Tomohiro Kiire

Doppler phase-shifting digital holography and its application to surface shape measurement

Digital holography utilizing the optical Doppler effect is proposed in which the time variation of interference fringes is recorded using a high-speed CMOS camera. The complex amplitude diffracted from the object wave is extracted by time-domain Fourier transforming the recorded interference fringes. The method was used to measure the surface shape of a concave mirror under a disturbed environment.

Simultaneous two-wavelength Doppler phase-shifting digital holography.

This paper presents a method based on the use of an image sensor for obtaining the complex amplitudes of beams diffracted from an object at two different wavelengths. The complex amplitude for each wavelength is extracted by the Doppler phase-shifting method. The principle underlying the proposed method is experimentally verified by using the method with two lasers having different wavelengths to measure the surface shape of a concave mirror.

Color digital holography using a single monochromatic imaging sensor

Color digital holography utilizing the Doppler effect is proposed. The time variation of holograms produced by superposing images at three wavelengths is recorded using a high-speed monochromatic imaging sensor. The complex amplitude at each wavelength can be extracted from frequency information contained in the Fourier transforms of the recorded holograms. An image of the object is reconstructed by the angular spectrum method. Reconstructed monochromatic images at the three wavelengths are combined to produce a color image for display.

Three-dimensional displacement measurement for diffuse object using phase-shifting digital holography with polarization imaging camera

The amount of displacement of a diffused object can be measured using phase-shifting digital holography with a polarization imaging camera. Four digital holograms in quadrature are extracted from the polarization imaging camera and used to calculate the phase hologram. Two Fourier transforms of the phase holograms are calculated before and after the displacement of the object. A phase slope is subsequently obtained from the phase distribution of division between the two Fourier transforms. The slope of the phase distribution is proportional to the lateral displacement of the object. The sensitivity is less than one pixel size in the lateral direction of the movement. The longitudinal component of the displacement can be also measured separately from the intercept on the phase axis along the phase distribution of the division between two Fourier transforms of the phase holograms.

Selectable-wavelength low-coherence digital holography with chromatic phase shifter

We propose a new digital holography method using an ultra-broadband light source and a chromatic phase-shifter. The chromatic phase-shifter gives different frequency shifts for respective spectral frequencies so that the spectrum of the light reflected from the object can be measured to reveal the spectral property of the object, and arbitrary selection of signals in the temporal frequency domain enables single- and multi-wavelength measurements with wide dynamic range. A theoretical analysis, computer simulations, and optical experiments were performed to verify the advantages of the proposed method.

Ultraweak background scattered light reveals structure of a diffractive element

Background scattered light should normally be reduced in industrial fabrication processes; however, we demonstrated that background scattered light from an optical element contains significant structural information about the element. This was revealed by quite good agreement between the measured scattering intensity distribution of a sample and a computer simulation of the light intensity from the sample. The intensity distribution from a carefully fabricated sample with artificially controlled defects was obtained with a measurement system designed to measure ultraweak background scattered light covering an intensity range of over 10(10).

Simultaneous Two-Wavelength Digital Holography and Its Application to Surface Shape Measurement

Two-wavelength digital holography with Doppler-phase shifting method was proposed. Two laser beams with different wavelength was simultaneously illuminated onto a target object, and the surface shape was reconstructed from the digital holograms.

3D velocity measurement by a single camera using Doppler phase-shifting holography

In order to understand the details of the flow field in micro- and nano-fluidic devices, it is necessary to measure the 3D velocities under a microscopy. Thus, there is a strong need for the development of a new measuring technique for 3D velocity by a single camera. One solution is the use of holography, but it is well known that the accuracy in the depth direction is very poor for the commonly used in-line holography. At present, the Doppler phase-shifting holography is used for the 3D measurement of an object. This method extracts the signal of a fixed frequency caused by the Doppler beat between the object light and the reference light. It can measure the 3D shape precisely. Here, the frequency of the Doppler beat is determined by the velocity difference between the object light and the reference light. This implies that the velocity of an object can be calculated by the Doppler frequency. In this study, a Japanese 5 yen coin was traversed at a constant speed and its holography has been observed by a high-speed camera. By extracting only the first order diffraction signal at the Doppler frequency, a precise measurement of the shape and the position of a 5 yen coin has been achieved. At the same time, the longitudinal velocity of a 5 yen coin can be measured by the Doppler frequency. Furthermore, the lateral velocities are obtained by particle image velocimetry (PIV) method. A 5 yen coin has been traversed at different angles and its shapes and the 3D velocities have been measured accurately. This method can be applied to the particle flows in the micro- or nano-devices, and the 3D velocities will be measured under microscopes.

Photon-counting scatterometer with illumination adjusting and intensity stitching

For scattered light measurements, a novel scatterometer that is based on photon counting using extinction filters is proposed. A photon counter is used as the detector, with the advantages of high dynamic range, high linearity, and detection of light down to the single-photon level. Extinction filters expand the dynamic range of the detector, and data acquired using the filters are stitched together. The use of the photon counting device and the extinction filters results in a dynamic range extending over 11 orders of magnitude. Light scattered by three parallel slabs of glass are measured, and compared with roughness parameters obtained by atomic force microscopy.

Calculation method for a quadrature phase-shifting interferometer and its applications

A calculation method for a quadrature phase-shifting interferometer is presented, and its applications to specular and speckle interferometers and digital holography are described. Two sets of quadrature phase-shifted interferograms are acquired, and the calculation method proposed gives the phase distribution of the interferograms. The principle of the calculation method with error analysis and experimental results for specular and speckle interferometers and digital holography are also given.