reflecting neural activity, i.e., functional OCT (fOCT) could be obtained. Experiments have been done to obtain a depth resolved stimulus-specific profile of activation in cat visual cortex.

Speckle-shearing interferometry using a liquid-crystal cell as a phase modulator

Fundamental properties of the phase-modulation ability of a nematic liquid-crystal cell are studied. Based on these phase-modulation properties of the liquid-crystal cell, a new type of speckle-shearing interferometer is proposed and studied experimentally. A liquid-crystal cell is employed as a phase shifter to implement the phase-shifting method for the conventional speckle-shearing interferometer. From the experiments used to measure the deformation of an object, the usefulness of the method is confirmed. Finally, a compensation method for phase-shift error is proposed on the basis of the statistical properties of the fully developed speckle field. In this method the speckle phase is regarded, in a statistical sense, as a standard phase object used to calibrate the measuring system. Experiments to confirm the error-compensation method are performed, and it is shown that the phase-shift error can be determined with an accuracy of as much as λ/100.

Statistical interferometry based on a fully developed speckle field: an experimental demonstration with noise analysis

A novel interferometric method named statistical interferometry is proposed and studied. In the method, in contrast to the conventional deterministic interferometry, the complete randomness of the two interfering light fields, i.e., the random interference of the fully developed speckle fields, plays an essential role and is used as a standard of phase in a statistical sense. Preliminary experiments were conducted to verify the validity of the method, followed by a computer simulation. As an experimental result, the accuracy of the measurements of an out-of-plane displacement was confirmed up to lambda/800 by comparison with the heterodyne interferometer. The method has the advantage of simplicity of the optical system required, while at the same time providing high accuracy.

Dynamic ESPI with subtraction–addition method for obtaining the phase

Abstract Dynamic electronic speckle pattern interferometry (DESPI) was developed for in situ observations. The quantitative evaluation of the deformation field was performed through 2-D subtraction–addition method (SAM) for phase analysis. This method does not require additional phase modulation, which makes it applicable to studying dynamic events. The method utilizes the ratio of the subtraction and addition correlation fringe patterns. The bias intensity that has to be removed from the addition images was determined by temporal averaging of a sequence of speckle patterns. To reduce the error, Gaussian filter was applied to the correlation fringe patterns. The deformation field was evaluated with accuracy of λ /10.

Power spectral analysis for evaluating optical near-field images of 20 nm gold particles

Abstract We have demonstrated that the power spectral analysis could be employed as a potential tool for evaluation of near-field optical images of colloidal gold particles of size 20 nm fixed on a glass substrate. It has been found that the power spectral density of the near-field image resembles the Airy pattern due to a circular dot with the first minimum occurring at the spatial frequency 5.6 × 10 7 m −1 . This corresponds to an estimated particle diameter of 22 nm. From a comparison with the theoretical power spectral density of randomly distributed circular dots, the transfer function of the near-field optical microscope has been estimated. It has been found that the resolution estimated using −3 dB cutoff value obtained from the transfer function was 0.8 nm which is much higher than that determined by aperture size. The high resolving ability of the present microscopic system is believed to be due to the confinement of very high spatial frequency evanescent fields around the edges of the metal-glass fiber boundary at the apex of the apertured probe.

Use of dynamic electronic speckle pattern interferometry with the Hilbert transform method to investigate thermal expansion of a joint material

A dynamic electronic speckle pattern interferometry method is applied to investigate thermal expansion of a joint material (ceramic-stainless steel) as a practical industrial object. The speckle interference signal is considered in the temporal domain and the phase is analyzed by the Hilbert transform method. Errors caused by the bias and modulation variations over the phase values are first examined by numerical simulation. Two experiments are performed with in-plane and out-of-plane sensitive systems to study the 3D deformation field thoroughly. The deformation field showed clearly the difference between the thermal expansions of the stainless steel and ceramic. It was also revealed that the boundary of materials and its vicinity suffer very large thermal strain due to the significantly large difference in the linear coefficient of thermal expansions.

Color classification by vector subspace method and its optical implementation using liquid crystal spatial light modulator

Abstract Color classification by a statistical pattern recognition method, the subspace method, was implemented by a simple optical system including a liquid crystal spatial light modulator. Area coding filters corresponding to eigenvectors were drawn on the liquid crystal panel. The vector inner products between the filter functions of each class and the input spectral distributions were calculated instantaneously. To construct low-dimensional subspaces for each class, the recognition system for arbitrary spectral distributions was used and efficient classification filters were constructed after several iterative learning cycles. Samples with small hue differences were correctly classified.