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David Huang - One of the best experts on this subject based on the ideXlab platform.
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Repeatability of pachymetric mapping using Fourier Domain optical coherence tomography in corneas with opacities.
Cornea, 2012Co-Authors: Nehal Maher Samy El Gendy, Xinbo Zhang, David HuangAbstract:PURPOSE To evaluate the repeatability of Fourier-Domain optical coherence tomography (OCT) pachymetric mapping in patients with corneal opacities, and to assess the reliability of Fourier-Domain OCT with 830 nm wavelength as a pachymetric measurement tool in opaque corneas.
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corneal power measurement with Fourier Domain optical coherence tomography
Journal of Cataract and Refractive Surgery, 2010Co-Authors: Maolong Tang, Alex Y Chen, David HuangAbstract:Purpose To study the accuracy and repeatability of anterior, posterior, and net corneal power measured by Fourier-Domain optical coherence tomography (OCT). Setting Doheny Eye Institute, Los Angeles, California, USA. Design Cross-sectional study. Methods A Fourier-Domain OCT system (RTVue) was used to scan normal eyes, eyes after myopic laser in situ keratomileusis (LASIK), and keratoconic eyes. After the corneal surfaces were delineated, the system calculated anterior and posterior corneal powers by curve fitting over the central 3.0 mm diameter area. Net corneal power was calculated using a thick-lens formula. The repeatability of the calculations was evaluated by the pooled standard deviation of 3 measurements from the same visit. The net corneal power values were compared with standard automated keratometry measurements (IOLMaster). Results The repeatability of Fourier-Domain OCT net corneal power was 0.19 diopters (D), 0.26 D, and 0.30 D in the normal, post-LASIK, and keratoconus groups, respectively. The Fourier-Domain OCT net corneal power was significantly lower than keratometry by a mean of −1.21 D, −2.89 D, and −3.07 D, respectively (P Conclusions Corneal power measured by Fourier-Domain OCT achieved good repeatability in all 3 groups. The repeatability was better than slower time-Domain OCT systems. Because Fourier-Domain OCT directly measures both anterior and posterior corneal surfaces, it may produce more consistent results than standard keratometry in post-LASIK and keratoconic eyes in which the anterior–posterior corneal curvature ratios are altered by surgery or disease. Financial Disclosure No author has a financial or proprietary interest in any material or method mentioned. Additional disclosures are found in the footnotes.
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Pachymetric mapping with Fourier-Domain optical coherence tomography.
Journal of cataract and refractive surgery, 2010Co-Authors: Maolong Tang, Xinbo Zhang, Camila Haydée Rosas Salaroli, Jose Luiz Branco Ramos, David HuangAbstract:Purpose To evaluate the repeatability of Fourier-Domain optical coherence tomography (OCT) pachymetric mapping and compare central corneal thickness (CCT) measurements by OCT, ultrasound pachymetry, and scanning-slit tomography. Setting Doheny Eye Institute, University of Southern California, Los Angeles, California, USA. Methods A Fourier-Domain OCT system was used to map the corneal thickness in normal eyes with scans centered on the corneal vertex or the pupil. Repeatability of central and pericentral map sectors was assessed by pooled standard deviation. The CCT measurements were compared between the OCT, ultrasound, and scanning-slit devices. Results Pupil centration (SD: 1.3 μm central, 1.8 to 3.8 μm pericentral) provided better repeatability than vertex centration (SD: 1.7 μm central, 2.4 to 5.7 μm pericentral) in all sectors ( P P = .007; mean difference −19.7 μm; 95% limits of agreement [LoA], −40.4 to 0.9 μm) but not than by scanning-slit tomography ( P = .2637; mean difference −0.3 μm; 95% LoA, −24.0 to 23.5 μm). The CCT by OCT correlated well with ultrasound and scanning-slit CCTs ( r = 0.940 and r = 0.934, respectively). Conclusion Pachymetric mapping with Fourier-Domain OCT was highly repeatable. Repeatability was better with pupil-centered scans than with corneal vertex–centered scans. Ultrasound pachymetry, Fourier-Domain OCT, and scanning-slit tomography should not be used interchangeably for CCT assessment. Financial Disclosure No author has a financial or proprietary interest in any material or method mentioned. Additional disclosures are found in the footnotes.
Jay S. Duker - One of the best experts on this subject based on the ideXlab platform.
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Ultrahigh-resolution, high-speed, Fourier Domain optical coherence tomography and methods for dispersion compensation
Optics Express, 2004Co-Authors: Maciej Wojtkowski, Tony H Ko, Vivek J. Srinivasan, Andrzej Kowalczyk, James G Fujimoto, Jay S. DukerAbstract:Ultrahigh-resolution optical coherence tomography uses broadband light sources to achieve axial image resolutions on the few micron scale. Fourier Domain detection methods enable more than an order of magnitude increase in imaging speed and sensitivity, thus overcoming the sensitivity limitations inherent in ultrahigh-resolution OCT using standard time Domain detection. Fourier Domain methods also provide direct access to the spectrum of the optical signal. This enables automatic numerical dispersion compensation, a key factor in achieving ultrahigh image resolutions. We present ultrahigh-resolution, high-speed Fourier Domain OCT imaging with an axial resolution of 2.1 ìm in tissue and 16,000 axial scans per second at 1024 pixels per axial scan. Ultrahigh-resolution spectral Domain OCT is shown to provide a ~100x increase in imaging speed when compared to ultrahigh-resolution time Domain OCT. In vivo imaging of the human retina is demonstrated. We also present a general technique for automatic numerical dispersion compensation, which is applicable to spectral Domain as well as swept source embodiments of Fourier Domain OCT.
Maciej Wojtkowski - One of the best experts on this subject based on the ideXlab platform.
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Spectrometer calibration for spectroscopic Fourier Domain optical coherence tomography.
Biomedical optics express, 2016Co-Authors: Maciej Szkulmowski, Szymon Tamborski, Maciej WojtkowskiAbstract:We propose a simple and robust procedure for Fourier Domain optical coherence tomography (FdOCT) that allows to linearize the detected FdOCT spectra to wavenumber Domain and, at the same time, to determine the wavelength of light for each point of detected spectrum. We show that in this approach it is possible to use any measurable physical quantity that has linear dependency on wavenumber and can be extracted from spectral fringes. The actual values of the measured quantity have no importance for the algorithm and do not need to be known at any stage of the procedure. As example we calibrate a spectral OCT spectrometer using Doppler frequency. The technique of spectral calibration can be in principle adapted to of all kind of Fourier Domain OCT devices.
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Fourier Domain OCT imaging of American cockroach nervous system
Optical Coherence Tomography and Coherence Domain Optical Methods in Biomedicine XVI, 2012Co-Authors: Joanna Wyszkowska, Andrzej Kowalczyk, Iwona Gorczynska, Daniel Ruminski, Karol Karnowski, Maria Stankiewicz, Maciej WojtkowskiAbstract:In this pilot study we demonstrate results of structural Fourier Domain OCT imaging of the nervous system of Periplaneta americana L. (American cockroach). The purpose of this research is to develop an OCT apparatus enabling structural imaging of insect neural system. Secondary purpose of the presented research is to develop methods of the sample preparation and handling during the OCT imaging experiments. We have performed imaging in the abdominal nerve cord excised from the American cockroach. For this purpose we have developed a Fourier Domain / spectral OCT system operating at 820 nm wavelength range.
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Ultrahigh-resolution, high-speed, Fourier Domain optical coherence tomography and methods for dispersion compensation
Optics Express, 2004Co-Authors: Maciej Wojtkowski, Tony H Ko, Vivek J. Srinivasan, Andrzej Kowalczyk, James G Fujimoto, Jay S. DukerAbstract:Ultrahigh-resolution optical coherence tomography uses broadband light sources to achieve axial image resolutions on the few micron scale. Fourier Domain detection methods enable more than an order of magnitude increase in imaging speed and sensitivity, thus overcoming the sensitivity limitations inherent in ultrahigh-resolution OCT using standard time Domain detection. Fourier Domain methods also provide direct access to the spectrum of the optical signal. This enables automatic numerical dispersion compensation, a key factor in achieving ultrahigh image resolutions. We present ultrahigh-resolution, high-speed Fourier Domain OCT imaging with an axial resolution of 2.1 ìm in tissue and 16,000 axial scans per second at 1024 pixels per axial scan. Ultrahigh-resolution spectral Domain OCT is shown to provide a ~100x increase in imaging speed when compared to ultrahigh-resolution time Domain OCT. In vivo imaging of the human retina is demonstrated. We also present a general technique for automatic numerical dispersion compensation, which is applicable to spectral Domain as well as swept source embodiments of Fourier Domain OCT.
Ninghua Zhu - One of the best experts on this subject based on the ideXlab platform.
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dual chirp Fourier Domain mode locked optoelectronic oscillator
Optics Letters, 2019Co-Authors: Tengfei Hao, Jian Tang, Nuannuan Shi, Ninghua ZhuAbstract:Optoelectronic oscillators (OEOs) have been widely investigated to generate ultra-pure single-frequency microwave signals. Here, we propose and experimentally demonstrate a dual-chirp Fourier Domain mode-locked (FDML) OEO to generate dual-chip microwave waveforms. In the proposed FDML OEO, a frequency-scanning dual-passband microwave photonics filter based on phase-modulation-to-intensity-modulation conversion using an optical notch filter and two laser diodes is incorporated into the OEO cavity. Fourier Domain mode-locking operation is achieved by synchronizing the scanning period of the filter to the cavity round-trip time. Tunable dual-chirp microwave waveforms with a large time-bandwidth product are generated directly from the FDML OEO cavity in the experiment, which can be used in modern radar systems to improve its range-Doppler resolution.
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Harmonically Fourier Domain Mode-Locked Optoelectronic Oscillator
IEEE Photonics Technology Letters, 2019Co-Authors: Tengfei Hao, Jian Tang, Ninghua ZhuAbstract:We experimentally demonstrate a harmonically Fourier Domain mode-locked optoelectronic oscillator (FDML OEO). Harmonic Fourier Domain mode locking operation is achieved when the cavity round-trip time is equal to integer multiples of the scanning period of the filter in the OEO loop. Compared with a fundamentally FDML OEO, frequency scanning microwave signals with increased tuning speed and large bandwidth can be easily generated by the proposed harmonically FDML OEO, which can find applications in spread-spectrum communication and modern radar systems. Up to fourth order, harmonic Fourier Domain mode locking operation is achieved in the experiment. The time-bandwidth product of the harmonically FDML OEO is as large as tens of thousands. The maximum chirp rate of the generated $X$ -band linearly chirped microwave waveform is 0.725 GHz/ $\mu \text{s}$ , which is four times higher than that of the fundamentally FDML OEO.
Changsoo Shin - One of the best experts on this subject based on the ideXlab platform.
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regularized laplace Fourier Domain full waveform inversion using a weighted l 2 objective function
Pure and Applied Geophysics, 2017Co-Authors: Hyunggu Jun, Changsoo Shin, Jungmin Kwon, Hongbo Zhou, Mike CoganAbstract:Full waveform inversion (FWI) can be applied to obtain an accurate velocity model that contains important geophysical and geological information. FWI suffers from the local minimum problem when the starting model is not sufficiently close to the true model. Therefore, an accurate macroscale velocity model is essential for successful FWI, and Laplace–Fourier-Domain FWI is appropriate for obtaining such a velocity model. However, conventional Laplace–Fourier-Domain FWI remains an ill-posed and ill-conditioned problem, meaning that small errors in the data can result in large differences in the inverted model. This approach also suffers from certain limitations related to the logarithmic objective function. To overcome the limitations of conventional Laplace–Fourier-Domain FWI, we introduce a weighted l2 objective function, instead of the logarithmic objective function, as the data-Domain objective function, and we also introduce two different model-Domain regularizations: first-order Tikhonov regularization and prior model regularization. The weighting matrix for the data-Domain objective function is constructed to suitably enhance the far-offset information. Tikhonov regularization smoothes the gradient, and prior model regularization allows reliable prior information to be taken into account. Two hyperparameters are obtained through trial and error and used to control the trade-off and achieve an appropriate balance between the data-Domain and model-Domain gradients. The application of the proposed regularizations facilitates finding a unique solution via FWI, and the weighted l2 objective function ensures a more reasonable residual, thereby improving the stability of the gradient calculation. Numerical tests performed using the Marmousi synthetic dataset show that the use of the weighted l2 objective function and the model-Domain regularizations significantly improves the Laplace–Fourier-Domain FWI. Because the Laplace–Fourier-Domain FWI is improved, the frequency-Domain FWI, in which the Laplace–Fourier-Domain FWI result is used as the starting model, yields inversion result much closer to the true velocity.
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Regularized Laplace–Fourier-Domain Full Waveform Inversion Using a Weighted l _2 Objective Function
Pure and Applied Geophysics, 2017Co-Authors: Hyunggu Jun, Changsoo Shin, Jungmin Kwon, Hongbo Zhou, Mike CoganAbstract:Full waveform inversion (FWI) can be applied to obtain an accurate velocity model that contains important geophysical and geological information. FWI suffers from the local minimum problem when the starting model is not sufficiently close to the true model. Therefore, an accurate macroscale velocity model is essential for successful FWI, and Laplace–Fourier-Domain FWI is appropriate for obtaining such a velocity model. However, conventional Laplace–Fourier-Domain FWI remains an ill-posed and ill-conditioned problem, meaning that small errors in the data can result in large differences in the inverted model. This approach also suffers from certain limitations related to the logarithmic objective function. To overcome the limitations of conventional Laplace–Fourier-Domain FWI, we introduce a weighted l _2 objective function, instead of the logarithmic objective function, as the data-Domain objective function, and we also introduce two different model-Domain regularizations: first-order Tikhonov regularization and prior model regularization. The weighting matrix for the data-Domain objective function is constructed to suitably enhance the far-offset information. Tikhonov regularization smoothes the gradient, and prior model regularization allows reliable prior information to be taken into account. Two hyperparameters are obtained through trial and error and used to control the trade-off and achieve an appropriate balance between the data-Domain and model-Domain gradients. The application of the proposed regularizations facilitates finding a unique solution via FWI, and the weighted l _2 objective function ensures a more reasonable residual, thereby improving the stability of the gradient calculation. Numerical tests performed using the Marmousi synthetic dataset show that the use of the weighted l _2 objective function and the model-Domain regularizations significantly improves the Laplace–Fourier-Domain FWI. Because the Laplace–Fourier-Domain FWI is improved, the frequency-Domain FWI, in which the Laplace–Fourier-Domain FWI result is used as the starting model, yields inversion result much closer to the true velocity.
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Laplace–Fourier-Domain Full Waveform Inversion of Deep-Sea Seismic Data Acquired with Limited Offsets
Pure and Applied Geophysics, 2016Co-Authors: Yongchae Cho, Changsoo Shin, Youngseo Kim, Satish Singh, Eunjin ParkAbstract:Laplace–Fourier-Domain full waveform inversion is considered one of the most reliable schemes to alleviate the drawbacks of conventional frequency-Domain inversion, such as local minima. Using a damped wavefield, we can reduce the possibility of converging to local minima and produce an accurate long-wavelength velocity model. Then, we can obtain final inversion results using high-frequency components and low damping coefficients. However, the imaging area is limited because this scheme uses a damped wavefield that makes the magnitudes of the gradient and residual small in deep areas. Generally, the imaging depth of Laplace–Fourier-Domain full waveform inversion is half the streamer length. Thus, dealing with seismic data in the deep-sea layer is difficult. The deep-sea layer reduces the amplitude of signals and acts as an obstacle for computing an exact gradient image. To reduce the water layer’s effect, we extrapolated the wavefield with a downward continuation and performed refraction tomography. Then, we performed Laplace–Fourier-Domain full waveform inversion using the refraction tomography results as an initial model. After obtaining a final velocity model, we verified the inversion results using Kirchhoff migration. We presented common image gathers and a synthetic seismogram of Sumatra field data to prove the reliability of the velocity model obtained by Laplace–Fourier-Domain full waveform inversion. Through the test, we concluded that Laplace–Fourier-Domain full waveform inversion with refraction tomography of the downward-continued wavefield recovers the subsurface structures located at depth despite a relatively short streamer length compared to the water depth.
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laplace Fourier Domain elastic full waveform inversion using time Domain modeling
Geophysics, 2014Co-Authors: Hyunggu Jun, Changsoo Shin, Youngseo Kim, Jungkyun Shin, Dongjoo MinAbstract:To obtain subsurface information from onshore seismic exploration data using full-waveform inversion (FWI) based on the acoustic wave equation, elastic waves, such as ground roll and mode-converted waves, should be suppressed through heavy preprocessing. However, such preprocessing deforms not only the elastic waves but also the acoustic waves. Moreover, it is not easy to separate body waves from surface waves in seismic traces. For these reasons, we need to generate both types of waves in the modeling step to obtain seismic waves that are similar to real seismic waves. Therefore, elastic FWI using the elastic wave equation is necessary to achieve a more accurate FWI. In addition, elastic FWI can provide better geologic information than acoustic FWI because it inverts the P-wavevelocity, S-wave velocity, and density. Laplace-Fourier-Domain elastic FWI is an effective method because it inverts these multiple parameters and can be applied to real seismic data that lack low-frequency components. However, the conventional Laplace-Fourier-Domain FWI requires a matrix solver with a huge memory cost to perform the modeling in the Laplace-FourierDomain, and memory usage becomes more intensive in the elastic case. In the present study, we combined time-Domain wave propagation modeling and Laplace-Fourier-Domain elastic FWI to invert multiple parameters with less memory cost. By using time-Domain modeling, which does not require a matrix solver, we obtainedthe forwardand adjoint wavefields withless memory cost. The residuals between the recorded and modeled data, the virtual sources, the Hessian matrices, and the gradient directions were calculated in the Laplace-Fourier Domain. To validate the proposed algorithm, we performed numerical tests with Model 94 synthetic data and Benjamin Creek real seismic data.
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Temporal windowing and inverse transform of the wavefield in the Laplace-Fourier Domain
Geophysics, 2013Co-Authors: Sangmin Kwak, Wansoo Ha, Changsoo ShinAbstract:Temporal windowing is a valuable process, which can help us to focus on a specific event in a seismogram. However, applying the time window is difficult outside the time Domain. We suggest a windowing method which is applicable in the Laplace-Fourier Domain. The window function we adopt is defined as a product of a gain function and an exponential damping function. The Fourier transform of a seismogram windowed by this function is equivalent to the partial derivative of the Laplace-Fourier Domain wavefield with respect to the complex damping constant. Therefore, we can obtain a windowed seismogram using the partial derivatives of the Laplace-Fourier Domain wavefield. We exploit the time-windowed wavefield, which is modeled directly in the Laplace-Fourier Domain, to reconstruct subsurface velocity model by waveform inversion in the Laplace-Fourier Domain. We present the windowed seismograms by introducing an inverse Laplace-Fourier transform technique and demonstrate the effect of temporal windowing in a synthetic Laplace-Fourier Domain waveform inversion example.