Noboru Nakatani

Laser Doppler velocimetry using polarization‐preserving optical fibers for simultaneous measurement of multidimensional velocity components

Laser Doppler velocimeter (LDV) systems using polarization‐preserving optical fibers are developed by the polarization and the color methods for simultaneous measurement of multidimensional velocity components. By the use of fibers the construction of the optical systems are made easy, the systems are made more flexible, and remote measurement is realized. A signal processor for the LDVs is developed using only a single frequency counter, a high‐speed electronic scanner, and a computer. Using Karman vortex flow and turbulent premixed flame, it is demonstrated that these systems are powerful tools for fluid mechanic measurements.

Simultaneous Measurement of Flow Velocities in Multipoint by the Laser Doppler Velocimeter

An important shortcoming of the conventional laser Doppler method is that it furnishes the instantaneous velocity at a single point in the fluid. To obtain the complete flow pattern, the experiment has to be repeated for different regions of the flow field (1) ~ (6).

Measurement of flow velocity distributions by pulse luminescence method

To visualize and measure quantitatively flow velocity distributions, a luminescence method which uses a pulse light was investigated. A method by which flow velocity can be measured regardless of the widths of excitation time and photograph exposure time was developed. Some optical and fluid characteristics of glycerin aqueous solutions in which luminescence particles are well dispersed are reported. The spatial resolving power of this method was found to be 100-300 mu m. The apparatus which has mechanical shutters with rotating discs was used to measure the velocity distribution of two-dimensional flows.

Atomic Force Microscope Using an Optical Fiber Heterodyne Interferometer Free from External Disturbances

This paper describes an atomic force microscope (AFM) using an optical fiber heterodyne interferometer free from external disturbances in the optical paths of two fibers used for measuring the small displacements of a cantilever. For the elimination of disturbances, the phases of beat signals for two points are differentiated. A special plano-convex lens with double optical axes is used for obtaining two beams at two points. The noise level of this interferometer is 4.5×10-3 nm/√Hz and is limited by the phase noise of the phase meter used. It is confirmed, by measuring the electric force between the tip and the sample, that the interferometer can be used for measuring the displacement of the cantilever. By inspecting the surface of an echellete grating, it is demonstrated that this AFM is a powerful tool for measurement of the surface on the nanometer order.

Optical fiber heterodyne interferometer probe free from external disturbance

A fiber heterodyne interferometer probe free from external disturbance, which can measure small displacement of an object as a pressure transducer or a vibrational object, is described. The phase fluctuation in the fiber due to external disturbance is excluded using a reference beat signal of light beams from a reflection mirror on a graded index rod lens. A graded index‐type multimode fiber connected to one end of a single‐mode fiber by arc fusion splicing is used to improve acceptance of the light. The characteristics of the fiber probe are also described.

Turbulence measurement by the pulse luminescence method using a nitrogen pulse laser.

In order to visualize and measure with ease the velocity distribution and diffusion of turbulent flow, the pulse luminescence method was investigated. Turbulence intensity was obtained from the turbulent diffusion patterns by Taylors diffusion theory. Apparatus was developed for easier measurement. A nitrogen pulse laser was used for instantaneous, high-power excitation. With the use of a night vision scope a bright image was recorded by a TV camera and video tape recorder. The optimum concentration of LC-G1A luminescent particles was about 0-05% wt for the measurement. High fidelity of the particles as an indicator of the fluid velocity was confirmed. It was demonstrated that the turbulence intensity could be visualized and measured quantitatively by the pulse luminescence method.

Optimization of the receiving optical system in the laser Doppler velocimeter for reduction of the sampling volume

A receiving optical system is described in which the scattered light beams are received through a projection lens by the pinhole of a photomultiplier. The depth Delta Z of the sampling volume in which the received scattered light beams are produced is calculated as a function of R/F, where R and F are the radius and focal length of the projection lens. Delta Z was also measured as a function of R/F; it is found that if the intersection angle of the laser beams is constant, then in order to make the sampling volume smaller, the magnification (M) of the lens should be increased, the radius (r) of the pinhole should be decreased, and R/F should be an optimum value.

Flow visualization by an improved double exposure method in holography

Abstract Some optical differential interferometric techniques were investigated in order to measure the density distribution in the air jet flow of a small scale flow cell while excluding the effects of the optical distortion of its glass windows. A technique using moire fringe formation by rotating the reference wave in one exposure of the double exposure method was found to be most suitable for this purpose. High contrast and multiplication of moire fringes can be obtained by a moire fringe sharpening and multiplication method.

Development of a New Aqueous Solution Highly Sensitive to Flow Birefringence

An aqueous solution, highly sensitive to flow birefringence and showing Newtonian viscosity is developed from the standpoint of material design to make the flow measurement easier using the flow birefringence method. The highly sensitive solution to flow birefringence is obtained by dissolving Congo Red (CR), which has a large optical anisotropy, into the aqueous solution of polyvinyl alcohol (PVA) of the average degree of polymerization 500 and of the average degree of saponification 98.8%, which can be easily oriented under the action of shear rate. The birefringence and viscosity of PVA–CR aqueous solutions are measured for a range of shear rate from 0 to 2000 s-1. The dispersion of flow birefringence with various wavelengths is also determined. The sensitivity of flow birefringence for shear rate is higher by one or two orders of magnitude than that in aqueous solutions employed by previous investigators. The relation between viscosity and shear rate in the solution is Newtonian.

Atomic force detection system with a differential heterodyne interferometer using an optical fiber array

This paper describes an atomic force microscope (AFM) using an optical fiber heterodyne interferometer free from external disturbances in optical paths of two fibers used for measuring small displacement of a microcantilever. An array of two fibers is used for superimposing interference beams easily and obtaining two closely spaced spots on a microcantilever. For eliminating the disturbances, the phase of the two fibers and the two spots of measurement beat signal and the phase of the two fibers of reference beat signal are differentiated. The noise level of this interferometer is 1.1×10−3 nm/√Hz and is limited by the phase noise of the phasemeter used. It is confirmed, by measuring thickness of a thin Au film, that the interferometer has a suitable performance for measuring the displacement of the microcantilever. By observing a surface of an optical disk, it is demonstrated that this AFM is a powerful tool for measuring a surface in the order of nanometer.