Low-coherence Interferometer Research Articles

A Method for Detecting Small Oscillations Based on Homodyne Demodulation with a Tandem Low-Coherence Interferometer

A Method for Detecting Small Oscillations Based on Homodyne Demodulation with a Tandem Low-Coherence Interferometer

Low-Coherence Homodyne Interferometer for Sub-Megahertz Fiber Optic Sensor Readout.

This study proposes a method for interferometric fiber optic sensor readouts. The method utilizes the advantages of the active homodyne demodulation technique and low-coherence interferometry. The usage of the tandem low-coherence interferometer enables modulating the reference interferometer without any changes to the sensor. This achieves high sensitivity, high stability, and a wide frequency band. A sensitivity of up to 0.1 nm (RMS) in the frequency range of 5 kHz is demonstrated by detecting acoustic signals with a fiber Michelson interferometer as a sensor.

Polarization state tomography technique based on coherent synthesis of polarization state and orthogonal polarization state separation method for comprehensive optical imaging.

Comprehensive optical imaging of the intensity, phase, and birefringent information of the biological sample is important because important physical or pathological changes always accompany the changes in multiple optical parameters. Current studies lack such a metric that can present the comprehensive optical property of the sample in one figure. In this paper, a polarization state synthesis tomography (PoST) method, which is based on the principle of polarization state coherent synthesis and demodulation, is proposed to achieve full-field tomographic imaging of the comprehensive information (i.e., intensity, phase, and birefringence) of the biological sample. In this method, the synthesis of the polarization state is achieved by the time-domain full-field low coherence interferometer, where the polarization states of the sample beam and the reference beam are set to be orthogonal for the synthesis of the polarization state. The synthesis of the polarization state enables two functions of the PoST system: (1) Depth information of the sample can be encoded by the synthesized polarization state because only when the optical path length difference between the two arms is within the coherence length, a new polarization state can be synthesized; (2) Since the scattering coefficient, refractive index and the birefringent property of the sample can modulate the intensity and phase of the sample beam, the synthesized polarization state is sensitive to all these three parameters and can provide the comprehensive optical information of the sample. In this work, the depth-resolved ability and the comprehensive optical imaging metric have been demonstrated by the standard samples and the onion cells, demonstrating the potential application value of this method for further investigation of the important physical or pathological process of the biological tissues.

Low Coherence Interferometry Measurement: An Algorithm for fast processing with low noise and phase linearisation

This work proposes a signal processing algorithm to analyse the optical signal from a Low Coherence Interferometric (LCI) system. The system uses a Mach-Zehnder (MZ) interferometer to interrogate a Fabry-Perot cavity, working as an optical sensor. This algorithm is based on the correlation and convolution operations, which allows the signal to be reconstructed based on itself, as well as, on the linearization of the signal phase, allowing the non-linearities of the actuator incorporated on the MZ interferometer to be compensated. The results show a noise reduction of 30 dB in the signal acquired. As a result, a reduction of 8.2 dB in the uncertainty of the measurement of the physical measurand is achieved. It is also demonstrated that the phase linearization made it possible to obtain a coefficient of determination (namely, R-squared) higher than 0.999.

Extended centroid position method in liquid-crystal-based interrogation system for electric-field-modulated Fabry-Perot sensing

A polarized low-coherence interferometer (PLCI) based on a liquid crystal (LC) wedge is designed, and an associated demodulation method encompassing the tunability feature is proposed for tunable, standalone optical sensing. The application of an electric field to the LC material effectively decreases the birefringence value and the related dispersion relation, which in turn enhances the resolution of detection. The effect of the electric field on the demodulation of the cavity length is addressed by the successive determinations of the centroid positions of the PLCI interferograms. Through a comprehensive study of numerical simulations, the effectiveness of the proposed approach is explored relative to the conventional envelope detection methodology. In order to verify this method, an experiment with a Fabry-Perot-based fiber optic displacement sensor is carried out using a 5CB LC wedge-based PLCI setup in the presence of an electric field. The measurement accuracy of the cavity length is found to be 0.74% of full scale, rendering it more precise and robust than the conventional envelope detection method.

Optical coherence tomography based diabetic – ophthalmic disease classification and measurement using bilateral filter and transfer learning approach

Optical Coherence Tomography (OCT) is a smooth application of low coherence interferometer with high air resolution and highly sensitive heterodyne detection technology to tomographic image measurement of living organisms. Currently, clinical applications are becoming more widespread in ophthalmology, cardiovascular system, dermatology, and dentistry. The problem with OCT is that the measurement area is as narrow as a few mm compared to other tomographic image measurement techniques, and it was initially applied to ophthalmology. Since then, various researches and developments have been carried out to expand clinical applications. Michelson type fiber optic interferometer is used for image acquisition. This paper presents a classification of ophthalmic diseases caused by diabetes. Bilateral filter is used for image pre-processing and noise removal. A transfer learning approach is implemented which uses AlexNet and Support vector machine (SVM) to classify the images. The AlexNet model is used to extract the features form the images and these features are classified using SVM model. The novelty of the proposed model lies in the use of image denoising using bilateral filter and then classification of the AlexNet features using SVM thereby achieving better classification accuracy with less training data. This also leads to better resource utilization. The ailments under study are Choroidal Neovascularization (CNV), Diabetic Macular Edema (DME), DRUSEN, and NORMAL. The proposed approach produced a higher classification accuracy of 99 % when compared to other deep learning algorithms like CNN, AlexNet and GoogleNet. The precision, sensitivity and specificity are recorded as 0.98, 0.99, and 0.99 respectively.

Low-coherence and broadband confocal refractometry: reducing the measurement time

This paper describes the steps taken to improve the measurement speed of a combined low-coherence and confocal refractive index measurement system. The instrument measures the refractive index and thickness of transparent plates using a fibre-based low-coherence interferometer with a line-scan spectrometer. The spectrometer allows on-line dispersion measurement which is necessary to derive the sample thickness t as well as both the phase and group refractive indices n p, n g. The measurements were performed on a sample with six surfaces consisting of three glass windows mounted in a lens tube. Experimental results show that a measurement time of 4.4 s for the multi-layered object, which has a total thickness of approximately 10.5 mm, can be achieved whilst maintaining an accuracy of better than 0.1% for n p, n g, and t. This represents an approximately hundredfold improvement over previously published measurements.

Complex optically computed phase microscopy for label-free sub-cellular imaging.

In this study, we investigated a complex optically computed phase microscopy (complex-OCPM) technology. Based on a low coherence interferometer and an innovative optical computation approach, the complex-OCPM imaging system achieves depth resolved quantitative phase measurement. Particularly, the complex-OCPM imaging system directly measures the complex amplitude of the optical field emerging from the sample, extracts the phase as the argument of a complex signal, and achieves a high spatial resolution in phase imaging. We evaluated the performance of complex-OCPM imaging using resolution targets and live cells. Our results show that the complex-OCPM system achieves quantitative phase imaging with sub-cellular resolution on label-free cells.

Self-Vernier Effect-Assisted Optical Fiber Sensor Based on Microwave Photonics and Its Machine Learning Analysis

Optical Vernier effect has been recently demonstrated as a tool to enhance the sensitivity of optical fiber interferometric sensors and has become a hot topic in the last few years. The generation of the Vernier effect relies on the superposition of interferograms of two interferometers (a sensing one and a reference one) with marginally different optical path differences (OPDs), where an amplitude modulation-like signal is sustained in the output spectrum. The Vernier modulation envelope exhibits significantly magnified sensitivity in response to external perturbations, compared to the individual sensing interferometer, providing a new route to new generations of ultra-sensitive optical fiber sensors. However, the construction of a Vernier effect-based sensor needs careful integration of two interferometers with a precise OPD deviation, which is challenging if not impossible sometimes. Additionally, the interrogation of a Vernier effect-based sensor requires the acquisition of its spectrum in a broad wavelength (frequency) window with a large number of sampling points, followed by cumbersome processing for the extraction of the Vernier envelope signal. In this article, we propose a new concept of the self-Vernier effect for the sensitivity improvement of optical fiber sensors. A proof-of-concept demonstration is performed using a single low-coherent microwave-photonic Fabry-Perot interferometer (FPI) with an addition of an auxiliary delay path. The self-Vernier effect is obtained by the overlap between the microwave interferogram of the FPI and its slightly time-delayed one. Detailed analyses regarding the principle, mathematical modeling, and experimental investigation are given. Moreover, we propose and demonstrate a new demodulation approach to the Vernier effect-based sensor through machine learning analysis by directly and statistically learning the embedded relation between the measurand of interest and the output spectrum. It is appreciated that the necessity of the frequency observation window and the number of sampling points can be remarkably reduced through machine learning analysis. The present interrogation strategy is highly generalizable and can be readily applied to optical-domain Vernier effect-based optical fiber sensors.

Liquid crystal based polarized low coherence interferometer for optical demodulation in sensors.

The resolution of the measurement detection and sensitivity of a polarized low coherence interferometer (PLCI) can be pre-engineered by optimizing the key parameters of the birefringent wedge, which is rarely reported. In this work, we introduce a liquid crystal (LC) wedge in the PLCI and use it to demodulate Fabry-Perot (FP) cavity length. The birefringence property of the nematic LC is used to convert the optical path difference (OPD) of the sensor into a spatial distribution. This results in the production of localized interference fringe patterns. The formation of PLCI fringes and the related shift of the interferogram with a variation in the displacement of the FP displacement sensor is explained with reference to the OPD matching between an LC wedge and the FP cavity. The displacement value is demodulated from the obtained fringe pattern by tracking the centroid position of the fringe envelope and also considering the birefringence dispersion. An additional simulation study shows that the spatial position of the interferogram signal coupled with the dispersion coefficient is almost identical to the experimental data. The demodulated results from both the simulation and experimental investigations are found to be consistent with each other and closely agree with the actual cavity length. Further, the possibility to enhance the sensing resolution is examined by modulating the interferogram fringes using an electric field. Compared to birefringent crystals, the LC wedge presented here is found to be advantageous for high precision and tunability of the measurement range, which is useful for robust fiber optic sensing applications.

Fiber Optic Impact Location System Based on a Tracking Tandem Low-Coherence Interferometer

This study proposes a method for detecting small-length fluctuations for fiber-optic sensors (FOS). The method is based on a tracking tandem low-coherence interferometer and enables the ability to compensate for temperature and deformation drifts in FOS. As a result, the constant high sensitivity of FOS over a wide frequency range is guaranteed. Sensitivity to the level of 2 nm in the frequency range of 200 kHz has been demonstrated. The operation of the circuit is demonstrated on the example of the 2D location of acoustic signals using a correlation algorithm for signal processing, known as the time reversal method. It is shown that this system enables us to determine the place of the impact on the sample under the test with an accuracy of about 2 cm using a single sensor.

Tunable image-mapping optical coherence tomography.

We present tunable image-mapping optical coherence tomography (TIM-OCT), which can provide optimized imaging performance for a given application by using a programmable phase-only spatial light modulator in a low-coherence full-field spectral-domain interferometer. The resultant system can provide either a high lateral resolution or a high axial resolution in a snapshot without moving parts. Alternatively, the system can achieve a high resolution along all dimensions through a multiple-shot acquisition. We evaluated TIM-OCT in imaging both standard targets and biological samples. Additionally, we demonstrated the integration of TIM-OCT with computational adaptive optics in correcting sample-induced optical aberrations.

Метод детектирования малых колебаний на основе гомодинной демодуляции с тандемным низкокогерентным интерферометром

The paper proposes a method for detecting small oscillations in the length of interference fiber-optic sensors, which makes it possible to compensate for the problem of slow drift of the working point with a remote sensor. The result is achieved by combining homodyne demodulation methods with a tandem low-coherence interferometer. Theoretically and experimentally, the possibility of detecting acoustic effects in the operating frequency band of 4 kHz with a sensitivity of up to 0.3 nm has been shown.

High-precision polarized dual low-coherence scanning interferometry for rapid step height measurements.

In this investigation, we propose a polarized dual low-coherence scanning interferometer. The spatial phase-shifting technique and the concept of the dual low coherence are adopted to overcome the scanning conditions of typical low-coherence scanning interferometers and reduce the measurement time. In the proposed interferometer, the scanning interval is not critical because the visibility and the phase are immediately obtained during a scanning procedure, and the whole scanning distance can be significantly reduced by the dual low coherence. To verify the system performance, the surface profiles of a plane mirror and a step height specimen were measured, and it was confirmed that the whole scanning distance was reduced 10 times without any scanning conditions.

Control of silicon dioxide etching rate in hydrogen microwave plasma by addition of oxygen

Hydrogen plasma treatment of thin films and bulk silica is important process for surface cleaning, smoothing and patterning. We studied etching of SiO2 plates in H2 + O2 microwave plasma at moderate pressures at high temperatures in the range of 790–1300 °C. A low-coherent optical interferometer was used for simultaneous in situ measurement of etch rate (ER) and the SiO2 temperature. We show that even a small addition of O2 (<1%) in microwave H2 plasma can effectively reduce the SiO2 ER due to redox reaction. Using an optical profilometry and atomic force microscopy we observed not only a strong etching inhibition by O2 addition in gas, but a significantly smoother etched surface as well. The activation energies Ea = 128 ± 8 kJ/mol and 276 ± 23 kJ/mol are found for the etching in pure H2 and H2 + 0.4 %O2 plasmas, respectively. An exponential dependence of ER on O2 content in gas is established, particularly, the O2 concentration as small as 2.6 % is sufficient to reduce the ER by 100 times. The effect of oxygen induced etching slow-down is supported by thermodynamic consideration of the surface reactions involved. The experiments were complimented with the plasma diagnostics by optical emission plasma spectroscopy to detect volatile etching products, and other species such as OH, and correlate the Si emission intensity with the measured ER. Our results confirm the efficiency of oxygen to control in a wide range the etch rate and surface topography upon atomic hydrogen etching in a microwave plasma.

Tunable image projection spectrometry.

We present tunable image projection spectrometry (TIPS), a Fourier-domain line-scan spectral imager with a tunable compression ratio. Compared to state-of-the-art spatial-domain pushbroom hyperspectral cameras, TIPS requires much fewer measurements and provides a higher light throughput. Using a rotating Dove prism and a cylindrical field lens, TIPS scans an input scene in the Fourier domain and captures a subset of multi-angled one-dimensional (1D) en face projections of the input scene, allowing a tailored data compression ratio for a given scene. We demonstrate the spectral imaging capability of TIPS with a hematoxylin and eosin (H&E) stained pathology slide. Moreover, we showed the spectral information obtained can be further converted to depths when combining TIPS with a low-coherence full-field spectral-domain interferometer.

Imposed positive defocus changes choroidal blood flow in young human subjects

PurposeIt has previously been found that imposing positive defocus changes axial length and choroidal thickness after only 30 min. In the present study, we investigated whether these changes may result from an altered choroidal blood flow.MethodsEighteen young adult subjects watched a movie from a large screen (65 in.) in a dark room at 2 m distance. A 15-min wash-out period was followed by 30 min of watching the movie with a monocular positive defocus (+ 2.5D). Changes in axial length and ocular blood flow were measured before and after the defocus, by using low-coherent interferometer (LS 900, Haag-Streit, Switzerland) and a laser speckle flowgraphy (LSFG) RetFlow unit (Nidek Co., LTD, Japan), respectively. Three regions were analyzed: (1) the macular area, where choroidal blood flow can be measured, (2) the optic nerve head (ONH), and (3) retinal vessel segments.ResultsChanges in choroidal blood flow were significantly and negatively correlated with changes in axial length that followed positive defocus in exposed eyes (R = − 0.67, p < 0.01). The absolute values of changes in choroidal blood flow in the defocused eyes were significantly larger than in the fellow control eyes (2.35 ± 2.16 AU vs. 1.37 ± 1.44 AU, respectively, p < 0.05). ONH and retinal blood flow were not associated with the induced changes in axial length.ConclusionsPositive defocus selectively alters choroidal, but not retinal or ONH blood flow in young human subjects after short-term visual exposure. The results suggest that blood flow modulation is involved in the mechanism of choroidal responses to optical defocus.

High-aperture low-coherence interferometer with a diffraction reference wave.

A high-aperture low-coherence interferometer with a diffraction reference wave based on a tipped single-mode optical fiber was proposed and investigated. Due to the usage of the central least-aberrated region of the diffracted wavefront, the interferometer comprise record working aperture among known PDIs. The interferometer makes it possible to study samples with a reflectance that varies over a wide range. A demonstration of the use of this interferometer for high-precision measurements of a spherical mirror is presented. Sub-nanometer reproducibility of measurements in terms of the peak to valley (PV) parameter and sub-angstrom reproducibility in terms of the rms are demonstrated.

Aladdin HW2.0 Optik Düşük Koherens İnterferometre ile Oküler Biyometri ve Pupillometrinin Yeniden Üretilebilirliği

Objective: The purpose of the study was to evaluate the reproducibility of ocular biometry and pupillometry with the Aladdin HW2.0 (Topcon, Tokyo, Japan) in patients with cataracts and healthy subjects.Materials and Methods: This prospective study was performed in eyes of healthy subjects and patients with cataracts. After a full ophthalmological examination; axial length (AL), anterior chamber depth (ACD), keratometry (K values), white-to-white (WTW), intraocular lens (IOL) powers, and pupillometric parameters were measured with the Aladdin biometer by two operators.Results: 72 eyes of 40 patients with cataracts and 57 eyes of 29 healthy subjects were evaluated. AL, ACD, K values, WTW and IOL power formulas were highly reproducible [intraclass correlation coefficient (ICC)&amp;gt;0.900] in two groups. AL was the most reproducible parameter in patients with cataracts, ACD in the healthy subjects. The ICC values of pupillometry were lower than 0.900 (range from 0.100 to 0.882) in patients with cataracts. The worst reproducible parameter was the maximum diameter of dynamic pupillometry. Except for the photopic pupil diameter from the healthy group, pupillometry parameters were within a quite wide range for 95% LoA.Conclusion: The Aladdin HW2.0 optical low coherence interferometer showed excellent inter-operator reproducibility for AL, ACD, K values, WTW and IOL power formulas except for pupillometry measurements.

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.