Nobukatsu Takai

Digital watermarking by a holographic technique

A holographic technique is applied for digital watermarking by a computer. A digital-watermark image to be hidden is phase modulated in a random fashion, and its Fourier-transformed hologram is superposed on a content image. The watermark is reconstructed by means of a holographic-reconstruction technique from the bit-map image that hides it. In the study the processes of constructing and reconstructing a digital hologram are described on the basis of the theory of Fourier optics. The conditions for superposing the hologram onto the content images are investigated in detail. The validity of the present method is verified by changing the weighting of the hologram relative to that of the content image. The effect of image size is also discussed with respect to reconstruction of the watermark, and it is shown that watermark information in a form of a diffuse-type Fourier-transform hologram cannot be removed by cutting it out of the host image.

Real-time velocity measurement for a diffuse object using zero-crossings of laser speckle

This paper proposes a new method for measuring the in-plane velocity of a moving diffuse object by using the technique of zero-crossings for the intensity fluctuation of spatially integrated laser speckles. The scattered speckle pattern is detected in the diffraction field by a finite-aperture photodetector whose output current is analyzed, after removal of its dc component, by counting zero-crossings. The number of zero-crossings per second for the signal is investigated theoretically and found to depend linearly on the object’s velocity. The theoretical results are confirmed experimentally for translational speckles detected by the photodetector having circular apertures of various sizes. The excellent agreement between theoretical and experimental results shows that the new method allows measurement of the velocity of a moving object in real time with good accuracy.

Statistical properties of laser speckles produced under illumination from a multimode optical fiber

In this paper we study the statistical properties of both speckle patterns incident upon and emerging from a diffuse object under illumination of the laser light generated from a multimode optical fiber. The speckle field resulting from the modal noise at the exit face of the optical fiber is treated as a quasi-homogeneous monochromatic source that describes the quasi-stationary state within a finite region and, simultaneously, has the correlation coefficient that remains to be strictly stationary. The speckle pattern at the object plane in the Fresnel-diffraction field is first investigated by analyzing the propagation of the correlation function of the optical fields emerging from the source and found also to be quasi-homogeneous. Next, the correlation function of the optical field produced at the observation plane by scattering from a moving diffuse object under illumination of the above speckle pattern is analyzed in connection with the source parameters.

New common-path phase shifting interferometer using a polarization technique.

A new type of phase shifting interferometer with a common-path arrangement using a polarization technique is proposed and discussed. In the interferometer, the dc (specular) component of an object beam is separated in the Fourier transform plane and used as a reference beam for its ac component. The phase of the dc component used as the reference beam is shifted by using a polarization technique for phase shifting interferometry. The present interferometer is different from a shearing type in that the phase distribution of an object beam is directly analyzed from the acquired intensity variations obtained by a 2-D detector such as a TV camera. Some experiments were conducted to verify the validity of the present phase shifting interferometer. They showed that high stability of the phase measurements is achieved up to lambda/200 with an accuracy of lambda/40 for wavelength lambda light. The interferometer is suitable for obtaining 2-D phase information about the surface structure of small objects.

Evaluation of blood flow at ocular fundus by using laser speckle.

We report a method for noninvasively evaluating blood flow at the ocular fundus by using laser speckle phenomena. The intensity fluctuation of speckles scattered from a 1-mm-diameter illuminated area at the fundus is detected and analyzed by the photon-correlation technique, which gives us the relative degree of total blood flows within the probe area. The method is used to evaluate blood flows at the ocular fundus of a rabbit and normal human volunteers. The experimental results show that the present laser speckle method is useful for the relative evaluation of blood flows in the ocular fundus tissue.

Correlation distance of dynamic speckles.

The translational and boiling motions of dynamic speckles produced in the Fresnel diffraction field under illumination of a Gaussian beam are investigated in detail. The speckle motion is analyzed from the space-time cross-correlation function of speckle intensity fluctuations detected at the two points in the receiving plane. The correlation distance of time-varying speckles is compared with the translation distance of the spatial speckle pattern. The optical conditions for the translational and boiling motions of dynamic speckles are examined and expressed in a diagram. The characteristics for the correlation distance of time-varying speckle intensity fluctuations are finally verified by several experiments.

Zero-crossing study on dynamic properties of speckles

The zero-crossing problem is studied for the purpose of investigating the dynamic properties of speckle produced in the diffraction field by a moving diffuse object under illumination of coherent light. The zero-crossing method is applied to the time-differentiated speckle intensity fluctuations and studied in some detail with respect to the influence of noise and the low-pass filter used for suppressing the noise. From the theoretical and experimental studies, the velocity of the moving object is found to be accurately determined by measuring the number of zero-crossings per second of the time-differentiated speckle intensity fluctuations when both the optical configuration used for producing speckles and the characteristic of the low-pass filter for suppressing the noise are known beforehand.

Dynamic statistical properties of laser speckle due to longitudinal motion of a diffuse object under Gaussian beam illumination

The dynamic statistical properties of laser speckle produced in the far-field diffraction region from a diffuse object moving longitudinally along the optical axis under illumination of a Gaussian beam are investigated theoretically and experimentally. Although the spatial structure of speckle patterns varies in time owing to the longitudinal motion of the object, the time-varying speckle intensity detected at the center of the far-field diffraction plane is found to follow a statistically stationary variation. The autocorrelation function and power spectral density of the speckle intensity variations are studied with relation to the illuminating condition of the Gaussian beam.

Velocity measurement of the diffuse object based on time-differentiated speckle intensity fluctuations

Abstract This paper proposes a new velocity measuring method for a moving diffuse object on the basis of the autocorrelation of time-differentiated speckle intensity fluctuations. The theoretical background and experimental verification of this method are given. The correlation length of time-differentiated speckle intensity fluctuations is defined by the point at which the correlation becomes zero. By means of the correlation length the object velocity is found to be accurately measured.

The Autocorrelation Function of the Speckle Intensity Fluctuation Integrated Spatially by a Detecting Aperture of Finite Size

The autocorrelation function of the spatially integrated speckle intensity fluctuation is investigated theoretically and experimentally by using both the circular hard aperture and the circular gaussian soft aperture for detection. This investigation shows that the use of the circular gaussian soft aperture simplifies an evaluation of the autocorrelation function of the spatially integrated speckle intensity fluctuation and that the resultant autocorrelation functions obtained using the circular gaussian soft aperture sufficiently approximate to the ones obtained using the circular hard aperture which is usually employed in real situations. The autocorrelation function of the speckle intensity fluctuation integrated spatially by an elliptic detecting aperture is also studied theoretically and experimentally as an extension of the circular hard and gaussian soft detecting apertures.