Since the 1970’s, the focused laser beam has become a familiar tool to manipulate neutral, dielectric micro-objects. A number of authors, including Higurashi and Gauthier, have described the effects of radiation pressure from laser light on microrotors. Collett, et al. developed a wave, rather than a ray optic, approach in the calculation of such forces on a microrotor for the first time. This paper describes a modification to the design of a laser driven, radiation pressure microrotor, intended to improve the optically generated torque. Employing the wave approach, the electric and magnetic fields in the vicinity of the rotor are calculated using the finite difference time domain (FDTD) method, which takes into account the wave nature of the incident light. Forces are calculated from the application of Maxwell’s stress tensor over the surfaces of the rotor. Results indicate a significant increase in torque when the index of refraction of the microrotor is changed from a single value to an inhomogeneous profile. The optical fiber industry has successfully employed a variation in the index of refraction across the cross section of a fiber for the purpose of increasing the efficiency of light transmission. Therefore, it is hoped that various fabrication methods can be utilized for causing desired changes in the index of refraction of an optically driven microrotor. Various profiles of the index of refraction inside a microrotor are considered for optimization of torque. Simulation methodology and results of torque on a microrotor for various profiles of the index of refraction are presented. Guidelines for improvised fabrication of efficient microrotors may then be obtained from these profiles.
This report presents a summary of signal strength testing conducted with the metal detector (MD) subsystem of the Mine H/K (hunter/killer) vehicular mine detection system. An overview of the operational characteristics of the MD subsystem, the VMV16, is provided. Tests are described that assess the variation in sensitivity across the MD coil array. Absolute sensitivity measurements of the MD array are also presented. Results presented show that the array has sufficient sensitivity to detect low metal (LM) mines provided the mines are not located further than 3.5 inches from the plane of array. Laboratory experiments indicate that saturation and a limited temporal sampling window severely restrict any opportunity for discrimination based on eddy current decay predictions/comparisions.