Scanning Interferometer Research Articles

The Calatabiano landslide (southern Italy): preliminary GB-InSAR monitoring data and remote 3D mapping

On 24 October 2015, following a period of heavy rainfall, a landslide occurred in the Calatabiano Municipality (Sicily Island, Southern Italy), causing the rupture of a water pipeline supplying water to the city of Messina. Following this event, approximately 250,000 inhabitants of the city suffered critical water shortages for several days. Consequently, on 6 November 2015, a state of emergency was declared (O.C.D.P. 295/2015) by the National Italian Department of Civil Protection (DPC). During the emergency management phase, a provisional by-pass, consisting of three 350-m long pipes passing through the landslide area, was constructed to restore water to the city. Furthermore, on 11 November 2015, a landslide remote-sensing monitoring system was installed with the following purposes: (i) analyse the landslide geomorphological and kinematic features in order to assess the residual landslide risk and (ii) support the early warning procedures needed to ensure the safety of the personnel involved in the by-pass construction and the landslide stabilization works. The monitoring system was based on the combined use of Ground-Based Interferometric Synthetic Aperture Radar (GB-InSAR) and terrestrial laser scanning (TLS). In this work, the preliminary results of the monitoring activities and a remote 3D map of the landslide area are presented.

Measurement of the characteristics of ultrahigh-frequency optical modulators by a scanning interferometer

Results of studying an original method of the modulated optical radiation spectrum analysis and testing a simple detector based on the Fabry–Perot interferometer are reported. The theoretical analysis of detector operation is compared with model experiment results. A possibility of using this analyzer for measuring the characteristics of ultrahigh-frequency modulators of optical radiation operating at modulation frequencies from 10 to 100 GHz and higher is noted.

RF plasma based selective modification of hydrophilic regions on super hydrophobic surface

Selective modification and regional alterations of the surface property have gained a great deal of attention to many engineers. In this paper, we present a simple, a cost-effective, and amendable reforming method for disparate patterns of hydrophilic regions on super-hydrophobic surfaces. Uniform super-hydrophobic layer (Contactangle; CA>150°, root mean square (RMS) roughness ∼0.28nm) can be formed using the atmospheric radio frequency (RF) plasma on top of the selective hydrophilic (CA∼70°, RMS roughness ∼0.34nm) patterns imprinted by electrohydrodynamic (EHD) jet printing technology with polar alcohols (butyl carbitol or ethanol). The wettability of the modified surface was investigated qualitatively utilizing scanning electron microscopy (SEM), atomic force microscopy (AFM), and wavelength scanning interferometer (WSI). Secondary ion mass spectroscopy (SIMS) analysis showed that the alcohol addiction reaction changed the types of radicals on the super-hydrophobic surface. The wettability was found to depend sensitively on chemical radicals on the surface, not on surface morphology (particle size and surface roughness). Furthermore, three different kinds of representative hydrophilic samples (polystyrene nano-particle aqueous solution, Salmonella bacteria medium, and poly(3,4-ethylenediocythiophene) ink) were tested for uniform deposition onto the desired hydrophilic regions. This simple strategy would have broad applications in various research fields that require selective deposition of target materials.

A 17 GHz molecular rectifier.

Molecular electronics originally proposed that small molecules sandwiched between electrodes would accomplish electronic functions and enable ultimate scaling to be reached. However, so far, functional molecular devices have only been demonstrated at low frequency. Here, we demonstrate molecular diodes operating up to 17.8 GHz. Direct current and radio frequency (RF) properties were simultaneously measured on a large array of molecular junctions composed of gold nanocrystal electrodes, ferrocenyl undecanethiol molecules and the tip of an interferometric scanning microwave microscope. The present nanometre-scale molecular diodes offer a current density increase by several orders of magnitude compared with that of micrometre-scale molecular diodes, allowing RF operation. The measured S11 parameters show a diode rectification ratio of 12 dB which is linked to the rectification behaviour of the direct current conductance. From the RF measurements, we extrapolate a cut-off frequency of 520 GHz. A comparison with the silicon RF-Schottky diodes, architecture suggests that the RF-molecular diodes are extremely attractive for scaling and high-frequency operation.

Influence of etch pit development on the surface area and dissolution kinetics of the orthoclase (001) surface

The (001) orthoclase surface was dissolved at 180°C and at far from equilibrium conditions with an alkaline solution (pH180°C=9) in a titanium open flow reactor. Vertical scanning interferometer (VSI) and atomic force microscope (AFM) surface monitoring were periodically used during the reaction process in order to quantify the surface topography evolution. The dissolution of the (001) orthoclase face occurs with the formation of diamond shape etch pits. Diamond pit diagonals are parallel to the [100] and [010] axes, and the pit walls are parallel to (6 5 6), 65¯6, (6¯511) and 6¯5¯11 planes. The etch pit size and global surface retreat of the (001) surface were found to increase linearly with time. Based on statistical treatments of etch pit development monitoring by AFM, we designed a numerical model aimed at reproducing and quantifying the total surface evolution. Numerical results show that the stabilization of etch pits doubles the calculated dissolution rate, partly due to the intrinsically higher reactivity of pit walls, consistent with a dissolution process in line with the periodic bond chain (PBC) theory. In addition, normalizing the dissolution rate by the initial surface area of the (001) orthoclase surface induces a 20% overestimation of the calculated dissolution rate, while the total surface area of the dissolving face reaches a steady state after a few days of reaction. Additional simulations conducted to assess the impact of defect parameters revealed a weak dependence of the dissolution rate on dislocation density, consistent with previous experimental observations. Overall, the combined effect of the various defect parameters does not affect the dissolution rate by more than an order of magnitude, and probably contributes to a moderate extent to the dispersion of mineral dissolution rate data reported in the literature.

Image Reconstruction and Evaluation: Applications on Micro-Surfaces and Lenna Image Representation

This article develops algorithms for the characterization and the visualization of micro-scale features using a small number of sample points, with the goal of mitigating the measurement shortcomings, which are often destructive or time consuming. The popular measurement techniques that are used in imaging of micro-surfaces include the 3D stylus or interferometric profilometry and Scanning Electron Microscopy (SEM), where both could represent the micro-surface characteristics in terms of 3D dimensional topology and greyscale image, respectively. Such images could be highly dense; therefore, traditional image processing techniques might be computationally expensive. We implement the algorithms in several case studies to rapidly examine the microscopic features of micro-surface of Microelectromechanical System (MEMS), and then we validate the results using a popular greyscale image; i.e., “Lenna” image. The contributions of this research include: First, development of local and global algorithm based on modified Thin Plate Spline (TPS) model to reconstruct high resolution images of the micro-surface’s topography, and its derivatives using low resolution images. Second, development of a bending energy algorithm from our modified TPS model for filtering out image defects. Finally, development of a computationally efficient technique, referred to as Windowing, which combines TPS and Linear Sequential Estimation (LSE) methods, to enhance the visualization of images. The Windowing technique allows rapid image reconstruction based on the reduction of inverse problem.

All-fiber upconversion high spectral resolution wind lidar using a Fabry-Perot interferometer.

An all-fiber, micro-pulse and eye-safe high spectral resolution wind lidar (HSRWL) at 1.5 μm is proposed and demonstrated by using a pair of upconversion single-photon detectors and a fiber Fabry-Perot scanning interferometer (FFP-SI). In order to improve the optical detection efficiency, both the transmission spectrum and the reflection spectrum of the FFP-SI are used for spectral analyses of the aerosol backscatter and the reference laser pulse. Taking advantages of high signal-to-noise ratio of the detectors and high spectral resolution of the FFP-SI, the center frequencies and the bandwidths of spectra of the aerosol backscatter are obtained simultaneously. Continuous LOS wind observations are carried out on two days at Hefei (31.843 °N, 117.265 °E), China. The horizontal detection range of 4 km is realized with temporal resolution of 1 minute. The spatial resolution is switched from 30 m to 60 m at distance of 1.8 km. In a comparison experiment, LOS wind measurements from the HSRWL show good agreement with the results from an ultrasonic wind sensor (Vaisala windcap WMT52). An empirical method is adopted to evaluate the precision of the measurements. The standard deviation of the wind speed is 0.76 m/s at 1.8 km. The standard deviation of bandwidth variation is 2.07 MHz at 1.8 km.

Cr:YAG passively Q-switched single-frequency Nd:YVO_4 ring cavity laser

We design and demonstrate a thermally stable four-mirror discrete-component planar ring cavity for Q-switched laser output. An 880 nm fiber-coupled laser diode is used to pump a-cut Nd:YVO4 directly. A magnetic rotation glass (MR3-2) inserted into a permanent cylinder magnet, in combination with a half-wave plate and the polarized emission properties of Nd:YVO4, is used to obtain unidirectional laser operation. With this setup, for a pump power of 5 W, 2.75 W of continuous 1064 nm single-frequency laser output is obtained. Next, a Cr:YAG with small signal transmittance of 75% is inserted in between the two concave mirrors where the beam waist is located. Consequently, a passive Q-switching single-frequency linear polarization pulse with an average power of 1.02 W, pulse width of 60 ns, frequency of 21 kHz, peak power of 810 W, and single-pulse energy of 48 μJ is obtained. Further, we also propose a new method of using a Fabry–Perot (FP) scanning interferometer to observe high-repetition rate Q-switched pulse output longitudinal-mode signals.

Group delay dispersion measurements in the mid-infrared spectral range of 2-20 µm.

We present two measurement devices which both allow the direct measurement of the group delay (GD) and group delay dispersion (GDD) of laser optics, covering the near- and mid-infrared (MIR) spectral range from 2 to 20 µm (500-5,000 cm-1). Two different kinds of devices were developed to measure the GDD of multilayer interference coatings. One is a resonant scanning interferometer (RSI) and the other is a white light interferometer (WLI). The WLI is also capable of measuring the GDD in transmission, for instance of bulk material. GDD measurements of a high dispersive mirror for wavelengths from 2.0 to 2.15 µm and one of a multilayer mirror from 8.5 to 12.0 µm are presented. A measurement of a zinc selenide (ZnSe) substrate in transmission was made with the WLI demonstrating the full bandwidth of the device from 1.9 to 20 µm. The comparison of all measurements with their related theoretical values shows a remarkable correspondence.

Brillouin optical time-domain reflectometry using up-conversion single-photon detector

A direct-detection Brillouin optical time-domain reflectometry (BOTDR) using an up-conversion photon-counting detector and an all-fiber structure Fabry–Perot scanning interferometer is demonstrated with shot-noise limited performance. Taking advantage of ultra-low noise equivalent power of the up-conversion photon-counting detector and high spectral resolution of the interferometer, the Brillouin spectra along a polarization maintaining fiber (PMF) are analyzed in the optical frequency domain directly. In contrast with heterodyne BOTDR, photon-counting BOTDR has better EM compatibility and faster speed in data processing. In experiments, using peak input power of 20dBm, temperature profile along a 9km PMF is retrieved according to the Brillouin shifts, with spatial/temporal resolution of 2m/15s. The precision is 0.7°C at the leading end and 1.2°C at the trailing end.

Design of a New Microcontroller-Based Vernier Fringe Counter for Interferometric Measurement of Laser Wavelength

This paper presents a new electronic Vernier fringe counter for the wavelength measurement of a diode laser in a scanning interferometer. The system is intended to be a low cost alternative to commercial systems used for gauge block calibration. The counter stage and phase coincidence detector are made with a microcontroller and high-speed CMOS logic to achieve the required resolution. The microcontroller also can synchronize with other elements to make a fully automated measurement system. This electronic design improves the resolution of the electronic counters in the previous designs.

Pulsed laser linewidth measurement using Fabry–Pérot scanning interferometer

We apply the Fabry–Pérot (FP) scanning interferometer, which is normally used for continuous wave (CW) laser linewidth measurement, for the measurement of pulsed laser linewidths. We analyze the response of the FP interferometer to continuous and pulsed lasers, also different detectors and suitable oscilloscope test parameters being selected for the measurement. For low-speed detectors, we set our oscilloscope to 1-MΩ impedance matching in the sampling mode. For high-speed detectors, we use the same oscilloscope test parameters or 50-Ω impedance matching with the peak-detection mode. With our setup, we achieve on-line linewidth measurement of a nanosecond pulsed laser for single-longitudinal and multi-longitudinal modes. Meanwhile, the linewidth measurement at different pulse repetition rates as low as 1Hz is also demonstrated. The possibility of detecting the linewidth for pulse widths larger than 100ps in the 1-μm band is discussed. The application range of the FP scanning interferometer is thus extended to the measurement of pulsed laser linewidths.

A method of suppressing vibration for high precision broadband laser frequency scanning interferometry

In the paper we study the method of reducing environmental influence in broadband laser frequency scanning interferometer. Target displacement caused by vibration will result in Doppler shift in measurement beat frequency. The extent of frequency shift is usually much larger than the actual target displacement. So the direct calculating of the target distance will cause ranging precision to decrease. In this paper, we establish a model for the influence of environmental vibration on the measurement and analyze the influence of the vibration on ranging result. To suppress the vibration effect, the Kalman filter is combined with the overlapping Chirp Z transform to estimate the measured data. The general process is described as follows. Firstly, the tuning nonlinearity will lead to the frequency spectrum broadening, so this paper we use the frequency sampling method to correct the frequency modulation nonlinearity of the laser. The frequency sampling method has the advantages of high speed and high precision. Secondly, the measurement system has the dispersion mismatch effect due to the use of broadband frequency swept laser. To solve this problem, the influence of the dispersion on the measurement is reduced by using the method of dispersion chirp slope calibration. Thirdly, because of the long frequency sweep period of the external cavity swept frequency laser, the vibration process of the target cannot be recorded in real time by single sweep, so in this paper we propose segmenting the measurement signal of single sweep and conducting Chirp Z transform to calculate target distance at different times. Compared with FFT algorithm, Chirp Z transform can achieve arbitrary narrow band spectrum subdivision, with the advantages of high accuracy and fast frequency measurement. Lastly, the Chirp Z ranging result is further combined with the method of Kalman filter to estimate the state of the target distance information. The experimental results indicate that the measurement standard is reduced from 185.4 μm to 9 μm by the proposed method. Without changing the absolute distance measuring device of broadband laser frequency scanning interferometer, this method provides a solution for further improving the ranging accuracy in the vibration environment, and reduces the complexity and cost of the device.

An international comparison of surface texture parameters quantification on polymer artefacts using optical instruments

An international comparison of optical instruments measuring polymer surfaces with arithmetic mean height values in the sub-micrometre range has been carried out. The comparison involved sixteen optical surface texture instruments (focus variation instruments, confocal microscopes and coherent scanning interferometers) from thirteen research laboratories worldwide. Results demonstrated that: (i) Agreement among different instruments could be achieved to a limited extent; (ii) standardised guidelines for uncertainty evaluation of areal surface parameters are needed for users; (iii) it is essential that the performance characteristics (and especially the spatial frequency response) of an instrument is understood prior to a measurement.

Modeling and de-embedding the interferometric scanning microwave microscopy by means of dopant profile calibration

This paper presents the full modeling and a methodology for de-embedding the interferometric scanning microwave microscopy measurements by means of dopant profile calibration. A Si calibration sample with different boron-doping level areas is used to that end. The analysis of the experimentally obtained S11 amplitudes based on the proposed model confirms the validity of the methodology. As a specific finding, changes in the tip radius between new and used tips have been clearly identified, leading to values for the effective tip radius in the range of 45 nm to 85 nm, respectively. Experimental results are also discussed in terms of the effective area concept, taking into consideration details related to the nature of tip-to-sample interaction.

An effort in planarising microwave plasma CVD grown polycrystalline diamond (PCD) coated 4 in. Si wafers

Large area CVD grown diamond coatings should have very smooth surface in many of its applications, like microwave power transmission windows etc. Combination of mechanical and chemical forces during polishing helps to achieve desired surface roughness of the polycrystalline diamond (PCD) coatings. Authors report the variation of pressure, slurry feeding rate, addition of chemicals and the time of polishing to observe the efficiency of chemo-mechanical polishing (CMP) technique in planarising CVD diamond material grown over 100mm diameter silicon wafers. PCD were found to be polished by bringing down the as-grown surface roughness from 1.62µm to 46nm at some points on large areas. One coherence scanning interferometer was used to check the roughness at different stages of CMP. Raman spectroscopy was used to evaluate the polished PCD samples in terms of their quality, internal stress at different positions on the same wafer surface after CMP process. It was found that the well polished regions were of better quality than the less polished regions on the same wafer surface. But due to coating non-uniformity of the deposited PCD grown by microwave plasma CVD over large area, CMP could not produce uniform surface roughness over the entire 100mm diameter wafer surface. We concluded that CMP could effectively but differentially polish large area PCD surfaces, and further process improvements were needed.

Characterization of surface topography of a newly developed metrological gloss scale

In the European Joint Research Project ‘Multidimensional Reflectometry for Industry’, a new gloss scale was developed with the aim to represent different levels of gloss, hue, roughness, and refractive indices. In this paper, the surfaces of six selected samples were thoroughly investigated using various measuring techniques in order to verify the outcome of the novel manufacturing processes in terms of distinct levels as well as types of surface roughness. The aim of the evaluation was to capture surface structures in different wavelength intervals utilizing a confocal microscope, a coherence scanning interferometer, and an atomic force microscope. Power spectral density functions were also calculated from the measurements and used to determine suitability of techniques for different roughness scales. The measurements show that the expected surface characteristics as well as different RMS roughness values are intimately connected to the perceived glossiness.

Errors and uncertainty in the topography gained via frequency-domain analysis

Frequency Domain Analysis of measurement results from coherence scanning interferometers provides an estimate of the topography of the surface scattering the light, with claims of nm levels of accuracy being achieved. In the following work we use simulations of the measurement result to show that the limited set of spatial frequencies passed by the instrument can lead to errors in excess of 200 nm for surfaces with curvature. In addition we present a method that takes the uncertainty in the amplitude and phase of each element of the transfer function and provides the upper and lower limit on the location of the surface provided by the Frequency Domain Analysis method. This provides an idea of the level of accuracy that the spatial frequency components must be known to in order to reproduce curved surfaces well.

Impact study of anisotropic optical fibers winding with different tension value on the H-parameter invariance degree

Subject of Research. We have investigated the effect of anisotropic optical fibers winding with an elliptical sheath subjecting to stress on the H-parameter invariance degree. This type of optical fiber is used in the manufacture of fiber loop in fiber-optic gyroscopes. Method of Research. The method of research is based on the application of Michelson polarization scanning interferometer as a measuring device. Superluminescent diode with a central wavelength of 1575 nm and a half-width of the spectrum equal to 45 nm is used as a radiation source. The studies were carried out with anisotropic optical fiber with 50 m long elliptical sheath subjecting to stress. The fiber was wound with one layer turn to turn on the coil with a diameter of 18 cm, which is used in the design of fiber-optic gyroscope. The tension force of the optical fiber was controlled during winding on a special machine. Main Results. It was found that at the increase of tension force from 0.05 N to 0.8 H the value of H -parameter increases from 7×10-6 1/m up to 178×10-6 1/m, respectively; i.e. the coupling coefficient of orthogonal modes in the test fiber is being increased. Thus, it is necessary to consider the longitudinal tension force of fiber in the design and manufacture of the fiber-optic sensors of high accuracy class: the less the fiber winding power, the higher invariance degree of distributed H -parameter. The longitudinal tension force of anisotropic optical fiber with elliptical sheath subjecting to stress equal to 0.2 N is recommended in the process of designing fiber-optic gyroscopes. Practical Relevance. The proposed method of Michelson scanning interferometer is usable in the production process for quality determination of the optical fiber winding: no local defects, value controlling of fiber H -parameter.

New Textures of Chocolate Are Formed by Polymorphic Crystallization and Template Effects: Velvet Chocolate

We report new types of chocolate textures with a soft mouth-feel (velvet effect) formed by controlling the crystallization and polymorphic transformation of cocoa butter (CB) with thermal treatment and template effects. A quick spray of molten chocolate liquor on chilled substrates (normally formed chocolate or metal) caused the crystallization of metastable forms of CB, which transformed to a stable form during subsequent heating processes. Cryo-scanning electron microscopy and confocal interferometric scanning microscopy were employed to observe the surface structures of the velvet chocolate. We characterized the domain sizes of the βV form for the CB crystals in velvet chocolate, which were much smaller and exhibited a lower melting temperature and softer mouth-feel than those in normally tempered chocolate. Polymorphic crystallization and transformation of CB were monitored in situ by X-ray diffraction by changing the temperatures of the substrates. It was obvious that the velvet effect was induced solely by decreasing the temperature of the substrates below 16 °C, because crystallization of the metastable γI form and subsequent transformation to βV of CB are prerequisites for forming velvet chocolate. The chocolate substrate was much more effective than the metal substrate in forming the velvet chocolate because of the template effect.