Wavelength Of Fringes Research Articles

Absolute phase recovery with multiple-wavelength fringes

In a recent work, a technique was proposed to recover the absolute phase maps of fringe patterns with two or three selected fringe wavelengths with the aim to improve anti-error capability. To further increase the anti-error capability associated with the phase unwrapping operation, multiple fringe patterns with different wavelengths may also be used. In this paper, we present a general rule to select the wavelengths and the strategy to increase phase error tolerance bound. Based on this rule, it is concluded that to maximize the error tolerance, four-wavelength method should be selected, where the maximum upper bound of the tolerable phase error is π/6. To minimize the required number of fringe patterns, three-wavelength method with three-step PSP should be selected, that is exactly the case the previous paper has demonstrated. These theoretical conclusions are verified by experimental results.

Calibration of Mie scattering imaging for microbubble measurement in hydrodynamic test facilities

Calibration of the Mie Scattering Imaging (MSI) technique for microbubble size and concentration measurement in hydrodynamic test facilities is investigated. Monodisperse bubbles are generated by a microfluidic ‘T’ junction, and individual bubbles simultaneously imaged with shadowgraphy and MSI. Nominal bubble diameters between 30 and $$150\,\upmu{\hbox {m}}$$ were tested. The influence of fringe uniformity and intensity for each polarisation on measurement precision was investigated. Parallel polarisation was chosen over perpendicular for its more uniform spacing despite the lower intensity. The linear relation between fringe wavelength and bubble diameter was demonstrated at a measurement angle of $$90^\circ$$. The calibration was derived from constants for light scattering, and for the imaging optics. The wavelength of the scattered fringe pattern is predicted using the Lorentz–Mie theory. A practical method for the calibration of interference patterns is presented. Using this approach, the measured bubble diameters from the shadowgraphy and MSI compare to within $$1\,\upmu{\hbox {m}}$$. A method for determining the size-dependent measurement volume for axisymmetric and arbitrary beam profiles is also presented.

Error analysis in the absolute phase maps recovered by fringe patterns with three different wavelengths

In a recently published work, we proposed a technique to recover the absolute phase maps from the wrapped ones with three different fringe wavelengths. It was noticed that when the wrapped phase maps are obtained by three-step phase shifting method, errors in fringe order still occur, leading to significant error in the recovered absolute phase maps. In this paper, the characteristic and reason of the errors are analysed, and a method to avoid these errors is also proposed. The theoretical conclusions in this paper are validated with experimental results.

Optical interference probe of biofilm hydrology: label-free characterization of the dynamic hydration behavior of native biofilms.

Biofilm produced by Escherichia coli (E. coli) or Pseudomonas aeruginosa (P. aeruginosa) on quartz or polystyrene is removed from the culture medium and drained. Observed optical interference fringes indicate the presence of a layer of uniform thickness with refractive index different from air-dried biofilm. Fringe wavelengths indicate that layer optical thickness is < 20 ?? ? m or 1 to 2 orders of magnitude thinner than the biofilm as measured by confocal Raman microscopy or fluorescence imaging of the bacteria. Raman shows that films have an alginate-like carbohydrate composition. Fringe amplitudes indicate that the refractive index of the interfering layer is higher than dry alginate. Drying and rehydration nondestructively thins and restores the interfering layer. The strength of the 1451-nm near infrared water absorption varies in unison with thickness. Absorption and layer thickness are proportional for films with different bacteria, substrates, and growth conditions. Formation of the interfering layer is general, possibly depending more on the chemical nature of alginate-like materials than bacterial processes. Films grown during the exponential growth phase produce no observable interference fringes, indicating requirements for layer formation are not met, possibly reflecting bacterial activities at that stage. The interfering layer might provide a protective environment for bacteria when water is scarce.

Recovery of absolute phases for the fringe patterns of three selected wavelengths with improved anti-error capability

In a recent published work, we proposed a technique to recover the absolute phase maps of fringe patterns with two selected fringe wavelengths. To achieve higher anti-error capability, the proposed method requires employing the fringe patterns with longer wavelengths; however, longer wavelength may lead to the degradation of the signal-to-noise ratio (SNR) in the surface measurement. In this paper, we propose a new approach to unwrap the phase maps from their wrapped versions based on the use of fringes with three different wavelengths which is characterized by improved anti-error capability and SNR. Therefore, while the previous method works on the two-phase maps obtained from six-step phase-shifting profilometry (PSP) (thus 12 fringe patterns are needed), the proposed technique performs very well on three-phase maps from three steps PSP, requiring only nine fringe patterns and hence more efficient. Moreover, the advantages of the two-wavelength method in simple implementation and flexibility in the use of fringe patterns are also reserved. Theoretical analysis and experiment results are presented to confirm the effectiveness of the proposed method.

Fast-response fiber-optic anemometer with temperature self-compensation.

We report a novel fiber-optic anemometer with self-temperature compensation capability based on a Fabry-Pérot interferometer (FPI) formed by a thin silicon film attached to the end face of a single-mode fiber. Guided in the fiber are a visible laser beam from a 635 nm diode laser used to heat the FPI and a white-light in the infrared wavelength range as the signal light to interrogate the optical length of the FPI. Cooling effects on the heated sensor head by wind is converted to a wavelength blueshift of the reflection spectral fringes of the FPI. Self-temperature-compensated measurement of wind speed is achieved by recording the difference in fringe wavelengths when the heating laser is turned on and then off. Large thermal-optic coefficient and thermal expansion coefficient of silicon render a high sensitivity that can also be easily tuned by altering the heating laser power. Furthermore, the large thermal diffusivity and the small mass of the thin silicon film endow a fast sensor response.

Approach for making visible and stable stripes in a spin-orbit-coupled Bose-Einstein superfluid

The striped phase exhibited by a spin-$1/2$ Bose-Einstein condensate with spin-orbit coupling is characterized by the spontaneous breaking of two continuous symmetries: gauge and translational symmetry. This is a peculiar feature of supersolids and is the consequence of interaction effects. We propose an approach to produce striped configurations with high-contrast fringes, making their experimental detection in atomic gases a realistic perspective. Our approach, whose efficiency is directly confirmed by three-dimensional Gross-Pitaevskii simulations, is based on the space separation of the two spin components into a two-dimensional bilayer configuration, causing the reduction of the effective interspecies interaction and the increase of the stability of the striped phase. We also explore the effect of a $\pi/2$ Bragg pulse, causing the increase of the fringe wavelength, and of a $\pi/2$ rf pulse, revealing the coherent nature of the order parameter in the spin channel.

Absolute phase map recovery of two fringe patterns with flexible selection of fringe wavelengths

A novel approach is proposed to unwrap the phase maps of two fringe patterns in fringe pattern projection-based profilometry. In contrast to existing techniques, where spatial frequencies (i.e., the number of fringes on a pattern) of the two fringe patterns must be integers and coprime, the proposed method is applicable for any two fringe patterns with different fringe wavelengths (i.e., the number of pixels in a fringe) and thus provides more flexibility in the use of fringe patterns. Moreover, compared to the existing techniques, the proposed method is simpler in its implementation and has better antierror capability. Theoretical analysis and experiment results are presented to confirm the effectiveness of the proposed method.

Semiempirical method of suppressing interference effects in photoluminescence spectra of GaN heterostructures

Interference fringes are generally found in the photoluminescence (PL) spectrum of heterostructures with large refractive index differences between layers and with a smooth interface. To eliminate the interference effects in an air/GaN/InGaN/GaN/Al2O3 structure, the measured interference fringe wavelengths of the PL spectrum are used to deduce the frequency dependent interference function of the system. This interference function is then used to numerically remove interference fringes from the as-measured PL spectrum. This versatile semiempirical method allows the derivation of the true PL spectrum from the measured data, including angular dependent spectra, without complicated calculations or additional measurements.

A Miniaturized Spectrometer Based on Optical Modulator Composed of Grating and Tunable Fabry-Perot

In this paper an innovative miniaturized spectrometer has been presented. The structure is similar to a grating spectrometer, but its optical modulator is composed of a grating and a tunable Fabry-Perot (FP). The collimated light is diffracted by the grating and filtered by the FP, and interference fringes distribute separately on the focal plane. The position of fringes moves when the FP is tuned, this helps in the scanning and detecting of the peak wavelength of fringes. The principle and characteristics of the spectrometer are discussed in detail.

Direct Evidence for Surface Plasmon-Mediated Enhanced Light Transmission through Metallic Nanohole Arrays

This paper provides direct evidence for the role of surface plasmons in the enhanced optical transmission of light through metallic nanoscale hole arrays. Near-field optical images directly confirmed the presence of surface plasmons on gold nanohole arrays with interhole spacings larger than the surface plasmon wavelength. A simple interference model provides an intuitive explanation of the two types of fringe wavelengths observed in the near-field optical images. Far-field spectroscopy revealed a surface plasmon band that contributed a factor > 8 to the transmission enhancement. Furthermore, silicon nanohole arrays did not exhibit any features in the near-field, which demonstrates that metallic materials are necessary for enhanced light transmission through nanohole arrays.

Two-Wavelength Phase-Shifting Projection $Moir\acute{e}$ Topography for Measurement of Three-Dimensional Profiles with High Step Discontinuities

[] technique is now being extensively investigated as a fast non-contact means of three-dimensional profile measurement especially for reverse engineering. One problem with technique is so called -ambiguity problem that limits the maximum step height difference between two neighboring sampling points to be less than half the equivalent wavelength of fringes. In this investigation, a new two-wavelength scheme of projection topography is proposed and tested to cope with the -ambiguity problem. Experimental results are discussed to assess the new method in measuring large objects with high step discontinuities.

Multiple-beam interferometry with thin metal films and unsymmetrical systems

Numerical calculations using the multilayer matrix method show that the interference fringes in the visible spectrum of light transmitted through two back-silvered transparent sheets separated by a dielectric film is greatly modified when a thin metal layer is present on each sheet surface. The layers cause a modulation of fringe intensity and a shift in fringe wavelength which depend on the refractive index and thickness of the layer. Fringes will be well resolved for <40 nm layers of metals with high reflectivity and low absorption. In regard to the use of this interferometer to determine surface separations in the Israelachvili surface forces apparatus, Israelachvili's equation is found to be no longer valid, and a multilayer matrix method is presented. Some unsymmetrical interferometers are also considered.

Gunn oscillator used as a phased-array aerial element

This letter describes an investigation into the feasibility of using the bias voltage as the phase-varying parameter in an array of synchronised Gunn oscillators to steer an antenna. The variation in bias changes the frequency of an unsynchronised oscillator, which, when locked, becomes a change in phase while it is oscillating at the pilot frequency. A simple experiment is described in which the interference pattern from two horns is made to sweep through a fringe wavelength with a voltage variation of 400mV in the bias of one oscillator.

Thickness Measurement of Epitaxial Films by the Infrared Interference Method

The infrared interference measurement has proved to be an accurate and nondestructive means for evaluating the thickness of epitaxially grown films on silicon. The measuring technique is discussed and the necessary relationships are presented. A fringe wavelength vs. film thickness chart is constructed, which enables rapid thickness determination without calculation. The chart is derived for use with silicon films of relatively low carrier concentration deposited on silicon substrates of high carrier concentration (0.007 ohm‐cm‐N‐type). The chart is most useful for low order interference fringes including 12th order. Similar charts are applicable for other semiconductor materials.