Stephen W. McKnight

Integrated self-biased hexaferrite microstrip circulators for millimeter-wavelength applications

Planar microstrip Y-junction circulators have been fabricated from metallized 130-/spl mu/m-thick self-biased strontium hexaferrite ceramic die, and then bonded onto silicon die to yield integrated circulator circuits. The impedance matching networks needed to transform the low-impedance circulator outputs were deployed on low-loss alumina or glass dielectrics to minimize circuit losses. These magnetically self-biased circulators show a normalized isolation and insertion loss of 33 and 2.8 dB, respectively, and a 1% bandwidth for an isolation of 20 dB. Application of small (H<1.5 kOe) magnetic bias fields improved the isolation and insertion loss values to 50 and 1.6 dB, respectively. This design may form the basis for future monolithic millimeter-wave integrated circulator circuits that do not require magnets.

Attenuation and dispersion for high-T/sub c/ superconducting microstrip lines

The attenuation and dispersion of microstrip lines of the high-T/sub c/ superconductor YBa/sub 2/Cu/sub 3/O/sub 7/ (YBCO) on yttria-stabilized zirconia substrates as a function of frequency and temperature are calculated. The effect on pulse propagation of superconducting and model dispersion in addition to the attenuation is demonstrated. At 60 K, microstrip lines of YBCO are significantly less attenuating at frequencies below 500 GHz than microstrip lines of copper at the same temperature. This advantage is particularly significant at the higher attenuations that result as the substrate thickness is made smaller for miniaturization or to improve the microstrip line bandwidth. The application of YBCO for microstrip lines appears to be most useful at frequencies above 100 GHz and dielectric thicknesses less than 100 mu m, where the attenuation of cooled copper is prohibitively large. Cooled to temperatures below 20 K, YBCO may make possible a new generation of extremely high bandwidth ( approximately 5 THz), small-feature-size ( approximately 5 mu m) circuits and devices. >

Theory and experiment of thin-film junction circulator

We have calculated the S-parameters and losses in ferrite-film-junction circulators using a new effective-field theory assuming TEM-like propagation. Conductivity loss dominates the dielectric and magnetic losses in Y-junction circulators fabricated on ferrite films with thicknesses less than 200 /spl mu/m. It is plausible to fabricate Y-junction thin-film circulator at X-band with insertion loss less than 0.5 dB if the film thickness is larger than 100 /spl mu/m. The quality of the conductor plane is important in reducing the overall insertion loss of the thin-film-junction circulator.

Picosecond-pulse and millimeter-wave spectroscopy of barium ferrite

Transmission measurements of a barium ferrite pressed-powder sample have been made with an optically switched picosecond-pulse spectrometer. By comparison to millimeter-wave spectroscopy of the same sample, features related to the ferrimagnetic resonance have been identified in the pulse spectrum. Time-domain spectra predicted from models of the permeability indicate significant discrepancies with the experimental short-pulse line shape.

Influence of nonuniform magnetic field on a ferrite junction circulator

We have analytically formulated the problem that a ferrite circulator junction is biased by a nonuniform magnetic field. Interport impedances of the junction can, therefore, be solved numerically. Nonuniform-bias field will reduce the transmission bandwidth, and the circulation condition is apt to be altered if the bias field shows nonuniformity near the center of the junction. Our calculation compares very well with measurements.

A Unified Discipline of Subsurface Sensing and Imaging Systems

Subsurface sensing and imaging seeks to locate and identify objects or conditions underneath an obscuring media by monitoring a probe or wave outside the surface. Many of the mathematical and physical models used in this process are common to underground and underwater environmental exploration, medical imaging, and three-dimensional microscopies, allowing a common framework of physic-based signal processing (PBSP) to be applied. The basis for a unified discipline of subsurface sensing and imaging can be identified from a few general subsurface information extraction strategies. These strategies and their related families of PBSP algorithms can be used to guide a systems-oriented approach to subsurface solutions.

Measurement of magnetic fields using the magneto‐optic Kerr effect

A technique for measuring small magnetic fields using a thin amorphous magnetic film and the magneto‐optic Kerr effect has been developed. A thin film having in‐plane uniaxial anisotropy is aligned such that an external magnetic field rotates the film magnetization in a controllable manner. This magnetization rotation is ‘‘read’’ optically by a coherent homodyne measurement of the amount of polarization rotation caused by the magneto‐optic Kerr effect during reflectance of polarized light from the film surface, using one polarization as the local oscillator. The resulting signal is linear in magnetic field. Initial noise equivalent magnetic field sensitivity measurements of the technique yield 2 mOe/Hz1/2 at a frequency of 8 mHz. Calculations indicate that sensitivities of order 10−6 Oe/Hz1/2 are obtainable using this technique.

Laser-induced acoustic imaging of buried land mines: experiment and modeling

The use for subsurface buried object detection of high-frequency (15-30 kHz) acoustic waves generated by CO2 laser pulses incident on the surface of dry sand has been demonstrated previously. In this work, field tests of the technique have demonstrated imaging of landmine simulants buried 2.5 cm below the surface in an outdoor test track. Acoustic finite-difference time-domain calculations have given insight into the observed acoustic lineshapes and verified that the over-estimate of the target dimensions in the outdoor field trials may be related to the lower frequency detector used in these measurements. The models also suggest that a large increase in detected signal may potentially be gained by the use of a Laser Doppler Vibrometer interfacial velocity detector in the place of the present airborne microphone.

Laser-Induced Acoustic Imaging of Buried Objects

A 100 mJ, 100 ns pulsed CO2 laser incident on the surface of soil is used as a localized acoustic source for the detection and imaging of underground objects. The acoustic pulse produced by the impulsive heating of the soil surface due to the absorbed 10.6 μm radiation is detected with an acoustic transducer suspended over the surface. Application of a Fourier domain filter enables the separation of the direct acoustic return from the faint echo from an object buried 3–25 mm below the surface. Scanning of the laser pulses across the position of a buried object allows the resolution of the shape and the depth of the buried object. The application of this technique to image buried landmines is demonstrated in trials at an outdoor test track.

Laser-induced acoustic generation for buried object detection

Mechanisms for the production of acoustic energy in soil by pulsed CO2 laser excitation of the surface are reported. When the laser pulse in unfocused with a spot size about 1 cm in diameter, a single narrow acoustic pulse is observed with a spectral content near the detector limit of 100 kHz and a velocity of 255 m/s, close to the speed of sound in air. Whenthe laser is focused to a spot size on the order of 1 mm diameter, the audible acoustic intensity in greatly increased and we observe a second broad acoustic feature. This feature has a much lower frequency and velocity. We have tentatively identified the fast mode as a normal compressive mode and the slow mode as a Biot slow-wave. A study of visible light emission when the focused CO2 laser beam strikes the sand surface indicates ionized nitrogen, oxygen, and silicon are present. This implies that the mechanism for sound production with the focused beam involves ionization by the optical electric field, expansion, and subsequence collapse of the air. The mechanisms for sound production by the unfocused beam, which produces better imaging of underground objects, appears to be quite different.