Tarek A. Al-Saeed

Dispersion compensation in moving-optical-wedge Fourier transform spectrometer

In this paper we study the effect of material dispersion on the performance of a moving-optical-wedge Fourier transform spectrometer. The spectrum is thus evaluated numerically using a test spectrum source. The obtained numerical results show that the classical technique for numerical dispersion compensation, usually used with a Michelson interferometer, cannot be efficiently used with wedge interferometers as it is limited to the cases of weak dispersion. The error in this technique is thus evaluated in different cases and a new numerical technique is proposed to overcome this error. We also notice shrinkage in the interferogram spread in the spatial domain in contradiction with the normal dispersion effect in a Michelson interferometer.

Diffraction effects in optical microelectromechanical system Michelson interferometers

We study the effect of diffraction on the performance of microelectromechanical system Michelson interferometers. By using a simple Gaussian model, we calculate the degradation of the interferometer visibility due to the diffraction effect. We then use this model to estimate the optimum detector diameter to maximize the fringe visibility at the interferometer output and study its effect on the resolution of Fourier transform spectrometers based on Michelson interferometers.

Signal-to-noise ratio calculation in a moving-optical-wedge spectrometer

In this paper we study the signal-to-noise ratio degradation in a moving-optical-wedge interferometer when used as an optical spectrometer. Both the mechanical vibration and temperature fluctuation effects are studied, and the effects are compared to their counterparts in a conventional Michelson interferometer. While the wedge interferometer is found to be more immune to linear translational vibration, it shows much higher sensitivity to rotational vibration.

Spot size effects in miniaturized moving-optical-wedge interferometer

In this paper we study the effect of diffraction on the performance of a miniaturized moving-optical-wedge interferometer. By using the Gaussian model, we calculate the degradation of the interferometer visibility due to diffraction effects. We use this model to optimize the detector size required to obtain maximum visibility and study its effect on resolution of Fourier transform spectrometers based on a moving-optical-wedge interferometer. A comparison between these effects in Michelson and wedge interferometers is also presented showing the advantage of the moving-optical-wedge interferometer in suppressing the diffraction effects with respect to the Michelson interferometer.

Dispersion compensation in Fourier domain optical coherence tomography

In this work, we propose a numerical technique to compensate for errors due to dispersion effects in Fourier domain optical coherence tomography. The proposed technique corrects for errors in depth measurements and resolution loss due to dispersion. The results show that, by using this technique, errors in thickness measurement are reduced from about 5% to less than 0.1% depending on the sample length and the amount of dispersion. Also, an improvement in the resolution from about 50 μm to less than 10 μm is demonstrated.

A Method for Medical Diagnosis Based on Optical Fluence Rate Distribution at Tissue Surface

Optical differentiation is a promising tool in biomedical diagnosis mainly because of its safety. The optical parameters’ values of biological tissues differ according to the histopathology of the tissue and hence could be used for differentiation. The optical fluence rate distribution on tissue boundaries depends on the optical parameters. So, providing image displays of such distributions can provide a visual means of biomedical diagnosis. In this work, an experimental setup was implemented to measure the spatially-resolved steady state diffuse reflectance and transmittance of native and coagulated chicken liver and native and boiled breast chicken skin at 635 and 808 nm wavelengths laser irradiation. With the measured values, the optical parameters of the samples were calculated in vitro using a combination of modified Kubelka-Munk model and Bouguer-Beer-Lambert law. The estimated optical parameters values were substituted in the diffusion equation to simulate the fluence rate at the tissue surface using the finite element method. Results were verified with Monte-Carlo simulation. The results obtained showed that the diffuse reflectance curves and fluence rate distribution images can provide discrimination tools between different tissue types and hence can be used for biomedical diagnosis.

Symmetric moving-optical-wedge interferometer for Fourier transform spectroscopy: a step toward miniaturization

In this paper we propose a modification to the ordinary moving-optical-wedge interferometer used for Fourier transform spectrometry. The ordinary wedge interferometer suffers from asymmetry in the visibility pattern, asymmetry in the interferogram, large volume, large weight, and greater motion of the wedge. Our proposed interferometer is based on using two pair of wedges. This results in symmetry in visibility pattern, symmetry in interferogram, halved size, halved motion, and more throughput.

The use of optical fluence rate distribution for the differentiation of biological tissues

diffuse optical imaging is a recent, safe, non-invasive, functional, and very promising medical imaging technique that employs near infrared light to characterize biological tissue. Absorption and scattering properties of biological tissues give very important information about physiological changes associated with vasculature, cellularity, and oxygen consumption in normal and/or diseased tissue. Obtaining the optical fluence rate distribution is important when constructing the image in diffuse imaging. In this work, the significance of using the fluence rate distribution for differentiating tissue types was investigated. An experimental setup for measuring spatially-resolved steady state diffuse reflectance spectra of breast chicken skin at 660nm laser irradiation was implemented. The measured values were then used to predict the optical parameters of the samples using a combination of a modified Kubelka-Munk method and Bouguer-Beer-Lambert law. These parameters were used with a finite element model to solve the radiative transfer equation to obtain the fluence rate at the sample boundary. The results revealed that the fluence rate distribution of a 660nm laser is significant in differentiating biological tissues.

Fourier transform spectrometer based on Fabry-Perot interferometer.

We analyze the Fourier transform spectrometer based on a symmetric/asymmetric Fabry-Perot interferometer. In this spectrometer, the interferogram is obtained by recording the intensity as a function of the interferometer length. Then, we recover the spectrum by applying the discrete Fourier transform (DFT) directly on the interferogram. This technique results in spectral harmonic overlap and fictitious wavenumber components outside the original spectral range. For this purpose, in this work, we propose a second method to recover the spectrum. This method is based on expanding the DFT of the interferogram and the spectrum by a Haar or box function. By this second method, we recovered the spectrum and got rid of the fictitious spectral components and spectral harmonic overlap.

Study of dual-source Fourier-domain optical coherence tomography

Abstract. Optical coherence tomography (OCT) is a very high-resolution imaging technique whose resolution depends on source bandwidth. Improving resolution is an important topic of research in OCT. Thus, to improve resolution, the bandwidth of the source must be increased. Practical sources have finite bandwidth. Therefore, it is suggested to use more sources. Previous work expected that resolution will be inevitably improved without mentioning to what extent it will be improved and without any referencing to the effect of spectral separation between the sources. We study the resolution of a Fourier-domain OCT (FD-OCT) system based on two sources. First, we show to what extent resolution is improved and we show that this improvement not only depends on the spectral widths of the sources but also on spectral separation of the sources. Second, we show that in most cases resolution will become poorer and discuss mathematically the origin of resolution worsening and reveal the problems encountered in such a system. Third, we propose two techniques to overcome these problems. One of them is by shifting spectral interferograms and the other is by multiplying the spatial interferogram. Then, we clarify their advantages, disadvantages, major drawbacks, and limitations.