Spectral Interference Signal Research Articles

Large-Area Thickness Measurement of Transparent Films Based on a Multichannel Spectral Interference Sensor

Thickness measurement of thin films is essential for quality control in the manufacturing process of the semiconductor and display industries. Real-time monitoring of film thickness during production is an urgent technical problem to be solved. In this study, a method for large-area thickness measurement of transparent films based on a multichannel spectral interference sensor is proposed. The sensor simultaneously acquires multichannel spectral interference signals through a combination of fan-out fiber optic bundles, detection probes, and an imaging spectrometer. The spectral data are calibrated and transformed into the wavenumber dimension, and then the power spectral density estimation method is used to demodulate the data frequency to swiftly derive the film thickness. The thickness measurement capacity of the proposed system is successfully validated on two standard film samples with a relative deviation of less than 0.38% and a relative standard deviation of less than 0.044%. The total spectral acquisition and calculation time for a single multichannel measurement was approximately 7.5 ms. The experimental results on polyimide films show that the measurement efficiency of the system is at least 4 times higher than that of the traditional system, indicating the potential of the multichannel spectral interference sensor for online monitoring in film production.

Improved Algorithms of Data Processing for Dispersive Interferometry Using a Femtosecond Laser

Two algorithms of data processing are proposed to shorten the unmeasurable dead-zone close to the zero-position of measurement, i.e., the minimum working distance of a dispersive interferometer using a femtosecond laser, which is a critical issue in millimeter-order short-range absolute distance measurement. After demonstrating the limitation of the conventional data processing algorithm, the principles of the proposed algorithms, namely the spectral fringe algorithm and the combined algorithm that combines the spectral fringe algorithm with the excess fraction method, are presented, together with simulation results for demonstrating the possibility of the proposed algorithms for shortening the dead-zone with high accuracy. An experimental setup of a dispersive interferometer is also constructed for implementing the proposed data processing algorithms over spectral interference signals. Experimental results demonstrate that the dead-zone using the proposed algorithms can be as small as half of that of the conventional algorithm while measurement accuracy can be further improved using the combined algorithm.

Accuracy improvement of surface measurement through phase correction in spectrally resolved interferometer

The spectrally resolved interferometer (SRI) is used to measure the surface profile by the linear fitting line of its phase distribution of spectral interference signal. However, the noise existed in the phase cannot be ignored if the nano-scale measurement accuracy is required. In this paper, a novel algorithm is proposed to improve the measurement accuracy by using the constant term oflinear fitting line of phase distribution in wavenumber domain. The simulations indicate that the algorithm achieves the similar measurement accuracy under the influence of three kinds of noise. Even though the proposed algorithm has the possibility of 4% to bring the additional minor error, it provides a better performance on measurement of entire surface in different SNRs compared with the other two common algorithms. In experiments, the variations of surface less than 10 nm and 30 nm and the repeatability of 2.2 nm and 4.9 nm are obtained in the measurements of a protected aluminium mirror with and without proposed algorithm. A smooth profile with a step shape of nearly 3 μm height is tested, where the maximum repeatability of step height and maximum magnitude of variations of the surfaces reaches to 3.8 nm and 10 nm from the 15.3 nm and 30 nm by using this algorithm. Both the simulations and experimental results demonstrate that the proposed algorithm is feasible for high-precision measurement.

Spectral interferometric depth-resolved photoacoustic viscoelasticity imaging.

Viscoelasticity is closely related to the physiological characteristics of biological tissues. In this Letter, we propose a novel spectral interferometric depth-resolved photoacoustic viscoelasticity imaging (SID-PAVEI) method, to the best of our knowledge for the first time, which breaks the plight of surface viscoelasticity imaging and achieves an internal visible microscale SID-PAVEI in a noncontact fashion. In this work, we employ a high-sensitive and depth-resolved spectral domain low coherence interferometry (SDLCI) to remotely track photoacoustic-induced strain response of absorbers in situ. By decoupling the phase and amplitude of the photoacoustic-encoded spectral interference signal, the SID-PAVEI and scattering structure imaging (SSI) can be obtained simultaneously. Depth-resolved performance of the SID-PAVEI and the SSI in one scan were demonstrated by imaging biological tissues. The method opens new perspectives for three-dimensional microscale viscoelasticity imaging and provides a great potential in multi-parametric characterizing pathological information.

Single input state, single mode fiber based spectral domain polarization sensitive optical coherence tomography using a single linear-in-wavenumber spectral camera

A single input state, single mode fiber (SMF) based spectral domain polarization sensitive optical coherence tomography (SD-PS-OCT) system using a single linear-in-wavenumber spectral camera is presented. With the proposed dual channel linear-in-wavenumber spectral camera and the polarization alignment procedure, the orthogonal polarization components of the spectral interference signal can be detected using the all SMF based SD-PS-OCT system without any bulk polarization optical elements. To evaluate the performance of the developed system, the polarization properties of a Babinet-Soleil compensator and a quarter waveplate are quantitatively measured with high accuracy. Furthermore, the depth-resolved images of the intensity, the phase retardation and the optic axis orientation of the ex-vivo tendon tissue and the chicken muscle tissue are reconstructed to verify the feasibility of the proposed method.

Precise selenium isotope measurement in seawater by carbon-containing hydride generation-Desolvation-MC-ICP-MS after thiol resin preconcentration

The isotopic compositions of Se in natural materials can be measured by multiple collector inductively coupled plasma mass spectrometry (MC-ICP-MS) in combination with hydride generation and desolvation device. Although oceans are important targets for research, precise isotopic analysis of Se in seawater is generally difficult owing to its low concentrations and the much higher concentrations of other elements. We developed a robust, accurate method for the precise determination of Se isotopic composition in seawater. A hydride generator combined with a desolvation device with methane addition was coupled to MC-ICP-MS to measure Se isotopes precisely. Two new thiol resins were used to preconcentrate and separate Se from the seawater matrix efficiently, and Se was quickly eluted by small amounts of concentrated HNO3. No detectable Se isotope fractionation was observed during the preconcentration and separation procedure. The carbon-containing plasma increased Se sensitivity, suppressed the spectral interference signal, and improved precision by >3 times. The precisions (2sd, n=24) of Merck Se standard solutions for δ82/78Se, δ82/77Se, and δ82/76Se were 0.07‰, 0.10‰, and 0.16‰, respectively. The δ82/76Se values of standard solutions agreed well with results obtained by other research groups. The method was used to obtain a depth profile for dissolved inorganic Se (DISe) isotopes in seawater from the Northwestern Pacific Ocean. The range of δ82/76DISe was 0.41‰ to 1.59‰, indicating that Se isotopes fractionate sufficiently to clarify the biogeochemical cycling of this element in the ocean.

Optimization of linear-wavenumber spectrometer for high-resolution spectral domain optical coherence tomography

Nonlinear detection of the spectral interferograms in wavenumber (k) space degrades the depth-dependent signal sensitivity in conventional linear-in-λ spectrometer based spectral domain optical coherence tomography (SDOCT). Linear-k spectrometer enables high sensitivity SDOCT imaging without the need of resampling the digitized non-linear-in-k data. Here we report an effective optimization method for linear-k spectrometer used in a high-resolution SDOCT system. The design parameters of the linear-k spectrometer, including the material of the dispersive prism, the prism vertex angle, and the rotation angle between the grating and prism, are optimized through the numerical simulation of the spectral interference signal. As guided by the optimization results, we constructed the linear-k spectrometer based SDOCT system and evaluated its imaging performances. The axial resolution of the system can be maintained to be higher than 9.1μm throughout the imaging depth range of 2.42 mm. The sensitivity was experimentally measured to be 91 dB with −6 dB roll-off within the depth range of 1.2 mm.

Denoising of partial discharge signal using rapid sparse decomposition

Summary Partial discharge (PD) detection is essential for electrical equipment. Focusing on the noise suppression of PD signal, a PD signal denoising method based on rapid sparse decomposition was proposed. Through the analysis of PD signal time-frequency characteristics, the PD signal correlated atom and PD signal correlated overcomplete dictionary were designed, which were correlated to the original PD pulse signal while uncorrelated or weak-correlated to the random noise and discrete spectral interference (DSI) signal. Denoising the noisy PD signal by sparse decomposition based on matching pursuit algorithm and correlation ratio iteration termination condition, only the original PD pulse signal can be expressed by the best atoms extracted from the PD signal correlated overcomplete dictionary; thus, the goal of denoising is achieved. What is more, the searching progress of best atoms was accelerated by quantum genetic algorithm. After finishing the above denoising process, the extracted noiseless pulse signals can be projected onto a Time–Frequency Map (TF Map) to overcome pulse-shape interference influence. The denoising method presented in this article is applied to the simulated, laboratory measured and field detected PD signals. The results are critical compared with denoising method based on db2 and db8 wavelet transform. The results show that the denoising method of this article is available to precisely suppress PD signal noise jamming and the denoising effect is superior to traditional wavelet methods, which has less amplitude error as well as less waveform destination. Copyright © 2016 John Wiley & Sons, Ltd.

High-resolution, dual-depth spectral-domain optical coherence tomography with interlaced detection for whole-eye imaging.

Dual-depth spectral-domain optical coherence tomography (SD-OCT) enables high-resolution in vivo whole-eye imaging. Two orthogonally polarized beams from a source are focused simultaneously on two axial positions of the anterior segment and the retina. For the detector arm, a 1×2 ultrafast optical switch sequentially delivers two spectral interference signals to a single spectrometer, which extends the in-air axial depth range up to 9.44 mm. An off-pivot complex conjugate removal technique doubles the depth range for all anterior segment imaging. The graphics-processing-unit-based parallel signal processing algorithm supports fast two- and three-dimensional image displays. The obtained high-resolution anterior and retinal images are measured biometrically. The dual-depth SD-OCT system has an axial resolution of ∼6.4 μm in air, and the sensitivity is 91.79 dB at 150 μm from the zero-delay line.

Spectral estimation optical coherence tomography for axial super-resolution.

The depth reflectivity profile of Fourier domain optical coherence tomography (FD-OCT) is estimated from the inverse Fourier transform of the spectral interference signals (interferograms). As a result, the axial resolution is fundamentally limited by the coherence length of the light source. We demonstrate that using the autoregressive spectral estimation technique instead of the inverse Fourier transform, to analyze the spectral interferograms can improve the axial resolution. We name this method spectral estimation OCT (SE-OCT). SE-OCT breaks the coherence length limitation and improves the axial resolution by a factor of up to 4.7 compared with FD-OCT. Furthermore, SE-OCT provides complete sidelobe suppression in the depth point-spread function, further improving the image quality. We demonstrate that these technical advances enables clear identification of corneal endothelium anatomical details ex vivo that cannot be identified using the corresponding FD-OCT. Given that SE-OCT can be implemented in the FD-OCT devices without any hardware changes, the new capabilities provided by SE-OCT are likely to offer immediate improvements to the diagnosis and management of diseases based on OCT imaging.

Peak-to-peak tracking method for measurement of optical path difference: Revisited

This paper reports on the measurement of optical path difference of spectral interference signal by using the peak-to-peak tracking method in the wavelength domain. Numerical simulations are performed to produce the interference spectrum based on an all fibre Mach–Zehnder interferometer with a white light source. The peak-to-peak tracking method is improvised by measuring the wavelength value corresponding to bright fringes from the phase plot for determination of the optical path difference. The results are compared to the conventional peak-to-peak tracking method and measurement of the slope of phase against wavenumber plot. Optical path difference determined from the improvised method yields higher accuracy with lower uncertainty.

Measurement of Step Height Using White Light Spectral Interferometry

The development of micro-nanomanufacturing technology leads to higher requirement for measurement technology, which gives birth to the ultra-precision measurement technology. As a kind of low coherence interferometry technology, spectral interferometry has been proposed. In this paper, the theory and system of white light spectral interferometry were described in detail, and the measurement principle of micro-structure was analyzed. The spectral interference signal was recorded and the phase information was retrieved from it by using five-step phase shifting algorithm, which includes the phase change on reflection from the stucture. So the optical path difference (OPD) can be obtained, which represents the height information. Three methods including slope method, single wavelength method and least square method were discussed based on the theory deduction and the experimental analysis. A step height standard was measured and the results show that this method has a good accuracy.

Surface Plasmon Resonance Sensor Using Spectral Interference

This paper presents a novel surface plasmon resonance (SPR) sensor based on whitelight spectral interferometry. The experimental arrangement employs a white-light source (bandwidth from 350 nm to 1000 nm), a polarizer and a birefringent quartz crystal for SPR excitation. SF10 Kretschmann SPR sensor prism is inserted into the beam path and one measures both the corresponding SPR phase change and reflectivity at a suitable fixed angle of incidence across the visible spectrum. The prism (sensing surface) is coated with gold film of 44 nm nominal thickness and the incident angle is approximately 58 . In our case of the SPR effect, the useful phase information is encoded in the spectral interferogram. Thereby, p- and s-polarized spectra interfere among themselves and the interferograms are obtained by using an analyzer when only p-polarized waves are affected by the SPR effect. Spectral interference signals are then used in corresponding phase retrieval [1]. In order to calculate the SPR induced differential phase change, a windowed Fourier transform (WFT) [2] was adopted to extract the phases from two spectral interferograms, one corresponding to the reference material and the other corresponding to the analyte. The pilot measurements were tested with water as an analyte and air as a reference material and the measurement results were compared with the results of theoretical models. The first results show good agreement between a calculated resonance wavelength and a resonance wavelength determined from reflectivity measurements. The second results show good agreement for phase change measurements. The proposed technique offers a much simpler experimental arrangement than was recently presented in a paper [3].

Full-range swept source optical coherence tomography based on carrier frequency by transmissive dispersive optical delay line

A high speed swept source optical coherence tomography (SS-OCT) system capable of full-range imaging is presented. Wave-number carrier frequency is introduced into the spectral interference signal by a transmissive dispersive optical delay line (TDODL). High carrier frequency in the spectral interference signal corresponding to an equivalent distance-shift is exploited to obtain full-range OCT imaging. Theoretical development is conducted with the instantaneous coherence function introduced for a complete description of a spectral interference signal. Performance advantage of the TDODL-based method over the conventional approach where only one side (positive or negative path length difference) is used for imaging to avoid overlaying mirror artifacts is confirmed by the measured envelopes of spectral interference signal. Feasibility of the proposed method for full-range imaging is validated in a custom-built SS-OCT system by in vivo imaging of a biological sample.

A spectral interferometric method to measure thickness with large range

We present a spectral interferometric method to measure the thickness of an optical plate or a film with a single layer. The system is based on the Michelson interferometer configuration, and the spectral interference signal of a broadband light source is recorded by a spectrometer. The optical path difference (OPD) between two interfering beams can be obtained by Fourier transform from the spectral interferogram. Gaussian fitting is used to find the exact peak of fringe to enhance the precision of the measurements. When the sample is inserted into the sample beam, the film’s thickness can be calculated by comparing the change of OPD, provided that the sample group refractive index is known. Only a single measurement is needed to determine a film’s thickness after the initial OPD of the system is calibrated. As no moving parts are required, the system has good stability. In particular, a large range of thicknesses, from micrometers up to several millimeters, can be measured. Such a large range is valuable for optical measurements. For demonstration, we measured the thicknesses of preservative film, cover glass and carrier glass, which were 9.6 ± 0.55, 156.1 ± 0.75, 1008.44 ± 0.96 μm, respectively.

利用白光干涉系统测量薄膜反射相位

A novel method to measure the absolute phase shift on reflection of thin film is presented utilizing a white-light interferometer in spectral domain. By applying Fourier transformation to the recorded spectral interference signal, we retrieve the spectral phase function \phi which is induced by three parts: the path length difference in air L the effective thickness of slightly dispersive cube beam splitter T_{eff} and the nonlinear phase function due to multi-reflection of the thin film structure. We utilize the fact that the overall optical path difference (OPD) is linearly dependent on the refractive index of the beam splitter to determine both L and T_{eff}. The spectral phase shift on reflection of thin film structure can be obtained by subtracting these two parts from \phi. We show theoretically and experimentally that our new method can provide a simple and fast solution in calculating the absolute spectral phase function of optical thin films, while still maintaining high accuracy.

Windowed Fourier transform applied in the wavelength domain to process the spectral interference signals

We report on processing the spectral interference signals by a new method based on a windowed Fourier transform applied in the wavelength domain. First, the numerical simulations are performed to demonstrate high precision of the phase retrieval from the spectral signal. Second, the feasibility of the method is confirmed in processing experimental data from a dispersive Michelson interferometer comprising a cube beamsplitter made of BK7 glass. From the retrieved spectral phase difference, the effective thickness of the beamsplitter is determined precisely.

Phase retrieval from the spectral interference signal used to measure thickness of SiO2 thin film on silicon wafer

A new method of phase retrieval from the spectral interference signal is presented, which is based on the use of a windowed Fourier transform in the wavelength domain. The phase retrieved by the method is utilized for measuring the thickness of SiO2 thin film on a silicon wafer. The numerical simulations are performed to demonstrate high precision of the phase retrieval. The feasibility of the method is confirmed in processing experimental data from a slightly dispersive Michelson interferometer with one of the mirrors replaced by SiO2 thin film on the silicon wafer. We determine the thin-film thickness for four samples provided that the optical constants for all the materials involved in the experiment are known. We confirm very good agreement with the previous results obtained by the fitting of the recorded channelled spectra to the theoretical ones.

Observation and analysis of spectral interference modulations in thin film light scattering experiments using synchrotron radiation

Light scattering in thin films represents several essential microstructural, morphological and compositional parameters in thin film systems. Under the present investigation certain gadolinia, silica thin films and multilayer systems have been probed for their spectral scattering signals using the photophysics beamline of INDUS-1 synchrotron radiation source. The pulsed structure of synchrotron radiation and its interaction with certain non-wedge optical component has however led to the spectral interference (SI) modulations in scattering signals, which was tackled by an appropriate modelling and analysis technique. Under this technique the geometric mean of the SI amplitudes were computed and then de-convoluted into several Gaussian peaks. Each Gaussian peak was made to interfere with an appropriate temporally delayed peak having certain transformed amplitude factors. Superposition of all these Gaussian interference was computed and compared with the experimental profile in an iterative manner to determine the mean pulse delay introduced by the optical element. Finally the inverse fast Fourier transform of the experimental SI signal was carried out to compare the modelling results and both the approaches demonstrated a closed match. Spectral scattering signals recorded for various thin film layer combinations depicted close resemblances with the specular characteristics recorded using a spectrophotometric technique. This distinctly highlights the dominance of multilayer thin film interference phenomena on the spectral light scattering characteristics.

Fourier-domain low-coherence interferometry for differential mode delay analysis of an optical fiber

We propose a novel mode analysis and differential mode delay measurement method for an optical fiber using Fourier-domain low-coherence interferometry. A spectral interferometer based on a Mach-Zehnder interferometer setup was used with a broadband source and an optical spectrum analyzer to detect relative temporal delays between the guided modes of a few-mode optical fiber by analyzing spectral interference signals. We have shown that experimental results of the proposed method agree well with those results obtained by using a conventional time-domain measurement method. We have demonstrated that this new mode analysis technique has high sensitivity (<60 dB) and very good resolution (<1 ps/m).