Christopher C. Davis Christopher C. DavisFree 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 been constructing novel diode-pumped Nd:YAG lasers that operate in as dual-frequency mode, and are making detailed studies of their noise performance, control, and stabilization. To provide compact, high-brightness pump source for these lasers we have been studying 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.
Mechanisms of interaction of non-ionizing radiation with biomaterials.
Engineering support for life-scientists studying the biological effects of non-ionizing radiation. Current collaborations exist with the University of Maryland at Baltimore (Drs. George Harrison and Elizabeth Balcer-Kubiczek), Polytechnic University (Professor Shirley Motazkin), and the Food and Drug Administration (Drs. Russell Owen and Ewa Czerska).
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 [1] 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.
For details of our recent dielectric measurements on tissue click below
Dielectric
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