Former Director of the Gemstone Program
Research in the laser sensor group covers a broad range of topics in the area of fiber and free-space laser interferometry scanning microscopy, and optical wireless. Coherent, hybrid, homodyne and heterodyne fiber sensors have been studied that can be used for remote electrical and magnetic field measurement, for mapping the surface structure of optical components with high spatial resolution, and for identifying patterns of birefringence in GaAs and related electronic and photonic devices. We have extended the ideas of Near-Field Scanning Optical Microscopy (NSOM) with our heterodyne single-mode fiber probes. By tapering an uncoated fiber to a taper on the order of 100nm in diameter we have been able to build an interferometric scanning microscopy, with sub-optical (20 -50 nm) resolution.
Most of our recent NSOM work has been aimed at studies of fundamental phenomena that occur at or near surfaces. This includes investigation of surface plasmon dynamics, nonlinear generation at surfaces, nanoscale direct lithography, and studies of ferroelectric and ferromagnetic materials.
Free space interferometry is being applied to studies of atmospheric turbulence and to the detection of interesting molecules in the atmosphere at very low concentrations (< 1ppb). Our research is also directed at improving, and understanding better, the laser sources that perform best in ultra-sensitive coherent sensors. To this end we have constructed novel diode-pumped Nd:YAG lasers that operate in as dual-frequency mode, and have made detailed studies of their noise performance, control, and stabilization. To provide compact, high-brightness pump source for these lasers we have studied the injection locking of broad-area diode lasers with the production of substantial power in a diffraction-limited lobe. We have also been developing incoherent and coherent fiber optic probes for biomedical applications.
Our principal sensor project in the biomedical area involves development of a practical magnetostrictive fiber sensor that can record, with good signal-to-noise ratio, the magnetocardiagram (MCG). The beating heart produces a field of about 100 picoTesla at the outer surface of the chest and this magnetic field variation with time can reveal cardiac abnormalities that do not show up on the electrocardiagram (ECG). Currently, the only reliable way to record the MCG is with a SQUID magnetometer, which is expensive technology and functions at cryogenic temperatures. Our sensor uses the commercial magnetostrictive material Metglas and has current sensitivity of 3pT per root Hertz.
We are engaged in a program of research designed to improve the performance of line of sight optical communication links (optical wireless) through the atmosphere along paths relatively close to the ground. These links must perform in the face of varying degrees of atmospheric turbulence and obscuration. The principal difficulties in achieving high data rate and low bit-error-rate performance with such links are discussed. A series of studies is described, which involve new modulation approaches, sources, and coding schemes to deal with problems such as fading, tracking and pointing, and various system engineering issues. Specifically, the program of research can be summarized as follows:
Studies of fade statistics on realistic urban-based line-of-sight ranges.
Comparison of link performance at 1.3 micrometers and 1.55 micrometers.
Studies of atmospheric chiralityStudies of Polarization Shift Keying and Polarization Diversity for fade resistance, and channel capacity doubling.
Tests of delayed, orthogonal channel polarization diversity for fade resistance.
Bit-error-rate measurements at high (1Gb/s) data rates.
Development and testing of forward error correcting codes for turbulent channels.
System engineering involving transmitter/receiver design, and aperture averaging.
Tests of system concepts with an artificial turbulence generator.
Research in this area includes:
Mechanisms of interaction of non-ionizing radiation with biomaterials.
Engineering support for life-scientists studying the biological effects of non-ionizing radiation. Past collaborations with the University of Maryland at Baltimore (Drs. George Harrison and Elizabeth Balcer-Kubiczek), and Polytechnic University (Professor Shirley Motzkin)
In a collaboration with Professor Leonard S. Taylor of this department and Dr. Edward C. Elson of the Walter Reed Army Institute of Research we have been making a comprehensive series of measurements of the complex dielectric constants of tissues in the head. This research is supported by Wireless Technology Research LLC.
In order to assess human exposure to the radiation from cellular telephones complex numerical modelling of the energy absorbed in models of the human head with realistic models of the cellular phone itself included are being carried out. These numerical models generally use detailed volume pixellated subdivisions of the head by tissue type determined from, for example, magnetic resonance imagery. In order to make the theoretical analysis meaningful reliable values of the complex dielectric constants of different tissues in the head are required. We have completed a comprehensive series of such measurements from 500 MHz to 10 GHz on 27 different tissues. The tissues measured were taken from the heads of freshly killed pigs, and measurements were made as soon as possible after death. Pigs were used as a model since there is general consensus that their tissues are similar to human tissue. We have monitored the change in dielectric properties of various tissues over time, and as a function of temperature, in order to determine as reliably as possible the likely value of the dielectric constants in-vivo. We were able to make measurement of the external tissues on human subjects. The measurements were made by the ``open-probe" technique  and the reliability of the data checked by measurements on standard materials.
The tissues that we have measured include: cortical and cancellous bone, human skin in various locations on the head, muscle, cartilage, white and grey brain matter in various locations, the medulla, cornea, aqueous humor, sclera, meningeal tissue, tongue, pons, peduncle, ventrical lining surface, and subcutaneous fat. Measurements on soft tissue are relatively easy to make, and the results generally agree with the recent data of Gabriel et al. However, the measurements on bone show great variability and are very strongly influenced by the degree of drying of the sample that has occurred. There is strong evidence that the real part of the dielectric constant of bone is higher than previously thought, perhaps as high as 40, and that this may vary from location to location. The conductivity of bone varies up to about 0.5S/m at 900 MHz and about 0.8S/m at 1800 MHz. It would be very desirable for additional measurements of the complex dielectric constant of bone to be made by an independent technique.
Jian-Zhong Bao, Mays L. Swicord, and Christopher C. Davis, ``Microwave dielectric characterization of binary mixtures of water, methanol, and ethanol" J. Chem. Phys. 104, 4441-4450, 1995.
For details of our recent dielectric measurements on tissue click below
For more details of the research in the laser sensor laboratory and laboratory personnel click below.
For details of Professor Davis's courses offered at the University of Maryland click below.
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