Todd W. Du Bosq

Millimeter wave imaging system for land mine detection

Applications using millimeter wave (mmW) and THz radiation have increased during the past few years. One of the principal applications of these technologies is the detection and identification of objects buried beneath the soil, in particular land mines and unexploded ordnances. A novel active mmW scanning imaging system was developed for this purpose. It is a hyperspectral system that collects images at different mmW frequencies from 90 to 140 GHz using a vector network analyzer collecting backscattering mmW radiation from the buried sample. A multivariate statistical method, principal components analysis, is applied to extract useful information from these images. This method is applied to images of different objects and experimental conditions.

Multilayer silicon cavity mirrors for the far-infrared p-Ge laser

Multilayer mirrors capable of > 99.9% reflectivity in the far infrared (70-200 microm wavelengths) were constructed using thin silicon etalons separated by empty gaps. Calculations indicate that only three periods are required to produce 99.9% reflectivity because of the large difference between the index of refraction of silicon (3.384) and the vacuum (1). The mirror was assembled from high-purity silicon wafers, with resistivity over 4000 omega cm to reduce free-carrier absorption. Wafers were double-side polished with faces parallel within 10 arc sec. The multilayer mirror was demonstrated as a cavity mirror for the far-infrared p-Ge laser. Dependence of reflectivity on design accuracy was considered.

High-reflectivity intracavity Bragg mirrors for the far-infrared p-Ge laser

Multi-layer mirrors capable of >99.9% reflectivity at ~100 micron wavelengths were constructed using thin silicon etalons separated by empty gaps. Due to the large difference between the index of refraction of silicon (3.384) and vacuum (1), calculations indicate that only three periods are required to produce 99.9% reflectivity. The mirror was assembled from high purity silicon wafers, with resistivity over 4000 ohm-cm to reduce free carrier absorption. Wafers were double side polished with faces parallel within 10 arc seconds. The multi-layer mirror was demonstrated as a cavity mirror for the far-infrared p-Ge laser.

Terahertz and millimeter wave transmission of soils

Transmission spectra were measured over the range 90-140 GHz and 300-4200 GHz for 20 soil samples that span a number of soil orders that have extensive worldwide distribution. A vector network analyzer equipped with 16 degree horn antennas covered the spectral range 90-140 GHz. Transmission measurements were also taken for some organic materials in the 90-140 GHz range. A Fourier spectrometer equipped with Hg arc lamp, pellicle beamsplitter, and Si bolometer collected transmission spectra over the range 300 to 4200 GHz. Transmittance ranged from 10-7 to almost 1. In all cases, transmission drops to zero for wavelengths shorter than the characteristic particle size of the sample as a consequence of scattering. In samples of mixed particle size, low transmittance in the 90-140 GHz range was found to be caused by the coarse component. This work is relevant to mine detection using THz and millimeter wave (mmW) radiation.

Scanning Fabry-Perot filter for terahertz spectroscopy based on silicon dielectric mirrors

A scanning Fabry-Perot transmission filter composed of a pair of dielectric mirrors has been demonstrated at millimeter and sub-millimeter wavelengths. The mirrors are formed by alternating quarter-wave optical thicknesses of silicon and air in the usual Bragg configuration. Detailed theoretical considerations are presented for determining the optimum design. Characterization was performed at sub-mm wavelengths using a gas laser together with a Golay cell detector and at mm-wavelengths using a backward wave oscillator and microwave power meter. High resistivity in the silicon layers was found important for achieving high transmittance and finesse, especially at the longer wavelengths. A finesse value of 411 for a scanning Fabry-Perot cavity composed of three-period Bragg mirrors was experimentally demonstrated. Finesse values of several thousand are considered to be within reach. This suggests the possibility of a compact terahertz Fabry-Perot spectrometer that can operate in low resonance order to realize high free spectral range while simultaneously achieving a high spectral resolution. Such a device is directly suitable for airborne/satellite and man-portable sensing instrumentation.

Millimeter wave imaging system for the detection of nonmetallic objects

With over 110 million landmines buried throughout the world, the ability to detect and identify objects beneath the soil is crucial. The increased use of plastic landmines requires the detection technology to be able to locate both metallic and non-metallic targets. A novel active mmW scanning imaging system was developed for this purpose. It is a hyperspectral system that collects images at different mmW frequencies from 90-140 GHz using a vector network analyzer collecting backscattering mmW radiation from the buried sample. A multivariate statistical method, Principal Components Analysis, is applied to extract useful information from these images. This method is applied to images of different objects and experimental conditions.

Fixed wavelength selection for the far-infrared p-Ge laser using thin silicon intracavity etalon

A thin two-side polished silicon etalon is demonstrated as a fixed-wavelength intracavity selector for the far-infrared p-Ge laser. The active cavity finesse is ~ 0.1. The wavelength position and spectral purity are maintained over a wide range of laser operating fields. A p-Ge laser with such a selector may find application in chemical sensing, THz imaging, or non-destructive testing.

Gain improvement for the THz p-Ge laser using neutron transmutation doped active crystal

A far-infrared p-type germanium laser with active crystal prepared from ultra pure single-crystal Ge by neutron transmutation doping (NTD) is demonstrated. Calculations show that the high uniformity of Ga acceptor distribution achieved by NTD significantly improves average gain. The negative factor of stronger ionized impurity scattering due to high compensation in NTD Ge is shown to be unremarkable for the gain at moderate doping concentrations sufficient for laser operation. Experimentally, this first NTD laser is found to have lower current-density lasing threshold than the best of a number of melt-doped laser crystals studied for comparison.

Fixed wavelength selection for the far-infrared p-Ge laser using patterned silicon etched mirrors

An etched silicon gold plated lamellar mirror is demonstrated as a fixed-wavelength intracavity selector for the far-infrared p-Ge laser, facilitating spectroscopic applications. The depth of the selective mirror, which defines the laser operation wavelength, can be precisely controlled during the etching process. The third-order Fabry-Perot resonance of this selector yields an active cavity finesse of at least 0.06.