Scanning White Light Interferometer Research Articles

Signal correction by detection of scanning position in a white-light interferometer for exact surface profile measurement.

In order to perform an exact surface profile measurement with a white-light scanning interferometer (WLSI), an actual optical path difference (OPD) changing with time is detected with an additional interferometer in which the light source of the WLSI and an optical band-pass filter are used. This interferometer is simply equipped in the WLSI and does not negatively influence the WLSI. The real OPD is easily calculated from an interference signal with the same signal processing as that in the WLSI. The interference signal of the WLSI is corrected with the real OPD values or the real scanning position values. The corrected interference signal with a constant sampling interval is obtained with an interpolation method. With this correction method, a surface profile with a step shape of 3-μm height is measured accurately with an error less than 2nm.

Design and manufacture of eyeball protrusion measuring device using white light scanning interferometer

Design and manufacture of eyeball protrusion measuring device using white light scanning interferometer

Opto-Electron Eng, 2018, 45(9)] Recent progress of accurate measurement for distributed polarization crosstalk of fiber optic polarization component and device

The polarization crosstalk of a fiber optic polarization component and device refers to the optical power coupling that occurs at a disturbance point between the two orthogonal polarized modes propagating in it. The distributed polarization crosstalk along with the light propagation direction is directly responsible for the optical polarization properties, for example, the polarization, elliptical polarization, and depolarization properties. It also indirectly reflects the manufacturing technique and the state of the ambient environment, for example, the stress and strain at the joint and fixed position, as well as the temperature. Thus, it is the comprehensive embodiment of the intrinsic performance of the fiber optic polarization component and device and the influence of environment. It is expected to be a general characteristic parameter for online testing, diagnosis, and evaluation of the performance of the fiber optic polarization component and device. The best measurement method for distributed polarization crosstalk till now is the optical coherence domain polarimetry (OCDP). It is based on the white light interferometry and accurately measures the position and amplitude of the distributed polarization crosstalk using a scanning white light interferometer to realize interference between different polarized modes. It has the merits of ultrahigh sensitivity, ultra-wide dynamic range, and ultra-long measurable length. This review paper takes the polarization maintaining fiber coil and multifunctional integrated optical modulator as examples of distributed polarization crosstalk measurement and application. Firstly, the measurement principle of distributed polarization crosstalk based on the OCDP is introduced. Secondly, the measurement error sources and corresponding suppression methods are reviewed. Thirdly, the accurate measurement results of the fiber optic polarization component and device at different temperature are demonstrated. In the end, it outlooks the development of distributed polarization crosstalk measurement considering the complicated and changeable operation environment of the fiber optic polarization component and device.

Investigation on machinability of the oxide layer in anodic oxidation of reaction-sintered silicon carbide by pure-water

Oxidation-assisted polishing has been developed as an efficient method for precision machining of reaction-sintered silicon carbide (RS-SiC), and characteristic of the oxide layer is the critical factor to obtain a fine surface property. Machinability of the oxide layer obtained in anodic oxidation of RS-SiC by the pure-water under high-frequency-square-wave potential is investigated by the ceria slurry polishing in this study. Quantitative analysis of surface quality of the oxidized RS-SiC sample is conducted by the scanning white light interferometer (SWLI) measurement. Along with increase of the oxidation time, surface quality of the oxidized RS-SiC is changing better at the beginning, and rapidly deteriorated in further oxidation process. Surface qualities of the RS-SiC sample before oxidation, after oxidation, after HF etching, and after abrasive polishing, are compared by the SWLI measurement. Surface roughness rms after anodic oxidation for 60min is 189.004nm. After removing the oxide by ceria slurry polishing, the rms can reach 3.688nm. Meanwhile, there is no visible scratch on the new revealed surface. Therefore, combination of anodic oxidation of RS-SiC by pure water and abrasive polishing of oxide layer by ceria slurry can be considered as an efficient method to machine RS-SiC.

Fabrication and Characterization of Silicon (100) Membranes for a Multi-beam Superconducting Heterodyne Receiver

We fabricated silicon (100) membranes of 3 mm in diameter on the surface of silicon-on-insulator (SOI) substrates and investigated the characteristics of the membranes. The handle layer of one SOI substrate was etched using deep reactive ion etching process with the buried oxide (BOX) layer that remained together with the device layer. The BOX layer of the other SOI substrate was removed using C4F8-based plasma etching after the handle layer etching. The surfaces of both silicon (100) membranes were observed using the scanning white light interferometer system at room temperature. Both silicon (100) membranes have dome-like deformations. The silicon (100) membranes are effectively flattened by etching the BOX layer under the device layer. Both silicon (100) membranes were cooled from room temperature to 4 K by a Gifford–McMahon refrigerator. Wrinkles appeared on the surfaces of both silicon (100) membranes when the temperature dropped to about 200 K. However, the wrinkles disappeared below about 180 K. This phenomenon indicates the wrinkles at low temperature would depend on the properties of the silicon (100) of the device layers and independent of the properties of the BOX layers under the silicon (100) membranes.

Residual vibration control based on a global search method in a high-speed white light scanning interferometer.

To achieve high-speed measurements using white light scanning interferometers, the scanning devices used need to have high feedback gain in closed-loop operations. However, flexure hinges induce a residual vibration that can cause a misidentification of the fringe order. The reduction of this residual vibration is crucial because the highly nonlinear distortions in interferograms lead to clearly incorrect measured profiles. Input shaping can be used to control the amplitude of the residual vibration. The conventional method uses continuous wavelet transform (CWT) to estimate parameters of the scanning device. Our proposed method extracts equivalent modal parameters using a global search algorithm. Due to its simplicity, ease of implementation, and response speed, this global search method outperforms CWT. The delay time is shortened by searching, because fewer modes are needed for the shaper. The effectiveness of the method has been confirmed by the agreement between simulated shaped responses and experimental displacement information from the capacitive sensor inside the scanning device, and the intensity profiles of the interferometer have been greatly improved. An experiment measuring the surface of a silicon wafer is also presented. The method is shown to be effective at improving the intensity profiles and recovering accurate surface topography. Finally, frequency localizations are found to be almost stable with different proportional gains, but their energy distributions change.

Exact surface profile measurement without subtracting dispersion phase through Fourier transform in a white-light scanning interferometer.

A new signal processing is proposed in which the dispersion phase is not subtracted from the detected spectral phase distribution. The linear and bias components in the spectral phase distribution are used to calculate the complex-valued interference signal (CVIS). The simulations verify that the dispersion phase generates an inclination in the measured surface profile along one direction in which the magnitude of the dispersion phase changes linearly. The simulations also show that the position of zero phase nearest the position of amplitude maximum in the CVIS almost does not change due to the bias component, although the random phase noise contained in the interference signal changes the slope of the linear component. Measured surface profiles show that the new signal processing achieves highly accurate measurement by the CVIS.

Study on Surface Quality factors of 10B / Al Composite In Ultra - Precision Turning

In order to study the factors of the nano-scale machining precision in the precision physical experiment, different contents level of 10B in the 10B/Al composite materials and high purity Al ultra-precision turning experiments with the natural diamond tool are compared, and the surface morphology of the material is analyzed using scanning electron microscopy and white light interferometer. Then the effects of different cutting parameters and different contents level of 10B on the surface roughness of 10B/Al composites are investigated by orthogonal cutting experiments. The experimental results show that under the same cutting parameters the higher of the content level of 10B, the surface roughness of the composite material is worse; and the cutting surface quality of different contents level of 10B are closer at high rotational speed; under the same condition, increasing the spindle speed and reducing the feed rate can improve the surface roughness of 10B/Al composites.

First Quantitative Determination of Birefringence Variations Induced by Axial-Strain in Polarization Maintaining Fibers

We report what we believe to be the first quantitative determination of strain-induced birefringence variations in polarization maintaining fibers (PMFs). The measurement method is based on the ghost-peak-free scanning white light interferometer based distributed polarization crosstalk analysis technique with two fusion-splicing-induced polarization crosstalk reference peaks, as the strain-induced birefringence variation causes a change of the spacing between the two peaks. Two types of commercial PMF samples (Panda and Bow-tie) are measured and the experimental data are fitted to a linear equation using the least squares method with a high goodness of fit. The strain coefficients of birefringence $1.60 \times {10^{ - 8}}/\mu \varepsilon $ and $1.06 \times {10^{ - 8}}/\mu \varepsilon $ are obtained for the Panda and Bow-tie PMFs, with uncertainties of 4.84% and 7.81%, respectively. Our error analysis indicates that the measurement uncertainty can be readily reduced to be less than 1% if the accuracy of the variable delay line used in the interferometer of the distributed polarization crosstalk analyzer can be improved to be ≤0.6 μm and the temperature variations can be compensated in final calculations by actively monitoring the temperature during the measurement.

Surface topography acquisition method for double-sided near-right-angle structured surfaces based on dual-probe wavelength scanning interferometry.

This paper proposes an approach to measure double-sided near-right-angle structured surfaces based on dual-probe wavelength scanning interferometry (DPWSI). The principle and mathematical model is discussed and the measurement system is calibrated with a combination of standard step-height samples for both probes vertical calibrations and a specially designed calibration artefact for building up the space coordinate relationship of the dual-probe measurement system. The topography of the specially designed artefact is acquired by combining the measurement results with white light scanning interferometer (WLSI) and scanning electron microscope (SEM) for reference. The relative location of the two probes is then determined with 3D registration algorithm. Experimental validation of the approach is provided and the results show that the method is able to measure double-sided near-right-angle structured surfaces with nanometer vertical resolution and micrometer lateral resolution.

Investigation of working pressure on the surface roughness controlling technology of glow discharge polymer films based on the diagnosed plasma

The effects of working pressure on the component, surface morphology, surface roughness, and deposition rate of glow discharge polymer (GDP) films by a trans-2-butene/hydrogen gas mixture were investigated based on plasma characteristics diagnosis. The composition and ion energy distributions of a multi-carbon (C4H8/H2) plasma mixture at different working pressures were diagnosed by an energy-resolved mass spectrometer (MS) during the GDP film deposition process. The Fourier transform infrared spectroscopy (FT–IR), field emission scanning electron microscope (SEM) and white-light interferometer (WLI) results were obtained to investigate the structure, morphology and roughness characterization of the deposited films, respectively. It was found that the degree of ionization of the C4H8/H2 plasma reduces with an increase in the working pressure. At a low working pressure, the C–H fragments exhibited small-mass and high ion energy in plasma. In this case, the film had a low CH3/CH2 ratio, and displayed a smooth surface without any holes, cracks or asperities. While the working pressure increased to 15 Pa, the largest number of large-mass fragments led to the deposition rate reaching a maximum of 2.11 μm h−1, and to hole defects on the film surface. However, continuing to increase the working pressure, the film surface became smooth again, and the interface between clusters became inconspicuous without etching pits.

Utilization of complex-valued signals in a white-light scanning interferometer for accurate measurement of a surface profile.

Complex-valued interference signals (CVISs) of a white-light scanning interferometer (WLSI) and a spectrally resolved interferometer (SRI) are obtained from their real-valued interference signals through Fourier transform. First the phase distribution in the CVIS of the SRI indicates a dispersion phase caused by two sides of unequal length in a cubic beam splitter, and the magnitude of the dispersion phase changes linearly along a horizontal direction of the beam splitter. Next the dispersion phase with a different magnitude is subtracted from the spectral phase in Fourier transform of the CVIS of the WLSI. Through inverse Fourier transform of this spectral distribution, a dispersion-free CVIS is obtained, and the position of zero phase nearest to the position of amplitude maximum provides a surface profile measured accurately with an error less than 4nm after 2π corrections, while a position calculated by the linear component of the spectral phase causes measurement error less than 12nm.

Facile one-step chemical deposition process to fabricate superhydrophobic porous Cu films on Al alloy surface

Herein, we demonstrated a facile one-step fabrication strategy combining both chemical deposition and self-assembly of stearic acid for the fabrication of superhydrophobic porous Cu films on the Al substrates. The scanning electron microscopy and white light interferometer showed that the multi-scale porous structures comprising micron-sized pores and nano-sized fishbone-like dendrites were formed on the sample. X-ray diffraction and Fourier-transform infrared spectrophotometer revealed the deposition of Cu and the grafting process of stearic acid on the films. In addition, the formation mechanism of the superhydrohobic surface was explained, and the processing conditions were investigated to determine their effects on the wettability. The resultant sample revealed superhydrophobicity with a water contact angle of 159 ± 1°, a sliding angle of ca. 4° and great bouncing behaviour of water droplets. Moreover, such a surface exhibited a stable superhydrophobicity even after six weeks of ambient air exposure.

Infrared measurement of the temperature at the tool–chip interface while machining Ti–6Al–4V

The challenges associated with machining titanium alloys (e.g., Ti–6Al–4V) are directly related to high cutting tool temperatures due to the low thermal conductivity of these alloys and the heat generated in the primary shear zone and at the tool–chip interface. Transparent yittrium aluminum garnet (YAG) tools are used in the current study to orthogonally machine a Ti–6Al–4V disk. Although YAG tools are not industrially relevant, they permit the temperature on the tool–chip interface to be measured. These measurements are relevant because they can be used to validate cutting models, which are in-turn used by industry to improve cutting processes. An infrared camera, using a high frame rate (700Hz) and a large field of view (20mm2), observes the tool–chip interface through these tools and measures the temperature distribution and records the chip curl and breakage while cutting with a feed rate of 50μm/rev and cutting speeds between 20m/min and 100m/min. In addition to the temperature measurements, cutting forces are recorded and the chip formation is documented using a high-speed (3kHz) visible-light camera. Results show that radiant temperature increases with speed while the cutting and thrust forces show no significant trend. Analysis of the temperature distribution from one edge of the chip to the other reveals differences from 6 % to 21 %, indicating that caution must be used when performing thermographic measurements of chip temperatures from the side of the cutting zone. Finally, post process measurements are performed using a scanning white-light interferometer to investigate any correlation between the tool condition and cutting temperature. Although the qualitative analysis of some cases appears to reveal a correlation between the condition of the YAG tool and the measured temperature distribution, further work work is required to understand this relationship.

Elimination of dispersion effect in a white-light scanning interferometer by using a spectral analyzer

A general equation of the interference signal of white-light scanning interferometer (WSI) and its Fourier transform are derived. Based on these equations, a new method for elimination of a dispersion effect in WSI is proposed to measure exactly a reflecting surface position. A dispersion phase caused by the two sides of unequal length in a beam splitter is detected with a spectrally resolved interferometer (SRI). A spectral distribution is obtained by using Fourier transform from an interference signal detected with a WSI. The spectral phase of the SRI is subtracted from the spectral phase of the WSI to get a dispersion-free spectral phase, which provides an improved complex-valued interference signal where a position of zero phase is almost equal to a position of maximum amplitude. An accurate measurement is achieved by using the position of zero phase.

Preliminary study on atmospheric-pressure plasma-based chemical dry figuring and finishing of reaction-sintered silicon carbide

Reaction-sintered silicon carbide (RS-SiC) is a research focus in the field of optical manufacturing. Atmospheric-pressure plasma-based chemical dry figuring and finishing, which consist of plasma chemical vaporization machining (PCVM) and plasma-assisted polishing (PAP), were applied to improve material removal rate (MRR) in rapid figuring and ameliorate surface quality in fine finishing. Through observing the processed RS-SiC sample in PCVM by scanning white-light interferometer (SWLI), the calculated peak-MRR and volume-MRR were 0.533 μm/min and 2.78×10−3 mm3/min, respectively. The comparisons of surface roughness and morphology of the RS-SiC samples before and after PCVM were obtained by the scanning electron microscope and atomic force microscope. It could be found that the processed RS-SiC surface was deteriorated with surface roughness rms 382.116 nm. The evaluations of surface quality of the processed RS-SiC sample in PAP corresponding to different collocations of autorotation speed and revolution speed were obtained by SWLI measurement. The optimal surface roughness rms of the processed RS-SiC sample in PAP was 2.186 nm. There were no subsurface damages, scratches, or residual stresses on the processed sample in PAP. The results indicate that parameters in PAP should be strictly selected, and the optimal parameters can simultaneously obtain high MRR and smooth surface.

Improved phase-shifting diffraction interferometer for microsphere topography measurements

In this study, an improved phase-shifting diffraction interferometer for measuring the surface topography of a microsphere is developed. A common diode-pumped solid state laser is used as the light source to facilitate apparatus realization, and a new polarized optical arrangement is designed to filter the bias light for phase-shifting control. A pinhole diffraction self-calibration method is proposed to eliminate systematic errors introduced by optical elements. The system has an adjustable signal contrast and is suitable for testing the surface with low reflectivity. Finally, a spherical ruby probe of a coordinate measuring machine is used as an example tested by the new phase-shifting diffraction interferometer system and the WYKO scanning white light interferometer for experimental comparison. The measured region presents consistent overall topography features, and the resulting peak-to-valley value of 84.43 nm and RMS value of 18.41 nm are achieved. The average roughness coincides with the manufacturer’s specification value.

Traceable methods for vertical scale characterization of dynamic stroboscopic scanning white-light interferometer measurements.

Stroboscopic scanning white-light interferometry (SSWLI) can be used for 3D imaging of oscillating samples. It allows measurement of micrometer to millimeter size samples with nanometer vertical precision. Unlike coherent light source systems the SSWLI can measure unambiguously samples with vertical steps. Traceability of the vertical displacement measurement is important with SSWLI since the height measurement is not related to any specific monochromatic light wavelength. For static measurements SSWLI can be calibrated using, e.g., step height standards, but to characterize dynamic measurements traceable samples with accurate motion are needed due to error sources related to the frequency response of the SSWLI. In the presented method, SSWLI measurements are performed on dynamic transfer standards, which are characterized using a laser interferometer traceable to the SI meter. In this work dynamic SSWLI measurements at subkilohertz to 10.7kHz frequencies with micrometer range displacement are characterized. The expanded uncertainty (k=2) was 9.6nm for a measured displacement of 766nm at 10.7kHz. The methodology can be used up to the frequency limits of the SSWLI using suitable samples.

Efficient processing of reaction-sintered silicon carbide by anodically oxidation-assisted polishing

Reaction-sintered silicon carbide (RS-SiC) is a promising optical material for the space telescope systems. Anodically oxidation-assisted polishing is a method to machine RS-SiC. The electrolyte used in this study is a mixture of hydrogen peroxide (H2O2) and hydrochloric acid (HCl), and the oxidation potential has two modes: constant potential and high-frequency-square-wave potential. Oxide morphologies are compared by scanning electron microscope/energy dispersive x-ray spectroscopy and scanning white-light interferometer. The results indicate that anodic oxidation under constant potential can not only obtain a relatively smooth surface but also be propitious to obtain high material removal rate. The oxidation depth in anodic oxidation under constant potential is calculated by comparing surface morphologies before and after hydrofluoric acid etching. The theoretical oxidation rate is 5.3 nm/s based on the linear Deal–Grove model. Polishing of the oxidized RS-SiC is conducted to validate the machinability of the oxide layer. The obtained surface roughness root-mean-square is around 4.5 nm. Thus, anodically oxidation-assisted polishing can be considered as an efficient method, which can fill the performance gap between the rough figuring and fine finishing of RS-SiC. It can improve the machining quality of RS-SiC parts and promote the application of RS-SiC products.

Estimation of measurement uncertainty caused by surface gradient for a white light interferometer

Although the scanning white light interferometer can provide measurement results with subnanometer resolution, the measurement accuracy is far from perfect. The surface roughness and surface gradient have significant influence on the measurement uncertainty since the corresponding height differences within a single CCD pixel cannot be resolved. This paper presents an uncertainty estimation method for estimating the measurement uncertainty due to the surface gradient of the workpiece. The method is developed based on the mathematical expression of an uncertainty estimation model which is derived and verified through a series of experiments. The results show that there is a notable similarity between the predicted uncertainty from the uncertainty estimation model and the experimental measurement uncertainty, which demonstrates the effectiveness of the method. With the establishment of the proposed uncertainty estimation method, the uncertainty associated with the measurement result can be determined conveniently.