Ahmad Safaai-Jazi

Novel techniques for the fabrication of holey optical fibers

Recently developed optical fibers rely on an array of air holes in the cladding to confine light to the fiber core as opposed to conventional telecommunications fibers that require a refractive index difference produced by different composition glasses in the core and cladding regions. Holey fibers have been fabricated by drawing an array of tubes stacked around a solid central core. In this paper, we describe a new technique to produce the holes (or pores) in the cladding region. These new fibers have been made by drawing a preform, consisting of a porous outer cladding region surrounding a solid central core region, into a fiber. During the fiber drawing process, the pores initially present in the preform cladding region are drawn into small, long, thin tubular pores. Controlling the dimensions and distribution of the pores in the preform can control the physical dimensions and distribution of the pores in the fiber. In some of the preforms, the porous cladding region in the preform was prepared by sol gel techniques. The preform fabrication process and fiber drawing process used to produce these new holey fibers as well as the results of the morphological study elucidating the size, shape and distribution of the porous phase are presented.

A new formulation for the design of Chebyshev arrays

A new formulation for the design of Chebyshev arrays is presented which makes no direct use of Chebyshev polynomials. The array factor is expressed in terms of cosine/cosine-hyperbolic functions based on which all analysis and design steps are carried out. Zeros of the array factor are used to obtain a system of equations for excitation currents. Solving this system of equations, current magnitudes are determined in terms of the current of one element chosen as the independent variable. A general formulation for an even or odd but otherwise arbitrary number of elements is presented. The minimum number of elements required to achieve the desired beamwidth and side lobe level is obtained in a single step without resorting to an iterative process. The optimum spacing between elements for broadside and end fire arrays is also addressed. Numerical results for example cases are provided. >

A new formulation for characterization of materials based on measured insertion transfer function

A widely used method for noncontact electromagnetic characterization of materials is based on the measurement of an insertion transfer function. This function, defined as the ratio of two phasor signals measured in the presence and absence of the material under test, is related to the dielectric constant of the material through a complex transcendental equation. Solving this equation requires a numerical two-dimensional root search technique, which is often time consuming due to slow convergence and the existence of spurious solutions. In this paper, a new formulation is presented, which facilitates the evaluation of complex dielectric constant of low-loss materials by means of real equations, thus requiring only one-dimensional root search techniques. Two sample materials are measured, and it is shown that their dielectric constants obtained from the exact complex equation and the new formulation are in excellent agreement. The new formulation reduces the computation time significantly and is highly accurate for the characterization of low-loss materials.

Path-loss and time dispersion parameters for indoor UWB propagation

The propagation of ultra wideband (UWB) signals in indoor environments is an important issue with significant impacts on the future direction and scope of the UWB technology and its applications. The objective of this work is to obtain a better assessment of the potentials of UWB indoor communications by characterizing the UWB indoor communication channels. Channel characterization refers to extracting the channel parameters from measured data. An indoor UWB measurement campaign is undertaken. Time-domain indoor propagation measurements using pulses with less than 100 ps width are carried out. Typical indoor scenarios, including line-of-sight (LOS), non-line-of-sight (NLOS), room-to-room, within-the-room, and hallways, are considered. Results for indoor propagation measurements are presented for local power delay profiles (local PDP) and small-scale averaged power delay profiles (SSA-PDP). Site-specific trends and general observations are discussed. The results for path-loss exponent and time dispersion parameters are presented.

Microstructural analysis of random hole optical fibers

A new type of optical fiber, the random hole optical fiber, has been fabricated and consists of a solid silica central core region surrounded by a porous silica cladding region. The holes present in the cladding region are random in both spatial location and in size, varying from approximately 25 nm to 2.5 microns in radius. Over a thousand holes are present in the fiber. The spectral attenuation in the fiber was measured to be about 2.0 dB/m at 1550 nm. The fibers were prepared by the incorporation of a gas producing agent in the optical fiber preform, which generated the holes in situ during the fiber drawing process.

UWB applications for through-wall detection

Preliminary experimental investigations on the application of UWB electromagnetic signals for through-wall detection are presented. Measurements are conducted using pulses with a full-width half-maximum (FWHM) nearly equal to 85 ps. This corresponds to a spatial resolution of about 26 mm. A simple signal processing approach is also introduced to extract the target image from the reflected fields.

UWB channel impulse response characterization using deconvolution techniques

UWB Channels can be measured by sounding the channel with pulses, and thereby obtain the impulse response. Multipath components have different waveforms depending on the type of transmitter and receiver antennas used and the angles of transmission and reception. A modified deconvolution technique is introduced to extract the UWB channel response. The application of deconvolution techniques results in resolving multipath components with waveforms different from that of the sounding pulse. Resolving more components should improve the design of the rake receiver. Accurate characterization for the impulse response of a UWB communication system facilitates performance evaluation studies such as simulating the effect of pulse shaping.

Results from the Virginia Tech propagation experiment using the Olympus satellite 12, 20 and 30 GHz beacons

A comprehensive set of propagation experiments was performed using the Olympus satellite 12, 20, and 30 GHz beacons. This set of experiments is unique in North America because of simultaneous reception of signals spanning the Ku- and Ka-bands from the same orbital slot, which permits direct inference of the frequency behavior of signal variations. The elevation angle from the receiving site in Blacksburg, VA, to the satellite was 14 degrees. Beacon, radiometric, and weather data for one year were analyzed. The statistical results for rain rate, beacon attenuation, attenuation ratios, radiometrically derived attenuation, fade duration and fade slope are presented. They are important to the design of Ku- and Ka-band satellite communication systems. The beacon attenuation results include cumulative statistics for attenuation with respect to free space and with respect to clear air. Attenuation ratio data are presented using attenuation with respect to clear air to focus on rain effects. Instantaneous attenuation ratios computed from instantaneous beacon levels were found to be nearly identical to statistical attenuation ratios obtained from cumulative attenuation statistics at each frequency. >

Ultra wideband material characterization for indoor propagation

The information on electromagnetic properties of building materials in the ultra wideband (UWB) frequency range provides valuable insights in assessing the capabilities and limitations of UWB technology. This research examines propagation through typical construction materials and their ultra wideband characterization. Ten commonly used construction materials are chosen for this investigation. Results for the insertion loss and the dielectric constant of each material over a frequency range of 0.5 to 15 GHz are presented.

Single-crystal sapphire-based optical high-temperature sensor for harsh environments

An optical temperature-sensing instrument based on broadband polarimetric differential interferometry (BPDI) technology is developed for ultrahigh temperature measurement in chemically corrosive environments. A single-crystal sapphire disk with inherent birefringence is employed as a sensing element in this instrument. Absolute temperature information is obtained and the measurement results are relatively immune to optical source power fluctuations and transmission fiber losses. Other advantages of this sensor are its mechanical simplicity, high resolution, and great accuracy over a wide measurement range.