A grating-based interferometer for 6-DOF displacement and angle measurement is proposed in this study. The proposed interferometer is composed of three identical detection parts sharing the same light source. Each detection part utilizes three techniques: heterodyne, grating shearing, and Michelson interferometries. Displacement information in the three perpendicular directions (X, Y, Z) can be sensed simultaneously by each detection part. Furthermore, angle information (θX, θY, θZ) can be obtained by comparing the displacement measurement results between two corresponding detection parts. The feasibility and performance of the proposed grating-based interferometer are evaluated in displacement and angle measurement experiments. In comparison with the internal capacitance sensor built into the commercial piezo-stage, the measurement resolutions of the displacement and angle of our proposed interferometer are about 2 nm and 0.05 μrad.

Development of a grating-based interferometer for six-degree-of-freedom displacement and angle measurements

Optical Measurement Systems for Industrial Inspection VII

The control of the liquid crystal (LC) alignment is very important for both academic research and practical applications. LC molecules aligned on the ZnO nanoparticle arrays (ZnO NPAs) are demonstrated and the pretilt angles of LCs can be controlled by using ZnO NPAs with different surface wettability. The wettability of ZnO NPAs fabricated by the solution-based hydrothermal method can be controlled by changing the annealing temperature of the as-prepared ZnO NPAs. The measurements of the energy-dispersive spectra and photoluminescence have shown that the chemical properties of ZnO NPAs have been changed with the annealing temperature. Our results show that the pretilt angle of LCs can be tuned continuously from ∼0 to ∼90° as the contact angle of water on ZnO NPAs changes from 33 to 108°.

Tunable Surface Wettability of ZnO Nanoparticle Arrays for Controlling the Alignment of Liquid Crystals.

A grating interferometer based on the wavelength-modulated phase-shifting method for displacement measurements is proposed A laser beam with sequential phase shifting can be accomplished using a wavelength-modulated light passing through an unequal-path-length optical configuration The optical phase of the moving grating is measured by the wavelength-modulated phase-shifting technique and the proposed time-domain quadrature detection method The displacement of the grating is determined by the grating interferometry theorem with the measured phase variation Experimental results reveal that the proposed method can detect a displacement up to a large distance of 1 mm and displacement variation down to the nanometer range

Displacement measurement using a wavelength-phase-shifting grating interferometer.

https://ir.nctu.edu.tw:443/bitstream/11536/21904/1/000318961100016.pdf

Liquid crystal alignment by polyhedral oligomeric silsesquioxane (POSS)-polyimide nanocomposites

This work proposes a simple method, based on the crystal rotation technique and heterodyne interferometry, to simultaneously determine the pretilt angle and cell gap of nematic liquid crystal cells. When heterodyne light passes through a nematic liquid crystal cell, the phase retardation given by the characteristic parameters of the cell can be measured accurately by heterodyne interferometry. This phase retardation relates to the pretilt angle, cell gap, and angle of incidence on the cell. By using the measured phase retardations at two incident angles, the pretilt angle and cell gap of the nematic liquid crystal cell can be estimated by numerical analysis. This method is feasible, requiring only two incident angles and prior knowledge of two characteristic parameters—extraordinary and ordinary refractive indices of the liquid crystal. It is characterized by the advantages of simplicity of installation, ease of operation, high stability, high accuracy, and high resolution.

Measurement of the pretilt angle and the cell gap of nematic liquid crystal cells by heterodyne interferometry

Absolute distance measurement by using modified dual-wavelength heterodyne Michelson interferometer

In this study, the optical activity of cholesteric liquid crystal and common-path heterodyne interferometry are used in a simple measurement technique that was developed to measure small wavelength differences. A circularly polarized heterodyne light passes through a cholesteric liquid crystal cell and an analyzer. Consequently, an interference signal is generated. When the cholesteric liquid crystal cell is properly chosen at circular regime, it owns strongly optical activity. Accordingly, the phase difference between the s- and p-polarized components of the interference signal depends strongly on the wavelength. As the wavelength changed, a variation of the phase difference can be accurately detected by heterodyne interferometry. Substituting the variation of phase difference into specially derived equations, the wavelength variations can be estimated accurately. The feasibility of this method was demonstrated and this method provides the advantages of a simple structure, easy operations, rapid measurement, high stability, and high sensitivity.

Simple method for the measurement of small wavelength differences by optical activity of cholesteric liquid crystal and heterodyne interferometer

Based on the technique of dual-wavelength and principle of heterodyne interferometry, a modified method for measuring the absolute distance is proposed. Because two test lights suffer from the same influence of wavelength drift in the measurement setup, the minus effect coming from the wavelength drift can be offset. Therefore, the measurement accuracy can be significantly increased. The feasibility of this method was demonstrated with a measurement resolution of about 1.50 μm. This method provides the advantages of a simple optical setup, easy operation and rapid measurement.

Alternative method of wavelength drift free dual-wavelength heterodyne interferometry for the absolute distance measurement

In this study a non-contact method for accurately measuring small concentration of solutions by surface plasmon resonance heterodyne interferometer is proposed. Firstly, a linearly polarized heterodyne light source is transmitted through a test box filled with pure water. The transmitted light is incident on the base of a hemi-spherical prism of a surface plasmon resonance apparatus. Then the reflected light passes through an analyzer and generates an interference signal on a photo-detector. Secondly, when the incident angle is chosen at resonant angle, a significant phase difference between the s- and p-polarized components occurs. This phase difference is a function of the incident angle at the base of the hemi-spherical prism. Finally, when the test box is filled with a test solution, the incident angle at the base of the hemi-spherical prism is changed. This causes a variation in the phase difference that can be detected by the heterodyne interferometry. Therefore, the concentration of the tested solution can be accurately determined with special derived equations. The validity of this method was demonstrated experimentally. The advantages of the propose method include a simple apparatus, rapid measurement, high stability, and high resolution. Due to the introduction of a common-path structure, the interference signal is not affected by surrounding fluctuations and can be captured easily.

Wei-Yao Chang

Papers

Measurement of the pretilt angle and the cell gap of nematic liquid crystal cells by heterodyne interferometry

Absolute distance measurement by using modified dual-wavelength heterodyne Michelson interferometer

Simple method for the measurement of small wavelength differences by optical activity of cholesteric liquid crystal and heterodyne interferometer

Alternative method of wavelength drift free dual-wavelength heterodyne interferometry for the absolute distance measurement

Study of the measurement of solution concentration by surface plasmon resonance heterodyne interferometer