Scanning white-light interferometer for measurement of the thickness of a transparent oil film on water

Highly precise thickness measurement of multilayer films based on the cross-correlation algorithm using a widely tunable MG-Y laser

Inspired by the demodulation algorithm of Fabry-Perot composite sensors in the field of fiber-optic sensing, this paper proposes a method based on a widely tunable modulated grating Y-branch (MG-Y) laser combined with the cross-correlation algorithm to achieve a highly precise measurement of the optical thickness of each layer of a multilayer optical sample. A sample consisting of a double glass stack was selected, and the interference spectrum of the stacked sample was acquired using a widely tunable MG-Y laser. A fast Fourier transform (FFT) algorithm combined with a finite impulse response (FIR) bandpass filter was utilized to separate the different frequency components of the multilayer optical sample. The normalized spectra of each layer were reconstructed using the Hilbert transform. Subsequently, a cross-correlation algorithm was employed to process the normalized spectrum and determine the optical thickness of each layer with high precision. The samples were measured at predetermined locations, with 150 consecutive measurements performed to assess the repetition of the thickness. The standard deviation of these measurements was found to be lower than 1.5 nm. The results show that the cross-correlation algorithm is advantageous in the optical thickness measurement of multilayer films.

Observation of Coherent Perfect Absorption in Oil Film on Water Surface and Sensitive Detection of Refractive Index Anisotropy in the Film

Sharp reflection dips of 50% were observed when white light was incident from the side of a cell on a 1 μm thick film of silicone oil (polydimethylsiloxane, PDMS, nearly transparent in visible light, with the extinction coefficient κ ≈ 0.0001) above a water surface in the cell so that the total reflection condition was satisfied at the oil-air interface. This is the first observation of a coherent perfect absorption (CPA) phenomenon in liquid. The experimental results can be reproduced by the Fresnel reflectance of the monolayer film, but the wavelength positions at which the dip appears for s-polarized and p-polarized light are reversed if the refractive index of the oil film is assumed to be isotropic. The experimental results were correctly reproduced by assuming that the extraordinary-ray refractive index (light polarized perpendicular to the interface) is 1% larger than the ordinary-ray refractive index (light polarized parallel to the interface). This indicates that the polarization dependence of the CPA phenomenon is extremely sensitive to the difference between the in-plane and out-of-plane refractive indices of the thin film.

Fabry-Perot interferometric sensor demodulation system utilizing multi-peak wavelength tracking and neural network algorithm

For FPI sensor demodulation systems to be used in actual engineering measurement, they must have high performance, low cost, stability, and scalability. Excellent performance, however, necessitates expensive equipment and advanced algorithms. This research provides a new absolute demodulation system for FPI sensors that is high-performance and cost-effective. The reflected light from the sensor was demultiplexed into distinct channels using an array waveguide grating (AWG), with the interference spectrum features change translated as the variation of the transmitted intensity in each AWG channel. This data was fed into an end-to-end neural network model, which was utilized to interrogate multiple interference peaks' absolute peak wavelengths simultaneously. This architecturally simple network model can achieve remarkable generalization capabilities without training large-scale datasets using an appropriate data augmentation strategy. Experiments show that in simultaneous multi-wavelength and cavity length interrogations, the proposed system has the precision of up to ± 14 pm and ± 0.07 µm, respectively. The interrogation resolution can theoretically reach the pm level benefit from the neural network method. Furthermore, the system's outstanding demodulation repeatability and suitability were demonstrated. The system is expected to provide a high-performance and cost-effective, reliable solution for practical engineering applications.

In situ ground settlement sensor for oil-tank monitoring by combining a fiber-optic low-coherent interferometry with a fine mechanical design

An in situ robust ground settlement (IR-GS) sensor was designed to meet the requirements for oil-tank health monitoring by combining a low-coherent fiber-optic interferometry with a fine mechanical spline shaft. A floating mirror was mounted on the shaft and moved up and down along with the liquid surface. The liquid-contained chambers were hydraulically connected at the bottom by using a liquid-filled tube. The liquid level inside each chamber was initially at equal level. One of the chambers was fixed on a steady ground point, which was chosen in a surveying point of view and served as a reference. The others were distributed around an oil tank and separated the tank's perimeter into eight equal spans. Thereby, the health states of the oil tank were able to be evaluated based on these sensing results. Interrogation of the sensor was employed via a low-coherent fiber-optic Michelson interferometer. One path of the interferometer was composed by the floating mirror, whereupon a light was reflected. The other path was projected to a mirror that was fixed on a stepping motor. Therefore, the corresponding liquid level could be optically surveyed. Differential settlements between each chamber and the reference served as a measure of how much the liquid level was changed from its initials. Experimental tests demonstrated that this IR-GS design, with the optimized shape and weights of the spline shaft, could overcome the error caused by dust, hysteresis, temperature, etc. and meet the practical requirement in the accuracy of ±0.5mm. A practical application was carried out, and its long-term stability has been proved.

Implementation of a Load Sensitizing Bridge Spherical Bearing Based on Low-Coherent Fiber-Optic Sensors Combined with Neural Network Algorithms

Low-coherent fiber-optic sensors combined with neural network algorithms were designed to carry out a load-sensitizing spherical bearing. Four sensing fibers were wound around the outside of the pot support of the spherical bearing uniformly deployed from upper to bottom. The upper three were configured in a distributed way to respond to the applied load as a function of the three strain sensors. The bottom one was employed as a temperature compensation sensor. A loading experiment was implemented to test the performance of the designed system. The results showed that there was a hysteresis in all the three sensors between loading and unloading process. The neural network algorithm is proposed to set up a function of the three sensors, treated as a set of input vectors to establish the input-output relationship between the applied loads and the constructed input vectors, in order to overcome the hysteresis existing in each sensor. An accuracy of 6% for load sensing was approached after temperature compensation.

Simultaneous measurement of the surface shape and thickness for an optical flat with a wavelength-tuning Fizeau interferometer with suppression of drift error

Two types of phase-shifting algorithms were developed for simultaneous measurement of the surface and thickness variation of an optical flat. During wavelength tuning, phase-shift nonlinearity can cause a spatially nonuniform error and spatially uniform DC drift error. A 19-sample algorithm was developed that eliminates the effect of the spatially uniform error by expanding the 17-sample algorithm with characteristic polynomial theory. The 19-sample algorithm was then altered to measure the surface shape of the optical flat by rotation of the characteristic diagram. The surface shape and thickness variation were measured with these two algorithms and a wavelength-tuning Fizeau interferometer.

Performance of the fiber-optic low-coherent ground settlement sensor: From lab to field

A fiber-optic low-coherent interferometry sensor was developed to measure the ground settlement (GS) in an accuracy of the micrometer. The sensor combined optical techniques with liquid-contained chambers that were hydraulically connected together at the bottom by using a water-filled tube. The liquid surface inside each chamber was at the same level initially. The optical interferometry was employed to read out the liquid level changes, which following the GS happened at the place where the chamber was put on and, thereby, the GS information was calculated. The laboratory effort had demonstrated its potential in the practical application. Here, the denoising algorithms on the measurement signal were carried out based on the specific environment to ensure the accuracy and stability of the system in field applications. After that, we extended this technique to the high-speed railway. The 5-days continuous measurement proved that the designed system could be applied to monitor the GS of the high-speed railway piers and approached an accuracy of ±70 μm in the field situation with a reference compensation sensor. So the performance of the sensor was suitable to the GS monitoring problem in the high-speed railway. There, the difficulties were to meet the monitoring requirement of both a large span in space and its quite tiny and slow changes.

Tilt performance of the ground settlement sensor configured in a fiber-optic low-coherent interferometer

Ground settlement (GS) is one of the causes that destroy the durability of reinforced concrete structures. It could lead to a deterioration in the structural basement and increase the risk of collapse. The methods used for GS monitoring were mostly electronic-based sensors for reading the changes in resistance, resonant frequencies, etc. These sensors often bear low accuracy in the long term. Our published work demonstrated that a fiber-optic low-coherent interferometer configured in a Michelson interferometer was designed as a GS sensor, and a micro-meter resolution in the room environment was approached. However, the designed GS sensor, which in principle is based on a hydraulic connecting vessel, has to suffer from a tilt degeneration problem due to a strictly vertical requirement in practical installment. Here, we made a design for the GS sensor based on its robust tilt performance. The experimental tests show that the sensor can work well within a ±5° tilt. This could meet the requirements in most designed GS sensor installment applications.

Surface measurement of indium tin oxide thin film by wavelength-tuning Fizeau interferometry

Indium-tin oxide (ITO) thin films have been widely used in displays such as liquid crystal displays and touch panels because of their favorable electrical conductivity and optical transparency. The surface shape and thickness of ITO thin films must be precisely measured to improve their reliability and performance. Conventional measurement techniques take single point measurements and require expensive systems. In this paper, we measure the surface shape of an ITO thin film on top of a transparent plate using wavelength-tuning Fizeau interferometry. The surface shape was determined by compensating for the phase error introduced by optical interference from the thin film, which was calculated using the phase and amplitude distributions measured by wavelength-tuning. The proposed measurement method achieved noncontact, large-aperture, and precise measurements of transparent thin films. The surface shape of the sample was experimentally measured to an accuracy of 5.13 nm.

Fiber-optic ground settlement sensor based on low-coherent interferometry

Ground settlement (GS) monitoring is a basic prerequisite in civil engineering. A commercialized instrument to meet this requirement has been available with millimeter accuracy. Major difficulties to improve this to micrometer scale, which are needed in special cases such as in high-speed railways, are challenged by the long stability of the sensor in the condition of the extremely slow settlement. A fiber-optic GS methodology was proposed by using a scanning low-coherent Michelson interferometer. One of the paths of the interferometer is formed by the liquid surface, and therefore the readout of the interferometer can make the measurement of the surface approach a micrometer scale. The liquid-contained chambers are hydraulically connected together at the bottom by using a water-filled tube. The liquid surface inside each chamber is at the same level initially. One of the chambers is located on stable ground or at a point that can be easily surveyed, too. The others are located at the points where settlement or heave is to be measured. Differential settlement, or heave, between the chambers will result in an apparent rise or fall of the liquid level, which biased the initial equal status. The experimental results demonstrated that the best accuracy of ±20  μm for GS monitoring was obtained with a reference compensation sensor.

Designing an optical set-up of differential laser triangulation for oil film thickness measurement on water

Based on the differential laser triangulation principle, an optical system configuration for measuring the oil film thickness on water is designed and developed. A semiconductor laser of 650 nm wavelength with the maximum power of 5 mW is used as a light source, the magnification of the imaging system is 1.4; the range of the measurement is 0.1 mm-10 mm; the resolution is 2.3 μm and the measurement accuracy is 10 μm theoretically. Experiments are conducted with block gauges and feeler gauges, and the experimental results, with absolute error less than ±25 μm and the maximal measurable thickness 12 mm, indicate that this system presented in this paper can fulfill high accuracy.

Holographic interferometry of oil films and droplets in water with a single-beam mirror-type scheme

Application of single-beam reflective laser optical interferometry for oil films and droplets in water detection and characterization is discussed. Oil films can be detected by the appearance of characteristic interference patterns. Analytical expressions describing intensity distribution in these interference patterns allow determination of oil film thickness, size of oil droplets, and distance to the oil film from the observation plane. Results from these analyses indicate that oil spill aging and breakup can be monitored in real time by analyzing time-dependent holographic fringe patterns. Interferometric methods of oil spill detection and characterization can be automated using digital holography with three-dimensional reconstruction of the time-changing oil spill topography. In this effort, the interferometric methods were applied to samples from Chevron oil and British Petroleum MC252 oil obtained during the Deep Water Horizon oil spill in the Gulf of Mexico.

Simultaneous interferometric measurement of corrosive or demineralizing bacteria and their mineral interfaces

Here, we report simultaneous surface profile measurements of several bacterial species involved in microbially influenced corrosion and their solid-surface interfaces by using vertical scanning interferometry. The capacity to nondestructively quantify microscale topographic changes beneath a single bacterium without its removal offers a unique opportunity to examine in vivo microbe-surface interactions.