Heba Yuksel

Aperture Averaging Analysis and Aperture Shape Invariance of Received Scintillation in Free Space Optical Communication Links

Intensity scintillation and beam wander caused by atmospheric turbulence are two significant phenomena that affect free space optical (FSO) communication links. We have constructed an imaging system for measuring the effects of atmospheric turbulence and obscuration on FSO links. A He-Ne laser beam propagates over a range of 863 meters in atmospheric turbulence conditions that vary diurnally and seasonally from weak to strong. A high performance digital camera with a frame-grabbing computer interface is used to capture received laser intensity distributions at rates up to 30 frames per second and various short shutter speeds, down to 1/16,000s per frame. The captured image frames are analyzed in Labview to evaluate the turbulence index parameter, temporal and spatial intensity variances, and aperture averaging. The aperture averaging results demonstrate the expected reduction in intensity fluctuations with increasing aperture diameter, and show quantitatively the differences in behavior between various strengths of turbulence. This paper will present the most accurate empirical data to date for the weak and intermediate turbulence regime. Such results can help build upon existing empirical data and lead to the development of new theories. Aperture averaging of the received irradiance is also shown to be independent of the shape of the receiver aperture, and depends only on its area. This finding allows the use of refractive or catadioptric receivers, whichever is convenient, and the same amount of aperture averaging will be achieved for equal unobscured aperture areas. This can make the telescope design for an FSO receiver more compact.

Fourier transform optical profilometry using fiber optic Lloyd’s mirrors

A fiber optic Lloyds mirror assembly is used to obtain various optical interference patterns for the detection of 3D rigid body shapes. Two types of fiber optic Lloyds systems are used in this work. The first consists of a single-mode optical fiber and a highly reflecting flat mirror to produce bright and dark strips. The second is constructed by locating a single-mode optical fiber in a v-groove, which is formed by two orthogonal flat mirrors to allow the generation of square-type interference patterns for the desired applications. The structured light patterns formed by these two fiber Lloyds techniques are projected onto 3D objects. Fringe patterns are deformed due to the objects surface topography, which are captured by a digital CCD camera and processed with a Fourier transform technique to accomplish 3D surface topography of the object. It is demonstrated that the fiber-optic Lloyds technique proposed in this work is more compact, more stable, and easier to configure than other existing surface profilometry systems, since it does not include any high-cost optical tools such as aligners, couplers, or 3D stages. The fringe patterns are observed to be more robust against environmental disturbances such as ambient temperature and vibrations.

Aperture Averaging and Correlation Function Measurements in Strong Atmospheric Turbulence for Optical Wireless Applications

The performance of free space optical (FSO) links in a clear atmosphere is affected by the non-ideal characteristics of the communication channel. Atmospheric turbulence causes fluctuations in the received signal level, which increase the bit errors in a digital communication link. In order to quantify performance limitations, a better understanding of the effect of the intensity fluctuations on the received signal at all turbulence levels is needed. Theory reliably describes the behavior in the weak turbulence regime, but theoretical descriptions in the intermediate and strong turbulence regimes are less well developed. We have developed a flexible empirical approach for characterizing link performance in strong turbulence conditions through image analysis of intensity scintillation patterns coupled with frame aperture averaging on an FSO communication link. These measurements are complemented with direct measurements of temporal and spatial correlation functions. A He-Ne laser beam propagates 106 meters in free-space over flat terrain about a meter above the ground to provide strong atmospheric turbulence conditions. A high performance digital camera with a frame-grabbing computer interface is used to capture received laser intensity distributions at rates up to 30 frames per second and various short shutter speeds, down to 1/16,000s per frame. A scintillometer is used for accurate measurements of the turbulence parameter Cn2. Laboratory measurements use a local strong turbulence generator, which mimics a strong phase screen. Spatial correlation functions are measured using laterally separated point detectors placed in the receiver plane. Correlations and captured image frames are analyzed in Labview to evaluate correlation functions, Cn2, and the aperture averaging factor. The aperture averaging results demonstrate the expected reduction in intensity fluctuations with increasing aperture diameter, and show quantitatively the differences in behavior between various strengths of turbulence. This paper will present accurate empirical data in the strong turbulence regime. Such results can help build upon existing empirical data and lead to the development of new theories.

Power analysis based side-channel attack on visible light communication

Abstract Visible light communication (VLC) technology has emerged as a significant and necessary improvement to the wireless communication technologies; VLC systems are considered to be more secure than wireless fidelity (Wi-Fi) systems by design, since light cannot penetrate most walls and obstacles. This paper presents a design for a bidirectional VLC system, and it also introduces a method to jeopardize the security aspect of this system. The methodology in this paper is side-channel attack which is defined as exploiting data based on the information gained from physical implementation. It is not a cyber attack on the theoretical weaknesses in the algorithms, but rather is a sideway approach to a fully-functioning physical system. Thus, the authors became the attackers and implemented a current sensor on the power lines from the power supply to the VLC system; the instantaneous power consumed by the communication system has become their main source to obtain the binary data transmitted by visible light. The work has resulted in effective attacks which could successfully extract the information and it has demonstrated side channel attacks on VLC systems through their instantaneous power consumption analysis.

Optical propagation in a turbulent atmosphere using the split step method

It is well known that optical signals propagating through the atmosphere are subject to random fluctuations in phase and amplitude. These fluctuations are caused by random temperature distributions in the atmosphere, which manifests themselves as a random index of refraction changes along the propagation path. We introduce a simulation method for modeling atmospheric turbulence effects, which is based on a split-step approach to numerically solve the parabolic wave equation. Atmospheric turbulence effects are modeled by a number of phase screens. These phase screens are generated on a numerical grid of finite size, which corresponds to a narrow spatial area of atmospheric turbulence.

3D Phase Stepping Optical Profilometry Using a Fiber Optic Lloyd's Mirror

This study defines measurements of three-dimensional rigid-body shapes by using a fiber optic Lloyd’s mirror. A fiber optic Lloyds mirror assembly is basically a technique to create an optical interference pattern using the real light point sources and their images. The generated fringe pattern thanks to this technique is deformed when it is projected on an objects surface. The introduced surface profilometry algorithm depends on a multi-step phase shifting process. The deformed fringe patterns containing information of the objects surface profile are captured by a digital CCD camera. While each frames are captured, required π∕2 phase shifts for interference fringe pattern are obtained by mechanically sliding the Lloyd assembly via an ordinary micrometer stage. Some preprocess algorithms are applied to the frames and are processed with an algorithm to accomplish 3D topographies. Finally, the continuous data determines the depth information and the surface topography of the object. The experimental setup is simple and low cost to construct, and is insensitive to the ambient temperature fluctuations and environmental vibrations that cause unwanted effects on the projected fringe pattern. Such a fiber optic Lloyd’s system which provides an accurate non-contact measurement without contaminating and harming the object surface has a wide range of applications from laser interference based lithography in nano-scale to macro-scale interferometers.

Phase stepping optical profilometry using fiber optic Lloyd's mirrors.

A three-step phase stepping profilometry based on a fiber optic Lloyds mirror assembly is employed in the optical profilometry for the first time to measure the shapes of 3D objects. Required π/2 phase shifts for interference fringe pattern are obtained by mechanically sliding the Lloyd assembly via an ordinary micrometer stage. The experimental setup is simple and low cost to construct, and is insensitive to the ambient temperature fluctuations and environmental vibrations that cause unwanted effects on the projected fringe pattern. Consecutive interferograms are captured by a CCD camera and are processed with an algorithm to accomplish 3D topographies.

Four-core optical fiber as a calorimetric gauge

A four-core optical fiber is demonstrated as a calorimetric gauge for investigation of one-dimensional heat transfer measurements. Transient heat pulses from a Nd:YAG laser of 600 ms duration with a repetition rate of the order of 10 s are delivered onto the cleaved distal end face of the four-core fiber, aiming at one of the single cores only, which cause an optical path length difference between four guiding cores due to the temperature-induced change in the index of refraction and physical length of the targeted fiber core of concern. This results in a shift in the fringe pattern, which is operated in the reflection scheme. A phase shift of 0.43±0.015  rad is measured with a CMOS camera for 40 mW pulses. The thermal heat diffusion length in the selected fiber core is determined to be 2.8 mm, which contains 10.9±0.38  kJ/m2s heat, causing a temperature rise of 1.43±0.05  K.

3D optical profilometry using a fiber optic Lloyd's mirror

This study defines measurements of three-dimensional rigid-body shapes by using a fiber optic Lloyd’s mirror. A fiber optic Lloyds mirror assembly is basically a technique to create an optical interference pattern using real light point sources and their images. The generated fringe pattern thanks to this technique is deformed when projected on an objects surface. The deformed fringe pattern containing information of the objects surface profile is captured by a digital CCD camera. The two-dimensional Fourier transformation is applied to the image, which is digitized with a frame grabber card. After applying a band-pass filter to this transformed data in its spatial frequency domain, the twodimensional inverse Fourier transform is applied. Using the complex data obtained by the inverse Fourier transform, the phase information is determined. A phase unwrapping algorithm is applied to eliminate discontinuities in the phase information and to make the phase data continuous. Finally, the continuous data determines the depth information and the surface topography of the object. It is illustrated for the first time that the use of such a fiber optic Lloyds system increases the compactness and the stability of the fringe projection system. Such a fiber optic Lloyd’s system which provides an accurate non-contact measurement without contaminating and harming the object surface has a wide range of applications from laser interference lithography (LIL) in nano-scale to macro-scale interferometers.

Geometrical modeling of optical phase difference for analyzing atmospheric turbulence

Ways of calculating phase shifts between laser beams propagating through atmospheric turbulence can give us insight towards the understanding of spatial diversity in Free-Space Optical (FSO) links. We propose a new geometrical model to estimate phase shifts between rays as the laser beam propagates through a simulated turbulent media. Turbulence is simulated by filling the propagation path with spherical bubbles of varying sizes and refractive index discontinuities statistically distributed according to various models. The level of turbulence is increased by elongating the range and/or increasing the number of bubbles that the rays interact with along their path. For each statistical representation of the atmosphere, the trajectories of two parallel rays separated by a particular distance are analyzed and computed simultaneously using geometrical optics. The three-dimensional geometry of the spheres is taken into account in the propagation of the rays. The bubble model is used to calculate the correlation between the two rays as their separation distance changes. The total distance traveled by each ray as both rays travel to the target is computed. The difference in the path length traveled will yield the phase difference between the rays. The mean square phase difference is taken to be the phase structure function which in the literature, for a pair of collimated parallel pencil thin rays, obeys a five-third law assuming weak turbulence. All simulation results will be compared with the predictions of wave theory.