Phase-shifting Twyman-Green Interferometer Research Articles

Determination of the tangential surface profile of Toroidal beam line mirrors with polarization phase-shifting Twyman–Green interferometer

A new technique for the measurement of the tangential surface profile of Toroidal mirrors (TMs) used in synchrotron radiation beam lines is proposed. A cylindrical lens placed in the collimated input (He–Ne laser) beam of a polarization Twyman–Green interferometer (TGI) forms line foci in the reference and test arms. The effect of the sagittal curvature of the TM is eliminated by adjusting a line focus on the surface of the TM, along the central tangential section. Thus, a toric test wavefront (TTW) with sagittal curvature identical to that of the incident cylindrical wavefront and tangential curvature with slope twice that of the TM, is produced along the reflection direction. The TTW interferes with a reference cylindrical wavefront, generated by retroreflecting the incident cylindrical wavefront from a reference plane mirror in the reference arm of the TGI and having identical curvature in the sagittal direction. Polarization phase-shifting interferometry has been applied to obtain the tangential surface profile of the test TM. Results are compared with other methods. The advantages of the technique are that only a plane reference mirror is needed to generate the reference beam, and the technique is suitable for TMs with long tangential radii of curvature in the kilometer regime.

Correlations between deposition parameters, mechanical properties, and microstructure for diamond-like carbon films synthesized by RF-PECVD

Diamond-like carbon (DLC) films were synthesized on silicon substrates by radio frequency plasma enhanced chemical vapor deposition (RF-PECVD). The reactant gas was pure acetylene and the working pressure, working power, and deposition time were, respectively, varied to comprehensively investigate their influence on the microstructure and mechanical properties of DLC films. A digital phase-shifting Twyman–Green interferometer was utilized to measure the residual stress resulting from the deposition process. Raman spectrometer and nanoindentation system were, respectively, utilized to clarify the constituent microstructure, hardness, and elastic modulus of DLC films. The results reveal that the hardness and elastic modulus are both closely correlated to the sp 3 content (I D/I G ratio), which is greatly dependent on working power but much less on working pressure and deposition time. Furthermore, the self-bias voltage is more dependent on working power than on working pressure and deposition time, a tendency contrary to that of the thickness investigation. Finally, it was found that the residual stress may be influenced by the ion energy and film temperature as well as the sp 3 content.

Interferometer and analysis methods for the <italic>in vitro</italic> characterization of dynamic fluid layers on contact lenses

The anterior refracting surface of the eye when wearing a contact lens is the thin fluid layer that forms on the surface of the contact lens. Under normal conditions, this fluid layer is less than 10 μm thick. The fluid layer thickness and topography change over time and are affected by the material properties of the contact lens and may affect vision quality and comfort. An in vitro method of characterizing dynamic fluid layers applied to contact lenses mounted on mechanical substrates has been developed by use of a phase-shifting Twyman-Green interferometer. This interferometer continuously measures light reflected from the surface of the fluid layer, allowing precision analysis of the dynamic fluid layer. Movies showing this fluid layer behavior can be generated. Quantitative analysis beyond typical contact angle or visual inspection methods is provided. Different fluid and contact lens material combinations have been evaluated, and variations in fluid layer properties have been observed. This paper discusses the interferometer design and analysis methods used. Example measurement results of different contact lens are presented.

Appling of New Optical Measurement and Theory in Mechanical Properties of Thin Film

The thesis aims to characterize the mechanical properties and stresses for thin films deposited on the circular substrates. First, the thin films with the same deposition condition were successively deposited on the distinct substrates using the evaporation technique. The phase-shifting Twyman-Green interferometer (PSTGI) was then employed to measure the warpage of the film-substrate structures and therefore the intrinsic stresses and thermal stresses can be calculated from the well-known Stoney’s formula. The coefficients of thermal expansion (CTE) and Young’s modulus of thin films were also obtained from the Stoney’s theory. Furthermore, the merit of full-field character of optical interferometry was used to propose a novel methodology using the Chen and Ou’s theory to improve the accuracy and to reduce the experiment procedures in the traditional measurement of the aforementioned mechanical properties. Finally, the measured results corresponding to the traditional and proposed methods were respectively substituted into their adopted theories to compare their difference. The results reveal that the accuracy of proposed methodology is considerably improved and the experimental procedures are reduced to those of the traditional methods.

Thermal expansion coefficients of obliquely deposited MgF_2 thin films and their intrinsic stress

This study elucidates the effects of columnar angles and deposition angles on the thermal expansion coefficients and intrinsic stress behaviors of MgF2 films with columnar microstructures. The behaviors associated with temperature-dependent stresses in the MgF2 films are measured using a phase-shifting Twyman-Green interferometer with a heating stage and the application of a phase reduction algorithm. The thermal expansion coefficients of MgF2 films at various columnar angles were larger than those of glass substrates. The intrinsic stress in the MgF2 films with columnar microstructures was compressive, while the thermal stress was tensile. The thermal expansion coefficients of MgF2 films with columnar microstructures and their intrinsic stress evidently depended on the deposition angle and the columnar angle.

Biaxial stresses, surface roughness and microstructure in evaporated TiO 2 films with different deposition geometries

The residual stresses, surface roughness and microstructure in titanium oxide films prepared by electron-beam evaporation and deposited with different geometries were investigated, with particular focus on the in-plane anisotropy of the biaxial stresses and microstructures. Thin films were deposited with various deposition angles on B270 glass substrates and silicon wafers. Two different types of deposition geometries were studied. The residual stress in the thin films was examined by a phase-shifting Twyman-Green interferometer. The optical constants, biaxial stress and surface roughness were found to be related to the evolution of the anisotropic microstructures in the films. The results revealed that the anisotropic stresses that developed in the evaporated titanium oxide films were dependent upon the deposition geometry and microstructure of the films.

A Transparent Indenter Measurement Method for Mechanical Property Evaluation

An optical Transparent Indenter Measurement (TIM) method was developed for measurement of material surface mechanical properties. During the spherical indentation test, in-situ measurements of indentation-induced surface out-of-plane displacements were obtained using an integrated phase-shifting Twyman-Green interferometer. Based on elastic recovery theory and 2D finite element analyses, a procedure was developed to determine the material Young's modulus using the measured surface out-of-plane displacements. During the spherical indentation test, contact radii were also measured and used to estimate the material post-yielding true stress-strain curve using Tabor's empirical relation. An experimental TIM apparatus was assembled to test on two engineering alloys and the results showed good agreement with known material properties.

An apparatus for the measurement of internal stress and thermal expansion coefficient of metal oxide films

A measuring apparatus based on a phase shifting interferometry technique to determine the mechanical properties of metal oxide films was presented. Thin films were prepared by ion-beam sputter deposition at low substrate temperature. Quantitative determination of the mechanical properties such as the internal stress, biaxial elastic modulus, and thermal expansion coefficient of metal oxide films were investigated. A phase shifting Twyman–Green interferometer with the phase reduction algorithm was setup to measure the temperature-dependent stress in thin films. Two types of circular glass plates, with known Young’s moduli, Poisson’s ratios, and thermal expansion coefficients, were used as coating substrate. The temperature-dependent stress behavior of the metal oxide films was obtained by heating samples in the range from room temperature to 70 °C. The stresses of thin films deposited on two different substrates were plotted against the stress measurement temperature, showing a linear dependence. Four oxide films were reported for their film stresses and thermal expansion coefficients.

Making parabolic mirrors by electron-beam gun evaporation method with ion-assisted deposition

A spherical surface was modified to form a parabolic surface by the evaporation of Al 2O 3 films and an ion-assisted deposition process using a designed mask. The stress of the evaporated films was measured by a phase shifting Twyman–Green interferometer. Ion beam bombardment was proven to reduce the stress of Al 2O 3 films. The optimum ion beam voltage and ion beam current density for reducing the film stress were 300 V and 16 μA/cm 2 for Al 2O 3 films.

Modulation transfer function measurement of sparse-array sensors using a self-calibrating fringe pattern

A simple method for the measurement of the pixel modulation transfer function (MTF) of sparse-array (extended MTF) sensors has been developed. We use a phase-shifting Twyman-Green interferometer to generate a series of single spatial-frequency fringe patterns incident on the sensor The resulting signal modulation is measured. We achieve self-calibration by restricting the measured spatial frequencies to multiples of the Nyquist frequency. The aliased patterns at these frequencies are unique and easily identifiable. Spatial frequencies of 480 cycles/mm are generated and measured. This frequency value is more than ten times that of the sensor sampling frequency. The expected MTF shape is obtained at multiples of the sampling frequency. At odd multiples of the Nyquist frequency, the MTF's are affected by the electronic bandwidth and cross talk in the charge-injection device sensor.

Phase Shifting Twyman-Green Interferometer Utilizing Nematic and Cholesteric Liquid Crystal Cells

We show a phase shifting Twyman-Green interferometer utilizing a nematic liquid crystal (NLC) cell and a cholesteric liquid crystal (CLC) cell. NLC cell is used as a phase shifter (PS) and CLC cell is used as an intensity stabilizer. The NLC PS is designed to induce a phase shift of 2π rad for He-Ne laser beam (λ=632.8 nm) with the applied voltage of about 10 V, and a CLC cell which is designed to exhibit its selective reflection peak at 632.8 nm, is used to remove the fluctuation of the intensity of the interferogram. The interferometer system is evaluated by testing a concave mirror surface and the result is discussed.